Electron beam excitation of nonequilibrium effects in spatially separated superconductors

A superconductor driven into a nonequilibrium state under electron beam irradiation emits phonons. They can be detected by a spatially separated superconductor of the same material. This phenomenon, which is typical only to be superconducting state, is used to map a superconductor in a scanning electron microscope.     

电子型掺杂铜氧化物超导体准粒子激发谱研究

文中工作采用t-t′-t″-J模型研究了电子型掺杂铜氧化物超导体在超导序和反铁磁序共存情况下的准粒子激发谱,分别计算了在欠掺杂区、最佳掺杂区和过掺杂区的谱权重,计算结果与角分辨光电子谱实验结果符合较好.     

A correlation measure in non-equilibrium phase transitions

Bit-number cumulants of a probability distribution give ordered measures of correlations between subsystems. Especially the second one, the bit-number variance (which in equilibrium is connected with specific heat) is discussed with respect to its critical behaviour for special non-equilibrium phase transitions in chemical reaction systems. Also in non-equilibrium the critical behaviour indicates critical correlations.     

Electromagnetic excitation and dynamical response of ferromagnetic superconductors

Electromagnetic excitation modes in ferromagnetic superconductors are studied theoretically as the coupled modes of photon, electric current and magnetic moments, and their dispersion relations are obtained. With the use of these modes, the reflection coefficient is calculated for the electromagnetic wave incident on the surface of a semi-infinite crystal of ferromagnetic superconductor. It is shown that the reflection coefficient decreases rapidly in the neighborhood of the critical temperature where a spontaneous surface magnetization occurs. An anomaly in the surface impedance is also predicted.     

Relation between vortex excitation and thermal conductivity in superconductors

Thermal conductivity κ _xx(T) under a field is investigated in d _{x2 - y2}-wave superconductors and isotropic s-wave superconductors by the linear response theory, using a microscopic wave function of the vortex lattice states. To study the origin of the different field dependence of κ_xx(T) between higher and lower temperature regions, we analyze the spatially-resolved thermal conductivity around a vortex at each temperature, which is related to the spectrum of the local density of states. We also discuss the electric conductivity in the same formulation for a comparison. Received 8 December 2001 and Received in final form 20 March 2002 Published online 6 June 2002     

Optical relaxation effects on magnetic resonances in vapours oriented by broad-line laser excitation

Relaxation effects produced by the broad-band pumping laser light on magnetic resonances induced in oriented vapours are investigated. Experimental evidence of resonance bleaching by optical transverse relaxation in a sodium vapour pumped by multifrequency laser radiation is reported.     

Cross-over effects in heavy-fermion superconductors

The influence of correlation effects on superconductivity in lanthanide and actinide compounds is investigated theoretically. As driving mechanism for the formation of heavy-fermion Cooper pairs different electron-phonon interactions are considered and discussed within the Anderson lattice model. The elimination of the bare electron-phonon interactions and a BCS-like truncation procedure lead to the appearance of different types of Cooper pairs. The stability of these Cooper pairs in the Kondo lattice state is studied by using a semi-phenomenological parameter which effectively describes the correlation off electrons. It is found that hybrid pairing is less important and that superconductivity of conduction andf electrons always occurs simultaneously. Furthermore, in the case of strongly correlatedf electrons a cross-over takes place from mainly heavy-fermion superconductivity to conduction electron superconductivity.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday     

Magnetic resonance in the spin excitation spectrum of electron-doped cuprate superconductors

We study the emergence of a magnetic resonance in the superconducting state of the electron-doped cuprate superconductors. We show that the recently observed resonance peak in the electron-doped superconductor Pr0.88LaCe0.12CuO4-delta is consistent with an overdamped spin exciton located near the particle-hole continuum. We present predictions for the magnetic-field dependence of the resonance mode as well as its temperature evolution in those parts of the phase diagram where dx2-y2-wave superconductivity may coexist with an antiferromagnetic spin-density wave.     

Particle-particle correlation and fluctuations in non-equilibrium Fermi/Bose gases

A theory of fluctuations and correlations in non-equilibrium Fermi and Bose gases is developed. A specific quantum correlation is predicted in non-equilibrium collisional quasi-classical gases, created by collisions between particles. The expressions for correlation sources entering the equation for an equal-time two-particle correlation function are obtained and analyzed. The expressions are valid for arbitrary particle-particle interaction potentials. The possibility of display of the quantum correlation in the macroscopic characteristics of fluctuation phenomena in non-equilibrium gases (e.g., current fluctuations in semiconductors) is discussed. A comparison with correlations in a Lorentz gas is made.     

Optical excitation spectra of selenium-doped silicon

Infrared absorption spectra of selenium-doped silicon contain excitation lines indicative of three selenium donor centers with first (and second) ionization energies at 116(214), 206.5(388) and 306.7(589) meV. Cooling rate effects on the absorption strengths establish that the 306 meV center is due to isolated selenium and that the two lower energy centers are due to selenium complexes. The results suggest close similarities with those for sulfur-doped silicon. Selenium-doped samples containing & times; 10¹⁴ cm^-3 boron exhibit spectra of isolated boron and of ionized donor centers; boron apparently acts as an isolated (compensating) acceptor and is not a constituent in either selenium complex.     

Optical conductivity studies of high‐Tc superconductors

A study of the optical conductivity spectra of high‐T_c copper oxides elucidates the evolution of superconducting gap and magnetic penetration depth (superfluid density) with doping. It is complementary to the μSR study and provides microscopic basis for the μSR results. This revised version was published online in August 2006 with corrections to the Cover Date.     

Cascade multiplication of strongly non-equilibrium charge carriers in metals by low energy electron excitation

The steady-state distribution function of the energies of non-equilibrium electrons and holes formed as a result of a cascade of electron-electron collisions in the presence of a primary electron flux is found by solving the linearized transport equation in the isotropic scattering approximation. The distributions obtained in this way include dependences on the energy of primary electrons and on the characteristics of the medium. Near the Fermi level they have a singularity and are close to a power law far from this level. A study is made of the possibility of deriving distributions for crystals with a complex energy band structure (in particular, of tungsten).     

Optical nonreciprocity in magnetic structures related to high-Tc superconductors

Rotation of the plane of polarization of reflected light (Kerr effect) is a direct manifestation of broken time-reversal symmetry and is generally associated with the appearance of a ferromagnetic moment. Here I identify magnetic structures that may arise within the unit cell of cuprate superconductors that generate polarization rotation despite the absence of a net moment. For these magnetic symmetries the Kerr effect is mediated by magnetoelectric coupling, which can arise when antiferromagnetic order breaks inversion symmetry. The structures identified are candidates for a time-reversal breaking phase in the pseudogap regime of the cuprates.     

Finite size effects in layered superconductors

We investigate the transition temperature of layered superconductors by considering a stack ofL _z superconducting layers, separated by insulating material. We adopt a pairing Hamiltonian, invoke the variational principle and solve the resulting gap equations numerically. Our results confirm previous weak coupling and Ginzburg-Landau treatments and reveal a rise ofT _c withL _z and saturation at the bulk transition temperature. Thus, the rise ofT _c is traced back to a finite size effect, corresponding to a crossover from 2-d to 3-d superconductivity. The results also reveal a sizeable variation of the gap along the stack with pronounced variation at the ends.