Far-infrared and mid-infrared absorption in (Bi,Pb)-Sr-(Ca,Y)-Cu-O

The reflectivities of seven films belonging to the Bi-Sr-Ca-Cu-O family, with T_c ranging from 60 to 95 K, and of a single crystal of insulating Bi-Sr-Y-Cu-O, have been measured at room temperature in the normal phase. The optical conductivity has been analyzed in terms of Drude (D), mid-infrared (MIR), and charge-transfer (CT) contributions. As T_c increases, the spectral weight within the charge-transfer gap (D+MIR) increases. Meanwhile, the peak frequency of the MIR band moves towards the far-infrared and the optical conductivity becomes more similar to that of a normal metal.     

Superconducting gap in iron pnictides studied by optical spectroscopy

We investigated the in-plane optical spectrum of a Co-doped Ba(Fe_0.94Co_0.06)₂As₂ single crystal showing superconductivity below . In the normal state, the low-energy optical conductivity spectrum can be decomposed into a sharp Drude term and a broad “incoherent” term. Below T_c, an s-wave-like superconducting gap appears in both components. We also investigated a magnetic or spin-density-wave gap in the detwinned parent compound.     

Optical conductivity of cuprates from Yang–Rice–Zhang ansatz: Comparison with experiment

We report the results of a theoretical investigation on charge dynamics in weakly coupled CuO₂ planes of the cuprate Ca_2 − xNa_xCuO₂Cl₂ in the pseudogap regime by using the theory of Yang, Rice, and Zhang (YRZ) [1]. With a detailed numerical analysis at various impurity scattering rates (_γimp${\gamma }_{\mathit{imp}}$), we observe that YRZ model is not able to reproduce (in magnitude) the experimentally observed frequency evolution of optical conductivity at a fixed doping level. Further, to analyze the doping evolution, we have done a detailed comparison of calculated YRZ conductivity with the experimental one using Two-Component Drude–Lorentz model. We find that YRZ model is capable of reproducing (qualitatively) the experimentally observed doping evolution of Drude processes (low energy scale) and processes at the pseudogap (intermediate energy scale). We also discuss physical reasons of the discrepancy seen in magnitudes.     

Molecular beam epitaxy of superconducting Pr2CuO4 films

Recent reports on superconductivity in parent compounds [RE = Pr, Nd, Sm, Eu, Gd] prepared by metal-organic decomposition (MOD) shed new light on the electronic and magnetic phase diagram of electron-doped cuprates. A thorough reduction process is the key to inducing superconductivity in square-planar coordinated cuprates. In this work, Pr₂CuO₄ films were grown epitaxially by molecular beam epitaxy (MBE) on (001)SrTiO₃, (110)GdScO₃ and (110)DyScO₃ substrates. A modified two-step reduction process is required in order to obtain single-phase superconducting samples. The optimized superconducting properties are , while the room-temperature resistivity is about . Hence, we show that even amorphous Pr₂CuO_y films deposited on (110)DyScO₃ become superconducting after the application of the two-step annealing process. These results indicate that superconductivity is induced to Pr₂CuO₄ by systematic optimization of the reduction conditions, irrespective of the synthesis route.     

Doping dependent evolution of infrared spectra in underdoped YBa2Cu3Oy in terms of two-component optical conductivity

In access to optical spectroscopy of heavily underdoped detwinned YBa₂Cu₃O_y (YBCO) crystals, we re-examine the doping dependent evolution of infrared spectra in the CuO₂ plane of underdoped YBCO in terms of two-component optical conductivity. The extended Drude model analysis is applied to the two-component conductivity, and the results are compared with experimental data in the pseudogap state. We demonstrate that a model consisting of a Drude and Lorentz oscillator components reproduces characteristics of infrared spectra in underdoped YBCO.     

Correlation effects obtained from optical spectra of Fe-pnictides using an extended Drude-Lorentz model analysis

We introduce an analysis model, an extended Drude–Lorentz model, and apply it to Fe-pnictide systems to extract their electron–boson spectral density functions (or correlation spectra). The extended Drude–Lorentz model consists of an extended Drude mode for describing correlated charge carriers and Lorentz modes for interband transitions. The extended Drude mode can be obtained by a reverse process starting from the electron–boson spectral density function and extending to the optical self-energy and, eventually, to the optical conductivity. Using the extended Drude–Lorentz model, we obtained the electron–boson spectral density functions of K-doped BaFe₂As₂ (Ba-122) at four different doping levels. We discuss the doping-dependent properties of the electron–boson spectral density function of K-doped Ba-122. We also can include pseudogap effects in the model using this approach. Therefore, this approach is very helpful for understanding and analyzing measured optical spectra of strongly correlated electron systems, including high-temperature superconductors (cuprates and Fe-pnictides).     

Optical properties of the iron-chalcogenide superconductor FeTe0.55Se0.45

The complex optical properties of the iron-chalcogenide superconductor FeTe_0.55Se_0.45 with T_c=14 K have been examined over a wide frequency range for light polarized in the Fe-Te(Se) planes above and below T_c. At room temperature the optical response may be described by a weakly interacting Fermi liquid; however, just above T_c this picture breaks down and the scattering rate takes on a linear frequency dependence. Below T_c there is evidence for two gap features in the optical conductivity at and . Less than 20% of the free carriers collapse into the condensate for T?T_c, and this material is observed to fall on the universal scaling line for a BCS dirty-limit superconductor in the weak-coupling limit.     

Rare earth ion effects on the pseudo-gap in electron-doped superconductors and possible nodeless d-wave gap

We report angle resolved photoemission (ARPES) studies on electron-doped cuprate superconductor Sm_2-xCe_xCuO₄ (x=0.14 and 0.18). A wide energy range scan shows clear “waterfall” effect at an energy scale close to 500 meV which is consistent with the value found in Nd_2-xCe_xCuO₄ (NCCO) but larger than that from hole-doped superconductors. High resolution results from both dopings show pseudo-gap effects that were observed in NCCO. However, the effects are found to be stronger than that observed in optimally doped NCCO. The overall electronic structure is well understood within a simple model in which a static order is assumed. Both ARPES and optical measurements give the coupling strengths to the Q=(π/2,π/2) (due to the order) to be about 0.1 eV, compatible with each other. The effect is strong enough to push the band near the nodal region below the Fermi energy, resulting in possible nodeless d-wave superconductivity where zero energy quasi-particle excitation is inhibited.     

Polarons and bipolarons in oxides

The physical properties of Fröhlich polarons and bipolarons are reviewed in relation to the recent experimental study of the mid-infrared absorption spectra of high- Tc cuprate superconductors, in particular, of Nd₂CuO_4-y , YBa₂Cu₃O_6+x and La₂CuO_4+x . The spectral region studied is of particular relevance for the open question of the presence of polarons and bipolarons in these materials, since it coincides with the frequency region where polaron and bipolaron absorption is to be expected. In our view, the optical absorption spectra as observed in the doped cuprates are a manifestation of the polaron type relaxed excited final states.     

Anomalous charge transfer in La<Subscript>2</Subscript>CuO<Subscript>4</Subscript> near the Néel temperature: Evidence of the transition from spin liquid to two-sublattice antiferromagnetic state

The temperature dependence of the electrical resistance and thermopower of La_{2? x}Sr_xCuO_4+δ single crystals with x ? 0.003 and δ<0.05 has been studied in the temperature range from 100 to 400 K. All crystals exhibiting two-dimensional hopping conductivity via neighboring acceptor sites in the CuO₂ plane show a significant difference in the charge transfer below and above the Néel temperature T_N. This difference indicates that the loss of a two-sublattice antiferromagnetic order strongly affects the charge transport in the CuO₂ plane. The obtained data lead to a conclusion that the crystal above T_N occurs in a resonance valence bond state of the Bose type.     

Dc and Ac conductivity of La2CuO4+δ

The complex conductivity of La₂CuO_4+δ has been investigated for frequencies 20 Hz≤ν≤4 GHz and temperatures 1.5K≤T≤450 K. Two single crystals with δ≈0 and δ≈0.02 were investigated, using dc (four-probe), reflectometric and contact-free techniques. At high temperatures the dc conductivity is thermally activated with low values of the activation energy. For low temperatures Mott's variable range hopping dominates. The real and imaginary parts of the ac conductivity follow a power-law dependence σ～v ^s, typical for charge transport by hopping processes. A careful analysis of the temperature dependence of the ac conductivity and of the frequency exponents has been performed. It is not possible to explain all aspects of the ac conductivity in La₂CuO_4+δ by standart hopping models. However, the observed minimum in the temperature dependence of the frequency exponents strongly suggests tunneling of large polarons as dominant transport process.     

Temperature dependence of the spectral weight in p- and n-type cuprates: A study of normal state partial gaps and electronic kinetic energy

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 σ₁ (ω) 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−xCe_xCuO₄ (PCCO) but not of p-type Bi₂Sr₂CaCu₂O_8+δ (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 E_kin—deduced from the spectral weight—at the superconducting transition is found to cross over from a conventional BCS behavior (increase of E_kin below T_c) to an unconventional behavior (decrease of E_kin below T_c) 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.     

Specific features of the behavior of optical properties upon the metal-insulator transition in EuBaCo<Subscript>2</Subscript>O<Subscript>5+δ</Subscript>

The temperature dependences of the optical properties and the electrical resistivity for EuBaCo₂O_5+δ single crystals are investigated. At temperatures below the metal-insulator transition (T _MI = 340 K), the electrical resistivity is well approximated by the relationship ρ = ρ₀exp(T/T ₀)^1/4. The optical band gap E _g = 0.05 eV for the insulating phase is underestimated as compared to the theoretical value. The specific features in the dispersion of the optical conductivity and the real part of the complex permittivity upon the metal-insulator transition are determined. It is demonstrated that the optical response from charge carriers on the metal side of the metal-insulator transition is caused by the redistribution of the spectral weight of the optical conductivity from the high-energy range to the low-energy range and exhibits a strongly incoherent character. The revealed features are associated with the manifestation of the strongly correlated metallic state.     

Five-fold way to new high <Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> superconductors

Discovery of high T _c superconductivity in La_2?x Ba _x CuO₄ by Bednorz and Muller in 1986 was a breakthrough in the 75-year long search for new superconductors. Since then new high T _c superconductors, not involving copper, have also been discovered. Superconductivity in cuprates also inspired resonating valence bond (RVB) mechanism of superconductivity. In turn, RVB theory provided a new hope for finding new superconductors through a novel electronic mechanism. This article first reviews an electron correlation-based RVB mechanism and our own application of these ideas to some new noncuprate superconducting families. In the process we abstract, using available phenomenology and RVB theory, that there are five directions to search for new high T _c superconductors. We call them five-fold way. As the paths are reasonably exclusive and well-defined, they provide more guided opportunities, than before, for discovering new superconductors. The five-fold ways are (i) copper route, (ii) pressure route, (iii) diamond route, (iv) graphene route and (v) double RVB route. Copper route is the doped spin-½ Mott insulator route. In this route one synthesizes new spin-½ Mott insulators and dopes them chemically. In pressure route, doping is not external, but internal, a (chemical or external) pressure-induced self-doping suggested by organic ET-salts. In the diamond route we are inspired by superconductivity in boron-doped diamond and our theory. Here one creates impurity band Mott insulators in a band insulator template that enables superconductivity. Graphene route follows from our recent suggestion of superconductivity in doped graphene, a two-dimensional broadband metal with moderate electron correlations, compared to cuprates. Double RVB route follows from our recent theory of doped spin-1 Mott insulator for superconductivity in iron pnictide family.     

Existence of two types of perfect Bi<Subscript>2</Subscript>Sr<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>La<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CuO<Subscript>6+δ</Subscript> single crystals

It is found that perfect Bi₂Sr_2?x La _x CuO_6+δ single crystals with the same concentrations of lanthanum x = 0.64 and excess oxygen δ = 0.237 exist in two types. Single crystals of the first type are obtained by slow cooling (the synthesis time is 90–105 h). They have a monoclinic superlattice and exhibit no superconducting transition down to 2 K. Crystals of the second type are obtained by rapid cooling (the synthesis time is 30–40 h) and are characterized by a orthorhombic superlattice and T _c = 18 K. Thus, the superconducting transition temperature is determined not only by the concentration of carriers but also by the configuration of defects. A rhombic superlattice prevails in single crystals obtained by slow cooling in the lanthanum concentration range x = 0.3–0.5, while a monoclinic superlattice dominates in the range x = 0.75–0.85. This fact explains the high values of T _c at optimal doping (x = 0.4) and the absence of high-T _c superconductivity at p < 0.10.     

Optical and transport properties of defective non-crystalline solids

A method is proposed to analyze the optical absorption and electrical conductivity of non-crystalline materials having a high concentration of localized mid-gap states. The method is applied to dual ion beam deposited silicon nitride films containing various N contents. In this method, the optical absorption spectrum is fitted by using hypothetical functions of the density of states (DOS). The contribution from electron transitions between localized states is taken into account. The DOS at the Fermi level (E_f) was found to be very high (10¹⁹–10²⁰ eV^-1cm^-3), so that hopping of charge carriers around E_f dominates the transport properties. Combined with the data of electrical conductivity, the hopping distance and the spread of the electron wave function of the charge carriers at E_f are calculated. Furthermore, the temperature dependence of the mobility of the charge carriers at E_f is also deduced. PACS 72.20.-i; 72.80.-r; 73.61.Jc; 78.30.Ly     

Effect of pressure on the electronic structure of cuprates with strongly correlated electrons

For cuprates of the n and p types, the effect of pressure on the electronic structure of a CuO₂ layer is studied. In the calculations performed, a generalized tight-binding method is used taking into account the influence of strong electronic correlations on the electronic structure of cuprates. The results obtained demonstrate the unusual effect of pressure on the nature of quasiparticle states at the top of the valence band in p-type cuprates. As the pressure increases, the hole states in these materials cease to be Zhang-Rice singlets and become combined singlet-triplet states.