Non-monotonic anisotropy in charge conduction induced by antiferrodistortive transition in metallic SrTiO3_{3}

Cubic SrTiO$_{3}$ becomes tetragonal below 105 K. The antiferrodistortive (AFD) distortion leads to clockwise and counter-clockwise rotation of adjacent TiO$_{6}$ octahedra. This insulator becomes a metal upon the introduction of extremely low concentration of n-type dopants. However, signatures of the structural phase transition in charge conduction have remained elusive. Employing the Montgomery technique, we succeed in resolving the anisotropy of charge conductivity induced by the AFD transition, in the presence of different types of dopants. We find that the slight lattice distortion ($<6 \times 10^{-4}$) gives rise to a twenty percent anisotropy in charge conductivity, in agreement with the expectations of band calculations. Application of uniaxial strain amplifies the detectable anisotropy by disfavoring one of the three possible tetragonal domains. In contrast with all other known anisotropic Fermi liquids, the anisotropy has opposite signs for elastic and inelastic scattering. Increasing the concentration of dopants leads to a drastic shift in the temperature of the AFD transition either upward or downward. The latter result puts strong constraints on any hypothetical role played by the AFD soft mode in the formation of Cooper pairs and the emergence of superconductivity in SrTiO$_3$.Comment: 6 pages with 5 figure

Pseudogap et supraconductivité dans les cuprates : une cohabitation tumultueuse

Although high Tc superconductivity has been discovered more than 29 years, the mechanism of superconductivity is not yet understood and raises fundamental questions. We will focus on the link betwen superconductivity and the pseudogap, its origin remains mysterious.Here we report our recent progress to grow single crystals of HgBa2Ca2Cu3O8 (Hg-1223) cuprate with a new self-flux growth technique. The crystals thus synthesized are submilimetric, slightly underdoped and show high surface quality for spectroscopic measurements. Magnetometry and X-Ray diffraction have been performed. We show for the first time that with adequate heat treatment, we can largely under-dope the Hg-1223 compound and control its doping level.Our raman spectroscopy study of a slightly underdoped single crystal of Hg-1223 allowed us to observe that the superconducting pair-breaking peak is associated with a dip on its higher-energy side, disappearing together at Tc. This result reveals a key aspect of the unconventional pairing mechanism : spectral weight lost in the dip is transferred to the pair-breaking peak at lower energies. This conclusion is supported by cellular dynamical mean-field theory on the Hubbard model, which is able to reproduce all the main features of the Raman response and explain the peak-dip behavior in terms of a nontrivial relationship between the superconducting and the pseudo gaps. We conrmed this results on others compounds revealing his universality.We tracked this structure with doping on Bi2Sr2CaCu2O8. The pseudogap survive in the overdoped region and then disappear above p = 0.225, wich corresponds to the normal state disappearance of the pseudogap in coincidence with a Lifshitz transition. This suggests a vertical ending line of the pseudogap in the T-p phase diagram.Bien que la supraconductivité à haute température fut découverte il y a bientôt 30 ans, le mécanisme à l'origine de ce phénomène n'est toujours pas compris et de nombreuses questions fondamentales demeurent. Nous nous intéressons notamment au lien entre la supraconductivité et la phase pseudogap, dont l'origine reste inconnue.Nous exposons dans un premier temps nos récents progrès sur la synthèse de monocristaux de HgBa2Ca2Cu3O8 (Hg-1223) par croissance en auto-fux. Les cristaux obtenus sont submilimétriques, légèrement sous-dopés et présentent une surface idéale pour des mesures optiques. Nous les avons caractérisés par magnétométrie SQUID et diffraction des rayons X. De plus, nous avons réussi pour la première fois à fortement sous-doper ce composé via des traitements thermiques adéquates. Notre étude par spectroscopie Raman d'un monocristal légèrement sous-dopé du composé Hg-1223 a permis d'observer que le pic supraconducteur de paires brisées est suivi d'un creux à plus haute énergie. Cette structure qui disparait à Tc révèle le caractère non conventionnelle du mécanisme d'appariement des électrons : le poids spectral du creux est transféré au pic supraconducteur de plus basse énergie. Cette conclusion repose également sur des calculs en CDMFT se basant sur le modèle de Hubbard. Cette méthode est capable de reproduire toutes les principales caractéristiques de la réponse Raman et explique l'association pic-creux en terme d'une relation non-triviale entre la supraconductivité et le pseudogap. Nous avons vérifié la présence de cette structure sur d'autres cuprates, révélant ainsi son universalitéNous avons ensuite suivi cette structure avec le dopage sur Bi2Sr2CaCu2O8. Le pseudogap survit dans la région sur-dopée avant de disparaître au dessus de p = 0.225, ce qui correspond à la disparition du pseudogap dans l'état normal coïncidant avec une transition de Lifshitz. Cela suggère que la fin du pseudogap dans le diagramme T-p est une ligne verticale

Crystal Growth and Characterization of HgBa2Ca2Cu3O8+δ Superconductor with the Highest Critical Temperature at Ambient Pressure

International audienceWe report an original procedure for the elaboration of very high quality single crystals of superconducting HgBa2Ca2Cu3O8+δ mercury cuprates. These single crystals are unique with very high quality surface paving the way for spectroscopic, transport and thermodynamic probes in order to understand the hole-doped cuprate phase diagram. The superconductivity transition width of about 2 K indicates they are homogeneous. Annealing allows to optimize Tc up to Tc max = 133 K. We show for the first time that with adequate heat treatment, Hg-1223 can be largely under-doped and its doping level controlled. Importantly, the crystal structure was studied in detail by single crystal X-ray diffraction, and we have identified the signature of the under-doping by a detailed sample characterization and micro-Raman spectroscopy measurements

Crystal Growth and Characterization of HgBa₂Ca₂Cu₃O_8+δ Superconductors with the Highest Critical Temperature at Ambient Pressure

We report an original procedure for the elaboration of very high-quality single crystals of superconducting HgBa₂Ca₂Cu₃O_8+δ mercury cuprates. These single crystals are unique, with very high-quality surface paving the way for spectroscopic, transport, and thermodynamic probes in order to understand the hole-doped cuprate phase diagram. Annealing allows one to optimize <i>T</i>_c up to <i>T</i>_c^max = 133 K. The superconductivity transition width of about 2 K indicates that they are homogeneous. We show for the first time that, with adequate heat treatment, Hg-1223 can be largely underdoped and its doping level controlled. Importantly, the crystal structure was studied in detail by single-crystal X-ray diffraction, and we have identified the signature of the underdoping by a detailed sample characterization and micro-Raman spectroscopy measurements