Superconducting Fluctuations, Pseudogap and Phase Diagram in Cuprates

We report transport measurements using pulsed magnetic fields to suppress the superconducting fluctuations (SCF) conductivity in a series of YBa_2Cu_3O_(6+x) samples. These experiments allow us altogether to measure the temperature T'c at which SCF disappear, and the pseudogap temperature T*. While the latter are consistent with previous determinations of T*, we find that T'c is slightly larger than similar data taken by Nernst measurements. A careful investigation near optimal doping shows that T* becomes smaller than T'c, which is an unambiguous evidence that the pseudogap cannot be assigned to preformed pairs. Studies of the incidence of disorder on both T'c and T* allow us to propose a phase diagram including disorder which explains most observations done in other cuprate families, and to discuss the available knowledge on the pseudogap line in the phase diagram.Comment: New version with minor correction

An electronic instability in bismuth far beyond the quantum limit

We present a transport study of semi-metallic bismuth in presence of a magnetic field applied along the trigonal axis extended to 55 T for electric conductivity and to 45 T for thermoelectric response. The results uncover a new field scale at about 40 T in addition to the previously detected ones. Large anomalies in all transport properties point to an intriguing electronic instability deep in the ultraquantum regime. Unexpectedly, both the sheer magnitude of conductivity and its metallic temperature dependence are enhanced by this instability.Comment: 5 pages, 4 figure

Quantum oscillations in underdoped YBa_2Cu_3O_6.5

Shubnikov-de Haas and de Haas-van Alphen effects have been measured in the underdoped high temperature superconductor YBa$_2$Cu$_3$O$_{6.51}$. Data are in agreement with the standard Lifshitz-Kosevitch theory, which confirms the presence of a coherent Fermi surface in the ground state of underdoped cuprates. A low frequency $F = 530 \pm 10$ T is reported in both measurements, pointing to small Fermi pocket, which corresponds to 2% of the first Brillouin zone area only. This low value is in sharp contrast with that of overdoped Tl$_2$Ba$_2$CuO$_{6+\delta}$, where a high frequency $F = 18$ kT has been recently reported and corresponds to a large hole cylinder in agreement with band structure calculations. These results point to a radical change in the topology of the Fermi surface on opposing sides of the cuprate phase diagram.Comment: proceeding of the ECRYS-200

Le Betizu, une population bovine des montagnes basques : statut juridique et modalités de gestion

Les betizu sont des bovins vivant en liberté dans les montagnes du Pays Basque. Cette population bovine compte environ 900 individus en Espagne et seulement une centaine de têtes en France. Le statut juridique du Betizu est très différent de part et d'autre de la frontière : alors qu'en Espagne, il s'agit d'une race bovine domestique officiellement reconnue et dont la production est soutenue par le gouvernement, en France, ces bovins n'ont pas de propriétaire officiel, ne sont pas identifiés et ne subissent pas de suivi sanitaire. L'amélioration de la gestion de Betizu en France et la clarification du statut juridique semblent nécessaire à la survie de cette population bovine menacée de disparition

Fermi liquid behavior of the in-plane resistivity in the pseudogap state of YBa_2Cu_4O_8

Our knowledge of the ground state of underdoped hole-doped cuprates has evolved considerably over the last few years. There is now compelling evidence that inside the pseudogap phase, charge order breaks translational symmetry leading to a reconstructed Fermi surface made of small pockets. Quantum oscillations, [Doiron-Leyraud N, et al. (2007) Nature 447:564-568], optical conductivity [Mirzaei SI, et al. (2013) Proc Natl Acad Sci USA 110:5774-5778] and the validity of Wiedemann-Franz law [Grissonnache G, et al. (2016) Phys. Rev. B 93:064513] point to a Fermi liquid regime at low temperature in the underdoped regime. However, the observation of a quadratic temperature dependence in the electrical resistivity at low temperatures, the hallmark of a Fermi liquid regime, is still missing. Here, we report magnetoresistance measurements in the magnetic-field-induced normal state of underdoped YBa_2Cu_4O_8 which are consistent with a T^2 resistivity extending down to 1.5 K. The magnitude of the T^2 coefficient, however, is much smaller than expected for a single pocket of the mass and size observed in quantum oscillations, implying that the reconstructed Fermi surface must consist of at least one additional pocket.Comment: Main + SI : published versio

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