New Yb-based systems: From an intermediate-valent to a magnetically ordered state

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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 < x < 0.3, using a flux technique in closed Ta crucibles and investigated and analysed their resistivity, specific heat and susceptibility. From the analysis of the temperature dependence of the 4f entropy, we deduce a monotonous increase of the Kondo temperature, TK, from TK = 17 K in YbRh2Si2 to TK = 50 K in Yb0.7La0.3Rh2Si2, as expected from the chemical negative pressure due to the lattice expansion. Accordingly we did not observe any evidence for the magnetic order in the x = 0.05 sample down to 17 mK. Instead, C/T of the x = 0.05 and x = 0.1 samples increases logarithmically down to 50 mK, while in the samples with x ≥ 0.15, C/T levels off at a temperature-independent value at low T. In addition the temperature dependence of the resistivity at low temperatures shows a power law with an exponent increasing with x from n = 1 in pure YbRh2Si2 (above TN) to n = 2 as expected for a Fermi liquid for x ≥ 0.15. Thus, the increase of the Kondo scale with increasing La-content suppress the magnetic order at a critical concentration of around xcr = 0.05. As a result, the NFL behaviour extends down to the lowest investigated temperature for 0.05 ≤ x ≤ 0.10, and is then replaced by a heavy Fermi-liquid state for x ≥ 0.15.
For the determination of the Kondo temperature and of the excitation energies of the crystal field levels in YbRh2Si2, the contribution C4f of the 4f electrons to the specific heat was estimated and analysed. The presence of a local maximum in C4f between 40 and 80 K was confirmed. We analysed C4f more precisely using the combination of a simple CEF model and the theoretical prediction for the single-ion Kondo model. This analysis places the first excited CEF level at an energy (15 ± 2) meV, in good agreement with the neutron data, and therefore allows to settle the CEF energy scheme.
Very surprising is the Yb ESR signal observed in YbRh2Si2, which was the first case where the ESR-line of the Kondo ion has been seen in a dense Kondo system. In the La-doped samples, the ESR line width scales with the residual resistivity, suggesting that the bottle neck effect plays an important role for the observability of the ESR line.
Attempts to prepare the pure isoelectronic compound YbRh2Ge2 failed, instead single crystals of Yb4Rh7Ge6 were obtained and studied. The results evidence a stable Yb3+state, which orders presumably antiferromagnetically at 0.49 K.