Quasiparticle dispersion of Sr2CuO2Cl2

Recent angle resolved photoemission (ARPES) measurements for the insulating cuprate Sr₂CuO₂Cl₂ have provided the first experimental data which can be directly compared to the (theoretically) well-studied problem of a single hole propagating in an antiferromagnet. Some discrepancies withthe familiar 2D t— J model were observed. Here we discuss a comparison between the ARPES results and the quasiparticle dispersion of both (i) the extended t— t— J Hamiltonian and (ii) the three-band Hubbard model.     

Transient Gratings Formed by Nonequilibrium Quasiparticles in YBa2Cu3O6.5

We report transient grating measurements of nonequilibrium quasiparticles in untwinned single crystals of YBa₂Cu₃O_6.5. By interfering two laser beams on the sample, we introduce a sinusoidally varying density of nonequilibrium quasiparticles. A third laser beam incident on this pattern is both reflected and diffracted due to the sinusoidally varying index change caused by photoinduced quasiparticles. In this paper, we focus on the low excitation density limit. In this limit, measuring the reflected and diffracted beams as a function of the time and grating period yields the diffusion constant and elastic scattering rate for these nonequilibrium quasiparticles.     

超新星爆前天体和超新星条件下55Co的正电子俘获

关于伽莫夫-泰载(GT)强度分布的争论尚在继续.对于pf壳层核,GT强度分布是希望的,从天体物理的观点来看这是重要的.应用质子-中子准粒子随机相近似(pn-QRPA)理论,计算了55Co的伽莫夫-泰勒跃迁和电子俘获率.这些计算已在温度为0.01,0.1,0.2,0.4,0.7,1.5,2,3,7,10,30(以109K为单位)以及密度在10 g·cm-3和1011g·cm-3之间进行了.计算表明GT强度是在多终态上分段的.正电子俘获率与壳层模型计算的在低温和星体区是有差别的,在高温和星体核心区符合得很好.     

Spectrum of quasiparticles near the Bragg plane and influence of degeneracy points and open-form Fermi surface onTc in superconductors

The Bogoliubov quasiparticle spectrum near the Bragg plane in the reciprocal space of a superconducting crystal lattice is obtained in the weak binding approximation for an arbitrary electron dispersion law. In the framework of the BCS model the possibility of the average pair potential and critical temperature increase is shown for some isoenergetic surface configurations at the expense of the diffraction gap in the quasiparticle spectrum.     

Spin-Injection Quasiparticle Nonequilibrium in Cuprate/Manganite Heterostructures

Quasiparticle nonequilibrium due to spin-polarized current injection in perovskite superconductor/ferromagnet (S/F) thin-film heterostructures has been studied with the technique of cryogenic scanning tunneling spectroscopy. The spin-injection heterostructures consisted of epitaxial bilayers of the high-T _c superconductor YBa₂Cu₃O_{7 –} and the half-metallic ferromagnet La_0.7Ca_0.3MnO₃, with a spin-polarized quasiparticle current injected into the cuprate layer from the manganite layer. The tunneling conductance measured at 4.2 K on the cuprate layer showed a distinctly nonequilibrium quasiparticle spectrum as a result of the spin injection. Quantitative analysis of the tunneling spectral evolution versus the injection current yielded an estimate of the quasiparticle spin-diffusion depth and the spin-relaxation time in the superconducting cuprate.     

Transmission of Phonons with Anomalous Dispersion through the Interface of Two Continuous Media

We solve the problem of phonon transmission through the interface between a quantum fluid with anomalous dispersion and a solid, for arbitrary angles of incidence. The Wiener–Hopf method is applied to solve the equations of the quantum fluid in the half-space, and in particular the solution is obtained for the dispersion relation of a Bose–Einstein condensate (BEC) and of superfluid helium at small wave vectors. It is shown that the solutions are running waves deformed near the border by specific surface standing waves. Boundary conditions are used to derive the transmission and reflection coefficients for the phonons, incident on either side of the interface, as a function of the incidence angles and the phonon frequencies. The deformation of wave packets passing through the interface is described both far from and near to the critical incidence angles.     

超新星爆前天体和超新星条件下55Co的电子俘获

在超新星爆发的复杂动力学中,电子俘获起着关键的作用。利用质子-中子准粒子随机相近似(pn-QRPA)理论,55Co上的电子俘获率已经被计算。计算了星体在温度为0.01,0.1,0.2,0.4,0.7,1.5,2,3,5,10,30(以109K为单位)以及密度在10 g.cm-3和1011g.cm-3之间的电子俘获率,同时介绍了电子俘获率与大壳模型计算率的差别。     

Absence of Phase-Fluctuation Contribution to the Low-Frequency Optical Conductivity in d-Wave Superconductors at Zero Temperature

The behavior of the low-frequency optical conductivity _reg() in superconducting cuprates is, at the present, an open and interesting issue. In particular, since the zero-temperature and zero-frequency limit of _reg() attains a value much larger than the universal value expected within a self-consistent T-matrix calculation, an intriguing possibility is that the collective mode can also contribute to _reg(). By taking into account the effect of dissipation on the collective mode in a d-wave superconductor, we evaluate the phase-fluctuation contribution to _reg(0) within the formalism of the phase-only action. We show that even though the collective mode contributes to _reg() at finite frequencies, approaching the zero-temperature and zero-frequency regime the corrections at _reg() due to phase fluctuations vanish.