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[en] LaB6 and other hexaborides are inclusion compounds in which the rare earth or other metal ion is weakly bound and sits in an oversized ''cage'' of boron ions. Here we show that a simple model that treats the La ions as independent harmonic (Einstein) oscillators embedded in a Debye framework of boron ions successfully accounts for the anomalies in the specific heat and resistivity of LaB6. One of the nice features of the model is that the Einstein temperature of the La atoms and the Debye temperature of the boron framework are derived from room-temperature x-ray crystallography data. This feature makes the model easy to apply to other hexaborides and other materials that can be treated as inclusion compounds. The results from this work imply that local modes are likely to be important for understanding the physical properties of all the hexaborides
[en] Structural, magnetic, electrical and thermal transport, and heat-capacity measurements are reported on single crystals of Eu8Ga16Ge30, Sr8Ga16Ge30, and Ba8Ga16Ge30. These compounds all crystallize in a cubic type-I ice clathrate structure, and are of interest as potential thermoelectric materials. Neutron-diffraction measurements were made on a single crystal of Eu8Ga16Ge30 that was grown using isotopically pure Eu153. Nuclear density maps clearly show that Eu atoms at the 6d sites 1/4,0 can move away from the cage center to one of four nearby positions. Ferromagnetism is observed in Eu8Ga16Ge30 for temperatures below 32 K, with the preferred direction of the Eu spins along the (100) axis. Ferromagnetism in these heavily doped semiconductors (∼1021 electrons/cm3) is likely due to a Rudermann-Kittel-Kasuya-Yoshida-type interaction. A large (∼10% at 8 T) negative magnetoresistance was measured near the Curie temperature of Eu8Ga16Ge30. The lattice thermal conductivities of Eu8Ga16Ge30 and Sr8Ga16Ge30 single crystals show all of the characteristics of a structural glass. The thermal conductivity of Ba8Ga16Ge30 is low at room temperature (1.3 W/m K), but exhibits a temperature dependence characteristic of a crystal. A magnetic field has no significant effect on the thermal conductivity of any of the crystals for temperatures between 2 and 300 K. Heat-capacity measurements indicated Einstein contributions from each of the rattlers, with characteristic temperatures of 60, 53, and 30 K for Ba, Sr, and Eu atoms respectively. No superconductivity was observed in heavily doped single crystals of Ba8Ga16Ge30 for temperatures above 2 K, contrary to a previous report
[en] Magnetization and neutron diffraction studies of the 2D S=1/2 antiferromagnet, K2V 3O8 , indicate an ordered state exhibiting weak ferromagnetism and field-induced spin reorientations. Of particular interest is the behavior in a basal plane magnetic field where a unique spin reorientation is observed in which the spins rotate from the easy c axis to the basal plane while remaining normal to the applied field. The experimental observations are well described by a two spin exchange model incorporating Heisenberg and Dzyaloshinskii-Moriya interactions with an additional c-axis anisotropy
[en] Superconducting magnesium diboride films with Tc0∼24 K and sharp transition ∼1 K were prepared on Si by pulsed-laser deposition from stoichiometric MgB2 target. Contrary to previous reports, anneals at 630 C and a background of 2 x 10-4 Ar/4%H2 were performed without the requirement of Mg vapor or Mg cap layer. This integration of superconducting MgB2 film on Si may thus prove enabling in superconductor-semiconductor device applications. Images of surface morphology and cross-section profiles by scanning electron microscopy show that the films have a uniform surface morphology and thickness. Energy-dispersive spectroscopy study reveals these films were contaminated with oxygen, originating either from the growth environment or from sample exposure to air. The oxygen contamination may account for the low Tc for those in situ annealed films, while the use of Si as a substrate does not result in a decrease in Tc as compared to other substrates
[en] Cd2Os2O7 crystallizes in the pyrochlore structure and undergoes a metal-insulator transition (MIT) near 226 K. We have characterized the MIT in Cd2Os2O7 using x-ray diffraction, resistivity at ambient and high pressure, specific heat, magnetization, thermopower, Hall coefficient, and thermal conductivity. Both single crystals and polycrystalline material were examined. The MIT is accompanied by no change in crystal symmetry and a change in unit-cell volume of less than 0.05%. The resistivity shows little temperature dependence above 226 K, but increases by 3 orders of magnitude as the sample is cooled to 4 K. The specific heat anomaly resembles a mean-field transition and shows no hysteresis or latent heat. Cd2Os2O7 orders magnetically at the MIT. The magnetization data are consistent with antiferromagnetic order, with a small parasitic ferromagnetic component. The Hall and Seebeck coefficients are consistent with a semiconducting gap opening at the Fermi energy at the MIT. We have also performed electronic structure calculations on Cd2Os2O7. These calculations indicate that Cd2Os2O7 is metallic, with a sharp peak in the density of states at the Fermi energy. We interpret the data in terms of a Slater transition. In this scenario, the MIT is produced by a doubling of the unit cell due to the establishment of antiferromagnetic order. A Slater transition -- unlike a Mott transition -- is predicted to be continuous, with a semiconducting energy gap opening much like a BCS gap as the material is cooled below TMIT
[en] Crystallographic data, electrical and thermal transport measurements, and heat-capacity data are reported for several compounds with the filled skutterudite structure: TlxCo4-yFeySb12 and TlxCo4Sb12-ySny, where 0<(x,y)<1 and x≅y. These materials have potential use for thermoelectric power generation. The Tl atomic displacement parameters (ADP's) are large relative to the other elements in the compounds indicating substantial ''rattling'' of the Tl about its equilibrium position. A simple analysis of the ADP's and low-temperature heat-capacity data indicate an Einstein temperature for the Tl of 53±2 K. The resonant scattering of acoustic phonons by the Tl ''rattlers'' are believed to be the major cause of the rapid decrease in the lattice thermal conductivity as small amounts of Tl are inserted into the voids of the skutterudite structure. Thermal and electrical transport are investigated as a function of the void filling with monovalent Tl and they are compared to previous data reported for partial filling with trivalent rare-earth ions. For comparable filling fractions, the Tl compounds had higher electron mobilities and a similar depression of the thermal conductivity. Unlike the rare-earth skutterudites, however, within experimental error there was no evidence of mass-fluctuation scattering and the minimum thermal conductivity occurred near complete void filling. At elevated temperatures (800 K), the maximum thermoelectric figure of merit, ZT, for n-type samples is estimated from room-temperature data to be about 0.8 for both Tl-filled and rare-earth-filled Co4Sb12. (c) 2000 The American Physical Society
[en] Application of pressure or electron doping through Co substitution into Fe sites transforms the itinerant antiferromagnet BaFe2As2 into a superconductor with Tc exceeding 20 K. We carried out systematic transport measurements of BaFe2-xCoxAs2 superconductors in pressures up to 2.5 GPa, and elucidated the interplay between the effects of electron doping and pressure. For the underdoped sample with nominal composition x = 0.08, application of pressure strongly suppresses a magnetic instability while enhancing Tc by nearly a factor of two from 11 to 21 K. In contrast, the optimally doped x = 0.20 sample shows very little enhancement of Tc = 22 K under applied pressure. Our results strongly suggest that the proximity to a magnetic instability is the key to the mechanism of superconductivity in iron-pnictides. (fast track communication)
[en] In this paper, we report the field and temperature dependence of low-temperature specific heat down to 400 mK and in magnetic fields up to 9 T of the electron-doped Ba(Fe0.92Co0.08)2As2 superconductor. Using the phonon specific heat obtained from pure BaFe2As2, we found a normal state Sommerfeld coefficient of 18 mJ mol-1 K-2 and a condensation energy of 1.27 J mol-1. The temperature dependence of electronic specific heat clearly indicates the presence of low-energy excitations in the system. The magnetic field variation of field-induced specific heat cannot be described by single clean s- or d-wave models. Rather, the data require an anisotropic gap scenario that may or may not have nodes. We discuss the implications of these results.
[en] Single crystals of the spin-gap system (VO)2P2O7 have been investigated by means of Raman scattering. At temperatures below T=75 K ≅2Δ01(k=0), with Δ01 the singlet-triplet gap, a shoulder appears at 47 cm-1. An analysis of the polarization selection rules and the temperature dependence of the scattering intensity leads to its identification as a singlet bound state with very small binding energy. Therefore in contrast to recent theoretical models the importance of frustration for this compound must be rejected. The observation of a strong anharmonicity of three phonons with energies close to Δ01 at the zone boundary and additional quasielastic light scattering, however, both point to a strong spin-phonon coupling in this system. We propose this coupling to be the origin of the second bound triplet state observed in recent neutron-scattering experiments. (c) 2000 The American Physical Society
[en] We report the temperature dependence of the low-temperature specific heat down to 400 mK of the electron-doped Ba(Fe0.92Co0.08)2As2 superconductors. We have measured two samples extracted from the same batch: first sample has been measured just after preparation with no additional heat treatment. The sample shows Tc=20 K, residual specific heat γ0=3.6 mJ/mol K2 and a Schottky-like contribution at low temperatures. A second sample has been annealed at 800 deg. C for two weeks and shows Tc=25 K and γ0=1.4 mJ/mol K2. By subtracting the lattice specific heat, from pure BaFe2As2, the temperature dependence of the electronic specific heat has been obtained and studied. For both samples the temperature dependence of Cel(T) clearly indicate the presence of low-energy excitations in the system. Their specific heat data cannot be described by single clean s- or d-wave models and the data requires an anisotropic gap scenario which may or may not have nodes.