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Spin-mechanical coupling is vital in diverse fields including spintronics, sensing and quantum transduction. Two-dimensional (2D) magnetic materials provide a unique platform for investigating spin-mechanical coupling, attributed to their mechanical flexibility and novel spin orderings. However, studying spin-mechanical coupling in 2D magnets presents challenges in probing mechanical deformation and thermodynamic properties change at nanoscale. Here we use nano opto-electro-mechanical interferometry to mechanically detect the phase transition and magnetostriction effect in multilayer CrSBr, an air-stable antiferromagnets with large magnon-exciton coupling. The transitions among antiferromagnetism, spin-canted ferromagnetism and paramagnetism are visualized by optomechanical frequency anomalies. Nontrivial magnetostriction coefficient 2.3x10^(-5) and magnetoelastic coupling strength on the order of 10^6 J/m^3 have been found. Moreover, we demonstrate the substantial tunability of the magnetoelastic constant by nearly 50% via gate-induced strain. Our findings demonstrate the strong spin-mechanical coupling in CrSBr and paves the way for developing sensitive magnetic sensing and efficient quantum transduction at atomically thin limit.

Excitons, bound electron-hole pairs, influence the optical properties in strongly interacting solid state systems. Excitons and their associated many-body physics are typically most stable and pronounced in monolayer materials. Bulk systems with large exciton binding energies, on the other hand, are rare and the mechanisms driving their stability are still relatively unexplored. Here, we report an exceptionally large exciton binding energy in single crystals of the bulk van der Waals antiferromagnet CrSBr. Utilizing state-of-the-art angle-resolved photoemission spectroscopy and self-consistent ab-initio GW calculations, we present direct spectroscopic evidence that robust electronic and structural anisotropy can significantly amplify the exciton binding energy within bulk crystals. Furthermore, the application of a vertical electric field enables broad tunability of the optical and electronic properties. Our results indicate that CrSBr is a promising material for the study of the role of anisotropy in strongly interacting bulk systems and for the development of exciton-based optoelectronics.

Transition metal dichalcogenides have received much attention in the past decade not only due to the new fundamental physics, but also due to the emergent applications in these materials. Currently chalcogenide deficiencies in TMDs are commonly believed either during the high temperature growth procedure or in the nanofabrication process resulting significant changes of their reported physical properties in the literature. Here we perform a systematic study involving pristine stochiometric HfSe2, Se deficient HfSe1.9 and HfSe1.8. Stochiometric HfSe2 transport results show semiconducting behavior with a gap of 1.1eV. Annealing HfSe2 under high vacuum at room temperature causes the Se loss resulting in HfSe1.9, which shows unconventionally large magnetoresistivity following the extended Kohler's rule at low temperatures below 50 K. Moreover, a clear electrical resistivity crossover, mimicking the metal-insulator transition, is observed in the HfSe1.9 single crystal. Further increasing the degree of deficiency in HfSe1.8 results in complete metallic electrical transport at all temperatures down to 2K. Such a drastic difference in the transport behaviors of stoichiometric and Se-deficient HfSe2 further emphasizes that defect control and engineering could be an effective method that could be used to tailor the electronic structure of 2D materials, potentially unlock new states of matter, or even discover new materials., Comment: 9 pages, 4 figures

Electronic topological phases are renowned for their unique properties, where conducting surface states exist on the boundary of an insulating three-dimensional bulk. While the transport response of the surface states has been extensively studied, the response of the topological hinge modes remains elusive. Here, we investigate a layered topological insulator $\alpha$-Bi$_4$Br$_4$, and provide the first evidence for quantum transport in gapless topological hinge states existing within the insulating bulk and surface energy gaps. Our magnetoresistance measurements reveal pronounced h/e periodic (where h denotes Planck's constant and e represents the electron charge) Aharonov-Bohm oscillation. The observed periodicity, which directly reflects the enclosed area of phase-coherent electron propagation, matches the area enclosed by the sample hinges, providing compelling evidence for the quantum interference of electrons circumnavigating around the hinges. Notably, the h/e oscillations evolve as a function of magnetic field orientation, following the interference paths along the hinge modes that are allowed by topology and symmetry, and in agreement with the locations of the hinge modes according to our scanning tunneling microscopy images. Remarkably, this demonstration of quantum transport in a topological insulator can be achieved using a flake geometry and we show that it remains robust even at elevated temperatures. Our findings collectively reveal the quantum transport response of topological hinge modes with both topological nature and quantum coherence, which can be directly applied to the development of efficient quantum electronic devices., Comment: Nature Physics, in press (2023)

While the surface-bulk correspondence has been ubiquitously shown in topological phases, the relationship between surface and bulk in Landau-like phases is much less explored. Theoretical investigations since 1970s for semi-infinite systems have predicted the possibility of the surface order emerging at a higher temperature than the bulk, clearly illustrating a counterintuitive situation and greatly enriching phase transitions. But experimental realizations of this prediction remain missing. Here, we demonstrate the higher-temperature surface and lower-temperature bulk phase transitions in CrSBr, a van der Waals (vdW) layered antiferromagnet. We leverage the surface sensitivity of electric dipole second harmonic generation (SHG) to resolve surface magnetism, the bulk nature of electric quadrupole SHG to probe bulk spin correlations, and their interference to capture the two magnetic domain states. Our density functional theory calculations show the suppression of ferromagnetic-antiferromagnetic competition at the surface responsible for this enhanced surface magnetism. Our results not only show unexpected, richer phase transitions in vdW magnets, but also provide viable ways to enhance magnetism in their 2D form.

Anisotropy plays a key role in science and engineering. However, the interplay between the material and engineered photonic anisotropies has hardly been explored due to the vastly different length scales. Here we demonstrate a matter-light hybrid system, exciton-polaritons in a 2D antiferromagnet, CrSBr, coupled with an anisotropic photonic crystal (PC) cavity, where the spin, atomic lattice, and photonic lattices anisotropies are strongly correlated, giving rise to unusual properties of the hybrid system and new possibilities of tuning. We show exceptionally strong coupling between engineered anisotropic optical modes and anisotropic excitons in CrSBr, which is stable against excitation densities a few orders of magnitude higher than polaritons in isotropic materials. Moreover, the polaritons feature a highly anisotropic polarization tunable by tens of degrees by controlling the matter-light coupling via, for instance, spatial alignment between the material and photonic lattices, magnetic field, temperature, cavity detuning and cavity quality-factors. The demonstrated system provides a prototype where atomic- and photonic-scale orders strongly couple, opening opportunities of photonic engineering of quantum materials and novel photonic devices, such as compact, on-chip polarized light source and polariton laser.

In this work, we report multiple intercalation stages and universal Tc enhancement of superconductivity in 1T-SnSe2 through Li and organic molecules cointercalation. We observe significantly increased lattice parameters up to 40 {\AA} and dramatically enlarged interlayer distance up to ~11{\AA} in Li and N,N-dimethylformamide (DMF) cointercalated SnSe2. Well-separated cointercalation stages with different stacking patterns have been discovered by carefully controlled reaction time and concentration of solutions. These cointercalation stages are superconductors showing different superconducting signals. In addition, Li and various organic species such as Acetone, Dimethyl sulfoxide (DMSO) and Tetrahydrofuran (THF) have been cointercalated into SnSe2 crystals, all of which show enhanced superconducting Tc compared to solely Li intercalated SnSe2. Our findings may provide more insight to effectively tune electronic structure of the lamellar structure through organic molecules co-regulation, and open a new strategy to engineer the physical properties of these layered materials by controlling their different intercalation stages., Comment: 21 pages,5 figures

Quasi-one-dimensional electron systems display intrinsic instability towards long-range ordered phases at sufficiently low temperatures. The superconducting orders are of particular interest as they can possess either singlet or triplet pairing symmetry and frequently compete with magnetism. Here we report on muon spin rotation and relaxation ($\mathrm{\mu}$SR) study of Rb$_2$Mo$_3$As$_3$ characterised by one of the highest critical temperatures $T_{\rm c}=10.4\ \mathrm{K}$ among quasi-one-dimensional superconductors. The transverse-field $\mathrm{\mu}$SR signal shows enhanced damping below $T_{\rm c}$ due to the formation of vortex lattice. Comparison of vortex lattice broadening against single gap $s-$, $p-$ and $d-$wave models shows the best agreement for the $s-$wave scenario but with the anomalously small superconducting gap, $\Delta_0$, to $T_{\rm c}$ ratio of $2\Delta_0/k_{\rm B}T_{\rm c}=2.74(1)$. The alternative nodal $p-$wave or $d-$wave scenarios with marginally worse goodness of fit would yield more realistic $2\Delta_0/k_{\rm B}T_{\rm c}=3.50(2)$ and $2\Delta_0/k_{\rm B}T_{\rm c}=4.08(1)$, respectively, and thus they cannot be ruled out when accounting for the superconducting state in Rb$_2$Mo$_3$As$_3$., Comment: 6 pages

The paradigm of classifying three-dimensional (3D) topological insulators into strong and weak ones (STI and WTI) opens the door for the discovery of various topological phases of matter protected by different symmetries and defined in different dimensions. However, in contrast to the vast realization of STIs, very few materials have been experimentally identified as being close to WTI. Even amongst those identified, none exists with topological surface states (TSS) exposed in a global bulk band gap that is stable at all temperatures. Here we report the design and observation of an ideal WTI in a quasi-one-dimensional (quasi-1D) bismuth halide, Bi$_{4}$I$_{1.2}$Br$_{2.8}$ (BIB). Via angle-resolved photoemission spectroscopy (ARPES), we identify that BIB hosts TSS on the (100)$\prime$ side surface in the form of two anisotropic $\pi$-offset Dirac cones (DCs) separated in momentum while topologically dark on the (001) top surface. The ARPES data fully determine a unique side-surface Hamiltonian and thereby identify two pairs of non-degenerate helical saddle points and a series of four Lifshitz transitions. The fact that both the surface Dirac and saddle points are in the global bulk band gap of 195 meV, combined with the small Dirac velocities, nontrivial spin texture, and the near-gap chemical potential, qualifies BIB to be not only an ideal WTI but also a fertile ground for topological many-body physics.

In this paper, the electrical transport, thermal transport, and thermoelectric properties of three new Janus STe$_{2}$, SeTe$_{2}$, and Se$_{2}$Te monolayers are systematically studied by first-principles calculations, as well as the comparative with available literature's results using different methods. It is found that the Seebeck coefficient and conductivity have opposite dependence on temperature, and we illustrate this phenomenon in detail. The decrease of the thermoelectric power factor (PF) with temperature originates from the decrease in conductivity. To obtain accurate and convergent lattice thermal conductivity, the root mean square (RMS) is calculated to obtain a reasonable cutoff radius for the calculation of third-order forces. Janus STe$_{2}$, SeTe$_{2}$, and Se$_{2}$Te monolayers exhibit ultra-low lattice thermal conductivity of 0.2, 0.133, and 4.81$\times10^{-4}$ W/mK at 300 K, which result from the strong coupling effect between the acoustic mode and the low-frequency optical branch, low phonon group velocity, small phonon lifetime, and large anharmonicity. Consequently, ultra-high $\textit{ZT}$ values of 2.11 (2.09), 3.28 (4.24), and 3.40 (6.51) for n-type(p-type) carrier doping of STe$_{2}$, SeTe$_{2}$, and Se$_{2}$Te are obtained, indicating that they are promising thermoelectric materials., Comment: 7 pages, 7 figures

Majorana zero modes form as intrinsic defects in an odd-orbital one-dimensional superconductor thus motivating the search for such materials in the pursuit of Majorana physics. Here, we present combined experimental results and first principles calculations which suggest that quasi-one-dimensional K$_2$Cr$_3$As$_3$ may be such a superconductor. Using inelastic neutron scattering we probe the dynamic spin-susceptibilities of K$_2$Cr$_3$As$_3$ and K$_2$Mo$_3$As$_3$ and show the presence of antiferromagnetic spin-fluctuations in both compounds. Below the superconducting transition, these fluctuations gap in K$_2$Mo$_3$As$_3$ but not in K$_2$Cr$_3$As$_3$. Using first principles calculations, we show that these fluctuations likely arise from nesting on one dimensional features of the Fermi surface. Considering these results we propose that while K$_2$Mo$_3$As$_3$ is a conventional superconductor, K$_2$Cr$_3$As$_3$ is likely a spin-triplet, and consequently, topological superconductor., Comment: 8 pages, 4 figures

van der Waals (vdW) magnets are receiving ever-growing attention nowadays due to their significance in both fundamental research on low-dimensional magnetism and potential applications in spintronic devices. High crystalline quality of vdW magnets is key for maintaining intrinsic magnetic and electronic properties, especially when exfoliated down to the 2D limit. Here, ultrahigh-quality air-stable vdW CrSBr crystals are synthesized using the direct vapor-solid synthesis method. The high single crystallinity and spatial homogeneity have been thoroughly evidenced at length scales from sub-mm to atomic resolution by X-ray diffraction, second harmonic generation, and scanning transmission electron microscopy. More importantly, specific heat measurements of these ultrahigh quality CrSBr crystals show three thermodynamic anomalies at 185K, 156K, and 132K, revealing a stage-by-stage development of the magnetic order upon cooling, which is also corroborated with the magnetization and transport results. Our ultrahigh-quality CrSBr can further be exfoliated down to monolayers and bilayers easily, paving the way to integrate them into heterostructures for spintronic and magneto-optoelectronic applications., Comment: 13 pages, 5 figures

We report a layered ternary selenide BaPt4Se6 featuring sesqui-selenide Pt2Se3 layers sandwiched by Ba atoms. The Pt2Se3 layers in this compound can be derived from the Dirac-semimetal PtSe2 phase with Se vacancies that form a honeycomb structure. This structure results in a Pt (VI) and Pt (II) mixed-valence compound with both PtSe6 octahedra and PtSe4 square net coordination configurations. Temperature dependent electrical transport measurements suggest two distinct anomalies: a resistivity crossover, mimic to the metal-insulator (M-I) transition at ~150K, and a resistivity plateau at temperatures below 10K. The resistivity crossover is not associated with any structural, magnetic or charge order modulated phase transitions. Magnetoresistivity, Hall and heat capacity measurements concurrently suggest an existing hidden state below 5K in this system. Angle-resolved photoemission spectroscopy measurements reveal a metallic state and no dramatic reconstruction of the electronic structure up to 200K., Comment: 25 pages, 5 figures

Quasi-one-dimensional (1D) materials provide a superior platform for characterizing and tuning topological phases for two reasons: i) existence for multiple cleavable surfaces that enables better experimental identification of topological classification, and ii) stronger response to perturbations such as strain for tuning topological phases compared to higher dimensional crystal structures. In this paper, we present experimental evidence for a room-temperature topological phase transition in the quasi-1D material Bi$_4$I$_4$, mediated via a first order structural transition between two distinct stacking orders of the weakly-coupled chains. Using high resolution angle-resolved photoemission spectroscopy on the two natural cleavable surfaces, we identify the high temperature $\beta$ phase to be the first weak topological insulator with gapless Dirac cones on the (100) surface and no Dirac crossing on the (001) surface, while in the low temperature $\alpha$ phase, the topological surface state on the (100) surface opens a gap, consistent with a recent theoretical prediction of a higher-order topological insulator beyond the scope of the established topological materials databases that hosts gapless hinge states. Our results not only identify a rare topological phase transition between first-order and second-order topological insulators but also establish a novel quasi-1D material platform for exploring unprecedented physics., Comment: 9 pages, 4 figures, accepted for publication in Phys. Rev. X

We report a combined experimental and theoretical study of the PdSe2-xTex system. With increasing Te fraction, structural evolutions, first from an orthorhombic phase (space group Pbca) to a monoclinic phase (space group C2/c) and then to a trigonal phase (space group P-3m1), are observed accompanied with clearly distinct electrical transport behavior. The monoclinic phase (C2/c) is a completely new polymorphic phase and is discovered within a narrow range of Te composition (0.3 \leq x \leq 0.8). This phase has a different packing sequence from all known transition metal dichalcogenides to date. Electronic calculations and detailed transport analysis of the new polymorphic PdSe1.3Te0.7 phase are presented. In the trigonal phase region, superconductivity with enhanced critical temperature is also observed within a narrow range of Te content (1.0 \leq x \leq 1.2). The rich phase diagram, new polymorphic structure as well as abnormally enhanced superconductivity could further stimulate more interest to explore new types of polymorphs and investigate their transport and electronic properties in the transition metal dichalcogenides family that are of significant interest., Comment: 16 pages, 7 figures

Two-dimensional transition metal dichalcogenide PdTe$_2$ recently attracts much attention due to its phase coexistence of type-II Dirac semimetal and type-I superconductivity. Here we report a 67 % enhancement of superconducting transition temperature in the 1T-PdSeTe in comparison to that of PdTe2 through partial substitution of Te atoms by Se. The superconductivity has been unambiguously confirmed by the magnetization, resistivity and specific heat measurements. 1T-PdSeTe shows type-II superconductivity with large anisotropy and non-bulk superconductivity nature with volume fraction ~ 20 % estimated from magnetic and heat capacity measurements. 1T-PdSeTe expands the family of superconducting transition metal dichalcogenides and thus provides additional insights for understanding superconductivity and topological physics in the 1T-PdTe$_2$ system, Comment: 7 pages, 6 figures

In this work, we have thoroughly studied the effects of flux composition and temperature on the crystal growth of the BaCu2As2 compound. While Pb and CuAs self-flux produce the well-known {\alpha}-phase ThCr2Si2-type structure (Z=2), a new polymorphic phase of BaCu2As2 (\b{eta} phase) with a much larger c lattice parameter (Z=10), which could be considered an intergrowth of the ThCr2Si2- and CaBe2Ge2-type structures, has been discovered via Sn flux growth. We have characterized this structure through single-crystal X-ray diffraction, transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM) studies. Furthermore, we compare this new polymorphic intergrowth structure with the {\alpha}-phase BaCu2As2 (ThCr2Si2 type with Z=2) and the \b{eta}-phase BaCu2Sb2 (intergrowth of ThCr2Si2 and CaBe2Ge2 types with Z=6), both with the same space group I4/mmm. Electrical transport studies reveal p-type carriers and magnetoresistivity up to 22% at 5 K and under a magnetic field of 7 T. Our work suggests a new route for the discovery of new polymorphic structures through flux and temperature control during material synthesis., Comment: 15 pages, 4 figures

We report the synthesis and physical properties studies of quais-1D iron chalcogenide $\rm BaFe_2Se_4$ which shares the $\rm FeSe_4$ tetrahedra building motif commonly seen in the iron chalcogenide superconductors. A high-quality polycrystalline sample was achieved by solid-state reaction method and characterized by X-ray diffraction, electrical resistivity, magnetic susceptibility and neutron diffraction measurements. $\rm BaFe_2Se_4$ is a narrow gap semiconductor that magnetically orders at $\sim$ 310 K. Both neutron powder diffraction results and isothermal M-H loops suggest a canted antiferromagnetic structure where Fe sublattice are antiferromagnetically ordered along the c-axis quasi-1D chain direction, resulting in a net ferromagnetic moment in the perpendicular direction along the a-axis with tilted angle of 18.7$^\circ$ towards the b-axis., Comment: 8 pages, 4 figures

Anisotropic materials, with orientation-dependent properties, have attracted more and more attention due to their compelling tunable and flexible performance in electronic and optomechanical devices. So far, two-dimensional (2D) black phosphorus shows the largest known anisotropic behavior, which is highly desired for synaptic and neuromorphic devices, multifunctional directional memories, and even polarization-sensitive photodetector, whereas it is unstable at ambient conditions. Recently, 2D few-layered As2S3 with superior chemical stability was successfully exfoliated in experiments. However, the electronic and mechanical properties of monolayer and bilayer As2S3 is still lacking. Here, we report the large anisotropic electronic and mechanical properties of As2S3 systems through first-principles calculations and general angle-dependent Hooke's law. Monolayer and bilayer As2S3 exhibit anisotropic factors of Young's modulus of 3.15 and 3.32, respectively, which are larger than the black phosphorous with experimentally confirmed and an anisotropic factor of 2. This study provides an effective route to flexible orientation-dependent nanoelectronics, nanomechanics, and offers implications in promoting related experimental investigations., Comment: 13 pages, 6 figures