The recently discovered (Rb,Cs)EuFe 4 As 4 compounds exhibit an unusual combination of superconductivity (T c ∼ 35 K) and ferromagnetism (T m ∼ 15 K). We have performed a series of x-ray diffraction, ac magnetic susceptibility, dc magnetization, and electrical resistivity measurements on both RbEuFe 4 As 4 and CsEuFe 4 As 4 to pressures as high as ∼ 30 GPa. We find that the superconductivity onset is suppressed monotonically by pressure while the magnetic transition is enhanced at initial rates of dT m /dP ∼ 1.7 K/GPa and 1.5 K/GPa for RbEuFe 4 As 4 and CsEuFe 4 As 4 respectively. Near 7 GPa, T c onset and T m become comparable. At higher pressures, signatures of bulk superconductivity gradually disappear. Room temperature x-ray diffraction measurements suggest the onset of a transition from tetragonal (T) to a half collapsed-tetragonal (hcT) phase at ∼ 10 GPa (RbEuFe 4 As 4) and ∼ 12 GPa (CsEuFe 4 As 4). The ability to tune T c and T m into coincidence with relatively modest pressures highlights (Rb,Cs)EuFe 4 As 4 compounds as ideal systems to study the interplay of superconductivity and ferromagnetism.

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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.

ZrSiS has recently gained attention due to its unusual electronic properties: nearly perfect electron-hole compensation, large, anisotropic magneto-resistance, multiple Dirac nodes near the Fermi level, and an extremely large range of linear dispersion of up to ∼ 2 eV. We have carried out a series of high pressure electrical resistivity measurements on single crystals of ZrSiS. Shubnikov-de Haas measurements show two distinct oscillation frequencies. For the smaller orbit, we observe a change in the phase of ∼0.5, which occurs between 0.16 − 0.5 GPa. This change in phase is accompanied by an abrupt decrease of the cross-sectional area of this Fermi surface. We attribute this change in phase to a possible topological quantum phase transition. The phase of the larger orbit exhibits a Berry phase of π and remains roughly constant up to ∼2.3 GPa. Resistivity measurements to higher pressures show no evidence for pressure-induced superconductivity to at least ∼ 20 GPa.

At ambient pressure, BiTeI exhibits a giant Rashba splitting of the bulk electronic bands. At low pressures, BiTeI undergoes a transition from trivial insulator to topological insulator. At still higher pressures, two structural transitions are known to occur. We have carried out a series of electrical resistivity and AC magnetic susceptibility measurements on BiTeI at pressure up to  ∼40 GPa in an effort to characterize the properties of the high-pressure phases. A previous calculation found that the high-pressure orthorhombic P4/nmm structure BiTeI is a metal. We find that this structure is superconducting with T values as high as 6 K. AC magnetic susceptibility measurements support the bulk nature of the superconductivity. Using electronic structure and phonon calculations, we compute T and find that our data is consistent with phonon-mediated superconductivity.