Far-infrared absorption and the metal-to-insulator transition in hole-doped cuprates

By studying the optical conductivity of BSLCO and YCBCO, we show that the metal-to-insulator transition (MIT) in these hole-doped cuprates is driven by the opening of a small gap at low T in the far infrared. Its width is consistent with the observations of Angle-Resolved Photoemission Spectroscopy in other cuprates, along the nodal line of the k-space. The gap forms as the Drude term turns into a far-infrared absorption, whose peak frequency can be approximately predicted on the basis of a Mott-like transition. Another band in the mid infrared softens with doping but is less sensitive to the MIT.Comment: To be published on Physical Review Letter

Optical properties of V2O3 in its whole phase diagram

Vanadium sesquioxide V2O3 is considered a textbook example of Mott-Hubbard physics. In this paper we present an extended optical study of its whole temperature/doping phase diagram as obtained by doping the pure material with M=Cr or Ti atoms (V1-xMx)2O3. We reveal that its thermodynamically stable metallic and insulating phases, although macroscopically equivalent, show very different low-energy electrodynamics. The Cr and Ti doping drastically change both the antiferromagnetic gap and the paramagnetic metallic properties. A slight chromium content induces a mesoscopic electronic phase separation, while the pure compound is characterized by short-lived quasiparticles at high temperature. This study thus provides a new comprehensive scenario of the Mott-Hubbard physics in the prototype compound V2O3

High-temperature optical spectral weight and Fermi liquid renormalization in Bi-based cuprates

The optical conductivity and the spectral weight W(T) of two superconducting cuprates at optimum doping, Bi2Sr2-xLaxCuO6 and Bi2Sr2CaCu2O8, have been first measured up to 500 K. Above 300 K, W(T) deviates from the usual T2 behavior in both compounds, even though the zero-frequency extrapolation of the optical conductivity remains larger than the Ioffe-Regel limit. The deviation is surprisingly well described by the T4 term of the Sommerfeld expansion, but its coefficients are enhanced by strong correlation. This renormalization is due to strong correlation, as shown by the good agreement with dynamical mean field calculations.Comment: 5 pages, 3 figures, Physical Review Letters in pres

The THz Radiation Source at the SPARC Facility

The interest for Terahertz (THz) radiation is rapidly growing, both as it is a powerful tool for investigating the behavior of matter at low energy, and as it allows for a number of possible spectroscopic applications spanning from medical science to security. The linacdriven THz source at the SPARC facility can deliver broadband THz pulses with femtosecond shaping and can be used for electron beam diagnostics to fully reconstruct the longitudinal charge distribution. Beyond this application, the possibility to store much more energy in a single THz pulse than table-top sources renders the SPARC THz source very interesting for a spectroscopic use. In addition, taking advantage from electron beam manipulation techniques, high power, narrow-band THz radiation can be also generated. Those source characteristics provide a unique chance to realize THz-pump/THz-probe spectroscopy, a technique practically unexplored up to now

Observation of Dirac plasmons in a topological insulator

Plasmons are the quantized collective oscillations of electrons in metals and doped semiconductors. The plasmons of ordinary, massive electrons are since a long time basic ingredients of research in plasmonics and in optical metamaterials. Plasmons of massless Dirac electrons were instead recently observed in a purely two-dimensional electron system (2DEG)like graphene, and their properties are promising for new tunable plasmonic metamaterials in the terahertz and the mid-infrared frequency range. Dirac quasi-particles are known to exist also in the two-dimensional electron gas which forms at the surface of topological insulators due to a strong spin-orbit interaction. Therefore,one may look for their collective excitations by using infrared spectroscopy. Here we first report evidence of plasmonic excitations in a topological insulator (Bi2Se3), that was engineered in thin micro-ribbon arrays of different width W and period 2W to select suitable values of the plasmon wavevector k. Their lineshape was found to be extremely robust vs. temperature between 6 and 300 K, as one may expect for the excitations of topological carriers. Moreover, by changing W and measuring in the terahertz range the plasmonic frequency vP vs. k we could show, without using any fitting parameter, that the dispersion curve is in quantitative agreement with that predicted for Dirac plasmons.Comment: 11 pages, 3 figures, published in Nature Nanotechnology (2013

The THz radiation source at SPARC

The linac driven coherent THz radiation source at the SPARC facility is able to deliver broadband THz pulses with femtosecond shaping. In addition, high peak power, narrow-band THz radiation can be also generated, taking advantage from advanced electron beam manipulation techniques, able to generate a train of short electron bunches with THz repetition rate

PRESENT AND PERSPECTIVES OF THE SPARC THz SOURCE

The development of radiation sources in the Terahertz (THz) spectral region has become more and more interesting because of the peculiar characteristics of this radiation: it is non ionizing, it penetrates dielectrics, it is highly absorbed by polar liquids, highly reflected by metals and reveals specific “fingerprint”absorption spectra arising from fundamentals physical processes. The THz source at SPARC is a linac-based source for both longitudinal beam diagnostics and research investigations. Its measured peak power is of the order of 108 W, very competitive with respect to other present sources. The status of the THz radiation source, in particular its generation and properties, is presented and future perspectives are discusse

An extended infrared study of the (p,T) phase diagram of the p-doped Cu-O plane

The ab-plane optical conductivity of eleven single crystals, belonging to the families Sr2-xCuO2Cl2, Y1-xCaxBa2Cu3O6, Bi2Sr2-xLaxCuO6, and Bi2Sr2CaCu2O8 has been measured with hole concentrations p between 0 and 0.18, and for 6 K < T < 500 K to obtain an infrared picture of the p,T phase diagram of the Cu-O plane. At extreme dilution (p = 0.005), a narrow peak is observed at 1570 cm-1 (195 meV), that we assign to a single-hole bound state. For increasing doping, that peak broadens into a far-infrared (FIR) band whose low-energy edge sets the insulating gap. The insulator-to-metal transition (IMT) occurs when the softening of the FIR band closes the gap thus evolving into a Drude term. In the metallic phase, a multi-band analysis identifies a mid-infrared band which weakly depends on temperature and softens for increasing p, while the extended-Drude analysis leads to an optical scattering rate larger than the frequency, as found in other cuprates. The infrared spectral weight W(T) is consistent with a Fermi liquid renormalized by strong correlations, provided that the T^4 term of the Sommerfeld expansion is included above 300 K. In the superconducting phase, the optical response of single-layer Bi2Sr2-xLaxCuO6 at optimum doping is similar to that of the corresponding optimally-doped bilayer Bi2Sr2CaCu2O8.Comment: 28 pages, 14 figure