Breaking Transition Research Articles

Crystal structure of the inversion-breaking metal Cd2Re2O7

Second harmonic generation (SHG) on the pyrochlore metal Cd2Re2O7 indicates the presence of three order parameters setting in below an inversion breaking transition. Here, we explore a possible structural explanation and relate it not only to the SHG data, but also to neutron and x-ray diffraction, where we find that such a structural scenario can explain certain reflection extinctions observed in single crystal x-ray data. From this analysis, we suggest future experiments that could be done to resolve this matter. Finally, we comment on the Landau-violating nature of the inversion breaking transition and its relation to similar phenomena observed in improper ferroelectrics.

Type-II Dirac states in full Heusler compounds XInPd2 ( X = Ti, Zr, and Hf)

We predict three full Heusler compounds XInPd2 (X = Zr, Hf and Ti) to be potential candidates for type-II Dirac semimetals. The crystal symmetry of these compounds have appropriate chemical environment with a unique interplay of inversion, time reversal and mirror symmetry. These symmetries help to give six pairs of type-II Dirac nodes on the C_4 rotation axis, closely located at/near the Fermi level. Using first principle calculations, symmetry arguments and crystal field splitting analysis, we illustrate the occurrence of such Dirac nodes in these compounds. Bulk Fermi surfaces have been studied to understand the Lorentz symmetry breaking and Lifshitz transition (LT) of Fermi surfaces. Bulk nodes are projected on the (001) and (111) surfaces which form the surface Fermi arcs, that can further be detected by probes such as angle resolved photo-emission and scanning tunneling spectroscopy. By analyzing the evolution of arcs with changing chemical potential, we prove the fragile nature and the absence of topological protection of the Dirac arcs. Our predicted compounds overcome the limitations of the previously reported PtTe2 class of compounds.

Multiple minimum-energy paths and scenarios of unwinding transitions in chiral nematic liquid crystals.

We apply the minimum-energy paths (MEPs) approach to study the helix unwinding transition in chiral nematic liquid crystals. A mechanism of the transition is determined by a MEP passing through a first order saddle point on the free energy surface. The energy difference between the saddle point and the initial state gives the energy barrier of the transition. Two starting approximations for the paths are used to find the MEPs representing different transition scenarios: (a) the director slippage approximation with in-plane helical structures and (b) the anchoring breaking approximation that involves the structures with profound out-of-plane director deviations. It is shown that, at sufficiently low voltages, the unwinding transition is solely governed by the director slippage mechanism with the planar saddle-point structures. When the applied voltage exceeds its critical value below the threshold of the Fréedericksz transition, the additional scenario through the anchoring breaking transitions is found to come into play. For these transitions, the saddle-point structure is characterized by out-of-plane deformations localized near the bounding surface. The energy barriers for different paths of transitions are computed as a function of the voltage and the anchoring energy strengths.

Helix breaking transition in the S4 of HCN channel is critical for hyperpolarization-dependent gating.

In contrast to most voltage-gated ion channels, hyperpolarization- and cAMP gated (HCN) ion channels open on hyperpolarization. Structure-function studies show that the voltage-sensor of HCN channels are unique but the mechanisms that determine gating polarity remain poorly understood. All-atom molecular dynamics simulations (~20 μs) of HCN1 channel under hyperpolarization reveals an initial downward movement of the S4 voltage-sensor but following the transfer of last gating charge, the S4 breaks into two sub-helices with the lower sub-helix becoming parallel to the membrane. Functional studies on bipolar channels show that the gating polarity strongly correlates with helical turn propensity of the substituents at the breakpoint. Remarkably, in a proto-HCN background, the replacement of breakpoint serine with a bulky hydrophobic amino acid is sufficient to completely flip the gating polarity from inward to outward-rectifying. Our studies reveal an unexpected mechanism of inward rectification involving a linker sub-helix emerging from HCN S4 during hyperpolarization.

Determination of the star-spot covering fraction as a function of stellar age from observational data

ABSTRACT The association of star-spots with magnetic fields leads to an expectation that quantities which correlate with magnetic field strength may also correlate with star-spot coverage. Since younger stars spin faster and are more magnetically active, assessing whether star-spot coverage correlates with shorter rotation periods and stellar youth tests these principles. Here, we analyse the star-spot covering fraction versus stellar age for M-, G-, K-, and F-type stars based on previously determined variability and rotation periods of over 30 000 Kepler main-sequence stars. We determine the correlation between age and variability using single and dual power-law best fits. We find that star-spot coverage does indeed decrease with age. Only when the data are binned in an effort to remove the effects of activity cycles of individual stars, do statistically significant power-law fits emerge for each stellar type. Using bin averages, we then find that the star-spot covering fraction scales with the X-ray to bolometric ratio to the power λ with 0.22 ± 0.03 < λ < 0.32 ± 0.09 for G-type stars of rotation period below 15 d and for the full range of F- and M-type stars. For K-type stars, we find two branches of λ separated by variability bins, with the lower branch showing nearly constant star-spot coverage and the upper branch λ ∼ 0.35 ± 0.04. G-type stars with periods longer than 15 d exhibit a transition to steeper power law of λ ∼ 2.4 ± 1.0. The potential connection to previous rotation-age measurements suggesting a magnetic breaking transition at the solar age, corresponding to period of 24.5 is also of interest.

Stability analysis of parallel channels in flow boiling system: Flow maldistribution and density wave oscillations

In a flow boiling thermal-hydraulic system, Density wave oscillations (DWO) and Flow maldistribution (FMD) are two important instability phenomena. These instabilities can lead to undesirable consequences of system operation. Thermal-hydraulic system with an odd number of channels behave differently as compared to an even number of channels. Following FMD for a two channel system, it leads to a new steady state operating condition. Here, one channel receives higher flowrate and another channel receives lower flowrate for a forced flow system. It may be possible that the new operating point is in a DWO zone. Post FMD, the individual flowrate of the channel starts to oscillate. It shall be noted that under constant mass flowrate condition, such local behaviour in the channels will be difficult to detect. For a twin-channel forced flow system, DWO which may be observed with/without FMD is related to out-of-phase oscillations only. It is thus interesting to study the interaction of DWO and FMD phenomena for a tri-channel flow boiling system. Symmetry breaking transition in the context of excursion instability (i.e., Ledinegg instability) is analyzed in this work. Here, a two and three channel system is used to analyze the two phenomena, FMD and DWO. The difference between the two and three channel flow boiling system is brought out in this work. Unlike two channel system, it is observed that a region in three channel system exists where both symmetrical and asymmetrical equilibrium solutions are unstable. Further, post FMD the solution starts to oscillate but the total system inflow and outflow remains constant.

Quantum state tomography across the exceptional point in a single dissipative qubit

Open systems with gain and loss, described by non-trace-preserving, non-Hermitian Hamiltonians, have been a subject of intense research recently. The effect of exceptional-point degeneracies on the dynamics of classical systems has been observed through remarkable phenomena such as the parity-time symmetry breaking transition, asymmetric mode switching, and optimal energy transfer. On the other hand, consequences of an exceptional point for quantum evolution and decoherence are hitherto unexplored. Here, we use post-selection on a three-level superconducting transmon circuit with tunable Rabi drive, dissipation, and detuning to carry out quantum state tomography of a single dissipative qubit in the vicinity of its exceptional point. Quantum state tomography reveals the PT symmetry breaking transition at zero detuning, decoherence enhancement at finite detuning, and a quantum signature of the exceptional point in the qubit relaxation state. Our observations demonstrate rich phenomena associated with non-Hermitian physics such as non-orthogonality of eigenstates in a fully quantum regime and open routes to explore and harness exceptional point degeneracies for enhanced sensing and quantum information processing.

Finite-size and finite-time effects in large deviation functions near dynamical symmetry breaking transitions

We introduce and study a class of particle hopping models consisting of a single box coupled to a pair of reservoirs. Despite being zero-dimensional, in the limit of large particle number and long observation time, the current and activity large deviation functions of the models can exhibit symmetry-breaking dynamical phase transitions. We characterize exactly the critical properties of these transitions, showing them to be direct analogues of previously studied phase transitions in extended systems. The simplicity of the model allows us to study features of dynamical phase transitions which are not readily accessible for extended systems. In particular, we quantify finite-size and finite-time scaling exponents using both numerical and theoretical arguments. Importantly, we identify an analogue of critical slowing near symmetry breaking transitions and suggest how this can be used in the numerical studies of large deviations. All of our results are also expected to hold for extended systems.

Kaon oscillations and baryon asymmetry of the Universe

Baryon asymmetry of the universe (BAU) is naturally explained with $K^0-K^{0'}$ oscillations of a newly developed mirror-matter model and new understanding of quantum chromodynamics (QCD) phase transitions. A consistent picture for the origin of both BAU and dark matter is presented with the aid of $n-n'$ oscillations of the new model. The global symmetry breaking transitions in QCD are proposed to be staged depending on condensation temperatures of strange, charm, bottom, and top quarks in the early universe. The long-standing BAU puzzle can then be understood with $K^0-K^{0'}$ oscillations that occur at the stage of strange quark condensation and baryon number violation via a non-perturbative sphaleron-like (coined "quarkiton") process. Similar processes at charm, bottom, and top quark condensation stages are also discussed including an interesting idea for top quark condensation to break both the QCD global $U_t(1)_A$ symmetry and the electroweak gauge symmetry at the same time. Meanwhile, the $U(1)_A$ or strong CP problem of particle physics is simply solved under the same framework.

Symmetry breaking transition towards directional locomotion in Physarum microplasmodia

True slime mold Physarum polycephalum has been widely used as a model organism to study flow-driven amoeboid locomotion as well as the dynamics of its complex mechanochemical self-oscillations. The aim of this work is to quantify the mechanical aspects of symmetry breaking and its transition into directional flow-driven amoeboid locomotion in small (<m) fragments of Physarum polycephalum. To this end, we combined measurements of traction stresses, fragment morphology, and ectoplasmic microrheology with experimental manipulations of cell–substrate adhesion, cortical strength, and microplasmodium size. These measurements show that initiation of locomotion is accompanied by the symmetry breaking of traction stresses and the polarization of ectoplasmic mechanical properties, with the rear part of the microplasmodium becoming significantly stiffer after the onset of locomotion. Our experimental data suggest that the initiation of locomotion in Physarum could be analogous to an interfacial instability process and that microplasmodial size is a critical parameter governing the instability. Specifically, our results indicate that the instability driving the onset of locomotion is strengthened by substrate adhesiveness and weakened by cortical stiffness. Furthermore, the Fourier spectral analysis of morphology revealed lobe number n = 2 as the consistent dominant mode number across various experimental manipulations, suggesting that the instability mechanism driving the onset of Physarum locomotion is robust with respect to changes in environmental conditions and microplasmodial properties.

Vinylidene Homologation of Boronic Esters and its Application to the Synthesis of the Proposed Structure of Machillene

Alkenyl boronic esters are important reagents in organic synthesis. Herein, we report that these valuable products can be accessed by the homologation of boronic esters with lithiated epoxysilanes. Aliphatic and electron-rich aromatic boronic esters provided vinylidene boronic esters in moderate to high yields, while electron-deficient aromatic and vinyl boronic esters were found to give the corresponding vinyl silane products. Through DFT calculations, this divergence in mechanistic pathway has been rationalized by considering the stabilization of negative charge in the C-Si and C-B bond breaking transition states. This vinylidene homologation was used in a short six-step stereoselective synthesis of the proposed structure of machillene, however, synthetic and reported data were found to be inconsistent.

Vinylidene Homologation of Boronic Esters and its Application to the Synthesis of the Proposed Structure of Machillene

AbstractAlkenyl boronic esters are important reagents in organic synthesis. Herein, we report that these valuable products can be accessed by the homologation of boronic esters with lithiated epoxysilanes. Aliphatic and electron‐rich aromatic boronic esters provided vinylidene boronic esters in moderate to high yields, while electron‐deficient aromatic and vinyl boronic esters were found to give the corresponding vinyl silane products. Through DFT calculations, this divergence in mechanistic pathway has been rationalized by considering the stabilization of negative charge in the C−Si and C−B bond breaking transition states. This vinylidene homologation was used in a short six‐step stereoselective synthesis of the proposed structure of machillene, however, synthetic and reported data were found to be inconsistent.

Thermomechanical properties of PEGDA in combination with different photo-curable comonomers

Abstract The technology of pharmaceutical drug delivery systems (DDS) as an individual and adjustable tool for drug administration has been intensively developed in the last years. Additive manufacturing (AM) techniques, such as stereolithography, are a promising approach towards DDS scaffold manufacturing. Stereolithography, by using layerby- photo-polymerisation, creates DDS scaffolds with highly controlled 3D geometry. Combined with inkjet printing it allows a very precise positioning of the drug depot in the basic scaffold and therefore also a better control of the drug release. Furthermore, this hybrid AM technique also allows for the creation of a multi-drug DDS with a several drug depots localized in desired positions within the scaffold. Determination of the scaffold and drug depot material properties is one of the initial steps for such novel DDS development. Basic characteristics, such as stiffness, elasticity or glass transition temperature (Tg), are important for designing and adapting the material for biomedical application. The photosensitive poly(ethylene glycol) diacrylate (PEGDA) can be easily formed into a desired biocompatible scaffold geometry via stereolithography. In this study we have focused on the evaluation of PEGDA (Mn=700 g/mol) as a pure and copolymer system in combination with other acrylates (butanediol diacrylate, pentaerythritol triacrylate) as possible materials for DDS using this novel hybrid AM technique. Irgacure 2959, a biocompatible photoinitiator (PI), was used as a radical starter for photopolymerisation. Samples varying in PI and coacrylate concentration were prepared by conventional photopolymerisation. Physico-chemical analyses of the samples were performed and several parameters, such as stiffness, elongation at break and glass transition temperatures, were determined.

Bipartitioning of directed and mixed random graphs

We show that an intricate relation of cluster properties and optimal bipartitions, which takes place in undirected random graphs, extends to directed and mixed random graphs. In particular, the satisfability threshold coincides with the relative size of the giant OUT component reaching 1/2. Moreover, when counting undirected links as two directed ones, the partition cost, and cluster properties, as well as location of the replica symmetry breaking transition for these random graphs depend primarily on the total number of directed links and not on their specific distribution.

Invariant theory and orientational phase transitions.

The Landau theory of phase transitions has been productively applied to phase transitions that involve rotational symmetry breaking, such as the transition from an isotropic fluid to a nematic liquid crystal. It even can be applied to the orientational symmetry breaking of simple atomic or molecular clusters that are not true phase transitions. In this paper, we address fundamental problems that arise with the Landau theory when it is applied to rotational symmetry breaking transitions of more complex particle clusters that involve order parameters characterized by larger values of the l index of the dominant spherical harmonic that describes the broken symmetry state. The problems are twofold. First, one may encounter a thermodynamic instability of the expected ground state with respect to states with lower symmetry. A second problem concerns the proliferation of quartic invariants that may or may not be physical. We show that the combination of a geometrical method based on the analysis of the space of invariants, developed by Kim to study symmetry breaking of the Higgs potential, with modern visualization tools provides a resolution to these problems. The approach is applied to the outcome of numerical simulations of particle ordering on a spherical surface and to the ordering of protein shells.

Inertial torques and a symmetry breaking orientational transition in the sedimentation of slender fibres

Experimental measurements of the force and torque on freely settling fibres are compared with predictions of the slender-body theory of Khayat &amp; Cox (J. Fluid Mech., vol. 209, 1989, pp. 435–462). Although the flow is viscous dominated at the scale of the fibre diameter, fluid inertia is important on the scale of the fibre length, leading to inertial torques which tend to rotate symmetric fibres toward horizontal orientations. Experimentally, the torque on symmetric fibres is inferred from the measured rate of rotation of the fibres using a quasi-steady torque balance. It is shown theoretically that fibres with an asymmetric radius or mass density distribution undergo a supercritical pitch-fork bifurcation from vertical to oblique settling with increasing Archimedes number, increasing Reynolds number or decreasing asymmetry. This transition is observed in experiments with asymmetric mass density and we find good agreement with the predicted symmetry breaking transition. In these experiments, the steady orientation of the oblique settling fibres provides a means to measure the inertial torque in the absence of transient effects since it is balanced by the known gravitational torque.

Phase transition and vacuum stability in the classically conformal B\u2013L model

Within classically conformal models, the spontaneous breaking of scale invariance is usually associated to a strong first order phase transition that results in a gravitational wave background within the reach of future space-based interferometers. In this paper we study the case of the classically conformal gauged B–L model, analysing the impact of this minimal extension of the Standard Model on the dynamics of the electroweak symmetry breaking and derive its gravitational wave signature. Particular attention is paid to the problem of vacuum stability and to the role of the QCD phase transition, which we prove responsible for concluding the symmetry breaking transition in part of the considered parameter space. Finally, we calculate the gravitational wave signal emitted in the process, finding that a large part of the parameter space of the model can be probed by LISA.

Determination of symmetry breaking transitions and polymorphism in Au[sbnd]Cu nanostructures by nano-beam electron diffraction

Alloyed nanostructures of AuCu in varying compositions were synthesized by wet chemistry routes. Synthetic products in all the compositions are FCC solid solutions of AuCu with slightly varying lattice parameter. Variation in shape, size, and nature of attachment has been studied through electron microscopy and localized surface plasmon resonance measurements. FCC solid solutions upon heat-treatment at ~400 °C for 1 h under N2 atmosphere, transform to orthorhombic AuCu (oP8) and many superlattice structures based on it. Nano-beam electron diffraction reveals multiplication of periodicities along [31¯0], [210] and [201] directions, mostly originating due to the chemical or vacancy ordering. Symmetry breaking transitions of orthorhombic AuCu (oP8) from Pbam to P21212 has been observed. The phase with P21212 space group also shows polymorphic transition to P21221. All these transitions have been confirmed through nano-beam electron diffraction coupled with computed diffraction patterns based on Wyckoff positions. Commonly observed orthorhombic (oI40) AuCu upon heat-treatment has not been found. The multiplication of lattice periodicity along certain crystallographic directions, symmetry breaking transition and polymorphism in the same sub-group are important findings of this work. These might have far-reaching implications in electron transport properties of such nanostructures.

Interaction effects on the PT -symmetry-breaking transition in atomic gases

Non-Hermitian systems having parity-time ($\mathcal {PT}$) symmetry can undergo a transition, spontaneously breaking the symmetry. Ultracold atomic gases provide an ideal platform to study interaction effects on the transition. We consider a model system of $N$ bosons of two components confined in a tight trap. Radio frequency and laser fields are coupled to the bosons such that the single particle Non-Hermitian Hamiltonian $h_{\mathcal PT}=-i \Gamma\sigma_z+J\sigma_x$, which has $\mathcal {PT}$-symmetry, can be simulated in a \emph{passive} way. We show that when interatomic interactions are tuned to maintain the symmetry, the $\mathcal {PT}$-symmetry breaking transition is affected only by the SU(2) variant part of the interactions parameterized by $\delta g$. We find that the transition point $\Gamma_{\rm tr}$ decreases as $|\delta g|$ or $N$ increases; in the large $|\delta g|$ limit, $\Gamma_{\rm tr}$ scales as $\sim|\delta g|^{-(N-1)}$. We also give signatures of the $\mathcal {PT}$-symmetric and the symmetry breaking phases for the interacting bosons in experiment.

Entropic Analysis of Mirror Symmetry Breaking in Chiral Hypercycles.

Replicators are fundamental to the origin of life and evolvability. Biology exhibits homochirality: only one of two enantiomers is used in proteins and nucleic acids. Thermodynamic studies of chemical replicators able to lead to homochirality shed valuable light on the origin of homochirality and life in conformity with the underlying mechanisms and constraints. In line with this framework, enantioselective hypercyclic replicators may lead to spontaneous mirror symmetry breaking (SMSB) without the need for additional heterochiral inhibition reactions, which can be an obstacle for the emergence of evolutionary selection properties. We analyze the entropy production of a two-replicator system subject to homochiral cross-catalysis which can undergo SMSB in an open-flow reactor. The entropy exchange with the environment is provided by the input and output matter flows, and is essential for balancing the entropy production at the non-equilibrium stationary states. The partial entropy contributions, associated with the individual elementary flux modes, as defined by stoichiometric network analysis (SNA), describe how the system’s internal currents evolve, maintaining the balance between entropy production and exchange, while minimizing the entropy production after the symmetry breaking transition. We validate the General Evolution Criterion, stating that the change in the chemical affinities proceeds in a way as to lower the value of the entropy production.