Daisuke Okuyama

Collective bulk carrier delocalization driven by electrostatic surface charge accumulation

In the classic transistor, the number of electric charge carriers—and thus the electrical conductivity—is precisely controlled by external voltage, providing electrical switching capability. This simple but powerful feature is essential for information processing technology, and also provides a platform for fundamental physics research. As the number of charges essentially determines the electronic phase of a condensed-matter system, transistor operation enables reversible and isothermal changes in the system’s state, as successfully demonstrated in electric-field-induced ferromagnetism and superconductivity. However, this effect of the electric field is limited to a channel thickness of nanometres or less, owing to the presence of Thomas–Fermi screening. Here we show that this conventional picture does not apply to a class of materials characterized by inherent collective interactions between electrons and the crystal lattice. We prepared metal–insulator–semiconductor field-effect transistors based on vanadium dioxide—a strongly correlated material with a thermally driven, first-order metal–insulator transition well above room temperature—and found that electrostatic charging at a surface drives all the previously localized charge carriers in the bulk material into motion, leading to the emergence of a three-dimensional metallic ground state. This non-local switching of the electronic state is achieved by applying a voltage of only about one volt. In a voltage-sweep measurement, the first-order nature of the metal–insulator transition provides a non-volatile memory effect, which is operable at room temperature. Our results demonstrate a conceptually new field-effect device, extending the concept of electric-field control to macroscopic phase control.

Versatile helimagnetic phases under magnetic fields in cubic perovskite SrFeO 3

A helical spin texture is of great current interest for a host of novel spin-dependent transport phenomena. We report a rich variety of nontrivial helimagnetic phases in the simple cubic perovskite

Epitaxial-strain effect on charge/orbital order in Pr0.5Ca0.5MnO3 films

{\mathrm{SrFeO}}_{3}

Magnetically-Driven Ferroelectric Atomic Displacements in perovskite like YMnO3

under magnetic fields up to 42 T. Magnetic and resistivity measurements revealed that the proper-screw spin phase proposed for

Electron doping in the cubic perovskiteSrMnO3: Isotropic metal versus chainlike ordering of Jahn-Teller polarons

{\mathrm{SrFeO}}_{3}

Gate-tunable gigantic lattice deformation in VO2

can be subdivided into at least five kinds of ordered phases. Near the multicritical point, an unconventional anomalous Hall effect was found to show up and was interpreted as due to a possible long-period noncoplanar spin texture with scalar spin chirality.

Non-Collinear Magnetic Structure of TbB4

Effect of growth orientation on charge- and orbital-ordering (CO-OO) phenomena has been studied for Pr0.5Ca0.5MnO3 epitaxial thin films fabricated on (LaAlO3)0.3-(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates by means of resistivity, synchrotron x-ray diffraction, and polarized optical microscopy measurements. CO-OO transition is observed around 220 K for a film grown on an LSAT (011) substrate ((011)-film), similarly to a bulk sample, while a film grown on a (001) plane of LSAT ((001)-film) shows much higher transition temperature around 300 K. The domain size of OO is approximately 3 times as large in the (011)-film as in the (001)-film. These results demonstrate that various properties of CO-OO phenomena can be controlled with the growth orientation via the epitaxial strain from the substrate.

All-in-all-out magnetic domains: x-ray diffraction imaging and magnetic field control.

by a single-crystal synchrotron x-ray diffraction. Therefined polar structure shows the characteristic bond alternation driven by the exchange strictionin staggered Mn-O-Mn arrays with ↑↑↓↓ type ordering, giving rise to a spontaneous polarizationalong a-axis. First-principles calculations based on the Berry phase method as well as on theexperimentally refined crystal structure can reproduce the observed polarization value.

Magnetically Tunable Metal–Insulator Superlattices

Single crystals of electron-doped SrMnO3 with a cubic perovskite structure have been systematically investigated as the most canonical (orbital-degenerate) double-exchange system, whose ground states have been still theoretically controversial. With only 1-2% electron doping by Ce substitution for Sr, a G-type antiferromagnetic metal with a tiny spin canting in a cubic lattice shows up as the ground state, where the Jahn-Teller polarons with heavy mass are likely to form. Further electron doping above 4%, however, replaces this isotropic metal with an insulator with tetragonal lattice distortion, accompanied by a quasi-one-dimensional 3z^2-r^2 orbital ordering with the C-type antiferromagnetism. The self-organization of such dilute polarons may reflect the critical role of the cooperative Jahn-Teller effect that is most effective in the originally cubic system.

X-ray anomalous scattering of diluted magnetic oxide semiconductors : Possible evidence of lattice deformation for high temperature ferromagnetism

We examined the impact of electric field on crystal lattice of vanadium dioxide (VO2) in a field-effect transistor geometry by in-situ synchrotron x-ray diffraction measurements. Whereas the c-axis lattice parameter of VO2 decreases through the thermally induced insulator-to-metal phase transition, the gate-induced metallization was found to result in a significant increase of the c-axis length by almost 1% from that of the thermally stabilized insulating state. We also found that this gate-induced gigantic lattice deformation occurs even at the thermally stabilized metallic state, enabling dynamic control of c-axis lattice parameter by more than 1% at room temperature.