Perturbed Sachdev-Ye-Kitaev Model: A Polaron in the Hyperbolic Plane

We study the Sachdev-Ye-Kitaev (SYK_{4}) model with a weak SYK_{2} term of magnitude Γ beyond the simplest perturbative limit considered previously. For intermediate values of the perturbation strength, J/N≪Γ≪J/sqrt[N], fluctuations of the Schwarzian mode are suppressed, and the SYK_{4} mean-field solution remains valid beyond the timescale t_{0}∼N/J up to t_{*}∼J/Γ^{2}. The out-of-time-order correlation function displays at short time intervals exponential growth with maximal Lyapunov exponent 2πT, but its prefactor scales as T at low temperatures T≤Γ.

Sachdev-Ye-Kitaev Model with Quadratic Perturbations: The Route to a Non-Fermi Liquid

We study stability of the Sachdev-Ye-Kitaev (SYK_{4}) model with a large but finite number of fermions N with respect to a perturbation, quadratic in fermionic operators. We develop analytic perturbation theory in the amplitude of the SYK_{2} perturbation and demonstrate stability of the SYK_{4} infrared asymptotic behavior characterized by a Green function G(τ)∝1/τ^{3/2}, with respect to weak perturbation. This result is supported by exact numerical diagonalization. Our results open the way to build a theory of non-Fermi-liquid states of strongly interacting fermions.

Microwave Properties of Superconductors Close to the Superconductor-Insulator Transition

Strongly disordered pseudogapped superconductors are expected to display arbitrarily high values of kinetic inductance close to the superconductor-insulator transition (SIT), which make them attractive for the implementation of large dissipationless inductance. We develop the theory of the collective modes in these superconductors and discuss associated dissipation at microwave frequencies. We obtain the collective mode spectra dependence on the disorder level and conclude that collective modes become a relevant source of dissipation and noise in the outer proximity of the SIT.

Magnetic field-induced giant enhancement of electron-phonon energy transfer in strongly disordered conductors

Relaxation of soft modes (e.g., charge density in gated semiconductor heterostructures, spin density in the presence of magnetic field) slowed down by disorder may lead to giant enhancement of energy transfer (cooling power) between overheated electrons and phonons at low bath temperature. We show that in strongly disordered systems with time-reversal symmetry broken by external or intrinsic exchange magnetic field the cooling power can be greatly enhanced. The enhancement factor as large as 10(2) at magnetic field B~10 T in 2D InSb films is predicted. A similar enhancement is found for the ultrasound attenuation.

Universal broadening of the Bardeen-Cooper-Schrieffer coherence peak of disordered superconducting films

In disordered superconductors, the local pairing field fluctuates in space, leading to the smearing of the BCS peak in the density of states and the appearance of the subgap tail states. We analyze the universal mesoscopic contributions to these effects and show that they are enhanced by the Coulomb repulsion. In the vicinity of the quantum critical point, where superconductivity is suppressed by the "fermionic mechanism," strong smearing of the peak due to mesoscopic fluctuations is predicted.

Anomalous Josephson current via Majorana bound states in topological insulators

We propose a setup involving Majorana bound states (MBS) hosted by a vortex on a superconducting surface of a 3D topological insulator (TI). We consider a narrow channel drilled across a TI slab with both sides covered by s-wave superconductor. In the presence of a vortex pinned to such a channel, it acts as a ballistic nanowire connecting the superconducting surfaces, with a pair of MBS localized in it. The energies of the MBS possess a 4π-periodic dependence on the superconductive phase difference φ between the surfaces. It results in the appearance of an anomalous term in the current-phase relation I(a)(φ) for the supercurrent flowing along the channel between the superconductive surfaces. We have calculated the shape of the 4π-periodic function I(a)(φ), as well as the dependence of its amplitude on temperature and system parameters.

Power-law spin correlations in a perturbed spin model on a honeycomb lattice

We consider the spin-1/2 model on the honeycomb lattice in the presence of a weak magnetic field hα << 1. Such a perturbation destroys the exact integrability of the model in terms of gapless fermions and static Z2 fluxes. We show that it results in the appearance of a long-range tail in the irreducible dynamic spin correlation function: <<s(z)(t,r)s(z)(0,0)>> ∝ h(z)(2)f(t,r), where f(t,r) ∝ [max(t,r)]-4 is proportional to the density polarization function of fermions.

Quantum spin metal state on a decorated honeycomb lattice

We present a modification of the exactly solvable spin-(1/)2 Kitaev model on the decorated honeycomb lattice, with a ground state of "spin metal" type. The model is diagonalized in terms of Majorana fermions; the latter form a 2D gapless state with a Fermi circle whose size depends on the ratio of exchange couplings. Low-temperature heat capacity C(T) and dynamic spin susceptibility χ(ω,T) are calculated in the case of small Fermi circle. Whereas, C(T)∼T at low temperatures as it is expected for a Fermi liquid, spin excitations are gapped and χ(ω,T) demonstrates unusual behavior with a power-law peak near the resonance frequency. The corresponding exponent as well as the peak shape are calculated.

Eigenfunction fractality and pseudogap state near the superconductor-insulator transition

We develop a theory of a pseudogap state appearing near the superconductor-insulator (SI) transition in strongly disordered metals with an attractive interaction. We show that such an interaction combined with the fractal nature of the single-particle wave functions near the mobility edge leads to an anomalously large single-particle gap in the superconducting state near SI transition that persists and even increases in the insulating state long after the superconductivity is destroyed. We give analytic expressions for the value of the pseudogap in terms of the inverse participation ratio of the corresponding localization problem.

Superconductivity in disordered thin films: giant mesoscopic fluctuations

We discuss the intrinsic inhomogeneities of superconductive properties of uniformly disordered thin films with a large dimensionless conductance g. It is shown that mesoscopic fluctuations, which usually contain a small factor 1/g, are crucially enhanced near the critical conductance g(cF) >> 1 where superconductivity is destroyed at T = 0 due to Coulomb suppression of the Cooper attraction. This leads to strong spatial fluctuations of the local transition temperature and thus to the percolative nature of the thermal superconductive transition.

Quantum percolation in granular metals

Theory of quantum corrections to conductivity of granular metal films is developed for the realistic case of large randomly distributed tunnel conductances. Quantum fluctuations of intergrain voltages (at energies E much below the bare charging energy scale E(C)) suppress the mean conductance g (E) much more strongly than its standard deviation sigma(E). At sufficiently low energies E(*) any distribution becomes broad, with sigma(E(*)) approximately g (E(*)), leading to strong local fluctuations of the tunneling density of states. The percolative nature of the metal-insulator transition is established by a combination of analytic and numerical analysis of the matrix renormalization group equations.

Dephasing in disordered metals with superconductive grains

The temperature dependence of electron dephasing time tau(phi)(T) is calculated for a disordered metal with a small concentration of superconductive grains. Above the macroscopic superconducting transition line, when electrons in the metal are normal, Andreev reflection from the grains leads to a nearly temperature-independent contribution to the dephasing rate. In a broad temperature range tau(-1)(phi)(T) strongly exceeds the prediction of the classical theory of dephasing in normal disordered conductors, whereas magnetoresistance is dominated (in two dimensions) by the Maki-Tompson correction and is positive.

Coulomb blockade of proximity effect at large conductance

We consider the proximity effect in a normal dot coupled to a bulk superconducting reservoir by the tunnel contact with large normal conductance. Coulomb interaction in the dot suppresses the proximity minigap induced in the normal part of the system. We find exact expressions for the thermodynamic and tunneling minigaps as functions of the junction's capacitance. The tunneling minigap interpolates between its proximity-induced value in the regime of weak Coulomb interaction to the Coulomb gap in the regime of strong interaction. In the intermediate case a nonuniversal two-step structure of the tunneling density of states is predicted. The charge quantization in the dot is also studied.

Superconducting tetrahedral quantum bits

We propose a design for a qubit with four superconducting islands in the topology of a symmetric tetrahedron, uniformly frustrated with one-half flux quantum per loop and one-half Cooper pair per island. This structure emulates a noise-resistant spin-1/2 system in a vanishing magnetic field. The flux frustration boosts quantum fluctuations and relieves the constraints on junction fabrication. Variability of manipulation and optimized readout are additional benefits of this design.

Topological order in the insulating Josephson junction array

We propose a Josephson junction array which can be tuned into an unconventional insulating state by varying external magnetic field. This insulating state retains a gap to half-vortices; as a consequence, such an array with nontrivial global geometry exhibits a ground state degeneracy. This degeneracy is protected from the effects of external noise. We compute the gaps, separating higher energy states from the degenerate ground state, and we discuss experiments probing the unusual properties of this insulator.

Topologically protected quantum bits using Josephson junction arrays

All physical implementations of quantum bits (or qubits, the logical elements in a putative quantum computer) must overcome conflicting requirements: the qubits should be manipulable through external signals, while remaining isolated from their environment. Proposals based on quantum optics emphasize optimal isolation, while those following the solid-state route exploit the variability and scalability of nanoscale fabrication techniques. Recently, various designs using superconducting structures have been successfully tested for quantum coherent operation, however, the ultimate goal of reaching coherent evolution over thousands of elementary operations remains a formidable task. Protecting qubits from decoherence by exploiting topological stability is a qualitatively new proposal that holds promise for long decoherence times, but its physical implementation has remained unclear. Here we show how strongly correlated systems developing an isolated twofold degenerate quantum dimer liquid ground state can be used in the construction of topologically stable qubits; we discuss their implementation using Josephson junction arrays. Although the complexity of their architecture challenges the technology base available today, such topological qubits greatly benefit from their built-in fault-tolerance.

Defect-unbinding and the Bose-glass transition in layered superconductors

The low-field Bose-glass transition temperature in heavy-ion irradiated Bi(2)Sr(2)CaCu(2)O(8+delta) increases progressively with increasing density n(d) of irradiation-induced columnar defects, but saturates for n(d) greater or = 1.5 x 10(9) cm(-2). The maximum Bose-glass temperature corresponds to that above which diffusion of two-dimensional pancake vortices between vortex lines becomes possible, and the "linelike" character of vortices is lost. We develop a description of the Bose-glass line that quantitatively describes experiments on crystals with widely different track densities and material parameters.

Quantum superconductor-metal transition in a proximity array

A theory of the zero-temperature superconductor-metal transition is developed for an array of superconductive islands (of size d) coupled via a disordered two-dimensional conductor with the dimensionless conductance g = Planck's over 2 pi/e(2)R(square)>>1. At T = 0 the macroscopically superconductive state of the array with lattice spacing b>>d is destroyed at g<g(c) approximately 0.1ln(2)(b/d). At high temperatures the normal-state resistance between neighboring islands at b = b(c) is much smaller than R(Q) = h/4e(2).