Macroscopic Spin Research Articles

Non-equilibrium dynamics of an unstable quantum pendulum explored in a spin-1 Bose–Einstein condensate

A pendulum prepared perfectly inverted and motionless is a prototype of unstable equilibrium and corresponds to an unstable hyperbolic fixed point in the dynamical phase space. Here, we measure the non-equilibrium dynamics of a spin-1 Bose-Einstein condensate initialized as a minimum uncertainty spin-nematic state to a hyperbolic fixed point of the phase space. Quantum fluctuations lead to non-linear spin evolution along a separatrix and non-Gaussian probability distributions that are measured to be in good agreement with exact quantum calculations up to 0.25 s. At longer times, atomic loss due to the finite lifetime of the condensate leads to larger spin oscillation amplitudes, as orbits depart from the separatrix. This demonstrates how decoherence of a many-body system can result in apparent coherent behaviour. This experiment provides new avenues for studying macroscopic spin systems in the quantum limit and for investigations of important topics in non-equilibrium quantum dynamics.

Arbitrary magnetosonic solitary waves in spin 1/2 degenerate quantum plasma

Linear and nonlinear compressional magnetosonic waves are studied in magnetized degenerate spin-1/2 Fermi plasmas. Starting from the basic equations of a quantum magnetoplasma we develop the system of quantum magnetohydrodynamic (QMHD) equations. Spin effects are incorporated via spin force and macroscopic spin magnetization current. Sagdeev potential approach is employed to derive the nonlinear energy integral equation which admits the rarefactive solitary structure in the subAlfvenic region. The quantum diffraction due to Bohm potential does not affect the amplitude of soliton but has a direct effect on its width. The width of soliton is broadened with the increase in the quantization of the system due to quantum diffraction. However, the nonlinear wave amplitude is reduced with the increase in the value of magnetization energy due to electron spin-1/2 effects. The degeneracy effect due to quantum plasma beta enhances the amplitude of magnetosonic soliton. The importance of the work relevant to compact astrophysical bodies is pointed out.

Diamond nitrogen-vacancy center as a probe of random fluctuations in a nuclear spin ensemble

New schemes that exploit the unique properties of nitrogen-vacancy (NV) centers in diamond are presently being explored as a platform for high-resolution magnetic sensing. Here we focus on the ability of a NV center to monitor an adjacent mesoscopic nuclear spin bath. For this purpose, we conduct comparative experiments where the NV spin evolves under the influence of surrounding ${}^{13}$C nuclei or, alternatively, in the presence of asynchronous alternating current fields engineered to emulate bath fluctuations. Our study reveals substantial differences that underscore the limitations of the semi-classical picture when interpreting and predicting the outcome of experiments designed to probe small nuclear spin ensembles. In particular, our study elucidates the NV center response to bath fluctuations under common pulse sequences, and explores a detection protocol designed to probe time correlations of the nuclear spin bath dynamics. Further, we show that the presence of macroscopic nuclear spin order is key to the emergence of semi-classical spin magnetometry.

Spin-Polarized Transient Electron Trapping in Phosphorus-Doped Silicon

Experimental evidence of electron spin precession during travel through the phosphorus-doped Si channel of an all-electrical device simultaneously indicates two distinct processes: (i) short time scales (≈50 ps) due to purely conduction-band transport from injector to detector and (ii) long time scales (≈1 ns) originating from delays associated with capture or reemission in shallow impurity traps. The origin of this phenomenon, examined via temperature, voltage, and electron density dependence measurements, is established by means of a comparison to a numerical model and is shown to reveal the participation of metastable excited states in the phosphorus-impurity spectrum. This work therefore demonstrates the potential to make the study of macroscopic spin transport relevant to the quantum regime of individual spin interactions with impurities as envisioned for quantum information applications.

Long-Lived Ultrafast Spin Precession in Manganese Alloys Films with a Large Perpendicular Magnetic Anisotropy

Spin precession with frequencies up to 280 GHz is observed in Mn(3-δ)Ga alloy films with a perpendicular magnetic anisotropy constant K(u)∼15 M erg/cm(3). The damping constant α, characterizing macroscopic spin relaxation and being a key factor in spin-transfer-torque systems, is not larger than 0.008 (0.015) for the δ=1.46 (0.88) film. Those are about one-tenth of α values for known materials with large K(u). First-principles calculations well describe both low α and large K(u) for these alloys.

Fast and slow magnetosonic waves in two-dimensional spin-1/2 quantum plasma

Using the spin-1/2 resistive quantum magnetohydrodynamics model, linear and nonlinear relations for slow and fast magnetosonic modes are derived. Spin effects are incorporated via spin force and macroscopic spin magnetization current. The plasma resistivity is shown to play a role of dissipation in the system. With the aid of tanh method the traveling wave solution of Kadomstev–Petviashvili–Burgers is obtained. The solution shows a general shock wave profile superposed by a perturbative solitary-wave contribution. The dynamics of fast and slow magnetosonic shock and soliton, respectively, in the presence and absence of dissipation is investigated with respect to electron spin magnetization, quantum diffraction, and plasma statistic. It is found that results obtained from the spin quantum plasmas differ significantly from the nonspin quantum plasmas. The relevance of the present work to dense astrophysical plasmas such as pulsar magnetosphere is pointed out.

Photoinduced magnetization of magnetic polarons in Cd0.8Mn0.2Te under high‐density resonant excitation of free exciton

AbstractThe pump‐probe Kerr rotation measurement of Cd0.8Mn0.2Te has revealed that the resonant excitation of free excitons is a more suitable method for angular momentum transfer from laser pulse to the exciton magnetic polaron than the band‐to‐band excitation. In the case of band‐to‐band excitation, a formation of macroscopic spin alignment of exciton magnetic polarons is suppressed by heat originating from excess energy of hot electron‐hole pairs and scatterings among the electron‐hole pairs. By contrast, in the case of the resonant excitation, the angular momentum of the laser is transferred to the spins of the exciton magnetic polarons even though the excitation density is higher than the Mott density. These results are consistent and supportive for our suggestion that the selective excitation of localized excitons with energy lower than that of free exciton leads to the high‐spin state of the high‐density exciton magnetic polaron. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Explanation of the Meissner effect and prediction of a spin Meissner effect in low and high [formula omitted] superconductors

I argue that the conventional BCS–London theory of superconductivity does not explain the most fundamental property of superconductors, the Meissner effect: how is the Meissner current generated, and how is it able to defy Faraday’s law? How is its mechanical angular momentum compensated? I propose that superconductivity is impossible unless the metal expels charge from its interior towards the surface in the transition to superconductivity. As a consequence, superconductors in their ground state are predicted to possess a macroscopic electric field in their interior, as well as excess negative charge and a macroscopic spin current near the surface. The system is driven normal when the applied magnetic field is strong enough to bring the spin current to a stop. High temperature superconductivity occurs in systems that have too much negative charge.

Theory of Impedance Dynamics for Magnetic Multilayers

On the basis of a macroscopic spin wave-diffusion theory we study high-frequency spin transport through a junction composed of a magnetic and a nonmagnetic layer. We illustrate the consequences of the spin wave-diffusion theory for the spatio-temporal dynamics of a spin current, and apply the theory to the calculation of the dynamical impedance of the junction. For low modulation frequencies, our results are in good agreement with those from spin-diffusion theory, but the wave character of spin transport leads to different predictions for the impedance in the high-frequency regime.

Macroscopic spin and charge transport theory

According to the general principle of non-equilibrium thermodynamics, we propose a set of macroscopic transport equations for the spin transport and the charge transport. In particular, the spin torque is introduced as a generalized ‘current density’ to describe the phenomena associated with the spin non-conservation in a unified framework. The Einstein relations and the Onsager relations between different transport phenomena are established. Specifically, the spin transport properties of the isotropic non-magnetic and the isotropic magnetic two-dimensional electron gases are fully described by using this theory, in which only the macroscopic-spin-related transport phenomena allowed by the symmetry of the system are taken into account.

Charge Expulsion, Spin Meissner Effect, and Charge Inhomogeneity in Superconductors

Superconductivity occurs in systems that have a lot of negative charge: the highly negatively charged (CuO2)= planes in the cuprates, negatively charged (FeAs)− planes in the iron arsenides, and negatively charged B − planes in magnesium diboride. And, in the nearly filled (with negative electrons) bands of almost all superconductors, as evidenced by their positive Hall coefficient in the normal state. No explanation for this charge asymmetry is provided by the conventional theory of superconductivity, within which the sign of electric charge plays no role. Instead, the sign of the charge carriers plays a key role in the theory of hole superconductivity, according to which metals become superconducting because they are driven to expel negative charge (electrons) from their interior. This is why NIS tunneling spectra are asymmetric, with larger current for negatively biased samples. The theory also offers a compelling explanation of the Meissner effect: as electrons are expelled towards the surface in the presence of a magnetic field, the Lorentz force imparts them with azimuthal velocity, thus generating the surface Meissner current that screens the interior magnetic field. In type II superconductors, the Lorentz force acting on expelled electrons that do not reach the surface gives rise to the azimuthal velocity of the vortex currents. In the absence of applied magnetic field, expelled electrons still acquire azimuthal velocity, due to the spin–orbit interaction, in opposite direction for spin-up and spin-down electrons: the “Spin Meissner effect.” This results in a macroscopic spin current flowing near the surface of superconductors in the absence of applied fields, of magnitude equal to the critical charge current (in appropriate units). Charge expulsion also gives rise to an interior outward-pointing electric field and to excess negative charge near the surface. In strongly type II superconductors this physics should give rise to charge inhomogeneity and spin currents throughout the interior of the superconductor, to large sensitivity to (non-magnetic) disorder and to a strong tendency to phase separation.

Electric Manipulation of Spin Relaxation Using the Spin Hall Effect

Using the spin Hall effect, magnetization relaxation in a Ni_{81}Fe_{19}/Pt film is manipulated electrically. An electric current applied to the Pt layer exerts spin torque on the entire magnetization of the Ni81Fe19 layer via the macroscopic spin transfer induced by the spin Hall effect and modulates the magnetization relaxation in the Ni81Fe19 layer. This method allows us to tune the magnetization dynamics regardless of the film size without applying electric currents directly to the magnetic layer.

Collective spin tunneling in spinor Bose-Einstein condensates

We investigate a novel aspect of rotational tunneling of the macroscopic spin for multicomponent spinor Bose-Einstein condensate (BEC). The Lagrangian is deduced from the multi-component BEC system formalism, and is written in terms of spin coherent states. From the effective Hamiltonian for the collective spin, the tunneling rate is obtained through a functional integral of the spin variable. It is pointed out that the cooperative effect between the Zeeman energy and the anisotropic nature of the spin-dependent inter-atomic interaction plays a key role for occurrence of collective spin tunneling.

Spin dynamics in a tunnel junction between ferromagnets

The dynamics of a single spin embedded in the tunnel junction (quantum point contact) between ferromagnets is addressed. Using the Keldysh technique, we derive a quantum Langevin equation. As a consequence of the spin-polarization in the leads, the spin displays a rich and unusual dynamics. Parallel configured and equally strong magnetic moments in the leads yield an ordinary spin precession with a Larmor frequency given by the effective magnetic field. Unequal and/or non-parallel configured magnetization, however, causes nutation of the spin in addition to the precession. Our predictions may be directly tested for macroscopic spin clusters.

Dynamics and thermalization of the nuclear spin bath in the single-molecule magnetMn12−ac: Test for the theory of spin tunneling

The description of the tunneling of a macroscopic variable in the presence of a bath of localized spins is a subject of great fundamental and practical interest, and is relevant for many solid-state qubit designs. Most of the attention is usually given to the dynamics of the ``central spin'' (i.e., the qubit), while little is known about the spin bath itself. Here, we present a detailed study of the dynamics of the nuclear spin bath in the ${\mathrm{Mn}}_{12}\text{\ensuremath{-}}\mathrm{ac}$ single-molecule magnet, probed by NMR experiments down to very low temperatures $(T\ensuremath{\simeq}20\phantom{\rule{0.3em}{0ex}}\mathrm{mK})$. The results are critically analyzed in the framework of the Prokof'ev-Stamp theory of nuclear-spin-mediated quantum tunneling. We find that the longitudinal relaxation rate of the $^{55}\mathrm{Mn}$ nuclei in ${\mathrm{Mn}}_{12}\text{\ensuremath{-}}\mathrm{ac}$ becomes roughly $T$ independent below $T\ensuremath{\simeq}0.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and can be strongly suppressed with a longitudinal magnetic field. This is consistent with the nuclear relaxation being caused by quantum tunneling of the molecular spin, and we attribute the tunneling fluctuations to the minority of fast-relaxing molecules present in the sample. The transverse nuclear relaxation is also $T$ independent for $T<0.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and can be explained qualitatively and quantitatively by the dipolar coupling between like nuclei in neighboring molecules. This intercluster nuclear spin diffusion mechanism is an essential ingredient for the global relaxation of the nuclear spin bath. We also show that the isotopic substitution of $^{1}\mathrm{H}$ by $^{2}\mathrm{H}$ leads to a slower nuclear longitudinal relaxation, consistent with the decreased tunneling probability of the molecular spin. Finally, we demonstrate that even at the lowest temperatures---where only $T$-independent quantum tunneling fluctuations are present---the nuclear spins remain in thermal equilibrium with the lattice phonons, and we investigate the time scale for their thermal equilibration. After a review of the theory of macroscopic spin tunneling in the presence of a spin bath, we argue that most of our experimental results are consistent with that theory, but the thermalization of the nuclear spins is not. This calls for an extension of the spin-bath theory to include the effect of spin-phonon couplings in the nuclear-spin-mediated tunneling process.

Current‐induced switching in spin‐valve structures

AbstractSpin transfer between conduction electrons and local magnetic moments can switch spin valves between different magnetic configurations. In some conditions, however, the spin torque can induce magnetic switching to stationary precessional modes. In this paper we consider current‐induced magnetic switching in nanopillar spin valves including two thin magnetic films separated by a nonmagnetic layer, and in molecular‐based spin valves. In the former case the spin torque is calculated from the macroscopic spin diffusion equations, while in the latter one we use quantum description based on the Fermi golden rule. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ionizing radiation from superconductors in the theory of hole superconductivity

We point out that large superconducting bodies described by the theoryof hole superconductivity will emit ionizing radiation in non-equilibriumsituations. This remarkable prediction, involving an energy scale a factor of1012 larger than the low energy scale usually associated with superconductivity, is unique to thetheory of hole superconductivity. The phenomenon is a consequence of the macroscopicinhomogeneous charge distribution predicted to exist in superconducting bodies, and theresulting intrinsic macroscopic spin currents in the superconducting state in the absence ofapplied fields. For superconducting bodies of sufficiently large size, the speed of thespin–current carriers approaches the speed of light, and in addition real electron–positronpair production is expected to occur in the interior. When the superconducting state isdestroyed, electromagnetic radiation with frequencies up to should arise from bremsstrahlung and electron–positron annihilation. In support of thisrather unconventional theory we point out that it is the only existing theory that proposesexplanations for two fundamental universal effects associated with superconductivity: theMeissner effect and the Tao effect.

Spin-Hall interface resistance in terms of Landauer-type spin dipoles

We considered the nonequlibrium spin dipoles induced around spin independent elastic scatterers by the intrinsic spin-Hall effect associated with the Rashba spin-orbit coupling. The normal to 2DEG spin polarization has been calculated in the diffusion range around the scatterer. We found that although around each impurity this polarization is finite, the corresponding macroscopic spin density, obtained via averaging of individual spin dipole distributions over impurity positions is zero in the bulk. At the same time, the spin density is finite near the boundary of 2DEG, except for a special case of a hard wall boundary. The boundary value of the spin polarization can be associated with the interface spin-Hall resistance determining the additional energy dissipation due to spin accumulation.

Minority spin condensate in the spin-polarized superfluid 3He A1 phase

The magnetic properties of (3)He in its various phases originate from the interactions among the nuclear spins. The spin-polarized 'ferromagnetic' superfluid (3)He A(1) phase (which forms below 3 mK between two transition temperatures, T(c1) and T(c2), in an external magnetic field) serves as a material in which theories of fundamental magnetic processes and macroscopic quantum spin phenomena may be tested. Conventionally, the superfluid component of the A(1) phase is understood to contain only the majority spin condensate, having energetically favoured paired spins directed along the external field and no minority spin condensate having paired spins in the opposite direction. Because of difficulties in satisfying both the ultralow temperature and high magnetic field required to produce a substantial phase space, there exist few studies of spin dynamics phenomena that could be used to test the conventional view of the A(1) phase. Here we develop a mechanical spin density detector that operates in the required regime, enabling us to perform measurements of spin relaxation in the A(1) phase as a function of temperature, pressure and magnetic field. Our mechanical spin detector is based in principle on the magnetic fountain effect; spin-polarized superfluid motion can be induced both magnetically and mechanically, and we demonstrate the feasibility of increasing spin polarization by a mechanical spin filtering process. In the high temperature range of the A(1) phase near T(c1), the measured spin relaxation time is long, as expected. Unexpectedly, the spin relaxation rate increases rapidly as the temperature is decreased towards T(c2). Our measurements, together with Leggett-Takagi theory, demonstrate that a minute presence of minority spin pairs is responsible for this unexpected spin relaxation behaviour. Thus, the long-held conventional view that the A(1) phase contains only the majority spin condensate is inadequate.

Energy and multipartite entanglement in multidimensional and frustrated spin models

We investigate the relation between the entanglement properties of a quantum state and its energy for macroscopic spin models. To this aim, we develop a general method to compute energy bounds for states without certain forms of multipartite entanglement. Violation of these bounds implies the presence of these types of multipartite entanglement. As examples, we investigate the Heisenberg model in different dimensions, the Ising model and the $XX$ model in the presence of a magnetic field. Finally, by studying the Heisenberg model on a triangular lattice, we demonstrate that our techniques can be applied also to frustrated systems.