Variable-range Hopping Conduction Research Articles

Role of Fe supplantation in tailoring structural, electrical conductivity, and magnetic properties in La2Ni1-xFexMnO6 double perovskites

Double perovskite system La2Ni1-xFexMnO6 (x = 0, 0.2, 0.5, 0.8 & 1.0) were developed using sol-gel method. Rietveld refinement revealed monoclinic structure (ordered) along with occurrence of rhombohedral and orthorhombic structures suggesting antisite cationic disorder. Crystallite size and average grain size of samples varied between 454 and 1058 Å and 0.774–1.429 μm respectively. XPS revealed the dual oxidation states of Ni, Fe, and Mn ions. La2NiMnO6 attained the highest DC conductivity among all compositions. The increase in DC conductivity with temperature suggested the semiconducting behavior of the samples, that explained by Thermal Activation (TA) and Variable Range Hopping (VRH) conduction mechanisms. Obtained activation energy(Edc) showed increasing trend with temperature. M-H loops exhibit a ferromagnetic nature accompanied by antiferromagnetic interactions as suggested by exchange bias and partially saturated loops. La2NiMnO6 attained the highest saturation magnetization (29.81emu/g) and decreased with Fe supplantation. The possible mechanism of tuning conductivity and magnetic properties of La2Ni1-xFexMnO6 has been discussed in detail.

Structural, magnetic, magnetocaloric behavior and magneto-transport, electrical polarization study in 3d based bulk and nano-crystalline multiferroic double perovskite Dy2MnCoO6

In this paper, we report a detailed investigation of the crystal structure, magnetic, magnetocaloric, magneto-transport and electrical polarization properties of a new multiferroic material in the polycrystalline and nanocrystalline form of the Dy2MnCoO6 double perovskite. Both compounds crystallized in the monoclinic structure with P21/n space group. The magnetic properties of both systems are mainly dominant ferromagnetic (FM) and weak antiferromagnetic (AFM). The FM/AFM coupling is related by the competing and combining functions of the radius and the magnetic moments of rare earth ions (i.e. 3d–4f exchange interactions). The reduction of the saturation magnetization in the isothermal magnetization curves can be explained by the existence of anti-phase boundaries and local anti-site defects in the system. Moreover, these materials hold reasonable values of magnetocaloric parameters and the absence of hysteresis makes the system a potential candidate for magnetic refrigeration. These compounds revealed two magnetic phase transitions, according to the appearance of two peaks in the temperature dependence of magnetic entropy change curves. The temperature dependent resistivity data for both the systems display semiconductor nature near room temperature and insulating like behavior at low temperature regime. The variable-range hopping conduction mechanism is used to best understand their transport mechanism. In addition, the electrical polarization loop at low temperature confirms the presence of ferroelectricity for both the studied systems. The decreases polarization under an external magnetic field evidence the weak magnetoelectric coupling. The coexistence of FM ordering with insulating behavior and ferroelectricity at low temperature promises new opportunities and improvements in next generation applications for information storage, spintronic, and sensors.

Yttrium ion influence on the structural, electrical, dielectric, and magnetic properties of α-Fe2O3 nanoparticles synthesized via Sol-Gel method

In this study, pure and Y3+ substituted Hematite (α-Fe2O3) nanostructure with various compositions, α-Fe2-xYxO3 (x = 0.00, 0.02, 0.05, 0.08 & 0.10), synthesized by using the Sol-Gel method. The X-ray diffraction (XRD) patterns of both pure and Y-substituted α-Fe2O3 samples revealed a rhombohedral phase with the R3¯c space group, having crystallite sizes ranging from 23 to 48 nm. Moreover, the XRD findings align closely with Raman observations. The variation in grain size observed via FE-SEM correlates well with the crystallite size and strain determined by XRD measurements. The resistivity plots exhibited two distinct regions corresponding to two different conduction mechanisms. Analysis of the resistivity data suggests the presence of variable range hopping and nearest-neighbour hopping conduction mechanisms within temperature ranges of 100 K to 300 K and 300 K to 400 K, respectively. The dielectric and magnetic properties exhibit a fluctuating behaviour with the increasing concentration of the Y3+ doping. We observed a shift towards weakly ferromagnetic conduct-like behaviour with the introduction of Y3+ into the hematite nanostructures.

Inducing Crystal Phase Transformation of Titanium Oxide by Pulse Laser Irradiation

To selectively pattern titanium oxide with desired properties, we report inducing crystal phase transformation from rutile TiO2 to γ/λ-Ti3O5 and corundum Ti2O3 by pulse laser irradiation. The crystal phase of the produced titanium oxide could be systematically controlled by laser fluence. This crystal phase transformation simultaneously increased the electrical conductivity with changing the electrical conduction mechanism from band conduction to variable-range hopping conduction. This report also proposes the mechanism of the crystal phase transformation. Comparing thermodynamic calculations and finite element simulations of the temperature rise due to laser irradiation showed that this phase transformation can be interpreted as a reduction reaction caused by laser heating. Our findings indicate that laser irradiation can spatially pattern various crystal phases of titanium oxides on a substrate, and this technique will enable the regioselective, fine, and highly productive fabrication process of titanium oxide-based electronic devices.

Double perovskite material of highly active, monoclinic, ordered Ni2ZnMoO6 for an effective quasi-solid state supercapacitor

In the present study, monoclinic Ni2ZnMoO6 double perovskites (monoclinic NZMO DP) electrodes were synthesized using hydrothermal method for supercapacitors application. The structural, elemental, vibrational, and optical characterization were analyzed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy-energy Dispersive X-ray analysis (HR-SEM & EDX), Transmission Electron Microscopy (TEM), Brunauer-Emmett-Teller (BET), Fourier Transform Infrared spectroscopy(FTIR) and UV-Visible Diffusion reflectance Spectroscopy (UV-DRS). A monoclinic structure with a space group of P 2/m and lattice parameters of a = 11.4377, b = 9.2961, c = 6.8301, and β = 97.710 was fitted using Rietveld refinement, and the cell volume was 719.6458 Å3. The surface area of monoclinic NZMO DP was found to be 50.45 m2 g−1 with a pore volume of 0.15 cm3 g−1. Electrochemical characterization was done using Cyclic voltammetry (CV), Electrochemical Impedance spectroscopy (EIS), and galvanostatic charge-discharge (GCD) measurements. An equivalent-circuit model R(CR)(CR) is iterated to interpret the EIS responses. The Rct found to be less than 100 ohms in the present study can be attributed to Mott variable range hopping (VRH) conduction. The GCD analysis of ASC monoclinic NZMO DP//AC in 6 M KOH at 0.5 Ag−1 in the potential window of 0–1.6 V yields a maximum specific capacitance of 160.45 Fg−1. Excellent electrochemical stability of the monoclinic NZMO DP was ascertained by the specific capacitance retention of the electrode material at 91.37 % after 6000 cycles. The ASC-NZMO monoclinic DP shows an excellent power density of 646.25 W/kg with an energy density of 20.53 Wh/kg. The excellent power storage efficiency and application possibilities of supercapacitors make them a promising technology in various fields.

Structural, dielectric and magnetic study of double perovskite La2CoMnO6

Polycrystalline double perovskite La2CoMnO6 sample was prepared via solid state reaction route and its structural, dielectric and magnetic properties were investigated. XRD confirmed the phase formation without any impurity. Rietveld refinement analysis suggested the presence of monoclinic (P21/n, ordered) as well as rhombohedral and orthorhombic (R3c &Pbnm, disordered) structures. Microstructural analysis using FESEM confirmed that average grain size lies in micrometer range. Dielectric constant (log ε′) and loss tangent (tanδ) both exhibited large values at lower frequencies as well as in higher temperature range. Nyquist impedance plot proposed NTCR (negative temperature coefficient of resistance) nature of double perovskite sample. DC conductivity showed increasing trend with increase in temperature and also, the conductivity is of the order of 10−2 S/m, which represent semiconducting behavior of prepared sample. Also, the analysis suggested the co-existence of thermal activation (TA), small polaron hopping (SPH) and variable range hopping (VRH) conduction mechanisms. Temperature dependent magnetic measurements (M-T) suggested that sample exhibit single magnetic transition at 226 K. Field dependent magnetic measurements (M-H) confirmed the ferromagnetic nature of the prepared sample. M-H loop is not well saturated even at 60 kOe which suggested the presence of anti-site disorder. Multi-domain magnetic structure of the prepared sample suggests its suitability for memory device applications.

Negative Hall coefficient in band conduction region in heavily Al-doped 4H-SiC

At low temperatures, the Hall coefficients in heavily Al-doped 4H-SiC are reported to be negative in the band conduction region as well as in the hopping conduction regions (i.e., nearest-neighbor hopping conduction region and variable-range hopping conduction region). A physical model was proposed to explain the negative sign of RH(T) in the hopping conduction regions. However, the negative value of RH(T) in the conduction band region remains unexplained. This study proposed a physical model to explain the negative value of RH(T) in the conduction band region. In addition to the valence band, doping copious amounts of Al acceptors in 4H-SiC causes a strong overlap of the wave functions of the excited states of Al acceptors, which results in the formation of allowed bands, referred to as allowed minibands. Although the holes can flow freely through the valence band as well as the allowed minibands, the energy–momentum relationship in the valence band and the allowed minibands determines the sign of RH(T). As elucidated here, if the holes flow primarily in the lower parts of the allowed minibands, the RH(T) in the band conduction region becomes negative, whereas if the holes flow primarily in the upper parts of the allowed minibands and the valence band, the RH(T) becomes positive.

Carrier transport across PtSe2/n-type GaN heterojunction

We fabricated PtSe2/n-type GaN heterojunction diode by transferring a large-scale grown two-dimensional (2D) PtSe2 layered film onto a 3D GaN substrate. The PtSe2 film transferred onto GaN was ∼5.84-nm thick and had 11 layers with interlayer spacing of 0.52 nm. An exploration of the temperature-dependence of the current–voltage (I–V) characteristics of the PtSe2/n-type GaN heterojunction in the range 125–400 K revealed a strong dependency of the ideality factor and barrier height on measurement temperature. This indicates a presence of low- and high-barrier-height patches resembling barrier inhomogeneities along the interface. A barrier inhomogeneity analysis with a consideration for the Gaussian barrier heights distribution demonstrated the existence of a double Gaussian barrier distribution having a mean barrier height of 0.97 and 1.41 eV in low- and high-temperature regimes, respectively. An examination of the temperature-dependency of reverse leakage current indicated the dominance of variable-range hopping conduction in lower temperature region (<225 K) and Poole–Frenkel emission mechanism in higher temperature region (>250 K). The interface state density Nss of the PtSe2/n-type GaN heterojunction determined from the capacitance method was lesser than that attained from the forward I–V characteristics that can be accredited to the inhomogeneous spreading of the Nss.

Carrier transport mechanism of Mg/Pt/Au Ohmic contact on p-GaN/AlGaN/GaN platform with ultra-low resistivity

In this work, the carrier transport mechanism of Mg/Pt/Au Ohmic contact on p-GaN/AlGaN/GaN with a very low specific contact resistivity of 1.98 × 10−5 Ω cm2 is investigated. Secondary ion mass spectroscopy measurement results show that the Mg concentration near the p-GaN surface increases form 8 × 1019 to 7 × 1020/cm3 for Mg/Pt/Au contact after annealing. It indicates that Mg atoms from the Mg/Pt/Au metal stack diffuse into the p-GaN during annealing, forming a heavily Mg doped p++-GaN layer with a depth of about 3 nm. The sheet resistance Rsh depends on temperature for Mg/Pt/Au contacts on p-GaN/GaN, indicating that the influence of 2DHG on carrier transport mechanisms at the metal/p-GaN interface was eliminated. For Mg/Pt/Au contacts at ≥360 K, specific contact resistivity reasonably follows T−1, which indicates that the band conduction of Schottky theory dominates the carrier transport. For Mg/Pt/Au contacts at 200–360 K, the electrical resistivity reasonably follows T−1/4, indicating variable-range hopping (VRH) conduction through Mg-related deep-level defects (DLD). Based on the VRH conduction model, the effective barrier height (qφ) of the Mg-related DLD band is extracted as 0.265 eV, which well matches the excellent Ohmic contact.

Electrical Properties of Heavily Al-Doped 4H-SiC

We investigate the temperature-dependent resistivity (ρ(T)) and Hall coefficient (RH(T)) of heavily Al-doped 4H-SiC and discuss the underlying conduction mechanisms. The sign of RH(T) changes from positive to negative in nearest-neighbor hopping (NNH) and variable-range hopping (VRH) conduction, whereas it is positive in band conduction because Al-doped 4H-SiC is a p-type semiconductor. We propose a general physical model to explain why RH(T) in hopping conduction becomes negative at low temperatures, which is applicable to both NNH and VRH conduction. Moreover, we elucidate why the activation energy for negative RH(T) becomes similar to that of ρ(T) in NNH conduction.

Highly conductive quasi-defect-free reduced graphene oxide for qualitative scalable production

Herein, we present a potential approach for the qualitative production of quasi-defect-free reduced graphene oxide (QrGO) with semimetallic transport behavior. In rGO, the formation of defects is highly dependent on the oxidation process of graphite. Subsequent exfoliation of graphite oxide, the quasi-defect-free graphene oxide (QGO) shows abundant epoxy and hydroxyl groups which confirmed the suppressed structural deformation. Typical rGO and QrGO are compared to characterize the remarkable electrical, optical, and thermal properties of QrGO. In particular, under low temperature, the unusual carrier transport phenomena of QrGO exhibits a metal-like behavior, unlike typical rGO, which shows an insulator-to-semimetal transition with variable-range hopping conduction behavior. This conduction mechanism, resembling that of mechanically exfoliated graphene, facilitates the fabrication of a QrGO-based conducting film with a high electrical conductivity of ∼1000 S/cm. Moreover, during a 200-h test with temperature and moisture conditions of 85 °C and 85%, respectively, QrGO shows environmental stability with minimal fluctuations in resistance. Furthermore, a conducting paste was fabricated from QrGO in an aqueous solution, showing a highly concentrated solid content and excellent electrical conductivity. It shows great potential for the large-scale production of high-quality chemically exfoliated graphene for practical applications.

Effect of Fe supplantation on structural and dielectric properties of La2CoMnO6

Double Perovskite system La2Co1-xFexMnO6 (x = 0, 0.2, 0.5, 0.8 and 1.0) has been synthesized by sol-gel route. XRD profiles confirmed the absence of any external phase in pure La2CoMnO6 phase. Rietveld refinement analysis confirmed the existence of monoclinic (ordered) as well as rhombohedral and orthorhombic (disordered) structures. Lattice parameters and unit cell volume are found to increase with increase in Fe content. Morphological and compositional analysis was done using FESEM with EDS. Frequency and temperature dependent dielectric characterization revealed the dispersion of both dielectric constant as well as loss tangent at low frequency and high temperature. The Nyquist plot of impedance (cole-cole) revealed negative temperature coefficient of resistance (NTCR) behavior of prepared samples and suggest a non-Debye type relaxation in these samples. An increasing trend with increase in frequency and temperature was exhibited by ac conductivity. Variation of dc electrical conductivity with temperature suggested a semiconducting behavior which can be explained by both small polaron hopping (SPH) and variable range hopping (VRH) conduction mechanisms.

Structural, electrical and magnetic properties of orthorhombic GdMn1-xNixO3 (0 ≤ x ≤ 0.1): Influence of Ni doping

GdMn1-xNixO3 (0 ≤ x ≤ 0.1) samples were synthesized by traditional solid state reaction method to understand the role of Ni substitution on structural, microstructural, electrical and magnetic properties. Powder X-ray diffraction data reveals the perovskite type structure with pnma space group for all the prepared samples. Scanning electron micrographs shows combined island and disc like granular structures. Raman spectroscopy reveals enhancement in asymmetric stretching mode due to out of phase MnO6 bending and bending mode due to O2 anti-stretching. The temperature dependent dc electrical resistivity confirms the semiconducting nature of the samples, supported by small polaron hopping (SPH) and variable range hopping (VRH) conduction models. The activation energies first increases and then decreases with Ni content. Temperature dependent magnetization under ZFC and FC regimes shows decrement in Tirr with Ni concentration. Currie-Weiss law fittings to inverse of susceptibility data indicates the paramagnetic nature of the samples with presence of antiferromagnetic interactions at 5 K.

Electronic properties of Cu2(Zn, Cd)SnS4 determined by the high-field magnetotransport

Resistivity, ρ(T), and magnetoresistance (MR) are investigated in the Cu2Zn1−x Cd x SnS4 single crystals for compositions x ≡ Cd/(Zn + Cd) = 0.15–0.24, in the temperature range of T ∼ 50–300 K in pulsed magnetic fields of B up to 20 T. The Mott variable-range hopping (VRH) conductivity is established within wide temperature intervals lying inside ΔT M ∼ 60–190 K for different x. The deviations from the VRH conduction, observable above and below ΔT M, are connected to the nearest-neighbor hopping regime and to the activation on the mobility threshold of the acceptor band (AB) with width W ≈ 16–46 meV. The joint analysis of ρ(T) and positive MR permitted determination of other important electronic parameters. These include the localization radius, α ≈ 19–30 Å, the density of the localized states, g(μ) ≈ (1.6–21) × 1017 meV−1 cm−3 at the Fermi level μ, and the acceptor concentration, N A ∼ (6–8) × 1019 cm−3, for various x and in conditions of different vicinity of the investigated samples to the metal–insulator transition. In addition, details of the AB structure, including positions of μ and of the mobility threshold, E c, are found depending on the alloy composition.

Multi-band simultaneous fits of transport data on p-type ScNiSb with including impurity conduction

The experimental data of the electrical conductivity [Formula: see text] and the Seebeck coefficient [Formula: see text] in the temperature range from 2 K to 950 K, together with those of the Hall coefficient [Formula: see text] in the temperature range from 2 K to 300 K, on the polycrystalline sample of [Formula: see text]-type ScNiSb reported by Ciesielski et al. [Phys. Rev. Appl. 14, 054046 (2020)] have been critically analyzed with including both nearest-neighbor and Mott variable-range hopping (VRH) conduction. Through the simultaneous fits to the experimental data of [Formula: see text], [Formula: see text] and [Formula: see text], various transport parameters such as the bandgap energy, the effective masses of holes and electrons, and the electron-to-hole mobility ratio, are deduced together with the ionization energy and the concentration of the acceptor level. It is shown that nearest-neighbor hopping (NNH) conduction exhibits a significant effect on the Seebeck coefficient at low temperatures while VRH conduction has an only small effect on it. Contrary to the speculation by Ciesielski et al., the electron mobility is proved to be lower than the hole mobility.

Thermoelectric Effect in Mott Variable-Range Hopping

On the basis of the Kubo-Luttinger linear response theory combined with the scaling theory of Anderson localization predicting the energy dependence of localization length near the mobility edge, we have studied the thermoelectric response of a disordered system exhibiting Mott variable-range hopping (VRH). We found that the low-tempetature Seebeck coefficient for VRH conduction in a $d$-dimensional system varies as $S\propto T^{d/(d+1)}$, which is different from the widely used expression, $S\propto T^{(d-1)/(d+1)}$, based on the energy-independent localization length. It is seen that the low-$T$ behavior of $S(T)$ of the thiospinel CuCrTiS$_4$ is in full agreement with the new scheme.

Electrical characterization and VRH conduction mechanism in Calcium Copper Titanate materials

Variable range hopping (VRH) conduction of the Mott type, where the electrical resistivity (ρ) follows the law (ln(ρ)∝T1/4), is observed in Strontium (Sr) doped Calcium Copper Titanate (CCTO) ceramics below 170 K at different frequency values up to 1 MHz. Two different methods of a) minimizing the percentage deviation between experimental data of ρ and equation (ρ=ρoexp(ToT)p) with the exponent p varying between zero to 1 and b) of analyzing the local activation energy (Eloc(T)) are used to confirm the Mott-VRH mechanism in these ceramics. The effect of frequency on the pre-exponential factor ρo, the localization temperature To and thus on the localization length ξ of charge carriers is analyzed.

Electrical Conduction Mechanism of β‐MnTe Thin Film with Wurtzite‐Type Structure Using Radiofrequency Magnetron Sputtering

Manganese telluride (MnTe) compound is known to be a polymorphic chalcogenide. Recently, it has been reported that the MnTe shows nonvolatile memory properties with a significant change in resistance via a polymorphic transition between NiAs‐type (NC) structure (low resistance) and wurtzite‐type (WZ) structure (high resistance). This crystalline polymorphic MnTe is expected to realize a phase‐change memory with fast operation speed and ultralow operation energy. While the NC‐MnTe, generally designated as α‐MnTe, is intensively studied, WZ‐MnTe is still poorly understood. Herein this study, electrical conduction mechanism of a β‐MnTe film with a WZ‐type structure is studied. A resistivity, Hall mobility, and Seebeck coefficient of the WZ‐MnTe film are measured at various temperatures. The temperature dependence of resistivity in the temperature range 120–300 K clearly indicates that the WZ‐MnTe film shows a variable‐range hopping (VRH) conduction. In this temperature region, with decreasing temperature, the conduction mechanism changes from Mott–VRH conduction to Efros–Shklovskii VRH conduction at about 210 K. Furthermore, the low thermally activated Hall mobility, occurrence of Hall‐effect sign anomaly, and relatively low activation energy for thermopower, which are the observed results, suggest that the small polaron hopping conduction is dominant above 310 K.

Activated Hopping Transport in Nematic Conducting Aerogels

The transport properties of nematic aerogels, which consist of highly oriented Al$_2$O$_3\cdot$SiO$_2$ nanofibers coated with a graphene shell with a large number of defects, are studied. The temperature dependences of the electrical resistivity in the range of 9-40K strictly follow the formula derived to describe the variable range hopping (VRH) conductivity, in which exponent $\alpha$ changes from 0.4 to 0.9 when the number of layers in the graphene shell decreases from 4-6 to 1-2. The dependence of $\alpha$ on the shell thickness can be explained by a simultaneous change in the dimensionality of hopping transport and the character of the energy dependence of the density of localized states near the Fermi level. The fact that $\alpha$ approaches unity at the minimum graphene shell thickness indicates a gradual transition from VRH transport to nearest neighbor hopping (NNH) transport. The magnetoresistance measured at T = 4.2 K is negative, increases significantly with decreasing graphene shell thickness, and is approximated by a formula for the case of weak localization with a good accuracy. The phase coherence lengths are in a reasonable relation with the graphene grain sizes. The conducting aerogels under study complement the well-known set of materials that exhibit hopping electron transport at low temperatures, which is characteristic of media with strong carrier localization, and also a negative magnetoresistance, which usually manifests itself under weak localization conditions.

Mott and Efros–Shklovskii Variable-Range Hopping Conduction in Films Formed by Silicon Nanoparticles Doped with Phosphorus and Boron

Mott and Efros–Shklovskii Variable-Range Hopping Conduction in Films Formed by Silicon Nanoparticles Doped with Phosphorus and Boron