Faculty Publications: February, 2017

Query Results from the Smithsonian/NASA Astrophysics Data System (ADS)

The following are Harvard Physics faculty members' publications, added to the ADS database last month. Please note that some publications which appeared in print last month may not be included in the database (and therefore may not appear on this list) until the following month.


Title:
Bootstrapping the Spectral Function: On the Uniqueness of Liouville and the Universality of BTZ
Authors:
Collier, Scott; Kravchuk, Petr; Lin, Ying-Hsuan; Yin, Xi
Publication:
eprint arXiv:1702.00423
Publication Date:
02/2017
Origin:
ARXIV
Keywords:
High Energy Physics - Theory
Comment:
44 pages, 15 figures
Bibliographic Code:
2017arXiv170200423C

Abstract

We introduce spectral functions that capture the distribution of OPE coefficients and density of states in two-dimensional conformal field theories, and show that nontrivial upper and lower bounds on the spectral function can be obtained from semidefinite programming. We find substantial numerical evidence indicating that OPEs involving only scalar Virasoro primaries in a c>1 CFT are necessarily governed by the structure constants of Liouville theory. Combining this with analytic results in modular bootstrap, we conjecture that Liouville theory is the unique unitary c>1 CFT whose primaries have bounded spins. We also use the spectral function method to study modular constraints on CFT spectra, and discuss some implications of our results on CFTs of large c and large gap, in particular, to what extent the BTZ spectral density is universal.

 

Title:
Revealing Hidden Antiferromagnetic Correlations in Doped Hubbard Chains via String Correlators
Authors:
Hilker, Timon A.; Salomon, Guillaume; Grusdt, Fabian; Omran, Ahmed; Boll, Martin; Demler, Eugene; Bloch, Immanuel; Gross, Christian
Publication:
eprint arXiv:1702.00642
Publication Date:
02/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Quantum Gases
Bibliographic Code:
2017arXiv170200642H

Abstract

Topological phases, like the celebrated Haldane phase in spin-1 chains, defy characterization through local order parameters. Instead, non-local string order parameters can be employed to reveal their hidden order. Similar diluted magnetic correlations appear in doped one-dimensional lattice systems due to the phenomenon of spin-charge separation. Here we report on the direct observation of such hidden magnetic correlations via quantum gas microscopy of hole-doped ultracold Fermi-Hubbard chains. The measurement of non-local spin-density correlation functions reveals a hidden finite-range antiferromagnetic order, a direct consequence of spin-charge separation. Our technique demonstrates how topological order can directly be measured in experiments and it can be extended to higher dimensions to study the complex interplay between magnetic order and density fluctuations.

 

Title:
Characterizing time-irreversibility in disordered fermionic systems by the effect of local perturbations
Authors:
Vardhan, Shreya; De Tomasi, Giuseppe; Heyl, Markus; Heller, Eric J.; Pollmann, Frank
Publication:
eprint arXiv:1702.01622
Publication Date:
02/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Disordered Systems and Neural Networks, Condensed Matter - Quantum Gases, Condensed Matter - Statistical Mechanics, Condensed Matter - Strongly Correlated Electrons
Bibliographic Code:
2017arXiv170201622V

Abstract

We study the effects of local perturbations on the dynamics of disordered fermionic systems in order to characterize time-irreversibility. We focus on three different systems, the non-interacting Anderson and Aubry-Andr\'e-Harper (AAH-) models, and the interacting spinless disordered t-V chain. First, we consider the effect on the full many-body wave-functions by measuring the Loschmidt echo (LE). We show that in the extended/ergodic phase the LE decays exponentially fast with time, while in the localized phase the decay is algebraic. We demonstrate that the exponent of the decay of the LE in the localized phase diverges proportionally to the single-particle localization length as we approach the metal-insulator transition in the AAH model. Second, we probe different phases of disordered systems by studying the time expectation value of local observables evolved with two Hamiltonians that differ by a spatially local perturbation. Remarkably, we find that many-body localized systems could lose memory of the initial state in the long-time limit, in contrast to the non-interacting localized phase where some memory is always preserved.

 

Title:
Thermal stiffening of clamped elastic ribbons
Authors:
Wan, Duanduan; Nelson, David R.; Bowick, Mark J.
Publication:
eprint arXiv:1702.01863
Publication Date:
02/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Soft Condensed Matter, Condensed Matter - Statistical Mechanics
Comment:
7 pages, 5 figures
Bibliographic Code:
2017arXiv170201863W

Abstract

We use molecular dynamics to study the vibrations of a thermally fluctuating two-dimensional elastic membrane clamped at both ends. We directly extract the eigenmodes from resonant peaks in the frequency domain of the time-dependent height and measure the dependence of the corresponding eigenfrequencies on the microscopic bending rigidity of the membrane, taking care also of the subtle role of thermal contraction in generating a tension when the projected area is fixed. At finite temperatures we show that the effective (macroscopic) bending rigidity tends to a constant as the bare bending rigidity vanishes, consistent with theoretical arguments that the large-scale bending rigidity of the membrane arises from a strong thermal renormalization of the microscopic bending rigidity. Experimental realizations include covalently-bonded two-dimensional atomically thin membranes such as graphene and molybdenum disulfide or soft matter systems such as the spectrin skeleton of red blood cells or diblock copolymers.

 

Title:
Photonic tensor networks produced by a single quantum emitter
Authors:
Pichler, Hannes; Choi, Soonwon; Zoller, Peter; Lukin, Mikhail D.
Publication:
eprint arXiv:1702.02119
Publication Date:
02/2017
Origin:
ARXIV
Keywords:
Quantum Physics
Bibliographic Code:
2017arXiv170202119P

Abstract

We propose and analyze a protocol to generate two dimensional tensor network states using a single quantum system that sequentially interacts with a 1D string of qubits. This is accomplished by using parts of the string itself as a quantum queue memory. As a physical implementation, we consider a single atom or atom like system coupled to a 1D waveguide with a distant mirror, where guided photons represent the qubits while the mirror allows the implementation of the queue memory. We identify the class of many-body quantum states that can be produced using this approach. These include universal resources for measurement based quantum computation and states associated with topologically ordered phases. We discuss an explicit protocol to deterministically create a 2D cluster state in a quantum nanophotonic experiment, that allows for a realization of a quantum computer using a single atom coupled to light.

 

Title:
The theory of parametrically amplified electron-phonon superconductivity
Authors:
Babadi, Mehrtash; Knap, Michael; Martin, Ivar; Refael, Gil; Demler, Eugene
Publication:
eprint arXiv:1702.02531
Publication Date:
02/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Superconductivity, Condensed Matter - Materials Science, Condensed Matter - Strongly Correlated Electrons
Comment:
42 pages, 17 figures
Bibliographic Code:
2017arXiv170202531B

Abstract

The ultrafast optical manipulation of ordered phases in strongly correlated materials is a topic of significant theoretical, experimental, and technological interest. Inspired by a recent experiment on light-induced superconductivity in fullerenes [Mitrano et al., Nature 530, 2016], we develop a comprehensive theory of light-induced superconductivity in driven electron-phonon systems with lattice nonlinearities. In analogy with the operation of parametric amplifiers, we show how the interplay between the external drive and lattice nonlinearities lead to significantly enhanced effective electron-phonon couplings. We provide a detailed and unbiased study of the nonequilibrium dynamics of the driven system using the real-time Green's function technique. To this end, we develop a Floquet generalization of the Migdal-Eliashberg theory and derive a numerically tractable set of quantum Floquet-Boltzmann kinetic equations for the coupled electron-phonon system. We study the role of parametric phonon generation and electronic heating in destroying the transient superconducting state. Finally, we predict the transient formation of electronic Floquet bands in time- and angle-resolved photo-emission spectroscopy experiments as a consequence of the proposed mechanism.

 

Title:
Mean-field scaling of the superfluid to Mott insulator transition in a 2D optical superlattice
Authors:
Thomas, Claire K.; Barter, Thomas H.; Leung, Tsz-Him; Okano, Masayuki; Jo, Gyu-Boong; Guzman, Jennie; Kimchi, Itamar; Vishwanath, Ashvin; Stamper-Kurn, Dan M.
Publication:
eprint arXiv:1702.04433
Publication Date:
02/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Quantum Gases, Physics - Atomic Physics, Quantum Physics
Bibliographic Code:
2017arXiv170204433T

Abstract

The mean-field treatment of the Bose-Hubbard model predicts properties of non-zero temperature lattice-trapped gases to be insensitive to the specific lattice geometry once system energies are scaled by the lattice coordination number $z$. We test this predicted scaling directly and quantitatively by comparing the coherence properties of quantum gases prepared identically and loaded into optical lattices of either the kagome or triangular geometry, so that $z$ is either four or six, as we increase interactions and drive a phase transition between the superfluid and Mott insulating states. We find that the coherent fraction is suppressed in the kagome lattice throughout the phase transition and our data are consistent with the scaling prediction. We also study the response of the gas to a change in lattice geometry, and observe out-of-equilibrium dynamics when the kagome-lattice Mott insulator is suddenly "hole-doped" by introducing the additional sites of the triangular lattice.

 

Title:
QED$_3$ with quenched disorder: quantum critical states with interactions and disorder
Authors:
Thomson, Alex; Sachdev, Subir
Publication:
eprint arXiv:1702.04723
Publication Date:
02/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Strongly Correlated Electrons
Comment:
58 pages, 18 figures
Bibliographic Code:
2017arXiv170204723T

Abstract

Quantum electrodynamics in 2+1-dimensions (QED$_3$) is a strongly coupled conformal field theory (CFT) of a U(1) gauge field coupled to $2N$ two-component massless fermions. The $N=2$ CFT has been proposed as a ground state of the spin-1/2 kagome Heisenberg antiferromagnet. We study QED$_3$ in the presence of weak quenched disorder in its two spatial directions. When the disorder explicitly breaks the fermion flavor symmetry from SU($2N$)$\rightarrow$U(1)$\times$SU($N$) but preserves time-reversal symmetry, we find that the theory flows to a non-trivial fixed line at non-zero disorder with a continuously varying dynamical critical exponent $z>1$. We determine the zero-temperature flavor (spin) conductivity along the critical line. Our calculations are performed in the large-$N$ limit, and the disorder is handled using the replica method.

 

Title:
Dark Catalysis
Authors:
Agrawal, Prateek; Cyr-Racine, Francis-Yan; Randall, Lisa; Scholtz, Jakub
Publication:
eprint arXiv:1702.05482
Publication Date:
02/2017
Origin:
ARXIV
Keywords:
Astrophysics - Cosmology and Nongalactic Astrophysics, High Energy Physics - Phenomenology
Comment:
24 pages + appendices, 7 figures
Bibliographic Code:
2017arXiv170205482A

Abstract

Recently it was shown that dark matter with mass of order the weak scale can be charged under a new long-range force, decoupled from the Standard Model, with only weak constraints from early Universe cosmology. Here we consider the implications of an additional charged particle $C$ that is light enough to lead to significant dissipative dynamics on galactic times scales. We highlight several novel features of this model, which can be relevant even when the $C$ particle constitutes only a small fraction of the number density (and energy density). We assume a small asymmetric abundance of the $C$ particle whose charge is compensated by a heavy $X$ particle so that the relic abundance of dark matter consists mostly of symmetric $X$ and $\bar{X}$, with a small asymmetric component made up of $X$ and $C$. As the universe cools, it undergoes asymmetric recombination binding the free $C$s into $(XC)$ dark atoms efficiently. Even with a tiny asymmetric component, the presence of $C$ particles catalyzes tight coupling between the heavy dark matter $X$ and the dark photon plasma that can lead to a significant suppression of the matter power spectrum on small scales and lead to some of the strongest bounds on such dark matter theories. We find a viable parameter space where structure formation constraints are satisfied and significant dissipative dynamics can occur in galactic haloes but show a large region is excluded. Our model shows that subdominant components in the dark sector can dramatically affect structure formation.

 

Title:
Deciphering the MSSM Higgs Mass at Future Hadron Colliders
Authors:
Agrawal, Prateek; Fan, JiJi; Reece, Matthew; Xue, Wei
Publication:
eprint arXiv:1702.05484
Publication Date:
02/2017
Origin:
ARXIV
Keywords:
High Energy Physics - Phenomenology
Comment:
19 pages, 13 figures
Bibliographic Code:
2017arXiv170205484A

Abstract

Future hadron colliders will have a remarkable capacity to discover massive new particles, but their capabilities for precision measurements of couplings that can reveal underlying mechanisms have received less study. In this work we study the capability of future hadron colliders to shed light on a precise, focused question: is the higgs mass of 125 GeV explained by the MSSM? If supersymmetry is realized near the TeV scale, a future hadron collider could produce huge numbers of gluinos and electroweakinos. We explore whether precision measurements of their properties could allow inference of the scalar masses and $\tan \beta$ with sufficient accuracy to test whether physics beyond the MSSM is needed to explain the higgs mass. We also discuss dark matt

 

Title:
Pretty good state transfer in graphs with an involution
Authors:
Kempton, Mark; Lippner, Gabor; Yau, Shing-Tung
Publication:
eprint arXiv:1702.07000
Publication Date:
02/2017
Origin:
ARXIV
Keywords:
Mathematics - Combinatorics, Quantum Physics, 05C50
Comment:
14 pages
Bibliographic Code:
2017arXiv170207000K

Abstract

We study pretty good quantum state transfer (i.e., state transfer that becomes arbitrarily close to perfect) between vertices of graphs with an involution in the presence of an energy potential. In particular, we show that if a graph has an involution that fixes at least one vertex or at least one edge, then there exists a choice of potential on the vertex set of the graph for which we get pretty good state transfer between symmetric vertices of the graph. We show further that in many cases, the potential can be chosen so that it is only non-zero at the vertices between which we want pretty good state transfer. As a special case of this, we show that such a potential can be chosen on the endpoints of a path to induce pretty good state transfer in paths of any length. This is in contrast to the result of [6], in which the authors show that there cannot be perfect state transfer in paths of length 4 or more, no matter what potential is chosen.

 

Title:
Direct Observation of Entropic Stabilization of bcc Crystals Near Melting
Authors:
Sprakel, Joris; Zaccone, Alessio; Spaepen, Frans; Schall, Peter; Weitz, David A.
Publication:
eprint arXiv:1702.07873
Publication Date:
02/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Soft Condensed Matter, Condensed Matter - Materials Science
Comment:
Phys. Rev. Lett. 118, 088003 (2017); doi:10.1103/PhysRevLett.118.088003
Bibliographic Code:
2017arXiv170207873S

Abstract

Crystals with low latent heat are predicted to melt from an entropically stabilized body-centered cubic symmetry. At this weakly first-order transition, strongly correlated fluctuations are expected to emerge, which could change the nature of the transition. Here we show how large fluctuations stabilize bcc crystals formed from charged colloids, giving rise to strongly power-law correlated heterogeneous dynamics. Moreover, we find that significant nonaffine particle displacements lead to a vanishing of the nonaffine shear modulus at the transition. We interpret these observations by reformulating the Born-Huang theory to account for nonaffinity, illustrating a scenario of ordered solids reaching a state where classical lattice dynamics fail.

 

Title:
Universal many-body response of heavy impurities coupled to a Fermi sea
Authors:
Schmidt, Richard; Knap, Michael; Ivanov, Dmitri A.; You, Jhih-Shih; Cetina, Marko; Demler, Eugene
Publication:
eprint arXiv:1702.08587
Publication Date:
02/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Quantum Gases, Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Strongly Correlated Electrons, Quantum Physics
Comment:
24+12 pages, 21 figures; comments welcome!
Bibliographic Code:
2017arXiv170208587S

Abstract

We study the dynamical response of heavy quantum impurities immersed in a Fermi gas at zero and at finite temperature. By investigating both the frequency and the time domain, we identify interaction regimes that are characterized by distinct many-body dynamics. From our theoretical study a picture emerges in which impurity dynamics is universal on essentially all time scales, and where the high-frequency few-body response is related to the long-time dynamics of the Anderson orthogonality catastrophe by Tan relations. In our theoretical description we rely on three complementary approaches: (1) functional determinants give an exact numerical solution for time- and frequency-resolved responses, (2) bosonization provides accurate analytical expressions at low temperatures, and (3) the theory of Toeplitz determinants allows us to analytically predict response up to high temperatures. Using these approaches we calculated the thermal decoherence rate of the fermionic system and we prove that, within the considered model, the fastest rate of long-time decoherence is given by $\gamma=\pi k_BT/4$. We show that Feshbach resonances in cold atomic systems give access to new interaction regimes where quantum effects can prevail even in the thermal regime of many-body dynamics. The key signature of this phenomenon is a crossover between different exponential decay rates of the real-time Ramsey signal. We show that the physics of the orthogonality catastrophe can be observed in a power-law dependence of thermal spectral weight on temperature that is reminiscent of the physics of heavy ions in liquid $^3$He and that is experimentally observable at temperatures $T/T_F\lesssim 0.2$. Our results are in excellent agreement with recent experiments on LiK mixtures, and we predict several phenomena that can be tested using currently available experimental technology.

 

Title:
Insulators and metals with topological order and discrete symmetry breaking
Authors:
Sachdev, Subir; Chatterjee, Shubhayu
Publication:
eprint arXiv:1703.00014
Publication Date:
02/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Strongly Correlated Electrons, High Energy Physics - Theory
Comment:
30 pages, 4 figures; (v2) added figure and clarifications
Bibliographic Code:
2017arXiv170300014S

Abstract

Numerous experiments have reported discrete symmetry breaking in the high temperature pseudogap phase of the hole-doped cuprates, including breaking of one or more of lattice rotation, inversion, or time-reversal symmetries. In the absence of translational symmetry breaking or topological order, these conventional order parameters cannot explain the gap in the charged fermion excitation spectrum in the anti-nodal region. Zhao et al. (1601.01688) and Jeong et al. (arXiv:1701.06485) have also reported inversion and time-reversal symmetry breaking in insulating Sr2IrO4 similar to that in the metallic cuprates, but co-existing with Neel order. We extend an earlier theory of topological order in insulators and metals, in which the topological order combines naturally with the breaking of these conventional discrete symmetries. We find translationally-invariant states with topological order co-existing with both Ising-nematic order and spontaneous charge currents. The link between the discrete broken symmetries and the topological-order-induced pseudogap explains why the broken symmetries do not survive in the confining phases without a pseudogap at large doping. Our theory also connects to the O(3) non-linear sigma model and CP1 descriptions of quantum fluctuations of the Neel order. In this framework, the optimal doping criticality of the cuprates is primarily associated with the loss of topological order.

 

Title:
Mach-Zehnder interferometry using spin- and valley-polarized quantum Hall edge states in graphene
Authors:
Wei, Di S.; van der Sar, Toeno; Sanchez-Yamagishi, Javier D.; Watanabe, Kenji; Taniguchi, Takashi; Jarillo-Herrero, Pablo; Halperin, Bertrand I.; Yacoby, Amir
Publication:
eprint arXiv:1703.00110
Publication Date:
02/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Mesoscale and Nanoscale Physics
Comment:
18 pages, 11 figures
Bibliographic Code:
2017arXiv170300110W

Abstract

Confined to a two-dimensional plane, electrons in a strong magnetic field travel along the edge in one-dimensional quantum Hall channels that are protected against backscattering. These channels can be used as solid-state analogues of monochromatic beams of light, providing a unique platform for studying electron interference. Electron interferometry is regarded as one of the most promising routes for studying fractional and non-Abelian statistics and quantum entanglement via two-particle interference. However, creating an edge-channel interferometer in which electron-electron interactions play an important role requires a clean system and long phase coherence lengths. Here we realize electronic Mach-Zehnder interferometers with record visibilities of up to 98% using spin- and valley-polarized edge channels that co-propagate along a PN junction in graphene. We find that inter-channel scattering between same-spin edge channels along the physical graphene edge can be used to form beamsplitters, while the absence of inter-channel scattering along gate-defined interfaces can be used to form isolated interferometer arms. Surprisingly, our interferometer is robust to dephasing effects at energies an order of magnitude larger than observed in pioneering experiments on GaAs/AlGaAs quantum wells. Our results shed light on the nature of edge-channel equilibration and open up new possibilities for studying exotic electron statistics and quantum phenomena.

 

Title:
A self-calibrating polarimeter to measure Stokes parameters
Authors:
Andreev, V.; Panda, C. D.; Hess, P. W.; Spaun, B.; Gabrielse, G.
Publication:
eprint arXiv:1703.00963
Publication Date:
02/2017
Origin:
ARXIV
Keywords:
Physics - Instrumentation and Detectors, Physics - Optics
Bibliographic Code:
2017arXiv170300963A

Abstract

An easily constructed and operated polarimeter precisely determines the relative Stokes parameters that characterize the polarization of laser light. The polarimeter is calibrated in situ without removing or realigning its optical elements, and it is largely immune to fluctuations in the laser beam intensity. The polarimeter's usefulness is illustrated by measuring thermally-induced birefringence in the indium-tin-oxide coated glass field plates used to produce a static electric field in the ACME collaboration's measurement of the electron electric dipole moment.

 

Title:
Search for lepton-flavour-violating decays of the Higgs and Z bosons with the ATLAS detector
Authors:
Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2839 coauthors
Publication:
The European Physical Journal C, Volume 77, Issue 2, article id. #70, 31 pp. (EPJC Homepage)
Publication Date:
02/2017
Origin:
SPRINGER
Abstract Copyright:
(c) 2017: CERN for the benefit of the ATLAS collaboration
DOI:
10.1140/epjc/s10052-017-4624-0
Bibliographic Code:
2017EPJC...77...70A

Abstract

Direct searches for lepton flavour violation in decays of the Higgs and Z bosons with the ATLAS detector at the LHC are presented. The following three decays are considered: H→ eτ , H→ μ τ , and Z→ μ τ . The searches are based on the data sample of proton-proton collisions collected by the ATLAS detector corresponding to an integrated luminosity of 20.3 fb^{-1} at a centre-of-mass energy of √{s}=8 TeV. No significant excess is observed, and upper limits on the lepton-flavour-violating branching ratios are set at the 95% confidence level: Br(H→ eτ )<1.04%, Br(H→ μ τ )<1.43%, and Br(Z→ μ τ )<1.69× 10^{-5}.

 

Title:
Analysis of Scanned Probe Images for Magnetic Focusing in Graphene
Authors:
Bhandari, Sagar; Lee, Gil-Ho; Kim, Philip; Westervelt, Robert M.
Publication:
Journal of Electronic Materials, Online First
Publication Date:
02/2017
Origin:
SPRINGER
Keywords:
Scanning probe microscopy theory, ballistic transport, graphene, simulation, magnetic focusing, electron trajectories
Abstract Copyright:
(c) 2017: The Author(s)
DOI:
10.1007/s11664-017-5350-y
Bibliographic Code:
2017JEMat.tmp..143B

Abstract

We have used cooled scanning probe microscopy (SPM) to study electron motion in nanoscale devices. The charged tip of the microscope was raster-scanned at constant height above the surface as the conductance of the device was measured. The image charge scatters electrons away, changing the path of electrons through the sample. Using this technique, we imaged cyclotron orbits that flow between two narrow contacts in the magnetic focusing regime for ballistic hBN-graphene-hBN devices. We present herein an analysis of our magnetic focusing imaging results based on the effects of the tip-created charge density dip on the motion of ballistic electrons. The density dip locally reduces the Fermi energy, creating a force that pushes electrons away from the tip. When the tip is above the cyclotron orbit, electrons are deflected away from the receiving contact, creating an image by reducing the transmission between contacts. The data and our analysis suggest that the graphene edge is rather rough, and electrons scattering off the edge bounce in random directions. However, when the tip is close to the edge, it can enhance transmission by bouncing electrons away from the edge, toward the receiving contact. Our results demonstrate that cooled SPM is a promising tool to investigate the motion of electrons in ballistic graphene devices.

 

Title:
Measurements of charge and CP asymmetries in b-hadron decays using top-quark events collected by the ATLAS detector in pp collisions at √{s}=8 TeV
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2849 coauthors
Publication:
Journal of High Energy Physics, Volume 2017, Issue 2, article id. #71, 49 pp.
Publication Date:
02/2017
Origin:
SPRINGER
Keywords:
Hadron-Hadron scattering (experiments)
Abstract Copyright:
(c) 2017: The Author(s)
DOI:
10.1007/JHEP02(2017)071
Bibliographic Code:
2017JHEP...02..071A

Abstract

Same- and opposite-sign charge asymmetries are measured in lepton+jets toverline{t} events in which a b-hadron decays semileptonically to a soft muon, using data corresponding to an integrated luminosity of 20.3 fb-1 from proton-proton collisions at a centre-of-mass energy of √{s}=8 TeV collected with the ATLAS detector at the Large Hadron Collider at CERN. The charge asymmetries are based on the charge of the lepton from the top-quark decay and the charge of the soft muon from the semileptonic decay of a b-hadron and are measured in a fiducial region corresponding to the experimental acceptance. Four CP asymmetries (one mixing and three direct) are measured and are found to be compatible with zero and consistent with the Standard Model.

 

Title:
Brane brick models in the mirror
Authors:
Franco, Sebastián; Lee, Sangmin; Seong, Rak-Kyeong; Vafa, Cumrun
Publication:
Journal of High Energy Physics, Volume 2017, Issue 2, article id. #106, 63 pp.
Publication Date:
02/2017
Origin:
SPRINGER
Keywords:
Brane Dynamics in Gauge Theories, D-branes, Supersymmetric gauge theory
Abstract Copyright:
(c) 2017: The Author(s)
DOI:
10.1007/JHEP02(2017)106
Bibliographic Code:
2017JHEP...02..106F

Abstract

Brane brick models are Type IIA brane configurations that encode the 2 d N=(0,2) gauge theories on the worldvolume of D1-branes probing toric Calabi-Yau 4-folds. We use mirror symmetry to improve our understanding of this correspondence and to provide a systematic approach for constructing brane brick models starting from geometry. The mirror configuration consists of D5-branes wrapping 4-spheres and the gauge theory is determined by how they intersect. We also explain how 2 d (0 , 2) triality is realized in terms of geometric transitions in the mirror geometry. Mirror symmetry leads to a geometric unification of dualities in different dimensions, where the order of duality is n - 1 for a Calabi-Yau n-fold. This makes us conjecture the existence of a quadrality symmetry in 0 d. Finally, we comment on how the M-theory lift of brane brick models connects to the classification of 2 d (0 , 2) theories in terms of 4-manifolds.

 

Title:
Measurements of top-quark pair to Z-boson cross-section ratios at √{s}=13 , 8 , 7 TeV with the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2827 coauthors
Publication:
Journal of High Energy Physics, Volume 2017, Issue 2, article id. #117, 54 pp.
Publication Date:
02/2017
Origin:
SPRINGER
Keywords:
Hadron-Hadron scattering (experiments)
Abstract Copyright:
(c) 2017: The Author(s)
DOI:
10.1007/JHEP02(2017)117
Bibliographic Code:
2017JHEP...02..117A

Abstract

Ratios of top-quark pair to Z-boson cross sections measured from proton-proton collisions at the LHC centre-of-mass energies of √{s}=13 TeV, 8 TeV, and 7 TeV are presented by the ATLAS Collaboration. Single ratios, at a given √{s} for the two processes and at different √{s} for each process, as well as double ratios of the two processes at different √{s} , are evaluated. The ratios are constructed using previously published ATLAS measurements of the toverline{t} and Z-boson production cross sections, corrected to a common phase space where required, and a new analysis of Z →ℓ+- where ℓ = e, μ at √{s}=13 TeV performed with data collected in 2015 with an integrated luminosity of 3 .2 fb-1. Correlations of systematic uncertainties are taken into account when evaluating the uncertainties in the ratios. The correlation model is also used to evaluate the combined cross section of the Z → e + e - and the Z → μ + μ - channels for each √{s} value. The results are compared to calculations performed at next-to-next-to-leading-order accuracy using recent sets of parton distribution functions. The data demonstrate significant power to constrain the gluon distribution function for the Bjorken- x values near 0.1 and the light-quark sea for x < 0.02.

 

Title:
Population Genetics with Fluctuating Population Sizes
Authors:
Chotibut, Thiparat; Nelson, David R.
Publication:
Journal of Statistical Physics, Online First
Publication Date:
02/2017
Origin:
SPRINGER
Keywords:
Population genetics, Fluctuating population sizes, Dynamical system, Stochastic process
Abstract Copyright:
(c) 2017: Springer Science+Business Media New York
DOI:
10.1007/s10955-017-1741-y
Bibliographic Code:
2017JSP...tmp...36C

Abstract

Standard neutral population genetics theory with a strictly fixed population size has important limitations. An alternative model that allows independently fluctuating population sizes and reproduces the standard neutral evolution is reviewed. We then study a situation such that the competing species are neutral at the equilibrium population size but population size fluctuations nevertheless favor fixation of one species over the other. In this case, a separation of timescales emerges naturally and allows adiabatic elimination of a fast population size variable to deduce the fluctuation-induced selection dynamics near the equilibrium population size. The results highlight the incompleteness of the standard population genetics with a strictly fixed population size.

 

Title:
Records of Magnetic Fields in the Chondrule Formation Environment
Authors:
Fu, R. R.; Weiss, B. P.; Kehayias, P.; Schrader, D. L.; Walsworth, R. L.
Publication:
Workshop on Chondrules and the Protoplanetary Disk, held February 27-28, 2017 in London, United Kingdom. LPI Contribution No. 1963, id.2043
Publication Date:
02/2017
Origin:
LPI
Bibliographic Code:
2017LPICo1963.2043F

Abstract

Paleomagnetic measurements can potentially constrain the formation mechanism and location of chondrules. We will present results on LL and CR chondrites, which appear to have experienced strong and weak magnetic fields, respectively.

 

Title:
Eliminating light shifts for single atom trapping
Authors:
Hutzler, Nicholas R.; Liu, Lee R.; Yu, Yichao; Ni, Kang-Kuen
Publication:
New Journal of Physics, Volume 19, Issue 2, article id. 023007 (2017).
Publication Date:
02/2017
Origin:
IOP
DOI:
10.1088/1367-2630/aa5a3b
Bibliographic Code:
2017NJPh...19b3007H

Abstract

Microscopically controlled neutral atoms in optical tweezers and lattices have led to exciting advances in the study of quantum information and quantum many-body systems. The light shifts of atomic levels from the trapping potential in these systems can result in detrimental effects such as fluctuating dipole force heating, inhomogeneous detunings, and inhibition of laser cooling, which limits the atomic species that can be manipulated. In particular, these light shifts can be large enough to prevent loading into optical tweezers directly from a magneto-optical trap. We implement a general solution to these limitations by loading, as well as cooling and imaging the atoms with temporally alternating beams, and present an analysis of the role of heating and required cooling for single atom tweezer loading. Because this technique does not depend on any specific spectral properties, it should enable the optical tweezer platform to be extended to nearly any atomic or molecular species that can be laser cooled and optically trapped.

 

Title:
Search for dark matter in association with a Higgs boson decaying to b-quarks 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 2856 coauthors
Publication:
Physics Letters B, Volume 765, p. 11-31.
Publication Date:
02/2017
Origin:
ELSEVIER
Abstract Copyright:
(c) 2017 Elsevier Science B.V. All rights reserved.
DOI:
10.1016/j.physletb.2016.11.035
Bibliographic Code:
2017PhLB..765...11A

Abstract

A search for dark matter pair production in association with a Higgs boson decaying to a pair of bottom quarks is presented, using 3.2 fb-1 of pp collisions at a centre-of-mass energy of 13 TeV collected by the ATLAS detector at the LHC. The decay of the Higgs boson is reconstructed as a high-momentum b b bar system with either a pair of small-radius jets, or a single large-radius jet with substructure. The observed data are found to be consistent with the expected backgrounds. Results are interpreted using a simplified model with a Z‧ gauge boson mediating the interaction between dark matter and the Standard Model as well as a two-Higgs-doublet model containing an additional Z‧ boson which decays to a Standard Model Higgs boson and a new pseudoscalar Higgs boson, the latter decaying into a pair of dark matter particles.

 

Title:
Search for new resonances decaying to a W or Z boson and a Higgs boson in the ℓ+- b b bar , ℓνb b bar , and ν ν bar b b bar channels with 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 2848 coauthors
Publication:
Physics Letters B, Volume 765, p. 32-52.
Publication Date:
02/2017
Origin:
ELSEVIER
Abstract Copyright:
(c) 2017 Elsevier Science B.V. All rights reserved.
DOI:
10.1016/j.physletb.2016.11.045
Bibliographic Code:
2017PhLB..765...32A

Abstract

A search is presented for new resonances decaying to a W or Z boson and a Higgs boson in the ℓ+- b b bar, ℓνb b bar , and ν ν bar b b bar channels in pp collisions at √{ s} = 13 TeV with the ATLAS detector at the Large Hadron Collider using a total integrated luminosity of 3.2 fb-1. The search is conducted by looking for a localized excess in the WH / ZH invariant or transverse mass distribution. No significant excess is observed, and the results are interpreted in terms of constraints on a simplified model based on a phenomenological Lagrangian of heavy vector triplets.

 

Title:
Measurement of W boson angular distributions in events with high transverse momentum jets at √{ s} = 8 TeV using the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2850 coauthors
Publication:
Physics Letters B, Volume 765, p. 132-153.
Publication Date:
02/2017
Origin:
ELSEVIER
Abstract Copyright:
(c) 2017 Elsevier Science B.V. All rights reserved.
DOI:
10.1016/j.physletb.2016.12.005
Bibliographic Code:
2017PhLB..765..132A

Abstract

The W boson angular distribution in events with high transverse momentum jets is measured using data collected by the ATLAS experiment from proton-proton collisions at a centre-of-mass energy √{ s} = 8 TeV at the Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb-1. The focus is on the contributions to W +jets processes from real W emission, which is achieved by studying events where a muon is observed close to a high transverse momentum jet. At small angular separations, these contributions are expected to be large. Various theoretical models of this process are compared to the data in terms of the absolute cross-section and the angular distributions of the muon from the leptonic W decay.

 

Title:
Shear viscosity at the Ising-nematic quantum critical point in two-dimensional metals
Authors:
Eberlein, Andreas; Patel, Aavishkar A.; Sachdev, Subir
Publication:
Physical Review B, Volume 95, Issue 7, id.075127 (PhRvB Homepage)
Publication Date:
02/2017
Origin:
APS
Abstract Copyright:
2017: American Physical Society
DOI:
10.1103/PhysRevB.95.075127
Bibliographic Code:
2017PhRvB..95g5127E

Abstract

In an isotropic strongly interacting quantum liquid without quasiparticles, general scaling arguments imply that the dimensionless ratio (kB/ℏ )η /s , where η is the shear viscosity and s is the entropy density, is a universal number. We compute the shear viscosity of the Ising-nematic critical point of metals in spatial dimension d =2 by an expansion below d =5 /2 . The anisotropy associated with directions parallel and normal to the Fermi surface leads to a violation of the scaling expectations: η scales in the same manner as a chiral conductivity, and the ratio η /s diverges at low temperature (T ) as T-2 /z, where z is the dynamic critical exponent for fermionic excitations dispersing normal to the Fermi surface.

 

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:
Physical Review B, Volume 95, Issue 7, id.075420 (PhRvB Homepage)
Publication Date:
02/2017
Origin:
APS
Abstract Copyright:
2017: American Physical Society
DOI:
10.1103/PhysRevB.95.075420
Bibliographic Code:
2017PhRvB..95g5420C

Abstract

The ability in experiments to control the relative twist angle between successive layers in two-dimensional (2D) materials offers an 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 MoS2, a representative of the transition-metal dichalcogenide family of 2D semiconductors, to illustrate the application of our method, which enables fast and efficient simulation of multilayered stacks in the presence of local disorder and external fields. We comment on the interesting features of their density of states as a function of twist angle and local configuration and on how these features can be experimentally observed.

 

Title:
Search for anomalous electroweak production of W W /W Z in association with a high-mass dijet system in p p collisions at √{s }=8 TeV with the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2843 coauthors
Publication:
Physical Review D, Volume 95, Issue 3, id.032001 (PhRvD Homepage)
Publication Date:
02/2017
Origin:
APS
Abstract Copyright:
2017: CERN
DOI:
10.1103/PhysRevD.95.032001
Bibliographic Code:
2017PhRvD..95c2001A

Abstract

A search is presented for anomalous quartic gauge boson couplings in vector-boson scattering. The data for the analysis correspond to 20.2 fb-1 of √{s }=8 TeV p p collisions and were collected in 2012 by the ATLAS experiment at the Large Hadron Collider. The search looks for the production of W W or W Z boson pairs accompanied by a high-mass dijet system, with one W decaying leptonically and a W or Z decaying hadronically. The hadronically decaying W /Z is reconstructed as either two small-radius jets or one large-radius jet using jet substructure techniques. Constraints on the anomalous quartic gauge boson coupling parameters α4 and α5 are set by fitting the transverse mass of the diboson system, and the resulting 95% confidence intervals are -0.024 <α4<0.030 and -0.028 <α5<0.033.

 

Title:
Magnetic resonance spectroscopy of an atomically thin material using a single-spin qubit
Authors:
Lovchinsky, I.; Sanchez-Yamagishi, J. D.; Urbach, E. K.; Choi, S.; Fang, S.; Andersen, T. I.; Watanabe, K.; Taniguchi, T.; Bylinskii, A.; Kaxiras, E.; Kim, P.; Park, H.; Lukin, M. D.
Publication:
Science, Volume 355, Issue 6324, pp. 503-507 (2017). (Sci Homepage)
Publication Date:
02/2017
Category:
APP PHYSICS
Origin:
SCIENCE
Abstract Copyright:
(c) 2017: Science
DOI:
10.1126/science.aal2538
Bibliographic Code:
2017Sci...355..503L

Abstract

Two-dimensional (2D) materials offer a promising platform for exploring condensed matter phenomena and developing technological applications. However, the reduction of material dimensions to the atomic scale poses a challenge for traditional measurement and interfacing techniques that typically couple to macroscopic observables. We demonstrate a method for probing the properties of 2D materials via nanometer-scale nuclear quadrupole resonance (NQR) spectroscopy using individual atomlike impurities in diamond. Coherent manipulation of shallow nitrogen-vacancy (NV) color centers enables the probing of nanoscale ensembles down to approximately 30 nuclear spins in atomically thin hexagonal boron nitride (h-BN). The characterization of low-dimensional nanoscale materials could enable the development of new quantum hybrid systems, combining atomlike systems coherently coupled with individual atoms in 2D materials.

 

Title:
Observation of the Wigner-Huntington transition to metallic hydrogen
Authors:
Dias, Ranga P.; ; Silvera, Isaac F.
Publication:
Science, Volume 355, Issue 6326, pp. 715-718 (2017). (Sci Homepage)
Publication Date:
02/2017
Category:
PHYSICS
Origin:
SCIENCE
Abstract Copyright:
(c) 2017: Science
DOI:
10.1126/science.aal1579
Bibliographic Code:
2017Sci...355..715D

Abstract

Producing metallic hydrogen has been a great challenge in condensed matter physics. Metallic hydrogen may be a room-temperature superconductor and metastable when the pressure is released and could have an important impact on energy and rocketry. We have studied solid molecular hydrogen under pressure at low temperatures. At a pressure of 495 gigapascals, hydrogen becomes metallic, with reflectivity as high as 0.91. We fit the reflectance using a Drude free-electron model to determine the plasma frequency of 32.5 ± 2.1 electron volts at a temperature of 5.5 kelvin, with a corresponding electron carrier density of 7.7 ± 1.1 × 1023 particles per cubic centimeter, which is consistent with theoretical estimates of the atomic density. The properties are those of an atomic metal. We have produced the Wigner-Huntington dissociative transition to atomic metallic hydrogen in the laboratory.

 

Title:
Imaging electron motion in graphene
Authors:
Bhandari, Sagar; Westervelt, Robert M.
Publication:
Semiconductor Science and Technology, Volume 32, Issue 2, article id. 024001 (2017).
Publication Date:
02/2017
Origin:
IOP
DOI:
10.1088/1361-6641/32/2/024001
Bibliographic Code:
2017SeScT..32b4001B

Abstract

A cooled scanning probe microscope (SPM) is an ideal tool to image electronic motion in graphene: the SPM tip acts as a scanning gate, which interacts with the electron gas below. We introduce the technique using our group’s previous work on imaging electron flow from a quantum point contact in a GaAs 2DEG and tuning an InAs quantum dot in an InAs/InP nanowire. Carriers in graphene have very different characteristics: electrons and holes travel at a constant speed with no bandgap, and they pass through potential barriers via Klein tunneling. In this paper, we review the extension of SPM imaging techniques to graphene. We image the cyclotron orbits passing between two narrow contacts in a single-atomic-layer graphene device in a perpendicular magnetic field. Magnetic focusing produces a peak in transmission between the contacts when the cyclotron diameter is equal to the contact spacing. The charged SPM tip deflects electrons passing from one contact to the other, changing the transmission when it interrupts the flow. By displaying the change in transmission as the tip is raster scanned above the sample, an image of flow is obtained. In addition, we have developed a complementary technique to image electronic charge using a cooled scanning capacitance microscope (SCM) that uses a sensitive charge preamplifier near the SPM tip to achieve a charge noise level 0.13 e Hz-1/2 with high spatial resolution 100 nm. The cooled SPM and SCM can be used to probe the motion of electrons on the nanoscale in graphene devices.

 


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