Strong spin-orbit interaction and time-reversal symmetry in topological insulators generate novel quantum states called topological surface states. Their study provides unique opportunities to explore exotic phenomena such as spin Hall effects and topological phase transitions, relevant to the development of quantum devices for spintronics and quantum computation. Although ultrahigh-vacuum surface probes can identify individual topological surface states, standard electrical and optical experiments have so far been hampered by the interference of bulk and quantum well states. Here, with terahertz time-domain spectroscopy of ultrathin Bi 2 Se 3 films, we give evidence for topological phase transitions, a single conductance quantum per topological surface state, and a quantized terahertz absorbance of 2.9% (four times the fine structure constant). Our experiment demonstrates the feasibility to isolate, detect and manipulate topological surface states in the ambient at room temperature for future fundamental research on the novel physics of topological insulators and their practical applications.

Typical Kondo insulators (KIs) can have a nontrivial Z_{2} topology because the energy gap opens at the Fermi energy (E_{F}) by a hybridization between odd- and even-parity bands. SmB_{6} deviates from such KI behavior, and it has been unclear how the insulating phase occurs. Here, we demonstrate that charge fluctuations are the origin of the topological insulating phase in SmB_{6}. Our angle-resolved photoemission spectroscopy results reveal that with decreasing temperature the bottom of the d-f hybridized band at the X[over ¯] point, which is predicted to have odd parity and is required for a topological phase, gradually shifts from below to above E_{F}. We conclude that SmB_{6} is a charge-fluctuating topological insulator.

Electronic devices based on a series of synthesized block copolymers are demonstrated. In particular, a block copolymer system with a lamellar structure exhibits unipolar switching behavior. This study provides a simple strategy based on the adjustment of the block ratio in block copolymers to control the polymer morphology and thus the electrical and switching properties of polymer-based memory devices.

We carried out the anionic polymerization of styrene derivatives containing the triphenylamine moieties, 4,4′-vinylphenyltriphenylamine (A) and 4,4′-vinylphenyl- N,N -bis(4-tert-butylphenyl)benzenamine (B), with a variety of initiators in tetrahydrofuran (THF) at −78 °C. The anionic polymerization of A was performed with sec-butyllithium, sec-butyllithium with additives, and (diphenylmethyl)potassium in THF at −78 °C. In all cases, an intermolecular side reaction took place between the polymer chains during polymerization. In contrast, the anionic polymerization of B was carried out successfully with sec-butyllithium and potassium naphthalenide in THF at −78 °C for 0.5 h without any observed intermolecular side reaction. Well-defined poly(B)s with predictable molecular weights and narrow molecular weight distributions (M w/M n = 1.05−1.11) were obtained. The sequential block copolymerization of B with styrene and 2-vinylpyridine was attempted subsequently, and the well-defined block copolymers, polystyrene-b-poly(B) and poly(B)-b-poly(2-vinylpyridine), were synthesized quantitatively. The solubilities as well as the thermal and optical properties of the resulting poly(B) were investigated.

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