Semimetal Phase Research Articles

Fermi Arcs Dominating the Electronic Surface Properties of Trigonal PtBi2

AbstractMaterials combining topologically non‐trivial behavior and superconductivity offer a potential route for quantum computation. However, the set of available materials intrinsically realizing these properties are scarce. Recently, surface superconductivity has been reported in PtBi2 in its trigonal phase and an inherent Weyl semimetal phase has been predicted. Here, based on scanning tunneling microscopy experiments, the signature of topological Fermi arcs are revealed in the normal state patterns of the quasiparticle interference. It is shown that the scattering between Fermi arcs dominates the interference spectra, providing conclusive evidence for the relevance of Weyl fermiology for the surface electronic properties of trigonal PtBi2.

Fully spin-polarized two dimensional polar semi-metallic phase in Eu substituted GdI2 monolayer

Abstract The coexistence of seemingly mutually exclusive properties such as ferromagnetism, ferroelectricity and metallicity in atomically thin materials is the requirement of the hour in electronics as the Moore’s law faces an impending end. Only a few 2D multiferroic materials have been predicted/realized so far. The polar metals with simultaneous presence of polarity and conductivity are also equally rare. Here, we predict, based on first-principles calculations that an Eu-substituted rare-earth halide GdI2 monolayer showcases ferromagnetism, ferroelasticity while being polar and a fully spin-polarized semi-metal at the same time. The ferroelasticity and polarity are shown to be coupled making it possible to switch the polar direction using external mechanical stress. Further, it is observed that an application of biaxial tensile strain of 5% causes the spin easy-axis to shift from out-of-plane to in-plane direction. Thus, spin easy axis gets coupled with the direction of polarization in the strained monolayer making the switching of magnetization also possible using external strain. Simultaneous coexistence and coupling of the ferroic orders in a metallic 2D material makes the Eu substituted GdI2 monolayer an incredibly rare material for nano-electronics and spintronics applications.

Pressure-induced magnetic and topological transitions in non-centrosymmetric MnIn2Te4

In recent years, the study of magnetic topological materials, with their variety of exotic physics, has significantly flourished. In this work, we predict the interplay of magnetism and topology in the non-centrosymmetric ternary manganese compound MnIn2Te4under external hydrostatic pressure, using first-principles calculations and symmetry analyses. At ambient pressure, the ground state of the system is an antiferromagnetic insulator. With the application of a small hydrostatic pressure (∼0.50 GPa), it undergoes a magnetic transition, and the ferromagnetic state becomes energetically favorable. At ∼2.92 GPa, the ferromagnetic system undergoes a transition into a Weyl semimetallic phase, which hosts multiple Weyl points in the bulk. The presence of non-trivial Weyl points have been verified by Wilson bands computations and the presence of characteristic surface Fermi arcs. Remarkably, we discover that the number of Weyl points in this system can be controlled by pressure and that these manifest in an anomalous Hall conductivity (AHC). In addition to proposing a new candidate magnetic topological material, our work demonstrates that pressure can be an effective way to induce and control topological phases, as well as AHC, in magnetic materials. These properties may allow our proposed material to be used as a novel pressure-controlled Hall switch.

Holographic description of an anisotropic Dirac semimetal

Holographic quantum matter exploits the AdS/CFT correspondence to study systems in condensed matter physics. An example of these systems are strongly correlated semimetals, which feature a rich phase diagram structure. In this work, we present a holographic model for a Dirac semimetal in 2 + 1 dimensions that features a topological phase transition. Our construction relies on deforming a relativistic UV fixed point with some relevant operators that explicitly break rotations and some internal symmetries. The phase diagram for different values of the relevant coupling constants is obtained. The different phases are characterized by distinct dispersion relations for probe fermionic modes in the AdS geometry. We find semi-metallic phases characterized by the presence of Dirac cones and an insulating phase featuring a mass gap with a mild anisotropy. Remarkably, we find as well an anisotropic semi-Dirac phase characterized by a massless a fermionic excitation dispersing linearly in one direction while quadratically in the other.

Heterocontact-Triggered 1H to 1T' Phase Transition in CVD-Grown Monolayer MoTe2: Implications for Low Contact Resistance Electronic Devices.

Single-layer molybdenum ditelluride (MoTe2) has attracted attention due to the smaller energy difference between the semiconducting (1H) and semimetallic (1T') phases with respect to other two-dimensional transition metal dichalcogenides (TMDs). Understanding the phenomenon of polymorphism between these structural phases is of great fundamental and practical importance. In this paper, we report a 1H to 1T' phase transition occurring during the chemical vapor deposition (CVD) synthesis of single-layer MoTe2 at 730 °C. The transformation originates at the heterocontact between monoclinic and hexagonal crystals and progresses to either yield a partial or complete 1H to 1T' phase transition. Microscopic and spectroscopic analyses of the MoTe2 crystals reveal the presence of Te vacancies and mirror twin boundaries (MTB) domains in the hexagonal phase. The experimental observations and theoretical simulations indicate that the combination of heterocontact formation and Te vacancies are relevant triggering mechanisms in the observed transformation. By advancing in the understanding and controlling of the direct synthesis of lateral 1T'/1H heterostructures, this work contributes to the development of MoTe2-based electronic and optoelectronic devices with low contact resistance.

Weak topological insulators, nodal-line semimetals, and Dirac semimetals in phononic crystal plates

Topological phases of matter help us to modulate elastic waves at the boundaries of mechanical systems with robustness against defects. Here, we propose to realize weak topological insulator phases, nodal-line semimetal phases, and Dirac semimetal phases in 2D mechanical systems. We implement this idea by theoretical analysis of tight-binding models and numerical simulations of phononic crystal plates based on the two-dimensional Su-Schrieffer-Heeger models. The topological nature of these topological phases is characterized by the topological invariants. The edge states, the visualization feature of the weak topological insulators, are observed unambiguously in the phononic crystal plates. Particularly, the robustness of these edge states against structural defects is verified by removing four single square plates from the center. Experimentally, we realize two types of weak topological insulators in 2D mechanical systems. This study fertilizes the topological phases in mechanical systems and opens excellent avenues for manipulating elastic wave propagation.

Molecular beam epitaxy growth and doping modulation of topological semimetal NiTe2

In this study, high-quality thin films of the topological semimetal phase NiTe2 were prepared using the molecular beam epitaxy technique, confirmed through x-ray diffraction with pronounced Laue oscillations. Electrical transport experiments reveal that thick films have properties similar to bulk materials. By employing co-deposition, we introduced either magnetic or non-magnetic elements during the growth of thinner films, significantly altering their electrical properties. Notably, the magnetic element Cr induces long-range ferromagnetic ordering, leading to the observation of a significant anomalous Hall effect in NiTe2 thin films. The Hall conductivity remains nearly constant well below the Curie temperature, indicating the correlation with the intrinsic topological nature of the band structure. Theoretical first principles band calculations support the generation of the Weyl semimetal state in the material through magnetic doping. These findings pave the way for exploring more magnetic Weyl semimetals and related low-dimensional quantum devices based on topological semimetals.

Weyl semimetallic phase in high pressure CrSb2 and structural compression studies of its high pressure polymorphs

In this study, high pressure synchrotron powder X-ray diffraction is used to investigate the compression of two high pressure polymorphs of CrSb2. The first is the CuAl2-type polymorph with an eight-fold coordinated Cr, which can be quenched to ambient conditions from high-pressure high-temperature conditions. The second is the recently discovered MoP2-type polymorph, which is induced by compression at room temperature, with a seven-fold coordinated Cr. Here, the assigned structure is unambiguously confirmed by solving it from single-crystal X-ray diffraction. Furthermore, the electrical properties of the MoP2-type polymorph were investigated theoretically and the resistance calculations under pressure were accompanied by resistance measurements under high pressure on a single crystal of CrSb2. The calculated electronic band structure for the MoP2-type phase is discussed and we show that the polymorph is semimetallic and possesses type-I Weyl points. No further phase transitions were observed for the CuAl2-type structure up to 50 GPa and 40 GPa for the MoP2-type structure. Even though the CuAl2-phase has the highest coordination number of Cr, it was found to be less compressible than the MoP2-phase having a seven-fold coordinated Cr, which was attributed to the longer Cr-Sb distance in the CuAl2-type phase. The discovery of a type-I Weyl semimetallic phase in CrSb2 opens up for discovering other Weyl semimetals in the transition metal di-pnictides under high pressure.

Deconfined quantum criticality of nodal d -wave superconductivity, Néel order, and charge order on the square lattice at half-filling

We consider a SU(2) lattice gauge theory on the square lattice, with a single fundamental complex fermion and a single fundamental complex boson on each lattice site. Projective symmetries of the gauge-charged fermions are chosen so that they match with those of the spinons of the π-flux spin liquid. Global symmetries of all gauge-invariant observables are chosen to match with those of the particle-hole symmetric electronic Hubbard model at half-filling. Consequently, both the fundamental fermion and fundamental boson move in an average background π-flux, their gauge-invariant composite is the physical electron, and eliminating gauge fields in a strong gauge-coupling expansion yields an effective extended Hubbard model for the electrons. The SU(2) gauge theory displays several confining/Higgs phases: a nodal d-wave superconductor, and states with Néel, valence-bond solid, charge, or staggered current orders. There are also a number of quantum phase transitions between these phases that are very likely described by (2+1)-dimensional deconfined conformal gauge theories, and we present large flavor expansions for such theories. These include the phenomenologically attractive case of a transition between a conventional insulator with a charge gap and Néel order, and a conventional d-wave superconductor with gapless Bogoliubov quasiparticles at four nodal points in the Brillouin zone. We also apply our approach to the honeycomb lattice, where we find a bicritical point at the junction of Néel, valence bond solid (Kekulé), and Dirac semimetal phases. Published by the American Physical Society 2024

Even-Odd Layer-Dependent Exchange Bias Effect in MnBi2Te4 Chern Insulator Devices.

Magnetic topological materials with coexisting magnetism and nontrivial band structures exhibit many novel quantum phenomena, including the quantum anomalous Hall effect, the axion insulator state, and the Weyl semimetal phase. As a stoichiometric layered antiferromagnetic topological insulator, thin films of MnBi2Te4 show fascinating even-odd layer-dependent physics. In this work, we fabricate a series of thin-flake MnBi2Te4 devices using stencil masks and observe the Chern insulator state at high magnetic fields. Upon magnetic field training, a large exchange bias effect is observed in odd but not in even septuple layer (SL) devices. Through theoretical calculations, we attribute the even-odd layer-dependent exchange bias effect to the contrasting surface and bulk magnetic properties of MnBi2Te4 devices. Our findings reveal the microscopic magnetic configuration of MnBi2Te4 thin flakes and highlight the challenges in replicating the zero magnetic field quantum anomalous Hall effect in odd SL MnBi2Te4 devices.

Cladding-free Fermi arc surface states and topological directional couplers in ideal photonic Weyl metamaterials

Photons can freely propagate in a vacuum, making it not a simple insulator but rather a conductor for photons. Consequently, in topological photonics, domain wall structures with opposing effective mass terms are used as cladding to confine electromagnetic waves. This approach is necessary to demonstrate topological edge/surface waves and Fermi arc surface states (FASS). Here, we show that the cladding-free FASS with high field localization at the boundary can be achieved using ideal Weyl gyromagnetic metamaterials (GMs). In these GMs, the ideal Weyl semimetal phase exists due to the dispersionless longitudinal modes. At the boundary of the GMs-vacuum system, the cladding-free FASS connects the projections of Weyl nodes with opposite chirality, thanks to the bulk-boundary correspondence principle. We further confirm that chiral boundary modes can propagate without experiencing scattering or backward reflection, i.e., they can advance seamlessly approximately various types of defects. Remarkably, various types of topological directional couplers are achieved by utilizing cladding-free FASS in an ideal gyromagnetic medium. Our theoretical analysis reveals that the underlying operational principle for accomplishing these nonreflecting directional couplers is due to the single coupling channel between the cladding-free FASS and the multi-type scatterers of the continuous media. Furthermore, the controllable propagation and topological directional coupling of cladding-free FASS can be further explored by adjusting the ideal gyromagnetic medium and boundary configurations of the continuous media system. This research offers increased flexibility for the development of cladding-free and directionally coupled topological devices.

Novel hard and unusual superconducting monoclinic phase of FeB2C2: An ab initio evolutionary study

This study focuses on conducting an ab initio evolutionary investigation to search for stable polymorphs of iron diborocarbides with the formula FeB2C2. We also examined other forms of C contents, including FeB3C and FeBC3. Our findings reveal that the lowest energetic structure of FeB2C2 is a semimetallic monoclinic phase with a space group (s.g.) of C2/m and a metastable metallic phase of FeB2C2 is an orthorhombic structure with s.g. of Pmmm. In addition, structural and relative properties of FeB3C and FeBC3 are performed and discussed to compare with FeB2C2. All predicted structures are dynamically and elastically stable, verified without negative phonon frequency and Born criteria, respectively. We also analyzed the energetic stability through calculated cohesive and formation energies, which showed that C2/m-FeB2C2 is stable at low pressure. Interestingly, the C2/m and Pmmm phases of FeB2C2 are hard materials with Vickers hardness (Hv) of 22.40 and 27.52 GPa, respectively. Additionally, we examined the electron–phonon coupling of both FeB2C2 phases. Unexpectedly, we found that the semimetallic C2/m-FeB2C2 phase is a superconductor with a significant superconducting temperature (Tc) exceeding 6 K. These findings provide some novel results for the Fe–B–C system and pave the way for investigating other metal borocarbides and related ternary compounds.

3D topological semimetal phases of strained [formula omitted]-Sn on insulating substrate

α-Sn is an elemental topological material, whose topological phases can be tuned by strain and magnetic field. Such tunability offers a substantial potential for topological electronics. However, InSb substrates, commonly used to stabilize α-Sn allotrope, suffer from parallel conduction, restricting transport investigations and potential applications. Here, the successful MBE growth of high-quality α-Sn layers on insulating, hybrid (001) CdTe/GaAs substrates, with bulk electron mobility approaching 20000 cm2V−1s−1 is reported. The electronic properties of the samples are systematically investigated by independent complementary techniques, enabling thorough characterization of the 3D Dirac (DSM) and Weyl (WSM) semimetal phases induced by the strains and magnetic field, respectively. Magneto-optical experiments, corroborated with band structure modelling, provide an exhaustive description of the bulk states in the DSM phase. The modelled electronic structure is directly observed in angle-resolved photoemission spectroscopy, which reveals linearly dispersing bands near the Fermi level. The first detailed study of negative longitudinal magnetoresistance relates this effect to the chiral anomaly and, consequently, to the presence of WSM. Observation of the π Berry phase in Shubnikov-de Haas oscillations agrees with the topologically non-trivial nature of the investigated samples. Our findings establish α-Sn as an attractive topological material for exploring relativistic physics and future applications.

Ultrafast creation of a light-induced semimetallic state in strongly excited 1T-TiSe2.

Screening, a ubiquitous phenomenon associated with the shielding of electric fields by surrounding charges, has been widely adopted as a means to modify a material's properties. While most studies have relied on static changes of screening through doping or gating thus far, here we demonstrate that screening can also drive the onset of distinct quantum states on the ultrafast timescale. By using time- and angle-resolved photoemission spectroscopy, we show that intense optical excitation can drive 1T-TiSe2, a prototypical charge density wave material, almost instantly from a gapped into a semimetallic state. By systematically comparing changes in band structure over time and excitation strength with theoretical calculations, we find that the appearance of this state is likely caused by a dramatic reduction of the screening length. In summary, this work showcases how optical excitation enables the screening-driven design of a nonequilibrium semimetallic phase in TiSe2, possibly providing a general pathway into highly screened phases in other strongly correlated materials.

Photoinduced Floquet higher-order Weyl semimetal in C6 symmetric Dirac semimetals

Abstract Topological Dirac semimetals are a parent state from which other exotic topological phases of matter, such as Weyl semimetals and topological insulators, can emerge. In this study, we investigate a Dirac semimetal possessing sixfold rotational symmetry and hosting higher-order topological hinge Fermi arc states, which is irradiated by circularly polarized light. Our findings reveal that circularly polarized light splits each Dirac node into a pair of Weyl nodes due to the breaking of time-reversal symmetry, resulting in the realization of the Weyl semimetal phase. This Weyl semimetal phase exhibits rich boundary states, including two-dimensional surface Fermi arc states and hinge Fermi arc states confined to six hinges. Furthermore, by adjusting the incident direction of the circularly polarized light, we can control the degree of tilt of the resulting Weyl cones, enabling the realization of different types of Weyl semimetals.

Electron-Phonon Interaction Mediated Gigantic Enhancement of Thermoelectric Power Factor Induced by Topological Phase Transition.

We propose an effective strategy to significantly enhance the thermoelectric power factor (PF) of a series of 2D semimetals and semiconductors by driving them toward a topological phase transition (TPT). Employing first-principles calculations with an explicit consideration of electron-phonon interactions, we analyze the electronic transport properties of germanene across the TPT by applying hydrogenation and biaxial strain. We reveal that the nontrivial semimetal phase, hydrogenated germanene with 8% biaxial strain, achieves a considerable 4-fold PF enhancement, attributed to the highly asymmetric electronic structure and semimetallic nature of the nontrivial phase. We extend the strategy to another two representative 2D materials (stanene and HgSe) and observe a similar trend, with a marked 7-fold and 5-fold increase in PF, respectively. The wide selection of functional groups, universal applicability of biaxial strain, and broad spectrum of 2D semimetals and semiconductors render our approach highly promising for designing novel 2D materials with superior thermoelectric performance.

Effect of annealing on the electrical resistivity of Kondo lattice CeRh2Ga2

CeRh2Ga2, which crystallizes in CaBe2Ge2-type primitive tetragonal structure (space group P4/nmm), is known to exhibit Kondo lattice heavy fermion behavior and is proposed to be a potential candidate for Weyl-Kondo semimetal phase. Here we examine the effect of annealing, particularly on the electrical resistivity of polycrystalline CeRh2Ga2. A comparative study of the powder x-ray diffraction (XRD), magnetic susceptibility χ(T), heat capacity C p(T) and electrical resistivity ρ(T) data of both as-arc-melted and annealed CeRh2Ga2 samples are presented. The XRD patterns of both as-arc-melted and annealed samples look similar. No marked effect of annealing could be clearly seen in the temperature dependences of χ and C p data. However, the effect of annealing is clearly manifested in the T dependence of ρ, particlularly at low temperatures. At low-T the ρ(T) data of as-arc-melted CeRh2Ga2 follow a T 2 temperature dependence (Fermi-liquid feature), whereas the ρ(T) data of annealed CeRh2Ga2 exhibit an upturn (semimetal-like feature).

Engineering Two-Dimensional Nodal Semimetals in Functionalized Biphenylene by Fluorine Adatoms.

We propose a band engineering scheme on the biphenylene network, a newly synthesized carbon allotrope. We illustrate that the electronic structure of the biphenylene network can be significantly altered by controlling conditions affecting the symmetry and destructive interference of wave functions through periodic fluorination. First, we investigate the mechanism for the appearance of a type-II Dirac fermion in a pristine biphenylene network. We show that the essential ingredients are mirror symmetries and stabilization of the compact localized eigenstates via destructive interference. While the former is used for the band-crossing point along high symmetry lines, the latter induces highly inclined Dirac dispersions. Subsequently, we demonstrate the transformation of the biphenylene network's type-II Dirac semimetal phase into various Dirac phases such as type-I Dirac, gapped type-II Dirac, and nodal line semimetals through the deliberate disruption of mirror symmetry or modulation of destructive interference by varying the concentration of fluorine atoms.

Observation of an acoustic topological Euler insulator with meronic waves

Topological band theory has conventionally been concerned with the topology of bands around a single gap. Only recently non-Abelian topologies that thrive on involving multiple gaps were studied, unveiling a new horizon in topological physics beyond the conventional paradigm. Here, we report on the first experimental realization of a topological Euler insulator phase with unique meronic characterization in an acoustic metamaterial. We demonstrate that this topological phase has several nontrivial features: First, the system cannot be described by conventional topological band theory, but has a nontrivial Euler class that captures the unconventional geometry of the Bloch bands in the Brillouin zone. Second, we uncover in theory and probe in experiments a meronic configuration of the bulk Bloch states for the first time. Third, using a detailed symmetry analysis, we show that the topological Euler insulator evolves from a non-Abelian topological semimetal phase via. the annihilation of Dirac points in pairs in one of the band gaps. With these nontrivial properties, we establish concretely an unconventional bulk-edge correspondence which is confirmed by directly measuring the edge states via. pump-probe techniques. Our work thus unveils a nontrivial topological Euler insulator phase with a unique meronic pattern and paves the way as a platform for non-Abelian topological phenomena.

Non-Hermitian Strongly Interacting Dirac Fermions.

Exotic quantum phases and phase transition in the strongly interacting Dirac systems have attracted tremendous interests. On the other hand, non-Hermitian physics, usually associated with dissipation arising from the coupling to environment, emerges as a frontier of modern physics in recent years. In this Letter, we investigate the interplay between non-Hermitian physics and strong correlation in Dirac-fermion systems. We generalize the projector quantum Monte-Carlo (PQMC) algorithm to the non-Hermitian interacting fermionic systems. Employing PQMC simulation, we decipher the ground-state phase diagram of the honeycomb Hubbard model with spin resolved non-Hermitian asymmetric hopping processes. The antiferromagnetic (AFM) ordering induced by Hubbard interaction is enhanced by the non-Hermitian asymmetric hopping. Combining PQMC simulation and renormalization group analysis, we reveal that the quantum phase transition between Dirac semi-metal and AFM phases belongs to Hermitian chiral XY universality class, implying that a Hermitian Gross-Neveu transition is emergent at the quantum critical point although the model is non-Hermitian.