arXiv subjects

Roland Hayn

Publications and source records attributed to Roland Hayn.

17 recordsLinked to original sources

arXiv2025

Topological Hall Response from Canted Antiferromagnetic Order in $d$-Electron Kagome Systems

In a two-dimensional kagome monolayer, a nontrivial intrinsic Berry curvature may arise in the $d$-electron system from the interaction with a non-collinear spin order induced by an underlying antiferromagnetic exchange. This opens the route for a quantum anomalous Hall effect in the multi-orbital system, even without an external magnetic field, explicit spin-orbit coupling or relativistic effects. For spin orders with an out-of-plane component, the scalar spin chirality is finite, and the integration of the Berry curvature over the Brillouin zone may yield integer Hall conductivities in units of $e^2/h$. For a Fermi level within a nontrivial gap, the canted configuration offers, at least in principle, the possibility of a maximal Chern number, $C=\pm 5$. Candidate materials are considered in this paper. In existing materials, the electron hopping is generally highly anisotropic, leading to a quantum anomalous Hall effect with smaller Chern numbers. A topological phase transition between Hall plateaus of opposite $C$ can be driven by flipping the out-of-plane component of the spin order, alluding to the potential of this system to applications in quantum information.

cond-mat.mes-hall↗

arXiv2025

Quantum dot-based device for high-performance magnetic microscopy and spin filtering in the Kondo regime

We propose a nanoscale device consisting of a double quantum dot with a full exchange and pair hopping interaction. In this design, the current can only flow through the upper dot, but is sensitive to the spin state of the lower dot. The system is immersed in a highly inhomogeneous magnetic field, and only the bottom dot feels a substantial magnetic field, while the top dot experiences only a residual one. We show that our device exhibits very interesting magnetic field-dependent transport properties at low temperatures. The Kondo effect partially survives the presence of the magnetic field and allows to obtain conductances that differ by several orders of magnitude for the two spin types across the top dot. Interestingly, as a function of the magnetic field, our two-dot device changes from a spin singlet state to a spin triplet state, in which the amplitudes of the spin-dependent conductances are reversed. Our device is able to discriminate between positive and negative magnetic fields with a high sensitivity and is therefore particularly interesting for imaging the surface of anti-ferromagnetic (AF) insulating materials with alternated surface magnetic field, as well as for spin filtering applications.

cond-mat.mes-hall↗

arXiv2024

Quantum Anomalous Hall Effect in $d$-Electron Kagome Systems: Chern Insulating States from Transverse Spin-Orbit Coupling

The possibility of quantum anomalous Hall effect (QAHE) in two-dimensional kagome systems with $d$-orbital electrons is studied within a multi-orbital tight-binding model. We concentrate on the case of isotropic Slater-Koster integrals which is realized in a recently discovered class of metal-organic frameworks TM$_3$C$_6$O$_6$ with transition metals (TM) in the beginning of the 3$d$ series. Furthermore, in the absence of exchange-type spin-orbit coupling, only isotropic Slater-Koster integrals give a perfect flatband in addition to the two dispersive bands hosting relativistic (Dirac) and quadratic band crossing points at high symmetry spots in the Brillouin zone. A quantized topological invariant requires a flux-creating spin-orbit coupling, giving Chern number (per spin sector) $C=1$ not only from the familiar Dirac points at the six corners of the Brillouin zone, but also from the quadratic band crossing point at the center $\Gamma$. In the case of isotropic Slater-Koster integrals the on-site spin-orbit coupling (SOC) is ineffective to create the QAHE and it is only the transfer or exchange-type SOC which can lead to a QAHE. Surprisingly, this QAHE comes from the nontrivial effective flux induced by the \textit{transverse} part of the spin-orbit coupling, exhibited by electrons in the $d$-orbital state with $m_l=0$ ($d_{z^2}$ orbital), in stark contrast to the more familiar form of QAHE due to the $d$-orbitals with $m_l \neq 0$, driven by the Ising part of spin-orbit coupling. The $C=1$ Chern plateau (per spin sector) due to Dirac point extends over a smaller region of Fermi energy than that due to quadratic band crossing. Our result hints at the promising potential of kagome $d$-electron systems as a platform for dissipationless electronics by virtue of its unique QAHE.

cond-mat.str-el↗

arXiv2023

Energetics, electronic structure and electric polarization of basal stacking faults in wurtzite GaN and ZnO

We investigate the effect of basal-plane stacking faults on the structural, electronic, and polarization properties of wurtzite GaN and ZnO. This theoretical study is performed within density-functional theory (DFT) using periodic hexagonal supercells. Both formation energies and band structures are obtained by means of total-energy calculations. The type-I stacking fault is observed to have the lowest formation energy, followed by type-II and finally the extrinsic stacking fault. In order to overcome the inherent shortcoming of DFT in reproducing band gaps, the generalized-gradient approximation is used in combination with the modified Becke-Johnson functional. It is shown that all stacking faults studied maintain a direct gap whose value is lower than that in the ideal defect-free crystals. The lowering in the band gap allows the creation of quantum-well regions at wurtzite/zincblende interfaces. In addition, we provide a consistent set of polarization parameters derived from the Berry-phase method. We find a trend of decreasing (increasing) spontaneous polarization and piezoelectric coefficient (polarization charge) in going from type-I to type-II to extrinsic stacking faults. We compare our results to experimental and theoretical data available from the literature and explain the observed trends in terms of the properties of the wurtzite and zincblende polytypes of both materials.

cond-mat.mtrl-sci↗

arXiv2023

Metal-organic kagome systems as candidates to study spin liquids, spin ice or the quantum anomalous Hall effect

We present the results of first-principle calculations using the Vienna Ab-initio Simulation Package (VASP) for a new class of organometallics labeled TM3C6O6 (TM =Sc, Ti, V, Cr, Fe, Co, Ni and Cu) in the form of planar, two-dimensional, periodic free-standing layers. These materials, which can be produced by on-surface coordination on metallic surfaces, have a kagome lattice of TM ions. Calculating the structural properties, we show that all considered materials have local magnetic moments in the ground state, but four of them (with Fe, Co, Ni and Cu) show spin-crossover behavior by changing the lattice constant, which could be valuable for possible epitaxy routes on various substrates. Surprisingly, we find a very large richness of electronic and magnetic properties, qualifying these materials as highly promising metal-organic topological quantum materials. We find semi-conductors with nearest-neighbor ferromagnetic (FM) or antiferromagnetic (AFM) couplings for V, and Sc and Cr, respectively, being of potential interest to study spin ice or spin liquids on the 2D kagome lattice. Other TM ion systems combine AFM couplings with metallic behavior (Ti, Fe and Ni) or are ferromagnetic kagome metals like Cu3C6O6 with symmetry protected Weyl crossings at the Fermi surface. For the latter compound, the spin orbit coupling is shown to be responsible for small gaps which should allow the observation of the quantum anomalous Hall effect (QAHE).

cond-mat.mtrl-sci↗

arXiv2022

The trigonal structure as a reference to access the spontaneous polarization of wurtzite crystals

The spontaneous polarization of wurtzite III-V nitrides XN (X=Al, Ga, In) and II-VI oxides YO (Y=Be, Zn) is investigated via first-principles computational methods. The modern treatment defines this quantity as the polarization difference between the investigated system and an appropriate reference state. We demonstrate that the trigonal structure can be used as a reference to determine the spontaneous polarization of wurtzite materials. We compare the current values with the widely-known zincblende results reported in the literature and find a very good agreement. It is shown that the electronic contribution of polarization is greater than the ionic one. Furthermore, we reproduce the experimental value of the spontaneous polarization of wurtzite GaN reported in a previous study. In order to do so, we calculate the spontaneous polarization for each type of stacking faults using periodic supercells and the Berry-phase method. This theoretical analysis leads to a value nearly identical to the experimental measurement.

cond-mat.mtrl-sci↗

arXiv2022

Self-organized Kagome-lattice in a metal-organic monolayer

We report on the successful on-surface synthesis of metal-organic covalent coordination networks with a dense Kagome lattice of metallic centers. In the case of Mn centers ab-initio calculations show that the adsorbed monolayer on Ag(111) has all the characteristic features of a strictly two-dimensional (2D) ferromagnetic Kagome metal. Tetrahydroxyquinone (THQ) and metal atoms (M=Cu or Mn) are co-deposited on the Ag(111) substrate to build well-ordered 2D lattices M$_3$C$_6$O$_6$. The surface is studied by scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS) to optimize the growth conditions like fluxes and temperatures. The details of the atomic, electronic and magnetic structures are clarified by density functional theory (DFT) calculations. XPS and DFT reveal a Cu$^+$ charge state and no local magnetic moments for the Cu-organic network. For the Mn-organic network, we find the charge state Mn$^{2+}$ and a local spin S=5/2. Charge transfer stabilizes the Cu$^+$ and Mn$^{2+}$ charge states. We find two different modifications of the M$_3$C$_6$O$_6$ lattice. DFT calculations which neglect the small spin-orbit coupling show a Dirac point, i.e. a band crossing with linear electron dispersion at the K-point of the Brillouin zone. This Dirac point is at the Fermi level if there is no charge transfer but drops by 100 meV if electron doping of Cu$_3$C$_6$O$_6$ on Ag(111) surface is acknowledged. We predict the magnetic couplings of an isolated M$_3$C$_6$O$_6$ monolayer to be short range and antiferromagnetic leading to high frustration at the Kagome lattice and a tendency towards a spin-liquid ground state. In the case of hole transfer from the substrates ferromagnetic ordering is introduced, making M$_3$C$_6$O$_6$ an interesting candidate for the quantum anomalous Hall effect.

cond-mat.mtrl-sci↗

arXiv2021

Kondo-assisted switching between three conduction states in capacitively coupled quantum dots

We propose a nanoscale device consisting of a double quantum dot with strong intra- and inter- dot Coulomb repulsions. In this design, the current can only flow through the lower dot, but is triggered by the gate-controlled occupancy of the upper dot. At low temperatures, our calculations predict the double dot to pass through a narrow Kondo regime, resulting in highly sensitive switching characteristics between three well-defined states : insulating, normal conduction and resonant conduction.

cond-mat.mes-hall↗

arXiv2020

Plasmons in anisotropic Dirac systems

We consider the plasmon excitations in anisotropic two-dimensional Dirac systems, be it either anisotropic graphene or surfaces of topological insulators. Generalizing the exact density-density response function one finds a plasmon dispersion that is anisotropic already at the lowest frequencies. Asymptotic expressions are obtained for the dispersion in this regime. We show that the plasmon properties of the complete material class of anisotropic Dirac systems are characterized by just two dimensionless material parameters. The strong anisotropy can be used to guide the plasmon modes, introducing new functionalities to the field of Dirac plasmonics.

cond-mat.mtrl-sci↗

arXiv2018

Interface magnetism and electronic structure: ZnO(0001)/Co3O4(111)

We have studied the structural, electronic and magnetic properties of spinel $\rm Co_3O_4$(111) surfaces and their interfaces with ZnO (0001) using density functional theory (DFT) within the Generalized Gradient Approximation with on-site Coulomb repulsion term (GGA+U). Two possible forms of spinel surface, containing $\rm Co^{2+} $ and $\rm Co^{3+} $ ions and terminated with either cobalt or oxygen ions were considered, as well as their interface with zinc oxide. Our calculations demonstrate that $\rm Co^{3+} $ ions attain non-zero magnetic moments at the surface and interface, in contrast to the bulk, where they are not magnetic, leading to the ferromagnetic ordering. Since heavily Co-doped ZnO samples can contain $\rm Co_3O_4 $ secondary phase, such a magnetic ordering at the interface might explain the origin of the magnetism in these diluted magnetic semiconductors (DMS).

cond-mat.mtrl-sci↗

arXiv2017

Exchange coupling and Mn valency in GaN doped with Mn and co-doped with Mg

We study 1 or 2 neighboring Mn impurities, as well as complexes of 1 Mn and 1 or 2 Mg ions in a 64 atoms supercell of GaN by means of density functional calculations. Taking into account the electron correlation in the local spin density approximation with explicit correction of the Hubbard term (the LSDA+U method) and full lattice relaxation we determine the nearest neighbor exchange J for a pair of Mn impurities. We find J to be ferromagnetic and of the order of about 18 meV in the Hamiltonian H=-2*J1*J2. That J is only weakly influenced by the U parameter (varying between 2 and 8 eV) and by the lattice relaxation. From a detailed analysis of the magnetization density distribution we get hints for a ferromagnetic super-exchange mechanism. Also the Mn valence was found to be 3+ without any doubt in the absence of co-doping with Mg. Co-doping with Mg leads to a valence change to 4+ for 1 Mg and to 5+ for 2 Mg. We show that the valence change can already be concluded from a careful analysis of the density of states of GaN doped with Mn without any Mg.

cond-mat.mtrl-sci↗

arXiv2013

Absence of helical surface states in bulk semimetals with broken inversion symmetry

Whereas the concept of topological band-structures was developed originally for insulators with a bulk bandgap, it has become increasingly clear that the prime consequences of a non-trivial topology -- spin-momentum locking of surface states -- can also be encountered in gapless systems. Concentrating on the paradigmatic example of mercury chalcogenides HgX (X = Te, Se, S), we show that the existence of helical semimetals, i.e. semimetals with topological surface states, critically depends on the presence of crystal inversion symmetry. An infinitesimally small broken inversion symmetry (BIS) renders the helical semimetallic state unstable. The BIS is also very important in the fully gapped regime, renormalizing the surface Dirac cones in an anisotropic manner. As a consequence the handedness of the Dirac cones can be flipped by a biaxial stress field.

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↗

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↗

arXiv2009

Transport through a band insulator with Rashba spin-orbit coupling: metal-insulator transition and spin-filtering effects

We calculate the current-voltage characteristic of a one-dimensional band insulator with magnetic field and Rashba spin-orbit coupling which is connected to nonmagnetic leads. Without spin-orbit coupling we find a complete spin-filtering effect, meaning that the electric transport occurs in one spin channel only. For a large magnetic field which closes the band gap, we show that spin-orbit coupling leads to a transition from metallic to insulating behavior. The oscillations of the different spin-components of the current with the length of the transport channel are studied as well.

cond-mat.mes-hall↗

arXiv2006

Exchange integrals and magnetization distribution in BaCu2X2O7 (X=Ge,Si)

Estimating the intrachain and interchain exchange constants in BaCu2X2O7 (X=Ge,Si) by means of density-functional calculations within the local spin-density approximation (LSDA) we find the Ge compound to be a more ideal realization of a one-dimensional spin chain with Dzyaloshinskii-Moriya interaction than its Si counterpart. Both compounds have a comparable magnitude of interchain couplings in the range of 5-10 K, but the nearest neighbor intrachain exchange of the Ge compound is nearly twice as large as for the Si one. Using the LSDA+U method we predict the detailed magnetization density distribution and especially remarkable magnetic moments at the oxygen sites.

cond-mat.str-el↗

arXiv2006

The energy-scale-dependent Composite Operator Method for the single-impurity Anderson model

The recently developed energy-scale-dependent Composite Operator Method is applied to the single-impurity Anderson model. A fully self-consistent solution is given and analyzed. At very low temperatures, the density of states presents, on the top of the high-energy background, a Kondo-like peak whose parameter dependence is discussed in detail. The proposed method reproduces the exact results known in the literature with very low numerical effort and it is applicable for arbitrary values of the external parameters.

cond-mat.str-el↗