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Register a new userOptical Spectroscopy as a Probe of Gaps and Kinetic Electronic Energy in p- and n-type cuprates
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Authors
Nicole Bontemps
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The real part of the optical in-plane conductivity of p-- and n--type cuprates thin films at various doping levels was deduced from highly accurate reflectivity measurements. We present here a comprehensive set of optical spectral weight data as a function of the temperature $T (> T_c$), for underdoped and overdoped samples. The temperature dependence of the spectral weight is not universal. Using various cut-off frequencies for the spectral weight, we show that n--type Pr$_{2-x}$Ce$_x$CuO$_4$ and p--type Bi$_2$Sr$_2$CaCu$_2$O$_{8+delta}$ exhibit both similarities and striking differences. The Fermi surface is closed in overdoped metallic samples. In underdoped Pr$_{2-x}$Ce$_x$CuO$_4$ samples, it clearly breaks into arcs, giving rise to a pseudogap signature. It is argued that such a signature is subtle in underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+delta}$.
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The optical conductivity of CuO2 (copper-oxygen) planes in p- and n-type cuprates thin films at various doping levels is deduced from highly accurate reflectivity data. The temperature dependence of the real part sigma1(omega) of this optical conductivity and the corresponding spectral weight allow to track the opening of a partial gap in the normal state of n-type Pr{2-x}Ce(x)CuO4 (PCCO), but not of p-type Bi2Sr2CaCu2O(8+delta} (BSCCO) cuprates. This is a clear difference between these two families of cuprates, which we briefly discuss. In BSCCO, the change of the electronic kinetic energy Ekin - deduced from the spectral weight- at the superconducting transition is found to cross over from a conventional BCS behavior (increase of Ekin below Tc to an unconventional behavior (decrease of Ekin below Tc) as the free carrier density decreases. This behavior appears to be linked to the energy scale over which spectral weight is lost and goes into the superfluid condensate, hence may be related to Mott physics.
The low-frequency dielectric response of hole-doped insulators La_{2}Cu_{1-x}Li_{x}O_{4} and La_{2-x}Sr_{x}NiO_{4} shows a large dielectric constant epsilon ^{} at high temperature and a step-like drop by a factor of 100 at a material-dependent low temperature T_{f}. T_{f} increases with frequency and the dielectric response shows universal scaling in a Cole-Cole plot, suggesting that a charge glass state is realized both in the cuprates and in the nickelates.
The three-band model relevant to high temperature copper-oxide superconductors is solved using single-site dynamical mean field theory and a tight-binding parametrization of the copper and oxygen bands. For a band filling of one hole per unit cell the metal/charge-transfer-insulator phase diagram is determined. The electron spectral function, optical conductivity and quasiparticle mass enhancement are computed as functions of electron and hole doping for parameters such that the corresponding to the paramagnetic metal and charge-transfer insulator sides of the one hole per cell phase diagram. The optical conductivity is computed using the Peierls phase approximation for the optical matrix elements. The calculation includes the physics of Zhang-Rice singlets. The effects of antiferromagnetism on the magnitude of the gap and the relation between correlation strength and doping-induced changes in state density are determined. Three band and one band models are compared. The two models are found to yield quantitatively consistent results for all energies less than about 4eV, including energies in the vicinity of the charge-transfer gap. Parameters on the insulating side of the metal/charge-transfer insulator phase boundary lead to gaps which are too large and near-gap conductivities which are too small relative to data. The results place the cuprates clearly in the intermediate correlation regime, on the paramagnetic metal side of the metal/charge-transfer insulator phase boundary.
Ultrafast optical spectroscopy is used to study the antiferromagnetic f-electron system USb2. We observe the opening of two charge gaps at low temperatures (<45 K), arising from renormalization of the electronic structure. Analysis of our data indicates that one gap is due to hybridization between localized f-electron and conduction electron bands, while band renormalization involving magnons leads to the emergence of the second gap. These experiments thus enable us to shed light on the complex electronic structure emerging at the Fermi surface in f-electron systems.
We show that important anomalous features of the normal-state thermoelectric power S of high-Tc materials can be understood as being caused by doping dependent short-range antiferromagnetic correlations. The theory is based on the fluctuation-exchange approximation applied to Hubbard model in the framework of the Kubo formalism. Firstly, the characteristic maximum of S as function of temperature can be explained by the anomalous momentum dependence of the single-particle scattering rate. Secondly, we discuss the role of the actual Fermi surface shape for the occurrence of a sign change of S as a function of temperature and doping.
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