A remarkable manifestation of the quantum character of electrons in matter is offered by graphene, a single atomic layer of graphite. Unlike conventional solids where electrons are described with the Schrödinger equation, electronic excitations in graphene are governed by the Dirac hamiltonian 1 . Some of the intriguing electronic properties of graphene, such as massless Dirac quasiparticles with linear energy-momentum dispersion, have been confirmed by recent observations 2-5 . Here, we report an infrared spectromicroscopy study of charge dynamics in graphene integrated in gated devices. Our measurements verify the expected characteristics of graphene and, owing to the previously unattainable accuracy of infrared experiments, also uncover significant departures of the quasiparticle dynamics from predictions made for Dirac fermions in idealized, freestanding graphene. Several observations reported here indicate the relevance of many-body interactions to the electromagnetic response of graphene.We investigated the reflectance R(ω) and transmission T (ω) of graphene samples on a SiO 2 /Si substrate (inset of Fig. 1a) as a function of gate voltage V g at 45 K (see the Methods section). We start with data taken at the charge-neutrality point V CN : the gate voltage corresponding to the minimum d.c. conductivity and zero total charge density (inset of Fig. 1c). Figure 1a shows R(ω) of a graphene gated structure (graphene/SiO 2 /Si) at V CN = 3 V normalized by reflectance of the substrate R sub (ω). R sub (ω) is dominated by a minimum around 5,500 cm −1 due to interference effects in SiO 2 . A remarkable observation is that a monolayer of undoped graphene markedly modifies the interference minimum of the substrate leading to a suppression of R sub (ω) by as much as 15%. This observation is significant because it enables us to evaluate the conductivity of graphene near the interference structure, as will be discussed below.Both reflectance and transmission spectra of graphene structures can be modified by a gate voltage. Figure 1b,c shows these modifications at various gate voltages normalized by data atThese data correspond to the Fermi energy E F on the electron side and similar behaviour was observed with E F on the hole side (not shown). At low voltages (<17 V), we found a dip in R(V )/R(V CN ) spectra. With increasing bias, this feature evolves into a peak-dip structure and systematically shifts to higher frequency. The T (V )/T (V CN ) spectra reveal a peak at all voltages, which systematically hardens with increasing bias. A voltage-induced increase in transmission (T (V )/T (V CN ) > 1) signals a decrease of the absorption with bias. Most interestingly, we observed that the frequencies of the main features in R(V )/R(V CN ) and T (V )/T (V CN ) all evolve approximately as √ V . To explore the quasiparticle dynamics under applied voltages, it is imperative to first discuss the two-dimensional (2D) optical conductivity of charge-neutral graphene, σ 1 (ω, V CN ) + iσ 2 (ω, V CN ), extracted from a multilayer analy...

The quantum Hall (QH) effect in two-dimensional electrons and holes in high quality graphene samples is studied in strong magnetic fields up to 45 T. QH plateaus at filling factors nu = 0, +/-1, +/-4 are discovered at magnetic fields B > 20 T, indicating the lifting of the fourfold degeneracy of the previously observed QH states at nu = +/-4(absolute value(n) + 1/2), where n is the Landau-level index. In particular, the presence of the nu = 0, +/-1 QH plateaus indicates that the Landau level at the charge neutral Dirac point splits into four sublevels, lifting sublattice and spin degeneracy. The QH effect at nu = +/-4 is investigated in a tilted magnetic field and can be attributed to lifting of the spin degeneracy of the n = 1 Landau level.

Graphene nanoribbons will be essential components in future graphene nanoelectronics. However, in typical nanoribbons produced from lithographically patterned exfoliated graphene, the charge carriers travel only about ten nanometres between scattering events, resulting in minimum sheet resistances of about one kilohm per square. Here we show that 40-nanometre-wide graphene nanoribbons epitaxially grown on silicon carbide are single-channel room-temperature ballistic conductors on a length scale greater than ten micrometres, which is similar to the performance of metallic carbon nanotubes. This is equivalent to sheet resistances below 1 ohm per square, surpassing theoretical predictions for perfect graphene by at least an order of magnitude. In neutral graphene ribbons, we show that transport is dominated by two modes. One is ballistic and temperature independent; the other is thermally activated. Transport is protected from back-scattering, possibly reflecting ground-state properties of neutral graphene. At room temperature, the resistance of both modes is found to increase abruptly at a particular length--the ballistic mode at 16 micrometres and the other at 160 nanometres. Our epitaxial graphene nanoribbons will be important not only in fundamental science, but also--because they can be readily produced in thousands--in advanced nanoelectronics, which can make use of their room-temperature ballistic transport properties.

We investigate the quantum Hall (QH) states near the charge-neutral Dirac point of a high mobility graphene sample in high magnetic fields. We find that the QH states at filling factors nu=+/-1 depend only on the perpendicular component of the field with respect to the graphene plane, indicating that they are not spin related. A nonlinear magnetic field dependence of the activation energy gap at filling factor nu=1 suggests a many-body origin. We therefore propose that the nu=0 and +/-1 states arise from the lifting of the spin and sublattice degeneracy of the n=0 Landau level, respectively.

We report on infrared spectroscopy of bilayer graphene integrated in gated structures. We observed a significant asymmetry in the optical conductivity upon electrostatic doping of electrons and holes. We show that this finding arises from a marked asymmetry between the valence and conduction bands, which is mainly due to the inequivalence of the two sublattices within the graphene layer. From the conductivity data, the energy difference of the two sublattices is determined.Recently there has been unprecedented interest in carbon-based materials due to the discovery of graphene [1]. Among all carbon systems, bilayer graphene stands out due to its remarkable properties such as the formation of a tunable band gap between the valence and con-: a property not attainable in common semiconductors. The vast majority of previous experimental and theoretical studies of bilayer graphene assumed a symmetric band structure that is governed by the interlayer coupling energy γ 1 . This is in contrast with a significant electron-hole asymmetry observed in cyclotron resonance [6] and cyclotron mass experiments [2]. Several theoretical proposals have been put forward to explain these results [2] [7]. Yet, the microscopic origin of the observed effects remains unknown.Here we present the first investigation of the optical conductivity of bilayer graphene via infrared spectroscopy. We observed dramatic differences in the evolution of the conductivity for electron and hole polarities of the gate voltage. We show that small band parameters other than γ 1 give rise to an asymmetry between the valence and conduction bands, in contrast to the commonly assumed symmetric band structure. The systematic character of our IR data enables us to extract an energy difference between the A and B sublattices within the same graphene layer (Fig 1(b)) of δ AB ≈18meV. We analyze some of the implications of these findings for other properties of bilayer graphene.Infrared (IR) reflectance R(ω) and transmission T(ω) measurements were performed on bilayer graphene samples on SiO 2 /Si substrate [6] as a function of gate voltage V g at 45K employing synchrotron radiation, as described in [8]. We find that both R(ω) [9,10] and T(ω) spectra of the bilayer graphene device can be strongly modified by a gate voltage. Figure 1 shows the transmission ratio data at several voltages normalized by data at the charge neutrality voltage V CN : T(V)/T(V CN ), where V CN is the voltage corresponding to the minimum DC conductivity, and V= V g −V CN . The T(V)/T(V CN

Living cells continually generate reactive oxygen species (ROS) through the respiratory chain during energetic metabolism. ROS at low or moderate concentration can play important physiological roles. However, an excessive amount of ROS under oxidative stress would be extremely deleterious. The central nervous system (CNS) is particularly vulnerable to oxidative stress due to its high oxygen consumption, weakly antioxidative systems and the terminal-differentiation characteristic of neurons. Thus, oxidative stress elicits various neurodegenerative diseases. In addition, chemotherapy could result in severe side effects on the CNS and peripheral nervous system (PNS) of cancer patients, and a growing body of evidence demonstrates the involvement of ROS in drug-induced neurotoxicities as well. Therefore, development of antioxidants as neuroprotective drugs is a potentially beneficial strategy for clinical therapy. In this review, we summarize the source, balance maintenance and physiologic functions of ROS, oxidative stress and its toxic mechanisms underlying a number of neurodegenerative diseases, and the possible involvement of ROS in chemotherapy-induced toxicity to the CNS and PNS. We ultimately assess the value for antioxidants as neuroprotective drugs and provide our comments on the unmet needs.

We present the first measurements of cyclotron resonance of electrons and holes in bilayer graphene. In magnetic fields up to B=18 T, we observe four distinct intraband transitions in both the conduction and valence bands. The transition energies are roughly linear in B between the lowest Landau levels, whereas they follow square root[B] for the higher transitions. This highly unusual behavior represents a change from a parabolic to a linear energy dispersion. The density of states derived from our data generally agrees with the existing lowest order tight binding calculation for bilayer graphene. However, in comparing data to theory, a single set of fitting parameters fails to describe the experimental results.

We discuss the ultrafast evolution of the surface electronic structure of the topological insulator Bi(2)Te(3) following a femtosecond laser excitation. Using time and angle-resolved photoelectron spectroscopy, we provide a direct real-time visualization of the transient carrier population of both the surface states and the bulk conduction band. We find that the thermalization of the surface states is initially determined by interband scattering from the bulk conduction band, lasting for about 0.5 ps; subsequently, few picoseconds are necessary for the Dirac cone nonequilibrium electrons to recover a Fermi-Dirac distribution, while their relaxation extends over more than 10 ps. The surface sensitivity of our measurements makes it possible to estimate the range of the bulk-surface interband scattering channel, indicating that the process is effective over a distance of 5 nm or less. This establishes a correlation between the nanoscale thickness of the bulk charge reservoir and the evolution of the ultrafast carrier dynamics in the surface Dirac cone.