Determining ground states of correlated electron systems is fundamental to understanding novel phenomena in condensed matter physics. A difficulty, however, arises in a geometrically frustrated system in which the incompatibility between the global topology of an underlying lattice and local spin interactions gives rise to macroscopically degenerate ground states 1 , potentially prompting the emergence of quantum spin states, such as resonating valence bond (RVB) and valence bond solid (VBS). Although theoretically proposed to exist in a kagome lattice -one of the most highly frustrated lattices in two dimensions (2D) being comprised of corner-sharing triangles -such quantum-fluctuation-induced states have not been observed experimentally. Here we report the first realization of the "pinwheel" VBS ground state in the S = 1 2 deformed kagome lattice antiferromagnet Rb 2 Cu 3 SnF 12 . In this system, a lattice distortion breaks the translational symmetry of the ideal kagome lattice and stabilizes the VBS state.

We investigated the crystal structure of Rb 2 Cu 3 SnF 12 and its magnetic properties using single crystals. This compound is composed of Kagomé layers of corner-sharing CuF 6 octahedra with a 2a  2a enlarged cell as compared with the proper Kagomé layer. Rb 2 Cu 3 SnF 12 is magnetically described as an S ¼ 1=2 modified Kagomé antiferromagnet with four kinds of neighboring exchange interaction. From magnetic susceptibility and high-field magnetization measurements, it was found that the ground state is a disordered singlet with the spin gap, as predicted from a recent theory. Exact diagonalization for a 12-site Kagomé cluster was performed to analyze the magnetic susceptibility, and individual exchange interactions were evaluated. Antiferromagnets on highly frustrated lattices produce a rich variety of physics.1,2) In particular, a two-dimensional Heisenberg Kagomé antiferromagnet (2D HKAF) is of great interest from the viewpoint of the interplay of the frustration and quantum effects. There are many theoretical studies on the 2D HKAF. The spin wave theory for a large spin value predicted an ordered ground state with the so-called ffiffi ffi 3 p  ffiffi ffi 3 p structure, which is selected by quantum fluctuation from infinite classical ground states, 3,4) whereas for a small spin value, a disordered ground state was observed by various approaches.5-9) Recent careful analyses and numerical calculations for an S ¼ 1=2 case demonstrated that the ground state is a spin liquid state composed of singlet dimers only, and that the ground state is gapped for triplet excitations, but gapless for singlet excitations.10-12) Consequently, magnetic susceptibility has a rounded maximum at T $ ð1=6ÞJ=k B and decreases exponentially toward zero with decreasing temperature, while specific heat exhibits a power law behavior at low temperatures. 8,13) Specific heat also shows an additional structure, peak or shoulder at low temperatures after exhibiting a broad maximum at T $ ð2=3ÞJ=k B .The experimental studies of the S ¼ 1=2 HKAF have been limited, and the above-mentioned intriguing predictions have not been verified experimentally. The cupric com- 27) Unfortunately, these systems undergo structural phase transitions at T t ¼ 220 and 170 K, respectively, and also magnetic phase transitions at T N ' 24 K. 27) However, the magnetic susceptibilities observed at T > T t can be perfectly described using theoretical results for an S ¼ 1=2 HKAF with large exchange interactions J=k B $ 250 K. 28)In the present work, we synthesized the new hexagonal compound Rb 2 Cu 3 SnF 12 with a similar crystal structure as Cs 2 Cu 3 ZrF 12 and performed magnetic susceptibility and high-field magnetization measurements using single crystals. As shown below, we found that the ground state is a disordered singlet with a finite gap for magnetic excitations.Rb 2 Cu 3 SnF 12 crystals were synthesized via the chemical reaction 2RbF þ 3CuF 2 þ SnF 4 ! Rb 2 Cu 3 SnF 12 . RbF, CuF 2 , and SnF 4 were dehydrated by heating in vacuum at 60 -100 C for three days....

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We demonstrate local manipulation and detection of nuclear spin coherence in semiconductor quantum wells by an optical pump-probe technique combined with pulse rf NMR. The Larmor precession of photoexcited electron spins is monitored by time-resolved Kerr rotation (TRKR) as a measure of nuclear magnetic field. Under the irradiation of resonant pulsed rf magnetic fields, Rabi oscillations of nuclear spins are traced by TRKR signals. The intrinsic coherence time evaluated by a spin-echo technique reveals the dependence on the orientation of the magnetic field with respect to the crystalline axis as expected by the nearest neighbor dipole-dipole interaction.