33 research outputs found

    Electronic structure of CeRu4Sn6: a density functional plus dynamical mean field theory study

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    The Kondo system CeRu4_4Sn6_6 shows a strong anisotropy in its electric, optic and magnetic properties. We employ density functional theory plus dynamical mean field theory and show that the predominant Ce-ff state has total angular moment J=5/2J=5/2 and zz-component mJ=¬Ī1/2m_J=\pm 1/2 in agreement with recent X-ray absorption experiments. Even though CeRu4_4Sn6_6 has the direct gap of a Kondo insulator through most of the Brillouin zone it remains weakly metallic. This is because of (i) a band crossing in the zz-direction and (ii) a negative indirect gap.Comment: 6 pages, 9 figure

    Anisotropic optical conductivity of the putative Kondo insulator CeRu4_4Sn6_6

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    Kondo insulators and in particular their non-cubic representatives have remained poorly understood. Here we report on the development of an anisotropic energy pseudogap in the tetragonal compound CeRu4_4Sn6_6 employing optical reflectivity measurements in broad frequency and temperature ranges, and local density approximation plus dynamical mean field theory calculations. The calculations provide evidence for a Kondo insulator-like response within the a‚ąíaa-a plane and a more metallic response along the c axis and qualitatively reproduce the experimental observations, helping to identify their origin

    Effects of electronic correlations and disorder on the thermopower of NaxCoO2

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    For the thermoelectric properties of NaxCoO2, we analyze the effect of local Coulomb interaction and (disordered) potential differences for Co-sites with adjacent Na-ion or vacancy. The disorder potential alone increases the resistivity and reduces the thermopower, while the Coulomb interaction alone leads only to minor changes compared to the one-particle picture of the local density approximation. Only combined, these two terms give rise to a substantial increase of the thermopower: the number of (quasi-)electrons around the Fermi level is much more suppressed than that of the (quasi-)holes. Hence, there is a particle-hole imbalance acting in the same direction as a similar imbalance for the group velocities. Together, this interplay results in a large positive thermopower. Introducing a thermoelectric spectral density, we located the energies and momenta regions most relevant for the thermopower and changes thereof.Comment: 23 pages, 27 figures, accepted at PR

    Charge Kondo Effect in Thermoelectric Properties of Lead Telluride doped with Thallium Impurities

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    We investigate the thermoelectric properties of PbTe doped with a small concentration xx of Tl impurities acting as acceptors and described by Anderson impurities with negative on-site (effective) interaction. The resulting charge Kondo effect naturally accounts for a number of the low temperature anomalies in this system, including the unusual doping dependence of the carrier concentration, the Fermi level pinning and the self-compensation effect. The Kondo anomalies in the low temperature resistivity at temperatures T‚ȧ10‚ÄČKT\leq 10\, {\rm K} and the xx-dependence of the residual resistivity are also in good agreement with experiment. Our model also captures the qualitative aspects of the thermopower at higher temperatures T>300‚ÄČKT>300\, {\rm K} for high dopings (x>0.6x>0.6%) where transport is expected to be largely dominated by carriers in the heavy hole band of PbTe.Comment: chapter contributed to 'New Materials for Thermoelectric Applications: Theory and Experiment' Springer Series: NATO Science for Peace and Security Series - B: Physics and Biophysics, pp. 67-80, ed. Veljko Zlati\'c, and Alex Hewson (Editor). ISBN: 978-94-007-4983-2 (2013

    First-principles quantum transport modeling of thermoelectricity in single-molecule nanojunctions with graphene nanoribbon electrodes

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    We overview nonequilibrium Green function combined with density functional theory (NEGF-DFT) modeling of independent electron and phonon transport in nanojunctions with applications focused on a new class of thermoelectric devices where a single molecule is attached to two metallic zigzag graphene nanoribbons (ZGNRs) via highly transparent contacts. Such contacts make possible injection of evanescent wavefunctions from ZGNRs, so that their overlap within the molecular region generates a peak in the electronic transmission. Additionally, the spatial symmetry properties of the transverse propagating states in the ZGNR electrodes suppress hole-like contributions to the thermopower. Thus optimized thermopower, together with diminished phonon conductance through a ZGNR/molecule/ZGNR inhomogeneous structure, yields the thermoelectric figure of merit ZT~0.5 at room temperature and 0.5<ZT<2.5 below liquid nitrogen temperature. The reliance on evanescent mode transport and symmetry of propagating states in the electrodes makes the electronic-transport-determined power factor in this class of devices largely insensitive to the type of sufficiently short conjugated organic molecule, which we demonstrate by showing that both 18-annulene and C10 molecule sandwiched by the two ZGNR electrodes yield similar thermopower. Thus, one can search for molecules that will further reduce the phonon thermal conductance (in the denominator of ZT) while keeping the electronic power factor (in the nominator of ZT) optimized. We also show how often employed Brenner empirical interatomic potential for hydrocarbon systems fails to describe phonon transport in our single-molecule nanojunctions when contrasted with first-principles results obtained via NEGF-DFT methodology.Comment: 20 pages, 6 figures; mini-review article prepared for the special issue of the Journal of Computational Electronics on "Simulation of Thermal, Thermoelectric, and Electrothermal Phenomena in Nanostructures", edited by I. Knezevic and Z. Aksamij

    Ten millennia of hepatitis B virus evolution

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    Hepatitis B virus (HBV) has been infecting humans for millennia and remains a global health problem, but its past diversity and dispersal routes are largely unknown. We generated HBV genomic data from 137 Eurasians and Native Americans dated between similar to 10,500 and similar to 400 years ago. We date the most recent common ancestor of all HBV lineages to between similar to 20,000 and 12,000 years ago, with the virus present in European and South American hunter-gatherers during the early Holocene. After the European Neolithic transition, Mesolithic HBV strains were replaced by a lineage likely disseminated by early farmers that prevailed throughout western Eurasia for similar to 4000 years, declining around the end of the 2nd millennium BCE. The only remnant of this prehistoric HBV diversity is the rare genotype G, which appears to have reemerged during the HIV pandemic.Molecular Technology and Informatics for Personalised Medicine and Healt

    Ten millennia of hepatitis B virus evolution

    Get PDF
    Hepatitis B virus (HBV) has been infecting humans for millennia and remains a global health problem, but its past diversity and dispersal routes are largely unknown. We generated HBV genomic data from 137 Eurasians and Native Americans dated between ~10,500 and ~400 years ago. We date the most recent common ancestor of all HBV lineages to between ~20,000 and 12,000 years ago, with the virus present in European and South American hunter-gatherers during the early Holocene. After the European Neolithic transition, Mesolithic HBV strains were replaced by a lineage likely disseminated by early farmers that prevailed throughout western Eurasia for ~4000 years, declining around the end of the 2nd millennium BCE. The only remnant of this prehistoric HBV diversity is the rare genotype G, which appears to have reemerged during the HIV pandemic
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