Do electrons change their c-axis kinetic energy upon entering the superconducting state?

The interlayer tunneling mechanism of the cuprate high temperature superconductors involves a conversion of the confinement kinetic energy of the electrons perpendicular to the CuO-planes (c-axis) in the normal state to the pair binding energy in the superconducting state. This mechanism is discussed and the arguments are presented from the point of view of general principles. It is shown that recent measurements of the c-axis properties support the idea that the electrons substantially lower their c-axis kinetic energy upon entering the superconducting state, a change that is nearly impossible in any conventional mechanism. The proper use of a c-axis conductivity sum rule is shown to resolve puzzles involving the penetration depth and the optical measurements. Received: 5 January 1998 / Accepted: 17 March 1998     

Optical and dc conductivities in high-T c superconductors: Spin-fluctuation scattering in the Emery model

On the basis of the Emery model for the CuO₂ plane, the optical conductivity and resistivity due to the inelastic scattering of oxygen holes by antiferromagnetic fluctuations of copper spins are calculated. For moderate hole doping, the electrical conductivity obeys a generalized Drude law. Using a phenomenological model for the dynamic spin susceptibility, the in-plane resistivity reveals a crossover from a quadratic to a linear temperature dependence at the scale of the spin-fluctuation energy. The frequency dependence of the scattering rate changes from a quadratic to a linear increase over a wide frequency range. The theory is compared with experiments on LSCO and YBCO compounds, where the spin dynamics is described within the model by Millis et al. A good quantitative agreement (in particular of the frequency-dependent scattering rate) with experiments is found. We conclude that the spin-fluctuation scattering may play a dominant role in the transport properties of Cu-oxide superconductors.     

Magnetic fluctuations in coupled inequivalent Hubbard layers as a model for

We investigate, within the fluctuation-exchange approximation, a correlated-electron model for represented by two inequivalent Hubbard layers coupled by an interlayer hopping . An energy offset is introduced in order to produce a different charge carrier concentration in the two layers. We compare several single-particle and magnetic excitations, namely, the single particle scattering rate, the spectral function and the spin lattice as well as spin-spin relaxation times in the two layers as a function of . We show that the induced interlayer magnetic coupling produces a tendency to “equalization” of the magnetic properties in the two layers whereby antiferromagnetic fluctuations are suppressed in the less doped layer and enhanced in the heavily doped one.The strong antiferromagnetic bilayer coupling causes the charge carriers in the plane with larger doping concentration to behave similar to those of the underdoped layer, they are coupled to. This effect grows for decreasing temperature. For high temperatures or if both layers are optimally or overdoped, i.e. when the antiferromagnetic correlation length becomes of the order or smaller than one lattice site the charge carrier and magnetic dynamics of the two layers is disconnected and the equalization effect disappears. These results are in good agreement with NMR experiments on by Stern et al. [Phys. Rev B 51, 15478 (1995)]. We also compare the results with calculations on bilayer systems with equivalent layers as models for the constituent compounds and . Received: 28 August 1998     

Indications of c-axis Charge Transport in Hole Doped Triangular Antiferromagnets

The c-axis charge transport of the hole doped triangular antiferromagnet is investigated within the tJ model by considering the incoherent interlayer hopping.It is shown that the c-axis charge transport of the hole doped triangular antiferromagnet is essentially determined by the scattering from the in-plane fluctuation.The c-axis conductivity spectrum shows a lov-energy peak and the unusual high-energy broad band,while the c-axis resistivity is characterized by a crossover from the high temperature metallic-like behavior to the Iow temperature insulating-like behavior,which is qualitatively consistent with those of the hole doped square lattice antiferromagnet.     

Effects of uniaxial pressure and annealing on the resistivity of Ba(Fe1−xCox)2As2

Single crystals of underdoped Ba(Fe_1−xCo_x)₂As₂ were detwinned by applying uniaxial pressure. The anisotropic in-plane resistivity was measured using the Montgomery method without releasing pressure. The resistivity along the a-axis shows metallic behavior down to 5 K, while the resistivity along the b-axis shows an insulator-like behavior in some temperature range. Annealing the sample radically reduces the residual resistivity for x=0, and at the same time the anisotropy becomes much smaller at low temperatures.     

Synthesis and microwave properties of TI2Ba2CaCu2O8 superconducting films grown by MOCVD

We report on the synthesis, structural and electrical characterization of high quality Tl₂Ba₂Ca₁Cu₂O₈ (Tl-2212) superconducting films. The samples have been grown ex-situ on mm² LaAlO₃ (100) substrates by a combined approach of metal-organic chemical vapor deposition (MOCVD) and thallium vapor diffusion. The morphological and compositional nature of the c-axis oriented films has been investigated by SEM and X-ray analyses. Typical values of K and MA/cm² at 77 K have been measured. Microwave measurements have been performed at f = 87 GHz inserting the film in a copper cavity and at f =1.5 GHz on patterned samples using a microstrip resonator technique. A penetration depth nm is evaluated by fitting the microwave data with phenomenological equations. The minimum value of the surface resistance measured at 4.2 K is 60 and 6 m at 1.5 GHz and 87 GHz respectively. The microwave data are described in the context of a modified two fluid model. An evaluation of the temperature dependence of the scattering rate has been performed through the simultaneous measurement of the surface resistance and the penetration depth. Received 16 December 1999 and Received in final form 17 March 2000     

Spectral properties of incommensurate charge-density wave systems

The concept of frustrated phase separation is applied to investigate its consequences for the electronic structure of the high T _c cuprates. The resulting incommensurate charge density wave (CDW) scattering is most effective in creating local gaps in k-space when the scattering vector connects states with equal energy. Starting from an open Fermi surface we find that the resulting CDW is oriented along the (10)- and (or) (01)-direction which allows for a purely one-dimensional or a two-dimensional “eggbox type” charge modulation. In both cases the van Hove singularities are substantially enhanced, and the spectral weight of Fermi surface states near the M-points, tends to be suppressed. Remarkably, a leading edge gap arises near these points, which, in the eggbox case, leaves finite arcs of the Fermi surface gapless. We discuss our results with repect to possible consequences for photoemission experiments. Received 14 June 1999     

Metastable phenomena in lutetium nitrate crystal

The ac conductivities along the c-axis of the lutetium nitrate crystal were measured successively in the frequency range from 0.5 to 10⁵ Hz at temperatures from 290 to 210 K. The metastable behaviors of the conductivity were found in the temperature region above 200 K. The fluctuation was given by the successive measurements of the time series of the ac conductivities and represented by non-Gaussian probability distribution function. The effect of the metastable singularity on Raman scattering spectra was observed in the metastable temperature region.     

Fluctuation conductivity in layered d-wave superconductors near critical disorder

We consider the fluctuation conductivity in the critical region of a disorder induced quantum phase transition in layered d-wave superconductors. We specifically address the fluctuation contribution to the systems conductivity in the limit of large (quasi-two-dimensional system) and small (quasi-three-dimensional system) separation between adjacent layers of the system. Both in-plane and c-axis conductivities were discussed near the point of insulator-superconductor phase transition. The value of the dynamical critical exponent, z = 2, permits a perturbative treatment of this quantum phase transition under the renormalization group approach. We discuss our results for the system conductivities in the critical region as function of temperature and disorder.Received: 10 October 2003, Published online: 23 December 2003PACS: 74.40. + k Fluctuations (noise, chaos, nonequilibrium superconductivity, localization, etc.) - 73.43.Nq Quantum phase transitions     

Aqueous mixtures of spherical and rodlike (flexible) macroparticles: structure and dynamics of latex spheres in saltfree solutions

This paper resumes light scattering investigations of saltfree aqueous solutions of two component mixtures of charged spheres by extending those measurements to systems in which one component is replaced by essentially stiff rodlike particles. In a second step of investigations these were replaced by linear flexible particles. Fd-virus particles (length l=883 nm) or macromolecules of NaPSS of four different contour lengths have been used as representatives. Mostly the concentration of latex spheres was fixed at 0.02 Vol%. The concentration of the other component was varied over a wide range. Concerning the scattering intensity, the contribution of the latex spheres dominates, in particular in the systems containing NaPSS particles. This simplifies the interpretation of data considerably. A rearrangement of the spheres is observed, depending on the shape of the other sort of particles. These conclusions can be drawn from the shift of measured static structure factor with concentration c. A power law is found for the q-value of the maximum. The exponent depends on the properties of the second component. For the lower molecular weight (MW) samples of NaPSS below a critical concentration, the exponent is smaller than 1/3, decreasing the more the smaller the MW of the samples is. A tentative explanation in terms of charge number of NaPSS particles is given. The short time dynamics has been explored too. From the data a “dynamically determined structure factor” can be derived, that can be compared with the measured static structure factor. Good (fd) and fair (NaPSS) agreement is obtained respectively. Only at small wavenumbers below the maximum of deviations occur which increase with concentration; they are consistent with hydrodynamic interaction. Received 30 July 1998 and Received in final form 14 December 1998     

Fluctuation conductivity along the c-axis and parallel to the ab-planes in melt-textured YBa2Cu3O7−δ samples doped with Y211 phase

We report measurements of in-plane and out-of-plane fluctuation conductivity under low applied magnetic fields up to 500 Oe in two melt-textured YBa₂Cu₃O_7−δ (YBCO) samples. 3D-Gaussian and genuine critical 3D-XY-E fluctuation regimes were identified in the conductivity parallel to the ab plane. In addition, a regime beyond 3D-XY was observed in the immediate vicinity of the superconducting transition of the in-plane fluctuation conductivity. The 3D-XY-E scaling was also identified in the fluctuation conductivity along the c-axis in the sample with smaller content of the Y₂BaCuO₅ (Y211) phase. This result indicates that the superconducting state in YBCO has a three-dimensional character, in contradiction to some studies suggesting that critical phenomenology is fundamentally distinct in orientations parallel or perpendicular to the Cu-O₂ planes of the high-temperature superconductors. However, the results suggests the presence of a sub-dominant order-parameter component in YBCO that has an appreciable projection along the c-axis.     

Coulomb drag experiments in low density 2D hole bilayers

We performed experiments studying the Coulomb drag in low density 2D hole bilayers, with r_s ranging from roughly 10 to 20. As the carrier density is lowered into the dilute regime, we observe a significant enhancement of the drag resistivity, such that the interlayer carrier–carrier scattering rate constitutes a major component of the single layer resistivity. In addition, anomalies to the expected temperature and in-plane magnetic field dependences are observed, and are found to correlate with similar anomalies in the single layer resistivity. These results suggests that the origin of the 2D metal–insulator transition phenomena affects both transport properties in a very similar fashion.     

A dynamic scattering approach for a gated interacting wire

A scattering approach for correlated one-dimensional systems is developed. The perfect contact to charge reservoirs is encoded in time-dependent boundary conditions. The conductance matrix for an arbitrary gated wire, respecting charge conservation, is expressed through a dynamic scattering matrix. Two applications are developed. First, it is shown that the dc conductance is equal to e ²/h for any model with conserved total left- and right-moving charges. Second, the ac conductance matrix is explicitly computated for the Tomonaga-Luttinger model (TLL). Received 31 August 1998     

Magnetization dynamics below of EuO and EuS

Between 4.2 K and the Curie temperatures of the cubic Heisenberg ferromagnets EuS and EuO, their homogeneous dynamic susceptibilities have been investigated by means of a broad-band reflectometer operating from 0.1 GHz to 40 GHz. For internal magnetic fields larger than the anisotropy fields H A ( T ) of both materials, their static susceptibilities exhibit a -divergence, which reveals quantitatively the dominance of dipolar-anisotropic spin-wave fluctuations. displays a Lorentzian shape the damping frequency of which obeys scaling in terms of .The scaling function agrees quantitatively with work by Frey and Schwabl [#!FS88!#] for dipolar Heisenberg ferromagnets at temperatures above T_c. Building upon their approach, the resonance frequency of the Lorentzian can be related to a memory effect in the damping determined by the large value of the relaxation rate of the longitudinal magnetization fluctuations . For EuS, this relation is substantiated directly by inelastic neutron scattering. All these features reveal the hitherto uncovered importance of the dipolar anisotropic fluctuations below T_c of ferromagnets. Received: 4 March 1998 / Accepted: 12 May 1998     

Small-angle X-ray scattering from salt-free solutions of star-branched polyelectrolytes

The dispersion state of sodium-sulphonated polystyrene ( NaPSS) star-branched polyelectrolytes was investigated in salt-free aqueous solutions, by use of the small-angle X-ray scattering technique. With respect to polystyrene (PS) star-branched polymers of identical functionality, the ordering phenomenon occurring in the neighborhood of the overlap concentration c^* is reinforced and observed in a larger range of concentrations. Moreover, the degree of order is no longer maximum at c^* and is improved as the concentration decreases. The dispersion state is then mainly controlled by the electrostatic interaction. A crystalline order should therefore be achieved with stars of lower functionality, provided the electrostatic interaction is added to the osmotic repulsion. On the other hand, unusual scattering patterns are measured for aqueous solutions of NaPSS star polyelectrolytes. Indeed, a diffuse scattering is revealed at high angles, in addition to the regular diffraction rings related to preferred interstar distances. It is similar to the broad scattering peak produced by semidilute solutions of NaPSS linear polyelectrolytes and associated to the electrostatic correlation hole within the isotropic model. In the dilute regime (c < c ^*), it is just an intramolecular characteristic and represents the electrostatic repulsion between arms belonging to the same star. In the semidilute regime (c > c ^*), it also reflects the electrostatic repulsion between arms of distinct stars. So, as the concentration increases, it is mainly caused by the interpenetration of NaPSS stars. Such an observation is in agreement with the composite structure earlier proposed by Daoud and Cotton for star semidilute solutions. For c > c ^*, NaPSS star aqueous solutions can therefore be pictured as effective stars immersed in a matrix formed by the overlap of the arm ends. With respect to the dilute regime, the effective stars are smaller; the higher the concentration the smaller the size. Received 14 May 1999 and Received in final form 15 March 2000     

Structural properties of high-quality sputtered Fe films on Al2O3(1120) and MgO(001) substrates

High-quality thin Fe films were deposited on MgO(001) and Al₂O₃(1120) substrates in the thickness range from 7 to 50 nm. The structural properties have been studied by out-of-plane and in-plane X-ray scattering experiments. From the out-of-plane measurements the electron density profile was determined together with interface and surface roughness parameters. Fe on Al₂O₃ grows along the [110]-direction with a structural coherence length comprising about the total film thick ness and a very small mosaicity. From in-plane scattering experiments a three-domain structure was observed. On MgO(001) substrates Fe grows in the [001]-direction, with the Fe [100]-axis parallel to the MgO [110]-axis. On both substrates, the Fe films exhibit a very small surface and interface roughness, indicative for a high quality of the sputtered samples.     

Simulation study of the anomalous Hall effect in high-T c superconductors

Including influence of the thermal fluctuation on flux motion in the Wang-Ting model, we use the numerical simulation method to investigate the anomalous Hall effect in high-T _c superconductors. The negative Hall resistivity has still been found in a certain range of temperature at low magnetic fields, which is qualitatively consistent with the experimental observations. Our results support the view point that the negative Hall effect is caused, most probably, by the flux motion with the pinning.     

Doping effects arising from Ni for Fe in PrFeO3 ceramic thin films

Structural, morphological and transport properties of PrFe_{1? x} Ni _x O₃ (x?=?0.1, 0.2, 0.3, 0.4 and 0.5) thin films grown on LaAlO₃ substrate by pulsed laser deposition were studied experimentally. Structural analysis of the samples showed that they have in-plane compressive strain and single-phase epitaxial growth along with c-axis (001) orientation having orthorhombic structure with space group Pbnm. The observed strain is reduced with Ni substitution. The resistivity as a function of temperature follows the variable range hopping (VRH) model up to certain amount of Ni substitution (x?=?0.3) but fails for higher values of x. From the above model, parameters such as density of states at the Fermi level, N(E _F), hopping energy, E _h, and hopping distance R _h, were calculated. Ni substitution leads to an increase in conductivity and this conduction is controlled by disorder-induced localization of charge carriers. With Ni substitution the gap parameter is found to decrease. The enhancement in conductivity and the failure of VRH model for higher doped compositions at high temperature is discussed.