Studies of unconventional superconductors
Biswas, P. K. (Pabitra K.) (2012) Studies of unconventional superconductors. PhD thesis, University of Warwick.
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In this thesis, the superconducting properties of some unconventional superconductors
have been investigated using low temperature magnetic, thermal
and transport measurements, small angle neutron scattering, and muon spin rotation/
relaxation techniques. The aim was to correlate the symmetry and structure
of the superconducting gap with the unusual properties in these superconductors.
These studies have required the preparation of high quality samples using
different growth techniques. Good quality polycrystalline and single crystal samples
of FeSe1-xTex and FeTe1-xSx were grown using a self-
ux method. Polycrystalline
samples of Lu2Fe3Si5 and Re3W were made using the arc furnace. We have also
grown single crystals of ZrB12 using the optical
oating zone method in a 4 mirror
image furnace, and CaAlSi crystal using the Bridgman method. All the compounds
have been characterized with a combination of X-ray, neutron diffraction, EDX,
magnetization, resistivity or specific heat measurements.
In order to investigate the pairing symmetry of the iron chalcogenide superconductors,
low temperature muon spin rotation/relaxation (μSR) measurements
have been performed on FeTe0:5Se0:5. The temperature dependence of the in-plane
magnetic penetration depth is found to be compatible with either a two gap s + s-
wave or an anisotropic s-wave model. This result is consistent with our heat capacity
data collected on the same sample. μSR results of FeTe1-xSx show an antiferromagnetic
transition at low temperature and also suggest the presence of excess S in the
samples. A similar magnetic transition has also been observed in the magnetization
The symmetry of the superconducting gap of Lu2Fe3Si5 with Tc = 6:1 K has
been investigated using low-temperature transverse-field μSR and specific heat measurements.
The temperature dependence of the magnetic penetration depth, λ(T)
is consistent with a two gap s+s-wave model. Low-temperature specific heat measurements
on the same sample also show evidence of two distinct superconducting
gaps and hence support the muon results.
To resolve whether CaAlSi is a single band or multiband superconductor, we
have studied the
ux line lattice in CaAlSi using small angle neutron scattering. A
well defined hexagonal
ux line lattice is seen just above Hc1 in an applied field of
only 54 Oe. A 30° reorientation of this vortex lattice has been observed in a very
low field of 200 Oe. This reorientation transition appears to be of first-order and could be explained by non-local effects. The magnetic field dependence of the form
factor is well described by a single penetration depth and a single coherence length.
The penetration depth anisotropy has also been estimated with the field applied at
different angles to the c-axis.
The B-T phase diagram of superconducting ZrB12 has been investigated by
means of μSR spectroscopy using a mosaic of single crystal. The local field distribution
for different applied fields and temperatures shows evidence of the Meissner,
mixed, and intermediate states in ZrB12. The intermediate state indicates that this
material has some of the characteristics of a type-I superconductor, while the mixed
state is typical of a type-II superconductor. Regions of coexistence have also been
observed between the different states. We have not observed any distinct features
of two-band or two-gap superconductivity in this material.
Two different superconducting phases of Re3W have been found with different
physical properties. One phase crystallizes in a non-centrosymmetric cubic
(α-Mn) structure and has a superconducting transition temperature, Tc, of 7.8 K.
The other phase has a hexagonal centrosymmetric structure and is superconducting
with a Tc of 9.4 K. Switching between the two phases is possible by annealing
the sample or remelting it. The zero-field μSR results indicate that time reversal
symmetry is preserved for both structures of Re3W. For both phases of Re3W, the
temperature dependence of the penetration depth can be explained using a singlegap
s-wave BCS model. Low temperature specific heat data also provide evidence
for an s-wave gap-symmetry for the two phases of Re3W. Both the μSR and heat capacity
data show that the CS material has a higher Tc and a larger superconducting
gap (0) at 0 K than the NCS compound.
The experimental work detailed in this thesis provides new information on the
superconducting properties of FeSe0:5Te0:5, FeTe1-xSx, Lu2Fe3Si5, CaAlSi, ZrB12,
and two different superconducting phases of Re3W and contributes to our overall
understanding of the physics of the different exotic superconducting features in these
|Item Type:||Thesis or Dissertation (PhD)|
|Subjects:||Q Science > QC Physics|
|Library of Congress Subject Headings (LCSH):||Superconductors -- Analysis, Superconductors -- Thermal properties, Superconductors -- Magnetic properties|
|Official Date:||June 2012|
|Institution:||University of Warwick|
|Theses Department:||Department of Physics|
|Supervisor(s)/Advisor:||Lees, Martin ; Paul, Don McK.|
|Sponsors:||Midlands Physics Alliance Graduate School ; Advantage West Midlands (AWM) ; European Regional Development Fund (ERDF)|
|Extent:||xix, 147 leaves : ill., charts|
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