Angular and temperature‐related specific features of averaging of hole effective masses in p‐Ge at low temperatures

Microwave magnetoresistance of lightly doped (nondegenerate) p‐Ge has been studied by the electron spin resonance method, which can record the derivative of the microwave absorption with respect to the magnetic field. The change in the absorption is proportional to that in the conductivity of the semiconductor in the magnetic field (magnetoresistance). It was found that the averaging time of the light and heavy holes effective masses depends on temperature and on the magnetic field direction in a sample. An analysis of the derivative made it possible to determine regions of the fastest effective mass averaging.     

Large positive magnetoresistance of insulating organic crystals in the non-ohmic region

We report a large positive magnetoresistance ratio in insulating organic crystals theta-(ET)(2)CsZn(SCN)(4) at low temperatures at which they exhibit highly nonlinear current-voltage characteristics. Despite the nonlinearity, the magnetoresistance ratio is independent of the applied voltage. The magnetoresistance ratio depends little on the magnetic field direction and is described by a simple universal function of mu(B)B/k(B)T, where mu(B) is the Bohr magneton. The positive magnetoresistance may be caused by magnetic-field-induced parallel alignment of spins of mobile and localized electrons, and a resulting blockade of electrical conduction due to the Pauli exclusion principle.     

Magnetic shielding property for cylinder with circular,square, and equilateral triangle holes

The shielding property of cylinder with circular, square, and equilateral triangle holes was investigated by finite element analysis(FEA). The hole area(S_(hole)) plays an important role in magnetic circuit on the surface of cylinder. When Sholeis less than the critical area(S_H), cylinder with three shapes of holes obtained the same remanent magnetization inside,indicating that the shielding property is unaffected by the shape of the hole. Hence, high-permeability material is the major path of the magnetic field. On the condition of S_(hole) S_H, the sequence of the shielding property is equilateral triangle square circular, resulting from magnetoresistance of leakage flux in air dielectric. Besides, the anisotropy of shielding property caused by hole structural differences of the cylinder is evaluated. We find that a good shielding effectiveness is gained in the radial direction, compared with the axis direction. This research focuses on providing a theoretical support for the design of magnetic shield and improvement on the magnetic shielding ability.     

The analysis of high field magnetoresistance data

In magnetoresistance measurements on compensated metals, both maxima and minima have been observed as the magnetic field B becomes perpendicular to the open orbit direction. In this paper we show that this is a natural consequence of the transformation of well known magnetoresistance tensors to the appropriate set of axes. The condition for observing a maximum or minimum is derived. A similar analysis applied to uncompensated metals indicates that there will always be a maximum in the magnetoresistance for B in an open orbit direction, unless B approaches the longitudinal situation, when the presence of open orbits can not be observed in either compensated or uncompensated metals.     

High-pressure thermopower of sulfur

First measurements of the thermopower and transverse magnetoresistance of sulfur at ultrahigh pressures of up to ～40 GPa are reported. The conductivity of sulfur, as that of other elemental Group VI semiconductors (Te, Se), is shown to be due to valence band holes. The variation of band gap width is derived from the pressure dependence of thermopower. The observed negative magnetoresistance of sulfur at P ～ 30 GPa indicates a low hole mobility and suggests the existence of an indirect minimum gap in the electronic spectrum. The pressure-induced variation of the electronic structure of sulfur is discussed in terms of the Peierls lattice instability model.     

块体TaP 极大磁阻及量子振荡特性研究

Weyl 半金属因其载流子满足外尔运动方程, 表现出高迁移率、 极大磁阻等新奇量子物性, 从而在无耗散电子器件应用中具有广泛应用前景. 在本文中, 我们系统研究了块体 TaP 样品的磁电输运特性, 获得了高达106 %极大的磁阻特性和显著的SdH 振荡特性. 结合TaP 样品载流子随温度的变化行为, 我们进一步揭示了块体TaP 样品的极大磁阻的物理起源, 在低温下, 其主要来源于样品费米面附近近似补偿的空穴和电子, 而在高温下则主要来源自块体TaP 样品中增强的电子散射作用. 我们的实验结果为理解 Weyl 半金属新奇量子输运特性和器件设计开发提供了实验参考.     

Magnetoresistance anisotropy in La0.67Ba0.33MnO3 films laterally compressed by a neodymium gallate substrate

The mismatch in the crystal lattice parameters induces biaxial lateral compression of 35-nm La_0.67Ba_0.33MnO₃ films coherently grown on neodymium gallate substrates. Mechanical stresses emerging during the nucleation and growth of the manganite layer facilitate the depletion of this layer in the alkali-earth element. This results in an increase in the unit cell volume in the grown films and a decrease in temperature T _M at which the resistivity attains the maximal value. The extremal values of the negative magnetoresistance (MR ≈ 17% for μ₀ H = 1 T) of the grown films are observed at temperatures close to room temperature. At T < T _M , the response of the resistivity of the films to the magnetic field depends on the direction of this field relative to the normal to the substrate plane and to the direction of the measuring current. At T = 95 K, scattering of holes from 90°-domain walls leads to an increase in the resistivity of the manganite films by approximately 1.1%, while the negative anisotropic magnetoresistance reaches 1.5%.     

Separating positive and negative magnetoresistance in organic semiconductor devices

We study the transition between positive and negative organic magnetoresistance (OMAR) in tris-(8 hydroxyquinoline) aluminium (Alq_{3}), in order to identify the elementary mechanisms governing this phenomenon. We show how the sign of OMAR changes as function of the applied voltage and temperature. The transition from negative to positive magnetoresistance (MR) is found to be accompanied by an increase in slope of log(I) versus log(V). ac admittance measurements show this transition coincides with the onset of minority charge (hole) injection in the device. All these observations are consistent with two simultaneous contributions with opposite sign of MR, which may be assigned to holes and electrons having different magnetic field responses.     

Sr2RuO4正常态的c方向的磁阻的研究

研究了典型的层状钙钛矿结构超导单晶Sr₂RuO₄在c方向的磁阻(Δρ/ρ₀)(H∥ab,J∥c)的变化.实验发现,磁阻表现出强烈的各向异性,并且随着温度T的降低,磁阻效应越明显;当在平面ab内旋转磁场H的方向时,磁阻成周期性变化;实验表明,磁场沿(110)方向时,出现磁阻的极大值.分别从Sr₂RuO₄的费米面的各向异性、载流子散射率、c方向能带色散的各向异性等方面来解释这些输运性质. 关键词： 2RuO₄')" href="#">Sr₂RuO₄ 磁阻     

Hopping conductivity and Coulomb correlations in 2D arrays of Ge/Si quantum dots

The temperature and magnetic-field dependences of the conductivity associated with hopping transport of holes over a 2D array of Ge/Si(001) quantum dots with various filling factors are studied experimentally. A transition from the Éfros-Shklovski? law for the temperature dependence of hopping conductivity to the Arrhenius law with an activation energy equal to 1.0–1.2 meV is observed upon a decrease in temperature. The activation energy for the low-temperature conductivity increases with the magnetic field and attains saturation in fields exceeding 4 T. It is found that the magnetoresistance in layers of quantum dots is essentially anisotropic: the conductivity decreases in an increasing magnetic field oriented perpendicularly to a quantum dot layer and increases in a magnetic field whose vector lies in the plane of the sample. The absolute values of magnetoresistance for transverse and longitudinal field orientations differ by two orders of magnitude. The experimental results are interpreted using the model of many-particle correlations of holes localized in quantum dots, which lead to the formation of electron polarons in a 2D disordered system.     

Hopping magnetotransport via nonzero orbital momentum states and organic magnetoresistance

In hopping magnetoresistance of doped insulators, an applied magnetic field shrinks the electron (hole) s-wave function of a donor or an acceptor and this reduces the overlap between hopping sites resulting in the positive magnetoresistance quadratic in a weak magnetic field, B. We extend the theory of hopping magnetoresistance to states with nonzero orbital momenta. Different from s states, a weak magnetic field expands the electron (hole) wave functions with positive magnetic quantum numbers, m>0, and shrinks the states with negative m in a wide region outside the point defect. This together with a magnetic-field dependence of injection/ionization rates results in a negative weak-field magnetoresistance, which is linear in B when the orbital degeneracy is lifted. The theory provides a possible explanation of a large low-field magnetoresistance in disordered π-conjugated organic materials.     

多层膜巨磁电阻特性及电流最佳测量

测量了不同方向外磁场和温度下多层膜巨磁电阻的磁阻特性,给出了巨磁电阻模拟传感器用于电流测量的最佳磁偏置.结果表明:外磁场强度相同但方向不同,对巨磁电阻的作用效果不同,巨磁电阻饱和时,阻值与外磁场方向无关.温度不同,巨磁电阻的阻值不同,磁电阻变化率也有改变.     

Magnetoresistance effect in vertical NiFe/graphene/NiFe junctions

For convenient and efficient verification of the magnetoresistance effect in graphene spintronic devices, vertical magnetic junctions with monolayer graphene sandwiched between two NiFe electrodes are fabricated by a relatively simple way of transferring CVD graphene onto the bottom ferromagnetic stripes. The anisotropic magnetoresistance contribution is excluded by the experimental result of magnetoresistance (MR) ratio dependence on the magnetic field direction. The spin-dependent transport measurement reveals two distinct resistance states switching under an in-plane sweeping magnetic field. A magnetoresistance ratio of about 0.17 % is obtained at room temperature and it shows a typical monotonic downward trend with the bias current increasing. This bias dependence of MR further verifies that the spin transport signal in our device is not from the anisotropic magnetoresistance. Meanwhile, the I—V curve is found to manifest a linear behavior, which demonstrates the Ohmic contacts at the interface and the metallic transport characteristic of vertical graphene junction.     

The tunneling magnetoresistance in GaMnAs/GaAs/GaMnAs junctions

The tunneling magnetoresistance (TMR) in GaMnAs/GaAs/GaMnAs magnetic tunnel junctions is studied under an extended coherent tunneling approach where both the contributions of the light holes and the heavy holes and their mutual competitions are investigated. It is shown that the TMR ratio can increase with decreasing the barrier strength, which is different from the results in the conventional magnetic tunnel junctions but a good news for the applications. It is also shown that the presence of the pinholes in the thin barrier layer gives a possible explanation of the peak in the barrier thickness dependence of the TMR ratio.     

Search for linear magnetoresistance in aluminum

The induced torque from high-purity aluminum for magnetic field rotation in a (100) plane exhibits little high-field linear magnetoresistance except for a narrow angular region (±3°) around 〈100〉 directions. The dimensionless slope of the high-field magnetoresistance determined from the induced torque is up to 25 times smaller than that determined previously by four-probe measurements. This suggests that the generally observed high-field linear magnetoresistance is not an intrinsic effect. An increase in the induced torque at 〈100〉 suggests the existence of open or extended orbits at high fields for this magnetic field direction.     

Interstitial vortex in superconducting film with periodic hole arrays

The response of superconducting Nb films with a diluted triangular and square array of holes to a perpendicular magnetic field are investigated.Due to small edge-to-edge separation of the holes,the patterned films are similar to multi-connected superconducting islands.Two regions in the magnetoresistance R(H) curves can be identified according to the field intervals of the resistance minima.Moreover,in between these two regions,variation of the minima spacing was observed.Our results provide strong evidence of the coexistence of interstitial vortices in the islands and fluxoids in the holes.     

EDESR and ODMR of Impurity Centers in Nanostructures Inserted in Silicon Microcavities

We present the first findings of the new electrically and optically detected magnetic resonance technique [ED electron spin resonance (EDESR) and ODMR], which reveal single point defects in the ultra-narrow silicon quantum wells (Si-QW) confined by the superconductor δ-barriers. This technique allows the ESR identification without application of an external cavity, as well as a high frequency source and recorder, and with measuring only the magnetoresistance (EDESR) and transmission (ODMR) spectra within the frameworks of the excitonic normal-mode coupling caused by the microcavities embedded in the Si-QW plane. The new resonant positive magnetoresistance data are interpreted here in terms of the interference transition in the diffusive transport of free holes, respectively, between the weak antilocalization regime in the region far from the ESR of a paramagnetic point defect located inside or near the conductive channel and the weak localization regime in the nearest region of the ESR of that defect.     

Giant magnetoresistance effect in hybrid ferromagnetic/semiconductor nanosystems

We theoretically investigate the giant magnetoresistance (GMR) effect in general magnetically modulated semiconductor nanosystems, which can be realized experimentally by depositing two parallel ferromagnetic strips on the top of a heterostructure. Here the exact magnetic profiles and arbitrary magnetization direction of ferromagnetic strips are emphasized. It is shown that a considerable GMR effect can be achieved in such nanosystems due to the significant transmission difference for electrons tunneling through parallel and antiparallel magnetization configurations. It is also shown that the magnetoresistance ratio is strongly influenced by the magnetization direction of ferromagnetic strips in nanosystems, thus possibly leading to tunable GMR devices.     

Size effect of the longitudinal magnetoresistance of thin films of n- type Ge

In a study of the longitudinal magnetoresistance as a function of the magnetic field in samples of n-type Ge of various thicknesses at 300°K and 77°K, the surface of the samples was oriented parallel to the (001) plane. The directions of the current and the magnetic field were along either the <100> direction or the <110> direction. As the thickness of the samples was reduced, regardless of the orientation of the current and the magnetic field with respect to the crystallographic directions, a decrease was observed in the magnetoresistance over the entire range of magnetic fields studied, 1–280 kOe. The experimental results are attributed to a screening-length size effect and to “sliding” orbits in the enriched surface layers, which lead to an increase in the electron mobility in a strong magnetic field.     

First principles investigation of magnesium antimonite semiconductor compound in two different phases under hydrostatic pressure

We investigate the electronic and the structural properties of Mg₃Sb₂ in cubic and hexagonal phases using the full potential-linearized augmented plane wave (FP-LAPW) method within the framework of density functional theory. The effects of hydrostatic pressure on band gap, bandwidths of bands under Fermi energy labeled by B1 and B2 from the top, the energy gap between B1 and B2 (anti-symmetry gap) and also effective masses of electrons and holes are studied using optimized lattice parameters. We observe that the hydrostatic pressure decreases the band gap and the anti-symmetry gap while it increases the bandwidths of all bands below the Fermi energy. The effective masses of electrons and holes for the hexagonal phase depend on pressure in the Γ→Λ direction. In the cubic phase the effective mass of electrons is independent of pressure and the effective mass of holes depend on the pressure in the Γ→N direction.