Decrease Of Localization Length Research Articles

Sign reversal of magnetoresistance in a perovskite nickelate by electron doping

We present low temperature resistivity and magnetotransport measurements conducted on pristine and electron doped ${\mathrm{SmNiO}}_{3}$ (SNO). The low temperature transport in both pristine and electron-doped SNO shows a Mott variable range hopping with a substantial decrease in localization length of carriers by one order in the case of doped samples. Undoped SNO films show a negative magnetoresistance (MR) at all temperatures characterized by spin fluctuations with the evolution of a positive cusp at low temperatures. In striking contrast, upon electron doping of the films via hydrogenation, we observe a crossover to a linear nonsaturating positive $\mathrm{MR}\ensuremath{\sim}0.2%$ at 50 K. The results signify the role of localization phenomena in tuning the magnetotransport response in doped nickelates. Ionic doping is therefore a promising approach to tune magnetotransport in correlated perovskites.

Investigation of Brewster anomalies in one-dimensional disordered media having Lévy-type distribution

In this paper, we calculate the localization length of a TM electromagnetic wave in unit of system length versus incident angle in a disordered layered structure where the refractive index of one of its constituents follows a Levy-type distribution with a power exponent α. The incident angle at which the localization length takes the maximum value is called the generalized Brewster angle as before. However, in contrast to previous works with a weak disorder, the wave incident at generalized Brewster angle is not always in the extended regime. For special values of α and the frequency, the system is in a localized state at this angle. But, the localization length at this Brewster angle is always larger than that at other angles. The effects of α variation on the localization length at this Brewster angle and its position are investigated for different frequencies. The localization at this angle degrades with increasing α for all frequencies. At some working frequencies, the generalized Brewster angle is a decreasing function of α. However, at other frequencies, the dependence of generalized Brewster angle on α is not monotonic. For incident angles smaller than a specific angle, the localization length increases with increasing α. However, for incident angles larger than this specific angle, there are incident angles at which any increase of α leads to the decrease of localization length. In other words, for these incident angles, the improvement of Anderson localization surprisingly happens with decrease of disorder strength and the refractive index contrast.

Room temperature positive magnetoresistance via charge trapping in polyaniline-iron oxide nanoparticle composites

We demonstrate a polyaniline-iron oxide nanoparticle (PANI-NP) organic hybrid composite device with room temperature positive magnetoresistance of 85.7%. Temperature dependent resistivity measurements attribute this observation to the decrease in localization length of the charge carriers in the presence of an external magnetic field which result in them being trapped within the device between the insulating PANI layer, hence allowing the device to maintain its resistive state even when the power is switched off, thus exhibiting a memory effect.

Optical and electrical resistivity studies of isovalent and aliovalent3dtransition metal ion doped ZnO

In this paper, we report on the optical and electrical transport studies of TM ion (Ni, Ti, V, Fe, Cr, Mn, and Co) doped ZnO polycrystalline samples. Diffuse reflectance spectroscopy of doped ZnO showed the existence of absorption bands which were attributed to the $d\text{\ensuremath{-}}d$ transitions of respective dopants. Electrical resistivity was found to decrease in case of Ti-, V-, Fe-, and Ni-doped ZnO bulk samples as compared to undoped. We explain the above behavior on the basis of impurity $d$-band splitting model. It is observed that with increase in dopant content the temperature range where variable-range hopping is valid shrinks to lower values and the activation energy lowers. Increase in characteristic temperature and decrease in localization length was observed with increase in TM ion content pointing towards the delocalization of electrons that sets in with doping.

Metal-insulator transition below the Curie temperature in layered manganites

In contrast to the traditional viewpoint that a metal-insulator transition (MIT) occurs just at the resistivity peak temperature ${T}_{p},$ we propose that in perovskite manganites with temperature-dependent disorder, ${T}_{\mathrm{MI}}$ at which the MIT occurs may be well below the Curie temperature ${T}_{C}(\ensuremath{\simeq}{T}_{p}).$ At temperatures between ${T}_{\mathrm{MI}}$ and ${T}_{C},$ due to a decrease in localization length upon heating, the resistivity of the localized electrons behaves with metallic temperature dependence. Only above ${T}_{C}$ does the resistivity exhibit semiconductorlike temperature behavior. Such an idea that the MIT occurs below ${T}_{C}$ can account for the anomalous resistivity behavior in the layered manganite crystal ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ with $x=0.3.$

LOCALIZATION EFFECTS IN HIGH‐TEMPERATURE SUPERCONDUCTORS

Basic facts on the interplay of Anderson loalization and superconductivity in high‐T c oxides are presented. The ‘‘minimal metallic conductivity’’ for the quasi‐two‐dimensional case is enhanced owing to a small overlap of electronic states on the nearest neighbor conducting planes. This leads to a much stronger influence of localization effects than in ordinary (three‐dimensional) superconductors. From this point of view high‐temperature oxides are very close to the Anderson transition even for rather weak disorder. Anomalies of the upper critical field are also analyzed as well as degradation of T c under disordering, due to the enhanced Coulomb effects caused by the disorder‐induced decrease of localization length.