On the topology of the Fermi surface in dense neutron matter

The model of dense neutron matter has been considered, where the topological rearrangement of the ground state of the system of Landau quasiparticles, which is associated with the appearance of the second sheet of the Fermi surface, occurs through two different scenarios. The rearrangement scenario depends on the relation between the wave vector q _c of critical spin-isospin fluctuations and the Fermi momentum p _F. Rearrangement at q _c < p _F occurs continuously with vanishing of the topological rigidity, whereas rearrangement at q _c > p _F occurs with the stepwise appearance of a bubble with a radius of about 0.5p _F in the filled Fermi sphere.     

Calculated electronic and magnetic structure of UNi2Al3 and UPd2Al3

The electronic band structure of UNi₂Al₃ and UPd₂Al₃ is calculated in the local spin-density functional approximation using the self-consistent scalar relativistic ASW-method including spin-orbit coupling. The chemical bond is discussed qualitatively and an explanation is given for the small observed magnetic moments ofU in terms of nearly compensating spin and orbital moments. Calculated total energies are used to discuss a number of possible ground state magnetic-moment configurations.     

Antiferromagnetism and the node structure of the superconducting order parameter of UPd Al

The node structure of the superconducting order parameter of the heavy-fermion system is analyzed within the weak-coupling theory. A pairing interaction induced by the exchange of antiferromagnetic spin excitations is assumed as suggested by recent inelastic neutron scattering experiments and tunneling spectroscopy. The multi-sheeted Fermi surface is taken into account. Based on a model susceptibility for the simple antiferromagnetic structure of , line nodes result at the rim of the magnetic Brillouin zone. Received 29 July 1999     

Pressure induced effects on the Fermi surface of superconducting 2H-NbSe2

The pressure dependence of the critical temperature T(c) and upper critical field H(c2)(T) has been measured up to 19 GPa in the layered superconducting material 2H-NbSe2. T(c)(P) has a maximum at 10.5 GPa, well above the pressure for the suppression of the charge density wave (CDW) order. Using an effective two-band model to fit H(c2)(T), we obtain the pressure dependence of the anisotropy in the electron-phonon coupling and Fermi velocities, which reveals the peculiar interplay between CDW order, Fermi surface complexity, and superconductivity in this system.     

Dual model for magnetic excitations and superconductivity in UPd 2 Al 3

The Heavy Fermion state in UPd₂Al₃ may be approximately described by a dual model where two of the three U-5 f electrons are in a localized state split by the crystalline electric field into two low lying singlets with a splitting energy Δ≃ 6 meV. The third 5 f electron has itinerant character and forms the Heavy Electron bands. Inelastic neutron scattering and tunneling experiments suggest that magnetic excitons, the collective propagating crystal field excitations of the localized 5 f electrons, mediate superconducting (sc) pairing in UPd₂Al₃. A theory for this novel mechanism is developed within a nonretarded approach. A model for the magnetic exciton bands is analyzed and compared with experiment. The sc pair potential which they mediate is derived and the gap equations are solved. It is shown that this mechanism favors an odd parity state which is nondegenerate due to the combined symmetry breaking by the crystalline electric field and the AF order parameter. A hybrid model including the spin fluctuation contribution to the pairing is also discussed. Received 22 October 2001 and Received in final form 28 February 2002     

Heavy-fermion superconductivity atT c =2K in the antiferromagnet UPd2Al3

UPd₂Al₃ is a new heavy-fermion superconductor with a recordT _c of 2 K. In addition, it shows a transition to long-range antiferromagnetic order atT _N =14 K. Its Sommerfeld coefficient is reduced from _p =210mJ/K² mole in the paramagnetic to ₀=150mJ/K² mole in the antiferromagnetic phase.     

Distinct fermi surface topology and nodeless superconducting gap in a (Tl0.58Rb0.42)Fe1.72Se2 superconductor

High resolution angle-resolved photoemission measurements have been carried out to study the electronic structure and superconducting gap of the (Tl0.58Rb0.42)Fe1.72Se2 superconductor with a T(c) = 32 K. The Fermi surface topology consists of two electronlike Fermi surface sheets around the Γ point which is distinct from that in all other iron-based superconductors reported so far. The Fermi surface around the M point shows a nearly isotropic superconducting gap of ～12 meV. The large Fermi surface near the Γ point also shows a nearly isotropic superconducting gap of ～15 meV, while no superconducting gap opening is clearly observed for the inner tiny Fermi surface. Our observed new Fermi surface topology and its associated superconducting gap will provide key insights and constraints into the understanding of the superconductivity mechanism in iron-based superconductors.     

Fermi surface topology and the upper critical field in two-band superconductors: application to MgB2

Recent measurements of the anisotropy of the upper critical field B(c2) on MgB2 single crystals have shown a puzzling strong temperature dependence. Here, we present a calculation of the upper critical field based on a detailed modeling of band structure calculations that takes into account both the unusual Fermi surface topology and the two gap nature of the superconducting order parameter. Our results show that the strong temperature dependence of the B(c2) anisotropy can be understood as an interplay of the dominating gap on the sigma band, which possesses a small c-axis component of the Fermi velocity, with the induced superconductivity on the pi-band possessing a large c-axis component of the Fermi velocity. We provide analytic formulas for the anisotropy ratio at T=0 and T=T(c) and quantitatively predict the distortion of the vortex lattice based on our calculations.     

Band structure,Fermi surface,and superconducting gap in FeAs-based superconductors revealed by angle-resolved photoemission spectroscopy

In this paper, we present a brief review on our angle-resolved photoemission measurements on the band structure, Fermi surface, and superconducting gap of the newly-discovered FeAs-based high temperature superconductors. (1) The Fermi surface of the FeAs-based compounds are characterized by the hole-like Fermi surface sheets near Γ (0, 0) and the existence of singular Fermi spots near M(π,     

Fermi surface of an important nanosized metastable phase: Al3Li

Nanoscale particles embedded in a metallic matrix are of considerable interest as a route towards identifying and tailoring material properties. Al-Li alloys, which form ordered nanoscale precipitates of Al(3)Li for a range of concentrations, have been deployed successfully in the aerospace industry owing to their superior strength-to-weight ratio. The precipitates are metastable and their electronic structure has so far been inaccessible through conventional techniques. Here, we take advantage of the strong positron affinity of Li to probe the Fermi surface of nanoscale Al(3)Li precipitates.     

Line nodes in the superconducting gap function of noncentrosymmetric CePt3Si

The superconducting gap structure of recently discovered heavy fermion CePt(3)Si without spatial inversion symmetry was investigated by thermal transport measurements down to 40 mK. In zero field a residual T-linear term was clearly resolved as T --> 0, with a magnitude in good agreement with the value expected for a residual normal fluid with a nodal gap structure, together with a T2 dependence at high temperatures. With an applied magnetic field, the thermal conductivity grows rapidly, in dramatic contrast to fully gapped superconductors, and exhibits one-parameter scaling with T/sqrt[H]. These results place an important constraint on the order parameter symmetry; that is, CePt(3)Si is most likely to have line nodes.     

Pressure-induced change of the Fermi surface topology in Al-based solid solutions

An ab initio study of the electronic structure and the Fermi surface is carried out for random Al-Si and Al-Ge solid solutions. At a 10 at. % Si content, a topological transition of the neck-formation type is revealed, which can account for the experimentally observed peculiarities of the transport properties of the Al-Si system. A similar transition is also found in the Al-Ge system, and the appearance of the anomalous transport coefficients at Ge concentrations of about 10 at. % is predicted. In addition, it is shown that the increase in the concentration of the dopants gives rise to nesting of the Fermi surface sheets (superposition of electron-hole pockets). This peculiarity of the Fermi surface can be responsible for the enhancement of the superconductivity and the instability of the crystal structure observed in the Al_1?xSi_x and Al_1?xGe_x solid solutions.