Photoinduced Change in the Spin State of Itinerant Correlated Electron Systems

Abstract

Photo-induced spin-state change in itinerant correlated-electron system is studied. The model Hamiltonians before and after photon-pumping are derived from the two-orbital Hubbard model with crystalline field splitting. A photon introduced in the low-spin band insulator induces a bound state of the high-spin state and a photoexcited hole. This bound state brings a characteristic peak in the optical absorption spectra in the photo-excited state. The present results well explain the recent experimental results of the ultrafast optical spectroscopy in perovskite cobaltites. 71.10.w, 71.30.+h, 78.20.Bh A number of electronic phases and the phase transition between them are one of the central issues in correlated electron systems. In particular, materials with multi-degrees of freedom, such as charge, spin, orbital and so on, exhibit various exotic phenomena related to the phase transition [1]. Recently developed ultrafast optical techniques open up a new frontier for research of the phase transition [2]. Irradiation of a pump laser pulse into a system in a vicinity of the phase boundary triggers an abrupt change in the electronic structures. This is the so-called photo-induced phase transition (PIPT). In contrast to the conventional phase transitions caused by controlling temperature (T ), doping carriers and so on, photoinduced phase is transient and highly nonequilibrium. Photoinduced phenomena in correlated electron systems offer large possibility of new hidden phases [3,4], which do not realize in the thermal equilibrium state, and prompt several theoretical challenges [5][6][7][8][9][10][11].The spin-state transition is one of the targets in recent PIPT studies. This is a transition between the states with different magnitudes of the spin-angular moment in transition-metal ions. Different spin states are realized owing to a delicate balance of the intra-ion Hund coupling and the crystallinefield splitting. Some examples have been seen in the insulating organometallic complexes, such as the Prussian-blue analog, where magnitudes of the localized spins in Co or Fe ions are switched by photon irradiation [12,13]. Another type of the photo-induced spin-state transition is suggested in the correlated electron systems, the cobalt oxides with a perovskite structure, R 1−x A x CoO 3 and RACo 2 O 6−δ (R: a rare-earth ion, A: an alkaline-earth ion). Possible three spin states in Co 3+ are the low-spin (LS) in the (e g ) 0 (t 2g ) 6 configuration, the intermediate-spin (IS) in (e g ) 1 (t 2g ) 5 , and the high-spin (HS) in (e g ) 2 (t 2g ) 4 [14,15]. Temperature induced spin-state transition from the low-T LS insulating state into the high-T HS or IS metallic one is commonly observed in these cobaltites. The ultrafast optical measurements in the low-T LS insulators show transient metallic spectra which are completely different from the spectra in the high-T phase [16][17][18]. The electron conduction and the spin-state are strongly coupled with each other, in highly contrast to the spin-crossover organometallic comple...

Phase diagram and collective excitations in an excitonic insulator from an orbital physics viewpoint

“…In an isolated ion, there are the relations U = U + 2J and I = J. The electronic structures in the two-orbital Hubbard model have been investigated so far from the several points of view [25,26,[34][35][36][37][38].…”

Photoinduced Hund excitons in the breakdown of a two-orbital Mott insulator

“…The breakdown depends not only on the field strength but also on the Hubbard-U interaction. Light-induced phenomena in two-orbital models have been studied focusing mainly on quarter-filled Mott insulators using double-exchange models [23][24][25][26][27] . A fieldgenerated high-low spin insulator-ferromagnet transition was observed, as well as bound states, and orbital coherent oscillations.…”

Femtosecond Reflection Spectroscopy in La1.5Sr0.5CoO4

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