40 research outputs found
Meissner response of a bulk superconductor with an embedded sheet of reduced penetration depth
We calculate the change in susceptibility resulting from a thin sheet with
reduced penetration depth embedded perpendicular to the surface of an isotropic
superconductor, in a geometry applicable to scanning Superconducting QUantum
Interference Device (SQUID) microscopy, by numerically solving Maxwell's and
London's equations using the finite element method. The predicted stripes in
susceptibility agree well in shape with the observations of Kalisky et al. of
enhanced susceptibility above twin planes in the underdoped pnictide
superconductor Ba(Fe1-xCox)2As2 (Ba-122). By comparing the predicted stripe
amplitudes with experiment and using the London relation between penetration
depth and superfluid density, we estimate the enhanced Cooper pair density on
the twin planes, and the barrier force for a vortex to cross a twin plane. Fits
to the observed temperature dependence of the stripe amplitude suggest that the
twin planes have a higher critical temperature than the bulk, although stripes
are not observed above the bulk critical temperature.Comment: 16 pages, 9 figure
Mixed superconducting state without applied magnetic field
A superconducting (SC) mixed state occurs in type-II superconductors where
the upper critical field Hc2 is higher than the thermodynamic critical field
Hc. When an applied field is in between these fields, the free energy depends
weakly on the order parameter which therefore can be small (SC state) or zero
(normal state) at different parts of the sample. In this paper we demonstrate
how a normal state along a line traversing a superconductor can be turned on
and off externally in zero field. The concept is based on a long,
current-carrying excitation coil, piercing a ringshaped superconductor. The
ring experiences zero field, but the vector potential produced by the coil
generates a circular current that destroys superconductivity along a radial
line starting at preexisting nucleation points in the sample. 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 cooldowns yield the same
normal lines. We suggest potential applications of this magnetic-field-free
mixed state.Comment: 5 pages, 7 figure
Multiferroicity in plastically deformed SrTiO
A major challenge in the development of quantum technologies is to induce
additional types of ferroic orders into materials that exhibit other useful
quantum properties. Various techniques have been applied to this end, such as
elastically straining, doping, or interfacing a compound with other materials.
Plastic deformation introduces permanent topological defects and large local
strains into a material, which can give rise to qualitatively new
functionality. Here we show via local magnetic imaging that plastic deformation
induces robust magnetism in the quantum paraelectric SrTiO3, in both conducting
and insulating samples. Our analysis indicates that the magnetic order is
localized along dislocation walls and coexists with polar order along the
walls. The magnetic signals can be switched on and off in a controllable manner
with external stress, which demonstrates that plastically deformed SrTiO3 is a
quantum multiferroic. These results establish plastic deformation as a
versatile platform for quantum materials engineering
Enhanced superfluid density on twin boundaries in Ba(Fe1-xCox)2As2
Superconducting quantum interference device (SQUID) microscopy shows stripes
of increased diamagnetic susceptibility in underdoped, but not overdoped,
single crystals of Ba(Fe1-xCox)2As2. These stripes of increased diamagnetic
susceptibility are consistent with enhanced superfluid density on twin
boundaries. Individual vortices avoid pinning on or crossing the stripes, and
prefer to travel parallel to them. These results indicate a relationship
between superfluid density, local strain, and frustrated magnetism, and
demonstrate two mechanisms for enhancing critical currents.Comment: 16 pages, 4 figure
Imaging and tuning polarity at SrTiO3 domain walls.
Electrostatic fields tune the ground state of interfaces between complex oxide materials. Electronic properties, such as conductivity and superconductivity, can be tuned and then used to create and control circuit elements and gate-defined devices. Here we show that naturally occurring twin boundaries, with properties that are different from their surrounding bulk, can tune the LaAlO3/SrTiO3 interface 2DEG at the nanoscale. In particular, SrTiO3 domain boundaries have the unusual distinction of remaining highly mobile down to low temperatures, and were recently suggested to be polar. Here we apply localized pressure to an individual SrTiO3 twin boundary and detect a change in LaAlO3/SrTiO3 interface current distribution. Our data directly confirm the existence of polarity at the twin boundaries, and demonstrate that they can serve as effective tunable gates. As the location of SrTiO3 domain walls can be controlled using external field stimuli, our findings suggest a novel approach to manipulate SrTiO3-based devices on the nanoscale