YbRh2Si2: Pronounced Non-Fermi-Liquid Effects above a Low-Lying Magnetic Phase Transition

O. Trovarelli, C. Geibel, S. Mederle, C. Langhammer, F. M. Grosche, P. Gegenwart, M. Lang, G. Sparn, F. Steglich
2000 Physical Review Letters  
We report the first observation of non-Fermi-liquid (NFL) effects in a clean Yb compound at ambient pressure and zero magnetic field. The electrical resistivity and the specific-heat coefficient of high-quality single crystals of YbRh 2 Si 2 present a linear and a logarithmic temperature dependence, respectively, in more than a decade in temperature. We ascribe this NFL behavior to the presence of (presumably) quasi-2D antiferromagnetic spin fluctuations related to a very weak magnetic phase
more » ... k magnetic phase transition at T N Ӎ 65 mK. Application of hydrostatic pressure induces anomalies in the electrical resistivity, indicating the stabilization of magnetic order. An increasing number of f-electron systems, mostly Cebased heavy fermion (HF) metals, show pronounced deviations from the properties of a conventional Landau Fermi liquid (LFL) when they are tuned through an antiferromagnetic (AF) quantum critical point (QCP) by varying a control parameter such as doping, pressure, or magnetic field [1] [2] [3] . It is generally assumed that, close to the value of the control parameter at which T N ! 0, the abundance of low-lying and extended spin fluctuations mediating the interactions between the quasiparticles gives rise to strongly temperature-dependent quasiparticle masses and scattering cross sections [4] [5] [6] . This is manifested, for instance, in the electrical resistivity Dr r 2 r 0 (r 0 , residual resistivity) and in the 4f-derived increment to the specific heat DC, which do not behave as expected for a LFL (Dr~T 2 , DC͞T const), but instead follow "non-Fermi-liquid" (NFL) temperature dependencies such as power laws (Dr~T´, with 1 #´, 2) or logarithmic divergencies ͑DC͞T~2 lnT ͒ over a substantial temperature range, in a few cases as large as one decade in T . The observation of quantum-critical phenomena is, in principle, expected to be present in the paramagnetic regime not only at but also on both sides of the QCP provided the temperature is sufficiently close to absolute zero [4] . This has been observed in alloys such as CeCu 62x R x (R Au, Ag) [1,3], in which the heavy LFL at x 0 is driven to a magnetically ordered state via doping, or in AF-ordered HF compounds such as CeIn 3 [2] or CePd 2 Si 2 [2,7], in which magnetic order is suppressed by applying hydrostatic pressure p [8]. So far, the observation of NFL effects in undoped compounds at ambient pressure is restricted to only a few prototypical HF metals, e.g., normal-state UBe 13 [9], CeNi 2 Ge 2 [10,11], and CeCu 2 Si 2 [12]. So far, the existence of an AF-QCP could be established only in the case of CeCu 2 Si 2 , either via choosing slight off-stoichiometry or via applying a tiny hydrostatic pressure [12] . In comparison to Ce-and U-based materials, quantumcritical phenomena in Yb compounds have been rather less investigated. The hole-electron analogy between the 4f 13 -Yb 31 and the 4f 1 -Ce 31 electronic configurations offers an interesting alternative way of studying the physics close to a QCP: contrary to the Ce case, in Yb-based compounds the exchange interaction between the local 4f moments and the conduction electrons decreases upon increasing pressure. It is, therefore, possible to drive a nonmagnetic Yb system into a magnetically ordered state under pressure and to follow the evolution of magnetism in the vicinity of the QCP. Pressure-induced onset of magnetism has indeed been observed in the nonmagnetic compounds Yb 2 Ni 2 Al [13], YbCuAl [14] , and YbCu 2 Si 2 [15]. Since for these materials the critical pressure p c necessary to induce magnetism is as large as or even larger than 8 GPa, the thermodynamic measurements necessarily required for a study of quantum-critical behavior [1, 3, 9, 11, 12] are inaccessible at present. Thus no conclusive evidence of NFL behavior has yet been reported in any stoichiometric Yb-based compound close to p c . Recent doping experiments on YbCu 52x Al x were successful in "tuning" a valence change of Yb towards 31, accompanied by the occurrence of AF order, and NFL effects were noticed close to a critical concentration x c ഠ 1.5 [16] . However, disorder may strongly influence or even produce [17] the observed NFL phenomena. It is fair to state that stoichiometric Yb-based HF compounds are lacking, which could be taken as suitable "starting systems" to explore the physics near a magnetic instability. This is mainly due to difficulties in the sample preparation owing to the high vapor pressure of Yb [18] . In this Letter we report the first observation of NFL effects at ambient pressure in high-quality single crystals of an undoped Yb-based HF compound: YbRh 2 Si 2 . Below T , 10 K, r͑T ͒ shows an almost linear temperature dependence, whereas DC͑T ͒͞T increases logarithmically upon cooling over more than a decade in temperature. These NFL effects are in fact related to the proximity of a magnetic instability: evidence for AF order at p 0 is observed at the extremely low ordering temperature of Ӎ65 mK. Using hydrostatic pressure as a control 626 0031-9007͞00͞85(3)͞626(4)$15.00
doi:10.1103/physrevlett.85.626 pmid:10991356 fatcat:jsw5646qtrc3flwkunyfjvpqt4