11/13 Development of ferrite-based electronic-phase-change devices Tanaka lab. Tatsuya Hori

What is Mott insulator ? Hubbard model As U increases Localization (U) vs Delocalization (t) U > t insulator with immobile electrons U : Coulomb repulsion t : Transfer integral

External stimuli (T, H, E, n) Electrons : InsulatorMetal Electronic phase change New promising principles to create devices H 2 O :

Electronic-phase-change devices However, device operations have so far been limited at low temperature (<<RT) ! Fe 3 O 4 : S. Lee et al., Nature Mater., 7, 130 (2008). PCMO: A. Asamitsu et al., Nature, 388, 50 (1997). Pr 0.7 Ca 0.3 MnO 3 Fe 3 O 4

A candidate material : layered ferrite N. Kimizuka et al., Solid State Commun., 15, 1321 (1974). 吉井 et al., 日本結晶学会誌, 51, 162 (2009). RE = Y, Dy…Yb, Lu LuFe 2 O 4 CO3D2D No order ~320 K ~500 K RE Fe O RE/O layer Fe/O bilayer Fe 2.5+ (Fe 3+ :Fe 2+ =1:1) ? + Interaction Charge-ordered (immobilized) state at room temperature

Electric-field-induced current switching L. J. Zeng et al., EPL 84, (2008). Electronic phase change Current switching Bulk crystal

Charge-ordered LuFe 2 O 4 films Out-of-plane 2  /  scan (003)(006)(009) * LuFe 2 O 4 Grown by pulsed-laser deposition c-axis oriented growth Thermaly activated conduction

Current switching in the thin-film structure V sample 3D 2D 310K 300K LuFe 2 O 4 A significant step to device applications K. Fujiwara, T. Hori, H. Tanaka, J. Phys. D: Appl.Phys., 46, (2013).

Summary : current switching function ・ Fabricated charge-ordered LuFe 2 O 4 and YbFe 2 O 4 thin films. ・ Induced the current switching effect in the thin film structure.

Current work What’s my next target ? Another way to control the electronic phases by electric fields. It’s electrostatic carrier doping. E

What is electrostatic doping ? Gate Source Drain Substrate Gate insulator Insulator Metal Mott insulator External control of the number of charge carriers and resulting electronic states (order vs disorder) VGVG

Collapse of CO by chemical carrier doping LuFe 2 O 4 Lu(Fe 1.85 Mg 0.15 )O 4 Superlattice reflections Y. B. Qin et al., J. Phys. Condens. Mater. 21, (2009). Very sensitive to external carrier doping ! Fe +2.5 (Fe +2 : Fe +3 = 1 : 1) Fe   Mg +2

Key component : gate insulator Q = CV C =  r  0 S/d n 2D = Q/S =  r  0 V/d Gate Source Drain Substrate Gate insulator

Mott-transition needs large n 2D at 0 K C. H. Ahn et al., Nature 424, 1018 (2003).

Ionic liquid – giant carrier accumulation capability M. Nakano et al., Nature 487, 459 (2012). Suitable for Mott-insulator systems ・ High n 2D ~ 10 13~15 /cm 2 ・ No structural mismatching VO 2

Toward purely electronic Mott-transistors Structural phase transition NdNiO 3 : MIT control (Charge-Disproportionation) S. Asanuma et al., APL 97, (2010). Structural phase transition MIT Pursue and investigate MIT in the system without struct. phase trans.

Device structure YSZ(111) Substrate Separator SiO 2 DSG Ionic liquid* REFe 2 O 4 A A IDID VDVD IGIG VGVG Cation Anion *N,N-Diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide

Ionic liquid* Fabrication processes Masking process Electrode attaching Milling process Separator attaching Thin film fabrication Masking process Dropping ionic liquid YSZ(111) Substrate DS G REFe 2 O 4 Separator SiO 2

Summary Through fabricating and evaluating Mott-transistors with REFe 2 O 4 ・ Reveal the nature of the field effect in Mott insulator systems.