Faculty Publications: January, 2017

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

 go to Most Recent Faculty Publications

Title:
New Frontiers in Cosmology
Authors:
Dvorkin, Cora
Affiliation:
AA(Harvard University)
Publication:
APS April Meeting 2017, abstract #Q1.002
Publication Date:
01/2017
Origin:
APS
Bibliographic Code:
2017APS..APR.Q1002D

Abstract

Observations of the Cosmic Microwave Background and the Large-Scale Structure of the Universe carry immense promise for measurements of new physics beyond the Standard Models of cosmology and particle physics. In this talk, I will explain how we can use these observations to reconstruct the detailed physics of the Universe's earliest epoch, when it was only a tiny fraction of a second old.

 

Title:
Probing dim point sources in the inner Milky Way using PCAT
Authors:
Daylan, Tansu; Portillo, Stephen K. N.; Finkbeiner, Douglas P.
Publication:
APS April Meeting 2017, abstract #R5.005
Publication Date:
01/2017
Origin:
APS
Bibliographic Code:
2017APS..APR.R5005D

Abstract

Poisson regression of the Fermi-LAT data in the inner Milky Way reveals an extended gamma-ray excess. An important question is whether the signal is coming from a collection of unresolved point sources, possibly old recycled pulsars, or constitutes a truly diffuse emission component. Previous analyses have relied on non-Poissonian template fits or wavelet decomposition of the Fermi-LAT data, which find evidence for a population of dim point sources just below the 3FGL flux limit. In order to be able to draw conclusions about the flux distribution of point sources at the dim end, we employ a Bayesian trans-dimensional MCMC framework by taking samples from the space of catalogs consistent with the observed gamma-ray emission in the inner Milky Way. The software implementation, PCAT (Probabilistic Cataloger), is designed to efficiently explore that catalog space in the crowded field limit such as in the galactic plane, where the model PSF, point source positions and fluxes are highly degenerate. We thus generate fair realizations of the underlying MSP population in the inner galaxy and constrain the population characteristics such as the radial and flux distribution of such sources.

 

Title:
CMB B-mode Polarization Measurements and constraints on Primordial Gravitational Waves from the BICEP/Keck Program at South Pole
Authors:
Kovac, John; Bicep/Keck Collaboration
Publication:
APS April Meeting 2017, abstract #Y5.008
Publication Date:
01/2017
Origin:
APS
Bibliographic Code:
2017APS..APR.Y5008K

Abstract

The BICEP/Keck Array cosmic microwave background (CMB) polarization experiments located at the South Pole are a series of small-aperture refracting telescopes designed to probe the degree-scale B-mode signature of primordial gravitational waves. These highly-targeted experiments have produced the world's deepest maps of CMB polarization, leading to the most stringent constraints on the tensor-to-scalar ratio to date: r < 0 . 09 from B-modes alone, and r < 0 . 07 in combination with other datasets. These limits are rapidly improving with ongoing measurements at the multiple frequencies needed to separate Galactic foregrounds from the CMB, and in combination with higher-resolution experiments to remove B-modes induced by gravitational lensing. I will review the current status of measurements and results, and will discuss the challenges that will be confronted as measurements reach sensitivities to primordial gravitational waves at a level r < 0 . 01 and below within the next several years.

 

Title:
Loop-Corrected Virasoro Symmetry of 4D Quantum Gravity
Authors:
He, Temple; Kapec, Daniel; Raclariu, Ana-Maria; Strominger, Andrew
Publication:
eprint arXiv:1701.00496
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
High Energy Physics - Theory, General Relativity and Quantum Cosmology
Comment:
12 pages
Bibliographic Code:
2017arXiv170100496H

Abstract

Recently a boundary energy-momentum tensor $T_{zz}$ has been constructed from the soft graviton operator for any 4D quantum theory of gravity in asymptotically flat space. Up to an "anomaly" which is one-loop exact, $T_{zz}$ generates a Virasoro action on the 2D celestial sphere at null infinity. Here we show by explicit construction that the effects of the IR divergent part of the anomaly can be eliminated by a one-loop renormalization that shifts $T_{zz}$.

 

Title:
Argyres-Douglas matter and N=2 dualities
Authors:
Xie, Dan; Yau, Shing-Tung
Publication:
eprint arXiv:1701.01123
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
High Energy Physics - Theory
Comment:
38 pages, 24 figures
Bibliographic Code:
2017arXiv170101123X

Abstract

We study S duality of four dimensional N=2 Argyres-Douglas (AD) theory engineered from 6d A_{N-1} (2,0) theory. We find a (p,q) sequence of SCFTs, here (p,q) is co-prime and class S theory defined on sphere corresponds to class (0,1) theory. We represent these theories by a sphere with marked points, and S duality is interpreted as different pants decompositions of the same punctured sphere. The weakly coupled gauge theory description involves gauging AD matter which is represented by three punctured sphere.

 

Title:
Radical chiral Floquet phases in a periodically driven Kitaev model and beyond
Authors:
Po, Hoi Chun; Fidkowski, Lukasz; Vishwanath, Ashvin; Potter, Andrew C.
Publication:
eprint arXiv:1701.01440
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Strongly Correlated Electrons, Condensed Matter - Disordered Systems and Neural Networks, Condensed Matter - Quantum Gases
Comment:
4+8 pages, 3 figures
Bibliographic Code:
2017arXiv170101440P

Abstract

Time periodic driving serves not only as a convenient way to engineer effective Hamiltonians, but also as a means to produce intrinsically dynamical phases that do not exist in the static limit. A recent example of the latter are 2D chiral Floquet (CF) phases exhibiting anomalous edge dynamics that pump discrete packets of quantum information along one direction. In non-fractionalized systems with only bosonic excitations, this pumping is quantified by a dynamical topological index that is a rational number -- highlighting its difference from the integer valued invariant underlying equilibrium chiral phases (e.g. quantum Hall systems). Here, we explore CF phases in systems with emergent anyon excitations that have fractional statistics (Abelian topological order). Despite the absence of mobile non-Abelian particles in these systems, external driving can supply the energy to pump otherwise immobile non-Abelian defects (sometimes called twist defects or genons) around the boundary, thereby transporting an irrational fractional number of quantum bits along the edge during each drive period. This enables new CF phases with chiral indices that are square roots of rational numbers, inspiring the label: "radical CF phases". We demonstrate an unexpected bulk-boundary correspondence, in which the radical CF edge is tied to bulk dynamics that exchange electric and magnetic anyon excitations during each period. We construct solvable, stroboscopically driven versions of Kitaev's honeycomb spin model that realize these radical CF phases, and discuss their stability against heating in strongly disordered many-body localized settings or in the limit of rapid driving as an exponentially long-lived pre-thermal phenomena.

 

Title:
Many-body interferometry of magnetic polaron dynamics
Authors:
Ashida, Yuto; Schmidt, Richard; Tarruell, Leticia; Demler, Eugene
Publication:
eprint arXiv:1701.01454
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Quantum Gases, Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Strongly Correlated Electrons, Quantum Physics
Comment:
6+6 pages, 3+2 figures
Bibliographic Code:
2017arXiv170101454A

Abstract

Strongly imbalanced mixtures of ultracold atoms open new possibilities for exploring impurity physics. Recent studies of impurities interacting with a single-component, degenerate gas provided new insights into the physics of Bose and Fermi polarons. Here we demonstrate that an impurity atom immersed in a two-component Bose-Einstein condensate provides a unique platform to study magnetic polaron dynamics. We show that Ramsey interference of bath atoms enables a direct measurement of spin-wave excitations generated in the environment, which allows to study the formation of magnetic polarons in real time. Furthermore, such an interferometric approach reveals an interplay of few- and many-body physics that is signified by single- and multi-frequency oscillatory spin dynamics corresponding to the formation of many-body bound states. We discuss a concrete experimental implementation for the observation of the predicted phenomena.

 

Title:
2D spin-orbit coupling for ultracold atoms in optical lattices
Authors:
Grusdt, Fabian; Li, Tracy; Bloch, Immanuel; Demler, Eugene
Publication:
eprint arXiv:1701.02111
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Quantum Gases, Physics - Atomic Physics, Quantum Physics
Comment:
4 pages, 4 figures, 4 pages supplementary material
Bibliographic Code:
2017arXiv170102111G

Abstract

Spin-orbit coupling (SOC) is at the heart of many exotic band-structures and can give rise to many-body states with topological order. Here we present a general scheme based on a combination of microwave driving and lattice shaking for the realization of time-reversal invariant 2D SOC with ultracold atoms in systems with inversion symmetry. We show that the strengths of Rashba and Dresselhaus SOC can be independently tuned in a spin-dependent square lattice. More generally, our method can be used to open gaps between different spin states without breaking time-reversal symmetry. We demonstrate that this allows for the realization of topological insulators in the presence of SOC, which is closely related to the Kane-Mele model.

 

Title:
Accurate formation energies of charged defects in solids: a systematic approach
Authors:
Vinichenko, Dmitry; Gokhan Sensoy, M.; Friend, Cynthia M.; Kaxiras, Efthimios
Publication:
eprint arXiv:1701.02521
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Mesoscale and Nanoscale Physics
Bibliographic Code:
2017arXiv170102521V

Abstract

Defects on surfaces of semiconductors have a strong effect on their reactivity and catalytic properties. The concentration of different charge states of defects is determined by their formation energies. First-principles calculations are an important tool for computing defect formation energies and for studying the microscopic environment of the defect. The main problem associated with the widely used supercell method in these calculations is the error in the electrostatic energy, which is especially pronounced in calculations that involve surface slabs and 2D materials. We present an internally consistent approach for calculating defect formation energies in inhomogeneous and anisotropic dielectric environments, and demonstrate its applicability to the cases of the positively charged Cl vacancy on the NaCl (100) surface and the negatively charged S vacancy in monolayer MoS2.

 

Title:
Spectrum of the Wilson-Fisher conformal field theory on the torus
Authors:
Whitsitt, Seth; Schuler, Michael; Henry, Louis-Paul; Läuchli, Andreas M.; Sachdev, Subir
Publication:
eprint arXiv:1701.03111
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Strongly Correlated Electrons, Condensed Matter - Statistical Mechanics, High Energy Physics - Theory
Comment:
52 pages, 10 figures
Bibliographic Code:
2017arXiv170103111W

Abstract

We study the finite-size spectrum of the O($N$) symmetric Wilson-Fisher conformal field theory (CFT) on the $d=2$ spatial-dimension torus using the expansion in $\epsilon=3-d$. This is done by deriving a set of universal effective Hamiltonians describing fluctuations of the zero momentum modes. The effective Hamiltonians take the form of $N$-dimensional quantum anharmonic oscillators, which are shown to be strongly coupled at the critical point for small $\epsilon$. The low-energy spectrum is solved numerically for $N = 1,2,3,4$. Using exact diagonalization (ED), we also numerically study explicit lattice models known to be in the O($2$) and O($3$) universality class, obtaining estimates of the low-lying critical spectrum. The analytic and numerical results show excellent agreement and the critical low energy torus spectra are qualitatively different among the studied CFTs, identifying them as a useful fingerprint for detecting the universality class of a quantum critical point.

 

Title:
Ultracold Molecular Assembly
Authors:
Liu, Lee R.; Zhang, Jessie T.; Yu, Yichao; Hutzler, Nicholas R.; Liu, Yu; Rosenband, Till; Ni, Kang-Kuen
Publication:
eprint arXiv:1701.03121
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
Physics - Atomic Physics, Condensed Matter - Quantum Gases, Physics - Chemical Physics
Bibliographic Code:
2017arXiv170103121L

Abstract

Chemical reactions can be surprisingly efficient at ultracold temperatures ( < 1mK) due to the wave nature of atoms and molecules. The study of reactions in the ultracold regime is a new research frontier enabled by cooling and trapping techniques developed in atomic and molecular physics. In addition, ultracold molecular gases that offer diverse molecular internal states and large electric dipolar interactions are sought after for studies of strongly interacting many-body quantum physics. Here we propose a new approach for producing ultracold molecules in the absolute internal and motional quantum ground state, where single molecules are assembled one by one from individual atoms. The scheme involves laser cooling, optical trapping, Raman sideband cooling, and coherent molecular state transfer. As a crucial initial step, we demonstrate quantum control of constituent atoms, including 3D ground-state cooling of a single Cs atom, in a simple apparatus. As laser technology advances to shorter wavelengths, additional atoms will be amenable to laser-cooling, allowing more diverse, and eventually more complex, molecules to be assembled with full quantum control.

 

Title:
One dimensional magneto-optical compression of a cold CaF molecular beam
Authors:
Chae, Eunmi; Anderegg, Loic; Augenbraun, Benjamin L.; Ravi, Aakash; Hemmerling, Boerge; Hutzler, Nicholas R.; Collopy, Alejandra L.; Ye, Jun; Ketterle, Wolfgang; Doyle, John M.
Publication:
eprint arXiv:1701.03254
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
Physics - Atomic Physics
Bibliographic Code:
2017arXiv170103254C

Abstract

We demonstrate with a RF-MOT the one dimensional, transverse magneto-optical compression of a cold beam of calcium monofluoride (CaF). By continually alternating the magnetic field direction and laser polarizations of the magneto-optical trap, a photon scattering rate of $2\pi \times$0.4 MHz is achieved. A 3D model for this RF-MOT, validated by agreement with data, predicts a 3D RF-MOT capture velocity for CaF of 5 m/s.

 

Title:
Conductivity and Dissociation in Metallic Hydrogen: Implications for Planetary Interiors
Authors:
Zaghoo, Mohamed; Silvera, Isaac F.
Publication:
eprint arXiv:1701.03532
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Materials Science
Bibliographic Code:
2017arXiv170103532Z

Abstract

Liquid metallic hydrogen (LMH) was recently produced under static compression and high temperatures in bench-top experiments. Here, we report a study of the optical reflectance of LMH in the pressure region of 1.4-1.7 Mbar and use the Drude free-electron model to determine its optical conductivity. We find static electrical conductivity of metallic hydrogen to be 11,000-15,000 S/cm. A substantial dissociation fraction is required to best fit the energy dependence of the observed reflectance. LMH at our experimental conditions is largely atomic and degenerate, not primarily molecular. We determine a plasma frequency and the optical conductivity. Properties are used to analyze planetary structure of hydrogen rich planets such as Jupiter.

 

Title:
Measurement of the neutrino mixing angle $\theta_{23}$ in NOvA
Authors:
The NOvA Collaboration; Adamson, P.; Allakhverdian, V.; Ambrose, D.; ... Feldman, G. J.;... and 175 coauthors
Publication:
eprint arXiv:1701.05891
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
High Energy Physics - Experiment
Comment:
7 pages, 3 figures
Bibliographic Code:
2017arXiv170105891T

Abstract

This Letter reports new results on muon neutrino disappearance from NOvA, using a 14 kton detector equivalent exposure of $6.05\times10^{20}$ protons-on-target from the NuMI beam at the Fermi National Accelerator Laboratory. The measurement probes the muon-tau symmetry hypothesis that requires maximal mixing ($\theta_{23} = \pi/4$). Assuming the normal mass hierarchy, we find $\Delta m^2 = (2.67 \pm 0.11)\times 10^{-3}$ eV$^2$ and $\sin^2 \theta_{23}$ at the two statistically degenerate values $0.404^{+0.030}_{-0.022}$ and $0.624^{+0.022}_{-0.030}$, both at the 68% confidence level. Our data disfavor the maximal mixing scenario with 2.6 $\sigma$ significance.

 

Title:
Probing dark excitons in atomically thin semiconductors via near-field coupling to surface plasmon polaritons
Authors:
Zhou, You; Scuri, Giovanni; Wild, Dominik S.; High, Alexander A.; Dibos, Alan; Jauregui, Luis A.; Shu, Chi; de Greve, Kristiaan; Pistunova, Kateryna; Joe, Andrew; Taniguchi, Takashi; Watanabe, Kenji; Kim, Philip; Lukin, Mikhail D.; Park, Hongkun
Publication:
eprint arXiv:1701.05938
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Mesoscale and Nanoscale Physics
Comment:
4 figures
Bibliographic Code:
2017arXiv170105938Z

Abstract

Transition metal dichalcogenide (TMD) monolayers are direct bandgap semiconductors that feature tightly bound excitons, strong spin-orbit coupling, and spin-valley degrees of freedom. Depending on the spin configuration of the electron-hole pairs, intra-valley excitons of TMD monolayers can be either optically bright or dark. Dark excitons involve nominally spin-forbidden optical transitions with zero in-plane transition dipole moment, making their detection with conventional far-field optical techniques challenging. Here, we introduce a new method for probing the optical properties of two-dimensional (2D) materials via near-field coupling to surface plasmon polaritons (SPPs), which selectively enhances optical transitions with dipole moments normal to the 2D plane. We utilize this method to directly detect dark excitons in monolayer TMDs. When a WSe2 monolayer is placed on top of a single-crystal silver film, its emission into near-field-coupled SPPs displays new spectral features whose energies and dipole orientations are consistent with dark neutral and charged excitons. The SPP-based near-field spectroscopy significantly enhances experimental capabilities for probing and manipulating exciton dynamics of atomically thin materials.

 

Title:
Pairing in Luttinger Liquids and Quantum Hall States
Authors:
Kane, Charles L.; Stern, Ady; Halperin, Bertrand I.
Publication:
eprint arXiv:1701.06200
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Strongly Correlated Electrons
Comment:
16 pages
Bibliographic Code:
2017arXiv170106200K

Abstract

We study spinless electrons in a single channel quantum wire interacting through attractive interaction, and the quantum Hall states that may be constructed by an array of such wires. For a single wire the electrons may form two phases, the Luttinger liquid and the strongly paired phase. The Luttinger liquid is gapless to one- and two-electron excitations, while the strongly paired state is gapped to the former and gapless to the latter. In contrast to the case in which the wire is proximity-coupled to an external superconductor, for an isolated wire there is no separate phase of a topological, weakly paired, superconductor. Rather, this phase is adiabatically connected to the Luttinger liquid phase. The properties of the one dimensional topological superconductor emerge when the number of channels in the wire becomes large. The quantum Hall states that may be formed by an array of single-channel wires depend on the Landau level filling factors. For odd-denominator fillings $\nu=1/(2n+1)$, wires at the Luttinger phase form Laughlin states while wires in the strongly paired phase form bosonic fractional quantum Hall state of strongly-bound pairs at a filling of $1/(8n+4)$. The transition between the two is of the universality class of Ising transitions in three dimensions. For even-denominator fractions $\nu=1/2n$ the two single-wire phases translate into four quantum Hall states. Two of those states are bosonic fractional quantum Hall states of weakly- and strongly- bound pairs of electrons. The other two are non-Abelian quantum Hall states, which originate from coupling wires close to their critical point. One of these non-Abelian states is the Moore-Read state. The transition between all these states are of the universality class of Majorana transitions. We point out some of the properties that characterize the different phases and the phase transitions.

 

Title:
BPS spectra and 3-manifold invariants
Authors:
Gukov, Sergei; Pei, Du; Putrov, Pavel; Vafa, Cumrun
Publication:
eprint arXiv:1701.06567
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
High Energy Physics - Theory, Mathematical Physics, Mathematics - Geometric Topology, Mathematics - Quantum Algebra
Comment:
72 pages, 6 figures
Bibliographic Code:
2017arXiv170106567G

Abstract

We provide a physical definition of new homological invariants $\mathcal{H}_a (M_3)$ of 3-manifolds (possibly, with knots) labeled by abelian flat connections. The physical system in question involves a 6d fivebrane theory on $M_3$ times a 2-disk, $D^2$, whose Hilbert space of BPS states plays the role of a basic building block in categorification of various partition functions of 3d $\mathcal{N}=2$ theory $T[M_3]$: $D^2\times S^1$ half-index, $S^2\times S^1$ superconformal index, and $S^2\times S^1$ topologically twisted index. The first partition function is labeled by a choice of boundary condition and provides a refinement of Chern-Simons (WRT) invariant. A linear combination of them in the unrefined limit gives the analytically continued WRT invariant of $M_3$. The last two can be factorized into the product of half-indices. We show how this works explicitly for many examples, including Lens spaces, circle fibrations over Riemann surfaces, and plumbed 3-manifolds.

 

Title:
Imaging electron flow and quantum dot formation in MoS2 nanostructures
Authors:
Bhandari, Sagar; Wang, Ke; Watanabe, Kenji; Taniguchi, Takashi; Kim, Philip; Westervelt, Robert M.
Publication:
eprint arXiv:1701.07532
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Mesoscale and Nanoscale Physics
Comment:
17 pages, 5 figures
Bibliographic Code:
2017arXiv170107532B

Abstract

Among newly discovered two-dimensional (2D) materials, semiconducting ultrathin sheets of MoS2 show potential for nanoelectronics. However, the carrier mobility in MoS2 is limited by scattering from surface impurities and the substrate. To probe the sources of scattering, we use a cooled scanning probe microscope (SPM) to image the flow of electrons in a MoS2 Hall bar sample at 4.2 K. Capacitive coupling to the SPM tip changes the electron density below and scatters electrons flowing nearby; an image of flow can be obtained by measuring the change in resistance between two contacts as the tip is raster scanned across the sample. We present images of current flow through a large contact that decay exponentially away from the sample edge. In addition, the images show the characteristic "bullseye" pattern of Coulomb blockade conductance rings around a quantum dot as the density is depleted with a back gate. We estimate the size and position of these quantum dots using a capacitive model.

 

Title:
Asymmetric resonances in micro-ring resonators
Authors:
Reshef, Orad; Moebius, Michael G.; Mazur, Eric
Publication:
eprint arXiv:1701.08464
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
Physics - Optics
Comment:
7 pages, 4 figures
Bibliographic Code:
2017arXiv170108464R

Abstract

Propagation losses in micro-ring resonator waveguides can be determined from the shape of individual resonances in their transmission spectrum. The losses are typically extracted by fitting these resonances to an idealized model that is derived using scattering theory. Reflections caused by waveguide boundaries or stitching errors, however, cause the resonances to become asymmetric, resulting in poor fits and unreliable propagation loss coefficients. We derive a model that takes reflections into account and, by performing full-wave simulations, we show that this model accurately describes the asymmetric resonances that result from purely linear effects, yielding accurate propagation loss coefficients.

 

Title:
Nested Hilbert schemes on surfaces: Virtual fundamental class
Authors:
Gholampour, Amin; Sheshmani, Artan; Yau, Shing-Tung
Publication:
eprint arXiv:1701.08899
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
Mathematics - Algebraic Geometry
Comment:
47 pages
Bibliographic Code:
2017arXiv170108899G

Abstract

We construct natural virtual fundamental classes for nested Hilbert schemes on a nonsingular projective surface $S$. This allows us to define new invariants of $S$ that recover some of the known important cases such as Poincare invariants of Durr-Kabanov-Okonek and the stable pair invariants of Kool-Thomas. In the case of the nested Hilbert scheme of points, we can express these invariants in terms of integrals over the products of Hilbert scheme of points on $S$, and relate them to the vertex operator formulas found by Carlsson-Okounkov. The virtual fundamental classes of the nested Hilbert schemes play a crucial role in the Donaldson-Thomas theory of local-surface-threefolds that we study in [GSY17b].

 

Title:
Localized Donaldson-Thomas theory of surfaces
Authors:
Gholampour, Amin; Sheshmani, Artan; Yau, Shing-Tung
Publication:
eprint arXiv:1701.08902
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
Mathematics - Algebraic Geometry
Comment:
28 pages
Bibliographic Code:
2017arXiv170108902G

Abstract

Let $S$ be a projective simply connected complex surface and $\mathcal{L}$ be a line bundle on $S$. We study the moduli space of stable compactly supported 2-dimensional sheaves on the total spaces of $\mathcal{L}$. The moduli space admits a $\mathbb{C}^*$-action induced by scaling the fibers of $\mathcal{L}$. We identify certain components of the fixed locus of the moduli space with the moduli space of torsion free sheaves and the nested Hilbert schemes on $S$. We define the localized Donaldson-Thomas invariants of $\mathcal{L}$ by virtual localization in the case that $\mathcal{L}$ twisted by the anti-canonical bundle of $S$ admits a nonzero global section. When $p_g(S)>0$, in combination with Mochizuki's formulas, we are able to express the localized DT invariants in terms of the invariants of the nested Hilbert schemes defined by the authors in [GSY17a], the Seiberg-Witten invariants of $S$, and the integrals over the products of Hilbert schemes of points on $S$. When $\mathcal{L}$ is the canonical bundle of $S$, the Vafa-Witten invariants defined recently by Tanaka-Thomas, can be extracted from these localized DT invariants. VW invariants are expected to have modular properties as predicted by S-duality.

 

Title:
Optimal design of experiments by combining coarse and fine measurements
Authors:
Lee, Alpha A.; Brenner, Michael P.; Colwell, Lucy J.
Publication:
eprint arXiv:1702.06001
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
Physics - Data Analysis, Statistics and Probability, Condensed Matter - Soft Condensed Matter, Condensed Matter - Statistical Mechanics, Physics - Chemical Physics
Bibliographic Code:
2017arXiv170206001L

Abstract

In many contexts it is extremely costly to perform enough high quality experimental measurements to accurately parameterize a predictive quantitative model. However, it is often much easier to carry out experiments that indicate whether a particular sample is above or below a given threshold. Can many such binary or "coarse" measurements be combined with a much smaller number of higher resolution or "fine" measurements to yield accurate models? Here, we demonstrate an intuitive strategy, inspired by statistical physics, wherein the coarse measurements identify the salient features of the data, while fine measurements determine the relative importance of these features. We illustrate our strategy by considering the problem of solubility prediction for small organic molecule from their 2D molecular structure.

 

Title:
Higher spin realization of the DS/CFT correspondence
Authors:
Anninos, Dionysios; Hartman, Thomas; Strominger, Andrew
Publication:
Classical and Quantum Gravity, Volume 34, Issue 1, article id. 015009 (2017). (CQGra Homepage)
Publication Date:
01/2017
Origin:
IOP
DOI:
10.1088/1361-6382/34/1/015009
Bibliographic Code:
2017CQGra..34a5009A

Abstract

We conjecture that Vasiliev’s theory of higher spin gravity in four-dimensional de Sitter space (dS4) is holographically dual to a three-dimensional conformal field theory (CFT3) living on the spacelike boundary of dS4 at future timelike infinity. The CFT3 is the Euclidean Sp(N) vector model with anticommuting scalars. The free CFT3 flows under a double-trace deformation to an interacting CFT3 in the IR. We argue that both CFTs are dual to Vasiliev dS4 gravity but with different future boundary conditions on the bulk scalar field. Our analysis rests heavily on analytic continuations of bulk and boundary correlators in the proposed duality relating the O(N) model with Vasiliev gravity in AdS4.

 

Title:
Evaluating the paleomagnetic potential of single zircon crystals using the Bishop Tuff
Authors:
Fu, Roger R.; Weiss, Benjamin P.; Lima, Eduardo A.; Kehayias, Pauli; Araujo, Jefferson F. D. F.; Glenn, David R.; Gelb, Jeff; Einsle, Joshua F.; Bauer, Ann M.; Harrison, Richard J.; Ali, Guleed A. H.; Walsworth, Ronald L.
Publication:
Earth and Planetary Science Letters, Volume 458, p. 1-13. (E&PSL Homepage)
Publication Date:
01/2017
Origin:
ELSEVIER
Keywords:
paleomagnetism, rock magnetism, zircons, Jack Hills, geodynamo, mineralogy
Abstract Copyright:
(c) 2017 Elsevier Science B.V. All rights reserved.
DOI:
10.1016/j.epsl.2016.09.038
Bibliographic Code:
2017E&PSL.458....1F

Abstract

Zircon crystals offer a unique combination of suitability for high-precision radiometric dating and high resistance to alteration. Paleomagnetic experiments on ancient zircons may potentially constrain the history of the earliest geodynamo, which would hold broad implications for the early Earth's interior and atmosphere. However, the ability of zircons to record accurately the geomagnetic field has not been demonstrated. Here we conduct thermal and alternating field (AF) paleointensity experiments on 767.1 thousand year old (ka) zircons from the Bishop Tuff, California. The rapid emplacement of these zircons in a well-characterized magnetic field provides a high-fidelity test of the zircons' intrinsic paleomagnetic recording accuracy. Successful dual heating experiments on eleven zircons measured using a superconducting quantum interference device (SQUID) microscope yield a mean paleointensity of 54.1 ± 6.8μT (1σ; 42.6 ± 5.3μT after excluding possible maghemite-bearing zircons), which is consistent with high-precision results from Bishop Tuff whole rock (43.0 ± 3.2μT). High-resolution quantum diamond magnetic (QDM) mapping, electron microscopy, and X-ray tomography indicate that the bulk of the remanent magnetization in Bishop Tuff zircons is carried by Fe oxides associated with apatite inclusions, which may be susceptible to destruction via metamorphism and aqueous alteration in older zircons. As such, while zircons can reliably record the geomagnetic field, robust zircon-derived paleomagnetic results require careful characterization of the ferromagnetic carrier and demonstration of their occurrence in primary inclusions. We further conclude that a combination of quantum diamond magnetometry and high-resolution imaging can provide detailed, direct characterization of the ferromagnetic mineralogy of geological samples.

 

Title:
A measurement of the calorimeter response to single hadrons and determination of the jet energy scale uncertainty using LHC Run-1 pp-collision data with the ATLAS detector
Authors:
Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2845 coauthors
Publication:
The European Physical Journal C, Volume 77, Issue 1, article id.26, 47 pp. (EPJC Homepage)
Publication Date:
01/2017
Origin:
SPRINGER
Abstract Copyright:
(c) 2017: CERN for the benefit of the ATLAS collaboration
DOI:
10.1140/epjc/s10052-016-4580-0
Bibliographic Code:
2017EPJC...77...26A

Abstract

A measurement of the calorimeter response to isolated charged hadrons in the ATLAS detector at the LHC is presented. This measurement is performed with 3.2 nb^{-1} of proton-proton collision data at √{s}=7 TeV from 2010 and 0.1 nb^{-1} of data at √{s}=8 TeV from 2012. A number of aspects of the calorimeter response to isolated hadrons are explored. After accounting for energy deposited by neutral particles, there is a 5% discrepancy in the modelling, using various sets of Geant4 hadronic physics models, of the calorimeter response to isolated charged hadrons in the central calorimeter region. The description of the response to anti-protons at low momenta is found to be improved with respect to previous analyses. The electromagnetic and hadronic calorimeters are also examined separately, and the detector simulation is found to describe the response in the hadronic calorimeter well. The jet energy scale uncertainty and correlations in scale between jets of different momenta and pseudorapidity are derived based on these studies. The uncertainty is 2-5% for jets with transverse momenta above 2 TeV, where this method provides the jet energy scale uncertainty for ATLAS.

 

Title:
Cores in Dwarf Galaxies from Fermi Repulsion
Authors:
Randall, Lisa; Scholtz, Jakub; Unwin, James
Publication:
Monthly Notices of the Royal Astronomical Society, Advance Access (MNRAS Homepage)
Publication Date:
01/2017
Origin:
OUP
Astronomy Keywords:
Cosmology: theory, dark matter, elementary particles, galaxies: dwarf
Abstract Copyright:
2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
DOI:
10.1093/mnras/stx161
Bibliographic Code:
2017MNRAS.tmp..169R

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 size ≳ 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 α 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:
Controlled finite momentum pairing and spatially varying order parameter in proximitized HgTe quantum wells
Authors:
Hart, Sean; Ren, Hechen; Kosowsky, Michael; Ben-Shach, Gilad; Leubner, Philipp; Brüne, Christoph; Buhmann, Hartmut; Molenkamp, Laurens W.; Halperin, Bertrand I.; Yacoby, Amir
Publication:
Nature Physics, Volume 13, Issue 1, pp. 87-93 (2017).
Publication Date:
01/2017
Origin:
NATURE
Abstract Copyright:
(c) 2017: Nature Publishing Group
DOI:
10.1038/nphys3877
Bibliographic Code:
2017NatPh..13...87H

Abstract

Conventional s-wave superconductivity arises from singlet pairing of electrons with opposite Fermi momenta, forming Cooper pairs with zero net momentum. Recent studies have focused on coupling s-wave superconductors to systems with an unusual configuration of electronic spin and momentum at the Fermi surface, where the nature of the paired state can be modified and the system may even undergo a topological phase transition. Here we present measurements and theoretical calculations of HgTe quantum wells coupled to aluminium or niobium superconductors and subject to a magnetic field in the plane of the quantum well. We find that this magnetic field tunes the momentum of Cooper pairs in the quantum well, directly reflecting the response of the spin-dependent Fermi surfaces. In the high electron density regime, the induced superconductivity evolves with electron density in agreement with our model based on the Hamiltonian of Bernevig, Hughes and Zhang. This agreement provides a quantitative value for g ˜/vF, where g ˜ is the effective g-factor and vF is the Fermi velocity. Our new understanding of the interplay between spin physics and superconductivity introduces a way to spatially engineer the order parameter from singlet to triplet pairing, and in general allows investigation of electronic spin texture at the Fermi surface of materials.

 

Title:
Search for heavy resonances decaying to a Z boson and a photon 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 764, p. 11-30.
Publication Date:
01/2017
Origin:
ELSEVIER
Abstract Copyright:
(c) 2017 Elsevier Science B.V. All rights reserved.
DOI:
10.1016/j.physletb.2016.11.005
Bibliographic Code:
2017PhLB..764...11A

Abstract

This Letter presents a search for new resonances with mass larger than 250 GeV, decaying to a Z boson and a photon. The dataset consists of an integrated luminosity of 3.2 fb-1 of pp collisions collected at √{ s} = 13 TeV with the ATLAS detector at the Large Hadron Collider. The Z bosons are identified through their decays either to charged, light, lepton pairs (e+e-, μ+μ-) or to hadrons. The data are found to be consistent with the expected background in the whole mass range investigated and upper limits are set on the production cross section times decay branching ratio to Zγ of a narrow scalar boson with mass between 250 GeV and 2.75 TeV.

 

Title:
Strain effects on the behavior of isolated and paired sulfur vacancy defects in monolayer MoS2
Authors:
Sensoy, Mehmet Gokhan; Vinichenko, Dmitry; Chen, Wei; Friend, Cynthia M.; Kaxiras, Efthimios
Publication:
Physical Review B, Volume 95, Issue 1, id.014106 (PhRvB Homepage)
Publication Date:
01/2017
Origin:
APS
Abstract Copyright:
2017: American Physical Society
DOI:
10.1103/PhysRevB.95.014106
Bibliographic Code:
2017PhRvB..95a4106S

Abstract

We investigate the behavior of sulfur vacancy defects, the most abundant type of intrinsic defect in monolayer MoS2, using first-principles calculations based on density functional theory. We consider the dependence of the isolated defect formation energy on the charge state and on uniaxial tensile and compressive strain up to 5%. We also consider the possibility of defect clustering by examining the formation energies of pairs of vacancies at various relative positions, and their dependence on charge state and strain. We find that there is no driving force for vacancy clustering, independent of strain in the material. The barrier for diffusion of S vacancies is larger than 1.9 eV in both charged and neutral states regardless of the presence of other nearby vacancies. We conclude that the formation of extended defects from S vacancies in planar monolayer MoS2 is hindered both thermodynamically and kinetically.

 

Title:
Electron spin-flip correlations due to nuclear dynamics in driven GaAs double dots
Authors:
Pal, Arijeet; Nichol, John M.; Shulman, Michael D.; Harvey, Shannon P.; Umansky, Vladimir; Rashba, Emmanuel I.; Yacoby, Amir; Halperin, Bertrand I.
Publication:
Physical Review B, Volume 95, Issue 3, id.035306 (PhRvB Homepage)
Publication Date:
01/2017
Origin:
APS
Abstract Copyright:
2017: American Physical Society
DOI:
10.1103/PhysRevB.95.035306
Bibliographic Code:
2017PhRvB..95c5306P

Abstract

We present experimental data and associated theory for correlations in a series of experiments involving repeated Landau-Zener sweeps through the crossing point of a singlet state and a spin-aligned triplet state in a GaAs double quantum dot containing two conduction electrons, which are loaded in the singlet state before each sweep, and the final spin is recorded after each sweep. The experiments reported here measure correlations on time scales from 4 μ s to 2 ms. When the magnetic field is aligned in a direction such that spin-orbit coupling cannot cause spin flips, the correlation spectrum has prominent peaks centered at zero frequency and at the differences of the Larmor frequencies of the nuclei, on top of a frequency-independent background. When the spin-orbit field is relevant, there are additional peaks, centered at the frequencies of the individual species. A theoretical model which neglects the effects of high-frequency charge noise correctly predicts the positions of the observed peaks, and gives a reasonably accurate prediction of the size of the frequency-independent background, but gives peak areas that are larger than the observed areas by a factor of 2 or more. The observed peak widths are roughly consistent with predictions based on nuclear dephasing times of the order of 60 μ s . However, there is extra weight at the lowest observed frequencies, which suggests the existence of residual correlations on the scale of 2 ms. We speculate on the source of these discrepancies.

 

Title:
Search for flavor-changing nonstandard neutrino interactions using νe appearance in MINOS
Authors:
Adamson, P.; Anghel, I.; Aurisano, A.;... Feldman, G. J.;... and 117 coauthors; Minos Collaboration
Publication:
Physical Review D, Volume 95, Issue 1, id.012005 (PhRvD Homepage)
Publication Date:
01/2017
Origin:
APS
Abstract Copyright:
2017: American Physical Society
DOI:
10.1103/PhysRevD.95.012005
Bibliographic Code:
2017PhRvD..95a2005A

Abstract

We report new constraints on flavor-changing nonstandard neutrino interactions from the MINOS long-baseline experiment using νe and ν¯e appearance candidate events from predominantly νμ and ν¯μ beams. We used a statistical selection algorithm to separate νe candidates from background events, enabling an analysis of the combined MINOS neutrino and antineutrino data. We observe no deviations from standard neutrino mixing, and thus place constraints on the nonstandard interaction matter effect, |ɛe τ|, and phase, (δC Pe τ) , using a 30-bin likelihood fit.

 

Title:
Supersymmetric Sachdev-Ye-Kitaev models
Authors:
Fu, Wenbo; Gaiotto, Davide; Maldacena, Juan; Sachdev, Subir
Publication:
Physical Review D, Volume 95, Issue 2, id.026009 (PhRvD Homepage)
Publication Date:
01/2017
Origin:
APS
Abstract Copyright:
2017: American Physical Society
DOI:
10.1103/PhysRevD.95.026009
Bibliographic Code:
2017PhRvD..95b6009F

Abstract

We discuss a supersymmetric generalization of the Sachdev-Ye-Kitaev (SYK) model. These are quantum mechanical models involving N Majorana fermions. The supercharge is given by a polynomial expression in terms of the Majorana fermions with random coefficients. The Hamiltonian is the square of the supercharge. The N =1 model with a single supercharge has unbroken supersymmetry at large N , but nonperturbatively spontaneously broken supersymmetry in the exact theory. We analyze the model by looking at the large N equation, and also by performing numerical computations for small values of N . We also compute the large N spectrum of "singlet" operators, where we find a structure qualitatively similar to the ordinary SYK model. We also discuss an N =2 version. In this case, the model preserves supersymmetry in the exact theory and we can compute a suitably weighted Witten index to count the number of ground states, which agrees with the large N computation of the entropy. In both cases, we discuss the supersymmetric generalizations of the Schwarzian action which give the dominant effects at low energies.

 

Title:
Discrete Time Crystals: Rigidity, Criticality, and Realizations
Authors:
Yao, N. Y.; Potter, A. C.; Potirniche, I.-D.; Vishwanath, A.
Publication:
Physical Review Letters, Volume 118, Issue 3, id.030401 (PhRvL Homepage)
Publication Date:
01/2017
Origin:
APS
Abstract Copyright:
2017: American Physical Society
DOI:
10.1103/PhysRevLett.118.030401
Bibliographic Code:
2017PhRvL.118c0401Y

Abstract

Despite being forbidden in equilibrium, spontaneous breaking of time translation symmetry can occur in periodically driven, Floquet systems with discrete time-translation symmetry. The period of the resulting discrete time crystal is quantized to an integer multiple of the drive period, arising from a combination of collective synchronization and many body localization. Here, we consider a simple model for a one-dimensional discrete time crystal which explicitly reveals the rigidity of the emergent oscillations as the drive is varied. We numerically map out its phase diagram and compute the properties of the dynamical phase transition where the time crystal melts into a trivial Floquet insulator. Moreover, we demonstrate that the model can be realized with current experimental technologies and propose a blueprint based upon a one dimensional chain of trapped ions. Using experimental parameters (featuring long-range interactions), we identify the phase boundaries of the ion-time-crystal and propose a measurable signature of the symmetry breaking phase transition.

 

Title:
Holography of the Dirac Fluid in Graphene with Two Currents
Authors:
Seo, Yunseok; Song, Geunho; Kim, Philip; Sachdev, Subir; Sin, Sang-Jin
Publication:
Physical Review Letters, Volume 118, Issue 3, id.036601 (PhRvL Homepage)
Publication Date:
01/2017
Origin:
APS
Abstract Copyright:
2017: American Physical Society
DOI:
10.1103/PhysRevLett.118.036601
Bibliographic Code:
2017PhRvL.118c6601S

Abstract

Recent experiments have uncovered evidence of the strongly coupled nature of graphene: the Wiedemann-Franz law is violated by up to a factor of 20 near the charge neutral point. We describe this strongly coupled plasma by a holographic model in which there are two distinct conserved U(1) currents. We find that our analytic results for the transport coefficients for the two current model have a significantly improved match to the density dependence of the experimental data than the models with only one current. The additive structure in the transport coefficients plays an important role. We also suggest the origin of the two currents.

 

Title:
Statistical Mechanics of Thin Spherical Shells
Authors:
Košmrlj, Andrej; Nelson, David R.
Publication:
Physical Review X, Volume 7, Issue 1, id.011002
Publication Date:
01/2017
Origin:
APS
Abstract Copyright:
2017: authors
DOI:
10.1103/PhysRevX.7.011002
Bibliographic Code:
2017PhRvX...7a1002K

Abstract

We explore how thermal fluctuations affect the mechanics of thin amorphous spherical shells. In flat membranes with a shear modulus, thermal fluctuations increase the bending rigidity and reduce the in-plane elastic moduli in a scale-dependent fashion. This is still true for spherical shells. However, the additional coupling between the shell curvature, the local in-plane stretching modes, and the local out-of-plane undulations leads to novel phenomena. In spherical shells, thermal fluctuations produce a radius-dependent negative effective surface tension, equivalent to applying an inward external pressure. By adapting renormalization group calculations to allow for a spherical background curvature, we show that while small spherical shells are stable, sufficiently large shells are crushed by this thermally generated "pressure." Such shells can be stabilized by an outward osmotic pressure, but the effective shell size grows nonlinearly with increasing outward pressure, with the same universal power-law exponent that characterizes the response of fluctuating flat membranes to a uniform tension.

 

Title:
Striated populations in disordered environments with advection
Authors:
Chotibut, Thiparat; Nelson, David R.; Succi, Sauro
Publication:
Physica A: Statistical Mechanics and its Applications, Volume 465, p. 500-514.
Publication Date:
01/2017
Origin:
ELSEVIER
Keywords:
Population dynamics, Spatially quenched disorder, Generalized FKPP equation, Pattern formation
Abstract Copyright:
(c) 2017 Elsevier B.V.
DOI:
10.1016/j.physa.2016.08.059
Bibliographic Code:
2017PhyA..465..500C

Abstract

Growth in static and controlled environments such as a Petri dish can be used to study the spatial population dynamics of microorganisms. However, natural populations such as marine microbes experience fluid advection and often grow up in heterogeneous environments. We investigate a generalized Fisher-Kolmogorov-Petrovsky-Piscounov (FKPP) equation describing single species population subject to a constant flow field and quenched random spatially inhomogeneous growth rates with a fertile overall growth condition. We analytically and numerically demonstrate that the non-equilibrium steady-state population density develops a flow-driven striation pattern. The striations are highly asymmetric with a longitudinal correlation length that diverges linearly with the flow speed and a transverse correlation length that approaches a finite velocity-independent value. Linear response theory is developed to study the statistics of the steady states. Theoretical predictions show excellent agreement with the numerical steady states of the generalized FKPP equation obtained from Lattice Boltzmann simulations. These findings suggest that, although the growth disorder can be spatially uncorrelated, correlated population structures with striations emerge naturally at sufficiently strong advection.

 


 

 go to the Top
 go to Most Recent Faculty Publications