32 research outputs found
Signature of a pair of Majorana zero modes in superconducting gold surface states
Under certain conditions, a fermion in a superconductor can separate in space
into two parts known as Majorana zero modes, which are immune to decoherence
from local noise sources and are attractive building blocks for quantum
computers. Promising experimental progress has been made to demonstrate
Majorana zero modes in materials with strong spin-orbit coupling proximity
coupled to superconductors. Here we report signatures of Majorana zero modes in
a new material platform utilizing the surface states of gold. Using scanning
tunneling microscope to probe EuS islands grown on top of gold nanowires, we
observe two well separated zero bias tunneling conductance peaks aligned along
the direction of the applied magnetic field, as expected for a pair of Majorana
zero modes. This platform has the advantage of having a robust energy scale and
the possibility of realizing complex designs using lithographic methods
Evidence of Ising pairing in superconducting NbSe atomic layers
Two-dimensional transition metal dichalcogenides with strong spin-orbit
interactions and valley-dependent Berry curvature effects have attracted
tremendous recent interests. Although novel single-particle and excitonic
phenomena related to spin-valley coupling have been extensively studied,
effects of spin-momentum locking on collective quantum phenomena remain
unexplored. Here we report an observation of superconducting monolayer NbSe
with an in-plane upper critical field over six times of the Pauli paramagnetic
limit by magneto-transport measurements. The effect can be understood in terms
of the competing Zeeman effect and large intrinsic spin-orbit interactions in
non-centrosymmetric NbSe monolayers, where the electronic spin is locked to
the out-of-plane direction. Our results provide a strong evidence of
unconventional Ising pairing protected by spin-momentum locking and open up a
new avenue for studies of non-centrosymmetric superconductivity with unique
spin and valley degrees of freedom in the exact two-dimensional limit
Visualizing the Localized Electrons of a Kagome Flat Band
Destructive interference between electron wavefunctions on the
two-dimensional (2D) kagome lattice induces an electronic flat band, which
could host a variety of interesting many-body quantum states. Key to realize
these proposals is to demonstrate the real space localization of kagome flat
band electrons. In particular, the extent to which the often more complex
lattice structure and orbital composition of realistic materials counteract the
localizing effect of destructive interference, described by the 2D kagome
lattice model, is hitherto unknown. We used scanning tunneling microscopy (STM)
to visualize the non-trivial Wannier states of a kagome flat band at the
surface of CoSn, a kagome metal. We find that the local density of states
associated with the flat bands of CoSn is localized at the center of the kagome
lattice, consistent with theoretical expectations for their corresponding
Wannier states. Our results show that these states exhibit an extremely small
localization length of two to three angstroms concomitant with a strongly
renormalized quasiparticle velocity, which is comparable to that of moir\'e
superlattices. Hence, interaction effects in the flat bands of CoSn could be
much more significant than previously thought. Our findings provide fundamental
insight into the electronic properties of kagome metals and are a key step for
future research on emergent many-body states in transition metal based kagome
materials
Nematic topological superconducting phase in Nb-doped Bi2Se3
A nematic topological superconductor has an order parameter symmetry, which
spontaneously breaks the crystalline symmetry in its superconducting state.
This state can be observed, for example, by thermodynamic or upper critical
field experiments in which a magnetic field is rotated with respect to the
crystalline axes. The corresponding physical quantity then directly reflects
the symmetry of the order parameter. We present a study on the superconducting
upper critical field of the Nb-doped topological insulator NbxBi2Se3 for
various magnetic field orientations parallel and perpendicular to the basal
plane of the Bi2Se3 layers. The data were obtained by two complementary
experimental techniques, magnetoresistance and DC magnetization, on three
different single crystalline samples of the same batch. Both methods and all
samples show with perfect agreement that the in-plane upper critical fields
clearly demonstrate a two-fold symmetry that breaks the three-fold crystal
symmetry. The two-fold symmetry is also found in the absolute value of the
magnetization of the initial zero-field-cooled branch of the hysteresis loop
and in the value of the thermodynamic contribution above the irreversibility
field, but also in the irreversible properties such as the value of the
characteristic irreversibility field and in the width of the hysteresis loop.
This provides strong experimental evidence that Nb-doped Bi2Se3 is a nematic
topological superconductor similar to the Cu- and Sr-doped Bi2Se3
Nematic Topological Superconducting Phase in Nb-Doped Bi₂Se₃
A nematic topological superconductor has an order parameter symmetry, which spontaneously breaks the crystalline symmetry in its superconducting state. This state can be observed, for example, by thermodynamic or upper critical field experiments in which a magnetic field is rotated with respect to the crystalline axes. The corresponding physical quantity then directly reflects the symmetry of the order parameter. We present a study on the superconducting upper critical field of the Nb-doped topological insulator NbxBi2Se3 for various magnetic field orientations parallel and perpendicular to the basal plane of the Bi2Se3 layers. The data were obtained by two complementary experimental techniques, magnetoresistance and DC magnetization, on three different single crystalline samples of the same batch. Both methods and all samples show with perfect agreement that the in-plane upper critical fields clearly demonstrate a two-fold symmetry that breaks the three-fold crystal symmetry. The two-fold symmetry is also found in the absolute value of the magnetization of the initial zero-field-cooled branch of the hysteresis loop and in the value of the thermodynamic contribution above the irreversibility field, but also in the irreversible properties such as the value of the characteristic irreversibility field and in the width of the hysteresis loop. This provides strong experimental evidence that Nb-doped Bi2Se3 is a nematic topological superconductor similar to the Cu- and Sr-doped Bi2Se3