The Seebeck coefficient of a metal is expected to display a linear temperature-dependence in the zero-temperature limit. To attain this regime, it is often necessary to cool the system well below 1K. We put under scrutiny the magnitude of this term in different families of strongly-interacting electronic systems. For a wide range of compounds (including heavy-fermion, organic and various oxide families) a remarkable correlation between this term and the electronic specific heat is found. We argue that a dimensionless ratio relating these two signatures of mass renormalisation contains interesting information about the ground state of each system. The absolute value of this ratio remains close to unity in a wide range of strongly-correlated electron systems.
The origin of superconductivity in bulk SrTiO3 is a mystery, since the nonmonotonous variation of the critical transition with carrier concentration defies the expectations of the crudest version of the BCS theory. Here, employing the Nernst effect, an extremely sensitive probe of tiny bulk Fermi surfaces, we show that down to concentrations as low as 5.5 × 10 17 cm −3 , the system has both a sharp Fermi surface and a superconducting ground state. The most dilute superconductor currently known has therefore a metallic normal state with a Fermi energy as little as 1.1 meV on top of a band gap as large as 3 eV. Occurrence of a superconducting instability in an extremely small, single-component and barely anisotropic Fermi surface implies strong constraints for the identification of the pairing mechanism.
We present a study of angle-resolved quantum oscillations of electric and thermoelectric transport coefficients in semi-metallic WTe2, which has the particularity of displaying a large B 2 magnetoresistance. The Fermi surface consists of two pairs of electron-like and hole-like pockets of equal volumes in a "Russian doll" structure. Carrier density, Fermi energy, mobility and the mean-freepath of the system are quantified. An additional frequency is observed above a threshold field and attributed to magnetic breakdown across two orbits. In contrast to all other dilute metals, the Nernst signal remains linear in magnetic field even in the high-field (ωcτ ≫ 1) regime. Surprisingly, none of the pockets extend across the c-axis of the first Brillouin zone, making the system a three-dimensional metal with moderate anisotropy in Fermi velocity yet a large anisotropy in mean-free-path. 2 ) was reported with no sign of saturation up to 60 T[6]. This is the expected behavior of a perfectly-compensated semi-metal [7], but has not been seen in bismuth [8] or graphite [9], two compensated semi-metals whose Fermi surface is accurately known. A first step towards uncovering the ultimate reason behind the quadratic magnetoresistance of WTe 2 is a quantitative determination of the structure of the Fermi surface and the components of the mobility tensor.In this letter, we report on a study of quantum oscillations of resistivity, Seebeck and Nernst coefficients in high-quality single crystals of WTe 2 and find that the Fermi surface consists of two pairs of electron-like and hole-like pockets. Each pair is concentric with identical structure like a set of Russian dolls. The anisotropy is much smaller than one would naively expect in a layered system. The longer axis of the pockets is much shorter than the height of the Brillouin zone, in contrast to the theoretical expectations. Moreover, we find another distinctive feature of this semi-metal in addition to quadratic magnetoresistance, which is a Nernst response linear in magnetic field deep inside the high-field limit. Our results quantify carrier concentration of the system and set plausible quantitative windows for mobilities and Fermi energies leading to the huge quadratic magnetoresistance and large field-linear Nernst signal.
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