Search for spin-dependent macroscopic short-range interactions using neutron spin precession close to a mirror

Free neutrons can be employed as a sensitive probe to search for spin-dependent macroscopic short-range interactions induced by axion-like particles. In this Letter it is proposed to use pseudomagnetic precession of ultracold neutrons propagating close to a massive mirror of a trap. The method should be several orders of magnitude more sensitive than other methods proposed so far.     

Nonlinear optical spin Hall effect and long-range spin transport in polariton lasers

We report on the experimental observation of the nonlinear analogue of the optical spin Hall effect under highly nonresonant circularly polarized excitation of an exciton-polariton condensate in a GaAs/AlGaAs microcavity. The circularly polarized polariton condensates propagate over macroscopic distances, while the collective condensate spins coherently precess around an effective magnetic field in the sample plane performing up to four complete revolutions.     

Two-dimensional small particles coupled by dipolar interactions

We study the effect of the dipolar coupling on the magnetic properties of two small interacting ferromagnetic particles. Each particle is a two-dimensional array of Ising spins with a central spin surrounded by a variable number of shells. The coupling between spins inside each particle is ferromagnetic and the dipolar interaction between the particles is determined as a function of the number of shells, temperature, and distance between their centers. We investigate the system by mean-field approximation and Monte Carlo simulations. The dipolar interaction is calculated in two ways, one assuming effective spins in the centers of the particles, and the other directly computing the interactions among all the pairs of spins, one in each particle. We show that the difference in the corresponding dipolar energies is a power law on the distance with exponent 5. We calculate the magnetization and susceptibility as a function of temperature, number of shells and distance between the particles’ centers. We show that the critical temperature increases with the number of spins in each particle, and it is more noticeable in the mean-field calculations than in the Monte Carlo simulations.     

Entanglement in spin chains and lattices with long-range Ising-type interactions

We consider N initially disentangled spins, embedded in a ring or d-dimensional lattice of arbitrary geometry, which interact via some long-range Ising-type interaction. We investigate relations between entanglement properties of the resulting states and the distance dependence of the interaction in the limit N-->infinity. We provide a sufficient condition when bipartite entanglement between blocks of L neighboring spins and the remaining system saturates and determine S(L) analytically for special configurations. We find an unbounded increase of S(L) as well as diverging correlation and entanglement length under certain circumstances. For arbitrarily large N, we can efficiently calculate all quantities associated with reduced density operators of up to ten particles.     

Fibrillar templates and soft phases in systems with short-range dipolar and long-range interactions

We analyze the thermal fluctuations of particles that have a short-range dipolar attraction and a long-range repulsion. In an inhomogeneous particle density region, or "soft phase," filamentary patterns appear which are destroyed only at very high temperatures. The filaments act as a fluctuating template for correlated percolation in which low-energy excitations can move through the stable pattern by local rearrangements. At intermediate temperatures, dynamically averaged checkerboard states appear. We discuss possible implications for doped cuprate oxides and related materials.     

Quantitative analysis of long-range interactions between adsorbed dipolar molecules on Cu(111)

Highly dispersed superstructures of a dipolar iridium complex are formed on a Cu(111) surface. We show that the dilute superstructures with density-controlled intermolecular separations are stabilized by the strong and long-range repulsive intermolecular interactions. The repulsive intermolecular interactions are quantitatively evaluated by using low-temperature scanning tunneling microscopy, which are characterized by the surface-enhanced dipole-dipole interactions.     

Stabilization of the long-range magnetic ordering by dipolar and magnetoelastic interactions in two-dimensional ferromagnets

The possibility of the long-range magnetic order stabilization in two-dimensional ferromagnets with the account of dipolar and magnetoelastic interactions is investigated. The mechanisms of the magnetic order stabilization by both types of interactions are studied. The Curie temperature is estimated. The comparisons with experimental data are made. Received 22 June 2001 and Received in final form 17 January 2002     

Random versus nonrandom competing bonds for spin glasses with long-range interactions

The mapping of random onto nonrandom competing bonds recently suggested is here implemented for spin glasses of long-ranged interactions. Exact results are obtained which support previous conclusions from high-temperature series.     

Squeezing in a spin chain with site-dependent periodic and long-range interactions

Numerical and analytical results for the squeezing factor, ζ², in a pseudo-spin s-1/2 chain. The open chain is composed by N two-level atoms with site-dependent interactions. The time evolution of the squeezing factor is studied, as well as its dependence on the number of atoms and on the interactions. It is found that long-range interactions may optimize the degree of spin squeezing.     

Classical spin systems with long-range interactions: universal reduction of mixing

We present the numerical study of chaos in a classical model of N coupled rotators on a lattice, in dimensions d=2,3. The coupling constants decay with distance as r_ij^−α (α⩾0). The thermodynamics of the model is extensive if α/d>1 and nonextensive otherwise. For energies above a critical threshold U_c the largest Lyapunov exponent scales as N^−κ, where κ is a universal function of α/d. The function κ decreases from 1/3 to 0 when α/d increases from 0 to 1, and vanishes above 1. We conjecture that this scaling law is related to the nonextensivity of the model, through a power-law sensitivity to initial conditions (weak mixing).     

Langevin dynamics simulation of a one-dimensional linear spin chain with long-range interactions

In this paper we study the critical behavior of a simple one-dimensional rotor spin in the form of a linear chain with long-range interactions, using the mean field Langevin dynamics approach and in the presence of fluctuations added by a heat bath. We have computed the specific heat, the magnetic susceptibility, the Binder fourth-order cumulant, and the magnetization, and then we have calculated the critical exponents using finite-size scaling. In addition, we provide a relation between the thermal bath temperature and the temperature of the system. Our results confirm the existence of a second-order critical temperature in the one-dimensional chain of spins with long-range interaction.     

On the existence of Fröhlich's long-range interactions in dipolar chains

A chain of N identical polarizable molecules interacting via dipole-dipole forces is investigated. A careful mathematical analysis results in a proof of the existence of long-range forces analogous to those described by Fröhlich for two giant dipoles which is a special case in our model. The energy spectrum is more complicated and depends on the structure constants of the system. In addition, formulas have been provided for temperature dependence of the effective interaction energy. At room temperature Fröhlich's expression underestimates the strength of the interactions by at least an order of magnitude.     

Coherent spin precession via photoinduced antiferromagnetic interactions in La0.67Ca0.33MnO3

Pronounced spin precessions are observed in a geometry with negligible canting of the magnetization in ferromagnetic La(0.67)Ca(0.33)MnO(3) thin films using the time-resolved magneto-optical Kerr effect. The precession amplitude monotonically decreases with increasing field, indicating that the coherent spin rotation may be triggered by a transient exchange field and not by demagnetization and/or anisotropy field modulation. We attribute the transient exchange field to emergent antiferromagnetic interactions due to charge transfer and modification of the kinetic energy of e(g) electrons under optical excitation.     

Yangian symmetry and Virasoro character in a lattice spin system with long-range interactions

We consider the symmetry of lattice su(n) spin systems with inverse square exchange; the Haldane-Shastry model and the Polychronakos-Frahm model. It is shown that both systems have yangian symmetry for a finite number of sites. Based on the yangian symmetry the energy spectrum is considered. Furthermore the representation for motifs and the relations with the Virasoro character for su(n) level-1 Wess-Zumino- Witten theory are discussed.     

Monte Carlo simulations of the equilibrium and non-equilibrium properties of low-dimensional magnetic systems with long-range dipolar interactions

The equilibrium and non-equilibrium properties of a two-dimensional Ising-like spin system with both ferromagnetic exchange and long-range dipolar interactions are studied. Implementing the Ewald Sums (ES) for the calculation of the dipolar interaction we reproduced the results of the literature concerning the equilibrium-phase diagram and the stability of the phases. We also investigated the aging regime and the Fluctuation–Dissipation Theorem (FDT) violations in the system and we showed that, despite usual claims in the literature, the dynamical behavior of the system is independent of the strength of the dipolar interaction, at least in the zone of the phase diagram under study.     

Nonlinear damping in spin systems: long range spin-spin interactions

The effective field theory of nonlinear damping for strongly interacting spins driven by external fields and weakly coupled to a bath is derived starting from the microscopic dynamics. The use of a block-spin model with long range spin-spin interactions makes it possible to treat the spin dynamics exactly and to eliminate the microscopic degrees of freedom. The results are presented in form of a nonlinear integro-differential equation for the time and space dependent magnetization. This equation is of memory type, applies to spin-spin interactions of any symmetry and is valid for inhomogeneous external fields of arbitrary strength and time dependence. For an isotropic ferromagnet in weak and slow external fields the general integro-differential equation is shown to reduce to a differential equation of motion of Landau-Lifshitz-Bloch type.This work was performed within a program of the Sonderforschungsbereich 185 Nichlineare Dynamik Frankfurt-Darmstadt     

Dephasing of spin echoes by multiple heteronuclear dipolar interactions in rotational echo double resonance NMR experiments

The application of rotational echo double resonance (REDOR) nuclear magnetic resonance (NMR) for accurate distance measurements has thus far been largely restricted to isolated heteronuclear two-spin systems. In the present paper, the informational content of REDOR curves is explored for systems characterized by multi-spin interactions. To this end, numerical REDOR simulations are presented for cases in which single observe spins S are dipolarly coupled to groups of spins I in distinct geometries. To develop the utility of REDOR for characterizing dipolar couplings in unknown and/or ill-defined geometries, the validity ranges and systematic errors of certain analytical approximations are studied. In the limit of short dipolar evolution times where 0 < deltaS/S0 < or = 0.2 to 0.3, the REDOR difference signal intensity increases approximately proportional to the square of the dipolar evolution time. Here, the curvature depends simply on the second moment M2 characterizing the overall strength of the heterodipolar coupling, irrespective of specific molecular geometries. Fitting experimental REDOR data in this manner produces slight systematic underestimates of M2. However, these errors tend to be counterbalanced by additional systematic errors made by neglecting weak couplings to more remote spins and distribution effects caused by disorder. Based on these findings, the results suggest a convenient method of obtaining site-resolved second moment information in disordered materials.