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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., Comment: 20 pages, 5 figures

The layered honeycomb material BaCo$_2$(AsO$_4$)$_2$ (BCAO) is of topical interest because its magnetic state is related to that of the Kitaev magnet $\alpha$-RuCl$_3$. Using thermal transport to probe how magnetic excitations interact with phonons in the magnetically disordered regime, we have uncovered an unusually large enhancement of the thermal conductivity $\kappa_{xx}$ in an in-plane magnetic field ${\bf H}$. Just above the N\'{e}el temperature $T_{\rm N}$, a field of 13 T increases $\kappa_{xx}$ by a factor $\sim 211$, much larger than reported previously in any magnetic insulator. Interestingly, $\kappa_{xx}(H,T)$ exhibits a scaling behavior in the entire magnetically disordered region that surrounds the ordered zigzag state. The ratio $\Delta \kappa_{xx}(H,T)/\kappa_{xx}(13,T)$, measured throughout the disordered region, collapses to a one-parameter scaling function ${\rm exp}(-1/gx)$ (where $x = \mu_{\rm B}B/k_{\rm B}T$ and $g$ is a constant)., Comment: 8 pages, 4 figures

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 $\alpha$-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.

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.

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., Comment: PRB accepted 11 pages main text + 4 pages SI

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., Comment: 13 pages, 4 figures

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.

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., Comment: 17 pages, 2 tables, 6 figures excluding supplemental information. Supplemental information included at end

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.

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., Comment: 11 pages, 6 figures

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 $\varphi$ across the junction, we demonstrate that a Josephson supercurrent ${\bf J}_{\rm s}\sim \sin\varphi$ 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., Comment: 12 pages, 6 figures

The Kitaev model predicts that quantum spin liquids (QSLs) will form at low temperatures under certain special conditions, and materials hosting the QSL state are frequently sought. The layered honeycomb lattice material {\alpha}-RuCl3 has emerged as a prime candidate for displaying the Kitaev QSL state. Here we describe a new polymorph of RuI3 with a layered honeycomb lattice structure, synthesized at moderately high pressures and stable under ambient conditions. Preliminary characterization reveals metallic, paramagnetic behavior, the absence of long-range magnetic order down to 0.35 K and an unusually large T-linear contribution to the heat capacity at low temperatures. We propose that this RuI3 phase, with a layered honeycomb lattice and strong spin-orbit coupling, provides a new route for the characterization of quantum materials.

We report high-resolution angular magnetoresistance (AMR) experiments performed on crystals of ReO$_3$ with high mobility (90,000 cm$^2$/Vs at 2 K) and extremely low residual resistivity (5-8 n$\Omega$cm). The Fermi surface, comprised of intersecting cylinders, supports open orbits. The resistivity $\rho_{xx}$ in a magnetic field $B$ = 9 T displays a singular pattern of behavior. With $\bf E\parallel \hat{x}$ and $\bf B$ initially $\parallel\bf\hat{z}$, tilting $\bf B$ in the longitudinal $k_z$-$k_x$ plane leads to a steep decrease in $\rho_{xx}$ by a factor of 40. However, if $\bf B$ is tilted in the transverse $k_y$-$k_z$ plane, $\rho_{xx}$ increases steeply by a factor of 8. Using the Shockley tube integral approach, we show that, in ReO$_3$, the singular behavior results from the rapid conversion of closed to open orbits, resulting in opposite signs for AMR in orthogonal planes. The floor values of $\rho_{xx}$ in both AMR scans are identified with specific sets of open and closed orbits. Also, the "completion angle" $\gamma_c$ detected in the AMR is shown to be an intrinsic geometric feature that provides a new way to measure the Fermi radius $k_F$. However, additional sharp resonant features which appear at very small tilt angles in the longitudinal AMR scans are not explained by the tube integral approach., Comment: 12 pages, 7 figures

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.

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., Comment: 6 pages, 4 figures (published version) + supplemental material (4 pages)

Radio-frequency (14.6 MHz) AC magnetic susceptibility, $\chi^{\prime}_{AC}$, of \dytio\ was measured using a self-oscillating tunnel-diode resonator. Measurements were made with the excitation AC field parallel to the superimposed DC magnetic field up 5 T in a wide temperature range from 50 mK to 100 K. At 14.6 MHz a known broad peak of $\chi^{\prime}_{AC}(T)$ from kHz - range audio-frequency measurements around 15~K for both [111] and [110] directions shifts to 45~K, continuing the Arrhenius activated behavior with the same activation energy barrier of $E_a \approx 230$~K. Magnetic field dependence of $\chi^{\prime}_{AC}$ along [111] reproduces previously reported low-temperature two-in-two-out to three-in-one-out spin configuration transition at about 1~T, and an intermediate phase between 1 and 1.5~T. The boundaries of the intermediate phase show reasonable overlap with the literature data and connect at a critical endpoint of the first-order transition line, suggesting that these low-temperature features are frequency independent. An unusual upturn of magnetic susceptibility at $T \to 0$ was observed in magnetic fields between 1.5~T and 2~T for both magnetic field directions, before fully polarized configuration sets in above 2~T.

We report the low temperature magnetic properties of Nd$^{3+}$ pyrochlore $\rm Nd_2ScNbO_7$. Susceptibility and magnetization show an easy-axis moment, and heat capacity reveals a phase transition to long range order at $T_N=371(2)$ mK with a fully recovered $\Delta S = R \ln(2)$, 53\% of it recovered for $T>T_N$. Elastic neutron scattering shows a long-range all-in-all-out magnetic order with low-$Q$ diffuse elastic scattering. Inelastic neutron scattering shows a low-energy flat-band, indicating a magnetic Hamiltonian similar to $\rm Nd_2Zr_2O_7$. Nuclear hyperfine excitations measured by ultra-high-resolution neutron backscattering indicates a distribution of static electronic moments below $T_N$, which may be due to B-site disorder influencing Nd crystal electric fields. Analysis of heat capacity data shows an unexpected $T$-linear or $T^{3/2}$ term which is inconsistent with conventional magnon quasiparticles, but is consistent with fractionalized spinons or gapless local spin excitations. We use legacy data to show similar behavior in $\rm Nd_2Zr_2O_7$. Comparing local static moments also reveals a suppression of the nuclear Schottky anomaly in temperature, evidencing a fraction of Nd sites with nearly zero static moment, consistent with exchange-disorder-induced random singlet formation. Taken together, these measurements suggest an unusual fluctuating magnetic ground state which mimics a spin-liquid -- but may not actually be one., Comment: Main text: 11 pages, 9 figures. Appendices: 6 pages and 7 figures

When two superconductors are connected across a ferromagnet, the spin configuration of the transferred Cooper pairs can be modulated due to magnetic exchange interaction. The resulting supercurrent can reverse its sign across the Josephson junction (JJ) [1-4]. Here we demonstrate Josephson phase modulation in van der Waals heterostructures when Cooper pairs from superconducting NbSe$_2$ tunnel through atomically thin magnetic insulator (MI) Cr$_2$Ge$_2$Te$_6$. Employing a superconducting quantum interference device based on MI JJs, we probe a doubly degenerate non-trivial JJ phase ($\phi$) originating from the magnetic barrier. This $\phi$-phase JJ is formed by momentum conserving tunneling of Ising Cooper pairs [5] across magnetic domains in the Cr$_2$Ge$_2$Te$_6$ barrier. The doubly degenerate ground states in MI JJs provide a two-level quantum system that can be utilized as a new disipationless component for superconducting quantum devices, including phase batteries [6], memories [7,8], and quantum Ratchets [9,10].

In the present work, we investigate the electronic structure of the two-dimensional (2D) ferromagnet Cr2Ge2Te6 by photoemission spectroscopy and ab initio calculations. Our results demonstrate the presence of multiple hole-type bands in the vicinity of the Fermi level indicating that the material can support high electrical conductivity by manipulating the chemical potential. Also, our photon energy dependent angle resolved photoemission experiment revealed that several of the hole bands exhibit weak dispersion with varied incident photon energy providing experimental signature for its two dimensionality. These findings can pave the way for further studies towards the application of Cr2Ge2Te6 in electronic devices., Comment: 8 pages, 3 figures

The Ising chain in transverse field is a paradigmatic model for a host of physical phenomena, including spontaneous symmetry breaking, topological defects, quantum criticality, and duality. Although the quasi-1D ferromagnet CoNb$_2$O$_6$ has been put forward as the best material example of the transverse field Ising model, it exhibits significant deviations from ideality. Through a combination of THz spectroscopy and theory, we show that CoNb$_2$O$_6$ in fact is well described by a different model with strong bond dependent interactions, which we dub the {\it twisted Kitaev chain}, as these interactions share a close resemblance to a one-dimensional version of the intensely studied honeycomb Kitaev model. In this model the ferromagnetic ground state of CoNb$_2$O$_6$ arises from the compromise between two distinct alternating axes rather than a single easy axis. Due to this frustration, even at zero applied field domain-wall excitations have quantum motion that is described by the celebrated Su-Schriefer-Heeger model of polyacetylene. This leads to rich behavior as a function of field. Despite the anomalous domain wall dynamics, close to a critical transverse field the twisted Kitaev chain enters a universal regime in the Ising universality class. This is reflected by the observation that the excitation gap in CoNb$_2$O$_6$ in the ferromagnetic regime closes at a rate precisely twice that of the paramagnet. This originates in the duality between domain walls and spin-flips and the topological conservation of domain wall parity. We measure this universal ratio `2' to high accuracy -- the first direct evidence for the Kramers-Wannier duality in nature., Comment: 6 pages, 3 figures; 5 pages, 7 figures (Supplemental)

We report the synthesis and initial characterization of Ba6Y2Rh2Ti2O17-{\delta}, a previously unreported material with a hexagonal symmetry structure. Face-sharing RhO6 octahedra form triangular planes of Rh2O9 dimers that are widely separated in the perpendicular direction. The material displays a small effective magnetic moment, due to the Rh ions present, and a negative Curie-Weiss temperature. The charge transport and optical band gaps are very similar, near 0.16 eV. A large upturn in the heat capacity at temperatures below 1 K, suppressed by applied magnetic fields larger than {{\mu}0H = 2 Tesla, is observed. A large T-linear term in the specific heat ({\gamma}=166 mJ/mol f.u-K2) is seen, although the material is insulating at low temperatures. These results suggest the possibility of a spin liquid ground state in this material., Comment: 5 figures, 2 tables

Atom-like defects in solid-state hosts are promising candidates for the development of quantum information systems, but despite their importance, the host substrate/defect combinations currently under study have almost exclusively been found serendipitously. Here we systematically evaluate the suitability of host materials by applying a combined four-stage data mining and manual screening process to all entries in the Materials Project database, with literature-based experimental confirmation of band gap values. We identify 580 viable host substrates for quantum defect introduction and use in quantum information systems. While this constitutes a significant increase in the number of known and potentially viable material systems, it nonetheless represents a significant (99.54%) reduction from the total number of known inorganic phases, and the application of additional selection criteria for specific applications will reduce their number even further. The screening principles outlined may easily be applied to previously unrealized phases and other technologically important materials systems., Comment: Currently under consideration at npj Computational Materials

Hexagonal oxide perovskites, in contrast to the more familiar perovskites, allow for face-sharing of metal-oxygen octahedra or trigonal prisms within their structural frameworks. This results in dimers, trimers, tetramers, or longer fragments of chains of face-sharing octahedra in the crystal structures, and consequently in much shorter metal-metal distances and lower metal-oxygen-metal bond angles than are seen in the more familiar perovskites. The presence of the face-sharing octahedra can have a dramatic impact on magnetic properties of these compounds, and dimer-based materials, in particular, have been the subjects of many quantum-materials-directed studies in materials physics. Hexagonal oxide perovskites are of contemporary interest due to their potential for geometrical frustration of the ordering of magnetic moments or orbital occupancies at low temperatures, which is especially relevant to their significance as quantum materials. As such, several hexagonal oxide perovskites have been identified as potential candidates for hosting the quantum spin liquid state at low temperatures. In our view, hexagonal oxide perovskites are fertile ground for finding new quantum materials. This review briefly describes the solid state chemistry of many of these materials., Comment: "Submitted for publication in a special issue on quantum materials at the journal Chemical Reviews. "

Cobalt pyrochlore fluoride NaCaCo$_2$F$_7$ is a disordered frustrated magnet composed of Co$^{2+}$ ions with an effective spin-$\frac{1}{2}$ magnetic moment and exhibits spin freezing below $T_f \sim$2.4 K. We perform ultrasound velocity measurements on a single crystal of the cubic NaCaCo$_2$F$_7$. The temperature dependence of the bulk modulus (the breathing elastic mode) exhibits Curie-type softening upon cooling below $\sim$20 K down to $T_f$, which is suppressed by the magnetic field. This Curie-type softening should be a precursor to the enhancement of the strength of exchange disorder via the spin-lattice coupling, which causes the spin freezing. In contrast to the magnetic-field-suppressed Curie-type softening in the bulk modulus, the trigonal shear modulus exhibits softening with a characteristic minimum upon cooling, which is enhanced by the magnetic field at temperatures below $\sim$20 K. This magnetic-field-enhanced elastic anomaly in the trigonal shear modulus suggests a coupling of the lattice to the dynamical spin-cluster state. For NaCaCo$_2$F$_7$, the observed elastic anomalies reveal an occurrence of magnetic-field-induced crossover from an isostructural lattice instability toward the spin freezing to a trigonal lattice instability arising from the emergent dynamical spin-cluster state., Comment: 8 pages, 5 figures

The crystal structures of the Sr1-xCaxRuO3 perovskites are investigated using both long range and local structural probes. High resolution synchrotron powder X-ray diffraction characterization at ambient temperature shows that the materials are orthorhombic to high precision, and we support previous work showing that Ca2+ substitution for Sr2+ primarily changes the tilting of rigid corner-sharing RuO6 octahedra at their shared oxygen vertices. X-ray pair distribution function analysis for SrRuO3, CaRuO3 and one intermediate composition show them to be locally monoclinic, and no long range or local phase transitions are observed between 80 and 300 K for materials with intermediate compositions. High-resolution transmission electron microscopy shows that the Sr/Ca distribution is random at the nanoscale. We plot magnetic characteristics such as the ferromagnetic Tc, Curie-Weiss theta, effective moment, and ambient temperature susceptibility vs. the octahedral tilt and unit cell volume., Comment: 20 Figures

Edge supercurrents in superconductors have long been an elusive target. Interest in them has reappeared in the context of topological superconductivity. We report the observation of a robust edge supercurrent in the Weyl superconductor MoTe2. In a magnetic field B, fluxoid quantization generates a periodic modulation of the edge condensate observable as a "fast-mode" oscillation of the critical current Ic versus B. Remarkably, the fast-mode frequency is distinct from the conventional Fraunhofer oscillation displayed by the bulk supercurrent. We confirm that the fast mode frequency increases with crystal area as expected for an edge supercurrent. In addition, weak excitation branches are resolved which display an unusual broken symmetry., Comment: 8 pages, 4 figures

High entropy alloys (HEAs) are a recently-opened research area in materials science and condensed matter physics. Although 3d-metal-based HEAs have already been the subject of many investigations, studies of HEA superconductors, which tend to be based on 4d metals, are relatively fewer. Here we provide a short update of the progress made in studies of superconducting HEAs. We aim to summarize their current status and describe some of the key factors that appear to influence their superconducting transition temperatures and properties, including crystal structure, atomic makeup, valence electron count, molar volume and mixing entropy. Many opportunities and challenges remain for expanding our knowledge of HEA superconductors, finding new types of HEA superconductors, and their potential for applications; these are also briefly discussed., Comment: 32 pages, 5 figures

Spin liquids are quantum phases of matter that exhibit a variety of novel features associated with their topological character. These include various forms of fractionalization - elementary excitations that behave as fractions of an electron. While there is not yet entirely convincing experimental evidence that any particular material has a spin liquid ground state, in the past few years, increasing evidence has accumulated for a number of materials suggesting that they have characteristics strongly reminiscent of those expected for a quantum spin liquid.

We present present a quantitative experimental investigation of the scalar chiral magnetic order with in $\rm{Nd_3Sb_3Mg_2O_{14}}$. Static magnetization reveals a net ferromagnetic ground state, and inelastic neutron scattering from the hyperfine coupled nuclear spin reveals a local ordered moment of 1.76(6) $\mu_B$, just 61(2)% of the saturated moment size. The experiments exclude static disorder as the source of the reduced moment. A 38(1) $\mu$eV gap in the magnetic excitation spectrum inferred from heat capacity rules out thermal fluctuations and suggests a multipolar explanation for the moment reduction. We compare $\rm{Nd_3Sb_3Mg_2O_{14}}$ to Nd pyrochlores and show that it is close to a moment fragmented state., Comment: 5 pages, 4 figures, 3 pages supplemental information

Symmetry and topology play key roles in the identification of phases of matter and their properties. Both concepts are central to understanding quantum Hall ferromagnets (QHFMs), two-dimensional electronic phases with spontaneously broken spin or pseudospin symmetry whose wavefunctions also have topological properties. Domain walls between distinct broken symmetry QHFM phases are predicted to host gapless one-dimensional (1D) modes that emerge due to a topological change of the underlying electronic wavefunctions at such interfaces. Although a variety of QHFMs have been identified in different materials, probing interacting electronic modes at these domain walls has not yet been accomplished. Here we use a scanning tunneling microscope (STM) to directly visualize the spontaneous formation of boundary modes, within a sign-changing topological gap, at domain walls between different valley-polarized quantum Hall phases on the surface of bismuth. By changing the valley occupation and the corresponding number of modes at the domain wall, we can realize different regimes where the valley-polarized channels are either metallic or develop a spectroscopic gap. This behavior is a consequence of Coulomb interactions constrained by the symmetry-breaking valley flavor, which determines whether electrons in the topological modes can backscatter, making these channels a unique class of interacting Luttinger liquids.

A recurring theme in topological matter is the protection of unusual electronic states by symmetry, for example, protection of the surface states in Z2 topological insulators by time reversal symmetry [1-3]. Recently interest has turned to unusual surface states in the large class of nonsymmorphic materials [4-11]. In particular KHgSb is predicted to exhibit double quantum spin Hall (QSH) states [10]. Here we report observation of a novel feature of the Hall conductivity in KHgSb in strong magnetic field B. In the quantum limit, the Hall conductivity is observed to fall exponentially to zero, but the diagonal conductivity is finite. A large gap protects this unusual zero-Hall state. We propose that, in this limit, the chemical potential drops into the bulk gap, intersecting equal numbers of right and left-moving QSH surface modes to produce the zero-Hall state., Comment: 7 pages, 4 figures

Currently, there is a flurry of research interest on materials with an unconventional electronic structure, and we have already seen significant progress in their understanding and engineering towards real-life applications. The interest erupted with the discovery of graphene and topological insulators in the previous decade. The electrons in graphene simulate massless Dirac Fermions with a linearly dispersing Dirac cone in their band structure, while in topological insulators, the electronic bands wind non-trivially in momentum space giving rise to gapless surface states and bulk bandgap. Weyl semimetals in condensed matter systems are the latest addition to this growing family of topological materials. Weyl Fermions are known in the context of high energy physics since almost the beginning of quantum mechanics. They apparently violate charge conservation rules, displaying the "chiral anomaly", with such remarkable properties recently theoretically predicted and experimentally verified to exist as low energy quasiparticle states in certain condensed matter systems. Not only are these new materials extremely important for our fundamental understanding of quantum phenomena, but also they exhibit completely different transport phenomena. For example, massless Fermions are susceptible to scattering from non-magnetic impurities. Dirac semimetals exhibit non-saturating extremely large magnetoresistance as a consequence of their robust electronic bands being protected by time reversal symmetry. These open up whole new possibilities for materials engineering and applications including quantum computing. In this review, we recapitulate some of the outstanding properties of WTe$_2$, namely, its non-saturating titanic magnetoresistance due to perfect electron and hole carrier balance up to a very high magnetic field observed for the very first time. (Continued. Please see the main article)., Comment: 33 pages, 7 figures

Two-dimensional (2D) materials are promising candidates for next-generation electronic devices. In this regime, insulating 2D ferromagnets, which remain rare, are of special importance due to their potential for enabling new device architectures. Here we report the discovery of ferromagnetism in a layered van der Waals semiconductor, VI3, which is based on honeycomb vanadium layers separated by an iodine-iodine van der Waals gap. It has a BiI3-type structure (R-3, No.148) at room temperature, and our experimental evidence suggests that it may undergo a subtle structural phase transition at 78 K. VI3 becomes ferromagnetic at 49 K, below which magneto-optical Kerr effect imaging clearly shows ferromagnetic domains, which can be manipulated by the applied external magnetic field. The optical band gap determined by reflectance measurements is 0.6 eV, and the material is highly resistive.

Ba4NbIr3O12, a previously unreported material with a triangular planar geometry of Ir3O12 trimers, is described. Magnetic susceptibility measurements show no magnetic ordering down to 1.8 K despite the Curie-Weiss temperature of -13 K. The material has a very low effective magnetic moment of 0.80 {\mu}B/f.u. To look at the lower temperature behavior, the specific heat (Cp) was measured down to 0.35 K; it shows no indication of magnetic ordering and fitting a power law to Cp vs. T below 2 K yields the power {\alpha} = 3/4. Comparison to the previously unreported trimer compound made with the 4d element Rh in place of the 5d element Ir, Ba4NbRh3O12, is presented. The analysis suggests that Ba4NbIr3O12 is a candidate spin liquid material., Comment: 19 pages, 7 main figures, 2 SI figures

The magnetic properties of Ba3TiRu2O9 , whose crystal structure is based on stacked triangular planar lattices of MO6 dimers and single MO6 octahedra, are reported. The system is magnetically disturbed by a substantial amount of Ti/Ru chemical disorder. The Weiss temperature and effective magnetic moment were found to be -29.5 K and 1.82 {\mu}B/f.u. respectively, and a bifurcation in the zero field cooled and field cooled magnetic susceptibility is observed below 4.7 K, suggesting that this is a compositionally-disordered spin-glass system. The material is a semiconductor with an activation energy for charge transport of approximately 0.14 eV., Comment: 11 pages, 3 figures

Time- and angle-resolved photoemission spectroscopy accesses the ultrafast evolution of quasiparticles and many-body interactions in solid-state systems. However, the momentum- and energy-resolved transient photoemission intensity may not be unambiguously related to the intrinsic relaxation dynamics of photoexcited electrons. In fact, interpretation of the time-dependent photoemission signal can be affected by the transient evolution of both the one-electron removal spectral function as well as the photoemission dipole matrix elements. Here we investigate the topological insulator Bi$_{1.1}$Sb$_{0.9}$Te$_2$S to demonstrate, by means of a careful probe-polarization study, the transient contribution of matrix elements to the time-resolved photoemission signal., Comment: 7 pages, 3 figures

We report high-frequency/high-field electron spin resonance (ESR) and high-field magnetization studies on single crystals of the bond-disordered pyrochlore NaCaCo$_2$F$_7$. Frequency- and temperature-dependent ESR investigations above the freezing temperature $T_f \sim 2.4$ K reveal the coexistence of two distinct magnetic phases. A cooperative paramagnetic phase, evidenced by a gapless excitation mode, is found as well as a spin-glass phase developing below 20 K which is associated with a gapped low-energy excitation. Effective $g$-factors close to 2 are obtained for both modes in line with pulsed high-field magnetization measurements which show an unsaturated isotropic behavior up to 58 T at 2 K. In order to describe the field-dependent magnetization in high magnetic fields, we propose an empirical model accounting for highly anisotropic ionic $g$-tensors expected for this material and taking into account the strongly competing interactions between the spins which lead to a frustrated ground state. As a detailed quantitative relation between effective $g$-factors as determined from ESR and the local $g$-tensors obtained by neutron scattering [Ross et al., Phys. Rev. B 93, 014433 (2016)] is still sought after, our work motivates further theoretical investigations of the low-energy excitations in bond-disordered pyrochlores., Comment: 9 pages, 6 figures

Ba4NbMn3O12 is reported, synthesized by a solid state method in air. The crystal structure, determined by performing refinements on room temperature powder X-ray diffraction data by the Rietveld method, consists of Mn3O12 trimers in the configuration of three face-sharing MnO6 octahedra, with the trimers arranged in triangular planes. An effective moment of 4.82 {\mu}B/f.u is observed and competing antiferromagnetic and ferromagnetic interactions between Mn ions are inferred from the Weiss temperature of -4 K and the ferrimagnetic ordering transition of approximately 42 K. Ba4NbMn3O12 is a semiconductor with a transport activation energy of 0.37 eV., Comment: 19 pages, 7 figures

We report the crystal field levels of several newly-discovered rare-earth kagome compounds: $\rm{Nd_3Sb_3Mg_2O_{14}}$, $\rm{Nd_3Sb_3Zn_2O_{14}}$, and $\rm{Pr_3Sb_3Mg_2O_{14}}$. We determine the CEF Hamiltonian by fitting to neutron scattering data using a point-charge Hamiltonian as an intermediate fitting step. The fitted Hamiltonians accurately reproduce bulk susceptibility measurements, and the results indicate easy-axis ground state doublets for $\rm{Nd_3Sb_3Mg_2O_{14}}$ and $\rm{Nd_3Sb_3Zn_2O_{14}}$, and a singlet ground state for $\rm{Pr_3Sb_3Mg_2O_{14}}$. These results provide the groundwork for future investigations of these compounds and a template for CEF analysis of other low-symmetry materials., Comment: 10 pages, 10 figures, and 10 pages supplemental materials

We report the magnetic structure and electronic properties of the honeycomb antiferromagnet $\rm NaNi_2BiO_{5.66}$. We find magnetic order with moments along the $c$ axis for temperatures below $T_{c1}=6.3(1)\>{\rm K}$ and then in the honeycomb plane for $T < T_{c2}=4.8(1)\>{\rm K}$ with a counterrotating pattern and an ordering wave vector ${\bf q}=(\frac{1}{3},\> \frac{1}{3},\> 0.15(1))$. Density functional theory and electron spin resonance indicate this is high-spin Ni$^{3+}$ magnetism near a high to low spin transition. The ordering wave vector, in-plane magnetic correlations, missing entropy, spin state, and superexchange pathways are all consistent with bond-dependent Kitaev-$\Gamma$-Heisenberg exchange interactions in $\rm NaNi_2BiO_{6-\delta}$., Comment: 12 pages, 11 figures, 8 pages of appendices

A new pyrochlore compound, NaCaNi$_2$F$_7$, was recently synthesized and has a single magnetic site with spin-1 Ni$^{2+}$. We present zero field (ZF) and longitudinal field (LF) muon spin rotation ($\mu$SR) measurements on this pyrochlore. Density functional theory (DFT) calculations show that the most likely muon site is located between two fluorine ions, but off-centre. A characteristic F-$\mu$-F muon spin polarization function is observed at high temperatures where Ni spin fluctuations are sufficiently rapid. The Ni$^{2+}$ spins undergo spin freezing into a disordered ground state below 4~K, with a characteristic internal field strength of 140~G. Persistent Ni spin dynamics are present to our lowest temperatures (75~mK), a feature characteristic of many geometrically frustrated magnetic systems.

The synthesis, structural description, temperature dependent magnetic susceptibilities and field dependent magnetizations of a series of rare earth garnets of the form Li3RE3Te2O12 (RE = Y, La, Pr, Nd, Sm-Lu) are reported. The structure of Li3Dy3Te2O12 is refined from powder X-ray diffraction data. The field dependent magnetizations saturate for some of the members by 9 Tesla at 2 K. Of particular interest for further study in this family is the behavior of the Pr, Tb, Dy and Yb garnets., Comment: 23 pages, 4 tables, 17 figures

The single ion physics of Ho$_2$Ti$_2$O$_7$ is well-understood to produce strong Ising anisotropy, which is an essential ingredient to its low-temperature spin ice state. We present inelastic neutron scattering measurements on Ho$_2$Ti$_2$O$_7$ that reveal a clear inconsistency with its established single ion Hamiltonian. Specifically, we show that a crystal field doublet near 60~meV is split by approximately 3~meV. Furthermore, this crystal field splitting is not isolated to Ho$_2$Ti$_2$O$_7$ but can also be found in its chemical pressure analogs, Ho$_2$Ge$_2$O$_7$ and Ho$_2$Sn$_2$O$_7$. We demonstrate that the origin of this effect is a vibronic bound state, resulting from the entanglement of a phonon and crystal field excitation. We derive the microscopic Hamiltonian that describes the magneto-elastic coupling and provides a quantitative description of the inelastic neutron spectra.

Novel broken symmetry states can spontaneously form due to Coulomb interactions in electronic systems with multiple internal degrees of freedom. Multi-valley materials offer an especially rich setting for the emergence of such states, which have potential electronic and optical applications. To date, identification of these broken symmetry phases has mostly relied on the examination of macroscopic transport or optical properties. Here we demonstrate a powerful direct approach by visualizing the wave functions of bismuth surface states with a scanning tunneling microscope. Strong spin-orbit coupling on the surface of bismuth leads to six degenerate teardrop-shaped hole pockets. Our spectroscopic measurements reveal that this degeneracy is fully lifted at high magnetic field as a result of exchange interactions, and we are able to determine the nature of valley ordering by imaging the broken symmetry Landau level (LL) wave functions. The remarkable spatial features of singly degenerate LL wave functions near isolated defects contain unique signatures of interference between spin-textured valleys, which identify the electronic ground state as a quantum Hall ferroelectric. Our observations confirm the recent prediction that interactions in strongly anisotropic valley systems favor the occupation of a single valley, giving rise to emergent ferroelectricity in the surface state of bismuth., Comment: Accepted to Nature Physics, available through Advance Online Publication