A. P. Pikul

Violation of Critical Universality at the Antiferromagnetic Phase Transition of YbRh2Si2

We report on precise low-temperature specific-heat measurements, C(T), of YbRh2Si2 in the vicinity of the antiferromagnetic phase transition on a single crystal of superior quality (residual resistivity ratio of approximately 150). We observe a very sharp peak at T_{N}=72 mK with absolute values as high as C/T=8 J/mol K2. A detailed analysis of the critical exponent alpha around T_{N} reveals alpha=0.38 which differs significantly from those of the conventional universality classes in the Ginzburg-Landau theory, where alpha< or =0.11. Thermal-expansion measurements corroborate this large positive critical exponent. These results provide insight into the nature of the critical magnetic fluctuations at a temperature-driven phase transition close to a quantum critical point.

Magnetic properties and electronic structures of intermediate valence systems CeRhSi2 and Ce2Rh3Si5

The crystal structures and the physical (magnetic, electrical transport and thermodynamic) properties of the ternary compounds CeRhSi(2) and Ce(2)Rh(3)Si(5) (orthorhombic CeNiSi(2)- and U(2)Co(3)Si(5)-type structures, respectively) were studied over wide ranges of temperature and magnetic field strength. The results revealed that both materials are valence fluctuating systems, in line with previous literature reports. Direct evidence for valence fluctuations was obtained by means of Ce L(III)-edge x-ray absorption spectroscopy and Ce 3d core-level x-ray photoelectron spectroscopy. The experimental data were confronted with the results of ab initio calculations of the electronic band structures in both compounds.

Non-Fermi-liquid behaviour close to the disappearance of ferromagnetism in CePd1?xRhx

The orthorhombic CePd1?xRhx system exhibits a continuous evolution from ferromagnetic order in CePd (TC = 6.6?K) to an intermediate-valence ground state in CeRh. Here we report low-temperature (T?0.08?K) specific-heat measurements for concentrations 0.80?x?0.95 close to the disappearance of magnetic order (~0.87). In contrast to x = 0.8, still demonstrating a smeared phase transition signature at 0.27?K, non-Fermi-liquid behaviour is observed in the electronic specific-heat coefficient at x = 0.85. For higher Rh concentrations a weak power-law dependence with ??0.4 is found. Upon applying magnetic fields Fermi-liquid-like behaviour ?C(T)/T~const is recovered in all the different samples. The magnetic entropy as well as the concentration of unscreened magnetic moments at 2?K indicate a broad distribution of local TK values ranging from well below 2?K to far above 50?K.

Single-crystal study of highly anisotropic CeNiGe2

High quality single crystals of CeNiGe2 have been investigated by means of magnetic susceptibility, magnetization, electrical resistivity, magnetoresistivity and thermoelectric power measurements, carried out along all three principal crystallographic directions. The compound is an antiferromagnetic Kondo system that orders magnetically at TN = 3.9 K and undergoes a spin structure rearrangement at T1 = 3.2 K. The magnetic behaviour is strongly anisotropic with the easy magnetic direction parallel to the crystallographic a-axis. The Kondo temperature and the total crystal field splitting are of the order of 20 and 100 K, respectively.

Low-temperature thermodynamic properties of the heavy-fermion compound YbAgGe close to the field-induced quantum critical point

We present temperature and field dependent heat capacity and magnetization data taken at temperatures down to 50 mK and in an applied magnetic field up to 11.5 Tesla for YbAgGe, a heavy-fermion compound with a field induced quantum critical point. These data clearly indicate that the same electronic degrees of freedom are responsible for the features seen in both specific heat and magnetization data. In addition, they further refine the different boundaries suggested for the H - T phase diagram of YbAgGe through previous, magneto-transport measurements, and allow for further understanding of different phases on the H - T phase diagram, in particular, clearly disconnecting the field-induced quantum critical point in YbAgGe from any sort of saturation of the Yb moment in higher applied magnetic field.

Possible field-induced quantum criticality in Ce3Pd20Si6

Ce3Pd20Si6 forms in a cubic crystal structure with the cerium atoms located on two different sites, both of cubic point symmetry, resulting in two interleaved cubic cerium sublattices. We present here the results of detailed low-temperature specific heat and electrical resistivity measurements taken in magnetic fields up to 14 T, with the purpose of shedding light on the complex low-temperature behaviour that was alluded to in previous literature reports on this compound. Our specific heat data show two phase transitions, at TL(B = 0) = 0.31 K and at TU(B = 0) = 0.5 K respectively, probably of magnetic origin, and with distinctly different response to applied magnetic fields. TL is shifted towards lower temperatures and becomes indiscernable in fields above 1 T. TU on the other hand is shifted upward and disappears in fields above 8 T. The electrical resistivity data follow a Δρ ∝ T5/3 power-law behaviour below TU for a limited range in field, suggestive of spin fluctuations near a three-dimensional ferromagnetic quantum phase transition.

Single-ion Kondo Scaling of the Coherent Fermi Liquid Regime in Ce1 xLaxNi2Ge2

Thermodynamic and transport properties of the La-diluted Kondo lattice CeNi(2)Ge(2) were studied in a wide temperature range. The Ce-rich alloys Ce(1-x)La(x)Ni(2)Ge(2) were found to exhibit distinct features of the coherent heavy Fermi liquid. At intermediate compositions (0.7≤x≤0.9), non-Fermi liquid properties have been observed, followed by the local Fermi liquid behavior in the dilute limit. The 4f-electron contribution to the specific heat was found to follow the predictions of the Kondo-impurity model in both the local as well as the coherent regimes, with the characteristic Kondo temperature decreasing rapidly from about 30 K for the parent compound CeNi(2)Ge(2) to about 1 K in the most dilute samples. The specific heat does not show any evidence for the emergence of a new characteristic energy scale related to the formation of the coherent Kondo lattice.