Anomalous plasma diffusion transverse to high magnetic fields in InSb

Investigations on the ambipolar diffusion of an electron-hole plasma transverse to a magnetic field have been carried out in InSb. A plasma layer, produced at the surface of the sample by a short laser pulse, was moved through the sample in crossed electric and magnetic fields by the Lorentz force. From the broadening of the plasma layer we found at 80K an enhanced diffusion coefficient which decreased proportional to 1/B for magnetic fields higher than 1T, constrary to the expected classical 1/B ² dependence. Furthermore, the diffusion coefficient was strongly dependent on the electric field. The ambipolar drift velocity, measured simultaneously showed a classical behaviour. Together with the enhanced diffusion we observed instabilites in the electric potential. The instability threshold decreased towards the cathode.     

Spin relaxation of Mn ions in (CdMn)Te/(CdMg)Te quantum wells under picosecond optical pumping

Spin relaxation of Mn ions in a Cd_0.97Mn_0.03Te/Cd_0.75Mg_0.25Te quantum well with photogenerated quasi-two-dimensional electron-hole plasma at liquid helium temperatures in an external magnetic field has been investigated. Heating of Mn ions by photogenerated carriers due to spin and energy exchange between the hot electron-hole plasma and Mn ions through direct sd-interaction between electron and Mn spins has been detected. This process has a short characteristic time of about 4 ns, which leads to appreciable heating of the Mn spin subsystem in about 0.5 ns. Even under uniform excitation of a dense electron-hole plasma, the Mn heating is spatially nonuniform, and leads to formation of spin domains in the quantum well magnetic subsystem. The relaxation time of spin domains after pulsed excitation is measured to be about 70 ns. Energy relaxation of excitons in the random exchange potential due to spin domains results from exciton diffusion in magnetic field B=14 T with a characteristic time of 1 to 4 ns. The relaxation time decreases with decreasing optical pump power, which indicates smaller dimensions of spin domains. In weak magnetic fields (_B=2 T) a slow down in the exciton diffusion to 15 ns has been detected. This slow down is due to exciton binding to neutral donors (formation of bound excitons) and smaller spin domain amplitudes in low magnetic fields. The optically determined spin-lattice relaxation time of Mn ions in a magnetic field of 14 T is 270±10 and 16±7 ns for Mn concentrations of 3% and 12%, respectively. Zh. éksp. Teor. Fiz. 112, 1440–1463 (October 1997)     

Allgemeine Berechnung der Elektronenverteilungsfunktion von schwach ionisierten Plasmen in beliebig zeitabhängigen elektromagnetischen Feldern

The behaviour of a weakly ionized plasma in external, arbitrarily time-dependent, electromagnetic fields is treated within the framework of kinetic theory. The Boltzmann kinetic equation is solved using the Lorentz ansatz, taking into account elastic collisions between electrons and neutral particles and assuming that the collision frequency is independent of the electron velocity. The drift velocity of electrons enters into the isotropic part f₀ and into the direction-dependent part f₁ of the electron distribution function. A method is given for the calculation of the drift velocity, which is calculated explicitly for the important but difficult case of a sinusoidal electric field in the presence of a magnetic switching field. f₀ and f₁ are calculated; f₀ is investigated generally. f₀ consists of an expansion in generalized Laguerre polynomials. The influence of the electromagnetic fields on the distribution function and its time variation is discussed and the relaxation behaviour is shown. The following two special cases are calculated explicitly: a linear rising electric field and a sinusoidal electric field, both in the presence of a constant magnetic field.     

Phonon and electron-hole plasma effects on binding energies of excitons in wurtzite GaN/In<Subscript>x</Subscript>Ga<Subscript>1−x</Subscript>N quantum wells

The binding energy of an exciton screened by the electron-hole plasma in a wurtzite GaN/In _x Ga_1−x N quantum well (in the case of 0.1 < x < 1 within which the interface phonon modes play a dominant role) is calculated including the exciton-phonon interaction by a variational method combined with a self-consistent procedure. The coupling between the exciton and various longitudinal-like optical phonon modes is considered to demonstrate the polaronic effect which strongly depends on the exciton wave function. All of the built-in electric field, the exciton-phonon interaction and the electron-hole plasma weaken the Coulomb coupling between an electron and a hole to reduce the binding energy since the former separates the wave functions of the electron and hole in the z direction and the later two enlarge the exciton Bohr radius. The electron-hole plasma not only restrains the built-in electric field, but also reduces the polaronic effect to the binding energy.     

ϑ-pinch in extrinsic InSb

The compression of an electron-hole plasma, caused by a ?-pinch in extrinsic InSb of 140 K, was investigated by measuring the absorption of 10·6 μm radiation and the change of the magnetic flux in the sample. A plasma density of 5 × 10¹⁵ cm^?3 was hereby found. The temporal development of the plasma density indicates that the electrons and holes resulted from impact ionization in the electric field when the magnetic field was low. Since the electric field is highest at the sample surface, the ionization was limited to the outer region of the sample and the plasma was transmitted to the inner sample volume by the ?-pinch.     

Magnetoelectric effect in perovskite quantum paraelectric EuTiO3

On the basis of successful theoretical explanation of the observed large magnetic-field effect (by ∼7% with 1.5 T) on the dielectric constant below the Néel temperature T_N of 5.5 K, we have demonstrated convincingly the magnetoelectric effect in an antiferromagnetic quantum paraelectric EuTiO₃ system. The mutual control of electric and magnetic properties is revealed by the variation of the electric-field-induced polarization with applied magnetic fields as well as the change of the magnetic-field-induced spin moments under the control of electric fields. It is found that the applied electric field (magnetic field) acts like a fictitious magnetic field (electric field) on the EuTiO₃ system. The magnetoelectric susceptibility is deduced to be proportional to the product of the magnetization, electrical polarization, magnetic susceptibility and dielectric susceptibility.     

Applications of photoconductivity measurements in n-InSb under crossed fields

The photocurrent in n-InSb at 85K was measured as function of the applied longitudinal voltage and an additionally applied transverse magnetic field. In the driftconfiguration the photoionized electron-hole plasma was driven into the sample volume, and negative photocurrents resulted from the negative feedback of this motion. With the magnetic field being reversed, in the Suhl configuration, only positive photocurrents were measured. All results are in agreement with the theory in [1]. They were used to determine recombination coefficients in n-InSb, and, principally, the transverse diffusion coefficientD _⊥. As first observed in [13],D _⊥ proved to be anomalously enhanced above classical values if plasma instabilities were generated in the samples.     

Behaviour of gluons at high temperature

The screening of colour electric and magnetic fields and plasma oscillations in a high-temperature gluon plasma are investigated using linear response theory and self-consistent nonperturbative solutions to the Schwinger-Dyson equation. Static electric fields are screened, with

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. This result is proven to be gauge invariant in two ways: by computing π₀₀ in temporal axial, Coulomb and covariant gauges, and by computing the physical free energy of a heavy quark pair in the plasma in temporal axial gauge. To order g³ static magnetic fields are not screened.     

Stabilization of electron-hole liquid in uniaxially strained germanium in a strong magnetic field

Luminescence spectra of uniaxially and uniformly strained high-purity germanium crystals at liquid-helium temperatures in a magnetic field of up to 14 T have been investigated. In strongly strained Ge crystals, a new line has been detected on the low-energy side of the excitonline in magnetic fields higher than 4 T. Studies of this line’s characteristics as functions of pressure, temperature, and magnetic field have led us to conclude that its presence is due to recombination of electron-hole pairs in an electron-hole liquid. The experimental data suggest that the metallic electron-hole liquid is stabilized in a strong magnetic field. By approximating the shape of the newly detected line using the model of metallic electron-hole liquid, we have obtained the electron-hole liquid density n _EHL(B) and Fermi energies E _Fe,h of electrons and holes. The liquid binding energy ø as a function of magnetic field has been estimated.     

Low-frequency instability of an inhomogeneous weakly ionized plasma in crossed electric and inhomogeneous magnetic fields

The paper discusses the drift dissipative instability that arises in a weakly ionized inhomogeneous plasma in crossed electric and magnetic fields. The potential perturbations are studied in the WKB (Wentzel-Krammers-Brillouin) approximation, i.e. for k = d lnn ₀/dx. The critical magnetic field, beyond the value of which the plasma is again stable against the studied perturbations, is computed. The effect of the inhomogeneity of the magnetic field modelled by a gravitational field is analyzed and the effect of a magnetic field, growing in the direction opposite to that of the plasma density, is pointed out. Under certain circumstances similar phenomena also arise in the current-convective instability.The author wishes to express his thanks to J. Václavík, K. Jungwirth and J. Preinhaelter for their stimulating discussions and comments.     

Electronic structure of paramagnetic In<Subscript>1-x</Subscript>Mn<Subscript>x</Subscript> As nanowires

The electronic structure, spin splitting energies, and g factors of paramagnetic In_1-xMn_xAs nanowires under magnetic and electric fields are investigated theoretically including the sp-d exchange interaction between the carriers and the magnetic ion. We find that the effective g factor changes dramatically with the magnetic field. The spin splitting due to the sp-d exchange interaction counteracts the Zeeman spin splitting. The effective g factor can be tuned to zero by the external magnetic field. There is also spin splitting under an electric field due to the Rashba spin-orbit coupling which is a relativistic effect. The spin-degenerated bands split at nonzero k_z (k_z is the wave vector in the wire direction), and the spin-splitting bands cross at k_z = 0, whose k_z-positive part and negative part are symmetrical. A proper magnetic field makes the k_z-positive part and negative part of the bands asymmetrical, and the bands cross at nonzero k_z. In the absence of magnetic field, the electron Rashba coefficient increases almost linearly with the electric field, while the hole Rashba coefficient increases at first and then decreases as the electric field increases. The hole Rashba coefficient can be tuned to zero by the electric field.     

Evolution of electron-hole avalanches and streamers in indirect gap semiconductors

A numerical simulation of the origination and evolution of streamers in semiconductors has been performed using the diffusion-drift approximation including the impact and tunnel ionization. It is assumed that an external electric field E ₀ is static and uniform, an avalanche and a streamer are axisymmetric, background electrons and holes are absent, and all their kinetic coefficients are identical. The linear evolution of an electron-hole avalanche, an avalanche-to-streamer transition, and two successive stages of the evolution of the streamer—intermediate “diffusion” and main exponentially self-similar—have been examined in detail. It has been shown that a streamer is similar to a dumbbell with conical weights. The bases of these cones, streamer fronts, are thin shells, which contain almost the entire streamer charge and are close in shape to the halves of ellipsoids of revolution. A front propagates so that its shape and the shape of the weight of the dumbbell, the maximum field on the front, and the electron-hole plasma density in weights remain unchanged. The field strength behind the front is much smaller than E ₀, but increases with approaching the bar of the dumbbell whose diameter increases with the time t owing to the transverse diffusion. The electron and hole densities in the bar increase due to the impact ionization in an almost uniform field, which is only slightly lower than E ₀. At the diffusion stage, the length of the streamer and the curvature radius of its front increase with constant rates, which are determined not only by the impact ionization and drift, but also by diffusion. In relatively low fields (E ₀ ≲ 0.4 MV/cm for silicon) this stage ends due to the appearance of the instability of the front. In higher fields, the tunnel ionization is manifested before the appearance of instability and gives rise to the appearance of a new-type streamer. Its main feature is the stable exponential increase in all spatial scales with the same response time t _R , so that the charge-carrier density and field strength at large times t depend only on one vector variable $ \hat R $ \hat R = Rexp(−t/t _R ). This means that the solution of a Cauchy problem describing the evolution of the streamer in the uniform field is asymptotically exponentially self-similar.     

Stimulated raman scattering from plasma modes in magnetoactive semiconductors

Stimulated scattering off electron plasma mode is investigated analytically for the case when the pump wave is an intense circularly polarised electromagnetic wave propagating parallel to a homogeneous dc magnetic field in an isotropic semiconductor-plasma. The threshold electric field of the pump necessary for the stimulated Raman scattering and the growth rate of the parametrically unstable mode have been obtained for two cases (i)B ₀=0 and (ii) B₀ ≠ 0. It is seen that the magnetic field does not significantly affect the threshold electric field as well as the growth rate provided the cyclotron frequency is small compared to the frequency of the pump wave. The threshold conditions are also found to be insensitive to the electron thermal velocity.     

A method to separate conservative and magnetically-induced electric fields in calculations for MRI and MRS in electrically-small samples

This work presents a method to separately analyze the conservative electric fields (E_c, primarily originating with the scalar electric potential in the coil winding), and the magnetically-induced electric fields (E_i, caused by the time-varying magnetic field B1) within samples that are much smaller than one wavelength at the frequency of interest. The method consists of first using a numerical simulation method to calculate the total electric field (E_t) and conduction currents (J), then calculating E_i based on J, and finally calculating E_c by subtracting E_i from E_t. The method was applied to calculate electric fields for a small cylindrical sample in a solenoid at 600 MHz. When a non-conductive sample was modeled, calculated values of E_i and E_c were at least in rough agreement with very simple analytical approximations. When the sample was given dielectric and/or conductive properties, E_c was seen to decrease, but still remained much larger than E_i. When a recently-published approach to reduce heating by placing a passive conductor in the shape of a slotted cylinder between the coil and sample was modeled, reduced E_c and improved B1 homogeneity within the sample resulted, in agreement with the published results.     

Dynamics of electron-hole plasma in semiconductor opening switches for ultradense currents

A physicomathematical model for calculating the dynamics of the electron-hole plasma in semiconductor opening switches for ultradense currents is developed. The model takes account of the real doping profile of a semiconductor p ⁺-p-n-n ⁺ structure and the following elementary processes in the electron-hole plasma: current-carrier diffusion and drift in high electric fields, recombination on deep impurities and Auger recombination, and collisional ionization in a dense plasma. The electrical pumping circuit of the opening switch is calculated by solving the Kirchhoff equations. The motion of the plasma in the semiconductor structure is analyzed on the basis of the model. It is shown that for ultrahigh pumping levels the interruption of the current in the opening switch occurs in the heavily doped regions of the p ⁺-p-n-n ⁺ structure and is due to saturation of the particle drift velocity in high electric fields. Zh. Tekh. Fiz. 67, 64–70 (October 1997)     

Intense laser field effect on impurity states in a semiconductor quantum well: transition from the single to double quantum well potential

In this work are studied the intense laser effects on the impurity states in GaAs-Ga_{1− x} Al _x As quantum wells under applied electric and magnetic fields. The electric field is taken oriented along the growth direction of the quantum well whereas the magnetic field is considered to be in-plane. The calculations are made within the effective mass and parabolic band approximations. The intense laser effects have been included through the Floquet method by modifying the confinement potential associated to the heterostructure. The results are presented for several configurations of the dimensions of the quantum well, the position of the impurity atom, the applied electric and magnetic fields, and the incident intense laser radiation. The results suggest that for fixed geometry setups in the system, the binding energy is a decreasing function of the electric field intensity while a dual monotonic behavior is detected when it varies with the magnitude of an applied magnetic field, according to the intensity of the laser field radiation.     

Low-frequency instabilities of electron-hole plasmas in crossed fields

Using local point-contact probes, we observed two types of low-frequency instabilities inn-InSb at 85 K if the samples were exposed to crossed fields. One is a local density instability with threshold frequencies off = 1 20 Mc, the other a more turbulent current instability. The threshold values ofU ₀ andB for the onset of these instabilities and the dependence of their amplitudes on the fields have been measured.If a rectangular semiconductor slab is placed in crossed fields, regions of high electric field strength at opposite edges of the contacts are caused by the distortion of the Hall field, giving rise to the generation of electron-hole plasmas by impact ionization. These plasmas are the sources of the observed instabilities. This is especially evident in the case of the local density instability, which originates at the anode high field corner. Several possible reasons for the development of the instabilities are discussed.     

On galvanomagnetic size effects in metals

The importance of open electron trajectories formed by specular reflections of charge carriers by the sample boundary to the metal electric conductivities in the strong magnetic fieldH is analyzed. It is shown that the electric conductivity of the near-surface layer _skin is rather sensitive to the state of the conductor surface. The electron-hole Umklapp processes during the surface scattering of charge carriers do not change the dependence _skin(H), while skipping from the closed Fermi surface section to the open one is able to affect _skin essentially only in bulk samples. The method is proposed to restore the indicatrix of conduction electron scattering by the sample boundary through an experimental investigation of the Sondheimer effect and the static skin effect.     

Quasistatic magnetic fields generated by a nonrelativistic intense laser pulse in uniform underdense plasma

Quasistatic magnetic fields generated by nonrelativistic intense linearly polarized (LP) and circularly polarized (CP) laser pulses in an initially uniform underdense plasma in the collision-dominated limit are investigated analytically. Using a selfconsistent analytical model, we perform a detailed derivation of quasistatic magnetic fields in the laser pulse envelope in the collision-dominated limit to obtain exact analytical expressions for magnetic fields and discuss the dependence of magnetic fields on laser and plasma parameters. Equations for quasistatic magnetic fields including both axial component B_z and the azimuthal one B_θ are derived simultaneously from such a selfconsistent model. The dependence of quasistatic magnetic field on incident laser intensity, transverse focused radius of laser pulse, electron density and electron temperature is discussed.     

Self-consistent equilibria in a cylindrical reversed-field pinch

Summary A previous investigation by one of us, concerning the self-consistent equilibria of a two-region (plasma+gas) cylindrical Tokamak, is extended to the similar equilibria of a Reversed-Field Pinch, where a significant current density is driven by a dynamo electric field due to turbulence. The previous model has been generalized under the following basic assumptions:a) to the lowest order, the turbulent dynamo electric fieldE ^t is expressed as a homogeneous function of degree 1 of the magnetic fieldB, sayE ^t =α·B, with α being a 2nd-rank tensor, homogeneous of degree 0 inB, and generally depending on the plasma state;b)E ^t does not appear in the plasma power balance, as if it were produced by a Maxwell demon able to extract the needed power from the plasma internal energy. In particular we show that, in the simplest case when both α and the plasma resistivity η are isotropic and constant, the magnetic field turns out force-free with constant abnormality αμ₀/η for vanishing axial electric fieldE _z . This case has also been solved analytically, for whateverE _z , under circular, besides cylindrical, symmetry.