N. G. Patil

Anomalous peak effect in CeRu2 and 2H-NbSe2: Fracturing of a flux line lattice

CeRu 2 and 2H-NbSe 2 display remarkable similarities in their magnetic response, reflecting the manner in which the weakly pinned flux line lattice (FLL) loses spatial order in the peak-effect (PE) regime. We present evidence for discontinuous changes in the screening response near the onset of the PE in these systems, and demonstrate history-dependent effects. We attribute these features to a disorder-induced fracturing and entanglement of the FLL, as an alternative to the appearance of a spatially modulated ground state for CeRu 2 .

Metastability and switching in the vortex state of 2H-NbSe2

History-dependent metastable states with different bulk properties are formed in the vortex state of the type-II superconductor 2H-NbSe2. Magnetic measurements demonstrate the difference between the shielding responses of a field- and a zero-field-cooled state, and provide a procedure for switching the system from one state to the other.

Effect of field inhomogeneity on the magnetisation measurements in the peak effect region of CeRu2 superconductor

Abstract Detailed magnetisation measurements are presented for a cubic Laves phase superconducting single crystal CeRu 2 . These measurements have been performed in the peak effect regime using Quantum Design SQUID Magnetic Property Measurement System (MPMS). To understand the influence of magnetic field inhomogeneity of this set up, on the measured magnetisation, all the data is recorded for different scan lengths. We clearly demonstrate that some of the anomalous magnetisation results reported in literature are artefacts of field inhomogeneity. We also present a model based on Critical State Model to simulate the effect of field inhomogeneity on the measured magnetisation.

Magnetic phase diagram of anisotropic superconductor 2H-NbSe2

Abstract We report the magnetic phase diagram of a clean single crystal of 2H-NbSe 2 and present an update on the determination of its intrinsic superconducting parameters.

Re-entrant peak effect in an anisotropic superconductor 2H − NbSe2: Role of disorder

– The influence of disorder (induced by pinning centres) on the peak effect (PE) phenomenon, namely, an anomalous increase in Jc vs. H (or T ), has been studied in the anisotropic superconductor 2H-NbSe2. Results from new electrical transport as well as dc and ac magnetic response studies are presented to demonstrate that increasing disorder shrinks the (H, T ) parameter space over which the vortex lattice retains spatial order. We find that the re-entrant nature of PE is clearly manifested only in crystals which have moderate amount of disorder. We also find that the upper branch of the PE curve is fairly robust, whereas the lower re-entrant branch is strongly affected by disorder. The advent of high-temperature superconductors (HTSC) focused attention on the influence of thermal fluctuations on Abrikosov flux line lattice (FLL) or vortex lattice (VL) and led to the prediction [1] of a vortex liquid state with an unusual re-entrant melting (Hm, Tm) phase boundary in the field-temperature (H, T ) plane such that at a fixed T , the phase boundary Hm(T ) is encountered twice upon increasing H [2]. A dilute vortex liquid phase is expected to form just above the lower critical field Hc1, where the lattice constant a0(∝ 1 √ Field ) ≥ λ, and a highly dense vortex liquid phase is expected below the upper critical field Hc2, where a0 ≥ 2ξ. The two branches comprising the re-entrant melting phase boundary join around the so-called “nose” temperature above which the vortex array is thermally melted at all field values. The () E-mail: satya@tifrc2.tifr.res.in () Present and permanent address: NEC Research Institute 4 Independence Way, Princeton, New Jersey 08540, USA. () Present address: Department of Physics, Indian Institute of Technology, Kanpur, 208016, India.

Magnetism in R5Rh4Sn10 (RCe, Pr, Nd) systems

Abstract Ternary rare-earth intermetallic compounds of the type R 5 T 4 M 10 (RDy to Lu, TCo, Rh, Ir, Os and MSi, Ge) exhibit unusual magnetic and superconducting properties with charge density wave transition (CDW). In this paper, we report the magnetic properties of a new series of R 5 Rh 4 Sn 10 compounds, where RCe, Pr and Nd. We find magnetic ordering at 4.3, 5.6 and 7.3 K for Ce, Pr and Nd compounds, respectively. The fit to the normal state heat capacity data on Ce 5 Rh 4 Sn 10 yields 0.1 J/Ce-mol K 2 for γ. A large anomaly in the heat-capacity data is observed for Nd 5 Rh 4 Sn 10 at 6.3 K, which could be due a possible structural transition driven by the antiferromagnetic ordering.

Study of magnetism in R2Rh3Si5 (R Gd, Tb, Dy, Ho, Er, Tm) system

Abstract It is well known that the compounds which belong to the R2Fe3Si5 series exhibit unusual superconducting and magnetic properties. In this study, we present resistivity, susceptibility and heat capacity studies of compounds which crystallize in R2Rh3Si5-type structure which is closely related to R2Fe3Si5. We find that the antiferromagnetic ordering temperature TN decreases from 8.5 to 3 K for Gd2Rh3Si5 and Ho2Rh3Si5, respectively. We find multiple magnetic transitions for Tb and Dy samples of these series whereas no magnetic ordering was observed in the Tm sample down to 1.5 K. Heat capacity studies show considerable contribution from the crystal field effects at low temperatures.

AC and DC magnetisation studies of peak effect in a clean crystal of CeRu2

New magnetisation results on a clean crystal of cubic CeRu2 reveal that the magnetisation hysteresis anomaly below Hc2(T) involves two distinct steps.

Thermodynamic evidence for reentrant peak effect in a clean single crystal of 2H-NbSe2 and the effect of disorder and thermomagnetic history on it

Abstract New results in 2H-NbSe2 system confirm the thermodynamic nature of reentrant characteristic of PE curve in very clean samples and show that disorder shrinks the (H,T) region of vortex solid phase. All thermomagnetic history effects cease at a Peak Effect boundary which we identify with onset of pinned vortex liquid state.

Study of superconductivity and antiferromagnetism in La2−xNdxRh3Si5 system

Abstract It is well-known that the ternary rare earth silicides of the type R 2 Fe 3 Si 5 exhibit interesting superconducting and magnetic properties. Unlike R 2 Fe 3 Si 5 which forms in the tetragonal (space group P4/mnc) structure, R 2 Rh 3 Si 5 forms in the orthorhombic (Ibram or Iba2) structure. In this work, we establish the bulk superconductivity ( T c = 4.4 K) and bulk antiferromagnetism ( T N = 2.7 K) in La 2 Rh 3 Si 5 and Nd 2 Rh 3 Si 5 , respectively, from resistivity, susceptibility and heat-capacity studies. The heat-capacity data of Nd 2 Rh 3 Si 5 reveal significant presence of crystal-field contributions. These results are compared with the heat-capacity study of Gd 2 Rh 3 Si 5 which shows bulk antiferromagnetic ordering below 8.5 K. We have also analyzed the variation of T c with Nd substitution in the La 2− x Nd x Rh 3 Si 5 system.