The discovery of a new family of high-T(C) materials, the iron arsenides (FeAs), has led to a resurgence of interest in superconductivity. Several important traits of these materials are now apparent: for example, layers of iron tetrahedrally coordinated by arsenic are crucial structural ingredients. It is also now well established that the parent non-superconducting phases are itinerant magnets, and that superconductivity can be induced by either chemical substitution or application of pressure, in sharp contrast to the cuprate family of materials. The structure and properties of chemically substituted samples are known to be intimately linked; however, remarkably little is known about this relationship when high pressure is used to induce superconductivity in undoped compounds. Here we show that the key structural features in BaFe2As2, namely suppression of the tetragonal-to-orthorhombic phase transition and reduction in the As-Fe-As bond angle and Fe-Fe distance, show the same behaviour under pressure as found in chemically substituted samples. Using experimentally derived structural data, we show that the electronic structure evolves similarly in both cases. These results suggest that modification of the Fermi surface by structural distortions is more important than charge doping for inducing superconductivity in BaFe2As2.

Intense experimental and theoretical studies have demonstrated that the anisotropic triangular lattice as realized in the κ-(BEDT-TTF)2X family of organic charge transfer (CT) salts yields a complex phase diagram with magnetic, superconducting, Mott insulating and even spin liquid phases. With extensive density functional theory (DFT) calculations we refresh the link between manybody theory and experiment by determining hopping parameters of the underlying Hubbard model. This leads us to revise the widely used semiempirical parameters in the direction of less frustrated, more anisotropic triangular lattices. The implications of these results on the systems' description are discussed.PACS numbers: 74.70. Kn,71.10.Fd,71.15.Mb,71.20.Rv A strong research trend of the new millennium has been the desire to understand complex manybody phenomena like superconductivity and magnetism by realistic modelling, i.e. to employ precise first principles calculations to feed the intricate details of real materials into the parameter sets of model Hamiltonians that are then solved with increasingly powerful manybody techniques. The κ-(BEDT-TTF) 2 X [1] organic charge transfer salts are a perfect example for a class of materials with such fascinating properties that they drive progress in experimental and manybody methods alike. Experimentally, the phase diagram shows Mott insulating, superconducting, magnetic and spin liquid phases [2,3,4,5]. Theoretically, the underlying anisotropic triangular lattice is a great challenge due to effects of frustration and the intense efforts to get a grip on the problem include studies with path integral renormalization group (PIRG) [6], exact diagonalization [7], variational Monte Carlo [8], cluster dynamical mean field theory [9, 10] and dual Fermions [11] to cite a few. In this rapidly expanding field of research, electronic structure calculations play the decisive role of mediating between the complex underlying structure and phenomenology of organic charge transfer salts and the models used for understanding the physics [12], and in this work, we will provide the perspective of precise, state of the art electronic structure calculations.Previously, κ-type CT salts have been investigated by semi-empiricial and first principles electronic structure calculations. The most commonly used t, t ′ , U parameter sets derive from extended Hückel molecular orbitals calculations [13,14] performed on different constellations of BEDT-TTF dimers.The main result reported here is that our first principles study shows all four considered κ-type CT salts to be less frustrated than previously assumed based on semiempirical theory. Most importantly, the often cited value of t ′ /t = 1.06 [13] for the spin liquid material κ-(ET) 2 Cu 2 (CN) 3 should be replaced by the significantly smaller value t ′ /t = 0.83 ± 0.08. This has fundamental implications on the systems' model description as we shall see below.In this Letter, we employ the Car-Parrinello [15] projector-augmented wave [16] molecular dynamics (CPMD)...

We study metal-insulator transitions between Mott insulators and metals. The transition mechanism completely different from the original dynamical mean field theory (DMFT) emerges from a cluster extension of it. A consistent picture suggests that the quasiparticle weight Z remains nonzero through metals and suddenly jumps to zero at the transition, while the gap opens continuously in the insulators. This is in contrast with the original DMFT, where Z continuously vanishes but the gap opens discontinuously. The present results arising from electron differentiation in momentum space agree with recent puzzling bulk-sensitive experiments on CaVO3 and SrVO3.PACS numbers: PACS: 71.30.+h; 71.10.Fd; 71.10.Pm Mechanisms and nature of correlation-induced metalinsulator transitions (MIT) are fundamental challenges in condensed matter physics 1 . Although the metals and insulators far away from the MIT are relatively well understood, electronic states dominated and controlled by the proximity of the MIT are far from complete understanding. We find many challenging phenomena such as the high-T c superconductivity in this region, which waits for better understanding of underlying proximity of MIT.The dynamical mean-field theory (DMFT) offers a clear picture of the MIT by taking the limit of high dimensions 2 . In the DMFT, the MIT is approached from metals by the reduction of the quasiparticle weight Z at the Fermi level and the transition is driven by vanishing Z 3 . At the transition, a nonzero insulating gap is already open in the density of states (DOS) as a result of the vanishing Z of the coherent peak isolated from the well-separated upper and lower Hubbard bands. Early photoemission spectroscopy (PES) and inverse-PES of Ca 1−x Sr x VO 3 4 showed a three peak structure, called a sharp coherent band at E F and the incoherent ones corresponding to the upper and lower Hubbard bands at a few electron volts above and below E F , which is consistent with the above DMFT scenario. However, recent more bulk-sensitive PES have revealed a qualitatively different and puzzling feature with a new broad peak at around 0.2eV connected to the lower Hubbard band around 1.6eV and a pseudogap formation around the Fermi level 5 . This is in marked contrast with the DMFT results.In this paper, we show that a scenario of the MIT completely different from the original DMFT emerges in two and three dimensional systems by extending the DMFT to allow the momentum dependence of the self-energy. The MIT is now not driven by a continuous reduction of Z to zero, but by the quasiparticle poles moving away from the Fermi surface, leading to a discontinuous jump of Z to zero at the Fermi level. Such a MIT is the consequence of an anisotropic extinction of the Fermi surface with the topological change, namely the electron differentiation in momentum space. We show the results by considering a cluster extension of the DMFT up to four sites for the Hubbard model on the square and cubic lattices. Our results indicate an opening of pseudogap in the DOS a...

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From a combination of high resolution angle-resolved photoemission spectroscopy and density functional calculations, we show that BaFe2As2 possesses essentially two-dimensional electronic states, with a strong change of orbital character of two of the Γ-centered Fermi surfaces as a function of kz. Upon Co doping, the electronic states in the vicinity of the Fermi level take on increasingly three-dimensional character. Both the orbital variation with kz and the more three-dimensional nature of the doped compounds have important consequences for the nesting conditions and thus possibly also for the appearance of antiferromagnetic and superconducting phases. 74.25.Jb, Since the discovery of high T c superconductivity in Fepnictides [1], many experiments have been carried out to reveal the physical and electronic properties of these materials [2,3,4,5]. The parent compounds of Fepnictide superconductors are antiferromagnetic (AFM) metals. Both electron and hole doping suppresses the AFM order and leads to a superconducting phase. The AFM ordering is supposed to occur by nesting of hole pockets at the center of the Brillouin zone (BZ) and electron pockets at the zone corner. Nesting may be also important for the pairing mechanism in these compounds [6] although there are alternative scenarios based on the high polarizability of the As ions [7]. The nesting scenario could explain why in the SmFeAsO-based superconductors [8], predicted to have an almost twodimensional electronic structure [9, 10], higher superconducting transition temperatures T c are observed than in BaFe 2 As 2 -based systems [2] which are predicted to have a more three-dimensional electronic structure [11]. In general, reduction of the dimensionality increases the number of states that could be considered to be well nested. Furthermore, we point out that the orbital character of the states at the Fermi level E F is very important for the nesting conditions as the interband transitions which determine the electronic susceptibility, as calculated by the Lindhard function, are (in weak coupling scenarios) by far strongest when the two Fermi surfaces have the same orbital character [12]. The admixture of threedimensionality, arising from interlayer coupling, makes the materials potentially more useful in devices and other applications. Thus the dimensionality of the electronic structure, i.e., the k z dispersion of the electronic states is of great importance for the understanding and application of these new superconductors.Although angle-resolved photoemission spectroscopy (ARPES) is an ideal tool to study the dispersion of bands parallel and perpendicular to the FeAs layers there exist only a few experimental studies of these issues [13,14,15]. In this letter, we report a systematic study of the dimensionality of the electronic structure of BaFe 2−x Co x As 2 (x= 0 to 0.4) using polarization dependent ARPES, uncovering two new factors which are of great signi cance for the nesting of the Fermi surfaces of these systems. Firstly we show that the Co d...

We report a combined experimental and theoretical investigation of the layered antimonide PrMnSbO which is isostructural to the parent phase of the iron pnictide superconductors. We find linear resistivity near room temperature and Fermi liquid-like T^{2} behaviour below 150 K. Neutron powder diffraction shows that unfrustrated C-type Mn magnetic order develops below \sim 230 K, followed by a spin-flop coupled to induced Pr order. At T \sim 35 K, we find a tetragonal to orthorhombic (T-O) transition. First principles calculations show that the large magnetic moments observed in this metallic compound are of local origin. Our results are thus inconsistent with either the itinerant or frustrated models proposed for symmetry breaking in the iron pnictides. We show that PrMnSbO is instead a rare example of a metal where structural distortions are driven by f-electron degrees of freedom

We report high resolution angle-resolved photoemission spectroscopy ͑ARPES͒ studies of the electronic structure of BaFe 2 As 2 , which is one of the parent compounds of the Fe-pnictide superconductors. ARPES measurements have been performed at 20 and 300 K, corresponding to the orthorhombic antiferromagnetic phase and the tetragonal paramagnetic phase, respectively. Photon energies between 30 and 175 eV and polarizations parallel and perpendicular to the scattering plane have been used. Measurements of the Fermi surface yield two hole pockets at the ⌫ point and an electron pocket at each of the X points. The topology of the pockets has been concluded from the dispersion of the spectral weight as a function of binding energy. Changes in the spectral weight at the Fermi level upon variation in the polarization of the incident photons yield important information on the orbital character of the states near the Fermi level. No differences in the electronic structure between 20 and 300 K could be resolved. The results are compared with density functional theory band structure calculations for the tetragonal paramagnetic phase.

Using ab initio molecular dynamics we investigate the electronic and lattice structure of AFe 2 As 2 ͑A =Ca,Sr,Ba͒ under pressure. We find that the structural phase transition ͑orthorhombic to tetragonal symmetry͒ is always accompanied by a magnetic phase transition in all the compounds while the nature of the transitions is different for the three systems. Our calculations explain the origin of the existence of a collapsed tetragonal phase in CaFe 2 As 2 and its absence in BaFe 2 As 2 . We argue that changes in the Fermi-surface nesting features dominate the phase transitions under pressure rather than spin frustration or a Kondo scenario. The consequences for superconductivity are discussed.The discovery of iron pnictide superconductors 1 with critical temperatures T c up to 57.4 K ͑Ref. 2͒ upon doping has strongly revived the interest in high-T c superconductivity. The undoped Fe-based parent compound undergoes at low temperatures a structural transition from tetragonal to orthorhombic symmetry accompanied by a magnetic phase transition to a stripe-type spin-density-wave state. 3-7 While the nature of these two transitions is different between LaFeAsO ͑1111 compound͒ and AFe 2 As 2 ͑122 compound͒ with A = ͑Ba, Sr, Ca͒, superconductivity appears in both material classes only when the lattice distortion and magnetic ordering are suppressed, indicating a strong competition between the structural distortion, magnetic ordering, and superconductivity in iron pnictides.Recently, superconductivity in the parent compounds 1111 and 122 was reported under application of pressure. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] In LaFeAsO, 8 resistivity measurements show superconductivity at Ϸ12 GPa with T c = 21 K. In BaFe 2 As 2 superconductivity is found to appear gradually with increasing pressure while in SrFe 2 As 2 the onset of superconductivity occurs abruptly. 22 In CaFe 2 As 2 , 12-14 detailed neutron-and x-ray diffraction analysis shows that the system undergoes a first-order phase transition from a magnetic orthorhombic to a nonmagnetic "collapsed" tetragonal phase under pressure. The possible appearance of superconductivity in this collapsed tetragonal phase is presently under debate. 15,16 While various experiments give different values of critical pressures 17-25 due to the fact that the phase transition is sensitive to possible nonhydrostatic pressure effects, Sn content in some samples, or the use of single crystals or polycrystalline material for structure determination, it is claimed that BaFe 2 As 2 and SrFe 2 As 2 do not manifest a collapsed tetragonal phase at elevated pressure. 13,17,18,23,24 Therefore, the fact that structurally similar compounds exhibit phase transitions of different nature urgently calls for a theoretical understanding. Moreover, it is still under intensive debate which is the driving mechanism of the collinear stripe-type antiferromagnetic ordering; whether the Fermi-surface nesting or the competition of exchange antiferromagnetic interactions between the ...