Unconventional superconductivity originating from disconnected Fermi surfaces in correlated superconductors : A case study for iron pnictides

We first generally summarize the effect of disconnected Fermi surfaces in spin fluctuation mediated superconductivity. We argue that disconnected Fermi surfaces are favorable in that the sign of the superconducting gap can be changed without nodal lines intersecting the Fermi surface. Then, as an example of actual materials that have disconnected Fermi surfaces, we focus on the iron-based high T_c superconductors. We construct a model that contains all of the five Fe d bands, and apply random-phase approximation. We find that multiple spin fluctuation modes develop due to the nesting between disconnected Fermi surfaces, and the superconductivity originating from the cooperation or competition between these multiple spin fluctuation modes depends on the lattice structure. In particular, the appearance of the Fermi surface around (π, π) in the unfolded Brillouin zone is sensitive to the pnictogen height h_Pn measured from the Fe plane, and the height can act as a switch between high T_c nodeless and low T_c nodal pairings. In the high T_c case, the superconducting gap is fully open on all of the five Fermi surfaces, but changes sign across the nesting vectors that bridge the disconnected Fermi surfaces.     

Fermi surface of Ba1−xKxFe2As2 as probed by angle-resolved photoemission

Here we apply high resolution angle-resolved photoemission spectroscopy (ARPES) using a wide excitation energy range to probe the electronic structure and the Fermi surface topology of the Ba_1?xK_xFe₂As₂ (T_c = 32 K) superconductor. We find significant deviations in the low energy band structure from that predicted in calculations. A set of Fermi surface sheets with unexpected topology is detected at the Brillouin zone boundary. At the X-symmetry point the Fermi surface is formed by a shallow electron-like pocket surrounded by four hole-like pockets elongated in Γ?X and Γ?Y directions.     

Jastrow correlations and the hard sphere fermi gas

A diagrammatic expansion for the variational energy of a Fermi fluid using state-dependent correlations is described. It is used to obtain the leading order terms (up to (k_Fa)²) in the low density expansion of a hard sphere Fermi gas using both state-dependent and state-independent correlations.     

de Haas van Alphen (dHvA) effect in two-dimensional (2D) conductors: susceptibility oscillations

A new scheme for analyzing the de Haas van Alphen (dHvA) effect in nearly two dimensional (2D) metals (i.e. with nearly cylindrical Fermi surface) is presented. The envelope of the magnetic susceptibility oscillations is calculated in the entire range of magnetic fields and temperatures. The resulting envelope function is found to be proportional to a universal function of the dimensionless parameter Q=hω_c/k _B T. The upper (i.e. paramagnetic) branch of the susceptibility envelope has a maximum at a certain Q = 5.45. This universal value may be useful for determining the effective cyclotron mass and the Fermi energy of nearly 2D metals. A simple relation between magnetization oscillations amplitude and calculated susceptibility amplitudes is derived. The corresponding limiting formulae for the magnetization oscillations envelope are found to match smoothly around the value X = 2π²/Q?2 of the Lifshitz-Kosevich (LK) smearing parameter. The influence of Fermi surface sheets with open orbits on magneto-quantum oscillations is considered. Triangle-like rather than saw-tooth-like oscillations at ultralow temperatures are obtained and substantially diminished magnetization and susceptibility amplitudes are calculated. This suggests the possibility of estimating the band structure parameters of Fermi surface sheets from magneto-quantum oscillations measurements.     

Neutrons scattering as a method for fermi surface investigations

High-T_c A15-compounds are suggested as suitable materials for which Fermi surface investigations by means of thermal neutron scattering might be possible. The structure of the differential neutron—d-electron cross section at small scattering angles is connected in a simple way with parameters defining the Fermi surface of the linear chain model. The magnitude of the cross section is estimated are compared with the phonon background.     

Finite Fermi systems theory and self-consistency relations

The self-consistent theory of the finite Fermi systems is outlined. This approach is based on the same Fermi liquid theory principles as the familiar theory for finite Fermi systems (FFS) by Migdal. We show that the basic Fermi system properties can be evaluated in terms of the quasiparticle Lagrangian L_q which incorporates the energy dependency effects. This Lagrangian is defined so that the corresponding Lagrange equations should coincide with the FFS theory equations of motion of the quasiparticles. The quasiparticle energy E_q defined in the terms of t he quasiparticle Lagrangian L_q according to the usual canonical rules is shown to be equal to the binding energy E_o of the system. For a given Lagrangian L_q the particle densities in nuclei, the nuclear single-particle spectra, the low-lying collective states (LCS) properties, and the amplitude of the interquasiparticle interaction are also evaluated. The suggested approach is compared with the Hartree-Fock theory with effective forces.     

On the fermi velocity and static conductivity of epitaxial graphene

The models of the energy density of states of a metallic or semiconductor substrate, which does not further lead to divergences, have been proposed to calculate the characteristics of epitaxial graphene. The Fermi velocity of epitaxial graphene formed on a metal has been shown to be greater than that in free-standing graphene irrespective of the position of the Fermi level. On the contrary, the Fermi velocity of graphene formed on a semiconductor is lower so that the lower is the Fermi velocity, the closer is the Fermi level to the center of the band gap of the semiconductor. The zero-temperature static conductivity σ of epitaxial graphene has been calculated according to the Kubo-Greenwood formula. The quantity σ_m of undoped graphene on metal has been shown to decrease with an increase in the deviation of the Dirac point ?_D (which coincides with the Fermi level of the system) from the center of the conduction band of the substrate. In the case of the semiconductor substrate, the static conductivity σ_sc turns out to be nonzero and amounts to σ_sc = 2e ²/π?-only under the condition ?_F =?′_D, where ?′_D is the Dirac-point energy renormalized by the interaction with the substrate.     

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.     

Hidden Fermi Surface in K<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>2–<Emphasis Type="Italic">y</Emphasis></Subscript>Se<Subscript>2</Subscript>: LDA + DMFT Study

In this paper we provide theoretical LDA + DMFT support of recent angle-resolved photoemission spectroscopy (ARPES) observation of the so-called hidden hole-like band and corresponding hidden Fermi surface sheet near Γ-point in the K_0.62Fe_1.7Se₂ compound. To some extent, this is a solution to the long-standing riddle of Fermi surface absence around Γ-point in the K_xFe_2–ySe₂ class of iron chalcogenide superconductors. In accordance with the experimental data, Fermi surface was found near the Γ-point within LDA + DMFT calculations. Based on the LDA + DMFT analysis in this paper it is shown that the largest of the experimental Fermi surface sheets is actually formed by a hybrid Fe-3d ( xy, xz, yz )quasiparticle band. It is also shown that the Fermi surface is not a simple circle as DFT-LDA predicts, but has (according to the LDA + DMFT) a more complicated “propeller”-like structure due to correlations and multiorbital nature of the K_xFe_2–ySe₂ materials. While the smallest experimental Fermi surface around Γ-point is in some sense fictitious, since it is formed by the summation of the intensities of the spectral function associated with “propeller” loupes and is not connected to any of quasiparticle bands.     

Mirror nesting: A superconducting pairing of carriers with a large momentum

The coincidence of Fermi contour portions with isolines of zero kinetic energy of the relative pair motion (the pair Fermi contour) is a necessary condition for superconducting pairing of carriers with a large total pair momentum. In high?T _c cuprates, this situation can occur either due to the formation of a stripe structure or in the absence of the stripe structure when the Fermi contour satisfies the mirror nesting condition. A gradual deviation from this condition leads to a decrease in the superconducting energy gap to zero.     

Fermi momentum in an electron fluid

The Fermi momentum of an electron fluid with Coulomb interaction is determined as a function of density and temperature. Due to the many body interaction, the actual Fermi momentum decreases withr _s faster than the case without interaction, wherer _s is the electron radius measured in the units of the Bohr radius. This depression effect is enhanced at lower temperatures.     

Electronic properties of iron arsenic high temperature superconductors revealed by angle resolved photoemission spectroscopy (ARPES)

We present an overview of the electronic properties of iron arsenic high temperature superconductors with emphasis on low energy band dispersion, Fermi surface and superconducting gap. ARPES data is compared with full-potential linearized plane wave (FLAPW) calculations. We focus on single layer NdFeAsO_0.9F_0.1 (R1111) and two layer Ba_1?xK_xFe₂As₂ (B122) compounds. We find general similarities between experimental data and calculations in terms of character of Fermi surface pockets, and overall band dispersion. We also find a number of differences in details of the shape and size of the Fermi surfaces as well as the exact energy location of the bands, which indicate that magnetic interaction and ordering significantly affects the electronic properties of these materials. The Fermi surface consists of several hole pockets centered at Γ and electron pockets located in zone corners. The size and shape of the Fermi surface changes significantly with doping. Emergence of a coherent peak below the critical temperature T_c and diminished spectral weight at the chemical potential above T_c closely resembles the spectral characteristics of the cuprates, however the nodeless superconducting gap clearly excludes the possibility of d-wave order parameter. Instead it points to s-wave or extended s-wave symmetry of the order parameter.     

Field-emission from high-index crystallographic faces: A scattering approach

We reconsider the usual theory of electron field-emission to deal with the case of high index crystallographic directions. The general requirement for an electron to be emitted is k_∥ + G ≈ 0, where k_∥ is the component of the electron wavevector parallel to the crystal surface, and G is a vector of the two-dimensional reciprocal lattice of the surface. In the case of high index crystallographic directions this requirement can be fulfilled for a large number of k points on the Fermi surface. A scattering approach is proposed to solve the problem of wavefunction matching at the crystal boundary. The calculation is carried out to the first Born approximation for the case of free electrons. Then the extension to arbitrary band structures in discussed, and a detailed treatment is given in the case of a nearly free electron model. The main result is that every k on the Fermi surface can contribute to the emitted current, and not only those with k_∥ ≈ 0 as in usual “specular” field-emission theory. This can yield considerable changes in such cases where there is no k vector normal to the crystal surface at the Fermi level.     

Pseudogap phenomena in ultracold atomic Fermi gases

The pairing and superfluid phenomena in a two-component ultracold atomic Fermi gas is an analogue of Cooper pairing and superconductivity in an electron system, in particular, the high Tc superconductors. Owing to the various tunable parameters that have been made accessible experimentally in recent years, atomic Fermi gases can be explored as a prototype or quantum sinmlator of superconductors. It is hoped that, utilizing such an analogy, the study of atomic Fermi gases may shed light to the mysteries of high Tc superconductivity. One obstacle to the ultimate understand- ing of high Tc superconductivity, from day one of its discovery, is the anomalous yet widespread pseudogap phenomena, for which a consensus is yet to be reached within the physics comnnmity, after over 27 years of intensive research efforts. In this article, we shall review the progress in the study of pseudogap phenomena in atomic Fermi gases in terms of both theoretical understanding and experimental observations. We show that there is strong, unambiguous evidence for the existence of a pseudogap in strongly interacting Fermi gases. In this context, we shall present a pairing fuctuation theory of the pseudogap physics and show that it is indeed a strong candidate theory for high Tc superconductivity.     

Magnetic moments of spherical nuclei: Status of the problem and unsolved issues

Dipole magnetic moments of more than 100 odd spherical nuclei are calculated within the theory of finite Fermi systems. For the effective interaction of nucleons within the theory of finite Fermi systems, use is made of a version that takes into account nuclear-medium-modified amplitudes for the exchange of one pion and one rho meson. A new tensor local charge ζ _t is incorporated in the theory of finite Fermi systems in addition to the known orbital (ζ _l ) and spin (ζ _s ) local charges. Good agreement with experimental data, at a level of 0.1 to 0.2μ _N , is obtained for the overwhelming majority of the nuclei considered here. Several cases of a significant discrepancy with experimental data, at a level of 0.3 to 0.5μ _N , are revealed. Possibilities for removing these discrepancies are discussed. A detailed comparison with known results obtained within the multiparticle shell model is performed for 2p-to 1f-shell nuclei. Cases where the standard theory of finite Fermi systems must be extended by taking into account multiparticle configurations are found. Magnetic moments are analyzed for a number of long isotopic chains. Several new experimental values of magnetic moments for copper isotopes far from the beta-stability valleys are known. For the example of the copper-isotope chain, it is shown how the emergence of a deformation in the ground state of a nucleus can be revealed on the basis of a systematic analysis of magnetic moments.     

Fermi sea effects on pion and kaon properties in the Nambu-Jona-Lasinio model

We study modifications of hadronic properties in dense nuclear and neutron matter, in the context of a generalized three-flavor Nambu-Jona-Lasinio model, focussing mainly on Fermi sea effects. In symmetric nuclear matter, we observe the splitting between K^± and the occurrence of a low-lying K⁻ branch at baryonic densities around 0.4 times normal nuclear matter density ϱ₀. This branch is not significantly affected by the inclusion of strange matter in the medium while the flavor asymmetry is preserved. A similar π⁺ branch is found in neutron matter at neutron densities around 0.3ϱ₀. These low-lying modes are particle-hole excitations of the Fermi sea and decrease smoothly with density. In any case, the vacuum does not exhibit signs of instability associated with boson condensation.     

General properties of a magnetic-field-induced landau Fermi liquid in high-temperature superconductors and heavy fermion metals

It has been shown that the magnetic-field-induced transition from a non-Fermi-liquid state to a Fermi liquid state in the Tl₂Ba₂CuO_{6 + x} high-temperature superconductor is similar to a transition observed in heavy fermion metals. This behavior is explained in the theory of the Fermi condensate quantum-phase transition implying the existence of Landau quasiparticles. The Fermi condensate quantum-phase transition can be considered as a universal cause of the strongly correlated behavior observed in various metals and liquids such as high-temperature superconductors, heavy fermion metals, and two-dimensional Fermi systems.     

Electronic structure reconstruction of CaFe2As2 in the spin density wave state

The electronic structure of CaFe₂As₂, a parent compound of iron-based superconductors, is studied with high-resolution angle-resolved photoemission spectroscopy. The electronic structure of CaFe₂As₂ in the paramagnetic state is consistent with that of density-functional theory calculations. We show that the electronic structure of this compound is significantly reconstructed when entering the spin density wave state. We could resolve two hole-like pockets and four electron-like pockets around the (0, 0) point, and one electron-like pocket surrounded with a pair of electron- and hole-like pockets around the (π, π) point in the spin density wave state. Therefore, the complicated Fermi surface topology and electronic structure near Fermi surface of CaFe₂As₂ illustrate that there exists unconventional electronic reconstruction in the spin density wave state, which cannot be explained by the band folding and Fermi surface nesting pictures.     

Magnetic properties of TiZr alloys

Magnetic susceptibility measurements of TiZr alloys are reported. Zirconium, by alloying, acts on the Néel temperature giving rise to a complicated magnetic phase diagram. The main mechanism in the T_N shift is the change of the Fermi level. The experimental dependence between T_N and X₀ is in good agreement with the theoretical dependence obtained on the Fedders-Martin model by taking into account the effect of nonmagnetic impurity on the Fermi level.