Enhancement of Magnetic Fluctuations on Passing belowTcin the Heavy Fermion SuperconductorUPd2

Abstract

The heavy fermion superconductor UPd 2 Al 3 exhibits the unusual combination of an antiferromagnetic phase transition, at T N 14.3 K, followed by a superconducting phase transition below 2 K without destruction of the ordered magnetic moment. Polarized inelastic neutron scattering reveals the presence of two coupled modes, both transverse to the sublattice magnetization. On passing into the superconducting phase an abrupt change is observed in the magnetic inelastic response. We show that it is reasonable to consider the superconducting state as arising out of interactions between quasiparticles which are strongly renormalized by the low-frequency exchange field. [S0031-9007 (98)07531-0] PACS numbers: 74.70.Tx, 78.70.NxThe discovery of the heavy fermion superconductor UPd 2 Al 3 [1] which exhibits both an antiferromagnetic phase transition, T N 14.3 K, and a superconducting phase transition below T c 2 K has aroused great interest. The compound crystallizes in the hexagonal PrNi 2 Al 3 structure (space group P6͞mmm) with lattice constants a 5.350 Å and c 4.185 Å at room temperature. Neutron and x-ray scattering measurements have revealed unusually large low temperature ordered moments of 0.85m B which are coupled in ferromagnetic sheets in the basal plane; these ferromagnetic planes are then stacked along the c axis with a wave vector Q 0 ͑0 0 0.5͒ [2-4]. The measured discontinuity in the heat capacity at T c is large, DC 1.2gT c ͑g 140 mJ͞mole K 2 ͒ [1,5] and suggests that the superconducting ground state evolves out of interactions between quasiparticles located in strongly renormalized states in a low energy shell around the Fermi surface.In the hope of finding a connection between antiferromagnetism and superconductivity there have been many investigations in this compound since its discovery. Changes in antiferromagnetic Bragg peak intensities on passing through T c were reported [6]; however, these results are questioned by independent measurements using both neutron diffraction and resonant magnetic x-ray scattering [3,7]. Pioneering inelastic neutron scattering experiments under relatively coarse energy resolution [8] revealed no changes in the spectrum around the antiferromagnetic zone center on cooling through T c . An additional low energy component to the spectral response was first reported by Sato et al. [9], and Metoki et al. [10] have reported a gap opening below T c and the disappearance of the low energy response when a magnetic field is applied.In this Letter we report on new, high resolution, inelastic neutron scattering experiments carried out on the IN14 spectrometer at the Institut Laue-Langevin, Grenoble. With full neutron polarization analysis we have been able to identify the transverse nature of the dominant fluctuations at all temperatures below T N and the opening of a gap in the energy spectrum of magnetic fluctuations at the lowest temperatures (150 mK) in the superconducting state. The spectra consist of a damped spin wave coupled to a low energy mode. In the normal state we develo...

Evidence for Weak Itinerant Long-Range Magnetic Correlations inUGe2

“…These apparently conflicting results can be understood if the 5f electrons are viewed as two electron subsets. This picture has already been argued for UCu 5 [23] and UPd 2 Al 3 [24][25][26][27][28]. However, for UGe 2 the signatures of both subsets are found at a single temperature, the Curie temperature, whereas for UCu 5 and UPd 2 Al 3 the temperatures at which the two subsets are detected are far apart.…”

Magnétisme de bande et Supraconductivité : l'apport de la diffusion inélastique des neutrons

“…La seconde est par exemple utilisée pour interpréter la présence d'un pic de résonance magnétique dans le composé UPd 2 Al 3 [31][32][33]. Ce composé à fermions lourds est caractérisé par la présence de deux types d'électrons f : les uns itinérants et les autres localisés induisant la réponse magnétique localisée.…”

“…Des calculs similaires ont été réalisés dans le composé UPd 2 Al 3 [47], reliant l'observation d'un pic de résonance magnétique mesuré par diffusion de neutrons [31] aux anomalies dans ( ) mesurés par effet tunnel [46]. Dans le plomb, qui est un supraconducteur conventionnel, les anomalies dans ( ) sont directement reliés à la structure de la densité d'états de phonons mesuré par diffusion inélastique des neutrons [48].…”

Novel Coexistence of Superconductivity with Two Distinct Magnetic Orders

“…In cases where superconductivity and magnetic order coexist, such as rareearth based molybdenum chalcogenides, rhodium borides and borocarbides, the superconducting d electrons and localized f electrons are weakly coupled, and the competition between superconductivity and magnetic order is understood [2,3] in terms of the theory of Abrikosov and Gorkov [1]. In contrast, strong magnetic fluctuations have been observed in heavy fermion, cuprate and ruthenate superconductors [4,5,6,7], and magnetic excitations have been proposed to mediate the Cooper pairing in these unconventional superconductors [8,9,10,11,12,13] Recently a new family of Ce-based heavy fermion materials has been discovered, which sets a record superconducting transition temperature of T C =2.3 K for heavy fermion materials [24,25,26]. Coexistence of magnetic order and superconductivity is observed in a wide composition range in CeRh 1−x Ir x In 5 [27] [see Fig.…”