Since the discovery of superconductivity in 1911, the journey towards the realization of a room
temperature superconductor and the understanding of the nature of unconventional superconductivity
is still ongoing up to date. Despite tremendous theoretical and experimental efforts, the regular
emerging of new unconventional superconducting materials provides regularly new challenges to
describe their unknown mechanisms and novel phenomena. Nevertheless, many researchers are
continuously trying to overcome the high mountain. Among them, uncovering the mechanism
of unconventional superconductivity is of primary interest. In this thesis work, exotic states and
pairing gap symmetry are studied in newly discovered unconventional superconductors.
First, we investigate the re-entrant tet...[
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Since the discovery of superconductivity in 1911, the journey towards the realization of a room
temperature superconductor and the understanding of the nature of unconventional superconductivity
is still ongoing up to date. Despite tremendous theoretical and experimental efforts, the regular
emerging of new unconventional superconducting materials provides regularly new challenges to
describe their unknown mechanisms and novel phenomena. Nevertheless, many researchers are
continuously trying to overcome the high mountain. Among them, uncovering the mechanism
of unconventional superconductivity is of primary interest. In this thesis work, exotic states and
pairing gap symmetry are studied in newly discovered unconventional superconductors.
First, we investigate the re-entrant tetragonal phase in the Fe-based superconductor Ba
0.76K
0.24Fe
2As
2 by thermodynamic and thermoelectrical measurements. The reversible DC magnetization confirms by a thermodynamic method that the spin alignment in the re-entrant C
4 phase is out-of-plane, in agreement with an itinerant double-Q magnetic order. The Nernst coefficient shows the typical unusually large negative value in the stripe-type spin density wave (SDW) state owing to the Fermi surface reconstruction associated with SDW and nematic order. At the transition into the re-entrant C
4 tetragonal phase it hardly changes, which indicates that instead of a complete vanishing of the associated charge order, the spin reorientation triggers a redistribution of the charges to form a secondary charge order, e.g. in form of a chequerboard-like pattern that no longer breaks the rotational C
4 symmetry.
Second, the magnetic phase diagram near the upper critical field is studied in the KF
2As
2 superconductor by resistance, magnetic torque and specific heat experiments using a high-resolution piezorotary positioner to precisely control the parallel alignment of the applied magnetic field with respect to the FeAs layers. We observe a clear double transition when the field is strictly aligned in the plane and a characteristic upturn of the upper critical field line, which goes far beyond the Pauli limit at 4.8 T. This provides firm evidence that a Fulde-Ferrell-Ovchinnikov (FFLO) state exists in this iron-based KFe
2As
2 superconductor.
Third, we study the gap symmetry of the newly discovered topological superconductor Nb
xBi
2Se
3 by DC magnetization, specific heat and thermal expansion experiments. We find a clear two-fold nematic superconducting state in field-angle-resolved experiments including DC magnetization data in combination with magnetoresistance data. We find that broad maxima and minima appear in the in-plane angular dependence of the upper critical field, which vary among different samples, which may be due to the nematic multi-domain effect. By measuring the field-angle resolved specific heat, we further confirm the existence of multiple nematic-domains. In addition, we
observe the highly coupled structural distortion that coincides with the superconducting transition
in high-resolution thermal expansion experiments.
Finally, the pairing mechanism and gap symmetry in the Ising superconductor NbSe
2 is examined with field-angle resolved magnetoresistance experiments. We observe a mixture of a six-and two-fold angular dependence of the upper critical field within the plane in a monolayer NbSe
2, which is inconsistent with the crystal lattice symmetry. This provides firm evidence that novel superconducting phases are induced by a strong in-plane magnetic field. We experimentally confirm
the existence of two distinct topological superconducting phases, which will need further studies and stimulate intensive theoretical works.
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