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To change the velocity of an electron requires that a Lorentz force acts on it, through an electric or a magnetic field. We point out that within the conventional understanding of superconductivity electrons appear to change their velocity in the absence of Lorentz forces. This indicates a fundamental problem with the conventional theory of superconductivity. A hypothesis is proposed to resolve this difficulty. This hypothesis is consistent with the theory of hole superconductivity. [Copyright &y& Elsevier]

We consider the behavior of strongly correlated electron liquid in high-temperature superconductors within the framework of the fermion condensation model. We show that at low temperatures the normal state recovered by the application of a magnetic field larger than the critical field can be viewed as the Landau Fermi liquid induced by the magnetic field. In this state, the Wiedemann–Franz law and the Korringa law are held and the elementary excitations are the Landau Fermi liquid quasiparticles. Contrary to what might be expected from the Landau theory, the effective mass of quasiparticles depends on the magnetic field. The recent experimental verifications of the Wiedemann–Franz law in heavily hole-overdoped, overdoped and optimally doped cuprates and the verification of the Korringa law in the electron-doped copper oxide superconductor strongly support the existence of fermion condensate in high-Tc metals. [Copyright &y& Elsevier]

For superconducting cylinders with sufficiently small sizes, normal states exist around half-integer flux quanta even at zero temperature. Coupling between coaxial superconducting cylinders could merge the phase boundaries of the outer cylinder between n2=0 and n2=1 states and the zero resistance fields for the outer cylinder can be enhanced from around 0.5φ0 to around 1.5φ0. [Copyright &y& Elsevier]

We construct asymptotic expansions of Laplace type for the time-dependent quantum averages for Bose systems with many degrees of freedom, initially populated in coherent states. These solutions are localized in phase space, and they are different from the usual oscillating asymptotics for the quasi-classical wave functions. These expansions are valid on any fixed time interval, and caustics do not contribute to the asymptotics. [Copyright &y& Elsevier]

A one-dimensional system of nonintersecting Brownian particles is constructed as the diffusion scaling limit of Fisher''s vicious random walk model. N Brownian particles start from the origin at time t=0 and undergo mutually avoiding motion until a finite time t=T. Dynamical correlation functions among the walkers are exactly evaluated in the case with a wall at the origin. Taking an asymptotic limit N→∞, we observe discontinuous transitions in the dynamical correlations. It is further shown that the vicious walk model with a wall is equivalent to a parametric random matrix model describing the crossover between the Bogoliubov–deGennes universality classes. [Copyright &y& Elsevier]

We present a semi-perturbative approach which yields an infrared-stable fixed point in the Ginzburg–Landau for N=2, where N/2 is the number of complex components. The calculations are done in d=3 dimensions and below Tc, where the renormalization group functions can be expressed directly as functions of the Ginzburg parameter κ which is the ratio between the two fundamental scales of the problem, the penetration depth λ and the correlation length ξ. We find a charged fixed point for κ>1/√ of 2, that is, in the type II regime, where Δκ≡κ−1/2, that is, in the type II regime, where Δκ≡κ−1/√ of 2 is shown to be a natural expansion parameter. This parameter controls a momentum space instability in the two-point correlation function of the order field. This instability appears at a non-zero wave-vector p0 whose magnitude scales like ∼Δκβ¯, with a critical exponent β¯=1/2 in the one-loop approximation, a behavior known from magnetic systems with a Lifshitz point in the phase diagram. This momentum space instability is argued to be the origin of the negative η-exponent of the order field. [Copyright &y& Elsevier]

For the one-dimensional t–J model the optical conductivity σ(ω,T) is examined in the limit of small ω and T. The treatment is based on string operators appropriate for description of spin–charge separation. In the limit of small hole doping the inverse relaxation time Γ(ω,T) is proportional to ω2ln&z.sfnc;max(ω,T)/ω&z.sfnc;. Such a behaviour of Γ(ω,T) manifests the presence of a finite Drude weight D(T) at T>0. [Copyright &y& Elsevier]

Charge ordering instabilities are studied in a multiorbital model of cuprate superconductors. A known, key feature of this model is that the large local Coulomb interaction in the Cu ${{d}_{{{x}^{2}}-{{y}^{2}}}}$ orbitals generates local moments with short range antiferromagnetic (AF) correlations. The strong simplifying ansatz that these moments are static and ordered allows us to explore a regime not generally accessible to weak-coupling approaches. The AF correlations lead to a pseudogap-like reconstruction of the Fermi surface. We find that the leading charge instability within this pseudogap-like state is to a phase with a spatially modulated transfer of charge between neighbouring oxygen p _x and p _y orbitals accompanied by weak modulations of the charge density on the Cu ${{d}_{{{x}^{2}}-{{y}^{2}}}}$ orbitals. As a prime result of the AF Fermi-surface reconstruction, the wavevectors of the charge modulations are oriented along the crystalline axes with a periodicity that agrees quantitatively with experiments. This suggests a resolution to a discrepancy between experiments, which find axial order, and previous theoretical calculations, which find modulation wavevectors along the Brillouin zone diagonal. The axial order is stabilized by hopping processes via the Cu4 s orbital, which is commonly not included in model analyses of cuprate superconductors. The main implication of our results is that charge order emerges from the pseudogap state, and is not the primary source of the pseudogap.

We identify a novel type of periodic charge-density modulation in triangular-lattice materials near an incommensurate filling. These charge modulations are suppressed by a strong pairing interaction, leaving the pairing modulations with the same periodicity. We also explore the competition between charge-density waves and superconductivity in chiral $d+id$ or f -wave superconductors, and we discuss the implications for cobaltate and organic superconductors. Furthermore, we self-consistently calculate the local density of states around impurities and show that the gap anisotropy of the chiral $d+id$ state is doping-dependent, which naturally explains the close-to-nodal gap features of cobaltate superconductors as indicated by experiments.

We discuss a two-color SASE free-electron laser (FEL) amplifier where the time and energy separation of two separated radiation pulses are controlled by manipulation of the electron beam phase space. Two electron beamlets with adjustable time and energy spacing are generated in an RF photo-injector illuminating the cathode with a comb-like laser pulse followed by RF compression in the linear accelerator. We review the electron beam manipulation technique to generate bunches with time and energy properties suitable for driving two-color FEL radiation. Experimental measurements at the SPARC-LAB facility illustrate the flexibility of the scheme for the generation of two-color FEL spectra.

The electron band around the M point in the $({\text{Ba}_{1-x}}{{\text{K}}_{x}}){\text{Fe}_{2}}{\text{As}_{2}}$ compound, which is completely lifted up above the Fermi level for $x>0.7$ and hence its Fermi surface (FS) disappears, can still play the role of the main pairing resource by exchanging inter-band repulsive interaction with the main hole band ( h1 ) around the $\Gamma $ point. This hidden electron band, which develops the superconducting order parameter (OP) ${{\Delta }_{e}}$ but has no FS, displays a shadow gap feature which is easily detected by various experimental probes such as angle-resolved photoemission spectroscopy (ARPES) and tunneling measurements. We also show that the formation of the nodal gap ${{\Delta }_{\text{nodal}}}$ with ${{A}_{1g}}$ symmetry on another hole pocket ( h2 ) around the $\Gamma $ point with a larger FS is stabilized due to the balance of the inter-band repulsive interactions from the main hole band ( h1 ) with the OP ${{\Delta }_{h1}}={{\Delta }_{+}}$ , and the hidden electron band with the OP ${{\Delta }_{e}}={{\Delta }_{-}}$ .

Abstract: Superconductivity is not considered as a Bose–Einstein condensation, because the creation and annihilation operators of Cooper pairs do not satisfy bosonic commutation relations. However, collective pairs can be constructed by a linear combination of Cooper pairs and we demonstrate in this Letter that these collective Cooper pairs have bosonic nature. In addition, the Bardeen–Cooper–Schrieffer (BCS) superconducting ground state can be built by means of these pairs and in consequence, could be treated as a Bose–Einstein condensate. [Copyright &y& Elsevier]

An extensive theoretical study of the superconducting state parameters (SSPs), that is, electron-phonon coupling strength λ, Coulomb pseudopotential μ*, transition temperature T C , isotope effect exponent α, and effective interaction strength N 0 V of Pb-Tl-Bi alloys, including Tl0.90Bi0.10, Pb0.40Tl0.60, Pb0.60Tl0.40, Pb0.80Tl0.20, Pb0.60Tl0.20Bi0.20, Pb0.90Bi0.10, Pb0.80Bi0.20, Pb0.70Bi0.30, Pb0.65Bi0.35, and Pb0.45Bi0.55, has been conducted for the first time using model potential formalism. A considerable influence of various exchange and correlation functions on λ and μ* is found. The obtained values of the SSPs are in qualitative agreement with the available experimental data. [ABSTRACT FROM AUTHOR]

Abstract: Recent angle resolved photoemission experiments by Kanigel et al., [Nature Physics 2 (2006) 447] show that the low energy electronic excitations in the normal state of underdoped cuprate superconductors exist on open Fermi arcs which shrink upon cooling, and appear to extrapolate to nodal points at zero temperature. We show that this non-Fermi liquid phenomenon could result from the underdoped normal state lying in the quantum critical regime of a superconductor–insulator transition, and present computational results for the electron spectral function which are in reasonable agreement with the experimental results. [Copyright &y& Elsevier]

Abstract: With in mind, we study a half-filled-band Hubbard model on an anisotropic triangular lattice, using an optimization variational Monte Carlo method. As a trial function, we use a state having antiferromagnetic (AF) and -wave singlet gaps simultaneously, and allow for a band renormalization effect in it. It is found that, at a fairy small value of , a first-order conductor-to-AF insulator transition takes place even for remarkably large strength of frustration. In the optimized insulating states, the staggered magnetization is always robust, even for fully frustrated case. Thereby, a regime of robust superconductor as well as of a nonmagnetic insulator vanishes in this wave function. [Copyright &y& Elsevier]

We suggest that the high-temperature superconductivity can be attributed to the director-roles of the van Hove singularity between an electron-electron interaction and an electron-phonon interaction. The difference between the critical temperature and the pairing temperature is presented, and the Fermi arc, the d-wave symmetry and the poor conductivity, etc., are discussed. In particular, the non- s-wave symmetry is predicted to have the highest T c for superconductors. [ABSTRACT FROM AUTHOR]

We study the ferromagnetic superconductor of UGe2 applying our previous model [Phys. Rev. B 61 (2000), 4289] for the high transition temperature superconductivity (HTSC). The Coulomb interaction for triplet electron pairs is reduced by a difference of the exchange interaction. In the case of UGe2 including other heavy fermion superconductors, coexistence of triplet superconductivity and ferromagnetism is possible in the case of our scheme. We also investigate the pressure-dependence of Curie temperature, Tc and superconducting temperature, Tsc. [Copyright &y& Elsevier]

Gutzwiller variational method is applied to an effective 2D Hubbard model to examine the recently proposed gossamer superconductivity [LANL cond-mat/0209269 (2002)]. The ground state at half filled is a gossamer superconductor for smaller intrasite Coulomb repulsion U and a Mott insulator for larger U. The transition is of first type. Away from half filled, the gossamer superconductor is adiabatically evolved into the RVB superconducting state. [Copyright &y& Elsevier]