The authors report on a design of terahertz quantum-cascade lasers based on three-well active modules. Each module consists of two tunnel-coupled wells for the two lasing states and another well for both resonant-phonon depopulation and carrier injection. This design is the simplest so far among the various published working devices. The test device has a lasing frequency of 3.4THz and maximum operating temperature of 142K.
It is shown that a temperature window between the Curie temperatures of martensite and austenite phases around the room temperature can be obtained by a vacancy-tuning strategy in Mn-poor Mn 1-x CoGe alloys (0 ≤ x ≤ 0.050). Based on this, a martensitic transformation from paramagnetic austenite to ferromagnetic martensite with a large magnetization difference can be realized in this window. This gives rise to a magnetic-field-induced martensitic transformation and a large magnetocaloric effect in the Mn 1-x CoGe system. The decrease of the transformation temperature and of the thermal hysteresis of the transformation, as well as the stable Curie temperatures of martensite and austenite, are discussed on the basis of the Mn-poor Co-vacancy structure and the corresponding valence-electron concentration.
The electronic structure and magnetism of the Mn2FeZ (Z=Al, Ga, Si, Ge, Sb) Heusler alloys have been studied by density functional calculations. Two half-metallic ferromagnets, namely, Mn2FeAl and Mn2FeSb, are predicted. It is found that a small expansion of the crystal lattice can restore the half-metallicity in Mn2FeSi. The calculated total magnetic moments Mtot are 1μB/f.u. for Mn2FeAl and Mn2FeGa, 2μB/f.u. for Mn2FeSi and Mn2FeGe, and 3μB/f.u. for Mn2FeSb, which agree with the Slater–Pauling curve quite well. The moments of Mn (A) and Mn (B) are large and antiparallel to each other, which is indicative of ferrimagnetism in Mn2FeZ alloys. Fe shows only a small moment and its moment is parallel to that of Mn (B). By investigating the effect of lattice distortion on the half-metallicity and magnetic moments of Mn2FeZ, it is found that the half-metallic properties of Mn2FeSb are insensitive to the lattice distortion and a 100% spin polarization can be obtained within the wide range of 5.4–6.05 Å. This is preferable in practical applications.
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