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DOE OSTI2024

Dipolar Spin Ice Regime Proximate to an All-In-All-Out Néel Ground State in the Dipolar-Octupolar Pyrochlore Ce 2 Sn 2 O 7

The dipolar-octupolar (DO) pyrochlores, R 2 M 2 O 7 ( R = Ce , Sm , Nd ), are key players in the search for realizable novel quantum spin liquid (QSL) states as a large parameter space within the DO pyrochlore phase diagram is theorized to host QSL states of both dipolar and octupolar nature. New single crystals and powders of Ce 2 Sn 2 O 7 , synthesized by hydrothermal techniques, present an opportunity for a new characterization of the exchange parameters in Ce 2 Sn 2 O 7 using the near-neighbor X Y Z model Hamiltonian associated with DO pyrochlores. Utilizing quantum numerical linked cluster expansion fits to heat capacity and magnetic susceptibility measurements, and classical Monte Carlo calculations to the diffuse neutron diffraction of the new hydrothermally grown Ce 2 Sn 2 O 7 samples, we place Ce 2 Sn 2 O 7 ’s ground state within the ordered dipolar all-in-all-out (AIAO) Néel phase, with quantum Monte Carlo calculations showing a transition to long-range order at temperatures below those accessed experimentally. Indeed, our new neutron diffraction measurements on the hydrothermally grown Ce 2 Sn 2 O 7 powders show a broad signal at low scattering wave vectors, reminiscent of a spin ice, in striking contrast from previous powder neutron diffraction on samples grown from solid-state synthesis, which found diffuse scattering at high scattering wave vectors associated with magnetic and suggested an octupolar quantum spin ice state. We conclude that new hydrothermally grown Ce 2 Sn 2 O 7 samples host a finite-temperature proximate dipolar spin ice phase, above the expected transition to AIAO Néel order. Published by the American Physical Society 2024

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Phase diagram of superconductivity in the integer quantum Hall regime

An interplay between pairing and topological orders has been predicted to give rise to superconducting states supporting exotic emergent particles, such as Majorana particles obeying non-Abelian braid statistics. We consider a system of spin polarized electrons on a Hofstadter lattice with nearest-neighbor attractive interaction and solve the mean-field Bogoliubov–de Gennes equations in a self-consistent fashion, leading to gauge-invariant observables and a rich phase diagram as a function of the chemical potential, the magnetic field, and the interaction. As the strength of the attractive interaction is increased, the system first makes a transition from a quantum Hall phase to a skyrmion lattice phase that is fully gapped in the bulk but has topological chiral edge current, characterizing a topologically nontrivial state. This is followed by a vortex phase in which the vortices carrying Majorana modes form a lattice; the spectrum contains a low-energy Majorana band arising from the coupling between neighboring vortex-core Majorana modes but does not have chiral edge currents. For some parameters, a dimer vortex lattice occurs with no Majorana band. The experimental feasibility and the observable consequences of skyrmions as well as Majorana modes are indicated.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Charge disproportionation in the spin-liquid candidate κ – ( ET ) 2 Cu 2 ( CN ) 3 at 6 K revealed by Cu 63 NQR measurements

The spin-liquid candidate κ – ( ET ) 2 Cu 2 ( CN ) 3 [ET: bis(ethylenedithio)tetrathiafulvalene] does not exhibit magnetic ordering down to a very low temperature, but shows a mysterious anomaly at 6 K. The origin of the so-called 6-K anomaly is still under debate. We carried out nuclear quadrupole resonance (NQR) measurements on the copper sites of the insulating layers, which are sensitive to the charge dynamics unlike conventional spin- 1 / 2 nuclear magnetic resonance (NMR). The main finding of this Rapid Communication is that the observation of a sharp peak behavior in the nuclear spin-lattice relaxation rate T 1 – 1 of Cu 63 NQR at 6 K while T 1 – 1 of both C 13 and H 1 NMR show no clear anomaly. This behavior can be understood as a second-order phase transition related to charge disproportionation in the ET layers.

36 MATERIALS SCIENCE↗
DOE OSTI2020

$${\mathcal{P}}{\mathcal{T}}$$-Symmetric Effective Model for Nonequilibrium Phase Transitions in a Dissipative Fermionic Mott Insulator Chain

Nonequilibrium phase transitions in open dissipative systems can be described as instabilities in the spectra and wavefunctions of effective non-Hermitian Hamiltonians invariant under simultaneous parity (P) and time-reversal (T) transformations. The degree of non-Hermiticity reflects the strength of the external drive and dissipation, and the transition is described as a loss of the PT symmetry of the solutions corresponding to stationary low-drive dynamics. This approach has been successfully applied to spin, superconducting, and Mott insulator systems. However, the microscopic foundations for the employed phenomenological models are currently lacking. Here we propose a microscopic mechanism leading to the PT-symmetric effective model in the context of the nonequilibrium Mott transition in a dissipative Hubbard chain. Our model comprises a half-filled fermionic Hubbard chain subject to a constant electric field. The dissipation is introduced via the electron-phonon coupling. We obtain the explicit expressions for the non-Hermitian parameter in terms of the electron-phonon coupling strength and driving field. Analyzing the implications of microscopic model, we find a re-entrant Mott insulator with the increasing electric field for phonon density of states that increases slower than the square of the energy (such as in one or two dimensions), or varies non-monotonously with energy.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Inhomogeneous time-reversal symmetry breaking in Sr 2 RuO 4

We show that the observed time-reversal symmetry breaking (TRSB) of the superconducting state in Sr 2 RuO 4 can be understood as originating from inhomogeneous strain fields near edge dislocations of the crystal. Specifically, we argue that, without strain inhomogeneities, Sr 2 RuO 4 is a single-component, time-reversal symmetric superconductor, likely with d x 2 -y 2 symmetry. However, due to the strong strain inhomogeneities generated by dislocations, a slowly decaying subleading pairing state contributes to the condensate in significant portions of the sample. As it phase winds around the dislocation, time-reversal symmetry is locally broken. Global phase locking and TRSB occur at a sharp Ising transition that is not accompanied by a change of the single-particle gap and yields a very small heat capacity anomaly. Our model thus explains the puzzling absence of a measurable heat capacity anomaly at the TRSB transition in strained samples and the dilute nature of the time-reversal symmetry broken state probed by muon spin rotation experiments. We propose that plastic deformations of the material may be used to manipulate the onset of broken time-reversal symmetry.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Charge density waves on a half-filled decorated honeycomb lattice

Tight binding models like the Hubbard Hamiltonian are most often explored in the context of uniform intersite hopping $\textit{t}$. The electron-electron interactions, if sufficiently large compared to this translationally invariant $\textit{t}$, can give rise to ordered magnetic phases and Mott insulator transitions, especially at commensurate filling. The more complex situation of nonuniform $\textit{t}$ has been studied within a number of situations, perhaps most prominently in multiband geometries where there is a natural distinction of hopping between orbitals of different degree of overlap. In this paper we explore related questions arising from the interplay of multiple kinetic energy scales and electron-phonon interactions. Specifically, we use determinant quantum Monte Carlo (DQMC) to solve the half-filled Holstein Hamiltonian on a “decorated honeycomb lattice,” consisting of hexagons with internal hopping $\textit{t}$ coupled together by $\textit{t'}$. This modulation of the hopping introduces a gap in the Dirac spectrum and affects the nature of the topological phases. Here, we determine the range of $\textit{t/t'}$ values which support a charge density wave phase about the Dirac point of uniform hopping $\textit{t = t'}$, as well as the critical transition temperature $T_c$. The QMC simulations are compared with the results of mean field theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Total absorption spectroscopy of the β decay of 101,102 Zr and 109 Tc

The β decay of 101,102 Zr and 109 Tc was studied using the technique of total absorption spectroscopy. The experiment was performed at the National Superconducting Cyclotron Laboratory using the Summing NaI(Tl) (SuN) detector in the first-ever application of total absorption spectroscopy with a fast beam produced via projectile fragmentation. The β -decay feeding intensity and Gamow-Teller transition strength distributions were extracted for these three decays. The extracted distributions were compared to three different quasiparticle random-phase approximation (QRPA) models based on different mean-field potentials. A comparison with calculations from one of the QRPA models was performed to learn about the ground-state shape of the parent nucleus. For 101 Zr and 102 Zr, calculations assuming a pure shape configuration (oblate or prolate) were not able to reproduce the extracted distributions. These results may indicate that some type of mixture between oblate and prolate shapes is necessary to reproduce the extracted distributions. For 109 Tc, a comparison of the extracted distributions with QRPA calculations suggests a dominant oblate configuration. The other two QRPA models are commonly used to provide β-decay properties in r-process network calculations. This work shows the importance of making comparisons between the experimental and theoretical β-decay distributions, rather than just half-lives and β-delayed neutron emission probabilities, as close to the r-process path as possible.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Clean 2D superconductivity in a bulk van der Waals superlattice

Alternative route to a 2D superconductor Single layers of transition metal dichalcogenides exhibit exotic properties, including superconductivity. The usual route to obtaining such samples is to exfoliate a three-dimensional (3D) crystal. Devarakonda et al. instead grew a superlattice comprising alternating layers of the transition metal dichalcogenide hexagonal NbS 2 and the material Ba 3 NbS 5 (see the Perspective by Schoop). The inert Ba 3 NbS 5 layers serve to dissociate the superconducting NbS 2 layers from one another, resulting in 2D superconductivity with high carrier mobility. The combination of high mobility and reduced dimensionality may give rise to exotic quantum phases. Science , this issue p. 231 see also p. 170

Science & Technology - Other Topics↗
DOE OSTI2023

Charge density wave fluctuation driven composite order in layered kagome metals

The newly discovered kagome metals AV 3 Sb 5 (A = K, Rb, Cs) offer an exciting route to study exotic phases arising due to interplay between electronic correlations and topology. Besides superconductivity, these materials exhibit a charge-density wave (CDW) phase occurring at ~ 100 K, whose origin still remains elusive. The robust multi-component 2 × 2 CDW phase in these systems is of great interest due to the presence of an unusually large anomalous Hall effect. In quasi-2D systems with weak inter-layer coupling fluctuation driven exotic phases may appear. In particular in systems with multi-component order parameters fluctuations may lead to establishment of composite order when only products of individual order parameters condense while the individual ones themselves remain disordered. Here we argue that such fluctuation-driven regime of composite CDW order may exist in thin films of kagome metals above the CDW transition temperature. It is suggested that the melting of the Trihexagonal state in the material doped way from the van Hove singularities gives rise to a pseudogap regime where the spectral weight is concentrated in small pockets and most of the original Fermi surface is gapped. Our findings suggest possible presence of exotic phases in the weakly coupled layered kagome metals, more so in the newly synthesized thin films of kagome metals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Absence of E 2 g Nematic Instability and Dominant A 1 g Response in the Kagome Metal CsV 3 Sb 5

Ever since the discovery of the charge density wave (CDW) transition in the kagome metal CsV 3 Sb 5 , the nature of its symmetry breaking has been under intense debate. While evidence suggests that the rotational symmetry is already broken at the CDW transition temperature ( T CDW ), an additional electronic nematic instability well below T CDW has been reported based on the diverging elastoresistivity coefficient in the anisotropic channel ( m E 2 g ). Verifying the existence of a nematic transition below T CDW is not only critical for establishing the correct description of the CDW order parameter, but also important for understanding low-temperature superconductivity. Here, we report elastoresistivity measurements of CsV 3 Sb 5 using three different techniques probing both isotropic and anisotropic symmetry channels. Contrary to previous reports, we find the anisotropic elastoresistivity coefficient m E 2 g is temperature independent, except for a step jump at T CDW . The absence of nematic fluctuations is further substantiated by measurements of the elastocaloric effect, which show no enhancement associated with nematic susceptibility. On the other hand, the symmetric elastoresistivity coefficient m A 1 g increases below T CDW , reaching a peak value of 90 at T * = 20 K . Our results strongly indicate that the phase transition at T * is not nematic in nature and the previously reported diverging elastoresistivity is due to the contamination from the A 1 g channel. Published by the American Physical Society 2024

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Supercurrent-Induced Weyl Superconductivity

Here, we show that Weyl superconductivity can be induced by finite supercurrent in noncentrosymmetric spin–orbit-coupled superconductors with line nodes. We introduce a three-dimensional tight-binding model of a tetragonal superconductor in a D+p-wave pairing state with a finite center-of-mass momentum, and elucidate that a line-nodal to point-nodal spectral transition occurs by applying an infinitesimal supercurrent. We also clarify that the higher-order effect in spin–orbit coupling is particularly important for this phenomenon. The point nodes are protected by topologically nontrivial Weyl charges, and therefore gapless arc states appear on the surface of the superconductor. Furthermore, both the positions and the Weyl charges of the point nodes depend on the direction of the current. In addition, a quantized Berry phase defined on high-symmetry planes characterizes the Weyl nodes when the in-plane supercurrent is considered. Our proposition paves a new way for controlling the superconducting gap structures by using an external field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Thermodynamic and electron transport properties of Ca 3 Ru 2 O 7 from first-principles phonon calculations and Boltzmann transport theory

Here, this work demonstrates a first-principles-based approach to obtaining finite temperature thermal and electronic transport properties which can be employed to model and understand mesoscale structural evolution during electronic, magnetic, and structural phase transitions. A computationally tractable model was introduced to estimate electron relaxation time and its temperature dependence. The model is applied to Ca 3 Ru 2 O 7 with a focus on understanding its electrical resistivity across the electronic phase transition at 48 K. A quasiharmonic phonon approach to the lattice vibrations was employed to account for thermal expansion while the Boltzmann transport theory including spin-orbit coupling was used to calculate the electron-transport properties, including the temperature dependence of electrical conductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Thermal effects on the electronic properties of sodium electride under high pressures

Sodium is a simple metal at ambient conditions, while it transits to an electride phase at pressures above ~160 GPa along the room temperature isotherm. Here we explore the thermal effects on the electronic properties of the $hP4$ phase of sodium along the $ρ$ = 5.872 g/cm 3 isochore. We quantitatively classify this phase as an insulator based on the criterion of nearsightedness of the one-particle density matrix. Ab initio calculations suggest that the band gap of this insulator decreases with increasing temperature along the isochore, primarily because of ionic distortions; culminating in an insulator-to-metal transition upon melting at $T_m \approx 2100$ $\hbox{K}$. This transition is accompanied by residual electronic localization (in d orbitals) in the form of dynamic electron bubbles and a change in hybridization form $p–d$ to $s–p$ upon melting. This transition is explored by tracking the electronic and electro-optical properties along the isochore under consideration.

36 MATERIALS SCIENCE↗
NASA NTRS2000

Helium-4 Experiments near T-lambda in a Low-Gravity Simulator

We report our studies of gravity cancellation in a liquid helium sample cell along the lambda-line using a low-gravity simulator facility. The simulator consists of a superconducting magnet capable of producing B(delta-B/delta-z) = 22squareT)/cm. We have verified experimentally that the simulator can cancel gravity to about 0.01g in a cylindrical sample volume of 0.5 cm in diameter and 0.5 cm in height. This allows us to approach more closely the superfluid transition without entering the normal-superfluid two phase region induced by gravity. We also present the measurements of T-c(Q,P): depression of the superfluid transition temperature by a heat current(Q) along the lambda-line (P). The results are consistent with the Renormalization-group theory calculation. Measurements of thermal expansion coefficient in a heat current will also be discussed. The work has been carried out by JPL, California Institute of Technology under contract to NASA.

Liu, Yuanming↗
DOE OSTI2020

Soluble limit and criticality of fermions in $\mathbb Z_2$ gauge theories

Quantum information theory and strongly correlated electron systems share a common theme of macroscopic quantum entanglement. In both topological error correction codes and theories of quantum materials (spin liquid, heavy fermion and high-$T_c$ systems), entanglement is implemented by means of an emergent gauge symmetry. Inspired by these connections, in this paper we introduce a simple model for fermions moving in the deconfined phase of a $\mathbb Z_2$ gauge theory by coupling Kitaev's toric code to mobile fermions. This permits us to exactly solve the ground state of this system and map out its phase diagram. Reversing the sign of the plaquette term in the toric code permits us to tune the ground state between an orthogonal metal and an orthogonal semimetal in which gapless quasiparticles survive despite a gap in the spectrum of original fermions. The small-to-large Fermi surface transition between these two states occurs in a stepwise fashion with multiple intermediate phases. By using a diagrammatic technique, we are able to explore physics beyond the integrable point to examine various instabilities of the deconfined phase and to derive the critical theory at the transition between deconfined and confined phases. We outline how the fermionic toric code can be implemented as a quantum circuit, thus providing an important link between quantum materials and quantum information theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗