Awadhesh Mani

Pressure-induced superconductivity in BaFe2As2 single crystal

The evolution of pressure-induced superconductivity in single crystal as well as polycrystalline samples of BaF e2As2 have been investigated through temperature-dependent electrical resistivity studies in the 0–7 GPa pressure range. While the superconducting transition remains incomplete in the polycrystalline sample, a clear pressure-induced superconductivity with zero resistivity at the expense of magnetic transition, associated with spin density wave (SDW), is observed in the single-crystal sample. The superconducting transition temperature (TC) is seen to increase upto a moderate pressure of about ~1.5 GPa and decreases monotonically beyond this pressure. The SDW transition temperature TSDW decreases rapidly with increasing pressure and vanishes above ~1.5 GPa.

Pressure-induced metallization of BaMn2As2

The temperature and pressure dependent electrical resistivity rho(T,P) studies have been performed on BaMn2As2 single crystal in the 4.2 to 300 K range upto of 8.2 GPa to investigate the evolution of its ground state properties. The rho(T) shows a negative co-efficient of resistivity under pressure upto 3.2 GPa. The occurrence of an insulator to metal transition (MIT) in an external P ~4.5 GPa is indicated by a change in the temperature co-efficient in the rho(T) data at ~36 K . However complete metallization in entire temperature range is seen at a P~5.8 GPa. High pressure XRD studies carried out at room temperature also shows an anomaly in the pressure versus volume curve around P ~ 5 GPa, without a change in crystal structure, indicative of an electronic transition. Further, a clear precipitous drop in rho(T) at ~17 K is seen for P ~5.8 GPa which suggests the possibility of the system going over to a superconducting ground state.

Intricacies of strain and magnetic field induced charge order melting in Pr0.5Ca0.5MnO3 thin films

Thin films of the half-doped manganite Pr0.5Ca0.5MnO3 were grown on (1 0 0) oriented MgO substrates by pulsed laser deposition technique. In order to study the effect of strain on the magnetic field induced charge order melting, films of different thicknesses were prepared and their properties were studied by X-ray diffraction, electrical resistivity and magnetoresistance measurements. A field induced charge order melting is observed for films with very small thicknesses. The charge order transition temperature and the magnetic field induced charge order melting are observed to depend on the nature of strain that is experienced by the film.

Role of Se vacancies on Shubnikov-de Haas oscillations in Bi2Se3: A combined magneto-resistance and positron annihilation study

Magneto resistance measurements coupled with positron lifetime measurements, to characterize the vacancy type defects, have been carried out on the topological insulator (TI) system Bi2Se3, of varying Se/Bi ratio. Pronounced Shubnikov de Haas (SdH) oscillations are seen in nominal Bi2Se3.1 crystals for measurements performed in magnetic fields up to 15 T in the 4 K to 10 K temperature range, with field applied perpendicular to the (001) plane of the crystal. The quantum oscillations, characteristic of 2D electronic structure, are seen only in the crystals that have a lower concentration of Se vacancies, as inferred from positron annihilation spectroscopy.

Large magnetocaloric effect in hexagonal Yb1−xHoxMnO3

Magnetocaloric properties of polycrystalline hexagonal Yb1−xHoxMnO3 (x = 0.1, 0.2, and 0.3) compounds are studied through magnetization measurements. Temperature dependence of Zero Field Cooled magnetic moment measurements shows Neel temperature (TN1) of ∼83 K, corresponding to the Mn3+ antiferromagnetic ordering. At low temperatures (TN2 ∼ 5 K), all compounds show ferromagnetic ordering due to alignment of the Yb moments and the field induced magnetic transition is observed in the isothermal magnetization measurements. The maximum entropy change |ΔSMmax| and the relative cooling power (RCP) of Yb1−xHoxMnO3 are 3.75 ± 0.78 J/(mol K) and 90.0 ± 27 J/mol for x = 0.3 at ΔH = 100 kOe. Values of both |ΔSMmax| and RCP found to increase with increasing Ho content.

Thickness controlled proximity effects in C-type antiferromagnet/superconductor heterostructure.

Modulation of the superconducting state possessing a C-type antiferromagnetic phase in the Nd0.35Sr0.65MnO3/YBa2Cu3O7 heterostructure is investigated, with the Nd0.35Sr0.65MnO3 thickness (t) varying from 40 to 200 nm. Both the superconducting transition temperature and the upper critical field along the c-axis decrease with increasing t; while the in-plane coherence length increases from 2.0 up to 3.6 nm. Meanwhile, the critical current density exhibits a field-independent behavior, indicating an enhanced flux pinning effect. Furthermore, low-temperature spin canting induces a breakdown and re-entrance of the superconductivity, demonstrating a dynamic completion between the superconducting pairing and the exchange field. An unexpected colossal magnetoresistance is observed below the superconducting re-entrance temperature at t = 200 nm, which is attributed to the dominant influence of the exchange field over the pairing energy.

Studies on the magnetoelastic and magnetocaloric properties of Yb1−xMgxMnO3 using neutron diffraction and magnetization measurements

We report the magnetic ordering and magnetoelastic coupling of polycrystalline hexagonal Yb1−xMgxMnO3 (x = 0.00 and 0.05) compounds by using neutron diffraction measurements. The magnetocaloric properties of these Yb1−xMgxMnO3 compounds are also studied using magnetization measurements. The temperature dependence of the lattice parameters (a and c/a ratio) and unit cell volume V show anomalous behavior near TN1 ∼ 85 K (the Mn ordering temperature) due to the magnetoelastic effect. Also all the Mn–O bond distances display considerable variation at TN1. Isothermal magnetization curves measured near the Yb long range ordering temperatures indicate a field induced magnetic transition with applied field. The isothermal magnetic entropy change (−ΔSM) is calculated from the magnetization curves measured for different temperatures. Values of maximum entropy change (−ΔSmaxM), the adiabatic temperature change (ΔTad) and the relative cooling power (RCP) for these compounds are found to be 3.02 ± 0.37 J mol−1 K−1, 8.6 ± 0.95 K and 41 ± 9 J mol−1 for x = 0.00, and 2.63 ± 0.36 J mol−1 K−1, 9.06 ± 0.96 K and 40.0 ± 10 J mol−1 for x = 0.05, respectively, for ΔH = 100 kOe. Rescaling of the −ΔSM vs. T curves for various fields fit into a single curve, implying the second-order phase transition.

Structural, magnetic and magnetocaloric properties of hexagonal multiferroic Yb1−xScxMnO3 (x = 0.1 and 0.2)

We have studied the effect of Sc doping on the structural, magnetic and magnetocaloric properties of multiferroic Yb1−xScxMnO3 (x = 0.1 and 0.2). X-ray powder diffraction shows that both samples crystallize in the hexagonal phase with P63cm space group. The structural analysis shows a decrease in the lattice parameter a, a decrease in the cell volume of the hexagonal unit cell and a decrease in the average bond length between Mn–O, with Sc substitution. Magnetic measurements show that the Neel temperature (TN) increases from 90 K for x = 0.1 to 94 K for x = 0.2 samples. Isothermal magnetic curves show that the field variation in magnetization generates a metamagnetic transition. The maximum entropy change −ΔSmaxM and the relative cooling power (RCP) of Yb1−xScxMnO3 are found to be 2.46 ± 0.40 J mol−1 K−1 and 38.5 ± 9 J mol−1 for x = 0.1 and 1.87 ± 0.31 J mol−1 K−1 and, 30.1 ± 8 J mol−1 for x = 0.2 with ΔH = 10 T. The rescaled magnetic entropy change curves for different applied fields collapse onto a single curve for materials with second-order phase transitions.

Evolution of ferromagnetic clustering in Pr0.5Ca0.5MnO3 and its effect on the critical temperature of YBa2Cu3O7 thin film

A thin film bilayer of Pr0.5Ca0.5MnO3 and YBa2Cu3O7 has been deposited on LaAlO3 substrate by pulsed laser deposition. XRD pattern confirms the formation of the bilayer. Current and magnetic field dependent resistance measurements have been carried out to study the proximity effect. These measurements have shown that the superconducting transition temperature (Tc) of YBa2Cu3O7 is suppressed to lower temperatures with a progressive increase in current. In addition, a hump like feature appears in resistivity near Tc with increase in applied current. The results are explained on the basis of the formation and growth of ferromagnetic clusters on account of charge order melting in Pr0.5Ca0.5MnO3 upon application of high current and magnetic field.

X-ray photoemission study in Re0.7Ca0.3MnO3 epitaxial films

We have performed the x-ray photoemission experiments in the colossal magnetoresistive manganites Re0.7Ca0.3MnO3 with Re=La, Pr, and Nd in order to study the Re dependence of electronic structures in this system. The samples are epitaxial films grown on LaAlO3 substrates with pulsed laser deposition system. It is found that the binding energies of Mn 2p3/2 are 641.7 and 643.1 eV for La0.7Ca0.3MnO3 and Nd0.7Ca0.3MnO3, respectively. The shift of this energy level is related to the change in bond angle and/or bond length between Mn and oxygen. Moreover, two lines of O 1s core level are assigned as Mn–O (530 eV) and Re–O (533 eV). The intensity ratio of these two lines in Nd0.7Ca0.3MnO3 is different from those in La0.7Ca0.3MnO3 and Pr0.7Ca0.3MnO3, suggesting that the ion redistribution between Re–O and Mn–O sites occurs with the Re substitution in these films.