Superconducting Carrier Concentration Research Articles

Microscopic parameters from high-resolution specific heat measurements on superoptimally substitutedBaFe2(As1−xPx)2single crystals

We investigate the electronic specific heat of overdoped BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ single crystals in the superconducting state using high-resolution nanocalorimetry. From the measurements, we extract the doping dependence of the condensation energy, superconducting gap $\Delta$, and related microscopic parameters. We find that the anomalous scaling of the specific heat jump $\Delta C \propto T_{\mathrm{c}}^3$, found in many iron-based superconductors, in this system originates from a $T_\mathrm{c}$-dependent ratio $\Delta/k_\mathrm{B}T_\mathrm{c}$ in combination with a doping-dependent density of states $N(\varepsilon_\mathrm{F})$. A clear enhancement is seen in the effective mass $m^{*}$ as the composition approaches the value that has been associated with a quantum critical point at optimum doping. However, a simultaneous increase in the superconducting carrier concentration $n_\mathrm{s}$ maintains the superfluid density, yielding an apparent penetration depth $\lambda$ that decreases with increasing $T_\mathrm{c}$ without sharp divergence at the quantum critical point. Uemura scaling indicates that $T_\mathrm{c}$ is governed by the Fermi temperature $T_\mathrm{F}$ for this multi-band system.

Microscopic study of the superconducting state of the iron pnictideRbFe2As2via muon spin rotation

A study of the temperature and field dependence of the penetration depth \lambda of the superconductor RbFe_2As_2 (T_c=2.52 K) was carried out by means of muon-spin rotation measurements. In addition to the zero temperature value of the penetration depth \lambda(0)=267(5) nm, a determination of the upper critical field B_c2(0)=2.6(2) T was obtained. The temperature dependence of the superconducting carrier concentration is discussed within the framework of a multi-gap scenario. Compared to the other "122" systems which exhibit much higher Fermi level, a strong reduction of the large gap BCS ratio 2\Delta/k_B T_c is observed. This is interpreted as a consequence of the absence of interband processes. Indications of possible pair-breaking effect are also discussed.

Electrodynamics of molecular organic superconductors studied by μ SR

The response of the superconducting charge carriers to an applied magnetic field has been probed in a number of organic superconductors using μSR. Systematic studies have been made of the in-plane superconducting penetration depth λ in superconductors based on the ET and BETS molecules. These studies have revealed a strong correlation between λ and T c . This type of correlation was first noted in high T c cuprates by Uemura et al who found a linear relation between T c and λ -2 . In the case of organic superconductors, however, we find that the overall correlation is better described as T c λ -3 . Understanding this characteristic trend should provide some important clues about the nature of the superconductivity in the organic superconductors; several possible models are considered here for describing this behaviour. It is concluded that models in which the superconducting carrier concentration is tied to the nominal band filling are inconsistent with the observed scaling behaviour.

The effect of the YBCO–PST composite composition on the superconducting carrier concentration determined by microwave studies under high pressure

The effect of hydrostatic pressure ( p<0.6 GPa) on the superconducting critical temperature T c in YBa 2Cu 3O 7− δ –Pb(Sc 0.5Ta 0.5)O 3 (YBCO–PST) composite is measured by the method of magnetically modulated microwave absorption (MMMA). The T c dependence on the PST fraction in weight x (0, 0.25, 0.5 and 0.75) is approximated by an inverted parabola function whereas the influence of pressure on T c is represented by the equation: d T c/d p=0.61(2)−1.72(6) x. The result may be explained assuming that PST phase in YBCO–PST composite influences the superconducting carrier concentration similar to the chemical substitution in YBa 2Cu 3O 7 [J.J. Neumeier, H.A. Zimmermann, Phys. Rev. B 47 (1993) 8385]. It is suggested that ions from PST diffuse to YBCO cell during the sintering of the composite.

Generalization of the Kink Solution for Superconductors with Large Penetration Depths in the Ginzburg–Landau Formalism

Exact solutions of the anisotropic Ginzburg–Landau equation are derived in the elliptic function formalism. The general class of solutions is described by sn(u|k), of argument u and modulus k, which, in the limit k<<1, is approximated by sinus and, in the limit k→1, by the well-known kink solution. The superconducting carrier concentration, ns, penetration depth, λ and critical field, Hc are calculated. The concordance with experimental data and other theories is in good agreement in the range from 0.8Tc until the critical temperature.

Elliptic function formalism description of anisotropic Ginzburg–Landau equation

By using the elliptic function formalism, we derived exact solutions of anisotropic Ginzburg–Landau equation for materials with very large penetration depth. The solutions are described by sn( u, k) of argument u and modulus k, in the limit k→0 being approximated by sinus and in the limit k→1 we obtain the hyperbolic tangent-kink solution. The kink energy Δ E (energy gap), the topological charge S (total spin), superconducting carrier concentration n s, penetration depth λ and the critical field H c are calculated. The dependence on temperature of these parameters is achieved by the means of k. The equations of superconducting parameters are in agreement with experimental data in the temperature limit T∈(0.8 T c, T c).

Doping dependence of the thermopower of high-Tc cuprates: Tight-binding band model.

Tight-binding calculatoins are performed which include both Cu-O and O-O interactions in the ${\mathrm{CuO}}_{2}$ plane. These calculations reconcile inconsistencies in observed behaviors of the thermopower S and the Hall coefficient ${\mathit{R}}_{\mathit{H}}$: the sign of S of high-${\mathit{T}}_{\mathit{c}}$ cuprates at room temperature becomes negative in the overdoped regime, while ${\mathit{R}}_{\mathit{H}}$ remains positive. A striking feature of the ${\mathrm{CuO}}_{2}$ antibonding band is that a holelike Fermi surface is formed even when the band is less than half-filled. This brings about an unusual electron state in which the Hall (cyclotron) mass parallel to the Fermi surface is holelike (0) but the transport mass perpendicular to it is electronlike (&gt;0). This electronlike transport mass contributes to negative S, while the holelike Hall mass results in positive ${\mathit{R}}_{\mathit{H}}$. In such a state, the electron on the Fermi surface has complete duality: it is holelike in one direction, but electronlike in another. In the overdoped regime, where ${\mathit{R}}_{\mathit{H}}$&gt;0 and S0, the hole doping increases the carrier concentration defined as \ensuremath{\propto} ${\mathit{R}}_{\mathit{H}}^{\mathrm{\ensuremath{-}}1}$, but it decreases the carrier concentration defined as (n/${\mathit{m}}^{\mathrm{*}}$${)}_{\mathit{D}}$ in Drude's formula. This qualitatively explains the recent muon-spin-rotation (\ensuremath{\mu}SR) results that the superconducting carrier concentration ${\mathit{n}}_{\mathit{s}}$/${\mathit{m}}^{\mathrm{*}}$\ensuremath{\sim}(n/${\mathit{m}}^{\mathrm{*}}$${)}_{\mathit{D}}$ decreases with hole doping in the overdoped regime.

Dependence of Tc on carrier concentration for (Bi 1.8Pb 0.3)(Sr 2−xLa x) Ca 0.9Cu 2O 8+δ

The muon spin relaxation rate( μSR- σ( T), a measure of the superconducting carrier concentration) due to the vortex lattice was measured for ceramic (Bi 1.8Pb 0.3)(Sr 2− x La x )Ca 0.9Cu 2O 8+δ samples with T c = 26 → 85 K. It was found that both T c( x) and σ 0( T c) are almost-linear functions, showing that these materials are in the low-doping range of the T c vs carrier concentration curve. This brings μSR results in line with other studies of the T c-dependence on hole concentration in the BiSrCaCuO family.

Effect of hydrogen in YBa 2Cu 3O 7 studied with the muon spin rotation technique

Polycrystalline superconducting YBa 2Cu 3O 7 samples were doped with hydrogen and investigated by the muon spin rotation ( μ + SR) technique. For a hydrogen concentration above 0.5 per formula unit YBa 2Cu 3O 7 we find a well defined muon spin precession signal in zero external magnetic field. This is a clear indication of magnetic ordering in these samples. In samples with a smaller hydrogen content no magnetic ordering was found. For these samples the magnetic penetration depth which is related to the superconducting carrier concentration was measured. We find a strong reduction of the carrier concentration by hydrogen doping. Our data are consistent with the assumption that hydrogen acts as an electron donor filling the hole states in YBa 2Cu 3O 7.