421 research outputs found
Spin Polarization Phenomena and Pseudospin Quantum Hall Ferromagnetism in the HgTe Quantum Well
The parallel field of a full spin polarization of the electron gas in a
\Gamma8 conduction band of the HgTe quantum well was obtained from the
magnetoresistance by three different ways in a zero and quasi-classical range
of perpendicular field component Bper. In the quantum Hall range of Bper the
spin polarization manifests in anticrossings of magnetic levels, which were
found to strongly nonmonotonously depend on Bper.Comment: to be published in AIP Conf. Proc.: 15-th International Conference on
Narrow Gap Systems (NGS-15
Effect of exchange electron-electron interaction on conductivity of InGaAs single and double quantum wells in ballistic regime
We report an experimental study of quantum conductivity corrections for
two-dimensional electron gas in a GaAs/InGaAs/GaAs single and double quantum
wells in a wide temperature range (1.8-100) K. We perform a comparison of our
experimental data for the longitudinal conductivity at zero magnetic field to
the theory of interaction-induced corrections to th transport coefficients. In
the temperature range from 10 K up to (45-60) K, wich covers the ballistic
interaction regimes for our samples, a rather good agreement between the theory
and our experimental results has been found
Magnetotransport in Double Quantum Well with Inverted Energy Spectrum: HgTe/CdHgTe
We present the first experimental study of the double-quantum-well (DQW)
system made of 2D layers with inverted energy band spectrum: HgTe. The
magnetotransport reveals a considerably larger overlap of the conduction and
valence subbands than in known HgTe single quantum wells (QW), which may be
regulated by an applied gate voltage . This large overlap manifests itself
in a much higher critical field separating the range above it where the
quantum peculiarities shift linearly with and the range below with a
complicated behavior. In the latter case the -shaped and double--shaped
structures in the Hall magnetoresistance are observed with their
scale in field pronouncedly enlarged as compared to the pictures observed in an
analogous single QW. The coexisting electrons and holes were found in the whole
investigated range of positive and negative as revealed from fits to the
low-field -shaped and from the Fourier analysis of
oscillations in . A peculiar feature here is that the found
electron density remains almost constant in the whole range of investigated
while the hole density drops down from the value a factor of 6 larger
than at extreme negative to almost zero at extreme positive
passing through the charge neutrality point. We show that this difference
between and stems from an order of magnitude larger density of states
for holes in the lateral valence band maxima than for electrons in the
conduction band minimum. We interpret the observed reentrant sign-alternating
between electronic and hole conductivities and its zero
resistivity state in the quantum Hall range of fields on the basis of a
calculated picture of magnetic levels in a DQW.Comment: 15 pages, 13 figure
The variation of the magnetic field of the Ap star HD~50169 over its 29 year rotation period
Context. The Ap stars that rotate extremely slowly, with periods of decades
to centuries, represent one of the keys to the understanding of the processes
leading to the differentiation of stellar rotation.
Aims. We characterise the variations of the magnetic field of the Ap star HD
50169 and derive constraints about its structure.
Methods. We combine published measurements of the mean longitudinal field
of HD 50169 with new determinations of this field moment from circular
spectropolarimetry obtained at the 6-m telescope BTA of the Special
Astrophysical Observatory of the Russian Academy of Sciences. For the mean
magnetic field modulus , literature data are complemented by the analysis of
ESO spectra, both newly acquired and from the archive. Radial velocities are
also obtained from these spectra.
Results. We present the first determination of the rotation period of HD
50169, Prot = (29.04+/-0.82) y. HD 50169 is currently the longest-period Ap
star for which magnetic field measurements have been obtained over more than a
full cycle. The variation curves of both and have a significant degree
of anharmonicity, and there is a definite phase shift between their respective
extrema. We confirm that HD 50169 is a wide spectroscopic binary, refine its
orbital elements, and suggest that the secondary is probably a dwarf star of
spectral type M.
Conclusions. The shapes and mutual phase shifts of the derived magnetic
variation curves unquestionably indicate that the magnetic field of HD 50169 is
not symmetric about an axis passing through its centre. Overall, HD 50169
appears similar to the bulk of the long-period Ap stars.Comment: 10 pages, 3 figures, accepted for publication in A&
Temperature dependence of quantum lifetime in n-InGaAs/GaAs structures with strongly coupled double quantum wells
Longitudinal ρxx(B) and Hall ρxy(B) magnetoresistances are experimentally investigated as a function of in-plane and transverse magnetic fields in n-InGaAs/GaAs nanostructures with strongly-coupled double quantum wells in the temperature range T = 1.8-70 K and magnetic fields B = 0-9.0 T. Experimental data on the temperature dependence of quantum lifetime in diffusive (kBT/τtr ≪ 1) and ballistic (kBT/τtr ≫ 1) regimes are reported. It has been found that in the ballistic regime in the temperature range where kBT/EF < 0.1, the observed quadratic temperature dependence of quantum lifetime is determined by inelastic electron-electron scattering. However, the temperature dependence of quantum lifetime cannot be quantitatively described by the existing theories in the whole temperature range. © 2013 American Institute of Physics
Spatial structure of an individual Mn acceptor in GaAs
The wave function of a hole bound to an individual Mn acceptor in GaAs is
spatially mapped by scanning tunneling microscopy at room temperature and an
anisotropic, cross-like shape is observed. The spatial structure is compared
with that from an envelope-function, effective mass model, and from a
tight-binding model. This demonstrates that anisotropy arising from the cubic
symmetry of the GaAs crystal produces the cross-like shape for the hole
wave-function. Thus the coupling between Mn dopants in GaMnAs mediated by such
holes will be highly anisotropic.Comment: 3 figures, submitted to PR
Effects of spin polarization in the HgTe quantum well
Magnetoresistivity features connected with the spin level coincidences under tilted fields in a Γ 8 conduction band of the HgTe quantum well were found to align along straight trajectories in a (B ⊥,B) plane between the field components perpendicular and parallel to the layer, meaning a linear spin polarization dependence on a magnetic field. Among the trajectories is a noticeable set of lines descending from a single point on the Baxis, which is shown to yield a field of the full spin polarization of the electronic system, in agreement with the data on the electron redistribution between spin subbands obtained from Fourier transforms of oscillations along circle trajectories in the (B ⊥,B) plane and with the point on the magnetoresistivity under pure Bseparating a complicated weak field dependence from the monotonous one. The whole picture of coincidences is well described by the isotropic g factor, although its value is twice as small as that obtained from oscillations under pure perpendicular fields. The discrepancy is attributed to different manifestations of spin polarization phenomena in the coincidences and within the exchange-enhanced spin gaps. In the quantum Hall range of B ⊥, the spin polarization manifests in anticrossings of magnetic levels, which were found to depend dramatically nonmonotonously on B ⊥. © 2012 American Physical Society
Search for stellar spots in field blue horizontal-branch stars
Blue horizontal-branch stars are Population II objects which are burning
helium in their core and possess a hydrogen-burning shell and radiative
envelope. Because of their low rotational velocities, diffusion has been
predicted to work in their atmospheres. In many respects, blue
horizontal-branch stars closely resemble the magnetic chemically peculiar stars
of the upper main sequence, which show photometric variability caused by
abundance spots on their surfaces. These spots are thought to be caused by
diffusion and the presence of a stable magnetic field. However, the latter does
not seem to be axiomatic. We searched for rotationally induced variability in
30 well-established bright field blue horizontal-branch stars in the solar
neighbourhood and searched the literature for magnetic fields measurements of
our targets. We employed archival photometric time series data from the ASAS,
ASAS-SN, and SuperWASP surveys. The data were carefully reduced and processed,
and a time series analysis was applied using several different techniques. We
also synthesized existing photometric and spectroscopic data of magnetic
chemically peculiar stars in order to study possible different surface
characteristics producing lower amplitudes. In the accuracy limit of the
employed data, no significant variability signals were found in our sample
stars. The resulting upper limits for variability are given. We conclude that
either no stellar surface spots are present in field blue horizontal-branch
stars, or their characteristics (contrast, total area, and involved elements)
are not sufficient to produce amplitudes larger than a few millimagnitudes in
the optical wavelength region. New detailed models taking into account the
elemental abundance pattern of blue horizontal-branch stars are needed to
synthesize light curves for a comparison with our results.Comment: 6 pages, 2 figures, 2 tables, accepted for publication in Astronomy &
Astrophysic
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