ANNUAL REPORT - MTA SzFKI

C. ELECTRONIC STATES IN SOLIDS
J. Kollár, P. Fazekas , K. Kádas, B. Lazarovits, I. Tüttő, B. Újfalussy, A. Virosztek + , L.
Vitos, V. Zólyomi
Traditionally, new materials have been developed by empirically correlating their
chemical composition, and the manufacturing processes used to form them, with their
properties. Until recently, metallurgists have not used quantum theory for practical
purposes. However, the development of modern density functional methods means that
today, computational quantum mechanics can help engineers to identify and develop novel
materials. Besides, we demonstrate that computational modeling based on modern firstprinciples
alloy theory can yield fundamental physical parameters with high accuracy. We
illustrate this in a few examples.
Using first-principles quantum-mechanical theory, we demonstrate that the surface
chemistry of Fe-Cr alloys follows the peculiar threshold behavior characteristic to ferritic
stainless steels. We find that in dilute alloys the surfaces are covered exclusively by Fe,
whereas for bulk Cr concentration above ~10 % the Cr-containing surfaces become
favorable. The two distinctly dissimilar surface regimes appear as a consequence of two
competing magnetic effects: the magnetically induced immiscibility in bulk Fe-Cr alloys
and the stability of magnetic surfaces.
Using density-functional theory formulated within the framework of the exact muffin-tin
orbitals method, we investigate the stability of the body-centered-cubic phase of Be (β-
Be). The elastic constants and Debye temperature of β-Be are calculated over a wide
volume range and compared to those obtained for the low temperature hexagonal phase
(α-Be). A significant difference in the anisotropy of the bcc and hcp structures is found. In
line with experiments, we predict that the hcp→bcc phase transition occurs at 240 GPa at 0
K and 239 GPa at ambient temperature. We find that the cubic shear constant
C’=(C 11 −C 12 )/2 rapidly decreases for volumes above ~1.05V 0 , where V 0 is the zero
temperature equilibrium volume for β-Be. At 1.17V 0 , the stability condition C’>0 is
violated and the bcc phase becomes mechanically unstable. We demonstrate that at 0 K,
the softening of β-Be near its experimental volume of 1.063V 0 is related to an electronic
topological transition due to the increased number of occupied s states near the Fermi level
compared to that at V 0 . This softening turns out to be important for the stability of the bcc
phase before melting. The disclosed electronic topological transition is found to be present
in other analogous hexagonal metals as well.
First-principles calculations show that measured surface core-level shifts (SCLSs) of the
GaAs(100)(2x4) surfaces can be described within the initial state effects. The calculated
As 3d and Ga 3d SCLSs for the β2 and α2 reconstructions of the GaAs(100) (2x4)
surfaces are in reasonable agreement with recent measurements. In particular, the results
confirm that both the lower and the higher binding energy SCLSs, relative to the bulk
emission in the As 3d photoelectron spectra, are intrinsic properties of the GaAs(100)
(2x4) surfaces. The most positive and most negative As shifts are attributed to the third
layer As atoms, which differs from the previous intuitive suggestions. In general,
calculations show that significant SCLSs arise from deep layers, and that there are more
than two SCLSs. Our previously measured As 3d spectra are fitted afresh using the
calculated SCLSs. The intensity ratios of the SCLSs, obtained from the fits, show that as
the heating temperature of the GaAs(100) (2x4) surface is increased gradually, the area of
+ Permanent position: Budapest University of Technology and Economics
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