Distinguishing a Majorana zero mode using spin resolved measurements
We use spin-polarized scanning tunneling microscopy to show that MZMs realized in self-assembled Fe chains on the surface of Pb have a spin polarization that exceeds that stemming from the magnetism of these chains. This feature is a direct consequence of the nonlocality of the Hilbert space of MZMs emerging from a topological band structure. Our study establishes spin polarization measurements as a diagnostic tool to distinguish topological MZMs from trivial in-gap states of a superconductor.

full sory: Science 10.1126/science.aan3670 (Oct. 2017)

High-resolution studies of the Majorana atomic chain platform
Recent experiments have shown that chains of Fe atoms on Pb generically have the required electronic characteristics to form a 1D topological superconductor and have revealed spatially resolved signatures of localized Majorana Fermion quasiparticle (MQP) end modes. With significantly better energy resolution...

full story: Nature Physics 13 (June 2016)


Observation of a nematic quantum Hall liquid on the surface of bismuth
Nematic quantum fluids with wavefunctions that break the underlying crystalline symmetry can form in interacting electronic systems. We examine the quantum Hall states that arise in high magnetic fields from multiple anisotropic hole pockets on the Bi(111) surface. Spectroscopy performed with a...
full story: Science 354 (October 2016)


Quasiparticle interference of the Fermi arcs and surface-bulk connectivity of Weyl semimetals
Weyl semimetals host topologically protected surface states, with arced Fermi surface contours that are predicted to propagate through the bulk when their momentum matches that of the surface projections of the bulk’s Weyl nodes. We use spectroscopic mapping with a scanning tunneling microscope...
full story: Science 351 (March 2016)

Weyl semimetal TaAs

Observation of Majorana fermions in ferromagnetic atomic chains on a superconductor
Majorana fermions are predicted to localize at the edge of a topological superconductor, a state of matter that can form when a ferromagnetic system is placed in proximity to a conventional superconductor with strong spin-orbit interaction. With the goal of realizing a one-dimensional topological...
full story: Science 346 (Oct. 2014)

One-dimensional topological edge states of bismuth bilayers
Bismuth bilayers have been among the first systems theoretically predicted to be a two-dimensional topological insulator. We have been able to study the topological edge modes of bismuth bilayer system by carrying out scanning tunneling spectroscopy measurements on bulk bismuth samples. By...
full story: Nature Physics 10 (Aug. 2014)

Landau Quantization and Quasiparticle Interference in the Three-Dimensional Dirac Semimetal Cd3As2
A pair of Weyl fermions protected by crystalline symmetry has been proposed to appear as low-energy excitations in a number of materials termed 3D Dirac semimetals. Cd3As2 known to have a high mobility is one of the first candidates to exhibit a bulk 3D Dirac semimetal phase which is protected by...
full story: Nature Materials 13 (Sept. 2014)


Ubiquitous interplay between charge ordering and High-Tc Superconductivity
Combining spectroscopic mapping with the scanning tunneling microscope and resonant elastic x-ray scattering we have been able to demonstrate the ubiquitous interplay between charge ordering and superconductivity in high-Tc cuprates. Our results also show that strong electronic correlation play a...
full story: Science 343 (Jan. 2014)


Visualizing nodal heavy fermion superconductivity
Enabled by our recent development of high resolution, dilution fridge-based STM, we have uncovered new insights on the superconductivity of heavy fermions, a class of materials whose unconventional superconductivity may share a common origin as the high-Tc cuprates. By examining the pair-breaking...

cover story: Nature Physics 9 (Aug. 2013) 


Evidence for a universal minimum superfluid response in field-tuned disordered superconducting films
Our measurements of the ac conductivity of disordered thin films near the field-tuned superconductor-insulator transition show a sudden drop in the superfluid response with either increased temperature or with applied magnetic field. Surprisingly, this abrupt drop, seen in two different material...
full story: Phys Rev Lett vol 110 issue 3 (Jan. 18, 2013)

SIT(superconductor-insulator transition)

Visualizing heavy fermions emerging in a quantum critical Kondo lattice
Cool electrons to far below room temperature in certain solids with f orbitals, and they gain mass, acting like much heavier particles. In a new study our group has shown, for the first time, how these heavy electrons emerge from entanglement between conduction and f electrons and probed their...
full story: Nature vol 486 issue 7402 (June 2012)


Spatial fluctuations of helical Dirac fermions on the surface of topological insulators
Helical Dirac fermions on the surface topological insulators are a new class of electronic states that could enable dissipation-free spintronics and robust quantum information processors. Our recent study of the influence of disorder on these states shows that although they are resilient against...
full story: Nature Physics vol 7 (Dec. 2011)


Fluctuating stripes at the onset of the pseudogap in the high-Tc superconductor Bi2Sr2CaCu2O8+x
An important clue has been discovered in the mystery of the pseudogap state of the high temperature superconducting cuprates. A basic understanding of cuprates has been lacking because scientists do not understand the state out of which superconductivity develops, the so-called pseudogap state...
full story: Nature vol 468 issue 7324 (Dec. 2010)


Transmission of topological surface states through surface barriers
Topological surface states are a class of novel electronic states that are of potential interest in quantum computing or spintronic applications. Unlike conventional two-dimensional electron states, these surface states are expected to be immune to localization and to overcome barriers caused by...
full story: Nature vol 466 issue 7304 (July 2010)


Visualizing the formation of the Kondo lattice and the hidden order in URu2Si2
Heavy electronic states originating from the f atomic orbitals underlie a rich variety of quantum phases of matter. We use atomic scale imaging and spectroscopy with the scanning tunneling microscope to examine the novel electronic states that emerge from the uranium f states in...
full story: PNAS vol 107 no. 23 (June 2010)


Nanoscale Proximity Effect in the High-Temperature Superconductor Bi2Sr2Ca2CuO8+δ Using a Scanning Tunneling Microscope
High-temperature cuprate superconductors exhibit extremely local nanoscale phenomena and strong sensitivity to doping. While other experiments have looked at nanoscale interfaces between layers of different dopings, we focus on the interplay between naturally inhomogeneous nanoscale...
full story: Phys Review Lett vol 104 issue 11


Visualizing Critical Correlations near the Metal-Insulator Transition in Ga1‑xMnxAs
Electronic states in disordered conductors on the verge of localization are predicted to exhibit critical spatial characteristics indicative of the proximity to a metal‑insulator phase transition. We have used scanning tunneling microscopy to visualize electronic states in...

full story: Science 5 vol 327 (Feb. 2010)

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Topological Surface States Protected From Backscattering by Chiral Spin Texture
We have used scanning tunnelling spectroscopy and angle-resolved photoemission spectroscopy to visualize the gapless surface states in the three-dimensional topological insulator Bi1‑xSbx, and examine in detail the influence of scattering from disorder caused by random...
full story: Nature 460 issue 7259 (Aug. 2009)


Extending Universal Nodal Excitations Optimizes Superconductivity in Bi2Sr2Ca2CuO8+δ
We have studied high-temperature superconducting cuprates samplesthroughout the doping-temperature phase diagram. We find that the low energy excitations of these systems havea surprising, universal character at low doping. We have developed a procedure to extract the angular dependence of the...

full story: Science 26 vol 324 (June 2009)

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Electronic Origin of the Inhomogeneous Pairing Interaction in the High-Tc Superconductor Bi2Sr2Ca2CuO8+δ
We investigate the source of the variation of the pairing strength in high‑Tc superconductors. We have developed a new technique that allows us to study the same nanoscale region of the material through a broad range of temperatures. Using this technique, we can probe the...
full story: Science 11 vol 320 (April 2008)


Visualizing Pair Formation on the Atomic Scale in the High-Tc Superconductor Bi2Sr2Ca2CuO8+δ
We take an atom-by-atom look at where pairing gaps form in the cuprate superconductor Bi2Sr2Ca2CuO8+δ. When pairing occurs in a superconductor, a gap forms in the density of states (DOS) that can be probed by...

full story: Nature 447 issue 7144 (May 2007)


Atom-by-Atom Substitution of Mn in GaAs and Visualization of their Hole-Mediated Interactions
A novel technique developed by our group uses a scanning tunneling microscope (STM) to substitute atoms into a semiconductor one atom at a time. This technique has been used to assemble a magnetic semiconductor, manganese-doped gallium arsenide (Ga1‑xMnxAs), atom by...

cover story: Nature 442 issue 7101 (July 2006)


Local Ordering in the Pseudogap State of a High-Tc Superconductor
We performed a scanning tunneling spectrocopy characterization of the electronic correlations in the pseudogap state in a high‑Tc superconductor. Heating a normal superconductor above its critical temperature results in a normal metallic behavior, but heating a high-temperature...

full story: Science 303 no. 5666 (March 2004)


Atomic Scale Imaging and Spectroscopy of a CuO2 plane at the Surface of a High-Tc Superconductor
Using a scanning tunneling microscope, we demonstrated that a single copper-oxide plane can form a stable layer at the superconductor's surface. This plane behaves differently when exposed at the surface than when buried inside the crystal, offering additional insight into the behavior of...

full story: Phys Review Lett vol 89 no. 8 (Aug 2002)


Mapping the One-Dimensional Electronic States of Nanotube Peapod Structures
Scientists recently discovered that nanoscopic peapods — the latest class of nanomaterials created by filling the cores of single‑wall nanotubes — have tunable electronic properties. Our measurements show that encapsulation of molecules can dramatically modify the electronic...

cover story: Science vol 295 no. 5556 (Feb. 2002)