Hindered Quadrupole Order in PrMgNi₄ with a Nonmagnetic Doublet Ground State

Abstract

Structural, transport and magnetic properties of single-crystalline samples of a praseodymium-based cubic compound PrMgNi 4 were studied. The single-crystal X-ray structural analysis revealed that Mg atoms are substituted for the Pr atoms at the 4a site by 4.5%. The χ(T ) data follow the Curie-Weiss law with an effective moment for the Pr 3+ ion. The magnetic specific heat divided by temperature, C m /T , shows a broad maximum at around 3 K, which is reproduced by a two-level model with a ground state doublet. On cooling below 1 K, C m /T approaches a constant value, which behavior is reproduced by a random two-level model. The twofold degeneracy of the ground state is lifted by symmetry lowering due to the substituted Mg atoms for the Pr atoms or strong hybridizations between the 4 f 2 electron states and conduction bands, which hinders the long-range quadrupole order.Praseodymium-based cubic compounds with a 4 f 2 configuration have attracted much attention because a variety of phenomena manifest themselves at low temperatures. Pr-filled skutterudites exhibit correlated electronic phenomena due to strong hybridization between the 4 f electron and conduction electrons (cf hybridization), e.g., heavy-fermion superconductivity, a metal-insulator transition, and higher-order multipolar ordering.[1-4] These phenomena are attributed to the interplay between the conduction electrons and multipolar degrees of freedom in quasi-degenerated ground states of the Pr ions under the crystalline electric field (CEF).In case the CEF ground state is the non-Kramers doublet of the Pr ion under cubic symmetry, the magnetic dipole is quenched but electric quadrupole and magnetic octupole become active. Thereby, the multipoles in the non-Kramers doublet could govern the low-temperature properties. A typical system is PrPb 3 , which exhibits an antiferroquadrupole (AFQ) order at T Q = 0.4 K [5][6][7]. Below T Q , the quadrupoles are modulated sinusoidally by the correlation between the quadrupoles and the conduction electrons [8,9]. Another example is PrInNi 4 , which presents a ferromagnetic transition at T C = 0.75 K.[10-12] The nonmagnetic ground state doublet could mix with the excited magnetic triplet by the ferromagnetic exchange interaction inducing a magnetic moment in the ground state which orders ferromagnetically.On the contrary, PrAg 2 In and PrMg 3 exhibit no quadrupole order. [13][14][15] The absence of the quadrupole order was discussed by considering the Kondo effect due to strong c-f hybridization, because the specific heat divided by temperature, C/T , is largely enhanced at low temperatures. However, the thermoelectric power is not enhanced and the absolute value is comparable to that of the La counterpart, indicating the conduction electrons at the Fermi level could not correlate strongly with the 4 f 2 electrons. [16,17] Moreover, in the specific heat measurements using a high-quality single crystal of PrAg 2 In with the residual resistivity ratio of RRR = 14, a cusp-type anomaly manifests itself at 0.33 K, indic...

Effects of Magnetic Field on the Nonmagnetic Doublet Ground State in PrMgNi4

“…Figures 1(a) and 1(b) show the temperature dependences of C m (T ) of PrMgNi 4 in magnetic fields of B ≤ 14 T applied along the [100] and [110] directions, respectively. A broad maximum appears in C m (T ) at around 5 K in B = 0, which can be reproduced by a two-level model with the ground state doublet and an excited-triplet separated by 12 K [16]. Thereby, the CEF ground state is the Γ 3 doublet.…”

“…Thereby, the CEF ground state is the Γ 3 doublet. The solid curve is a calculation using CEF parameters of W = −2 K and x = 0.77 [17], where the nine-fold J multiplet is split into Γ 3 doublet (0), Γ 4 triplet (12.2 K), Γ 1 singlet (29.3 K), and Γ 5 triplet (84.8 K) [16].…”

“…Single crystals of PrMgNi 4 and LaMgNi 4 were grown by the Mg self-flux method [16]. The samples were characterized by the electron-probe microanalysis (EPMA) and the powder x-ray diffraction technique.…”

“…No impurity phase was detected in either backscattered electron images nor the powder x-ray diffraction patterns. The single-crystal x-ray structural analysis revealed that 4.5% of the Pr atoms at the 4a site are replaced by Mg atoms [16].…”

“…Measurements of the specific heat and the magnetization of single-crystalline samples revealed that the CEF ground state is the non-Kramers Γ 3 doublet [16]. On cooling, however, the magnetic specific heat divided by temperature, C m /T , shows no anomaly due to a phase transition down to 0.1 K but approaches a constant value for T < 0.4 K. This result was moderately explained by using a random two-level (RTL) model with an energy width of 4.5 K. Thereby, the twofold degeneracy of the nonmagnetic ground state could be lifted by the symmetry lowering due to atomic disorder as detected by the x-ray structural refinement and/or strong c-f hybridization between the 4 f 2 electron state and conduction bands [16].…”