Speaker
Jan Kuriplach
(Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic)
Description
Positronium can be very helpful when studying the electronic structure of materials. Indeed, the recent experiment [1], where the Ps emission from a copper (110) surface was examined, allowed for the precise determination of the electron chemical potential of copper by means of measuring the Ps affinity. This affinity ($A_{Ps}$) [1] is defined formally via the electron ($\Phi_-$) and positron ($\Phi_+$) work functions as $A_{Ps} = \Phi_- + \Phi_+ - E_{Ps}$, with $E_{Ps}$ ($\doteq$ 6.803 eV) being the Ps ground state binding energy. In the Ps emission experiment, the maximum kinetic energy ($E_K$) of emitted Ps atoms is measured via the Ps time of flight. Since $E_K = -A_{Ps}$, the Ps affinity can be determined. Alternatively, $\Phi_- + \Phi_+$ can be represented via the sum of the electron and positron chemical potentials, which can be obtained using density functional theory for electrons and positrons. The Ps affinity is, therefore, a bulk property. When the accurate correlation functional for positrons [2] is employed, one can check various possibilities for the electron exchange-correlation (XC) functional and compare the resulting electron chemical potential with that deduced from the Ps affinity.
Such a procedure was shown to work for Cu [1] where we could find the proper XC functional with a precision of order 10 meV. In this contribution, we investigate computationally other materials, which are expected to show interesting features in their electronic structure like 2D or 3D Dirac cones and Weyl points (half-metals), and check whether the Ps affinity is a negative number so that Ps atoms may escape materials surfaces, allowing thus for the precise measurement of $A_{Ps}$. Several XC functionals are tested for electrons, including the recently introduced meta-generalized-gradient approximation [3]. As for materials, we examine the Heusler alloy/compound Co$_2$MnAl, topological insulator Bi$_2$Se$_3$, and Dirac metal candidate Zr$_2$Te$_2$P. In addition, we inspect the Na$_3$Bi system exhibiting several topological features in its electronic structure; Na$_3$Bi is further considered as a Na-ion battery (anode) material [4]. The possibility of studying the electronic structure of high-entropy alloys via the Ps emission is also discussed. A working positron beam is a necessary condition – but by far not the only one – to perform such experiments. Spin-polarized positron beams may bring further research possibilities (e.g. for spintronics). In this respect, we discuss approaches to the spin-polarized theory of electron-positron correlations [5].
[1] A.C.L. Jones et al., Phys. Rev. Lett. 117, 216402 (2016).
[2] B. Barbiellini and J. Kuriplach, Phys. Rev. Lett. 114, 147401 (2015).
[3] J. Sun et al., Phys. Rev. Lett. 115, 036402 (2015).
[4] J. Sottmann et al., Chem. Mater. 28, 2750 (2016).
[5] H. Li et al., J. Phys.: Condens. Matter 27, 246001 (2015).
Primary author
Jan Kuriplach
(Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic)
Co-author
Dr
Bernardo Barbiellini
(Department of Physics, Northeastern University, Boston, MA, USA)
