arXiv subjects

Pavan Hosur

Publications and source records attributed to Pavan Hosur.

At least 55 records · Page 3Linked to original sources

arXiv2017

Larkin-Ovchinnikov state of superconducting Weyl metals: Fundamental differences between pairings restricted and extended in the $\it{\textbf{k}}$-space

Two common approaches of studying theoretically the property of a superconductor are shown to have significant differences, when they are applied to the Larkin-Ovchinnikov state of Weyl metals. In the first approach the pairing term is restricted by a cutoff energy to the neighborhood of the Fermi surface, whereas in the second approach the pairing term is extended to the whole Brillouin zone. We explore their difference by considering two minimal models for the Weyl metal. For a model giving a single pair of Weyl pockets, both two approaches give a partly-gapped (fully-gapped) bulk spectrum for small (large) pairing amplitude. However, for very small cutoff energy, a portion of the Fermi surface can be completely unaffected by the pairing term in the first approach. For the other model giving two pairs of Weyl pockets, while the bulk spectrum for the first approach can be fully gapped, the one from the second approach has a robust line node, and the surface states are also changed qualitatively by the pairing. We elucidate the above differences by topological arguments and analytical analyses. A factor common to both of the two models is the tilting of the Weyl cones which leads to asymmetric normal state band structure with respect to the Weyl nodes. For the Weyl metal with two pairs of Weyl pockets, the band folding leads to a double degeneracy in the effective model, which distinguishes the pairing of the second approach from all others.

cond-mat.supr-con↗

arXiv2016

Chaos in quantum channels

We study chaos and scrambling in unitary channels by considering their entanglement properties as states. Using out-of-time-order correlation functions to diagnose chaos, we characterize the ability of a channel to process quantum information. We show that the generic decay of such correlators implies that any input subsystem must have near vanishing mutual information with almost all partitions of the output. Additionally, we propose the negativity of the tripartite information of the channel as a general diagnostic of scrambling. This measures the delocalization of information and is closely related to the decay of out-of-time-order correlators. We back up our results with numerics in two non-integrable models and analytic results in a perfect tensor network model of chaotic time evolution. These results show that the butterfly effect in quantum systems implies the information-theoretic definition of scrambling.

hep-th↗

arXiv2016

Time-reversal asymmetry without local moments via directional scalar spin chirality

Invariably, time-reversal symmetry (TRS) violation in a state of matter is identified with static magnetism in it. Here, a directional scalar spin chiral order (DSSCO) phase is introduced that disobeys this basic principle: it breaks TRS but has no density of static moments. It can be obtained by melting the spin moments in a magnetically ordered phase but retaining residual broken TRS. Orbital moments are then precluded by the spatial symmetries of the spin rotation symmetric state. It can exist in one, two and three dimensions under different conditions of temperature and disorder. Recently, polar Kerr effect experiments in the mysterious pseudogap phase of the underdoped cuprates hinted at a strange form of broken TRS below a temperature $T_{K}$, that exhibits a hysteretic "memory effect" above $T_{K}$ and begs reconciliation with nuclear magnetic resonance (which sees no moments), X-ray diffraction (which finds charge ordering tendencies) and the Nernst effect (which detects nematicity). Remarkably, the DSSCO provides a phenomenological route for reconciling all these observations, and it is conceivable that it onsets at the pseudogap temperature $\sim T^{*}$. A testable prediction of the existence of the DSSCO in the cuprates is a Kerr signal above $T_{K}$ triggered and trainable by a current driven along one of the in-plane axes, but not by a current along the other.

cond-mat.str-el↗

arXiv2015

Quantum oscillations from generic surface Fermi arcs and bulk chiral modes in Weyl semimetals

We re-examine the question of quantum oscillations from surface Fermi arcs and chiral modes in Weyl semimetals. By introducing two tools - semiclassical phase-space quantization and a numerical implementation of a layered construction of Weyl semimetals - we discover several important generalizations to previous conclusions that were implicitly tailored to the special case of identical Fermi arcs on top and bottom surfaces. We show that the phase-space quantization picture fixes an ambiguity in the previously utilized energy-time quantization approach and correctly reproduces the numerically calculated quantum oscillations for generic Weyl semimetals with distinctly curved Fermi arcs on the two surfaces. Based on these methods, we identify a 'magic' magnetic-field angle where quantum oscillations become independent of sample thickness, with striking experimental implications. We also analyze the stability of these quantum oscillations to disorder, and show that the high-field oscillations are expected to persist in samples whose thickness parametrically exceeds the quantum mean free path.

cond-mat.mes-hall↗

arXiv2015

Elastoconductivity as a probe of broken mirror symmetries

We propose the possible detection of broken mirror symmetries in correlated two-dimensional materials by elastotransport measurements. Using linear response theory we calculate the shearconductivity $Γ_{xx,xy}$, defined as the linear change of the longitudinal conductivity $σ_{xx}$ due to a shear strain $ε_{xy}$. This quantity can only be non-vanishing when in-plane mirror symmetries are broken and we discuss how candidate states in the cuprate pseudogap regime (e.g. various loop current or charge orders) may exhibit a finite shearconductivity. We also provide a realistic experimental protocol for detecting such a response.

cond-mat.str-el↗

arXiv2015

Characterizing eigenstate thermalization via measures in the Fock space of operators

The eigenstate thermalization hypothesis (ETH) attempts to bridge the gap between quantum mechanical and statistical mechanical descriptions of isolated quantum systems. Here, we define unbiased measures for how well the ETH works in various regimes, by mapping general interacting quantum systems on regular lattices onto a single particle living on a high-dimensional graph. By numerically analyzing deviations from ETH behavior in the non-integrable Ising model, we propose a quantity that we call the $n$-$weight$ to democratically characterize the average deviations for all operators residing on a given number of sites, irrespective of their spatial structure. It appears to have a simple scaling form, that we conjecture to hold true for all non-integrable systems. A closely related quantity, that we term the $n$-$distinguishability$, tells us how well two states can be distinguished if only $n$-site operators are measured. Along the way, we discover that complicated operators on average are worse than simple ones at distinguishing between neighboring eigenstates, contrary to the naive intuition created by the usual statements of the ETH that few-body (many-body) operators acquire the same (different) expectation values in nearby eigenstates at finite energy density. Finally, we sketch heuristic arguments that the ETH originates from the limited ability of simple operators to distinguish between quantum states of a system, especially when the states are subject to constraints such as roughly fixed energy with respect to a local Hamiltonian.

cond-mat.stat-mech↗

arXiv2015

Unified Topological Response Theory for Gapped and Gapless Free Fermions

We derive a scheme for systematically enumerating the responses of gapped as well as gapless systems of free fermions to electromagnetic and strain fields starting from a common parent theory. Using the fact that position operators in the lowest Landau level of a quantum Hall state are canonically conjugate, we consider a massive Dirac fermion in $2n$ spatial dimensions under $n$ mutually orthogonal magnetic fields and reinterpret physical space in the resulting zeroth Landau level as phase space in $n$ spatial dimensions. The bulk topological responses of the parent Dirac fermion, given by a Chern-Simons theory, translate into quantized insulator responses, while its edge anomalies characterize the response of gapless systems. Moreover, various physically different responses are seen to be related by the interchange of position and momentum variables. We derive many well-known responses, and demonstrate the utility of our theory by predicting spectral flow along dislocations in Weyl semimetals.

cond-mat.mes-hall↗

arXiv2015

Tunable circular dichroism due to the chiral anomaly in Weyl semimetals

Weyl semimetals are a three dimensional gapless topological phase in which bands intersect at arbitrary points -- the Weyl nodes -- in the Brillouin zone. These points carry a topological quantum number known as the \emph{chirality} and always appear in pairs of opposite chiralities. The notion of chirality leads to anomalous non-conservation of chiral charge, known as the \emph{chiral anomaly}, according to which charge can be pumped between Weyl nodes of opposite chiralities by an electromagnetic field with non-zero $\boldsymbol{E}\cdot\boldsymbol{B}$. Here, we propose probing the chiral anomaly by measuring the optical activity of Weyl semimetals via circular dichroism. In particular, we observe that applying such an electromagnetic field on this state gives it a non-zero gyrotropic coefficient or a Hall-like conductivity, which may be detectable by routine circular dichroism experiments. This method also serves as a diagnostic tool to discriminate between Weyl and Dirac semimetals; the latter will give a null result. More generally, any experiment that probes a bulk correlation function that has the same symmetries as the gyrotropic coefficient can detect the chiral anomaly as well as differentiate between Dirac and Weyl semimetals.

cond-mat.str-el↗

arXiv2014

Kerr effect as evidence of gyrotropic order in the cuprates - revisited

Recent analysis has confirmed earlier general arguments that the Kerr response vanishes in any time-reversal invariant system which satisfies the Onsager relations. Thus, the widely cited relation between natural optical activity (gyrotropy) and the Kerr response, employed in Hosur \textit{et al}, Phys. Rev. B \textbf{87}, 115116 (2013), is incorrect. However, there is increasingly clear experimental evidence that, as argued in our paper, the onset of an observable Kerr-signal in the cuprates reflects point-group symmetry rather than time-reversal symmetry breaking.

cond-mat.supr-con↗

arXiv2014

Crisscrossed stripe order from interlayer tunneling in hole-doped cuprates

Motivated by recent observations of charge order in the pseudogap regime of hole-doped cuprates, we show that {\it crisscrossed} stripe order can be stabilized by coherent, momentum-dependent interlayer tunneling, which is known to be present in several cuprate materials. We further describe how subtle variations in the couplings between layers can lead to a variety of stripe ordering arrangements, and discuss the implications of our results for recent experiments in underdoped cuprates.

cond-mat.str-el↗

arXiv2014

Time-reversal invariant topological superconductivity in doped Weyl semimetals

Time-reversal invariant topological superconductors are a new state of matter which have a bulk superconducting gap and robust Majorana fermion surface states. These have not yet been realized in solid state systems. In this paper, we propose that this state can be realized in doped Weyl semimetals or Weyl metals. The Fermi surfaces of a Weyl metal carry Chern numbers, which is a required ingredient for such a topological superconductor. By applying the fluctuation-exchange approach to a generic model of time-reversal invariant Dirac and Weyl semimetals, we investigate what microscopic interactions can supply the other ingredient, viz., sign changing of the superconducting gap function between Fermi surfaces with opposite Chern numbers. We find that if the normal state is inversion symmetric, onsite repulsive and exchange interactions induce various nodal phases as well as a small region of topological superconductivity on the phase diagram. Unlike the He$^{3}$B topological superconductor, the phase here does not rely on any special momentum dependence of the pairing amplitude. Breaking inversion symmetry precludes some of the nodal phases and the topological superconductor becomes much more prominent, especially at large ferromagnetic interaction. Our approach can be extended to generic Dirac or Weyl metals.

cond-mat.str-el↗

arXiv2013

Recent developments in transport phenomena in Weyl semimetals

The last decade has witnessed great advancements in the science and engineering of systems with unconventional band structures, seeded by studies of graphene and topological insulators. While the band structure of graphene simulates massless relativistic electrons in two dimensions, topological insulators have bands that wind non-trivially over momentum space in a certain abstract sense. Over the last couple of years, enthusiasm has been burgeoning in another unconventional and topological (although, not quite in the same sense as topological insulators) phase -- the Weyl Semimetal. In this phase, electrons mimic Weyl fermions that are well-known in high-energy physics, and inherit many of their properties, including an apparent violation of charge conservation known as the Chiral Anomaly. In this review, we recap some of the unusual transport properties of Weyl semimetals discussed in the literature so far, focusing on signatures whose roots lie in the anomaly. We also mention several proposed realizations of this phase in condensed matter systems, since they were what arguably precipitated activity on Weyl semimetals in the first place.

cond-mat.str-el↗

arXiv2012

Kerr effect as evidence of gyrotropic order in the cuprates

The Kerr effect can arise in a time-reversal invariant dissipative medium that is "gyrotropic", i.e. one that breaks inversion ($\mathcal I$) and all mirror symmetries. Examples of such systems include electron analogs of cholesteric liquid crystals, and their descendants, such as systems with chiral charge ordering. We present arguments that the striking Kerr onset seen in the pseudogap phase of a large number of cuprate high temperature superconductors is evidence of chiral charge ordering. We discuss additional experimental consequences of a phase transition to a gyrotropic state, including the appearance of a zero field Nernst effect.

cond-mat.str-el↗

arXiv2012

Friedel oscillations due to Fermi arcs in Weyl semimetals

Weyl semimetals harbor unusual surface states known as Fermi arcs, which are essentially disjoint segments of a two dimensional Fermi surface. We describe a prescription for obtaining Fermi arcs of arbitrary shape and connectivity by stacking alternate two dimensional electron and hole Fermi surfaces and adding suitable interlayer coupling. Using this prescription, we compute the local density of states -- a quantity directly relevant to scanning tunneling microscopy -- on a Weyl semimetal surface in the presence of a point scatterer and present results for a particular model that is expected to apply to pyrochlore iridate Weyl semimetals. For thin samples, Fermi arcs on opposite surfaces conspire to allow nested backscattering, resulting in strong Friedel oscillations on the surface. These oscillations die out as the sample thickness is increased and Fermi arcs from the bottom surface retreat and weak oscillations, due to scattering between the top surface Fermi arcs alone, survive. The surface spectral function -- accessible to photoemission experiments -- is also computed. In the thermodynamic limit, this calculation can be done analytically and separate contributions from the Fermi arcs and the bulk states can be seen.

cond-mat.str-el↗

arXiv2011

Charge Transport in Weyl Semimetals

We study transport in three dimensional Weyl semimetals with N isotropic Weyl nodes in the presence of Coulomb interactions or disorder at temperature T. In the interacting clean limit, we determine the conductivity by solving a quantum Boltzmann equation within a `leading log' approximation and find it to be proportional to T, upto logarithmic factors arising from the flow of couplings. In the noninteracting disordered case, we compute the finite-frequency Kubo conductivity and show that it exhibits distinct behaviors for omega << T and omega >> T: in the former regime we recover the results of a previous analysis, of a finite conductivity and a Drude width that vanishes as NT^2; in the latter, we find a conductivity that vanishes linearly with omega whose leading contribution as T -> 0 is the same as that of the clean, non-interacting system sigma(omega, T=0) = N(e^2/12h)(|omega|/v_F). We compare our results to experimental data on Y2Ir2O7 and also comment on the possible relevance to recent transport data on Eu2Ir2O7.

cond-mat.str-el↗

arXiv2011

Ultracold Atoms in a Tunable Optical Kagome Lattice

Geometrically frustrated systems with a large degeneracy of low energy states are of central interest in condensed-matter physics. The kagome net - a pattern of corner-sharing triangular plaquettes - presents a particularly high degree of frustration, reflected in the non-dispersive orbital bands. The ground state of the kagome quantum antiferromagnet, proposed to be a quantum spin liquid or valence bond solid, remains uncertain despite decades of work. Solid-state kagome magnets suffer from significant magnetic disorder or anisotropy that complicates the interpretation of experimental results. Here, we realize the kagome geometry in a two-dimensional optical superlattice for ultracold $^{87}$Rb atoms. We employ atom optics to characterize the lattice as it is tuned between various geometries, including kagome, one-dimensional stripe, and decorated triangular lattices, allowing for a sensitive control of frustration. The lattices implemented in this work offer a near-ideal realization of a paradigmatic model of many-body quantum physics.

cond-mat.quant-gas↗

arXiv2011

Majorana modes at the ends of superconductor vortices in doped topological insulators

Recent experiments have observed bulk superconductivity in doped topological insulators. Here we ask whether vortex Majorana zero modes, previously predicted to occur when superconductivity is induced on the surface of topological insulators, survive even in these doped systems with metallic normal states. Assuming inversion symmetry, we find that Majorana zero modes indeed appear but only below a critical doping. The critical doping is associated with a topological phase transition of the vortex line, where it supports gapless excitations along its length. The critical point depends only on the orientation of the vortex line, and a Berry phase property, the SU(2) Berry phase of the Fermi surface in the metallic normal state. By calculating this phase for available band structures we determine that materials candidates like $p$-doped Bi$_{2}$Te$_{3}$ under pressure supports vortex end Majorana modes. Surprisingly, even superconductors derived from topologically trivial band structures can support Majorana modes, providing a promising route to realizing them.

cond-mat.supr-con↗

arXiv2010

Optical characterization of topological insulator surface states: Berry curvature-dependent response

We study theoretically the optical response of the surface states of a topological insulator, especially the generation of helicity-dependent direct current by circularly polarized light. Interestingly, the dominant current, due to an interband transition, is controlled by the Berry curvature of the surface bands. This extends the connection between photocurrents and Berry curvature beyond the quasiclassical approximation where it has been shown to hold. Explicit expressions are derived for the (111) surface of the topological insulator Bi_{2}Se_{3} where we find significant helicity dependent photocurrents when the rotational symmetry of the surface is broken by an in-plane magnetic field or a strain. Moreover, the dominant current grows linearly with time until a scattering occurs, which provides a means for determining the scattering time. The dc spin generated on the surface is also dominated by a linear-in-time, Berry curvature dependent contribution.

cond-mat.mtrl-sci↗