15 research outputs found
Topology- and symmetry-protected domain wall conduction in quantum Hall nematics
We consider domain walls in nematic quantum Hall ferromagnets predicted to
form in multivalley semiconductors, recently probed by scanning tunnelling
microscopy experiments on Bi(111) surfaces. We show that the domain wall
properties depend sensitively on the filling factor of the underlying
(integer) quantum Hall states. For and in the absence of impurity
scattering we argue that the wall hosts a single-channel Luttinger liquid whose
gaplessness is a consequence of valley and charge conservation. For , it
supports a two-channel Luttinger liquid, which for sufficiently strong
interactions enters a symmetry-preserving thermal metal phase with a charge gap
coexisting with gapless neutral intervalley modes. The domain wall physics in
this state is identical to that of a bosonic topological insulator protected by
symmetry, and we provide a formal mapping between these
problems. We discuss other unusual properties and experimental signatures of
these `anomalous' one-dimensional systems.Comment: 11 pages, 3 figures, published versio
Visualizing Heavy Fermion Confinement and Pauli-Limited Superconductivity in Layered CeCoIn5
Layered material structures play a key role in enhancing electron-electron
interactions to create correlated metallic phases that can transform into
unconventional superconducting states. The quasi-two-dimensional electronic
properties of such compounds are often inferred indirectly through examination
of their bulk properties. Here we use scanning tunneling microscopy and
spectroscopy to directly probe in cross section the quasi-two-dimensional
correlated electronic states of the heavy fermion superconductor CeCoIn5. Our
measurements reveal the strong confined nature of heavy quasi-particles,
anisotropy of tunneling characteristics, and layer-by-layer modulated behavior
of the precursor pseudogap gap phase in this compound. Examining the interlayer
coupled superconducting state at low temperatures, we find that the orientation
of line defects relative to the d-wave order parameter determines whether
in-gap states form due to scattering. Spectroscopic imaging of the anisotropic
magnetic vortex cores directly characterizes the short interlayer
superconducting coherence length and shows an electronic phase separation near
the upper critical in-plane magnetic field, consistent with a Pauli-limited
first-order phase transition into a pseudogap phase
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 anisotropic hole
pockets on the Bi(111) surface. Spectroscopy performed with a scanning
tunneling microscope shows that a combination of local strain and many-body
Coulomb interactions lift the six-fold Landau level (LL) degeneracy to form
three valley-polarized quantum Hall states. We image the resulting anisotropic
LL wavefunctions and show that they have a different orientation for each
broken-symmetry state. The wavefunctions correspond precisely to those expected
from pairs of hole valleys and provide a direct spatial signature of a nematic
electronic phase
Interacting multi-channel topological boundary modes in a quantum Hall valley system
Symmetry and topology play key roles in the identification of phases of
matter and their properties. Both concepts are central to understanding quantum
Hall ferromagnets (QHFMs), two-dimensional electronic phases with spontaneously
broken spin or pseudospin symmetry whose wavefunctions also have topological
properties. Domain walls between distinct broken symmetry QHFM phases are
predicted to host gapless one-dimensional (1D) modes that emerge due to a
topological change of the underlying electronic wavefunctions at such
interfaces. Although a variety of QHFMs have been identified in different
materials, probing interacting electronic modes at these domain walls has not
yet been accomplished. Here we use a scanning tunneling microscope (STM) to
directly visualize the spontaneous formation of boundary modes, within a
sign-changing topological gap, at domain walls between different
valley-polarized quantum Hall phases on the surface of bismuth. By changing the
valley occupation and the corresponding number of modes at the domain wall, we
can realize different regimes where the valley-polarized channels are either
metallic or develop a spectroscopic gap. This behavior is a consequence of
Coulomb interactions constrained by the symmetry-breaking valley flavor, which
determines whether electrons in the topological modes can backscatter, making
these channels a unique class of interacting Luttinger liquids
Bulk crystal growth and electronic characterization of the 3D Dirac Semimetal Na3Bi
High quality hexagon plate-like Na3Bi crystals with large (001) plane
surfaces were grown from a molten Na flux. The freshly cleaved crystals were
analyzed by low temperature scanning tunneling microscopy (STM) and
angle-resolved photoemission spectroscopy (ARPES), allowing for the
characterization of the three-dimensional (3D) Dirac semimetal (TDS) behavior
and the observation of the topological surface states. Landau levels (LL) were
observed, and the energy-momentum relations exhibited a linear dispersion
relationship, characteristic of the 3D TDS nature of Na3Bi. In transport
measurements on Na3Bi crystals the linear magnetoresistance and Shubnikov-de
Haas (SdH) quantum oscillations are observed for the first time.Comment: To be published in a special issue of APL Material
Tuning interactions between spins in a superconductor
Novel many-body and topological electronic phases can be created in assemblies of interacting spins coupled to a superconductor, such as one-dimensional topological superconductors with Majorana zero modes (MZMs) at their ends. Understanding and controlling interactions between spins and the emergent band structure of the in-gap Yu-Shiba-Rusinov (YSR) states they induce in a superconductor are fundamental for engineering such phases. Here, by precisely positioning magnetic adatoms with a scanning tunneling microscope (STM), we demonstrate both the tunability of exchange interaction between spins and precise control of the hybridization of YSR states they induce on the surface of a bismuth (Bi) thin film that is made superconducting with the proximity effect. In this platform, depending on the separation of spins, the interplay among Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, spin-orbit coupling, and surface magnetic anisotropy stabilizes different types of spin alignments. Using high-resolution STM spectroscopy at millikelvin temperatures, we probe these spin alignments through monitoring the spin-induced YSR states and their energy splitting. Such measurements also reveal a quantum phase transition between the ground states with different electron number parity for a pair of spins in a superconductor tuned by their separation. Experiments on larger assemblies show that spin-spin interactions can be mediated in a superconductor over long distances. Our results show that controlling hybridization of the YSR states in this platform provides the possibility of engineering the band structure of such states for creating topological phases