Subhrangsu Sarkar

The second magnetization peak and the peak effect phenomenon in the superconductor Ca3Rh4Sn13

Abstract We report on the observation of the anomalous second magnetization peak (SMP) and the peak effect (PE) in juxtaposition to each other in the isothermal magnetization hysteresis loops at T⩽4 K in a weakly pinned single crystal of an isotropic superconductor Ca3Rh4Sn13 (Tc≈8.2 K). The position of the SMP does not display any temperature variation, whereas the PE appearing at the edge of the irreversibility line progressively moves towards lower fields as temperature increases. At T≈5 K, the PE nearly swamps the SMP, and above this temperature only the PE can be observed. If the SMP and the PE can be construed as representing changes in the spatial order of the flux line lattice (FLL), the apparent merger of loci of the peak fields of the SMP and the PE in the neighborhood of H≈15 kOe at a reduced temperature t [=T/T c (0)] of about 0.6 could imply the existence of a multi-critical point.

Peak effect in a superconducting DyBa2Cu3O7-δ film at microwave frequencies

We report the observation of a peak in the microwave (9.55 GHz) surface resistance in an epitaxial DyBa2Cu3O7–δ superconducting film in magnetic fields (parallel to the c axis) ranging between 0.2 to 0.9 T. Such a peak is absent in the measurements done in zero field. The temperature and field dependence of the peak suggests that this peak could be associated with the peak effect (PE) phenomenon reflecting the order-disorder transformation in the flux-line lattice (FLL). A strong frequency dependence of the PE is observed close to the depinning frequency of the FLL.

Two-Dimensional Nanostrips of Hydrophobic Copper Tetradecanoate for Making Self-Cleaning Glasses

We report a simple, solution-based technique for coating arbitrary surfaces with thin layers of self-assembled copper tetradecanoate CTD nanostrips, resulting in an optically transparent, superhydrophobic coating. The nanostrip-coated surfaces show water contact angles close to 150° and roll-off angles as small as 2°-3°. Importantly, CTD retains its hydrophobic nature even after annealing the self-assembled nanostrips at 200°C, which does not alter the crystal structure but “melts” the surface microstructure. This clearly indicates that the hydrophobicity in CTD is likely to be intrinsic in nature and not induced by the surface microstructure as has been suggested earlier. Strong hydrophobicity in CTD over a relatively wide temperature range presumably results from the presence of the long aliphatic tetradecanoate chains in its structure. Importantly, the self-assembled copper tetradecanoate nanostrips can be dip-coated on glass to render it hydrophobic and at the same time retain a significant level of transparency over the entire visible region. Such nanostructured thin films may be expected to find applications not only as a self-cleaning glass, but also as a corrosion resistant coating, in gas storage due to the layered structure, and as an active catalyst because of the visible absorbance.

Critical current in a spin injection device

We report on direct evidence of the suppression of critical current due to pair-breaking in a superconducting microbridge when the measurement is carried out by injecting spin-polarized carriers instead of normal electrons. A thin layer of La0.7Ca0.3MnO3 was used as the source of spin-polarized carriers. The microbridge was formed on the DyBa2Cu3O7−δ thin film by photolithographic techniques. The design of our spin-injection device allowed us to inject spin-polarized carriers from the La0.7Ca0.3MnO3 layer directly to the DyBa2Cu3O7−δ microbridge (without any insulating buffer layer) making it possible to measure the critical current when polarized electrons alone are injected into the superconductor. Our results confirm the role of polarized carriers in breaking the Cooper pairs in the superconductor.

Critical current of a superconductor measured via injection of spin-polarized carriers

In this paper we report direct evidence of the suppression of critical current due to pair breaking in a superconducting micro-bridge when the measurement is carried out by injecting spin-polarized carriers instead of normal electrons. A thin layer of La0.7Ca0.3MnO3 was used as the source of spin-polarized carriers. The micro-bridge was formed on the DyBa2Cu3O7-δ thin film by photo-lithographic techniques. The design of our spin-injection device allowed us to inject spin-polarized carriers from the La0.7Ca0.3MnO3 layer directly to the DyBa2Cu3O7-δ micro-bridge (without any insulating buffer layer), making it possible to measure the critical current when polarized electrons alone are injected into the superconductor. Our results confirm the role of polarized carriers in breaking the Cooper pairs in the superconductor.

Mass selection in laser-plasma ion accelerator on nanostructured surfaces

When an intense laser pulse interacts with a solid surface, ions get accelerated in the laser-plasma due to the formation of transient longitudinal electric field along the target normal direction. However, the acceleration is not mass-selective. The possibility of manipulating such ion acceleration scheme to enhance the energy of one ionic species (either proton or carbon) selectively over the other species is investigated experimentally using nanopore targets. For an incident laser intensity of approximately 5×1017 W/cm2, we show that the acceleration is optimal for protons when the pore diameter is about 15-20 nm, while carbon ions are optimally accelerated when the pore diameter is close to 40-50 nm. The observed effect is due to tailoring targetry by the pulse pedestal of the laser prior to the arrival of the main pulse.

Pulverization of the flux line lattice, the phase coexistence and the spinodal temperature of the order—disorder transition in a weakly pinned crystal of Yb3Rh4Sn13

We have studied metastability effects pertaining to the peak effect (PE) in critical current density (Jc) via isofield scans in AC susceptibility measurements in a weakly pinned single crystal of Yb3Rh4Sn13 (Tc(0) ≈ 7.6 K). The order-disorder transition in this specimen proceeds in a multi-step manner. The phase coexistence regime between the onset temperature of the PE and the spinodal temperature (where metastability effects cease) seems to comprise two parts, where ordered and disordered regions dominate the bulk behavior, respectively. The PE line in the vortex phase diagram is argued to terminate at the low field end at a critical point in the elastic (Bragg) glass phase.

Observation of size dependent orbital magnetism of single Fe impurity in nanocrystalline Pb hosts

Employing the time differential perturbed angular distribution (TDPAD) technique we have investigated the local magnetism of single Fe impurity in nanocrystalline Pb matrices, by measuring the local susceptibility and spin relaxation rate of 54Fe nuclei. Compared to the nonmagnetic behavior of Fe in bulk Pb host, characterized by independent local susceptibility χloc(T), the magnetic response of Fe in the nanocrystalline samples with particle size of 10 nm and below exhibit Curie-Weiss like χloc(T) with positive slope, indicating the presence of large orbital magnetic moment on the Fe impurity. The orbital moment of Fe has been estimated to be ∼ 1 µB for 6 nm nano-Pb. Further investigations are being carried out to understand the emergence of orbital magnetism of Fe in nano-Pb.Employing the time differential perturbed angular distribution (TDPAD) technique we have investigated the local magnetism of single Fe impurity in nanocrystalline Pb matrices, by measuring the local susceptibility and spin relaxation rate of 54Fe nuclei. Compared to the nonmagnetic behavior of Fe in bulk Pb host, characterized by independent local susceptibility χloc(T), the magnetic response of Fe in the nanocrystalline samples with particle size of 10 nm and below exhibit Curie-Weiss like χloc(T) with positive slope, indicating the presence of large orbital magnetic moment on the Fe impurity. The orbital moment of Fe has been estimated to be ∼ 1 µB for 6 nm nano-Pb. Further investigations are being carried out to understand the emergence of orbital magnetism of Fe in nano-Pb.

A Reduction in Particle Size Generally Causes Body-Centered-Cubic Metals to Expand but Face-Centered-Cubic Metals to Contract

From a careful analysis of existing data as well as new measurements, we show that the size dependence of the lattice parameters in metal nanoparticles with face-centered cubic (fcc) and body-centered cubic (bcc) symmetries display opposite trends: nanoparticles with fcc structure generally contract with decreasing particle size, while those with bcc structure expand. We present a microscopic explanation for this apparently puzzling behavior based on first-principles simulations. Our results, obtained from a comparison of density functional theory calculations with experimental data, indicate that the nanoparticles are capped by a surface monolayer of oxygen atoms, which is routinely detected by surface-sensitive techniques. The bcc- and fcc-based nanoparticles respond in contrasting fashion to the presence of the oxygen capping layer, and this dictates whether the corresponding lattice parameter would increase or decrease with size reduction. The metal-oxygen bonds at the surface, being shorter and stronger than typical metal-metal bonds, pull the surface metal atoms outward. This outward movement of surface atoms influences the core regions to a larger extent in the relatively open bcc geometry, producing a rather large overall expansion of the cluster, compared to the bulk. In case of fcc clusters, on the other hand, the outward movement of surface metal atoms does not percolate too far inside, resulting in either a smaller net expansion or contraction of the cluster depending on the extent of surface oxygen coverage. Our study therefore provides a convincing physicochemical basis for the correlation between the underlying geometry and the nature of change of the lattice parameters under size reduction.

Efficient fast electron generation in an interaction of Intense, ultrashort laser with metal nanoparticle coated dielectric target

Hot electron generation in intense laser-matter interaction studies is a topic of great interest due in significant part to its applications in fast ignitor scheme in Inertial Confinement Fusion (ICF). We measure the hot electron energy spectrum from Ag nanoparticle coated fused silica target (100 μm thick) interacting with an intense (I~1018W/cm2), short pulse (τ~ 30× 10-15s) laser and compare the results with those of an uncoated fused silica. Enhancement in hot electron energy and hard x-ray yield is measured as a function of thickness of Ag nano-coating, varied from tens of nm to hundreds of nm. The hot electron temperatures and integrated x-ray yield are observed to be greater for subwavelength film thicknesses for the case of a p-polarized laser. Such results indicate that metal nanoparticle layers have an important role to play in the enhancement of laser-plasma coupling efficiency for short scale-length plasmas created in femtosecond laser interactions.