We report Curie temperatures up to 150 K in annealed Ga 1-x Mn x As epilayers grown with a relatively low As:Ga beam equivalent pressure ratio. A variety of measurements (magnetization, Hall effect, magnetic circular dichroism and Raman scattering) show that the higher ferromagnetic transition temperature results from an enhanced free hole density. The data also indicate that, in addition to the carrier concentration, the sample thickness limits the maximum attainable Curie temperature in this material -suggesting that the free surface of G a 1-x Mn x As epilayers is important in determining their physical properties.(a) nsamarth@psu.edu
We have investigated strong optical
nonlinearity of monolayer MoS2(1–x)Se2x
across the exciton resonance, which
is directly tunable by Se doping. The quality of monolayer alloys
prepared by chemical vapor deposition is verified by atomic force
microscopy, Raman spectroscopy, and photoluminescence analysis. The
crystal symmetry of all of our alloys is essentially D
3h
, as confirmed by polarization-dependent
second-harmonic generation (SHG). The spectral structure of the exciton
resonance is sampled by wavelength-dependent SHG (λ = 1000–1800
nm), where the SHG resonance red-shifts in accordance with the corresponding
optical gap. Surprisingly, the effect of compositional variation turns
out to be much more dramatic owing to the unexpected increase of B-exciton-induced SHG, which indeed dominates over the A-exciton resonance for x ≥ 0.3.
The overall effect is therefore stronger and broader SHG resonance
where the latter arises from different degrees of red-shift for the
two exciton states. We report the corresponding absolute SHG dispersion
of monolayer alloys, χ(2), as a function of Se doping.
We believe that our finding is a critical step toward engineering
highly efficient nonlinear optical van der Waals materials working
in a broader performance range.
Raman spectrum of a quantum dot (QD) is characterized by transverse (TO) and longitudinal (LO) optical modes as well as surface optical modes, occurring between the TO and LO modes. We have studied in detail the size-dependence of the Raman spectrum of InP QD of diameter larger than 35 Å. The LO phonon frequency decreases while the TO phonon frequency increases with decreasing QD size. The linewidth of the LO phonon broadens and the broadening becomes increasingly asymmetrical towards the low frequency side as the QD size decreases. By analyzing the Raman intensity ratio of the LO phonon to its overtone, we find that the electron-phonon coupling decreases with decreasing QD size.
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