Structural evidence for an edge-contaminated vortex phase in a Nb crystal using neutron diffraction

H. A. Hanson, X. Wang, I. K. Dimitrov, J. Shi, X. S. Ling, B. B. Maranville, C. F. Majkrzak, M. Laver, U. Keiderling, and M. Russina

Phys. Rev. B 84, 014506 – Published 20 July 2011

Abstract

We report the structural observation of a disordered vortex matter phase existing near the edge of a bulk type-II superconductor Nb using a novel position-sensitive neutron diffraction technique. This “edge-contaminated” vortex state was implicated in previous studies using transport techniques and magneto-optics and is postulated to play a significant role in the behavior of vortex dynamics in a wide range of type-II superconductors. In this sample, thermal annealing causes the vortex matter in the interior to reorder implying that the edge-contaminated bulk state is metastable. However, the edge vortex structure remains disordered after repeated thermal annealing indicating the spatial coexistence of a vortex glass with a Bragg glass. This observation resolves outstanding issues concerning the peak effect in type-II superconductors.

Received 30 April 2011

DOI:https://doi.org/10.1103/PhysRevB.84.014506

Authors & Affiliations

H. A. Hanson, X. Wang, I. K. Dimitrov^*, J. Shi^†, and X. S. Ling^‡

Department of Physics, Brown University, Providence RI 02912, USA

B. B. Maranville and C. F. Majkrzak

NIST Center for Neutron Research, Gaithersburg, MD 20899, USA

M. Laver

Paul Scherrer Institut, Villigen PSI, CH 5232 Switzerland

U. Keiderling and M. Russina

Helmholtz Zentrum Berlin fr Materialien und Energie GmbH, 14109 Berlin, Germany

^*Current address: Brookhaven National Laboratories, Upton, NY 11973, USA
^†Permanent address: School of Physics, Wuhan University, Wuhan 430072, PRC
^‡xsling@brown.edu

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Issue

Vol. 84, Iss. 1 — 1 July 2011

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Images

Figure 1
(a) The magnetic field, $H$ , versus $T$ phase diagram for the Nb crystal used in this study. PE is remnant of a peak effect. The solid lines are fits to the data. The values of the critical magnetic fields are very similar to a previous Nb crystal but the peak effect line is very different.[27] The growth procedures for FC and ZFC are sketched on the diagram. (b) The scattering geometry for conventional small-angle neutron scattering (SANS). (c,d) SANS images, summed over the rocking angle, for the FC and the ZFC states at $T = 1.5$ K and $H = 1400$ Oe.Reuse & Permissions
Figure 2
(a) The scattering geometry for AND/R in real space and reciprocal space. Radial scans probe the direction parallel to the reciprocal lattice vector, $Q$ , and azimuthal scans measure the direction perpendicular. Note the magnetic field, $H$ , is applied parallel to the cylindrical axis of the sample. (b) The scattering intensity as a function of $2 θ$ plotted for the ZFC and FC vortex matter radial Bragg peaks at $T = 3.5$ K and $H = 1400$ Oe. Solid lines are Gaussian best fits. (c) The normalized intensity versus $2 θ$ for the radial FC Bragg peak and the main beam measurement. Note that the FC (and the ZFC) Bragg peak is (are) resolution-limited. (d) The scattering intensity divided by the summed intensity ( $I / SI$ ) versus $ω$ for the vortex matter in (b) azimuthal Bragg peaks. Solid lines are Lorentzian fits. Here, neither peak is resolution-limited. The error bars are the statistical standard deviation from the neutron counts.Reuse & Permissions
Figure 3
(a) $I$ versus $ω$ for the annealed Bragg peaks compared to the as-grown Bragg peaks: ZFC (bottom) and FC (top) at $T = 3.5$ K and $H = 1400$ Oe. Solid lines are Lorentzian fits. (b) The half-width, $σ$ , (bottom) and Bragg peak center, $ω_{○}$ , (top) from the Lorentzian fits at $H = 1400$ Oe are plotted as a function of the annealing temperature, $T_{A}$ . $T_{A}$ is the highest temperature the vortex lattice is heated to before returning to the measurement temperature, $T = 3.5$ K. Solid lines are guides for the eyes.Reuse & Permissions
Figure 4
(a, top) A top view of the location of the two Bragg peaks in the Nb crystal. (a, main) $I / SI$ , versus rocking angle minus the location of the Bragg condition, $ω - ω_{○}$ , of the Bragg scattering from the edge of the sample and from the sample center for an annealed FC vortex matter scaled to $4.5$ K and $1400$ Oe. The neutron beam measurement is also plotted to show that both peaks are much broader than the instrument resolution. (b) The radial scan of the center Bragg peak and the main beam measurement showing that the center radial peak is resolution-limited.Reuse & Permissions
Figure 5
(a, top) The scattering location for the five Bragg peaks from an AFC vortex lattice. (a, main) 3D plot of the AFC Bragg peaks at $4.5$ K and $1400$ Oe: $I / SI$ versus $ω$ versus sample position, $x$ , with a top view of the respective spatial positions in the Nb crystal. The solid lines are guides for the eyes. (b, top) The scattering location in the Nb crystal. Notice here the scattering moves from the center to one edge. (b, main) 3D plot of the ZFC Bragg peaks for the as-grown Nb sample compared to the surface oxidized sample at $4.5$ K and $1400$ Oe: $I / SI$ versus $ω - ω_{○}$ versus $x$ . The peaks are all shifted by the same $ω_{○}$ for each history: The centers of the center Bragg peaks is shifted to zero. The solid lines are guides for the eyes.Reuse & Permissions
Figure 6
The impact of surface oxidation on the FC and ZFC vortex matter is shown. (a) The intensity divided by the summed intensity ( $I / SI$ ) is plotted versus $ω - ω_{○}$ for the FC versus FC $_{o}$ vortex matter (top) and ZFC versus ZFC $_{o}$ vortex matter (bottom). (b) The radial scans of FC $_{o}$ and ZFC $_{o}$ in the center of the Nb crystal are shown. (top). The $B$ value is plotted versus the sample position, $x$ , for ZFC and FC vortex matter(bottom). The measurements are at $T = 4.5$ T and $H = 1400$ Oe.Reuse & Permissions

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