R. C. Budhani

Two-dimensional superconductivity at a Mott insulator/band insulator interface LaTiO3/SrTiO3

Transition metal oxides show a great variety of quantum electronic behaviours where correlations often have an important role. The achievement of high-quality epitaxial interfaces involving such materials gives a unique opportunity to engineer artificial structures where new electronic orders take place. One of the most striking result in this area is the recent observation of a two-dimensional electron gas at the interface between a strongly correlated Mott insulator LaTiO(3) and a band insulator SrTiO(3). The mechanism responsible for such a behaviour is still under debate. In particular, the influence of the nature of the insulator has to be clarified. In this article, we show that despite the expected electronic correlations, LaTiO(3)/SrTiO(3) heterostructures undergo a superconducting transition at a critical temperature T(c)(onset)~300 mK. We have found that the superconducting electron gas is confined over a typical thickness of 12 nm and is located mostly on the SrTiO(3) substrate.

Enhanced thermoelectric figure-of-merit in spark plasma sintered nanostructured n-type SiGe alloys

We report a significant enhancement in the thermoelectric figure-of-merit of phosphorous doped nanostructured n-type Si80Ge20 alloys, which were synthesized employing high energy ball milling followed by rapid-heating using spark plasma sintering. The rapid-heating rates, used in spark plasma sintering, allow the achievement of near-theoretical density in the sintered alloys, while retaining the nanostructural features introduced by ball-milling. The nanostructured alloys display a low thermal conductivity (2.3 W/mK) and a high value of Seebeck coefficient (−290 μV/K) resulting in a significant enhancement in ZT to about 1.5 at 900 °C, which is so far the highest reported value for n-type Si80Ge20 alloys.

Multiple quantum criticality in a two-dimensional superconductor.

The diverse phenomena associated with the two-dimensional electron gas (2DEG) that occurs at oxide interfaces include, among others, exceptional carrier mobilities, magnetism and superconductivity. Although these have mostly been the focus of interest for potential future applications, they also offer an opportunity for studying more fundamental quantum many-body effects. Here, we examine the magnetic-field-driven quantum phase transition that occurs in electrostatically gated superconducting LaTiO3/SrTiO3 interfaces. Through a finite-size scaling analysis, we show that it belongs to the (2+1)D XY model universality class. The system can be described as a disordered array of superconducting puddles coupled by a 2DEG and, depending on its conductance, the observed critical behaviour is single (corresponding to the long-range phase coherence in the whole array) or double (one related to local phase coherence, the other one to the array). A phase diagram illustrating the dependence of the critical field on the 2DEG conductance is constructed, and shown to agree with theoretical proposals. Moreover, by retrieving the coherence-length critical exponent ν, we show that the quantum critical behaviour can be clean or dirty according to the Harris criterion, depending on whether the phase-coherence length is smaller or larger than the size of the puddles.

Synthesis of superconducting films of the Y‐Ba‐Cu‐O system by a screen printing method

A screen printing method has been used to deposit superconducting films of Y‐Ba‐Cu‐O materials on Al2O3 substrates. The electrical resistivity, microstructure, and adhesion of these films are highly sensitive to the post‐printing heat treatment. Films heat treated at 950 °C and below are poorly adherent, whereas those annealed above 1000 °C undergo phase separation to Y2Cu2O5 and BaCuO2 and also react with the substrate. Nearly single phase, highly adherent, and smooth films showing a zero resistance state at 79 K have been synthesized by optimizing the annealing conditions.

Raman, infrared and x-ray diffraction study of phase stability in La1−xBaxMnO3 doped manganites

Formation of different crystallographic phases and their characteristic Raman and infrared spectra are studied in the manganite system La1−xBaxMnO3 for x in the range of 0⩽x⩽1, and with variations in oxygen stoichiometry. Synthesis of the end member LaMnO3 in pure argon environment leads to the formation of Jahn–Teller distorted, antiferromagnetic orthomanganite. While the observed Raman modes in this compound are primarily due to internal vibrations of MnO6 octahedra, the infrared (IR) spectra show an absorption edge in addition to the IR active phonons. The oxygen rich LaMnO3 is rhombohedral and has fewer zero-wave-vector phonon modes. In the barium substituted compositions with 0<x⩽0.25, a single phase rhombohedral compound of decreasing rhombicity is formed. A further increase in x leads to the ideal cubic perovskite structure for which a factor group analysis yields no Raman active and three IR active phonons of F1u symmetry. The compound with x=0.35, shows faint Raman modes of hexagonal BaMnO3 (P63m...

Kinetics of structural relaxation and hydrogen evolution from plasma deposited silicon nitride

Infrared absorption measurements and the temperature dependence of stress have been used to establish the kinetics of structural relaxation and hydrogen evolution from plasma deposited a‐SixNy :H films. The Arrhenius rate law describes the dissociation of N–H and Si–H bonds which occurs on annealing the films above 600 °C. The activation energies deduced from the infrared data are lower than the respective bond dissociation energies. The films undergo a rapid stress relaxation in the temperature range 400–650 °C. The discussion of the experimental results highlights possible mechanisms for the evolution of hydrogen from a‐SixNy: H networks.

Structural order in Si–N and Si–N–O films prepared by plasma assisted chemical vapor deposition process

Local bonding modes and the nearest‐neighbor environment of the constituent atoms in glow discharge silicon nitride and oxynitride films have been probed with Fourier‐transform infrared spectroscopy. The silicon nitride films prepared under a wide range of (NH3+N2)/SiH4 ratio and rf power levels all show characteristic absorption bands of N–H, Si–H, and Si–N bonds. The Si–H stretch mode absorption peak shifts to higher wave numbers with the increasing number of N–H bonds in the material. This behavior is observed for all power levels and gas flow combinations. The observed universal dependence of the Si–H stretch frequency on the concentration of the N–H bonds, and the sign of the shift indicate that the change in the frequency is caused by the increasing number of nitrogen atoms at the Si site. Arguments advanced on the basis of the electronegativity of the constituents suggest that the formation of N–H and Si–H bonds at the Si–N site leads to opposite shifts in the Si–N frequency. Thermal annealing stud...

Mg3Sb2-based Zintl compound: a non-toxic, inexpensive and abundant thermoelectric material for power generation

The deployment of thermoelectric materials for deriving an enhanced figure of merit (ZT) for power generation in inexpensive, non-toxic and relatively abundant bulk homogeneous solid relies on the extent of achieving the “phonon-glass electron crystal” (PGEC) characteristics. Here, a proof of principal has been established experimentally in the present work for a Zintl compound of Mg3Sb2 and its derivative of isoelectronically Bi doped Bi; Mg3Sb2−xBix (0 ≤ x ≤ 0.4) alloys in Mg3Sb2. Single phase p-type Mg3Sb2 compounds, with Mg and Sb powders as starting materials, have been prepared directly by spark plasma sintering (SPS) in a one step process. The structural refinements of this hexagonal Zintl compound by X-ray diffraction analysis (XRD) and high resolution transmission electron microscopy (HRTEM) investigation reveal that they are single phase devoid of any oxides or Sb precipitates. Transport measurements indicate low thermoelectric figure of merit (ZT = 0.26 at 750 K) for Mg3Sb2. However, an optimum doping of 0.2 at% with iso-electronic Bi ions at the Sb site enhances the ZT to 0.6 at 750 K, which is comparable with the present day industrial materials such as Bi based tellurides and selenides which are toxic. We note that the system becomes metal with carrier density exceeding 15 × 1020/cm3 for x ≥0.25. The substantial increase in ZT in Mg3Sb2−xBix (0 ≤ x ≤ 0.4) owes to a partial decoupling of the electronic and phonon subsystem, as expected for a Zintl phase compound. While the reduction in thermal conductivity in Mg3Sb2−xBix (0 ≤ x ≤ 0.4) accounts to mass fluctuations and grain boundary scattering, the enhancement in the electronic power-factor is attributed to the presence of heavy and light bands in its valence band structure. The latter has been confirmed by means of both X-ray photo electron spectroscopy studies and first-principles density functional based calculations. These measurements established that a high figure of merit can be achieved in this class of materials with appropriate doping. Further, relative abundance of the material ingredients combined with its one step synthesis leads to a cost effective production and less toxicity makes the material an environmentally benign system for thermoelectric power generation.

Antiferromagnetic coupling and enhanced magnetization in all-ferromagnetic superlattices

The structural and magnetic properties of a series of superlattices consisting of two ferromagnetic metals La0.7Sr0.3MnO3 (LSMO) and SrRuO3 (SRO) grown on (001) oriented SrTiO3 are studied. Superlattices with a fixed LSMO layer thickness of 20unit cells and varying SRO layer thickness show a sudden drop in magnetization on cooling through a temperature where both LSMO and SRO layers are ferromagnetic. This behavior suggests an antiferromagnetic coupling between the layers. In addition, the samples having thinner SRO layers (n<6) exhibit enhanced saturation magnetization at 10K. These observations are attributed to the possible modification in the stereochemistry of the Ru and Mn ions in the interfacial region.

Magnetically measured irreversibility temperatures in superconducting oxides and alloys

Irreversibility temperatures Tr(H) were magnetically measured for the aligned powders of superconducting oxides, La1.85Sr0.15CuO4, YBa2Cu3O7, Bi2Sr2CaCu2O8, and Bi2Sr2Ca2Cu3O10 and for the metallic superconductors Nb, Nb-Ti, Nb3Sn and PbMo6S8. The results are briefly discussed in terms of the thermally activated flux creep and the lattice or the glass melting models for the irreversibility temperatures. Also, the measurement procedures for Tr(H) using a SQUID magnetometer are described in detail, since the measuring methods and the criteria which are used for Tr(H) are important in the determination and interpretation of the temperatures.