In0.53Ga0.47As/In0.52Al0.48As量子阱中的正磁电阻效应

在低温强磁场条件下,对In_0.53Ga_0.47As/In_0.52Al_0.48As量子阱中的二维电子气进行了磁输运测试.在低磁场范围内观察到正磁电阻效应,在高磁场下这一正磁电阻趋于饱和,分析表明这一现象与二维电子气中的电子占据两个子带有关.在考虑了两个子带之间的散射效应后,通过分析低磁场下的正磁电阻,得到了每个子带电子的迁移率,结果表明第二子带电子的迁移率高于第一子带电子的迁移率.进一步分析表明,这主要是由两个子带之间的 关键词： 二维电子气 正磁电阻 子带散射     

Transport properties of n-type ferromagnetic semiconductor HgCr2−xInxSe4(x = 0.100)

Measurements of the electrical conductivity, magnetoresistance, and Hall effect were performed on a n-type ferromagnetic semiconductor HgCr_2?xIn_xSe₄(x = 0.100) single crystal from 6.3 to 296 K in magnetic fields up to 1.19×l0⁶A/m. The conductivity decreases rapidly near the Curie temperatureT_c (≈120 K) as the temperature is raised. A large peak in the magnetoresistance is observed near T_c. The Hall effect measurements indicate that the temperature dependence of the conductivity and the magnetoresistance are due mostly to a change in electron mobility. The electron mobility is 1.2 × 10^?2 m²/V · s at 6.3 K, and decreases rapidly near T_c with the rise in temperature. Then it increases slowly from 5.5 × 10^?4 m²/V · s at 160 K to 7.5 × 10^?4 m²/V · s at 241 K. This temperature dependence of the electron mobility can be explained in terms of the spin-disorder scattering which takes into account the exchange interaction between charge carriers and localized magnetic moments.     

A magneto-transport anomaly in semiconductors at low carrier-densities,a wigner condensation?

Transport properties of the narrow-gap semiconductors PbTe, PbSnTe, and HgCdTe were studied in the extreme magnetic quantum limit. With low carrier densities an anomalous behaviour was found for the longitudinal (?_□) and the transverse (?_⊥) magnetoresistance, for the Hall effect, and in the IU-characteristic. ?_□ increased near a critical magnetic field by almost two orders of magnitude. A pronounced kink in the ?_⊥ ∝ B^α dependence took place at the critical field. This critical magnetic field of the anomaly shows the features predicted for a Wigner condensation, i.e. the dependence on carrier density in all the three semiconductors. The new state at high magnetic fields is obviously a free carrier effect since it can be annihilated by heating the electrons in an electric field.     

Quantum limit behavior of the magnetoresistance in Hg1-x-yCdxMnyTe alloys subjected to a high pulsed magnetic field

The transverse magnetoresistance and the Hall effect of a quaternary alloy semimagnetic semiconductor Hg_1-x-yCd_xMn_yTe were measured in high pulsed magnetic fields up to 30T. The investigation of the asymptotic power law (‘_p(B) ∝ B^α) in a quantum limit regime revealed that the short range type scattering is enhanced with increasing Mn composition, suggesting the scattering associated with the localized magnetic moment of Mn.     

Critical currents of LaMo6S8 thin films

The thin films of LaMo₆S₈ were prepared by d.c. getter sputtering method. Critical current have been measured vs applied magnetic field B_⊥ up to 8 T at the temperature ranged from 1.7 to 4.2 K. The highest critical current density reached was J_c = 1.7 × 10⁷ Am^?2 at B = 0 and T = 1.7 K. Based on the measurements performed, it was possible to indicate that depending on the heat treatment applied, the pinning forces created in the LaMo₆S₈ thin films obey the scaling law.     

Carrier concentration dependence and temperature dependence of mobility in silicon (100) N-channel inversion layers at low temperatures

The carrier concentration (N_s) dependence of electron mobility in Si (100) inversion layers has been measured at temperatures T = 1.5?70K for high- and low-mobility MOSFETs. An extrinsic term is observed in the T-dependent part of the scattering probability, τ^?1_T. At T = 4.4 K, τ^?1_T depends on N_s as N^?1.9_s in low mobility samples. In high-mobility samples, τ^?1_T increases with increasing N_s in high N_s region while τ^?1_T ∝ N^?1.6_s in low N_s region. The N_s-dependence of τ^?1_T becomes weaker with increasing T in both of low- and high-mobility samples. At N_s = 3 × 10¹² cm^?2, τ^?1_T depends on T as T^1.8 in low-mobility samples and τ^?1_T ∝ T^2.0 in high- mobility samples at T 5 K.     

High field Shubnikov-De-Haas effect and magnetoresistance in the organic metal (TMTSF)2 ClO4

The transverse magnetoresistance of (TMTSF)₂ ClO₄ has been investigated in magnetic fields up to 32 T at several temperatures down to 4.2 K and different angles between B and the crystalline axis. Shubnikov-de-Haas oscillations are observed directly on the magnetoresistance at fields higher than 10 T giving a fundamental field of 259± 10 T in a direction close to c¹ axis.     

Magnetoresistance of a highly correlated electron liquid

The behavior in a magnetic field of a highly correlated electron liquid approaching the fermion condensation quantum phase transition from the disordered phase is considered. We show that, at sufficiently high temperatures T≥T*(x), the effective mass starts to depend on T, M* ∝T^?1/2. This T^?1/2 dependence of the effective mass at elevated temperatures leads to the non-Fermi liquid behavior of the resistivity, σ(T) ∝ T and at higher temperatures σ(T) ∝ T^3/2. The application of a magnetic field B restores the common T² behavior of the resistivity. The effective mass depends on the magnetic field, M*(B) ∝ B^?2/3, being approximately independent of the temperature at T≤T*(B) ∝ B^4/3. At T≥T*(B), the T^?1/2 dependence of the effective mass is reestablished. We demonstrate that this B-T phase diagram has a strong impact on the magnetoresistance (MR) of the highly correlated electron liquid. The MR as a function of the temperature exhibits a transition from negative values of MR at T→0 to positive values at T ∝ B^4/3. Thus, at T≥T*(B), MR as a function of the temperature possesses a node at T ∝ B^4/3.     

Magnetic and ESR studies of Er3+ in lanthanum dihydride LaH2

Er³⁺ electron spin resonance ESR and magnetic susceptibility have been studied in metallic lanthanum dihydride host. The ESR spectrum contains a single asymmetrical line with g-factor g = 6.68 ± 0.05 close to that expected for Γ₇ as ground state. The experimental magnetic susceptibility was interpreted on the base of LLW cubic crystal field Hamiltonian. The best fit of the experimental data has been obtained for the following B₄ and B₆ crystal field parameters: B₄ = ?5.2 × 10^?3 K; B₆ = 3.8 × 10^?5 K which support the anionic-like character hydridic model of hydrogen atoms in this hydride.     

Hall effect and magnetoresistance of (SN)x films

Hall effect and electrical conductivity have been investigated between 77 K and 300 K and the magnetoresistance at 4.2 K for a number of (SN)_x films deposited at substrate temperatures between — 10 and 50°C. The small magnitude of the Hall mobility (? 1 cm² Vsec^?1 at 300 K) and its activated temperature dependence are interpreted in terms of a heterogenous model for (SN)_x films with thin depletion layers separating highly conductive islands. The hole concentration in these islands (p ≈ 10²¹ cm^?3, the microscopic mobility (μ ≈ 500 cm² Vsec^?1 at 4.2 K) and the temperatures dependence of μ are found to be close to values for (SN)_x crystals.     

Ferrimagnetic damping of epitaxial garnet film Yb1.7Eu1.3Ga0.5Fe4.5O12

The criterion for selecting rare-earth ions in iron garnet to achieve low damping and high magnetic mobility is established using the slow ferrimagnetic relaxation theory of Van Vleck and Orbach. Ytterbium and europium are excellent candidates to form mixed rare-earth iron garnet for magnetic bubble applications. An epitaxial film of Yb_1.7Eu_1.3Ga_0.5Fe_4.5O₁₂ was grown on a {111} gadolinium gallium substrate, and its low Landau-Lifshitz damping parameter λ/γ² = 1.1 × 10^?7 oe²-sec-rad^?1 was demonstrated by measuring the ferrimagnetic resonance line width.     

Electrical resistivity and magnetoresistance of CeB6

Electrical resistivity and magnetoresistance of CeB₆ single crystal have been measured in the temperature range from 1.3 to 300 K under the magnetic field up to 85 kOe. Three characteristic phases are distinguished consistently with other measurements. The Kondo like behaviour in the resistivity observed in the high temperature phase is fitted by the conventional form for the dilute Kondo state with the Kondo temperature T_K = 5 ～ 10K and the unitarity limit resistivity $\text{?}{}_{\mathrm{u}}\text{? 110 μΩ}$ cm/Ce-atom. The negative magnetoresistance in the middle phase is stronger with increasing magnetic field and with decreasing temperature suggesting rapid destruction of the Kondo state. The magnetoresistance in the low temperature phase exhibits some anomalies suggesting sub-phases corresponding to several kinds of spin ordering.     

Observation of large magnetic resistance in weak magnetic fields using CuPt/SiO2/Si/SiO2/CuPt structure

The magnetoresistive effect of CuPt(8 nm)/SiO₂(5 nm)/Si(50,000 nm)/SiO₂(5 nm)/CuPt(8 nm) structure made by e-beam evaporation technique is studied in this work. Variation in magnetoresistance obtained by I-V measurements at 77 K and in the presence of less than 5 mT magnetic field applied in parallel to the surface is investigated. We have found that this structure exhibit large magnetoresistance in low magnetic fields (i.e. <5 mT). Our results also indicate that the variation in magnetoresistance in the presence of external magnetic field has oscillatory behavior and has the maximum value of 3295%. This structure due to its high sensitivity to low magnetic fields can also be used as an active element in magnetic field sensor devices.     

Specific features of magnetoresistance during the antiferromagnet—paramagnet transition in Tm1 − x Yb x B12

The transverse magnetoresistance Δρ/ρ(H, T) of Tm_{1 ? x} Yb _x B₁₂ single crystals is studied in the ytterbium concentration range corresponding to the antiferromagnet-paramagnet transition in a magnetic field up to 80 kOe at low temperatures. A magnetic H-T phase diagram is constructed for the antiferromagnetic state of substitutional Tm_{1 ? x} Yb _x B₁₂ solid solutions with x ≤ 0.1. The contributions to the magnetoresistance in the antiferromagnetic and paramagnetic phases of the dodecaborides under study are separated. Along with negative quadratic magnetoresistance -Δρ/ρ ∝ H ₂, the magnetically ordered phase of these compounds is found to have component Δρ/ρ ∝ H that linearly changes in a magnetic field. The negative contribution to the magnetoresistance of Tm_{1 ? x} Yb _x B₁₂ is analyzed in terms of the Yosida model for a local magnetic susceptibility.     

Manganese-doped CdGeAs2, ZnGeAs2 and ZnSiAs2 chalcopyrites: A new materials for spintronics

Based on Mn-doped chalcopyrites CdGeAs₂, ZnGeAs₂ and ZnSiAs₂ the new dilute magnetic semiconductors with p-type conductivity were produced. Magnetization, electrical resistivity, magnetoresistance and Hall effect of mentioned compositions were studied. Their curves of temperature dependence of magnetization have the similar form in spite of complicated character, for which the concentration and mobility of the charge carriers are responsible. Thus, for T<15 K, these curves are characteristic for superparamagnetics and for T>15 K for a frustrated ferromagnetics. In compounds with Zn these two states dilute by spinglass-like state. This specific feature is assigned to an attraction of Mn ions occupying neighboring sites and to the competition between the carrier-mediated exchange and superexchange interactions. Curie temperatures of these compounds are above room temperature. These are the highest Curie temperatures in the A^IIB^IVC^V₂:Mn systems.     

Galvanomagnetic effects in Fe40Ni40B20

Hall effect and magnetoresistance measurements on the amorphous ferromagnet Fe₄₀Ni₄₀B₂₀ are reported. Both properties are approximately independent of temperature (1.5–300 K) and related to magnetization. The extraordinary Hall coefficient is 3.5 × 10^?8 m³/As. The temperature dependence of the resistivity (1.5–30 K) is also reported.     

Pulsed magnetic field study of the spin gap in intermediate valence compound SmB6

In this work, we report the behavior of electrical resistivity of SmB₆ at temperatures between 2.2 and 70 K in pulsed magnetic fields up to 54 T. A strong negative magnetoresistance was detected with increasing magnetic field, when lowering the temperature in the range T<30 K. We show that the amplitude of negative magnetoresistance reaches its maximum dR/R~70% at B=54 T, in the vicinity of phase transition occurring in this strongly correlated electron system at T_C~5 K. The crossover from negative magnetoresistance to positive magnetoresistance found at intermediate temperatures at T>30 K is discussed within the framework of exciton-polaron model of local charge fluctuations in SmB₆ proposed by Kikoin and Mishchenko. It seems that these exciton-polaron in-gap states are influenced both by temperature and magnetic field.     

Absorption spectrum of Cr3+ ion doped in lithium ammonium sulphate single crystal

The absorption spectrum of Cr³⁺ ion doped in lithium ammonium sulphate single crystal has been studied both at room (300 K) and liquid nitrogen (77 K) temperatures. From the nature and position of the observed bands, O_h symmetry is assumed for the ion. The spectroscopic parameters derived for the ion in the crystal at 77 K are Dq = 1655 cm^?1, B = 735 cm^?1 and C/B = 4.4     

Plasmon reflectivity of n-type ferromagnetic semiconductor HgCr2Se4

The reflectivity of n-type HgCr₂Se₄ was measured between 86 and 320 K for photon wavenumber from 250 to 1000 cm^?1. A critical behaviour of the plasmon parameters (plasmon enegy and plasmon scattering energy ) was observed at about 160 K but not in the vicinity of the Curie temperature T_c = 110 K. The effective mass and the optical mobility exhibit a rapid variation between 150 and 170 K.     

The relation between magnetoresistance and magnetocaloric effect in La0.85Ag0.15MnO3 manganite

We present the temperature dependence of La_0.85Ag_0.15MnO₃ resistivity in the temperature interval between 77 and 340 K and magnetic fields up to 26 kOe. We offer a method of separating tunnel magnetoresistance from total magnetoresistance. A change in both the magnetic entropy, which is caused by the magnetocaloric effect (MCE), and the magnetoresistance are shown to be connected through a simple relationship to La_0.85Ag_0.15MnO₃.