arXiv subjects
Publications and source records attributed to M. Strongin.
Following a short discussion of the granular model for an inhomogeneous superconductor, we review the Uemura and Homes correlations and show how both follow in two limits of a simple granular superconductor model. Definite expressions are given for the almost universal coefficients appearing in these relationships in terms of known constants.
In many cases inhomogeneities are known to exist near the metal (or superconductor)-insulator transition, as follows from well-known domain-wall arguments. If the conducting regions are large enough (i.e. when the T=0 superconducting gap is much larger than the single-electron level spacing), and if they have superconducting correlations, it becomes energetically favorable for the system to go into a Josephson-coupled zero-resistance state before (i.e. at higher resistance than) becoming a "real" metal. We show that this is plausible by a simple comparison of the relevant coupling constants. For small grains in the above sense, the electronic grain structure is washed out by delocalization and thus becomes irrelevant. When the proposed "Josephson state" is quenched by a magnetic field, an insulating, rather then a metallic, state should appear. This has been shown to be consistent with the existing data on oxide materials as well as ultra-thin films. We discuss the Uemura correlations versus the Homes law, and derive the former for the large-grain Josephson array (inhomogenous superconductor) model. The small-grain case behaves like a dirty homogenous metal. It should obey the Homes law provided that the system is in the dirty supeconductivity limit. A speculation why that is typically the case for d-wave superconductors is presented.
A scaling relation ρ_s \simeq 35σ_{dc}T_c has been observed in the copper-oxide superconductors, where ρ_s is the strength of the superconducting condensate, T_c is the critical temperature, and σ_{dc} is the normal-state dc conductivity close to T_c. This scaling relation is examined within the context of a clean and dirty-limit BCS superconductor. These limits are well established for an isotropic BCS gap 2Δand a normal-state scattering rate 1/τ; in the clean limit 1/τ\ll 2Δ, and in the dirty limit 1/τ> 2Δ. The dirty limit may also be defined operationally as the regime where ρ_s varies with 1/τ. It is shown that the scaling relation ρ_s \propto σ_{dc}T_c is the hallmark of a BCS system in the dirty-limit. While the gap in the copper-oxide superconductors is considered to be d-wave with nodes and a gap maximum Δ_0, if 1/τ> 2Δ_0 then the dirty-limit case is preserved. The scaling relation implies that the copper-oxide superconductors are likely to be in the dirty limit, and that as a result the energy scale associated with the formation of the condensate is scaling linearly with T_c. The a-b planes and the c axis also follow the same scaling relation. It is observed that the scaling behavior for the dirty limit and the Josephson effect (assuming a BCS formalism) are essentially identical, suggesting that in some regime these two effects may be viewed as equivalent. This raises the possibility that electronic inhomogeneities in the copper-oxygen planes may play an important role in the nature of the superconductivity in the copper-oxide materials.
We present a new method of extracting electron-boson spectral function $α^2$F($ω$) from infrared and photoemission data. This procedure is based on inverse theory and will be shown to be superior to previous techniques. Numerical implementation of the algorithm is presented in detail and then used to accurately determine the doping and temperature dependence of the spectral function in several families of high-T$_c$ superconductors. Principal limitations of extracting $α^2$F($ω$) from experimental data will be pointed out. We directly compare the IR and ARPES $α^2$F($ω$) and discuss the resonance structure in the spectra in terms of existing theoretical models.
In many cases inhomogeneities are known to exist near the metal (or superconductor)- insulator transition, as follows from well-known domain-wall arguments. If the conducting regions are large enough, and if they have superconducting correlations, it becomes energetically favorable for the system to go into a Josephson- coupled zero-resistance state before (i.e. at higher resistance than) the material becomes a real metal. We show that this is plausible by a simple comparison of the relevant coupling constants. We also illustrate using data in the literature on oxide materials as well as ultra-thin films, that when this proposed Josephson state is quenched by a magnetic field, an insulating, rather then a metallic, state indeed appears.
Scaling laws express a systematic and universal simplicity among complex systems in nature. For example, such laws are of enormous significance in biology. Scaling relations are also important in the physical sciences. The seminal 1986 discovery of high transition-temperature (high-T_c) superconductivity in cuprate materials has sparked an intensive investigation of these and related complex oxides, yet the mechanism for superconductivity is still not agreed upon. In addition, no universal scaling law involving such fundamental properties as T_c and the superfluid density ρ_s, a quantity indicative of the number of charge carriers in the superconducting state, has been discovered. Here we demonstrate that the scaling relation ρ_s \propto σ_{dc} T_c, where the conductivity σ_{dc} characterizes the unidirectional, constant flow of electric charge carriers just above T_c, universally holds for a wide variety of materials and doping levels. This surprising unifying observation is likely to have important consequences for theories of high-T_c superconductivity.
The optical properties of optimally-doped Bi_2Sr_2CaCu_2O_{8+δ} (Bi2212) have been measured normal to the edge planes [ac plane, bc plane, and (1\bar{1}0) plane], for light polarized parallel to nodal and anti-nodal (gap) directions, respectively. While the superfluid contribution can be obtained from the optical conductivities in the (1\bar{1}0)-plane, it is unobservable in the ac and bc-planes. This apparent asymmetry implies that the edge region of high-T_c cuprates is unusual and further supports a d-wave symmetry of the superconducting order parameter.
Temperature dependent optical conductivities and DC resistivity of c-axis oriented superconducting (Tc = 39.6 K) MgB2 films (~ 450 nm) have been measured. The normal state ab-plane optical conductivities can be described by the Drude model with a temperature independent Drude plasma frequency of ω_{p,D}=13,600 +/- 100 cm-1 or 1.68 +/- 0.01 eV. The normal state resistivity is fitted by the Bloch-Gruneisen formula with an electron-phonon coupling constant λ_{tr} = 0.13 +/- 0.02. The optical conductivity spectra below T_c of these films suggest that MgB2 is a multi-gap superconductor.
Carrier dynamics in amorphous a-Si$_{1-x}$RE$_{x}$ (RE=Gd, Y) films has been studied in the doping regime close to the metal-insulator transition by means of infrared spectroscopy. Optical constants throughout the entire intra-gap region ($\hbar ω<$ 1 eV) have been found to be anomalously sensitive to changes of temperature and/or magnetic field. The observed behavior is consistent with the model of hopping transport where the interaction of carriers with both the lattice and large core spin of Gd ions is taken into account.
An analysis of the frequency-dependent scattering rate reveals signatures of the superconducting gap as well as the pseudogap in high-T_c cuprates. These features can be identified in the temperature-dependent spectral function W(omega), inverted from the optical data of the optimally-doped Bi2212 using an experimentally unambiguous method that shows the behavior of both the maxima and minima in the spectral function.
We report on generic trends in the behavior of the interlayer penetration depth $λ_c$ of several different classes of quasi two-dimensional superconductors including cuprates, Sr$_2$RuO$_4$, transition metal dichalcogenides and organic materials of the $(BEDT-TTF)_2X$-series. Analysis of these trends reveals two distinct patterns in the scaling between the values of $λ_c$ and the magnitude of the DC conductivity: one realized in the systems with a Fermi liquid (FL) ground state and the other seen in systems with a marked deviation from the FL response. The latter pattern is found primarily in under-doped cuprates and indicates a dramatic enhancement (factor $\simeq 10^2$) of the energy scale $Ω_C$ associated with the formation of the condensate compared to the data for the FL materials. We discuss implications of these results for the understanding of pairing in high-$T_c$ cuprates.
In this paper we report the first experimental measurement of the infrared conductivity of ultra-thin quenched-condensed Pb films. For dc sheet resistances such that $\omega \tau \ll 1$ the ac conductance increases with frequency but is in disagreement with the predictions of weak localization. We attribute this behavior to the effects of an inhomogeneous granular structure of these films, which is manifested at the very small probing scale of infrared measurements. Our data are consistent with predictions of two-dimensional percolation theory.