Propriétés de l'état normal des cuprates sous-dopés sous champ magnétique intense

Dans cette thèse, nous nous sommes intéressés à la physique des supraconducteurs à haute température critique et à leur diagramme de phase. Depuis leur découverte en 1986, ces matériaux ont suscité un fort engouement notamment à cause de leur température critique élevée. La phase supraconductrice, dans le diagramme de phase de ces matériaux, présente un dôme avec une température critique maximale qui délimite le côté sous dopé (à gauche) du coté sur dopé (à droite). Ce dôme est situé entre une phase isolante et une phase métallique (type liquide de Fermi). Afin de comprendre l'origine de la supraconductivité, il est important de bien caractériser la phase normale à partir de laquelle elle se développe. Nous nous sommes particulièrement intéressés aux propriétés à basse température. Pour cela, des champs magnétiques intenses ont été nécessaires afin de détruire la supraconductivité et de restaurer l'état normal à basse température. Les oscillations quantiques ont été découvertes en 2007, dans ces matériaux, par des mesures de résistivité et d'aimantation sous champ magnétique intense et à basse température. La présence d'oscillations quantiques de faibles fréquences associées à des coefficients de Hall et Seebeck négatifs ont mis en évidence la présence d'une petite poche d'électrons dans la surface de Fermi couvrant seulement 1.9% de la première zone de Brillouin (PZB) du côté sous dopé. Ceci est incompatible avec les calculs de structure de bandes. Ce résultat est en fort contraste avec les oscillations quantiques mesurées dans les cuprates sur-dopés qui correspondent à la large orbite de type trou prédite par les calculs de structure de bandes et qui occupe 65% de la PZB. La présence de cette petite poche d'électrons dans des composés sous-dopés en trous peut être expliquée par une reconstruction de la surface de Fermi. En effet, des mesures de RMN et de rayons X ont montré la présence d'une onde de densité de charge en compétition avec la supraconductivité. Pour étudier plus en détail cette reconstruction et la relation entre la surface de Fermi déduite des oscillations quantiques et l'ordre de charge, nous avons réalisé différentes séries de mesures à 70T dans une large gamme de dopage, de champs magnétiques et de températures pour des échantillons d'YBa2Cu3Oy. Nous avons également utilisé la pression hydrostatique pour induire un changement de dopage sur un même échantillon. Ces mesures ont révélé la présence d'une nouvelle série d'oscillations de plus faible fréquence que nous avons associé à une poche de trou. Ce résultat permet d'expliquer plusieurs propriétés de transport mesurées dans le composé YBa2Cu3Oy. En parallèle, nous avons également réalisé des mesures de transport dans des échantillons de HgBa2CuO4+?, considéré comme un matériau modèle de la famille des cuprates (absence de chaîne et structure monoplan). Nous avons réussi à observer des oscillations quantiques, similaires à celles observées dans YBa2Cu3Oy. Ceci a permis de démontrer l'universalité de ce phénomène dans les cuprates et de confirmer de manière indiscutable que les plans conducteurs sont le siège de la reconstruction de la surface de Fermi à l'origine de ces oscillations quantiques. Enfin, nous avons effectué différentes simulations de reconstruction de la surface de Fermi basées sur un ordre de charge bi-axial qui a récemment été mis en évidence par des mesures de RMN, de rayons X et de vitesse du son. Les surfaces de Fermi obtenues ont été comparées aux mesures expérimentales.In this thesis, we have studied in high temperature superconductors and their phase diagram. Since their discovery in 1986, these materials have generated great interest, especially because of their high critical temperature. In the generic phase diagram of these materials, the superconducting phase is a dome with a maximal critical temperature, which defines the under-doped side (left-side of the phase diagram) and the over-doped side (right-side of the phase diagram). The dome is located between an insulating phase and a metallic phase (Fermi-liquid like). In order to understand the origin of superconductivity, it is essential to characterize the normal phase from which superconductivity arises, in particular at low temperature. Therefore, high magnetic fields were needed to destroy superconductivity and restore the low temperature normal state. Quantum oscillations have been discovered in 2007 in these materials, thanks to resistivity and magnetization measurements under high magnetic fields and low temperature. Quantum oscillations and negative Hall and Seebeck coefficients have revealed the presence of a small electron pocket in the Fermi surface of the underdoped side, covering only 2% of the first Brillouin zone (FBZ). This is in disagreement with band-structure calculations. In contrast quantum oscillations in overdoped cuprates revealed the large hole-like orbit, covering 65% of the FBZ, as predicted by the band structure calculations. The presence of this small electron pocket in underdoped compounds can be explained by a Fermi surface reconstruction. Indeed, RMN and X-rays measurements have shown the presence of a charge density wave in competition with superconductivity. To study the relationship between the Fermi surface deduced from quantum oscillations and the charge order, we have followed the evolution of the oscillation frequencies as a function of doping. Therefore, we have performed different measurements on YBa2Cu3Oy in a wide range of doping, magnetic fields (up to 70T) and temperatures. We also used hydrostatic pressure to induce a change of doping on the same sample. These measurements have revealed the presence of a new series of oscillations with a lower frequency which has been associated to a holepocket. This result allows us to explain some of the transport properties measured in YBa2Cu3Oy compound. In addition, we have also performed transport measurements in HgBa2CuO4+? samples, considered as a textbook material of the cuprate family (monolayer structure and no CuO chain). We have succeeded to observe quantum oscillations similar to those observed in YBa2Cu3Oy. This demonstrates the universality of this phenomenon in cuprates and clearly demonstrates that reconstruction of the Fermi surface involves the quintessential CuO2 planes. Finally, we have performed various simulations of Fermi surface reconstruction based on a biaxial charge order recently discovered by NMR, X-ray and sound velocity measurements. The reconstructed Fermi surface has been compared with experimental measurements

Inverse correlation between quasiparticle mass and Tc in a cuprate high-Tc superconductor

Close to a zero-temperature transition between ordered and disordered electronic phases, quantum fluctuations can lead to a strong enhancement of electron mass and to the emergence of competing phases such as superconductivity. A correlation between the existence of such a quantum phase transition and superconductivity is quite well established in some heavy fermion and iron-based superconductors, and there have been suggestions that high-temperature superconductivity in copper-oxide materials (cuprates) may also be driven by the same mechanism. Close to optimal doping, where the superconducting transition temperature Tc is maximal in cuprates, two different phases are known to compete with superconductivity: a poorly understood pseudogap phase and a charge-ordered phase. Recent experiments have shown a strong increase in quasiparticle mass m* in the cuprate YBa2Cu3O7-δ as optimal doping is approached, suggesting that quantum fluctuations of the charge-ordered phase may be responsible for the high-Tc superconductivity. We have tested the robustness of this correlation between m* and Tc by performing quantum oscillation studies on the stoichiometric compound YBa2Cu4O8 under hydrostatic pressure. In contrast to the results for YBa2Cu3O7-δ, we find that in YBa2Cu4O8, the mass decreases as Tc increases under pressure. This inverse correlation between m* and Tc suggests that quantum fluctuations of the charge order enhance m* but do not enhance Tc

Thermopower across the phase diagram of the cuprate La1.6−xNd0.4SrxCuO4 : signatures of the pseudogap and charge-density-wave phases

The Seebeck coefficient (thermopower) S of the cuprate superconductor La1.6−xNd0.4SrxCuO4 was measured across its doping phase diagram (from p = 0.12 to p = 0.25), at various temperatures down to T ≃ 2 K, in the normal state accessed by suppressing superconductivity with a magnetic field up to H = 37.5 T. The magnitude of S/T in the T = 0 limit is found to suddenly increase, by a factor ≃ 5, when the doping is reduced below p ⋆ = 0.23, the critical doping for the onset of the pseudogap phase. This confirms that the pseudogap phase causes a large reduction of the carrier density n, consistent with a drop from n = 1 + p above p ⋆ to n = p below p⋆, as previously inferred from measurements of the Hall coefficient, resistivity and thermal conductivity. When the doping is reduced below p = 0.19, a qualitative change is observed whereby S/T decreases as T → 0, eventually to reach negative values at T = 0. In prior work on other cuprates, negative values of S/T at T → 0 were shown to result from a reconstruction of the Fermi surface caused by charge- density-wave (CDW) order. We therefore identify pCDW ≃ 0.19 as the critical doping beyond which there is no CDW-induced Fermi surface reconstruction. The fact that pCDW is well separated from p ⋆ reveals that there is a doping range below p⋆ where the transport signatures of the pseudogap phase are unaffected by CDW correlations, as previously found in YBa2Cu3Oy and La2−xSrxCuO4.Center for Dynamics and Control of Material

Universal quantum oscillations in the underdoped cuprate superconductors

The metallic state of the underdoped high-Tc cuprates has remained an enigma: How may seemingly disconnected Fermi surface segments, observed in zero magnetic field as a result of the opening of a partial gap (the pseudogap), possess conventional quasiparticle properties? How do the small Fermi-surface pockets evidenced by the observation of quantum oscillations (QO) emerge as superconductivity is suppressed in high magnetic fields? Such QO, discovered in underdoped YBa2Cu3O6.5 (Y123) and YBa2Cu4O8 (Y124), signify the existence of a conventional Fermi surface (FS). However, due to the complexity of the crystal structures of Y123 and Y124 (CuO2 double-layers, CuO chains, low structural symmetry), it has remained unclear if the QO are specific to this particular family of cuprates. Numerous theoretical proposals have been put forward to explain the route toward QO, including materials-specific scenarios involving CuO chains and scenarios involving the quintessential CuO2 planes. Here we report the observation of QO in underdoped HgBa2CuO4+{\delta} (Hg1201), a model cuprate superconductor with individual CuO2 layers, high tetragonal symmetry, and no CuO chains. This observation proves that QO are a universal property of the underdoped CuO2 planes, and it opens the door to quantitative future studies of the metallic state and of the Fermi-surface reconstruction phenomenon in this structurally simplest cuprate.Comment: 17 pages, 5 figure

Publisher Correction: Truncated FGFR2 is a clinically actionable oncogene in multiple cancers.

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Change of carrier density at the pseudogap critical point of a cuprate superconductor

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Critical Doping for the Onset of Fermi- Surface Reconstruction by Charge-Density-Wave Order in the Cuprate Superconductor La2_xSrxCuO4

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Critical Doping for the Onset of Fermi- Surface Reconstruction by Charge-Density-Wave Order in the Cuprate Superconductor La2_xSrxCuO4

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Critical Doping for the Onset of Fermi-Surface Reconstruction by Charge-Density-Wave Order in the Cuprate Superconductor La 2 − x Sr x CuO 4

International audienceThe Seebeck coefficient S of the cuprate superconductor La2−xSrxCuO4 (LSCO) was measured in magnetic fields large enough to access the normal state at low temperatures, for a range of Sr concentrations from x=0.07 to x=0.15. For x=0.11, 0.12, 0.125 and 0.13, S/T decreases upon cooling to become negative at low temperatures. The same behavior is observed in the Hall coefficient RH(T). In analogy with other hole-doped cuprates at similar hole concentrations p, the negative S and RH show that the Fermi surface of LSCO undergoes a reconstruction caused by the onset of charge-density-wave modulations. Such modulations have indeed been detected in LSCO by X-ray diffraction in precisely the same doping range. Our data show that in LSCO this Fermi-surface reconstruction is confined to 0.085<p<0.15. We argue that in the field-induced normal state of LSCO, charge-density-wave order ends at a critical doping pCDW=0.15±0.005, well below the pseudogap critical doping p⋆≃0.19