arXiv subjects

Gil Refael

Publications and source records attributed to Gil Refael.

At least 163 records · Page 9Linked to original sources

arXiv2009

Vortex Synchronization in Bose-Einstein Condensates: A Time-Dependent Gross-Pitaevskii Equation Approach

In this work we consider vortex lattices in rotating Bose-Einstein Condensates composed of two species of bosons having different masses. Previously [1] it was claimed that the vortices of the two species form bound pairs and the two vortex lattices lock. Remarkably, the two condensates and the external drive all rotate at different speeds due to the disparity of the masses of the constituent bosons. In this paper we study the system by solving the full two-component Gross-Pitaevskii equations numerically. Using this approach we verify the stability of the putative locked state which is found to exist within a disk centered on the axis of rotation and which depends on the mass ratio of the two bosons. We also show that an analytic estimate of this locking radius based on a two-body force calculation agrees well with the numerical results.

cond-mat.quant-gas↗

arXiv2009

Superfluid-insulator transition of disordered bosons in one-dimension

We study the superfluid-insulator transition in a one dimensional system of interacting bosons, modeled as a disordered Josephson array, using a strong randomness real space renormalization group technique. Unlike perturbative methods, this approach does not suffer from run-away flows and allows us to study the complete phase diagram. We show that the superfluid insulator transition is always Kosterlitz- Thouless like in the way that length and time scales diverge at the critical point. Interestingly however, we find that the transition at strong disorder occurs at a non universal value of the Luttinger parameter, which depends on the disorder strength. This result places the transition in a universality class different from the weak disorder transition first analyzed by Giamarchi and Schulz [Europhys. Lett. {\bf 3}, 1287 (1987)]. While the details of the disorder potential are unimportant at the critical point, the type of disorder does influence the properties of the insulating phases. We find three classes of insulators which arise for different classes of disorder potential. For disorder only in the charging energies and Josephson coupling constants, at integer filling we find an incompressible but gapless Mott glass phase. If both integer and half integer filling factors are allowed then the corresponding phase is a random singlet insulator, which has a divergent compressibility. Finally in a generic disorder potential the insulator is a Bose glass with a finite compressibility.

cond-mat.dis-nn↗

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

Criticality and entanglement in random quantum systems

We review studies of entanglement entropy in systems with quenched randomness, concentrating on universal behavior at strongly random quantum critical points. The disorder-averaged entanglement entropy provides insight into the quantum criticality of these systems and an understanding of their relationship to non-random ("pure") quantum criticality. The entanglement near many such critical points in one dimension shows a logarithmic divergence in subsystem size, similar to that in the pure case but with a different universal coefficient. Such universal coefficients are examples of universal critical amplitudes in a random system. Possible measurements are reviewed along with the one-particle entanglement scaling at certain Anderson localization transitions. We also comment briefly on higher dimensions and challenges for the future.

cond-mat.dis-nn↗

arXiv2008

Permutation Symmetric Critical Phases in Disordered Non-Abelian Anyonic Chains

Topological phases supporting non-abelian anyonic excitations have been proposed as candidates for topological quantum computation. In this paper, we study disordered non-abelian anyonic chains based on the quantum groups $SU(2)_k$, a hierarchy that includes the $ν=5/2$ FQH state and the proposed $ν=12/5$ Fibonacci state, among others. We find that for odd $k$ these anyonic chains realize infinite randomness critical {\it phases} in the same universality class as the $S_k$ permutation symmetric multi-critical points of Damle and Huse (Phys. Rev. Lett. 89, 277203 (2002)). Indeed, we show that the pertinent subspace of these anyonic chains actually sits inside the ${\mathbb Z}_k \subset S_k$ symmetric sector of the Damle-Huse model, and this ${\mathbb Z}_k$ symmetry stabilizes the phase.

cond-mat.str-el↗

arXiv2008

Infinite Randomness Phases and Entanglement Entropy of the Disordered Golden Chain

Topological insulators supporting non-abelian anyonic excitations are at the center of attention as candidates for topological quantum computation. In this paper, we analyze the ground-state properties of disordered non-abelian anyonic chains. The resemblance of fusion rules of non-abelian anyons and real space decimation strongly suggests that disordered chains of such anyons generically exhibit infinite-randomness phases. Concentrating on the disordered golden chain model with nearest-neighbor coupling, we show that Fibonacci anyons with the fusion rule $τ\otimesτ={\bf 1}\oplus τ$ exhibit two infinite-randomness phases: a random-singlet phase when all bonds prefer the trivial fusion channel, and a mixed phase which occurs whenever a finite density of bonds prefers the $τ$ fusion channel. Real space RG analysis shows that the random-singlet fixed point is unstable to the mixed fixed point. By analyzing the entanglement entropy of the mixed phase, we find its effective central charge, and find that it increases along the RG flow from the random singlet point, thus ruling out a c-theorem for the effective central charge.

cond-mat.str-el↗

arXiv2008

Sagnac interference in Carbon nanotubes

The Sagnac interference mode arises when two interfering counterpropogating beams traverse a loop, but with their velocities detuned by a small amount $2u$, with $v_{R/L}=v_F\pm u$. In this paper we perform a perturbative non-equilibrium calculation of Sagnac interference in single channel wires as well as armchair nanotube loops. We study the dependence of the Sagnac conductance oscillations on temperature and interactions. We find that the Sagnac interference is not destroyed by strong interactions, but becomes weakly dependent on the velocity detuning $u$. In armchairs nanotubes with typical interaction strength, $0.25 \leq g \leq 0.5$, we find that the necessary temperature for observing the interference effect, $T_{SAG}$ is also only weakly dependent on the interaction, and is enhanced by a factor of 8 relative to the temperature necessary for observing Fabry-Perot interference in the same system, $T_{FP}$.

cond-mat.mes-hall↗

arXiv2008

Dissipation-driven quantum phase transition in superconductor-graphene systems

We show that a system of Josephson junctions coupled via low-resistance tunneling contacts to graphene substrate(s) may effectively operate as a current switching device. The effect is based on the dissipation-driven superconductor-to-insulator quantum phase transition, which happens due to the interplay of the Josephson effect and Coulomb blockade. Coupling to a graphene substrate with gapless excitations further enhances charge fluctuations favoring superconductivity. The effect is shown to scale exponentially with the Fermi energy in graphene, which can be controlled by the gate voltage. We develop a theory, which quantitatively describes the quantum phase transition in a two-dimensional Josephson junction array, but it is expected to provide a reliable qualitative description for one-dimensional systems as well. We argue that the local effect of dissipation-induced enhancement of superconductivity is very robust and a similar sharp crossover should be present in finite Josephson junction chains.

cond-mat.supr-con↗

arXiv2008

Superconductor-to-normal transition in finite nanowires

In this paper we discuss the interplay of quantum fluctuations and dissipation in uniform superconducting nanowires. We consider a phenomenological model with superconducting and normal components, and a finite equilibration rate between these two-fluids. We find that phase-slip dipoles proliferate in the wire, and decouple the two-fluids within its bulk. This implies that the the normal fluid only couples to the superconductor fluid through the leads at the edges of the wire, and the {\it local} dissipation is unimportant. Therefore, while long wires have a superconductor-metal transition tuned by local properties of the superconducting fluid, short wires have a transition when the {\it total} resistance is $R_{total}=R_Q=h/4e^2$.

cond-mat.supr-con↗

arXiv2008

Vortex Lattice Locking in Rotating Two-Component Bose-Einstein Condensates

The vortex density of a rotating superfluid, divided by its particle mass, dictates the superfluid's angular velocity through the Feynman relation. To find how the Feynman relation applies to superfluid mixtures, we investigate a rotating two-component Bose-Einstein condensate, composed of bosons with different masses. We find that in the case of sufficiently strong interspecies attraction, the vortex lattices of the two condensates lock and rotate at the drive frequency, while the superfluids themselves rotate at two different velocities, whose ratio is the ratio between the particle mass of the two species. In this paper, we characterize the vortex-locked state, establish its regime of stability, and find that it surives within a disk smaller than a critical radius, beyond which vortices become unbound, and the two Bose-gas rings rotate together at the frequency of the external drive.

cond-mat.str-el↗

arXiv2007

Superfluid-insulator transition in Fermi-Bose mixtures and the orthogonality catastrophe

The superfluid-insulator transition of bosons is strongly modified by the presence of Fermions. Through an imaginary-time path integral approach, we derive the self-consistent mean-field transition line, and account for both the static and the dynamic screening effects of the fermions. We find that an effect akin to the fermionic orthogonality catastrophe, arising from the fermionic screening fluctuations, suppresses superfluidity. We analyze this effect for various mixture parameters and temperatures, and consider possible signatures of the orthogonality catastrophe effect in other measurables of the mixture.

cond-mat.supr-con↗

arXiv2007

Effect of inhomogeneous coupling on superconductivity

We investigate the influence of inhomogeneity in the pairing coupling constant $U(\vec r)$ on dirty BCS superconductors, focusing on $T_c$, the order parameter $Δ(\vec r)$, and the energy gap $E_g(\vec r)$. Within mean-field theory, we find that when the length-scale of the inhomogeneity is comparable to, or larger than the coherence length, the ratio $2E_g/T_c$ is significantly reduced from that of a homogeneous superconductor, while in the opposite limit this ratio stays unmodified. In two dimensions, when strong phase fluctuations are included, the Kosterlitz-Thouless temperature $T_{KT}$ is also studied. We find that when the inhomogeneity length scale is much larger than the coherence length, $2E_g/T_{KT}$ can be larger than the usual BCS value. We use our results to qualitatively explain recent experimental observation of a surprisingly low value of $2E_g/T_c$ in thin films.

cond-mat.supr-con↗

arXiv2007

The insulating phases and superfluid-insulator transition of disordered boson chains

Using a strong disorder real-space renormalization group (RG), we study the phase diagram of a fully disordered chain of interacting bosons. Since this approach does not suffer from run-away flows, it allows a direct study of the insulating phases, which are not accessible in a weak disorder perturbative treatment. We find that the universal properties of the insulating phase are determined by the details and symmetries of the onsite chemical-potential disorder. Three insulating phases are possible: (i) an incompressible Mott glass with a finite superfluid susceptibility, (ii) a random-singlet glass with diverging compressibility and superfluid susceptibility, (iii) a Bose glass with a finite compressibility but diverging superfluid susceptibility. In addition to characterizing the insulating phases, we show that the superfluid-insulator transition is always of the Kosterlitz-Thouless universality class.

cond-mat.dis-nn↗

arXiv2007

Sharp Superconductor-Insulator Transition in Short Wires

Recent experiments on short MoGe nanowires show a sharp superconductor-insulator transition tuned by the normal state resistance of the wire, with a critical resistance of $R_c\approx R_Q= h/(4e^2)$. These results are at odds with a broad range of theoretical work on Josephson-like systems that predicts a smooth transition, tuned by the value of the resistance that shunts the junction. We develop a self-consistent renormalization group treatment of interacting phase-slips and their dual counterparts, correlated cooper pair tunneling, beyond the dilute approximation. This analysis leads to a very sharp transition with a critical resistance of $R_Q$. The addition of the quasi-particles' resistance at finite temperature leads to a quantitative agreement with the experimental results. This self-consistent renormalization group method should also be applicable to other physical systems that can be mapped onto similar sine-Gordon models, in the previously inaccessible intermediate-coupling regime.

cond-mat.supr-con↗

arXiv2007

Universal Periods in Quantum Hall Droplets

Using the hierarchy picture of the fractional quantum Hall effect, we study the the ground state periodicity of a finite size quantum Hall droplet in a quantum Hall fluid of a different filling factor. The droplet edge charge is periodically modulated with flux through the droplet and will lead to a periodic variation in the conductance of a nearby point contact, such as occurs in some quantum Hall interferometers. Our model is consistent with experiment and predicts that superperiods can be observed in geometries where no interfering trajectories occur. The model may also provide an experimentally feasible method of detecting elusive neutral modes and otherwise obtaining information about the microscopic edge structure in fractional quantum Hall states.

cond-mat.mes-hall↗

arXiv2007

Superfluidity and magnetism in multicomponent ultracold fermions

We study the interplay between superfluidity and magnetism in a multicomponent gas of ultracold fermions. Ward-Takahashi identities constrain possible mean-field states describing order parameters for both pairing and magnetization. The structure of global phase diagrams arises from competition among these states as functions of anisotropies in chemical potential, density, or interactions. They exhibit first and second order phase transition as well as multicritical points, metastability regions, and phase separation. We comment on experimental signatures in ultracold atoms.

cond-mat.supr-con↗

arXiv2007

Entanglement entropy of the random spin-1 Heisenberg chain

Random spin chains at quantum critical points exhibit an entanglement entropy between a segment of length L and the rest of the chain that scales as log_2 L with a universal coefficient. Since for pure quantum critical spin chains this coefficient is fixed by the central charge of the associated conformal field theory, the universal coefficient in the random case can be understood as an effective central charge. In this paper we calculate the entanglement entropy and effective central charge of the spin-1 random Heisenberg model in its random-singlet phase and also at the critical point at which the Haldane phase breaks down. The latter is the first entanglement calculation for an infinite-randomness fixed point that is not in the random-singlet universality class. Our results are consistent with a c-theorem for flow between infinite-randomness fixed points. The formalism we use can be generally applied to calculation of quantities that depend on the RG history in s>=1 random Heisenberg chains.

cond-mat.dis-nn↗

arXiv2006

Sagnac interference in Carbon nanotube loops

In this paper we study electron interference in nanotube loops. The conductance as a function of the applied voltage is shown to oscillate due to interference between electron beams traversing the loop in two opposite directions, with slightly different velocities. The period of these oscillations with respect to the gate voltage, as well as the temperatures required for the effect to appear, are shown to be much larger than those of the related Fabry-Perot interference. This effect is analogous to the Sagnac effect in light interferometers. We calculate the effect of interactions on the period of the oscillations, and show that even though interactions destroy much of the near-degeneracy of velocities in the symmetric spin channel, the slow interference effects survive.

cond-mat.mes-hall↗