arXiv subjects

Claudia Felser

Publications and source records attributed to Claudia Felser.

At least 361 records · Page 20Linked to original sources

arXiv2015

Magnetic dichroism study on Mn$_{1.8}$Co$_{1.2}$Ga thin film using a combination of X-ray absorption and photoemission spectroscopy

Using circularly polarised radiation and a combination of bulk-sensitive hard X-ray photoelectron spectroscopy and X-ray-absorption spectroscopy (XAS) we studied the electronic and magnetic structure of epitaxial Mn$_{1.8}$Co$_{1.2}$Ga thin films. Spin resolved Bloch spectral functions, density of states as well as charge and magnetisation densities were investigated by a first-principles analysis of full potential, fully relativistic Korringa--Kohn--Rostoker calculations of the electronic structure. The valence states were experimentally investigated by using linear dichroism in the angular distribution and comparing the results to spin-resolved densities of states. The linear dichroism in the valence band enabled a symmetry analysis of the contributing states. The spectra were in good agreement with the theoretical partial density of states. The element-specific, spin-resolved, unoccupied densities of states for Co and Mn were analysed by using XAS and X-ray magnetic circular dichroism (XMCD) at the $L_{3,2}$ edges. The spectra were influenced by strong correlation effects. XMCD was used to extract the site resolved magnetic moments. The experimental values of $m_{\rm Mn}=0.7\:μ_B$ and $m_{\rm Co}=1.05\:μ_B$ agree very well with the calculated magnetic moments. Magnetic circular dichroism in angle-resolved photoelectron spectroscopy at the Mn and Co $2p$ core level exhibited a pronounced magnetic dichroism and confirmed the localised character of the Mn $d$ valence states.

cond-mat.mtrl-sci↗

arXiv2015

State of Co and Mn in half-metallic ferromagnet Co$_2$MnSi explored by magnetic circular dichroism in hard X-ray photoelectron emission and soft X-ray absorption spectroscopies

The half-metallic Heusler compound Co$_2$MnSi is a very attractive material for spintronic devices because it exhibits very high tunnelling magnetoresistance ratios. This work reports on a spectroscopic investigation of thin Co$_2$MnSi films as they are used as electrodes in magnetic tunnel junctions. The investigated films exhibit a remanent in-plane magnetisation with a magnetic moment of about 5~$μ_B$ when saturated, as expected. The low coercive field of only 4~mT indicates soft magnetic behaviour. Magnetic dichroism in emission and absorption was measured at the Co and Mn $2p$ core levels. The photoelectron spectra were excited by circularly polarised hard X-rays with an energy of of 6~keV and taken from the remanently magnetised film. The soft X-ray absorption spectra were taken in an induction field of 4~T. Both methods yielded large dichroism effects. An analysis reveals the localised character of the electrons and magnetic moments attributed to the Mn atoms, whereas the electrons related to the Co atoms contribute an itinerant part to the total magnetic moment.

cond-mat.mtrl-sci↗

arXiv2014

Chemical disorder as engineering tool for spin-polarizationin Mn3Ga-based Heusler systems

Our study highlights spin-polarization mechanisms in metals, by focusing on the mobilities of conducting electrons with different spins instead of their quantities. Here, we engineer electron mobility by applying chemical disorder induced by non-stoichiometric variations. As a practical example, we discuss the scheme that establishes such variations in tetragonal Mn3Ga Heusler material. We justify this approach using first-principles calculations of the spin-projected conductivity components based on the Kubo-Greenwood formalism. It follows that, in majority of the cases, even a small substitution of some other transition element instead of Mn may lead to a substantial increase in spin-polarization along the tetragonal axis.

cond-mat.mtrl-sci↗

arXiv2014

Quantum Anomalous Hall Effect in Magnetic Insulator Heterostructure

Based on ab initio calculations, we predict that a monolayer of Cr-doped (Bi,Sb)2Te3 and GdI2 heterostructure is a quantum anomalous Hall insulator with a non-trivial band gap up to 38 meV. The principle behind our prediction is that the band inversion between two topologically trivial ferromagnetic insulators can result in a non-zero Chern number, which offers a better way to realize the quantum anomalous Hall state without random magnetic doping. In addition, a simple effective model is presented to describe the basic mechanism of spin polarized band inversion in this system. Moreover, we predict that 3D quantum anomalous Hall insulator could be realized in (Bi2/3Cr1/3)2Te3/GdI2 superlattice.

cond-mat.mes-hall↗

arXiv2014

Non-collinear Antiferromagnets and the Anomalous Hall Effect

The anomalous Hall effect is investigated theoretically by means of density functional calculations for the non-collinear antiferromagnetic order of the hexagonal compounds Mn$_3$Ge and Mn$_3$Sn using various planar triangular magnetic configurations as well as unexpected non-planar configurations. The former give rise to anomalous Hall conductivities (AHC) that are found to be extremely anisotropic. For the planar cases the AHC is connected with Weyl-points in the energy-band structure, which are described in detail. If this case were observable in Mn$_3$Ge, a large AHC of about 900 S/cm should be expected. However, in Mn$_3$Ge it is the non-planar configuration that is energetically favored, in which case it gives rise to an AHC of 100 S/cm. The non-planar configuration allows a quantitative evaluation of the topological Hall effect that is seen to determine this value of the conductivity to a large extent. For Mn$_3$Sn it is the planar configurations that are predicted to be observable. In this case the AHC can be as large as 250 S/cm.

cond-mat.mtrl-sci↗

arXiv2014

Guidelines for understanding cubic manganese-rich Heusler compounds

Manganese-rich Heusler compounds are attracting much interest in the context of spin transfer torque and rare-earth free hard magnets. Here we give a comprehensive overview of the magnetic properties of non-centrosymmetric cubic Mn$_2$-based Heusler materials, which are characterized by an antiparallel coupling of magnetic moments on Mn atoms. Such a ferrimagnetic order leads to the emergence of new properties that are absent in ferromagnetic centrosymmetric Heusler structures. In terms of the band structure calculations, we explain the formation of this magnetic order and the Curie temperatures. This overview is intended to establish guidelines for a basic understanding of magnetism in Mn2 -based Heusler compounds.

cond-mat.mtrl-sci↗

arXiv2014

On the superconductivity of TiNCl and ZrNCl: A local bonding perspective

We analyze the superconductors TiNCl and ZrNCl from a local bonding perspective. Although TiNCl crystallizes in an orthorhombic structure and ZrNCl crystallizes in a hexagonal structure, both compounds show significant structural similarities, for example that both consist of layered metal-Nitrogen networks. The local bonding in those two structures is very similar, giving rise to a dispersive conduction band mostly consisting of metal-d -states. Upon doping both compounds show structural changes, which lead to short metal-metal distances, indicating a bonding interaction that might be important for the appearance of superconductivity in these systems. We furthermore draw analogies to other superconductors that are close to a charge density wave instability around a d 1 -configuration and offer a different perspective on this class of superconductors, which show non-BCS-like superconductivity.

cond-mat.supr-con↗

arXiv2014

Paramagnetic to ferromagnetic phase transition in lightly Fe-doped Cr$_2$B

Cr$_2$B displays temperature independent paramagnetism. We induce ferromagnetism by replacing less than $3\,\%$ of the Cr atoms by Fe. By the lowest Fe doping level made, Curie-Weiss behavior is observed; $Θ_{CW}$ changes from $-20\,$K for $0.5\,\%$ Fe-doped Cr$_2$B to positive values of about 50 K by $5\,\%$ Fe doping. The ferromagnetic T$_C$ is 8 K for $2.5\,\%$ Fe doping and increases linearly to 46 K by $5\,\%$ doping; we infer that a quantum phase transition occurs near the $2.0\,\%$ Fe level. Magnetic fluctuations at the intermediate doping levels are reflected in the linear resistance and an anomalous heat capacity at low temperatures. Imaging and chemical analysis down to the atomic scale show that the Fe dopant is randomly distributed.

cond-mat.str-el↗

arXiv2014

New Family of Robust 2D Topological Insulators in van der Waals Heterostructures

We predict a new family of robust two-dimensional (2D) topological insulators in van der Waals heterostructures comprising graphene and chalcogenides BiTeX (X=Cl, Br and I). The layered structures of both constituent materials produce a naturally smooth interface that is conducive to proximity induced new topological states. First principles calculations reveal intrinsic topologically nontrivial bulk energy gaps as large as 70-80 meV, which can be further enhanced up to 120 meV by compression. The strong spin-orbit coupling in BiTeX has a significant influence on the graphene Dirac states, resulting in the topologically nontrivial band structure, which is confirmed by calculated nontrivial Z2 index and an explicit demonstration of metallic edge states. Such heterostructures offer an unique Dirac transport system that combines the 2D Dirac states from graphene and 1D Dirac edge states from the topological insulator, and it offers new ideas for innovative device designs.

cond-mat.mtrl-sci↗

arXiv2014

Evidence of surface transport and weak anti-localization in single crystal of Bi2Te2Se topological insulator

Topological insulators are known to their metallic surface states, a result of strong-spin-orbital coupling, that show unique surface transport phenomenon. But these surface transports are buried in presence of metallic bulk conduction. We synthesized very high quality Bi$_2$Te$_2$Se single crystals by modified Bridgman method, that possess high bulk resistivity of $>$20~$Ω$cm below 20~K, whereas the bulk is mostly inactive and surface transport dominates. Temperature dependence resistivity follows the activation law like a gap semiconductor in temperature range 20-300~K. We designed a special measurement geometry, which aims to extract the surface transport from the bulk. This special geometry is applied to measure the resistance and found that Bi$_2$Te$_2$Se single crystal exhibits a cross over from bulk to surface conduction at 20~K. Simultaneously, the material also shows strong evidence of weak anti-localization in magneto-transport due to the protection against scattering by conducting surface states. This novel simple geometry is an easy route to find the evidence of surface transport in topological insulators, which are the promising materials for future spintronic applications.

cond-mat.mtrl-sci↗

arXiv2014

Theoretical search for half-Heusler topological insulators

We have performed ab-initio band structure calculations on more than two thousand half-Heusler compounds in order to search for new candidates for topological insulators. Herein, LiAuS and NaAuS are found to be the strongest topological insulators with the bulk band gap of 0.20 and 0.19 eV, respectively, different from the zero band gap feature reported in other Heusler topological insulators. Due to the inversion asymmetry of the Heusler structure, their topological surface states on the top and bottom surfaces exhibit p-type and n-type carriers, respectively. Thus, these materials may serve as an ideal platform for the realization of topological magneto-electric effects as polar topological insulators. Moreover, these topological surface states exhibit the right-hand spin-texture in the upper Dirac cone, which distinguish them from currently known topological insulator materials. Their topological nontrivial character remains robust against in-plane strains, which makes them suitable for epitaxial growth of films.

cond-mat.mtrl-sci↗

arXiv2014

Spin Texture and Mirror Chern number in Hg-Based Chalcogenides

The unique feature of surface states in topological insulators is the so-called "spin-momentum locking", which means that electron spin is oriented along a fixed direction for a given momentum and forms a texture in the momentum space. In this work, we study spin textures of two typical topological insulators in Hg-Based Chalcogenides, namely HgTe and HgS, based on both the first principles calculation and the eight band Kane model. We find opposite helicities of spin textures between these two materials, originating from the opposite signs of spin-orbit couplings. Furthermore, we reveal that different mirror Chern numbers between HgTe and HgS characterize different topological natures of the systems with opposite spin textures and guarantee the existence of gapless interface states.

cond-mat.mes-hall↗

arXiv2014

Ab initio study of topological surface states of strained HgTe

The topological surface states of mercury telluride (HgTe) are studied by ab initio calculations assuming different strains and surface terminations. For the Te-terminated surface, a single Dirac cone exists at the $Γ$ point. The Dirac point shifts up from the bulk valence bands into the energy gap when the substrate-induced strain increases. At the experimental strain value (0.3%), the Dirac point lies slightly below the bulk valence band maximum. A left-handed spin texture was observed in the upper Dirac cone, similar to that of the Bi$_2$Se$_3$-type topological insulator. For the Hg-terminated surface, three Dirac cones appear at three time-reversal-invariant momenta, excluding the $Γ$ point, with nontrivial spin textures.

cond-mat.mtrl-sci↗

arXiv2014

Lattice-site-specific spin-dynamics in double perovskite Sr2CoOsO6

We have studied the magnetic properties and spin-dynamics of the structurally-ordered double perovskite Sr$_2$CoOsO$_6$. Our neutron diffraction, muon spin relaxation and ac-susceptibility measurements reveal two antiferromagnetic (AFM) phases on cooling from room temperature down to 2 K. In the first AFM phase, with transition temperature $T_{\mathrm{N}}=108$ K, cobalt (3$d^7$, $S=3/2$) and osmium (5$d^2$, $S=1$) moments fluctuate dynamically, while their \textit{average} effective moments undergo long-range order. In the second AFM phase with $T_{\mathrm{N}}=67$ K, cobalt moments first become frozen and induce a noncollinear spin-canted AFM state, while dynamically fluctuating osmium moments are later frozen into a randomly canted state at $T\approx 5$ K. Our \textit{ab initio} calculations indicate that the effective exchange coupling between cobalt and osmium sites is rather weak, so that cobalt and osmium sublattices exhibit different ground states and spin-dynamics, making Sr$_2$CoOsO$_6$ distinct from previously reported double-perovskite compounds.

cond-mat.mtrl-sci↗

arXiv2014

Ab-inito study of low temperature magnetic properties of double perovskite Sr2FeOsO6

Using density-functional theory calculations, we investigated the electronic structure and magnetic exchange interactions of the ordered 3d-5d double perovskite Sr2FeOsO6, which has recently drawn attention for interesting antiferromagnetic transitions. Our study reveals the vital role played by long-range magnetic exchange interactions in this compound. The competition between the ferromagnetic nearest neighbor Os-O-Fe interaction and antiferromagnetic next nearest neighbor Os-O-Fe-O-Os interaction induces strong frustration in this system, which explains the lattice distortion and magnetic phase transitions observed in experiments.

cond-mat.mtrl-sci↗

arXiv2013

Exchange bias up to room temperature in the antiferromagnetic bulk hexagonal Mn3Ge

This work reports an exchange bias (EB) effect up to room temperature in the binary intermetallic bulk compound Mn3.04Ge0.96. The sample annealed at 700 K crystallizes in a tetragonal structure with ferromagnetic ordering, whereas, the sample annealed at 1073 K crystallizes in a hexagonal structure with antiferromagnetic ordering. The hexagonal Mn3.04Ge0.96 sample exhibits an EB of around 70 mT at 2 K that continues with a non-zero value up to room temperature. The exchange anisotropy is proposed to be originating from the exchange interaction between the triangular antiferromagnetic host and the embedded ferrimagnetic like clusters. The ferrimagnetic clusters develop when excess Mn atoms occupy empty Ge sites in the original triangular antiferromagnet structure of Mn3Ge.

cond-mat.mtrl-sci↗

arXiv2013

Graphene-based topological insulator with an intrinsic bulk band gap above room temperature

Topological insulators (TIs) represent a new quantum state of matter characterized by robust gapless states inside the insulating bulk gap. The metallic edge states of a two-dimensional (2D) TI, known as quantum spin Hall (QSH) effect, are immune to backscattering and carry fully spin-polarized dissipationless currents. However, existing 2D TIs realized in HgTe and InAs/GaSb suffer from small bulk gaps (<10 meV) well below room temperature, thus limiting their application in electronic and spintronic devices. Here, we report a new 2D TI comprising a graphene layer sandwiched between two Bi2Se3 slabs that exhibits a large intrinsic bulk band gap of 30 to 50 meV, making it viable for room-temperature applications. Distinct from previous strategies for enhancing the intrinsic spin-orbit coupling effect of the graphene lattice, the present graphene-based TI operates on a new mechanism of strong inversion between graphene Dirac bands and Bi2Se3 conduction bands. Strain engineering leads to effective control and substantial enhancement of the bulk gap. Recently reported synthesis of smooth graphene/Bi2Se3 interfaces demonstrates feasibility of experimental realization of this new 2D TI structure, which holds great promise for nanoscale device applications.

cond-mat.mes-hall↗

arXiv2013

Lattice Instability and Competing Spin Structures in the Double Perovskite Insulator Sr2FeOsO6

The semiconductor Sr2FeOsO6, depending on temperature, adopts two types of spin structures that differ in the spin sequence of ferrimagnetic iron - osmium layers along the tetragonal c-axis. Neutron powder diffraction experiments, 57Fe Mössbauer spectra, and density-functional theory calculations suggest that this behavior arises because a lattice instability resulting in alternating iron-osmium distances fine-tunes the balance of competing exchange interactions. Thus, Sr2FeOsO6 is an example for a double perovskite, in which the electronic phases are controlled by the interplay of spin, orbital, and lattice degrees of freedom.

cond-mat.mtrl-sci↗