International Journal of Modern Physics B

Intercalating polyatomic molecules into a superconductor strongly modifies its superconducting properties. A mechanism for a large increase in T_c is proposed that explains recent experimental findings on C₆₀/CHX₃(X=Cl, Br, I). The increase of T_c upon intercalation as well as upon Cl→Br substitution is described in terms of the additional contribution to pairing interaction arising from the coupling of holes (or electrons) to the vibrational manifold of the intercalated haloform molecules. We also present first optimization of the crystal structure and calculations of the band structure for neutral C₆₀/CHCl₃.

In a recent paper [J. H. Schön, Ch. Kloc, R. C. Haddon and B. Batlogg, Nature408, 549 (2000)] C₆₀ doped with holes by application of an intense electric field was found to superconduct at remarkably high temperature. In this paper we show that the observed dependence of T_c on doping can be reproduced by solving the (four) s-wave Eliashberg equations in the case of a finite, non-half-filled energy band, with the only initial hypothesis that the Coulomb pseudopotential depends on the filling in a very simple and plausible way. The experimental data are reproduced by using very reasonable values of the physical parameters involved. We also show that the application of the same approach to new experimental data [ J. H. Schön, Ch. Kloc and B. Batlogg, Science293, 2432 (2001)] also gives good results.

The electronic structure and the superconductivity in field-doped polyacenes are considered. Within a modified Thomas–Fermi approach for typical experimental values of the surface charge density the injected charge is confined to a monolayer. The electron–phonon coupling constant for internal modes λ_intra is estimated using the work of Devos and Lanoo (Ref. 4) and the density of states N(0) estimated from a 2D tight-binding model derived from a full potential LDA band structure calculation for bulk anthracene. The empirical values of the Coulomb pseudopotentials are significantly enhanced. The strong Coulomb interaction is considered as a key quantity which determines the large differences in the critical temperatures achieved for n-doped polyacenes and C₆₀.

We present a series of results concerning the determination of the gap(s) in MgB₂ by means of break-junction and point-contact measurements. We were able to observe the small 3D gap predicted by the two-band models for MgB₂ in the clean limit as well as the gap in dirty limit. We were also able to follow the temperature evolution of the latter, from 4.2 K up to a reduced critical temperature. We show that all our results can be explained in the framework of the two-band model provided that the existence of a damaged surface in dirty limit proximized by the clean bulk as well as the occurrence of parallel contacts are admitted.

We report a detailed study of the electronic and dynamical properties of MgB₂, BeB₂ and of the AlMgB₄ superlattice, within the local density approximation to the density functional theory. On the basis of our results we discuss the superconducting properties of these systems, and point to the high T_c in MgB₂ as a fortunate combination of events.

We report experimental results on microwave surface resistance in MgB₂ superconductors. The results have been obtained using the cavity perturbation method. Two samples have been investigated: one was made up of commercial Alfa-Aesar powder and the other one was obtained by direct reaction from elements. The temperature and dc magnetic field dependence of the surface resistance has been investigated down to 4.2 K and up to 10 kOe, respectively. Our results show that microwave losses are strongly affected by the magnetic field even by a relatively weak one. In particular, an unusually enhanced field dependence of the low-T residual surface resistance has been detected in both samples.

Scanning Tunneling Spectroscopy is used at low temperature in an extensive study of the recently discovered superconducting compound MgB₂. Both powder- and crystal-based samples show a very unusual double-gap structure in the tunneling conductance spectra. The spectral weights of the gaps may vary from one junction to another but their energy positions remain constant. Both gaps vanish at the critical temperature of the bulk material. These findings confirm the existence of two-gap superconductivity in MgB₂, which opens a new class of multiple-gap superconductors.

A two-band scheme of MgB₂ superconductivity is developed. It comprises repulsive interband transfer of intraband pairs between effective σ- and π-bands together with σ-intraband electron–phonon attraction and Coulomb interaction. The interaction constants are found by fitting T_c, specific heat jump and zero-temperature gaps ratio. The gaps and specific heat dependences on temperature besides the T_c isotope effect exponent, calculated without any additional free parameters, agree with the experiment. T_c versus chemical potential shows a broad maximum to which the "autodoped" MgB₂ belongs.

It is shown that the process of T_c amplification in diborides occurs in a particular region of the (ρ,ε) phase diagram, where ρ is the charge density and ε is the micro-strain in the metallic boron plane. The T_c(ρ,ε) shows that the superconducting phase occurs while the chemical potential is tuned near the "shape resonance" of the diboride superlattice and the micro-strain is in a critical range. The range of the high T_c phase is determined by the modulation amplitude Δ_shape of the shape resonance energy due to the zero-point lattice vibrations Δu_rms(ε), pointing towards an electronic or vibronic pairing mechanism. It has been discussed that the McMillan's formula breaks down for the diborides.

We present a number of experimental results on the temperature dependence of the magnetic penetration depth λ and on the temperature and field dependence of the microwave surface impedance Z_s=R_s+iX_s in both pellets and thin films of MgB₂, exhibiting critical temperatures ranging between 26 and 38 K. Accurate measurements of Z_s(H,T) were performed by means of a sapphire dielectrically loaded cavity operating in the microwave region (20 GHz). The study of λ(T) was carried out employing a single coil mutual inductance technique in the MHz region. An anisotropic s-wave BCS model can account for the temperature dependence experimentally observed in the penetration depth data of the best films, confirming previous reports on the conventional nature of superconductivity in diborides. On the contrary, films having a reduced value of the critical temperature and pellets show no evidence of saturation, and the experimental results strictly follow a quadratic dependence down to the lowest temperatures. We explain this behavior with the presence of metallic Mg inclusions that may locally depress the gap. The study of the surface impedance versus temperature and field shows also that the source of microwave loss can be markedly different, depending on the structural and transport properties of the samples.

The MgB₂ system is studied by self consistent multiple-scattering calculations at the B K-edge X-ray absorption near edge structure (XANES). We find that the features in the B K-edge XANES are strongly affected by medium and long range order with large influence of the core hole potential suggesting that the band structure calculations are not enough to provide a quantitative interpretation of the spectral features. We have investigated evolution of density of states in the conduction band as a function of Al substitution in place of the Mg. The results are consistent with the fact that the Al substitution influences mainly the density of states in the B σ-band and hence suppresses the superconductivity.

We report layer-Korringa–Kohn–Rostocker calculations for bulk and surface states (SFS) as well as the corresponding photoemission intensities of MgB₂. Our theoretical results reproduce very well the recent angle resolved photoemission data by Uchiyama et al. At least two SFS are assigned. Consequences of SFS on the anisotropy of the upper critical fields and other properties in the superconducting state of grains in micro-powder samples are discussed.

We have measured B K-edge X-ray absorption spectra of Mg_1-xAl_xB₂ (x=0.0 ~ 0.4) using bulk-sensitive total fluorescence yield (TFY) at room temperature and 30 K. In contrast to previous reports no peaks near 192 eV and 194 eV could be observed after repeated scraping, indicating their origin being due to surface contamination. These clean spectra show a peak right above the absorption threshold, anticipated from transition to the unoccupied B 2p states. Except in the intermediate Al doping range of x ~ 0.1 to 0.28 the threshold peak intensity decreases, consistent with p_σ-hole filling. The increase of peak intensity in the intermediate range is correlated with structural phase transition.

We have studied effect of X-ray illumination on charge ordering superstructures in oxygen doped La₂CuO_4+δ superconducting system. Formation of long range crystalline phase of polarons is favored in this system by mobile counter-ions (interstitial oxygen ions in the La₂O₂ layer) in the temperature range 200–350 K. We find a clear signature of disorder to order like X-ray induced phase transition. This transition appears to show up beyond a threshold dose of about 10¹⁶ photons/cm², the value which increases with decreasing temperature.

The La_2-xSr_xNiO₄ system is isostructural with the high T_C superconducting cuprate La_2-xSr_xCuO₄ and is a prototypical system for the understanding of strongly correlated electron-phonon coupling, and the resultant effects on material properties. X-ray scattering studies have been performed on La_5/3Sr_1/3NiO₄ that demonstrate the two-dimensional nature of these charge stripes. Such studies, demonstrate the very high correlation length of the stripes (~ 2000 Å) at low temperatures. We have undertaken a series of experiments measuring the wavevector and charge stripe correlation length on a variety of crystals with the compositions La_2-xSr_xNiO₄ (x=0.20, 0.25, 0.275, 0.30 and 0.33) using ~10 keV X-rays. The results demonstrate that for x=0.275, and above, the charge stripes are highly correlated in a well-ordered crystalline lattice. Measurements of the incommensurability, ε, as a function of temperature for the series revealed that it is commensurate and temperature independent for the x=0.33 sample. For other compositions it is incommensurate and also temperature dependent. However for the x=0.20 and 0.25 crystals a much reduced correlation length was observed suggestive of a charge stripe glass. However, such experiments are sensitive to such charge ordering only in the near (top few micron) surface region. High energy X-rays however can probe the charge stripe ordering within the bulk of the single crystal by utilising the dramatic increase in penetration depth. We have used 130 keV X-rays and demonstrate that in La_5/3Sr_1/3NiO₄ the charge stripes are far less correlated in the bulk than in the near surface region. This reduced correlation length (~300 Å), consistent with neutron scattering measurements, is indicative of a charge stripe glass, reminiscent of that observed below x=0.25, in the near surface region.

Temperature dependence of local structure of the CuO₂ plane has been studied by in plane polarized (E//ab) Cu K-edge extended X-ray absorption fine structure (EXAFS) experiments on an under-doped single crystal of superconducting La_1.87Sr_0.13CuO_4-y system with T_c=32 K. The temperature dependence of the Debye–Waller factors for the Cu-O and Cu-La/Sr pairs are determined. We find that the motion of Cu and O atoms in the Cu-O bond is partially correlated at higher temperatures, going over to an uncorrelated motion at lower temperatures. The results are discussed in terms of the local lattice instabilities and stripes as reported in other systems.

Exchange couplings are calculated for Li₂VOSiO₄ using local density approximation (LDA). From a two band tight-binding model fitted to the LDA band structure and mapped to a Heisenberg model, the in-plane and inter-plane exchange integrals are obtained. While the sum of in-plane couplings J₁+J₂=9.5±1.5 K and the inter-plane coupling J_⊥ ~ 0.2-0.3 K agree with recent experimental data, the ratio J₂/J₁ ~ 12 exceeds the reported value by an order of magnitude. Using geometrical considerations, high temperature expansions and perturbative mean field theory, we show that the LDA derived exchange constants lead to a remarkably accurate description of the properties of these materials including specific heat, susceptibility and Neél temperatures.

Stoichiometric LiMn₂O₄ undergoes a structural phase transition around room temperature from cubic to orthorhombic, induced by a partial charge ordering on the manganese sites, similar to the Verwey transition occurring in Fe₃O₄. We studied the charge ordering transition of LiMn₂O₄ by means of anelastic spectroscopy, measuring the Young's modulus, E, and the elastic energy loss function, Q^-1. Our measurements confirm the occurrence of a phase transition at 292 K on cooling, showing a large hysteresis on heating. The anelastic spectroscopy results also indicate that the transformation is characterised by the coexistence of two phases. In addition, the Young's modulus reveals presence of an additional anomaly, on cooling, around the onset of the phase transformation.

The interplay between charge, spin and orbital degrees of freedom in transition metal oxides has been a matter of much interest in recent years. In this paper we present X-ray scattering results of charge and orbital ordering in the electron doped manganite Bi_0.24Ca_0.76MnO₃. Using High Energy X-ray Scattering (HEXS) the charge ordering, structural modulation due to the Jahn–Teller ordering and the harmonic of the structural modulation have been measured. The superlattice reflections due to the Jahn–Teller ordering are found to maximise in intensity at the Neél point. We have confirmed using resonant X-ray scattering at the K-edge of manganese and the Anisotropic Tensor of Susceptibility technique (ATS) that superlattice reflections occurring at a modulation wavevector of (0.5, 0, 0) are due to charge ordering in a Mn³⁺/Mn⁴⁺ ordering pattern. In addition superlattice reflections corresponding to the orbital order were measured at a modulation wavevector of (0.25, 0, 0). The temperature dependence of both the Jahn–Teller structural distortion peak (1.75, 2, 0) and the charge ordering peak (3 0 0) were observed to behave identically, gradually increasing with decreasing temperature.

It has been proposed, that unconventional density waves (UDW) are possible candidates for systems with hidden order parameter. Unlike in conventional density waves, no periodic modulation of either the charge-, or the spin-density is present in UDW, in spite of a clear thermodynamic signal. Although the unconventional spin density wave (USDW) has been suggested for the "antiferromagnetic" phase of URu₂Si₂, the micromagnetism seen by neutron scattering has not been understood. We present here the calculation of the local spin density due to impurities in USDW, which describes quantitatively the neutron scattering data by Amitsuka et al. Further, we propose that the pseudogap phase in high temperature superconductors (HTSC) should also be USDW. Strong evidence for this are the micromagnetism seen by Sidis et al., and the optical dichroism seen by Campuzano et al.

A crystal-chemical study of trioctahedral micas previously characterized by single-crystal XRD has been performed by XANES spectroscopy at the Si and Al K edges. XANES, being a local structural probe, can investigate distortion and modification of the tetrahedral sheet with increasing Fe for Mg substitution in the octahedral sheet. Comparison of XANES spectra allows determining the size of the tetrahedral site occupied by either Si or Al. The Si-O distance remains essentially unchanged whereas the Al-O distance appears to increase. The behavior may be interpreted as a tilt of the tetrahedra, initially rotated to match the ideal mica geometry, with increasing Fe substitution in the octahedral sheet.

High resolution angle-resolved photoelectron spectroscopy (ARPES) has been performed on magnetic nanostructures which were in-situ grown hetero-epitaxially on GaAs substrates by low temperature molecular beam epitaxy. Sulfur termination of GaAs surfaces enabled the formation of NiAs-type MnSb dots and zinc-blende (zb) type MnAs dots, showing huge magnetoresistance at RT and half-metallic behavior, respectively. ARPES analysis revealed that the zb-MnAs dots show unique valence band feature with Mn 3d-related peak at about 4 eV of binding energy which is quite similar to (Ga,Mn)As DMS films. Zb-CrAs thin films on GaAs with T_c higher than RT are found to show the band dispersion within 2 eV near the Fermi edge, which is in good agreement with the FLAPW calculation. We have also found that (Ga,Cr)As DMS thin films have the similar valence band features to zb-CrAs

The electronic structures of the nearly optimally doped single-layer, bilayer and trilayer Bi-based cuprates are investigated by angle-resolved photoemission spectroscopy. A lineshape analysis of data taken for different photon energies indicates that the interlayer coupling within the trilayer is not stronger than its counterpart in the bilayer system. This suggests that the higher T_c of the trilayer cuprate superconductors is not due to an enhancement of the coupling strength between the neighboring CuO₂ planes within each unit cell.

Structural compliance is defined as the ability of a crystal structure to accommodate variations in local atomic bond-lengths without incurring large strain energies. The structural compliance of cuprates is relatively small. In the presence of charge stripes short, highly doped, Cu-O-Cu bonds become subject to a tensile misfit strain. We show that this leads to nanoscale microstructures. This explains a number of experimental observations as we discuss here.

It is well documented that SDW in (TMTSF)₂PF₆ undergoes another phase transition at T* ≈ 4 K, though the nature of the new phase has not been identified. In particular the angular dependence of the magnetoresistance for B‖(a, b′) plane below 4.2 K is dramatically different from the one above 4.2 K. We propose that below T^* the new phase unconventional SDW (USDW) appears which modifies drastically the quasiparticle spectrum. The present model describes the observed angular dependence of the magnetoresistance reasonably well, which cannot be interpreted in terms of the conventional SDW.

We report local structure of Nb₃Ge intermetallic superconductor by Ge K-edge extended X-ray absorption fine structure (EXAFS) measurements performed in the temperature range of 6–300 K, with an emphasis to determine the local and instantaneous atomic displacements across the superconducting transition temperature T_c. We find that the temperature dependent correlated Debye–Waller factor of the Ge-Nb bonds shows a drop at the T_c while cooling the sample, similar to the one observed in the high-T_c cuprate superconductors. The results provide a clear indication of an intimacy between the local atomic displacements and the short coherence superconductivity, and suggests that local electron-lattice interaction should be considered to explain the high-T_c superconductivity in these materials.

We describe X-ray absorption spectroscopy (XAS) technique to probe the local structure of photo-induced phase. Photo-induced melting of chalcogenide glass (amorphous selenium, a-Se) was investigated. A grazing-incidence fluorescence excitation and a high efficiency X-ray detector were used to detect a small change of coordination number due to photo-induced transformation. It was shown that photo-induced three-fold coordinated sites are formed upon light irradiation forming cross-linking between selenium chains (photo-induced melting). A simple model is proposed which describes the reorientation of selenium chain perpendicular to the polarization direction of excitation light leading to photo-induced dichroism (anisotropy). Potential of XAS as a local probe of nanoscale structural modifications induced by photo-excitation is clearly demonstrated.

Recently discovered new type of high temperature superconductors have circular molecular orbits in each unit site of 2D s/p electron system. We discuss a new model of superconductivity caused by the correlated state of electrons in the 2D interconnection of circular orbits. This model gives an estimation of the superfluidity transition temperature: T_c ≈ 70-200 K for fcc C₆₀, and T_c ≈50-150 K for MgB₂.

A model of a "defect" band which expands by doping above the itinerant hole band is used to describe the cuprate superconductivity by the interband scattering of pairs. Bands overlap reached by progressive doping determines the under/optimal-doping separation line and the reset of the Fermi-liquid behaviour. A two-band scheme with the interband scattering of intraband pairs reproduces the observed peculiar behaviour of gaps. A pseudogap appears as the minimal excitation energy of the itinerant band at underdoping. Doping forms the electronic structure of cuprates on which the interband superconductivity can occur.

The charge and spin freezing phenomena in high T_cYBa₂Cu₃O_6+x superconductor is investigated in the low doping region (0<x<0.4) by the zero-field muon spin relaxation (ZF-μSR) technique. For x>0.2, the observed spin freezing is correlated to the charge freezing of the holes in the Cu(2)O₂ planes. The data may be fitted to a model of size-dependent reduction of the staggered magnetization implied in a stripe scenario. The hole concentration dependence of these freezing phenomena is mapped onto the phase diagram of YBa₂Cu₃O_6+x.