ABSTRACT
We investigate the superconducting critical current density (J c), transition temperature (T c), and flux pinning properties under hydrostatic pressure (P) for Cr0.0009NbSe2 single crystal. The application of P enhances T c in both electrical resistivity (â¼0.38 K GPa-1: 0 ≤ P ≤ 2.5 GPa) and magnetization (â¼0.98 K GPa-1: 0 ≤ P ≤ 1 GPa) measurements, which leads to a monotonic increase in J c and flux pinning properties. The field-dependent J c at various temperatures under P is analyzed within the collecting pinning theory and it shows that δT c pinning is the crossover to δl pinning above the critical pressure (P c â¼0.3 GPa). Our systematic analysis of the flux pinning mechanism indicates that both the density of pinning centers and pinning forces greatly increase with the application of P, which leads to an enhancement in the vortex state. Structural studies using synchrotron X-ray diffraction under pressure illustrate a stable hexagonal phase without any significant impurity phase and lattice parameter reduction with P shows highly anisotropic nature.
ABSTRACT
Superconducting properties of Cr0.0005NbSe2 (Tc~6.64 K) single crystals have been investigated through the temperature dependent resistivity (~8 GPa) and DC magnetization (~1 GPa) measurements. Further, the critical current density (Jc) as a function of applied magnetic field has been studied from magnetic isotherms. The vortex pinning mechanisms have also been systematically analyzed using weak collective pinning theory as a function of pressure. The Jc corresponds to the flux flow enhanced by the application of pressure due to increase of Tc and vortex changes. We found that the pressure is responsible for the spatial variations in the charge carrier mean free path (δl pinning). We find that core point pinning is more dominant than surface pinning which is caused by the application of pressure. In addition, Jc(H = 0) increases from 3.9 × 105 (0 GPa) to 1.3 × 106 (1.02 GPa) A/cm2 at 2 K as the pressure is increased from normal pressure to 1.02 GPa. The pressure dependence of Tc (dTc/dP) becomes 0.91 K/GPa and 0.75 K/GPa from magnetization and resistivity measurements respectively. We found that the pressure promotes the anisotropy nature, and decrease of coherence length and resulting in pathetic interface of the vortex core with pinning centers.
ABSTRACT
The impact of hydrostatic pressure (P) up to 1 GPa on T c , J c and the nature of the pinning mechanism in FexNbSe2 single crystals have been investigated within the framework of the collective theory. We found that the pressure can induce a transition from the regime where pinning is controlled by spatial variation in the critical transition temperature (δT c ) to the regime controlled by spatial variation in the mean free path (δâ). Furthermore, T c and low field J c are slightly induced, although the J c drops more rapidly at high fields than at ambient P. The pressure effect enhances the anisotropy and reduces the coherence length, resulting in weak interaction of the vortex cores with the pinning centers. Moreover, the P can induce the density of states, which, in turn, leads to enhance in T c with increasing P. P enhances the T c with the rates of dT c /dP of 0.86, 1.35 and 1.47 K/GPa for FexNbSe2, respectively. The magnetization data are used to establish a vortex phase diagram. The nature of the vortices has been determined from the scaling behaviour of the pinning force density extracted from the J c -H isotherms and demonstrates the point pinning mechanism.
ABSTRACT
Magnetization measurements have been used to determine the effect of magnetic impurities (Fe) on the Larkin-Ovchinnikov (LO) 3D collective pinning model in NbSe2 single crystals. Upon increasing the concentration of Fe impurities, the superconducting critical current density enhances appreciably compared to pure NbSe2 reflecting the fact that the addition of magnetic impurities assists in improving the practical applicability of NbSe2. The random pinning potential that is introduced by the Fe impurities also shows a considerable change in the interaction between the vortices and the core region, resulting in a competitive nature of single vortex, small bundle and large bundle pinning regimes in the H-T phase diagram. The intrinsic disorder in pure NbSe2 single crystals shows δTc flux pinning; however, the extrinsic disorder created by Fe atoms in NbSe2 shows δl flux pinning. Furthermore, the field dependence of the pinning force on both NbSe2 and Fe-incorporated NbSe2 represents the existence of point pinning and the surface pinning mechanism with a broadening of the fp curves in the Fe-incorporated single crystals.
