Parity Breaking Research Articles

Edge current and pairing order transition in chiral bacterial vortices

Bacterial suspensions show turbulence-like spatiotemporal dynamics and vortices moving irregularly inside the suspensions. Understanding these ordered vortices is an ongoing challenge in active matter physics, and their application to the control of autonomous material transport will provide significant development in microfluidics. Despite the extensive studies, one of the key aspects of bacterial propulsion has remained elusive: The motion of bacteria is chiral, i.e., it breaks mirror symmetry. Therefore, the mechanism of control of macroscopic active turbulence by microscopic chirality is still poorly understood. Here, we report the selective stabilization of chiral rotational direction of bacterial vortices in achiral circular microwells sealed by an oil/water interface. The intrinsic chirality of bacterial swimming near the top and bottom interfaces generates chiral collective motions of bacteria at the lateral boundary of the microwell that are opposite in directions. These edge currents grow stronger as bacterial density increases, and, within different top and bottom interfaces, their competition leads to a global rotation of the bacterial suspension in a favored direction, breaking the mirror symmetry of the system. We further demonstrate that chiral edge current favors corotational configurations of interacting vortices, enhancing their ordering. The intrinsic chirality of bacteria is a key feature of the pairing order transition from active turbulence, and the geometric rule of pairing order transition may shed light on the strategy for designing chiral active matter.

Mirror Optical Activity: Nanophotonic Chiral Sensing from Parity Indefiniteness

Mirror symmetry is among the most fundamental concepts of physics, and its spontaneous breaking at the molecular level allows chiral molecules to exist in two enantiomers that are mirror images of each other. The majority of the chiro-optical effects routinely used to detect enantiomers in mixtures, such as circular dichroism, rely on chiral sensitivity to photon circular polarization, and thus do not harness the full potential of mirror-symmetry breaking, which also involves the spatial profile of the radiation. Here we show that the parity indefiniteness of an electromagnetic field interacting with chiral matter supports mirror optical activity, a chiro-optical effect where a chiral film, when it is probed by the mirror-symmetric field of a nanoemitter, produces a near field whose spatial profile has broken mirror symmetry. The detection of near-field dissymmetry can provide an efficient chiral sensing technique. We specialize the discussion to nanofilms with infrared chirality by using a swift electron in an aloof configuration as the nanoemitter and an off-axis transparent conducting nanoparticle as the near-field probe; the spatial dissymmetry factor of the nanoparticle cathodoluminescence is 1 order of magnitude larger than the dimensionless circular dichroism, and is further enhanced to 2 orders of magnitude if an additional graphene sheet is deposited on the film interface.

Nonlinear shallow water dynamics with odd viscosity

In this letter, we derive the Korteweg-de Vries (KdV) equation corresponding to the surface dynamics of a shallow depth ($h$) two-dimensional fluid with odd viscosity ($\nu_o$) subject to gravity ($g$) in the long wavelength weakly nonlinear limit. In the long wavelength limit, the odd viscosity term plays the role of surface tension albeit with opposite signs for the right and left movers. We show that there exists two regimes with a sharp transition point within the applicability of the KdV dynamics, which we refer to as weak $(|\nu_o|< \sqrt{gh^3}/6)$ and strong $(|\nu_o|> \sqrt{gh^3}/6)$ parity-breaking regimes. While the `weak' parity breaking regime results in minor qualitative differences in the soliton amplitude and velocity between the right and left movers, the `strong' parity breaking regime on the contrary results in solitons of depression (negative amplitude) in one of the chiral sectors.

Metal-insulator transition in Cr-doped hollandite vanadate K2V8O16

The hollandite K2V8O16 is a metal with ferromagnetic spin fluctuations in the high temperature (tetragonal I4/m) phase and encounters metal-insulator transition (MIT) at 170 K. The mechanism behind the MIT is still controversial due to inadequate information on the electronic structure. Furthermore, substitution of Rb for K or Ti for V increases the transition temperature. We have investigated whether break in the mirror symmetry is responsible for the insulating ground state of K2V8O16 and since the Cr counterpart is a robust ferromagnet, whether Cr doping which leads to break in mirror symmetry can help in driving a gap. We find that both 25% and 75% Cr doping break the mirror symmetry but only the former leads to an insulating ground state. Ferromagnetism (FM) remains intact in the insulating phase of K2V6Cr2O16. The structural, electronic and magnetic properties of pure and doped K2V8O16 were investigated within first-principles calculations using density functional theory (DFT). Electron correlation suppresses orbital fluctuations between the partially occupied Cr and V-3dt2g states. Consequently, transfer of charge (electron) from V-3d to Cr-3dt2g states is observed which facilitates Cr3+-V4+ charge ordering. Furthermore, Peierls like structural distortion is associated to the breaking of mirror symmetry in the Cr-V rectangular four chain columns within the crystal. Therefore, the simultaneous effect of Peierls instability, charge ordering and Coulomb correlation is responsible for the MIT in K2V6Cr2O16. Besides, Cr-3d O-2p hybridization and Cr-O coupling increases with Cr doping. These two effects are cooperatively responsible for the observed FM in insulating K2V6Cr2O16. More interestingly, the strength of FM is augmented with Cr concentration in K2V8O16.

P not PQ

Parity solutions to the strong CP problem are a compelling alternative to approaches based on Peccei-Quinn symmetry, particularly given the expected violation of global symmetries in a theory of quantum gravity. The most natural of these solutions break parity at a low scale, giving rise to a host of experimentally accessible signals. We assess the status of the simplest parity-based solution in light of LHC data and flavor constraints, highlighting the prospects for near-future tests at colliders, tabletop experiments, and gravitational wave observatories. The origin of parity breaking and associated gravitational effects play crucial roles, providing new avenues for discovery through EDMs and gravity waves. These experimental opportunities underline the promise of generalized parity, rather than Peccei-Quinn symmetry, as a robust and testable solution to the strong CP problem.

Quasi-bound states in the continuum in metal complementary periodic cross-shaped resonators at terahertz frequencies.

We numerically and experimentally demonstrate quasi-bound states in the continuum (BICs) in free-standing metal complementary periodic cross-shaped resonators (CPCRs) at terahertz frequencies. Such induced quasi-BICs arise from the broken symmetry in CPCRs. By slightly breaking mirror symmetry through reducing the length of one arm in CPCRs, the measured Q-factor of the quasi-BIC can reach 102, which is significantly high compared with that of conventional modes. Further study shows that the high Q-factor of the quasi-BIC is mainly determined not by the radiative loss but the material loss, as the asymmetry of the structures is unremarkable. The sharp quasi-BICs realized in the proposed structure may immediately boost the exploration of ultra-narrowband filters and ultra-sensitive sensors at terahertz frequencies.

Complete structural characterization of single carbon nanotubes by Rayleigh scattering circular dichroism.

Non-invasive, high-throughput spectroscopic techniques can identify chiral indices (n,m) of carbon nanotubes down to the single-tube level1-6. Yet, for complete characterization and to unlock full functionality, the handedness, the structural property associated with mirror symmetry breaking, also needs to be identified accurately and efficiently7-14. So far, optical methods fail in the handedness characterization of single nanotubes because of the extremely weak chiroptical signals (roughly 10-7) compared with the excitation light15,16. Here we demonstrate the complete structure identification of single nanotubes in terms of both chiral indices and handedness by Rayleigh scattering circular dichroism. Our method is based on the background-free feature of Rayleigh scattering collected at an oblique angle, which enhances the nanotube's chiroptical signal by three to four orders of magnitude compared with conventional absorption circular dichroism. We measured a total of 30 single-walled carbon nanotubes including both semiconducting and metallic nanotubes and found that their absolute chiroptical signals show a distinct structure dependence, which can be qualitatively understood through tight-binding calculations. Our strategy enables the exploration of handedness-related functionality of single nanotubes and provides a facile platform for chiral discrimination and chiral device exploration at the level of individual nanomaterials.

Spin response and topology of a staggered-Rashba superconductor

Inversion symmetry is a key symmetry in unconventional superconductors, and even its local breaking can have profound implications. For inversion-symmetric systems, there is a competition on a microscopic level between the spin-orbit coupling associated with the local lack of inversion and hybridizing terms that ``restore'' inversion. Investigating a layered system with alternating mirror-symmetry breaking, we study this competition considering the spin response of different superconducting order parameters for the case of strong spin-orbit coupling. We find that signatures of the local noncentrosymmetry, such as an increased spin susceptibility in spin-singlet superconductors for $T\ensuremath{\rightarrow}0$, persist even into the quasi-three-dimensional regime. This leads to a direction-dependent spin response that allows us to distinguish different superconducting order parameters. Furthermore, we identify several regimes with possible topological superconducting phases within a symmetry-indicator analysis. Our results may have direct relevance for the recently reported Ce-based superconductor ${\mathrm{CeRh}}_{2}{\mathrm{As}}_{2}$ and beyond.

Active terahertz spin state and optical chirality in liquid crystal chiral metasurface

Dynamic control of photonic spin state and chirality plays a vital role in various applications, such as polarization control, polarization-sensitive imaging, and biosensing. Here, we present a scheme for the flexible and dynamic manipulation of terahertz spin state conversion and optical chirality by combining two achiral structures: an asymmetric metasurface and a layer of anisotropic liquid crystal. The proposed asymmetric metasurface can realize the polarization conversion effect. For the circularly polarized incidence, it exhibits the asymmetric transmission of the spin-flipped states but no spin-locked optical chirality since its geometry is mirror symmetric along with the wave propagation. The introduction of the liquid crystal makes the composite metasurface not only exhibit the spin state conversion but also spin-locked chirality and spin-flipped chirality on account of breaking mirror symmetry, which realizes an electrically active terahertz chiral device. The experimental results show that the asymmetric transmission of the terahertz spin states can be dynamically manipulated, resulting in a large controllable range 83.8% to \ensuremath{-}30.7% of spin-locked circular dichroism at 0.76 THz and \ensuremath{-}98.2% to 44.7% of spin-flipped circular dichroism at 0.73 THz. This work paves the way for the development of terahertz meta devices capable of enabling active photonic spin state and chirality manipulation.

Absolute Asymmetric Catalysis, from Concept to Experiment: A Narrative

AbstractThe generally accepted hypothesis to explain the origin of biological homochirality (that is to say, the fact that proteinogenic amino acids are left-handed, and carbohydrates right-handed, in all living beings) is to assume, in the course of prebiotic chemical evolution, the appearance of an initial enantiomeric excess in a set of chiral molecular entities by spontaneous mirror-symmetry breaking (SMSB), together with suitable amplification and replication mechanisms that overcome the thermodynamic drive to racemization. However, the achievement of SMSB in chemical reactions taking place in solution requires highly specific reaction networks showing nonlinear dynamics based on enantioselective autocatalysis, and examples of its experimental realization are very rare. On the other hand, emergence of net supramolecular chirality by SMSB in the self-assembly of achiral molecules has been seen to occur in several instances, and the chirality sign of the resulting supramolecular system can be controlled by the action of macroscopic chiral forces. These considerations led us to propose a new mechanism for the generation of net chirality in molecular systems, in which the SMSB takes place in the formation of chiral supramolecular dissipative structures from achiral monomers, leading to asymmetric imbalances in their composition that are subsequently transferred to a standard enantioselective catalytic reaction, dodging in this way the highly limiting requirement of finding suitable reactions in solution that show enantio­selective autocatalysis. We propose the name ‘absolute asymmetric catalysis’ for this approach, in which an achiral monomer is converted into a nonracemic chiral aggregate that is generated with SMSB and that is catalytically active.Our aim in this Account is to present a step-by-step narrative of the conceptual and experimental development of this hitherto unregarded, but prebiotically plausible, mechanism for the emergence of net chirality in molecular reactions.1 Introduction: The Origin of Biological Homochirality and Spontaneous Mirror-Symmetry Breaking2 Experimental Chemical Models for Spontaneous Mirror-Symmetry Breaking: The Soai Reaction and Beyond3 Spontaneous Mirror-Symmetry Breaking in Supramolecular Chemistry: Plenty of Room at the Top4 Absolute Asymmetric Catalysis: An Alternative Mechanism for the Emergence of Net Chirality in Molecular Systems 5 Experimental Realization of Top-Down Chirality Transfer to the Molecular Level6 Conclusions and Outlook

Defect-engineered room-temperature ferromagnetism in quasi-two-dimensional nitrided CoTa2O6

Thermal ammonolysis of quasi-two-dimensional (quasi-2D) $\mathrm{Co}{\mathrm{Ta}}_{2}{\mathrm{O}}_{6}$ yields the ${\mathrm{O}}^{2\ensuremath{-}}/{\mathrm{N}}^{3\ensuremath{-}}$ and anionic vacancy-ordered ${\mathrm{Co}}^{2+}{\mathrm{Ta}}_{2}^{5+}{\mathrm{O}}_{6\ensuremath{-}x}{\mathrm{N}}_{2x/3}{\ensuremath{\square}}_{x/3}$ ($x\ensuremath{\le}0.15$), which exhibits a transition from antiferromagnetism to defect-engineered above-room-temperature ferromagnetism. First-principles calculations reveal that the origin of ferromagnetism is a particular $\mathrm{Co}{\mathrm{O}}_{5}\mathrm{N}$ configuration with N located at Wyckoff position $8j$, which breaks mirror symmetry about the $ab$ plane. A pressure-induced electronic phase transition is also predicted at $\ensuremath{\sim}24.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, accompanied by insulator-to-metal transition and magnetic moment vanishing.

Mirror symmetry breaking and lateral stacking shifts in twisted trilayer graphene

We construct a continuum model of twisted trilayer graphene using {\it ab initio} density-functional-theory calculations, and apply it to address twisted trilayer electronic structure. Our model accounts for moir\'e variation in site energies, hopping between outside layers and within layers. We focus on the role of a mirror symmetry present in ABA graphene trilayers with a middle layer twist. The mirror symmetry is lost intentionally when a displacement field is applied between layers, and unintentionally when the top layer is shifted laterally relative to the bottom layer. We use two band structure characteristics that are directly relevant to transport measurements, the Drude weight and the weak-field Hall conductivity, and relate them via the Hall density to assess the influence of the accidental lateral stacking shifts currently present in all experimental devices on electronic properties, and comment on the role of the possible importance of accidental lateral stacking shifts for superconductivity in twisted trilayers.

Spontaneous chiral self-assembly of CdSe@CdS nanorods

Spontaneous chiral self-assembly of CdSe@CdS nanorods

Mirror-symmetry breaking mitigates finite-size related performance degradation in guided mode resonance filters

Guided mode resonances in subwavelength patterned thin-films endow them with narrow-linewidth near-unity reflectance peaks. Their ultrathin profile is particularly attractive when mated with image sensor arrays that enables compact field-deployable spectral filtering and sensing systems. While this approach enjoys several advantages over other approaches, a well known limitation is the trade-off between the lateral footprint and spectral linewidth. Mirroring strategies involving metallic or distributed Bragg reflectors have been explored in the past to improve lateral confinement at the expense of increased fabrication complexity, footprint, and insertion loss. Here, we numerically study mirrorless grating modification strategies and predict the mitigation of finite-size related performance degradation. Specifically, we consider mirror symmetry broken miniaturized medium refractive index contrast (silicon nitride) gratings, which exhibit quasi bound states in the continuum (QBIC) resonances. For the same lateral footprint, a nearly 2 fold improvement in quality factor is predicted for the proposed design in comparison to a simple grating surrounded by aluminium mirrors. Numerical study of the design and operational performance of visible-wavelength arrayed filters and multiplexed refractive index sensors is presented. For a typical lateral device footprint of 8 µm, the gratings span wavelengths ranging from 560 nm–800 nm with a coupling efficiency of 43–60%, and a full width half maximum (FWHM) of 4 nm–12 nm. Besides this, the proposed geometry gives a four times better figure of merit (FOM) than the unperturbed medium contrast grating in surface refractometric sensing.

Theory of multi-orbital topological superconductivity in transition metal dichalcogenides

We study possible superconducting states in transition metal dichalcogenide (TMD) monolayers, assuming an on-site pairing potential that includes both intra- and inter-orbital terms. We find that if the mirror symmetry with respect to the system’s plane is broken (e.g., by a substrate), this type of pairing can give rise to unconventional superconductivity, including time-reversal-invariant nodal and fully gapped topological phases. Using a multi-orbital renormalization group procedure, we show how these phases may result from the interplay between the local Coulomb repulsion, Hund’s rule coupling, and phonon-mediated attraction. In particular, for a range of interaction parameters, the system transitions from a trivial phase to a nodal phase and finally to a gapped topological phase upon increasing the strength of the mirror symmetry breaking term.

Spin–orbit correlations and exchange-bias control in twisted Janus dichalcogenide multilayers

Janus dichalcogenide multilayers provide a paradigmatic platform to engineer electronic phenomena dominated by spin–orbit coupling. Their unique spin–orbit effects stem from the mirror symmetry breaking in each layer, which induces a colossal Rashba spin–orbit effect in comparison with the conventional dichalcogenide counterparts. Here we put forward twisted Janus dichalcogenide bilayers as a simple platform to realize spin–orbit correlated states. We demonstrate the emergence of flat bands featuring strong spin-momentum locking and the emergence of symmetry broken states with associated non-coplanar magnetization when interactions are included. We further show that the symmetry broken states can be controlled by means of a magnetic substrate, strongly impacting the non-coplanar magnetic texture of the moire unit cell. Our results put forward twisted Janus multilayers as a powerful platform to explore spin–orbit correlated physics, and highlighting the versatility of magnetic substrates to control unconventional moire magnetism.

Bosonization study of a generalized statistics model with four Fermi points

We study a one-dimensional lattice model of fractional statistics in which particles have next-nearest-neighbor hopping between sites which depends on the occupation number at the intermediate site and a statistical parameter $\phi$. The model breaks parity and time-reversal symmetries and has four-fermion interactions if $\phi \ne 0$. We first analyze the model using mean field theory and find that there are four Fermi points whose locations depend on $\phi$ and the filling $\eta$. We then study the modes near the Fermi points using the technique of bosonization. Based on the quadratic terms in the bosonized Hamiltonian, we find that the low-energy modes form two decoupled Tomonaga-Luttinger liquids with different values of the Luttinger parameters which depend on $\phi$ and $\eta$; further, the right and left moving modes of each system have different velocities. A study of the scaling dimensions of the cosine terms in the Hamiltonian indicates that the terms appearing in one of the Tomonaga-Luttinger liquids will flow under the renormalization group and the system may reach a non-trivial fixed point in the long distance limit. We examine the scaling dimensions of various charge density and superconducting order parameters to find which of them is the most relevant for different values of $\phi$ and $\eta$. Finally we look at two-particle bound states that appear in this system and discuss their possible relevance to the properties of the system in the thermodynamic limit. Our work shows that the low-energy properties of this model of fractional statistics have a rich structure as a function of $\phi$ and $\eta$.

Signatures of dephasing by mirror-symmetry breaking in weak-antilocalization magnetoresistance across the topological transition in Pb1−xSnxSe

Many conductors, including recently studied Dirac materials, show saturation of coherence length on decreasing temperature. This surprising phenomenon is assigned to external noise, residual magnetic impurities or two-level systems specific to non-crystalline solids. Here, by considering the SnTe-class of compounds as an example, we show theoretically that breaking of mirror symmetry deteriorates Berry's phase quantization, leading to additional dephasing in weak-antilocalization magnetoresistance (WAL-MR). Our experimental studies of WAL-MR corroborate these theoretical expectations in (111) Pb$_{1-x}$Sn$_x$Se thin film with Sn contents $x$ corresponding to both topological crystalline insulator and topologically trivial phases. In particular, we find the shortening of the phase coherence length in samples with intentionally broken mirror symmetry. Our results indicate that the classification of quantum transport phenomena into universality classes should encompass, in addition to time-reversal and spin-rotation invariances, spatial symmetries in specific systems.

Quantum phases of spin-1 system on 3/4 and 3/5 skewed ladders

We study the quantum phase transitions of frustrated antiferromagnetic Heisenberg spin-1 systems on the 3/4 and 3/5 skewed two leg ladder geometries. These systems can be viewed as arising by periodically removing rung bonds from a zigzag ladder. We find that in large systems, the ground state (gs) of the 3/4 ladder switches from a singlet to a magnetic state for J1≥1.82; the gs spin corresponds to the ferromagnetic alignment of effective S=2 objects on each unit cell. The gs of antiferromagnetic exchange Heisenberg spin-1 system on a 3/5 skewed ladder is highly frustrated and has spiral spin arrangements. The amplitude of the spin density wave in the 3/5 ladder is significantly larger compared to that in the magnetic state of the 3/4 ladder. The gs of the system switches between singlet state and low spin magnetic states multiple times on tuning J1 in a finite size system. The switching pattern is nonmonotonic as a function of J1 and depends on the system size. It appears to be the consequence of a higher J1 favoring a higher spin magnetic state and the finite system favoring a standing spin wave. For some specific parameter values, the magnetic gs in the 3/5 system is doubly degenerate in two different mirror symmetry subspaces. This degeneracy leads to spontaneous spin-parity and mirror symmetry breaking, giving rise to spin current in the gs of the system.

Rabi oscillations of bound states in the continuum.

Photonic bound states in the continuum (BICs) are special localized and non-decaying states of a photonic system with a frequency embedded into the spectrum of scattered states. The simplest photonic structure displaying a single BIC is provided by two waveguides side-coupled to a common waveguide lattice, where the BIC is protected by symmetry. Here we consider such a simple photonic structure and show that by breaking mirror symmetry and allowing for non-nearest neighbor couplings, a doublet of quasi-BIC states can be sustained, enabling weakly damped embedded Rabi oscillations of photons between the waveguides.