In this paper we discuss D-branes in the Melvin background and its supersymmetric generalizations. In particular we determine the D-brane spectra in these backgrounds by constructing their boundary states explicitly, where some of the D-branes are supersymmetric. The results sensitively depend on whether the value of magnetic flux in the Melvin background is rational or irrational. For the rational case the D-branes are regarded as the generalizations of fractional D-branes in abelian orbifolds C n /Z N of type-II or type-0 string theory. For the irrational case we found a very limited spectrum. Since the background includes the non-trivial H-flux, the D-branes will provide interesting examples from the viewpoint of the non-commutative geometry.Keywords: D-branes, Conformal Field Models in String Theory. A. T-duality for Kaluza-Klein background 34 B. Mode expansion 36 C. Formulae of θ-functions 38 JHEP11(2001)036unstable and thus non-supersymmetric D-brane systems (for a review see [2,3]). The key idea of understanding this phenomenon is the Sen's conjecture, which says that the unstable vacuum with D-branes decays to stable lower-dimensional D-branes or the closed string vacuum. From this we can expect that the original vacuum with the unstable D-brane system belongs to the spontaneous supersymmetry broken phase. Indeed the non-linear supersymmetries can be realized on such systems [4,5]. From this we expect that the above observation about open string unstable systems may be applied to closed string unstable vacua such as type-0 string etc. (for the type-0 theory see [6]). If we naively translate the above statement into those examples, such closed string "theories" are not different theories from type II but only different vacua. One attempt in this direction will be the conjecture that type-0A string is equivalent to S 1 compactification of M-theory with the antiperiodic boundary condition for all fermions [7]. Moreover we may speculate that the type-0 string is a spontaneously broken phase of the original thirty two supersymmetries in type II, where the gravitino may obtain the infinite mass and thus disappear. However, in reality it seems difficult to explain it completely within the framework of classical supergravity. For example, one should explain the presence of tachyon in type-0 string and the doubling of RR-fields. These intriguing questions about unstable closed string vacua will also be closely related to the closed string tachyon condensation, which remains to be well understood (for latest discussions see e.g [8]- [14]). Recently the Kaluza-Klein Melvin backgrounds [15]-[18] have been intensively studied both in M-theory (or superstring with RR flux) [9] and [19]-[28] and in NS-NS superstring model [12,13,14,19,29,30]. Both backgrounds will provide useful materials for the studies of unstable closed string vacua since these are known to connect type-0 string with type-II string in the flat space [19]. In the former context the Melvin-like solutions are called flux-branes (Fp-brane) [9,20,21,22]. ...

A phase change optical disk using a new quaternary system of Ag-In-Sb-Te as the active layer will be reported. With this disk, “complete erasure” of the recorded signal has been achieved. A carrier-to-noise ratio of 47 dB and an erase ratio of -47 dB were obtained in the two-pass writing-erasing mode with a linear velocity of 7 m/s and a writing power of 11 mW. In the one-pass overwriting mode, a carrier-to-noise ratio greater than 40 dB was obtained and this recorded signal was also completely erased. The peak power and the bias power for the overwriting were 10 mW and 5 mW, respectively. The facts that the widths of the written marks are narrow and that no large crystalline grains are observed in the periphery of the written marks are considered to be the major causes for the high erasability. We also found that the existence of fine crystalline particles in the initialized active layer can be considered to be of great advantage to high sensitivity.

The Ag-In-Sb-Te system is widely used for phase-change discs such as compact disc rewritable (CD-RW) and rewritable digital versatile disc (DVD+RW). To clarify the effect of Ag and In in the Ag-In-Sb-Te system, we studied the local structure of Ag-In-Sb-Te phase-change material by extended X-ray absorption fine structure (EXAFS). The results suggest that the existence of Ag contributes to the thermal stability of amorphous marks and existence of In contributes to high-speed phase-change.

The local structures of three phases; stable compound Sb2Te3, metastable crystalline c-SbTe and amorphous a-SbTe films having the atomic ratio Sb/Te=3: in Sb-Te system have been studied by EXAFS. The c-SbTe has a partly similar local structure to crystalline Sb2Te3. The a-SbTe film has a local structure of NaCl-type, which is topologically analogous to the crystalline form. The amorphous phase has shorter bond distances 2.86A (around Sb-site) and 2.83A (around Te-site) than the corresponding distances 2.89A and 2.87A in the crystalline phase. This unbalanced bond distances between the Sb-site and Te-site implies that site-disordering occurs. Shortening of bond distances in the amorphous phase is due to the relaxation of locally distorted crystalline structure.

We studied phase-change recording using a 405 nm blue laser diode, an objective lens of numerical aperture (NA)=0.6 and Ag–In–Sb–Te phase-change material. We report land and groove recording on a conventional disk structure with a 0.6-mm-thick and 120-mm-diameter PC substrate. We confirmed the possibility of achieving a high density, 12 GB capacity and a high data-transfer rate of 30 Mbps, corresponding to that of a read-only digital versatile disk (DVD-ROM) under the conditions of 6.5 m/s recording speed, 0.18 µm/bit linear density (minimum mark length 0.27 µm) and 0.42 µm trackpitch.