Quantum Fluctuations and the Closing of the Coulomb Gap in a Correlated Insulator

Abstract

The ''switchable mirror'' yttrium hydride is one of the few strongly correlated systems with a continuous Mott-Hubbard metal-insulator transition. We systematically map out the low temperature electrical transport from deep in the insulator to the quantum critical point using persistent photoconductivity as a drive parameter. Both activated hopping over a Coulomb gap and power-law quantum fluctuations must be included to describe the data. Collapse of the data onto a universal curve within a dynamical scaling framework (with corrections) requires z 6:0 0:5, where and z are the static and dynamical critical exponents, respectively. DOI: 10.1103/PhysRevLett.89.276402 PACS numbers: 71.30.+h, 71.27.+a, 72.20.-i, 73.50.-h The metal-insulator (MI) transition is only defined properly at temperature T 0, where the electrical conductivity is identically zero in the insulator and assumes a finite value in the metal. Nonetheless, experiments at finite temperature can reveal fundamental properties of the transition by probing quantum fluctuations in the immediate vicinity of the T 0 critical point [1]. Quantum fluctuations inextricably link the static and dynamical response [2], and have led to predictions for new exponents and scaling forms [3] for T; n on both sides of the MI transition. Here n is the distance from the quantum critical point for a transition tuned by electron density, pressure, magnetic field, or any such nonthermal means.The T 0 MI transition in the limit of strong electronelectron correlations has remained resistant to both theoretical understanding and experimental characterization. Progress in this direction would be welcome for illuminating the Mott-Hubbard MI transition itself as well as for shedding light on the physics of cuprate superconductors, colossal magnetoresistance perovskites, rare earth actinides, and transition metal oxides. Moreover, systematic studies of the development of correlations and fluctuations in the insulator are sorely lacking when compared to investigations of the metal. We address these shortcomings through a high-resolution study of the insulating state in yttrium hydride as we approach the T 0 MI transition through successive illuminations under ultraviolet light. Unlike most highly correlated materials, YH x does not have a first-order structural phase transition that cuts off the critical behavior. It retains a continuous Mott-Hubbard MI transition, permitting full access to the quantum critical point.Yttrium hydride and lanthanum hydride are the original ''switchable mirrors' ' [4] with remarkable electronic and optical properties [5]. The reversible transition from shiny, metallic dihydride to transparent, insulating trihydride can be triggered at room temperature simply by changing the surrounding hydrogen gas pressure or an electrolytic cell potential [6]. A wide range of possible applications follows, from solid state displays to smart windows. A key aspect of the technology is the opening of an optical gap of order 3 eV in the insulator [7] -a Hubbard gap -...

Metal-insulator transition in a semiconductor nanocrystal network

“…Results of a previous study of near-MIT transport in ZnO NCs appeared consistent with vz = 2 ( 7 ), although the present data suggest that this was due to underestimated distance from the MIT (see note S2). High vz has been observed previously in materials considerably different from ours, namely, the highly correlated Mott-Hubbard systems NiS 2− x Se x and YH x , which exhibited vz ≈ 5 ( 30 ) and vz ≈ 6 ( 31 ), respectively. Currently, we are unable to identify the cause of high vz in ZnO NC networks, although we note that the critical frequency exponent of σ—and hence the critical temperature exponent, μ/ vz —depends on dimensionality ( 32 , 33 ).…”

Thin‐Film Metal Hydrides

“…The insulating nature of the solid at this high vacancy concentration of 0.2 per 3 H is a first hint for a very localized, that is, correlated, interaction of electrons (Mott's criterion). [60,61] After the electron is excited from the octahedral vacancy site, it will occupy the lowest Y 4d orbitals in the conduction band. Several experimental papers underlined these ideas, [45,62] however, it was Raman spectroscopy on YH x thin films, providing direct evidence for the correlation models.…”

Strong Correlations inYH3−δEvidenced by Raman Spectroscopy

“…Infrared Fourier transform measurements by Rode et al [19] determine a significant charge transfer from yttrium to hydrogen of approximately 0:5 e ÿ per hydrogen atom. Electrical conductivity data of Hoekstra et al [20] and Roy et al [21] over a wide temperature and hydrogen concentration range both in the metallic and insulating phases collapse onto a universal curve with unusually large critical exponents, pointing to the important role played by electron correlations. Angular resolved photoemission spectroscopy data for YH 3 by Hayoz et al [22] have also been interpreted in terms of a strongly correlated electron system.…”