New Yb-based systems: From an intermediate-valent to a magnetically ordered state
Author | : Julia Ferstl |
Publisher | : Cuvillier Verlag |
Total Pages | : 208 |
Release | : 2007-09-25 |
ISBN-10 | : 9783736923751 |
ISBN-13 | : 3736923759 |
Rating | : 4/5 (51 Downloads) |
Book excerpt: Heavy-fermion systems belong to the group of strongly correlated electron systems and have attracted considerable attention because of their unusual physical behaviour at low temperatures. Only a few Yb-based heavy-fermion compounds were discovered and investigated in the past twenty years in comparison to Ce-based ones, because of the difficulty in synthesising Yb-compounds due to the high vapour pressure of Yb. In this thesis, we present the first thorough investigation of the compound YbFe2Ge2, and a study of the physical properties upon crossing the quantum critical point in the heavy-fermion system Yb1-xLaxRh2Si2. We grew polycrystals and single crystals of YbFe2Ge2 and of its reference compound LuFe2Ge2 and investigated their physical properties. Surprisingly, our results evidence in both compounds the presence of a paramagnetic Fe moment with μeff ~ 3 μB/Fe at high temperatures, in contrast to the old belief of a non-magnetic transition metal state for all RT2X2-compounds with T = Fe, Co, Ni and X = Si, Ge. Anomalies in the susceptibility and in the specific heat suggest AF-ordering of these Fe moments at TN = 9 K in LuFe2Ge2. No evidence for magnetic order was found in YbFe2Ge2 , instead the magnetic properties at low temperatures are dominated by the effect of an intermediate-valent Yb state, at the border to the Kondo regime with a rather low characteristic energy of the order of TK = 80 K. Both compounds are Fermi-liquid systems at low temperatures with an effective mass slightly enhanced by spin fluctuations in LuFe2Ge2 and more strongly enhanced in YbFe2Ge2 due to the intermediate-valent Yb state, at the border to the Kondo regime. The heavy-fermion system YbRh2Si2, has attracted strong attention because it is located very close to a quantum critical point (QCP) connected with the transition from a magnetically ordered ground state to a non-magnetic one. At ambient pressure it orders antiferromagnetically at a very low temperature, TN = 70 mK. Upon applying a small magnetic field (60 mT) or a slight negative chemical pressure using Ge doping, TN disappears at a QCP where the effective mass of the quasiparticles diverges. The aim of this work was to reach and to cross the quantum critical point by using negative chemical pressure in La-doped YbRh2Si2. We succeeded with the single-crystal growth of Yb1-xLaxRh2Si2 with 0