The materials with spin-polarized electronic states have attracted a huge amount of interest due to their potential applications in spintronics. Based on first-principles calculations, we study the electronic characteristics of a series of AB2X4 chalcogeniden spinel structures and propose two promising candidates, VZn2O4 and VCd2S4, are spin-polarized semimetal materials. Both of them have ferromagnetic ground states. Their bands near the Fermi level are completely spin-polarized and form two types of nodal rings in the spin-up channel, and the large gaps in the spin-down channel prevent the spin-flip. Further symmetry analysis reveals that the nodal rings are protected by the glide mirror or mirror symmetries. Significantly, these nodal rings connect with each other and form a nodal chain structure, which can be well described by a simple four-band tight-binding (TB) model. The two ternary chalcogeniden spinel materials with a fully spin polarized nodal chain can serve as a prominent platform in the future applications of spintronic.
Diamond-structure materials have been extensively studied for decades, which form the foundation for most semiconductors and their modern day electronic devices. Here, we discover a e$_g$-orbital ($d_{z^2}$,$d_{x^2-y^2}$ ) model within the diamond lattice (e$_g$-diamond model) that hosts novel topological states. Specifically, the e$_g$-diamond model yields a 3D nodal cage (3D-NC), which is characterized by a $d$-$d$ band inversion protected by two types of degenerate states (i.e., e$_g$-orbital and diamond-sublattice degeneracies). We demonstrate materials realization of this model in the wel…
Semiconducting Electronic Structure of the Ferromagnetic Spinel $\mathbf{Hg}\mathbf{Cr}_2\mathbf{Se}_4$ Revealed by Soft-X-Ray Angle-Resolved Photoemission Spectroscopy
Topological semimetals in ferromagnetic materials have attracted enormous attention due to the potential applications in spintronics. Using the first-principles density functional theory together with an effective lattice model, here we present a new family of topological semimetals with a fully spin-polarized nodal loop in alkaline-metal monochalcogenide $MX$ ($M$ = Li, Na, K, Rb, Cs; $X$ = S, Se, Te) monolayers. The half-metallic ferromagnetism can be established in $MX$ monolayers, in which one nodal loop formed by two crossing bands with the same spin components is found at the Fermi energ…
Topological Weyl semimetals (WSMs) have attracted widespread interests due to the chiral Weyl fermions and surface Fermi arcs that enable unique optical and transport phenomena. In this work, we present angle-resolved Raman spectroscopy of TaP, a prototypical noncentrosymmetric WSM, for five excitation wavelengths ranging from 364 to 785 nm. The Raman active modes, $A_1$, $B_1^1$, and $B_1^2$ modes, exhibit two main unique features beyond the conventional Raman theory. First, the relative intensities of Raman active modes change as a function of the excitation wavelength. Second, angle-resolve…
Recently, magnetic topological semimetals have received a lot of attention due to their potential applications in the field of spintronics. By using first-principles calculations, we propose that two ferromagnetic spinel materials of X2MnO4 (X=Li, Na) have Weyl-type nodal chains around the Fermi level. Their stabilities are validated by cohesive energies, phonon dispersions, and elastic constants. The nodal chains are composed of two types of nodal loops, which are protected by the glide operation Mz, the mirror operation M101 and their equivalent. The drumhead surface states are observed on t…
We investigate the surface electronic structures of polar 1T'-MoTe2, the Weyl semimetal candidate realized through the nonpolar-polar structural phase transition, by utilizing the laser angle-resolved photoemission spectroscopy (ARPES) combined with first-principles calculations. Two kinds of domains with different surface band dispersions are observed from a single-crystalline sample. The spin-resolved measurements further reveal that the spin polarizations of the surface and the bulk-derived states show the different domain-dependences, indicating the opposite bulk polarity. For both domains…
We study the electronic state of an inverse spinel compound MnMg$_{2}$O$_{4}$ based on first-principles calculations. The high-spin state is realized in Mn ions on the diamond lattice, resulting in that this material is found to be a half-metallic semimetal with the minority spin-gap about 3eV, and also with line nodes in the Brillouin zone. The intrinsic anomalous Hall conductivity (AHC) is also computed as a function of the chemical potential of the system assuming the rigid band structure, and is found to exhibit a peak structure with a maximum value of 200 S/cm at only 15 meV above the Fer…
Spin-gapless semimetals (SGSMs), which generate 100\% spin polarization, are viewed as promising semi-half-metals in spintronics with high speed and low consumption. We propose and characterize a new $\mathbb{Z_{\mathrm{2}}}$ class of topological nodal line (TNL) in SGSMs. The proposed TNLSGSMs are protected by space-time inversion symmetry or glide mirror symmetry with two-dimensional (2D) fully spin-polarized nearly flat surface states. Based on first-principles calculations and effective model analysis, a series of high-quality materials with $\textit{R}\overline{3}\textit{c}$ and $\textit{…
We study the photoresponse of two-dimensional semi-Dirac semimetals and three-dimensional Dirac semimetals to off-resonant circularly polarized light. For two-dimensional semi-Dirac semimetals we find that incident light does not open a gap in the spectrum, in contrast to the case of purely linear dispersion. For the three-dimensional case we find that applying a circularly polarized light, one can tune from a trivial insulator to three-dimensional Dirac semimetal with an inverted gap. We propose and show that application of light leads to an intriguing platform for generation, motion and merg…
Recently discovered high-quality nodal chain spin-gapless semimetals $M$F$_3$ ($M$ = Pd, Mn) feature an ultra-clean nodal chain in the spin up channel residing right at the Fermi level and displaying a large spin gap leading to a 100\% spin-polarization of transport properties. Here, we investigate both intrinsic and extrinsic contributions to anomalous and spin transport in this class of materials. The dominant intrinsic origin is found to originate entirely from the gapped nodal chains without the entanglement of any other trivial bands. The side-jump mechanism is predicted to be negligibly …
The superconductor SnTaS$_{2}$ is theoretically predicted to be an intriguing topological nodal line semimetal without consideration of spin-orbit coupling. By carrying out angle-resolved photoemission (ARPES) and spin-resolved ARPES measurements combined with band structure calculations, we have provided a complete picture of the electronic structure and spin polarization property for the prominent surface states of SnTaS$_{2}$. The low-energy electronic states are dominated by surface states; two of them are from the S-terminated surface, while four of them are from the Sn-terminated surface…
The layered 2D-material MoTe$_2$ in the T$_d$ crystal phase is a semimetal which has theoretically been predicted to possess topologically non-trivial bands corresponding to Weyl fermions. Clear experimental evidence by angle-resolved photoemission spectroscopy (ARPES) is, however, lacking, which calls for a careful examination of the relation between ground state band structure calculations and ARPES intensity plots. Here we report a study of the near Fermi-energy band structure of MoTe$_2$(T$_d$) by means of ARPES measurements, density functional theory, and one-step-model ARPES calculations…
Weyl semimetals are characterized by the presence of massless band dispersion in momentum space. When a Weyl semimetal meets magnetism, large anomalous transport properties emerge as a consequence of its topological nature. Here, using $in-situ$ spin- and angle-resolved photoelectron spectroscopy combined with $ab\ initio$ calculations, we visualize the spin-polarized Weyl cone and flat-band surface states of ferromagnetic Co$_2$MnGa films with full remanent magnetization. We demonstrate that the anomalous Hall and Nernst conductivities systematically grow when the magnetization-induced massiv…
A bulk superconductor possessing a topological surface state at the Fermi level is a promising system to realize long-sought topological superconductivity. Although several candidate materials have been proposed, experimental demonstrations concurrently exploring spin textures and superconductivity at the surface have remained elusive. Here we perform spectroscopic-imaging scanning tunnelling microscopy on the centrosymmetric superconductor $β$-PdBi$_2$ that hosts a topological surface state. By combining first-principles electronic-structure calculations and quasiparticle interference experim…
Most of the 2D transition metal dichalcogenides (TMDC) are nonmagnetic in pristine form. However, 2D pristine VX2 (X=S, Se, Te) materials are found to be ferromagnetic. Using spin polarized density functional theory (DFT) calculations, we have studied the electronic, magnetic and surface properties of this class of materials in both trigonal prismatic 2H- and octahedral 1T-phase. Our calculations reveal that they exhibit materially different properties in those two polymorphs. Most importantly, detailed investigation of electronic structure explored the quantum size effect in 2H-phase of these…
Weyl semimetal Td-phase WTe2 possesses the spin-resolved band structure with strong spin-orbit coupling, holding promises as a useful spin source material. The noncentrosymmetric crystalline structure of Td-WTe2 endows the generation of the out-of-plane polarized spin, which is of great interest in magnetic memory applications. Previously, WTe2 was explored in spin devices based on mechanically exfoliated single crystal flakes with a size of micrometers. For practical spintronics applications, it is highly desirable to implement wafer-scale thin films. In this work, we utilize centimeter-scale…
We theoretically investigated the dependence of higher-order harmonic generation (HHG) in solid-state materials on the ellipticity of the electric field. We found that in the multiphoton absorption and ac Zener regimes, the intensity of HHG monotonically decreases with increasing ellipticity of the incident electric field, while in the semimetal regime, the intensity reaches a maximum for finite values of ellipticity. Moreover, the characteristics of the polarization of the emitted HHG change depending on the field intensity; only parallel emissions with respect to the major axis exist in the …
LaBi is a three-dimensional rocksalt-type material with a surprisingly quasi-two-dimensional electronic structure. It exhibits excellent electronic properties such as the existence of nontrivial Dirac cones, extremely large magnetoresistance, and high charge-carrier mobility. The cigar-shaped electron valleys make the charge transport highly anisotropic when the magnetic field is varied from one crystallographic axis to another. We show that the electrons can be polarized effectively in these electron valleys under a rotating magnetic field. We achieved a polarization of 60% at 2 K despite the…
We study the electric-field tunable electronic properties of phosphorene thin films, using the framework of density functional theory. We show that phosphorene thin films offer a versatile material platform to study two dimensional Dirac fermions on application of a transverse electric field. Increasing the strength of the transverse electric field beyond a certain critical value in phosphorene leads to the formation of two symmetry protected gapless Dirac fermions states with anisotropic energy dispersion. The spin-orbit coupling splits each of these Dirac state into two spin- polarized Dirac…
We report a polarized Raman study of Weyl semimetal TaAs. We observe all the optical phonons, with energies and symmetries consistent with our first-principles calculations. We detect additional excitations assigned to multiple-phonon excitations. These excitations are accompanied by broad peaks separated by 140~cm$^{-1}$ that are also most likely associated with multiple-phonon excitations. We also noticed a sizable B$_1$ component for the spectral background, for which the origin remains unclear.
We propose a new topological quantum state of matter---the two-dimensional (2D) Weyl half semimetal (WHS), which features 2D Weyl points at Fermi level belonging to a single spin channel, such that the low-energy electrons are described by fully spin-polarized 2D Weyl fermions. We predict its realization in the ground state of monolayer PtCl$_3$. We show that the material is a half metal with an in-plane magnetization, and its Fermi surface consists of a pair of fully spin-polarized Weyl points protected by a mirror symmetry, which are robust against spin-orbit coupling. Remarkably, we show th…
The layered ternary compound TaIrTe4 is an important candidate to host the recently predicted type-II Weyl Fermions. However, a direct and definitive proof of the absence of inversion symmetry in this material, a prerequisite for the existence of Weyl Fermions, has so far remained evasive. Herein, an unambiguous identification of the broken inversion symmetry in TaIrTe4 is established using angle-resolved polarized Raman spectroscopy. Combining with high-resolution transmission electron microscopy, we demonstrate an efficient and nondestructive recipe to determine the exact crystallographic or…
Weyl semimetals and nodal line semimetals are characterized by linear band touching at zero-dimensional points and one-dimensional lines, respectively. We predict that a circularly polarized light drives nodal line semimetals into Weyl semimetals. The Floquet Weyl points thus obtained are tunable by the incident light, which enables investigations of them in a highly controllable manner. The transition from nodal line semimetals to Weyl semimetals is accompanied by the emergence of a large and tunable anomalous Hall conductivity. Our predictions are experimentally testable by transport measure…
CeBi has an intricate magnetic phase diagram whose fully-polarized state has recently been suggested as a Weyl semimetal, though the role of $f$ states in promoting strong interactions has remained elusive. Here we focus on the less-studied, but also time-reversal symmetry-breaking ferrimagnetic phase of CeBi, where our density functional theory (DFT) calculations predict additional Weyl nodes near the Fermi level $E_\mathrm{F}$. We use spin-polarized scanning tunneling microscopy and spectroscopy to image the surface ferrimagnetic order on the itinerant Bi $p$ states, indicating their orbital…
The kagome lattice Co$_3$Sn$_2$S$_2$ exhibits the quintessential topological phenomena of a magnetic Weyl semimetal such as the chiral anomaly and Fermi-arc surface states. Probing its magnetic properties is crucial for understanding this correlated topological state. Here, using spin-polarized scanning tunneling microscopy/spectroscopy (STM/S), we report the discovery of localized spin-orbit polarons (SOPs) with three-fold rotation symmetry nucleated around single-S vacancies in Co$_3$Sn$_2$S$_2$. The SOPs carry a spin-polarized magnetic moment and a large orbital magnetization of a topologic…
We have studied helicity dependent photocurrent (HDP) in Bi-based Dirac semimetal thin films. HDP increases with film thickness before it saturates, changes its sign when the majority carrier type is changed from electrons to holes and takes a sharp peak when the Fermi level lies near the charge neutrality point. These results suggest that irradiation of circularly polarized light to Dirac semimetals induces an effective magnetic field that aligns the carrier spin along the light spin angular momentum and generates a spin current along the film normal. The effective magnetic field is estimated…
The past decade has seen a proliferation of topological materials for both insulators and semimetals in electronic systems and classical waves. Topological semimetals exhibit topologically protected band degeneracies, such as nodal points and nodal lines. Dirac nodal line semimetals (DNLS), which own four-fold line degeneracy, have drawn particular attention. DNLSs have been studied in electronic systems but there is no photonic DNLS. Here in this work, we provide a new mechanism which is unique for photonic systems to investigate a stringent photonic DNLS. When truncated, the photonic DNLS ex…
Weyl points with monopole charge $\pm 1$ have been extensively studied, however, real materials of multi-Weyl points, whose monopole charges are higher than $1$, have yet to be found. In this Rapid Communication, we show that nodal-line semimetals with nontrivial line connectivity provide natural platforms for realizing Floquet multi-Weyl points. In particular, we show that driving crossing nodal lines by circularly polarized light generates double-Weyl points. Furthermore, we show that monopole combination and annihilation can be observed in crossing-nodal-line semimetals and nodal-chain semi…
Using spin-resolved and angle-resolved photoemission spectroscopy and first-principles calculations, we have identified bulk band inversion and spin polarized surface state evolved from a weak topological insulator (TI) phase in van der Waals materials Nb3XTe6 (X = Si, Ge). The fingerprints of weak TI homologically emerge with hourglass fermions, as multi nodal chains composed by the same pair of valence and conduction bands gapped by spin orbit coupling. The novel topological state, with a pair of valence and conduction bands encoding both weak TI and hourglass semimetal nature, is essential …
Synergic effect of electronic correlation and spin-orbit coupling is an emerging topic in topological materials. Central to this rapidly developing area are the prototypes of strongly correlated heavy-fermion systems. Recently, some Ce-based compounds are proposed to host intriguing topological nature, among which the electronic properties of CeSb are still under debate. In this paper, we report a comprehensive study combining magnetic and electronic transport measurements, and electronic band structure calculations of this compound to identify its topological nature. Quantum oscillations are …
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 vec…
We present how to detect type-$1$ Weyl nodes in a material by inelastic neutron scattering. Such an experiment first of all allows one to determine the dispersion of the Weyl fermions. We extend the reasoning to produce a quantitative test of the Weyl equation taking into account realistic anisotropic properties. These anisotropies are mostly contained in the form of the emergent magnetic moment of the excitations, which determines how they couple to the neutron. Although there are many material parameters, we find several quantitative predictions that are universal and demonstrate that the ex…
The nature of the interaction between magnetism and topology in magnetic topological semimetals remains mysterious, but may be expected to lead to a variety of novel physics. We present $ab$ $initio$ band calculations, electrical transport and angle-resolved photoemission spectroscopy (ARPES) measurements on the magnetic semimetal EuAs$_3$, demonstrating a magnetism-induced topological transition from a topological nodal-line semimetal in the paramagnetic or the spin-polarized state to a topological massive Dirac metal in the antiferromagnetic (AFM) ground state at low temperature, featuring a…
We theoretically investigate possible photoinduced topological phase transitions in the organic salt $α$-(BEDT-TTF)$_2$I$_3$, which possesses a pair of inclined massless Dirac-cone bands between the conduction and valence bands under uniaxial pressure. The Floquet analyses of a driven tight-binding model for this material reveal rich photoinduced variations of band structures, Chern numbers, and Hall conductivities under irradiation with elliptically polarized light. The obtained phase diagrams contain a variety of nonequilibrium steady phases, e.g., the Floquet Chern insulator, Floquet semime…
Quantum materials with strong transport responses to disparate physical quantities are of great fundamental significance and may hold technological potentials. The interplay between interactions and topology drive such responses through the effects of spontaneous symmetry breaking and the associated domain configurations on quantum transport. Here we provide a comprehensive description of the magnetism of Mn3Ge, an antiferromagnetic kagomebased semimetal with room temperature transport anomalies associated with topologically protected Weyl nodes. Using polarized neutron diffraction, we show th…
