arXiv subjects

J. Henk

Publications and source records attributed to J. Henk.

15 recordsLinked to original sources

arXiv2025

Improved imaging of magnetic domains with a photoelectron emission microscope by utilizing symmetry and momentum selection

Imaging of magnetic domains with a photoelectron emission microscope operated with photon energies in the threshold regime often suffers from low contrast. In this work we show by symmetry considerations, photoemission calculations, and imaging experiments, how the contrast can be improved significantly. The key to both domain selectivity and sizable intensity asymmetries is, guided by symmetry considerations, selecting the momenta of the photoelectrons by a properly positioned contrast aperture. By comparing computational with experimental results for an Fe(001) surface we prove the feasibility of the approach.

cond-mat.mtrl-sci↗

arXiv2022

Magnon-Polaron Driven Thermal Hall Effect in a Heisenberg-Kitaev Antiferromagnet

The thermal Hall effect, defined as a heat current response transversal to an applied temperature gradient, is a central experimental probe of exotic electrically insulating phases of matter. A key question is how the interplay between magnetic and structural degrees of freedom gives rise to a nonzero thermal Hall conductivity (THC). Here, we present evidence for an intrinsic thermal Hall effect in the Heisenberg-Kitaev antiferromagnet and spin-liquid candidate Na$_2$Co$_2$TeO$_6$ brought about by the quantum-geometric Berry curvature of so-called magnon polarons, resulting from magnon-phonon hybridization. At low temperatures, our field- and temperature-dependent measurements show a negative THC for magnetic fields below 10 T and a sign change to positive THC above. Theoretically, the sign and the order of magnitude of the THC cannot be solely explained with magnetic excitations. We demonstrate that, by incorporating spin-lattice coupling into our theoretical calculations, the Berry curvature of magnon polarons counteracts the purely magnonic contribution, reverses the overall sign of the THC, and increases its magnitude, which significantly improves agreement with experimental data. Our work highlights the crucial role of spin-lattice coupling in the thermal Hall effect.

cond-mat.str-el↗

arXiv2014

Origin of Rashba-splitting in the quantized subbands at Bi2Se3 surface

We study the band structure of the $\text{Bi}_2\text{Se}_3$ topological insulator (111) surface using angle-resolved photoemission spectroscopy. We examine the situation where two sets of quantized subbands exhibiting different Rashba spin-splitting are created via bending of the conduction (CB) and the valence (VB) bands at the surface. While the CB subbands are strongly Rashba spin-split, the VB subbands do not exhibit clear spin-splitting. We find that CB and VB experience similar band bending magnitudes, which means, a spin-splitting discrepancy due to different surface potential gradients can be excluded. On the other hand, by comparing the experimental band structure to first principles LMTO band structure calculations, we find that the strongly spin-orbit coupled Bi 6$p$ orbitals dominate the orbital character of CB, whereas their admixture to VB is rather small. The spin-splitting discrepancy is, therefore, traced back to the difference in spin-orbit coupling between CB and VB in the respective subbands' regions.

cond-mat.mes-hall↗

arXiv2013

Exchange interaction and its tuning in magnetic binary chalcogenides

Using a first-principles Green's function approach we study magnetic properties of the magnetic binary chalcogenides Bi2Te3, Bi2Se3, and Sb2Te3. The magnetic coupling between transition-metal impurities is long-range, extends beyond a quintuple layer, and decreases with increasing number of d electrons per 3d atom. We find two main mechanisms for the magnetic interaction in these materials: the indirect exchange interaction mediated by free carriers and the indirect interaction between magnetic moments via chalcogen atoms. The calculated Curie temperatures of these systems are in good agreement with available experimental data. Our results provide deep insight into magnetic interactions in magnetic binary chalcogenides and open a way to design new materials for promising applications.

cond-mat.mtrl-sci↗

arXiv2013

Pressure-induced Topological Phase Transitions in Rock-salt Chalcogenides

By means of a comprehensive theoretical investigation, we show that external pressure can induce topological phase transitions in IV-VI semiconducting chalcogenides with rock-salt structure. These materials satisfy mirror symmetries that are needed to sustain topologically protected surface states, at variance with time-reversal symmetry responsible for gapless edge states in $\mathcal{Z}_{2}$ topological insulators. The band inversions at high-symmetry points in the Brillouin zone that are related by mirror symmetry, are brought about by an "asymmetric" hybridization between cation and anion $sp$ orbitals. By working out the microscopic conditions to be fulfilled in order to maximize this hybridization, we identify materials in the rock-salt chalcogenide class that are prone to undergo a topological phase transition induced by pressure and/or alloying. Our model analysis is fully comfirmed by complementary advanced \textit{first-principles} calculations and \textit{ab initio}-based tight-binding simulations.

cond-mat.mtrl-sci↗

arXiv2010

Correlating transmission and local electronic structure in planar junctions: A tool for analyzing spin-dependent transport calculations

We propose to correlate transmittance maps and spectral-density maps of planar junctions, in order to analyze quantitatively and in detail spin-dependent transport calculations. Since spectral-density maps can be resolved with respect to atom, angular momentum, and spin, the resulting correlation coefficients reveal unequivocally, e.g., which layers or which orbitals determine the tunnel conductances. Our method can be used for transport calculations within the Landauer-Büttiker formalism. Its properties and features will be discussed by means of a pure bcc Fe(001) lead as well as an extensively studied Fe(001)/MgO/Fe(001) planar tunnel junction.

cond-mat.mes-hall↗

arXiv2010

Strong influence of the complex bandstructure on the tunneling electroresistance: A combined model and ab-initio study

The tunneling electroresistance (TER) for ferroelectric tunnel junctions (FTJs) with BaTiO_{3} (BTO) and PbTiO}_{3} (PTO) barriers is calculated by combining the microscopic electronic structure of the barrier material with a macroscopic model for the electrostatic potential which is caused by the ferroelectric polarization. The TER ratio is investigated in dependence on the intrinsic polarization, the chemical potential, and the screening properties of the electrodes. A change of sign in the TER ratio is obtained for both barrier materials in dependence on the chemical potential. The inverse imaginary Fermi velocity describes the microscopic origin of this effect; it qualitatively reflects the variation and the sign reversal of the TER. The quantity of the imaginary Fermi velocity allows to obtain detailed information on the transport properties of FTJs by analyzing the complex bandstructure of the barrier material.

cond-mat.mtrl-sci↗

arXiv2010

Tuning independently Fermi energy and spin splitting in Rashba systems: Ternary surface alloys on Ag(111)

By detailed first-principles calculations we show that the Fermi energy and the Rashba splitting in disordered ternary surface alloys (BiPbSb)/Ag(111) can be independently tuned by choosing the concentrations of Bi and Pb. The findings are explained by three fundamental mechanisms, namely the relaxation of the adatoms, the strength of the atomic spin-orbit coupling, and band filling. By mapping the Rashba characteristics,i.e.the splitting and the Rashba energy, and the Fermi energy of the surface states in the complete range of concentrations. Our results suggest to investigate experimentally effects which rely on the Rashba spin-orbit coupling in dependence on spin-orbit splitting and band filling.

cond-mat.mtrl-sci↗

arXiv2010

Tailoring tunnel magnetoresistance by ultrathin Cr and Co interlayers: A first-principles investigation of Fe/MgO/Fe junctions

We report on systematic ab-initio investigations of Co and Cr interlayers embedded in Fe(001)/MgO/Fe(001) magnetic tunnel junctions, focusing on the changes of the electronic structure and the transport properties with interlayer thickness. The results of spin-dependent ballistic transport calculations reveal options to specifically manipulate the tunnel magnetoresistance ratio. The resistance area products and the tunnel magnetoresistance ratios show a monotonous trend with distinct oscillations as a function of the Cr thickness. These modulations are directly addressed and interpreted by means of magnetic structures in the Cr films and by complex band structure effects. The characteristics for embedded Co interlayers are considerably influenced by interface resonances which are analyzed by the local electronic structure.

cond-mat.mtrl-sci↗

arXiv2009

Silicon surface with giant spin-splitting

We demonstrate the induction of a giant Rashba-type spin-splitting on a semiconducting substrate by means of a Bi trimer adlayer on a Si(111) wafer. The in-plane inversion symmetry is broken so that the in-plane potential gradient induces a giant spin-splitting with a Rashba energy of about 140 meV, which is more than an order of magnitude larger than what has previously been reported for any semiconductor heterostructure. The separation of the electronic states is larger than their lifetime broadening, which has been directly observed with angular resolved photoemission spectroscopy. The experimental results are confirmed by relativistic first-principles calculations. We envision important implications for basic phenomena as well as for the semiconductor based technology.

cond-mat.mtrl-sci↗

arXiv2009

Structural phase transitions and fundamental band gaps of Mg(x)Zn(1-x)O alloys from first principles

The structural phase transitions and the fundamental band gaps of Mg(x)Zn(1-x)O alloys are investigated by detailed first-principles calculations in the entire range of Mg concentrations x, applying a multiple-scattering theoretical approach (Korringa-Kohn-Rostoker method). Disordered alloys are treated within the coherent potential approximation (CPA). The calculations for various crystal phases have given rise to a phase diagram in good agreement with experiments and other theoretical approaches. The phase transition from the wurtzite to the rock-salt structure is predicted at the Mg concentration of x = 0.33, which is close to the experimental value of 0.33 - 0.40. The size of the fundamental band gap, typically underestimated by the local density approximation, is considerably improved by the self-interaction correction. The increase of the gap upon alloying ZnO with Mg corroborates experimental trends. Our findings are relevant for applications in optical, electrical, and in particular in magnetoelectric devices.

cond-mat.mtrl-sci↗

arXiv2009

Magnetic Anisotropy and Magnetization Dynamics of Individual Atoms and Clusters of Fe and Co on Pt(111)

The recently discovered giant magnetic anisotropy of single magnetic Co atoms raises the hope of magnetic storage in small clusters. We present a joint experimental and theoretical study of the magnetic anisotropy and the spin dynamics of Fe and Co atoms, dimers, and trimers on Pt(111). Giant anisotropies of individual atoms and clusters as well as lifetimes of the excited states were determined with inelastic scanning tunneling spectroscopy. The short lifetimes due to hybridization-induced electron-electron scattering oppose the magnetic stability provided by the magnetic anisotropies.

cond-mat.mtrl-sci↗

arXiv2003

Spin polarization of the L-gap surface states on Au(111)

The electron spin polarization (ESP) of the L-gap surface states on Au(111) is investigated theoretically by means of first-principles electronic-structure and photoemission calculations. The surface states show a large spin-orbit induced in-plane ESP which is perpendicular to the in-plane wavevector, in close analogy to a two-dimensional electron gas with Rashba spin-orbit interaction. The surface corrugation leads to a small ESP component normal to the surface, being not reported so far. The surface-states ESP can be probed qualitatively and quantitatively by spin- and angle-resolved photoelectron spectroscopy, provided that the initial-state ESP is retained in the photoemission process and not obscured by spin-orbit induced polarization effects. Relativistic photoemission calculations provide detailed information on what photoemission set-ups allow to conclude from the photoelectron ESP on that of the surface states.

cond-mat↗

arXiv2003

Bias-voltage dependence of the magneto-resistance in ballistic vacuum tunneling: Theory and application to planar Co(0001) junctions

Motivated by first-principles results for jellium and by surface-barrier shapes that are typically used in electron spectroscopies, the bias voltage in ballistic vacuum tunneling is treated in a heuristic manner. The presented approach leads in particular to a parameterization of the tunnel-barrier shape, while retaining a first-principles description of the electrodes. The proposed tunnel barriers are applied to Co(0001) planar tunnel junctions. Besides discussing main aspects of the present scheme, we focus in particular on the absence of the zero-bias anomaly in vacuum tunneling.

cond-mat↗

arXiv2002

Spin Motion in Electron Transmission through Ultrathin Ferromagnetic Films Accessed by Photoelectron Spectroscopy

Ab initio and model calculations demonstrate that the spin motion of electrons transmitted through ferromagnetic films can be analyzed in detail by means of angle- and spin-resolved core-level photoelectron spectroscopy. The spin motion appears as precession of the photoelectron spin polarization around and as relaxation towards the magnetization direction. In a systematic study for ultrathin Fe films on Pd(001) we elucidate its dependence on the Fe film thickness and on the Fe electronic structure. In addition to elastic and inelastic scattering, the effect of band gaps on the spin motion is addressed in particular.

cond-mat↗