Crossover from Coulomb glass to Fermi glass in Si:P

Phys. Status Solidi (c)

2008

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At low energy scales charge transport in the insulating Si:P is dominated by activated hopping between the localized donor electron states. Thus, theoretical models for a disordered system with electron‐electron interaction are appropriate to interpret the electric conductivity spectra.With a newly developed technique we have measured the complex broadband microwave conductivity of Si:P from 100 MHz to 5 GHz in a broad range of phosphorus concentration n /nc from 0.56 to 0.95 relative to the critical value nc = 3:5 × 1018 cm–3 corresponding to the metal‐insulator transition driven by doping. At our base temperature of T = 1.1 K the samples are in the zero‐phonon regime where they show a super‐linear frequency dependence of the conductivity indicating the influence of the Coulomb gap in the density of the impurity states. At higher doping n → nc , an abrupt drop in the conductivity power law σ1(ω) ∼ ωα is observed. The dielectric function ε1 increases upon doping following a power law in (1 – n /nc ). Dynamic response at elevated temperatures has also been investigated. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Section: Frequency Dependence Of the Conductivity In The Zero-phonon supporting

confidence: 93%

Influence of electronic correlations on the frequency‐dependent hopping transport in Si:P

Ritz

Phys. Status Solidi (c)

2008

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“…A super-linear conductivity power law was previously observed in Si:As and Si:P by Castner and collaborators 32,33 using resonator techniques at certain frequency within the range of the present work. Our results are in accord with the measurements on similar samples at higher frequencies (30 GHz to 3 THz) using optical techniques 34,35 . In contrast, a sub-linear frequency dependence in the zero-phonon regime has been reported by Lee and Stutzmann 36 based on experiments on Si:B in the microwave range and by Helgren et al 37 using quasi-optical experiments.…”

Section: Application To the Hopping Transport In Si:p A Dynamical Con...supporting

confidence: 92%

Analysis of broadband microwave conductivity and permittivity measurements of semiconducting materials

Ritz

Journal of Applied Physics

2008

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We perform broadband phase sensitive measurements of the reflection coefficient from 45 MHz up to 20 GHz employing a vector network analyzer with a 2.4 mm coaxial sensor which is terminated by the sample under test. While the material parameters (conductivity and permittivity) can be easily extracted from the obtained impedance data if the sample is metallic, no direct solution is possible if the material under investigation is an insulator. Focusing on doped semiconductors with largely varying conductivity, here we present a closed calibration and evaluation procedure for frequencies up to 5 GHz, based on the rigorous solution for the electromagnetic field distribution inside the sample combined with the variational principle; basically no limiting assumptions are necessary. A simple static model based on the electric current distribution proves to yield the same frequency dependence of the complex conductivity up to 1 GHz. After a critical discussion we apply the developed method to the hopping transport in Si:P at temperature down to 1 K.

“…We measured the transmission and phase shift at frequencies below 1 THz ≈ 33 cm −1 with a THz spectrometer using a Mach–Zehnder arrangement and backward wave oscillators as radiation sources 16. Since our sample is big (1 cm 2 ) and transparent, such experiments can be performed down to very low frequencies, of the order of 60 GHz ≈ 2 cm −1 7, 8, 17.…”

Section: Methodsmentioning

confidence: 99%

Observing the anisotropic optical response of the heavy‐fermion compound UNi₂Al₃

Ostertag

Scheffler

Physica Status Solidi (b)

et al. 2010

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The optical conductivity of heavy fermions can reveal fundamental properties of the charge carrier dynamics in these strongly correlated electron systems. Here we extend the conventional techniques of infrared optics on heavy fermions by measuring the transmission and phase shift of THz radiation that passes through a thin film of UNi 2 Al 3 , a material with hexagonal crystal structure. We deduce the optical conductivity in a previously not accessible frequency range, and furthermore we resolve the anisotropy of the optical response (parallel and perpendicular to the hexagonal planes). At frequencies around 7 cm À1, we find a strongly temperature-dependent and anisotropic optical conductivity that -surprisingly -roughly follows the dc behavior.