Photoinduced nano and micron structures K. Nasu Solid state theory division, Institute of materials structure science, High energy accelerator research organization (KEK) Graduate university for advanced study, Tsukuba, Japan

Photoinduced nano and micron structures There discovered a new class of insulating solids, which, being shined by only a few visible photons, become pregnant with an excited nano or micron domain, that has new structural and electronic orders (, charge, spin, gauge ), quite different from the starting ground state. Purpose Clarify, 1) conditions of its occurrence (, hidden multi-stability ), 2) its mechanism (, criticality, initial condition sensitivity ), 3) difference from thermally excited nano domain, and finally establish the way how to control nano domain. K.Nasu et. al J.P.CM, 13 (2001) R693-R721.

Microscopic Order parameter of New lattice structure and electronic order hν Thermal energy Ground state False ground state Excited nano domain lattice Phase transition Lattice relaxation Franck- Condon state distortion ExcitedExcited Photo- induced structural change Hidden multistability Proliferation

Neutral-ionic phase transition in TTF-CA T c =84K DDimerization Molecules at z=1/2 Molecules at z=0 Neutral Phase (D 0 A 0 ) P12 1 /n1 Molecules at z=1/2 Molecules at z=0 Ionic Phase (D + A  ) P1n1 b c a  90° D+D+ D+D+ AA AA AA AA D+D+ AA D0D0 D0D0 D0D0 D0D0 A0A0 A0A0 A0A0 A0A0 A0A0 Neutral (D 0 A 0 )  Ionic(D + A  ) 0.3  0.8 Charge Transfer  : Monomer phase

Diamagnetic Phase Red ( S = 0 ), T < 120K Paramagnetic phase Yello ( S = 2 ), T > 120K Organo-metallic complex crystal Difference between photoinduced phase and thermally induced phase They are two equilibrium phases of this material.

Red equilibrium diamagnetic phase (S=0) Yellow nonequilibrium paramagnetic phase (S=2) Yellow paramagnetic phase can also be generated as a nonequilibrium phase by light, even at low temperatures. By S.Koshihara

Intensity ( arb. uinit ) Thermal para- magnetic phase Thermal dia- magnetic phase Photoinduced paramagnetic phase Low temperature photoinduced phase is different from the thermal one of high temperature. New parity violation only in photoinduced phase Raman spectra Photoinduced phase can make a new broken symmetry appear, even if it can never appear in any equilibrium phases. Our knowledge on materials based only on equilibrium phases is insufficient. T. Tayagaki and K. Tanaka, Phys.Rev.Letters.86 (2001) K 70K 30K

Spring-8, BL 2B2, by Y. Moritomo, J.Phys.Soc.Jpn 71(202)2609. Lattice constant change in photoinduced phase of metal complex crystal Fe(ptz), X-ray structure analysis

It can never appear as an equilibrium state.

Nasu, Phys. Rev. B 67 (2003)

Ground state Franck-Condon states Start with a large excess energy Nonlinear proliferation Initial condition sensitivity Fractal Pattern formation Photoinduced Nano domain Lattice distortion (, total exciton number, domain size ) Theory for real time dynamics of 1- and 2- D many-exciton phonon systems

Many-exciton system in a reservoir Quantum Transfer Attraction, or Interaction Proliferation Large lattice Non- Master equation under Markov approximation Bistability relaxation adiabaticity

Time evolution after single photon pulse excitation Total Total Energy/ω Total Exciton Number

Successive photon pulse excitation

Time evolution of spatial pattern

Isotropy and anisotropy of interexciton interaction anisotropic isotropic

Fractal pattern analysis Perimeter – area relation Fractal dimension D

Perimeter – area relation Case(2 ) Case(3) R.Yabuki, Phys. Letters, N total N per Anisotropic case is more efficient than isotropic case.

(1) -> (2) -> (3) Anisotropy becomes strong. 1D → 2D crossover point

Interexciton interactions

Mean field theory for frozen exciton

Decay of frozen exciton ・・ ・・ ・