Metallic Temperature Dependence Of Resistivity Research Articles

Anisotropy and size effects in Bi1-xSbx semiconductor wires in a magnetic field

The electron transport and longitudinal and transverse magnetoresistance (MR) of glass-insulated Bi0.92Sb0.08 single-crystal wires with diameters of 180 nm to 2.2 μm and the (1011) orientation along the wire axis have been studied. The wires have been prepared by liquid-phase casting. It has been first found that the energy gap ΔE increases by a factor of 4 with a decrease in wire diameter d due to the manifestation of the quantum size effect, which can occur under conditions of a linear energy–momentum dispersion law characteristic of both the gapless state and the surface states in topological insulators (TIs). It has been revealed that, in strong magnetic fields at low temperatures, a semiconductor–semimetal transition occurs, which is evident as an anomalous decrease in the transverse MR anisotropy and the appearance of a metallic temperature dependence of resistance at T < 100 K. It has been found that the effect of negative MR, the appearance of an anomalous maximum in the longitudinal MR, and the dependence of Hmax ~ d-1 at 4.2 K is a manifestation of the classical MacDonald–Chambers size effect. The calculated value of the component of the Fermi momentum perpendicular to the magnetic induction vector H is 2 times higher than the value for pure bismuth wires. The features of the manifestation of the quantum size effect in Bi0.92Sb0.08 wires, semiconductor–semimetal electronic transitions induced by a magnetic field, and a decrease in the transverse MR anisotropy indicate the occurrence of new effects in low-dimensional structures based on semiconductor wire TIs, which require new scientific approaches and applications.

Peierls-type metal-insulator transition in carbon nanostructures

We report the observation of Peierls-type metal-insulator transition in carbon nanostructures formed by chemical vapor deposition inside the pore network of the ZSM-5 zeolite. The Raman spectrum of this nanocarbon@ZSM-5 indicates a clear signature of the radial breathing mode (RBM) for (3,0) carbon nanotubes that can constitute the carbon network segments. Electrical transport measurements on multiple few-micron-sized nanocarbon@ZSM-5 crystals showed metallic temperature dependence of resistance down to 30 K, at which point the resistance exhibited a sharp upturn that is accompanied by the opening of a quasigap at the Fermi level as indicated by the differential resistance measurements. Further Hall measurements have yielded both the sign of the charge carrier and its density. The latter demonstrated excellent consistency with the quasigap data. We employed first-principles calculations to verify that there can indeed be softening of the phonon modes in the (3,0) carbon nanotubes.

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.

Pressure-induced metallization in Erbium trihydride

Electrical resistivity and Raman spectra of ErH3 were studied in a diamond anvil cell under high pressure up to 140GPa in the temperature range 4–300K. A crossover from a semiconductor-like to a metallic temperature dependence of resistivity at fixed pressures was observed at about 50GPa. In the pressure range 80–140GPa a resistivity maximum was observed at the R(T) dependencies. The temperature corresponding to this maximum linearly increased with pressure increase, reaching 26K at 140GPa. No superconductivity was observed in the studied pressure-temperature range.

Metallic Transport in a Monatomic Layer of In on a Silicon Surface

We have succeeded in detecting metallic transport in a monatomic layer of In on an Si(111) surface, Si(111)-sqrt[7]×sqrt[3]-In surface reconstruction, using the micro-four-point probe method. The In layer exhibited conductivity higher than the minimum metallic conductivity (the Ioffe-Regel criterion) and kept the metallic temperature dependence of resistivity down to 10K. This is the first example of a monatomic layer, with the exception of graphene, showing metallic transport without carrier localization at cryogenic temperatures. By introducing defects on this surface, a metal-insulator transition occurred due to Anderson localization, showing hopping conduction.

Temperature dependent conductivity and thermoelectric power of the iodine doped poly(vinyl alcohol)–Cu 2+ chelate

We have investigated the temperature dependence of electrical conductivity and thermoelectric power (TEP) at 1.7 K < T < 300 K in an organo metallic complex, the iodine doped poly(vinyl alcohol)–Cu 2+ chelate. We observed intrinsic metallic temperature dependence of resistivity from room temperature to 68 K with a broad minimum [ ρ(68 K)/ ρ(300 K) ∼0.75], which has not been observed previously in similar organo metallic complexes. There occurs an unusual metal-insulator transition at T ∼68 K and the resisitivity increases upon cooling below 68 K. However, the low temperature resistivity becomes finite (instead of going to infinity), [ρ(1.7 K)/ ρ(300 K) ∼0.98] indicating that a quantum mechanical tunneling conduction is dominant at this low temperature. It is remarkable that the resistivity at 1.7 K is as small as that of room temperature. Such unusual temperature dependence of conductivity could be understood as thermally assisted hopping conduction between metallic islands. However, the observed intrinsic metallic temperature dependence of resistivity implies that such hopping conduction barrier is not important at high temperature ( T > 68 K). The intrinsic metallic characteristics are confirmed by the quasi-linear temperature dependence of TEP for the whole measured temperature range (1.7 K < T < 300 K) with a small slope change at low temperature, T < 68 K, which is understood as an effect of variable range hopping (VRH) conduction at low temperature. The results of magneto resistance (MR) and magneto thermoelectric power (MTEP) are consistent with the above interpretation.

Observation of Metallic Resistance Down to 56 K in Langmuir–Blodgett Films Based on Ditetradecyldimethylammonium–Au(dmit)2 Salt

A metallic temperature dependence of the resistance of ditetradecyldimethylammonium–Au(dmit)2 [2C14–Au(dmit)2] Langmuir–Blodgett (LB) film is observed in a wide temperature range of 56–293 K. While a weak metallic temperature dependence of resistance has been observed down to 216 K in a case where the electrode gap for the measurement is 0.5 mm (500 µm), an unambiguous metallic behavior is observed down to 56 K when utilizing a narrower interdigitated gap of 5 µm for the measurement.

The relationship between quantum transport and microstructure evolution incarbon-sheathed Pt granular metal nanowires

The carbon-sheathed Pt nanowires fabricated by focused ion beam induced deposition werePt grains in a Ga-doped carbon matrix. The dimensions of the Pt quantum dots and theintergrain coupling strength were modified through annealing the nanowires, and thereforeour investigation demonstrates a tunable ‘quantum metal’ system by experiment viaadjusting both the effects of electron–electron (e–e) interaction and the quantuminterference. At low temperatures, the as-deposited samples display a temperature dependence of resistivity, while the500 °C annealedsamples exhibit a ln(T) temperature dependence of conductivity. For the900 °C annealed samples, the conductivity was enhanced by one order of magnitude, accompaniedby a metallic temperature dependence of resistivity and a negative magnetoresistance. Theinteresting transitions from e–e interactions to local voltage fluctuations, from electronlocalization to delocalization, from tunneling transport between isolated Pt grains todiffusive transport in continuously conductive metal, and from weak antilocalization withspin–orbital scattering to weak localization, are discussed with reference to the evolution ofthe sample microstructures. The results and discussions may be valuable for understandinghow the microstructures influence the manner of electron transport through controlling theintergrain coupling strength, intragrain confinements, and the degree of disorder.

Pseudogap and metal-insulator transitions in doped semiconductors

The electronic spectrum of a doped semiconductor described by the Anderson-Holstein impurity model and its conductivity derived from the Kubo linear response theory are calculated. Two characteristic temperatures depending on the doping level x are found in the phase diagram, T{sub PG} and T{sub {lambda}}(x). The pseudogap that opens in the single-particle spectrum at low doping levels and temperatures closes at the lower one, T{sub PG}. The pseudogap state of an insulator is attributed to spin fluctuations in a doped compound. At the higher characteristic temperature T{sub {lambda}}(x),, spin fluctuations vanish and the doped compound becomes a paramagnetic poor metal. Two distinct metal-insulator crossovers between semiconductor-like and metallic temperature dependence of resistivity are found. An insulator-to-poor-metal transition occurs at T{sup *}(x) {approx} T{sub {lambda}}(x). A poor-metal-to-insulator transition at a lower temperature is attributed to the temperature dependence of density of states in the pseudogap. It is shown that both transitions are observed in La{sub 2-x}Sr{sub x}CUO{sub 4}.

Collapse of Coherent Quasiparticle States inθ−(BEDT−TTF)2I3Observed by Optical Spectroscopy

Optical conductivity measurements on the organic metal theta-(BEDT-TTF)2I3 revealed that the system crosses over rapidly from a coherent quasiparticle state to an incoherent state with temperature. Despite the metallic temperature dependence of resistivity, a well-defined Drude peak at low temperatures turns into a far-infrared peak with temperature. The peak energy shifts to higher frequencies and, simultaneously, the spectral weight is transferred to high frequencies beyond the electron band width. These characteristics imply that theta-(BEDT-TTF)2I3, so far believed to be a typical metal, is in fact a strongly correlated electron system with "bad-metal" character.

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.

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.

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.

Giant Shubnikov-de Haas oscillation and the new metallic state in the organic τ-type conductors

Two-dimensional organic τ-type conductors show metallic temperature dependence of resistivity down to T min =30–50 K, below which a resistivity upturn is observed. A large negative magnetoresistance appears below T min suddenly in temperature. Below about 4 K, magnetoresistance in any direction is extremely hysteretic in field-, temperature- or field-angle-sweep, and ρ ⊥( H//plane) shows switching between two- and four-fold depending on magnetic and thermal history, indicating magnetic domain generation. Static susceptibility shows slight ferromagnetic component. NMR is in favor of anti-ferromagnetism below about 15 K. These two results imply that the system is a weak ferromagnetic one. Preliminary optical reflectance and specific heat studies implied metallic state at low temperature, but no clear-cut proof has been shown whether or not the ground state is metallic. This report presents giant Shubnikov de Haas (SdH) oscillation in 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), which is a clear evidence for the metallic ground state. However, the observed Fermi surfaces (FS) are 0.65 and 6.1% of the first Brillouin zone for τ-(EDO- S, S-DMEDT-TTF) 2(AuBr 2) 1+ y , and 2.3 and 6.9% for τ-(P- S, S-DMEDT-TTF) 2(AuBr 2) 1+ y , ( y∼0.75), which suggest extremely small FS pockets, and were not expected by the previous band calculation. Further, it is surprising that the Dingle temperature was around 1 K, indicating unexpectedly clean, while residual resistivity ratio is about 1. We note that the fundamental oscillation of 0.65% is already n=2 state, which is already next to the Landau quantum limit.

Metal–insulator transition and spin degree of freedom in Silicon 2D electron systems

Magnetotransport in 2DESs formed in Si-MOSFETs and Si/SiGe quantum wells at low temperatures is reported. Metallic temperature dependence of resistivity is observed for the n-Si/SiGe sample even in a parallel magnetic field of 9 T, where the spins of electrons are expected to be polarized completely. Correlation between the spin polarization and minima in the diagonal resistivity observed by rotating the samples for various total strength of the magnetic field is also investigated.

Structure and properties of TTP conductors substituted with ethylenedioxy group

Radical cation salts of EOPT-TTP, where EOPT-TTP is 2-(4,5-ethylenedioxy-1,3-dithiol-2-ylidene)-5-(4,5-propylenedithio-l,3 dithiol-2-ylidene) -1,3,4,6-tetrathiapentalene, have been prepared. All the salts obtained so far exhibited metallic temperature dependence of resistivity down to low temperature regardless of shape and size of counter anions.

Magnetoresistance of the metallic polyacetylene

The temperature dependence of the zero field resistivity, ρ( T), and the magnetoresistance (MR) of polyacetylene (PA) doped with iodine, metal-halide (AuCl 3, FeCl 3) and perchlorate are measured. The results of doped PA are different for each dopant and for the degree of aging. The ρ( T) of the metal-halide and the perchlorate doped PAs show a resistivity minimum near T*≈200 K and the weak temperature dependence at low temperature. In particular, some of the perchlorate doped PA samples show the positive temperature coefficient of resistivity (TCR) from T=1.5 K to T=300 K with resistivity ratio ρ( T=1.5 K)/ ρ( T=300 K)=0.5∼0.7 depending on the samples. The MR is negative for all of the metallic PA samples. Both transverse ( J⊥ H) and longitudinal ( J‖ H) MR show similar magnetic field dependencies, although the magnitude of the transverse MR is larger than that of the longitudinal MR. For the aged samples, the ρ( T) increase more rapidly upon cooling than that of the fresh samples. The MR of the aged samples is positive. The localization–interaction theory is examined for the observed ρ( T) and MR data of doped PA. But the model of interchain charge transfer bridged by the dopants seems to explain the negative MR as well as the metallic temperature dependence of resistivity for the perchlorate doped polyacetylene.

Metallic temperature dependence of resistivity in perchlorate doped polyacetylene

We have measured the electrical resistivity (ρ) and thermoelectric power (TEP) of the perchlorate (ClO 4 −) doped stretch oriented polyacetylene (PA) film. For the highly conducting samples ( σ RT≈41 000 S/cm), the temperature dependence of the four-probe resistivity shows positive temperature coefficient of resistivity (TCR) from T = 1.5 to 300 K. For the less conducting samples, the four-probe resistivity data show the crossover of TCR with a broad minimum peak at T = T ∗ ≈ 200 K . For samples of σ RT ≥ 20 000 S/cm, ρ(1.5 K) ρ(300 K) < 1 , i.e., the resistivity at 1.5 K is lower than the room temperature resistivity value. The temperature dependence of the TEP shows diffusive linear metallic TEP becoming temperature independent below 40 K. Unlike the others who used Cu(ClO 4) 2 for the ClO 4 − doping, the initial doping material we used is anhydrous Fe(ClO 4) 3 which is crucial to obtain positive TCR from T = 1.5 to 300 K.

Metallic temperature dependence of resistivity in polypyrrole and poly(3-methylthiophene) at low temperatures with and without pressure

The temperature dependence of resistivity (TDR) has been studied in PF 6− doped poly(3-methylthiophene) (PMeT(PF 6−) and AsF 6− doped polypyrrole (PPy(AsF 6−)). For several samples of PMeT(PF 6−) metallic TDR was observed below 8K similar to that in PPy(PF 6−). For PPyCAsF 6−), application of a pressure of 13.5 Kbar induced the metallic TDR below 9K, despite of the semiconducting TDR at ambient pressure throughout the whole temperatures. We have also observed a clear transition from semiconducting to metallic TDR in PMeT(PF 6−) with increasing pressure. From our observations, it is indicated that the metallic TDR is not an exceptional phenomenon only in the PPy(PF 6−).

Metallic temperature dependence of resistivity in heavily doped polyacetylene by NMR

1H-NMR spin-lattice relaxation rate T 1 −1 of heavily doped polyacetylene was measured in an attempt to deduce the intrinsic resistivity of its metallic phase. Against our intention, in all of the samples a metallic behavior in the T 1 −1 expected for a quasi-one-dimensional metal was not found for some period after doping, and it was revealed that those T 1 −1's were governed by a relaxation mechanism due to residual Curie spins. In the bromine-doped sample, however, a change in the temperature dependence of T 1 −1 from the non-metallic behavior to the metallic one was found as time passed. It was confirmed that this change was caused by the partial substitution of hydrogen atoms by bromine atoms in the polyacetylene chain. We succeeded in deducing the temperature dependence of intrinsic resistivity for metallic polyacetylene from the data of T 1 −1.