High Transverse Magnetic Field Research Articles

Size dependence- and induced transformations- of fractional quantum Hall effects under tilted magnetic fields

Two-dimensional electron systems subjected to high transverse magnetic fields can exhibit Fractional Quantum Hall Effects (FQHE). In the GaAs/AlGaAs 2D electron system, a double degeneracy of Landau levels due to electron-spin, is removed by a small Zeeman spin splitting, g mu _B B, comparable to the correlation energy. Then, a change of the Zeeman splitting relative to the correlation energy can lead to a re-ordering between spin polarized, partially polarized, and unpolarized many body ground states at a constant filling factor. We show here that tuning the spin energy can produce fractionally quantized Hall effect transitions that include both a change in nu for the R_{xx} minimum, e.g., from nu = 11/7 to nu = 8/5, and a corresponding change in the R_{xy}, e.g., from R_{xy}/R_{K} = (11/7)^{-1} to R_{xy}/R_{K} = (8/5)^{-1}, with increasing tilt angle. Further, we exhibit a striking size dependence in the tilt angle interval for the vanishing of the nu = 4/3 and nu = 7/5 resistance minima, including “avoided crossing” type lineshape characteristics, and observable shifts of R_{xy} at the R_{xx} minima- the latter occurring for nu = 4/3, 7/5 and the 10/7. The results demonstrate both size dependence and the possibility, not just of competition between different spin polarized states at the same nu and R_{xy}, but also the tilt- or Zeeman-energy-dependent- crossover between distinct FQHE associated with different Hall resistances.

Heat transfer in mixed convection of molten salt in the presence of magnetic fields

This paper discusses the results of an experimental study and numerical simulation of a down flow of heat exchange of molten salt in a uniformly heated pipe under the action of a high transverse magnetic field. This article also presents a comparison of the results of the experiment and numerical simulation. The impact of transverse magnetic field on a flow structure and heat transfer in a vertical pipe was studied. Experimental studies were performed in the Reynolds number range (Re = 4000-20000) under the influence of a magnetic field (Ha = 13-14). Numerical calculations were estimate in the same ranges of Reynolds numbers and Hartman numbers which were 13–30. The paper is about comparison of results.

Crystallization in the Fractional Quantum Hall Regime Induced by Landau-Level Mixing.

The interplay between strongly correlated liquid and crystal phases for two-dimensional electrons exposed to a high transverse magnetic field is of fundamental interest. Through the nonperturbative fixed-phase diffusion MonteCarlo method, we determine the phase diagram of the Wigner crystal in the ν-κ plane, where ν is the filling factor and κ is the strength of Landau-level (LL) mixing. The phase boundary is seen to exhibit a striking ν dependence, with the states away from the magic filling factors ν=n/(2pn+1) being much more susceptible to crystallization due to Landau-level mixing than those at ν=n/(2pn+1). Our results explain the qualitative difference between the experimental behaviors observed in n- and p-doped gallium arsenide quantum wells and, in particular, the existence of an insulating state for ν<1/3 and also for 1/3<ν<2/5 in low-density p-doped systems. We predict that, in the vicinity of ν=1/5 and ν=2/9, increasing LL mixing causes a transition not into an ordinary electron Wigner crystal, but rather into a strongly correlated crystal of composite fermions carrying two vortices.

Thermal mixing enhancement of liquid metal MHD free-surface flow by optimizing vortex generator arrays

Liquid divertor concepts in fusion reactor research have been widely studied. Recently, thermal mixing enhancement of the locally heated free-surface of liquid metal MHD (Magneto-Hydro-Dynamics) film flow using one vortex generator was proposed and investigated by the authors. In this study, an inline arrangement of many hemispherical protrusions to generate many vortices in their wakes was proposed, and the transport characteristics of high temperature fluid at the free-surface to the bottom (i.e., thermal mixing) in liquid metal MHD film flow was investigated experimentally. The experiments for three streamwise pitch lengths between the protrusions under various transverse magnetic field strengths (B = 0.00–0.33T) for two flowrate conditions were conducted. As the results, the heat flux distributions on the bottom wall of the film flow and the efficiency of heat transport showed the existence of the optimum streamwise pitch length for the thermal mixing enhancement even under relatively high transverse magnetic field. Therefore, the inline arrangement of many hemispherical protrusions can be considered as one of the candidates for thermal mixing enhancement methods of liquid metal MHD free-surface flow application.

Coupling of magnetic orders inLa2CuO4+x

High transverse magnetic field and zero field muon spin rotation and relaxation measurements have been carried out in a lightly oxygen-doped high-Tc parent compound La2CuO4 in a temperature range from 2 K to 300 K. As in the stoichiometric compound, muon spin rotation spectra reveal, along with the antiferromagnetic local field, the presence of an additional source of magnetic field at the muon. The results indicate that this second magnetic order is driven by the antiferromagnetism at low temperature but the two magnetic orders decouple at higher temperature. The ability of muSR to detect this additional magnetism deteriorates with doping, thus rendering the technique impotent to reveal time-reversal symmetry breaking in superconductors.

Combining an obstacle and electrically driven vortices to enhance heat transfer in a quasi-two-dimensional MHD duct flow

The design of vortex promoters in a heated-wall duct is often limited by the considerations of practicality, especially in complex systems such as fusion blankets. The present study investigates the use of current injection to invoke a street of vortices in quasi-two-dimensional high transverse magnetic field magnetohydrodynamic duct flows to enhance instability behind a cylinder. The intent is to generate intensive flow vorticity parallel to a magnetic field downstream of a field-aligned cylinder. Electric current enters the flow through an electrode embedded in one of the Hartmann walls, radiates outward, imparting a rotational forcing around the electrode due to the Lorentz force. The quasi-two-dimensional nature of these flows then promotes a vortical rotation across the interior of the duct with axis aligned to the magnetic field. The hot and cold walls are parallel to the magnetic field. Electric current amplitude and pulse width, excitation frequency and electrode position are systematically varied to explore their influences on the convective heat transport phenomenon. This investigation builds on a recommendation from previous work of Bühler (J. Fluid Mech., vol. 326, 1996, pp. 125–150) dedicated to understanding of the flow stability in a similar configuration. This study provides supportive evidence for the use of current injection as an alternative to the conventional mechanically actuated turbuliser, with heat transfer almost doubled for negligible additional pumping power requirements.

Comment on “Direct observation of the magnetic polaron”

Storchak et al. have reported a muon spin-rotation/relaxation $(\ensuremath{\mu}$SR) investigation of the ferromagnetic semiconductor EuS. The muon frequency spectra in a high transverse magnetic field in the paramagnetic state show a structure which is attributed to a magnetic polaron bound to the positive muon. In this Comment, the spectroscopic signature for a bound magnetic polaron is reexamined. In the fast fluctuation limit, which appears to be appropriate for EuS, one expects the magnetic polaron spin $\mathcal{S}$ and resulting hyperfine field on the muon to be rapidly fluctuating. This leads to a single frequency, which is shifted relative to the Larmor frequency by an amount proportional to a Brillouin function associated with a large spin $\mathcal{S}$. In the case of EuS there is no evidence for any large attendant spin $S$. The anomalous signal in EuS may be due to a small fraction of muons occupying the face-center interstitial site which is occupied in isostructural compounds.

Measurements of the Lorentz angle with a Micromegas detector in high transverse magnetic fields

We measured the Lorentz angle in an argon + 10 % isobutane mixture using a Micromegas detector placed inside the CLAS-DVCS solenoid of Hall B at the Jefferson Laboratory. The primary goal of these tests was to validate Magboltz-based simulations developed for the implementation of Micromegas bulk detectors in the Central Tracker of the future CLAS12 spectrometer. We used a UV laser to extract electrons from the drift electrode of the detector. After amplification, the electrons were collected on the readout strips and the signals recorded. The difference of position observed between runs with and without magnetic field provided a direct measurement of the Lorentz angle. Scans in the drift electric field were performed for the first time in very high magnetic fields, up to 4.2 T, and a good agreement was observed with the Magboltz predictions.

Muon spin relaxation study of superconductingBi2Sr2−xLaxCuO6+δ

We have performed transverse-field (TF) and zero-field (ZF) $\ensuremath{\mu}\mathrm{SR}$ measurements of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}\mathrm{Cu}{\mathrm{O}}_{6+\ensuremath{\delta}}$ (Bi2201) systems with $x=0.2$, 0.4, 0.6, and 1.0, using ceramic specimens with modest $c$-axis alignment and single-crystal specimens. The absence of static magnetic order has been confirmed in underdoped $(x=0.6)$ and optimally doped $(x=0.4)$ systems at $T=2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, while only a very weak signature towards static magnetism has been found at $T=2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the $x=1.0$ system, which is a lightly hole-doped nonsuperconducting insulator. In the superconducting ($x=0.6$, 0.4, and 0.2) systems, the relaxation rate $\ensuremath{\sigma}$ in TF-$\ensuremath{\mu}\mathrm{SR}$, proportional to ${n}_{s}∕{m}^{*}$ (superconducting carrier density and effective mass), followed a general trend found in other cuprate systems in a plot of ${T}_{c}$ vs ${n}_{s}∕{m}^{*}(T\ensuremath{\rightarrow}0)$. Assuming the in-plane effective mass ${m}^{*}$ for Bi2201 to be comparable to three to four times the bare electron mass ${m}_{e}$ as found in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ (LSCO) and ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) systems, we obtain ${n}_{s}\ensuremath{\sim}0.15--0.2$ per Cu for the $x=0.4$ Bi2201 system. This carrier density is much smaller than the Hall number ${n}_{\mathit{\text{Hall}}}\ensuremath{\sim}10$ per Cu obtained at $Tl1.6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in high magnetic fields $(40--60\phantom{\rule{0.3em}{0ex}}\mathrm{T})$ along the $c$ axis applied to suppress superconductivity. The present results of the superfluid density $({n}_{s}∕{m}^{*})$ in Bi2201 are compared with those from other cuprate systems, including YBCO systems with very much reduced ${T}_{c}l20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ studied by microwave, ${H}_{c1}$, and inductance methods. Additional muon-spin-relaxation $(\ensuremath{\mu}\mathrm{SR})$ measurements have been performed on a single-crystal specimen of Bi2201 $(x=0.4)$ in a high transverse magnetic field of $5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ parallel to the $c$ axis, in order to search for the field-induced muon spin relaxation recently found in LSCO and some other high-temperature superconducting cuprate (HTSC) systems well above ${T}_{c}$. The nearly temperature-independent and very small relaxation rate observed in Bi2201 above ${T}_{c}$ rules out a hypothesis that the field-induced relaxation is directly proportional to the magnitude of the Nernst coefficient, which is a measure of the strength of dynamic superconductivity. We also describe a procedure for angular averaging of $\ensuremath{\sigma}$ in $\ensuremath{\mu}\mathrm{SR}$ measurements using ceramic specimens with modest alignment of $c$-axis orientations, together with the neutron-scattering results obtained for determining the orientation distribution of microcrystallites in the present ceramic specimens.

Muon Spin Relaxation Studies of Magnetic-Field-Induced Effects in High-TcSuperconductors

Muon spin relaxation measurements in high transverse magnetic fields [FORMULA: SEE TEXT] revealed strong field-induced quasistatic magnetism in the underdoped and Eu-doped (La,Sr)2CuO4 and La1.875Ba0.125CuO4, existing well above Tc and TN. The susceptibility counterpart of Cu spin polarization, derived from the muon spin relaxation rate, exhibits a divergent behavior towards T approximately 25 K. No field-induced magnetism was detected in overdoped La1.81Sr0.19CuO4, optimally doped Bi2212, and Zn-doped YBa2Cu3O7.

Superconducting and Resistive Tilted Coil Magnets for Generation of High and Uniform Transverse Magnetic Field

Mathematical foundation is laid for a relatively new type of magnets generating uniform transverse magnetic field-tilted coil magnets. These consist of concentric nested coils with elliptical turns tilted at a certain angle to the central axis and current flowing in opposite directions in the coils tilted at opposite angles, generating a perfectly uniform transverse field. Both superconducting wire-wound and resistive Bitter tilted coils are discussed. An original analytical method is used to prove that the wire-wound tilted coils have the ideal distribution of the axial current density-"cosine-theta". To program coil fabrication tooling, a parametric description of the helical path of the conductor within a tilted coil is given. Magnetic fields are calculated for a tilted Bitter coil magnet using an original exact solution for current density in an elliptical Bitter disk.

Hydrogen states in CuInSe2—a μSR study

Implanted positive muons, used to mimic protons, are observed to form a diamagnetic state in the chalcopyrite semiconductor CuInSe2. A shift of the muon precession frequency is observed at a high transverse magnetic field (1600G) and is interpreted as being associated to the presence of a transient neutral state. A high charge or spin flip rate of the electron bound to the muon is suggested to be the cause for the absence of a hyperfine splitting, i.e., a direct signature of a neutral state. This observation is consistent with the presence of a hydrogen donor level close to or above the conduction band minimum.

Influence of mutual drag of the carrier-phonon system on the thermopower and transverse Nernst-Ettingshausen effect

The thermopower and Nernst-Ettingshausen (NE) effect in degenerate semiconductors and semimetals placed in high electric and magnetic fields are calculated by taking into account the heating of both electrons and phonons as well as their thermal and mutual drags. The magnetic and electric field dependences of the thermoelectric power and the transverse NE voltage are found in analytical forms. It is shown that in weak and high transverse magnetic fields, the electronic and phonon parts of the NE coefficients change their sign for some scattering mechanisms.

Light hydrogen isotopeμ+in Sc andα−ScHxsolid solutions

The response of Sc and $\ensuremath{\alpha}\ensuremath{-}{\mathrm{ScH}}_{x}$ systems $(0&lt;~x&lt;~0.25)$ to the implantation of positive muons, i.e., light H isotopes, has been studied by ${\ensuremath{\mu}}^{+}\mathrm{SR}$ spectroscopy in high and low transverse magnetic field, and in zero field (ZF). The investigation covered a temperature range from 300 K down to 2 K, for various orientations of the single-crystal samples with respect to the field or initial ${\ensuremath{\mu}}^{+}$ polarization. Below 30 K the positive muons occupy, in pure Sc, a localized ``fast-tunneling'' state, extending over two adjacent, interstitial-tetrahedral (T) sites above and below the $\mathrm{ab}$-atomic plane of the hcp lattice. In $\ensuremath{\alpha}\ensuremath{-}{\mathrm{ScH}}_{x},$ a fraction of the muons stays in this tunneling state, while for the other part, the muons pair with a H atom, to form the analog of the T-pair state H-Sc-H aligned along the c axis, specific to the ${\ensuremath{\alpha}}^{*}$ phase. The electric-field gradients, of muonic as well as intrinsic-crystalline origin, acting on the nuclei near the ${\ensuremath{\mu}}^{+}$ influence the results at low and zero magnetic fields. Their characteristics and the H-concentration dependences are evaluated. For the ZF-data analysis a newly developed uniaxial version of the Kubo-Toyabe formula was applied. Above 30 K the long-range diffusion of positive muons becomes noticeable; a two-state model explains very well the data. It assumes the ${\ensuremath{\mu}}^{+}$ to be, alternatively, in a state of free diffusion between localized-tunneling states or in a ${\ensuremath{\mu}}^{+}$-Sc-H trap. The model describes the repeated ${\ensuremath{\mu}}^{+}$-capture and release processes with three characteristic correlation times, which follow Arrhenius laws. The parameters, e.g., activation energies and preexponential factors, are compared to the parameters of the H and D dynamics in $\ensuremath{\alpha}\ensuremath{-}\mathrm{ScH}{(D)}_{x}.$ From $\ensuremath{\mu}\mathrm{SR}$ measurements in a deuterated sample $(\ensuremath{\alpha}\ensuremath{-}{\mathrm{ScD}}_{0.10})$ one infers very similar behaviors of the ${\ensuremath{\mu}}^{+}$ in D and H solid solutions in the Sc host.

Magnetoexcitons in coupled quantum wells

Excitons in coupled quantum wells with spatially separated electrons (e) and holes (h) in a high transverse magnetic field ( H) are considered. The exciton energies are calculated for various Landau levels and arbitrary interwell separations ( D). The change of parameter D l (where l= hc eH ) leads to a reconstruction of the exciton dispersion laws ε( P) ( P is the conserved “magnetic” momentum of the magnetoexciton, being proportional to the mean distance between e and h along the wells). Noncentral (“roton”) extrema exist only for interlayer separations D l smaller than the critical one (the latter depends on exciton quantum numbers). Impurity states in CQWs are also calculated.

(TM) 2X organic superconductors: interplay between 1-D charge localization and higher dimensionality cross-over

A survey of transport and magnetic properties of the (TM) 2X series together with a recent investigation in high magnetic fields shows that the charge localization occurring at low temperature in a 1-D half-filled band is suppressed by the 1-D to 2-D (3-D) cross-over in selenium compounds. However, a localization around 30 K in the non-ordered phase can be reactivated by the application of a high transverse magnetic field. The intermediate (strong) coupling limit is valid for the spin degree of freedom whereas the charge is governed by the strength of the Umklapp scattering varying by a large factor from sulfur to selenium compounds and under pressure.

Correlations, dimensionality and instabilities in organic superconductors

We discuss the role of Coulombic repulsion in organic superconductors exhibiting quasi-one-dimensional transport properties. A recent investigation in high magnetic fields shows that the charge localization occurring at low temperature in a 1-D half-filled band is suppressed by the 1-D to 2-D (3-D) cross-over in selenium compounds. However, a localization around 30 K in the non-ordered phase can be reactivated by the application of a high transverse magnetic field. This phenomenon provides an interpretation for the large transverse magnetoresistance observed in quasi-1-D conductors with open Fermi surfaces. The intermediate (strong) coupling limit is valid for the spin degree of freedom whereas the charge is governed by the strength of the Umklapp scattering varying by a large factor from sulfur to selenium compounds and under pressure.

Methods to measure the charge of the quasiparticles in the fractional quantum Hall effect.

We propose various experimental circumstances in which the longitudinal resistance of a two-dimensional electron gas in a high transverse magnetic field depends in a simple and characteristic way on the charge of the quasiparticle excitations. We propose that experiments of this sort could be used to directly measure the charge of the quasiparticle excitations which carry the dissipative part of the current. While it has been persuasively argued by Laughlin that the Hall conductance itself measures the quasiparticle charge, the connection is indirect, since the Hall current is carried by the condensate, not by the quasiparticles.

Spatial characteristics of the EML plasma armature and the dependence on plasma composition

A simplified picture of a radiation-dominated plasma armature of an electromagnetic launcher (EML) is developed to evaluate in quasi-steady-state its spatial characteristics and their dependence on plasma composition, applied electric and magnetic fields, and possible temporal changes. A graphical solution of the energy balance equation is found which permits quick visualization of the effects of changing rail voltage or bore diameter. It is found that metallic vapor armatures will have plasmas extending to the highest expected pressures near the projectile. However, armatures consisting of the ablation products of G-9 or Lexan insulators or other nonmetallic plasmas such as hydrogen or argon cannot satisfy the energy balance conditions at high pressures unless the electric field is sufficiently high. Such armatures may not develop the desired magnetic pressure gradient in their leading edge to accelerate the projectile, with possible performance degradation. At low pressures (1-10 atm) in the trailing edge of the armature, the electrical conductivity can be significantly reduced by the high transverse magnetic field. It is concluded that this extended tail of low temperature plasma could be of importance in restrike phenomena.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effect of a high transverse magnetic field on the tunneling through barriers between semiconductors and superlattices.

We present a quantum-mechanical analysis of magnetotunneling in a high transverse magnetic field B. We use a transfer-Hamiltonian technique for computing the current density through a barrier as a function of both B and the bias voltage V. Several systems of the type semiconductor-barrier-semiconductor and superlattice-barrier-superlattice are studied. From our results quantum oscillations and negative differential conductance are interpreted in terms of variations in the available channels for tunneling. Special attention is devoted to the comparison with previously reported experimental information.