Vortex Lattice Structural Transitions in a Type-II Superconductor
The mixed state in a type-II superconductor is characterized by the properties of the vortex or flux lattice that forms in the presence of an applied magnetic field. Vortices are formed at points where the magnetic field penetrates the superconductor in a flux tube through the sample. The flux penetrates the superconductor in quantized units of the flux quantum (Φ0=h2e) with the result that the region near the core of the vortex acts as a normal metal. As the density of vortices in the material increases, a vortex lattice is formed by the interactions between the flux tubes or vortices. Many of the important applications of superconductors rely on the formation and stabilization of the vortex lattice at high values of applied field. For example, the optimization of vortex pinning in high-TC materials controls flux creep and gives high critical currents required for the operation of superconducting magnets. Making progress in probing the underlying physics of the vortex lattice is important not only because it relates directly to such technological applications, but also because it is the key to understanding more complex interactions, such as the coexistence of superconductivity and magnetism.
Vortex flux lattice in V3Si observed in STM Fermi-level conductance image at H=3 T and T=2.3 K.
The peaks indicate the location of a vortex with a single flux quantum of magnetic flux. The image is 500 x 500 nm2.
Real space measurements of vortex lattices are difficult because the length scale of the vortex unit cell is in the nanometer range for field strengths on the order of 1 T. Nanometer real space measurements have now become possible using cryogenic scanning tunneling microscopy, which can probe the electronic structure of the superconductor on the atomic scale. The first real-space measurements of the symmetry transition in V3Si were made by recording spatial maps of the local density of states (LDOS) of the superconductor as a function of magnetic field, using the low temperature scanning tunneling microscope of the Nanoscale Physics Facility in the Electron Physics Group at NIST. At the location of the vortex, the superconductor is a normal metal and has a much higher density of states for energies inside the superconducting gap. Thus, spatial maps of the LDOS show a bright spot at the location of the vortex. Measuring the vortex lattice as a function of magnetic field shows that vortex-vortex interactions are important in determining the symmetry of the vortex lattice. Moreover, the measurements reveal that symmetries in the electronic structure of the superconductor play an important role as they directly link the symmetry of the vortex lattice to the underlying crystal structure.
(a-d, top row) STM Fermi-level conductance images of the vortex lattice of
V3Si as a function of applied magnetic field at 2.3 K.
(e-f, bottom row) Corresponding auto-correlation images showing the unit
cell of the vortex lattice
undergoing the hexagonal-to-square symmetry transition.
Nanoscale Physics Facility
Real-Space Imaging of Structural Transitions in the Vortex Lattice of V3Si
Joseph A. Stroscio
Robert J. Celotta
Mark D. Stiles
Steven R. Blankenship
Aaron Fein - NIST (PL)
Eric Hudson - MIT
Chad Sosolik - Clemson University
Supported in part by the Office of Naval Research
Online: September 2004
Last Updated: February 2008
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