Novel Phase Transition Near the Quantum Critical Point in the Filled-Skutterudite Compound CeOs₄Sb₁₂: An Sb-NQR Study

Abstract

We report on a novel phase transition at T = 0.9 K in the Ce-based filled-skutterudite compound CeOs4Sb12 via measurements of the nuclear-spin lattice relaxation rate 1/T1 and nuclear quadrupole resonance (NQR) spectrum of Sb nuclei. The temperature (T ) dependence of 1/T1 behaves as if approaching closely an antiferromagnetic (AFM) quantum critical point (QCP), following the relation 1/T1 ∝ T /(T − TN) 1/2 with TN = 0.06 K in the range of T = 1.3 − 25 K. The onset of either the spin-density-wave (SDW) or charge-density-wave (CDW) order at T0 = 0.9 K, that is, of the first order, is evidenced by a broadening of the NQR spectrum and a marked reduction in 1/T1 just below T0. The f -electron-derived correlated band realized in CeOs4Sb12 is demonstrated to give rise to the novel phase transition on the verge of AFM QCP.KEYWORDS: filled skutterudite, Kondo semiconductor, quantum critical point, SDW/CDW, NQR, CeOs4Sb12Filled-skutterudite compounds ReT 4 Pn 12 (Re = rare earth; T = Fe, Ru or Os; Pn = P, As or Sb) have attracted much attention because of their rich physical phenomena that are not yet fully understood. For instance, PrFe 4 P 12 shows a heavy fermion (HF)-like behavior with a large mass of m * ∼ 70 m e under a magnetic field (H) 1 and undergoes an anomalous transition at a temperature T = 6.4 K at H = 0, indicative of a quadrupolar ordering. [2][3][4] Note that the large C/T value for this compound is due not only to low-energy degrees of freedom such as either magnetic or quadrupolar fluctuations, but also to the Schottky anomaly originating from some low-lying CEF splitting. PrOs 4 Sb 12 is the first Pr-based HF superconductor to reveal the large jump in the specific heat at T c = 1.85 K, the slope of the upper critical field near T c , and the electronic specific heat coefficient γ ∼ 350 − 500 mJ/K 2 mol in the normal state. 5 The CEF-energy scheme of PrOs 4 Sb 12 has recently been determined to be the singlet Γ 1 ground state accompanying the very low lying excited state Γ (2) 4 , 6, 7 and it was suggested that the HF-like behavior exhibited by PrOs 4 Sb 12 may be relevant to a very close energy level between Γ 1 and Γ 4 . Thus, in the Pr-based compound, the 4f 2 -derived CEF effect plays an important role in their rich physical phenomena.In contrast, most Ce-based filled-skutterudite compounds show semiconducting behavior, on the basis of which they are called hybridization-gap semiconductors. A series of CeT 4 P 12 (T = Fe, Ru or Os) compounds have a hybridization gap of 400∼1500 K, and *

Spin Susceptibility of CeOs4Sb12Probed by121Sb-NMR

“…Thus, the T dependence of 1/T 1 above ∼60 K can be interpreted by using the c-f hybridization gap model with a rectangular shape of the DOS with parameters 2D = 3000 K and 2∆ = 320 K as shown in Fig. 4(a) [4]. For 1/T 1 , at temperatures above ∼60 K there is no apparent magnetic field dependence; therefore, the parameters do not depend on a magnetic field [6].…”

“…The NMR spectrum was obtained by integrating a spin-echo signal point by point as a function of the magnetic field. In the filled skutterudite compound, the electric field gradient (EFG) at the Sb site is quite large and non-axially symmetric; therefore, the nuclear quadrupole frequency increases [4]. Thus, a broad, complex NMR spectrum would be expected for a powdered sample, as shown in Fig.…”

“…Filled skutterudite compounds ReTx 4 Pn 12 (Re = rare earth; Tx = Fe, Ru, and Os; Pn = P, As, and Sb) shows various ground states in many combinations of Re, Tx and Pn atoms. Among filledskutterudite compounds, most Ce-based compounds show insulating or semiconducting behavior and are commonly referred to as hybridization-gap semiconductors.…”

“…The magnitude of the gap is related to the lattice constant of the crystal, which collapses for Sb-based compounds [1]. However, CeOs 4 Sb 12 , which has the largest lattice constant among the CeTx 4 Pn 12 compounds, exhibits Kondo insulating behavior in electric resistivity with a very small gap ∆/k B ∼ 10 K [2]. A large specific heat coefficient γ ∼ 92 mJ/(K 2 mol) was also reported [3].…”

“…We previously reported that CeOs 4 Sb 12 has a hybridization gap of 2∆/k B ∼ 320 K, although a large residual density of states (DOS) exists within the gap, as determined from measurement of the nuclear spin-lattice relaxation rate 1/T 1 [4]. Furthermore, the observed 1/T 1 showed an increase in spin fluctuation on the verge of an antiferromagnetic (AFM) quantum critical point below 20 K. Moreover, CeOs 4 Sb 12 has been reported to exhibit AFM order at approximately T ∼ 1 K and H = 0 [5]. A discriminative H − T phase diagram was obtained from resistivity, specific heat and 121 Sb-NMR 1/T 1 T measurements [1,3,6].…”