Region Of Metal-insulator Transition Research Articles

Modeling and performance of vanadium–oxide transition edge microbolometers

The performance of a VO2 thin-film microbolometer has been investigated. The device is operated within 35°C<T<60°C, in the hysteretic metal-insulator transition region. An algebraic hysteresis model has been used to model the resistance-temperature characteristic of the sensor. It accurately describes the resistance versus temperature characteristics of the material. Employing this model, and in conjunction with established bolometer theory, the responsivity of a VO2 film is calculated and compared with experimental data. Superior performance of the device is achievable under conditions of single pulse incident radiation where the operating point remains on the major hysteresis loop. This results in a pronounced responsivity peak within the center of the metal-insulator transition. Continuous periodic excitation, in contrast, leads to a steadily decreasing and much lower sensitivity at higher temperature, due to the formation of minor hysteresis loops and the loop accommodation process.

Electric, magnetic, and calorimetric properties and phase diagram ofPr1−xCaxCoO3(0<~x<~0.55)

Resistivity, dc magnetization, ac susceptibility, and heat-capacity measurements were carried out for polycrystalline ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{CoO}}_{3}$ $(0l~xl~0.55).$ Insulating, metallic, and metal-insulator transition (MIT) regions appear successively with increasing x. The boundary compositions of neighboring regions are between $x=0.1$ and 0.2 and between $x=0.45$ and 0.5. In the insulating region, the sample is an insulator and shows no phase transition in the range of 2--300 K. In the metallic region, the sample is a metal and shows a ferromagnetic ordering below about 80 K. In the MIT region, the sample shows a metal-insulator transition above about 80 K and a ferromagnetic ordering at around 5 K. The electronic and spin states of each phase are discussed based on the experimental results.

FeCl4− doped polyacetylene near the metalinsulator transition: Anomalous magnetoresistance at high fields up to H30T

We report on the anomalous high field magnetoresistance of FeCl 4 - doped polyacetylene films near the metal-insulator transition in the temperature range T = 0.6-1.4K and the magnetic field range H = 0-30T. When doped polyacetylene approaches to the metal-insulator transition region from the metallic side, the magnetoresistance of FeCl 4 - doped polyacetylene shows two positive peaks, which is different from the case of iodine or ClO 4 - doped polyacetylene showing only one broad peak. The results could he understood in terms of the compensation for the internal exchange field of a FeCl 4 - ion by the external high magnetic field.

Suppression of universal conductance fluctuations by an electric field in doped Si(P,B) near the metal–insulator transition

We present results of 1/ f noise measurements at low frequency ( 10 −3<f<10 Hz ) and at low temperatures ( 1 K<T<20 K ) in single crystals of Si doped with P and B. The doping concentration n is close to the critical composition n c of the metal–insulator transition (MIT). We observed that the noise which originates from the universal conductance fluctuation, can be suppressed effectively by an electric field of moderate magnitude at T<20 K . Near the critical region of MIT ( n≈ n c) the suppression is extremely large. We show that this effect can originate by dephasing arising from an electric field in presence of electron–electron interaction.

Phase diagram of partially deuterated κ-(BEDT-TTF)2Cu[N(CN)2]Br

The phase diagram of κ-(BEDT-TTF)2Cu[N(CN)2]Br around the metal–insulator (MI) boundary controlled by partially deuteration and by cooling rate through the 80 K anomaly has been investigated by resistivity measurements under magnetic fields. The humps of resistance were observed at 33.5 K corresponding to the T* anomaly and at 9.5 K with the hysteresis of resistance in the partially deuterated sample for slowly cooled. According to approach to the critical region of MI transition from the metallic side by the increase of cooling rate: (a) the hump of resistance at the T* anomaly is shifted towards a lower temperature and (b) the temperature of the resistance hump with the hysteresis increases. Our results suggest that these anomalies are related to the phase separation of the metallic and insulator phase around the MI boundary.

Overhauser shift of the electron spin resonance line of Si:P at the metal-insulator transition:31Pcontribution

The electron spin resonance (ESR) line of delocalized electron spins shifts upon saturation due to the hyperfine interaction with the dynamically polarized nuclear spins. The ${}^{31}\mathrm{P}$ contribution to the ESR line's Overhauser shift is separated for phosphorus doped silicon (Si:P) in the concentration range $(2.7--7.3)\ifmmode\times\else\texttimes\fi{}{10}^{18}/{\mathrm{cm}}^{3}$ covering the metal-insulator transition. The Overhauser shift profiles recorded versus ${}^{31}\mathrm{P}$ nuclear magnetic resonance (NMR) frequency are nonsymmetrical and vary with P concentration. With increasing P content, their width decreases, the maximum increases, and shifts towards the bare ${}^{31}\mathrm{P}$ nuclear Zeeman frequency. These data are compared with published results obtained by conventional ${}^{31}\mathrm{P}$ spin-echo NMR and an early Overhauser shift measurement for Si:P. Most remarkable is the continuous variation of the frequency position of the Overhauser shift maximum across the metal-insulator transition region.

Investigation of Magnetoresistance in Crystalline Semiconductors in the Metal–Insulator Transition Region under Hydrostatic Compression

Investigation of Magnetoresistance in Crystalline Semiconductors in the Metal–Insulator Transition Region under Hydrostatic Compression

Investigation of Magnetoresistance in Crystalline Semiconductors in the Metal-Insulator Transition Region under Hydrostatic Compression

Investigation of Magnetoresistance in Crystalline Semiconductors in the Metal-Insulator Transition Region under Hydrostatic Compression

Metal-insulator transition inRbC60polymer fulleride studied by ESR and electron-spin relaxation

The ESR intensity, line shape, and longitudinal electron-spin relaxation in the polymer phase of the ${\mathrm{RbC}}_{60}$ fulleride are investigated in the temperature range $4.2&lt;T&lt;300 \mathrm{K}.$ Most attention is focused on the metal-insulator transition region (25--50 K). It is found that below 50 K the ESR line can be separated into two Lorentzian components ascribed to conduction electrons and some localized paramagnetic centers (with concentration of about 0.03 per formula unit) with allowance made for the relaxation bottleneck. The decrease of the conduction-electron susceptibility obeys an activation law with the characteristic energy $\ensuremath{\Delta}{/k}_{B}=80\ifmmode\pm\else\textpm\fi{}10 \mathrm{K}$ related to the opening of a gap $2\ensuremath{\Delta}\ensuremath{\approx}100 {\mathrm{cm}\mathrm{}}^{\mathrm{\ensuremath{-}}1}.$ The same quantity is found by analyzing both longitudinal and transverse relaxation caused by fluctuations of internal fields with correlation time ${\ensuremath{\tau}}_{c}\ensuremath{\propto}\mathrm{exp}(2\ensuremath{\Delta}{/k}_{B}T).$ Below 25 K, the temperature dependencies of the linewidth and the relaxation times change abruptly, revealing the development of a new ordered state. The nature of this state is discussed.

Mid-infrared properties of a VO2 film near the metal-insulator transition.

A ${\mathrm{VO}}_{2}$ film was grown on a sapphire(0001) substrate using pulsed laser deposition. The film showed a first-order metal-insulator (MI) transition and its dc conductivity started to increase drastically near 68 \ifmmode^\circ\else\textdegree\fi{}C and changed by three orders of magnitude. Mid-infrared transmittance and reflectance spectra of the ${\mathrm{VO}}_{2}$ film were measured between 20 \ifmmode^\circ\else\textdegree\fi{}C and 90 \ifmmode^\circ\else\textdegree\fi{}C. Using the intensity transfer-matrix method, the frequency-dependent dielectric constant ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$(\ensuremath{\omega}) and the conductivity ${\mathrm{\ensuremath{\sigma}}}_{\mathit{f}}$(\ensuremath{\omega}) of the film were obtained between 1600 and 4000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ from the measured transmittance and reflectance spectra. With the ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$(\ensuremath{\omega}) and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{f}}$(\ensuremath{\omega}) spectra, mid-infrared properties of the ${\mathrm{VO}}_{2}$ film near the MI transition region were investigated in detail. Above 78 \ifmmode^\circ\else\textdegree\fi{}C, ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$(\ensuremath{\omega})0 and d${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$/d\ensuremath{\omega}g0, which is a typical metallic behavior. In particular, ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$(\ensuremath{\omega}) and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{f}}$(\ensuremath{\omega}) at 88 \ifmmode^\circ\else\textdegree\fi{}C were analyzed in terms of extended Drude model in which the frequency-dependent scattering rate and the effective mass could be obtained. The mean free path of charge carriers in the dc limit was estimated to be larger by an order of magnitude than the previously reported value, i.e., 4 \AA{}. Below 74 \ifmmode^\circ\else\textdegree\fi{}C, ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$(\ensuremath{\omega})g0 and d${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$/d\ensuremath{\omega}\ensuremath{\approxeq}0, which is characteristic of an insulator. Interestingly, ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$ in the insulating region increased as the temperature approached the MI transition temperature. To explain this anomalous behavior, the MI transition of the ${\mathrm{VO}}_{2}$ film was modeled with coexistence of metallic and insulating domains and their dynamic evolution. Then the behaviors of ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$(\ensuremath{\omega}) and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{f}}$(\ensuremath{\omega}) were explained using the effective medium approximation, which is a mean-field theory predicting a percolation transition. This work clearly demonstrates that the transport and optical properties near the MI transition region are strongly influenced by the connectivity of the metallic domains. \textcopyright{} 1996 The American Physical Society.

Raman-active intramolecular CH2 and C=C vibrations and microwave surface impedance studies of the organic superconductor kappa -

We report a systematic Raman study of intramolecular vibrations for single crystals of the organic superconductor \ensuremath{\kappa}-(BEDT-TTF${)}_{2}$Cu(N(CN${)}_{2}$)Br in both the normal and superconducting states. The study has focused on the C=C stretching vibrations and the ${\mathrm{CH}}_{2}$ scissoring modes. From the frequency of the C=C-ring-stretching Raman mode ${\ensuremath{\nu}}_{3}$(${\mathit{A}}_{\mathit{g}}$) we estimate the average charge transfer \ensuremath{\rho}=0.4 on the BEDT-TTF donor at 295 K while a metal-insulator transition region around 190 K is confirmed by a frequency discontinuity of the C=C ring vibrations. Frequency of ${\mathrm{CH}}_{2}$ vibrational modes show a discontinuity at ${\mathit{T}}_{\mathit{c}}$ that indicates a selective change in the interaction between the ethylene groups of the donor molecule BEDT-TTF and the anion (CU(N(CN${)}_{2}$)Br${)}^{\mathrm{\ensuremath{-}}}$ while the frequency of the C=C-ring-stretching mode is blue shifted just above ${\mathit{T}}_{\mathit{c}}$, suggesting a possible renormalization of the electron-molecular interaction. Also, in order to complement our Raman experiments and to appreciate both the crystal quality and the nature of the superconducting state in our studied samples, we present measurements, as a function of temperature, of the microwave surface impedance.

Transport in polyaniline near the critical regime of the metal-insulator transition.

Critical behavior of the electrical conductivity near the disorder-induced metal-insulator (MI) transition has been observed in polyaniline (PANI) doped with camphor sulfonic acid (CSA). The temperature dependence of the resistivity (\ensuremath{\rho}) depends on the degree of disorder present in the PANI-CSA films. In the most metallic samples the resistivity is nearly temperature independent; whereas in the critical region of MI transition, \ensuremath{\rho}(T) is characterized by a power-law temperature dependence, \ensuremath{\rho}(T)\ensuremath{\propto}${\mathit{T}}^{\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\beta}}}$. Resistivity ratios \ensuremath{\rho}(1.4 K)/\ensuremath{\rho}(300 K) as low as 1.6 have been observed in the most metallic samples of PANI-CSA. In the metallic regime, the conductivity is characterized by \ensuremath{\sigma}(T)=\ensuremath{\sigma}(0)+${\mathit{mT}}^{1/2}$ at low temperatures; the dependence of m(H) on magnetic field provides information on the role of electron-electron interactions in the transport near the MI transition. The ${\mathit{T}}^{\mathrm{\ensuremath{-}}1}$ dependence found for inelastic-scattering time (${\mathrm{\ensuremath{\tau}}}_{\mathrm{in}}$) is in agreement with that predicted for metallic systems near the MI transition. For the samples initially in the critical regime, a magnetic field of 8--10 T induces the transition to variable-range hopping. The typical localization length in PANI-CSA just on the insulating side of the MI transition is about 80--130 \AA{}. The magnitude of the positive magnetoresistance increases considerably as the system moves from the metallic to the insulating regime. The magnitude and quasilinear temperature dependence of the thermopower near the critical regime of the MI transition is typical of that expected for a metal.

Electronic states near the metal-insulator transition in oxide superconductors

Abstract The metal-insulator transition region of high Tc superconducting oxides is studied in the p-d mixing model. A new narrow band arises spontaneously inside the charge transfer gap due to a many-body effect via spin fluctuations and it broadens with hole doping. Various observed features of the optical conductivity are interpreted in terms of intra- and inter-band transitions involving the new narrow band. The carriers in this band may be responsible for superconductivity.

A new narrow band of highly correlated electrons in oxide superconductors

The electronic state in the metal-insulator transition region of the high T c superconducting oxides is theoretically studied using the d-p mixing model. It is found that a new narrow band arises spontaneously inside the charge transfer gap due to a many body effect, and develops with hole doping. The formation of the band explains many unusual experimental results in the normal state of the oxides.

Optical Absorptions in a Half-Filled Narrow Band

The optical absorptions in the system with a single half-filled narrow band are studied in the metal-insulator transition region. It is shown that the system shows a strong absorption near the frequency \(I/\hbar\) for both the metallic and insulating states, where I is the intra-atomic Coulomb repulsion.