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Register a new userAre multiphase competition & order-by-disorder the keys to understanding Yb2Ti2O7?
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Added by
Ludovic D.C. Jaubert
Publication date
2015
fields
Physics
and research's language is
English
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If magnetic frustration is most commonly known for undermining long-range order, as famously illustrated by spin liquids, the ability of matter to develop new collective mechanisms in order to fight frustration is no less fascinating, providing an avenue for the exploration and discovery of unconventional properties of matter. Here we study an ideal minimal model of such mechanisms which, incidentally, pertains to the perplexing quantum spin ice candidate Yb2Ti2O7. Specifically, we explain how thermal and quantum fluctuations, optimized by order-by-disorder selection, conspire to expand the stability region of an accidentally degenerate continuous symmetry U(1) manifold against the classical splayed ferromagnetic ground state that is displayed by the sister compound Yb2Sn2O7. The resulting competition gives rise to multiple phase transitions, in striking similitude with recent experiments on Yb2Ti2O7 [Lhotel et al., Phys. Rev. B 89 224419 (2014)]. Considering the effective Hamiltonian determined for Yb2Ti2O7, we provide, by combining a gamut of numerical techniques, compelling evidence that such multiphase competition is the long-sought missing key to understanding the intrinsic properties of this material. As a corollary, our work offers a pertinent illustration of the influence of chemical pressure in rare-earth pyrochlores.
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We document the coexistence of ferro- and anti-ferromagnetism in pyrochlore $rm Yb_2Ti_2O_7$ using three neutron scattering techniques on stoichiometric crystals: elastic neutron scattering shows a canted ferromagnetic ground state, neutron scattering shows spin wave excitations from both a ferro-and an antiferro-magnetic state, and field and temperature dependent small angle neutron scattering reveals the corresponding anisotropic magnetic domain structure. High-field $langle 111 rangle$ spin wave fits show that $rm Yb_2Ti_2O_7$ is extremely close to an antiferromagnetic phase boundary. Classical Monte Carlo simulations based on the interactions inferrred from high field spin wave measurements confirm $psi_2$ antiferromagnetism is metastable within the FM ground state.
Motivated by recent neutron scattering experiments, we derive and study an effective pseudo-dipolar spin-1/2 model for the XY pyrochlore antiferromagnet Er2Ti2O7. While a bond-dependent in-plane exchange anisotropy removes any continuous symmetry, it does lead to a one-parameter `accidental classical degeneracy. This degeneracy is lifted by quantum fluctuations in favor of the non-coplanar spin structure observed experimentally -- a rare experimental instance of quantum order by disorder. A non-Goldstone low-energy mode is present in the excitation spectrum in accordance with inelastic neutron scattering data. Our theory also resolves the puzzle of the experimentally observed continuous ordering transition, absent from previous models.
Several rare earth magnetic pyrochlore materials are well modeled by a spin-1/2 quantum Hamiltonian with anisotropic exchange parameters Js. For the Er2Ti2O7 material, the Js were recently determined from high-field inelastic neutron scattering measurements. Here, we perform high-temperature (T) series expansions to compute the thermodynamic properties of this material using these Js. Comparison with experimental data show that the model describes the material very well including the finite temperature phase transition to an ordered phase at Tc~1.2 K. We show that high temperature expansions give identical results for different q=0 xy order parameter susceptibilities up to 8th order in beta=1/T (presumably to all orders in beta). Conversely, a non-linear susceptibility related to the 6th power of the order parameter reveals a thermal order-by-disorder selection of the same non-colinear psi_2 state as found in Er2Ti2O7.
In frustrated magnetic systems with competing interactions fluctuations can lift the residual accidental degeneracy. We argue that the state selection may have different outcomes for quantum and thermal order by disorder. As an example, we consider the semiclassical Heisenberg fcc antiferromagnet with only the nearest-neighbor interactions. Zero-point oscillations select the type 3 collinear antiferromagnetic state at T=0. Thermal fluctuations favor instead the type 1 antiferromagnetic structure. The opposite tendencies result in a finite-temperature transition between the two collinear states. Competition between effects of quantum and thermal order by disorder is a general phenomenon and is also realized in the J1-J2 square-lattice antiferromagnet at the critical point J2 = 0.5 J1.
We examine the Si(111) multi-valley quantum Hall system and show that it exhibits an exceptionally rich interplay of broken symmetries and quantum Hall ordering already near integer fillings $ u$ in the range $ u=0-6$. This six-valley system has a large $[SU(2)]^3rtimes D_3$ symmetry in the limit where the magnetic length is much larger than the lattice constant. We find that the discrete ${D}_3$ factor breaks over a broad range of fillings at a finite temperature transition to a discrete nematic phase. As $T rightarrow 0$ the $[SU(2)]^3$ continuous symmetry also breaks: completely near $ u =3$, to a residual $[U(1)]^2times SU(2)$ near $ u=2$ and $4$ and to a residual $U(1)times [SU(2)]^2$ near $ u=1$ and $5$. Interestingly, the symmetry breaking near $ u=2,4$ and $ u=3$ involves a combination of selection by thermal fluctuations known as order by disorder and a selection by the energetics of Skyrme lattices induced by moving away from the commensurate fillings, a mechanism we term order by doping. We also exhibit modestly simpler analogs in the four-valley Si(110) system.
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