Stable and Metastable vortex states and the first order transition across the peak effect region in weakly pinned 2H-NbSe_2

The peak effect in weakly pinned superconductors is accompanied by metastable vortex states. Each metastable vortex configuration is characterized by a different critical current density J_c, which mainly depends on the past thermomagnetic history of the superconductor. A recent model [G. Ravikumar, et al, Phys. Rev. B 61, R6479 (2000)] proposed to explain the history dependent J_c postulates a stable state of vortex lattice with a critical current density J_c^{st}, determined uniquely by the field and temperature. In this paper, we present evidence for the existence of the stable state of the vortex lattice in the peak effect region of 2H-NbSe_2. It is shown that this stable state can be reached from any metastable vortex state by cycling the applied field by a small amplitude. The minor magnetization loops obtained by repeated field cycling allow us to determine the pinning and "equilibrium" properties of the stable state of the vortex lattice at a given field and temperature unambiguously. The data imply the occurence of a first order phase transition from an ordered phase to a disordered vortex phase across the peak effect.Comment: 20 pages, 10 figures. Corresponding author: S. Ramakrishna

4-[(E)-(Hy­droxy­imino)­meth­yl]-N,N-di­methyl­anilinium chloride

In the title compound, C9H13N2O+·Cl−, the cation, apart from the methyl groups, is almost planar, with a maximum deviation of 0.040 (1) Å; the methyl C atoms deviate by 0.389 (2) and −1.247 (1) Å, from the mean plane. In the crystal, cations and anions associate through C—H⋯Cl hydrogen bonds, forming a helical arrangement. In addition, inter­molecular O—H⋯Cl, N—H⋯Cl and C—H⋯N inter­actions are observed

A study of supercooling of the disordered vortex phase via minor hysteresis loops in 2H-NbSe_2

We report on the observation of novel features in the minor hysteresis loops in a clean crystal of NbSe_2 which displays a peak effect. The observed behavior can be explained in terms of a supercooling of the disordered vortex phase while cooling the superconductor in a field. Also, the extent of spatial order in a flux line lattice formed in ascending fields is different from (and larger than) that in the descending fields below the peak position of the peak effect; this is attributed to unequal degree of annealing of the state induced by a change of field in the two cases.Comment: 5 pages of text + 6 figures, submitted to Phys. Rev.

Critical points in the Bragg glass phase of a weakly pinned crystal of Ca3_3Rh4_4Sn13_{13}

New experimental data are presented on the scan rate dependence of the magnetization hysteresis width $\Delta M(H)$ ($\propto$ critical current density $J_c(H)$) in isothermal $M-H$ scans in a weakly pinned single crystal of Ca$_3$Rh$_4$Sn$_{13}$, which displays second magnetization peak (SMP) anomaly as distinct from the peak effect (PE). We observe an interesting modulation in the field dependence of a parameter which purports to measure the dynamical annealing of the disordered bundles of vortices injected through the sample edges towards the destined equilibrium vortex state at a given $H$. These data, in conjunction with the earlier observations made while studying the thermomagnetic history dependence in $J_c(H)$ in the tracing of the minor hysteresis loops, imply that the partially disordered state heals towards the more ordered state between the peak field of the SMP anomaly and the onset field of the PE. The vortex phase diagram in the given crystal of Ca$_3$Rh$_4$Sn$_{13}$ has been updated in the context of the notion of the phase coexistence of the ordered and disordered regions between the onset field of the SMP anomaly and the spinodal line located just prior to the irreversibility line. A multi-critical point and a critical point in the ($H,T$) region of the Bragg glass phase have been marked in this phase diagram and the observed behaviour is discussed in the light of recent data on multi-critical point in the vortex phase diagram in a single crystal of Nb.Comment: To appear in Current trends in Vortex State Studies - Pramana J. Physic

Enhancing the heat transfer of triangular pyramid solar still using phase change material as storage material

This paper presents the method of improvement of enhancing the performance of triangular pyramid solar still with and without latent heat energy storage. For comparing the productivity of solar still with and without LHTESS a solar still is designed, fabricated. Experiments are conducted in hot and humid climate of Chennai, India. Paraffin wax is used as LHTESS due to its feasible general and economic properties. The hourly productivity is slightly higher in case of solar still without LHTESS during sunny days. There is an increase of about 35% in production of fresh water with LHTESS than that of solar still without LHTESS. Also it was found that during the off shine period the fresh water produced from the still is higher. The solar still with and without LHTESS were found to be 4.5 L/m2day and 3.5 L/m2day

Generalised coherent point drift for group-wise registration of multi-dimensional point sets

In this paper we propose a probabilistic approach to group-wise registration of unstructured high-dimensional point sets. We focus on registration of generalised point sets which encapsulate both the positions of points on surface boundaries and corresponding normal vectors describing local surface geometry. Richer descriptions of shape can be especially valuable in applications involving complex and intricate variations in geometry, where spatial position alone is an unreliable descriptor for shape registration. A hybrid mixture model combining Student’s t and Von-Mises-Fisher distributions is proposed to model position and orientation components of the point sets, respectively. A group-wise rigid and non-rigid registration framework is then formulated on this basis. Two clinical data sets, comprising 27 brain ventricle and 15 heart shapes, were used to assess registration accuracy. Significant improvement in accuracy and anatomical validity of the estimated correspondences was achieved using the proposed approach, relative to state-of-the-art point set registration approaches, which consider spatial positions alone

Peak Effect in Superconductors: Absence of Phase Transition and Possibility of Jamming in Vortex Matter

The magnetic field $B$ dependence of the critical current $I_c$ for the vortex phase of a disordered superconductor is studied numerically at zero temperature. The $I_{c}(B)$ increases rapidly near the upper critical field $B_{c2}$ similar to the peak effect (PE) phenomenon observed in many superconductors. The real space configuration across the PE changes continuously from a partially ordered domain (polycrystalline) state into an amorphous state. The topological defect density $n_{d}(B)\sim e^{\alpha B^{k}}$ with $k>1$ for $B\geq 0.4B_{c2}$. There is no evidence of a phase transition in the vicinity of the PE suggesting that an order-disorder transition is not essential for the occurrence of the PE phenomenon. An alternative view is presented wherein the vortex system with high dislocation density undergoes jamming at the onset of the PE.Comment: 8 pages and 5 figure

Reentrant Peak Effect in an anisotropic superconductor 2H-NbSe_2 : Role of disorder

The reentrant nature of Peak Effect is established in a single crystal of 2H-NbSe_2 via electrical transport and dc magnetisation studies. The role of disorder on the reentrant branch of PE has been examined in three single crystals with varying levels of quenched random disorder. Increasing disorder presumably shrinks the (H,T) parameter space over which vortex array retains spatial order. Although, the upper branch of the PE curve is somewhat robust, the lower reentrant branch of the same curve is strongly affected by disorder.Comment: 5 Pages of text, 4 figure

Calculation of minor hysteresis loops under metastable to stable transformations in vortex matter

We present a model in which metastable supercooled phase and stable equilibrium phase of vortex matter coexist in different regions of a sample. Minor hysteresis loops are calculated with the simple assumption of the two phases of vortex matter having field-independent critical current densities. We use our earlier published ideas that the free energy barrier separating the metastable and stable phases reduces as the magnetic induction moves farther from the first order phase transition line, and that metastable to stable transformations occur in local regions of the sample when the local energy dissipation exceeds a critical value. Previously reported anomalous features in minor hysteresis loops are reproduced, and calculated field profiles are presented.Comment: 9pages, 7 figure