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We performed optical pulse propagation experiments in a system in which two ultrahigh-Q silica microspheres of different diameters were coupled in tandem to a fiber taper to yield coupled-resonator-induced transparency. Nearly Gaussian-shaped optical pulses propagated with a large positive delay of 8.5 ns through a transparent frequency window, without significant attenuation, amplification, or pulse deformation, demonstrating classical analogy of the extremely slow light obtained with electromagnetically induced transparency.

We study the Heisenberg antiferromagnet on the Shastry-Sutherland lattice under magnetic fields, to clarify the magnetic properties of SrCu2(BO3)2. Treating magnetic excitations promoted by the field as Bose particles and using strong coupling expansion, we derive an effective Hamiltonian for the effective magnetic particles. Anisotropic repulsive interactions between effective particles induce 'insulating' states with a stripe SDW structure at magnetization m/msat = 1/3 and a checkerboard structure at 1/2, and thereby form magnetization plateaus. Supersolid phases appear around insulating SDW phases by changing the magnetic field. Nature of these supersolid phases is discussed in detail. We also demonstrate how the geometry of the Shastry-Sutherland lattice affects dynamical properties of magnetic excitations significantly and makes a novel type of quintuplet (S = 2) boundstates condense for very small magnetization.

We study the magnetization process in two-dimensional S = 1/2 spin systems, to discuss the appearance of a plateau structure. The following three cases are considered: 1) the Heisenberg antiferromagnet and multiple-spin exchange model on the triangular lattice, 2) Shastry-Sutherland type lattice, (which is a possible model for SrCu2(BO3)2,) 3) 1/5-depleted lattice (for CaV4O9). We find in these systems that magnetization plateaus can appear owing to a transition from superfluid to a Mott insulator of magnetic excitations. The plateau states have CDW order of the excitations. The magnetizations of the plateaus depend on components of the magnetic excitations, range of the repulsive interaction, and the geometry of the lattice.PACS numbers: 75.60. Ej, 75.10.Jm In some one-dimensional spin systems, spin-densitywave states with finite spin gap appear under a finite magnetic field accompanying plateau structures in the magnetization process. Magnetization plateaus were observed in some quasi one-dimensional materials. 1 Theoretical arguments clarify that the appearance of the plateau is explained by an insulator-conductor transition of magnetic excitations. 2 In two-or higher-dimensional systems, magnetization plateaus have been also found in both theoretical 3-5 -and experimental studies. [6][7][8] In this paper, we propose a rather general picture that these twodimensional plateaus are formed owing to field-induced insulator-superfluid transitions of magnetic excitations. To demonstrate how it works, we discuss three examples in details.The first example is a family of antiferromagnets on a triangular lattice. For the S = 1/2 antiferromagnet on a triangular lattice (AFT), Nishimori and Miyashita 3 found a magnetization plateau at m/m sat = 1/3, which comes from the appearance of a collinear state with three sublattices, i.e., the so-called "uud" state. This plateau was actually observed in AFT materials like C 6 Eu(Ref. 6) and CsCuCl 3 (Ref. 7). Recently in a multiple-spin exchange (MSE) model, which is a possible model 9 for solid 3 He films, a magnetization plateau was predicted 5 at m/m sat = 1/2. In this case, the plateau is attributed to the formation of a similar collinear state but with four sublattices. The magnetization processes of these systems have been studied extensively and here we just attempt interpreting the known results to test the new picture.We take as the second example the S = 1/2 Heisenberg antiferromagnet (HAF) on the Shastry-Sutherland lattice (Shastry-Sutherland model, hereafter. See Fig. 1), 10 which is known to have an exact dimer ground state. Recently Kageyama et al. 8 found that SrCu 2 (BO 3 ) 2 realizes a lattice structure equivalent to that discussed in Ref. 10 and that it has a gapful ground state. The magnetization measurements show plateaus at m/m sat = 1/8 and 1/4. The last is the S = 1/2 HAF on the 1/5-depleted square lattice (Fig. 2), which includes a model Hamiltonian for CaV 4 O 9 . In this system, the plaquette singlet state is realized in the ground state. 11In our ...

We give a simple method to reduce the vss-type stares, usually expressed by the valence bonds. to the matrix-product representation. In this representation. peculiar properlies of the vss-type states are quite manif&. Using it. we investigate hidden order in the spin4 VBS model. The possibility that the Haldane phase for higher spin S is characterized by string order p m m e ~r s is discussed. A picture of an approximate excitation is also presented.