K. Miyano and N. Takubo RCAST, U. of Tokyo Bidirectional optical phase control between a charge-ordered insulator and a metal in manganite thin films What is it? motivation history of photoinduced transition sample development results future

What is optical bidirectional phase control? manganite thin film photoexcitation COOI: charge- and orbital-ordered insulating state FMM: ferromagnetic metallic state COOI ’  FMM  ’  COOI ’ …… pulseCWpulse ……

motivation to understand ‘colossal’ response in strongly-correlated electron systems => “inhomogeneity” is the keyword* in particular, the effect of photoexcitation => ‘high energy’ excitation, far from equilibrium, instantaneous, high density, time-resolved spectroscopy …etc. => a ‘novel phase’ not accessible with ‘low energy’ stimuli e.g., T, H, E, … * E.Dagotto, New J. Phys. 7, 67 (2005)

history1: material Tomioka et al. PRB53, R1689 (1996) P: paramagnetic F: ferromagnetic AF: antiferromagnetic I: insulator

2: discovery ↓ Pr 0.7 Ca 0.3 MnO 3 Laser pulse (5 ns) 100μ Time ( s) Resistance (Ω) Tomioka et al. J.Phys.Soc.Jpn. 64, 3626 (1995) ‘colossal effect’ AF COOI => FMM

3: problems not persistent = conducting state remains only while current is kept => phase transition? needs potential across electrodes at photoinduced transition => driven by current or photoexcitation? but WHY? local transition = IMT is 1 st -order with lattice distortion => stress from the surrounding COOI undo the transition? => make it small or thin

4: ‘film’ is not good enough Pr 0.5 Ca 0.5 Mn 0.96 Cr 0.04 O 3 /MgO(001) 20K, 633nm, 1mW/cm 2 7m7m photoinduced persistent conductivity MFM observation 80K film thickness H. Oshima, M. Nakamura, and K. Miyano, Phys. Rev. B63, (2001). H. Oshima et al., PRB 63, and (2001).

sample development need a thin film with a clear 1 st -order phase transition (IMT) i.e., forgive large lattice distortion => use (110) substrates (001) tetragonal symmetry conserved (110) shear deformation allowed Y. Ogimoto et al., Phys. Rev. B 71, (R) (2005) Y. Ogimoto et al., Appl. Phys. Lett. 86, (2005) M. Nakamura et al., Appl. Phys. Lett. 86, (2005).

tetragonal distortion predetermines the orbital = electronic states Z. Fang et al. PRL (2000) x=0.5 on (001) substrates: C-type AFM FMA-type AFM < c a ~ b ~ c > c

clear transition magnetic transport structural Nd 0.5 Sr 0.5 MnO 3 Pr 0.5 Sr 0.5 MnO 3 a: in-plane b,c: tilted Wakabayashi et al., cond-mat/

bicritical point Chaikin and Lubensky “Principles of condensed matter physics” Y.Tomioka and Y.Tokura, Phys.Rev.B 66,104416(2002) Pr 1-x (Ca 1-y Sr y ) x MnO 3 (Single Crystals)

bicritical point in thin film y=0.20 y=0.25 y=0.30 y=0.40 COO

phase diagrams Pr 0.55 (Ca 1-y Sr y ) 0.45 MnO 3 Y.Tomioka and Y.Tokura,Phys.Rev.B 66,104416(2002) bulk vs. thin films TCTC T CO ( y=0.25) 5 T 3 T 1 T 0 T 2 T 4 T

photoinduced phase transition (to lower T phase) YAG OPO Pulse Laser λ=637 nm 0.5 mJ/pulse rep rate 10 Hz ( y=0.25) Laser (100 pulses) T=77 K Laser stable, persistent, no assisting field

photoinduced phase transition: shot by shot single shot one shot is not enough multiple shots · each shot is stable · effect of shots are cumulative I (mJ/cm 2 ) R(  ) 1.95 eV threshold

photoinduced phase transition: dynamics need to destroy charge gap = triggered by collapse of COO nucleation and growth <= pumping rate percolation essential physics transition heating all-optical write-erase memory

future clear case of 1st-order phase transition involving charge + spin + lattice parameters: U + V + t (s,  + J + g + T + H +  study: nucleation and growth scaling time-resolved (pump and probe) dynamics  electronic  magnetic

summary establish film growth technique => clear COOI to FMM transition separate electronic excitation from heating => bidirectional phase control “soft and complex matter in solid form” => looks and feels hard but deformable colossal response vs inhomogeneity => understanding strongly-correlated electron systems? (optical measurement is a way to go)

collaborators Naoko Takubo Yusuke Uozu Hiroharu Tamaru Makoto Izumi Yoshinori Tokura Yasuhide Tomioka Hideki Kuwahara Yoichi Murakami current work force Yasushi Ogimoto Manfred Fiebig Takao Mori Toru Tonogai Mikhail Milyaev visitors in the past Many students in the past other institutions