Weak Coupling Strength Research Articles

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.

Design and simulation of a binary full adder based on quantum rings

In this paper, a binary full adder circuit is designed with eight quantum rings where each ring is threaded by a magnetic flux which is an odd multiple of half of the magnetic flux quantum. The quantum rings are divided into two categories with four rings connected to each other in parallel. They are attached symmetrically to two semi-infinite one-dimensional metallic electrodes , and three gate voltages are applied as three inputs of the full adder. The Hamiltonian of the system is approximated by the tight-binding method, and the calculations are performed by using Green's function formalism. The current-voltage characteristics are obtained for different values of applied gate voltages in the strong and weak-coupling strengths between the quantum rings and source and drain electrodes. The results are in accordance with the truth table of a binary full adder and show this quantum circuit behaves as a binary full adder. • This work proposes a binary full adder consisting of eight quantum rings where a constant magnetic flux is passing through each of them. • The designed full adder with eight quantum rings is divided into two separate parts (Sum and Carry-out) which are connected symmetrically to the source and drain electrodes. • The values of the applied gate voltages are considered as the full adder inputs and the electric current is the output of the system. • The output current of the proposed circuit based on the system input voltages is in accordance with the truth table of a binary full adder.

Reversible Conversion of Odd/Even One-Way Modes in Magneto-Optical Photonic Crystal Double-Channel Waveguides.

We have studied the transmission properties of odd/even one-way modes and their reversible conversion in a double-channel waveguide consisting of two magneto-optical photonic crystals (MOPCs) sandwiched with Al2O3 PC. There exist two pairs of even and odd modes, i.e., M1(even)/M2(odd) or M3(odd)/M4(even) modes, for the double-channel waveguides with one- or two-stranded coupling layer of Al2O3 rods, respectively. Among them, the M1, M2, and M3 modes are caused by the weak coupling strength of two sub-waveguides, while the M4 mode results from the strong coupling effect and supports dispersionless slow-light propagation. Furthermore, we realize the reversible conversion between odd and even modes (i.e., between M1 and M2 modes, or M3 and M4 modes) in the one- or two-stranded structure, respectively, by adjusting the length and position of the perfect electric conductor (PEC) defect properly to cause the desired significant phase delay along the upper and lower equivalent transmission paths. Additionally, we find that the robustness of the M1 even mode is poor because of extra excitations of counter-propagation modes near the right Brillouin boundary, while the other three modes have extremely strong robustness against PEC defects and their one-way transmittances are nearly 100%. These results hold promise for many fields, such as slow-light modulation and the design of topological devices.

Pressure Control of the Structure and Multiferroicity in a Hydrogen-Bonded Metal-Organic Framework.

Multiferroic materials with the cross-coupling of magnetic and ferroelectric orders provide a new platform for physics study and designing novel electronic devices. However, the weak coupling strength of ferroelectricity and magnetism is the main obstacle for potential applications. The recent research focuses on enhancing the coupling effect via synthesizing novel materials in a chemical route or tuning the multiferroicity in the physical way. Among them, pressure is an effective method to modify multiferroic materials, especially when the chemical doping has reached its tuning limit. In this work, we systemically studied the multiferroic properties in a hydrogen-bonded metal-organic framework (MOF) [(CH3)2NH2]Ni(HCOO)3 under high pressure. X-ray diffraction and Raman scattering reveal that a structural phase transition occurs in a pressure region of 6-9 GPa, and the crystal structure is greatly modified by pressure. With the ac magnetic susceptibility, pyroelectric current, and dielectric constant measurements, we obtain the multiferroic property evolution under high pressure and create a temperature-pressure phase diagram. Our study demonstrates that the pressure can modify the magnetic superexchange interaction and hydrogen bonding simultaneously in these perovskite-like MOFs. The multiferroic phase region has been expanded to higher temperature due to the pressure-enhanced spin-phonon coupling effect.

Echo effect in brain networks

Echo behavior has been found in many systems and takes roles in different aspects. By checking the time series of terminating processes of dynamics events, we find that echo behavior may also exist in brain networks but its mechanism remains unclear. We here study the echo effect on real brain networks by presenting a new framework, which replaces the two perturbations in previous studies by only one perturbation and is thus convenient for theoretical analysis. We interestingly find that echo effect depends on both the phase-resetting parameter and coupling strength, and there is an optimal area of echo effect in phase diagram where the optimal phase-resetting parameter and weak coupling strength are the two necessary conditions. We further reveal that echo comes from a new mechanism of perturbation selected correlation, i.e. a reverse deduced correlation, which explains a more fundamental aspect of echo. Moreover, a theoretical analysis is provided to explain the echo effect, based on the uncoupled brain network. These findings uncover the underlying mechanism why the echoes in brain networks can only happen during the terminating processes, i.e. the condition of optimal phase-resetting or the needed perturbations is provided right before the terminating process while the condition of weak coupling is provided by the terminating process.

Nonperturbative renormalization of quantum thermodynamics from weak to strong couplings

By solving the exact master equation of open quantum systems, we formulate the quantum thermodynamics from weak to strong couplings. The open quantum systems exchange matters, energies and information with their reservoirs through quantum particles tunnelings that are described by the generalized Fano-Anderson Hamiltonians. We find that the exact solution of the reduced density matrix of these systems approaches a Gibbs-type state in the steady-state limit for both the weak and strong system-reservoir coupling strengths. When the couplings become strong, thermodynamic quantities of the system must be renormalized. The renormalization effects are obtained nonperturbatively after exactly traced over all reservoir states through the coherent state path integrals. The renormalized system Hamiltonian is characterized by the renormalized system energy levels and interactions, corresponding to the quantum work done by the system. The renormalized temperature is introduced to characterize the entropy production counting the heat transfer between the system and the reservoir. We further find that only with the renormalized system Hamiltonian and other renormalized thermodynamic quantities, can the exact steady state of the system be expressed as the standard Gibbs state. Consequently, the corresponding exact steady-state particle occupations in the renormalized system energy levels obey the Bose-Einstein and the Fermi-Dirac distributions for bosonic and fermionic systems, respectively. Thus, the conventional statistical mechanics and thermodynamics are thereby rigorously deduced from quantum dynamical evolution.

Mineral-Inspired Approach Mediated Design, Solution Synthesis, and Characterization and Luminescence Study of Blue Emitting Phosphor BaSc2Si3O10:Eu2+

A series of new silicate based luminescence materials, Ba1-xEuxSc2Si3O10 (BSS:Eu2+) (x = 0.005, 0.01, 0.02, 0.05, 0.07), which can be efficiently excited by UV light and generates a broadband bright blue emission, with ‘Internal Quantum Efficiency’ 45% have been developed from a water-soluble propylene glycol modified silane (PGMS) based silicon precursor by a systematic solution based bottom up approach. The concept as well as the methodology of using a ‘Mineral Inspired Approach’ emerges as a new blueprint for the strategic design of novel phosphors for phosphor converted white light emitting diode (pc-WLEDs). The detailed crystal structure of monoclinic BSS:Eu2+ was determined from the XRD profile. Steady state photoluminescence measurement as well as the thermal quenching properties of the phosphor has been investigated in detail. The optimum concentration of Eu2+ in this matrix was found to be 2% and the ‘Concentration Quenching’ mechanism was verified to be the dipole-dipole interaction and the critical distance was calculated to be 25.01 Å. A highly efficient broadband blue emission centred at λmax ~ 446 nm shows very low thermal quenching; 71.2% at 150 degrees C with respect to that measured at room temperature. The remarkable thermal stability of the phosphor was attributed to the rigid structural network of the host and weak electron-phonon coupling strength. The interesting photoluminescence properties and robust thermal performance reflect the potential application of this phosphor for UVLED driven solid-state lighting.

Spin texture in doped Mott insulators with spin-orbit coupling

A hole injected into a Mott insulator will gain an internal structure as recently identified by exact numerics, which is characterized by a nontrivial quantum number whose nature is of central importance in understanding the Mott physics. In this work, we show that a spin texture associated with such an internal degree of freedom can explicitly manifest after the spin degeneracy is lifted by a \emph{weak} Rashba spin-orbit coupling (SOC). It is described by an emergent angular momentum $J_{z}=\pm3/2$ as shown by both exact diagonalization (ED) and variational Monte Carlo (VMC) calculations, which are in good agreement with each other at a finite size. In particular, as the internal structure such a spin texture is generally present in the hole composite even at high excited energies, such that a corresponding texture in momentum space, extending deep inside the Brillouin zone, can be directly probed by the spin-polarized angle-resolved photoemission spectroscopy (ARPES). This is in contrast to a Landau quasiparticle under the SOC, in which the spin texture induced by SOC will not be protected once the excited energy is larger than the weak SOC coupling strength, away from the Fermi energy. We point out that the spin texture due to the SOC should be monotonically enhanced 1with reducing spin-spin correlation length in the superconducting/pseudogap phase at finite doping. A brief discussion of a recent experiment of the spin-polarized ARPES will be made.

Giant Enhancement of Nonlinear Harmonic Generation in a Silicon Topological Photonic Crystal Nanocavity Chain

AbstractStrongly enhanced third‐harmonic generation (THG) by the topological localization of an edge mode in a Su‐Schrieffer‐Heeger (SSH) chain of silicon photonic crystal nanocavities is demonstrated. The edge mode of the nanocavity chain not only naturally inherits resonant properties of the single nanocavity, but also exhibits the topological feature with mode robustness extending well beyond individual nanocavity. By engineering the SSH nanocavities with alternating strong and weak coupling strengths on a silicon slab, the edge mode formation that entails a strong THG signal similar to that obtained from a single nanocavity is observed, which both show three orders of magnitude enhancement compared with that in a trivial SSH structure. The results indicate that the photonic crystal nanocavity chain can provide a promising on‐chip platform for topology‐driven nonlinear photonics.

Spin transport properties in DNA & electrically doped iron QD organo-metallic junction

The DNA sensor has emerged as strong candidates for next generation ultra low power application due to its self assemble technique. Non-Equilibrium Green’s Function (NEGF) along with Density Functional Theory (DFT) based First Principle approach is used in investigation of spin transport properties along with quantum scattering transmission characteristics of DNA sensor via Iron (Fe) quantum dots (QD) electrodes at room temperature. Electrically doped Fe QD plays an important role in spin transport mechanism. This electrical doping concentration and weak coupling strength between DNA and Fe QD organo-metallic junction effect into the tunneling contact resistance (TCR) along with quantum-ballistic transmission and junction conductivity of parallel and anti-parallel configuration of this analytical model representation. It has been observed that higher current has been achieved for parallel configuration when compare with anti-parallel configuration at same bias voltage. This voltage-current characteristic is significantly modulated due to the electrical doping effect. This spin transport property shows that this system can well perform for anti parallel configuration. High tunnel organo-metallic resistance approximately 99.9% is observed even at 0 V bias voltage. Tunneling Organo Metallic Contact Resistance (TOMCR) remains large at upper bias voltage.

Perspective of Direct Search for Dark Components in the Universe with Multi-Wavelengths Stimulated Resonant Photon-Photon Colliders

We explore a possibility to detect dark components in the Universe via stimulated photon–photon collisions by focusing two-frequency coherent electromagnetic fields in a vacuum. Those fields are assumed to be pulsed reaching Fourier transform limits in near-infrared, THz, and GHz frequency bands, respectively. The numbers of signal photons as a result of exchange of a pseudoscalar-type pseudo Nambu–Goldstone boson have been evaluated in the individual frequency bands. Within presently available beam intensities, we found that the QCD axion scenarios are thoroughly testable in the mass range 10−6–100 eV based on the common method. Furthermore, we show a possibility to reach the weak coupling domain even beyond the gravitationally weak coupling strength if pulse compression in the GHz band is realized in the near future development.

Mesoscale Surface Wind‐SST Coupling in a High‐Resolution CESM Over the KE and ARC Regions

AbstractA strong positive correlation exists between mesoscale sea surface temperature (SST) and surface wind stress perturbations in the extratropical oceans, which is revealed by high‐resolution satellite but is difficult to be accurately simulated in low‐resolution coupled ocean‐atmosphere models. The extent to which the mesoscale coupling is captured in a high‐resolution Community Earth System Model (CESM‐HR) is assessed in this study, with a focus on two regions, the Kuroshio Extensions (KE) and Agulhas Return Current (ARC), respectively. Compared with satellite observations, it is found that the CESM‐HR can well depict the characteristics of the mesoscale air‐sea coupling, with its strength being comparable to that observed; but in the KE region, the simulated strength is only half of the observed during summer. Mechanisms for the atmospheric responses to mesoscale SST perturbations through the pressure adjustment (PA) and the downward momentum transport (DMT) are analyzed. The results highlight different mechanisms for controlling the atmospheric responses over the KE and ARC regions, which are regionally and seasonally dependent. In the ARC region, pronounced dipole patterns of sea surface pressure (SLP) and vertical velocity perturbation responses indicate that the DMT exerts a dominant effect in both winter and summer. In the KE region, monopole patterns show the main role played by the PA mechanism for the atmospheric adjustment in summer, while dipole patterns indicate the main role played by the DMT mechanism in winter. The weak coupling strength in summer over the KE is closely related to the inadequate parameterization of vertical heat mixing.

Influence of parameters inhomogeneity on the existence of chimera states in a ring of nonlocally coupled maps

The aim of the research is to study the influence of inhomogeneity in a control parameter of all partial elements in a ring of nonlocally coupled chaotic maps on the possibility of observing chimera states in the system and to compare the changes in regions of chimera realization using different methods of introducing the inhomogeneity. Methods. In this paper, snapshots of the system dynamics are constructed for various values of the parameters, as well as spatial distributions of cross-correlation coefficient values, which enable us to determine the regime observed in the system for these parameters. To improve the accuracy of the obtained results, the numerical studies are carried out for fifty different realizations of initial conditions of the ring elements. Results. It is shown that a fixed inhomogeneous distribution of the control parameters with increasing noise intensity leads to an increase in the range of the coupling strength where chimera states are observed. With this, the boundary lying in the region of strong coupling changes more significantly as compared with the case of weak coupling strength. The opposite effect is provided when the control parameters are permanently affected by noise. In this case increasing the noise intensity leads to a decrease in the interval of existence of chimera states. Additionally, the nature of the random variable distribution (normal or uniform one) does not strongly influence the observed changes in the ring dynamics. The regions of existence of chimera states are constructed in the plane of «coupling strength – noise intensity» parameters. Conclusion. We have studied how the region of existence of chimeras changes when the coupling strength between the ring elements is varied and when different characteristics of the inhomogeneous distribution of the control parameters are used. It has been shown that in order to increase the region of observing chimera states, the control parameters of the elements must be distributed inhomogeneously over the entire ensemble. To reduce this region, a constant noise effect on the control parameters should be used.

Superconducting and normal state properties of quasi-one-dimensional Nb2Pd(S0.9Te0.1)5 system

A comprehensive study of superconducting and normal state properties of Nb2Pd(S0.9Te0.1)5 system which consists of solid chunks embedded with mesh of fibers has been presented here. Below the superconducting transition temperature, in different iso-magnetic fields, the motion of vortices in mixed state has been analyzed using thermally activated flux flow (TAFF) model and the obtained thermal activation energy (TAE) follows Kramer’s scaling which suggests the dominance of mechanism similar to grain boundary pinning in these fibers. Bulk nature of superconductivity is clearly seen in specific heat data, however with a jump of ∼0.97, less than the BCS estimate, places these fibers in weak coupling strength limit. Low temperature electronic heat capacity down to 2 ​K point towards a fully gapped s-wave superconductivity in these fibers. Moreover, normal state study using thermoelectric power data suggests a hole dominated transport in this system.

Evolutionary game on a growing multilayer network

Numerous complex systems display the multilayer feature rather than the single or isolated one. It is thus significant to study how these structures emerge and the mechanisms behind them. In this paper we propose a novel growing model to explain the formation of multilayer networks, where the network growth and strategy evolution in prisoner’s dilemma game interact and coevolve. We introduce the growing rate to control the ratio of strategy evolution to network growth, and the coupling strength to adjust the connecting density between layers. The simulation results show that besides the scale-free property observed in the resulting networks, the coupling strength is significant to cooperative behaviors. Under the weak coupling strength, the role of network reciprocity works and the resulting networks can preserve a relatively high cooperation level; however, with the increase of coupling strength, the role of network reciprocity fades away or disappears. The strong coupling strength, denoting individuals can obtain more extra payoff from their external circumstance, induces a low cooperation level. In addition, the growing rate also influences cooperative behaviors and a fast growing rate is beneficial to cooperation.

Nanoscale electric-field imaging based on a quantum sensor and its charge-state control under ambient condition

Nitrogen-vacancy (NV) centers in diamond can be used as quantum sensors to image the magnetic field with nanoscale resolution. However, nanoscale electric-field mapping has not been achieved so far because of the relatively weak coupling strength between NV and electric field. Here, using individual shallow NVs, we quantitatively image electric field contours from a sharp tip of a qPlus-based atomic force microscope (AFM), and achieve a spatial resolution of ~10 nm. Through such local electric fields, we demonstrated electric control of NV’s charge state with sub-5 nm precision. This work represents the first step towards nanoscale scanning electrometry based on a single quantum sensor and may open up the possibility of quantitatively mapping local charge, electric polarization, and dielectric response in a broad spectrum of functional materials at nanoscale.

Exact eigenenergies of a model of vibronically coupled electron transfer reactions.

We present the first exact solution to the time-independent Schrödinger equation of a model Hamiltonian consisting of a vibrational mode coupled to three electronic states. This Hamiltonian serves as a generic model for photo-induced electronic transfer reactions. The solution is non-perturbative and can be applied to ET reactions with weak and strong electronic and vibrational coupling strengths. This work suggests a new direction towards understanding the vibronic effects in ET dynamics beyond the non-adiabatic limit and Condon approximation.

Mapping the Cavity Optomechanical Interaction with Subwavelength-Sized Ultrasensitive Nanomechanical Force Sensors

In a canonical optomechanical system, mechanical vibrations are dynamically encoded on an optical probe field which reciprocally exerts a backaction force. Due to the weak single photon coupling strength achieved with macroscopic oscillators, most of existing experiments were conducted with large photon numbers to achieve sizeable effects, thereby causing a dilution of the original optomechanical non-linearity. Here, we investigate the optomechanical interaction of an ultrasensitive suspended nanowire inserted in a fiber-based microcavity mode. This implementation allows to enter far into the hitherto unexplored ultrastrong optomechanical coupling regime, where one single intracavity photon can displace the oscillator by more than its zero point fluctuations. To fully characterize our system, we implement nanowire-based scanning probe measurements to map the vectorial optomechanical coupling strength, but also to reveal the intracavity optomechanical force field experienced by the nanowire. This work establishes that the single photon cavity optomechanics regime is within experimental reach.

Aspects of the disordered harmonic chain

We discuss the driven harmonic chain with fixed boundary conditions subject to weak coupling strength disorder. We discuss the evaluation of the Liapunov exponent in some detail expanding on the dynamical system theory approach by Levi et al. We show that including mass disorder the mass and coupling strength disorder can be combined in a renormalised mass disorder. We review the method of Dhar regarding the disorder-averaged heat current, apply the approach to the disorder-averaged large deviation function and finally comment on the validity of the Gallavotti–Cohen fluctuation theorem. The paper is also intended as an introduction to the field and includes detailed calculations.

Fast universal two-qubit gate for neutral fermionic atoms in optical tweezers

An array of ultracold neutral atoms held in optical micro-traps is a promising platform for quantum computation. One of the major bottlenecks of this platform is the weak coupling strength between adjacent atoms, which limits the speed of two-qubit gates. Here, we present a method to perform a fast universal square-root-SWAP gate with fermionic atoms. The basic idea of the gate is to release the atoms into a harmonic potential positioned in between the two atoms. By properly tailoring the interaction parameter, the collision process between the atoms generates entanglement and yields the desired gate. We prove analytically that in the limit of broad atomic wave-packets, the fidelity of the gate approaches unity. We demonstrate numerically that with typical experimental parameters, our gate can operate on a microsecond timescale and achieves a fidelity higher than 0.998. Moreover, the gate duration is independent of the initial distance between the atoms. A gate with such features is an important milestone towards all-to-all connectivity and fault tolerance in quantum computation with neutral atoms.