摘要:

Magnetic relaxation processes in superconducting fullerideK3C60are investigated on monocrystalline samples in a wide range of temperatures and magnetic fields. A logarithmic time dependenceM(t)of magnetization is observed in samples with100&percent;of superconducting phase. The normalized relaxation rate increases with temperature in the entire range of magnetic fields. The flux creep activation energy ranges from 10 to80 meV,and its temperature dependence has a peak. It has been concluded from the results of measurements on samples with nonideal stoichiometry that inhomogeneities strongly affect the relaxation processes and can mask the logarithmic dependenceM(t)completely. ©1997 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593394

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The peculiarities of electrodynamics of layered HTS materials in a longitudinal magnetic field, which are associated with fluctuations of the shape of an Abrikosov vortex formed by a set of weakly coupled2d-vortices centered in equilibrium in superconducting planes, are studied. The value of the longitudinal critical current of the layered media is estimated, and its temperature dependence is determined. ©1997 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593395

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The hopping conductivity ofLa2CuO4+&dgr;single crystal with excess oxygen has been studied. It is found that the transition from variable-range hopping (with the temperature dependence of resistanceR∝exp(T0/T)1/4) to simple activation conductivity(R∝exp(&Dgr;/kT))occurs at a temperature lower than20 K.This transition is accompanied with significant changes in the behavior of magnetoresistance and current–voltage characteristics. It is shown that such a behavior of hopping conductivity below20 Kis due to the presence of superconducting inclusions in insulating sample due to phase separation ofLa2CuO4+&dgr;.The observed peculiarities in the conducting properties are in good agreement with the known effects of competition between electron localization and superconductivity in homogeneous systems. ©1997 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593396

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

YBaCuO films fabricated by laser ablation method were subjected to a thermocycling. Our investigations showed that the degradation process of the current transport properties of films is accompanied by the appearance of a maximum on the curve of the critical current density, depending on the number of thermocycles. This effect manifests itself most strongly in the films with degraded superconducting properties. The critical temperature and the transition width vary only slightly in this case. ©1997 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593397

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The nonlinear interaction of radiowaves with different frequencies in a superconducting plate under unilateral excitation is studied theoretically. The analysis of wave interaction is carried out in the framework of the critical state model. It is shown that the nonlinear nature of this model leads to a peculiar phenomenon in the wave interaction, namely, to jumps in the time dependence of electric field at the sample surface. The necessary conditions for the existence of these jumps are formulated. The role of dielectric properties of the substrate in the predicted phenomenon is formulated. ©1997 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593398

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A thermodynamic analysis of the influence of non-Heisenberg interionic spin-spin interactions on the temperature dependence of the NMR frequency of aNiCl2crystal is carried out. ©1997 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593399

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Abragam &etal; (PhysicaB81, 245 (1976)) proposed a relaxation mechanism for far nuclei via dipole–dipole reservoir (DDR) of own nuclei of paramagnetic centers which do not “freeze” at ultra low temperatures. In order to verify the possibility of such a process, they proved that the matrix elements of flip-flops of own nuclei are the electron matrix elements due to the mixing of electron and nuclear spin states as a result of superfine interaction. This is equivalent to an increase in heat capacity (“increase in weight”) of the DDR of own nuclei. Such a “heavier” DDR subsequently plays the role of a thermodynamic subsystem through which far nuclei relax to the lattice, a narrow bottleneck being observed on the second relaxation region. Since this mechanism can play a dominating role in low-temperature nuclear relaxation as well as in other processes, it is important to obtain the Hamiltonian of a “heavier” DDR in explicit form, to calculate its heat capacity, and to estimate the effective relaxation rate of far nuclei via DDR, which has been accomplished in the present publication. ©1997 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593478

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A magnetic study of perovskitesRMnO3+x(R = Nd, Eu, Gd, Tb, Dy) is reported. The compoundsNdMnO2.99andEuMnO2.99are weak ferromagnets withTN=88and 49 K respectively. It is shown thatGdMnO3undergoes a transition from antiferromagnetic to weak ferromagnetic state upon cooling. The antiferromagnetic ordering of the magnetic moments in the Gd sublattice is observed below 6 K. The magnetic moments ofTbMnO3.01andDyMnO3.01are ordered below7 K. The external field induces an antiferromagnet–ferromagnet transition in the rare-earth sublattice. The critical fields forGdMnO3,TbMnO3.01,andDyMnO3.01are 5, 10, and7 kOe,respectively. ©1997 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593400

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Complex studies of weak electron localization, electron–electron interaction, and electron overheating in Si crystals containing a&dgr;⟨Sb⟩-layer with various concentrations of Sb atoms are carried out in order to obtain information on the characteristic times of inelastic electron relaxation. The temperature dependence of the electron–phonon relaxation time&tgr;epderived from the electron overheating effect can be described by the dependence&tgr;ep∝T−p,wherep≅3.7±0.3,which corresponds to the caseqTl<1(qTis the wave vector of the thermal phonon andlthe electron mean free path). ©1997 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593462

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Peculiar conductivity oscillations of a2DCorbino disk in a magnetic field in a magnetic field, which have been observed in recent experiments (V. T. Dolgopolov &etal;, Pis’ma Zh. E´ksp. Teor. Fiz.63b, 55 (1996) [JETP Lett.63, 63 (1996)]), are considered on the basis of existing concepts concerning specific properties of contacts of a two-dimensional electron system with “external” metallic electrodes, which lead to violation of spatial uniformity of the2Delectron density. ©1997 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593512

出版商:AIP    

年代:1997

数据来源: AIP