We have investigated the influence of potassium intercalation on the formation of the charge-density wave (CDW) instability in 2H-tantalum diselenide by means of Electron Energy-Loss Spectroscopy and density functional theory. Our observations are consistent with a filling of the conduction band as indicated by a substantial decrease of the plasma frequency in experiment and theory. In addition, elastic scattering clearly points to a destruction of the CDW upon intercalation as can be seen by a vanishing of the corresponding superstructures. This is accompanied by a new superstructure, which can be attributed to the intercalated potassium. Based on the behavior of the c-axis upon intercalation we argue in favor of interlayer-sites for the alkali-metal and that the lattice remains in the 2H-modification.

A. König K. Koepernik R. Schuster R. Kraus M. Knupfer B. Büchner H. Berger

Itinerant magnets generally exhibit pressure induced transitions towards non magnetic states. Using synchrotron based X-ray diffraction and emission spectroscopy, the evolution of the lattice and spin moment in the chiral magnet MnGe was investigated in the paramagnetic state and under pressures up to 38 GPa. The collapse of spin-moment takes place in two steps. A first-order transition with a huge hysteresis around 7 GPa transforms the system from the high-spin at ambient pressure to a low-spin state. The coexistence of spin-states and observation of history-depending irreversibility is explained as effect of long-range elastic strains mediated by magnetovolume coupling. Only in a second transition, at about 23 GPa, the spin-moment collapses.

N. Martin I. Mirebeau M. Deutsch J. -P. Itié J. -P. Rueff U. K. Rössler K. Koepernik L. N. Fomicheva A. V. Tsvyashchenko

Bandstructure calculations with different spin arrangement for the spin-chain compound MgVO_3 have been performed, and paramagnetic as well as magnetic solutions with ferro- and antiferromagnetically ordered chains are found, the magnetic solutions being by 0.22 eV per formula unit lower than the paramagnetic one. The orbital analysis of the narrow band crossing the Fermi level in the paramagnetic solution reveals that the band has almost pure vanadium 3d character, the lobes of the relevant d-orbitals at the neighboring in-chain sites being directed towards each other, which suggests direct exchange. The tight-binding analysis of the band confirms the strong exchange transfer between neighboring in-chain V-ions. Besides, some additional superexchange transfer terms are found, which give rise both to in-plane coupling between the chains and to frustration, the dominant frustration occuring due to the interchain interaction.

I. Chaplygin R. Hayn K. Koepernik

We calculate the magnetic interactions between two nearest neighbor substitutional magnetic ions (Co or Mn) in ZnO by means of density functional theory and compare it with the available experimental data. Using the local spin density approximation we find a coexistence of ferro- and antiferromagnetic couplings for ZnO:Co, in contrast to experiment. For ZnO:Mn both couplings are antiferromagnetic but deviate quantitatively from measurement. That points to the necessity to account better for the strong electron correlation at the transition ion site which we have done by applying the LSDA+U method. We show that we have to distinguish two different nearest neighbor exchange integrals for the two systems in question which are all antiferromagnetic with values between -1.0 and -2.0 meV in reasonable agreement with experiment.

Thomas Chanier M. Sargolzaei I. Opahle R. Hayn K. Koepernik

We unambiguously demonstrate, based on high-energy x-ray diffraction data and LDA calculations, that sodium-density stripes are formed in Na0.75CoO2 at low temperatures and rule out the previously proposed Na-ordering models. The LDA calculations prove, that the sodium-density stripes lead to a sizeable dip in the density of the Co-states at the Fermi level, pointing to band structure effects as a driving force for the stripe formation. This indicates that the sodium ordering is connected to stripe-like charge correlations within the CoO2 layers, leading to an astonishing similarity between the doped cuprates and the NaxCoO2 compounds.

J. Geck M. v. Zimmermann H. Berger S. V. Borisenko H. Eschrig K. Koepernik M. Knupfer B. Büchner

We report an angular resolved photoemission study of Na0.73CoO2 where it is found that the renormalization of the quasiparticle (QP) dispersion changes dramatically upon a rotation from GM to GK. The comparison of the experimental data to the calculated band structure reveals that the QP-renormalization is most pronounced along the GK-direction, while it is significantly weaker along the GM-direction. We discuss the observed anisotropy in terms of multiorbital effects and point out the relevance of magnetic correlations for the band structure of Na0.73CoO2.

J. Geck S. V. Borisenko H. Berger H. Eschrig J. Fink M. Knupfer K. Koepernik A. Koitzsch A. A. Kordyuk V. B. Zabolotnyy B. Buechner

We have studied the reflectance of the recently discovered superconductor LaO_0.9F0.FeAs in a wide energy range from the far infrared to the visible regime. We report on the observation of infrared active phonons, the plasma edge (PE) and possible interband transitions. On the basis of this data and the reported in-plane penetration depth lambda_L(0) about 254 nm [H. Luetkens et al., Phys. Rev. Lett. v. 101, 0970009 (2008)] a disorder sensitive relatively small value of the total electron electron-boson coupling constant lambda_tot=lambda_e-ph+lambda_e-sp ~ 0.6 +- 0.35 can be estimated adopting an effective single-band picture.

S. -L. Drechsler M. Grobosch K. Koepernik G. Behr A. Koehler J. Werner A. Kondrat N. Leps Ch. Hess R. Klingeler R. Schuster B. Buechner M. Knupfer

First principles calculations of magnetic and, to a lesser extent, electronic properties of the novel LaFeAsO-based superconductors show substantial apparent controversy, as opposed to most weakly or strongly correlated materials. Not only do different reports disagree about quantitative values, there is also a schism in terms of interpreting the basic physics of the magnetic interactions in this system. In this paper, we present a systematic analysis using four different first principles methods and show that while there is an unusual sensitivity to computational details, well-converged full-potential all-electron results are fully consistent among themselves. What makes results so sensitive and the system so different from simple local magnetic moments interacting via basic superexchange mechanisms is the itinerant character of the calculated magnetic ground state, where very soft magnetic moments and long-range interactions are characterized by a particular structure in the reciprocal (as opposed to real) space. Therefore, unravelling the magnetic interactions in their full richness remains a challenging, but utterly important task.

I. I. Mazin M. D. Johannes L. Boeri K. Koepernik D. J. Singh

It is demonstrated by means of density functional and ab-initio quantum chemical calculations, that transition metal - carbon systems have the potential to enhance the presently achievable area density of magnetic recording by three orders of magnitude. As a model system, Co_2-benzene with a diameter of 0.5 nm is investigated. It shows a magnetic anisotropy in the order of 0.1 eV per molecule, large enough to store permanently one bit of information at temperatures considerably larger than 4 K. A similar performance can be expected, if cobalt dimers are deposited on graphene or on graphite. It is suggested that the subnanometer bits can be written by simultaneous application of a moderate magnetic and a strong electric field.

Ruijuan Xiao Daniel Fritsch Michael D. Kuz'min Klaus Koepernik Helmut Eschrig Manuel Richter Knut Vietze Gotthard Seifert

High-sensitivity $^{27}$Al nuclear magnetic resonance (NMR) measurements of aluminum metal under hydrostatic pressure of up to 10.1 GPa reveal an unexpected negative curvature in the pressure-dependence of the electronic density of states measured through shift and relaxation, which violates free electron behavior. A careful analysis of the Fermiology of aluminum shows that pressure induces an electronic topological transition (Lifshitz transition) that is responsible for the measured change in the density of states. The experiments also reveal a sudden increase in the NMR linewidth above 4.2 GPa from quadrupole interaction, which is not in agreement with the metal's cubic symmetry.

Thomas Meissner Swee K. Goh Jürgen Haase Manuel Richter Klaus Koepernik Helmut Eschrig