Structure and d.c. conductivity of amorphous Si1−xSnx alloys

Amorphous Si_1?xSn_x alloys have been prepared by vapor deposition at a pressure of about 10^?8 Torr on substrates maintained at 77 K. Density measurements and electron diffraction show that Sn atoms are substituted for Si in a random continuous network. The d.c. resistivity of samples of stabilized structure is correctly described by the variable range hopping formula. Structural changes are revealed by the variation of the resistivity at 77 K of samples annealed from 77 K to the crystallization temperature.     

Effect of spin fluctuations on the electrical resistivity in the pressure-induced metallic phase of 7 at.% Co-doped NiS2

The electrical resistivity measurements were made from 4.2 K to room temperature on 7 at.% Co-doped NiS₂ at pressures from 19 to 71 kbar. T²-dependence of electrical resistivity due to the spin fluctuations was found, and the pressure dependence of its coefficient was determined.     

Pressure enhancement of charge density wave formation in VSe2; The role of coulomb correlations

We have followed the pressure dependence of the onset and lock-in CDW transitions in VSe₂ by measurements of resistivity and Hall effect up to 30 kbar. (The onset transition appears as a break in the slope of the resistivity, and the lock-in as ≈ 1% temperature hysteresis). Both transition temperatures rise at 0.8 K kbar^?. We attribute this behaviour to pressure broadening of the d conduction band in the presence of strong electron Coulomb repulsion.     

Electrical measurements of the electron irradiation induced E- and H-traps in GaAs under hydrostatic pressure

The change of resistivity of the 2.3 MeV-electron-irradiated bulk n- and p-GaAs have been measured at hydrostatic pressure up to 5 kbar at RT. Corrections for the changes in free electron and hole mobilities with pressure have been neglected. The resistivity changes are explained by a dependence on pressure of the ionisation energy of the radiation-induced E- and H-traps. The results indicate that most from these radiation- induced levels moves away from the conduction-band edge (γ_c-point) at a rate approximately (0.8?1.0)γ_G, here γ_G=11.6×10^?6 eV bar^?1 is the energy gap pressure coefficient for GaAs at RT. The high changes in ionization energies of E2 to E5-traps upon pressure are to be compared with the lower changes in ionization energies found for the deep-lying impurity levels. In accordance with the theoretical investigation it was suggested that most of the investigated radiation-induced levels in GaAs are t₂-states of Ga- and As-vacancies.     

Anomalous electrical properties of linear chain mercury compounds

The electrical resistivity of Hg_2.86AsF₆ has been studied as a function of temperature. At room temperature, the resistivity along the chain direction is 10^?4 Ω-cm with an anisotropy of about 10². This incommensurate linear chain system remains metallic at low temperatures with resistance ratio ?_ab(300 K)/ ?_ab(1.4 K) ? 3000 and still increasing with no apparent sign of residual resistivity. A large anisotropic magnetic field dependence of the resistivity is observed below 30 K. Near 4 K, the c-axis resistance drops abruptly more than three orders of magnitude, apparently to zero, while ?_ab is continuous. The c-axis transition is suppressed in a small magnetic field.     

Staggered spin order of localized π-electrons in the insulating state of the organic conductor κ-(BETS)<Subscript>2</Subscript>Mn[N(CN)<Subscript>2</Subscript>]<Subscript>3</Subscript>

The magnetic properties of the conduction π-electron system of κ-(BETS)₂Mn[N(CN)₂]₃ have been probed using ¹³C NMR. At ambient pressure, the metal-insulator transition observed in the resistivity measurements below T ? 23 K is shown to be accompanied by ordering of the π-spins in a long-range staggered structure. As the metal-insulator transition is suppressed by applying a small pressure of ～0.5 kbar, the π-spin system maintains the properties of the metallic state down to 5 K.     

A new organic low temperature conductor: HMTSF-TNAP

Conductivity and optical data on a new organic, conducting charge transfer salt Δ^{2, 2′}-Bi-(4,5-trimethylene-1,3-diselenole) 11,11′,12,12′-tetracyano-2, 6-napthoquinodimethane (HMTSF-TNAP) are given. $\text{σ(300}\text{K}\text{)= 2400 ± 600 Ω}{}^{-1}\text{cm}{}^{-1}$. A maximum in σ(T) is found at T_M = 47 K with σ(T_M)/σ(300 K) = 6.0 ± 10%, and $\text{σ(1.5}\text{K}\text{) > 250 Ω}{}^{-1}$. σ(T) is well defined in the high temperature region, but is sample dependent at low temperatures. The optical data indicate a bandwidth and carrier density comparable to that of HMTSF-TCNQ.     

Phase transitions in N—n-propylpyridinium-TCNQ2 and N—n-butylpyridinium-TCNQn at high pressures

The electrical resistivity of N-n-propylpyridinium-TCNQ₂ (NPPy-TCNQ₂) and N-n-butylpyridinium-TCNQ_n (NBPy-TCNQ_n) has been measured as a function of temperature and pressure. Phase transitions in these salts have been studied at high pressures. The transition temperature (T_c) in NPPy-TCNQ₂ at atmospheric pressure increased with increasing pressure at the rate of dT_c/dP = + 12.0 degkbar^?1. The value of volume change calculated from the Clapeylon-Clausius relation was + 4.4 cm³ mol^?1. The electrical resistivity along the a- and c-axis increased with increasing pressure below 7 kbar. This anomalous electrical behaviour is closely related to the crystal structure of NPPy-TCNQ₂. The resistivity dropped sharply at about 11 kbar. This abrupt change may be due to a new pressure induced phase transition.The T_c of the NBPy-TCNQ_n increased remarkably with increasing pressure up to 0.7 kbar, above which the phase transition disappeared. The phase transitions of N-n-alkyl-substituted pyridinium TCNQ salts depend strongly on the nature of cations.     

Magnetic and related properties of GdCu5.1In6.9

Magnetic measurements performed between 1.7 and 300K show that GdCu_5.1In_6.9 is an antiferromagnet below T _N = 13.4(1) K. At temperatures 17–300K the Curie-Weiss law is followed with p _eff = 7.57 μ _B and θ = -33.8K. Electrical resistivity measurements do not show any pronounced anomaly, but the resistivity is rather low with RRR ≈ 59. The EPR experiment shows that the resonance exists between 10 and 300 K. A temperature independent g-shift (Δg = -0.036(10)) and a Korringa.type linewidth broadening of 2.6 G/K was observed at temperatures higher than 55 K. The linewith increase found at high temperature well above T _N can suggest a significant amount of short. range order. ¹⁵⁵Gd Mössbauer effect examinations show that H _hf is perpendicular to the c-axis and therefore magnetic moments of Gd are located within the ab-plane.     

The D.C. voltage-dependent resistivity of CuCl powder compacts under pressure

The CuCl powder compacts are found to exhibit d.c. voltage dependent resistivity. The resistivity drops by six orders of magnitudes when the applied voltage exceeds a critical value V_c.V_c is found to decrease with the increase in pressure. The high conducting phase shows a positive temperature coefficient of resistance at 1 atm of 0.004°C^-1 which increases with pressure at a rate 0.0004°C^?1 GPa^?1. It is suggested that the pressure induced transition reported at 4 GPa could arise from the application of d.c. voltage.     

Heavy fermion state and pressure-induced resistive behaviour in CeRu2Si2

Pressure-induced variation in the resistive behaviour of CeRu₂Si₂, a Kondo-lattice system, has been studied in the temperature interval 1.5K – 300K. At low temperatures, resistivity exhibits (i) a linear temperature dependence at low applied pressures (P ?2.26 kbar) and (ii) a quadratic temperature dependence

(T) ～ AT² at higher pressures (6.5 kbar ? P ? 16.8 kbar). The coefficient A decrease, which in turn implies that the spin fluctuation temperature T_sf increases, with increasing applied pressure. The temperature coefficient γ of specific heat C = γ T at low temperature (0.3K ? T ? 1.6K) is quite large (385 mJ/mole-K²) confirming the heavy fermi liquid nature of the material.     

Electronic structure and lattice dynamics of domeykite Cu<Subscript>3</Subscript>As according to nuclear quadrupolar resonance of<Superscript>75</Superscript>As and<Superscript>63,65</Superscript>Cu

The temperature dependences of nuclear quadrupole resonance (NQR) frequencies, the line width and nuclear relaxation of⁷⁵As and^63,65Cu, as well as the electrical resistivity in domeykite Cu₃As are studied in the temperature range of 4.2-300 K. The comparison of the calculated with the measured lattice contribution to the NQR frequencies points at a substantial role played by the conduction electrons in creating the electric field gradient at the nuclei sites. The temperature dependence of the copper and arsenic nuclear spin-lattice relaxation linear at 4.2<T<200 K and that of the electric resistivity (30<T<200 K) prove the metallic character of the conductivity of domeykite. The enhancement of nuclear relaxation, the narrowing of copper and arsenic NQR line widths are considered as arising due to the ionic movement starting beyond 200 K. This movement influences the electric resistivity, most likely due to the inreasing density of states at the Fermi surface.     

Observation of new Raman lines in GeSe and SnSe at low temperatures

Polarized Raman spectra are investigated in GeSe and SnSe at low temperatures. New Raman lines which can not be expected by a group theoretical analysis for the known crystal structure of the orthorhombic D¹⁶_2h have been observed typically in the (a, a) and (b, b) polarization configurations. With decreasing temperature, three lines in GeSe grow weakly at 89,201 and strongly at 226 cm^-1 below 150 K, accompanied by enhancement of layer breathing mode (175 cm^-1 at 273 K) intensity. One line in SnSe grows weakly at 193 cm^-1 below 50 K. The appearance of the new Raman lines as well as a resistivity anomaly suggests a novel structural phase transition in this system.     

Low temperature magnetoresistance in La1.32Sr1.68Mn2O7 layered manganite under hydrostatic pressure

The La_1.32Sr_1.68Mn₂O₇ layered manganite system has been studied by the low temperature electrical resistance and magnetoresistance under hydrostatic pressure up to 25 kbar. We have observe both, a Curie temperature (T_C) and a metal-insulator transition (T_MI) at 118 K in the ambient pressure. The applied pressure shifts the T_MI to higher temperature values and induces a second metal-insulator transition (T²_MI) at 90 K, in the temperature dependence of resistivity measurements. Also, the pressure suppresses the peak resistance abruptly at T_C. When an external field of 5 T is applied, we have observed a large negative magnetoresistance of 300% at the transition temperature and a 128% at 4.5 K. However, the increased pressure decreases the magnetoresistance ratio gradually. When the pressure reaches its maximum available value of 25 kbar, the magnetoresistance ratio decreases at a rate of 1.3%/kbar. From our experimental results, the decrease of magnetoresistance ratio with pressure is explained by the pressure induced canted spin state which is not favor for the spin polarized intergrain tunneling in layered manganites.     

Effects of uniaxial pressure and annealing on the resistivity of Ba(Fe1−xCox)2As2

Single crystals of underdoped Ba(Fe_1−xCo_x)₂As₂ were detwinned by applying uniaxial pressure. The anisotropic in-plane resistivity was measured using the Montgomery method without releasing pressure. The resistivity along the a-axis shows metallic behavior down to 5 K, while the resistivity along the b-axis shows an insulator-like behavior in some temperature range. Annealing the sample radically reduces the residual resistivity for x=0, and at the same time the anisotropy becomes much smaller at low temperatures.     

The decomposition pressure,congruent melting point and electrical resistivity of cerium nitride

The melting point of CeN in nitrogen was determined at pressures between 10⁻² and 21 atmospheres. Congruent melting was observed at T = 2575 ± 20°C and P = 5 ± 1 atmospheres. With lower nitrogen pressures, the melting point was related to the applied (or decomposition) pressure by the equation log P_atmos(N₂) = 18·5 − 5·1 × 10⁴/T. The electrical resistivities of samples prepared from these melts were measured at temperatures between 150–900°K. The room temperature resistivity of congruently melted samples was 21 μΩ-cm, which may be compared with the value of 75 μΩ-cm for cerium metal at this temperature. These samples had a positive (metallic) temperature dependence of resistivity of 0·11 μΩ-cm/°C at room temperature and increased with temperature to a value of 0·22 μΩ-cm/°C at a temperature of 700°K. The noncongruently melted samples had a similar behavior, but with somewhat higher values of resistivity. The room temperature Seebeck coefficients for both types of samples were approximately + 5 μV/°C, relative to platinum.     

Curie temperature,crystallization temperature and electrical resistivity as a function of composition for Fe80-xMoxB20 metallic glasses

The influence of the substitution of small amounts of Fe with Mo (0–10 at.%) in a Fe₈₀B₂₀ metallic glass on the Curie temperature, crystallization temperature and room temperature electrical resistivity is reported. A decrease in Curie temperature of approximately 40 K/at.% Mo is observed. The crystallization temperatures show a small increase with increasing Mo-content, and the room temperature electrical resistivity of the as quenched samples is essentially independent of the Mo-content (?^am(295 K) ～ 128 μχ-cm).     

Large unsaturated positive and negative magnetoresistance in Weyl semimetal TaP

After successfully growing single-crystal Ta P, we measured its longitudinal resistivity(ρxx) and Hall resistivity(ρyx) at magnetic fields up to 9 T in the temperature range of 2-300 K. At 8 T, the magnetoresistance(MR) reached 3.28 × 105% at 2 K, 176%at 300 K. Neither value appeared saturated. We confirmed that Ta P is a hole-electron compensated semimetal with a low carrier concentration and high hole mobility of μh=3.71 × 10~5cm~2/V s, and found that a magnetic-field-induced metal-insulator transition occurs at room temperature. Remarkably, because a magnetic field(H) was applied in parallel to the electric field(E), a negative MR due to a chiral anomaly was observed and reached-3000% at 9 T without any sign of saturation, either, which is in contrast to other Weyl semimetals(WSMs). The analysis of the Shubnikov-de Haas(Sd H) oscillations superimposed on the MR revealed that a nontrivial Berry's phase with a strong offset of 0.3958, which is the characteristic feature of charge carriers enclosing a Weyl node. These results indicate that Ta P is a promising candidate not only for revealing fundamental physics of the WSM state but also for some novel applications.     

Effect of hydrostatic pressure on the Neel temperature and the electrical residual resistivity of α-manganese

The electrical resistivity of α-Mn was measured from 4.2 K to 290 K at pressures up to 8 kbar. The residual resistivity was found not to depend on pressure. The Néel temperature decreases linearly with pressure at a slope dT_N/dP = − 1.7 ± 0.2 K/kbar.     

Electrical properties and spin fluctuations studies of Y(Co1−xNix)2 compounds

Synthesis by arc melting, the structural and the electric properties of Y(Co_1−xNi_x)₂ alloys were studied by X-ray diffraction (XRD) and four probe dc electrical measurements. XRD analysis (300 K) shows that all samples crystallize in a cubic MgCu₂-type structure. The lattice parameters linearly decrease with Ni content. Electrical resistivity for the Y(Co_1−xNi_x)₂ intermetallic series was measured in a temperature range of 15-1100 K. The parameters involved in the dependence of resistivity on temperature were determined. Residual, phonon and spin fluctuations resistivity were separated from electrical resistivity using both the Matthiesen formula and the Bloch-Gruneisen formula. The spin fluctuations resistivity of the Y(Co_1−xNi_x)₂ series are compared to the mean square amplitudes of spin fluctuations previously calculated by the Linear Muffin Tin Orbital-Tight Binding Approach method for these series in the literature. The contribution of spin fluctuations to total resistivity ρ_sf is proportional to T² at low temperatures. The proportionality parameter strongly reduces across the Y(Co_1−xNi_x)₂ series.