arXiv subjects

Brad Ramshaw

Publications and source records attributed to Brad Ramshaw.

3 recordsLinked to original sources

arXiv2020

Elastic properties of hidden order in URu$_{\text2}$Si$_{\text2}$ are reproduced by a staggered nematic

We develop a phenomenological mean field theory describing the hidden order phase in URu$_{\text2}$Si$_{\text2}$ as a nematic of the $\text B_{\text{1g}}$ 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 $(C_{11}-C_{12})/2$ above the transition at ambient pressure, and orthorhombic symmetry breaking in the high-pressure antiferromagnetic phase. In this scenario, hidden order is characterized by broken rotational symmetry that is modulated along the $c$-axis, the primary order of the high-pressure phase is an unmodulated nematic, and the triple point joining those two phases with the high-temperature paramagnetic phase is a Lifshitz point.

cond-mat.str-el↗

arXiv2018

Topological Excitations of Hidden Order in URu$_2$Si$_2$ Under Extreme Electric Fields

Quantum materials are epitomized by the influence of collective modes upon their macroscopic properties. Relatively few examples exist, however, whereby coherence of the ground-state wavefunction directly contributes to the conductivity. Notable examples include the quantizing effects of high magnetic fields upon the 2D electron gas, the collective sliding of charge density waves subject to high electric fields, and perhaps most notably the macroscopic phase coherence that enables superconductors to carry dissipationless currents. Here we reveal that the low temperature hidden order state of URu$_2$Si$_2$ exhibits just such a connection between the quantum and macroscopic worlds -- under large voltage bias we observe non-linear contributions to the conductivity that are directly analogous to the manifestation of phase slips in one-dimensional superconductors [1], suggesting a complex order parameter for hidden order

cond-mat.str-el↗

arXiv2015

Magnetic torque anomaly in the quantum limit of the Weyl semi-metal NbAs

Electrons in materials with linear dispersion behave as massless Weyl- or Dirac-quasiparticles, and continue to intrigue physicists due to their close resemblance to elusive ultra-relativistic particles as well as their potential for future electronics. Yet the experimental signatures of Weyl-fermions are often subtle and indirect, in particular if they coexist with conventional, massive quasiparticles. Here we report a large anomaly in the magnetic torque of the Weyl semi-metal NbAs upon entering the "quantum limit" state in high magnetic fields, where topological corrections to the energy spectrum become dominant. The quantum limit torque displays a striking change in sign, signaling a reversal of the magnetic anisotropy that can be directly attributed to the topological properties of the Weyl semi-metal. Our results establish that anomalous quantum limit torque measurements provide a simple experimental method to identify Weyl- and Dirac- semi-metals.

cond-mat.mtrl-sci↗