Enhancement of the colossal magnetoresistance in Eu1 − x Ca x B6

Seebeck effect near quantum MIT in Eu1 − xCaxB6

“…The sign inversion of the Hall and Seebeck effects to positive values found for x > 0.2 points to the transition to hole‐like conductivity, which has no explanation within the scenario of quantum MIT in Eu 1 − x Ca x B 6 based on the double‐exchange approach 6, 7. As a result, the abnormal dispersion of the x MIT values and the inconsistency between the results of the optical and transport studies of Eu‐rich compounds ( x ∼ 0.3–0.7) 8–11 do not allow identification of the origin of the insulating ground state at x > x MIT . In this respect it is of great interest to discover the peculiarities of the band‐structure evolution near quantum MIT in Eu 1 − x Ca x B 6 that could be provided by the study of the Seebeck effect.…”

“…The analysis of optical conductivity in the series of Eu 1 − x Ca x B 6 10 favors the suppression of the density of free carriers at x > 0.35 as well. However, the recent study of charge transport in Eu 1 − x Ca x B 6 revealed the onset of a low‐temperature “insulating” state at a lower value of x MIT ∼ 0.2 11. The sign inversion of the Hall and Seebeck effects to positive values found for x > 0.2 points to the transition to hole‐like conductivity, which has no explanation within the scenario of quantum MIT in Eu 1 − x Ca x B 6 based on the double‐exchange approach 6, 7.…”

Reentrant Metal-Insulator Transition in Ca_{1-x}Eu_{x}B_{6}

“…The data of Hall resistivity show that positive Hall eect is detected only in the paramagnetic phase in a narrow range of Eu doping (0.7 < x < 0.8), changing to the negative one in moderate magnetic eld [3]. A standard procedure was applied to extract normal (R H ) and anomalous (R A H ) Hall coecients using the magnetic data of Ref.…”

“…SEM analysis showed that these solid solutions are homogeneous within ±0.5 at.% of Eu (∆x ∼ 0.005). The experimental setups for measurement of the transport parameters have been earlier described in [3]. * corresponding author; e-mail: glushkov@lt.gpi.ru Fig.…”

“…The scenario of magnetic polaron percolation, which is used to explain the temperature driven metal-insulator transition (MIT) in EuB 6 [4] and the exponential decrease of resistivity under applied magnetic eld in Ca 0.4 Eu 0.6 B 6 [5], fails to describe the onset of hole-like conductivity and huge CMR enhancement under transition into the insulating ground state in Eu-rich solid solutions (x < x M IT ≈ 0.8) [3]. In this respect the study of charge transport in Ca 1−x Eu x B 6 (x < x M IT ) is of great importance for understanding the nature of CMR in this strongly correlated electron system.…”