We discuss the character of states near the Fermi level in Mn-doped GaAs, as revealed by a survey of dc transport and optical studies over a wide range of Mn concentrations. A thermally activated valence-band contribution to dc transport, a midinfrared peak at energy ប Ϸ 200 meV in the ac conductivity, and the hot photoluminescence spectra indicate the presence of an impurity band in low-doped ͑Ӷ1% Mn͒ insulating GaAs:Mn materials. Consistent with the implications of this picture, both the impurity-band ionization energy inferred from the dc transport and the position of the midinfrared peak move to lower energies, and the peak broadens with increasing Mn concentration. In metallic materials with Ͼ2% doping, no traces of Mn-related activated contribution can be identified in dc transport, suggesting that the impurity band has merged with the valence band. No discrepancies with this perception are found when analyzing optical measurements in the high-doped GaAs:Mn. A higher-energy ͑ប Ϸ 250 meV͒ midinfrared feature which appears in the metallic samples is associated with inter-valence-band transitions. Its redshift with increased doping can be interpreted as a consequence of increased screening, which narrows the localized-state valence-band tails and weakens higher-energy transition amplitudes. Our examination of the dc and ac transport characteristics of GaAs:Mn is accompanied by comparisons with its shallow acceptor counterparts, confirming the disordered valence-band picture of high-doped metallic GaAs:Mn material.

We report pressure-dependent transient picosecond and continuous wave photomodulation studies of disordered and ordered films of 2-methoxy-5-(2-ethylhexyloxy) poly(para-phenylenevinylene) (MEH-PPV). Photoinduced absorption (PA) bands in the disordered film exhibit very weak pressuredependence and are assigned to intrachain excitons and polarons. In contrast, the ordered film exhibits two additional transient PA bands in the mid-infrared that blueshift dramatically with pressure. Based on high-order configuration interaction calculations we ascribe the PA bands in the ordered film to excimers. Our work brings new insight to the exciton binding energy in ordered films versus disordered films and solutions.Ordered π-conjugated polymer films exhibit photophysics remarkably different from dilute solutions or disordered films [1][2][3][4]. Explanations given for these differences include photoexcitation branching into intrachain excitons and polarons in the ordered films [5], as well as formation of a variety of intermolecular species [1-4, 6, 7]. The distinctive behavior of the ordered films are due to strong interchain interaction absent in dilute solutions or disordered films. It follows that the ability to vary the extent of interchain interactions in a controlled manner would provide an ideal tool for understanding the role of morphology on the photophysics in these materials.Here we report such a study: we probe pressure effects on the transient picosecond (ps) and continuous wave (cw) PM spectra of disordered and ordered MEH-PPV films up to 119 kbar. The ordered film exhibits two correlated PA bands missing in the disordered films, which dramatically blue-shift with pressure. We further show, both experimentally and theoretically, that this key experimental result cannot be explained within scenarios involving exciton delocalization, or photogeneration of bound polaron-pairs. Our calculations establish unambiguously that the primary photoexcitated species in ordered MEH-PPV films are excimers, whose PA bands are expected to show pressure-induced blueshift. Our wavefunction analysis of initial and final states of PA bands also gives physical understanding behind the reduced exciton binding energy in ordered films [8].We used thin films of polymer samples drop-cast on quartz substrates from powder as received from ADS. Transient photomodulation (PM) spectroscopy was utilized to resolve the primary photoexcitations. Specifically, we used femtosecond (fs) two-color pump-probe correlation technique with a low-power (energy/pulse ∼ 0.1 nJ), high repetition rate (∼ 80 MHz) laser system based on a Ti:sapphire (Tsunami, Spectra-Physics) laser having a temporal pulse resolution of 150 fs [9]. The pumphω was frequency doubled tohω = 3.1 eV; the output beam of an optical parametric oscillator, OPO (Opal, Spectra-Physics) was used as a probe withhω ranging from 0.24 to 1.1 eV [9]. The pump and probe beams were focused on the film surface inside the cryostat or diamond anvil pressure cell to a spot ∼ 50 µm in diameter; and t...

Using femtosecond transient photomodulation, photoluminescence, and electro-absorption spectroscopies, we studied the ultrafast photoexcitation dynamics and nonlinear optical properties of ordered poly(thienylenevinylene) (PTV), which belongs to a rare class of nonluminescent, nondegenerate-ground-state π-conjugated polymers. We show that the ordered PTV films contain abundant nanocrystalline domains that substantially influence the optical spectra as a result of aggregates formation. We demonstrate that the primary intrachain exciton (1 1 B u) decays within ∼500 fs to the more stable "dark" exciton (2 1 A g), and the released energy results in both static strain and propagating strain wave that bounces back and forth in the polymer film.

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Physical properties of lithium under extreme pressures continuously reveal unexpected features. These include a sequence of structural transitions to lower symmetry phases, metal-insulator-metal transition, superconductivity with one of the highest elemental transition temperatures, and a maximum followed by a minimum in its melting line. The instability of the bcc structure of lithium is well established by the presence of a temperature-driven martensitic phase transition. The boundaries of this phase, however, have not been previously explored above 3 GPa. All higher pressure phase boundaries are either extrapolations or inferred based on indirect evidence. Here we explore the pressure dependence of the martensitic transition of lithium up to 7 GPa using a combination of neutron and X-ray scattering. We find a rather unexpected deviation from the extrapolated boundaries of the hR3 phase of lithium. Furthermore, there is evidence that, above ∼3 GPa, once in fcc phase, lithium does not undergo a martensitic transition.