Avoided valence transition in a plutonium superconductor

Ramshaw, B. J.; Shekhter, Arkady; McDonald, R. D.; Betts, J. B.; Mitchell, Jeanette; Tobash, Paul H.; Mielke, C. H.; Bauer, E. D.; Migliori, A.

doi:10.1073/pnas.1421174112

Cited by 50 publications

(46 citation statements)

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“…This proves that for CeCoIn 5 at p L the spin fluctuations responsible for the intrinsic Kondo liquid T 1 exhibit quantum critical magnetic behavior, and suggests a possibly similar situation in PuCoGa 5 . We note that recent elastic moduli measurements have observed an anomalous softening of the bulk modulus in PuCoGa 5 and suggested that valence fluctuations may play a critical role in its superconductivity [20]. Combining their results and the NMR measurement [1] suggests that both valence and spin fluctuations may be involved to give rise to the particularly high T c in this interesting compound.…”

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confidence: 61%

Quantum critical scaling and superconductivity in heavy electron materials

2015

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We use the two fluid model to determine the conditions under which the nuclear spin-lattice lattice relaxation rate, T1, of candidate heavy quantum critical superconductors can exhibit scaling behavior and find that it can occur if and only if their "hidden" quantum critical spin fluctuations give rise to a temperature-independent intrinsic heavy electron spin-lattice relaxation rate. The resulting scaling of T1 with the strength of the heavy electron component and the coherence temperature, T * , provides a simple test for their presence at pressures at which the superconducting transition temperature, Tc, is maximum and is proportional to T * . These findings support the previously noted partial scaling of the spin-lattice relaxation rate with Tc in a number of important heavy electron materials and provide additional evidence that in these materials their optimal superconductivity originates in the quantum critical spin fluctuations associated with a nearby phase transition from partially localized to fully itinerant quasiparticles.PACS numbers: 71.27.+a, 74.70.Tx, 76.60.-k A tantalizing hint that the spin fluctuations seen in the nuclear spin relaxation rate for a number of unconventional superconductors might be the magnetic glue responsible for their superconductivity appears in a scaling relation between that rate and the optimal superconducting transition temperature, T c , that was first noted by Curro et al [1]. In the present communication we focus on understanding this scaling relation for one important member of this family, the heavy electron materials, for which some experimental results are given in Fig. 1 [1, 2]. As may be seen in Fig. 2, finding such a relation appears at first sight highly problematic because the scaling covers a range of temperatures (T c < T < T * ) in the normal state in which both hybridized localized spins and the itinerant heavy electron Kondo liquid contribute to the spin-lattice relaxation rate. However, we find that rigorous Curro T c scaling can become possible if three conditions are met: (1) the maximum in T c occurs at the pressure p L , at which the line marking the boundary between partially localized and fully itinerant behavior for heavy electron quasiparticles, T L , intersects with T c , so that T max c = T L (p L ); (2) at p L the total spin-lattice relaxation rate scales with the coherence temperature, T

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confidence: 61%

Quantum critical scaling and superconductivity in heavy electron materials

2015

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“…The occurrence of valence fluctuations in PuCoGa 5 has been recently suggested by resonant ultrasound spectroscopy measurements, showing that the three compressional elastic moduli exhibit anomalous softening upon cooling, which is truncated at the SC transition [14]. These results have been interpreted as evidence for a valence transition at a T V < T c that is avoided by the superconducting state [14].…”

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confidence: 75%

“…These results have been interpreted as evidence for a valence transition at a T V < T c that is avoided by the superconducting state [14]. On the other hand, the relaxation…”

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confidence: 88%

“…In this framework, the 5f local magnetic moment is quenched by the combination of intermediate valence and hybridization with the surrounding cloud of conduction electrons. On the other hand, for X = In, the predicted Pu ground state is magnetic as a result of a weaker hybridization strength [11].The occurrence of valence fluctuations in PuCoGa 5 has been recently suggested by resonant ultrasound spectroscopy measurements, showing that the three compressional elastic moduli exhibit anomalous softening upon cooling, which is truncated at the SC transition [14]. These results have been interpreted as evidence for a valence transition at a T V < T c that is avoided by the superconducting state [14].…”

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confidence: 95%

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

et al. 2017

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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...

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“…A consequence of this realization is that no single-configuration electronic-structure model has succeeded or even can succeed in accounting for either large changes in the bulk modulus with temperature or the negative thermal expansion. In the superconductor PuCoGa 5 , measurements have shown that multiple plutonium valence configurations lead to unusual elastic softening (7). Earlier inelastic neutron-scattering measurements by Trouw et al (8), confirmed by the statistically improved recent neutron-scattering measurements by Janoschek et al (6), reveal that multiple electronic configurations are present in δ-Pu that are also thermally accessible with the higher energy state approximately 1,400 K above the ground state.…”

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Origin of the multiple configurations that drive the response of δ-plutonium’s elastic moduli to temperature

Migliori

Söderlind

Landa

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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The electronic and thermodynamic complexity of plutonium has resisted a fundamental understanding for this important elemental metal. A critical test of any theory is the unusual softening of the bulk modulus with increasing temperature, a result that is counterintuitive because no or very little change in the atomic volume is observed upon heating. This unexpected behavior has in the past been attributed to competing but never-observed electronic states with different bonding properties similar to the scenario with magnetic states in Invar alloys. Using the recent observation of plutonium dynamic magnetism, we construct a theory for plutonium that agrees with relevant measurements by using density-functional-theory (DFT) calculations with no free parameters to compute the effect of longitudinal spin fluctuations on the temperature dependence of the bulk moduli in δ-Pu. We show that the softening with temperature can be understood in terms of a continuous distribution of thermally activated spin fluctuations.T he plutonium δ-phase (face-centered cubic, fcc), depending on gallium concentration, can on heating expand (above ∼2 at % Ga), contract (less than ∼2 at % Ga), or maintain volume independent of temperature from approximately 500 to 800 K (∼2 at % Ga) (1). Reminiscent of the Invar effect (2, 3), Lawson et al. (1) made the critically important observation that this could be modeled by assuming that two configurations were thermodynamically accessible in δ-Pu. This assumption introduces a strong constraint on a microscopic theory. Separated by 1,400 K, the higher energy configuration is assumed to have smaller volume than the lower energy one. Thus, as temperature rises, the Boltzmann factor increases the occupied fraction of the higher energy state, compensating for ordinary thermal expansion effects. This model could be made to fit δ-Pu's volume versus temperature dependence as measured by elastic neutron scattering for the range of Ga concentrations typically used to stabilize the δ-phase (4). However, measurements of the elastic moduli of polycrystalline Pu-Ga alloys by Suzuki et al. (5) showed that the bulk modulus softened substantially on warming in temperature regions where the atomic volume remained fixed. In an attempt to include elastic softening as temperature increased and volume decreased, Lawson et al.(1) made the unusual conjecture that the higher energy state with smaller volume, required to get the thermal expansion correct, must make no contribution to the bulk modulus. The usual situation is that a higher energy state with lower volume is stiffer, not softer as required here. Although the multiple-configuration assumption is now strongly supported by recent measurements (6), the yet-unexplained anomalous behavior (softening on warming with no volume change) remains a critical missing component of a fundamental understanding of δ-Pu.The detailed issue then is that the bulk and shear moduli of δ-Pu soften with temperature at a rate an order of magnitude greater than other metals with similar...

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Avoided valence transition in a plutonium superconductor

Cited by 50 publications

References 45 publications

Quantum critical scaling and superconductivity in heavy electron materials

Quantum critical scaling and superconductivity in heavy electron materials

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

Origin of the multiple configurations that drive the response of δ-plutonium’s elastic moduli to temperature

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