Dirac Semimetal Phase Research Articles

Surface plasmon polaritons in a waveguide composed of Weyl and Dirac semimetals

We investigate features of the surface plasmon polaritons (SPPs) hosted by a slot waveguide comprised of a Weyl semimetal in Voigt/Faraday configuration and a Dirac semimetal connected via a dielectric layer. In the Voigt configuration, SPP dispersion is nonreciprocal possessing unidirectional SPP modes in a wide range of frequency. However, the reciprocal SPP dispersion of the Faraday configuration composes of two distinct dispersion bands existing below or above the bulk plasmon frequency depending on the selection of the materials realizing Weyl and Dirac semimetal phases. We explain that these SPP modes are tunable with the thickness of the dielectric layer, the chemical potentials of the semimetals and the separation of the Weyl nodes in the Weyl semimetal. These structures provide a high level of functionality and tunability and may be employed in optical devices based on the unidirectional SPPs. • The slot waveguide allows unidirectional propagation of surface plasmon polaritons. • Surface plasmon polariton modes show new features in the Faraday configuration. • Thickness of the dielectric layer can be used to control surface plasmon polariton modes. • Chiral anomaly effect of the Weyl semimetal can be utilized to control the surface plasmon polariton modes.

Photogalvanic effects in symmetry broken nodal ring materials

Nodal ring semimetals are a class of topological material characterized by a one-dimensional circular region of band crossing in momentum space. The presence of spin-orbit coupling, whether extrinsic or intrinsic, may change the parent nodal ring phase to a Weyl semimetal, Dirac semimetal, or topological insulator child phase. We investigate second harmonic generation and circular photogalvanic effect in the mid-infrared region of nodal ring materials where spin-orbit coupling produces a Weyl semimetal child phase (such as in $\mathrm{Zr}{\mathrm{Te}}_{5}$ and $\mathrm{Ca}{\mathrm{P}}_{3}$). Spin-orbit coupling breaks the symmetries protecting the nodal ring, inducing a nontrivial Berry curvature which gives rise to colossal photocurrents up to the order of ${10}^{3} \ensuremath{\mu}\mathrm{A}/{\text{V}}^{2}$ at the interband harmonic. Our results are found to be rather robust to parameters such as Fermi level, residual scattering rate, and the number of Weyl points. However, decreasing temperature tends to destroy the harmonic peaks and changing the nodal ring radius drastically alters the harmonic condition, shifting the peak frequency. Equivalent calculations and experiments have been carried out for intrinsic Weyl semimetals such as TaAs where the photocurrents calculated and observed were at least one order of magnitude smaller, highlighting that the parent nodal ring phase enhances these optical nonlinear phenomena.

Nonlinear Hall Effect in Antiferromagnetic Half-Heusler MaterialsSupported by the National Natural Science Foundation of China (Grant Nos. 11834006, 12074181, and 11674165), the Natural Science Foundation of Jiangsu Province (Grant No. BK20200007), the Fok Ying-Tong Education Foundation of China (Grant No. 161006), and the Fundamental Research Funds for the Central Universities (Grant No. 020414380149).

It has recently been demonstrated that various topological states, including Dirac, Weyl, nodal-line, and triple-point semimetal phases, can emerge in antiferromagnetic (AFM) half-Heusler compounds. However, how to determine the AFM structure and to distinguish different topological phases from transport behaviors remains unknown. We show that, due to the presence of combined time-reversal and fractional translation symmetry, the recently proposed second-order nonlinear Hall effect can be used to characterize different topological phases with various AFM configurations. Guided by the symmetry analysis, we obtain expressions of the Berry curvature dipole for different AFM configurations. Based on the effective model, we explicitly calculate the Berry curvature dipole, which is found to be vanishingly small for the triple-point semimetal phase, and large in the Weyl semimetal phase. Our results not only put forward an effective method for the identification of magnetic orders and topological phases in AFM half-Heusler materials, but also suggest these materials as a versatile platform for engineering the nonlinear Hall effect.

Large Damping Enhancement in Dirac‐Semimetal–Ferromagnetic‐Metal Layered Structures Caused by Topological Surface States

AbstractThis article reports damping enhancement in a ferromagnetic NiFe thin film due to an adjacent α‐Sn thin film. Ferromagnetic resonance studies show that an α‐Sn film separated from a NiFe film by an ultrathin Ag spacer can cause an extra damping in the NiFe film that is three times bigger than the intrinsic damping of the NiFe film. Such an extra damping is absent in structures where the α‐Sn film interfaces directly with a NiFe film, or is replaced by a β‐Sn film. The data suggest that the extra damping is associated with topologically nontrivial surface states in the topological Dirac semimetal phase of the α‐Sn film. This work suggests that, like topological insulators, topological Dirac semimetal α‐Sn may have promising applications in spintronics.

Low-energy electrodynamics of Dirac semimetal phases in the doped Mott insulator Sr2IrO4

Correlated Dirac semimetal phases emerge in lightly doped (Tb- or La-doped) Mott insulator Sr$_2$IrO$_4$, where a d-wave symmetry-breaking order underlying a pseudogap plays a crucial role in determining the nature of Dirac degeneracy, i.e., whether it is a Dirac line node or Dirac point node. Here, using a realistic five-orbital tight-binding model with a Hubbard U and a semiclassical Boltzmann transport theory, we systematically study the low-energy electrodynamic properties of the Dirac semimetal phases in the paramagnetic lightly doped Sr$_2$IrO$_4$. We investigate the effects of the d-wave electronic order and electron doping concentration on the electronic band structures and optical properties of various Dirac semimetal phases. We calculate the intraband optical conductivity and obtain electrodynamic parameters of dc conductivity, scattering rate, and Drude weight for three Dirac semimetal phases: two are Dirac point-node states observed in the 3% Tb-doped and 5% La-doped Sr$_2$IrO$_4$, and the other is a Dirac line-node state. Our results show that the temperature dependence of the electrodynamic parameters is strong in the Tb-doped system while weak in the La-doped and Dirac line-node systems, which are consistent with available experimental data. Moreover, using the low-energy effective theory, we also compare the temperature-dependent screening effect in the Tb- and La-doped systems using graphene as a reference. Our paper provides valuable insight for understanding the transport and optical properties of correlated Dirac semimetal phases in the doped Sr$_2$IrO$_4$.

Spin-Orbit Coupling-Determined Topological Phase: Topological Insulator and Quadratic Dirac Semimetals.

Our work reveals a class of three-dimensional materials whose main features are dominated by d-orbital states. Their unique properties are derived from the low-energy states t2g. Without spin-orbital coupling (SOC), we find a triple degenerate point with a quadratic dispersion, demonstrated by an effective Hamiltonian. When SOC is included, the sign of SOC could determine the topological phases of materials: a negative SOC contributes a Dirac semimetal phase with a quadratic energy dispersion, whereas a positive SOC leads to a strong topological insulator phase. There exist clear surface states for the corresponding topological phases. Very interestingly, by application of a triaxial strain, the sequence of bands can be exchanged, as do the topological phases. In particular, there exists a 6-fold degenerate point under a critical strain. Furthermore, we use a uniaxial compressive/tensile strain, changing the quadratic Dirac point into a linear Dirac/strong topological insulator phase.

Impurity-induced topological phase transitions in Cd3As2 and Na3Bi Dirac semimetals

Using first-principles density functional theory calculations, combined with a topological analysis, we have investigated the electronic properties of $Cd_3As_2$ and $Na_3Bi$ Dirac topological semimetals doped with non-magnetic and magnetic impurities. Our systematic analysis shows that the selective breaking of the inversion, rotational and time-reversal symmetry, controlled by specific choices of the impurity doping, induces phase transitions from the original Dirac semimetal to a variety of topological phases such as, topological insulator, trivial semimetal, non-magnetic and magnetic Weyl semimetal, and Chern insulator. The Dirac semimetal phase can exist only if the rotational symmetry $C_n$ with $n > 2$ is maintained. One particularly interesting phase emerging in doped $Cd_3As_2$ is a coexisting Dirac-Weyl phase, which occurs when only inversion symmetry is broken while time-reversal symmetry and rotational symmetry are both preserved. To further characterize the low-energy excitations of this phase, we have complemented our density functional results with a continuum four-band $k\cdot p$ model, which indeed displays nodal points of both Dirac and Weyl type. The coexisting phase appears as a transition point between two topologically distinct Dirac phases, but may also survive in a small region of parameter space controlled by external strain.

Boundary Criticality of PT-Invariant Topology and Second-Order Nodal-Line Semimetals.

For conventional topological phases, the boundary gapless modes are determined by bulk topological invariants. Based on developing an analytic method to solve higher-order boundary modes, we present PT-invariant 2D topological insulators and 3D topological semimetals that go beyond this bulk-boundary correspondence framework. With unchanged bulk topological invariants, their first-order boundaries undergo transitions separating different phases with second-order boundary zero modes. For the 2D topological insulator, the helical edge modes appear at the transition point for two second-order topological insulator phases with diagonal and off-diagonal corner zero modes, respectively. Accordingly, for the 3D topological semimetal, the criticality corresponds to surface helical Fermi arcs of a Dirac semimetal phase. Interestingly, we find that the 3D system generically belongs to a novel second-order nodal-line semimetal phase, possessing gapped surfaces but a pair of diagonal or off-diagonal hinge Fermi arcs.

Topologically Nontrivial Phase-Change Compound GeSb2Te4.

Chalcogenide phase-change materials show strikingly contrasting optical and electrical properties, which has led to their extensive implementation in various memory devices. By performing spin-, time-, and angle-resolved photoemission spectroscopy combined with the first-principles calculation, we report the experimental results that the crystalline phase of GeSb2Te4 is topologically nontrivial in the vicinity of the Dirac semimetal phase. The resulting linearly dispersive bulk Dirac-like bands that cross the Fermi level and are thus responsible for conductivity in the stable crystalline phase of GeSb2Te4 can be viewed as a 3D analogue of graphene. Our finding provides us with the possibility of realizing inertia-free Dirac currents in phase-change materials.

Phase Controlled Growth of Cd3As2 Nanowires and Their Negative Photoconductivity.

The bottom-up synthesis process often allows the growth of metastable phase nanowires instead of the thermodynamically stable phase. Herein, we synthesized Cd3As2 nanowires with a controlled three-dimensional Dirac semimetal phase using a chemical vapor transport method. Three different phases such as the body centered tetragonal (bct), and two metastable primitive tetragonal (P42/nbc and P42/nmc) phases were identified. The conversion between three phases (bct → P42/nbc → P42/nmc) was achieved by increasing the growth temperature. The growth direction is [110] for bct and P42/nbc and [100] for P42/nmc, corresponding to the same crystallographic axis. Field effect transistors and photodetector devices showed the nearly same electrical and photoelectrical properties for three phases. Differential conductance measurement confirms excellent electron mobility (2 × 104 cm2/(V s) at 10 K). Negative photoconductance was first observed, and the photoresponsivity reached 3 × 104 A/W, which is ascribed to the surface defects acting as trap sites for the photogenerated electrons.

Topological Dirac Semimetal Phase in Bismuth Based Anode Materials for Sodium-Ion Batteries

Bismuth has recently attracted interest in connection with Na-ion battery anodes due to its high volumetric capacity. It reacts with Na to form Na 3 Bi which is a prototypical Dirac semimetal with a nontrivial electronic structure. Density-functional-theory based first-principles calculations are playing a key role in understanding the fascinating electronic structure of Na 3 Bi and other topological materials. In particular, the strongly-constrained-and-appropriately-normed (SCAN) meta-generalized-gradient-approximation (meta-GGA) has shown significant improvement over the widely used generalized-gradient-approximation (GGA) scheme in capturing energetic, structural, and electronic properties of many classes of materials. Here, we discuss the electronic structure of Na 3 Bi within the SCAN framework and show that the resulting Fermi velocities and s-band shift around the Γ point are in better agreement with experiments than the corresponding GGA predictions. SCAN yields a purely spin-orbit-coupling (SOC) driven Dirac semimetal state in Na 3 Bi in contrast with the earlier GGA results. Our analysis reveals the presence of a topological phase transition from the Dirac semimetal to a trivial band insulator phase in Na 3 Bi x Sb 1 − x alloys as the strength of the SOC varies with Sb content, and gives insight into the role of the SOC in modulating conduction properties of Na 3 Bi.

Tunable quantum interference effect on magnetoconductivity in few-layer black phosphorus

In this study, we develop a systematic weak localization/antilocalization theory fully considering the anisotropy and Berry phase of the system, and apply it to various phases of few-layer black phosphorus (BP), which has a highly anisotropic electronic structure with an electronic gap size tunable even to a negative value. The derivation of a Cooperon ansatz for the Bethe-Salpeter equation in a general anisotropic system is presented, revealing the existence of various quantum interference effects in different phases of few-layer BP, including a crossover from weak localization to antilocalization. We also predict that the magnetoconductivity at the semi-Dirac transition point will exhibit a nontrivial power-law dependence on the magnetic field, while following the conventional logarithmic field-dependence of 2D systems in the insulator and Dirac semimetal phases. Notably, the ratio between the magnetoconductivity and Boltzmann conductivity turns out to be independent of the direction, even in strongly anisotropic systems. Finally, we discuss the tunability of the quantum corrections of few-layer BP in terms of the symmetry class of the system.

Light-Driven Raman Coherence as a Nonthermal Route to Ultrafast Topology Switching in a Dirac Semimetal

A grand challenge underlies the entire field of topology-enabled quantum logic and information science: how to establish topological control principles driven by quantum coherence and understand the time-dependence of such periodic driving? Here we demonstrate a THz pulse-induced phase transition in Dirac materials that is periodically driven by vibrational coherence due to excitation of the lowest Raman-active mode. Above a critical field threshold, there emerges a long-lived metastable phase with unique Raman coherent phonon-assisted switching dynamics, absent for optical pumping. The switching also manifest itself by non-thermal spectral shape, relaxation slowing down near the Lifshitz transition where the critical Dirac point (DP) occurs, and diminishing signals at the same temperature that the Berry curvature induced Anomalous Hall Effect varnishes. These results, together with first-principles modeling, identify a mode-selective Raman coupling that drives the system from strong to weak topological insulators, STI to WTI, with a Dirac semimetal phase established at a critical atomic displacement controlled by the phonon pumping. Harnessing of vibrational coherence can be extended to steer symmetry-breaking transitions, i.e., Dirac to Weyl ones, with implications on THz topological quantum gate and error correction applications.

Superconductivity and Shubnikov - de Haas effect in polycrystalline Cd3As2 thin films

In this study we observed the reproducible superconducting state in Cd3As2 thin films without any external stimuli. Comparison with our previous results reveals similar qualitative behavior for films synthesized by different methods, while the difference in the values of the critical parameters clearly shows the possibility to control this state. The X-ray diffraction measurements demonstrate the presence of the tetragonal Cd3As2 crystal phase in studied films. Measurements of high-field magnetoresistance reveal pronounced Shubnikov - de Haas oscillations. The analysis of these oscillations suggests that, due to high carrier concentration in studied Cd3As2 films, the initial Dirac semimetal phase may be partially suppressed, which, however, does not contradict with possible topological nature of the observed superconductivity.

Semi-quantized Spin Pumping and Spin-Orbit Torques in Topological Dirac Semimetals

We study the time-development processes of spin and charge transport phenomena in a topological Dirac semimetal attached to a ferromagnetic insulator with a precessing magnetization. Compared to conventional normal metals, topological Dirac semimetals manifest a large inverse spin Hall effect when a spin current is pumped from the attached ferromagnetic insulator. It is shown that the induced charge current is semi-quantized, i.e., it depends only on the distance between the two Dirac points in momentum space and hardly depends on the disorder strength when the system remains in the topological Dirac semimetal phase. As an inverse effect, we show that the electric field applied to the topological Dirac semimetal exerts a spin torque on the local magnetization in the ferromagnetic insulator via the exchange interaction and the semi-quantized spin Hall effect. Our study demonstrates that the topological Dirac semimetal offers a less-dissipative platform for spin-charge conversion and spin switching.

Hybrid Dispersion Dirac Semimetal and Hybrid Weyl Phases in Luttinger Semimetals: A Dynamical Approach

AbstractIt is shown that hybrid Dirac and Weyl semimetals can be realized in a 3D Luttinger semimetal with quadratic band touching (QBT). This is illustrated using a periodic kicking scheme. In particular, the focus is on a momentum‐dependent driving (nonuniform driving) and the realization of various hybrid Dirac and Weyl semimetals is demonstrated. A unique hybrid dispersion Dirac semimetal with two nodes is identified, where one of the nodes is linear while the other is dispersed quadratically. Next, it is shown that by tilting QBT via periodic driving and in the presence of an external magnetic field, one can realize various single/double hybrid Weyl semimetals depending on the strength of external field. Finally, it is noted that in principle, phases that are found in this work can also be realized by employing the appropriate electronic interactions.

Topological crystalline superconductivity in Dirac semimetal phase of iron-based superconductors

In iron-based superconductors, band inversion of $d$- and $p$-orbitals yields Dirac semimetallic states. We theoretically investigate their topological properties in normal and superconducting phases, based on the tight-binding model involving full symmetry of the materials. We demonstrate that a Cooper pair between electrons with $d$- and $p$-orbitals relevant to the band structure yields odd-parity superconductivity. Moreover, we present the typical surface states by solving the Bogoliubov-de Gennes equation and characterize them by topological invariants defined with crystal symmetry. It is found that there appear various types of Majorana fermions such as surface flat band, Majorana quartet and M\"{o}bius twisted surface state. Our theoretical results show that iron-based superconductors are promising platforms to realize rich topological crystalline phases.

Synthesis and magnetoresistance of (Cd1−x Znx)3As2 (x = 0,007) crystals

The vapor phase growth of Cd3As2—Zn3As2 (in the following (Cd1−x Znx)3As2 solid solutions process is described. The (Cd0,993 Zn0,007)3As2 solid solution single crystals were synthesized. Scanning electron microscopy and electron diffraction data suggest high crystalline quality of studied sample. Its structure and surface morphology, indicating the presence of growth nuclei and cleavage planes, were investigated. Giant anisotropic magnetoresistance and Shubnikov — de Haas oscillations were observed at low temperatures. Obtained results suggests that peculiarities of Dirac semimetal phase persist in (Cd1−x Znx)3As2 solid solution at low zinc content. At the same time, there are indications of some differences with initial Cd3As2 properties.

Basal-plane growth of cadmium arsenide by molecular beam epitaxy

(001)-oriented thin films of the three-dimensional Dirac semimetal cadmium arsenide can realize a quantum spin Hall insulator and other kinds of topological physics, all within the flexible architecture of epitaxial heterostructures. Here, we report a method for growing (001) cadmium arsenide films using molecular beam epitaxy. The introduction of a thin indium arsenide wetting layer improves surface morphology and structural characteristics, as measured by x-ray diffraction and reflectivity, atomic force microscopy, and scanning transmission electron microscopy. The electron mobility of 50-nm-thick films is found to be 9300 cm2/Vs at 2 K, comparable to the highest-quality films grown in the conventional (112) orientation. This work demonstrates a simple experimental framework for exploring topological phases that are predicted to exist in proximity to the three-dimensional Dirac semimetal phase.

A Dirac semimetal phase diagram of the binary compound CuI(R-3m)

Based on first-principles calculations, we study the properties and interactions of the existing binary compound CuI with the space group of R-3m. A pair of Dirac points with asymmetry high Fermi velocity is found to be distributed on the unique C3 symmetry axis in k space, which are formed by band inversion between I- |32,±32〉 and |32,±12〉 type states. CuI(R-3m) is located at the boundary of different topological phases. Anisotropic strain breaking C3 symmetry could switch CuI(R-3m) between topological Dirac semimetal state and strong topological insulator states. A Dirac semimetal phase diagram is obtained, and show that binary compound with the space group of R-3m is a good platform for getting different semi-metals states. We reveal that various topological phases can be achieved in the binary compounds with the space group of R-3m (NO.166) by regulating the on-site energy difference and the SOC strength. The compounds family of XY (X = Cu, Ag, Au; YI, Br, Cl, F) with the atomic structure of CuI(R-3m) are topological semimetals with one or two pairs of Dirac points around Fermi energy, except for AgCl and AgF.