We studied the field induced magnetic order in R(2)Ti(2)O(7) pyrochlore compounds with either uniaxial (R=Ho, Tb) or planar (R=Er, Yb) anisotropy, by polarized neutron diffraction. The determination of the local susceptibility tensor {chi(parallel to),chi(perpendicular)} provides a universal description of the field induced structures in the paramagnetic phase (2-270 K), whatever the field value (1-7 T) and direction. Comparison of the thermal variations of chi(parallel to) and chi(perpendicular) with calculations using the rare earth crystal field shows that exchange and dipolar interactions must be taken into account. We determine the molecular field tensor in each case and show that it can be strongly anisotropic.

In this work, we show that the zero field excitation spectra in the quantum spin ice candidate pyrochlore compound Yb2Ti2O7 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, Yb2Ti2O7 lies at the border between a ferro and an antiferromagnetic phase. These results suggest that the unconventional ground state of Yb2Ti2O7 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. arXiv:1506.01729v1 [cond-mat.str-el]

(2016) Antiferroquadrupolar correlations in the quantum spin ice candidate Pr2Zr2O7. Physical Review B, 94 (16). 165153 Permanent WRAP URL: http://wrap.warwick.ac.uk/87826 Copyright and reuse:The Warwick Research Archive Portal (WRAP) makes this work by researchers of the University of Warwick available open access under the following conditions. Copyright © and all moral rights to the version of the paper presented here belong to the individual author(s) and/or other copyright owners. To the extent reasonable and practicable the material made available in WRAP has been checked for eligibility before being made available.Copies of full items can be used for personal research or study, educational, or not-for-profit purposes without prior permission or charge. Provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. Publisher statement: © 2016 American Physical Society A note on versions:The version presented here may differ from the published version or, version of record, if you wish to cite this item you are advised to consult the publisher's version. Please see the 'permanent WRAP URL' above for details on accessing the published version and note that access may require a subscription. We present an experimental study of the quantum spin ice candidate pyrochlore compound Pr 2 Zr 2 O 7 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 [110] directions, 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 [110], the ordered moment appears on the so-called α chains only. The spin excitation spectrum is essentially 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 [110] (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 an 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 interactions. We then propose a range of acceptable parameters for Pr 2 Zr 2 O 7 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.

The dynamical magnetic correlations in Tb2Ti2O7 have been investigated using polarized inelastic neutron scattering. Dispersive excitations are observed, emerging from pinch points in reciprocal space and characterized by an anisotropic spectral weight. Anomalies in the crystal field and phonon excitation spectrum at Brillouin zone centers are also reported. These findings suggest that Coulomb phases, although they present a disordered ground state with dipolar correlations, allow the propagation of collective excitations. They also point out a strong spin-lattice coupling, which likely drives effective interactions between the 4f quadrupolar moments.

The pyrochlore oxides (A2B2O7) exhibit a remarkable range of structural, physical, and magnetic properties related to their various chemical compositions. This article reports the phase transformations induced by high electronic excitation in pyrochlores of the Gd2(ZrxTi1−x)2O7 family irradiated with swift ions. The structural changes, investigated by using several analytical techniques (x-ray diffraction, Raman spectroscopy, and transmission electron microscopy), strongly depend on the chemical composition. The high electronic excitation along the ion trajectory results in the amorphization of ion tracks for Gd2Ti2O7 and Gd2TiZrO7, whereas a defective fluorite structure is formed in Gd2Zr2O7. Moreover, the results underline the existence of an electronic stopping power threshold of 6 keV/nm for amorphizable compounds and 10 keV/nm for Gd2Zr2O7, below which phase transformations do not occur. Finally, the study of the thermal recovery of irradiated pyrochlores provides the recrystallization temperature for amorphized samples and reveals differences in the recovery process which are related to the chemical composition.

We have studied the low energy spin dynamics between 4.6 K and 0.07 K in a Tb2Ti2O7 single crystal sample, by means of inelastic neutron scattering experiments. The spectra consist in a dual response, with a static and an inelastic contribution, showing striking Q-dependences. We propose an interpretation involving an anisotropic exchange interaction in combination with a breaking of the threefold symmetry at the rare earth site. Simulations of the Q-dependent scattering in the Random Phase Approximation account well for the inelastic response.Spin liquids now attract considerable attention in modern condensed matter physics [1,2]. In a classical picture, the localized magnetic moments in such cooperative paramagnets keep fluctuating in a correlated manner, failing to develop long range order down to very low temperature. From a quantum point of view, a spin liquid ground state can be described by entangled spin wavefunctions and supports exotic fractionalized excitations also called spinons. Geometrically frustrated magnets are good candidates in the pursuit of such disordered quantum ground states [3] and one of the celebrated examples is the Tb 2 Ti 2 O 7 pyrochlore. It remains in a spin liquid state, with short range correlated fluctuating moments, down to a temperature as low as 20 mK [4,5]. Since 1999, it has been the subject of many theoretical as well as experimental works, and the origin of its spin liquid ground state is still puzzling.Tb 2 Ti 2 O 7 belongs to the same family as the Ho 2 Ti 2 O 7 and Dy 2 Ti 2 O 7 spin ices, characterized by an Ising anisotropy along local < 111 > axes [6]. However, the Tb 3+ crystal electric field (CEF) with trigonal symmetry [7,8] has a much lower energy gap between the ground state doublet and the first excited doublet than in spin ices (18 K instead of 200 to 300 K). It was suggested that, unlike in spin ices, this gap is small enough to allow admixture of excited crystal field states, that produces an effective ferromagnetic contribution which competes with the original antiferromagnetic interactions, and that moves Tb 2 Ti 2 O 7 towards a "quantum spin-ice" regime [9,10]. More recent general descriptions of pyrochlores introduce a minimal Hamiltonian, based on symmetry grounds, for pseudospins 1/2 (the subspace spanned by the ground doublet states |ψ ± ) [11][12][13]. These models involve an Ising exchange constant J zz responsible for the spin-ice behavior, as well as three "quantum" terms J ± , J z± and J ±± that lift the macroscopic degeneracy of the spin-ice manifold and stabilize a so called Coulomb phase or U(1) spin liquid phase, describable by an emergent U(1) gauge field. For large quantum couplings, conventional phases are stabilized against the spin liquid. Interestingly, in the particular case of non-Kramers ions (like Tb 3+ ), these phases are characterized by ordering of the 4f quadrupoles [14,15], breaking spontaneously the threefold symmetry of the crystal field. Recently, we proposed a somehow more phenomenological route to point out the relevanc...

Examples of materials where an "order by disorder" mechanism is at play to select a particular ground state are scarce. It has recently been proposed, however, that the antiferromagnetic XY pyrochlore Er 2 Ti 2 O 7 reveals a most convincing case of this mechanism, with the observation of a spin gap at zone centers having recently been interpreted as a corroboration of this physics. Here we argue, however, that the anisotropy generated by the interaction-induced admixing between the crystal-field ground and excited levels provides for an alternative mechanism. It especially predicts the opening of a spin gap of about 15 μeV, which is of the same order of magnitude as the one observed experimentally. We report high-resolution inelastic neutron scattering data which can be well understood within this scenario.Geometrically frustrated magnetism is a forefront research topic within condensed matter physics, as testified by the wealth of exotic phenomena discovered over the past years [1][2][3]. For instance, the problem of an XY antiferromagnet on the pyrochlore lattice (the celebrated lattice of corner-sharing tetrahedra) has been considered with much interest since this model displays an extensive classical degeneracy [4,5] along with classical and quantum order by disorder (ObD) effects [4][5][6][7][8][9][10][11][12][13]. The elegant concept of ObD [14,15] is a cornerstone of ordering in frustrated condensed matter systems. ObD comes into play by selecting a ground state, either because fluctuations away from this particular configuration allow for a relative gain of entropy compared to other classically degenerate states, or because quantum mechanical zero point fluctuations define a minimum in the total energy.Until now, the number of confirmed examples for ObD in real materials have remained scarce [16]. For ObD to be an efficient selection mechanism, the classical groundstate degeneracy must be extremely robust and the minimal theoretical model not openly subject to additional terms that would spoil the accidental emerging symmetry and lift the degeneracy. Recently, the XY pyrochlore antiferromagnet Er 2 Ti 2 O 7 has been proposed as a candidate that satisfies these conditions in a rather compelling way [7][8][9][10]. Given the unique position of Er 2 Ti 2 O 7 among frustrated quantum magnets, it is of foremost importance to scrutinize the soundness of this proposal.The crystal electric field (CEF) acting on the Kramers Er 3+ ion is responsible for a strong XY-like anisotropy, with easy magnetic planes perpendicular to the local 111 ternary axes [2,13]. Combined with antiferromagnetic interactions, an extensive classical degeneracy is expected [4][5][6]13]. Despite this degeneracy, Er 2 Ti 2 O 7 undergoes a second-order phase transition towards an antiferromagnetic noncollinear k = 0 Néel phase at T N = 1.2 K [13,17-19]. In this configuration, denoted ψ 2 and depicted in Fig. 1(a), the magnetic moments are perpendicular to the 111 axes [13,20] and make a zero net magnetic moment per tetrahedron.A theory based o...

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