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Register a new userStrong quantum fluctuations due to competition between magnetic phases in a pyrochlore iridate
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Henrik Jacobsen Henrik Jacobse
Publication date
2019
fields
Physics
and research's language is
English
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We report neutron diffraction measurements of the magnetic structures in two pyrochlore iridates, Yb2Ir2O7 and Lu2Ir2O7. Both samples exhibit the all-in-all-out magnetic structure on the Ir4+ sites below TN~ 150,K, with a low temperature moment of around 0.45 muB/Ir. Below 2,K, the Yb moments in Yb2Ir2O7 begin to order ferromagnetically. However, even at 40 mK the ordered moment is only 0.57(3)muB/Yb, well below the saturated moment of the ground state doublet of Yb3+ (1.9 muB/Yb), deduced from magnetization measurements and from a refined model of the crystal field environment, and also significantly smaller than the ordered moment of Yb in Yb2Ti2O7 (0.9 muB/Yb). A mean-field analysis shows that the reduced moment on Yb is a consequence of enhanced phase competition caused by coupling to the all-in-all-out magnetic order on the Ir sublattice.
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Fractonic phases of matter are novel quantum ground states supporting sub-dimensional emergent excitations with mobility restrictions and/or immobile fractons. The ground state degeneracy of such phases is sub-extensive and depends on the geometry of the underlying lattice. Due to these unusual properties, fractonic phases are considered as models for quantum memory or as examples of quantum glassy behaviors. While there exist a number of exactly solvable models with interactions between multiple particles/spins (twelve or more), the realization of such models in real materials is extremely challenging. In this work, we provide a realistic quantum model of quadratic spin interactions on the breathing pyrochlore lattice, inspired by a classical spin model studied earlier. We show that the emergent excitations in this model are immobile when they are present alone. They can only move as a cluster or when they reside at the corners of a membrane excitation. Using the membrane operators acting on the ground state manifold, we construct degenerate ground states with periodic boundary conditions. It is shown that the ground state degeneracy explicitly depends on the lattice geometry. We discuss the implications of these results in light of the rank-2 tensor gauge theory.
The pyrochlore Eu$_2$Ir$_2$O$_7$ has recently attracted significant attention as a candidate Weyl semimetal. The previous reports on this compound unanimously show a thermally induced metal to insulator (MI) transition, concomitant with antiferromagnetic (AFM) long-range ordering of the Ir-moments below T$_textit{N} sim $120 K. However, there are contradictory reports concerning the slope d$rho/$dT of the resistivity plots ($rho$) in the metallic state above the metal-insulator (MI) transition, and the value of $rho$ in the insulating state, both of which show significant sample dependence. Here, we explore this issue by investigating six different Eu$_2$Ir$_2$O$_7$ samples with slightly varying Eu:Ir ratio. High-resolution synchrotron powder diffraction are done to probe minor variations in the cell parameters of the various Eu$_2$Ir$_2$O$_7$ samples investigated here. Specific heat (C$ _p $) and magnetic susceptibility of all the samples showed long-range antiferromagnetic ordering upon cooling below T$ _textit{N} sim $120 K. The transitions are, however, found to be smeared out for the off-stoichiometric samples. We show that the sign of d$rho/$dT above the metal-insulator (MI) transition is highly sensitive to the unit cell length, which, in turn, depends on the level of Eu-stuffing at the Ir-site. Samples with composition close to the ideal stoichiometry (Eu : Ir $ = $ 1) showed a change of sign of d$rho/$dT from negative to positive upon cooling below a certain temperature T $^*$ $>$ T$_textit{MI}$. With increasing Eu-stuffing T$ ^* $ decreased until a negative d$rho/$dT persisted without any sign change down to T$_textit{MI}$.
We report a combined muon spin relaxation/rotation, bulk magnetization, neutron scattering, and transport study of the electronic properties of the pyrochlore iridate Nd2Ir2O7. We observe the onset of strongly hysteretic behavior in the temperature dependent magnetization below 120 K, and an abrupt increase in the temperature dependent resistivity below 8 K. Zero field muon spin relaxation measurements show that the hysteretic magnetization is driven by a transition to a magnetically disordered state, and that below 8 K a complex magnetically ordered ground state sets in, as evidenced by the onset of heavily damped spontaneous muon precession. Our measurements point toward the absence of a true metal-to-insulator phase transition in this material and suggest that Nd2Ir2O7 lies either within or on the metallic side of the boundary of the Dirac semimetal regime within its topological phase diagram.
Quantum spin liquids (QSL) have generated considerable excitement as phases of matter with emergent gauge structures and fractionalized excitations. In this context, phase transitions out of QSLs have been widely discussed as Higgs transitions from deconfined to confined phases of a lattice gauge theory. However the possibility of a wider range of novel phases, occuring between these two limits, has yet to be systematically explored. In this Letter, we develop a formalism which allows for interactions between fractionalised quasiparticles coming from the constraint on the physical Hilbert space, and can be used to search for exotic, hidden phases. Taking pyrochlore spin ice as a starting point, we show how a U(1) QSL can give birth to abundant daughter phases, without need for fine--tuning of parameters. These include a (charged--) $mathbb{Z}_2$ QSL, and a supersolid. We discuss implications for experiment, and numerical results which support our analysis. These results are of broad relevance to QSL subject to a parton description, and offer a new perspective for searching exotic hidden phases in quantum magnets.
Scaling relations are used to study cross-overs, due to anisotropic spin interactions or single ion anisotropy, and due to disorder, in the thermodynamics and correlation functions near quantum-critical transitions. The principal results are simple with a wide range of applications. The region of attraction to the stable anisotropic fixed point in the quantum-critical region is exponentially enhanced by the dynamical critical exponent $z$ compared to the region of attraction to the fixed point in the quantum disordered region. The result implies that, even for small anisotropy, the region of attraction to the stable incommensurate Ising or planar metallic anti-ferromagnetic critical points, which belong to the universality class of the XY model with $z to infty$, covers the entire quantum-critical region. In crossovers due to disorder, the instability of the pure fixed point in the quantum disordered region is exponentially enhanced by $z$ compared to that in the quantum-critical region. This result suggests that for some classes of disorder and for large enough $z$, one may find singularities in the correlations as a function of frequency and temperature down to very low temperatures even though the correlation length in space remains short range.
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