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, $S(T)$, exhibits two sign changes, at temperatures $T=T_{1}$ and $T=T_{2}>T_{1}$. We find that $T_{2}$ is of order the level width $\Gamma$ and $T_{1}> T_{p}\approx T_{K}$, where $T_{p}$ is the position of the Kondo induced peak in the thermopower and $T_{K}$ is the Kondo scale. No sign change is found outside the Kondo regime, or, for weak correlations, making a sign change in $S(T)$ 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

Unbiased solar energy storage: Photoelectrochemical redox flow battery

Electricity from renewable energy sources is craving for efficient storage technologies, in particularsolar industry, to enable practical small-scale solutions for residential and offices use. The beststationary technology is probably the redox flow batteries. This article proposes the directconversion of sunlight into electrochemical energy stored in a redox flow battery. A photoelectrochemicalcell is used to charge a vanadium redox flow cell (CdS(s)|V3+, VO2+||V3+, V2+|CarbonFelt(s), E0=0.6 VNHE). A CdS thin film photoelectrode is prepared to directly charge the cell, pairsV3+/VO2+. CdS photoanode exhibits competitive photocurrents, when compared to other photoelectrochemicaldevices, and yields enough photovoltage to charge the vanadium battery up to 75%with no external bias. An overlayer of CdSe improves the performance of CdS with current densitiesup to ca. 1.4 mA cm2. Finally, a tandem configuration is tested using a dye-sensitized solar cell anda CdS photoanoade creating 1.3 V of photovoltage. This tandem arrangement proves to chargeconventional all vanadium redox cell (DSC/CdS(s)|VO2+, VO2+||V3+, V2+|Carbon Felt(s), E0=1.2 VNHE)without external bias

BoltzTraP. A code for calculating band-structure dependent quantities

A program for calculating the semi-classic transport coefficients is described. It is based on a smoothed Fourier interpolation of the bands. From this analytical representation we calculate the derivatives necessary for the transport distributions. The method is compared to earlier calculations, which in principle should be exact within Boltzmann theory, and a very convincing agreement is found

CeRu4_4Sn6_6: 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 CeRu$_4$Sn$_6$, which has been proposed as a rare example of a tetragonal Kondo-insulator (KI). The NMR spin-latticerelaxation rate $1/T_1$ deviates from the Korringa law below 100 K signaling the onset of an energy gap $\Delta E_g1/k_B \simeq 30$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 $\gamma = C(T)/T$ : In zero field, $\gamma$ increases steeply below 5 K, reaching a maximum at 0.1 K, and then saturates at $\gamma = 0.6$ J/molK$^2$. This maximum is shifted to higher temperatures with increasing field suggesting a residual density of states at the Fermi level developing a spin gap $\Delta E_g2$. 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 $\mu = \Delta E_g1/\mu_{0H} \simeq 1 \mu_B$, 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 $\gamma$ in CeRu$_4$Sn$_6$ 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

Unbiased, complete solar charging of a neutral flow battery by a single Si photocathode

Solar redox flow batteries have attracted attention as a possible integrated technology for simultaneous conversion and storage of solar energy. In this work, we review current efforts to design aqueous solar flow batteries in terms of battery electrolyte capacity, solar conversion efficiency and depth of solar charge. From a materials cost and design perspective, a simple, cost-efficient, aqueous solar redox flow battery will most likely incorporate only one semiconductor, and we demonstrate here a system where a single photocathode is accurately matched to the redox couples to allow for a complete solar charge. The single TiO(2) protected Si photocathode with a catalytic Pt layer can fully solar charge a neutral TEMPO-sulfate/ferricyanide battery with a cell voltage of 0.35 V. An unbiased solar conversion efficiency of 1.6% is obtained and this system represents a new strategy in solar RFBs where a single silicon photocathode is paired with energetically suitable redox couples to build an integrated solar energy conversion and storage device with full realization of the energy storage capacity

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 Ba8_8Ga16_{16}Ge30_{30}

The optical conductivity spectra of the rattling phonons in the clathrate Ba$_8$Ga$_{16}$Ge$_{30}$ 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)$^{2+}$ 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