Confined spin waves reveal an assembly of nanosize domains in ferromagnetic La(1-x)CaxMnO3 (x=0.17,0.2)

We report a study of spin-waves in ferromagnetic La$_{1-x}$Ca$_{x}$MnO$_3$, at concentrations x=0.17 and x=0.2 very close to the metallic transition (x=0.225). Below T$_C$, in the quasi-metallic state (T=150K), nearly q-independent energy levels are observed. They are characteristic of standing spin waves confined into finite-size ferromagnetic domains, defined in {\bf a, b) plane for x=0.17 and in all q-directions for x=0.2. They allow an estimation of the domain size, a few lattice spacings, and of the magnetic coupling constants inside the domains. These constants, anisotropic, are typical of an orbital-ordered state, allowing to characterize the domains as "hole-poor". The precursor state of the CMR metallic phase appears, therefore, as an assembly of small orbital-ordered domains.Comment: 4 pages, 5 figure

Expanding Semiflows on Branched Surfaces and One-Parameter Semigroups of Operators

We consider expanding semiflows on branched surfaces. The family of transfer operators associated to the semiflow is a one-parameter semigroup of operators. The transfer operators may also be viewed as an operator-valued function of time and so, in the appropriate norm, we may consider the vector-valued Laplace transform of this function. We obtain a spectral result on these operators and relate this to the spectrum of the generator of this semigroup. Issues of strong continuity of the semigroup are avoided. The main result is the improvement to the machinery associated with studying semiflows as one-parameter semigroups of operators and the study of the smoothness properties of semiflows defined on branched manifolds, without encoding as a suspension semiflow

Approach to the metal-insulator transition in La(1-x)CaxMnO3 (0<x<.2): magnetic inhomogeneity and spin wave anomaly

We describe the evolution of the static and dynamic spin correlations of La$_{1-x}$Ca$_x$MnO$_3$, for x=0.1, 0.125 and 0.2, where the system evolves from the canted magnetic state towards the insulating ferromagnetic state, approaching the metallic transition (x=0.22). In the x=0.1 sample, the observation of two spin wave branches typical of two distinct types of magnetic coupling, and of a modulation in the elastic diffuse scattering characteristic of ferromagnetic inhomogeneities, confirms the static and dynamic inhomogeneous features previously observed at x$<$0.1. The anisotropic q-dependence of the intensity of the low-energy spin wave suggests a bidimensionnal character for the static inhomogeneities. At x=0.125, which corresponds to the occurence of a ferromagnetic and insulating state, the two spin wave branches reduce to a single one, but anisotropic. At this concentration, an anomaly appears at {\bf q$_0$}=(1.25,1.25,0), that could be related to an underlying periodicity, as arising from (1.5,1.5,0) superstructures. At x=0.2, the spin-wave branch is isotropic. In addition to the anomaly observed at q$_0$, extra magnetic excitations are observed at larger q, forming an optical branch. The two dispersion curves suggest an anti-crossing behavior at some {\bf q$_0$'} value, which could be explained by a folding due to an underlying perodicity involving four cubic lattice spacings

Evidence of anisotropic magnetic polarons in la0.94_{0.94}Sr0.06_{0.06}MnO3_3 by neutron scattering and comparison with Ca-doped manganites

Elastic and inelastic neutron scattering experiments have been performed in a La$_{0.94}$Sr$_{0.06}$MnO$_3$ untwinned crystal, which exhibits an antiferromagnetic canted magnetic structure with ferromagnetic layers. The elastic small q scattering exhibits a modulation with an anisotropic q-dependence. It can be pictured by ferromagnetic inhomogeneities or polarons with a platelike shape, the largest size ($\approx17\AA$) and largest inter-polaron distance ($\approx$ 38$\AA$) being within the ferromagnetic layers. Comparison with observations performed on Ca-doped samples, which show the growth of the magnetic polarons with doping, suggests that this growth is faster for the Sr than for the Ca substitution. Below the gap of the spin wave branch typical of the AF layered magnetic structure, an additional spin wave branch reveals a ferromagnetic and isotropic coupling, already found in Ca-doped samples. Its q-dependent intensity, very anisotropic, closely reflects the ferromagnetic correlations found for the static clusters. All these results agree with a two-phase electronic segregation occurring on a very small scale, although some characteristics of a canted state are also observed suggesting a weakly inhomogeneous state.Comment: 11 pages, 11 figure

Phason modes in spin-density wave in the presence of long-range Coulomb interaction

We study the effect of long-range Coulomb interaction on the phason in spin-density wave (SDW) within mean field theory. In the longitudinal limit and in the absence of SDW pinning the phason is completely absorbed by the plasmon due to the Anderson-Higgs mechanism. In the presence of SDW pinning or when the wave vector {\bf q} is tilted from the chain direction, though the plasmon still almost exhausts the optical sum rule, another optical mode appears at $\omega < 2\Delta(T)$, with small optical weight. This low frequency mode below the SDW gap may be accessible to electron energy loss spectroscopy (EELS).Comment: 7 pages, Revtex 2.1, SZFKI 102/9

Convergence to stable laws for multidimensional stochastic recursions: the case of regular matrices

Given a sequence $(M_{n},Q_{n})_{n\ge 1}$ of i.i.d.\ random variables with generic copy $(M,Q) \in GL(d, \R) \times \R^d$, we consider the random difference equation (RDE) $$R_{n}=M_{n}R_{n-1}+Q_{n},$$ $n\ge 1$, and assume the existence of $\kappa >0$ such that \lim_{n \to \infty}(\E{\norm{M_1 ... M_n}^\kappa})^{\frac{1}{n}} = 1 . We prove, under suitable assumptions, that the sequence $S_n = R_1 + ... + R_n$, appropriately normalized, converges in law to a multidimensional stable distribution with index $\kappa$. As a by-product, we show that the unique stationary solution $R$ of the RDE is regularly varying with index $\kappa$, and give a precise description of its tail measure. This extends the prior work http://arxiv.org/abs/1009.1728v3 .Comment: 15 page

High-field AFMR in single-crystalline La_{0.95}Sr_{0.05}MnO_3: Experimental evidence for the existence of a canted magnetic structure

High-field antiferromagnetic-resonance (AFMR) spectra were obtained in the frequency range 60 GHz < \nu < 700 GHz and for magnetic fields up to 8 T in twin-free single crystals of La_{0.95}Sr_{0.05}MnO_3. At low temperatures two antiferromagnetic modes were detected, which reveal different excitation conditions and magnetic field dependencies. No splitting of these modes was observed for any orientation of the static magnetic field excluding the phase-separation scenario for this composition. Instead, the full data set including the anisotropic magnetization can be well described using a two-sublattice model of a canted antiferromagnetic structure.Comment: 4 pages, 3 figure