Topological semimetals are systems in which conduction and valence bands cross each other and the crossings are protected by topological constraints. These materials provide intriguing tests for fundamental theories, while their unique physical properties promise a wide range of possible applications in low-power spintronics, optoelectronics, quantum computing and green energy harvesting. Here we report our study of the thermoelectric power of single-crystalline ZrSiS that is believed to be a topological nodal-line semimetal. We show that the thermoelectric power is an extremely sensitive probe of multiple quantum oscillations that are visible in ZrSiS at temperatures as high as 100 K. Two of these oscillations are shown to arise from three- and two-dimensional electronic bands, each with linear dispersion and the additional Berry phase predicted theoretically for materials with non-trivial topology. Our work not only provides further information on ZrSiS but also suggests a different route for studying other topological semimetals.

The properties of cuprate high-temperature superconductors are largely shaped by competing phases whose nature is often a mystery. Chiefly among them is the pseudogap phase, which sets in at a doping p* that is material-dependent. What determines p* is currently an open question. Here we show that the pseudogap cannot open on an electron-like Fermi surface, and can only exist below the doping p FS at which the large Fermi surface goes from hole-like to electron-like, so that p* ≤ p FS. We derive this result from high-magnetic-field transport measurements in La1.6−xNd0.4SrxCuO4 under pressure, which reveal a large and unexpected shift of p* with pressure, driven by a corresponding shift in p FS. This necessary condition for pseudogap formation, imposed by details of the Fermi surface, is a strong constraint for theories of the pseudogap phase. Our finding that p* can be tuned with a modest pressure opens a new route for experimental studies of the pseudogap.

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We report a quantum oscillation study of Tl 2 Ba 2 CuO 6+␦ for two different doping levels ͑T c = 10 and 26 K͒ and determine the Fermi-surface size and topology in considerable detail. Our results show that Fermi-liquid behavior is not confined to the edge of the superconducting dome and is robust up to at least 0.3T c max . Superconductivity is found to survive up to a larger doping p c = 0.31 than in La 2−x Sr x CuO 4 . Our data imply that electronic inhomogeneity does not play a significant role in the loss of superconductivity and superfluid density in overdoped cuprates, and point toward a purely magnetic or electronic pairing mechanism.

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The temperature dependences of the Hall-Lorenz numbers ͑L xy ͒ in a EuBa 2 Cu 3 O y ͑Eu-123͒ single crystal before and after oxygen reduction are reported. The study is based on data on the normal state longitudinal and transversal transport coefficients. Namely, the temperature dependences of the electrical resistivity, Hall coefficient, longitudinal thermal conductivity, and transverse thermal conductivity are presented. The set of measurements was performed for an optimally doped sample ͑y Ϸ 7͒, then the oxygen content in the the same crystal was reduced to y Ϸ 6.65, and the measurements were repeated. For both cases L xy 's are about two times larger than the Sommerfeld value of the Lorenz number and depend weakly on temperature in a range 300-160 K. Below T Ϸ 160 K the Hall-Lorenz number for the optimally doped sample slowly drops, while the value of L xy for the oxygen reduced sample begins to rise. Such results suggest that the electronic system in the investigated compound may be considered as a metallic one with pseudogaps that open at the Fermi level.