Basic experimental data are presented for a new class of high-temperature superconductors — iron-based layered compounds of the types REOFeAs (RE = La, Ce, Nd, Pr, Sm, ...), AFe2As2 (A = Ba, Sr, ...), AFeAs (A = Li, ...), and FeSe(Te). The structure of electronic spectra in these compounds is discussed, including the correlation effects, as is the spectrum and role of collective excitations (phonons and spin waves). Basic models for describing various types of magnetic ordering and Cooper pairing are reviewed.

https://iopscience.iop.org/article/10.1070/PU2008v051n12ABEH006820/pdf

High-temperature superconductivity in iron-based layered compounds

High-Tc superconductivity in FeAs-based (pnictide) multilayers, evading temperature decoherence effects in a quantum condensate, is assigned to a Feshbach resonance (also called shape resonance) in the exchange-like interband pairing. The resonance is switched on by tuning the chemical potential at an electronic topological transition (ETT) near a band edge, where the Fermi surface topology of one of the subbands changes from one-dimensional (1D) to two-dimensional (2D) topology. We show that the tuning is realized by changing (i) the misfit strain between the superconducting planes and the spacers, (ii) the charge density, and (iii) the disorder. The system is at the verge of a catastrophe, i.e. near a structural and magnetic phase transition associated with the order-to-disorder phase transition of the stripes (analogous to the 1/8 stripe phase in cuprates). Fine tuning of both the chemical potential and the disorder pushes the critical temperature Ts of this phase transition to zero, giving a quantum critical point. Here the quantum lattice and magnetic fluctuations promote the Feshbach resonance of the exchange-like anisotropic pairing. This superconducting phase that resists the attacks of temperature is shown to be controlled by the interplay of the hopping energy between stripes and the quantum fluctuations. The superconducting gaps in the multiple Fermi surface spots reported by the recent ARPES experiment of Evtushinsky et al (2008 arXiv: 0809.4455) are shown to support the Feshbach scenario.

Feshbach resonance and mesoscopic phase separation near a quantum critical point in multiband FeAs-based superconductors

We report experimental data showing the Feshbach shape resonance in the electron doped MgB2 where the chemical potential is tuned by Al, Sc, and C substitutions. The scaling of the critical temperature Tc as a function of the Lifshitz parameter z = EΓ−EF, where EF is the chemical potential and EΓ is the energy of the Γ critical point where the σ Fermi surface changes from the 3D to a 2D topology, is reported. The resonant amplification of Tc(z) driven by the interband pairing is assigned to a Feshbach shape resonance characterized by quantum superposition of pairs in states corresponding to different spatial location and different parity. It is centered at z = 0 where the chemical potential is tuned to a Van Hove-Lifshits feature for the change of Fermi surface dimensionality in the electronic energy spectrum in one of the subbands. In this heterostructure at atomic limit the multiband superconductivity is in the clean limit because the disparity and negligible overlap between electron wavefunctions in different subbands suppresses the single electron interband impurity scattering rate. The emerging scenario from these experimental data suggests that the Feshbach shape resonance could be the mechanism for high Tc in particular nanostructured architectures.

Feshbach shape resonance in multiband superconductivity in heterostructures

High-temperature superconductivity in layered iron compounds

The two-gap structure in the superconducting state of MgB 2 gives rise to unusual thermodynamic properties which depart markedly from the isotropic single-band BCS model, both in their temperature- and field dependence. We report and discuss measurements of the specific heat up to 16 T on ceramic, and up to 14 T on single crystal samples, which demonstrate these effects in the bulk. The behavior in zero field is described in terms of two characteristic temperatures, a crossover temperature T c,π ≅13 K, and a critical temperature T c = T c,σ ≅38 K, whereas the mixed-state specific heat requires three characteristic fields, an isotropic crossover field μ 0 H c2,π ≅0.35 T, and an anisotropic upper critical field with extreme values μ 0 H c2,σ, c ≅3.5 T and μ 0 H c2,σ, ab ≅19 T, where the indexes π and σ refer to the three-dimensional and two-dimensional sheets of the Fermi surface. Irradiation-induced interband scattering tends to move the gaps toward a common value, and increases the upper critical field up to ∼28 T when T c ≅30 K.

Unusual effects of anisotropy on the specific heat of ceramic and single crystal MgB2

The possibility is indicated of applying the theory of superconductors with overlapping energy bands to describe the thermodynamic and electromagnetic properties of the high-temperature compounds La2–x(Ba,Sr)xCuO4 and YBa2Cu3O7–δ. The two-band model was used to obtain high values of Tc, two energy gaps 2Δ1/Tc > 3.5 and 2Δ2/Tc < 3.5, large negative values of d ln Tc/d ln V (V is the volume) in lanthanum ceramics, small values of the jump in the electron heat capacity at T = Tc, negative curvature of the upper critical magnetic field H2c near the transition temperature, etc. Such behavior of the above quantities is observed experimentally. A description is also obtained of the decrease in Tc as the disordering of oxygen increases, and also as copper atoms are replaced by a nonmagnetic impurity (Al, Zn, etc.). The main mechanism responsible for this decrease is the interband scattering of electrons by impurities and by randomly distributed oxygen vacancies. A theory has been developed of multiband superconductors which takes into account the points of high symmetry in momentum space. On the basis of this theory one can explain the existence of a plateau in the dependence of Tc on δ for YBa2Cu3O7–δ, and also in the dependence of Tc on x for La2–x(Ba,Sr)xCuO4, that has been observed in a number of experiments. Moreover this theory also explains the presence of two maxima in the dependence of Tc on pressure for Bi2Sr2CaCu2O8.

https://iopscience.iop.org/article/10.1070/PU1991v034n08ABEH002466/pdf

High-temperature superconductivity and the characteristics of the electronic energy spectrum

The possibility is indicated of applying the theory of superconductors with overlapping energy bands to describe the thermodynamic and electromagnetic properties of the high-temperature compounds $La_{2-x}(Ba,Sr)_xCuO_4$ and $YBa_2Ci_3O_{7-\delta}$. The two-band model was used to obtain high values of $T_c$, two energy gaps $2\Delta_1/T_c > 3.5$ and $2\Delta_2/T_c < 3.5$, large negative values of $d\ln T_c/d\ln V$ ($V$ is the volume) in lanthanum ceramics, small values of the jump in the electron heat capacity at $T=T_c$, negative curvature of the upper critical magnetic field $H_{c2}$ near the transition temperature, etc. Such behavior of the above quantities is observed experimentally. A description is also obtained of the decrease in $T_c$ as the disordering of oxygen increases, and also as copper atoms are replaced by a nonmagnetic impurity (Al, Zn, etc.). The main mechanism responsible for this decrease is the interband scattering of electrons by impurities and by randomly distributed oxygen vacancies. A theory has been developed of multiband superconductors which takes into account the points of high symmetry in momentum space. On the basis of this theory one can explain the existence of a plateau in the dependence of $T_c$ on $\delta$ for $YBa_2Cu_3O_{7-\delta}$, and also in the dependence of $T_c$ on $x$ for $La_{2-x}(Ba,Sr)_xCuO_4$, that has been observed in a number of experiments. Moreover, this theory also explains the presence of two maxima in the dependence of $T_c$ on pressure for $Bi_2Sr_2CaCu_2O_8$.

V. M. Vakalyuk

Papers

High-temperature superconductivity and the characteristics of the electronic energy spectrum

High-temperature superconductivity and the characteristics of the electronic energy spectrum