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Microscopic parameters of a type-II superconductor measured by small-angle neutron scattering

D. Alba Venero, A. -M. Valente-Feliciano, O. O. Bernal, V. Kozhevnikov

Abstract

A necessary condition for understanding and predicting the properties of any material is knowledge of microscopic parameters which control these properties. In superconductors these parameters are the radius of the orbital motion of electrons bound in Cooper pairs $R_0$ and the radius of the field-induced currents $r_i$ caused by precession of the pairs; one more parameter, associated with $r_i$, is the number density of Cooper pairs $n_{cp}$. In this communication we report on the first measurements of these parameters in a type-II superconductor (niobium) by SANS (small-angle neutron scattering). Other approaches potentially applicable for measuring the microscopic parameters are considered.

Microscopic parameters of a type-II superconductor measured by small-angle neutron scattering

Abstract

A necessary condition for understanding and predicting the properties of any material is knowledge of microscopic parameters which control these properties. In superconductors these parameters are the radius of the orbital motion of electrons bound in Cooper pairs and the radius of the field-induced currents caused by precession of the pairs; one more parameter, associated with , is the number density of Cooper pairs . In this communication we report on the first measurements of these parameters in a type-II superconductor (niobium) by SANS (small-angle neutron scattering). Other approaches potentially applicable for measuring the microscopic parameters are considered.

Paper Structure

This paper contains 4 figures.

Figures (4)

  • Figure 1: (a) Equilibrium structure of the flux lines (FLs) in type-II superconductors in the mixed state. (b) Zoomed area with a FL passing through the network of micro-whirls; the FL takes the space originally occupied by the micro-whirl. $R_\perp$ is the rms radius of cylindrical volumes filled with CPs of different orientations; $r_i$ is the rms radius of micro-whirls equal to the radius of the field-induced currents in CPs and, respectively, to the radius of the FLs; $d$ is a parameter of the FL lattice (distance between neighboring scattering planes). Arrows in (b) depict induced currents. $\textbf{H}$ is the intensity of the magnetic field directed into the page.
  • Figure 2: (a) magnetization curves measured with a $dc$ SQUID magnetometer on the twin's sample used in this work. (b) the sample/field configuration in the SANS experiment. $H_0$ is the applied field directed parallel to the sample plane.
  • Figure 3: Experimental data and Laue-grams for the scattered neutron intensity $I$ (arb. units) vs the scattering vector $Q$ (1/$\mathring{A}$) at $T=3.5$ K and different applied fields. Solid (dashed) arrows mark the $Q$-vectors at the maxima of the first (second) order.
  • Figure 4: (a) Scattering vector at maxima of the first ($Q_{m1}$) and second ($Q_{m2}$) orders. (b) The flux line lattice parameter $d$ calculated from $Q_{m1}$ and $Q_{m2}$. The dashed line designates the minimum $d$. $H_0$ is the applied field.