Direct Domain Excitation by Photoemission
and Its Dynamical Features Manifesting in ARPES spectra
K. Iwano
Photon Factory, Institute of Materials Structure Science, KEK, Tsukuba 305-0801, JapanThe theoretical and experimental studies ofangle-resolved photoemission (ARPES) have focusedon quasi-particle properties in various kinds ofmaterials.
Among them, heavily renormalized
nature of quasi-particles typically seen instrongly-correlated electron systems have collectedmuch attention, since the nature can provide
important information on the many-body propertiesof those systems. However, even in such cases thequasi-particle basically consists of one hole; thestrong renormalizaiton only increases the particle
mass and the lifetime. In this sense, what weobserve in ARPES is inevitably of one-hole nature,
however it is modified by interactions.
In this paper, we report on a completely different typeof excitation that can be detected in the ARPES. InFig. 1, we show a schematic picture in which asystem that is located close to a phase boundary
absorbed one high-energy photon. Assuming avalence photoemission, we are left with one hole inthe valence band. In ordinary insulators, the process
stops at this point. However, in this system, it does
not stop but proceeds to form a domain [1]. We here
think of a one-dimensional system whose groundstate is expressed as in Fig. 1(a).
TTF-CA, anorganic molecular solid, is a system appropriate forthis situation, having nearly degenerate neutral (N)and ionic (I) phases. In the process shown in Fig.
1(b), we expect that a domain of the I phase is createdin the background of the N phase. It is emphasizedthat the driving force is the proximity to the N-I phaseboundary combined with inherent electron itineracy,
and that the detection of this process requires nopump-probe experiment but only an ordinary type ofphotoemission measurement.
In Fig. 2, we show calculated ARPES spectra (blacklines) at two momenta, which are obtained very close
to the phase boundary [2]. What is quite unique is thateach spectrum exhibits a special spectral shape; thespectrum at k=0 takes a cusp-like shape, while theleft-hand side of the spectrum at k=/2 is almost
fitted to a straight line. We also tried deriving aneffective model that retains the two basic degrees offreedom of a single domain, namely, its center ofgravity and its spatial size, plus spin degrees offreedom inside the domain. As a result of its
analysis, we find that the effective model reproduces
the spectral features, as shown by the red lines. In
particular, the above-mentioned spectra shapes aregoverned by the spin degrees of freedom, in a waythat domain states with various spin configurations
form a Hilbert space that can be accessed via theARPES measurement.
This type of domain excitation is new in the ARPES
studies, in the sense that it has never been observed sofar. In addition to this, it also has importantmeaning in the context of photoinduced phasetransitions.
We hope for future experimental
attempts that try to identify them in actual materials.

Fig. 1. (a) Neutral ground state and (b) schematic
picture of an I-domain formation process.

Fig. 2. Calculated ARPES spectra.
[1] K. Iwano, Phys. Rev. Lett., 97, 226404 (2006).
[2] K. Iwano, Phys. Rev. Lett. 102, 106405 (2009).