A method to measure the superconducting (SC) stiffness tensor
, without subjecting the sample to external magnetic field, is applied to La
1.875
Sr
0.125
CuO
4
. The method is based on the London equation
, where
J
is the current density and
A
is the vector potential which is applied in the SC state. Using rotor free
A
and measuring
J
via the magnetic moment of superconducting rings,
at
T
→
T
c
is extracted. The technique is sensitive to very small stiffnesses (penetration depths on the order of a few millimeters). The method is applied to two different rings: one with the current running only in the CuO
2
planes, and another where the current must cross planes. We find different transition temperatures for the two rings, namely, there is a temperature range with two-dimensional stiffness. Additional low energy muon spin rotation measurements on the same sample determine the stiffness anisotropy at
T
<
T
c
.
Josephson junctions and superconducting quantum interference devices (SQUID) are important electronic elements, which are based on normal conductor sandwiched between two superconductors. These junctions are produced by evaporation techniques, and once they are embedded in an electronic circuit, their properties are fixed. Using SQUIDs as a tunable component requires the ability to generate Josephson junctions in situ in a reversible controllable manner. In this work we demonstrated how a normal (metallic) region along a line traversing a superconductor can be turned on and off externally thus potentially generating a controllable Josephson junction or a SQUID. The concept is based on a long, current-carrying excitation coil, piercing a ring shaped superconductor with nucleation points. The vector potential produced by this coil generates a circular current that destroys superconductivity along a radial line starting at the nucleation point. Unlike the destruction of superconductivity with magnetic field, the vector potential method is reversible and reproducible; full superconductivity is recovered upon removing the current from the coil and different cool-downs yield the same normal lines.
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