109 research outputs found
Clustering induced suppression of ferromagnetism in diluted magnets
Ferromagnetism in diluted magnets in the compensated regime p << x is shown
to be suppressed by the formation of impurity spin clusters. The majority bulk
spin couplings are shown to be considerably weakened by the preferential
accumulation of holes in spin clusters, resulting in low-energy magnon
softening and enhanced low-temperature decay of magnetic order. A locally
self-consistent magnon renormalization analysis of spin dynamics shows that
although strong intra-cluster correlations tend to prolong global order, T_c is
still reduced compared to the ordered case.Comment: published version, 5 pages, 4 figure
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
Carrier induced ferromagnetism in diluted magnetic semi-conductors
We present a theory for carrier induced ferromagnetism in diluted magnetic
semi-conductor (DMS). Our approach treats on equal footing quantum fluctuations
within the RPA approximation and disorder within CPA. This method allows for
the calculation of , magnetization and magnon spectrum as a function of
hole, impurity concentration and temperature. It is shown that, sufficiently
close to , and within our decoupling scheme (Tyablicov type) the CPA for
the itinerant electron gas reduces to the Virtual Crystal Approximation. This
allows, in the low impurity concentration and low density of carriers to
provide analytical expression for . For illustration, we consider the case
of and compare our results with available experimental data.Comment: 5 figures included. to appear in Phys. Rev. B (brief report
Theory of Magnetic Properties and Spin-Wave Dispersion for Ferromagnetic (Ga,Mn)As
We present a microscopic theory of the long-wavelength magnetic properties of
the ferromagnetic diluted magnetic semiconductor (Ga,Mn)As. Details of the host
semiconductor band structure, described by a six-band Kohn-Luttinger
Hamiltonian, are taken into account. We relate our quantum-mechanical
calculation to the classical micromagnetic energy functional and determine
anisotropy energies and exchange constants. We find that the exchange constant
is substantially enhanced compared to the case of a parabolic heavy-hole-band
model.Comment: 9 pages, 4 figure
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
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
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.
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.
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
320 Gb/s Nyquist OTDM received by polarization-insensitive time-domain OFT
We have demonstrated the generation of a 320 Gb/s Nyquist-OTDM signal by rectangular filtering on an RZ-OTDM signal with the filter bandwidth (320 GHz) equal to the baud rate (320 Gbaud) and the reception of such a Nyquist-OTDM signal using polarization-insensitive time-domain optical Fourier transformation (TD-OFT) followed by passive filtering. After the time-to-frequency mapping in the TD-OFT, the Nyquist-OTDM signal with its characteristic sinc-shaped time-domain trace is converted into an orthogonal frequency division multiplexing (OFDM) signal with sinc-shaped spectra for each subcarrier. The subcarrier frequency spacing of the converted OFDM signal is designed to be larger than the transform-limited case, here 10 times greater than the symbol rate of each subcarrier. Therefore, only passive filtering is needed to extract the subcarriers of the converted OFDM signal. In addition, a polarization diversity scheme is used in the four-wave mixing (FWM) based TD-OFT, and less than 0.5 dB polarization sensitivity is demonstrated in the OTDM receiver.</p
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