Recent studies of the electromagnetic response of high-Tc superconductors using terahertz, infrared, and optical spectroscopies are reviewed. In combination these experimental techniques provide a comprehensive picture of the low-energy excitations and charge dynamics in this class of materials. These results are discussed with an emphasis on conceptual issues, including evolution of the electronic spectral weight in doped Mott-Hubbard insulators, the d-wave superconducting energy gap and the normal-state pseudogap, anisotropic superfluid response, electronic phase segregation, emergence of coherent electronic state as a function of both temperature and doping, the vortex state, and the energetics of the superconducting transition. Because the theoretical understanding of these issues is still evolving the review is focused on the analysis of the universal trends that are emerging out of a large body of work carried on by many research teams. Where possible data generated by infrared/optical techniques are compared with the data from other spectroscopic and transport methods.

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Electrodynamics of high- T c superconductors

This paper presents simple correct models of high-temperature superconductor (HTS) film parameters at microwave frequencies. The models are based on the enhanced two-fluid model of a superconductor. The quasi-particle scattering and peculiarities of the normal conductivity of the HTS at microwaves, including residual resistance of a material, are taken into account. The difference between the known Gorter and Casimir two-fluid model of low-temperature superconductor and the enhanced two-fluid model of HTS is proven. A simple quasi-static model of current distribution across the microstrip line and coplanar waveguide is used for a simulation of a contribution of the superconductor transport processes into impedance per unit length of the transmission lines considered. The models developed were applied to a simulation of microstrip line and coplanar waveguide resonators. Good agreement of simulated results and measurements in a wide temperature range has been demonstrated. The model presented can be considered as a starting point for the formation of the computer-aided design (CAD) package of HTS microwave components.

Empirical model of the microwave properties of high-temperature superconductors

The paper presents a survey on the dielectric analysis of integrated ceramic capacitors. The thin film examples are based on high–permittivity paraelectric SrTiO3, solid–solution (Ba,Sr)TiO3, ferroelectric Pb(Zr,Ti)O3, and superparaelectric BaTiO3. The current state of knowledge concerning the temperature dependence, the voltage dependence, and the frequency dependence of the permittivity and the losses is reviewed. The relation of the dielectric properties to the processing and microstructure of the ceramic thin films is outlined. With respect to the understanding of the leakage mechanism, further evidence for a complete depletion of the titanate films is presented and a refined quantitative model of the field dependence is sketched.

Dielectric analysis of intergrated ceramic thin film capacitors

The microwave properties of single crystalline TiO2 (rutile) were investigated. At a frequency of 7.5 GHz the loss tangent tan δ was found to increase from 1.4×10−7 at 4 K to 4×10−6 at 70 K for electric fields parallel to the crystallographic a,b plane. The high permittivity of 105 and the small tanδ in combination with the low microwave losses of high temperature superconductors (HTS) were utilized to construct a miniaturized X‐band resonator with a high quality factor Q. An assembly of two YBa2Cu3O7 films of 8 mm in diameter separated by a rutile cylinder of 2 mm height provides a TE011 resonance at 9.7 GHz with Qs ranging from 6×105 at 10 K to 105 at 70 K. Frequency scaling of the losses in rutile and in the HTS films indicates Qs in excess of 106 at 1.8 GHz using YBa2Cu3O7 films of two inches in diameter. Such resonators are considered to be key elements for high‐power filters in mobile communications.

Dielectric properties of rutile and its use in high temperature superconducting resonators

We have fabricated 12 dc SQUIDs by laser depositing YBa[sub 2]Cu[sub 3]O[sub 7-x] on a SrTiO[sub 3] bicrystal substrate with a misorientation angle of 24*. At 77K all twelve devices had acceptable values of critical current, resistance and voltage modulation produced by an external magnetic field. The white noise energy of one device with an estimated inductance of 41 pH was 1.8 [times] 10[sup [minus]30] JHz[sup [minus]1]. The noise power scaled as 1/f at frequencies below about 1kHz, however, by using a bias current reversal scheme we were able to reduce this noise by two orders of magnitude at 1 Hz, to a value of about 1.5 [times] 10[sup [minus]29] JHz[sup [minus]1]. We made a magnetometer by coupling the SQUID to a flux transformer with a 5-turn input coil. The measured magnetic field gain was 60, and the white noise 36fT Hz[sup [minus][1/2]]. However, the transformer produced relatively large levels of 1/f flux noise, not reduced by the bias reversal scheme, that limited the noise at 1 Hz to 1.7 pT Hz[sup [minus][1/2]]. A single-layer magnetometer with a single-turn pick up loop is briefly described.

Bicrystal YBCO dc squids with low noise

A millimeter wave spectrometer for frequencies between 100 and 350 GHz consisting of continuously tunable backward wave oscillators as sources and a quasioptical interferometer in the Mach-Zehnder configuration was used to measure the transmittivity in phase and amplitude of YBa{sub 2}Cu{sub 3}O{sub 7} thin films on NdGaO{sub 3} substrates. From the measured spectra the authors derived the real and imaginary part of the dynamic conductivity {sigma} = {sigma}{sub 1} + {sigma}{sub 2} in the superconducting state as a function of temperature. The {sigma}{sub 1}(T) and {sigma}{sub 2}(T) values at 300 GHz were compared to corresponding values at 19 GHz determined by surface impedance measurements of the same films using a shielded dielectric resonator. The observed frequency dependence of both {sigma}{sub 1}(T) and {sigma}{sub 2}(T) is consistent with a strong reduction of the quasiparticle scattering rate {tau}{sup {minus}1}(T) with decreasing temperature below {Tc}.

Frequency and temperature dependence of the millimeter wave conductivity of epitaxial YBa2Cu3O7 films

The technique of the edge preparation for multilayer Josephson edge type junctions on the basis of c-axis oriented YBa/sub 2/Cu/sub 3/O/sub 7/ thin films with the help of etching in Br-ethanol solution was improved. Microwave induced Shapiro steps, Josephson radiation, and characteristics of dc-SQUIDs were investigated. The developed technique prevents the junctions and device properties from degradation due to aging or thermal cycling. >

Application of Josephson edge type junctions with a PrBa/sub 2/Cu/sub 3/O/sub 7/ barrier prepared with Br-ethanol etching or cleaning

A millimeterwave spectrometer using continuously tunable backward wave oscillators as sources and a quasioptical interferometer in the Mach-Zehnder configuration was used to measure the transmittivity in amplitude and phase of 60nm thin YBa2Cu3O7 and 250nm thin Bi2Sr2Ca1Cu2O8 films on NdGaO3 substrates at 300GHz for temperatures between 4K and 300K. From the spectra we derived the real and imaginary part of the dynamic conductivity σ = σ1 + iσ2 in the normal and superconducting state. From σ2 we got absolute values for the London penetration depth λ. Beside the lower σ1(T) values for the BSCCO films over the entire temperature range a clear maximum in σ1(T) was measured at 60K for both compounds. The λ(T) values of the BSCCO films are more than a factor of two higher in comparison to YBCO. Using a Drude-model for the normal and superconducting state a strong temperature decrease of the quasiparticle scattering rate τ−(T) below TC was observed for both compounds, whereas the absolute τ−1(T) values are smaller for the BSCCO-films.

Yu . Goncharov

Papers

Frequency and temperature dependence of the millimeter wave conductivity of epitaxial YBa2Cu3O7 films

Application of Josephson edge type junctions with a PrBa/sub 2/Cu/sub 3/O/sub 7/ barrier prepared with Br-ethanol etching or cleaning

Temperature dependence of the millimeterwave conductivity of YBa2Cu3O7 and Bi2Sr2Ca1Cu2O8 Thin films at 300 GHZ