Semimetal Phase Research Articles

Thickness-dependent ultrafast nonlinear absorption properties of PtSe2 films with both semiconducting and semimetallic phases

The bandgap, electrical, and optical properties of PtSe2 depend dramatically on the vertical stacking and fabrication method. Here, we study the nonlinear absorption properties of the PtSe2 films composed of both semiconducting and semimetallic phases in a single film. These PtSe2 films exhibit remarkable thickness-dependent saturable absorption for femtosecond pulses at 400 nm and 800 nm. The saturation intensities decrease with the increase in the film thickness due to the accompanied increase in the semimetallic component and are much smaller than the reported values of PtSe2 synthesized by thermally assisted conversion. The saturable absorption characteristics are confirmed by time-resolved spectroscopies. The nonlinear refractive indexes of these PtSe2 films should be smaller than 1 × 10–12 cm2/W. Our results imply that the optical nonlinearities of PtSe2 could be flexibly tuned by the synthesis method and thickness.

Topological Insulator-to-Weyl Semimetal Transition in Strongly Correlated Actinide System UNiSn

Although strong electronic correlations are known to be responsible for some highly unusual behaviors of solids such as, metal--insulator transitions, magnetism and even high--temperature superconductivity, their interplay with recently discovered topological states of matter awaits full exploration. Here we use a modern electronic structure method combining density functional theory of band electrons with dynamical self-energies of strongly correlated states to predict that two well-known phases of actinide compound UNiSn, a paramagnetic semiconducting and antiferromagnetic metallic, correspond to Topological Insulator (TI) and Weyl semimetal (WSM) phases of topological quantum matter. Thus, the famous unconventional insulator-metal transition observed in UNiSn is also a TI-to-WSM transition. Driven by a strong hybridization between U f-electron multiplet transitions and band electrons, multiple energy gaps open up in the single-particle spectrum whose topological physics is revealed using the calculation of Z2 invariants in the strongly correlated regime. A simplified physical picture of these phenomena is provided based on a periodic Anderson model of strong correlations and multiple band inversions that occur in this fascinating compound. Studying the topology of interacting electrons reveals interesting opportunities for finding new exotic phase transitions in strongly correlated systems.

Chiral vortical conductivity across a topological phase transition from holography

We study the chiral vortical conductivity in a holographic Weyl semimetal model, which describes a topological phase transition from the strongly coupled topologically nontrivial phase to a trivial phase. We focus on the temperature dependence of the chiral vortical conductivity where the mixed gauge-gravitational anomaly plays a crucial role. After a proper renormalization of the chiral vortical conductivity by the anomalous Hall conductivity and temperature squared, we find that at low temperature in both the Weyl semimetal phase and the quantum critical region this renormalized ratio stays as universal constants. More intriguingly, this ratio in the quantum critical region depends only on the emergent Lifshitz scaling exponent at the quantum critical point.

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.

Topological electronic states in HfRuP family superconductors

Based on the first-principles calculations and experimental measurements, we report that the hexagonal phase of ternary transition metal pnictides TT’X (T=Zr, Hf; T’=Ru; X=P, As), which are well-known noncentrosymmetric superconductors with relatively high transition temperatures, host nontrivial bulk topology. Before the superconducting phase transition, we find that HfRuP belongs to a Weyl semimetal phase with 12 pairs of type-II Weyl points, while ZrRuAs, ZrRuP and HfRuAs belong to a topological crystalline insulating phase with trivial Fu-Kane {{mathbb{Z}}}_{2} indices but nontrivial mirror Chern numbers. High-quality single crystal samples of the noncentrosymmetric superconductors with these two different topological states have been obtained and the superconductivity is verified experimentally. The wide-range band structures of ZrRuAs have been identified by ARPES and reproduced by theoretical calculations. Combined with intrinsic superconductivity, the nontrivial topology of the normal state may generate unconventional superconductivity in both bulk and surfaces. Our findings could largely inspire the experimental searching for possible topological superconductivity in these compounds.

Observation of a topological nodal surface and its surface-state arcs in an artificial acoustic crystal

Three-dimensional (3D) gapless topological phases can be classified by the dimensionality of the band degeneracies, including zero-dimensional (0D) nodal points, one-dimensional (1D) nodal lines, and two-dimensional (2D) nodal surfaces. Both nodal points and nodal lines have been realized recently in photonics and acoustics. However, a nodal surface has never been observed in any classical-wave system. Here, we report on the experimental observation of a twofold symmetry-enforced nodal surface in a 3D chiral acoustic crystal. In particular, the demonstrated nodal surface carries a topological charge of 2, constituting the first realization of a higher-dimensional topologically-charged band degeneracy. Using direct acoustic field measurements, we observe the projected nodal surface and its Fermi-arc-like surface states and demonstrate topologically-induced robustness of the surface states against disorders. This discovery of a higher-dimensional topologically-charged band degeneracy paves the way toward further explorations of the physics and applications of new topological semimetal phases.

Photon-Induced Weyl Half-Metal Phase and Spin Filter Effect from Topological Dirac Semimetals.

Recently discovered Dirac semimetals (DSMs) with two Dirac nodes, such as Na_{3}Bi and Cd_{2}As_{3}, are regarded as carrying the Z_{2} topological charge in addition to the chiral charge. We study the Floquet phase transition of Z_{2} topological DSMs subjected to a beam of circularly polarized light. Owing to the resulting interplay of the chiral and Z_{2} charges, the Weyl nodes are not only chirality dependent but also spin dependent, which constrains the behavior in creation and annihilation of the pair of Weyl nodes. Interestingly, we find a novel phase: One spin band is in the Weyl semimetal phase while the other is in the insulator phase, and we dub it the Weyl half-metal (WHM) phase. We further study the spin-dependent transport in a Dirac-Weyl semimetal junction and find a spin filter effect as a fingerprint of the existence of the WHM phase. The proposed spin filter effect, based on the WHM bulk band, is highly tunable in a broad parameter regime and robust against magnetic disorder, which is expected to overcome the shortcomings of the previously proposed spin filter based on the topological edge or surface states. Our results offer a unique opportunity to explore the potential applications of topological DSMs in spintronics.

Tuning Insulator-Semimetal Transitions in 3D Topological Insulator thin Films by Intersurface Hybridization and In-Plane Magnetic Fields.

A pair of Dirac points (analogous to a vortex-antivortex pair) associated with opposite topological numbers (with ±π Berry phases) can be merged together through parameter tuning and annihilated to gap the Dirac spectrum, offering a canonical example of a topological phase transition. Here, we report transport studies on thin films of BiSbTeSe_{2}, which is a 3D topological insulator that hosts spin-helical gapless (semimetallic) Dirac fermion surface states for sufficiently thick samples, with an observed resistivity close to h/4e^{2} at the charge neutral point. When the sample thickness is reduced to below ∼10 nm thick, we observe a transition from metallic to insulating behavior, consistent with the expectation that the Dirac cones from the top and bottom surfaces hybridize (analogous to a "merging" in the real space) to give a trivial gapped insulator. Furthermore, we observe that an in-plane magnetic field can drive the system again towards a metallic behavior, with a prominent negative magnetoresistance (up to ∼-95%) and a temperature-insensitive resistivity close to h/2e^{2} at the charge neutral point. The observation is consistent with a predicted effect of an in-plane magnetic field to reduce the hybridization gap (which, if small enough, may be smeared by disorder and give rise to a metallic behavior). A sufficiently strong magnetic field is predicted to restore and split again the Dirac points in the momentum space, inducing a distinct 2D topological semimetal phase with two single-fold Dirac cones of opposite spin-momentum windings.

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.

The Study of Atmospheric Pressure CVD Growth Process of MoxW1-xTe2 Nanobelts for Tuneable Chemical Composition

Transition metal dichalcogenides nanomaterials with topological semimetallic phase (MoTe2, WTe2 and MoxW1-xTe2) are expected to realize no-consumption electronic transportation due to its Dirac point. Especially, the various structure existence in MoxW1-xTe2, like hexagonal (2H), monoclinic (1T’) and othorombic (Td), provides opportunities for phase engineering, which is beneficial for future novel electronic and spintronic devices. Plenty works have focused on the synthesis of MoTe2, WTe2 and MoxW1-xTe2 nanomaterials. Unfortunately, the understanding of the growth process of MoxW1-xTe2 nanomaterials is still absent and the exact control of the atomic ratio between Mo and W in MoxW1-xTe2 nanomaterials is still a huge task up to now. Here, we study the growth process of MoxW1-xTe2 nanomaterials by the growing of binary MoTe2 and WTe2 nanomaterials. Through detailed structural and compositional characterization, same growth mode, crystallography, and morphology have been observed among MoTe2, WTe2 and MoxW1-xTe2 nanomaterials. Through analysis, we suggest that the heating temperature of Mo and W precursors during the CVD process is the key parameters to achieve the tuneable chemical composition of MoxW1-xTe2 nanomaterials. This study dissected the growth process of MoxW1-xTe2 nanomaterials and provided a possible method to control the chemical composition of MoxW1-xTe2 nanomaterials by APCVD, which is beneficial to realize the phase engineering in Mo-W-Te system and also for future topological application.

Weyl semimetal phase in the noncentrosymmetric superlattice W2XY(X,Y=S,Se,Te,X≠Y)

We investigated the topological properties and strain modulation of transition-metal monochalcogenides ${\mathrm{W}}_{2}XY(X,Y=\mathrm{S},\phantom{\rule{0.16em}{0ex}}\mathrm{Se},\phantom{\rule{0.16em}{0ex}}\mathrm{Te},X\ensuremath{\ne}Y)$ by combining the first-principles method and the Wannier-based tight-binding method. The number and chirality of Weyl node, surface spectra, and surface Fermi arcs are calculated to identify their topological properties. The ${\mathrm{W}}_{2}XY$ compounds are nodal line semimetals in the absence of spin-orbit coupling (SOC). When SOC turns on, the nodal lines split into 24 Weyl nodes on the ${k}_{z}\ensuremath{\ne}0$ planes, which are related by time-reversal, ${C}_{3\mathrm{z}}$ rotational, and mirror symmetries. Weyl points' energies are close to the Fermi level and the maximal separation between two Weyl points with opposite handedness in reciprocal space is on the order of magnitude of $0.1\phantom{\rule{0.16em}{0ex}}{\AA{}}^{\ensuremath{-}1}$, which can be accessed by angle-resolved photoemission spectroscopy. Interestingly, the Weyl semimetal phase exhibits great robustness and asymmetric response under uniaxial/biaxial compression and tensile strain. The positions of Weyl points change significantly and lead to a striking modulation of topological properties under in-plane biaxial strain. These findings are important for studying the behavior of Weyl fermions in layered materials and useful for realizing their applications in topological electronic devices.

Magnetotransport evidence for topological phase transition in a Dirac semimetal

Dirac semimetals transform into Weyl semimetals by breaking the time reversal symmetry, resulting in interesting physical properties, such as the chiral anomaly induced negative magnetoresistance and the Weyl orbit related quantum transport. Here, we report the disappearance of the Weyl semimetal phase in Dirac semimetal Cd3As2 nanoplates with a (112) surface plane when applying a perpendicular high magnetic field. The Landau fan extracted from the notable quantum oscillations shows the nontrivial Berry phase π for B<9.4 T and its disappearance at around 1/B∼0.1 T−1. Our observations indicate the topological phase transition from nontrivial states to gap-opening states induced by the C4 rotational symmetry breaking under a high magnetic field.

Nontrivial topological phases on the stuffed honeycomb lattice

We report the appearance of nontrivial topological phases in a tight-binding model on the stuffed honeycomb lattice. The model contains nearest neighbor and next nearest neighbor hopping terms coupled with an additional phase depending on the direction of hopping. Chern insulating and semi-metallic phases emerge with the change of hopping parameters. Nonzero Chern numbers characterizing the bands and the existence of topologically protected edge states in the gap between the relevant bands confirm the presence of those phases. We show that adding an extra basis to Haldane’s honeycomb model can lead to an additional topological phase characterized by Chern number . Transition between different topological phases driven by the hopping parameters has been illustrated in the topological phase diagram of the system. Zero temperature Hall conductivity along with density of states is evaluated. Topological properties of another tight-binding model on the stuffed square lattice are also reported in this article.

Magnetic Textures and Dynamics in Magnetic Weyl Semimetals

AbstractRecent theoretical and experimental attempts have been successful in finding magnetic Weyl semimetal phases, which show nodal‐point structure in the electronic bands as well as magnetic orders. Beyond uniform ferromagnetic or antiferromagnetic orders, nonuniform magnetic textures, such as domain walls and skyrmions, enrich the properties of the Weyl electrons even more in such materials. Here, a topical review on interplay between Weyl electrons and magnetic textures in those magnetic Weyl semimetals is given. The basics of magnetic textures in nontopological magnetic metals are reviewed first, and then the effect of magnetic textures in Weyl semimetals is discussed, regarding the recent theoretical and experimental progress therein. The idea of the fictitious “axial gauge fields” is pointed out, which effectively describes the effect of magnetic textures on the Weyl electrons and can well account for the properties of the electrons localized around magnetic domain walls.

Topological semimetallic phase in PbO2 promoted by temperature

Materials exhibiting topological order host exotic phenomena that could form the basis for novel developments in areas ranging from low-power electronics to quantum computers. The past decade has witnessed multiple experimental realizations and thousands of predictions of topological materials. However, it has been determined that increasing temperature destroys topological order, restricting many topological materials to very low temperatures and thus hampering practical applications. Here, we propose a material realization of temperature promoted topological order. We show that a semiconducting oxide that has been widely used in lead-acid batteries, $\beta$-PbO$_2$, hosts a topological semimetallic phase driven by both thermal expansion and electron-phonon coupling upon increasing temperature. We identify the interplay between the quasi-two-dimensional nature of the charge distribution of the valence band with the three-dimensional nature of the charge distribution of the conduction band as the microscopic mechanism driving this unconventional temperature dependence. Thus, we propose a general principle to search for and design novel topological materials whose topological order is stabilized by increasing temperature. This provides a clear roadmap for taking topological materials from the laboratory to technological devices.

Quantum MonteCarlo Simulation of the Chiral Heisenberg Gross-Neveu-Yukawa Phase Transition with a Single Dirac Cone.

We present quantum MonteCarlo simulations for the chiral Heisenberg Gross-Neveu-Yukawa quantum phase transition of relativistic fermions with N=4 Dirac spinor components subject to a repulsive, local four fermion interaction in (2+1)D. Here we employ a two-dimensional lattice Hamiltonian with a single, spin-degenerate Dirac cone, which exactly reproduces a linear energy-momentum relation for all finite size lattice momenta in the absence of interactions. This allows us to significantly reduce finite size corrections compared to the widely studied honeycomb and π-flux lattices. A Hubbard term dynamically generates a mass beyond a critical coupling of U_{c}=6.76(1) as the system acquires antiferromagnetic order and SU(2) spin rotational symmetry is spontaneously broken. At the quantum phase transition, we extract a self-consistent set of critical exponents ν=0.98(1), η_{ϕ}=0.53(1), η_{ψ}=0.18(1), and β=0.75(1). We provide evidence for the continuous degradation of the quasiparticle weight of the fermionic excitations as the critical point is approached from the semimetallic phase. Finally, we study the effective "speed of light" of the low-energy relativistic description, which depends on the interaction U, but is expected to be regular across the quantum phase transition. We illustrate that the strongly coupled bosonic and fermionic excitations share a common velocity at the critical point.

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.

In Situ Probing Molecular Intercalation in Two-Dimensional Layered Semiconductors.

The electrochemical molecular intercalation of two-dimensional layered materials (2DLMs) produces stable and highly tunable superlattices between monolayer 2DLMs and self-assembled molecular layers. This process allows unprecedented flexibility in integrating highly distinct materials with atomic/molecular precision to produce a new generation of organic/inorganic superlattices with tunable chemical, electronic, and optical properties. To better understand the intercalation process, we developed an on-chip platform based on MoS2 model devices and used optical, electrochemical, and in situ electronic characterizations to resolve the intermediate stages during the intercalation process and monitor the evolution of the molecular superlattices. With sufficient charge injection, the organic cetyltrimethylammonium bromide (CTAB) intercalation induces the phase transition of MoS2 from semiconducting 2H phase to semimetallic 1T phase, resulting in a dramatic increase of electrical conductivity. Therefore, in situ monitoring the evolution of the device conductance reveals the electrochemical intercalation dynamics with an abrupt conductivity change, signifying the onset of the molecule intercalation. In contrast, the intercalation of tetraheptylammonium bromide (THAB), a branched molecule in a larger size, resulting in a much smaller number of charges injected to avoid the 2H to 1T phase transition. Our study demonstrates a powerful platform for in situ monitoring the molecular intercalation of many 2DLMs (MoS2, WSe2, ReS2, PdSe2, TiS2, and graphene) and systematically probing electronic, optical, and optoelectronic properties at the single-nanosheet level.

Multiple fermions in MoP

Based on the first-principles calculation, we report that the WC-type MoP is a new type of topological semimetal. It can host Weyl semimetal phase with six pairs of Weyl points (WPs) in the first whole Brillouin zone (BZ). In the absence of spin-orbit coupling (SOC), band inversion forms a gapless nodal ring protected by horizontal mirror symmetry [Formula: see text]. The strong SOC in MoP makes the nodal ring fully gapped in the mirror plane, and generates multiple WPs away from the plane. As one hallmark of Weyl semimetals, the unique Fermi arcs are also observed by calculating the projected surface states and Fermi surfaces. Besides the above prodigious features, it also possess an additional four pairs of massless fermions with triply degenerate points (TDPs) in the vicinity of the Fermi level. Each TDP is a band crossing between degenerate and non-degenerate bands along the high-symmetry line in the Brillouin zone, and is protected by crystalline symmetries. Such new type of topological semimetal realizes multiple fermions states and further provides a fancy platform to study emergent physical phenomena.

Raman scattering from current-stabilized nonequilibrium phases in Ca2RuO4

We used Raman light scattering to study the current-stabilized nonequilibrium semimetallic and metallic phases in Ca$_2$RuO$_4$. By determining the local temperature through careful analysis of the Stokes and anti-Stokes intensities, we find that Joule heating can be completely avoided by supplying sufficient cooling power in a helium-flow cryostat, and that the current induces the semimetallic state without inducing any significant heating. We further investigate the current-induced semimetallic state as a function of temperature and current. We confirm the absence of long-range antiferromagnetic order and identify a substantial Fano broadening of several phonons, which suggests coupling to charge and orbital fluctuations. Our results demonstrate that the semimetallic state is a genuine effect of the applied electrical current and that the current-induced phases have characteristics distinct from the equilibrium ones.