2H-NbSe2 Single Crystals Research Articles

Negative dynamic creep in the peak-effect regime in type-II superconductors

We report the observation of a negative dynamic creep rate, i.e., increasing irreversibility in the magnetic hysteresis loop for decreasing sweep rate of the magnetic field, in both YBa2Cu3O72d and 2H-NbSe2 single crystals. This phenomenon is found to appear on the increasing branch of the peak effect that corresponds to a state which is intermediate between the dislocation-free Bragg glass and a highly disordered vortex phase. The origin of this anomalous creep is shown to be connected to a negative differential resistance resulting from the N-like shape of the current-voltage characteristics.

Thermal diffusion of indium into 2H-NbSe2

The authors have studied the thermal diffusion of indium into 2H-NbSe2 single crystals parallel to the layers in situ in a scanning electron microscope equipped with an X-ray energy dispersive system. The distance of the diffusing indium front into the crystal as a function of time was determined from the secondary electron image as well as from X-ray line scans for indium taken at different time periods. The diffusion coefficients, D, obtained by fitting these data to (r2)=2Dt where (r2) is the mean square displacement in time t were: 10-9, 6*10-9, 11*10-9, 23*10-9 and 30*10-9 cm2 s-1 at 22, 63, 72, 100 and 120 degrees C respectively with experimental uncertainty of about 10%. The activation energy for diffusion of indium into NbSe2 is calculated to be 0.4 eV per atom. The results show that indium atoms put severe stress on the layers as they intercalate between them. This stress is relieved by buckling of the layers and these buckling features are visible in secondary electron images. Because of the large stresses involved, indium diffuses faster in the top layers than in the deeper layers of the crystal.

An experimental investigation of the reflection of low-energy electrons from basal surfaces of 2H-NbSe2

The energy dependence of the reflection of low-energy electrons in the 0-11 eV range from basal plane surfaces of 2H-NbSe2 single crystals is presented. The method used was total current spectroscopy. The results have been explained in terms of inelastic electron-electron scattering accompanied by interband transitions, using published densities of states for NbSe2.

Temperature and angular dependences of upper critical fields for the layer structure superconductor 2H-NbSe2

The temperature and angular dependences of upper critical fieldsH c2 have been measured for several 2H-NbSe2 single crystals by use of an electrical conduction method in magnetic fields up to 150 kOe. As the temperature approaches the transition temperatureT c , the value ofH c2‖ (parallel to the layer planes) decreases with a positive curvature, while the value ofH c2⊥ (perpendicular to the layer planes) decreases almost linearly. The ratio ofH c2‖ toH c2⊥ increases monotonically from 2.4 nearT c with decreasing temperature and reaches the constant value of 3.2 at the lowest temperature. It becomes clear that the simple effective mass model based on the anisotropic Ginzburg-Landau theory does not explain our experimental results. The anisotropic behavior ofH c2 can be accounted for by the Takanaka theory, which includes anisotropies of both the Fermi velocity and the energy gap and the effect of nonlocality. Agreement between experimental results and the theoretical prediction is obtained by the use of values of 0.16≲e ≲0.25 and −0.6≲e 2 ≲−0.3, where e 1 is the mass anisotropy parameter and e 2 the gap anisotropy parameter. The coupling strength between layers is too strong to be explained by the Josephson phase coupling model proposed for quasi-two-dimensional layer superconductors.