Photoelectron spectroscopy study of PuCoGa 5 thin films R. Eloirdi a, * , L. Havela b , T. Gouder a , A. Shick c , J. Rebizant a , F. Huber a , R. Caciuffo a a European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, D-76175 Karlsruhe, Germany b Department of Condensed Matter Physics, Charles University, Ke Karlovu 5, CZ-12116, Prague 2, Czech Republic c Institute of Physics, Academy of Sciences of the Czech Republic, Prague 8, Czech Republic article info abstract Thin layers of PuCoGa x (x = 4 to 18) have been prepared by dc sputtering from a PuCoGa 5 single crystal target, and investigated in situ by X-ray and ultraviolet photoelectron spectroscopy. We could achieve broad composition variability (monitored by the Pu-4f, Co-2p and Ga-2p core-level spectra). The results are compared to the valence band spectra reported previously for PuCoGa 5 . Our experiments reveal that some Ga excess (PuCoGa 7 ) was likely for those original data. We demonstrate that there is a tendency to the segregation of Ga at the surface, which has an important effect on the valence band spectra. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction The discovery of the first known Pu-based superconductor, PuC- oGa 5 (T c = 18.5 K) [1], initiated further studies of physical proper- ties [2,3] aimed to clarify the nature of the superconductivity and the role played by the 5f states. The mechanism of superconductiv- ity is still a matter of dispute, as evidenced by studies reporting on one side an almost magnetic state with an unconventional pairing mechanism [4] while others are assuming the prominence of elec- tron–phonon coupling [5]. Photoemission spectroscopy (PES) is a powerful tool to study the nature of the 5f states. The main difficulty is to obtain a clean surface. One can cleave a single crystal, or clean the surface e.g. by ion sputtering or by laser ablation, however this may also modify the surface composition. An alternative solution is to prepare in situ thin layers, e.g. by sputter deposition, while monitoring the composition by PES. Joyce et al. [6] presented the first PES data on PuCoGa 5 obtained after cleaning by laser ablation at T = 77 K. The valence band spectra were interpreted here as due to Pu-5f electrons in two configurations, one well removed from Fermi level (E F ) and one directly at E F . The mixed-level model calculation, in relatively good agreement with the PES data, found a total energy minimized for an atomic 5f 4 configuration and roughly one 5f elec- tron in delocalised Bloch states. In the present work, we describe the synthesis and PES (UPS and XPS) study of thin films obtained by the sputter deposition from PuCoGa 5 target. It was found that depending on conditions, large variations of Ga stoichiometry could be achieved, leading to PuCoGa x (x = 4 to 18). A tendency to Ga segregation at the surface was revealed especially when tem- peratures were elevated at the substrate. 2. Experimental Thin layers of PuCoGa x (4 6 x 6 18) were prepared in situ by dc sputtering using a small single crystal of PuCoGa 5 prepared by flux method [7] as a target. The plasma in the diode source was main- tained by injection of electrons at 50–100 eV. The sputter gas, ultrahigh-purity Ar (99.9999%), was used. The deposition rate was approximately one monolayer per second and a typical depo- sition time of 300 s. The background pressure of the plasma cham- ber was 4  10 7 Pa. The material was deposited on Si (1 1 1) single crystal cleaned by Ar ion bombardment at a temperature of T = 523 K. The deposition currents were typically 1–2 mA. Photo- electron spectra were recorded using a Leybold LHS-10 hemispher- ical analyser. X-ray photoelectron spectra (XPS) were taken using Mg Ka (1253.6 eV) radiation with an approximate resolution of 1 eV. Ultraviolet photoelectron spectroscopy (UPS) used HeI and HeII (hm = 21.22 and 40.81 eV, respectively) excitation produced by a high-intensity UV source (SPECS). The total resolution in UPS was greater than 0.05 eV. The background pressure in the analysis chamber was greater than 10 8 Pa. The film composition was determined from the ratio of the Pu-4f, Co-2p, and Ga-2p core-level spectra. 3. Results 3.1. Valence band study Fig. 1 shows the HeII valence band spectra of PuCoGa x thin films for two compositions, PuCoGa 16 and PuCoGa 5 . The layers deposited 0022-3115/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2008.08.059 * Corresponding author. Tel.: +49 7247 951803; fax: +49 7247 951599. E-mail address: rachel.eloirdi@ec.europa.eu (R. Eloirdi). Journal of Nuclear Materials 385 (2009) 8–10 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat