59 research outputs found
Giant Faraday rotation in single- and multilayer graphene
Optical Faraday rotation is one of the most direct and practically important
manifestations of magnetically broken time-reversal symmetry. The rotation
angle is proportional to the distance traveled by the light, and up to now
sizeable effects were observed only in macroscopically thick samples and in
two-dimensional electron gases with effective thicknesses of several
nanometers. Here we demonstrate that a single atomic layer of carbon - graphene
- turns the polarization by several degrees in modest magnetic fields. The
rotation is found to be strongly enhanced by resonances originating from the
cyclotron effect in the classical regime and the inter-Landau-level transitions
in the quantum regime. Combined with the possibility of ambipolar doping, this
opens pathways to use graphene in fast tunable ultrathin infrared
magneto-optical devices
Observation of an electrically tunable band gap in trilayer graphene
A striking feature of bilayer graphene is the induction of a significant band
gap in the electronic states by the application of a perpendicular electric
field. Thicker graphene layers are also highly attractive materials. The
ability to produce a band gap in these systems is of great fundamental and
practical interest. Both experimental and theoretical investigations of
graphene trilayers with the typical ABA layer stacking have, however, revealed
the lack of any appreciable induced gap. Here we contrast this behavior with
that exhibited by graphene trilayers with ABC crystallographic stacking. The
symmetry of this structure is similar to that of AB stacked graphene bilayers
and, as shown by infrared conductivity measurements, permits a large band gap
to be formed by an applied electric field. Our results demonstrate the critical
and hitherto neglected role of the crystallographic stacking sequence on the
induction of a band gap in few-layer graphene.Comment: 10 pages, 5 figures, including the supplementary information on the
electron-hole asymmetry of ABA-stacked trilaye
Microscopic Polarization in Bilayer Graphene
Bilayer graphene has drawn significant attention due to the opening of a band
gap in its low energy electronic spectrum, which offers a promising route to
electronic applications. The gap can be either tunable through an external
electric field or spontaneously formed through an interaction-induced symmetry
breaking. Our scanning tunneling measurements reveal the microscopic nature of
the bilayer gap to be very different from what is observed in previous
macroscopic measurements or expected from current theoretical models. The
potential difference between the layers, which is proportional to charge
imbalance and determines the gap value, shows strong dependence on the disorder
potential, varying spatially in both magnitude and sign on a microscopic level.
Furthermore, the gap does not vanish at small charge densities. Additional
interaction-induced effects are observed in a magnetic field with the opening
of a subgap when the zero orbital Landau level is placed at the Fermi energy
Revealing the high-energy electronic excitations underlying the onset of high-temperature superconductivity in cuprates
In strongly-correlated systems the electronic properties at the Fermi energy (EF) are intertwined with those at high energy scales. One of the pivotal challenges in the field of high-temperature superconductivity (HTSC) is to understand whether and how the high energy scale physics associated with Mott-like excitations (|E-EF|>1 eV) is involved in the condensate formation. Here we show the interplay between the many-body high-energy CuO2 excitations at 1.5 and 2 eV and the onset of HTSC. This is revealed by a novel optical pump supercontinuum-probe technique, which provides access to the dynamics of the dielectric function in Y-Bi2212 over an extended energy range, after the photoinduced suppression of the superconducting pairing. These results unveil an unconventional mechanism at the base of HTSC both below and above the optimal hole concentration required to attain the maximum critical temperature (Tc)
Electronic correlations in the iron pnictides
In correlated metals derived from Mott insulators, the motion of an electron
is impeded by Coulomb repulsion due to other electrons. This phenomenon causes
a substantial reduction in the electron's kinetic energy leading to remarkable
experimental manifestations in optical spectroscopy. The high-Tc
superconducting cuprates are perhaps the most studied examples of such
correlated metals. The occurrence of high-Tc superconductivity in the iron
pnictides puts a spotlight on the relevance of correlation effects in these
materials. Here we present an infrared and optical study on single crystals of
the iron pnictide superconductor LaFePO. We find clear evidence of electronic
correlations in metallic LaFePO with the kinetic energy of the electrons
reduced to half of that predicted by band theory of nearly free electrons.
Hallmarks of strong electronic many-body effects reported here are important
because the iron pnictides expose a new pathway towards a correlated electron
state that does not explicitly involve the Mott transition.Comment: 10 page
A Tunable Phonon-Exciton Fano System in Bilayer Graphene
Interference between different possible paths lies at the heart of quantum
physics. Such interference between coupled discrete and continuum states of a
system can profoundly change its interaction with light as seen in Fano
resonance. Here we present a unique many-body Fano system composed of a
discrete phonon vibration and continuous electron-hole pair transitions in
bilayer graphene. Mediated by the electron-phonon interactions, the excited
state is described by new quanta of elementary excitations of hybrid
phonon-exciton nature. Infrared absorption of the hybrid states exhibit
characteristic Fano lineshapes with parameters renormalized by many-body
interactions. Remarkably, the Fano resonance in bilayer graphene is
continuously tunable through electrical gating. Further control of the
phonon-exciton coupling may be achieved with an optical field exploiting the
excited state infrared activity. This tunable phonon-exciton system also offers
the intriguing possibility of a 'phonon laser' with stimulated phonon
amplification generated by population inversion of band-edge electrons.Comment: 21 pages, 3 figure
Graphene plasmonics
Two rich and vibrant fields of investigation, graphene physics and
plasmonics, strongly overlap. Not only does graphene possess intrinsic plasmons
that are tunable and adjustable, but a combination of graphene with noble-metal
nanostructures promises a variety of exciting applications for conventional
plasmonics. The versatility of graphene means that graphene-based plasmonics
may enable the manufacture of novel optical devices working in different
frequency ranges, from terahertz to the visible, with extremely high speed, low
driving voltage, low power consumption and compact sizes. Here we review the
field emerging at the intersection of graphene physics and plasmonics.Comment: Review article; 12 pages, 6 figures, 99 references (final version
available only at publisher's web site
Graphene Photonics and Optoelectronics
The richness of optical and electronic properties of graphene attracts
enormous interest. Graphene has high mobility and optical transparency, in
addition to flexibility, robustness and environmental stability. So far, the
main focus has been on fundamental physics and electronic devices. However, we
believe its true potential to be in photonics and optoelectronics, where the
combination of its unique optical and electronic properties can be fully
exploited, even in the absence of a bandgap, and the linear dispersion of the
Dirac electrons enables ultra-wide-band tunability. The rise of graphene in
photonics and optoelectronics is shown by several recent results, ranging from
solar cells and light emitting devices, to touch screens, photodetectors and
ultrafast lasers. Here we review the state of the art in this emerging field.Comment: Review Nature Photonics, in pres
A Novel Role of Three Dimensional Graphene Foam to Prevent Heater Failure during Boiling
We report a novel boiling heat transfer (NBHT) in reduced graphene oxide (RGO) suspended in water (RGO colloid) near critical heat flux (CHF), which is traditionally the dangerous limitation of nucleate boiling heat transfer because of heater failure. When the heat flux reaches the maximum value (CHF) in RGO colloid pool boiling, the wall temperature increases gradually and slowly with an almost constant heat flux, contrary to the rapid wall temperature increase found during water pool boiling. The gained time by NBHT would provide the safer margin of the heat transfer and the amazing impact on the thermal system as the first report of graphene application. In addition, the CHF and boiling heat transfer performance also increase. This novel boiling phenomenon can effectively prevent heater failure because of the role played by the self-assembled three-dimensional foam-like graphene network (SFG).open2
Properties of Graphene: A Theoretical Perspective
In this review, we provide an in-depth description of the physics of
monolayer and bilayer graphene from a theorist's perspective. We discuss the
physical properties of graphene in an external magnetic field, reflecting the
chiral nature of the quasiparticles near the Dirac point with a Landau level at
zero energy. We address the unique integer quantum Hall effects, the role of
electron correlations, and the recent observation of the fractional quantum
Hall effect in the monolayer graphene. The quantum Hall effect in bilayer
graphene is fundamentally different from that of a monolayer, reflecting the
unique band structure of this system. The theory of transport in the absence of
an external magnetic field is discussed in detail, along with the role of
disorder studied in various theoretical models. We highlight the differences
and similarities between monolayer and bilayer graphene, and focus on
thermodynamic properties such as the compressibility, the plasmon spectra, the
weak localization correction, quantum Hall effect, and optical properties.
Confinement of electrons in graphene is nontrivial due to Klein tunneling. We
review various theoretical and experimental studies of quantum confined
structures made from graphene. The band structure of graphene nanoribbons and
the role of the sublattice symmetry, edge geometry and the size of the
nanoribbon on the electronic and magnetic properties are very active areas of
research, and a detailed review of these topics is presented. Also, the effects
of substrate interactions, adsorbed atoms, lattice defects and doping on the
band structure of finite-sized graphene systems are discussed. We also include
a brief description of graphane -- gapped material obtained from graphene by
attaching hydrogen atoms to each carbon atom in the lattice.Comment: 189 pages. submitted in Advances in Physic
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