摘要 :
Self-organization of charged particles on a two-dimensional lattice, subject to an anisotropic Jahn-Teller-type interaction and three-dimensional Coulomb repulsion, is investigated. In the mean-field approximation without Coulomb...
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Self-organization of charged particles on a two-dimensional lattice, subject to an anisotropic Jahn-Teller-type interaction and three-dimensional Coulomb repulsion, is investigated. In the mean-field approximation without Coulomb interaction, the system displays a phase transition of first order. In the presence of the Coulomb repulsion, the global phase separation becomes unfavorable and the system shows a mesoscopic phase separation, where the size of the charged regions is determined by the competition between the ordering energy and the Coulomb energy. The phase diagram of the system as a function of particle density and temperature is obtained by systematic Monte Carlo simulations. With decreasing temperature, a crossover from a disordered state to a state composed of mesoscopic charged clusters is observed. In the phase separated state, charged clusters with even number of particles are more stable than those with odd number of particles in a large range of particle densities. With increasing particle density at low temperatures, a series of crossovers between states with different cluster sizes is observed. Above half-filling, in addition to the low temperature clustering, another higher temperature scale, which corresponds to orbital ordering of particles, appears. We suggest that the diverse functional behavior observed in transition metal oxides can be thought to arise from the self-organization of this type.
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摘要 :
Magnetic properties of high purity stoichiometric La_2CuO_4 nanoparticles are systematically investigated as a function of particle size. Ferromagnetic single-domain spin clusters are shown to spontaneously form at the surface of ...
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Magnetic properties of high purity stoichiometric La_2CuO_4 nanoparticles are systematically investigated as a function of particle size. Ferromagnetic single-domain spin clusters are shown to spontaneously form at the surface of fine grains as well as paramagnetic defects. Hysteresis loops and thermomagnetic irreversibility are observed in a wide temperature range 5-350 K with the remnant moment and coercivity gradually decreasing with increasing temperature. Possible origins of the spontaneous surface ferromagnetic clusters and the relation of our data to the appearance of unusual magnetic phenomena and phase separation of doped cuprates are discussed.
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摘要 :
Optical reflectivity and absorbance measurements of oriented Mo_6S_3I_6 nanowire thin films and dispersions in different solvents are presented extending from the far infrared to the ultraviolet. In spite of the highly one-dimensi...
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Optical reflectivity and absorbance measurements of oriented Mo_6S_3I_6 nanowire thin films and dispersions in different solvents are presented extending from the far infrared to the ultraviolet. In spite of the highly one-dimensional character of the nanowire material and narrow electronic valence and conduction subbands, as predicted by the density-functional theory calculations, sharp Van Hove features in the optical absorption spectra are not observed, partly because of the large density of interpenetrating electron subbands and partly due to damping and disorder. The optically measured electrical conductivity extrapolated to zero frequency σ_1(ω→0) and the calculated conductivity are significantly higher than the typical dc value from resistance measurements, indicating that disorder limits electron transport, a feature characteristic of strongly one-dimensional systems.
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The coupling of phonons with collective modes and single-particle gap excitations associated with one-(1d) and two-directional (2d) electronically driven charge-density wave (CDW) ordering in metallic RTe_3 is investigated as a fu...
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The coupling of phonons with collective modes and single-particle gap excitations associated with one-(1d) and two-directional (2d) electronically driven charge-density wave (CDW) ordering in metallic RTe_3 is investigated as a function of rare-earth ion chemical pressure (R = Tb, Dy, Ho) using femtosecond pump-probe spectroscopy. From the T dependence of the CDW gap △_(CDW) and the amplitude mode, we find that while the transition to a 1d-CDW ordered state at T_(c1) initially proceeds in an exemplary mean-field-like fashion, below T_(c1), A_(CDW) is depressed and departs from the mean-field behavior. The effect is apparently triggered by resonant mode mixing of the amplitude mode with a totally symmetric phonon at 1.75 THz. At low temperatures, when the state evolves into a 2d-CDW ordered state at T_(c2) in the DyTe_3 and HoTe_3, additional much weaker mode mixing is evident but no soft mode is observed.
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摘要 :
Bulk magnetization in Pr_(0.6)Ca_(0.4)MnO_3 thin films with tensile (SrTiO_3) and compressive (LaAlO_3) substrate-induced strain is compared to the magneto-optical Kerr effect (MOKE) measurements. In the absence of an external mag...
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Bulk magnetization in Pr_(0.6)Ca_(0.4)MnO_3 thin films with tensile (SrTiO_3) and compressive (LaAlO_3) substrate-induced strain is compared to the magneto-optical Kerr effect (MOKE) measurements. In the absence of an external magnetic field, in both films, a stable ferromagnetic insulating majority phase coexists with an antiferromagnetic insulating phase below ～120 K. The MOKE measurements indicate that at 5 K a metastable ferromagnetic metallic (FM) phase is formed at the surface of the stretched film in a magnetic field below 1.1 T already; while in the bulk the FM phase starts to form in the field above ～4 T in both films.
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