PhD Thesis: Unraveling the cuprate superconductor phase diagram
by
Thorsten Jacobs(Stockholm University, Department of Physics)
→
Europe/Stockholm
FB54
FB54
Description
High-temperature superconductors belong to the group of strongly correlated materials. In these compounds, complex
repulsive electron interactions and a large number of degrees of freedom lead to a rich variety of states of matter. Exotic
phases like the pseudogap, charge-, spin- and pair-density waves, but also the remarkable phenomenon of superconductivity
emerge, depending on doping level and temperature. However, up to now it is unclear what exactly causes these states,
to what extent they are coexisting or competing, and where their borders in the phase diagram lie. A better understanding
could help in finding the mechanism behind high-temperature superconductivity, but would also provide a better insight
into the puzzling behavior of strongly correlated materials.
This thesis tries to resolve some of these questions with focus on the underdoped pseudogap regime. Mesa
structures of bismuth-based cuprate superconductors were studied using intrinsic tunneling, which allows spectroscopic
characterizations of electronic density of states inside the material. A micro/nano fabrication method was developed to
further reduce mesa areas into the sub square-micrometer range, in order to minimize the effect of crystal defects and
measurement artifacts caused by heating induced by the measurement current.
The comparison of energy scales in Bi-2201 and Bi-2212 cuprates shows that the pseudogap phenomenon is not
connected to superconductivity, but possibly represents a competing spin-singlet order that is universal to all cuprates. The
analysis of the upper critical field in Bi-2201 reveals a low anisotropy, which gives evidence of paramagnetically limited
superconductivity. Furthermore, a new electrical doping method is demonstrated, which enables the reversible tuning the
doping level of Bi-2212 and study a broad doping range upon a single sample. Using this method, two distinct critical
points were observed under the superconducting dome in the phase diagram: one at the overdoped side, associated with
the onset of the pseudogap and a metal to insulator transition, and one at optimal doping, associated with an enhanced
"dressed" electron energy. Finally, a novel angular-dependent magnetotunneling technique is introduced, which allows for
the separation of the superconducting and non-superconducting contributions to the pseudogap phenomenon. The method
reveals that after an abrupt decay of the energy gap for T→Tc, weak superconducting correlations persist up to several
tens of degrees above Tc.
