High Magnetic Field NMR Studies of LiVGe2_2O6_6, a quasi 1-D Spin S=1S = 1 System

We report $^{7}$Li pulsed NMR measurements in polycrystalline and single crystal samples of the quasi one-dimensional S=1 antiferromagnet LiVGe$_2$O$_6$, whose AF transition temperature is $T_{\text{N}}\simeq 24.5$ K. The field ($B_0$) and temperature ($T$) ranges covered were 9-44.5 T and 1.7-300 K respectively. The measurements included NMR spectra, the spin-lattice relaxation rate ($T_1^{-1}$), and the spin-phase relaxation rate ($T_2^{-1}$), often as a function of the orientation of the field relative to the crystal axes. The spectra indicate an AF magnetic structure consistent with that obtained from neutron diffraction measurements, but with the moments aligned parallel to the c-axis. The spectra also provide the $T$-dependence of the AF order parameter and show that the transition is either second order or weakly first order. Both the spectra and the $T_1^{-1}$ data show that $B_0$ has at most a small effect on the alignment of the AF moment. There is no spin-flop transition up to 44.5 T. These features indicate a very large magnetic anisotropy energy in LiVGe$_2$O$_6$ with orbital degrees of freedom playing an important role. Below 8 K, $T_1^{-1}$ varies substantially with the orientation of $B_0$ in the plane perpendicular to the c-axis, suggesting a small energy gap for magnetic fluctuations that is very anisotropic.Comment: submitted to Phys. Rev.

Anisotropic behaviour of human gallbladder walls

Inverse estimation of biomechanical parameters of soft tissues from non-invasive measurements has clinical significance in patient-specific modelling and disease diagnosis. In this paper, we propose a fully nonlinear approach to estimate the mechanical properties of the human gallbladder wall muscles from in vivo ultrasound images. The iteration method consists of a forward approach, in which the constitutive equation is based on a modified Hozapfel–Gasser–Ogden law initially developed for arteries. Five constitutive parameters describing the two orthogonal families of fibres and the matrix material are determined by comparing the computed displacements with medical images. The optimisation process is carried out using the MATLAB toolbox, a Python code, and the ABAQUS solver. The proposed method is validated with published artery data and subsequently applied to ten human gallbladder samples. Results show that the human gallbladder wall is anisotropic during the passive refilling phase, and that the peak stress is 1.6 times greater than that calculated using linear mechanics. This discrepancy arises because the wall thickness reduces by 1.6 times during the deformation, which is not predicted by conventional linear elasticity. If the change of wall thickness is accounted for, then the linear model can used to predict the gallbladder stress and its correlation with pain. This work provides further understanding of the nonlinear characteristics of human gallbladder

The effect of disorder on the critical points in the vortex phase diagram of YBCO

The effect of line disorder induced by heavy ion irradiation and of point disorder induced by proton and electron irradiation on the upper and lower critical points in the vortex phase diagram of YBCO is presented. The authors find that dilute line disorder induces a Bose glass transition at low fields which is replaced at the lower critical point by first order melting at higher fields. Strong pinning point defects raise the lower critical point, while weak pinning point defects have little or no effect on the lower critical point. The upper critical point is lowered by point disorder, but raised by line disorder. First order melting is suppressed by point disorder in two ways, by lowering of the upper critical point only for weak point pins, or by merging of the upper and lower critical points for strong point pins. The differing responses of the upper and lower critical points to line and point disorder can be understood in a picture of transverse and longitudinal spatial fluctuations

High magnetic field NMR investigation of the spin density wave phase of TMTSF2_2PF6_6

We report proton NMR measurements of the effect of very high magnetic fields up to 44.7 T (1.9 GHz) on the spin density wave (SDW) transition of the organic conductor TMTSF$_2$PF$_6$. Up to 1.8 GHz, no effect of critical slowing close to the transition is seen on the proton relaxation rate (1/T$_1$), which is determined by the SDW fluctuations associated with the phase transition at the NMR frequency. Thus, the correlation time for such fluctuations is less than $1O^{-10}$s. A possible explanation for the absence of longer correlation times is that the transition is weakly first order, so that the full critical divergence is never achieved. The measurements also show a dependence of the transition temperature on the orientation of the magnetic field and a quadratic dependence on its magnitude that agrees with earlier transport measurements at lower fields. The UCLA part of this work was supported by NSF Grant DMR-0072524

Competing Orders In Underdoped (ba1-xkx)fe 2as2

We report 75As Nuclear Magnetic Resonance (NMR) measurements in the high-Tc superconductor (Ba1-xKx)Fe 2As2 in the underdoped regime. A structural transition at Ts ≃110 K is followed by an antiferromagnetic (AFM) order at TN ≃102 K for our x = 0.16 single crystal [1]. Superconductivity (SC) also appears at Tc ≃20 K. We find that the ordered Fe moment (S) is reduced upon hole-doping. Both spectrum analysis and relaxation measurements indicate that pinned vortices are present below Tc and SC is coexisting with AFM fluctuations. © Published under licence by IOP Publishing Ltd.2731Urbano R R, Green E L, Moulton W G et al 2010 arXiv:1005.3718v1 [cond-mat.supr-con]Kamihara, Y., Watanabe, T., Hirano, M., Hosono, H., (2008) J. Am. Chem. Soc., 130 (11), pp. 3296-3297Rotter, M., Tegel, M., Johrendt, D., (2008) Phys. Rev. Lett., 101 (10), p. 107006Ni, N., Tillman, M.E., Yan, J.-Q., (2008) Phys. Rev., 78 (21), p. 214515Pratt, D.K., Tian, W., Kreyssig, A., (2009) Phys. Rev. Lett., 103 (8), p. 087001Lester, C., Chu, J.-H., Analytis, J.G., (2009) Phys. Rev., 79 (14), p. 144523Johnston D C 2010 arXiv:1005.4392v1 [cond-mat.supr-con]Park, J.T., Inosov, D.S., Niedermayer, Ch., (2009) Phys. Rev. Lett., 102 (11), p. 117006Kitagawa, K., Katayama, N., Ohgushi, K., (2008) J. Phys. Soc. Jpn, 77 (11), p. 114709Julien, M.H., Mayaffre, H., Horvatic, M., (2009) Europhys. Lett., 87 (3), p. 37001Mukuda, H., Terasaki, N., Yashima, M., (2009) Physica, 469 (9-12), p. 559Fukazawa, H., Yamazaki, T., Kondo, K., (2009) J. Phys. Soc. Japan., 78 (3), p. 033704Nakai, Y., Ishida, K., Kamihara, Y., Hirano, M., Hosono, H., (2008) J. Phys. Soc. Japan., 78, p. 033704Moriya, T., (1985) Spin Fluctuations in Itinerant Electron Magnetism, , Berlin, Springer-VerlagBachman, H.N., Reyes, A.P., Mitrovic, V.F., (1998) Phys. Rev. Lett., 80 (8), p. 172