arXiv subjects

Claudia Felser

Publications and source records attributed to Claudia Felser.

At least 379 records · Page 21Linked to original sources

arXiv2013

A large energy-gap oxide topological insulator based on the superconductor BaBiO3

Mixed-valent perovskite oxides based on BaBiO3 (BBO) are, like cuperates, well-known high-Tc superconductors. Recent ab inito calculations have assigned the high-Tc superconductivity to a correlation-enhanced electron--phonon coupling mechanism, stimulating the prediction and synthesis of new superconductor candidates among mixed-valent thallium perovskites. Existing superconductivity has meant that research has mainly focused on hole-doped compounds, leaving electron-doped compounds relatively unexplored. Here we demonstrate through ab inito calculations that BBO emerges as a topological insulator (TI) in the electron-doped region, where the spin-orbit coupling (SOC) effect is significant. BBO exhibits the largest topological energy gap of 0.7 eV among currently known TI materials, inside which Dirac-type topological surface states (TSSs) exit. As the first oxide TI, BBO is naturally stable against surface oxidization and degrading, different from chalcoginide TIs. An extra advantage of BBO lies in its ability to serve an interface between the TSSs and the superconductor for the realization of Majorana Fermions.

cond-mat.mtrl-sci↗

arXiv2013

Weak topological insulators induced by the inter-layer coupling: A first-principles study of stacked Bi2TeI

Based on first-principles calculations, we predict Bi2TeI, a stoichiometric compound synthesized, to be a weak topological insulator (TI) in layered subvalent bismuth telluroiodides. Within a bulk energy gap of 80 meV, two Dirac-cone-like topological surface states exist on the side surface perpendicular to BiTeI layer plane. These Dirac cones are relatively isotropic due to the strong inter-layer coupling, distinguished from those of previously reported weak TI candidates. Moreover, with chemically stable cladding layers, the BiTeI-Bi2-BiTeI sandwiched structure is a robust quantum spin Hall system, which can be obtained by simply cleaving the bulk Bi2TeI.

cond-mat.mtrl-sci↗

arXiv2013

Synthesis, Crystal Structure and Physical Properties of Sr2FeOsO6

In the exploration of new osmium based double perovskites, Sr2FeOsO6 is a new insertion in the existing family. The polycrystalline compound has been prepared by solid state synthesis from the respective binary oxides. PXRD analysis shows the structure is pseudo-cubic at room temperature, whereas low-temperature synchrotron data refinements reveal the structure to be tetragonal, space group I4/m. Heat capacity and magnetic measurements of Sr2FeOsO6 indicated the presence of two magnetic phase transitions at T1 = 140 K and T2 = 67 K. Band structure calculations showed the compound as a narrow energy gap semiconductor, which supports the experimental results obtained from the resistivity measurements. The present study documents significant structural and electronic effects of substituting Fe3+ for Cr3+ ion in Sr2CrOsO6.

cond-mat.mtrl-sci↗

arXiv2013

Loss of anisotropy in strained ultrathin epitaxial L10 Mn-Ga films

In this work we are investigating the effect of strain in ultrathin Mn-Ga thin films on the magnetic properties at room temperature. Two different Mn-Ga compositions of which one is furthermore doped with Co were grown on Cr buffered MgO (001) substrates. Films with a thickness below 12nm are highly strained and it was observed that the ratio c/a vs. thickness is depending on composition. Using c/a as an order parameter the PMA is shown to be drastically reduced with increasing strain. These findings should be considered when generalizing and downscaling results obtained from films >20nm. Furthermore it has been shown, that the strain can be reduced by introducing an additional Pt buffer and thus maintaining a high PMA for a thickness as low as 6nm.

cond-mat.mtrl-sci↗

arXiv2013

Kinetic arrest related to a first-order ferrimagnetic to antiferromagnetic transition in the Heusler compound Mn2PtGa

We report a magnetization study of the Heusler compound Mn2PtGa that shows the existence of a magnetic-glass state. Mn2PtGa shows a first-order ferromagnetic (FM)/ferrimagnetic (FI) to antiferromagnetic (AFM) transition in contrast to the martensitic structural transition observed in several Heusler alloys. The kinetic arrest of this first-order FM (FI) to AFM transition leads to the observed magnetic-glass behavior. We show that the strength of the applied magnetic field, which is the primary parameter to induce the magnetic-glass state, is also responsible for the stability of the supercooled FM (FI) phase in time.

cond-mat.mtrl-sci↗

arXiv2013

Lone Pair Effect, Structural Distortions and Potential for Superconductivity in Tl Perovskites

Drawing the analogy to BaBiO3, we investigate via ab-initio electronic structure calculations potential new superconductors of the type ATlX3 with A = Rb, Cs and X = F, Cl, and Br, with a particular emphasis on RbTlCl3. Based on chemical reasoning, supported by the calculations, we show that Tl-based perovskites have structural and charge instabilities driven by the lone pair effect, similar to the case of BaBiO3, effectively becoming A2Tl1+Tl3+X6. We find that upon hole doping of RbTlCl3, structures without Tl1+, Tl3+ charge disproportionation become more stable, although the ideal cubic perovskite, often viewed as the best host for superconductivity, should not be the most stable phase in the system. The known superconductor (Sr,K)BiO3 and hole doped RbTlCl3, predicted to be most stable in the same tetragonal structure, display highly analogous calculated electronic band structures.

cond-mat.supr-con↗

arXiv2013

Disorder-induced cubic phase in Fe$_2$-based Heusler alloys

Based on first-principles electronic structure calculations, we analyze the chemical and magnetic mechanisms stabilizing the cubic phase in Fe$_2$-based Heusler materials, which were previously predicted to be tetragonal when being chemically ordered. In agreement with recent experimental data, we found that these compounds crystallize within the so-called "inverted" cubic Heusler structure perturbed by a certain portion of the intrinsic chemical disorder. Understanding these mechanisms is a necessary step to guide towards the successful future synthesis of the stable Fe$_2$-based tetragonal phases, which are very promising candidates for the fabrication of rare-earth-free permanent magnets.

cond-mat.mtrl-sci↗

arXiv2012

First-principles study of the structural stability of Mn3Z (Z=Ga, Sn and Ge) Heusler compounds

We investigate the structural stability and magnetic properties of cubic, tetragonal and hexagonal phases of Mn3Z (Z=Ga, Sn and Ge) Heusler compounds using first-principles density-functional theory. We propose that the cubic phase plays an important role as an intermediate state in the phase transition from the hexagonal to the tetragonal phases. Consequently, Mn3Ga and Mn3Ge behave differently from Mn3Sn, because the relative energies of the cubic and hexagonal phases are different. This result agrees with experimental observations from these three compounds. The weak ferromagnetism of the hexagonal phase and the perpendicular magnetocrystalline anisotropy of the tetragonal phase obtained in our calculations are also consistent with experiment.

cond-mat.mtrl-sci↗

arXiv2012

Topological insulators and thermoelectric materials

Topological insulators (TIs) are a new quantum state of matter which have gapless surface states inside the bulk energy gap. Starting with the discovery of two dimensional TIs, the HgTe-based quantum wells, many new topological materials have been theoretically predicted and experimentally observed. Currently known TI materials can possibly be classified into two families, the HgTe family and the Bi2Se family. The signatures found in the electronic structure of a TI also cause these materials to be excellent thermoelectric materials. On the other hand, excellent thermoelectric materials can be also topologically trivial. Here we present a short introduction to topological insulators and thermoelectrics, and give examples of compound classes were both good thermoelectric properties and topological insulators can be found.

cond-mat.mtrl-sci↗

arXiv2012

Fabrication and characterization of semiconducting half Heusler YPtSb thin films

The semiconducting half Heusler compound YPtSb was predicted theoretically to be capable of changing into topological insulator under proper strain. In this work, p type semiconducting half-Heusler YPtSb thin films were prepared by magnetron co-sputtering method from a specially designed target for the first time. Textured structure with (111) plane paralleling with (001) of MgO substrate was observed when YPtSb thin films were grown on MgO (100) substrate at 600°C.Electrical measurements show that the resistivity of YPtSb films decreases with increasing temperature, indicating a semiconductor-like behavior. The carrier density is as high as 1.15 X 10^21 cm-3 at 300 K. The band gap of YPtSb thin films obtained by infrared spectroscopy is around 0.1 - 0.15 eV, which is well in agreement with the theoretical prediction and the value measured in bulk YPtSb.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic and transport properties of tetragonal- or cubic-Heusler-type Co-substituted Mn-Ga epitaxial thin films

The composition dependence of the structural, magnetic, and transport properties of epitaxially grown Mn-Co-Ga films were investigated. The crystal structure was observed to change from tetragonal to cubic as the Co content was increased. In terms of the dependence of saturation magnetization on the Co content, relatively small value was obtained for the Mn$_{2.3}$Co$_{0.4}$Ga$_{1.3}$ film at a large {\it K}$_\textrm u$ value of 9.2 Merg/cm$^3$. Electrical resistivity of Mn-Co-Ga films was larger than that of pure Mn-Ga film. The maximum value of the resistivity was 490 $μΩ$cm for Mn$_{2.2}$Co$_{0.6}$Ga$_{1.2}$ film. The high resistivity of Mn-Co-Ga might be due to the presence of localized electron states in the films due to chemical disordering caused by the Co substitution.

cond-mat.mtrl-sci↗

arXiv2012

Perpendicularly magnetized Mn-Co-Ga-based thin films with high coercive field

Mn$_{3-x}$Co$_{x}$Ga epitaxial thin films were grown on MgO substrates by magnetron co-sputtering. Structures were tetragonal or cubic depending on Co content. Composition dependence of saturation magnetization and uniaxial magnetic anisotropy $K_u$ of the films were investigated. A high $K_u$ (1.2 MJ m$^{-3}$) was achieved for the Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$ film with the magnetic moment 0.84$μ_B$. Valence band spectra were obtained by hard X-ray photoelectron spectroscopy. Sharp peaks in the cubic case, which were absent in the tetragonal case, prove that a van Hove singularity causes a band Jahn-Teller effect with tetragonal distortion. Observations agree well with the first-principles calculations.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic dichroism in angular resolved hard X-ray photoelectron spectroscopy from buried magnetic layers

This work reports on the measurement of magnetic dichroism in angular-resolved photoelectron spectroscopy from in-plane magnetized buried thin films. The high bulk sensitivity of hard X-ray photoelectron spectroscopy (HAXPES) in combination with circularly polarized radiation enables the investigation of the magnetic properties of buried layers. Angular distributions of high kinetic energy (7 to 8 keV) photoelectrons in a range of about 60 deg were recorded in parallel to the energy distribution. Depending on purpose, energy and angular resolutions of 150 to 250 meV and 0.17 to 2 deg can be accomplished simultaneously in such experiments. Experiments were performed on exchange-biased magnetic layers covered by thin oxide films. More specifically, the angular distribution of photoelectrons from the ferromagnetic layer Co2FeAl layer grown on MnIr exchange-biasing layer was investigated where the magnetic structure is buried beneath a MgO layer. Pronounced magnetic dichroism is found in the Co and Fe 2p states for all angles of emission. A slightly increased magnetic dichroism was observed for normal emission in agreement with theoretical considerations.

cond-mat.mtrl-sci↗

arXiv2012

Topological surface states of Bi2Se3 with the coexistence of Se vacancies

Although topological surface states are known to be robust against non-magnetic surface perturbations, their band dispersions and spatial distributions are still sensitive to the surface defects. Take Bi2Se3 as an example, we demonstrated that Se vacancies modifies the surface band structures considerably. When large numbers of Se vacancies exist on the surface, topological surface states may sink down from the first to second quintuple layer and get separated from the vacancies. We simulated STM images to distinguish the surfaces with Se- and Bi-terminations.

cond-mat.mtrl-sci↗

arXiv2012

Hard X-ray photoelectron spectroscopy on buried, off-stoichiometric CoxMnyGez (x : z = 2 : 0.38) Heusler thin films

Fully epitaxial magnetic tunnel junctions (MTJs) with off-stoichiometric Co2-based Heusler alloy shows a intense dependency of the tunnel magnetoresistance (TMR) on the Mn composition, demonstrating giant TMR ratios of up to 1995% at 4.2 K for 1. This work reports on the electronic structure of non-stoichiometric CoxMnyGez thin films with a fixed Co/Ge ratio of x : z = 2 : 0.38. The electronic structure was investigated by high energy, hard X-ray photoelectron spectroscopy combined with first-principles calculations. The high-resolution measurements of the valence band of the non-stoichiometric CoxMnyGez films close to the Fermi energy indicate a shift of the spectral weight compared to bulk Co2MnGe. This is in agreement with the changes in the density of states predicted by the calculations. Furthermore it is shown that the co-sputtering of Co2MnGe together with additional Mn is an appropriate technique to adjust the stoichiometry of the CoxMnyGez film composition. The resulting changes of the electronic structure within the valence band will allow to tune the magnetoresistive characteristics of CoxMnyGez based tunnel junctions as verified by the calculations and photoemission experiments.

cond-mat.mtrl-sci↗

arXiv2012

Realization of spin gapless semiconductors: the Heusler compound Mn2CoAl

Recent studies have reported an interesting class of semiconductor materials that bridge the gap between semiconductors and halfmetallic ferromagnets. These materials, called spin gapless semiconductors, exhibit a bandgap in one of the spin channels and a zero bandgap in the other and thus allow for tunable spin transport. Here, a theoretical and experimental study of the spin gapless Heusler compound Mn2CoAl is presented. It turns out that Mn2CoAl is a very peculiar ferrimagnetic semiconductor with a magnetic moment of 2 μB and a high Curie temperature of 720 K. Below 300 K, the compound exhibits nearly temperature-independent conductivity, very low, temperature-independent carrier concentration, and a vanishing Seebeck coefficient. The magnetoresistance changes sign with temperature. In high fields, it is positive and non-saturating at low temperatures, but negative and saturating at high temperatures. The anomalous Hall effect is comparatively low, which is explained by the close antisymmetry of the Berry curvature for kz of opposite sign.

cond-mat.mtrl-sci↗

arXiv2012

Topological phase transitions in bulk

We consider the analogy between the topological phase transition which occurs as a function of spatial coordinate on a surface of a non-trivial insulator, and the one which occurs in the bulk due to the change of internal parameters (such as crystal field and spin-orbit coupling). In both cases the system exhibits a Dirac cone, which is the universal manifestation of topological phase transition, independently on the type of driving parameters. In particular, this leads to a simple way of determining the topological class based solely on the bulk information even for the systems with translational symmetry broken by atomic disorder or by strong electron correlations. Here we demonstrate this on example of the zinc-blende related semiconductors by means of the {\it ab-initio} fully-relativistic band structure calculations involving the coherent potential approximation (CPA) technique.

cond-mat.mtrl-sci↗

arXiv2012

A p-type Heusler compound: Growth, structure, and properties of epitaxial thin NiYBi films on MgO(100)

Epitaxial semiconducting NiYBi thin films were directly prepared on MgO(100) substrates by magnetron sputtering. The intensity ratio of the (200) and (400) diffraction peaks, I(200)/I(400) = 2.93, was close to the theoretical value (3.03). The electronic structure of NiYBi was calculated using WIEN2k and a narrow indirect band gap of width 210 meV was found. The valence band spectra of the films obtained by linear dichroism in hard X-ray photoelectron spectroscopy exhibit clear structures that are in good agreement with the calculated band structure of NiYBi.

cond-mat.mtrl-sci↗