We have observed Shubnikov-de Haas oscillations in FeSe. The Fermi surface deviates significantly from predictions of band-structure calculations and most likely consists of one electron and one hole thin cylinder. The carrier density is in the order of 0.01 carriers/ Fe, an order-of-magnitude smaller than predicted. Effective Fermi energies as small as 3.6 meV are estimated. These findings call for elaborate theoretical investigations incorporating both electronic correlations and orbital ordering.

Here, we report the discovery of superconductivity in a new transition metal-chalcogenide compound, i.e. Nb2Pd0.81S5, with a transition temperature Tc ≅ 6.6 K. Despite its relatively low Tc, it displays remarkably high and anisotropic superconducting upper critical fields, e.g. μ0Hc2 (T → 0 K) > 37 T for fields applied along the crystallographic b-axis. For a field applied perpendicularly to the b-axis, μ0Hc2 shows a linear dependence in temperature which coupled to a temperature-dependent anisotropy of the upper critical fields, suggests that Nb2Pd0.81S5 is a multi-band superconductor. This is consistent with band structure calculations which reveal nearly cylindrical and quasi-one-dimensional Fermi surface sheets having hole and electron character, respectively. The static spin susceptibility as calculated through the random phase approximation, reveals strong peaks suggesting proximity to a magnetic state and therefore the possibility of unconventional superconductivity.

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Lattice constant of Bi 2 Se 3 and Sb 2 Te 3 crystals is determined by X-ray powder diffraction measurement in a wide temperature range. Linear thermal expansion coefficients (α) of the crystals are extracted, and considerable anisotropy between α and α ⊥ is observed. The low temperature values of α can be fit well by the Debye model, while an anomalous behavior at above 150 K is evidenced and explained. Grüneisen parameters of the materials are also estimated at room temperature.Recently, much attention has been given to an intriguing class of materials, the so-called topological insulators (TIs). This type of material exhibits a band gap in the bulk, but gapless states on the edge or surface, which are protected by topological order and cannot be analogized to conventional semiconductors or insulators 1,2 . Bi 2 Se 3 , Bi 2 Te 3 and Sb 2 Te 3 are among the most interested compounds of three-dimensional TIs, owing to their robust and simple surface states 3 . Although these compounds were under extensive studies in 1950s and 1960s as excellent thermoelectric materials, some basic physical properties still remain unexplored. In this letter, we present the measurements of the temperature dependent linear thermal expansion coefficients of Bi 2 Se 3 and Sb 2 Te 3 crystals using X-ray powder diffraction (XRD). Thermal expansion is the tendency of materials to change in size and shape as they heat and cool. It is essential to device design and engineering, as the induced strain could cause the deformation of the device and affect its phonon dynamics. Indeed, our recent Raman spectroscopy study of TIs has uncovered significant contributions in the temperature dependent phonon frequency shifts from the thermal expansion of the materials 4 . In addition, the knowledge of thermal expansion coefficients is necessary for the directional growth of TI crystals and the understanding of the high thermoelectric efficiency 5 .Large grain polycrystalline Bi 2 Se 3 materials (single crystal grain size >1 mm) were synthesized at Sandia National Laboratories. First, Bi 2 Se 3 pieces (99.999%, from VWR international, LLC.) were placed in an evacuated (<10 −7 Torr) quartz ampoule and melted at 800 • C for 16 hours. The melt was then cooled at 10 • C/h to 550 • C, held for 3 days at this temperature, and finally allowed to furnace cool to room temperature. Single crystals of Sb 2 Te 3 were grown by Bridgman method at Purdue University. Stoichiometric amount of high purity antimony and tellurium (99.999%) was deoxidized and purified by multiple vacuum distillations under dynamic vacuum of <10 −7 Torr, and then heated up to 900 • C.This was followed by a slow cool down under a controlled pressure to minimize tellurium defects. Afterwards, the crystals were grown at a speed of 0.5-1.5 mm/h with a linear temperature gradient set to 5 • C/cm. Bi 2 Se 3 and Sb 2 Te 3 crystals have similar rhombohedral structure with five atoms in the trigonal primitive cell. A straightforward way to visualize the structure is to use a hexagonal lattice with th...

We report Shubnikov-de Haas (SdH) oscillation measurements on FeSe under high pressure up to P = 16.1 kbar. We find a sudden change in SdH oscillations at the onset of the pressure-induced antiferromagnetism at P ∼ 8 kbar. We argue that this change can be attributed to a reconstruction of the Fermi surface by the antiferromagnetic order. The negative dTc/dP observed in a range between P ∼ 8 and 12 kbar may be explained by the reduction in the density of states due to the reconstruction. The ratio of the transition temperature to the effective Fermi energy remains high under high pressure: kBTc/EF ∼ 0.1 even at P = 16.1 kbar.

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We report the influence of glassing solvent deuteration and Gd 3+ doping on 13 C dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) performed on [1-13 C] sodium acetate at B 0 ¼ 5 T and 1.2 K. Our data reveal that at 5 T, glassing solvent deuteration still results in a 40% improvement of the 13 C DNP signal when a large electron spin resonance (ESR) linewidth 4-oxo-TEMPO free radical is used, but results in a 60% decrease of the DNP signal in the case of a sample doped with small ESR linewidth trityl OX063. An addition of a trace amount of the Gd 3+ complex Gd-HP-DO3A led to a negligible slight decrease on the 13 C polarization TEMPO-doped sample, but is still relatively beneficial for the trityl-doped sample with 30% improvement of the DNP-enhanced 13 C polarization. These findings indicate that while these DNP optimization steps are still valid at 5 T, the effects are not as pronounced as observed in 13 C DNP at B 0 ¼ 3.35 T. These DNP results at 5 T are discussed thermodynamically within the framework of the thermal mixing model of DNP.

The magnetic field-induced changes in the conductivity of metals are the subject of intense interest, both for revealing new phenomena and as a valuable tool for determining their Fermi surface. Here we report a hitherto unobserved magnetoresistive effect in ultra-clean layered metals, namely a negative longitudinal magnetoresistance that is capable of overcoming their very pronounced orbital one. This effect is correlated with the interlayer coupling disappearing for fields applied along the so-called Yamaji angles where the interlayer coupling vanishes. Therefore, it is intrinsically associated with the Fermi points in the field-induced quasi-one-dimensional electronic dispersion, implying that it results from the axial anomaly among these Fermi points. In its original formulation, the anomaly is predicted to violate separate number conservation laws for left- and right-handed chiral (for example, Weyl) fermions. Its observation in PdCoO2, PtCoO2 and Sr2RuO4 suggests that the anomaly affects the transport of clean conductors, in particular near the quantum limit.