233 research outputs found
Enhancement of electron-hole superfluidity in double few-layer graphene
We propose two coupled electron-hole sheets of few-layer graphene as a new
nanostructure to observe superfluidity at enhanced densities and enhanced
transition temperatures. For ABC stacked few-layer graphene we show that the
strongly correlated electron-hole pairing regime is readily accessible
experimentally using current technologies. We find for double trilayer and
quadlayer graphene sheets spatially separated by a nano-thick hexagonal
boron-nitride insulating barrier, that the transition temperature for
electron-hole superfluidity can approach temperatures of 40 K.Comment: 17 pages, 5 figure
Pairing effects in the normal phase of a two-dimensional Fermi gas
In a recent experiment [M. Feld et al., Nature 480, 75 (2011); B. Froehlich
et al., Phys. Rev. Lett. 109,130403 (2012)], a pairing gap was detected in a
two-dimensional (2D) Fermi gas with attractive interaction at temperatures
where superfluidity does not occur. The question remains open as to whether
this gap is a pseudogap phenomenon or is due to a molecular state. In this
paper, by using a t-matrix approach, we reproduce quite well the experimental
data for a 2D Fermi gas, and set the boundary between the pseudogap and
molecular regimes. We also show that pseudogap phenomena occurring in 2D and 3D
can be related through a variable spanning the BCS-BEC crossover in a universal
way.Comment: 10 pages, 9 figures; final versio
Temperature and coupling dependence of the universal contact intensity for an ultracold Fermi gas
Physical properties of an ultracold Fermi gas in the temperature-coupling
phase diagram can be characterized by the contact intensity C, which enters the
pair-correlation function at short distances and describes how the two-body
problem merges into its surrounding. We show that the local order established
by pairing fluctuations about the critical temperature Tc of the superfluid
transition considerably enhances the contact C in a temperature range where
pseudogap phenomena are maximal. Our ab initio results for C in a trap compare
well with recently available experimental data over a wide coupling range. An
analysis is also provided for the effects of trap averaging on C.Comment: 5 pages, 5 figure
Competition between final-state and pairing-gap effects in the radio-frequency spectra of ultracold Fermi atoms
The radio-frequency spectra of ultracold Fermi atoms are calculated by
including final-state interactions affecting the excited level of the
transition, and compared with the experimental data. A competition is revealed
between pairing-gap effects which tend to push the oscillator strength toward
high frequencies away from threshold, and final-state effects which tend
instead to pull the oscillator strength toward threshold. As a result of this
competition, the position of the peak of the spectra cannot be simply related
to the value of the pairing gap, whose extraction thus requires support from
theoretical calculations.Comment: 4 pages, 3 figures, final version published in Phys. Rev. Let
Shape-resonant superconductivity in nanofilms: from weak to strong coupling
Ultrathin superconductors of different materials are becoming a powerful
platform to find mechanisms for enhancement of superconductivity, exploiting
shape resonances in different superconducting properties. Here we evaluate the
superconducting gap and its spatial profile, the multiple gap components, and
the chemical potential, of generic superconducting nanofilms, considering the
pairing attraction and its energy scale as tunable parameters, from weak to
strong coupling, at fixed electron density. Superconducting properties are
evaluated at mean field level as a function of the thickness of the nanofilm,
in order to characterize the shape resonances in the superconducting gap. We
find that the most pronounced shape resonances are generated for weakly coupled
superconductors, while approaching the strong coupling regime the shape
resonances are rounded by a mixing of the subbands due to the large energy gaps
extending over large energy scales. Finally, we find that the spatial profile,
transverse to the nanofilm, of the superconducting gap acquires a flat behavior
in the shape resonance region, indicating that a robust and uniform multigap
superconducting state can arise at resonance.Comment: 7 pages, 4 figures. Submitted to the Proceedings of the Superstripes
2016 conferenc
Multicomponent Electron-Hole Superfluidity and the BCS-BEC Crossover in Double Bilayer Graphene
Superfluidity in coupled electron-hole sheets of bilayer graphene is predicted here to be multicomponent because of the conduction and valence bands. We investigate the superfluid crossover properties as functions of the tunable carrier densities and the tunable energy band gap E_g. For small band gaps there is a significant boost in the two superfluid gaps, but the interaction-driven excitations from the valence to the conduction band can weaken the superfluidity, even blocking the system from entering the Bose-Einstein condensate (BEC) regime at low densities. At a given larger density, a band gap E_g∼80-120 meV can carry the system into the strong-pairing multiband BCS-BEC crossover regime, the optimal range for realization of high-T_c superfluidity
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