K G Lisunov

Asymmetry of a complex gap near the Fermi level, determined from measurements of the thermopower in La1−xCaxMn1−yFeyO3

The thermopower, S, of La0.7Ca0.3Mn1−yFeyO3 (y = 0–0.09), measured for T = 25–310 K and magnetic fields of B = 0–10 T, exhibits strong sensitivity to doping with Fe. The weak increase of S observed in the undoped material with lowering temperature is enhanced considerably in the sample with y = 0.03 until a steep decrease takes place in the vicinity of the paramagnetic–ferromagnetic transition temperature, TC. S increases with further growth of y and its maximum follows the decrease of TC from 228 K at y = 0.03 to 119 K at y = 0.09. Application of a magnetic field reduces S strongly. The experimental data are analysed quantitatively with a percolation model using results of our recent investigations of the Shklovskii–Efros-like variable-range hopping conductivity in the same set of La1−xCaxMn1−yFeyO3 samples with evidence for a soft Coulomb gap and a rigid gap in the density of states near the Fermi level (Laiho et al 2002 J. Phys.: Condens. Matter 14 8043). In the paramagnetic insulating phase the temperature dependence of S is determined by competing contributions from two types of asymmetry of the gap: a small shift of its centre with respect to the Fermi level and cubic non-parabolicity. Numerical values of the shift and the cubic asymmetry terms are determined.

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.

Low-field magnetic properties of LaMnO3+δ with 0.065≤δ≤0.154

In a weak ðB ¼ 2G Þ magnetic field LaMnO3þd exhibits at d ¼ 0:065 below the paramagnetic-to-ferromagnetic (FM) Curie temperature, TC; a mixed (spin-glass and FM) phase followed by a frustrated FM phase at d between 0.100 and 0.154. The same behavior is observed in La12xCaxMnO3 with x between 0 and 0.3. This can be understood by the similar variation of the Mn 4þ concentration, c between < 0.13 and 0.34, in both materials when x or d is increased. On the other hand, considerable differences are found between these compounds in the values of the magnetic irreversibility, in the dependencies of TCðcÞ and the magnetic susceptibility, xðcÞ; as well as in the critical behavior of xðTÞ near TC: These differences can be explained by distortions of the cubic perovskite structure, by the reduced lattice disorder and by the more homogeneous hole distribution in LaMnO3þd than in La12xCaxMnO3.

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.

Magnetic properties of the new diluted magnetic semiconductor : evidence of MnAs clusters

The preparation and magnetic properties of the diluted magnetic semiconductor , opening a novel group of compounds alloyed with transition metal elements, are reported for the first time. Single crystals of are obtained with a modified Bridgman method. For their structure is isomorphic to the parent compound. Magnetic properties of are investigated for x = 0.01, 0.05 and 0.1 between T = 2 and 500 K in fields up to 60 kG. All the samples show a steep decrease of magnetization well above 300 K. In low fields (2-200 G) the temperature dependence of the magnetic susceptibility is strongly irreversible below K. The magnetization displays nonlinear field dependence starting from -2 kG and reaching full saturation above 40 kG. Its temperature dependence is weak between 2 and 300 K. The magnetic properties are explained by the presence of MnAs clusters. The distribution of cluster sizes is described by three overlapping Gaussian functions with the maxima at 1.7, 2.4 and 3.3 nm. At 300 K the corresponding magnetic moments are found to be 1.5, 4.1 and 10.4 in units of .

Shubnikov-de Haas effect in (Cd1−x−yZnxMny)3As2 far from the zero-gap state

Abstract The Shubnikov-de Haas effect in the type II 3 V 2 diluted magnetic semiconductor alloy ( Cd 1− x − y Zn x Mn y ) 3 As 2 was investigated in the region of 0⩽ y ⩽0.08, x + y = 0.3. Single period oscillations of the conductivity were observed in the temperature interval of 4.2–31 K and magnetic fields up to 11 T. In one of the samples ( y = 0.02) a well-resolved spin-splitting was seen. Values of the cyclotron effective mass and the g -factor were determined. A strong dependence of the cyclotron mass on the magnetic field was observed in crystals with a high Mn concentration.

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)

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.

Magnetic MnAs nanoclusters in the diluted magnetic semiconductor (Zn1-xMnx)3As2

Magnetic properties of the II-V diluted magnetic semiconductor .Zn1 x Mnx/3As2 are investigated for xD 0:08, 0.10 and 0.13, betweenT D 3 and 500 K and in fields up to 60 kG. All samples show a steep decrease of the magnetization above 280 K. In low fields (5-80 G) the temperature dependence of the magnetic susceptibility is strongly irreversible below the blocking temperatureTb 250 K. Above 15 kG the difference between the zero-field-cooled and the field- cooled susceptibilities disappears. These features give evidence for the presence of two magnetic subsystems: (i) paramagnetic centres including a single Mn ion and an open or closed triple antiferromagnetic cluster of Mn 2+ and (ii) ferromagnetic MnAs nanoclusters. The size distribution of the MnAs clusters is described by two overlapping Gaussian functions with the maxima at R1D 2:6-3.1 nm andR2D 3:3-3.8 nm, depending onx.