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The characteristics of topological insulators are manifested in both their surface and bulk properties, but the latter remain to be explored. Here we report bulk signatures of pressure-induced band inversion and topological phase transitions in ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}\mathrm{Se}$ ($x=0.00$, 0.15, and 0.23). The results of infrared measurements as a function of pressure indicate the closing and the reopening of the band gap as well as a maximum in the free carrier spectral weight. The enhanced density of states near the band gap in the topological phase gives rise to a steep interband absorption edge. The change of density of states also yields a maximum in the pressure dependence of the Fermi level. Thus, our conclusive results provide a consistent picture of pressure-induced topological phase transitions and highlight the bulk origin of the novel properties in topological insulators.

We report the observation of two signatures of a pressure-induced topological quantum phase transition in the polar semiconductor BiTeI using x-ray powder diffraction and infrared spectroscopy. The x-ray data confirm that BiTeI remains in its ambient-pressure structure up to 8 GPa. The lattice parameter ratio c/a shows a minimum between 2.0-2.9 GPa, indicating an enhanced c-axis bonding through pz band crossing as expected during the transition. Over the same pressure range, the infrared spectra reveal a maximum in the optical spectral weight of the charge carriers, reflecting the closing and reopening of the semiconducting band gap. Both of these features are characteristics of a topological quantum phase transition, and are consistent with a recent theoretical proposal., revised final version

We report the generation of tunable, narrow-band, few-cycle and multicycle coherent terahertz (THz) pulses from a temporally modulated relativistic electron beam. We demonstrate that the frequency of the THz radiation and the number of the oscillation cycles of the THz electric field can be tuned by changing the modulation period of the electron beam through a temporally shaped photocathode drive laser. The central frequency of the THz spectrum is tunable from $\ensuremath{\sim}0.26$ to 2.6 THz with a bandwidth of $\ensuremath{\sim}0.16\text{ }\text{ }\mathrm{THz}$.

The in-plane optical phonons around 200 meV in few-layer graphene are investigated utilizing infrared absorption spectroscopy. The phonon spectra exhibit unusual asymmetric features characteristic of Fano resonances, which depend critically on the layer thickness and stacking order of the sample. The phonon intensities in samples with rhombohedral (ABC) stacking are significantly higher than those with Bernal (AB) stacking. These observations reflect the strong coupling between phonons and interband electronic transitions in these systems and the distinctive variation in the joint density of electronic states in samples of differing thickness and stacking order., 20 pages, 7 figures, supplementary material included

We report the complex optical conductivity of a superconducting thin film of Nb 0.5 Ti 0.5 N in an external magnetic field. The field was applied parallel to the film surface and the conductivity extracted from far-infrared transmission and reflection measurements. The real part shows the superconducting gap, which we observe to be suppressed by the applied magnetic field. We compare our results with the pair-breaking theory of Abrikosov and Gor'kov and confirm directly the theory's validity for the optical conductivity.

We report the complex optical conductivity of a superconducting thin film of Nb 0.5 Ti 0.5 N in an external magnetic field. The field was applied parallel to the film surface and the conductivity extracted from far-infrared transmission and reflection measurements. The real part shows the superconducting gap, which we observe to be suppressed by the applied magnetic field. We compare our results with the pair-breaking theory of Abrikosov and Gor'kov and confirm directly the theory's validity for the optical conductivity.

The low-frequency conductivity ${\mathrm{\ensuremath{\sigma}}}_{1}$(\ensuremath{\omega}) of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ has a Drude-like component below ${\mathit{T}}_{\mathit{c}}$. Interpreting the width of this component as the quasiparticle relaxation rate (${\mathrm{\ensuremath{\tau}}}^{\mathrm{\ensuremath{-}}1}$), we find that ${\mathrm{\ensuremath{\tau}}}^{\mathrm{\ensuremath{-}}1}$ decreases dramatically just below ${\mathit{T}}_{\mathit{c}}$, in sharp contrast with the T-linear ${\mathrm{\ensuremath{\tau}}}^{\mathrm{\ensuremath{-}}1}$ above ${\mathit{T}}_{\mathit{c}}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{6}$. This decrease causes a peak in ${\mathrm{\ensuremath{\sigma}}}_{1}$(\ensuremath{\omega}\ensuremath{\rightarrow}0,T) for T just below ${\mathit{T}}_{\mathit{c}}$, a peak which is due to the scattering rate and not to pair coherence effects, consistent with the lack of a coherence peak in the NMR relaxation rate. This result implies that the excitations which scatter the carriers are suppressed below ${\mathit{T}}_{\mathit{c}}$.

We have demonstrated nonlinear cross-phase modulation in electro-optic crystals using intense, single-cycle terahertz (THz) radiation. Individual THz pulses, generated by coherent transition radiation emitted by subpicosecond electron bunches, have peak energies of up to 100 microJ per pulse. The time-dependent electric field of the intense THz pulses induces cross-phase modulation in electro-optic crystals through the Pockels effect, leading to spectral shifting, broadening, and modulation of copropagating laser pulses. The observed THz-induced cross-phase modulation agrees well with a time-dependent phase-shift model.

Temperature dependent optical conductivities and dc resistivity of c-axis oriented superconducting ( ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}39.6\mathrm{K}$) ${\mathrm{MgB}}_{2}$ films ( $\ensuremath{\sim}450\mathrm{nm}$) have been measured. The normal state ab-plane optical conductivities can be described by the Drude model with a temperature independent Drude plasma frequency of ${\ensuremath{\omega}}_{p,D}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}13600\ifmmode\pm\else\textpm\fi{}100{\mathrm{cm}}^{\ensuremath{-}1}$ or $1.68\ifmmode\pm\else\textpm\fi{}0.01\mathrm{eV}$. The normal state resistivity is fitted by the Bloch-Gr\"uneisen formula with an electron-phonon coupling constant ${\ensuremath{\lambda}}_{\mathrm{tr}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.13\ifmmode\pm\else\textpm\fi{}0.02$. The optical conductivity spectra below ${T}_{c}$ of these films suggest that ${\mathrm{MgB}}_{2}$ is a multigap superconductor.

We have examined the recombination of excess quasiparticles in superconducting Pb by time-resolved far-infrared spectroscopy using a pulsed synchrotron source. The energy gap shift calculated by Owen and Scalapino [Phys. Rev. Lett. 28, 1559 (1972)] is directly observed, as is the associated reduction in the Cooper pair density. The relaxation process involves a two-component decay; the faster ( approximately 200 ps) is associated with the actual (effective) recombination process, while the slower ( approximately 10 to 100 ns) is due to heat transport across the film/substrate interface. The temperature dependence of the recombination process between 0. 5T(c) and 0.85T(c) is in good agreement with theory.

Granular superconductors are shown to have a far-infrared absorption that is larger when the samples are superconducting than when they are normal. By constrast, theoretical models for these materials predict that when the samples become superconducting, the absorption should decrease.