Superconductivity arises from pairing of electrons on the Fermi surface. Here we use Fourier transform STM to map a small-to-large Fermi surface
transition in the cuprate superconductor Bi2-yPbySr2-xLaxCuO6+d, demonstrating a quantum critical point near optimal
doping at zero field. Furthermore, we show that superconductivity coexists with the pseudogap on the recovered antinodal Fermi surface above optimal doping. Finally, we
demonstrate the nanoscale decoherence effect of the pseudogap, despite the robust magnitude of the superconducting order parameter. Our work suggests the importance of
mitigating this decoherence at the nanoscale, to optimize superconductivity.
We combined resonant x-ray scattering (REXS), scanning tunneling microscopy (STM), and angle-resolved photoemission spectroscopy (ARPES) to
observe a charge order that appears consistently in surface and bulk, and in momentum and real space within one cuprate family,
Bi2Sr2-xLaxCuO6+d. Combined with earlier observations of electronic order in other cuprate families, these findings suggest the
existence of a generic charge-ordered state in underdoped cuprates and uncover its intimate connection to the pseudogap regime.
We use STM to conduct the first atomic resolution spectroscopic study of the cleaved surface of SmB6, and to reveal a robust
hybridization gap that universally spans the Fermi level. Employing a cotunneling model, we separate the density of states of the hybridized bands from which the predicted
topological surface states must be disentangled. Our technique lays the groundwork for understanding the first strongly correlated topological insulator, and implements a
general method to quantitatively understand a wider class of Kondo insulators.
We demonstrated the first complementary use of two momentum-resolved STM techniques, quasiparticle interference (QPI) and Landau level spectroscopy,
to measure the nanoscale band structure of the topological semimetal Sb, and to quantify with ~10nm spatial resolution the three important metrics
for topological spintronics devices: spin-orbit coupling, mean free path, and g-factor.
We used STM to discover a 1D (stripe) CDW smoothly interfacing with the familiar 2D (triangular) CDW on the surface of the superconductor
NbSe2. We use this strain-tuned quantum phase transition to resolve two longstanding debates about the anomalous spectroscopic gap and the role of Fermi surface
nesting in the CDW phase of NbSe2. Our results highlight the importance of local strain in governing phase transitions and competing phenomena, and suggest a
promising direction of inquiry for resolving similarly longstanding debates in cuprate superconductors and other strongly correlated materials.
Single oxygen atoms in superconducting Bi2Sr2CaCu2O8+x
We developed techniques to double the available energy range for scanning tunneling spectroscopy on fragile surfaces. We employ these
techniques on the cuprate high-Tc superconductor Bi2+ySr2-yCaCu2O8+x to locate the atomic defects which control the
local hole concentration, govern the local energy scale of the pseudogap, and pin the periodic charge modulation.
We implemented a new algorithm for improved spatial resolution scanning tunneling microscopy, which allows us to image a subtle ~5 picometer inversion-symmetry-breaking
structural distortion in superconducting Bi2Sr2Can-1CunO2n+4+x. Although this structural distortion can impact electronic
measurements, we show from its insensitivity to temperature, magnetic field and doping, that it cannot be the long-sought pseudogap state.
VO2 undergoes a sharp insulator-to-metal transition near room temperature. We demonstrate controlled local
phase switching of a VO2 film using a biased conducting atomic force microscope tip. Our nanoscale phase manipulation technique opens up the possibility for an
understanding of the microscopic mechanism of phase transition in VO2 as well as its potential relevance to solid state devices.
We performed the first atomic resolution spectroscopy, impurity imaging, and vortex imaging in the new family of Fe-based high-Tc superconductors. We extract the
spatial variations of the superconducting gap, the superconducting coherence length, and properties of the vortex pinning.