102 research outputs found
Optical properties of metallic (III,Mn)V ferromagnetic semiconductors in the infrared to visible range
We report on a study of the ac conductivity and magneto-optical properties of
metallic ferromagnetic (III,Mn)V semiconductors in the infrared to visible
spectrum. Our analysis is based on the successful kinetic exchange model for
(III,Mn)V ferromagnetic semiconductors. We perform the calculations within the
Kubo formalism and treat the disorder effects pertubatively within the Born
approximation, valid for the metallic regime. We consider an eight-band
Kohn-Luttinger model (six valence bands plus two conduction bands) as well as a
ten-band model with additional dispersionless bands simulating
phenomenologically the upper-mid-gap states induced by antisite and
interstitial impurities. These models qualitatively account for
optical-absorption experiments and predict new features in the mid-infrared
Kerr angle and magnetic-circular-dichroism properties as a function of Mn
concentration and free carrier density.Comment: 10 pages, 7 figures, some typos correcte
A Substellar Companion to the Intermediate-Mass Giant 11 Com
We report the detection of a substellar companion orbiting the
intermediate-mass giant star 11 Com (G8 III). Precise Doppler measurements of
the star from Xinglong station and Okayama Astrophysical Observatory (OAO)
revealed Keplerian velocity variations with an orbital period of 326.03 +/-
0.32 days, a semiamplitude of 302.8 +/- 2.6 m/s, and an eccentricity of 0.231
+/- 0.005. Adopting a stellar mass of 2.7 +/- 0.3 M_solar, the minimum mass of
the companion is 19.4 +/- 1.5 M_Jup, well above the deuterium burning limit,
and the semimajor axis is 1.29 +/- 0.05 AU. This is the first result from the
joint planet search program between China and Japan aiming at revealing
statistics of substellar companions around intermediate-mass giants. 11 Com b
emerged from 300 targets of the planet search program at OAO. The current
detection rate of a brown dwarf candidate seems to be comparable to that around
solar-type stars within orbital separations of 3 AU.Comment: 19 pages, 4 figures, accepted by Ap
Attributes Affecting User Decision to Adopt a Virtual Private Network (VPN) App
© 2020, Springer Nature Switzerland AG. A Virtual Private Network (VPN) helps to mitigate security and privacy risks of data transmitting on unsecured network such as public Wi-Fi. However, despite awareness of public Wi-Fi risks becoming increasingly common, the use of VPN when using public Wi-Fi is low. To increase adoption, understanding factors driving user decision to adopt a VPN app is an important first step. This study is the first to achieve this objective using discrete choice experiments (DCEs) to elicit individual preferences of specific attributes of a VPN app. The experiments were run in the United Kingdom (UK) and Japan (JP). We first interviewed participants (15 UK, 17 JP) to identify common attributes of a VPN app which they considered important. The results were used to design and run a DCE in each country. Participants (149 UK, 94 JP) were shown a series of two hypothetical VPN apps, varying in features, and were asked to choose one which they preferred. Customer review rating, followed by price of a VPN app, significantly affected the decision to choose which VPN app to download and install. A change from a rating of 3 to 4–5 stars increased the probability of choosing an app by 33% in the UK and 14% in Japan. Unsurprisingly, price was a deterrent. Recommendations by friends, source of product reviews, and the presence of in-app ads also played a role but to a lesser extent. To actually use a VPN app, participants considered Internet speed, connection stability, battery level on mobile devices, and the presence of in-app ads as key drivers. Participants in the UK and in Japan prioritized these attributes differently, suggesting possible influences from cultural differences
Desorption of CO from individual ruthenium porphyrin molecules on a copper surface via an inelastic tunnelling process.
The coordination of CO to metalloporphyrins changes their electronic and magnetic properties. Here we locally desorb CO molecules from a single ruthenium tetraphenylporphyrin carbonyl (CO-RuTPP) on Cu(110) using STM. The desorption is triggered by the injection of holes into the occupied states of the adsorbate using an unusual two-carrier process
High-Temperature Hall Effect in Ga(1-x)Mn(x)As
The temperature dependence of the Hall coefficient of a series of
ferromagnetic Ga(1-x)Mn(x)As samples is measured in the temperature range 80K <
T < 500K. We model the Hall coefficient assuming a magnetic susceptibility
given by the Curie-Weiss law, a spontaneous Hall coefficient proportional to
rho_xx^2(T), and including a constant diamagnetic contribution in the
susceptibility. For all low resistivity samples this model provides excellent
fits to the measured data up to T=380K and allows extraction of the hole
concentration (p). The calculated p are compared to alternative methods of
determining hole densities in these materials: pulsed high magnetic field (up
to 55 Tesla) technique at low temperatures (less than the Curie temperature),
and electrochemical capacitance- voltage profiling. We find that the Anomalous
Hall Effect (AHE) contribution to rho_xy is substantial even well above the
Curie temperature. Measurements of the Hall effect in this temperature regime
can be used as a testing ground for theoretical descriptions of transport in
these materials. We find that our data are consistent with recently published
theories of the AHE, but they are inconsistent with theoretical models
previously used to describe the AHE in conventional magnetic materials.Comment: 6 pages, 5 figures, 1 table. Accepted to Phys.Rev.
Anisotropic Magnetoresistance and Magnetic Anisotropy in High-quality (Ga,Mn)As Films
We have performed a systematic investigation of magnetotransport of a series
of as-grown and annealed Ga1-xMnxAs samples with 0.011 <= x <= 0.09. We find
that the anisotropic magnetoresistance (AMR) generally decreases with
increasing magnetic anisotropy, with increasing Mn concentration and on low
temperature annealing. We show that the uniaxial magnetic anisotropy can be
clearly observed from AMR for the samples with x >= 0.02. This becomes the
dominant anisotropy at elevated temperatures, and is shown to rotate by 90o on
annealing. We find that the in-plane longitudinal resistivity depends not only
on the relative angle between magnetization and current direction, but also on
the relative angle between magnetization and the main crystalline axes. The
latter term becomes much smaller after low temperature annealing. The planar
Hall effect is in good agreement with the measured AMR indicating the sample is
approximately in a single domain state throughout most of the magnetisation
reversal, with a two-step magnetisation jump ascribed to domain wall nucleation
and propagation.Comment: 27 pages, 8 figures, accepted by Phys. Rev.
Anisotropic Magnetoresistance in GaMnAs
We have measured the magnetoresistance in a series of GaMnAs
samples with 0.033 0.053 for three mutually orthogonal orientations
of the applied magnetic field. The spontaneous resistivity anisotropy (SRA) in
these materials is negative (i.e. the sample resistance is higher when its
magnetization is perpendicular to the measuring current than when the two are
parallel) and has a magnitude on the order of 5% at temperatures near 10K and
below. This stands in contrast to the results for most conventional magnetic
materials where the SRA is considerably smaller in magnitude for those few
cases in which a negative sign is observed. The magnitude of the SRA drops from
its maximum at low temperatures to zero at T in a manner that is consistent
with mean field theory. These results should provide a significant test for
emerging theories of transport in this new class of materials.Comment: 4 pages with 4 figures. Submitted to Physical Review
Prospects of high temperature ferromagnetism in (Ga,Mn)As semiconductors
We report on a comprehensive combined experimental and theoretical study of
Curie temperature trends in (Ga,Mn)As ferromagnetic semiconductors. Broad
agreement between theoretical expectations and measured data allows us to
conclude that T_c in high-quality metallic samples increases linearly with the
number of uncompensated local moments on Mn_Ga acceptors, with no sign of
saturation. Room temperature ferromagnetism is expected for a 10% concentration
of these local moments. Our magnetotransport and magnetization data are
consistnent with the picture in which Mn impurities incorporated during growth
at interstitial Mn_I positions act as double-donors and compensate neighboring
Mn_Ga local moments because of strong near-neighbor Mn_Ga-Mn_I
antiferromagnetic coupling. These defects can be efficiently removed by
post-growth annealing. Our analysis suggests that there is no fundamental
obstacle to substitutional Mn_Ga doping in high-quality materials beyond our
current maximum level of 6.2%, although this achievement will require further
advances in growth condition control. Modest charge compensation does not limit
the maximum Curie temperature possible in ferromagnetic semiconductors based on
(Ga,Mn)As.Comment: 13 pages, 12 figures, submitted to Phys. Rev.
Magnetotransport properties of (Ga,Mn)As investigated at low temperature and high magnetic field
Magnetotransport properties of ferromagnetic semiconductor (Ga,Mn)As have
been investigated. Measurements at low temperature (50 mK) and high magnetic
field (<= 27 T) have been employed in order to determine the hole concentration
p = 3.5x10^20 cm ^-3 of a metallic (Ga0.947Mn0.053)As layer. The analysis of
the temperature and magnetic field dependencies of the resistivity in the
paramagnetic region was performed with the use of the above value of p, which
gave the magnitude of p-d exchange energy |N0beta | ~ 1.5 eV.Comment: PDF file, 8 pages, 4 figure
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