Metallization of vanadium dioxide driven by large phonon entropy (original) (raw)
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Origin of the transition entropy in vanadium dioxide
Physical Review B, 2019
The reversible metal-insulator transition in VO2 at TC ≈ 340 K has been closely scrutinized yet its thermodynamic origin remains ambiguous. We discuss the origin of the transition entropy by calculating the electron and phonon contributions at TC using density functional theory. The vibration frequencies are obtained from harmonic phonon calculations, with the soft modes that are imaginary at zero temperature renormalized to real values at TC using experimental information from diffuse x-ray scattering at high-symmetry wavevectors. Gaussian Process Regression is used to infer the transformed frequencies for wavevectors across the whole Brillouin zone, and in turn compute the finite temperature phonon partition function to predict transition thermodynamics. Using this method, we predict the phase transition in VO2 is driven five to one by phonon entropy over electronic entropy, and predict a total transition entropy that accounts for 95% of the calorimetric value.
Non-congruence of thermally driven structural and electronic transitions in VO2
Journal of Applied Physics, 2012
Coupled structural and electronic phase transitions underlie the multifunctional properties of strongly-correlated materials. For example, colossal magnetoresistance 1,2 in manganites involves phase transition from paramagnetic insulator to ferromagnetic metal linked to a structural Jahn-Teller distortion 3. Vanadium dioxide (VO 2) likewise exhibits an insulator-to-metal transition (IMT) at ~67 o C with abrupt changes in transport and optical properties and coupled to a structural phase transition (SPT) from monoclinic to tetragonal 4. The IMT and SPT hystereses are signatures of first-order phase transition tracking the nucleation to stabilization of a new phase. Here we have for the first time measured independently the IMT and SPT hystereses in epitaxial VO 2 films, and shown that the hystereses are not congruent. From the measured volume fractions of the two phases in the region of strong correlation, we have computed the evolving dielectric function under an effective-medium approximation. But the computed dielectric functions could not reproduce the measured IMT, implying that there is a strongly correlated metallic phase that is not in the stable rutile structure, consistent with Qazilbash et al 5. Search for a corresponding macroscopic structural intermediate also yielded negative result. The complex physics of VO 2 phase transition has long been debated 6-10. Unlike other strongly correlated materials 11 exhibiting IMT, such as V 2 O 3 12,13 , where phase transitions are satisfactorily explained by the Mott mechanism alone, the IMT in VO 2 is complicated by accompanying spin-Peierls instability 14 that leads via strong electron-phonon coupling
Scientific reports, 2016
Materials that undergo reversible metal-insulator transitions are obvious candidates for new generations of devices. For such potential to be realised, the underlying microscopic mechanisms of such transitions must be fully determined. In this work we probe the correlation between the energy landscape and electronic structure of the metal-insulator transition of vanadium dioxide and the atomic motions occurring using first principles calculations and high resolution X-ray diffraction. Calculations find an energy barrier between the high and low temperature phases corresponding to contraction followed by expansion of the distances between vanadium atoms on neighbouring sub-lattices. X-ray diffraction reveals anisotropic strain broadening in the low temperature structure's crystal planes, however only for those with spacings affected by this compression/expansion. GW calculations reveal that traversing this barrier destabilises the bonding/anti-bonding splitting of the low tempera...
Phonon triggered rhombohedral lattice distortion in vanadium at high pressure
Scientific Reports, 2016
In spite of the simple body-centered-cubic crystal structure, the elements of group V, vanadium, niobium and tantalum, show strong interactions between the electronic properties and lattice dynamics. Further, these interactions can be tuned by external parameters, such as pressure and temperature. We used inelastic x-ray scattering to probe the phonon dispersion of single-crystalline vanadium as a function of pressure to 45 GPa. Our measurements show an anomalous high-pressure behavior of the transverse acoustic mode along the (100) direction and a softening of the elastic modulus C 44 that triggers a rhombohedral lattice distortion occurring between 34 and 39 GPa. Our results provide the missing experimental confirmation of the theoretically predicted shear instability arising from the progressive intra-band nesting of the Fermi surface with increasing pressure, a scenario common to all transition metals of group V. Although body-centered-cubic (bcc) metals have one of the simplest crystal structures in the periodic table, they display a rich variety of physical properties and thus provide an important benchmark for the validation of modern first-principle theory 1. In particular, the lattice dynamics of bcc transition metals have attracted great scientific attention. The Kohn anomaly in the phonon dispersion of bcc transition metals, and its dependence upon pressure and temperature, has been a challenge for first principle calculations to capture 2,3. The strong differences displayed by the phonon dispersion of the various elements of group V (vanadium, niobium and tantalum) suggest that there is a profound dependence of the phonon energies on the electronic structure and the topology of the Fermi surface 4,5. The high superconducting temperature (T c = 9.25 K for Nb and T c = 5.3 K for V) and its notable increase with pressure have also been suggested to be due to electron-phonon coupling and Fermi-surface properties 6-8. The stability at high pressure of the bcc structure is speculated to critically hinge on the topology of the Fermi surface as well, and an intra-band nesting is theoretically predicted to give rise to shear phonon instabilities 9. Focusing on vanadium, calculations of shear instabilities arising from phonon softening 9 have prompted the reinvestigation of the structural stability of V under high pressure. X-ray powder diffraction showed a transition from the bcc to a rhombohedral phase at 69 GPa 10 and subsequent calculations have confirmed the nature of the rhombohedral distortion-even though different transition pressures were proposed 5,11-13. Interestingly, under hydrostatic conditions the transition is hindered, and non-hydrostaticity helps in overcoming the energy barrier associated with the structural phase change 14. Irrespective of the exact pressure at which the transition occurs, the bulk of theoretical work points towards a common mechanism: the progressive intra-band nesting at the Fermi surface that eventually leads to an electronic topological transition (ETT) with a concomitant transverse acoustic phonon mode softening. Specifically, at a critical pressure, parts of the 3rd electronic, partially occupied, conduction band of d symmetry move into the close vicinity of the Fermi level. The nesting vector, already responsible for the Kohn anomaly in the transverse acoustic phonon mode along the (ξ, 0, 0) direction at ξ = 0.25 at ambient pressure 8 , reduces to zero and the ETT takes place, with instability in the shear elastic constant C 44 9. This anomalous softening of the elastic response causes an energy gain that counterbalances the standard elastic strain energy
Physical Review B, 2016
Recent experiments have revealed an intriguing pressure-induced isostructural transition of the low temperature monoclinic VO 2 and hinted to the existence of a new metallization mechanism in this system. The physics behind this isostructural phase transition and the metallization remains unresolved. In this work, we show that the isostructural transition is a result of pressure-induced instability of a phonon mode that relates to a CaCl 2-type of rotation of the oxygen octahedra which alleviates, but does not completely remove, the dimerization and zigzagging arrangement of V atoms in the M1 phase. This phonon mode shows an increasing softening with pressure, ultimately leading to an isostructural phase transition characterized by the degree of the rotation of the oxygen octahedra. We also find that this phase transition is accompanied by an anisotropic compression, in excellent agreement with experiments. More interestingly, in addition to the experimentally identified M1' phase, we find a closely related M1" phase which is nearly degenerate with the M1' phase. Unlike the M1' phase which has a nearly pressure-independent electronic band gap, the gap of the M1" drops quickly at high pressures and vanishes at a theoretical pressure of about 40 GPa.
Physical Review B, 2012
In addition to its metal-insulator transition (MIT), VO 2 exhibits a rich phase behavior of insulating monoclinic (M1,M2) and triclinic (T) phases. By using micro-Raman spectroscopy and independent control of temperature and uniaxial strain in individual single-crystal microbeams, we map these insulating phases with their associated structural changes as represented by their respective phonon frequencies. The competition between these structural forms is dictated by the internal strain due to differing lattice constants, the experimentally applied external strain, and the temperature-dependent phase stability. We identify the nature of the triclinic phase as a continuously distorted variant of the M1 monoclinic phase, while a discontinuous transition into the M2 phase occurs from both the M1 and T phases. The results suggest that understanding the driving forces that determine the interplay between M1, M2, and T phases near the MIT could be critical for the identification of the underlying mechanism behind the MIT itself.
The metal-insulator transition of M1 vanadium dioxide
arXiv: Materials Science, 2016
Materials that undergo reversible metal-insulator transitions are obvious candidates for new generations of devices. For such potential to be realised, the underlying microscopic mechanisms of such transitions must be fully determined. In this work we probe the correlation between the energy landscape and electronic structure of the metal-insulator transition of vanadium dioxide and the atomic motions occurring using first principles calculations and high resolution X-ray diffraction. Calculations find an energy barrier between the high and low temperature phases corresponding to contraction followed by expansion of the distances between vanadium atoms on neighbouring sublattices. X-ray diffraction reveals anisotropic strain broadening in the low temperature structure's crystal planes, however only for those with spacings affected by this compression/expansion. GW calculations reveal that traversing this barrier destabilises the bonding/anti-bonding splitting of the low temperature phase. This precise atomic description of the origin of the energy barrier separating the two structures will facilitate more precise control over the transition characteristics for new applications and devices.
Condensed Matter
Among transition metal oxides, VO 2 is a particularly interesting and challenging correlated electron material where an insulator to metal transition (MIT) occurs near room temperature. Here we investigate a 16 nm thick strained vanadium dioxide film, trying to clarify the dynamic behavior of the insulator/metal transition. We measured (resonant) photoemission below and above the MIT transition temperature, focusing on heating and cooling effects at the vanadium L 23-edge using X-ray Absorption Near-Edge Structure (XANES). The vanadium L 23-edges probe the transitions from the 2p core level to final unoccupied states with 3d orbital symmetry above the Fermi level. The dynamics of the 3d unoccupied states both at the L 3-and at the L 2-edge are in agreement with the hysteretic behavior of this thin film. In the first stage of the cooling, the 3d unoccupied states do not change while the transition in the insulating phase appears below 60 • C. Finally, Resonant Photoemission Spectra (ResPES) point out a shift of the Fermi level of~0.75 eV, which can be correlated to the dynamics of the 3d // orbitals, the electron-electron correlation, and the stability of the metallic state.
Nano Letters, 2010
The ability to synthesize VO 2 in the form of single-crystalline nanobeams and nano-and microcrystals uncovered a number of previously unknown aspects of the metal-insulator transition (MIT) in this oxide. In particular, several reports demonstrated that the MIT can proceed through competition between two monoclinic (insulating) phases M1 and M2 and the tetragonal (metallic) R phase under influence of strain. The nature of such phase behavior has been not identified. Here we show that the competition between M1 and M2 phases is purely lattice-symmetry-driven. Within the framework of the Ginzburg-Landau formalism, both M phases correspond to different directions of the same four-component structural order parameter, and as a consequence, the M2 phase can appear under a small perturbation of the M1 structure such as doping or stress. We analyze the strain-controlled phase diagram of VO 2 in the vicinity of the R-M2-M1 triple point using the Ginzburg-Landau formalism and identify and experimentally verify the pathways for strain-control of the transition. These insights open the door toward more systematic approaches to synthesis of VO 2 nanostructures in desired phase states and to use of external fields in the control of the VO 2 phase states. Additionally, we report observation of the triclinic T phase at the heterophase domain boundaries in strained quasi-two-dimensional VO 2 nanoplatelets, and theoretically predict phases that have not been previously observed.