Cd3As2 is a candidate three-dimensional Dirac semi-metal which has exceedingly high mobility and nonsaturating linear magnetoresistance that may be relevant for future practical applications. We report magnetotransport and tunnel diode oscillation measurements on Cd3As2, in magnetic fields up to 65 T and temperatures between 1.5 K to 300 K. We find the non-saturating linear magnetoresistance persist up to 65 T and it is likely caused by disorder effects as it scales with the high mobility, rather than directly linked to Fermi surface changes even when approaching the quantum limit. From the observed quantum oscillations, we determine the bulk three-dimensional Fermi surface having signatures of Dirac behaviour with non-trivial Berry's phase shift, very light effective quasiparticle masses and clear deviations from the band-structure predictions. In very high fields we also detect signatures of large Zeeman spin-splitting (g ∼ 16).

The nematic electronic state and its associated nematic critical fluctuations have emerged as potential candidates for superconducting pairing in various unconventional superconductors. However, in most materials their coexistence with other magnetically-ordered phases poses significant challenges in establishing their importance. Here, by combining chemical and hydrostatic physical pressure in FeSe0.89S0.11, we provide a unique access to a clean nematic quantum phase transition in the absence of a long-range magnetic order. We find that in the proximity of the nematic phase transition, there is an unusual non-Fermi liquid behavior in resistivity at high temperatures that evolves into a Fermi liquid behaviour at the lowest temperatures. From quantum oscillations in high magnetic fields, we trace the evolution of the Fermi surface and electronic correlations as a function of applied pressure. We detect experimentally a Lifshitz transition that separates two distinct superconducting regions: one emerging from the nematic electronic phase with a small Fermi surface and strong electronic correlations and the other one with a large Fermi surface and weak correlations that promotes nesting and stabilization of a magnetically-ordered phase at high pressures. The lack of mass divergence suggests that the nematic critical fluctuations are quenched by the strong coupling to the lattice. This establishes that superconductivity is not enhanced at the nematic quantum phase transition in the absence of magnetic order.

Shubnikov-de Haas (SdH) oscillations and upper critical magnetic field (Hc2) of the iron-based superconductor FeSe (Tc = 8.6 K) have been studied by tunnel diode oscillatorbased measurements in magnetic fields of up to 55 T and temperatures down to 1.6 K. Several Fourier components enter the SdH oscillations spectrum with frequencies definitely smaller than predicted by band structure calculations indicating band renormalization and reconstruction of the Fermi surface at low temperature, in line with previous ARPES data. The Werthamer-Helfand-Hohenberg model accounts for the temperature dependence of Hc2 for magnetic field applied both parallel (H ab) and perpendicular (H c) to the iron conducting plane, suggesting that one band mainly controls the superconducting properties in magnetic fields despite the multiband nature of the Fermi surface. Whereas Pauli pair breaking is negligible for H c, a Pauli paramagnetic contribution is evidenced for H ab with Maki parameter α = 2.1, corresponding to Pauli field HP = 36.5 T.

Analytical formulae for de Haas-van Alphen (dHvA) oscillations in linear chain of coupled two-dimensional (2D) orbits (Pippard's model) are derived systematically taking into account the chemical potential oscillations in magnetic field. Although corrective terms are observed, basic (α) and magnetic-breakdown-induced (β and 2β − α) orbits can be accounted for by the Lifshits-Kosevich (LK) and Falicov-Stachowiak semiclassical models in the explored field and temperature ranges. In contrast, the "forbidden orbit" β − α amplitude is described by a non-LK equation involving a product of two classical orbit amplitudes. Furthermore, strongly non-monotonic field and temperature dependence may be observed for the second harmonics of basic frequencies such as 2α and the magnetic breakdown orbit β + α, depending on the value of the spin damping factors. These features are in agreement with the dHvA oscillation spectra of the strongly 2D organic metal θ-(ET)4CoBr4(C6H4Cl2).

We present measurements of the resistivity ρx,x of URu2Si2 high-quality single crystals in pulsed high magnetic fields up to 81 T at a temperature of 1.4 K and up to 60 T at temperatures down to 100 mK. For a field H applied along the magnetic easy-axis c, a strong sample-dependence of the low-temperature resistivity in the hidden-order phase is attributed to a high carrier mobility. The interplay between the magnetic and orbital properties is emphasized by the angle-dependence of the phase diagram, where magnetic transition fields and crossover fields related to the Fermi surface properties follow a 1/cos θ-law, θ being the angle between H and c. For H c, a crossover defined at a kink of ρx,x, as initially reported in [Shishido et al., Phys. Rev. Lett. 102, 156403 (2009)], is found to be strongly sample-dependent: its characteristic field µ0H* varies from ≃ 20 T in our best sample with a residual resistivity ratio RRR = ρx,x(300K)/ρx,x(2K) of 225 to ≃ 25 T in a sample with a RRR of 90. A second crossover is defined at the maximum of ρx,x at the sample-independent characteristic field µ0H LT ρ,max ≃ 30 T. Fourier analyzes of Shubnikov-de Haas oscillations show that H LT ρ,max coincides with a sudden modification of the Fermi surface, while H * lies in a regime where the Fermi surface is smoothly modified. For H a, i) no phase transition is observed at low temperature and the system remains in the hidden-order phase up to 81 T, ii) quantum oscillations surviving up to 7 K are related to a new and almost-spherical orbit -for the first time observed here -at the frequency F λ ≃ 1400 T and associated with a low effective mass m * λ = (1 ± 0.5) · m0, where m0 is the free electron mass, and iii) no Fermi surface modification occurs up to 81 T.

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The H -T phase diagrams of single crystalline electron-doped K 0.83 Fe 1.83 Se 2 (KFS1), K 0.8 Fe 2 Se 2 (KFS2) and hole-doped Eu 0.5 K 0.5 Fe 2 As 2 (EKFA) have been deduced from tunnel diode oscillator-based contactless measurements in pulsed magnetic fields up to 57 T for the interplane (H c) and in-plane (H ab) directions. The temperature dependence of the upper critical magnetic field H c2 (T ) relevant to EFKA is accounted for by Pauli model including an anisotropic Pauli paramagnetic contribution (μ B H p = 114 T for H ab and 86 T for H c). This is also the case of KFS1 and KFS2 for H ab whereas a significant upward curvature, accounted for by a two-gap model, is observed for H c. Despite the presence of antiferromagnetic lattice order within the superconducting state of the studied compounds, no influence of magnetic ordering on the temperature dependence of H c2 (T ) is observed.

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