Prediction of first-order martensitic transitions in strained epitaxial films

Abstract

Coherent epitaxial growth allows us to produce strained crystalline films with structures that are unstable in the bulk. Thereby, the overlayer lattice parameters in the interface plane, (a, b), determine the minimum-energy out-of-plane lattice parameter, c a b ( , ) min . We show by means of density-functional total energy calculations that this dependence can be discontinuous and predict related firstorder phase transitions in strained tetragonal films of the elements V, Nb, Ru, La, Os, and Ir. The abrupt change of c min can be exploited to switch properties specific to the overlayer material. This is demonstrated for the example of the superconducting critical temperature of a vanadium film which we predict to jump by 20% at a discontinuity of c min . MethodWe model the structures of the considered films by the epitaxial Bain path (EBP) [11]. The EBP model applies to the bulk part (i.e., the interior) of a coherently grown thick film [12]. It assumes that the film can be described by OPEN ACCESS RECEIVED

Metastable cubic and tetragonal phases of transition metals predicted by density-functional theory

“…The EBP assumes those bct structures for which the lattice parameter c min is relaxed for every lattice parameter a. Hence, the EBP c min (a) is defined by the minimization of the total energy E(a, c) at fixed a [27,28],…”

“…Thus, provided that the two points of cubic symmetry belong to the EBP, E EBP (a) is not only extremal at these two points but there is at least one additional extremal point with noncubic (bct) symmetry [24,26]. In some cases, one of the cubic points does not belong to the EBP but E EBP (a) assumes a maximum close to this point [28]. In the absence of spin-orbit coupling, the symmetry property also holds for ferromagnetically ordered states since the magnetization density preserves the full symmetry of the lattice.…”

“…The EBP is one possible realization of a Bain path, the principles of which were discussed by Alippi et al 24 and Marcus et al 25 The EBP assumes those bct structures for which the lattice parameter c min is relaxed for every lattice parameter a. Hence, the EBP c min (a) is dened by the minimization of the total energy E(a, c) at xed a, 28,29 Eða; c min Þ ¼ min c Eða; cÞ:…”

“…25,27 In some cases, one of the cubic points does not belong to the EBP but E EBP (a) assumes a maximum close to this point. 29 In the absence of spin-orbit coupling, the symmetry property also holds for ferromagnetically ordered states since the magnetization density preserves the full symmetry of the lattice.…”