Promising emulsion-detection technology for ultrafast time-resolved study of structural phase transitions one of the most exciting fields of condensed matter science Katsumi Tanimura, Condensed Matter Science Division The Institute of Scientific and Industrial Research, Osaka University ultrafast crystallography by MeV-electron diffraction

a system of charged particles with high velocities v <<c solids (many-body system in complete dark) photons lattice system (phonons) electronic system electron-phonon interaction electron-photon interaction example: structures of Si

Solid State Physics (traditional) physical understanding of several properties of given solids (crystals) structural, electronic, magnetic, optical, etc targets: solids, given by nature, or fabricated thermodynamically limited functions for device performance (for human life) Nanotechnology (current Condensed Matter Science) artificial fabrication of solid structures from atomic levels in order to generate desired functions one needs “fight” against nature (equilibrium, adiabatic processes) how ?

Photons in Action !! photoinduced structural phase transitions photoinduced new surface-phase creation controlling bonding orders in solids

several modes of interactions phase transition: one of the most typical phenomena of solids （ phase, electronic, magnetic, structural, etc ） traditional studies ： 1 ） structures and electronic properties of thermodynamically stable (metastable) phases 2 ） transition dynamics on quasi-static processes solids: quantum many-body system structural phase transition ： dramatic changes in symmetry, structures, and electronic properties an example: graphite diamond electronic structural no way to control modes of condensation !! （ multi-stability of condensed phase ） thermodynamicl transitions ： simultaneous excitation of all degrees of freedom! impossible to detect, elucidate, and control a given fundamental process involved

Photoinduced Phase transitions 2) ”hidden phases” unable to be reached by thermodynamical processes ！ new structures (phases) creation light lattice transitions in non-equilibrated systems by external stimulation initial changes only in the electronic systems: selective excitation real-time detection of a given fundamental process triggered significance electrons thermal excitation photoexcitation Energy metastable stable ( s) ( s) photoinduced Graphite-to-Diamond transition revealing the crucial process that governs the modes of condensation 1) microscopic study (atomic, temporal) of macroscopic transitions

a direct probe of atomic structures of solids: Scanning Tunneling Microscope (STM) surface atomic arrangement of InP(110) spatial resolution (<10 -8 cm ） atomic resolution! STM tip graphite 100~1kHz 798nm ( 1.57eV ) 80 fs (hexagonal graphite)

STM observation of photoinduced phase transition on Graphite surface ex =800 nm  =80 fs p-polarized prior to irradiation after irradiation depressed periodically ! STM-image structures changes in a large-scale area (d~20nm) including ~10 4 C atoms photoinduced phase transition completely absent prior to irradiation

a new phase of diamond (?) photoinduced structural transformation on graphite surface back bonding via sp 2 sp 3 bond-order change not conventional Diamond formed thermodynamically! “ Diaphite ” mode of atomic displacements a new phase of C specific to photoexcitation !

Diaphite signature of ultrafast transformation processes probed by optical technique (reflection changes) fs-laser pulse dynamics of phase transition direct structural information in fs-temporal domain issues to be solved STM observation of new phases  R/R

faulted half unfaulted half adatom rest atom structural determination by electron diffraction: typical examples Si(111)-(7x7): a typical example of surface reconstruction K. Takayanagi et al. J. Vac. Sci Technol A3 (1985) )transmission electron diffraction (TED) detecting bulk properties 2) single-shot detection for studying irreversible processes 3) time resolution less than 100 fs ( s) for ultrafast dynamics Our goal: 1) a new type of photocathode RF gun to generate MeV e - beams 2) highly sensitive detection of MeV electrons

examples of the research efforts Femtosecond time-resolved electron diffraction studies: a trend

advantages over X-ray diffraction challenges: space-charge induced broadening effects ! advantages and issues in the femtosecond time-resolved electron diffraction B. J. Siwick et al (2004) ultrafast and direct structural determination ！ 1 ） time resolution better than 100fs: possible 2 ） elastic scattering cross-section: larger than X rays by a factor of 1000 highly sensitive detection and significant reduction of any inelastic effects 3 ） high precision in beam control (great progress in electron microscope technology) generation of electron pulses with width less than 100 fs: easy propagation dynamics of short electron pulses: destroying temporal and energy characteristics

Science, 318, 788 (2007) 1)reflection mode only surface regions 2) high repetition rate (1 kHz) only for reversible processes issues to be overcome (CALTECH)

Ultrafast Transmission Electron Diffraction system under construction beam characteristics for UTED 1)temporal width ＜ 100 fs  E/E<0.05% 3)emittance <0.1 mm-mrad 4)Energy ： 1~4 MeV 5) number of electrons per pulse ： as many as possible

RF photocathode femtosecond laser 2856MHz 、 ~MW ＠ 4  s =262, 266nm RF cavity ~100MV/m solenoid magnet temporal width ： < 100 fs emittance ： 0.02mm-mrad  E/E ： <2x10 -4 beam energy (E): ~2 MeV beam size: r=0.2 cm number of e - ： <10 6 /pulse (~0.1pC/pulse ) summary of particle simulation

final message ( love call ) to nuclear emulsion technology! challenge highly sensitive detection of MeV electrons !  t< s ultrafast (< s) single-shot structural analysis: emulsion detection; sensitivity of one-electron detection! answer (promising!!) 3D-automatic imaging technique (Niwa’s group; Ngoya Univ.) This is our hope!