M A Shakhov

Variable-range hopping conductivity in La1-xCaxMn1-yFeyO3: evidence of a complex gap in density of states near the Fermi level

The resistivity, ρ, of ceramic La1−xCaxMn1−yFeyO3 with x = 0.3 and y = 0.0–0.09 is found to obey, between a temperature Tv ≈ 310–330 K and the ferromagnetic-to-paramagnetic transition temperature, TC = 259–119 K (decreasing with y), the Shklovskii–Efros-type variable-range hopping conductivity law, ρ(T) = ρ0 (T) exp [(T0 /T)1/2 ]. This behaviour is governed by generation of a soft Coulomb gap Δ ≈ 0.42 eV in the density of localized states and a rigid gap δ(T) ≈ δ(Tv)(T/Tv)1/2 with δ(Tv) ≈ 0.16, 0.13 and 0.12 eV at y = 0.03, 0.07 and 0.09, respectively. Deviations from the square root dependence of δ(T), decreasing when y is increased, are observed as T → TC. The prefactor of the resistivity follows the law ρ0 (T) ~ Tm, where m changes from 9/2 at y = 0 to 5/2 in the investigated samples with y = 0.03, 0.07 and 0.09, which is connected to introduction of an additional fluctuating short-range potential by doping with Fe.

Non-universal low-field magnetic scaling and variable-range hopping conductivity as a consequence of disorder in La1−xCaxMn1−yFeyO3

Abstract Low-field (B=2−80 G ) dc magnetic susceptibility, χ , investigated in hole-doped La 1− x Ca x Mn 1− y Fe y O 3 (LCMFO) with x =0.3 and y =0.05−0.10, exhibits non-uniform critical behavior near the paramagnetic (PM)-to-ferromagnetic (FM) transition temperature, T C . We observe the scaling law, χ −1 ( T )− χ −1 ( T C )∼( T / T C −1) γ ≡ τ γ , with γ ≈1.4 corresponding to a 3D Heisenberg spin system below τ cr ∼0.1–0.2, and with γ ≈1.7 characterizing a 3D percolation system, above τ cr . The non-universal scaling results from strongly inhomogeneous distribution of holes leading to formation of percolation clusters from the hole-rich nanoscale FM particles, embedded in the host PM matrix. The resistivity, ρ , of LCMFO exhibits between a temperature T v ≈310–330 K and T C the Shklovskii–Efros-type variable-range hopping conductivity law, ρ ( T )= ρ 0 ( T )exp[( T 0 / T ) 1/2 ], governed by generation of a complex gap in the density of states. The prefactor follows the law ρ 0 ( T )∼ T m , where m changes from 9/2 at y =0 to 5/2 at y =0.03, 0.07 and 0.09, reflecting an additional fluctuating short-range potential induced by doping with Fe.

Mechanisms of hopping conductivity in weakly doped La1−xBaxMnO3

The resistivity, ?, of ceramic La1?xBaxMnO3 with x = 0.02?0.10 corresponding to the concentrations of holes c?0.15?0.17 displays an activated behaviour both above and below the paramagnetic to ferromagnetic transition temperature TC = 175?209?K, obtained from measurements of the magnetization. Above T~310?390?K ?(T,x) is determined by nearest-neighbour hopping of small polarons with activation energy Ea = 0.20?0.22?eV. Below the onset temperature Tv = 250?280?K, depending on x, a Shklovskii?Efros-like variable-range hopping conductivity mechanism, governed by a soft temperature independent Coulomb gap, ??0.44?0.46?eV, and a rigid gap, ?(T), is found. For the range T~50?120?K, ?(T) is connected to the formation of small lattice polarons in conditions of strong electron?phonon interaction and lattice disorder. The rigid gap obeys a law ?(T)~T1/2 within two temperature intervals above and below TC, exhibits an inflection at TC and reaches at Tv a value of ?v?0.14?0.18?eV. Such behaviour suggests a spin dependent contribution to ?(T). The localization radius of the charge carriers, a, has different constant values within the temperature intervals where ?(T)~T1/2. With further decrease of T, a increases according to the law expected for small lattice polarons.

Hopping conductivity of Ni-doped p-CdSb

Mechanisms of the resistivity, ρ, of single crystal samples oriented along the [100] (No 1), [010] (No 2) and [001] (No 3) axes of anisotropic semiconductor p-CdSb doped with 2 at.% of Ni are investigated. In zero magnetic field the Mott type variable-range hopping (VRH) conductivity is observed in No 2 and the Shklovskii‐Efros type in No 1 and No 3 at T 2. 5K . The magnetoresistance (MR) of the samples obeys the law ln ρ ∼ B 2 up to B ∼ 6T . However, the temperature dependence of MR gives evidence for the Mott-VRH conductivity in No 1 at T 4.2 K and the nearest-neighbor hopping conductivity in No 2 between T = 3 and 4.2 K and in No 3 between 1.5 and 4.2 K. From the experimental data the values of the localization radius and dielectric permittivity and details of their critical behavior near the metal‐insulator transition, as well as the widths and the values of the density of the localized states, the acceptor energies, their concentrations and the anisotropy coefficients, are obtained. (Some figures in this article are in colour only in the electronic version)

Shallow donor states of Ag impurity in ZnSe single crystals

The Hall coefficient, electrical conductivity and electron mobility are investigated in Ag-doped n-ZnSe single crystals at temperatures between 8 and 500 K immediately after doping the samples and after storing them for 5 years under normal room conditions in darkness. The formation of donor-type Agi interstitial defects stimulated by time is found for the first time. After 5 years storage, the samples show a dramatic increase in the electrical conductivity, free electron concentration and concentration of shallow donors. A model that explains these changes by the displacement of Ag ions into interstitial sites due to lattice deformation forces is proposed. The formation of an Ag-donor impurity band in n-ZnSe:Zn:Ag crystals stored at room temperature is also studied.

Hall effect and band structure of p-CdSb in strong magnetic field

The Hall effect in the anisotropic II–V group semiconductor p-CdSb is investigated at temperatures between T = 3.6 and 200 K and pulsed magnetic fields up to B = 25 T in unintentionally doped samples oriented along the crystallographic axes [100] and [010]. The Hall coefficient, R(B, T), with B ∥ [001] exhibits in low fields a flat region followed by a descending interval when B is increased. This behaviour is attributed to the presence of two groups of holes with concentrations p2(T) > p1(T) and mobilities μ2(T) Tcr p1 and p2 are related to the holes activated to the light- and heavy-hole bands, respectively. The analysis of μ1(T) and μ2(T) confirms the existence of the heavy-hole band or a non-equivalent maximum and two equivalent maxima of the light-hole valence band.

The Hall effect and electron energy spectrum near the conduction band edge of n-CdSb in magnetic fields up to 25 T

The Hall effect in the II–V group semiconductor n-CdSb doped with In is investigated in pulsed magnetic fields up to B = 25 T and temperatures between T = 2 and 77 K for samples oriented along the [0 1 0] and [0 0 1] crystallographic axes. The Hall coefficient, R (B, T), exhibits a sequence of an almost flat region followed by a descending interval and an upturn when B is increased. The decrease of R (B, T) is interpreted by the presence of two groups of electrons with concentrations n2(T) > n1(T) and mobilities μ2(T) < μ1(T). Analysis of n1(T) and n2(T) demonstrates that the high-mobility carriers n1 are the conduction band (CB) electrons, whereas the low-mobility carriers n2 are itinerant electrons of a lower resonant impurity band (IB), having at low T energies of Ei ~ 3–4 meV above the CB edge. In addition, near the CB edge lies a higher IB containing only localized electron states. The IBs are split by spin to states differing by an energy ΔEi ≈ 0.9 meV. The upturn of R (B, T) in the high-field region is explained by the redistribution of the electrons between the IBs due to the decrease of ΔEi when B is increased. The mobility of the CB electrons is determined presumably by strong anisotropic scattering on neutral impurity centres, accompanied at high T by isotropic scattering on acoustic phonons, whereas scattering from ionized impurities is small.

The Hall effect in Ni-doped p-CdSb in a strong magnetic field

The Hall effect in single crystals of the group II–V semiconductor p-CdSb doped with 2 at% of Ni is investigated between T = 1.5 and 300 K in pulsed magnetic fields up to B = 25 T. The Hall resistivity, ρH, exhibits a nonlinear dependence on B, which is strongly pronounced below ~10 K but is still observed even up to 300 K. The analysis of ρH(B) gives evidence for the presence of a positive normal and a negative anomalous contribution, ρN = R0B and ρA, respectively. The temperature dependence of the (normal) Hall coefficient R0 is determined by the activation of holes into the valence band with a small contribution of the itinerant holes from the acceptor band at lowest T. The dependence of ρA on T is quite different within two temperature intervals, being weak between ~50 and 300 K and very strong below ~50 K, the latter resembling that of the resistivity, ρ. Analysis of ρA below ~77 K demonstrates that it scales approximately as ρn with n = 1.6 ± 0.1 within four decades of ρA and more than two decades of ρ. The anomalous Hall effect in p-CdSb:Ni is attributable to the presence of magnetic Ni-rich nanoclusters, whose properties have previously been investigated by the analysis of the magnetization data.

Influence of the magnetic field on the conductivity within the Coulomb gap of n-ZnSe single crystals doped with Ag

The conductivity and magnetoresistance of undoped and Ag-doped n-ZnSe single crystals are investigated in pulsed magnetic fields up to 20 T at temperatures between 1.5 and 300 K. In fields below 10 T, the resistivity is found to obey the temperature dependence ρ(T) = ρ0exp{T0/T}1/2 indicating variable range hopping (VRH) conductivity over the states of the Coulomb gap in the vicinity of the Fermi level. In low fields, a region of negative magnetoresistivity (NMR) is observed and attributed to the increasing density of states around the Fermi level in a magnetic field. As the magnetic field increases, NMR is gradually cancelled by exponentially increasing the contribution of positive magnetoresistivity (PMR) caused by shrinking of the impurity electron wavefunctions. Values of the localization radius of the electron wavefunction, a, and the dielectric constant, κ, are obtained from analysis of the PMR data. Both a and κ increase when approaching the metal–insulator (MI) transition, at which the ratio of the critical indices νκ/νa = 3.5 is found.

Magnetophonon resonance in quantum wells in tilted field: what is concealed behind its angular dependence?

Magnetophonon resonance in quantum wells in a tilted magnetic field B is investigated. Measurements of the Hall coefficient and correspondingly of the carrier concentration as a functions of magnetic field and temperature are simultaneously performed. It is shown that the experimental data can be interpreted in terms of a great sensitivity to the effects of varying the two dimensional carrier concentration ns in a certain concentration interval. In other words, the observed angular dependence of the MPR amplitudes is a manifestation of dependence of ns on the magnitude of the magnetic field B. We believe that such a dependence can be relevant in general for the interpretation of magnetotransport in nondegenerate 2D electron gas.