Electron-phonon interaction in NbB2: a comparison with MgB2

We present a comparison of electron-phonon interaction in NbB₂ and MgB₂, calculated using full-potential, density-functional-based methods in P6/mmm crystal structure. Our results, described in terms of (i) electronic structure, (ii) phonon density of states F(ω), (iii) Eliashberg function α²F(ω), and (iv) the solutions of the isotropic Eliashberg gap equation, clearly show significant differences in the electron-phonon interaction in NbB₂ and MgB₂. We find that the average electron-phonon coupling constant λ is equal to 0.59 for MgB₂ and 0.43 for NbB₂, leading to superconducting transition temperatures T_c at around 22 K for MgB₂ and 3 K for NbB₂.     

Carbon doping in MgB2: role of boron and carbon px(y) bands

We have studied the changes in the electronic structure and the superconducting transition temperature T_c of Mg(B_1−xC_x)₂ alloys as a function of x with 0≤x≤0.3. Our density-functional-based approach uses the coherent-potential approximation to describe the effects of disorder, the Gaspari-Gyorffy formalism to estimate the electron-phonon matrix elements and the Allen-Dynes equation to calculate T_c in these alloys. We find that the changes in the electronic structure of Mg(B_1−xC_x)₂ alloys, especially near the Fermi energy E_F, come mainly from the outward movement of E_F with increasing x, and the effects of disorder in the B plane are small. In particular, our results show a sharp decline in both B and C p_x(y) states for 0.2≤x≤0.3. Our calculated variation in T_c of Mg(B_1−xC_x)₂ alloys is in qualitative agreement with the experiments.     

A simple theory of 40 K superconductivity in MgB2: first-principles calculations of Tc, its dependence on boron mass and pressure

We have studied the superconducting properties of MgB₂ from first-principles under isotropic, uniaxial, and biaxial compressions. We find that the in-plane boron phonons near the zone-center are very anharmonic and strongly coupled to the planar B σ bands near the Fermi level. This mode is found to be the key to quantitatively explain the observed high T_c, the total isotope effect and the pressure dependence of T_c. We propose that a stringent test on the hole and phonon based theories of the superconductivity in MgB₂ would be a measurement of the biaxial ab-compression dependence of T_c.     

Resonant Raman scattering in the superconducting cuprates: Frozen-phonon versus perturbational approach

We consider in detail Raman scattering by vibration of the apical oxygen ions in the RBa₂Cu₃O₇ superconducting cuprates. The scattering intensity is very sensitive to the ratio of diagonal and off-diagonal matrix elements of electron-phonon coupling, bandstructure, and carrier concentration. Our results show a large quantitative difference between the results of frozen-phonon and perturbational approach to the Raman process. The discrepancy becomes especially large when interband transitions to the states near the Fermi level are close to resonance with the incident light. The calculation of phonon-induced ion charge fluctuations shows an analogous discrepancy. The reason for these effects is the possibility of carrier redistribution between different parts of the Fermi surface arising in the frozen-phonon approximation. Our results show that Raman scattering in superconducting superlattices is very sensitive to the properties of the states near the Fermi level. For this reason experiments performed on the superlattices can help to resolve the discrepancy. Received 8 December 1999     

AuB2 in comparison to superconducting MgB2-an ab initio study

The noble metal diboride AuB₂, a potential candidate for superconductor, is studied by an ab initio method in comparison to the superconducting MgB₂. The results, described in terms of equilibrium lattice constants, bulk modulus, pressure derivative of bulk modulus and their in- and out-of-plane linear values, volume coefficient of T_c, density of states, band structure, show some similarity as well as dissimilarity between the behaviour of the two compounds. The implications for the behaviour are discussed.     

Calculated phonon dispersion of infinite-layer compounds and the effects of charge fluctuations

In a first step we use an ab initio rigid-ion model (RIM) to calculate the lattice parameters and the phonon dispersion of the infinite-layer compounds CaCuO₂, SrCuO₂, and BaCuO₂. We find an increase of both the planar and the axial lattice constant when going from CaCuO₂ through SrCuO₂ to BaCuO₂. The rate of increase of the planar lattice constant with respect to the alkaline-earth ionic radius is calculated to be smaller for the replacement of Sr by Ba than for the replacement of Ca by Sr. Both results are in accordance with experimental studies. The phonon dispersion in the RIM exhibits several unstable branches mainly related to axial displacements of the oxygens, indicating the tendency of the crystal to reconstruct in a lower-symmetry structure. The structural stability increases, however, towards BaCuO₂; simultaneously, the maximum phonon frequency decreases. AnA _2u zone-center mode with very large LO-TO-splitting exists in all three compounds (ferroelectric mode). In a second step charge fluctuations (CF) are taken into account at the copper- and oxygen ions, using SrCuO₂ as an example. Due to the vanishing of the ferroelectric split a branch with very steep dispersion forms in the [001] direction in the metallic phase whereas the zone-centerA _2u modes are unchanged in the insulating phase because of the two-dimensional (2D) electronic structure assumed. Characteristic nonlocal electron-phonon-interaction effects are associated with theZ-point Sr-axial-breathing mode: CF of uniform sign within the CuO planes but alternating sign in consecutive planes do occur in the metalic phase. This interplane charge transfer is, on the other hand, suppressed in the insulating phase due to the 2D electronic structure assumed. Instead, large induced site-potential changes emerge in this case.     

The selected thermodynamic properties of the strong-coupled superconductors in the van Hove scenario

In the present paper, the exact numerical solutions of the Eliashberg equations within the van Hove scenario are obtained. The order function, the wave function renormalization factor, the energy shift function and the chemical potential are calculated. It is shown that the van Hove singularity change considerably the relationship between the order function and the occupation number. In particular, the order function have strong maximum in hole-doped region. Additionally, the critical temperature is calculated. It is shown that the observed maximum for the order function in hole-doped region induces a very high value of the critical temperature despite the fact that superconductivity originates from phonon-induced pairing. Finally, the ratio of the zero temperature energy gap to the critical temperature is found.     

On the possibility of a phonon mediated transport anomaly in MgB2 under compression

Results of electrical resistance measurements on MgB₂ at ambient temperature up to 25 GPa are presented. An abrupt reduction of nearly 30% in resistance around 18 GPa is observed. Band structure calculations in the presence of a frozen-in distortion of the E_2g phonon mode reveal that one of the closed Fermi sheets corresponding to the σ-band opens along the Γ-A direction at this pressure. It is suggested that the anomaly observed in the resistance is due to this phonon mediated electronic topological transition (ETT).     

Polar oscillation and dispersion properties of quasi-confined optical phonon modes in a wurtzite GaN/AlxGa1−xN nanowire

Under the dielectric continuum model and Loudon’s uniaxial crystal model, the properties of the quasi-confined (QC) optical phonon dispersions and the electron-QC phonons coupling functions in a cylindrical wurtzite nanowire are deduced via the method of electrostatic potential expanding. Numerical computations on a GaN/Al_0.15Ga_0.85N wurtzite nanowire are performed. Results reveal that, for a definite axial wave number k_z and a certain azimuthal quantum number m, there are infinite branches of QC modes. The frequencies of these QC modes fall into two regions, i.e. a high frequency region and a low frequency region. The dispersion of the QC modes are quite apparant only when k_z and m are small. The lower-order QC modes in the higher frequency region play more important role in the electron-QC phonon interactions. Moreover, for the higher-order QC modes in the high frequency region, the electrostatic potentials “escaping” out of the well-layer material nearly could be ignored.     

Mn and Fe impurities in MgB2

Based on first principles calculations, we show that Mn impurities are magnetic in MgB₂ due to exchange-splitting of band. Thus, Mn impurities could act as strong magnetic scattering centres leading to pair-breaking effects in MgB₂. In contrast, we find Fe impurities in MgB₂ to be nearly non-magnetic.     

Calculation of Tc and the ratio 2Δ(0)/kBTc of MgB2 within a two-band model of superconductivity

Superconductivity in the MgB₂ superconductor is described within the framework of a two-band Eliashberg formalism. Different gaps are assumed to open on the different parts of the Fermi surface of this compound. Separation of the order parameter (OP) into two components is achieved by taking the Fourier transform of the OP using the momentum states of the σ- and π-bands of MgB₂. Expressions for the T_c and the ratio 2Δ_σ(0)/k_BT_c for this superconductor are obtained. Numerical values for these two properties are obtained for a range of values of the cut-off frequency of the phonons responsible for the superconductivity and for a range of values of the ratio between the two energy gaps. This was done for various values of the normalized partial densities of states on the σ-sheet of the Fermi surface.     

Core level photoemission study of polycrystalline MgB2

X-ray photoemission measurements have been made on Mg 2p and B 1s core levels from MgB₂ as well as on the O 1s core level from the contaminant oxygen. The motivation for this study came for the discrepancy in the reported papers on the energy values of the B 1s and Mg 2p core levels, ostensibly due to the oxygen, present as an impurity, reacting and forming oxides and other compounds. Sputtering leads to a significant reduction of oxygen on and in the sample. The in situ sputter-cleaned sample gives the best results. The measurements are compared with results reported earlier and discrepancies discussed.     

Lattice Dynamics and Superconductivity of RuB2: A First-Principles Study

We report the first-principles linear response calculations on lattice dynamics and electron-phonon coupling （EPC） of superhard material RuB2. Phonon frequencies and eigenvectors are obtained throughout the whole Brillouin zone. The calculated EPC paracneters for the optical phonon modes at F indicate that the d electrons of transition metal play the most important role in deciding the superconducting behaviour, and there are sizeable contributions from B p electrons to EPC. Our calculated EPC constant is 0.41, and the estimated superconducting transition temperature Tc is 1.6K using the Coulomb pseudopotential u^＊ = 0.12, in excellent agreement with the experimental ones.     

First-principles study on the electronic structure and magnetic properties of metallic antiferromagnet C7H3S2N2

First-principles full potential linearized augmented plane wave (FPLAPW) calculations have been performed to study the electronic structure and the magnetic properties of 3-Cyanobenzo-1,3,2-dithiazolyl,C₇H₃S₂N₂. The density of states (DOS), the total energy of the cell, and the spontaneous magnetic moment of C₇H₃S₂N₂ were all calculated. The calculations reveal that the low-temperature phase of the compound C₇H₃S₂N₂ has a stable metal-antiferromagnetic ground state, and there exists an antiferromagnetically coupled interactions between the dithiazolyl radical(1), which is in good agreement with experiment.     

Enhanced superconducting properties of Eu2O3-doped MgB2

Bulk polycrystalline samples of Eu₂O₃-doped MgB₂ have been synthesized by a standard solid state reaction route and their structural and superconducting properties have been investigated. As a function of Eu₂O₃ content we have found a significant increase in the critical current density (J_c) and the irreversibility field (H_irr) in the magnetic field range 0–6 T. The XRD results reveal the presence of MgO and EuB₆ secondary phases along with the main hexagonal phase of MgB₂. The strain values and the lattice distortions have been found to increase almost linearly with the nominal Eu₂O₃ content. The observed significant improvement in J_c(H) and H_irr in the Eu₂O₃-doped MgB₂ samples, thus is mainly attributed to the lattice distortions introduced by Eu₂O₃ doping.     

Properties of hot pressed MgB2/Ti tapes

Hot axial and hot isostatic pressing was applied for single-core MgB₂/Ti tapes. Differences in transport current density, n-exponents and critical current anisotropy are discussed and related to the grain connectivity influenced by pressing. The magnetic Hall probe scanning measurements allowed observing the isolated regions for axially hot pressed sample attributed to the longitudinally oriented cracks introduced by pressing. The highest current densities were measured for the tape subjected to hot isostatic pressing due to improved connectivity.     

Pinning mechanism in carbon nanotube doping of MgB2 superconductor

MgB_1.9C_0.1 samples are synthesized under the ambient pressure (AP) and high pressure (HP), respectively. The further studies demonstrate different field-dependence of the critical current density J_c(H) in each sample. In the view of two-gap superconductivity in these samples, δT_c pinning (resulting from the spatial fluctuations of the transition temperature) is dominant in the AP sample, while in the HP sample, both δT_c and δl pinning (due to the mean-free-path fluctuations) act together and their contributions vary with temperature. Besides the improvement of H_c2(0), due to the different pinning mechanism, J_c(H) of the HP sample shows a slower decay with the increasing fields than that of the AP sample in high fields, which suggests a possible method of retarding the rapid decay of J_c(H) under elevated fields.     

Raman scattering in organic superconductor (BEDT-TTF)2 I 3: A model study

An interpretation of the Raman data of the organic superconductor (BEDT-TTF)₂ I ₃ is given, based on the modified charge bag model for superconductivity. The Raman intensities are calculated at zero temperature both in the normal as well as the superconducting (SC) states. The scattering due to charge carriers as well as the phonons are taken into account. The results show a constant intensity background which reduces on going from the normal to the superconducting state. Similarly, the loss of intensity, broadening and softening of the frequency of a low lying phonon on going from the normal to the SC state are predicted. All these features are in qualitative agreement with the observed Raman data.     

Correlated vortex pinning in Si-nanoparticle doped MgB2

The magnetoresistivity and critical current density of well characterized Si-nanoparticle doped and undoped Cu-sheathed MgB₂ tapes have been measured at temperatures T≥28 K in magnetic fields B≤0.9 T. The irreversibility line B_irr(T) for doped tape shows a stepwise variation with a kink around 0.3 T. Such B_irr(T) variation is typical for high-temperature superconductors with columnar defects (a kink occurs near the matching field B_?) and is very different from a smooth B_irr(T) variation in undoped MgB₂ samples. The microstructure studies of nanoparticle doped MgB₂ samples show uniformly dispersed nanoprecipitates, which probably act as a correlated disorder. The observed difference between the field variations of the critical current density and pinning force density of the doped and undoped tape supports the above findings.     

Fuchs-Kliewer phonon spectrum of Co3O4(110) single crystal surfaces by high resolution electron energy loss spectroscopy

The Fuchs-Kliewer phonon spectrum of single crystal Co₃O₄(110) has been analyzed by high resolution electron energy loss spectroscopy (HREELS) and the four fundamental phonon losses have been identified at 26.8, 47.5, 71.1 and 84.7 meV (216, 383, 573 and 683 cm⁻¹). This is the first HREELS study reported for an intrinsic spinel single-crystal surface with primary focus on the Fuchs-Kliewer phonon structure. The Co₃O₄ crystal is first characterized by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED), which establish the composition, cleanliness, and order of the (110) surface. Electron scattering is then used to obtain a series of well-resolved Fuchs-Kliewer phonon spectra over 2.25-14.25 eV incident electron energy range. The variation in phonon intensity with primary beam energy is shown to agree with that predicted by dielectric theory.