arXiv subjects

Manuel Richter

Publications and source records attributed to Manuel Richter.

At least 37 records · Page 2Linked to original sources

arXiv2015

Metastable cubic and tetragonal phases of transition metals predicted by density-functional theory

By means of density-functional calculations, we systematically investigated 24 transition metals for possible metastable phases in body-centered tetragonal structure (bct), including face-centered cubic (fcc) and body-centered cubic (bcc) geometries. A total of 36 structures not coinciding with equilibrium phases were found to minimize the total energy for the bct degrees of freedom. Among these, the fcc structures of Sc, Ti, Co, Y, Zr, Tc, Ru, Hf, Re, and Os, and bct Zr with $c/a=0.82$ were found to be metastable according to their computed phonon spectra. Eight of these predicted phases are not known from the respective pressure-temperature phase diagrams. Possible ways to stabilize the predicted metastable phases are illustrated.

cond-mat.mtrl-sci↗

arXiv2015

Prediction of first-order martensitic transitions in strained epitaxial films

Coherent epitaxial growth allows to produce crystalline films with strained structures which are unstable in the bulk. Thereby, the relationship between the lattice parameters of the overlayer in the interface plane, $(a,b)$, and its minimum-energy out-of-plane lattice parameter, $c_{\text{min}}(a,b)$, need not be continuous. This general statement is proven by examples of total energy calculations. As a consequence, $c_{\text{min}}$, which is determined by the choice of the substrate, and $c_{\text{min}}$-dependent intrinsic properties of the overlayer cannot always be tuned in a continuous way as one may aim to do by means of strained epitaxy. Employing the model of the epitaxial Bain path we predict that such discontinuities occur in films of the elements V, Nb, Ru, La, Os, and Ir. The abrupt change of $c_{\text{min}}$ can be exploited to switch properties specific to the overlayer material. This is demonstrated for the example of the superconducting critical temperature of a V film which we predict to jump by 20% at a discontinuity of $c_{\text{min}}$

cond-mat.mtrl-sci↗

arXiv2015

Sub-nm wide electron channels protected by topology

Helical locking of spin and momentum and prohibited backscattering are the key properties of topologically protected states. They are expected to enable novel types of information processing such as spintronics by providing pure spin currents, or fault tolerant quantum computation by using the Majorana fermions at interfaces of topological states with superconductors. So far, the required helical conduction channels used to realize Majorana fermions are generated through application of an axial magnetic field to conventional semiconductor nanowires. Avoiding the magnetic field enhances the possibilities for circuit design significantly. Here, we show that sub-nanometer wide electron channels with natural helicity are present at surface step-edges of the recently discovered topological insulator Bi14Rh3I9. Scanning tunneling spectroscopy reveals the electron channels to be continuous in both energy and space within a large band gap of 200 meV, thereby, evidencing its non-trivial topology. The absence of these channels in the closely related, but topologically trivial insulator Bi13Pt3I7 corroborates the channels' topological nature. The backscatter-free electron channels are a direct consequence of Bi14Rh3I9's structure, a stack of 2D topologically insulating, graphene-like planes separated by trivial insulators. We demonstrate that the surface of Bi14Rh3I9 can be engraved using an atomic force microscope, allowing networks of protected channels to be patterned with nm precision.

cond-mat.mes-hall↗

arXiv2013

Engineering topological surface-states: HgS, HgSe and HgTe

Using density functional electronic structure calculations, we establish the consequences of surface termination and modification on protected surface-states of metacinnabar (beta-HgS). Whereas we find that the Dirac cone is isotropic and well-separated from the valence band for the (110) surface, it is highly anisotropic at the pure (001) surface. We demonstrate that the anisotropy is modified by surface passivation because the topological surface-states include contributions from dangling bonds. Such dangling bonds exist on all pure surfaces within the whole class HgX with X = S, Se, or Te and directly affect the properties of the Dirac cone. Surface modifications also alter the spatial location (depth and decay length) of the topologically protected edge-states which renders them essential for the interpretation of photoemission data.

cond-mat.mes-hall↗

arXiv2013

Electric field control of magnetic exchange coupling

Electric control of magnetism is a vision which drives intense research on magnetic semiconductors and multiferroics. Recently, also ultrathin metallic films were reported to show magnetoelectric effects at room temperature. Here we demonstrate much stronger effects by exploiting reduction/oxidation reactions in a naturally grown oxide layer exchange coupled to an underlying ferromagnet. For the exemplarily studied FePt/iron oxide composite in an electrolyte, a large and reversible change of magnetization and anisotropy is obtained. The principle can be transferred to various metal/oxide combinations. It represents a novel approach towards multifunctionality.

cond-mat.mtrl-sci↗

arXiv2013

Stacked topological insulator built from bismuth-based graphene sheet analogues

Commonly materials are classified as either electrical conductors or insulators. The theoretical discovery of topological insulators (TIs) in 2005 has fundamentally challenged this dichotomy. In a TI, spin-orbit interaction generates a non-trivial topology of the electronic band-structure dictating that its bulk is perfectly insulating, while its surface is fully conducting. The first TI candidate material put forward -graphene- is of limited practical use since its weak spin-orbit interactions produce a band-gap of ~0.01K. Recent reinvestigation of Bi2Se3 and Bi2Te3, however, have firmly categorized these materials as strong three-dimensional TI's. We have synthesized the first bulk material belonging to an entirely different, weak, topological class, built from stacks of two-dimensional TI's: Bi14Rh3I9. Its Bi-Rh sheets are graphene analogs, but with a honeycomb net composed of RhBi8-cubes rather than carbon atoms. The strong bismuth-related spin-orbit interaction renders each graphene-like layer a TI with a 2400K band-gap.

cond-mat.mtrl-sci↗

arXiv2012

Nuclear magnetic resonance at up to 10.1 Giga-Pascal pressure detects an electronic topological transition in aluminum metal

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.

cond-mat.mtrl-sci↗

arXiv2012

Highly anisotropic magnetic states of Co dimers bound to graphene-vacancies

The adsorption behavior and the magnetic states of cobalt atoms and dimers on single vacancies in a graphene sheet are investigated by means of relativistic density functional calculations. It is found that local magnetic moments are formed in both cases, despite strong chemical binding. Of particular interest are kinetically stable isomers with two cobalt atoms attached to the same side of the graphene sheet. Magnetic bi-stability with an anisotropy barrier of about 50 meV is possible in this geometry. The feasibility of its preparation is discussed.

cond-mat.mtrl-sci↗

arXiv2011

Metacinnabar (β-HgS): a strong 3D topological insulator with highly anisotropic surface states

We establish the presence of topologically protected edge states on the (001) surface of HgS in the zinc-blende structure using density-functional electronic structure calculations. The Dirac point of the edge state cone is very close to the bulk valence band maximum. The Dirac cone is extremely anisotropic with a very large electron velocity along one diagonal of the surface elementary cell x' and a nearly flat dispersion in the perpendicular direction y'. The strong anisotropy originates from a broken fourfold rotoinversion symmetry at the surface.

cond-mat.mtrl-sci↗

arXiv2011

The effect of chemical disorder on the magnetic anisotropy of strained Fe-Co films

Strained Fe-Co films have recently been demonstrated to exhibit a large magnetocrystalline anisotropy (MCA) and thus to be of potential interest as magnetic storage material. Here, we show by means of density-functional (DF) calculations, that chemical order can remarkably enhance the MCA. We also investigate the effect of relaxation perpendicular to the applied strain and evaluate the strain energy as a function of Co concentration and substrate lattice parameter. On this basis, favourable preparation routes for films with a large perpendicular anisotropy are suggested.

cond-mat.mtrl-sci↗

arXiv2010

CoIr-carbon complexes with magnetic anisotropies larger than 0.2 eV: a density-functional-theory prediction

We report a density-functional study of the heteronuclear CoIr dimer adsorbed on benzene or graphene. In either case CoIr prefers an upright position above the center of a carbon hexagon with the Co atom next to it. The Ir atom stays away from the carbon ring and thus preserves its free-atom-like properties. This results in a very large magnetic anisotropy of more than 0.2 eV per dimer. So high a value should suffice for long-term data storage at the temperature of liquid nitrogen.

cond-mat.mes-hall↗

arXiv2010

Prediction of huge magnetic anisotropies of transition-metal dimer-benzene complexes

Based on numerically accurate density functional theory (DFT) calculations, we systematically investigate the ground-state structure and the spin and orbital magnetism including the magnetic anisotropy energy (MAE) of 3d- and 4d-transition-metal dimer benzene complexes (TM2Bz, TM = Fe, Co, Ni, Ru, Rh, Pd; Bz = C6H6). These systems are chosen to model TM-dimer adsorption on graphene or on graphite. We find that Fe2, Co2, Ni2, and Ru2 prefer the upright adsorption mode above the center of the benzene molecule, while Rh2 and Pd2 are adsorbed parallel to the benzene plane. The ground state of Co2Bz (with a dimer adsorption energy of about 1 eV) is well separated from other possible structures and spin states. In conjunction with similar results obtained by ab initio quantum chemical calculations, this implies that a stable Co2Bz complex with C6v symmetry is likely to exist. Chemical bonding to the carbon ring does not destroy the magnetic state and the characteristic level scheme of the cobalt dimer. Calculations including spin- orbit coupling show that the huge MAE of the free Co dimer is preserved in the Co2Bz structure. The MAE predicted for this structure is much larger than the MAE of other magnetic molecules known hitherto, making it an interesting candidate for high-density magnetic recording. Among all the other investigated complexes, only Ru2Bz shows a potential for strong-MAE applications, but it is not as stable as Co2Bz. The electronic structure of the complexes is analyzed and the magnitude of their MAE is explained by perturbation theory.

cond-mat.mes-hall↗

arXiv2009

Magnetic properties of small Pt-capped Fe, Co and Ni clusters: A density functional theory study

Theoretical studies on M$_{13}$ (M = Fe, Co, Ni) and M$_{13}$Pt$_n$ (for $n$ = 3, 4, 5, 20) clusters including the spin-orbit coupling are done using density functional theory. The magnetic anisotropy energy (MAE) along with the spin and orbital moments are calculated for M$_{13}$ icosahedral clusters. The angle-dependent energy differences are modelled using an extended classical Heisenberg model with local anisotropies. From our studies, the MAE for Jahn-Teller distorted Fe$_{13}$, Mackay distorted Fe$_{13}$ and nearly undistorted Co$_{13}$ clusters are found to be 322, 60 and 5 $μ$eV/atom, respectively, and are large relative to the corresponding bulk values, (which are 1.4 and 1.3 $μ$eV/atom for bcc Fe and fcc Co, respectively.) However, for Ni$_{13}$ (which practically does not show relaxation tendencies), the calculated value of MAE is found to be 0.64 $μ$eV/atom, which is approximately four times smaller compared to the bulk fcc Ni (2.7 $μ$eV/atom). In addition, MAE of the capped cluster (Fe$_{13}$Pt$_4$) is enhanced compared to the uncapped Jahn-Teller distorted Fe$_{13}$ cluster.

cond-mat.mes-hall↗

arXiv2009

Co dimers on hexagonal carbon rings proposed as subnanometer magnetic storage bits

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.

cond-mat.mtrl-sci↗

arXiv2009

Structural, Magnetic, and Electronic Properties of the Monometallofullerene Gd@C_82 : Theory

The structural, electronic, and magnetic properties of Gd@C_82 endohedral metallofullerene have been studied by employing on-site correlation corrected, scalar-relativistic and full-relativistic density functional theory within the local density and generalized gradient approximations. The experimentally observed reduction of the magnetic moment of Gd@C_82 with respect to that of a free Gd^+3 ion can be explained by a tiny hybridization between unoccupied Gd-4f states and carbon-pi states, resulting in a generic antiferromagnetic coupling of the Gd-4f spin with the remaining unpaired spin in the hybridized molecular orbital.

physics.atm-clus↗

arXiv2008

Jahn-Teller like origin of the tetragonal distortion in disordered Fe-Pd magnetic shape memory alloys

The electronic structure and magnetic properties of disordered Fe$_{x}$Pd$_{100-x}$ alloys $(50 < x < 85)$ are investigated in the framework of density functional theory using the full potential local orbital method (FPLO). Disorder is treated in the coherent potential approximation (CPA). Our calculations explain the experimental magnetization data. The origin of the tetragonal distortion in the Fe-Pd magnetic shape memory alloys is found to be a Jahn-Teller like effect which allows the system to reduce its band energy in a narrow composition range. Prospects for an optimization of the alloys' properties by adding third elements are discussed.

cond-mat.mtrl-sci↗

arXiv2008

Transition metal dimers as potential molecular magnets: A challenge to computational chemistry

Dimers are the smallest chemical objects that show magnetic anisotropy. We focus on 3$d$ and 4$d$ transition metal dimers that have magnetic ground states in most cases. Some of these magnetic dimers have a considerable barrier against re-orientation of their magnetization, the so-called magnetic anisotropy energy, MAE. The height of this barrier is important for technological applications, as it determines, e.g., the stability of information stored in magnetic memory devices. It can be estimated by means of relativistic density functional calculations. Our approach is based on a full-potential local-orbital method (FPLO) in a four-component Dirac-Kohn-Sham implementation. Orbital polarization corrections to the local density approximation are employed. They are discussed in the broader context of orbital dependent density functionals. Ground state properties (spin multiplicity, bond length, harmonic vibrational frequency, spin- and orbital magnetic moment, and MAE) of the 3$d$ and 4$d$ transition metal dimers are evaluated and compared with available experimental and theoretical data. We find exceptionally high values of MAE, close to 0.2 eV, for four particular dimers: Fe$_2$, Co$_2$, Ni$_2$, and Rh$_2$.

physics.atm-clus↗

arXiv2007

Lifshitz transitions and elastic properties of Osmium under pressure

Topological changes of the Fermi surface under pressure may cause anomalies in the low-temperature elastic properties. Our density functional calculations for elemental Osmium evidence that this metal undergoes three such Lifshitz transitions in the pressure range between 70 GPa and 130 GPa. The related elastic anomalies are, however, invisibly weak. The critical pressures considerably exceed the values for recently measured and calculated anomalies in the pressure (P) dependence of the hexagonal c/a lattice parameter ratio close to 25 GPa. We demonstrate that the latter anomalies are statistically not significant and that (c/a)(P) can be fitted equally well by a smooth dependence.

cond-mat.mtrl-sci↗