Strong Pairing Fluctuations Research Articles

Photoemission spectrum and effect of inhomogeneous pairing fluctuations in the BCS-BEC crossover regime of an ultracold Fermi gas

We investigate the photoemission-type spectrum in a cold Fermi gas which was recently measured by JILA group [J. T. Stewart {\it et al}., Nature \textbf{454}, 744 (2008)]. This quantity gives us very useful information about single-particle properties in the BCS-BEC crossover. In this letter, including pairing fluctuations within a $T$-matrix theory, as well as effects of a harmonic trap within the local density approximation, we show that spatially inhomogeneous pairing fluctuations due to the trap potential is an important key to understand the observed spectrum. In the crossover region, while strong pairing fluctuations lead to the so-called pseudogap phenomenon in the trap center, such strong-coupling effects are found to be weak around the edge of the gas. Our results including this effect are shown to agree well with the recent photoemission data by JILA group.

Pseudogap behaviors of atomic Fermi gases above [formula omitted] in the BCS–BEC crossover

We investigate pseudogap behaviors of an ultracold Fermi gas in the BCS–BEC crossover above the superfluid transition temperature Tc. We calculate the single-particle density of states (DOS), as well as the spectral weight, over the entire BCS–BEC crossover region, including strong pairing fluctuations within a T-matrix approximation. We show that the pseudogap structure in DOS develops from the weak-coupling BCS region, and continuously changes into a fully gapped one in the strong-coupling BEC region. We determine the pseudogap temperature T∗ at which the dip structure in DOS disappears, and identify the pseudogap region in the phase diagram in terms of temperatures and pairing interactions.

Pairing fluctuations and pseudogap effects in the BCS–BEC crossover regime of a superfluid Fermi gas

We study effects of strong pairing fluctuations on single-particle properties of a superfluid Fermi gas in the BCS–BEC crossover region. Including phase and amplitude fluctuations of the order parameter within the T-matrix approximation, we calculate the superfluid density of states below the superfluid phase transition temperature Tc. We show how the pseudogap appearing in the normal phase above Tc changes into the superfluid gap over the entire BCS–BEC crossover region, as one passes through Tc.

Pairing fluctuation theory of high-Tcsuperconductivity in the presence of nonmagnetic impurities

The pseudogap phenomena in the cuprate superconductors requires a theory beyond the mean-field BCS level. A natural candidate is to include strong pairing fluctuations, and treat the two-particle and single-particle Green's functions self-consistently.At the same time, impurities are present in even the cleanest samples of the cuprates. Some impurity effects can help reveal whether the pseudogap has a superconducting origin and thus test various theories. Here we extend the pairing fluctuation theory for a clean system [Chen et al., Phys. Rev. Lett. 81, 4708 (1998)] to the case with nonmagnetic impurities. Both the pairing and the impurity T matrices are included and treated self-consistently. We obtain a set of three equations for the chemical potential \ensuremath{\mu}, ${T}_{c},$ the excitation gap $\ensuremath{\Delta}{(T}_{c})$ at ${T}_{c},$ or \ensuremath{\mu}, the order parameter ${\ensuremath{\Delta}}_{\mathrm{sc}},$ and the pseudogap ${\ensuremath{\Delta}}_{\mathrm{pg}}$ at temperature $T<{T}_{c},$ and study the effects of impurity scattering on the density of states, ${T}_{c}$ and the order parameter, and the pseudogap. Both ${T}_{c}$ and the order parameter as well as the total excitation gap are suppressed, whereas the pseudogap is not for given $T<~{T}_{c}.$ Born scatterers are about twice as effective as unitary scatterers in suppressing ${T}_{c}$ and the gap. In the strong pseudogap regime, pair excitations contribute a new ${T}^{3/2}$ term to the low-$T$ superfluid density. The initial rapid drop of the zero-$T$ superfluid density in the unitary limit as a function of impurity concentration ${n}_{i}$ also agrees with experiment.

Photoemission, inverse photoemission and superconducting correlations in Hubbard and t-J ladders: role of the anisotropy between legs and rungs

Several experiments in the context of ladder materials have recently shown that the study of simple models of anisotropic ladders (i.e. with different couplings along legs and rungs) is important for the understanding of these compounds. In this paper Exact Diagonalization studies of the one-band Hubbard and t-J models are reported for a variety of densities, couplings, and anisotropy ratios. The emphasis is given to the one-particle spectral function A(q,\omega) which presents a flat quasiparticle dispersion at the chemical potential in some region of parameter space. This is correlated with the existence of strong pairing fluctuations, which themselves are correlated with an enhancement of the bulk-extrapolated value for the two-hole binding energy as well as with the strength of the spin-gap in the hole-doped system. Part of the results for the spectral function are explained using a simple analytical picture valid when the hopping along the legs is small. In particular, this picture predicts an insulating state at quarter filling in agreement with the metal-insulator transition observed at this special filling for increasing rung couplings. The results are compared against previous literature, and in addition pair-pair correlations using extended operators are also here reported.

Pairing Fluctuation Theory of Superconducting Properties in Underdoped to Overdoped Cuprates

We propose a theoretical description of the superconducting state of under- to overdoped cuprates, based on the short coherence length of these materials and the associated strong pairing fluctuations. The calculated $T_c$ and the zero temperature excitation gap $\Delta(0)$, as a function of hole concentration $x$, are in semi-quantitative agreement with experiment. Although the ratio $T_c/\Delta(0)$ has a strong $x$ dependence, different from the universal BCS value, and $\Delta(T)$ deviates significantly from the BCS prediction, we obtain, quite remarkably, quasi-universal behavior, for the normalized superfluid density $\rho_s(T)/\rho_s(0)$ and the Josephson critical current $I_c(T)/I_c(0)$, as a function of $T/T_c$. While experiments on $\rho_s(T)$ are consistent with these results, future measurements on $I_c(T)$ are needed to test this prediction.

Competition between pairing-assisted tunneling and single-particle alignment in the decay of high-K isomeric states.

In an axially symmetric, strongly rotating quadrupole deformed nucleus, the projection K of the angular momentum along the symmetry axis is a partially conserved quantity. A K isomer (K=25) at low excitation energy has been observed in $^{182}\mathrm{Os}$ which decays into the ground-state band (K=0). The associated electromagnetic lifetime at spin I\ensuremath{\approxeq}25 can be interpreted in terms of tunneling of a system displaying strong pairing fluctuations across a barrier which separates configurations in which the nucleus rotates about the symmetry axis and about an axis perpendicular to it. A hindrance factor \ensuremath{\approxeq}${10}^{\mathrm{\ensuremath{-}}6}$--${10}^{\mathrm{\ensuremath{-}}9}$ for the process is calculated, in overall agreement with the data.