arXiv subjects

G. Bihlmayer

Publications and source records attributed to G. Bihlmayer.

At least 37 records · Page 2Linked to original sources

arXiv2014

Intra- and Interband Electron Scattering in the Complex Hybrid Topological Insulator Bismuth Bilayer on Bi$_2$Se$_3$

The band structure, intra- and interband scattering processes of the electrons at the surface of a bismuth-bilayer on Bi$_2$Se$_3$ have been experimentally investigated by low-temperature Fourier-transform scanning tunneling spectroscopy. The observed complex quasiparticle interference patterns are compared to a simulation based on the spin-dependent joint density of states approach using the surface-localized spectral function calculated from first principles as the only input. Thereby, the origin of the quasiparticle interferences can be traced back to intraband scattering in the bismuth bilayer valence band and Bi$_2$Se$_3$ conduction band, and to interband scattering between the two-dimensional topological state and the bismuth-bilayer valence band. The investigation reveals that the bilayer band gap, which is predicted to host one-dimensional topological states at the edges of the bilayer, is pushed several hundred milli-electronvolts above the Fermi level. This result is rationalized by an electron transfer from the bilayer to Bi$_2$Se$_3$ which also leads to a two-dimensional electron state in the Bi$_2$Se$_3$ conduction band with a strong Rashba spin-splitting, coexisting with the topological state and bilayer valence band.

cond-mat.mes-hall↗

arXiv2013

Co atoms on Bi$_{2}$Se$_{3}$ revealing a coverage dependent spin reorientation transition

We investigate Co nanostructures on Bi$_{2}$Se$_{3}$ by means of scanning tunneling microscopy and spectroscopy [STM/STS], X-ray absorption spectroscopy [XAS], X-ray magnetic dichroism [XMCD] and calculations using the density functional theory [DFT]. In the single adatom regime we find two different adsorption sites by STM. Our calculations reveal these to be the fcc and hcp hollow sites of the substrate. STS shows a pronounced peak for only one species of the Co adatoms indicating different electronic properties of both types. These are explained on the basis of our DFT calculations by different hybridizations with the substrate. Using XMCD we find a coverage dependent spin reorientation transition from easy-plane toward out-of-plane. We suggest clustering to be the predominant cause for this observation.

cond-mat.mes-hall↗

arXiv2013

Tuning the Curie temperature of FeCo compounds by tetragonal distortion

Combining density-functional theory calculations with a classical Monte Carlo method, we show that for B2-type FeCo compounds tetragonal distortion gives rise to a strong reduction of the Curie temperature $T_{\mathrm{C}}$. The $T_{\mathrm{C}}$ monotonically decreases from 1575 K (for $c/a=1$) to 940 K (for $c/a=\sqrtwo$). We find that the nearest neighbor Fe-Co exchange interaction is sufficient to explain the $c/a$ behavior of the $T_{\mathrm{C}}$. Combination of high magnetocrystalline anisotropy energy with a moderate $T_{\mathrm{C}}$ value suggests tetragonal FeCo grown on the Rh substrate with $c/a=1.24$ to be a promising material for heat-assisted magnetic recording applications.

cond-mat.mtrl-sci↗

arXiv2013

Elemental Topological Insulator with a Tunable Fermi Level: Strained α-Sn on InSb(001)

We report on the epitaxial fabrication and electronic properties of a topological phase in strained α-Sn on InSb. The topological surface state forms in the presence of an unusual band order not based on direct spin-orbit coupling, as shown in density functional and GW slab-layer calculations. Angle-resolved photoemission including spin detection probes experimentally how the topological spin-polarized state emerges from the second bulk valence band. Moreover, we demonstrate the precise control of the Fermi level by dopants.

cond-mat.mes-hall↗

arXiv2012

Probing two topological surface bands of Sb2Te3 by spin-polarized photoemission spectroscopy

Using high resolution spin- and angle-resolved photoemission spectroscopy, we map the electronic structure and spin texture of the surface states of the topological insulator Sb2Te3. In combination with density functional calculations (DFT), we directly show that Sb2Te3 exhibits a partially occupied, single spin-Dirac cone around the Fermi energy, which is topologically protected. DFT obtains a spin polarization of the occupied Dirac cone states of 80-90%, which is in reasonable agreement with the experimental data after careful background subtraction. Furthermore, we observe a strongly spin-orbit split surface band at lower energy. This state is found at 0.8eV below the Fermi level at the gamma-point, disperses upwards, and disappears at about 0.4eV below the Fermi level into two different bulk bands. Along the gamma-K direction, the band is located within a spin-orbit gap. According to an argument given by Pendry and Gurman in 1975, such a gap must contain a surface state, if it is located away from the high symmetry points of the Brillouin zone. Thus, the novel spin-split state is protected by symmetry, too.

cond-mat.mes-hall↗

arXiv2012

On the nature of the spin polarization limit in the warped Dirac cone of the Bi2Te3

The magnitude of electron spin polarization in topologically protected surface states is an important parameter with respect to spintronics applications. In order to analyze the warped spin texture in Bi$_2$Te$_3$ thin films, we combine angle- and spin-resolved photoemission experiments with theoretical \textit{ab initio} calculations. We find an \textit{in-plane} spin polarization of up to $\sim$~45\% in the topologically protected Dirac cone states near the Fermi level. The Fermi surface of the Dirac cone state is warped and shows an \textit{out-of-plane} spin polarization of $\sim$~15\%. These findings are in quantitative agreement with dedicated simulations which find electron density of the Dirac cone delocalized over the first quintuple layer with spin reversal occurring in the surface atomic layer.

cond-mat.mes-hall↗

arXiv2012

Graphene for spintronics: giant Rashba splitting due to hybridization with Au

Graphene in spintronics has so far primarily meant spin current leads of high performance because the intrinsic spin-orbit coupling of its pi-electrons is very weak. If a large spin-orbit coupling could be created by a proximity effect, the material could also form active elements of a spintronic device such as the Das-Datta spin field-effect transistor, however, metal interfaces often compromise the band dispersion of massless Dirac fermions. Our measurements show that Au intercalation at the graphene-Ni interface creates a giant spin-orbit splitting (~100 meV) in the graphene Dirac cone up to the Fermi energy. Photoelectron spectroscopy reveals hybridization with Au-5d states as the source for the giant spin-orbit splitting. An ab initio model of the system shows a Rashba-split dispersion with the analytically predicted gapless band topology around the Dirac point of graphene and indicates that a sharp graphene-Au interface at equilibrium distance will account for only ~10 meV spin-orbit splitting. The ab initio calculations suggest an enhancement due to Au atoms that get closer to the graphene and do not violate the sublattice symmetry.

cond-mat.mtrl-sci↗

arXiv2011

Direct observation of spin-polarized surface states in the parent compound of topological insulator Bi-Sb using spin-resolved-ARPES in a 3D Mott-polarimetry spin mode

We report high-resolution spin-resolved photoemission spectroscopy (Spin-ARPES) measurements on the parent compound Sb of the first discovered 3D topological insulator Bi{1-x}Sb{x} [D. Hsieh et al., Nature 452, 970 (2008) Submitted 2007]. By modulating the incident photon energy, we are able to map both the bulk and (111) surface band structure, from which we directly demonstrate that the surface bands are spin polarized by the spin-orbit interaction and connect the bulk valence and conduction bands in a topologically non-trivial way. A unique asymmetric Dirac surface state gives rise to a $k$-splitting of its spin polarized electronic channels. These results complement our previously published works on this materials class and re-confirm our discovery of first bulk (3D) topological insulator - topological order in bulk solids. [Invited article for NJP-IOP Focus issue on "Topological Insulators"]

cond-mat.mes-hall↗

arXiv2010

Well-localized edge states in two-dimensional topological insulators: ultrathin Bi films

We theoretically study the generic behavior of the penetration depth of the edge states in two-dimensional quantum spin Hall systems. We found that the momentum-space width of the edge-state dispersion scales with the inverse of the penetration depth. As an example of well-localized edge states, we take the Bi(111) ultrathin film. Its edge states are found to extend almost over the whole Brillouin zone. Correspondingly, the bismuth (111) 1-bilayer system is proposed to have well-localized edge states in contrast to the HgTe quantum well.

cond-mat.mtrl-sci↗

arXiv2009

Observation of topologically protected Dirac spin-textures and πBerry's phase in pure Antimony (Sb) and topological insulator BiSb

A topologically ordered material is characterized by a rare quantum organization of electrons that evades the conventional spontaneously broken symmetry based classification of condensed matter. Exotic spin transport phenomena such as the dissipationless quantum spin Hall effect have been speculated to originate from a novel topological order whose identification requires a spin sensitive measurement. Using Spin-resolved-ARPES, we probe the spin degrees of freedom and demonstrate that topological quantum numbers are uniquely determined from spin-texture Berry Phase imaging measurements. Applying this method to pure antimony (Sb) and Bi-Sb, we identify the origin of its novel Topological Order and the negative value of the mirror Chern number. These results taken together constitute the first observation of surface electrons collectively carrying a topological Berry's phase and definite mirror Chern chirality in pure Antimony (Sb) which are the key electronic properties for realizing topological quantum computing via the interface Majorana fermion framework. This paper contains the details of the above mentioned previously reported (Science \textbf{323}, 919 (2009)) results.

cond-mat.mes-hall↗

arXiv2009

First direct observation of Spin-textures in Topological Insulators : Spin-resolved ARPES as a probe of topological quantum spin Hall effect and Berry's phase

A topologically ordered material is characterized by a rare quantum organization of electrons that evades the conventional spontaneously broken symmetry based classification of condensed matter. Exotic spin transport phenomena such as the dissipationless quantum spin Hall effect have been speculated to originate from a novel topological order whose identification requires a spin sensitive measurement, which does not exist to this date in any system (neither in Hg(Cd)Te quantum wells nor in the topological insulator BiSb). Using Mott polarimetry, we probe the spin degrees of freedom of these quantum spin Hall states and demonstrate that topological quantum numbers are uniquely determined from spin texture imaging measurements. Applying this method to the Bi{1-x}Sb{x} series, we identify the origin of its novel order and unusual chiral properties. These results taken together constitute the first observation of surface electrons collectively carrying a geometrical quantum (Berry's) phase and definite chirality (mirror Chern number, n_M =-1), which are the key electronic properties for realizing topological computing bits with intrinsic spin Hall-like topological phenomena. Our spin-resolved results not only provides the first clear proof of a topological insulating state in nature but also demonstrate the utility of spin-resolved ARPES technique in measuring the quantum spin Hall phases of matter.

cond-mat.mes-hall↗

arXiv2008

Intriguing magnetism of Fe monolayers on hexagonal transition-metal surfaces

Using first-principles calculations, we demonstrate that an Fe monolayer can assume very different magnetic phases on hexagonal hcp (0001) and fcc (111) surfaces of 4d- and 5d-transition metals. Due to the substrates' d-band filling, the nearest-neighbor exchange coupling of Fe changes gradually from antiferromagnetic (AFM) for Fe films on Tc, Re, Ru and Os to ferromagnetic on Rh, Ir, Pd, and Pt. In combination with the topological frustration on the triangular lattice of these surfaces the AFM coupling results in a 120-degree Neel structure for Fe on Re and Ru and an unexpected double-row-wise AFM structure on Rh, which is a superposition of a left- and right-rotating 90-degree spin spiral.

cond-mat.mtrl-sci↗

arXiv2007

Magnetic Phase Control in Monolayer Films by Substrate Tuning

We propose to tailor exchange interactions in magnetic monolayer films by tuning the adjacent non-magnetic substrate. As an example, we demonstrate a ferromagnetic-antiferromagnetic phase transition for one monolayer Fe on a Ta(x)W(1-x)(001) surface as a function of the Ta concentration. At the critical Ta concentration, the nearest-neighbor exchange interaction is small and the magnetic phase space is dramatically broadened. Complex magnetic order such as spin-spirals, multiple-Q, or even disordered local moment states can occur, offering the possibility to store information in terms of ferromagnetic dots in an otherwise zero-magnetization state matrix.

cond-mat.mtrl-sci↗

arXiv2007

Interplay of structure and spin-orbit strength in magnetism of metal-benzene sandwiches: from single molecules to infinite wires

Based on first-principles density functional theory calculations we explore electronic and magnetic properties of experimentally producible sandwiches and infinite wires made of repeating benzene molecules and transition-metal atoms of V, Nb, and Ta. We describe the bonding mechanism in the molecules and in particular concentrate on the origin of magnetism in these structures. We find that all the considered systems have sizable magnetic moments and ferromagnetic spin-ordering, with the single exception of the V3-Bz4 molecule. By including the spin-orbit coupling into our calculations we determine the easy and hard axes of the magnetic moment, the strength of the uniaxial magnetic anisotropy energy (MAE), relevant for the thermal stability of magnetic orientation, and the change of the electronic structure with respect to the direction of the magnetic moment, important for spin-transport properties. While for the V-based compounds the values of the MAE are only of the order of 0.05-0.5 meV per metal atom, increasing the spin-orbit strength by substituting V with heavier Nb and Ta allows to achieve an increase in anisotropy values by one to two orders of magnitude. The rigid stability of magnetism in these compounds together with the strong ferromagnetic ordering makes them attractive candidates for spin-polarized transport applications. For a Nb-benzene infinite wire the occurrence of ballistic anisotropic magnetoresistance is demonstrated.

cond-mat.mtrl-sci↗

arXiv2006

First-principles stabilization of an unconventional collinear magnetic ordering in distorted manganites

First-principles calculations have been performed for different collinear magnetic orderings in orthorhombic manganites, such as HoMnO3, TbMnO3 and YMnO3, showing large GdFeO3-like distortions. Our results suggest that the AFM-E type ordering, experimentally observed in HoMnO3 and recently proposed from model hamiltonian studies as a potentially novel phase, is indeed the magnetic ground state. Its stability is strongly connected with octahedral distortions and points to the relevance of structural more than chemical effects. The calculated exchange constants, extracted from a Heisenberg model used to fit the first-principles total energies, show that the ferromagnetic in-plane nearest-neighbour coupling is reduced compared to less-distorted manganites, such as LaMnO3. In parallel, the antiferromagnetic next-nearest-neighbour coupling along planar Mn-O-O-Mn paths in highly-distorted manganites plays a relevant role in the stabilization of the AFM-E spin configuration. In agreement with experiments, the density of states shows that this phase is insulating with an indirect band-gap of about 0.5 eV.

cond-mat.mtrl-sci↗

arXiv2006

Structural determination of a low-symmetry surface by low-energy electron diffraction and ab initio calculations: Bi(110)

The surface structure of Bi(110) has been investigated by low-energy electron diffraction (LEED) intensity analysis and by first-principles calculations. Diffraction patterns at a sample temperature of 110 K and normal incidence reveal a bulk truncated (1$\times$1) surface without indication of any structural reconstruction despite the presence of dangling bonds on the surface layer. Good agreement is obtained between the calculated and measured diffraction intensities for this complex, low-symmetry surface containing only one mirror-plane symmetry element. No significant interlayer spacing relaxations are found. The Debye temperature for the surface layer is found to be lower than in the bulk, which is indicative of larger vibrational atomic amplitudes at the surface. Meanwhile, the second layer shows a Debye temperature close to the bulk value. The experimental results for the relaxations agree well with those of our first-principles calculation.

cond-mat.other↗

arXiv2005

Thermal collapse of spin-polarization in half-metallic ferromagnets

The temperature dependence of the magnetization and spin-polarization at the Fermi level is investigated for half-metallic ferromagnets. We reveal a new mechanism, where the hybridization of states forming the half-metallic gap depends on thermal spin fluctuations and the polarization can drop abruptly at temperatures much lower than the Curie point. We verify this for NiMnSb by ab-initio calculations. The thermal properties are studied by mapping ab-initio results to an extended Heisenberg model which includes longitudinal fluctuations and is solved by a Monte Carlo method.

cond-mat.mtrl-sci↗

arXiv2005

Surface-electronic structure of La(0001) and Lu(0001)

Most spectroscopic methods for studying the electronic structure of metal surfaces have the disadvantage that either only occupied or only unoccupied states can be probed, and the signal is cut at the Fermi edge. This leads to significant uncertainties, when states are very close to the Fermi level. By performing low-temperature scanning tunneling spectroscopy and ab initio calculations, we study the surface-electronic structure of La(0001) and Lu(0001), and demonstrate that in this way detailed information on the surface-electronic structure very close to the Fermi energy can be derived with high accuracy.

cond-mat.mtrl-sci↗