Pairing fluctuations and pseudogaps in the attractive Hubbard model

The two-dimensional attractive Hubbard model is studied in the weak to intermediate coupling regime by employing a non-perturbative approach. It is first shown that this approach is in quantitative agreement with Monte Carlo calculations for both single-particle and two-particle quantities. Both the density of states and the single-particle spectral weight show a pseudogap at the Fermi energy below some characteristic temperature T*, also in good agreement with quantum Monte Carlo calculations. The pseudogap is caused by critical pairing fluctuations in the low-temperature renormalized classical regime $\omega < T$ of the two-dimensional system. With increasing temperature the spectral weight fills in the pseudogap instead of closing it and the pseudogap appears earlier in the density of states than in the spectral function. Small temperature changes around T* can modify the spectral weight over frequency scales much larger than temperature. Several qualitative results for the s-wave case should remain true for d-wave superconductors.Comment: 20 pages, 12 figure

Scaling of an Infrared Absorption with Tc in a Cuprate Superconductor

We have studied the complex optical conductivity of Ba2YCu3O6+x crystals for

Reflectivity of Ba2YCu3O7-Σnormal State Dynamics

Measurements of reflectivity, R, as a function of temperature, T, and frequency, ω, in crystals of Ba2YCu3O7-δ, with σ ≅ 0.1, 0.3, and 0.8, are reported. The frequency dependent conductivity, σ(ω, T), obtained by analysis of R extrapolates at low ω to values consistent with previously reported σ(O,T). To explain the shape of σ(ω, T) requires the quasiparticle mass and lifetime to be strongly ω-dependent, withh¯/τ ≈ 2kBT and m*/mo ≈ 7 at low ω, crossingg over toh¯/τ ≈ 0.35 eV and m*/mo ≈ 1 forh¯/τ 〉 0.3 eV

Correlation of Infrared Absorption Intensity with Tc

The magnitude of Tc in Ba2YCu3O 6+x for

Ba2YCu3O7-Ieeelectrodynamics of Crystals with High Reflectivity

We present results for the reflectivity, R of two bulk crystals of Ba2YCu3O7- which, below the superconducting transition at Tc, show behavior consistent with perfect reflectivity up to an energy of (3-4)kBTc. We interpret the reproducibility and high R values (especially R1/41 below Tc) as indicative of intrinsic behavior in the highly conducting crystal planes probed by the light. Results above Tc show that the charge carriers have an enhanced effective mass and low scattering rate at low frequency and temperature, indicating a strong interaction with some excitation

Variations in the Far-Infrared Reflectivity of Ba2YCu3Ox

We present an empirical summary of the far-infrared reflectivity of 18 samples of Ba 2YCu3Ox, with emphasis on 10 oriented, microtwinned crystals with x nominally near 7. The results vary among samples, indicating that further work will be necessary to identify the intrinsic behavior to our satisfaction. At this stage the general behavior allows us to rule out a constant relaxation rate for charge carriers and to make only general remarks concerning the superconducting energy gap

Properties of Optical Features in YBa2Cu3O7-

Characteristic optical features in the far-infrared conductivity of YBa2Cu3O7- crystals have been studied as a function of doping. We focus particularly on a reflectivity edge near 435 cm-1 which we find to be both insensitive to carrier concentration (i.e., Tc), and present in the normal state of some samples. These results place tight constraints on possible interpretations of this feature

Cytopathic Killing of Peripheral Blood CD4(+) T Lymphocytes by Human Immunodeficiency Virus Type 1 Appears Necrotic rather than Apoptotic and Does Not Require env

An important unresolved issue of AIDS pathogenesis is the mechanism of human immunodeficiency virus (HIV)-induced CD4(+) T-lymphocyte destruction. We show here that HIV type 1 (HIV-1) exerts a profound cytopathic effect upon peripheral blood CD4(+) T lymphocytes that resembles necrosis rather than apoptosis. Necrotic cytopathology was found with both laboratory-adapted strains and primary isolates of HIV-1. We carefully investigated the role of env, which has been previously implicated in HIV cytopathicity. HIV-1 stocks with equivalent infectivity were prepared from constructs with either an intact or mutated env coding region and pseudotyped with the glycoprotein of vesicular stomatitis virus (VSV-G) so that the HIV envelope was not rate-limiting for infection. Infected Jurkat T cells died whether or not env was intact; however, the expression of env accelerated death significantly. The accelerated death was blocked by protease inhibitors, indicating that it was due to reinfection by newly produced virus in env(+) cultures. Accordingly, we found no disparity in kinetics in CD4(lo) Jurkat cells. In highly infected peripheral blood T cells, profound necrosis occurred equivalently with both env(+) and env(−) stocks of HIV-1. We also found that HIV-1 cytopathicity was undiminished by the absence of nef. However, viral stocks made by complementation or packaging of HIV-1 genomes with the natural protein-coding sequences replaced by the green fluorescent protein were highly infectious but not cytopathic. Thus, env can accelerate cell death chiefly as an entry function, but one or more viral functions other than env or nef is essential for necrosis of CD4(+) T cells induced by HIV-1