Probing Magnetism in the Vortex Phase of PuCoGa5 by X-Ray Magnetic Circular Dichroism

Abstract

We have measured X-ray magnetic circular dichroism (XMCD) spectra at the Pu M4,5 absorption edges from a newly-prepared high-quality single crystal of the heavy fermion superconductor 242 PuCoGa5, exhibiting a critical temperature Tc = 18.7 K. The experiment probes the vortex phase below Tc and shows that an external magnetic field induces a Pu 5f magnetic moment at 2 K equal to the temperature-independent moment measured in the normal phase up to 300 K by a SQUID device. This observation is in agreement with theoretical models claiming that the Pu atoms in PuCoGa5 have a nonmagnetic singlet ground state resulting from the hybridization of the conduction electrons with the intermediate-valence 5f electronic shell. Unexpectedly, XMCD spectra show that the orbital component of the 5f magnetic moment increases significantly between 30 and 2 K; the antiparallel spin component increases as well, leaving the total moment practically constant. We suggest that this indicates a low-temperature breakdown of the complete Kondo-like screening of the local 5f moment.PuCoGa 5 is a prototypical heavy-fermion compound that becomes a superconductor below T c ≃ 18.5 K [1], the highest critical temperature of any heavy-fermion material. Fifteen years on its discovery, the nature of the pairing boson in PuCoGa 5 remains an open question. Superconductivity (SC) mediated by spin fluctuations (SFs) associated with the proximity to an antiferromagnetic (AFM) quantum critical point (QCP) was initially proposed. The hypothesis was supported by the observation in the normal phase of a Curie-Weiss (CW) behavior of the magnetic susceptibility χ m , suggesting the presence of Pu atoms carrying a local magnetic moment. Further arguments in favor of SFs-controlled SC were provided by NMR studies [2], revealing a nodal-gap function separating the condensate from the unpaired states. Subsequent point-contact spectroscopy measurements confirmed that the wavefunction of the paired electrons has an unconventional d-wave symmetry [3]. However, the SF conjecture was questioned [4,5] after polarized neutron diffraction failed to observe a local magnetic moment in the normal state of PuCoGa 5 [6], pointing to an extrinsic origin of the reported temperature dependent χ m . This observation is confirmed in the present article by showing that the magnetic susceptibility of an almost defect-free PuCoGa 5 single crystal is weak and temperature-independent from T c up to room temperature.Other members of the PuM X 5 family (M = Co, Rh, and X = Ga, In) also become superconductors, with T c ranging from ∼1.7 K in the case of PuRhIn 5 to ∼8.7 K for PuRhGa 5 [7]. The much larger T c of PuCoGa 5 could indicate that a different pairing mechanism is acting in the various compounds of the family. Indeed, Bauer et al.[8] proposed that SC in PuCoIn 5 (T c = 2.5 K) is related to an AFM QCP, whereas PuCoGa 5 would reside on a larger SC dome in the temperature-pressure (hybridization strength) phase diagram, around a valence fluctuation (VF) QCP. The hypothesis is suppor...

X-ray synchrotron radiation studies of actinide materials

“…Applying the sum rules, the moments and their ratio can be extracted from the experimental data. The values obtained are in excellent agreement with dynamical mean field theories (DMFT) predictions of a non-magnetic ground state for Pu in PuCoGa 5 [see Table I of Magnani et al (2017)]. This surprising result is a consequence of two effects: intermediate valence driven by 5f-6d Coulomb interaction mixes a magnetic 5f 5 sextet with a non-magnetic 5f 6 singlet, reducing the magnetic moment; a complete quenching is then produced by a Kondo-like screening promoted by the hybridization between 5f and conduction electron states (Pezzoli et al, 2011;Shick et al, 2013).…”

“…XMCD spectra have been measured above and below T c on a 2 mg single-crystal sample of 242 PuCoGa 5 (99.99 wt% 242 Pu) (Magnani et al, 2017). The c-axis of the crystal was oriented along the incident photon beam, and therefore parallel to the applied magnetic field.…”

“…As an example, Fig. 11 shows XANES and XMCD spectra measured in PuCoGa 5 (Magnani et al, 2017), a tetragonal heavy fermion superconductor with a critical temperature T c = 18.5 K (Sarrao et al, 2002). The origin of superconductivity in this compound remains puzzling, despite years of intensive investigations (Jutier et al, 2008;Das et al, actinide physics and chemistry J. Synchrotron Rad.…”