Universal spin-dependent variable range hopping in wide-band-gap oxide ferromagnetic semiconductors

This paper proposes a universal spin-dependent variable range hopping theoretical model to describe various experimental transport phenomena observed in wide-band-gap oxide ferromagnetic semiconductors with high transition metal concentration. The contributions of the ＇hard gap＇ energy, Coulomb interaction, correlation energy, and exchange interaction to the electrical transport are considered in the universal variable range hopping theoretical model. By fitting the temperature and magnetic field dependence of the experimental sheet resistance to the theoretical model, the spin polarization ratio of electrical carriers near the Fermi level and interactions between electrical carriers can be obtained.

Ferromagnetism,variable range hopping,and the anomalous Hall effect in epitaxial Co:ZnO thin film

A series of high quality single crystalline epitaxial Zn 0.95 Co 0.05 O thin films is prepared by molecular beam epitaxy.Superparamagnetism and ferromagnetism are observed when the donor density is manipulated in a range of 10 18 cm 3-10 20 cm 3 by changing the oxygen partial pressure during film growth.The conduction shows variable range hopping at low temperature and thermal activation conduction at high temperature.The ferromagnetism can be maintained up to room temperature.However,the anomalous Hall effect is observed only at low temperature and disappears above 160 K.This phenomenon can be attributed to the local ferromagnetism and the decreased optimal hopping distance at high temperatures.

Variable range hopping conduction in n-CdSe samples at very low temperature

We reanalyzed experimental data already published in Friedman J R, Zhang Y, Dai P, et al. Phys Rev B, 1996, 53（15）： 9528. Variable range hopping （VRH） conduction in the insulating three-dimensional n-CdSe samples has been studied over the entire temperature range from 0.03 to 1 K. In the absence of a magnetic field, the low temperature conductivity a of the three samples （A, B and C） obeys the Mott VRH conduction with an appropriate temperature dependence in the prefactor （a = σ0 exp [- （T0/T）]^p with p ≈ 0.25）. This behavior can be explained by a VRH model where the transport occurs by hopping between localized states in the vicinity of the Fermi level, EF, without creation of the Coulomb gap （CG）. On the contrary, no Efros-Shklovskii VRH is observed, suggesting that the density is constant in the vicinity of the EF.

Localization effect in single crystal of RuAs_(2)

We report the magnetotransport and thermal properties of RuAs_(2) single crystal.RuAs_(2) exhibits semiconductor behavior and localization effect.The crossover from normal state to diffusive transport in the weak localization(WL)state and then to variable range hopping(VRH)transport in the strong localization state has been observed.The transitions can be reflected in the measurement of resistivity and Seebeck coefficient.Negative magnetoresistance(NMR)emerges with the appearance of localization effect and is gradually suppressed in high magnetic field.The temperature dependent phase coherence length extracted from the fittings of NMR also indicates the transition from WL to VRH.The measurement of Hall effect reveals an anomaly of temperature dependent carrier concentration caused by localization effect.Our findings show that RuAs_(2) is a suitable platform to study the localized state.

A crossover from Efros–Shklovskii hopping to activated transport in a GaAs two-dimensional hole system at low temperatures

In this paper, we discuss low-temperature hopping-conductivity behavior in the insulating phase, in the absence of a magnetic field. We conduct a theoretical study of the crossover from hopping to activated transport in a GaAs two-dimensional hole system at low temperatures, finding that a crossover takes place from the Efros-Shklovskii variable-range hopping(VRH)regime to an activated regime in this system. This conductivity behavior in p-GaAs quantum wells is qualitatively consistent with the laws laid down in theories of localized electron interactions. Given sufficiently strong interactions, the holes in the localized states are able to hop collectively.

Electronic mobility in the high-carrier-density limit of ion gel gated IDTBT thin film transistors

Indacenodithiophene-co-benzothiadiazole（IDTBT） has emerged as one of the most exciting semiconducting polymers in recent years because of its high electronic mobility and charge transport along the polymer backbone. By using the recently developed ion gel gating technique we studied the charge transport of IDTBT at carrier densities up to 10^21cm^-3.While the conductivity in IDTBT was found to be enhanced by nearly six orders of magnitude by ionic gating, the charge transport in IDTBT was found to remain 3D Mott variable range hopping even down to the lowest temperature of our measurements, 12 K. The maximum mobility was found to be around 0.2 cm^2·V^-1·s^-1, lower than that of Cytop gated field effect transistors reported previously. We attribute the lower mobility to the additional disorder induced by the ionic gating.

Substituting Effect of Y in La_(0.5) Ba_(0.5) CoO_3

La 0.5- x Y x Ba 0.5 CoO 3 polycrystals were prepared by solid state reaction. The substituting effects of Y for La on the magnetic and transport properties of the materials were studied systematically. The results indicate that substitution of Y induces two effects. Firstly, the charge transfer from Y to 3d orbital of Co happens. This causes the molecular magnetic moment to decrease. Secondly, the antiferromagnetic exchange interaction of Co ions appears. When the content of Y is less than or equal to 30%, the non colinear structure of spins in materials is observed. When the content of Y is greater than 30%, the materials transit from predominant ferromagnetic state to predominant antiferromagnetic one. The conductive mechanism for the materials with different content of Y belongs to the variable range hopping conduction of polarons. The resistivity of materials increases sharply with increasing Y content.

Doping dependent metal to insulator transition in the(Bi,Pb)-2212 system:The evolution of structural and electronic properties with europium substitution

The present work investigates the effect of europium substitution on the （Bi, Pb）-2212 system in the concentration range 0.5 ≤ x ≤1.0. Phase analysis and lattice parameter calculations on the powder diffraction data and the elemental analysis of EDX show that the Eu atoms are successfully substituted into the （Bi, Pb）-2212 system. Resistivity measurements （64-300 K） reveal that the system exhibits superconductivity at x ≤ 0.5 and semiconductivity at x 〉 0.5. With the complete suppression of superconductivity which is known to be a quasi-two dimensional phenomenon in these materials, a metal to insulator transition takes place at x = 0.6 and the predominant conduction mechanism is found to be variable range hopping between localized states, resulting in macroscopic semiconducting behaviour. The results of electrical and structural properties of the doped （Bi, Pb）-2212 compounds suggest that the decrease of charge carrier concentration and the induced structural disorder are the more effective and dominant mechanisms in the origin of the metal to insulator transition and suppression of superconductivity due to Eu substitution at its Sr site.

Charge transport in conducting polyaniline co-doped with sulfosalicylic acid and dodecylbenzoyl sulfonic acid

We prepared conducting polyaniline （PAn） co-doped with sulfosalicylic acid （SSA） and dodecylbenzoyl sultonic acid （DBSA） in micro-emulsive polymerization, and studied its charge transport behaviors based on the measurement of its electrical conductivity in the temperature range between 203 K and 298 K. The conductivity was found to increase with temperature, similar to the case in semiconductors. Analyzing the experimental data with three models, namely the charge-energy-limitedtunneling model, Kivelson model and the three-dimensional variable range hopping （3D-VRH） model demonstrated that these models all describe well the charge transport behaviors of PAn co-doped with SSA and DBSA within the mentioned temperature range. From calculation with the 3D-VRH model, the hopping distance of the conducting PAn is obviously larger than its localization length. The PAn doped with SSA and DBSA enjoys desirable crystallinity due to the co-doping of two functional sulfonic acids. The macroscopic conductivity may correspond to three-dimensional transport in the network of the bundles, and the metallic islands may be attributed to quasi-one-dimensional bundles.

Doping Effect of Poly(vinylidene fluoride)on Carbon Nanofibers Deduced by Thermoelectric Analysis of Their Melt Mixed Films

The effect of temperature on the electrical conductivity(σ)and Seebeck coefficient(S)of n-type vapor grown carbon nanofibers(CNFs)and poly(vinylidene fluoride)(PVDF)melt-mixed with 15 wt%of those CNFs is analyzed.At 40°C,the CNFs show stable n-type character(S=-4.8μV·K^(-1))with anσof ca.165 S·m^(-1),while the PVDF/CNF composite film shows anσof ca.9 S·m^(-1)and near-zero S(S=-0.5μV·K^(-1)).This experimental reduction in S is studied by the density functional tight binding(DFTB)method revealing a contact electron transfer from the CNFs to the PVDF in the interface.Moreover,in the temperature range from 40°C to 100°C,theσ(T)of the CNFs and PVDF/CNF film,successfully described by the 3D variable range hopping(VRH)model,is explained as consequence of a thermally activated backscattering mechanism.On the contrary,the S(T)from 40°C to 100°C of the PVDF/CNF film,which satisfactorily matches the model proposed for some multi-walled carbon nanotube(MWCNT)doped mats;however,it does not follow the increase in S(T)found for CNFs.All these findings are presented with the aim of discerning the role of these n-type vapor grown carbon nanofibers on theσand S of their melt-mixed polymer composites.

Buildup of Sn@CNT nanorods by in-situ thermal plasma and the electronic transport behaviors

Monocrystal Sn nanorods encapsulated in the multi-walled carbon nanotubes(Sn@CNT NRs), were fabricated by a facile arc-discharge plasma process, using bulk Sn as the raw target and methane as the gaseous carbon source. The typical Sn@CNT NRs are 40–90 nm in diameter and400–500 nm in length. The CNTs protect the inner Sn nanorods from oxidation. Temperature dependent I–V curve and electronic resistance reveal that the dielectric behavior of Sn@CNT NRs is attributed to the multi-wall CNTs shell and follows Mott-David variable range hopping [ln R(T)∝T-1/4]model above the superconducting critical temperature of3.69 K, with semiconductor–superconductor transition(SST).Josephson junction of Sn/CNT/Sn layered structure is responsible for the superconducting behavior of Sn@CNT NRs.