Femtosecond Carrier Relaxation Dynamics and Photoinduced Phase Separation inκ−(BEDT−TTF)2Cu[N

Abstract

We investigate the relaxation dynamics of nonequilibrium carriers in organic conductors -ðBEDT-TTFÞ 2 Cu½NðCNÞ 2 X (X ¼ Br and Cl) using ultrafast time-resolved optical spectroscopy. The dynamics for both salts show similar temperature dependences, which is well characterized by the carrier relaxation across the pseudogap (PG) of the magnitude Á PG % 16 meV for Br salt and 7.0 meV for Cl salt. On the other hand, only the Br salt shows an abrupt increase of the decay time at low temperature, indicating an additional decay component associated with the superconducting (SC) gap below T c . The fluence dependent dynamics at low temperature evidences the superposition of the SC component onto the PG component. These results indicate a metallic-insulating phase separation in the Br salt triggered by photoexcited nonequilibrium carriers. DOI: 10.1103/PhysRevLett.107.227002 PACS numbers: 74.70.Kn, 71.30.+h, 78.47.jg Investigations of nonequilibrium carrier dynamics in strongly correlated electron systems (SCES) have provided new insights into the underlying physics of the correlation and the interactions driving the phase transition. Timeresolved optical pump-probe technique is one of the major experimental techniques to measure such dynamics, where the nonequilibrium distributions of carriers and phonons are induced by an intense pump pulse and their relaxation is traced by a weak probe pulse after the pump pulse. Owing to the high sensitivity in the time domain, the technique allows us to resolve the electronic state stationary points and band extrema near the Fermi energy (E F ). Recently, the framework for analyzing the transient data has being elucidated for various SCES such as high-T c superconductors (SCs) [1][2][3], heavy fermions [4-6], transition metal oxides [7], and other related compounds [8]. The application to organic SCES is especially interesting because of their rich variety of electronic ground states which can be tuned by (chemical and/or physical) pressure, temperature, and external fields including photoexcitation [9][10][11][12][13]. Since such variety of electronic ground states leads to a complexity of the transient data even within the short time, none of the previous reports have yet been successful in identifying the relaxation processes.In this Letter, we report the relaxation dynamics of photoexcited carriers in organic molecular salts -ðBEDT-TTFÞ 2 Cu½NðCNÞ 2 X [BEDT-TTF denotes bis-(ethylenedithio)tetrathiafulvalene and X is a counteranion] using the optical pump-probe technique. We compare the dynamics of an isostructural pair of organic salts, X ¼ Br and Cl [ Fig. 1(b)], as a function of temperature (T) and photoexcitation fluence (F ). Except for the SC phase of Br salt, T-dependent dynamics for the two salts are well characterized by a carrier relaxation across the pseudogap (PG). On the other hand, only the Br salt shows an abrupt increase of the decay time when lowering T below T c , indicating an additional decay component associated with the SC phase. F -dependent dynamics at...

Photoinduced phase separation with local structural ordering in organic molecular conductors

“…In κ-Br, the direction of anisotropy and its temperature dependence agree with those in a previous work [15]. In this study, the metallic-insulating PIPS has been suggested as a possible origin for the PG response.…”

“…• , which was inferred from the previous report [15]. Optical penetration depth is roughly estimated as 13 μm for pump and 3 μm for probe in κ-NCS, respectively, which are almost identical to those in κ-Br, meaning that bulk electronic properties are measurable.…”

“…In this study, the metallic-insulating PIPS has been suggested as a possible origin for the PG response. According to this scenario, the irradiation of pump pulse will lead to modulation of t/U [15], which can act as a negative pressure on the electronic state. As a result, a part of the sample changes from metal to insulator, leading to microscopic or mesoscopic phase separation in real space and/or a gap opens at different points in the momentum space.…”

Pump- and Probe-polarization Analyses of Ultrafast Carrier Dynamics in Organic Superconductors

“…Such coexistence of the different states can be explained by phase separation, which has been observed in other superconductors [15]. In κ-Br the phase separation induced by photon irradiation [13] has been suggested. In this case irradiation of a pump pulse modulates the t/U, acting the electronic state as negative pressure.…”

“…The diameters of the pulses were roughly estimated ∼ 13 µm for the pump and ∼ 3 µm for the probe. The relative angle between optical pulse polarization and a crystal axis was measured from the direction tilted by 45 • from b (c) axis for κ-NCS (κ-Br, which was inferred from the previous report [13]) using a half-wave plate. To investigate relationship between the anisotropic response and pump polarization we measured ∆ R/R for various pump polarizations, where the pump fluence was fixed to be constant.…”