We present uniaxial pressure structural relaxations for CaFe2As2 and BaFe2As2 within density functional theory and compare them with calculations under hydrostatic pressure as well as available experimental results. We find that CaFe2As2 shows a unique phase transition from a magnetic orthorhombic phase to a nonmagnetic collapsed tetragonal phase for both pressure conditions and no indication of a tetragonal phase is observed at intermediate uniaxial pressures. In contrast, BaFe2As2 shows for both pressure conditions two phase transitions from a magnetic orthorhombic to a collapsed tetragonal phase through an intermediate nonmagnetic tetragonal phase. We find that the critical transition pressures under uniaxial conditions are much lower than those under hydrostatic conditions manifesting the high sensitivity of the systems to uniaxial stress. We discuss the origin of this sensitivity and its relation to experimental observations. The discovery of superconductivity in iron pnictides [1] has initiated an enourmous amount of activities related to these materials. Superconductivity can be triggered either by chemical doping or by application of pressure on the undoped parent compounds. One of the families that has been intensively studied under pressure is the 122 family AEFe 2 As 2 (AE = Ca, Sr, and Ba). CaFe 2 As 2 at ambient pressure undergoes a first order phase transition from a tetragonal to an orthorhombic phase at 172 K accompanied by a magnetic transition. Initial reports on pressure experiments showed that at P ∼ 0.23 GPa the orthorhombic and antiferromagnetic phases are suppressed and the system superconducts at low temperatures [2,3]. Moreover, a compressed tetragonal phase -also called 'collapsed' tetragonal phase -was identified at higher pressures. Subsequent susceptibility and transport measurements under hydrostatic conditions showed at low temperatures and P ∼ 0.35 GPa a sharp orthorhombic to collapsed tetragonal phase but no signature of superconductivity [4]. In contrast, recent neutron diffraction experiments on CaFe 2 As 2 under uniaxial pressure along the c axis [5] indicate for pressures above 0.06 GPa and low temperatures the presence of an intermediate nonmagnetic tetragonal phase between the magnetic orthorhombic and the nonmagnetic collapsed tetragonal phases. This phase was identified by the authors as the phase responsible for superconductivity at T = 10 K. Other reports based on muon spin-relaxation measurements suggest the existence of superconductivity in the orthorhombic phase, raising the question whether superconductivity and magnetism can coexist [6].BaFe 2 As 2 shows an even more complex behavior under pressure. At ambient pressure it undergoes a phase transition from a metallic tetragonal phase to an orthorhombic antiferromagnetic phase at T = 140 K. Under pressure the gradual appearance of a superconducting dome has been observed by various groups [7] though the role of nonhydrostatic conditions is not yet well understood [8]. Recent synchrotron X-ray diffraction experiments ...
Potassium-doped picene (Kxpicene) has recently been reported to be a superconductor at x = 3 with critical temperatures up to 18 K. Here we study the electronic structure of K-doped picene films by photoelectron spectroscopy and ab initio density functional theory combined with dynamical mean-field theory (DFT+DMFT). Experimentally we observe that, except for spurious spectral weight due to the lack of a homogeneous chemical potential at low K-concentrations (x ≈ 1), the spectra always display a finite energy gap. This result is supported by our DFT+DMFT calculations which provide clear evidence that Kxpicene is a Mott insulator for integer doping concentrations x = 1, 2, and 3. We discuss various scenarios to understand the discrepancies with previous reports of superconductivity and metallic behavior.
We use angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations to study the electronic structure of CaFe2As2 in previously unexplored collapsed tetragonal (CT) phase. This unusual phase of the iron arsenic high temperature superconductors was hard to measure as it exists only under pressure. By inducing internal strain, via the post growth, thermal treatment of the single crystals, we were able to stabilize the CT phase at ambient-pressure. We find significant differences in the Fermi surface topology and band dispersion data from the more common orthorhombic-antiferromagnetic or tetragonal-paramagnetic phases, consistent with electronic structure calculations. The top of the hole bands sinks below the Fermi level, which destroys the nesting present in parent phases. The absence of nesting in this phase along with apparent loss of Fe magnetic moment, are now clearly experimentally correlated with the lack of superconductivity in this phase. The AFe 2 As 2 (A = Ca, Sr, Ba) materials have become one of the key systems for the study of Fe-based high temperature superconductivity 1-10 . Detailed substitution and pressure studies on these systems have revealed that superconductivity is intimately linked to the magnetic state of iron and can be turned on or off by manipulating the nature of the order, the fluctuations or even the existence of Fe-magnetism 6-15 . Moreover, in contrast to cuprate superconductors, the Fe-based superconductors are multiorbital systems and orbital degrees of freedom have been also proposed to be important for the understanding of the structural, magnetic and superconducting properties of these materials [16][17][18] . At ambient pressure CaFe 2 As 2 manifests a strongly first order, coupled structural / magnetic phase transition at 170 K that is exceptionally pressure sensitive with a remarkably large pressure derivative 9,10 . For pressures as low as 0.4 GPa another dramatic, first order phase transition to a non-moment bearing collapsed tetragonal phase 9 is stabilized near 100 K and rapidly increases in temperature for higher applied pressures. Whereas the electronic properties of CaFe 2 As 2 in the ambient pressure phases 19,20 were found to be similar to the other parent compounds of 122 family 21-28 , the electronic behavior of the CT phase is so far largely unexplored due to the need for at least 0.4 GPa of external pressure.Post growth annealing/quenching of CaFe 2 As 2 samples grown from excess FeAs can be used to control the degree of internal strain due to Fe-As precipitates associated with a small width of formation [29][30][31] . One of the important findings of these works is that the nonmagnetic, collapsed tetragonal phase can be stabilized in an ambient pressure sample by using internal strain. Key spectroscopic tools, such as ARPES and STM, which normally cannot be combined with pressures even at the 0.1 GPa level, can now be brought to play in order to understand the electronic properties of the collapsed tetragonal phase.In this...
One- and two-dimensional nuclear magnetic resonance (NMR) methods were used to determine a three-dimensional model of an eight-base-pair DNA fragment (d-GGGTACCC) cross-linked with psoralen in solution. Two-dimensional nuclear Overhauser effect experiments were used to assign the spectrum and estimate distances for 171 proton pairs in the cross-linked DNA. The NMR-derived model shows a 53 degree bend into the major groove that occurs primarily at the site of drug addition and a 56 degree unwinding that spans the eight-base-pair duplex.
We revisit the problem that relevant parts of bandstructures for a given cell choice can reflect exact or approximate higher symmetries of subsystems in the cell and can therefore be significantly simplified by an unfolding procedure that recovers the higher symmetry. We show that bandstructure unfolding can be understood as projection onto induced irreducible representations of a group obtained by extending the original group of translations with a number of additional symmetry operations. The resulting framework allows us to define a generalized unfolding procedure which includes the point group operations and can be applied to any quantity in the reciprocal space. The unfolding of the Brillouin zone follows naturally from the properties of the induced irreducible representations. In this context, we also introduce a procedure to derive tight-binding models of reduced dimensionality by making use of point group symmetries. Further, we show that careful consideration of unfolding has important consequences on the interpretation of angle resolved photoemission experiments. Finally, we apply the unfolding procedure to various representative examples of Fe-based superconductor compounds and show that the one iron picture arises as an irreducible representation of the glide mirror group and we comment on the consequences for the interpretation of one-iron versus two-iron Brillouin zone representations.
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