Tokyo Institute of Technology · Materials Science
Professor Takayoshi Katase's research lab specializes in the development and fundamental study of oxide-based superconducting thin films and functional oxides, with a focus on iron-based superconductors such as Co-doped BaFe₂As₂. The lab explores epitaxial film growth on textured substrates to achieve high critical current densities and engineer quantum devices like Josephson junctions and dc-SQUIDs. A key research direction involves manipulating electronic properties through ion doping (e.g., La, Co) and protonic gating, enabling metal-insulator transitions and electrochromic switching in oxides like WO₃ and VO₂. The lab also pioneers novel three-terminal, solid-state oxide devices for smart window and energy-efficient optoelectronic applications.
Figures are computed from collected data and may differ slightly.
High critical current densities (Jc)>1 MA/cm2 were realized in cobalt-doped BaFe2As2 (BaFe2As2:Co) films on flexible metal substrates with biaxially textured MgO base-layers fabricated by an ion-beam assisted deposition technique. The BaFe2As2:Co films showed small in-plane crystalline misorientations (ΔϕBaFe2As2:Co) of ∼3° regardless of twice larger misorientations of the MgO base-layers (ΔϕMgO=7.3°), and exhibited high self-field Jc up to 3.5 MA/cm2 at 2 K. These values are comparable t
Josephson junctions were fabricated in epitaxial films of cobalt-doped BaFe2As2 on [001]-tilt (La,Sr)(Al,Ta)O3 bicrystal substrates. 10-μm-wide microbridges spanning a 30°-tilted bicrystal grain boundary (BGB bridge) exhibited resistively-shunted-junction (RSJ)-like current–voltage characteristics up to 17 K, and the critical current was suppressed remarkably by a magnetic field. Microbridges without a BGB did not show the RSJ-like behavior, and their critical current densities were 20 times lar
DC superconducting quantum interference devices (dc-SQUIDs) were fabricated in Co-doped BaFe2As2 epitaxial films on (La, Sr)(Al, Ta)O3 bicrystal substrates with 30deg misorientation angles. The 18 x 8 micro-meter^2 SQUID loop with an estimated inductance of 13 pH contained two 3 micro-meter wide grain boundary junctions. The voltage-flux characteristics clearly exhibited periodic modulations with deltaV = 1.4 micro-volt at 14 K, while the intrinsic flux noise of dc-SQUIDs was 7.8 x 10^-5 fai0/Hz
Proton and hydroxyl ion play an essential role for tuning functionality of oxides because their electronic state can be controlled by modifying oxygen off-stoichiometry and/or protonation. Tungsten trioxide (WO3), a well-known electrochromic (EC) material for smart window, is a wide bandgap insulator, whereas it becomes a metallic conductor HxWO3 by protonation. Although one can utilize electrochromism together with metal-insulator (MI) switching for one device, such EC-MI switching cannot be ut
Electron doping of a 122-type iron pnictide BaFe${}_{2}$As${}_{2}$ by substituting the Ba site with an aliovalent ion (indirect doping), which had been unsuccessful by conventional solid-state synthesis methods, was achieved by a nonequilibrium film growth process. The substitution with La was substantiated by a systematic shrinkage of the $c$-axis lattice parameter due to the smaller ionic radius of La${}^{3+}$ than that of Ba${}^{2+}$. A negative Hall coefficient indicated that the majority ca
Room-temperature protonation and metal–insulator conversion of a transition metal oxide (TMO) is demonstrated by all-solid-state thin-film transistor with vanadium dioxide using a water-infiltrated nanoporous glass as a solid electrolyte. This promising result can provide a novel route for TMO-based solid-state electro-optical devices, in particular, smart windows for on-demand infrared shielding. As a service to our authors and readers, this journal provides supporting information supplied by t
A(1-x)Fe(2-y)Se2 (A = K, Cs, Rb, Tl) are recently discovered iron-based superconductors with critical temperatures (Tc) ranging up to 32 K. Their parent phases have unique properties compared with other iron-based superconductors; e.g., their crystal structures include ordered Fe vacancies, their normal states are antiferromagnetic (AFM) insulating phases, and they have extremely high Néel transition temperatures. However, control of carrier doping into the parent AFM insulators has been difficu
Insulator-to-metal (MI) phase transition in vanadium dioxide $(\mathrm{V}{\mathrm{O}}_{2})$ thin films with controlled lattice distortion was investigated by thermopower measurements. $\mathrm{V}{\mathrm{O}}_{2}$ epitaxial films with different crystallographic orientations, grown on $(0001)\phantom{\rule{0.28em}{0ex}}\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}$, $(11\overline{2}0)\phantom{\rule{0.28em}{0ex}}\phantom{\rule{4pt}{0ex}}\ensuremath{\alpha}\text{
The electronic structure across the metal-insulator (MI) transition of electron-doped ${\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{W}}_{x}{\mathrm{O}}_{2}$ epitaxial films ($x=\phantom{\rule{0.16em}{0ex}}0\ensuremath{-}0.06$) grown on $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}$ substrates was studied by means of thermopower ($S$) measurements. Significant increase of $|S|$ values accompanied by MI transition was observed, and the transition temperatures of $S
Abstract The development of environmentally benign thermoelectric materials with high energy conversion efficiency ( ZT ) continues to be a long‐standing challenge. So far, high ZT has been achieved using heavy elements to reduce lattice thermal conductivity (κ lat ). However, it is not preferred to use such elements because of their environmental load and high material cost. Here a new approach utilizing hydride anion (H − ) substitution to oxide ion is proposed for ZT enhancement in thermoelec
Tin mono-selenide (SnSe) exhibits the world record of thermoelectric conversion efficiency ZT in the single crystal form, but the performance of polycrystalline SnSe is restricted by low electronic conductivity (σ) and high thermal conductivity (κ), compared to those of the single crystal. Here an effective strategy to achieve high σ and low κ simultaneously is reported on p-type polycrystalline SnSe with isovalent Te ion substitution. The nonequilibrium Sn(Se<sub>1-</sub> <sub>x</sub> Te<sub>x<
Abstract Dynamic control of thermal transport in solid materials is highly desired for thermal management technology. However, the development of a material exhibiting large modulation of thermal conductivity (κ) by external stimuli remains a major challenge. Here, the large κ modulation is reported by the reversible 3D to 2D crystal structure transition in a nonequilibrium solid solution of (Pb 1− x Sn x )Se, where Pb 2+ stabilizes a 3D cubic structure while Sn 2+ does a 2D layered structure. T
In order to understand the optoelectronic properties of amorphous niobium oxide (a-NbO x ), we have investigated the valence states, local structures, electrical resistivity, and optical absorption of a-NbO x thin films with various oxygen contents. It was found that the valence states of Nb ion in a-NbO x films can be controlled from 5+ to 4+ by reducing oxygen pressure during film deposition at room temperature, together with changing the oxide-ion arrangement around Nb ion from Nb2O5-like to
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