Spin-momentum locking from topological quantum chemistry: applications to multifold fermions

In spin-orbit coupled crystals, symmetries can protect multifold degeneracies with large Chern numbers and Brillouin zone spanning topological surface states. In this work, we explore the extent to which the nontrivial topology of chiral multifold fermions impacts the spin texture of bulk states. To do so, we formulate a definition of spin-momentum locking in terms of reduced density matrices. Using tools from the theory of topological quantum chemistry, we show how the reduced density matrix can be determined from the knowledge of the basis orbitals and band representation forming the multifold fermion. We show how on-site spin orbit coupling, crystal field splitting, and Wyckoff position multiplicity compete to determine the spin texture of states near chiral fermions. We compute the spin texture of multifold fermions in several representative examples from space groups $P432$ (207) and $P2_13$ (198). We show that the winding number of the spin around the Fermi surface can take many different integer values, from zero all the way to $\pm 7$. Finally, we conclude by showing how to apply our theory to real materials using the example of PtGa in space group $P2_13$.Comment: 28 pages, 6 figure

Emergence of Dirac-like bands in the monolayer limit of epitaxial Ge films on Au(111)

After the discovery of Dirac fermions in graphene, it has become a natural question to ask whether it is possible to realize Dirac fermions in other two-dimensional (2D) materials as well. In this work, we report the discovery of multiple Dirac-like electronic bands in ultrathin Ge films grown on Au(111) by angle-resolved photoelectron spectroscopy. By tuning the thickness of the films, we are able to observe the evolution of their electronic structure when passing through the monolayer limit. Our discovery may signify the synthesis of germanene, a 2D honeycomb structure made of Ge, which is a promising platform for exploring exotic topological phenomena and enabling potential applications

Weyl-fermions, Fermi-arcs, and minority-spin carriers in ferromagnetic CoS2

The pyrite compound CoS2 has been intensively studied in the past due to its itinerant ferromagnetism and potential for half-metallicity, which make it a promising material for spintronic applications. However, its electronic structure remains only poorly understood. Here we use complementary bulk- and surface-sensitive angle-resolved photoelectron spectroscopy and ab-initio calculations to provide a complete picture of its band structure. We discover Weyl-cones at the Fermi-level, which presents CoS2 in a new light as a rare member of the recently discovered class of magnetic topological metals. We directly observe the topological Fermi-arc surface states that link the Weyl-nodes, which will influence the performance of CoS2 as a spin-injector by modifying its spin-polarization at interfaces. Additionally, we are for the first time able to directly observe a minority-spin bulk electron pocket in the corner of the Brillouin zone, which proves that CoS2 cannot be a true half-metal. Beyond settling the longstanding debate about half-metallicity in CoS2, our results provide a prime example of how the topology of magnetic materials can affect their use in spintronic applications

Controllable orbital angular momentum monopoles in chiral topological semimetals

The emerging field of orbitronics aims at generating and controlling currents of electronic orbital angular momentum (OAM) for information processing. Structurally chiral topological crystals could be particularly suitable orbitronic materials because they have been predicted to host topological band degeneracies in reciprocal space that are monopoles of OAM. Around such a monopole, the OAM is locked isotopically parallel or antiparallel to the direction of the electron's momentum, which could be used to generate large and controllable OAM currents. However, OAM monopoles have not yet been directly observed in chiral crystals, and no handle to control their polarity has been discovered. Here, we use circular dichroism in angle-resolved photoelectron spectroscopy (CD-ARPES) to image OAM monopoles in the chiral topological semimetals PtGa and PdGa. Moreover, we also demonstrate that the polarity of the monopole can be controlled via the structural handedness of the host crystal by imaging OAM monopoles and anti-monopoles in the two enantiomers of PdGa, respectively. For most photon energies used in our study, we observe a sign change in the CD-ARPES spectrum when comparing positive and negative momenta along the light direction near the topological degeneracy. This is consistent with the conventional view that CD-ARPES measures the projection of the OAM monopole along the photon momentum. For some photon energies, however, this sign change disappears, which can be understood from our numerical simulations as the interference of polar atomic OAM contributions, consistent with the presence of OAM monopoles. Our results highlight the potential of chiral crystals for orbitronic device applications, and our methodology could enable the discovery of even more complicated nodal OAM textures that could be exploited for orbitronics.Comment: 16 pages, 8 figure

Surface termination and electronic reconstruction in YBa2Cu3O7−δ\mathrm{YBa_{2}Cu_{3}O_{7−δ}}

We present a spatially resolved angle-resolved photoemission spectroscopy (ARPES) study on the Y-based high-T$_c$ cuprate superconductor YBa$_2$Cu$_3$O$_{7−δ}$ to disentangle surface electronic inhomogeneity. Two surface terminations consisting of either a CuO or BaO layer are identified through a chemical-states-specified core-level intensity distribution. This enables us to perform termination-selective ARPES measurements that uncover the different charge fillings and electronic configurations depending on the surface termination. By combining the real-space and electronic information, we propose a simple model to explain the termination-dependent surface electronic reconstruction. Our results demonstrate a significant importance of the spatially resolved ARPES in order to obtain intriguing electronic information, particularly from surface-inhomogeneous systems

Theoretical study of topological properties of ferromagnetic pyrite CoS2

Since the discovery of the first topological material 15 years ago, the search for material realizations of novel topological phases has become the driving force of the field. While oftentimes we search for new materials, we forget that well established materials can also display very interesting topological properties. In this work, we revisit CoS${}_2$, a metallic ferromagnetic pyrite that has been extensively studied in the literature due to its magnetic properties. We study the topological features of its electronic band structure and identify Weyl nodes and nodal lines, as well as a symmetry-protected fourfold fermion close to the Fermi level. Looking at different surface cleavage planes, we observe both spin polarized Fermi arcs in the majority channel and drumhead states. These findings suggest that CoS2 is a promising platform to study topological phenomena, as well as a good candidate for spintronic applications.M G V and I R acknowledge the Spanish Ministerio de Ciencia e Innovacion (Grant Number PID2019109905GB-C21), Programa Red Guipuzcoana de Ciencia, Tecnología e Innovación 2021 No. 2021-CIEN-000070-01 Gipuzkoa Next and the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) GA 3314/1-1—FOR 5249 (QUAST). A R S is grateful to PASPA-DGAPA-UNAM for the sabbatical scholarship and to the DIPC for their support during the research stay. A B acknowledges financial support from the Spanish Ministry of Science and Innovation (PID2019-105488GBI00). N B M S was supported by Microsoft. This work was supported by the Gordon and Betty Moore Foundation through Grant GBMF9064 to L M S.Peer reviewe

Dataset for article "Determination of interatomic coupling between two-dimensional crystals using angle-resolved photoemission spectroscopy"

This dataset contains information about the results of angle-resolved photoemission spectroscopy (ARPES) measurements of twisted bilayer graphene as well as theoretical modelling of this experiment. It accompanies a peer reviewed academic publication in which the experiment and theoretical model are described in more detail.The ARPES measurements were performed at the Spectromicroscopy beamline at the Elettra synchrotron (Trieste, Italy). Before measurements, the samples were annealed at 350 °C for 30 minutes. The experiment was then performed at a base pressure of 10^-10 mbar in ultrahigh vacuum and at the temperature of 110 K. We used photons with energy of 74 eV and estimate our energy and angular resolution as 50 meV and 0.5°, respectively. For each sample, we determined the twist angle by measuring the distance between the nearest BZ corners of the two layers. The theoretical graphs and spectra were produced using equations given in the text of the manuscript.Experimental ARPES data was transferred to Matlab for further analysis and processing. Theoretical simulations were also performed using Matlab. All the enclosed data is saved in Matlab formats.The following describes content of the files in the archive (below, "(x)" stands for numerical part of the name of one of the files in the archive; figure numbers refer to figures in the accompanying publication): - Cut_9_64.mat - experimental ARPES map for Fig 2(a). - couplinggraphs.mat - plots in Fig 2(b). - Cut_9_06.mat - experimental ARPES map for Fig 3(a). - Cut_19.mat – experimental ARPES mao for Fig. 3(b). - (x)energyworkspace.mat - calculated ARPES maps for Fig 4 at energy (x). - experimental(x)workspace.mat - experimental data for Fig 4 at energy (x). - Bilayer_19_1.mat – original experimental ARPES data for twisted bilayer graphene in Fig 3 (b); workspace is organised in the same way as “E_contour_(x).mat” described below. - E_contour_(x).mat - original experimental ARPES data for twisted trilayer with twist angle (x); within this file, the structures "SMPM12058_(x)" contain information about photoelectrons with energy (x); the variables "x" and "y" correspond to kx and ky, respectively, and "value" to measured ARPES intensity; also contained within these structure is variable "info" which describes experimental details of the ARPES measurement. - IntensityCutTBLG134 and IntensityCutTBLG096 correspond to the supplementary figure 1, sub-figures (a) and (b) respectively

Electronic Structure of InAs and InSb Surfaces:Density Functional Theory and Angle‐Resolved Photoemission Spectroscopy

The electronic structure of surfaces plays a key role in the properties of quantum devices. However, surfaces are also the most challenging to simulate and engineer. Here, the electronic structure of InAs(001), InAs(111), and InSb(110) surfaces is studied using a combination of density functional theory (DFT) and angle-resolved photoemission spectroscopy (ARPES). Large-scale first principles simulations are enabled by using DFT calculations with a machine-learned Hubbard U correction [npj Comput. Mater. 6, 180 (2020)]. To facilitate direct comparison with ARPES results, a "bulk unfolding" scheme is implemented by projecting the calculated band structure of a supercell surface slab model onto the bulk primitive cell. For all three surfaces, a good agreement is found between DFT calculations and ARPES. For InAs(001), the simulations clarify the effect of the surface reconstruction. Different reconstructions are found to produce distinctive surface states, which may be detected by ARPES with low photon energies. For InAs(111) and InSb(110), the simulations help elucidate the effect of oxidation. Owing to larger charge transfer from As to O than from Sb to O, oxidation of InAs(111) leads to significant band bending and produces an electron pocket, whereas oxidation of InSb(110) does not. The combined theoretical and experimental results may inform the design of quantum devices based on InAs and InSb semiconductors, for example, topological qubits utilizing the Majorana zero modes