Electric field distribution in the base of semiconductor <Emphasis Type="Italic">p-n</Emphasis> junction diode

An expression for distribution of the electric field in the base of a semiconductor p-n junction diode is obtained valid for arbitrary injection levels in the approximation of exponential distribution of non-equilibrium carriers in the diode base. It is shown that at a certain current the field is constant across the base which allows one to determine the mobility of majority carriers (whose concentration is determined from the barrier capacity measurements). For commercial D223B and D219A diodes the current-voltage characteristics and the differential resistance are measured and the mobility of electrons is calculated. The electric field distribution in the D223B diode is derived at three injection levels.     

Pair creation by a photon in an electric field

The process of pair creation by a photon in a constant and homogeneous electric field is investigated basing on the polarization operator in the field. The total probability of the process is found in a relatively simple form. At high energy the quasiclassical approximation is valid. The corrections to the standard quasiclassical approximation (SQA) are calculated. In the region of relatively low photon energies, where SQA is unapplicable, the new approximation is used. It is shown that in this energy interval the probability of pair creation by a photon in electric field exceeds essentially the corresponding probability in a magnetic field. This approach is valid at the photon energy much larger than the “vacuum” energy in electric field: ω?eE/m. For smaller photon energies the low energy approximation is developed. At ω?eE/m the found probability describes the absorption of soft photon by the particles created by an electric field.     

Pion properties at finite isospin chemical potential with isospin symmetry breaking

Pion properties at finite temperature, finite isospin and baryon chemical potentials are investigated within the SU(2) NJL model. In the mean field approximation for quarks and random phase approximation fpr mesons, we calculate the pion mass, the decay constant and the phase diagram with different quark masses for the u quark and d quark, related to QCD corrections, for the first time. Our results show an asymmetry between μI 0 and μI 0 in the phase diagram, and different values for the charged pion mass(or decay constant) and neutral pion mass(or decay constant) at finite temperature and finite isospin chemical potential. This is caused by the effect of isospin symmetry breaking, which is from different quark masses.     

Time-dependent mean field S-matrix theory

By using the path integral approach to many-body systems, we formulate a time-dependent mean field S-matrix theory of nuclear reactions. Many-body channel eigenstates are constructed by using projection techniques. In this way the S-matrix between the channel eigenstates is expressed as a superposition of S-matrix elements between wave-packet-like states localized in space and time. A field operator representation of the interaction picture S-matrix is derived which enables one to apply the path integral approach. Applying the stationary phase approximation to the path integral representation of the interaction picture S-matrix between the localized states an asymptotically constant time-dependent mean field approximation to this S-matrix is obtained. Finally, the S-matrix between the projected channel eigenstates is obtained by evaluating the integral, arising from the projections, over the space-time positions of the localized states in the stationary phase approximation. The stationary phase conditions select those localized states from the projected channel states for which the mean field values of energy and momentum coincide with their corresponding channel eigenvalues.     

Thermodynamic properties of the φ4 lattice field theory near the Ising limit

For a d-dimensional φ⁴ lattice field theory consisting of N spins with nearest-neighbor interactions, the partition function is transformed for large bare coupling constant λ into an Ising-like system with additional neighbor interactions. For d = 2 a mean field approximation is then used to estimate the difference in critical temperature between the lattice φ⁴ field theory and its Ising limit (λ = ∞). Expansions are obtained for the susceptibility and specific heat. The critical exponents are shown to be identical to the Ising exponents.     

TCP, quantum gravity,the cosmological constant and all that...

We study cosmology from the point of view of quantum gravity. Some light is thrown on the nature of time, and it is suggested that the cosmological arrow of time is generated by a spontaneous breakdown of TCP. Conventional cosmological models in which quantum fields interact with a time-dependent gravitational field are shown to describe an approximation to the quantum gravitational wave function which is valid in the long-wavelength limit. Two problems with initial conditions are resolved in models in which a negative bare cosmological constant is cancelled by the classical excitation of a Bose field η with a very flat potential. These models can also give a natural explanation for the observed value of the cosmological constant.     

Solution of a symmetry conserving chiral soliton model for nucleon and delta

The linear chiral soliton model is solved for nucleon and delta by constructing Fock states in the coherent pair approximation with correct spin and isospin properties. The quark configurations are those arising from theSU(2)×SU(2) coupling of three quarks in 1s-orbits. The overall Fock state is formed by the vector coupling of the quark configurations with the pion coherent state and thus avoids the use of the hedgehog ansatz. The sigma field is treated in the mean field approximation. Equations of motion for the quark, sigma and pion fields are solved in the static approximation. Soliton solutions are found with properties that are in reasonable agreement with those observed for the nucleon and delta including the axial vector coupling constant. With only components having zero and one unpaired pion in the coherent pair approximation the nucleon mass is found to be larger than that using the projected hedgehog approach.     

A new method to solve the non-perturbative renormalization group equations

We propose a method to solve the non-perturbative renormalization group equations for the n-point functions. In leading order, it consists in solving the equations obtained by closing the infinite hierarchy of equations for the n-point functions. This is achieved: (i) by exploiting the decoupling of modes and the analyticity of the n  -point functions at small momenta: this allows us to neglect some momentum dependence of the vertices entering the flow equations; (ii) by relating vertices at zero momenta to derivatives of lower order vertices with respect to a constant background field. Although the approximation is not controlled by a small parameter, its accuracy can be systematically improved. When it is applied to the O(N)$O(N)$ model, its leading order is exact in the large-N limit; in this case, one recovers known results in a simple and direct way, i.e., without introducing an auxiliary field.     

The theory of nonlinear dispersion interferometers in the constant intensity approximation

The operation of a dispersion interferometer with intracavity location of a medium under study is analyzed in the constant intensity approximation taking into account the reciprocal influence of a harmonic on the phase of the exciting wave. This approximation allows the effects of phase mismatch and losses in a medium on the course of a nonlinear process to be taken into account simultaneously. Intensity minima of the harmonic (as opposed to the constant field approximation) in the constant intensity approximation are shown to depend on dispersion of the refractive index of the medium under study, which makes it possible to determine dispersion of the refractive index of the substance under study directly and accurately     

Phase effects at second harmonic generation in the layer media

An analysis is made of quasi-phase-matched interaction at second harmonic generation in a regular domain structure with accounts for losses and change of phases of all the interacting waves. The constant-intensity approximation of basic radiation is applied for this purpose, not to regular domain structure as a whole, but to each separately taken domain. It allows to carry out more strict analysis of quasi-synchronous interaction during the frequency conversion to the polydomain consisting of n layers, forming “grating” periods of modulation of the nonlinear susceptibility. With this, the values of complex amplitudes of basic radiation and second harmonic wave at the outlet of each domain are entrance values of the corresponding complex amplitudes to the following domain. The analytical expression is given for the case of n domains and the factors limiting the efficiency of the process of frequency conversion are analyzed. In the constant-intensity approximation, in contrast to the constant field approximation, the coherent length of domain depends on pump intensity. With increasing pump intensity the optimum length decreases. In a regular domain structure at frequency conversion from a layer to a layer, intensity of the basic radiation changes. Also the optimum length of domains at which conversion efficiency is maximal therefore changes. Thus, it is possible to obtain the high values of conversion frequency at the outlet of a regular domain structure by choice of optimum parameters of a task (length of domains, phase mismatch, pump intensity), as well as using the layers-domains of high quality.     

Chiral thermodynamics in a magnetic field

The phase structure of the QCD vacuum in a magnetic field H is investigated at low temperatures T. The free energy of the hadronic phase in a constant homogeneous magnetic field is calculated in the one-loop approximation of chiral perturbation theory. The quark and the gluon condensate are found as functions of temperature and the field strength. It is shown that the order parameter $\left\langle {\bar qq} \right\rangle $ for the chiral phase transition remains constant when the temperature T and the magnetic field H change in such a way that H=const×T ².     

Radiation field of an optically finite homogeneous atmosphere with internal sources

The equation of radiative transfer in an optically finite homogeneous atmosphere with different internal sources is solved using the method of kernel approximation the essence of which is to approximate the kernel in the equation for the Sobolev resolvent function by a Gauss-Legendre sum. This approximation allows to solve the equation exactly for the resolvent function while the solution is a weighted sum of exponents. Since the resolvent function is closely connected with the Green function of the integral radiative transfer equation, the radiation field for different internal sources can be found by simple integration. In order to simplify the obtained formulas we have defined the x and y functions as the generalization of the well-known Ambarzumian-Chandrasekhar X and Y functions.For some types of internal sources the package of codes in Fortran-77 can be found at http://www.aai.ee/∼viik/HOMOGEN.FOR.     

Landau coefficients and the critical electric field in a KDP crystal

Analytical expressions for the Landau coefficients of deuterated potassium dihydrogen phosphate (DKDP) crystal are obtained in a four-particle cluster approximation. The critical electric field strengths calculated within the phenomenological and microscopic theories for DKDP are compared to experimental data. Calculations of the critical electric field in potassium dihydrogen phosphate (KDP) crystal in the cluster approximation with allowance for proton tunneling on hydrogen bonds allows us to determine the tunneling constant and the geometric factor associated with the change in the effective dipole moment upon deuteration. Good agreement with the experimental data is obtained only for positive values of the long-range interaction parameter.     

Free energy in the (2+1)-dimensional SU(2) QCD model against the background of a gauge-field condensate

The thermodynamic potential (free energy) for quarks and gluons in (2+1)-dimensional spacetime is calculated in the one-loop approximation at finite temperatures against the background of a constant uniform color magnetic field H and a constant uniform A ₀ condensate. The problem of interpreting the tachyon mode in the gluon energy spectrum is discussed. The question of whether the free energy may develop a minimum at nonzero values of H and A ₀ is analyzed.     

A vector type Gürsey model and its renormalization group analysis

We start with a model classically equivalent to the vector Gürsey model. The model is coupled with a non-abelian SU(N) gauged field. For the one loop approximation, the renormalization group equations are constructed and the coupling constant solutions are found. The coupling constants behaviors are briefly investigated and non-triviality conditions of the model are presented. Under certain conditions, a non-trivial field theoretical model is obtained.     

Retarded transverse current-current response functions of a two-dimensional electron gas

The retarded transverse current-current response functions of an ideal two-dimensional electron gas at zero temperature are calculated. Introducing a new approximation scheme which allows one to take account of the reaction of the electromagnetic field to the induced current, i.e. the influence of the electronic induced current on the dynamics of the electromagnetic field, within the framework of the conventional linear response theory in a self-consistent way, a possible form of the dispersion relation for the transverse plasmon in the system is obtained. It is found that the energy of the transverse plasmon has a gap at k=0, if the thickness of the system is finite, in contrast to the longitudinal case.     

The 2D effective field theory of interfaces derived from 3D field theory

The one-loop determinant computed around the kink solution in the 3D φ⁴ theory, in cylindrical geometry, allows one to obtain the partition function of the interface separating coexisting phases. The quantum fluctuations of the interface around its equilibrium position are described by a c = 1 two-dimensional conformal field theory, namely a 2D free massless scalar field living on the interface. In this way the capillary wave model conjecture for the interface free energy in its gaussian approximation is proved.     

Ground state of the SU(2) Yang-Mills theory

The problem of the ground state of the SU(2) Yang-Mills theory is investigated within the one-loop approximation. It is shown that an assumed initial constant magnetic field will be diminished to realize a state of lower energy (vacuumI) by a condensation of some components of Yang-Mill fields. We also find a Lorentz invariant state (vaccumII) through a singlet scalar condensation. However, the vaccumI which is Lorentz noinvariant is found to have a lower energy density than the vacuumII.