Normal-state Transport Properties Research Articles

Unusual Hall effect in quasi two-dimensional strongly correlated metal CeCoIn5

We report on the systematic study of Hall effect in quasi two-dimensional strongly correlated metal CeCoIn5, ranging from non-Fermi liquid to Fermi liquid regime by using pressure as a tuning parameter. We found that unusual normal-state transport properties in CeCoIn5 bear a striking resemblance to that in high-Tc cuprates. We also found that at ambient pressure in the vicinity of quantum critical point (QCP), the magnitude of Hall coefficient RH dramatically increases with decreasing temperature. The enhancement of RH is strongly suppressed by applying pressure and magnetic field. These results indicate that the anomalous transport properties are governed by anisotropic scattering due to strong spin fluctuations in the vicinity of the QCP.

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 & 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.

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.

A study of aging effects in YBa 2Cu 3O 7− y films by means of AC susceptibility and paraconductivity

Superconducting and normal-state transport properties are investigated for YBa 2Cu 3O 7− y (YBCO) films with different T c values (80–90 K), obtained by thermal cycling and aging processes. Film properties are characterized mainly by means of AC susceptibility and paraconductivity analyses, and also by the measurements on the X-ray diffractometry and the normal-state resistivity. The imaginary part of the AC susceptibility normalized by the magnitude of its real part is found to provide a criterion on the sample quality. The paraconductivity data are analyzed in terms of the Aslamazov–Larkin theory with and without cutoff effects in the fluctuation spectrum, from which we show that anisotropic and cutoff parameters are correlated with T c values. The T c of the films is also well correlated with the room-temperature resistivity and c-axis lattice parameter. The overall features of our results are consistent with the behavior observed in oxygen-controlled YBCO films.

Pressure-induced electronic topological transitions in low dimensional superconductors**EHPRG Award Lecture.

The high-Tc cuprate superconductors are characterized by a quasi-two-dimensionallayered structure where most of the physics relevant forhigh-Tc superconductivity is believed to take place. In such compounds, the unusual dependence of the criticaltemperature Tc on external pressure results from the combination of the nonmonotonic dependence ofTc on hole doping or hole-doping distribution among inequivalent layers, and froman ‘intrinsic’ contribution. After reviewing our work on the interplay amongTc, hole content, and pressure in the bilayered and multilayered cuprate superconductors,we will discuss how the proximity to an electronic topological transition (ETT)may give a microscopic justification of the ‘intrinsic’ pressure dependence ofTc in the cuprates. An ETT takes place when some external agent, such as doping,hydrostatic pressure, or anisotropic strain, modifies the topology of theFermi surface of an electronic system. As a function of the critical parameterz, measuring the distance of the chemical potential from the ETT,we recover a nonmonotonic behaviour of the superconducting gap atT = 0,regardless of the pairing symmetry of the order parameter. This is in agreement with the trend observedfor Tc as a function of pressure and other material specific quantities in severalhigh-Tc cuprates and other low dimensional superconductors. In the case of epitaxiallystrained cuprate thin films, we argue that an ETT can be driven by a strain-inducedmodification of the in-plane band structure, at constant hole content, atvariance with a doping-induced ETT, as is usually assumed. We also find thatan increase of the in-plane anisotropy enhances the effect of fluctuations aboveTc on the normal-state transport properties, which is a fingerprint of quantum criticality atT = 0.

Correlations between transport coefficients and electronic structure for 1-2-3(Ca0.1La0.9)(Ba1.65La0.35)Cu3Oy superconductors

Normal-state transport properties of(Ca0.1La0.9)(Ba1.65La0.35)Cu3Oy ceramic superconductors have been studied. The oxygen content(y) was varied from 6.75 to 7.23 by annealing in atmospheres of various oxygen partialpressures (up to 1330 bar). That way the concentration of mobile charge carriers wasvaried throughout the entire doping range. The most reduced sample in our series(y = 6.75) is non-superconducting. The first superconducting, underdoped sample(y = 7.00) has a criticaltemperature of Tc = 16.9 K.The Tc rises furtherto 30.1 K for y = 7.10 (optimal doping) and then continuously drops to 8.8 K fory = 7.23 (overdoped region).We present temperature dependences of the resistivity(ρ), thermoelectricpower (S) andHall coefficient (RH), and their interpretation in terms of a simple metal theory.We have found a correlation between the transition temperature(Tc) and the density of the electronic states (EDOS) at the Fermi surface(gEF). This result agrees with previous calculations and experiments.

Through-thickness superconducting and normal-state transport properties revealed by thinning of thick film ex situ YBa2Cu3O7−x coated conductors

A rapid decrease in the critical current density (Jc) of YBa2Cu3O7−x (YBCO) films with increasing film thickness has been observed for multiple YBCO growth processes. While such behavior is predicted from two-dimensional collective pinning models under certain assumptions, empirical observations of the thickness dependence of Jc are believed to be largely processing dependent at present. To investigate this behavior in ex situ YBCO films, 2.0- and 2.9-μm-thick YBCO films on ion beam assisted deposition-yttria stabilized zirconia substrates were thinned and repeatedly measured for ρ(T) and Jc(H). The 2.9 μm film exhibited a constant Jc[77 K, self-field (SF)] through thickness of ∼1 MA/cm2 while the 2.0 μm film exhibited an increase in Jc(77 K, SF) as it was thinned. Neither film offered evidence of significant dead layers, suggesting that further increases in critical current can be obtained by growing thicker YBCO layers.

Evidence for the coupling between γ-band carriers and the incommensurate spin fluctuations inSr2RuO4

The normal-state transport properties are investigated for ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ both in and out of the ${\mathrm{RuO}}_{2}$ plane. It is shown that a quantitatively consistent explanation of ${\ensuremath{\rho}}_{\mathrm{ab}}$ and ${\ensuremath{\rho}}_{c}$ can be obtained by assuming that, in addition to the normal scattering of electrons by impurities and the electron-electron scattering, there is a strong coupling between the carriers of \ensuremath{\gamma} band and the incommensurate spin fluctuations peaked at ${\mathbf{Q}}_{i}=(\ifmmode\pm\else\textpm\fi{}0.6,\ifmmode\pm\else\textpm\fi{}0.6)\ensuremath{\pi}/a.$ In order to test this model, we suggest a pressure-dependent resistivity measurement to be performed.

Effects of proximity to an electronic topological transition on normal-state transport properties of thehigh−Tcsuperconductors

Within the time dependent Ginzburg-Landau theory, the effects of the superconducting fluctuations on the transport properties above the critical temperature are characterized by a non-zero imaginary part of the relaxation rate gamma of the order parameter. Here, we evaluate Im gamma for an anisotropic dispersion relation typical of the high-Tc cuprate superconductors (HTS), characterized by a proximity to an electronic topological transition (ETT). We find that Im gamma abruptly changes sign at the ETT as a function of doping, in agreement with the universal behavior of the HTS. We also find that an increase of the in-plane anisotropy, as is given by a non-zero value of the next-nearest to nearest hopping ratio r=t'/t, increases the value of | Im gamma | close to the ETT, as well as its singular behavior at low temperature, therefore enhancing the effect of superconducting fluctuations. Such a result is in qualitative agreement with the available data for the excess Hall conductivity for several cuprates and cuprate superlattices.

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.

Transport and magnetic properties in MgB2 single crystals

We report measurements of the transport and the magnetic properties of high-quality MgB 2 single crystals with clear hexagonal-plate shapes. The Debye temperature of Θ D ∼ 1100 K, obtained from the zero-field resistance curve, suggests that the normal-state transport properties are dominated by electron-phonon interactions. The resistivity ratio between 40 K and 300 K is about 5. The superconducting anisotropy, γ, increases from a value around 2 near T c to about 3.3 at 26 K. The low-field magnetization and the magnetic hysteresis curves show the bulk pinning of these crystals to be very weak.

The Transport Properties of Li (1+x) Ti 2 O 4 in High Magnetic Field

The spinel oxide superconductor LiTi 2 O 4 shows intricate transport properties in both normal state and superconducting state. The Hall effect measurements exhibit an abrupt change in carrier density near T = 170K where the temperature derivative of resistivity changes sign from negative to positive on cooling. In addition, the sample shows significant positive magnetoresistance below 40K, e.g., ΔR(H)/R(H = 0) ∼ 3% at T = 14K and H = 16T. A most interesting feature is the magnetic field effect on the resistivity near the superconducting state. In addition to the field-induced broadening of R(T), there exhibits an anomalous upturn in R(T) near the superconducting regime when magnetic field is above 3T. The temperature T 1 at which R(T) shows upturn linearly correlates with magnetic field H and can be expressed as T 1 /T C(H=0) = 1 - 0.04H for 3T < H < 12T. The superconductivity is completely suppressed at field above 14T and the low temperature resistance behaves semiconductor-like.

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.

Normal-state dielectric and transport properties of In-doped Bi–Pb–Sr–Ca–Cu–O

The effect of indium doping on the normal-state transport and dielectric properties of the Bi 1.84Pb 0.34Sr 1.91Ca 2.03Cu 3.06In x O y (0≤ x≤0.15) has been investigated. The dielectric constant, loss tangent, carrier concentration and electrical resistivity were measured at room temperature as a function of indium concentration. The dielectric constant is found to increase with increasing dopant concentration possibly due to increase in polarization. Impurities in any crystalline lattice generally cause deformation of the surrounding volume and modification in the local fields. The loss tangent of the indium doped-system decreased with increasing dopant concentration. Decrease in the loss tangent with increase in the dopant concentration indicates the decrease in the dielectric losses in the sample during doping. The carrier concentration is found to decrease with increasing indium concentration. This could be due to charge imbalance, which leads to a decrease in the number of holes in the system. Consequently, the electrical resistivity at 50 °C increased with the dopant concentration. Also, above x=0.05 dopant concentration there was a rapid rise in electrical resistivity, possibly due to disorder in the Cu–O 2 planes. The temperature dependence of the electrical resistivity above room temperature is consistent with metallic behavior.

Effect of Ag admixture on the electron localization mechanism in the normal state of dip-coated YBa 2Cu 3O 7− δ thick films

The effects of superconducting grain boundary disorder on the normal-state transport properties of cuprate films was investigated. Dip-coated granular YBa 2Cu 3O 7− δ (YBaCuO) thick films on polycrystalline MgO substrates were synthesized and networked grains were systematically made less disordered in order to probe the cross-over from strong to weak inter-grain (between grains) disorder. Grain boundary passivation was achieved by AgNO 3 metallic inclusions. The normal state of samples was shown to exhibit a semi-conducting behaviour and then changes to metallic with sharper transitions to the superconducting state as the grain-interfaces disorder is decreased, i.e. increase metallic inclusion content. The normal-state conductivity is thus shown to undergo a dimensional cross-over from 3D to 2D in the frame of the variable range-hopping regime.

Superconducting properties of well-shapedMgB2single crystals

We report measurements of the transport and the magnetic properties of high-quality, sub-millimeter-sized ${\mathrm{MgB}}_{2}$ single crystals with clear hexagonal-plate shapes. The low-field magnetization and the magnetic hysteresis curves show the bulk pinning of these crystals to be very weak. The Debye temperature of ${\ensuremath{\Theta}}_{D}\ensuremath{\sim}1100 \mathrm{K},$ obtained from the zero-field resistance curve, suggests that the normal-state transport properties are dominated by electron-phonon interactions. The resistivity ratio between 40 K and 300 K was about 5, and the upper critical field anisotropy ratio was $3.0\ifmmode\pm\else\textpm\fi{}0.2$ at temperatures around 32 K.

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.