Nanomagnetism & Oxides Laboratory
High-Tc supercoductors
People: A. Legros (PhD), A.Forget (Permanent staff), Dorothée Colson (Permanent staff)
Alumni: B. Loret (PhD), F. Rullier-Albenque (Permanent Staff), P. Bonville (Permanent Staff), A. Olariu (Post-Doc)  
Collaborations: S. Poissonnet, P. Bonnaillie (CEA-Saclay), G. Collin (LPS-Orsay), P.Thuéry (CEA-Saclay), V. Brouet (LPS-Orsay), J.-P. Rueff (Soleil), S. Bierman (GPhT-Polytechnique), Y. Gallais (MPQ-P7), A. Sacuto, (MPQ-P7), C. Proust (LCMI-Toulouse), M.-H. Julien (LCMI-Grenoble), L. Taillefer (Sherbrooke Univ.)…
- 2010-2014: ANR project PNICTIDES, Supraconductivité, magnétisme et propriétés électroniques des nouveaux pnictides à base de fer. Project coordinated by CEA Saclay-SPEC (F. Rullier-Albenque).  
- 2010: MagCorPnic, Magnétisme et corrélations électroniques dans les pnictures supraconducteurs, funded by RTRA-Triangle de la Physique. Project managed by SPEC-Saclay (D. Colson). 
- 2005-2009: ANR OXYFONDA (ANR-05-NT05-4_41913), Etats fondamentaux originaux dans les oxydes: de nouveaux matériaux pour une nouvelle physique. Project coordinated by LPS-Orsay (P. Mendels). 

Crystal Growth of HgBa2Ca2Cu3O8+d Superconductor with the Highest Critical Temperature at Ambient Pressure 
An original procedure for the synthesis of very high quality single crystals of superconducting HgBa2Ca2Cu3O8+δ mercury cuprates has been developped. These single crystals are unique with high quality surfaces paving the way for spectroscopic, transport and thermodynamic probes in order to understand the hole-doped cuprate phase diagram. Annealing allows to optimize Tc up to Tcmax = 133 K. The superconductivity transition width of about 2 K indicates they are homogeneous. 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. 

After 30 years of research on high-Tc superconductors, many questions remain open, particularly those concerning the mechanism involved in electron pairing, the high critical temperature (Tc) and its dependence on doping. Up to now, the relationship between structure, properties and the effect of pressure on superconductivity is still unknown. Yet this understanding is crucial for achieving higher Tc values that would allow the development of significant technological applications. Mercury cuprates could be good candidates for this challenge. Indeed, since their discovery in 1993, they still display the highest critical temperature at ambient pressure, 133 K (160 K at 30 GPa) for HgBa2Ca2Cu3O8 + δ (Hg-1223). Unfortunately, very few studies have been conducted because of the lack of single crystals, although mercury cuprates can be considered as model structures to understand the mechanisms of high-Tc, because of their tetragonal symmetry and their record Tc values. 
In this perspective, we have developed an original procedure for the elaboration of very high quality single crystals of superconducting Hg-1223 and a method for a precise control of their oxygen content, which will make it possible to probe, in a wide range of composition, their phase diagram.  
A low melting point region in a pseudo-ternary BaO-CaO-CuO phase diagram was determined and the crystal growth was made in a melt from an excess (or flux) of BaO and CuO. After a high temperature heat treatment followed by a slow cooling in vacuum sealed bulb, submillimetric platelet crystals of Hg1223 are mechanically extracted from the solidified flux (Figure 1).  
Remarkable points are that the surfaces of the crystals are extremely clean with high optical quality. The crystallographic axes are easily identifiable: a-axis lies along the edges and c-axis lies perpendicular to the largest face. Note that the crystals elaborated by the method described here have already enabled recent Raman studies in Sacuto’s group at MPQ-Paris7.2,3 
Figure 1. SEM micrography of a single crystal of superconducting HgBa2Ca2Cu3O8+δ mercury cuprates grown by the self-flux growth procedure. We control finely the oxygen content which will enable to probe, in a large range of composition, the phase diagram of these compounds. Superimposed to the photography, magnetic susceptibility curves shown for several doping p levels (from p=0.07, Tc=42K to p=0.14, Tc=129K). 
A low-temperature vacuum annealing procedure made it possible to obtain the first crystals with very low doping, thus widening three times the doping range available for this phase. The Raman spectroscopy measurements reveal for the first time the effect of the hole doping on the phonon spectrum of optimally and underdoped Hg-1223 and confirms the efficiency of the annealing process.1 This work is an important advance that should allow a deeper understanding of the phase diagram of cuprates, in particular the origin of the superconducting dome described by Tc, the true nature of the pseudogap and its relationship to the superconducting state. More generally, this flux growth technique can be applied to other compounds for which there is no phase diagram and / or growth in the air is prohibited due to volatile components. 
2Loret et al., Phys. Rev. Lett. 116 (2016) 197001. 
3Loret et al., Phys. Rev. B 96 (2017) 094525.