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Register a new userOn the metal-insulator-transition in vanadium dioxide
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Vanadium dioxide (VO$_2$) undergoes a metal-insulator transition (MIT) at 340 K with the structural change between tetragonal and monoclinic crystals as the temperature is lowered. The conductivity $sigma$ drops at MIT by four orders of magnitude. The low-temperature monoclinic phase is known to have a lower ground-state energy. The existence of a $k$-vector ${boldsymbol k}$ is prerequisite for the conduction since the ${boldsymbol k}$ appears in the semiclassical equation of motion for the conduction electron (wave packet). Each wave packet is, by assumption, composed of the plane waves proceeding in the ${boldsymbol k}$ direction perpendicular to the plane. The tetragonal (VO$_2$)$_3$ unit cells are periodic along the crystals $x$-, $y$-, and z-axes, and hence there are three-dimensional $k$-vectors. The periodicity using the non-orthogonal bases does not legitimize the electron dynamics in solids. There are one-dimensional ${boldsymbol k}$ along the c-axis for a monoclinic crystal. We believe this decrease in the dimensionality of the $k$-vectors is the cause of the conductivity drop. Triclinic and trigonal (rhombohedral) crystals have no $k$-vectors, and hence they must be insulators. The majority carriers in graphite are electrons, which is shown by using an orthogonal unit cell for the hexagonal lattice.
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We use polarization- and temperature-dependent x-ray absorption spectroscopy, in combination with photoelectron microscopy, x-ray diffraction and electronic transport measurements, to study the driving force behind the insulator-metal transition in VO2. We show that both the collapse of the insulating gap and the concomitant change in crystal symmetry in homogeneously strained single-crystalline VO2 films are preceded by the purely-electronic softening of Coulomb correlations within V-V singlet dimers. This process starts 7 K (+/- 0.3 K) below the transition temperature, as conventionally defined by electronic transport and x-ray diffraction measurements, and sets the energy scale for driving the near-room-temperature insulator-metal transition in this technologically-promising material.
Vanadium dioxide(VO$_2$) is a paradigmatic example of a strongly correlated system that undergoes a metal-insulator transition at a structural phase transition. To date, this transition has necessitated significant post-hoc adjustments to theory in order to be described properly. Here we report standard state-of-the-art first principles quantum Monte Carlo (QMC) calculations of the structural dependence of the properties of VO$_2$. Using this technique, we simulate the interactions between electrons explicitly, which allows for the metal-insulator transition to naturally emerge, importantly without ad-hoc adjustments. The QMC calculations show that the structural transition directly causes the metal-insulator transition and a change in the coupling of vanadium spins. This change in the spin coupling results in a prediction of a momentum-independent magnetic excitation in the insulating state. While two-body correlations are important to set the stage for this transition, they do not change significantly when VO$_2$ becomes an insulator. These results show that it is now possible to account for electron correlations in a quantitatively accurate way that is also specific to materials.
In vanadium dioxide, the interplay between coherent lattice transformation and electronic correlation drives an insulator-to-metal transition (IMT). This phase commutation can be triggered by temperature, pressure, doping or deposition of optical energy. Here we demonstrate that an atomically-strong terahertz electric field initiates a metastable ultrafast IMT in vanadium dioxide without a concomitant lattice transformation. The free-space terahertz field acts as off-resonant excitation with photon energy below the lattice phonons and the interband transitions. Differently from optical and infrared excitation, terahertz interaction leads to a full IMT by interband Zener tunneling with a negligible entropy deposition. In previous experiments the temporal dynamics of IMT in VO2 could be only indirectly inferred. We disentangle the electronic and lattice contributions to the IMT on a sub-picosecond timescale. Near the critical temperature the IMT becomes dissipative and the terahertz field concludes the lattice-assisted metallic nucleation initiated by heating. The method of strong-field induced phase transition presented here is applicable to a wide class of strongly correlated systems and will enable the discovery of novel metastable phases.
We present a detailed infrared study of the insulator-to-metal transition (IMT) in vanadium dioxide (VO2) thin films. Conventional infrared spectroscopy was employed to investigate the IMT in the far-field. Scanning near-field infrared microscopy directly revealed the percolative IMT with increasing temperature. We confirmed that the phase transition is also percolative with cooling across the IMT. We present extensive near-field infrared images of phase coexistence in the IMT regime in VO2. We find that the coexisting insulating and metallic regions at a fixed temperature are static on the time scale of our measurements. A novel approach for analyzing the far-field and near-field infrared data within the Bruggeman effective medium theory was employed to extract the optical constants of the incipient metallic puddles at the onset of the IMT. We found divergent effective carrier mass in the metallic puddles that demonstrates the importance of electronic correlations to the IMT in VO2. We employ the extended dipole model for a quantitative analysis of the observed near-field infrared amplitude contrast and compare the results with those obtained with the basic dipole model.
Amorphous vanadium dioxide (VO$_{2}$) films deposited by atomic layer deposition (ALD) were crystallized with an ex situ anneal at 660-670 ${deg}$C for 1-2 hours under a low oxygen pressure (10$^{-4}$ to 10$^{-5}$ Torr). Under these conditions the crystalline VO$_{2}$ phase was maintained, while formation of the V$_{2}$O$_{5}$ phase was suppressed. Electrical transition from the insulator to the metallic phase was observed in the 37-60 ${deg}$C range, with a R$_{ON}$/R$_{OFF}$ ratio of up to about 750 and critical transition temperature of 7-10 ${deg}$C. Electric field applied across two-terminal device structures induced a reversible phase change, with a room temperature transition field of about 25 kV/cm in the VO$_{2}$ sample processed with the 2 hr long anneal. Both the width and slope of the field induced MIT hysteresis were dependent upon the VO$_{2}$ crystalline quality.
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