Mobility of 2D-electrons in scattering on a correlated distribution of impurity ions in doped thin layers

The spatial correlations of impurity ions in doped thin layers have been considered. A model of hard spheres on the plane has been developed for describing the correlations. In this model, an analytical expression has been obtained for the structure factor of 2D-electrons. The concentration dependences of the mobility of 2D-electrons in heterostructures with separate doping have been investigated using Al _x Ga_{1 − x} As/GaAs as an example.     

Influence of the form of structure factor on the mobility of nondegenerate two-dimensional electrons

The temperature dependences of the mobility of nondegenerate two-dimensional electrons in scattering by a correlated distribution of impurity ions in Al _x Ga_{1 ? x} As/GaAs heterostructures have been investigated. The cases where the influence of the first maximum of the structure factor on the scattering of electrons begins to dominate with increasing temperature have been considered. It has been found that the mobility of two-dimensional electrons decreases with increasing correlations in the spatial distribution of impurity ions. The influence exerted by the correlations and the width of the spacer layer on this effect has been analyzed.     

Influence of correlations in a flat distribution of impurity ions on the mobility of two-dimensional electrons at low temperatures

The temperature dependences of the electrical resistivity of degenerate two-dimensional electrons in scattering by impurity ions in heterostructures with a spacer of arbitrary width have been investigated using the Al _x Ga_{1 ? x} As/GaAs heterostructure as an example. Correlations in the arrangement of impurity ions have been taken into account in the model of hard spheres on a plane.     

Mobility of nondegenerate two-dimensional electrons in scattering by a correlated distribution of impurity ions

The temperature dependences of the inverse mobility of nondegenerate two-dimensional electrons in scattering by impurity ions in heterostructures with a narrow spacer have been investigated using the Al _x Ga_{1 ? x} As/GaAs heterostructure as an example. Correlations in the arrangement of impurity ions have been taken into account in the model of hard spheres on a plane. The influence of the form of the structure factor in the electron mobility has been considered.     

Effect of impurities on thermoelectric power due to phonon drag in metals

The effect of inelastic impurity scattering of electrons on the thermoelectric power due to phonon drag in metals has been studied. It is shown that this is the main cause of the thermoelectric power suppression due to doping at low temperatures. The thermoelectric power in a metal with a quadratic electron spectrum has been calculated as a function of temperature and impurity concentration. In addition to the impurity concentration, the correction to the thermoelectric power due to inelastic scattering contains the large factor Θ_D/T. Zh. éksp. Teor. Fiz. 111, 2237–2242 (June 1997)     

EPR study of exchange interactions in the nonmagnetic Kondo system La1−x CexCu6

The EPR of paramagnetic impurities Gd³⁺ and Mn²⁺ was studied in nonmagnetic Kondo system La_1−x Ce_xCu₆ containing in the 1.6–200 K range. The exchange interaction parameters of gadolinium and manganese ions with conduction electrons, of cerium ions with conduction electrons and with one another, the Kondo temperature of cerium ions, and the temperature behavior of cerium-ion spin-fluctuation rate have been determined. A pseudogap in the density of states at the Fermi level has been detected in the CeCu₆ regular system, which is apparently due to s-f hybridization. This pseudogap can be destroyed by introducing an aluminum impurity, which induces strong conduction-electron scattering. It was also found that RKKY interaction among manganese ions in CeCu_6−y Mn_y is considerably stronger than it is in LaCu_6−y Mn_y, which implies enhancement of nonlocal spin susceptibility due to an f band contribution to conduction-electron states. Fiz. Tverd. Tela (St. Petersburg) 40, 593–599 (April 1998)     

Bulk Sn1−xMnxO2 magnetic semiconductors without room-temperature ferromagnetism

Polycrystalline Sn_1−xMn_xO₂ (0≤x≤0.05) diluted magnetic semiconductors were prepared by solid-state reaction method and their structural and magnetic properties had been investigated systematically. The three Mn-doped samples (x=0.01, 0.03, 0.05) undergo paramagnetic to ferromagnetic phase transitions upon cooling, but their Curie temperatures are far lower than room temperature. The magnetization cannot be attributed to any identified impurity phase. It is also found that the magnetization increases with increasing Mn doping, while the ratio of the Mn ions contributing to ferromagnetic ordering to the total Mn ions decreases.     

Influence of unstable valence of cerium ions on the crystal field in ReNi compounds

To examine the effect of hybridization of 4f electrons with conduction electrons on the crystal field potential using neutron spectroscopy, we studied the effects of the crystal electric field (CEF) in intermetallic compounds of the type ReNi, in which chemical substitution is followed by a transition of the cerium ions from an intermediate valence state to the Kondo state. Measurements were performed both on cerium ions in the compounds Ce_1−x La_xNi (x=0.5, 0.8), where they have a whole-number population of the 4f shell, and on the paramagnetic impurity ion Nd in the series of compounds Re_1−x Nd_xNi (Re=Ce, La, Y), in which the cerium ions are found either in an intermediate valence state or in the Kondo state. From the neutron inelastic magnetic scattering spectra on Nd ions, we have reconstructed the crystal field parameters in ReNi compounds and calculated the CEF level diagram of Ce ions in these compounds as functions of the interion distances Re-Ni. The results of our calculations are in good agreement with the experimentally determined splitting diagram of the ground-state multiplet of the Ce ions. We have determined that as the degree of hybridization with the conduction electrons grows the CEF potential varies considerably and the effective splitting of the 4f shell of the cerium ions increases. The estimated energy scale of the splitting of the ground-state multiplet of the Ce³⁺ ions in the ReNi CEF (Δ_CEF∼15 meV) turns out to be commensurate with the Kondo temperature (T _K ;140 K for CeNi. Analysis indicates that the CEF potential has a substantial effect on the formation of the valence-unstable ground state of the f shell in this compound. Zh. éksp. Teor. Fiz. 113, 1731–1747 (May 1998)     

低补偿度n-Hg1-xCdxTe的磁致金属-绝缘体相变和相变后的温度激活输运行为

报道了ｘ＝０．２１４组份、低补偿度（Ｋ《１）ｎ－Ｈｇ_1-xＣｄ_xＴｅ晶体在０．３─３０Ｋ温度范围，０─７Ｔ强磁场下的横向磁阻、电子霍耳迁移率、霍耳系数测量结果，观测到了磁致金属－绝缘体相变和相变后的温度激活输运行为。分析实验数据，提出：低补偿度、组份：ｘ＝０．２附近的ｎ－Ｈｇ_1-xＣｄ_xＴｅ，磁致金属－绝缘体相变（ＭＩＴ）发生的机理是载流子在浅施主杂质态上的磁冻结；发生磁冻结的前提是热冻出（ｔｈｅｒｍａｌ ｆｒｅｅｚｅ 关键词：     

Density of states in the impurity d band and inelastic electron scattering by a system of mixed-valence iron ions in HgSe: Fe crystals

The resistivity and the Hall coefficient of HgSe: Fe crystals with various iron content are experimentally studied in the temperature range 1.3≤T≤300 K and in magnetic fields up to 60 kOe. The temperature dependences of the density and mobility of conduction electrons in these crystals are determined. The influence of spatial charge ordering in the system of mixed-valence iron ions on impurity states in HgSe: Fe crystals is considered. The density of states in the impurity d band is theoretically analyzed, and inelastic electron scattering in which bi-and trivalent iron ions are recharged is discussed. It is shown that the experimentally detected features in the dependence of the density and mobility of electrons on temperature and iron impurity content can be explained by the influence of Coulomb correlations in the mixed-valence iron ion system on the structure of the impurity d band.

     

Transverse Nernst-Ettingshausen effect, resonant scattering, and superconductivity in SnTe: In

The temperature dependences of the coefficient of the transverse Nernst-Ettingshausen effect in SnTe: In samples with different indium contents (1–16 at %) in the temperature range 100–300 K and the electrical resistivity at temperatures of 1.2–4.2 K and in magnetic fields of up to 10 kOe are investigated. The data obtained indicate the presence of resonant hole scattering into the band of quasi-local In impurity states in Sn_1?x In _x Te samples with In content x ≥ 0.05 and a superconducting transition with a critical temperature T _c ～ 1.5–2.2 K. The SnTe: In samples with the degree of filling of impurity states by electrons, which is close to 1/2, and the Fermi level ?_F pinned in the vicinity of the minimum energy dependence of the relaxation time τ(?) are characterized by inhomogeneities of a new type, i.e., inhomogeneities of the scattering parameter r = ?lnτ/?ln?| $ \varepsilon _F The temperature dependences of the coefficient of the transverse Nernst-Ettingshausen effect in SnTe: In samples with different indium contents (1–16 at %) in the temperature range 100–300 K and the electrical resistivity at temperatures of 1.2–4.2 K and in magnetic fields of up to 10 kOe are investigated. The data obtained indicate the presence of resonant hole scattering into the band of quasi-local In impurity states in Sn_1−x In _x Te samples with In content x ≥ 0.05 and a superconducting transition with a critical temperature T _c ∼ 1.5–2.2 K. The SnTe: In samples with the degree of filling of impurity states by electrons, which is close to 1/2, and the Fermi level ɛ_F pinned in the vicinity of the minimum energy dependence of the relaxation time τ(ɛ) are characterized by inhomogeneities of a new type, i.e., inhomogeneities of the scattering parameter r = ϖlnτ/∂lnɛ|, which are induced by fluctuations of the degree of filling of quasi-local states by electrons. Original Russian Text ? S.A. Nemov, V.I. Proshin, G.L. Tarantasov, R.V. Parfen’ev, D.V. Shamshur, A.V. Chernyaev, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 3, pp. 461–464.     

Kondo effect and impurity-impurity interaction in (La,Ce)B6 alloys

Measurements of the magnetic susceptibility between 0.03 and 300 K and of the magnetization between 0.05 and 10K for magnetic fields up to 60kOe have been used to investigate effects from the interaction between the conduction electrons and local magnetic moments in (La_1–x Ce _x )B₆ alloys (0.0007x0.10). For Ce concentrationsx<0.006 the data show Kondo-type single impurity behavior at low temperatures with a transition from a magnetic to a non-magnetic regime of the Ce ions. In the magnetic regime the impurity susceptibility follows a Curie-Weiss law, and in the non-magnetic regime it varies withT ². An external magnetic field gradually restores the free-ion behavior of the Ce impurities. For more concentrated alloys interactions between the impurities are observed. The RKKY interaction strength derived is more than two orders of magnitude smaller than in the Kondo systemCuFe. Values of thes–f exchange integral,J, estimated from both the Kondo effect and the RKKY interactions are in good agreement. The relatively high Kondo temperature, in spite of a smallJ, can be explained by a density-of-states argument. The influence of crystal field excitation on the susceptibility is also discussed.     

Impurity absorption in indium-doped Pb1−xSnxTe

Optical absorption spectra of indium doped Pb_1−xSn_xTe (x = 0–0.22) have been measured at 90 and 300 K. It was found that doping gives rise to a broad absorption band with its maximum at photon energies much greater than the thermal ionization energy of the indium impurity. The energetic position and the shape of impurity absorption band change little with alloy composition, but the band broadens substantially with temperature. The results are explained by a strong lattice relaxation around the indium impurity.     

Comparing the physical properties of Pr/Gd and Pr/Ce substitutions in Ru(Gd<Subscript>1.5</Subscript>Ce<Subscript>0.5</Subscript>)Sr<Subscript>2</Subscript>Cu<Subscript>2</Subscript>O<Subscript>10−<Emphasis Type="Italic">δ</Emphasis></Subscript>

We have compared the electrical and magnetic properties of Ru(Gd_1.5−x Pr _x )Ce_0.5Sr₂Cu₂O_10−δ (Pr/Gd samples) with x = 0.0, 0.01, 0.03, 0.033, 0.035, 0.04, 0.05, 0.06, 0.1 and RuGd_1.5(Ce_0.5−x Pr _x ) Sr₂ Cu₂O_10−δ (Pr/Ce samples) with x = 0.0, 0.01, 0.03, 0.05, 0.08, 0.1, 0.15, 0.2 prepared by the standard solid-state reaction technique. We obtained the XRD patterns for different samples with various x. The lattice parameters versus x for different substitutions have been obtained from Rietveld analysis. To determine how the magnetic and superconducting properties of these layered cuprate systems can be affected by Pr substitution, the resistivity, and magnetoresistivity, with H _ext varying from 0.0 to 15 kOe, have been measured at various temperatures. Superconducting transition temperature T _c and magnetic transition T _irr , have been obtained through resistivity and ac susceptibility measurements. The T _c suppression due to Pr/Gd and Pr/Ce substitutions show competition between pair breaking by magnetic impurities, hole doping due to different valances of ions, difference in ionic radii, and oxygen stoichiometry. Pr/Gd substitution suppresses superconductivity more rapidly than for Pr/Ce, showing that the effect of hole doping and magnetic impurity pair breaking is stronger than the difference in ionic radii. In Pr/Gd substitution, the small difference between the ionic radii of Pr^3+,4+ and Gd³⁺, and absorption of more oxygen due to the higher valence of Pr with respect to Gd, decreases the mean Ru-Ru distance, and as a result, the magnetic exchange interaction becomes stronger with the increase of x. However, Pr/Ce substitution has the opposite effect. The magnetic parameters such as H _c , obtained through magnetization measurements versus applied magnetic field isotherm at 77 K and room temperatures, become stronger with x in Pr/Gd and weaker with x in Pr/Ce substitution.     

尖晶石铁氧体TixNi1-xFe2O4中阳离子分布和Ti离子磁矩的实验研究

采用固相反应法制备了系列样品Ti_xNi_1-xFe₂O₄ (x=0.0, 0.1, 0.2, 0.3, 0.4). 室温下的X射线衍射谱表明样品全部为(A)[B]₂O₄型单相立方尖晶石结构, 属于空间群Fd3m. 样品的晶格常数随Ti掺杂量的增加而增大. 样品在10 K温度下的比饱和磁化强度σ_S随着Ti掺杂量x的增加逐渐减小. 研究发现, 当Ti掺杂量x≥ 0.2时, 磁化强度σ随温度T的变化曲线出现两个转变温度T_L和T_N. 当温度低于T_N时, 磁化强度明显减小; 当温度达到T_N时, dσ/dT具有最大值. σ-T曲线的这些特征表明, 由于Ti掺杂在样品中出现了附加的反铁磁结构. 这说明样品中的Ti离子不是无磁性的+4价离子, 而是以+2和+3价态存在, 其离子磁矩的方向与Fe和Ni离子的磁矩方向相反. 利用本课题组提出的量子力学方势垒模型拟合样品在10 K温度下的磁矩, 得到了Ti, Fe和Ni三种阳离子在(A)位和[B]位的分布情况, 并发现在所有掺杂样品中, 80%的Ti离子以+2价态占据尖晶石结构的[B]位.     

Specific features of depth distribution profiles of implanted cobalt ions in rutile TiO<Subscript>2</Subscript>

This paper reports on the results of the calculation of the depth distribution profiles of the concentration of the impurity implanted into an anisotropic crystalline material. The sputtering of the irradiated material, fast one-dimensional diffusion of the impurity along structural channels, and accumulation of the implanted impurity at different depths have been taken into account. The results of the calculations have been compared with the experimental distribution profiles of cobalt ions implanted into the crystal structure of rutile TiO₂ along and across structural channels at different temperatures of the irradiated substrate. A comparison of the model and experimental profiles has made it possible to evaluate the precipitation rate of cobalt in the TiO₂ matrix on different precipitation centers. A model has been proposed for explaining the unusual shift in the peak of the concentration distribution of implanted ions deep into the sample with an increase in the temperature of the irradiated substrate. The model has allowed one to separate the contributions from different phases of the impurity (nanoparticles and solid solution) to the magnetism of the Co: TiO₂ system.     

An alternative approach for optimal carrier concentration towards ideal thermoelectric performance

Optimization of the carrier concentration of any thermoelectric material is a prime factor for the enhancement of the thermoelectric figure of merit. An alternative approach for achieving optimal carrier concentration is presented here. We introduce impurity levels of ytterbium (Yb) near the valence band edge of Pb_1–xSn_x Te. The temperature‐dependent redistribution of electrons between the Yb‐impurity levels and the valence band is found to optimize the excess hole concentration at low temperature and negating the effect of intrinsic conduction at higher temperature leading to significantly improved thermoelectric performance in Pb_1–xSn_x Te. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Jahn-Teller distortion and magnetoresistance in electron doped Sr1-xCexMnO3 (x = 0.1, 0.2, 0.3 and 0.4)

Neutron and electron diffraction, electrical transport and magnetic measurements have been carried out on a newly synthesized electron doped Sr_1-xCe _x MnO₃ (x = 0.1, 0.2, 0.3 and 0.4) system. For x=0.1, while cooling, it undergoes a first-order metal-insulator transition at 315 K which is associated with a structural transition from cubic (Pm3m) to tetragonal (I4/mcm) due to Jahn-Teller ordering () which stabilizes a chain like (C-type) antiferromagnetic ground state with . The antiferromagnetic insulator state is insensitive to an applied magnetic field of 7 T. With increase of x, while the nuclear structure at room temperature for x=0.2 and 0.3 remains tetragonal, for x=0.4 it becomes orthorhombic (Imma) where the doping electrons seem to occupy mainly the d _x2-y2 symmetry. Further, the JT distortion and the antiferromagnetic interactions decrease with doping and a small negative magnetoresistance appears for . Magnetic measurements show that the dilution of antiferromagnetic interaction results into a spin glass like behaviour at low temperature for the samples with x=0.3 and 0.4. This behaviour is in contrast with the CMR properties of calcium based electron doped systems and hole doped manganites. The stability of C-type antiferromagnetic ordering in the electron doped system with large A-site cationic size may be responsible for the absence of double exchange ferromagnetism and CMR effect. Received 10 September 1999     

Effect of Cd impurity on the electrical properties of a-Se80Te20

Temperature dependence of dark and photoconductivity is studied in amorphous films of Se₈₀Te_20−xCd_x in the temperature range (300 K to 380 K) with a view to see the effect of Cd impurity on a-Se₈₀Te₂₀ binary alloy. It is observed that, at low concentration of Cd (x = 0.5), the dark and photoconductivity increase at all temperatures. However, at higher concentration of Cd (x = 10), an appreciable reduction in these parameters occurs in the same temperature range. The photosensitivity (σ_ph/σ_d) remains unchanged at x = 0.5 but decreases quite significantly at x = 10. The results are explained in terms of impurity doping at x = 0.5 and alloying effect at x = 10.     

Pseudogap and metal-insulator transitions in doped semiconductors

The electronic spectrum of a doped semiconductor described by the Anderson-Holstein impurity model and its conductivity derived from the Kubo linear response theory are calculated. Two characteristic temperatures depending on the doping level x are found in the phase diagram, T _PG and T _λ(x). The pseudogap that opens in the single-particle spectrum at low doping levels and temperatures closes at the lower one, T _PG. The pseudogap state of an insulator is attributed to spin fluctuations in a doped compound. At the higher characteristic temperature T _λ(x),, spin fluctuations vanish and the doped compound becomes a paramagnetic poor metal. Two distinct metal-insulator crossovers between semiconductor-like and metallic temperature dependence of resistivity are found. An insulator-to-poor-metal transition occurs at T ^*(x) ≈ T _λ(x). A poor-metal-to-insulator transition at a lower temperature is attributed to the temperature dependence of density of states in the pseudogap. It is shown that both transitions are observed in La_2?x Sr_xCUO₄.