Using high magnetic fields up to 60 T, we report magneto-transport and photoluminescence (PL) studies of a two-dimensional electron gas (2DEG) in a GaN/AlGaN heterojunction grown by molecular-beam epitaxy. Transport measurements demonstrate that the quantum limit can be exceeded (Landau level filling factor ν < 1) and show evidence for the ν = 2 / 3 fractional quantum Hall state. Simultaneous optical and transport measurements reveal synchronous quantum oscillations of both the PL intensity and the longitudinal resistivity in the integer quantum Hall regime.

We develop a phenomenological mean-field theory describing the hidden-order phase in URu2Si2 as a nematic of the B1g representation staggered along the c axis. Several experimental features are reproduced by this theory: The topology of the temperature-pressure phase diagram, the response of the elastic modulus (C11-C12)/2 above the transition at ambient pressure, and orthorhombic symmetry breaking in the high-pressure antiferromagnetic phase.

A variety of precise experiments have been carried out to establish the character of the superconducting state in Sr2RuO4. Many of these appear to imply contradictory conclusions concerning the symmetries of this state. Here we propose that these results can be reconciled if we assume that there is a near-degeneracy between a dx2−y2 (B1g in group theory nomenclature) and a gxy(x2−y2) (A2g) superconducting state.

High magnetic fields have revealed a surprisingly small Fermi surface in underdoped cuprates, possibly resulting from Fermi-surface reconstruction due to an order parameter that breaks translational symmetry of the crystal lattice. A crucial issue concerns the doping extent of such a state and its relationship to the principal pseudogap and superconducting phases. We employ pulsed magnetic-field measurements on the cuprate HgBa2CuO4+δ to identify signatures of Fermi-surface reconstruction from a sign change of the Hall effect and a peak in the temperature-dependent planar resistivity.

The unusual correlated state that emerges in URu2Si2 below THO = 17.5 K is known as “hidden order†because even basic characteristics of the order parameter, such as its dimensionality (whether it has one component or two), are “hidden.†We use resonant ultrasound spectroscopy to measure the symmetry-resolved elastic anomalies across THO. We observe no anomalies in the shear elastic moduli, providing strong thermodynamic evidence for a one-component order parameter.

The Acknowledgements section in this Article is incomplete. “The authors wish to thank A. Shekhter for useful discussions. B.J.R. acknowledges funding from LANL LDRD 20160616ECR ‘New States of Matter in Weyl Semimetals’, from the DOE-BES ‘Science of 100 Tesla’ program, and from the National Science Foundation under Grant No. 1752784. Work at Los Alamos National Laboratory was supported by the LDRD Program. T.M. is funded by Deutsche Forschungsgemeinschaft through GRK 1621, SFB 1143, and the Emmy-Noether program ME 4844/1. P.J.M.

Uniaxial pressure applied along a Ru-O-Ru bond direction induces an elliptical distortion of the largest Fermi surface of Sr2RuO4, eventually causing a Fermi surface topological transition, also known as a Lifshitz transition, into an open Fermi surface. There are various anomalies in low-temperature properties associated with this transition, including maxima in the superconducting critical temperature and in resistivity.

Layered heavy-metal square-lattice compounds have recently emerged as potential Dirac fermion materials due to bonding within those sublattices. We report quantum transport and spectroscopic data on the layered Sb square-lattice material LaCuSb2. Linearly dispersing band crossings, necessary to generate Dirac fermions, are experimentally observed in the electronic band structure observed using angle-resolved photoemission spectroscopy, along with a quasi-two-dimensional Fermi surface.

Although crystals of strongly correlated metals exhibit a diverse set of electronic ground states, few approaches exist for spatially modulating their properties. In this study, we demonstrate disorder-free control, on the micrometer scale, over the superconducting state in samples of the heavy-fermion superconductor CeIrIn5. We pattern crystals by focused ion beam milling to tailor the boundary conditions for the elastic deformation upon thermal contraction during cooling.

Even a small amplitude charge-density wave (CDW) can reconstruct a Fermi surface, giving rise to new quantum oscillation frequencies. Here, we investigate quantum oscillations when the CDW has a finite correlation length ζ - a case relevant to the hole-doped cuprates. By considering the Berry phase induced by a spatially varying CDW phase, we derive an effective Dingle factor that depends exponentially on the ratio of the cyclotron orbit radius, Rc, to ζ.