Appearance Of Negative Magnetoresistance Research Articles

Особенности поглощения тонких пленок La-=SUB=-0.69-=/SUB=-Ba-=SUB=-0.31-=/SUB=-MnO-=SUB=-3-delta-=/SUB=-, полученных методом импульсного лазерного осаждения

The electrical, optical, and magneto-optical properties of La0.69Ba0.31MnO3 – δ thin films with a Curie temperature near room temperature are studied. It is shown that in the region of the phase transition there is a sharp change in the behavior of the temperature dependences of the electrical resistance and optical transparency of the films, which is associated with the insulator-metal transition. In a narrow temperature region near the transition, a change in the external magnetic field leads to the appearance of negative magnetoresistance and magnetotransmission of unpolarized light in the spectral range from 1 to 12 μm. It is shown that the effect of magnetic transmission (magnetoabsorption) in films is mainly due to the contribution of free charge carriers. The magnetic transmission spectra are sensitive to the magnetic and electronic inhomogeneities of the films.

Features of absorption of thin films La-=SUB=-0.69-=/SUB=-Ba-=SUB=-0.31-=/SUB=-MnO-=SUB=-3-delta-=/SUB=- obtained using the method of pulsed laser deposition

The electrical, optical, and magneto-optical properties of La0.69Ba0.31MnO3-delta thin films with a Curie temperature near room temperature are studied. It is shown that in the region of the phase transition there is a sharp change in the behavior of the temperature dependences of the electrical resistance and optical transparency of the films, which is associated with the insulator-metal transition. In a narrow temperature region near the transition, a change in the external magnetic field leads to the appearance of negative magnetoresistance and magnetotransmission of unpolarized light in the spectral range from 1 to 12 μm. It is shown that the effect of magnetic transmission (magnetoabsorption) in films is mainly due to the contribution of free charge carriers. The magnetic transmission spectra are sensitive to the magnetic and electronic inhomogeneities of the films. Keywords: Thin films, manganites, magnetoabsorption, colossal magnetoresistance, IR spectroscopy, metal-insulator transition, magnetic inhomogeneities.

Spin-orbit coupling in strained Ge whiskers

The field dependences of the magnetoresistance for germanium whiskers with gallium doping concentration of 2 ⋅ 1017 cm-3 were studied in the magnetic field range 0–14 T at temperature 4.2 K under compressive strain up to 0.2%. The strain influence on a spin-orbit splitting on the valence band spectrum was studied. As a result, the effective mass and the energies of spin-orbit splitting for light and heavy holes were found under the compressive strain according to kp method. From an analysis of the Shubnikov—de Haas magnetoresistance oscillations the effective mass of heavy holes mc = 0.25 m0 was calculated. The spin-orbit splitting energy of heavy holes ΔHH = 15 meV, the Lande factor in direction [111] g* = 4.8 as well as the Rashba cubic parameter of spin-orbit interaction βSO=1⋅10−28eV⋅m3 were found due to the studying of longitudinal magnetoconductance in strained Ge whiskers at low temperatures. The appearance of negative magnetoresistance in the magnetic field range up to 7 T likely results from the effect of charge carrier interaction.

Electronic transport properties of Co cluster-decorated graphene**Project supported by the National Basic Research Program of China (Grant Nos. 2013CB921900 and 2014CB920900), the National Natural Science Foundation of China (Grant No. 11374021), and the National Key Research and Development Program of China (Grant No. 2018YFA0305604).

Interactions of magnetic elements with graphene may lead to various electronic states that have potential applications. We report an in-situ experiment in which the quantum transport properties of graphene are measured with increasing cobalt coverage in continuous ultra-high vacuum environment. The results show that e-beam deposited cobalt forms clusters on the surface of graphene, even at low sample temperatures. Scattering of charge carriers by the absorbed cobalt clusters results in the disappearance of the Shubnikov–de Haas (SdH) oscillations and the appearance of negative magnetoresistance (MR) which shows no sign of saturation up to an applied magnetic field of 9 T. We propose that these observations could originate from quantum interference driven by cobalt disorder and can be explained by the weak localization theory.

Large Oxygen Nonstoichiometry in La0.77Sr3.23Co2.75C0.25O8.40+δ Oxide (δ = 0, 1.3) Related to n = 3 RP Series

An original Ruddlesden-Popper phase, La(0.77)Sr(3.23)Co(2.75)C(0.25)O(8.40+δ), was isolated and studied by electron, X-ray, and neutron diffraction. This structure has complex crystal chemistry resulting from a high degree of flexibility in the structure, comprising the disordered introduction of carbonates into a cobalt layer and an important oxygen deficiency with a preferential repartition of vacancies along the layers stacking sequence. The former is necessary for the stabilization of the system, while the latter can be tuned by postsynthetic treatment, yielding in a large variety of cobalt species formal oxidation states ranging from Co(2+)/Co(3+) in the as-made phase to Co(3+)/Co(4+) when annealed under oxygen pressure. The potential richness deriving from this flexibility is illustrated in terms of the magnetotransport properties and includes a resistivity that varies within a range of 5 orders of magnitude after modulation of the oxygen content with the appearance of negative magnetoresistance and ferromagnetic interactions due to Co(3+)/Co(4+) mixed-valence state.

Enhancement of the retrapping current of superconducting microbridges of finite length

We theoretically find that the resistance of a superconducting microbridge/nanowire {\it decreases} while the retrapping current $I_r$ for the transition to the superconducting state {\it increases} when one suppresses the magnitude of the order parameter $|\Delta|$ in the attached superconducting leads. This effect is a consequence of the increased energy interval for diffusion of the 'hot' nonequilibrium quasiparticles (induced by the oscillations of $|\Delta|$ in the center of the microbridge) to the leads. The effect is absent in short microbridges (with length less than the coherence length) and it is relatively weak in long microbridges (with length larger than the inelastic relaxation length of the nonequilibrium distribution function). A nonmonotonous dependence of $I_r$ on the length of the microbridge is predicted. Our results are important for the explanation of the enhancement of the critical current and the appearance of negative magnetoresistance observed in many recent experiments on superconducting microbridges/nanowires.

Role of electron “lakes” in the negative magnetoresistance effect in the region of Mott hopping conductivity

It is shown for doped and compensated germanium that the appearance of negative magnetoresistance under the conditions of Mott hopping conductivity may be due to the presence of a nonuniform spatial distribution of the electron density, the temperature at which the effect appears apparently being determined by the temperature at which the electron gas condenses into electron “lakes.” A “dead zone” effect was also observed in weak magnetic fields, the threshold field increasing with the nonuniformity of the electron distribution.

Microtexture and magnetoresistance of glass-like carbons

Microtextures of glass-like carbons commercially available were investigated through scanning electron microscopy and measurement of their magnetoresistance. A field-emission electron gun type SEM, which is characterized by a specimen chamber equipped in its objective lens, was operated with the acceleration voltage of 2 kV and high fidelity observations were thereby achieved with a resolving power of 2 nm. All the specimens exhibit granular microtextures with the mean grain diameter spanning 6–13 nm. The mean grain diameter correlates to the mean crystallite size L a (110) determined by the X-ray powder diffractometry. The density of each sample calculated by using the mean diameter and the X-ray parameters reproduces well its catalog value. Negative magnetoresistance was observed just for the specimens whose average grain sizes are not smaller than 7.3 nm. The crystallite size L a(110) of the specimen whose grain size shows the minimum value for appearance of negative magnetoresistance is estimated to be 2.7 nm. The absolute value of negative magnetoresistance under a constant magnetic field increases with increasing the average grain size.