We perform infrared conductivity measurements on a series of CaCuO$_2$/SrTiO$_3$ heterostructures made by the insulating cuprate CaCuO$_2$ (CCO) and the insulating perovkite SrTiO$_3$ (STO). We estimate the carrier density of various heterostructures
, with different level of hole doping from the integral of the optical conductivity and we measure the corresponding degree of correlation by estimating the ratio between the Drude weight and the integral of the infrared spectrum. The analysis demonstrates a large degree of correlation, which increases as the doping is reduced. The experimental results can be reproduced by Dynamical Mean-Field Theory calculations which strongly support the role of correlations in the CCO/STO heterostructures and their similarities with the most common cuprate superconductors. Our results suggest that cuprate superconductors can be looked at as natural superlattices, where the properties of the CuO$_2$ conducting planes and charge reservoir blocks can be completely disentangled.
V2O3 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 i
nsulator (PI). 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 V2O3 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 which drives a genuine Mott transition to an eventually homogeneous metallic state.
We present reflectance measurements in the infrared region on a single crystal the rare earth scandate DyScO3. Measurements performed between room temperature and 10 K allow to determine the frequency of the infrared-active phonons, never investigate
d experimentally, and to get information on their temperature dependence. A comparison with the phonon peak frequency resulting from ab-initio computations is also provided. We finally report detailed data on the frequency dependence of the complex refractive index of DyScO3 in the terahertz region, which is important in the analysis of terahertz measurements on thin films deposited on DyScO3.
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 observat
ions 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.
We report the first optical study of CaAlSi, a superconductor which displays both the crystal structure of MgB2 and the electronic band structure of intercalated graphites. The reflectivity of a CaAlSi single crystal was measured down to sub-THz freq
uencies and to 3.3 K, with the use of Coherent Synchrotron Radiation. A superconducting gap in the hexagonal planes, two gaps along the c axis were found and measured, as expected from the structure of the CaAlSi Fermi surface. The anisotropic optical parameters of the normal state were also determined.
Heavily-boron-doped diamond films become superconducting with critical temperatures $T_c$ well above 4 K. Here we first measure the reflectivity of such a film down to 5 cm$^{-1}$, by also using Coherent Synchrotron Radiation. We thus determine the o
ptical gap, the field penetration depth, the range of action of the Ferrell-Glover-Tinkham sum rule, and the electron-phonon spectral function. We conclude that diamond behaves as a dirty BCS superconductor.
The reflectivity $R (omega)$ of both the $ab$ plane and the c axis of two single crystals of La$_{1.875}$Ba$_{0.125-y}$Sr$_{y}$CuO$_4$ has been measured down to 5 cm$^{-1}$, using coherent synchrotron radiation below 30 cm$^{-1}$. For $y$ = 0.085, a
Josephson Plasma Resonance is detected at $T ll T_c$ = 31 K in $R_{c} (omega)$, and a far-infrared peak (FIP) appears in the optical conductivity below 50 K, where non-static charge ordering (CO) is reported by X-ray scattering. For $y$ = 0.05 ($T_c$ = 10 K), a FIP is observed in the low-temperature tetragonal phase below the ordering temperature $T_{CO}$. At 1/8 doping the peak frequency scales linearly with $T_{CO}$, confirming that the FIP is an infrared signature of CO, either static or fluctuating.
Most of the novel superconductors are uniaxial crystals, with metallic planes ($ab$) orthogonal to an insulating axis ($c$). Far-infrared measurements of the reflectivity $R_{ab} (omega)$ provide valuable information on their low-energy electrodynami
cs, but involve delicate experimental issues. Two of them are a possible contamination of $R_{ab} (omega)$ from the c axis and the extrapolation of the $R_{ab}$ data to $omega$ =0, both above and below $T_c$. Here we discuss quantitatively these issues with particular regard to La$_{2-x}$Sr$_x$CuO$_{4+y}$, one of the most studied high-$T_c$ materials.
The optical conductivity of single crystals of Na_xCoO_2 for $x$ = 0.57 is first reported between 295 and 30 K. In the far infrared, an anomalous Drude analysis leads to a carrier effective mass of 5 electron masses. That this high value is due to st
rong electron-phonon coupling is suggested by the Fano distortion of a phonon at 570 cm^-1. A peak at 8800 cm^-1 scales with the charge transfer band of several high-$T_c$ cuprates by simply replacing the in-plane Cu-O bond length with that for Co-O.
The in-plane optical conductivity of seven La(2-x)Sr(x)CuO(4) single crystals with x between 0 and 0.15 has been studied from 30 to 295 K. All doped samples exhibit strong peaks in the far-infrared, which closely resemble those observed in Cu-O ladde
rs with one-dimensional charge-ordering. The behavior with doping and temperature of the peak energy, width, and intensity allows us to conclude that we are observing charge stripes dynamics in La(2-x)Sr(x)CuO(4) on the fast time scale of infrared spectroscopy.
