Low Temperature Electrical Transport Research Articles

Tellurium doped n-type Zintl Zr3Ni3Sb4 thermoelectric materials: Balance between carrier-scattering mechanism and bipolar effect

Thermoelectric figure of merit ZT has been greatly improved in the past decade via band engineering to enhance power factor or nanostructuring to reduce thermal conductivity, but less attention has been paid to other significant factors, e.g., carrier scattering mechanism, bipolar effect, etc. Here we show that Te doping on the Sb site, as an n-type strong donor, could significantly suppress the high-temperature bipolar effect in the nanostructured Zintl Zr3Ni3Sb4, which can be ascribed to the combination of high majority-carrier concentration and enlarged band gap. A relatively good ZT of ∼0.6 at 773 K for Te doping can be achieved and that is almost double of the previous reported ZT by Cu doping. In addition, the role of carrier scattering mechanism on the low-temperature electrical transport properties is also pointed out, where both carrier mobility and power factor of Te doping, due to the detrimental effect of ionized impurity scattering, are lower than that of Cu doping in which the mixed acoustic phonon and ionized impurity scattering dominates.

Electrostatic tuning of magnetism at the conducting (111) (La0.3Sr0.7)(Al0.65Ta0.35)/SrTiO3 interface

We present measurements of the low temperature electrical transport properties of the two dimensional carrier gas that forms at the interface of (111) (La0.3Sr0.7)(Al0.65Ta0.35)/SrTiO3 as a function of applied back gate voltage, Vg. As is found in (111) LaAlO3/SrTiO3 interfaces, the low-field Hall coefficient is electron-like but shows a sharp reduction in magnitude below V g ∼ 20 V, indicating the presence of hole-like carriers in the system. This same value of Vg correlates approximately with the gate voltage below which the magnetoresistance evolves from nonhysteretic to hysteretic behavior at millikelvin temperatures, signaling the onset of magnetic order in the system. We believe our results can provide insight into the mechanism of magnetism in SrTiO3 based systems.

Anisotropic multicarrier transport at the (111) LaAlO3/SrTiO3 interface

The conducting gas that forms at the interface between LaAlO$_3$ and SrTiO$_3$ has proven to be a fertile playground for a wide variety of physical phenomena. The bulk of previous research has focused on the (001) and (110) crystal orientations. Here we report detailed measurements of the low-temperature electrical properties of (111) LAO/STO interface samples. We find that the low-temperature electrical transport properties are highly anisotropic, in that they differ significantly along two mutually orthogonal crystal orientations at the interface. While anisotropy in the resistivity has been reported in some (001) samples and in (110) samples, the anisotropy in the (111) samples reported here is much stronger, and also manifests itself in the Hall coefficient as well as the capacitance. In addition, the anisotropy is not present at room temperature and at liquid nitrogen temperatures, but only at liquid helium temperatures and below. The anisotropy is accentuated by exposure to ultraviolet light, which disproportionately affects transport along one surface crystal direction. Furthermore, analysis of the low-temperature Hall coefficient and the capacitance as a function of back gate voltage indicates that in addition to electrons, holes contribute to the electrical transport.

Accessing Rashba states in electrostatically gated topological insulator devices

We study the low temperature electrical transport in gated BiSbTe1.25Se1.75/hexagonal-Boron Nitride van der Waals heterostructure devices. Our experiments indicate the presence of Rashba spin-split states confined to the sample surface. While such states have been observed previously in photo-emission spectroscopy and STM experiments, it has not been possible to unambiguously detect them by electrical means and their transport properties remain mostly unknown. We show that these states support high mobility conduction with Hall effect mobilities ∼2000 to 3000 cm2/V-s that are paradoxically much larger than the mobilities of the topological surface states ∼300 cm2/V-s at T = 2 K. The spin-split nature of these states is confirmed by magneto-resistance measurements that reveal multi-channel weak anti-localization. Our work shows that Rashba spin split states can be electrically accessed in Topological insulators paving the way for future spintronic applications.

Structural and low temperature electrical transport properties of Mo-doped vanadium oxide NTC ceramic thin films

Mo doped V2O3 [V1−xMoxO2−x/2 (x = 0, 0.5–1)] ceramic thin films were prepared on metal substrates by sol-gel dip coating and the influence of Mo addition on their microstructure, negative temperature coefficient (NTC) electrical transport properties and metal to insulator phase transition behavior were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), and resistance-temperature measurements. Resistivity-temperature curves (over a temperature range of 273.15–253.15 K) indicated that all of the prepared thin films have NTC effects, after annealing with 20 sccm N2 at 673.15 K. It was demonstrated through microstructure analysis at Mo high concentration, (i.e., x > 0.07) it segregates at the V2O3 grain boundaries, causing scattering and distortion of the crystal lattice. Compared with the other V2O3 films, the films prepared at Mo x > 0.07 offered the high resistivity and moderate thermal constant (B) values. In particular, V2O3 doped with 10 mol % Mo showed excellent NTC properties and high resistivity (0.072 Ω cm). At sub-zero temperatures, the variation of electrical transport properties of the V2O3 films is correlated with Mo concentration, micro-structure and Joule effect.

The low temperature specific heat and electrical transport, magnetic properties of Pr0.65Ca0.35MnO3

The magnetic properties, electrical transport properties, and low temperature specific heat of polycrystalline perovskite manganese oxide Pr0.65Ca0.35MnO3 have been investigated experimentally. It is found that there exists cluster glass state in the sample at low temperature besides the antiferromagnetic insulating state. With the increase of magnetic field, antiferromagnetic insulating state converts to ferromagnetic metal state and the Debye temperature decreases gradually. In addition, the low temperature electron specific heat in zero magnetic field is obviously larger than that of ordinary rare-earth manganites oxide and this phenomenon is related to the itinerant electrons in ferromagnetic cluster state and the disorder in Pr0.65Ca0.35MnO3.

Synthesis, structure, thermal, transport and magnetic properties of VN ceramics

We present a detailed study investigating structure, thermal, transport and magnetic properties of vanadium mononitride. Pure vanadium mononitride ceramics with a stoichiometry very close to ideal composition VN was synthesized by ammonolysis of ammonium vanadate followed by sintering in NH3/N2 atmosphere. The detailed chemical and thermochemical analysis as well as transport properties measurement reveal stoichiometric composition of VN. The low temperature electrical and thermal transport measurements show superconducting transition at 8K, a gradual structural phase transition below 250K and a high temperature phase decomposition accompanied by V2N formation. The electronic structure calculations are correlated with the analysis of transport and thermal properties revealing an essential role of electron-phonon coupling and strong anharmonic effects.

Characteristics of low-resistivity aluminum-doped zinc oxide films deposited at room temperature by off-axis radio-frequency sputtering on flexible plastic substrates

The crystalline structure, morphology, composition, electrical transport, and optical properties of aluminum-doped zinc oxide (AZO) films are studied for applications in transparent electronics and optoelectronic devices. AZO thin films of c-axis-oriented growth and with different thickness were deposited on PET flexible plastic substrates at room temperature by rf magnetron sputtering. A larger grain size with a decreased strain e value is observed in a thicker film, while changes in composition for films with different thicknesses are insignificant. Moreover, the resistivity of film decreases with increasing thickness, and the low-temperature electrical transport properties can be described by the scenario of quantum corrections to conductivity. With the room-temperature growth conditions, the resistivity of 4.5 × 10−4 Ω cm, carrier concentration of 6.4 × 1020 cm−3, and transmittance of 80 % for the 1100-nm-thick film are obtained. In addition, the optical bandgap energy decreases with increasing film thickness, which can be attributed to the bandgap renormalization and crystallite size effects.

CdS quantum dots: growth, microstructural, optical and electrical characteristics

Cadmium sulfide (CdS) quantum dots (QDs) with cubic phase were prepared using simple precursors by chemical precipitation technique, and their thin films were grown on glass substrates by chemical bath deposition. The obtained quantum dots were characterized for their structural, morphological, optical, thermal and electrical properties using X-ray diffraction (XRD), field emission transmission electron microscopy, UV–visible absorption spectroscopy, Raman spectroscopy, photoluminescence, thermogravimetric analysis/differential thermal analysis and low-temperature electrical transport measurements, respectively. XRD pattern reveals that the prepared CdS QDs are highly pure and crystalline in nature with cubic phase. The average particle size, estimated to be ~2 nm, is almost in agreement with the values calculated by Brusis formula. Selected area electron diffraction also recognizes the cubic structure of CdS quantum dots. The UV–visible spectra exhibit a blueshift with respect to that of bulk sample which is attributed to the quantum size effect of electrons and holes. The band gap of CdS QDs is calculated from absorption data using Tauc plot and found to be 2.84 eV. Energy-dispersive X-ray analysis reveals the presence of Cd and S in almost stoichiometric ratio in the prepared CdS QDs. Micro-Raman spectroscopic studies also yield convincing evidence for the transformation of structure. The emission spectra of CdS QDs show peak centered at 541 nm, which is attributed to the presence of cadmium vacancies in the lattice. The DC resistivity data at low temperatures are qualitatively consistent with the variable-range hopping model, and the density of states at the Fermi level is determined.

Giant magnetoresistance in the cluster glass regime of Co-Ga alloys

A detailed study of low temperature electrical transport properties of CoxGa100−x (x = 54, 55.5, 57) alloy has been carried out. The origin of the resistivity anomalies and correlation between magnetic and electrical transport properties are identified through an elaborate analysis. The weak localization and enhanced electron-electron interaction effects partially support the electrical transport properties of the system. Further, the observed magnetoresistance can be well represented by localized model along with quantum corrections. The low temperature magnetoresistance value near critical composition is comparable to that reported in giant magnetoresistance materials.

P.1.b.002 The EEG specificity in children with attention deficit hyperactivity disorder (ADHD)

Nanocrystalline thin films of silicon (Si) were grown on glass substrates using pulsed laser deposition technique. Si nanoparticles thin films were investigated employing X-ray diffraction, field emission scanning electron microscopy, atomic force microscopy, low temperature electrical transport measurements, and UV–visible absorption spectroscopy. XRD data show that Si films are polycrystalline having cubic crystal structure with preferred (111) orientation. SEM and AFM micrographs exhibit dense and slightly rough surface morphology with well defined columnar grains. Temperature dependence of the resistivity has been fitted with ln ρ(T) ∞ T −1/4 which indicates three dimensional variable range hopping conduction mechanism as well as semiconducting behavior of films. Finally, the optical absorbance edge of Si thin films is described in terms of indirect transition model proposed by Tauc in the strong absorption region, and according to Urbach׳s rule in the medium absorption region.

Low temperature electrical transport in modified carbon nanotube fibres

Carbon nanotube fibres are a new class of materials highly promising for many electrical/electronic applications. The range of applications could be extended through the modification of their electrical transport properties by inclusions of foreign materials. However, the changes in electrical transport are often difficult to assess. Here, we propose that the analysis of resistance–temperature dependencies of modified fibres supported by a recently developed theoretical model may aid research in this area and accelerate real life applications of the fibres.

High-temperature soft magnetic properties of antiperovskite nitrides ZnNFe 3 and AlNFe 3

We report the synthesis, crystal structure, magnetic and electrical/thermal transport properties of antiperovskite nitrides ZnNFe3 and AlNFe3. Both compounds display ferromagnetic (FM) ground state and transform into the paramagnetic phase at 762 K and 770 K, respectively. At FM phase, for ZnNFe3 and AlNFe3, the saturated magnetizations at 5 K are 163 and 188 emu/g, which is the largest value among all the antiperovskite compounds; the minimal coercive forces (HC) are as low as 18.6 and 16.3 Oe; the low HC is almost constant covering a broad temperature range of 5–650 K. These characteristics indicate that both ZnNFe3 and AlNFe3 may be promising candidates of high-temperature soft magnetic material. In addition, electrons are dominant carriers owing to the negative Seebeck coefficient, the Fermi-liquid behavior is well presented via the investigation of low-temperature electrical transport, and the lattice thermal conductivity estimated by Wiedemann–Franz law is the main part of total thermal conductivity at high temperatures for both ZnNFe3 and AlNFe3.

Structural, electrical and optical properties of nanocrystalline silicon thin films deposited by pulsed laser ablation

Nanocrystalline thin films of silicon (Si) were grown on glass substrates using pulsed laser deposition technique. Si nanoparticles thin films were investigated employing X-ray diffraction, field emission scanning electron microscopy, atomic force microscopy, low temperature electrical transport measurements, and UV–visible absorption spectroscopy. XRD data show that Si films are polycrystalline having cubic crystal structure with preferred (111) orientation. SEM and AFM micrographs exhibit dense and slightly rough surface morphology with well defined columnar grains. Temperature dependence of the resistivity has been fitted with ln ρ(T) ∞ T−1/4 which indicates three dimensional variable range hopping conduction mechanism as well as semiconducting behavior of films. Finally, the optical absorbance edge of Si thin films is described in terms of indirect transition model proposed by Tauc in the strong absorption region, and according to Urbach׳s rule in the medium absorption region.

Low temperature electrical transport behavior of La0.7Ba0.3MnO3 thin Films on LaAlO3 substrates

The high-quality La0.7Ba0.3MnO3 (LBMO) epitaxial films were deposited on (001) oriented LaAlO3 (LAO) substrates by magnetron sputtering technique. The low temperature electrical transport behavior of the films was investigated in details. In the low temperature ferromagnetic metallic state, a common feature, i.e., the minimum of electrical resistivity, was found in both of the films post annealed in oxygen and argon atmosphere. By taking the unusual combination of factors including spin dependent Kondo-like scattering, electron–electron and electron–phonon interactions into account, the resistivity minimum can be fitted well. It was found that the spin dependent Kondo-like scattering plays an important role in both the films, indicating the intrinsic spin disorder in strongly correlated manganites. Furthermore, in a lower temperature interval 4–25K, a stronger electron–electron interaction was observed in the Ar annealed films, which can be attributed to the enhanced disorder arising from oxygen vacancy.

Low-temperature electrical transport in B-doped ultrananocrystalline diamond film

B-doped ultrananocrystalline diamond (UNCD) films are grown using hot-filament chemical vapor deposition method, and their electrical transport properties varying with temperature are investigated. When the B-doped concentration of UNCD film is low, a step-like increase feature of the resistance is observed with decreasing temperature, reflecting at least three temperature-modified electronic state densities at the Fermi level according to three-dimensional Mott's variable range hopping transport mechanism, which is very different from that of reported B-doped nanodiamond. With increasing B-doped concentration, a superconductive transformation occurs in the UNCD film and the highest transformation temperature of 5.3 K is observed, which is higher than that reported for superconducting nanodiamond films. In addition, the superconducting coherence length is about 0.63 nm, which breaks a reported theoretical and experimental prediction about ultra-nanoscale diamond's superconductivity.

Low temperature electrical transport and photoresponsive properties of H-doped MoO3nanosheets

α-MoO3 nanosheets were synthesized by a water bath method using ammonium heptamolybdenum tetrahydrate and concentrated nitric acid as precursors. Hydrogen was doped by a chemical reduction in aqueous acidic media, with hydrazine hydrate used as the reducing agent. Temperature dependent resistance showed that the low temperature Peierls transition of H-doped MoO3 nanosheets breaks below 50 K, and its resistance is satisfied at temperatures lower than 37 K (37–10 K). This phenomenon was induced by thermal disturbance and the dominance of defects in low temperature transport, which was confirmed by photoresponse measurements taken before and after the break of the new phase.

Renormalized bands and low-temperature colossal thermopower induced by Ir doping in Ca3Co4O9 system

The effects of Ir doping on the low-temperature thermal and electrical transport properties of Ca3Co4−xIrxO9 (0 ≤ x ≤ 0.4) have been investigated. As Ir ions are doped into the system, a low-temperature thermopower upturn is introduced, and the temperature corresponding to the minimum thermopower TSmin increases monotonously with increasing x. Moreover, a low-temperature colossal thermopower is observed in the Ir-doped samples, which is beneficial to the low-temperature thermoelectric refrigeration in superconducting devices. For the x = 0.4 sample, its maximum thermopower Smax at the lowest measured temperature ∼24 K reaches 1500 μV/K. The appearance of such a colossal thermopower phenomenon is suggested to be closely related to the variations of the transport mechanism and the band structure induced by 5d-Ir doping.

Tunable Electronics in Large-Area Atomic Layers of Boron–Nitrogen–Carbon

We report on the low-temperature electrical transport properties of large area boron and nitrogen codoped graphene layers (BNC). The temperature dependence of resistivity (5 K < T < 400 K) of BNC layers show semiconducting nature and display a band gap which increases with B and N content, in sharp contrast to large area graphene layers, which shows metallic behavior. Our investigations show that the amount of B dominates the semiconducting nature of the BNC layers. This experimental observations agree with the density functional theory (DFT) calculations performed on BNC structures similar in composition to the experimentally measured samples. In addition, the temperature dependence of the electrical conductivity of these samples displays two regimes: at higher temperatures, the doped samples display an Arrhenius-like temperature dependence thus indicating a well-defined band gap. At the lowest temperatures, the temperature dependence of the conductivity deviates from activated behavior and displays a conduction mechanism consistent with Mott's two-dimensional (2D) variable range hopping (2D-VRH). The ability to tune the electronic properties of thin layers of BNC by simply varying the concentration of B and N will provide a tremendous boost for obtaining materials with tunable electronic properties relevant to applications in solid state electronics.

A facile chemical synthesis of a novel photo catalyst: SWCNT/titania nanocomposite

A simple chemical precipitation technique is reported for the synthesis of a hybrid nanostructure of single-wall carbon nanotubes (SWCNT) and titania (<TEX>$TiO_2$</TEX>) nanocrystals of average size 5 nm, which may be useful as a prominent photocatalytic material with improved functionality. The synthesized hybrid structure has been characterized by transmission electron microscopy (HRTEM), energy-dispersive X-ray analysis (EDAX), powder X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. It is clearly revealed that nearly monodispersed titania nanocrystals (anatase phase) of average size 5 nm decorate the surfaces of SWCNT bundles. The UV-vis absorption study shows a blue shift of 16 nm in the absorbance peak position of the composite material compared to the unmodified SWCNTs. The photoluminescence study shows a violet-blue emission in the range of 325-500 nm with a peak emission at 400 nm. The low temperature electrical transport property of the synthesized nanomaterial has been studied between 77-300 K. The DC conductivity shows semiconductor-like characteristics with conductivity increasing sharply with temperature in the range of 175-300 K. Such nanocomposites may find wide applications as improved photocatalyst due to transfer of photo-ejected electrons from <TEX>$TiO_2$</TEX> to SWCNT, thus reducing recombination, with the SWCNT scaffold providing a firm and better positioning of the catalytic material.