Influence of s±s_{\pm} symmetry on unconventional superconductivity in pnictides above the Pauli limit -- two-band model study

The theoretical analysis of the Cooper pair susceptibility shows the two-band Fe-based superconductors (FeSC) to support the existence of the phase with nonzero Cooper pair momentum (called the Fulde--Ferrel--Larkin--Ovchinnikov phase or shortly FFLO), regardless of the order parameter symmetry. Moreover this phase for the FeSC model with $s_{\pm}$ symmetry is the ground state of the system near the Pauli limit. This article discusses the phase diagram $h-T$ for FeSC in the two-band model and its physical consequences. We compare the results for the superconducting order parameter with s-wave and $s_{\pm}$-wave symmetry -- in first case the FFLO phase can occur in both bands, while in second case only in one band. We analyze the resulting order parameter in real space -- showing that the FeSC with $s_{\pm}$-wave symmetry in the Pauli limit have typical properties of one-band systems, such as oscillations of the order parameter in real space with constant amplitude, whereas with s-wave symmetry the oscillations have an amplitude modulation. Discussing the free energy in the superconducting state we show that in absence of orbital effects, the phase transition from the BCS to the FFLO state is always first order, whereas from the FFLO phase to normal state is second order.Comment: European Physical Journal B (2013

Unconventional superconductivity in iron-base superconductors in a three-band model

Iron-base superconductors exhibits features of systems where the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) phase, a superconducting state with non-zero total momentum of Cooper pairs, is actively sought. Experimental and theoretical evidence points strongly to the FFLO phase in these materials above the Pauli limit. In this article we discuss the ground state of iron-base superconductors near the critical magnetic field and the full $h-T$ phase diagram for pnictides in case of intra-band pairing, in a three-band model with $s_{\pm}$ symmetry.Comment: RevTeX, 5 pages, 3 figures. Presented on "XVI National Conference of Superconductivity", October 7-12, 2013, Zakopane, Polan

The Fulde-Ferrell-Larkin-Ovchinnikov Superconductivity in Disordered Systems

The Fulde–Ferrell–Larkin–Ovchinnikov phase, with a spatially oscillating order parameter, may be induced by strongly magnetic field at low temperature. It is believed that the Fulde–Ferrell–Larkin–Ovchinnikov phase can exist only in homogeneous superconductors, and even weak impurity potential can lead to its destruction. The analysis of the Fulde–Ferrell–Larkin–Ovchinnikov phase in the Bogoliubov–de Gennes equation shows however, that this phase can exist in the presence of weak disorder. Using Bogoliubov–de Gennes equations, we discussed the influence of diagonal and off-diagonal disorder on the Fulde–Ferrell–Larkin–Ovchinnikov phase

Ruthenium dioxide RuO2_{2}: effect of the altermagnetism on the physical properties

Ruthenium oxide with the rutile structure is one of example of altermagnets. These systems are characterized by compensated magnetic moments (typical for antiferromagnets) and strong time reversal symmetry breaking (typical for ferromagnets). However, in such cases, the electronic band structure exhibit strong spin splitting along some directions in the momentum space. Occurrence of the compensated magnetic textures allows for realization of surfaces with specific magnetization, which dependent on the surface orientation and/or its termination. Here, we study interplay between the electronic surface states and the surface magnetization. We show that the spin-resolved spectra strongly depends on a direction in reciprocal space. Such properties can be used for the experimental confirmation of the altermagnetism in RuO$_{2}$ within the spectroscopic techniques. Additionally, we show that the most modified orbitals in the system are $d_{z^{2}}$ and $d_{xy}$ orbitals of Ru. Similarly, the Ru $e_{g}$ states are most sensitive on epitaxial strain, what can suggest some link between altermagnetism and strain.Comment: 7 pages, 8 figure

First principles study of topological phase in chains of 3d3d transition metals

Recent experiments have shown the signatures of Majorana bound states at the ends of magnetic chains deposited on a superconducting substrate. Here, we employ first principles calculations to directly investigate the topological properties of $3d$ transition metal nanochains (i.e., Mn, Cr, Fe and Co). In contrast to the previous studies [Nadj-Perge et al. Science 346, 602 (2014) and Ruby et al. Nano Lett. 17, 4473 (2017)], we found the exact tight binding models in the Wannier orbital basis for the isolated chains as well as for the surface--deposited wires. Based on these models, we calculate topological invariant of $\mathbb{Z}_2$ phase for all systems. Our results for the isolated chains demonstrate the existence of the topological phase only in the Mn and Co systems. We considered also a non-collinear magnetic order as a source of the non--trivial topological phase and found that this type of magnetic order is not a stable ground state in the Fe and Co isolated chains. Further studies showed that a coupling between the chain and substrate leads to strong modification of the band structure. Moreover, the analysis of the topological invariant indicates a possibility of emergence of the topological phase in all studied nanochains deposited on the Pb surface. Therefore, our results demonstrate an important role of the coupling between deposited atoms and a substrate for topological properties of nanosystems.Comment: 11 pages, 7 figure