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Among the materials with remarkable electronic and magnetic properties, few are more extraordinary than the
superconductors. Apart from the perfect loss of electrical resistance, they also possess intriguing magnetic
properties. When a type-II superconductor is placed in a magnetic field, it is threaded by swirling whirlpools of
electric current known as vortices or flux-lines. The vortices are truly fascinating creatures which behave like massive entities,
and provide a unique probe into the nature of the superconducting state in the host material.
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In a broader perspective, vortices in superconductors are of crucial relevance to the use of superconductors, and are
often the limiting factor to practical applications. Furthermore, it appears that some of the most significant advances
in superconductivity will be precisely in those materials which are most complex to understand and control. This is
largely owing to the fact that in many such materials the superconducting order parameter has unconventional
character.
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Our research is centered around studies of vortices in unusual and/or unconventional superconductors. In the absence of
disorder, the vortices arrange themselves in a periodic vortex lattice (VL).
During the last couple of years the main areas of research within our group has been focussed on VL studies within the
three areas listed below. Further details can be found in the references.
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Pauli paramagnetic effects
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The interaction of superconductivity with local moment magnetism is a field which still has many open questions. A prominent
example is the prediction of an inhomogenous superconducting state; the so-called Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase.
Despite vigorous efforts clear experimental proof for the existence of the FFLO phase is still missing. However, we have recently
observed strong effects of Pauli paramagnetism on the vortices in both TmNi2B2C and CeCoIn5.
In both cases the effect of Pauli paramagnetism on the superconducting state is enhanced by a polarization of the local moments,
which are coupled to the conduction electrons through an exchange intercation. As a consequence the unpaired electrons in the
vortex cores are spin-polarized, adding a substantial periodic magnetization to the field modulation.
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References:
L. DeBeer-Schmitt et al., Phys. Rev. Lett. 99, 167001 (2007).
A. D. Bianchi et al., Science 319, 177 (2008).
J. S. White et al., New J. Phys. 12, 023026 (2010).
P. Das et al., arXiv:1201.1880
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Vortex lattice metastability
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Metastable phases of matter are well-known, with famous examples including supercooling and superheating of liquids
and diamond which is one of the many allotropes of carbon. Metastability is almost exclusively observed in connection
with first-order transitions, and is often found in frustrated systems where the energy difference between the states
is small.
The structure of the vortex lattice (VL) is known to be highly sensitive to changes in external parameters such as
temperature and magnetic field and can therefore be expected to display metastability, for example, in connection with
first order transitions such as the VL melting or the reorientation transition of the rhombic VL found in most
superconductors with a four-fold in-plane anisotropy.
We have recently discovered an unprecedented degree of VL metastability in MgB2 in connection with a
second-order rotation transition. This allows us, for the first time, to perform structural studies of a well-ordered,
non-equilibrium VL. Presently the mechanism responsible for the longevity of the metastable states is not resolved, but
is speculated to be due to a jamming of VL domains, preventing a rotation to the ground state orientation.
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References:
P. Das et al., submitted.
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Direct space field reconstruction from SANS
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One often seeks to parameterize the field modulation due to the vortex lattice in terms of two characteristic length
scales: the penetration depth (λ) and the coherence length (ξ). While such an approach provides a simplified
method of analyzing experimental results, it also requires the implicit acceptance of a particular theoretical model.
Recently we have begun a more complete, model independent analysis of small-angle neutron scattering (SANS) measurements
of the VL in member of the nickelborocarbide superconductors, extended significantly beyond the first-order Bragg
reflection, which is customarily the only one measured. This allows for a real-space reconstruction of the VL magnetic
field profile, which among other things will provide information about the in-plane anisotropy of these materials.
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References:
J. M. Densmore et al., Phys. Rev. B 79, 174522 (2009).
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The pnictide superconductor Ba(Fe,Co)2As2
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The recent discovery of superconductivity in the pnictide and chalcogenide superconductors with elevated critical
temperatures increasing, has sparked a strong interest in these materials. Furthermore, the possibility of growing large
high-quality single crystals of Ba(Fe,Co)2As2 from flux allow small-angle neutron scattering (SANS)
studies of the VL in this material. At all fields and temperatures the SANS measurements show a ring of scattering,
indicating a highly disordered vortex configuration, and with a rocking curve extending beyond the measurable range. The
magnitude of the VL scattering vector indicates either small, randomly oriented domains of rhombic symmetry at all
fields or a vortex glass with short-range hexagonal order. An analysis of the radial width provides an estimate of the
radial in-plane correlation length, which is found to be only a few vortex spacings at all fields, indicating that a
single-vortex pinning regime.
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References:
M. R. Eskildsen et al., Phys. Rev. B 79, 100501(R) (2009).
M. R. Eskildsen et al., Physica C 469, 529 (2009).
P. Das et al., Supercond. Sci. Technol. 23, 054007 (2010).
M. R. Eskildsen, E. M. Forgan, and H. Kawano-Furukawa, Rep. Prog. Phys. 74, 124504 (2011).
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Our group's research is currently supported by:
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The U. S. National Science Foundation, Directorate for Mathematical & Physical Sciences, Materials Research
grant no. DMR-0804887 entitled "Vortices and the Interplay between Superconductivity and Magnetism".
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The U. S. Department of Energy, Office of Science, Basic Energy Sciences
grant no. DE-FG02-10ER46783 entitled "Metastable Vortex Lattices - Properties and Applications".
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Argonne National Laboratory, Materials Science Division award no. 5J-00022-0004A entitled
"Characterization of the vortex properties of underdoped YBCO and iron pnictide superconductors"
(support for off-campus graduate student Carlos Chaparro).
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Previous support includes:
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The Alfred P. Sloan Foundation
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