arXiv subjects

Manuel Richter

Publications and source records attributed to Manuel Richter.

At least 19 recordsLinked to original sources

arXiv2024

Large anomalous Hall conductivity in Weyl ferrimagnet Cs$_{2}$Co$_{3}$S$_4$ predicted by density-functional calculations

The identification of topological Weyl semimetals has recently gained considerable attention. Here, we report the results of density-functional theory calculations regarding the magnetic properties, the electronic structure, and the intrinsic anomalous Hall conductivity of the title compound, which was synthesized already 50 years ago but received little attention, hitherto. We found Cs$_{2}$Co$_{3}$S$_4$ to be a ferrimagnetic half-metal with a total spin magnetic moment of about 3 $\mu_B$ per formula unit. It shows energy band gap of 0.36 eV in the majority-spin channel and a pseudo-gap at the Fermi level in the minority-spin channel. We identified several sets of low-energy Weyl points and traced their dependence on the direction of magnetization. The intrinsic anomalous Hall conductivity is predicted to reach a magnitude up to 500 $\Omega^{-1}$cm$^{-1}$, which is comparable to values obtained in other celebrated Weyl semimetals.

cond-mat.mtrl-sci↗

arXiv2022

Field-dependent Shubnikov-de Haas oscillations in ferromagnetic Weyl semimetal Co3Sn2S2

We report a study of Shubnikov-de Haas oscillations in high quality single crystals of ferromagnetic Weyl semimetal Co$_3$Sn$_2$S$_2$. The Fermi surfaces resolved in our experiments are three-dimensional and reflect an underlying trigonal crystallographic symmetry. Combined with density functional theoretical calculations, we identify that the majority of the Fermi surfaces in the system -- of both electron and hole nature -- arise from the strong energy dispersion of the (spin-orbit gapped) mirror-protected nodal rings. We observe that an in-plane magnetic field induces a continuous evolution of Fermi surfaces, in contrast to field perpendicular to the kagome lattice planes which has little effect. Viewed alongside the easy-axis anisotropy of the system, our observation reveals an evolution of the electronic structure of Co$_3$Sn$_2$S$_2$ -- including the Weyl points -- with the ferromagnetic moment orientation. Through the case study of Co$_3$Sn$_2$S$_2$, our results provide concrete experimental evidence of an anisotropic interplay via spin-orbit coupling between the magnetic degrees of freedom and electronic band singularities, which has long been expected in semimetallic and metallic magnetic topological systems.

cond-mat.mtrl-sci↗

arXiv2021

Revised crystal structure and electronic properties of high dielectric Ba(Fe$_{1/2}$Nb$_{1/2}$)O$_{3}$ ceramics

Ba(Fe$_{1/2}$Nb$_{1/2}$)O$_3$ (BFN) ceramics are considered to be promising for technological applications owing to their high dielectric constant over a wide range of temperatures. However, there exists considerable discrepancy over the structural details. We address this discrepancy through a combined x-ray diffraction at room temperature and neutron powder diffraction measurements in the range from 5K up to room temperature, supplemented by a comparative analysis of the earlier reported structures. Our study reveals a cubic structure with space group Pm$\bar{3}$m at all measured temperatures. Further, the x-ray near edge structure and the extended x-ray absorption fine structure studies on the local environment of the Fe ions is consistent with the cubic symmetry. An appropriate value of $U$ for DFT+$U$ calculations is obtained by comparison with x-ray absorption spectroscopy, which agrees well with the earlier reported electronic properties.

cond-mat.str-el↗

arXiv2021

Ferromagnetic helical nodal line and Kane-Mele spin-orbit coupling in kagome metal Fe3Sn2

The two-dimensional kagome lattice hosts Dirac fermions at its Brillouin zone corners K and K', analogous to the honeycomb lattice. In the density functional theory electronic structure of ferromagnetic kagome metal Fe$_3$Sn$_2$, without spin-orbit coupling we identify two energetically split helical nodal lines winding along $z$ in the vicinity of K and K' resulting from the trigonal stacking of the kagome layers. We find that hopping across A-A stacking introduces a layer splitting in energy while that across A-B stacking controls the momentum space amplitude of the helical nodal lines. The effect of spin-orbit coupling is found to resemble that of a Kane-Mele term, where the nodal lines can either be fully gapped to quasi-two-dimensional massive Dirac fermions, or remain gapless at discrete Weyl points depending on the ferromagnetic moment orientation. Aside from numerically establishing Fe$_3$Sn$_2$ as a model Dirac kagome metal, our results provide insights into materials design of topological phases from the lattice point of view, where paradigmatic low dimensional lattice models often find realizations in crystalline materials with three-dimensional stacking.

cond-mat.mtrl-sci↗

arXiv2020

Tunable chirality of noncentrosymmetric magnetic Weyl semimetals

Even if Weyl semimetals are characterized by quasiparticles with well-defined chirality, exploiting this experimentally is severely hampered by Weyl lattice-fermions coming in pairs with opposite chirality, typically causing the net chirality picked up by experimental probes to vanish. Here we show this issue can be circumvented in a controlled manner when both time-reversal- and inversion- symmetry are broken. To this end, we investigate chirality-disbalance in the carbide family RMC$_2$ (R a rare-earth and M a transition metal), showing several members to be Weyl semimetals. Using the noncentrosymmetric ferromagnet NdRhC$_2$ as an illustrating example, we show that an odd number of Weyl nodes can be stabilized at its Fermi surface by properly tilting its magnetization. The tilt direction determines the sign of the resulting net chirality, opening up a simple route to control it.

cond-mat.mes-hall↗

arXiv2020

Potassium-intercalated bulk HfS$_2$ and HfSe$_2$: Phase stability, structure, and electronic structure

We have studied potassium-intercalated bulk HfS$_2$ and HfSe$_2$ by combining transmission electron energy loss spectroscopy, angle-resolved photoemission spectroscopy and density functional theory calculations. Calculations of the formation energies and the evolution of the energies of the charge carrier plasmons as a function of the potassium content show that certain, low potassium concentrations $x$ are thermodynamically unstable. This leads to the coexistence of undoped and doped domains if the provided amount of the alkali metal is insufficient to saturate the whole crystal with the minimum thermodynamically stable potassium stoichiometry. Beyond this threshold concentration the domains disappear, while the alkali metal and charge carrier concentrations increase continuously upon further addition of potassium. At low intercalation levels, electron diffraction patterns indicate a significant degree of disorder in the crystal structure. The initial order in the out-of-plane direction is restored at high $x$ while the crystal layer thicknesses expand by 33-36%. Superstructures emerge parallel to the planes which we attribute to the distribution of the alkali metal rather than structural changes of the host materials. The in-plane lattice parameters change by not more than 1%. The introduction of potassium causes the formation of charge carrier plasmons. The observation of this semiconductor-to-metal transition is supported by calculations of the density of states (DOS) and band structures as well as angle-resolved photoemission spectroscopy. The calculated DOS hint at the presence of an almost ideal two-dimensional electron gas at the Fermi level for $x<0.6$. The plasmons exhibit quadratic momentum dispersions which is in agreement with the behavior expected for an ideal electron gas.

cond-mat.mes-hall↗

arXiv2020

Strong and Weak Three-Dimensional Topological Insulators Probed by Surface Science Methods

We review the contributions of surface science methods to discover and improve 3D topological insulator materials, while illustrating with examples from our own work. In particular, we demonstrate that spin-polarized angular-resolved photoelectron spectroscopy is instrumental to evidence the spin-helical surface Dirac cone, to tune its Dirac point energy towards the Fermi level, and to discover novel types of topological insulators such as dual ones or switchable ones in phase change materials. Moreover, we introduce procedures to spatially map potential fluctuations by scanning tunneling spectroscopy and to identify topological edge states in weak topological insulators.

cond-mat.mtrl-sci↗

arXiv2019

Topological electronic structure and intrinsic magnetization in MnBi$_4$Te$_7$: a Bi$_2$Te$_3$-derivative with a periodic Mn sublattice

Combinations of non-trivial band topology and long-range magnetic order hold promise for realizations of novel spintronic phenomena, such as the quantum anomalous Hall effect and the topological magnetoelectric effect. Following theoretical advances material candidates are emerging. Yet, a compound with a band-inverted electronic structure and an intrinsic net magnetization remains unrealized. MnBi$_2$Te$_4$ is a candidate for the first antiferromagnetic topological insulator and the progenitor of a modular (Bi$_2$Te$_3$)$_n$(MnBi$_2$Te$_4$) series. For $n$ = 1, we confirm a non-stoichiometric composition proximate to MnBi$_4$Te$_7$ and establish an antiferromagnetic state below 13 K followed by a state with net magnetization and ferromagnetic-like hysteresis below 5 K. Angle-resolved photoemission experiments and density-functional calculations reveal a topological surface state on the MnBi$_4$Te$_7$(0001) surface, analogous to the non-magnetic parent compound Bi$_2$Te$_3$. Our results render MnBi$_4$Te$_7$ as a band-inverted material with an intrinsic net magnetization and a complex magnetic phase diagram providing a versatile platform for the realization of different topological phases.

cond-mat.mtrl-sci↗

arXiv2019

Large off diagonal exchange couplings and spin liquid states in $\mathbf{C_3}$ symmetric iridates

Iridate oxides on a honeycomb lattice are considered promising candidates for realization of quantum spin liquid states. We investigate the magnetic couplings in a structural model for a honeycomb iridate K$_2$IrO$_3$, with $C_3$ point group symmetry at the Ir sites, which is an end member of the recently synthesized iridate family K$_x$Ir$_y$O$_2$. Using \textit{ab-initio} quantum chemical methods, we elucidate the subtle relationship between the real space symmetry and magnetic anisotropy and show that the higher point group symmetry leads to high frustration with strong magnetic anisotropy driven by the unusually large off-diagonal exchange couplings ($Γ$'s) as opposed to other spin-liquid candidates considered so far. Consequently, large quantum fluctuations imply lack of magnetic ordering consistent with the experiments. Exact diagonalization calculations for the fully anisotropic $K$-$J$-$Γ$ Hamiltonian reveal the importance of the off-diagonal anisotropic exchange couplings in stabilizing a spin liquid state and highlight an alternative route to stabilize spin liquid states for ferromagnetic $K$.

cond-mat.str-el↗

arXiv2019

Dirac fermions and flat bands in the ideal kagome metal FeSn

The kagome lattice based on 3d transition metals is a versatile platform for novel topological phases hosting symmetry-protected electronic excitations and exotic magnetic ground states. However, the paradigmatic states of the idealized two-dimensional (2D) kagome lattice - Dirac fermions and topological flat bands - have not been simultaneously observed, partly owing to the complex stacking structure of the kagome compounds studied to date. Here, we take the approach of examining FeSn, an antiferromagnetic single-layer kagome metal with spatially-decoupled kagome planes. Using polarization- and termination-dependent angle-resolved photoemission spectroscopy (ARPES), we detect the momentum-space signatures of coexisting flat bands and Dirac fermions in the vicinity of the Fermi energy. Intriguingly, when complemented with bulk-sensitive de Haas-van Alphen (dHvA) measurements, our data reveal an even richer electronic structure that exhibits robust surface Dirac fermions on specific crystalline terminations. Through band structure calculations and matrix element simulations, we demonstrate that the bulk Dirac bands arise from in-plane localized Fe-3d orbitals under kagome symmetry, while the surface state realizes a rare example of fully spin-polarized 2D Dirac fermions when combined with spin-layer locking in FeSn. These results highlight FeSn as a prototypical host for the emergent excitations of the kagome lattice. The prospect to harness these excitations for novel topological phases and spintronic devices is a frontier of great promise at the confluence of topology, magnetism, and strongly-correlated electron physics.

cond-mat.str-el↗

arXiv2019

Creating Weyl nodes and controlling their energy by magnetization rotation

As they do not rely on the presence of any crystal symmetry, Weyl nodes are robust topological features of an electronic structure that can occur at any momentum and energy. Acting as sinks and sources of Berry curvature, Weyl nodes have been predicted to strongly affect the transverse electronic response, like in the anomalous Hall or Nernst effects. However, to observe large anomalous effects the Weyl nodes need to be close to or at the Fermi-level, which implies the band structure must be tuned by an external parameter, e.g. chemical doping or pressure. Here we show that in a ferromagnetic metal tuning of the Weyl node energy and momentum can be achieved by rotation of the magnetization. Taking Co$_3$Sn$_2$S$_2$ as an example, we use electronic structure calculations based on density-functional theory to show that not only new Weyl fermions can be created by canting the magnetization away from the easy axis, but also that the Weyl nodes can be driven exactly to the Fermi surface. We also show that the dynamics in energy and momentum of the Weyl nodes strongly affect the calculated anomalous Hall and Nernst conductivities.

cond-mat.mes-hall↗

arXiv2018

Indole moiety induced biological potency in pseudo- peptides derived from 2-amino-2-(1H-indole-2-yl) based acetamides: synthesis, structure and computational investigations

We report the synthesis and theoretical investigations of three novel pseudo-peptide molecules derived from 2-amino-2-(1H-indole-2-yl) acetamides. The compounds were subjected to spectroscopic characterization ($^1$H, $^{13}$C-NMR and MS) and their chemical, electronic, and optical properties have been investigated. To ascertain their potential pharmacological applicability, the prospective reactive centers and molecular sites prone to interaction with water were identified along with possible sensitivity to autoxidation. Further, we have studied the optical response in the presence of different solvents and compared the electronic and optical properties of the pristine molecules. We highlight the subtle dependence of the properties on the structure and composition of these pseudo-peptides. Our results indicate that these molecules have high pharmaceutical potential and could serve as lead components in new drug formulations.

cond-mat.mtrl-sci↗

arXiv2017

Chemical Gating of a Weak Topological Insulator: Bi14Rh3I9

The compound Bi14Rh3I9 has recently been suggested as a weak three-dimensional topological insulator on the basis of angle-resolved photoemission and scanning-tunneling experiments in combination with density functional (DF) electronic structure calculations. These methods unanimously support the topological character of the headline compound, but a compelling confirmation could only be obtained by dedicated transport experiments. The latter, however, are biased by an intrinsic n-doping of the materials surface due to its polarity. Electronic reconstruction of the polar surface shifts the topological gap below the Fermi energy, which would also prevent any future device application. Here, we report the results of DF slab calculations for chemically gated and counter-doped surfaces of Bi14Rh3I9. We demonstrate that both methods can be used to compensate the surface polarity without closing the electronic gap.

cond-mat.mes-hall↗

arXiv2017

Effects of octahedral tilting on the electronic structure and optical properties of $d^0$ double perovskites $\mathbf{\rm A_2ScSbO_6}$ ($\mathbf{\rm A=Sr, Ca}$)

With increasing temperature, ${\rm Sr}_2{\rm ScSbO}_6$ undergoes three structural phase transitions at approximately ${\rm 400K}$, ${\rm 560K}$ and ${\rm 650K}$, leading to the following sequence of phases: $P2_1/n \rightarrow I2/m \rightarrow I4/m \rightarrow Fm\bar{3}m$, making it an ideal candidate to study the effects of octahedral tilting keeping other parameters fixed. To ascertain the isolated effects of octahedral distortions, the electronic and optical properties of the monoclinic $P2_1/n$ (at room temperature), monoclinic $I2/m$ (at ${\rm 430K}$), tetragonal $I4/m$ (at ${\rm 613K}$) and the cubic $Fm\bar{3}m$ (at ${\rm 660K}$) phases have been studied in terms of the electronic structure, dielectric constant, optical conductivity and electron energy loss spectroscopy using density functional theory. ${\rm Ca}_2{\rm ScSbO}_6$, on the other hand, shows only a $P2_1/n$ phase at room temperature and its properties have been been compared with the corresponding ${\rm Sr}$ compound. UV-Vis spectroscopic studies of the optical properties of the room-temperature phase of these $d^0$ double perovskite have been performed and presence of large direct bandgap for both the compounds have been reported. The electronic bandgaps for the room temperature phases is found to be in good agreement with the corresponding experimental values obtained using the Kubelka-Munk function. Interestingly, in contrast to other Sc-based $d^0$ double perovskites, with increasing octahedral distortions, the effective $t_{\rm 2g}$ bandwidth remains unaffected while the states forming the band change due to changes in unit cell orientation, leading to small effects on the electronic and optical properties.

cond-mat.mtrl-sci↗

arXiv2016

Ba2NiOsO6: A Dirac-Mott insulator with ferromagnetism near 100 K

The ferromagnetic semiconductor Ba2NiOsO6 (Tmag ~100 K) was synthesized at 6 GPa and 1500 °C. It crystallizes into a double perovskite structure [Fm-3m; a = 8.0428(1) Å], where the Ni2+ and Os6+ ions are perfectly ordered at the perovskite B-site. We show that the spin-orbit coupling of Os6+ plays an essential role in opening the charge gap. The magnetic state was investigated by density functional theory calculations and powder neutron diffraction. The latter revealed a collinear ferromagnetic order in a >21 kOe magnetic field at 5 K. The ferromagnetic gapped state is fundamentally different from that of known dilute magnetic semiconductors such as (Ga,Mn)As and (Cd,Mn)Te (Tmag < 180 K), the spin-gapless semiconductor Mn2CoAl (Tmag ~720 K), and the ferromagnetic insulators EuO (Tmag ~70 K) and Bi3Cr3O11 (Tmag ~220 K). It is also qualitatively different from known ferrimagnetic insulator/semiconductors, which are characterized by an antiparallel spin arrangement. Our finding of the ferromagnetic semiconductivity of Ba2NiOsO6 should increase interest in the platinum group oxides, because this new class of materials should be useful in the development of spintronic, quantum magnetic, and related devices.

cond-mat.str-el↗

arXiv2016

Electronic Structure of the Dark Surface of the Weak Topological Insulator Bi14Rh3I9

The compound Bi14Rh3I9 consists of ionic stacks of intermetallic [(Bi4Rh)3I]2+ and insulating [Bi2I8]2- layers and has been identified to be a weak topological insulator. Scanning tunneling microscopy revealed the robust edge states at all step edges of the cationic layer as a topological fingerprint. However, these edge states are found 0.25 eV below the Fermi level which is an obstacle for transport experiments. Here, we address this obstacle by comparing results of density functional slab calculations with scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy. We show that the n-type doping of the intermetallic layer is intrinsically caused by the polar surface and is well screened towards the bulk. In contrast, the anionic "spacer" layer shows a gap at the Fermi level, both, on the surface and in the bulk, i.e. it is not surface-doped due to iodine desorption. The well screened surface dipole implies that a buried edge state, probably already below a single spacer layer, is located at the Fermi level. Consequently, a multilayer step covered by a spacer layer could provide access to the transport properties of the topological edge states. In addition, we find a lateral electronic modulation of the topologically non-trivial surface layer which is traced back to the coupling with the underlying zigzag chain structure of the spacer layer.

cond-mat.mtrl-sci↗

arXiv2015

Correlation between topological band character and chemical bonding in a $\mathbf{Bi_{14}Rh_{3}I_{9}}$-based family of insulators

Recently the presence of topologically protected edge-states in Bi$_{14}$Rh$_3$I$_9$ was confirmed by scanning tunnelling microscopy consolidating this compound as a weak 3D topological insulator (TI). Here, we present a density-functional-theory-based study on a family of TIs derived from the Bi$_{14}$Rh$_3$I$_9$ parent structure via substitution of Ru, Pd, Os, Ir and Pt for Rh. Comparative analysis of the band-structures throughout the entire series is done by means of a unified minimalistic tight-binding model that evinces strong similarity between the quantum-spin-Hall (QSH) layer in Bi$_{14}$Rh$_3$I$_9$ and graphene in terms of $p_z$-molecular orbitals. Topologically non-trivial energy gaps are found for the Ir-, Rh-, Pt- and Pd-based systems, whereas the Os- and Ru-systems remain trivial. Furthermore, the energy position of the metal $d$-band centre is identified as the parameter which governs the evolution of the topological character of the band structure through the whole family of TIs. The $d$-band position is shown to correlate with the chemical bonding within the QSH layers, thus revealing how the chemical nature of the constituents affects the topological band character.

cond-mat.mtrl-sci↗

arXiv2015

Metastable cubic and tetragonal phases of transition metals predicted by density-functional theory

By means of density-functional calculations, we systematically investigated 24 transition metals for possible metastable phases in body-centered tetragonal structure (bct), including face-centered cubic (fcc) and body-centered cubic (bcc) geometries. A total of 36 structures not coinciding with equilibrium phases were found to minimize the total energy for the bct degrees of freedom. Among these, the fcc structures of Sc, Ti, Co, Y, Zr, Tc, Ru, Hf, Re, and Os, and bct Zr with $c/a=0.82$ were found to be metastable according to their computed phonon spectra. Eight of these predicted phases are not known from the respective pressure-temperature phase diagrams. Possible ways to stabilize the predicted metastable phases are illustrated.

cond-mat.mtrl-sci↗