Faculty Publications: November, 2016

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Title:
Direct numerical simulations of vortex ring collisions
Authors:
Ostilla Monico, Rodolfo; Pumir, Alain; Brenner, Michael
Publication:
APS Division of Fluid Dynamics (Fall) 2016, abstract #H3.007
Publication Date:
11/2016
Origin:
APS
DOI:
10.1103/BAPS.2016.DFD.H3.7
Bibliographic Code:
2016APS..DFD.H3007O

Abstract

We numerically simulate the ring vortex collision experiment of Lim and Nickels in an attempt to understand the rapid formation of very fine scale turbulence (or 'smoke') from relatively smooth initial conditions. Reynolds numbers of up to Re = Γ / ν = 7500 , where Γ is the vortex ring circulation and ν the kinematic viscosity of the fluid are reached, which coincide with the highest Reynolds number case of the experiments. Different perturbations to the ring vortex are added, and their effect on the generation and amplification of turbulence is quantified. The underlying dynamics of the vortex core is analyzed, and compared to the dynamics arising from a simple Biot-Savart filament model for the core.

 

Title:
Phototactic guidance of a tissue-engineered soft-robotic ray
Authors:
Park, Sung-Jin; Gazzola, Mattia; Park, Kyung Soo; Park, Shirley; di Santo, Valentina; Deisseroth, Karl; Lauder, George V.; Mahadevan, L.; Parker, Kevin Kit
Publication:
APS Division of Fluid Dynamics (Fall) 2016, abstract #G19.008
Publication Date:
11/2016
Origin:
APS
DOI:
10.1103/BAPS.2016.DFD.G19.8
Bibliographic Code:
2016APS..DFDG19008P

Abstract

Inspired by the relatively simple morphological blueprint provided by batoid fish such as stingrays and skates, we created a biohybrid system that enables an artificial animal, a tissue-engineered ray - to swim and phototactically follow a light cue. By patterning dissociated rat cardiomyocytes on an elastomeric body enclosing a microfabricated gold skeleton, we replicated fish morphology at 1/10 scale and captured basic fin deflection patterns of batoid fish. Optogenetics allows for phototactic guidance, steering, and turning maneuvers. Optical stimulation induced sequential muscle activation via serpentine-patterned muscle circuits, leading to coordinated undulatory swimming. The speed and direction of the ray was controlled by modulating light frequency and by independently eliciting right and left fins, allowing the biohybrid machine to maneuver through an obstacle course.

 

Title:
Outsourcing neural active control to passive composite mechanics: a tissue engineered cyborg ray
Authors:
Gazzola, Mattia; Park, Sung Jin; Park, Kyung Soo; Park, Shirley; di Santo, Valentina; Deisseroth, Karl; Lauder, George V.; Mahadevan, L.; Parker, Kevin Kit
Publication:
APS Division of Fluid Dynamics (Fall) 2016, abstract #G19.009
Publication Date:
11/2016
Origin:
APS
DOI:
10.1103/BAPS.2016.DFD.G19.9
Bibliographic Code:
2016APS..DFDG19009G

Abstract

Translating the blueprint that stingrays and skates provide, we create a cyborg swimming ray capable of orchestrating adaptive maneuvering and phototactic navigation. The impossibility of replicating the neural system of batoids fish is bypassed by outsourcing algorithmic functionalities to the body composite mechanics, hence casting the active control problem into a design, passive one. We present a first step in engineering multilevel "brain-body-flow" systems that couple sensory information to motor coordination and movement, leading to behavior. This work paves the way for the development of autonomous and adaptive artificial creatures able to process multiple sensory inputs and produce complex behaviors in distributed systems and may represent a path toward soft-robotic "embodied cognition".

 

Title:
Enhanced oil recovery with polymer flooding
Authors:
Parsa, Shima; Weitz, David
Publication:
APS Division of Fluid Dynamics (Fall) 2016, abstract #G22.004
Publication Date:
11/2016
Origin:
APS
DOI:
10.1103/BAPS.2016.DFD.G22.4
Bibliographic Code:
2016APS..DFDG22004P

Abstract

Polymer flooding is a method for enhanced oil recovery, however the mechanism responsible for the effectiveness of polymer flooding is not well understood. We use confocal microscopy and bulk transport measurements to probe the effectiveness of different molecular weight and concentrations of Polyacrylamide solution in imbibition of crude oil in 3D micromodel. We show that large molecular weight and moderate to high concentration of polymer is required for enhanced recovery. By directly measuring the pore level velocities in the medium, we show that polymer retention in the medium results in diversion of flow in some pores. The inhomogeneous changes in the flow velocities result in redistribution of viscous forces and enhanced recovery of oil.

 

Title:
Simulating wave-turbulence on thin elastic plates with arbitrary boundary conditions
Authors:
van Rees, Wim M.; Mahadevan, L.
Publication:
APS Division of Fluid Dynamics (Fall) 2016, abstract #L14.005
Publication Date:
11/2016
Origin:
APS
DOI:
10.1103/BAPS.2016.DFD.L14.5
Bibliographic Code:
2016APS..DFDL14005V

Abstract

The statistical characteristics of interacting waves are described by the theory of wave turbulence, with the study of deep water gravity wave turbulence serving as a paradigmatic physical example. Here we consider the elastic analog of this problem in the context of flexural waves arising from vibrations of a thin elastic plate. Such flexural waves generate the unique sounds of so-called thunder machines used in orchestras - thin metal plates that make a thunder-like sound when forcefully shaken. Wave turbulence in elastic plates is typically investigated numerically using spectral simulations with periodic boundary conditions, which are not very realistic. We will present the results of numerical simulations of the dynamics of thin elastic plates in physical space, with arbitrary shapes, boundary conditions, anisotropy and inhomogeneity, and show first results on wave turbulence beyond the conventionally studied rectangular plates. Finally, motivated by a possible method to measure ice-sheet thicknesses in the open ocean, we will further discuss the behavior of a vibrating plate when floating on an inviscid fluid.

 

Title:
Buckling of thin viscous sheets with inhomogenous viscosity under extensional flows
Authors:
Srinivasan, Siddarth; Wei, Zhiyan; Mahadevan, L.
Publication:
APS Division of Fluid Dynamics (Fall) 2016, abstract #M18.001
Publication Date:
11/2016
Origin:
APS
DOI:
10.1103/BAPS.2016.DFD.M18.1
Bibliographic Code:
2016APS..DFDM18001S

Abstract

We investigate the dynamics, shape and stability of a thin viscous sheet subjected to an extensional flow under an imposed non-uniform temperature field. Using finite element simulations, we first solve for the stretching flow to determine the pre-buckling sheet thickness and in-plane flow velocities. Next, we use this solution as the base state and solve the linearized partial differential equation governing the out-of-plane deformation of the mid-surface as a function of two dimensionless operating parameters: the normalized stretching ratio α and a dimensionless width of the heating zone β. We show the sheet can become unstable via a buckling instability driven by the development of localized compressive stresses, and determine the global shape and growth rates of the most unstable mode. The growth rate is shown to exhibit a transition from stationary to oscillatory modes in region upstream of the heating zone. Finally, we investigate the effect of surface tension and present an operating diagram that indicates regions of the parameter space that minimizes or entirely suppresses the instability while achieving desired outlet sheet thickness. Therefore, our work is directly relevant to various industrial processes including the glass redraw & float-glass method.

 

Title:
Dynamics of water uptake in spreading bacterial colonies
Authors:
Kaplan, C. Nadir; Mahadevan, L.
Publication:
APS Division of Fluid Dynamics (Fall) 2016, abstract #M39.004
Publication Date:
11/2016
Origin:
APS
DOI:
10.1103/BAPS.2016.DFD.M39.4
Bibliographic Code:
2016APS..DFDM39004K

Abstract

Bacteria can colonize a moist, nutrient-rich surface by secreting osmolytes to recruit water from the underlying substrate. We consider the outermost region of an expanding Escherichia coli biofilm, where the rim width is set by the cell growth rate and the colony expansion speed. Based on the hypothesis that sliding due to the mechanical contact between cells governs their speed, we model the interplay between the flow of cells and the water uptake via osmolyte production. This allows us to determine the front expansion speed and the non-uniform biofilm thickness, in agreement with experiments.

 

Title:
Twistronics: Manipulating the Electronic Properties of Two-dimensional Layered Structures through their Twist Angle
Authors:
Carr, Stephen; Massatt, Daniel; Fang, Shiang; Cazeaux, Paul; Luskin, Mitchell; Kaxiras, Efthimios
Publication:
eprint arXiv:1611.00649
Publication Date:
11/2016
Origin:
ARXIV
Keywords:
Condensed Matter - Mesoscale and Nanoscale Physics
Comment:
7 pages, 4 figures, to be submitted to Phys. Rev. B
Bibliographic Code:
2016arXiv161100649C

Abstract

The ability in experiments to control the relative twist angle between successive layers in two-dimensional (2D) materials offers a new approach to manipulating their electronic properties; we refer to this approach as "twistronics". A major challenge to theory is that, for arbitrary twist angles, the resulting structure involves incommensurate (aperiodic) 2D lattices. Here, we present a general method for the calculation of the electronic density of states of aperiodic 2D layered materials, using parameter-free hamiltonians derived from ab initio density-functional theory. We use graphene, a semimetal, and MoS$_2$, a representative of the transition metal dichalcogenide (TMDC) family of 2D semiconductors, to illustrate the application of our method, which enables fast and efficient simulation of multi-layered stacks in the presence of local disorder and external fields. We comment on the interesting features of their Density of States (DoS) as a function of twist-angle and local configuration and on how these features can be experimentally observed.

 

Title:
Imaging the Spin Texture of a Skyrmion Under Ambient Conditions Using an Atomic-Sized Sensor
Authors:
Dovzhenko, Yuliya; Casola, Francesco; Schlotter, Sarah; Zhou, Tony X.; Büttner, Felix; Walsworth, Ronald L.; Beach, Geoffrey S. D.; Yacoby, Amir
Publication:
eprint arXiv:1611.00673
Publication Date:
11/2016
Origin:
ARXIV
Keywords:
Condensed Matter - Strongly Correlated Electrons, Condensed Matter - Mesoscale and Nanoscale Physics
Comment:
Paper and supplementary informaion: 17 pages, 8 figures total
Bibliographic Code:
2016arXiv161100673D

Abstract

Magnetic skyrmions are two-dimensional non-collinear spin textures characterised by an integer topological number. They commonly crystallise at low temperatures in bulk noncentrosymmetric ferromagnets where the lack of inversion symmetry leads to an antisymmetric component of the exchange interaction. Recently, stable room-temperature skyrmions were reported in stacks of thin magnetic films where antisymmetric exchange results from broken symmetry at the interface. Determining the spin structure of these technologically-relevant skyrmions in the presence of external magnetic fields has remained a key experimental challenge. Here, we use the single electron spin of a Nitrogen-Vacancy (NV) centre in diamond to perform scanning magnetometry of skyrmions in Pt/Co/Ta multilayers under ambient conditions. We introduce a novel method to assess the manifold of spin textures compatible with the measured local magnetic fields and identify physically allowed configurations based on the topology of the resulting solution. We determine that the underlying magnetization pattern for the skyrmion is consistent with a cycloid (or N\'eel)-like spin texture, in agreement with theoretical predictions for interfacial antisymmetric exchange interaction. Our results open up wide-ranging possibilities in quantitative model-free imaging of two-dimensional spin structures using scanning NV magnetometry.

 

Title:
Rare region effects and dynamics near the many-body localization transition
Authors:
Agarwal, Kartiek; Altman, Ehud; Demler, Eugene; Gopalakrishnan, Sarang; Huse, David A.; Knap, Michael
Publication:
eprint arXiv:1611.00770
Publication Date:
11/2016
Origin:
ARXIV
Keywords:
Condensed Matter - Disordered Systems and Neural Networks
Comment:
Review, 14 pages + acknowledgements + Refs
Bibliographic Code:
2016arXiv161100770A

Abstract

The low-frequency response of systems near the many-body localization phase transition, on either side of the transition, is dominated by contributions from rare regions that are locally "in the other phase", i.e., rare localized regions in a system that is typically thermal, or rare thermal regions in a system that is typically localized. Rare localized regions affect the properties of the thermal phase, especially in one dimension, by acting as bottlenecks for transport and the growth of entanglement, whereas rare thermal regions in the localized phase act as local "baths" and dominate the low-frequency response of the MBL phase. We review recent progress in understanding these rare-region effects, and discuss some of the open questions associated with them: in particular, whether and in what circumstances a single rare thermal region can destabilize the many-body localized phase.

 

Title:
Perfect state transfer on graphs with a potential
Authors:
Kempton, Mark; Lippner, Gabor; Yau, Shing-Tung
Publication:
eprint arXiv:1611.02093
Publication Date:
11/2016
Origin:
ARXIV
Keywords:
Mathematics - Combinatorics, Quantum Physics, 05C50
Comment:
We mistakenly attributed a conjecture to Godsil in the first version. This is now corrected, as well are a few typos
Bibliographic Code:
2016arXiv161102093K

Abstract

In this paper we study quantum state transfer (also called quantum tunneling) on graphs when there is a potential function on the vertex set. We present two main results. First, we show that for paths of length greater than three, there is no potential on the vertices of the path for which perfect state transfer between the endpoints can occur. In particular, this answers a question raised by Godsil in Section 20 of [8]. Second, we show that if a graph has two vertices that share a common neighborhood, then there is a potential on the vertex set for which perfect state transfer will occur between those two vertices. This gives numerous examples where perfect state transfer does not occur without the potential, but adding a potential makes perfect state transfer possible. In addition, we investigate perfect state transfer on graph products, which gives further examples where perfect state transfer can occur.

 

Title:
Hot Particles Attract in a Cold Bath
Authors:
Tanaka, Hidenori; Lee, Alpha A.; Brenner, Michael P.
Publication:
eprint arXiv:1611.02234
Publication Date:
11/2016
Origin:
ARXIV
Keywords:
Condensed Matter - Soft Condensed Matter, Condensed Matter - Statistical Mechanics, Physics - Biological Physics
Bibliographic Code:
2016arXiv161102234T

Abstract

Controlling interactions out of thermodynamic equilibrium is crucial for designing addressable and functional self-organizing structures. These active interactions also underpin collective behavior in biological systems. Here we study a general setting of active particles in a bath of passive particles, and demonstrate a novel mechanism for long ranged attraction between active particles. The mechanism operates when the translational persistence length of the active particle motion is smaller than the particle diameter. In this limit, the system reduces to particles of higher diffusivity ("hot" particles) in a bath of particles with lower diffusivity ("cold" particles). This attractive interaction arises as a hot particle pushes cold particles away to create a large hole around itself, and the holes interact via a depletion-like attraction even though all particles have the same size. Although the mechanism occurs outside the parameter range of typical biological organisms, the mechanism could be realized in the laboratory.

 

Title:
Landau level evolution driven by band hybridization in mirror symmetry broken ABA-stacked trilayer graphene
Authors:
Shimazaki, Yuya; Yoshizawa, Toru; Borzenets, Ivan V.; Wang, Ke; Liu, Xiaomeng; Watanabe, Kenji; Taniguchi, Takashi; Kim, Philip; Yamamoto, Michihisa; Tarucha, Seigo
Publication:
eprint arXiv:1611.02395
Publication Date:
11/2016
Origin:
ARXIV
Keywords:
Condensed Matter - Mesoscale and Nanoscale Physics
Comment:
9 pages, 5 figures
Bibliographic Code:
2016arXiv161102395S

Abstract

Layer stacking and crystal lattice symmetry play important roles in the band structure and the Landau levels of multilayer graphene. ABA-stacked trilayer graphene possesses mirror-symmetry-protected monolayer-like and bilayer-like band structures. Broken mirror symmetry by a perpendicular electric field therefore induces hybridization between these bands and various quantum Hall phases emerge. We experimentally explore the evolution of Landau levels in ABA-stacked trilayer graphene under electric field. We observe a variety of valley and orbital dependent Landau level evolutions. These evolutions are qualitatively well explained by considering the hybridization between multiple Landau levels possessing close Landau level indices and the hybridization between every third Landau level orbitals due to the trigonal warping effect. These observations are consistent with numerical calculations. The combination of experimental and numerical analysis thus reveals the entire picture of Landau level evolutions decomposed into the monolayer- and bilayer-like band contributions in ABA-stacked trilayer graphene.

 

Title:
Rotation of an immersed cylinder sliding near a thin elastic coating
Authors:
Rallabandi, Bhargav; Saintyves, Baudouin; Jules, Theo; Salez, Thomas; Schönecker, Clarissa; Mahadevan, L.; Stone, Howard A.
Publication:
eprint arXiv:1611.03552
Publication Date:
11/2016
Origin:
ARXIV
Keywords:
Physics - Fluid Dynamics
Bibliographic Code:
2016arXiv161103552R

Abstract

It is well known that an object translating parallel to a soft wall produces viscous stresses and a pressure field that deform the wall, which, in turn, results in a lift force on the object. Recent experiments on cylinders sliding near a soft incline under gravity confirmed previously developed theoretical arguments, but also reported an unexplained rotation of the cylinder at steady state (Saintyves et al. \emph{PNAS} 113(21), 2016). Here, we use the Lorentz reciprocal theorem to calculate the angular velocity of an infinite cylinder sliding near a soft incline, in the lubrication limit. Our results show that the softness-induced angular velocity of the cylinder is quadratic in the deformation of the elastic layer. This implies that a cylinder sliding parallel to a soft wall without rotation experiences an elastohydrodynamic torque that is proportional to the cube of the sliding speed. We compare the theoretical predictions of the rotation speed with experimental measurements. We then develop scaling and symmetry arguments that are generally applicable to hydrodynamically mediated interactions between soft systems, such as those in biological and geophysical settings.

 

Title:
Cores in Dwarf Galaxies from Fermi Repulsion
Authors:
Randall, Lisa; Scholtz, Jakub; Unwin, James
Publication:
eprint arXiv:1611.04590
Publication Date:
11/2016
Origin:
ARXIV
Keywords:
Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics, High Energy Physics - Phenomenology
Comment:
12 pages, 7 figures
Bibliographic Code:
2016arXiv161104590R

Abstract

We show that Fermi repulsion can lead to cored density profiles in dwarf galaxies for sub-keV fermionic dark matter. We treat the dark matter as a quasi-degenerate self-gravitating Fermi gas and calculate its density profile assuming hydrostatic equilibrium. We find that suitable dwarf galaxy cores of larger than 130 pc can be achieved for fermion dark matter with mass in the range 70 eV - 400 eV. While in conventional dark matter scenarios, such sub-keV thermal dark matter would be excluded by free streaming bounds, the constraints are ameliorated in models with dark matter at lower temperature than conventional thermal scenarios, such as the Flooded Dark Matter model that we have previously considered. Modifying the arguments of Tremaine and Gunn we derive a conservative lower bound on the mass of fermionic dark matter of 70 eV and a stronger lower bound from Lyman-$\alpha$ clouds of about 470 eV, leading to slightly smaller cores than have been observed. We comment on this result and how the tension is relaxed in dark matter scenarios with non-thermal momentum distributions.

 

Title:
Investigating the Transition Region in Scanned Probe Images of the Cyclotron Orbit in Graphene
Authors:
Bhandari, Sagar; Lin, Andrew; Westervelt, Robert M.
Publication:
eprint arXiv:1611.04865
Publication Date:
11/2016
Origin:
ARXIV
Keywords:
Condensed Matter - Mesoscale and Nanoscale Physics
Comment:
4 pages, 3 figures
Bibliographic Code:
2016arXiv161104865B

Abstract

A cooled scanning probe microscope (SPM) has been used to image cyclotron orbits of electrons through high-mobility graphene in a magnetic field.1-5 In a hBN-graphene-hBN device patterned into a hall bar geometry, the magnetic field focuses a current Ii injected from one narrow contact into another narrow contact located an integer number of cyclotron diameters away, creating a voltage Vc. The degree of focusing is measured by the transresistance Rm = Vc/Ii. In SPM, the tip can either enhance or decrease conductance in the sample by deflecting electrons into or away from the second contact, respectively.3,4 Our SPM images of magnetic focusing feature a region in which the tip transitions from enhancing to decreasing the conductance in the sample where the change in transresistance caused by the tip is equal to zero. In this paper, we investigate how the location of this region in the graphene sample changes as we modulate the electron density n and magnetic field B. By plotting line-cuts of the change in trans-resistance for different electron densities and magnetic fields, we identify trends in the inflection point where the tip changes from enhancing to decreasing the conductance in the sample. From the location of each transition region, we show that the cyclotron diameter of the electron trajectories can be obtained, and explain the trends in inflection point location for different electron densities and magnetic fields.

 

Title:
Duality and bosonization of (2+1)d Majorana fermions
Authors:
Metlitski, Max A.; Vishwanath, Ashvin; Xu, Cenke
Publication:
eprint arXiv:1611.05049
Publication Date:
11/2016
Origin:
ARXIV
Keywords:
Condensed Matter - Strongly Correlated Electrons
Comment:
11 pages
Bibliographic Code:
2016arXiv161105049M

Abstract

We construct a dual bosonized description of a massless Majorana fermion in $(2+1)d$. In contrast to Dirac fermions, for which a bosonized description can be constructed using a flux attachment procedure, neutral Majorana fermions call for a different approach. We argue that the dual theory is an $SO(N)_1$ Chern-Simons gauge theory with a critical $SO(N)$ vector bosonic matter field ($N \geq 3$). The monopole of the $SO(N)$ gauge field is identified with the Majorana fermion. We provide evidence for the duality by establishing the correspondence of adjacent gapped phases and by a parton construction. We also propose a generalization of the duality to $N_f$ flavors of Majorana fermions, and discuss possible resolutions of a caveat associated with an emergent global $Z_2$ symmetry. Finally, we conjecture a dual description of an $\mathcal{N} = 1$ supersymmetric fixed point in $(2+1)d$, which is realized by tuning a single flavor of Majorana fermions to an interacting (Gross-Neveu) critical point.

 

Title:
Nanodiamond-enhanced MRI
Authors:
Waddington, David E. J.; Sarracanie, Mathieu; Zhang, Huiliang; Salameh, Najat; Glenn, David R.; Rej, Ewa; Gaebel, Torsten; Boele, Thomas; Walsworth, Ronald L.; Reilly, David J.; Rosen, Matthew S.
Publication:
eprint arXiv:1611.05167
Publication Date:
11/2016
Origin:
ARXIV
Keywords:
Condensed Matter - Mesoscale and Nanoscale Physics
Bibliographic Code:
2016arXiv161105167W

Abstract

Nanodiamonds are of interest as nontoxic substrates for targeted drug delivery and as highly biostable fluorescent markers for cellular tracking. Beyond optical techniques however, options for noninvasive imaging of nanodiamonds in vivo are severely limited. Here, we demonstrate that the Overhauser effect, a proton-electron double resonance technique developed to detect free radicals in vivo, can enable high contrast magnetic resonance imaging (MRI) of nanodiamonds in water at room temperature and ultra-low magnetic field. The technique transfers spin polarization from paramagnetic impurities at nanodiamond surfaces to $^1$H spins in the surrounding water solution, creating MRI contrast on-demand. We further examine the conditions required for maximum enhancement as well as the ultimate sensitivity of the technique. The ability to perform continuous hyperpolarization via the Overhauser mechanism, in combination with excellent in vivo stability, raises the possibility of performing noninvasive tracking of nanodiamonds over indefinitely long periods of time.

 

Title:
Topological Materials Discovery Through the Lens of Electron Filling Constraints
Authors:
Chen, Ru; Po, Hoi Chun; Neaton, Jeffrey B.; Vishwanath, Ashvin
Publication:
eprint arXiv:1611.06860
Publication Date:
11/2016
Origin:
ARXIV
Keywords:
Condensed Matter - Materials Science, Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Strongly Correlated Electrons
Comment:
4.5 + 2 pages; 4 figures
Bibliographic Code:
2016arXiv161106860C

Abstract

Nodal semimetals, materials systems with nodal-point or -line Fermi surfaces, are much sought after for their novel transport and topological properties. Identification of experimental materials candidates, however, has mainly relied on exhaustive database searches. Here we show how recent studies on the interplay between electron filling and nonsymmorphic space-group symmetries can guide the search for nodal semimetals which are `filling-enforced'. We recast the previously-derived constraints on the allowed electron fillings for band insulators in the 230 space groups into a new form, which enables effective screening of materials candidates based solely on their (1) space group, (2) electron count in the formula unit, and (3) multiplicity of the formula unit. This criterion greatly reduces the number of candidate materials in a database of previously-synthesized compounds, and helps us to efficiently identify a handful of candidates, including the monoclinic crystals Ca$_2$GaPt$_2$, AgF$_2$, Ca$_2$InOsO$_6$ and Ru(PO$_3$)$_3$, and the orthorhombic crystal CsHg$_2$, from sampling only a few nonsymmorphic space groups. Based on ab initio calculations, we show that these materials have filling-enforced Dirac nodes near the Fermi energy. In addition, we also identify CaGaPt as a promising filling-enforced Dirac-ring semimetal candidate.

 

Title:
Control and Local Measurement of the Spin Chemical Potential in a Magnetic Insulator
Authors:
Du, Chunhui; Van der Sar, Toeno; Zhou, Tony X.; Upadhyaya, Pramey; Casola, Francesco; Zhang, Huiliang; Onbasli, Mehmet C.; Ross, Caroline A.; Walsworth, Ronald L.; Tserkovnyak, Yaroslav; Yacoby, Amir
Publication:
eprint arXiv:1611.07408
Publication Date:
11/2016
Origin:
ARXIV
Keywords:
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Materials Science
Comment:
18 pages, 4 figures
Bibliographic Code:
2016arXiv161107408D

Abstract

The spin chemical potential characterizes the tendency of spins to diffuse. Probing the spin chemical potential could provide insight into materials such as magnetic insulators and spin liquids and aid optimization of spintronic devices. Here, we introduce single-spin magnetometry as a generic platform for non-perturbative, nanoscale characterization of spin chemical potentials. We use this platform to investigate magnons in a magnetic insulator, surprisingly finding that the magnon chemical potential can be efficiently controlled by driving the system's ferromagnetic resonance. We introduce a symmetry-based two-fluid theory describing the underlying magnon processes, realize the first e

 

Title:
Superrotation Charge and Supertranslation Hair on Black Holes
Authors:
Hawking, Stephen W.; Perry, Malcolm J.; Strominger, Andrew
Publication:
eprint arXiv:1611.09175
Publication Date:
11/2016
Origin:
ARXIV
Keywords:
High Energy Physics - Theory
Comment:
42 pages
Bibliographic Code:
2016arXiv161109175H

Abstract

It is shown that black hole spacetimes in classical Einstein gravity are characterized by, in addition to their ADM mass $M$, momentum $\vec P$, angular momentum $\vec J$ and boost charge $\vec K$, an infinite head of supertranslation hair. The distinct black holes are distinguished by classical superrotation charges measured at infinity. Solutions with supertranslation hair are diffeomorphic to the Schwarzschild spacetime, but the diffeomorphisms are part of the BMS subgroup and act nontrivially on the physical phase space. It is shown that a black hole can be supertranslated by throwing in an asymmetric shock wave. A leading-order Bondi-gauge expression is derived for the linearized horizon supertranslation charge and shown to generate, via the Dirac bracket, supertranslations on the linearized phase space of gravitational excitations of the horizon. The considerations of this paper are largely classical augmented by comments on their implications for the quantum theory.

 

Title:
More on the admissible condition on differentiable maps $\varphi: (X^{\!A\!z},E;\nabla)\rightarrow Y$ in the construction of the non-Abelian Dirac-Born-Infeld action $S_{DBI}(\varphi,\nabla)$
Authors:
Liu, Chien-Hao; Yau, Shing-Tung
Publication:
eprint arXiv:1611.09439
Publication Date:
11/2016
Origin:
ARXIV
Keywords:
High Energy Physics - Theory, Mathematics - Algebraic Geometry, Mathematics - Differential Geometry, Mathematics - Symplectic Geometry, 81T30, 53C05, 16S50, 14A22, 35R01
Comment:
15+2 pp, 2+1 figures
Bibliographic Code:
2016arXiv161109439L

Abstract

In D(13.1) (arXiv:1606.08529 [hep-th]), we introduced an admissible condition on differentiable maps $\varphi: (X^{\!A\!z}, E;\nabla)\rightarrow Y$ from an Azumaya/matrix manifold $X^{\!A\!z}$ (with the fundamental module $E$) with a connection $\nabla$ on $E$ to a manifold $Y$ in order to resolve a pull-push issue in the construction of a non-Abelian-Dirac-Infeld action $S_{DBI}$ for $(\varphi,\nabla)$ and to render $\nabla$ massless from the aspect of open strings. The admissible condition ibidem consists of two parts: Condition (1) and Condition (2). In this brief note, we examine these two conditions in more detail and bring their geometric implications on $(\varphi,\nabla)$ and the full action $S_{DBI}(\varphi,\nabla)+S_{CS/WZ}(\varphi,\nabla)$ more transparent. In particular, we show that Condition (1) alone already implies masslessness of $\nabla$ from open-string aspect; and that the additional Condition (2) implies a decoupling of the nilpotent fuzzy cloud of $\varphi(X^{\!A\!z})$ to the dynamics of $(\varphi, \nabla)$. We conclude with a refined definition of admissible $(\varphi,\nabla)$ and a remark on the anomaly factor in the integrand of the Chern-Simons/Wess-Zumino term $S_{CS/WZ}(\varphi,\nabla)$ for D-branes based on the current study.

 

Title:
Stability of metallic edges and Fermi-level pinning in transition-metal dichalcogenide nanoribbons
Authors:
Davelou, Daphne; Kopidakis, Georgios; Kaxiras, Efthimios; Remediakis, Ioannis N.
Publication:
eprint arXiv:1611.09535
Publication Date:
11/2016
Origin:
ARXIV
Keywords:
Condensed Matter - Materials Science
Comment:
8 pages, 5 figures
Bibliographic Code:
2016arXiv161109535D

Abstract

Nanoribbons of MoS$_2$ present a unique electronic structure that consists of a semiconducting bulk bounded by metallic edges; same holds for other Transition-Metal Dichalcogenides (TMDs) (Mo-,W-,S$_{2}$,Se$_{2}$). We perform first-principles calculations for TMD nanoribbons with reconstructed zig-zag metal terminated edges that contain chalcogen adatoms. All nanorobbons have possitive edge energies when the chemical potential of chalcogens is close to the energy of solids, and negative edge energies for high chemical potential. The reconstruction with two chalcogen adatoms is expected to be the most stable one. In all nanoribbons, a metallic phase is found near their edges, with the Fermi level of this metallic phase being lower than the Fermi level of the 2D material.

 

Title:
Exploring Cosmic Origins with CORE: Cosmological Parameters
Authors:
Di Valentino, Eleonora; Brinckmann, Thejs; Gerbino, Martina; Poulin, Vivian;... ; Dvorkin, Cora;... and 59 coauthors; the CORE collaboration
Publication:
eprint arXiv:1612.00021
Publication Date:
11/2016
Origin:
ARXIV
Keywords:
Astrophysics - Cosmology and Nongalactic Astrophysics, General Relativity and Quantum Cosmology, High Energy Physics - Phenomenology, High Energy Physics - Theory
Comment:
90 pages, 25 Figures
Bibliographic Code:
2016arXiv161200021D

Abstract

We forecast the main cosmological parameter constraints achievable with the CORE space mission which is dedicated to mapping the polarisation of the Cosmic Microwave Background (CMB). CORE was recently submitted in response to ESA's fifth call for medium-sized mission proposals (M5). Here we report the results from our pre-submission study of the impact of various instrumental options, in particular the telescope size and sensitivity level, and review the great, transformative potential of the mission as proposed. Specifically, we assess the impact on a broad range of fundamental parameters of our Universe as a function of the expected CMB characteristics, with other papers in the series focusing on controlling astrophysical and instrumental residual systematics. In this paper, we assume that only a few central CORE frequency channels are usable for our purpose, all others being devoted to the cleaning of astrophysical contaminants. On the theoretical side, we assume LCDM as our general framework and quantify the improvement provided by CORE over the current constraints from the Planck 2015 release. We also study the joint sensitivity of CORE and of future Baryon Acoustic Oscillation and Large Scale Structure experiments like DESI and Euclid. Specific constraints on the physics of inflation are presented in another paper of the series. In addition to the six parameters of the base LCDM, which describe the matter content of a spatially flat universe with adiabatic and scalar primordial fluctuations from inflation, we derive the precision achievable on parameters like those describing curvature, neutrino physics, extra light relics, primordial helium abundance, dark matter annihilation, recombination physics, variation of fundamental constants, dark energy, modified gravity, reionization and cosmic birefringence. (ABRIDGED)

 

Title:
Performance of pile-up mitigation techniques for jets in pp collisions at √{s}=8 TeV using the ATLAS detector
Authors:
Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2851 coauthors
Publication:
The European Physical Journal C, Volume 76, Issue 11, article id.581, 36 pp. (EPJC Homepage)
Publication Date:
11/2016
Origin:
SPRINGER
Abstract Copyright:
(c) 2016: CERN for the benefit of the ATLAS collaboration
DOI:
10.1140/epjc/s10052-016-4395-z
Bibliographic Code:
2016EPJC...76..581A

Abstract

The large rate of multiple simultaneous proton-proton interactions, or pile-up, generated by the Large Hadron Collider in Run 1 required the development of many new techniques to mitigate the adverse effects of these conditions. This paper describes the methods employed in the ATLAS experiment to correct for the impact of pile-up on jet energy and jet shapes, and for the presence of spurious additional jets, with a primary focus on the large 20.3 fb^{-1} data sample collected at a centre-of-mass energy of √{s} = 8 TeV. The energy correction techniques that incorporate sophisticated estimates of the average pile-up energy density and tracking information are presented. Jet-to-vertex association techniques are discussed and projections of performance for the future are considered. Lastly, the extension of these techniques to mitigate the effect of pile-up on jet shapes using subtraction and grooming procedures is presented.

 

Title:
Search for minimal supersymmetric standard model Higgs Bosons H / A and for a Z^' } boson in the τ τ final state produced in pp collisions at √{s}= 13 TeV with the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.;... Franklin, M.;... Huth, J.;... Morii, M.; ... and 2852 coauthors
Publication:
The European Physical Journal C, Volume 76, Issue 11, article id.585, 30 pp. (EPJC Homepage)
Publication Date:
11/2016
Origin:
SPRINGER
Abstract Copyright:
(c) 2016: The Author(s)
DOI:
10.1140/epjc/s10052-016-4400-6
Bibliographic Code:
2016EPJC...76..585A

Abstract

A search for neutral Higgs bosons of the minimal supersymmetric standard model (MSSM) and for a heavneutral Z^' } boson is performed using a data sample corresponding to an integrated luminosity of 3.2 fb^{-1} from proton-proton collisions at √{s} = 13 {TeV} recorded by the ATLAS detector at the LHC. The heavy resonance is assumed to decay to a τ ^+ τ ^- pair with at least one τ lepton decaying to final states with hadrons and a neutrino. The search is performed in the mass range of 0.2-1.2 {TeV} for the MSSM neutral Higgs bosons and 0.5-2.5 {TeV} for the heavy neutral Z^' } boson. The data are in good agreement with the background predicted by the Standard Model. The results are interpreted in MSSM and Z^' } benchmark scenarios. The most stringent constraints on the MSSM m_A-tan β space exclude at 95 % confidence level (CL) tan β > 7.6 for m_A = 200 {GeV} in the mh^{ {mod+}} MSSM scenario. For the Sequential Standard Model, a Z^' }_SSM mass up to 1.90 {TeV} is excluded at 95 % CL and masses up to 1.82-2.17 {TeV} are excluded for a Z^' }_{SFM} of the strong flavour model.

 

Title:
Search for the Higgs boson produced in association with a W boson and decaying to four b-quarks via two spin-zero particles in pp collisions at 13 TeV with the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.;... Franklin, M.;... Huth, J.;... Morii, M.; ... and 2848 coauthors
Publication:
The European Physical Journal C, Volume 76, Issue 11, article id.605, 31 pp. (EPJC Homepage)
Publication Date:
11/2016
Origin:
SPRINGER
Abstract Copyright:
(c) 2016: CERN for the benefit of the ATLAS collaboration
DOI:
10.1140/epjc/s10052-016-4418-9
Bibliographic Code:
2016EPJC...76..605A

Abstract

This paper presents a dedicated search for exotic decays of the Higgs boson to a pair of new spin-zero particles, H → aa, where the particle a decays to b-quarks and has a mass in the range of 20-60 GeV. The search is performed in events where the Higgs boson is produced in association with a W boson, giving rise to a signature of a lepton (electron or muon), missing transverse momentum, and multiple jets from b-quark decays. The analysis is based on the full dataset of pp collisions at √{s} = 13 {TeV} recorded in 2015 by the ATLAS detector at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 3.2 { fb }^{-1}. No significant excess of events above the Standard Model prediction is observed, and a 95 % confidence-level upper limit is derived for the product of the production cross section for pp → WH times the branching ratio for the decay H → aa → 4b. The upper limit ranges from 6.2 pb for an a-boson mass m_a = 20 {GeV} to 1.5 pb for m_a = 60 {GeV}.

 

Title:
The future of primordial features with large-scale structure surveys
Authors:
Chen, Xingang; Dvorkin, Cora; Huang, Zhiqi; Namjoo, Mohammad Hossein; Verde, Licia
Publication:
Journal of Cosmology and Astroparticle Physics, Issue 11, article id. 014 (2016). (JCAP Homepage)
Publication Date:
11/2016
Origin:
IOP
DOI:
10.1088/1475-7516/2016/11/014
Bibliographic Code:
2016JCAP...11..014C

Abstract

Primordial features are one of the most important extensions of the Standard Model of cosmology, providing a wealth of information on the primordial Universe, ranging from discrimination between inflation and alternative scenarios, new particle detection, to fine structures in the inflationary potential. We study the prospects of future large-scale structure (LSS) surveys on the detection and constraints of these features. We classify primordial feature models into several classes, and for each class we present a simple template of power spectrum that encodes the essential physics. We study how well the most ambitious LSS surveys proposed to date, including both spectroscopic and photometric surveys, will be able to improve the constraints with respect to the current Planck data. We find that these LSS surveys will significantly improve the experimental sensitivity on features signals that are oscillatory in scales, due to the 3D information. For a broad range of models, these surveys will be able to reduce the errors of the amplitudes of the features by a factor of 5 or more, including several interesting candidates identified in the recent Planck data. Therefore, LSS surveys offer an impressive opportunity for primordial feature discovery in the next decade or two. We also compare the advantages of both types of surveys.

 

Title:
Study of hard double-parton scattering in four-jet events in pp collisions at √{s}=7 TeV with the ATLAS experiment
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.; Abdallah, J.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2849 coauthors
Publication:
Journal of High Energy Physics, Volume 2016, Issue 11, article id.110, 52 pp.
Publication Date:
11/2016
Origin:
SPRINGER
Keywords:
Hadron-Hadron scattering (experiments)
Abstract Copyright:
(c) 2016: The Author(s)
DOI:
10.1007/JHEP11(2016)110
Bibliographic Code:
2016JHEP...11..110A

Abstract

Inclusive four-jet events produced in proton-proton collisions at a centre-of-mass energy of √{s}=7 TeV are analysed for the presence of hard double-parton scattering using data corresponding to an integrated luminosity of 37 .3 pb-1, collected with the ATLAS detector at the LHC. The contribution of hard double-parton scattering to the production of four-jet events is extracted using an artificial neural network, assuming that hard double-parton scattering can be approximated by an uncorrelated overlaying of dijet events. For events containing at least four jets with transverse momentum p T ≥ 20 GeV and pseudorapidity | η| ≤ 4 .4, and at least one having p T ≥ 42 .5 GeV, the contribution of hard double-parton scattering is estimated to be f DPS = 0.092 - 0.011 + 0.005 (stat.) - 0.037 + 0.033 (syst.). After combining this measurement with those of the inclusive dijet and four-jet cross-sections in the appropriate phase space regions, the effective cross-section, σ eff , was determined to be σ eff = 14. 9 - 1.0 + 1.2 (stat.) - 3.8 + 5.1 (syst.) mb. This result is consistent within the quoted uncertainties with previous measurements of σ eff, performed at centre-of-mass energies between 63 GeV and 8 TeV using various final states, and it corresponds to 21 - 6 + 7 % of the total inelastic cross-section measured at √{s}=7 TeV. The distributions of the observables sensitive to the contribution of hard double-parton scattering, corrected for detector effects, are also provided. [Figure not available: see fulltext.]

 

Title:
Search for the Standard Model Higgs boson produced by vector-boson fusion and decaying to bottom quarks in √{s}=8 TeV pp collisions with the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.; ... Franklin, M.;... Huth, J.;... Morii, M.;... ; and 2846 coauthors
Publication:
Journal of High Energy Physics, Volume 2016, Issue 11, article id.112, 37 pp.
Publication Date:
11/2016
Origin:
SPRINGER
Keywords:
Hadron-Hadron scattering (experiments), Higgs physics, proton-proton scattering
Abstract Copyright:
(c) 2016: The Author(s)
DOI:
10.1007/JHEP11(2016)112
Bibliographic Code:
2016JHEP...11..112A

Abstract

A search with the ATLAS detector is presented for the Standard Model Higgs boson produced by vector-boson fusion and decaying to a pair of bottom quarks, using 20.2 fb-1 of LHC proton-proton collision data at √{s}=8 TeV. The signal is searched for as a resonance in the invariant mass distribution of a pair of jets containing b-hadrons in vector-boson-fusion candidate events. The yield is measured to be -0.8 ± 2.3 times the Standard Model cross-section for a Higgs boson mass of 125 GeV. The upper limit on the cross-section times the branching ratio is found to be 4.4 times the Standard Model cross-section at the 95% confidence level, consistent with the expected limit value of 5.4 (5.7) in the background-only (Standard Model production) hypothesis. [Figure not available: see fulltext.]

 

Title:
A measurement of material in the ATLAS tracker using secondary hadronic interactions in 7 TeV pp collisions
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.; ... Franklin, M.;... Huth, J.;... Morii, M.;... ; and 2852 coauthors
Publication:
Journal of Instrumentation, Volume 11, Issue 11, pp. P11020 (2016).
Publication Date:
11/2016
Origin:
IOP
DOI:
10.1088/1748-0221/11/11/P11020
Bibliographic Code:
2016JInst..11P1020A

Abstract

Knowledge of the material in the ATLAS inner tracking detector is crucial in understanding the reconstruction of charged-particle tracks, the performance of algorithms that identify jets containing b-hadrons and is also essential to reduce background in searches for exotic particles that can decay within the inner detector volume. Interactions of primary hadrons produced in pp collisions with the material in the inner detector are used to map the location and amount of this material. The hadronic interactions of primary particles may result in secondary vertices, which in this analysis are reconstructed by an inclusive vertex-finding algorithm. Data were collected using minimum-bias triggers by the ATLAS detector operating at the LHC during 2010 at centre-of-mass energy √s = 7 TeV, and correspond to an integrated luminosity of 19 nb-1. Kinematic properties of these secondary vertices are used to study the validity of the modelling of hadronic interactions in simulation. Secondary-vertex yields are compared between data and simulation over a volume of about 0.7 m3 around the interaction point, and agreement is found within overall uncertainties.

 

Title:
Holographic maps of quasiparticle interference
Authors:
Dalla Torre, Emanuele G.; He, Yang; Demler, Eugene
Publication:
Nature Physics, Volume 12, Issue 11, pp. 1052-1056 (2016).
Publication Date:
11/2016
Origin:
NATURE
Abstract Copyright:
(c) 2016: Nature Publishing Group
DOI:
10.1038/nphys3829
Bibliographic Code:
2016NatPh..12.1052D

Abstract

The analysis of Fourier-transformed scanning tunnelling microscopy images with subatomic resolution is a common tool for studying the properties of quasiparticle excitations in strongly correlated materials. Although Fourier amplitudes are generally complex valued, earlier analysis primarily focused on their absolute values. Their complex phases were often deemed random, and thus irrelevant, due to the unknown positions of the impurities in the sample. Here we show how to factor out these random phases by analysing overlaps between Fourier amplitudes that differ by reciprocal lattice vectors. The resulting holographic maps provide important and previously unknown information about the electronic structures. When applied to superconducting cuprates, our method solves a long-standing puzzle of the dichotomy between equivalent wavevectors. We show that d-wave Wannier functions of the conduction band provide a natural explanation for experimental results that were interpreted as evidence for competing unconventional charge modulations. Our work opens a new pathway to identify the nature of electronic states in scanning tunnelling microscopy.

 

Title:
Establishing the limits of efficiency of perovskite solar cells from first principles modeling
Authors:
Grånäs, Oscar; Vinichenko, Dmitry; Kaxiras, Efthimios
Publication:
Nature Scientific Reports, Volume 6, id. 36108 (2016).
Publication Date:
11/2016
Origin:
NATURE
Abstract Copyright:
(c) 2016: The Author(s)
DOI:
10.1038/srep36108
Bibliographic Code:
2016NatSR...636108G

Abstract

The recent surge in research on metal-halide-perovskite solar cells has led to a seven-fold increase of efficiency, from ~3% in early devices to over 22% in research prototypes. Oft-cited reasons for this increase are: (i) a carrier diffusion length reaching hundreds of microns; (ii) a low exciton binding energy; and (iii) a high optical absorption coefficient. These hybrid organic-inorganic materials span a large chemical space with the perovskite structure. Here, using first-principles calculations and thermodynamic modelling, we establish that, given the range of band-gaps of the metal-halide-perovskites, the theoretical maximum efficiency limit is in the range of ~25-27%. Our conclusions are based on the effect of level alignment between the perovskite absorber layer and carrier-transporting materials on the performance of the solar cell as a whole. Our results provide a useful framework for experimental searches toward more efficient devices.

 

Title:
Strong quantum scarring by local impurities
Authors:
Luukko, Perttu J. J.; Drury, Byron; Klales, Anna; Kaplan, Lev; Heller, Eric J.; Räsänen, Esa
Publication:
Nature Scientific Reports, Volume 6, id. 37656 (2016).
Publication Date:
11/2016
Origin:
NATURE
Abstract Copyright:
(c) 2016: The Author(s)
DOI:
10.1038/srep37656
Bibliographic Code:
2016NatSR...637656L

Abstract

We discover and characterise strong quantum scars, or quantum eigenstates resembling classical periodic orbits, in two-dimensional quantum wells perturbed by local impurities. These scars are not explained by ordinary scar theory, which would require the existence of short, moderately unstable periodic orbits in the perturbed system. Instead, they are supported by classical resonances in the unperturbed system and the resulting quantum near-degeneracy. Even in the case of a large number of randomly scattered impurities, the scars prefer distinct orientations that extremise the overlap with the impurities. We demonstrate that these preferred orientations can be used for highly efficient transport of quantum wave packets across the perturbed potential landscape. Assisted by the scars, wave-packet recurrences are significantly stronger than in the unperturbed system. Together with the controllability of the preferred orientations, this property may be very useful for quantum transport applications.

 

Title:
Measurement of the W±Z boson pair-production cross section in pp collisions at √{ s} = 13 TeV with the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.;... Franklin, M.;... Huth, J.;... Morii, M.;... ; and 2857 coauthors
Publication:
Physics Letters B, Volume 762, p. 1-22.
Publication Date:
11/2016
Origin:
ELSEVIER
Abstract Copyright:
(c) 2016 Elsevier Science B.V. All rights reserved.
DOI:
10.1016/j.physletb.2016.08.052
Bibliographic Code:
2016PhLB..762....1A

Abstract

The production of W± Z events in proton-proton collisions at a centre-of-mass energy of 13 TeV is measured with the ATLAS detector at the LHC. The collected data correspond to an integrated luminosity of 3.2 fb-1. The W± Z candidates are reconstructed using leptonic decays of the gauge bosons into electrons or muons. The measured inclusive cross section in the detector fiducial region for leptonic decay modes is σW±Z →ℓ νℓℓ fid. = 63.2 ± 3.2(stat.) ± 2.6(sys.) ± 1.5(lumi.) fb. In comparison, the next-to-leading-order Standard Model prediction is 53.4-2.8+3.6 fb. The extrapolation of the measurement from the fiducial to the total phase space yields σW±Ztot. = 50.6 ± 2.6(stat.) ± 2.0(sys.) ± 0.9(th.) ± 1.2(lumi.) pb, in agreement with a recent next-to-next-to-leading-order calculation of 48.2-1.0+1.1 pb. The cross section as a function of jet multiplicity is also measured, together with the charge-dependent W+ Z and W- Z cross sections and their ratio.

 

Title:
Search for new resonances in events with one lepton and missing transverse momentum in pp collisions at √{ s} = 13 TeV with the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.;... Franklin, M.;... Huth, J.;... Morii, M.;... ; and 2857 coauthors
Publication:
Physics Letters B, Volume 762, p. 334-352.
Publication Date:
11/2016
Origin:
ELSEVIER
Abstract Copyright:
(c) 2016 Elsevier Science B.V. All rights reserved.
DOI:
10.1016/j.physletb.2016.09.040
Bibliographic Code:
2016PhLB..762..334A

Abstract

A search for W* bosons in events with one lepton (electron or muon) and missing transverse momentum is presented. The search uses 3.2 fb-1 of pp collision data collected at √{ s} = 13 TeV by the ATLAS experiment at the LHC in 2015. The transverse mass distribution is examined and no significant excess of events above the level expected from Standard Model processes is observed. Upper limits on the W* boson cross-section times branching ratio to leptons are set as a function of the W* mass. Within the Sequential Standard Model W* masses below 4.07 TeV are excluded at the 95% confidence level. This extends the limit set using LHC data at √{ s} = 8 TeV by around 800 GeV.

 

itle:
Epsilon-near-zero behavior from plasmonic Dirac point: Theory and realization using two-dimensional materials
Authors:
Mattheakis, Marios; Valagiannopoulos, Constantinos A.; Kaxiras, Efthimios
Publication:
Physical Review B, Volume 94, Issue 20, id.201404 (PhRvB Homepage)
Publication Date:
11/2016
Origin:
APS
Abstract Copyright:
2016: American Physical Society
DOI:
10.1103/PhysRevB.94.201404
Bibliographic Code:
2016PhRvB..94t1404M

Abstract

The electromagnetic response of a two-dimensional metal embedded in a periodic array of a dielectric host can give rise to a plasmonic Dirac point that emulates epsilon-near-zero (ENZ) behavior. This theoretical result is extremely sensitive to structural features like periodicity of the dielectric medium and thickness imperfections. We propose that such a device can actually be realized by using graphene as the two-dimensional metal and materials like the layered semiconducting transition-metal dichalcogenides or hexagonal boron nitride as the dielectric host. We propose a systematic approach, in terms of design characteristics, for constructing metamaterials with linear, elliptical, and hyperbolic dispersion relations which produce ENZ behavior, normal or negative diffraction.

 

Title:
Fractionalized Fermi liquid with bosonic chargons as a candidate for the pseudogap metal
Authors:
Chatterjee, Shubhayu; Sachdev, Subir
Publication:
Physical Review B, Volume 94, Issue 20, id.205117 (PhRvB Homepage)
Publication Date:
11/2016
Origin:
APS
Abstract Copyright:
2016: American Physical Society
DOI:
10.1103/PhysRevB.94.205117
Bibliographic Code:
2016PhRvB..94t5117C

Abstract

Doping a Mott-insulating Z2 spin liquid can lead to a fractionalized Fermi liquid (FL*). Such a phase has several favorable features that make it a candidate for the pseudogap metal for the underdoped cuprates. We focus on a particular, simple Z2-FL* state which can undergo a confinement transition to a spatially uniform superconductor which is smoothly connected to the "plain vanilla" BCS superconductor with d -wave pairing. Such a transition occurs by the condensation of bosonic particles carrying +e charge but no spin ("chargons"). We show that modifying the dispersion of the bosonic chargons can lead to confinement transitions with charge density waves and pair density waves at the same wave vector K , coexisting with d -wave superconductivity. We also compute the evolution of the Hall number in the normal state during the transition from the plain vanilla FL* state to a Fermi liquid, and argue, following Coleman, Marston, and Schofield [Phys. Rev. B 72, 245111 (2005), 10.1103/PhysRevB.72.245111], that it exhibits a discontinuous jump near optimal doping. We note the distinction between these results and those obtained from models of the pseudogap with fermionic chargons.

 

Title:
Measurement of top quark pair differential cross sections in the dilepton channel in p p collisions at √{s }=7 and 8 TeV with ATLAS
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.;... Franklin, M.;... Huth, J.;... Morii, M.;... ; and 2843 coauthors
Publication:
Physical Review D, Volume 94, Issue 9, id.092003 (PhRvD Homepage)
Publication Date:
11/2016
Origin:
APS
Abstract Copyright:
2016: CERN
DOI:
10.1103/PhysRevD.94.092003
Bibliographic Code:
2016PhRvD..94i2003A

Abstract

Measurements of normalized differential cross sections of top quark pair (t t ¯) production are presented as a function of the mass, the transverse momentum and the rapidity of the t t ¯ system in proton-proton collisions at center-of-mass energies of √{s }=7 and 8 TeV. The data set corresponds to an integrated luminosity of 4.6 fb-1 at 7 TeV and 20.2 fb-1 at 8 TeV, recorded with the ATLAS detector at the Large Hadron Collider. Events with top quark pair signatures are selected in the dilepton final state, requiring exactly two charged leptons and at least two jets with at least one of the jets identified as likely to contain a b hadron. The measured distributions are corrected for detector effects and selection efficiency to cross sections at the parton level. The differential cross sections are compared with different Monte Carlo generators and theoretical calculations of t t ¯ production. The results are consistent with the majority of predictions in a wide kinematic range.

 

Title:
Universal Signatures of Quantum Critical Points from Finite-Size Torus Spectra: A Window into the Operator Content of Higher-Dimensional Conformal Field Theories
Authors:
Schuler, Michael; Whitsitt, Seth; Henry, Louis-Paul; Sachdev, Subir; Läuchli, Andreas M.
Publication:
Physical Review Letters, Volume 117, Issue 21, id.210401 (PhRvL Homepage)
Publication Date:
11/2016
Origin:
APS
Abstract Copyright:
2016: American Physical Society
DOI:
10.1103/PhysRevLett.117.210401
Bibliographic Code:
2016PhRvL.117u0401S

Abstract

The low-energy spectra of many body systems on a torus, of finite size L , are well understood in magnetically ordered and gapped topological phases. However, the spectra at quantum critical points separating such phases are largely unexplored for (2 +1 )D systems. Using a combination of analytical and numerical techniques, we accurately calculate and analyze the low-energy torus spectrum at an Ising critical point which provides a universal fingerprint of the underlying quantum field theory, with the energy levels given by universal numbers times 1 /L . We highlight the implications of a neighboring topological phase on the spectrum by studying the Ising* transition (i.e. the transition between a Z2 topological phase and a trivial paramagnet), in the example of the toric code in a longitudinal field, and advocate a phenomenological picture that provides qualitative insight into the operator content of the critical field theory.

 

Title:
A low-temperature external cavity diode laser for broad wavelength tuning
Authors:
Tobias, William G.; Rosenberg, Jason S.; Hutzler, Nicholas R.; Ni, Kang-Kuen
Publication:
Review of Scientific Instruments, Volume 87, Issue 11, id.113104 (RScI Homepage)
Publication Date:
11/2016
Origin:
AIP
Abstract Copyright:
2016: Author(s)
DOI:
10.1063/1.4967231
Bibliographic Code:
2016RScI...87k3104T

Abstract

We report on the design and characterization of a low-temperature external cavity diode laser (ECDL) system for broad wavelength tuning. The performance achieved with multiple diode models addresses the scarcity of commercial red laser diodes below 633 nm, which is a wavelength range relevant to the spectroscopy of many molecules and ions. Using a combination of multiple-stage thermoelectric cooling and water cooling, the operating temperature of a laser diode is lowered to -64 °C, more than 85 °C below the ambient temperature. The laser system integrates temperature and diffraction grating feedback tunability for coarse and fine wavelength adjustments, respectively. For two different diode models, single-mode operation is achieved with 38 mW output power at 616.8 nm and 69 mW at 622.6 nm, more than 15 nm below their ambient temperature free-running wavelengths. The ECDL design can be used for diodes of any available wavelength, allowing individual diodes to be tuned continuously over tens of nanometers and extending the wavelength coverage of commercial laser diodes.

 

Title:
An integrated diamond nanophotonics platform for quantum-optical networks
Authors:
Sipahigil, A.; Evans, R. E.; Sukachev, D. D.; Burek, M. J.; Borregaard, J.; Bhaskar, M. K.; Nguyen, C. T.; Pacheco, J. L.; Atikian, H. A.; Meuwly, C.; Camacho, R. M.; Jelezko, F.; Bielejec, E.; Park, H.; Lončar, M.; Lukin, M. D.
Publication:
Science, Volume 354, Issue 6314, pp. 847-850 (2016). (Sci Homepage)
Publication Date:
11/2016
Category:
PHYSICS
Origin:
SCIENCE
Abstract Copyright:
(c) 2016: Science
DOI:
10.1126/science.aah6875
Bibliographic Code:
2016Sci...354..847S

Abstract

Efficient interfaces between photons and quantum emitters form the basis for quantum networks and enable optical nonlinearities at the single-photon level. We demonstrate an integrated platform for scalable quantum nanophotonics based on silicon-vacancy (SiV) color centers coupled to diamond nanodevices. By placing SiV centers inside diamond photonic crystal cavities, we realize a quantum-optical switch controlled by a single color center. We control the switch using SiV metastable states and observe optical switching at the single-photon level. Raman transitions are used to realize a single-photon source with a tunable frequency and bandwidth in a diamond waveguide. By measuring intensity correlations of indistinguishable Raman photons emitted into a single waveguide, we observe a quantum interference effect resulting from the superradiant emission of two entangled SiV centers.

 

Title:
Atom-by-atom assembly of defect-free one-dimensional cold atom arrays
Authors:
Endres, Manuel; Bernien, Hannes; Keesling, Alexander; Levine, Harry; Anschuetz, Eric R.; Krajenbrink, Alexandre; Senko, Crystal; Vuletic, Vladan; Greiner, Markus; Lukin, Mikhail D.
Publication:
Science, Volume 354, Issue 6315, pp. 1024-1027 (2016). (Sci Homepage)
Publication Date:
11/2016
Category:
PHYSICS
Origin:
SCIENCE
Abstract Copyright:
(c) 2016: Science
DOI:
10.1126/science.aah3752
Bibliographic Code:
2016Sci...354.1024E

Abstract

The realization of large-scale fully controllable quantum systems is an exciting frontier in modern physical science. We use atom-by-atom assembly to implement a platform for the deterministic preparation of regular one-dimensional arrays of individually controlled cold atoms. In our approach, a measurement and feedback procedure eliminates the entropy associated with probabilistic trap occupation and results in defect-free arrays of more than 50 atoms in less than 400 milliseconds. The technique is based on fast, real-time control of 100 optical tweezers, which we use to arrange atoms in desired geometric patterns and to maintain these configurations by replacing lost atoms with surplus atoms from a reservoir. This bottom-up approach may enable controlled engineering of scalable many-body systems for quantum information processing, quantum simulations, and precision measurements.


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