When holes of about 0.018% are induced into a conduction band (breakdown of critical on-site Coulomb energy), an abrupt first-order Mott metal–insulator transition (MIT) rather than a continuous Hubbard MIT near a critical on-site Coulomb energy U/Uc=1, where U is on-site Coulomb energy between electrons, is observed on an inhomogeneous VO2 film, a strongly correlated Mott insulator. As a result, discontinuous jumps of the density of states on the Fermi surface are observed and inhomogeneity inevitably occurs. The off-current and temperature dependences of the abrupt MIT in a two-terminal device and the gate effect in a three-terminal device are clear evidence that the abrupt Mott MIT was induced by the excitation of holes. Raman spectra measured by a micro-Raman system show an MIT without the structural phase transition. Moreover, the magnitude of the observed jumps ΔJobserved at the abrupt MIT is an average over an inhomogeneous measurement region of the maximum true jump, ΔJtrue, deduced from the Brinkman–Rice picture. A brief discussion of whether VO2 is a Mott insulator or a Peierls insulator is presented.

https://iopscience.iop.org/article/10.1088/1367-2630/6/1/052/pdf

Mechanism and observation of Mott transition in VO2-based two- and three-terminal devices

Mott insulators are identified here with ordinary magnetic insulators. The insulating gap, local moment, and effective spin hamiltonian aspects are qualitatively explained by means of a novel set of solutions of the Hartree-Fock equations. The apparent conflict between Bloch's theorem and localized-electron phenomenology is thereby resolved in an elementary manner. This Hartree-Fock approach also sheds considerable light on the physical mechanisms responsible for the associated metal-insulator (Mott) and other related phase transitions, as observed in V2O3 and several other materials. With some generalizations and refinements, this theoretical picture is shown to also account semiquantitatively for a number of detailed properties of NiO and CoO, two of the most extensively studied Mott insulator materials. A wide variety of experimental data for NiO is surveyed in order to determine reasonable values for its effective Hubbard hamiltonian parameters, suitably generalized for the 3d electrons. The ...

Electronic structure of Mott insulators

In femtosecond pump-probe measurements, the appearance of coherent phonon oscillations at 4.5 and 6.0 THz indicating the rutile metal phase of ${\mathrm{VO}}_{2}$ does not occur simultaneously with the first-order metal-insulator transition (MIT) near $68\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. The monoclinic and correlated metal (MCM) phase between the MIT and the structural phase transition (SPT) is generated by a photoassisted hole excitation, which is evidence of the Mott transition. The SPT between the MCM phase and the rutile metal phase occurs due to subsequent Joule heating. The MCM phase can be regarded as an intermediate nonequilibrium state.

https://arxiv.org/pdf/cond-mat/0608085.pdf

Monoclinic and correlated metal phase in VO(2) as evidence of the Mott transition: coherent phonon analysis.

https://escholarship.org/content/qt2xf0c3gx/qt2xf0c3gx.pdf?t=pxd659

Recent progresses on physics and applications of vanadium dioxide

The metal–insulator transition in vanadium dioxide can be reversibly suppressed to cryogenic temperatures by doping with atomic hydrogen.

Hydrogen stabilization of metallic vanadium dioxide in single-crystal nanobeams

The Raman scattering spectrum of V${\mathrm{O}}_{2}$ is reported between 4.2 and 600\ifmmode^\circ\else\textdegree\fi{}K. In the semiconducting monoclinic phase only sharp phonon structure is observed. The metallic rutile phase exhibits a pronounced softening and damping in the observed phonon spectrum. This evidence supports a recently proposed phenomenological mechanism for the semiconductor-to-metal transition.

Ramakant Srivastava

Papers

Raman Spectrum of Semiconducting and Metallic V O 2

Raman spectra of CrO2 and MoO2 single Crystals