Electrical transport measurements in a quasi-onedimensional semiconductor ZrS3

We have used the technique of chemical vapour transport to prepare needle shaped single crystal of ZrS₃. Results of the measurements of d.c. resistivity. Hall coefficient and thermoelectric power of the temperature range 100–500 K are reported. All the samples exhibited semiconducting behaviour with a room temperature resistivity of about 15 Ω-cm and an activation energy of 0.20±0.02 eV. Room temperature thermoelectric power is -850 μVK^?1 and the dominant carriers are electrons. The thermoelectric power varies as (1/T), a behaviour associated with a typical semiconductor. Mobility at low temperatures is limited by ionized impurity scattering and is given by μ₁ = 6.5 × 10^?2T^3/2 cm²V^7-1 sec^?1. At high temperatures, phonon scattering is dominant and the mobility is given by μ₂ = 1.35 × 10⁺⁵T^?32 cm²V^?1 sec^?1.     

Galvanomagnetic properties of La0.7Sr0.3Mn0.9Cu0.1O3

La_0.7Sr_0.3Mn_0.9Cu_0.1O₃ ceramic samples have been obtained by the conventional method of the solid-phase reaction, and their resistivity ρ has been investigated in a temperature range from 50 to 300 K in magnetic fields B = 0–20 T. Dependences are typical of perovskite manganites with a maximum at T _max = 140–150 K and an increase in ρ near T _max with increasing external magnetic field B. It has been established that the behavior of resistivity is caused by the variable range hopping conduction mechanism ρ(T) = ρ₀(T)exp[(T ₀/T)^1/4], where ρ₀(T) ～ T ^25/4. The Mott variable range hopping conduction has been observed below the Curie temperature for La_0.7Sr_0.3Mn_0.9Cu_0.1O₃ samples (T _C ～ 300 K) in a temperature range from 300 to 200 K. The influence of Cu doping on the properties of La_0.7Sr_0.3MnO₃ samples is apparently caused by an additional distortion introduced into the crystal lattice of the material and by a weakening of the double-exchange mechanism.     

Magnetotransport of the low-carrier density one-dimensional S=1/2 antiferromagnet Yb4As3

The transport properties of the semimetallic quasi-one-dimensional S=1/2 antiferromagnet Yb₄As₃ have been studied by performing low-temperature (T≥0.02 K) and high magneticfield (B≤60 T) measurements of the electrical resistivity ρ(T, B). For T ≿ 2 K a ‘heavy-fermion’-like behavior Δρ(T)=AT ² with huge and nearly field-independent coefficient A ≈ 3 μΩ cm/K² is observed, whereas at lower temperatures ρ(T) deviates from this behavior and slightly increases to the lowest T. In B>0 and T ≾ 6 K the resistivity shows an anomalous magnetic-history dependence together with an unusual relaxation behavior. In the isothermal resistivity Shubnikov-de Haas (SdH) oscillations, arising from a low-density system of mobile As-4p holes, with a frequency of 25 T have been recorded. From the T- and B-dependence of the SdH oscillations an effective carrier mass of (0.275±0.005)m ₀ and a charge-carrier mean-free path of 215 ? are determined. Furthermore, in B≥15 T, the system is near the quantum limit and spin-splitting effects are observed.     

Electrical conduction and 1ƒ noise in Li-DOPED MnO

Electrical resistivity, thermoelectric power and current noise were measured on Li-doped MnO single crystals in the temperature range from 300 to 1000 K. Below 700 K the crystals are p-type and the activation energy of the resistivity is 0.75 eV. Around 700 K the activation energy changes from 0.75 to 1.25 eV owing to a change from p- to n-type conduction. The depth of the Li acceptor is found to be 0.65 eV. From resistivity and thermoelectric power data it is concluded that the bandgap in first approximation can be written as E_s(T) = E_o ? γT between 750 and 1000 K, with E_o = 1.9 eV and γ = 6 × 10^?4 eV/K. The current noise spectra show $\text{1}\text{?}$ noise. The magnitude of the $\text{1}\text{?}$ noise is strongly temperature dependent. From the noise data it is deduced that E_o = 2.2 eV and γ = 10^?3 eV/K in the temperature range 430–700 K.     

The electronic properties of chromium borides

Measurements of magnetic susceptibility between 300 and 100°K and of resistivity between 300 and 4.2°K are reported for the five intermediate compounds Cr₂B, Cr₃B₃, CrB, Cr₃B₄ and CrB₂. With the exception of CrB₂ (antiferromagnetic T_N = 88°K) all the compounds were found to be weakly temperature dependent paramagnets. Large, orbital contributions to the susceptibility are proposed for Cr₂B and Cr₅B₃. It is suggested that with the formation of discrete boron networks and the associated change in band structure in the higher borides this contribution diminishes very rapidly. At low temperatures Cr₂B₁ CrB and CrB₂ were all found to have a prominent T² behaviour in ρ(T). At high temperatures the resistivities of CrB and CrB₂ were found to vary linearly with temperature, the resistivity of Cr₂B on the other hand seemed to follow a T(1 ? BT²) law.     

非晶态(Fe1-xWx)84.5B15.5合金的低温电阻率反常

研究了(Fe_1-xW_x)_84.5B_15.5(x=0—0.1)非晶态合金的电阻率ρ与温度T(4.2—300K)的关系。实验结果表明,在所研究浓度区域内均出现电阻率与温度关系的极小值,电阻率极小值的温度T_min在x=0.06时出现峰值。用x=0.02—0.1浓度区域内,当Tmin时,又出现了电阻率与温度关系的极大值,电阻率极大值的温度T_max在26—35K之间。低温电阻率反常现象与类Kondo效应及局域磁矩之间RKKY相互作用有关。 关键词：     

TEP studies on Fe80B20 under pressure

The amorphous to crystalline transformation in the ferromagnetic metallic glass Fe₈₀B₂₀ has been studied up to 30 kbar pressure and 1000K. A previous study at ambient pressure revealed no change in thermoelectric power (TEP) at the crystallization temperature (T_x) while the resistivity showed a sudden decrease at the same temperature. The present experimental results show a distinct anomaly in TEP at T_x even at ambient pressure. This anomaly gets enhanced under pressure.     

Thermoelectric power of potassium doped lanthanum manganites at low temperatures

The temperature-dependent resistivity and thermoelectric power of monovalent (K) doped La_1−xK_xMnO₃ polycrystalline pellets (x=0.05, 0.10 and 0.15) between 50 and 300 K are reported. K substitution enhances the conductivity of this system. Curie temperature (T_C) also increases from 260 to 309 K with increasing K content. In the paramagnetic region (T>T_C), the electrical resistivity is well represented by adiabatic polaron hopping, while in the ferromagnetic region (T<T_C), the resistivity data show a nearly perfect fit for all the samples to an expression containing, the residual resistivity, spin-wave and two-magnon scattering and the term associated with small-polaron metallic conduction, which involves a relaxation time due to a soft optical phonon mode. Small polaron hopping mechanism is found to fit well to the thermoelectric power (S) data for T>T_C whereas at low temperatures (T<T_C) in ferromagnetic region (S_FM), S_FM is well explained with the spin-wave fluctuation and electron–magnon scattering. Both, resistivity and thermopower data over the entire temperature range (50–300 K) are also examined in light of a two-phase model based on an effective medium approximation.     

Thermoelectric properties of sol-gel derived cobaltite Bi2Ca2.4Co2Oy

Layered misfit cobaltite Bi₂Ca_2.4Co₂O_y has been synthesized by a sol-gel method. This compound exhibits large thermoelectric (TE) power (S_300 K∼170 μV K⁻¹), low resistivity (ρ_300 K∼42 mΩ cm) and relatively small thermal conductivity (κ_300 K∼2.8 W K⁻¹ m⁻¹) at room temperature. Furthermore, the resistivity of this compound displays a metallic behavior above T^?∼150 K with a semiconducting behavior below this temperature. This abnormal behavior in resistivity is analogous to those observed in Sr and Ba based misfit cobaltites. The observed features of the TE have been discussed based on the narrow band model.     

Spin fluctuations in hydrogen-stabilized Laves phase UTi2H5

Uranium Laves phase UTi₂ does not exist in a pure form, but can be stabilised by the presence of hydrogen, which can be absorbed in concentration exceeding 5?H atoms/f.u. Low temperature specific heat, magnetic susceptibility, and resistivity indicate that UTi₂H₅ is a spin fluctuator close to the verge of magnetic ordering. Its susceptibility follows at high temperatures the Curie–Weiss law with U effective moment µ_eff[ ?= 3.1?µ_B/U and paramagnetic Curie temperature Θ_p = ?200 K. The temperature dependence of specific heat exhibits a pronounced and weakly field dependent upturn in C_p/T versus T below 10 K reflecting the effect of spin fluctuations. It can be described by an additional T^½ term. The Sommerfeld coefficient γ = 256?mJ/mol K² classifies the compound as a mid-weight heavy fermion. Spin fluctuations are affecting also electrical and thermal transport and thermoelectric power, which all resemble UAl₂. A lattice anomaly at ≈ 240?K, attributed to the melting of hydrogen sublattice, reflects in most of bulk properties.     

Synthesis and excess conductivity analysis of TlBa2(Ca3−y Be y )Cu4O12−δ superconductors

TlBa₂(Ca_3?y Be _y )Cu₄O_12?δ (y = 0, 0.5, 1.0, 1.5, 2) samples are synthesized at normal pressure and the influence of doped Be-atoms on the superconductivity parameters at the microscopic level is investigated by carrying out excess conductivity analyses of conductivity data. The samples have shown tetragonal structure and the unit cell volume decreases with increased Be-doping. The onset temperature of superconductivity [T _c(onset)] and zero resistivity critical temperature [T _c(R = 0)] decrease with increased Be, however, the magnitude of diamagnetism is enhanced with Be (except for Be-doping of y = 1.0). The apical oxygen mode of the type Tl–O_A–Cu(2) and CuO₂ planar oxygen mode are softened as observed in FTIR absorption measurements. The FIC analyses of conductivity data have shown an increase in the coherence length along the c-axis and inter-plane coupling. The values of B _c0(T), B _c1(T), J _c(0), τ _φ are improved with the doping of Be. These observations suggested that due to the proximity effect there is less suppression in the value of the order parameter of the Cooper pairs from |ψ|² = 1 value in the CuO₂ planes in Be-doped samples that maintains the density of carriers in the conducting CuO₂ planes [since the |ψ|² = n/2] which promotes enhancement in the magnitude of superconductivity.     

Degradation of superconducting properties in MgB2 by Cu addition

The (MgB₂)_2−xCu_x (x=0-0.5) superconducting system was prepared by a solid-state reaction technique. Microstructural evolution and transport properties including resistivity versus temperature up to a magnetic field of 6 T, activation energy, thermoelectric power and Fermi energy, E_F, and the corresponding velocity, V_F, values of the samples prepared were also investigated. The XRD analysis showed a multiphase formation and no detectable solution of Cu in MgB₂. Two different impurity phases, MgCu₂ and CuB₂₄, have been identified and their peak intensity increased when the Cu concentration increased. The temperature dependence of the resistivity of the samples showed a metallic behavior down to T_c. But, for the Cu concentrations above 0.3 the superconducting phase transition completely disappeared. The magnetic field strongly affects the electrical properties. For x=0.0 samples, the transition is found to be sharp, ΔT∼1 K, but it becomes broader with increasing magnetic field and Cu concentration. The calculated values of carrier concentration, n, of the samples are showed a sharp decrease with increasing Cu content. For x=0.0 sample the n was calculated to be 12×10²¹ cm⁻³, but for the x=0.5 sample it decreased to 1.3×10²¹ cm⁻³. We found that the activation energy, U(B), decreased sharply with increasing magnetic field. According to thermoelectric power and Fermi energy, E_F, calculations the decrease of the carrier concentration by the additions of Cu into MgB₂ gives a decrease in E_F and this could be attributed to a shift of the Fermi level towards the top of the σ-hole band.     

Transport properties of dilute magnetic alloys

The electrical resistivity, thermopower, and the electronic part of the thermal resistivity of dilute magnetic alloys are calculated in the framework of the Suhl-Nagoaka theory. Using Bloomfield's and Hamann's solution of the Nagaoka equations, we derive expressions for the transport quantities in the limitT? ¯ T_K andT?¯ T_K to order (In ¯T _K /T)^?4 where ¯T _K is the Kondo temperature which may depend on the spin independent scattering. We find that the thermopower and deviations from the Wiedemann-Franz law in this limit decrease as ¦In ¯T_K/T¦^?3 if one neglects a trivial temperature dependence of the thermopower due to the electron-phonon interaction.     

Electronic structure and magnetic properties of the compound CeCuIn

Magnetization, magnetic susceptibility, electrical resistivity, thermoelectric power and X-ray photoemission measurements were performed on a polycrystalline sample of CeCuIn. This compound crystallizes in a hexagonal structure of the ZrNiAl type. The magnetic data indicate that CeCuIn remains paramagnetic down to 1.9 K with a paramagnetic Curie temperature of −13 K and an effective magnetic moment equal to 2.5 μ_B. The electrical resistivity has metallic character, yet in the entire temperature range studied here, it is a strongly nonlinear function of temperature. The temperature dependence of the thermoelectric power is dominated by a small positive maximum near 76 K and a deep negative minimum at about 16 K. Above 150 K the thermopower exhibits a Mott's type behavior. The positive sign of the Seebeck coefficient in this temperature region indicates that the holes are dominant charge and heat carriers. The structure of Ce 3d_5/2 and Ce 3d_3/2 XPS spectra has been interpreted in terms of the Gunnarsson-Schönhammer theory. Three final-state contributions f⁰, f¹ and f² are clearly observed, which exhibit a spin-orbit splitting Δ_SO≈18.7 eV. The appearance of the 3d⁹f⁰ component is a clear evidence of the intermediate valence behavior of Ce. From the intensity ratio I(f⁰)/[I(f⁰)+I(f¹)+I(f²)] the 4f-occupation number is estimated to be 0.95. In turn, the ratio I(f²)/[I(f¹)+I(f²)]=0.08 yields a measure of the hybridization energy that is equal to 45 meV.     

Study of the semi-conducting properties of pyrolusite

The electrical conductivity σ of the specimens is found to obey a relation of the type σ=C exp (?B/k T) over a temperature range 300 to 500° K whereC andB are constants. The experimentally determined values ofB and C are (1)B≈1490 to 2199 (2)C ≈ 0·72 × 10^?3 to 4·9 × 10^?3 ohms^?1 cms^?1. The value of the activation energy determined from the values ofB are ≈0·13 to 0·19 e.V. In A.C measurements σ is found to vary with voltage applied across the specimen at a given temperature. The i?V characteristics of metal point semi-conductor contacts are non-linear symmetrical curves and are strongly temperature dependant. The value of the Hall constant (?0·14 cm³/coul) yields carrier concentration as 4·3 × 10¹⁹/cm³, and mobility 1·2 cm² volt^?1 second^?1. Δ?/? for the specimen is found to vary asH ² where ? is the resistivity andH the value of magnetic field. The specimens develop a thermo-electric power of magnitude 200 μV/K to 500 μV/K which is fairly constant over the temperature range 300 to 800° K. The sign of the thermo-e.m.f. and of the Hall constant indicate that the specimens are “n” type.     

Zum Kondo-Effekt in Zink-Mangan: Der Einfluß der Manganverteilung auf den elektrischen Widerstand im Temperaturbereich bis 0,4 °K

The electrical resistivity of dilute Zn-Mn alloys (c=0–0.6 at% Mn) has been measured in a temperature range from room temperature (RT) down to 0.4 °K. Three different series of samples are investigated: 1. annealed 72 h, 400 °C, 2. annealed and aged 1 year atRT, 3. coldworked atRT. All samples show a minimum in the residual resistance atT _min∝c ^1/5 followed by an increase of the resistivityρ∝k _s · lnT (Kondo-effect). For some alloys the resistivity minimum is followed by a resistivity maximum atT _max.T _max mainly depends on the distribution of the Mnatoms in the Zn-matrix. The coefficientk _s is also strongly influenced by changes of the Mn-distribution. For alloys withc>0.02 at% (max. solubility of Mn in Zn atRT) the slopek _s =?ρ/? lnT decreases with increasing degree of precipitation of the manganese. Samples withc<0.02 at% however, show the opposite effect, increasingk _s with increasing Mn-precipitation. Moreover, during the aging process, we observe an increase in the “residual resistance” (resistance ratioR _T/R ₂₉₃ atT _min) for these alloys. The effect can be explained by assuming that the precipitation leads to the formation of superparamagnetic clusters.     

Ultra-low temperature transport anomalies in air annealed Pt

Measurements of electrical resistivity ? and thermoelectric ratio G on air annealed reference grade Pt samples from 4 K down to 40 mK reveal ultra-low temperature anomalies in both properties. The observed T² components of ? are consistent with values obtained by previous investigators from measurements above 1 K.     

Electrical and thermal properties of Pr2/3(Ba1−xCsx)1/3MnO3 manganites

Electrical and thermal properties of Pr_2/3(Ba_1?x Cs _x )_1/3MnO₃ (0 ≤ x ≤ 0.25) manganite perovskites are reported here. Two insulator-metal (I-M) transitions (T _P1 &T _P2) are observed in the electrical resistivity (ρ) of the pristine Pr_2/3Ba_1/3MnO₃ (PBMO) sample, and they are systematically shifted to lower temperatures with increasing Cs substitution. An upturn in ρ is noticed below 50 K in these perovskites, presumably due to the combined effect of weak localization, electron-electron and electron-phonon scattering. It is found that the absolute value of room-temperature thermoelectric power (TEP) gradually decreases with increasing Cs content, implying the annihilation of the charge carriers with doping. An analysis of the electrical resistivity and thermoelectric power data indicates that the paramagnetic insulating state above T _P1 is governed by the small polaron hopping due to a non-adiabatic process. It is argued that the electron-magnon scattering processes are responsible for low temperature metallic behavior of TEP. A distinct specific heat peak below T _P1 is observed, attributed to the magnetic ordering, and its broadening with Cs-doping corresponds to the increase of magnetic inhomogeneity. Further, the temperature variation of thermal conductivity and the low temperature plateau in κ has been associated with the delocalization of Jahn-Teller polarons and transition from Umklapp scattering to a defect-limited scattering, respectively.     

Transport phenomena in metal nanocomposites

Ferromagnetic nanocomposites are the special case of metal composites; they are of practical interest for spintronics. Temperature dependences of resistivity ρ(T) and thermoelectric power α(T) of ferromagnetic nanocomposites with the composition Co _x (Al₂O₃)_{100 ? x} (36.6 ≤ x ≤ 52.5 at %) are investigated near the percolation threshold (x _p ≈ 43.3 at %) in a temperature range of 77–300 K. Sizes of Co nanoparticles are no larger than 25 nm. Specific features are observed in the dependences α(T) in the form of a kink at T _b ≈ 170 K. The analysis of the structural and electrical schematics as well as energy diagrams of percolation channels of electrons shows that only the diffusion thermoelectric power appears in Co nanoparticles, at which α(T) is the linear function. No mechanisms of the thermoelectric power caused by nanosizes of Co particles or by electron tunneling between them are found. The kink of the α(T) linear dependence is explained by the existence of the oxide shell of Co nanoparticles. It is assumed that the temperature dependences of energy barriers of oxide shells of metal nanoparticles (including ferromagnetic ones) in oxygen-containing dielectric matrices determine the features of α(T) and ρ(T) dependences of such nanocomposites.     

Transport properties of SmO

The electrical resistivity and thermoelectric power of SmO are measured between 4.2 and 300 K. The existence of a low temperature T² law in the resistivity and the large and roughly constant magnetic susceptibility suggest that SmO presents large spin fluctuations.