# Faculty Publications: July, 2017

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

 Title: Rare-region effects and dynamics near the many-body localization transition Authors: Agarwal, Kartiek; Altman, Ehud; Demler, Eugene; Gopalakrishnan, Sarang; Huse, David A.; Knap, Michael Publication: Annalen der Physik, vol. 529, issue 7, p. 1600326 (AnP Homepage) Publication Date: 07/2017 Origin: CROSSREF DOI: 10.1002/andp.201600326 Bibliographic Code: 2017AnP...52900326A

### Abstract

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

 Title: Structure and Topology of Band Structures in the 1651 Magnetic Space Groups Authors: Watanabe, Haruki; Po, Hoi Chun; Vishwanath, Ashvin Publication: eprint arXiv:1707.01903 Publication Date: 07/2017 Origin: ARXIV Keywords: Condensed Matter - Strongly Correlated Electrons, Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Materials Science Comment: (8 + 34) pages; 2 figures; (2+19) tables Bibliographic Code: 2017arXiv170701903W

### Abstract

The properties of electrons in magnetically ordered crystals are of interest both from the viewpoint of realizing novel topological phases, such as magnetic Weyl semimetals, and from the applications perspective of creating energy-efficient memories. A systematic study of symmetry and topology in magnetic materials has been challenging given that there are 1651 magnetic space groups (MSGs). Here, by using an efficient representation of allowed band structures, we obtain a systematic description of several basic properties of free electrons in all MSGs in three dimensions as well as in the 528 magnetic layer groups relevant to two dimensional magnetic materials. We compute constraints on electron fillings and band connectivity compatible with insulating behavior. Also, by contrasting with atomic insulators, we identify band topology entailed by the symmetry transformation of bands, as determined by the MSG alone. We give an application of our results to identifying topological semimetals arising in periodic arrangements of hedgehog-like magnetic textures.

 Title: Intertwined and vestigial order with ultracold atoms in multiple cavity modes Authors: Gopalakrishnan, Sarang; Shchadilova, Yulia E.; Demler, Eugene Publication: eprint arXiv:1707.03907 Publication Date: 07/2017 Origin: ARXIV Keywords: Condensed Matter - Quantum Gases, Quantum Physics Comment: 12 pages, 6 figures Bibliographic Code: 2017arXiv170703907G

### Abstract

Atoms in transversely pumped optical cavities "self-organize" by forming a density wave and emitting superradiantly into the cavity mode(s). For a single-mode cavity, the properties of this self-organization transition are well characterized both theoretically and experimentally. Here, we explore the self-organization of a Bose-Einstein condensate in the presence of two cavity modes---a system that was recently experimentally realized [Leonard \emph{et al.}, \emph{Nature} {\bf 543}, 87 (2017)]. We argue that this system can exhibit a "vestigially ordered" phase in which neither cavity mode exhibits superradiance but the cavity modes are mutually phase-locked by the atoms. We argue that this vestigially ordered phase should generically be present in multimode cavity geometries.

 Title: Probing many-body dynamics on a 51-atom quantum simulator Authors: Bernien, Hannes; Schwartz, Sylvain; Keesling, Alexander; Levine, Harry; Omran, Ahmed; Pichler, Hannes; Choi, Soonwon; Zibrov, Alexander S.; Endres, Manuel; Greiner, Markus; Vuletić, Vladan; Lukin, Mikhail D. Publication: eprint arXiv:1707.04344 Publication Date: 07/2017 Origin: ARXIV Keywords: Quantum Physics, Condensed Matter - Quantum Gases, Physics - Atomic Physics Comment: 17 pages, 13 figures Bibliographic Code: 2017arXiv170704344B

### Abstract

Controllable, coherent many-body systems provide unique insights into fundamental properties of quantum matter, allow for the realization of novel quantum phases, and may ultimately lead to computational systems that are exponentially superior to existing classical approaches. Here, we demonstrate a novel platform for the creation of controlled many-body quantum matter. Our approach makes use of deterministically prepared, reconfigurable arrays of individually controlled, cold atoms. Strong, coherent interactions are enabled by coupling to atomic Rydberg states. We realize a programmable Ising-type quantum spin model with tunable interactions and system sizes of up to 51 qubits. Within this model we observe transitions into ordered states (Rydberg crystals) that break various discrete symmetries, verify high-fidelity preparation of ordered states, and investigate dynamics across the phase transition in large arrays of atoms. In particular, we observe a novel type of robust many-body dynamics corresponding to persistent oscillations of crystalline order after a sudden quantum quench. These observations enable new approaches for exploring many-body phenomena and open the door for realizations of novel quantum algorithms.

 Title: On the Power Spectrum of Dark Matter Substructure in Strong Gravitational Lenses Authors: Diaz Rivero, Ana; Cyr-Racine, Francis-Yan; Dvorkin, Cora Publication: eprint arXiv:1707.04590 Publication Date: 07/2017 Origin: ARXIV Keywords: Astrophysics - Cosmology and Nongalactic Astrophysics, High Energy Physics - Phenomenology Comment: 14 pages + appendices, 7 figures Bibliographic Code: 2017arXiv170704590D

### Abstract

Studying the smallest self-bound dark matter structure in our Universe can yield important clues about the fundamental particle nature of dark matter. Galaxy-scale strong gravitational lensing provides a unique way to detect and characterize dark matter substructures at cosmological distances from the Milky Way. Within the cold dark matter (CDM) paradigm, the number of low-mass subhalos within lens galaxies is expected to be large, implying that their contribution to the lensing convergence field is approximately Gaussian and could thus be described by their power spectrum. We develop here a general formalism to compute from first principles the substructure convergence power spectrum for different populations of dark matter subhalos. As an example, we apply our framework to two distinct subhalo populations: a truncated Navarro-Frenk-White subhalo population motivated by standard CDM, and a truncated cored subhalo population motivated by self-interacting dark matter (SIDM). We study in detail how the subhalo abundance, mass function, internal density profile, and concentration affect the amplitude and shape of substructure power spectrum. We determine that the power spectrum is mostly sensitive to a specific combination of the subhalo abundance and moments of the mass function, as well as to the average tidal truncation scale of the largest subhalos included in the analysis. Interestingly, we show that the asymptotic slope of the substructure power spectrum at large wavenumber reflects the internal density profile of the subhalos. In particular, the SIDM power spectrum exhibits a characteristic steepening at large wavenumber absent in the CDM power spectrum, opening the possibility of using this observable, if at all measurable, to discern between these two scenarios.

 Title: Dynamics of quantum information in many-body localized systems Authors: Bañuls, M. C.; Yao, N. Y.; Choi, S.; Lukin, M. D.; Cirac, J. I. Publication: eprint arXiv:1707.05051 Publication Date: 07/2017 Origin: ARXIV Keywords: Condensed Matter - Disordered Systems and Neural Networks, Condensed Matter - Statistical Mechanics, Quantum Physics Comment: 10 pages, 7 figures Bibliographic Code: 2017arXiv170705051B

### Abstract

We characterize the information dynamics of strongly disordered systems using a combination of analytics, exact diagonalization, and matrix product operator simulations. More specifically, we study the spreading of quantum information in three different scenarios: thermalizing, Anderson localized, and many-body localized. We qualitatively distinguish these cases by quantifying the amount of remnant information in a local region. The nature of the dynamics is further explored by computing the propagation of mutual information with respect to varying partitions. Finally, we demonstrate that classical simulability, as captured by the magnitude of MPO truncation errors, exhibits enhanced fluctuations near the localization transition, suggesting the possibility of its use as a diagnostic of the critical point.

 Title: Variational Study of Fermionic and Bosonic Systems with Non-Gaussian States: Theory and Applications Authors: Shi, Tao; Demler, Eugene; Cirac, J. Ignacio Publication: eprint arXiv:1707.05902 Publication Date: 07/2017 Origin: ARXIV Keywords: Quantum Physics, Condensed Matter - Statistical Mechanics Comment: 45 pages, 14 figures Bibliographic Code: 2017arXiv170705902S

### Abstract

We present a new variational method for investigating the ground state and out of equilibrium dynamics of quantum many-body bosonic and fermionic systems. Our approach is based on constructing variational wavefunctions which extend Gaussian states by including generalized canonical transformations between the fields. The key advantage of such states compared to simple Gaussian states is presence of non-factorizable correlations and the possibility of describing states with strong entanglement between particles. In contrast to the commonly used canonical transformations, such as the polaron or Lang-Firsov transformations, we allow parameters of the transformations to be time dependent, which extends their regions of applicability. We derive equations of motion for the parameters characterizing the states both in real and imaginary time using the differential structure of the variational manifold. The ground state can be found by following the imaginary time evolution until it converges to a steady state. Collective excitations in the system can be obtained by linearizing the real-time equations of motion in the vicinity of the imaginary time steady-state solution. Our formalism allows us not only to determine the energy spectrum of quasiparticles and their lifetime, but to obtain the complete spectral functions and to explore far out of equilibrium dynamics such as coherent evolution following a quantum quench. We illustrate and benchmark this framework with several examples: a single polaron in the Holstein and Su-Schrieer-Heeger models, non-equilibrium dynamics in the spin-boson and Kondo models, the superconducting to charge density wave phase transitions in the Holstein model.

 Title: Micrometer-scale Magnetic Imaging of Geological Samples Using a Quantum Diamond Microscope Authors: Glenn, David R.; Fu, Roger R.; Kehayias, Pauli; Le Sage, David; Lima, Eduardo A.; Weiss, Benjamin P.; Walsworth, Ronald L. Publication: eprint arXiv:1707.06714 Publication Date: 07/2017 Origin: ARXIV Keywords: Quantum Physics Comment: 41 pages, 14 figures Bibliographic Code: 2017arXiv170706714G

### Abstract

Remanent magnetization in geological samples may record the past intensity and direction of planetary magnetic fields. Traditionally, this magnetization is analyzed through measurements of the net magnetic moment of bulk millimeter to centimeter sized samples. However, geological samples are often mineralogically and texturally heterogeneous at submillimeter scales, with only a fraction of the ferromagnetic grains carrying the remanent magnetization of interest. Therefore, characterizing this magnetization in such cases requires a technique capable of imaging magnetic fields at fine spatial scales and with high sensitivity. To address this challenge, we developed a new instrument, based on nitrogen-vacancy centers in diamond, which enables direct imaging of magnetic fields due to both remanent and induced magnetization, as well as optical imaging, of room-temperature geological samples with spatial resolution approaching the optical diffraction limit. We describe the operating principles of this device, which we call the quantum diamond microscope (QDM), and report its optimized image-area-normalized magnetic field sensitivity (20 uT.um/Hz^1/2), spatial resolution (5 um), and field of view (4 mm), as well as trade-offs between these parameters. We also perform an absolute magnetic field calibration for the device in different modes of operation, including three-axis (vector) and single-axis (projective) magnetic field imaging. Finally, we use the QDM to obtain magnetic images of several terrestrial and meteoritic rock samples, demonstrating its ability to resolve spatially distinct populations of ferromagnetic carriers.

 Title: Color Memory Authors: Pate, Monica; Raclariu, Ana-Maria; Strominger, Andrew Publication: eprint arXiv:1707.08016 Publication Date: 07/2017 Origin: ARXIV Keywords: High Energy Physics - Theory, High Energy Physics - Phenomenology Comment: 7 pages Bibliographic Code: 2017arXiv170708016P

### Abstract

A transient color flux across null infinity in classical Yang-Mills theory is considered. It is shown that a pair of test `quarks' initially in a color singlet generically acquire net color as a result of the flux. A nonlinear formula is derived for the relative color rotation of the quarks. For weak color flux the formula linearizes to the Fourier transform of the soft gluon theorem. This color memory effect is the Yang-Mills analog of the gravitational memory effect.

 Title: Scale Invariant Instantons and the Complete Lifetime of the Standard Model Authors: Andreassen, Anders; Frost, William; Schwartz, Matthew D. Publication: eprint arXiv:1707.08124 Publication Date: 07/2017 Origin: ARXIV Keywords: High Energy Physics - Phenomenology, High Energy Physics - Theory Comment: References added Bibliographic Code: 2017arXiv170708124A

### Abstract

In a classically scale-invariant quantum field theory, tunneling rates are infrared divergent due to the existence of instantons of any size. While one expects such divergences to be resolved by quantum effects, it has been unclear how higher-loop corrections can resolve a problem appearing already at one loop. With a careful power counting, we uncover a series of loop contributions that dominate over the one-loop result and sum all the necessary terms. We also clarify previously incomplete treatments of related issues pertaining to global symmetries, gauge fixing and finite mass effects. In addition, we produce exact closed-form solutions for the functional determinants over scalars, fermions and vector bosons around the scale-invariant bounce, demonstrating manifest gauge invariance in the vector case. With these problems solved, we produce the first complete calculation of the lifetime of our universe: 10^139 years. With 95% confidence, we expect our universe to last more than 10^58 years. The uncertainty is part experimental uncertainty on the top quark mass and on ${\alpha}s$ and part theory uncertainty from electroweak threshold corrections. Using our complete result, we provide phase diagrams in the $mt/mh$ and the $mt/{\alpha}s$ planes, with uncertainty bands. To rule out absolute stability to $3{\sigma}$ confidence, the uncertainty on the top quark pole mass would have to be pushed below 250 MeV or the uncertainty on ${\alpha}s(mZ)$ pushed below 0.00025.

 Title: Pileup Mitigation with Machine Learning (PUMML) Authors: Komiske, Patrick T.; Metodiev, Eric M.; Nachman, Benjamin; Schwartz, Matthew D. Publication: eprint arXiv:1707.08600 Publication Date: 07/2017 Origin: ARXIV Keywords: High Energy Physics - Phenomenology, High Energy Physics - Experiment, Statistics - Machine Learning Comment: 20 pages, 8 figures Bibliographic Code: 2017arXiv170708600K

### Abstract

Pileup involves the contamination of the energy distribution arising from the primary collision of interest (leading vertex) by radiation from soft collisions (pileup). We develop a new technique for removing this contamination using machine learning and convolutional neural networks. The network takes as input the energy distribution of charged leading vertex particles, charged pileup particles, and all neutral particles and outputs the energy distribution of particles coming from leading vertex alone. The PUMML algorithm performs remarkably well at eliminating pileup distortion on a wide range of simple and complex jet observables. We test the robustness of the algorithm in a number of ways and discuss how the network can be trained directly on data.

 Title: Measurement of cosmic-ray reconstruction efficiencies in the MicroBooNE LArTPC using a small external cosmic-ray counter Authors: MicroBooNE collaboration; Acciarri, R.; Adams, C.; An, R.; Anthony, J.;... Guenette, R.;... and 146 coauthors Publication: eprint arXiv:1707.09903 Publication Date: 07/2017 Origin: ARXIV Keywords: High Energy Physics - Experiment Comment: 19 pages, 12 figures Bibliographic Code: 2017arXiv170709903M

### Abstract

The MicroBooNE detector is a liquid argon time projection chamber at Fermilab designed to study short-baseline neutrino oscillations and neutrino-argon interaction cross-section. Due to its location near the surface, a good understanding of cosmic muons as a source of backgrounds is of fundamental importance for the experiment. We present a method of using an external 0.5 m (L) x 0.5 m (W) muon counter stack, installed above the main detector, to determine the cosmic-ray reconstruction efficiency in MicroBooNE. Data are acquired with this external muon counter stack placed in three different positions, corresponding to cosmic rays intersecting different parts of the detector. The data reconstruction efficiency of tracks in the detector is found to be $\epsilon_{\mathrm{data}}=(97.1\pm0.1~(\mathrm{stat}) \pm 1.4~(\mathrm{sys}))\%$, in good agreement with the Monte Carlo reconstruction efficiency $\epsilon_{\mathrm{MC}} = (97.4\pm0.1)\%$. This analysis represents a small-scale demonstration of the method that can be used with future data coming from a recently installed cosmic-ray tagger system, which will be able to tag $\approx80\%$ of the cosmic rays passing through the MicroBooNE detector.

 Title: Jet reconstruction and performance using particle flow with the ATLAS Detector Authors: Aaboud, M.; Aad, G.; Abbott, B.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2850 coauthors Publication: The European Physical Journal C, Volume 77, Issue 7, article id.466, 47 pp. (EPJC Homepage) Publication Date: 07/2017 Origin: SPRINGER Abstract Copyright: (c) 2017: CERN for the benefit of the ATLAS collaboration DOI: 10.1140/epjc/s10052-017-5031-2 Bibliographic Code: 2017EPJC...77..466A

### Abstract

This paper describes the implementation and performance of a particle flow algorithm applied to 20.2 fb^{-1} of ATLAS data from 8 TeV proton-proton collisions in Run 1 of the LHC. The algorithm removes calorimeter energy deposits due to charged hadrons from consideration during jet reconstruction, instead using measurements of their momenta from the inner tracker. This improves the accuracy of the charged-hadron measurement, while retaining the calorimeter measurements of neutral-particle energies. The paper places emphasis on how this is achieved, while minimising double-counting of charged-hadron signals between the inner tracker and calorimeter. The performance of particle flow jets, formed from the ensemble of signals from the calorimeter and the inner tracker, is compared to that of jets reconstructed from calorimeter energy deposits alone, demonstrating improvements in resolution and pile-up stability.

 Title: Measurements of electroweak Wjj production and constraints on anomalous gauge couplings with the ATLAS detector Authors: Aaboud, M.; Aad, G.; Abbott, B.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2831 coauthors Publication: The European Physical Journal C, Volume 77, Issue 7, article id.474, 74 pp. (EPJC Homepage) Publication Date: 07/2017 Origin: SPRINGER Abstract Copyright: (c) 2017: CERN for the benefit of the ATLAS collaboration DOI: 10.1140/epjc/s10052-017-5007-2 Bibliographic Code: 2017EPJC...77..474A

### Abstract

Measurements of the electroweak production of a W boson in association with two jets at high dijet invariant mass are performed using √{s} = 7 and 8 {TeV} proton-proton collision data produced by the Large Hadron Collider, corresponding respectively to 4.7 and 20.2 fb^{-1} of integrated luminosity collected by the ATLAS detector. The measurements are sensitive to the production of a W boson via a triple-gauge-boson vertex and include both the fiducial and differential cross sections of the electroweak process.

 Title: Representations in neural network based empirical potentials Authors: Cubuk, Ekin D.; Malone, Brad D.; Onat, Berk; Waterland, Amos; Kaxiras, Efthimios Publication: The Journal of Chemical Physics, Volume 147, Issue 2, id.024104 (JChPh Homepage) Publication Date: 07/2017 Origin: AIP Abstract Copyright: 2017: Author(s) DOI: 10.1063/1.4990503 Bibliographic Code: 2017JChPh.147b4104C

### Abstract

Many structural and mechanical properties of crystals, glasses, and biological macromolecules can be modeled from the local interactions between atoms. These interactions ultimately derive from the quantum nature of electrons, which can be prohibitively expensive to simulate. Machine learning has the potential to revolutionize materials modeling due to its ability to efficiently approximate complex functions. For example, neural networks can be trained to reproduce results of density functional theory calculations at a much lower cost. However, how neural networks reach their predictions is not well understood, which has led to them being used as a "black box" tool. This lack of understanding is not desirable especially for applications of neural networks in scientific inquiry. We argue that machine learning models trained on physical systems can be used as more than just approximations since they had to "learn" physical concepts in order to reproduce the labels they were trained on. We use dimensionality reduction techniques to study in detail the representation of silicon atoms at different stages in a neural network, which provides insight into how a neural network learns to model atomic interactions.

 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, Volume 46, Issue 7, pp.3837-3841 Publication Date: 07/2017 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..46.3837B

### 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: Quadrality for supersymmetric matrix models Authors: Franco, Sebastián; Lee, Sangmin; Seong, Rak-Kyeong; Vafa, Cumrun Publication: Journal of High Energy Physics, Volume 2017, Issue 7, article id.53, 42 pp. Publication Date: 07/2017 Origin: SPRINGER Keywords: Brane Dynamics in Gauge Theories, D-branes, Supersymmetric Gauge Theory Abstract Copyright: (c) 2017: The Author(s) DOI: 10.1007/JHEP07(2017)053 Bibliographic Code: 2017JHEP...07..053F

### Abstract

We introduce a new duality for N = 1 supersymmetric gauged matrix models. This 0 d duality is an order 4 symmetry, namely an equivalence between four different theories, hence we call it Quadrality. Our proposal is motivated by mirror symmetry, but is not restricted to theories with a D-brane realization and holds for general N = 1 matrix models. We present various checks of the proposal, including the matching of: global symmetries, anomalies, deformations and the chiral ring. We also consider quivers and the corresponding quadrality networks. Finally, we initiate the study of matrix models that arise on the worldvolume of D(-1)-branes probing toric Calabi-Yau 5-folds.

 Title: Fivebranes and 3-manifold homology Authors: Gukov, Sergei; Putrov, Pavel; Vafa, Cumrun Publication: Journal of High Energy Physics, Volume 2017, Issue 7, article id.71, 82 pp. Publication Date: 07/2017 Origin: SPRINGER Keywords: Chern-Simons Theories, Topological Field Theories, M-Theory, Topological Strings Abstract Copyright: (c) 2017: The Author(s) DOI: 10.1007/JHEP07(2017)071 Bibliographic Code: 2017JHEP...07..071G

### Abstract

Motivated by physical constructions of homological knot invariants, we study their analogs for closed 3-manifolds. We show that fivebrane compactifications provide a universal description of various old and new homological invariants of 3-manifolds. In terms of 3d/3d correspondence, such invariants are given by the Q-cohomology of the Hilbert space of partially topologically twisted 3d N=2 theory T[ M 3] on a Riemann surface with defects. We demonstrate this by concrete and explicit calculations in the case of monopole/Heegaard Floer homology and a 3-manifold analog of Khovanov-Rozansky link homology. The latter gives a categorification of Chern-Simons partition function. Some of the new key elements include the explicit form of the S-transform and a novel connection between categorification and a previously mysterious role of Eichler integrals in Chern-Simons theory.

 Title: Physical limits to biomechanical sensing in disordered fibre networks Authors: Beroz, Farzan; Jawerth, Louise M.; Münster, Stefan; Weitz, David A.; Broedersz, Chase P.; Wingreen, Ned S. Publication: Nature Communications, Volume 8, id. 16096 (2017). Publication Date: 07/2017 Origin: NATURE Abstract Copyright: (c) 2017: The Author(s) DOI: 10.1038/ncomms16096 Bibliographic Code: 2017NatCo...816096B

### Abstract

Cells actively probe and respond to the stiffness of their surroundings. Since mechanosensory cells in connective tissue are surrounded by a disordered network of biopolymers, their in vivo mechanical environment can be extremely heterogeneous. Here we investigate how this heterogeneity impacts mechanosensing by modelling the cell as an idealized local stiffness sensor inside a disordered fibre network. For all types of networks we study, including experimentally-imaged collagen and fibrin architectures, we find that measurements applied at different points yield a strikingly broad range of local stiffnesses, spanning roughly two decades. We verify via simulations and scaling arguments that this broad range of local stiffnesses is a generic property of disordered fibre networks. Finally, we show that to obtain optimal, reliable estimates of global tissue stiffness, a cell must adjust its size, shape, and position to integrate multiple stiffness measurements over extended regions of space.

 Title: Inducing superconducting correlation in quantum Hall edge states Authors: Lee, Gil-Ho; Huang, Ko-Fan; Efetov, Dmitri K.; Wei, Di S.; Hart, Sean; Taniguchi, Takashi; Watanabe, Kenji; Yacoby, Amir; Kim, Philip Publication: Nature Physics, Volume 13, Issue 7, pp. 693-698 (2017). Publication Date: 07/2017 Origin: NATURE Abstract Copyright: (c) 2017: Nature Publishing Group DOI: 10.1038/nphys4084 Bibliographic Code: 2017NatPh..13..693L

### Abstract

The quantum Hall (QH) effect supports a set of chiral edge states at the boundary of a two-dimensional system. A superconductor (SC) contacting these states can provide correlations of the quasiparticles in the dissipationless edge states. Here we fabricated highly transparent and nanometre-scale SC junctions to graphene. We demonstrate that the QH edge states can couple via superconducting correlations through the SC electrode narrower than the superconducting coherence length. We observe that the chemical potential of the edge state exhibits a sign reversal across the SC electrode. This provides direct evidence of conversion of the incoming electron to the outgoing hole along the chiral edge state, termed crossed Andreev conversion (CAC). We show that CAC can successfully describe the temperature, bias and SC electrode width dependences. This hybrid SC/QH system could provide a novel route to create isolated non-Abelian anyonic zero modes, in resonance with the chiral edge states.

 Title: Measurement of the cross section for inclusive isolated-photon production in pp collisions at √{ s} = 13 TeV using the ATLAS detector Authors: Aaboud, M.; Aad, G.; Abbott, B.;... Franklin, M.;... Huth, J.;... Morii, M.;... and 2831 coauthors Publication: Physics Letters B, Volume 770, p. 473-493. Publication Date: 07/2017 Origin: ELSEVIER Abstract Copyright: (c) 2017 Elsevier Science B.V. All rights reserved. DOI: 10.1016/j.physletb.2017.04.072 Bibliographic Code: 2017PhLB..770..473A

### Abstract

Inclusive isolated-photon production in pp collisions at a centre-of-mass energy of 13 TeV is studied with the ATLAS detector at the LHC using a data set with an integrated luminosity of 3.2 fb-1. The cross section is measured as a function of the photon transverse energy above 125 GeV in different regions of photon pseudorapidity. Next-to-leading-order perturbative QCD and Monte Carlo event-generator predictions are compared to the cross-section measurements and provide an adequate description of the data.

 Title: Strong-coupling Bose polarons in a Bose-Einstein condensate Authors: Grusdt, F.; Schmidt, R.; Shchadilova, Y. E.; Demler, E. Publication: Physical Review A, Volume 96, Issue 1, id.013607 (PhRvA Homepage) Publication Date: 07/2017 Origin: APS Abstract Copyright: 2017: American Physical Society DOI: 10.1103/PhysRevA.96.013607 Bibliographic Code: 2017PhRvA..96a3607G

### Abstract

We use a nonperturbative renormalization group approach to develop a unified picture of the Bose polaron problem, where a mobile impurity is strongly interacting with a surrounding Bose-Einstein condensate (BEC). A detailed theoretical analysis of the phase diagram is presented and the polaron-to-molecule transition is discussed. For attractive polarons we argue that a description in terms of an effective Fröhlich Hamiltonian with renormalized parameters is possible. Its strong-coupling regime is realized close to a Feshbach resonance, where we predict a sharp increase of the effective mass. Already for weaker interactions, before the polaron mass diverges, we predict a transition to a regime where states exist below the polaron energy and the attractive polaron is no longer the ground state. On the repulsive side of the Feshbach resonance we recover the repulsive polaron, which has a finite lifetime because it can decay into low-lying molecular states. We show for the entire range of couplings that the polaron energy has logarithmic corrections in comparison with predictions by the mean-field approach. We demonstrate that they are a consequence of the polaronic mass renormalization which is due to quantum fluctuations of correlated phonons in the polaron cloud.

 Title: Thermal stiffening of clamped elastic ribbons Authors: Wan, Duanduan; Nelson, David R.; Bowick, Mark J. Publication: Physical Review B, Volume 96, Issue 1, id.014106 (PhRvB Homepage) Publication Date: 07/2017 Origin: APS Abstract Copyright: 2017: American Physical Society DOI: 10.1103/PhysRevB.96.014106 Bibliographic Code: 2017PhRvB..96a4106W

### 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: Quantum correlations at infinite temperature: The dynamical Nagaoka effect Authors: Kanász-Nagy, Márton; Lovas, Izabella; Grusdt, Fabian; Greif, Daniel; Greiner, Markus; Demler, Eugene A. Publication: Physical Review B, Volume 96, Issue 1, id.014303 (PhRvB Homepage) Publication Date: 07/2017 Origin: APS Abstract Copyright: 2017: American Physical Society DOI: 10.1103/PhysRevB.96.014303 Bibliographic Code: 2017PhRvB..96a4303K

### Abstract

Do quantum correlations play a role in high-temperature dynamics of many-body systems? A common expectation is that thermal fluctuations lead to fast decoherence and make dynamics classical. In this paper we provide a striking example that a single particle created in a featureless, infinite temperature spin bath not only exhibits nonclassical dynamics but it also induces strong long-lived correlations between the surrounding spins. We study the nonequilibrium dynamics of a hole created in a Mott insulator in the atomic limit, which corresponds to a degenerate spin system. In the absence of interactions, the spin correlations arise purely from quantum interference. Furthermore, these correlations are both antiferromagnetic and ferromagnetic, in striking contrast to the equilibrium Nagaoka effect. These results are relevant for a number of condensed matter spin systems and should be observable using state of the art bosonic or fermionic quantum gas microscopes.

 Title: Theory of parametrically amplified electron-phonon superconductivity Authors: Babadi, Mehrtash; Knap, Michael; Martin, Ivar; Refael, Gil; Demler, Eugene Publication: Physical Review B, Volume 96, Issue 1, id.014512 (PhRvB Homepage) Publication Date: 07/2017 Origin: APS Abstract Copyright: (c) 2017: American Physical Society DOI: 10.1103/PhysRevB.96.014512 Bibliographic Code: 2017PhRvB..96a4512B

### Abstract

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 [M. Mitrano et al., Nature (London) 530, 461 (2016), 10.1038/nature16522], 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 photoemission spectroscopy experiments as a consequence of the proposed mechanism.

 Title: Rotation of an immersed cylinder sliding near a thin elastic coating Authors: Rallabandi, Bhargav; Saintyves, Baudouin; Jules, Theo; Salez, Thomas; Schönecker, Clarissa; Mahadevan, L.; Stone, Howard A. Publication: Physical Review Fluids, Volume 2, Issue 7, id.074102 Publication Date: 07/2017 Origin: APS Abstract Copyright: 2017: American Physical Society DOI: 10.1103/PhysRevFluids.2.074102 Bibliographic Code: 2017PhRvF...2g4102R

### Abstract

It is known that an object translating parallel to a soft wall in a viscous fluid produces hydrodynamic stresses that deform the wall, which in turn results in a lift force on the object. Recent experiments with cylinders sliding under gravity near a soft incline, which confirmed theoretical arguments for the lift force, also reported an unexplained steady-state rotation of the cylinders [B. Saintyves et al., Proc. Natl. Acad. Sci. USA 113, 5847 (2016), 10.1073/pnas.1525462113]. Motivated by these observations, we show, in the lubrication limit, that an infinite cylinder that translates in a viscous fluid parallel to a soft wall at constant speed and separation distance must also rotate in order to remain free of torque. Using the Lorentz reciprocal theorem, we show analytically that for small deformations of the elastic layer, the angular velocity of the cylinder scales with the cube of the sliding speed. These predictions are confirmed numerically. We then apply the theory to the gravity-driven motion of a cylinder near a soft incline and find qualitative agreement with the experimental observations, namely, that a softer elastic layer results in a greater angular speed of the cylinder.

 Title: Critical Time Crystals in Dipolar Systems Authors: Ho, Wen Wei; Choi, Soonwon; Lukin, Mikhail D.; Abanin, Dmitry A. Publication: Physical Review Letters, Volume 119, Issue 1, id.010602 (PhRvL Homepage) Publication Date: 07/2017 Origin: APS Abstract Copyright: 2017: American Physical Society DOI: 10.1103/PhysRevLett.119.010602 Bibliographic Code: 2017PhRvL.119a0602H

### Abstract

We analyze the quantum dynamics of periodically driven, disordered systems in the presence of long-range interactions. Focusing on the stability of discrete time crystalline (DTC) order in such systems, we use a perturbative procedure to evaluate its lifetime. For 3D systems with dipolar interactions, we show that the corresponding decay is parametrically slow, implying that robust, long-lived DTC order can be obtained. We further predict a sharp crossover from the stable DTC regime into a regime where DTC order is lost, reminiscent of a phase transition. These results are in good agreement with the recent experiments utilizing a dense, dipolar spin ensemble in diamond [Nature (London) 543, 221 (2017), 10.1038/nature21426]. They demonstrate the existence of a novel, critical DTC regime that is stabilized not by many-body localization but rather by slow, critical dynamics. Our analysis shows that the DTC response can be used as a sensitive probe of nonequilibrium quantum matter.

 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: Physical Review Letters, Volume 119, Issue 1, id.016802 (PhRvL Homepage) Publication Date: 07/2017 Origin: APS Abstract Copyright: 2017: American Physical Society DOI: 10.1103/PhysRevLett.119.016802 Bibliographic Code: 2017PhRvL.119a6802V

### 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 noninteracting Anderson and Aubry-André-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 or 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 noninteracting localized phase where some memory is always preserved.

 Title: Topological Quantum Optics in Two-Dimensional Atomic Arrays Authors: Perczel, J.; Borregaard, J.; Chang, D. E.; Pichler, H.; Yelin, S. F.; Zoller, P.; Lukin, M. D. Publication: Physical Review Letters, Volume 119, Issue 2, id.023603 (PhRvL Homepage) Publication Date: 07/2017 Origin: APS Abstract Copyright: 2017: American Physical Society DOI: 10.1103/PhysRevLett.119.023603 Bibliographic Code: 2017PhRvL.119b3603P

### Abstract

We demonstrate that two-dimensional atomic emitter arrays with subwavelength spacing constitute topologically protected quantum optical systems where the photon propagation is robust against large imperfections while losses associated with free space emission are strongly suppressed. Breaking time-reversal symmetry with a magnetic field results in gapped photonic bands with nontrivial Chern numbers and topologically protected, long-lived edge states. Due to the inherent nonlinearity of constituent emitters, such systems provide a platform for exploring quantum optical analogs of interacting topological systems.

 Title: Thermal Transport Signatures of Broken-Symmetry Phases in Graphene Authors: Pientka, Falko; Waissman, Jonah; Kim, Philip; Halperin, Bertrand I. Publication: Physical Review Letters, Volume 119, Issue 2, id.027601 (PhRvL Homepage) Publication Date: 07/2017 Origin: APS Abstract Copyright: 2017: American Physical Society DOI: 10.1103/PhysRevLett.119.027601 Bibliographic Code: 2017PhRvL.119b7601P

### Abstract

In the half filled zero-energy Landau level of bilayer graphene, competing phases with spontaneously broken symmetries and an intriguing quantum critical behavior have been predicted. Here we investigate signatures of these broken-symmetry phases in thermal transport measurements. To this end, we calculate the spectrum of spin and valley waves in the ν =0 quantum Hall state of bilayer graphene. The presence of Goldstone modes enables heat transport even at low temperatures, which can serve as compelling evidence for spontaneous symmetry breaking. By varying external electric and magnetic fields, it is possible to determine the nature of the symmetry breaking. Temperature-dependent measurements may yield additional information about gapped modes.

 Title: Pairing in Luttinger Liquids and Quantum Hall States Authors: Kane, Charles L.; Stern, Ady; Halperin, Bertrand I. Publication: Physical Review X, Volume 7, Issue 3, id.031009 Publication Date: 07/2017 Origin: APS Abstract Copyright: 2017: authors DOI: 10.1103/PhysRevX.7.031009 Bibliographic Code: 2017PhRvX...7c1009K

### 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 ν =1 /(2 n +1 ), wires at the Luttinger phase form Laughlin states, while wires in the strongly paired phase form a bosonic fractional quantum Hall state of strongly bound pairs at a filling of 1 /(8 n +4 ). The transition between the two is of the universality class of Ising transitions in three dimensions. For even-denominator fractions ν =1 /2 n , 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 transitions between all of 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.

go to Most Recent Faculty Publications