Nigel E. Hussey

Unconventional mass enhancement around the Dirac nodal loop in ZrSiS

The topological properties of fermions arise from their low-energy Dirac-like band dispersion and associated chirality. Initially confined to points, extensions of the Dirac dispersion to lines, and even loops, have now been uncovered, and semimetals hosting such features have been identified. However, experimental evidence for the enhanced correlation effects predicted to occur in these topological semimetals has been lacking. Here, we report a quantum oscillation study of the nodal-loop semimetal ZrSiS in high magnetic fields that reveals significant enhancement in the effective mass of the quasiparticles residing near the nodal loop. Above a threshold field, magnetic breakdown occurs across gaps in the loop structure with orbits that enclose different windings around its vertices, each winding accompanied by an additional π Berry phase. The amplitudes of these breakdown orbits exhibit an anomalous temperature dependence. These findings demonstrate the emergence of novel, correlation-driven physics in ZrSiS associated with the Dirac-like quasiparticles. A significant enhancement in the effective mass of Dirac-like quasiparticles residing near a nodal loop in the electronic band structure provides evidence for strong correlation effects in a topological semimetal.

Inverse correlation between quasiparticle mass and Tc in a cuprate high-Tc superconductor

Contrary to what is expected near a quantum critical point, pressure decreases the quasiparticle mass of a high-Tc superconductor. Close to a zero-temperature transition between ordered and disordered electronic phases, quantum fluctuations can lead to a strong enhancement of electron mass and to the emergence of competing phases such as superconductivity. A correlation between the existence of such a quantum phase transition and superconductivity is quite well established in some heavy fermion and iron-based superconductors, and there have been suggestions that high-temperature superconductivity in copper-oxide materials (cuprates) may also be driven by the same mechanism. Close to optimal doping, where the superconducting transition temperature Tc is maximal in cuprates, two different phases are known to compete with superconductivity: a poorly understood pseudogap phase and a charge-ordered phase. Recent experiments have shown a strong increase in quasiparticle mass m* in the cuprate YBa2Cu3O7-δ as optimal doping is approached, suggesting that quantum fluctuations of the charge-ordered phase may be responsible for the high-Tc superconductivity. We have tested the robustness of this correlation between m* and Tc by performing quantum oscillation studies on the stoichiometric compound YBa2Cu4O8 under hydrostatic pressure. In contrast to the results for YBa2Cu3O7-δ, we find that in YBa2Cu4O8, the mass decreases as Tc increases under pressure. This inverse correlation between m* and Tc suggests that quantum fluctuations of the charge order enhance m* but do not enhance Tc.

Linear Magnetoresistance in a Quasifree Two-Dimensional Electron Gas in an Ultrahigh Mobility GaAs Quantum Well

We report a high-field magnetotransport study of an ultrahigh mobility (μ[over ¯]≈25×10^{6}u2009u2009cm^{2}u2009V^{-1}u2009s^{-1}) n-type GaAs quantum well. We observe a strikingly large linear magnetoresistance (LMR) up to 33xa0T with a magnitude of order 10^{5}% onto which quantum oscillations become superimposed in the quantum Hall regime at low temperature. LMR is very often invoked as evidence for exotic quasiparticles in new materials such as the topological semimetals, though its origin remains controversial. The observation of such a LMR in the simplest system-with a free electronlike band structure and a nearly defect-free environment-excludes most of the possible exotic explanations for the appearance of a LMR and rather points to density fluctuations as the primary origin of the phenomenon. Both, the featureless LMR at high T and the quantum oscillations at low T follow the empirical resistance rule which states that the longitudinal conductance is directly related to the derivative of the transversal (Hall) conductance multiplied by the magnetic field and a constant factor α that remains unchanged over the entire temperature range. Only at low temperatures, small deviations from this resistance rule are observed beyond ν=1 that likely originate from a different transport mechanism for the composite fermions.

High-temperature quantum Hall effect in finite gapped HgTe quantum wells

We report on the observation of the quantum Hall effect at high temperatures in HgTe quantum wells with a finite band gap and a thickness below and above the critical thickness

Anomalous Magnetothermopower in a Metallic Frustrated Antiferromagnet

d_textnormal{c}

Full superconducting dome of strong Ising protection in gated monolayer WS2

that separates a conventional semiconductor from a two-dimensional topological insulator. At high carrier concentrations we observe a quantized Hall conductivity up to 60,K with energy gaps between Landau Levels of the order of 25,meV, in good agreement with the Landau Level spectrum obtained from

Construction and Performance of a 38-T Resistive Magnet at the Nijmegen High Field Magnet Laboratory

mathbf{kcdot p}

Simultaneous loss of interlayer coherence and long-range magnetism in quasi-two-dimensional PdCrO2

-calculations. Using the scaling approach for the plateau-plateau transition at

Fermi liquid behavior of the in-plane resistivity in the pseudogap state of YBa2Cu4O8

nu=2rightarrow 1

Progress in the Development of the HFML 45 T Hybrid Magnet

, we find the scaling coefficient