The non-equilibrium control of emergent phenomena in solids is an important research frontier, encompassing effects like the optical enhancement of superconductivity 1 . Recently, nonlinear excitation 2 , 3 of certain phonons in bilayer cuprates was shown to induce superconducting-like optical properties at temperatures far above T c 4,5,6 . This effect was accompanied by the disruption of competing charge-density-wave correlations 7,8 , which explained some but not all of the experimental results. Here, we report a similar phenomenon in a very different compound. By exciting metallic K 3 C 60 with mid-infrared optical pulses, we induce a large increase in carrier mobility, accompanied by the opening of a gap in the optical conductivity. Strikingly, these sameReprints and permissions information is available online at www.nature.com/reprints.Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use:http://www.nature.com/authors/editorial_policies/license.html#termsCorrespondence and request for materials should be addressed to An.C. (andrea.cavalleri@mpsd.mpg.de). Author Contributions

Plasmons are quantized collective oscillations of electrons and have been observed in metals and doped semiconductors. The plasmons of ordinary, massive electrons have been the basic ingredients of research in plasmonics and in optical metamaterials for a long time. However, plasmons of massless Dirac electrons have only recently been observed in graphene, a purely two-dimensional electron system. Their properties are promising for novel tunable plasmonic metamaterials in the terahertz and mid-infrared frequency range. Dirac fermions also occur in the two-dimensional electron gas that forms at the surface of topological insulators as a result of the strong spin-orbit interaction existing in the insulating bulk phase. One may therefore look for their collective excitations using infrared spectroscopy. Here we report the first experimental evidence of plasmonic excitations in a topological insulator (Bi2Se3). The material was prepared in thin micro-ribbon arrays of different widths W and periods 2W to select suitable values of the plasmon wavevector k. The linewidth of the plasmon was found to remain nearly constant at temperatures between 6 K and 300 K, as expected when exciting topological carriers. Moreover, by changing W and measuring the plasmon frequency in the terahertz range versus k we show, without using any fitting parameter, that the dispersion curve agrees quantitatively with that predicted for Dirac plasmons.

Raman and combined trasmission and reflectivity mid infrared measurements have been carried out on monoclinic VO2 at room temperature over the 0-19 GPa and 0-14 GPa pressure ranges, respectively. The pressure dependence obtained for both lattice dynamics and optical gap shows a remarkable stability of the system up to P*∼10 GPa. Evidence of subtle modifications of V ion arrangements within the monoclinic lattice together with the onset of a metallization process via band gap filling are observed for P>P*. Differently from ambient pressure, where the VO2 metal phase is found only in conjunction with the rutile structure above 340 K, a new room temperature metallic phase coupled to a monoclinic structure appears accessible in the high pressure regime, thus opening to new important queries on the physics of VO2. PACS numbers:Since the first observation of the metal to insulator transition (MIT) in several vanadium oxides, these materials attracted considerable interest because of the huge and abrupt change of the electrical properties at the MIT. As usual in transition metal oxides, electronic correlation strongly affects the conduction regime of vanadium oxides, although, in some compounds, lattice degrees of freedom seem to play an important role. This is the case of VO 2 , which undergoes a first order transition from a high temperature metallic rutile (R) phase to a low temperature insulating monoclinic (M1) one. At the MIT temperature, T c =340 K, the opening of an optical gap in the mid-infrared (MIR) conductivity and a jump of several order of magnitude in the resistivity are observed [1]. The interest on this compound is thus mainly focused on understanding the role and the relative importance of the electron-electron and the electron-lattice interaction in driving the MIT. Despite the great experimental and theoretical efforts [2], the understanding of this transition is still far from being complete [3,4,5,6,7]. In the R phase the V atoms, each surrounded by an oxygen octahedron, are equally spaced along linear chains in the c-axis direction and form a body-centered tetragonal lattice. On entering the M1 insulating phase the dimerization of the vanadium atoms and the tilting of the pairs with respect to the c axis lead to a doubling of the unit cell, with space group changing from C 5 2h (R) to D 14 4h (M1) [8,9]. As first proposed by Goodenough [10], the V-V pairing and the off-axis zig-zag displacement of the dimers lead to a band splitting with the formation of a Peierls-like gap at the Fermi level. First principle electronic structure calculations based on local density approximation (LDA) showed the band splitting on entering the monoclinic phase, but failed to yield the opening of the band gap [11,12]. In fact, as early pointed out [13], the electron-electron correlation has to be taken into account to obtain the insulating phase. A recent theoretical paper where the electronic Coulomb repulsion U is properly accounted for, shows that calculations carried out joining dynamical mean field theory with...

V 2 O 3 is the prototype system for the Mott transition, one of the most fundamental phenomena of electronic correlation. Temperature, doping or pressure induce a metal-to-insulator transition (MIT) between a paramagnetic metal (PM) and a paramagnetic insulator. This or related MITs have a high technological potential, among others, for intelligent windows and field effect transistors. However the spatial scale on which such transitions develop is not known in spite of their importance for research and applications. Here we unveil for the first time the MIT in Cr-doped V 2 O 3 with submicron lateral resolution: with decreasing temperature, microscopic domains become metallic and coexist with an insulating background. This explains why the associated PM phase is actually a poor metal. The phase separation can be associated with a thermodynamic instability near the transition. This instability is reduced by pressure, that promotes a genuine Mott transition to an eventually homogeneous metallic state.

Raman and infrared transmission and reflectivity measurements were carried out at room temperature and high pressure ͑0-15 GPa͒ on V 1−x Cr x O 2 compounds. Raman spectra were collected at ambient conditions on the x = 0.007 and 0.025 materials, which are characterized by different insulating monoclinic phases ͑M3 and M2, respectively͒, while infrared spectra were collected on the x = 0.025 sample only. The present data were compared with companion results on undoped VO 2 ͓E. Arcangeletti et al., Phys. Rev. Lett. 98, 196406 ͑2007͔͒, which is found at ambient conditions in a different, third insulating monoclinic phase, named M1. This comparison allowed us to investigate the effects of different extents of structural distortions ͑Peierls distortion͒ on the lattice dynamics and the electronic properties of this family of compounds. The pressure dependence of the Raman spectrum of VO 2 and Cr-doped samples shows that all the systems retain the monoclinic structure up to the highest explored pressure, regardless the specific monoclinic structure ͑M1, M2, and M3͒ at ambient condition. Moreover, the Raman spectra of the two Cr-doped samples, which exhibit an anomalous behavior over the low-pressure range ͑P Ͻ 8 GPa͒, merge into that of VO 2 in the high-pressure regime and are all found into a common monoclinic phase ͑a possible fourth kind phase͒. Combining Raman and infrared results on both the VO 2 and the present data, we found that a common metallic monoclinic phase appears accessible in the high-pressure regime at room temperature for both undoped and Cr-doped samples independently of the different extents of Peierls distortion at ambient conditions. This finding differs from the behavior observed at ambient pressure, where the metallic phase is found only in conjunction with the rutile structure. The whole of these results suggests a major role of the electron correlations, rather than of the Peierls distortion, in driving the metal-insulator transition in vanadium dioxide systems, thus opening to new experimental and theoretical queries.

In La2-xSrxCuO4 (LSCO) the spectral weight W=integralOmega0sigma(ab)1(omega,T)domega [where sigma(ab)1(omega,T) is the ab-plane conductivity] obeys the same law W=W0-BOmegaT2 as in a conventional metal such as gold, for any Omega up to the plasma edge. However, in LSCO BOmega points toward correlation effects and, unlike in gold, is related to an energy scale tT<

The optical conductivity σ1(ω) and the spectral weight SW of four topological insulators with increasing chemical compensation (Bi2Se3,Bi2Se2Te,Bi 2-xCaxSe3, and Bi2Te2Se) have been measured from 5 to 300 K and from subterahertz to visible frequencies. The effect of compensation is clearly observed in the infrared spectra through the suppression of an extrinsic Drude term and the appearance of strong absorption peaks that we assign to electronic transitions among localized states. From the far-infrared spectral weight SW of the most compensated sample (Bi2Te2Se), one can estimate a density of charge carriers on the order of 1017/cm3 in good agreement with transport data. Those results demonstrate that the low-energy electrodynamics in single crystals of topological insulators, even at the highest degree of compensation presently achieved, is still influenced by three-dimensional charge excitations. © 2012 American Physical Society

Electrons with a linear energy/momentum dispersion are called massless Dirac electrons and represent the low-energy excitations in exotic materials such as graphene and topological insulators. Dirac electrons are characterized by notable properties such as a high mobility, a tunable density and, in topological insulators, a protection against backscattering through the spin–momentum locking mechanism. All those properties make graphene and topological insulators appealing for plasmonics applications. However, Dirac electrons are expected to present also a strong nonlinear optical behaviour. This should mirror in phenomena such as electromagnetic-induced transparency and harmonic generation. Here we demonstrate that in Bi2Se3 topological insulator, an electromagnetic-induced transparency is achieved under the application of a strong terahertz electric field. This effect, concomitantly determined by harmonic generation and charge-mobility reduction, is exclusively related to the presence of Dirac electron at the surface of Bi2Se3, and opens the road towards tunable terahertz nonlinear optical devices based on topological insulator materials.