Noncentrosymmetric Systems Research Articles

Spontaneous skyrmionic lattice from anisotropic symmetric exchange in a Ni-halide monolayer

Topological spin structures, such as magnetic skyrmions, hold great promises for data storage applications, thanks to their inherent stability. In most cases, skyrmions are stabilized by magnetic fields in non-centrosymmetric systems displaying the chiral Dzyaloshinskii-Moriya exchange interaction, while spontaneous skyrmion lattices have been reported in centrosymmetric itinerant magnets with long-range interactions. Here, a spontaneous anti-biskyrmion lattice with unique topology and chirality is predicted in the monolayer of a semiconducting and centrosymmetric metal halide, NiI2. Our first-principles and Monte Carlo simulations reveal that the anisotropies of the short-range symmetric exchange, when combined with magnetic frustration, can lead to an emergent chiral interaction that is responsible for the predicted topological spin structures. The proposed mechanism finds a prototypical manifestation in two-dimensional magnets, thus broadening the class of materials that can host spontaneous skyrmionic states.

Multitype Dirac fermions protected by orthogonal glide symmetries in a noncentrosymmetric system

Compared to inversion-symmetric systems, emerging bulk Dirac point (DP) in noncentrosymmetric systems is much harder and its symmetry protection mechanism is poorly understood. In this work, we propose that orthogonal glide symmetries can protect two distinct types of bulk anisotropic DPs. One is the ordinary anisotropic DP that is doubly degenerate only along one invariant axis, which is resulted from symmetry-protected accidental band crossing. The other one is a symmetry-enforced DP fixed at a non-time-reversal-invariant point, which is doubly degenerate at three orthogonal directions. This unique topological phase is exemplified by KSnSe$_{2}$ in space group 108. Our work not only unveils an unique symmetry protection mechanism but also provides the first material candidate for exploring multi-type Dirac fermions in noncentrosymmetric systems.

Attosecond transient absorption spectroscopy without inversion symmetry

Transient absorption is a very powerful observable in attosecond experiments on atoms, molecules and solids and is frequently used in experiments employing phase-locked few-cycle infrared and XUV laser pulses derived from high harmonic generation. We show numerically and analytically that in non-centrosymmetric systems, such as many polyatomic molecules, which-way interference enabled by the lack of parity conservation leads to new spectral absorption features, which directly reveal the laser electric field. The extension of attosecond transient absorption spectroscopy (ATAS) to such targets hence becomes sensitive to global and local inversion symmetry. We anticipate that ATAS will find new applications in non-centrosymmetric systems, in which the carrier-to-envelope phase of the infrared pulse becomes a relevant parameter and in which the orientation of the sample and the electronic symmetry of the molecule can be addressed.

Nanometric square skyrmion lattice in a centrosymmetric tetragonal magnet.

Magnetic skyrmions are topologically stable spin swirls with a particle-like character and are potentially suitable for the design of high-density information bits. Although most known skyrmion systems arise in non-centrosymmetric systems with a Dzyaloshinskii-Moriya interaction, centrosymmetric magnets with a triangular lattice can also give rise to skyrmion formation, with a geometrically frustrated lattice being considered essential in this case. Until now, it remains an open question if skyrmions can also exist in the absence of both geometrically frustrated lattice and inversion symmetry breaking. Here we discover a square skyrmion lattice state with 1.9 nm diameter skyrmions in the centrosymmetric tetragonal magnet GdRu2Si2 without a geometrically frustrated lattice by means of resonant X-ray scattering and Lorentz transmission electron microscopy experiments. A plausible origin of the observed skyrmion formation is four-spin interactions mediated by itinerant electrons in the presence of easy-axis anisotropy. Our results suggest that rare-earth intermetallics with highly symmetric crystal lattices may ubiquitously host nanometric skyrmions of exotic origins.

Very large Dzyaloshinskii-Moriya interaction in two-dimensional Janus manganese dichalcogenides and its application to realize skyrmion states

The Dzyaloshinskii-Moriya interaction (DMI), which only exists in noncentrosymmetric systems, is responsible for the formation of exotic chiral magnetic states. The absence of DMI in most two-dimensional (2D) magnetic materials is due to their intrinsic inversion symmetry. Here, using first-principles calculations, we demonstrate that significant DMI can be obtained in a series of Janus monolayers of manganese dichalcogenides MnXY in which the difference between X and Y on the opposites sides of Mn breaks the inversion symmetry. In particular, the DMI amplitudes of MnSeTe and MnSTe are comparable to those of state-of-the-art ferromagnet/heavy metal (FM/HM) heterostructures. In addition, by performing Monte Carlo simulations, we find that at low temperatures the ground states of the MnSeTe and MnSTe monolayers can transform from ferromagnetic states with worm-like magnetic domains into the skyrmion states by applying external magnetic field. At increasing temperature, the skyrmion states starts fluctuating above 50 K before an evolution to a completely disordered structure at higher temperature. The present results pave the way for new device concepts utilizing chiral magnetic structures in specially designed 2D ferromagnetic materials.

Effect of interfacial Dzyaloshinskii-Moriya interaction on polarized neutrons reflection

The antisymmetric Dzyaloshinskii-Moriya interaction (DMI) in noncentrosymmetric systems leads to various nonuniform chiral magnetic textures. Polarized neutron scattering is a powerful method for investigation such chiral distributions. The most used technique is a small-angle neutrons scattering (SANS). Multilayered magnetic films with the interface induced DMI (iDMI) can’t be investigated by SANS because of their small volume. The appropriate technique is a polarized neutron reflectometry. Within the framework of the continuum theory of micromagnetics, we explore the impact of the iDMI on the polarized neutrons reflection from multilayers with random magnetic anisotropy. It is shown that the iDMI gives rise to a polarization-dependent asymmetric term in the reflection.

Nonreciprocal Landau\u2013Zener tunneling

Application of strong dc electric field to an insulator leads to quantum tunneling of electrons from the valence band to the conduction band, which is a famous nonlinear response known as Landau-Zener tunneling. One of the growing interests in recent studies of nonlinear responses is nonreciprocal phenomena where transport toward the left and the right differs. Here, we theoretically study Landau-Zener tunneling in noncentrosymmetric systems, i.e., the crystals without spatial inversion symmetry. A generalized Landau-Zener formula is derived, taking into account the geometric nature of the wavefunctions. The obtained formula shows that nonreciprocal tunneling probability originates from the difference in the Berry connections of the Bloch wavefunctions across the band gap, i.e., shift vector. We also discuss application of our formula to tunneling in a one-dimensional model of a ferroelectrics.

Can second order nonlinear spectroscopies selectively probe optically "dark" surface states in small semiconductor nanocrystals?

Second order nonlinear responses such as sum frequency and second harmonic generation arise from the response of a material system to the second power of an incident electromagnetic field through the material's first hyperpolarizability or second-order optical susceptibility. These quantities are nonzero only for noncentrosymmetric systems, but different length scales of the noncentrosymmetry give rise to second harmonic or sum frequency radiation with different spatial and coherence characteristics. This perspective discusses the possible contributions to the second-order signal from films of small semiconductor quantum dots and addresses whether such experiments are expected to selectively enhance transitions to surface defects or trap states in such systems. It points out how "surface" and "bulk" contributions to the sum frequency or the second harmonic signal should be distinguishable through their angular dependence in a scattering geometry. It also explores possible mechanisms whereby second order spectroscopies might provide access to surface states that are very weak or absent in other forms of optical spectroscopy.

Unconventional crystal-field splitting in noncentrosymmetric BaTiO3 thin films

Understanding the crystal-field splitting and orbital polarization in noncentrosymmetric systems such as ferroelectric materials is fundamentally important. In this study, taking ${\mathrm{BaTiO}}_{3}$ as a representative material, we investigate titanium crystal-field splitting and orbital polarization in noncentrosymmetric ${\mathrm{TiO}}_{6}$ octahedra with resonant x-ray linear dichroism at the Ti ${L}_{2,3}$ edge. The high-quality ${\mathrm{BaTiO}}_{3}$ thin films were deposited on ${\mathrm{DyScO}}_{3}$ (110) single crystal substrates in a layer-by-layer way by pulsed laser deposition. The reflection high-energy electron diffraction and element specific x-ray absorption spectroscopy were performed to characterize the structural and electronic properties of the films. In sharp contrast to conventional crystal-field splitting and orbital configuration (${d}_{xz}/{d}_{yz}<{d}_{xy}<{d}_{3{z}^{2}\ensuremath{-}{r}^{2}}<{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ or ${d}_{xy}<{d}_{xz}/{d}_{yz}<{d}_{{x}^{2}\ensuremath{-}{y}^{2}}<{d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$) expected from compressive or tensile epitaxial strain, respectively, it is revealed that ${d}_{xz}$, ${d}_{yz}$, and ${d}_{xy}$ orbitals are nearly degenerate, whereas ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbitals are split with an energy gap $\ensuremath{\sim}100$ meV in the epitaxial ${\mathrm{BaTiO}}_{3}$ films. We find that the unexpected degenerate orbitals ${d}_{xz}/{d}_{yz}/{d}_{xy}$ result from the competition between the orbital splitting induced by epitaxial strain and that induced by polar distortions of ${\mathrm{BaTiO}}_{3}$ films. Our results provide a route to manipulate orbital degree of freedom by switching electric polarization in ferroelectric materials.

Nonreciprocal transport of a super-Ohmic quantum ratchet

Nonreciprocal transport, which refers to directional transport, is known to occur in noncentrosymmetric systems with broken time-reversal symmetry. In this paper, we study the nonreciprocal motion of a super-Ohmic dissipative particle---with a quadratic spectral density function of the environment ($s=2$)---in an asymmetric periodic potential. Using the Keldysh formalism, we derive general expressions for the perturbative corrections to finite-frequency mobility up to third order in potential strength. We find from numerical integration that as a function of temperature, second-order mobility in the low-frequency limit scales exponentially in the low-temperature region, ${\ensuremath{\mu}}_{2}^{(3)}(T;{\ensuremath{\omega}}_{1},{\ensuremath{\omega}}_{2})\ensuremath{\sim}exp(\ensuremath{-}b/T),\phantom{\rule{0.16em}{0ex}}(bg0)$ and scales as a power law in the high-temperature region, ${\ensuremath{\mu}}_{2}^{(3)}(T;{\ensuremath{\omega}}_{1},{\ensuremath{\omega}}_{2})\ensuremath{\sim}{T}^{\ensuremath{-}3}$. As a function of frequency, we show that second-order mobility scales according to ${\ensuremath{\mu}}_{2}^{(3)}({\ensuremath{\omega}}_{1},{\ensuremath{\omega}}_{2};T)\ensuremath{\sim}i(\ensuremath{\omega}log(\ensuremath{\omega}/{\ensuremath{\omega}}_{c}){)}^{\ensuremath{-}3}$ for the $s=2$ case, but scales as a power law, ${\ensuremath{\mu}}_{2}^{(3)}({\ensuremath{\omega}}_{1},{\ensuremath{\omega}}_{2};T)\ensuremath{\sim}i{\ensuremath{\omega}}^{\ensuremath{-}3}$, for $s\ensuremath{\ge}3,(s\ensuremath{\in}{\mathbb{Z}}^{+})$.

Orbitally dominated Rashba-Edelstein effect in noncentrosymmetric antiferromagnets

Efficient manipulation of magnetic order with electric current pulses is desirable for achieving fast spintronic devices. The Rashba-Edelstein effect, wherein spin polarization is electrically induced in noncentrosymmetric systems, provides a mean to achieve staggered spin-orbit torques. Initially predicted for spin, its orbital counterpart has been disregarded up to now. Here we report a generalized Rashba-Edelstein effect, which generates not only spin polarization but also orbital polarization, which we find to be far from being negligible. We show that the orbital Rashba-Edelstein effect does not require spin-orbit coupling to exist. We present first-principles calculations of the frequency-dependent spin and orbital Rashba-Edelstein tensors for the noncentrosymmetric antiferromagnets CuMnAs and Mn{}_{2}Au. We show that the electrically induced local magnetization can exhibit Rashba-like or Dresselhaus-like symmetries, depending on the magnetic configuration. We compute sizable induced magnetizations at optical frequencies, which suggest that electric-field driven switching could be achieved at much higher frequencies.

Band splitting with vanishing spin polarizations in noncentrosymmetric crystals

The Dresselhaus and Rashba effects are well-known phenomena in solid-state physics, in which spin–orbit coupling splits spin-up and spin-down energy bands of nonmagnetic non-centrosymmetric crystals. Here, we discuss a phenomenon we dub band splitting with vanishing spin polarizations (BSVSP), in which, as usual, spin-orbit coupling splits the energy bands in nonmagnetic non-centrosymmetric systems. Surprisingly, however, both split bands show no net spin polarization along certain high-symmetry lines in the Brillouin zone. In order to rationalize this phenomenon, we propose a classification of point groups into pseudo-polar and non-pseudo-polar groups. By means of first-principles simulations, we demonstrate that BSVSP can take place in both symmorphic (e.g., bulk GaAs) and non-symmorphic systems (e.g., two dimensional ferroelectric SnTe). Furthermore, we identify a linear magnetoelectric coupling in reciprocal space, which could be employed to tune the spin polarization with an external electric field. The BSVSP effect and its manipulation could therefore form the basis for future spintronic devices.

Photonic Weyl points due to broken time-reversal symmetry in magnetized semiconductor

Weyl points are discrete locations in the three-dimensional momentum space where two bands cross linearly with each other. They serve as the monopoles of Berry curvature in the momentum space, and their existence requires breaking of either time-reversal or inversion symmetry1–16. Although various non-centrosymmetric Weyl systems have been reported15, demonstration of Weyl degeneracies due to breaking of the time-reversal symmetry remains scarce and is limited to electronic systems17,18. Here, we report the experimental observation of photonic Weyl degeneracies in a magnetized semiconductor—InSb, which behaves as a magnetized plasma19 for electromagnetic waves at the terahertz band. By varying the magnetic field strength, Weyl points and the corresponding photonic Fermi arcs have been demonstrated. Our observation establishes magnetized semiconductors as a reconfigurable20 terahertz Weyl system, which may prompt research on novel magnetic topological phenomena such as chiral Majorana-type edge states and zero modes in classic systems21,22. Photonic Weyl points—topologically chiral singularity points in three-dimensional momentum space—have been realized in a homogeneous non-reciprocal material without a crystal lattice structure.

Determining GaN Nanowire Polarity and its Influence on Light Emission in the Scanning Electron Microscope.

The crystal polarity of noncentrosymmetric wurtzite GaN nanowires is determined nondestructively in the scanning electron microscope using electron backscatter diffraction (EBSD). The impact of the nanowire polarity on light emission is then investigated using cathodoluminescence (CL) spectroscopy. EBSD can determine polarity of noncentrosymmetric crystals by interrogating differences in the intensity distribution of bands of the EBSD pattern associated with semipolar planes. Experimental EBSD patterns from an array of GaN nanowires are compared with theoretical patterns produced using dynamical electron simulations to reveal whether they are Ga- or N-polar or, as in several cases, of mixed polarity. CL spectroscopy demonstrates the effect of the polarity on light emission, with spectra obtained from nanowires of known polarity revealing a small but measurable shift (≈28 meV) in the GaN near band edge emission energy between those with Ga and N polarity. We attributed this energy shift to a difference in impurity incorporation in nanowires of different crystal polarity. This approach can be employed to nondestructively identify polarity in a wide range of noncentrosymmetric nanoscale material systems and provide direct comparison with their luminescence.

Current-Voltage Characteristic and Shot Noise of Shift Current Photovoltaics.

We theoretically study the current-voltage relation, the I-V characteristic, of the photovoltaics due to the shift current, i.e., the photocurrent generated without the external dc electric field in noncentrosymmetric crystals through the Berry connection of the Bloch wave functions. We find that the I-V characteristic and shot noise are controlled by the difference of group velocities between conduction and valence bands, i.e., v_{11}-v_{22}, and the relaxation time τ. Since the shift current itself is independent of these quantities, there are a wide variety of possibilities to design it to maximize the energy conversion rate and also to suppress the noise. We propose that the Landau levels in noncentrosymmetric two-dimensional systems are a promising candidate for energy conversion.

Drumhead surface states and their signatures in quasiparticle scattering interference

We consider a two-orbital tight-binding model defined on a layered three-dimensional hexagonal lattice to investigate the properties of topological nodal lines and their associated drumhead surface states. We examine these surface states in centrosymmetric systems, where the bulk nodal lines are of Dirac type (i.e., four-fold degenerate), as well as in non-centrosymmetric systems with strong Rashba and/or Dresselhaus spin-orbit coupling, where the bulk nodal lines are of Weyl type (i.e., two-fold degenerate). We find that in non-centrosymmetric systems the nodal lines and their corresponding drumhead surface states are fully spin polarized due to spin-orbit coupling. We show that unique signatures of the topologically nontrivial drumhead surface states can be measured by means of quasiparticle scattering interference, which we compute for both Dirac and Weyl nodal line semimetals. At the end, we analyze the possible crystal structures with a symmetry that supports flat surface states which are effectively ringlike.

Current-induced dynamics of skyrmion strings.

Dynamics of string-like objects is an important issue in a broad range of physical systems, including vortex lines in superconductors, viscoelastic polymers, and superstrings in elementary particle physics. In noncentrosymmetric magnets, string forms of magnetic skyrmions are present as topological spin objects, and their current-induced dynamics has recently attracted intense interest. We show in the chiral magnet MnSi that the current-induced deformation dynamics of skyrmion strings results in transport response associated with the real-space Berry phase. Prominent nonlinear Hall signals emerge above the threshold current only in the skyrmion phase. We clarify the mechanism for these nonlinear Hall signals by adopting spin density wave picture to describe the moving skyrmion lattice; deformation of skyrmion strings occurs in an asymmetric manner due to the Dzyaloshinskii-Moriya interaction, which leads to the nonreciprocal nonlinear Hall response originating from an emergent electromagnetic field. This finding reveals the dynamical nature of string-like objects and consequent transport outcomes in noncentrosymmetric systems.

Giant photovoltaic response in band engineered ferroelectric perovskite

Recently the solar energy, an inevitable part of green energy source, has become a mandatory topics in frontier research areas. In this respect, non-centrosymmetric ferroelectric perovskites with open circuit voltage (VOC) higher than the bandgap, gain tremendous importance as next generation photovoltaic materials. Here a non-toxic co-doped Ba1−x(Bi0.5Li0.5)xTiO3 ferroelectric system is designed where the dopants influence the band topology in order to enhance the photovoltaic effect. In particular, at the optimal doping concentration (xopt ~ 0.125) the sample reveals a remarkably high photogenerated field EOC = 320 V/cm (VOC = 16 V), highest ever reported in any bulk polycrystalline non-centrosymmetric systems. The band structure, examined through DFT calculations, suggests that the shift current mechanism is key to explain the large enhancement in photovoltaic effect in this family.

First-Principles Evaluation of the Dzyaloshinskii–Moriya Interaction

We review recent developments of formulations to calculate the Dzyaloshinskii--Moriya (DM) interaction from first principles. In particular, we focus on three approaches. The first one evaluates the energy change due to the spin twisting by directly calculating the helical spin structure. The second one employs the spin gauge field technique to perform the derivative expansion with respect to the magnetic moment. This gives a clear picture that the DM interaction can be represented as the spin current in the equilibrium within the first order of the spin-orbit couplings. The third one is the perturbation expansion with respect to the exchange couplings and can be understood as the extension of the Ruderman--Kittel--Kasuya--Yosida (RKKY) interaction to the noncentrosymmetric spin-orbit systems. By calculating the DM interaction for the typical chiral ferromagnets Mn$_{1-x}$Fe$_x$Ge and Fe$_{1-x}$Co$_x$Ge, we discuss how these approaches work in actual systems.

Identifying Two-Dimensional Z2 Antiferromagnetic Topological Insulators

We revisit the question of whether a two-dimensional topological insulator may arise in a commensurate Neel antiferromagnet, where staggered magnetization breaks the symmetry with respect to both elementary translation and time reversal, but retains their product as a symmetry. In contrast to the so-called Z2 topological insulators, an exhaustive characterization of antiferromagnetic topological phases with the help of topological invariants has been missing. We analyze a simple model of an antiferromagnetic topological insulator and chart its phase diagram, using a recently proposed criterion for centrosymmetric systems [13]. We then adapt two methods, originally designed for paramagnetic systems, and make antiferromagnetic topological phases manifest. The proposed methods apply far beyond the particular examples treated in this work, and admit straightforward generalization. We illustrate this by two examples of non-centrosymmetric systems, where no simple criteria have been known to identify topological phases. We also present, for some cases, an explicit construction of edge states in an antiferromagnetic topological insulator.