495 research outputs found
Theory of Ferromagnetism in Diluted Magnetic Semiconductor Quantum Wells
We present a mean field theory of ferromagnetism in diluted magnetic
semiconductor quantum wells. When subband mixing due to exchange interactions
between quantum well free carriers and magnetic impurities is neglected,
analytic result can be obtained for the dependence of the critical temperature
and the spontaneous magnetization on the distribution of magnetic impurities
and the quantum well width. The validity of this approximate theory has been
tested by comparing its predictions with those from numerical self-consistent
field calculations. Interactions among free carriers, accounted for using the
local-spin-density approximation, substantially enhance the critical
temperature. We demonstrate that an external bias potential can tune the
critical temperature through a wide range.Comment: 4 pages, 3 figures, submitted to Phys. Rev.
Blue Quantum Fog: Chiral Condensation in Quantum Helimagnets
It is shown that a condensation transition involving a chiral order parameter
can occur in itinerant helimagnets, in analogy to the transition between the
isotropic phase and the phase known as blue fog or blue phase III in
cholesteric liquid crystals. It is proposed that such a transition is the
explanation for recent neutron scattering results in MnSi. Predictions are made
that will allow to experimentally test this proposal.Comment: 4pp, 1 eps fi
Exciton Diamagnetic Shifts and Valley Zeeman Effects in Monolayer WS and MoS to 65 Tesla
We report circularly-polarized optical reflection spectroscopy of monolayer
WS and MoS at low temperatures (4~K) and in high magnetic fields to
65~T. Both the A and the B exciton transitions exhibit a clear and very similar
Zeeman splitting of approximately 230~eV/T (), providing
the first measurements of the valley Zeeman effect and associated -factors
in monolayer transition-metal disulphides. These results complement and are
compared with recent low-field photoluminescence measurements of valley
degeneracy breaking in the monolayer diselenides MoSe and WSe. Further,
the very large magnetic fields used in our studies allows us to observe the
small quadratic diamagnetic shifts of the A and B excitons in monolayer WS
(0.32 and 0.11~eV/T, respectively), from which we calculate exciton
radii of 1.53~nm and 1.16~nm. When analyzed within a model of non-local
dielectric screening in monolayer semiconductors, these diamagnetic shifts also
constrain and provide estimates of the exciton binding energies (410~meV and
470~meV for the A and B excitons, respectively), further highlighting the
utility of high magnetic fields for understanding new 2D materials.Comment: 9 pages, 5 figure
Magneto-reflection spectroscopy of monolayer transition-metal dichalcogenide semiconductors in pulsed magnetic fields
We describe recent experimental efforts to perform polarization-resolved
optical spectroscopy of monolayer transition-metal dichalcogenide
semiconductors in very large pulsed magnetic fields to 65 tesla. The
experimental setup and technical challenges are discussed in detail, and
temperature-dependent magneto-reflection spectra from atomically thin tungsten
disulphide (WS) are presented. The data clearly reveal not only the valley
Zeeman effect in these 2D semiconductors, but also the small quadratic exciton
diamagnetic shift from which the very small exciton size can be directly
inferred. Finally, we present model calculations that demonstrate how the
measured diamagnetic shifts can be used to constrain estimates of the exciton
binding energy in this new family of monolayer semiconductors.Comment: PCSI-43 conference (Jan. 2016; Palm Springs, CA
On the use of a sunward-libration-point orbiting spacecraft as an IMF monitor for magnetospheric studies
Magnetospheric studies often require knowledge of the orientation of the IMF. In order to test the accuracy of using magnetometer data from a spacecraft orbiting the sunward libration point for this purpose, the angle between the IMF at ISEE 3, when it was positioned around the libration point, and at ISEE 1, orbiting Earth, has been calculated for a data set of two-hour periods covering four months. For each period, a ten-minute average of ISEE 1 data is compared with ten-minute averages of ISEE 3 data at successively lagged intervals. At the lag time equal to the time required for the solar wind to convect from ISEE 3 to ISEE 1, the median angle between the IMF orientation at the two spacecraft is 20 deg, and 80% of the cases have angles less than 38 deg. The results for the angles projected on the y-z plane are essentially the same
Monica’s Designer Handbags: Creative Marketing Decision-Making Based on Financial Analysis—A Case Study
Monica learned much about the designer apparel trade as an intern with a major retailer, and started a designer handbag business, selling through independent retailers. She practiced making sound marketing decisions using financial analysis techniques learned in college. These techniques proved useful when a regional discount chain offered a deal to sell her handbags through their stores on a trial basis. She was faced with a tough decision to accept the deal, reject it, or renegotiate it on mutually acceptable terms. Students are asked to analyze case data and to advise Monica on how to proceed with the prospective deal
Ultrafast geometric manipulation of electron spin and detection of the geometric phase via Faraday rotation spectroscopy
Time-resolved Faraday rotation spectroscopy is currently exploited as a
powerful technique to probe spin dynamics in semiconductors. We propose here an
all-optical approach to geometrically manipulate electron spin and to detect
the geometric phase by this type of extremely sensitive experiment. The global
nature of the geometric phase can make the quantum manipulation more stable,
which may find interesting application in quantum devices.Comment: 4 pages, 3 figures, to appear in Phys. Rev.
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