21 research outputs found
Surface states and Rashba-type spin polarization in antiferromagnetic MnBiTe
The layered van der Waals antiferromagnet MnBiTe has been predicted
to combine the band ordering of archetypical topological insulators such as
BiTe with the magnetism of Mn, making this material a viable candidate
for the realization of various magnetic topological states. We have
systematically investigated the surface electronic structure of
MnBiTe(0001) single crystals by use of spin- and angle-resolved
photoelectron spectroscopy experiments. In line with theoretical predictions,
the results reveal a surface state in the bulk band gap and they provide
evidence for the influence of exchange interaction and spin-orbit coupling on
the surface electronic structure.Comment: Revised versio
Momentum-space signatures of Berry flux monopoles in the Weyl semimetal TaAs
Since the early days of Dirac flux quantization, magnetic monopoles have been sought after as a potential corollary of quantized electric charge. As opposed to magnetic monopoles embedded into the theory of electromagnetism, Weyl semimetals (WSM) exhibit Berry flux monopoles in reciprocal parameter space. As a function of crystal momentum, such monopoles locate at the crossing point of spin-polarized bands forming the Weyl cone. Here, we report momentum-resolved spectroscopic signatures of Berry flux monopoles in TaAs as a paradigmatic WSM. We carried out angle-resolved photoelectron spectroscopy at bulk-sensitive soft X-ray energies (SX-ARPES) combined with photoelectron spin detection and circular dichroism. The experiments reveal large spin- and orbital-angular-momentum (SAM and OAM) polarizations of the Weyl-fermion states, resulting from the broken crystalline inversion symmetry in TaAs. Supported by first-principles calculations, our measurements image signatures of a topologically non-trivial winding of the OAM at the Weyl nodes and unveil a chirality-dependent SAM of the Weyl bands. Our results provide directly bulk-sensitive spectroscopic support for the non-trivial band topology in the WSM TaAs, promising to have profound implications for the study of quantum-geometric effects in solids
Author Correction: Non-local effect of impurity states on the exchange coupling mechanism in magnetic topological insulators
A Correction to this paper has been published: https://doi.org/10.1038/s41535-021-00314-
Spectroscopic and Quantum Chemical Investigation of Benzene-1,2-dithiolate-Coordinated Diiron Complexes with Relevance to Dinitrogen Activation
In this work, a benzene-1,2-dithiolate (bdt) pentamethylcyclopentadienyl di-iron complex [Cp*Fe(ÎźâΡ2:Ρ4-bdt)FeCp*] and its [Cp*Fe(bdt)(X)FeCp*] analogues (where X = N2H2, N2H3â, Hâ, NH2â, NHCH3â, or NO+) were investigated through spectroscopic and computational studies. These complexes are of relevance as model systems for dinitrogen activation in nitrogenase and share with its active site the presence of iron, sulfur ligands, and a very flexible electronic structure. On the basis of a combination of X-ray emission spectroscopy (XES), X-ray crystallography, MĂśssbauer, NMR, and EPR spectroscopy, the geometric and electronic structure of the series has been experimentally elucidated. All iron atoms were found to be in a local low-spin configuration. When no additional X ligand is bound, the bdt ligand is tilted and features a stabilizing Ď-interaction with one of the iron atoms. The number of lone-pair orbitals provided by the nitrogen-containing species is crucial to the overall electronic structure. When only one lone-pair is present and the iron atoms are bridged by one atom, a three-center bond occurs, and a direct FeâFe bond is absent. If the bridging atom provides two lone-pairs, then an FeâFe bond is formed. A recurring theme for all ligands is Ď-donation into the unoccupied eg manifolds of both iron atoms and back-donation from the t2g manifolds into the ligand Ď* orbitals. The latter results in a weakening of the double bond of the bound ligand, and in the case of NO+, it results in a weakening of all bonds that comprise triple bond. The electron-rich thiolates further amplify this effect and can also serve as bases for proton binding. While the above observations have been made for the studied di-iron complexes, they may be of relevance for the active site in nitrogenase, where a similar N2 binding mode may occur allowing for the simultaneous weakening of the N2 Ď bond and Ď bonds
Time-domain evidence for an excitonic insulator
Time- and angle-resolved photoemission spectroscopy with a highâharmonic- generation source is used to classify the potential excitonic insulator 1T-TiSe2 and the reference Peierls-Mott insulator 1T-TaS2 on the basis of the melting times of âspectroscopic order parametersâ
Time-domain evidence for an excitonic insulator
Time- and angle-resolved photoemission spectroscopy with a highâharmonic- generation source is used to classify the potential excitonic insulator 1T-TiSe2 and the reference Peierls-Mott insulator 1T-TaS2 on the basis of the melting times of âspectroscopic order parametersâ
Ultrafast Melting of a Charge-Density Wave in the Mott Insulator
Femtosecond time-resolved core-level photoemission spectroscopy with a
free-electron laser is used to measure the atomic-site specific charge-order
dynamics in the charge-density-wave/Mott insulator 1T-TaS2. After strong
photoexcitation, a prompt loss of charge order and subsequent fast
equilibration dynamics of the electron-lattice system are observed. On the time
scale of electron-phonon thermalization, about 1 ps, the system is driven
across a phase transition from a long-range charge ordered state to a
quasi-equilibrium state with domain-like short-range charge and lattice order.
The experiment opens the way to study the nonequilibrium dynamics of condensed
matter systems with full elemental, chemical, and atomic site selectivity
Momentum-space signatures of Berry flux monopoles in the Weyl semimetal TaAs
Weyl semimetals exhibit Berry flux monopoles in momentum-space, but direct experimental evidence has remained elusive. Here, the authors reveal topologically non-trivial winding of the orbital-angular-momentum at the Weyl nodes and a chirality-dependent spin-angular-momentum of the Weyl bands, as a direct signature of the Berry flux monopoles in TaAs
Enhanced ultrafast relaxation rate in the Weyl semimetal phase of MoTe2 measured by time- and angle-resolved photoelectron spectroscopy
MoTe2 has recently been shown to realize in its low-temperature phase the type-II Weyl semimetal (WSM). We investigated by time- and angle- resolved photoelectron spectroscopy (tr-ARPES) the possible influence of the Weyl points on the electron dynamics above the Fermi level EF, by comparing the ultrafast response of MoTe2 in the trivial and topological phases. In the low-temperature WSM phase, we report an enhanced relaxation rate of electrons optically excited to the conduction band, which we interpret as a fingerprint of the local gap closure when Weyl points form. By contrast, we find that the electron dynamics of the related compound WTe2 is slower and temperature independent, consistent with a topologically trivial nature of this material. Our results shows that tr-ARPES is sensitive to the small modifications of the unoccupied band structure accompanying the structural and topological phase transition of MoTe2