Broken Inversion Symmetry Research Articles

Theoretical design and discovery of two‐dimensional materials for next‐generation flexible piezotronics and energy conversion

AbstractTwo‐dimensional (2D) materials are at the epicenter of the current nanotechnology research for its intriguing properties. It has the advantage of high flexibility, high surface to volume ratio, robust mechanical strength and are stackable in contrary to bulk materials. Piezoelectric effect is exploited in 2D layered nanomaterials with broken inversion symmetry and non‐zero band gap for applications in energy harvesting and energy conversion. Density functional theory (DFT) is a powerful tool to predict the piezoelectric properties of 2D materials. This mini review emphasizes the importance and discusses the challenges of different methods within DFT‐based framework to compute piezoelectric properties. Recent simulation works of piezoelectric engineering in novel 2D semiconductors for in‐plane and out‐of‐plane piezoelectricity are reviewed. Various controlling parameters and techniques for piezoelectric effect are also discussed. Theoretical predictions pave the path for experimental exploration of 2D materials exhibiting strong piezoelectric properties and to replace the commercially used lead‐based toxic materials in future devices.

PVD growth of spiral pyramid-shaped WS2 on SiO2/Si driven by screw dislocations.

Atomically thin layered transition metal dichalcogenides (TMDs), such as MoS2 and WS2, have been getting much attention recently due to their interesting electronic and optoelectronic properties. Especially, spiral TMDs provide a variety of candidates for examining the light-matter interaction resulting from the broken inversion symmetry, as well as the potential new utilization in functional optoelectronic, electromagnetic and nanoelectronics devices. To realize their potential device applications, it is desirable to achieve controlled growth of these layered nanomaterials with a tunable stacking. Here, we demonstrate the Physical Vapor Deposition (PVD) growth of spiral pyramid-shaped WS2 with ∼200 in size and the interesting optical properties via AFM and Raman spectroscopy. By controlling the precursors concentration and changing the initial nucleation rates in PVD growth, WS2 in different nanoarchitectures can be obtained. We discuss the growth mechanism for these spiral-patterned WS2 nanostructures based on the screw dislocations. This study provides a simple, scalable approach of screw dislocation-driven (SDD) growth of distinct TMD nanostructures with varying morphologies, and stacking.

Origin of the magnetoelectric couplings in the spin dynamics of molecular magnets

We present a simple but attractive tool to describe and model the spin states of single molecule magnets. It is presented through a generalized Landau-Lifshitz-Gilbert equation with bias-dependent couplings that can be externally controlled. We provide a complete account of the various magnetic couplings [magnetic anisotropy, exchange, Dzyaloshinskii-Moriya interactions (DMI), damping, and magnetic noise's correlations] within a dimer of localized moments. The inversion symmetry breaking, ensued by the bias potential, induces a DMI between the magnetic elements that can be tuned accordingly. Through a calculation of the evolution of the spin, we conclude that such DMI is the dominant interaction during the reversal process. Along with the prescription to describe the time evolution of the spin moments, our results provide a qualitatively complete and integrated picture of various systems of interest in nanomagnetism.

Moiré phonons in graphene/hexagonal boron nitride moiré superlattice

We theoretically study in-plane acoustic phonons of graphene/hexagonal boron nitride moir\'e superlattice by using a continuum model. We demonstrate that the original phonon bands of individual layers are strongly hybridized and reconstructed into moir\'e phonon bands consisting of dispersive bands and flat bands. The phonon band structure can be effectively described by a spring-mass network model to simulate the motion of moir\'e domain walls, where the flat-band modes are interpreted as vibrations of independent, decoupled strings. We also show that the moir\'e phonon has angular momentum due to the inversion symmetry breaking by hBN, with high amplitudes concentrated near narrow gap region. Finally, we apply the same approach to twisted bilayer graphene, and we find a notable difference between the origins of the flat-band modes in G/hBN and TBG, reflecting distinct geometric structures of domain pattern.

Ferroic Berry Curvature Dipole in a Topological Crystalline Insulator at Room Temperature.

The physics related to Berry curvature is now a central research topic in condensed matter physics. The Berry curvature dipole (BCD) is a significant and intriguing condensed matter phenomenon that involves inversion symmetry breaking. However, the creation and controllability of BCDs have so far been limited to far below room temperature (RT), and nonvolatile (i.e., ferroic) BCDs have not yet been discovered, hindering further progress in topological physics. In this work, we demonstrate a switchable and nonvolatile BCD effect at RT in a topological crystalline insulator, Pb1-xSnxTe (PST), which is attributed to ferroic distortion. Surprisingly, the magnitude of the ferroic BCD is several orders of magnitude greater than that of the nonferroic BCDs that appear, for example, in transition metal dichalcogenides. The discovery of this ferroic and extraordinarily large BCD in PST could pave the way for further progress in topological materials science and the engineering of novel topological devices.

Nonlinear Hall effect in monolayer phosphorene with broken inversion symmetry

Nonlinear Hall effect (NLHE), a new member of the family of Hall effects, in monolayer phosphorene is investigated. We find that phosphorene exhibits pronounced NLHE, arising from the dipole moment of the Berry curvature induced by the proximity effect that breaks the inversion symmetry of the system. Remarkably, the nonlinear Hall response exhibits central minimum with a width on the order of the band gap, followed by two resonance-like peaks. Interestingly, each resonance peak of the Hall response shifts in the negative region of the chemical potential which is consistent with the shift of valence and conduction bands in the energy spectrum of monolayer phosphorene. It is observed that the two peaks are asymmetric, originated from anisotropy in the band structure of phosphorene. It is shown that the NLHE is very sensitive to the band gap and temperature of the system. Moreover, we find that a phase transition occurs in the nonlinear Hall response and nonlinear spin Hall conductivity of the system under the influence of spin–orbit interaction, tuned by the strength of interaction and band gap induced in the energy spectrum of monolayer phosphorene with broken inversion symmetry.

High-frequency diode effect in superconducting Nb3Sn microbridges

The superconducting diode effect has recently been reported in a variety of systems and different symmetry-breaking mechanisms have been examined. However, the frequency range of these potentially important devices still remains obscure. We investigated superconducting microbridges of ${\mathrm{Nb}}_{3}\mathrm{Sn}$ in out-of-plane magnetic fields; optimum magnetic fields of $\ensuremath{\sim}$10 mT generate $\ensuremath{\sim}10%$ diode efficiency, while higher fields of $\ensuremath{\sim}$15--20 mT quench the effect. The diode changes its polarity with magnetic field reversal. We documented superconductive diode rectification at frequencies up to 100 kHz, the highest reported as of today. Interestingly, the bridge resistance during diode operation reaches a value that is a factor of two smaller than in its normal state, which is compatible with the vortex-caused mechanism of resistivity. This is confirmed by finite-element modeling based on time-dependent Ginzburg-Landau equations. To explain experimental findings, no assumption of lattice thermal inequilibrium has been required. Dissimilar edges of the superconductor strip can be responsible for the inversion symmetry breaking by the vortex penetration barrier; visual evidence of this opportunity was revealed by scanning electron microscopy. Estimates are in favor of a much higher (GHz) range of frequencies for this type of diode.

Electric field manipulation of spin chirality and skyrmion dynamic.

The Dzyaloshinskii-Moriya interaction (DMI) is an antisymmetric exchange interaction that stabilizes spin chirality. One scientific and technological challenge is understanding and controlling the interaction between spin chirality and electric field. In this study, we investigate an unconventional electric field effect on interfacial DMI, skyrmion helicity, and skyrmion dynamics in a system with broken inversion symmetry. We design heterostructures with a 3d-5d atomic orbital interface to demonstrate the gate bias control of the DMI energy and thus transform the DMI between opposite chiralities. Furthermore, we use this voltage-controlled DMI (VCDMI) to manipulate the skyrmion spin texture. As a result, a type of intermediate skyrmion with a unique helicity is created, and its motion can be controlled and made to go straight. Our work shows the effective control of spin chirality, skyrmion helicity, and skyrmion dynamics by VCDMI. It promotes the emerging field of voltage-controlled chiral interactions and voltage-controlled skyrmionics.

Layered Semiconductor Cr0.32Ga0.68Te2.33 with Concurrent Broken Inversion Symmetry and Ferromagnetism: A Bulk Ferrovalley Material Candidate.

The valleytronic state found in group-VI transition-metal dichalcogenides such as MoS2 has attracted immense interest since its valley degree of freedom could be used as an information carrier. However, valleytronic applications require spontaneous valley polarization. Such an electronic state is predicted to be accessible in a new ferroic family of materials, i.e., ferrovalley materials, which features the coexistence of spontaneous spin and valley polarization. Although many atomic monolayer materials with hexagonal lattices have been predicted to be ferrovalley materials, no bulk ferrovalley material candidates have been reported or proposed. Here, we show that a new non-centrosymmetric van der Waals (vdW) semiconductor Cr0.32Ga0.68Te2.33, with intrinsic ferromagnetism, is a possible candidate for bulk ferrovalley material. This material exhibits several remarkable characteristics: (i) it forms a natural heterostructure between vdW gaps, a quasi-two-dimensional (2D) semiconducting Te layer with a honeycomb lattice stacked on the 2D ferromagnetic slab comprised of the (Cr, Ga)-Te layers, and (ii) the 2D Te honeycomb lattice yields a valley-like electronic structure near the Fermi level, which, in combination with inversion symmetry breaking, ferromagnetism, and strong spin-orbit coupling contributed by heavy Te element, creates a possible bulk spin-valley locked electronic state with valley polarization as suggested by our DFT calculations. Further, this material can also be easily exfoliated to 2D atomically thin layers. Therefore, this material offers a unique platform to explore the physics of valleytronic states with spontaneous spin and valley polarization in both bulk and 2D atomic crystals.

Direct observation of electric field-induced magnetism in a molecular magnet

We report the direct observation of an electrically-induced magnetic susceptibility in the molecular nano- magnet [Fe3O(O2CPh)6(py)3]ClO4·py, an Fe3 trimer. This magnetoelectric effect results from the breaking of spatial inversion symmetry due to the spin configurations of the antiferromagnetic trimer. Both static and very low frequency electric fields were used. Fractional changes of the magnetic susceptibility of 11 ppbpm 2 per kVm-1 for the temperature range 8.5 < T < 13.5 K were observed for applied electric fields up to 62 kV m−1. The changes in susceptibility were measured using a tunnel diode oscillator operating at liquid helium temperatures while the sample is held at a higher regulated temperature.

Local inversion-symmetry breaking in a bismuthate high-Tc superconductor

The doped perovskite BaBiO3 exhibits a maximum superconducting transition temperature (Tc) of 34 K and was the first high-Tc oxide to be discovered, yet pivotal questions regarding the nature of both the metallic and superconducting states remain unresolved. Although it is generally thought that superconductivity in the bismuthates is of the conventional s-wave type, the pairing mechanism is still debated, with strong electron-phonon coupling and bismuth valence or bond disproportionation possibly playing a role. Here we use diffuse x-ray scattering and Monte Carlo modeling to study the local structure of Ba1-xKxBiO3 across its insulator-metal boundary. We find no evidence for either long- or short-range disproportionation, which resolves a major conundrum, as disproportionation and the related polaronic effects are likely not relevant for the metallic and superconducting states. Instead, we uncover nanoscale structural correlations that break inversion symmetry, with far-reaching implications for the electronic physics. This unexpected finding furthermore establishes that the bismuthates belong to the broader classes of materials with hidden spin-orbit coupling and a tendency towards inversion-breaking displacements.

Twist-Induced Near-Field Radiative Heat Switch in Hyperbolic Antiferromagnets

We study the twisted control of the near-field radiative heat transfer between two hyperbolic antiferromagnetic insulators under external magnetic fields. We show that the near-field heat flux can be affected by both the twist angle $\theta$ and the magnitude of the applied magnetic field with different broken symmetries. Irrespective of twist angle, the external magnetic field causes the radiative heat flux to change nonmonotonically, and the minimum heat flux can be found with the magnetic fields of about 1.5 T. Such nonmonotonic behavior is due to the fact that the magnetic field can radically change the nature of the magnon polaritons with time reversal symmetry breaking. The field not only affects the topological structure of surface magnon polaritons, but also induces the volume magnon polaritons that progressively dominate the heat transfer as the field increases. We further propose a twist-induced thermal switch device with inversion symmetry breaking, which can severely regulate radiative heat flux through different magnetic fields. Our findings account for a characteristic modulation of radiative heat transfer with implications for applications in dynamic thermal management.

Valley-dependent conductivity and dispersion relation of surface magnetoplasmons

Electron hydrodynamics attracts increasing interest in solid state physics, offering a universal long-wavelength description of collective modes in a two-dimensional system. In two-dimensional materials with broken inversion symmetry and two degenerate valleys, a perpendicular magnetic field lifts the valley degeneracy via a Zeeman effect due to the orbital magnetic moments arising from electron rotations, relevant for developing valleytronic devices. In this work, we use a two-component hydrodynamic model to investigate the valley-dependent conductivity tensor and valley-dependent dispersion relation of surface magnetoplasmons under the modulations of the perpendicular magnetic field, Berry curvature and topological orbital magnetic moment. It is shown that the valley splitting of conductivity is large and tunable in the presence of the valley Zeeman effect. Valley splitting of frequency appears on the order of terahertz, which is preferable for realizing sub-wavelength nonreciprocal magneto-optical devices in the terahertz range.

Intrinsic layer-polarized anomalous Hall effect in bilayer MnBi2Te4

Layer-polarized anomalous Hall effect (LP-AHE) is an attractive phenomenon in condensed-matter physics from the standpoints of both fundamental interest and device applications. The current LP-AHE research is based on the extrinsic paradigm of using external electric fields, in which the generation and control of LP-AHE are not straightforward. Here, we propose a novel mechanism that realizes intrinsic LP-AHE in bilayer lattices, through the mediation of sliding physics and Berry curvature. Moreover, this mechanism could render the LP-AHE in a controllable and reversable fashion. We analyze the symmetry requirements for a system to host such intrinsic LP-AHE. Its validity is further demonstrated in a real material of bilayer MnBi2Te4. By stacking with broken inversion symmetry, the layer-locked Berry curvature enables the intrinsic LP-AHE in bilayer MnBi2Te4, and the switchable control of its LP-AHE is achieved by sliding ferroelectricity. Our work opens a significant new direction for LP-AHE and two-dimensional (2D) materials research.

Topological spin textures in 1T -phase Janus magnets: Interplay between Dzyaloshinskii-Moriya interaction, magnetic frustration, and isotropic higher-order interactions

The search for topological magnetism in two-dimensional (2D) magnetic materials is one of the hot topics in spintronics. We present comprehensive studies of magnetic phases in a series of Janus monolayers $\mathrm{Mn}XZ$ (MnAsBr, MnAsI, MnPBr, and MnPCl) and $\mathrm{Cr}YZ$ ($Y=\mathrm{Se}$, Te; and $Z=\mathrm{Cl}$, Br, I) via combining first-principles calculations and atomistic spin model simulations. Sizable Dzyaloshinskii-Moriya interaction can be realized in the $\mathrm{Mn}XZ$ and $\mathrm{Cr}YZ$ monolayers due to their intrinsic inversion symmetry breaking. More interestingly, the $\mathrm{Mn}XZ$ and $\mathrm{Cr}YZ$ monolayers exhibit different degrees of magnetic frustration and isotropic higher-order interactions. Lastly, our atomistic spin model simulations demonstrate that a variety of topological spin textures can be generated by the interplay among complex magnetic interactions. These results provide valuable information and fundamental understanding for topological magnetism in 2D magnets.

2D janus niobium oxydihalide NbOXY: Multifunctional piezoelectric semiconductor for electronics, photonics, sensing and sustainable energy applications

Two-dimensional (2D) niobium oxydihalide NbOI2 has been recently demonstrated as an excellent in-plane piezoelectric and nonlinear optical material. Here we show that Janus niobium oxydihalide, NbOXY (X, Y = Cl, Br, I and X ≠ Y), is a multifunctional anisotropic semiconductor family with exceptional piezoelectric, electronic, photocatalytic and optical properties. NbOXY are stable and flexible monolayers with band gap around the visible light regime of ∼1.9 eV. The carrier mobility of NbOXY exhibits an exceptionally strong anisotoropy ratio of ∼400, which is larger than most of the anisotropic 2D semiconductors. The exceptionally large deformation potential of NbOXYmonolayers leads to strong piezoresistive effect useful for strain sensing application. Inversion symmetry breaking in Janus NbOXY generates sizable out-of-plane d31 piezoelectric response while still retaining a strong in-plane piezoelectricity. Remarkably, NbOXY exhibits an additional out-of-plane piezoelectric response, d32 as large as 0.55 pm/V. G0W0-BSE calculation further reveals the strong linear optical dichroism of NbOXY in the visible-to-ultraviolet regime. The optical absorption peaks of 14 ∼ 18% in the deep UV regime (5 ∼ 6 eV) outperform the vast majority of other 2D materials. The high carrier mobility, strong optical absorption, sizable built-in electric field and band alignment compatible with overall water splitting further suggest the strengths of NbOXY in solar-to-hydrogen conversion. We further propose a directional stress sensing device to demonstrate how the out-of-plane piezoelectricity can be harnessed for functional device applications. Our findings unveil NbOXY as an exceptional multifunctional 2D semiconductor for flexible electronics, optoelectronics, UV photonics, piezoelectronics and sustainable energy applications.

Nonlinear spin current of photoexcited magnons in collinear antiferromagnets

We study the nonlinear magnon spin current induced by an ac electric field under light irradiation in collinear antiferromagnets with broken inversion symmetry. For linearly polarized light, we find that a dc spin current appears through ``the magnon spin shift current" mechanism, which is driven by a spin polarization generation in the two magnon creation process and has a close relationship to the geometry of magnon bands through Berry connection. For circularly polarized light, a dc spin current appears through ``the spin injection current" mechanism, which is proportional to the relaxation time of magnons and can be large when the magnon lifetime is long. We demonstrate generation of the magnon spin shift and injection currents, based on a few toy models and a realistic model for a multiferroic material M$_2$Mo$_3$O$_8$.

Berezinskii-Kosterlitz-Thouless Transition in the Type-I Weyl Semimetal PtBi2.

Symmetry breaking in topological matter has become in recent years a key concept in condensed matter physics to unveil novel electronic states. In this work, we predict that broken inversion symmetry and strong spin-orbit coupling in trigonal PtBi2 lead to a type-I Weyl semimetal band structure. Transport measurements show an unusually robust low dimensional superconductivity in thin exfoliated flakes up to 126 nm in thickness (with Tc ∼ 275-400 mK), which constitutes the first report and study of unambiguous superconductivity in a type-I Weyl semimetal. Remarkably, a Berezinskii-Kosterlitz-Thouless transition with TBKT ∼ 310 mK is revealed in up to 60 nm thick flakes, which is nearly an order of magnitude thicker than the rare examples of two-dimensional superconductors exhibiting such a transition. This makes PtBi2 an ideal platform to study low dimensional and unconventional superconductivity in topological semimetals.

Valley Hall effect in graphene-like SnX ( X=Si , Ge) buckled monolayers with high charge carrier mobility and low lattice thermal conductivity

A systematic analysis of the electronic and thermal properties of two semiconducting monolayers, SnSi and SnGe, is conducted through the lens of density functional theory. Unlike centrosymmetric graphene, these two monolayers are noncentrosymmetric and buckled in the structure; however, their band dispersion resembles that of graphene with a finite small band gap occurring at the $K$ point of the Brillouin zone. Unlike transition metal dichalcogenide (TMDC) monolayers, the band edges in the studied semiconducting monolayers are devoid of $d$ orbitals and contain $s$ and $p$ orbitals, and thus, the valley spin splitting occurring in these semiconductors is a nontrivial topological effect originating from broken inversion symmetry in the crystal structure, as observed earlier in bilayer graphene or graphene-based van der Waals heterostructures. The electronic properties are found to be tunable via external perturbations like an electric field and biaxial strain. These monolayers are highly flexible, as indicated by the small value of Young's modulus, $&lt;50\phantom{\rule{0.28em}{0ex}}\mathrm{N}/\mathrm{m}$. The charge carrier mobility is calculated within the Boltzmann transport equation and found to be $\ensuremath{\sim}1000\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.28em}{0ex}}{\mathrm{V}}^{\ensuremath{-}}1\phantom{\rule{0.28em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. The phonon dynamics study indicates a very low value of lattice thermal conductivity at room temperature (i.e., ${\ensuremath{\kappa}}_{l}&lt;1.5\phantom{\rule{0.28em}{0ex}}\mathrm{W}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$), which arises from the domination of Umklapp scattering, small group velocity, and low Debye temperature. Such a low value of ${\ensuremath{\kappa}}_{l}$ will facilitate better thermal control than TMDCs or graphene-based devices. In this paper, we indicate that the inherent properties of SnSi and SnGe monolayers, such as valley contrasting properties, high charge carrier mobility, low lattice thermal conductivity, and mechanical flexibility, surpass that of well-known TMDCs (${\mathrm{MoS}}_{2}, {\mathrm{WS}}_{2}$) and boron pnictides (BP, BAs, BSb), thereby motivating their synthesis.

Orbital contribution to the regulation of the spin-valley coupling in antiferromagnetic monolayer MnPTe3

In addition to the ferromagnetic materials with inversion symmetry breaking, the symmetric antiferromagnetic materials also exhibit intrinsic valley splitting due to the spin-valley coupling. Using first-principles calculations, we investigated the manipulation of valley splitting of antiferromagnetic monolayer ${\mathrm{MnPTe}}_{3}$ via biaxial strain. It is shown that two ${\mathrm{MnPTe}}_{3}$ monolayers with different structures are both stable antiferromagnetic semiconductors, and exhibit valley splitting between $K$ and ${K}^{\ensuremath{'}}$. The spin-valley coupling strength can be greatly tuned by in-plane strain, which is due to the changes of orbital composition of electronic state and the different contribution of two sublattice atoms. The proportions of $d$ orbitals of Mn atoms determine the orbital angular momentum of the electronic state, and the different contribution of different sublattices results in the change of Berry curvature at $K$ and ${K}^{\ensuremath{'}}$ points. The combination of two factors leads to the same changes of valley splitting and the proportion of the ${d}_{xz}$, ${d}_{yz}$ orbitals. These results of are some significance for the design of materials with greater valley splitting and the understanding of its mechanism.