Quantum textures of the many-body wavefunctions in magic-angle graphene

Here we use high-resolution scanning tunnelling microscopy to study the wavefunctions of the correlated phases in MATBG.

full story: Nature (Aug. 2023)

Broken symmetries and excitation spectra of interacting electrons in partially filled Landau levels

Interacting electrons in flat bands give rise to a variety of quantum phases. One fundamental aspect of such states is the ordering of the various flavours—such as spin or valley—that the electrons can possess and the excitation spectrum of the broken-symmetry states that they form.
full story: Nature Physics (July 2023)

Hunting for Majoranas

The past decade has witnessed considerable progress toward the creation of new quantum technologies. Substantial advances in present leading qubit technologies, which are based on superconductors, semiconductors, trapped ions, or neutral atoms, will undoubtedly be made in the years ahead.
full story: Science (June 2023)

Visualizing broken symmetry and topological defects in a quantum Hall ferromagnet

The interaction between electrons in graphene under high magnetic fields drives the formation of a rich set of quantum Hall ferromagnetic phases (QHFM), with broken spin or valley symmetry. Visualizing atomic scale electronic wavefunctions with scanning tunneling spectroscopy (STS), we resolve microscopic signatures of valley ordering in QHFM and spectral features of fractional quantum Hall phases of graphene.
full story: Science (December 2021)

Evidence for unconventional superconductivity in twisted bilayer graphene

The emergence of superconductivity and correlated insulators in magic-angle twisted bilayer graphene (MATBG) has raised the intriguing possibility that its pairing mechanism is distinct from that of conventional superconductors, as described by the Bardeen-Cooper-Schrieffer (BCS) theory. However, recent studies have shown that superconductivity persists even when Coulomb interactions are partially screened.
full story: Nature (October 2021). Free link.

Detecting and distinguishing Majorana zero modes with the scanning tunnelling microscope

One of the most exciting areas of research in quantum condensed matter physics is the push to create topologically protected qubits using non-Abelian anyons. The focus of these efforts has been Majorana zero modes (MZMs)...
cover story: Nature Reviews Physics, vol. 3 (August 2021)

Strongly correlated Chern insulators in magic-angle twisted bilayer graphene

We demonstrate that strong electron–electron interactions alone can produce...unexpected Chern insulating phases in MATBG. The full sequence of phases that we observe can be understood by postulating that strong correlations favor breaking time-reversal symmetry to form Chern insulators that are stabilized by weak magnetic fields. 
full story: Nature 588 (December 2020).

Observation of backscattering induced by magnetism in a topological edge state

The boundary modes of topological insulators are protected by the symmetries of the nontrivial bulk electronic states...They can give rise to novel phenomena...where quasiparticle backscattering is suppressed by time-reversal symmetry (TRS).
full story: PNAS 117 (June 2020)

Cascade of electronic transitions in magic-angle twisted bilayer graphene

... The cascade of transitions we report here characterizes the correlated high-temperature parent phase from which various insulating and superconducting ground-state phases emerge at low temperatures in MATBG.
full story: Nature 582 (June 2020)

Spectroscopic signatures of many-body correlations in magic-angle twisted bilayer graphene

...We show that a phenomenological extended-Hubbard-model cluster calculation, which is motivated by the nearly localized nature of the relevant electronic states of MATBG, produces spectroscopic features that are similar to those that we observed experimentally...
full story: Nature 572 (August 2019)

Observation of a Majorana zero mode in a topologically protected edge channel

... Consistent with model calculations, our measurements revealed the emergence of a localized MZM at the interface between the superconducting helical edge channel and the iron clusters, with a strong magnetization component along the edge. Our experiments also resolve the MZM’s spin signature, which distinguishes it from trivial.... 
full story: Science 364 (June 2019)

Interacting multi-channel topological boundary modes in a quantum Hall valley system

...Here we use a scanning tunnelling microscope to directly visualize the spontaneous formation of boundary modes at domain walls between QHFM phases with different valley polarization  (that is, the occupation of equal-energy but quantum mechanically distinct valleys in the electronic structure) on the surface of bismuth...
full story:  Nature 566 (February 2019)

Higher-order topology in bismuth

We establish that the electronic structure of bismuth, an element consistently described as bulk topologically trivial, is in fact topological and follows a generalized bulk–boundary correspondence of higher-order: not the surfaces of the crystal, but its hinges host topologically protected conducting modes. 
cover story: Nature Physics 14 (July 2018) 
(The crystal in the cover photo was grown by Princeton undergraduate, Adam Bowman, who worked in the Yazdani Lab and completed his Senior Thesis in 2017.)

Ferroelectric quantum Hall phase revealed by visualizing Landau level wavefunction interferenc

Quantum materials are a rich platform in which to explore novel phenomena that emerge due to interactions between electrons, giving rise to new phases of matter that cannot be explained by the properties of isolated, non-interacting electrons. Just like electrons have a spin degree of freedom, they can also be associated with...
full story: Nature Physics 14 (May 2018)

Imaging Anyons with Scanning Tunneling Microscopy

We combine analytical calculations with numerical simulations to demonstrate that an STM could visualize the structure of interacting electronic states of FQH states. Spectroscopy mapping with the STM near individual impurity defects can be used to image fractional statistics in FQH states, identifying unique signatures in such...
full story: Physical Review X 8 (March 2018)

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...
full story: Science 10 (Nov. 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 (Nov. 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)

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 (June 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)

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 (Oct. 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)

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)

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)