Transition to Fulde-Ferrel-Larkin-Ovchinnikov phases near the tricritical point : an analytical study

We explore analytically the nature of the transition to the Fulde-Ferrel-Larkin-Ovchinnikov superfluid phases in the vicinity of the tricritical point, where these phases begin to appear. We make use of an expansion of the free energy up to an overall sixth order, both in order parameter amplitude and in wavevector. We first explore the minimization of this free energy within a subspace, made of arbitrary superpositions of plane waves with wavevectors of different orientations but same modulus. We show that the standard second order FFLO phase transition is unstable and that a first order transition occurs at higher temperature. Within this subspace we prove that it is favorable to have a real order parameter and that, among these states, those with the smallest number of plane waves are prefered. This leads to an order parameter with a $\cos({\bf q}_{0}. {\bf r})$ dependence, in agreement with preceding work. Finally we show that the order parameter at the transition is only very slightly modified by higher harmonics contributions when the constraint of working within the above subspace is released.Comment: 11 pages, revte

Effective bosonic hamiltonian for excitons : a too naive concept

Excitons, being made of two fermions, may appear from far as bosons. Their close-to-boson character is however quite tricky to handle properly. Using our commutation technique especially designed to deal with interacting close-to-boson particles, we here calculate the exact expansion in Coulomb interaction of theexciton-exciton correlations, and show that a naive effective bosonic hamiltonian for excitons cannot produce these X-X correlations correctly

Transitions to the Fulde-Ferrell-Larkin-Ovchinnikov phases at low temperature in two dimensions

We explore the nature of the transition to the Fulde-Ferrell-Larkin- Ovchinnikov superfluid phases in the low temperature range in two dimensions, for the simplest isotropic BCS model. This is done by applying the Larkin-Ovchinnikov approach to this second order transition. We show that there is a succession of transitions toward ever more complex order parameters when the temperature goes to zero. This gives rise to a cascade with, in principle, an infinite number of transitions. Except for one case, the order parameter at the transition is a real superposition of cosines with equal weights. The directions of these wavevectors are equally spaced angularly, with a spacing which goes to zero when the temperature goes to zero. This singular behaviour in this $T = 0$ limit is deeply linked to the two-dimensional nature of the problem.Comment: 18 pages, 6 figure

"Gray" BCS condensate of excitons and internal Josephson effect

It has been recently suggested that the Bose-Einstein condensate formed by excitons in the dilute limit must be dark, i.e., not coupled to photons. Here, we show that, under a density increase, the dark exciton condensate must acquire a bright component due to carrier exchange in which dark excitons turn bright. This however requires a density larger than a threshold which seems to fall in the forbidden region of the phase separation between a dilute exciton gas and a dense electron-hole plasma. The BCS-like condensation which is likely to take place on the dense side, must then have a dark and a bright component - which makes it "gray". It should be possible to induce an internal Josephson effect between these two coherent components, with oscillations of the photoluminescence as a strong proof of the existence for this "gray" BCS-like exciton condensate.Comment: 4 pages, typo correcte

Shift of the molecular bound state threshold in dense ultracold Fermi gases with Feshbach resonance

We consider a dense ultracold Fermi gas in the presence of a Feshbach resonance. We investigate how the treshold for bound state formation, which is just at the Feshbach resonance for a dilute gas, is modified due to the presence of the Fermi sea. We make use of a preceding framework of handling this many-body problem. We restrict ourselves to the simple case where the chemical potential $\mu$ is negative, which allows us to cover in particular the classical limit where the effect is seen to disappear. We show that, within a simple approach where basically only the effect of Pauli exclusion is included, the Fermi sea produces a large shift of the threshold, which is of order of the width of the Feshbach resonance. This is in agreement with very recent experimental findings.Comment: one reference adde

The low temperature Fulde-Ferrell-Larkin-Ovchinnikov phases in 3 dimensions

We consider the nature of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phases in three dimensions at low temperature. We introduce a new method to handle the quasiclassical equations for superconductors with space dependent order parameter, which makes use of a Fourier expansion. This allows us to show that, at T=0, an order parameter given by the linear combination of three cosines oscillating in orthogonal directions is preferred over the standard single cosine solution. The transition from the normal state to this phase is first order, and quite generally the transition below the tricritical point to the FFLO phases is always first order.Comment: 4 pages, revtex, 1 figur

Perturbation of Tunneling Processes by Mechanical Degrees of Freedom in Mesoscopic Junctions

We investigate the perturbation in the tunneling current caused by non-adiabatic mechanical motion in a mesoscopic tunnel junction. A theory introduced by Caroli et al. \cite{bi1,bi2,bi3} is used to evaluate second order self-energy corrections for this non-equilibrium situation lacking translational invariance. Inelastic signatures of the mechanical degrees of freedom are found in the current-voltage $I(V)$ characteristics. These give rise to sharp features in the derivative spectrum, $d^2I/dV^2$.Comment: 22 pages LaTeX + 3 uuencoded PS picture

On X-ray-singularities in the f-electron spectral function of the Falicov-Kimball model

The f-electron spectral function of the Falicov-Kimball model is calculated within the dynamical mean-field theory using the numerical renormalization group method as the impurity solver. Both the Bethe lattice and the hypercubic lattice are considered at half filling. For small U we obtain a single-peaked f-electron spectral function, which --for zero temperature-- exhibits an algebraic (X-ray) singularity ($|\omega|^{-\alpha}$) for $\omega \to 0$. The characteristic exponent $\alpha$ depends on the Coulomb (Hubbard) correlation U. This X-ray singularity cannot be observed when using alternative (Keldysh-based) many-body approaches. With increasing U, $\alpha$ decreases and vanishes for sufficiently large U when the f-electron spectral function develops a gap and a two-peak structure (metal-insulator transition).Comment: 8 pages, 8 figures, revte

Trion dynamics in coupled double quantum wells. Electron density effects

We have studied the coherent dynamics of injected electrons when they are either free or bounded both in excitons and in trions (charged excitons). We have considered a remotely doped asymmetric double quantum well where an excess of free electrons and the direct created excitons generate trions. We have used the matrix density formalism to analyze the electron dynamics for different concentration of the three species. Calculations show a significant modification of the free electron inter-sublevel oscillations cWe have studied the coherent dynamics of injected electrons when they are aused by electrons bound in excitons and trions. Based on the present calculations we propose a method to detect trions through the emitted electromagnetic radiation or the current density.Comment: 14 pages, 13 figure