45 research outputs found
In-plane orientation effects on the electronic structure, stability and Raman scattering of monolayer graphene on Ir(111)
We employ angle-resolved photoemission spectroscopy (ARPES) to investigate
the electronic structures of two rotational variants of epitaxial, single-layer
graphene on Ir(111). As grown, the more-abundant R0 variant is nearly
charge-neutral, with strong hybridization between graphene and Ir bands near
the Fermi level. The graphene Fermi surface and its replicas exactly coincide
with Van Hove singularities in the Ir Fermi surface. Sublattice symmetry
breaking introduces a small gap-inducing potential at the Dirac crossing, which
is revealed by n-doping the graphene using K atoms. The energy gaps between
main and replica bands (originating from the moir\'e interference pattern
between graphene and Ir lattices) is shown to be non-uniform along the mini-
zone boundary due to hybridization with Ir bands. An electronically mediated
interaction is proposed to account for the stability of the R0 variant. The
variant rotated 30{\deg} in-plane, R30, is p-doped as grown and K doping
reveals no band gap at the Dirac crossing. No replica bands are found in ARPES
measurements. Raman spectra from the R30 variant exhibit the characteristic
phonon modes of graphene, while R0 spectra are featureless. These results show
that the film/substrate interaction changes from chemisorption (R0) to
physisorption (R30) with in-plane orientation. Finally, graphene-covered Ir has
a work function lower than the clean substrate but higher than graphite.Comment: Manuscript plus 7 figure
Giant Spin-splitting in the Bi/Ag(111) Surface Alloy
Surface alloying is shown to produce electronic states with a very large
spin-splitting. We discuss the long range ordered bismuth/silver(111) surface
alloy where an energy bands separation of up to one eV is achieved. Such strong
spin-splitting enables angular resolved photoemission spectroscopy to directly
observe the region close to the band edge, where the density of states shows
quasi-one dimensional behavior. The associated singularity in the local density
of states has been measured by low temperature scanning tunneling spectroscopy.
The implications of this new class of materials for potential spintronics
applications as well as fundamental issues are discussed.Comment: 4 pages, 4 figure
Small scale rotational disorder observed in epitaxial graphene on SiC(0001)
Interest in the use of graphene in electronic devices has motivated an
explosion in the study of this remarkable material. The simple, linear Dirac
cone band structure offers a unique possibility to investigate its finer
details by angle-resolved photoelectron spectroscopy (ARPES). Indeed, ARPES has
been performed on graphene grown on metal substrates but electronic
applications require an insulating substrate. Epitaxial graphene grown by the
thermal decomposition of silicon carbide (SiC) is an ideal candidate for this
due to the large scale, uniform graphene layers produced. The experimental
spectral function of epitaxial graphene on SiC has been extensively studied.
However, until now the cause of an anisotropy in the spectral width of the
Fermi surface has not been determined. In the current work we show, by
comparison of the spectral function to a semi-empirical model, that the
anisotropy is due to small scale rotational disorder ( 0.15)
of graphene domains in graphene grown on SiC(0001) samples. In addition to the
direct benefit in the understanding of graphene's electronic structure this
work suggests a mechanism to explain similar variations in related ARPES data.Comment: 5 pages, 4 figure
Highly p-doped graphene obtained by fluorine intercalation
We present a method for decoupling epitaxial graphene grown on SiC(0001) by
intercalation of a layer of fluorine at the interface. The fluorine atoms do
not enter into a covalent bond with graphene, but rather saturate the substrate
Si bonds. This configuration of the fluorine atoms induces a remarkably large
hole density of p \approx 4.5 \times 1013 cm-2, equivalent to the location of
the Fermi level at 0.79 eV above the Dirac point ED .Comment: 4 pages, 2 figures, in print AP
Linearly dispersive bands at the onset of correlations in KC films
Molecular crystals are a flexible platform to induce novel electronic phases.
Due to the weak forces between molecules, intermolecular distances can be
varied over relatively larger ranges than interatomic distances in atomic
crystals. On the other hand, the hopping terms are generally small, which
results in narrow bands, strong correlations and heavy electrons. Here, by
growing KC fullerides on hexagonal layered BiSe, we show
that upon doping the series undergoes a Mott transition from a molecular
insulator to a correlated metal, and an in-gap state evolves into highly
dispersive Dirac-like fermions at half filling, where superconductivity occurs.
This picture challenges the commonly accepted description of the low energy
quasiparticles as appearing from a gradual electron doping of the conduction
states, and suggests an intriguing parallel with the more famous family of the
cuprate superconductors. More in general, it indicates that molecular crystals
offer a viable route to engineer electron-electron interactions.Comment: 5 pages, 4 figures. Accepted at Physical Review Researc
Tunable Electronic Structure in Gallium Chalcogenide van der Waals Compounds
Transition metal monochalcogenides comprise a class of two-dimensional
materials with electronic band gaps that are highly sensitive to material
thickness and chemical composition. Here, we explore the tunability of the
electronic excitation spectrum in GaSe using angle-resolved photoemission
spectroscopy. The electronic structure of the material is modified by
potassium deposition as well as by forming
GaSSe alloy compounds. We find that potassium decouples the
top-most tetra-layer of the GaSe unit cell, leading to a substantial change of
the dispersion around the valence band maximum (VBM). The observed band
dispersion of a single tetralayer is consistent with a transition from the
direct gap character of the bulk to the indirect gap character expected for
monolayer GaSe. Upon alloying with sulfur, we observe a phase transition from
AB to stacking. Alloying also results in a rigid energy
shift of the VBM towards higher binding energies which correlates with a blue
shift in the luminescence. The increase of the band gap upon sulfur alloying
does not appear to change the dispersion or character of the VBM appreciably,
implying that it is possible to engineer the gap of these materials while
maintaining their salient electronic properties
Universal Mechanism of Band-Gap Engineering in Transition-Metal Dichalcogenides
Two-dimensional (2D) van-der-Waals semiconductors have emerged as a class of
materials with promising device characteristics owing to the intrinsic bandgap.
For realistic applications, the ideal is to modify the bandgap in a controlled
manner by a mechanism that can be generally applied to this class of materials.
Here, we report the observation of a universally tunable bandgap in the family
of bulk 2H transition metal dichalcogenides (TMDs) by in situ surface doping of
Rb atoms. A series of angle-resolved photoemission spectra unexceptionally
shows that the bandgap of TMDs at the zone corners is modulated in the range of
0.8 ~ 2.0 eV, which covers a wide spectral range from visible to near infrared,
with a tendency from indirect to direct bandgap. A key clue to understand the
mechanism of this bandgap engineering is provided by the spectroscopic
signature of symmetry breaking and resultant spin splitting, which can be
explained by the formation of 2D electric dipole layers within the surface
bilayer of TMDs. Our results establish the surface Stark effect as a universal
mechanism of bandgap engineering based on the strong 2D nature of van-der-Waals
semiconductors
Electronic structure of graphene on single crystal copper substrates
The electronic structure of graphene on Cu(111) and Cu(100) single crystals
is investigated using low energy electron microscopy, low energy electron
diffraction and angle resolved photoemission spectroscopy. On both substrates
the graphene is rotationally disordered and interactions between the graphene
and substrate lead to a shift in the Dirac crossing of -0.3 eV and the
opening of a 250 meV gap. Exposure of the samples to air resulted in
intercalation of oxygen under the graphene on Cu(100), which formed a
()R45 superstructure. The effect of this
intercalation on the graphene bands is to increase the offset of the
Dirac crossing ( -0.6 eV) and enlarge the gap ( 350 meV). No such
effect is observed for the graphene on Cu(111) sample, with the surface state
at not showing the gap associated with a surface superstructure. The
graphene film is found to protect the surface state from air exposure, with no
change in the effective mass observed