120 research outputs found
Thermoelectric effects in correlated quantum dots and molecules
We investigate thermoelectric properties of correlated quantum dots and
molecules, described by a single level Anderson model coupled to conduction
electron leads, by using Wilson's numerical renormalization group method. In
the Kondo regime, the thermopower, , exhibits two sign changes, at
temperatures and . We find that is of order
the level width and , where is the
position of the Kondo induced peak in the thermopower and is the Kondo
scale. No sign change is found outside the Kondo regime, or, for weak
correlations, making a sign change in a particularly sensitive signature
of strong correlations and Kondo physics. For molecules, we investigate the
effect of screening by conduction electrons on the thermoelectric transport. We
find that a large screening interaction enhances the figure of merit in the
Kondo and mixed valence regimes.Comment: 4 pages, 3 figures; to appear in the Proceedings of the International
Conference on Strongly Correlated Electron Systems, Santa Fe 2010; revised
version: typos corrected and references update
CeRuSn: heavy fermions emerging from a Kondo-insulating state
The combination of low-temperature specific-heat and
nuclear-magnetic-resonance (NMR) measurements reveals important information of
the ground-state properties of CeRuSn, which has been proposed as a
rare example of a tetragonal Kondo-insulator (KI). The NMR
spin-latticerelaxation rate deviates from the Korringa law below 100 K
signaling the onset of an energy gap K. This gap is
stable against magnetic fields up to 10 T. Below 10 K, however, unusual
low-energy excitations of in-gap states are observed, which depend strongly on
the field H. The specific heat C detects these excitations in the form of an
enhanced Sommerfeld coefficient : In zero field,
increases steeply below 5 K, reaching a maximum at 0.1 K, and then saturates at
J/molK. This maximum is shifted to higher temperatures with
increasing field suggesting a residual density of states at the Fermi level
developing a spin gap . A simple model, based on two narrow
quasiparticle bands located at the Fermi level - which cross the Fermi level in
zero field at 0.022 states/meV f.u. - can account qualitatively as well as
quantitatively for the measured observables. In particular, it is demonstrated
that fitting our data of both specific heat and NMR to the model, incorporating
a Ce magnetic moment of , leads to
the prediction of the field dependence of the gap. Our measurements rule out
the presence of a quantum critical point as the origin for the enhanced
in CeRuSn and suggest that this arises rather from correlated,
residual in-gap states at the Fermi level. This work provides a fundamental
route for future investigations into the phenomenon of narrow-gap formation in
the strongly correlated class of systemComment: 11 pages, 13 figure
Significance of Off-Center Rattling for Emerging Low-lying THz Modes in type-I Clathrates
We show that the distinct differences of low-lying THz-frequency dynamics
between type-I clathrates with on-center and off-center guest ions naturally
follow from a theoretical model taking into account essential features of the
dynamics of rattling guest ions. Our model analysis demonstrates the drastic
change from the conventional dynamics shown by on-center systems to the
peculiar dynamics of off-center systems in a unified manner. We claim that
glass-like plateau thermal conductivities observed for off-center systems stem
from the flattening of acoustic phonon dispersion in the regime |k|<|G|/4. The
mechanism is applicable to other systems such as glasses or relaxers
Quinone-mediated extracellular electron transfer processes in ex situ biomethanation reactors
Redox mediators are used in a wide diversity of systems including biological ones. We investigated the effect of adding an artificial quinone (2,7-AQDS) as external redox molecule to an anaerobic digester system dominated by hydrogenotrophic methanogens. When oxidized AQDS was present, the methanogens diverted electrons from H2 to reduce AQDS instead of CO2. The AQDS reduction process was accompanied by a temporary CH4 inhibition, which was re-established several days after the full reduction of AQDS to AH2QDS. The presence of AQDS furthermore resulted in a community shift from Methanobacterium as the dominant methanogen to a more diverse community of methanogens. Protein expression profiles showed a shift in cofactor preference of the adapted community, as a potential response mechanism to AQDS inhibition. AH2QDS was only used as electron donor to a limited extent. Stable isotope incorporation experiments here indicated that the acetogen Acetoanaerobium used AH2QDS to reduce CO2 into acetate.</p
Organic Redox Species in Aqueous Flow Batteries: Redox Potentials, Chemical Stability and Solubility
Organic molecules are currently investigated as redox species for aqueous low-cost redox flow batteries (RFBs). The envisioned features of using organic redox species are low cost and increased flexibility with respect to tailoring redox potential and solubility from molecular engineering of side groups on the organic redox-active species. In this paper 33, mainly quinone-based, compounds are studied experimentially in terms of pH dependent redox potential, solubility and stability, combined with single cell battery RFB tests on selected redox pairs. Data shows that both the solubility and redox potential are determined by the position of the side groups and only to a small extent by the number of side groups. Additionally, the chemical stability and possible degradation mechanisms leading to capacity loss over time are discussed. The main challenge for the development of all-organic RFBs is to identify a redox pair for the positive side with sufficiently high stability and redox potential that enables battery cell potentials above 1 V
Anomalous infrared spectra of hybridized phonons in type-I clathrate BaGaGe
The optical conductivity spectra of the rattling phonons in the clathrate
BaGaGe are investigated in detail by use of the terahertz
time-domain spectroscopy. The experiment has revealed that the lowest-lying
vibrational mode of a Ba(2) ion consists of a sharp Lorentzian peak at
1.2 THz superimposed on a broad tail weighted in the lower frequency regime
around 1.0 THz. With decreasing temperature, an unexpected linewidth broadening
of the phonon peak is observed, together with monotonic softening of the phonon
peak and the enhancement of the tail structure. These observed anomalies are
discussed in terms of impurity scattering effects on the hybridized phonon
system of rattling and acoustic phonons.Comment: Submitted to JPS
Structural and optical studies of FeSb2 under high pressure
Nanostructured orthorhombic FeSb2 and an amorphous phase were formed by
mechanical alloying starting from a mixture of high purity elemental Fe and Sb
powders. The effects of high pressures on structural and optical properties
were studied using X-ray diffraction (XRD) and Raman spectroscopy (RS). XRD
patterns showed the presence of the orthorhombic FeSb2 phase up to the maximum
pressure applied (28.2 GPa). The XRD patterns showed also an increase in the
amount of the amorphous phase with increasing pressure up to 23.3 GPa. At 14.3
GPa, together with the former phases, a new phase was observed and indexed to a
tetragonal FeSb2 phase, but its volume fraction is small at least up to 23.3
GPa. For the orthorhombic FeSb2 phase, the pressure dependence of the volume
fitted to a Birch-Murnaghan equation of state gave a bulk modulus = 74.2 +- 3.0
GPa and its pressure derivative = 7.5 +- 0.6. RS measurements were performed
from atmospheric pressure up to 45.2 GPa. For the orthorhombic FeSb2 phase, the
Raman active mode was observed up to the maximum pressure applied, while the
mode disappeared at 16.6 GPa. For pressures higher than 21 GPa, the Raman
active mode of a tetragonal FeSb2 phase was observed, confirming ab initio
calculations reported in the literature.Comment: 31 pages, 11 figures and 2 tables. Already submitted for publicatio
Thermopower of the Correlated Narrow Gap Semiconductor FeSi and Comparison to RuSi
Iron based narrow gap semiconductors such as FeSi, FeSb2, or FeGa3 have
received a lot of attention because they exhibit a large thermopower, as well
as striking similarities to heavy fermion Kondo insulators. Many proposals have
been advanced, however, lacking quantitative methodologies applied to this
problem, a consensus remained elusive to date. Here, we employ realistic
many-body calculations to elucidate the impact of electronic correlation
effects on FeSi. Our methodology accounts for all substantial anomalies
observed in FeSi: the metallization, the lack of conservation of spectral
weight in optical spectroscopy, and the Curie susceptibility. In particular we
find a very good agreement for the anomalous thermoelectric power. Validated by
this congruence with experiment, we further discuss a new physical picture of
the microscopic nature of the insulator-to-metal crossover. Indeed, we find the
suppression of the Seebeck coefficient to be driven by correlation induced
incoherence. Finally, we compare FeSi to its iso-structural and iso-electronic
homologue RuSi, and predict that partially substituted Fe(1-x)Ru(x)Si will
exhibit an increased thermopower at intermediate temperatures.Comment: 14 pages. Proceedings of the Hvar 2011 Workshop on 'New materials for
thermoelectric applications: theory and experiment
Thermal Conductivity of Methane-Hydrate
The thermal conductivity of the methane hydrate CH4 (5.75 H2O) was measured
in the interval 2-140 K using the steady-state technique. The thermal
conductivity corresponding to a homogeneous substance was calculated from the
measured effective thermal conductivity obtained in the experiment. The
temperature dependence of the thermal conductivity is typical for the thermal
conductivity of amorphous solids. It is shown that after separation of the
hydrate into ice and methane, at 240 K, the thermal conductivity of the ice
exhibits a dependence typical of heavily deformed fine-grain polycrystal. The
reason for the glass-like behavior in the thermal conductivity of clathrate
compounds has been discussed. The experimental results can be interpreted
within the phenomenological soft-potential model with two fitting parameters.Comment: 13 pages, 3 figure
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