Abstract
In the periodic table, only a few pure metals exhibit lattice or magnetic instabilities associated with Fermi surface nesting, the classical examples being α-U and Cr. Whereas α-U displays a strong Kohn anomaly in the phonon spectrum that ultimately leads to the formation of charge density waves (CDWs), Cr is known for its nesting-induced spin density waves (SDWs). Recently, it has become clear that a pronounced Kohn anomaly and the corresponding softening in the elastic constants is also the key factor that controls structural transformations and mechanical properties in compressed group VB metals—materials with relatively high superconducting critical temperatures. This article reviews the current understanding of the structural and mechanical behavior of these metals under pressure with an introduction to the concept of the Kohn anomaly and how it is related to the important concept of Peierls instability. We review both experimental and theoretical results showing different manifestations of the Kohn anomaly in the transverse acoustic phonon mode TA (ξ00) in V, Nb, and Ta. Specifically, in V the anomaly triggers a structural transition to a rhombohedral phase, whereas in Nb and Ta it leads to an anomalous reduction in yield strength.
Highlights
In 1959 Walter Kohn discovered [1] that in metallic systems the phonon spectrum ω(q) exhibits singularities or anomalies associated with the existence of the sharp Fermi surface (FS)
The real part, Re χ0 (q), exhibits a very weak peak near QCDW. All these results suggest that charge density waves (CDWs) in 2H–NbSe2 and 2H–TaSe2 are not nesting derived [5,10,11]
The chemical bonding of the elemental transition metals was believed to be well understood as originating from a gradual occupation of first bonding and later antibonding states as one proceeds through the transition series [101]
Summary
In 1959 Walter Kohn discovered [1] that in metallic systems the phonon spectrum ω(q) exhibits singularities or anomalies associated with the existence of the sharp Fermi surface (FS). Antonangeli et al [91] have recently applied inelastic X-ray scattering to investigate the phonon dispersion of the single-crystal V as a function of compression up to 450 kbar These experiments found an unusual high-pressure behavior of the TA mode along the (100) direction and softening of the trigonal elastic constant C44 that triggers a rhombohedral lattice distortion occurring between 340 and 390 kbar. Tracing the pressure dependence of the position of the maximum on the generalized partial susceptibility, Landa et al [76] plotted (Figure 5) the magnitude of the nesting vector, or the position of the Kohn anomaly, as a function of pressure These calculations show that the nesting vector decreases as pressure increases and the termination of a so-called “jungle-gym” hole-tube occurs at 2.50, 0.75, and 2.75 Mbar for V, Nb, and Ta, respectively. For V and Nb, their calculations revealed a lower termination pressure for Ta (~2.25 Mbar)
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