39 research outputs found
Emergence of superconductivity in the cuprates via a universal percolation process
A pivotal step toward understanding unconventional superconductors would be
to decipher how superconductivity emerges from the unusual normal state upon
cooling. In the cuprates, traces of superconducting pairing appear above the
macroscopic transition temperature , yet extensive investigation has led
to disparate conclusions. The main difficulty has been the separation of
superconducting contributions from complex normal state behaviour. Here we
avoid this problem by measuring the nonlinear conductivity, an observable that
is zero in the normal state. We uncover for several representative cuprates
that the nonlinear conductivity vanishes exponentially above , both with
temperature and magnetic field, and exhibits temperature-scaling characterized
by a nearly universal scale . Attempts to model the response with the
frequently evoked Ginzburg-Landau theory are unsuccessful. Instead, our
findings are captured by a simple percolation model that can also explain other
properties of the cuprates. We thus resolve a long-standing conundrum by
showing that the emergence of superconductivity in the cuprates is dominated by
their inherent inhomogeneity
Percolative nature of the dc paraconductivity in the cuprate superconductors
We present an investigation of the planar direct-current (dc)
paraconductivity of the model cuprate material HgBaCuO in the
underdoped part of the phase diagram. The simple quadratic
temperature-dependence of the Fermi-liquid normal-state resistivity enables us
to extract the paraconductivity above the macroscopic with great
accuracy. The paraconductivity exhibits unusual exponential temperature
dependence, with a characteristic temperature scale that is distinct from
. In the entire temperature range where it is discernable, the
paraconductivity is quantitatively explained by a simple superconducting
percolation model, which implies that underlying gap disorder dominates the
emergence of superconductivity
Demonstrating the model nature of the high-temperature superconductor HgBaCuO
The compound HgBaCuO (Hg1201) exhibits a simple tetragonal
crystal structure and the highest superconducting transition temperature
(T) among all single Cu-O layer cuprates, with T = 97 K (onset) at
optimal doping. Due to a lack of sizable single crystals, experimental work on
this very attractive system has been significantly limited. Thanks to a recent
breakthrough in crystal growth, such crystals have now become available. Here,
we demonstrate that it is possible to identify suitable heat treatment
conditions to systematically and uniformly tune the hole concentration of
Hg1201 crystals over a wide range, from very underdoped (T = 47 K, hole
concentration p ~ 0.08) to overdoped (T = 64 K, p ~ 0.22). We then present
quantitative magnetic susceptibility and DC charge transport results that
reveal the very high-quality nature of the studied crystals. Using XPS on
cleaved samples, we furthermore demonstrate that it is possible to obtain large
surfaces of good quality. These characterization measurements demonstrate that
Hg1201 should be viewed as a model high-temperature superconductor, and they
provide the foundation for extensive future experimental work.Comment: 15 pages, 6 Figure
Two-component uniform spin susceptibility in superconducting HgBaCuO single crystals determined with Cu and Hg NMR
Cu and Hg NMR shifts for an optimally and underdoped
HgBaCuO single crystal are reported, and the temperature
dependence dictates a two-component description of the uniform spin
susceptibility. The first component, associated with the pseudogap phenomenon
in the NMR shifts, decreases already at room temperature and continues to drop
as the temperature is lowered, without a drastic change at the transition
temperature into the superconducting state. The second component is temperature
independent above the superconducting transition temperature and vanishes
rapidly below it. It increases with doping and is a substantial part of the
total spin susceptibility measured at both nuclei