首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 93 毫秒
1.
A carrier transport model to explain the high-frequency response in high-speed MQW lasers is described. The ambipolar approximation, which is unsuitable for dealing with the high-speed carrier dynamics in MQW structures, was not adopted for small-signal analysis. The carrier transport effect can be characterized by four time constants: the electron transport time, bmn; the hole transport time, bmp; the electron escape time, wbn; and the hole escape time, wbp. The frequency response was interpreted as the sum of the constant response term due to the fast electron current and the roll-off term due to the slow hole transport time. The ratio of the electron contribution to the total response was proportional to the ratio of electron contribution to the total differential gain, , and reciprocally proportional to n0 = 1 + bmn/wbn. The value of was calculated to be about 0.5 for typical MQW lasers. The roll-off frequency is mainly determined by . The ratio p0 = 1 + bmp/wbp affects the resonant frequency and the damping rate in the high-bias condition.  相似文献   

2.
Simulation has been applied to NaCl-lattice crystals to examine the effects from the strength and relative concentration of point obstacles on sliding-dislocation movement through a composite ensemble of forest dislocations and point obstacles. The range in the parameter = cr p /cr=1.5–2.5, corresponds to the threshold value * of the proportion of forest dislocations in the composite ensemble above which the point obstacles affect the motion of the sliding dislocations through the composite ensemble, where cr and cr p are correspondingly the critical stresses for the passage of sliding dislocations through the one-component ensembles of forest dislocations and point obstacles in the composite. The sum of the squares of cr f and cr corresponds best to the square of the total stress for the passage of sliding dislocations through the composite ensemble.Kaluga Branch, N. É. Bauman Moscow State Technical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 105–111, September, 1993.  相似文献   

3.
The stationary photoconductivity, the photomagnetic effect, and the relaxation kinetics of photoconductivity in n-InAs1–x–ySbxPy crystals (x=0.06, y=0.11) with n0 = 8·1015 and 3·1016 cm–3 were measured and the lifetimes of nonequilibrium current carriers in the temperature interval T=78–295 K were determined. The possible mechanisms of recombination, which limit the lifetimes (radiative R, Auger recombination A, and recombination through centers with Ef=0.13 eV), which, as is demonstrated, are determined by interband recombination processes with RA = RA/(R + A), are calculated theoretically. The contribution of the 0.13 eV recombination centers can be significant when n01014 cm–3.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 52–54, April, 1991.  相似文献   

4.
A qualitative change in the topology of the joint probability densityP(,x), which occurs for strongly colored noise in multistable systems, has recently been observed first by analog simulation (F. Moss and F. Marchesoni,Phys. Lett. A 131:322 (1988)) and confirmed by matrix continued fraction methods (Th. Leiber and H. Riskin, unpublished), and by analytic theory (P. Hänggi, P. Jung, and F. Marchesoni,J. Stat. Phys., this issue). Systems studied were of the classx=–U(x)/x+(t,), whereU(x) is a multistable potential and (t, ) is a colored, Gaussian noise of intensityD, for which =0, and (t) (s)=(D/)exp(–t–s/). When the noise correlation time is smaller than some critical value 0, which depends onD, the two-dimensional densityP(,x) has the usual topology [P. Jung and H. Risken,Z. Phys. B 61:367 (1985); F. Moss and P. V. E. McClintock,Z. Phys. B 61:381 (1985)]: a pair of local maxima ofP(,x), which correspond to a pair of adjacent local minima ofU(x), are connected by a single saddle point which lies on thex axis. When >0, however,the single saddle disappears and is replaced by a pair of off-axis saddles. A depression, or hole, which is bounded by the saddles and the local maxima thus appears. The most probable trajectory connecting the two potential wells therefore does not pass through the origin for >0, but instead must detour around the local barrier. This observation implies that successful mean-first-passage-time theories of strongly colored noise driven systems must necessarily be two dimensional (Hänggiet al.). We have observed these holes for several potentialsU(x): (1)a soft, bistable potential by analog simulation (Moss and Marchesoni); (2) a periodic potential [Th. Leiber, F. Marchesoni, and H. Risken,Phys. Rev. Lett. 59:1381 (1987)] by matrix continued fractions; (3) the usual hard, bistable potential,U(x)=–ax 2/2+bx 4/4, by analog simulations only; and (4) a random potential for which the forcingf(x)=–U(x)/x is an approximate Gaussian with nonzero correlation length, i.e., colored spatiotemporal noise, by analog simulation. There is a critical curve 0(D) in the versusD plane which divides the two topological behaviors. For a fixed value ofD, this curve is shifted toward larger values of 0 for progressively weaker barriers between the wells. Therefore, strong barriers favor the observation of this topological transformation at smaller values of . Recently, an analytic expression for the critical curve, valid asymptotically in the small-D limit, has been obtained (Hänggiet al.).This paper will appear in a forthcoming issue of theJournal of Statistical Physics.  相似文献   

5.
Let be an action of a compact abelian groupG on aC*-algebraA, and assume that the fixed-point subalgebraA is an AF-algebra. We show that if is a closed *-derivation onA commuting with , and the restriction of toA generates a one-parameter group of *-automorphisms, then itself is a generator. In particular, the result applies if is an infinite product action ofG on a UHF algebra. Furthermore, if in this situation 1 and 2 are two derivations both satisfying the hypotheses on , and 1 and 2 have the same restriction toA , then there exists a one-parameter subgroup of the action with generator 0 such thatD(1)D(2)D(0) is a joint core for the three derivations, and 2=1+0 on this core.  相似文献   

6.
We investigated the magnetic structure of NdCu2 by means of neutron diffraction as a function of temperature between 1.5 K and 8 K in zero external field. The diffraction data were obtained on two single crystals with different orientations using the triple-axis-spectrometer TAS6 at the DR-3 reactor at Risø. Two magnetic phases were observed between 1.5K andT N =6.5K. From 1.5 K to 4.1 K the magnetic reflections can be described by the commensurate wave vector =(3/5 0 0) and its higher harmonics 3 and 5. Below 2.5K the structure is completely squared-up. For 4.1 KT6.5 K the magnetic structure is incommensurate with the chemical lattice and can be described by the wave vector=(3/5 0 0) and its higher harmonies 3 and 5M. Below 2.5 K the structure is completely squared-up. For 4.1 K T 6.5 K the magnetic structure is incommensurate with the chemical lattice and can be described by the wave vector *=(0.62 0.044 0). In both phases the Nd-moments are oriented along the easyb-direction.  相似文献   

7.
This paper discuses the , decay process in a field formed by the super-position of two fields: a crossed constant field and and electromagnetic plane-wave field. An expression is derived for the decay probability under the assumption of a nonzero -neutrino mass. The dependence of the decay probability on the lepton spin and on the external-field parameters is analyzed. The variation of the decay probability with the neutrino mass and lepton spin yields information which may be useful for ascertaining the neutrino mass.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 27–32, December, 1985.The authors are indebted to Professor V. R. Khalilov for a formulation of the problem and many valuable comments.  相似文献   

8.
The variation of the convergence time, as a function of the storage capacity is studied numerically for systems ranging in size fromN=1000 toN=16,000 neurons. is found to increase likeexp[–A(c–)] as one nears the critical storage capacity c =0.142=0.002.  相似文献   

9.
The paramagnetic state (+e) in Si and Te was observed in a longitudinal magnetic field. The mean lifetimes of these states were obtained: Si = 1.45(3) s, Te = 12.5(8) s at 290 K, Te = 12(2) s at 250 K.  相似文献   

10.
We consider the mechanism of macroscopic polarization of semiconducting plates owing to the interaction of free carriers with an impurity level, in which role the level of the residual impurity of compensated semiconductors may appear. This mechanism, in combination with the diffusion-drift mechanism of polarization, results in additional dispersion of the real () and imaginary () parts of the dielectric permittivity, this being particularly significant for semiconductors of thickness smaller than the screening length Ls of free carriers. The character of the behavior of and depends on the relation between the Maxwell relation time M and the times of carrier capture: c and ejection e by an impurity center. For cetm and (/Ls) e/c1 the dispersion of and is the same as for thick plates (/Ls1). For c m e and (/Ls)e/c1 the () curve has a characteristic kink in the region 1/e, indicating additional absorption associated with the ejection of carriers into the surface region.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 11–13, October, 1981.  相似文献   

11.
/M1 ( — ,M — ) (= 1/137) - . 0,2% –v/c, , .
Radiative corrections to the electron polarization in the beta decay of a free neutron
An expression for the electromagnetic radiative corrections of the order (=1/137) to the longitudinal polarization degree of the electron in the beta decay of a free neutron is derived in the approximation /M1 ( is the neutron-proton mass difference,M — mass of the neutron). The corrections are not larger than 0·2% and it seems that they cannot explain the deviation of the experimental value for the electron polarization from the value –v/c (measured in the beta decay of nuclei).


. . . . . .  相似文献   

12.
Let : [0, 1][0, 1] be a piecewise monotonie expanding map. Then admits an absolutely continuous invariant measure. A result of Kosyakin and Sandler shows that can be approximated by a sequence of absolutely continuous measures n invariant under piecewise linear Markov maps itn. Each itn is constructed on the inverse images of the turning points of . The easily computable measures n are used to estimate the Liapunov exponent of . The idea of using Markov maps for estimating the Liapunov exponent is applied to both expanding and nonexpanding maps.  相似文献   

13.
For a space-time which admits a closed timelike smooth curve it is estimated that 2 · 10–24 · l 2, where is the real time andl the spatial length associated with the timelike curve, and is the density of material.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 16, No. 9, pp. 33–36, September, 1973.  相似文献   

14.
In the macroscopic electrodynamics (MED) of good conductors (metals) based on Ohm's law j=E, the momentum relaxation time =m/ne2 of the electrons limits the application to electromagnetic (EM) processes with characteristic timest. An interesting physical difficulty occurs in MED since the EM field damping time R=/ of metals is very small compared with the minimum macroscopic time scale, R. Consequently, the damping and propagation of EM waves and pulses in good conductors cannot be correctly described within the frame of conventional MED. New hyperbolic EM wave equations with relaxation and memory are proposed, which no longer exhibit the R deficiency. The latter is caused by Ohm's law, which breaks down for short-time processes, due to neglect of electron inertia. The advantages of the proposed and the disadvantages of the conventional EM wave equations for good conductors are discussed in applications.  相似文献   

15.
A physical framework has been proposed which describes manifestly covariant relativistic evolution using a scalar time . Studies in electromagnetism, measurement, and the nature of time have demonstrated that in this framework, electromagnetism must be formulated in terms of -dependent fields. Such an electromagnetic theory has been developed. Gravitation must also use of -dependent fields, but many references do not take the metric's dependence on fully into account. Others differ markedly from general relativity in their formulation. In contrast, this paper outlines steps towards a -dependent classical intrinsic formulation of gravitation, patterned after general relativity, which we call parametrized general relativity (PGR). Given the existence of a preferred foliation, the Hamiltonian constraint is removed. We find that some nonmetricity in the connection is allowed, unlike in general relativity. Conditions on the allowable nonmetricity are found. Consideration of the initial value problem confirms that the metric signature should normally be O(3, 2) rather than O(4, 1). Following the lead of earlier works, we argue that concatenation (integration over ) is unnecessary for relating parametrized physics to experience, and propose an alternative to it. Finally, we compare and contrast PGR with other relevant gravitational theories.  相似文献   

16.
On the basis of an arbitrary (V, A) structure of the neutral weak ¯ee and LL currents (L=, M0) a study is made of the processes of production in colliding electron-positron beams of pairs of heavy leptons with subsequent decays in accordance with the schemes e+evµv) + +( anything) and e+eM0e+ve) + M0( anything). The energy spectrum and asymmetry of the distribution of the produced muons are investigated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 66–70, February, 1981.  相似文献   

17.
The laser plasma produced by irradiation of a graphite target simultaneously by the first and second harmonics of two Nd3+:YAG lasers has been studied by emission spectroscopy methods. The delay between radiation pulses () varied from 0 to 700 sec. It is established that in the absence of a delay between pulses ( = 0) the increase in the intensity of plasma emission at the wavelengths corresponding to the radiative transitions of the C2 and C3 molecules is of nonradiative character. The plasma produced by laser radiation at the wavelength = 1064 nm exerts its influence on the radiation spectrum of the plasma produced by laser radiation at the wavelength = 532 nm, if the magnitude of the delay between laser pulses does not exceed 30 sec. The most probable reason for this character of influence of the prepulse on the laser plasma radiation spectrum is sublimation of soot particles caused by laser radiation at = 532 nm.  相似文献   

18.
Relations have been derived here between the macroscopic characteristics of liquid metal embrittlement (durability c under creep and strain c prior to rupture under tension) and the parameters which characterize the micromechanism of fracture (surface energy at the crystal bath interface, energy of grain boundaries, temperature, structure of the crystal-bath interface, etc.), on the basis of test data indicating that the subcritical stage of microcrack development governs the fracture process, and on the assumption that transition to supercritical fracture occurs when the crack angle at the tip opens to its critical width c. It is also shown here that, as the rate of subcritical crack development changes by three orders of magnitude, the magnitude of the critical angle c changes only by a factor of 3.0 and may, to the first approximation, be regarded as independent of the bath composition. The values of c and c calculated according to this approximation agree closely enough with values based on tests.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 22–29, July, 1976.  相似文献   

19.
Conductance fluctuations are studied in twodimensional mesoscopic electron system with a two-hold valley degeneracy (n v =2), which corresponds to the inversion layer of Si-MOSFET formed in (1,0,0) plane. It is shown that the intervalley scattering modifies conductance fluctuations depending on the ratio, Min { c , T }/ v , where v = ( – 2)/2 and c , T , and are, respectively, system traversal time, thermal diffusion time, intervalley scattering time and total life time of electrons. Conductance fluctuations are no longer universal and vary from G univ 0.862·e 2/h to {ie223-5} at low temperatures even for isotropic systems. The conductance fluctuations increase with decreasing system size, increasing electron density and increasing intervalley scattering time. The effect of intervalley scattering is essentially the same as that of intersubband scattering as previously reported. At finite temperatures where T c , the intervalley scattering modifies the fluctuations through the change in the energy correlation range to results in the reduction of the conductance fluctuations. In Si-MOSFET formed in (1, 1, 1) plane, wheren v =6, more enhanced fluctuations are expected. Experimental studies are desired on theoretically predicted points.  相似文献   

20.
Field-emission fluctuations of the coadsorption system W(112)Ni-K were investigated by the cross-correlation technique in two perpendicular crystallographic directions at various temperatures. In- and cross-channel movements show a considerable reduction of max which is the time delay corresponding to the cross-correlation function maximum with increasing Ni coverage at a constant potassium probe concentration. max, its activation energy and the signal velocity are discussed in terms of the well-known W(112)Ni surface structure.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号