HALL AND TRANSVERSE EVEN EFFECTS IN THE VICINITY OF A QUANTUM CRITICAL POINT IN Tm_1-x Yb_x B₁₂
Sluchanko N.E., Azarevich A.N., Bogach A.V., Glushkov V.V., Demishev S.V., Levchenko A.V., Filipov V.B., Shitsevalova N.Yu., Stankewicz J., Moshchalkov V.V.

Received: July 19, 2011

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The angular, temperature, and magnetic field dependences of the resistance recorded in the Hall effect geometry are studied for the rare-earth dodecaboride Tm_1-x Yb_x B₁₂ solid solutions where the metal-insulator and antiferromagnetic-paramagnetic phase transitions are observed in the vicinity of the quantum critical point . The measurements performed on high-quality single crystals in the temperature range 1.9-300 K for the first time allow revealing the appearance of the second harmonic contribution, a transverse even effect in these fcc compounds near the quantum critical point. This contribution is found to increase drastically both under the Tm-to-ytterbium substitution in the range x>x_c and with an increase in the external magnetic field. Moreover, as the Yb concentration x increases, a negative peak of a significant amplitude appears on the temperature dependences of the Hall coefficient R_H(T) for the Tm_1-xYb_xB₁₂ compounds, in contrast to the invariable behavior R_H(T)\approx const found for TmB₁₂. The complicated activation-type behavior of the Hall coefficient is observed at intermediate temperatures for with activation energies K and E_a/k_B=55-75 K, and the sign inversion of R_H(T) is detected at liquid-helium temperatures in the coherent regime. Renormalization effects in the electron density of states induced by variation of the Yb concentration are analyzed. The anomalies of the charge transport in Tm_1-x Yb_x B₁₂ solid solutions in various regimes (charge gap formation, intra-gap many-body resonance, and coherent regime) are discussed in detail and the results are interpreted in terms of the electron phase separation effects in combination with the formation of nanosize clusters of rare earth ions in the cage-glass state of the studied dodecaborides. The data obtained allows concluding that the emergence of Yb-Yb dimers in the Tm_1-x Yb_x B₁₂ cage-glass matrix is the origin of the metal-insulator transition observed in the achetypal strongly correlated electron system YbB₁₂.