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Register a new userEnhanced thermopower via spin-state modification
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We investigated the effect of pressure on the magnetic and thermoelectric properties of Sr$_{3.1}$Y$_{0.9}$Co$_{4}$O$_{10+delta }$. The magnetization is reduced with the application of pressure, reflecting the spin-state modification of the Co$^{3+}$ ions into the nonmagnetic low-spin state. Accordingly, with increasing pressure, the Seebeck coefficient is enhanced, especially at low temperatures, at which the effect of pressure on the spin state becomes significant. These results indicate that the spin-orbital entropy is a key valuable for the thermoelectric properties of the strongly correlated cobalt oxides.
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High-temperature thermopower is interpreted as entropy that a carrier carries. Owing to spin and orbital degrees of freedom, a transition metal perovskite exhibits large thermopower at high temperatures. In this paper, we revisit the high-temperature thermopower in the perovskites to shed light on the degrees of freedom. Thus, we theoretically derive an expression of thermopower in one-dimensional octahedral-MX6-clusters chain using linear-response theory and electronic structure calculation of the chain based on the tight-binding approximation. The derived expression of the thermopower is consistent with the extended Heikes formula and well reproduced experimental data of several perovskite oxides at high temperatures. In this expression, a degeneracy of many electron states in octahedral ligand field (which is characterized by multiplet term) appears instead of the spin and orbital degeneracies. Complementarity in between our expression and the extended Heikes formula is discussed.
We report giant thermopower S = 2.5 mV/K in CoSbS single crystals, a material that shows strong high-temperature thermoelectric performance when doped with Ni or Se. Changes of low temperature thermopower induced by magnetic field point to mechanism of electronic diffusion of carriers in the heavy valence band. Intrinsic magnetic susceptibility is consistent with the Kondo- Insulator-like accumulation of electronic states around the gap edges. This suggests that giant thermopower stems from temperature-dependent renormalization of the non-interacting bands and buildup of the electronic correlations on cooling.
Graphene is the first model system of two-dimensional topological insulator (TI), also known as quantum spin Hall (QSH) insulator. The QSH effect in graphene, however, has eluded direct experimental detection because of its extremely small energy gap due to the weak spin-orbit coupling. Here we predict by ab initio calculations a giant (three orders of magnitude) proximity induced enhancement of the TI energy gap in the graphene layer that is sandwiched between thin slabs of Sb2Te3 (or MoTe2). This gap (1.5 meV) is accessible by existing experimental techniques, and it can be further enhanced by tuning the interlayer distance via compression. We reveal by a tight-binding study that the QSH state in graphene is driven by the Kane-Mele interaction in competition with Kekule deformation and symmetry breaking. The present work identifies a new family of graphene-based TIs with an observable and controllable bulk energy gap in the graphene layer, thus opening a new avenue for direct verification and exploration of the long-sought QSH effect in graphene.
We report on a resonant soft X-ray spectroscopy study of the electronic and magnetic structure of the cuprate-manganite interface. Polarized X-ray spectroscopy measurements taken at the Cu L edge reveal up to a five-fold increase in the dichroic signal as compared to past experimental and theoretical values. Furthermore an increase in the degree of interlayer charge transfer up to 0.25e (where e is charge of an electron) per copper ion is observed leading to a profound reconstruction in the orbital scheme for these interfacial copper ions. It is inferred that these enhancement are related to an increase in TMI observed for manganite layers grown with rapidly modulated flux.
The increasing worldwide energy consumption calls for the design of more efficient energy systems. Thermoelectrics could be used to convert waste heat back to useful electric energy if only more efficient materials were available. The ideal thermoelectric material combines high electrical conductivity and thermopower with low thermal conductivity. In this regard, the intermetallic type-I clathrates show promise with their exceedingly low lattice thermal conductivities [1]. Here we report the successful incorporation of cerium as guest atom into the clathrate crystal structure. In many simpler intermetallic compounds, this rare earth element is known to lead, via the Kondo interaction, to strong correlation phenomena including the ocurrence of giant thermopowers at low temperatures [2]. Indeed, we observe a 50% enhancement of the thermopower compared to a rare earth-free reference material. Importantly, this enhancement occurs at high temperatures and we suggest that a `rattling enhanced Kondo interaction [3] underlies this effect.
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