Abstract
We describe experiments aimed at probing the sticking of positrons to the surfaces of topological insulators using the Positron Annihilation induced Auger Electron Spectrometer (PAES). A magnetically guided beam was used to deposit positrons at the surface of Bi2Te2Se sample at energy of ∼2eV. Peaks observed in the energy spectra and intensities of electrons emitted as a result of positron annihilation showed peaks at energies corresponding to Auger peaks in Bi, Teand Se providing clear evidence of Auger emission associated with the annihilation of positrons in a surface bound state. Theoretical estimates of the binding energy of this state are compared with estimates obtained by measuring the incident beam energy threshold for secondary electron emission and the temperature dependence positronium(Ps) emission. The experiments provide strong evidence for the existence of a positron bound state at the surface of Bi2Te2Se and indicate the practicality of using positron annihilation to selectively probe the critically important top most layer of topological insulator system.
Highlights
1.1 Overview Positrons have proven to be a very effective tool in probing surfaces and reduced dimensional systems such as nano-particles which possess high surface to volume ratios
We present two pieces of evidence for the existence of positron surface state. 1.) The positron annihilation induced Auger electron spectrum using a 9eV beam 2.) Calculating the positronium fraction formed at the surface of the Topological insulator as function of temperature
If the positrons are in the bulk state at the time of annihilation only a tiny fraction of the positrons will annihilate close enough to the surface for an electron to escape without losing energy
Summary
1.1 Overview Positrons have proven to be a very effective tool in probing surfaces and reduced dimensional systems such as nano-particles which possess high surface to volume ratios. A means of selectively sampling the top most layers, by trapping the positrons in an image potential surface state before annihilation in a material or nanostructure. This is due to fact that the image potential surface state extends about one atomic layer below the surface. The existence of a positron surface state would reveal surface specific information regarding electronic momentum and spin dependent properties. Localization of positrons to the surface of topological insulators can lead to probing other positron spectroscopic techniques such as a.) Two dimensional – Angular Correlated Annihilation Radiation (2D- ACAR): gives information about the Fermi surface and electron momentum distribution of the topmost layers of the sample. The experiments performed were a.) Positron Annihilation Induced Auger Electron Spectroscopy (PAES). b.) Auger Mediated Sticking (AMS) Measurement. c.) Ps Fraction Measurement
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