Quantum relativistic theory of an electron in terms of local observables

Dirac equation is reformulated in terms of real local observables, which are mean values of the wave function . The quadrivector current is shown to be a function of the potential vector and of other local observables. The equations describe the evolution of a four dimensional system T, X, Y, Z, and of two scalars, in the coordinate system ct, x, y, z. The current is proportional to the T vector. The Z vector is associated with the spin of the electron. Energy and gauge transformations correspond to rotations in the plane (X, Y). In the presence of a static field, the (real) solutions of the equations appear as eigenfunctions associated with energy eigenvalues. Received 7 September 1998     

Coexistence of antiferromagnetic and ferromagnetic phase for ferromagnetic Kondo lattice model

To study the proposed phase separations in doped manganites, we performed Monte-Carlo calculations for the ferromagnetic Kondo lattice model with strong Hund's coupling between conduction electrons and localized spins. For the practical calculations, we adopted a one dimensional lattice and treated the spins of the localized t_2g electrons semi-classically. A direct evidence of the phase separation is observed from a snapshot of the spatial dependence of localized spins. No indication of the canted or spiral phases is found in the results of simulations. Further, the calculated results of the spin structure factor in the phase separation region are well compared with recent experiments. Received: 1st September 1998 / Revised: 30 October 1998 / Accepted: 27 November 1998     

Theory of site-disordered magnets

In realistic spinglasses, such as , and , magnetic atoms are located at random positions. Their couplings are determined by their relative positions. For such systems a field theory is formulated. In certain limits it reduces to the Hopfield model, the Sherrington-Kirkpatrick model, and the Viana-Bray model. The model has a percolation transition, while for RKKY couplings the “concentration scaling” occurs. Within the Gaussian approximation the Ginzburg-Landau expansion is considered in the clusterglass phase, that is to say, for not too small concentrations. Near special points, the prefactor of the cubic term, or the one of the replica-symmetry-breaking quartic term, may go through zero. Around such points new spin glass phases are found. Received: 27 April 1998 / Received in final form: 27 July 1998 / Accepted: 13 August 1998     

Critical dynamics in clusters of noble gas atoms

The multi-fragmentation dynamics of noble gas atomic clusters is considered for different statistically distributed deposited energies. The conditions giving rise to the development of criticality in the cluster evolution are revealed from an analysis of the signals in the fragment mass distribution. The time dependence of the observables related to critical exponents is studied. It is demonstrated that in a certain regime the cluster exhibits a behavior which can be identified as the precursor of a second-order liquid-gas phase transition. Received 1st September 1998 and Received in final form 14 January 1999     

Novel massive ground states of spin chains in a magnetic field

Novel massive quantum states appearing in spin chains under a strong magnetic field are discussed. These states lead to plateaus in magnetization curves. When the systems are axially symmetric and the field is applied parallel to the symmetry-axis, the phenomena are analogous to metal-insulator transitions. Striking features of the plateau phenomena - exactness and rationality - are explained as consequences the commensurability condition to the underlying lattice. The effects of the planar anisotropy are also discussed in detail. Received: 16 February 1998 / Revised: 20 April 1998 / Accepted: 30 April 1998     

Spontaneous SU(2) symmetry breaking in one-dimensional quantum antiferromagnet

We present a Bethe Ansatz based investigation of a one-dimensional (1D) Heisenberg spin chain in a real 3D crystal lattice. We have shown that due to an influence of the lattice distortion on a crystalline field of ligands of magnetic ions, a Heisenberg antiferromagnetic spin chain is unstable under the appearance of a magnetic anisotropy of the “easy-plane” type. The effects of an external magnetic field and nonzero temperature onto such a phase transition are studied. Received: 19 January 1998 / Revised: 16 March 1998 / Accepted: 17 March 1998     

First excitations of the spin 1/2 Heisenberg antiferromagnet on the kagomé lattice

We study the exact low energy spectra of the spin 1/2 Heisenberg antiferromagnet on small samples of the kagomé lattice of up to N=36 sites. In agreement with the conclusions of previous authors, we find that these low energy spectra contradict the hypothesis of Néel type long range order. Certainly, the ground state of this system is a spin liquid, but its properties are rather unusual. The magnetic () excitations are separated from the ground state by a gap. However, this gap is filled with nonmagnetic () excitations. In the thermodynamic limit the spectrum of these nonmagnetic excitations will presumably develop into a gapless continuum adjacent to the ground state. Surprisingly, the eigenstates of samples with an odd number of sites, i.e. samples with an unsaturated spin, exhibit symmetries which could support long range chiral order. We do not know if these states will be true thermodynamic states or only metastable ones. In any case, the low energy properties of the spin 1/2 Heisenberg antiferromagnet on the kagomé lattice clearly distinguish this system from either a short range RVB spin liquid or a standard chiral spin liquid. Presumably they are facets of a generically new state of frustrated two-dimensional quantum antiferromagnets. Received: 27 November 1997 / Accepted: 29 January 1998     

Mixed spin transverse Ising model with longitudinal random crystal field interactions

The effect of a longitudinal random crystal field interaction on the phase diagrams of the mixed spin transverse Ising model consisting of spin-1/2 and spin-1 is investigated within the finite cluster approximation based on a single-site cluster theory. In order to expand a cluster identity of spin-1, we transform the spin-1 to spin-1/2 representation containing Pauli operators. We derive the state equations applicable to structures with arbitrary coordination number N. The phase diagrams obtained in the case of a honeycomb lattice (N=3) and a simple-cubic lattice (N=6), are qualitatively different and examined in detail. We find that both systems exhibit a variety of interesting features resulting from the fluctuation of the crystal field interactions. Received: 13 February 1998 / Accepted: 17 March 1998     

Theory of resonant Raman scattering from low-energy collective excitations of interacting electrons in quantum wires

The Raman spectra of quantum wires in the region of electronic intra-band excitations are investigated using one- and two-band models based on the Luttinger approximation with spin. Structures related to charge and spin density modes are identified, and analyzed with respect to their behavior with photon energy and temperature. It is found that the low-energy peaks in the polarized spectra, close to resonance that are commonly assigned to “single particle excitations”, can be interpreted as the signature of spin density excitations. A broad structure in the resonant depolarized spectrum is predicted above the frequency of the spin density excitations. This is due to simultaneous but independent propagation of spin and charge density modes. The results, when compared with experiment, show, that the electronic collective excitations of quantum wires at low energies are characteristic for a non-Fermi liquid. Received: 25 March 1998 / Accepted: 3 June 1998     

Three types of maximally entangled states for a two-particle system

Foratwo-particle(spin1/2)systemthespinstateisdescribedusingavectorinafour-dimensionalHilbertspace.Inangularmomentumcouplingtheory[1],theangularmomentumuncouplingandcouplingrepresentationsareusuallyadopted.WhilethesimultaneouseigenstatesoftheCSCO(s1z,s2z),12,12,12and12,arechosenasthebasisofangularmomentumuncouplingscheme,thesimultaneouseigenstatesoftheCSCO(2,zSS),,SM(12=+Sss),arechosenasthebasisofangularmomentumcouplingscheme,1212100,2轾=-臌1212110,2轾=+臌121211,1,1.=-=Thefo…     

Martensitic transformations: first-principles calculations combined with molecular-dynamics simulations

Results are presented of first-principles total-energy calculations and molecular-dynamics simulations of structural transformations in magnetic transition metal alloys like Fe_1-xNi_x. While first-principles calculations allow to identify those structures having the lower total energy, molecular-dynamics simulations can be used to trace out the dependence of the transformation on temperature, composition, concentration of defects etc. We have used the method of the semi-empiric embedded-atom potential in the molecular-dynamics simulations which yields remarkable good results for the structural changes. Received: 3 February 1998 / Revised: 4 May 1998 / Accepted: 24 June 1998     

Spin correlations in the two-dimensional spin-5/2 Heisenberg antiferromagnet

We report a neutron scattering study of the instantaneous spin correlations in the two-dimensional spin S =5/2 square-lattice Heisenberg antiferromagnet Rb₂MnF₄. The measured correlation lengths are quantitatively described, with no adjustable parameters, by high-temperature series expansion results and by a theory based on the quantum self-consistent harmonic approximation. Conversely, we find that the data, which cover the range from about 1 to 50 lattice constants, are outside of the regime corresponding to renormalized classical behavior of the quantum non-linear model. In addition, we observe a crossover from Heisenberg to Ising critical behavior near the Néel temperature; this crossover is well described by a mean-field model with no adjustable parameters. Received: 3 March 1998 / Received in final form: 4 May 1998 / Accepted: 19 May 1998     

Monte-Carlo study of correlations in quantum spin chains at non-zero temperature

Antiferromagnetic Heisenberg spin chains with various spin values (S=1/2,1,3/2,2,5/2) are studied numerically with the quantum Monte-Carlo method. Effective spin S chains are realized by ferromagnetically coupling n=2S antiferromagnetic spin chains with S=1/2. The temperature dependence of the uniform susceptibility, the staggered susceptibility, and the static structure factor peak intensity are computed down to very low temperatures, . The correlation length at each temperature is deduced from numerical measurements of the instantaneous spin-spin correlation function. At high temperatures, very good agreement with exact results for the classical spin chain is obtained independent of the value of S. For the S=2 chain which has a gap , the correlation length and the uniform susceptibility in the temperature range are well predicted by the semi-classical theory of Damle and Sachdev. Received: 23 December 1997 / Revised and Accepted: 11 March 1998     

The temperature dependence of the perpendicular giant magnetoresistance in Co/Cu multilayered nanowires

A theory, based on earlier work by Valet and Fert, is first presented to describe the influence of temperature on the perpendicular giant magnetoresistance (GMR) in multilayers. Then we present GMR measurements performed at T=77 K and at room temperature on Co/Cu multilayered nanowires with layer thicknesses ranging from a few nm to 1 μm. We use our model to obtain a good quantitative fit to the experimental results in both the short spin diffusion length limit and out of this limit. We discuss the temperature dependence of the bulk parameters, the scattering spin asymmetry coefficient and spin diffusion length in the Co layers. Received: 25 January 1998 / Accepted: 6 May 1998     

Nucleon–nucleon scattering observables in large-Nc QCD

Nucleon–nucleon scattering observables are considered in the context of the large N_c limit of QCD for initial states with moderately high momenta (pN_c). The scattering is studied in the framework of the time-dependent mean-field approximation. We focus on the dependence of those observables on the spin and isospin of the initial state which may be computed using time-dependent mean-field theory. We show that, up to corrections, all such observables must be invariant under simultaneous spin and isospin flips (i.e., rotations through π/2 in both spin and isospin) acting on either particle. All observables of this class obtained from spin unpolarized measurements must be isospin independent up to 1/N_c corrections. Moreover, it can be shown that the leading correction is of relative order 1/N_c² rather than 1/N_c.     

Magnetic susceptibility, heat capacity, and optical conductivity of LiPc and LiPcI

The magnetic susceptibility, using dc and electron spin resonance (ESR) methods, the specific heat, and the infrared properties of the one-dimensional molecular semiconductors lithium phthalocyanine (LiPc) and the iodinated compound LiPcI have been investigated for temperatures K. LiPc has a half-filled conduction band and is expected to be an organic metal. However, due to the strong Coulomb repulsion the system is a one-dimensional Mott-Hubbard insulator with a Hubbard gap of 0.75 eV as inferred from optical measurements. The localized electrons along the molecular stacks behave like a S = 1/2 antiferromagnetic spin chain. The spin susceptibility, as determined by ESR experiments, and the magnetic contribution to the heat capacity show a Bonner-Fisher type of behavior with an exchange constant K. LiPcI is an intrinsic narrow-gap semiconductor with an optical gap of 0.43 eV. In ESR experiments it is silent, indicating that all the unpaired electrons have been removed from the macrocycle via doping with iodine. Received: 16 June 1998 / Accepted: 14 July 1998