116 research outputs found
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Controlling a Van Hove singularity and Fermi surface topology at a complex oxide heterostructure interface.
The emergence of saddle-point Van Hove singularities (VHSs) in the density of states, accompanied by a change in Fermi surface topology, Lifshitz transition, constitutes an ideal ground for the emergence of different electronic phenomena, such as superconductivity, pseudo-gap, magnetism, and density waves. However, in most materials the Fermi level, [Formula: see text], is too far from the VHS where the change of electronic topology takes place, making it difficult to reach with standard chemical doping or gating techniques. Here, we demonstrate that this scenario can be realized at the interface between a Mott insulator and a band insulator as a result of quantum confinement and correlation enhancement, and easily tuned by fine control of layer thickness and orbital occupancy. These results provide a tunable pathway for Fermi surface topology and VHS engineering of electronic phases
Orbital character effects in the photon energy and polarization dependence of pure C60 photoemission
Recent direct experimental observation of multiple highly-dispersive C
valence bands has allowed for a detailed analysis of the unique photoemission
traits of these features through photon energy- and polarization-dependent
measurements. Previously obscured dispersions and strong photoemission traits
are now revealed by specific light polarizations. The observed intensity
effects prove the locking in place of the C molecules at low
temperatures and the existence of an orientational order imposed by the
substrate chosen. Most importantly, photon energy- and polarization-dependent
effects are shown to be intimately linked with the orbital character of the
C band manifolds which allows for a more precise determination of the
orbital character within the HOMO-2. Our observations and analysis provide
important considerations for the connection between molecular and crystalline
C electronic structure, past and future band structure studies, and for
increasingly popular C electronic device applications, especially those
making use of heterostructures
Observation of a flat and extended surface state in a topological semimetal
A topological flatband, also known as drumhead states, is an ideal platform
to drive new exotic topological quantum phases. Using angle-resolved
photoemission spectroscopy experiments, we reveal the emergence of a highly
localized possible drumhead surface state in a topological semimetal BaAl4 and
provide its full energy and momentum space topology. We find that the observed
surface state is highly localized in momentum, inside a square-shaped bulk
Dirac nodal loop, and in energy, leading to a flat band and a peak in the
density of state. These results establish this class of materials as a possible
experimental realization of drumhead surface states and provide an important
reference for future studies of fundamental physics of topological quantum
phase transition
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Chemical Analysis of Impurity Boron Atoms in Diamond Using Soft X-ray Emission Spectroscopy
To analyze the local structure and/or chemical states of boron atoms in boron-doped diamond, which can be synthesized by the microwave plasma-assisted chemical vapor deposition method (CVD-B-diamond) and the temperature gradient method at high pressure and high temperature (HPT-B-diamond), we measured the soft X-ray emission spectra in the CK and BK regions of B-diamonds using synchrotron radiation at the Advanced Light Source (ALS). X-ray spectral analyses using the fingerprint method and molecular orbital calculations confirm that boron atoms in CVD-B-diamond substitute for carbon atoms in the diamond lattice to form covalent B-C bonds, while boron atoms in HPT-B-diamond react with the impurity nitrogen atoms to form hexagonal boron nitride. This suggests that the high purity diamond without nitrogen impurities is necessary to synthesize p-type B-diamond semiconductors
Robust Luttinger liquid state of 1D Dirac fermions in a van der Waals system NbSiTe
We report on the Tomonaga-Luttinger liquid (TLL) behavior in fully degenerate
1D Dirac fermions. A ternary van der Waals material NbSiTe
incorporates in-plane NbTe chains, which produce a 1D Dirac band crossing
Fermi energy. Tunneling conductance of electrons confined within NbTe2 chains
is found to be substantially suppressed at Fermi energy, which follows a power
law with a universal temperature scaling, hallmarking a TLL state. The obtained
Luttinger parameter of ~0.15 indicates strong electron-electron interaction.
The TLL behavior is found to be robust against atomic-scale defects, which
might be related to the Dirac electron nature. These findings, as combined with
the tunability of the compound and the merit of a van der Waals material, offer
a robust, tunable, and integrable platform to exploit non-Fermi liquid physics
Evidence for quasi-one-dimensional charge density wave in CuTe by angle-resolved photoemission spectroscopy
We report the electronic structure of CuTe with a high charge density wave
(CDW) transition temperature Tc = 335 K by angle-resolved photoemission
spectroscopy (ARPES). An anisotropic charge density wave gap with a maximum
value of 190 meV is observed in the quasi-one-dimensional band formed by Te px
orbitals. The CDW gap can be filled by increasing temperature or electron
doping through in situ potassium deposition. Combining the experimental results
with calculated electron scattering susceptibility and phonon dispersion, we
suggest that both Fermi surface nesting and electron-phonon coupling play
important roles in the emergence of the CDW
Beyond Triplet: Unconventional Superconductivity in a Spin-3/2 Topological Semimetal
In all known fermionic superfluids, Cooper pairs are composed of spin-1/2
quasi-particles that pair to form either spin-singlet or spin-triplet bound
states. The "spin" of a Bloch electron, however, is fixed by the symmetries of
the crystal and the atomic orbitals from which it is derived, and in some cases
can behave as if it were a spin-3/2 particle. The superconducting state of such
a system allows pairing beyond spin-triplet, with higher spin quasi-particles
combining to form quintet or septet pairs. Here, we report evidence of
unconventional superconductivity emerging from a spin-3/2 quasiparticle
electronic structure in the half-Heusler semimetal YPtBi, a low-carrier density
noncentrosymmetric cubic material with a high symmetry that preserves the
-like manifold in the Bi-based band in the presence of
strong spin-orbit coupling. With a striking linear temperature dependence of
the London penetration depth, the existence of line nodes in the
superconducting order parameter is directly explained by a
mixed-parity Cooper pairing model with high total angular momentum, consistent
with a high-spin fermionic superfluid state. We propose a
model of the fermions to explain how a dominant =3 septet pairing
state is the simplest solution that naturally produces nodes in the mixed
even-odd parity gap. Together with the underlying topologically non-trivial
band structure, the unconventional pairing in this system represents a truly
novel form of superfluidity that has strong potential for leading the
development of a new generation of topological superconductors.Comment: 12 pages, 10 figures, supplementary info include
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