Screening of the magnetic field by magnetic monopoles in spin ice

The screening of the external magnetic field by magnetic monopoles in spin ice has been considered. The polarization of the magnetic system with moving monopoles has been shown to result in the incomplete screening of the external magnetic field. The static permeability of spin ice and the magnetic-field screening length have been calculated and numerically estimated and the physical meaning of introducing monopoles is discussed.     

Room-temperature electric polarization induced by phase separation in multiferroic GdMn<Subscript>2</Subscript>O<Subscript>5</Subscript>

It is theoretically shown that the Coulomb interaction between violations of the Bernal–Fowler rules leads to a temperature-induced stepwise increase in their concentration by 6–7 orders of magnitude. This first-order phase transition is accompanied by commensurable decrease in the relaxation time and can be interpreted as melting of the hydrogen bond network. The new phase with the melted hydrogen lattice and survived oxygen one is unstable in the bulk of ice, and further drastic increase in the concentrations of oxygen interstitials and vacancies accomplishes the ice melting. The fraction of broken hydrogen bonds immediately after the melting is about 0.07 of their total number that implies an essential conservation of oxygen lattice in water.     

Trapping energy of a magnetic monopole in magnetic materials

Analytical methods to investigate the interaction of magnetic monopoles with known magnetic media have been developed. Trapping energies of monopoles inside ferro-magnetic or super onducting materials of size greater than about 10⁻⁶ cm are found to be of the order of several kiloelectron volts. These are two to three orders of magnitude higher than in paramagnetic materials. Thus if stable magnetic monopoles exist at all in the universe, they are perhaps trapped in these magnetic materials. The effect of the finite size of the magnetic bodies is taken into account explicitly in our calculations of the trapping energy.     

四端双量子点系统中的自旋和电荷能斯特效应

基于非平衡态格林函数方法,理论研究了与四个电极耦合的双量子点系统中的自旋和电荷能斯特效应,考虑了不同电极的磁动量结构和量子点内以及量子点间电子的库仑相互作用对热电效应的影响.结果表明铁磁端口中的磁化方向能够有效地调节能斯特效应:当电极1和电极3中的磁化方向反平行排列时,通过施加横向的温度梯度,系统中将会出现纯的自旋能斯特效应;当电极4从普通金属端口转变为铁磁金属端口时,将同时观测到电荷和自旋能斯特效应.研究发现,能斯特效应对于铁磁电极极化强度的依赖程度较弱,但对库仑排斥作用十分敏感.在量子点内和点间库仑排斥作用的影响下,自旋及电荷能斯特系数有望提高两个数量级.     

Critical behavior of two-dimensional spin models and charge asymmetry in the Coulomb gas

Many two-dimensional spin models can be transformed into Coulomb-gas systems in which charges interact via logarithmic potentials. For some models, such as the eight-vertex model and the Ashkin-Teller model, the Coulomb-gas representation has added significantly to the insight in the phase transitions. For other models, notably theXY model and the clock models, the equivalence has been instrumental for almost our entire understanding of the critical behavior. Recently it was shown that theq-state Potts model and then-vector model are equivalent to a Coulomb gas with an asymmetry between positive and negative charges. Fieldlike operators in these spin models transform noninteger charges and magnetic monopoles. With the aid of exactly solved models the Coulombgas representation allows analytic calculation of some critical indices.     

Dynamics of magnetic charges in artificial spin ice

Artificial spin ice has been recently implemented in two-dimensional arrays of mesoscopic magnetic wires. We propose a theoretical model of magnetization dynamics in artificial spin ice under the action of an applied magnetic field. Magnetization reversal is mediated by domain walls carrying two units of magnetic charge. They are emitted by lattice junctions when the local field exceeds a critical value Hc required to pull apart magnetic charges of opposite sign. Positive feedback from Coulomb interactions between magnetic charges induces avalanches in magnetization reversal.     

Possibility of increasing the efficiency of spin injection by current in magnetic junctions

The nonequilibrium spin polarization of electrons is calculated under the conditions of spin injection by current passing from one ferromagnetic film to another in a magnetic junction. It is shown that the nonequilibrium spin concentration may substantially increase with an appropriate choice of parameters of the films and operation conditions. This leads to a decrease in the threshold current density required for switching the magnetic junction by several orders of magnitude.     

Two-stage ordering of spins in dipolar spin ice on the kagome lattice

Spin ice, a peculiar thermal state of a frustrated ferromagnet on the pyrochlore lattice, has a finite entropy density and excitations carrying magnetic charge. By combining analytical arguments and Monte Carlo simulations, we show that spin ice on the two-dimensional kagome lattice orders in two stages. The intermediate phase has ordered magnetic charges and is separated from the paramagnetic phase by an Ising transition. The transition to the low-temperature phase is of the three-state Potts or Kosterlitz-Thouless type, depending on the presence of defects in the charge order.     

Heavy and light monopoles in magnetic reversion in artificial spin ice

This work makes a theoretical study of the dynamics of emergent elemental excitations in artificial spin ice systems with hexagonal geometry during the magnetic reversion of the system. The magnetic and physical parameters of the nanoislands that form the array are considered as variables in the study. The parameters considered are: the energy barrier for the inversion of each nanoisland, the magnetic moment of the nanomagnets and the possible disorder in the sample. Our results show that the reversion dynamic presents two distinct mechanisms of magnetic reversion, with different elemental excitations for each mechanism. The first mechanism presents a reversion with the appearance of magnetic monopoles that do not move in the samples (heavy monopoles) and the absence of Dirac chains. In the other mechanism elemental magnetic excitations (light monopoles) appear that move great distances in the sample, giving rise to extensive Dirac chains during the magnetic reversion.     

Apparent absence of Pauli paramagnetism in metallic polyparaphenylene

The magnetic susceptibility of doped and undoped polyparaphenylene has been measured in the temperature range from 5 to 300 K by the static Faraday method and by ESR. In the static experiments, a Curielike paramagnetism has been observed superimposed on a temperature-independent diamagnetic susceptibility. The Pauli paramagnetism of the doped samples is surprisingly small. Its upper limit has been determined to be only 2?10^?7 emu cm³/mole-carbon. —The concentration of magnetic centers in the doped samples is several orders of magnitude smaller than the dopant concentration and does not scale with it. — These results indicate a conduction mechanism involving nearly complete spin compensation of the charge carriers.     

Energy-loss mechanisms and the annihilation of confined monopoles

The annihilation process for a superheavy monopole-antimonopole pair connected by a relatively light magnetic flux tube (which is possibly relevant to the cosmological monopole problem) is studied. Three energy-loss mechanisms are considered in some detail. The first two are the radiation of photons and weak bosons due to the linear and Coulomb parts of the potential, respectively. Then the energy loss due to the frictional drag of monopoles moving through a relativistic plasma is analyzed. We argue that for the parameters relevant to standard theories this last mechanism dominates by many orders of magnitude and leads to an almost instantaneous annihilation at the time confinement takes place. The implications of this work for the dilution of the baryon to entropy ratio are described.     

Modeling of magnetic susceptibility of an antiferromagnetic system with disorder-driven quantum critical behavior

A simple model has been proposed for an antiferromagnetic system in which the quantum critical regime is induced by disorder. The proposed model makes it possible to find the magnetic susceptibility for an arbitrary ratio of the temperature T to the characteristic magnitude of the exchange integral J in a disordered magnet. The model analytically describes the crossover from the power dependence χ ～ 1/T ^ξ, which is characteristic of the ground state (the Griffiths phase, T ? J), to a dependence of the Curie-Weiss type (T ≥ J) with the effective parameters dependent on the characteristics of the distribution function of the exchange integrals. The characteristic size of spin clusters forming the Griffiths phase has been estimated. It has been demonstrated using iron-doped germanium cuprate as an example that, within the proposed approach, the experimental data can be adequately described over a wide temperature range in which the temperature changes by more than two orders of magnitude and the magnetic properties of the Griffiths phase are determined by spin clusters of nanometer size.     

The Kondo lattice state in the presence of Van Hove singularities: Next-to-leading order scaling

Renormalization group analysis of the Kondo model with a logarithmic Van Hove singularity in the electron density of states has been carried out in the next-to-leading scaling approximation in different magnetic phases. The effective coupling constant remains small, while the renormalized magnetic moment and the frequency of spin fluctuations decrease by several orders of magnitude. In this way, broad regions of non-Fermi-liquid behavior are found from scaling trajectories in a large interval of the bare coupling constant. Applications to the physics of itinerant magnetism are considered.     

Analysis of a fully packed loop model arising in a magnetic Coulomb phase

The Coulomb phase of spin ice, and indeed the I(c) phase of water ice, naturally realize a fully packed two-color loop model in 3D. We present a detailed analysis of the statistics of these loops: we find loops spanning the system multiple times hosting a finite fraction of all sites while the average loop length remains finite. We contrast the behavior with an analogous 2D model. We connect this body of results to properties of polymers, percolation and insights from Schramm-Loewner evolution processes. We also study another extended degree of freedom, called worms, which appear as "Dirac strings" in spin ice. We discuss implications of these results for the efficiency of numerical cluster algorithms, and address implications for the ordering properties of a broader class of magnetic systems, e.g., with Heisenberg spins, such as CsNiCrF(6) or ZnCr(2)O(4).     

Striped antiferromagnetic order and electronic properties of stoichiometric LiFeAs from first-principles calculations

We use state-of-the-arts first-principles method to investigate the structural, electronic, and magnetic properties of stoichiometric LiFeAs. We optimize fully all the structures, including lattice constants and internal position parameters, for different magnetic orders. We find the magnetic ground state by comparing the total energies among all the possible magnetic orders. Our calculated lattice constants and As internal position are in good agreement with experiment. The experimental fact that no magnetic phase transition has been observed at finite temperature can be attributed to the tiny inter-layer spin coupling. Our results show that stoichiometric LiFeAs has almost the same striped antiferromagnetic spin order as other FeAs-based parent compounds and tetragonal FeSe do, which may imply that all Fe-based superconductors have the same mechanism of superconductivity.     

Effective magnetic monopoles and universal conductance fluctuations

The observation of isolated positive and negative charges, but not isolated magnetic north and south poles, is an old puzzle. Instead, evidence of effective magnetic monopoles has been found in the abstract momentum space. Apart from Hall-related effects, few observable consequences of these abstract monopoles are known. Here, we show that it is possible to manipulate the monopoles by external magnetic fields and probe them by universal conductance fluctuation measurements in ferromagnets with strong spin-orbit coupling. The observed fluctuations are not noise, but reproducible quasiperiodic oscillations as a function of magnetization direction, a novel Berry phase fingerprint of the magnetic monopoles.     

High fidelity single qubit operations using pulsed electron paramagnetic resonance

Systematic errors in spin rotation operations using simple rf pulses place severe limitations on the usefulness of the pulsed magnetic resonance methods in quantum computing applications. In particular, the fidelity of quantum logic operations performed on electron spin qubits falls well below the threshold for the application of quantum algorithms. Using three independent techniques, we demonstrate the use of composite pulses to improve this fidelity by several orders of magnitude. The observed high-fidelity operations are limited by pulse phase errors, but nevertheless fall within the limits required for the application of quantum error correction.     

磁单极子进展概述

自磁单极子概念被狄拉克提出以来,不管是理论还是实验物理学家都一直在努力寻找,但迄今仍然没能找到它们存在的确凿证据.最近,一些凝聚态物理学家声称在动量空间以及自旋冰材料中找到了磁单极子存在的有力证据,并通过磁单极子的集体激发行为解释了一些新颖的物理现象.这使得磁单极子艰难的探索之路出现了一丝新的曙光.作为电动力学教学内容的补充,本文拟把磁单极子的最新进展做一个概述,让大学生对此有一个全面的认识,从而激发他们学习和科研的兴趣.     

Spin polarization of the current in a correlated one-dimensional conductor with an impurity

The spin transport as the current flows through an impurity in a one-dimensional conductor is analyzed. The interacting electrons are described in terms of the Luttinger liquid theory. Both the Coulomb and short-range interactions are considered; the latter appears when the gate screens the long-range part of the Coulomb potential. The cases of magnetic and nonmagnetic impurities are considered. It has been revealed that, for a magnetic impurity, the electric current flow induces the generation of the spin current, which has direct and alternating components. At low temperatures and voltages, the current can be completely spin-polarized. For a nonmagnetic impurity, the spin current generation is absent. The spin current flowing though the wire affects the current-voltage characteristic for both magnetic and nonmagnetic impurities. The results have been obtained for a rather strong electron-electron interaction.     

Macrospin in ferromagnetic nanojunctions

We study the passage of transverse current through a ferromagnetic nanojunctions, viz., a layered nanostructure of the spin-valve type containing two ferromagnetic layers separated by a spacer that prevents exchange coupling between the layers in the absence of current, but does not affect spin polarization of the current. The conditions for a high level of injection of spins by current are derived at which the concentration of injected nonequilibrium spins can reach or even exceed their equilibrium concentration. In such conditions, a number of new effects are observed. The threshold of exchange switching by current is lowered by several orders of magnitude due to matching of spin resistances of the layers. The application of an external magnetic field in the vicinity of the orientation phase transition additionally lowers this threshold. This leads to multistability, in which one value of the current corresponds to two (or more) stable noncollinear orientations of magnetization, and switching itself becomes irreversible. A methodical feature of this research is that the calculation is performed in the so-called macrospin approximation, which is in good agreement with most of known experiments. In this approximation, the equations of motion taking into account the torque as well as spin injection are derived for the first time and solved.