The identification of topological Weyl semimetals has recently gained considerable attention. Here, we report the results of density-functional theory calculations regarding the magnetic properties, the electronic structure, and the intrinsic anomalous Hall conductivity of the title compound, which was synthesized already 50 years ago but received little attention, hitherto. We found Cs$_{2}$Co$_{3}$S$_4$ to be a ferrimagnetic half-metal with a total spin magnetic moment of about 3 $μ_B$ per formula unit. It shows energy band gap of 0.36 eV in the majority-spin channel and a pseudo-gap at the Fermi level in the minority-spin channel. We identified several sets of low-energy Weyl points and traced their dependence on the direction of magnetization. The intrinsic anomalous Hall conductivity is predicted to reach a magnitude up to 500 $Ω^{-1}$cm$^{-1}$, which is comparable to values obtained in other celebrated Weyl semimetals.
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 tetragonal ferrimagnetic Mn$_3$Ga exhibits a wide range of intriguing magnetic properties. Here, we report the emergence of topologically nontrivial nodal lines in the absence of spin orbit coupling (SOC) which are protected by both mirror and $C_{4z}$ rotational symmetries. In the presence of SOC we demonstrate that the doubly degenerate nontrivial crossing points evolve into $C_{4z}$-protected Weyl nodes with chiral charge of $\pm$2. Furthermore, we have considered the experimentally reported noncollinear ferrimagnetic structure, where the magnetic moment of the Mn$_I$ atom (on the Mn-Ga…
We report the results of an experimental study of thermal and magnetic properties of nanostructured ferrimagnetic iron oxide composites with graphene and graphite fillers synthesized via the current activated pressure assisted densification. The thermal conductivity was measured using the laser-flash and transient plane source techniques. It was demonstrated that addition of 5 wt. % of equal mixture of graphene and graphite flakes to the composite results in a factor of x2.6 enhancement of the thermal conductivity without significant degradation of the saturation magnetization. The microscopy …
We predict a magnetic Weyl semimetal in the inverse Heusler Ti2MnAl, a compensated ferrimagnet with a vanishing net magnetic moment and a Curie temperature of over 650 K. Despite the vanishing net magnetic moment, we calculate a large intrinsic anomalous Hall effect (AHE) of about 300 S/cm. It derives from the Berry curvature distribution of the Weyl points, which are only 14 meV away from the Fermi level and isolated from trivial bands. Different from antiferromagnets Mn3X (X= Ge, Sn, Ga, Ir, Rh, and Pt), where the AHE originates from the non-collinear magnetic structure, the AHE in Ti2MnAl s…
Magnetic topological quantum materials have attracted great attention due to their exotic topological quantum physics induced by the interplay among crystalology, magnetism, and topology, which is of profound importance to fundamental research and technology applications. However, limited materials are experimentally available, most of whom are realized by magnetic impurity doping or heterostructural constructions. In this work, based on the first-principles calculations, we predict that double perovskite Ba2CdReO6 is an intrinsic ferromagnetic topological semi-half-metal, while the ferrimagne…
We study dc conductivity of a Weyl semimetal with uniaxial anisotropy (Fermi velocity ratio $ξ= v_\bot/v_\parallel\neq1$) considering the scattering of charge carriers by a wide class of impurity potentials, both short- and long-range. We obtain the ratio of transverse and longitudinal (with respect to the anisotropy axis) conductivities as a function of both $ξ$ and temperature. We find that the transverse and longitudinal conductivities exhibit different temperature dependence in the case of short-range disorder. For general long-range disorder, the temperature dependence ($\sim T^4$) of the…
Tungsten ditelluride WTe$_2$ is a type-II Weyl semimetal with electronic properties highly sensitive to external pressure, as demonstrated by the superconductivity emerging under pressure. Here, we study the optical conductivity of the type-II Weyl semimetal WTe$_2$ under external pressure at room temperature. With increasing pressure, a pronounced spectral weight transfer occurs from the high-energy to the low-energy interband transitions, with drastic changes in the profile of the optical conductivity spectrum indicating a high sensitivity of the electronic band structure to external pressur…
Employing high-pressure infrared spectroscopy we unveil the Weyl semimetal phase of elemental Te and its topological properties. The linear frequency dependence of the optical conductivity provides clear evidence for metallization of trigonal tellurium (Te-I) and the linear band dispersion above 3.0 GPa. This semimetallic Weyl phase can be tuned by increasing pressure further: a kink separates two linear regimes in the optical conductivity (at 3.7 GPa), a signature proposed for Type-II Weyl semimetals with tilted cones; this however reveals a different origin in trigonal tellurium. Our density…
Mn$_{3}$Sn is a non-collinear antiferromagnet which displays a large anomalous Hall effect at room temperature. It is believed that the principal contribution to its anomalous Hall conductivity comes from Berry curvature. Moreover, dc transport and photoemission experiments have confirmed that Mn$_{3}$Sn may be an example of a time-reversal symmetry breaking Weyl semimetal. Due to a small, but finite moment in the room temperature inverse triangular spin structure, which allows control of the Hall current with external field, this material has garnered much interest for next generation memory …
TaIrTe$_4$ is an example of a candidate Weyl type-II semimetal with a minimal possible number of Weyl nodes. Four nodes are reported to exist a single plane in $k$-space. The existence of a conical dispersion linked to Weyl nodes has yet to be shown experimentally. Here we use optical spectroscopy as a probe of the band structure on a low-energy scale. Studying optical conductivity allows us to probe intraband and interband transitions with zero momentum. In TaIrTe$_4$, we observe a narrow Drude contribution and an interband conductivity that may be consistent with a tilted linear band dispers…
The inverse spinel ferrimagnetic NiCo$_2$O$_4$ possesses high magnetic Curie temperature $T_C$, high spin polarization, and strain-tunable magnetic anisotropy. Understanding the thickness scaling limit of these intriguing magnetic properties in NiCo$_2$O$_4$ thin films is critical for their implementation in nanoscale spintronic applications. In this work, we report the unconventional magnetotransport properties of epitaxial (001) NiCo$_2$O$_4$ films on MgAl$_2$O$_4$ substrates in the ultrathin limit. Anomalous Hall effect measurements reveal strong perpendicular magnetic anisotropy for films …
We report the magnetic and electronic transport properties of the inversion and time-reversal symmetry breaking Weyl semimetal NdAlGe. This material is analogous to NdAlSi, whose helical magnetism presents a rare example of a Weyl-mediated collective phenomenon, but with a larger spin-orbit coupling. Our neutron diffraction experiments revealed that NdAlGe, similar to NdAlSi, supports an incommensurate Ising spin density wave ($T_{\text{inc}}=6.8$ K) with a small helical spin canting of 3$^\circ$ and a long-wavelength of $\sim$ 35 nm, which transitions to a commensurate ferrimagnetic state bel…
We study transport in a Weyl semimetal with donor and acceptor impurities. At sufficiently high temperatures transport is dominated by electron-electron interactions, while the low-temperature resistivity comes from the scattering of quasiparticles on screened impurities. Using the diagrammatic technique, we calculate the conductivity $σ(T,ω,n_A,n_D)$ in the impurities-dominated regime as a function of temperature $T$, frequency $ω$, and the concentrations $n_A$ and $n_D$ of donors and acceptors and discuss the crossover behaviour between the regimes of low and high temperatures and impurity c…
We theoretically study the thermal Hall effect by magnons in skyrmion crystal phases of ferrimagnets in the vicinity of the angular momentum compensation point (CP). To this end, we start by deriving the equation of motion for magnons in the background of an arbitrary equilibrium spin texture, which gives rise to the fictitious electromagnetic field for magnons. As the net spin density varies, the resultant equation of motion interpolates between the relativistic Klein-Gordon equation at CP and the nonrelativistic Schr{ö}dinger-like equation away from it. In skyrmion crystal phases, the right-…
The Weyl semimetal NbP exhibits an extremely large magnetoresistance (MR) and an ultra-high mobility. The large MR originates from a combination of the nearly perfect compensation between electron- and hole-type charge carriers and the high mobility, which is relevant to the topological band structure. In this work we report on temperature- and field-dependent thermopower and thermal conductivity experiments on NbP. Additionally, we carried out complementary heat capacity, magnetization, and electrical resistivity measurements. We found a giant adiabatic magnetothermopower with a maximum of 80…
The optical properties of (001)-oriented NbP single crystals have been studied in a wide spectral range from 6 meV to 3 eV from room temperature down to 10 K. The itinerant carriers lead to a Drude-like contribution to the optical response; we can further identify two pronounced phonon modes and interband transitions starting already at rather low frequencies. By comparing our experimental findings to the calculated interband optical conductivity, we can assign the features observed in the measured conductivity to certain interband transitions. In particular, we find that transitions between t…
Berry curvature in Weyl semimetals results in very intriguing magneto-electric and magneto-thermal transport properties. In this paper, we explore the impact of the tilting of the Weyl nodes, which breaks the time-reversal symmetry, on the magneto-electric conductivity of type-I and type-II Weyl semimetals. Using the Berry curvature connected Boltzmann transport formalism, we find that in addition to the quadratic-B corrections induced by the tilt, there are also anisotropic and B-linear corrections in several elements of the conductivity matrix. For the case of magnetic field applied perpendi…
Within a Kubo formalism, we study dc transport and ac optical properties of 3D Dirac and Weyl semimetals. Emphasis is placed on the approach to charge neutrality and on the differences between Dirac and Weyl materials. At charge neutrality, the zero-temperature limit of the dc conductivity is not universal and also depends on the residual scattering model employed. However, the Lorenz number L retains its usual value L_0. With increasing temperature, the Wiedemann-Franz law is violated. At high temperatures, L exhibits a new plateau at a value dependent on the details of the scattering rate. S…
We present a systematic study of both the temperature and frequency dependence of the optical response in TaAs, a material that has recently been realized to host the Weyl semimetal state. Our study reveals that the optical conductivity of TaAs features a narrow Drude response alongside a conspicuous linear dependence on frequency. The width of the Drude peak decreases upon cooling, following a $T^{2}$ temperature dependence which is expected for Weyl semimetals. Two linear components with distinct slopes dominate the 5-K optical conductivity. A comparison between our experimental results and …
The Weyl semimetal exhibits various interesting physical phenomena because of the Weyl points, i.e., linear band-crossings. We show by Floquet theory that a linearly polarized light applied to a band insulator can induce controllable Weyl points. In a tight-binding model, we classify different types of photoinduced Weyl points that lead to a rich phase diagram characterized by the Chern number defined on each momentum slices of the bulk states. Taking account of the nonequilibrium electron distribution, we calculate and explain the nonmonotonous anomalous Hall conductivity in terms of the ligh…
We discuss the dynamical polarization, plasmon dispersion, relaxation time, and the Friedel oscillation of screened potential of the two-dimension Dirac and three-dimension Weyl system (which are gapped) in the low-energy tigh-binding model. The results, like the Fermi wavevector, Thomas-Fermi wavevector, and longitudinal conductivity are obtained in different dimensions. Some important conclusions are detailedly discussed in this paper, including the screening character under short or long range Coulomb interaction, and the longitudinal conductivity in two- or three-dimensions. The longitudin…
We study transport in three dimensional Weyl semimetals with N isotropic Weyl nodes in the presence of Coulomb interactions or disorder at temperature T. In the interacting clean limit, we determine the conductivity by solving a quantum Boltzmann equation within a `leading log' approximation and find it to be proportional to T, upto logarithmic factors arising from the flow of couplings. In the noninteracting disordered case, we compute the finite-frequency Kubo conductivity and show that it exhibits distinct behaviors for omega << T and omega >> T: in the former regime we recover the results …
The interfacial phase-change memory (iPCM) GeTe/Sb2Te3, promising candidates for the next generation non-volatile random-access memories, exhibits fascinating topological properties. Depending on the atomic-layer-stacking sequence of the GeTe block, the iPCM can be either in the SET (Ge-Te-Ge-Te) or RESET (Te-Ge-Ge-Te) states, where the former exhibits ferroelectric polarization and electrical conductivity two orders of magnitude larger than that of the RESET state. But, its origin remains elusive. Here, we predict the emergence of a Weyl semimetal phase in the SET state induced by the ferroel…
Anomalous Hall Conductivity as an Effective Means of Tracking the Floquet Weyl Nodes in Quasi-One-Dimensional $β$-Bi$_4$I$_4$
Hongshen Honors School of Chongqing University（重庆大学弘深荣誉学院）；Center for Computational Science and Engineering, Southern University of Science and Technology（南方科技大学计算科学与工程中心）；Center of Quantum materials and devices, Chongqing University（重庆大学量子材料与器件中心）
The peculiar band structure of semimetals exhibiting Dirac and Weyl crossings can lead to spectacular electronic properties such as large mobilities accompanied by extremely high magnetoresistance. In particular, two closely neighbouring Weyl points of the same chirality are protected from annihilation by structural distortions or defects, thereby significantly reducing the scattering probability between them. Here we present the electronic properties of the transition metal diphosphides, WP2 and MoP2, that are type-II Weyl semimetals with robust Weyl points. We present transport and angle res…
Molybdenum ditelluride, MoTe2, is a versatile material where the topological phase can be readily tuned by manipulating the associated structural phase transition. The fine details of the band structure of MoTe2, key to understanding its topological properties, have proven difficult to disentangle experientially due to the multi-band character of the material. Through experimental optical conductivity spectra, we detect two strong low-energy interband transitions. Both are linked to excitations between spin-orbit split bands. The lowest interband transition shows a strong thermal shift, pointi…
We study the ac anomalous Hall conductivity $σ_{xy}(ω)$ of a Weyl semimetal with broken time-reversal symmetry. Even in the absence of free carriers these materials exhibit a "universal" anomalous Hall response determined solely by the locations of the Weyl nodes. We show that the free carriers, which are generically present in an undoped Weyl semimetal, give an additional contribution to the ac Hall conductivity. We elucidate the physical mechanism of the effect and develop a microscopic theory of the free carrier contribution to $σ_{xy}(ω)$. The latter can be expressed in terms of a small nu…
We investigate the topological nodal structure of three-dimensional (3D) ZrTe$_5$ driven by Zeeman splitting as a function of the direction of external magnetic (B) field by using a Wannier-function-based tight-binding (WFTB) model obtained from first-principles calculations. It is known that small external stimuli can drive 3D ZrTe$_5$ into different topological phases including Dirac semimetal. In order to emphasize the effect of Zeeman splitting, we consider 3D ZrTe$_5$ in a strong TI phase with a small band gap. With Zeeman splitting greater than the band gap, the WFTB model suggests that …
Multiple Floquet Chern insulator phases in the spin-charge coupled triangular-lattice ferrimagnet: Crucial role of higher-order terms in the high-frequency expansion
We point out that, contrary to an assertion by Chen, Bergman and Burkov [Phys. Rev. B 88, 125110 (2013)], the non-quantized part of the intrinsic anomalous Hall conductivity can indeed be expressed as a Fermi-surface property even when Weyl points are present in the bandstructure.
Quantum anomalies in Weyl semimetal (for either ${\bf E}\cdot{\bf B} \neq 0$ or ${\bf \nabla}T\cdot{\bf B} \neq 0$) leads to chiral charge and energy pumping between the opposite chirality nodes. This results in chiral charge and energy imbalance between the Weyl nodes which manifests in several intriguing magneto-transport phenomena. Here, we investigate the role of electrical-, thermal-, and gravitational chiral anomaly on magneto-transport in Weyl semimetals. We predict the planar Ettinghausen and Righi-Leduc effect to be a distinct signature of these quantum anomalies. We also demonstrate …
Kagome lattice, made of corner-sharing triangles, provides an excellent platform for hosting exotic topological quantum states. Here we systematically studied the magnetic and transport properties of RMn6Sn6 (R = Tb, Dy, Ho) with clean Mn kagome lattice. All the compounds have a collinear ferrimagnetic structure with different easy axis at low temperature. The low-temperature magnetoresistance (MR) is positive and has no tendency to saturate below 7 T, while the MR gradually declines and becomes negative with the increasing temperature. A large intrinsic anomalous Hall conductivity about 250 Ω…
We study the origin of the strong spin Hall effect (SHE) in a recently discovered family of Weyl semimetals, LaAl$X$ ($X$=Si, Ge) via a first-principles approach with maximally localized Wannier functions. We show that the strong intrinsic SHE in LaAl$X$ originates from the multiple slight anticrossings of nodal lines and points near $E_F$ due to their high mirror symmetry and large spin-orbit interaction. It is further found that both electrical and thermal means can enhance the spin Hall conductivity ($σ_{SH}$). However, the former also increases the electrical conductivity ($σ_{c}$), while …
We report measurement of terahertz anomalous Hall conductivity and Faraday rotation in the magnetic Weyl semimetal Co$_2$MnGa thin films as a function of the magnetic field, temperature and thickness, using time-domain terahertz spectroscopy. The terahertz conductivity shows a thickness-independent anomalous Hall conductivity of around 600 $Ω^{-1}\cdot cm^{-1}$ at room temperature, and it is also frequency-independent from 0.2-1.5 THz. The magnitude of the longitudinal and Hall conductivities, the weak spin-orbit coupling, and the very close positioning of Weyl points to the chemical potential…
We predict a large anomalous Nernst effect in the inverse Heusler compensated ferrimagnets Ti$_2$Mn$X$ ($X$=Al,Ga,In) with vanishing net magnetic moments. Though the net magnetic moment is zero, the Weyl points in these systems lead to a large anomalous Nernst conductivity (ANC) due to the lack of a magnetic sublattice that inverses the sign of the Berry curvature. In comparison to the noncollinear antiferromagnets Mn$_3$Sn and Mn$_3$Ge, the high ANC stems almost entirely from the Weyl points in this class of compounds, and thus, it is topologically protected. This work shows for the first tim…
Unusually high phonon thermal conductivity in the Weyl semimetal TaP: A comparative study with TaAs
College of Physics and Optoelectronic Engineering, Chongqing Normal University（重庆师范大学物理与光电工程学院）；School of Science, Chongqing University of Posts and Telecommunications（重庆邮电大学理学院）；Center for Computational Science and Engineering, Southern University of Science and Technology（南方科技大学计算科学与工程中心）；College of Physics and Electronic Engineering, Chongqing Normal University（重庆师范大学物理与电子工程学院）；College of Physics and Center of Quantum Materials and Devices, Chongqing University（重庆大学物理学院量子材料与器件中心）；Chongqing Key Laboratory for Strongly Coupled Physics, Chongqing University（重庆大学强耦合物理重庆市重点实验室）
The Casimir interaction is induced by electromagnetic fluctuations between objects and it is strongly dependent upon the electronic and optical properties of the materials making up the objects. Here we investigate this ubiquitous interaction between Weyl semimetals, a class of 3D systems with low energy linear dispersion and nontrivial topology due to symmetry conditions and stemming from separated energy cones. A comprehensive examination of all components of the bulk conductivity tensor as well as the surface conductivity due to the Fermi arc states in real and imaginary frequency domains i…
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 of electronic correlations in a kagome magnet Co3Sn2S2 and the influence of electronic correlations on its WSM state expected within a single-particle picture. The electronic kinetic energy estimated from our optical data is abou…
