Search Results
Saturation of the anomalous Hall effect at high magnetic fields in altermagnetic RuO2
Observations of the anomalous Hall effect in RuO2 and MnTe have demonstrated unconventional time-reversal symmetry breaking in the electronic structure of a recently identified new class of compensated collinear magnets, dubbed altermagnets. While in MnTe, the unconventional anomalous Hall signal accompanied by a vanishing magnetization is observable at remanence, the anomalous Hall effect in RuO2 is excluded by symmetry for the Néel vector pointing along the zero-field [001] easy-axis. Guided by a symmetry analysis and ab initio calculations, a field-induced reorientation of the Néel vector from the easy-axis toward the [110] hard-axis was used to demonstrate the anomalous Hall signal in this altermagnet. We confirm the existence of an anomalous Hall effect in our RuO2 thin-film samples, whose set of magnetic and magneto-transport characteristics is consistent with the earlier report. By performing our measurements at extreme magnetic fields up to 68 T, we reach saturation of the anomalous Hall signal at a field Hc ≃ 55 T that was inaccessible in earlier studies but is consistent with the expected Néel-vector reorientation field.
Two-dimensional ferromagnetic extension of a topological insulator
Inducing a magnetic gap at the Dirac point of the topological surface state (TSS) in a three-dimensional (3D) topological insulator (TI) is a route to dissipationless charge and spin currents. Ideally, magnetic order is present only at the surface, as through proximity of a ferromagnetic (FM) layer. However, experimental evidence of such a proximity-induced Dirac mass gap is missing, likely due to an insufficient overlap of TSS and the FM subsystem. Here, we take a different approach, namely ferromagnetic extension (FME), using a thin film of the 3D TI Bi2Te3, interfaced with a monolayer of the lattice-matched van der Waals ferromagnet MnBi2Te4. Robust 2D ferromagnetism with out-of-plane anisotropy and a critical temperature of Tc≈15 K is demonstrated by x-ray magnetic dichroism and electrical transport measurements. Using angle-resolved photoelectron spectroscopy, we observe the opening of a sizable magnetic gap in the 2D FM phase, while the surface remains gapless in the paramagnetic phase above Tc. Ferromagnetic extension paves the way to explore the interplay of strictly 2D magnetism and topological surface states, providing perspectives for realizing robust quantum anomalous Hall and chiral Majorana states.
Multiscale simulations of the electronic structure of III-nitride quantum wells with varied indium content: Connecting atomistic and continuum-based models
Carrier localization effects in III-N heterostructures are often studied in the frame of modified continuum-based models utilizing a single-band effective mass approximation. However, there exists no comparison between the results of a modified continuum model and atomistic calculations on the same underlying disordered energy landscape. We present a theoretical framework that establishes a connection between atomistic tight-binding theory and continuum-based electronic structure models, here a single-band effective mass approximation, and provide such a comparison for the electronic structure of (In,Ga)N quantum wells. In our approach, in principle, the effective masses are the only adjustable parameters since the confinement energy landscape is directly obtained from tight-binding theory. We find that the electronic structure calculated within effective mass approximation and the tight-binding model differ noticeably. However, at least in terms of energy eigenvalues, an improved agreement between the two methods can be achieved by adjusting the band offsets in the continuum model, enabling, therefore, a recipe for constructing a modified continuum model that gives a reasonable approximation of the tight-binding energies. Carrier localization characteristics for energetically low lying, strongly localized states differ, however, significantly from those obtained using the tight-binding model. For energetically higher lying, more delocalized states, good agreement may be achieved. Therefore, the atomistically motivated continuum-based single-band effective mass model established provides a good, computationally efficient alternative to fully atomistic investigations, at least at when targeting questions related to higher temperatures and carrier densities in (In,Ga)N systems.
Influence of multi-walled carbon nanotubes in polytetrafluoroethylene on the parameters of electronic structure and absorption of ultra-high-frequency radiation
Using the methods of angular correlation of annihilation radiation (ACAR), attenuation of electromagnetic radiation in 1.5–2.2 GHz frequency range, and optical ellipsometry, it was shown that in composites of polytetrafluoroethylene (PTFE) + multi-walled carbon nanotubes (MWCNTs), a 2% decrease in the probability of annihilation of positrons in free volumes in PTFE leads to changes in other parameters of electronic structure of composites by 8–29%. Polytetrafluoroethylene is transparent to electromagnetic radiation, but after the addition of 10 wt.% or more of MWCNTs, the composites demonstrate 200–410-fold decrease in the electromagnetic radiation intensity when the radiation passes through a specimen with a thickness of ≈2 mm. It was found that the average radius of the free volumes and the probability of annihilation of positrons are determined by the defect and electronic structures of the polymer matrix only. The Fermi angle and the probability of positrons annihilation with free electrons are determined by the analogous structures of MWCNTs only. Since the electronic characteristics of the atoms and defects in the polymer matrix (at least outside the interphase) do not change, the changes in the other ACAR parameters are mainly due to changes in the imperfect MWCNTs’ atomic and electronic structures. The average radius of free volumes reaches its maximum value in the composite with 10 wt.% MWCNTs. It was found that in a specimen with 10 wt.% MWCNTs, the highest density of free electrons is observed due to charge transfer from free volumes to MWCNTs, and the highest electron density is observed on defects. A disorder of MWCNTs and their branched conductive network can form the ‘tails’ of electronic density of states in a band gap. Thus, composite with 10 wt.% MWCNTs has the highest absorption coefficient for electromagnetic radiation.
Stabilizing a three-center single-electron metal–metal bond in a fullerene cage
Trimetallic carbide clusterfullerenes (TCCFs) encapsulating a quinary M3C2 cluster represent a special family of endohedral fullerenes with an open-shell electronic configuration. Herein, a novel TCCF based on a medium-sized rare earth metal, dysprosium (Dy), is synthesized for the first time. The molecular structure of Dy3C2@Ih(7)-C80 determined by single crystal X-ray diffraction shows that the encapsulated Dy3C2 cluster adopts a bat ray configuration, in which the acetylide unit C2 is elevated above the Dy3 plane by ∼1.66 Å, while Dy–Dy distances are ∼3.4 Å. DFT computational analysis of the electronic structure reveals that the endohedral cluster has an unusual formal charge distribution of (Dy3)8+(C2)2−@C806− and features an unprecedented three-center single-electron Dy–Dy–Dy bond, which has never been reported for lanthanide compounds. Moreover, this electronic structure is different from that of the analogous Sc3C2@Ih(7)-C80 with a (Sc3)9+(C2)3−@C806− charge distribution and no metal–metal bonding.
Autocorrected off-axis holography of two-dimensional materials
The reduced dimensionality in two-dimensional materials leads to a wealth of unusual properties, which are currently explored for both fundamental and applied sciences. In order to study the crystal structure, edge states, the formation of defects and grain boundaries, or the impact of adsorbates, high-resolution microscopy techniques are indispensable. Here we report on the development of an electron holography (EH) transmission electron microscopy (TEM) technique, which facilitates high spatial resolution by an automatic correction of geometric aberrations. Distinguished features of EH beyond conventional TEM imaging are gap-free spatial information signal transfer and higher dose efficiency for certain spatial frequency bands as well as direct access to the projected electrostatic potential of the two-dimensional material. We demonstrate these features with the example of h-BN, for which we measure the electrostatic potential as a function of layer number down to the monolayer limit and obtain evidence for a systematic increase of the potential at the zig-zag edges.
Modulating the luminance of organic light-emitting diodes: Via optical stimulation of a photochromic molecular monolayer at transparent oxide electrode
Self-assembled monolayers (SAMs) deposited on bottom electrodes are commonly used to tune charge carrier injection or blocking in optoelectronic devices. Beside the enhancement of device performance, the fabrication of multifunctional devices in which the output can be modulated by multiple external stimuli remains a challenging target. In this work, we report the functionalization of an indium tin oxide (ITO) electrode with a SAM of a diarylethene derivative designed for optically control the electronic properties. Following the demonstration of dense SAM formation and its photochromic activity, as a proof-of-principle, an organic light-emitting diode (OLED) embedding the light-responsive SAM-covered electrode was fabricated and characterized. Optically addressing the two-terminal device by irradiation with ultraviolet light doubles the electroluminescence. The original value can be restored reversibly by irradiation with visible light. This expanded functionality is based on the photoinduced modulation of the electronic structure of the diarylethene isomers, which impact the charge carriers' confinement within the emissive layer. This approach could be successfully exploited in the field of opto-communication technology, for example to fabricate opto-electronic logic circuits. © 2020 The Royal Society of Chemistry.
Electronic correlations and magnetic interactions in infinite-layer NdNiO2
The large antiferromagnetic exchange coupling in the parent high-Tc cuprate superconductors is believed to play a crucial role in pairing the superconducting carriers. The recent observation of superconductivity in hole-doped infinite-layer (IL-) NdNiO2 brings to the fore the relevance of magnetic coupling in high-Tc superconductors, particularly because no magnetic ordering is observed in the undoped IL-NdNiO2, unlike in parent copper oxides. Here, we investigate the electronic structure and the nature of magnetic exchange in IL-NdNiO2 using state-of-the-art many-body quantum chemistry methods. From a systematic comparison of the electronic and magnetic properties with isostructural cuprate IL-CaCuO2, we find that the on-site dynamical correlations are significantly stronger in IL-NdNiO2 compared to the cuprate analog. These dynamical correlations play a critical role in the magnetic exchange resulting in an unexpectedly large antiferromagnetic nearest-neighbor isotropic J of 77 meV between the Ni1+ ions within the ab plane. While we find many similarities in the electronic structure between the nickelate and the cuprate, the role of electronic correlations is profoundly different in the two. We further discuss the implications of our findings in understanding the origin of superconductivity in nickelates. © 2020 authors.
Magnetic warping in topological insulators
We analyze the electronic structure of topological surface states in the family of magnetic topological insulators MnBi2nTe3n+1. We show that, at natural-cleavage surfaces, the Dirac cone warping changes its symmetry from hexagonal to trigonal at the magnetic ordering temperature. In particular, an energy splitting develops between the surface states of the same band index but opposite surface momenta upon formation of the long-range magnetic order. As a consequence, measurements of such energy splittings constitute a simple protocol to detect the magnetic ordering via the surface electronic structure, alternative to the detection of the surface magnetic gap. Interestingly, while the latter signals a nonzero surface magnetization, the trigonal warping predicted here is, in addition, sensitive to the direction of the surface magnetic flux. Our results may be particularly useful when the Dirac point is buried in the projection of the bulk states, caused by certain terminations of the crystal or in hole-doped systems, since in both situations the surface magnetic gap itself is not accessible in photoemission experiments.
Photoemission electron microscopy of magneto-ionic effects in La0.7Sr0.3MnO3
Magneto-ionic control of magnetism is a promising route toward the realization of non-volatile memory and memristive devices. Magneto-ionic oxides are particularly interesting for this purpose, exhibiting magnetic switching coupled to resistive switching, with the latter emerging as a perturbation of the oxygen vacancy concentration. Here, we report on electric-field-induced magnetic switching in a La0.7Sr0.3MnO3 (LSMO) thin film. Correlating magnetic and chemical information via photoemission electron microscopy, we show that applying a positive voltage perpendicular to the film surface of LSMO results in the change in the valence of the Mn ions accompanied by a metal-to-insulator transition and a loss of magnetic ordering. Importantly, we demonstrate that the voltage amplitude provides granular control of the phenomena, enabling fine-tuning of the surface electronic structure. Our study provides valuable insight into the switching capabilities of LSMO that can be utilized in magneto-ionic devices. © 2020 Author(s).