Pressure-induced insulator-to-metal transition in VO₂ studied by near-infrared pump – mid-infrared probe spectroscopy

The strongly correlated electron material VO₂ shows an insulator-to-metal transition (IMT) when heated above Tc = 340 K accompanied by a lattice transformation from monoclinic to rutile structure. The mechanism of the temperature-driven IMT in VO₂ is very complex and involves a Peierls instability facilitated by a strong Coulomb interaction. However, the hierarchy of these processes during the IMT is still under discussion.

Ultrafast time-resolved techniques allow to study the photo-induced IMT in VO₂ that occurs on a femtosecond time scale. At sufficient pump fluences the insulating gap closes almost instantaneously due to the screening of the Coulomb repulsion [1] and the lattice evolves on a sub-picosecond timescale to a rutile structure indicating the existence of a transient monoclinic metallic phase [2].

On the other hand, application of external pressure also can induce an equilibrium monoclinic metallic phase [3]. Therefore, the mechanism of the pressure-induced IMT in VO₂ must be qualitatively different compared to the temperature-driven IMT. However, until now the information about the electronic band structure of the pressurized metallic phase remains obscure due to incompatibility of photoemission spectroscopy with high-pressure techniques.

Here we use ultrafast near-infrared pump – mid-infrared probe spectroscopy in order to unravel the changes in the electronic structure of VO₂ across the pressure-induced IMT. The nonlinear spectroscopy allows us to extract information about the response of localized and free charge carriers. In this case the non-degenerate pump-probe scheme is essential: The probe photon energy of 0.12 eV is well below the band gap (0.6 eV) at ambient conditions while the pumping with photon energies of 1.5 eV photo-excites additional charge carriers across the band gap.

The probe radiation is focused on a VO₂ single crystal inside a diamond anvil cell to a nearly diffraction limited spot. We measure the transient reflectivity change in mid-infrared induced by the near-infrared pumping. In agreement with previous studies the pump-probe traces indicate the onset of a long-living metallic state when the excitation fluence exceeds a certain threshold 𝛷th [4]. The results for three independent experimental runs of different VO₂ crystals are shown in Figure 1 together with the linear transmissivity of the probe beam. Initially the threshold grows with pressure increase, but at a critical pressure pc of 6-8 GPa a sudden drop is observed. It coincides with the vanishing of the linear transmissivity (measured without pumping) indicating the pressure-induced IMT in the sample. Remarkably, there is a remnant threshold behavior even for pressures above pc. By pumping it is still possible to enhance the conductivity of the pressure-induced metallic phase. Such behavior is fundamentally different from the temperature-driven IMT, where all t2g bands overlap with the Fermi level leading to the vanishing of the pump-probe response in the metallic phase.

We suggest that the pressure-induced changes of the threshold and linear transmission agree well to a scenario of a bandwidth-driven Mott-Hubbard transition [5]. In this scenario of a purely electronic IMT, pressure-induced increase of the bandwidth leads to a spectral weight transfer from the Hubbard bands to a quasiparticle peak at the Fermi level causing metallic conductivity and a gradual filling of the insulating gap. As a large portion of the spectral weight is still located in the Hubbard bands, the number of charge carriers still can be increased by photoexcitation.

Figure 1. Threshold fluence 𝛷th for three samples and linear transmissivity (bottom curve) as functions of pressure.

[1] D. Wegkamp et al., Phys. Rev. Lett. 2014, 113, 216401.
[2] V. R. Morrison et al., Science 2014, 346, 445.
[3] E. Arcangeletti et al., Phys. Rev. Lett. 2007, 98, 196406.
[4] C. Kübler et al., Phys. Rev. Lett. 2007, 99, 116401.
[5] J. M. Braun et al., in preparation (2017).