T. V. Chandrasekhar Rao

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 .

Ferromagnetism in cobalt-doped n-GaN

Ferromagnetic ordering is reported in the postannealed samples of Co doped n-GaN formed by Co+ implantation. A maximum Curie temperature ∼250K is recorded for the sample with 8at.% Co. Particle induced x-ray emission–channeling study confirmed the substitutional Co in Ga lattice site. Local atomic arrangement around magnetic impurities is also analyzed using Raman study. A disordered model with carrier mediated coupling of localized magnetic moments is made responsible for the observed ferromagnetic ordering.

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.

Characterization of carriers in GaSb∕InAs superlattice grown on conductive GaSb substrate

We report on mobility spectrum analysis of electrical transport in a GaSb∕InAs superlattice (SL) grown on GaSb substrate. Despite domineering contribution to conduction from the substrate, it was possible to discern and characterize carriers from SL. A single electron specie with an ambient temperature mobility of ∼104cm2∕Vs was found to emanate from SL. We show that this carrier has an activation energy of 0.27eV and is associated with the SL band gap.

Disordered type-II superconductors: a universal phase diagram for low-Tc systems

A universal phase diagram for weakly pinned low-Tc type-II superconductors is revisited and extended with new proposals. The low-temperature “Bragg glass” phase is argued to transform first into a disordered, glassy phase upon heating. This glassy phase, a continuation of the high-field equilibrium vortex glass phase, then melts at higher temperatures into a liquid. This proposal provides an explanation for the anomalies observed in the peak effect regime of 2H-NbSe2 and several other low-Tc materials which is independent of the

Magnetic phase diagram of YNi2B2C

Abstract Structurally, YNi 2 B 2 C is a layered material. However, measurements on a single crystal show no anisotropy in the magnetic response. Accordingly, we have determined the values of H c1 ( T ), H r ( T ) and H c2 ( T ) ( T c = 15.45 K) through detailed magnetisation and resistivity measurements using a polycrystalline sample. The values of the intrinsic parameters H c1 (0), H c2 (0), H c (0), λ GL (0) ξ GL (0), κ 0 are estimated to be 220 Oe, 52 kOe, 1.8 kOe, 1630 A, 80 A and 20.4, respectively. The H c1 ( T ) data is found to conform to the quadratic relation H c1 (0)[1 − ( T / T c ) 2 ] for T H r ( T ) data fit to a power-law response, [1 − ( T / T c )] 1.6 , and the H c2 ( T ) curve shows an unusual curvature, namely, a concave upward feature. Our sample is brittle and electron microscopy reveals that it could comprise weakly coupled grains. As a result, our sample exhibits intergranularity effects near T c via the negative peak effect in field-cooled warm up data and asymmetry in the hysteresis loop recorded after trapping of flux. The deviation of H c1 ( T ) values from the quadratic behaviour near T c and the concave upward feature in the H c2 curve are viewed as the consequence of the presence of “weak links” as the temperature approaches T c .

Optimal electron irradiation as a tool for functionalization of MoS2: Theoretical and experimental investigation

We demonstrate the utility of electron irradiation as a tool to enhance device functionality of graphene-analogous MoS2. With the help of first-principles based calculations, vacancy-induced changes of various electronic properties are shown to be a combined result of crystal-field modification and spin-orbital coupling. A comparative theoretical study of various possible vacancy configurations both in bulk and monolayer MoS2 and related changes in their respective band-structures help us to explain plausible irradiation induced effects. Experimentally, various structural forms of MoS2 in bulk, few layered flakes, and nanocrystals are observed to exhibit important modification of their magnetic, transport, and vibrational properties, following low doses of electron irradiation. While irradiated single crystals and nanocrystals show an enhanced magnetization, transport properties of few-layered devices show a significant increase in their conductivity, which can be very useful for fabrication of electronic...

Surface barrier effect and the crossover in magnetization relaxation in 2H-NbSe2

We have studied the low field magnetization hysteresis and relaxation process in pure 2H-NbSe 2 single crystal. Our results show an order of magnitude difference in the relaxation rate for the flux entry and the flux exit. Also, we have observed a characteristic change of slope in M(ln t) curve in the flux-exit case. We analyse this asymmetry in the relaxation rate and the change of slope in M(ln t) curve in the framework of surface barrier controlled irreversible magnetic behaviour.

Quantitative mobility spectrum analysis of carriers in GaSb/InAs/GaSb superlattice

The authors have investigated electrical transport in a type II GaSb/InAs superlattice grown on GaSb using “quantitative mobility spectrum analysis.” Their results indicate that the superlattice contributes a lone electron specie with an ambient temperature mobility of ∼104 cm2/V s. Variable temperature studies in the range 50–300 K show that the carrier is associated with an activation energy of 0.27 eV, which is very close to the superlattice band gap.

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.