252 research outputs found

    Classical magnetotransport of inhomogeneous conductors

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    We present a model of magnetotransport of inhomogeneous conductors based on an array of coupled four-terminal elements. We show that this model generically yields non-saturating magnetoresistance at large fields. We also discuss how this approach simplifies finite-element analysis of bulk inhomogeneous semiconductors in complex geometries. We argue that this is an explanation of the observed non-saturating magnetoresistance in silver chalcogenides and potentially in other disordered conductors. Our method may be used to design the magnetoresistive response of a microfabricated array.Comment: 12 pages, 13 figures. Minor typos correcte

    Anomalous temperature behavior of resistivity in lightly doped manganites around a metal-insulator phase transition

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    An unusual temperature and concentration behavior of resistivity in La0.7Ca0.3Mn1xCuxO3La_{0.7}Ca_{0.3}Mn_{1-x}Cu_xO_3 has been observed at slight CuCu doping (0x0.050\leq x \leq 0.05). Namely, introduction of copper results in a splitting of the resistivity maximum around a metal-insulator transition temperature T0(x)T_0(x) into two differently evolving peaks. Unlike the original CuCu-free maximum which steadily increases with doping, the second (satellite) peak remains virtually unchanged for x<xcx<x_c, increases for xxcx\ge x_c and finally disappears at xm2xcx_m\simeq 2x_c with xc0.03x_c\simeq 0.03. The observed phenomenon is thought to arise from competition between substitution induced strengthening of potential barriers (which hamper the charge hopping between neighboring MnMn sites) and weakening of carrier's kinetic energy. The data are well fitted assuming a nonthermal tunneling conductivity theory with randomly distributed hopping sites.Comment: 10 REVTEX pages, 2 PostScript figures (epsf.sty); to be published in JETP Letter

    Non-saturating magnetoresistance of inhomogeneous conductors: comparison of experiment and simulation

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    The silver chalcogenides provide a striking example of the benefits of imperfection. Nanothreads of excess silver cause distortions in the current flow that yield a linear and non-saturating transverse magnetoresistance (MR). Associated with the large and positive MR is a negative longitudinal MR. The longitudinal MR only occurs in the three-dimensional limit and thereby permits the determination of a characteristic length scale set by the spatial inhomogeneity. We find that this fundamental inhomogeneity length can be as large as ten microns. Systematic measurements of the diagonal and off-diagonal components of the resistivity tensor in various sample geometries show clear evidence of the distorted current paths posited in theoretical simulations. We use a random resistor network model to fit the linear MR, and expand it from two to three dimensions to depict current distortions in the third (thickness) dimension. When compared directly to experiments on Ag2±δ_{2\pm\delta}Se and Ag2±δ_{2\pm\delta}Te, in magnetic fields up to 55 T, the model identifies conductivity fluctuations due to macroscopic inhomogeneities as the underlying physical mechanism. It also accounts reasonably quantitatively for the various components of the resistivity tensor observed in the experiments.Comment: 10 pages, 7 figure

    Kinetics of diffusion-limited catalytically-activated reactions: An extension of the Wilemski-Fixman approach

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    We study kinetics of diffusion-limited catalytically-activated A+BBA + B \to B reactions taking place in three dimensional systems, in which an annihilation of diffusive AA particles by diffusive traps BB may happen only if the encounter of an AA with any of the BBs happens within a special catalytic subvolumen, these subvolumens being immobile and uniformly distributed within the reaction bath. Suitably extending the classical approach of Wilemski and Fixman (G. Wilemski and M. Fixman, J. Chem. Phys. \textbf{58}:4009, 1973) to such three-molecular diffusion-limited reactions, we calculate analytically an effective reaction constant and show that it comprises several terms associated with the residence and joint residence times of Brownian paths in finite domains. The effective reaction constant exhibits a non-trivial dependence on the reaction radii, the mean density of catalytic subvolumens and particles' diffusion coefficients. Finally, we discuss the fluctuation-induced kinetic behavior in such systems.Comment: To appear in J. Chem. Phy

    Low-temperature specific heat of real crystals: Possibility of leading contribution of optical and short-wavelength acoustical vibrations

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    We point out that the repeatedly reported glass-like properties of crystalline materials are not necessarily associated with localized (or quasilocalized) excitations. In real crystals, optical and short-wavelength acoustical vibrations remain damped due to defects down to zero temperature. If such a damping is frequency-independent, e.g. due to planar defects or charged defects, these optical and short-wavelength acoustical vibrations yield a linear-in-TT contribution to the low-temperature specific heat of the crystal lattices. At low enough temperatures such a contribution will prevail over that of the long-wavelength acoustical vibrations (Debye contribution). The crossover between the linear and the Debye regime takes place at TNT^* \propto \sqrt N, where NN is the concentration of the defects responsible for the damping. Estimates show that this crossover could be observable.Comment: 5 pages. v4: Error in Appendix corrected, which does not change the main results of the pape

    Survival probability of a particle in a sea of mobile traps: A tale of tails

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    We study the long-time tails of the survival probability P(t)P(t) of an AA particle diffusing in dd-dimensional media in the presence of a concentration ρ\rho of traps BB that move sub-diffusively, such that the mean square displacement of each trap grows as tγt^{\gamma} with 0γ10\leq \gamma \leq 1. Starting from a continuous time random walk (CTRW) description of the motion of the particle and of the traps, we derive lower and upper bounds for P(t)P(t) and show that for γ2/(d+2)\gamma \leq 2/(d+2) these bounds coincide asymptotically, thus determining asymptotically exact results. The asymptotic decay law in this regime is exactly that obtained for immobile traps. This means that for sufficiently subdiffusive traps, the moving AA particle sees the traps as essentially immobile, and Lifshitz or trapping tails remain unchanged. For γ>2/(d+2)\gamma > 2/(d+2) and d2d\leq 2 the upper and lower bounds again coincide, leading to a decay law equal to that of a stationary particle. Thus, in this regime the moving traps see the particle as essentially immobile. For d>2d>2, however, the upper and lower bounds in this γ\gamma regime no longer coincide and the decay law for the survival probability of the AA particle remains ambiguous

    Effective conductivity of 2D isotropic two-phase systems in magnetic field

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    Using the linear fractional transformation, connecting effective conductivities sigma_{e} of isotropic two-phase systems with and without magnetic field, explicit approximate expressions for sigma_{e} in a magnetic field are obtained. They allow to describe sigma_{e} of various inhomogeneous media at arbitrary phase concentrations x and magnetic fields. the x-dependence plots of sigma_e at some values of inhomogeneity and magnetic field are constructed. Their behaviour is qualitatively compatible with the existing experimental data. The obtained results are applicable for different two-phase systems (regular and nonregular as well as random), satisfying the symmetry and self-duality conditions, and admit a direct experimental checking.Comment: 9 pages, 2 figures, Latex2e, small corrections and new figure

    Pascal Principle for Diffusion-Controlled Trapping Reactions

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    "All misfortune of man comes from the fact that he does not stay peacefully in his room", has once asserted Blaise Pascal. In the present paper we evoke this statement as the "Pascal principle" in regard to the problem of survival of an "A" particle, which performs a lattice random walk in presence of a concentration of randomly moving traps "B", and gets annihilated upon encounters with any of them. We prove here that at sufficiently large times for both perfect and imperfect trapping reactions, for arbitrary spatial dimension "d" and for a rather general class of random walks, the "A" particle survival probability is less than or equal to the survival probability of an immobile target in the presence of randomly moving traps.Comment: 4 pages, RevTex, appearing in PR

    A new structural relaxation pathway of low-density amorphous ice

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    Low-density amorphous ice (LDA) is involved in critical cosmological processes and has gained prominence as one of the at least two distinct amorphous forms of ice. Despite these accolades, we still have an incomplete understanding of the structural diversity that is encompassed within the LDA state and the dynamic processes that take place upon heating LDA. Heating the high-pressure ice VIII phase at ambient pressure is a remarkable example of temperature-induced amorphisation yielding LDA. We investigate this process in detail using X-ray diffraction and Raman spectroscopy, and show that the LDA obtained from ice VIII is structurally different from the more 'traditional' states of LDA which are approached upon thermal annealing. This new structural relaxation pathway involves an increase of structural order on the intermediate range length scale. In contrast with other LDA materials the local structure is more ordered initially and becomes slightly more disordered upon annealing. We also show that the cascade of phase transitions upon heating ice VIII at ambient pressure includes the formation of ice IX which may be connected with the structural peculiarities of LDA from ice VIII. Overall, this study shows that LDA is a structurally more diverse material than previously appreciated.Comment: 12 pages, 5 figure
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