arXiv subjects

Daniel Podolsky

Publications and source records attributed to Daniel Podolsky.

At least 55 records · Page 3Linked to original sources

arXiv2013

Modulated Floquet Topological Insulators

Floquet topological insulators are topological phases of matter generated by the application of time-periodic perturbations on otherwise conventional insulators. We demonstrate that spatial variations in the time-periodic potential lead to localized quasi-stationary states in two-dimensional systems. These states include one-dimensional interface modes at the nodes of the external potential, and fractionalized excitations at vortices of the external potential. We also propose a setup by which light can induce currents in these systems. We explain these results by showing a close analogy to px+ipy superconductors.

cond-mat.str-el↗

arXiv2013

Cusp singularities in boundary-driven diffusive systems

Boundary driven diffusive systems describe a broad range of transport phenomena. We study large deviations of the density profile in these systems, using numerical and analytical methods. We find that the large deviation may be non-differentiable, a phenomenon that is unique to non-equilibrium systems, and discuss the types of models which display such singularities.\ The structure of these singularities is found to generically be a cusp, which can be described by a Landau free energy or, equivalently, by catastrophe theory. Connections with analogous results in systems with finite-dimensional phase spaces are drawn.

cond-mat.stat-mech↗

arXiv2012

Visibility of the Amplitude (Higgs) Mode in Condensed Matter

The amplitude mode is a ubiquitous collective excitation in condensed matter systems with broken continuous symmetry. It is expected in antiferromagnets, short coherence length superconductors, charge density waves, and lattice Bose condensates. Its detection is a valuable test of the corresponding field theory, and its mass gap measures the proximity to a quantum critical point. However, since the amplitude mode can decay into low-energy Goldstone modes, its experimental visibility has been questioned. Here we show that the visibility depends on the symmetry of the measured susceptibility. The longitudinal susceptibility diverges at low frequency as χ_{σσ} ~ i/ω(d=2) or log(1/|ω|) (d=3), which can completely obscure the amplitude peak. In contrast, the scalar susceptibility is suppressed by four extra powers of frequency, exposing the amplitude peak throughout the ordered phase. We discuss experimental setups for measuring the scalar susceptibility. The conductivity of the O(2) theory (relativistic superfluid) is a scalar response and therefore exhibits suppressed absorption below the Higgs mass threshold, σ~ ω^{2d+1}. In layered, short coherence length superconductors, (relevant e.g. to cuprates) this threshold is raised by the interlayer plasma frequency.

cond-mat.supr-con↗

arXiv2012

Spectral functions of the Higgs mode near two-dimensional quantum critical points

We study the Higgs excitation in the Goldstone phase of the relativistic O(N) model in two spatial dimensions at zero temperature. The response functions of the order parameter, and its magnitude-squared, become universal functions of frequency in the vicinity of the quantum critical point described by the Wilson-Fisher fixed point, and we compute them to next-to-leading order in 1/N. The Higgs particle has an infrared singular decay to gapless Goldstone excitations, and its response functions are characterized by a pole in the lower-half of the complex frequency plane. The pole acquires a non-zero real part only at next-to-leading order in 1/N, demonstrating that the Higgs excitation has an oscillatory component even in the scaling limit. Both the real and imaginary parts of the pole position vanish with the correlation length exponent νupon approaching the critical point. We present evidence that the spectral density of the O(N)-invariant amplitude-squared of the order parameter has a peak at a non-zero frequency in the scaling limit. We connect our results to recent experimental studies of the superfluid-insulator quantum phase transition of ultracold bosonic atoms in optical lattices.

cond-mat.quant-gas↗

arXiv2012

Non differentiable large-deviation functionals in boundary-driven diffusive systems

We study the probability of arbitrary density profiles in conserving diffusive fields which are driven by the boundaries. We demonstrate the existence of singularities in the large-deviation functional, the direct analog of the free-energy in non-equilibrium systems. These singularities are unique to non-equilibrium systems and are a direct consequence of the breaking of time-reversal symmetry. This is demonstrated in an exactly-solvable model and also in numerical simulations on a boundary-driven Ising model. We argue that this singular behavior is expected to occur in models where the compressibility has a deep enough minimum. The mechanism is explained using a simple model.

cond-mat.stat-mech↗

arXiv2012

Large deviations in boundary-driven systems: Numerical evaluation and effective large-scale behavior

We study rare events in systems of diffusive fields driven out of equilibrium by the boundaries. We present a numerical technique and use it to calculate the probabilities of rare events in one and two dimensions. Using this technique, we show that the probability density of a slowly varying configuration can be captured with a small number of long wave-length modes. For a configuration which varies rapidly in space this description can be complemented by a local equilibrium assumption.

cond-mat.stat-mech↗

arXiv2010

Magnetic breakdown in ortho-II high-temperature cuprates

The recent confirmation of closed Fermi surface in the high temperature cuprates via de Haas-van Alphen and Shubnikov-de haas oscillations calls for a theoretical investigation of the origin of such oscillations. We study de Haas-van Alphen oscillations and magnetic breakdown in the context of ortho-II high temperature cuprates to understand the origin of the multiple frequencies. We find that the magnetic breakdown is highly sensitive not only to the ortho-II potential, but also to the Fermi surface topology, and is thus useful to distinguish between various theoretical proposals related to quantum oscillations.

cond-mat.supr-con↗

arXiv2010

Spin-orbit coupling in the metallic and spin-liquid phases of Na4Ir3O8

It has recently been proposed that Na4Ir3O8 is a weak Mott insulator at ambient pressure, supporting a three-dimensional spin liquid phase with a spinon Fermi surface. This proposal is consistent with recent experimental findings that the material becomes a metal upon increasing pressure or doping. In this work, we investigate the effect of the spin-orbit coupling arising from 5d Ir moments both in the metallic and spin liquid phases of Na4Ir3O8. The effective Hubbard model in terms of pseudospin j=1/2 Ir states is derived and its consequences to both metallic and spin liquid phases are studied. In particular, the model leads to enhanced Wilson ratio and strong temperature dependence of the Hall coefficient.

cond-mat.str-el↗

arXiv2009

Phase Transitions of S=1 Spinor Condensates in an Optical Lattice

We study the phase diagram of spin-one polar condensates in a two dimensional optical lattice with magnetic anisotropy. We show that the topological binding of vorticity to nematic disclinations allows for a rich variety of phase transitions. These include Kosterlitz-Thouless-like transitions with a superfluid stiffness jump that can be experimentally tuned to take a continuous set of values, and a new cascaded Kosterlitz-Thouless transition, characterized by two divergent length scales. For higher integer spin bosons S, the thermal phase transition out of the planar polar phase is strongly affected by the parity of S.

cond-mat.stat-mech↗

arXiv2009

A geometrical approach to the dynamics of spinor condensates I: Hydrodynamics

In this work, we derive the equations of motion governing the hydrodynamics of spin-F spinor condensates. We pursue a description based on standard physical variables (total density and superfluid velocity), alongside 2F `spin-nodes': unit vectors that describe the spin F state, and also exhibit the point-group symmetry of a spinor condensate's mean-field ground state. The hydrodynamic equations of motion consist of a mass continuity equation, 2F Landau-Lifshitz equations for the spin-nodes, and a modified Euler equation. In particular, we provide a generalization of the Mermin-Ho relation to spin one, and find an analytic solution for the skyrmion texture in the incompressible regime of a spin-half condensate. These results exhibit a beautiful geometrical structure that underlies the dynamics of spinor condensates.

cond-mat.other↗

arXiv2009

The interaction between the magnetic and superconducting order parameters in a La1.94Sr0.06CuO4 wire

We investigate the coupling between the magnetic and superconducting order parameters in an 8 m long meander line ("wire") made of a La1.94Sr0.06CuO4 film with a cross section of 0.5 x 100 um^2. The magnetic order parameter is determined using the Low-Energy muon spin relaxation technique. The superconducting order parameter is characterized by transport measurements and modified by high current density. We find that, when the superconducting order parameter is suppressed by the current, the magnetic transition temperature, Tm, increases. The extracted sign and magnitude of the Ginzburg-Landau coupling constant indicate that the two orders are repulsive, and that our system is located close to the border between first and second order phase transition.

cond-mat.supr-con↗

arXiv2009

Nematicity as a route to a magnetic field-induced spin density wave order; application to the high temperature cuprates

The electronic nematic order characterized by broken rotational symmetry has been suggested to play an important role in the phase diagram of the high temperature cuprates. We study the interplay between the electronic nematic order and a spin density wave order in the presence of a magnetic field. We show that a cooperation of the nematicity and the magnetic field induces a finite coupling between the spin density wave and spin-triplet staggered flux orders. As a consequence of such a coupling, the magnon gap decreases as the magnetic field increases, and it eventually condenses beyond a critical magnetic field leading to a field-induced spin density wave order. Both commensurate and incommensurate orders are studied, and the experimental implications of our findings are discussed.

cond-mat.str-el↗

arXiv2009

Collective modes and emergent SO(6) symmetry in the iron pnictides

We show the existence of an emergent SO(6) symmetry in the low energy description of the iron pnictides. This approximate symmetry provides a unifying framework for the occurrence of spin density wave (SDW) and superconductivity (SC) in these materials. We use this symmetry to make several predictions for future experiments, including the topology of the phase diagram and the presence of various resonant modes in neutron scattering experiments in both the SC and SDW phases. We also predict the existence of a new "Orbital Density Wave" state, which competes with both SDW and SC orders.

cond-mat.supr-con↗

arXiv2008

Quantum oscillations of ortho-II YBa_2Cu_3O_6.5

Motivated by quantum oscillations observed in highly ordered ortho-II YBa_2Cu_3O_6.5, we study the Fermi surface topology of d-density wave (ddw) or antiferromagnetic (AF) ordering in the presence of the ortho-II potential. We find that the electron pocket is unaffected by the presence of the ortho-II potential. This further strengthens the proposal that quantum oscillations observed in ortho-II-free YBa_2Cu_4O_8 arise from an electron pocket. On the other hand, the hole pocket topology is sensitive to the ortho-II potential. We show there exist generically three distinct quantum oscillations associated with one electron-like and two hole-like Fermi pockets. We compare our results to the quantum oscillations observed in experiments. We discuss possible ways to distinguish between ddw and antiferromagnetic orders in a potential single-layer ortho-II material.

cond-mat.supr-con↗

arXiv2008

Mott transition between a spin-liquid insulator and a metal in three dimensions

We study a bandwidth controlled Mott metal-insulator transition (MIT) between a Fermi liquid metal and a quantum spin-liquid insulator at half-filling in three dimensions (3D). Using a slave rotor approach, and incorporating gauge field fluctuations, we find a continuous MIT and discuss the finite temperature crossovers around this critical point. We show that the specific heat C=T ln ln (1/T) at the MIT and argue that the electrical transport on the metallic side near the transition should exhibit a `conductivity minimum' as a function of temperature. A possible candidate to test these predictions is the 3D spin liquid insulator Na4Ir3O8 which exhibits a pressure-tuned transition into a metallic phase. We also present the electron spectral function of Na4Ir3O8 at the transition.

cond-mat.str-el↗

arXiv2008

Halperin-Saslow modes as the origin of the low temperature anomaly in $NiGa_2S_4$

The absence of magnetic long range order in the triangular lattice spin-1 antiferromagnet $NiGa$_2$S$_4$ has prompted the search for a novel quantum ground state. In particular, several experiments suggest the presence of a linearly dispersing mode despite no long-range magnetic order. We show that the anomalous low temperature properties of NiGa$_2$S$_4$ can naturally be explained by the formulation developed by Halperin and Saslow where the linearly dispersing Halperin-Saslow mode may exist in the background of frozen spin moments and zero net magnetization. We provide highly non-trivial consistency checks on the existing experimental data and suggest future experiments that can further confirm the existence of the Halperin-Saslow mode. Our results place strong constraints on any microscopic theory of this material.

cond-mat.str-el↗

arXiv2007

Thermoelectric transport near pair breaking quantum phase transition out of d-wave superconductivity

We study electric, thermal, and thermoelectric conductivities in the vicinity of a z=2 superconductor-diffusive metal transition in two dimensions, both in the high and low frequency limits. We find violation of the Wiedemann-Franz law and a dc thermoelectric conductivity $α$ that does not vanish at low temperatures, in contrast to Fermi liquids. We introduce a Langevin equation formalism to study critical dynamics over a broad region surrounding the quantum critical point.

cond-mat.str-el↗

arXiv2006

Nernst effect and diamagnetism in phase fluctuating superconductors

When a superconductor is warmed above its critical temperature $T_c$, long range order is destroyed by fluctuations in the order parameter. These fluctuations can be probed by measurements of conductivity, diamagnetism, and of the Nernst effect. Here, we study a regime where superconductivity is destroyed by phase fluctuations arising from a dilute liquid of mobile vortices. We find that the Nernst effect and diamagnetic response differ significantly from Gaussian fluctuations -- in particular, a much sharper decay with temperature is obtained. We predict a rapid onset of Nernst signal at a temperature T$_{\rm onset}$ that tracks $T_c$, rather than the pairing temperature. We also predict a close quantitative connection with diamagnetism -- the ratio of magnetization to transverse thermoelectric conductivity $α_{xy}$ reaches a universal value at high temperatures. We interpret Nernst effect measurements on the underdoped cuprates in terms of a dilute vortex liquid over a wide temperature range above $T_c$.

cond-mat.supr-con↗