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Topological nodal-line semimetals (TNLSMs) are materials whose conduction and valence bands cross each other, meeting a topologically protected closed loop rather than discrete points in the Brillouin zone (BZ). The anticipated properties for TNLSMs, include drumhead-like nearly flat surface states, unique Landau energy levels, special collective modes, long-range Coulomb interactions, or the possibility of realizing high-temperature superconductivity. Recently, SrAs3 has been theoretically proposed and then experimentally confirmed to be a TNLSM. Here, we report high-pressure experiments on SrAs3, identifying a Lifshitz transition below 1 GPa and a superconducting transition accompanied by a structural phase transition above 20 GPa. A topological crystalline insulator (TCI) state is revealed by means of density functional theory (DFT) calculations on the emergent high-pressure phase. As the counterpart of topological insulators, TCIs possess metallic boundary states protected by crystal symmetry, rather than time reversal. In consideration of topological surface states (TSSs) and helical spin texture observed in the high-pressure state of SrAs3, the superconducting state may be induced in the surface states, and is most likely topologically nontrivial, making pressurized SrAs3 a strong candidate for topological superconductor.

This paper is concerned with the asymptotic behavior of solutions for a class of non-autonomous fractional FitzHugh-Nagumo equations deriven by additive white noise. We first provide some sufficient conditions for the existence and uniqueness of solutions, and then prove the existence and uniqueness of tempered pullback random attractors for the random dynamical system generated by the solutions of considered equations in an appropriate Hilbert space. The proof is based on the uniform estimates and the decomposition of dynamical system.

The electronic structure of the natural topological semimetal Co3Sn2S2crystals was studied by using near-edge X-ray absorption spectroscopy (NEXAFS) and resonant photoelectron spectroscopy (ResPES). Although, the significant increase of the Co 3d valence band emission is observed at the Co 2p absorption edge in the ResPES experiments, the spectral weight at these photon energies is dominated by the normal Auger contribution. This observation indicates the delocalized character of photoexcited Co 3d electrons and is supported by the first-principle calculations. Our results on the investigations of the element- and orbital-specific electronic states near the Fermi level of Co3Sn2S2are of importance for the comprehensive description of the electronic structure of this material, which is significant for its future applications in different areas of science and technology, including catalysis and water splitting.

We report magnetic, transport, neutron diffraction, and muon spin rotation data showing that Pb$_2$CoOsO$_6$, a newly synthesized metallic double-perovskite with a centrosymmetric space group at room temperature, exhibits a continuous second-order phase transition at 45 K to a magnetically ordered state with a non-centrosymmetric space group. The absence of inversion symmetry is very uncommon in metals, particularly metallic oxides. In contrast to the recently reported ferroelectric-like structural transition in LiOsO$_3$, the phase transition in Pb$_2$CoOsO$_6$ is driven by a long-range collinear antiferromagnetic order, with propagation vector $\textbf{k} = (\frac{1}{2},0,\frac{1}{2})$, which relieves the frustration associated with the symmetry of themagnetic exchanges. This magnetically-driven loss of inversion symmetry represents a new frontier in the search for novel metallic behavior., 5 pages, 3 figures

Dirac semimetals (DSMs) are classified into different phases based on the types of the Dirac fermions. Tuning the transition among different types of the Dirac fermions in one system remains challenging. Recently, KMgBi was predicted to be located at a critical state that various types of Dirac fermions can be induced owing to the existence of a flat band. Here, we carried out systematic studies on the electronic structure of KMgBi single crystal by combining angle-resolve photoemission spectroscopy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS). The flat band was clearly observed near the Fermi level. We also revealed a small bandgap of ~ 20 meV between the flat band and the conduction band. These results demonstrate the critical state of KMgBi that transitions among various types of Dirac fermions can be tuned in one system., 15 pages, 4 figures

The magnetic van der Waals crystals MnBi_{2}Te_{4}/(Bi_{2}Te_{3})_{n} have drawn significant attention due to their rich topological properties and the tunability by external magnetic field. Although the MnBi_{2}Te_{4}/(Bi_{2}Te_{3})_{n} family have been intensively studied in the past few years, their close relatives, the MnSb_{2}Te_{4}/(Sb_{2}Te_{3})_{n} family, remain much less explored. In this work, combining magnetotransport measurements, angle-resolved photoemission spectroscopy, and first principles calculations, we find that MnSb_{4}Te_{7}, the n=1 member of the MnSb_{2}Te_{4}/(Sb_{2}Te_{3})_{n} family, is a magnetic topological system with versatile topological phases that can be manipulated by both carrier doping and magnetic field. Our calculations unveil that its A-type antiferromagnetic (AFM) ground state stays in a Z_{2} AFM topological insulator phase, which can be converted to an inversion-symmetry-protected axion insulator phase when in the ferromagnetic (FM) state. Moreover, when this system in the FM phase is slightly carrier doped on either the electron or hole side, it becomes a Weyl semimetal with multiple Weyl nodes in the highest valence bands and lowest conduction bands, which are manifested by the measured notable anomalous Hall effect. Our work thus introduces a new magnetic topological material with different topological phases that are highly tunable by carrier doping or magnetic field.

Using angle-resolved photoemission spectroscopy and band structure calculation, we have investigated the three-dimensional nature and the orbital character of the low-energy electronic structure in the layered ternary boride ${\mathrm{AlMn}}_{2}{\mathrm{B}}_{2}$, in which ferromagnetic Mn $ab$ layers couple antiferromagnetically along the $c$ axis (N\'eel temperature ${\mathrm{T}}_{\mathrm{N}}\ensuremath{\sim}320\phantom{\rule{0.28em}{0ex}}\mathrm{K}$). The calculation indicates that electronic bands in the vicinity of the Fermi energy ${E}_{\mathrm{F}}$ are dominated by the Mn $3d$ orbitals. The ${e}_{g}$ orbitals especially contribute high density of states (DOS) with large effective electron masses near ${E}_{\mathrm{F}}$. The calculated bands are renormalized by a factor of $\ensuremath{\sim}1.5$ to match the overall experimental observations, indicating moderate electronic correlation. Considering that the neighbor compound ${\mathrm{AlFe}}_{2}{\mathrm{B}}_{2}$, with one more $3d$ electron, is ferromagnetic, the high DOS near ${E}_{\mathrm{F}}$ from the different $3d$ orbitals should be associated with the different magnetic orders along the special directions in these layered ternary borides. On the other hand, our studies of electronic structure of ${\mathrm{AlMn}}_{2}{\mathrm{B}}_{2}$ would further prompt its potential applications as a layered room-temperature antiferromagnetic material.

Topological materials host fascinating low dimensional gapless states at the boundary. As a prominent example, helical topological edge states (TESs) of two-dimensional topological insulators (2DTIs) and their stacked three-dimensional (3D) equivalent, weak topological insulators (WTIs), have sparked research enthusiasm due to their potential application in the next generation of electronics/spintronics with low dissipation. Here, we propose layered superconducting material CaSn as a WTI with nontrivial Z2 as well as nodal line semimetal protected by crystalline non-symmorphic symmetry. Our systematic angle-resolved photoemission spectroscopy (ARPES) investigation on the electronic structure exhibits excellent agreement with the calculation. Furthermore, scanning tunnelling microscopy/spectroscopy (STM/STS) at the surface step edge shows signatures of the expected TES. These integrated evidences from ARPES, STM/STS measurement and corresponding ab initio calculation strongly support the existence of TES in the non-symmorphic nodal line semimetal CaSn, which may become a versatile material platform to realize multiple exotic electronic states as well as topological superconductivity.

The noncentrosymmetric RAlPn (R = rare earth, Pn = Si, Ge) family, predicted to host nonmagnetic and magnetic Weyl states, provide an excellent platform for investigating the relation between magnetism and Weyl physics. By using high field magnetotransport measurements and first principles calculations, we have unveiled herein both type-I and type-II Weyl states in the nonmagnetic LaAlSi. By a careful comparison between experimental results and theoretical calculations, nontrivial Berry phases associated with the Shubnikov-de Haas oscillations are ascribed to the electron Fermi pockets related to both types of Weyl points located ~ 0.1 eV above and exactly on the Fermi level, respectively. Under high magnetic field, signatures of Zeeman splitting are also observed. These results indicate that, in addition to the importance for exploring intriguing physics of multiple Weyl fermions, LaAlSi as a comparison with magnetic Weyl semimetals in the RAlPn family would also yield valuable insights into the relation between magnetism and Weyl physics., Comment: 18 pages, 5 figures, 1 table

We present extensive investigations of the crystallographic phase diagram and electronic properties of the Fe-based superconductor FeSe under extreme conditions (high pressure (HP) and low temperature (LT)) by synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). An isostructural phase transition (Tetragonal (T) →high-pressure Tetragonal (Tʹ)) is discovered in FeSe at ∼2.8 GPa based on the axial ratio c/a with finer pressure step as observed in Fe-As-based superconductor such as EuFe2As2. We also find a pressure-induced Tʹ → MnP-type phase transition at 7.6 GPa in FeSe, which is consistent with the documented pressure-induced high-spin → low-spin transition (∼6–7 GPa). These results reveal the pressure-induced structural phase transition sequence in FeSe at room temperature to be T → Tʹ → Tʹ+MnP-type at pressures of 0–10.6 GPa, enriching the crystallographic phase diagram. The HPLT XRD data also indicate that a sluggish structural phase transition (Cmma → Pnma) begins at 7.5 GPa, and these two phases coexist up to 26.5 GPa. The HP X-ray absorption near-edge spectroscopy (XANES) measurement shows that Eo of Se experiences a pressure-induced shift to high energy, evidencing strongly charge transfer between Fe and Se under high pressure. Our results shed lights on the correlation between crystallographic/electronic structure and superconductivity in this material.

Recently, layered copper chalcogenides ${\mathrm{Cu}}_{2}X$ family $(X=\mathrm{S}, \mathrm{Se}, \mathrm{Te})$ has attracted tremendous research interests due to their high thermoelectric performance, which is partly due to the superionic behavior of mobile Cu ions, making these compounds ``phonon liquids.'' Here, we systematically investigate the electronic structure and its temperature evolution of the less studied single crystal ${\mathrm{Cu}}_{2\ensuremath{-}x}\mathrm{Te}$ by the combination of angle resolved photoemission spectroscopy (ARPES) and scanning tunneling microscope/spectroscopy (STM/STS) experiments. While the band structure of the ${\mathrm{Cu}}_{2\ensuremath{-}x}\mathrm{Te}$ shows agreement with the calculations, we clearly observe a $2\ifmmode\times\else\texttimes\fi{}2$ surface reconstruction from both our low temperature ARPES and STM/STS experiments which survives up to room temperature. Interestingly, our low temperature STM experiments further reveal multiple types of reconstruction patterns, which suggests the origin of the surface reconstruction being the distributed deficiency of liquidlike Cu ions. Our findings reveal the electronic structure and impurity level of ${\mathrm{Cu}}_{2}\mathrm{Te}$, which provides knowledge about its thermoelectric properties from the electronic degree of freedom.

The newfound two-dimensional antiferromagnetic $\mathrm{Gd}{\mathrm{Te}}_{3}$ has great potential in novel magnetic twistronic and spintronic devices because it has the highest carrier mobility among all known layered magnetic materials. Here, we used high-resolution angle-resolved photoemission spectroscopy to investigate its Fermi-surface topology and low-lying electronic band structure. The Fermi surface is partially gapped by charge-density waves below the transition temperature. Very steep and nearly linear band dispersion near the Fermi energy contributes to the high carrier mobility in $\mathrm{Gd}{\mathrm{Te}}_{3}$. We find that the scattering rate of the quasiparticle increases linearly as a function of binding energy within a wide energy range, indicating that $\mathrm{Gd}{\mathrm{Te}}_{3}$ is a non-Fermi-liquid metal. Our results in this paper provide a fundamental understanding of this layered antiferromagnetic material to guide future studies on it.

Layered structural materials have been a fertile playground to investigate mechanisms of fundamental physics and explore potential applications. Here, we report investigations on ferromagnetic van der Waals $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_{3}$ via high-pressure synchrotron x-ray diffraction, electrical resistance, and magnetoresistance measurements. Under compression, $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_{3}$ undergoes an insulator-metal transition and a structural transition at $\ensuremath{\sim}7.5$ GPa. Concomitantly with the structural transition, the magnetoresistance changes sign, the negative Hall coefficient increases dramatically, and superconductivity emerges at 3 K. The superconductivity persists up to the highest measured pressure of 47.1 GPa with a maximum ${T}_{c}\ensuremath{\approx}$ 4.5 K at $\ensuremath{\sim}30$ GPa. Our results suggest that $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_{3}$ is paramagnetic in the pressure range of superconductivity. The discoveries of superconductivity and magnetic transition in ferromagnetic $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_{3}$ under pressure provide new perspectives to explore the interplay between superconductivity and magnetism in Cr-based two-dimensional van der Waals materials.

${\mathrm{Pb}}_{2}\mathrm{Co}\mathrm{Os}{\mathrm{O}}_{6}$ is a newly synthesized polar metal in which inversion symmetry is broken by the magnetic frustration in an antiferromagnetic ordering of Co and Os sublattices. The coupled magnetic and structural transition occurs at 45 K at ambient pressure. Here, we perform transport measurements and first-principles calculations to study the pressure effects on the magnetic/structural coupled transition of ${\mathrm{Pb}}_{2}\mathrm{Co}\mathrm{Os}{\mathrm{O}}_{6}$. Experimentally, we monitor the resistivity anomaly at ${T}_{N}$ under various pressures up to 11 GPa in a cubic anvil cell apparatus. We find that ${T}_{N}$ determined from the resistivity anomaly first increases quickly with pressure in a large slope of $d{T}_{N}/dP=+6.8(8)\phantom{\rule{0.28em}{0ex}}\mathrm{K}/\mathrm{GPa}$ for $Pl4\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$ and, then, increases with a much reduced slope of 1.8(4) K/GPa above 4 GPa. Our first-principles calculations suggest that the observed discontinuity of $d{T}_{N}/dP$ around 4 GPa may be attributed to the vanishing of the Os magnetic moment under pressure. Pressure substantially reduces the Os moment and completely suppresses it above a critical value, which relieves the magnetic frustration in the antiferromagnetic ordering of ${\mathrm{Pb}}_{2}\mathrm{Co}\mathrm{Os}{\mathrm{O}}_{6}$. The Co and Os polar distortions decrease with the increasing pressure and simultaneously vanish at the critical pressure. Therefore, above the critical pressure, a new centrosymmetric antiferromagnetic state emerges in ${\mathrm{Pb}}_{2}\mathrm{Co}\mathrm{Os}{\mathrm{O}}_{6}$, distinct from the one under ambient pressure, thus, showing a discontinuity in $d{T}_{N}/dP$.

There has been an upsurge in the discovery of topological quantum materials, where various topological insulators and semimetals have been theoretically predicted and experimentally observed. However, very few of them contain silicon, the most widely used element in the electronics industry. Recently, ternary compound $\mathrm{CaA}{\mathrm{l}}_{2}\mathrm{S}{\mathrm{i}}_{2}$ has been predicted to be a topological Dirac semimetal, hosting Lorentz-symmetry-violating quasiparticles with a strongly tilted conical band dispersion. In this work, by using high-resolution angle-resolved photoemission spectroscopy, we investigated the comprehensive electronic structure of $\mathrm{CaA}{\mathrm{l}}_{2}\mathrm{S}{\mathrm{i}}_{2}$. A pair of topological Dirac crossings is observed along the ${k}_{z}$ direction, in good agreement with the ab initio calculations, confirming the topological Dirac semimetal nature of the compound. Our study expands the topological material family on Si-containing compounds, which have great application potential in realizing low-cost, nontoxic electronic devices with topological quantum states.

We have investigated the magnetic correlations in the candidate Weyl semimetals ${\mathrm{EuCd}}_{2}P{n}_{2}$ ($Pn=\mathrm{As}$, Sb) by resonant elastic x-ray scattering at the ${\mathrm{Eu}}^{2+}$ ${M}_{5}$ edge. The temperature and field dependence of the diffuse scattering of ${\mathrm{EuCd}}_{2}{\mathrm{As}}_{2}$ provide direct evidence that the Eu moments exhibit slow ferromagnetic (FM) correlations well above the N\'eel temperature. By contrast, the diffuse scattering in the paramagnetic phase of isostructural ${\mathrm{EuCd}}_{2}{\mathrm{Sb}}_{2}$ is at least an order of magnitude weaker. The FM correlations present in the paramagnetic phase of ${\mathrm{EuCd}}_{2}{\mathrm{As}}_{2}$ could create short-lived Weyl nodes.

We reveal an ultrafast purely electronic phase transition in ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$, which is a plausible excitonic insulator, after excited by an ultrafast infrared laser pulse. Specifically, the order parameter of the strong electron-hole binding shrinks with enhancing the pump pulse, and above a critical pump fluence, a photoexcited semimetallic state is experimentally identified with the absence of ultrafast structural transition. In addition, the bare valence and conduction bands and also the effective masses in ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ are determined. These findings and detailed analysis suggest a bare nonequilibrium semimetallic phase in ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ and the strong electron-hole binding cannot be exclusively driven by Coulomb interaction.

${\mathrm{AlFe}}_{2}{\mathrm{B}}_{2}$, a layered ferromagnet with a Curie temperature near room temperature (${T}_{c}\ensuremath{\sim}270$ K), has attracted growing attention recently in condensed-matter physics and materials science. Utilizing angle-resolved photoemission spectroscopy and first-principles calculations, we provide evidence of a linear band and a saddlelike flat band in the vicinity of the Fermi energy (${E}_{F}$) in ${\mathrm{AlFe}}_{2}{\mathrm{B}}_{2}$, consisting of slabs of ${\mathrm{Fe}}_{2}{\mathrm{B}}_{2}$ atoms separated by layers of Al. The anomalous scattering rate of the linear band varies linearly with the kinetic energy up to 120 meV, showing non-Fermi-liquid behavior consistent with T-linear dependence of resistivity. The flat band with ${d}_{xy}$ orbital character and with strong ${k}_{y}$ dispersion may be associated with spin orientation along the $a$ axis in layered ${\mathrm{AlFe}}_{2}{\mathrm{B}}_{2}$. Electronic and magnetic correlation can be modulated by tuning the flat band near ${E}_{F}$ in layered ternary borides. Our findings have important implications to exploit emergent physics and quantum phases in $3d$ transition metals.

We report herein the results of magnetotransport measurements and ab initio calculations on single crystalline $\mathrm{CaA}{\mathrm{l}}_{2}\mathrm{S}{\mathrm{i}}_{2}$ semimetal. The transport properties could be understood in connection with the two-band model, agreeing well with the theoretical calculations indicating four main sheets of Fermi surface consisting of three hole pockets centered at the $\mathrm{\ensuremath{\Gamma}}$ point and one electron pocket centered at the $M$ point in the Brillouin zone. Magnetotransport measurements showed striking Shubnikov--de Haas oscillations associated with a nontrivial Berry phase, which originate from a hole Fermi pocket indicated by the ab initio calculations. The calculations also unveiled that the hole Fermi surface encloses a nodal line setting around the $\mathrm{\ensuremath{\Gamma}}$ point close to the Fermi level without considering the spin-orbit coupling (SOC). Once the SOC is included, the fragile nodal-line will be gapped and a pair of Dirac points emerge along the high symmetric $\mathrm{\ensuremath{\Gamma}}\text{\ensuremath{-}}A$ direction located at the Brillouin zone coordinates (0, 0, ${k}_{z}^{D}\ensuremath{\approx}\ifmmode\pm\else\textpm\fi{}0.278\phantom{\rule{4pt}{0ex}}\ifmmode\times\else\texttimes\fi{}\frac{2\ensuremath{\pi}}{c}$), about 1.22 eV below the Fermi level. In addition, the SOC can also induce a topological insulator state along the $\mathrm{\ensuremath{\Gamma}}\text{\ensuremath{-}}A$ direction with a gap of about 3 meV. The results demonstrate $\mathrm{CaA}{\mathrm{l}}_{2}\mathrm{S}{\mathrm{i}}_{2}$ as an excellent platform for the study of novel topological physics with multiple nontrivial topological states.

The van der Waals ferromagnet $\mathrm{F}{\mathrm{e}}_{3}\mathrm{GeT}{\mathrm{e}}_{2}$ has attracted great research attention recently due to its extraordinary properties. Here, using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the temperature evolution of the electronic structure of bulk $\mathrm{F}{\mathrm{e}}_{3}\mathrm{GeT}{\mathrm{e}}_{2}$. We observe largely dispersive energy bands with exchange splitting that are in overall agreement with our density-functional theory calculation. Interestingly, the band dispersions barely change upon heating towards the ferromagnetic transition near 225 K, except for the reduction of quasiparticle coherence, which strongly deviates from the itinerant Stoner model. We suggest that the local magnetic moments may play a crucial role in the ferromagnetic ordering and the electronic structure of $\mathrm{F}{\mathrm{e}}_{3}\mathrm{GeT}{\mathrm{e}}_{2}$, which will shed light on the generic understanding of itinerant magnetism in correlated materials.

High-magnetic-field transport measurements and ab initio calculations on the layered superconductor TaSe3 have provided evidence for the existence of a three-dimensional strong topological state. Longitudinal magnetotransport measurements up to similar to 33 T unveiled striking Shubnikov-de Haas oscillations with two fundamental frequencies at 100 and 175 T corresponding to a nontrivial hole Fermi pocket at the Gamma point and a nontrivial electron Fermi pocket at the B point, respectively, in the Brillouin zone. However, calculations revealed one more electron pocket at the B point which was not detected by the magnetotransport measurements, presumably due to the limited carrier momentum relaxation time. Angle-dependent quantum oscillations by rotating the sample with respect to the magnetic field revealed clear changes in the two fundamental frequencies, indicating anisotropic electronic Fermi pockets. The ab initio calculations gave the topological Z(2) invariant of (1; 100) and revealed a single Dirac cone on the (1 0 -1) surface at the (X) over bar point with helical spin texture at a constant energy contour, suggesting a strong topological state. The results demonstrate TaSe3 as an excellent platform to study the interplay between the topological phase and superconductivity and a promising system for the exploration of topological superconductivity.

Pyrite-structured PdSb2 with the nonsymmorphic crystalline symmetry, has long been predicted to host sixfold-degenerate exotic fermions beyond Dirac and Weyl. Although magnetotransport measurements on PdSb2 have suggested its topologically nontrivial character, the direct spectroscopic evidence still remains absent. By utilizing high-resolution angle-resolved photoemission spectroscopy, we present a systematic study on the low-energy bulk and surface electronic structure of pyrite-structured PdSb2. Through careful comparison with first-principles calculations, we verify the existence of sixfold fermions in this compound, which are formed by three doubly degenerate bands centered at the R point in the Brillouin zone. These bands exhibit parabolic dispersion close to sixfold fermion nodes, in sharp contrast to previously reported sixfold double spin-1 chiral fermions. Furthermore, our findings reveal no protected Fermi arcs in PdSb2, which is compatible with its achiral structure. Our findings suggest that pyrite-structured PdSb2 provides an ideal platform for the investigation of fermions and their potential applications.

The three-dimensional and temperature-dependent electronic structures of the heavy-fermion superconductor ${\mathrm{CePt}}_{2}{\mathrm{In}}_{7}$ are investigated. Angle-resolved photoemission spectroscopy using variable photon energy establishes the existence of quasi-two- and three-dimensional Fermi surface topologies. Temperature-dependent $4d\text{\ensuremath{-}}4f$ on-resonance photoemission spectroscopies data reveal that heavy quasiparticle bands begin to form at a temperature well above the characteristic (coherence) temperature ${T}^{*}$. The emergence of low-lying crystal electric field excitation may be responsible for the ``relocalization'' or the precursor to the establishment of heavy electrons coherence in heavy-fermion compounds. These findings provide critical insight into understanding the hybridization in heavy-fermion systems.

The van der Waals ferromagnet Fe5GeTe2 has a Curie temperature TC of about 270 K, which can be raised above room temperature by tuning the Fe deficiency content. To achieve insights into its ferromagnetic exchange, we have studied the critical behavior by measuring the magnetization in bulk Fe5GeTe2 crystal around the ferromagnetic to paramagnetic phase transition. The analysis of the magnetization by employing various techniques including the modified Arrott plot, Kouvel-Fisher plot and critical isotherm analysis achieved a set of reliable critical exponents with TC = 273.7 K, beta = 0.3457, gamma = 1.40617, and delta = 5.021, suggesting a three-dimensional magnetic exchange with the distance decaying as J(r) ~ (r)$^-4.916, which is close to that of a three-dimensional Heisenberg model with long-range magnetic coupling., 22 pages, 2 tables, 6 figures

Triangular lattice of rare-earth ions with interacting effective spin-$1/2$ local moments is an ideal platform to explore the physics of quantum spin liquids (QSLs) in the presence of strong spin-orbit coupling, crystal electric fields, and geometrical frustration. The Yb delafossites, NaYbCh$_2$ (Ch=O, S, Se) with Yb ions forming a perfect triangular lattice, have been suggested to be candidates for QSLs. Previous thermodynamics, nuclear magnetic resonance, and muon spin rotation measurements on NaYbCh$_2$ have supported the suggestion of the QSL ground states. The key signature of a QSL, the spin excitation continuum, arising from the spin quantum number fractionalization, has not been observed. Here we perform both elastic and inelastic neutron scattering measurements as well as detailed thermodynamic measurements on high-quality single-crystalline NaYbSe$_2$ samples to confirm the absence of long-range magnetic order down to 40 mK, and further reveal a clear signature of magnetic excitation continuum extending from 0.1 to 2.5 meV. The comparison between the structure of the magnetic excitation spectra and the theoretical expectation from the spinon continuum suggests that the ground state of NaYbSe$_2$ is a QSL with a spinon Fermi surface., Comment: 18 pages, 6 figures

The antiferromagnet Mn${}_{3}$Ge has been recently shown to exhibit a large spontaneous anomalous Hall effect, which can be switched by a small magnetic field and which has been attributed to topologically protected Weyl nodes. Up till now, the true ground-state magnetic structure of the compound has eluded countless experimental studies, which began some fifty years ago. Here, the authors report the use of an advanced polarized neutron scattering technique to resolve unambiguously the magnetic structure of this compound.

We report on the structural, magnetic, and electronic properties of two new double-perovskites synthesized under high pressure; Pb2CaOsO6 and Pb2ZnOsO6. Upon cooling below 80 K, Pb2CaOsO6 simultaneously undergoes a metal--insulator transition and develops antiferromagnetic order. Pb2ZnOsO6, on the other hand, remains a paramagnetic metal down to 2 K. The key difference between the two compounds lies in their crystal structure. The Os atoms in Pb2ZnOsO6 are arranged on an approximately face-centred cubic lattice with strong antiferromagnetic nearest-neighbor exchange couplings. The geometrical frustration inherent to this lattice prevents magnetic order from forming down to the lowest temperatures. In contrast, the unit cell of Pb2CaOsO6 is heavily distorted up to at least 500 K, including antiferroelectric-like displacements of the Pb and O atoms despite metallic conductivity above 80 K. This distortion relieves the magnetic frustration, facilitating magnetic order which in turn drives the metal--insulator transition. Our results suggest that the phase transition in Pb2CaOsO6 is spin-driven, and could be a rare example of a Slater transition., Comment: 14 pages, 9 figures. Accepted as a regular article in Phys. Rev. B

The intrinsic magnetic topological insulator MnBi2Te4 exhibits rich topological effects such as quantum anomalous Hall effect and axion electrodynamics. Here, by combining the use of synchrotron and laser light sources, we carry out comprehensive and high-resolution angle-resolved photoemission spectroscopy studies on MnBi2Te4 and clearly identify its topological electronic structure. In contrast to theoretical predictions and previous studies, we observe topological surface states with diminished gap forming a characteristic Dirac cone. We argue that the topological surface states are mediated by multidomains of different magnetization orientations. In addition, the temperature evolution of the energy bands clearly reveals their interplay with the magnetic phase transition by showing interesting differences between the bulk and surface states, respectively. The investigation of the detailed electronic structure of MnBi2Te4 and its temperature evolution provides important insight into not only the exotic properties of MnBi2Te4, but also the generic understanding of the interplay between magnetism and topological electronic structure in magnetic topological quantum materials.

The existence and approximation of manifolds for the Swift-Hohenberg equation with a proper parameter have mainly been studied. Using the backward-forward systems from Swift-Hohenberg equation, the existence and specific representation forms of manifolds for Swift-Hohenberg equation with a parameter have been obtained. Meanwhile, we make use of technique of deposition of lower and higher frequency spaces of solutions and assume the reduced system to obtain the main numeration approximation system of approximation solution for the original system Swift-Hohenberg equation with a proper parameter.

We investigated the magnetic structure and dynamics of YbMnBi$_2$, with elastic and inelastic neutron scattering, to shed light on the topological nature of the charge carriers in the antiferromagnetic phase. We confirm C-type antiferromagnetic ordering of the Mn spins below $T_{\rm N} = 290$ K, and determine that the spins point along the $c$-axis to within about $3^\circ$. The observed magnon spectrum can be described very well by the same effective spin Hamiltonian as was used previously to model the magnon spectrum of CaMnBi$_2$. Our results show conclusively that the creation of Weyl nodes in YbMnBi$_2$ by the time-reversal-symmetry breaking mechanism can be excluded in the bulk.

We have unveiled a magnetic exchange-induced topological phase transition in a bulk natural van der Waals crystal MnSb2Te4, based on magnetization and magnetotransport measurements and first principles calculations. At the A-type antiferromagnetic ground state, MnSb2Te4 is a topologically trivial insulator with a bandgap of ∼ 42 meV at the Γ point of the Brillouin zone. A small magnetic field less than 1.4 T along the c axis can drive the system into a spin fully polarized state, which hosts only a single pair of Weyl points setting near the Γ point at the Fermi level without other band mixing, supported by both the first principles calculations and the measured anomalous Hall effect. The results would setup an excellent paradigm for the study of interplay between magnetism and nontrivial topology of the electronic band structure.

We report systematic magneto-transport measurements and ab initio calculations on single-crystalline TaP2, a new member of the transition-metal dipnictides. We observed unsaturated magnetoresistance (MR) reaching ~ 700% at a magnetic field (B) of 9 T at 2 K along with striking Shubnikov-de Hass (SdH) oscillations. Our analysis on the SdH oscillations reveals nonzero Berry phase, indicating nontrivial band topology. The analysis also uncovers three fundamental magnetic oscillation frequencies of 72 T, 237 T, and 356 T, consistent with the theoretical calculations which reveal one hole pocket and two electron pockets at the L point and one electron pocket at the Z point of the Brillouin zone. We also found negative longitudinal MR (n-MR) within a narrow window of the angles between B and the electric current (I). The n-MR could be fitted with the Adler-Bell-Jackiw chiral anomaly equation but the origin remains yet ambiguous. The ab inito calculations suggest TaP2 as a weak topological insulator with the Z2 indices of (0; 111), which exhibits topological surface states on the (001) surface., 22 pages, 6 figures, 1 table

We report a systematic NMR study on [Sr4Sc2O6]Fe-2(As1-xPx)(2), for which the local lattice parameters of the iron-pnictogen (FePn) layer are similar to those of the series LaFe(As1-x'Px')O, which exhibits two segregated antiferromagnetic (AFM) order phases, AFM1 at x' = 0 - 0.2 and AFM2 at x' = 0.4 - 0.7. Our results reveal that the parent AFM1 phase at x = 0 disappears at x = 0.3 - 0.4, corresponding to a pnictogen height (h(pn)) from the Fe plane of 1.3-1.32 angstrom, which is similar to that of LaFe(As1-x'Px')O and various parent Fe pnictides. By contrast, the AFM2 order reported for LaFe(As0.4P0.6)O does not appear at x similar to 0.8, although the local lattice parameters of the FePn layer and the microscopic electronic states are quite similar. Despite the absence of the static AFM2 order, reemergent dynamical AFM spin fluctuations were observed at approximately x similar to 0.8, which can be attributed to the instability of the AFM2 phase. We suggest this re-enhancement of AFM spin fluctuations plays a significant role in enhancing the T-c to 17 K for x = 0.8 - 1. Finally, we discuss the universality and diversity of the complicated magnetic ground states from a microscopic point of view, including the difference in the origins of the AFM1 and AFM2 phases, and their relations with the high superconducting transitions in Fe pnictides.

The recent discovery of intrinsic ferromagnetic order in the atomically thin van der Waals crystal CrXTe_{3} (X=Si, Ge) stimulates intensive studies on the nature of low-dimensional magnetism because the presence of long-range magnetic order in two-dimensional systems with continuous symmetry is strictly prohibited by thermal fluctuations. By combining advanced many-body calculations with angle-resolved photoemission spectroscopy we investigate CrSiTe_{3} single crystals and unveil the pivotal role played by the strong electronic correlations at both high- and low-temperature regimes. Above the Curie temperature (T_{c}), Coulomb repulsion (U) drives the system into a charge transfer insulating phase. In contrast, below T_{c} the crystal field arranges the Cr-3d orbitals such that the ferromagnetic superexchange profits, giving rise to the bulk ferromagnetic ground state with which the electronic correlations compete. The excellent agreement between theory and experiment establishes CrSiTe_{3} as a prototype low-dimensional crystal with the cooperation and interplay of electronic correlation and ferromagnetism.

We present AC and DC magnetometry, heat capacity, muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$), and resonant inelastic x-ray scattering (RIXS) studies of the pyrochlore osmate ${\mathrm{Y}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$. Consistent with previous results, we observe a small Curie-Weiss effective moment of approximately 0.4 ${\ensuremath{\mu}}_{\mathrm{B}}$ and spin freezing at a temperature ${T}_{f}\ensuremath{\simeq}5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. On the other hand, our analysis of the RIXS spectra with single-ion energy-level calculations implies a nonmagnetic ${J}_{\mathrm{eff}}=0$ ground state on the ${\mathrm{Os}}^{4+}$ sites, with the spin-orbit interaction, Hund's coupling, and trigonal distortion of ${\mathrm{OsO}}_{6}$ octahedra, all important in modeling the observed spectra. From high-field magnetization data, we show that the paramagnetic moment is most likely due to large moments ${\ensuremath{\mu}}_{\mathrm{eff}}\ensuremath{\simeq}3\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ on only a small fraction $f\ensuremath{\simeq}0.02$ of Os sites. We suggest disorder-related effects, such as oxygen nonstoichiometry or site interchange between Os and Y ions, as the most likely explanation for the magnetic response of this material.

Magnetic Weyl semimetals (WSMs) bearing long-time pursuing are still very rare. We herein identified magnetic exchange induced Weyl state in EuCd2Sb2, a semimetal in type IV magnetic space group, via performing high magnetic field (B) magneto-transport measurements and ab initio calculations. For the A-type antiferromagnetic (AFM) structure of EuCd2Sb2, external B larger than 3.2 T can align Eu spins to be fully polarized along the c-axis and consequently drive the system into a ferromagnetic (FM) state. Measurements up to B ~ 55 T revealed a striking Shubnikov-de Hass oscillation imposed by a nontrivial Berry phase. We unveiled a phase transition from a small-gap AFM topological insulator into a FM WSM in which Weyl points emerged along the {\Gamma}-Z path. Fermi arcs on (100) and (010) surfaces are also revealed. The results pave a way towards realization of various topological states in a single material through magnetic exchange manipulation., Comment: 31 papges,10 figures, 2 tables

The extremely large magnetoresistance (XMR) in nonmagnetic semimetals has inspired growing interest owing to both intriguing physics and potential applications. We report results of synchrotron X-ray diffraction (SXRD) and electrical transport measurements on TaAs2 under pressure up to ~ 37 GPa, which revealed an anisotropic compression of the unit cell, formation of unusual As-As bonds above 9.5 GPa, and enhancement of metallicity. Interestingly, the MR of TaAs2 under pressure changed gently, which at 1.7 GPa is 96.6% and at 36.6 GPa is still 36.7%. The almost robust MR under pressure could be related to the nearly stable electronic structure unveiled by the ab initio calculations. The discovery would expand the potential use of XMR even under high pressure., Comment: 15 pages, 7 figures including those in supporting information

Weyl semimetals exhibit exceptional quantum electronic transport due to the presence of topologically-protected band crossings called Weyl nodes. The nodes come in pairs with opposite chirality, but their number and location in momentum space is otherwise material specific. Following the initial discoveries there is now a need for better material realizations, ideally comprising a single pair of Weyl nodes located at or very close to the Fermi level and in an energy window free from other overlapping bands. Here we propose the layered intermetallic EuCd$_2$As$_2$ to be such a system. We show that Weyl nodes in EuCd$_2$As$_2$ are magnetically-induced via exchange coupling, emerging when the Eu spins are aligned by a small external magnetic field. The identification of EuCd$_2$As$_2$ as a model magnetic Weyl semimetal, evidenced here by ab initio calculations, photoemission spectroscopy, quantum oscillations and anomalous Hall transport measurements, opens the door to fundamental tests of Weyl physics.

Van der Waals (vdW) ferromagnetic materials have attracted considerable attention in the nanomaterial community, which could provide a unique platform to study magnetism at the nanoscale. Along this direction, many interesting results have been reported, including the electric field control of magnetism and topological spin textures. In this report, we present a rapid and spatially resolved imaging method to study the dimensionality-dependent magnetic properties of Fe3GeTe2 (FGT) nanoflakes. Our method is named as polar magneto-optical Kerr imaging microscopy magnetometry (p-MIMM), which is made possible by analyzing the intensity evolution of wide-field polar magneto-optical Kerr effect (MOKE) images that were collected by varying magnetic fields, thicknesses, and temperatures. In particular, spatially resolved MOKE hysteresis loops can be acquired in the FGT nanoflakes with a submicrometer resolution. By analyzing the evolution of the relative (saturated) MOKE intensity as a function of temperature, we further study the critical exponent and universality class and its dependence on the FGT nanoflake thickness. Combining the polar MOKE images with the calculated MOKE hysteresis loops, a detailed magnetic phase diagram summarizing an evolution of the stripe domain, single domain, and paramagnetic state is further validated. Our results suggest that the wide-field p-MIMM can be conveniently used for rapidly examining the magnetic properties of versatile vdW magnetic materials.

Approximation representation of parameterizing manifold and non-Markovian reduced systems for a stochastic Swift–Hohenberg equation with additive noise has been investigated. The corresponding backward–forward systems have been proposed, which can give such stochastic parameterizing manifold as pullback limits depending through the nonlinear terms on the time–history of the dynamics of the low modes when the latter is simply approximated by its stochastic linear component in a mean square sense. Then approximation representation of parameterizing manifold is given. Furthermore, according to it, the non-Markovian reduced systems can be derived to reach good modeling performances in practice.

Characterization of Fermi surface of the BaSn3 superconductor (T c ∼ 4.4 K) by de Haas–van Alphen (dHvA) effect measurement reveals its non-trivial topological properties. Analysis of non-zero Berry phase is supported by the ab initio calculations, which reveals a type-II Dirac point setting and tilting along the high symmetric K–H line of the Brillouin zone, about 0.13 eV above the Fermi level, and other two type-I Dirac points on the high symmetric Γ–A direction, but slightly far below the Fermi level. The results demonstrate BaSn3 as an excellent example hosting multiple Dirac fermions and an outstanding platform for studying the interplay between nontrivial topological states and superconductivity.

We have used the de Hass-van Alphen (dHvA) effect to investigate the Fermi surface of high-quality crystalline TiSb2, which unveiled a nontrivial topologic nature by analyzing the dHvA quantum oscillations. Moreover, our analysis on the quantum oscillation frequencies associated with nonzero Berry phase when the magnetic field is parallel to both of the ab-plane and c-axis of TiSb2 finds that the Fermi surface topology has a three-dimensional (3D) feature. The results are supported by the first-principle calculations which revealed a symmetry-protected Dirac point appeared along the {\Gamma}-Z high symmetry line near the Fermi level. On the (001) surface, the bulk Dirac points are found to project onto the -{\Gamma} point with nontrivial surface states. Our finding will substantially enrich the family of 3D Dirac semimetals which are useful for topological applications., Comment: 14 pages, including main text + SI, 5 figures

Materials with giant spin splitting are desired for spintronic applications. The fabrications of spintronic devices from half metals with one spin direction are often hampered, however, by stray magnetic fields, domain walls, short spin coherence times, scattering on magnetic atoms or magnetically active interfaces, and other characteristics that come along with the magnetism. The surfaces of topological insulators, or Dirac/Weyl semimetals, could be an alternative, but production of high-quality thin films without the presence of the bulk states at the Fermi energy remains very challenging. Here, by utilizing angle-resolved photoemission spectroscopy, a record-high Dresselhaus spin-orbit splitting of the bulk state in the nonmagnetic IrBiSe is found. The band structure calculations indicate that the splitting band is fully spin-polarized with 3D chiral spin texture. As a source of spin-polarized electrons, lightly doped IrBiSe is expected to generate electric-field-controlled spin-polarized currents, free from back scattering, and could host triplet and Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) superconductivity.

Half-metals are a class of materials that are metallic only for one spin direction, and are essential for spintronics applications where one needs to read, write, store and transfer spin-data. This spin sensitivity appears to restrict them to be magnetic, and the known examples indeed are. The fabrication of real spintronic devices from such materials is often hampered, however, by stray magnetic fields, domain walls, short spin coherence times, scattering on magnetic atoms or magnetically active interfaces, and other characteristics that come along with the magnetism. The surfaces of topological insulators, or Dirac or Weyl semimetals, could be an alternative, but production of high-quality thin films without the presence of the bulk states at the Fermi level remains very challenging. Here we introduce non-magnetic half-metals and demonstrate that this state is realized in IrBiSe. Using angle-resolved photoemission spectroscopy and band structure calculations we find a record-high Dresselhaus spin-orbit splitting, fully spin-polarized remnant Fermi surfaces and a chiral 3D spin-texture, all with no magnetism present. Promising applications include using IrBiSe as a source of spin-polarized electrons, and lightly doped IrBiSe is expected to generate electric-field-controlled spin-polarized currents, free from back scattering, and could host triplet superconductivity.

Vanadium diselenide (VSe2) is a van der Waals layered transitional metal dichalcogenide, which has attracted intensive attention due to the coexistence of charge density wave (CDW) transition and magnetic property down to two-dimension limit. We apply Spin- and angle-resolved photoemission spectroscopy measurement to explore the evolution of the valence eletronic states of VSe2 accompanied with the CDW transition. The V 3d-derived-band in the vicinity of Fermi energy shows negligible spin polarization both below and above the CDW transition temperature. Furthermore, a sizable spin polarization of Se 4p electronic states has been clearly observed, indicating an atomic-layer-dependent spin polarization in VSe2. Our results provide new clues to understand the spin polarized electronic states in transitional metal dichalcogenides.

We use resonant elastic x-ray scattering to determine the evolution of magnetic order in EuCd$_2$As$_2$ below $T_\textrm{N}=9.5$\,K, as a function of temperature and applied magnetic field. We find an A-type antiferromagneticstructure with in-plane magnetic moments, and observe dramatic magnetoresistive effects associated with field-induced changes in the magnetic structure and domain populations. Our \textit{ab initio} electronic structure calculations indicate that the Dirac dispersion found in the nonmagnetic Dirac semimetal Cd$_3$As$_2$ is also present in EuCd$_2$As$_2$, but is gapped for $T < T_\textrm{N}$ due to the breaking of $C_3$ symmetry by the magnetic structure., Supplemental information attached to preprint

LiOsO$_3$ is the first example of a new class of material called a ferroelectric metal. We performed zero-field and longitudinal-field $\mu$SR, along with a combination of electronic structure and dipole field calculations, to determine the magnetic ground state of LiOsO$_3$. We find that the sample contains both static Li nuclear moments and dynamic Os electronic moments. Below $\approx 0.7\,$K, the fluctuations of the Os moments slow down, though remain dynamic down to 0.08$\,$K. We expect this could result in a frozen-out, disordered ground state at even lower temperatures., Comment: 6 pages, 1 figure

Different two-dimensional materials, when combined together to form heterostructures, can exhibit exciting properties that do not exist in individual components. Therefore, intensive research efforts have been devoted to their fabrication and characterization. Previously, vertical and in-plane two-dimensional heterostructures have been formed by mechanical stacking and chemical vapor deposition. Here we report a new material system that can form in-plane p-n junctions by thermal conversion of n-type SnSe2 to p-type SnSe. Through scanning tunneling microscopy and density functional theory studies, we find that these two distinctively different lattices can form atomically sharp interfaces and have a type II to nearly type III band alignment. We also demonstrate that this method can be used to create micron sized in-plane p-n junctions at predefined locations. These findings pave the way for further exploration of the intriguing properties of the SnSe2-SnSe heterostructure., Comment: 30 pages, 6 figures

In this paper, we investigate a global random attractor for a stochastic local modified Swift-Hohenberg equation with multiplicative noise in Stratonovich sense. Through the Ornstein-Uhlenbeck (O-U) transformation, we obtain the random dynamical system associated with the stochastic local modified Swift-Hohenberg equation. Using the properties of the O-U process, we derive the specific uniform a priori estimates, using which we prove the existence of global random attractor for the corresponding random dynamical system.

Plasma treatment is a powerful tool to tune the properties of two-dimensional materials. Previous studies have utilized various plasma treatments on two-dimensional materials. We find a new effect of plasma treatment. After controlled oxygen-plasma treatment on field-effect transistors based on two-dimensional SnSe 2 , the capacitive coupling between the silicon back gate and the channel through the 300nm SiO 2 dielectric can be dramatically enhanced by about two orders of magnitude (from 11 nF/cm 2 to 880 nF/cm 2 ), reaching good efficiency of ion-liquid gating. At the same time, plasma treated devices show large hysteresis in the gate sweep demonstrating memory behavior. We reveal that this spontaneous ion gating and hysteresis are achieved with the assistance of a thin layer of water film automatically formed on the sample surface with water molecules from the ambient air, due to the change in hydrophilicity of the plasma treated samples. The water film acts as the ion liquid to couple the back gate and the channel. Thanks to the rich carrier dynamics in plasma-treated two-dimensional transistors, synaptic functions are realized to demonstrate short- and long-term memories in a single device. This work provides a new perspective on the effects of plasma treatment and a facile route for realizing neuromorphic devices.