Quasiquartet crystal-electric-field ground state with possible quadrupolar ordering in the tetragonal compoundYbRu2Ge2

Abstract

We have investigated the magnetic properties of YbRu 2 Ge 2 by means of magnetic susceptibility χ(T), specific heat C(T) and electrical resistivity ρ(T) measurements performed on flux grown single crystals. The Curie-Weiss behavior of χ(T) along the easy plane, the large magnetic entropy at low temperatures and the weak Kondo like increase in ρ(T) proves a stable trivalent Yb state.Anomalies in C(T), ρ(T) and χ(T) at T 0 = 10.2 K, T 1 = 6.5 K and T 2 = 5.7 K evidence complex ordering phenomena, T 0 being larger than the highest Yb magnetic ordering temperature found up to now. The magnetic entropy just above T 0 amounts to almost Rln4, indicating that the crystal electric field (CEF) ground state is a quasiquartet instead of the expected doublet. The behavior at T 0 is rather unusual and suggest that this transition is related to quadrupolar ordering, being a consequence of the CEF quasiquartet ground state. The combination of a quasiquartet CEF ground state, a high ordering temperature, and the relevance of quadrupolar interactions makes YbRu 2 Ge 2 a rather unique system among Yb based compounds.

Pressure Dependence of the Magnetic and Quadrupolar Ordering in YbRu2Ge2

“…Above H = 70 kOe for H a T 0 shifts strongly to lower temperatures, from 10.8 K at ambient pressure to 8.8 K at 140 kOe, while for H c T 0 cannot be detected unambiguously above 70 kOe. In contrast to the reported behavior at atmospheric pressure T 0 (H ) decreases for both field orientations [1]. The field dependence of T 0 for H c is in reasonable agreement with the p = 0 data.…”

“…Above H = 70 kOe no clear feature is visible in the data anymore. To determine T 0 , T 1 , and T 2 we used the derivative (∂ρ(T )/∂T ) H , which is in good correspondence to the specific-heat data at atmospheric pressure [1]. The kinks at T 1 and T 2 stay clearly visible up to the highest field of our experiment (H = 140 kOe).…”

“…Open symbols correspond to data obtained in a piston-cylinder cell, solid symbols to data from a Bridgman-type cell at T 0 = 10.2 K, T 1 = 6.5 K, and T 2 = 5.7 K evidence complex ordering phenomena [1]. The entropy around T 0 obtained from integrating the specific heat data is very close to R ln 4, which corresponds to a quasiquartet crystal-field ground-state.…”

“…This is rather unique among the YbT 2 X 2 compounds. The anomaly at T 0 is discussed to be associated to a quadrupolar ordering, while the anomaly at T 1 is related to an antiferromagnetic ordering and the anomaly at T 2 likely to a change of the magnetic structure [1,2].…”

Incommensurate short-range multipolar order parameter of phase II in Ce3Pd20Si6

“…Such hidden-order phases have been observed in a variety of compounds containing 4 f and 5 f elements, for example, URu 2 Si 2 [1,2], NpO 2 [3], skutterudites [4,5], YbRu 2 Ge 2 [6], or CeB 6 [7][8][9]. It is often assumed that the multipolar moments of the f electrons in their specific crystal-field environment play a decisive role in the formation of these phases [10][11][12].…”