[en] We present the upper critical fields μ₀H_c2(T) and Hall effect in β-FeSe single crystals. The μ₀H_c2(T) increases as the temperature is lowered for fields applied parallel and perpendicular to (101), the natural growth facet of the crystal. The μ₀H_c2(T) for both field directions and the anisotropy at low temperature increase under pressure. Hole carriers are dominant at high magnetic fields. However, the contribution of electron-type carriers is significant at low fields and low temperature. Our results show that multiband effects dominate μ₀H_c2(T) and electronic transport in the normal state.

[en] We present transport measurements on a tunable three-layer graphene single electron transistor (SET). The device consists of an etched three-layer graphene flake with two narrow constrictions separating the island from source and drain contacts. Three lateral graphene gates are used to electrostatically tune the device. An individual three-layer graphene constriction has been investigated separately showing a transport gap near the charge neutrality point. The graphene tunneling barriers show a strongly nonmonotonic coupling as a function of gate voltage indicating the presence of localized states in the constrictions. We show Coulomb oscillations and Coulomb diamond measurements proving the functionality of the graphene SET. A charging energy of ∼0.6 meV is extracted.

[en] We report high-pressure skin-depth measurements on the heavy fermion material CeIn₃ in magnetic fields up to 64 T using a self-resonant tank circuit based on a tunnel diode oscillator. At ambient pressure, an anomaly in the skin depth is seen at 45 T. The field where this anomaly occurs decreases with applied pressure until approximately 1.0 GPa, where it begins to increase before merging with the antiferromagnetic phase boundary. Possible origins for this transport anomaly are explored in terms of a Fermi surface reconstruction. The critical magnetic field at which the Neel-ordered phase is suppressed, is also mapped as a function of pressure and extrapolates to the previous ambient-pressure measurements at high magnetic fields and high-pressure measurements at zero magnetic field.

[en] Thermal expansion, or dilation, is closely related to the specific heat, and provides useful information regarding material properties. The accurate measurement of dilation in confined spaces coupled with other limiting experimental environments such as low temperatures and rapidly changing high magnetic fields requires a new sensitive millimeter size dilatometer that has little or no temperature and field dependence. We have designed an ultracompact dilatometer using an atomic force microscope piezoresistive cantilever as the sensing element and demonstrated its versatility by studying the charge density waves in alpha uranium to high magnetic fields (up to 31 T). The performance of this piezoresistive dilatometer was comparable to that of a titanium capacitive dilatometer.

[en] X-ray diffraction, magnetic susceptibility, magnetization, heat capacity and electrical resistivity results are reported for single crystals of two structural variants of EuNi_2−δSb₂ that crystallize in the CaBe₂Ge₂ and ThCr₂Si₂-type structures. While the former occurs with a stoichiometric ratio, the latter exhibits a Ni site vacancy (δ = 0.36). Both systems exhibit similar magnetic behavior at elevated temperatures, where there is an isotropic Curie–Weiss temperature dependence that indicates an antiferromagnetic exchange interaction between divalent europium ions, although it is stronger for the CaBe₂Ge₂-variant. At low temperatures, the differing structural environments that surround the Eu ions result in distinct ordering behavior. The CaBe₂Ge₂-variant orders antiferromagnetically near T _N1 = 6.9 K and then undergoes a first order phase transition at T _M = 4.6 K. The ThCr₂Si₂-variant exhibits simpler behavior, with antiferromagnetic ordering at T _N2 = 5.6 K. For both compounds, an applied magnetic field suppresses the ordering temperatures and induce metamagnetic phase transitions, while applied pressure causes the ordering temperatures to increase. From these results, EuNi_2−δSb₂ emerges as a useful system in which to study the impact of structural variation on magnetism in a Eu-based metal. (paper)

[en] We report magnetotransport properties of BaZnBi₂ single crystals. Whereas electronic structure features Dirac states, such states are removed from the Fermi level by spin-orbit coupling (SOC) and consequently electronic transport is dominated by the small hole and electron pockets. Our results are consistent with not only three-dimensional, but also with quasi-two-dimensional portions of the Fermi surface. The SOC-induced gap in Dirac states is much larger when compared to isostructural SrMnBi₂. This suggests that not only long-range magnetic order, but also mass of the alkaline-earth atoms A in ABX₂ (A = alkaline-earth, B = transition-metal, and X = Bi/Sb) are important for the presence of low-energy states obeying the relativistic Dirac equation at the Fermi surface.

[en] Topological materials which are also superconducting are of great current interest, since they may exhibit a non-trivial topologically-mediated superconducting phase. Although there have been many reports of pressure-tuned or chemical-doping-induced superconductivity in a variety of topological materials, there have been few examples of intrinsic, ambient pressure superconductivity in a topological system having a stoichiometric composition. Here, we report that the pure intermetallic CaSn₃ not only exhibits topological fermion properties, but also has a superconducting phase at ∼1.178 K under ambient pressure. The topological fermion properties, including the nearly zero quasi-particle mass and the non-trivial Berry phase accumulated in cyclotron motions, were revealed from the de Haas–van Alphen (dHvA) quantum oscillation studies of this material. Although CaSn₃ was previously reported to be superconducting with T _c  =  4.2 K, our studies show that the T _c  =  4.2 K superconductivity is extrinsic and caused by Sn on the degraded surface, whereas its intrinsic bulk superconducting transition occurs at 1.178 K. These findings make CaSn₃ a promising candidate for exploring new exotic states arising from the interplay between non-trivial band topology and superconductivity, e.g. topological superconductivity (TSC). (paper)

[en] Magnetic susceptibility, electrical resistivity, and heat capacity results are reported for the chemical substitution series URu₂Si_2−xP_x for $0<x\lesssim <!--\; ???\; -->0.5$. This study expands in detail on work recently reported in Gallagher et al (2016 Nat. Commun. 10712), which focused on the small x region of this substitution series. Measurements presented here reveal persistent hybridization between the f- and conduction electrons and strong variation of the low temperature behavior with increasing x. Hidden order and superconductivity are rapidly destroyed for $x\lesssim <!--\; ???\; -->0.03$ and are replaced for $0.03\lesssim <!--\; ???\; -->x\lesssim <!--\; ???\; -->0.26$ by a region with Kondo coherence but no ordered state. Antiferromagnetism abruptly appears for $x\gtrsim <!--\; ???\; -->0.27$. This phase diagram differs significantly from those produced by most other tuning strategies in URu₂Si₂, including applied pressure, high magnetic fields, and isoelectronic chemical substitution (i.e. Ru  →  Fe and Os), where hidden order and magnetism share a common phase boundary. Besides revealing an intriguing evolution of the low temperature states, this series provides a setting in which to investigate the influence of electronic tuning, where probes that are sensitive to the Fermi surface and the symmetry of the ordered states will be useful to unravel the anomalous behavior of URu₂Si₂. (paper)

[en] We report measurements of Shubnikov–de Haas oscillations in the giant Rashba semiconductor BiTeI under applied pressures up to ∼2 GPa. We observe one high frequency oscillation at all pressures and one low frequency oscillation that emerges between ∼0.3–0.7 GPa indicating the appearance of a second small Fermi surface. BiTeI has a conduction band bottom that is split into two sub-bands due to the strong Rashba coupling, resulting in a ‘Dirac point’. Our results suggest that the chemical potential starts below the Dirac point in the conduction band at ambient pressure and moves upward, crossing it as pressure is increased. The presence of the chemical potential above this Dirac point results in two Fermi surfaces. We present a simple model that captures this effect and can be used to understand the pressure dependence of our sample parameters. These extracted parameters are in quantitative agreement with first-principles calculations and other experiments. The parameters extracted via our model support the notion that pressure brings the system closer to the predicted topological quantum phase transition. (fast track communication)

[en] We have synthesized K_0.95(1)Ni_1.86(2)Se₂ single crystals. The single crystals contain K and Ni deficiencies not observed in KNi₂Se₂ polycrystals. Unlike KNi₂Se₂ polycrystals, the superconductivity is absent in single crystals. The detailed physical property study indicates that the K_0.95Ni_1.86Se₂ single crystals exhibit heavy-fermion-like characteristics. The transition to a heavy fermion state below T ∼ 30 K results in an enhancement of the electron-like carrier density whereas the magnetic susceptibility shows little anisotropy and suggests the presence of both itinerant and localized Ni orbitals. (paper)