arXiv subjects

Michael Merz

Publications and source records attributed to Michael Merz.

42 records · Page 3Linked to original sources

arXiv2020

Band Engineering of Dirac cones in Iron Chalcogenides

By band engineering the iron chalcogenide Fe(Se,Te) via ab-initio calculations, we search for topological surface states and realizations of Majorana bound states. Proposed topological states are expected to occur for non-stoichiometric compositions on a surface Dirac cone where issues like disorder scattering and charge transfer between relevant electronic states have to be addressed. However, this surface Dirac cone is well above the Fermi-level. Our goal is to theoretically design a substituted crystal in which the surface Dirac cone is shifted towards the Fermi-level by modifying the bulk material without disturbing the surface. Going beyond conventional density functional theory (DFT), we apply the coherent potential approximation (BEB-CPA) in a mixed basis pseudo-potential framework to scan the substitutional phase-space of co-substitutions on the Se-sites. We have identified iodine as a promising candidate for intrinsic doping. Our specific proposal is that FeSe$_{0.325}$I$_{0.175}$Te$_{0.5}$ is a very likely candidate to exhibit a Dirac cone right at the Fermi energy without inducing strong disorder scattering.

cond-mat.mtrl-sci↗

arXiv2020

On the microscopic origin of reversible and irreversible reactions of LiNixCoyMnx cathode materials: Ni-O hybrid bond formation vs. cationic and anionic redox

Energy density limitations of layered oxides with different Ni contents, i.e., of the conventional cathode materials in Li-ion batteries, are investigated across the first discharge cycle using advanced spectroscopy and state-of-the-art diffraction. For the first time unambiguous experimental evidence is provided, that redox reactions in NCMs proceed via a reversible oxidation of Ni and a hybridization with O, and not, as widely assumed, via pure cationic or more recently discussed, pure anionic redox processes. Once Ni-O hybrid states are formed, the sites cannot be further oxidized. Instead, irreversible reactions set in which lead to a structural collapse and thus, the lack of ionic Ni limits the reversible capacity. Moreover, the degree of hybridization, which varies with the Ni content, triggers the electronic structure and the operation potential of the cathodes. With an increasing amount of Ni, the covalent character of the materials increases and the potential decreases.

cond-mat.mtrl-sci↗

arXiv2020

Band hybridisation at the semimetal-semiconductor transition of Ta$_2$NiSe$_5$ enabled by mirror-symmetry breaking

We present a combined study from angle-resolved photoemission and density-functional theory calculations of the temperature-dependent electronic structure in the excitonic insulator candidate Ta$_2$NiSe$_5$. Our experimental measurements unambiguously establish the normal state as a semimetal with a significant band overlap of $>$100~meV. Our temperature-dependent measurements indicate how these low-energy states hybridise when cooling through the well-known 327~K phase transition in this system. From our calculations and polarisation-dependent photoemission measurements, we demonstrate the importance of a loss of mirror symmetry in enabling the band hybridisation, driven by a shear-like structural distortion which reduces the crystal symmetry from orthorhombic to monoclinic. Our results thus point to the key role of the lattice distortion in enabling the phase transition of Ta$_2$NiSe$_5$.

cond-mat.str-el↗

arXiv2018

Nodal gaps in the nematic superconductor FeSe from heat capacity

Superconductivity in FeSe has recently attracted a great deal of attention because it emerges out of an electronic nematic state of elusive character. Here we study both the electronic normal state and the superconducting gap structure using heat-capacity measurements on high-quality single crystals. The specific-heat curve, from 0.4 K to Tc = 9.1 K, is found to be consistent with a recent gap determination using Bogoliubov quasiparticle interference [P. O. Sprau et al., Science 357, 75 (2017)], however only if nodes are introduced on either the electron or the hole Fermi-surface sheets. Our analysis, which is consistent with quantum-oscillation measurements, relies on the presence of only two bands, and thus the fate of the theoretically predicted second electron pocket remains mysterious.

cond-mat.supr-con↗

arXiv2018

Large nematic susceptibility in the double-Q $C_{4}$ magnetic phase of Ba$_{1-x}$Na$_{x}$Fe$_{2}$As$_{2}$

The nematic susceptibility of Ba$_{1-x}$Na$_{x}$Fe$_{2}$As$_{2}$ single crystals is studied by measuring the Young's modulus using a three-point-bending setup in a capacitance dilatometer over a wide doping range. Particular emphasis is placed on the behavior within the double-Q antiferromagnetic $C_{4}$ re-entrant phase. Here, we surprisingly still observe a sizable nematic susceptibility in spite of the well-developed magnetic order, consistent with recent theoretical calculations. Outside the $C_{4}$ re-entrant region, the behavior is very similar to that of K-doped BaFe$_{2}$As$_{2}$. A significant coupling of superconductivity to the shear modulus is observed and is discussed in terms of strong competition between the superconducting and magnetic phases.

cond-mat.supr-con↗

arXiv2015

Quantum criticality in AFe2As2 with A = K, Rb, and Cs suppresses superconductivity

Superconductors close to quantum phase transitions often exhibit a simultaneous increase of electronic correlations and superconducting transition temperatures. Typical examples are given by the recently discovered iron-based superconductors. We investigated the band-specific quasiparticle masses of AFe2As2 single crystals with A = K, Rb, and Cs and determined their pressure dependence. The evolution of electronic correlations could be tracked as a function of volume and hole doping. The results indicate that with increasing alkali-metal ion radius a quantum critical point is approached. The critical fluctuations responsible for the enhancement of the quasiparticle masses appear to suppress the superconductivity.

cond-mat.supr-con↗