Thermopower Anomalies Research Articles

Temperature dependence of conductance and thermopower anomalies of quantum point contacts

It has been shown within the Landauer single-channel approach that the presence of the 0.7 anomaly in the conductance of a ballistic microcontact and the respective plateau in the thermopower implies unusual pinning of the potential barrier height $U$ at a depth of $k_BT$ below the Fermi level $E_F$. A simple way of taking into account the effect of electron-electron interaction on the profile and temperature dependence of a smooth one-dimensional potential barrier in the lower spin degeneracy subband of the microcontact has been proposed. The calculated temperature dependences of the conductance and Seebeck coefficient agree with the experimental gate-voltage dependences, including the emergence of anomalous plateaux with an increase in temperature.

Effect of Glass Composition on the Thermal Expansion of Relict Crystals of RuO<sub>2</sub> in Doped Lead-Silicate Glasses (Thick Film Resistors)

The thermal expansion coefficients (TEC) of RuO2 crystallits in thick film resistor (TFR) composites, consisting of RuO2 dispersed in lead-silicate glass of various compositions, were evaluated from X-ray diffraction patterns at temperatures 298; 773; 973 and 1123 K corresponding to characteristic temperatures of resistivity and thermopower anomalies of the TFRs. It has been found that TEC of free RuO2 powder along a-axis has an anomaly at T > 973 K (expansion is replaced by constriction), whereas constriction along c-axes remains for all temperatures. This anomaly disappears in doped glass of simplest composition (2SiO2.PbO) but occurs in glasses of some complex compositions. Symmetry of unit cell of RuO2 is not changed in the temperature range investigated.

Sr x Ba 1 − x Nb 2 O 6 − δ Ferroelectric-thermoelectrics: Crystal anisotropy, conduction mechanism, and power factor

Nonstoichiometric tungsten bronze-structured ferroelectric SrxBa1−xNb2O6−δ (SBN) single crystals were found to be a promising n-type thermoelectric oxide. Thermopower anomalies were observed at the phase transition temperatures, depending on the degree of reduction as well as crystal anisotropy. Above 500 K, heavily reduced SBN crystals show high thermoelectric power factors (∼20 μW/cm K2 at 516 K) with both thermopower and electrical conductivity higher parallel to the c-axis. It is noted that the power factor increases with temperature due to the semiconducting behavior with high carrier concentration. The carrier transport mechanism also varies with the degree of reduction and temperature.

Thermopower in highly reducedn-type ferroelectric and related perovskite oxides and the role of heterogeneous nonstoichiometry

Nonstoichiometric perovskite-structured alkaline-earth titanates with ferroelectric, paraelectric, and paraelastic phases were investigated for thermoelectric properties. Depending on the degree of reduction, different trends are noted. In ferroelectric ${\text{BaTiO}}_{3}$, thermopower anomalies are observed in and around the paraelectric $(Pm3m)$-ferroelectric $(P4mm)$ and ferroelectric $(P4mm)$-ferroelectric $(Cmm2)$ phase transition temperatures, and the nature of these trends was found to depend on the degree of reduction. This indicates a coupling between the thermoelectric effect and the ferroelectric phase transition, a phenomena also noted in the recent work of Kolodiazhnyi [T. Kolodiazhnyi, Phys. Rev. B 78, 045107 (2008)]. Heavily reduced ${\text{SrTiO}}_{3\ensuremath{-}\ensuremath{\delta}}$ showed a strong metallic behavior in the thermopower and conductivity data without anomalies as the phase is paraelastic in the temperature range studied. The nature of the reduction from the low oxygen partial pressure anneals is heterogeneous; clusters of defects $\ensuremath{\sim}3\text{ }\text{nm}$ wide meander through the crystallites. The defective regions have high oxygen vacancy concentrations, and the chemical nature of the Ti changes from ${\text{Ti}}^{4+}$ to ${\text{Ti}}^{3+}$. The complex nature of the thermochemical reduction near the metal-insulator transition will challenge simple physical models for oxide thermoelectrics. Traditional thermopower models are discussed in relation to the reported thermopower and the conductivity in the paraelectric and ferroelectric phases.

Magnetic, electrical resistivity, heat-capacity, and thermopower anomalies inCeCuAs2

The results of magnetic susceptibility, electrical resistivity ($\rho$), heat-capacity (C) and thermopower (S) measurements on CeCuAs2, forming in ZrCuSi2-type tetragonal structure, are reported. Our investigations reveal that Ce is trivalent and there is no clear evidence for long range magnetic ordering down to 45 mK. The $\rho$ behavior is notable in the sense that (i) the temperature (T)-coefficient of $\rho$ is negative in the entire range of measurement (45 mK to 300 K) with large values of $\rho$, while S behavior is typical of metallic Kondo lattices, and (ii) $\rho$ is proportional to T-0.6 at low temperatures, without any influence on the exponent by the application of a magnetic field, which does not seem to classify this compound into hither-to-known non-Fermi liquid (NFL) systems. In contrast to the logarithmic increase known for NFL systems, C/T measured down to 0.5 K exhibits a fall below 2 K. The observed properties of this compound are unusual among Ce systems.

The Influence of Ni on the Transport Properties of CoSb3

ABSTRACTThe effect of Ni doping on the Co site of the binary skutterudite CoSb3 is investigated. We measured resistivity, Hall effect, magnetoresistance, thermopower, thermal conductivity, and magnetization of a series of samples of the form Co1-xNixSb3 with x in the range x=0 to x=0.01. We find that Ni takes the tetravalent state Ni4+, assumes the d6 electronic configuration for the lower energy non-bonding orbitals, and gives an electron to the conduction band. Ni doping dramatically suppresses the thermal conductivity, changes the temperature dependence of the thermopower, and increases the carrier concentration. Low temperature anomalies in thermopower, Hall coefficient and magnetoresistance are found.

Electronic transport properties of KxC70 thin films. II.

The transport properties of ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{70}$ thin films doped to their maximum conductivity are reported. At this doping level, ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{70}$ (nominally ${\mathrm{K}}_{4}$${\mathrm{C}}_{70}$), with room-temperature-conductivity (\ensuremath{\sigma}) values ranging between \ensuremath{\sim}10--600 S/cm, is characteristic of a disordered metal near the Anderson localization regime, whose \ensuremath{\sigma} values are larger than the minimum metallic conductivity. The conduction bandwidth is estimated as 0.2--0.6 eV and the charge transport is modeled by the fluctuation-induced-tunneling model. The temperature dependence of the thermopower (S) and Hall coefficient (${\mathit{R}}_{\mathit{H}}$) are similar to that for superconducting ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ and graphite intercalation compounds. Thermopower anomalies in S(T) are observed in the same temperature range 80T100 K as the sign change of ${\mathit{R}}_{\mathit{H}}$(T), reflecting a possible change in structure or scattering process in this T range. The temperature and magnetic field dependence of the magnetoresistance of ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{70}$, also similar to that of ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$, is compared with weak-localization theory, electron-electron-interaction theory and a two-band-metal model. Based on these models, we extract information about the intrinsic conductivity of the metallic regions. No superconducting transition in ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{70}$ is observed above 1.35 K.

Thermoelectric power of Ce-based Kondo alloys

Abstract The Kondo lattice compounds CePdSn, CePdGa and CePtGa display pronounced anomalies in thermopower as a function of temperature, with a characteristic inverted tilda ( ∼ ) shape. We have attempted to correlate these anomalies with the dynamical susceptibility derived from inelastic neutron scattering. The TEP of CePdSb has a large positive slope, while that of CeRhSb shows a large peak at 20 K, signalling the onset of the Kondo insulator state.

Topological transition and its connection with the conductivity and thermopower anomalies in two-dimensional systems.

The conductivity and thermopower of a two-dimensional electron gas in the case when the Fermi level is placed near the origin of one of the size-quantization subbands are considered. The intersection of the Fermi level with the size-quantization level causes the topological change of the Fermi surface, which results in anomalies in the conductivity and thermopower. It is shown that a consistent quantitative description of the anomaly requires that the distortion of the electronic states in the populating subband by the random potential be taken into account. A general expression for the electron relaxation time in the vicinity of the transition is derived with the use of diagrammatic techniques. Detailed calculations are performed for two models of the random potential: Gaussian white noise and the screened Coulomb potential.

The electrical properties of MnFe2O4 at the Néel temperature

Abstract The conductivity and thermoelectric power of single crystals of manganese ferrite, MnFe2O4, have been measured in the temperature range 300–1000 K. Resistivity and thermopower anomalies are observed at the Neel temperature and are explained as being due to displacements of the bands of localized states and the Fermi level as magnetic ordering occurs. The polaron hopping energy at 800 K is found to be equal to approximately 0·18eV.

Exponential temperature dependence of the resistivity, thermopower, and susceptibility of Y9Co7: Evidence for local mode (abstract)

The magnetic superconductor Y9Co7 is unique in that its magnetism coexists with superconductivity at a temperature (∼5.5 K) before it transforms into the superconducting state at a lower temperature (∼2.7 K) [B. V. B. Sarkissian, J. Appl. Phys. 53, 8070 (1982)]. There is now ample evidence that the compound has a puzzling array of magnetic properties. The magnetism is consistent with the lack of long-range order, and shows some evidence of dominant incipient ferromagnetic ordering. Apart from these striking effects this compound shows anomalous temperature dependence in a variety of physical properties at high temperatures, in particular, the ‘‘saturation’’ of the resistivity and thermopower with increasing temperature and marked departure of the susceptibility from the usual Curie–Weiss behavior. The conventional view of these effects is that the resistivity and the thermopower anomalies are a consequence of the s-d scattering mechanism [E. Gratz et al., J. Magn. Magn. Mater. 21, 191 (1980)], whereas the magnetic susceptibility is viewed as a composite of a Curie–Weiss and a temperature independent part [A. Kolodziejczyk and J. Spalek, J. Phys. F 14, 1277 (1984)]. We have carried out measurements of the variation in resistivity, thermopower, and susceptibility with temperature in Y9Co7. All the results show a temperature dependence dominated by an exponential-type behavior with a well-defined characteristic temperature. The latter defines the crossover from the region dominated by strong T dependence to that dominated by weak T dependence. The presence of similar unusual high T properties in many intermetallic compounds—particularly the A15 compounds—has been evident for many years. Many of them show one or more of the properties of the Y9Co7; however, at lower temperatures their behavior differs considerably from that of Y9Co7. In interpreting the anomalous exponential behavior shown by Y9Co7 a mechanism, closely related to localized phonon mode formation, that suggests itself is one developed by Yu and Anderson [Phys. Rev, B 29, 6165 (1984)] to explain the high-T anomalous physical properties of high T superconductors, such as V3Si. It thereby implies that the unusual high-T features of Y9Co7 are due to dominating effects of the ‘‘hopping’’ mechanism of the Co atoms from one site to another along the c-axis chains. The phenomenon should occur in Y9Co7 because of the reduced spatial symmetry associated with the potential sites of Co atoms along the chains (chains of less rigidly confined Co atoms). The mechanism we are postulating for the observed high-T anomaly of Y9CO7 is also in agreement with the unusual low-T magnetic and superconducting properties of this system. The emphasis, therefore, is on behavior related to the c-axis Co atom hopping rate, which we expect to become progressively more important on cooling, due to renormalization of the electron-phonon coupling. The interesting point to be made is that this hopping mechanism undoubtedly leads to mode softening, as well as to a broadening in the effective d-level width of magnetic Co atoms. From this point of view it can be inferred that when (in the magnetic region) the hopping rate is low compared to the effective d-level width of the Co atoms then Co will conserve its magnetism. If the hopping rate is high, on the other hand, magnetism of Co may be weakened, and, with a suitable mode softening effect, the system may then condense to a true superconducting state. The magnetic character, as we perceive it, in Y9Co7 is akin to the finite lifetime spin fluctuations (quasi localized), and is the result of the balance of the durability of the Co spin against the fluctuations in configuration space accompanying the hopping process. The latter has obvious attraction as an explanation of the observed hysteresis [B. V. B. Sarkissian and A. K. Grover, J. Phys, F 12, L107 (1982)] and strong pressure effects [B. V. B. Sarkissian and J. Beille, J. Appl. Phys. 55, 2004 (1984)] in Y9Co7.

Dibenzo-TTF-dichloro-TCNQ: a quasi-one-dimensional magnetic semiconductor

An experimental study of the 1:1 charge transfer salt of Delta 2,2'-bi-d-benzo-1,3-dithiol and 2,5-dichloro-7,7',8,8'-tetracyano-p-quinodimethane (DBTTF-TCNQCl2), is presented. Despite a crystal structure with uniform and segregated stacks of molecules, the room temperature conductivity is rather low, approximately 40 Omega -1 cm-1, and increases only slightly upon cooling. The conductivity decreases rapidly below 200K. The thermopower is found to increase monotonically with decreasing temperature. Anomalies in thermopower as well as in conductivity at TC1=180K separate conducting and non-conducting regimes. The magnetic susceptibility is high ( chi (300K) approximately=7*10-4 emu/mole) and increases slowly down to 100K. There is no anomaly near TC1. A drastic drop in chi below 40K indicates an essentially magnetic phase transition with transition temperature TC2=38K. Optical measurements of the plasma edge are consistent with a rather small bandwidth approximately 0.3 eV. The behaviour around TC1 is discussed in terms of a Coulomb driven localisation of the carriers.

Thermopower anomalies in PtCo in the temperature range 4 K to 300 K

The absolute thermopower of PtCo alloys is shown to contain negative peaks which decrease in magnitude and move rapidly to higher temperature as the Co concentration increases from 0.31 to 3.74 at.%. The extrema of the peaks track rather well with magnetic ordering temperatures determined from measurements of other properties of PtCo.

Low temperature resistivity and thermopower anomalies in pure gold

Minute quantities of magnetic impurities, particularly iron, can change the low temperature transport properties of gold beyond recognition. Careful measurements of the resistivity and thermopower can yield a valuable insight into the type of impurity present. It has been possible to remove iron selectively by a high temperature chlorine treatment, and remanent iron concentrations as low as 1 part per billion (10 −9)have been obtained.

Crystal field induced thermopower anomalies in Van Vleck paramagnets

We report here strong thermopower anomalies in two Van Vleck paramagnets TmCd and PrPb 3, due to crystal field levels. The results can only be understood on the basis of electron-ion interactions involving orbital degrees of freedom, and therefore differ markedly from the dilute alloy case.

Electronic and magnetic states in metallic compounds—I. Kondo effect and ferromagnetism in CoAl

Magnetic susceptibility and electron transport properties were investigated in CoAl alloys in the vicinity of exact stoichiometry. Resistance minima, negative magnetoresistance, and thermo-power anomalies were observed in the liquid helium region. The concentration, temperature, and field dependence of the transport properties were interpreted in terms of localized magnetic moments formed on Co atoms on Al sites (Co x atoms) in the lattice. The effective moment on the Co x atoms in the paramagnetic state is 5·9 μ R . Analysis of the data in terms of the theories of the Kondo effect led to estimates of the exchange parameter and Kondo temperature of ∼ −1·3 to −2·2 eV and ∼8°K. respectively. At excess Co concentrations of more than 0·6 at.%. ferromagnetic interactions between the Co x atoms become apparent in that the resistance minima disappear, the Weiss temperature changes from negative to positive, and remanent magnetization is observed. The nature of the ferromagnetic state is discussed along with some evidence for spin-fluctuation effects in the nearly ferromagnetic alloys.