Magnetic Field Spectrum Research Articles

Photoluminescence of InAs quantum dots grown on disoriented GaAs substrates

The photoluminescence spectra in an external magnetic field of an ensemble of InAs quantum dots grown by molecular beam epitaxy on a (001) GaAs substrate with a disorientation in the [010] direction are studied. A redistribution of the photoexcited carriers among different groups of dots under the influence of the magnetic field is observed. The concentration of quantum dots is determined by analyzing the data.

Reflectance difference spectroscopy and magneto-optical analysis of digital magnetic heterostructures

We report on the investigation of CdTe/Cd0.75Mg0.25Te quantum wells grown by molecular-beam epitaxy and containing a digital distribution of magnetic MnTe fractional monolayers in the well. The influence of electronic confinement and of exciton primary localization on the formation of ferromagnetic ordering on the scale of small spin clusters (polarons) has been investigated by performing selective excitation of the exciton luminescence. The calculated temperature dependence of the free magnetic polarons binding energy F is found to qualitatively agree with the photoluminescence excitation data and the ferromagnetic clusters result to be more stable in these magnetic structures of reduced dimensionality, with respect to the three-dimensional case. The magneto-optical Kerr effect is discussed in the case of reflectance difference spectroscopy (RDS) and from the angular dependence of the RDS spectra in magnetic field, signals due to interband Faraday rotation and signals originating from crystal anisotropy could be distinguished. The Faraday rotation in these structures is found to follow the Curie–Weiss law with a transition temperature of −6 K corresponding to the nominal ion spin concentration in the magnetic barriers.

Back Reaction to the Spectrum of Magnetic Field in the Kinetic Dynamo Theory

We take account of the lowest order back reaction on a fluid and modify the KulsrudAnderson equation, ∂tEM =2 γEM , obtained in the kinetic dynamo theory, where EM is the energy density of the magnetic field. We apply our results to certain astrophysical stages where the magnetic field is expected to be amplified by the dynamo mechanism.

Investigation of Magnetic Oscillation of Turbulent Plasma Flow in Mirror Traps

On the installations AMBAL-M and MAL in turbulent plasma column the longitudinal current of some kiloampers was measured. The fluctuation spectrum of magnetic field was measured by azimuthal magnetic probes and expansion of plasma current by spatial modes was carried out.According to the experimental data the amplitudes of currents In, sizes Rn and shifts from axis of simmetry ΔRn for 3rd (40kHz) and 5th (72kHz) basic modes were calculated.

A helically symmetric stellarator (HSX)

The Helically Symmetric Experiment (HSX) Is a quasi-helically symmetric (QHS) stellarator currently under construction at the Torsatron/Stellarator Laboratory of the University of Wisconsin-Madison. This device is unique in its magnetic design in that the magnetic field spectrum possesses only a single dominant (helical) component. This design avoids the large direct orbit losses associated with conventional stellarators. The restoration of an ignorable coordinate in the confining magnetic field makes this device analogous to an axisymmetric q=1/3 tokamak with respect to neoclassical confinement.

Many-body effects in photoluminescence of a two-dimensional electron gas in high magnetic field

The behaviour of a two-dimensional electron gas has been studied in a series of modulation doped quantum wells of the type In0.53Ga0.47As/InP with different electron densities. The low energy sidebands in the optical emission spectra in high magnetic field due to many-body effects have been investigated. In a sample with a high electron density the first shake-up sideband is followed up to 20 T, where it reappears after crossing with phonon replicas. Its intensity profile in high magnetic field confirms previous interpretations on the filling factor dependence. In a sample with a lower electron density, polarization measurements illustrated the emptying of the higher spin state in the first Landau level, reaching a high degree of polarization at a filling factor below one.

Inverse cascade of primordial magnetic field in MHD turbulence

The feature of the spectrum of primordial magnetic field is studied by using renormalization group analysis in magnetohydrodynamics. Taking account of the renormalized resistivity at the fixed point, we show that the scaling of the typical scale with time obeys L( t)∼ t 2/5 for random initial condition.

Magnetic field strength distributions and spectra in the heliosphere and their significance for cosmic ray modulation: Voyager 1, 1980–1994

Annual variations of the hourly averages of the heliospheric magnetic field strengthBobserved by Voyager 1 (V1) have been analyzed for the 15‐year interval 1980–1994 inclusive. During this period, V1 moved from a radial distance of 6.9–58.1 AU and from a heliolatitude of −5.4 to 32.7°. The annual distribution function ofBis nearly lognormal. The width of the distribution, w, does not change systematically with time, distance, latitude or solar activity: 〈w〉 = 0.61 ± 0.02. The standard deviation ofB, SD(B), normalized by the average,Bav, is also invariant: 〈SD(B)/Bav〉 = 0.67 ± 0.03. There is a decadal frequency range for which the power spectral density (PSD) ofB(t) is well described by a power law with an exponent s that ranges from −2.0 near solar maximum (1980, 1990) when GMIRs were present to −1.5 near solar minimum (1987–1988). A Global Merged Interaction Region (GMIR) is a shell‐like region that encircles the Sun, propagates away from the Sun, has a radial thickness of the order of a few AU and contains intense, disturbed magnetic fields. The lower limit of the scaling range corresponds to a period of ≈ 5 days and is invariant in the 15 years studied. The relative amplitude of the PSD at the lower limit of this range is also invariant. However, the relative amplitude of the fluctuations at the solar rotation period of 26 days decreases with increasing distance from the sun asR−(0–9 ± 0.2). These derived observational results in time and frequency domains require theoretical models of the solar wind expansion into the distant heliosphere that include both physical and statistical features. Such models could be based on (1) energy and/or entropy principles or (2) conventional MHD along with temporally complex but statistically simple input functions. Our results suggest an anticorrelation between the annual magnetic field strength and cosmic ray intensity, as is observed during the 15‐year period considered here. Statistical models of the magnetic field might provide a basis for new models of cosmic ray modulation.

Microwave background anisotropies from Alfvén waves

We investigate microwave background anisotropies in the presence of primordial magnetic fields. We show that a homogeneous field with fixed direction can amplify vector perturbations. We calculate the correlations of δT/T explicitly and show that a large scale coherent field induces correlations between al-1,m and al+1,m. We discuss constraints on the amplitude and spectrum of a primordial magnetic field imposed by observations of CMB anisotropies.

Statistical analysis of magnetic-field spectra

We have calculated and statistically analyzed the magnetic-field spectrum (the ``B-spectrum'') at fixed electron Fermi energy for two quantum dot systems with classically chaotic shape. This is a new problem which arises naturally in transport measurements where the incoming electron has a fixed energy while one tunes the magnetic field to obtain resonance conductance patterns. The ``B-spectrum'', defined as the collection of values ${B_i}$ at which conductance $g(B_i)$ takes extremal values, is determined by a quadratic eigenvalue equation, in distinct difference to the usual linear eigenvalue problem satisfied by the energy levels. We found that the lower part of the ``B-spectrum'' satisfies the distribution belonging to Gaussian Unitary Ensemble, while the higher part obeys a Poisson-like behavior. We also found that the ``B-spectrum'' fluctuations of the chaotic system are consistent with the results we obtained from random matrices.

Advanced antenna system for Alfvén wave plasma heating and current drive in TCABR tokamak

An advanced antenna system that has been developed for investigation of Alfvén wave plasma heating and current drive in the TCABR tokamak is described. The main goal was the development of such a system that could insure the excitation of travelling single helicity modes with predefined wave mode numbers M and N. The system consists of four similar modules with poloidal windings. The required spatial spectrum is formed by proper phasing of the RF feeding currents. The impedance matching of the antenna with the four-phase oscillator is accomplished by resonant circuits which form one assembly unit with the RF feeders. The characteristics of the antenna system design with respect to the antenna-plasma coupling and plasma wave excitation, for different phasing of the feeding currents, are summarised. The antenna complex impedance Z= Z R+ Z I is calculated taking into account both the plasma response to resonant excitation of fast Alfvén waves and the nonresonant excitation of vacuum magnetic fields in conducting shell. The matching of the RF generator with the antenna system during plasma heating is simulated numerically, modelling the plasma response with mutually coupled effective inductances with corresponding active Z R and reactive Z I impedances. The results of the numerical simulation of the RF system performance, including both the RF magnetic field spectrum analysis and the modelling of the RF generator operation with plasma load, are presented.

Finite-Size Bosonization of 2-Channel Kondo Model: A Bridge between Numerical Renormalization Group and Conformal Field Theory

We generalize Emery and Kivelson's (EK) bosonization-refermionization treatment of the 2-channel Kondo model to finite system size and on the EK line analytically construct its exact eigenstates and finite-size spectrum. The latter crosses over to conformal field theory's (CFT) universal non-Fermi-liquid spectrum (and yields the most-relevant operators' dimensions), and further to a Fermi-liquid spectrum in a finite magnetic field. Our approach elucidates the relation between bosonization, scaling techniques, the numerical renormalization group (NRG), and CFT. All CFT Green's functions are recovered with remarkable ease from the model's scattering states.

Magneto-photoluminescence spectra of GaP/AlP short-period superlattices in high magnetic fields and uniaxial pressures

A GaP/AlP multiple-superlattice (SL) containing four kinds of (GaP) m /(AlP) n SLs with different periodicity, (m,n)=(4,4), (5,4), (6,4), (7,4) was grown on a GaP (1 0 0) substrate by means of gas source molecular beam epitaxy. A clamp cell for generating uniaxial pressure was developed allowing an optical measurement in pulsed high magnetic field. We measured magneto-PL spectra in high magnetic fields up to 40 T and uniaxial pressure up to 0.5 GPa and found an intensity enhancement of the PL originating from the (4,4) SL, associated with the indirect to pseudo-direct band-gap transition. The magnetic field dependence at 0.5 GPa is slightly changed from that at atmospheric pressure: the decrease of the PL intensity with increasing magnetic field perpendicular to the SL layer becomes small. This feature indicates that the magnetic field dependence of PL deeply concerns the exciton recombination at the hetero-interfaces.

Marine magnetotellurics for petroleum exploration Part I: A sea‐floor equipment system

Induction in electrically conductive seawater attenuates the magnetotelluric (MT) fields and, coupled with a minimum around 1 Hz in the natural magnetic field spectrum, leads to a dramatic loss of electric and magnetic field power on the sea floor at periods shorter than 1000 s. For this reason the marine MT method traditionally has been used only at periods of 103 to 105 s to probe deep mantle structure; rarely does a sea‐floor MT response extend to a 100-s period. To be useful for mapping continental shelf structure at depths relevant to petroleum exploration, however, MT measurements need to be made at periods between 1 and 1000 s. This can be accomplished using ac-coupled sensors, induction coils for the magnetic field, and an electric field amplifier developed for marine controlled‐source applications. The electrically quiet sea floor allows the attenuated electric field to be amplified greatly before recording; in deep (1-km) water, motional noise in magnetic field sensors appears not to be a problem. In shallower water, motional noise does degrade the magnetic measurement, but sea‐floor magnetic records can be replaced by land recordings, producing an effective sea‐surface MT response. Field trials of such equipment in 1-km-deep water produced good‐quality MT responses at periods of 3 to 1000 s; in shallower water, responses to a few hertz can be obtained. Using an autonomous sea‐floor data logger developed at Scripps Institution of Oceanography, marine surveys of 50 to 100 sites are feasible.

Cascade and dynamo action in a shell model of magnetohydrodynamic turbulence

A shell model of magnetohydrodynamic turbulence, which allows one to conserve all the integrals of motion in both two and three dimensions, is proposed and studied. We demonstrate that this model reproduces basic facts known in the small-scale turbulent dynamo theory. In particular, we consider a process of redistribution of magnetic helicity generated by the mean-field dynamo, described in the model as magnetic forcing, into a small-scale magnetic field. We argue that the resulting equilibrium magnetic field spectrum strongly depends on the level of magnetic helicity and cross helicity, introduced by the large scales. The spectra with spectral index ``-5/3'' dominate if the cross helicity vanishes. If the level of cross helicity is high (correlated velocity and magnetic field) the spectra depend on the magnetic helicity: the strong magnetic helicity suppresses any cascade providing steep spectra, while the vanishing helicity of turbulent magnetic fields results in the occurrence of Kraichnan-Iroshnikov spectral index ``-3/2.''

A statistical study of auroral electromagnetic ion cyclotron waves

This paper discusses results from a statistical study of narrowband auroral electromagnetic ion cyclotron (EMIC) waves recorded by the Freja magnetic field experiment from December 1, 1993, to November 30, 1994. The study involves data collected at all magnetic local times (MLT), invariant latitudes between 40° and 75°, and altitudes between 1200 and 1750 km. Wave events have been identified using a two‐step process. First, magnetic field spectra in the frequency range between 5 and 256 Hz were surveyed using an automated procedure which detects spectral peaks. Second, the results of this automated procedure were confirmed by visually inspecting wave spectrograms which contained these spectral peaks. A total of 316 auroral EMIC wave events were identified. The occurrence of these waves peak at auroral latitudes in the premidnight sector (1800–0100 MLT). The events have a strong seasonal dependence with peak occurrence in the winter. These properties are the same as those of auroral electron precipitation, which is suggested to be the free energy source of these waves. The spectral properties of the waves were also analyzed. The wave usually contained either one or two spectral peaks. In the cases where two spectral peaks were observed one peak was below the local O+ gyrofrequency (ƒO+) and one was above the He+ gyrofrequency (ƒHe+). The observations suggest that auroral EMIC waves can be generated below ƒO+. The wave spectra were consistent with wave generation at altitudes near or above Freja's altitude. Analysis of the spectra suggest that the auroral EMIC waves originated from a source altitude between 1500 and 5500 km.

Pulsed ESR investigations of anisotropic interactions in M@C 82 (M=Sc,Y,La)

, Y@C82, and La@C82 in frozen solutions. We were able to determine the g tensors of these molecules by analysing magnetic field spectra at X-band (9.5 GHz) and W-band (94 GHz) frequencies. Moreover, in Y@C82 we have investigated the hyperfine interaction of the 89Y nuclear spin (I=1/2) with the electron spin on the C82 cage. The principal values of the hyperfine tensor A and the relative orientation of g and A tensors were determined by applying three- and four-pulse electron spin echo envelope modulation techniques (ESEEM).

Primordial Magnetic Fields, Anomalous Matter-Antimatter Fluctuations, and Big Bang Nucleosynthesis

We show that the presence of primordial stochastic (hypercharge) magnetic fields before the electroweak (EW) phase transition induces isocurvature fluctuations (baryon number inhomogeneities). Depending on the details of the magnetic field spectrum and on the particle physics parameters (such as the strength of the EW phase transition and electron Yukawa couplings) these fluctuations may survive until the Big Bang nucleosynthesis (BBN). Their lenghtscale may exceed the neutron diffusion length at that time, while their magnitude can be so large that sizable antimatter domains are present. This provides the possibility of a new type of initial conditions for non-homogeneous BBN or, from a more conservative point of view, stringent bounds on primordial magnetic fields.

Three-dimensional tokamak equilibria and stellarators with two-dimensional magnetic symmetry

Three-dimensional computer codes have been developed to simulate equilibrium, stability and transport in tokamaks and stellarators. Bifurcated solutions of the tokamak problem suggest that three-dimensional effects may be more important than has generally been thought. Extensive calculations have led to the discovery of a stellarator configuration with just two field periods and with aspect ratio 3.2 that has a magnetic field spectrum with toroidal symmetry. Numerical studies of equilibrium, stability and transport for this new device, called the Modular Helias-like Heliac 2 (MHH2), will be presented.

Effect of Polymethylene-Chain Dynamics on the Lifetime of a Charge-Separated Biradical Studied by RYDMR Spectroscopy

The novel solvent-viscosity dependence of the time-resolved reaction-yield-detected magnetic resonance (RYDMR) spectra is observed for the polymethylene-linked biradical generated in the photoinduced intramolecular electron-transfer reaction between N,N-dimethylaniline (DMA) and phenanthrene (Ph) in the compound of the type Ph−(CH2)10−O−(CH2)2−DMA. With decreasing the solvent viscosity, the line width and peak intensity of the RYDMR B0 (external magnetic field) spectrum increases and decreases, respectively. This result is interpreted in terms of the decrease of the lifetime of intermediate ionic biradicals at lower viscosity. This explicitly shows that the lifetime of the ionic biradical is governed by the reencounter frequency of the two terminal radicals determined by the polymethylene-chain dynamics. The lifetime of the spin states in the ionic biradical is estimated to be less than 2 ns.