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Jeudi 27 Juin 2013, 14h
Amphi Holweck, Esc C, 1ème etage
Torque magnetometry studies of high-temperature superconductors and
magnetic materials
Saskia Bosma
Université de Zurich
The underlying mechanism of high-temperature superconductivity is
still not fully understood, despite the 27 years elapsed since its
discovery. All the high-temperature superconductors have a layered
structure ; measuring the anisotropy parameter of those systems is
therefore a privileged method to investigate their superconducting
properties. Our custom torque sensors and experimental setup allow us
to reach a very high sensitivity for anisotropy measurements. The
anisotropy is related to the superconducting gap through the magnetic
field penetration depth. The presence of multiple gaps has been
proposed to explain the high critical temperatures of unconventional
superconductors, and has also been linked to a temperature dependence
of the anisotropy. We therefore studied the temperature dependence of
the anisotropy of the cuprate high-temperature superconductor
YBa$_2$Cu$_3$O$_7-\delta$. Accurate measurements are further enabled
by the so-called shaking technique. The resulting gain in sensitivity
allowed us to directly observe the lock-in of the vortices in the same
YBa$_2$Cu$_3$O$_7-\delta$ crystal. The lock-in is especially
interesting because it is related to the dimensionality of the
superconductor. The dimensionality is pertinent to the fundamental
understanding of the material, but also to vortex pinning, a feature
which is relevant for industrial applications. Observing how
superconductivity interacts with other ordered states can help to
understand its basic principles. In the iron-based high-temperature
superconductor Rb$_x$Fe$_2-y$Se$_2$, the anisotropy does not depend
on temperature, but strongly depends on field, although the
temperatures studied are too low to excite any change in the
antiferromagnetic order. In order to extend the temperature and field
domain accessible to our torque measurements, we built a new torque
magnetometer that can be used in a 2 K cryostat equipped with a
high-field superconducting magnet.