Light-Induced Metal-Insulator Transition in a Switchable Mirror

Abstract

Rare earth hydride films can be converted reversibly from metallic mirrors to insulating windows simply by changing the surrounding hydrogen gas pressure at room temperature. At low temperatures, in situ doping is not possible in this way as hydrogen cannot diffuse. However, our finding of persistent photoconductivity under ultraviolet illumination offers an attractive possibility to tune yttrium hydride through the T 0 metal-insulator transition. Conductivity and Hall measurements are used to determine critical exponents. The unusually large value for the product of the static and dynamical critical exponents appears to signify the important role played by electron-electron interactions. DOI: 10.1103/PhysRevLett.86.5349 PACS numbers: 71.30.+h, 71.27. +a, 72.15.-v, 73.50. -h Trivalent rare earth hydrides stabilized in thin film form demonstrate spectacular optical and electronic properties [1][2][3][4][5][6][7][8]. Dihydrides such as YH 2 and LaH 2 are good metals; the trihydrides are large gap semiconductors. A thin film of YH x or LaH x can be transformed rapidly from metal to insulator, from shiny mirror to transparent window, simply by changing the surrounding hydrogen gas pressure or an electrolytic cell potential.The continuous and reversible nature of the transformation, as well as the fine-tuning of materials properties afforded by the periodic table, provide a compelling basis for technological development. These same characteristics make switchable mirrors also attractive for scientific scrutiny. The fundamental nature of the metal-insulator transition with hydrogen concentration x is not well understood, although it underlies the attention-getting optical and electronic changes. Pronounced electron-electron interactions have been posited to lead to the opening of the large optical gap [9,10]. If experimentally confirmed as the causative agent [11,12], metal hydride films would be placed in the broad class of highly correlated materials that include transition metal oxides, cuprate superconductors, and colossal magnetoresistance perovskites. Unlike nearly all Mott-Hubbard systems, however, the metalinsulator transition in YH x and LaH x is continuous, unaccompanied by a structural phase transition, and could provide a rare window on critical behavior in the strong electron interactions limit.We systematically investigate the magnetotransport properties of YH x for temperatures T between 0.35 and 293 K in magnetic fields H up to 14 T for chosen values of x. Metal-insulator transitions are properly defined only at T 0, where the electrical conductivity is finite in the metal and zero in the insulator. Hydrogen diffusion is ineffective as a means to drive the transition at low T , but we find that charge carriers created by UV irradiation at T , 1 K persist for days at temperatures as high as 200 K. This persistent photoconductivity enables us to finely tune the conductivity s and the Hall coefficient R H through the quantum critical point. The metal-insulator transition takes place in the hcp g phase...

Crystal structure, electronic structure, and vibrational properties ofMAlSiH(M=Ca,Sr,Ba)<

“…This is notably different compared to hitherto reported transitions. [18][19][20][21][22] Potential applications of the reversible transition MAlSi-MAlSiH, however, will be hampered by the relatively high formation and desorption temperatures of the hydrides. On the other hand, the hydrides MAlSiH could reveal interesting properties inherent to the noncentrosymmetric layer structure.…”

“…17 Reversible hydrogen-induced metal-nonmetal transitions occur rarely and systems displaying this phenomenon have attracted attention as switchable mirror systems when the nonmetal is a large band-gap semiconductor. [18][19][20][21] This is the case for YH 2 -YH 3−x or LaH 2 -LaH 3−x , where the dihydrides are good metals, or Mg 2 Ni-Mg 2 NiH 4 . Although the metallic component absorbs hydrogen near ambient conditions, the transition is usually accompanied with a major reconstruction of the metal-atom arrangement because the nonmetallic state implies the presence of covalently bonded or hydridic hydrogen.…”

Photochromic mechanism in oxygen-containing yttrium hydride thin films: An optical perspective

“…In addition, yttrium hydride films also exhibit photochromic (PC) behavior, i.e., the optical properties of the films change reversibly when illuminated by light of adequate energy (wavelengths in the blue or UV range). Early works by Hoekstra et al [11] have shown a light induced metal insulator transition in yttrium hydrides at low temperature, and Ohmura et al [6,12] accidentally discovered PC behavior in yttrium hydride films subjected to high pressure. Later, Mongstad et al [13,14] reported PC behavior in transparent oxygen-rich yttrium hydride films under atmospheric conditions and at room temperature.…”

Optical properties of d.c. magneto sputtered tantalum and titanium nanostructure thin film metal hydrides

“…The special characteristics of these mirrors is that they can be continuously and reversibly switched between a metallic reflecting state to an insulating transparent state by the absorption of hydrogen. Effect of hydrogen on optical properties used in different applications such as gasochromic (Huiberts et al 1996), electrochromic (Notten et al 1996;Armitage et al 1999) or chemochromic (Vander Sluis et al 1999), photochromic (Hoekstra et al 2001), piezochromic (Wijngaarden et al 2000) and thermochromic (Giebles et al 2002). These materials are also interesting for hydrogen storage applications (Schlapbach and Zuttel 2002;Farangis et al 2003).…”