Transport properties of quinolinium (TCNQ)2 under pressure

We have measured the electrical conductivity and thermoelectric power of quinolinium (TCNQ)₂ in the pressure range from atmospheric to 20 kilobars at room temperature and for 15 kilobars from 300 K to 30 K. The room temperature conductivity is enhanced by a factor of seven at 20 kilobars in a manner similar to TTF-TCNQ, whereas the thermopower is only slightly reduced (～ 30%). At 15 kilobars the low temperature behavior remains dominated by disorder and Coulomb correlations.     

Electron diffusion thermopower of alkali metals

The electron diffusion thermopower of K, Rb and Cs is calculated and analysed by using an accurate phonon spectrum generated from neutron scattering data. It is found that electron-phonon normal scattering generates a negative thermopower which is linear in temperature. On the other hand, Umklapp scattering produces a large positive and extremely non-linear thermopower. Since the individual contributions are weighted by the respective thermal resistivities, the positive Umklapp component dominates for Rb and Cs, whereas the negative normal component does so for K. Although the observed sign anomaly of the thermopower is generally attributed to the phonon drag effect, we see that the electron diffusion thermopower also plays a significant role.     

Thermopower of doped semiconducting hydrogenated amorphous carbon films

The thermopower S(T) and electrical conductivity have been measured from 25 to 250C for semiconducting a-C:H films doped with boron or phosphorus. S has the expected sign (positive for B-doping and negative for P-doping), is low for all films (10–50 μ V/K), and increases nearly linearly with T. This behavior, along with that observed for the electrical conductivity, is consistent with conduction via hopping at or near the Fermi level which has been shifted via doping from near mid-gap into broad bands of tail states at the appropriate band edges.     

The energy-band gaps in tetrathiafulvalene-tetracyanoquinodimethane measured by far-infrared photoconductivity

The b-axis photoconductivity of TTF-TCNQ at 5 K has been measured with polarized radiation. The bolometric signal from vibrational modes was removed by increasing the chopping frequency and reducing the temperature. Band gaps of 403 K (E ∥ a) and 453 K (E ∥ b) were obtained, the former agreeing with d.c. conductivity results above 13 K. Impurity photoconductivity was observed with E ∥ b and an energy gap of 130 K explaining d.c. conductivity results below 13 K.     

Effect of tellurium on electrical and structural properties of sintered silicon nitride ceramics

The effects of tellurium (Te) additives on electrical conductivity, dielectric constant and structural properties of sintered silicon nitride ceramics have been studied. Different amounts of Te (10% and 20%) were added as sintering additives to silicon nitride ceramic powders and sintering was performed. Microstructure and composition were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrical conductivity and dielectric constant (ε′) increase exponentially with temperature greater than 800 K. The electrical conductivity and dielectric constant increase but activation energy decreases from 0.72 to 0.33 eV with the increase of Te concentration. However, the conductivity increases five orders of magnitude at the concentration of 10% of Te in Si₃N₄. As the Te concentration increases the sintered silicon nitride ceramics become denser. These types of samples can be used as high temperature semiconducting materials.     

Magnetic anomalies in Gd2PdSi3

The results of ac and dc magnetic susceptibility, thermopower and Hall effect measurements of a compound, Gd₂PdSi₃, establish that this compound orders magnetically below T_n = 20 K. Though the ordering appears to be of an antiferromagnetic-type, the paramagnetic Curie temperature is positive with the magnitude being nearly the same as that of T_N, suggestive of the existence of ferromagnetic correlations. The thermopower at 300 K is large, apparently due to Pd 4d electrons, decreasing monotonically with temperature. There is a change in the sign of Hall constant well below T_N Also considering the observation of Kondo-like characteristics above 21K earlier by us, the overall thermal, transport and magnetic behaviour of this compound is interesting.     

Electrical and switching properties of NiAlxFe2−xO4 ferrites synthesized by chemical method

Nickel-aluminum ferrite system NiAl_xFe_2−xO₄ has been synthesized by wet chemical co-precipitation method. The samples were studied by means of X-ray diffraction, d.c. electrical resistivity, a.c. electrical resistivity, a.c. conductivity and switching properties. The XRD patterns confirm the cubic spinel structure for all the synthesized samples. The crystallite size calculated from XRD data which confirm the nano-size dimension of the prepared samples. Electrical properties such as a.c. and d.c. resistivities as function of temperature were studied for various Al substitution in nickel ferrite. The dielectric constant and dielectric loss tangent were also studied as a function of frequency. The dielectric constant follows the Maxwell-Wagner interfacial polarization. A.C. conductivity increases with increase in applied frequency. The d.c. resistivity decreases as temperature increases, which indicate that the sample have semi-conducting nature. Verwey hoping mechanism explains the observed variation in resistivity. The activation energy is derived from the temperature variation of resistivity. Electrical switching properties were studied as I-V measurements. The current controlled negative resistance type switching is observed in all the samples. The Al substitution in nickel ferrite decreases the required switching field.     

Non-ohmic properties and electric breakdown of quasi one-dimensional organic conductors

The dependence of the d.c. conductivity of single crystals of TTF-TCNQ on the electric field strength has been investigated. At 4.2 K drastic deviations from Ohm's law are observed. At an electric field strength of 300–600 V/cm, dependent on the room temperature conductivity, a reversible breakdown occurs connected with a rise of the conductivity by about three orders of magnitude. Similar effects have been found in single crystals of (TTF)₇J₅ and (TTF)J₂ in the temperature range around 100 K. Possible mechanisms responsible for this large conductivity changes are discussed.     

The structure,conductivity, and thermopower of HMTTF-TCNQ

The structure and some transport properties of a new organic conductor, HMTTF-TCNQ, are reported. A metallic conductivity and thermopower are found between room temperature and 50°K, with insulating behavior at lower temperatures. The conductivity data suggest that two phase transitions occur, one at 43±1°K and the other at 50±°K. Comparison with the properties of TTF-TCNQ and HMTSeF-TCNQ is made.     

Thermal transport in the oxygenated magnetic superconductor, Eu1.5Ce0.5RuSr2Cu2O10+δ

The thermal conductivity and thermopower are reported for a hole doped Eu_1.5Ce_0.5RuSr₂Cu₂O_10+δ sample that has been annealed at 1100 K under an oxygen pressure of 54 atm. At T_c=45 K superconductivity and weak ferromagnetism coexist (T_m=180 K). Weak features in the thermopower, S(T), and thermal conductivity, κ(T), are observed both at T_m and at T^*=140 K. The thermopower begins to decrease sharply toward zero at T_c, and there is an extremely sharp increase of about 30% in the thermal conductivity at T_c. This “first order” transition may be related to the sudden appearance of a spontaneous vortex phase at T_c. A small shoulder is observed in κ(T) in the temperature range T=5–13 K.     

Temperature dependent X-ray observations of TTF-TCNQ

Single crystal X-ray oscillation photographs of the organic solid TTF-TCNQ have been recorded at temperatures above and below the conductivity maximum. No evidence of a multiplicative enlargement of the unit cell in a direction parallel to the conducting chains has been observed.     

Anisotropic thermoelectric power of TTF-TCNQ

We have measured the thermoelectric power along the a-axis as well as the highly conducting b-axis in TTF-TCNQ crystals. Measurements were taken both parallel and perpendicular to the long axis on two sets of crystals. One set grew along the a-axis and the other along the b-axis. The thermopowers are of opposite sign except near 60°K where both cross zero at slightly different temperatures. The a-axis thermopower is consistent with non-metallic diffusive transport in the a direction.     

Electrical properties of a-antimony selenide

This paper reports conduction mechanism in a-Sb₂Se₃ over a wide range of temperature (238 to 338 K) and frequency (5 Hz to 100 kHz). The d.c. conductivity measured as a function of temperature shows semiconducting behaviour with activation energy ΔE=0.42 eV. Thermally induced changes in the electrical and dielectric properties of a-Sb₂Se₃ have been examined. The a.c. conductivity in the material has been explained using modified CBH model. The band conduction and single polaron hopping is dominant above room temperature. However, in the lower temperature range the bipolaron hopping dominates.     

Anisotropic dielectric constant of TTF-TCNQ observed by dielectric resonance

Independent measurements at low temperatures of the anisotropic dielectric constant (∈₁) of TTF-TCNQ are reported. The measurements utilize dielectric resonance techniques to determine ∈^B₁ and ∈^a₁ on the same crystal. The results confirm the unusually large value for ∈^b₁(4.2 K) > 10³ and the anisotropic behavior ∈^b₁(4.2 K) > 10² found earlier in cavity perturbation studies of TTF-TCNQ.     

Far infrared photoconductivity of TTF-TCNQ

The spectral dependence of the far-infrared photoconductivity of TTF-TCNQ at low temperatures shows an interband transition with an edge at approximately 22 meV. We associate this gap with the d.c. conductivity maximum near 70 K, and suggest that at least two distinct mean-field transition temperatures are important for the observed electrical and magnetic properties of the material.     

Sr2FeMoO6 nanosized compound with dielectric sheaths for magnetically sensitive spintronic devices

Single-phase agglomerated Sr₂FeMoO₆-_δ powders with the iron and molybdenum cations superstructural ordering of 88% were synthesized by sol-gel technique from the Sr(NO₃)₂ and Fe(NO₃)₃·9H₂O solutions with pH = 4. The ultrasound dispersion enabled us to obtain 75 nm grains. Powders were pressed with 4 GPa to receive the ceramics. The additional annealing at 700 K promoted the appearance of 7.5% SrMoO₄ phase. The nanocomposite with dielectric sheaths around the grains was obtained. Magnetization temperature dependences in zero-field cooled mode revealed inhomogeneous magnetic states. At temperature below 19 K, the superparamagnetic state is observed. Temperature increase leads to a realization of the stable superparamagnetic and metastable ferrimagnetic states, blocked by magnetic anisotropy energy. The resistivity temperature dependences have the semiconducting conductivity type. The charge transfer due to the hopping conductivity on the localized states in the energy band near the Fermi level dominates at 260–300 K. At 130–200 K the charge transfer is realized by electrons tunneling through the energy barrier. The electrons inelastic tunneling on conducting channels between grains, through the localized states in the dielectic interlayer dominates at low temperatures. The resistivity decreases in magnetic fields and the negative tunneling magnetoresistive effect reaching 41% occurs.     

Electrical transport properties of consolidated ZnSe quantum dots at and above room temperature

Here we report a comprehensive study on the prevailing conduction mechanism and dielectric relaxation behavior of consolidated Zinc Selenide quantum dots in the frequency range of 1 kHz ≤ f ≤ 1.5 MHz and in the temperature range of 298K ≤ T ≤ 573 K. The ac conductivity increases either with increase in temperature or with increase in frequency, which is explained by the Jonscher Power law. At higher temperatures, correlated barrier hopping is found to be the prevalent charge transport mechanism with a maximum barrier height of 0.88 eV. The dielectric constant of the sample is found to exhibit weak temperature dependence. DC conductivity study reveals the semiconducting nature of the sample and it is discussed in the light of polaron hopping conduction. From the impedance spectroscopic study, role of the grains and grain boundaries in the overall electrical transport properties have been elucidated by considering an electrical equivalent circuit (composed of resistances and constant phase elements). Electric modulus study reveals non-Debye responses of the sample in the experimental range.     

Giant dielectric constant in the one-dimensional semiconductor Meφ3As (TCNQ)2

We have measured a large highly anisotropic dielectric constant in methyltriphenylarsonium ditetracyanoquinodimethanide as a function of frequency and temperature. We present a model based upon finite barriers in the conducting strands. Magnetic resonance and electrical conductivity are used in testing the model; good agreement is obtained.     

Elastic and kinetic properties of single-crystal La<Subscript>0.75</Subscript>Ba<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript>

An experimental study of the temperature behavior of longitudinal sound velocity, internal friction, electrical resistivity, and thermopower of single-crystal La_0.75Ba_0.25MnO₃ is reported. A structural transition accompanied by a large jump (18%) in the sound velocity was found to occur at T _S ≈170 K. Within the interval 156–350 K, the temperature dependences of the sound velocity and internal friction reveal a temperature hysteresis. An internal-friction peak due to relaxation processes was detected. The metallic and semiconducting regions are separated by a transition domain about 80 K wide lying below the Curie temperature T _C =300 K.     

UV laser-induced transport properties change in silver metaphosphate glass

Samples of AgPO₃ were prepared from analar oxides (BDH) using standard sintering ceramic technique. The effect of UV Nd:YAG pulsed laser irradiation with wavelength of 355 nm on the transport properties of the prepared glassy samples AgPO₃ was studied. The frequency and temperature dependences of the A.C. conductivity and dielectric properties were analyzed in the temperature range (300 K ≤ T ≤ 393 K) throughout a range of applied frequencies (0.1 kHz ≤ f ≤ 400 kHz). The activation energies were calculated at different temperatures for the unirradiated and irradiated samples. Comparison between the A.C. electrical conductivity, dielectric constant, and dielectric loss, for unirradiated and irradiated samples was performed. The bulk conductivity has been measured with complex impedance method. Cole-Cole diagram has been investigated to obtain the bulk activation energy which corresponds to D.C. conductivity. Results indicate the semiconducting like behavior. At low temperatures and frequencies, random diffusion of the ionic charge carriers via activated hopping gives rise to a frequency independent conductivity characterizing the D.C. conductivity. It was observed that the A.C. and D.C. activation energies have the same order of magnitude.