Attractive Fermi gases with unequal spin populations in highly elongated traps

We investigate two-component attractive Fermi gases with imbalanced spin populations in trapped one dimensional configurations. The ground state properties are determined within local density approximation, starting from the exact Bethe-ansatz equations for the homogeneous case. We predict that the atoms are distributed according to a two-shell structure: a partially polarized phase in the center of the trap and either a fully paired or a fully polarized phase in the wings. The partially polarized core is expected to be a superfluid of the FFLO type. The size of the cloud as well as the critical spin polarization needed to suppress the fully paired shell, are calculated as a function of the coupling strength.Comment: Final accepted versio

Pair formation and collapse in imbalanced Fermion populations with unequal masses

We present an exact Quantum Monte Carlo study of the effect of unequal masses on pair formation in Fermionic systems with population imbalance loaded into optical lattices. We have considered three forms of the attractive interaction and find in all cases that the system is unstable and collapses as the mass difference increases and that the ground state becomes an inhomogeneous collapsed state. We also address the question of canonical vs grand canonical ensemble and its role, if any, in stabilizing certain phases

Crystal growth, structural studies and superconducting properties of beta-pyrochlore KOs2O6

Single crystals of KOs2O6 have been grown in a sealed quartz ampoule. Detailed single crystal X-ray diffraction studies at room temperature show Bragg peaks that violate Fd-3m symmetry. With a comparative structure refinement the structure is identified as non-centrosymmetric (F-43m). Compared to the ideal beta-pyrochlore lattice (Fd-3m), both Os tetrahedral and O octahedral network exhibit breathing mode like volume changes accompanied by strong anisotropic character of the K channels. The crystals show metallic conductivity and a sharp transition to the superconducting state at Tc = 9.65 K. Superconducting properties have been investigated by magnetization measurements performed in a temperature range from 2 to 12 K and in magnetic fields from 0 to 60 kOe. The temperature dependence of the upper critical field Hc2(T) has been determined and the initial slope (dHc2/dT)Tc = -33.3 kOe/K has been obtained near Tc. The upper critical field at zero temperature was estimated to be Hc2(0) \cong 230 kOe, which is a value close to the Pauli paramagnetic limiting field Hp(0)\cong 250 kOe. Then, the Ginzburg-Landau (GL) coherence length xi GL(0) \approx 3.8 nm was calculated, and the Maki parameter alpha \approx \sqrt 2 was obtained, suggesting the possibility that KOs2O6 might behave unconventionally at low temperatures and high magnetic fields

Near-Zero Modes in Superconducting Graphene

Vortices in the simplest superconducting state of graphene contain very low energy excitations, whose existence is connected to an index theorem that applies strictly to an approximate form of the relevant Bogoliubov-deGennes equations. When Zeeman interactions are taken into account, the zero modes required by the index theorem are (slightly) displaced. Thus the vortices acquire internal structure, that plausibly supports interesting dynamical phenomena.Comment: 9 pages, to appear in Proceedings of the Nobel Symposium on Graphene and Quantum Matte

Comment on ``Texture in the Superconducting Order Parameter of CeCoIn5_5 Revealed by Nuclear Magnetic Resonance''

The study of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state has been of considerable recent interest. Below the temperature $T^*$ which is believed to be the transition temperature ($T$) to the FFLO phase in CeCoIn$_5$, K. Kakuyanagi et al. (Phys. Rev. Lett. 94, 047602 (2005)) reported a composite NMR spectrum with a tiny component observed at frequencies corresponding to the normal state signal. The results were interpreted as evidence for the emergence of an FFLO state. This result is inconsistent with two other NMR studies of V. F. Mitrovi{\'c} et al. (Phys. Rev. Lett. 97, 117002 (2006)) and B.-L. Young et al. (Phys. Rev. Lett. 98, 036402 (2007)). In this comment we show that the findings of K. Kakuyanagi et al. do not reflect the true nature of the FFLO state but result from excess RF excitation power used in that experiment.Comment: 1 page, to appear in PR

Integrable models for asymmetric Fermi superfluids: Emergence of a new exotic pairing phase

We introduce an exactly-solvable model to study the competition between the Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) and breached-pair superfluid in strongly interacting ultracold asymmetric Fermi gases. One can thus investigate homogeneous and inhomogeneous states on an equal footing and establish the quantum phase diagram. For certain values of the filling and the interaction strength, the model exhibits a new stable exotic pairing phase which combines an inhomogeneous state with an interior gap to pair-excitations. It is proven that this phase is the exact ground state in the strong coupling limit, while numerical examples demonstrate that also at finite interaction strength it can have lower energy than the breached-pair or LOFF states.Comment: Revised version accepted for publicatio

Free-energy distribution functions for the randomly forced directed polymer

We study the $1+1$-dimensional random directed polymer problem, i.e., an elastic string $\phi(x)$ subject to a Gaussian random potential $V(\phi,x)$ and confined within a plane. We mainly concentrate on the short-scale and finite-temperature behavior of this problem described by a short- but finite-ranged disorder correlator $U(\phi)$ and introduce two types of approximations amenable to exact solutions. Expanding the disorder potential $V(\phi,x) \approx V_0(x) + f(x) \phi(x)$ at short distances, we study the random force (or Larkin) problem with $V_0(x) = 0$ as well as the shifted random force problem including the random offset $V_0(x)$; as such, these models remain well defined at all scales. Alternatively, we analyze the harmonic approximation to the correlator $U(\phi)$ in a consistent manner. Using direct averaging as well as the replica technique, we derive the distribution functions ${\cal P}_{L,y}(F)$ and ${\cal P}_L(F)$ of free energies $F$ of a polymer of length $L$ for both fixed ($\phi(L) = y$) and free boundary conditions on the displacement field $\phi(x)$ and determine the mean displacement correlators on the distance $L$. The inconsistencies encountered in the analysis of the harmonic approximation to the correlator are traced back to its non-spectral correlator; we discuss how to implement this approximation in a proper way and present a general criterion for physically admissible disorder correlators $U(\phi)$.Comment: 16 pages, 5 figure

Cooper-pair resonances and subgap Coulomb blockade in a superconducting single-electron transistor

We have fabricated and measured superconducting single-electron transistors with Al leads and Nb islands. At bias voltages below the gap of Nb we observe clear signatures of resonant tunneling of Cooper pairs, and of Coulomb blockade of the subgap currents due to linewidth broadening of the energy levels in the superconducting density of states of Nb. The experimental results are in good agreement with numerical simulations.Comment: 4 pages, 3 figure

Long-time behavior of the momentum distribution during the sudden expansion of a spin-imbalanced Fermi gas in one dimension

We study the sudden expansion of spin-imbalanced ultracold lattice fermions with attractive interactions in one dimension after turning off the longitudinal confining potential. We show that the momentum distribution functions of majority and minority fermions approach stationary values quickly due to a quantum distillation mechanism that results in a spatial separation of pairs and majority fermions. As a consequence, Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) correlations are lost during the expansion. Furthermore, we argue that the shape of the stationary momentum distribution functions can be understood by relating them to the integrals of motion in this integrable quantum system. We discuss our results in the context of proposals to observe FFLO correlations, related to recent experiments by Liao et al., Nature 467, 567 (2010).Comment: 8 pages including supplementary material, 9 eps figures, revised version as published, some text moved to the supplemental materia