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1.INTRODUCTIONIt is a remarkable fact that carefully designed pump-probe experiments, using wavelengths from infrared to X-ray, can be used to characterize and even control a wide variety of properties in important materials. Here we very briefly review a few narrow topics in this vast area of current research, with an even more narrow selection of important results. 2.ULTRAFAST BANDGAP COLLAPSE IN SEMICONDUCTORSExperimental studies of ultrafast bandgap collapse in semiconductors like GaAs1-3 indicated a nonthermal transition from semiconducting to metallic phases, and this was confirmed by the theoretical studies4-8, as illustrated in Figs. 1-3. Note three aspects in the behavior of the dielectric function shown in Fig. 1:
Figure 2 shows the corresponding behavior of the nonlinear susceptibility, which probes the atomic structure of the material and is thus complementary to the linear dielectric function, which probes the electronic structure. Notice that Im χ(2) falls to zero over the entire range of photon energies, signaling a loss of the original symmetry of the GaAs lattice. This is also consistent with the experimental results of Refs. 1-3. The power of realistic simulations is that one can study the behavior of the system in microscopic detail, as well as obtaining the quantities, like those in Figs. 1 and 2, which are experimentally accessible. For example, Fig. 3 shows how rapidly the atoms move from their initial positions following femtosecond-scale laser pulses for which the amplitude of the electric field is given by A; A =1.0 corresponds to a fluence of 0.815 kJ/m2 for a full-width-at-half-maximum duration of 70 femtoseconds (which was used in the simulations of Figs. 1-3). 3.C60 AND GRAPHENEFigure 4 represents experimental observations of excitation of the breathing mode in C60 buckyballs by an ultrafast laser pulse. In Figure 5, both the breathing mode and the pentagonal pinch mode at higher frequency were observed. As can be seen in Fig. 6, the calculations by Torralva11 resolve what might appear to be a discrepancy between the results of the two experimental groups, by demonstrating that only the breathing mode is seen at high laser intensity, whereas both the breathing mode and the pentagonal-pinch mode are seen at lower intensity. At still higher intensity, there is fragmentation with the emission of dimers, in agreement with experiment, as shown in Fig. 7. The related work of Zhao et al.13 and Jiang et al.14 demonstrated the potential for enhancing the relative response of selected modes by the proper choice of pulse duration and the interval between applied sequential pulses. One motivation is to emphasize those modes which might be most useful for identification of a chemical species or biological agent. Figure 8 demonstrates such enhancement for a C60 molecule: In (a), the originally small response of the Ag(1) mode becomes totally dominant. In (b), the Hg(1) mode is dominant. In (c), the Hg(1) mode is almost completely suppressed, with the combined Ag(1) and Hg(4) modes enhanced. Figure 9 shows an interesting discovery of Long et al.15 These simulations – which, like all the others discussed in this section, are density-functional-based – demonstrated that atomic hydrogen can be encapsulated in C60 via collisions of either H or H2 with a C60 molecule whose breathing mode has been properly excited by an optimized femtosecond-scale laser pulse. The basic mechanism is a “breathing trap’’, with the C60 first expanding, to admit the H, and then contracting, to contain it. Figure 10 illustrates a remarkable discovery of Zhibin Lin16: In our technique, electrons equilibrate with each other in about 100 femtoseconds, because they are indirectly coupled to each other via the nuclear motion – even though the electronic temperature is one or two orders of magnitude higher than the temperature associated with nuclear motion (and even though correlation effects are omitted in these mean-field calculations). This means the description is physically correct, and also justifies a 2-temperature model for simulations on larger scales. 4.LIGHT-INDUCED SUPERCONDUCTIVITYHere we use “light” in a very broad sense, to include infrared and terahertz electromagnetic radiation. One can imagine several hypothetical mechanisms for (transient) light-induced superconductivity. An ultrafast laser pulse might lead to, e.g., one of the following on roughly a picosecond time scale:
It appears that the group of Cavalleri and coworkers have observed the first two effects, in cuprates18-21 and K3 C6022 respectively. Suda et al.23 appear to have observed the third in a strongly correlated molecular conductor, on a vastly longer time scale (seconds rather than femtoseconds) as shown in Figs. 11 and 12, where isomerization of a spiropyran monolayer dopes the κ-Br with holes, with both resistance and diamagnetism acting as signatures of induced superconductivity. In Fig. 13, a Josephson plasma resonance edge in the reflectance, near 60 cm-1, is interpreted as a signature of 3D superconductivity in the relevant cuprates, associated with Josephson coupling between CuO2 planes. It is seen in superconducting La1.84Sr0.16CuO4 (LSCO0.16), for example, but not in the non-superconducting phases of this material or La1.675Eu0.2Sr0.125CuO4 (LESCO1/8). The interpretation is shown in the schematic drawing of Fig. 14, which depicts for La1.875Ba0.125CuO4 the charge, spin, and lattice arrangement within a CuO2 plane and set of planes in the stripe-ordered, low-temperature tetragonal (LTT) phase below 55 K. Cu atoms are blue and oxygen atoms red. Holes form stripes which separate domains of oppositely phased antiferromagnetic domains, with spins indicated by arrows. The stripe orientation of periodic CuO2 planes rotates by 90° between layers. Then, when an ultrafast laser pulse is applied, melting of the stripes permits coherent 3D superconductivity, according to this interpretation. 5.CONCLUSIONHere we considered a tiny selection of the ways in which studies of the electronic and structural response to ultrafast laser pulses can elucidate the behavior of important materials. Carefully designed pump-probe experiments can address a vast number of other topics24 including, e.g., the coupled dynamics of electrons, nuclear motion, and the order parameters associated with magnetism, charge density waves, spin density waves, superconductivity, and even more exotic phases. We are currently developing a hybrid technique for treating such problems, which combines a density-functional-based approach for the gross electronic structure, on a 1eV energy scale, with semiempirical models for interacting order parameters on a 100 K (or 0.01 eV) scale. A key point is that superconductivity, charge density waves, and spin density waves all involve pairing of electronic states near the Fermi surface, with quasiparticle energies having the basic form E = (ε2 + Δ2)1/2 (referenced to the chemical potential μ). The order parameter Δ can be determined by a semiempirical free energy, and the scheme can be extended to make everything time-dependent (and to include collective modes). Similar ideas can be used for other magnetic and correlation effects. REFERENCESGlezer, E. N., Siegal, Y., Huang, L., and Mazur, E.,
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