Transport Properties and Electronic Specific Heat of La2−x Ba x Cu1−y Zn y O4+d Bulk

The effect of the partial substitution of Cu by Zn and La by Ba in La: 214 prepared by conventional solid state reaction method has been investigated by using Hall coefficient, Seebeck coefficient and electronic specific heat measurements, over the wide temperature range between 4.2 K and 300 K, in magnetic fields up to 5 T. The samples with y=0 and d=0, showed superconductivity for x between 0.055 and 0.30. The critical temperature, the Hall and Seebeck coefficients strongly depend on Zn content. The phonon specific heat C _ph and electronic specific heat C _el have been extracted from the total specific heat C. Zn-doping effect on the (T)=C _el/T behavior suggests that the large range stripe order and the fluctuating stripe suppress the singlet formation and pseudo gap.     

Seebeck Coefficient Measurement and Its Narrow Band Model Interpretation in FeTe<Subscript>0.5</Subscript>Se<Subscript>0.5</Subscript> Superconductor

We have measured Seebeck coefficient (S) of FeTe_0.5Se_0.5 superconducting sample from 10 to 300 K. The variation of Seebeck coefficient with temperature of this system was found to be very anomalous, and the overall experimental observation of the S(T) was studied in the outline of a narrow-band model. In high-temperature region, the Seebeck coefficient is almost independent of temperature. Further, from the study of high-temperature magnitude of S, sample undergoes a change in sign in the Seebeck coefficient, wherein, appearance of a negative peak around 22 K and subsequently, its Seebeck coefficient goes to zero in the superconducting transition temperature regime around 11 to 13 K. It is revealed that the bandwidth and a small asymmetry involved in narrow bands give a realistic explanation to the anomalous temperature dependence of Seebeck coefficient of FeTe_0.5Se_0.5 system.     

Seebeck coefficient of Ca<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Pr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3–δ</Subscript> paramagnetic manganites

The Seebeck coefficient (S) of Ca_1–x Pr _x MnO_3–δ (х = 0, 0.05, 0.10, 0.15) manganites with a perovskite-like structure has been measured in air at temperatures (T) from 300 to 1200 K. The negative sign of their S indicates that all of the samples have n-type conductivity. The observed increase in the magnitude of the Seebeck coefficient with increasing T is interpreted in terms of small-polaron transport with allowance for the decrease in Mn³⁺ concentration as a result of the disproportionation reaction 2Mn³⁺ = Mn²⁺ + Mn⁴⁺. Based on a theoretical analysis of experimental S(T) data, we calculated equilibrium constants for the disproportionation reaction, carrier concentration, and the concentration of sites available for carrier migration as functions of temperature. It has been shown that, for an adequate analysis of electron hopping and calculation of the Seebeck coefficient of the electron-doped manganites, the spin state of the Mn⁴⁺ ions should be taken into account.     

Electrical properties of Si4+ substituted copper ferrite

The electrical resistivity, , and Seebeck coefficient, , for the system Cu_1+xSi_xFe_2–2xO₄ (where x = 0.05, 0.1, 0.15, 0.2 and 0.3) have been studied as a function of temperature. Temperature variation of the resistivity exhibits two breaks. Each break is associated with a change in activation energy. The conduction process at low temperature is governed by the reaction Cu _A ¹⁺ + Cu _A ²⁺ Cu _A ²⁺ + Cu _A ¹⁺ . However, at higher temperature, it is due to intersite cation exchange and reoxidation such as Cu _A ²⁺ + Fe _B ³⁺ Cu _B ²⁺ + Fe _A ³⁺ . Measurement of the Seebeck coefficient, , from room temperature to 800 K reveals n-type conduction for the sample with x= 0.05, while the measurements for other samples show p-type conduction for lower temperatures and n-type conduction at higher temperatures. The activation energies in the paramagnetic region are found to be less than those in the ferrimagnetic region.     

Equation of State for Molten Alkali Metals from Surface Tension. Part II

This work presents a new method for predicting the equation of state for molten alkali metals, based on statistical–mechanical perturbation theory from two scaling constants that are available from measurements at ordinary pressures and temperatures. The scaling constants are the surface tension and the liquid density at the boiling temperature (_b, _b). Also, a reference temperature, T _Ref, is presented at which the product (T _Ref T _b ^1/2 ) is an advantageous corresponding temperature for the second virial coefficient, B ₂(T). The virial coefficient of alkali metals cannot be expected to obey a law of corresponding states for normal fluids, because two singlet and triplet potentials are involved. The free parameter of the Ihm–Song–Mason equation of state compensates for the uncertainties in B ₂(T). The vapor pressure of molten alkali metals at low temperatures is very low and the experimental data for B ₂(T) of these metals are scarce. Therefore, an equation of state for alkali metals from the surface tension and liquid density at boiling temperature (_b, _b) is a suitable choice. The results, the density of Li through Cs from the melting point up to several hundred degrees above the boiling temperature, are within 5%.     

Equation of state for compressed liquids from surface tension

A method for predicting an analytical equation of state for liquids from the surface tension and the liquid density at the freezing temperature ( ₁, ₁) as scaling constants is presented. The reference temperature. T_ref. is introduced and the product (T _ref T ₁ ^{1 2} ) is shown to be an advantageous corresponding temperature for the second virial coeflicienls. B₂(T). of spherical and molecular fluids. Thus, B₂(T) follows a promising corresponding states principle and then calculations for(T) andb(T), the two other temperature-dependent constants of the equation of state, are made possible by scaling. As a result, ( ₁, ₁) are sufficient for the determination of thermophysical properties of fluids from the freezing line up to the critical temperature. The present procedure has the advantage that it can also be used in cases whereT _c andP _c are not known or the vapor pressure is too small to allow accurate measurements. We applied the procedure to predict the density of Lennard-Jones liquids over an extensive range of temperatures and pressures. The results for liquids with a wide range of acentric factor values are within 5%.     

Thermoelectric properties of β-FeSi2 with B4C and BN dispersion by mechanical alloying

The B₄C- and BN-dispersed -FeSi₂ thermoelectric materials were synthesized by mechanical alloying and subsequent hot pressing. The effects of the B₄C and BN dispersion on the thermoelectric properties, such as Seebeck coefficient, electrical resistivity and thermal conductivity etc., of the -FeSi₂ were investigated. For the sample with B₄C and BN addition, a larger amount of the residual phase was detected in the X-ray diffraction patterns than the sample without addition. In the case of the BN addition, the Seebeck coefficient was enhanced by BN addition above 700 K, and the electrical resistivity also increased with increasing amount of BN. This is considered to result from doping of a small amount of B into the phase due to partial decomposition of the BN phase. The fine dispersion of BN particles in the phase matrix was quite effective for reducing the thermal conductivity as compared to the B₄C addition over the entire temperature range. The figure of merit, Z, of the -FeSi₂ was significantly enhanced by BN addition.     

Adsorption isotherms of helium on activated charcoal

Adsorption of helium on activated charcoal has been measured in the pressure range 1–30 atm¹ and in the temperature range 15–70 K. A Clapeyron like law (InP_s = A + B/T) for fictitious saturation vapour pressure P_s above T_cr is deduced experimentally from the measurements. This law leads to a fairly good generalized correlation for all experimental data in the framework of Dubinin theory:

     

Electrical transport in light rare-earth orthochromites

Electrical conductivity, , Seebeck coefficient, S, and dielectric constant, , measurements on the pressed pellets of six light rare-earth orthochromites, RCrO₃, where R = La, Pr, Nd, Sm, Eu and Gd, have been carried out in the temperature range 300 to 1000 K. These are essentially electronic conductors, exhibiting p-type extrinsic semiconducting nature in the studied temperature range. The extrinsic charge carriers (holes) originate from Cr⁴⁺ centres which are present due to native defects in these solids. Their room-temperature electrical conductivities lie in the range 10^–7 to 10^{–5 –1} cm^–1, which become of the order of 10^{–2 –1} cm^–1 near 1000 K. The conductivity is a maximum in LaCrO₃ and drops across the RCrO₃ series, with SmCrO₃ being an exception. The mechanism involved in the electrical transport is the hopping of holes from Cr⁴⁺ centres to neighbouring Cr³⁺ ions. The activation energy of transport is nearly 0.3 eV. Typical hopping mobility of the holes is of the order of 10² cm^–2 V^–1 sec^–1 at 325 K and of the order of 10 cm² V^–1 sec^–1 at 1000 K. The mobility activation energy of the holes in a typcial RCrO₃ decreases with temperature due to the smoothing of the potential barriers between Cr⁴⁺ and Cr³⁺ sites. Several discontinuities are observed in the T against T ^–1 and S against T ^–1 plots of RCrO₃. The anomalies which these discontinuities reflect here have also been indicated.     

Electrical conduction in narrow band trivalent transition metal tungstates

The measurements of electrical conductivity () and Seebeck coefficient (S) of trivalent transition metal tungstates M₂(WO₄)₃ with M = Cr, Fe and Ni have been reported in the temperature range of 450 to 1250 K. The results have been explained on the basis of a schematic energy band diagram appropriate for these solids. The estimated value of the energy band gap is slightly more than 3 eV.     

Electrical transport in heavy rare-earth vanadates

This paper reports measurements of electrical conductivity () and Seebeck coefficient (S) between 300 and 1250 K and differential thermal analysis (DTA) and thermogravimetric analysis (TGA) between 300 and 1200 K, together with X-ray diffraction studies of heavy rare-earth vanadates (RVO₄ with R=Tb, Dy, Ho, Er and Yb). All these vanadates have been found to have a tetragonal unit cell. The DTA study shows a flat dip in the temperature interval 1075 to 1300 K, indicating a possible structural phase transition of these compounds. Practically no weight loss has been observed in TGA from 300 to 1200 K in any of the vanadates. All RVO₄ are semiconducting materials with the room-temperature value lying in the range 10^–12 to 10^{–3 –1} m^–1, becoming of the order of 10^{–2 –1} m^–1 around 1000 K. The electrical conductivity of all vanadates exhibits an exponential increase in the temperature intervals 420 K toT ₁ andT ₁ toT ₂, with different values of the activation energy. A log againstT ^–1 plot shows a peak aroundT ₃ and drops to a minimum value aroundT ₄, before increasing again with temperature.T ₄ >T ₃ >T ₂ >T ₁ are different for different vanadates and these are termed break temperatures.T ₄ lies well within the temperature range of the DTA peak and can be termed the phase transition temperature. In the lower temperature interval the electrical conduction is essentially extrinsic. The localized charge carriers on defect centres conduct by a hopping mechanism. The defect centres are V⁴⁺ ions in all vanadates with R⁴⁺ centres in some of them. It is concluded that in the temperature intervalT ₁ <T <T ₂ the conduction mechanism is of the intrinsic band type, with oxygen 2p and vanadium 3d as the valence and conduction bands, respectively. Related parameters like the energy band gap and the mobilities of the charge carriers have also been evaluated. The low values of mobility suggest that large polarons with intermediate coupling are the charge carriers rather than bare electrons in the intrinsic region. All these vanadates tend to become metallic, but before this is achieved the phase change makes the conductivity smaller.     

Pyroelectric and dielectric properties of some heavy rare-earth orthochromites

The measurement of pyroelectric coefficient (p) and dielectric constant (K) of rare-earth orthochromites RCrO₃, where R = Tb, Dy, Ho, Er and Yb are reported for the temperature range 300–600 K. Pyroelectric data show that all the studied orthochromites have ferroelectric phase in this temperature range. The dielectric data in some cases support this conclusion. Spontaneous polarization (P _s) and Curie temperature (T _c) have also been evaluated. The maximum value of P _s varies from 0.041 Cm^–2 to 0.30 Cm^–2 in the studied orthochromites, which is very small.     

Electrical resistivity and Seebeck coefficient of La1−x M x MnO3 (M = Ca,Sr) single crystals

The temperature dependence of the electrical resistivity and Seebeck coefficient was measured on single crystals of La_1–x Ca _x MnO₃(0 <x 0.3) and La_1–x Sr _x MnO 3 (0 <x 0.4) grown by the arc-image floating zone method. The electrical conduction for La_1–x crystals withx 0.2 was of the activation type aboveT _c and of the degenerate type belowT _c, while that for the crystal withx = 0.1 was of the activation type over the whole measured temperature range between –170 and 400°C. The conduction behaviour of La_{1– x} Sr _x MnO₃ was essentially the same as that of La_1–x Ca _x MnO₃ except that the conduction of the crystals withx = 0.3 and 0.4 was of the degenerate type aboveT _c. A distinct difference in Seebeck data was observed between the calcium and the strontium compounds.     

Electrical properties of vanadium pentoxide doped with lithium and sodium in the α-phase range

The electrical conductivity and Seebeck coefficients have been measured from room temperature to 500° C for polycrystalline V₂O₅ and V₂O₅ doped with lithium and sodium in the -phase range. The conductivity increases with doping and the energy of activation decreases. The Seebeck coefficient indicates that electrons are the majority carriers. The results have been discussed in terms of the two-level hopping model.     

Low-temperature properties of the heavy-fermion compound CeRu2Si2 at the metamagnetic transition

Magnetization measurements performed below 1 K on a single-crystalline sample of the heavy fermion compound CeRu₂Si₂ are reported. The field variation of the linear term of the specific heat derived from these data (via a Maxwell relation) exhibits a large peak at the metamagnetic-like transition at B* (=7.7 atT 0). Thermal-expansion measurements, show a drastic drop of the temperature where a Fermi liquid behavior is reached whenB B*. Similar experiments performed on a single-crystalline Ce_0.95La_0.05Ru₂Si₂ sample show a broadening of all the anomalies atB* and a reduction of their intensities. These results strongly suggest that pure CeRu₂Si₂ approaches a magnetic instability atB* forT 0.     

Temperature dependence of the flow stress and ductility of annealed and unannealed amorphous Fe40Ni40P14B6

The flow stress and ductility of amorphous Fe₄₀Ni₄₀P₁₄B₆ was investigated on asprepared and annealed ribbons over the temperature range 78 to 573 K. The embrittlement is characterized by a rise in the ductile-to-brittle transition temperatureT _dtb, such thatT _dtbTanneal and sets in at temperatures as low as 423 K. The flow stress of Fe₄₀Ni₄₀P₁₄B₆ at room temperature is 2.21 GPa (320 ksi), in good agreement with the extrapolation from hardness measurements. The flow stress increases with decreasing temperature. The increase can be accounted for in dislocation-based models of the deformation of a metallic glass but is more difficult to understand in terms of a free volume model.     

Electrical Properties and Ferromagnetism in (Ga,Cr)As

MBE-grown (Ga,Cr)As has interesting electric and magnetic properties. Ga_1–x Cr _x As with x = 0.1 exhibits short-range ferromagnetic behavior at low temperatures. This is manifest in several anomalous properties: magnetization does not scale with B/T; fitting M(B) requires a model of distributed magnetic cluster or polarons; and inverse susceptibility is nonliner in T (non-Curie–Weiss) at low fields. At room temperature, the conductivity is activated and Hall measurements yield a hole concentration of 10²⁰ cm^–3, indicating that chromium acts as an acceptor similar to Mn in GaAs. For decreasing temperature, the conductivity decreases by eight orders of magnitude and follows exp(1/T ^1/2).     

Field induced magnetic phase transition as a magnon Bose Einstein condensation

We report specific heat, magnetocaloric effect and magnetization measurements on single crystals of the frustrated quasi-2D spin antiferromagnet Cs₂CuCl₄ in the external magnetic field along a-axis and in the temperature range . Decreasing the applied magnetic field B from high fields leads to the closure of the field induced gap in the magnon spectrum at a critical field and a long-range incommensurate state below B_c. In the vicinity of B_c, the phase transition boundary is well described by the power law T_N∼(B_c-B)^1/φ with the measured critical exponent φ?1.5. These findings provide experimental evidence that the scaling law of the transition temperature T_N can be described by the universality class of 3D Bose-Einstein condensation (BEC) of magnons.     

Low Temperature Specific Heat of CeRu2Si2 at the Field Induced Metamagnetic Instability

We report specific heat measurements in a broad temperature range between 0.06 and 20 K on single crystalline CeRu₂Si₂ in magnetic fields applied parallel to the c-direction around the critical field B _M , where the metamagnetic transition from the itinerant to the localized state takes place. In the critical field B _M c=7.8 T a distinct deviation from the usual Fermi- liquid behavior is found down to our lowest temperature. Below 1.8 K C/T varies as (1–aT), while for 1.8 K<T<20 K a power law T ^–1 with =0.66 is found. Small deviations from the critical magnetic field leads to the recovery of a Fermi-liquid ground state. The observed behavior of the specific heat at the field induced magnetic instability is discussed in the context of a non-Fermi-liquid state and compared with those for the field-induced nFl CeCu_6–x Ag _x and the recent observation of nFl behavior at B _M in UPt₃.