Parity Breaking Research Articles

Phonon Thermal Hall Conductivity from Scattering with Collective Fluctuations

Because electrons and ions form a coupled system, it is a priori clear that the dynamics of the lattice should reflect symmetry breaking within the electronic degrees of freedom. Recently, this has been clearly evidenced for the case of time-reversal and mirror symmetry breaking by observations of a large phononic thermal Hall effect in many strongly correlated electronic materials. However, the mechanism by which time-reversal breaking and chirality is communicated to the lattice is far from evident. In this paper, we discuss how this occurs via many-body scattering of phonons by collective modes: a consequence of non-Gaussian correlations of the latter modes. We derive fundamental new results for such skew (i.e., chiral) scattering and the consequent thermal Hall conductivity. We emphasize that these results apply to any collective variables in any phase of matter: electronic, magnetic, or neither; highly fluctuating and correlated, or not. As a proof of principle, we compute general formulas for the above quantities for ordered antiferromagnets. From the latter, we obtain the scaling behavior of the phonon thermal Hall effect in clean antiferromagnets. The calculations show several different regimes and give quantitative estimates of similar order to that seen in recent experiments.Received 21 February 2022Revised 3 July 2022Accepted 26 September 2022DOI:https://doi.org/10.1103/PhysRevX.12.041031Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasHall effectMagnetismPhononsSpin-phonon couplingThermal conductivityPhysical SystemsStrongly correlated systemsTechniquesBoltzmann theoryCondensed Matter, Materials & Applied Physics

Correlated insulating states in carbon nanotubes controlled by magnetic field

Recent experiments have shown that nearly metallic nanotubes remain insulating throughout magnetic field sweeps, in which the magnetic flux through the nanotube is expected to pass through a critical point where the single-particle band gap closes in one valley. Here, the authors investigate correlated insulating states of zigzag carbon nanotubes, which emerge from the interplay of electron-electron interactions, magnetic flux, strain, and spin-orbit coupling. They find that the gap for charged excitations generically remains open near the critical flux value, and predict a novel mirror symmetry breaking phase that may arise in the experimentally relevant regime of spin-orbit coupling.

Theory of linewidth narrowing in Fano lasers

We present a general theory for the coherence of Fano lasers based on a bound state in the continuum. We find that such lasers enable an orders-of-magnitude reduction in the quantum-limited linewidth and, by introducing mirror symmetry breaking, the linewidth can be further reduced. In contrast to ordinary macroscopic lasers, though, the linewidth may rebroaden due to optical nonlinearities enhanced by the strong light localization. This leads to the identification of optimal material systems. We also show that the coherence of this new type of microscopic laser can be understood intuitively using a simple, effective potential model. Based on this model, we examine the laser stability and deduce the dependence of the laser linewidth on the general Fano line shape. Our model facilitates the incorporation of other degrees of design freedom and can be applied to a general class of lasers with strongly dispersive mirrors.

Bulk generalized Dzyaloshinskii-Moriya interaction in PT -symmetric antiferromagnets

The Dzyaloshinskii-Moriya interaction (DMI) in magnetic materials plays an important role in spintronics, giving rise to chiral spin textures such as the nucleation and propagation of domain walls and skyrmions. The necessary ingredient for the emergence of the DMI is the lack of inversion symmetry combined with elements with strong spin-orbit coupling. We report on a first-principles investigation of generalized DMIs in bulk centrosymmetric crystals with noncentrosymmetric local sublattices, where we consider a prototype family of $\mathcal{PT}$-symmetric antiferromagnets, including, ${\mathrm{Mn}}_{2}\mathrm{Au}$, ${\mathrm{MnPd}}_{2}$ and MnCuAs in the tetragonal and orthorhombic phases. We employ a Green's function approach to calculate the interatomic relativistic exchange coupling which can, in turn, determine the sublattice elements of the generalized fifth-order DMI tensor ${D}_{\ensuremath{\alpha}\ensuremath{\beta},\ensuremath{\gamma}}^{s{s}^{\ensuremath{'}}}$. We demonstrate that the breaking of the local mirror symmetry and the resulting local Rashba-type spin-momentum locking yield local DMIs with opposite signs on the two antiparallel sublattices. We present numerical results and provide analytical expressions for the magnon dispersion in the presence of the sublattice DMI and show that the intra- (inter-) sublattice DMIs result in identical (opposite) contributions to the nonreciprocal components of the two low-frequency antiferromagnetic magnon modes.

Energy Harvesting Using a Nonlinear Resonator with Asymmetric Potential Wells

This paper presents the results of numerical simulations of a nonlinear bistable system for harvesting energy from ambient vibrating mechanical sources. Detailed model tests were carried out on an inertial energy harvesting system consisting of a piezoelectric beam with additional springs attached. The mathematical model was derived using the bond graph approach. Depending on the spring selection, the shape of the bistable potential wells was modified including the removal of wells’ degeneration. Consequently, the broken mirror symmetry between the potential wells led to additional solutions with corresponding voltage responses. The probability of occurrence for different high voltage/large orbit solutions with changes in potential symmetry was investigated. In particular, the periodicity of different solutions with respect to the harmonic excitation period were studied and compared in terms of the voltage output. The results showed that a large orbit period-6 subharmonic solution could be stabilized while some higher subharmonic solutions disappeared with the increasing asymmetry of potential wells. Changes in frequency ranges were also observed for chaotic solutions.

Two-dimensional quadratic double Weyl semimetal

Unconventional Weyl semimetals have attracted intensive research interest in condensed-matter physics and materials science, but they are very rare in two dimensions. In this work, based on symmetry analysis and first-principles electronic structure calculations, we predict that the Si/Bi van der Waals heterostructure is a two-dimensional unconventional quadratic double Weyl semimetal with strong spin-orbit coupling (SOC). Although unprotected by the ${C}_{3v}$ double group symmetry of the heterostructure, the two-dimensional quadratic double Weyl semimetal is stable for compressive strains up to 6.64%. The system transforms into a trivial semimetal with further increasing strain, where the phase boundary is a two-dimensional triply degenerate semimetal state. Furthermore, the Kane-Mele tight-binding model calculations show that the quadratic double Weyl phase is derived from the competition between the Rashba SOC and the proximity-effect-enhanced intrinsic SOC. On the other hand, by breaking mirror symmetry, the quadratic double Weyl semimetal transforms into a quantum spin Hall insulator as well as a quantum valley Hall insulator phase. Thus, the Si/Bi heterostructure is an excellent platform for studying the exotic physics of the two-dimensional double Weyl semimetal and other novel topological phases.

Probing parity-odd bispectra with anisotropies of GW V modes

It is well known that non-trivial squeezed tensor bispectra can lead to anisotropies in the inflationary stochastic gravitational wave (GW) background, providing us with an alternative and complementary window to primordial non-Gaussianities (NGs) with respect to the CMB. Previous works have highlighted the detection prospects of parity-even tensor NGs via the GW I-mode anisotropies. In this work we extend this by analysing for the first time the additional information carried by GW V-mode anisotropies due to squeezed NGs. We show that GW V modes allow us to probe parity-odd squeezed 〈 tts 〉 and 〈 ttt 〉 bispectra. These bispectra break parity at the non-linear level and can be introduced by allowing alternative symmetry breaking patterns during inflation, like those comprised in solid inflation. Considering a BBO-like experiment, we find that a non-zero detection of squeezed 〈 tts 〉 parity-odd bispectra in the V modes dipole is possible without requiring any short-scale enhancement of the GW power spectrum amplitude over the constraints set by the CMB. We also briefly discuss the role of V-CMB cross-correlations. Our work can be extended in several directions and motivates a systematic search for polarized GW anisotropies in the next generations of GW experiments.

Tremendous Acceleration of Plant Growth by Applying a New Sunlight Converter Sr4 Al14- x Gax O25 :Mn4+ Breaking Parity Forbidden Transition.

Majority of Mn4+ activated oxide phosphors have the wavelength of excitation and emission suitable for acceleration of plant growth as light converter from sunlight to deep red. Here, it is observed that 60% increase of red emission of Sr4 Al14 O25 :0.01Mn4+ is found by substituting 0.1Ga3+ . It is clarified that the increase is originated from a unique mechanism of breaking parity forbidden transition under the substitution of cation in d-d transition by using the tool of special aberration corrected transmission electron microscope(AC-STEM), pre-edge peak (1s→3d) Mn K-edge X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), Rietveld analysis of X-ray diffraction (XRD) patterns, and reflection spectra. Further, a combination of substituted Ga, Mg, and special double flux H3 BO3 /AlF3 is found to tremendously increase the emission intensity (355% up). Actual growth of chlorella and rose is examined by a combination of the cheap Sr4 Al14 O25 :0.01Mn4+ ,0.007Mg2+ ,0.1Ga3+ and a unique reflection typed phosphor-film system as sunlight converting system. Optical density of chlorella and height of rose grass is increased by 36±14% and 174±80% compared with nonphosphor-film, respectively.

Terahertz Spin-Conjugate Symmetry Breaking for Nonreciprocal Chirality and One-Way Transmission Based on Magneto-Optical Moiré Metasurface.

In this work, the gyrotropic semiconductor InSb into the twisted bilayer metasurface to form a magneto-optical moiré metasurface is introduced. Through the theoretical analysis, the "moiré angle" is developed in which case the nonreciprocity and chirality with the spin-conjugate asymmetric transmission are obtained due to the simultaneous breaking of both time-reversal symmetry and spatial mirror symmetry. The experiments confirm that the chirality can be actively manipulated by rotating the twisted angle and the external magnetic field, realizing spin-conjugate asymmetric transmission. Meanwhile, the two spin states also realize the nonreciprocal one-way transmission, and their isolation spectra are also spin-conjugate asymmetric: one is enhanced up to 48 dB, and the other's bandwidth is widened to over 730 GHz. This spin-conjugate symmetry-breaking effect in the MOMM brings a combination of time-space asymmetric transmission, and it also provides a new scheme for the implementation of high-performance THz chirality controllers and isolators.

Moiré phonons and impact of electronic symmetry breaking in twisted trilayer graphene

Twisted trilayer graphene is a particularly promising moir\'e superlattice system, due to its tunability, strong superconductivity, and complex electronic symmetry breaking. Motivated by these properties, we study lattice relaxation and the long-wavelength phonon modes of this system. We show that mirror-symmetric trilayer graphene hosts, aside from the conventional acoustic phonon modes, two classes of shear modes, which are even and odd under mirror reflection. The mirror-even modes are found to be gapless and equivalent to the "phason" modes of twisted bilayer graphene, with appropriately rescaled parameters. The modes odd under mirror symmetry have no analogue in twisted bilayer graphene and exhibit a finite gap, which we show is directly proportional to the degree of lattice relaxation. We also discuss the impact of mirror-symmetry breaking, which can be tuned by a displacement field or result from a stacking shift, and of rotational- as well as time-reversal-symmetry breaking, resulting from spontaneous electronic order. We demonstrate that this can induce finite angular momentum to the phonon branches. Our findings are important to the interpretation of recent experiments, concerning the origin of superconductivity and of linear-in-$T$ resistivity.

Janus Type Monolayers of S-MoSiN2 Family and Van Der Waals Heterostructures with Graphene: DFT-Based Study.

Novel representative 2D materials of the Janus type family X-M-ZN2 are studied. These materials are hybrids of a transition metal dichalcogenide and a material from the MoSi2N4 family, and they were constructed and optimized from the MoSi2N4 monolayer by the substitution of SiN2 group on one side by chalcogen atoms (sulfur, selenium, or tellurium), and possibly replacing molybdenum (Mo) to tungsten (W) and/or silicon (Si) to germanium (Ge). The stability of novel materials is evaluated by calculating phonon spectra and binding energies. Mechanical, electronic, and optical characteristics are calculated by methods based on the density functional theory. All considered 2D materials are semiconductors with a substantial bandgap (>1 eV). The mirror symmetry breaking is the cause of a significant built-in electric field and intrinsic dipole moment. The spin-orbit coupling (SOC) is estimated by calculations of SOC polarized bandstructures for four most stable X-M-ZN2 structures. The possible van der Waals heterostructures of considered Janus type monolayers with graphene are constructed and optimized. It is demonstrated that monolayers can serve as outer plates in conducting layers (with graphene) for shielding a constant external electric field.

Valley Hall elastic topological insulator with large Chern numbers

We present a reconfigurable electro-elastic topological insulator that produces a new valley phase with a large Chern number. This piezoelectric metamaterial comprises a hexagon-like honeycomb structure and periodic bonded piezoelectric beams. The composite piezoelectric Mindlin plate formulation in the isogeometric analysis is employed to calculate the band structure. A Dirac cone can emerge at the K point due to the C3v symmetry of the primitive element. Nontrivial topologically protected bandgaps develop after breaking mirror symmetry with the adding-on negative capacitance circuit. Additionally, we demonstrate that the topological quantity in this system arises in a large valley Chern number of one. The domain wall will exhibit two edge states between different topological microstructures according to the bulk-edge correspondence principle. The shielded transport of flexural waves is highly resistant to structural disturbances such as sharp corners or cavity flaws in the proposed metamaterial, resulting in a novel strategy for the reconfigurable electro-elastic topological insulator.

Engineering electronic structures of Janus monolayer group-III monochalcogenides via biaxial strain

We investigate the effects of biaxial strain and spin-orbit coupling on the electronic structures of Janus monolayer group-III monochalcogenides based on first-principle calculations. Due to the spin-orbit coupling and mirror symmetry breaking, Rashba band splitting appears at Γ point of the bottom conduction band and the band gap decreases significantly. The spin-orbit coupling strength and electronic structures can be greatly tuned by biaxial strain, which originate from the change of interaction between pz orbitals. As the lattice constant decreases, the interaction between pz orbitals is enhanced, leading to the decrease (increase) of the energy of corresponding bonding (anti-bonding) state, which is the origin of the indirect-direct-indirect transition of the band gap. In addition, when tensile strain is applied, the pz orbital component of Te atoms increases, leading to the enhancement of the Rashba effect. These results are of great significance for the band engineering and spin-orbitronics of two-dimensional materials.

Enhancing light emission near low-refractive-index contrast nanostructures via mirror-symmetry breaking

A platform of recent interest and large application potential is one in which the light emitters are directly integrated with an optical metasurface. Plasmonic metals and high-refractive-index, dielectric Mie metasurfaces have been explored in this connection but have their challenges. We propose low-refractive-index, contrast nanostructured thin films for studying metasurface-emitter systems, specifically focusing on two configurations of practical importance: LED white light generation using color converting polymers and fluorescence-based sensing in aqueous media. To achieve light emission enhancement in a low-index contrast, low optical absorption setting, we exploit the excitation of quasi-bound states in the continuum modes in a mirror-symmetry broken grating. Our numerical study predicts widely tuneable sharp-linewidth emission enhancements, near-zero quenching, and large and controllable active volume in the grating vicinity, which are significant improvements in comparison with both plasmonic and high-index contrast, all-dielectric platforms. When compared with simple gratings, mirror-symmetry broken gratings give four times larger radiative enhancement. Our results are of interest in furthering experimental activity and realizing applications such as light converter for efficient white LEDs and smart detection electronics-integrated substrates for sensing.

Symmetry Breaking Slows Convergence of the ADAPT Variational Quantum Eigensolver.

Because quantum simulation of molecular systems is expected to provide the strongest advantage over classical computing methods for systems exhibiting strong electron correlation, it is critical that the performance of VQEs be assessed for strongly correlated systems. For classical simulation, strong correlation often results in symmetry breaking of the Hartree-Fock reference, leading to Löwdin's well-known "symmetry dilemma", whereby accuracy in the energy can be increased by breaking spin or spatial symmetries. Here, we explore the impact of symmetry breaking on the performance of ADAPT-VQE using two strongly correlated systems: (i) the "fermionized" anisotropic Heisenberg model, where the anisotropy parameter controls the correlation in the system, and (ii) symmetrically stretched linear H4, where correlation increases with increasing H-H separation. In both of these cases, increasing the level of correlation of the system leads to spontaneous symmetry breaking (parity and , respectively) of the mean-field solutions. We analyze the role that symmetry breaking in the reference states and orbital mappings of the fermionic Hamiltonians have in the compactness and performance of ADAPT-VQE. We observe that improving the energy of the reference states by breaking symmetry has a deleterious effect on ADAPT-VQE by increasing the length of the ansatz necessary for energy convergence and exacerbating the problem of "gradient troughs".

Hartree-Fock approach to dynamical mass generation in the generalized ( 2+1 )-dimensional Thirring model

The (2+1)-dimensional generalized massless Thirring model with 4-component Fermi-fields is investigated by the Hartree-Fock method. The Lagrangian of this model is constructed from two different four-fermion structures. One of them takes into account the vector$\times$vector channel of fermion interaction with coupling constant $G_v$, the other - the scalar$\times$scalar channel with coupling $G_s$. At some relation between bare couplings $G_s$ and $G_v$, the Hartree-Fock equation for self-energy of fermions can be renormalized, and dynamical generation of the Dirac and Haldane fermion masses is possible. As a result, phase portrait of the model consists of two nontrivial phases. In the first one the chiral symmetry is spontaneously broken due to dynamical appearing of the Dirac mass term, while in the second phase a spontaneous breaking of the spatial parity $\mathcal P$ is induced by Haldane mass term. It is shown that in the particular case of pure Thirring model, i.e. at $G_s=0$, the ground state of the system is indeed a mixture of these phases. Moreover, it was found that dynamical generation of fermion masses is possible for any finite number of Fermi-fields.

On the Definition of Chirality and Enantioselective Fields

AbstractIn solid state physics, any symmetry breaking is known to be associated with emergence of an order parameter. However, the order parameter for molecular and crystal chirality, which is a consequence of parity and mirror symmetry breaking, has not been known since its discovery. In this article, the authors show that the order parameter for chirality can be defined by electric toroidal monopole . By this definition, one becomes able to discuss external field that can distinguish two different enantiomers only by physical fields. In addition, dynamics and fluctuations of the order parameter can be discussed, with which one can obtain fruitful insights on a spin filtering effect called CISS (Chirality Induced Spin Selectivity). Emergence of time‐reversal‐odd dipole by time propagation of quantities is discussed to explain the enantioselective effect (chiral resolution) at a ferromagnetic surface.

Nonlinear Chiroptical Holography with Pancharatnam–Berry Phase Controlled Plasmonic Metasurface

AbstractMetasurface optical holography represents an important technique for developing ultrathin diffractive optical elements. Among various designs, the metasurface devices with strong chiroptical responses offer a new degree of freedom to manipulate light fields. However, in a linear optical regime, it is usually complicated to realize strong chiroptical properties with metasurfaces. Here, nonlinear chiroptical holography using the Pancharatnam–Berry phase controlled achiral metasurface is reported. The metasurface consists of mirror symmetry breaking plasmonic meta‐atoms. Under the pumping of a fundamental wave, the achiral meta‐atoms radiate second harmonic waves with strong circular dichroism and spin dependent Pancharatnam–Berry phase. Both the amplitude and phase of the second harmonic waves can be independently and simultaneously controlled at the subwavelength scale. As a proof of concept, the spin selective nonlinear chiroptical holography is experimentally demonstrated through the second harmonic process on gold plasmonic metasurfaces. The proposed nonlinear metasurface with Pancharatnam–Berry phase and chiroptical properties open new avenues for multifunctional wavefront engineering and optical information processing.

Multiband topologically protected states realized by elastic honeycomb structures based on fundamental mechanical elements

Topological metamaterials with novel properties of topologically protected states have been developed vigorously inspired by the recent emergence of topological phases in electronic systems. In mechanical community, however, the vast majority of such investigations have either focused on discrete mass–spring systems, whose operating bands are single and are difficult to be transformed into practice, or on various carefully designed continuous elastic systems, suffering from the drawbacks of structural complexity and the presence of modal hybridization. In this study, we design an elastic system composed of clamped-free Euler–Bernoulli (EB) beams coupled via slender rods capable of supporting multiband uni-mode topological states, which overcomes the shortcomings of both discrete lattice and continuous structure to a great extent. Through a theoretical beam-spring model together with finite element analysis, we reveal that the symmetry of the underlying lattice structure and the nature of multi-order natural modes of the beam ensure the deterministic existence of multiple Dirac points. Splittings of these degeneracies induced by the mirror symmetry breaking give rise to two kinds of valley Hall phases characterized by opposite valley Chern numbers. Numerical simulations demonstrate that the domain walls formed by structures with distinct topological indices support elastic interface waves at multiple frequency ranges, which are topologically protected manifested by the features of being immune to backscattering of sharp corners and defects. In addition, we show that the frequency range of interest can be easily tuned by varying strength of symmetry breaking. Our results provide an entirely novel paradigm to design topological mechanical systems with multiple working frequencies. Moreover, thanks to the structural simplicity, the proposed elastic system can be exploited in practice as an excellent and versatile platform to explore other intriguing wave phenomena brought about by richer topological notions.

Emergence of Rashba splitting and spin-valley properties in Janus MoGeSiP2As2 and WGeSiP2As2 monolayers

First-principles calculations are performed to study the structural stability and spintronics properties of Janus MoGeSiP2As2 and WGeSiP2As2 monolayers. The high cohesive energies and the stable phonon modes confirm that both these structures are experimentally accessible. In contrast to pristine MoSi2P4, the Janus monolayers demonstrate reduced direct bandgaps and large spin-split states at K/K’. For the monolayered Janus structure, the broken mirror symmetry with respect to the Mo/W-plane gives rise to a potential gradient normal to the basal plane, which causes difference in the work function for the two surfaces. In addition, the spin textures exposed that breaking the mirror symmetry brings Rashba-type spin splitting in the systems which can be increased by using higher atomic spin–orbit coupling. The large valley spin splitting together with the Rashba splitting in these Janus monolayer structures can make a remarkable contribution to semiconductor valleytronics and spintronics.