Muon-Spin Rotation Measurements of an Unusual Vortex-Glass Phase in the Layered Superconductor Bi2.15Sr1.85CaCu2O8+δ

Muon-spin rotation measurements, performed on the mixed state of the classic anisotropic superconductor Bi2.15Sr1.85CaCu2O8+δ, obtain quantities directly related to two- and three-body correlations of vortices in space. A novel phase diagram emerges from such local probe measurements of the bulk, revealing an unusual glassy state at intermediate fields which appears to freeze continuously from the equilibrium vortex liquid but differs both from the lattice and the conventional high-field vortex glass state in its structure.Publisher PDFPeer reviewe

Resonant transport in a highly conducting single molecular junction via metal-metal covalent bond

Achieving highly transmitting molecular junctions through resonant transport at low bias is key to the next-generation low-power molecular devices. Although, resonant transport in molecular junctions was observed by connecting a molecule between the metal electrodes via chemical anchors by applying a high source-drain bias (> 1V), the conductance was limited to < 0.1 G$_0$, G$_0$ being the quantum of conductance. Here, we report electronic transport measurements by directly connecting a Ferrocene molecule between Au electrodes at the ambient condition in a mechanically controllable break junction setup (MCBJ), revealing a conductance peak at ~ 0.2 G$_0$ in the conductance histogram. A similar experiment was repeated for Ferrocene terminated with amine (-NH2) and cyano (-CN) anchors, where conductance histograms exhibit an extended low conductance feature including the sharp high conductance peak, similar to pristine ferrocene. Statistical analysis of the data along with density functional theory-based transport calculation suggests the possible molecular conformation with a strong hybridization between the Au electrodes and Fe atom of Ferrocene molecule is responsible for a near-perfect transmission in the vicinity of the Fermi energy, leading to the resonant transport at a small applied bias (< 0.5V). Moreover, calculations including Van der Waals/dispersion corrections reveal a covalent like organometallic bonding between Au and the central Fe atom of Ferrocene, having bond energies of ~ 660 meV. Overall, our study not only demonstrates the realization of an air-stable highly transmitting molecular junction, but also provides an important insight about the nature of chemical bonding at the metal/organo-metallic interface.Comment: 23 pages, 6 figures, supplementary include

Nanoscale investigation of superconductivity and magnetism using neutrons and muons

The work presented in this thesis was broadly focussed on the investigation of the magnetic behaviour of different superconducting materials in the form of bulk (single crystals and pellets) and thin films (nanomagnetic devices like superconducting spin valves etc). Neutrons and muons were extensively used to probe the structural and magnetic behaviour of these systems at the nanoscale along with bulk characterisation techniques like high-sensitive magnetic property measurements, scanning probe microscopy and magneto-transport measurements etc. The nanoscale interplay of Superconductivity and Ferromagnetism was studied in the thin film structures using a combination of Polarised Neutron Reflectivity (PNR) and Low Energy Muon Spin Rotation (LE-µSR) techniques while bulk Muon Spin Rotation (µSR) technique was used for microscopic magnetic investigation in the bulk materials. In the Fe/Pb heterostructure, evidence of the Proximity Effect was observed in the form of an enhancement of the superconducting penetration depth (λs) with an increase in the ferromagnetic layer thickness (dF) in both the bilayered and the trilayered structures. The existence of an Inverted Magnetic Region was also detected at the Ferromagnet-Superconductor (F/S) interface in the normal state possibly originating from the induced spin polarisation within the Pb layer in the presence of the neighbouring Fe layer(s). The spatial size (height and width) of the Inverted Magnetic Region did not change much while cooling the sample below the superconducting transition temperature(Tc)and it also stayed unaffected by an increase in the Fe layer thickness and by a change of the applied magnetic field. In the superconducting spin valve structure containing Permalloy (Py) as ferromagnetic layer and Nb as the superconducting layer, LE-µSR measurements revealed the evidence of the decay of magnetic flux density (as a function of thickness) within the Nb layer symmetrically from the Py/Nb interfaces towards the centre of the Nb layer in the normal state. The thickness dependent magnetisation decay occurred over two characteristic length scales in the normal state that stayed of similar values in the superconducting state also. In the superconducting state, an additional contribution towards the magnetisation was found in the vicinity of the Py/Nb interfaces possibly originating from the spin polarisation of the singlet Cooper pairs in these areas. The nanoscale magnetic investigation on a highly engineered F/S/F structure (where each of the F blocks made of multiple Co/Pd layers with magnetic moments aligned perpendicular to the plane of these layers and neighbouring magnetic blocks separated by Ru layers giving rise to antiferromagnetic alignment) using LE-µSR showed an antisymmetric thickness dependent magnetic flux density profile with two characteristic length scales. In the superconducting state, the magnetic flux density profile got modified within the superconducting Nb₆₇Ti₃₃ layer near the F/S interfaces in a way similar to that of observed in the case of Py/Nb system, most likely because of the spin polarisation of the superconducting electron pairs. The vortex magnetic phase diagram of Bi₂Sr₂Ca₂Cu₃O10-δ was studied using the Muon Spin Rotation (µSR) technique to explore the effects of vortex lattice melting and rearrangements for vortex transitions and crossover as a function of magnetic field and temperatures. At low magnetic fields, the flux vortices undergo a first order melting transition from a vortex lattice to a vortex liquid state with increasing temperature while another transition also occurred with increasing field at fixed temperature to a vortex glass phase at the lowest temperatures. Evidence of a frozen liquid phase was found in the intermediate field region at low temperature in the form of a lagoon in the superconducting vortex state which is in agreement with earlier observations made in BiSCCO-2212. The magnetic behaviour of the unconventional superconductor Sr₂RuO₄ was investigated using µSR to find the evidence of normal state magnetism and the nature of the vortex state. In the normal state, a weak hysteretic magnetic signal was detected over a wide temperature and field range believed to be supporting the evidence of a chiral order parameter. The nature of the vortex lattice structure was obtained in different parts of the magnetic phase diagram and the evidence of magnetic field driven transition in the lattice structure was detected from a Triangular→Square structure while the vortex lattice stayed Triangular over the entire temperature region below Tc at low fields with a disappearance of pinning at higher temperatures

Noncovalent Catalysis for Enantioselective Direct Aldol Reaction of 3-Acetylcoumarins to Pyrazole-4,5-diones

The first catalytic enantioselective direct aldol reaction of 3-acetylcoumarins to pyrazole-4,5-diones is reported and shown to proceed through noncovalent substrate activation by a quinine-derived bifunctional tertiary amino-amide as the catalyst. The densely functionalized products, consisting of an oxygen-containing quaternary stereocenter and bearing two biologically relevant heterocycles, are generally obtained in high yields with moderate to excellent enantioselectivities

Electronic Properties of Twisted hBN/NbSe₂ Hetero-structure and Its Application as an Electrode in Lithium-Ion Battery: First-Principle Study

Despite the availability of a variety of two-dimensional (2D) materials for potential use as Li-ion battery electrodes, it is difficult to find all the desirable qualities of an electrode in a single material. Therefore, research efforts are ongoing in designing a heterostructure to incorporate the desirable characteristics that are not available in the parent structures. In our work, we have designed a van der Waals heterostructure made of a conducting 2D NbSe2-layer and insulating hexagonal boron nitride (h-BN) and applied interlayer twist at different twist angles for potential applications as an electrode in the Li-ion battery. The heterostructure offers a metallic character, which makes the insulating h-BN capable of battery application. The adsorption site changes for different twist angles. For the twist angles of 5.21 and 54.79°, the H-site is the most favorable adsorption site, but for all other twist angles, T-site stays the most favorable adsorption site. When the angle between surfaces is 19.11°, the heterostructure shows better stability as compared to all other configurations in different twist angles. The adsorption energy gets enhanced compared to the individual monolayers, indicating better intercalation. At a twist angle of 19.11°, our structure shows a minimum diffusion barrier of 0.6 eV, whereas at all other twist angles, it shows a nearly 0.9 eV barrier. The open circuit voltage is found to be 0.62 V. The structure shows a specific capacity of 185 mA h g m–1

Muon-Spin Rotation Measurements of an Unusual Vortex-Glass Phase in the Layered Superconductor Bi_2.15Sr_1.85CaCu₂O_8+δ

Muon-spin rotation measurements, performed on the mixed state of the classic anisotropic superconductor Bi2.15Sr1.85CaCu2O8+δ, obtain quantities directly related to two- and three-body correlations of vortices in space. A novel phase diagram emerges from such local probe measurements of the bulk, revealing an unusual glassy state at intermediate fields which appears to freeze continuously from the equilibrium vortex liquid but differs both from the lattice and the conventional high-field vortex glass state in its structure

Instantaneous Gelation of a Self-Healable Wide-Bandgap Semiconducting Supramolecular Mg(II)-Metallohydrogel: An Efficient Nonvolatile Memory Design with Supreme Endurance

An efficient strategy for room-temperature, atmospheric-pressure synthesis of a supramolecular metallohydrogel of the Mg(II) ion, i.e., Mg@3AP, using the metal-coordinating organic ligand 3-amino-1-propanol as a low-molecular-weight gelator (LMWG) in a water medium has been developed. Through a rheological analysis, we looked into the mechanical properties of the supramolecular Mg(II)-metallohydrogel. The self-healing nature of the metallohydrogel is confirmed along with the thixotropic characteristics. Investigation using field emission scanning electron microscopy revealed the hierarchical network of the supramolecular metallohydrogel. The EDX elemental mapping confirms the primary chemical constituents of the metallohydrogel. The possible metallohydrogel formation strategy has been analyzed through FT-IR spectroscopic studies. In this work, Schottky diode structures in a metal–semiconductor–metal geometry structures based on a magnesium(II) metallohydrogel (Mg@3AP) have been constructed, and charge transport behavior has been observed. Furthermore, here, it is demonstrated that the resistive random access memory (RRAM) device based on Mg@3AP exhibits bipolar resistive switching behavior at room temperature and ambient conditions. We have also looked into the switching mechanism through the formation (rupture) of conductive filaments between the metal electrodes to understand the process of resistive switching behavior. With a high on/off ratio (∼100), this RRAM device exhibits remarkable switching endurance over 10,000 switching cycles. These structures are suitable for use in nonvolatile memory design, neuromorphic computing, flexible electronics, and optoelectronics, among other fields, due to their simple fabrication procedures, reliable resistive switching behavior, and stability of the current system