Biomedical subjects

B C Sales

Publications and source records attributed to B C Sales.

10 recordsLinked to original sources

PubMed2005

Observation of a new magnetic anomaly below the ferromagnetic Curie temperature in Yb14MnSb11.

Yb14MnSb11 is an unusual ferromagnet with a Curie temperature of 52 +/- 1 K. Recent optical, Hall, magnetic, and thermodynamic measurements indicate that Yb14MnSb11 may be a rare example of an underscreened Kondo lattice. We report the first experimental observation of a new magnetic anomaly in this system at around 47 K, a few degrees below T(c). Systematic investigations of the ac and dc susceptibilities of Yb14MnSb11 single crystals reveal features associated with possible spin reorientation at this temperature. This new anomaly is extremely sensitive to the applied measurement field and is absent in temperature-dependent dc magnetization data for fields above 50 Oe. The origin of this could be due to decoupling of two distinct magnetic sublattices associated with MnSb4 tetrahedra.

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PubMed2005

Fermi surface evolution and Luttinger theorem in Na(x)CoO2: a systematic photoemission study.

We report a systematic angle-resolved photoemission study on Na(x)CoO2 for a wide range of Na concentrations (0.3 < or = x < or = 0.72). In all the metallic samples at different x, we observed (i) only a single holelike Fermi surface centered around gamma and (ii) its area changes with x according to the Luttinger theorem. We also observed a surface state that exhibits a larger Fermi surface area. The e'(g) band and the associated small Fermi surface pockets near the K points predicted by band calculations are found to sink below the Fermi energy in a manner almost independent of the doping and temperature.

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PubMed2005

Goldstone-mode phonon dynamics in the pyrochlore Cd2Re2O7.

We have measured the polarized Raman scattering spectra of Cd2Re2O7, the first superconducting pyrochlore, as a function of temperature. For temperatures below the cubic-to-tetragonal structural phase transition (SPT) at 200 K, a peak with B1 symmetry develops at zero frequency with divergent intensity. We identify this peak as the first observation of the Goldstone phonon in a crystalline solid. The Goldstone phonon is a collective excitation that exists due to the breaking of the continuous symmetry with the SPT. Its emergence coincides with that of a Raman-active soft mode. The order parameter for both features derives from an unstable doubly degenerate vibration (with Eu symmetry) of the O1 atoms which drives the SPT.

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PubMed2005

Optical study of interactions in a d-electron Kondo lattice with ferromagnetism.

We report on a comprehensive optical, transport, and thermodynamic study of the Zintl compound Yb(14)MnSb(11), demonstrating that it is the first ferromagnetic Kondo lattice compound in the underscreened limit. We propose a scenario whereby the combination of Kondo and Jahn-Teller effects provides a consistent explanation of both transport and optical data.

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PubMed2004

ARPES on Na0.6CoO2: Fermi surface and unusual band dispersion.

The electronic structure of single crystals Na0.6CoO2, which are closely related to the superconducting Na0.3CoO2.yH(2)O (T(c) approximately 5 K), is studied by angle-resolved photoelectron spectroscopy. While the measured Fermi surface (FS) is consistent with the large FS enclosing the Gamma point from the band theory, the predicted small FS pockets near the K points are absent. In addition, the band dispersion is found to be highly renormalized, and anisotropic along the two principal axes (Gamma-K, Gamma-M). Our measurements also indicate that an extended flatband is formed slightly above E(F) along Gamma-K.

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PubMed2004

Four-well tunneling States and elastic response of clathrates.

We present resonant ultrasound elastic constant measurements of the Eu8Ga16Ge30 and Sr8Ga16Ge30 clathrates. The elastic response of the Eu clathrate provides clear evidence for the existence of a new type of four-well tunneling states, described by two nearly degenerate four level systems (FLS). The FLS's are closely linked with the fourfold split positions of Eu known from neutron diffraction density profiles. Using a realistic potential we estimate the tunneling frequencies and show that the energy gap between the two FLS's explains the observed harmonic oscillator type specific heat. In addition the quadrupolar interaction of FLS's with elastic strains explains the pronounced depression observed in elastic constant measurements. In the case of the Sr clathrate, we explain the elastic properties assuming the same type of interaction, but with Sr Einstein mode.

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PubMed2004

Metallic "ferroelectricity" in the pyrochlore Cd2Re2O7.

A class of materials known as "ferroelectric metals" was discussed theoretically by Anderson and Blount in 1965 [Phys. Rev. Lett. 14, 217 (1965)]], but to date no examples of this class of materials have been reported. Here we present measurements of the elastic moduli of Cd2Re2O7 through the 200 K cubic-to-tetragonal phase transition. A Landau analysis of the moduli reveals that the transition is consistent with Cd2Re2O7 being classified as a ferroelectric metal in the weaker sense described by Anderson and Blount (loss of a center of symmetry). First-principles calculations of the lattice instabilities indicate that the dominant lattice instability corresponds to a twofold degenerate mode with E(u) symmetry and that motions of the O ions forming the O octahedra dominate the energetics of the transition.

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PubMed2003

Observation of bulk superconductivity in NaxCoO2.yH(2)O and NaxCoO2.yD(2)O powder and single crystals.

Poly- and single-crystalline NaxCoO2 has been successfully intercalated with H2O and D2O as confirmed by x-ray diffraction and thermogravimetric analysis. Resistivity, magnetic susceptibility, and specific heat measurements show bulk superconductivity with T(c) close to 5 K in both cases. The substitution of deuterium for hydrogen has an effect on T(c) of less than 0.2 K. Investigation of the resistivity anisotropy of NaxCoO2.yH(2)O single crystals shows (a). almost zero resistivity below T(c), and (b). an abrupt upturn at T(*) approximately 52 K in both the ab plane and the c direction. The implications of our results on the possible superconducting mechanism will be discussed.

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PubMed2003

In-plane thermal conductivity of Nd2CuO4: evidence for magnon heat transport.

We report the temperature and magnetic field dependence of the in-plane thermal conductivity (kappa(ab)) of high-quality monocrystalline Nd2CuO4. Isothermal measurements of the field dependence of kappa(ab) at low temperatures (2 K<or=T<or=5 K) show no change in kappa(ab) below a critical magnetic field H(c) (H(c) approximately 4.5 T for H parallel [100]) and H(c) approximately 2.5 T for H approximately parallel [110]). Above H(c), kappa(ab) more than doubles as H is increased to 9 T. At H(c), there is a transition from a noncollinear to a collinear arrangement of the Nd and Cu spins and a collapse of the gap, Delta, in an acoustic magnon branch at k=0. Closure of this gap appears to allow the conduction of substantial amounts of heat by acoustic magnons.

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PubMed2002

Magnetic field enhancement of heat transport in the 2D Heisenberg antiferromagnet K2V3O8.

The thermal conductivity and heat capacity of single crystals of the spin 1/2 quasi-2D Heisenberg antiferromagnet K(2)V(3)O(8) have been measured from 1.9 to 300 K in magnetic fields from 0 to 8 T. The zero field thermal conductivity data are consistent with resonant scattering of phonons by magnons near the zone boundary. Application of a magnetic field greater than 1 T, however, produces a new magnetic ground state with substantial heat transport by long wavelength magnons.

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