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Research in topological matter has expanded to include the Dirac and Weyl semimetals 1-10 , which feature three-dimensional Dirac states protected by symmetry. Zirconium pentatelluride has been of recent interest as a potential Dirac or Weyl semimetal material. Here, we report the results of experiments performed by in situ three-dimensional double-axis rotation to extract the full 4π solid angular dependence of the transport properties. A clear anomalous Hall effect is detected in every sample studied, with no magnetic ordering observed in the system to the experimental sensitivity of torque magnetometry. Large anomalous Hall signals develop when the magnetic field is rotated in the plane of the stacked quasi-two-dimensional layers, with the values vanishing above about 60 K, where the negative longitudinal magnetoresistance also disappears. This suggests a close relation in their origins, which we attribute to the Berry curvature generated by the Weyl nodes.

The high-pressure behavior of germanates is of interest as these compounds serve as analogs for silicates of the deep Earth. Current theoretical and experimental studies of iron germanate, FeGeO3, are limited. Here, we have examined the behavior of FeGeO3 to 127 GPa using the laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction. Upon compression at room temperature, the ambient-pressure clinopyroxene phase transforms to a disordered triclinic phase [FeGeO3 (II)] at ~ 18 GPa in agreement with earlier studies. An additional phase transition to FeGeO3 (III) occurs above 54 GPa at room temperature. Laser-heating experiments (~ 1200–2200 K) were conducted at three pressures (33, 54, and 123 GPa) chosen to cover the stability regions of different GeO2 polymorphs. In all cases, we observe that FeGeO3 dissociates into GeO2 + FeO at high pressure and temperature conditions. Neither the perovskite nor the post-perovskite phase was observed up to 127 GPa at ambient or high temperatures. The results are consistent with the behavior of FeSiO3, which also dissociates into a mixture of the oxides (FeO + SiO2) at least up to 149 GPa.

We present time-of-flight neutron total scattering and polarized neutron scattering measurements of the magnetically frustrated compounds NaCaCo2F7 and NaSrCo2F7, which belong to a class of recently discovered pyrochlore compounds based on transition metals and fluorine. The magnetic pair distribution function (mPDF) technique is used to analyze and model the total scattering data in real space. We find that a previously proposed model of short-range XY-like correlations with a length scale of 10-15 Å, combined with nearest-neighbor collinear antiferromagnetic correlations, accurately describes the mPDF data at low temperature, confirming the magnetic ground state in these materials. This model is further verified by the polarized neutron scattering data. From an analysis of the temperature dependence of the mPDF and polarized neutron scattering data, we find that short-range correlations persist on the nearest-neighbor length scale up to 200 K, approximately two orders of magnitude higher than the spin freezing temperatures of these compounds. These results highlight the opportunity presented by these new pyrochlore compounds to study the effects of geometric frustration at relatively high temperatures, while also advancing the mPDF technique and providing an opportunity to investigate a genuinely short-range-ordered magnetic ground state directly in real space.

We report the experimental discovery of intrinsic ferromagnetism in Cr2Ge2Te6 atomic layers by scanning magneto-optic Kerr microscopy. In this 2D van der Waals ferromagnet, unprecedented control of transition temperature is realized via small magnetic fields.

The realization of long-range ferromagnetic order in two-dimensional van der Waals crystals, combined with their rich electronic and optical properties, could lead to new magnetic, magnetoelectric and magneto-optic applications. In two-dimensional systems, the long-range magnetic order is strongly suppressed by thermal fluctuations, according to the Mermin-Wagner theorem; however, these thermal fluctuations can be counteracted by magnetic anisotropy. Previous efforts, based on defect and composition engineering, or the proximity effect, introduced magnetic responses only locally or extrinsically. Here we report intrinsic long-range ferromagnetic order in pristine Cr2Ge2Te6 atomic layers, as revealed by scanning magneto-optic Kerr microscopy. In this magnetically soft, two-dimensional van der Waals ferromagnet, we achieve unprecedented control of the transition temperature (between ferromagnetic and paramagnetic states) using very small fields (smaller than 0.3 tesla). This result is in contrast to the insensitivity of the transition temperature to magnetic fields in the three-dimensional regime. We found that the small applied field leads to an effective anisotropy that is much greater than the near-zero magnetocrystalline anisotropy, opening up a large spin-wave excitation gap. We explain the observed phenomenon using renormalized spin-wave theory and conclude that the unusual field dependence of the transition temperature is a hallmark of soft, two-dimensional ferromagnetic van der Waals crystals. Cr2Ge2Te6 is a nearly ideal two-dimensional Heisenberg ferromagnet and so will be useful for studying fundamental spin behaviours, opening the door to exploring new applications such as ultra-compact spintronics.

A long-standing issue in topological insulator research has been to find a bulk single crystal material that provides a high-quality platform for characterizing topological surface states without interference from bulk electronic states. This material would ideally be a bulk insulator, have a surface state Dirac point energy well isolated from the bulk valence and conduction bands, display quantum oscillations from the surface state electrons and be growable as large, high-quality bulk single crystals. Here we show that this material obstacle is overcome by bulk crystals of lightly Sn-doped Bi1.1Sb0.9Te2S grown by the vertical Bridgman method. We characterize Sn-BSTS via angle-resolved photoemission spectroscopy, scanning tunnelling microscopy, transport studies, X-ray diffraction and Raman scattering. We present this material as a high-quality topological insulator that can be reliably grown as bulk single crystals and thus studied by many researchers interested in topological surface states.

Topological insulators (TIs) are novel materials that manifest spin-polarizedDirac states on their surfaces or at interfaces made with conventional matter.We have measured the electron kinetics of bulk doped TI Bi$_2$Se$_3$ with angleresolved photoemission spectroscopy while depositing cathodic and anodicadatoms on the TI surfaces to add charge carriers of the opposite sign frombulk dopants. These P-N overlayer interfaces create Dirac point transportregimes and larger interface potentials than previous N-N type surfacedeposition studies, revealing unconventional Rashba-like and surface-bulkelectron interactions, and an unusual characteristic distribution of spectralweight near the Dirac point in TI Dirac point interfaces. The electronicstructures of P-N doped topological interfaces observed in these experimentsare an important step towards the understanding of solid interfaces withtopological materials.

We show that the strongly spin-orbit coupled materials Bi2Te3 and Sb2Te3(first non-Bi topological insulator) and their derivatives belong to the Z2(Time-Reversal-Protected, elastic backscattering suppressed)topological-insulator class. Using a combination of first-principlestheoretical calculations and photoemission spectroscopy, we directly show thatBi2Te3 is a large spin-orbit-induced indirect bulk band gap (about 150 meV)semiconductor whose surface is characterized by a single topological spin-Diraccone. The electronic structure of self-doped Sb2Te3 exhibits similar Z2topological properties. We demonstrate that the dynamics of surface spin-onlyDirac fermions can be controlled through systematic Mn doping, making thesematerials classes potentially suitable for exploring novel topological physics.We emphasize (theoretically and experimentally) that the Dirac node is wellwithin the bulk-gap and not degenerate with the bulk valence band.

Three dimensional (3D) topological insulators are quantum materials with aspin-orbit induced bulk insulating gap that exhibit quantum-Hall-like phenomenain the absence of applied magnetic fields. The proposed applications oftopological insulators in device geometries rely on the ability to tune thechemical potential on their surfaces in the vicinity of the Dirac node. Here,we demonstrate a suite of surface control methods based on a combination ofphoto-doping and molecular-doping to systematically tune the Dirac fermiondensity on the topological (111) surface of Bi2Te3. Their efficacy isdemonstrated via direct electronic structure topology measurements using highresolution angle-resolved photoemission spectroscopy (ARPES). These resultsopen up new opportunities for probing topological behavior of Dirac electronson the Bi2Te3 surface. At least one of the methods demonstrated here can besuccessfully applied to other topological insulators such as the Bi{1-x}Sb{x},Sb2Te3 and Bi2Se3 which will be shown elsewhere. More importantly, our methodsof topological surface state manipulation demonstrated here are highly suitablefor future spectroscopic studies of topological phenomena which will complementthe transport results gained from the traditional electrical gating techniques.

Electron systems that possess light-like dispersion relations or the conicalDirac spectrum, such as graphene and bismuth, have recently been shown toharbor unusual collective states in high magnetic fields. Such states arepossible because their light-like electrons come in spin pairs that arechiral,which means that their direction of propagation is tied to a quantitycalled pseudospin that describes their location in the crystal lattice. Anemerging direction in quantum materials research is the manipulation of atomicspin-orbit coupling to simulate the effect of a spin dependent magneticfield,in attempt to realize novel spin phases of matter. This effect has beenproposed to realize systems consisting of unpaired Dirac cones that arehelical, meaning their direction of propagation is tied to the electron spinitself, which are forbidden to exist in graphene or bismuth. The experimentalexistence of topological order can not be determined without spin-resolvedmeasurements. Here we report a spin-and angle-resolved photoemission study ofthe hexagonal surface of the Bi2Te3 and Bi{2-x}MnxTe3 series, which is found toexhibit a single helical Dirac cone that is fully spin-polarized. Ourobservations of a gap in the bulk spin-degenerate band and a spin-resolvedsurface Dirac node close to the chemical potential show that the low energydynamics of Bi2Te3 is dominated by the unpaired spin-helical Dirac modes. Ourspin-texture measurements prove the existence of a rare topological phase inthis materials class for the first time, and suggest its suitability for novel2D Dirac spin device applications beyond the chiral variety or traditionalgraphene.

Through-focus electron holography and quantitative electron diffraction have been applied to determine the structure of the 21 K superconductor ThPdx B6-2x with x ≈ 0.65. This compound adopts a cubic structure (a = 0.42nm; space group, Pm 3 m), which is an intermediate between the structures of the CaB6 and Cu3Au. Short-range ordering results in a superstructure as is evident from diffuse streaks in the electron diffraction pattern. © 1995 Taylor & Francis Ltd.

Through-focus electron holography and quantitative electron diffraction have been applied to determine the structure of the 21 K superconductor ThPdx B6-2x with x ≈ 0.65. This compound adopts a cubic structure (a = 0.42nm; space group, Pm 3 m), which is an intermediate between the structures of the CaB6 and Cu3Au. Short-range ordering results in a superstructure as is evident from diffuse streaks in the electron diffraction pattern. © 1995 Taylor & Francis Ltd.

Several specimens in the system Th-Pd-B-C with nominal compositions ThPd3B2C and ThPd3B3C (as melted and annealed), containing superconducting phases exhibiting Tc's of 14.5 K and 21 K, have been studied with EPMA, electron diffraction, EDX element analysis and HREM. A number of phases (approximate compositions are given) have been observed: (1) ThPd2B2C, I-centered tetragonal, a = 0.375, c = 1.07 nm, which can be related to the Tc of 14.5 K; (2) ThPd0.65B4.7, P-type cubic, a = 0.42 nm, with short-range ordered superstructure resulting in diffuse streaks in the electron-diffraction pattern; this phase is suggested to be the 21 K superconducting phase; (3) ThPd3 P-type hexagonal, a = 0.58, c = 0.98 nm; (4) ThPd8B3, I-centred orthorhombic a = 0.84, b = 0.90, c = 1.67 nm; (5) ThPd3B2C, with unknown structure; (6) ThB4, tetragonal a = 0.72, c = 0.41 nm; (7) graphitic C. Annealing of the specimen ThPd3B2C at 1050°C results in the disappearance of ThPd2B2C and the loss of the Tc at 14.5 K but a strong increase of the 21 K superconducting fraction with a corresponding strong increase of the phases ThPd0.65B4.7 and ThPd3. HREM indicates that ThPd0.65B4.7 adopts a modified CaB6 structure in which the cube face, 1 2, 1 2, 0, which is vacant in CaB6, is partially occupied by Pd with the removal of some B atoms. The superstructure indicates a short-range ordering (clustering) of the Pd atoms. © 1994.

The compound ThPd2B2C has been studied with high-resolution electron microscopy and electron diffraction. ThPd2B2-xC adopts the LuNi2B2C structure with a layer sequence (ByNi2BzLuC)n in which the B composition and the exact position of the C are still uncertain. No intergrowths or planar defects are observed. The c-axis in the thinner parts of the crystals (

We report the observation of superconductivity in intermetallic thorium-based boride carbides. For ThM2B2C, superconducting Tc's of 6 K (M=Ni), 6.5 K (M=Pt), and 14.5 K (M=Pd) have been measured. These compounds are isostructural with the recently reported LnNi2B2C superconductors. We have also observed bulk superconductivity in an as-yet-undetermined thorium-palladium-boron-carbon quaternary phase at 21.5 K. Critical-field measurements on the two palladium-based superconductors, as well as the pressure dependence of Tc for the 21.5 K superconductor, are reported. © 1994.

Experiments on quantum materials have uncovered many interesting quantum phases ranging from superconductivity to a variety of topological quantum matter including the recently observed fractional quantum anomalous Hall insulators. The findings have come in parallel with the development of approaches to probe the rich excitations inherent in such systems. In contrast to observing electrically charged excitations, the detection of charge-neutral electronic excitations in condensed matter remains difficult, although they are essential to understanding a large class of strongly correlated phases. Low-energy neutral excitations are especially important in characterizing unconventional phases featuring electron fractionalization, such as quantum spin liquids, spin ices and insulators with neutral Fermi surfaces. In this Perspective, we discuss searches for neutral fermionic, bosonic or anyonic excitations in unconventional insulators, highlighting theoretical and experimental progress in probing excitonic insulators, new quantum spin liquid candidates and emergent correlated insulators based on two-dimensional layered crystals and moiré materials. We outline the promises and challenges in probing and using quantum insulators, and discuss exciting new opportunities for future advancements offered by ideas rooted in next-generation quantum materials, devices and experimental schemes., Competing Interests: Competing interests The authors declare no competing interests., (© 2024. Springer Nature Limited.)

Pyroxenes (AMX 2 O 6 ) consisting of infinite one-dimensional edge-sharing MO 6 chains and bridging XO 4 tetrahedra are fertile ground for finding quantum materials. Thus, here, we have studied calcium cobalt germanate (CaCoGe 2 O 6 ) and calcium cobalt silicate (CaCoSi 2 O 6 ) crystals in depth. Heat capacity data show that the spins in both compounds are dominantly Ising-like, even after being manipulated by high magnetic fields. On cooling below the Néel temperatures, a sharp field-induced transition in magnetization is observed for CaCoGe 2 O 6 , while multiple magnetization plateaus beneath the full saturation moment are spotted for CaCoSi 2 O 6 . Our analysis shows that these contrasting behaviors potentially arise from the different electron configurations of germanium and silicon, in which the 3d orbitals are filled in the former but empty in the latter, enabling electron hopping. Thus, silicate tetrahedra can aid the interchain superexchange pathway between cobalt(II) ion centers, while germanate ones tend to block it during magnetization.

Charge-transfer excitations are of paramount importance for understanding the electronic structure of copper-oxide based high-temperature superconductors. In this study, we investigate the response of a Bi 2 Sr 2 CaCu 2 O 8 + δ crystal to the charge redistribution induced by an infrared ultrashort pulse. Element-selective time-resolved core-level photoelectron spectroscopy with a high energy resolution allows disentangling the dynamics of oxygen ions with different coordination and bonds thanks to their different chemical shifts. Our experiment shows that the O 1s component arising from the Cu-O planes is significantly perturbed by the infrared light pulse. Conversely, the apical oxygen, also coordinated with Sr ions in the Sr-O planes, remains unaffected. This result highlights the peculiar behavior of the electronic structure of the Cu-O planes. It also unlocks the way to study the out-of-equilibrium electronic structure of copper-oxide-based high-temperature superconductors by identifying the O 1s core-level emission originating from the oxygen ions in the Cu-O planes. This ability could be critical to gain information about the strongly-correlated electron ultrafast dynamical mechanisms in the Cu-O plane in the normal and superconducting phases., (© 2024. The Author(s).)

Nitrogen-vacancy (NV) centers in diamond are a promising platform for nanoscale NMR sensing. Despite significant progress toward using NV centers to detect and localize nuclear spins down to the single spin level, NV-based spectroscopy of individual, intact, arbitrary target molecules remains elusive. Such sensing requires that target molecules are immobilized within nanometers of NV centers with long spin coherence. The inert nature of diamond typically requires harsh functionalization techniques such as thermal annealing or plasma processing, limiting the scope of functional groups that can be attached to the surface. Solution-phase chemical methods can be readily generalized to install diverse functional groups, but they have not been widely explored for single-crystal diamond surfaces. Moreover, realizing shallow NV centers with long spin coherence times requires highly ordered single-crystal surfaces, and solution-phase functionalization has not yet been shown with such demanding conditions. In this work, we report a versatile strategy to directly functionalize C-H bonds on single-crystal diamond surfaces under ambient conditions using visible light, forming C-F, C-Cl, C-S, and C-N bonds at the surface. This method is compatible with NV centers within 10 nm of the surface with spin coherence times comparable to the state of the art. As a proof-of-principle demonstration, we use shallow ensembles of NV centers to detect nuclear spins from surface-bound functional groups. Our approach to surface functionalization opens the door to deploying NV centers as a tool for chemical sensing and single-molecule spectroscopy., Competing Interests: Competing interests statement:L.V.H.R., S.T.N., R.R.K., and N.P.d.L. have filed for a patent that relates to this work. The authors declare no other competing interests.

Superconducting states onsetting at moderately high temperatures have been observed in epitaxially stabilized RE NiO 2 -based thin films. However, recently, it has also been reported that superconductivity at high temperatures is observed in bulk La 3 Ni 2 O 7-δ at high pressure, opening further possibilities for study. Here we report the reduction profile of La 3 Ni 2 O 7 in a stream of 5% H 2 /Ar gas and the isolation of the metastable intermediate phase La 3 Ni 2 O 6.45 , which is based on Ni 2+ . Although this reduced phase does not superconduct at ambient or high pressures, it offers insights into the Ni-327 system and encourages future study of nickelates as a function of oxygen content.

We report superconductivity in the full Heusler compound LiPd 2 Si (space group Fm3̅m , No. 225) at a critical temperature of T c = 1.3 K and a normalized heat capacity jump at T c , Δ C /γ T c = 1.1. The low-temperature isothermal magnetization curves imply type-I superconductivity, as previously observed in LiPd 2 Ge. We show, based on density functional theory calculations and using the molecular orbital theory approach, that while LiPd 2 Si and LiPd 2 Ge share the Pd cubic cage motif that is found in most of the reported Heusler superconductors, they show distinctive features in the electronic structure. This is due to the fact that Li occupies the site which, in other compounds, is filled with an early transition metal or a rare-earth metal. Thus, while a simple valence electron count-property relationship is useful in predicting and tuning Heusler materials, inclusion of the symmetry of interacting frontier orbitals is also necessary for the best understanding., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

van der Waals materials provide a versatile toolbox for the emergence of new quantum phenomena and fabrication of functional heterostructures. Among them, the trihalide VI 3 stands out for its unique magnetic and structural landscape. Here we investigate the spin and orbital magnetic degrees of freedom in the layered ferromagnet VI 3 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) μ B /V atom) in the ferromagnetic state and (b) an instability of the orbital moment in the 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 VI 3 as a prime candidate for the study of orbital quantum effects.

We synthesize four novel quasi-one-dimensional organic-inorganic hybrid iron chloride compounds (CH 3 NH 3 FeCl 3 , CH(NH 2 ) 2 FeCl 3 , C(NH 2 ) 3 FeCl 3 , and C 3 H 5 N 2 FeCl 3 ) and characterize their structural and magnetic properties. These materials crystallize in a hexagonal perovskite-type structure, constituting a triangular array of face-sharing iron chloride octahedra chains running along the c -axis, isolated from one another by the organic cation. Through magnetization and heat capacity measurements, we find that the intrachain coupling is weakly ferromagnetic for each variant. Importantly, this work underscores the utility of solid-state chemistry approaches in synthesizing new organic-inorganic hybrid materials.

A layered rhombohedral polymorph of indium(III) triiodide is synthesized at high pressure and temperature. The unit cell symmetry and approximate dimensions are determined by single crystal X-ray diffraction. Its R 3̄ crystal structure, with a = 7.214 Å, and c = 20.47 Å, is refined by the Rietveld method on powder X-ray diffraction data. The crystal structure is based on InI 6 octahedra sharing edges to form honeycomb lattice layers, though with considerable stacking defects. Different from ambient pressure InI 3 , which has a monoclinic molecular structure and a light-yellow color, high pressure InI 3 is layered and has an orange color. The band gaps of both the monoclinic and rhombohedral variants of InI 3 are estimated from diffuse reflectance measurements.

We report the synthesis and magnetic characterization of stuffed rare-earth gallium garnets, RE 3+ x Ga 5- x O 12 (RE = Lu, Yb, Er, Dy, Gd), for x up to 0.5. The excess rare-earth ions partly fill the octahedral sites normally fully occupied by Ga 3+ , forming disordered pairs of corner-shared face-sharing magnetic tetrahedra. The Curie-Weiss constants and observed effective moments per rare-earth are smaller than are seen for the unstuffed gallium garnets. No significant change in the field-dependent magnetization is observed but missing entropy is seen when integrating the heat capacity down to 0.5 K.

Magnetic toroidicity is an uncommon type of magnetic structure in solid-state materials. Here, we experimentally demonstrate that collinear spins in a material with R-3 lattice symmetry can host a significant magnetic toroidicity, even parallel to the ordered spins. Taking advantage of a single crystal sample of CoTe 6 O 13 with an R-3 space group and a Co 2+ triangular sublattice, temperature-dependent magnetic, thermodynamic, and neutron diffraction results reveal A-type antiferromagnetic order below 19.5 K, with magnetic point group -3' and k = (0,0,0). Our symmetry analysis suggests that the missing mirror symmetry in the lattice could lead to the local spin canting for a toroidal moment along the c axis. Experimentally, we observe a large off-diagonal magnetoelectric coefficient of 41.2 ps/m that evidences the magnetic toroidicity. In addition, the paramagnetic state exhibits a large effective moment per Co 2+ , indicating that the magnetic moment in CoTe 6 O 13 has a significant orbital contribution. CoTe 6 O 13 embodies an excellent opportunity for the study of next-generation functional magnetoelectric materials., (© 2023. The Author(s).)

Interacting electrons in one dimension (1D) are governed by the Luttinger liquid (LL) theory in which excitations are fractionalized. Can a LL-like state emerge in a 2D system as a stable zero-temperature phase? This question is crucial in the study of non-Fermi liquids. A recent experiment identified twisted bilayer tungsten ditelluride (tWTe 2 ) as a 2D host of LL-like physics at a few kelvins. Here we report evidence for a 2D anisotropic LL state down to 50 mK, spontaneously formed in tWTe 2 with a twist angle of ~ 3 o . While the system is metallic-like and nearly isotropic above 2 K, a dramatically enhanced electronic anisotropy develops in the millikelvin regime. In the anisotropic phase, we observe characteristics of a 2D LL phase including a power-law across-wire conductance and a zero-bias dip in the along-wire differential resistance. Our results represent a step forward in the search for stable LL physics beyond 1D., (© 2023. The Author(s).)

We report the high-pressure synthesis (6 GPa, 1200 °C) and ambient-pressure characterization of hexagonal HP-BaCoO 3 . The material (with a 2H crystal structure) has a short intrachain Co-Co distance of about 2.07 Å. Our magnetization investigation revealed robust diamagnetic behavior below approximately 130 K when the material was exposed to weak applied magnetic fields (10 Oe) and a distinct "half-levitation" phenomenon below that temperature, as is often observed for superconductors. Its field-dependent magnetization profile, however, unveils the characteristics of ferromagnetism, marked by a substantial magnetic retentivity of 0.22(1) μ B /Co at a temperature of 2 K. Electrical resistivity measurements indicate that HP-BaCoO 3 is a ferromagnetic insulator, not a superconductor.

A series of garnets of formula Er 3+ x Ga 5- x O 12 are 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 dodecahedral (24c) garnet site plus some of the octahedral site (16a) in place of the Ga normally present. Long-range antiferromagnetic order with spin-ice-like frustration is suggested by the transition temperature ( T N ≈ 0.8 K) being 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. The field-dependent magnetic entropy trend is consistent with the reported behavior for frustrated triangular magnetic systems: an increasing transition temperature with a broader hump as the applied field increases [Xing, J.; Phys. Rev. Mater. 2019, 3(11), 114413;Filippi, J.; Solid State Commun. 1977, 23(9), 613-616; Bloxsom, J. A. Thermal and Magnetic Studies of Spin Ice Compounds. University College London, 2016].

Atomic defects in the solid state are a key component of quantum repeater networks for long-distance quantum communication 1 . Recently, there has been significant interest in rare earth ions 2-4 , in particular Er 3+ for its telecom band optical transition 5-7 that allows long-distance transmission in optical fibres. However, the development of repeater nodes based on rare earth ions has been hampered by optical spectral diffusion, precluding indistinguishable single-photon generation. Here, we implant Er 3+ into CaWO 4 , a material that combines a non-polar site symmetry, low decoherence from nuclear spins 8 and is free of background rare earth ions, to realize significantly reduced optical spectral diffusion. For shallow implanted ions coupled to nanophotonic cavities with large Purcell factor, we observe single-scan optical linewidths of 150 kHz and long-term spectral diffusion of 63 kHz, both close to the Purcell-enhanced radiative linewidth of 21 kHz. This enables the observation of Hong-Ou-Mandel interference 9 between successively emitted photons with a visibility of V = 80(4)%, measured after a 36 km delay line. We also observe spin relaxation times T 1,s  = 3.7 s and T 2,s  > 200 μs, with the latter limited by paramagnetic impurities in the crystal instead of nuclear spins. This represents a notable step towards the construction of telecom band quantum repeater networks with single Er 3+ ions., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

We report on the far-infrared, temperature-dependent optical properties of a CrI3 transition metal halide single crystal, a van der Waals ferromagnet (FM) with a Curie temperature of 61 K. In addition to the expected phonon modes determined by the crystalline symmetry, the optical reflectance and transmittance spectra of CrI3 single crystals show many other excitations as a function of temperature as a consequence of the combination of a strong lattice anharmonicity and spin-phonon coupling. This complex vibrational spectrum highlights the presence of entangled interactions among the different degrees of freedom in CrI3.

Over the past decades, superconducting qubits have emerged as one of the leading hardware platforms for realizing a quantum processor. Consequently, researchers have made significant effort to understand the loss channels that limit the coherence times of superconducting qubits. A major source of loss has been attributed to two level systems that are present at the material interfaces. It is recently shown that replacing the metal in the capacitor of a transmon with tantalum yields record relaxation and coherence times for superconducting qubits, motivating a detailed study of the tantalum surface. In this work, the chemical profile of the surface of tantalum films grown on c-plane sapphire using variable energy X-ray photoelectron spectroscopy (VEXPS) is studied. The different oxidation states of tantalum that are present in the native oxide resulting from exposure to air are identified, and their distribution through the depth of the film is measured. Furthermore, it is shown how the volume and depth distribution of these tantalum oxidation states can be altered by various chemical treatments. Correlating these measurements with detailed measurements of quantum devices may elucidate the underlying microscopic sources of loss., (© 2023 The Authors. Advanced Science published by Wiley-VCH GmbH.)

Here we report the first observation of the concurrent breakdown of the strange metal (SM) normal state and superconductivity at a pressure-induced quantum critical point in Ca 10 (Pt 4 As 8 )((Fe 0.97 Pt 0.03 ) 2 As 2 ) 5 superconductor. We find that, upon suppressing the superconducting state, the power exponent (α) changes from 1 to 2, and the slope of the temperature-linear resistivity per FeAs layer (A □ ) gradually diminishes. At a critical pressure, A □ and superconducting transition temperature (T c ) go to zero concurrently, where a quantum phase transition from a superconducting state with a SM normal state to a non-superconducting Fermi liquid state occurs. Scaling analysis reveals that the change of A □ with T c obeys the relation of T c  ~ (A □ ) 0.5 , similar to what is seen in other chemically doped unconventional superconductors. These results suggest that there is a simple but powerful organizational principle of connecting the SM normal state with the high-T c superconductivity., (© 2023. The Author(s).)

Neutral silicon vacancy centers (SiV^{0}) in diamond are promising candidates for quantum applications; however, stabilizing SiV^{0} requires high-purity, boron-doped diamond, which is not a readily available material. Here, we demonstrate an alternative approach via chemical control of the diamond surface. We use low-damage chemical processing and annealing in a hydrogen environment to realize reversible and highly stable charge state tuning in undoped diamond. The resulting SiV^{0} centers display optically detected magnetic resonance and bulklike optical properties. Controlling the charge state tuning via surface termination offers a route for scalable technologies based on SiV^{0} centers, as well as charge state engineering of other defects.

Electronic structure calculations indicate that the Sr 2 FeSbO 6 double perovskite has a flat-band set just above the Fermi level that includes contributions from ordinary subbands with weak kinetic electron hopping plus a flat subband that can be attributed to the lattice geometry and orbital interference. To place the Fermi energy in that flat band, electron-doped samples with formulas Sr 2- x La x FeSbO 6 (0 ≤ x ≤ 0.3) were synthesized, and their magnetism and ambient temperature crystal structures were determined by high-resolution synchrotron X-ray powder diffraction. All materials appear to display an antiferromagnetic-like maximum in the magnetic susceptibility, but the dominant spin coupling evolves from antiferromagnetic to ferromagnetic on electron doping. Which of the three subbands or combinations is responsible for the behavior has not been determined.

Quantum spin liquids (QSLs) are topologically ordered states of matter that host fractionalized excitations. A particular route towards a QSL is via strongly bond-dependent interactions on the hexagonal lattice. A number of Ru- and Ir-based candidate Kitaev QSL materials have been pursued, but all have appreciable non-Kitaev interactions. Using time-domain terahertz spectroscopy, we observed a broad magnetic continuum over a wide range of temperatures and fields in the honeycomb cobalt-based magnet BaCo 2 (AsO 4 ) 2 , which has been proposed to be a more ideal version of a Kitaev QSL. Applying an in-plane magnetic field of ~0.5 T suppresses the magnetic order, and at higher fields, applying the field gives rise to a spin-polarized state. Under a 4 T magnetic field that was oriented principally out of plane, a broad magnetic continuum was observed that may be consistent with a field-induced QSL. Our results indicate BaCo 2 (AsO 4 ) 2 is a promising QSL candidate., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)