We have investigated the Shubnikov‐de Haas effect in single crystals of the group II‐V diluted magnetic semiconductors (Cd1−x−yZnxMny)AS2(x + y = 0.3, y = 0.06 and 0.08) at temperatures T = 4.2 to 16 K and magnetic fields B = 0 to 6.5 T under hydrostatic pressure p up to 14 kbar. Values of the cyclotron mass mc, the effective g‐factor g*, and the Dingle temperature TD have been determined. In both specimens a linear field dependence mc(B) = m c* + αB is observed, where m c* is independent of B but increases with pressure and where a does not depend on p and B. Additionally, pressure dependencies of g*, TD and the Hall mobility μR are observed. The dependence of m c* and g*, on p is interpreted within the three‐band Kane model. The decrease of μR and TD with increasing pressure is consistent with an increase of m c*.
The temperature dependence of the resistivity,
ρ, of
ceramic LaMnO3+δ
with δ = 0.065
and 0.154 (or simply # 065 and # 154), is investigated under hydrostatic pressure,
p, between the
temperatures T = 80
and 320 K in magnetic fields up to 10 T. The data for
ρ(T)
are analysed within a Shklovskii–Efros-like variable-range
hopping conductivity model, governed by a soft Coulomb gap,
Δ, and a
rigid gap, γ, in the density of localized states. The validity of this approach to the resistivity of
LaMnO3+δ
under ambient pressure was established in recent investigations (Laiho et al 2005 J. Phys:
Condens. Matter 17, 105). In the present work it is shown that upon increasing
p
up to 11 kbar (# 154) and 13 kbar (# 065) the magnetic Curie temperature,
TC, is increased
by ∼12–13%.
Correspondingly, Δ(p)
is decreased by ∼7–8%
and γ(p)
by ∼8%
in # 154 and 12% in # 065. Similarly, the electron localization radius
a(p) is increased
by ∼4%
in # 154 and 8% in # 065, respectively. The influence of pressure on
TC,
Δ,
γ
and a
is interpreted by increasing the electron bandwidth and decreasing the
polaron potential well, which stimulates the delocalization of electrons when
p
is increased.
Resistivity of La0.7−δ Ca0.3Mn1−yFeyO3 (LCMFO) with y=0–0.05, δ=0 and 0.017 is investigated between 4.2–350 K, showing that La vacancies increase the effect of Fe doping by influencing electronic states through additional microscope disorder. The Shklovskii–Efros variable-range hopping conductivity, governed by generation of a soft parabolic gap in the spectrum of localized states due to Coulomb interaction between charge carriers, is observed both above and well below the ferromagnetic-to-paramagnetic transition temperature. Our results give evidence for absence of a true metal–insulator transition both in undoped material and in LCMFO.
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