Welcome to EleMag at LPEM, ESPCI Paris!

Our group focuses on the study of strongly-correlated electron systems (typically oxides), which feature a large array of exotic electronic states such as high-temperature superconductivity and anomalous metallic states. Using several experimental techniques (transport, thermodynamics, Nuclear Magnetic Resonance...), we aim at characterizing and better understanding these electronic states, both at low (cryogenic) temperature and in high magnetic fields.

We are part of the LPEM lab, itself a joint research unit of ESPCI Paris (our host), CNRS, and UPMC (Pierre and Marie Curie University).

HIGHLIGHT: Superionic conductor with colossal dielectric constant

Electrical conductivity and high dielectric constant are in principle self-excluding, which makes the terms insulator and dielectric usually synonymous. This is certainly true when the electrical carriers are electrons, but not necessarily in a material where ions are extremely mobile, electronic conduction is negligible, and the charge transfer at the interface is immaterial. We have recently demonstrated in a perovskite-derived structure containing five-coordinated Ti atoms, a colossal dielectric constant (up to 109) together with very high ionic conduction 10-3 S.cm-1 at room temperature. Coupled investigations of I-V and dielectric constant behavior have allowed us to demonstrate that, due to ion migration and accumulation, this material behaves like a giant dipole, exhibiting colossal electrical polarization (of the order of 0.1 C.cm-2). Therefore it may be considered as a "ferro-ionet" and is extremely promising in terms of applications.

Rb2Ti2O5: Superionic conductor with colossal dielectric constant
R. Federicci, S. Holé, A. F. Popa, L. Brohan, B. Baptiste, S. Mercone, and B. Leridon, Phys. Rev. Materials 1, 032001(R) (2017) - Editor’s suggestion

The crystal structure of Rb2Ti2O5
R. Federicci, B. Baptiste, F. Finocchi, F. Popa, L. Brohan, K. Beneut, P. Giura, G. Rousse, A. Descamps-Mandine, T. Douillard, A. Shukla, and B. Leridon, Acta Cristallographica B (in the press)

HIGHLIGHT: Competition for nanoscale territory between magnetism and superconductivity

Figure publi

The most promising superconductors (materials which display ideal electrical conduction and magnetic field cancellation) are often chemically similar to magnetic materials, suggesting that it is crucial to understand the cross-over from magnetic to superconducting behavior. The corresponding phase diagram (the evolution of the electronic properties when changing the doping, i.e., the number of available electrons) can however be misleading due to the frequent uncertainty on the actual composition of the studied compounds. Here, we show that the nanometer-scale electronic variation intrinsic to some iron-based superconductors allows us to redefine the doping with high accuracy and precision, yielding a detailed reference phase diagram. From the then quantitatively correct variations of the structural, magnetic, and superconducting quantities, we reveal the spatial competition, on a nanometer scale, of superconductivity and magnetism: the weakening and loss of magnetism through dilution allows for the apparition of unconventional superconductivity, which tends to spill over (’proximity effect’) the whole material. In addition to this comprehensive experimental description of the competition of electronic states in a family of superconductors, our study provides a reference frame for future work and to revisit previous results.

Spatial competition of the ground states in 1111 iron pnictides
G. Lang, L. Veyrat, U. Gräfe, F. Hammerath, D. Paar, G. Behr, S. Wurmehl, and H.-J. Grafe, Phys. Rev. B 94, 014514 (2016)

HIGHLIGHT: Confinement of superconducting fluctuations due to emergent electronic inhomogeneities

Figure publi

How a disordered superconductor evolves into an insulator is still a matter of intense debate. Disorder and reduced screening of Coulombic repulsion may simply break the pairs or, alternatively, the Cooper pairs themselves may localize. Between these two extreme cases there can be intermediate behaviors that emerge giving rise to intermediate scale inhomogeneity with anomalous properties. Thin NbN films indeed display such ambiguous character as revealed by Scanning Tunneling Spectroscopy and transport experiments. While STS shows inhomogeneity on a scale much larger than that of the intrinsic nanocristalline structure, conductivity fluctuations near the SC-I transition have on the same large scale a zero-dimensional character similar to the one previously found only in granular systems. This can be interpreted in terms of incipient localization of the pairs before, closer to the superconducting transition temperature, they eventually escape from the incipient localization and evolve into the superconducting condensate.

Confinement of superconducting fluctuations due to emergent electronic inhomogeneities
C. Carbillet, S. Caprara, M. Grilli, C. Brun, T. Cren, F. Debontridder, B. Vignolle, W. Tabis, D. Demaille, L. Largeau, K. Ilin, M. Siegel, D. Roditchev, and B. Leridon, Phys. Rev. B 93, 144509 (2016) Editors’ Suggestion

HIGHLIGHT: Electron states in mixed valence, electron doped manganites

Figure publi

The manganites are strongly correlated materials which display a great variety of electronic properties when doped by charge carriers. In order to shed light on the difficult problem of the electron states in mixed valence, electron doped manganites, we have used 17O NMR in oxygen-deficient SrMnO3-x compounds. In the paramagnetic state, we show that when increasing the electron doping, SrMnO3-x is shifted towards a ferromagnetic (FM) metallic state. Below TN, the Néel temperature, some of the doped electrons are no more fast moving creating FM domains which are considered as small size magnetic polarons. The trend of our results indicates that the de Gennes metallic ground state cannot be achieved when increasing the electron doping. Besides, the transition in SrMnO3 is generally considered as an order-disorder transition of the local Mn-t2g spins in the isotropic 3D Heisenberg system. Nevertheless, the bulk susceptibility shows no sign of the conventional Curie-Weiss law behavior, even far above TN. Furthermore, we show that the thermal behavior of the isotropic and axial contributions to the local magnetic susceptibility measured with 17O NMR demonstrate a rather gapped behavior, indicative of low-D short-range spin order in the paramagnetic state.

17O NMR study of the doped electrons in lightly oxygen-deficient cubic SrMnO3-x
A. Trokiner, S. Verkhovskii, Z. Volkova, A. Gerashenko, K. Mikhalev, A. Germov, A. Yakubovskii, A. Korolev, B. Dabrowski, and A. Tutunick, Phys. Rev. B 93, 174413 (2016)