Seebeck effect near quantum MIT in Eu_1 − xCa_xB₆

Abstract

We report the results of a Seebeck effect study carried out on the single crystals of Eu 1 À x Ca x B 6 (0 x 1) at temperatures 1.8-300 K. The Seebeck coefficient is shown to be determined by the sum of the linear term S ¼ AT and a phonon-drag contribution, which is induced by the local phonon mode of europium (calcium) with the characteristic frequency 7.3 meV (9.1 meV). The crossover from negative (A % À0.23 mV K À2 for x ¼ 0) to positive (A % þ1.2 mV K À2 for x ¼ 0.244) values of the diffusion thermopower indicates the smooth change of the band structure when approaching the hole-like conductivity found in Eu 1 À x Ca x B 6 at x > x C % 0.2.

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

“…The maximal values of ρ(0)/ρ(7 T) ≈ 2.4 × 10 7 , measured for x = 0.635 at T = 2 K, seem to be the record ones for this class of materials. The increase of Ca content restores the metallic ground state with moderate values of resistivity (ρ ∼ 1 ÷ 5 mΩ·cm) and CMR eect (ρ(0)/ρ(7 T) < 7) comparable with that of ρ(0)/ρ(7 T) ∼ 11 found for EuB 6 .…”

“…1). At the same time, a positive slope of S(T ) ∼ AT , corresponding to diusive thermopower in Ca 1−x Eu x B 6 [6], is observed in a wider range of concentrations 0.6 < x < 0.85 (Fig. 2a).…”

“…The onset of insulating ground state, appearing due to strong interaction between itinerant electrons and localized magnetic moments in Ca 1−x Eu x B 6 [13], provides an interesting opportunity for identifying the microscopic mechanisms of colossal magnetoresistance (CMR) in these rare earth hexaborides. 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].…”

Bulk and surface electron transport in topological insulator candidate YbB6-δ

“…b). While the S ( T ) behavior for crystal 1 is typical for the undoped hexaborides R 2+ B 6 (R = Eu, Ca) , the Seebeck coefficient of the crystal 2 depends on the temperature in a complicated manner changing twice the sign at ∼20 K and ∼270 K (Fig. b).…”

Evolution of thermoelectric properties in EuxYb1−xB6 family

“…Note that both the form of S(T ) curve as well as the temperature interval of S d dominance may drastically change for the diluted Eu x Yb 1−x B 6 systems (e.g. for x ∼ 0.5) as proven by the analogy with the isostructural Eu x Ca 1−x B 6 solid solutions [41].…”