Small-q Phonon Mediated Unconventional Superconductivity in the Iron Pnictides

We report self-consistent calculations of the gap symmetry for the iron-based high-temperature superconductors using realistic small-q phonon mediated pairing potentials and four-band energy dispersions. When both electron and hole Fermi surface pockets are present, we obtain the nodeless $s_\pm$ state that was first encountered in a spin-fluctuations mechanism picture. Nodal gap structures such as $d_{x^2-y^2}$ and $s_\pm+d_{x^2-y^2}$ and even a p-wave triplet state, are accessible upon doping within our phononic mechanism. Our results resolve the conflict between phase sensitive experiments reporting a gap changing sign attributed previously only to a non-phononic mechanism and isotope effect measurements proving the involvement of phonons in the pairing.Comment: Final version. Corrected typos. Reference adde

Nematicity from mixed S_{+-} + d_{x^2-y^2} states in iron-based superconductors

We demonstrate that in iron-based superconductors, the extended S_{+-} SC state coexists with the d_{x^2-y^2} state under generic conditions. The mixed S_{+-} + d_{x^2-y^2} SC is a natural nematic state in which the tetragonal symmetry C_4 is broken to C_2 explaining puzzling findings of nematic SC in FeSe films [Science 332, 1410 (2011)]. Moreover, we report the possibility of a first order transition at low-T from the nematic S_{+-} + d_{x^2-y^2} state to the pure d_{x^2-y^2} state induced by the Zeeman magnetic field proposing an original experimental strategy for identifying our mixed nematic state in FeSe films. Extrapolating our findings, we argue that nematicity in non superconducting states of underdoped and undoped pnictides may reflect mixed S_{+-} + d_{x^2-y^2} Density Wave states.Comment: Improvements and corrections in the texte, references adde

Evolution of multi-gap superconductivity in the atomically thin limit: Strain-enhanced three-gap superconductivity in monolayer MgB2_2

Starting from first principles, we show the formation and evolution of superconducting gaps in MgB$_2$ at its ultrathin limit. Atomically thin MgB$_2$ is distinctly different from bulk MgB$_2$ in that surface states become comparable in electronic density to the bulk-like $\sigma$- and $\pi$-bands. Combining the ab initio electron-phonon coupling with the anisotropic Eliashberg equations, we show that monolayer MgB$_2$ develops three distinct superconducting gaps, on completely separate parts of the Fermi surface due to the emergent surface contribution. These gaps hybridize nontrivially with every extra monolayer added to the film, owing to the opening of additional coupling channels. Furthermore, we reveal that the three-gap superconductivity in monolayer MgB$_2$ is robust over the entire temperature range that stretches up to a considerably high critical temperature of 20 K. The latter can be boosted to $>$50 K under biaxial tensile strain of $\sim$ 4\%, which is an enhancement stronger than in any other graphene-related superconductor known to date.Comment: To appear in Phys. Re

Magnetic-field-induced chiral hidden order in URu2Si2

Two of the most striking and yet unresolved manifestations of the hidden order (HO) in URu2Si2, are associated on one hand with the double-step metamagnetic transitions and on the other with the giant anomalous Nernst signal. Both are observed when a magnetic field is applied along the c-axis. Here we provide for the first time a unified understanding of these puzzling phenomena and the related field-temperature (B-T) phase diagram. We demonstrate that the HO phase at finite fields can be explained with a chiral dxy+idx2-y2 spin density wave, assuming that the zero field HO contains only the time-reversal symmetry preserving idx2-y2 component. We argue that the presence of the field-induced chiral HO can be reflected in a distinctive non-linear B-dependence of the Kerr angle, when a Kerr experiment is conducted for finite fields. This fingerprint can be conclusive for the possible emergence of chirality in the HO.Comment: 8 pages and 9 figures main text + 6 pages supplementary material. Philosophical Magazine: Special Issue: Focused Issue on Hidden Order in URu2Si2 (May 2014

Magnetic field induced pattern of coexisting condensates in the superconducting state of CeCoIn5_5

In usual superconductors the carriers form Cooper pairs that have zero total momentum meaning that the superfluid density is homogeneous. We know for decades that it is a priori possible to observe at high fields the Fulde-Ferrel-Larkin-Ovchinikov (FFLO) state in which the superfluid density exhibits a field dependent modulation. The search for such inhomogeneous condensates in materials is the subject of tremendous excitement especially after the discovery of a promising high-field superconducting (HFSC) phase in CeCoIn$_5$. However, subsequent neutron and NMR experiments contradicted the FFLO picture in CeCoIn$_5$ establishing a puzzling coupling of the distinct HFSC state with a magnetic modulation. Here we show that, a novel type of exotic state is observed at high fields in CeCoIn$_5$ in which a pattern of coexisting condensates manifests. The specific pattern includes the d-wave singlet SC state, the staggered $\pi$-triplet SC state and Spin Density Waves (SDW). Because of particle-hole asymmetry these three condensates may either appear separately or all three together providing a new perspective on antiferromagnetic superconductors that may include high-$T_c$ cuprates and pniktides. The field induced transition to the pattern state in CeCoIn$_5$ is only a paradigm of a generic mechanism that prevents the elimination of the dominating condensate by the field via the formation of a pattern of condensates. This opens new avenues for the search of exotic states not only in electronic systems, but also in other systems where inhomogeneous condensates are actively discussed like trapped atomic fermi gases and dense quark matter

Unconventional superconducting phases in a correlated two-dimensional Fermi gas of nonstandard quasiparticles: a simple model

We discuss a detailed phase diagram and other microscopic characteristics on the applied magnetic field - temperature (H_a-T) plane for a simple model of correlated fluid represented by a two-dimensional (2D) gas of heavy quasiparticles with masses dependent on the spin direction and the effective field generated by the electron correlations. The consecutive transitions between the Bardeen-Cooper-Schrieffer (BCS) and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phases are either continuous or discontinuous, depending on the values of H_a and T. In the latter case, weak metamagnetic transitions occur at the BCS-FFLO boundary. We single out two different FFLO phases, as well as a reentrant behaviour of one of them at high fields. The results are compared with those for ordinary Landau quasiparticles in order to demonstrate the robustness of the FFLO states against the BCS state for the case with spin-dependent masses (SDM). We believe that the mechanism of FFLO stabilization by SDM is generic: other high-field low-temperature (HFLT) superconducting phases benefit from SDM as well.Comment: 10 pages, 4 figure

Fulde-Ferrell-Larkin-Ovchinnikov phase in the presence of pair hopping interaction

The recent experimental support for the presence of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase in the CeCoIn5 directed the attention towards the mechanisms responsible for this type of superconductivity. We investigate the FFLO state in a model where on--site/inter--site pairing coexists with repulsive pair hopping interaction. The latter interaction is interesting in that it leads to pairing with nonzero momentum of the Cooper pairs even in the absence of the external magnetic field (the so-called eta-pairing). It turns out that depending on the strength of the pair hopping interaction the magnetic field can induce one of two types of the FFLO phase with different spatial modulations of the order parameter. It is argued that the properties of the FFLO phase may give information about the magnitude of the pair hopping interaction. We also show that eta-pairing and d-wave superconductivity may coexist in the FFLO state. It holds true also for superconductors which in the absence of magnetic field are of pure d-wave type.Comment: 16 pages, 8 figure

Antiferromagnetic Phases in the Fulde-Ferrell-Larkin-Ovchinnikov State of CeCoIn_5

The antiferromagnetic (AFM) order in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state is analyzed on the basis of a Ginzburg-Landau theory. To examine the possible AFM-FFLO state in CeCoIn_5, we focus on the incommensurate AFM order characterized by the wave vector Q = Q_{0} \pm q_inc with Q_0 =(\pi,\pi,\pi) and q_inc \parallel [110] or [1-10] in the tetragonal crystal structure. We formulate the two component Ginzburg-Landau theory and investigate the two degenerate incommensurate AFM order. We show that the pinning of AFM moment due to the FFLO nodal planes leads to multiple phases in magnetic fields along [100] or [010]. The phase diagrams for various coupling constants between the two order parameters are shown for the comparison with CeCoIn_5. Experimental results of the NMR and neutron scattering measurements are discussed.Comment: 6pages, Proceedings of ICHE2010, To appear in J. Phys. Soc. Jpn. Supp

Ginzburg-Landau Analysis for the Antiferromagnetic Order in the Fulde-Ferrell-Larkin-Ovchinnikov Superconductor

Incommensurate antiferromangetic (AFM) order in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductor is investigated on the basis of the Ginzburg-Landau theory. We formulate the two component Ginzburg-Landau model to discuss two degenerate incommensurate AFM states in the tetragonal crystal structure. Owing to the broken translation symmetry in the FFLO state, a multiple phase diagram of single-q phase and double-q phase is obtained under the magnetic field along [100] or [010] direction. Magnetic properties in each phase are investigated and compared with the neutron scattering and NMR measurements for a heavy fermion superconductor CeCoIn_5. An ultrasonic measurement is proposed for a future experimental study to identify the AFM-FFLO state. The field orientation dependence of the AFM order in CeCoIn_5 is discussed.Comment: 8 page