106 research outputs found
Pseudogap in fermionic density of states in the BCS-BEC crossover of atomic Fermi gases
We study pseudogap behaviors of ultracold Fermi gases in the BCS-BEC
crossover region. We calculate the density of states (DOS), as well as the
single-particle spectral weight, above the superfluid transition temperature
including pairing fluctuations within a -matrix approximation.
We find that DOS exhibits a pseudogap structure in the BCS-BEC crossover
region, which is most remarkable near the unitarity limit. We determine the
pseudogap temperature at which the pseudogap structure in DOS disappears.
We also introduce another temperature at which the BCS-like
double-peak structure disappears in the spectral weight. While one finds
in the BCS regime, becomes higher than in the
crossover and BEC regime. We also determine the pseudogap region in the phase
diagram in terms of temperature and pairing interaction.Comment: 6 pages, 4 figures, Proceedings of QFS 200
Observation of pseudogap behavior in a strongly interacting Fermi gas
Ultracold atomic Fermi gases present an opportunity to study strongly
interacting Fermi systems in a controlled and uncomplicated setting. The
ability to tune attractive interactions has led to the discovery of
superfluidity in these systems with an extremely high transition temperature,
near T/T_F = 0.2. This superfluidity is the electrically neutral analog of
superconductivity; however, superfluidity in atomic Fermi gases occurs in the
limit of strong interactions and defies a conventional BCS description. For
these strong interactions, it is predicted that the onset of pairing and
superfluidity can occur at different temperatures. This gives rise to a
pseudogap region where, for a range of temperatures, the system retains some of
the characteristics of the superfluid phase, such as a BCS-like dispersion and
a partially gapped density of states, but does not exhibit superfluidity. By
making two independent measurements: the direct observation of pair
condensation in momentum space and a measurement of the single-particle
spectral function using an analog to photoemission spectroscopy, we directly
probe the pseudogap phase. Our measurements reveal a BCS-like dispersion with
back-bending near the Fermi wave vector k_F that persists well above the
transition temperature for pair condensation
Evolution of Charge-Lattice Dynamics across the Kuramoto Synchronization Phase Diagram of Quantum Tunneling Polarons in Cuprate Superconductors
Because of its sensitivity to the instantaneous structure factor, S(Q,t = 0), Extended X-ray Absorption Fine Structure (EXAFS) is a powerful tool for probing the dynamic structure of condensed matter systems in which the charge and lattice dynamics are coupled. When applied to hole-doped cuprate superconductors, EXAFS has revealed the presence of internal quantum tunneling polarons (IQTPs). An IQTP arises in EXAFS as a two-site distribution for certain Cu–O pairs, which is also duplicated in inelastic scattering but not observed in standard diffraction measurements. The Cu–Sr pair distribution has been found to be highly anharmonic and strongly correlated to both the IQTPs and to superconductivity, as, for example, in YSr2Cu2.75Mo0.25O7.54
(Tc=84
K). In order to describe such nontrivial, anharmonic charge-lattice dynamics, we have proposed a model Hamiltonian for a prototype six-atom cluster, in which two Cu-apical-O IQTPs are charge-transfer bridged through Cu atoms by an O atom in the CuO2
plane and are anharmonically coupled via a Sr atom. By applying an exact diagonalization procedure to this cluster, we have verified that our model indeed produces an intricate interplay between charge and lattice dynamics. Then, by using the Kuramoto model for the synchronization of coupled quantum oscillators, we have found a first-order phase transition for the IQTPs into a synchronized, phase-locked phase. Most importantly, we have shown that this transition results specifically from the anharmonicity. Finally, we have provided a phase diagram showing the onset of the phase-locking of IQTPs as a function of the charge-lattice and anharmonic couplings in our model. We have found that the charge, initially confined to the apical oxygens, is partially pumped into the CuO2
plane in the synchronized phase, which suggests a possible connection between the synchronized dynamic structure and high-temperature superconductivity (HTSC) in doped cuprates
Quantitative comparison between theoretical predictions and experimental results for the BCS-BEC crossover
Theoretical predictions for the BCS-BEC crossover of trapped Fermi atoms are
compared with recent experimental results for the density profiles of Li.
The calculations rest on a single theoretical approach that includes pairing
fluctuations beyond mean field. Excellent agreement with experimental results
is obtained. Theoretical predictions for the zero-temperature chemical
potential and gap at the unitarity limit are also found to compare extremely
well with Quantum Monte Carlo simulations and with recent experimental results.Comment: 4 pages, 3 eps figure
Two-gap model for underdoped cuprate superconductors
Various properties of underdoped superconducting cuprates, including the
momentum-dependent pseudogap opening, indicate a behavior which is neither BCS
nor Bose-Einstein condensation (BEC) like. To explain this issue we introduce a
two-gap model. This model assumes an anisotropic pairing interaction among two
kinds of fermions with small and large Fermi velocities representing the
quasiparticles near the M and the nodal points of the Fermi surface
respectively. We find that a gap forms near the M points resulting into
incoherent pairing due to strong fluctuations. Instead the pairing near the
nodal points sets in with phase coherence at lower temperature. By tuning the
momentum-dependent interaction, the model allows for a continuous evolution
from a pure BCS pairing (in the overdoped and optimally doped regime) to a
mixed boson-fermion picture (in the strongly underdoped regime).Comment: 5 pages, 1 enclosed figure. For further information see
http://htcs.or
Three-dimensional electron-hole superfluidity in a superlattice close to room temperature
Although there is strong theoretical and experimental evidence for
electron-hole superfluidity in separated sheets of electrons and holes at low
, extending superfluidity to high is limited by strong 2D fluctuations
and Kosterlitz-Thouless effects. We show this limitation can be overcome using
a superlattice of alternating electron- and hole-doped semiconductor
monolayers. The superfluid transition in a 3D superlattice is not topological,
and for strong electron-hole pair coupling, the transition temperature
can be at room temperature. As a quantitative illustration, we show can
reach K for a superfluid in a realistic superlattice of transition metal
dichalcogenide monolayers.Comment: 5 pages, 3 figures, supplementary material (3 pages) includes 1 table
and 1 figur
Superfluid phase transition and strong-coupling effects in an ultracold Fermi gas with mass imbalance
We investigate the superfluid phase transition and effects of mass imbalance
in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation)
crossover regime of an cold Fermi gas. We point out that the Gaussian
fluctuation theory developed by Nozi\`eres and Schmitt-Rink and the -matrix
theory, that are now widely used to study strong-coupling physics of cold Fermi
gases, give unphysical results in the presence of mass imbalance. To overcome
this problem, we extend the -matrix theory to include higher-order pairing
fluctuations. Using this, we examine how the mass imbalance affects the
superfluid phase transition. Since the mass imbalance is an important key in
various Fermi superfluids, such as K-Li Fermi gas mixture, exciton
condensate, and color superconductivity in a dense quark matter, our results
would be useful for the study of these recently developing superfluid systems.Comment: 7 pages, 4 figures, Proceedings of QFS-201
Pairing-gap, pseudo-gap, and no-gap phases in the radio-frequency spectra of a trapped unitary 6Li gas
Radio frequency spectra of a trapped unitary 6Li gas are reported and
analyzed in terms of a theoretical approach that includes both final-state and
trap effects. Final-state effects play a crucial role in evidencing two main
peaks both above and below the critical temperature Tc as being associated with
two distinct phases that reside in different trap regions. These are the
pairing-gap and pseudo-gap phases below Tc, which evolve into the pseudo-gap
and no-gap phases above Tc. In this way, a long standing puzzle about the
interpretation of rf spectra for 6Li in a trap is solved.Comment: 5 pages, 6 figures (final version
Pseudogap and spectral function from superconducting fluctuations to the bosonic limit
The crossover from weak to strong coupling for a three dimensional continuum
model of fermions interacting via an attractive contact potential is studied
above the superconducting critical temperature. The pair-fluctuation
propagator, the one-loop self-energy, and the spectral function are
investigated in a systematic way from the superconducting fluctuation regime
(weak coupling) to the bosonic regime (strong coupling). Analytic and numerical
results are reported. In the strong-coupling regime, where the pair fluctuation
propagator has bosonic character, two quite different peaks appear in the
spectral function, a broad one at negative frequencies and a narrow one at
positive frequencies. By decreasing coupling, the two-peak structure evolves
smoothly. In the weak-coupling regime, where the fluctuation propagator has
diffusive Ginzburg-Landau character, the overall line-shape of the spectral
function is more symmetric. The systematic analysis of the spectral function
identifies specific features which allow one to distinguish by ARPES whether a
system is in the weak- or strong-coupling regime. Connection of the results of
our analysis with the phenomenology of cuprate superconductors is also
attempted and rests on the recently introduced two-gap model.Comment: 19 pages, 18 figure
Density-induced BCS to Bose-Einstein crossover
We investigate the zero-temperature BCS to Bose-Einstein crossover at the
mean-field level, by driving it with the attractive potential and the particle
density.We emphasize specifically the role played by the particle density in
this crossover.Three different interparticle potentials are considered for the
continuum model in three spatial dimensions, while both s- and d-wave solutions
are analyzed for the attractive (extended) Hubbard model on a two-dimensional
square lattice. For this model the peculiar behavior of the crossover for the
d-wave solution is discussed.In particular, in the strong-coupling limit when
approaching half filling we evidence the occurrence of strong correlations
among antiparallel-spin fermions belonging to different composite bosons, which
give rise to a quasi-long-range antiferromagnetic order in this limit.Comment: 10 pages, 5 enclosed figure
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