Superconductivity in water intercalated Na CoO2 thin films
Zusammenfassung der Projektergebnisse
The project on p-wave superconductivity in water intercalated NaxCoO2 thin films failed to reach its original goal. The starting point of our project was the observation of superconductivity in Na0.3CoO2 · 1.3 D2O thin films at 4.2 K. These films were stable on a time scale of the order of a week. However, in the project we were not able to improve this result and stabilize superconductivity in thin films by water intercalation. As a conclusion, the dream to have for the first time a solid state thin film material allowing to investigate for example Josephson junctions and heterostructures based on a p-wave superconductor cannot be fulfilled by using the cobaltate Na0.3CoO2 · 1.3 H2O thin films. The chemical reason is the metastability of the phase with 1.3 water molecules intercalated per unit cell. Thus, the result of our project is a negative one: We exclude the possibility of reproducible and stable water intercalation in sodium cobaltate thin films for further experiments or applications in Josephson junctions. As a consequence, one of the authors (LA) designed a new project aiming to realize p-wave superconducting thin films in Sr2RuO4. Meanwhile, it has been shown that superconductivity can be observed in Sr2RuO4 thin films, however, with a critical temperature of about half the bulk value. Nevertheless, this is a promising starting point for a stable solid state system with p-wave superconductivity without a metastable water intercalation phase. Oxide molecular beam epitaxy might be the most promising technique to obtain single-crystal like thin films of in Sr2RuO4. Within this project we have learnt more about the thin film growth of sodium cobaltate. We have shown that the growth parameters can be adjusted in such a way that sodium contents between 0.38 and 1 can be directly obtained without additional (electro)chemical treatment. This result is important with respect to the use NaxCoO2 as thermoelectric thin films. Another surprising side result was the interest in NaxCoO2 as a battery material where the sustainable element Na substitutes the expensive and critical Li in the isostructural compound. Preliminary measurements together with the group of Prof. Jaegermann at TU Darmstadt who is working on thin film Li-ion batteries have shown promising results. Based on these first experiments we are currently preparing another project to explore the possibility to use Nax(Co,Mn,Ni)O2 as an alternative battery material. We have learnt during this project, how such thin films with mixed occupancy at the transition metal site can be grown using pulsed laser deposition.
Projektbezogene Publikationen (Auswahl)
- Anomalous electronic Raman scattering in NaxCoO2 ∙y H2O. Phys. Rev. Lett. 96, 167204 (2006)
P. Lemmens, K.-Y. Choi, V. Gnezdilov, E.Ya. Sherman, D.P. Chen, C.T. Lin, F.C. Chou, and B. Keimer
- Sodium Cobaltates: Crystal Growth, Structure, Thermoelectricity, and Superconductivity. J. Cryst. Growth. 292, 422-428 (2006)
C.T. Lin, D.P. Chen, A. Maljuk, and P. Lemmens
- Comment on ”Raman spectroscopy study of NaxCoO 2 and superconducting NaxCoO2 · yH2O”. Phys. Rev. B 75, 106501 (2007)
P. Lemmens, P. Scheib, Y. Krockenberger, L. Alff, F. C. Chou, C. T. Lin, H.-U. Habermeier, and B. Keimer
- Substrate-induced anisotropy of c-axis textured NaxCoO2 thin films. Prog. Solid State Ch. 35, 545 (2007).
L. Yu, Y. Krockenberger, I. Fritsch and H.-U. Habermeier
- Electron-phonon interaction in the lamellar cobaltate NaxCoO2. Phys. Rev. B 77, 100504(R) (2008)
A. Donkov, M. M. Korshunov, I. Eremin, P. Lemmens, V. Gnezdilov, F. C. Chou, C. T. Lin
- Epitaxial layered cobaltite NaxCoO2 thin films grown on planar and vicinal cut substrates. J. Cryst. Growth 328, 34 (2011)
L. Yu, L. Gu, Y. Wang, P.-X. Zhang, and H.-U. Habermeier
- ol-gel synthesis of sodium and lithium based materials. J. Sol Gel Sci. Technol. (2012)
S. Hildebrandt, A. Eva, P. Komissinskiy, C. Fasel, I. Fritsch, and L. Alff
(Siehe online unter https://doi.org/10.1007/s10971-012-2789-4)
