Metal-like Temperature Dependence Research Articles

Electronic transport in reactively sputtered Mn3GaN films prepared under optimized nitrogen flow

Abstract Mn-based nitrides with antiperovskite structures have several properties that can be utilized for antiferromagnetic spintronics. Their magnetic properties depend on the structural quality, composition and doping of the cubic antiperovskite structure. Such nitride thin films are usually produced by reactive physical vapor deposition, where the deposition rate of N can only be controlled by the N2 gas flow. We show that the tuning of the N content can be optimized using low temperature resistivity measurements, which serve as an indicator of the degree of structural disorder. Several Mn3GaN x films were prepared by reactive magnetron sputtering under different N2 gas flows. Under optimized gas flow conditions, we obtain films that exhibit a metal-like temperature dependence of the resistivity, a vanishing logarithmic increase of the resistivity towards zero, the highest resistivity ratio, and a lattice contraction of 0.4% along the growth direction when heated above the Néel temperature T N . The retarded formation of an additional magnetic phase appearing at a temperature T ∗ ≪ T N gives rise to a large thermal hysteresis of the resistivity and anomalous Hall effect.

Phase transformation from FeSe to Fe3Se4

We report on thermal atomic layer deposition of iron selenide (FeSe and Fe3Se4) thin films with four-inch wafer-scale uniformity using two precursors of iron bis(N, N′-diisopropylacetamidinato) [Fe(i-Pr-Me-AMD)2] and bis(triethysilyl) selenide [(Et3Si)2Se]. The temperature window is determined to be 350–400 °C. The transformation from tetragonal FeSe to monoclinic Fe3Se4 has been found. The tetragonal FeSe thin films exhibit a strong (00 l) texture deposited on amorphous silica glass, suggesting that the surface is partially parallel to the layered ab plane. By adjusting the ratio of iron and selenium, the thin films exhibit from conductors to semiconductors, and finally to insulators. All as-grown conductive samples appear metallic-like temperature dependence by low-temperature electrical transport measurements. This study opens the way to prepare various metal selenides by atomic layer deposition.

Anisotropic superconducting properties of FeSe0.5Te0.5 single crystals

We investigated the anisotropic electrical transport and magnetic properties of FeSe0.5Te0.5 single crystals grown by the self-flux method. The in-plane resistivity shows a metallic-like temperature dependence, while the out-of-plane resistivity shows a broad hump with a maximum at around 64 K. The magnetization loops for H//c-axis and H//ab-plane are also different, for example, there is a typical second peak for H//c-axis. The in-plane critical current density is larger than the out-of-plane one. The coherence length and penetration depth were estimated by the Ginzburg–Landau theory. The anisotropic parameter γ depends on the applied magnetic field and the temperature. The coupling of superconducting FeSe(Te) layers and the flux pinning mechanism relevant to anisotropy are also discussed.

Models for sensing by nanowire networks: application to organic vapour detection by multiwall carbon nanotube—DNA films

Electronic sensors for volatile organic compounds have been prepared by drop-casting dispersions of multi-wall carbon nanotubes (MWCNTs) in aqueous solutions of λ-DNA onto Pt microband electrodes. The MWCNTs themselves show a metal-like temperature dependence of the conductance, but the conductance of DNA/MWCNT composites has an activated component that corresponds to inter-tube tunneling. The resistance of the composite was modelled by a series combination of a term linear in temperature for the nanotubes and a stretched exponential form for the inter-tube junctions. The resistance may increase or decrease with temperature according to the composition and may be tuned to be almost temperature-independent at 67% by mass of DNA. Upon exposure to organic vapours, the resistance of the composites increases and the time-dependence of this signal is consistent with diffusion of the vapour into the composite. The fractional change in resistance at steady-state provides an analytical signal with a linear calibration and the presence of DNA enhances the signal and adjusts the selectivity in favour of polar analytes. The temperature dependence of the signal is determined by the enthalpy of adsorption of the analyte in the inter-tube junctions and may be satisfactorily modelled using the Langmuir isotherm. Temperature and pressure-dependent studies indicate that neither charge injection by oxidation/reduction of the analyte nor condensation of analyte on the device is responsible for the signal. We suggest that the origin of the sensing response is an adsorption of the analyte in the inter-tube regions that modulates the tunneling barriers. This suggests a general route to tuning the selectivity of MWCNT gas sensors using non-conductive polymers of varying chemical functionality.

High-performance NdSrCo2O5+δ–Ce0.8Gd0.2O2-δ composite cathodes for electrolyte-supported microtubular solid oxide fuel cells

NdSrCo2O5+δ (NSCO) is a perovskite with an electrical conductivity of 1551.3 S cm−1 at 500 °C and 921.7 S cm−1 at 800 °C and has a metal-like temperature dependence. This perovskite is used as the cathode material for Ce0.8Gd0.2O2-δ (GDC)-supported microtubular solid oxide fuel cells (MT-SOFCs). The MT-SOFCs fabricated in this study consist of a bilayer anode, comprising a NiO–GDC composite layer and a NiO layer, and a NSCO–GDC composite cathode. Three cell designs with different outer tube diameters, GDC thicknesses, and NSCO/GDC ratios are designed. The MT-SOFC with an outer tube diameter of 1.86 mm, an electrolyte thickness of 180 μm, and a 5NSCO–5GDC composite cathode presents the best performance. The flexural strength of the aforementioned cell is 177 MPa, which is sufficient to confer mechanical integrity to the cell. Moreover, the ohmic and polarization resistance values of the cell are 0.22 and 0.09 Ω cm2 at 700 °C, respectively, and 0.15 and 0.03 Ω cm2 at 800 °C, respectively. These results indicate that the NSCO-GDC composite exhibits high electrochemical activity. The maximum power densities of the cell at 700 and 800 °C are 0.46 and 0.67 W cm−2, respectively, exceeding those of existing electrolyte-supported MT-SOFCs with similar electrolyte thicknesses.

Organic Conductors with Narrow Bandwidth Based on 2-(Pyran-4-ylidene)-1,3-dithiole

Abstract A π-electron donor incorporating pyran-4-ylidene moiety, 2-(pyran-4-ylidene)-1,3-dithiole (PDT, 1a), and its derivatives (1b–e) have been synthesized. Cyclic voltammetry revealed that the derivatives of 1 exhibited two pairs of redox waves. Comparison of the first redox potentials (E1) indicated that the donating ability of PDT (E1 = −0.16 V vs. Fc/Fc+, in benzonitrile) is stronger than that of TTF (E1 = −0.09 V), but is weaker than the sulfur analog TPDT (E1 = −0.19 V). X-ray structure analyses of radical cation salts based on the ethylendithio derivative (1d)2X (X = ClO4, ReO4, and GaCl4) revealed that the donors form two-dimensional conducting sheets, in which the donors adopt the so-called β-type packing with a uniform head-to-tail stacking. Calculation of the overlap integrals of the HOMOs suggest that (1d)2X has a small intrastack overlap compared to the TTF-type conductors because of a head-to-tail stacking of the unsymmetrical π-electron framework. A tight-binding band calculation suggested that all the salts have quasi-one-dimensional Fermi surfaces. They exhibited relatively high conductivity of σrt = 0.79–20 S cm−1 on a single crystal and showed weak but metal-like temperature dependence of resistivity.

Structure analysis and electrical resistivity of the high- and low-temperature phases of NaPd3Ge2 single crystal

Rod-shaped single crystals of NaPd3Ge2 with a length of ∼1 mm were grown using the self-flux method. The electrical resistivity exhibits a metal-like temperature dependence with a kink at 115 K, which is due to a structural phase transition not observed in the previous study using polycrystalline samples. The high-temperature phase of NaPd3Ge2 crystallizes in an orthorhombic structure of the space group Imma, while the low-temperature phase is isostructural to NaPd3Si2 (space group: Imm2) as determined by the single-crystal X-ray diffraction analyses at 90 K. The density of states and total energies obtained by the density functional theory calculations reveal that both phases are metallic, and the low-temperature structure is more stable.

KCu7P3: A Two-Dimensional Noncentrosymmetric Metallic Pnictide

We report a 2D material, KCu7P3, with a noncentrosymmetric structure (trigonal space group P31m, a = 6.9637(2) Å, c = 24.1338 (10) Å), which forms both from a molten potassium polyphosphide flux and from the elements. This phase consists of infinite [Cu7P3]- layers with hexagonal P sheets separated by K+ ions. The structure of the layers is unique but related to both Cu3P and the CaCu4P2 structure-types. Single-crystal refinement reveals extensive disorder within the Cu3P-like slabs. KCu7P3 is paramagnetic and exhibits a room temperature resistivity of ∼335 μΩ cm with a metal-like temperature dependence. The metallic character is supported by density functional theory electronic structure calculations. Hall and Seebeck effect measurements yield p-type behavior with a hole mobility of ∼15 cm2 V-1 s-1 at 300 K and a carrier concentration on the order of 1021 cm-3. KCu7P3 is chemically stable in ambient conditions, as well as in aqueous neutral and acidic solutions.

Electronic Conductivity in Biomimetic α-Helical Peptide Nanofibers and Gels.

Examples of long-range electronic conductivity are rare in biological systems. The observation of micrometer-scale electronic transport through protein wires produced by bacteria is therefore notable, providing an opportunity to study fundamental aspects of conduction through protein-based materials and natural inspiration for bioelectronics materials. Borrowing sequence and structural motifs from these conductive protein fibers, we designed self-assembling peptides that form electronically conductive nanofibers under aqueous conditions. Conductivity in these nanofibers is distinct for two reasons: first, they support electron transport over distances orders of magnitude greater than expected for proteins, and second, the conductivity is mediated entirely by amino acids lacking extended conjugation, π-stacking, or redox centers typical of existing organic and biohybrid semiconductors. Electrochemical transport measurements show that the fibers support ohmic electronic transport and a metallic-like temperature dependence of conductance in aqueous buffer. At higher solution concentrations, the peptide monomers form hydrogels, and comparisons of the structure and electronic properties of the nanofibers and gels highlight the critical roles of α-helical secondary structure and supramolecular ordering in supporting electronic conductivity in these materials. These findings suggest a structural basis for long-range electronic conduction mechanisms in peptide and protein biomaterials.

Magnetostructural properties and electric transport in Mn48+xCr3-xNi38Sn11 (x = 0, 1) ribbons: Mn/Ni ratio versus Cr doping

We report the structural, electrical and magnetic properties of Cr substituted Mn48+xCr3-xNi38Sn11 (x = 0 and 1) Heusler ribbons synthesised by melt spinning of the arc-melted bulk alloy. Composition analysis employing EDAX reveals same Ni and Sn concentrations with difference in the Mn/Cr ratio in both the ribbons. Mn48Cr3Ni38Sn11 ribbons have pure cubic-austenite phase whereas Mn49Cr2Ni38Sn11 show a mixture of the dominant cubic-austenite with traces of the orthorhombic-martensitic phase as confirmed by X-ray analysis. Ribbons having higher Cr content show (i) pure cubic austenite structure (L21 type) at room temperature (ii) canonical paramagnetic-ferromagnetic transition in the austenite phase and (iii) metallic temperature dependence of resistivity. Small decrease in the Cr content produces dramatic changes in the structure-property profile. Hence ribbons having lower Cr content show (i) a mixture of cubic and orthorhombic phases, (ii) paramagnetic-ferromagnetic transition in the austenite phase above the room temperature which is followed by strongly hysteretic martensitic transition and (iii) metal-like temperature dependence of resistivity followed by typical martensitic resistivity and re-entrant metallic behaviour in the lowest temperature region. AC susceptibility measurements show absence of a spin glass like behaviour in Mn48Cr3Ni38Sn11 while the lower Cr variant demonstrates nearly ideal spin glass like characteristics. The high sensitivity of structure-property correlation towards small compositional variations is attributed to the twin effect of Cr substitution at Mn sites and the Mn/Ni ratio.

A study of electron and thermal transport in layered titanium disulphide single crystals

We present a detailed study of thermal and electrical transport behavior of single crystal titanium disulphide flakes, which belong to the two dimensional, transition metal dichalcogenide class of materials. In-plane Seebeck effect measurements revealed a typical metal-like linear temperature dependence in the range of 85–285 K. Electrical transport measurements with in-plane current geometry exhibited a nearly T2 dependence of resistivity in the range of 42–300 K. However, transport measurements along the out-of-plane current geometry showed a transition in temperature dependence of resistivity from T2 to T5 beyond 200 K. Interestingly, Au ion-irradiated TiS2 samples showed a similar T5 dependence of resistivity beyond 200 K, even in the current-in-plane geometry. Micro-Raman measurements were performed to study the phonon modes in both pristine and ion-irradiated TiS2 crystals.

Examination of the temperature dependent electronic behavior of GeTe for switching applications

The DC and RF electronic behaviors of GeTe-based phase change material switches as a function of temperature, from 25 K to 375 K, have been examined. In its polycrystalline (ON) state, GeTe behaved as a degenerate p-type semiconductor, exhibiting metal-like temperature dependence in the DC regime. This was consistent with the polycrystalline (ON) state RF performance of the switch, which exhibited low resistance S-parameter characteristics. In its amorphous (OFF) state, the GeTe presented significantly greater DC resistance that varied considerably with bias and temperature. At low biases (<1 V) and temperatures (<200 K), the amorphous GeTe low-field resistance dramatically increased, resulting in exceptionally high amorphous-polycrystalline (OFF-ON) resistance ratios, exceeding 109 at cryogenic temperatures. At higher biases and temperatures, the amorphous GeTe exhibited nonlinear current-voltage characteristics that were best fit by a space-charge limited conduction model that incorporates the effect of a defect band. The observed conduction behavior suggests the presence of two regions of localized traps within the bandgap of the amorphous GeTe, located at approximately 0.26–0.27 eV and 0.56–0.57 eV from the valence band. Unlike the polycrystalline state, the high resistance DC behavior of amorphous GeTe does not translate to the RF switch performance; instead, a parasitic capacitance associated with the RF switch geometry dominates OFF state RF transmission.

High temperature transport properties of co-substituted Ca1−xLnxMn1−xNbxO3 (Ln = Yb, Lu; 0.02 ≤ x ≤ 0.08)

The co-substituted Ca1−xLnxMn1−xNbxO3 (Ln=Yb, Lu; 0.02≤x≤0.08) are synthesized by solid-state reaction and the electronic transport properties are investigated. Rietveld refinement confirms the formation of single phase orthorhombic structure with gradual increase of cell parameters with doping level. The electronic transport properties such as Seebeck coefficient and electrical resistivity decrease with increasing the dopant concentration for both the co-substituted compositions. All the compositions of Ca1−xLnxMn1−xNbxO3 show nonmetal-like temperature dependence of resistivity; whereas metal-like temperature dependence of thermopower. This inconsistency is explained by the formation of oxygen vacancy associated defect centres that originates from partial reduction of Mn4+ to Mn3+ due to co-substitution. The defect centres act as extrinsic carriers and cause additional contribution to the entropy of the system, leading to increase of Seebeck coefficient as a function of temperature. The transport mechanism of charge carriers is explained in the framework of Mott’s small polaron hopping mechanism.

Enhanced Thermoelectric Response of Ca0.96Dy0.02Re0.02MnO3 Ceramics (Re = La, Nd, Sm) at High Temperature

Perovskite-type Ca0.98Dy0.02MnO3, Ca0.96Dy0.04MnO3, and Ca0.96Dy0.02 Re0.02MnO3 (Re = La, Nd, Sm) were prepared by solid-state reaction, and their thermoelectric properties were evaluated between 300 and 1000 K. All were single-phase, with an orthorhombic structure, and had metal-like temperature dependence of resistivity and Seebeck coefficient. The second doping element, Re = La, Nd, or Sm, introduced a larger carrier concentration, leading to a decrease in both resistivity and Seebeck coefficient. This contributed to lower thermal conductivity by introducing a second element into the system. The highest figure of merit, 0.20, was obtained for Re = La at 973 K; this was an increase of almost 100% compared with Ca0.98Dy0.02MnO3 at the same temperature.

Metal-insulator transition in a HgTe quantum well under hydrostatic pressure

The 2D semimetal in a 20 nm (100) HgTe quantum well is characterized by a comparatively low overlap between the conduction and the valence bands induced by lattice mismatch. In the present paper we report the results of transport measurements in this quantum well under hydrostatic pressure of 14.4 kbar. By applying pressure we have further reduced the band overlap, thereby creating favorable conditions for the formation of the excitonic insulator state. As a result, we observed that the metallic-like temperature dependence of the conductivity at lowering temperature sharply changes to the activated behavior, signaling the onset of an excitonic insulator regime.

Anomalous metallic-like transport of Co–Pd ferromagnetic nanoparticles cross-linked with π-conjugated molecules having a rotational degree of freedom

We investigated the electron transport in Co-Pd ferromagnetic nanoparticles (Co 16%) cross-linked with oligo(phenyleneethynylene)diethanethiolate, which consists of three rotary phenylene moieties bridged by two acetylene groups, or icosane-1,20-dithiol, which consists of one alkane chain. Although the nanoparticles cross-linked with the alkane dithiols (the latter) have extremely high electrical resistance in electron transport, the resistance of the nanoparticles cross-linked with the conjugated molecules (the former) demonstrates a linear temperature dependence from room temperature to ca. 20 K; below that temperature, it has a weak temperature-dependent residual contribution with a resistance minimum around 7 K. Computational simulations suggest that the apparent metallic-like temperature dependence at high temperatures can be explained in terms of the rotational degree of freedom of the linker molecule. The rotational motion of the constituent phenylene groups, which hinders π-conjugation along the linker molecule, becomes less excited as the temperature is lowered. The successive development of a ballistic transport path through the π-conjugated linker molecule with decreasing temperature yields the metallic-like temperature dependence observed for the bridged nanoparticles. The low-temperature resistance behaviour with a minimum is a consequence of carrier scattering by the localized Co spins of Co-Pd nanoparticles randomly ordered in a ferromagnetic state that develops below the temperature of the resistance minimum.

High-temperature transport properties of Ca0.98RE0.02MnO3−δ (RE=Sm, Gd, and Dy)

We report measurements of electrical resistivity and thermopower on CaMnO3−δ and Ca0.98RE0.02MnO3−δ (RE=Sm, Gd, and Dy) prepared by solid state reaction. CaMnO3−δ exhibits nonmetal-like temperature dependence of resistivity while metal-like temperature dependence of thermopower. This inconsistency can be explained by the extrinsic carriers arising from oxygen defects using two-band model. Ca0.98RE0.02MnO3−δ exhibits metal-like temperature dependence in both resistivity and thermopower. The transition to metal-like behavior resembles the case in degenerate semiconductors and can be attributed to an impurity band formation with characteristic of hybridized Mn 3d eg and O 2p states due to electron doping via partial substitution of lanthanides for Ca2+ and oxygen defects.

Metal-insulator transition of the LaAlO3-SrTiO3interface electron system

We report on a metal-insulator transition in the LaAlO${}_{3}$-SrTiO${}_{3}$ interface electron system, of which the carrier density is tuned by an electric gate field. Below a critical carrier density ${n}_{c}$ ranging from 0.5 to $1.5\ifmmode\times\else\texttimes\fi{}{10}^{13}/{\mathrm{cm}}^{2}$, LaAlO${}_{3}$-SrTiO${}_{3}$ interfaces, forming drain-source channels in field-effect devices, are nonohmic. The differential resistance at zero channel bias diverges within a 2$%$ variation of the carrier density. Above ${n}_{c}$, the conductivity of the ohmic channels has a metal-like temperature dependence, while below ${n}_{c}$ conductivity sets in only above a threshold electric field. For a given thickness of the LaAlO${}_{3}$ layer, the conductivity follows a ${\ensuremath{\sigma}}_{0}\ensuremath{\propto}(n\ensuremath{-}{n}_{c})/{n}_{c}$ characteristic. The metal-insulator transition is found to be distinct from that of the semiconductor 2D systems.

Chemical Bonding in Compounds of the CuAl2 Family: MnSn2, FeSn2 and CoSn2

A model for the chemical bonding in the isostructural intermetallic compounds MnSn(2), FeSn(2) and CoSn(2), crystallising in the CuAl(2)-type structure, is developed. The description is based on quantum-chemical calculations applying the electron localisability approach as well as on experimental results obtained from Raman spectroscopy, Hall effect and electrical resistivity measurements on oriented single crystals. The analysis of the chemical bonding reveals four different covalent interactions leading to the formation of interpenetrating 6(3) nets of tin and chains of transition-metal atoms T (T=Mn, Fe or Co) along [001], which are interconnected by three-centre bonds. Polarised Raman measurements on oriented single crystals allowed the determination of the bond strengths, resulting in a bond order of 0.5 within the 6(3) nets, while the three-centre interactions show bond orders of up to 1. Measurements show a metal-like temperature dependence of the resistivity. A comparison of the results with the bonding models obtained for the isostructural compounds CuAl(2), TiSb(2) and VSb(2) reveals the influence of the main-group element on the connectivity pattern.

Crystal structure and transport properties of Ba8Ge43□3

The single phase clathrate-I Ba(8)Ge(43)square(3) (space group Ia3d (no. 230), a = 21.307(1) A) was synthesized by quenching the melt between cold steel plates. Specimens for physical property measurements were characterized by microstructure analysis and X-ray diffraction on polycrystalline samples as well as single crystals. Transport properties including thermopower, electrical resistivity, thermal conductivity and specific heat were investigated in a temperature range of 2-673 K. The electrical resistivity exhibits a metal-like temperature dependence below 300 K turning into a semiconductor-like behaviour above 300 K. The analysis of the specific heat at low temperature indicates a finite density of states at the Fermi level, thus corroborating the metallic character below 300 K. The temperature dependence of the specific heat was modelled assuming Einstein-like localized vibrations of Ba atoms inside the cages of the Ge framework. A conventional crystal-like behaviour of the thermal conductivity with a low lattice contribution (kappa(l)(300 K) = 2.7 W m(-1) K(-1)) has been evidenced.