Publication Highlights

Fermi Surface & Pseudogap in a Cuprate

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 Bi_2-yPb_ySr_2-xLa_xCuO_6+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.

CDW on the Surface & Bulk of Bi₂Sr_2-xLa_xCuO_6+d

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, Bi₂Sr_2-xLa_xCuO_6+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.

Kondo Insulating Gap on SmB₆

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.

Landau levels & QPI on Topological Sb(111)

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.

Discovery of 1D CDW in NbSe₂

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 Bi₂Sr₂CaCu₂O_8+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-T_c superconductor Bi_2+ySr_2-yCaCu₂O_8+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.

Picometer structural distortion in Bi₂Sr₂CuO_6+x

Nanoscale Control of Resistance Switching in VO₂

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.

Vortex Imaging in Superconducting BaFe_1.8Co_0.2As₂

We performed the first atomic resolution spectroscopy, impurity imaging, and vortex imaging in the new family of Fe-based high-T_c superconductors. We extract the spatial variations of the superconducting gap, the superconducting coherence length, and properties of the vortex pinning.