Metallic Temperature Dependence Research Articles

Structures and electrical properties of molecular conductors based on bis-fused TTF donors with bis(alkylthio) groups

Abstract Several bis(alkyldithio) derivatives of BDT-TTP (BTC n -TTP), where BDT-TTP is 2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene, have been synthesized. Cyclic voltammograms of new donors consist of four pairs of single-electron transfer waves, and the first redox potentials ( E 1 ) are higher by 0.04–0.05 V than that of unsubstituted BDT-TTP. The TaF 6 - salt of bis(methylthio) derivative (BTM-TTP) have the so-called β-type array of the donors. A tight-binding band calculation of (BTM-TTP) 2 TaF 6 suggests that this salt has a quasi one-dimensional Fermi surface. The BTM-TTP salts with the PF 6 - and TaF 6 - anions exhibit metallic temperature dependence down to 90 K, while the SbF 6 - salt is a semiconductor with the activation energy of 0.13 eV.

Screening-theory-based description of the metallic behavior inSi∕SiGetwo-dimensional electron systems

We calculate the temperature, density, and parallel magnetic field dependence of low temperature electronic resistivity in 2D high-mobility Si/SiGe quantum structures, assuming the conductivity limiting mechanism to be carrier scattering by screened random charged Coulombic impurity centers. We obtain comprehensive agreement with existing experimental transport data, compellingly establishing that the observed 2D metallic behavior in low-density Si/SiGe systems arises from the peculiar nature of 2D screening of long-range impurity disorder. In particular, our theory correctly predicts the experimentally observed metallic temperature dependence of 2D resistivity in the fully spin-polarized system.

Structural chemistry, magnetism and electrical properties of binary Gd silicides and Ho 3Si 4

Binary gadolinium silicides were thoroughly investigated with respect to their structural chemistry and physical properties as well. Formation and structure types of all known binary phases were confirmed. In addition, a new binary compound Gd 3Si 4 belonging to the Ho 3Si 4 type was obtained through a sluggish peritectoidal reaction between the binary phases GdSi and GdSi 2− x . In addition, the limits of the homogeneity range of the GdSi 2− x phase (AlB 2 type) were also confirmed. Moreover, the crystal structures of Gd 5Si 3 (Mn 5Si 3 type) and GdSi (FeB type) were determined from X-ray single crystal data. The magnetic and electrical properties for most of the binary Gd–Si phases, encountered within this system, have been studied. The magnetic properties are characterised by the onset of magnetic ordering below 100 K. Gd 5Si 3 shows an antiferromagnetic transition at T N = 75 K, GdSi orders weakly ferromagnetically at T C = 78 K followed by an antiferromagnetic spin alignment at T N = 54 K, GdSi 2− x reveals also two consecutive transitions at T N = 54 K and T C = 35 K and GdSi 2 exhibits the same behaviour but at T N = 26 K and T C = 14 K. In addition, magnetic measurements were also performed for the binary silicide Ho 3Si 4 (antiferromagnetic behaviour, T N = 16 K), structurally related to Gd 3Si 4. The electrical resistivities for all silicides resemble the temperature dependence of metals, with characteristic changes of slope in the resistivity curves due to the reduced electron scattering in the magnetically ordered states.

Anisotropic conductance of Pb?induced chain structures on Si(557) in the monolayer regime

We present data of a study of four-point conductance of adsorbed Pb films on Si(557) in the thickness range between 0.6 up to several monolayers (ML) at various annealing stages. These measurements are combined with tunneling microscopy (STM). Onset of conductance is found close to the percolation limit. Pb layers annealed to room temperature are characterized by activated contributions to conductance up to 3 ML, a purely metallic temperature dependence at thicker layers, and an anisotropy of at most a factor of 2. On the contrary, annealing to 640 K, leaving only the first monolayer on the Si(557) surface results in extremely high surface state conductance which is quasi one–dimensional below a critical temperature of Tc=78 K, associated with an order–disorder phase transition. Induced by a 10-fold superperiodicity along the Pb chains and their lateral ordering, the system switches from low to high conductance anisotropy, with a metal–insulator transition in the direction perpendicular to the chain structure, while in the direction along the chains conductance with a 1/T + const. temperature dependence was found.

Ba2Ir3O9: A New 5d Mixed‐Valence Metallic Oxide with KSbO3‐Type Structure.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Preparation and characterization of La0.8Sr0.2MnO3 thin films by an excimer laser MOD process for a bolometer

La0.8Sr0.2MnO3 (LSMO) films were prepared on LaAlO3 substrates by excimer laser metal organic deposition (ELMOD) at 500 °C. The temperature dependence of resistance of the LSMO films was investigated by changing the laser fluence, irradiation time, and film thickness. It was found that the resistance of the LSMO films 80 nm in thickness that were irradiated by an ArF laser at a fluence of 100 mJ/cm2 for 60 min showed a metallic temperature dependence, and the maximum temperature coefficient of resistance of the films (defined as 1/R×dR/dT) was 3.4% at 265 K.

The Synthesis of Difluoro and Dimethyl Derivatives of 2,6-Bis(dicyanomethylene)-2,6-dihydro-4H-cyclopenta[2,1-b:3,4-b′]dithiophen-4-one (CPDT-TCNQ) and the Conducting Properties of the Metallic Salts Based on the Dimethyl Derivative

Abstract In order to investigate how the sheet-like network along the side-by-side direction constructed by the strong inter-column S···N and O···H atom–atom contacts influences the stability of the metallic state of novel one-dimensional metallic anion radical salts of 2,6-bis(dicyanomethylene)-2,6-dihydro-4H-cyclopenta[2,1-b:3,4-b′]dithiophen-4-one (CPDT-TCNQ), the 3,5-difluoro (F2CPDT-TCNQ) and 3,5-dimethyl (Me2CPDT-TCNQ) derivatives of CPDT-TCNQ, not possessing hydrogen atoms at the 3,5-positions to construct the inter-column O···H contacts, have been synthesized. The anion radical salts, Me4X(Me2CPDT-TCNQ)2 (X = P and As), showed a metallic temperature dependence of the resistivity from room temperature down to around 240 K and 200 K, respectively, whereas Me4X(Me2CPDT-TCNQ)2 (X = N and Sb) were semiconducting. The metal–insulator transition temperatures (TMI) of Me4X(Me2CPDT-TCNQ)2 (X = P and As) are significantly higher than those of MX4(CPDT-TCNQ)2 (X = P and As). Moreover, the extent of destabilization of the metallic state is much more significant in Me4N(Me2CPDT-TCNQ)2, since MeN4(CPDT-TCNQ)2 is metallic down to 130 K. Although electric conduction can not occur along the side-by-side direction in either salt of CPDT-TCNQ and Me2CPDT-TCNQ, these facts suggest that the two-dimensional sheet-like molecular network along the a-axis may contribute to stabilize the one-dimensional metallic states of the anion radical salts of CPDT-TCNQ.

Preparation of Bi 2Sr 2Ca n−1 Cu nO y thick films by chemical solution deposition

We prepared Bi 2Sr 2Ca n−1 Cu n O y ( n=2: Bi-2212) thick films on MgO (1 0 0) substrates by a chemical solution deposition of acetic acid metal salts, where a partial melt-process was included. From the X-ray diffraction results, we found that the optimum maximum temperature ( T max) and period of sintering for obtaining c-axis-oriented Bi-2212 thick films were 910 °C and 15 min, respectively. The film showed a metallic temperature dependence of resistance in the normal state, and its T c (critical temperature) was about 88 K, which became about 90 K by annealing at 700 °C.

Low-temperature growth of La 0.8Sr 0.2MnO 3 thin films on LaAlO 3 and SrTiO 3 substrates by an excimer laser metal organic deposition (ELMOD) process

The preparation of La 0.8Sr 0.2MnO 3 (LSMO) films on LaAlO 3 (LAO) and SrTiO 3 (STO) substrates was examined by an excimer laser MOD processes (ELMOD). Epitaxial LSMO films on the LAO substrate were obtained by the ArF laser irradiation at a fluence of 100 mJ/cm 2 for 10, 30, 60 and 90 min with heating at 500 °C. In the case of the STO substrate with heating at 500 °C, epitaxial LSMO was obtained by the ArF laser irradiation at 70 mJ/cm 2 whereas no film was formed at 100 mJ/cm 2. The effects of the laser fluence, irradiation time and film thickness on the temperature dependence of the resistance and TCR (defined as 1/R·dR/DT) of the LSMO films on the LAO substrate were investigated. It was found that the resistance of the 80 nm thick LSMO film that was irradiated by the ArF laser at a fluence of 100 mJ/cm 2 for 60 min showed a metallic temperature dependence from 310 to 100 K, and the maximum temperature coefficient of resistance of the film (TCR: defined as 1/R·dR/DT) was 3.4% at 265 K. In the case of the STO substrate, the resistance of the 80 nm thick LSMO film that was irradiated by the ArF laser at a fluence of 70 mJ/cm 2 for 60 min showed a Tc at 280 K, and the maximum TCR of the film was 2.9% at 220 K. The formation mechanisms of the epitaxial LSMO films obtained by the ELMOD process also are discussed.

Ba2Ir3O9: a new 5d mixed-valence metallic oxide with KSbO3-type structure

Crystal structure, magnetic and transport properties of new ternary mixed-valence iridium oxide Ba2Ir3O9 are reported. A hydrothermal reaction of IrO2 and BaO2 under a pressure of 150MPa at 650°C yields small black crystals with cubic shape. Energy dispersive X-ray analysis on an as-grown crystal gives the atomic ratio Ba:Ir = 2:3. X-ray powder diffraction shows a pattern of cubic lattice with a cell parameter a = 9.42Å and further Rietveld analysis reveals the KSbO3-related structure. Although the structure of Ba2Ir3O9 is almost identical to Ba0.5IrO3 [Mat. Res. Bull. 9 (1974) 1777], there is a significant difference in the composition between them. Because the crystallographic site for Ir is unique, Ba2Ir3O9 is classified into class III mixed-valence compound where the average valence of Ir is +4.67. The magnetic susceptibility shows almost temperature independent Pauli paramagnetism of 2.9 × 10−4emu/mol Ir at room temperature. The resistivity for a pellet sample is measured to be 1.5 × 10−1Ωcm at room temperature and shows metallic temperature dependence. At low temperature, the temperature dependent component of the resistivity is almost proportional to T2. This indicates that Ba2Ir3O9 is a bad metal because of strong electron-electron scattering. The narrow 5d-band feature characteristic of the KSbO3-type structure is related to the strong electron correlation.

Quantum Hall and insulating states of a 2-D electron–hole system

We review recent results from magneto-transport studies of an InAs/GaSb based electron–hole system. In high magnetic field, the system shows two types of overall behaviour depending on whether the effective occupancy ν e− ν h= ν eff is zero or finite. When ν e− ν h is finite the resistivity has a metallic temperature dependence and the Hall resistance is quantized at a finite value. When ν e− ν h=0 however, the resistivity shows an insulating temperature dependence. Since the condition ν e− ν h= ν eff for a given value of ν eff can be achieved with different values of the individual occupancies, the system can oscillate between insulating and metallic behaviour with magnetic field. In the insulating states where ν e− ν h=0, the system exhibits novel behaviour, where the Hall resistance becomes symmetric under field reversal, and fluctuates reproducibly with magnetic field. Geometry dependence measurements show that in this state the sample interior becomes extremely insulating, and conduction is dominated by the mesa edge. The transport is one dimensional, and the chirality usually associated with the Hall effect is lost. Current driven breakdown is studied for quantum Hall states where ν e− ν h is finite. There is a strong dependence of the breakdown behaviour on channel width and the relative electron and hole densities. This suggests a strong involvement of the sample interior in stark contrast to the insulating states.

Physical properties of double perovskite compounds ALaVMoO 6 (A=Ca, Sr, Ba)—a possible half-metallic antiferromagnetic system

Double perovskite compounds ALaVMoO 6 (A=Ca, Sr, Ba) have been synthesized and their electrical and magnetic properties have been investigated. Magnetization measurements have indicated the possible antiferromagnetic transitions at 120 and 130 K for A=Ca and Sr samples, respectively. Electrical resistivity ρ for this system shows metallic temperature dependence from 300 to 20 K, though the sample with A=Ca shows weak semiconducting behavior in the low temperature region (<70 K). Considering the magnetic and electrical properties and assuming the V 3+ S=1 and Mo 4+ S=1 valence and spin states, the samples with A=Ca and Sr can be promising candidates for half-metallic antiferromagnets.

The binary gallide Eu 5Ga 9 – crystal structure, chemical bonding and physical properties

The title compound was prepared from the elements by reaction in a sealed tantalum tube at 1320 K followed by slow cooling to 970 K or, alternatively, in glassy carbon crucibles with HF melting. The crystal structure of Eu 5Ga 9 was refined from single-crystal data: Cmcm, a=4.613(1) Å, b=10.902(3) Å, c=26.097(6) Å, Z=4, R F=0.036, 811 structure factors and 46 variables. The structure is described as a three-dimensional network formed by gallium atoms with europium atoms embedded in the cavities. The bonding analysis (LMTO, ELF) confirmed this representation of the structure. Magnetic susceptibility measurements show Curie–Weiss behavior above 60 K with a magnetic moment per Eu atom of 8.12(1) μ B, indicating divalent europium. Eu 5Ga 9 orders antiferromagnetically at 19.0(5) K with re-ordering at 6.0(5) K. The electrical resistivity shows a metallic temperature dependence and magnetic scattering. 151Eu Mössbauer spectroscopic experiments are compatible with divalent europium and show complex magnetic hyperfine field splitting below the ordering temperature.

Preparation, crystal structure and physical properties of ternary compounds ( R3N)In, R=rare-earth metal

The compounds ( R 3N)In ( R=Sc, La–Nd, Sm, Gd–Tm, Lu) were synthesized by arc melting and sintering reactions from the binaries RN and R 2In. Combined Rietveld refinements on X-ray and neutron powder diffraction data on the Ce-phase and X-ray diffraction for the other phases reveal the compounds ( R 3N)In to crystallize in the perovskite structure ((Ce 3N)In: Pm 3 ̄ m , a=504.89(2) pm; Rietveld: neutron powder diffraction: R F=0.056, R Bragg=0.104, X-ray powder diffraction: R F=0.069, R Bragg=0.115; refined composition Ce 3InN 0.92(1), chemical analysis Ce 3InN 0.91(2)O 0.05(1)). Compounds R 3In (AuCu 3 structure type) are known for R=La–Sm. Nitrogen is able to stabilize this AuCu 3-substructure with R=Gd–Tm, Lu and Sc. In measurements of the magnetic susceptibility (Ce, Pr, Nd, Sm, Tb: antiferromagnetic ordering at low temperature) and X-ray absorption spectroscopy at the R- L III-edges the rare-earth atoms behave like R 3+ species in insulating compounds. The electrical resistivity of the Sc, Ce and Nd compounds shows metallic temperature dependence. Chemical bonding was analyzed via LDA-COHP calculations for (La 3N)In showing ionic polyhedra NLa 6/2 embedded in a metallic matrix.

Anomalous metallic phase and magnetism in a high-mobility and strongly correlated 2D electron system

We report the transport properties of a low disorder two-dimensional electron system in the Si/SiGe heterostructure, where the strong e–e interaction causes the anomalous metallic temperature dependence of resistivity. The metallic behavior was observed even in a high magnetic field where spin degree of freedom is frozen.

Temperature dependence of superconductor-correlated metal–superconductor Josephson junctions

Josephson junctions, with the barrier composed of a correlated metal (or insulator) tuned to lie close to the metal–insulator transition, show promise to provide the fastest operating speeds for digital electronics based on rapid single-flux quantum logic. We provide theoretical calculations that indicate that these devices have a small enough temperature derivative of Ic(T) within the junction operating range to allow them to be employed as elements in complex digital circuits.

Shubnikov–de Haas oscillations in the two-dimensional organic conductorτ−(EDO−S,S−DMEDT−TTF)2(AuBr2)1+y(y∼0.75)

The quasi-two-dimensional organic τ-type conductor, τ-(EDO-S,S-DMEDT-TTF) 2 (AuBr 2 ) 1 + y (y ∼0.75), shows metallic temperature dependence of the resistivity down to T m i n ∼50 K, below which resistance upturn is observed. There has been no evidence of the phase transition through T m i n . In this work, in pursuit of direct evidence for metal at low temperature by transport measurement, we show an observation of the Shubnikov-de Haas (SdH) oscillations for this salt. The observed frequencies of the oscillations are inconsistent with the previous band calculation. Further, it is remarkable that the amplitude of the oscillation was so giant (‖Δρ/ρ 0 ‖∼30%), and the oscillation above 20 T corresponds to a N = 2 Landau state, i.e., next to a quantum limit. We also observed the SdH oscillations in other τ-type conductor, τ-(P-S,S-DMEDT-TTF) 2 (AuBr 2 ) 1 + y (y∼0.75), which almost reproduced the previous report by Storr et al. [Phys. Rev. B 64, 045107 (2001)].

Electrical and Magnetic Properties on Perovskite Compounds SrFe(Cr)0.5-xMo0.5+xO3and Sr2Fe(Cr)0.5-xMo0.5+xO4

We have studied the electrical and magnetic properties on perovskite compounds SrFe(Cr) 0.5- x Mo 0.5+ x O 3 and two-dimensional analogue Sr 2 Fe(Cr) 0.5- x Mo 0.5+ x O 4 . Magnetization measurements have indicated the possibility of ferromagnetism for Sr 2 Fe(Cr) 0.5- x Mo 0.5+ x O 4 as well as three-dimensional systems. Electrical resistivity ρ and ferromagnetic magnetization at 5 K decrease with increasing Mo content x for all titled compounds. Ferromagnetism seems to vanish at around x =0.4, 0.3 and 0.35 for SrFe 0.5- x Mo 0.5+ x O 3 , SrCr 0.5- x Mo 0.5+ x O 3 and Sr 2 Fe 0.5- x Mo 0.5+ x O 4 systems, respectively. The ρ's of SrFe(Cr) 0.5- x Mo 0.5+ x O 3 systems show metallic temperature dependence for x ≧0.375, while weak localized feature remains at low temperature for Sr 2 Fe(Cr) 0.5- x Mo 0.5+ x O 4 systems. No significant anomaly such as phase transition has been observed in ρ down to 0.5 K for SrFe 0.5- x Mo 0.5+ x O 3 with x =0.4, 0.425 and SrCr 0.5- x Mo 0.5+ x O 3 with x =0.35.

Magnetic, Electronic Transport, and Calorimetric Investigations of La1-xCaxCoO3in Comparison with La1-xSrxCoO3

The magnetic and electronic transport properties and the specific heat of La 1- x Ca x CoO 3 (0.05≤ x ≤0.5) have been investigated. In the whole composition range, the Co atoms are in an intermediate-spin (IS) state, and a spin glass or a ferromagnetic ordered state appears at low temperatures. The conductivity increases with x , and shows a metallic temperature dependence for x ≥0.4. The overall resemblance of the Ca substituted system with the Sr substituted one implies that the Ca substitution introduces holes into Co–O bonds as in the case of the Sr substitution. The present results reveal that the hole-doping, not the ionic size, is essential to stabilize the IS state of Co atoms in La 1- x M x CoO 3 with M being a divalent ion. The quantitative difference, lower T C and less conductivity for M = Ca than M = Sr with the same x , suggests less mobility and/or less number of holes in the system with M = Ca. The electronic specific heat coefficient, null for LaCoO 3 , increases sharply around x =0.2 for M = Sr and x =0.25 for M = Ca, and saturates at higher concentrations. The observation indicates the presence of the Fermi surface in the deeply substituted specimens. The magneto-transport properties of the metallic specimens, especially a large amplitude of the side-jump scattering in the extraordinary Hall resistivity, are characteristic of itinerant ferromagnetic metals, which supports the double exchange mechanism in the present system.

Evidence for a metallic but unusual ground state in τ-conductors

The band calculation for the τ-conductors predicted the existence of a Fermi surface, which has a two-dimensional star-shaped 4-fold symmetry. Actually, both τ-(EDO- S, S-DMEDT-TTF) 2(AuBr 2) 1+ y and τ-(P- S, S-DMEDT-TTF) 2(AuBr 2) 1+ y ( y∼0.75) show metallic temperature dependence of resistivity (d R/d T>0) below 300 K with a resistance upturn at 30–50 K. Below the temperature of resistance minimum, a large negative magnetoresistance (MR) suddenly appears in temperature, accompanied by a remarkable features such as the switching of the periodicity of the angular dependence of MR, large electronic specific heat coefficient γ (0.06 J/mol K 2). Lots of works have been devoted to clarify whether or not the low temperature electronic state is metal. Recently we could demonstrate a giant Shubnikov de Haas oscillation in both materials, which is a direct evidence for a metallic state. We note that the observed FSs were not consistent with the previous calculation. How this metallic state explains those remarkable features is still unsolved.