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Register a new userAntiferro-quadrupolar correlations in the quantum spin ice candidate Pr2Zr2O7
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We present an experimental study of the quantum spin ice candidate pyrochlore coumpound przr by means of magnetization measurements, specific heat and neutron scattering up to 12 T and down to 60 mK. When the field is applied along the $[111]$ and $[1bar{1}0]$ directions, ${bf k}=0$ field induced structures settle in. We find that the ordered moment rises slowly, even at very low temperature, in agreement with macroscopic magnetization. Interestingly, for $H parallel [1bar{1}0]$, the ordered moment appears on the so called $alpha$ chains only. The spin excitation spectrum is essentially {it inelastic} and consists in a broad flat mode centered at about 0.4 meV with a magnetic structure factor which resembles the spin ice pattern. For $H parallel [1bar{1}0]$ (at least up to 2.5 T), we find that a well defined mode forms from this broad response, whose energy increases with $H$, in the same way as the temperature of the specific heat anomaly. We finally discuss these results in the light of mean field calculations and propose a new interpretation where quadrupolar interactions play a major role, overcoming the magnetic exchange. In this picture, the spin ice pattern appears shifted up to finite energy because of those new interactions. We then propose a range of acceptable parameters for przr, that allow to reproduce several experimental features observed under field. With these parameters, the actual ground state of this material would be an antiferroquadrupolar liquid with spin-ice like excitations.
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In this work, we show that the zero field excitation spectra in the quantum spin ice candidate pyrochlore compound ybti is a continuum characterized by a very broad and almost flat dynamical response which extends up to $1-1.5$ meV, coexisting or not with a quasi-elastic response depending on the wave-vector. The spectra do not evolve between 50 mK and 2 K, indicating that the spin dynamics is only little affected by the temperature in both the short-range correlated and ordered regimes. Although classical spin dynamics simulations qualitatively capture some of the experimental observations, we show that they fail to reproduce this broad continuum. In particular, the simulations predict an energy scale twice smaller than the experimental observations. This analysis is based on a careful determination of the exchange couplings, able to reproduce both the zero field diffuse scattering and the spin wave spectrum rising in the field polarized state. According to this analysis, ybti lies at the border between a ferro and an antiferromagnetic phase. These results suggest that the unconventional ground state of ybti is governed by strong quantum fluctuations arising from the competition between those phases. The observed spectra may correspond to a continuum of deconfined spinons as expected in quantum spin liquids.
Recent work has highlighted remarkable effects of classical thermal fluctuations in the dipolar spin ice compounds, such as artificial magnetostatics, manifesting as Coulombic power-law spin correlations and particles behaving as diffusive magnetic monopoles. In this paper, we address quantum spin ice, giving a unifying framework for the study of magnetism of a large class of magnetic compounds with the pyrochlore structure, and in particular discuss Yb2Ti2O7 and extract its full set of Hamiltonian parameters from high field inelastic neutron scattering experiments. We show that fluctuations in Yb2Ti2O7 are strong, and that the Hamiltonian may support a Coulombic Quantum Spin Liquid ground state in low field and host an unusual quantum critical point at larger fields. This appears consistent with puzzling features in prior experiments on Yb2Ti2O7. Thus Yb2Ti2O7 is the first quantum spin liquid candidate in which the Hamiltonian is quantitatively known.
We report a positive muon spin relaxation and rotation (muSR) study of the quantum spin ice materials Yb2Ti2O7 and Yb2Sn2O7 focusing on the low field response. In agreement with earlier reports, data recorded in small longitudinal fields evidence anomalously slow spin dynamics in the microsecond range below the temperature T_c at which the specific heat displays an intense peak, namely T_c = 0.24 K and 0.15 K, respectively, for the two systems. We found that slow dynamics extends above T_c up to at least 0.7 K for both compounds. The conventional dynamical Gaussian Kubo-Toyabe model describes the muSR spectra recorded above T_c. At lower temperatures a published analytical extension of the Gaussian Kubo-Toyabe model provides a good description, consistent with the existence of short-range magnetic correlations. While the physical response of the two systems is qualitatively the same, Yb2Ti2O7 exhibits a much larger local magnetic susceptibility than Yb2Sn2O7 below T_c. Considering previously reported ac susceptibility, neutron scattering and muSR results, we suggest the existence of anomalously slow spin dynamics to be a common physical property of pyrochlore magnetic materials. The possibility of molecular spin substructures to be associated to the slow dynamics and therefore the short-range correlations is mentioned. The slow spin dynamics observed under field does not exclude the presence of much faster dynamics detected in extremely low or zero field.
We report on diffuse neutron scattering experiments providing evidence for the presence of random strains in the quantum spin ice candidate Pr2Zr2O7. Since Pr is a non-Kramers ion, the strain deeply modifies the picture of Ising magnetic moments governing the low temperature properties of this material. It is shown that the derived strain distribution accounts for the temperature dependence of the specific heat and of the spin excitation spectra. Taking advantage of mean field and spin dynamics simulations, we argue that the randomness in Pr2Zr2O7, promotes a new state of matter, which is disordered, yet characterized by short range antiferroquadrupolar correlations, and from which emerge spin-ice like excitations. This study thus opens an original research route in the field of quantum spin ice.
A promising route to realize entangled magnetic states combines geometrical frustration with quantum-tunneling effects. Spin-ice materials are canonical examples of frustration, and Ising spins in a transverse magnetic field are the simplest many-body model of quantum tunneling. Here, we show that the tripod kagome lattice material Ho3Mg2Sb3O14 unites an ice-like magnetic degeneracy with quantum-tunneling terms generated by an intrinsic splitting of the Ho3+ ground-state doublet, realizing a frustrated transverse Ising model. Using neutron scattering and thermodynamic experiments, we observe a symmetry-breaking transition at T*~0.32 K to a remarkable quantum state with three peculiarities: a continuous magnetic excitation spectrum down to T~0.12K; a macroscopic degeneracy of ice-like states; and a fragmentation of the spin into periodic and aperiodic components strongly affected by quantum fluctuations. Our results establish that Ho3Mg2Sb3O14 realizes a spin-fragmented state on the kagome lattice, with intrinsic quantum dynamics generated by a homogeneous transverse field.
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