Collective Precession Research Articles

Larmor precession in strongly correlated itinerant electron systems

Many-electron systems undergo a collective Larmor precession in the presence of a magnetic field. In a paramagnetic metal, the resulting spin wave provides insight into the correlation effects generated by the electron-electron interaction. Here, we use dynamical mean-field theory to investigate the collective Larmor precession in the strongly correlated regime, where dynamical correlation effects such as quasiparticle lifetimes and non-quasiparticle states are essential. We study the spin excitation spectrum, which includes a dispersive Larmor mode as well as electron-hole excitations that lead to Stoner damping. We also extract the momentum-resolved damping of slow spin waves. The accurate theoretical description of these phenomena relies on the Ward identity, which guarantees a precise cancellation of self-energy and vertex corrections at long wavelengths. Our findings pave the way towards a better understanding of spin wave damping in correlated materials.

Three-terminal magnonic demultiplexer, power divider, and circulator

Spin waves, the collective precession of spins in ordered magnets, have attractive properties and have aroused extensive investigation of using them as data carriers. Lots of signal processing devices based on spin waves have been investigated recently, in which one structure normally realized a single function. Here, we propose and numerically demonstrate a conceptual prototype of one three-terminal magnonic device with three functions: demultiplexer, power divider, and circulator. These magnonic functions in “tuning fork” shaped magnetic waveguide are realized by the interfacial Dzyaloshinsky-Moriya interaction induced chiral spin wave modes of the circular cavity, which results in the difference between the intensity and phase of propagating magnetostatic forward volume spin waves. For the demultiplexer, the dual-frequency spin waves can be effectively separated from the fork handle into the two different fork arms. For the power divider, the single-frequency spin wave can propagated from the fork handle into fork arms with equal amplitude but different phases. For circulators, spin waves excited in different ports propagate unidirectionally and nonreciprocally with a field-controlled chirality of the circulation. Our findings can enrich the investigation of magnonic and spintronic devices.

Stationary solutions for fast flavor oscillations of a homogeneous dense neutrino gas

We present a method to find the stationary solutions for fast flavor oscillations of a homogeneous dense neutrino gas. These solutions correspond to collective precession of all neutrino polarization vectors around a fixed axis in the flavor space on average, and are conveniently studied in the co-rotating frame. We show that these solutions can account for the numerical results of explicit evolution calculations, and that even with the simplest assumption of adiabatic evolution, they can provide the average survival probabilities to good approximation. We also discuss improvement of these solutions and their use as estimates of the effects of fast oscillations in astrophysical environments.

Nonexponential decoherence of collective spin states in 2DES probed by time-resolved Kerr rotation

The spin coherence of two-dimensional electrons is determined by two independent mechanisms: a single particle relaxation owing to spatially fluctuating magnetic field, and a many-particle exchange interaction maintaining collective precession of the electron spins with a common Larmor frequency. In this study, we investigate the structure of a time-resolved Kerr rotation signal for the different spin states of two-dimensional electron system subjected to the quantizing magnetic field. At low temperatures, when spin–spin correlations define the ground state of the two-dimensional electron system, our data show a nonlinear damping of Larmor oscillations. The amplitude and the correlation length of the fluctuating magnetic field acting on individual electron spins are estimated.

Quantum Spin-Wave Materials, Interface Effects and Functional Devices for Information Applications

With the continuous miniaturization of electronic devices and the increasing speed of their operation, solving a series of technical issues caused by high power consumption has reached an unprecedented level of difficulty. Fortunately, magnons (the quanta of spin waves), which are the collective precession of spins in quantum magnetic materials, making it possible to replace the role of electrons in modern information applications. In the process of information transmission, nano-sized spin-wave devices do not transport any physical particles; therefore, the corresponding power consumption is extremely low. This review focuses on the emerging developments of the spin-wave materials, tunable effects, and functional devices applications. In the materials front, we summarize the magnetic properties and preparation characteristics of typical insulating single-crystalline garnet films or metallic alloy films, the development of new spin-wave material system is also introduced. Afterward, we introduce the emerging electric control of spin-wave effects originating from the interface transitions, physical or chemical, among these films including, voltage-controlled magnetic anisotropy, magneto-ionic transport, electric spin-torque, and magnon-torque. In the functional devices front, we summarize and elaborate on the low dispassion information processing devices and sensors that are realized based on spin waves.

Recent advances in key elements of spin-wave logic gates

Traditional charge-based device development is approaching the limit of Moores law, thus, there is a desperate need for new information processing principles or architectures. Replacing traditional charge-based devices with non-charge-based devices for information processing shows promise. Spin waves, which are generated from the collective precession of electron spins, are thought to be an important information carrier for the transmission and processing of information in a beyond-Moore era. Spin-wave logic gates possess incomparable advantages over traditional logic devices in energy consumption, device scale, application frequency, and logical operation, among other factors. This article explains the operating principles of spin-wave logic gates based on a Mach-Zehnder interferometer structure, and then analyzes the realization theory of key spin-wave logic gate elements at a device level. Recent advances in these key elements are also reviewed systematically. Lastly, future prospects of spin-wave logic gates are discussed.

Negative-Mass Instability of the Spin and Motion of an Atomic Gas Driven by Optical Cavity Backaction.

We realize a spin-orbit interaction between the collective spin precession and center-of-mass motion of a trapped ultracold atomic gas, mediated by spin- and position-dependent dispersive coupling to a driven optical cavity. The collective spin, precessing near its highest-energy state in an applied magnetic field, can be approximated as a negative-mass harmonic oscillator. When the Larmor precession and mechanical motion are nearly resonant, cavity mediated coupling leads to a negative-mass instability, driving exponential growth of a correlated mode of the hybrid system. We observe this growth imprinted on modulations of the cavity field and estimate the full covariance of the resulting two-mode state by observing its transient decay during subsequent free evolution.

Electromagnon resonance in a collinear spin state of the polar antiferromagnet Fe2Mo3O8

Magnetic excitations are investigated for a hexagonal polar magnet Fe2Mo3O8 by terahertz spectroscopy. We observed magnon modes including an electric-field active magnon, electromagnon, in the collinear antiferromagnetic phase with spins parallel to the c axis. We unravel the nature of these excitations by investigating the correlation between the evolution of the mode profile and the magnetic transition from antiferromagnetic to ferrimagnetic order induced by magnetic field or Zn-doping. We propose that the observed electromagnon mode involves the collective precession of the spins with oscillating in-plane electric polarization through the mechanism of the linear magnetoelectric effect.

Carrier-impurity spin transfer dynamics in paramagnetic II-VI diluted magnetic semiconductors in the presence of a wave-vector-dependent magnetic field

Quantum kinetic equations of motion for carrier and impurity spins in paramagnetic II-VI diluted magnetic semiconductors in a $\mathbf{k}$-dependent effective magnetic field are derived, where the carrier-impurity correlations are taken into account. In the Markov limit, rates for the electron-impurity spin transfer can be derived for electron spins parallel and perpendicular to the impurity spins corresponding to measurable decay rates in Kerr experiments in Faraday and Voigt geometry. Our rigorous microscopic quantum kinetic treatment automatically accounts for the fact that, in an individual spin flip-flop scattering process, a spin flip of an electron is necessarily accompanied by a flop of an impurity spin in the opposite direction and the corresponding change of the impurity Zeeman energy influences the final energy of the electron after the scattering event. This shift in the electron energies after a spin flip-flop scattering processes, which usually has been overlooked in the literature, turns out to be especially important in the case of extremely diluted magnetic semiconductors in an external magnetic field. As a specific example for a $\mathbf{k}$-dependent effective magnetic field the effects of a Rashba field on the dynamics of the carrier-impurity correlations in a Hg$_{1-x-y}$Cd$_y$Mn$_x$Te quantum well are described. It is found that, although accounting for the Rashba interaction in the dynamics of the correlations leads to a modified $\mathbf{k}$-space dynamics, the time evolution of the total carrier spin is not significantly influenced. Furthermore, a connection between the present theory and the description of collective carrier-impurity precession modes is presented.

Theory of Anderson pseudospin resonance with Higgs mode in superconductors

A superconductor illuminated by an ac electric field with frequency $\Omega$ is theoretically found to generate a collective precession of Anderson's pseudospins, and hence a coherent amplitude oscillation of the order parameter, with a doubled frequency $2\Omega$ through a nonlinear light-matter coupling. We provide a fundamental theory, based on the mean-field formalism, to show that the induced pseudospin precession resonates with the Higgs amplitude mode of the superconductor at $2\Omega=2\Delta$ with $2\Delta$ being the superconducting gap. The resonant precession is accompanied by a divergent enhancement of the third-harmonic generation (THG). By decomposing the THG susceptibility into the bare one and vertex correction, we find that the enhancement of the THG cannot be explained by individual quasiparticle excitations (pair breaking), so that the THG serves as a smoking gun for an identification of the collective Higgs mode. We further explore the effect of electron-electron scattering on the pseudospin resonance by applying the nonequilibrium dynamical mean-field theory to the attractive Hubbard model driven by ac electric fields. The result indicates that the pseudospin resonance is robust against electron correlations, although the resonance width is broadened due to electron scattering, which determines the lifetime of the Higgs mode.

Modulation Rate Study in a Spin-Torque Oscillator-Based Wireless Communication System

Wireless transmission requires a circuitry to generate GHz range signal. Spin transfer torque [1,2] devices are one of the promising candidates for generating stable signal in the GHz range through collective precession of the local spins in one of the ferromagnetic layer, known as free layer. Another ferromagnetic layer, fixed layer, acts as a medium to convert the incoming electric current into spin polarized current. This spin polarized current drives local spins of the free layer into constant precession. Factors like high linewidth, low output power and modulation rate are yet to be optimized to make these devices ready for use in practical wireless communication system. Recently, H. S. Choi et al. [4], has demonstrated the first ever complete wireless communication system using spin torque nano-oscillators (STNOs) through the amplitude shift keying (ASK) modulation. Here, we present a more detailed study of wireless communication as a function of the modulation frequency and distance between receiver and transmitting antennas (T x ) in order to determine modulation rate limits.

Light-induced collective pseudospin precession resonating with Higgs mode in a superconductor.

Superconductors host collective modes that can be manipulated with light. We show that a strong terahertz light field can induce oscillations of the superconducting order parameter in NbN with twice the frequency of the terahertz field. The result can be captured as a collective precession of Anderson's pseudospins in ac driving fields. A resonance between the field and the Higgs amplitude mode of the superconductor then results in large terahertz third-harmonic generation. The method we present here paves a way toward nonlinear quantum optics in superconductors with driving the pseudospins collectively and can be potentially extended to exotic superconductors for shedding light on the character of order parameters and their coupling to other degrees of freedom.

Pure collective precession motion of a high-spin torus isomer

We investigate the precession motion of the exotic torus configuration in high-spin excited states of ${}^{40}$Ca. For this aim, we use the three-dimensional time-dependent Hartree-Fock (TDHF) method. Although the high-spin torus isomer is a unique quantum object characterized by the alignment of angular momenta of independent single-particle motions, we find that the obtained moment of inertia for rotations about an axis perpendicular to the symmetry axis is close to the rigid-body value. We also analyze the microscopic structure of the precession motion using the random-phase approximation (RPA) method for high-spin states. In the RPA calculation, the precession motion of the torus isomer is generated by coherent superposition of many one-particle--one-hole excitations across the sloping Fermi surface that strongly violates the time-reversal symmetry. By comparing results of the TDHF and the RPA calculations, we find that the precession motion obtained by the TDHF calculation is a pure collective motion well decoupled from other collective modes.

Collective spin precession excitations in a two-dimensional quantum Hall ferromagnet

Using the inelastic light scattering technique we observe the spin excitations for a quantum Hall ferromagnet with charged defects. The lowest energy spin excitation branch behaves as the theoretically predicted collective spin precession (``cyclotron'') mode of the spin-texture liquid. A coupling between the spin-exciton and the ``cyclotron'' mode indicates that the existing theory of noninteracting spin texture liquid does not provide a fully adequate description for the ground state of a quantum Hall ferromagnet with charged defects.

A “nano-windmill” driven by a flux of water vapour: a comparison to the rotating ATPase

We measure the frequency of collective molecular precession as a function of temperature in the ferroelectric liquid crystalline monolayer at the water-air interface. This movement is driven by the unidirectional flux of evaporating water molecules. The collective rotation in the monolayer with angular velocities ω ~ 1 s(-1) (at T = 312 K) to 10(-2) s(-1) (at T = 285.8 K) is 9 to 14 orders of magnitude slower than rotation of a single molecule (typically ω ~ 10(9) to 10(12) s(-1)). The angular velocity reaches 0 upon approach to the two dimensional liquid-to-solid transition in the monolayer at T = 285.8 K. We estimate the rotational viscosity, γ1, in the monolayer and the torque, Γ, driving this rotation. The torque per molecule equals Γ = 5.7 × 10(-8) pN nm at 310 K (γ1 = 0.081 Pa s, ω = 0.87 s(-1)). The energy generated during one turn of the molecule at the same temperature is W = 3.5 × 10(-28) J. Surprisingly, although this energy is 7 orders of magnitude smaller than the thermal energy, kBT (310 K) = 4.3 × 10(-21) J, the rotation is very stable. The potential of the studied effect lies in the collective motion of many (>10(12)) "nano-windmills" acting "in concerto" at the scale of millimetres. Therefore, such systems are candidates for construction of artificial molecular engines, despite the small energy density per molecular volume (5 orders of magnitude smaller than for a single ATPase).

Symmetries in collective neutrino oscillations

We discuss the relationship between a symmetry in the neutrino flavour evolution equations and neutrino flavour oscillations in the collective precession mode. This collective precession mode can give rise to spectral swaps (splits) when conditions can be approximated as homogeneous and isotropic. Multi-angle numerical simulations of supernova neutrino flavour transformation show that when this approximation breaks down, non-collective neutrino oscillation modes decohere kinematically, but the collective precession mode is still expected to stand out. We provide a criterion for significant flavour transformation to occur if neutrinos participate in a collective precession mode. This criterion can be used to understand the suppression of collective neutrino oscillations in anisotropic environments in the presence of a high matter density. This criterion is also useful in understanding the breakdown of the collective precession mode when neutrino densities are small.

Collective single-mode precession of electron spins in an ensemble of singly charged (In,Ga)As/GaAs quantum dots

We show that the spins of all electrons, each confined in a quantum dot of an (In,Ga)As/GaAs dot ensemble, can be driven into a single mode of precession about a magnetic field. This regime is achieved by allowing only a single mode within the electron spin precession spectrum of the ensemble to be synchronized with a train of periodic optical excitation pulses. Under this condition a nuclei induced frequency focusing leads to a shift of all spin precession frequencies into the synchronized mode. The macroscopic magnetic moment of the electron spins that is created in this regime precesses without dephasing.

Spatiotemporal patterns in a Langmuir monolayer due to driven molecular precession

Langmuir monolayers of chiral liquid crystals on the surface of water exhibit orientational waves with complex spatiotemporal patterns. These patterns arise from a collective precession of the mesogenic molecules, driven by the evaporation of water through the monolayer. We investigate the behavior of these orientational waves around topological defects in the molecular orientation. Through Brewster angle microscopy, we find that the waves form a reversing spiral pattern, which rotates about the central vortex. With increasing relative humidity, the rotation slows and then stops. We model the system theoretically, and show that predicted patterns are in good agreement with the experiments.

Coherent magnetization precession in GaMnAs induced by ultrafast optical excitation

The authors use femtosecond optical pulses to induce, control, and monitor magnetization precession in ferromagnetic Ga0.965Mn0.035As. At temperatures below ∼40K, they observe coherent oscillations of the local Mn spins, triggered by an ultrafast photoinduced reorientation of the in-plane easy axis. The amplitude saturation of the oscillations above a certain pump intensity indicates that the easy axis remains unchanged above ∼TC∕2. The authors find that the observed magnetization precession damping (Gilbert damping) is strongly dependent on pump laser intensity, but independent of ambient temperature. They provide a physical interpretation of the observed light-induced collective Mn-spin precession and relaxation.

Analysis of collective neutrino flavor transformation in supernovae

We study the flavor evolution of a dense gas initially consisting of pure monoenergetic ${\ensuremath{\nu}}_{e}$ and ${\overline{\ensuremath{\nu}}}_{e}$. Using adiabatic invariants and the special symmetry in such a system we are able to calculate the flavor evolution of the neutrino gas for the cases with slowly decreasing neutrino number densities. These calculations give new insights into the results of recent large-scale numerical simulations of neutrino flavor transformation in supernovae. For example, our calculations reveal the existence of what we term the ``collective precession mode.'' Our analyses suggest that neutrinos which travel on intersecting trajectories subject to destructive quantum interference nevertheless can be in this mode. This mode can result in sharp transitions in the final energy-dependent neutrino survival probabilities across all trajectories, a feature seen in the numerical simulations. Moreover, this transition is qualitatively different for the normal and inverted neutrino mass hierarchies. Exploiting this difference, the neutrino signals from a future galactic supernova can potentially be used to determine the actual neutrino mass hierarchy.