217 research outputs found
Interlayer interaction and electronic screening in multilayer graphene
The unusual transport properties of graphene are the direct consequence of a
peculiar bandstructure near the Dirac point. We determine the shape of the pi
bands and their characteristic splitting, and the transition from a pure 2D to
quasi-2D behavior for 1 to 4 layers of graphene by angle-resolved
photoemission. By exploiting the sensitivity of the pi bands to the electronic
potential, we derive the layer-dependent carrier concentration, screening
length and strength of interlayer interaction by comparison with tight binding
calculations, yielding a comprehensive description of multilayer graphene's
electronic structure
Massive enhancement of electron-phonon coupling in doped graphene by an electronic singularity
The nature of the coupling leading to superconductivity in layered materials
such as high-Tc superconductors and graphite intercalation compounds (GICs) is
still unresolved. In both systems, interactions of electrons with either
phonons or other electrons or both have been proposed to explain
superconductivity. In the high-Tc cuprates, the presence of a Van Hove
singularity (VHS) in the density of states near the Fermi level was long ago
proposed to enhance the many-body couplings and therefore may play a role in
superconductivity. Such a singularity can cause an anisotropic variation in the
coupling strength, which may partially explain the so-called nodal-antinodal
dichotomy in the cuprates. Here we show that the topology of the graphene band
structure at dopings comparable to the GICs is quite similar to that of the
cuprates and that the quasiparticle dynamics in graphene have a similar
dichotomy. Namely, the electron-phonon coupling is highly anisotropic,
diverging near a saddle point in the graphene electronic band structure. These
results support the important role of the VHS in layered materials and the
possible optimization of Tc by tuning the VHS with respect to the Fermi level.Comment: 8 page
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
Quasiparticle Transformation During a Metal-Insulator Transition in Graphene
Here we show, with simultaneous transport and photoemission measurements,
that the graphene terminated SiC(0001) surface undergoes a metal-insulator
transition (MIT) upon dosingwith small amounts of atomic hydrogen. We find the
room temperature resistance increases by about 4 orders of magnitude, a
transition accompanied by anomalies in the momentum-resolved spectral function
including a non-Fermi Liquid behaviour and a breakdown of the quasiparticle
picture. These effects are discussed in terms of a possible transition to a
strongly (Anderson) localized ground state.Comment: 11 pages, 4 figure
Experimental Determination of the Spectral Function of Graphene
A number of interesting properties of graphene and graphite are postulated to
derive from the peculiar bandstructure of graphene. This bandstructure consists
of conical electron and hole pockets that meet at a single point in momentum
(k) space--the Dirac crossing, at energy . Direct
investigations of the accuracy of this bandstructure, the validity of the
quasiparticle picture, and the influence of many-body interactions on the
electronic structure have not been addressed for pure graphene by experiment to
date. Using angle resolved photoelectron spectroscopy (ARPES), we find that the
expected conical bands are distorted by strong electron-electron,
electron-phonon, and electron-plasmon coupling effects. The band velocity at
and the Dirac crossing energy are both renormalized by these
many-body interactions, in analogy with mass renormalization by electron-boson
coupling in ordinary metals. These results are of importance not only for
graphene but also graphite and carbon nanotubes which have similar
bandstructures.Comment: pdf file, 10 pages, 4 figure
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Understanding the Mechanism of Electronic Defect Suppression Enabled by Nonidealities in Atomic Layer Deposition.
Silicon germanium (SiGe) is a multifunctional material considered for quantum computing, neuromorphic devices, and CMOS transistors. However, implementation of SiGe in nanoscale electronic devices necessitates suppression of surface states dominating the electronic properties. The absence of a stable and passive surface oxide for SiGe results in the formation of charge traps at the SiGe-oxide interface induced by GeOx. In an ideal ALD process in which oxide is grown layer by layer, the GeOx formation should be prevented with selective surface oxidation (i.e., formation of an SiOx interface) by controlling the oxidant dose in the first few ALD cycles of the oxide deposition on SiGe. However, in a real ALD process, the interface evolves during the entire ALD oxide deposition due to diffusion of reactant species through the gate oxide. In this work, this diffusion process in nonideal ALD is investigated and exploited: the diffusion through the oxide during ALD is utilized to passivate the interfacial defects by employing ozone as a secondary oxidant. Periodic ozone exposure during gate oxide ALD on SiGe is shown to reduce the integrated trap density (Dit) across the band gap by nearly 1 order of magnitude in Al2O3 (<6 × 1010 cm-2) and in HfO2 (<3.9 × 1011 cm-2) by forming a SiOx-rich interface on SiGe. Depletion of Ge from the interfacial layer (IL) by enhancement of volatile GeOx formation and consequent desorption from the SiGe with ozone insertion during the ALD growth process is confirmed by electron energy loss spectroscopy (STEM-EELS) and hypothesized to be the mechanism for reduction of the interfacial defects. In this work, the nanoscale mechanism for defect suppression at the SiGe-oxide interface is demonstrated, which is engineering of diffusion species in the ALD process due to facile diffusion of reactant species in nonideal ALD
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
Intrinsic Insulating Ground State in Transition Metal Dichalcogenide TiSe2
The transition metal dichalcogenide TiSe has received significant
research attention over the past four decades. Different studies have presented
ways to suppress the 200~K charge density wave transition, vary low temperature
resistivity by several orders of magnitude, and stabilize magnetism or
superconductivity. Here we give the results of a new synthesis technique
whereby samples were grown in a high pressure environment with up to 180~bar of
argon gas. Above 100~K, properties are nearly unchanged from previous reports,
but a hysteretic resistance region that begins around 80~K, accompanied by
insulating low temperature behavior, is distinct from anything previously
observed. An accompanying decrease in carrier concentration is seen in Hall
effect measurements, and photoemission data show a removal of an electron
pocket from the Fermi surface in an insulating sample. We conclude that high
inert gas pressure synthesis accesses an underlying nonmetallic ground state in
a material long speculated to be an excitonic insulator.Comment: 11 pages, 7 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
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