Speaker
Ophir Auslaender (Technion - Israel Institute of Technology)
Description
We use a low temperature magnetic force microscope (MFM) to
study
superconductors. The interaction between the magnetic tip and
individual vortices allows us to both image vortices and to
manipulate
them. The manipulation results depend on sample thickness
and on the
superconducting properties. Here I concentrate on YBCO
samples and on
an underdoped pnictide sample. In thin films, if the force
exerted by
the tip is strong enough to overcome the pinning potential a
vortex
jumps as a whole to a new pinning site. The behavior in
thick YBCO
single crystals depends on the doping level. In a slightly
overdoped
sample vortices stretch rather than jump when we perturb them
strongly. The dragging distance in this crystal is
anisotropic: it is
easier to drag vortices along the Cu-O chains than across them,
consistent with the tilt modulus and the pinning potential being
weaker along the chains. We also find that when we
``wiggle'' the top
of a vortex we can drag it significantly farther than when
we do not,
giving rise to a striking dynamic anisotropy between the
fast and the
slow directions of the scan pattern. In underdoped YBCO single
crystals, where superconductivity is so anisotropic that a
vortex
should be viewed as a stack of two dimensional pancakes, we
show that
vortices kink rather than tilt when we perturb them. I will also
present preliminary data on the behavior of pancake vortices
near a
twin boundary (TB). Our measurements indicate that it is
easier to
drag a vortex along a TB than across it. Surprisingly, we find a
difference between the two opposite directions normal to a
TB in terms
of how hard it is to drag a vortex. Since the discovery of the
pnictides, a new family of high temperature superconductors,
we have
also been developing ways to determine the absolute value of the
magnetic penetration depth, which is notoriously difficult
to measure,
as well as its dependence on temperature. For that we either
use the
Meissner repulsion of the magnetic MFM tip from the sample
or the
magnetic interaction between the tip and the magnetic field
from a
vortex. The temperature dependence that we find allows us to
comment
on the symmetry of the order parameter.
