Abstract:Using scanning tunneling microscopy with Fe-coated W tips and first-principles calculations, we show that the interface of epitaxial graphene/SiC(0001) is a warped graphene layer with the periodic inclusion of hexagon-pentagon-heptagon (H 5,6,7 ) defects that break the honeycomb symmetry, thereby inducing a gap and states below E F near the K point. Although the next graphene layer assumes the perfect honeycomb lattice, its interaction with the warped layer modifies the dispersion at the Dirac point. These results explain recent angle-resolved photoemission and carbon core-level shift data, and solve the long-standing problem of the interfacial structure of epitaxial graphene on SiC(0001).
We investigate the atomic and electronic structures of Mn-incorporated GaN͑0001͒ surface by scanning tunneling microscopy and first-principles calculations. The incorporation of Mn at room temperature initially leads to a disordered phase, and subsequently an ordered ͑3 ϫ 3͒ reconstruction at ϳ0.5 ML Mn coverage. The ͑3 ϫ 3͒ exhibits a honeycomb structure for sample bias below −0.5 V, and appears as a closed-packed structure above −1.2 V, which is attributed to antiferromagnetic ordering induced by Mn atoms. Incorporation of the same amount of Mn at an elevated substrate temperature of 300°C produces an additional phase with local ͑5 ϫ 5͒ periodicity, consisting of two types of trimers: one with three spots of equal intensity and the other with uneven intensities. This disparity in intensity can be resolved using a Fe-coated W tip, which is attributed to enhanced spin-dependent tunneling between the Mn-induced minority states and the Fe-induced states of the functionalized W tip.
Delayed self-heterodyne/homodyne measurements based on an unbalanced interferometer are the most used methods for measuring the linewidth of narrow-linewidth lasers. They typically require the service of a delay of six times (or greater) than the laser coherence time to guarantee the Lorentzian characteristics of the beat notes. Otherwise, the beat notes are displayed as a coherent envelope. The linewidth cannot be directly determined from the coherence envelope. However, measuring narrow linewidths using traditional methods introduces significant errors due to the 1/f frequency noise. Here, a short fiber-based linewidth measurement scheme was proposed, and the influence of the noise floor on the measurement of the laser linewidth using this scheme was studied theoretically and experimentally. The results showed that this solution and calibration process is capable of significantly improving the measurement accuracy of narrow linewidth.
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