We found that an Al-Si-Ru cubic quasicrystalline approximant has a semiconducting band structure by performing an orbital analysis based on density functional theory. These semiconducting transport properties have been confirmed in an experimentally synthesized sample. The temperature dependences of the electrical conductivity and the Seebeck coefficient were consistent with the trends of an intrinsic semiconductor with a band gap of 0.15 eV above 350 K. The lattice thermal conductivity had a low value of approximately 1.0 W m −1 K −1 above 400 K, which is close to the theoretical minimum.Since the discovery of the quasicrystal (QC) in 1984 [1], many of the unique properties of QCs have been revealed, including their crystal structures [2] and electrical properties [3]. However, the question of whether or not semiconducting or insulating QCs exist remains one of the fundamental problems to be solved in solid state physics, although the QC concept has been established as one of the categories of solid-state structures.Semiconducting QCs have attracted attention, not only from the perspective of academic interest, but also from the viewpoint of application to thermoelectric (TE) materials to realize a direct conversion between the thermal and electrical energies. The performance of a TE material can be evaluated using the dimensionless figure of merit zT = S 2 σ T /(κ el + κ lat ), where S, σ , T , κ el , and κ lat are the Seebeck coefficient, the electrical conductivity, the absolute temperature, the electronic thermal conductivity, and the lattice thermal conductivity, respectively. The highest zT between QCs achieved to date is 0.26, which is only approximately one quarter of the general target value of unity, at 500 K in an Al-Ga-Pd-Mn QC [4]. The main problem is that the value of S (≈100 μV K −1 ) is only approximately one half of that of typical practical materials. To obtain a sufficiently large S at a target temperature T , a semiconductor with a band gap of 6-10k B T , where k B is the Boltzmann constant, is generally required [5]. Therefore, the discovery of a semiconducting QC is necessary to enable the breakthrough of QCs for use as TE materials.While semiconductorlike properties that were attributed to a combination of the pseudogap in the density of states (DOS) and electronic weak localization were previously reported in some aluminum-transition metal (Al-TM) QCs [6-10] and quasicrystalline approximants (QCAs) [10,11], a finite band gap has yet to be observed experimentally in these materials to date. There have been several theoretical works on semiconducting QCAs based on density functional theory (DFT) that have thus far contributed to the understanding of the problem of complex semiconducting QCs. Krajčí and Hafner investigated the electronic structures of hypothetical QCAs that were constructed using a model of Al-TM icosahedral QCs [12][13][14]. While they predicted the existence of some QCAs with semiconducting electronic structures and indicated the presence of an energy gap in QCs, these typ...

A laser scanning differential interference contrast microscope using a differential detection method is proposed. This microscope permits observers to control the differential image contrast with a simple operation. Utilizing this function, the observers are capable of obtaining the optimized differential image whose contrast is most favorable for observation. A simple theoretical analysis and experimental considerations are described.

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We show here the possibility of optical quasi-phase matched secondary harmonic generation in crystal quartz that is a non-ferroelectric crystal. By utilizing a twinning phenomenon, for the first time, we have succeeded in fabricating periodic modulation of the nonlinear coefficient by hot pressing. In this method, stress is applied vertically on a quartz surface. We also devised a special press system. The numbers of twins we have achieved are 10 for 500 μm and 17 for 250 μm periods, and the depth of the twinned area is about 50 μm. Our results suggest that the twinned regions can be controlled artificially, and over a few tens of microns of twinned depth can be realized.

Search for high-order semiconducting quasicrystalline approximants can play an essential role in finding clues to the discovery of semiconducting quasicrystals. According to the previous theoretical work, a model of AlPdCo 1/1 cubic quasicrystalline approximant was predicted to be semiconductor from a calculation based on the density functional theory. We noticed that the F phase in the AlPdCo system is a 2 © 2 © 2 superlattice structure of the calculated model. To verify this prediction, we synthesized the F phase sample, and measured its thermoelectric properties. The measured electrical conductivity linearly increases with increasing temperature. The magnitude of measured Seebeck coefficient is smaller than the typical semiconductor. These properties indicate that the prepared sample of the F phase has a pseudogap rather than a finite band gap. To investigate this discrepancy between the theoretical prediction and experimental results, we calculated the electronic structure for the three structural models using density functional theory. The most energetically stable model has a semimetallic electronic structure.