Temperature-dependent Resistivity Measurements Research Articles

Magnetotransport Irreversibility in Single Crystalline La0.18Pr0.40Ca0.42MnO3 Thin Films

Magnetotransport irreversibility in single crystalline La0.18Pr0.40Ca0.42MnO3 thin films is probed with respect to intrinsic electronic phase separation (IEPS). Temperature‐dependent magnetization and resistivity measurements show that (i) the high temperature non‐hysteretic regime is dominated by the antiferromagnetic insulator (AFMI) and the charge ordered (CO) phases; (ii) at intermediate temperatures hysteretic regime is akin to a spin liquid; and (iii) the glass transition occurs at temperature Tg below which the spin liquid freezes. The suppression of the ferromagnetic and insulator–metal transitions (TC and TIM) during cooling confirms supercooled magnetic liquid. Magnetic field‐dependent resistivity (ρ–H) measured during cooling and warming highlights the differences in the spin‐ordered structures through (i) reversible behavior at T < Tg; (ii) colossal irreversibility in the isothermal cooling and warming ρ–H at Tg < T < TC/TIM; and (iii) reversible ρ–H at low/moderate fields but irreversible behavior at high fields at T > TIM (warming). The present study demonstrates that the scaling of area between the isothermal cooling and warming cycle ρ–H curves with temperature mimics the ρ–T behavior and hence also reflects the insulator–metal transition. The observed irreversibility and the area scaling in the different spin regimes have been explained in terms of the intrinsic electronic phase separation.

Strain induced extrinsic magnetocaloric effects in La0.67Sr0.33MnO3 thin films, controlled by magnetic field

The magnetic control of strain-induced giant caloric properties of epitaxial La0.67Sr0.33MnO3 (LSMO) thin films grown on (0 0 1) BaTiO3 (BTO) single crystal substrates is investigated. Arising from the crystallographic structural transformation of the BTO substrate during its 3-phase transitions, there is a drastic change of 3D film-lattice strain, which affects the local magnetic anisotropy of the LSMO at the interface and changes the entropy of the film. The thermal hysteresis observed in temperature dependent resistivity measurement can be manipulated by applying a magnetic field. Detailed transport and magnetic studies in different crystal orientations reveal the origin of such field-dependent magnetocaloric properties induced by the different phase transitions of the BTO substrate. Finally, a large entropy change of 1.03–1.95 J · Kg−1 · K−1 · T−1 was obtained at the monoclinic–tetragonal (M–T) transition at 283 K which compares to <1 J · Kg−1 · K−1 · T−1 for previously reported films.

Observation of impurity band conduction and variable range hopping in heavily doped (010) β-Ga2O3

Temperature-dependent resistivity and Hall measurements were performed on a heavily Sn-doped (010) β-Ga2O3 substrate grown by edge-defined film-fed method. At room temperature, the Hall electron mobility was 77.7 cm2 V−1 s−1. A maximum mobility of 95.2 cm2 V−1 s−1 was achieved at T = 185 K. Because the doping concentration is near the Mott critical value of 2.48 × 1018 cm−3, Hall electron concentration and mobility data were analyzed using the two-band model, which considers transport through both the conduction band and an impurity band. At T > 200 K, charge transport is limited by free carriers in the conduction band. The donor ionization energy was extracted to be ECD = 4 meV. At T ≤ 200 K, the net conductivity is due to electrons in the impurity band, with a concentration 7.6 × 1018 cm−3 and a low mobility of ∼0.09–2.27 cm2 V−1 s−1. Over a very wide temperature range, 1.7–200 K, transport in the impurity band was found to occur via variable range hopping, where the average hopping length increases with decreasing temperature.

Electrical properties of pulsed laser deposited BaBiO3−δ thin films and their non-volatile resistive switching

Thin films (50 nm, 100 nm and 300 nm) of multifunctional BaBiO3-δ (BBO) are grown in Au/BBO/Pt architecture using pulsed laser deposition (PLD) technique. Temperature dependent resistivity and dielectric measurements show a first order transformation with a thermal hysteresis of ∼40 K (between ∼150 K and ∼190 K) in Au/BBO300/Pt device. Non-volatile resistive switching (RS) with a resistance window of ∼8.5X is observed in Au/BBO300/Pt device at room temperature. The hysteretic J-V behavior exists in I2/m phase only. The application of a magnetic field (∼0.5 T) doubles the J-V hysteresis area. Thus, we demonstrate that the Au/BBO300/Pt is a non-volatile RS device and J-V hysteresis area of this device can be appreciably improved by applied magnetic field.

Enhancement of superconducting properties and flux pinning mechanism on Cr0.0005NbSe2 single crystal under Hydrostatic pressure

Superconducting properties of Cr0.0005NbSe2 (Tc~6.64 K) single crystals have been investigated through the temperature dependent resistivity (~8 GPa) and DC magnetization (~1 GPa) measurements. Further, the critical current density (Jc) as a function of applied magnetic field has been studied from magnetic isotherms. The vortex pinning mechanisms have also been systematically analyzed using weak collective pinning theory as a function of pressure. The Jc corresponds to the flux flow enhanced by the application of pressure due to increase of Tc and vortex changes. We found that the pressure is responsible for the spatial variations in the charge carrier mean free path (δl pinning). We find that core point pinning is more dominant than surface pinning which is caused by the application of pressure. In addition, Jc(H = 0) increases from 3.9 × 105 (0 GPa) to 1.3 × 106 (1.02 GPa) A/cm2 at 2 K as the pressure is increased from normal pressure to 1.02 GPa. The pressure dependence of Tc (dTc/dP) becomes 0.91 K/GPa and 0.75 K/GPa from magnetization and resistivity measurements respectively. We found that the pressure promotes the anisotropy nature, and decrease of coherence length and resulting in pathetic interface of the vortex core with pinning centers.

Electronic and Magnetic Properties of Pt Based Intermetalic LaPtAs and LaPt2As Compounds

We report the synthesis of polycrystalline LaPtAs and LaPt2As via solid state reactions and their temperature dependent magnetic and electrical transport properties. Crystal structures of both samples are suggested to be orthorhombic with a Pmmm space group. Temperature and field dependent magnetization measurements show that the splitting of field cooling and zero field cooling curves (initiation of irreversibility) take place at temperatures just below the Curie temperatures for both samples. This behavior is a characteristic of magnetic nanoparticles. Therefore, we propose that the magnetization arises from nanosized platinum clusters due to partial filling of the available interstitial sites. Temperature (T) dependent resistivity measurements of both alloys show metallic behavior at high temperatures and resistivity curve bends away from classical linear T dependence toward the temperature axis in the intermediate temperature range. This behavior is attributed to phonon induced interband s–d scattering. The main contribution to the resistivity at low temperatures is suggested to arise from the scattering of conduction electrons by spin-waves, in the presence of strong spin–orbit coupling due to Pt atoms. We discuss the role of spin–orbit effect on the electronic structure and the magnetic state of both samples based on various nano-scale structures.

Effects of CNTs blending on the superconducting parameters of YBCO superconductor

We report the structural modifications and variation of superconducting parameters of high temperature superconductor (HTSc) YBa2Cu3O7-δ (YBCO) by addition of different wt% of CNTs (0.0, 0.2, 0.4, 0.6, 0.8 & 1.0 wt%) synthesized by standard solid state reaction route. The orthorhombic structure of all the samples with Pmmm space group was confirmed from the X-ray Diffraction (XRD) analysis through Rietveld refinements. No significant variation is observed in the lattice parameters of the orthorhombic structure of YBCO superconducting matrix with increasing wt% of CNTs. The Field emission scanning electron microscope (FESEM) analysis shows better connectivity between the grains of CNTs doped samples. From the temperature dependent resistivity measurement, it is observed that there is no chemical reaction between superconducting YBCO and CNTs, rather addition of CNTs enhances the room temperature resistivity and more importantly reduces the critical temperature (TCon) for lower doped sample and then increases for higher doped sample but not for the pristine one. The paraconductivity studies reveal the 2D-3D transition in parent YBCO and the composites, with a signature of percolative behavior in the composites. Inclusion of CNTs to the YBCO enhances the value of critical current density and gives maximum value of JC which is 12 times more than that of pure YBCO sample for 0.8 wt% of YBCO.

Enhanced Néel temperature in EuSnP under pressure.

We present the combined results of single crystal X-ray diffraction, physical properties characterization, and theoretical assessment of EuSnP under high pressure. Single crystals of EuSnP prepared using Sn self-flux crystallize in the tetragonal NbCrN-type crystal structure (S.G. P4/nmm) at ambient pressure. Previous studies have shown that for Eu ions, seven unpaired electrons impart a 2+ oxidation state. Assuming the oxidation states of Eu to be +2 and P to be -3, each Sn will donate one electron, with one p valence electron left for forming a weak Sn-Sn bond. According to the high-pressure single crystal X-ray diffraction measurements, no structural phase transition was observed up to ∼6.2 GPa. Temperature-dependent resistivity measurements up to 2.15 GPa on single crystals indicate that the phase-transition temperature occurring at the Néel temperature (TN) is significantly enhanced under high pressure. The robust crystallography and enhanced antiferromagnetic transition temperatures can be rationalized by the electronic structure calculations and chemical bonding analysis. The increasing Eu-P bonding interaction is consistent with the lattice parameter changing and enhanced TN. Moreover, the molecular orbital diagram shows that the weak Sn-Sn bond can be squeezed under pressure, acting as a compression buffer to stabilize the structure.

Optical and electrical properties of all-inorganic Cs2AgBiBr6 double perovskite single crystals.

In this work, we explored the possibility of using Cs2AgBiBr6, a double perovskite crystal, for radiation detection. Cs2AgBiBr6 crystals were grown using the solution growth technique. The resistivity of the as-grown Cs2AgBiBr6 crystal is larger than 1010 Ω cm, which is high enough to ensure low leakage current for fabrication of semiconductor radiation detectors. Using the temperature-dependent resistivity measurements, we estimated that the Fermi level is at 0.788 eV above the valence band and the material is a p-type semiconductor. From the low-temperature cathodoluminescence measurements, two near band gap energies at 1.917 eV and 2.054 eV were revealed.

Influence of pyrolytic temperature on optoelectronic properties and the energy harvesting applications of high pressure TiO2 thin films

The present work deals with the development of a novel photocatalyst to decompose most of the organic pollutants. This work provides a strategy for reproducible high-pressure phases of TiO2 at ambient conditions through nebulizer spray pyrolysis (NSP) technique by varying the substrate temperatures of 200–500 °C using titanium (IV) isopropoxide as a titanium precursor. The structural, morphological, optical and electrical properties of the deposited TiO2 thin films were investigated by using X-ray diffraction, field emission scanning electron microscopy, UV–Visible spectroscopy, Hall effect and temperature dependent resistivity measurements, respectively. The structural analysis confirms that the films deposited at higher substrate temperature are crystallized in TiO2–II phase having polycrystalline nature with good crystalline quality and also pressure induced amorphous phases were also observed for the films deposited at lower substrate temperatures. Deposited films have the uniform surface with homogeneity and porous nature which is suitable for enhanced photocatalytic applications. Electrical measurements show that the films are having n-type behavior with appreciable carrier mobility and concentration. The use of high-pressure TiO2-II phase for higher photocatalytic efficiency was demonstrated.

Mn valence state mediated room temperature ferromagnetism in nonpolar Mn doped GaN

In this work, we demonstrate room temperature ferromagnetism in nonpolar, m-plane manganese doped gallium nitride (m-GaN:Mn) epitaxial thin films synthesized using plasma-assisted molecular beam epitaxy on nonpolar, m-plane sapphire (Al2O3) substrates. We observed that Mn doping concentration plays a key role in stabilizing the valence state of Mn ions thereby influencing the magnetic and transport properties. Chemical characterization by X-ray photoelectron spectroscopy, X-ray absorption near edge spectra, and energy dispersive X-ray spectroscopy affirm homogeneous valence state Mn+3 in lightly doped samples, while higher doping levels cause a heterogeneous Mn+0 and Mn+2 mixed valence states. Magnetic measurements indicate that films with low doping levels show the signature of room temperature ferromagnetism, while films with higher doping show antiferromagnetic/spin glass behavior besides the indication of MnGa alloys. Temperature dependent resistivity measurements show that both the samples are semiconducting with deep level defect states. Our findings suggest the possibility of incorporating spin functionality in non-polar optoelectronic devices.

On the extraction of resistivity and area of nanoscale interconnect lines by temperature-dependent resistance measurements

Several issues concerning the applicability of the temperature coefficient of the resistivity (TCR) method to scaled interconnect lines are discussed. The central approximation of the TCR method, i.e. the substitution of the interconnect wire TCR by the bulk TCR becomes doubtful when the resistivity of the conductor metal is strongly increased by finite size effects. Semiclassical calculations for thin films show that the TCR deviates from bulk values when the surface roughness scattering contribution to the total resistivity becomes significant with respect to grain boundary scattering, an effect that might become even more important in nanowires due to their larger surface-to-volume ration. In addition, the TCR method is redeveloped to account for line width roughness. It is shown that for rough wires, the TCR method yields the harmonic average of the cross-sectional area as well as, to first order, the accurate value of the resistivity at the extracted area. Finally, the effect of a conductive barrier or liner layer on the TCR method is discussed. It is shown that the liner or barrier parallel conductance can only be neglected when it is lower than about 5–10% of the total conductance. It is furthermore shown that neglecting the liner/barrier parallel conductance leads mainly to an overestimation of the cross-sectional area of the center conductor whereas its resistivity is less affected.

Modulated vacancy ordering in SrGe6− x (x≈0.45)

Abstract The structural properties of modulated SrGe6− x (x≈0.45) were investigated by means of single-crystal and powder X-ray diffraction combined with quantum chemical calculations. The framework compound SrGe6− x adopts a defect variant of the EuGa2Ge4-type crystal structure. Samples of the binary compound with nominal compositions 0≤x≤0.5 were synthesized at pressures from 5 to 6 GPa and a temperature of typically 1400 K. The product reveals diffraction peaks of the EuGa2Ge4-type subcell plus additional reflections indicating an ordered superstructure. Detailed crystal structure analysis evidences the incommensurate nature of the superstructure and a modulation of the vacancy ordering in the germanium network. The computations have shown that the non-stoichiometric composition of the framework with its local defect organization affects the calculated charge of the strontium anions. Although the chemical composition is close to a charge-balanced situation, temperature-dependent resistivity measurements showed metal-type conductivity. At ambient pressure SrGe6− x decomposes exothermally and irreversibly at T=680(10) K into SrGe2 and germanium, indicating its metastable nature at ambient pressure.

2D organic molecular metallic soft material derived from BEDO-TTF with electrochromic and rectifying properties

In this article we demonstrate that a migration of iodine species and chemical transformation in a moist environment induced by a voltage-biased Pt electrode is able to alter the color and degree of charge transfer in a layer of the 2D organic molecular metal (BEDO-TTF)2.4I3 [BEDO = bis(ethylenedioxy)tetrathiafulvalene] self-assembled at the surface of a polycarbonate film. These effects produce a reversible electrochromic behavior of the layer with low operating voltages and fast operation times. Adjuvant with electrochromism, this flexible material exhibits rectifying behavior whose I-V curves are dependent on the voltage sweep directions. These results open new possibilities for the design and fabrication of organic flexible materials for soft electrochromic and rectifying components. The easy working principle ensures reliability, low power consumption, and versatility through its implementation into simple devices. Such working principle has been confirmed by temperature dependent resistance measurements, X-Ray, EDX-SEM, and conducting-AFM studies.

Reversible metallization and carrier transport behavior of In2S3 under high pressure

The electrical transport properties of indium trisulfide (In2S3) under high pressure were investigated using the in situ Hall-effect and temperature dependent resistivity measurements. Resistivity, Hall coefficient, carrier concentration, and mobility were obtained at pressures up to 41.6 GPa. Pressure induced metallization of In2S3 occurred at approximately 6.8 GPa. This was determined by measuring temperature dependent resistivity. The metallization transition was also determined from compression electrical parameters, and the decompression electrical parameters indicated that the metallization was a reversible transition. The main cause of the sharp decline in resistivity was the increase in carrier concentration at 6.8 GPa. Superconductivity was not observed at the pressures (up to 32.5 GPa) and temperatures (100–300 K) used in the experiment.

Thermoelectric properties and EPR analysis of Fe doped Cu12Sb4S13

Recently tetrahedrite compounds have attracted great attention due to their potential applications on the mid-temperature thermoelectric technologies. In this work, structural, Electron Paramagnetic Resonance (EPR) and the thermoelectric properties of tetrahedrite (Cu12-xFexSb4S13 where x = 0.0, 1.0, 1.5, 2.5) were investigated. The crystal structural, morphological properties and elemental compositions of the samples were investigated by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS-EDX) tools respectively. Magnetic properties of the samples were performed by EPR techniques using electron spin resonance (ESR) spectrometer with the model of Bruker EMX series at X-band (9.5 GHz). The thermoelectric properties were assisted based on Seebeck coefficient, electrical resistivity, and thermal conductivity measurement between 300 and 600 K temperature range. PXRD patterns and SEM-EDX analysis confirmed the main phase of tetrahedrite structure for all compounds. The Seebeck coefficient sign indicated that holes were dominant carriers in all compounds. Electrical resistivity measurement showed an increment with increasing Fe concentration likely due to decreasing in carrier density. A typical behavior of intrinsic semiconductor was observed from temperature-dependent electrical resistivity measurements. The maximum ZT value of 0.6 was achieved for the sample Cu11Fe1Sb4S13 at 550 K. Electron Paramagnetic Resonance (EPR) and thermoelectric measurements exhibited that the maximum Fe concentration might be ≤ 1.0 for achieving high ZT value in Cu12-xFexSb4S13 compounds.

Superconductivity in the superhard boride WB4.2

We show that the superhard boride WB4.2 is a superconductor with a Tc of 2.05(5) K. Temperature-dependent magnetic susceptibility, electrical resistivity, and specific heat measurements were used to characterize the superconducting transition. The Sommerfeld constant γ for WB4.2 is 2.07(3) mJ mol−1 K−2 and the ΔC/γTc = 1.56, which is somewhat higher than what is expected for weakly coupled Bardeen–Cooper–Schrieffer type superconductors. The Hc2 versus T plot is linear over a wide temperature range but does show signs of flattening by the lowest temperatures studied and therefore the zero temperature upper critical field (μ0Hc2(0)) for WB4.2 lies somewhere between the linear extrapolation of μ0Hc2(T) to 0 K and expectations based on the Werthamer–Helfand–Hohenberg model.

The σ-phase superconductors Nb20.4Rh5.7Ge3.9 and Nb20.4Rh5.7Si3.9

We show that the previously unreported ternary σ phases Nb20.4Rh5.7Ge3.9 and Nb20.4Rh5.7Si3.9 are both superconductors with Tc values of approximately 1.9 K. The superconducting transitions were characterized through temperature-dependent magnetic susceptibility, electrical resistance, and specific heat measurements. The Sommerfeld constants, γ, for Nb20.4Rh5.7Ge3.9 and Nb20.4Rh5.7Si3.9 are 89(1) mJ mol-f.u.−1K−2 and 86(1) mJ mol-f.u.−1K−2 (about 3.0 and 2.9 mJ mol-atom−1 K−2), respectively. The normalized specific heat jumps at Tc, ΔC/γTc, are approximately 1.39 for Nb20.4Rh5.7Ge3.9 and 1.30 for Nb20.4Rh5.7Si3.9, consistent with both materials being weakly coupled BCS type superconductors.

Effects of rare-earth elements and alkali metals on the superconductivity of (Tl0.7M0.3)Sr2Ca0.8Cr0.2Cu2O7 with M = Gd, Er, La, Li, Na, K, and Rb

The effects of rare-earth elements M = Gd, Er, and La and alkali metals M = K, Li, Na, and Rb substitutions on Tl0.7M0.3Sr2Ca0.8Cr0.2Cu2O7 (Tl-1212) were investigated. The characterization includes X-ray diffraction method, scanning electron microscopy, electrical resistance and AC susceptibility measurements. X-ray diffraction patterns showed that almost all samples consisted of major Tl-1212 and minor Tl-1201 and Ca0.3Sr0.7CuO2 phase. Rare-earth elemental substitution improved the formation of the Tl-1212 phase but suppressed the transition temperature. Scanning electron micrographs showed smaller grain size in the substituted samples compared with non-substituted sample. The temperature-dependent electrical resistance measurements showed metallic normal state behavior for all samples. Alkali metals substitutions showed higher zero transition temperature, Tc-zero compared with the rare-earth elemental substitution. AC susceptibility measurements showed a higher superconducting transition, Tcχ′ for alkali metals substitutions (84–93 K) compared with the rare-earth elemental substitutions (50–61 K). The inter-grain critical current density at the peak temperature Tp of the imaginary part χ”, Jc(Tp) measured using the Bean's model was between 17 and 22 A cm−2. The effects of rare-earth elements and alkali metals substitutions were discussed in terms of ionic radius and the concept of average Cu valence.

Structural defects and electronic phase diagram of topological insulator bismuth telluride epitaxial films

In this work, bismuth telluride films are grown by molecular beam epitaxy (MBE) on (111) BaF2 substrates, using stoichiometric Bi2Te3 and additional Te solid sources. The growth dynamics and structural defects are investigated in detail as function of substrate temperature, Bi2Te3 flux and extra Te supply, by means of atomic force microscopy, Raman spectroscopy and reciprocal space mapping. The growth rate increases linearly with the Bi2Te3 flux and the most appropriate conditions to grow high-quality Bi2Te3 single layers is found to be in a narrow window of MBE parameters. At low growth temperatures Te clusters are formed, while the Te deficit increases with raising substrate temperature and decreasing deposition rate. It results in films with Bi-richer phases due to the formation of Bi double layers in between Bi2Te3 quintuple layers. The electronic transport properties are also studied by temperature dependent resistivity and Hall measurements. By properly changing the substrate temperature and/or the extra Te supply, the behavior of the films can vary from insulating to metallic as well as the major carriers from p- to n-type. The electronic phase diagram presented here provides a fast route to control the bulk conductance properties of bismuth telluride, which enables the production of intrinsic bulk insulating films. In addition, the results suggest the possibility of growing intrinsic sharp p-n junctions of Bi2Te3 by properly monitoring the occurrence of structural defects, which is the first step for practical applications of this topological insulator material.