Kohn Anomaly and Phase Stability in Group VB Transition Metals

Abstract

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.

Electron-phonon properties, structural stability, and superconductivity of doped antimonene

“…2(d), which exhibits a hexagonal warping. This topology opens up a possibility for the scattering between the parallel regions of the surface, known as the Fermi surface nesting [47,68]. In this case one can expect an increase of the electronphonon coupling strength at the corresponding charge carrier concentrations (N h = 3.7 × 10 14 cm −2 ).…”

Temperature dependence of the Kohn anomaly in bcc Nb from first-principles self-consistent phonon calculations

“…(7), P c Km is the spectral weight calculated as in Eq. (6) and E c Km is the energy of band m at the wave vector K for the configuration c.…”

“…Kawashima [4] reported an anomalous decrease of the elastic modulus, C = (C 11 − C 12 )/2, of the normally conducting state below T c using a magnetic field higher than the critical field combined with ultrasonic measurements. Landa et al [5,6] and Koči et al [7] predicted a shear elastic softening of Nb from 20 to 150 GPa based on 0 K density functional theory (DFT) calculations. Based on the assumption that the thermal excitation of the electrons is the most significant temperature effect, Wang et al [8] performed static DFT calculations, varying the electronic temperature and including the thermoelectronic entropy to predict the temperature dependence of the C 44 shear elastic constant.…”

Comparison of dispersion-corrected exchange-correlation functionals using atomic orbitals

“…Semilocal XC functionals, which are all characterized by a localized XC hole density [5,6], can recognize various types of interactions and density regimes, such as covalent, metallic, and some short-ranged weak bonds [4,[7][8][9], can be utilized for an accurate description of surface asymptotics [10], and can describe the dimensional crossover of the XC energy [11][12][13] for various confinements [14]; but they cannot possibly describe correctly van der Waals (vdW) interactions even at short range [15]. Whenever vdW interactions play a role, one can use the so-called DFT-D semiempirical approach [16][17][18][19][20][21][22][23][24][25][26][27][28][29], which is based on the cancellation of various errors especially at a short range [15].…”