Broken Spatial Inversion Symmetry Research Articles

High Photocurrent Anisotropy in Domain‐Engineered Ferroelectrics for Visible‐Light Polarization Detection

AbstractVisible‐light polarization detection is a critical challenge for next‐generation light communication and imaging technologies. While low‐dimensional nanomaterials exhibit polarization‐dependent photocurrents originating from their anisotropic absorption, ferroelectric photovoltaics provide a photocurrent derived from spatial inversion symmetry breaking. Here, it is reported that domain‐engineered ferroelectrics are promising materials for visible‐light polarization detection. Polarization angle‐resolved photocurrent measurements demonstrate that the introduction of domain walls in ferroelectric BiFe0.95Mn0.05O3 films increases the photocurrent anisotropy ratio up to 7.9 under visible light with hν = 2.4 eV (λ = 515 nm), which is much higher than that of typical nanomaterials (≈2–3). Simulations of polarization‐dependent photocurrents show that the domain walls provide an additional degree of freedom for designing ferroelectrics with quite high photocurrent anisotropy. This study opens up the possibility of developing ferrophotovoltaic‐based visible‐light polarization detectors with simple device structures.

Current-driven dynamics of skyrmions in the presence of pinning at finite temperatures

Magnetic skyrmions are topological solitons in magnetic materials with broken spatial inversion symmetry. They offer potential as an information carrier for future low-power memory and computing systems owing to the small size and relatively low driving current compared to traditional devices. To develop applications such as racetrack memory at ambient temperature, the interaction of current-driven motion, pinning and thermal fluctuations must be investigated. The skyrmion dynamics at finite temperatures are shown using the expanded Thiele equation, and the relationship between critical depinning current and temperature is established with regard to different damping constants. Furthermore, we found that the Thiele equation still is insufficient to describe the dynamics of skyrmion precisely at low temperatures with the presence of pinning. Moreover, we have shown that the skyrmion Hall angle is dependent on the driving current in the presence of the pinning effect at various finite temperatures.

Defect-gradient-induced Rashba effect in van der Waals PtSe2 layers

Defect engineering is one of the key technologies in materials science, enriching the modern semiconductor industry and providing good test-beds for solid-state physics. While homogenous doping prevails in conventional defect engineering, various artificial defect distributions have been predicted to induce desired physical properties in host materials, especially associated with symmetry breakings. Here, we show layer-by-layer defect-gradients in two-dimensional PtSe2 films developed by selective plasma treatments, which break spatial inversion symmetry and give rise to the Rashba effect. Scanning transmission electron microscopy analyses reveal that Se vacancies extend down to 7 nm from the surface and Se/Pt ratio exhibits linear variation along the layers. The Rashba effect induced by broken inversion symmetry is demonstrated through the observations of nonreciprocal transport behaviors and first-principles density functional theory calculations. Our methodology paves the way for functional defect engineering that entangles spin and momentum of itinerant electrons for emerging electronic applications.

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.

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.

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.

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.

Prediction of unconventional magnetism in doped FeSb2

It is commonly believed that the energy bands of typical collinear antiferromagnets (AFs), which have zero net magnetization, are Kramers spin-degenerate. Kramers nondegeneracy is usually associated with a global time-reversal symmetry breaking (e.g., via ferromagnetism) or with a combination of spin-orbit interaction and broken spatial inversion symmetry. Recently, another type of spin splitting was demonstrated to emerge in some collinear magnets that are fully spin compensated by symmetry, nonrelativistic, and not even necessarily noncentrosymmetric. These materials feature nonzero spin density staggered in real space as seen in traditional AFs but also spin splitting in momentum space, generally seen only in ferromagnets. This results in a combination of materials characteristics typical of both ferromagnets and AFs. Here, we discuss this recently discovered class with application to a well-known semiconductor, FeSb2, and predict that with certain alloying, it becomes magnetic and metallic and features the aforementioned magnetic dualism. The calculated energy bands split antisymmetrically with respect to spin-degenerate nodal surfaces rather than nodal points, as in the case of spin-orbit splitting. The combination of a large (0.2-eV) spin splitting, compensated net magnetization with metallic ground state, and a specific magnetic easy axis generates a large anomalous Hall conductivity (∼150 S/cm) and a sizable magnetooptical Kerr effect, all deemed to be hallmarks of nonzero net magnetization. We identify a large contribution to the anomalous response originating from the spin-orbit interaction gapped anti-Kramers nodal surfaces, a mechanism distinct from the nodal lines and Weyl points in ferromagnets.

Ideal type-II Weyl phases and surface states for elastic waves in three-dimensional solid phononic crystals

The study of Weyl semimetals (WSs) has been attracting widely attention due to the fact that the Weyl point (WP) and associated topological invariants enable WSs to exhibit a variety of unusual properties, including robust surface wave and chiral anomaly. However, for the elastic analogue of ideal type-II WS, having the strongly tilted cone dispersion and conical Fermi surface, its realization still remains elusive. Here, we show an elastic type-II WS in three-dimensional (3D) solid phononic crystals (PnCs) with the broken spatial inversion symmetry. The ideal type-II WPs with opposite topological charges (±1), opened Fermi-arc-like surface states and associated robust transport for 3D elastic wave are demonstrated. Furthermore, we reveal the phase transition between the elastic type-II WSs and two valley topological insulators (TIs). At the domain wall of two 3D PnCs with different valley topological phase, the Fermi-circle-like surface states are obtained. Our research provides a platform for the exploration of Weyl physics on a macroscopic mechanical structure.

Electrically-tuned transition of band alignment in arsenene/MoTe[formula omitted] van der Waals heterostructures

Based on the first principles calculations, the electronic properties of single layer arsenene (As), MoTe2 and their van der Waals (vdW) heterostructures are studied. It shows that the electronic properties, especially the bandgaps of As/MoTe2 vdW heterostructures are effectively tuned by applied external fields due to the strong in-build electric field and the inter-layer electron transferring. The absorptions of H/O atom on the heterostructures are clearly tuned by the external electric fields. The transition from type I to type II band alignment in such heterostructures are realized by varying the applied electric field owing to the spatial inversion symmetry breaking. Our study provides the possibility for the applications of novel vdW heterostructures based on As/MoTe2.

Effect of symmetry breaking on short-wavelength acoustic phonons in the chiral magnet MnSi

We have investigated the effects of spatial-inversion and time-reversal symmetry breaking on the acoustic phonon branches in chiral MnSi using high-resolution inelastic x-ray scattering and first-principles calculations. We find a momentum-transfer-dependent ( $q$ -dependent) splitting between transverse phonon bands having angular momentum parallel or antiparallel to $q$ . This is understood by a phenomenological theory using a gyrotropic tensor. We observed no significant impact from the time-reversal symmetry breaking induced by a magnetic field (energy shifts 0.3 meV). This suggests the effect of time-reversal symmetry breaking is small or is restricted to a very-low-energy regime in this material, possibly due to small spin-orbit interaction.

Tailoring Dzyaloshinskii\u2013Moriya interaction in a transition metal dichalcogenide by dual-intercalation

Dzyaloshinskii–Moriya interaction (DMI) is vital to form various chiral spin textures, novel behaviors of magnons and permits their potential applications in energy-efficient spintronic devices. Here, we realize a sizable bulk DMI in a transition metal dichalcogenide (TMD) 2H-TaS2 by intercalating Fe atoms, which form the chiral supercells with broken spatial inversion symmetry and also act as the source of magnetic orderings. Using a newly developed protonic gate technology, gate-controlled protons intercalation could further change the carrier density and intensely tune DMI via the Ruderman–Kittel–Kasuya–Yosida mechanism. The resultant giant topological Hall resistivity {rho }_{{xy}}^{T} of 1.41{mathrm{mu}} Omega cdot {{mathrm{cm}}} at {V}_{g}=-5.2{mathrm{V}} (about 424 % larger than the zero-bias value) is larger than most known chiral magnets. Theoretical analysis indicates that such a large topological Hall effect originates from the two-dimensional Bloch-type chiral spin textures stabilized by DMI, while the large anomalous Hall effect comes from the gapped Dirac nodal lines by spin–orbit interaction. Dual-intercalation in 2H-TaS2 provides a model system to reveal the nature of DMI in the large family of TMDs and a promising way of gate tuning of DMI, which further enables an electrical control of the chiral spin textures and related electromagnetic phenomena.

Nonreciprocal electronic transport in PdCrO2 : Implication of spatial inversion symmetry breaking

Noncollinear spin structures, such as helimagnetic structures, have been gathering attention because of their nontrivial magnetic transport induced by topological effects. For a precise determination of spin structure, in addition to traditional neutron diffraction measurements, the time-reversal-odd rectification of electronic transport, which is referred to as nonreciprocal electronic transport, provides useful information because of the symmetry sensitivity. We observed nonreciprocal electronic transport below the N\'eel temperature in an itinerant antiferromagnet, $\mathrm{Pd}\mathrm{Cr}{\mathrm{O}}_{2}$, with a ${120}^{\ensuremath{\circ}}$ spin structure. This implies breaking of spatial inversion symmetry owing to the magnetic ordering. Possible spin structures are discussed based on the magnetic field dependence of nonreciprocal transport.

Narrow bandgap ferroelectric semiconductors within BiFeO<sub>3</sub>-based solid solution perovskites

<p indent=0mm>Solar energy is a clean and renewable source of energy. Various solar photovoltaic (PV) cell technologies have been developed, out of which perovskite PV cell technology is growing rapidly. Using organometal halide perovskite as an optical absorption component, the power conversion efficiency (PCE) of heterojunction solar PV cells increased from 3.8% in 2009 to 25.5% recently. Nonetheless, the bottleneck for CH₃NH₃PbI₃-based solar cells entering into commercial market is their poor stabilities relating to environmental, thermal, and ultraviolet light influence. In contrast, transition metal oxide perovskites are thermodynamically and chemically stable enough for solar cells’ commercialization, with more advantages such as bandgap (<italic>E</italic>_g) adjustability from <sc>0 to 6 eV,</sc> processing being compatible with transparent conducting oxides, and ferroelectric bulk PV effect. Owing to a wide <italic>E</italic>_g, traditional ferroelectric perovskite oxides exhibit poor visible optical absorption, low electrical conductivity, and thus extremely low PCE. As an alternative to p-n junction PV effect, ferroelectric bulk PV effect provides a new separation mechanism for photo-excited carriers, which is closely related to spatial inversion symmetry breaking and its resulting spontaneous electric polarization. Different from p-n junction, the active space for photo-excited carrier separation spans the whole ferroelectric body, and thus an above-<italic>E</italic>_g open-circuit voltage is produced. By narrowing <italic>E</italic>_g while maintaining ferroelectricity of oxide perovskites, ferroelectric semiconductors become available to monolithically integrate the p-n junction and ferroelectric bulk PV effects, which could, in principle, go beyond the Shockley-Queisser theoretical PCE limit of conventional<italic> </italic>p-n junction solar cells. Bismuth ferrite (BiFeO₃) has been experimentally demonstrated to exhibit ferroelectric PV effect. Meanwhile, it has a high ferroelectric Curie temperature (<italic>T</italic>_C) of 830°C and an intermediate <italic>E</italic>_g of <sc>~2.2 eV,</sc> providing a large chemical space for solid solution perovskite oxides to trade off both target properties of narrow <italic>E</italic>_g and <italic>T</italic>_C><sc>400 K.</sc> In this report, 0.50BiFeO₃-0.25A₁MnO₃-0.25A₂TiO₃ (A₁ = Ca, Sr, Ba, A₂ = Sr, Ba, Pb) and 0.49BiFeO₃-0.26BaTiO₃-0.25(Sr₁₋_{<italic>x</italic>}Ba_{<italic>x</italic>})(Co_1/3Nb_2/3)O₃ (<italic>x </italic>= 0 and 0.4) ferroelectric semiconductors with <italic>E</italic>_g of <sc>~0.9 eV</sc> were created by substituting Fe³⁺ with Mn⁴⁺ or Co²⁺ in BiFeO₃-based solid solution perovskites. Ceramic samples were prepared using a refined solid-state reaction electroceramic processing. X-ray diffraction measurements showed them crystallized in a single pseudo-cubic perovskite phase, while Raman scattering characterizations illustrated a breaking of spatial inversion symmetry at room temperature. Optical absorbance measurements found that these Mn⁴⁺- or Co²⁺-substituted BiFeO₃-based solid solution perovskites have a direct bandgap in a range of <sc>0.75–1.0 eV.</sc> Compared with CH₃NH₃PbI₃ perovskites, their optical absorption reaches near-infrared band of solar spectra. Temperature-dependent resistance measurements showed their resistivity on the magnitude of order of <sc>~10⁶ Ω cm</sc> with thermal excitation energy (<italic>E</italic>_a) of <sc>~0.5 eV.</sc> Combined with observation of frequency-dependent dielectric properties, A-site vacancies were proposed responsible for electrical conduction and dielectric relaxation. Through data-mining causal relationships between <italic>E</italic>_g and the filling number of d electron of B-site cations, <italic>μ</italic>×<italic>r</italic>_A/<italic>r</italic>_B ensemble descriptor of oxide perovskites (<italic>μ</italic>, <italic>r</italic>_A, and <italic>r</italic>_B are the reduced mass of primitive cell, A-site cation radius, and B-site cation radius, respectively), a physical model was proposed to predictively design chemical compositions of ferroelectric semiconducting oxide perovskites with <italic>E</italic>_g<sc>~0.9 eV</sc> for applications of solar PV cells. This essay provides an opportunity for developing novel solar PV cells to integrate monolithically p-n junction and ferroelectric bulk PV effects, with PCE beyond the Shockley-Queisser theoretical limit.

Optical-field induced SU(2) pair potential in caesium lead halide perovskites

Motivated by the controversial bright exciton splittings in inorganic cesium lead halide perovskite nanocrystals, we studied excitation state of perovskite CsPbBr3 by four-band model Hamiltonian. With spin–orbit coupling, the calculated band structure shows that the double degeneracy of each band around [Formula: see text] point is removed because of broken spatial inversion symmetry (SIS). With broken SIS, ring-shaped valence band maximum is formed in the band structure of ground state, and SU(2) vector potential in momentum is created with the excitation of one electron from valence band to conduction band by photon absorption. In the case of low density carrier, our theory also predicts that the energy splitting between the four binding electron-hole states by the vector potential is proportional to the power of laser light which certainly stimulates further experimental work in this intriguing topic.

High laser harmonics induced by the Berry curvature in time-reversal invariant materials

A nonlinear scheme of high-harmonic generation in a wide class of time-reversal invariant materials with broken spatial inversion symmetry (for which, recently, the nonlinear Hall effect was established) due to the nontrivial topology of bands is proposed. A microscopic quasiclassical theory describing the nonperturbative optical response of pseudorelativistic electrons with nonzero Berry curvature of bands to a strong laser field is developed. We analyze the harmonic content of the induced current and show that one can decouple induced laser harmonics solely by the Berry curvature of bands. We also study the dependence of the nonlinear response on the driving wave and system parameters.

Nonreciprocal thermal transport in a multiferroic helimagnet.

Breaking of spatial inversion symmetry induces unique phenomena in condensed matter. In particular, by combining this symmetry with magnetic fields or another type of time-reversal symmetry breaking, noncentrosymmetric materials can be made to exhibit nonreciprocal responses, which are responses that differ for rightward and leftward stimuli. However, the effect of spatial inversion symmetry breaking on thermal transport in uniform media remains to be elucidated. Here, we show nonreciprocal thermal transport in the multiferroic helimagnet TbMnO3 The longitudinal thermal conductivity depends on whether the thermal current is parallel or antiparallel to the vector product of the electric polarization and magnetization. This phenomenon is thermal rectification that is controllable with external fields in a uniform crystal. This discovery may pave the way to thermal diodes with controllability and scalability.

Dzyaloshinskii–Moriya Interaction between Multipolar Moments in 5d1 Systems

We propose a new type of Dzyaloshinskii-Moriya (DM) interactions which act on high-rank multipolar moments such as quadrupolar and octupolar moments. Here we consider 5d1 systems with broken spatial inversion symmetry, where the interplay of electron correlation, the spin-orbit coupling, and inversion symmetry breaking plays a crucial role. Using a numerical diagonalization on a two-site multiorbital Hubbard model, we reveal that anti-symmetric products of multipole operators have finite expectation values, indicating the existence of DM interactions for multipoles. We also find that the spin-orbit coupling dependences of DM interactions for multipoles are significantly different depending on the lattice structure. Finally, we discuss the numerical results for small and large spin-orbit coupling region by using perturbative analysis.

Topological Valley Hall Edge State Lasing

AbstractTopological lasers based on topologically protected edge states exhibit unique features and enhanced robustness of operation in comparison with conventional lasers, even in the presence of disorder, edge deformation, or local defects. Here a new class of topological lasers arising from the valley Hall edge states is proposed, which does not rely on magnetic fields. Specifically, topological lasing occurs at domain walls between two honeycomb waveguide arrays with broken spatial inversion symmetry. Two types of valley Hall edge lasing modes are found by shaping the gain landscape along the domain walls. In the presence of uniform losses and two‐photon absorption, the lasing results in the formation of stable nonlinear dissipative excitations localized on the edge of the structure, even if it has complex geometry and even if it is finite. Robustness of lasing edge states is demonstrated in both periodic and finite structures, where such states can circumvent certain corners without scattering losses or radiation into the bulk, as long as the intervalley scattering is absent. The photonic structure and mechanism proposed here for topological lasing is fundamentally different from those previously employed in topological lasers, and can be used for fabrication of practical topological lasers of various geometries.

Reentrant superconductivity in $$\hbox {YBa}_{2}\hbox {Cu}_{3}\hbox {O}_{7-\delta }$$ microstructured particles

A reentrant superconducting effect was observed in YBCO particles as a consequence of their size and microstructure. It rises from the coexistence between bulk superconductivity and surface ferromagnetism, where the enhanced magneto-electric coupling induced by broken spatial-inversion symmetry in layers near structural defects plays a very important role on surface ferromagnetism. The study assumes grain boundaries inside the particles act as additional surface regions that enhance the ferromagnetic properties. Magnetic and superconducting properties as a function of particle size were studied. Particularly, the 385 nm particles which were the ones that exhibited the reentrant superconducting effect. The Ginzburg-Landau parameter and the critical temperatures are $k \sim 1.29$, T$_{c1}$ = 92 $\pm$ 5 K, and T$_{c2}$ = 10 $\pm$ 3 K.