Session V5: Unusual Transport Phenomena in Chalcogenides

Quantum Linear Magnetoresistance: Solution of an Old Mystery

In the paper the history of the discovery of the linear magnetoresistance in metals by P. L. Kapitza in 1928 - 1929 and its explanations are presented. Actually, Kapitza discovered two different phenomena. One of them -- the linear magnetoresistance at classically large magnetic fields in polycrystalline samples of metals, having open Fermi surfaces, was explained by I. Lifshits and V. Peschansky in 1958. The other phenomenon is the quantum linear magnetoresistance, appearing in metals, or semimetals, with a small concentration of carriers and a small effective mass, when only the lowest Landau band participates in the conductivity. Manifestations of this unusual phenomenon in different materials are described.   

The quest for imperfection

The stoichiometric compounds Ag$_2$S, Ag$_2$Se and Ag$_2$Te are superionic conductors at higher temperature. Below 400 K, ion migration is effectively frozen and the compounds are non-magnetic semiconductors that exhibit no appreciable magnetoresistance. We showed that slightly altering the stoichiometry can lead to a marked increase in the magnetic response; up to 200 $\%$ at room temperature and in a magnetic field of 5.5T in Ag$_{2+\delta}$Se and Ag$_{2+\delta}$Te ($\delta$ about 10$^{-4}$) reaching a maximum of about 350 $\%$ at low temperature. But more importantly, the response can be almost linear in magnetic field even at low magnetic fields. Not only do these silver chalcogenides show linear magnetoresistance, this response also is still unsaturated up to 55 T showing no signs of saturation. M. Parish and P. Littlewood identified inhomogeneities as a key factor. Their theoretical model has initiated a project on artificially created structures in semiconductors to mimic transport in real materials with inhomogeneities. We processed a number of different geometric realizations in collaboration with L. Cohen's group at Imperial College, London, on their InSb epilayers on GaAs (001). However, the films themselves have a very large and almost linear and non-saturating (up to applied fields of 13 T) magnetoresistance intrinsically which made the interpretation of the results somewhat difficult. We then went back to the origin of this intrinsic magnetoresistance by comparing, at room temperature, undoped InSb epilayers grown on GaAs(001) by molecular-beam epitaxy with varying thickness from 100 to 2000nm. The question is whether these films and the silver chalcogenides share a similar physical origin for their magnetoresistance. Experiments to very high fields in InSb films are on the way.   

Non-saturating magnetoresistance in heavily disordered semiconductors

We present a classical model of the magnetotransport of strongly inhomogeneous semiconductors based on an array of coupled four-terminal elements. We show that this model generically yields non-saturating, quasi-linear magnetoresistance at large magnetic fields, in contrast to the resistance of a homogeneous semiconductor, which increases quadratically with magnetic field at low fields and, except in very special cases, saturates at fields much larger than the inverse of the carrier mobility. We argue that our model provides an explanation for the observed non-saturating magnetoresistance in doped silver chalcogenides and potentially in other macroscopically disordered conductors. Finally, our method may be used to design the magnetoresistive response of a microfabricated array and thus pave the way to the construction of magnetic field sensors with a controllable response.   

The Evolution from CDW to Superconductivity in Cu$_{x}$TiSe$_{2}$

Charge density waves (CDWs) are periodic modulations of the conduction electron density in solids: collective states that arise due to intrinsic instabilities often present in low dimensional electronic systems. The layered dichalcogenides are the most well known examples of CDW-bearing systems, and TiSe$_{2}$ was one of the first CDW-bearing materials known. The competition between CDW superconducting states at low temperatures has often been characterized and discussed, and yet no chemical system has been previously reported where finely controlled chemical tuning allows for this competition to be studied in detail. This talk will describe our work [1] reporting how, upon controlled intercalation of TiSe$_{2}$ with Cu to yield Cu$_{x}$TiSe$_{2}$, the CDW transition is continuously suppressed, and a new superconducting state emerges near x = 0.04, with a maximum Tc of 4.15 K found at x = 0.08. The anisotropic superconducting properties, obtained by characterization of the resistivity and magnetization of single crystals of Cu$_{0.07}$TiSe$_{2}$, will also be described. \newline \newline [1] E. Morosan, H. W. Zandbergen, B. S. Dennis, J. W. G. Bos, Y. Onose, T. Klimczuk, A.P. Ramirez, N. P. Ong, and R. J. Cava \textit{Nature Physics}\textbf{2,} 544 (2006).   

Current Jets and Non-saturating Magnetoresistance in Disordered Semiconductors

The transverse, positive magnetoresistance of doped silver telluride and silver selenide changes linearly with field by thousands of percent, with no sign of saturation up to MegaGauss. The inhomogeneous distribution of excess/deficient silver atoms lies behind this anomalous magnetoresistive response, introducing spatial conductivity fluctuations with length scales independent of the cyclotron radius. Theoretical simulations of two and three-dimensional random resistor networks reveal distorted current flows that provide a linear contribution to the transverse magnetoresistance, but a pronounced negative longitudinal magnetoresistance. We show that a systematic investigation of the resistivity tensor in longitudinal field could be used to identify the spatial inhomogeneities in the silver chalcogenides and determine the associated length scale of the current distortion. The incorporation of macroscopic inhomogeneities to other semiconductors, such as InSb, opens the gate to artificial fabrication of conducting networks with micron scale unit size for enhanced magnetoresistive sensitivity.