90 research outputs found
Emergence of unidirectional coherent quasiparticles from high-temperature rotational symmetry broken phase of AV3Sb5 kagome superconductors
Kagome metals AV3Sb5 display a rich phase diagram of correlated electron
states, including superconductivity and novel density waves. Within this
landscape, recent experiments reveal signs of a new transition below T ~ 35 K
attributed to the highly sought-after electronic nematic phase that
spontaneously breaks rotational symmetry of the lattice. We use
spectroscopic-imaging scanning tunneling microscopy to study atomic-scale
signatures of electronic symmetry breaking as a function of temperature across
several materials in this family: CsV3Sb5, KV3Sb5 and Sn-doped CsV3Sb5. We find
that rotational symmetry breaking onsets universally at a high temperature in
these materials, toward the 2 x 2 charge density wave (CDW) transition
temperature T*. At a significantly lower temperature of about 30 K, we discover
a striking emergence of the quantum interference of coherent quasiparticles, a
key signature for the formation of a coherent electronic state. These
quasiparticles display a pronounced unidirectional reciprocal-space
fingerprint, which strengthens on approaching the superconducting state. Our
experiments reveal that the high-temperature charge ordering states are
separated from the superconducting ground state by an intermediate-temperature
regime with coherent unidirectional quasiparticles. Their emergence that occurs
significantly below the onset of rotational symmetry breaking is
phenomenologically different compared to high-temperature superconductors,
shedding light on the complex nature of electronic nematicity in AV3Sb5 kagome
superconductors
Incommensurate charge-stripe correlations in the kagome superconductor CsVSbSn
We track the evolution of charge correlations in the kagome superconductor
CsVSb as its parent, long-ranged charge density order is destabilized.
Upon hole-doping doping, interlayer charge correlations rapidly become
short-ranged and their periodicity is reduced by half along the interlayer
direction. Beyond the peak of the first superconducting dome, the parent charge
density wave state vanishes and incommensurate, quasi-1D charge correlations
are stabilized in its place. These competing, unidirectional charge
correlations demonstrate an inherent electronic rotational symmetry breaking in
CsVSb, independent of the parent charge density wave state and reveal a
complex landscape of charge correlations across the electronic phase diagram of
this class of kagome superconductors. Our data suggest an inherent 2
charge instability and the phenomenology of competing charge instabilities is
reminiscent of what has been noted across several classes of unconventional
superconductors.Comment: 6 pages, 4 figure
Quantifying magnetic field driven lattice distortions in kagome metals at the femto-scale using scanning tunneling microscopy
A wide array of unusual phenomena has recently been uncovered in kagome
solids. The charge density wave (CDW) state in the kagome superconductor AV3Sb5
in particular intrigued the community -- the CDW phase appears to break the
time-reversal symmetry despite the absence of spin magnetism, which has been
tied to exotic orbital loop currents possibly intertwined with magnetic field
tunable crystal distortions. To test this connection, precise determination of
the lattice response to applied magnetic field is crucial, but can be
challenging at the atomic-scale. We establish a new scanning tunneling
microscopy based method to study the evolution of the AV3Sb5 atomic structure
as a function of magnetic field. The method substantially reduces the errors of
typical STM measurements, which are at the order of 1% when measuring an
in-plane lattice constant change. We find that the out-of-plane lattice
constant of AV3Sb5 remains unchanged (within 10^-6) by the application of both
in-plane and out-of-plane magnetic fields. We also reveal that the in-plane
lattice response to magnetic field is at most at the order of 0.05%. Our
experiments provide further constraints on time-reversal symmetry breaking in
kagome metals, and establish a new tool for higher-resolution extraction of the
field-lattice coupling at the nanoscale applicable to other quantum materials
Electronic nematicity without charge density waves in titanium-based kagome metal
Layered crystalline materials that consist of transition metal atoms on a
kagome network have emerged as a versatile platform to study unusual electronic
phenomena. For example, in the vanadium-based kagome superconductors AV3Sb5
(where A can stand for K, Cs, or Rb) there is a parent charge density wave
phase that appears to simultaneously break both the translational and the
rotational symmetry of the lattice. Here, we show a contrasting situation where
electronic nematic order - the breaking of rotational symmetry without the
breaking of translational symmetry - can occur without a corresponding charge
density wave. We use spectroscopic-imaging scanning tunneling microscopy to
study the kagome metal CsTi3Bi5 that is isostructural to AV3Sb5 but with a
titanium atom kagome network. CsTi3Bi5 does not exhibit any detectable charge
density wave state, but comparison to density functional theory calculations
reveals substantial electronic correlation effects at low energies. Comparing
the amplitudes of scattering wave vectors along different directions, we
discover an electronic anisotropy that breaks the six-fold symmetry of the
lattice, arising from both in-plane and out-of-plane titanium-derived d
orbitals. Our work uncovers the role of electronic orbitals in CsTi3Bi5,
suggestive of a hexagonal analogue of the nematic bond order in Fe-based
superconductors.Comment: This is the submitted version. Final manuscript will appear in Nature
Physic
Alternate cleavage structure and electronic inhomogeneity in Ca-doped YBaCuO
YBaCuO (YBCO) has favorable macroscopic superconducting
properties of up to 93 K and up to 150 T. However, its
nanoscale electronic structure remains mysterious because bulk-like electronic
properties are not preserved near the surface of cleaved samples for easy
access by local or surface-sensitive probes. It has been hypothesized that
Ca-doping at the Y site could induce an alternate cleavage plane that mitigates
this issue. We use scanning tunneling microscopy (STM) to study both Ca-free
and 10% Ca-doped YBCO, and find that the Ca-doped samples do indeed cleave on
an alternate plane, yielding a spatially-disordered partial (Y,Ca) layer. Our
density functional theory calculations support the increased likelihood of this
new cleavage plane in Ca-doped YBCO. On this surface, we image a
superconducting gap with average value 24 3 meV and characteristic length
scale 1-2 nm, similar to Bi-based high- cuprates, but the first
map of gap inhomogeneity in the YBCO family.Comment: corrected typo in metadata author nam
Small Fermi pockets intertwined with charge stripes and pair density wave order in a kagome superconductor
The kagome superconductor family AV3Sb5 (A=Cs, K, Rb) emerged as an exciting
platform to study exotic Fermi surface instabilities. Here we use
spectroscopic-imaging scanning tunneling microscopy (SI-STM) and angle-resolved
photoemission spectroscopy (ARPES) to reveal how the surprising cascade of
higher and lower-dimensional density waves in CsV3Sb5 is intimately tied to a
set of small reconstructed Fermi pockets. ARPES measurements visualize the
formation of these pockets generated by a 3D charge density wave transition.
The pockets are connected by dispersive q* wave vectors observed in Fourier
transforms of STM differential conductance maps. As the additional 1D charge
order emerges at a lower temperature, q* wave vectors become substantially
renormalized, signaling further reconstruction of the Fermi pockets.
Remarkably, in the superconducting state, the superconducting gap modulations
give rise to an in-plane Cooper pair-density-wave at the same q* wave vectors.
Our work demonstrates the intrinsic origin of the charge-stripes and the
pair-density-wave in CsV3Sb5 and their relationship to the Fermi pockets. These
experiments uncover a unique scenario of how Fermi pockets generated by a
parent charge density wave state can provide a favorable platform for the
emergence of additional density waves
Flat band separation and robust spin-Berry curvature in bilayer kagome metals
Kagome materials have emerged as a setting for emergent electronic phenomena
that encompass different aspects of symmetry and topology. It is debated
whether the XVSn kagome family (where X is a rare earth element), a
recently discovered family of bilayer kagome metals, hosts a topologically
non-trivial ground state resulting from the opening of spin-orbit coupling
gaps. These states would carry a finite spin-Berry curvature, and topological
surface states. Here, we investigate the spin and electronic structure of the
XVSn kagome family. We obtain evidence for a finite spin-Berry
curvature contribution at the center of the Brillouin zone, where the nearly
flat band detaches from the dispersing Dirac band because of spin-orbit
coupling. In addition, the spin-Berry curvature is further investigated in the
charge density wave regime of ScVSn, and it is found to be robust
against the onset of the temperature-driven ordered phase. Utilizing the
sensitivity of angle resolved photoemission spectroscopy to the spin and
orbital angular momentum, our work unveils the spin-Berry curvature of
topological kagome metals, and helps to define its spectroscopic fingerprint.Comment: 21 pages, 4 figure
Mapping the unconventional orbital texture in topological crystalline insulators
The newly discovered topological crystalline insulators (TCIs) harbor a
complex band structure involving multiple Dirac cones. These materials are
potentially highly tunable by external electric field, temperature or strain
and could find future applications in field-effect transistors, photodetectors,
and nano-mechanical systems. Theoretically, it has been predicted that
different Dirac cones, offset in energy and momentum-space, might harbor vastly
different orbital character, a unique property which if experimentally
realized, would present an ideal platform for accomplishing new spintronic
devices. However, the orbital texture of the Dirac cones, which is of immense
importance in determining a variety of materials properties, still remains
elusive in TCIs. Here, we unveil the orbital texture in a prototypical TCI
PbSnSe. By using Fourier-transform (FT) scanning tunneling
spectroscopy (STS) we measure the interference patterns produced by the
scattering of surface state electrons. We discover that the intensity and
energy dependences of FTs show distinct characteristics, which can directly be
attributed to orbital effects. Our experiments reveal the complex band topology
involving two Lifshitz transitions and establish the orbital nature of the
Dirac bands in this new class of topological materials, which could provide a
different pathway towards future quantum applications
YbVSb and EuVSb, vanadium-based kagome metals with Yb and Eu zig-zag chains
Here we present YbVSb and EuVSb, two new compounds exhibiting
slightly distorted vanadium-based kagome nets interleaved with zig-zag chains
of divalent Yb and Eu ions. Single crystal growth methods are
reported alongside magnetic, electronic, and thermodynamic measurements.
YbVSb is a nonmagnetic metal with no collective phase transitions
observed between 60mK and 300K. Conversely, EuVSb is a magnetic kagome
metal exhibiting easy-plane ferromagnetic-like order below =32K
with signatures of noncollinearity under low field. Our discovery of
YbVSb and EuVSb demonstrate another direction for the discovery
and development of vanadium-based kagome metals while incorporating the
chemical and magnetic degrees of freedom offered by a rare-earth sublattice
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