Regensburg 2013 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 53: Correlated Electrons: Heavy Fermions

TT 53.5: Talk

Thursday, March 14, 2013, 10:30–10:45, H19

Strong enhancement of the thermopower of YbRh₂Si₂ by La substitution — •Ulrike Stockert¹, Heike Pfau¹, Cornelius Krellner^1,2, Steffen Wirth¹, Christoph Geibel¹, and Frank Steglich¹ — ¹MPI for Chemical Physics of Solids, 01187 Dresden, Germany — ²Present address: Physikalisches Institut Goethe-University Frankfurt, 60438 Frankfurt (Main)

YbRh₂Si₂ orders antiferromagnetically below T_N = 70 mK and B_c ≈ 60 mT ⊥ c. In addition, a crossover from a small Fermi surface (FS) at low fields to a large FS incorporating the 4f electrons at high fields takes place at T^*(B). Rather large values are found for the absolute thermopower divided by temperature |S/T|. The different regions of the phase diagram are characterized by specific T and B dependencies, e.g. a broadened step-like increase of |S/T| upon crossing T^*(B).

Substitution of 2 % La on the Yb site of YbRh₂Si₂ results in a suppression of T_N to below 20 mK, while T^*(B) is not altered. Moreover, additional scattering centers are introduced. This leads to a lowering of the thermal conductivity in Yb_0.98La_0.02Rh₂Si₂. By contrast, |S/T| is strongly enhanced below 1 K, but falls rapidly below the value for YbRh₂Si₂ already in moderate magnetic fields. In order to evaluate the origin of this unexpected behavior we studied in detail the field-dependence of the low-T thermopower of Yb_0.98La_0.02Rh₂Si₂. Our results suggest, that the enhancement of the absolute thermopower upon La substitution and the strong field dependence are related to modifications of the band structure upon doping into the 4f sublattice.