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Register a new userMuon spin rotation investigation of the pressure effect on the magnetic penetration depth in YBa2Cu3Ox
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Added by
Alexander Maisuradze
Publication date
2011
fields
Physics
and research's language is
English
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The pressure dependence of the magnetic penetration depth in polycrystalline samples of YBa2Cu3Ox with different oxygen concentrations x = 6.45, 6.6, 6.8, and 6.98 was studied by muon spin rotation (muSR). The pressure dependence of the superfluid density (p_s) as a function of the superconducting transition temperature Tc is found to deviate from the usual Uemura line. The ratio (dTc/dP)/(dp_s/dP) is factor of 2 smaller than that of the Uemura relation. In underdoped samples, the zero temperature superconducting gap and the BCS ratio both increase with increasing external hydrostatic pressure, implying an increase of the coupling strength with pressure. The relation between the pressure effect and the oxygen isotope effect on the magnetic penetration depth is also discussed. In order to analyze reliably the muSR spectra of samples with strong magnetic moments in a pressure cell, a special model was developed and applied.
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We report temperature- and magnetic field-dependent bulk muon spin rotation measurements in a c-axis oriented superconductor CaC6 in the mixed state. Using both a simple second moment analysis and the more precise analytical Ginzburg-Landau model, we obtained a field independent in-plane magnetic penetration depth {lambda}ab (0) = 72(3) nm. The temperature dependencies of the normalized muon spin relaxation rate and of the normalized superfluid density result to be identical, and both are well represented by the clean limit BCS model with 2Delta/kB Tc = 3.6(1), suggesting that CaC6 is a fully gapped BCS superconductor in the clean limit regime.
A study of the pressure effect on the magnetic penetration depth $lambda$ in polycrystalline MgB$_{2}$ was performed by measuring the temperature dependence of the magnetization under an applied pressure of 0.15 and 1.13 GPa. We found that $lambda^{-2}$ at low temperature is only slightly affected by pressure [$frac{Delta lambda^{-2}}{lambda^{-2}} = 1.5(9)%$], in contrast to cuprate superconductors, where, in the same range of pressure, a very large effect on $lambda^{-2}$ was found. Theoretical estimates indicate that most of the pressure effect on $lambda^{-2}$ in MgB$_2$ arises from the electron-phonon interaction.
In the tetragonal heavy fermion system CeCoIn5 the unconventional superconducting state is probed by means of muon spin rotation. The pressure dependence (0-1 GPa) of the basal-plane magnetic penetration depth (lambda_a), the penetration depth anisotropy (gamma=lambda_c/lambda_a) and the temperature dependence of 1/lambda_i^2 (i=a,c) were studied in single crystals. A strong decrease of lambda_a with pressure was observed, while gamma and lambda_i^2(0)/lambda_i^2(T) are pressure independent. A linear relationship between 1/lambda_a^2(270 mK) and Tc was also found. The large decrease of lambda_a with pressure is the signature of an increase of the number of superconducting quasiparticles by a factor of about 2.
Muon-spin rotation spectroscopy has been used to measure the internal magnetic field distribution in NbSe2 for Hc1 << H < 0.25 Hc2. The deduced profiles of the supercurrent density indicate that the vortex-core radius in the bulk decreases sharply with increasing magnetic field. This effect, which is attributed to increased vortex-vortex interactions, does not agree with the dirty-limit microscopic theory. A simple phenomenological equation in which the core radius depends on the intervortex spacing is used to model this behaviour. In addition, we find for the first time that the in-plane magnetic penetration depth increases linearly with H in the vortex state of a conventional superconductor.
We have performed transverse field muon spin rotation measurements of single crystals of Ba(Fe$_{0.93}$Co$_{0.07})_2$As$_2$ with the applied magnetic field along the $hat{c}$ direction. Fourier transforms of the measured spectra reveal an anisotropic lineshape characteristic of an Abrikosov vortex lattice. We have fit the $mu$SRSR spectra to a microscopic model in terms of the penetration depth $lambda$ and the Ginzburg-Landau parameter $kappa$. We find that as a function of temperature, the penetration depth varies more rapidly than in standard weak coupled BCS theory. For this reason we first fit the temperature dependence to a power law where the power varies from 1.6 to 2.2 as the field changes from 200G to 1000G. Due to the surprisingly strong field dependence of the power and the superfluid density we proceeded to fit the temperature dependence to a two gap model, where the size of the two gaps is field independent. From this model, we obtained gaps of $2Delta_1=3.7k_BT_c$ and $2Delta_2=1.6k_BT_c$, corresponding to roughly 6 meV and 3 meV respectively.
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