Masatomo Uehara

Bio: Masatomo Uehara is an academic researcher from Yokohama National University. The author has contributed to research in topics: Magnetization & Superconductivity. The author has an hindex of 26, co-authored 118 publications receiving 3971 citations. Previous affiliations of Masatomo Uehara include Rutgers University & College of Industrial Technology.

Percolative phase separation underlies colossal magnetoresistance in mixed-valent manganites

Abstract: Colossal magnetoresistance1—an unusually large change of resistivity observed in certain materials following application of magnetic field—has been extensively researched in ferromagnetic perovskite manganites. But it remains unclear why the magnetoresistive response increases dramatically when the Curie temperature (T C) is reduced. In these materials, T C varies sensitively with changing chemical pressure; this can be achieved by introducing trivalent rare-earth ions of differing size into the perovskite structure2,3,4, without affecting the valency of the Mn ions. The chemical pressure modifies local structural parameters such as the Mn–O bond distance and Mn–O–Mn bond angle, which directly influence the case of electron hopping between Mn ions (that is, the electronic bandwidth). But these effects cannot satisfactorily explain the dependence of magnetoresistance on T C. Here we demonstrate, using electron microscopy data, that the prototypical (La,Pr,Ca)MnO3 system is electronically phase-separated into a sub-micrometre-scale mixture of insulating regions (with a particular type of charge-ordering) and metallic, ferromagnetic domains. We find that the colossal magnetoresistive effect in low-T C systems can be explained by percolative transport through the ferromagnetic domains; this depends sensitively on the relative spin orientation of adjacent ferromagnetic domains which can be controlled by applied magnetic fields.

Superconductivity in the Ladder Material Sr0.4Ca13.6Cu24O41.84

Abstract: We have observed superconductivity in the ladder material Sr 0.4 Ca 13.6 Cu 24 O 41.84 under pressures of 3 GPa and 4.5 GPa by means of electrical measurements. The superconducting transition temperatures T c (onset) are 12 K and 9 K at 3 and 4.5 GPa, respectively. The superconducting volume fraction was obtained to be about 5% from magnetization measurement under 3.5 GPa at 4.2 K, indicating the bulk nature of the superconductivity in this system.

Large room-temperature intergrain magnetoresistance in double perovskite srfe1- x(mo or re)xo3

Abstract: We report significant intergrain magnetoresistance (IMR) in polycrystalline double perovskites of SrFe1−x(Mo or Re)xO3 at room temperature. Systematics of the temperature dependence of IMR indicates that the observed large room-temperature IMR in SrFe1/2Mo1/2O3 originates from the ferrimagnetic nature of insulating grain boundaries as well as the half-metallicity of this perovskite. Our results indicate that a new avenue for spin-polarized tunneling junctions is to utilize insulating interface layers with ferromagnetic or ferrimagnetic coupling.

Thermal and Electronic Transport Properties and Two-Phase Mixtures in La 5 / 8 − x Pr x Ca 3 / 8 MnO 3

Abstract: We measured thermal conductivity $\ensuremath{\kappa}$, thermoelectric power $S$, and electric conductivity $\ensuremath{\sigma}$ of ${\mathrm{La}}_{5/8\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Ca}}_{3/8}{\mathrm{MnO}}_{3}$, showing an intricate interplay between metallic ferromagnetism (FM) and charge ordering (CO) instability. The change of $\ensuremath{\kappa}$, $S$, and $\ensuremath{\sigma}$ with temperature $(T)$ and $x$ agrees well with the effective medium theories for binary metal-insulator mixtures. This agreement clearly demonstrates that with the variation of $T$ as well as $x$, the relative volumes of FM and CO phases drastically change and percolative metal-insulator transition occurs in the mixture of FM and CO domains.

Pressure-induced dimensional crossover and superconductivity in the hole-doped two-leg ladder compound Sr14-xCaxCu24O41

Abstract: Anisotropic electrical resistivity under high pressure was measured for a single crystal of ${\mathrm{Sr}}_{2.5}{\mathrm{Ca}}_{11.5}\ensuremath{-}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$, a hole-doped two-leg ladder compound. Superconductivity was observed between 3.5 and 8 GPa, accompanied by metallic resistivity between the ladders, which indicates semiconducting behavior at ambient pressure. This in turn strongly suggests that the application of pressure brings about a dimensional crossover from one to two, and that superconductivity in this system is a consequence of an insulator to superconductor transition in the anisotropic two-dimensional system.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Electroluminescence from single monolayers of nanocrystals in molecular organic devices

Abstract: The integration of organic and inorganic materials at the nanometre scale into hybrid optoelectronic structures enables active devices that combine the diversity of organic materials with the high-performance electronic and optical properties of inorganic nanocrystals. The optimization of such hybrid devices ultimately depends upon the precise positioning of the functionally distinct materials. Previous studies have already emphasized that this is a challenge, owing to the lack of well-developed nanometre-scale fabrication techniques. Here we demonstrate a hybrid light-emitting diode (LED) that combines the ease of processability of organic materials with the narrow-band, efficient luminescence of colloidal quantum dots (QDs). To isolate the luminescence processes from charge conduction, we fabricate a quantum-dot LED (QD-LED) that contains only a single monolayer of QDs, sandwiched between two organic thin films. This is achieved by a method that uses material phase segregation between the QD aliphatic capping groups and the aromatic organic materials. In our devices, where QDs function exclusively as lumophores, we observe a 25-fold improvement in luminescence efficiency (1.6 cd A(-1) at 2,000 cd m(-2)) over the best previous QD-LED results. The reproducibility and precision of our phase-segregation approach suggests that this technique could be widely applicable to the fabrication of other hybrid organic/inorganic devices.

Complexity in Strongly Correlated Electronic Systems

Abstract: A wide variety of experimental results and theoretical investigations in recent years have convincingly demonstrated that several transition metal oxides and other materials have dominant states that are not spatially homogeneous. This occurs in cases in which several physical interactions-spin, charge, lattice, and/or orbital-are simultaneously active. This phenomenon causes interesting effects, such as colossal magnetoresistance, and it also appears crucial to understand the high-temperature superconductors. The spontaneous emergence of electronic nanometer-scale structures in transition metal oxides, and the existence of many competing states, are properties often associated with complex matter where nonlinearities dominate, such as soft materials and biological systems. This electronic complexity could have potential consequences for applications of correlated electronic materials, because not only charge (semiconducting electronic), or charge and spin (spintronics) are of relevance, but in addition the lattice and orbital degrees of freedom are active, leading to giant responses to small perturbations. Moreover, several metallic and insulating phases compete, increasing the potential for novel behavior.