Verner Thorsmolle

Morphology Effectively Controls Singlet-Triplet Exciton Relaxation and Charge Transport in Organic Semiconductors

We present a comparative study of ultrafast photoconversion dynamics in tetracene (Tc) and pentacene (Pc) single crystals and Pc films using optical pump-probe spectroscopy. Photoinduced absorption in Tc and Pc crystals is activated and temperature-independent, respectively, demonstrating dominant singlet-triplet exciton fission. In Pc films (as well as C60-doped films) this decay channel is suppressed by electron trapping. These results demonstrate the central role of crystallinity and purity in photogeneration processes and will constrain the design of future photovoltaic devices.

Ultrafast conductivity dynamics in pentacene probed using terahertz spectroscopy

We present measurements of the transient photoconductivity in pentacene single crystals using optical-pump terahertz-probe spectroscopy. We have measured the temperature and fluence dependence of the mobility of the photoexcited charge carriers with picosecond resolution. The pentacene crystals were excited at 3.0 eV, which is above the bandgap of ∼2.2 eV, and the induced change in the far-infrared transmission was measured. At 30 K, the carrier mobility is μ≈0.4 cm2/V s and decreases to μ≈0.2 cm2/V s at room temperature. The transient terahertz signal reveals the presence of free carriers that are trapped on the time scale of a few picoseconds or less, possibly through the formation of excitons, small polarons, or trapping by impurities.

C-axis Josephson plasma resonance observed in Tl 2 Ba 2 CaCu 2 O 8 superconducting thin films by use of terahertz time-domain spectroscopy

We have unambiguously observed the c -axis Josephson plasma resonance (JPR) in high-critical-temperature (T(c)) cuprate (Tl(2)Ba(2)CaCu(2)O(8)) superconducting thin films, employing terahertz time-domain spectroscopy in transmission as a function of temperature in zero magnetic field. These are believed to be the first measurements of the JPR temperature dependence of a high-T(c) material in transmission. With increasing temperature, the JPR shifts from 705 GHz at 10 K to ~170 GHz at 98 K, corresponding to an increase in c-axis penetration depth from 22.4+/-0.6mum to 94+/-9mum . The linewidth of the JPR peak increases with temperature, which indicates an increase in the quasi-particle scattering rate. We have probed the onset of the c -axis phase coherence to ~0.95T(c) . The JPR vanishes above T(c) as expected.

STRUCTURE OF VORTEX LIQUID PHASE IN IRRADIATED BI2SR2CACU2O8-DELTA CRYSTALS

The c-axis resistivity in irradiated and in pristine BSCCO(2212) crystals is measured as a function of the in-plane magnetic field component at fixed out-of-plane component B_\perp in the vortex liquid phase at T=67 K. From this data we extract the dependence of the phase difference correlation length inside layers on B_\perp and estimate the average length of pieces of vortex lines confined inside columnar defects as a function of the filling factor f=B_\perp / B_\phi. The maximum length, about 15 interlayer distances, is reached near f=0.35.

Photoexcited electron dynamics in kondo insulators and heavy fermions

We have studied the photoexcited carrier relaxation dynamics in the Kondo insulator SmB6 and the heavy fermion metal YbAgCu4 as a function of temperature and excitation level. The dynamic response is found to be both strongly temperature dependent and nonlinear. The data are analyzed with a Rothwarf-Taylor bottleneck model, where the dynamics are governed by the presence of a narrow gap in the density of states near the Fermi level. The remarkable agreement with the model suggests that carrier relaxation in a broad class of heavy electron systems (both metals and insulators) is governed by the presence of a (weakly temperature dependent) hybridization gap.

Ultrafast Spectroscopy of the Uranium(IV) and Thorium(IV) Bis(ketimide) Complexes (C5Me5)2An[-N=C(Ph)(CH2Ph)]2 (An = Th, U)

Ultrafast pump-probe spectroscopic studies have been performed on (C 5Me 5) 2U[- N=C(Ph)(CH 2Ph)] 2 and (C 5Me 5) 2Th[- N=C(Ph)(CH 2Ph)] 2 including, for the uranium complex, the first direct measurement of dynamics of electronic deactivation within a 5f-electron manifold. Evidence has been found for strong coupling between the electronic ground state and the f-electron manifold which dominates the dynamics of the excited states of the bis(ketimide) uranium complex. These also demonstrate strong singlet-f manifold coupling, which assists in the deactivation of the photoexcited state of the uranium complex, and provide information on intersystem crossing and internal conversion processes in both complexes.

Ultrafast (but Many-Body) Relaxation in a Low-Density Electron Glass

We present a study of the relaxation dynamics of the photoexcited conductivity of the impurity states in the low-density electronic glass, phosphorous-doped silicon Si:P. Using subband gap optical pump-terahertz probe spectroscopy we find strongly temperature- and fluence-dependent glassy power-law relaxation occurring over subnanosecond time scales. Such behavior is in contrast to the much longer time scales found in higher electron density glassy systems. We also find evidence for both multiparticle relaxation mechanisms and/or coupling to electronic collective modes and a low temperature quantum relaxational regime.

Evidence for linelike vortex liquid phase in Tl{sub 2}Ba{sub 2}CaCu(sub 2}O{sub 8} probed by the Josephson plasma resonance.

We measured the Josephson plasma resonance (JPR) in optimally doped Tl{sub 2}Ba{sub 2}CaCu{sub 2}O{sub 8+{delta}} thin films using terahertz time-domain spectroscopy in transmission. The temperature and magnetic field dependence of the JPR frequency shows that the c-axis correlations of pancake vortices remain intact at the transition from the vortex solid to the liquid phase. In this respect Tl{sub 2}Ba{sub 2}CaCu{sub 2}O{sub 8+{delta}} films, with anisotropy parameter {gamma}{approx}150, are similar to the less anisotropic YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} ({gamma}{approx}8) rather than to the most anisotropic Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} single crystals ({gamma}{approx}500).

Exciton dynamics in pentacene and tetracene studied using optical pump-probe spectroscopy

We present room temperature photoinduced reflection and transmission measurements in pentacene and tetracene single crystals using optical pump-probe spectroscopy. Singlet exciton recombination, singlet-triplet fission, excited singlet, and triplet state absorption is observed.

Josephson plasma resonance in Tl2Ba2CaCu2O8 in a magnetic field measured using THz spectroscopy

Abstract We report the first measurements of the c-axis Josephson plasma resonance (JPR) in Tl2Ba2CaCu2O8 as a function of temperature with and without a c-axis magnetic field using terahertz time-domain spectroscopy in transmission. The JPR is sensitive to the alignment of pancake vortices along the c-axis, and is observed to decrease when applying a magnetic field as expected.