다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

BoltzTraP. A code for calculating band-structure dependent quantities

日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

Physical Review B

Herein we cover the key concepts in the field of thermoelectric materials research, present the current understanding, and show the latest developments. Current research is aimed at increasing the thermoelectric figure of merit (ZT) by maximizing the power factor and/or minimizing the thermal conductivity. Attempts at maximizing the power factor include the development of new materials, optimization of existing materials by doping, and the exploration of nanoscale materials. The minimization of the thermal conductivity can come through solid-solution alloying, use of materials with intrinsically low thermal conductivity, and nanostructuring. Herein we describe the most promising bulk materials with emphasis on results from the last decade. Single-phase bulk materials are discussed in terms of chemistry, crystal structure, physical properties, and optimization of thermoelectric performance. The new opportunities for enhanced performance bulk nanostructured composite materials are examined and a look into the not so distant future is attempted.

New and Old Concepts in Thermoelectric Materials

The effect of pressure on the low-temperature states of the newly discovered clathrate Ba6Ge25 is investigated by means of measurements of the electrical resistivity. At ambient pressure, Ba6Ge25 undergoes a two-step structural phase transition between 230 and 180 K from metallic behaviour to a highresistivity state characterized by a mean free path of about 3 A. Interestingly, a Bardeen–Cooper–Schrieffer-like (BCS-like) superconducting transition occurs at TC ≈ 0.24 K fro mt he resulting ‘bad metal’. With increasing pressure, the structural phase transition is depressed but TC increases drastically. TC reaches am aximum value of 3.85 K at the critical pressure pC ≈ 2. 8G Pa, where the structural distortion is completely suppressed and the system exhibits metallic behaviour. Higher pressures lead to a slight decrease of TC.

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High-pressure studies on a new superconducting clathrate: Ba6Ge25

A ferromagnetic ground state was identified for the compounds CeCuGe (T c = 10 K), CeCuSi (T c = 15 K) [F. Yang, et al., J. Appl. Phys. 69 (1991) 4705] and CeAuGe (T c = 10K) [R. Pottgen, J. Magn. Magn. Mater. 152 (1996) 196]. The observed saturation magnetic moment values at low temperatures for all three compounds are considerably less than the theoretically expected value g J J = 2.14μ B for the free Ce 3+ ion involving the entire six-fold J = 5/2 multiplet, and thus provide a first indication of partial lifting of the f-electron level degeneracy in these compounds. Specific heat data yield crystal electric field (CEF) excitation energies (Δ Sch ) equivalent to 140 K for CeCuGe, 110 K for CeCuSi and 280 K for CeAuGe. To confirm the presence of CEF excitations directly, we have carried out inelastic neutron scattering (INS) measurements on all three compounds, using the HET spectrometer at ISIS Facility. Here, we present a detailed analysis of the INS spectra of CeCuSi on the basis of a CEF model and the detailed analysis of the INS of the other two compounds will be reported elsewhere.

Crystal electric field excitations in ferromagnetic CeTX compounds

The spin-density-wave scenario for quantum phase transitions in heavy fermion compounds has recently been challenged by a more radical unconventional scenario where the Kondo screening breaks down at the quantum critical point. Since the claim of Coleman and co-workers that Hall effect measurements are a promising tool to distinguish between these scenarios this technique has become increasingly popular among experimentalists. Experimental results on two representatives of these classes, Cr 1 - x V x and YbRh 2 Si 2 , are reviewed and put into perspective.

Hall effect for classification of quantum critical points

By substituting Ca for Eu in EuB 6 , a local-moment ferromagnet, the transition temperature of 16 K for the binary compound, is lowered substantially with increasing x . In spite of the divalency of both Eu and Ca, significant changes in the electronic properties are indicated by changes in the temperature and magnetic-field dependences of the electrical resistivity, particularly at low temperatures. For pure CaB 6 similar anomalies in the resistivity and the specific heat at very low temperatures as previously observed for SrB 6 , have been identified. Most of these properties most likely originate in the particular electronic structure of these hexaborides.

Electronic transport and thermal properties of CaB6 and Eu1−xCaxB6

Quantum criticality in certain heavy-fermion metals is believed to go beyond the Landau framework of order-parameter fluctuations. In particular, there is considerable evidence for Kondo destruction: a disappearance of the static Kondo singlet amplitude that results in a sudden reconstruction of Fermi surface across the quantum critical point and an extra critical energy scale. This effect can be analyzed in terms of a dynamical interplay between the Kondo and RKKY interactions. In the Kondo-destroyed phase, a well-defined Kondo resonance is lost, but Kondo singlet correlations remain at nonzero frequencies. This dynamical effect allows for mass enhancement in the Kondo-destroyed phase. Here, we elucidate the dynamical Kondo effect in Bose-Fermi Kondo/Anderson models, which unambiguously exhibit Kondo-destruction quantum critical points. We show that a simple physical quantity---the expectation value $\langle {\bf S}_{f} \cdot {\bf s}_{c} \rangle$ for the dot product of the local ($f$) and conduction-electron ($c$) spins---varies continuously across such quantum critical points. A nonzero $\langle {\bf S}_{f} \cdot {\bf s}_{c} \rangle$ manifests the dynamical Kondo effect that operates in the Kondo-destroyed phase. Implications are discussed for the stability of Kondo-destruction quantum criticality as well as the understanding of experimental results in quantum critical heavy-fermion metals.

Silke Paschen

Papers

High-pressure studies on a new superconducting clathrate: Ba6Ge25

Crystal electric field excitations in ferromagnetic CeTX compounds

Hall effect for classification of quantum critical points

Electronic transport and thermal properties of CaB6 and Eu1−xCaxB6

Dynamical Kondo effect and Kondo destruction in effective models for quantum-critical heavy fermion metals