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 31 毫秒
1.
V H Kulkarni  P V Naik 《Pramana》1989,32(5):619-625
The dispersion relation of electromagnetic waves in the presence of magnetic monopoles of electron mass in a uniform magnetic field is obtained. The waves of the frequencyω in the range ωϱii<ω e ϱa are analysed. It is shown that the monopole charges lead to observable effects. Finally, the results are applied to a typical pulsar.  相似文献   

2.
The microwave photoresistance of a double GaAs quantum well with two occupied size-quantization sub-bands E 1 and E 2 has been studied at the temperatures T = 1.6–4.2 K in the magnetic fields B < 0.5 T. The microwave photoresistance of such a system has been found to have a maximum amplitude when the maximum of the magneto-intersubband oscillations with the number k = (E 2E 1)ℏωc coincides with the maximum or minimum of the ω/ωc oscillations, where ω is the microwave frequency and ωc is the cyclotron frequency. It has been shown that the resonance photoresistance that appears in the kth maximum of the magneto-intersubband oscillations is determined by the condition ℏω/(E 2E 1) = (j ± 0.2)/k, where k and j are positive integers.  相似文献   

3.
We consider the effect of Coulomb interactions on the average density of states (DOS) of disordered low-dimensional metals for temperatures T and frequencies ω smaller than the inverse elastic life-time 1/τ. Using the fact that long-range Coulomb interactions in two dimensions (2d) generate ln2-singularities in the DOS ν(ω) but only ln-singularities in the conductivity σ(ω), we can re-sum the most singular contributions to the average DOS via a simple gauge-transformation. If σ(ω) > 0, then a metallic Coulomb gapν(ω) ∝ |ω|/e 4 appears in the DOS at T = 0 for frequencies below a certain crossover frequency Ω 2 which depends on the value of the DC conductivity σ(0). Here, - e is the charge of the electron. Naively adopting the same procedure to calculate the DOS in quasi 1d metals, we find ν(ω) ∝ (|ω|/Ω 1)1/2exp(- Ω 1/|ω|) at T = 0, where Ω 1 is some interaction-dependent frequency scale. However, we argue that in quasi 1d the above gauge-transformation method is on less firm grounds than in 2d. We also discuss the behavior of the DOS at finite temperatures and give numerical results for the expected tunneling conductance that can be compared with experiments. Received 28 August 2001 / Received in final form 28 January 2002 Published online 9 July 2002  相似文献   

4.
The available astrophysical S(E) factor data for the reaction 7Li (p,α)α at 10 < E < 1000keV exhibit an exponential increase at low energies due to the effects of electron screening. A parametrisation of the data using a non-resonant, direct process and two subthreshold resonances reproduces the data at energies E≥ 100keV, while at lower energies this calculated S b(E) factor curve for bare nuclides drops below the data, which in turn represent the case of electron-shielded nuclides, i.e. the electron-shielded S s(E) factor. The comparison between S b(E) and S s(E) leads to an electron-screening potential energy U e = 350eV, which is much higher than the adiabatic limit of 175eV and not understood at present. The deduced value of S b(0) is considerably smaller than the previously adopted value of 59keV b, significantly increasing the calculated abundance of 7Li in big-bang nucleosynthesis. The Trojan-horse method was applied to the reaction 7Li (p,α)α to determine the energy dependence of the S b(E) factor for 10 < E < 370keV, free from the effects of the Coulomb barrier and electron screening. The THM results are close to the calculated S b(E) curve and suggest that the THM may become a powerful way to obtain improved information on low-energy cross-sections and associated electron-screening effects in a model-independent way. Received: 9 September 2000 / Accepted: 5 December 2000  相似文献   

5.
Boltzmann-transport equation is analytically solved for two-component magnetoplasma using Chapman-Enskog analysis to include collisional diffusion transport having anisotropies in both streaming velocity and temperature components. The modified collisional integrals are analytically solved with flux integrals and perturbed kinetic equation to arrive at drift diffusion velocity and resulting transport coefficients which are markedly affected by both streaming and temperature anisotropy. The early isotropic results are recovered in the limit V 0 = 0 and T = T which reduce to eqs (11.30) and (11.31) of [1] and eqs (2.7) and (2.13) of [2]. The electrical resistivity (η) diminishes sharply in fusion temperature limit kT = 1 keV. The shape of the curves for both electrical resistivity and thermal conductivity is rectangular hyperbolic. However, for low thermal ratio (T /T < 1), the curves are raised up and for high thermal ratio (T /T > 1), they are lowered down the isotropic case (T /T > 1), showing comparatively diminished magnitudes of the quantities.   相似文献   

6.
The return current induced in a plasma by a relativisitc electron beam generates a new electron-ion two-stream instability (return current instability). Although the effect of these currents on the beam-plasma e-e instability is negligible, there exists a range of wave numbers which is unstable only to return current (RC) instability and not to e-e instability. The electromagnetic waves propagating along the direction of the external magnetic field, in which the plasma is immersed, are stabilized by these currents but the e.m. waves with frequencies,ω 2Ω e 2ω pe 2 (Ω e andω pe being cyclotron and plasma frequency for the electrons of the plasma respectively) propagating transverse to the magnetic field get destabilized. Heuristic estimates of plasma heating, due to RC instability and due to decay of ion-acoustic turbulence generated by the return current, are made. The fastest time scale on which the return current delivers energy to the plasma due to the scattering of ion-sound waves by the electrons can be ∼ω pi −1 (ω pi being the plasma frequency for the ions).  相似文献   

7.
An investigation is reported of stimulated two-photon emission by Bose-condensed excitons accompanied by a coherent two-exciton recombination, i.e., by simultaneous recombination of two excitons with opposite momenta leaving unchanged the occupation numbers of exciton states with momenta p≠0. Raman light scattering (RLS) accompanied by a similar two-exciton recombination (or production of two excitons) is also analyzed. The processes under consideration can occur only if a system contains Bose condensate, so their detection can be used as a new method to reveal Bose condensation of excitons. The recoil momentum, which corresponds to a change in the momentum of the electromagnetic field in the processes, is transferred to phonons or impurities. If the recoil momentum is transmitted to optical phonons with frequency ω 0 s , whose occupation numbers are negligible, and the incident light frequency satisfies ω<2Ω, where Ω_=Ω−ω 0 s is the difference frequency and Ω is the light frequency corresponding to the recombination of an exciton with zero momentum, stimulated two-photon emission and RLS with coherent two-exciton recombination give rise to a line at 2Ω_ −ω and an anti-Stokes component at ω+2Ω_, respectively. For ω2Ω_ the RLS spectrum contains Stokes and anti-Stokes components at frequencies ω±2Ω_, whereas stimulated two-photon emission is impossible. Formulas for the cross sections at finite temperatures are obtained for the processes under consideration. Our estimates indicate that a spectral line at 2Ω_−ω, corresponding to the stimulated two-photon emission accompanied by coherent optical phonon-assisted two-exciton recombination can be experimentally detected in Cu2O. Zh. éksp. Teor. Fiz. 115, 1353–1376 (April 1999)  相似文献   

8.
The spectral dependence of the electron-phonon relaxation rate γe−ph(ℏω) in metals is studied in pump-supercontinuum-probe (PSCP) experiments with femtosecond time resolution. Investigation of this spectral dependence, which exhibits a substantial slowing of the relaxation rate γe−ph(ℏω) near the Fermi level E F , using the parametrization γe−ph(ℏω)∝λ〈Ω2〉 (ℏω−E F )2 makes it possible to determine directly the electron-phonon interaction parameter λ〈Ω2〉. The parameter λ〈Ω2〉 for YBa2Cu3O7−δ is analyzed using this method. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 5, 329–332 (10 September 1999)  相似文献   

9.
The low-temperature dc mobility of the two-dimensional electron system localized above the surface of superfluid helium is determined by the slowest stage of the longitudinal momentum transfer to the bulk liquid, namely, by the interaction between the surface and volume excitations of liquid helium, which decreases rapidly with the temperature. Thus, the temperature dependence of the low-frequency mobility is μdc ≈ 8.4 × 10−11 n e T −20/3 cm4 K20/3/(V s), where n e is the surface electron density. The relation T 20/3 E−3 ≪ 2 × 10−7 between the pressing electric field (in kilovolts per centimeter) and temperature (in Kelvins) and the value ω ≲ 108 T 5 K−5 s−1 of the driving-field frequency have been obtained, at which the above effect can be observed. In particular, E ≃ 1 kV/cm corresponds to T ≲ 70 mK and ω/2π ≲ 30 Hz.  相似文献   

10.
We develop the reconstruction of the f(T) gravity model according to the holographic dark energy. T is the torsion scalar and its initial value from the teleparallel gravity is imposed for fitting the initial value of the function f(T). The evolutionary nature of the holographic dark energy is essentially based on two important parameters, Ω V  and ω V , respectively, the dimensionless dark energy and the parameter of the equation of state, related to the holographic dark energy. The result shows a polynomial function for f(T), and we also observe that, when Ω V →1 at the future time, ω V may cross −1 for some values of the input parameter b. Another interesting aspect of the obtained model is that it provides a unification scenario of dark matter with dark energy.  相似文献   

11.
We describe a new method to calculate the VLEED (very low energy electron diffraction) spectra within the Bloch waves approach. The method is based on the variational solution of the Schr?dinger equation for a semi-infinite crystal. Inside the solid the trial LEED functionΦ is a linear combination of propagating and evanescent Bloch waves, which are generated by the inverse ELAPW (extended linear augmented plane waves)-kp method. The trial function is smoothly continuous over the whole space, and it satisfies by construction the equation (Ĥ − E)Φ=0 both in the crystal and in the vacuum half-spaces. In the surface layer the equation δ‖(Ĥ − E)Φ‖=0 is solved. To illustrate the properties of the method we discuss its application to the 1D case. We have performed a self-consistent band structure calculation of the 1T chalcogenide VSe2 and obtained from the first principles the normal-incidence target current spectrum (TCS) for its (0001) surface. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999.  相似文献   

12.
Abstact: The elastic scattering cross sections, σ (E,θ), for the systems He+Ta and He+W have been measured at θlab=165° and E lab=76.1 keV to 3.988 MeV using targets with a thickness of a few atomic layers. The results are smaller than the results given by the Rutherford scattering law, σR(E,θ), due to the effects of electron screening and can be described by σ(E,θ)/σR(E,θ)=(1+Ue/E)−1, where U e is an atomic screening potential energy. The deduced average value, U e=28 ± 3 keV, is consistent with the Moliére- and Lenz-Jensen-models as well as electron binding energies. Received: 25 May 1998  相似文献   

13.
A theory of the shift current induced by direct two-photon and indirect one-photon absorption is developed for noncentrosymmetric crystals. A formula is derived for the microscopic shifts of Bloch electrons induced by two-quantum processes. It is shown that the ratio of the two-photon photocurrent at the photon frequency ω to the photocurrent induced by direct one-photon transitions at the photon frequency 2ω, as compared to the corresponding absorption-rate ratio, contains a large factor {ie152-1}ω/(2{ie152-2}ω − E g), where E g is the bandgap; i.e., these photocurrent can be comparable in order of magnitude. For crystals of T d symmetry, the photocurrents induced by one- and two-photon absorption are compared in terms of polarization dependence.  相似文献   

14.
The g-factor of the first excited state of 93Sr (E = 213 keV, T 1/2 = 4.6 ns) was measured by an on-line TDPAC technique with use of the strong hyperfine field in Fe metal. The Larmor frequency ω L = (2.60 ± 0.15) × 108 rad/s was obtained. The g-factor is derived as g = −0.227 ± 0.013 from g = −ℏω L/B hf μ N. If the spin of the first excited state of 93Sr is assumed to be 3/2, the g-factor is predicted by a simple core-excitation model as g = −0.22, which is in good agreement with the present experimental result.  相似文献   

15.
For the first time submillimetric microwaves (λ<1 mm) are used to observe Azbel' Kaner cyclotron resonance in metals. The very high frequency used (typicallyF≅400 GHz) gives a large value ofωτ (typically 500) and therefore very sharp peaks. The fundamental resonance fieldH c=m * cω/e is rather high (typically 200 KG), so subharmonicsH c/n can be observed at many values ofH in the field region 0–27 KG. If relatively few electrons participate in the resonance and ifω cτ≧50 (ω c=eH/m * c,τ relaxation time) thenChambers has shown that the line shapes are independent of relaxation time while the fractional linewidthΔH/H varies as l/ωτ. For the belly orbit in pure copper the conditions of Chambers' theory are satisfied forH≧20 KG parallel to [111] axis.m * is a minimum andτ=1.8×10−10 s.  相似文献   

16.
A pronounced step-like (kink) behavior in the temperature dependence of resistivity ρ(T) is observed in the optimally doped Sm1.85Ce0.15CuO4 thin films around T sf = 87 K and attributed to the manifestation of strong-spin fluctuations induced by Sm3+ moments with the energy ħωsf = k B T sf ≃ 7 meV. The experimental data are found to be well fitted by the residual (zero-temperature) ρres, electron-phonon ρe-ph(T) = AT, and electron-electron ρe-e(T) = BT 2 contributions in addition to the fluctuation-induced contribution ρsf(T) due to thermal broadening effects (of the width ωsf). According to the best fit, the plasmon frequency, impurity scattering rate, electron-phonon coupling constant, and Fermi energy are estimated as ωp = 2.1 meV, τ 0 −1 = 9.5 × 10−14 s−1, λ = 1.2, and E F = 0.2 eV, respectively. The text was submitted by the authors in English.  相似文献   

17.
For a given domain ω ⋐ ℝ2 with boundary γ = ∂ω, we study the cardinality of the set $ \mathfrak{A}_\eta \left( \Phi \right) $ \mathfrak{A}_\eta \left( \Phi \right) of pairs of numbers (a, b) for which there is a function u = u (a,b): ω → ℝ such that ∇2 u(x) = au(x) + b ⩾ 0 for xω, u| γ = 0, and ||∇u(s)| − Φ(s) ⩽ η for sγ. Here η ⩾ 0 stands for a very small number, Φ(s) = |∇(s)| / ∫ γ |∇v| d γ, and v is the solution of the problem ∇2 v = a 0 v + 1 ⩾ 0 on ω with v| γ = 0, where a 0 is a given number. The fundamental difference between the case η = 0 and the physically meaningful case η > 0 is proved. Namely, for η = 0, the set $ \mathfrak{A}_\eta \left( \Phi \right) $ \mathfrak{A}_\eta \left( \Phi \right) contains only one element (a, b) for a broad class of domains ω, and a = a 0. On the contrary, for an arbitrarily small η > 0, there is a sequence of pairs (a j , b j ) ∈ $ \mathfrak{A}_\eta \left( \Phi \right) $ \mathfrak{A}_\eta \left( \Phi \right) and the corresponding functions u j such that ‖f u j+1‖ − ‖f u j ‖ > 1, where ‖f u j = max x∈ω |f u j (x)| and f u j (x) = a j u j (x) + b j . Here the mappings f u j : ω → ℝ necessarily tend as j → ∞ to the δ-function concentrated on γ.  相似文献   

18.
The differential resistance r xx in a GaAs double quantum well with two occupied size-quantization subbands have been studied at temperatures T = 1.6–4.2 K in magnetic fields B < 0.5 T. It has been found that differential resistance r xx vanishes at the maxima of magneto-intersubband oscillations with an increase in the direct current I dc. It has been shown that the discovered r xx ≈ 0 state appears under the condition 2R c E H/ħωc < 1/2, where R c is the cyclotron radius of electrons at the Fermi level, E H is the Hall electric field induced by the current I dc, and ωc is the cyclotron frequency.  相似文献   

19.
The PHENIX experiment at RHIC has measured ω and ϕ meson production using di-electron decay mode over a psude-rapidity range of |η| ≤ 0.35 and a transverse momentum range of 0 < p T < 5 GeV/c in proton + proton collisions at $ \sqrt s $ \sqrt s = 200 GeV. The spectra of production cross section as a function of p T for ω and ϕe + e show good agreement with other hadornic decay channels ωπ 0 γ, ω 0 π + π and ϕK + K , respectively.  相似文献   

20.
The unparticle has been suggested as a candidate of dark matter. We investigated the growth rate of the density perturbation for unparticle dark matter in the flat Universe. First, we consider the model in which the unparticle is the sole dark matter and find that the growth factor can be approximated well by f=(1+3ω u )Ω u γ , where ω u is the equation of state of unparticle. Our results show that the presence of ω u modifies the behavior of the growth factor f. For the second model where the unparticle co-exists with cold dark matter, the growth factor has a new approximation f=(1+3ω u )Ω u γ +α Ω m and α is a function of ω u . Thus the growth factor of the unparticle is quite different from that of the usual dark matter. This information can help us know more about unparticle and the early evolution of the Universe.  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