Quantum matter in high magnetic fields

Abstract:

We present a comprehensive study of the critical current densities and the superconducting vortex phase diagram in the stoichiometric superconductor CaKFe4As4 which has a critical temperature of ∼35 K. We performed detailed magnetization measurements both of high quality single crystals for different orientations in an applied magnetic field up to 16 T and for a powder sample. We find an extremely large critical current density, Jc, up to 108 A/cm2 for single crystals when H(ab) at 5 K, which remains robust in fields up to 16 T, being the largest of any other iron-based superconductor. The critical current density is reduced by a factor 10 in single crystals when Hc at 5 K and significantly suppressed by the presence of grain boundaries in the powder sample. We also observe the presence of the fishtail effect in the magnetic hysteresis loops of single crystals when Hc. The flux pinning force density and the pinning parameters suggest that the large critical current could be linked to the existence of point core and surface pinning. Based on the vortex phase diagram and the large critical current densities, CaKFe4As4 is now established as a potential iron-based superconductor candidate for practical applications.

Abstract:

We report magnetotransport studies in thin (<100 nm) exfoliated films of CuxBi2Se3 and we detect an unusual electronic transition at low temperatures. Bulk crystals show weak superconductivity with Tc ∼ 3.5 K and a possible electronic phase transition around 200K. Following exfoliation, superconductivity is supressed and a strongly temperature dependent multi-band conductivity is observed for T < 30K. This transition between competing conducting channels may be enhanced due to the presence of electronic ordering, and could be affected by the presence of an effective internal stress due to Cu intercalation. By fitting to the weak antilocalisation conductivity correction at low magnetic fields we confirm that the low temperature regime maintains a quantum phase coherence length Lφ > 100 nm indicating the presence of topologically protected surface states.

Abstract:

FeSe is a fascinating superconducting material at the frontier of research in condensed matter physics. Here we provide an overview on the current understanding of the electronic structure of FeSe, focusing in particular on its low energy electronic structure as determined from angular resolved photoemission spectroscopy, quantum oscillations and magnetotransport measurements of single crystal samples. We discuss the unique place of FeSe amongst iron-based superconductors, being a multi-band system exhibiting strong orbitally-dependent electronic correlations and unusually small Fermi surfaces, prone to different electronic instabilities. We pay particular attention to the evolution of the electronic structure which accompanies the tetragonal-orthorhombic structural distortion of the lattice around 90 K, which stabilizes a unique nematic electronic state. Finally, we discuss how the multi-band multi-orbital nematic electronic structure has an impact on the understanding of the superconductivity, and show that the tunability of the nematic state with chemical and physical pressure will help to disentangle the role of different competing interactions relevant for enhancing superconductivity.

Abstract:

We report magnetotransport studies in thin (<100nm) exfoliated films of Cu_xBi₂Se₃ and we detect an unusual electronic transition at low temperatures. Bulk crystals show weak superconductivity with T_c∽3.5K and a possible electronic phase transition around 200K. Following exfoliation, superconductivity is supressed and a strongly temperature dependent multi-band conductivity is observed for T<30K. This transition between competing conducting channels may be enhanced due to the presence of electronic ordering, and could be affected by the presence of an effective internal stress due to Cu intercalation. By fitting to the weak antilocalisation conductivity correction at low magnetic fields we confirm that the low temperature regime maintains a quantum phase coherence length L_φ > 100nm indicating the presence of topologically protected surface states.

Abstract:

These data were created by recording the transport behaviour on thin flakes devices at low temperatures and in magnetic field. These are all DAT files easily readable