共查询到20条相似文献,搜索用时 31 毫秒
1.
Kecheng Xiao Hideo Ogawa Hideo Suzuki Yasuo Fukui 《International Journal of Infrared and Millimeter Waves》1998,19(1):15-31
A heterodyne receiver based on a 1/3 reduced height rectangular waveguide SIS mixer with two mechanical tuners has been built for astronomical observations of molecular transitions in the 230 GHz frequency band. The mixer used an untuned array (RnCj3, Rn70 ) of four Nb/AIOx/Nb tunnel junctions in series as a nonlinear mixing element. A reasonable balance between the input and output coupling efficiencies has been obtained by choosing the junction number N=4. The receiver exhibits DSB (Double Side Band) noise temperature around 50 K over a frequency range of more than 10 GHz centered at 230 GHz. The lowest system noise temperature of 38 K has been recorded at 232.5 GHz. Mainly by adjusting the subwaveguide backshort, the SSB (Single Side Band) operation with image rejection of 15 dB is obtained with the noise temperature as low as 50 K. In addition, the noise contribution from each receiver component has been studied further. The minimum SIS mixer noise temperature is estimated as 15 K, pretty close to the quantum limit v/k11 K at 230 GHz. It is believed that the receiver noise temperatures presented are the lowest yet reported for a 230 GHz receiver using untuned junctions. 相似文献
2.
Christoph Meingast Frank Gugenberger Georg Roth Miro Haluška Hans Kuzmany 《Zeitschrift für Physik B Condensed Matter》1994,95(1):67-71
The thermal expansion of vapor-grownC
70 single crystals ahs been investigated using high-resolution capacitance dilatometry from 5–380 K. Measurements were made both parallel and perpendicular to the hexagonalc-axis. Three first-order phase transitions which we associate with the consecutive disordering of theC
70 molecules are observed upon heating at 280 K (long-axis spinning), 300 K (long-axis precession) and 355 K (quasi-free rotation), respectively. The highest-temperature transition exhibits a very large (50 K) thermal hysteresis. Powder and single-crystal X-ray diffraction show that the crystals are predominantly hexagonal-close-packed (HCP) with an idealc/a1.63 above 360 K andc/a1.84 at 295 K. 相似文献
3.
M. Weiden J. Köhler G. Sparn M. Köppen M. Lang C. Geibel F. Steglich 《Zeitschrift für Physik B Condensed Matter》1995,98(2):167-169
We present an investigation of the spin-Peierls transition atT
SP=14.5 K in polycrystalline CuGeO3 through specific-heat and thermal-expansion measurements. Clear second-order phase-transition anomalies are found in both properties atT
SP, although only a small entropy of S0.1 Rn2 is released at the transition. Most of the entropy is released atT
SP<T<150 K, where the temperature dependence of the magnetic contribution to the specific heat as well as the thermal expansion exhibit extrema atT
*40 K. These are caused by one-dimensional antiferromagnetic fluctuations along the Cu chains, possibly accompanied by structural fluctuations. Using Ehrenfest's relation, a hydrostatic pressure coefficient (T
SP/p)p0 (0.45±0.06) K/kbar is derived. 相似文献
4.
Physical principles, design and operation characteristics of a negative mass cyclotron resonance maser inp-type germanium are considered in this paper. The formation of anisotropic inverted distributions of negative effective mass heavy holes in strong electric and magnetic fieldsE H [001], resulting in negative conductivity in the millimetre and submillimetre wavelength ranges, is discussed. The generation is observed at 0.9 to 8mm in low compensated germanium samples with the hole concentration rangeN
0 2×1012 to 2×1014 cm–3 at low temperaturesT 25 K in electric fieldsE 40 to 350Vcm–1. The maser frequency is tuned by the magnetic field corresponding to a cyclotron resonance frequency of carriers with an effective mass ofm
c 0.4m
0. The spectral width of the emission in single mode operation does not exceed several megahertz. A pulse duration of 1 to 200 s and a repetition rate off
rep 1 to 200 Hz has been obtained limited by sample heating. Possibilities of improving the maser characteristics inp-Ge as well as in other AIII Bv semiconductors and the perspectives of new frequency tuning methods due to the application of uniaxial stress and magnetic field are demonstrated. 相似文献
5.
The temperature dependence of the majority carrier lifetime was studied in single-crystal PbS films. In n- and p-type films in the range 300-200 ° K, increases exponentially with decreasing temperature, with an activation energy of e O. 17-0. 2 eV. This m dependence is assumed due to the trapping of minority carriers at deep levels. Below 160 ° K in the n-type films, does not depend on the temperature, while in the p-type films, decreases exponentially with an activation energy of 0. 11–0. 14 eV.Translated from Vysshikh Uchebnykh Zavedenii Fizika, No. 6, pp. 64–67, June, 1970. 相似文献
6.
V. N. Brudnyi D. L. Budnitskii M. A. Krivov E. A. Popova 《Russian Physics Journal》1981,24(7):663-666
The absorption spectra of n- and p-type GaAs bombarded with 2-MeV electrons at T=300K were studied in the spectral range from 0.2 eV to Eg. It was found that shallow radiation-defect levels Ec – 0.01 eV and Ec + (0.06–0.1) eV were formed. The structureless character of the absorption in the region h < Eg in electron-bombarded gallium arsenide specimens was shown to be due to the distinctive features of photoionization of deep levels and the strong electron-phonon interaction in this material.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 93–97, July, 1981. 相似文献
7.
The second harmonic pulse train from a mode-locked NdYAG Self-Filtering Unstable Resonator has been compressed by a factor of 2.3 using Raman backscattering. Single pulses of 7.4 picoseconds at=0.68m and with 70 percent success probability have been obtained. 相似文献
8.
C. Nölscher P. Müller H. Adrian M. Lehmann G. Saemann-Ischenko 《Zeitschrift für Physik B Condensed Matter》1981,41(4):291-299
Niobium films on sapphire were reacted in tin-vapour to Nb3Sn with resistance ratiosR(297 K)/R(18.3 K) up to 6 and resistively measured superconducting transition temperaturesT
c
up to 17.93 K. The composition Nb3+z
Sn1–z
H
x
of electrolytically hydrogenated samples was determined depth dependent by Rutherford backscattering of 30 MeV32S and simultaneous detection of recoiled protons. Considerable concentration gradients in the thin layers (0.27 m) were detected. The increase of resistivity with hydrogen content and the change in the temperature dependence of is analyzed. A correlation betweenT
c
and 0= is found: An increase of T
c
=0.2 K at 025cm andx0.03 is followed by a drastic decrease toT
c
<1.1 K at 080cm andx1. TheT
c
vs. 0 andT
c
vs. (T) characteristic correlations are different from universal irradiation or preparation induced correlations. The discrepancies can be interpreted by a stiffening of phonon modes and a band-shifting caused by the hydrogen. 相似文献
9.
W. Görtz E. Gerstenhauer P. Grosse 《Applied Physics A: Materials Science & Processing》1982,27(1):35-38
Undoped tellurium single crystals show at low temperatures (2.2 K) strong photoeffect under submillimeter irradiation. Therefore, tellurium was used as a FIR-detector. For a comparison with other detectors signal-to-noise ratio, response time, and noise equivalent power were determined. Tests were carried out with HCN-laser radiation ( = 337 m, = 30 cm- t). The sensitivity of the Tc-photoconductor is comparable to that of a TGS-bolometer but lower than that of a Ge-bolometer. The Te-detector is much faster than the bolometers. 相似文献
10.
Zaitsev V. V. Kislyakov A. G. Stepanov A. V. Urpo S. Shkelev E. I. 《Radiophysics and Quantum Electronics》2001,44(1-2):36-52
We analyze low-frequency intensity fluctuations of the microwave emission from solar flares at frequencies 22 and 37 GHz. The three microwave bursts of durations of about 1 h, observed at the Metsähovi Radio Observatory (Finland) with the time resolution of 0.1 and 0.05 s, are studied. To obtain spectral-temporal characteristics of the low-frequency fluctuations, we apply the Wigner-Ville method, i.e., the time-lag Fourier transform of the local autocorrelation function of an analytical signal. As a result, we obtain for the first time the dynamical spectra of the low-frequency fluctuations, which are identified as MHD eigenoscillations of coronal magnetic loops. The features of the dynamical spectra testify that several types of low-frequency pulsations are excited in coronal magnetic loops during solar flares: 1) Fast and slow magnetosonic oscillations with periods of 1-1.5 s and 200-280 s, respectively. Fast magnetosonic oscillations appear as pulse trains of duration 100-200 s and have the positive frequency drift d/ dt 0.125 Hz/min and the frequency splitting 0.05 Hz; 2) The eigenoscillations of a coronal magnetic loop as an equivalent electric circuit. The period of these oscillations is about 1 s during the initial stage of a microwave burst and increases gradually up to 4 s during the decay stage of the radio emission; and 3) Intensity modulation of the microwave radiation by a periodic pulse sequence with a period of about 1 s at the burst onset and about 2 s at its end. The parameters of the dynamical spectra and identification of the MHD pulsations allow us to obtain information on the loop parameters, such as the ratio of the loop radius to its length (r/L 0.1), the ratio of the gas pressure to the magnetic-field pressure inside the loop ( 3· 10-3), the ratio of plasma densities outside and inside the loop, and the electric current in the coronal loop (I 1.5 · 1012 A). 相似文献
11.
The effective mass of heavy holes has been determined on the basis of simultaneous analysis of the Hall coefficient and conductivity data obtained in the temperature region 100–300 K on well characterized p-type Hg1–x
CdxTe (x0·2) samples. Its value is 0·7m0. The calculation of intrinsic carrier concentration for 0·19 x0·3 and 50 Kg T 300 K has been carried out using the above value of the effective mass of holes, Hansen's expression for the band gap and momentum matrix element from magneto-optical measurements. 相似文献
12.
J.D. Pedarnig M. Peruzzi H. Salhofer R. Schwödiauer W. Reichl J. Runck 《Applied Physics A: Materials Science & Processing》2005,80(7):1401-1404
Piezoelectric gallium orthophosphate (GaPO4) thickness – shear mode resonators were patterned by F2-laser ablation and employed for gas sensing. The thickness of GaPO4 crystals was reduced from 215 m to 115 m by laser ablation and the piezoelectric fundamental resonance frequency in the thinned region increased thereby from 6 MHz to 12 MHz. The Q values of laser-thinned and pristine resonators in air were Q7000 and Q95000, respectively. The GaPO4 crystals were coated by thin polyimide layers that served as receptor for water vapour. The resonance frequency of coated crystals decreased linearly with increasing level of relative humidity (RH) and the sensitivity for laser-patterned 12 MHz GaPO4 resonators, SRH-98 Hz/%RH, was much larger than for the pristine 6 MHz GaPO4 resonators. PACS 61.10.-i; 68.37.-d; 81.15.Fg 相似文献
13.
M. V. Demidenko 《Russian Physics Journal》1972,15(6):877-880
High-resistance (dark 108 · cm) photosensitive CdS samples (light 102–103 · cm at saturation) were obtained from low-resistance ( 0.1 · cm) nonphotosensitive CdS single crystals by thermal diffusion of Cu, Ag, and In in vacuum at 2 · 10–5 torr. The spectral response of impurity and pure high-resistance photosensitive single-crystal CdS samples was studied in the wavelength range from 0.35 to 3. The short-wave and long-wave limits of the spectral curves, the wavelength range of maximum photoconductivity and the forbidden band width, the depth of impurity levels, the upper edge of the impurity band, and the depth of maximum-density activation centers have been determined from the spectral curves. The results are discussed and compared with published data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 117–121, June, 1972. 相似文献
14.
No Heading The uncertainty in the measured fluorescence decay lifetimes of 30 nm particles of YAG:Cc was used to evaluate the predictions of a novel form of the Heisenberg uncertainty principle suggested by de Sabbata and Sivaram, T t h/k. The worst-case uncertainty in temperature of 4.5 °K (as derived from the relationship between temperature and lifetime) and the measured uncertainty in decay lifetime, 0.45 ns, yielded an internal estimate of T t = 2.0 × 10–9 °K s, which is 263 times larger than /k = 7.6 × 10–12 °K s. An external estimate of T t = 4.5 × 1011 °K s (which is = 6 times /k) is derived from the independently measured uncertainty in the temperature of the sample and the experimentally determined uncertainty in lifetime. These results could be low by a factor of 5.6 if signal averaging must be taken into account. If valid, the findings are consistent with the predictions of this version of the uncertainty principle and they imply the existence of a type of thermal quantum limit. 相似文献
15.
G. Wiese 《Zeitschrift für Physik B Condensed Matter》1991,82(3):453-463
The fine structures of the threshold of subsidiary absorption and parallel pumping, observed on highly polished YIG spheres in 1975 by W. Jantz et al., are discussed with a model which uses instead of plane spin-waves the true modes regarding the shape of the sphere. These spherical modes are magnetostatic modes taking additionally the exchange interaction into account. At the closely spaced resonances of the fine structure of parallel pumping single spherical modes with angular-momentum quantum numberm=0 are excited, whereas at the resonances of subsidiary absorption pairs of spherical modes with big angular-momentum quantum numbers (|m|1000) satisfying the selection rulem+m=1 are excited. The analysis of the fine structure of subsidiary absorption shows that in regimes where the plane wave approximation predicts the excitation of spin-waves with formalk0, modes withk1.5·105 cm–1 have the lowest threshold. This discrepancy is discussed with the effect of surface pit scattering, which increases the threshold of modes withk < 2/pit, where pit is a typical size of the surface pits. Consequently modes withk22/pit instead ofk0 have the lowest threshold. 相似文献
16.
We present the results of studying the development features of nonlinear effects at the initial stage of interaction of powerful HF radio waves with the plasma in the ionospheric F region. Experimental measurements were performed at the Sura heating facility for a wide pump frequency range (4.5–9.0 MHz) and a variety of pulse durations (0.3–100 ms) and effective radiated powers (1–30 MW) at various times of a day. The performed measurements allow us to study the excitation thresholds and time–amplitude characteristics of the ponderomotive self-action of a pump wave as well as the relaxation characteristics of the stimulated electromagnetic emission of the ionosphere as functions of the pump parameters and ionospheric conditions. The measured development features of the ponderomotive parametric instability in the ionospheric plasma are compared with the calculation results. The instability threshold fields (Eth 220 mV/m) and the damping rates (e 450 s-1) of plasma waves, measured under evening-time conditions, are close to the estimates obtained on the assumption of collisional damping of Langmuir turbulence. A significant increase in the threshold field and the damping rate (by factors of up to 3 and 6, respectively) was observed under daytime conditions. In this case, the minimum values of these quantities (Eth 350 mV/m and e 600 s-1) were observed for pump-wave reflection heights of about z 230 km. The measurement and simulation results are indicative of the dominant effect of photoelectrons on the development features of ionospheric plasma turbulence under daytime conditions. We discuss the possibilities of using the developed method for comprehensive monitoring of the parameters of Langmuir turbulence and the background ionospheric plasma. 相似文献
17.
P. M. Lam 《Zeitschrift für Physik B Condensed Matter》1984,57(4):301-305
The statistics of true-self-avoiding walk model on two dimensional critical percolation clusters and lattice animals are studied using real-space renormalization group method. The correlation length exponents 's are found to be
TSAW
pc
0.576 and
TSAW
LA
0.623 respectively for the critical percolation clusters and lattice animals. 相似文献
18.
K. Jungwirth 《Czechoslovak Journal of Physics》1967,17(6):498-505
A study is made of instability at frequencies close to the electron cyclotron frequency
B and its multiples, subject to the presence of two different groups of electrons. It is shown that a mixture of hot and cold electrons (
ph
2
pc
2
) in the region of frequencies s
B, s2 can be unstable with respect to waves of the flute type (k
z=0) with maximum increment
max (
ph/pc).
B, if there exists an interval of transversal velocities
in whichF/
>0. When the curvature of the magnetic field is taken into account, even waves with
B can be unstable in such a plasma. The effect of spatial inhomogeneity of the hot component on flute-type instability and on two-beam cyclotron instability is also examined.The author extends his thanks to A. B. Mikhajlovskij for his valuable comments and discussions. 相似文献
19.
E. Duering R. Blumenfeld D. J. Bergman A. Aharony M. Murat 《Journal of statistical physics》1992,67(1-2):113-121
The current and logarithm-of-the-current distributionsn(i) andn(ln i) on bond diluted two-dimensional random-resistor networks at the percolation threshold are studied by a modified transfer matrix method. Thek th moment (–9k8) of n(ln i) i.e., ln i&k, is found to scale with the linear sizeL as (InL)(k). The exponents (k) are not inconsistent with the recent theoretical prediction (k)=k, with deviations which may be attributed to severe finitesize effects. For small currents, ln n(y)–, yielding information on the threshold below which the multifractality of
(i) breaks down. Our numerical results for the moments of the currents are consistent with other available results. 相似文献
20.
A Fabry-Perot-type interference filter for x-rays is proposed, where thin crystal plates are used as Bragg reflectors at normal incidence. The plates are cut from a perfect silicon crystal leaving a thin gap which may be filled with an appropriate immersion liquid. The calculated resolution of the transmitted beam isE/E5 × 107, corresponding toE4 ×10–4 eV. A possible layout of a high-resolution x-ray spectrometer based on Fabry-Perot-type interferometers is discussed. 相似文献