## Summary

Our lab designs and builds unique experiments to probe the fundamental transport and thermodynamic properties of quantum materials—systems that exhibit non-trivial quantum phenomena. Current research topics include the identification of unique phases of matter in topological semimetals, uncovering broken symmetries in high-Tc superconductors using ultrasound, and probing topological superconductivity using the unique experimental technique of resonant ultrasound spectroscopy.

## Research Focus

Much of our work looks at the interactions of systems such as superconductors, correlated metals, spin liquids, and topological semimetals, with intense magnetic fields. High fields are a versatile tuning parameter that can be used to probe the electronic structure of materials, suppress or enhance competing orders such as superconductivity, or induce phase transitions to new states of matter. Our lab has a 20 tesla DC magnet system, a 35 tesla pulsed magnet system, and we use fields up to 100 Tesla available at user facilities around the world. Below are just a few examples of the projects we are working on, and the techniques we currently use and are developing.

##### New phases of matter in topological semimetals

Field-induced phases, such as the fractional quantum Hall effect, have long driven both theoretical and experimental advances in condensed matter. We recently discovered that the Weyl semimetal TaAs enters a gapped state above 80 Tesla at low temperatures. This provides the first opportunity to study correlated electron physics embedded in the topological bandstructure of a Weyl semimetal. We have developed a unique method for measuring the sound velocity of a material in magnetic fields up to 100 Tesla, and are using this technique to investigate whether this high-field phase breaks any symmetries, what those broken symmetries are, and to explore the dynamics of the phase transition.

##### Broken symmetry in high-temperature superconductors

The cuprate high-Tc superconductors are famous for their superconducting transition temperatures above 100 Kelvin, but are even more interesting from a fundamental physics standpoint because they can simultaneously exhibit superconductivity, magnetism, and charge order. This `intertwining’ of orders may be fundamental to the high-Tc superconductivity itself, and/or it may be a symptom of the strongly correlated metallic state out of which they evolve. We recently showed that these orders terminate near optimal doping in a quantum critical point [3], and that at least one of the broken symmetries—broken rotational symmetry—is fundamentally connected to the Fermi surface [2]. We are currently developing new methods for measuring Fermi surface geometry in extremely high magnetic fields, and will use this to determine how the Fermi surface evolves out of the `pseudogap’ phase—where many orders are intertwined—into the Fermi liquid side of the phase diagram where the physics is relatively well understood.

##### Ultrasonic signatures of broken symmetries and topological phases

One of the great challenges in experimental condensed matter physics is finding the right `signature’ to identify and study a new phase of matter. For example, spin liquids have a perfectly well-defined theoretical meaning, but are largely identified experimentally by a lack of magnetic order. Identifying something by what it’s not is problematic. Similarly, many phase transitions can be discovered by measuring resistance, but identifying a new phase of matter—such as determining if it breaks a symmetry, and what that symmetry is—is generally more involved. Ultrasound—the measurement of a material’s elastic moduli (or equivalently, sound speeds)—is a relatively under-utilized probe that is particularly well suited for the study of phase transitions. Elastic moduli contain a great deal of information, with the potential to identify the symmetry of an order parameter, and to explore the dynamics of a phase transition through ultrasonic attenuation. Our lab has been at the cutting edge of developing resonant ultrasound spectroscopy (RUS)—a unique tool for measuring all elastic moduli in a single experiment [5,8]. We are preparing experiments that use RUS to determine the presence of spin-triplet superconductivity in certain unconventional superconductors, to explore spin-liquid physics in candidate materials, and other exciting experimental possibilities.

#### Graduate Students

Kursti DeLello

Yawen Fang

Sayak Ghosh

Patrick Hollister

I am currently accepting new graduate students.

## Publications

[1] KA Modic, Tobias Meng, Filip Ronning, Eric D Bauer, Philip JW Moll, and B. J. Ramshaw. Thermodynamic Signatures of Weyl Fermions in NbP. Scientific Reports, 9(1): 2095, 2019

[2] B. J. Ramshaw, K.A. Modic, Arkady Shekter, Yi Zhang, Eun-Ah Kim, Philip J. W. Moll, M.K. Chan, J.B. Betts, F. Balakirev, A. Migliori, N.J. Ghimire, E.D. Bauer, F. Ronning, and R.D. McDonald. Quantum limit transport and destruction of the Weyl nodes in TaAs. Nature Communications, **9**, June 2018.

[3] B. J. Ramshaw, N Harrison, SE Sebastian, S Ghannadzadeh, KA Modic, DA Bonn, WN Hardy, Ruixing Liang, and PA Goddard. Broken Rotational Symmetry on the Fermi Surface of a High-Tc Superconductor. *NPJ Quantum Materials*,** 2**, 2017

[4] B. J. Ramshaw, S. E. Sebastian, R. D. McDonald, James Day, B. S. Tan, Z. Zhu, J. B. Betts, Ruixing Liang, D. A. Bonn, W. N. Hardy, and N. Harrison. Quasiparticle Mass Enhancement Approaching Optimal Doping in a High-Tc Superconductor. Science, **348**:317–320, 2015

[5] B. J. Ramshaw, Arkady Shekhter, Ross D. McDonald, Jon B. Betts, J. N. Mitchell, P. H. Tobash, C. H. Mielke, E. D. Bauer, and Albert Migliori. Avoided Valence Transition in a Plutonium Superconductor. Proceedings of the National Academy of Sciences, **112**(11):3285–3289, 2015

[6] Philip JW Moll, Andrew C Potter, Nityan L Nair, B. J. Ramshaw, KA Modic, Scott Riggs, Bin Zeng, Nirmal J Ghimire, Eric D Bauer, Robert Kealhofer, Filip Ronning, and James G. Analytis. Magnetic Torque Anomaly in the Quantum Limit of Weyl Semimetals. Nature Communications, **7**, 2016

[7] Akash V. Maharaj, Yi Zhang, B. J. Ramshaw, and S. A. Kivelson. Quantum Oscillations in a Bilayer With Broken Mirror Symmetry: A Minimal Model for YBa_{2}Cu_{3}O_{6+x}. Phys. Rev. B, **93**:094503, Mar 2016

[8] A. Shekhter, B. J. Ramshaw, R. D. McDonald, J. B. Betts, F. Balakirev, Ruixing Liang,W. N. Hardy, D. A. Bonn, Scott C. Riggs, and Albert Migliori. Bounding the Pseudogap With a Line of Phase Transitions in YBa_{2}Cu_{3}O_{6+x}. Nature, **498**(7452):75–77, 2013

[9] B. J. Ramshaw, Baptiste Vignolle, James Day, Ruixing Liang, W. N. Hardy, Cyril Proust, and D. A. Bonn. Angle Dependence of Quantum Oscillations in YBa_{2}Cu_{3}O_{6.59} Shows Free-Spin Behaviour of Quasiparticles. Nature Physics,** 7**(3):234–238, Mar 2011