arXiv subjects

R. J. Cava

Publications and source records attributed to R. J. Cava.

At least 19 recordsLinked to original sources

arXiv2026

Crystal structure and basic properties of dirhenate quantum materials

The anhydrous divalent 3d-metal dirhenate quantum materials, M(ReO$_4$)$_2$,were synthesized using solid-state methods for M = Mn through Zn. Previously unreported Mg(ReO4)2 is also described. Their layered crystal structures, which feature an in-plane triangular lattice of M$^{2+}$, were refined using single crystal X-ray diffraction, and their optical absorption and several other physical properties were characterized. Their magnetism and heat capacity reveal long-range magnetic order at low temperatures in many of the M(ReO$_4$)$_2$ phases. Notably, many of these ordered states are sensitive to applied magnetic fields and can be readily suppressed by relatively small fields, suggesting competing magnetic interactions in a low-dimensional framework, which appear worthy of further study.

cond-mat.mtrl-sci↗

arXiv2026

Probing the topological protection of edge states in multilayer tungsten ditelluride with the superconducting proximity effect

The topology of WTe2, a transition metal dichalcogenide with large spin-orbit interactions, is thought to combine type II Weyl semimetal and second-order topological insulator (SOTI) character. The SOTI character should endow WTe2 multilayer crystals with topologically protected helical states at its hinges, and, indeed, 1D states have been detected thanks to Josephson interferometry. However, the immunity to backscattering conferred to those states by their helical nature has so far not been tested. To probe the topological protection of WTe2 edge states, we have fabricated Superconducting Quantum Interference Devices (SQUIDs) in which the supercurrent through a junction on the crystal edge interferes with the supercurrent through a junction in the bulk of the crystal. We find behaviors ranging from a Symmetric SQUID pattern to asymmetric SQUID patterns, including one in which the modulation by magnetic field reveals a sawtooth-like supercurrent versus phase relation for the edge junction, demonstrating that the supercurrent at the edge is carried by ballistic channels over 600 nm, a tell-tale sign of the SOTI character of WTe2.

cond-mat.mes-hall↗

arXiv2026

Proximate Spin Liquid Ground State Arising from Competing Stripy and 120$^{\circ}$ Spin Correlations in the Triangular Quantum Antiferromagnet ErMgGaO$_4$

ErMgGaO$_4$ is a quantum antiferromagnet wherein the pseudospin-1/2 degrees of freedom of Er$^{3+}$ decorate two-dimensional triangular planes separated by disordered non-magnetic bilayers of Mg$^{2+}$ and Ga$^{3+}$. Unlike its sister compound, YbMgGaO$_4$, our powder ErMgGaO$_4$ sample shows a clear spin glass transition near $T_g \sim 2.5$~K, about 1/6 of its Curie-Weiss temperature. We have carried out new inelastic neutron scattering measurements on these powder ErMgGaO$_4$ samples. At high energies, we observed crystalline electric field (CEF) transitions within the $J=15/2$ multiplet of Er$^{3+}$, but with the first excited CEF level sufficiently low in energy ($\sim$ 3~meV) so as to allow the possibility that virtual CEF transitions influence the exchange couplings. At E=0, we observe diffuse elastic scattering which is analysed using Warren lineshapes appropriate for two dimensional correlations. This reveals dominant 2D stripy correlations below $T_g$, coexisting with 2D 120$^\circ$-type correlations that persist above $T_g$. At low temperatures, the low energy inelastic component of the scattering shows a continuum with bandwidth of $\sim$ 0.8~meV. This dynamic magnetic spectral weight can be modeled at all $Q$, energies, and temperatures as the sum of high energy and low energy damped harmonic oscillators (DHO), with the high energy DHO defining the bandwidth of $\sim$ 0.8~meV. We use linear spin wave theory to model this inelastic scattering and to estimate its spin Hamiltonian parameters in terms of a $J_1-J_2-Δ$ model on the triangular lattice. This gives a good description of the low lying spectral weight for ErMgGaO4, and allows us to place it on the theoretical $J_1-J_2-Δ$ phase diagram with $\frac{J_1}{J_2}=0.13 \pm 0.03$ and $Δ=0.4 \pm 0.1$, which is close to the expected quantum phase boundary between the spin liquid and the stripy ordered phases.

cond-mat.str-el↗

arXiv2026

Stability of the Local Ni$^{2+}$ Electronic Structure to $A$-site Disorder in the Pyrochlore Antiferromagnet NaCaNi$_2$F$_7$

NaCaNi$_2$F$_7$ is a unique example of spin-1 Heisenberg antiferromagnet on the pyrochlore lattice, but the presence of Na$^{1+}$/Ca$^{2+}$ $A$-site disorder complicates the local electronic and magnetic environment of the Ni$^{2+}$ $B$-site. We utilize resonant inelastic X-ray scattering (RIXS) to study the influence of $A$-site disorder on the $B$-site electronic structure of NaCaNi$_2$F$_7$. Ni L-edge RIXS measurements reveal a Ni$^{2+}$ electronic structure in nearly ideal octahedral coordination, with only a small trigonal compression ($δ$ = -200$\;$meV) required to capture all spectral features. Within the $D_{3d}$ symmetry of the Ni local environment, we extract an anisotropic $g$-factor of $g_{\parallel} = 2.26$ and $g_{\perp} = 2.27$, and a corresponding paramagnetic moment of $μ_{\rm{eff}}=3.2\;μ_B$. To simulate disorder, RIXS spectra were calculated with realistic distributions of crystal field parameters; however, these spectra are invariant relative to a disorder-free model, demonstrating the robustness of the Ni$^{2+}$ electronic environment to the $A$-site disorder, within the resolution of our measurement.

cond-mat.str-el↗

arXiv2025

Single-gap two-band superconductivity well above the Pauli limit in non-centrosymmetric TaIr$_2$B$_2$

Non-centrosymmetric superconducting materials represent an exciting class of novel superconductors featuring a variety of unconventional properties, including mixed-parity pairing and very high upper critical fields. Here, we present a comprehensive study of TaIr$_2$B$_2$ (with $T_c$ = 5.1 K), using a set of complementary experimental methods, including bulk- and surface-sensitive techniques. We provide evidence that this system is a two-band, yet it behaves as a single-gap superconductor with a strong coupling. The upper critical field of TaIr$_2$B$_2$ significantly exceeds the Pauli limit and exhibits a nearly linear temperature dependence down to the lowest temperatures. This behavior, rarely seen in superconductors, is discussed in terms of anti-symmetric spin-orbit interaction, two-band-, and strong-coupling effects, as well as disorder.

cond-mat.supr-con↗

arXiv2025

Zeeman split Kramers doublets in spin-supersolid candidate Na$_{2}$BaCo(PO$_{4}$)$_{2}$

Na$_{2}$BaCo(PO$_{4}$)$_{2}$ is a triangular antiferromagnet that displays highly efficient adiabatic demagnetization cooling (J. Xiang $\textit{et al.}$ Nature ${\bf{625}}$, 270 (2024)) near a quantum critical point at $μ_{0}H_{c}\sim 1.6$ T, separating a low-field magnetically disordered from a high-field fully polarized ferromagnetic phase. We apply high resolution backscattering neutron spectroscopy in an applied field to study the magnetic excitations near $μ_{0}H_{c}$. At large fields we observe ferromagnetic fluctuations that gradually transition to being overdamped in energy below $μ_{0}H_{c}$ where the magnetism is spatially disordered. We parameterize the excitations in the high field polarized phase in terms of coupled Zeeman split Kramers doublets originating from the presence of spin-orbit coupling. On reducing the field, the splitting between the Kramers doublets is reduced and if done adiabatically, provides a mechanism for reducing temperature. On lowering the applied field through the $μ_{0}H_{c}$ the excitations characterize a textured phase that we suggest is inefficient for cooling. Low temperature disordered frustrated magnets built on Kramers doublets with nearby quantum critical points provide a route for efficient magnetocalorics.

cond-mat.str-el↗

arXiv2024

Insulator to Metal Transition under High Pressure in FeNb$_3$Se$_{10}$

Non-magnetic FeNb$_3$Se$_{10}$ has been demonstrated to be an insulator at ambient pressure through both theoretical calculations and experimental measurements and it does not host topological surface states. Here we show that on the application of pressure, FeNb$_3$Se$_{10}$ transitions to a metallic state at around 3.0 GPa. With a further increase in pressure, its resistivity becomes independent of both temperature and pressure. Its crystal structure is maintained to at least 4.4 GPa.

cond-mat.str-el↗

arXiv2024

Anisotropy of the zigzag order in the Kitaev honeycomb magnet $α$-RuBr$_3$

Kitaev materials often order magnetically at low temperatures due to the presence of non-Kitaev interactions. Torque magnetometry is a very sensitive technique for probing the magnetic anisotropy, which is critical in understanding the magnetic ground state. In this work, we report detailed single-crystal torque measurements in the proposed Kitaev candidate honeycomb magnet $α$-RuBr$_3$, which displays zigzag order below 34 K. Based on angular-dependent torque studies in magnetic fields up to 16 T rotated in the plane normal to the honeycomb layers, we find an easy-plane anisotropy with a temperature dependence of the torque amplitude following closely the behaviour of the powder magnetic susceptibility. The torque for field rotated in the honeycomb plane has a clear six-fold periodicity with a saw-tooth shape, reflecting the three-fold symmetry of the crystal structure and stabilization of different zigzag domains depending on the field orientation, with a torque amplitude that follows an order parameter form inside the zigzag phase. By comparing experimental data with theoretical calculations we identify the relevant anisotropic interactions and the role of the competition between different zigzag domains in this candidate Kitaev material.

cond-mat.str-el↗

arXiv2024

Evidence of temperature-dependent interplay between spin and orbital moment in van der Waals ferromagnet VI3

Van der Waals materials provide a versatile toolbox for the emergence of new quantum phenomena and the fabrication of functional heterostructures. Among them, the trihalide VI3 stands out for its unique magnetic and structural landscape. Here we investigate the spin and orbital magnetic degrees of freedom in the layered ferromagnet VI3 by means of temperature-dependent x-ray absorption spectroscopy and x-ray magnetic circular and linear dichroism. We detect localized electronic states and reduced magnetic dimensionality, due to electronic correlations. We furthermore provide experimental evidence of (a) an unquenched orbital magnetic moment (up to 0.66(7)) in the ferromagnetic state, and (b) an instability of the orbital moment in proximity of the spin reorientation transition. Our results support a coherent picture where electronic correlations give rise to a strong magnetic anisotropy and a large orbital moment, and establish VI3 as a prime candidate for the study of orbital quantum effects.

cond-mat.mtrl-sci↗

arXiv2023

Superconductivity in the Face Centered Cubic $\rm W_{n-x}Mo_{x}RhIrPt_{2}$ High Entropy Alloy

We report single phase superconducting face centered cubic (FCC) intermetallic high entropy alloys (HEAs) synthesized via splat cooling. The single phase materials fall at electron counts in the HEA superconductor alloy family where structural stability and optimal superconducting electron counts clash. The materials' superconducting properties follow the general trends published for metallic alloys. Many of the superconducting characteristics are summarized. Insights are provided as to why an FCC structure may be stable.

cond-mat.supr-con↗

arXiv2023

Superconductivity in electron doped PbBi2Te4

Single crystals of In-doped PbBi2Te4 are synthesized via a conventional solid-state method. Chemical analysis and hall measurements indicate that In replaces Pb, introducing n-type carriers, creating Pb1-xInxBi2Te4. A superconducting transition is observed with a maximum transition temperature around 2.06 K for Pb1-xInxBi2Te4. Field dependent transport measurements reveal type-II superconductivity and yield a maximum upper critical field around 1.55 T. Thermodynamic data indicates bulk superconductivity in the BCS weak coupling limit. Our findings establish an ambient-pressure superconducting system in the AM2X4 family, and doped PbBi2Te4 as a promising platform for the study of topological superconductivity.

cond-mat.supr-con↗

arXiv2023

Erbium-excess gallium garnets

A series of garnets of formula Er3+xGa5-xO12 is described, for which we report the crystal structures for both polycrystalline and single-crystal samples. The x limit in the garnet phase is between 0.5 and 0.6 under our conditions, with the Er fully occupying the normal garnet site plus half-occupying the octahedral site at x = 0.5 in place of the Ga normally present. Long-range antiferromagnetic order with spin ice-like frustration is suggested by the transition temperature (TN=0.8K) being much lower than the Curie-Weiss theta. The magnetic ordering temperature does not depend on the Er excess, but there is increasing residual entropy as the Er excess is increased, highlighting the potential for unusual magnetic behavior in this system.

cond-mat.mtrl-sci↗

arXiv2023

Eavesdropping on competing condensates by the edge supercurrent in a Weyl superconductor

In a topological insulator the metallic surface states are easily distinguished from the insulating bulk states (FuKane07). By contrast, in a topological superconductor (FuKane08,Qi,FuBerg,Oppen), much less is known about the relationship between an edge supercurrent and the bulk pair condensate. Can we force their pairing symmetries to be incompatible? In the superconducting state of the Weyl semimetal MoTe$_2$, an edge supercurrent is observed as oscillations in the current-voltage (\emph{I-V}) curves induced by fluxoid quantization (Wang). We have found that the $s$-wave pairing potential of supercurrent injected from niobium contacts is incompatible with the intrinsic pair condensate in MoTe$_2$. The incompatibility leads to strong stochasticity in the switching current $I_c$ as well as other anomalous properties such as an unusual antihysteretic behavior of the ``wrong'' sign. Under supercurrent injection, the fluxoid-induced edge oscillations survive to much higher magnetic fields \emph{H}. Interestingly, the oscillations are either very noisy or noise-free depending on the pair potential that ends up dictating the edge pairing. Using the phase noise as a sensitive probe that eavesdrops on the competiting bulk states, we uncover an underlying blockade mechanism whereby the intrinsic condensate can pre-emptively block proximitization by the Nb pair potential depending on the history.

cond-mat.supr-con↗

arXiv2023

Spin disorder in a stacking polytype of a layered magnet

Strongly correlated ground states and exotic quasiparticle excitations in low-dimensional systems are central research topics in the solid state research community. The present work develops a new layered material and explores the physical properties. Single crystals of 3R-Na2MnTeO6 were synthesized via a flux method. Single crystal x-ray diffraction and transmission electron microscopy reveal a crystal structure with ABC-type stacking and an R-3 space group, which establishes this material as a stacking polytype to previously reported 2H-Na2MnTeO6. Magnetic and heat capacity measurements demonstrate dominant antiferromagnetic interactions, the absence of long-range magnetic order down to 0.5 K, and field-dependent short range magnetic correlations. A structural transition at ~ 23 K observed in dielectric measurements may be related to displacements of the Na positions. Our results demonstrate that 3R-Na2MnTeO6 displays low-dimensional magnetism, disordered structure and spins, and the system displays a rich structure variety.

cond-mat.str-el↗

arXiv2022

A Layered Spin 1/2 polymorph of titanium triiodide

A previously unreported layered spin 1/2 triangular lattice polymorph of TiI3 is described, synthesized under 6 GPa of applied pressure at 900 C, but stable at atmospheric pressure. This air-sensitive material has a CdI2-type layered structure (P-3m1 (#164), a = 4.012 A and c = 6.641 A at 120 K, Z = 1 of Ti0.667I2) with an in-plane triangular lattice, related to that of TiI4 (Ti0.5I2). Although the TiI3 formula is consistent with expectations for a layered honeycomb lattice of spin 1/2 Ti(III), there appears to be disorder in the crystal structure. Magnetic susceptibility and heat capacity measurements suggest that the material undergoes several low temperature phase transitions.

cond-mat.mtrl-sci↗

arXiv2022

A magnetic continuum observed by terahertz spectroscopy in a quantum spin liquid candidate BaCo$_2$(AsO$_4$)$_2$

Quantum spin liquids (QSLs) are topologically ordered exotic states of matter that host fractionalized excitations. Kitaev proposed a particular route towards a QSL via strongly bond-dependent interactions on the hexagonal lattice. A number of candidate Kitaev QSL materials have been pursued, but all have appreciable non-Kitaev interactions, which put these systems far from the QSL regime. Using time-domain terahertz spectroscopy (TDTS) we observed a broad magnetic continuum over a wide range of temperature and field in the honeycomb cobalt-based magnet, BaCo$_2$(AsO$_4$)$_2$, which has been proposed to be more ideal versions of a Kitaev QSL. Applying a small in-plane magnetic field of $\sim$ 0.5 T suppresses the magnetic order and at at even higher fields gives rise to a spin-polarized state. With 4T magnetic field oriented principally out-of-plane, a broad magnetic continuum was observed that could be consistent with a field induced QSL. Our results indicate BaCo$_2$(AsO$_4$)$_2$ to be a promising QSL candidate.

cond-mat.str-el↗

arXiv2021

Phase tuning of multiple Andreev reflections of Dirac fermions and the Josephson supercurrent in Al-MoTe2-Al junctions

When a normal metal $N$ is sandwiched between two superconductors, the energy gaps in the latter act as walls that confine electrons in $N$ in a square-well potential. If the voltage $V$ across $N$ is finite, an electron injected into the well undergoes multiple Andreev reflections (MAR) until it gains enough energy to overcome the energy barrier. Because each reflection converts an electron to a hole (or vice versa), while creating (or destroying) a Cooper pair, the MAR process shuttles a stream of pairs across the junction. An interesting question is, given a finite $V$, what percentage of the shuttled pairs end up as a Josephson supercurrent? This fraction does not seem to have been measured. Here we show that, in high-transparency junctions based on the type II Dirac semimetal MoTe$_2$, the MAR leads to a stair-case profile in the current-voltage ($I$-$V$) response, corresponding to pairs shuttled incoherently by the $n^{th}$-order process. By varying the phase $φ$ across the junction, we demonstrate that a Josephson supercurrent ${\bf J}_{\rm s}\sim \sinφ$ co-exists with the MAR steps, even at large $V$. The observed linear increase in the amplitude of ${\bf J}_{\rm s}$ with $n$ (for small $n$) implies that ${\bf J}_{\rm s}$ originates from the population of pairs that are coherently shuttled. We infer that the MAR steps and the supercurrent are complementary aspects of the Andreev process. The experiment yields the percentage of shuttled pairs that form the supercurrent. At large $V$, the coherent fraction is initially linear in $n$. However, as $V\to 0$ ($n\gg 1$), almost all the pairs end up as the observed Josephson supercurrent.

cond-mat.supr-con↗

arXiv2021

Frustration enhanced by Kitaev exchange in a $\boldsymbol{\tilde{j}_{\text{eff}}=\frac12}$ triangular antiferromagnet

Triangular Heisenberg antiferromagnets are prototypes of geometric frustration, even if for nearest-neighbor interactions quantum fluctuations are not usually strong enough to destroy magnetic ordering: stronger frustration is required to stabilize a spin-liquid phase. On the basis of static magnetization and electron spin resonance measurements, we demonstrate the emergence of ${\tilde{j}_{\text{eff}}=\frac12}$ moments in the triangular-lattice magnet Na$_2$BaCo(PO$_4$)$_2$. These moments are subject to an extra source of frustration that causes magnetic correlations to set in far above both the magnetic ordering and Weiss temperatures. Corroborating the $\tilde{j}_{\text{eff}}=\frac12$ ground state, theory identifies ferromagnetic Kitaev exchange anisotropy as additional frustrating agent, altogether putting forward Na$_2$BaCo(PO$_4$)$_2$ as a promising Kitaev spin-liquid material.

cond-mat.str-el↗