Martensitic accommodation strain and the metal-insulator transition in manganites

In this paper, we report polarized optical microscopy and electrical transport studies of manganese oxides that reveal that the charge ordering transition in these compounds exhibits typical signatures of a martensitic transformation. We demonstrate that specific electronic properties of charge-ordered manganites stem from a combination of martensitic accommodation strain and effects of strong electron correlations. This intrinsic strain is strongly affected by the grain boundaries in ceramic samples. Consistently, our studies show a remarkable enhancement of low field magnetoresistance and the grain size effect on the resistivity in polycrystalline samples and suggest that the transport properties of this class of manganites are governed by the charge-disordered insulating phase stabilized at low temperature by virtue of martensitic accommodation strain. High sensitivity of this phase to strains and magnetic field leads to a variety of striking phenomena, such as unusually high magnetoresistance (10^10 %) in low magnetic fields.Comment: Short paper, 4 figures, to appear in Rapid Communicatio

Multiphase segregation and metal-insulator transition in single crystal La(5/8-y)Pr(y)Ca(3/8)MnO3

The insulator-metal transition in single crystal La(5/8-y)Pr(y)Ca(3/8)MnO3 with y=0.35 was studied using synchrotron x-ray diffraction, electric resistivity, magnetic susceptibility, and specific heat measurements. Despite the dramatic drop in the resistivity at the insulator-metal transition temperature Tmi, the charge-ordering (CO) peaks exhibit no anomaly at this temperature and continue to grow below Tmi. Our data suggest then, that in addition to the CO phase, another insulating phase is present below Tco. In this picture, the insulator-metal transition is due to the changes within this latter phase. The CO phase does not appear to play a major role in this transition. We propose that a percolation-like insulator-metal transition occurs via the growth of ferromagnetic metallic domains within the parts of the sample that do not exhibit charge ordering. Finally, we find that the low-temperature phase-separated state is unstable against x-ray irradiation, which destroys the CO phase at low temperatures.Comment: 9 pages, 9 encapsulated eps figure

Intrinsic charge transport on the surface of organic semiconductors

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, mu, and the growth of mu with cooling. The anisotropy of mu 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.Comment: 10 pages, 4 figure

Thermal Conductivity of the Spin Peierls Compound CuGeO_3

The thermal conductivity of the Spin-Peierls (SP) compound CuGeO_3 was measured in magnetic fields up to 16 T. Above the SP transition, the heat transport due to spin excitations causes a peak at around 22 K, while below the transition the spin excitations rapidly diminish and the heat transport is dominated by phonons; however, the main scattering process of the phonons is with spin excitations, which demonstrates itself in an unusual peak in the thermal conductivity at about 5.5 K. This low-temperature peak is strongly suppressed with magnetic fields in excess of 12.5 T.Comment: 6 pages, including 2 postscript figure

Thermodynamic Properties of the Incommensurate Phase of CuGeO_3

We present high resolution measurements of the specific heat and the thermal expansion of the inorganic spin--Peierls cuprate CuGeO_3 in a magnetic field of 16 Tesla. At the transition from the incommensurate to the uniform phase both quantities show pronounced anomalies, which allow to derive the uniaxial pressure dependencies of the transition temperature. In high magnetic fields the specific heat is dominated by magnetic excitations and follows a T^3 law at low temperatures. The thermal expansion measurements show the occurrence of spontaneous strains along all three lattice constants and yield high resolution measurements of the temperature dependence of the incommensurate structural distortion. The sizes of the spontaneous strains in the incommensurate phase are significantly reduced, but both their anisotropy as well as their temperature dependencies are very similar to those in zero field.Comment: 12 pages (Latex), 4 Figs. (PS), to appear in Phys. Rev. B54 (Vol.21

Universality in one dimensional orbital wave ordering in spinel and related compounds: an experimental perspective

Recent state-of-the-art crystallographic investigations of transition metal spinel compounds have revealed that the d- orbital charge carriers undergo ordering transitions with the formation of local "molecular bonding" units such as dimers in MgTi2O4, octomers in CuIr2S4, and heptamers in AlV2O4. Herein, we provide a unifying scheme involving one- dimensional orbital wave ordering applicable to all of these spinels. The relative phase of the orbitals in the chains is shown to be crucial to the formation of different local units, and thus both the amplitude and phase of the orbital wave play important roles. Examination of Horibe et al.'s [1] structure for AlV2O4 serves as the vehicle for developing the general behavior for such orbital wave ordering. Ordered AlV2O4 will be seen to organize into three equivalent chains in 2D Kagome planes coupled so as to form units of three dimer bonds. Three additional equivalent chains manifest a more complex tetramerization with three different charge states and two different bonding schemes. The orbital wave ordering scheme developed is extended to other spinel and related compounds with local triangular transition metal coordination and partial filling of the t2g-d orbitals

Spin-phonon coupled modes in the incommensurate phases of doped CuGeO3_{3}

The doping effect of the folded phonon mode at 98 cm$^{-1}$ was investigated on the Si-doped CuGeO$_3$ by magneto-optical measurements in far-infrared (FIR) region under high magnetic field. The folded phonon mode at 98 cm$^{-1}$ appears not only in the dimerized (D) phase but also in the dimerized-anitiferromagnetic (DAF) phase on the doped CuGeO$_3$. The splitting was observed in the incommensurate (IC) phase and the antiferromagnetically ordered incommensurate (IAF) phase above $H_C$. The split-off branches exhibit different field dependence from that of the pure CuGeO$_3$ in the vicinity of $H_C$, and the discrepancy in the IAF phase is larger than that in the IC phase. It is caused by the interaction between the solitons and the impurities.Comment: 7 pages, 4 figures, resubmitted to Phys. Rev.

Charge Ordering and Phase Competition in the Layered Perovskite Lasr2mn2o7

Charge-lattice fluctuations are observed in the layered perovskite manganite LaSr2Mn2O7 by Raman spectroscopy as high as 340 K and with decreasing temperature they become static and form a charge ordered (CO) phase below TCO=210 K. In the static regime, superlattice reflections are observed through neutron and x-ray diffraction with a propagation vector (h+1/4,k-1/4,l). Crystallographic analysis of the CO state demonstrates that the degree of charge and orbital ordering in this manganite is weaker than the charge ordering in three dimensional perovskite manganites. A TN=170K a type-A antiferromagnetism (AF) develops and competes with the charge ordering, that eventually melts below T*=100K. High resolution diffraction measurements suggest that that CO- and AF-states do not coincide within the same region in the material but rather co-exist as separate phases. The transition to type-A antiferromagnetism at lower temperatures is characterized by the competition between these two phases.Comment: 9 pages, 6 figure