Session Q64: UTe2 and Actinide Heavy Fermion Materials

Growth conditions, stoichiometry and the superconducting properties of UTe2

Uranium ditelluride (UTe2) shows several hallmarks of spin-triplet superconductivity, including ultra-high critical fields beyond the Pauli limit and miniscule Knight shift through the superconducting transition. However, the superconducting state in UTe2 has, for the most part, not displayed the exquisite sensitivity to disorder that is expected in triplet superconductors. In this talk I will review the differences in superconducting properties grown under different conditions and our ongoing attempts to control them. I will then present data from electrical and thermal transport, along with heat capacity and elemental analysis, that provide multiple evaluations of the cleanliness of these different samples. Finally, I will discuss how these different measures of cleanliness correlate with the key superconducting properties of UTe2 and how these observations inform the emerging picture of superconductivity in this system.   

Triplet pairing via Kondo hybridization - Applications to UTe2 and CeRh2As2

We explore pairing mechanisms for triplet superconductivity mediated by Kondo interactions, relevant for UTe₂ and CeRh₂As₂. In these systems, the upper critical field violates the Pauli limit by an order of magnitude, and there are multiple distinct superconducting phases which appear to be enhanced and even enabled by strong magnetic fields. Taken together, the experimental puzzles raised by these systems challenge our current understanding of heavy fermion superconductivity and raise the intriguing possibility of a very new kind of superconductivity driven by the local moments of U and Ce. We seek to unify the Kondo physics with the triplet pairing in a coherent framework. With this motivation, we present a mean-field analysis of a simple two-channel Kondo model which has two spins at locally-noncentrosymmetric sites in the unit cell, inspired by UTe₂ and CeRh₂As₂. This illustrates the emergence of local triplet correlations as a result of Kondo hybridization with spin-orbit coupled local moments. In the spirit of a resonating valence bond picture, the superconductivity can be understood as the condensation of spin-triplet pre-formed pairs which are odd under exchange of the two U/Ce sites.   

Measuring the critical field anisotropy of UTe2

The candidate spin-triplet superconductor UTe₂ is of interest both for its fundamental electronic properties and its potential applications in quantum computing. Among the intriguing properties of UTe₂ is the strong variation of its upper critical field, depending on the direction of the applied field with relation to its crystal axes. In this talk we will share some measurements of the anisotropy of the upper critical field of UTe₂ and explain their relevance in the context of the overall phase diagram of UTe₂.   

Magnetocaloric effect measurements of the high field superconducting states in UTe2.

UTe₂ has attracted significant attention in the last two years due to the discovery of potential chiral spin-triplet superconductivity evidenced by the unusually high critical fields and the small temperature variation of the NMR Knight shift. Even more intriguing a high field induced superconducting state was proposed to exist in UTe₂ for field orientations between H||b and H||c emerging under ambient pressure between 40 to 60T. Here we present magnetocaloric measurements in pulsed magnetic fields up to 55T on new high quality UTe₂ single crystals and show thermodynamic evidence of the high and low field superconducting phases in UTe₂ as well as the field polarized state. The magnetocaloric measurements are supported by proximity detector oscillator (PDO) measurement which are sensitive to changes in the skin depth (electrical resistivity) and magnetic susceptibility – revealing a phase diagram in agreement with previous studies. The results are discussed with respect to vortex physics within the superconducting phases of UTe₂.   

Weyl nodes in the odd-parity superconductor UTe2

UTe2 has recently been found to be an example of a spin-triplet superconductor that exhibits evidence for chiral Majorana surface states. Here we present general symmetry and topology arguments relating to Weyl points that can account for these surface states. This will be followed by an analysis of the superconducting state of a DFT+U inspired model for UTe2. This model supports a B2u+iB3u superconducting pairing state that is consistent with polar Kerr measurements. We find this state has four Weyl point nodes and we numerically examine the resultant surface Fermi arc states. Theses states reveal unexpected properties that can be accounted for by our general symmetry arguments.   

Heat capacity of URu2-xOsxSi2 at low temperatures

We will present the results of the measurements of the heat capacity as a function of temperature on URu_2-xOs_xSi₂ alloys.  Our experimental results show that the critical temperature of the second-order phase transition increases while the value of the Sommerfeld coefficient in the ordered state decreases with 

an increase in osmium concentration. We also observe the increase in the values of the heat capacity at the critical temperature as well as a broadening

of the critical fluctuations region with an increase in x. We analyze the experimental data using the Haule-Kotliar model which, in particular, identifies the 

'hidden order' transition in the parent material URu₂Si₂ as a transition to a state with nonzero hexadecapole moment. We will demonstrate that our experimental 

results are consistent with the predictions of that model.   

Anisotropy of the T vs. H phase diagram and the HO-LMAFM boundary in URu2-xFexSi2

We explored the role of angle, θ, as a tuning parameter in URu_2-xFe_xSi₂, where θ is the angle between the magnetic field, H, and the c-axis. We first investigated the parent compound URu₂Si₂ by measuring electrical resistance, R, vs. H, at several selected θ, for temperatures T = 1.5, 10, and 13 K. We observed that R(H) scaled with the projection of the magnetic field onto the c-axis, and therefore the θ- and H-dependent behavior of R, including the so-called "hidden order" (HO) transition and the Fermi surface reconstruction in URu₂Si₂, at fixed T, demonstrate that R(θ, H) is actually dependent on H//c = H cosθ, consistent with a similar study reported previously. We continued this investigation on URu_2-xFe_xSi₂ to observe whether θ becomes a tuning parameter as Fe substitution increases and the system is driven toward large-moment antiferromagnetic order (LMAFM). Measurements on R(θ) at μ₀H = 20, 33, 40, and 45 T were conducted at 0.33 ≤ T ≤ 20 K and showed θ-dependent behavior in the various phase transitions of URu_2-xFe_xSi₂ (HO, LMAFM, SDW, Fermi surface, etc.). These phase transitions, as functions of T, H, and θ were plotted in a phase diagram of T vs. H//c for multiple x and reaffirmed that H//c, not θ, is a tuning parameter of URu_2-xFe_xSi₂ throughout all Fe concentrations.    

No symmetry breaking at critical point driven by electronic entropy in URu2Si2

URu₂Si₂ exhibits a novel type of order below T_o ≈ 17K, with an apparent (BCS)-like phase transition in C_p(T), yet three decades of intense research have yielded no definitive signatures of any associated symmetry-breaking. Noticeably, a number experiments have also shown an electronic energy gap which closely resembles that resulting from hybridization between conduction electron and 5f-electron states. We argue here, using thermodynamic measurements and minimalistic modeling, that the above observations can be jointly understood by way of proximity to an entropy-driven critical point, in which the latent heat of a valence-type electronic instability at T_o is quenched. Salient features of the putative valence-type transition include a robust energy gap displaying highly degenerate features in the electronic DOS that is only weakly suppressed by temperature on approaching T_o, an elliptical (T_o,H_c) phase boundary in magnetic field and temperature emerging from the consequences of a clear Pauli-PM limit in the (same) electrons responsible for all T_o, quantum oscillations, and superconductivity and, importantly, the lifting of the requirement of a broken symmetry in the ground state below T_o in URu₂Si₂. Implications for other unusual aspects of strongly correlated electron physics will be discussed.   

Giant spin-orbit interactions in the actinides

We investigate the effect of many-body interactions on the spin-orbit coupling of anisotropic metals. We use the Underscreened Anderson Lattice Model that was proposed to describe actinide compounds. The Coulomb interactions induce off-diagonal correlations that enhance the components of the spin-orbit coupling. Modest values of the Coulomb interaction U can significantly enhance the spin-orbit coupling and effect the electronic spectrum. The enhancements are most pronounced for systems that are either magnetic or on the verge of becoming magnetic. The enhancement is anisotropic in crystals with non-cubic symmetries and can lead to giant magnetic anisotropies in paramagnetic states.   

Hybridization effect on the X-ray absorption spectra for actinide materials

Studying the local moment and 5f-electron occupations sheds insight into the electronic behavior of actinide materials. X-ray absorption spectroscopy (XAS) has been a powerful tool to reveal the valence electronic structure when assisted with theoretical calculations. In this work, we employed the DFT+Gutzwiller rotationally-invariant slave boson method to obtain the local Hamiltonian of the single-impurity Anderson model (SIAM), and used exact diagonalization (ED) method to calculate the XAS spectra from the model. An in-house computational code was developed for the ED method. By applying this technique to the recently discovered 5f-electron topological Kondo insulator plutonium tetraboride (PuB₄), we were able to determine the signature of 5f-electronic correlation effects in the theoretical X-ray spectra. We found that the Pu 5f-6d hybridization effect provides an extra channel to mix the j=5/2 and 7/2 orbitals in the 5f valence. As a consequence, the resultant electron occupation number and spin-orbit coupling strength deviate from the intermediate coupling regime.

LA-UR-21-30320   

Magnetotransport studies of single crystalline URhSn

Uranium compounds have been a great deal of interest for exclusively showing unusual phases, e. g. field boosted superconductivity, hidden order, multipolar order are some of the features which have been realized prevalently in uranium compounds. URhSn is one of such material which is yet to catch an interest for its enigmatic successive double phase transitions; remarkably the order parameter for the higher temperature transition occurring at T_O = 54 K is yet to known, while below T_C = 18 K a ferromagnetic ground state is confirmed. URhSn crystallizes in a non-centrosymmetric hexagonal unit cell of ZrNiAl-type with potentially frustrated quasi-kagome structure formed by the magnetic U-atoms.  We have grown high quality (RRR = 41) single crystal of URhSn by Czochralski method which furnishes quantum oscillations up to dHvA frequencies of 1.5 kT. High field magnetization shows field reinforcement of the order parameter at T_O. Electrical transport under pressure presents a unique type of P-T phase diagram composed of a pair of bicritical points and likely a quantum phase transition at 6.25 GPa accompanied by a Fermi surface reconstruction. In this meeting electrical transport in magnetic fields of the title compound will be highlighted.   

Listening to Phase Transitions - Ultrasound Techniques in the Study of Heavy Fermion Systems.

One of the early triumphs of the BCS theory of superconductivity was that it successfully predicted the behavior of ultrasound attenuation in tin and indium. Since then ultrasound techniques have been used to study conventional and unconventional superconductivity, as well as a range of other phases.

In this talk I would like to present results from ultrasound attenuation and speed of sound measurements on U₆Fe under hydrostatic pressures of up to ~7 kBar. U₆Fe is on the light side of what are considered heavy fermion systems with an effective electronic mass of ~20m_e, and has, at ambient pressure, a superconducting (SC) as well as a charge density wave (CDW) transition at about 4K and 9K respectively. 

By tuning the pressure we are able to gradually suppress the CDW, revealing an interesting interplay between it and SC. Upon the SC transition a peak is seen in the ultrasound attenuation, similar to what has been observed in much heavier fermionic systems, but unexpected from the point of view of BCS theory. We wish to investigate what these results imply for the SC gap structure.