Semimetallic State Research Articles

Symmetry Dictated Grain Boundary State in a Two-Dimensional Topological Insulator

Grain boundaries (GBs) are ubiquitous in solids and have been of central importance in understanding the nature of polycrystals. In addition to their classical roles, topological insulators (TIs) offer a chance to realize GBs hosting distinct topological states that can be controlled by their crystal symmetries. However, such roles of crystalline symmetry in two-dimensional (2D) TIs have not been definitively measured yet. Here, we present the first direct evidence of a symmetry-enforced metallic state along a GB in 1T'-MoTe2, a prototypical 2D TI. Using scanning tunneling microscopy, we show a metallic state along a GB with nonsymmorphic lattice symmetry and its absence along another boundary with symmorphic symmetry. Our atomistic simulations demonstrate in-gap Weyl semimetallic states for the former, whereas they demonstrate gapped states for the latter, explaining our observation well. The observed metallic state, tightly linked to its crystal symmetry, can be used to create a stable conducting nanowire inside TIs.

Non-Coulomb strong electron-hole binding in Ta2NiSe5 revealed by time- and angle-resolved photoemission spectroscopy

We reveal an ultrafast purely electronic phase transition in \TNS, which is a plausible excitonic insulator, after excited by an ultrafast infrared laser pulse. Specifically, the order parameter of the strong electron-hole binding shrinks with enhancing the pump pulse, and above a critical pump fluence, a photo-excited semimetallic state is experimentally identified with the absence of ultrafast structural transition. In addition, the bare valence and conduction bands and also the effective exciton binding energy in \TNS~are determined. These findings and detailed analysis suggest a bare nonequilibrium semimetallic phase in $Ta_2NiSe_5$ and the strong electron-hole binding cannot be exclusively driven by Coulomb interaction.

Studied and Forgotten. A Fresh Look at the Li–Mn–Ge System

The crystal structure of the ternary germanide Li2MnGe has been re‐evaluated from single‐crystal X‐ray diffraction data. This compound crystallizes in a non‐centrosymmetric superstructure of the ZrCuSiAs type (space group P4bm, Pearson code tP16), with the lattice parameters a = 6.088(4) Å, c = 6.323(4) Å. First‐principle calculations for the idealized structure predict antiferromagnetic exchange in the square Mn nets and semimetallic ground state. In addition, a new ternary phase with the composition Li2–xMn4+xGe5 (x ≈ 1.2) was discovered. It adopts the V6Si5 structure type (space group Ibam, Pearson code oI44), with the lattice parameters a = 7.570(2) Å, b = 16.323(3) Å, c = 5.057(1) Å. DSC/TG measurements show that this compound is thermally stable below 995 K.

Tuneable quantum spin Hall states in confined 1T' transition metal dichalcogenides

Investigation of quantum spin Hall states in 1T' phase of the monolayer transition metal dichalcogenides has recently attracted the attention for its potential in nanoelectronic applications. While most of the theoretical findings in this regard deal with infinitely periodic crystal structures, here we employ density functional theory calculations and k.p Hamiltonian based continuum model to unveil the bandgap opening in the edge-state spectrum of finite width molybdenum disulphide, molybdenum diselenide, tungsten disulphide and tungsten diselenide. We show that the application of a perpendicular electric field simultaneously modulates the band gaps of bulk and edge-states. We further observe that tungsten diselenide undergoes a semi-metallic intermediate state during the phase transition from topological to normal insulator. The tuneable edge conductance, as obtained from the Landauer-Büttiker formalism, exhibits a monotonous increasing trend with applied electric field for deca-nanometer molybdenum disulphide, whereas the trend is opposite for other cases.

Detection of decoupled surface and bulk states in epitaxial orthorhombic SrIrO3 thin films

We report the experimental evidence of evolving lattice distortion in high quality epitaxial orthorhombic SrIrO3(001) thin films fully strained on (001) SrTiO3 substrates. Angle-resolved X-ray photoemission spectroscopy studies show that the surface layer of 5 nm SrIrO3 films is Sr–O terminated, and subsequent layers recover the semimetallic state, with the band structure consistent with an orthorhombic SrIrO3(001) having the lattice constant of the substrate. While there is no band folding in the experimental band structure, additional super-periodicity is evident in low energy electron diffraction measurements, suggesting the emergence of a transition layer with crystal symmetry evolving from the SrTiO3 substrate to the SrIrO3(001) surface. Our study sheds light on the misfit relaxation mechanism in epitaxial SrIrO3 thin films in the orthorhombic phase, which is metastable in bulk.

Does β-PbO2 harbor topological states?

The electronic properties of β-PbO2, have been controversial for several decades. Experiments find behavior ranging from metallic, attributed to oxygen vacancies, to indirect semiconducting for stoichiometric samples with a gap of 0.61 eV. Theory leads to similar ambiguities, and predicts this phase to be metallic (PBE, HSE06) or to possess a small bandgap (HSE06). An area where this inconsistency is amplified, is when a material behavior depends on the electronic structure in the vicinity of the Fermi energy, such as topological states. In our work, we use a self-consistent DFT + U approach and find that stoichiometric β-PbO2 to be an indirect semiconductor with a band gap of ∼0.8 eV, similar to experiment. The larger bandgap requires at least ∼4% strain, to drive β-PbO2 into a nodal line semimetallic state, significantly larger strains than reported previously. Moreover, we find that the nodal line semimetallic state is not protected against spin-orbit-coupling. Also, the surface computations do not show any evidence for topologically protected states near the Fermi energy. Therefore, our results strongly suggest that β-PbO2 is a topologically trivial material, consistent with experiment, but in stark contrast to previous computations. Previously reported topologically protected states in β-PbO2 are attributed to an inaccurate description of the (bulk) optical properties.

Low-temperature properties of pyrochlore Lu2Ir2O7

Low-temperature properties of Lu2Ir2O7 were studied by means of magnetization, ac-susceptibility and specific heat measurements. Our low-temperature measurements reveal a bifurcation of ZFC and FC magnetization data below 140 K, which is further pronounced as a broad anomaly in the temperature development of specific heat and ac-susceptibility. The anomaly can be attributed to the same physical mechanism as discussed for other A2Ir2O7 with A = magnetic rare-earth element, i.e. to short-range correlations among Ir4+ magnetic moments, rather than to a long-range order of Ir sublattice reported earlier. The ac-susceptibility data revealed a strong frequency dependence of the signal, a paradigmatic sign of a spin-glass transition. The analysis of low-temperature specific heat data brought important information on physical properties of Lu2Ir2O7, in striking agreement with our density functional theoretical (DFT) calculations, concluding relatively weak electron-phonon interaction and semimetallic ground state in Lu2Ir2O7.

Copper-doped induced ferromagnetic half-metal zirconium diselenide single crystals

Two-dimensional (2D) magnetic layered materials have attracted considerable attention in memory storage devices due to their exciting magnetic ordering. Herein, the electronic and magnetic properties of high-quality single crystals zirconium diselenide and copper (Cu)-doped zirconium diselenide as grown via chemical vapor transport technique combined with first principle density functional theory calculations were investigated. A semimetallic state is recognized for Cu0.052Zr0.93Se2 as measured through resistance versus temperature measurements and angle resolved photoemission spectroscopy (ARPES). The magnetic measurement shows diamagnetic semiconducting behaviour for ZrSe2, whereas Cu0.052Zr0.93Se2 exhibits ferromagnetic character via applying perpendicular magnetic field. Cu0.052Zr0.93Se2 reveals the room temperature magnetic moment ∼0.0125 emu g−1, while the Curie temperature is ∼363.49 K. Furthermore, first principle density functional theory (DFT) calculations show energetically long range ferromagnetic ordering in a half-metallic Cu-doped ZrSe2, while a diamagnetic state in case of ZrSe2 agrees well with experiment results. These results suggest that due to strong interaction elements at the octahedral site of zirconium atoms when replaced by copper atoms, which can change the spin ordering of electrons and make zirconium vacancy, while their magnetic moment is increased. Very importantly the half-metallic character of Cu0.052Zr0.93Se2 promotes much spin polarized electrons around the Fermi level, suggesting significant potential in future memory devices and spintronic applications.

Synthetic Semimetals with van der Waals Interfaces.

The assembly of suitably designed van der Waals (vdW) heterostructures represents a new approach to produce artificial systems with engineered electronic properties. Here, we apply this strategy to realize synthetic semimetals based on vdW interfaces formed by two different semiconductors. Guided by existing ab initio calculations, we select WSe2 and SnSe2 mono- and multilayers to assemble vdW interfaces and demonstrate the occurrence of semimetallicity by means of different transport experiments. Semimetallicity manifests itself in a finite minimum conductance upon sweeping the gate over a large range in ionic liquid gated devices, which also offer spectroscopic capabilities enabling the quantitative determination of the band overlap. The semimetallic state is additionally revealed in Hall effect measurements by the coexistence of electrons and holes, observed by either looking at the evolution of the Hall slope with sweeping the gate voltage or with lowering temperature. Finally, semimetallicity results in the low-temperature metallic conductivity of interfaces of two materials that are themselves insulating. These results demonstrate the possibility to implement a state of matter that had not yet been realized in vdW interfaces and represent a first step toward using these interfaces to engineer topological or excitonic insulating states.

Investigation of electronic structure of transition metal silicides MnSi1.75 and CoSi for enhanced thermoelectric properties

Transition metal silicides that have low band gap or semimetallic states have gained huge interest due to enhanced thermoelectric efficiency. We present high resolution photoemission studies on two such systems like MnSi1.75 a low band gap semiconductor and CoSi a semimetal to understand the inherent electronic properties. Valence band studies on these systems reveal that there are both localized and delocalized 3d states present where as in the pure transition metals more localized states present near the Fermi level. In both MnSi1.75 and CoSi, the localized 3d states are found to be shifted to higher binding energy due to the strong hybridization with the Si 3s−3p states that gives rise to the hybridization gap in these systems. The delocalized 3d states in both the systems has much lower electronic density near the Fermi level than the pure transition metals that contributes to the conduction. Supporting evidences of delocalized screening in MnSi1.75 and weak correlated satellite in CoSi have been observed in the 2p core-level spectra. We find that the presence of both hybridization gap and the finite carrier concentration at the Fermi level in these transition metal silicides are necessary to have the enhanced thermoelectric properties.

Contrasted Sn substitution effects on Dirac line node semimetals SrIrO3 and CaIrO3

Perovskite-type iridates SrIrO3 and CaIrO3 are a Dirac line node semimetal protected by crystalline symmetry, providing an interesting playground to investigate electron correlation effects on topological semimetals. The effect of Sn doping was examined by growing SrIr1−xSnxO3 and CaIr1−xSnxO3 thin films epitaxially on the SrTiO3(001) substrate using pulsed laser deposition. Upon Sn doping, the semimetallic ground state switches into an insulator. As temperature is lowered, the resistivity, ρ(T), of SrIr1−xSnxO3 above a critical doping level (xc ∼ 0.1) shows a well-defined transition from the semimetal to a weakly ferromagnetic insulator at T = Tc. In contrast, ρ(T) of CaIr1−xSnxO3 with increasing x shows a rapid increase in magnitude but does not show a clear signature of metal-insulator transition in the temperature dependence. We argue that the contrasted behavior of the two closely related iridates reflects the interplay between the effects of electron correlation and disorder enhanced by Sn doping.

Magnetic field–induced type II Weyl semimetallic state in geometrically frustrated Shastry-Sutherland lattice GdB4

Weyl semimetal is a topologically non-trivial phase of matter with pairs of Weyl nodes in the k-space, which act as monopole and antimonopole pairs of the Berry curvature. Two hallmarks of the Weyl metallic state are the topological surface state called the Fermi arc and the chiral anomaly. It is known that the chiral anomaly yields anomalous magnetotransport phenomena. In this study, we report the emergence of the type II Weyl semimetallic state in the geometrically frustrated non-collinear antiferromagnetic Shastry-Sutherland lattice (SSL) GdB4 crystal. When we apply magnetic fields perpendicular to the non-collinear moments in the SSL plane, Weyl nodes are created above and below the Fermi energy along the M-A line (τ-band) because the spin tilting breaks the time-reversal symmetry and lifts band degeneracy while preserving C4z or C2z symmetry. The unique electronic structure of GdB4 under magnetic fields applied perpendicular to the SSL gives rise to a non-trivial Berry phase, detected in de Haas-van Alphen experiments, and chiral anomaly–induced negative magnetoresistance. The emergence of the magnetic field–induced Weyl state in the SSL presents a new guiding principle to develop novel types of Weyl semimetals in frustrated spin systems.

Pressure-induced structural transitions and metallization in hollow ZnMn2O4 microspheres

High-pressure in situ angle dispersive X-ray diffraction (ADXRD) measurements combined with first-principles calculations were performed on hollow ZnMn2O4 microspheres up to 43.5 GPa at room temperature, and two high-pressure phases emerged at 15.9 GPa. Phase III was assigned to the CaMn2O4 type structure (space group Pbcm). It is supposed that phase II probably has a MgAl2S4 inverted spinel type structure (space group Imma). Moreover, phase I, phase II and phase III were probably coexisted when pressure was released. The compressional behaviors of phase I (space group I41/amd) and phase III were all determined. Besides, based on electronic band structure calculations, it was uncovered that phase I probably experiences a pressure-induced electronic topological transition (ETT) to a semimetallic state upon pressure, and phase III is a semimetallic state. Our results show that pressure play a dramatic role in tuning ZnMn2O4’s crystalline structures, electrical behavior and spin-state.

Effect of point defects and functionalization on structural stability and electron properties of borophene as investigated by means of density functional theory

The discovery of a new two dimensional material is a new trend emerged after the discovery of graphene in 2004. Hundreds of new materials have been predicted, designed and investigated during last decade. Many of them have outstanding properties for potential application in new technologies. One of those materials is so-called borophene, being isolated atomically thin boron. This material can show different electronic properties, including semiconducting, semi-metallic, and metallic states due to its unique structure. Borophene demonstrates unusually high in-plane elasticity and strength. The crucial point for application of borophene is its low structural stability at room temperature. A possible way to solve this problem is the functionalization of its surface by external atoms like hydrogen and fluorine. This work presents a study on the effect of O dopants and the presence of vacancy defects on electronic properties of borophene by using first-principles calculations. It was revealed that unlike other 2D materials, the band structure of borophene is stable by respect to the above-mentioned factors. However, the doping by O atoms is found as an effective way to increase the stability of the borophene sheet.

Pressure-induced enhancement of two-dimensionality in LaO1−xFxBi(Se/S)2 superconductors

Application of high pressure (HP) induces evolution from metallic to semi-metallic state in the layered superconducting LaO1-xFxBiSe2 phase, as concluded from ab initio calculations based on experimental P-dependent structural data. These changes in conduction properties are associated with a reported decrease in the superconducting transition temperature, Tc. However, further increase in pressure induces structural transformation in LaO1-xFxBiSe2 which results in a strong enhancement of its two-dimensionality with a related improvement in superconducting performance. In addition, the HP structural anisotropy induces a loss of long-range order correlations along the stacking direction of the LaO/F and BiSe2 layers yielding formation of stacking faults. Similar but even more pronounced structural transition was observed in the related LaO1-xFxBiS2 phase which is also accompanied by an increase in Tc. This demonstrates that enhancement of two-dimensionality in layered superconducting systems acts in favor of their superconducting performance.

Unique Crystal Structure of Ca_{2}RuO_{4} in the Current Stabilized Semimetallic State.

The electric-current stabilized semimetallic state in the quasi-two-dimensional Mott insulator Ca_{2}RuO_{4} exhibits an exceptionally strong diamagnetism. Through a comprehensive study using neutron and x-ray diffraction, we show that this nonequilibrium phase assumes a crystal structure distinct from those of equilibrium metallic phases realized in the ruthenates by chemical doping, high pressure, and epitaxial strain, which in turn leads to a distinct electronic band structure. Dynamical mean field theory calculations based on the crystallographically refined atomic coordinates and realistic Coulomb repulsion parameters indicate a semimetallic state with partially gapped Fermi surface. Our neutron diffraction data show that the nonequilibrium behavior is homogeneous, with antiferromagnetic long-range order completely suppressed. These results provide a new basis for theoretical work on the origin of the unusual nonequilibrium diamagnetism in Ca_{2}RuO_{4}.

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.

Interplay of Dirac Nodes and Volkov-Pankratov Surface States in Compressively Strained HgTe

Preceded by the discovery of topological insulators, Dirac and Weyl semimetals have become a pivotal direction of research in contemporary condensed matter physics. While easily accessible from a theoretical viewpoint, these topological semimetals pose a serious challenge in terms of experimental synthesis and analysis to allow for their unambiguous identification. In this work, we report on detailed transport experiments on compressively strained HgTe. Due to the superior sample quality in comparison to other topological semimetallic materials, this enables us to resolve the interplay of topological surface states and semimetallic bulk states to an unprecedented degree of precision and complexity. As our gate design allows us to precisely tune the Fermi level at the Weyl and Dirac points, we identify a magnetotransport regime dominated by Weyl/Dirac bulk state conduction for small carrier densities and by topological surface state conduction for larger carrier densities. As such, similar to topological insulators, HgTe provides the archetypical reference for the experimental investigation of topological semimetals.

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.

Mixed topological semimetals driven by orbital complexity in two-dimensional ferromagnets

The concepts of Weyl fermions and topological semimetals emerging in three-dimensional momentum space are extensively explored owing to the vast variety of exotic properties that they give rise to. On the other hand, very little is known about semimetallic states emerging in two-dimensional magnetic materials, which present the foundation for both present and future information technology. Here, we demonstrate that including the magnetization direction into the topological analysis allows for a natural classification of topological semimetallic states that manifest in two-dimensional ferromagnets as a result of the interplay between spin-orbit and exchange interactions. We explore the emergence and stability of such mixed topological semimetals in realistic materials, and point out the perspectives of mixed topological states for current-induced orbital magnetism and current-induced domain wall motion. Our findings pave the way to understanding, engineering and utilizing topological semimetallic states in two-dimensional spin-orbit ferromagnets.