Nanoscale physics

Positron spectroscopy of nanoscale regions of homogeneous atomic and magnetic orderings in strongly correlated alloys
Len E.G.1,2, Shatnii T.D.1, Len T.S.3, Galstian I.Ye.1, Tsapko Ye.A.1, 
Lizunov V.V.1
1 G.V. Kurdyumov Institute for Metal Physics, NAS of Ukraine, 36 Academician Vernadsky Blvd., UA-03142 Kyiv, Ukraine.
E-mail: len@imp.kiev.ua
2 Kyiv Academic University, NAS and MES of Ukraine, 36 Academician Vernadsky Blvd., UA-03142 Kyiv, Ukraine.
3 National Aviation University, 1 Kosmonavt Komarov Ave., UA-03058 Kyiv, Ukraine.
Today, the most promising materials of electronic, energy, and aerospace industries are developed not only taking into account quantum effects due to the nanometer level of the element base of devices, but also complex collective phenomena in the electrons’ subsystem, which strongly interact with each other and other excitations (phonons, magnons, etc.) in solids. A very limited number of methods allows an experimental observation of these effects in the electronic structure. Such methods reproduce the Fermi surface in strong magnetic fields or permit a direct observation of the band structure (using angle-resolved photoemission spectroscopy (ARPES)) and require the magnetic fields, low temperatures and ultra-high vacuum (ARPES). They are applicable to systems with a well-defined band structure and Fermi surface only. All these experimental complexities and natural limitations are absent in the method of positron spectroscopy, namely, in the method of angular correlation of electron positron annihilation radiation (ACAR). In addition, this method allows to determine the size and concentration of defects in crystalline or non-crystalline matter and to study their electronic properties. Thus, the theoretical consideration of the influence of some many-particle effects on the momentum distributions of conduction electrons recorded by the ACAR method is actual.
At the framework of the single-band Hubbard model for binary substitutional alloys with strong electron correlations, the changes in momentum distributions of electrons are investigated depending on the presence and magnitude of pair correlations in the arrangement of atoms of different types and/or in the orientation of electron magnetic moments at nearest sites. These correlations determine the size of regions of homogeneous atomic and/or magnetic short-range (nanoscale) ordering. As shown at temperature of 0 K, the effects of pair correlations on these distributions are clearly defined, which allows the use ACAR for their experimental identification in the studied systems.