Milan Tomic

Absence of Metallicity in K-doped Picene: Importance of Electronic Correlations

Potassium-doped picene (K(x)picene) 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 K(x)picene 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.

Unfolding of electronic structure through induced representations of space groups: Application to Fe-based superconductors

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.

Uniaxial versus hydrostatic pressure-induced phase transitions in CaFe2As2and BaFe2As2

We present uniaxial pressure structural relaxations for CaFe2As2 and BaFe2Ase2 within density functional theory and compare them with calculations under hydrostatic pressure conditions 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 superconductivity.

Pressure-Induced Conductivity in a Neutral Nonplanar Spin-Localized Radical

There is a growing interest in the development of single-component molecular conductors based on neutral organic radicals that are mainly formed by delocalized planar radicals, such as phenalenyl or thiazolyl radicals. However, there are no examples of systems based on nonplanar and spin-localized C-centered radicals exhibiting electrical conductivity due to their large Coulomb energy (U) repulsion and narrow electronic bandwidth (W) that give rise to a Mott insulator behavior. Here we present a new type of nonplanar neutral radical conductor attained by linking a tetrathiafulvalene (TTF) donor unit to a neutral polychlorotriphenylmethyl radical (PTM) with the important feature that the TTF unit enhances the overlap between the radical molecules as a consequence of short intermolecular S···S interactions. This system becomes semiconducting upon the application of high pressure thanks to increased electronic bandwidth and charge reorganization opening the way to develop a new family of neutral radical conductors.

Dramatic changes in the electronic structure upon transition to the collapsed tetragonal phase in CaFe2As2

We use angle-resolved photoemission spectroscopy and density functional theory calculations to study the electronic structure of CaFe2As2 in the collapsed tetragonal (CT) phase. This unusual phase of iron arsenic high-temperature superconductors was hard to measure as it exists only under pressure. By inducing internal strain, via the postgrowth thermal treatment of 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 an apparent loss of Fe magnetic moment, are now clearly experimentally correlated with the lack of superconductivity in this phase.

Effects of Lifshitz transition on charge transport in magnetic phases of Fe-based superconductors.

The unusual temperature dependence of the resistivity and its in-plane anisotropy observed in the Fe-based superconducting materials, particularly Ba(Fe_{1-x}Co_{x})_{2}As_{2}, has been a long-standing puzzle. Here, we consider the effect of impurity scattering on the temperature dependence of the average resistivity within a simple two-band model of a dirty spin density wave metal. The sharp drop in resistivity below the Néel temperature T_{N} in the parent compound can only be understood in terms of a Lifshitz transition following Fermi surface reconstruction upon magnetic ordering. We show that the observed resistivity anisotropy in this phase, arising from nematic defect structures, is affected by the Lifshitz transition as well.

In-plane uniaxial stress effects on the structural and electronic properties of BaFe2As2 and CaFe2As2

Starting from the orthorhombic magnetically ordered phase, we investigate the effects of uniaxial tensile and compressive stresses along a, b, and the diagonal a+b directions in BaFe

Comparing charge transfer tuning effects by chemical substitution and uniaxial pressure in the organic charge transfer complex tetramethoxypyrene-tetracyanoquinodimethane

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Ab initio perspective on structural and electronic properties of iron‐based superconductors

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Publisher’s Note: Effects of Lifshitz Transitions on Charge Transport in Magnetic Phases of Fe-Based Superconductors [Phys. Rev. Lett. 114 , 097003 (2015)]

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