Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userGround State Phase Diagram of Generic XY Pyrochlore Magnets with Quantum Fluctuations
148
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Motivated by recent experimental and theoretical progress on the Er2Ti2O7 pyrochlore XY antiferromagnet, we study the problem of quantum order-by-disorder in pyrochlore XY systems. We consider the most general nearest-neighbor pseudo spin-1/2 Hamiltonian for such a system characterized by anisotropic spin-spin couplings J_e = [J_pm, J_{pmpm}, J_{zpm}, J_{zz}] and construct zero-temperature phase diagrams. Combining symmetry arguments and spin-wave calculations, we show that the ground state phase boundaries between the two candidate ground states of the Gamma_5 irreducible representation, the psi_2 and psi_3 (basis) states, are rather accurately determined by a cubic equation in J_{pm}J_{pmpm})/J_{zpm}^2. Depending on the value of J_{zz}, there can be one or three phase boundaries that separate alternating regions of psi_2 and psi_3 states. In particular, we find for sufficiently small J_{zz}/J_{pm} a narrow psi_2 sliver sandwiched between two psi_3 regions in the J_{pmpm}/J_pm vs J_{zpm}/J_pm phase diagram. Our results further illustrate the very large potential sensitivity of the ground state of XY pyrochlore systems to minute changes in their spin Hamiltonian. Using the experimentally determined J_3 and g-tensor values for Er2Ti2O7, we show that the heretofore neglected long-range 1/r^3 magnetostatic dipole-dipole interactions do not change the conclusion that Er2Ti2O7 has a psi_2 ground state induced via a quantum order-by-disorder mechanism. We propose that the CdDy2Se4 chalcogenide spinel, in which the Dy^{3+} ions form a pyrochlore lattice and may be XY-like, could prove interesting to investigate.
rate research
Read More
We report neutron scattering and muon spin relaxation measurements (muSR) on the pyrochlore antiferromagnet Yb2Ge2O7. Inelastic neutron scattering was used to probe the transitions between crystal electric field levels, allowing us to determine the eigenvalues and eigenvectors appropriate to the J=7/2 Yb3+ ion in this environment. The crystal electric field ground state doublet in Yb2Ge2O7 corresponds primarily to m_J = +/- 1/2 with local XY anisotropy, consistent with an S_eff = 1/2 description for the Yb moments. muSR measurements reveal the presence of an ordering transition at T_N = 0.57 K with persistent weak dynamics in the ordered state. Finally, we present neutron diffraction measurements that reveal a clear phase transition to the k = (000) Gamma_5 ground state with an ordered magnetic moment of 0.3(1) mu_B per Yb ion. We compare and contrast this phenomenology with the low temperature behavior of Yb2Ti2O7 and Er2Ti2O7, the prototypical S_eff = 1/2 XY pyrochlore magnets.
There is a growing family of rare-earth kagome materials with dominant nearest-neighbor interactions and strong spin orbit coupling. The low symmetry of these materials makes theoretical description complicated, with six distinct nearest-neighbor coupling parameters allowed. In this Article, we ask what kinds of classical, ordered, ground states can be expected to occur in these materials, assuming generic (i.e. non-fine-tuned) sets of exchange parameters. We use symmetry analysis to show that there are only five distinct classical ground state phases occurring for generic parameters. The five phases are: (i) a coplanar, 2-fold degenerate, state with vanishing magnetization (${sf A_1}$), (ii) a noncoplanar, 2-fold degenerate, state with magnetization perpendicular to the kagome plane (${sf A_2}$), (iii) a coplanar, 6-fold degenerate, state with magnetization lying within the kagome plane (${sf E}$-coplanar), (iv) a noncoplanar, 6-fold degenerate, state with magnetization lying within a mirror plane of the lattice (${sf E}$-noncoplanar$_{6}$), (v) a noncoplanar, 12-fold degenerate, state with magnetization in an arbitrary direction (${sf E}$-noncoplanar$_{12}$). All five are translation invariant (${bf q}=0$) states. Having found the set of possible ground states, the ground state phase diagram is obtained by comparing numerically optimized energies for each possibility as a function of the coupling parameters. The state ${sf E}$ noncoplanar$_{12}$ is extremely rare, occupying $<1%$ of the full phase diagram, so for practical purposes there are four main ordered states likely to occur in anisotropic kagome magnets with dominant nearest neighbor interactions. These results can aid in interpreting recent experiments on ``tripod kagome systems R$_3$A$_2$Sb$_3$O$_{14}$, as well as materials closer to the isotropic limit such as Cr- and Fe- jarosites.
The dipolar-octupolar pyrochlore oxides R$_2$M$_2$O$_7$ (R=Ce, Sm, Nd) represent an important opportunity in the search for three dimensional Quantum Spin Liquid (QSL) ground states. Their low energy physics is governed by an alluringly simple XYZ Hamiltonian, enabling theoretical description with only a small number of free parameters. Meanwhile, recent experiments on Ce pyrochlores strongly suggest QSL physics. Motivated by this, we present here a complete analysis of the ground state phase diagram of dipolar-octupolar pyrochlores. Combining cluster mean field theory, variational arguments and exact diagonalization we find multiple U(1) QSL phases which together occupy a large fraction of the parameter space. These results give a comprehensive picture of the ground state physics of an important class of QSL candidates and support the possibility of a $U(1)$ QSL ground state in Ce$_2$Zr$_2$O$_7$ and Ce$_2$Sn$_2$O$_7$.
We explore the field-temperature phase diagram of the XY pyrochlore antiferromagnet Er$_2$Ti$_2$O$_7$, by means of magnetization and neutron diffraction experiments. Depending on the field strength and direction relative to the high symmetry cubic directions $[001], [1bar{1}0]$ and $[111]$, the refined field induced magnetic structures are derived from the zero field $psi_2$ and $psi_3$ states of the $Gamma_5$ irreducible representation which describes the ground state of XY pyrochlore antiferromagnets. At low field, domain selection effects are systematically at play. In addition, for $[001]$, a phase transition is reported towards a $psi_3$ structure at a characteristic field $H_c^{001}=$ 43 mT. For $[1bar{1}0]$ and $[111]$, the spins are continuously tilted by the field from the $psi_2$ state, and no phase transition is found while domain selection gives rise to sharp anomalies in the field dependence of the Bragg peaks intensity. For $[1bar{1}0]$, these results are confirmed by high resolution inelastic neutron scattering experiments, which in addition allow us to determine the field dependence of the spin gap. This study agrees qualitatively with the scenario proposed theoretically by Maryasin {it et al.} [Phys. Rev. B {bf 93}, 100406(R) (2016)], yet the strength of the field induced anisotropies is significantly different from theory.
Conclusive evidence of order by disorder is scarce in real materials. Perhaps one of the strongest cases presented has been for the pyrochlore XY antiferromagnet Er2Ti2O7, with the ground state selection proceeding by order by disorder induced through the effects of quantum fluctuations. This identification assumes the smallness of the effect of virtual crystal field fluctuations that could provide an alternative route to picking the ground state. Here we show that this order by virtual crystal field fluctuations is not only significant, but competitive with the effects of quantum fluctuations. Further, we argue that higher-multipolar interactions that are generically present in rare-earth magnets can dramatically enhance this effect. From a simplified bilinear-biquadratic model of these multipolar interactions, we show how the virtual crystal field fluctuations manifest in Er2Ti2O7 using a combination of strong coupling perturbation theory and the random phase approximation. We find that the experimentally observed psi2 state is indeed selected and the experimentally measured excitation gap can be reproduced when the bilinear and biquadratic couplings are comparable while maintaining agreement with the entire experimental spin-wave excitation spectrum. Finally, we comments on possible tests of this scenario and discuss implications for other order-by-disorder candidates in rare-earth magnets.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
