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...

We present an experimental study of the infrared conductivity, transmission, and reflection of a gated bilayer graphene and their theoretical analysis within the Slonczewski-Weiss-McClure (SWMc) model. The infrared response is shown to be governed by the interplay of the interband and the intraband transitions among the four bands of the bilayer. The position of the main conductivity peak at the charge neutrality point is determined by the interlayer tunneling frequency. The shift of this peak as a function of the gate voltage gives information about less known parameters of the SWMc model, in particular, those responsible for the electron-hole and sublattice asymmetries. These parameter values are shown to be consistent with recent electronic structure calculations for the bilayer graphene and the SWMc parameters commonly used for the bulk graphite.

We report on the detailed analysis of the infrared (IR) conductivity of two prototypical high-Tc systems YBa2Cu3Oy and La2−xSrxCuO4 throughout the complex phase diagram of these compounds. Our focus in this work is to thoroughly document the electromagnetic response of the nodal metal state which is initiated with only few holes doped in parent antiferromagnetic systems and extends up to the pseudogap boundary in the phase diagram. The key signature of the nodal metal is the two-component conductivity: the Drude mode at low energies followed by a resonance in mid-IR. The Drude component can be attributed to the response of coherent quasiparticles residing on the Fermi arcs detected in photoemission experiments. The microscopic origin of the mid-IR band is yet to be understood. A combination of transport and IR data uncovers fingerprints of the Fermi liquid behavior in the response of the nodal metal. The comprehensive nature of the data sets presented in this work allows us to critically re-evaluate common approaches to the interpretation of the optical data. Specifically we re-examine the role of magnetic excitations in generating electronic self energy effects through the analysis of the IR data in high magnetic field.

We report on an infrared spectroscopy study of mobile holes in the accumulation layer of organic field-effect transistors based on rubrene single crystals. Our data indicate that both transport and infrared properties of these transistors at room temperature are governed by light quasiparticles in molecular orbital bands with the effective masses m* comparable to free electron mass. Furthermore, the m* values inferred from our experiments are in agreement with those determined from band structure calculations. These findings reveal no evidence for prominent polaronic effects, which is at variance with the common beliefs of polaron formation in molecular solids.

We report a novel aspect of the competition and coexistence between magnetism and superconductivity in the high-T(c) cuprate La(2-x)Sr(x)CuO4 (La214). With a modest magnetic field applied H parallel c axis, we monitored the infrared signature of pair tunneling between the CuO2 planes and discovered the complete suppression of interlayer coupling in a series of underdoped La214 single crystals. We find that the in-plane superconducting properties remain intact, in spite of enhanced magnetism in the planes.