H. Raffy

Evolution of the pseudogap from Fermi arcs to the nodal liquid

The pseudogap phase in the cuprates is a most unusual state of matter: it is a metal, but its Fermi surface is broken up into disconnected segments known as Fermi arcs. Using angle resolved photoemission spectroscopy, we show that the anisotropy of the pseudogap in momentum space and the resulting arcs depend only on the ratio T/T*(x), where T*(x) is the temperature below which the pseudogap first develops at a given hole doping x. In particular, the arcs collapse linearly with T/T* and extrapolate to zero extent as T goes to 0. This suggests that the T = 0 pseudogap state is a nodal liquid, a strange metallic state whose gapless excitations are located only at points in momentum space, just as in a d-wave superconductor.

Pairing in cuprates from high-energy electronic states

The in-plane optical conductivity of Bi2Sr2CaCu2O 8 + δ films with small carrier density (underdoped) up to large carrier density (overdoped) is derived from accurate reflectivity data. Integrating the conductivity up to increasingly higher frequencies yields the energy scale involved in the formation of the condensate. At least in the underdoped sample, states extending up to 2 eV contribute to the superfluid. This anomalously large energy scale may be assigned to a change of in-plane kinetic energy at the superconducting transition, and is compatible with an electronic pairing mechanism.

Absence of a loss of in-plane infrared spectral weight in the pseudogap regime of Bi(2)Sr(2)CaCu(2)O(8+delta).

The ab-plane reflectance of Bi2Sr2CaCu2O(8+d) thin films was measured in the 30-25000 cm^(-1) range for one underdoped (Tc = 70 K), and one overdoped sample (Tc = 63 K) as a function of temperature (10-300 K). We find qualitatively similar behaviors in the temperature dependence of the normal-state infrared response of both samples. Above Tc, the effective spectral weight, obtained from the integrated conductivity, does not decrease when T decreases, so that no opening of an optical pseudogap is seen. We suggest that these are consequences of the pseudogap opening first in the k=(0, pi) direction, according to ARPES, and of the in-plane infrared conductivity being mostly sensitive to the k=(pi, pi) direction.

Crossover from coherent to incoherent electronic excitations in the normal state of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+delta}.

Angle resolved photoemission spectroscopy (ARPES) and resistivity measurements are used to explore the overdoped region of the high temperature superconductor Bi(2)Sr(2)CaCu(2)O(8+delta). We find evidence for a new crossover line in the phase diagram between a coherent metal phase, for lower temperatures and higher doping, and an incoherent metal phase, for higher temperatures and lower doping. The former is characterized by two well-defined spectral peaks in ARPES due to coherent bilayer splitting and superlinear behavior in the resistivity, whereas the latter is characterized by a single broad spectral feature in ARPES and a linear temperature dependence of the resistivity.

Observation of a d -wave nodal liquid in highly underdoped Bi 2 Sr 2 CaCu 2 O 8+ δ

High-temperature superconductivity in the cuprates arises when charge carriers are added to an insulator. Between these states lies the so-called nodal liquid at low temperature. Photoemission spectroscopy suggests that superconductivity evolves smoothly from this nodal-liquid state.

Nondispersive Fermi Arcs and the Absence of Charge Ordering in the Pseudogap Phase of Bi2Sr2CaCu2O8+δ

The autocorrelation of angle resolved photoemission data from the high temperature superconductor Bi(2)Sr(2)CaCu(2)O(8+delta) shows distinct peaks in momentum space which disperse with binding energy in the superconducting state, but not in the pseudogap phase. Although it is tempting to attribute a nondispersive behavior in momentum space to charge ordering, a deconstruction of the autocorrelation reveals that the nondispersive peaks arise from the tips of the Fermi arcs, which themselves do not change with binding energy.

Electronic phase diagram of high-temperature copper oxide superconductors

In order to understand the origin of high-temperature superconductivity in copper oxides, we must understand the normal state from which it emerges. Here, we examine the evolution of the normal state electronic excitations with temperature and carrier concentration in Bi2Sr2CaCu2O8+δ using angle-resolved photoemission. In contrast to conventional superconductors, where there is a single temperature scale Tc separating the normal from the superconducting state, the high-temperature superconductors exhibit two additional temperature scales. One is the pseudogap scale T∗, below which electronic excitations exhibit an energy gap. The second is the coherence scale Tcoh, below which sharp spectral features appear due to increased lifetime of the excitations. We find that T∗ and Tcoh are strongly doping dependent and cross each other near optimal doping. Thus the highest superconducting Tc emerges from an unusual normal state that is characterized by coherent excitations with an energy gap.

Scaling of the mixed state magnetoresistance of high-Tc superconductors in terms of two-dimensional superconducting fluctuations

Abstract We show scaling properties of the magnetoresistance of Bi 2 Sr 2 CaCu 2 O 8 thin films below T c =89 K in fields up to 20 T. The functional dependence is that of two-dimensional superconducting fluctuations above a critical temperature of 30 K. We propose that the double CuO 2 sheets in each unit cell behave as 2D superconducting units with giant fluctuations.

Oxygen-induced superconducting, metallic or insulating behaviour in as-grown epitaxial Bi2Sr2CuOx thin films

Abstract Epitaxial Bi2Sr2CuOx thin films were prepared in situ by reactive RF magnetron sputtering. Superconducting films with Tc (R=0) up to 18 K were obtained which correspond to what we call an optimal oxygen content. It is the highest Tc reported for films of the 2201 phase. In the case of films made under the same sputtering conditions, the superconducting critical temperature value decreases and superconductivity disappears when the oxygen content in the films is decreased or increased during the cooling process. With reference to the optimal oxygen content, overdoped films tend to display a normal metal behaviour and underdoped films tend to display a semiconductor-like behaviour at low temperature.

Evolution of the resistivity of single-layer Bi2Sr1.6La0.4CuOy thin films with doping and phase diagram

Abstract The temperature dependence of the in-plane resistivity ρ ( T ) is measured on epitaxial c -axis oriented single-layer Bi 2 Sr 1.6 La 0.4 CuO y thin films at various oxygen concentrations. By successive annealing treatments, the oxygen content of the same film is changed from maximally overdoped to strongly underdoped non-superconducting state, passing through the optimal state with T c max =30 K . The underdoped states show a downturn of the resistivity from the high T -linear behavior below a characteristic temperature T *, signature of the pseudogap effect. T * appears near optimally doped state and increases sharply with decreasing carrier concentration. Two other characteristic temperatures are observed in ρ ( T ) for underdoped states: the temperature T I of the inflection point in ρ ( T ) ( T I ∼0.5 T * ) and the temperature T M corresponding to the onset of localization effects. A phase diagram T versus doping is established.