Fixed point behavior of cumulants in the three-dimensional Ising universality class

High-order cumulants and factorial cumulants of conserved charges are suggested for the study of the critical dynamics in heavy-ion collision experiments. In this paper, using the parametric representation of the three-dimensional Ising model which is believed to belong to the same universality class as quantum chromo-dynamics, the temperature dependence of the second- to fourth-order (factorial) cumulants of the order parameter is studied. It is found that the values of the normalized cumulants are independent of the external magnetic field at the critical temperature, which results in a fixed point in the temperature dependence of the normalized cumulants. In finite-size systems simulated using the Monte Carlo method, this fixed point behavior still exists at temperatures near the critical. This fixed point behavior has also appeared in the temperature dependence of normalized factorial cumulants from at least the fourth order. With a mapping from the Ising model to QCD, the fixed point behavior is also found in the energy dependence of the normalized cumulants (or fourth-order factorial cumulants) along different freeze-out curves.     

Zero-temperature nonequilibrium dynamics of the spherical Scherrington-Kirkpatrick model of a spin glass in an external field

The decay of the nonequilibrium spin-spin autocorrelation function is calculated analytically and numerically based on the Langevin dynamics for the spherical Scherrington-Kirkpatrick model (p = 2) of a spin glass in a constant magnetic field at zero temperature. The behavior of the spin system is analyzed for different amplitudes of the external constant magnetic field and different waiting times. In the mean-field approximation, the critical value of the external magnetic field is found. The occurrence of the spin-glass phase in weak magnetic fields is demonstrated. For stronger fields, the stationary behavior of the correlation function is revealed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 10–16, April, 2007.     

Mean field theory of a quantum heisenberg spin glass

A full mean-field solution of a quantum Heisenberg spin-glass model is presented in a large- N limit. A spin-glass transition is found for all values of the spin S. The quantum critical regime associated with the quantum transition at S = 0 and the various regimes in the spin-glass phase at high spin are analyzed. The specific heat is shown to vanish linearly with temperature. In the spin-glass phase, intriguing connections between the equilibrium properties of the quantum problem and the out-of-equilibrium dynamics of classical models are pointed out.     

Magnetic field effects on a classical anisotropic spin-glass model

The simple model of a classical spin-glass proposed by van Hemmen [1] is extended to the case of a generalS Ising spin and anisotropy energy. Due to its simplicity, the model is amenable to an exact solution and in the case of spin-1 the phase diagram is similar to that of the Sherrington-Kirkpatrick model. ForS=1 and for certain values of the anisotropy the phase diagram in the (H-T) plane shows continuous and discontinuous transitions, a critical point within the condensed phase, and a field-induced mixed phase in addition to the paramagnetic and spin-glass phases.Work partially supported by CNPq and FINEP (Brazilian Agencies). Part of this work was presented at the International Conference of Magnetism, San Francisco, 1985     

Non-fermi-liquid scaling in Ce(Ru0.5Rh)2Si2

We study the temperature and field dependence of the magnetic and transport properties of the non-Fermi-liquid (NFL) compound Ce(Ru0.5Rh0.5)2Si2. For fields H less, similar0.1 T the results suggest that the observed NFL behavior is disorder driven. For higher fields, however, magnetic and transport properties are dominated by the coupling of the conduction electrons to critical spin fluctuations. The temperature dependence of the susceptibility as well as the scaling properties of the magnetoresistance are in very good agreement with the predictions of recent dynamical mean-field theories of Kondo alloys close to a spin-glass quantum critical point.     

On equivalence of spin and field pictures of lattice systems

We investigate the spin and field systems on a lattice connected by the Kac-Siegert transform. It is shown that the structures of corresponding theories are equivalent (in the sense of isomorphy of space of Gibbs states and order parameters). Using the idea of equivalence of spin and field pictures, we exhibit a class of lattice systems possessing infinitely uncountably many ground states. The systems of this type with infinite-range, slow-decaying interactions are expected to have a spin-glass phase transition.     

Partial moments of electric field strength in the problem of conductivity of binary composites: Numerical experiment on a plane disordered lattice

Partial (calculated by the volume of separate components) moments of electric field strength of different orders are calculated within a linear problem on the electric conductivity of binary composites. A numerical experiment is carried out on a square lattice with a size of 401 ×401 sites with randomly distributed bonds. The moments (of orders n = 2, 3,…, 6) are calculated as a function of concentration for a number of fixed values of the parameter h—the ratio of the conductivities of the components. The behavior of partial moments near the metal-dielectric phase-transition point is studied, and the corresponding critical indices are determined.     

Spin-glass behavior of the polyvinyl pyrrolidone-protected Prussian blue analog K1.14Mn[Fe(CN)6]0.88 nanocubes

Prussian Blue analog K_1.14Mn[Fe(CN)₆]_0.88 nanocubes were synthesized by using polyvinyl pyrrolidone (PVP) as a protective matrix. The PVP-protected MnFe PBA nanocubes with face centered cubic structure are well dispersed with a narrow size distribution of around 50 nm. A spin-glass behavior (including hysteresis, a peak in the zero-field-cooled magnetization and frequency-dependent AC magnetic susceptibility) is observed in the nanoparticles. A possible origin of this spin-glass freezing is discussed. Spin disorder due to the structural defects may be the reason that causes the spin-glass freezing in the MnFe PBA nanoparticles.     

Universality class of absorbing phase transitions with a conserved field

We investigate the critical behavior of systems exhibiting a continuous absorbing phase transition in the presence of a conserved field coupled to the order parameter. The results obtained point out the existence of a new universality class of nonequilibrium phase transitions that characterizes a vast set of systems including conserved threshold transfer processes and stochastic sandpile models.     

A spin-glass model with random couplings

We define a frustrated spin-glass model for which the Migdal-Kadanoff renormalization group is exact. Our model has random couplings, and the renormalization group acts on these. We study the high and low temperature phases of the model, exhibit a critical fixed point (in high dimension), and show that the Edwards-Anderson parameter takes a non-zero value in the low-temperature phase.On leave from the University of Geneva     

Mott-Anderson transition controlled by a magnetic field in pyrochlore molybdate

The pyrochlore molybdate Gd2MO2O7 locates near the phase boundary between the ferromagnetic-metallic and the spin-glass insulating state. This metal-insulator transition is governed on a large energy scale by the electron-correlation effect, while the geometrical frustration causes the random potential. The magnetic field can tune the randomness of the potential and control, under a suitable pressure, the continuous Mott-Anderson transition precisely. The critical exponent (mu = 1.04 +/- 0.1) of the Mott-Anderson transition has been determined for this ferromagnetic orbital-degenerate electron system.     

On the iteration of a rational function: Computer experiments with Newton's method

Using Newton's method to look for roots of a polynomial in the complex plane amounts to iterating a certain rational function. This article describes the behavior of Newton iteration for cubic polynomials. After a change of variables, these polynomials can be parametrized by a single complex parameter, and the Newton transformation has a single critical point other than its fixed points at the roots of the polynomial. We describe the behavior of the orbit of the free critical point as the parameter is varied. The Julia set, points where Newton's method fail to converge, is also pictured. These sets exhibit an unexpected stability of their gross structure while the changes in small scale structure are intricate and subtle.     

Spin-glass like behaviors in cubic VOx

Magnetic susceptibility (d.c.) measurements were performed on the well characterized cubic VO_x(1.10 ?x?1.28). A sharp cusp-like peak in χ(T) curve was analyzed in terms of spin-glass behavior and the spin-glass order parameter, q(T) was extracted from χ cusp using Sherrington-Kirkpatrick model. This analysis as well as the other characteristic behaviors strongly suggests the possible existence of spin-glass state in this material.     

Effect of local field fluctuations on orientational ordering in random-site dipole systems

Some peculiarities of dipole ordering in systems with uniaxial or cubic anisotropy with an arbitrary degree of dilution are analyzed in terms of random local field theory. The approach takes into account the effect of thermal and spatial fluctuations of the local fields acting on each particle from its neighbors with an accuracy corresponding to that of the Bethe-Paierls pair clusters approach. We show that ferromagnetic (ferroelectric) structure for uniaxial Ising dipoles distributed on a simple cubic lattice is intrinsically unstable against the fluctuations of the local fields for any concentration of the dipoles. This result is quite different from the prediction of the mean-field theory which implies the possibility of ferromagnetic ordering as a metastable state in field-cooled experiments. The local field fluctuations do not exclude, however, antiferromagnetic ordering above a certain critical concentration. Ferromagnetic ordering is possible for other types of lattice geometries and for an amorphous-like dipole distribution above a certain critical concentration. A simple physical explanation of such behavior is given based on the specific angular dependence of the dipole-dipole interaction that results in a relatively high value of the local field second moment for simple cubic lattice.     

Bethe lattice spin glass: The effects of a ferromagnetic bias and external fields. II. Magnetized spin-glass phase and the de Almeida-Thouless line

In this and the companion paper, we analyze the ±J Ising spin-glass model on the Bethe lattice with fixed uncorrelated boundary conditions. Phase diagrams are derived as a function of temperature vs. concentration of ferromagnetic bonds and, for a symmetric distribution of bonds, external field vs. temperature. In this part we characterize magnetized spin-glass (MSG) phases by divergence of an appropriate susceptibility: at zero field this signals the existence of an intermediate MSG phase; at nonzero field, this is used to identify the de Almeida-Thouless line.     

Strong disorder fixed point in absorbing-state phase transitions

The effect of quenched disorder on nonequilibrium phase transitions in the directed percolation universality class is studied by a strong disorder renormalization group approach and by density matrix renormalization group calculations. We show that for sufficiently strong disorder the critical behavior is controlled by a strong disorder fixed point and in one dimension the critical exponents are conjectured to be exact: beta=(3-sqrt[5])/2 and nu( perpendicular )=2. For disorder strengths outside the attractive region of this fixed point, disorder dependent critical exponents are detected. Existing numerical results in two dimensions can be interpreted within a similar scenario.     

Conformal field theory representation and background-independent action for superstring field theory

The two-dimensional conformal field theory representation of Witten's open superstring field theory is discussed. We argue that the previously suggested cubic action for the superstring is actually dependent on the spacetime background, and suggest a modification which formally removes this dependence. The transformation from the new version of the superstring cubic action to Witten's action for open string states is discussed. A class of linearized solutions to the equations of motion of the open superstring cubic action are exhibited. These solutions are in one-to-one correspondence with the physical excitations (both massless and massive) of the open and closed type I superstrings.     

外应力场下铁磁/反铁磁双层膜系统中的自旋波

采用自由能极小的方法研究了铁磁/反铁磁双层膜系统在外应力场下的一致进动自旋波性质，即铁磁共振现象. 本模型中铁磁层很薄可看成单畴结构，但具有单轴磁晶各向异性和立方磁晶各向异性；而反铁磁层仅具有单轴磁晶各向异性，但其厚度趋于半无穷. 推导出了该系统的铁磁共振频率和频谱宽度的解析式. 结果表明，外应力场和界面交换耦合或反铁磁磁强度仅在弱磁场下对系统的铁磁共振有影响，且系统的铁磁共振行为按磁场强度可分为两支，其区分弱磁场和强磁场的临界场依赖于外应力场的方向. 另一方面，应力场方向的改变可借助于反铁磁层磁畴变化对铁磁层磁晶各向异性轴有影响. 关键词： 铁磁/反铁磁双层膜 界面耦合强度 铁磁共振 应力场