Quantum-Critical Conductivity Scaling for a Metal-Insulator Transition

Lee, H.-L.; Carini, John P.; Baxter, David V.; Henderson, William; Grüner, G.

doi:10.1126/science.287.5453.633

Cited by 48 publications

(35 citation statements)

References 17 publications

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“…If hyperscaling holds, i.e., m n, then z 6 for YH x . This value is much higher than z 2 derived from dynamical scaling analyses of the amorphous alloy NbSi [21] and the doped semiconductor Si:P [22], both of which lie closer to the strong disorder limit.…”

Section: -9007͞01͞86(23)͞5349(4)$1500mentioning

confidence: 64%

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Light-Induced Metal-Insulator Transition in a Switchable Mirror

et al. 2001

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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...

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Section: -9007͞01͞86(23)͞5349(4)$1500mentioning

confidence: 64%

“…The nine middle curves in Fig. 2 21 , deserve closer examination. Extrapolation to zero temperature, where the conductivity will assume a finite value solely in the metal, requires a linear as opposed to a logarithmic temperature scale.…”

Section: -9007͞01͞86(23)͞5349(4)$1500mentioning

confidence: 96%

Light-Induced Metal-Insulator Transition in a Switchable Mirror

et al. 2001

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“…This functional form has been invoked previously to describe both insulating Si:B [15] and -NbSi alloys [16]. We show in Fig.…”

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confidence: 72%

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

et al. 2002

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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 -...

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“…The primary goals of this work then are to present new data and to draw on results of previous work on other electron glass systems [4][5][6] to look at the insulating side of the MIT using AC conductivity data and a broad range of dopant concentrations in order to come up with a phenomenologically supported phase diagram outlining the general classification scheme for the electrodynamic response of electron glasses. DC conductivity data are presented as well and a side by side comparison of the AC and DC data provides for unique insight.…”

Section: Introductionmentioning

confidence: 99%

Frequency-dependent conductivity of electron glasses

2004

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Results of DC and frequency dependent conductivity in the quantum limit, i.e.hω > kBT , for a broad range of dopant concentrations in nominally uncompensated, crystalline phosphorous doped silicon and amorphous niobium-silicon alloys are reported. These materials fall under the general category of disordered insulating systems, which are referred to as electron glasses. Using microwave resonant cavities and quasi-optical millimeter wave spectroscopy we are able to study the frequency dependent response on the insulating side of the metal-insulator transition. We identify a quantum critical regime, a Fermi glass regime and a Coulomb glass regime. Our phenomenological results lead to a phase diagram description, or taxonomy, of the electrodynamic response of electron glass systems.

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Quantum-Critical Conductivity Scaling for a Metal-Insulator Transition

Cited by 48 publications

References 17 publications

Light-Induced Metal-Insulator Transition in a Switchable Mirror

Light-Induced Metal-Insulator Transition in a Switchable Mirror

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

Frequency-dependent conductivity of electron glasses

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