Ultrasonic study of dysprosium in a magnetic field

The ultrasonic attenuation of longitudinal waves propagating along the c axis of single crystal dyprosium is reported, as a function of the applied basal plane field in the paramagnetic region, and as a function of the temperature, at constant applied basal plane field, in the spin-spiral region. In the paramagnetic region, anomalous attenuation behavior is explained on the basis of competing spin-polarization and spin-fluctuation effects. Two anomalous maxima in the temperature dependence of the attenuation were observed: one near T_N is attributed to spin fluctuations associated with short range ferromagnetic ordering; another one at 130 K is attributed to a magnetic phase transition from a fanstructure phase, intermediate between the spin-spiral and ferromagnetically ordered phase     

General theory for the ferroelectric polarization induced by spin-spiral order

The ferroelectric polarization of triangular-lattice antiferromagnets induced by helical spin-spiral order is not explained by any existing model of magnetic-order-driven ferroelectricity. We resolve this problem by developing a general theory for the ferroelectric polarization induced by spin-spiral order and then by evaluating the coefficients needed to specify the general theory on the basis of density functional calculations. Our theory correctly describes the ferroelectricity of triangular-lattice antiferromagnets driven by helical spin-spiral order and incorporates known models of magnetic-order-driven ferroelectricity as special cases.     

Spin-spiral and domain wall structures in monolayer Fe/W(1 1 0)

The stability of spin-spiral and domain wall structures in an Fe monolayer on a W(1 1 0) substrate is theoretically investigated. By analyzing the exchange parameters obtained from first principles total energy calculations, we find that a competition between the nearest-neighbor ferromagnetic and long-distant antiferromagnetic exchange interactions leads to a stabilization of the spin-spiral structures. When the strong magnetocrystalline anisotropy (MCA) arising from the Fe/W(1 1 0) interface is introduced, however, the formation of the spin-spiral structures is suppressed and the ground state appears to be the ferromagnetic state—as observed in experiments. In addition, the strong MCA is found to play a key role in determining the domain wall structures.     

Spin-orbit coupling and ion displacements in multiferroic TbMnO3

The magnetic and ferroelectric (FE) properties of TbMnO3 are investigated on the basis of relativistic density functional theory calculations. We show that, due to spin-orbit coupling, the spin-spiral plane of TbMnO3 can be either the bc or ab plane, but not the ac plane. As for the mechanism of FE polarization, our work reveals that the "pure electronic" model by Katsura, Nagaosa, and Balatsky is inadequate in predicting the absolute direction of FE polarization. Our work indicates that to determine the magnitude and the absolute direction of FE polarization in spin-spiral states, it is crucial to consider the displacements of the ions from their centrosymmetric positions.     

Smectic H-smectic VI pretransitional effect by orientational ordering in TBBA

New neutron quasi-elastic scattering data on TBBA in its supercooled smectic H phase are presented and analyzed in terms of the elastic incoherent structure factor. Contrary to what was found in the normal H phase, the results support the idea of partial orientational ordering of the molecules. It is suggested that this ordering (of dynamical nature) is a precursory effect to the smectic H-smectic VI phase transition.     

Ising model on evolution networks and its application on opinion formation

Many phenomena show that in a favorable circumstance an agent still has an updating possibility, and in an unfavor- able circumstance an agent also has a possibility of holding its own state and reselecting its neighbors. To describe this kind of phenomena an Ising model on evolution networks was presented and used for consensus formation and separation of opinion groups in human population. In this model the state-holding probability p and selection-rewiring probability q were introduced. The influence of this mixed dynamics of spin flips and network rewiring on the ordering behavior of the model was investigated, p hinders ordering of opinion networks and q accelerates the dynamical process of networks. Influence of q on the ordering and separating stems from its effect on average path length of networks.     

Simulation of spin-polarized scanning tunneling microscopy images of nanoscale non-collinear magnetic structures

We apply an efficient method to calculate spin-polarized scanning tunneling microscopy (SP-STM) images of nanostructures with complex non-collinear magnetic order. The model is based on the spin-polarized version of the Tersoff–Hamann model of STM and the independent orbital approximation for the surface electronic structure. For its application, only the knowledge of the arrangement of the magnetic moments of the surface atoms is required. In spite of its simplifications, calculated SP-STM images of periodic collinear and non-collinear magnetic spin structures are in many cases in excellent agreement with those obtained from computationally much more demanding ab initio calculations. Especially for surfaces of chemically equivalent atoms, the atomic scale SP-STM images are dominated by the magnetic structure and depend much less on the accurate electronic structure. This suggests the application of the method to more complex non-collinear magnetic structures such as domain walls in antiferromagnets, spin-spiral states, spin glasses, or disordered states. Based on the model, we predict SP-STM images of helical spin-spiral states in ultra-thin films. PACS 68.37; 75.70; 75.30     

Conical spin-spiral state in an ultrathin film driven by higher-order spin interactions

We report a transverse conical spin spiral as the magnetic ground state of a double-layer Mn on a W(110) surface. Using spin-polarized scanning tunneling microscopy, we find a long-range modulation along the [001] direction with a periodicity of 2.4 nm coexisting with a local row-wise antiferromagnetic contrast. First-principles calculations reveal a transverse conical spin-spiral ground state of this system which explains the observed magnetic contrast. The canting of the spins is induced by higher-order exchange interactions, while the spiraling along the [001] direction is due to frustrated Heisenberg exchange and Dzyaloshinskii-Moriya interaction.     

Ab?initio study of the phonon spectrum, entropy and lattice heat capacity of disordered Re-W alloys

The Re(1-x)W(x) alloy is formed by continuous neutron bombardment of W, the core material making up the shield in fusion devices. Here, we present an ab?initio study of the lattice dynamical properties of this commercially important alloy. The dynamical (force constant) matrix was obtained through a first-principles, density functional perturbation theory. Various vibrational properties, such as fuzzy phonon dispersion relations, density of states (DOS), scattering life-times, vibrational entropy and specific heat are studied. The effects of short-range ordering is shown to be important in the 50-50 alloy.     

Correlations induced orbital ordering and cooperative Jahn–Teller distortion in the paramagnetic insulator KCrF<Subscript>3</Subscript>

We investigate the origin of the orbital ordering in the paramagnetic phase of KCrF₃. All previous studies described structural parameters of the paramagnetic phase using a magnetic ordering in the compound. Our simulations of real paramagnetic KCrF₃ were performed within an approach combining density functional theory and dynamical mean field theory (DFT+DMFT). As a result, it was found that the experimentally observed cooperative Jahn–Teller effect is successfully described in a lattice relaxation calculation for structure without any long-range magnetic ordering. It is established that the existence of the orbital ordering even in undistorted perovskite structure clearly confirms the electronic origin of the orbital ordering in KCrF₃.     

Dynamical ordering of driven stripe phases in quenched disorder

We examine the dynamics and stripe formation in a system with competing short and long-range interactions in the presence of both an applied dc drive and quenched disorder. Without disorder, the system forms stripes organized in a labyrinth state. We find that, when the disorder strength exceeds a critical value, an applied dc drive can induce a dynamical stripe ordering transition to a state that is more ordered than the originating undriven, unpinned pattern. We show that signatures in the structure factor and transport properties correspond to this dynamical reordering transition, and we present the dynamic phase diagram as a function of strengths of disorder and dc drive.     

Quantum mechanics and the direction of time

In recent papers the authors have discussed the dynamical properties of large Poincaré systems (LPS), that is, nonintegrable systems with a continuous spectrum (both classical and quantum). An interesting example of LPS is given by the Friedrichs model of field theory. As is well known, perturbation methods analytic in the coupling constant diverge because of resonant denominators. We show that this Poincaré catastrophe can be eliminated by a natural time ordering of the dynamical states. We obtain then a dynamical theory which incorporates a privileged direction of time (and therefore the second law of thermodynamics). However, it is only in very simple situations that this time ordering can be performed in an extended Hilbert space. In general, we need to go to the Liouville space (superspace) and introduce a time ordering of dynamical states according to the number of particles involved in correlations. This leads then to a generalization of quantum mechanics in which the usual Heisenberg's eigenvalue problem is replaced by a complex eigenvalue problem in the Liouville space.     

Scroll wave instability controlled by external fluctuations

Scroll waves, a characteristic dynamical pattern of three-dimensional excitable media, exhibit an instability under low excitability conditions. This unstable regime can be partially controlled by using random forcing in a clear manifestation of the ordering role of a stochastic external perturbation. Analytical and numerical results confirm this unexpected noise effect.     

Dynamical critical slowing down in CsNiF3

Dynamical critical slowing down in CsNiF₃ is studied using the a.c. susceptibility measurements at 9.5 GHz in zero external static magnetic field. The dynamical critical exponent for the relaxation time of the in-plane spin fluctuations is obtained for the temperature interval 6 K < T < 20 K. For this temperature interval where one-dimensional spin fluctuations are dominant, very good qualitative and quantitative agreement with the spinwave calculation of the dynamical response is obtained at long wavelength and low frequency. The dynamical critical exponent for the relaxation time is measured to be 0.96 ± 0.6. At higher temperature, a gradual crossover to an isotropic Heisenberg chain behaviour is observed. For temperatures close to the 3-d antiferromagnetic ordering temperature T_N, a crossover to 3-d fluctuation regime gives rise to a speeding-up of the spin relaxation rate.     

Dissipative quantum dynamics and optimal control using iterative time ordering: an application to superconducting qubits

We combine a quantum dynamical propagator that explicitly accounts for quantum mechanical time ordering with optimal control theory. After analyzing its performance with a simple model, we apply it to a superconducting circuit under so-called Pythagorean control. Breakdown of the rotating-wave approximation is the main source of the very strong time-dependence in this example. While the propagator that accounts for the time ordering in an iterative fashion proves its numerical efficiency for the dynamics of the superconducting circuit, its performance when combined with optimal control turns out to be rather sensitive to the strength of the time-dependence. We discuss the kind of quantum gate operations that the superconducting circuit can implement including their performance bounds in terms of fidelity and speed.     

Electronic Transport in Magnetic Domain-wall Structure

The resistivity due to the domain wall in the presence of impurities in a ferromagnetic metallic wire is calculated based on the linear response theory. Assuming the local spin distribution to be spin-spiral the modification of the band structure has been obtained exactly. With this modified band structure we calculate the conductivity of the system as a function of electronic filling and show that the domain-wall contribution to the resistance can be positive or negative depending on the dimensionality, the position of the Fermi energy on the band structure and the strength of the electron local spin interaction of the system.     

Anisotropic domain growth of the axial next-nearest-neighbor Ising model at low temperatures

We investigate the ordering kinetics for the axial next-nearest-neighbor Ising (ANNNI) model in one and two dimensions by the multispin heat bath dynamical simulation. This dynamics enables us to overcome the pinning effect and to observe the dynamical scaling law for domain growth in the ANNNI model at zero temperature. The domain growth exponent is 1/2 isotropically both in the ferromagnetic and the dry (commensurate) antiphase. In the wet (commensurate) antiphase, however, it is approximately 1/3 in the modulated direction, whereas it remains 1/2 in the nonmodulated direction. We suggest that these exponent values are dictated by 3- and 4-body diffusion-reaction processes of domain walls.     

Dynamical pattern formation for diblock copolymer films under stretch by moving walls

We propose a computational method that takes into account the dynamical influence of moving rigid walls over the pattern formation for thin films of diblock copolymers. The competition between the surface field energy and elastic stretching energy, and the effects of the molecular relaxation on pattern formation are studied. Finally, it is also observed that stretching the film enhances the ordering of patterns in it.     

Hierarchies of non-classical states in quantum optics

The conventional separation of states of the quantised radiation field into “classical” and “nonclassical” types is expressed in a dual operator form and then refined. This is based on new features of the normal ordering rule for passage from classical to quantum dynamical variables. The cases of single and two-mode radiation fields are discussed     

Effect of thermal fluctuations on magnetic phase separation and on the parameters of spin-spiral waves

Effects of temperature on magnetic phase separation and on the parameters of spin-spiral waves are studied. The study is performed using the two-dimensional single-band Hubbard model and the Hubbard-Stratonovich transformation. Both commensurate (antiferromagnetic, ferromagnetic) and incommensurate (spiral) magnetic phases are considered. The problem is solved using the static approximation with allowance for transverse fluctuations of the magnetic moment. It is shown that the temperature significantly affects the collinear and spiral magnetic phases. With an increase in the temperature, the phase-separation region near the half-filling is sufficiently reduced and substituted by the antiferromagnetic phase. The results are used for the interpretation of the magnetic properties of cuprates.