2.2. Materials 929 2.3. Factors Influencing Charge Mobility 931 2.3.1. Molecular Packing 931 2.3.2. Disorder 932 2.3.3. Temperature 933 2.3.4. Electric Field 934 2.3.5. Impurities 934 2.3.6. Pressure 934 2.3.7. Charge-Carrier Density 934 2.3.8. Size/molecular Weight 935 3. The Charge-Transport Parameters 935 3.1. Electronic Coupling 936 3.1.1. The Energy-Splitting-in-Dimer Method 936 3.1.2. The Orthogonality Issue 937 3.1.3. Impact of the Site Energy 937 3.1.4. Electronic Coupling in Oligoacene Derivatives 938

Charge transport in organic semiconductors.

BiFeO3 is perhaps the only material that is both magnetic and a strong ferroelectric at room temperature. As a result, it has had an impact on the field of multiferroics that is comparable to that of yttrium barium copper oxide (YBCO) on superconductors, with hundreds of publications devoted to it in the past few years. In this Review, we try to summarize both the basic physics and unresolved aspects of BiFeO3 (which are still being discovered with several new phase transitions reported in the past few months) and device applications, which center on spintronics and memory devices that can be addressed both electrically and magnetically.

Physics and Applications of Bismuth Ferrite

http://sces.phys.utk.edu/mostcited/mc1_1114.pdf

Colossal Magnetoresistant Materials: The Key Role of Phase Separation

Physical Review B

Electron and ambipolar transport in organic field-effect transistors.

Unidirectional electric current flow, such as that found in a diode, is essential for modern electronics. It usually occurs at asymmetric interfaces such as p-n junctions or metal/semiconductor interfaces with Schottky barriers. We report on a diode effect associated with the direction of bulk electric polarization in BiFeO3: a ferroelectric with a small optical gap edge of ∼2.2 electron volts. We found that bulk electric conduction in ferroelectric monodomain BiFeO3 crystals is highly nonlinear and unidirectional. This diode effect switches its direction when the electric polarization is flipped by an external voltage. A substantial visible-light photovoltaic effect is observed in BiFeO3 diode structures. These results should improve understanding of charge conduction mechanisms in leaky ferroelectrics and advance the design of switchable devices combining ferroelectric, electronic, and optical functionalities.

Switchable ferroelectric diode and photovoltaic effect in BiFeO3.

The air-gap field-effect technique enabled realization of the intrinsic (not limited by static disorder) polaronic transport on the surface of rubrene (C42H28) crystals over a wide temperature range. The signatures of this intrinsic transport are the anisotropy of the carrier mobility, mu, and the growth of mu with cooling. Anisotropy of mu vanishes in the activation regime at low temperatures, where the transport is dominated by shallow traps. The deep traps, introduced by x-ray radiation, increase the field-effect threshold without affecting mu, an indication that the filled traps do not scatter polarons.

Intrinsic Charge Transport on the Surface of Organic Semiconductors

Manganese oxides of the general formula A1–xBxMnO3 (where A and B are trivalent and divalent cations, respectively) have recently attracted considerable attention by virtue of their unusual magnetic and electronic properties1–9. For example, in some of these materials magnetic fields can drive insulator-to-metal transitions where both the conductivity and magnetization change dramatically—an effect termed 'colossal magneto-resistance'1–3—raising hopes for application of these materials in the magnetic recording industry1–9. Here we show that in one such compound, Pr0.7Ca0.3MnO3, a transition from the insulating antiferromagnetic state to the metallic ferromagnetic state can be driven by illumination with X-rays at low temperatures (<40 K). This transition is accompanied by significant changes in the lattice structure, and can be reversed by thermal cycling. This effect, undoubtedly a manifestation of the strong electron–lattice interactions believed to be responsible for the magnetoresistive properties of these materials6–9, provides insights into the physical mechanisms of persistent photoconductivity, and may also find applications in X-ray detection and X-ray lithographic patterning of ferromagnetic nanostructures.

An X-ray-induced insulator–metal transition in a magnetoresistive manganite

We report discovery of collinear-magnetism-driven ferroelectricity in the Ising chain magnet ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{2\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{O}}_{6}$ ($x\ensuremath{\approx}0.96$). Neutron diffraction shows that ${\mathrm{Co}}^{2+}$ and ${\mathrm{Mn}}^{4+}$ ions alternating along the chains exhibit an up-up-down-down ($\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}$) magnetic order. The ferroelectricity results from the inversion symmetry breaking in the $\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}$ spin chain with an alternating charge order. Unlike in spiral magnetoelectrics where antisymmetric exchange coupling is active, the symmetry breaking in ${\mathrm{Ca}}_{3}(\mathrm{Co},\mathrm{Mn}{)}_{2}{\mathrm{O}}_{6}$ occurs through exchange striction associated with symmetric superexchange.

/pdf/ferroelectricity-in-an-ising-chain-magnet-mw5s3l5vta.pdf

Ferroelectricity in an ising chain magnet.

Inorganic compounds with the AB2X4 spinel structure have been studied for many years, because of their unusual physical properties The spinel crystallographic structure, first solved by Bragg in 1915, has cations occupying both tetrahedral (A) and octahedral (B) sites Interesting physics arises when the B-site cations become mixed in valence Magnetite (Fe3O4) is a classic and still unresolved example, where the tendency to form ordered arrays of Fe2+ and Fe3+ ions competes with the topological frustration of the B-site network The CuIr2S4 thiospinel is another example, well known for the presence of a metal-insulator transition at 230 K with an abrupt decrease of the electrical conductivity on cooling accompanied by the loss of localized magnetic moments Here, we report the determination of the crystallographic structure of CuIr2S4 below the metal-insulator transition Our results indicate that CuIr2S4 undergoes a simultaneous charge-ordering and spin-dimerization transition-a rare phenomenon in three-dimensional compounds Remarkably, the charge-ordering pattern consists of isomorphic octamers of Ir83+S24 and Ir84+S24 (as isovalent bi-capped hexagonal rings) This extraordinary arrangement leads to an elegant description of the spinel structure, but represents an increase in complexity with respect to all the known charge-ordered structures, which are typically based on stripes, slabs or chequerboard patterns

Valery Kiryukhin

Papers

Switchable ferroelectric diode and photovoltaic effect in BiFeO3.

Intrinsic Charge Transport on the Surface of Organic Semiconductors

An X-ray-induced insulator–metal transition in a magnetoresistive manganite

Ferroelectricity in an ising chain magnet.

Formation of isomorphic Ir3+ and Ir4+ octamers and spin dimerization in the spinel CuIr2S4.