Imaging nanoscale Fermi-surface variations in an inhomogeneous superconductor
Nature Physics, Volume 5, Issue 3, pp. 213-216 (2009) [Preprint PDF]
Particle–wave
duality suggests we think of electrons as waves stretched across a sample,
with wavevector k proportional to their momentum. Their arrangement in 'k-space',
and in particular the shape of the Fermi surface, where the highest-energy
electrons of the system reside, determine many material properties. Here
we use a novel extension of Fourier-transform scanning tunnelling microscopy
to probe the Fermi surface of the strongly inhomogeneous Bi-based cuprate
superconductors. Surprisingly, we find that, rather than being globally
defined, the Fermi surface changes on nanometre length scales. Just as shifting
tide lines expose variations of water height, changing Fermi surfaces indicate
strong local doping variations. This discovery, unprecedented in any material,
paves the way for an understanding of other inhomogeneous characteristics
of the cuprates, such as the pseudogap magnitude, and highlights a new approach
to the study of nanoscale inhomogeneity in general.
Scanning tunneling microscopy of the 32 K superconductor (Sr1-xKx)Fe2As2
eprint arXiv:0806.4400 [Preprint PDF]
The
discovery of high temperature superconductivity in La[O1-xFx]FeAs
at the beginning of this year has generated much excitement and has led
to the rapid discovery of similar compounds with as high as 55 K transition
temperatures. The high superconducting transition temperatures are seemingly
incompatible with the electron-phonon driven pairing of conventional superconductors,
resulting in wide speculation as to the mechanism and nature of the superconductivity
in these materials. Here we report results of the first scanning tunneling
microscopy study of the 32 K superconductor (Sr1-xKx)Fe2As2.
We find two distinct topographic regions on the sample, one with no apparent
atomic corrugation, and another marked by a stripe-like modulation at double
the atomic periodicity. In the latter the stripes appear to modulate the
local density of states, occasionally revealing a Δ = 10 mV gap with
a shape consistent with unconventional (non-s wave) superconductivity.
Charge-density-wave origin of cuprate checkerboard visualized by scanning tunnelling microscopy
Nature Physics, Volume 4, Issue 9, pp. 696-699 (2008) [Preprint PDF]
One
of the main challenges in understanding high-Tc superconductivity is to
disentangle the rich variety of states of matter that may coexist, cooperate
or compete with d-wave superconductivity. At centre stage is the pseudogap
phase, which occupies a large portion of the cuprate phase diagram surrounding
the superconducting dome. Using scanning tunnelling microscopy, we find
that a static, non-dispersive, 'checkerboard'-like electronic modulation
exists in a broad regime of the cuprate phase diagram and exhibits strong
doping dependence. The continuous increase of checkerboard periodicity with
hole density strongly suggests that the checkerboard originates from charge-density-wave
formation in the antinodal region of the cuprate Fermi surface. These results
reveal a coherent picture for static electronic orderings in the cuprates
and shed important new light on the nature of the pseudogap phase.
Visualization of the interplay between high-temperature superconductivity, the pseudogap and impurity resonances
Nature Physics, Volume 4, Issue 2, pp. 108-111 (2008).
Introduction (News & Views): Superconductivity: Bring on the real resonance (W.-S. Lee & Z.-X. Shen)
In
conventional superconductors, the superconducting gap in the electronic
excitation spectrum prevents scattering of low-energy electrons. In high-temperature
superconductors (HTSs), an extra gap, the pseudogap, develops well above
the superconducting transition temperature TC. Here, we present
a new avenue of investigating the pseudogap state, using scanning tunnelling
microscopy (STM) of resonances generated by single-atom scatterers. Previous
studies on the superconducting state of HTSs have led to a fairly consistent
picture in which potential scatterers, such as Zn, strongly suppress superconductivity
in an atomic-scale region, while generating low-energy excitations with
a spatial distribution—as imaged by STM—indicative of the d-wave nature
of the superconducting gap. Surprisingly, we find that similar native impurity
resonances coexist spatially with the superconducting gap at low temperatures
and survive virtually unchanged on warming through TC. These
findings demonstrate that properties of impurity resonances in HTSs are
not determined by the nature of the superconducting state, as previously
suggested, but instead provide new insights into the pseudogap state.
Imaging the two gaps of the high-temperature superconductor Bi2Sr2CuO6+x
Nature Physics, Volume 3, Issue 11, pp. 802-806 (2007) [Preprint PDF]
The
nature and behaviour of electronic states in high-temperature superconductors
are the centre of much debate. The pseudogap state, observed above the superconducting
transition temperature, TC, is seen by some as a precursor to
the superconducting state. Others view it as a competing phase. Recently,
this discussion has focused on the number of energy gaps in the system.
Some experiments indicate a single energy gap, implying that the pseudogap
is a precursor state. Others indicate two, suggesting that it is a competing
or coexisting phase. Here, we use temperature-dependent scanning tunnelling
spectroscopy of (Bi1-yPby)2Sr2CuO6+x
to clarify the situation. We find a previously unobserved narrow and homogeneous
gap that vanishes near TC, superimposed on the typically observed
inhomogeneous and broad gap, which is only weakly temperature dependent.
These results not only support the two-gap picture, but also explain previously
troubling differences between scanning tunnelling microscopy and other experimental
measurements.