Non-zero Coupling Research Articles

Vortices in 𝒫𝒯-symmetric non-Hermitian superfluid

We discuss the properties of the non-Hermitian 𝒫𝒯-symmetric two–scalar fields model. We investigate stability areas of this system and properties of vortices that emerge in the system of two interacting scalar fields. The phase diagram of the model contains stable and unstable regions depending on 𝒫𝒯-symmetry breaking, which intercross the regions of U(1)-symmetric and U(1)-broken phases in a nontrivial way. At non-zero quartic couplings, the non-Hermitian model possesses classical vortex solutions in the 𝒫𝒯-symmetric regions. We also consider a close Hermitian analog of the theory and compare the results with the non-Hermitian model.

A new permutation-symmetry-adapted machine learning diabatization procedure and its application in MgH2 system.

This work introduces a new permutation-symmetry-adapted machine learning diabatization procedure, termed the diabatization by equivariant neural network (DENN). In this approach, the permutation symmetric and anti-symmetric elements in diabatic potential energy metrics (DPEMs) were simultaneously simulated by the equivariant neural network. The diabatization by deep neural network scheme was adopted for machine learning diabatization, and non-zero diabatic coupling was included to increase accuracy in the near degenerate region. Based on DENN, the global DPEMs for 11A' and 21A' states of MgH2 have been constructed. To the best of our knowledge, these are the first global DPEMs for the MgH2 system. The root-mean-square-errors (RMSEs) for diagonal elements (H11 and H22) and the off-diagonal element (H12) around the conical intersection region were 5.824, 5.307, and 5.796meV, respectively. The RMSEs of global adiabatic energies for two adiabatic states were 4.613 and 12.755meV, respectively. The spectroscopic calculations of the 11A' state in the linear HMgH region are in good agreement with the experiment and previous theoretical results. The differences between calculated frequencies and corresponding experiment values are 1.38 and 1.08 cm-1 for anti-symmetric stretching fundamental vibrational frequency and first overtone. The global DPEMs obtained in this work should be useful for further quantum mechanics dynamic simulations on the MgH2 system.

Looking forward to lepton-flavor-violating ALPs

We assess the status of past and future experiments on lepton flavor violating (LFV) muon and tau decays into a light, invisible, axion-like particle (ALP), a. We propose a new experimental setup for MEG II, the MEGII-fwd, with a forward calorimeter placed downstream from the muon stopping target. Searching for μ → ea decays MEGII-fwd is maximally sensitive to LFV ALPs, if these have nonzero couplings to right-handed leptons. The experimental set-up suppresses the (left-handed) Standard Model background in the forward direction by controlling the polarization purity of the muon beam. The reach of MEGII-fwd is compared with the present constraints, the reach of Mu3e and the Belle-II reach from τ → ℓa decays. We show that a dedicated experimental campaign for LFV muon decays into ALPs at MEG II and Mu3e will be able to probe the ALP parameter space in an unexplored region well beyond the existing astrophysical constraints. We study the implications of these searches for representative LFV ALP models, where the presence of a light ALP is motivated by neutrino masses, the strong CP problem and/or the SM flavor puzzle. To this extent we discuss the majoron in low-scale seesaw setups and introduce the LFV QCD axion, the LFV axiflavon and the leptonic familon, paying particular attention to the cases where the LFV ALPs constitute cold dark matter.

Changing Vibration Coupling Strengths of Liquid Acetonitrile with an Angle-Tuned Etalon.

This work is the first report on nonzero molecular vibration-vibration coupling in an infrared cavity-vibration experiment. Vibration-vibration coupling strength is determined as a cavity mode of parallel spaced mirrors (etalon mode or fringe) is angle-tuned in the region between two vibrations of liquid acetonitrile which are Fermi coupled, namely, a CN stretch dominated vibration and a nearby combination band dominated by the symmetric CH3 bend and C-C stretch. All other infrared cavity-vibration work to date involving more than one vibration has used a value of zero for vibration-vibration coupling; however, this work starts with Fermi coupled vibrations and reveals that there are changes in the vibration-vibration coupling and cavity-vibration couplings as the cavity mode is angle-tuned between the interacting vibrations. The ability to change fundamental vibrational dynamics within a cavity is an exciting result which helps to build a foundation for understanding molecular vibrational dynamics in parallel plate etalon cavities.

Conformations and diffusion of flexibly linked colloidal chains

For biologically relevant macromolecules such as intrinsically disordered proteins, internal degrees of freedom that allow for shape changes have a large influence on both the motion and function of the compound. A detailed understanding of the effect of flexibility is needed in order to explain their behavior. Here, we study a model system of freely-jointed chains of three to six colloidal spheres, using both simulations and experiments. We find that in spite of their short lengths, their conformational statistics are well described by two-dimensional Flory theory, while their average translational and rotational diffusivity follow the Kirkwood–Riseman scaling. Their maximum flexibility does not depend on the length of the chain, but is determined by the near-wall in-plane translational diffusion coefficient of an individual sphere. Furthermore, we uncover shape-dependent effects in the short-time diffusivity of colloidal tetramer chains, as well as non-zero couplings between the different diffusive modes. Our findings may have implications for understanding both the diffusive behavior and the most likely conformations of macromolecular systems in biology and industry, such as proteins, polymers, single-stranded DNA and other chain-like molecules.

Boundary effects in two-band superconductors

We present a microscopic study of the behavior of the order parameters near boundaries of a two-band superconducting material, described by the standard tight-binding Bardeen-Cooper-Schrieffer model. We find superconducting surface states. The relative difference between bulk and surface critical temperatures is a nontrivial function of the interband coupling strength. For superconductors with weak interband coupling, boundaries induce variations of the gaps with the presence of multiple length scales, despite non-zero interband Josephson coupling.

A coherent harmonic generation method for producing femtosecond coherent radiation in a laser plasma accelerator based light source.

The electron beam generated in laser plasma accelerators (LPAs) has two main initial weaknesses - a large beam divergence (up to a few milliradians) and a few percent level energy spread. They reduce the beam brightness and worsen the coherence of the LPA-based light source. To achieve fully coherent radiation, several methods have been proposed for generating strong microbunching on LPA beams. In these methods, a seed laser is used to induce an angular modulation into the electron beam, and the angular modulation is converted into a strong density modulation through a beamline with nonzero longitudinal position and transverse angle coupling. In this paper, an alternative method to generate microbunching into the LPA beam by using a seed laser that induces an energy modulation and transverse-longitudinal coupling beamlines that convert the energy modulation into strong density modulation is proposed. Compared with the angular modulation methods, the proposed method can use more than one order of magnitude lower seed laser power to achieve similar radiation performance. Simulations show that with the proposed method a coherent pulse of a few microjoules pulse energy and femtosecond duration can be generated with a typical LPA beam.

Trifundamental quartic model

We consider a multi-scalar field theory with either short-range or long-range free action and with quartic interactions that are invariant under $O(N_1)\times O(N_2) \times O(N_3)$ transformations, of which the scalar fields form a tri-fundamental representation. We study the renormalization group fixed points at two loops at finite $N$ and in various large-$N$ scaling limits for small $\epsilon$, the latter being either the deviation from the critical dimension or from the critical scaling of the free propagator. In particular, for the homogeneous case $N_i = N$ for $i=1,2,3$, we study the subleading corrections to previously known fixed points. In the short-range model, for $\epsilon N^2\gg 1$, we find complex fixed points with non-zero tetrahedral coupling, that at leading order reproduce the results of arXiv:1707.03866 ; the main novelty at next-to-leading order is that the critical exponents acquire a real part, thus allowing a correct identification of some fixed points as IR stable. In the long-range model, for $\epsilon N \ll 1 $, we find again complex fixed points with non-zero tetrahedral coupling, that at leading order reproduce the line of stable fixed points of arXiv:1903.03578; at next-to-leading order, this is reduced to a discrete set of stable fixed points. One difference between the short-range and long-range cases is that, in the former the critical exponents are purely imaginary at leading-order and gain a real part at next-to-leading order, while for the latter the situation is reversed.

Effective action of type II superstring theories at order \u03b1\u20323: NS-NS couplings

Recently, it has been shown that the minimum number of gauge invariant couplings for B-field, metric and dilaton at order α′3 is 872. These couplings, in a particular scheme, appear in 55 different structures. In this paper, up to an overall factor, we fix all parameters in type II supertirng theories by requiring the reduction of the couplings on a circle to be invariant under T-duality transformations. We find that there are 445 non-zero couplings which appear in 15 different structures. The couplings are fully consistent with the partial couplings that have been found in the literature by the four-point S-matrix element and by the non-linear Sigma model methods.

Quantum MonteCarlo Simulations of the 2D Su-Schrieffer-Heeger Model.

Over the past several years, a new generation of quantum simulations has greatly expanded our understanding of charge density wave phase transitions in Hamiltonians with coupling between local phonon modes and the on-site charge density. A quite different, and interesting, case is one in which the phonons live on the bonds, and hence modulate the electron hopping. This situation, described by the Su-Schrieffer-Heeger (SSH) Hamiltonian, has so far only been studied with quantum MonteCarlo in one dimension. Here we present results for the 2D SSH model, show that a bond ordered wave (BOW) insulator is present in the ground state at half filling, and argue that a critical value of the electron-phonon coupling is required for its onset, in contradistinction with the 1D case where BOW exists for any nonzero coupling. We determine the precise nature of the bond ordering pattern, which has hitherto been controversial, and the critical transition temperature, which is associated with a spontaneous breaking of Z_{4} symmetry.

Features of the implementation of fixed phase shifters on coupled microstrip transmission lines with a stub

Fixed phase shifters with a phase-shifting channel based on coupled transmission lines of class II with a stub have a number of advantages in comparison with previously known structures for similar purposes. However, their practical implementation causes difficulties associated with the topological features of the structure, which, in turn, leads to a deterioration in frequency characteristics. In this regard, a numerical analysis of inhomogeneities in a class II structure with a stub and an assessment of their influence on the frequency characteristics of fixed phase shifters is required. In this work, circuitry and electrodynamic modeling of class II structure elements has been carried out, which showed that the greatest parasitic effect on the frequency characteristics of the phase shifter has a nonzero coupling coefficient of single transmission lines and an inhomogeneity localized at the junction of the short-circuited stub and the output arms of the associated transmission lines. A fixed phase shifter with a phase-shifting channel based on a smoothly stepped structure with a stub has been developed. The presented results of the study of fixed phase shifters based on microstrip coupled transmission lines of class II with a stub can be useful to scientific and technical workers engaged in the development of functional devices for the phase shift of microwave signals.

Robust in-phase synchronization in repressor-based coupled gene oscillators

Abstract Inside living cells, proteins or mRNA can show oscillations even without a periodic driving force. Such genetic oscillations are precise timekeepers for cell-cycle regulations, pattern formation during embryonic development in higher animals, and daily cycle maintenance in most organisms. The synchronization between oscillations in adjacent cells happens via intercellular coupling, which is essential for periodic segmentation formation in vertebrates and other biological processes. While molecular mechanisms of generating sustained oscillations are quite well understood, how do simple intercellular coupling produces robust synchronizations are still poorly understood? To address this question, we investigate two models of coupled gene oscillators - activator-based coupled oscillators (ACO) and repressor-based coupled oscillators (RCO) models. In our study, a single autonomous oscillator (that operates in a single cell) is based on a negative feedback, which is delayed by intracellular dynamics of an intermediate species. For the ACO model (RCO), the repressor protein of one cell activates (represses) the production of another protein in the neighbouring cell after a intercellular time delay. We investigate the coupled models in the presence of intrinsic noise due to the inherent stochasticity of the biochemical reactions. We analyze the collective oscillations from stochastic trajectories in the presence and absence of explicit coupling delay and make careful comparison between two models. Our results show no clear synchronizations in the ACO model when the coupling time delay is zero. However, a non-zero coupling delay can lead to anti-phase synchronizations in ACO. Interestingly, the RCO model shows robust in-phase synchronizations in the presence and absence of the coupling time delay. Our results suggest that the naturally occurring intercellular couplings might be based on repression rather than activation where in-phase synchronization is crucial.

Experimental demonstration of valley-protected backscattering suppression and interlayer topological transport for elastic wave in three-dimensional phononic crystals

Topological insulator (TI) that possesses topologically protected characteristic of guiding the wave against disorders and structural perturbations without backscattering has attracted significant research interest in various fields including electromagnetic, acoustic and elastic system. However, for the mechanical system, realizing an elastic analogue of three-dimensional (3D) TI supporting the topologically protected wave propagation in two-dimensional (2D) plane is still a challenge due to the complicated mode polarization of elastic wave in 3D. This paper theoretically and experimentally investigates the robust and layer-selective transports of elastic wave in 3D monolayer- and bilayer-stacked plate-like metamaterial structures. Firstly, considering 3D monolayer-stacked structure, the 2D valley surface states are achieved numerically along the 2D projected plane based on the mechanism of quantum valley Hall effect. The simulation and experimental measurement are performed to confirm the robust transport of 3D elastic wave and backscattering immunity against the straight channel and sharp bends. Then, by stacking the monolayer into bilayer with a twisted angle of 60°, non-zero interlayer coupling of elastic valley layer is introduced and the layer-related topological phase is revealed in the 3D bilayer-stacked structure, giving rise to the 2D topological layer-dependent surface states. Finally, the 3D robust layer-selective transports of elastic wave are tested by experiment. This research provides exciting application perspectives for ultrasonic devices with robustness, lower-power consumption and high-dimensional manipulation.

S-Matrix and Anomaly of de Sitter

S-matrix formulation of gravity excludes de Sitter vacua. In particular, this is organic to string theory. The S-matrix constraint is enforced by an anomalous quantum break-time proportional to the inverse values of gravitational and/or string couplings. Due to this, de Sitter can satisfy the conditions for a valid vacuum only at the expense of trivializing the graviton and closed-string S-matrices. At non-zero gravitational and string couplings, de Sitter is deformed by corpuscular 1/N effects, similarly to Witten–Veneziano mechanism in QCD with N colors. In this picture, an S-matrix formulation of Einstein gravity, such as string theory, nullifies an outstanding cosmological puzzle. We discuss possible observational signatures which are especially interesting in theories with a large number of particle species. Species can enhance the primordial quantum imprints to potentially observable level even if the standard inflaton fluctuations are negligible.

On scalar products in higher rank quantum separation of variables

Using the framework of the quantum separation of variables (SoV) for higher rank quantum integrable lattice models , we introduce some foundations to go beyond the obtained complete transfer matrix spectrum description, and open the way to the computation of matrix elements of local operators. This first amounts to obtain simple expressions for scalar products of the so-called separate states, that are transfer matrix eigenstates or some simple generalization of them. In the higher rank case, left and right SoV bases are expected to be pseudo-orthogonal, that is for a given SoV co-vector \langle\underline{\mathbf{h}}\rangle⟨𝐡̲⟩, there could be more than one non-vanishing overlap \langle{\underline{\mathbf{h}}}|{\underline{\mathbf{k}}}\rangle⟨𝐡̲|𝐤̲⟩ with the vectors |{\underline{\mathbf{k}}}\rangle|𝐤̲⟩ of the chosen right SoV basis. For simplicity, we describe our method to get these pseudo-orthogonality overlaps in the fundamental representations of the \mathcal{Y}(gl_3)𝒴(gl3) lattice model with NN sites, a case of rank 2. The non-zero couplings between the co-vector and vector SoV bases are exactly characterized. While the corresponding SoV-measure stays reasonably simple and of possible practical use, we address the problem of constructing left and right SoV bases which do satisfy standard orthogonality (by standard we mean \langle{\underline{\mathbf{h}}}|{\underline{\mathbf{k}}}\rangle \propto \delta_{\underline{\mathbf{h}}, \underline{\mathbf{k}}}⟨𝐡̲|𝐤̲⟩∝δ𝐡̲,𝐤̲). In our approach, the SoV bases are constructed by using families of conserved charges. This gives us a large freedom in the SoV bases construction, and allows us to look for the choice of a family of conserved charges which leads to orthogonal co-vector/vector SoV bases. We first define such a choice in the case of twist matrices having simple spectrum and zero determinant. Then, we generalize the associated family of conserved charges and orthogonal SoV bases to generic simple spectrum and invertible twist matrices. Under this choice of conserved charges, and of the associated orthogonal SoV bases, the scalar products of separate states simplify considerably and take a form similar to the \mathcal{Y}(gl_2)𝒴(gl2) rank one case.

Random-mass disorder in the critical Gross-Neveu-Yukawa models

An important yet largely unsolved problem in the statistical mechanics of disordered quantum systems is to understand how quenched disorder affects quantum phase transitions in systems of itinerant fermions. In the clean limit, continuous quantum phase transitions of the symmetry-breaking type in Dirac materials such as graphene and the surfaces of topological insulators are described by relativistic (2+1)-dimensional quantum field theories of the Gross-Neveu-Yukawa (GNY) type. We study the universal critical properties of the chiral Ising, XY, and Heisenberg GNY models perturbed by quenched random-mass disorder, both uncorrelated or with long-range power-law correlations. Using the replica method combined with a controlled triple epsilon expansion below four dimensions, we find a variety of new finite-randomness critical and multicritical points with nonzero Yukawa coupling between low-energy Dirac fields and bosonic order parameter fluctuations, and compute their universal critical exponents. Analyzing bifurcations of the renormalization-group flow, we find instances of the fixed-point annihilation scenario—continuously tuned by the power-law exponent of long-range disorder correlations and associated with an exponentially large crossover length—as well as the transcritical bifurcation and the supercritical Hopf bifurcation. The latter is accompanied by the birth of a stable limit cycle on the critical hypersurface, which represents the first instance of fermionic quantum criticality with emergent discrete scale invariance.

Interplay between spin-orbit coupling and Van Hove singularity in the Hund's metallicity of Sr2RuO4

We investigate the dynamical properties of Sr$_2$RuO$_4$ at zero and very low temperature using density functional theory plus dynamical mean-field theory with an exact diagonalization solver. By considering rotationally invariant local interaction, we examine how Hund's coupling and spin-orbit coupling affect the correlated nature of the system. In the absence of Hund's coupling, the system shows a Fermi liquid behavior over the entire range of temperatures we consider. We confirm that the Fermi liquid persists at zero temperature even with nonzero Hund's coupling; however, at sufficient temperatures Hund's coupling significantly reduces the Fermi liquid regime and the system evolves into a typical Hund's metal. At the bare electronic occupancy of Sr$_2$RuO$_4$ ($t_{2g}^4$), a stronger Hund's metallicity accompanies a larger long-time correlator. Remarkably, electron doping further destabilizes the Fermi liquid even though the long-time correlator and magnetic fluctuations decrease upon doping. This suppression of the Fermi liquid is driven by the van Hove singularity above the Fermi level in Sr$_2$RuO$_4$, combined with an enhanced Van Vleck susceptibility by spin-orbit coupling. Such findings point to the important role that electronic structure plays in the behavior of Hund's metals, in addition to magnetic fluctuations.

Model-dependence of minimal-twist OPEs in d > 2 holographic CFTs

Following recent work on heavy-light correlators in higher-dimensional conformal field theories (CFTs) with a large central charge CT, we clarify the properties of stress tensor composite primary operators of minimal twist, [Tm], using arguments in both CFT and gravity. We provide an efficient proof that the three-point coupling leftlangle {mathcal{O}}_L{mathcal{O}}_Lleft[{T}^mright]rightrangle , where {mathcal{O}}_L is any light primary operator, is independent of the purely gravitational action. Next, we consider corrections to this coupling due to additional interactions in AdS effective field theory and the corresponding dual CFT. When the CFT contains a non-zero three-point coupling leftlangle TT{mathcal{O}}_Lrightrangle , the three-point coupling leftlangle {mathcal{O}}_L{mathcal{O}}_Lleft[{T}^2right]rightrangle is modified at large CT if leftlangle TT{mathcal{O}}_Lrightrangle sim sqrt{C_T} . This scaling is obeyed by the dilaton, by Kaluza-Klein modes of prototypical supergravity compactifications, and by scalars in stress tensor multiplets of supersymmetric CFTs. Quartic derivative interactions involving the graviton and the light probe field dual to {mathcal{O}}_L can also modify the minimal-twist couplings; these local interactions may be generated by integrating out a spin-ℓ ≥ 2 bulk field at tree level, or any spin ℓ at loop level. These results show how the minimal-twist OPE coefficients can depend on the higher-spin gap scale, even perturbatively.

Degenerate quasi-normal mode theory for near-field radiation between plasmonic structures.

Near-field radiation can exceed the blackbody radiation limit due to the contributions from evanescent waves. One promising approach to further enhance near-field radiation beyond existing bulk materials is to utilize metamaterials or metasurfaces made from subwavelength plasmonic structures. In this work, we investigate the near-field thermal radiation between complex plasmonic structures with higher-order symmetry and degeneracy, which is crucial for understanding the radiative heat exchange between metamaterials or metasurfaces at extremely small gaps. We demonstrate that the introduction of degeneracy can drastically boost near-field thermal radiation between plasmonic structures. The enhancement of near-field thermal radiation originates from the emergence of degenerate resonance modes and the secondary emission of thermal photons due to the nonzero coupling between the degenerate modes. Our study provides new pathways for designing high-intensity near-field thermal emitters and absorbers for thermophotovoltaics, thermal management, and infrared spectroscopy.

Enhanced CP asymmetries in B rightarrow K mu ^+ mu ^-

We show that the current values of R_K^mathrm {exp} and R_{K^*}^mathrm {exp} can be accommodated by allowing a nonzero New Physics coupling delta C_9^{mu mu } to be complex, both in the scenario in which only delta C_9^{mu mu } is affected, and in the scenario with complex delta C_{9,10}^{mu mu } satisfying delta C_{9}^{mu mu }=-delta C_{10}^{mu mu }. A presence of the weak CP-violating phase can then be tested by measuring the CP-asymmetry, mathcal {A}_mathrm {CP}. We show that this asymmetry is enhanced around the peak of each cbar{c}-resonance, and in fact more pronounced in the close vicinity of J/psi and psi (2S). Therefore, measuring mathcal {A}_mathrm {CP} before and after the resonances’ peak could be revelatory of the CP-violation that originates from beyond the Standard Model, or to be a significant constrain when building a realistic scenario of New Physics.