1,307 research outputs found

    Magnetic and Thermodynamic Properties of the Collective Paramagnet-Spin Liquid Pyrochlore Tb2Ti2O7

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    In a recent letter [Phys. Rev. Lett. {\bf 82}, 1012 (1999)] it was found that the Tb3+^{3+} magnetic moments in the Tb2_2Ti2_2O7_7 pyrochlore lattice of corner-sharing tetrahedra remain in a {\it collective paramagnetic} state down to 70mK. In this paper we present results from d.c. magnetic susceptibility, specific heat data, inelastic neutron scattering measurements, and crystal field calculations that strongly suggest that (1) the Tb3+^{3+} ions in Tb2_2Ti2_2O7_7 possess a moment of approximatively 5μB\mu_{\rm B}, and (2) the ground state gg-tensor is extremely anisotropic below a temperature of O(100)O(10^0)K, with Ising-like Tb3+^{3+} magnetic moments confined to point along a local cubic diagonal(e.g.towardsthemiddleofthetetrahedron).SuchaverylargeeasyaxisIsinglikeanisotropyalonga diagonal (e.g. towards the middle of the tetrahedron). Such a very large easy-axis Ising like anisotropy along a direction dramatically reduces the frustration otherwise present in a Heisenberg pyrochlore antiferromagnet. The results presented herein underpin the conceptual difficulty in understanding the microscopic mechanism(s) responsible for Tb2_2Ti2_2O7_7 failing to develop long-range order at a temperature of the order of the paramagnetic Curie-Weiss temperature θCW101\theta_{\rm CW} \approx -10^1K. We suggest that dipolar interactions and extra perturbative exchange coupling(s)beyond nearest-neighbors may be responsible for the lack of ordering of Tb2_2Ti2_2O7_7.Comment: 8 POSTSCRIPT figures included. Submitted to Physical Review B. Contact: [email protected]

    Ferroelectric and Dipolar Glass Phases of Non-Crystalline Systems

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    In a recent letter [Phys. Rev. Lett. {\bf 75}, 2360 (1996)] we briefly discussed the existence and nature of ferroelectric order in positionally disordered dipolar materials. Here we report further results and give a complete description of our work. Simulations of randomly frozen and dynamically disordered dipolar soft spheres are used to study ferroelectric ordering in non-crystalline systems. We also give a physical interpretation of the simulation results in terms of short- and long-range interactions. Cases where the dipole moment has 1, 2, and 3 components (Ising, XY and XYZ models, respectively) are considered. It is found that the Ising model displays ferroelectric phases in frozen amorphous systems, while the XY and XYZ models form dipolar glass phases at low temperatures. In the dynamically disordered model the equations of motion are decoupled such that particle translation is completely independent of the dipolar forces. These systems spontaneously develop long-range ferroelectric order at nonzero temperature despite the absence of any fined-tuned short-range spatial correlations favoring dipolar order. Furthermore, since this is a nonequilibrium model we find that the paraelectric to ferroelectric transition depends on the particle mass. For the XY and XYZ models, the critical temperatures extrapolate to zero as the mass of the particle becomes infinite, whereas, for the Ising model the critical temperature is almost independent of mass and coincides with the ferroelectric transition found for the randomly frozen system at the same density. Thus in the infinite mass limit the results of the frozen amorphous systems are recovered.Comment: 25 pages (LATEX, no macros). 11 POSTSCRIPT figures enclosed. Submitted to Phisical Review E. Contact: [email protected]

    Non-trivial fixed point structure of the two-dimensional +-J 3-state Potts ferromagnet/spin glass

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    The fixed point structure of the 2D 3-state random-bond Potts model with a bimodal (±\pmJ) distribution of couplings is for the first time fully determined using numerical renormalization group techniques. Apart from the pure and T=0 critical fixed points, two other non-trivial fixed points are found. One is the critical fixed point for the random-bond, but unfrustrated, ferromagnet. The other is a bicritical fixed point analogous to the bicritical Nishimori fixed point found in the random-bond frustrated Ising model. Estimates of the associated critical exponents are given for the various fixed points of the random-bond Potts model.Comment: 4 pages, 2 eps figures, RevTex 3.0 format requires float and epsfig macro

    Stability of the Bragg glass phase in a layered geometry

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    We study the stability of the dislocation-free Bragg glass phase in a layered geometry consisting of coupled parallel planes of d=1+1 vortex lines lying within each plane, in the presence of impurity disorder. Using renormalization group, replica variational calculations and physical arguments we show that at temperatures T<TGT<T_G the 3D Bragg glass phase is always stable for weak disorder. It undergoes a weakly first order transition into a decoupled 2D vortex glass upon increase of disorder.Comment: RevTeX. Submitted to EP

    Proposal for a [111] Magnetization Plateau in the Spin Liquid State of Tb2Ti2O7

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    Despite a Curie-Weiss temperature θCW14\theta_{\rm CW} \sim -14 K, the Tb2Ti2O7 pyrochlore magnetic material lacks long range magnetic order down to at least T50T^*\approx 50 mK. It has recently been proposed that the low temperature collective paramagnetic or spin liquid regime of this material may be akin to a spin ice state subject to both thermal and quantum fluctuations - a {\it quantum spin ice} (QSI) of sorts. Here we explore the effect of a magnetic field B{\bm B} along the [111][111] direction on the QSI state. To do so, we investigate the magnetic properties of a microscopic model of Tb2Ti2O7 in an independent tetrahedron approximation in a finite B{\bm B} along [111][111]. Such a model describes semi-quantitatively the collective paramagnetic regime where nontrivial spin correlations start to develop at the shortest lengthscale, that is over a single tetrahedron, but where no long range order is yet present. Our results show that a magnetization plateau develops at low temperatures as the system develops B=0{\bm B}=0 ferromagnetic spin-ice-like "two-in/two-out" correlations at the shortest lengthscale. From these results, we are led to propose that the observation of such a [111] magnetization plateau in Tb2Ti2O7 would provide compelling evidence for a QSI at B=0{\bm B}=0 in this material and help guide the development of a theory for the origin of its spin liquid state.Comment: 6 pages, 3 figure

    Neutron spectroscopic study of crystal field excitations in Tb2Ti2O7 and Tb2Sn2O7

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    We present time-of-flight inelastic neutron scattering measurements at low temperature on powder samples of the magnetic pyrochlore oxides Tb2Ti2O7 and Tb2Sn2O7. These two materials possess related, but different ground states, with Tb2Sn2O7 displaying "soft" spin ice order below Tn~0.87 K, while Tb2Ti2O7 enters a hybrid, glassy spin ice state below Tg~0.2 K. Our neutron measurements, performed at T=1.5 K and 30 K, probe the crystal field states associated with the J=6 states of Tb3+ within the appropriate Fd\bar{3}m pyrochlore environment. These crystal field states determine the size and anisotropy of the Tb3+ magnetic moment in each material's ground state, information that is an essential starting point for any description of the low-temperature phase behavior and spin dynamics in Tb2Ti2O7 and Tb2Sn2O7. While these two materials have much in common, the cubic stanate lattice is expanded compared to the cubic titanate lattice. As our measurements show, this translates into a factor of ~2 increase in the crystal field bandwidth of the 2J+1=13 states in Tb2Ti2O7 compared with Tb2Sn2O7. Our results are consistent with previous measurements on crystal field states in Tb2Sn2O7, wherein the ground state doublet corresponds primarily to m_J=|\pm 5> and the first excited state doublet to mJ=|\pm 4>. In contrast, our results on Tb2Ti2O7 differ markedly from earlier studies, showing that the ground state doublet corresponds to a significant mixture of mJ=|\pm 5>, |\mp 4>, and |\pm 2>, while the first excited state doublet corresponds to a mixture of mJ=|\pm 4>, |\mp 5>, and |\pm 1>. We discuss these results in the context of proposed mechanisms for the failure of Tb2Ti2O7 to develop conventional long-range order down to 50 mK.Comment: 12 pages, 6 figures. Version is the same as the published one, except for figure placement on page
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