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 $\mathrm{carriers}/\mathrm{Fe}$, 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.

Lattice constant of Bi₂Se₃ and Sb₂Te₃ 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.

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.

Studies of the structure, magnetization, and resistivity under pressure on stoichiometric normal spinel $\mathrm{Co}[{\mathrm{V}}_2]{\mathrm{O}}_4$ single crystals show (i) absence of a structural distortion, (ii) abnormal magnetic critical exponents, and (iii) metallic conductivity induced by pressures at low temperatures. All these results prove that $\mathrm{Co}[{\mathrm{V}}_2]{\mathrm{O}}_4$ sits on the edge of the itinerant-electron limit. Compared with similar measurements on $\mathrm{Fe}[{\mathrm{V}}_2]{\mathrm{O}}_4$ and other $A[{\mathrm{V}}_2]{\mathrm{O}}_4$ studies, it is shown that a critical V-V separation for a localized-itinerant electronic phase transition exists.

Here, we report the discovery of superconductivity in a new transition metal-chalcogenide compound, i.e. Nb₂Pd_0.81S₅, with a transition temperature T_c ≅ 6.6 K. Despite its relatively low T_c, it displays remarkably high and anisotropic superconducting upper critical fields, e.g. μ₀H_c2 (T → 0 K) > 37 T for fields applied along the crystallographic b-axis. For a field applied perpendicularly to the b-axis, μ₀H_c2 shows a linear dependence in temperature which coupled to a temperature-dependent anisotropy of the upper critical fields, suggests that Nb₂Pd_0.81S₅ 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.

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 $P\sim 8$ kbar. We argue that this change can be attributed to a reconstruction of the Fermi surface by the antiferromagnetic order. The negative $d{T}_c/dP$ observed in a range between $P\sim 8$ 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: $k_B{T}_c/E_F\sim 0.1$ even at $P=16.1$ kbar.