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Recent studies have shown that layered compound EuCd2As2 could exhibit diverse topological states depending on the different magnetic structures, such as Weyl semimetal, Dirac semimetal, topological insulator, etc. In order to further study the interplay between magnetism and topology of EuCd2As2, it is necessary to figure out its magnetic structure. Here, by magnetization (M) measurements and negative magnetostriction (λ) along the [001] direction measured by scanning tunneling microscope on EuCd2As2 single crystals, we observe field-induced metamagnetic phase transition from A-type antiferromagnetic (AFM) ground state to field-polarized state, with canted AFM (CAFM) state in between. Magnetization and magnetostriction are more sensitive to the in-plane field than the out-of-plane field, indicating the magnetic moments lying in the ab plane. The absence of abrupt jump on M-H and λ-H curves demonstrates the phase transition is a second-order type. In CAFM state, M increases linearly with the field and λ is proportional to M 2 . Tunneling conductance spectra show the field-induced evolution of the electronic density of states. Our results provide experimental evidence for understanding the magnetic structure of EuCd2As2.

Superhard semiconductors have been long sought after for electronic device applications enduring extreme conditions, such as astronautics, due to their intrinsic toughness, high thermal and chemical stability. Here, we report the superhard p-type semiconductor Al2.69B50 single crystal with the determined Vickers hardness of ~ 40.5 GPa under the load of 0.49 N, being one of the hardest semiconductor compounds that have been ever found. With the direct band gap of 2.3 eV, Al2.69B50 exhibits excellent optical transmittance (> 90%,) covering the visible range from 459 nm to 760 nm and part of infrared range, and also shows high intensity of photon emission in the visible light. Al2.69B50 is very stable, thermally and chemically, with ultra-low density of ～2.52 g/cm3, allowing for further extension of its applications. Such an assembly of various excellent properties within one material has great implication for high power electronic design and applications.

Description, A spin-triplet superconductor with ferromagnetic spin fluctuation superconducts at a critical temperature of 6.5 K., A spin-triplet superconductor can harbor Majorana bound states that can be used in topological quantum computing. Recently, K2Cr3As3 and its variants with critical temperature Tc as high as 8 kelvin have emerged as a new class of superconductors with ferromagnetic spin fluctuations. Here, we report a discovery in K2Cr3As3 single crystal that the spin susceptibility measured by 75As Knight shift below Tc is unchanged with the magnetic field H0 applied in the ab plane but vanishes toward zero temperature when H0 is along the c axis, which unambiguously establishes this compound as a spin-triplet superconductor described by a vector order parameter d→ parallel to the c axis. Combining with point nodal gap, we show that K2Cr3As3 is a new platform for the study of topological superconductivity and its possible technical application.

Superhard semiconductors have been long sought after for electronic device applications enduring extreme conditions, such as astronautics, due to their intrinsic toughness, high thermal and chemical stability. Here, we report the superhard p-type semiconductor Al

The layered EuSn2As2 crystal has been proved to be an intrinsic magnetic topological insulator with Bi2Te3-type rhombohedral structure at ambient conditions and undergoes a structural phase transition under pressure. Here, we report an experimental lattice vibration study of EuSn2As2 up to 30.5 GPa by in situ Raman scattering measurements. Four Raman-active modes (2A1g + 2Eg) at ambient pressure are observed experimentally and identified by first-principles calculations. Upon compression, the Eg2 and A1g2 modes merge together at 12.45 GPa, meanwhile the frequencies, intensities, and linewidths of all Raman modes show discontinuous changes, exhibiting a structural phase transition. No Raman-active modes are observed above 21.5 GPa, indicating the new high-pressure metallic phase is completely achieved. The evolution of all observed modes under pressure is also discussed with a two-stage reconstruction mechanism proposed recently by ab initio calculations and XRD measurements. These results provide a basic information about the lattice dynamics and expand our understandings on the structural evolution in layered magnetic topological insulators under high pressure.

In-plane transport properties, including anisotropic magnetoresistance (AMR) and planar Hall effect (PHE), are of great interest in electrical transport and spintronic applications. Unconventional transport behavior emerging from the topological physics has been intensively studied and very much desired. In this study, a large AMR of −18% at a very low magnetic field of 0.2 T is observed in a soft magnetic Weyl semimetal EuB6 based on the characteristics of both high magnetization and large magnetoresistance. Furthermore, the intrinsic antisymmetric AMR and PHE are unambiguously observed and interpreted as the modification in conductivity owing to the Berry curvature in a tilted Weyl system instead of the out-of-plane magnetic field component. Our study provides a strategy for low-magnetic-field applications of large AMR and enriches the transport physics of spintronic devices.

We report a unique type of gap in a magnetic Weyl semimetal ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ where the electrons are spin polarized and preserve the spin-momentum locking feature of Weyl fermions. Such spin-polarized gaps are associated with the Weyl node annihilation, where a pair of Weyl nodes with opposite chirality touch each other at \ensuremath{\sim}210 meV above the Fermi energy. These are revealed by both time- and spin-resolved ultrafast spectroscopy experiments, combined with first-principles calculations. The spin-polarized gap opening is accompanied by a topological phase transition, and the gap magnitude exhibits an unconventional temperature dependence originated from the Weyl physics. Furthermore, we propose possible circularly polarized terahertz-midinfrared radiation from such a spin-polarized gap. Our results shed light on exploring the topological properties of excited states and endow application potentials for gapped materials based on chirality.

The Kagome superconductors AV3Sb5 (A = K, Rb, Cs) have received enormous attention due to their nontrivial topological electronic structure, anomalous physical properties and superconductivity. Unconventional charge density wave (CDW) has been detected in AV3Sb5. High-precision electronic structure determination is essential to understand its origin. Here we unveil electronic nature of the CDW phase in our high-resolution angle-resolved photoemission measurements on KV3Sb5. We have observed CDW-induced Fermi surface reconstruction and the associated band folding. The CDW-induced band splitting and the associated gap opening have been revealed at the boundary of the pristine and reconstructed Brillouin zones. The Fermi surface- and momentum-dependent CDW gap is measured and the strongly anisotropic CDW gap is observed for all the V-derived Fermi surface. In particular, we have observed signatures of the electron-phonon coupling in KV3Sb5. These results provide key insights in understanding the nature of the CDW state and its interplay with superconductivity in AV3Sb5 superconductors., The impact of the charge density wave (CDW) state to the electronic structure in the Kagome superconductors AV3Sb5 remains unclear. Here, the authors observe CDW-induced Fermi surface reconstruction with a strongly anisotropic CDW gap and signatures of the electron-phonon coupling for all V-derived bands.

$\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$ has been proposed to have magnetic topological states as a potential Weyl semimetal. We synthesized single crystals of $\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$ and systematically investigated their structural and physical properties. $\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$ has an isostructural septuple-layered structure that is similar to the magnetic topological insulator $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ but possesses transpositional Mn and Sb atoms between the sublayers. Magnetic and specific-heat measurements revealed a ferrimagnetic phase transition with a Curie temperature ${T}_{\mathrm{C}}$ of \ensuremath{\sim}31 K in $\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$, which originates from the interaction of the interexchanged ${\mathrm{Mn}}^{2+}$ ions. As the temperature decreases below ${T}_{\mathrm{C}}$, negative longitudinal magnetoresistance and anomalous Hall effect are observed, implying a non-negligible connection between the magnetism and expected transport carriers that may be driven by topological bands. Our results indicate that ferrimagnetic $\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$ provides insights for further studies on magnetic topological materials.

$\mathrm{TaIr}{\mathrm{Te}}_{4}$ is a recently discovered type-II Weyl semimetal, hosting only four Weyl points. Here, we study the cleaved $\mathrm{TaIr}{\mathrm{Te}}_{4}$ crystal using scanning tunneling microscopy/spectroscopy and find that it also hosts a superconducting state with a transition temperature of 3.9 K. From Dynes function fitting, the superconducting phase is consistent with anisotropic $s$-wave pairing, with a superconducting gap of 1.31 meV. This value leads to a value of $2{\mathrm{\ensuremath{\Delta}}}_{\mathrm{max}}/{k}_{B}{T}_{C}=7.81$, much larger than the 3.53 predicted by Bardeen-Cooper-Schrieffer theory for weak-coupling superconductors. The critical field is found to be 0.7 T based on the analysis of tunneling conductance as a function of magnetic field. Two types of nonmagnetic defects on the $\mathrm{TaIr}{\mathrm{Te}}_{4}$ surface are observed, neither of which induce bound states inside the superconducting gap, further supporting conventional $s$-wave superconductivity in the $\mathrm{TaIr}{\mathrm{Te}}_{4}$ system.

The layered crystal of ${\mathrm{EuSn}}_{2}{\mathrm{As}}_{2}$ has a ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$-type structure in rhombohedral ($R\overline{3}m$) symmetry and has been confirmed to be an intrinsic magnetic topological insulator at ambient conditions. Combining ab initio calculations and in situ x-ray diffraction measurements, we identify a new monoclinic ${\mathrm{EuSn}}_{2}{\mathrm{As}}_{2}$ structure in $C2/m$ symmetry above $\ensuremath{\sim}14\text{ }\text{ }\mathrm{GPa}$. It has a three-dimensional network made up of honeycomblike Sn sheets and zigzag As chains, transformed from the layered ${\mathrm{EuSn}}_{2}{\mathrm{As}}_{2}$ via a two-stage reconstruction mechanism with the connecting of Sn-Sn and As-As atoms successively between the buckled SnAs layers. Its dynamic structural stability has been verified by phonon mode analysis. Electrical resistance measurements reveal an insulator-metal-superconductor transition at low temperature around 5 and 15 GPa, respectively, according to the structural conversion, and the superconductivity with a ${T}_{C}$ value of $\ensuremath{\sim}4\text{ }\text{ }\mathrm{K}$ is observed up to 30.8 GPa. These results establish a high-pressure ${\mathrm{EuSn}}_{2}{\mathrm{As}}_{2}$ phase with intriguing structural and electronic properties and expand our understandings about the layered magnetic topological insulators.

Topological crystalline insulators (TCIs) are insulating electronic states with nontrivial topology protected by crystalline symmetries. Recently, theory has proposed new classes of TCIs protected by rotation symmetries $$\hat C_n$$ C ̂ n , which have surface rotation anomaly evading the fermion doubling theorem, i.e., n instead of 2n Dirac cones on the surface preserving the rotation symmetry. Here, we report the first realization of the $$\hat C_2$$ C ̂ 2 rotation anomaly in a binary compound SrPb. Our first-principles calculations reveal two massless Dirac fermions protected by the combination of time-reversal symmetry $$\hat T$$ T ̂ and $$\hat C_{2y}$$ C ̂ 2 y on the (010) surface. Using angle-resolved photoemission spectroscopy, we identify two Dirac surface states inside the bulk band gap of SrPb, confirming the $$\hat C_2$$ C ̂ 2 rotation anomaly in the new classes of TCIs. The findings enrich the classification of topological phases, which pave the way for exploring exotic behavior of the new classes of TCIs.

As exemplified by the growing interest in the quantum anomalous Hall effect, the research on topology as an organizing principle of quantum matter is greatly enriched from the interplay with magnetism. In this vein, we present a combined electrical and thermoelectrical transport study on the magnetic Weyl semimetal ${\mathrm{EuCd}}_{2}{\mathrm{As}}_{2}$. Unconventional contribution to the anomalous Hall and anomalous Nernst effects were observed both above and below the magnetic transition temperature of ${\mathrm{EuCd}}_{2}{\mathrm{As}}_{2}$, indicating the existence of significant Berry curvature. ${\mathrm{EuCd}}_{2}{\mathrm{As}}_{2}$ represents a rare case in which this unconventional transverse transport emerges both above and below the magnetic transition temperature in the same material. The transport properties evolve with temperature and field in the antiferromagnetic phase in a different manner than in the paramagnetic phase, suggesting different mechanisms to their origin. Our results indicate ${\mathrm{EuCd}}_{2}{\mathrm{As}}_{2}$ is a fertile playground for investigating the interplay between magnetism and topology, and potentially a plethora of topologically nontrivial phases rooted in this interplay.

The layered crystal of EuSn_{2}As_{2} has a Bi_{2}Te_{3}-type structure in rhombohedral (R3[over ¯]m) symmetry and has been confirmed to be an intrinsic magnetic topological insulator at ambient conditions. Combining ab initio calculations and in situ x-ray diffraction measurements, we identify a new monoclinic EuSn_{2}As_{2} structure in C2/m symmetry above ∼14 GPa. It has a three-dimensional network made up of honeycomblike Sn sheets and zigzag As chains, transformed from the layered EuSn_{2}As_{2} via a two-stage reconstruction mechanism with the connecting of Sn-Sn and As-As atoms successively between the buckled SnAs layers. Its dynamic structural stability has been verified by phonon mode analysis. Electrical resistance measurements reveal an insulator-metal-superconductor transition at low temperature around 5 and 15 GPa, respectively, according to the structural conversion, and the superconductivity with a T_{C} value of ∼4 K is observed up to 30.8 GPa. These results establish a high-pressure EuSn_{2}As_{2} phase with intriguing structural and electronic properties and expand our understandings about the layered magnetic topological insulators.

Co$_3$Sn$_2$S$_2$ is a ferromagnetic semi-metal with Weyl nodes in its band structure and a large anomalous Hall effect below its Curie temperature of 177 K. We present a detailed study of its Fermi surface and examine the relevance of the anomalous transverse Wiedemann Franz law to it. We studied Shubnikov-de Haas oscillations along two orientations in single crystals with a mobility as high as $2.7\times$10$^3$ cm$^2$V$^{-1}$s$^{-1}$ subject to a magnetic field as large as $\sim$ 60 T. The angle dependence of the frequencies is in agreement with density functional theory (DFT) calculations and reveals two types of hole pockets (H1, H2) and two types of electron pockets (E1, E2). An additional unexpected frequency emerges at high magnetic field. We attribute it to magnetic breakdown between the hole pocket H2 and the electron pocket E2, since it is close to the sum of the E2 and H2 fundamental frequencies. By measuring the anomalous thermal and electrical Hall conductivities, we quantified the anomalous transverse Lorenz ratio, which is close to the Sommerfeld ratio ($L_0=\frac{\pi^2}{3}\frac{k_B^2}{e^2}$) below 100 K and deviates downwards at higher temperatures. This finite temperature deviation from the anomalous Wiedemann-Franz law is a source of information on the distance between the sources and sinks of the Berry curvature and the chemical potential., Comment: 7 pages, 6 figures, supplemental materials included

Magnetic topological materials, in which the time-reversal symmetry is broken, host various exotic quantum phenomena, including the quantum anomalous Hall effect, axion insulator states, and Majorana fermions. The study of magnetic topological materials is at the forefront of condensed matter physics. Recently, a variety of magnetic topological materials have been reported, such as Mn$_3$Sn, Co$_3$Sn$_2$S$_2$, Fe$_3$Sn$_2$, and MnBi$_2$Te$_4$. Here, we report the observation of a topological electronic structure in an antiferromagnet, HoSbTe, a member of the ZrSiS family of materials, by angle-resolved photoemission spectroscopy measurements and first-principles calculations. We demonstrate that HoSbTe is a Dirac nodal line semimetal when spin-orbit coupling (SOC) is neglected. However, our theoretical calculations show that the strong SOC in HoSbTe fully gaps out the nodal lines and drives the system to a weak topological insulator state, with each layer being a two-dimensional topological insulator. Because of the strong SOC in HoSbTe, the gap is as large as hundreds of meV along specific directions, which is directly observed by our ARPES measurements. The existence of magnetic order and topological properties in HoSbTe makes it a promising material for realization of exotic quantum devices., 5 pages, 3 figures

We report the experimental and theoretical studies of a magnetic topological nodal line semimetal candidate HoSbTe. Single crystals of HoSbTe are grown from Sb flux, crystallizing in a tetragonal layered structure (space group: P4/nmm, no.129), in which the Ho-Te bilayer is separated by the square-net Sb layer. The magnetization and specific heat present distinct anomalies at 4 K related to an antiferromagnetic (AFM) phase transition. Meanwhile, with applying magnetic field perpendicular and parallel to the crystallographic c axis, an obvious magnetic anisotropy is observed. Electrical resistivity undergoes a bad-metal-like state below 200 K and reveals a plateau at about 8 K followed by a drop due to the AFM transition. In addition, with the first-principle calculations of band structure, we find that HoSbTe is a topological nodal line semimetal or a weak topological insulator with or without taking the spin-orbit coupling into account, providing a platform to investigate the interplay between magnetic and topological fermionic properties., 7 pages, 6 figures, 2tables

We report a comprehensive neutron scattering study on the spin excitations in the magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$ with quasi-two-dimensional structure. Both in-plane and out-of-plane dispersions of the spin waves are revealed in the ferromagnetic state, similarly dispersive but damped spin excitations persist into the paramagnetic state. The effective exchange interactions have been estimated by a semi-classical Heisenberg model to consistently reproduce the experimental $T_C$ and spin stiffness. However, a full spin wave gap below $E_g=2.3$ meV is observed at $T=4$ K, much larger than the estimated magnetic anisotropy energy ($\sim0.6$ meV), while its temperature dependence indicates a significant contribution from the Weyl fermions. These results suggest that Co$_3$Sn$_2$S$_2$ is a three-dimensional correlated system with large spin stiffness, and the low-energy spin dynamics could interplay with the topological electron states., Comment: 15 pages, 10 figures, including Supplementary Materials

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

We report the synthesis of single-crystal NbC, a transition metal carbide with various unusual properties. Transport, magnetic susceptibility, and specific heat measurements demonstrate that NbC is a conventional superconductor with a superconducting transition temperature ($T_c$) of 11.5 K. Our theoretical calculations show that NbC is a type-II Dirac semimetal with strong Fermi surface nesting, which is supported by our ARPES measurement results. We also observed the superconducting gaps of NbC using angle-resolved photoemission spectroscopy (ARPES) and found some unconventional behaviors. These intriguing superconducting and topological properties, combined with the high corrosion resistance, make NbC an ideal platform for both fundamental research and device applications., Comment: to appear in Physical Review B

We report here a comprehensive study of the AFM structures of the Eu and Mn magnetic sublattices as well as the interplay between Eu and Mn magnetism in this compound by using both polarized and non-polarized single-crystal neutron diffraction. Magnetic susceptibility, specific heat capacity measurements and the temperature dependence of magnetic diffractions suggest that the AFM ordering temperature of the Eu and Mn moments is at 22 and 337 K, respectively. The magnetic moments of both Eu and Mn ions are oriented along the crystallographic $c$ axis, and the respective magnetic propagation vector is $\textbf{k}_{Eu} = (0,0,1)$ and $\textbf{k}_{Mn}=(0,0,0)$. With proper neutron absorption correction, the ordered moments are refined at 3 K as 7.7(1) $\mu_B$ and 4.1(1) $\mu_B$ for the Eu and Mn ions, respectively. In addition, a spin-flop (SF) phase transition of the Eu moments in an applied magnetic field along the $c$ axis was confirmed to take place at a critical field of B$_c$ $\sim$ 5.3 T. The evolution of the Eu magnetic moment direction as a function of the applied magnetic field in the SF phase was also determined. Clear kinks in both field and temperature dependence of the magnetic reflections ($\pm1$, 0, 1) of Mn were observed at the onset of the SF phase transition and the AFM order of the Eu moments, respectively. This unambiguously indicates the existence of a strong coupling between Eu and Mn magnetism. The interplay between two magnetic sublattices could bring new possibilities to tune Dirac fermions via changing magnetic structures by applied fields in this class of magnetic topological semimetals., Comment: 15 pages, 12 figures, accepted by Physical Review Research

We use scanning tunneling microscopy/spectroscopy (STM/S) to elucidate the atomically resolved electronic structure in strongly correlated topological kagome magnet Mn$_3$Sn. In stark contrast to its broad single-particle electronic structure, we observe a pronounced resonance with a Fano line shape at the Fermi level resembling the many-body Kondo resonance. We find that this resonance does not arise from the step edges or atomic impurities, but the intrinsic kagome lattice. Moreover, the resonance is robust against the perturbation of a vector magnetic field, but broadens substantially with increasing temperature, signaling strongly interacting physics. We show that this resonance can be understood as the result of geometrical frustration and strong correlation based on the kagome lattice Hubbard model. Our results point to the emergent many-body resonance behavior in a topological kagome magnet., Comment: Includes Supplementary Materials. To appear in Phys. Rev. Lett (2020)

Parity-time symmetry plays an essential role for the formation of Dirac states in Dirac semimetals. So far, all of the experimentally identified topologically nontrivial Dirac semimetals (DSMs) possess both parity and time reversal symmetry. The realization of magnetic topological DSMs remains a major issue in topological material research. Here, combining angle-resolved photoemission spectroscopy with density functional theory calculations, it is ascertained that band inversion induces a topologically nontrivial ground state in EuCd

Electron phonon coupling (EPC) between surface electronic states and surface phonons is significant for low-dimensional devices. This is especially important for topological materials with exotic surface states. The authors experimentally map out the surface phonon dispersions of a topological nodal line semimetal ZrSiS with exotic surface floating two-dimensional electronic bands. They reveal a prominent Kohn anomaly in a surface phonon branch, originating from the interactions with the two-dimensional electrons. This discovery provides an EPC picture, important for the applications of topological nodal line semimetals.

We report an experimental study of the magnetic order and electronic structure and transport of the layered pnictide ${\mathrm{EuMnSb}}_{2}$, performed using neutron diffraction, angle-resolved photoemission spectroscopy (ARPES), and magnetotransport measurements. We find that the Eu and Mn sublattices display antiferromagnetic (AFM) order below ${T}_{\mathrm{N}}^{\mathrm{Eu}}=21(1)\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}}^{\mathrm{Mn}}=350(2)\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, respectively. The former can be described by an A-type AFM structure with the Eu spins aligned along the $c$ axis (an in-plane direction), whereas the latter has a C-type AFM structure with Mn moments along the $a$ -axis (perpendicular to the layers). The ARPES spectra reveal Dirac-like linearly dispersing bands near the Fermi energy. Furthermore, our magnetotransport measurements show strongly anisotropic magnetoresistance and indicate that the Eu sublattice is intimately coupled to conduction electron states near the Dirac point.

The search for proximity-induced superconductivity in topological materials has generated widespread interest in the condensed matter physics community. The superconducting states inheriting nontrivial topology at interfaces are expected to exhibit exotic phenomena such as topological superconductivity and Majorana zero modes, which hold promise for applications in quantum computation. However, a practical realization of such hybrid structures based on topological semimetals and superconductors has hitherto been limited. Here, we report the strong proximity-induced superconductivity in type-II Weyl semimetal WTe2, in a van der Waals hybrid structure obtained by mechanically transferring NbSe2 onto various thicknesses of WTe2. When the WTe2 thickness (tWTe2) reaches 21 nm, the superconducting transition occurs around the critical temperature (Tc) of NbSe2 with a gap amplitude (Δp) of 0.38 meV and an unexpected ultralong proximity length (lp) up to 7 μm. With the thicker 42 nm WTe2 layer, however, the proxi...

A new multifunctional quaternary chalcogenide K2MnGe3S8 was discovered for the first time by using a conventional high-temperature solid-state reaction method. K2MnGe3S8 crystallizes in the space group P21 (no. 4) of the monoclinic system and features a 2∞[MnGe3S8]2− layer constructed by 1∞[MnGeS6]6− anionic chains and [Ge2S6]4− dimers. This air-stable compound exhibits a large band gap (about 2.95 eV) and a moderate nonlinear optical (NLO) response (about 0.6 × AgGaS2 with a particle size of 20–41 μm at a laser radiation of 2.09 mm). Moreover, despite the large Mn⋯Mn separation, namely, 7.1978 A for the shortest intralayer Mn⋯Mn and 6.6975 A for the neighboring interlayer Mn⋯Mn, K2MnGe3S8 exhibits an interesting antiferromagnetic transition at ∼8.1 K. The optical and magnetic properties are also investigated by first principles calculations. Our study demonstrates that the interplay between NLO-active structural units and transition metals may contribute to the discovery of new multifunctional materials.

Theories and experiments on dirty superconductors are sophisticated but important for both fundamentals and applications. It becomes more challenging when magnetic fields are present, because the field distribution, the electron density of states, and the superconducting pairing potentials are nonuniform. Here we present tunneling microspectroscopic experiments on NbC single crystals and show that NbC is a homogeneous dirty superconductor. When applying magnetic fields to the sample, we observe that the zero-energy local density of states and the pairing energy gap follow an explicit scale relation proposed by de Gennes for homogeneous dirty superconductors in high magnetic fields. Surprisingly, our experimental findings suggest that the validity of the scale relation extends to magnetic field strengths far below the upper critical field and call for new nonperturbative understanding of this fundamental property in dirty superconductors. On the practical side, we use the observed scale relation to drive a simple and straightforward experimental scheme for extracting the superconducting coherence length of a dirty superconductor in magnetic fields.

Magnetic two-dimensional (2D) van der Waals (vdWs) materials are receiving increased attention due to their exceptional properties and potential applications in spintronic devices. Because exchange bias and spin–orbit torque (SOT)-driven magnetization switching are basic ingredients for spintronic devices, it is of pivotal importance to demonstrate these effects in the 2D vdWs material-based magnetic heterostructures. In this work, we employ a vacuum exfoliation approach to fabricate Fe3GeTe2 (FGT)/Ir22Mn78 (IrMn) and FGT/Pt bilayers, which have high-quality interfaces. An out-of-plane exchange bias of up to 895 Oe is obtained in the former bilayer, which is larger than that of the previously studied bilayers. In the latter bilayer, the SOT switching of the perpendicularly magnetized FGT is realized, which exhibits higher SOT-driven switching performance compared to the previously studied FGT/Pt bilayer devices with interfacial oxidation. The minimum of SOT efficiency is further determined to be 0.18 ± 0.04, comparable to the previously reported values for the Pt/Co and Pt/CoFeB bilayers. This work highlights the importance of the high-quality interface for the exchange bias and SOT effect and may pave the way for implementing 2D vdWs in spintronic devices.

At sufficiently low temperatures, many quantum effects, such as weak localization, electron-electron interaction (EEI), and Kondo screening, can lead to pronounced corrections to the semiclassical electron transport. Although low temperature corrections to longitudinal resistivity, ordinary Hall resistivity, or anomalous Hall (AH) resistivity are often observed, the corrections to three of them have never been simultaneously detected in a single sample. Here, we report on the observation of $\sqrt{T}$-type temperature dependences of the longitudinal, ordinary Hall and AH resistivities at temperatures down to at least 20 mK in $n$-type ${\mathrm{HgCr}}_{2}{\mathrm{Se}}_{4}$, a half-metallic ferromagnetic semiconductor that can reach extremely low carrier densities. For the samples with moderate disorder, the longitudinal and ordinary Hall conductivities can be satisfactorily described by the EEI theory developed by Altshuler et al., whereas the large corrections to AH conductivity are inconsistent with the existing theory, which predicts vanishing and finite corrections to AH conductivity for EEI and weak localization, respectively.

The interplay between electronic correlations and topological protection may offer a rich avenue for discovering emergent quantum phenomena in condensed matter. However, electronic correlations have so far been little investigated in Weyl semimetals (WSMs) by experiments. Here, we report a combined optical spectroscopy and theoretical calculation study on the strength and effect of electronic correlations in a magnet Co3Sn2S2. The electronic kinetic energy estimated from our optical data is about half of that obtained from single-particle ab initio calculations in the ferromagnetic ground state, which indicates intermediate-strength electronic correlations in this system. Furthermore, comparing the energy and side-slope ratios between the interband-transition peaks at high energies in the experimental and single-particle-calculation-derived optical conductivity spectra with the bandwidth-renormalization factors obtained by many-body calculations enables us to estimate the Coulomb-interaction strength (U ∼ 4 eV) in Co3Sn2S2. Besides, a sharp experimental optical conductivity peak at low energy, which is absent in the single-particle-calculation-derived spectrum but is consistent with the optical conductivity peaks obtained by many-body calculations with U ∼ 4 eV, indicates that an electronic band connecting the two Weyl cones is flattened by electronic correlations and emerges near the Fermi energy in Co3Sn2S2. Our work paves the way for exploring flat-band-generated quantum phenomena in WSMs., How electron correlation interplays with topological states remains rarely explored. Here, the authors report flat band arising due to electron correlations in magnetic Weyl semimetal Co3Sn2S2 from a combined optical-spectroscopy and simulation study.

Although superconductivity in the vicinity of an antiferromagnetic (AFM) instability has been extensively explored in the last three decades or so, superconductivity in compounds with a background of ferromagnetic (FM) spin fluctuations is still rare. We report $^{75}\mathrm{As}$ nuclear quadrupole resonance measurements on the ${A}_{2}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}$ family, which is the first group of Cr-based superconductors at ambient pressure, with $A$ being alkali elements. From the temperature dependence of the spin-lattice relaxation rate ($1/{T}_{1}$), we find that by changing $A$ in the order of $A=\mathrm{Na}$, ${\mathrm{Na}}_{0.75}{\mathrm{K}}_{0.25}$, K, and Rb, the system is tuned to approach a possible FM quantum critical point (QCP). This may be ascribed to the Cr2-As2-Cr2 bond angle that decreases towards 90\ifmmode^\circ\else\textdegree\fi{}, which enhances the FM interaction via the Cr2-As2-Cr2 path. Upon moving away from the QCP, the superconducting transition temperature ${T}_{\mathrm{sc}}$ increases progressively up to 8.0 K in ${\mathrm{Na}}_{2}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}$, which is in sharp contrast to the AFM case where ${T}_{\mathrm{sc}}$ usually shows a maximum around a QCP. The $1/{T}_{1}$ decreases rapidly below ${T}_{\mathrm{sc}}$ with no Hebel-Slichter peak, and ubiquitously follows a ${T}^{5}$ variation below a characteristic temperature ${T}^{*}\ensuremath{\approx}0.6$ ${T}_{\mathrm{sc}}$, which indicates the existence of point nodes in the superconducting gap function commonly in the family. These results suggest that the ${A}_{2}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}$ family is a possible solid-state analog of superfluid $^{3}\mathrm{He}$.

The $n$-type ${\text{HgCr}}_{2}{\mathrm{Se}}_{4}$ exhibits a sharp semiconductor-to-metal transition (SMT) in resistivity accompanying the ferromagnetic order at ${T}_{C}=106\text{ }\text{ }\mathrm{K}$. Here, we investigate the effects of pressure and magnetic field on the concomitant SMT and ferromagnetic order by measuring resistivity, dc and ac magnetic susceptibility, as well as single-crystal neutron diffraction under various pressures up to 8 GPa and magnetic fields up to 8 T. Our results demonstrate that the ferromagnetic metallic ground state of $n$-type ${\text{HgCr}}_{2}{\mathrm{Se}}_{4}$ is destabilized and gradually replaced by an antiferromagnetic, most likely a spiral magnetic, and insulating ground state upon the application of high pressure. On the other hand, the application of external magnetic fields can restore the ferromagnetic metallic state again at high pressures, resulting in a colossal magnetoresistance (CMR) as high as $\ensuremath{\sim}\text{ }3\ifmmode\times\else\texttimes\fi{}{10}^{11}%$ under 5 T and 2 K at 4 GPa. The present study demonstrates that $n$-type ${\text{HgCr}}_{2}{\mathrm{Se}}_{4}$ is located at a peculiar critical point where the balance of competition between ferromagnetic and antiferromagnetic interactions can be easily tipped by external stimuli, providing a new platform for achieving CMR in a single-valent system.

Based on the first-principles calculations and experimental measurements, we report that the hexagonal phase of ternary transition metal pnictides TT'X (T=Zr, Hf; T'=Ru; X=P, As), which are well-known noncentrosymmetric superconductors with relatively high transition temperatures, host nontrivial bulk topology. Before the superconducting phase transition, we find that HfRuP belongs to a Weyl semimetal phase with 12 pairs of type-II Weyl points, while ZrRuAs, ZrRuP and HfRuAs belong to a topological crystalline insulating phase with trivial Fu-Kane $\mathbb Z_2$ indices but $nontrivial$ mirror Chern numbers. High-quality single crystal samples of the noncentrosymmetric superconductors with these two different topological states have been obtained and the superconductivity is verified experimentally. The wide-range band structures of ZrRuAs have been identified by ARPES and reproduced by theoretical calculations. Combined with intrinsic superconductivity, the nontrivial topology of the normal state may generate unconventional superconductivity in both bulk and surfaces. Our findings could largely inspire the experimental searching for possible topological superconductivity in these compounds., Published version, 15 pages, 4 figures

The intertwined charge, spin, orbital, and lattice degrees of freedom could endow 5d compounds with exotic properties. Current interest is focused on electromagnetic interactions in these materials, whereas the important role of lattice geometry remains to be fully recognized. For this sake, we investigate pressure-induced phase transitions in the spin-orbit Mott insulator Sr3Ir2O7 with Raman, electrical resistance, and x-ray diffraction measurements. We reveal an interesting magnetic transition coinciding with a structural transition at 14.4 GPa, but without a concurrent insulator-metal transition. The conventional correlation between magnetic and Mott insulating states is thereby absent. The observed softening of the one-magnon mode can be explained by a reduced tetragonal distortion, while the actual magnetic transition is associated with tilting of the IrO6 octahedra. This work highlights the critical role of lattice frustration in determining the high-pressure phases of Sr3Ir2O7. The ability to control electromagnetic properties via manipulating the crystal structure with pressure promises a new way to explore new quantum states in spin-orbit Mott insulators., 6 pages, 5 figures

Photoemission demonstrates that the Dirac-like band crossings form 1D nodal lines and 2D nodal surfaces in the bulk of ZrSiS., Topological semimetals are characterized by symmetry-protected band crossings, which can be preserved in different dimensions in momentum space, forming zero-dimensional nodal points, one-dimensional nodal lines, or even two-dimensional nodal surfaces. Materials harboring nodal points and nodal lines have been experimentally verified, whereas experimental evidence of nodal surfaces is still lacking. Here, using angle-resolved photoemission spectroscopy (ARPES), we reveal the coexistence of Dirac nodal surfaces and nodal lines in the bulk electronic structures of ZrSiS. As compared with previous ARPES studies on ZrSiS, we obtained pure bulk states, which enable us to extract unambiguously intrinsic information of the bulk nodal surfaces and nodal lines. Our results show that the nodal lines are the only feature near the Fermi level and constitute the whole Fermi surfaces. We not only prove that the low-energy quasiparticles in ZrSiS are contributed entirely by Dirac fermions but also experimentally realize the nodal surface in topological semimetals.

ZrCo

WTe2, as a type-II Weyl semimetal, has 2D Fermi arcs on the (001) surface in the bulk and 1D helical edge states in its monolayer. These features have recently attracted wide attention in condensed matter physics. However, in the intermediate regime between the bulk and monolayer, the edge states have not been resolved owing to its closed band gap which makes the bulk states dominant. Here, we report the signatures of the edge superconductivity by superconducting quantum interference measurements in multilayer WTe2 Josephson junctions and we directly map the localized supercurrent. In thick WTe2 (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$\sim 60{\rm{\ nm}})$\end{document}, the supercurrent is uniformly distributed by bulk states with symmetric Josephson effect (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$| {I_c^ + ( B )} | {=} | {I_c^ - ( B )} |\ $\end{document}). In thin WTe2 (10 nm), however, the supercurrent becomes confined to the edge and its width reaches up to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$1.4{\rm{\ \mu m\ }}$\end{document}and exhibits non-symmetric behavior \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$| {I_c^ + ( B )} | \ne | {I_c^ - ( B )} |$\end{document}. The ability to tune the edge domination by changing thickness and the edge superconductivity establishes WTe2 as a promising topological system with exotic quantum phases and a rich physics., We report the evidence of edge superconductivity in multilayer WTe2 Josephson junctions which remains rarely studied in type-II Weyl semimetal.