THESIS
2015
xvi, 120 pages : illustrations (some color) ; 30 cm
Abstract
This thesis focuses on several interesting aspects of various superconductors, which a special
focus on iron-based superconductors. The choice of experimental probes under the influence of
high pressure is very limited. In this thesis I am pioneering in measuring a large variety of
physical quantities in pressure up to 10 GPa, including heat capacity and thermoelectric
measurements.
Heat capacity and resistivity experiments for CaFe
2As
2 and underdoped Ba(Fe
1-xCo
x)
2As
2 were
performed under hydrostatic pressure conditions. The data supports the bulk nature of pressure-induced
superconductivity in these iron-based pnictides, and reveals details on the pressure-induced
structural transitions. It is observed that the maximum critical temperature and the
superconducting condensation...[
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This thesis focuses on several interesting aspects of various superconductors, which a special
focus on iron-based superconductors. The choice of experimental probes under the influence of
high pressure is very limited. In this thesis I am pioneering in measuring a large variety of
physical quantities in pressure up to 10 GPa, including heat capacity and thermoelectric
measurements.
Heat capacity and resistivity experiments for CaFe
2As
2 and underdoped Ba(Fe
1-xCo
x)
2As
2 were
performed under hydrostatic pressure conditions. The data supports the bulk nature of pressure-induced
superconductivity in these iron-based pnictides, and reveals details on the pressure-induced
structural transitions. It is observed that the maximum critical temperature and the
superconducting condensation energy are strongly increased under pressure. Furthermore,
detailed experimental data of various physical quantities are presented on Ba
1-xK
xFe
2As
2, which
are rarely performed under pressure. These comprise heat capacity, resistivity, Nernst effect, Hall
effect and Seebeck effect experiments. The data validates the existence of a new phase above the
superconducting state under pressure. The data provides evidence that in this phase a spin-density
wave order and filamentary superconductivity coexist.
The effect of thermal fluctuations is widely believed to be unobservable in classical
superconductors. It will be shown that in V
3Si fluctuations become significantly enhanced in magnetic fields of a few Teslas, because of the confinement of the electronic quasiparticles in
low Landau orbits. They become so strong that a vortex melting transition into a liquid vortex
phase is observed in high-resolution specific heat experiments.
Finally, Nernst effect data of a novel interfacial superconductor is presented, which occurs at the
interface of a Bi
2Te
3/Fe
1+yTe heterostructure. It is superconducting below 12 K and shows a
pronounced pseudogap up to 40 K. The data reveals that this pseudogap is of a normal state
origin, which likely competes with superconductivity.
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