State Transport Properties Research Articles

Universal intrinsic doping behavior of in-plane dc conductivity for hole-doped high-temperature cuprate superconductors

Understanding the normal state transport properties in hole-doped high-temperature cuprate superconductors (HTCSs) is a challenging task which has been widely believed to be one of the key steps toward revealing the pairing mechanism of high-temperature superconductivity. Here, we present a true intrinsic and universal doping dependence of in-plane dc conductivity for all underdoped HTCSs. The doping dependence of in-plane dc conductivity normalized to that at optimal doping can be represented by a simple exponential formula. The doping behavior of the square of the nodal Fermi velocity derived by the high-resolution laser-based angle-resolved photoemission spectroscopy in the superconducting state follows reasonably well the universal intrinsic doping behavior. Our findings suggest a commonality of the low-energy quasiparticles both in the normal and superconducting states that place a true universal and stringent constraint on the mechanism of high-temperature superconductivity for HTCSs.

Superconductivity and normal state magnetoresistance in superconducting FeSe:Sb

We prepared a series of β-FeSe samples with a nominal composition of Fe1.11Se1−x Sb x (0⩽x⩽0.5). The X-ray diffraction, transport and magnetic measurements were performed on these samples to investigate the structure, the superconducting properties and the normal state transport and magnetic properties. Although the X-ray diffraction data suggested that Sb atoms were not incorporated into the β-FeSe phase, the transport data showed observable changes of superconductivity, normal state resistivity and magnetoresistance. This was represented by the increase in the superconducting transition temperature and the upper critical field. Also, for the samples with a low level of Sb content, a clear decrease of the normal state resistivity and a substantial increase of the residual resistance ratio were observed. Furthermore, the samples showed a significant increase of the normal state magnetoresistance that appeared not to follow the Kohler’s rule. The results were discussed in the frame of reduction of excess Fe at interstitial sites of β-FeSe.

A semimetal model of the normal state magnetic susceptibility and transport properties of Ba(Fe 1−xCo x) 2As 2

A simple two-band 3D model of a semimetal is constructed to determine which normal state features of the Ba(Fe 1− x Co x ) 2As 2 superconductors can be qualitatively understood within this framework. The model is able to account in a semiquantitative fashion for the measured magnetic susceptibility, Hall, and Seebeck data, and the low temperature Sommerfeld coefficient for 0 < x < 0.3 with only three parameters for all x. The purpose of the model is not to fit the data but to provide a simple starting point for thinking about the physics of these interesting materials. Although many of the static magnetic properties, such as the increase of the magnetic susceptibility with temperature, are reproduced by the model, none of the spin-fluctuation dynamics are addressed. A general conclusion from the model is that the magnetic susceptibility of most semimetals should increase with temperatures.

The study on the ZnO and Zn 0.95Mn 0.05O added YBCO system: Investigation of microstructure and transport properties

The effect of nano-size Zn 0.95Mn 0.05O and ZnO (30 nm) addition on the microstructure and the normal state transport properties of polycrystalline YBa 2Cu 3O y (YBCO) was systematically studied. Samples were synthesized in air using a standard solid state reaction technique by adding nano-sized particles up to 10 wt.%. When Zn 0.95Mn 0.05O and ZnO are added to the YBCO the orthorhombic structure maintained even at the highest concentration. TEM and EDS analyses show the presence of inhomeginities embedded in the superconducting matrix. To interpret the normal state properties of the samples, the percolation theory based on localized states is applied. A cross-over between variable-range hopping and Coulomb gap mechanisms is observed as a result of increasing the nano-particles concentration. The ZnO addition modifies the electrical behavior of samples from metallic to insulating with a much lower concentration comparatively to Zn 0.95Mn 0.05O addition. The calculated values of the localization length, d, are greater in the case of Zn 0.95Mn 0.05O addition. This result can be interpreted by the internal structure defects.

Solid state synthesis, structure and transport properties of compositions in the CaRu 1− xTi xO 3− δ system

Double B perovskite-type CaRu 1− x Ti x O 3− δ series with x = 0, 0.1, 0.2…0.9, 1.0 were synthesized in air by solid state reactions from simple ruthenium and titanium oxides and CaCO 3. Crystal structures of CaRu 1− x Ti x O 3 samples have been refined in space group Pnma (GdFeO 3-type of structure) assuming statistical distribution of Ru and Ti species over the octahedral sites. Electrical conductivity and thermal expansion were investigated at t = 20–1000 °C and pO 2 = 1–21000 Pa. Apart from CaRuO 3 with a combined p-/n-type conductivity depending on temperature and pO 2, all other compositions are n-type semiconductors. The conductivity of CaRuO 3 decreases with increasing temperature (metal-type). The Ti-containing compositions with x = 0.1–0.2 change their conductivity dependence from metal-type at relatively low oxygen partial pressures ( pO 2) to semi-conducting-type at relatively high ones. The materials with x > 0.2 have the semi-conducting-type dependence at all investigated pO 2. Disproportionation of Ru 4+ into Ru 3+ and Ru 5+ cations is considered for explanation of the conductivity dependences on temperature and pO 2. The investigated compositions show high catalytic activity for propene oxidation and no activity for NO 2 reduction.

Strain state, microstructure and electrical transport properties of LaNiO3 films grown on Si substrates

LaNiO3 films with various thicknesses were deposited on Si substrates by sputtering. The film thickness dependent strain state, microstructure and electrical properties were investigated. X-ray diffraction analysis shows that the films are generally strained with larger lattice constants than the value for bulk LaNiO3, due to a large difference in thermal expansion coefficients between LaNiO3 and Si. However, with increasing film thickness, the internal strain is gradually relaxed with the lattice constant decreased and the crystallinity and electrical properties improved as well. The film thickness dependent resistivity can be quantitatively described by a combined Fuchs–Sondheimer and Mayadas–Shatzkes model, including both the grain boundary scattering and the surface scattering. Further low temperature resistivity measurement shows that the film of 600 nm is semiconducting. The higher activation energy acquired by the small polaron conduction fitting than by the Arrhenius fitting indicates relatively large effective masses for the polarons in the film.

The normal state transport properties of NdBa2−xLaxCu3O7−δ: Evidence of localization hole by La

The transport properties of sintered samples NdBa2−xLaxCu3O7−δ with 0⩽x⩽0.30 have been studied in the normal state by X-ray diffraction, the resistivity, and the thermoelectric power measurements. La doping results in a larger normal state resistivity, the thermoelectric power, a lower critical temperature Tc, a larger pseoudogap temperature Tg, and a change from a metal-like to a semiconductor-like behavior at low temperature and high doping concentration. The strong increase in the room temperature resistivity for x ⩾0.05 suggested that hole filling picture is not only mechanism to decrease hole concentration but there is also another mechanism such as mobile hole localization. The results of the resistivity as a function of temperature and doping concentration was analyzed within bipolaron model while the thermoelectric power was analyzed within a phenomenological narrow band model. A good agreement between models and data were obtained. An increased tendency for localization is found within both models. Therefore, the results of both models suggested that both hole localization and hole filling mechanisms decrease the hole concentration.

Apical oxygen, 3D‐2D cross over and superconductivity in Sm2–xCexCuO4–δ

AbstractIn spite of the vast amount of experimental and theoretical articles accumulated in HTSC, the mechanism of the interaction driving charge carriers to form Cooper pairs below Tc is still unknown. The comparison of the normal state transport properties of YBa2Cu3O7–δ and the Sm2–xCexCuO4–δ [1, 2] might shed some light on the microscopic origin of HTSC. In comparison to the YBCO, the apical oxygen in Sm2–xCexCuO4–δ [3] destroys the superconductivity via the vertical ionic bonding which localizes the charge in the Cu‐O squares, however the hole transfer by moving O(4) towards the CuO2 planes, leads to the optimization of YBCO properties. The behaviour of C axis parameter vs the oxygen content cannot be explained by a BSC mechanism. The high amount of anisotropy ratio [4] is explained by the sheer square planes in NCCO system, i.e. without apical oxygen (SC with Tc maximum). From the data of the resistivity in the normal state, we conclude the observation of a 3D‐2D cross over only in Sm2–xCexCuO4–δ [2] and Nd2–xCexCuO4–δ [5] which is also related to its high anisotropy. The competition between anisotropy and superconductivity destroys the superconducting state in the 2D limit even in the ground state. In this material the superconductivity cannot be enhanced at high temperature because the compound is a quasi 2D system (sheer square planes of CuO2) and the cuprate superconductors is a genuine three‐dimensional (3D) phenomenon [6]. The Josephson coupling between the different layers is S‐I‐S for NCCO and S‐N‐S for YBCO, thus the Lawrence and Doniach model (LD) [7] with neighbouring layers coupled by the Josephson tunnelling is appropriate. In summary the behaviour of apical oxygen is intrinsically different in the two kinds of cuprates. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Superconducting and normal state transport properties of epitaxial 10 at.% Ca substituted YBCO thin films

The improvement of the critical current density in YBCO coated conductor tapes necessarily concerns the study of grain boundary (GB) properties and the study of different pinning mechanisms involving both GB and the interior of the grain (IG). Recently, it has been proposed that the introduction of Ca substituting Y enhances Jc in YBCO grain boundaries (GB). On the other hand enhanced correlated pinning, as observed in YBCO thin films grown on CeO2 buffered substrates, has been ascribed to elongated defects along the c-axis of the film. In order to evaluate the role of calcium and CeO2 buffer layer on the superconducting transport properties of YBCO films, critical current density measurements as a function of temperature, magnetic field and angle between magnetic field direction and c-axis of the film have been performed on epitaxial thin films of unsubstituted and 10at.% Ca substituted YBCO grown on CeO2 buffered Al2O3 single crystal substrates. Results are compared with data obtained on 10at.% Ca substituted YBCO grown on SrTiO3 single crystal substrate.

Layer by layer approach to transport in noncollinear magnetic structures

By now a common method to solve for transport phenomena in a multilayered structure is to solve the equations of motion in each layer and use boundary conditions to match the solutions across interfaces. This has worked quite well for magnetic multilayers as long as the magnetic layers are collinear, however, when they are noncollinear one has to introduce an artifice to account for correlations between states in different layers that are left out in the layer by layer approach. We have calculated the steady state transport properties, i.e., the charge and spin currents, spin torque, and resistance, as a function of the angle between two magnetic layers separated by a normal (nonmagnetic) spacer, by using the Boltzmann equations of motion for spin transport in the layers and by including current induced spin flip scattering at interfaces (the artifice). We compare our results to those found when one does not include the artifice at the interfaces, and find they agree only at times before which the current driven spin accumulation in the spacer layer has been able to couple the scattering at its interfaces with the magnetic layers.

Thermoelectric power in Bi 2Sr 2− xK xCaCu 2O y

The superconducting and normal state transport properties of ceramic samples with composition Bi 2Sr 2− x K x CaCu 2O y ( x = 0, 0.1, 0.2, 0.3, 0.4) have been investigated by measuring thermoelectric power. The samples were prepared by the solid state route, and single phase Bi-2212 was found for all compositions. The T c(0) and thermoelectric power data were collected from 50 to 300 K. The thermoelectric power changes slightly with increasing doping level and increases linearly with decreasing temperature reaching a maximum value at T c(0). The T c(0), hole concentration and thermoelectric data were correlated. The results obtained are analysed in terms of phenomenological theoretical models that makes it possible to determine the band spectrum parameters in the normal state and to trace their changes with varying composition. Following models applied such as (a) Hubbard model, (b) two-band model, (c) Fermi-liquid behaviour. Applicability of these models to the present HTSC system is discussed and the parameters related to the models have been evaluated.

Electroluminescent devices based on cross-linked polymer blends

We report the electrical and optical properties of two-component blends of electron and hole transporting materials in single and bilayer structures for organic light emitting diode (OLED) applications. The materials considered were a blue-emitting bipolar transporting polyfluorene, poly(9,9-di-n-hexylfluorene) (DHF), and a hole-transporting material, poly-[4-nhexyltriphenylamine] (HTPA). We compare the steady state OLED performance, transport, and optical properties of devices and describe morphology studies of the polymer films based on cross-linkable (x) blends with the analogous non-cross-linkable blends. The cross-linkable blends exhibit highest efficiency at low concentrations of the hole transporting material. At these concentrations the single layer OLEDs reach efficiencies greater than 0.1%, and are higher than for single layer x-DHF or the binary non-cross-linkable blend by more than an order of magnitude. Bilayer structures with homogeneous x-HTPA as hole transport layer show efficiencies between 0.08% and 0.96%, depending on the blend concentration in the emitting layer and on the top contact. We interpret these results in terms of the relative degree of phase segregation in the cross-linked networks versus the non-cross-linkable blends.

The normal state scattering rate in high- cuprates

I present a new phenomenological model for the normal state transport properties of high-Tc cuprates. In particular, I identify a form of scattering rate that may account, qualitatively and quantitatively, for the normal state (magneto)-transport properties of Tl2Ba2CuO6+\delta and Bi2Sr2CaCu2O8+\delta from optimal doping through to the overdoped side of the phase diagram. The form of the scattering rate is also consistent with features seen in photoemission spectroscopy in Bi2Sr2CaCu2O8+\delta and offers a new intuitive way to understand the evolution of the temperature dependence of the inverse Hall angle with disorder and with carrier concentration.

Normal state transport properties of MgB 2

The transport properties of magnesium diboride in the normal state are briefly reviewed. The temperature dependence of resistivity is explained by taking into account the contribution of E 2g optical phonon mode. From the sign of Hall coefficient, thermoelectric power, the type of predominant carriers is determined for each direction. In the Hall coefficient, the multiband effect is clearly observed, as a strong anisotropy between in-plane and out-of-plane component. In the thermoelectric power, a phonon contribution to thermal transport manifests itself.

Electron localization induced by grain boundary disorder in the normal state of high- Tc superconductors

We have studied the effects of superconducting grain boundary disorder on the normal state transport properties of cuprate films. Dip-coated granular YBa 2Cu 3O 7− y (YBaCuO) thick films on polycrystalline MgO substrates were synthesized and networked grains were systematically made less disordered in order to probe the crossover from strong to weak inter-grain disorder. Grain boundary passivation was achieved by metallic inclusions of different forms. We have shown that the normal state of samples exhibit a semiconducting behavior and changes to ‘metallic’ with sharper transitions to the superconducting state as we reduce grain-interfaces disorder, i.e. increase metallic inclusion content. On the basis of electron localization mechanisms, the normal state conductivity is thus shown to undergo a dimensional crossover from 3D to 2D in the frame of the variable-range hopping (VRH) regime. The transition threshold was found to depend on the form of metallic inclusions.

Local balance of electron energy gain and loss in SST condition

AbstractSpatially resolved steady state (SST) transport properties of electrons in Ne including the electron flux and the energy flow under a high reduced electric field E/N of 300 Td are investigated with the numerical data obtained by SST‐FTI method. It is confirmed that the local net energy gain is always the same as the sum of the local energy loss and the divergence of energy flow. That is, the local conservation of energy and the continuity of energy flow are both explicitly verified. The features of SST and PT (pulsed Townsend) transport properties in conservative and nonconservative conditions for the electron number are also discussed. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 140(4): 1–8, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10000

Impurity and strain effects on the magnetotransport ofLa1.85Sr0.15Cu1−yZnyO4films

The influence of zinc doping and strain related effects on the normal state transport properties(the resistivity, the Hall angle and the orbital magneto- resistance(OMR) is studied in a series of La1.85Sr0.15Cu(1-y)Zn(y)O4 films with values of y between 0 and 0.12 and various degrees of strain induced by the mismatch between the films and the substrate. The zinc doping affects only the constant term in the temperature dependence of cotangent theta but the strain affects both the slope and the constant term, while their ratio remains constant.OMR is decreased by zinc doping but is unaffected by strain. The ratio delta rho/(rho*tan^2 theta) is T-independent but decreases with impurity doping. These results put strong constraints on theories of the normal state of high- temperature superconductors.

Multi-patch model for transport properties of cuprate superconductors

A number of normal state transport properties of cuprate superconductors are analyzed in detail using the Boltzmann equation. The momentum dependence of the electronic structure and the strong momentum anisotropy of the electronic scattering are included in a phenomenological way via a multi-patch model. The Brillouin zone and the Fermi surface are divided in regions where scattering between the electrons is strong and the Fermi velocity is low (hot patches) and in regions where the scattering is weak and the Fermi velocity is large (cold patches). We present several motivations for this phenomenology starting from various microscopic approaches. A solution of the Boltzmann equation in the case of N patches is obtained and an expression for the distribution function away from equilibrium is given. Within this framework, and limiting our analysis to the two patches case, the temperature dependence of resistivity, thermoelectric power, Hall angle, magnetoresistance and thermal Hall conductivity are studied in a systematic way analyzing the role of the patch geometry and the temperature dependence of the scattering rates. In the case of Bi-based cuprates, using ARPES data for the electronic structure, and assuming an inter-patch scattering between hot and cold states with a linear temperature dependence, a reasonable agreement with the available experiments is obtained.

Anomalous superconductivity in bismuthates

We report the anomalous superconductivity found in bismuthates with different compositions. In the Sr 1− x K x BiO 3 (SKBO) compound the field-induced superconductivity is observed, which shows a striking suppression of the reentrant resistance at T< T c by applying an external magnetic field ( H) or increasing the electrical transport current ( I). The magnetotransport measurements on several different samples reveal that the reentrant resistance behavior with H and I is related to the normal state transport properties. Disordered junction barriers between superconducting grains seem to be important for the observed anomalous reentrant resistance phenomena. In the Ba 1− x K x BiO 3 system, the whole region of the superconducting cubic phase diagram from x=0.4 up to x=1 was investigated for the first time by using the high pressure and high temperature synthesis technique. The superconducting temperature ( T c) of Ba 1− x K x BiO 3 (BKBO) samples shows a parabolic decrease of T c from the well-known Ba 0.63K 0.37BiO 3 composition with T c=30.4 K to Ba 0.24K 0.76BiO 3 with very low diamagnetic signal at nearly 3 K. However, we found that further increasing x gives an increase of T c up to 9 K for x=1. Possible physical origins of the field-induced superconductivity in SKBO and the anomalous superconducting region in high potassium concentration ( x>0.8) BKBO are discussed.

Surface and bulk normal state transport properties in K(3)C(60).

The temperature dependence of the surface resistivity for a metallic K(3)C(60) ordered film in the nonsuperconducting state has been obtained by reflection electron energy loss spectroscopy. We demonstrate that the normal state electronic and transport properties of the top molecular layer of K(3)C(60) are similar to the corresponding properties measured with bulk sensitive techniques. These observations strengthen and give a general character to the experimental results obtained with surface sensitive techniques on fullerene compounds. In addition, the transport properties may deviate from the Fermi-liquid behavior above 500 K.