Dynamical study of photoinduced structural phase transitions
Katsumi Tanimura,
The Institute of Scientific and Industrial Research, Osaka University, Japan
Abstract
Structural phase transition, which inherently reflects the multi-stability of condensed
matter, is governed by cooperative and non-linear interactions between electronic and latticesystems. Phase transition triggered by electronic photoexcitation, known as photo-induced
phase transition (PIPT), impacts multiple research fields because of following reasons. First,
studies of PIPT can directly reveal ultrafast dynamics of cooperative interaction in real- timeand space, using specific optically prepared excited states. Second, PIPT research opens apromising route to create new material phases which cannot be reached by conventional
excitation under thermal equilibrium [1]. For these reasons, PIPT has been extensively
studied world-wide. Here I review present status of the PIPT researches, with putting someemphasis on laser-induced order-disorder phase transitions in semiconductors and metals.
The central issue in PIPT is the dynamics of structural-order transformation under nonequilibrium conditions (electronically excited materials). In order to trace dynamics involved,
and to understand then from microscopic point of view throughly, one need to study following
three primary aspects of the whole PIPT process; one is the ultrafast dynamics of electronic
excited states generated by photoexcitation in fs-temporal domains, the second is the ultrafast
structural dynamics triggered by electronic-order changes, and the third is the atomic-scalecharacterization of photoinduced structural phases. In Fig.1, I show schematically themethodology typically used in PIPT studies.
In Fig.1, I would like to
Ultrafast dynamics of excitedStructural determinationelectronic systems
at atomic levels
emphasize the two points. First,
EE
thetime-resolved
two-photon
photoemission
spectroscopy
nonlinearprovides more direct information
Time-resolved tworelaxation
photonon the ultrafast dynamics ofphotoemission
transition
spectroscopy
STM observation ofdynamics newelectronic systems in time- energyphoto-induced newelectrons
structural phases
structural
holes
and momentum spaces, than any
orderphotoinduced
other optical methods [2]. In fact,
光誘起相
phase
this method has been applied to
Ultrafast time-resolved electron diffraction
study
photoinduced
metalof transmission mode
directly probing structural dynamics
insulator transitions in Mott
with fs-temporal resolution
insulators [3]. The second is thattime-resolved electron diffraction Fig.1 Experimental methods used in PIPT studies
in femtosecond temporal domains
is a powerful technique to capture ultrafast structural dynamics [4,5]. We have recently
constructed an ultrafast transmission electron diffractometer with outstanding performance[6]. This instrument, by overcoming serious space-charge effects in previous studies, has
shown 90 fs temporal resolution and a high sensitivity for single-shot diffraction
measurements. In order to demonstrate this excellent performance, we show in Fig.2 thediffraction pattern measured by a single shot for Au single crystals. The three lens system ofvac

CBM
VBM

BulkSurface

the diffractometer makes it possible to measure different image modes; wide-momentum
mode and high precision mode. Image (a) acquired by the former mode shows that themaximum scattering vector is greater than 1.56 Å-1, which is larger than any previously
reported. Image (b) acquired by the latter mode displays the diffraction-peak width as narrowas 0.02 A-1, showing an excellent spectral resolution, which makes it possible to determine
precisely small shifts of Bragg-peak widths and positions.
Capability of single-shot diffraction measurements is best suited to study irreducible
phase-transition
processes.
(b)(a)
Laser- induced melting (orderpump pulse
disorder transition) of solids isPLDL
CLa typical example of suchprocesses. We studied thelaser- induced melting processprobe pulseof Au, a typical fcc metal, toBFP
which extensive studies havebeen performed. Intensities ofBragg peaks from singlecrystalline Au were measured
as a function of time delay Fig.2 A schematic diagram of Ultrafast electron diffractometer and single-shot images for Au single crystals.
between pump and probe
pulses, and of the excitation laser intensity. The results were analyzed theoretically by
combining two-temperature model to describe non-equilibrium states of excited metals with
MD calculation. Electronic specific heat and electron-lattice coupling constant were obtainedby ab-initio calculation. The structural factor, which corresponds to the Bragg-peak intensity,
were calculated from pair density function at each time step of MD calculation.
The results show following important features of melting of Au, which qualitatively
depends on the excitation intensity:(1) When the absorbed energy is just above the total heat Qm required to thermal melting,melting starts from the surface, and melt front propagates inside crystals (heterogeneousmelting). It takes about 100 ps to melt completely the 10-nm Au film.
(2) When the absorbed energy increases to 1.5 times of Qm, we first observe a rapid expansionof samples with accompanying surface melting. This is followed by a rapid creation ofhomogenously distributed small molten pockets in the bulk (homogeneous melting). Within 20
ps after excitation, sample melts entirely.
(3) When the absorbed energy increases further to more than 3 times of Qm, melting process
shows an interplay between lattice heating effects by the electron-phonon coupling andnon-thermal effects which leads to variation of potential energy surface of Au nuclei.
References:
1) J. Kanasaki et al. Phys. Rev. Lett. 102, 087402 (2009).
2) T. Ichibayashi and K. Tanimura, Phys. Rev. Lett. 102, 087403 (2009).
3) L. Perfetti et al. Phys. Rev. Lett. 97, 067402 (2006).
4) A. H. Zewail, Science, 328, 187 (2010).
5) G. Sciaini and R.J.D. Miller, Rep. Prog. Phys. 74 096101 (2011).
6) Y. Murooka et al. Appl. Phys. Lett. 98, 251903 (2011).