Basic similarities among cuprate, bismuthate, organic, Chevrel-phase, and heavy-fermion superconductors shown by penetration-depth measurements

Uemura, Y. J.; Le, L. P.; Luke, G. M.; Sternlieb, B. J.; Wu, W. D.; Brewer, J.H.; Riseman, T. M.; Seaman, C. L.; Maple, M. B.; Ishikawa, M.; Hinks, D. G.; Jorgensen, J. D.; Saito, G.; Yamochi, Hideki

doi:10.1103/physrevlett.66.2665

Cited by 728 publications

(367 citation statements)

References 17 publications

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“…B) More importantly, this conclussion is supported on empirical grounds, if we use measured values of the London penetration depth to deduce T θ . Indeed, not only is T θ comparable in magnitude to T c , it often exhibits similar trends with doping and changes in other material properties, especially in the "underdoped region" where x is small; this important correlation was originally noticed by Uemura et al [4], and is called the "Uemura relation." On the otherhand, in "overdoped" materials, T θ overestimates T c by as much as a factor of three or four.…”

Section: Introductionmentioning

confidence: 93%

Making high Tc higher: a theoretical proposal

Kivelson

2002

Physica B: Condensed Matter

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There is considerable evidence that the highest Tc obtainable in a copper-oxide plane is limitted by the competition between two effects: On the one hand, as the concentration of doped-holes, x, is increased, the pairing scale, which is related to the properties of a doped Mott insulator, decreases. On the otherhand, the superfluid density, which controls the stiffness of the system to phase fluctuations, vanishes as x → 0, and increases with increasing x. Optimal Tc is obtained at a crossover from a phase ordering dominated regime at small x to a pairing dominated regime at large x. If this description is valid, then higher Tc's can be obtained in an array of coupled planes with different doped hole concentrations, such that a high pairing scale is derived from the underdoped planes and a large phase stiffness from the optimally or overdoped ones.

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Section: Introductionmentioning

confidence: 93%

Making high Tc higher: a theoretical proposal

Kivelson

2002

Physica B: Condensed Matter

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“…Originally Uemura et al [6] suggested a relation ρ s ∝ T c which works well in the case of underdoped cuprates. More recently, Homes et al [7] extended the scaling relation by an additional term covering the optimal and overdoped cuprates as well: ρ s = (125 ± 25)σ dc T c , where the σ dc is the value of dc conductivity just above T c .…”

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confidence: 99%

Superfluid density of Ba(Fe1-xMx)2<

Barišić

Drichko

et al. 2010

Physica C: Superconductivity and its Applications

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The temperature dependence of the ab-plane optical reflectivity of Ba(Fe0.92Co0.08)2As2 and Ba(Fe0.95Ni0.05)2As2 single crystals is measured in a wide spectral range. Upon entering the superconducting regime, the reflectivity in both compounds increases considerably at low frequency, leading to a clear gap in the optical conductivity below 100 cm −1 . From the analysis of the complex conductivity spectra we obtain the penetration depth λ(T ) = (3500 ± 350)Å for Ba(Fe0.92Co0.08)2As2 and (3000 ± 300)Å for Ba(Fe0.95Ni0.05)2As2. The calculated superfluid density ρs of both compounds nicely fits Homes' scaling relation ρs = (125 ± 25)σ dc Tc. . Here we present a comprehensive optical study on optimal electron-doped Ba(Fe 0.92 Co 0.08 ) 2 As 2 (T c =25 K) and Ba(Fe 0.95 Ni 0.05 ) 2 As 2 (T c =20 K) single crystals. The superconducting gap is clearly observed in reflectivity spectra for T < T c . The spectral weight analysis on optical conductivity provides information on the penetration depth and the superfluid density.Single crystals of Ba(Fe 0.92 Co 0.08 ) 2 As 2 and Ba(Fe 0.95 Ni 0.05 ) 2 As 2 were synthesized using selfflux method [3]. The platelets with a typical size of 2 mm × 2 mm × 0.1 mm have naturally flat and shiny surfaces. The resistivity and susceptibility evidence sharp superconducting transitions at 25 K and 20 K, indicating that samples are doped uniformly [4]. The temperature dependence of optical reflectivity was measured in a wide frequency range from 20 to 37 000 cm −1 using a coherentsource spectrometer in the THz range, infrared Fourier transform spectrometers (Bruker IFS 66v/s and IFS 113v) and a Woollam spectroscopic ellipsometer extending up to the ultraviolet. The lowfrequency extrapolation was done according to the dc conductivity measured on the same crystals by standard four-probe method. The optical conductivity was calculated from the reflectivity spectra using Kramers-Kronig analysis. Fig. 1 shows the temperature dependent reflectivity and conductivity of Ba(Fe 1−x M x ) 2 As 2 . The R(ω) spectra show a good metallic behavior above T c as the reflectivity goes towards unity at low frequencies and increases by cooling. Upon entering the superconducting regime, the reflectivity starts to increase rapidly with a change in curvature below 70 cm −1 and 60 cm −1 , respectively. This yields

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“…In the dirty limit where 1/[2∆ 0 τ imp (1 + λ)] ≫ 1, it is instead given by n dirty s (T = 0) = nπ∆ 0 τ imp with λ dropping out [13]. For fixed n, this gives immediately a relation between the superfluid density n s , T c , and σ n [17,18,19] The superfluid density so estimated is shown in Table I; its variation is due entirely to the change in T c . We note also the large peak at 2∆ 0 in 1 + λ op , predicted by theory and seen in the data.…”

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confidence: 99%

Unusual thickness dependence of the superconducting transition ofα-MoGethin films

et al. 2008

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Thin films of α-MoGe show progressively reduced Tc's as the thickness is decreased below 30 nm and the sheet resistance exceeds 100 Ω/ . We have performed far-infrared transmission and reflection measurements for a set of α-MoGe films to characterize this weakened superconducting state. Our results show the presence of an energy gap with ratio 2∆0/kBTc = 3.8 ± 0.1 in all films studied, slightly higher than the BCS value, even though the transition temperatures decrease significantly as film thickness is reduced. The material properties follow BCS-Eliashberg theory with a large residual scattering rate except that the coherence peak seen in the optical scattering rate is found to be strongly smeared out in the thinner superconducting samples. A peak in the optical mass renormalization at 2∆0 is predicted and observed for the first time.PACS numbers: 74.81.Bd, Disorder and reduced dimensionality affect the physical properties of metallic systems in a number of ways. Anomalous diffusion leads to localization of electrons and a related enhancement of the Coulomb interaction via reduced screening[1, 2], seen as an increase in µ * , the renormalized Coulomb interaction parameter. In a system of lower dimensions, the coupling to disorder increases, and pronounced effects are expected. Disorder-driven localization and the related enhancement of the Coulomb interaction inherently compete with the attractive interaction in superconducting metals [3,4], described by the electron-phonon spectral density α 2 F (ω) [5]. This competition reduces the transition temperature. Of particular interest are two-dimensional (2D) superconductors in which the degree of disorder can be adjusted by varying the appropriate parameters. In an ideal 2D system, the relevant parameter is normally considered to be the sheet resistance, R . The sheet resistance is determined by two factors: the (possibly thickness dependent) conductivity σ and the film thickness d.Amorphous MoGe (α-MoGe) thin films are thought to be a model system for studying the interplay between superconductivity and disorder. Several transport experiments have revealed a sharp reduction in the superconducting transition temperature T c with increasing R , even in the weakly localized regime [6,7,8,9]. The suppression of T c has been attributed to localization and an increase in the Coulomb interaction [3]. In this Letter, we explore the T c suppression in α-MoGe thin films with different thickness via temperature-dependent far-infrared transmittance and reflectance. A strong suppression of T c with increasing R is observed. The superconducting energy gap is also reduced, but the ratio of gap energy to transition temperature and the normal-state conductivity, both of which could be dependent on the disorderdriven Coulomb interaction, are not affected at all.Our films were prepared by co-magnetron sputtering

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Basic similarities among cuprate, bismuthate, organic, Chevrel-phase, and heavy-fermion superconductors shown by penetration-depth measurements

Cited by 728 publications

References 17 publications

Making high Tc higher: a theoretical proposal

Making high Tc higher: a theoretical proposal

Superfluid density of Ba(Fe1-xMx)2<

Unusual thickness dependence of the superconducting transition ofα-MoGethin films

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