The symmetry of the superconducting order parameter in quasi-two-dimensional BEDT-TTF organic superconductors is a subject of ongoing debate. We report ab initio density functional theory calculations for a number of organic superconductors containing κ-type layers. Using projective Wannier functions we derive parameters of a common low-energy Hamiltonian based on individual BEDT-TTF molecular orbitals. In a random phase approximation spin-fluctuation approach we investigate the evolution of the superconducting pairing symmetry within this model and point out a phase-transition between extended s + d x 2 −y 2 and dxy symmetry. We discuss the origin of the mixed order parameter and the relation between the realistic molecule description and the widely used dimer approximation. Based on our ab initio calculations we position the investigated materials in the obtained molecule model phase diagram and simulate scanning tunneling spectroscopy experiments for selected cases. Our calculations show that many κ-type materials lie close to the phase transition line between the two pairing symmetry types found in our calculation, possibly explaining the multitude of contradictory experiments in this field.

We report on a combined theoretical and experimental investigation of the superconducting state in the quasi-two-dimensional organic superconductor κ-(ET)_{2}Cu[N(CN)_{2}]Br. Applying spin-fluctuation theory to a low-energy, material-specific Hamiltonian derived from ab initio density functional theory we calculate the quasiparticle density of states in the superconducting state. We find a distinct three-peak structure that results from a strongly anisotropic mixed-symmetry superconducting gap with eight nodes and twofold rotational symmetry. This theoretical prediction is supported by low-temperature scanning tunneling spectroscopy on in situ cleaved single crystals of κ-(ET)_{2}Cu[N(CN)_{2}]Br with the tunneling direction parallel to the layered structure.

We report on measurements of the magnetic susceptibility and electrical resistance under He-gas pressure on single crystals of Ca(Fe1−xCox)2As2. We find that for properly heat-treated crystals with modest Co-concentration, x = 0.028, the salient ground states associated with iron-arsenide superconductors, i.e., orthorhombic/antiferromagnetic (o/afm), superconducting, and nonmagnetic collapsed-tetragonal (cT) states can be accessed all in one sample with reasonably small and truly hydrostatic pressure. This is possible owing to the extreme sensitivity of the o/afm (for T ≤ Ts,N ) and superconducting (T ≤ Tc) states against variation of pressure, disclosing pressure coefficients of dTs,N /dP = -(1100 ± 50) K/GPa and dTc/dP = -(60 ± 3) K/GPa, respectively. Systematic investigations of the various phase transitions and ground states via pressure tuning revealed no coexistence of bulk superconductivity (sc) with the o/afm state which we link to the strongly firstorder character of the corresponding structural/magnetic transition in this compound. Our results, together with literature results, indicate that preserving fluctuations associated with the o/afm transition to low enough temperatures is vital for sc to form.

In the recently synthesized Lix(NH2)y(NH3)zFe2Se2 family of iron chalcogenides a molecular spacer consisting of lithium ions, lithium amide and ammonia separates layers of FeSe. It has been shown that upon variation of the chemical composition of the spacer layer, superconducting transition temperatures can reach Tc ∼ 44 K, but the relative importance of the layer separation and effective doping to the Tc enhancement is currently unclear. Using state of the art band structure unfolding techniques, we construct eight-orbital models from ab-initio density functional theory calculations for these materials. Within an RPA spin-fluctuation approach, we show that the electron doping enhances the superconducting pairing, which is of s±-symmetry and explain the experimentally observed limit to Tc in the molecular spacer intercalated FeSe class of materials.PACS numbers: 71.15. Mb, 71.18.+y, 74.20.Pq, 74.24.Ha, 74.70.Xa After the discovery of iron based superconductors in 2008, transition temperatures were quickly improved to ∼ 56 K by chemical substitution 1 . Recently, the possible discovery of superconductivity with T c = 65 K 2 and even T c ∼ 100 K 3 in single-layer FeSe films grown by molecular beam epitaxy on SrTiO 3 showed that temperatures close to and above the boiling point of liquid nitrogen (77 K) might be achievable. These results have initiated an intensive debate regarding the origin of the high superconducting temperatures and the role played by electron doping via substrate, dimensionality and lattice strain.While bulk FeSe has a T c of only 8-10 K, it has been known for some time that it can be substantially enhanced, to 40 K or higher by alkali intercalation 4 . Materials with a single alkali A = K, Cs, Rb between FeSe layers of nominal form A x Fe 2−y Se 2 have been intensively studied, and shown to display a wide variety of unusual behaviors relative to the Fe pnictide superconducting materials 5 . These include likely phase separation into an insulating phase with block antiferromagnetism and ordered Fe vacancies, and a superconducting phase that is strongly alkali deficient and whose Fermi surface as measured by ARPES apparently contains no holelike Fermi surface pockets, in contrast to Fe-pnictides. Since the popular spin fluctuation scenario for s ± pairing relies on near nesting of hole and electron pockets, it has been speculated that a different mechanism for pairing might be present in these materials, and even within the spin fluctuation approach, different gap symmetries including d-wave pairing have been proposed 6-9 . The gap symmetry and structure is still controversial, however 10,11 .In addition to the unusual doping, speculation on the origin of the higher T c has centered on the intriguing possibility that enhancing the FeSe layer spacing improves the two-dimensionality of the band structure and hence Fermi surface nesting 12,13 . In an effort to investigate this latter effect, organic molecular complexes including alkalis were recently intercalated between the FeSe layers 12-19 ,...

We investigate the role of correlations in the tetragonal and collapsed tetragonal phases of CaFe2As2 by performing charge self-consistent DFT+DMFT (density functional theory combined with dynamical mean-field theory) calculations. While the topology of the Fermi surface is basically unaffected by the inclusion of correlation effects, we find important orbital-dependent mass renormalizations which show good agreement with recent angle-resolved photoemission (ARPES) experiments. Moreover, we observe a markedly different behavior of these quantities between the low-pressure tetragonal and the high-pressure collapsed tetragonal phase. We attribute these effects to the increased hybridization between the iron-and arsenic orbitals as one enters the collapsed tetragonal phase.

Materials with a perfect kagome lattice structure of magnetic ions are intensively sought for, because they may exhibit exotic ground states like the a quantum spin liquid phase. Barlowite is a natural mineral that features perfect kagome layers of copper ions. However, in barlowite there are also copper ions between the kagome layers, which mediate strong interkagome couplings and lead to an ordered ground state. Using ab initio density functional theory calculations we investigate whether selective isoelectronic substitution of the interlayer copper ions is feasible. After identifying several promising candidates for substitution we calculate the magnetic exchange couplings based on crystal structures predicted from first-principles calculations. We find that isoelectronic substitution with nonmagnetic ions significantly reduces the interkagome exchange coupling. As a consequence, interlayer-substituted barlowite can be described by a simple two-parameter Heisenberg Hamiltonian, for which a quantum spin liquid ground state has been predicted.

Recently, KFe2As2 was shown to exhibit a structural phase transition from a tetragonal to a collapsed tetragonal phase under applied pressure of about 15 GPa. Surprisingly, the collapsed tetragonal phase hosts a superconducting state with Tc ∼ 12 K, while the tetragonal phase is a Tc ≤ 3.4 K superconductor. We show that the key difference between the previously known non-superconducting collapsed tetragonal phase in AFe2As2 (A= Ba, Ca, Eu, Sr) and the superconducting collapsed tetragonal phase in KFe2As2 is the qualitatively distinct electronic structure. While the collapsed phase in the former compounds features only electron pockets at the Brillouin zone boundary and no hole pockets are present in the Brillouin zone center, the collapsed phase in KFe2As2 has almost nested electron and hole pockets. Within a random phase approximation spin fluctuation approach we calculate the superconducting order parameter in the collapsed tetragonal phase. We propose that a Lifshitz transition associated with the structural collapse changes the pairing symmetry from d-wave (tetragonal) to s± (collapsed tetragonal). Our DFT+DMFT calculations show that effects of correlations on the electronic structure of the collapsed tetragonal phase are minimal. Finally, we argue that our results are compatible with a change of sign of the Hall coefficient with pressure as observed experimentally. * guterding@itp.uni-frankfurt.de 1 M. Rotter, M. Tegel, and D. Johrendt, Superconductivity at

Unconventional superconductivity in iron pnictides and chalcogenides has been suggested to be controlled by the interplay of low-energy antiferromagnetic spin fluctuations and the particular topology of the Fermi surface in these materials. Based on this premise, one would also expect the large class of isostructural and isoelectronic iron germanide compounds to be good superconductors. As a matter of fact, they, however, superconduct at very low temperatures or not at all. In this work we establish that superconductivity in iron germanides is suppressed by strong ferromagnetic tendencies, which surprisingly do not originate from changes in bond-angles or -distances with respect to iron pnictides and chalcogenides, but are due to changes in the electronic structure in a wide range of energies happening upon substitution of atom species (As by Ge and the corresponding spacer cations). Our results indicate that superconductivity in iron-based materials may not always be fully understood based on d or dp model Hamiltonians only.