Simultaneous superconducting and antiferroquadrupolar transitions in PrRh2Zn20

Abstract

Superconducting and antiferroquadrupolar (AFQ) transitions in a Pr-based compoundPrRh 2 Zn 20 have been found to occur simultaneously at T c =T Q =0.06 K. The superconducting transition manifests itself by zero resistance and large diamagnetic susceptibility. The specific heat exhibits a Schottky anomaly peaking at 14 K and magnetization curves measured at 2 K show anisotropic behaviors. The analysis of these data indicates that the crystalline electric field (CEF) ground state of the trivalent Pr ion is the non-Kramers Γ 3 doublet with the quadrupolar degrees of freedom. A sharp peak in the specific heat at 0.06 K has been attributed not to the superconducting transition but to the AFQ transition because the ordering temperature T Q decreases in B || [100] but increases in B|| [110] and B || [111] with increasing B up to 6 T. This anisotropic behavior of T Q (B) can be well explained by a two-sublattice mean-field calculation, which corroborates the AFQ ordered state below T Q . The entropy release at T Q is only 10% of Rln2 expected for the Γ 3 doublet, suggesting possible interplay between the quadrupolar degrees of freedom and the superconductivity.

Suppression of Superconductivity in PrRh2Zn20by the Substitution of Co for Rh

“…Figure 4 shows the temperature dependence of the electrical resistivity ρ(T) divided by the values at 0.2 K of Pr(Rh 1-x Co x ) 2 Zn 20 for x ≤ 0.10. For x = 0, ρ(T) drops to zero at T c = 0.06 K as was reported previously [9]. With increasing x, T c decreases gradually and goes below 0.041 K for x ≥ 0.10.…”

“…It is noted that more relevant compound LaRh 2 Zn 20 does not form by the melting and annealing. There are broad peaks in C m (T) at around 13 K, which can be reasonably reproduced as a Schottky peak by a doublet-triplet two-levels model with an energy split of 32 K. For x = 0, a broad peak appears at around 0.25 K, probably resulting from the two-channel Kondo effect due to the quadrupolar degrees of freedom [9]. On the other hand, for 0.05 ≤ x ≤ 0.66, another broad peak manifests itself below 3 K. By increasing x from 0.05, the peak shifts to higher temperatures and the peak temperature becomes the highest at x = 0.47.…”

“…In fact, doublet ground states of Pr 3+ in PrT 2 Zn 20 (T = Rh and Ir) have been confirmed by ultrasonic and inelastic neutron scattering measurements [7,8]. The compounds with T = Rh and Ir undergo AFQ orders at T Q = 0.06 and 0.11 K and superconducting transitions at T c = 0.06 and 0.05 K, respectively [6,9]. The magnetic entropy at T Q estimated from the magnetic specific heat are only 10% and 20% of Rln2 for T = Rh and Ir, respectively.…”

“…In PrRh 2 Zn 20 , C(T) exhibits a Schottky anomaly peaking at 14 K, and magnetization curves measured at 1.8 K show anisotropic behaviors. The analysis of these data indicates a nonmagnetic doublet ground state of the Pr 3+ ion [9,10]. Furthermore, in inelastic neutron scattering experiments, magnetic excitations from the doublet ground state were observed [8].…”

“…Therefore, it is suggested that the quadrupole fluctuations play a role in formation of the superconducting Cooper pairs. No structural transition was found in PrIr 2 Zn 20 up to 600 K. For PrRh 2 Zn 20 , on the other hand, a first-order structural transition has been suggested by a hysteretic behavior in the electrical resistivity between 140 and 470 K [9,10,11]. In PrRh 2 Zn 20 , C(T) exhibits a Schottky anomaly peaking at 14 K, and magnetization curves measured at 1.8 K show anisotropic behaviors.…”