Breaking Of Space-inversion Symmetry Research Articles

Probing chiral discrimination in biological systems using atomic force microscopy: The role of van der Waals and exchange interactions

We analyze from a theoretical perspective recent experiments where chiral discrimination in biological systems was established using Atomic Force Microscopy (AFM). Even though intermolecular forces involved in AFM measurements have different origins, i.e., electrostatic, bonding, exchange, and multipole interactions, the key molecular forces involved in enantiospecific biorecognition are electronic spin exchange and van der Waals (vdW) dispersion forces, which are sensitive to spin–orbit interaction (SOI) and space-inversion symmetry breaking in chiral molecules. The vdW contribution to chiral discrimination emerges from the inclusion of SOI and spin fluctuations due to the chiral-induced selectivity effect, a result we have recently demonstrated theoretically. Considering these two enantiospecific contributions, we show that the AFM results regarding chiral recognition can be understood in terms of a simple physical model that describes the different adhesion forces associated with different electron spin polarization generated in the (DD), (LL), and (DL) enantiomeric pairs, as arising from the spin part of the exchange and vdW contributions. The model can successfully produce physically reasonable parameters accounting for the vdW and exchange interaction strength, accounting for the chiral discrimination effect. This fact has profound implications in biorecognition where the relevant intermolecular interactions in the intermediate-distance regime are clearly connected to vdW forces.

Pressure-induced superconductivity in the noncentrosymmetric Weyl semimetals LaAlX (X=Si,Ge)

In topological materials, Dirac fermions can split into two Weyl fermions with opposite chiralities due to the breaking of space-inversion symmetry, while in noncentrosymmetric superconductors, novel superconducting electron pairing mechanisms arise because of the antisymmetric spin-orbit coupling. In this work, we report the pressure-introduced superconductivity in a typical noncentrosymmetric Weyl semimetal $\mathrm{LaAl}X$ ($X=\mathrm{Si}$ and Ge). Superconductivity was observed at around 65 GPa without structural phase transition. A typical dome-shape phase diagram is obtained with the maximum ${T}_{\mathrm{c}}$ of 2.5 K (2.1 K) for LaAlSi (LaAlGe). Furthermore, the application of pressure does not destroy the nontrivial band topology of LaAlSi up to 80.4 GPa, making such materials potential candidates for realizing topological superconductivity. Our report of superconductivity in $\mathrm{LaAl}X$ ($X=\mathrm{Si}$ and Ge) will provide critical insight in noncentrosymmetric superconductors and stimulate further study on superconductivity in Weyl semimetals.

Nonreciprocal optical response in parity-breaking superconductors

Superconductivity, an emergence of a macroscopic quantum state, gives rise to unique electromagnetic responses leading to perfect shielding of magnetic field and zero electrical resistance. In this paper, we propose that superconductors with the space-inversion symmetry breaking host giant nonreciprocal optical phenomena, such as photocurrent generation and second harmonic generation. The nonreciprocal optical responses show divergent behaviors in the low-frequency regime and originate from two-fold indicators unique to parity-breaking superconductors, namely, the nonreciprocal superfluid density and the Berry curvature derivative. Furthermore, the mechanism and frequency dependence are closely tied to the preserved temporal symmetry in the superconductor. The relation is useful for probing the space-time structure of the superconducting symmetry. The indicators characterize the low-frequency property of nonreciprocal optical responses and hence quantify the performance of superconductors in nonreciprocal optics.

Seemingly asymmetric atom-localized electronic densities following laser-dissociation of homonuclear diatomics.

Recent experiments on laser-dissociation of aligned homonuclear diatomic molecules show an asymmetric forward-backward (spatial) electron-localization along the laser polarization axis. Most theoretical models attribute this asymmetry to interference effects between gerade and ungerade vibronic states. Presumably due to alignment, these models neglect molecular rotations and hence infer an asymmetric (post-dissociation) charge distribution over the two identical nuclei. In this paper, we question the equivalence that is made between spatial electron-localization, observed in experiments, and atomic electron-localization, alluded by these theoretical models. We show that (seeming) agreement between these models and experiments is due to an unfortunate omission of nuclear permutation symmetry, i.e., quantum statistics. Enforcement of the latter requires mandatory inclusion of the molecular rotational degree of freedom, even for perfectly aligned molecules. Unlike previous interpretations, we ascribe spatial electron-localization to the laser creation of a rovibronic wavepacket that involves field-free molecular eigenstates with opposite space-inversion symmetry i.e., even and odd parity. Space-inversion symmetry breaking would then lead to an asymmetric distribution of the (space-fixed) electronic density over the forward and backward hemisphere. However, owing to the simultaneous coexistence of two indistinguishable molecular orientational isomers, our analytical and computational results show that the post-dissociation electronic density along a specified space-fixed axis is equally shared between the two identical nuclei-a result that is in perfect accordance with the principle of the indistinguishability of identical particles.

Present and future trends in spin ARPES

Spin-orbit (SO) coupling combined with space inversion symmetry breaking is at the origin of the lifting of the spin degeneracy in the band structure of many materials. The momentum-dependent energy splitting and the degree of spin polarization are indicative of the strength and symmetry of the SO interaction, and enter in the figure of merit of essentially all conceptual designs aiming to control the electron spin degree of freedom in solids. Here we briefly review the role played by spin-and-angle–resolved photoemission (spin ARPES or SARPES) in revealing the spin polarization specifically in the topological states of 3-dimensional (3D) topological insulators (TIs), and we discuss how the progress in the technique can assist the rise of several active fields like spintronics, spin-orbitronics or the generation of spin-polarized beams.

Rectification of AC Electric Current at the Edge of 2D Electron Gas

Driving a 2D electron gas by AC electric field leads to a DC electric current flowing along the edge of the system. The effect is caused by the local breaking of space inversion symmetry at the edge. The current is generated in a narrow stripe determined by the screening length of the AC electric field and the mean free path of carriers. The developed microscopic theory of the effect shows that the excitation spectrum of the edge current and even the current direction depend on the mechanism of electron scattering. In an external magnetic field, the excitation spectrum of the current features the cyclotron resonance.

Symmetry-breaking and spin-blockage effects on carrier dynamics in single-layer tungsten diselenide

Understanding carrier creation and evolution in materials initiated by pulsed optical excitation is central to developing ultrafast optoelectronics. We demonstrate herein that the dynamic response of a system can be drastically modified when its physical dimension is reduced to the atomic scale, the ultimate limit of device miniaturization. A comparative study of single-layer (SL) tungsten diselenide(WSe2) relative to bulk WSe2 shows substantial differences in the transient response as measured by time- and angle-resolved photoemission spectroscopy (TRARPES). The conduction-band minimum in bulk WSe2, populated by optical pumping, decays promptly. The corresponding decay for SL WSe2 is much slower and exhibits two time constants. The results indicate the presence of two distinct decay channels in the SL that are correlated with the breaking of space inversion symmetry in the two-dimensional limit. This symmetry breaking lifts the spin degeneracy of the bands, which in turn causes the blockage of decay for one spin channel. The stark contrast between the single layer and the bulk illustrates the basic carrier scattering processes operating at different timescales that can be substantially modified by dimensional and symmetry-reduction effects.

Polarization-dependent and valley-protected Lamb waves in asymmetric pillared phononic crystals

We present the realization of the topological valley-protected zero-order antisymmetric (A0) or symmetric (S0) and zero-order shear-horizontal (SH0) Lamb waves at different domain walls based on topologically distinct asymmetric double-sided pillared phononic crystals. The elastic periodic structures have either the triangular or the honeycomb symmetry and give rise to a double-negative branch in the dispersion curves. By artificially folding the doubly negative branch, a degenerate Dirac cone is achieved. Different polarization-dependent propagation along the same primary direction along the constituent branches are presented. Moreover, divergent polarization-dependent phenomena along different primary directions along a given branch are also reported. By imposing two large space-inversion symmetry (SIS) breaking perturbations the topological phase transition is obtained. We show that the Berry curvature becomes strongly anisotropic when the wave vector gets away from the valleys. Further, we demonstrate the unidirectional transport of A0, S0, and SH0 Lamb waves at different domain walls in straight or Z-shaped wave guides. In the large SIS breaking case, we show negligible reflection at the zigzag outlet of the straight wave guide and occurrence of weak inter-valley scattering at the bending corners of the Z-shaped wave guide. For a larger strength of SIS breaking, the edge states are gapped and strong reflection at the zigzag outlet and bending corners is observed. The topological protection cannot be guaranteed anymore in that case.

Andreev reflections modulated by the breaking of space-inversion symmetry in graphene-typed materials

We theoretically perform a comprehensive analysis about the influences of the space-inversion symmetry breaking in graphene-based materials on the Andreev reflections (AR) in the normal-metal/superconductor (NS) and NSN heterojunctions. It is found that in the NS junction, the AR can be suppressed or be enhanced by the enhancement of space-inversion symmetry breaking, depending on the relationship among the coherence parameters. Following this result, the AR properties in the NSN structure are evaluated. It is readily observed that the local AR can be weakened for low space-inversion symmetry breaking, and can be enhanced for high space-inversion symmetry breaking. Alternatively, the efficiency of the crossed AR can be improved to a great degree, with the increase of space-inversion symmetry breaking. One can therefore understand the special role of space-inversion symmetry in modulating the AR, especially for the enhancement of crossed AR.

Topological phase transition with p orbitals in the exciton-polariton honeycomb lattice

We study the topological phase transition with the TE-TM splitting in the p-orbital exciton-polariton honeycomb lattice. We find that some Dirac points survive at the high-symmetry points with space-inversion symmetry breaking, which reflects the characteristic of p orbitals. A phase diagram is obtained by the gap Chern number, from which the topological phase transition takes place in the intermediate gap. There is no topological phase transition in the bottom or top gap, and its edge state has the potential application for transporting signals in optoelectronic devices. When taking into account the non-degenerate p orbitals, we find that the bottom gap arises owing to the competition between the Zeeman energy and rotating angular velocity, and topological phase transition also appears in the complete gaps. These results can facilitate the experimental investigations of the topological properties of p-orbital exciton-polariton lattice structure.

Many-body spin Hall effect with space-inversion symmetry

In contrast to the ordinary spin Hall effect (SHE), which is a single-body phenomenon caused by the spin-orbit interaction (SOI), we propose amany-body SHE induced by the dipole-dipole interaction (DDI) between particles and demonstrate it in a system of ultracold magnetic atoms. While the SOI usually requires the breaking of space-inversion symmetry, the DDI preserves it. The many-body SHE can, in principle, be observed in a wide range of systems with large dipole moments and offers a powerful tool to generate spin currents, an essential ingredient in spintronics and atomtronics.

Magnetic hexadecapole order and magnetopiezoelectric metal state in Ba1−xKxMn2As2

We study an odd-parity magnetic multipole order in Ba$_{1-x}$K$_x$Mn$_2$As$_2$ and related materials. Although BaMn$_2$As$_2$ is a seemingly conventional Mott insulator with G-type antiferromagnetic order, we identify the ground state as a magnetic hexadecapole ordered state accompanied by simultaneous time-reversal and space-inversion symmetry breaking. A symmetry argument and microscopic calculations reveal the ferroic ordering of leading magnetic hexadecapole moment and admixed magnetic quadrupole moment. Furthermore, we clarify electromagnetic responses characterizing the magnetic hexadecapole state of semiconducting BaMn$_2$As$_2$ and doped metallic systems. A magnetoelectric effect and antiferromagnetic Edelstein effect are shown. Interestingly, a counter-intuitive currentinduced nematic order occurs in the metallic state. The electric current along the \textit{z}-axis induces the \textit{xy}-plane nematicity in sharp contrast to the spontaneous nematic order in superconducting Febased 122-compounds. Thus, the magnetic hexadecapole state of doped BaMn$_2$As$_2$ is regarded as a magnetopiezoelectric metal. The anomalous responses stem from the peculiar symmetry of the parity-violating magnetic order. Other candidate materials for magnetic hexadecapole order are proposed.

Toroidal Dipolar Response in Metamaterials Composed of Metal–Dielectric–Metal Sandwich Magnetic Resonators

Toroidal metamaterials have been drawing increasing interest recently because of their unusual electromagnetic properties and a variety of potential applications. In this work, we have investigated numerically toroidal dipolar response at optical frequency in metamaterials whose unit cell includes three magnetic resonators. The magnetic resonators are metal–dielectric–metal sandwich nanostructures, which are composed of two Ag rods and a SiO 2 spacer. They have the same shape and dimension, but they are placed at different positions to break the space-inversion symmetry. The near-field plasmon coupling between magnetic resonators leads to the excitation of a toroidal dipolar mode, which is characterized by a head-to-tail distribution of magnetic dipoles within magnetic resonators. In our designed toroidal metamaterials, space-inversion symmetry breaking is needed only in the polarization direction of incident light, and light can be normally incident on the toroidal metamaterials.

Spin- and valley-coupled electronic states in monolayer WSe2 on bilayer graphene

We have fabricated a high-quality monolayer WSe2 film on bilayer graphene by epitaxial growth and revealed the electronic states by spin- and angle-resolved photoemission spectroscopy. We observed a direct energy gap at the Brillouin-zone corner in contrast to the indirect nature of gap in bulk WSe2, which is attributed to the lack of interlayer interaction and the breaking of space-inversion symmetry in monolayer film. A giant spin splitting of ∼0.5 eV, which is the largest among known monolayer transition-metal dichalcogenides, is observed in the energy band around the zone corner. The present results suggest a high potential applicability of WSe2 to develop advanced devices based with the coupling of spin- and valley-degrees of freedom.

Space inversion symmetry breaking and pattern selection in nonlinear optics

Pattern formation in nonlinear optical cavities, when an advection-like term is present, is analysed. This term breaks the space inversion symmetry causing the existence of a regime of convective instabilities, where noise-sustained structures can be found, and changing the pattern orientation and the selected wavevector. The concepts of convective and absolute instability, noise-sustained structures and the selection mechanisms in two dimensions are discussed in the case of optical parametric oscillators and a Kerr resonator. In the latter case, in which hexagons are the selected structure, we predict and observe that stripes are the most unstable structures in the initial linear transient. In the nonlinear regime of the absolute instability these stripes destabilize and hexagons form. Their orientation is dictated by that of the transient stripes and therefore by the advection term. In the convective regime we predict and observe disordered noise-sustained hexagons, preceded in space by noise-sustained stripes.