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 21 毫秒
1.
Using a model previously introduced to describe spin-spin asymmetries in large angle elastic proton-proton scattering, we have computed the helicity density matrix of ρ vector mesons produced in πp→ρp interactions, at 90° in the c.m. system. Contrary to most other schemes, we obtain some nondiagonal elements sizeably different from zero; when comparing the ρ? decay angular distribution with experimental data we have an excellent agreement. We also give predictions for the case of ρ+ production.  相似文献   

2.
Analysis of Soffer's size-effect theory for electrical resistivity shows, for measurements in such a T range for which the temperature dependent portion of the resistivity, ρi, is always much smaller than the residual bulk resistivity ρ(0) of the metal studied, that while size-effects leave the essential T dependence of ρi unchanged, it may increase its absolute value and the observed residual resistivity ρ(0), thus explaining recent results of Caplin et al. This also corrects the general conclusion arrived at by the latter authors, i.e. that the T dependence of ρ of a metal foil of given residual resistivity is the same as that of a bulk sample of the same residual resistivity provided that the latter is governed by impurity scattering, as being true for a narrow T range only, i.e. for which ρi(T) ? ρ(0). However, for this T range a procedure is outlined which allows one to extract values of the surface specularity parameter pS and also ρ of the metal foils studied.  相似文献   

3.
We show that the Wigner function W = Tr(△ρ) (an ensemble average of the density operator ρ, △ is theWigner operator) can be expressed as a matrix element of ρ in the entangled pure states. In doing so, converting fromquantum master equations to time-evolution equation of the Wigner functions seems direct and concise. The entangledstates are defined in the enlarged Fock space with a fictitious freedom.  相似文献   

4.
Measurements of the photoproduction processes γρ→ρ+ n and γρ→ρ-Δ++ (1236) are reported in the energy range 2.8 to 4.8 GeV. The data show shrinkage of the differential cross section in this energy region for the process γρ→ρ-Δ++ (1236); no shrinkage is observed for the ρ+ n process. The energy dependences of the ρ+ n and ρ-Δ++ (1236) total cross sections are much steeper than current model prediction. The ρ spin density matrices for each process are also presented.  相似文献   

5.
A. de Santis 《Physica A》1976,83(2):425-429
The predictions of the Van der Waals model for the self-diffusion in Kr are compared with the experimental results at 220 K and for 0.7ρc ? ρ ? 2.1ρc. The Van der Waals model fails to represent the Kr data while at the same temperature in the case of CH4 this model works quite well for ρc < ρ ? 3ρc.  相似文献   

6.
A. Wehrl 《Foundations of Physics》1979,9(11-12):939-946
We investigate to what extent theorems about quantum mechanical or classical entropy can be generalized to functionals of the type ρ→Tr f(ρ), or ψ→∫f(ψ)dμ, respectively, wheref is an arbitrary concave function.  相似文献   

7.
ABSTRACT

Explicit analytical expressions are presented for the density derivative, ?gHS(R; ρ)/?ρ, of the Percus–Yevick approximation to the hard-sphere radial distribution function for R ≤ 6σ, where σ is the hard-sphere diameter and ρ = (N/V3 is the reduced density, where N is the number of particles and V is the volume. A FORTRAN program is provided for the implementation of these for R ≤ 6σ, which includes code for the calculation of gHS(R; ρ) itself over this range. We also present and incorporate within the program code convenient analytical expressions for the numerical extrapolation of both quantities past R = 6σ. Our expressions are numerically tested against exact results.  相似文献   

8.
We consider the integrated density of states (IDS) ρ(λ) of random Hamiltonian Hω=?Δ+Vω, Vω being a random field on ? d which satisfies a mixing condition. We prove that the probability of large fluctuations of the finite volume IDS |Λ|?1ρ(λ, HΛ(ω)), Λ ? ? d , around the thermodynamic limit ρ(λ) is bounded from above by exp {?k|Λ|},k>0. In this case ρ(λ) can be recovered from a variational principle. Furthermore we show the existence of a Lifshitztype of singularity of ρ(λ) as λ → 0+ in the case where Vω is non-negative. More precisely we prove the following bound: ρ(λ)≦exp(?kλ?d/2) as λ → 0+ k>0. This last result is then discussed in some examples.  相似文献   

9.
The structure, electrical resistivity, and magnetoresistance of La0.67Sr0.33MnO3 heteroepitaxial films (120-nm thick) practically unstrained by lattice mismatch with the substrate were studied. A strong maximum of negative magnetoresistance of ≈27% (for μ0H = 4 T) was observed at T ≈360 K. While the magnetoresistance decreased monotonically in magnitude with decreasing temperature, it was still in excess of 2% at 150 K. For T < 250 K, the temperature dependence of the electrical resistivity ρ of La0.67Sr0.33MnO3 films is fitted well by the relation ρ = ρ0 + ρ 1(H)T2.3, where ρ0 = 1.1×10?4 Ω cm, ρ1(H = 0) = 1.8×10?9 Ω cm/K2.3, and ρ10H = 4 T)/ρ1(H = 0) ≈0.96. The temperature dependence of a parameter γ characterizing the extent to which the electrical resistivity of the ferromagnetic phase of La0.67Sr0.33MnO3 films is suppressed by a magnetic field (μ 0H = 5 T) was determined.  相似文献   

10.
An exact solution to the Schrödinger equation for the ground state of two-dimensional Pauli electrons in a nonuniform transverse magnetic field H is presented for two cases. In the first case, the field H depends on a single variable, H = H(y), while in the second case, the field is axially symmetric, H = H(ρ), ρ2=x 2+y 2. The electron density distributions n = n(y) and n = n(ρ) that correspond to a completely filled lower level are found. For quasiuniform fields of fixed sign, the functions n(y) and n(ρ) are locally related to the magnetic field: n(y) = H(y)/?0 and n(ρ) = H(ρ)/?0, where ?0 = hc/|e| is a magnetic flux quantum. Magnetic fields are considered that are periodic, singular, and bounded in the plane xy. Finite electron objects in a nonuniform magnetic field are analyzed.  相似文献   

11.
A qualitative analysis of the chiral phase transition in QCD with two massless quarks and nonzero baryon density is performed. It is assumed that, at zero baryonic density, ρ=0, the temperature phase transition is of the second order. Due to a specific power dependence of baryon masses on the chiral condensate, the phase transition becomes of the first order at the temperature T=Tph(ρ) for ρ>0. At temperatures Tcont(ρ)> T>Tph(ρ), there is a mixed phase consisting of the quark phase (stable) and the hadron phase (unstable). At the temperature T=Tcont(ρ), the system experiences a continuous transition to the pure chirally symmetric phase.  相似文献   

12.
Hydrodynamics is often applied to quantum phenomena such as heavy-ion collisions. But here it should be noted that local equilibrium is not always realized in these collision processes and also the quantum effect is not fully taken into account in hydrodynamics. In this sense, a fluid-dynamical treatment of quantum many-body systems which does not presuppose local equilibrium is required. As an attempt in this direction, we derive simultaneous equations governing the motion of local variables such as the particle density ρ(r, t) and velocity field ν(r, t) by averaging a many-body wave function. The equations obtained will be shown to unify into a single nonlinear Schrödinger-type equation. Hence this is worthy of being called a quantum fluid dynamics (QFD). In deriving the QFD, we have employed the time-dependent Hartree-Fock and the generalized scaling approximation. Particularly, in order to attain self-containedness, we have assumed a certain relation which is valid in the case of the locally isotropic strain tensor. The introduction of anisotropy requires other local variables reflecting explicitly the deviation from local equilibrium and thus has been left as a future task.  相似文献   

13.
The structure, electrical resistivity, and magnetoresistance of predominantly oriented La0.67Ca0.33MnO3(30 nm)/LaAlO3 films are investigated after partial relaxation of biaxial mechanical stresses. The negative magnetoresistance MR of the films reaches a maximum at T = 235–240 K. The full width at half-maximum of the peak in the curve MR(T) for a film is five to six times greater than that for a manganite layer grown on a substrate with a small lattice mismatch. For T < 150 K, the temperature dependence of the electrical resistivity ρ of the films is fitted well by the relationship ρ = ρ0 + ρ1 (H)T 4.5, where ρ0 ≡ ρ(T = 4.2 K) and ρ1(H) is a parameter that is independent of temperature but dependent on the magnetic field H. The parameter ρ1(H = 0) for the La0.67Ca0.33MnO3(30 nm)/LaAlO3 films is several times larger than that for thin manganite layers only weakly strained by the substrate. The electrical resistivity ρ1 decreases almost linear as the quantity μ0 H increases in the field range 1–5 T.  相似文献   

14.
At-channel factorization model is used to estimate cross sections for the processes γγ→V 1 V 2. WheneverV=ρ, the width of the ρ has been included in the calculations. The channels γγ→ρ0ρ0, ρ0?, ??, ωω, ρ0ω and ρ+ρ? are calculated for two quasi-real photons. Predictions are also given for the process γ*γ→ρ0ρ0 for virtual photon mass squaredQ 2<5 GeV2. Our results are consistent with all available experimental data.  相似文献   

15.
Relationship is established between the security of the BB84 quantum key distribution protocol and the forward and converse coding theorems for quantum communication channels. The upper bound Q c ≈ 11% on the bit error rate compatible with secure key distribution is determined by solving the transcendental equation $H(Q_c ) = \bar C(\rho )/2$ , where ρ is the density matrix of the input ensemble, $\bar C(\rho )$ is the classical capacity of a noiseless quantum channel, and H(Q) is the capacity of a classical binary symmetric channel with error rate Q.  相似文献   

16.
Y. Ozhigov 《JETP Letters》2002,76(11):675-680
Call a spectrum of Hamiltonian H sparse if each eigenvalue can be quickly restored within ε from its rough approximation within ε1 by means of some classical algorithm. It is shown how the behavior of a system with a sparse spectrum up to time T=(1?ρ)/14ε can be predicted on a quantum computer with the time complexity t=4/(1?ρ)ε1 plus the time of classical algorithm, where ρ is the fidelity. The quantum knowledge of Hamiltonian eigenvalues is considered as the new Hamiltonian W H whose action on each eigenvector of H gives the corresponding eigenvalue. Speedup of evolution for systems with a sparse spectrum is possible, because, for such systems, the Hamiltonian W H can be quickly simulated on the quantum computer. For an arbitrary system (even in the classical case), its behavior cannot be predicted on a quantum computer even for one step ahead. By this method, we can also restore the history with the same efficiency.  相似文献   

17.
S. N. Molotkov 《JETP Letters》2016,103(5):345-349
The trace distance is used as a security criterion in proofs of security of keys in quantum cryptography. Some authors doubted that this criterion can be reduced to criteria used in classical cryptography. The following question has been answered in this work. Let a quantum cryptography system provide an ε-secure key such that ½‖ρXE ? ρU ? ρE1 < ε, which will be repeatedly used in classical encryption algorithms. To what extent does the ε-secure key reduce the number of search steps (guesswork) as compared to the use of ideal keys? A direct relation has been demonstrated between the complexity of the complete consideration of keys, which is one of the main security criteria in classical systems, and the trace distance used in quantum cryptography. Bounds for the minimum and maximum numbers of search steps for the determination of the actual key have been presented.  相似文献   

18.
Using the typical generators of the special unitary groups S U(2), the concrete representation of symmetric quantum state is established, then the relations satisfied by those coefficients in the representation are presented. Based on the representation of density matrix, the PPT criterion and CCNR criterion are proved to be equivalent on judging the separability of symmetric quantum states. Moreover, it is showed that the matrix Γ ρ of symmetric quantum state only has five efficient entries, thus the calculation of ∥Γ ρ ∥ is simplified. Finally, the quantitative expressions of real symmetric quantum state under the ∥Γ ρ ∥ separability criterion are obtained.  相似文献   

19.
A study has been made of the electrical resistivity ρ, magnetoresistance Δρ/ρ, and magnetization of La0.35Nd0.35Sr0.3MnO3 epitaxial films on ZrO2(Y2O3), SrTiO3, LaAlO3, and MgO substrates. The first film can exist in four equivalent crystallographic orientations in the sample plane, while the other three have only one orientation. The maxima of ρ and Δρ/ρ of the first film are broadened considerably in the vicinity of the Curie point T C compared to the three others, the magnitude of ρ itself being larger by 1.5 orders of magnitude, and a large negative magnetoresistance (|Δρ/ρ| ~ 10% in a field of 8.4 kOe) is observed at temperatures 80≤T≤200 K. In all films, the magnetic moment per molecule at 5 K is ~46% smaller than the pure spin value, due to the existence of magnetically disordered regions. The larger value of ρ of the film deposited on ZrO2(Y2O3) is due to the electrical resistance of the boundaries separating regions with different crystallographic orientations, and the magnetoresistance is associated with polarized carriers tunneling through the boundaries coinciding with domain walls. The low-temperature magnetoresistance in fields above technical saturation is caused by the strong p-d exchange coupling within spin-ordered regions.  相似文献   

20.
It is shown, to leading order in the virial expansion, that unitarity of the ππ scattering amplitude implies that if the ρ-meson widthΓ ρ(T) increases with temperature, then the ρ-meson massM ρ(T) must also increase. The temperature dependence ofM ρ(T) is explicitly obtained from a simple, but physically motivated, parametrization of theI=J=1 ππ scattering amplitude.  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