Millimeter Wave Spectral Range Research Articles

Infrared Detector Arrays for Astronomy

Use of infrared detector arrays in astronomy began roughly 20 years ago, and our detection capabilities in parts of this spectral range have doubled about every seven months since then. A variety of approaches are now used for detector arrays operating from 1 μm to 1 mm and beyond. They include direct hybrid arrays of InSb and HgCdTe photodiodes that operate from 0.6 μm to 5 μm, and of Si:As impurity band conduction detectors from 5 μm to 28 μm; a number of approaches to photoconductive detector arrays in the far-infrared; and bolometer arrays read out by transistors or superconducting devices in the far-infrared through millimeter-wave spectral range. The underlying principles behind these approaches are discussed. The application of these principles is illustrated through detailed discussion of a number of state-of-the-art detector arrays.

Charge-density-wave dynamics in (Ta1-xNbxSe4)2I alloys.

Frequency-dependent conductivity and phase-resolved harmonic-mixing studies are reported on alloys of the linear-chain compound (${\mathrm{TaSe}}_{4}$${)}_{2}$I with Nb in the microwave and millimeter-wave spectral range. The frequency-dependent conductivity is evaluated in the charge-density-wave (CDW) state, and the collective-mode response is analyzed in terms of a harmonic-oscillator response with an effective mass ${\mathit{m}}^{\mathrm{*}}$, a pinning frequency ${\mathrm{\ensuremath{\omega}}}_{0}$, and relaxation time ${\mathrm{\ensuremath{\tau}}}^{\mathrm{*}}$ as parameters. We find that the magnitude, temperature, and concentration dependence of the effective mass ${\mathit{m}}^{\mathrm{*}}$ is well described in terms of the mean-field description of the dynamical CDW response. The pinning frequency ${\mathrm{\ensuremath{\omega}}}_{0}$ increases with increasing impurity concentration x, and the concentration dependence is suggestive of strong-impurity pinning. We also evaluate the temperature and concentration dependence of the relaxation time ${\mathrm{\ensuremath{\tau}}}^{\mathrm{*}}$, and compare this parameter with various models.

Spin-density-wave dynamics in (TMTSF)2PF6.

The electrodynamical response of the spin-density-wave state is examined in (TMTSF${)}_{2}$${\mathrm{PF}}_{6}$ by conducting experiments in the microwave and millimeter-wave spectral range. We present the first complete optical spectrum of the ground state, and show that both the collective-mode response and single-particle excitations contribute to the frequency-dependent conductivity. The analysis of the ac response leads to a significant spin-density-wave mass enhancement.

Relaxation-time enhancement in the heavy-fermion systems CePd3 and UPt3.

The frequency dependence of the electrical conductivity was measured at microwave- and millimeter-wave frequencies in the heavy-fermion materials ${\mathrm{CePd}}_{3}$ and ${\mathrm{UPt}}_{3}$. Although the conductivity is independent of the frequency at high temperatures, a substantial deviation from the dc conductivity develops in the low-temperature, ``coherent'' regime. The observed dependence on frequency agrees with a Drude expression ${\mathrm{\ensuremath{\sigma}}}_{1}$(\ensuremath{\omega})=${\mathrm{\ensuremath{\sigma}}}_{\mathit{dc}}$/(1+${\mathrm{\ensuremath{\omega}}}^{2}$${\mathrm{\ensuremath{\tau}}}^{\mathrm{*}2}$), incorporating a renormalized relaxation time ${\ensuremath{\tau}}^{\mathrm{*}}$ typically around ${10}^{\mathrm{\ensuremath{-}}12}$ s. From ${\ensuremath{\tau}}^{\mathrm{*}}$ and the measured ${\ensuremath{\sigma}}_{\mathrm{dc}}$, the renormalized plasma frequency, ${\mathrm{\ensuremath{\omega}}}_{\mathit{p}}^{\mathrm{*}}$=(${\mathrm{\ensuremath{\sigma}}}_{\mathit{dc}}$/${\mathrm{\ensuremath{\epsilon}}}_{0}$${\mathrm{\ensuremath{\tau}}}^{\mathrm{*}}$${)}^{1/2}$ is evaluated. By comparison of ${\ensuremath{\omega}}_{p}^{\mathrm{*}}$ with the plasma frequency obtained at optical frequencies, where renormalization effects do not occur, an enhanced mass is extracted. Similar results are found by comparison of ${\ensuremath{\omega}}_{p}^{\mathrm{*}}$ with the linear specific-heat coefficient. The enhanced relaxation time is approximately given by the expression ${\ensuremath{\tau}}^{\mathrm{*}}$/\ensuremath{\tau}=${m}^{\mathrm{*}}$/${m}_{e}$, where \ensuremath{\tau} and ${m}_{e}$ refer to the unrenormalized quantities, supporting the conjectures of Varma and Fukuyama, and the theoretical models advanced by Millis and co-workers, Auerbach and co-workers, and Coleman. An internally consistent analysis of \ensuremath{\sigma}(\ensuremath{\omega}) in terms of an enhanced relaxation time also suggests that a frequency-dependent density of states does not play a dominant role in the frequency-dependent response measured in the millimeter-wave spectral range. Possible Fermi-liquid and low-energy density-of-state contributions to the frequency-dependent conductivity are also discussed.

Frequency-dependent transport properties of UPt3.

Resistivity [\ensuremath{\rho}(\ensuremath{\omega})] measurements are reported in the millimeter-wave spectral range over the temperature interval from 1.3 to 300 K in the heavy-fermion compound ${\mathrm{UPt}}_{3}$. Below 148 GHz the low-temperature (T4 K) data are in agreement with a Drude expression, with an enhanced relaxation time. The effective mass is derived from \ensuremath{\rho}(\ensuremath{\omega}) and is compared with the thermodynamic effective mass.