Spectroscopic perspective on the interplay between electronic and magnetic properties of magnetically doped topological insulators

We combine low energy muon spin rotation (LE-$\mu$SR) and soft-X-ray angle-resolved photoemission spectroscopy (SX-ARPES) to study the magnetic and electronic properties of magnetically doped topological insulators, (Bi,Sb)$_2$Te$_3$. We find that one achieves a full magnetic volume fraction in samples of (V/Cr)$_x$(Bi,Sb)$_{2-x}$Te$_3$ at doping levels x $\gtrsim$ 0.16. The observed magnetic transition is not sharp in temperature indicating a gradual magnetic ordering. We find that the evolution of magnetic ordering is consistent with formation of ferromagnetic islands which increase in number and/or volume with decreasing temperature. Resonant ARPES at the V $L_3$ edge reveals a nondispersing impurity band close to the Fermi level as well as V weight integrated into the host band structure. Calculations within the coherent potential approximation of the V contribution to the spectral function confirm that this impurity band is caused by V in substitutional sites. The implications of our results on the observation of the quantum anomalous Hall effect at mK temperatures are discussed

Metamagnetic transition and a loss of magnetic hysteresis caused by electron trapping in monolayers of single-molecule magnet Tb2_{2}@C79_{79}N

Whereas bulk Tb$_{2}$@C$_{79}$N is a single-molecule magnet with broad hysteresis, its monolayers on different substrates show the prevalence of a non-magnetic ground state near zero magnetic field and a metamagnetic transition with the field increase. Realization of stable spin states in surface-supported magnetic molecules is crucial for their applications in molecular spintronics, memory storage or quantum information processing. In this work, we studied the surface magnetism of dimetallo-azafullerene Tb$_{2}$@C$_{79}$N, showing a broad magnetic hysteresis in a bulk form. Surprisingly, monolayers of Tb$_{2}$@C$_{79}$N exhibited a completely different behavior, with the prevalence of a ground state with antiferromagnetic coupling at low magnetic field and a metamagnetic transition in the magnetic field of 2.5–4 T. Monolayers of Tb$_{2}$@C$_{79}$N were deposited onto Cu(111) and Au(111) by evaporation in ultra-high vacuum conditions, and their topography and electronic structure were characterized by scanning tunneling microscopy and spectroscopy (STM/STS). X-ray photoelectron spectroscopy (XPS), in combination with DFT studies, revealed that the nitrogen atom of the azafullerene cage tends to avoid metallic surfaces. Magnetic properties of the (sub)monolayers were then studied by X-ray magnetic circular dichroism (XMCD) at the Tb-M$_{4,5}$ absorption edge. While in bulk powder samples Tb$_{2}$@C$_{79}$N behaves as a single-molecule magnet with ferromagnetically coupled magnetic moments and blocking of magnetization at 28 K, its monolayers exhibited a different ground state with antiferromagnetic coupling of Tb magnetic moments. To understand if this unexpected behavior is caused by a strong hybridization of fullerenes with metallic substrates, XMCD measurements were also performed for Tb2@C79N adsorbed on h-BN|Rh(111) and MgO|Ag(100). The co-existence of two forms of Tb$_{2}$@C$_{79}$N was found on these substrates as well, but magnetization curves showed narrow magnetic hysteresis detectable up to 25 K. The non-magnetic state of Tb$_{2}$@C$_{79}$N in monolayers is assigned to anionic Tb$_{2}$@C$_{79}$N− species with doubly-occupied Tb–Tb bonding orbital and antiferromagnetic coupling of the Tb moments. A charge transfer from the substrate or trapping of secondary electrons are discussed as a plausible origin of these species

Influence of the surface reorganization on the spin electronic structure of the Dirac cone of topological insulators

Resumen del trabajo presentado al New Trends in Topological Insulators (NTTI), celebrado en Donostia-San Sebastián (España) del 6 al10 de julio de 2015.Peer reviewe

Spin-orbit coupling induced gap in graphene on Pt(111) with intercalated Pb monolayer

Graphene is one of the most promising materials for nanoelectronics owing to its unique Dirac cone-like dispersion of the electronic state and high mobility of the charge carriers. However, to facilitate the implementation of the graphene-based devices, an essential change of its electronic structure, a creation of the band gap should controllably be done. Brought about by two fundamentally different mechanisms, a sublattice symmetry breaking or an induced strong spin-orbit interaction, the band gap appearance can drive graphene into a narrow-gap semiconductor or a 2D topological insulator phase, respectively, with both cases being technologically relevant. The later case, characterized by a spin-orbit gap between the valence and conduction bands, can give rise to the spin-polarized topologically protected edge states. Here, we study the effect of the spin-orbit interaction enhancement in graphene placed in contact with a lead monolayer. By means of angle-resolved photoemission spectroscopy, we show that intercalation of the Pb interlayer between the graphene sheet and the Pt(111) surface leads to formation of a gap of 200 meV at the Dirac point of graphene. Spin-resolved measurements confirm the splitting to be of a spin-orbit nature, and the measured near-gap spin structure resembles that of the quantum spin Hall state in graphene, proposed by Kane and Mele [ Phys. Rev. Lett. 2005, 95, 226801 ]. With a bandstructure tuned in this way, graphene acquires a functionality going beyond its intrinsic properties and becomes more attractive for possible spintronic applications.The work was partially supported by grants from Saint Petersburg State University for scientific investigations (Nos. 11.38.271.2014 and 15.61.202.2015) and DFG - SPbU grant No. 11.65.42.2017. We acknowledge financial support from the University of Basque Country UPV/EHU (Grant No. GIC07-IT-756-13), the Departamento de Educacion del Gobierno Vasco and the Spanish Ministerio de Ciencia e Innovacion (Grant No. FIS2010-19609-C02-01), the Spanish Ministry of Economy and Competitiveness MINECO (Grant No. FIS2013-48286-C2-1-P), and the Tomsk State University Academic D. I. Mendeleev Fund Program in 2015 (Research Grant No. 8.1.05.2015).Peer Reviewe

Reply to "comment on 'spin-orbit coupling induced gap in graphene on Pt(111) with intercalated Pb monolayer'"

In a recent article, we studied the electronic and spin structure of graphene on Pt(111) with intercalated Pb monolayer. By means of ARPES, we have shown that the Dirac cone of graphene is characterized by the gap between the π and π* states in this system. The spin texture of the π and π* states and its correspondence with the Kane and Mele model for graphene with spin−orbit gap have been unveiled by spinresolved ARPES. In the Comment on our paper, Dedkov and Voloshina claim that (1) the notation of the superstructure used in our work is incorrect, and (2) spin-resolved data treatment is inappropriate. In this Reply, we show that the superstructure notation is indeed correct. Moreover, the statistical analysis of the reported spin-resolved ARPES data and new experimental data with better statistics support the main conclusion of our article.The work was partially supported by grant of Saint Petersburg State University for scientific investigations (No. 15.61.202.2015) and DFG - SPbU grant No. 11.65.42.2017. We acknowledge support by the University of the Basque Country (Grant Nos. GIC07IT36607 and IT-756-13), the Spanish Ministry of Science and Innovation (Grant Nos. FIS2013-48286-C02-02-P, FIS2013-48286-C02-01-P, and FIS2016-75862-P), and Tomsk State University competitiveness improvement programme (Project No. 8.1.01.2017). The part of photoemission measurements was supported by Russian Science Foundation (Project No. 17-12-01047). The authors acknowledge support from the Russian−German laboratory at BESSY II and the “German−Russian Interdisciplinary Science Center”(G-RISC) program.Peer Reviewe

Spin-resolved band structure of heterojunction Bi-bilayer/3D topological insulator in the quantum dimension regime in annealed Bi2Te2.4Se0.6

Two-and three-dimensional topological insulators are the key materials for the future nanoelectronic and spintronic devices and quantum computers. By means of angle-and spin-resolved photoemission spectroscopy we study the electronic and spin structure of the Bi-bilayer/3D topological insulator in quantum tunneling regime formed under the short annealing of BiTeSe Owing to the temperature-induced restructuring of the topological insulator's surface quintuple layers, the hole-like spin-split Bi-bilayer bands and the parabolic electronic-like state are observed instead of the Dirac cone. Scanning Tunneling Microscopy and X-ray Photoemission Spectroscopy measurements reveal the appearance of the Bi terraces at the surface under the annealing. The experimental results are supported by density functional theory calculations, predicting the spin-polarized Bi-bilayer bands interacting with the quintuple-layers-derived states. Such an easily formed heterostructure promises exciting applications in spin transport devices and low-energy electronics.The work was partially supported by grant of Saint Petersburg State University for scientific investigations (No 15.61.202.2015). We acknowledge the financial support of the University of Basque Country UPV/EHU (Grant No. GIC07-IT-756-13), the Departamento de Educación del Gobierno Vasco and the Spanish Ministerio de Ciencia e Innovación (Grant No. FIS2010-19609-C02-01), the Spanish Ministry of Economy and Competitiveness MINECO (Grant No. FIS2013-48286-C2-1-P) and the Tomsk State University Competitiveness Improvement Program.Peer Reviewe

Spectroscopic perspective on the interplay between electronic and magnetic properties of magnetically doped topological insulators

We combine low energy muon spin rotation (LE-μSR) and soft-x-ray angle-resolved photoemission spectroscopy (SX-ARPES) to study the magnetic and electronic properties of magnetically doped topological insulators, (Bi,Sb)2Te3. We find that one achieves a full magnetic volume fraction in samples of (V/Cr)x(Bi,Sb)2-xTe3 at doping levels x=0.16. The observed magnetic transition is not sharp in temperature indicating a gradual magnetic ordering. We find that the evolution of magnetic ordering is consistent with formation of ferromagnetic islands which increase in number and/or volume with decreasing temperature. Resonant ARPES at the V L3 edge reveals a nondispersing impurity band close to the Fermi level as well as V weight integrated into the host band structure. Calculations within the coherent potential approximation of the V contribution to the spectral function confirm that this impurity band is caused by V in substitutional sites. The implications of our results on the observation of the quantum anomalous Hall effect at mK temperatures are discussed.The work at PSI was supported by the Swiss National Science Foundation (SNF-Grant No. 200021_165910). We acknowledge support from DFG through priority program SPP1666 (Topological Insulators), University of the Basque Country (Grants No. GIC07IT36607 and No. IT-756-13), Spanish Ministry of Science and Innovation (Grants No. FIS2013-48286-C02-02-P, No. FIS2013-48286-C02-01-P, and No. FIS2016-75862-P) and Tomsk State University competitiveness improvement programme (Project No. 8.1.01.2017). Partial support by the Saint Petersburg State University Project No. 15.61.202.2015 is also acknowledged. At MIT, C.Z.C. and J.S.M. acknowledge support from the STC Center for Integrated Quantum Materials under NSF grant DMR-1231319 as well as grants NSF (DMR-1207469, DMR1700137), ONR (N00014-13-1-0301 and N00014-16-1-2657). C.Z.C also acknowledges the support from the startup grant provided by Penn State University.Peer Reviewe

Mapping Orbital-Resolved Magnetism in Single Lanthanide Atoms

© Single lanthanide atoms and molecules are promising candidates for atomic data storage and quantum logic due to the long lifetime of their magnetic quantum states. Accessing and controlling these states through electrical transport requires precise knowledge of their electronic configuration at the level of individual atomic orbitals, especially of the outer shells involved in transport. However, no experimental techniques have so far shown the required sensitivity to probe single atoms with orbital selectivity. Here we resolve the magnetism of individual orbitals in Gd and Ho single atoms on MgO/Ag(100) by combining X-ray magnetic circular dichroism with multiplet calculations and density functional theory. In contrast to the usual assumption of bulk-like occupation of the different electronic shells, we establish a charge transfer mechanism leading to an unconventional singly ionized configuration. Our work identifies the role of the valence electrons in determining the quantum level structure and spin-dependent transport properties of lanthanide-based nanomagnets.11Nsciescopu