Research Interests

Currently, our laboratory is actively pursuing the following research and development themes:

Titanium oxides

　Titanium oxide has the smallest atomic number among electrically conductive transition-metal oxides and exhibits all electronic phases from superconductivity (TiO) to insulator (TiO2). In addition, it is a treasure trove of phase transitions, including metal-insulator transitions in Ti2O3 and Ti4O7, and photoinduced phase transitions in Ti3O5.

Because Ti is light and has a low tendency to ionize, titanium oxide is a system with strong electron-lattice interactions. By utilizing the lattice distortion caused by epitaxial thin films, it is thought that further control of the diverse electronic properties may be possible. Both Ti and O are elements with low Clarke numbers, making raw material costs low, and they can be said to be materials that have opened the way to practical application through the development of their functions.

Titanium oxides as electronic materials

Neuromorphic device utilizing metal-insulator transition of transition metal oxide

[P. Stoliar et al., Adv. Funct. Mater. 27, 1604740 (2017).]

　The information society has developed in tandem with silicon semiconductor electronics. However, due to the limitations of transistor miniaturization, there is a need to develop electronic devices with new operating principles.

By utilizing the phase transitions exhibited by transition metal oxides, novel electronic devices that would be impossible to realize with conventional semiconductors have been proposed. Among titanium oxides, Ti2O3 exhibits a metal-insulator transition at ~450 K, and Ti3O5 exhibits a phase transition between crystalline polymorphs when exposed to pulsed visible light at room temperature.

We are conducting research into electronic devices that utilize these phase transitions in titanium oxides, such as phase transition memory and brain-like memory.

Titanium oxides as energy and environmental materials

Water purification by photothermal conversion using Ti2O3 with ultra-small band gap

　Titanium dioxide (TiO2) is an environmental and energy material that has been put to practical use in photocatalysts [1] and dye-sensitized solar cells [2]. Taking advantage of its safe and inexpensive material properties, research into the application of other titanium oxides to the environmental and energy fields is also underway. Specifically, photothermal conversion [3] utilizing the extremely small band gap of Ti2O3 and heat storage devices utilizing the pressure-induced phase transition of Ti3O5 [4] have been proposed. Our laboratory is utilizing titanium oxide thin film technology to advance the fabrication of environmental and energy devices.

[1] TOTO: hydrotect　　[2] RICOH: Solid-state dye-sensitized solar cells

[3] J. Wang et al., Adv. Mater. 29, 1603730 (2016).　[4] S. Ohkoshi et al., Nat. Chem. 2, 539 (2010).

Titanium oxides as material chmeistry in a thin-film form

Ellingham diagram of Ti-O with the region of thin-film synthesis

In addition to the integer valences of 0, +2, +3, and +4, Ti is stable in non-integer valence states between +3 and +4. Additionally, Ti4+ oxides exhibit polymorphisms, including rutile, anatase, and brookite. In bulk synthesis following thermodynamic equilibrium, titanium oxides with various Ti:O composition ratios can be stably formed according to the Ellingham diagram (left).

On the other hand, the temperature and pressure region that can be achieved in thin films is limited to the upper right corner of the Ellingham diagram, and only titanium dioxide (TiO2) with a valence of +4 can be synthesized under thermodynamic equilibrium.

Therefore, we are working on the synthesis of titanium oxide thin films with various Ti:O composition ratios and the development of technology to control the crystal polymorphism using pulsed laser deposition, which has a strong kinetic equilibrium.

Original papers

K. Yoshimatsu et al., “Temperature-induced phase transition of λ-Ti3O5 films” Phys. Rev. Mater. 8, 035002 (2024). Editor’s suggestion
K. Yoshimatsu et al., “Direct synthesis of metastable λ-phase Ti3O5 film on LaAlO3 (110) substrates under high temperatures” Cryst. Growth Des. 22, 703 (2022).
K. Yoshimatsu et al., “Evidence of lattice deformation induced metal-insulator transition in Ti2O3” Phys. Rev. B 106, L081110 (2022).
N. Hasegawa, K. Yoshimatsu et al., “Direct observation of band structure for Ti2O3 thin films by soft X-ray angle resolved photoemission spectroscopy” Phys. Rev. B 105, 235137 (2022).
T. Soma, K. Yoshimatsu et al., “Superconducting dome underlying bipolaronic insulating state in charge-doped Ti4O7 epitaxial films” J. Phys. Soc. Jpn. 90, 023705 (2021).
J. Mizushiro, K. Yoshimatsu et al., “Optical and structural investigations on titanium oxynitride films for visible-UV photocatalytic applications” J. Appl. Phys. 127, 135301 (2020).
K. Yoshimatsu et al., “Metallic ground states of undoped Ti2O3 films induced by elongated c-axis lattice constant” Scientific Reports 10, 22109 (2020).
S. Sekiguchi K. Yoshimatsu et al., “High-pressure study of superconductivity in Ti4O7 film” J. Phys. Soc. Jpn. 88, 035001 (2019).
K. Yoshimatsu et al., “Large anisotropy in conductivity of Ti2O3 films” APL Mater. 6, 101101 (2018).
K. Yoshimatsu et al., “Superconductivity in Ti4O7 and γ-Ti3O5 films” Scientific Reports 7, 12544 (2017).
H. Kurokawa, K. Yoshimatsu et al., “Effects of phase fraction on superconductivity of low-valence eutectic titanate films” J. Appl. Phys. 122, 055302 (2017).

Electronic and structural analyzes using synchrotron light source

Band dispersion of SrVO3 quantum wells measured by synchrotron radiation ARPES.

K. Yoshimatsu et al., Science 333, 319 (2011).

　By using high-brilliance, high-resolution synchrotron radiation, we can reveal the true crystal structure and electronic state that cannot be observed with laboratory light sources. Using the conductive oxide SrVO3, we have observed phenomena unique to the quantized state of strongly correlated electrons.
Using the synchrotron radiation facilities at KEK-PF and SPring-8, we are conducting photoelectron spectroscopy, X-ray absorption spectroscopy, synchrotron radiation X-ray diffraction, and X-ray magnetic circular dichroism to analyze the electrical and magnetic properties and structure of synthesized thin film samples. Clarifying the properties of new materials is a fundamental and most important research theme in materials science. Furthermore, measuring the samples we have created ourselves also leads to feedback on sample synthesis.

Original papers (KEK-PF)

BL-2 (Angle-integrated PES, Soft x-ray ARPES and XAS)

N. Hasegawa, K. Yoshimatsu et al., “Direct observation of band structure for Ti2O3 thin films by soft X-ray angle resolved photoemission spectroscopy” Phys. Rev. B 105, 235137 (2022).
K. Yoshimatsu et al., “Large anisotropy in conductivity of Ti2O3 films” APL Mater. 6, 101101 (2018).
K. Yoshimatsu et al., “Electronic structures and photoanodic properties of ilmenite-type MTiO3 epitaxial films (M = Mn, Fe, Co, Ni)” J. Phys. Chem. C 121, 18717 (2017).
K. Yoshimatsu et al., “Direct growth of metallic TiH2 thin films by pulsed laser deposition” Appl. Phys. Express 8, 035801 (2015).
K. Yoshimatsu et al., “Dimensional-crossover-driven metal-insulator transition in SrVO3 ultrathin films” Phys. Rev. Lett. 104, 147601 (2010).
K. Yoshimatsu et al., "Origin of metallic states at the heterointerface between the band insulators LaAlO3 and SrTiO3" Phys. Rev. Lett. 101, 026802 (2008).

BL-4C (Four-circle XRD)

K. Yoshimatsu et al., “Temperature-induced phase transition of λ-Ti3O5 films” Phys. Rev. Mater. 8, 035002 (2024). Editor’s suggestion

BL-16 (XMCD)

K. Yoshimatsu et al., “Magnetic and electronic properties of B-site-ordered double-perovskite oxide La2CrMnO6 thin films” Phys. Rev. B. 99, 235129 (2019).
G. Shibata, K. Yoshimatsu et al., “Anisotropic spin-density distribution and magnetic anisotropy of strained La1–xSrxMnO3 thin films: angle-dependent x-ray magnetic circular dichroism” npj Quantum Mater. 3, 3 (2018).
K. Watarai, K. Yoshimatsu et al., “Epitaxial synthesis and physical properties of double-perovskite oxide Sr2CoRuO6 thin films” J. Phys. Condens.: Matter 28, 436005 (2016).
K. Yoshimatsu et al., “Spectroscopic studies on the electronic and magnetic states of Co-doped perovskite manganite Pr0.8Ca0.2Mn1–yCoyO3 thin films” Phys. Rev. B 88, 174423 (2013).

BL-28 (VUV ARPES)

K. Yoshimatsu et al., “Determination of the surface and interface phase shifts in metallic quantum well structures of perovskite oxides” Phys. Rev. B 88, 115308 (2013).
K. Yoshimatsu et al., “Metallic quantum well states in artificial structures of strongly correlated oxide” Science 333, 319 (2011).

Original papers (SPring-8)

BL15XU (Four-circle XRD)

K. Yoshimatsu et al., “Superconductivity in Ti4O7 and γ-Ti3O5 films” Scientific Reports 7, 12544 (2017).
K. Yoshimatsu et al., “Synthesis and magnetic properties of double-perovskite oxide La2MnFeO6 thin films” Phys. Rev. B 91, 054421 (2015).

BL15XU (HAXPES)

K. Yoshimatsu et al., “Strain-induced metal-insulator transition in t2g electron system of perovskite titanate Sm0.5Ca0.5TiO3 films” Phys. Rev. B 93, 195159 (2016).

BL47XU (HAXPES)

E. Sakai, K. Yoshimatsu et al., "Competition between instabilities of Peierls transition and Mott transition in W-doped VO2 thin films" Phys. Rev. B 84, 195132 (2011).
K. Yoshimatsu et al., “Thickness dependent electronic structure of La0.6Sr0.4MnO3 layer in SrTiO3/La0.6Sr0.4MnO3/SrTiO3 heterostructures studied by hard x–ray photoemission spectroscopy” Appl. Phys. Lett. 94, 071901 (2009).

Electronic-structure calculations using density functional theory

　The use of theoretical calculations can lead to a deeper understanding of experimental results obtained from physical property measurements and synchrotron radiation measurements. We are performing electronic state calculations using density functional theory with Quantum ESPRESSO to clarify the origin of band dispersion and metal-insulator transition in Ti2O3.

Partial Ti 3d DOS of Ti2O3 calculated based on DFT

Original paper

K. Yoshimatsu et al., “Evidence of lattice deformation induced metal-insulator transition in Ti2O3” Phys. Rev. B 106, L081110 (2022).
N. Hasegawa, K. Yoshimatsu et al., “Direct observation of band structure for Ti2O3 thin films by soft X-ray angle resolved photoemission spectroscopy” Phys. Rev. B 105, 235137 (2022).
K. Yoshimatsu et al., “Metallic ground states of undoped Ti2O3 films induced by elongated c-axis lattice constant” Scientific Reports 10, 22109 (2020).

Development of original experimental equipment

3D CAD images of PLD load lock and two AC Hall-effect measurement system

By independently developing synthesis and measurement equipment, one can conduct truly original research. In addition, equipment development leads to understanding of principles and the acquisition of peripheral knowledge. It also leads to a reduction in research costs.

By now our laboratory has constructed

and others. We continue to construct several experimental equipments now.

Development of LabVIEW program for controlling original equipment

PLD control program using LabVIEW software

We are developing GUI-based control and measurement programs using National Instruments' LabVIEW. To date, we have developed:

A VISA-based program for measuring the temperature dependence of resistivity and the Hall effect using instrument control
A PLD device program using RS-485/Modbus to control stepping motors and temperature controllers
An RHEED analysis program using a USB camera and NI Vision

Developing our own software prevents control and measurement from becoming a black box, which also leads to saving research cost.