Spatial Inversion Symmetry Research Articles

NdAlSi: A magnetic Weyl semimetal candidate with rich magnetic phases and atypical transport properties

Magnetic Weyl semimetals (MWSMs) have attracted significant attention due to their intriguing physical properties and potential applications in spin-electronic devices. Here we report the characterization of NdAlSi including transport, magnetization, and heat capacity on single crystals, as well as band structure calculation. It is a newly proposed MWSM candidate which breaks both time-reversal and spatial inversion symmetries. A temperature--magnetic field phase diagram is experimentally established. Remarkably, on the angular magnetoresistance, a twofold symmetric sharp peak instead of a smooth variation is observed across the phase boundary between the ferrimagnetic and antiferromagnetic phases. We argue that the tunability of both the electronic structure and magnetic properties in NdAlSi is crucial for realizing such a behavior. Our results indicate that $4f$-electron-based MWSM could provide a unique platform to explore new and intriguing quantum phenomena arising from the interaction between magnetism and electron topology.

Analysis of model-parameter dependences on the second-order nonlinear conductivity in PT -symmetric collinear antiferromagnetic metals with magnetic toroidal moment on zigzag chains

A magnetic toroidal moment is a fundamental electronic degree of freedom in the absence of both spatial inversion and time-reversal symmetries and gives rise to novel multiferroic and transport properties. We elucidate essential model parameters of the nonlinear transport in the space-time ($\mathcal{PT}$) symmetric collinear antiferromagnetic metals accompanying a magnetic toroidal moment. By analyzing the longitudinal and transverse components of the second-order nonlinear conductivity on a two-dimensionally stacked zigzag chain based on the nonlinear Kubo formula, we show that an effective coupling between the magnetic toroidal moment and the antisymmetric spin-orbit interaction is an essential source of the nonlinear conductivity. Moreover, we find that the nonreciprocal longitudinal current and nonlinear transverse current in a multi-band system are largely enhanced just below the transition temperature of the antiferromagnetic ordering. We also discuss the relevance of the nonlinear conductivity to the linear magnetoelectric coefficient and conductivity. Our result serves as a guide for exploring microscopic essence and clarifying the parameter dependence of the nonlinear conductive phenomena in ferrotoroidal metals.

Experimental and first-principles studies of superconductivity in topological nodal line semimetal SnTaS2

We report a detailed study of superconductivity in polycrystalline SnTaS2 using electrical transport, magnetization and heat capacity measurements. SnTaS2 crystallizes in centrosymmetric hexagonal structure with space group . Electrical resistivity, magnetization and specific heat data suggest SnTaS2 to be a weakly coupled, type-II superconductor with K. First-principles calculations show signature for nodal line topology in the electronic band structure, protected by the spatial-inversion and time-reversal symmetries, that strongly gapped out by the inclusion of spin–orbit coupling. Superconductivity in layered SnTaS2 with nodal line topological state makes it a strong candidate to be considered for a 3D topological superconductor.

Theory of Emergent Inductance with Spin-Orbit Coupling Effects.

We extend the theory of emergent inductance, which has recently been discovered in spiral magnets, to arbitrary magnetic textures by taking into account spin-orbit couplings arising in the absence of spatial inversion symmetry. We propose a new concept of spin-orbit emergent inductance, which can be formulated as originating from a dynamical Aharonov-Casher phase of an electron in ferromagnets. The spin-orbit emergent inductance universally arises in the coexistence of magnetism and the spin-orbit couplings, even with spatially uniform magnetization, allowing its stable operation in wide ranges of temperature and frequency. Revisiting the widely studied systems involving ferromagnets with spatial inversion asymmetry, with the new perspective offered by our work, will lead to opening a new paradigm in the study of spin-orbit physics and the spintronics-based power management in ultrawideband frequency range.

Topological nodal line semimetal in an all-sp 2 monoclinic carbon

Topological nodal line semimetal is an exotic class of quantum materials featuring the continuous line of nodes inside the first Brillouin zone. Here we identify by systematical ab initio calculations a new all-sp 2 hybridized carbon allotrope with monoclinic C2/c symmetry which is termed as bcm-C16. Total energy calculations show that our proposed bcm-C16 carbon is energetically comparable to or stable than the previously proposed bco-C16, bct-C16, and oP16 carbon. Its dynamical stability has been confirmed by phonon mode calculations. Detailed analysis of the electronic properties show that bcm-C16 carbon is a topological nodal line semimetal with a single closed nodal ring around the Γ high symmetric point, protected by spatial inversion and time-reversal symmetry. When the nodal ring is projected onto the (001) surface, a topologically protected drumhead-like surface state can be seen inside or outside the nodal ring depending on the different surface terminations. Moreover, we also examined the tensile-strain robustness of the electronic properties of bcm-C16 carbon. The nodal ring is robust under a tensile-strain along the crystalline x- and z-directions up to 20%. In addition, the simulated x-ray diffraction pattern (XRD) of bcm-C16 carbon matches with the experimental pattern found in the detonation and chimney soot experiment. The present proposal has enriched the family of carbon allotropes with topological nodal lines, and pave the way for further theoretical and experimental studies.

Valley modulation and single-edge transport of magnons in breathing kagome ferromagnets

Owing to charge-free property, magnon is promising to achieve dissipationless transport without Joule heating. Valley degree of freedom has also been fully expressed in electronic structures, enabling information coding and manipulation via valley-based qubits. Considering the conceptual importance and application potential, it is highly desirable to couple valley to magnon excitation and realize their modulations. We demonstrate valley magnons and valley modulations in a kagome ferromagnetic lattice, with Dzyaloshinskii-Moriya and staggered exchange interactions. The staggered exchange interaction breaks spatial inversion symmetry, leading to magnon valley Hall effect. When the Dzyaloshinskii-Moriya interaction is further included, the valley degeneracy is lifted, exhibiting net anomalous Hall effect and a series of topological phase transitions. Single-edge heat transport of magnons can occur, with propagating currents along the edge and local circulating currents. These findings give full play to spin and valley degrees of freedom and add another dimension to magnonic device paradigms.

Interfacial Pockels Effect of Solvents with a Larger Static Dielectric Constant than Water and an Ionic Liquid on the Surface of a Transparent Oxide Electrode

The optical Pockels effect is a change in the refractive index proportional to an applied electric field. As a typical example of the interfacial Pockels effect occurring at interfaces where the spatial inversion symmetry is broken, it is known that water in the electric double layer (EDL) on the transparent oxide electrode surface has a large Pockels coefficient, but the physical factors that determine its size are not clear. Therefore, we experimentally studied the Pockels effect of water and other characteristic liquids—formamide (FA), methylformamide (NMF) (these two have larger static dielectric constants than water), dimethylformamide (DMF), and an ionic liquid that is itself salts (IL, [BMIM] [BF4])—and evaluated their Pockels coefficients in the EDL on the transparent electrode surface. The magnitude of the Pockels coefficient was found to be in the order of water, DMF, FA, NMF, and IL, with the magnitude of the static dielectric constant not being an important factor.

Valley Hall Elastic Edge States in Locally Resonant Metamaterials.

This paper presents a locally resonant metamaterial periodically rearranged as a local resonator, that is hexagonal holes arranged in a thin plate replace the elastic local resonator to achieve the quantum valley Hall effect. Due to the symmetry in the primitive hexagonal lattice, one Dirac point emerges at high symmetry points in the Brillouin zone in the sub-wavelength area. Rotating the beam element of the resonator can break the spatial inversion symmetry to lift the Dirac degeneracy and form a new bandgap. Thus, the band inversion is discovered by computing the relationship between the associated bandgap and the rotational parameter. We also confirmed this result by analyzing the vortex chirality and calculating the Chern number. We can discover two kinds of edge states in the projected band obtained by computing the supercell composed of different topological microstructures. Finally, the propagation behavior in various heterostructures at low frequencies was analyzed. It is shown that these valley Hall elastic insulators can guide elastic waves along sharp interfaces and are immune to backscattering from defects or disorder. By utilizing elastic resonators, a simple reconfigurable topological elastic metamaterial is realized in the sub-wavelength area.

Coexistence of topological type-II Weyl and triply degenerate points in a chiral photonic metamaterial

In this work, we report the coexistence of type-II Weyl points and triply degenerate points while taking the nonlocal effect into account in a chiral photonic metamaterial system. The chiral effect introduces a break in the spatial inversion symmetry, which is necessary for the generation of nodal points. These nodal points are symmetrically distributed on the ${k}_{z}$ axis and possess the time-reversal symmetry-protection mechanism. Remarkably, the projections of all nodal points are connected by a Fermi arc surface state, and it agrees well with the single monopole charges of these nodal points, which demonstrates the topological characteristics of the band degenerate points. We theoretically show that the localized Fermi arc can be formed at the interface between the metamaterial and vacuum, which may improve the compactness of photonic devices. Especially, the nonreciprocal surface waves can propagate forward around the sharp corner without experiencing backscattering, which can be used for the robust transmission of information.

Proximity-driven ferromagnetism and superconductivity in the triangular Rashba-Hubbard model

Bilayer Moir\'e structures are a highly tunable laboratory to investigate the physics of strongly correlated electron systems. Moir\'e transition metal dichalcogenides at low-energies, in particular, are believed to be described by a single narrow band Hubbard model on a triangular lattice with spin-orbit coupling. Motivated by recent experimental evidence for superconductivity in twisted bilayer materials, we investigate the possible superconducting pairings in a two-dimensional single band Rashba-Hubbard model. Using a random-phase approximation in the presence of nearest and next-nearest neighbor hopping, we analyze the structure of spin fluctuations and the symmetry of the superconducting gap function. We show that Rashba spin-orbit coupling favors ferromagnetic fluctuations which strengthen triplet superconductivity. If parity is violated due to the absence of spatial inversion symmetry, singlet (d-wave) and triplet (p-wave) channels of superconductivity will be mixed. Moreover, we show that time-reversal symmetry can be spontaneously broken leading to a chiral superconducting state. Finally, we consider quasiparticle interference as a possible experimental technique to observe the superconducting gap symmetry.

Observation of Antiferromagnetic Order as Odd-Parity Multipoles inside the Superconducting Phase in CeRh_{2}As_{2}.

Spatial inversion symmetry in crystal structures is closely related to the superconducting (SC) and magnetic properties of materials. Recently, several theoretical proposals that predict various interesting phenomena caused by the breaking of the local inversion symmetry have been presented. However, experimental validation has not yet progressed owing to the lack of model materials. Here we present evidence for antiferromagnetic (AFM) order in CeRh_{2}As_{2} (SC transition temperature T_{SC}∼0.37 K), wherein the Ce site breaks the local inversion symmetry. The evidence is based on the observation of different extents of broadening of the nuclear quadrupole resonance spectrum at two crystallographically inequivalent As sites. This AFM ordering breaks the inversion symmetry of this system, resulting in the activation of an odd-parity magnetic multipole. Moreover, the onset of antiferromagnetism T_{N} within an SC phase, with T_{N}<T_{SC}, is quite unusual in systems wherein superconductivity coexists or competes with magnetism. Our observations show that CeRh_{2}As_{2} is a promising system to study how the absence of local inversion symmetry induces or influences unconventional magnetic and SC states, as well as their interaction.

Elastic valley Hall edge wave in a hierarchical hexagonal lattice

By exploiting topological phases, a new approach can be taken to design elastic waveguides with robust wave propagation against sharp bends or defects. In particular, the topological phase based on the quantum valley Hall effect, or valley topological phase, is an attractive candidate for elastic waveguide because its mechanism can be realized in a relatively simple way. In this work, we report a waveguide design with in-plane dynamics based on the acoustic valley Hall effect, leveraging a hierarchical hexagonal lattice built by replacing the vertices of a regular hexagonal lattice with smaller hexagons. These are purposely introduced because they provide better structural performance than the regular hexagonal lattice. We modified the size of the adjacent smaller hexagons using two different values to break spatial inversion symmetry, which leads to topologically distinct lattices depending on the sign of the modification parameter and the emergence of the valley-dependent edge modes at the interface of two lattice structures. We numerically demonstrated the robust propagation of the valley-dependent edge states through sharp bends. Notably, we aggressively exploited the ease of tuning the geometric parameters to design frequency-tailorable edge modes, to demonstrate wave splitting and frequency filtering with the waveguide. The proposed structure can be a pathway for practically utilizing the elastic topological phase, and can also widen the scope of multifunctional (elastic wave control and load-bearing) structures to include the topological phase.

Weyl nodes with higher-order topology in an optically driven nodal-line semimetal

Creating and manipulating topological states is a key goal of condensed matter physics. Periodic driving offers a powerful method to manipulate electronic states, and even to create topological states in solids. Here, we investigate the tunable Floquet states in a periodically driven higher-order nodal line semimetal with both spatial inversion and time-reversal symmetries. We found that the Floquet Weyl semimetal states, which support both one-dimensional hinge Fermi arc and two-dimensional surface Fermi arc states, can be induced in the higher-order nodal-line semimetal by shining circularly polarized light. Moreover, we show that the location of Weyl nodes and the curvature of surface Fermi arcs can be tuned by adjusting the propagation direction and incident angle of light.

Confirming the trilinear form of the optical magnetoelectric effect in the polar honeycomb antiferromagnet Co2Mo3O8

Magnetoelectric phenomena are intimately linked to relativistic effects and also require the material to break spatial inversion symmetry and time-reversal invariance. Magnetoelectric coupling can substantially affect light–matter interaction and lead to non-reciprocal light propagation. Here, we confirm on a fully experimental basis, without invoking either symmetry-based or material-specific assumptions, that the optical magnetoelectric effect in materials with non-parallel magnetization (M) and electric polarization (P) generates a trilinear term in the refractive index, δn ∝ k ⋅ (P × M), where k is the propagation vector of light. Its sharp magnetoelectric resonances in the terahertz regime, which are simultaneously electric and magnetic dipole active excitations, make Co2Mo3O8 an ideal compound to demonstrate this fundamental relation via independent variation of M, P, and k. Remarkably, the material shows almost perfect one-way transparency in moderate magnetic fields for one of these magnetoelectric resonances.

Essential model parameters for nonreciprocal magnons in multisublattice systems

We theoretically investigate the microscopic conditions for emergent nonreciprocal magnons toward unified understanding on the basis of a microscopic model analysis. We show that the products of the Bogoliubov Hamiltonian obtained within the linear spin wave approximation is enough to obtain the momentum-space functional form and the key ingredients in the nonreciprocal magnon dispersions in an analytical way even without solving the eigenvalue problems. We find that the odd order of an effective antisymmetric Dzyaloshinskii-Moriya interaction and/or the even order of an effective symmetric anisotropic interaction in the spin rotated frame can be a source of the antisymmetric dispersions. We present possible kinetic paths of magnons contributing to the antisymmetric dispersions in the one- to four-sublattice systems with the general exchange interactions. We also test the formula for both ferromagnetic and antiferromagnetic orderings in the absence of spatial inversion symmetry.

Detection and tuning of spin-orbit interactions on inclined-grown Bi2O2Se nanoplates

Inclined semiconductor Bi2O2Se nanoplates of a high crystal quality were prepared via chemical vapor deposition, and a circular photogalvanic effect (CPGE) current was detected in them. The CPGE current in the Bi2O2Se nanoplate reached 100 nA/W, which is far beyond what has been found in other two-dimensional (2D) materials, e.g., Bi2Se3, WSe2, and Sb2Te3, as well as some 2D electron systems such as AlxGa1-xN/GaN heterostructures, indicating the large spin–orbit coupling (SOC) in inclined Bi2O2Se. To tune the strength of the SOC, an ionic liquid gate was applied to the Bi2O2Se nanoplate. The significant increase in the CPGE current that appeared with the application of the positive gate voltage indicates that the gate can further break spatial inversion symmetry, leading to enhanced SOC. Unlike the in-plane Bi2O2Se, the bottom surface of which has electrostatic interactions with the mica substrate and hence is naturally asymmetric with the top surface, the inclined-grown nanoplate has two Bi2O2Se-air interfaces. Therefore, the large CPGE current in the inclined Bi2O2Se shows that strong SOC is intrinsic to the Bi2O2Se lattice instead of the interfaces, and Bi2O2Se is a prospective material in the application of spintronic devices.

Robust pure spin current induced by the photogalvanic effect in half-silicane with spatial inversion symmetry.

The spin-dependent photogalvanic (PG) effect in low-dimensional spin semiconductors has attracted great interest recently. Here, we have studied the spin semiconducting feature and spin-dependent photocurrent in a two-dimensional (2D) silicene-based device with spatial inversion symmetrical half-hydrogenation, in which half of the silicene is hydrogenated on the upper surface and half is hydrogenated on the lower surface. Because of the unique spin semiconductor properties and symmetry of the system, pure spin current can be robustly produced in both the zigzag and armchair directions for linearly and elliptically polarized light. The behavior of the spin-dependent photoresponse in the spin PG effect is highly anisotropic and can be tuned by the polarization/phase angles or photon energy (Eph). Moreover, the produced pure spin current in such a half-silicane device with spatial inversion symmetry via the PG effect is several orders of magnitude larger than that obtained in metal/semiconductor/metal systems. These findings suggest a promising approach for generating pure spin current by the PG effect and provide a new possibility for the application of 2D half-silicane in spintronics.

Carbon phosphide nanoribbons with spatial inversion symmetry: robust generators of pure spin current with a photogalvanic effect.

Our recent work has demonstrated that the spin-dependent photogalvanic effect (PGE) is an ideal way to induce pure spin current in certain centrosymmetric systems (X. Tao, P. Jiang, H. Hao, X. Zheng, L. Zhang and Z. Zeng, Phys. Rev. B, 2020, 102, 081402), and thus the search for appropriate materials or structures is the key to realize it. Here, we theoretically propose a spin optoelectric device with α-phase carbon phosphide nanoribbons (α-CPNRs) to generate pure spin current by PGE using density functional theory simulation. By designing an α-CPNR based device with a centrosymmetric structure, due to the structural inversion symmetry and real space spin polarization antisymmetry of the system, the PGE induced pure spin current without any accompanying charge current can be always obtained, independent of polarization type and polarization angle of the photons. Furthermore, the magnitude of pure spin current can be greatly increased (nearly by an order of magnitude) by applying antiparallel electrical fields in the two leads to tune the spin density and band structure. Furthermore, by applying parallel electrical fields, a fully spin-polarized photocurrent can be produced in this system, suggesting a fantastic scheme to achieve half-metallic transport, another important goal in spintronics. These investigations suggest that the optoelectric devices constructed by α-CPNRs will have great potential in spintronics.

Giant and tunable Rashba spin splitting in MoS2/Bi2Te3 heterostructures

The study of the spin-orbit coupling (SOC) effect is an active research field in physics and it is of great interest for materials with large Rashba spin splitting (RSS) in spintronics applications. In this report, based on first-principles calculations, we propose the heterostructure of MoS2/Bi2Te3 with giant RSS duo to the lack of spatial inversion symmetry and the intrinsic electric field perpendicular to the heterostructure. Furthermore, it is found that the electronic and RSS can be tuned effectively by layers of MoS2 and Bi2Te3. When the Bi2Te3 thickness is less than 2QL, these MoS2/Bi2Te3 heterojunction systems exhibit semiconductor properties. The magnitude of the RSS in these MoS2/Bi2Te3 heterojunctions changes with the thickness, and the largest band splitting is found in the (MoS2)1(Bi2Te3)2 heterostructure with a Rashba coefficient of ~2.5 eV Å, which is a very large value among two-dimensional (2D) materials. While the interlayer distance and charge transfer remain stable at about 3.5 Å and 0.1 e, respectively. Through layer-projected band structure analysis, it is proven that the electronic states of these quantum well states (QWSs) are mainly contributed by Bi2Te3, while the MoS2 layer only plays an inductive role and does not contribute to the QWSs electrons. In addition, strain only plays a minor role in the Rashba effect in the MoS2/Bi2Te3 heterojunction, which is mainly caused by the breaking of the spatial inversion symmetry. The giant and tunable Rashba-type SOC of the MoS2/Bi2Te3 heterostructure suggests that it is a type of promising materials for spintronic applications.

Functionalized tellurene; a candidate large-gap 2D topological insulator

The discovery of group IV and V elemental xene’s with topologically non-trivial characters in their honeycomb lattice structure (HLS) has led to extensive efforts in realising analogous behaviour in group VI elemental monolayers. Theoretically; it was concluded that, group VI elemental monolayers cannot exist in HLS. However, some recent experimental evidence suggests that group VI elemental monolayers can be realised in HLS. In this letter, we report HLS of group VI elemental monolayer (such as, tellurene) can be realised to be dynamically stable when functionzalised with oxygen. The functionalization leads to, peculiar orbital filtering effects and broken spatial inversion symmetry which gives rise to the non-trivial topological character. The exotic quantum behaviour of this system is characterized by, spin–orbit coupling induced large-gap (≈0.36 eV) with isolated Dirac cone along the edges indicating potential room temperature spin-transport applications. Further investigations of spin Hall conductivity and the Berry curvatures unravel high conductivity as compared to previously explored xene’s alongside the potential valley Hall effects. The non-trivial topological character is quantified in terms of the invariant as ν = 1 and Chern number C = 1. Also, for practical purposes, we report that, hBN/TeO/hBN quantum-wells can be strain engineered to realize a sizeable non-trivial gap (≈0.11 eV). We finally conclude that, functionalization of group VI elemental monolayer with oxygen gives rise to, exotic quantum properties which are robust against surface oxidation and degradations while providing viable electronic degrees of freedom for spintronic/valleytronic applications.