The novel technique based on air-gap transistor stamps enabled realization of the intrinsic (not dominated by static disorder) transport of the electric-field-induced charge carriers on the surface of rubrene crystals over a wide temperature range. The signatures of the intrinsic transport are the anisotropy of the carrier mobility, µ, and the growth of µ with cooling. The anisotropy of µ vanishes in the activation regime at lower temperatures, where the charge transport becomes dominated by shallow traps. The deep traps, deliberately introduced into the crystal by X-ray radiation, increase the field-effect threshold without affecting the mobility. These traps filled above the field-effect threshold do not scatter the mobile polaronic carriers.

Materials exhibiting low-field magnetoelectric and magnetodielectric (MD) effects are necessary for utilization of these effects in multifunctional devices. Since large magnetic fields (H) or electric fields (E) are required to produce any significant effect in existing single-phase magnetoelectrics, recent efforts have been largely devoted to the investigation of laminates or thin film composites made of piezoelectric and magnetostrictive materials. In this work, we report large MD effect Δε∕ε∼3% at remarkably low fields (H<2kOe) in a single-phase material, terbium iron garnet. Our results suggest a route towards future applications of the MD effect in advanced devices.

We report x-ray scattering studies of nanoscale structural correlations in Nd1−xSrxMnO3 and La1−x(Ca,Sr)xMnO3, x=0.2-0.5. We find that the correlated regions possess a temperatureindependent correlation length of 2-3 lattice constants which is the same in all samples. The period of the lattice modulation of the correlated regions is proportional to the Ca/Sr doping concentration x. Remarkably, the lattice modulation periods of these and several other manganites with a ferromagnetic ground state fall on the same curve when plotted as a function of x. Thus, the structure of the correlated regions in these materials appears to be determined by a single parameter, x. We argue that these observations provide important clues for understanding the Colossal Magnetoresistance phenomenon in manganites.PACS numbers: 75.30.Vn, 71.38.+i, 71.30.+h Manganite perovskites of the chemical formula A 1−x B x MnO 3 (where A is a rare earth, and B is an alkali earth atom) have recently attracted considerable attention because they exhibit a wide diversity of ground states and a number of interesting phase transitions [1]. Perhaps the most dramatic of these is the magnetic-fieldinduced insulator-metal transition which is referred to as the Colossal Magnetoresistance (CMR) effect. In its most widely studied form, the CMR effect is the transition from a paramagnetic insulating (PI) to a ferromagnetic metallic (FM) phase. The very large difference between the electrical resistivities of the PI and FM phases lies at the core of the CMR effect, and considerable efforts have been spent in order to explain this difference [1]. The metallic nature of the FM phase was explained in the 1950's in the framework of the double-exchange (DE) mechanism [1] in which the itinerant e g electrons have their spins aligned with the localized t 2g spins of the Mn atoms by virtue of a strong Hund coupling. The DE mechanism, however, does not explain the high resistivity of the PI phase [2]. While recent experimental [3,4] and theoretical [2] work suggests that the enhanced resistivity might in fact result from the presence of small lattice polarons, the anomalously large resistivity of the PI phase still remains largely unexplained.With the resurgent interest in the manganites in the 1990's, it was realized that the rich behavior of the manganites results from the complex interplay between the charge, spin, lattice, and orbital degrees of freedom [1]. A very important consequence of the competition between the various degrees of freedom is the large variety of inhomogeneous states exhibited by the manganites [5]. The characteristic length scale of these inhomogeneous states varies from microns in conventional phase-separated states down to nanometers in the materials exhibiting nanoscale charge/orbitally ordered re-gions [1,[5][6][7].There is a growing body of evidence that the large resistivity of the PI phase in manganites is associated with nanoscale inhomogeneities. In fact, it was recently found that the PI phase exhibits short-range structural correl...

At low temperatures, spinel CuIr2S4 is a charge-ordered spin-dimerized insulator with triclinic lattice symmetry. We find that x-rays induce a structural transition in which the local triclinic structure is preserved, but the average lattice symmetry becomes tetragonal. These structural changes are accompanied by a thousandfold reduction in the electrical resistivity. The transition is persistent, but the original state can be restored by thermal annealing. We argue that x-ray irradiation disorders the lattice dimerization pattern, producing a state in which the orientation of the dimers is preserved, but the translational long-range order is destroyed.PACS numbers: 61.10.Nz, 72.80.Ga, 61.80.Cb Spinel compounds AB 2 X 4 (X is O, S, Se, or Cl) have attracted much attention over the last decade because they exhibit a large variety of interesting ground states, including superconductivity, cooperative antiferromagnetism, heavy fermion, and charge-ordered and spin-dimerized states [1,2]. The panoply of different properties exhibited by spinels results from the interplay of Coulomb interactions, effects of frustrated magnetism, and electron-lattice interaction. The corner sharing tetrahedral network of B sites in the spinel structure can accommodate a large number of different charge ordering patterns. In fact, some of the most complex charge-ordering patterns reported to date are found in spinel compounds [3,4]. The same tetrahedral network gives rise to geometric magnetic frustration when the ions occupying the B site are magnetic. The electronic and magnetic states realized in such a complex environment are often multi-degenerate and strongly fluctuating. Because of these complexity, a number of properties of the spinels remain poorly understood. Spinels, therefore, are important subjects of research in the physics of strongly correlated materials. In addition, some of these compounds, such as the lithium manganese spinels used in battery cathodes and the ferrites used in microwave applications, are of substantial technological importance.Chalcogenide spinel CuIr 2 S 4 has attracted attention because this compound undergoes a sharp metalinsulator transition at T MI ≈230 K [5]. The lowtemperature insulating state is nonmagnetic. NMR and photoemission experiments have shown that the Cu ion is monovalent in the insulating phase, and therefore the nominal valence of the iridium atoms is 3.5 [6]. It was proposed that charge ordering of Ir 3+ (S=0) and Ir 4+(S=1/2) ions is the origin of the metal-insulator transition in CuIr 2 S 4 [5], and that some kind of spin dimerization is responsible for the nonmagnetic nature of the insulating phase. Recent experimental determination of the lowtemperature structure [3] has provided very strong evidence that this scenario is correct. Specifically, these experiments have shown that the Ir sublattice consists of two types of Ir bi-capped hexagonal rings, which were described as Ir

Neutron scattering measurements on a magnetoresistive manganite La0.75(Ca0.45Sr0.55)0.25MnO3 show that uncorrelated dynamic polaronic lattice distortions are present in both the orthorhombic (O) and rhombohedral (R) paramagnetic phases. The uncorrelated distortions do not exhibit any significant anomaly at the O-to-R transition. Thus, both the paramagnetic phases are inhomogeneous on the nanometer scale, as confirmed further by strong damping of the acoustic phonons and by the anomalous Debye-Waller factors in these phases. In contrast, recent x-ray measurements and our neutron data show that polaronic correlations are present only in the O phase. In optimally doped manganites, the R phase is metallic, while the O paramagnetic state is insulating (or semiconducting). These measurements therefore strongly suggest that the correlated lattice distortions are primarily responsible for the insulating character of the paramagnetic state in magnetoresistive manganites.