Type Variable Range Hopping Research Articles

Temperature dependent carrier transport in few-layered MoS2: from hopping to band transport

Understanding the carrier transport mechanisms is critical for electronic devices based on 2D semiconductors. Here, using a two-terminal device configuration, we show that the carrier transport behaviours in chemical vapour deposited few-layer MoS2 transition from resonant tunnelling to hopping, and eventually to band transport as the temperature increases from 5 K to 370 K. Specifically, the transport in the channel is dominated by resonant tunnelling when T < 30 K is reflected in the temperature-independent conductance. At 50 K < T < 110 K, the channel conductance exhibits a dependence of exp(T 1/2), a signature of Efros–Shklovskii type variable range hopping (VRH). At 110 K < T < 160 K, carrier transport behaves in a transition region with potential attribution to Mott-type VRH. At 160 K < T < 210 K, the nearest neighbour hopping mechanism is confirmed by the linear dependence from the resistance curve derivative analysis. For VRH, the localization length, hopping distance and energy, Coulomb gap energy and density of states are extracted. At T > 210 K, the carrier transport is dominated by thermally activated band transport based on AC conductance and mobility analysis. These findings are significant for revealing the material properties for future 2D semiconductor device applications.

Semiconducting nature and magnetoresistance behaviour of ZnO / La0.3Ca0.7MnO3 / SrTiO3 heterostructures

In this report, ZnO /La0.3Ca0.7MnO3 /SrTiO3 (ZnO /LCMO /STO) heterostructures, with various thicknesses of LCMO, were grown successfully using chemical solution deposition (CSD) method. X–ray diffraction (XRD) measurement confirms the presence of LCMO and ZnO phases only without any other unwanted phases. To understand the charge conduction mechanisms across ZnO /LCMO interface as well as for LCMO manganite thin layer, temperature dependent resistivity measurements have been investigated under different applied magnetic fields. The measured ρ (T) data of all the heterostructures in current perpendicular to plane (CPP; for ZnO /LCMO interface studies) mode and current in plane (CIP; for LCMO thin layer studies) mode show the semiconducting behaviour throughout the temperature and magnetic field range studied. The semiconducting state of all the heterostructures have been studied using Mott type variable range hopping (VRH) mechanism to understand the field dependence of charge carrier localization length. Observed negative magnetoresistance (MR) has been discussed in detail with its dependence on applied magnetic field, temperature and manganite layer thickness.

Transport properties, charge conduction mechanism and magnetic behavior of La0.3Ca0.7MnO3:ZnO Nanocomposites

Nanocomposites consist of ZnO and La0.3Ca0.7MnO3 (LCMO) nanoparticles, prepared by cost effective sol–gel method, have been studied for their transport and magnetoresistance (MR) behaviors. Different ZnO nanoparticles contents (weight ratio) were mixed with LCMO nanostructured manganite to understand the possible interactions between ZnO and LCMO nanoparticles. X–ray diffraction (XRD) measurement was performed for all composites and structural quality was verified by performing Rietveld refinements. Variation in an average crystallite size for LCMO and ZnO nanoparticles in different composites has been discussed. Temperature dependent resistivity measurements under different applied magnetic fields suggest the semiconducting nature of all nanocomposites under all applied magnetic fields. Mott type variable range hopping (VRH) mechanism has been employed to understand the dependence of charge carrier localization in LCMO nanostructured manganite lattice on the applied magnetic field and ZnO nanoparticles content within the nanocomposites. Observed MR behavior has been understood under the influence of temperature and ZnO nanoparticles content in the context of spin polarized tunneling (SPT) and magnetic field induced improved conduction across the manganite lattice. Grain and grain boundary contributions to the MR behavior have been explained by using theoretical model fits to obtained MR data at different temperatures for all studied LCMO:ZnO nanocomposites. Magnetic nature of studied LCMO:ZnO nanocomposites has been understood by performing zero field cooled (ZFC) and field cooled (FC) measurement protocols based magnetization and magnetic isotherms measurements that confirm the coexisting magnetic phase in all studied nanocomposites. Magnetic nature has been understood on the basis of weakening of zener double exchange (ZDE) mechanism within the magnetic lattice of LCMO manganite and magnetic contribution from the free charge carriers within the ZnO clusters.

Charge localization crossover from Mott to Efros-Shklovskii type variable range hopping mechanism in In1−xPbxTe compounds

We have investigated the electrical transport properties of the In1−xPbxTe compounds. Small Pb-doping is not incorporated in the In-site but randomly distributed in the matrix, found from the X-ray diffraction and elemental mapping by energy-dispersive X-ray spectroscopy measurements. The random distribution of Pb elements induces robust charge localization at low temperatures causing the variable range hopping (VRH) transport. The electrical resistivity ρ(T) of pristine InTe exhibits semiconducting to metal transition near 175 K. From the analysis of temperature exponent, we found that the VRH transport is changed from Mott to Efros-Shklovskii (ES) type with decreasing temperature, where the crossover temperatures are found as 14.4 K (x=0.01) and 13.36 (x=0.02), respectively. The magnetoresistance (MR) of the InTe shows that the weak antilocalization at low temperature (T≤3 K) and low magnetic field (H≤1 T) competes with weak localization with increasing temperature (T≥5 K). It is noteworthy that small Pb-doping exhibits unconventional negative MR (NMR) behavior because it is not a magnetic or topological material. The unconventional NMR behavior of Pb-doped compounds is attributed to the quantum mechanical interference under the magnetic field in ES type VRH transport. The charge localization crossover from Mott to ES type VRH transport mechanism suggests the strong electron-electron Coulomb interaction in the compounds, leading to the significant change of density of states and inducing the Coulomb gap at low temperatures.

Studies on transport properties of manganite based nano–micro particles–matrix composites

In this communication, low resistive bulk La0.7Ca0.3MnO3 (LCMO) was prepared by solid state reaction (SSR) route whereas high resistive nanoparticles of LaMnO3 (LMO) was prepared by cost effective sol–gel technique. Composites of LMO and LCMO were prepared by mixing them in different weight ratios, i.e. LMO = 10%, 20% and 30% in LCMO manganite matrix. In the present study, LMO have been used as nanoparticle fillers in the LCMO matrix lattice to study the transport properties of the composites. X–ray diffraction (XRD) measurement reveals the single phasic nature of the composites without any detectable impurity within the measurement range studied. XRD pattern of higher LMO content based composite possesses slightly different structural phases of LMO and LCMO. Resistivity measurement reveals that all the composites exhibit metal to insulator electronic phase transition at respective TP, wherein values of resistivity and TP get modified due to the cooling or heating cycles performed, measurement current used and LMO nanoparticle filler content in the composites. These variations have been discussed in the context of scattering centres created due to the presence of LMO fillers and complex phase separation scenario of the composites. A clear phase separation in higher filler content based composite has been displayed in its resistivity behaviour. To understand the charge transport mechanisms responsible for metallic and insulating (or semiconducting) regions in resistivity behaviours of the composites, most suitable zener double exchange polynomial law and Mott type variable range hopping mechanism were fitted theoretically to the obtained experimental results. Fittings show that spin fluctuations in the composites are highly dependent on the cycles performed, current applied and LMO nanoparticle filler content in the composites. Applied negative and positive current and voltage dependent resistance of the composites reveal the negative electroresistance. Both, resistance of the composites as well as electroresistance are highly affected by the cycles performed, current applied and LMO nanoparticle filler content in the composites understudy. Temperature dependent electroresistance was studied for better insight into an active role of phase separation near electronic phase transition temperatures for all manganite based composites.

AC conductivity and magneto-dielectric permittivity of GaV4S8 below structural transition

GaV4S8 is a Mott insulator having structural transition at ∼44 K and ferromagnetic ordering below ∼12 K. In this article, we present the results of the dc resistivity, ac conductivity, dielectric constant and the magneto-dielectric effect (MDE) measurements on this compound. We observed a crossover from Mott type to Effros-Sklovskii type variable range hopping in going from higher to lower temperature. Frequency dependence of conductivity suggests the hopping transport of the carriers between the pair centers only. The Magneto-dielectric coupling (maximum value of 20% near 22 K) is maximum in the intermediate region of structural and magnetic transitions where exchange splitting of the Fermi surface develops the energetically favorable state for ferromagnetic ordering.

Investigations on Charge Transport Mechanisms in Mixed Valent La0.5Pr0.2Ba0.3–xCaxMnO3 Manganites

In this communication, we report the results of the studies on the structural and transport properties of La0.5Pr0.2Ba0.3–xCaxMnO3 (LPBCMO; x = 0.00, 0.05, 0.15, 0.20 & 0.25) manganites, synthesized by conventional solid state reaction route. All the samples possess distorted orthorhombic structure verified by Rietveld refinements. Temperature dependent resistance of LPBCMO samples shows an exhibition of metal to insulator (semiconductor) phase transition at temperature TP. To understand the charge transport mechanisms responsible for these two electronic phases, zener double exchange (ZDE) polynomial law and Mott type variable range hopping (VRH) model have been used for metallic and semiconducting regions, respectively. Possible nature of the charge carrier interactions with the lattice and activation energy have been discussed on the basis of these theoretical fittings to the resistance data.

Crossover from diffusive to tunneling regime in NbN/DyN/NbN ferromagnetic semiconductor tunnel junctions

We have investigated NbN-DyN-NbN junctions with 1 to 10 nm thick DyN barriers. A crossover from diffusive (hopping) to tunneling-type transport was found in these junctions as the DyN thickness is reduced below $\ensuremath{\sim}$4 nm. We have also made a detailed study of magnetic and electrical properties of thicker DyN thin films deposited under similar conditions; DyN films were found to be ferromagnetic with ${T}_{\text{Curie}}\ensuremath{\sim}35\ifmmode\pm\else\textpm\fi{}5$ K. Electrical transport of the junctions with $\ensuremath{\sim}$10 nm DyN was understood in terms of Shklovskii-Efros (SE)-type variable range hopping (VRH) at low temperature between 90 and 35 K. We estimated localization length $\ensuremath{\xi}=5.6$ nm in this temperature range. Temperature dependence of resistance was found to deviate from SE-VRH below 35 K along with large suppression of resistance with magnetic field. This is correlated with onset of magnetism below 35 K. Large butterfly-shaped MR up to $\ensuremath{\sim}$40$%$ was found for the $\ensuremath{\sim}$10 nm thick DyN junction at 2 K. In the tunneling regime, barrier height of the tunnel junction was estimated $\ensuremath{\sim}$50 meV from the Simmons model. Signatures of spin filtering was found in temperature dependence of resistance in tunnel junction with $\ensuremath{\sim}$3 nm thick DyN. Cooper pair tunneling in these junctions below ${T}_{C}$ ($\ensuremath{\sim}$10.8 K) of NbN was understood according to S-I-S tunneling current model. We found coherent tunneling of Cooper pairs through a $\ensuremath{\sim}$1 nm thick DyN tunnel barrier with critical current ${I}_{C}\ensuremath{\sim}12$ $\ensuremath{\mu}$A. The critical current also showed modulation with magnetic field.

Polypyrrole decorated graphene nanostructure: Fabrication, depiction and anomalous dimensional crossover in electronic conduction

We had fabricated polypyrrole based graphene nanostructure by using CTAB (cetyl trimethylammonium bromide) surfactant with variation of graphene loading and characterized them by XRD, FT-IR, SEM, TEM, AFM, Raman etc. Variable range hopping (VRH) model had been applied for explication of temperature dependent D.C. conductivity scrutinized from 20 to 300K. A synergetic effect of π-electron flow in conducting polymer polypyrrole and 2D graphene layer was appeared to demonstrate the plausible charge-transport mechanism. Dramatically, two peculiar crossovers had been observed simultaneously in the composite samples with variation of temperature. A 3D–2D effective dimensional crossover had been observed at ≈100K with variation of hopping length which can be controlled by temperatures. Below ≈50K another cross-over between Mott and ES (Efrost & Shklovskii) type VRH mechanism had also been explored as resistivity increases with lowering of temperature. Here, charge transport occurred via VRH between intact graphene island and polypyrrole chain.

Electronic structure and electrical transport properties of LaCo1−xNixO3 (0 ≤ x ≤0.5)

The structural analysis of LaCo1−xNixO3 (0 ≤ x ≤ 0.5) samples assures single phase rhombohedral structure with space-group R3¯c. Electronic structure of these samples has been studied by x-ray absorption near edge spectroscopy (XANES) at K-edge of O, Ni, Co and M5,4 edge of La. These studies confirm the trivalent state of Co/Ni and La in all the compositions. Upon substitution of the Ni at the Co site in LaCoO3, the O K-edge spectra show a feature about 1.2 eV lower than that of LaCoO3. This feature keeps on growing as the concentration of the Ni is increasing. This is consistent with our resistivity data which shows drastic decrease in resistivity with the Ni substitution. The resistivity data have been analyzed using Arrhenius and Efros-Shklovski's type variable range hopping models in different temperature ranges. The activation energy decreases and localization length increases systematically with increase in the Ni concentration. Observed features have been explained on the basis of change in charge-carrier density with substitution. The disorder-induced localization of carriers is found to govern conduction mechanism and resistivity behavior in substituted sample.

Variable-range-hopping conduction and low thermal conductivity in chalcogenide spinel CuyFe4Sn12X32 (X = S, Se)

Low-temperature thermoelectric properties are reported for polycrystalline samples of chalcogenide spinel CuyFe4Sn12X32 (X = S, Se; y = 8.0 to 4.0). For all samples with X = S, the electrical resistivity ρ behaves similarly to that of a doped semiconductor at high temperatures and shows a variable-range-hopping-type (VRH) temperature dependence at low temperatures. The values of ρ and the thermopower S decrease concomitantly with decreasing initial Cu composition y. For y=6.0 of the sample with X = Se, ρ shows VRH behavior in a wide temperature range. Because of the monotonic increase of S as T1/2 and the exponential decrease of ρ with increasing temperature due to the the VRH conduction, the sample with X = Se can be a good high-temperature thermoelectric material. Furthermore, both systems show a low thermal conductivity of 1.5 W/Km because of their complex structures.

Importance of double-exchange interaction in low-doped ferromagnetic insulator [formula omitted

Transport and magnetic properties of ferromagnetic (FM) insulating La 0.875Ca 0.125MnO 3 (LCMO) manganite have been reported. Temperature variation of field cooled (FC) magnetization shows paramagnetic to ferromagnetic (PM–FM) transition at 168 K ( T C ) while ac susceptibility exhibits an additional cusp like downfall around 115 K ( T f ), which shifts with frequency indicating a frustrated magnetic state. Resistivity shows insulating behaviour with a distinct inflection near T C . The Shklovskii–Efros (S–E) type variable range hopping mechanism is seen to govern the conduction within the temperature range 156–282 K. Thermoelectric power (TEP) exhibits a peak at ∼ T C and the field dependant resistivity shows different curvatures above and below T C . Such behaviours suggest that the PM–FM transition is mediated by double exchange (DE) mechanism similar to that observed in FM metallic manganites. Unexpectedly, magnetoresistance data analysis shows that DE mechanism persists in this sample even down to 50 K. Thus the DE mechanism plays an important role even in case of ferromagnetic insulating manganite La 0.875Ca 0.125MnO 3, governing the magnetic and electrical properties.

A temperature-dependent pre-exponential factor in Efros–Shklovskii variable range hopping conduction in p-type CuInTe 2

We report on the temperature dependence of the electrical resistivity down to 0.4 K in a single crystal sample of CuInTe 2 with a hole concentration at room temperature of 2.32×10 18 cm −3 . Mott type variable range hopping (VRH) conduction is observed in the temperature range from 90 to 53 K . A crossover to Efros–Shklovskii (ES) type VRH conduction with a temperature-dependent pre-exponential factor, i.e. ρ= ρ 0 T α exp( T 0/ T) 0.5 with α=0.18, is seen below 25 K . This is the first time that a crossover from Mott to ES VRH with a pre-exponential factor has been reported.

Pressure effect on the electrical resistivity of the filled skutterudite compound CeOs4Sb12

Abstract The electrical resistivity of the filled skutterudite compound CeOs4Sb12 has been measured at temperature 2– 300 K under high pressure from 2 to 8 GPa . The resistivity shows semiconducting behavior below about 70 K and is proportional to exp [(T ∗ /T) 1/2 ] below about 20 K at 2 GPa . It is found that the reciprocal characteristic temperature 1/T ∗ is proportional to the pressure P. The dependences of resistivity on both temperature and pressure are consistent with Efros–Shklovskii type variable-range hopping (VRH) conduction. This fact indicates the importance of long range Columb correlations in the semiconducting resistivity in this compound.

Electron Assisted Variable Range Hopping in Strongly Correlated 2D Electron Systems

We have investigated the temperature dependence of longitudinal resistivity of high mobility two dimensional (2D) electron systems in the strongly correlated regime (rs ∼ 4–5). The area of the samples was relatively small, typically 2 × 2 μm2. The resistivity followed an Efros–Shklovskii (ES)-type variable range hopping (VRH) behavior, ϱ (T) = ϱ0 exp (T0/T)p, with p = 0.5. We found that in this regime, the pre-factor ϱ0 increases by discrete steps of h/2e2 as the carrier concentration (n) is increased, starting from ϱ0 = h/e2 at the lowest values of n close to the localization. The jumps in ϱ0 were associated with an oscillation in the activation energy defined by T0. We have discussed the observation of such quantized, temperature-independent resistivity pre-factors in the framework of electron assisted VRH.

Thermally induced variable-range-hopping crossover and ferromagnetism in the layered cobalt oxideSr2Y0.5Ca0.5Co2O7

Temperature-dependent transport and magnetic measurements on Sr{sub 2}Y{sub 0.5}Ca{sub 0.5}Co{sub 2}O{sub 7} indicate that ferromagnetism appears along with a crossover between two forms of variable-range-hopping (VRH) conductivity on cooling. Efros-Shklovskii (ES)-type VRH conduction was found below approximately 30 K, transformed from Mott-type VRH at higher temperature. The magnitude of the Coulomb gap and the Mott-ES VRH crossover temperature are {approx}57 K and {approx}170 K, respectively. These are unusually large compared to those of nonmagnetic disordered materials. The peculiar electronic state for the ferromagnetic Coulomb gap is probably due to Coulomb correlations among d electrons in the disordered system.