Manipulating the topological interface using molecular adsorbates

The need of smaller, faster, and less energy demanding electronic components for tomorrow computer and smart devices is calling for new paradigms in information technology. Among them, spintronics today seems the closest to massive production. In spintronics information is brought no more by the electrical charge of the electrons as in usual semiconductor electronics, but from their spin degree of freedom. Two main features characterize a spintronics system: the first one is the spin coherence length, that is the distance an electron can travel in a solid before losing its spin-information, and the second one is the spin relaxation time, that is a measure of how long an electron can retain its spin state. Today’s fundamental research is devoted in finding new materials exhibiting long coherence length and/or relaxation time, and interfaces capable of exploiting this two fundamental characteristic.
In this experiment we proposed a fundamental study on the electronic structure of a prototypical Cobalt Phthalocyanine (CoPc)/Bi₂Se₃ interface. The two components of the interface show on the organic side the typical long relaxation time of metalorganic complexes, while, on the other side, the topological insulator can provide long coherence length thanks to its topologically protected spin-resolved surface state (TSS). Electrons occupying this state can, in fact, travel just in one direction, according to their spin state, while phenomena of scattering are allowed only in presence of magnetic impurities capable of changing the spin direction.
In Figure 1 an overview of the experimental data acquired at the APE beamline is presented: increasing the CoPc coverage results at first in a general blurring of the Bi₂Se₃ electronic structure due to the increasing scattering. When coverage approaches one monolayer some non-dispersing features appear: these correspond to the highly localized molecular states of CoPc.
Focusing our attention on the TSS of Bi₂Se₃, we can see in figure 2a-b that the deposition of the organic overlayer actually perturbs their presence at the interface: moving from 0.73 monolayer (ML) to 1 ML of CoPc/Bi₂Se₃ they disappear from the surface band dispersion. In fact, while the bulk valence band (white dashed line) and the bulk conduction band (black dashed line) are still visible, there is no more evidence of the presence of the TSS (yellow line). This is an unexpected result, since TSS are protected by time reversal symmetry, meaning that just the presence of magnetic impurities can perturb them, opening a gap in correspondence of the Dirac point. However, no signs of gap opening are present, and, consequently, the topological protection still holds.

Figure 1. Overview of the modification induced by CoPc on the electronic structure of Bi₂Se₃ at various coverage. From left to right: pristine surface, 0.66 ML CoPc/Bi₂Se₃, 0.73 ML CoPc/Bi₂Se₃, 1.05 ML CoPc/Bi₂Se₃.

If TSS are no more at the interface, but their protection is not perturbed, it means that we should be able to find them in the subsurface region. Figure 2c shows in fact that repeating the same experiment, but using a lower photon energy (6 eV instead of 55 eV as in all other images), so having a higher penetration depth, we were able to retrieve their presence.
 

Figure 2. Comparison between 0.73 ML CoPc/ Bi₂Se₃ (a) and 1 ML CoPc/ Bi₂Se₃ (b) acquired with 55 eV photons, and 1 ML CoPc/ Bi₂Se₃ with 6 eV photons (c).

This experiment shows for the first time that TSS, even if protected by time reversal symmetry, are not completely insensitive to adsorbates. Special care has to be paid to this aspect in the design of future interface involving topological insulators.
 

This research was conducted by the following research team:

Marco Caputo¹, Lama Khalil¹, Evangelos Papalazarou¹, Marino Marsi¹, Mirko Panighel², Aitor Mugarza², Simone Lisi³, Giovanni Di Santo⁴, Andrea Goldoni⁴, Andrzej Hruban⁵, Marcin Konczykowski⁶, Luca Perfetti⁶, Lia Krusin Elbaum⁷,Ziya Aliev⁸, Mahammad Babanly⁸,Mikhail Otrokov⁹, Evgueni Chulkov⁹, Andres Arnau⁹, Antonio Politano¹⁰, Vera Marinova¹¹,Pranab Das¹², Jun Fujii¹², Ivana Vobornik^12
 
1 Laboratoire de Physique des Solides, CNRS, Universitè Paris-Sud, Université Paris-Saclay, France
² Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Bellaterra, Barcelona, Spain
³ Dipartimento di Fisica, Università di Roma La Sapienza, Roma, Italy
⁴ Laboratory Micro & Nano-Carbon, Elettra - Sincrotrone Trieste, Trieste, Italy
⁵ Institute of Electronic Materials Technology, Warsaw, Poland
⁶ Laboratoire des Solides Irradiés, Ecole Polytechnique, CNRS, CEA, Université Paris-Saclay, Palaiseau Cedex, France
⁷ Department of Physics, The City College of New York, CUNY, New York, New York, United States
⁸ Institute of Catalysis and Inorganic Chemistry, Institute of Physics, Azerbaijan National Academy of Sciences, Baku, Azerbaijan
⁹ Donostia International Physics Center, Donostia-San Sebastian, Spain
¹⁰ Department of Physics, University of Calabria, Rende (CS), Italy
¹¹ Institute of Optical Materials and Technologies, Sofia, Bulgaria
¹² Istituto Officina dei Materiali (IOM)-CNR, Laboratorio TASC, Trieste, Italy

Ivana Vobornik, email: