Colloquium : Graphene spectroscopy Basov, D. N.; Fogler, M. M.; Lanzara, A. ...
Reviews of modern physics,
07/2014, Volume:
86, Issue:
3
Journal Article
Peer reviewed
Open access
Graphene has become one of the largest fields in condensed matter research and is having a large impact on technological applications. This Colloquium summarizes the experimental knowledge on the ...spectroscopic properties of graphene, from angle-resolved photoemission, and Raman scattering, to scanning tunneling microscopy and optics. It is a fundamental text for a novice in this field or a graphene aficionado who wants to be updated on the latest developments. Spectroscopic studies of electronic phenomena in graphene are reviewed. A variety of methods and techniques are surveyed, from quasiparticle spectroscopies (tunneling, photoemission) to methods probing density and current response (infrared optics, Raman) to scanning probe nanoscopy and ultrafast pump-probe experiments. Vast complimentary information derived from these investigations is shown to highlight unusual properties of Dirac quasiparticles and many-body interaction effects in the physics of graphene.
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The capability to control the type and amount of charge carriers in a material and, in the extreme case, the transition from metal to insulator, is one of the key challenges of modern electronics. By ...employing angle-resolved photoemission spectroscopy we find that a reversible metal to insulator transition and a fine-tuning of the charge carriers from electrons to holes can be achieved in epitaxial bilayer and single layer graphene by molecular doping. The effects of electron screening and disorder are also discussed. These results demonstrate that epitaxial graphene is suitable for electronics applications, as well as provide new opportunities for studying the hole doping regime of the Dirac cone in graphene.
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Angle-resolved photoemission and x-ray diffraction experiments show that multilayer epitaxial graphene grown on the SiC(0001) surface is a new form of carbon that is composed of effectively isolated ...graphene sheets. The unique rotational stacking of these films causes adjacent graphene layers to electronically decouple leading to a set of nearly independent linearly dispersing bands (Dirac cones) at the graphene K point. Each cone corresponds to an individual macroscale graphene sheet in a multilayer stack where AB-stacked sheets can be considered as low density faults.
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Originating from relativistic quantum field theory, Dirac fermions have been invoked recently to explain various peculiar phenomena in condensed-matter physics, including the novel quantum Hall ...effect in graphene, the magnetic-field-driven metal-insulator-like transition in graphite, superfluidity in 3He (ref. 5) and the exotic pseudogap phase of high-temperature superconductors. Despite their proposed key role in those systems, direct experimental evidence of Dirac fermions has been limited. Here, we report the first direct observation of relativistic Dirac fermions with linear dispersion near the Brillouin zone (BZ) corner H, which coexist with quasiparticles that have a parabolic dispersion near another BZ corner K. In addition, we also report a large electron pocket that we attribute to defect-induced localized states. Thus, graphite presents a system in which massless Dirac fermions, quasiparticles with finite effective mass and defect states all contribute to the low-energy electronic dynamics.
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Graphene has shown great application potential as the host material for next-generation electronic devices. However, despite its intriguing properties, one of the biggest hurdles for graphene to be ...useful as an electronic material is the lack of an energy gap in its electronic spectra. This, for example, prevents the use of graphene in making transistors. Although several proposals have been made to open a gap in graphene's electronic spectra, they all require complex engineering of the graphene layer. Here, we show that when graphene is epitaxially grown on SiC substrate, a gap of approximately 0.26 eV is produced. This gap decreases as the sample thickness increases and eventually approaches zero when the number of layers exceeds four. We propose that the origin of this gap is the breaking of sublattice symmetry owing to the graphene-substrate interaction. We believe that our results highlight a promising direction for bandgap engineering of graphene.
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High-transition-temperature cuprate superconductors are characterized by a strong momentum-dependent anisotropy between the low-energy excitations along the Brillouin zone diagonal (nodal direction) ...and those along the Brillouin zone face (antinodal direction)--the most striking example of which is the d -wave superconducting gap, with the largest magnitude found in the antinodal direction and no gap in the nodal direction. Furthermore, whereas antinodal quasiparticle excitations occur only below the transition temperature (Tc), superconductivity is thought to be indifferent to nodal excitations that are regarded as robust and insensitive to Tc. Here we reveal an unexpected link between nodal quasiparticles and superconductivity using high-resolution time- and angle-resolved photoemission on optimally doped Bi2Sr2CaCu2O8+δ. We observe a suppression of the nodal quasiparticle spectral weight following pump laser excitation, and measure its recovery dynamics. This suppression is greatly enhanced in the superconducting state. These results reduce the nodal-antinodal dichotomy and challenge the conventional view of nodal excitation neutrality in superconductivity. PUBLICATION ABSTRACT
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This paper reports the synthesis and detailed characterization of graphite thin films produced by thermal decomposition of the (0
0
0
1) face of a 6H–SiC wafer, demonstrating the successful growth of ...single crystalline films down to approximately one graphene layer. The growth and characterization were carried out in ultrahigh vacuum (UHV) conditions. The growth process and sample quality were monitored by low-energy electron diffraction, and the thickness of the sample was determined by core level X-ray photoelectron spectroscopy. High-resolution angle-resolved photoemission spectroscopy shows constant energy map patterns, which are very sharp and fully momentum-resolved, but nonetheless not resolution limited. We discuss the implications of this observation in connection with scanning electron microscopy data, as well as with previous studies.
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In the underdoped copper-oxides, high-temperature superconductivity condenses from a nonconventional metallic "pseudogap" phase that exhibits a variety of non-Fermi liquid properties. Recently, it ...has become clear that a charge density wave (CDW) phase exists within the pseudogap regime. This CDW coexists and competes with superconductivity (SC) below the transition temperature Tc, suggesting that these two orders are intimately related. Here we show that the condensation of the superfluid from this unconventional precursor is reflected in deviations from the predictions of BSC theory regarding the recombination rate of quasiparticles. We report a detailed investigation of the quasiparticle (QP) recombination lifetime, τqp, as a function of temperature and magnetic field in underdoped HgBa2CuO(4+δ) (Hg-1201) and YBa2Cu3O(6+x) (YBCO) single crystals by ultrafast time-resolved reflectivity. We find that τqp(T) exhibits a local maximum in a small temperature window near Tc that is prominent in underdoped samples with coexisting charge order and vanishes with application of a small magnetic field. We explain this unusual, non-BCS behavior by positing that Tc marks a transition from phase-fluctuating SC/CDW composite order above to a SC/CDW condensate below. Our results suggest that the superfluid in underdoped cuprates is a condensate of coherently-mixed particle-particle and particle-hole pairs.
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