Intermediate Coupling Strength Research Articles

Dynamics of spin relaxation in nonequilibrium magnetic nanojunctions

We investigate nonequilibrium phenomena in magnetic nano-junctions using a numerical approach that combines classical spin dynamics with the hierarchical equations of motion technique for quantum dynamics of conduction electrons. Our focus lies on the spin dynamics, where we observe non-monotonic behavior in the spin relaxation rates as a function of the coupling strength between the localized spin and conduction electrons. Notably, we identify a distinct maximum at intermediate coupling strength, which we attribute to a competition that involves the increasing influence of the coupling between the classical spin and electrons, as well as the influence of decreasing local density of states at the Fermi level. Furthermore, we demonstrate that the spin dynamics of a large open system can be accurately simulated by a short chain coupled to semi-infinite metallic leads. In the case of a magnetic junction subjected to an external DC voltage, we observe resonant features in the spin relaxation, reflecting the electronic spectrum of the system. The precession of classical spin gives rise to additional side energies in the electronic spectrum, which in turn leads to a broadened range of enhanced damping in the voltage.

Microwave assisted switching in two-phase nanowires

Micromagnetic simulations are used to study the resonances and the microwave assisted magnetic switching (MAS) in two-phase Co/CoPt nanowires. This is chosen as an interesting model system in which the processes of nucleation, interfacial domain wall pinning, and propagation are well distinguished and can all come into play. The coupling strength is varied to cover both smooth (strong coupling) and stepped (weak coupling) hysteresis curves. At intermediate coupling strengths interfacial resonances play an important role in MAS and enable to find optimal conditions combining both low MAS frequencies and dc applied fields. There is always a tradeoff between using low fields and achieving short reversal time.

Synchronization and oscillation quenching in coupled three nonidentical Lorenz oscillators

In this paper, we introduce the heterogeneity in the parameter [Formula: see text] to three coupled Lorenz oscillators and investigate the effects of parameter heterogeneity on the coupling dynamics. In the presence of parameter heterogeneity, the complete synchronous state is replaced by lag synchronous state which owns the largest Lyapunov exponent exactly the same as that of the complete synchronous chaos. We find two types of oscillation quenching states induced by the parameter heterogeneity, homogeneous nontrivial equilibria and heterogeneous equilibria. In the homogeneous nontrivial equilibria, all oscillators fall onto a same nontrivial equilibrium of the isolated Lorenz oscillator, which requires low coupling strength. Depending on the coupling function, the heterogeneous equilibria may appear at intermediate coupling strength or large coupling strength. We numerically show that the transitions among lag synchronous state and different types of quenching states are always discontinuous ones. The stability diagram of the lag synchronous chaos is presented theoretically, which is compatible with those based on the synchronization error and Lyapunov exponents.

Determinants of collective failure in excitable networks.

We study collective failures in biologically realistic networks that consist of coupled excitable units. The networks have broad-scale degree distribution, high modularity, and small-world properties, while the excitable dynamics is determined by the paradigmatic FitzHugh-Nagumo model. We consider different coupling strengths, bifurcation distances, and various aging scenarios as potential culprits of collective failure. We find that for intermediate coupling strengths, the network remains globally active the longest if the high-degree nodes are first targets for inactivation. This agrees well with previously published results, which showed that oscillatory networks can be highly fragile to the targeted inactivation of low-degree nodes, especially under weak coupling. However, we also show that the most efficient strategy to enact collective failure does not only non-monotonically depend on the coupling strength, but it also depends on the distance from the bifurcation point to the oscillatory behavior of individual excitable units. Altogether, we provide a comprehensive account of determinants of collective failure in excitable networks, and we hope this will prove useful for better understanding breakdowns in systems that are subject to such dynamics.

Pinch-points to half-moons and up in the stars: The kagome skymap

Pinch point singularities, associated with flat band magnetic excitations, are tell-tale signatures of Coulomb spin liquids. While their properties in the presence of quantum fluctuations have been widely studied, the fate of the complementary non-analytic features -- shaped as half-moons and stars -- arising from adjacent shallow dispersive bands has remained unexplored. Here, we address this question for the spin $S=1/2$ Heisenberg antiferromagnet on the kagome lattice with second and third neighbor couplings, which allows one to tune the classical ground state from flat bands to being governed by shallow dispersive bands for intermediate coupling strengths. Employing the complementary strengths of variational Monte Carlo, pseudo-fermion functional renormalization group, and density-matrix renormalization group, we establish the quantum phase diagram. The U(1) Dirac spin liquid ground state of the nearest-neighbor antiferromagnet remains remarkably robust till intermediate coupling strengths when it transitions into a pinwheel valence bond crystal displaying signatures of half-moons in its structure factor. Our work thus identifies a microscopic setting that realizes one of the proximate orders of the Dirac spin liquid identified in a recent work [Song, Wang, Vishwanath, He, Nat. Commun. 10, 4254 (2019)]. For larger couplings, we obtain a collinear magnetically ordered ground state characterized by star-like patterns.

Asymmetric phase diagram of DPPC/DOPC to investigate inter-leaflet coupling.

Cell membranes are asymmetric, meaning that the bilayer is composed of leaflets with different lipid composition. Simplified models for the exoplasmic leaflet typically form coexisting Ld (liquid disordered) + Lo (liquid ordered) phases in symmetric bilayers. In contrast, the cytoplasmic leaflet lacks high-melting lipids, and models for this leaflet always form a single fluid phase in symmetric bilayers. Thus, among the major open questions is how the phase behavior of these two leaflets is coupled in an asymmetric bilayer. Here, we report the first experimentally determined asymmetric phase diagram for a binary lipid mixture, DPPC/DOPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine/1,2-dioleoyl-sn-glycero-3-phosphocholine) at room temperature. Different from cholesterol-containing model membranes that separate into coexisting liquid phases, this simpler mixture exhibits gel + fluid coexistence over a range of composition and temperature. We used calcium-induced hemifusion to engineer asymmetric GUVs (aGUVs), and we measured the extent of outer leaflet exchange through quantitative analysis of fluorescent probe intensities. Our experiments can be directly compared to theoretical phase diagrams calculated from mean field theory, where phase boundaries and tieline orientations depend on the relative strength of in-plane and out-of-plane lipid interaction energies. The experimental DPPC/DOPC asymmetric phase diagram is consistent with an intermediate coupling strength whereby anti-registered domains coexist with registered domains at lower levels of asymmetry, but phase separation is completely suppressed at higher levels of asymmetry. Our results allow us to estimate a mismatch free energy of 0.2-0.3 kT per square nanometer for a gel domain directly opposite a fluid domain.

Steady-state Peierls transition in nanotube quantum simulator

Quantum dots placed along a vibrating nanotube provide a quantum simulation platform that can directly address the electron-phonon interaction. This offers promising prospects for the search of new quantum materials and the study of strong correlation effects. As this platform is naturally operated by coupling the dots to an electronic reservoir, state preparation is straightforwardly achieved by driving into the steady state. Here we show that for intermediate electron-phonon coupling strength, the system with spin-polarized quantum dots undergoes a Peierls transition into an insulating regime which exhibits charge-density wave order in the steady state as a consequence of the competition between electronic Coulomb repulsive interactions and phonon-induced attractive interactions. The transport phenomena can be directly observed as fingerprints of electronic correlations. We also present powerful methods to numerically capture the physics of such an open electron-phonon system at large numbers of phonons. Our work paves the way to study and detect correlated electron-phonon physics in the nanotube quantum simulator with current experimentally accessible techniques.

Amplified signal response by cluster synchronization competition in rings with short-distance couplings.

Topological resonance has been revealed in degree-heterogeneous scale-free networks for weak signal amplification, but not in degree-homogeneous all-to-all networks [Acebrón etal., Phys. Rev. Lett. 99, 128701 (2007)0031-900710.1103/PhysRevLett.99.128701]. Here, we show that when the coupling distance of the all-to-all networks is reduced from global to local, i.e., converting all-to-all networks into rings, we can observe a resonant response to a weak signal similar to scale-free networks. We find that such a resonance effect induced by ring topology is robust across a wide range of ring sizes and signal frequencies. We further show that at intermediate coupling strength, oscillators in the rings can form separate synchronous clusters that compete with each other, resulting in large amplitude oscillations of boundary nodes between clusters and thus giving rise to the resonant signal amplification. Finally, we propose a structure of a three-node feed-forward motif simplified from the observed cluster synchronization competition to analyze the mechanism underlying the resonance behavior, which corresponds well with the numerical results.

Finite-temperature optical conductivity with density-matrix renormalization group methods for the Holstein polaron and bipolaron with dispersive phonons

A comprehensive picture of polaron and bipolaron physics is essential to understand the optical absorption spectrum in many materials with electron-phonon interactions. In particular, the finite-temperature properties are of interest since they play an important role in many experiments. Here, we combine the parallel two-site time-dependent variational principle algorithm (p2TDVP) with local basis optimization (LBO) and purification to calculate time-dependent current-current correlation functions. From this information, we extract the optical conductivity for the Holstein polaron and bipolaron with dispersive phonons at finite temperatures. For the polaron in the weak and intermediate electron-phonon coupling regimes, we analyze the influence of phonon dispersion relations on the spectra. For strong electron-phonon coupling, the known result of an asymmetric Gaussian is reproduced for a flat phonon band. For a finite phonon bandwidth, the center of the Gaussian is either shifted to larger or smaller frequencies, depending on the sign of the phonon hopping. We illustrate that this can be well understood by considering the Born-Oppenheimer surfaces. A similar behavior is seen for the bipolaron for strong coupling. For the bipolaron with weak and intermediate coupling strengths and a flat phonon band, we obtain two very different spectra. The latter also has a temperature-dependent resonance at a frequency below the phonon frequency.

Ground states of atomic Fermi gases in a two-dimensional optical lattice with and without population imbalance

We study the ground state phase diagram of population balanced and imbalanced ultracold atomic Fermi gases with a short range attractive interaction throughout the crossover from BCS to Bose-Einstein condensation (BEC), in a two-dimensional optical lattice (2DOL) comprised of two lattice and one continuum dimensions. We find that the mixing of lattice and continuum dimensions, together with population imbalance, has an extraordinary effect on pairing and the superfluidity of atomic Fermi gases. In the balanced case, the superfluid ground state prevails the majority of the phase space. However, for relatively small lattice hopping integral $t$ and large lattice constant $d$, a pair density wave (PDW) emerges unexpectedly at intermediate coupling strength, and the nature of the in-plane and overall pairing changes from particle-like to hole-like in the BCS and unitary regimes, associated with an abnormal increase in the Fermi volume with the pairing strength. In the imbalanced case, the stable polarized superfluid phase shrinks to only a small portion of the entire phase space spanned by $t$, $d$, imbalance $p$ and interaction strength $U$, mainly in the bosonic regime of low $p$, moderately strong pairing, and relatively large $t$ and small $d$. Due to the Pauli exclusion between paired and excessive fermions within the confined momentum space, a PDW phase emerges and the overall pairing evolves from particle-like into hole-like, as the pairing strength grows stronger in the BEC regime. In both cases, the ground state property is largely governed by the Fermi surface topology. These findings are very different from the cases of pure 3D continuum, 3D lattice or 1DOL.

Internal energy and information flows mediate input and output power in bipartite molecular machines.

Microscopic biological systems operate far from equilibrium, are subject to strong fluctuations, and are composed of many coupled components with interactions varying in nature and strength. Researchers are actively investigating the general design principles governing how biomolecular machines achieve effective free-energy transduction in light of these challenges. We use a model of two strongly coupled stochastic rotary motors to explore the effect of coupling strength between components of a molecular machine. We observe prominent thermodynamic characteristics at intermediate coupling strength, near that which maximizes output power: a maximum in power and information transduced from the upstream to the downstream system, and equal subsystem entropy production rates. These observations are unified through a bound on the machine's input and output power, which accounts for both the energy and information transduced between subsystems.

Pairing Phenomena and Superfluidity of Atomic Fermi Gases in a Two‐Dimensional Optical Lattice: Unusual Effects of Lattice‐Continuum Mixing

AbstractThe superfluid behavior of ultracold atomic Fermi gases with a short range attractive interaction in a two‐dimensional optical lattice (2DOL) is studied using a pairing fluctuation theory, within the context of BCS‐BEC crossover. It is found that the mixing of lattice and continuum dimensions leads to exotic phenomena. For relatively large lattice constant d and small hopping integral t, the superfluid transition temperature exhibits a remarkable reentrant behavior as a function of the interaction strength, and leads to a pair density wave ground state, where vanishes, for a range of intermediate coupling strength. In the unitary and BCS regimes, the nature of the in‐plane and overall pairing changes from particle‐like to hole‐like, with an unexpected nonmonotonic dependence of the chemical potential on the pairing strength. The Bose–Einstein condensation asymptotic behaviors exhibit distinct power law dependencies on the interaction strength compared to cases of pure 3D lattice, 3D continuum, and 1DOL. These predictions can be tested in future experiments.

Large random arrowhead matrices: Multifractality, semilocalization, and protected transport in disordered quantum spins coupled to a cavity

Large arrowhead matrices with randomly distributed entries describe a variety of important phenomena where a degree of freedom is nonlocally coupled to a disordered continuum of modes, including central-spin problems in condensed matter, molecular junctions, and quantum emitters in cavity QED. Here we provide an exact solution of random arrowhead Hamiltonians with diagonal disorder in the thermodynamic limit. For concreteness, we focus on the problem of $N$ emitters homogeneously coupled to a nonlocal cavity mode, corresponding to the disordered Tavis-Cummings model of cavity QED in the single-excitation limit, for which we provide asymptotically exact formulas for static and dynamical quantities of interest. By varying the coupling strength, we find that the distribution of energy spacing can be continuously tuned between Poisson statistics and a distribution that is very close to semi-Poisson statistics, the latter statistics being usually associated with the critical point of Anderson localization-delocalization transitions. We show that the system has a peculiar diffusivelike behavior with an escape probability growing linearly with time for any finite coupling strength and that the escape rate can be controlled by selecting the energy of the initial site. The escape rate averaged over the disorder configurations is found to exhibit a maximum for intermediate coupling strengths, before saturating at a lower, $g$-independent value for sufficiently large $N$ in the collective strong-coupling limit, a cavity protection effect. We investigate the system in a two-terminal configuration and show that the steady-state excitation current exhibits features similar to the escape probability, thereby extending our cavity-protected transport scenario to out-of-equilibrium situations. We finally demonstrate that dark states can provide a major contribution to long-distance transport in disordered systems.

Benchmarking the multiconfigurational Hartree method by the exact wavefunction of two harmonically trapped bosons with contact interaction

We consider two bosons in a one-dimensional harmonic trap, interacting by a contact potential, and compare the exact solution of this problem to a self-consistent numerical solution by using the multiconfigurational time-dependent Hartree (MCTDH) method. We thereby benchmark the predictions of the MCTDH method with a few-body problem that has an analytical solution for the most commonly experimentally realized interaction potential in ultracold quantum gases. It is found that exact ground state energy and first order correlations are accurately reproduced by MCTDH up to the intermediate dimensionless coupling strengths corresponding to typical background scattering lengths of magnetically trapped ultracold dilute Bose gases. For larger couplings, established for example by (a combination of) Feshbach resonances and optical trapping, the MCTDH approach overestimates the depth of the trap-induced correlation dip of first order correlations in position space, as well as overestimates the fragmentation, defined as the average relative occupation of orbitals other than the energetically lowest one. We anticipate that qualitatively similar features in the correlation function may arise for larger particle numbers, paving the way for a quantitative assessment of the accuracy of MCTDH by experiments with ultracold atoms.

Signal amplification enhanced by large phase disorder in coupled bistable units.

We study the maximum response of network-coupled bistable units to subthreshold signals focusing on the effect of phase disorder. We find that for signals with large levels of phase disorder, the network exhibits an enhanced response for intermediate coupling strength, while generating a damped response for low levels of phase disorder. We observe that the large phase-disorder-enhanced response depends mainly on the signal intensity but not on the signal frequency or the network topology. We show that a zero average activity of the units caused by large phase disorder plays a key role in the enhancement of the maximum response. With a detailed analysis, we demonstrate that large phase disorder can suppress the synchronization of the units, leading to the observed resonancelike response. Finally, we examine the robustness of this phenomenon to the unit bistability, the initial phase distribution, and various signal waveform. Our result demonstrates a potential benefit of phase disorder on signal amplification in complex systems.

Antiferromagnetic magnons and local anisotropy: Dynamical mean-field study

We present a dynamical mean-field study of antiferromagnetic magnons in a one-, two-, and three-orbital Hubbard model of square and bcc cubic lattice at intermediate coupling strength. We investigate the effect of anisotropy introduced by an external magnetic field or single-ion anisotropy. For the latter we tune continuously between the easy-axis and easy-plane models. We also analyze a model with spin-orbit coupling in cubic site-symmetry setting. The ordered states as well as the magnetic excitations are sensitive to even a small breaking of $SU(2)$ symmetry of the model and follow the expectations of spin-wave theory as well as general symmetry considerations.

Observation of Efimov Universality across a Nonuniversal Feshbach Resonance in ^{39}K.

We study three-atom inelastic scattering in ultracold ^{39}K near a Feshbach resonance of intermediate coupling strength. The nonuniversal character of such resonance leads to an abnormally large Efimov absolute length scale and a relatively small effective range r_{e}, allowing the features of the ^{39}K Efimov spectrum to be better isolated from the short-range physics. Meticulous characterization of and correction for finite-temperature effects ensure high accuracy on the measurements of these features at large-magnitude scattering lengths. For a single Feshbach resonance, we unambiguously locate four distinct features in the Efimov structure. Three of these features form ratios that obey the Efimov universal scaling to within 10%, while the fourth feature, occurring at a value of scattering length closest to r_{e}, instead deviates from the universal value.

Identifiability of structural networks of nonlinear electronic oscillators

The interplay between structure and function is critical in the understanding of complex systems, their dynamics and their behavior. We investigated the interplay between structural and functional networks by means of the differential identifiability framework, which here quantifies the ability of identifying a particular network structure based on (1) the observation of its functional network and (2) the comparison with a prior observation under different initial conditions. We carried out an experiment consisting of the construction of M=20 different structural networks composed of N=28 nonlinear electronic circuits and studied the regions where network structures are identifiable. Specifically, we analyzed how differential identifiability is related to the coupling strength between dynamical units (modifying the level of synchronization) and what are the consequences of increasing the amount of noise existing in the functional networks. We observed that differential identifiability reaches its highest value for low to intermediate coupling strengths. Furthermore, it is possible to increase the identifiability parameter by including a principal component analysis in the comparison of functional networks, being especially beneficial for scenarios where noise reaches intermediate levels. Finally, we showed that the regime of the parameter space where differential identifiability is the highest is highly overlapped with the region where structural and functional networks correlate the most.

Global and local performance metric with inertia effects

A complex system’s structural–dynamical interplay plays a profound role in determining its collective behavior. Irregular behavior in the form of macroscopic chaos, for instance, can be potentially exhibited by the Kuramoto model of coupled phase oscillators at intermediate coupling strength with frequency assortativity and this behavior is theoretically interesting. In practice, however, such irregular behavior is often not under control and is undesired for the system’s functioning. How the underlying structural and oscillators’ dynamical interplay affects a collective phenomenon (and its corresponding stability) after being subjected to disturbances, attracts great attention. Here, we exploit the concept of a coherency performance metric, as a sum of phase differences and frequency displacements, to evaluate the response to perturbations on network-coupled oscillators. We derive the performance metric as a quadratic form of the eigenvalues and eigenmodes corresponding to the unperturbed system and the perturbation vector, and analyze the influences of perturbation direction as well as strength on the metric. We further apply a computational approach to obtain the performance metric’s derivative with respect to the oscillators’ inertia. We finally extend the metric to a local definition which reflects the pairwise casual effects between any two oscillators. These results deepen the understanding of the combined effects of the structural (eigenmodes) and dynamical (inertia) effects on the system stability.

Electrical coupling controls dimensionality and chaotic firing of inferior olive neurons.

We previously proposed, on theoretical grounds, that the cerebellum must regulate the dimensionality of its neuronal activity during motor learning and control to cope with the low firing frequency of inferior olive neurons, which form one of two major inputs to the cerebellar cortex. Such dimensionality regulation is possible via modulation of electrical coupling through the gap junctions between inferior olive neurons by inhibitory GABAergic synapses. In addition, we previously showed in simulations that intermediate coupling strengths induce chaotic firing of inferior olive neurons and increase their information carrying capacity. However, there is no in vivo experimental data supporting these two theoretical predictions. Here, we computed the levels of synchrony, dimensionality, and chaos of the inferior olive code by analyzing in vivo recordings of Purkinje cell complex spike activity in three different coupling conditions: carbenoxolone (gap junctions blocker), control, and picrotoxin (GABA-A receptor antagonist). To examine the effect of electrical coupling on dimensionality and chaotic dynamics, we first determined the physiological range of effective coupling strengths between inferior olive neurons in the three conditions using a combination of a biophysical network model of the inferior olive and a novel Bayesian model averaging approach. We found that effective coupling co-varied with synchrony and was inversely related to the dimensionality of inferior olive firing dynamics, as measured via a principal component analysis of the spike trains in each condition. Furthermore, for both the model and the data, we found an inverted U-shaped relationship between coupling strengths and complexity entropy, a measure of chaos for spiking neural data. These results are consistent with our hypothesis according to which electrical coupling regulates the dimensionality and the complexity in the inferior olive neurons in order to optimize both motor learning and control of high dimensional motor systems by the cerebellum.