Three-dimensional Mott Variable-range Hopping Research Articles

Non-Centrosymmetric Sr2IrO4 Obtained Under High Pressure.

Sr2IrO4 with strong spin-orbit coupling and Hubbard repulsion (U) hosts Mott insulating states. The similar crystal structure and magnetic and electronic properties, particularly the d-wave gap observed in Sr2IrO4 enhanced the analogies to the cuprate high-Tc superconductor, La2CuO4. The incomplete analogy was due to the lack of broken inversion symmetry phases observed in Sr2IrO4. Here, under high-pressure and high-temperature conditions, we report a noncentrosymmetric Sr2IrO4. The crystal structure and its noncentrosymmetric character were determined by single-crystal X-ray diffraction and high-resolution scanning transmission electron microscopy. The magnetic characterization confirms the Ir4+ with S = 1/2 at low temperature in Sr2IrO4 with magnetic ordering occurring at around 86 K, where a larger moment is observed than the ambient pressure Sr2IrO4. Moreover, the resistivity measurement shows three-dimensional Mott variable-range hopping (VRH) existed in the system. This noncentrosymmetric Sr2IrO4 phase appears to be a unique material that offers a further understanding of high-Tc superconductivity.

Ordered phase transformation and Cu doping effects in room-temperature ferromagnetic Sr3YCo4O10.5+δ.

Sr3YCo4O10.5+δ (314-SYCO), with an unusual ordered structure and a high Curie temperature (Tc ≈ 335K), is attracting increasing attention. Herein, to improve the electrical performance of 314-SYCO, Cu-doped Sr3YCo4-x Cu x O10.5+δ (x = 0-0.8) ceramics were prepared using a solid-state reaction method. Systematic research was conducted on both the ordered phase transformation and the effects of Cu doping on the microstructure, electrical transport characteristics, and magnetic properties. For x = 0-0.4, the (103) and (215) planes were observed and combined with Rietveld refinement results for the X-ray diffraction data, confirming the formation of ordered tetragonal Sr3YCo4-x Cu x O10.5+δ . This phase was formed with a mass gain of ∼0.8% and heat released at ∼1,042°C. With increasing Cu content, the concentration of hole carriers also increased, leading to a substantial reduction in electrical resistivity. The electrical resistivity decreased by 92-99% at 300K. The polycrystalline materials have semiconducting behaviour with a three-dimensional Mott variable-range hopping mechanism. For the magnetic properties, a Hopkinson peak was observed at 319K, and the Tc was approximately 321K for x = 0. The magnetisation and Tc decreased with increasing Cu content, and a G-type antiferromagnetic-to-ferromagnetic phase transition occurred due to the spin state change for some Co3+ ions from high/intermediate spin to low/intermediate spin. These results lay the groundwork for refinement of the sintering procedure and doping parameters to enhance the performance of 314-SYCO in the context of current applications such as microwave absorbers and solid oxide fuel cell cathodes.

Three-Dimensional Variable Range Hopping and Thermally Activated Conduction Mechanism of Polypyrrole/Zinc Cobalt Oxide Nanocomposites

Zinc cobalt oxide (ZCO) nanoparticles and zinc cobalt oxide/polypyrrole (ZCO/PPy) nanocomposites were synthesized by hydrothermal and in situ chemical polymerization routes, respectively. X-ray diffraction (XRD) patterns of PPy and ZCO/PPy nanocomposites show the amorphous nature of PPy, while there is the presence of dominant crystalline peaks of ZCO in the PPy matrix. Field-emission scanning electron microscopy (FESEM) depicts a spherical geometry and shows a decrease in the particle size with an increase in the ZCO weight percentage, while the transmission electron microscopy (TEM) images depict the core–shell-type formation of particles with aggregated ZCO nanoparticles as a core embedded in the PPy shell. The Fourier transform infrared (FTIR) spectra of ZCO/PPy nanocomposites show all the asymmetrical stretching vibrations of PPy and ZCO along with an increase in intensities of the respective metal oxides peaks and slight shifts in a few bands. Further, the transport studies, i.e., direct current (DC) conductivities of the ZCO/PPy nanocomposites, were studied as a function of temperature (300–453 K) and reveal semiconducting transport behavior. The analysis of DC conduction in PPy and ZCO/PPy suggests that the conduction of charge carriers follows the three-dimensional Mott variable range hopping (3D-VRH) model at a lower temperature, while it follows a thermally activated conduction model at high temperature.

Crossover between Mott and Efros-Shklovskii variable-range hopping in Sr2IrO4 epitaxial thin films by misfit strain and isovalent doping

We report three-dimensional Mott variable-range hopping (3D Mott-VRH) transport of Sr2IrO4 epitaxial thin films, with a transition to Efros-Shklovskii variable-range hopping (ES-VRH) with increased misfit strain or isovalent doping. We have observed that the characteristic temperature of 3D Mott-VRH transport decreases under increased misfit strain, implying that the density of states near the Fermi energy is reconstructed. With further increased misfit strain (or doping with Ca or Ba ions), a crossover from the 3D Mott-VRH to ES-VRH transport takes place due to increased carrier localization by disorder, opening a Coulomb gap by increasing the effective electron-correlation. The results of magnetoresistance measurements also confirm that the disorder caused by misfit strain or isovalent doping plays an important role in the electronic transport of these Sr2IrO4 thin films. Our experimental observations propose that subtle external stimuli such as structural modifications can modulate the electronic properties of the relativistic Mott insulator, suggesting an unprecedented pathway for electronic device applications.

LaMn3Rh4O12: An Antiferromagnetic Quadruple Perovskite Synthesized at High Pressure.

A quadruple perovskite LaMn3Rh4O12 with A' = Mn and B = 4d transition metal was synthesized at high pressure (8 GPa) and temperature (1423 K) for the first time. Room temperature powder X-ray diffraction indicates that LaMn3Rh4O12 forms in cubic symmetry (Im3̅, a = 7.4997(1) Å). X-ray absorption near-edge spectroscopy shows predominantly Mn3+ and Rh3+ oxidation states. An antiferromagnetic transition at TN ∼ 41 K is corroborated by specific heat measurements. The resistivity measurements indicate a three-dimensional Mott variable-range hopping conduction mechanism between 300 and 160 K.

Growth of high quality Sr2IrO4 epitaxial thin films on conductive substrates**Project supported by the National Natural Science Foundation of China (Grant Nos. 51627901 and 11574287), the National Key Research and Development Program of China (Grant No. 2016YFA0401004), and Hefei Science Center-Chinese Academy of Sciences (Grant No. 2016HSC-IU004). X. Z. acknowledges the support of the Youth Innovation Promotion Association of Chinese

Ruddlesden–Popper iridium oxides have attracted considerable interest because of the many proposed novel quantum states that arise from the large spin–orbit coupling of the heavy iridium atoms in them. A prominent example is the single layer Sr2IrO4, in which superconductivity has been proposed under electron doping. However, the synthesis of Sr2IrO4 high quality thin films has been a huge challenge due to the easy formation of impurities associated with different numbers of SrO layers. Thus techniques to optimize the growth of pure phase Sr2IrO4 are urgently required. Here we report the deposition of high quality Sr2IrO4 thin films on both insulating SrTiO3 and conducting SrTiO3:Nb substrates using pulsed laser deposition assisted with reflective high-energy electron diffraction. The optimal deposition temperature of Sr2IrO4 epitaxial films on SrTiO3:Nb substrates is about 90 °C lower than that on SrTiO3 substrates. The electrical transports of high quality Sr2IrO4 films are measured, which follow the three-dimensional Mott variable-range hopping model. The film magnetizations are measured, which show weak ferromagnetism below ∼240 K with a saturation magnetization of /Ir at 5 K. This study provides applicable methods to prepare high quality 5d Sr2IrO4 epitaxial films, which could be extended to other Ruddlesden–Popper phases and potentially help the future study of exotic quantum phenomena in them.

Structural and electrical properties of HCl–polyaniline–Ag composites synthesized by polymerization using Ag-coated (NH4)2S2O8 powder

Ag nanoparticles were sputter-deposited on ammonium persulfate ((NH4)2S2O8) powder to obtain (NH4)2S2O8−Ag powder, which was used to synthesize the HCl-doped polyaniline−Ag (HCl−PANI−Ag) composite via a polymerization procedure. The Ag nanoparticles were dispersed in the HCl−PANI matrix, and their sizes mainly ranged from 3 to 6 nm. The Ag nanoparticles did not affect the structure of emeraldine salt in the composite, and they increased the ordered crystalline regions in the HCl−PANI matrix. The HCl−PANI−Ag composite had a conductivity of (6.8 ± 0.1) S/cm, which is about four times larger than that of the HCl−PANI. The charge transport mechanism in the composite is explained by the three-dimensional Mott variable-range hopping (3D-Mott-VRH).

Facile synthesis and characterizations of polypyrrole/BiOCl hybrid composites

Polypyrrole(PPy)/BiOCl hybrid composites were synthesized for the first time via one-step chemical oxidation process by addition of Bi2O3 nanoparticle in an aqueous solution of pyrrole monomer/FeCl3 oxidant agent. X-ray diffractions (XRD), field emission scanning electron microscopy (FESEM), and thermogravimetric technique confirmed the growth of BiOCl in PPy matrix. From the XRD, the amount of BiOCl in PPy matrix increased with increasing of Bi2O3 addition in pyrrole solution. The FESEM images indicated the presence of two phases related to PPy and BiOCl. Thermal stability of PPy/BiOCl hybrid composites has been improved in the range 300–800 K and degraded above 800 °C, i.e., decomposition point of BiOCl. Fourier transforms infrared spectroscopy point to a mutual interaction between PPy and BiOCl system. The characteristic optical absorption peaks of PPy shifted to higher wavelength in PPy/BiOCl(5%) composites and disappeared at PPy/BiOCl(20%). From electrical measurement, the PPy/BiOCl hybrid composites have higher conductivity than PPy, where the maximum conductivity observed was for PPy/BiOCl(5%). The conducting mechanism of PPy and PPy/BiOCl composites followed three-dimensional Mott variable range hopping in the range of 300–150 K and has involved fluctuation-assisted tunneling phenomenon below 150 K.

Modeling the electrical transport in epitaxial undoped and Ni-, Cr-, and W-doped TiO2 anatase thin films

Electrical transport in undoped and Ni-, Cr-, and W-doped TiO2 thin films on SrTiO3(001) is modelled either with the sum of two thermally activated processes with exponential temperature dependence of conductivity, or with the sum of three-dimensional Mott variable-range hopping (VRH) and an activated process with low activation energy. The latter is interpreted for both models as small polaron hopping (<θD/4). According to reduced chi-square values, the double activated model is superior for data of higher ordered films grown at 540 and 460 °C. For lower growth temperature, VRH plus activated conductivity fits partly better. For all dopants, n-type conductivity is observed.

Electron transport in a GaPSb film

We have performed transport measurements on a gallium phosphide antimonide (GaPSb) film grown on GaAs. At low temperatures (T), transport is governed by three-dimensional Mott variable range hopping (VRH) due to strong localization. Therefore, electron–electron interactions are not significant in GaPSb. With increasing T, the coexistence of VRH conduction and the activated behavior with a gap of 20 meV is found. The fact that the measured gap is comparable to the thermal broadening at room temperature (approximately 25 meV) demonstrates that electrons can be thermally activated in an intrinsic GaPSb film. Moreover, the observed carrier density dependence on temperature also supports the coexistence of VRH and the activated behavior. It is shown that the carriers are delocalized either with increasing temperature or magnetic field in GaPSb. Our new experimental results provide important information regarding GaPSb which may well lay the foundation for possible GaPSb-based device applications such as in high-electron-mobility transistor and heterojunction bipolar transistors.

Preparation, Electrical Conductivity, Photocurrent and Wettability of Carbon Microcoils

In this paper, we report on fabrication and physical properties of carbon microcoils, which are prepared by a chemical vapor deposition (CVD) process with Ni-catalyzed pyrolysis of acetylene, and characterized by a scanning electron microscope, a transmission electron microscope and an infrared spectrometer. The dark electrical conductivity of an isolated carbon microcoil is about 81 S/cm at room temperature, and its temperature dependence follows three-dimensional Mott variable-range hopping (VRH) model. Particularly, evident photocurrent is observed in the carbon microcoil upon cameral flash illumination. In addition, it is found that the surface of microcoil film is hydrophobic, showing a large water contact angle of about 135°. These results indicate that carbon microcoils have excellent physical properties, and can be used as optoelectronic and reinforced materials.

Quantum transport in indium nitride nanowires

Transport measurements are performed under the variations of temperature and magnetic field on single crystalline InN nanowires. Conduction at low temperature reveals a three-dimensional Mott variable range hopping mechanism. With rising temperature, a semiconductor-to-metal transition is observed around 80 K. In addition, the nanowire exhibits negative magnetoresistance under both parallel and perpendicular fields due to the suppression of the electron wave function interference. A field direction asymmetry on the change of magnetoresistance is examined, attributing to the conduction channel anisotropy.

Template-Free Synthesis and Properties of Polyaniline Nanostructures Doped with Different Oxidants

In this work, we report on conducting polyaniline (PANI) nanostructures synthesized by a simplified template-free self-assembly method, which are doped with different oxidants such as ammonium persulfate (APS), FeCl3, Fe(NO3)3, and Fe2(SO4)3. It is found that the morphologies of the as-prepared PANI nanowires are dependent on the oxidant and the molar ratio of aniline to oxidant. The PANI nanostructures are semiconducting with room-temperature conductivity ranging from 10-2to 100S/cm, and the temperature dependence of conductivity follows three-dimensional Mott variable range hopping (3D Mott-VRH) model. In addition, the PANI pellets exhibit hydrophilic behavior.

Temperature-dependent transport and1/fnoise mechanisms in single-walled carbon nanotube films

The temperature dependence of $1/f$ noise and resistivity in single-walled carbon nanotube (CNT) films are studied. We find that as the temperature decreases, resistivity monotonically increases whereas $1/f$ noise amplitude first decreases, then increases, reaching a minimum at around 40 K. At temperatures considerably smaller than 40 K, the temperature dependence of both resistivity and $1/f$ noise amplitude can be explained by three-dimensional Mott variable-range hopping, which is due to localization effects that result in an insulating behavior in CNT films. At higher temperatures, on the other hand, the dependence of resistivity on temperature can be explained by fluctuation-induced tunneling. In this high-temperature regime, we analyze the temperature dependence of the noise amplitude to extract the density of fluctuators as a function of their energy. Our results show a characteristic peak between 0.3 and 0.6 eV that is responsible for the majority of $1/f$ noise. We also find that, due to its correlation with the number of carriers, the noise amplitude is very sensitive to CNT film device dimensions, especially near the percolation threshold where the resistivity increases. These results not only provide fundamental physical insights about transport and $1/f$ noise mechanisms in CNT films at different temperatures but also help assess the suitability of these films for device applications.

Ag-doped La 2CuO 4 thin films grown on SrTiO 3 substrate by laser ablation

The effects of 5 wt.% Ag 2O addition on the structural and electrical properties of La 2CuO 4 (LCO) thin films have been examined. The Ag-containing films show smooth surface appearance which has been regarded as the result of the so-called rapid sequential pulsed laser deposition (RS-PLD). Fine fittings to the resistivity data indicate that pure LCO film exhibits the behavior of three-dimensional Mott variable range hopping (VRH). However, the doped film shows VRH behavior in the lower temperature range, while in higher temperature range obeys the law of ordinary thermal activation. This fact implies that Ag addition to LCO system creates strong disorder which suppresses superconductivity by means of destroying the effective coupling strength and phase coherence.

Ag-doped La2CuO4 thin films grown on SrTiO3 substrate by laser ablation

The effects of 5 wt.% Ag2O addition on the structural and electrical properties of La2CuO4 (LCO) thin films have been examined. The Ag-containing films show smooth surface appearance which has been regarded as the result of the so-called rapid sequential pulsed laser deposition (RS-PLD). Fine fittings to the resistivity data indicate that pure LCO film exhibits the behavior of three-dimensional Mott variable range hopping (VRH). However, the doped film shows VRH behavior in the lower temperature range, while in higher temperature range obeys the law of ordinary thermal activation. This fact implies that Ag addition to LCO system creates strong disorder which suppresses superconductivity by means of destroying the effective coupling strength and phase coherence.