Temperature-dependent Resistivity Measurements Research Articles

Growth and characterization of the sputtered type-II topological semimetal PdTe2 thin films and PdTe2/Co60Fe20B20 heterostructures

The topological phases such as Dirac/Weyl semimetals are recently observed in Layered transition metal dichalcogenides (TMDs). The family of materials XTe2 (with X = Pd, Pt and Ni) possesses a type-II topological semimetal (TSM) phase. Here, we report the synthesis of the layered XTe2 thin films using the industry-friendly sputtering technique. The structural and morphological qualities of the sputtered-grown XTe2 thin films are analysed using X-ray diffraction and Raman measurements. The crystallites are highly oriented along the c-axis of the sapphire substrate, which suggests that growth is driven by van der Waals epitaxy. The magneto-transport measurements namely temperature-dependent resistivity and magnetoresistance measurement in out-of-plane field configuration are performed on PdTe2 thin films. The semi-metallic trend and superconducting transition are observed in the temperature-dependent resistivity curve. The weak antilocalization behavior observed in the magnetoconductance curves at a low magnetic field signifies the possibility of a non-zero Berry phase, possibly arising from the Dirac fermionic nature of electronic states. The spin dynamics measurements are performed on the PdTe2/Co60Fe20B20 bilayers. The effective spin mixing conductance and spin current density are found to be ∼2×1020m-2and 2.5MAm-2, respectively in PdTe2/Co60Fe20B20 bilayer which are quite larger than those observed in heavy metal-ferromagnets bilayers, and comparable to MBE-grown topological insulator-ferromagnets heterostructures. This work paves the way to synthesize Te-based layered TMDs using the industrially viable sputtering technique, and demonstrates the prospects for exploring the topological superconductive phase in PdTe2 films and large spin-charge conversion efficiency in PdTe2/Co60Fe20B20 heterostructures.

Effect of the growth orientation on the physical properties of Sr2CoNbO6 thin films

We study the effect of the growth orientation on the structural, electronic, and hence transport properties of Sr2CoNbO6 thin films grown on the orthorhombic NGO(100) and cubic MgO(100) substrates. The x-ray diffraction patterns show the growth of the thin film along the a axis resulting in the asymmetric (b≠c) in-plane compressive strain in the case of NGO(100), whereas along the c axis with tensile strain in the case of MgO(100) substrate. The temperature dependent resistivity measurements indicate the lower electronic conductivity for the film grown on the NGO(100) substrate, which is found to be correlated with the higher degree of the oxygen deficiencies and, hence, a larger concentration of the insulating Co2+ in this sample. Further, the x-ray photoemission spectroscopy measurements show that Sr and Nb are present in the 2+ and 4+ valence state, whereas Co exists in the 2+, 3+, and 4+ states, a fraction of which was found to vary with the growth orientation. Moreover, the analysis of leakage current using the sum exponent model indicates the presence of two different relaxation mechanisms in these samples.

Increased phase coherence length in a porous topological insulator

The surface area of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ thin films was increased by introducing nanoscale porosity. Temperature dependent resistivity and magnetotransport measurements were conducted both on as-grown and porous samples (23 and 70 nm). The longitudinal resistivity of the porous samples became more metallic, indicating the increased surface area resulted in transport that was more surfacelike. Weak antilocalization was present in all samples, and remarkably the phase coherence length doubled in the porous samples. This increase is likely due to the large Fermi velocity of the Dirac surface states. Our results show that the introduction of nanoporosity does not destroy the topological surface states but rather enhances them, making these nanostructured materials promising for low energy electronics, spintronics and thermoelectrics.

Ion irradiation induced modification in the electrical properties and the electronic structures of vanadium dioxide thin films

Ion irradiation modifies the physical properties of oxide materials. The present study focuses on the effect of 150 keV Fe and Ni ion irradiations in VO2 thin films. These films were deposited by pulsed laser on an r-cut sapphire substrate (1 1‾ 02). The range of ion fluences was from 1 × 1014 to 1 × 1016 ions/cm2 and these samples were characterized by using grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy, Temperature-dependent resistivity measurements (R-T), and Hall effect measurement. While the presence of monoclinic phases is confirmed by GIXRD and Raman measurements, the ion irradiation on VO2 modifies the metal to insulator transition temperature (MIT). The R-T measurement with supporting analysis of carrier concentration authenticates a decrease in transition temperature and an increase in conductivity with an increment of ion-fluences. Synchrotron-based X-ray photoemission spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) revealed that Vanadium ions are in mixed oxidation states of 4+ and 5+ after ion irradiation. Further, the Photo-Electron Spectroscopy (PES) measurement indicates the metallization of VO2 after ion irradiation consistent with R-T measurements.

Controllable Synthesis and Charge Density Wave Phase Transitions of Two-Dimensional 1T-TaS2 Crystals.

1T-TaS2 has attracted much attention recently due to its abundant charge density wave phases. In this work, high-quality two-dimensional 1T-TaS2 crystals were successfully synthesized by a chemical vapor deposition method with controllable layer numbers, confirmed by the structural characterization. Based on the as-grown samples, their thickness-dependency nearly commensurate charge density wave/commensurate charge density wave phase transitions was revealed by the combination of the temperature-dependent resistance measurements and Raman spectra. The phase transition temperature increased with increasing thickness, but no apparent phase transition was found on the 2~3 nm thick crystals from temperature-dependent Raman spectra. The transition hysteresis loops due to temperature-dependent resistance changes of 1T-TaS2 can be used for memory devices and oscillators, making 1T-TaS2 a promising material for various electronic applications.

Experimental evidence of a size-dependent sign change of the Seebeck coefficient of Bi nanowire arrays

The electrical transport in bismuth nanowires is strongly influenced by both sample geometry and crystallinity. Compared to bulk bismuth, the electrical transport in nanowires is dominated by size effects and influenced by surface states, which gain increasing relevance with increasing surface-to-volume ratios, i.e. with decreasing wire diameter. Bismuth nanowires with tailored diameter and crystallinity constitute, therefore, excellent model systems, allowing to study the interplay of the different transport phenomena. Here, we present temperature-dependent Seebeck coefficient and relative electrical resistance measurements of parallel bismuth nanowire arrays with diameters between 40 and 400 nm synthesized by pulsed electroplating in polymer templates. Both electrical resistance and Seebeck coefficient exhibit a non-monotonic temperature dependence, with the sign of the Seebeck coefficient changing from negative to positive with decreasing temperature. The observed behavior is size-dependent and is attributed to limitations of the mean free path of the charge carriers within the nanowires. The observed size-dependent Seebeck coefficient and in particular the size-dependent sign change opens a promising avenue for single-material thermocouples with p- and n-legs made from nanowires with different diameters.

Effect of thickness on the superconducting properties of the thin films of FeTe0.55Se0.45 deposited on YSZ substrates using Pulsed laser deposition (PLD) technique

We report the successful fabrication of thin films of thickness ∼26 nm, 59 nm, 76 nm, and 158 nm of the composition FeTe0.55Se0.45 on the (100)-oriented Yttria-stabilized zirconia (YSZ) substrate using the pulsed laser deposition (PLD) technique. The effect of thickness on the structure and superconducting properties of the thin films has been investigated. X-ray diffraction study reveals the growth of thin films along the c-axis. The temperature-dependent resistivity measurements confirm superconductivity in all the fabricated thin films, and the highest ∼ 16.2 K has been observed for the thin film of thickness ∼59 nm, which is higher than the corresponding value of the polycrystalline target sample. The superconducting parameters such as upper critical magnetic field coherence length thermally activated energy (TAE) U0, and vortex phase state have been measured by magnetotransport measurements near in the different applied magnetic fields up to 12 T. The TAE values determined from the conventional and modified thermally activated flux flow (TAFF) models are discussed and compared. A crossover from 2-dimensional (2D) vortex to 3-dimensional (3D) vortex behaviour has been observed. The vortex phase diagrams are used to observe the transition from the vortex liquid to the vortex glass state in the grown thin films. Moreover, magnetoresistance (MR) measurements show non-saturating linear magnetoresistance (LMR) in the high field region, indicating the presence of the possible topological state in the grown thin films. The values of magnetic field dependent transport critical current density, have been evaluated from the current-voltage (I-V) curves. Furthermore, the normalised pinning force density has been used to characterize the pinning processes using the Dew Hughes’ scaling rule which reveals that the grown thin films contain δl-surface pinning centers.

Insights on the host precursor role play in the chemiresistive gas sensing properties of nebulizer spray-coated CuO films

Copper oxide (CuO) gas sensors have been designed via nebulizer spray pyrolysis onto the glass substrate using different cationic precursors. XRD analysis confirmed that the prepared CuO films with monoclinic structure and the porous morphology was observed for copper acetate precursor solution from SEM analysis. EDAX and XPS analysis revealed a near stoichiometric ratio of copper and oxygen in the films. The temperature-dependent resistivity measurements were measured using the four-probe technique. The optical band gap was calculated from 1.5 eV to 1.9 eV and the photoluminescence emission peak was recorded at 628 nm. From the TRPL analysis, the A: CuO film attained a maximum average lifetime (τavg) of 0.495 ns. The porous structure of the A: CuO sensor operating at room temperature exhibits high sensitivity, fast response and recovery speed for acetone, ethanol and toluene gas detection (16 s/47 s, 9 s/12 s and 6 s/38 s).

Magnetoelastic coupling with inverse magnetocaloric effect observed in Sr2YRuO6

Sr2YRuO6, a 4d transition metal, Ru ion based double perovskite crystallizes in monoclinic structure with Y and Ru octahedra ordering alternately. The relevance of lattice distortion to complex magnetic ground state is investigated from structural, vibrational, magnetic, transport and thermal properties. Raman modes due to monoclinic distortions are observed. From the temperature dependent magnetization studies, a magnetic transition is observed at 25 K and below that the presence of weak ferromagnetic contribution is noticed. A magnetic frustration parameter of six is evaluated from the magnetic results that suggest the presence of spin frustration on the Ru octahedra leading to geometric frustration in SYRO. Isothermal magnetization measurements are carried out for several temperatures from 8 K to 70 K in small temperature intervals to determine the magnetocaloric effect of Sr2YRuO6. The change in magnetic entropy around the Neel temperature, 25 K and Inverse magnetocaloric effect is observed in the system around this temperature. The change in isothermal magnetic entropy results indicates a canted AFM structure as the ground state. Neutron diffraction data refined using propagation vector, k = (0.5,0.5,0) reveal antiferromagnetic ordering at temperatures below 30 K down to 2 K. Variation of the lattice parameters with temperature from the neutron powder diffraction result signifies the presence of magnetoelastic coupling around 25 K. From the temperature dependent resistivity measurements, a small polaron hopping like conductivity is observed in the system. Activation energy reduces with the increase in temperature. Temperature dependent specific heat data shows no ordering, the absence of peak around magnetic transition along with magnetic contribution well above transition temperature is consistent with magnetic frustration in this system.

Low-temperature synthesis of crystalline vanadium oxide films using oxygen plasmas

Vanadium oxide (VOx) compounds feature various polymorphs, including V2O5 and VO2, with attractive temperature-tunable optical and electrical properties. However, to achieve the desired material property, high-temperature post-deposition annealing of as-grown VOx films is mostly needed, limiting its use for low-temperature compatible substrates and processes. Herein, we report on the low-temperature hollow-cathode plasma-enhanced atomic layer deposition (ALD) of crystalline vanadium oxide thin films using tetrakis(ethylmethylamido)vanadium and oxygen plasma as a precursor and coreactant, respectively. To extract the impact of the type of plasma source, VOx samples were also synthesized in an inductively coupled plasma-enhanced ALD reactor. Moreover, we have incorporated in situ Ar-plasma and ex situ thermal annealing to investigate the tunability of VOx structural properties. Our findings confirm that both plasma-ALD techniques were able to synthesize as-grown polycrystalline V2O5 films at 150 °C. Postdeposition thermal annealing converted the as-grown V2O5 films into different crystalline VOx states: V2O3, V4O9, and VO2. The last one, VO2 is particularly interesting as a phase-change material, and the metal-insulator transition around 70 °C has been confirmed using temperature-dependent x-ray diffraction and resistivity measurements.

Magnetic phase diagram and multiple field-induced states in the intermetallic triangular-lattice antiferromagnet NdAuAl4Ge2 with Ising-like spins

Geometrical frustration and the enhancement of strong quantum fluctuations in two-dimensional triangular antiferromagnets can lead to various intriguing phenomena. Here, we studied the spin-1/2 triangular lattice antiferromagnet NdAuAl$_4$Ge$_2$. Thermodynamic and transport properties, such as magnetization and specific heat together with the resistivity measurements were performed. In zero field, two successive phase transitions were observed at $T_{\rm N1}=1.75\pm 0.02$ K and $T_{\rm N2}=0.49\pm 0.02$ K, respectively. Under magnetic field, $\rm XXZ$-type anisotropy was revealed, with the moments pointing along the easy $c$ axis. For $B\parallel c$, multiple field-induced states were observed, and the magnetic phase diagram was established based on the specific heat and magnetization data. The temperature-dependent resistivity measurements indicate that NdAuAl$_4$Ge$_2$ is a good metal. It is very likely that both the long-range RKKY interactions and the geometrical frustration play an important roles in this case.

Fabrication and Properties of Epitaxial VO2 Thin Film on m-Al2O3 Substrate

A thin film of thermochromic VO2 was prepared on m-Al2O3 substrate using a radio frequency (RF) magnetron sputtering technique. The epitaxial growth of the monoclinic M1 phase of VO2 on the m-Al2O3 substrate was confirmed through synchrotron X-ray diffraction (XRD) measurements. The transformation of this monoclinic M1 phase into a rutile phase at ~68 °C was reflected in the temperature-dependent XRD measurements of the VO2 thin film. The temperature-dependent electrical resistance measurements of this sample also revealed an abrupt metal-to-insulator transition at ~68 °C, which is reversible in nature. Temperature-dependent X-ray absorption (XAS) measurements at V L-edge and O K-edge were performed to study the electronic structure of the epitaxial VO2/m-Al2O3 thin film during the metal-to-insulator (MIT) transition.

Effect of annealing conditions on VO2 thin films prepared by sol-gel method

Vanadium dioxide has attracted considerable attention because of the semiconductor-metal transition at 68 °C. This reversible transition changes crystal structure from monoclinic to rutile resulting in tremendously change of transmittance and reflectance of infrared wavelength and electrical resistance making VO2 thin film potential candidate for energy-saving smart windows—automatically block heat. However, the preparation is very challenging due to the multivalence of vanadium element. In this study, we explored different annealing conditions for thin films synthesized by sol-gel method which has advantage over other methods such as lower cost, simpler process, and ability to make large scale thin films. In the experiment, vanadium (V) triisopropoxy oxide (VO(OC3H7)3) was used as precursor. The annealing conditions explored are annealing temperature and annealing time of thin films in argon rich environment. The spin-coated films were annealed at 300-500 °C for 30 minutes, while another set was annealed at 450 °C for 30, 60, 90, and 120 minutes, respectively. Then the films were characterized with Raman spectroscopy and temperature dependent resistance measurement. The results show that the longer the annealing time and the higher the annealing temperature are better for film formation—closer to VO2 (higher V:O ratio). In addition, the effect of various substrate types was also explored, another set of thin films was coated on c-plane sapphire (c-Al2O3) and soda lime glass (mostly SiO2) to compare the effect. Then the films were characterized with Raman spectroscopy and X-Ray Diffraction (XRD). The result showed that sapphire substrate yielded better film formation.

Growth of rare-earth monopnictide DySb single crystal by novel self-flux method

A new method for the single crystal growth of rare-earth monopnictide DySb by self-flux technique is reported. Detailed structural, transport and magnetic characterisation of the crystal has been carried out by using X-ray diffraction (XRD), High resolution X-ray diffraction (HRXRD), resistivity and magnetisation measurements, respectively. The Rietveld refinement of powder XRD pattern reveals that the grown crystal is in single phase and crystallises in space group Fm3̄m (225) of rock- salt type crystal structure. The HRXRD and rocking curve analysis reveals the high-quality of the grown crystal. Temperature dependent resistivity and magnetisation measurements show a transition at 9.7 K from paramagnetic to antiferromagnetic state.

Electronic and optical properties of strain-locked metallic Ti2O3 films

We successfully grow corundum structured Ti2O3 films on c-plane sapphire substrates using pulsed laser deposition. Temperature dependent resistivity measurements show that a metal to insulator transition (MIT) is suppressed, showing conducting behavior at all temperatures. Samples still show an increase in resistivity as temperature is decreased, a characteristic indicative of a semiconducting phase. Our films exhibit grain size on the order of 30 nm which induce a strain consistent with nanoparticle Ti2O3 showing a (c/a) ratio of 2.7. The imposed strain causes an increase in the c-axis length as the temperature is decreased, and thereby suppresses the transition to an insulating phase. Our optical data agrees with this result, showing the lack of a band gap and the electronic structure consistent with bulk high temperature metallic Ti2O3 with the a1g - egπ interband transition shifted down to 0.7 eV from its bulk insulating value of ∼1 eV.

Structural, thermo-electric, and thermo-magnetic characteristics of non-stoichiometric L21-type Fe43Mn29Si28 Heusler structures

The structural, thermo-electrical, and thermo-magnetic characteristics of non-stoichiometric L21-type Fe43Mn29Si28 Heusler-type alloy, as bulk and glass-coated microwire by employing temperature-dependent electrical resistivity, relative permeability and magnetization measurements. Interestingly, the temperature dependence of electrical resistivity experiments on Fe43Mn29Si28 alloy under a heating-cooling cycle showed a clear temperature hysteresis in the temperature range of 149–226 K, corresponding to the martensitic-austenite phase transformation. During the martensitic transformation, the relative permeability of glass-coated Fe43Mn29Si28 microwire varies significantly by 2350% which much higher than the bulk Fe43Mn29Si28 alloy value of about 940%. Further magnetization experiments on both bulk and glass coated Fe43Mn29Si28 Heusler alloys measured at 0.05 kOe reveal a hysteresis of magnetization during heating and cooling process in the temperature range of 181–194 K attributed to martensitic transformation. Magnetic hysteresis measured at 5 K shows that the bulk Fe43Mn29Si28 alloy coercivity is 44 Oe which is magnetically softer than the glass-coated Fe43Mn29Si28 microwire with Hc = 110 Oe. The observation of martensitic transformation in bulk and glass-coated Fe43Mn29Si28 wire would be useful for developing common actuator and sensing applications.

Phase-transition-induced thermal hysteresis in Type-II weyl semimetals [formula omitted] and [formula omitted

The resistivity versus temperature measurement is commonly used for identifying temperature-induced phase change and the resulting hysteresis loop. While the resistance is influenced by both the density of states and the carrier lifetimes, the Seebeck coefficient is influenced predominantly by the density of states, and hence is a better probe of the phase of the material. Here, 1T′−Td temperature-induced phase transition in MoTe2 is studied using temperature-dependent X-ray diffraction, resistivity, and Seebeck coefficient measurements. A more distinct hysteresis is observed when measuring the Seebeck coefficient which is consistent with direct measurements of the crystallographic angle using the temperature-dependent X-ray diffraction. The Seebeck and electrical resistivity measurements indicate a competing contribution of the electrons and holes. The contribution of electron pockets becomes more dominant when molybdenum atoms are replaced by tungsten. In MoTe2, a topologically induced enhancement of the Nernst coefficient is observed at low temperatures, and a relatively large phase-transition induced Thomson coefficient of 111 μV⋅K−1 is measured at 254 K which is larger than the Seebeck coefficient measured in the entire temperature range.

(Ba1−x Na x )F(Zn1−x Mnx)Sb: A novel fluoride-antimonide magnetic semiconductor with decoupled charge and spin doping

We report the successful synthesis and characterization of a novel 1111-type magnetic semiconductor (Ba1−x Na x )F(Zn1−x Mn x )Sb (0.05 ≤ x ≤ 0.175) with tetragonal ZrSiCuAs-type structure, which is isostructural to the layered iron-based superconductor La(O,F)FeAs. Na substitutions for Ba and Mn substitutions for Zn introduce carriers and local magnetic moments, respectively. Ferromagnetic interaction is formed when Na and Mn are codoped, demonstrating that local magnetic moments are mediated by carriers. Iso-thermal magnetization shows that the coercive field is as large as ~ 12 000 Oe, which is also reflected in the large split between the temperature-dependent magnetization in zero-field-cooling and field-cooling condition. AC susceptibility under zero field demonstrates that samples evolve into spin-glass state below spin freezing temperature T f. The measurements of temperature-dependent resistivity indicate that (Ba1−x Na x )F(Zn1−x Mn x )Sb exhibits semiconducting behaviour.

Cobalt as a promising dopant for producing semi-insulating β-Ga2O3 crystals: Charge state transition levels from experiment and theory

Optical absorption and photoconductivity measurements of Co-doped β-Ga2O3 crystals reveal the photon energies of optically excited charge transfer between the Co related deep levels and the conduction or valence band. The corresponding photoionization cross sections are fitted by a phenomenological model considering electron–phonon coupling. The obtained fitting parameters: thermal ionization (zero-phonon transition) energy, Franck–Condon shift, and effective phonon energy are compared with corresponding values predicted by first principle calculations based on density functional theory. A (+/0) donor level ∼0.85 eV above the valence band maximum and a (0/−) acceptor level ∼2.1 eV below the conduction band minimum are consistently derived. Temperature-dependent electrical resistivity measurement at elevated temperatures (up to 1000 K) yields a thermal activation energy of 2.1 ± 0.1 eV, consistent with the position of the Co acceptor level. Furthermore, the results show that Co doping is promising for producing semi-insulating β-Ga2O3 crystals.

Effect of SiO2 buffer layer on phase transition properties of VO2 films fabricated by low-pressure chemical vapor deposition

Vanadium dioxide (VO2) is a promising material with semiconductor-metal phase transition for novel electronic and optic applications. One of the most widely used substrates for VO2 films is silicon (Si) covered with a silicon dioxide (SiO2) buffer layer because such a substrate is compatible with standard silicon technology. However, little attention has been paid to the effect of SiO2 buffer layer preparation and properties of the VO2/SiO2 interface to the VO2 phase-transition parameters. In the present paper, we investigate the phase-transition properties of VO2 films grown by low-pressure chemical vapor deposition on dry and wet thermally grown SiO2 buffer layers. The x-ray diffraction technique, scanning electron microscopy, and temperature-dependent resistance measurements showed a high crystal quality of the grown VO2 films. We identified the synthesis conditions for obtaining a sharp reversible semiconductor-metal phase transition in VO2 films on both SiO2 buffer layers. It was found that the resistance jump due to phase transition in VO2 films grown on dry and wet SiO2 was 6.5 × 103 and 5.6 × 103 with the hysteresis widths being equal to 2 and 3 °C, respectively. The calculated average size of VO2 crystallites on the dry SiO2 buffer layer proved to be 1.5–2 times larger than that on the wet SiO2 buffer layer. The field-effect measurements showed that the electric-current modulation in VO2 films synthesized on dry SiO2 is almost ten times higher than on wet SiO2 due to different interface layer properties. For 250 nm VO2 film on dry SiO2, the current modulation value reached 0.6%. The results of this work may be useful for the fabrication of new VO2-based functional devices compatible with standard silicon technology.