Abstract
Vanadium dioxide exhibits a first order metal to insulator transition (MIT) at 340 K (T MI ) from a rutile (R) structure to a monoclinic (M 1 ) structure. The mechanism of this transition interpreted as due either to a Peierls instability or to a Mott–Hubbard instability is controversial since half a century. However, in the last twenty years the study of chemical and physical properties of VO 2 and of its alloys, benefits of a renewed interest due to possible applications coming from the realization of devices made of thin films. We describe in this review the structural, electronic and magnetic properties of the different metallic (R) and insulating (M 1 , T, M 2 ) phases of VO 2 , of its solid solutions and under constraint. We present in a synthetic manner the various phase diagrams and their symmetry analysis. This work allows us to revisit older interpretation and to emphasize in particular the combined role of electron–electron interactions in the various phase of VO 2 and of structural fluctuations in the MIT mechanism. In this framework we show that the phase transition is surprisingly announced by anisotropic one-dimensional (1D) structural fluctuations revealing chain like correlations between the V due to an incipient instability of the rutile structure. This leads to an unexpected critical dynamics of the order–disorder (or relaxation) type. We describe how the two-dimensional (2D) coupling between these 1D fluctuations, locally forming uniform V 4+ zig-zag chains and V–V pairs, stabilizes the M 2 and M 1 insulating phases. These phases exhibit a 1D electronic anisotropy where substantial electron–electron correlations conduct to a spin–charge decoupling. The spin-Peierls ground state of M 1 is analyzed via a mechanism of dimerization, in the T phase, of the spin 1/2 V 4+ zig-zag Heisenberg chains formed in the M 2 phase. This review summarizes in a critical manner the main results of the large literature on fundamental aspects of the MIT of VO 2 . It is completed by unpublished old results. Interpretations are also placed in a large conceptual frame which is also relevant to interpret physical properties of other classes of materials.
Highlights
Jean-Paul Pouget (R) et isolantes (M1, T et M2) de VO2, de ses solutions solides et des modifications sous contrainte
In metallic VO2, the cR/aR ratio decreases in temperature, from 0.629 to 0.626 [34], reaching nearly the critical parameter ∼0.625 below which the rutile structure should be unstable with respect to a low symmetry distortion in which dimers, favoring direct M–M bonding, are formed [32]
The M2 structure (Figure 3) breaks the symmetry relating the two equivalent octahedral V chain of the rutile structure by stabilizing two different types of V shifts: namely the formation of zig-zag V2 chains running along cR and of a periodic array of V1–V1 pairs directed along cR
Summary
The study of the metal–insulator transition (MIT) has remained one of the most important field of research in solid state physics for more than half a century. Important progress occurred in the 1970s, when taking into account the influence of the M1 distortion on the t2g electronic levels of the R metallic phase, [19] proposed a mechanism for the MIT of VO2 This mechanism (more explicitly described in Section 5.1) combines an inter-band charge transfer process which leads to a half-filled quasi-1D d// band, within which a Peierls instability produces an insulating ground state through a lattice dimerization. The main reason was the lack, in the 1970’s, of ab-initio (for instance Local Density Approximation (LDA)) calculations of the true electronic structure and of an adequate formalism (for instance Dynamical Mean Field Theory (DMFT)) allowing, in presence of sizeable electron– electron repulsions, to consistently obtain the formation of Hubbard bands These first investigations raised a controversy between the so-called Peierls and Mott– Hubbard scenarios for the MIT of VO2, which is not completely closed these days (see for example [25, 26]).
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