F. Carbone

Universal optical conductance of graphite.

We find experimentally that the optical sheet conductance of graphite per graphene layer is very close to (pi/2)e2/h, which is the theoretically expected value of dynamical conductance of isolated monolayer graphene. Our calculations within the Slonczewski-Weiss-McClure model explain well why the interplane hopping leaves the conductance of graphene sheets in graphite almost unchanged for photon energies between 0.1 and 0.6 eV, even though it significantly affects the band structure on the same energy scale. The f-sum rule analysis shows that the large increase of the Drude spectral weight as a function of temperature is at the expense of the removed low-energy optical spectral weight of transitions between hole and electron bands.

Quantum critical behaviour in a high-Tc superconductor

Quantum criticality is associated with a system composed of a nearly infinite number of interacting quantum degrees of freedom at zero temperature, and it implies that the system looks on average the same regardless of the time- and length scale on which it is observed. Electrons on the atomic scale do not exhibit such symmetry, which can only be generated as a collective phenomenon through the interactions between a large number of electrons. In materials with strong electron correlations a quantum phase transition at zero temperature can occur, and a quantum critical state has been predicted, which manifests itself through universal power-law behaviours of the response functions. Candidates have been found both in heavy-fermion systems and in the high-transition temperature (high-Tc) copper oxide superconductors, but the reality and the physical nature of such a phase transition are still debated. Here we report a universal behaviour that is characteristic of the quantum critical region. We demonstrate that the experimentally measured phase angle agrees precisely with the exponent of the optical conductivity. This points towards a quantum phase transition of an unconventional kind in the high-Tc superconductors.In certain materials with strong electron correlations a quantum phase transition (QPT) at zero temperature can occur, in the proximity of which a quantum critical state of matter has been anticipated. This possibility has recently attracted much attention because the response of such a state of matter is expected to follow universal patterns defined by the quantum mechanical nature of the fluctuations. Forementioned universality manifests itself through power-law behaviours of the response functions. Candidates are found both in heavy fermion systems and in the cuprate high Tc superconductors. Although there are indications for quantum criticality in the cuprate superconductors, the reality and the physical nature of such a QPT are still under debate. Here we identify a universal behaviour of the phase angle of the frequency dependent conductivity that is characteristic of the quantum critical region. We demonstrate that the experimentally measured phase angle agrees precisely with the exponent of the optical conductivity. This points towards a QPT in the cuprates close to optimal doping, although of an unconventional kind.

Strong magnetic pair breaking in Mn-substituted MgB2 single crystals

Magnetic ions (Mn) were substituted in MgB2 single crystals resulting in a strong pair-breaking effect. The superconducting transition temperature, Tc, in Mg1−xMnxB2 has been found to be rapidly suppressed at an initial rate of 10 K∕%Mn, leading to a complete suppression of superconductivity at about 2% Mn substitution. This reflects the strong coupling between the conduction electrons and the 3d local moments, predominantly of magnetic character, since the nonmagnetic ion substitutions, e.g., with Al or C, suppress Tc much less effectively (e.g., 0.5 K∕%Al). The magnitude of the magnetic moment (≃1.7 μB per Mn), derived from normal state susceptibility measurements, uniquely identifies the Mn ions to be divalent, and to be in the low-spin state (S=1∕2). This has been found also in x-ray absorption spectroscopy measurements. Isovalent Mn2+ substitution for Mg2+ mainly affects superconductivity through spin-flip scattering reducing Tc rapidly and lowering the upper critical field anisotropy Hc2ab∕Hc2c at T=0 from 6 to 3.3 (x=0.88% Mn), while leaving the initial slope dHc2∕dT near Tc unchanged for both field orientations.

Doping dependence of the redistribution of optical spectral weight in Bi2Sr2CaCu2O8+δ

We present the ab-plane optical conductivity of four single crystals of

Optical and thermodynamic properties of the high-temperature superconductor HgBa 2 CuO 4+δ

{\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}\phantom{\rule{0.3em}{0ex}}(\mathrm{Bi}2212)

Temperature-modulation analysis of superconductivity-induced transfer of in-plane spectral weight in Bi 2 Sr 2 CaCu 2 O 8

with different carrier doping levels from the strongly underdoped to the strongly overdoped range with

In-plane optical spectral weight transfer in optimally doped Bi2Sr2Ca2Cu3O10

{T}_{c}=66

Evidence for strongly coupled charge-density-wave ordering in three-dimensional R 5 Ir 4 Si 10 compounds from optical measurements

, 88, 77, and

Intraband optical spectral weight in the presence of a van Hove singularity: Application to Bi 2 Sr 2 CaCu 2 O 8+δ

67\phantom{\rule{0.3em}{0ex}}\mathrm{K}

Optical properties of bcc transition metals in the range 0-40 eV

, respectively. We focus on the redistribution of the low frequency optical spectral weight (SW) in the superconducting and normal states. The temperature dependence of the low-frequency spectral weight in the normal state is significantly stronger in the overdoped regime. In agreement with other studies, the superconducting order is marked by an increase of the low frequency SW for low doping, while the SW decreases for the highly overdoped sample. The effect crosses through zero at a doping concentration