85 research outputs found
Anomalous spectral evolution with bulk sensitivity in BiPd
We investigate the electronic structure of a noncentrosymmetric
superconductor, BiPd using photoemission spectroscopy with multiple photon
energies ranging from ultraviolet to hard x-ray. Experimental data exhibit
interesting difference in the surface and bulk electronic structures of this
system. While the surface Bi core level peaks appear at lower binding energies,
the surface valence band features are found at the higher binding energy side
of the bulk valence band; valence band is primarily constituted by the Pd 4d
states. These changes in the electronic structure cannot be explained by the
change in ionicity of the constituent elements via charge transfer. Analysis of
the experimental data indicates that the Bi-Pd hybridization physics plays the
key role in deriving the anomalous spectral evolution and the electronic
properties of this system.Comment: Proceedings of DAE SSPS 201
Coexistence of multiple silicene phases in silicon grown on Ag(111)
Silicene, the silicon equivalent of graphene, is attracting increasing
scientific and technological attention in view of the exploitation of its
exotic electronic properties. This novel material has been theoretically
predicted to exist as a free-standing layer in a low-buckled, stable form, and
can be synthesized by the deposition of Si on appropriate crystalline
substrates. By employing low-energy electron diffraction and microscopy, we
have studied the growth of Si on Ag(111) and observed a rich variety of
rotationally non-equivalent silicene structures. Our results highlight a very
complex formation diagram, reflecting the coexistence of different and nearly
degenerate silicene phases, whose relative abundance can be controlled by
varying the Si coverage and growth temperature. At variance with other studies,
we find that the formation of single-phase silicene monolayers cannot be
achieved on Ag(111)
Spin Selective Evolution of Zhang-Rice State in Binary Transition Metal Oxide
The Zhang-Rice (ZR) state is a strongly hybridized bound state formed by the
transition metal and oxygen atoms. The spin-fluctuations within the ZR state
are known to play an important role in high- superconductivity in
cuprates. Here, we employ a combination of angle-resolved photoemission
spectroscopy (ARPES), X-ray photoemission spectroscopy (XPS), and {\it ab
initio} embedded dynamical mean-field theory (eDMFT) to investigate the
influence of magnetic ordering on the spectral characteristics of the valence
band and Mn 2 core-level in MnO (001) ultrathin films. Our results
demonstrate that a complex spin-selective evolution of Mn 3O 2
hybridization develops due to the long-range antiferromagnetic (AFM) ordering.
This hybridization significantly alters the spectral shape and weight of the ZR
state. Specifically, in the AFM phase, we observed the sharpening of the ZR
state and band folding with the periodicity of the AFM unit cell of MnO(001).
We also demonstrated a strong connection between the spectral evolution of the
ZR state and the non-local screening channels of the photoexcited core holes.
Further, our detailed temperature-dependent study reveals the presence of
short-range antiferromagnetic correlations that exist at much higher
temperatures than . Such comprehensive studies showing the
evolution of the ZR state across the magnetic transitions and its implication
to the core-hole screening have never been reported in any 3 binary
transition metal oxides.Comment: 8 pages, 4 figure
Is graphene on copper doped?
Angle-resolved photoemission spectroscopy (ARPES) and X-ray photoemission spectroscopy have been used to characterise epitaxially ordered graphene grown on copper foil by low-pressure chemical vapour deposition. A short vacuum anneal to 200 °C allows observation of ordered low energy electron diffraction patterns. High quality Dirac cones are measured in ARPES with the Dirac point at the Fermi level (undoped graphene). Annealing above 300 °C produces n-type doping in the graphene with up to 350 meV shift in Fermi level, and opens a band gap of around 100 meV.
Dirac cone dispersion for graphene on Cu foil after vacuum anneals (left: 200 °C, undoped; right: 500 °C, n-doped). Centre: low energy electron diffraction from graphene on Cu foil after 200 °C anneal. Data from Antares (SOLEIL)
Two Distinct Phases of Bilayer Graphene Films on Ru(0001)
By combining angle-resolved photoemission spectroscopy and scanning tunneling
microscopy we reveal the structural and electronic properties of multilayer
graphene on Ru(0001). We prove that large ethylene exposure allows to
synthesize two distinct phases of bilayer graphene with different properties.
The first phase has Bernal AB stacking with respect to the first graphene
layer, displays weak vertical interaction and electron doping. The long-range
ordered moir\'e pattern modulates the crystal potential and induces replicas of
the Dirac cone and minigaps. The second phase has AA stacking sequence with
respect to the first layer, displays weak structural and electronic modulation
and p-doping. The linearly dispersing Dirac state reveals the
nearly-freestanding character of this novel second layer phase
Indirect chiral magnetic exchange through Dzyaloshinskii–Moriya-enhanced RKKY interactions in manganese oxide chains on Ir(100)
Localized electron spins can couple magnetically via the Ruderman–Kittel–Kasuya–Yosida interaction even if their wave functions lack direct overlap. Theory predicts that spin–orbit scattering leads to a Dzyaloshinskii–Moriya type enhancement of this indirect exchange interaction, giving rise to chiral exchange terms. Here we present a combined spin-polarized scanning tunneling microscopy, angle-resolved photoemission, and density functional theory study of MnO_2 chains on Ir(100). Whereas we find antiferromagnetic Mn–Mn coupling along the chain, the inter-chain coupling across the non-magnetic Ir substrate turns out to be chiral with a 120° rotation between adjacent MnO_2 chains. Calculations reveal that the Dzyaloshinskii–Moriya interaction results in spin spirals with a periodicity in agreement with experiment. Our findings confirm the existence of indirect chiral magnetic exchange, potentially giving rise to exotic phenomena, such as chiral spin-liquid states in spin ice systems or the emergence of new quasiparticles
Controlling the topology of Fermi surfaces in metal nanofilms.
The properties of metal crystals are governed by the electrons of the highest occupied states at the Fermi level and determined by Fermi surfaces, the Fermi energy contours in momentum space. Topological regulation of the Fermi surface has been an important issue in synthesizing functional materials, which we found to be realized at room temperature in nanometer-thick films. Reducing the thickness of a metal thin film down to its electron wavelength scale induces the quantum size effect and the electronic system changes from three to two-dimensional, transforming the Fermi surface topology. Such an ultrathin film further changes its topology through one-dimensional (1D) structural deformation of the film when it is grown on a 1D substrate. In particular, when the interface has 1D metallic bands, the system is additionally stabilized by forming an electron energy gap by hybridization between 1D states of the film and substrate
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