Broken Space Inversion Symmetry Research Articles

Two-dimensional Rashba superconductivity in Ni/Bi bilayers evidenced by nonreciprocal transport

When two different materials are brought together, a plethora of quantum phenomena and functionalities can emerge. A prominent example is the superconductivity in Ni/Bi bilayers, which arises from the artificial layered structure composed of the non-superconducting ferromagnetic and heavy metals. Although this system has been shown to exhibit unconventional superconducting properties, the underlying mechanism of the superconductivity remains elusive. Here, we provide experimental evidence of the microscopic coexistence of two-dimensional (2D) superconductivity and broken space-inversion symmetry in the Ni/Bi bilayer. The evidence is obtained from nonreciprocal transport around the superconducting transition temperature, where the resistance depends on the direction of an applied current and an external magnetic field. We find that the nonreciprocal superconducting transport is most pronounced around the Berezinskii–Kosterlitz–Thouless transition temperature. These observations support the 2D Rashba superconductivity in the Ni/Bi bilayer, which will serve as a basis for advancing the understanding of unconventional superconductivity.

Unidirectional magnetoresistance and spin-orbit torque in NiMnSb

Spin-dependent transport phenomena due to relativistic spin-orbit coupling and broken space-inversion symmetry are often difficult to interpret microscopically, in particular when occurring at surfaces or interfaces. Here we present a theoretical and experimental study of spin-orbit torque and unidirectional magnetoresistance in a model room-temperature ferromagnet NiMnSb with inversion asymmetry in the bulk of this half-heusler crystal. Besides the angular dependence on magnetization, the competition of Rashba and Dresselhaus-like spin-orbit couplings results in the dependence of these effects on the crystal direction of the applied electric field. The phenomenology that we observe highlights potential inapplicability of commonly considered approaches for interpreting experiments. We point out that, in general, there is no direct link between the current-induced non-equilibrium spin polarization inferred from the measured spin-orbit torque and the unidirectional magnetiresistance. We also emphasize that the unidirectional magnetoresistance has not only longitudinal but also transverse components in the electric field -- current indices which complicates its separation from the thermoelectric contributions to the detected signals in common experimental techniques. We use the theoretical results to analyze our measurements of the on-resonance and off-resonance mixing signals in microbar devices fabricated from an epitaxial NiMnSb film along different crystal directions. Based on the analysis we extract an experimental estimate of the unidirectional magnetoresistance in NiMnSb.

Realization of valley polarization in monolayer WS2 via 3d transition metal atom adsorption

The great challenge in the development of valleytronics is the ability to induce and manipulate permanent valley polarization in an efficient way, which is crucial for theoretical and practical implications of valley-related physics and devices. Here, using first-principles calculation, valley polarization can be induced and controlled in a WS2 monolayer by adsorbing a 3d transition metal (TM, Sc–Zn) atom. Most 3d TM atoms have large adsorption energy, except for those with half-filled d5 and full d10 shells. With the introduction of the 3d TM atom, the system generates a local out-of-plane magnetic moment at the position of the 3d TM atom, forming an effective out-of-plane Zeeman effect. Meanwhile, the redistributed surface charges form an asymmetric electric field and imbalanced charge distribution in the K and K′ valleys. The lack of time inversion symmetry and the broken space inversion symmetry further enhance the spin–orbit coupling (SOC) interaction. A remarkable spin–orbit splitting and valley splitting occur in the K and K′ valleys in the conduction band minimum. The valley degeneracy is lifted by the effective out-of-plane Zeeman field originating from the TM adatom, which generates a permanent valley polarization. This is also expected for other honeycomb materials with strong SOC and absence of inversion symmetry.

Control of spin and valley Hall effects in monolayer transition metal dichalcogenides by magnetic proximity effect

Monolayer transition metal dichalcogenides have strong spin–orbit coupling and broken space inversion symmetry, which enable them to be the key building blocks in realizing spin and valley-related effects. Here, we report the spin and valley Hall conductivities of monolayer transition metal dichalcogenides in the presence of the magnetic proximity effect, which is introduced by a ferromagnetic substrate. It is found that the profile and magnitude of the spin and valley Hall conductivities in monolayer transition metal dichalcogenides are different with and without magnetic exchange interactions. This difference can be attributed to the asymmetrical band structure of monolayer transition metal dichalcogenides and chemical potential-dependent interband transitions. The former comes from the fact that the magnetic proximity effect can effectively break the time reversal symmetry and thus lead to the asymmetry of the band structures between K+ and K− valleys, which causes the final changes in the spin and valley Hall conductivities. Our findings demonstrate that the magnetic proximity effect can affect the spin as well as valley Hall behaviors in monolayer transition metal dichalcogenides, and this strategy is applicable for other two-dimensional layered materials, which is promising for spintronic and valleytronic devices.

Experimental observation of valley-Hall edge states in elastic waveguides based on diatomic-graphene-like phononic crystals

Inspired by recent discoveries of topological phases of matter in quantum physics, there has been a rapidly growing research effort in creating their analogs in other classical wave systems, including acoustics. Achieving robust wave transmission even in the presence of disorder and defects could have a profound impact on many practical applications and devices. In this study, we report on the design and experimental validation of a fully continuous and load-bearing phononic structural waveguide capable of one-directional guided modes along the walls of topologically distinct domains. The lattice structure of the waveguide is inspired by diatomic graphene which allows realizing an elastodynamic analog of the quantum valley Hall effect (QVHE). Despite the fact that individual bulk properties are topologically trivial (i.e., associated with a zero Chern number), the dynamic behavior acquires topological significance in the neighborhood of the high symmetry points in momentum space; the so-called valleys. Our theoretical and experimental results confirm the existence of protected edge states traveling along the walls of domains having broken space inversion symmetry.

Design and experimental validation on acoustic valley Hall edge states in reconfigurable phononic elastic waveguides

We report on the design and experimental validation of a tunable topological elastic lattice capable of supporting guided waves that are robust against back-scattering from disorder and defects. The topological lattice consists in a patterned aluminum thin plate having through-holes arranged in a hexagonal periodic configuration. The occurrence of the topological edge state is attributed to the acoustic analogue of the electron quantum valley Hall effect. By connecting two lattices having broken space inversion symmetry due to the application of opposite tunable strain fields, a topological transition emerges at the domain wall (i.e., the interface between them). Such a domain wall supports the formation and propagation of quasi-unidirectional edge states. The experimental validation of the topological waveguide is conducted on an aluminum plate that is already fabricated in a deformed (i.e., broken symmetry) configuration. The results confirm the existence of quasi-unidirectional edge states traveling along the domain wall and robust against sharp corners back-scattering. The experimental results also confirm the presence of mechanical energy flux vortices within the unit cell that are used to investigate the origin of the edge states robustness against disorder.

Design and experimental observation of valley-Hall edge states in diatomic-graphene-like elastic waveguides

We report on the design and experimental validation of a two-dimensional phononic elastic waveguide exhibiting topological Valley-Hall edge states. The lattice structure of the waveguide is inspired by the diatomic graphene and it is imprinted in an initially flat plate by means of geometric indentations. The indentations are distributed according to a hexagonal lattice structure which guarantees the existence of Dirac dispersion at the boundary of the Brillouin zone. Starting from this basic material, domain walls capable of supporting edge states can be obtained by contrasting waveguides having broken space inversion symmetry (SIS) achieved by using local resonant elements. Our theoretical study shows that such material maps into the acoustic analog of the quantum valley Hall effect (QVHE) while numerical and experimental results confirm the existence of protected edge states traveling along the walls of topologically distinct domains.

Prediction and control of spin polarization in a Weyl semimetallic phase of BiSb

Recent experimental realization of long sought Weyl fermions in non-magnetic crystals has greatly motivated condensed matter physicists to search for materials supporting Weyl fermions. Weyl fermions appear to be very promising for future electronics, often referred as Weyltronics. Here, by means of first-principle calculations, we report a stoichiometric crystal structure of BiSb with broken space-inversion symmetry. This structure is insulating in bulk and has non-trivial band topology. We observe a pressure driven Weyl semi-metallic phase transition in this crystal structure. The obtained Weyl semi-metallic phase exists in the 4.0 - 6.0 GPa pressure range. We find that a total 6 pairs of Weyl points, 6 monopoles and 6 antimonopoles, exist in the Brillouin zone. The Weyl points with opposite chirality are located at different energy values yielding separate electron and hole Fermi-surfaces. Additionally, the spin-texture of the bulk BiSb compound appears to be electrically controllable when the interlink between pressure and an electric field is exploited. This produces novel manipulable topological transport properties in this system which are very promising for implementation of this kind of materials in next-generation Weyltronics and spintronic devices.

Spectral characteristics of the sum-frequency generation from atomically thin hexagonal crystals lacking center-of-inversion symmetry

We calculate the second-order nonlinear optical susceptibility for the sum-frequency generation (SFG) from two-dimensional (2D) hexagonal crystals with broken space inversion symmetry, such as graphene, grown epitaxially on a SiC substrate, and boronitrene (a single atomic layer of hexagonal boron nitride (h-BN)). We show that in the presence of two normally incident pump beams at frequencies and , a remarkably large output signal at frequency is generated in those materials in the two-photon resonant regime where the frequencies and are matched to provide a resonance for the radiated photon energy with the bandgap energy Eg of the materials. In addition, it is found that the absolute magnitude of the is resonantly enhanced when the photon energy of one of the input beams is tuned to the fundamental absorption edge. These resonant behaviors lead to very large values of , reaching the order of 10−4 esu for the graphene/SiC overlayer system and 10−6 esu for monolayer h-BN. Furthermore, even outside the resonances, the magnitude of is fairly large ( esu for the overlying graphene and 10−8 esu for boronitrene), which opens up new opportunities for practical exploitation of the SFG effect in the examined 2D crystals.

X-ray Natural Circular Dichroism

Natural Circular Dichroism was only recently discovered in the x-ray range[1]. This effect stems from the interference terms which mix multipole transition moments of opposite parity: the Electric Dipole-Electric Quadrupole (E1.E2) and the Electric Dipole-Magnetic Dipole (E1.M1) exist only in structures with broken space inversion symmetry. The scalar E1.M1 term known to be responsible for Circular Dichroism at optical wavelengths is usually considered to be vanishingly small for core level spectroscopies. The E1.E2 interference term, on the contrary, can be large in the X-ray region, but it is a parity odd second rank tensor and therefore observable only in 13 non-centrosymmetric crystal classes. X-ray Natural Circular Dichroism has now been detected in the XANES region for several uniaxial and biaxial crystals. It can give access to the absolute configuration of chiral absorbing centers. On the other hand, Chiral-EXAFS, i.e. the analog of Magnetic-EXAFS for Natural Circular Dichroism has also been measured recently using a uniaxial optically active crystal of paratellurite (TeO2). Chiral-EXAFS originates from symmetry allowed multiple scattering paths. In this presentation, we wish to report on recent advances in X-ray natural circular dichroism and its applications. Determination of absolute configuration is illustrated with measurements of both E1.E2 and E1.M1 terms in chiral alpha-Ni(H2O)6·SO4 single crystals. Manifestation of X-ray optical acitivity in magnetoelectric crystals will be illustrated with various dichroisms measured at the Fe K-edge in multiferroic GaFeO3 crystal. Finally, we will review briefly the perspectives open by our experiments.

Large nonlinear optical rectification in atomic hexagonal layers with broken space inversion symmetry

Motivated by possible applications in optoelectronics, we consider nonlinear optical rectification (NOR) in two planar hexagonal lattice structures with broken space inversion symmetry—namely, in graphene epitaxially grown on a SiC substrate and in boronitrene (a monolayer of BN). For both structures, we calculate the second-order nonlinear optical susceptibility χ(2)(0;ω, − ω) relevant to the NOR effect and evaluate a bias voltage V0 appearing at the structure terminals under strong laser irradiation. We show that the reason for the χ(2)(0;ω, − ω) being nonvanishing in the examined structures is their sublattice (inversion) asymmetry combined with the trigonal symmetry of their π-electron energy bands near the corners of the hexagonal Brillouin zone of those structures. In spite of being rather small, the trigonal warping of the energy bands involved is found to provide a remarkably large magnitude of the NOR susceptibility, reaching the order of 5 × 10−4 esu for the graphene/SiC overlayer system when the pump photon energy ħω approaches the bandgap energy EG (≈0.26 eV) of the overlying graphene. For a graphene sample of a few microns length, irradiated by a normally incident laser beam with a relatively moderate power density of 10 kW cm−2, the corresponding optical rectification voltage V0 is estimated to be as large as several millivolts. Moreover, the sign of the voltage (i.e., its polarity) can be sharply reversed by sweeping the photon energy through the inter-π-band resonance condition ħω = EG. This frequency-controlled optical switching, if realized, will be a potent technique for graphene-based photonics and optoelectronics.

Optical second-harmonic generation from two-dimensional hexagonal crystals with broken space inversion symmetry

We propose a microscopic theory of the optical second-harmonic generation (SHG) from π electrons in two-dimensional (2D) honeycomb lattice structures with broken space inversion symmetry, such as graphene epitaxially grown on a SiC substrate and boronitrene (a single sheet of hexagonal boron nitride (h-BN)). The approach developed is based on a simple two-band π-electron tight-binding model combined with the original Genkin–Mednis formalism of the second-order nonlinear optical response theory, detailed in our recent paper (2010 Phys. Rev. B 82 235426). Within the framework of the approach, we derive an explicit expression for the SHG susceptibility , which involves two distinct contributions originating from a mixture of interband and intraband motion of π electrons. Both the contributions, and, hence, the on the whole, are found to tend to zero when the π-electron energy bands involved are treated at the simplest level of approximation, neglecting the effect of their trigonal warping around the corners of the Brillouin zone of the 2D hexagonal lattice. Through numerical calculations, it is shown that this effect, though rather small, leads to a fairly large magnitude of the SHG susceptibility, reaching the order of 10−4 esu for the graphene/SiC overlayer system and 10−7 esu for monolayer h-BN, when the pump photon energy ħω approaches half the bandgap energy Eg of those structures. These theoretical findings suggest that SHG can be used as a sensitive optical probe of the electronic structure of the examined 2D hexagonal crystals and simultaneously demonstrate that those crystals may be an appropriate material for practical uses in future optoelectronic nano-devices.

Rocking Subdiffusive Ratchets: Origin, Optimization and Efficiency

We study origin, parameter optimization, and thermodynamic efficiency of isothermal rocking ratchets based on fractional subdiffusion within a generalized non-Markovian Langevin equation approach. A corresponding multi-dimensional Markovian embedding dynamics is realized using a set of auxiliary Brownian particles elastically coupled to the central Brownian particle (see video on the journal web site). We show that anomalous subdiffusive transport emerges due to an interplay of nonlinear response and viscoelastic effects for fractional Brownian motion in periodic potentials with broken space-inversion symmetry and driven by a time-periodic field. The anomalous transport becomes optimal for a subthreshold driving when the driving period matches a characteristic time scale of interwell transitions. It can also be optimized by varying temperature, amplitude of periodic potential and driving strength. The useful work done against a load shows a parabolic dependence on the load strength. It grows sublinearly with time and the corresponding thermodynamic efficiency decays algebraically in time because the energy supplied by the driving field scales with time linearly. However, it compares well with the efficiency of normal diffusion rocking ratchets on an appreciably long time scale.

Tunable ferroelectricity in artificial tri-layer superlattices comprised of non-ferroic components

Heterostructured material systems devoid of ferroic components are presumed not to display ordering associated with ferroelectricity. In heterostructures composed of transition metal oxides, however, the disruption introduced by an interface can affect the balance of the competing interactions among electronic spins, charges and orbitals. This has led to the emergence of properties absent in the original building blocks of a heterostructure, including metallicity, magnetism and superconductivity. Here we report the discovery of ferroelectricity in artificial tri-layer superlattices consisting solely of non-ferroelectric NdMnO(3)/SrMnO(3)/LaMnO(3) layers. Ferroelectricity was observed below 40 K exhibiting strong tunability by superlattice periodicity. Furthermore, magnetoelectric coupling resulted in 150% magnetic modulation of the polarization. Density functional calculations indicate that broken space inversion symmetry and mixed valency, because of cationic asymmetry and interfacial polar discontinuity, respectively, give rise to the observed behaviour. Our results demonstrate the engineering of asymmetric layered structures with emergent ferroelectric and magnetic field tunable functions distinct from that of normal devices, for which the components are typically ferroelectrics.

Direct evidence of spin-polarized band structure of Sb(111) surface

We have directly characterized the spin-polarized energy-band structures of Sb(111) near the Fermi level by spin- and angle-resolved photoemission spectroscopy. The results clearly indicate that the energy bands are spin-split due to the spin-orbit coupling and the broken space inversion symmetry.

Phenomenology of magnetic second harmonic generation from low symmetry surfaces and interfaces

Low dimensional magnetic structures show interesting and novel phenomena such as oscillatory magnetic coupling and giant magnetoresistance. Magnetic second harmonic generation (MSHG) can provide unique information on magnetic surfaces and interfaces because, within the dipole approximation, broken space-inversion symmetry at the surface or interface of centrosymmetric media, and broken time-reversal symmetry arising from the magnetization, are both required in order to observe a magnetic-field-dependent second harmonic response. However, the additional reduction in symmetry arising from the magetization produces many non-zero susceptibility tensor components, particularly in the case of vicinal, stepped surfaces of 1m symmetry, and care is needed in designing experiments that will produce readily interpretable results. Phenomenological expressions for the MSHG response from systems of 1m symmetry are presented, where combinations of input and output polarizations and magnetic field orientations allow the essential physics of these systems to be explored, particularly in relation to distinguishing terrace and step contributions to the magnetization from vicinal surfaces and interfaces. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Nonlinear magneto-optics

The time-reversal symmetry breaking, that is the result of the presence of a magnetization, leads to a number of well-known magneto-optical effects like Faraday rotation in transmission and Kerr rotation in reflection. In nonlinear optics in the electric-dipole approximation optical effects, e.g. second-harmonic generation (SHG), are only allowed in media with a broken space-inversion symmetry. As a consequence, nonlinear magneto-optical phenomena like magnetization-induced second-harmonic generation (MSHG), can only be observed in materials in which both space-inversion and time-reversal symmetry are simultaneously broken. This makes nonlinear magneto-optical effects particularly attractive for the study of magnetic multilayers and nanostructures, that are the basis of a new generation of magnetic sensors and recording media. In this paper we will discuss some of the recent advances in this field, such as the observation of very large nonlinear magneto-optical effects, strong oscillations in the MSHG response from ultra-thin metallic overlayers and new modes of nonlinear magneto-optical imaging.

Circular dichroism in high-temperature superconductors: A characteristic of broken space-inversion symmetry.

The circular optical dichroism recently observed in a number of high-temperature superconductors has been commonly interpreted in terms of violation of both time-reversal invariance and space-inversion symmetry (the magnetoelectric effect). It is shown, however, that in view of numerous experimental observations of various piezoelectric or ferroelectric phenomena in these superconductors, the effect could be naturally interpreted in terms of a linear space dispersion of the dielectric tensor of a dissipative medium that lacks a center of inversion. Both bulk and surface contributions to the effect are discussed.