39 research outputs found
Critical Fields of mesoscopic superconductors
Recent measurements have shown oscillations in the upper critical field of
simply connected mesoscopic superconductors. A quantitative theory of these
effects is given here on the basis of a Ginzburg-Landau description. For small
fields, the phase boundary exhibits a cusp where the screening currents
change sign for the first time thus defining a lower critical field .
In the limit where many flux quanta are threading the sample, nucleation occurs
at the boundary and the upper critical field becomes identical with the surface
critical field .Comment: 5 pages (Revtex and 2 PostScript figures), to apppear in Z. Phys.
Bose-Einstein Condensation versus Dicke-Hepp-Lieb Transition in an Optical Cavity
We provide an exact solution for the interplay between Bose-Einstein
condensation and the Dicke-Hepp-Lieb self-organization transition of an ideal
Bose gas trapped inside a single-mode optical cavity and subject to a
transverse laser drive. Based on an effective action approach, we determine the
full phase diagram at arbitrary temperature, which features a bi-critical point
where the transitions cross. We calculate the dynamically generated band
structure of the atoms and the associated supression of the critical
temperature for Bose-Einstein condensation in the phase with a spontaneous
periodic density modulation. Moreover, we determine the evolution of the
polariton spectrum due to the coupling of the cavity photons and the atomic
field near the self-organization transition, which is quite different above or
below the Bose-Einstein condensation temperature. At low temperatures, the
critical value of the Dicke-Hepp-Lieb transition decreases with temperature and
thus thermal fluctuations can enhance the tendency to a periodic arrangement of
the atoms.Comment: 18 pages, 12 figure
FFLO strange metal and quantum criticality in two dimensions: theory and application to organic superconductors
Increasing the spin imbalance in superconductors can spatially modulate the
gap by forming Cooper pairs with finite momentum. For large imbalances compared
to the Fermi energy, the inhomogeneous FFLO superconductor ultimately becomes a
normal metal. There is mounting experimental evidence for this scenario in 2D
organic superconductors in large in-plane magnetic fields; this is complemented
by ongoing efforts to realize this scenario in coupled tubes of atomic Fermi
gases with spin imbalance. Yet, a theory for the phase transition from a metal
to an FFLO superconductor has not been developed so far and the universality
class has remained unknown. Here we propose and analyze a spin imbalance driven
quantum critical point between a 2D metal and an FFLO phase in anisotropic
electron systems. We derive the effective action for electrons and bosonic FFLO
pairs at this quantum phase transition. Using this action, we predict non-Fermi
liquid behavior and the absence of quasi-particles at a discrete set of hot
spots on the Fermi surfaces. This results in strange power-laws in
thermodynamics and response functions, which are testable with existing
experimental set-ups on 2D organic superconductors and may also serve as
signatures of the elusive FFLO phase itself. The proposed universality class is
distinct from previously known quantum critical metals and, because its
critical fluctuations appear already in the pairing channel, a promising
candidate for naked metallic quantum criticality over extended temperature
ranges.Comment: 3+1 figure
Efimov states near a Feshbach resonance and the limits of van der Waals universality at finite background scattering length
We calculate the spectrum of three-body Efimov bound states near a Feshbach
resonance within a model which accounts both for the finite range of
interactions and the presence of background scattering. The latter may be due
to direct interactions in an open channel or a second overlapping Feshbach
resonance. It is found that background scattering gives rise to substantial
changes in the trimer spectrum as a function of the detuning away from a
Feshbach resonance, in particular in the regime where the background channel
supports Efimov states on its own. Compared to the situation with negligible
background scattering, the regime where van der Waals universality applies is
shifted to larger values of the resonance strength if the background scattering
length is positive. For negative background scattering lengths, in turn, van
der Waals universality extends to even small values of the resonance strength
parameter, consistent with experimental results on Efimov states in K.
Within a simple model, we show that short-range three-body forces do not affect
van der Waals universality significantly. Repulsive three-body forces may,
however, explain the observed variation between around and of the
ratio between the scattering length where the first Efimov trimer appears and
the van der Waals length.Comment: 17 pages, 13 figures; final version as publishe
Superfluidity near phase separation in Bose-Fermi mixtures
We study the transition to fermion pair superfluidity in a mixture of
interacting bosonic and fermionic atoms. The fermion interaction induced by the
bosons and the dynamical screening of the condensate phonons due to fermions
are included using the nonperturbative Hamiltonian flow equations. We determine
the bosonic spectrum near the transition towards phase separation and find that
the superfluid transition temperature may be increased substantially due to
phonon damping.Comment: 7 pages and 3 figures, accepted versio