Long-lived Oscillations Research Articles

Pronounced exciton and coherent phonon dynamics in BiI3.

Trivalent bismuth compounds featuring octahedral halide coordination, such as BiI3, double-perovskite structures and vacancy-ordered perovskite derivatives, have recently attracted considerable attention as potential environmentally benign thin film materials for solar light-harvesting and optoelectronic applications aiming to replace toxic lead-based perovskites. Yet, their photovoltaic performance still lags considerably behind their lead-counterparts. Improving their performance requires a detailed knowledge of the photoinduced carrier dynamics and kinetics in these systems. In the current study, we present a detailed investigation of layered BiI3. The compound exhibits pronounced exciton dynamics which is associated with a characteristic second-derivative type feature in the transient absorption spectra. Accurate time constants are assigned to carrier cooling and recombination processes. Pronounced coherent oscillations are observed in the kinetic traces indicating strong electron-phonon coupling. Analysis of the coherences recovers the steady-state Raman spectrum of BiI3 with a dominant mode of Ag symmetry at 114 cm-1, associated with the zone center C(Γ) phonon. The line position is well reproduced by DFT calculations. In terms of the carrier lifetime, high absorption coefficient and band-gap position, BiI3 is a promising candidate as a photovoltaic material but challenges posed are the difficulty to split the strongly bound excitons which are confined in the 2D layers and the efficient extraction of electrons and holes at n- and p-selective contacts, beside well-known issues regarding high resistivity and the intrinsic softness of this material.

Photoinduced Enhancement of Excitonic Order.

We study the dynamics of excitonic insulators coupled to phonons using the time-dependent mean-field theory. Without phonon couplings, the linear response is given by the damped amplitude oscillations of the order parameter with a frequency equal to the minimum band gap. A phonon coupling to the interband transfer integral induces two types of long-lived collective oscillations of the amplitude, one originating from the phonon dynamics and the other from the phase mode, which becomes massive. We show that, even for small phonon coupling, a photoinduced enhancement of the exciton condensation and the gap can be realized. Using the Anderson pseudospin picture, we argue that the origin of the enhancement is a cooperative effect of the massive phase mode and the Hartree shift induced by the photoexcitation. We also discuss how the enhancement of the order and the collective modes can be observed with time-resolved photoemission spectroscopy.

Clock spectroscopy of interacting bosons in deep optical lattices

We report on high-resolution optical spectroscopy of interacting bosonic 174Yb atoms in deep optical lattices with negligible tunneling. We prepare Mott insulator phases with singly- and doubly-occupied isolated sites and probe the atoms using an ultra-narrow ‘clock’ transition. Atoms in singly-occupied sites undergo long-lived Rabi oscillations. Atoms in doubly-occupied sites are strongly affected by interatomic interactions, and we measure their inelastic decay rates and energy shifts. We deduce from these measurements all relevant collisional parameters involving both clock states, in particular the intra- and inter-state scattering lengths.

Bloch oscillations and resonant radiation of light propagating in arrays of nonlinear fibers with high-order dispersion

Bloch oscillations of spatio-temporal light wave packets in arrays of nonlinear fibers with high-order dispersion are studied. The light wave experiences discrete spatial diffraction along the waveguide array coordinate together with continuous temporal dispersion including higher order terms. When a gradient in the waveguide light confinement strength is considered, the wave packet features robust long-lived Bloch oscillations with temporal and spatial spreading in the presence of a Kerr nonlinearity. The effect of the spatial Bloch oscillations on the emission of the dispersive radiation by the solitary wave is studied. It is shown that Bloch oscillations result in the generation of new frequencies. The condition of resonant emission of dispersive waves is derived and it is demonstrated that it matches very well with the results of direct numerical simulations.

Ultra-sensitive graphene based mid-infrared plasmonic bio-chemical sensing using dielectric beads as a medium

Graphene is moving beyond the realm of simple electronic devices toward areas such as advanced bio-chemical sensing. The infrared (IR) response of graphene, characterized by collective long-lived charge-carrier oscillations, could be applied in IR-absorption spectroscopy, typically used for bio-chemical analysis. However, direct light absorption by propagating plasmons in graphene is forbidden due to the large momentum mismatch. Proposed methods to overcome this bottleneck come at a cost, the use of noble metal particles on graphene reduces the spectral bandwidth and nano-structuring graphene is expensive. Here, we propose a simple and cheap method to fabricate large scale ultra-sensitive graphene based mid-IR biosensors, by introducing dielectric beads to excite mid-IR range plasmons. Interference from waves scattered by the beads excite surface plasmon polaritons, which propagate several micrometers in graphene and enhance the interaction between the molecules and mid-IR light. This method opens an interesting window for the application of graphene in bio-chemical sensing.

Bloch oscillations sustained by nonlinearity

We demonstrate that nonlinearity may play a constructive role in supporting Bloch oscillations in a model which is discrete, in one dimension and continuous in the orthogonal one. The model can be experimentally realized in several fields of physics such as optics and Bose-Einstein condensates. We demonstrate that designing an optimal relation between the nonlinearity and the linear gradient strength provides extremely long-lived Bloch oscillations with little degradation. Such robust oscillations can be observed for a broad range of parameters and even for moderate nonlinearities and large enough values of linear potential. We also present an approximate analytical description of the wave packet’s evolution featuring a hybrid Bloch oscillating wave-soliton behavior that excellently corresponds to the direct numerical simulations.

The fully nonlinear stratified geostrophic adjustment problem

Abstract. The study of the adjustment to equilibrium by a stratified fluid in a rotating reference frame is a classical problem in geophysical fluid dynamics. We consider the fully nonlinear, stratified adjustment problem from a numerical point of view. We present results of smoothed dam break simulations based on experiments in the published literature, with a focus on both the wave trains that propagate away from the nascent geostrophic state and the geostrophic state itself. We demonstrate that for Rossby numbers in excess of roughly 2 the wave train cannot be interpreted in terms of linear theory. This wave train consists of a leading solitary-like packet and a trailing tail of dispersive waves. However, it is found that the leading wave packet never completely separates from the trailing tail. Somewhat surprisingly, the inertial oscillations associated with the geostrophic state exhibit evidence of nonlinearity even when the Rossby number falls below 1. We vary the width of the initial disturbance and the rotation rate so as to keep the Rossby number fixed, and find that while the qualitative response remains consistent, the Froude number varies, and these variations are manifested in the form of the emanating wave train. For wider initial disturbances we find clear evidence of a wave train that initially propagates toward the near wall, reflects, and propagates away from the geostrophic state behind the leading wave train. We compare kinetic energy inside and outside of the geostrophic state, finding that for long times a Rossby number of around one-quarter yields an equal split between the two, with lower (higher) Rossby numbers yielding more energy in the geostrophic state (wave train). Finally we compare the energetics of the geostrophic state as the Rossby number varies, finding long-lived inertial oscillations in the majority of the cases and a general agreement with the past literature that employed either hydrostatic, shallow-water equation-based theory or stratified Navier–Stokes equations with a linear stratification.

Effects of non-exciton components excited by broadband pulses on quantum beats in a GaAs/AlAs multiple quantum well

In this study, we report the effect of the excitation of non-exciton components caused by broadband pulses on quantum beat oscillation. Using a spectrally controlled pump pulse, a long-lived oscillation is clearly observed, and the pump-power dependence shows the suppression of the dephasing rate of the oscillation. Our results from incoherent carrier generation using a continuous wave laser demonstrate that the non-exciton components behaving as free carriers increase the oscillation dephasing rate.

Spectral hole burning and its application in microwave photonics

Ensembles of electron spins in hybrid microwave systems are powerful and versatile components for future quantum technologies. Quantum memories with high storage capacities are one such example which require long-lived states that can be addressed and manipulated coherently within the inhomogeneously broadened ensemble. This broadening is essential for true multimode memories, but induces a considerable spin dephasing and together with dissipation from a cavity interface poses a constraint on the memory's storage time. In this work we show how to overcome both of these limitations through the engineering of long-lived dark states in an ensemble of electron spins hosted by nitrogen-vacancy centres in diamond. By burning narrow spectral holes into a spin ensemble strongly coupled to a superconducting microwave cavity, we observe long-lived Rabi oscillations with high visibility and a decay rate that is a factor of forty smaller than the spin ensemble linewidth and thereby a factor of more than three below the pure cavity dissipation rate. This significant reduction lives up to the promise of hybrid devices to perform better than their individual subcomponents. To demonstrate the potential of our approach we realise the first step towards a solid-state microwave spin multiplexer by engineering multiple long-lived dark states. Our results show that we can fully access the "decoherence free" subspace in our experiment and selectively prepare protected states by spectral hole burning. This technique opens up the way for truly long-lived quantum memories, solid-state microwave frequency combs, optical to microwave quantum transducers and spin squeezed states. Our approach also paves the way for a new class of cavity QED experiments with dense spin ensembles, where dipole spin-spin interactions become important and many-body phenomena will be directly accessible on a chip.

The influence of ion dynamics on the breaking of plane electron oscillations

The influence of ion dynamics on the breaking of plane relativistic electron oscillations is studied by numerical modeling. An algorithm in Euler’s variables for finite-difference simulation is developed, and preliminary analytical study is performed. A new type of breaking of long-lived oscillations, different from the earlier-known one, is presented. Part of the computations were performed on the Chebyshev supercomputer (Lomonosov Moscow State University).

Antiferromagnetism and hidden order in isoelectronic doping ofURu2Si2

We present muon spin rotation $(\ensuremath{\mu}\mathrm{SR})$ and susceptibility measurements on single crystals of isoelectronically doped ${\mathrm{URu}}_{2\ensuremath{-}x}{\mathrm{T}}_{x}{\mathrm{Si}}_{2}$ (T = Fe, Os) for doping levels up to 50%. Zero field (ZF) $\ensuremath{\mu}\mathrm{SR}$ measurements show long-lived oscillations demonstrating that an antiferromagnetic state exists down to low doping levels for both Os and Fe dopants. The measurements further show an increase in the internal field with doping for both Fe and Os. Comparison of the local moment-hybridization crossover temperature from susceptibility measurements and our magnetic transition temperature shows that changes in hybridization, rather than solely chemical pressure, are important in driving the evolution of magnetic order with doping.

Dissipative dynamics at conical intersections: simulations with the hierarchy equations of motion method.

The effect of a dissipative environment on the ultrafast nonadiabatic dynamics at conical intersections is analyzed for a two-state two-mode model chosen to represent the S2(ππ*)-S1(nπ*) conical intersection in pyrazine (the system) which is bilinearly coupled to infinitely many harmonic oscillators in thermal equilibrium (the bath). The system-bath coupling is modeled by the Drude spectral function. The equation of motion for the reduced density matrix of the system is solved numerically exactly with the hierarchy equation of motion method using graphics-processor-unit (GPU) technology. The simulations are valid for arbitrary strength of the system-bath coupling and arbitrary bath memory relaxation time. The present computational studies overcome the limitations of weak system-bath coupling and short memory relaxation time inherent in previous simulations based on multi-level Redfield theory [A. Kühl and W. Domcke, J. Chem. Phys. 2002, 116, 263]. Time evolutions of electronic state populations and time-dependent reduced probability densities of the coupling and tuning modes of the conical intersection have been obtained. It is found that even weak coupling to the bath effectively suppresses the irregular fluctuations of the electronic populations of the isolated two-mode conical intersection. While the population of the upper adiabatic electronic state (S2) is very efficiently quenched by the system-bath coupling, the population of the diabatic ππ* electronic state exhibits long-lived oscillations driven by coherent motion of the tuning mode. Counterintuitively, the coupling to the bath can lead to an enhanced lifetime of the coherence of the tuning mode as a result of effective damping of the highly excited coupling mode, which reduces the strong mode-mode coupling inherent to the conical intersection. The present results extend previous studies of the dissipative dynamics at conical intersections to the nonperturbative regime of system-bath coupling. They pave the way for future first-principles simulations of femtosecond time-resolved four-wave-mixing spectra of chromophores in condensed phases which are nonperturbative in the system dynamics, the system-bath coupling as well as the field-matter coupling.

Universal single-frequency oscillations in a quantum impurity system after a local quench

Long-lived single-frequency oscillations in the local non-equilibrium dynamics of a quantum many-body system is an exceptional phenomenon. In fact, till now, it has never been observed, nor predicted, for the physically relevant case where a system is prepared to be quenched from one quantum phase to another. Here we show how the quench dynamics of the entanglement spectrum may reveal the emergence of such oscillations in a correlated quantum system with Kondo impurities. The oscillations we find are characterized by a single frequency. This frequency is independent of the amount of energy released by the local quench, and scales with the inverse system size. Importantly, the quench-independent frequency manifests itself also in local observables, such as the spin-spin correlation function of the impurities.

Electron dynamics following photoionization: Decoherence due to the nuclear-wave-packet width

The advent of attosecond techniques opens up the possibility to observe experimentally electron dynamics following ionization of molecules. Theoretical studies of pure electron dynamics at single fixed nuclear geometries in molecules have demonstrated oscillatory charge migration at a well-defined frequency but often neglecting the natural width of the nuclear wave packet. The effect on electron dynamics of the spatial delocalization of the nuclei is an outstanding question. Here, we show how the inherent distribution of nuclear geometries leads to dephasing. Using a simple analytical model, we demonstrate that the conditions for a long-lived electronic coherence are a narrow nuclear wave packet and almost parallel potential-energy surfaces of the states involved. We demonstrate with numerical simulations the decoherence of electron dynamics for two real molecular systems (paraxylene and polycyclic norbornadiene), which exhibit different decoherence time scales. To represent the quantum distribution of geometries of the nuclear wave packet, the Wigner distribution function is used. The electron dynamics decoherence result has significant implications for the interpretation of attosecond spectroscopy experiments since one no longer expects long-lived oscillations.

The Coherence of a Dipolar Condensate in a Harmonic Potential Superimposed to a Deep Lattice**Supported by the National Natural Science Foundation of China under Grant Nos 11274255 and 11305132, the Specialized Research Fund for the Doctoral Program of Higher Education of China under Grand No 20136203110001, the Natural Science Foundation of Gansu Province under Grant No 2011GS04358, and the Creation

Within the mean-field model, the coherent matter waves of a dipolar condensate in a harmonic potential superimposed to a deep lattice are investigated by the variational principle. It is shown that, in a harmonic potential superimposed to a deep lattice, it is possible to control the decoherence of Bloch oscillations due to the fact that the on-site and the inter-site dipolar interactions can not only damp out Bloch oscillations but also maintain long-lived Bloch oscillations under the certain condition. In particular, long-lived Bloch oscillations of dipolar condensate can be realized when the dipolar interaction, the contact interaction, the frequency of the harmonic potential and initial width of the wave packet satisfy an analytical condition. Thus the decoherence of Bloch oscillation can be controlled by adjusting the dipolar interaction, the contact interaction, the frequency of harmonic potential and the initial width of the wave packet.

Observation of coherent quench dynamics in a metallic many-body state of fermionic atoms.

Quantum simulation with ultracold atoms has become a powerful technique to gain insight into interacting many-body systems. In particular, the possibility to study nonequilibrium dynamics offers a unique pathway to understand correlations and excitations in strongly interacting quantum matter. So far, coherent nonequilibrium dynamics has exclusively been observed in ultracold many-body systems of bosonic atoms. Here we report on the observation of coherent quench dynamics of fermionic atoms. A metallic state of ultracold spin-polarized fermions is prepared along with a Bose-Einstein condensate in a shallow three-dimensional optical lattice. After a quench that suppresses tunnelling between lattice sites for both the fermions and the bosons, we observe long-lived coherent oscillations in the fermionic momentum distribution, with a period that is determined solely by the Fermi-Bose interaction energy. Our results show that coherent quench dynamics can serve as a sensitive probe for correlations in delocalized fermionic quantum states and for quantum metrology.

Interaction quench in the Holstein model: Thermalization crossover from electron- to phonon-dominated relaxation

We study the relaxation of the Holstein model after a sudden switch-on of the interaction by means of the nonequilibrium dynamical mean field theory, with the self-consistent Migdal approximation as an impurity solver. We show that there exists a qualitative change in the thermalization dynamics as the interaction is varied in the weak-coupling regime. On the weaker interaction side of this crossover, the phonon oscillations are damped more rapidly than the electron thermalization timescale, as determined from the relaxation of the electron momentum distribution function. On the stronger interaction side, the relaxation of the electrons becomes faster than the phonon damping. In this regime, despite long-lived phonon oscillations, a thermalized momentum distribution is realized temporarily. The origin of the thermalization crossover found here is traced back to different behaviors of the electron and phonon self-energies as a function of the electron-phonon coupling. In addition, the importance of the phonon dynamics is demonstrated by comparing the self-consistent Migdal results with those obtained with a simpler Hatree-Fock impurity solver that neglects the phonon self-energy. The latter scheme does not properly describe the evolution and thermalization of isolated electron-phonon systems.

RECOILING SUPERMASSIVE BLACK HOLES: A SEARCH IN THE NEARBY UNIVERSE

The coalescence of a binary black hole can be accompanied by a large gravitational recoil due to anisotropic emission of gravitational waves. A recoiling supermassive black hole (SBH) can subsequently undergo long-lived oscillations in the potential well of its host galaxy, suggesting that offset SBHs may be common in the cores of massive ellipticals. We have analyzed HST archival images of 14 nearby core ellipticals, finding evidence for small ($\lesssim 10$ pc) displacements between the AGN (locating the SBH) and the center of the galaxy (the mean photocenter) in 10 of them. Excluding objects that may be affected by large-scale isophotal asymmetries, we consider six galaxies to have detected displacements, including M87, where a displacement was previously reported by Batcheldor et al. 2010. In individual objects, these displacements can be attributed to residual gravitational recoil oscillations following a major or minor merger within the last few Gyr. For plausible merger rates, however, there is a high probability of larger displacements than those observed, if SBH coalescence took place in these galaxies. Remarkably, the AGN-photocenter displacements are approximately aligned with the radio source axis in four of the six galaxies with displacements, including three of the four having relatively powerful kpc-scale jets. This suggests intrinsic asymmetries in radio jet power as a possible displacement mechanism, although approximate alignments are also expected for gravitational recoil. Orbital motion in SBH binaries and interactions with massive perturbers can produce the observed displacement amplitudes but do not offer a ready explanation for the alignments.

Resonant optical control of the electrically induced spin polarization by periodic excitation

We show that the electron spin polarization generated by an electrical current may have its direction controlled and magnitude amplified by periodic optical excitation. The electrical and optical spin control methods were combined and implemented in a two-dimensional electron gas. By Kerr rotation in an external transverse magnetic field, we demonstrate unexpected long-lived coherent spin oscillations of the current-induced signal in a system with large spin-orbit interaction. Using a single linearly polarized pulse for spin manipulation and detection, we found a strong dependence on the pulse optical power and sample temperature indicating the relevance of the hole spin in the electron spin initialization. The signal was mapped in a Hall bar as function of the position relative to the injection contact. Finally, the presence of an in-plane spin polarization was directly verified by rotating the experimental geometry.

Long-range magnetic order in the 5d2 double perovskite Ba2CaOsO6: comparison with spin-disordered Ba2YReO6

The B-site ordered double perovskite Ba2CaOsO6 was studied by dc magnetic susceptibility, powder neutron diffraction and muon spin relaxation methods. The lattice parameter is a = 8.3619(6) Å at 280 K and cubic symmetry is retained to 3.5 K with a = 8.3462(7) Å. Curie-Weiss susceptibility behaviour is observed for T > 100 K and the derived constants are C = 0.3361(3) emu K mol−1 and ΘCW = −156.2(3) K, in excellent agreement with literature values. This Curie constant is much smaller than the spin-only value of 1.00 emu K mol−1 for a 5d2 Os6+ configuration, indicating a major influence of spin–orbit coupling. Previous studies had detected both susceptibility and heat capacity anomalies near 50 K but no definitive conclusion was drawn concerning the nature of the ground state. While no ordered Os moment could be detected by powder neutron diffraction, muon spin relaxation (µSR) data show clear long-lived oscillations indicative of a continuous transition to long-range magnetic order below TC = 50 K. An estimate of the ordered moment on Os6+ is ∼ 0.2 μB, based upon a comparison with µSR data for Ba2YRuO6 with a known ordered moment of 2.2 μB. These results are compared with those for isostructural Ba2YReO6 which contains Re5+, also 5d2, and has a nearly identical unit cell constant, a = 8.36278(2) Å—a structural doppelgänger. In contrast, Ba2YReO6 shows ΘCW = − 616 K, and a complex spin-disordered and, ultimately, spin-frozen ground state below 50 K, indicating a much higher level of geometric frustration than in Ba2CaOsO6. The results on these 5d2 systems are compared to recent theory, which predicts a variety of ferromagnetic and antiferromagnetic ground states. In the case of Ba2CaOsO6, our data indicate that a complex four-sublattice magnetic structure is likely. This is in contrast to the spin-disordered ground state in Ba2YReO6, despite a lack of evidence for structural disorder, for which theory currently provides no clear explanation.