Session F24: Probing Magnetic and Charge Degrees of Freedom in Spin-Triplet Superconductor UTe2

The curious case of the magnetic field-sensitive charge density waves in UTe2

In the ongoing search for new phases of matter, UTe₂ which combines strong correlations and unconventional superconductivity with possible non-trivial topology, is an extremely promising system. A plethora of surprising and confounding experimental phenomenology have been reported in this material, since its re-emergence in 2019. In this talk, I am going to present our discovery of a multicomponent, incommensurate charge density wave (CDW) order in UTe₂ using high resolution scanning tunneling microscopy and spectroscopy at 300 mK. The CDWs show an unexpected and unusual sensitivity to magnetic field, with the strength of the CDW order decreasing, and eventually disappearing close to the superconducting H_c2 indicating that the CDWs are strongly intertwined with superconductivity. In addition, the response of the CDWs is sensitive to the field orientation, similar to the behavior of superconductivity in this system. I will also discuss the possible implications and origin of such a CDW state and its connection to the underlying exotic superconducting state.    

Investigations of magnetism in pure and Th-doped UTe2 by μSR

The application of the muon spin relaxation/rotation (μSR) technique to condensed matter systems utilizes implanted positively charged spin-polarized muons as a local probe of static and dynamic magnetism. With our collaborators we have used μSR to investigate magnetism in the bulk of UTe₂ single crystals. At issue is the nature of the magnetic interactions and their role in the pairing mechanism that gives rise to what appears to be a rare topological spin-triplet superconducting state. I will first summarize our experimental results on pure UTe₂ that provide evidence for the occurrence of ubiquitous magnetic clusters, which gradually freeze upon cooling through the superconducting transition temperature (T_c) to form an inhomogeneous frozen state at low temperatures [1,2]. I will then present our recent μSR experimental results on Th-doped UTe₂ and measurements of pure UTe₂ in high magnetic field. Potential implications of our findings for understanding and establishing the intrinsic physical properties of UTe₂ will be discussed.   

Symmetry of magnetic correlations in spin-triplet superconductor UTe2

Uranium ditelluride UTe₂ is an unusual low-temperature superconductor that is a leading candidate for the realization of spin-triplet pairing and shows signs of multiple order parameters, time-reversal symmetry breaking, and in-gap chiral surface states. Moreover, UTe₂ also hosts several high-field reentrant superconducting phases. An outstanding question is what role magnetic interactions play in this system, especially with regard to the superconducting pairing.  In this talk, I will describe neutron scattering results and what they tell us about electron hybridization, magnetic interactions, and ramifications for superconductivity.    

de Haas-van Alphen effect and Fermi surfaces in spin-triplet superconductor UTe2

Unusual superconducting properties with the spin-triplet state of UTe2 are one of the hottest topics in condensed matter physics[1]. We present our recent studies on the de Haas-van Alphen (dHvA) effect in UTe2 using ultra-clean single crystals (RRR=200-800) [2]. Two kinds of cylindrical Fermi surfaces, which correspond to the main hole and electron Fermi surfaces, are clearly detected. The angular dependence of the dHvA frequencies is in very good agreement with that calculated by GGA+U (U=2eV). The detected cyclotron effective masses are very large, ranging from 32 to 57m0, revealing the strong mass enhancement from the band masses. The results of dHvA experiments are in less agreement with those of the LDA calculation for ThTe2 based on the 5f^2 localized model, while the bare LDA calculation with the 5f^3 itinerant model predicts a Kondo semiconducting behavior, suggesting that UTe2 has a mixed valence state.

[1] See review article, for example, D. Aoki, J. P. Brison, J. Flouquet, K. Ishida, G. Knebel, Y. Tokunaga, and Y. Yanase, J. Phys.: Condens. Matter, 34, 243002 (2022).

[2] D. Aoki, H. Sakai, P. Opletal, Y. Tokiwa, J. Ishizuka, Y. Yanase, H. Harima, A. Nakamura, D. Li, Y. Homma, Y. Shimizu, G. Knebel, J. Flouquet, and Y. Haga, J .Phys. Soc. Jpn., 91, 083704 (2022).   

Probing penetration depth and residual loss anisotropy of high-Tc superconducting UTe2

The recent discovery of unconventional superconductivity in UTe₂ has motivated careful studies of the electrodynamic properties of single crystals of this material [PRB 100, 220504 (2019); Nat. Comms 12:2644 (2021); arXiv:2105.13721].  The magnetic penetration depth shows a power-law temperature dependence at low temperatures in the superconducting state.  The microwave losses show a large residual value, and its origin may be due to surface states of potentially intrinsic character.  We extend the electrodynamics measurements to a new generation of UTe₂ crystals and extract the complex surface impedance Z_s and complex conductivity σ=σ₁-iσ₂ as a function of temperature from 100 mK to 20 K, over 5-12 GHz.  Recent studies [PRB 106, L060505, (2022)] indicate that the normal state and superconducting properties of UTe₂ are anisotropic.  We extend the cavity perturbation measurements to multiple resonant modes, each of which excites microwave currents in different crystallographic directions.  This allows extraction of the anisotropic electrodynamic properties as a function of temperature, including the anisotropic magnitude of the screening length.  We find that the penetration depth in new generation crystals (T_c=1.95 K) shows a robust power-law temperature dependence at low temperatures for currents in all directions, with exponents near 2, similar to the previous generation of UTe₂ crystals. Substantial residual resistance is seen in all UTe₂ crystals (old and new generation) and modes.  This is expected for nodal superconductors, but it appears to be stronger than expected for point nodes alone.  Our results are generally consistent with predictions of direction-independent power-law temperature dependence of the penetration depth in a generic topological superconductor having Weyl nodes and surface states [PRL 124, 067001 (2020)].  The results are also generally consistent with enhanced losses associated with the recently proposed topological anomalous skin effect [PRB 103, 104517 (2021)].  We will discuss the electrodynamic properties of the super- and normal-fluids in UTe₂ in light of this new data, and other measurements, as well as ongoing efforts to understand this unique material.