Publications


Approximating observables on eigenstates of large many-body localized systems

PHYSICAL REVIEW B 99 (2019) ARTN 104201

AK Kulshreshtha, A Pal, TB Wahl, SH Simon


NMR relaxation in Ising spin chains

PHYSICAL REVIEW B 99 (2019) ARTN 035156

J Steinberg, NP Armitage, FHL Essler, S Sachdev


Signatures of the many-body localized regime in two dimensions

NATURE PHYSICS 15 (2019) 164-169

TB Wahl, A Pal, SH Simon


Topological 'Luttinger' invariants for filling-enforced non-symmorphic semimetals.

Journal of physics. Condensed matter : an Institute of Physics journal 31 (2019) 104001-

SA Parameswaran

Luttinger's theorem is a fundamental result in the theory of interacting Fermi systems: it states that the volume inside the Fermi surface is left invariant by interactions, if the number of particles is held fixed. Although this is traditionally justified in terms of analytic properties of Green's functions, it can be viewed as arising from a momentum balance argument that examines the response of the ground state to the insertion of a single flux quantum (Oshikawa 2000 Phys. Rev. Lett. 84 3370). This reveals that the Fermi volume is a topologically protected quantity, whose change requires a phase transition. However, this sheds no light on the stability or lack thereof of interacting semimetals, that either lack a Fermi surface, or have perfectly compensated electron and hole pockets and hence vanishing net Fermi volume. Here, I show that semimetallic phases in non-symmorphic crystals possess additional topological 'Luttinger invariants' that can be nonzero even though the Fermi volume vanishes. The existence of these invariants is linked to the inability of non-symmorphic crystals to host band insulating ground states except at special fillings. I exemplify the use of these new invariants by showing that they distinguish various classes of two- and three-dimensional semimetals.


Active transport in a channel: stabilisation by flow or thermodynamics.

Soft matter (2019)

S Chandragiri, A Doostmohammadi, JM Yeomans, SP Thampi

Recent experiments on active materials, such as dense bacterial suspensions and microtubule-kinesin motor mixtures, show a promising potential for achieving self-sustained flows. However, to develop active microfluidics it is necessary to understand the behaviour of active systems confined to channels. Therefore here we use continuum simulations to investigate the behaviour of active fluids in a two-dimensional channel. Motivated by the fact that most experimental systems show no ordering in the absence of activity, we concentrate on temperatures where there is no nematic order in the passive system, so that any nematic order is induced by the active flow. We systematically analyze the results, identify several different stable flow states, provide a phase diagram and show that the key parameters controlling the flow are the ratio of channel width to the length scale of active flow vortices, and whether the system is flow aligning or flow tumbling.


Emergence of Active Nematic Behavior in Monolayers of Isotropic Cells.

Physical review letters 122 (2019) 048004-048004

R Mueller, JM Yeomans, A Doostmohammadi

There is now growing evidence of the emergence and biological functionality of liquid crystal features, including nematic order and topological defects, in cellular tissues. However, how such features that intrinsically rely on particle elongation emerge in monolayers of cells with isotropic shapes is an outstanding question. In this Letter, we present a minimal model of cellular monolayers based on cell deformation and force transmission at the cell-cell interface that explains the formation of topological defects and captures the flow-field and stress patterns around them. By including mechanical properties at the individual cell level, we further show that the instability that drives the formation of topological defects, and leads to active turbulence, emerges from a feedback between shape deformation and active driving. The model allows us to suggest new explanations for experimental observations in tissue mechanics, and to propose designs for future experiments.


Interacting multi-channel topological boundary modes in a quantum Hall valley system.

Nature 566 (2019) 363-367

MT Randeria, K Agarwal, BE Feldman, H Ding, H Ji, RJ Cava, SL Sondhi, SA Parameswaran, A Yazdani

Symmetry and topology are central to understanding quantum Hall ferromagnets (QHFMs), two-dimensional electronic phases with spontaneously broken spin or pseudospin symmetry whose wavefunctions also have topological properties1,2. Domain walls between distinct broken-symmetry QHFM phases are predicted to host gapless one-dimensional modes-that is, quantum channels that emerge because of a topological change in the underlying electronic wavefunctions at such interfaces. Although various QHFMs have been identified in different materials3-8, interacting electronic modes at these domain walls have not been probed. Here we use a scanning tunnelling microscope to directly visualize the spontaneous formation of boundary modes at domain walls between QHFM phases with different valley polarization (that is, the occupation of equal-energy but quantum mechanically distinct valleys in the electronic structure) on the surface of bismuth. Spectroscopy shows that these modes occur within a topological energy gap, which closes and reopens as the valley polarization switches across the domain wall. By changing the valley flavour and the number of modes at the domain wall, we can realize different regimes in which the valley-polarized channels are either metallic or develop a spectroscopic gap. This behaviour is a consequence of Coulomb interactions constrained by the valley flavour, which determines whether electrons in the topological modes can backscatter, making these channels a unique class of interacting one-dimensional quantum wires. QHFM domain walls can be realized in different classes of two-dimensional materials, providing the opportunity to explore a rich phase space of interactions in these quantum wires.


Fractional oscillations

Nature Physics (2019)

STEVEN SIMON


Topological states in chiral active matter: Dynamic blue phases and active half-skyrmions.

The Journal of chemical physics 150 (2019) 064909-064909

L Metselaar, A Doostmohammadi, JM Yeomans

We numerically study the dynamics of two-dimensional blue phases in active chiral liquid crystals. We show that introducing contractile activity results in stabilised blue phases, while small extensile activity generates ordered but dynamic blue phases characterised by coherently moving half-skyrmions and disclinations. Increasing extensile activity above a threshold leads to the dissociation of the half-skyrmions and active turbulence. We further analyse isolated active half-skyrmions in an isotropic background and compare the activity-induced velocity fields in simulations to an analytical prediction of the flow. Finally, we show that confining an active blue phase can give rise to a system-wide circulation, in which half-skyrmions and disclinations rotate together.


Kosterlitz-Thouless scaling at many-body localization phase transitions

Physical review B: Condensed matter and materials physics American Physical Society 99 (2019) 094205

P Dumitrescu, A Goremykina, SIDDHARTH ASHOK PARAMESWARAN, M Serbyn, R Vasseur


Twist-induced crossover from two-dimensional to three-dimensional turbulence in active nematics.

Physical review. E 98 (2018) 010601-

TN Shendruk, K Thijssen, JM Yeomans, A Doostmohammadi

While studies of active nematics in two dimensions have shed light on various aspects of the flow regimes and topology of active matter, three-dimensional properties of topological defects and chaotic flows remain unexplored. By confining a film of active nematics between two parallel plates, we use continuum simulations and analytical arguments to demonstrate that the crossover from quasi-two-dimensional (quasi-2D) to three-dimensional (3D) chaotic flows is controlled by the morphology of the disclination lines. For small plate separations, the active nematic behaves as a quasi-2D material, with straight topological disclination lines spanning the height of the channel and exhibiting effectively 2D active turbulence. Upon increasing channel height, we find a crossover to 3D chaotic flows due to the contortion of disclinations above a critical activity. Above this critical activity highly contorted disclination lines and disclination loops are formed. We further show that these contortions are engendered by twist perturbations producing a sharp change in the curvature of disclinations.


Exopolymer Dynamics Driven by Sessile Flagellates

BIOPHYSICAL JOURNAL 114 (2018) 514A-514A

TN Shendruk, AK Balin, A Zottl, JM Yeomans


Reply to "Comment on 'Interpretation of thermal conductance of the nu=5/2 edge' "

PHYSICAL REVIEW B 98 (2018) ARTN 167402

SH Simon


Behavior of 1-bits near the many-body localization transition

PHYSICAL REVIEW B 98 (2018) ARTN 184201

AK Kulshreshtha, A Pal, TB Wahl, SH Simon


Effective edge state dynamics in the fractional quantum Hall effect

PHYSICAL REVIEW B 98 (2018) ARTN 155321

R Fern, R Bondesan, SH Simon


Clustering of Magnetic Swimmers in a Poiseuille Flow.

Physical review letters 120 (2018) 188101-

F Meng, D Matsunaga, R Golestanian

We investigate the collective behavior of magnetic swimmers, which are suspended in a Poiseuille flow and placed under an external magnetic field, using analytical techniques and Brownian dynamics simulations. We find that the interplay between intrinsic activity, external alignment, and magnetic dipole-dipole interactions leads to longitudinal structure formation. Our work sheds light on a recent experimental observation of a clustering instability in this system.


Critical behavior of the extended Hubbard model with bond dimerization

PHYSICA B-CONDENSED MATTER 536 (2018) 474-478

S Ejima, F Lange, FHL Essler, H Fehske


John Cardy's scale-invariant journey in low dimensions: a special issue for his 70th birthday Preface

JOURNAL OF PHYSICS A-MATHEMATICAL AND THEORETICAL 51 (2018) ARTN 280301

P Calabrese, P Fendley, U Tauber


Far-field theory for trajectories of magnetic ellipsoids in rectangular and circular channels

IMA JOURNAL OF APPLIED MATHEMATICS 83 (2018) 767-782

D Matsunaga, A Zottl, F Meng, R Golestanian, JM Yeomans


Lattice Supersymmetry and Order-Disorder Coexistence in the Tricritical Ising Model.

Physical review letters 120 (2018) 206403-

E O'Brien, P Fendley

We introduce and analyze a quantum spin or Majorana chain with a tricritical Ising point separating a critical phase from a gapped phase with order-disorder coexistence. We show that supersymmetry is not only an emergent property of the scaling limit but also manifests itself on the lattice. Namely, we find explicit lattice expressions for the supersymmetry generators and currents. Writing the Hamiltonian in terms of these generators allows us to find the ground states exactly at a frustration-free coupling. These confirm the coexistence between two (topologically) ordered ground states and a disordered one in the gapped phase. Deforming the model by including explicit chiral symmetry breaking, we find the phases persist up to an unusual chiral phase transition where the supersymmetry becomes exact even on the lattice.

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