Collective Excitation Modes Research Articles

Vibrational model of thermal conduction for fluids with soft interactions.

A vibrational model of heat transfer in simple liquids with soft pairwise interatomic interactions is discussed. A general expression is derived, which involves an averaging over the liquid collective mode excitation spectrum. The model is applied to quantify heat transfer in a dense Lennard-Jones liquid and a strongly coupled one-component plasma. Remarkable agreement with the available numerical results is documented. A similar picture does not apply to the momentum transfer and shear viscosity of liquids.

Phase coherence in out-of-equilibrium supersolid states of ultracold dipolar atoms

A supersolid is a counterintuitive phase of matter that combines the global phase coherence of a superfluid with a crystal-like self-modulation in space. Recently, such states have been experimentally realized using dipolar quantum gases. Here we investigate the response of a dipolar supersolid to an interaction quench that shatters the global phase coherence. We identify a parameter regime in which this out-of-equilibrium state rephases, indicating superfluid flow across the sample as well as an efficient dissipation mechanism. We find a crossover to a regime where the tendency to rephase gradually decreases until the system relaxes into an incoherent droplet array. Although a dipolar supersolid is, by its nature, ‘soft’, we capture the essential behaviour of the de- and rephasing process within a rigid Josephson junction array model. Yet, both experiment and simulation indicate that the interaction quench causes substantial collective mode excitations that connect to phonons in solids and affect the phase dynamics. A supersolid is a phase of matter featuring both crystalline order as a solid and global phase coherence as a superfluid. Now an experiment shows how this global phase coherence can be established across the system in a non-equilibrium process.

DNA Transformation, Cell Epigenetic Landscape and Open Complex Dynamics in Cancer Development

Statistical thermodynamics allowed the formulation of mesoscopic approach of DNA transformation in course of the excitation of collective distortion modes (denaturation bubbles) associated with hydrogen bond breaking between the base pairs. Intermediate (non-continual limit) of DNA modeling (the Peyrard-Bishop model) is combined with the field description (generalized Ginzburg-Landau approach) to analyze the dynamics of collective open complex modes associated with mesodefects in the DNA ensemble. Collective modes dynamics describes different scenario of gene expression according to statistically predicted form of out-of-equilibrium potential (epigenetic landscape) reflecting specific type criticality of “soft matter” with mesodefects (open complexes) – the structural-scaling transition. Principal difference of thermodynamics of non-continual and continual models is thermalization conditions related to thermal fluctuations responsible for the DNA breathing (localized excitation with breather dynamics) and structural-scaling parameter responsible for spinodal decomposition of out-of-equilibrium potential metastability due to generation of open complex collective modes. Open complex collective modes have the nature of self-similar solutions (breathers, auto-solitary and blow-up modes) of open complex evolution equation accounting qualitative different types of potential metastabilities. Sub-sets of collective modes represent the phase variables of attractors associated with different scenario of expression dynamics, which allows the interpretation of multistability of the epigenetic landscape and the Huang diagram of gene expression. It was shown different epigenetic pathway in attractors phase space corresponding to normal and cancer expression scenario. These scenarios were supported by laser interference microscopy of living normal and cancer cells illustrating multi- and monofractal dynamics.

Dipole excitation of collective modes in viscous two-dimensional electron systems

We describe the structure of the time-harmonic electromagnetic field of a vertical Hertzian electric dipole source radiating over an infinite, translation invariant two-dimensional electron system. Our model for the electron flow takes into account the effects of shear and Hall viscosities as well as an external static magnetic field perpendicular to the sheet. We identify two wave modes, namely, a surface plasmon and a diffusive mode. In the presence of an external static magnetic field, the diffusive mode combines the features of both the conventional and Hall diffusion and may exhibit a negative group velocity. In our analysis, we solve exactly a boundary value problem for the time-harmonic Maxwell equations coupled with linearized hydrodynamic equations for the flat, two-dimensional material. By numerically evaluating the integrals for the electromagnetic field on the sheet, we find that the plasmon contribution dominates in the intermediate-field region of the dipole source. In contrast, the amplitude of the diffusive mode reaches its maximum value in the near-field region, and quickly decays with the distance from the source. We demonstrate that the diffusive mode can be distinguished from the plasmon in the presence of the static magnetic field, when the highly oscillatory plasmon is gapped and tends to disappear.

Coupled-oscillator collective mode of a magnetic chiral soliton lattice

The collective excitation modes of a magnetic chiral soliton lattice (CSL) are numerically studied. We perform micromagnetic simulations of a monoaxial chiral helimagnet with an external magnetic field that forms the CSL. Due to the repulsive interaction between solitons through a ferromagnet state, collective modes described by the coupled oscillator are excited. These modes exist at a frequency of $\ensuremath{\sim}1\phantom{\rule{0.28em}{0ex}}\mathrm{GHz}$ while the conventional ferromagnetic resonant mode exists at a frequency of $\ensuremath{\sim}10\phantom{\rule{0.28em}{0ex}}\mathrm{GHz}$ as measured in experiments [F. Goncalves et al., Phys. Rev. B 95, 104415 (2017)]. In contrast to ferromagnetic resonance, the resonant frequency of the coupled-oscillator mode decreases by increasing the magnetic field. We clarified that the coupling energy between solitons becomes weak by increasing the distance between solitons. We also investigate the resonant condition of a precessional mode of the CSL measured in experiments.

Spin susceptibilities of doped bilayer graphene in the presence of magnetic ordering

We study the dynamical spin susceptibilities of biased bilayer graphene due to spin polarization effects in the context of tight binding model Hamiltonian. Green's function approach has been implemented to find the behavior of dynamical spin susceptibility of bilayer graphene within linear response theory. We have found the frequency dependence of imaginary part of spin susceptibilities for different values of bias voltage. Also the dependence of both transverse and longitudinal spin susceptibilities on spin polarization has been investigated in details. The frequency position of sharp peaks in the frequency dependence of the susceptibilities introduce the collective magnetic excitation modes of itinerant electrons on bilayer system.

Collective excitations in α-helical protein structures interacting with the water environment

ABSTRACT Low-frequency vibrational excitations of protein macromolecules in the terahertz frequency region are suggested to contribute to many biological processes such as enzymatic catalysis, intra-protein energy/charge transport, recognition, and allostery. To explain high effectiveness of these processes, two possible mechanisms of the long-lived excitation were proposed by H. Fröhlich and A.S. Davydov, which relate to either vibrational modes or solitary waves, respectively. In this paper, we developed a quantum dynamic model of vibrational excitation in α-helical proteins interacting with the aqueous environment. In the model, we distinguished three coupled subsystems, i.e., (i) a chain of hydrogen-bonded peptide groups (PGs), interacting with (ii) the subsystem of the side-chain residuals which in turn interact with (iii) the environment, surrounding water responsible for dissipation and fluctuation in the system. It was shown that the equation of motion for phonon variables of the PG chain can be transformed to nonlinear Schrodinger equation which admits bifurcation into the solution corresponding to the weak-damped vibrational modes (Fröhlich-type regime) and Davydov solitons. A bifurcation parameter is derived through the strength of phonon–phonon interaction between the side-chains and hydration-shell water molecules. As shown, the energy of these excited states is pumped through the interaction of the side-chains with fluctuating water environment of the proteins. The suggested mechanism of the collective vibrational mode excitation is discussed in connection with the recent experiments on the long-lived collective protein excitations in the terahertz frequency region and vibrational energy transport pathways in proteins.

Study of collective mode excitations in Zr41Ti14Cu12.5Be22.5Fe10 bulk metallic glass

Abstract The collective mode excitations of Zr41Ti14Cu12.5Be22.5Fe10 bulk metallic glass (BMG) have been theoretically studied at room temperature by applying pseudopotential technique. A well-known Shaw’s optimized constant core model pseudopotential with five different forms of local field correction functions given by Hartree (H), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used to study the screening effects on the aforementioned work. The phonon dispersion curves (PDCs) are computed by using Hubbard-Beeby (HB) theoretical approach. While, elastic and thermodynamic properties are theoretically calculated from the elastic limit of the phonon dispersion curve. Such properties compared well with available data in the literature and are found qualitative agreement with them.

The collective excitation of nuclear matter in a bosonized Landau Fermi liquid model

The collective excitation of nuclear matter is analyzed in a bosonized Landau Fermi liquid model. When the nonlinear self-interacting terms of scalar mesons are included in Walecka model, the collective excitation energy of nuclear matter can be obtained self-consistently, and the calculation results are consistent with the corresponding experimental data of the nucleus P208b when the quantum number of the orientation of the total spin m is zero. The cases with the nonzero m values are also studied, and it is found that the collective excitation energy of nuclear matter decreases with the absolute value of m increasing when the total spin is conserved. Moreover, four kinds of collective excitation modes of nuclear matter are discussed when the isospin and spin of nucleons are taken into account. The direct interaction between two nucleons near Fermi surface only changes the effective nucleon mass and Fermi velocity, while the exchange interaction plays an critical role in the collective excitation of nuclear matter.

Many-body correlations in nuclear superfluidity

The two-fermion two-point correlation function in the pairing channel is discussed in the equation of motion framework. Starting from the bare two-fermion interaction, we derive the equation of motion for the two-fermion pair propagator in a strongly-correlated medium. The resulting equation is of the Dyson type with the kernel having a static and a one frequency dependent components and, thus, can be regarded as Dyson Bethe-Salpeter equation (Dyson-BSE). The many-body hierarchy generated by the dynamical interaction kernel is truncated on the level of two-body correlation functions, thus neglecting the explicit three-body and higher-rank correlations. The truncation is performed via a cluster expansion of the intermediate three-particle-one-hole correlation function irreducible in the particle-particle channel, that leads to the coupling between single fermions and emergent collective modes of excitation. The latter couplings are, thus, derived in terms of the exact mapping of the in-medium two-fermion correlation functions onto the domain of bosonic quasibound states (phonons) without introducing new parameters. The approach is applied to calculations of the pairing gaps in medium-mass nuclear systems, that include calcium, nickel and tin isotopic chains.

Coherent multimode conversion from microwave to optical wave via a magnon-cavity hybrid system

Coherent conversion from microwave to optical wave opens new research avenues towards a long distance quantum network covering quantum communication, computing, and sensing out of the laboratory. Especially a multimode enabled system is essential for practical applications. Here we experimentally demonstrate coherent multimode conversion from the microwave to optical wave via collective spin excitation in a single crystal yttrium iron garnet (YIG, ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$) which is strongly coupled to a microwave cavity mode in a three-dimensional rectangular cavity. Expanding a collective spin excitation mode of our magnon-cavity hybrid system from Kittel to multimagnetostatic modes, we verify that the size of the YIG sphere predominantly plays a crucial role for the microwave-to-optical multimode conversion efficiency at resonant conditions. We also find that the coupling strength between multimagnetostatic modes and a cavity mode is manipulated by the position of a YIG inside the cavity. It is expected to be valuable for designing a magnon-hybrid system that can be used for coherent conversion between microwave and optical photons.

Tight-binding method for the electronic and optical properties of C and BN nanotubes

We investigate the electronic and optical properties of carbon and boron nitride single-walled nanotubes (C-SWNT and BN-SWNT). At first, we use a minimal tight-binding (TB) model for the band structures of C-SWNT and BN-SWNT. We extract the hopping parameters and the onsite energies by fitting a TB empirical model to the DFT results. Then, we examine the collective π-electronic excitation modes of C-SWNT and BN-SWNT in terms of dynamic-dielectric-response function. Within this approach, we can easily calculate the electron-energy-loss function by the real and imaginary parts of the longitudinal dielectric function, ε(ω,q), where q is the transfer momentum along the tube axis and ω is the energy transfer. Our calculations show a blue-shift of plasmon peaks when q increases. Dispersion relations for the π-plasmon modes at angular momentum L = 1 are somewhat similar to L = 0, for BN-SWNT. Also, we find acoustic plasmons in C-SWNT and optical plasmons in BN-SWNT.

Radiative Toroidal Dipole and Anapole Excitations in Collectively Responding Arrays of Atoms

A toroidal dipole represents an often overlooked electromagnetic excitation distinct from the standard electric and magnetic multipole expansion. We show how a simple arrangement of strongly radiatively coupled atoms can be used to synthesize a toroidal dipole where the toroidal topology is generated by radiative transitions forming an effective poloidal electric current wound around a torus. We extend the protocol for methods to prepare a delocalized collective excitation mode consisting of a synthetic lattice of such toroidal dipoles and a nonradiating, yet oscillating charge-current configuration, dynamic anapole, for which the far-field radiation of a toroidal dipole is identically canceled by an electric dipole.

Charge structure factors of doped armchair nanotubes in the presence of electron–phonon interaction

We present the behaviors of both dynamical and static charge susceptibilities of doped armchair nanotubes using the Green function approach in the context of Holstein-model Hamiltonian. Specially, the effects of magnetization and gap parameter on the the plasmon modes of armchair nanotube are investigated via calculating correlation function of charge density operators. Random phase approximation has been implemented to find the interacting dynamical charge susceptibility. The electrons in this systems interacts with each other by mediation of dispersionless Holstein phonons. Our results show that the increase of gap parameter leads to decreasing intensity of charge collective mode. Also the frequency position of the collective mode tends to higher frequencies due to the gap parameter. Furthermore the number of collective excitation mode decreases with chemical potential in the presence of electron–phonon interaction. Finally the temperature dependence of static charge structure factor of armchair nanotubes is studied. The effects of the gap parameter, magnetization and electron–phonon interaction on the static structure factor are addressed in details.

Collective magnetic modes excitation in GaAs nanoclusters by azimuthally polarized vector beams

We investigate optical response of oligomers of dielectric nanodisks made of GaAs – anisotropic material with bulk quadratic nonlinearity. By using numerical approach, we analyse linear response of nanoparticle clusters in the vicinity of the magnetic dipole resonance for two types of incident radiation: linearly polarized plane wave and azimuthally polarized vector beams. Achieved results may be used to design highly efficient nonlinear optical nanoantennas with controllable radiation characteristics.

Manifestation of topological behaviors in interacting Weyl systems: One-body versus two-body correlations

Understanding correlation effects in topological phases of matter is at the forefront of current research in condensed matter physics. Here we try to clarify some subtleties in studying topological behaviors of interacting Weyl semimetals. It is well-known that there exist two topological invariants defined to identify their topological character. One is the many-body Chern number, which can be directly linked to the Hall conductivity and thus to the two-particle correlations. The other is the topological index constructed from the single-particle Green's functions. Because the information of Green's functions is easier to be achieved than the many-body wavefunctions, usually only the latter is employed in the literature. However, the approach based on the single-particle Green's function can break down in the strongly correlated phase. For illustration, an exactly solvable two-orbital model with momentum-local two-body interactions is discussed, in which both topological invariants can be calculated analytically. We find that the topological index calculated from the Green's function formalism can be nonzero even for a non-topological strongly correlated phase with vanishing many-body Chern number. In addition, we stress that the physical surface states implied by nonzero many-body Chern numbers should be the edge modes of particle-hole collective excitations, rather than those of quasiparticle nature derived from the Green's function formalism. Our observations thus demonstrate the limitation of the validity of Green's function formalism in the investigations of interacting topological materials.

Solid-state tube wakefield accelerator using surface waves in crystals

Solid-state or crystal acceleration has for long been regarded as an attractive frontier in advanced particle acceleration. However, experimental investigations of solid-state acceleration mechanisms which offer [Formula: see text] acceleration gradients have been hampered by several technological constraints. The primary constraint has been the unavailability of attosecond particle or photon sources suitable for excitation of collective modes in bulk crystals. Secondly, there are significant difficulties with direct high-intensity irradiation of bulk solids, such as beam instabilities due to crystal imperfections and collisions etc. Recent advances in ultrafast technology with the advent of submicron long electron bunches and thin-film compressed attosecond x-ray pulses have now made accessible ultrafast sources that are nearly the same order of magnitude in dimensions and energy density as the scales of collective electron oscillations in crystals. Moreover, nanotechnology enabled growth of crystal tube structures not only mitigates the direct high-intensity irradiation of materials, with the most intense part of the ultrafast source propagating within the tube but also enables a high degree of control over the crystal properties. In this work, we model an experimentally practicable solid-state acceleration mechanism using collective electron oscillations in crystals that sustain propagating surface waves. These surface waves are driven in the wake of a submicron long particle beam, ideally also of submicron transverse dimensions, in tube shaped nanostructured crystals with tube wall densities, [Formula: see text]. Particle-In-Cell (PIC) simulations carried out under experimental constraints demonstrate the possibility of accessing average acceleration gradients of several [Formula: see text] using the solid-state tube wakefield acceleration regime. Furthermore, our modeling demonstrates the possibility that as the surface oscillations and resultantly the surface wave transitions into a nonlinear or “crunch-in” regime under [Formula: see text], not only does the average gradient increase but strong transverse focusing fields extend down to the tube axis. This work thus demonstrates the near-term experimental realizability of Solid-State Tube Wakefield Accelerator (SOTWA). The ongoing progress in nanoengineering and attosecond source technology thereby now offers the potential to experimentally realize the promise of solid-state or crystal acceleration, opening up unprecedented pathways in miniaturization of accelerators.

The effects of electron-phonon coupling and magnetic field on charge structure factors of armchair graphene nanoribbons

We present the behaviors of both dynamical and static charge susceptibilities of undoped armchair graphene nanoribbon using the Green’s function approach in the context of Holstein model Hamiltonian. Specially, the effects of magnetic field and electron-phonon coupling strength on the plasmon modes of armchair graphene nanoribbon are investigated via calculating correlation function of charge density operators. Random phase approximation has been implemented to find the interacting dynamical charge susceptibility. The electrons in this systems interacts with each other by mediation of dispersionless Holstein phonons. Our results show the increase of electron phonon coupling strength leads to decrease intensity of charge collective mode. Also the frequency position of the collective mode tends to higher frequencies with electron phonon coupling. We also show that the frequency positions of plasmon mode of armchair nanoribbon are not affected by the increase of magnetic field. Furthermore the number of collective excitation mode increases with ribbon width in the presence of electron-phonon interaction. Finally the temperature dependence of static charge structure factor of armchair graphene nanoribbon is studied. The effects of magnetic field, ribbon width and electron-phonon interaction on the static structure factor are addressed in details.

Magnons at low excitations: Observation of incoherent coupling to a bath of two-level systems

Collective magnetic excitation modes, magnons, can be coherently coupled to microwave photons in the single excitation limit. This allows for access to quantum properties of magnons and opens up a range of applications in quantum information processing, with the intrinsic magnon linewidth representing the coherence time of a quantum resonator. Our measurement system consists of a yttrium iron garnet (YIG) sphere and a three-dimensional (3D) microwave cavity at temperatures and excitation powers typical for superconducting quantum circuit experiments. We perform spectroscopic measurements to determine the limiting factor of magnon coherence at these experimental conditions. Using the input-output formalism, we extract the magnon linewidth $\kappa_\mathrm{m}$. We attribute the limitations of the coherence time at lowest temperatures and excitation powers to incoherent losses into a bath of near-resonance two-level systems (TLSs), a generic loss mechanism known from superconducting circuits under these experimental conditions. We find that the TLSs saturate when increasing the excitation power from quantum excitation to multi-photon excitation and their contribution to the linewidth vanishes. At higher temperatures, the TLSs saturate thermally and the magnon linewidth decreases as well.

The antiferromagnetic phase of the Floquet-driven Hubbard model

A saddle point plus fluctuations analysis of the periodically driven half-filled two-dimensional Hubbard model is performed. For drive frequencies below the equilibrium gap, we find discontinuous transitions to time-dependent solutions. A highly excited, generically non-thermal distribution of magnons occurs even for drive frequencies far above the gap. Above a critical drive amplitude, the low-energy magnon distribution diverges as the frequency tends to zero and antiferromagnetism is destroyed, revealing the generic importance of collective mode excitations arising from a non-equilibrium drive.