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Register a new userLow Temperature Structural and Transport Studies of La0.175Pr0.45Ca0.375MnO3
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The temperature (T) dependent x-ray diffraction (XRD) and resistivity measurements of La0.175Pr0.45Ca0.375MnO3 (LPCMO) have been performed down to 2K to understand the structural and transport properties. From room temperature down to 220K, LPCMO exists in orthorhombic phase with Pnma structure and at 220K, it transforms to charge ordered (CO) monoclinic phase with P21/m structure and remains as it is down to 2K. The CO phase is evident from the occurrence of weak but well defined superlattice peaks in the XRD pattern. This structural transformation is of first order in nature as evident from the phase coexistence across the transition region. These results thus clearly illustrate that LPCMO undergoes a first order structural phase transition from charge disordered orthorhombic phase to CO monoclinic phase at 220K, consistent with temperature dependent resistivity results. Our structural analysis of T dependent XRD data using Rietveld refinement infers that below 220K, LPCMO forms commensurate CO monoclinic P21/m structure with four times structural modulation.
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The low-temperature and high-magnetic field (2K, 8T) powder x-ray diffraction (LTHM-XRD) measurements have been carried out at different temperatures (T) and magnetic fields (H) to investigate the structural phase diagram for phase separated La0.175Pr0.45Ca0.375MnO3 (LPCMO) manganite. The antiferromagnetic (AFM) P21/m insulating phase undergoes field induced transformation to ferromagnetic (FM) Pnma metallic ground state below its AFM ordering temperature (220K) in zero-field cooling (ZFC) from room temperature. At temperature greater than 25K, the field induced FM Pnma phase remained irreversible even after complete removal of field. However, for T ( 39-65K), the field induced transformation is partially reversible. This behaviour has been attributed to magnetic field induced devitrification of the glass-like arrested AFM P21/m phase to FM Pnma equilibrium phase. The devitrified FM Pnma phase starts transforming back to AFM P21/m phase around ~39K on heating the sample under zero field. Our results corroborate the evidence of strong magneto-structural coupling in this system. An H-T phase-diagram has been constructed based on LTHM-XTD data, which resembles with the one made from magnetic measurements. These results have been explained on the basis of kinetic arrest of first order phase transition and field induced devitrification of the arrested phase.
High- and low-field magneto-transport measurements, as well as SQUID measurements of magnetization, were carried out on Ga1-xMnxAs epilayers grown by low temperature molecular beam epitaxy, and subsequently annealed under various conditions. We observe a large enhancement of ferromagnetism when the samples are annealed at an optimal temperature, typically about 280 0C. Such optimal annealing leads to an increase of Curie temperature, accompanied by an increase of both the conductivity and the saturation magnetization. A decrease of the coercive field and of magnetoresistivity is also observed for Ga1-xMnxAs annealed at optimal conditions. We suggest that the experimental results reported in this paper are related to changes in the domain structure of Ga1-xMnxAs.
This work reports the electrical transport and temperature-dependent photoconductivity in tungsten diselenide (WSe2) thin films. The electrical conductivity analysis shows the presence of the three regions with temperature variation. At lower temperatures (<190K), carriers become localized to small regions in the film due to the Mott hopping mechanism. The middle region (190 to 273 K) follows Seto parameters and obtained low barrier height (0.0873 eV) may be responsible for the improved carrier mobility. At higher temperature (>273K) region, thermally activated conduction is dominated with two activation energies of ~138 meV and 98 meV. The peaks obtained in photoluminescent analysis attributes to the presence of mid-bandgap states or defect states which play an important role in the photoconductivity of WSe2. The transient photoconductivity measurements show consistent temperature-dependent behaviour. The effect of light intensity and wavelength variation on the photoconductivity of WSe2 thin films is also discussed. The photocurrent is 1.19*10-5 A at 125 K while at 350 K it was observed to be 3.12*10-4 A. The light-on/off current cycles show that the current can recover to its initial state which points to the stable and outstanding reversible properties of the WSe2 thin film device to be used in photodetector applications.
We have made thermal and electrical transport measurements of uncompressed pyrolytic graphite sheet (uPGS), a mass-produced thin graphite sheet with various thicknesses between 10 and 100 {mu}m, at temperatures between 2 and 300 K. Compared to exfoliated graphite sheets like Grafoil, uPGS has much higher conductivities by an order of magnitude because of its high crystallinity confirmed by X-ray diffraction and Raman spectroscopy. This material is advantageous as a thermal link of light weight in a wide temperature range particularly above 60 K where the thermal conductivity is much higher than common thermal conductors such as copper and aluminum alloys. We also found a general relationship between thermal and electrical conductivities in graphite-based materials which have highly anisotropic conductivities. This would be useful to estimate thermal conductance of a cryogenic part made of these materials from its electrical conductance more easily measurable at low temperature.
EuC$_2$ is a ferromagnet with a Curie-temperature of $T_C simeq 15,$K. It is semiconducting with the particularity that the resistivity drops by about 5 orders of magnitude on cooling through $T_C$, which is therefore called a metal-insulator transition. In this paper we study the magnetization, specific heat, thermal expansion, and the resistivity around this ferromagnetic transition on high-quality EuC$_2$ samples. At $T_C$ we observe well defined anomalies in the specific heat $c_p(T)$ and thermal expansion $alpha(T)$ data. The magnetic contributions of $c_p(T)$ and $alpha(T)$ can satisfactorily be described within a mean-field theory, taking into account the magnetization data. In zero magnetic field the magnetic contributions of the specific heat and thermal expansion fulfill a Gruneisen-scaling, which is not preserved in finite fields. From an estimation of the pressure dependence of $T_C$ via Ehrenfests relation, we expect a considerable increase of $T_C$ under applied pressure due to a strong spin-lattice coupling. Furthermore the influence of weak off stoichiometries $delta$ in EuC$_{2 pm delta}$ was studied. It is found that $delta$ strongly affects the resistivity, but hardly changes the transition temperature. In all these aspects, the behavior of EuC$_2$ strongly resembles that of EuO.
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