Investigation of the NMR enhancement factor in CuMn spin glasses

The NMR enhancement factor of a frozenCuMn spin glass has been measured at a temperatureT?T _g /5. The measurements were performed as function of static magnetic fields of different directions. A two component model of a spin glass has been outlined. One component being a “system of single spins” and the other one being a “subsystem of clusters”. Both components were attributed to different kinds of interaction being RKKY and dipole interaction respectively. The effective anisotropy field of the single spin system consists of two unidirectional contributionsH _a ^s andH _a ^c , which have been measured for different conditions. A second anisotropy fieldH _d binds the cluster system to the system of single spins. All anisotropy fields depend on the annealing temperature of the alloys.     

Monte Carlo study of small magnetic particles

In earlier computer simulation small magnetic particles with nearest neighbor Heisenberg interactions in zero magnetic field have been studied. We now continue these investigations including next nearest neighbor exchange and non zero magnetic fieldsH. The particles treated have spherical shape with a number of spinsN in the range from 33 to 3071. It is shown that the spontaneous magnetization of the particles is rather different from the bulk magnetization. The magnetization process can be accounted for by the Néel theory, if the correct spontaneous magnetization of the particle is used. The distribution of local magnetizations (the magnetic “profile”) was also obtained in various cases. It is shown that the magnetization of very small particles is much more depressed than predicted by the mean field approximation. We introduce an “effective magnetic radius” \(\hat R\) accounting for the reduction of the local magnetization. This magnetic radius is important for the interpretation of experimental results. A distinct dependence of \(\hat R\) on the magnetic field, temperature and the fraction of next nearest neighbor exchange is found. Finally a brief comparison is made with the recent study of magnetic surface properties by Binder and Hohenberg.     

Magnetic hyperfine field at Cr site in AgCrO2 given by Perturbed angular correlations

This work presents an electric field gradient and magnetic hyperfine field study, in the AgCrO₂ multiferroic with triangular spin lattice. The temperature dependence of the electric field gradient (efg) and magnetic hyperfine field (mhf) at Cr site was studied at isolde via perturbed angular correlation measurements with the ¹¹¹In probe, at room temperature and below the Néel temperature (T?≤?21 K) down to 12 K. The results show the presence of two distinct local environments. One axial symmetric efg with a very low mhf, and a non axially symmetric efg with a much higher one. The temperature dependences of mhf magnitude and of the angle between the mhf and the principle component of the efg are investigated.     

Spinwellen in Ferromagneten mit indirektem Austausch

We consider a lattice of localized, ferromagnetically ordered spins, exchange-inter-acting with a simple conduction band and give a unified theory of the elementary spin excitations. Spinwaves of the Heisenberg-model type and collective spinwaves, characteristic of a Fermigas with repulsive interactions, are coupled and give rise to an acoustical branch of predominantly localized and an optical branch of predominantly collective character which should appear in the infrared. In its most idealized version the model needs only three parameter: effective massm ?, interband exchange energy \(\tilde J\) and effective electron-electron repulsion \(\tilde v\) to describe this entire spectrum.     

Classial and quantum solitons in the transverse field Ising model

We investigate the shape and the dynamics of domain walls in the one-dimensional Ising model with spin S, exchange constant J and external transverse field Γ using numerical calculations up to S = 20 and analytical approximations. For $\tfrac{\Gamma } {{JS}}$ \] we describe classical domain walls as strongly localized excitations, which have either central spin or central bond symmetry. These symmetries are identified also in the quantum case, when solitary excitations develop into energy bands. In the classical limit S → ∞ localization results from the exponential vanishing of the bandwidth for the lowest bands. We describe the relation between the spectrum of moving classical solitons and the quantum band structure.     

Effective chiral symmetry restoration in excited mesons

Results on the spectrum of ${\bar q} q$ mesons in a model with a linear Coulomb-like instantaneous confining potential are presented. The single-quark Green function as well as the dynamical chiral symmetry breaking are obtained from the Dyson-Schwinger (gap) equation. Given the dressed quark propagator, the spectrum of “usual” mesons, i.e., ${\bar q} q$ states with nonexotic quantum numbers J ^PC , is obtained from the Bethe-Salpeter equation. Effective restoration of chiral symmetry at large spins and/or radial excitations is observed and the states fall into approximate linear radial and angular Regge trajectories.     

On a relation between the mobility edge problem and an isotropicXY model

The critical behaviour of noninteracting electrons in a disordered solid is investigated near the mobility edge. A two component “spin glass” order parameter which—in contrast to previous work—is gauge invariant, is introduced and it is shown by renormalization group arguments that the system can be mapped to anXY model. Using recent results on the jump in the spin wave stiffness of a two dimensionalXY model an universal exact minimum conductivity value \(\sigma _M = \frac{{e^2 }}{\hbar }\frac{1}{{\pi ^2 }}\) is found which agrees well with numerical predictions. An extrapolation of the proportionality between conductivity and spin wave stiffness to other spatial dimensions yields Wegner's power law, with an exponent related to theXY model.     

Atomic collective excitations in liquid lead

The atomic dynamics of liquid lead at the temperature T = 600 K has been simulated on the basis of the embedded atom model potential (the “embedded” atom model making it possible to effectively take into account the many-particle interactions) in order to study the mechanisms of formation of the atomic collective excitations. Spectra of the dynamic structure factor S(k, ω) and the spectral densities of the time correlation functions of the longitudinal \(\tilde C_L\) (k, ω) and transverse \(\tilde C_T\) (k, ω) currents have been calculated for the wavenumber region 0.11 Å^?1 ≤ k ≤ 2.01 Å^?1. It has been established that the dynamics of density fluctuations is characterized by two dispersion “acoustic-like” branches of the longitudinal and transverse polarization.     

The effect ofs-d exchange interaction on the nuclear spin relaxation in dilute alloys

The effect ofs-d exchange interaction on the relaxation rate of host nuclear spins has been investigated in dilute magnetic alloys. The leading logarithmic corrections to the Korringa relaxation rate have been calculated together with some lower order logarithmic terms to avoid a divergence at the Kondo temperature (T _k ). The obtained formulae are valid at and aboveT _k and in weak or strong magnetic fields (ω _i ?T or ω _i ?T, respectively, where ω _i is the Larmor frequency of the impurity spin). In second order of the perturbation theory our result reproduces that obtained by Giovannini and Heeger apart from a numerical factor of the order of unity and from some contribution of order (J/? _f )².     

A modified large number theory withconstant G

The inspiring “numerology” uncovered by Dirac, Eddington, Weyl,et al. can be explained and derived when it is slightly modified so to connect the “gravitational world” (cosmos) with the “strong world” (hadron), rather than with the electromagnetic one. The aim of this note is to show the following. In the present approach to the “Large Number Theory,” cosmos and hadrons are considered to be (finite)similar systems, so that the ratio \({{\bar R} \mathord{\left/ {\vphantom {{\bar R} {\bar r}}} \right. \kern-0em} {\bar r}}\) of the cosmos typical length \(\bar R\) to the hadron typical length \(\bar r\) is constant in time (for instance, if both cosmos and hadrons undergo an expansion/contraction cycle—according to the “cyclical bigbang” hypothesis—then \(\bar R\) and \(\bar r\) can be chosen to be the maximum radii, or the average radii). As a consequence, then gravitational constantG results to be independent of time. The present note is based on work done in collaboration with P. Caldirola, G. D. Maccarrone, and M. Pav?i?.     

Thermal Entanglement in a Two-Qubit Heisenberg XXZ Spin Chain Under a Single Pulse Magnetic Field

The thermal entanglement in a two-qubit Heisenberg XXZ spin chain is investigated under a single pulse magnetic field Bsinθ. The paper shows that the greater the contribution of the inhomogeneity on the exchange interaction, the higher thermal entanglement will be attained at the fixed temperature except the case that $\theta=2k\pi+\frac{3}{2}\pi$ . J _z  significantly disturb the thermal entanglement in uniform case. When the structure of the spin system are given, changing the external magnetic field B can induce controllable entanglement. Our study may provide a useful tool to change the entanglement of spin chain system.     

Benchmark cross sections forp \bar p collisions at 1.8 TeV

High-energyp \(\bar p\) collisions provide many quantitative tests of the standard model. Of particular interest are “hard scattering” processes, which test not only standard model matrix elements and higher order perturbative corrections, but also the distributions of quarks and gluons in the colliding hadrons. We present detailed comparisons of data from the CERNSp \(\bar p\) S collider with theory, incorporating up-to-date parton distributions derived from recent deep inelastic scattering data. Encouraged by the excellent agreement between data and theory at \(\sqrt s \) =630GeV, we present a complete set of “benchmark” predictions for the FNALp \(\bar p\) collider at \(\sqrt s \) =1.8TeV.     

Approximate Lifshitz Law for the Zero-Temperature Stochastic Ising Model in any Dimension

We study the Glauber dynamics for the zero-temperature stochastic Ising model in dimension d ≥ 4 with “plus” boundary condition. Let ${\mathcal{T}_+}$ be the time needed for an hypercube of size L entirely filled with “minus” spins to become entirely “plus”. We prove that ${\mathcal{T}_+}$ is O(L ²(log L) ^c ) for some constant c, not depending on the dimension. This brings further rigorous justification for the so-called “Lifshitz law” ${\mathcal{T}_{+} = O(L^{2})}$ (Fischer and Huse in Phys Rev B 35:6841–6848, 1987; Lifshitz in Sov Phys JETP 15:939–942, 1962) conjectured on heuristic grounds. The key point of our proof is to use the detailed knowledge that we have on the three-dimensional problem: results for fluctuation of monotone interfaces at equilibrium and mixing time for monotone interfaces dynamics extracted from Caputo et al. (Comm Pure Appl Math 64:778–831, 2011) to get the result in higher dimension.     

Topological pomeron andA-universality

The nuclear mass number dependence of inclusive spectra of secondaries with different quantum numbers in the projectile fragmentation region is analysed. We note that in models with topological pomeron, all the particle spectra fall into two main categories. The first one comprises particles which have a common “valence” quark with the projectile, the second one comprises all the other particles built of “sea” quarks. Thus, in the parameterization \(x\frac{{d\sigma }}{{dx}} \propto A^{\alpha (x)} \) the spectra of all “valence” hadrons (p, n, Λ, π^+,0,?,K ⁺, ... in thepA-interaction) atx→1 can be characterized by the single exponent α_υ =α(x?1) which differs slightly from α _s characterizing the spectra of “sea” hadrons ( \(\bar p, \bar \Lambda \) ,K ^?, ... forpA-interactions). This observation is essentially modelindependent and follows only from the topological structure of the pomeron and Gribov's space-time picture of soft hadronic interactions. Deviations from universality due to preasymptotic corrections and coherent particle production processes are estimated.     

Quantum control of 88Sr+ in a miniature linear Paul trap

We report on the construction and characterization of an apparatus for quantum information experiments using ⁸⁸Sr⁺ ions. A miniature linear radio-frequency (rf) Paul trap was designed and built. Trap frequencies above 1 MHz in all directions are obtained with 50 V on the trap end-caps and less than 1 W of rf power. We encode a quantum bit (qubit) in the two spin states of the S _1/2 electronic ground-state of the ion. We constructed all the necessary laser sources for laser cooling and full coherent manipulation of the ions’ external and internal states. Oscillating magnetic fields are used for coherent spin rotations. High-fidelity readout as well as a coherence time of 2.5 ms are demonstrated. Following resolved sideband cooling the average axial vibrational quanta of a single trapped ion is $\bar{n}=0.05$ and a heating rate of $\dot{\bar{n}}=0.016~\mathrm{ms}^{-1}$ is measured.     

Low-temperature relaxation in spin glasses

We analyze the mechanism governing the long-time, low-temperature relaxation of the thermoremanent magnetization (σ_TRM) in metallic spin glasses. Ideas of quantum tunneling and “screening” of the local magnetic moments are employed to explain the existence of a hierarchy of relaxation times. This results in a time decay which is described by anenhanced power law: $$\sigma _{TRM} = \sigma _0 \exp - A[\ln (\omega t)]^y = \sigma _0 (\omega t)^{ - A[\ln (\omega t)]y - 1} $$ withy≧1. A crossover is predicted below which most parameters become temperature-independent.     

The controversy in theγγ →ρρ process: potential scattering or qq\bar q};\bar q}; resonance?; resonance?

Theγγ →ρ ⁰ ρ ⁰ → 4π reaction shows a broad peak at 1.5 GeV in the (J ^P ,J _z )=(2⁺,2) channel which has no counterpart in theρ ⁺ ρ ^? channel. This “resonance” is considered as a candidate for a qq $\bar q$ ; $\bar q$ state in the “s-channel”. We show, however, that it can also be explained by potential scattering ofρ ⁰ ρ ⁰ via theσ- exchange in the “t-channel”.     

Domain wall theory of elementary excitations in anisotropic antiferromagneticS=1 chains

We present a theoretical investigation of elementary excitations in anisotropic antiferromagneticS=1 chains using the concept of domain walls in string (hidden) order. Domain walls are classified by the internal spin projectionS _dw ^z . We calculate energies and string correlation functions of low lying excited states of the domain wall type in the Haldane phase and compare the results to those of numerical computations. The boundaries of the Haldane phase are determined from the instability of these excitations with respect to the ground state. The interaction between two domain walls is found to be proportional to the productS _dw ^z , S _dw ^z ₂, it is effectively repulsive for equal spin projections.     

B \bar B widths of the diquoniums based on quark potential model

A detailed and systematic study is carried out of the \(B\bar B(N\bar N, N\bar \Delta , \Delta \bar \Delta )\) widths of the “True” (colour \(3 \times \bar 3\) ) diquoniums making use of the quark pair creation model supplemented by the idea of a quark-quark potential. The widths, which continue to be of the order of 10 MeV, help confirm our hypothesis of avoiding the need for the “Mock” (colour \(6 \times \bar 6\) ) diquoniums to explain the narrow width resonances observed in the production experiments. Another outcome concerns the criterion to select out the dominant ones among the numerous diquonium trajectories. It is proposed to replace the one based on “peripherality” withR～1/m _π with another more natural one based on the width pattern itself.