Frontier Condensed Matter Physics Seminar Series

1. Magnetic-field-controlled spin fluctuations and quantum critically in Sr3Ru2O7

Stephen Hayden (Bristol): August 24, 2021

ABSTRACT:  When the transition temperature of a continuous phase transition is tuned to absolute zero, new ordered phases and physical behaviour emerge in the vicinity of the resulting quantum critical point.
Sr3Ru2O7 can be tuned through quantum criticality with magnetic field at low temperature. Near its critical field B_c it displays the hallmark linear resistivity and a T \log(1/T) electronic heat capacity behaviour of strange metals. However, these behaviours have not been related to any critical fluctuations. Here we use inelastic neutron scattering to reveal the presence of collective spin fluctuations whose relaxation time and strength show a nearly singular variation with magnetic field as B_c is approached. The large increase in the electronic heat capacity and entropy near B_c can be understood quantitatively in terms of the scattering of conduction electrons by these spin-fluctuations.  On entering the spin density wave (SDW) phase present near B_c, the fluctuations become stronger suggesting that the SDW order is stabilised through an ``order-by-disorder'' mechanism.

2. Quantum triology in charge ordered kagome superconductors

Jia-Xin Yin (Princeton): August 31, 2021

ABSTRACT: The intertwining of lattice geometry, competing orders, and electronic topology is at the quantum frontier. Such a quantum triology has recently been discussed in the kagome superconductors AV3Sb5 (A = K, Cs, Rb). The van Hove singularities originating from the kagome lattice give rise to a high-temperature charge order, exhibiting dual electronic and magnetic anomalies. The charge order further interweaves with a tantalising unconventional superconducting phase. Both orders exhibit intricate connections with the underlying topological band structure, featuring Dirac fermions. This triology in kagome superconductors is inspiring manifold experimental and theoretical advances some of which exhibit common threads with the quantum anomalous Hall effect and the hidden phase in high temperature superconductors. I will discuss the status quo understanding of these instabilities that emerges from recent studies, with a particular focus on our scanning tunneling microscopy observation of the chiral charge order.

3. Spin-Triplet Pairing in Superfluids and Superconductors 

James A. Sauls (Northwestern): September 7, 2021

4. Triplet Superconductivity and Macroscopic Quantum states at Ultra-low Temperature

William Halperin (Northwestern University): September 14, 2021

5. Muon spin rotation insight into the topological kagomemagnets and superconductors

Zurab Guguchia (Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, Switzerland): September 21, 2021

ABSTRACT: The most intriguing properties of quantum materials are, arguably,
unconventional forms of magnetism as well as superconductivity on the one hand
and phenomena arising from topological electronic band structures on the other.
But even if topological materials rank among the most actively studied systems
in condensed matter physics over the past decade, materials in which the interplay
between band topology and magnetism/superconductivity can be studied
experimentally remain rare. One class of systems that is currently emerging as a
platform for such studies are so-called kagome magnets based on transition
metals. It has been gradually realized that materials which host such special lattice
structures can exhibit quantum diversity, ranging from spin-liquid phases,
topological matter to intertwined orders.

In this talk, I will make an introduction on a several novel topological kagome
metals and present our recent results on unconventional magnetic and
superconducting phases in these systems [1-5]. The systems include Weyl
semimetal Co3Sn2-xInxS2 (where the volume wise magnetic competition drives the
thermal and quantum evolution of the Berry curvature field, thus tuning its
topological state), quantum-limit Chern magnet TbMn6Sn6 (where formation of
quasi-static patches is coupled to the quantum-limit Chern gapped phase), and the
Z2 topological kagome metal KV3Sb5 (where time-reversal symmetry breaking
charge order intertwined with unconventional superconductivity is observed). I
will discuss these results from mostly the local-magnetic/superconducting probe
point of view such as muon-spin rotation.

[1] Z. Guguchia et. al., Nature Communications 11, 559 (2020).
[2] Z. Guguchia et. al., NPJ Quantum Materials 6, 50 (2021).
[3] C. Mielke III, ..., Z. Guguchia, Physical Review Materials 5, 034803 (2021).
[4] C. Mielke III, ..., Z. Guguchia, arXiv:2101.05763 (2021).
[5] C. Mielke III, ..., Z. Guguchia, arXiv:2106.13443 (2021).

6. Pseudospin-triplet superconductivity in CeRh2As2 and UTe2

Daniel Agterberg (University of Wisconsin-Milwaukee): September 28, 2021

Abstract: Recent measurements suggest that UTe2 is an odd parity Weyl superconductor [1] and that CeRh2As2 has field induced odd-parity superconductivity [2]. Here, after an overview of recent experimental developments, I will discuss how the underlying crystal structure links closely to superconductivity in both these materials. Specifically, I will argue that the local inversion symmetry breaking of the U and Ce atomic sites lead to a pseudospin Kramer’s band degeneracy that behaves differently from more usual spin ½ degeneracies. The consequences of this on odd-parity superconductivity in these materials will be highlighted.

[1] I.M. Hayes, D.S. Wei, T. Metz, J. Zhang, Y.S. Eo, S. Ran, S.R. Saha, J. Collini, N. P. Butch, D.F. Agterberg, A. Kapitulnik, and J. Paglione, Science 373, 797 (2021).

[2] S. Khim, J.F. Landaeta, J. Banda, N. Bannor, M. Brando, P.M.R. Brydon, D. Hafner, R. Kuchler, R. Cardoso-Gil, U. Stockert, A.P. Mackenzie, D.F. Agterberg, C. Geibel, and E. Hassinger, Science 373, 1012 (2021).

7. Kitaev Materials and Magnetic Excitations in a-RuCl3

Stephen Nagler (ORNL): October 5, 2021

ABSTRACT: This talk will briefly introduce the now famous Kitaev model on a honeycomb lattice, followed by a discussion of how the competing Ising interactions characteristic of the Kitaev Hamiltonian can be realized experimentally, and a succinct overview of known candidate materials. It will continue with a detailed case study of a-RuCl3 with emphasis on our group’s neutron scattering studies of this material, and a look at some of the more important evidence and open questions relating to the presence of Kitaev physics.

8. Multiple Superconducting Phases and Field-induced Phenomena in UTe2

Dai Aoki (Tohoku University): October 19, 2021

ABSTRACT: We present our recent results on heavy fermion paramagnet UTe2 as a candidate for novel spin-triplet superconductivity. The large specific heat jump at Tc=1.7 indicates a strong coupling superconductor. The huge upper critical field exceeding the Paul limit for all field directions suggests the spin-triplet state. One of the highlights in UTe2 is the field-reentrant superconductivity surviving up to the first-order metamagnetic transition field, 35T. The pressure also induces the exotic superconducting properties, revealing the split of Tc above ~0.3GPa and the multiple superconducting phases up to the critical pressure 1.5GPa. Finally, the magnetic order suppresses the superconducting phases at higher pressure. These unusual superconducting properties seem to be driven by complex fluctuations, such as antiferromagnetic, ferromagnetic, valence, and Fermi surface instabilities, with the unique crystal structure, which is discussed through the comparison to ferromagnetic superconductors, URhGe and UCoGe.

9. Pressure-induced high-temperature superconductivity retained at ambient pressure

Liangzi Deng (Univ of Houston): October 26, 2021

ABSTRACT: Pressure has played a crucial role in the development of material science due to the simplicity of varying the basic parameter of inter-atomic distance in a compound without introducing complications associated with altering its chemistry. Applying pressure to a material can tune its carrier concentration, shift its Fermi level, and even reshape its Fermi surface topology. In the pursuit of room-temperature superconductivity, all record superconducting transition temperatures (Tcs) reported since 1993 have been achieved under high pressure. While the record Tcs fall into practical cryogenic regimes for applications, the high pressure required to attain these superconducting states renders them impractical for significant applications and for comprehensive scientific inquiry. I will discuss recent breakthroughs in the design, synthesis, and kinetic stabilization of revolutionary materials with high Tcs and other novel properties, with a particular focus on understanding kinetic stabilization routes to retaining these properties under ambient pressure. Our recent work on FeSe demonstrates a possible path to retain lattice and/or electronic structures with desirable properties after removing the extreme conditions initially required to attain these properties.

10. Pressure-induced high-temperature superconductivity retained at ambient pressure

Topological materials and topological transport phenomena, Weizmann: Nov 23, 2021

ABSTRACT: Topological materials have been extensively studied in last decades. The Berry curvature usually characterizes the band structure topology and is known to induce intrinsic anomalous Hall effect (AHE). The topological Weyl semimetal shows a monopole-like Berry curvature field in band structure and thus exhibits giant AHE if the material is magnetic. Further, the topology can induce more exotic nonlinear electrical and optical phenomena. For example, Berry monopoles lead to a nonlinear version of AHE even in the presence of time-reversal symmetry and also a giant dc current in irritation of light.