Nodal Semimetals Research Articles

Effects of electronic correlation on topological properties of Kagome semimetal Ni3In2S2.

Kagome metals gain attention as they manifest a spectrum of quantum phenomena such as superconductivity, charge order, frustrated magnetism, and allied correlated states of condensed matter. With regard to electronic band structure, several of them exhibit non-trivial topological characteristics. Here, we present a thorough investigation on the growth and the physical properties of single crystals of Ni3In2S2 which is established to be a Dirac nodal line Kagome semimetal. Extensive characterization is attained through temperature and field-dependent resistivity, angle-dependent magnetoresistance and specific heat measurements. The central question we seek to address is the effect of electronic correlations in suppressing the manifestation of topological characteristics. In most metals, the Fermi liquid behaviour is restricted to a narrow range of temperatures. Here we show that Ni3In2S2 follows the Fermi-liquid behaviour up to 86K. This phenomenon is further supported by a high Kadowaki-Woods ratio obtained through specific heat analysis. Different interpretations of the magneto-transport study reveal that magnetoresistance exhibits linear behavior, suggesting the presence of Dirac semi-metallic signatures at lower temperatures. The angle-dependent magneto-transport study obeys the Voigt-Thomson formula. This, on the contrary, implies the classical origin of magnetoresistance. Thus, the effect of strong electron correlation in Ni3In2S2 manifests itself in the anisotropic magneto-transport. Furthermore, the magnetization measurement shows the presence of de-Haas van Alphen oscillations. Calculations of Berry phase provide insights into the topological features in the Kagome semimetal Ni3In2S2.&#xD.

Discovery of Higher-Order Nodal Surface Semimetals.

The emergent higher-order topological insulators significantly deepen our understanding of topological physics. Recently, the study has been extended to topological semimetals featuring gapless bulk band nodes. To date, higher-order nodal point and line semimetals have been successfully realized in different physical platforms. However, for the conceptually expected higher-order nodal surface semimetals, a concrete model has yet to be proposed, let alone experimentally observed. Here, we report an ingenious design route for constructing this unprecedented higher-order topological phase. The three-dimensional model, layer-stacked with a two-dimensional anisotropic Su-Schrieffer-Heeger lattice, exhibits appealing hinge arcs connecting the projected nodal surfaces. Experimentally, we realize this new topological phase in an acoustic metamaterial, and present unambiguous evidence for both the bulk nodal structure and hinge arc states, the two key manifestations of the higher-order nodal surface semimetal. Our findings can be extended to other classical systems such as photonic, elastic, and electric circuit systems, and open new possibilities for controlling waves.

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.

Monolayer Cu2Se: a topological catalysis in CO2 electroreduction

This investigation provides a comprehensive exploration into the intricate interplay between topological surface states (TSS) and catalytic performance in two-dimensional (2D) materials, with specific emphasis on monolayer Cu2Se. Leveraging the unique characteristics of nodal loop semimetals (NLSMs), we delve into the precise influence of TSS on catalytic activity, particularly in the domain of CO2 electrochemical reduction. Our findings illuminate the central role played by these TSS, arising from the underlying NLSM framework, in sculpting catalytic efficiency. The length of these surface states emerges as a critical determinant of surface density of states (DOSs), a fundamental factor governing catalytic behavior. Extension of these surface states correlates with heightened surface DOSs, yielding lower Gibbs free energies and consequently enhancing catalytic performance, particularly in the electrochemical reduction of CO2. Moreover, we underscore the profound importance of preserving symmetries that protect the nodal loop. The disruption of these symmetries is found to result in a significant degradation of catalytic efficacy, underscoring the paramount significance of topological features in facilitating catalytic processes. Therefore, this study not only elucidates the fundamental role of TSS in dictating the catalytic performance of topological 2D materials but also paves the way for harnessing these unique attributes to drive sustainable and highly efficient catalysis across a diverse spectrum of chemical processes.

Tight-binding model with sublattice-asymmetric spin-orbit coupling for square-net nodal line Dirac semimetals

Tight-binding model with sublattice-asymmetric spin-orbit coupling for square-net nodal line Dirac semimetals

Interacting nodal semimetals with nonlinear bands

Interacting nodal semimetals with nonlinear bands

Prediction of a two-dimensional high Curie temperature Weyl nodal line kagome semimetal.

Kagome lattices may have numerous exotic physical properties, such as stable ferromagnetism and topological states. Herein, combining the particle swarm structure search method with first-principles calculations, we identify a two-dimensional (2D) kagome Mo2Se3 crystal structure with space group P6/mmm. The results show that 2D kagome Mo2Se3 is a 100% spin-polarized topological nodal line semimetal and exhibits excellent ambient stability. The band crossing points form two nodal loops around the high-symmetry points Γ and K. On the other hand, Mo2Se3 shows intrinsic ferromagnetism with a large magnetic moment of 3.05 μB per Mo atom and magnetic anisotropy energy (MAE) of 4.78 meV. Monte Carlo simulations estimate that Mo2Se3 possesses a high Curie temperature of about 673 K. In addition, its ferromagnetic ground state can be well preserved under external strain, and the MAE can be improved by increasing the strain. More importantly, the position of each nodal line can be adjusted to the Fermi level through hole doping. This multifunctional 2D magnetic material that combines spin and topology has great potential in the field of nanoscale spintronic devices.

Interactions-disorder duality and critical phenomena in nodal semimetals, dilute gases, and other systems

Interactions-disorder duality and critical phenomena in nodal semimetals, dilute gases, and other systems

Hyperbolic Polaritons in Topological Nodal Ring Semimetals.

In mirror-symmetric systems, there is a possibility of the realization of extended gapless electronic states characterized as nodal lines or rings. Strain induced modifications to these states lead to the emergence of different classes of nodal rings with qualitatively different physical properties. Here we study optical response and the electromagnetic wave propagation in type I nodal ring semimetals, in which the low-energy quasiparticle dispersion is parabolic in momentum k_{x} and k_{y} and is linear in k_{z}. This leads to a highly anisotropic dielectric permittivity tensor in which the optical response is plasmonic in one spatial direction and dielectric in the other two directions. The resulting normal modes (polaritons) in the bulk material become hyperbolic over a broad frequency range, which is furthermore tunable by the doping level. The propagation, reflection, and polarization properties of the hyperbolic polaritons not only provide valuable information about the electronic structure of these fascinating materials in the most interesting region near the nodal rings but also pave the way to tunable hyperbolic materials with applications ranging from anomalous refraction and waveguiding to perfect absorption in ultrathin subwavelength films.

THz Generation from the Topological Nodal Line Semimetal Co2MnGa

Nonlinear optical spectroscopy is a fundamental probe for the investigation of topological effects in quantum materials. In this paper, we report on the terahertz (THz) emission from thin films at various thicknesses of the magnetic topological nodal semimetal Co2MnGa (CMG) when excited by femtosecond optical pulses. Experimental results suggest the presence of multiple THz generation mechanisms, originating from both bulk and surface states of CMG. The former is explained in terms of a photon-drag effect as induced by radiation pressure. The latter emission mechanism instead appears to be related to the photovoltaic effect coming from the topological surface states. This interplay between generation mechanisms indicates that Co2MnGa topological nodal semimetals are a valuable platform for THz emitter devices.

IrF4 : From tetrahedral compass model to topological semimetal

The intersection of topology, symmetry, and magnetism yields a rich structure of possible phases. In this work, we study theoretically the consequences of magnetism on ${\mathrm{IrF}}_{4}$, which was recently identified as a possible candidate topological nodal chain semimetal in the absence of magnetic order. We show that the spin-orbital nature of the Ir moments gives rise to strongly anisotropic magnetic couplings resembling a tetrahedral compass model on a diamond lattice. The predicted magnetic ground state preserves key symmetries protecting the nodal lines, such that they persist into the ordered phase at the mean-field level. The consequences for other symmetry reductions are also discussed.

Topological Nodal Surface and Quadratic Dirac Semimetal States and van Hove Singularities in ScH3 and LuH3 Superconductors.

The coexistence of non-trivial topology and superconductivity in a material may induce a novel physical phenomenon known as topological superconductivity. Topological superconductors have been the subject of intense research, yet there are severe limitations in their application due to a lack of suitable materials. Topological nodal surface semimetals with nearly flat nodal surfaces near the Fermi level can be promising materials to achieve topological superconductivity. Here, we use first-principles calculations to examine the topological electronic characteristics of two new superconductors, ScH3 and LuH3, at both ambient and high pressures. Our studies show that both ScH3 and LuH3 have van Hove singularities, which confirms their superconductivity. Interestingly, both materials host topological nodal surface states under the protection of time reversal and spatial inversion symmetries in the absence of spin-orbit coupling (SOC). These nodal surfaces are distinguished by a pair of unique drum-head-like surface states not previously observed in nodal surface semimetals. Moreover, the nodal surfaces transform into essential spin-orbit quadratic Dirac points when SOC is included. Our findings demonstrate that ScH3 and LuH3 are good candidates to investigate the exotic properties of both nodal surface semimetals (NSSMs) and quadratic Dirac semimetal states and also provide a platform to explore the coexistence of topology and superconductivity in NSSMs with promising applications in high-speed electronics and topological quantum computing.

Magneto-optical conductivity of nodal loop semimetals

In this paper, we calculate the Landau levels (LLs) and magneto-optical conductivity of nodal loop semimetals. We find the LLs have two inflection points at kz=0 and kz=2 with curved structure and linear structure, respectively. With the increase of magnetic field B, there are three different LLs structures. The longitudinal and Hall conductance are calculated in the three region of B with different chemical potential. Compared with other system, there are two different transition peaks from kz=0 and kz=2. We obtained that the positions of interband and intraband transitions of the conductivity occur at En+1±En/2 and En+1±En/2 with the chemical potential moving into the positive branch LLs. We find the interband peaks are redistributed for the Hall conductivity. Finally the magnetoresistivity and circular polarization are also distinguished as a measure for probing the properties of nodal loop semimetals in experimentally.

Delocalization of topological surface states by diagonal disorder in nodal loop semimetals

The effect of Anderson diagonal disorder on the topological surface (``drumhead'') states of a Weyl nodal loop semimetal is addressed. Since diagonal disorder breaks chiral symmetry, a winding number cannot be defined. Seen as a perturbation, the weak random potential mixes the clean exponentially localized drumhead states of the semimetal, thereby producing two effects: (i) the algebraic decay of the surface states into the bulk; (ii) a broadening of the low energy density of surface states of the open system due to degeneracy lifting. This behavior persists with increasing disorder, up to the bulk semimetal-to-metal transition at the critical disorder $W_{c}$. Above $W_{c}$, the surface states hybridize with bulk states and become extended into the bulk.

Topological nonmediocre nodes on two-leg superconducting quantum circuits

Topological gapless systems, as the connection of the different topological quantum phases, have received much attention. Topological nonmediocre nodes are typically observed in two- or three-dimensional gapless systems. In this paper, we demonstrate that the topological nonmediocre nodes are existent in a model that lies between one dimension and two dimensions. Superconducting circuits, as essential all-solid state quantum devices, have offered a promising platform for studying the macro-controlling quantum effects. Recently, experimental achievements have enabled the realization of tunable coupling strengths between transmon qubits and the implementation of a one-dimensional Su-Schrieffer-Heeger (SSH) model [Li X <i>et al.</i> <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1103/PhysRevApplied.10.054009">2018 <i>Phys. Rev. Appl.</i> <b>10</b> 054009</ext-link>]. According to this work, herein we present a two-leg SSH model implemented in superconducting circuits and demonstrate the existence of topological nonmediocre nodes. Firstly, two-leg superconducting circuit with transmon qubits which are coupled with their nearest-neighbor sites by capacitors is designed. To construct the two-leg SSH model, we introduce two alternating-current magnetic fluxes to drive each transmon qubit. We discover two types of phase boundaries in the SSH model and obtain the corresponding energy spectra and phase diagram. We identify two distinct topological insulating phases characterized by winding number ±1, and the corresponding edge states exhibit distinct characteristics. Moreover, we discuss the topological properties of the two phase boundaries. By representing the Bloch states as a vector field in <i>k</i> space, we demonstrate the existence of two kinks of nonmediocre nodes with first-type phase boundaries. These two nonmediocrenodes possess distinct topological charges of 1 and –1, respectively. On the other hand, the nonmediocre nodes with the second-type phase boundaries are topologically trivial. These results open the way for exploring novel topological states, ladder physical systems, and nodal point topological semimetals.

Multiple-symmetry-protected lantern-like nodal walls in lithium-rich compound LiRuO2

Topological semimetals have attracted wide attention due to their potential applications, such as electronic devices and electrocatalysis. Herein, based on the first-principles calculations and symmetry analysis, we first report that ternary compound pnnm-type LiRuO2 is a typical lantern-like nodal wall semimetal. Specifically, without considering spin-orbit coupling (SOC), one-dimensional (1D) two-fold degenerate bands on the ki = ±π (i = x, y) planes form the two-dimensional (2D) topological state (namely, nodal surface) under the constraint of multiple symmetry operations. In addition, the symmetry-enforced nodal network is formed on the kz = ±π planes. Finally, these nodal networks and nodal surfaces are coupled together to form lantern-like nodal walls. Remarkably, these topological states are protected by multiple symmetries, namely, nonsymmorphic two-fold screw-rotational symmetry [S2i (i = x, y)], time-reversal symmetry (T), inversion symmetry (I), glide plane symmetry (σz), and two-fold rotational symmetry (C2x/y). In addition, we further discuss the effect of spin-orbit coupling on the lantern-like nodal walls. We find that even if LiRuO2 contains S2z and T symmetries, these nodal surfaces and nodal networks are still broken. Then, due to the existence of I and T symmetries, Dirac nodal lines and Dirac points are formed in the low-energy region. Therefore, our work indicates that LiRuO2 is an excellent material platform for researching multiple topological states.

Quench dynamics and scaling laws in topological nodal loop semimetals

We employ quench dynamics as an effective tool to probe different universality classes of topological phase transitions. Specifically, we study a model encompassing both Dirac-like and nodal loop criticalities. Examining the Kibble-Zurek scaling of topological defect density, we discover that the scaling exponent is reduced in the presence of extended nodal loop gap closures. For a quench through a multicritical point, we also unveil a path-dependent crossover between two sets of critical exponents. Bloch state tomography finally reveals additional differences in the defect trajectories for sudden quenches. While the Dirac transition permits a static trajectory under specific initial conditions, we find that the underlying nodal loop leads to complex time-dependent trajectories in general. In the presence of a nodal loop, we find, generically, a mismatch between the momentum modes where topological defects are generated and where dynamical quantum phase transitions occur. We also find notable exceptions where this correspondence breaks down completely.

Band structures and electronic properties of edge-functionalized germanene nanoribbons

Free-standing germanene is a buckled 2D hexagonal semimetal with an intrinsic carrier mobility more than double that of graphene's. The germanene lattice allows for band gap opening via an applied electric field or adsorption of foreign atoms, enabling the creation of germanene based field-effect devices. While the formation of germanene nanoribbons has been shown to have a band gap, little research has been done on the electronic tunability of Ge nanoribbons. In this study, we analyzed the effect of H, C, P, As, S, Se, Te, F, Cl, Br, and I for edge-functionalization on the physical and electronic properties of germanene armchair (AGeNRs) and zigzag (ZGeNRs) nanoribbons. We found that the band gaps of AGeNRs are tunable through width and edge species, and alternate between semiconducting and semi-metallic. Through these methods the band gap can be tuned to any IR spectral band, while ZGeNRs are shown to be nodal semimetals at small atomic widths but transition to metallic behavior at large widths. This work indicates that the functionalized germanene nanoribbons are promising quantum materials with tunable band structures and electronic properties.

Magneto-optical conductivity of nodal link semimetals

Nodal link semimetals as a type of topological semimetals are characterized by the nodal link rings in the Brillouin zone. The magneto-optical conductivity of nodal link semimetals is investigated numerically based on a system with a magnetic field. We find that in the system, the Landau levels (LLs) and magnetic field show a relationship of three halves power, which can lead to an intricate curved LLs. Due to the rich structure of LLs, we divided the LLs into four different regions to calculate the magneto-optical conductivity. We calculated the absorption peaks of longitudinal and Hall conductance of a neutral system and with different chemical potential. We find that the absorption peaks of longitudinal conductance are in a curved background in the whole range of magnetic field. When chemical potential is tuned to pass through the LLs, there are not only the intraband transition but also weak redistributed peaks in the low frequency. From the transition peaks of Hall conductance, we can find that there are several negative peaks because of the striking LLs spectrum structure. This will provide a feasible way to distinguish nodal link semimetals from other materials.

Design of a higher-order nodal-line semimetal in a spring-shaped acoustic topological crystal

The concept of higher-order topology has prompted the development of topological insulators, and a series of theoretical and experimental schemes have been proposed and realized to demonstrate higher-order hinge or corner states. While these ideas thrive in topological semimetals such as higher-order Dirac or Weyl semimetals, other degeneracy types such as nodal lines or nodal loop semimetals still remain to be fully exploited. In this work we consider a setup that hosts an acoustic springlike higher-order nodal-line semimetal protected by a screw-rotational symmetry. The underlying sandwich architecture is engineered with different coupling modes between different layers to construct a specific space group configuration, which enables the emergence of higher-order nodal lines in momentum space. Numerical calculations of the band dispersion show that nodal lines in opposite directions result in linked double hinge arcs. A corresponding topological invariant index is introduced to characterize the different topological phases. Furthermore, a propagation simulation of hinge and surface waves in a multilayer sample is performed that matches well with theoretical computations. Our work thus presents a higher-order nodal-line semimetal in an acoustic crystal and may receive practical applications in manufacturing sonic devices.