Thermodynamical Study on the Heavy-Fermion Superconductor PrOs₄Sb₁₂: Evidence for Field-Induced Phase Transition

Abstract

We report measurements of low-temperature specific heat on the 4f 2 -based heavy-fermion superconductor PrOs 4 Sb 12 . In magnetic fields above 4.5 T in the normal state, distinct anomalies are found which demonstrate the existence of a field-induced ordered phase (FIOP). The Pr nuclear specific heat indicates an enhancement of the 4f magnetic moment in the FIOP. Utilizing a Maxwell relation, we conclude that anomalous entropy, which is expected for a single-site quadrupole Kondo model, is not concealed below 0.16 K in zero field. We also discuss two possible interpretations of the Schottky-like anomaly at $3 K, i.e., a crystalline-field excitation or a hybridization gap formation.The f -electron-related heavy fermion (HF) systems exhibiting superconductivity had previously been found only in Ce and U intermetallic compounds. Therefore, the recent observation of the first Pr-based heavy fermion superconductivity (HFSC) in a filled skutterudite PrOs 4 Sb 12 1) has profound scientific significance. The 4f 2 configuration of Pr ions in intermetallic compounds had been considered to be quite stable in view of there being no observation of strongly correlated electron behaviors until recent studies on PrInAg 2 2) and PrFe 4 P 12 , 3-5) in which this picture breaks down. In PrFe 4 P 12 , which is also a member of the filled skutterudites, we have shown by specific heat, 3) electrical resistivity 4) and de Haas-van Alphen (dHvA) effect measurements 5) that HF behaviors appear in high fields where a nonmagnetic ordered state, probably of quadrupole origin, is suppressed. To our knowledge, PrFe 4 P 12 is the only system for which such definitive evidence for the 4f 2 -based Fermi-liquid HF ground state has been obtained.Compelling evidence for the HFSC in PrOs 4 Sb 12 was given by a large specific heat jump ÁC=T ¼ 0:5 J/K 2 mol at T c ¼ 1:85 K on a pellet of compressed powdered single crystals. 1) The jump is superimposed on a Schottky-like anomaly appearing at $3 K. Bauer et al. 1) attributed this peak to a doublet-triplet (À 3 -À 5 in O h -type notation) crystalline-electric-field (CEF) thermal excitation, taking into account their magnetic susceptibility ðTÞ and inelastic neutron scattering data. Since the À 3 non-Kramers doublet ground state has quadrupole degrees of freedom, they pointed out the possibility that the HF behavior is associated with a quadrupolar Kondo effect 6) on the Pr-ion lattice. In order to confirm this scenario, it is essential to clarify how the entropy R ln 2 associated with the À 3 ground state is released and whether any residual entropy is hidden far below T c or not.In this letter, we report two important findings in PrOs 4 Sb 12 , based on specific heat and magnetization measurements on high-quality single crystalline samples:(1) clear evidence for the existence of a field-induced ordered phase (FIOP) and (2) confirmation that no anomalous entropy is concealed below 0.16 K in zero field.Single crystals of the filled skutterudite PrOs 4 Sb 12 and the reference compound LaOs 4 Sb 12 were gro...

Superconductivity and the high-field ordered phase in the heavy-fermion compound PrOs4Sb12

“…4). The high field ordered phase has also been observed by means of large peaks in the specific heat [10,13] and thermal expansion [11] and kinks in magnetization vs magnetic field curves [12,26] in magnetic fields > 4.5 tesla and temperatures < 1.5 K.…”

“…This would be the electric analogue of the exchange interaction between the magnetic dipole moments of Ce or U ions and the conduction electron spins that is widely believed to be responsible for the heavy fermion state in most Ce and U heavy fermion compounds. In fact, such a mechanism was proposed by Cox in 1987 [1] to account for the non-Fermi liquid temperature dependences of certain normal state physical properties of the heavy electron superconductor UBe 13 . The Pr compounds that display heavy fermion behavior include PrInAg 2 [2], PrFe 4 P 12 [3], and, possibly, PrFe 4 Sb 12 [4].…”

“…The superconducting state appears to be unconventional in nature and may consist of two distinct superconducting phases [7,8]. An ordered phase, presumably of magnetic or quadrupolar origin, occurs at high fields > 4.5 tesla and low temperatures < 1.5 K [7,9,10,11,12,13], suggesting that the superconductivity may occur in the vicinity of a magnetic or quadrupolar quantum critical point (QCP). In an effort to obtain information about the interactions that are responsible for the heavy fermion state and superconductivity in PrOs 4 Sb 12 , we have performed measurements of various normal and superconducting state properties of this compound as a function of temperature, pressure, and magnetic field [5,6,7,9].…”

Superconducting phase diagram of the filled skuterruditePrOs4Sb12

“…PrOs 4 Sb 12 is cubic with T h point group symmetry 2 , and has a nonmagnetic ground state, which in a single ion scheme can be either a Γ 23 doublet or a Γ 1 singlet, a question which remains a matter of controversy. Presently, most measurements in high field seem to favor a singlet ground state 3,4,5,6 . In any case, whatever the degeneracy of this ground state, the first excited state (at around 6K) is low enough to allow for an induced electric quadrupolar moment on the Pr 3+ ions 7,8 , that could explain the heavy fermion properties of this system by a quadrupolar Kondo effect 1 .…”

NMR Observation of Ferro-Quadrupole Order in PrTi2Al20

“…[1][2][3][4] In particular, non-Kramers rare-earth ions with an even number of f electrons in a cubic environment have been attracting strong interest recently. [5][6][7][8][9][10][11] In these systems, it is possible that the ground states of the J multiplet split by the cubic crystalline electric field (CEF) are nonmagnetic, i.e., the matrix elements of the magnetic dipole J are identically zero, yet have a degeneracy that allows active higher-order multipoles such as electric quadrupoles or magnetic octupoles. Then various interesting phenomena such as multipole orders [12][13][14] or multichannel Kondo effects 15,16) are expected at low temperatures.…”