Effect of strain-induced electronic topological transitions on the superconducting properties of [Formula: See Text] thin films

We propose a Ginzburg-Landau phenomenological model for the dependence of the critical temperature on microscopic strain in tetragonal high-[Formula: See Text] cuprates. Such a model is in agreement with the experimental results for LSCO under epitaxial strain, as well as with the hydrostatic pressure dependence of [Formula: See Text] in most cuprates. In particular, a nonmonotonic dependence of [Formula: See Text] on hydrostatic pressure, as well as on in-plane or apical microstrain, is derived. From a microscopic point of view, such results can be understood as due to the proximity to an electronic topological transition (ETT). In the case of LSCO, we argue that such an ETT can be driven by a strain-induced modification of the band structure, at constant hole content, at variance with a doping-induced ETT, as is usually assumed.     

Evidence for pseudogap and phase-coherence gap separation by Andreev reflection experiments in Au/ La 2 - x Sr x CuO 4 point-contact junctions

We present new Au/La_2-xSr_xCuO₄ (LSCO) point-contact conductance measures as a function of voltage and temperature in samples with 0.08 ? x ? 0.2. Andreev reflection features disappear at about the bulk T _c , giving no evidence of gap for T > T _c . The fit of the normalized conductance at any T < T _c supports a (s + d)-wave symmetry of the gap, whose dominant low-T s component follows the T _c ( x ) curve in contrast with recent angle-resolved photoemission spectroscopy and quasiparticle tunneling data. These results prove the separation between pseudogap and phase-coherence superconducting gap in LSCO at x 0.2. Received 14 June 2001     

Photoinduced IR absorption in WO 3 : determination of the polaron binding energy

Photoinduced IR absorption measurements are reported on WO₃. A photoinduced midinfrared small polaron peak centered at 4800 cm^-1 (0.59 eV) was observed. The data were analyzed in the framework of the photon-assisted small-polaron hopping theory and briefly compared to previously published infrared absorption measurements in WO₃ and photoinduced IR absorption measurements in high T _c cuprates. Received 20 April 2001 and Received in final form 13 July 2001     

Probing electron-phonon coupling in high-Tc superconductors by site-selective isotope substitution

We consider changes in the electron-phonon coupling in high-T _c cuprates caused by site-selective oxygen isotope substitution. Contrary to the total substitution, the site-selective replacement influences the coupling constant for each phonon mode due to the induced changes in the phonon eigenvectors. The relative changes for some modes can be larger than 100%. The measured properties sensitive to these changes are discussed. Received 9 August 2001 and Received in final form 11 January 2002     

High-pressure study of the non-Fermi liquid material U 2 Pt 2 In

The effect of hydrostatic pressure ( p ? 1.8 GPa) on the non-Fermi liquid state of U₂Pt₂In is investigated by electrical resistivity measurements in the temperature interval 0.3-300 K. The experiments were carried out on single-crystals with the current along ( I || c ) and perpendicular ( I || a ) to the tetragonal axis. The pressure effect is strongly current-direction dependent. For I || a we observe a rapid recovery of the Fermi-liquid T²-term with pressure. A comparison of the data with the magnetotransport theory of Rosch provides evidence for the location of U₂Pt₂In at an antiferromagnetic quantum critical point. For I || c the resistivity increases under pressure, indicating the enhancement of an additional scattering mechanism. Received 2 March 2001 and Received in final form 29 June 2001     

A simple theory for high Δ T c ratio in d-wave superconductors

We investigate a simple explanation for the high maximum gap to T _c ratio found experimentally in high T _c compounds. We ascribe this observation to the lowering of T _c by boson scattering of electrons between parts of the Fermi surface with opposite sign for the order parameter. We study the simplest possible model within this picture. Our quantitative results show that we can account for experiment for a rather small value of the coupling constant, all the other ingredients of our model being already known to exist in these compounds. A striking implication of this theory is the fairly high value of the critical temperature in the absence of boson scattering. Received 12 March 2001 and Received in final form 25 May 2001     

Coexistence of superconductivity and spin density wave orderings in the organic superconductor (TMTSF) 2 PF 6

The phase diagram of the organic superconductor (TMTSF)₂PF₆has been revisited using transport measurements with an improved control of the applied pressure. We have found a 0.8 kbar wide pressure domain below the critical point (9.43 kbar, 1.2 K) for the stabilisation of the superconducting ground state featuring a coexistence regime between spin density wave (SDW) and superconductivity (SC). The inhomogeneous character of the said pressure domain is supported by the analysis of the resistivity between T _SDW and T _SC and the superconducting critical current. The onset temperature T _SC is practically constant ( 1.20±0.01 K) in this region where only the SC/SDW domain proportion below T _SC is increasing under pressure. An homogeneous superconducting state is recovered above the critical pressure with T _SC falling at increasing pressure. We propose a model comparing the free energy of a phase exhibiting a segregation between SDW and SC domains and the free energy of homogeneous phases which explains fairly well our experimental findings. Received 3 September 2001 and Received in final form 9 November 2001     

Impurity scattering in f-wave superconductor UPt 3

We study theoretically the effect of impurity scattering in f-wave (or E_2u) superconductors. The quasi-particle density of states of f-wave superconductor is very similar to the one for d-wave superconductor as in hole-doped high T _c cuprates. Also in spite of anisotropy in Δ( ), both the reduced superfluid density and the reduced electronic thermal conductivity is completely isotropic. Received 11 October 2000     

Melting of the cooperative Jahn-Teller distortion in LaMnO 3 single crystal studied by Raman spectroscopy

We have studied the behavior of the Raman phonons of a stoichiometric LaMnO₃ single crystal as a function of temperature in the range between 77 K and 900 K. We focus on the three main phonon peaks of the Pbnm structure, related to the tilt, antisymmetric stretching (Jahn-Teller mode) and stretching modes of Mn-O octahedra. The phonon frequencies show a strong softening that can be fit taking into account their renormalization because of three phonon anharmonic effects in the pseudoharmonic approximation. Thermal expansion effects, in particular the variation of Mn-O bond lengths with temperature, are not relevant above 300 K. On the contrary, phonon width behavior deviates from the three phonon scattering processes well bellow T _c . The correlation between the magnitude of the cooperative Jahn-Teller distortion, that disappears at 800 K, and the amplitude of the Raman phonons in the orthorhombic phase is shown. Nevertheless, Pbnm phonons are still observable above this temperature. Phonon width and intensity behavior around T _c can be explained by local melting of the orbital order that begins quite below T _c and by fluctuations of the regular Mn-O octahedra that correspond to dynamic Jahn-Teller distortions. Received 25 January 2001 and Received in final form 14 March 2001     

Temperature dependence of the YBa2Cu3O7 energy gap in differently oriented tunnel junctions

We have applied the break-junction technique to highly biepitaxial c-axis oriented YBa₂Cu₃O₇ thin films with T _C (ρ=0) = 91 K. Mechanically adjustable junctions with a good stability and tunneling current favored along the ab-planes have been realized. The conductance characteristics of these junctions show the presence of gap related maxima that move towards zero bias for increasing temperatures. Considering the misorientation angle α≈ 45 ^° ± 5 ^° of the junction, a maximum gap value at the Fermi level Δ ≈ 22 meV is inferred at T = 13 K. The temperature dependence of the gap related structures, shows a quasilinear behavior for T > 0.4 T _C similar to that observed in c-axis oriented, S-I-N type YBa₂Cu₃O₇ planar junctions. Received 20 July 2001     

Unconventional antiferromagnetic correlations of the doped Haldane gapsystem Y 2 BaNi 1 - x Zn x O 5

We make a new proposal to describe the very low temperature susceptibility of the doped Haldane gap compound Y₂BaNi_1-xZn_xO₅. We propose a new mean field model relevant for this compound. The ground state of this mean field model is unconventional because antiferromagnetism coexists with random dimers. We present new susceptibility experiments at very low temperature. We obtain a Curie-Weiss susceptibility χ( T ) ∼ C /(Θ + T ) as expected for antiferromagnetic correlations but we do not obtain a direct signature of antiferromagnetic long range order. We explain how to obtain the “impurity” susceptibility ( T ) by subtracting the Haldane gap contribution to the total susceptibility. In the temperature range [1 K, 300 K] the experimental data are well fitted by T ( T ) = C _imp 1 + T _imp / T . In the temperature range [100 mK, 1 K] the experimental data are well fitted by T ( T ) = A ln( T / T _c ), where T _c increases with x. This fit suggests the existence of a finite Néel temperature which is however too small to be probed directly in our experiments. We also obtain a maximum in the temperature dependence of the ac-susceptibility ( T ) which suggests the existence of antiferromagnetic correlations at very low temperature. Received 17 July 2001     

Magnetization and neutron scattering studies of the pressure effect on the magnetic transition in Er Y Co

Neutron powder diffraction was employed to study the pressure effect on the magnetic transition in the pseudobinary Laves-phase compound Er_0.57Y_0.43Co₂ and to determine the magnetic moments of the Er- and Co-subsystems. Our studies reveal that the onset of long-range magnetic order for both the localized 4 f (Er) and itinerant 3 d (Co) electron moments appears at about the same temperature at ambient pressure. The pressure effect on T_c is found to be negative and equal for both sublattices, namely T _c / p ∼ - 0.4 K/kbar. The values of the magnetic moments of the Er and the Co ions are found = 5.40±0.15μ _B /atom, = 0.50±0.07μ _B /atom and 5.35±0.15μ _B /atom, 0.37±0.09μ _B /atom, for p = 0 and 6 kbar, respectively. Our experimental results give evidence for short-range magnetic order formation at temperatures already above T_c and for a coexistence short- and long-range order below T_c down to 4 K. Received 20 December 2001 / Received in final form 12 June 2002 Published online 31 October 2002 RID="a" ID="a"e-mail: andrew.podlesnyak@psi.ch     

The normal state scattering rate in high- cuprates

A detailed phenomenological model is developed for the normal state transport properties of optimal and overdoped high-T_c cuprates. In particular, an explicit form of scattering rate is identified that may account, qualitatively and quantitatively, for the normal state (magneto)-transport properties of Tl₂Ba₂CuO _{6 + δ} and Bi₂Sr₂CaCu₂O _{8 + δ} across the overdoped side of the phase diagram. The proscribed form of the scattering rate is consistent also with features seen in the photoemission spectroscopy of Bi₂Sr₂CaCu₂O _{8 + δ} and offers a new and intuitive way to understand the evolution of the temperature dependence of the inverse Hall angle in Bi-based cuprates with carrier concentration. Received 5 June 2002 / Received in final form 17 December 2002 Published online 6 March 2003 RID="a" ID="a"e-mail: n.e.hussey@bristol.ac.uk     

Enthalpy recovery of a glass-forming liquid constrained in a nanoporous matrix: Negative pressure effects

The T _g of organic liquids confined to nanoporous matrices and that of thin polymer films can decrease dramatically from the bulk value. One possible explanation for this phenomenon is the development of hydrostatic tension during vitrification under confinement that results in a concomitant increase in the free volume. Here we present experimental evidence and modeling results for ortho-terphenyl (o-TP) confined in pores as small as 11.6 nm that indicate that, although there is an important hydrostatic tension in the liquid in the pores, it does not develop until near the reduced T _g of the constrained material --well below the bulk T _g. Enthalpy recovery for the o-TP in the nanopores exhibits accelerated physical aging relative to the bulk, as well as a leveling off of the fictive temperature at equilibrium values greater than the aging temperature. An adaptation of the structural recovery model that incorporates vitrification under isochoric conditions is able to provide a quantitative explanation for the apparently anomalous aging observed in nanopore confined liquids and in thin polymeric films. The results strongly support the existence of an intrinsic size effect as the cause of the reduced T _g. Received 3 September 2001     

Low-temperature specific heat of Sr<Subscript>2</Subscript>RuO<Subscript>4</Subscript>

We report low-temperature measurements 0.07 K ? T ? 2 K of the specific heat, C, of the perovskite superconductor Sr₂RuO₄. Based on a detailed analysis of our data with respect to both sample quality (as measured by T _c = 0.43 K - 1.17 K) and magnetic-field dependence, it is shown that the electronic contribution to the specific heat, which contains the desired information on the gap structure, is superimposed by at least two additional contributions: a Schottky-type hump at T ≈ 0.1 - 0.2 K and a low-temperature upturn in C / T at T < 0.1 K. We discuss possible origins of these additional contributions and their implications for the interpretation of low-temperature C ( T ) data. Received 23 August 2001     

Magnetic properties of undoped YBa 2 Cu 3 O 6 + x system

In the present paper, we study the magnetic properties of bilayer cuprate antiferromagnets. In order to evaluate the expressions for spin-wave dispersion, sublattice magnetization, Néel temperature and the magnetic contribution to the specific heat, the double time Green's function technique has been employed in the random phase approximation (RPA). The spin wave dispersion curve for a bilayer antiferromagnetic system is found to consist of one acoustic and one optic branch. The “optical magnon gap” has been attributed solely to the intra-bilayer exchange coupling (J _⊥ ) as its magnitude does not change significantly with the inter-bilayer exchange coupling (J_z). However J_z is essential to obtain the acoustic mode contribution to the magnetization. The numerical calculations show that the Néel temperature (T _N ) of the bilayer antiferromagnetic system increases with the J_z and a small change in J_z gives rise to a large change in the Néel temperature of the system. The magnetic specific heat of the system follows a T² behaviour but in the presence of J_z it varies faster than T². Received 13 July 2000 and Received in final form 14 May 2001     

Element-specific magnetic long- and short-range order and competing interactions in Gd x Eu 1 - x S

We report on an investigation of the magnetic properties of Gd _x Eu _{1 - x} S mixed crystals with compositions in the range of 0.6 < x < 1. For the two samples Gd _0.8 Eu _0.2 S and Gd _0.73 Eu _0.27 S a long-range antiferromagnetic order was observed at low temperatures. Element-specific measurements exhibited a different temperature dependence of the reduced sublattice magnetisation of the two magnetic species. A model calculation and Monte Carlo simulations revealed that the different temperature dependence is due to frustration effects. These frustration effects lead to a breakdown of the long-range order for higher europium contents. For the Gd _0.67 Eu _0.33 S-sample we were able to observe a short-range antiferromagnetic order with correlation lengths of a few 10? with X-ray resonance exchange scattering. Received 25 April 2001 and Received in final form 3 December 2001     

Relation between vortex excitation and thermal conductivity in superconductors

Thermal conductivity κ _xx(T) under a field is investigated in d _{x2 - y2}-wave superconductors and isotropic s-wave superconductors by the linear response theory, using a microscopic wave function of the vortex lattice states. To study the origin of the different field dependence of κ_xx(T) between higher and lower temperature regions, we analyze the spatially-resolved thermal conductivity around a vortex at each temperature, which is related to the spectrum of the local density of states. We also discuss the electric conductivity in the same formulation for a comparison. Received 8 December 2001 and Received in final form 20 March 2002 Published online 6 June 2002     

Strong impact of the electron-photon matrix element on angle-resolved photoelectron spectra of Sr 2 CuO 2 Cl 2

In recent years insight has been gained into the electronic structure of layered cuprates using angle-resolved photoelectron spectroscopy. In many of these studies it is assumed that the electron-photon matrix element follows the trends set by the atomic photoionization cross sections and does not influence lineshape, dispersion and the k-dependence of the spectral intensity. In this study using Sr₂CuO₂Cl₂ as an example it will be shown that the electron-photon matrix element can have a strong impact on both strength and shape of a feature in an angle-resolved photoelectron spectrum of a layered cuprate which can strongly affect information on character and the momentum-dependence of the energy and spectral weight of a state deduced from the spectra. The results of this study put an emphasis on the need to employ the whole parameter range of the ARPES method to get reliable information on the spectral function of cuprates for which purpose synchrotron radiation is an uniquely suited tool. Received 10 January 2000 and Received in final form 15 January 2001     

Angular dependence of magnetoresistivity in c-oriented MgB thin film

The anisotropy of MgB₂ is still under debate: its value, strongly dependent on the kind of sample and on the measuring method, ranges between 1.2 and 13. In this work we present our results on MgB₂ c-oriented superconducting thin film. To evaluate the anisotropy, we followed two different approaches. Firstly, magnetoresistivity was measured as a function of temperature at selected magnetic fields applied both parallel and perpendicular to the c-axis; secondly, we measured magnetoresistivity at selected temperatures and magnetic fields, varying the angle θ between the magnetic field and the c-axis. The anisotropy estimated from the ratio between the upper critical fields parallel and perpendicular to the c-axis and the one obtained in the framework of the scaling approach within the anisotropic Ginzburg-Landau theory are different but show a similar trend in the temperature dependence. Some differences in the upper critical field and in its anisotropy of our film with respect to single crystals are emphasized: some of these aspects can be accounted for by an analysis of upper critical fields within a two-band model in presence of disorder and/or crystallographic strain. Received 12 July 2002 / Received in final form 17 September 2002 Published online 29 November 2002