Topological Hall Effect in Inhomogeneous Superconductors

We propose a possible mechanism of topological Hall effect in inhomogeneous superconducting states. In our scenario, the Berry phase effect associated with spatially modulated superconducting order parameter gives rise to a fictitious Lorentz force acting on quasiparticles. In the case of the Fulde-Ferrell-Larkin-Ovchinnikov state, the topological Hall effect is detected by applying an electromagnetic wave with a tuned wave number on a surface of the system.Comment: 4 page

Angular Fulde-Ferrell-Larkin-Ovchinnikov state in cold fermion gases in a toroidal trap

We study the possibility of angular Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, in which the rotation symmetry is spontaneously broken, in population imbalanced fermion gases near the BCS-BEC crossover. We investigate the superfluid gases at low temperatures on the basis of the Bogoliubov-de Gennes equation, and examine the stability against thermal fluctuations using the T-matrix approach beyond the local-density approximation (LDA). We find that the angular FFLO state is stabilized in the gases confined in the toroidal trap but not in the harmonic trap. The angular FFLO state is stable near the BCS-BEC crossover owing to the formation of pseudogap. Spatial dependences of number density and local population imbalance are shown for an experimental test.Comment: final version for publication in Phys. Rev. B Rapid Communicatio

Theory of the striped superconductor

We define a distinct phase of matter, a "pair density wave" (PDW), in which the superconducting order parameter $\phi$ varies periodically as a function of position such that when averaged over the center of mass position, all components of $\phi$ vanish identically. Specifically, we study the simplest, unidirectional PDW, the "striped superconductor," which we argue may be at the heart of a number of spectacular experimental anomalies that have been observed in the failed high temperature superconductor, La$_{2-x}$ Ba$_x$CuO$_4$. We present a solvable microscopic model with strong electron-electron interactions which supports a PDW groundstate. We also discuss, at the level of Landau theory, the nature of the coupling between the PDW and other order parameters, and the origins and some consequences of the unusual sensitivity of this state to quenched disorder.Comment: 16 pages, 3 figures, 1 table; Journal ref. adde

Topological exciton condensate of imbalanced electrons and holes

I study the effects of particle-hole imbalance on the exciton superfluid formed in a topological insulator thin-film and obtain the mean-field phase diagram. At finite imbalance a spatially modulated condensate is formed, akin to the Fulde-Ferrel-Larkin-Ovchinikov state in a superconductor, which preempts a first-order transition from the uniform condensate to the normal state at low temperatures. The imbalance can be tuned by changing the chemical potential at the two surfaces separately or, alternatively, by an asymmetric application of Zeeman fields at constant chemical potential. A vortex in the condensate carries a precisely fractional charge half of that of an electron. Possible experimental signatures for realistic parameters are discussed.Comment: 4 pages, 4 figures; (v2) Figures (especially Fig. 4) are improved; Fig. 1 now shows the transition lines correctly (results are the same as before); Text is updated and typos corrected; References are updated and added. To appear in PR

Fermi-liquid effects in the Fulde-Ferrell-Larkin-Ovchinnikov state of two-dimensional d-wave superconductors

We study the effects of Fermi-liquid interactions on quasi-two-dimensional d-wave superconductors in a magnetic field. The phase diagram of the superconducting state, including the periodic Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in high magnetic fields, is discussed for different strengths of quasiparticle many-body interactions within Landau's theory of Fermi liquids. Decreasing the Fermi-liquid parameter $F_0^a$ causes the magnetic spin susceptibility to increase, which in turn leads to a reduction of the FFLO phase. It is shown that a negative $F_0^a$ results in a first-order phase transition from the normal to the uniform superconducting state in a finite temperature interval. Finally, we discuss the thermodynamic implications of a first-order phase transition for CeCoIn$_5$.Comment: published version; removed direct comparison with experiment for the upper critical field, as required by the referee

Crystalline chiral condensates off the tricritical point in a generalized Ginzburg-Landau approach

We present an extensive study on inhomogeneous chiral condensates in QCD at finite density in the chiral limit using a generalized Ginzburg-Landau (GL) approach. Performing analyses on higher harmonics of one-dimensionally (1D) modulated condensates, we numerically confirm the previous claim that the solitonic chiral condensate characterized by Jacobi's elliptic function is the most favorable structure in 1D modulations. We then investigate the possibility of realization of several multidimensional modulations within the same framework. We also study the phase structure far away from the tricritical point by extending the GL functional expanded up to the eighth order in the order parameter and its spatial derivative. On the same basis, we explore a new regime in the extended GL parameter space and find that the Lifshitz point is the point where five critical lines meet at once. In particular, the existence of an intriguing triple point is demonstrated, and its trajectory consists of one of those critical lines.Comment: 17 pages, 20 eps figures; (v2) corrected an error in computation of eighth order coefficients; (v3) typos corrected, version to appear in Phys. Rev.

Vortex tilt modulus in Fulde-Ferrell-Larkin-Ovchinnikov state

Vortex tilt response in Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) vortex lattice is theoretically examined as a probe reflecting the spatial structure of this state. In the FFLO state with nodal planes perpendicular to the magnetic field in a quasi 2D material under a parallel field, the tilt modulus E_{2} {\it of the nodal planes} decreases as the paramagnetic effect is effectively enhanced, and this reduction of E_{2} in turn reduces the vortex tilt modulus. This reduction of vortex tilt modulus, more remarkable in more anisotropic systems, accompanying the FFLO transition may be an origin of the monotonous reduction of sound velocity detected upon cooling in a ultrasound measurement for CeCoIn5.Comment: 14 pages, 5 figures. Accepted for publication in Phys. Rev.

BEC-BCS crossover in "magnetized" Feshbach-resonantly paired superfluids

We map out the detuning-magnetization phase diagram for a ``magnetized'' (unequal number of atoms in two pairing hyperfine states) gas of fermionic atoms interacting via an s-wave Feshbach resonance (FR). For large positive FR detuning a normal magnetized Fermi gas is stable above an exponentially small value of the population difference. Below this critical value the phase diagram is dominated by coexistence of a magnetized normal gas and a singlet paired superfluid with the latter exhibiting a BCS-Bose Einstein condensate crossover with reduced detuning. On the BCS side of strongly overlapping Cooper pairs, a sliver of finite-momentum paired Fulde-Ferrell-Larkin-Ovchinnikov magnetized phase intervenes between the phase separated and normal states. In contrast, for large negative detuning a uniform, polarized superfluid, that is a coherent mixture of singlet Bose-Einstein-condensed molecules and fully magnetized single-species Fermi-sea, is a stable ground state.Comment: 4 RevTeX pages, 2 figures. Minor changes from previous versio

Modification of Born impurity scattering near the surface of d-wave superconductors and influence of external magnetic field

We study the influence of Born impurity scattering on the zero-energy Andreev bound states near the surface of a d-wave superconductor with and without an externally applied magnetic field. Without an external magnetic field we show that the effect of Born impurity scattering is stronger at the surface than in the bulk. In the presence of an external magnetic field the splitting of the zero-energy Andreev bound states is shown to have a nonmonotonous temperature dependence. Born impurity scattering does not wash out the peak splitting, but instead the peak splitting is shown to be quite robust against impurities. We also show that a nonzero gap renormalization appears near the surface.Comment: 9 pages, 17 figures; minor changes; new figure 11; accepted for publication in Phys. Rev.

Recovering of superconductivity in exchange fields exceeding Pauli limiting field under spin-dependent quasiparticle distribution

We study theoretically the simultaneous influence of spin accumulation potential $eV_\uparrow-eV_\downarrow$ and the Zeeman exchange field on singlet superconductivity. It is shown that the pair-breaking effect of the Zeeman field can be fully compensated by creation of the appropriate spin accumulation potential in the superconductor. Moreover, superconductivity can be recovered for exchange fields well exceeding the Pauli limiting field. It is proposed that the effect can be experimentally realized on the basis of voltage biased junction consisting of a thin superconducting film sandwiched between two half metals.Comment: 4 pages, 1 figure, published versio