Superconductivity in Pseudo-Binary Silicide SrNixSi2-x with AlB2-Type Structure

We demonstrate the emergence of superconductivity in pseudo-binary silicide SrNixSi2-x. The compound exhibits a structural phase transition from the cubic SrSi2-type structure (P4132) to the hexagonal AlB2-type structure (P6/mmm) upon substituting Ni for Si at approximately x = 0.1. The hexagonal structure is stabilized in the range of 0.1 < x < 0.7. The superconducting phase appears in the vicinity of the structural phase boundary. Ni acts as a nonmagnetic dopant, as confirmed by the Pauli paramagnetic behavior.Comment: 12 pages, 5 figure

^{63}Cu, ^{35}Cl, and ^{1}H NMR in the S=1/2 Kagom\'e Lattice ZnCu_{3}(OH)_{6}Cl_{2}

ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ ($S=1/2$) is a promising new candidate for an ideal Kagom\'e Heisenberg antiferromagnet, because there is no magnetic phase transition down to $\sim$50 mK. We investigated its local magnetic and lattice environments with NMR techniques. We demonstrate that the intrinsic local spin susceptibility {\it decreases} toward T=0, but that slow freezing of the lattice near $\sim$50 K, presumably associated with OH bonds, contributes to a large increase of local spin susceptibility and its distribution. Spin dynamics near T=0 obey a power-law behavior in high magnetic fields.Comment: Phys. Rev. Lett. (in press

Experimental Realization of Optimal Noise Estimation for a General Pauli Channel

We present the experimental realization of the optimal estimation protocol for a Pauli noisy channel. The method is based on the generation of 2-qubit Bell states and the introduction of quantum noise in a controlled way on one of the state subsystems. The efficiency of the optimal estimation, achieved by a Bell measurement, is shown to outperform quantum process tomography

Magnetic-field-induced transition in BaVS3

The metal-insulator transition (MIT) of BaVS3 is suppressed under pressure and above the critical pressure of p~2GPa the metallic phase is stabilized. We present the results of detailed magnetoresistivity measurements carried out at pressures near the critical value, in magnetic fields up to B=12T. We found that slightly below the critical pressure the structural tetramerization -- which drives the MIT -- is combined with the onset of magnetic correlations. If the zero-field transition temperature is suppressed to a sufficiently low value (T_MI<15K), the system can be driven into the metallic state by application of magnetic field. The main effect is not the reduction of T_MI with increasing B, but rather the broadening of the transition due to the applied magnetic field. We tentatively ascribe this phenomenon to the influence on the magnetic structure coupled to the bond-order of the tetramers.Comment: 5 pages, 5 figure

^{59}Co NMR evidence for charge ordering below T_{CO}\sim 51 K in Na_{0.5}CoO_2

The CoO$_{2}$ layers in sodium-cobaltates Na$_{x}$CoO$_{2}$ may be viewed as a spin $S=1/2$ triangular-lattice doped with charge carriers. The underlying physics of the cobaltates is very similar to that of the high $T_{c}$ cuprates. We will present unequivocal $^{59}$Co NMR evidence that below $T_{CO}\sim51 K$, the insulating ground state of the itinerant antiferromagnet Na$_{0.5}$CoO$_{2}$ ($T_{N}\sim 86 K$) is induced by charge ordering.Comment: Phys. Rev. Lett. 100 (2008), in press. 4 figure

Field-Induced Quasiparticle Excitation in Ca(Al0.5_{0.5}Si0.5_{0.5})2_2: Evidence for unconventional Superconductivity

The temperature ($T$) and magnetic field ($H$) dependence of the magnetic penetration depth, $\lambda(T,H)$, in Ca(Al$_{0.5}$Si$_{0.5}$)$_2$ exhibits significant deviation from that expected for conventional BCS superconductors. In particular, it is inferred from a field dependence of $\lambda(H)$ ($\propto H$) at 2.0 K that the quasiparticle excitation is strongly enhanced by the Doppler shift. This suggests that the superconducting order parameter in Ca(Al$_{0.5}$Si$_{0.5}$)$_2$ is characterized by a small energy scale $\Delta_S/k_B\le 2$ K originating either from anisotropy or multi-gap structure.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jp

Intra-Landau level polarization effect for a striped Hall gas

We calculate the polarization function including only intra-Landau level correlation effects of striped Hall gas. Using the polarization function, the dielectric function, the dispersion of the plasmon and the correlation energy are computed in a random phase approximation (RPA) and generalized random phase approximation (GRPA). The plasmon becomes anisotropic and gapless owing to the anisotropy of the striped Hall gas and two dimensionality of the quantum Hall system. The plasmon approximately agrees with the phonon derived before by the single mode approximation. The (G)RPA correlation energy is compared with other numerical calculations.Comment: 15 pages,15 figures, revtex4, published versio

Possible Magnetic Chirality in Optically Chiral Magnet [Cr(CN)6_6][Mn(SS)-pnH(H2_2O)](H2_2O) Probed by Muon Spin Rotation and Relaxation

Local magnetic fields in a molecule-based optically chiral magnet [Cr(CN)$_6$][Mn($S$)-pnH(H$_2$O)](H$_2$O) (GN-S) and its enantiomer (GN-R) are studied by means of muon spin rotation and relaxation (muSR). Detailed analysis of muon precession signals under zero field observed below T_c supports the average magnetic structure suggested by neutron powder diffraction. Moreover, comparison of muSR spectra between GN-S and GN-R suggests that they are a pair of complete optical isomers in terms of both crystallographic and magnetic structure. Possibility of magnetic chirality in such a pair is discussed.Comment: 5 pages, 5 figures, submitted to J. Phys. Soc. Jp