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Physica B 359–361 (2005) 1240–1242

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Roles of the strong spin–phonon coupling in the geometricallyfrustrated pyrochlore Y2Ru2O7

J.S. Leea, T.W. Noha,�, T. Takedab, R. Kannoc

aSchool of Physics and Research Center for Oxide Electronics, Seoul National University, Seoul 151-747, Republic of KoreabDivision of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Hokkaido 060-8628, JapancDepartment of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology,

Yokohama 226-8502, Japan

Abstract

We investigated temperature-dependent infrared-active transverse phonon modes of insulating pyrochlore Y2Ru2O7;which shows a spin-glass-like transition around 80K. We found quite abnormal temperature-dependent behaviors for

some phonon modes, which should be contributed by a strong antiferromagnetic correlation, not by a structural

distortion. Through quantitative analysis, we suggested that the strong spin–phonon coupling effects should play an

important role in the intriguing magnetic state of this geometrically frustrated compound.

r 2005 Elsevier B.V. All rights reserved.

PACS: 75.50.Lk; 78.30.�j; 63.20.Ls

Keywords: Geometrical frustration; Spin glass; Spin–phonon coupling; Y2Ru2O7; Pyrochlore

The tetrahedral geometry of the pyrochlorelattice antiferromagnet leads to intense frustrationof the magnetic bonds. As a ground state,however, they often show either antiferromagnetic(AF) ordering or signs of strong AF correlationaccompanying static local structural distortionsand/or related structural symmetry changes [1].Cubic pyrochlore ruthenate Y2Ru2O7 is an

unusual compound; although it has a very strong

e front matter r 2005 Elsevier B.V. All rights reserve

ysb.2005.01.371

ng author. Tel.: +82 2 880 6616; fax:

ss: twnoh@phya.snu.ac.kr (T.W. Noh).

AF correlation, it does not have any noticeablestructural change. This compound shows a spin-glass-like transition at TG �80K [2]. From theneutron scattering experiments, Ito et al. [2]argued that in the spin-glass state the magneticmoments at the Ru sites become aligned to forman almost long-range ordered state, but suchmagnetic ordering does not accompany any staticlattice symmetry change. In these respects, itis quite important to understand the relationbetween the spin and the lattice degrees of freedomin Y2Ru2O7:

d.

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(a) (b)

Fig. 1. T-dependencies of the 6th and the 7th phonon modes.

In both figures, the spectra from top to bottom are shown for

temperatures of 10, 40, 60, 70, 80, 90, 100, 120, 160, 200, 250,

and 300K. Dotted lines in each figure represent the fitting

results with the Fano-type function.

(a) (b)

(d)(c)

Fig. 2. T-dependent parameters of (a) 1/q, (b) DG=Gð300KÞ; (c)ð1=qÞ2=G; and (d) G=ð1=qÞ: The latter two parameters are

proportional to the density of continuum states r and the

interaction energy V between the continuum states and phonon,

respectively.

J.S. Lee et al. / Physica B 359– 361 (2005) 1240–1242 1241

In this paper, we tried to address this intriguingissue by investigating the temperature- (T-) depen-dencies of the seven infrared- (IR-) active phononmodes. High-density Y2Ru2O7 polycrystals weresynthesized at 3GPa with a solid state reactionmethod [3]. Near normal incident reflectivityspectra RðoÞ were measured in a wide photonenergy range from 5meV to 30 eV. To obtain theoptical conductivity spectra sðoÞ from the mea-sured RðoÞ; the Kramers–Kronig analysis wasused.In the far-IR region, we observed seven IR-

active phonon modes around 580, 490, 420, 325,253, 185, and 135 cm�1 [4], which is in goodagreement with the group theoretical analysis [5].Here, each phonon modes are numbered sequen-tially from the high to the low frequencies. WhileT-dependencies of the 1st, 4th, and 5th phononscan be well explained by the thermal effect, thoseof the other four-phonon modes of the 2nd, 3rd,6th, and 7th ones are quite unusual; as T decreasesthrough TG; their frequencies, strengths, andlinewidths show abrupt changes. These suggestthat the magnetic fluctuation contributes theabnormal behaviors of various lattice vibrations,namely, there should be a strong spin–phononcoupling effect in Y2Ru2O7:Here, let us focus on T-dependencies of the 6th

and the 7th phonon modes, from which we can geta good insight on the ground state nature of thisintriguing compound. In order to fit these twophonon modes, we have to use the Fano-typefunction, such as Aðq þ eÞ2=ð1þ e2Þ due to theirasymmetric lineshape [6]. The Fano-type dielectricfunction has been used to explain interferenceeffects between a sharp excitation of a phonon anda broad response. Here, e ¼ ðo� oT Þ=G; and 1=q

is the strength of the coupling between the phononand the broad response, and oT and G representthe frequency and the damping constant of thephonon mode, respectively. The dotted lines inFigs. 1(a) and (b) are ones of the fitting results foreach phonon mode at 10K.Figs. 2(a) and (b) show the T-dependent 1=q and

DG=G(300K) for the 6th and the 7th phonons,respectively. Here, DG ¼ GðTÞ � G(300K). As T

decreases below TG; 1=q becomes larger andDG=G(300K) starts to show a large decrease. In

the Fano-model, density of continuum states r andinteraction energy V between the continuum statesand phonon should be proportional to ð1=qÞ2=Gand G=ð1=qÞ [6], respectively. Figs. 2(c) and (d)show that, as T decreases below TG; while rincreases abruptly, V decreases.The coupling between the lattice and the spin

degrees of freedom, or the so-called dynamicspin–phonon coupling could provide one ofpossible explanations for such changes in phonondynamics [7]. First, the increase of r below TG can

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J.S. Lee et al. / Physica B 359– 361 (2005) 1240–12421242

be attributed to the enhancement of the AFcorrelations, which is in a good agreement withthe previous result of the neutron scatteringexperiment [2]. Second, such strong AF correlationbelow TG could increase the coupling strengthbetween the phonon and the continuum state of amagnetic origin, resulting in the increase in 1=q:Third, the abrupt decrease of G might beattributed to the reduction of the phonon decayingpaths due to occurrence of the nearly long-rangemagnetic ordering [7]. However, decrease of theinteraction energy V below TG could not be easilyunderstood yet.In summary, we investigated the temperature-

dependent behaviors of the transverse opticalphonon modes of Y2Ru2O7: We observed strongsignatures suggesting that the structural degrees offreedom are strongly coupled with the spin degreesof freedom, which should be closely related to thestrong antiferromagnetic correlation observed inthis geometrically frustrated system.

This work was supported by the Ministry ofScience and Technology through the CreativeResearch Initiative program.

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[3] R. Kanno, Y. Takeda, T. Yamamoto, Y. Kawamoto,

O. Yamamoto, J. Solid State Chem. 102 (1993) 106.

[4] J.S. Lee, T.W. Noh, J.S. Bae, In-Sang Yang, T. Takeda,

R. Kanno, Phys. Rev. B 69 (2004) 214428.

[5] E. Buixaderas, S. Kamba, J. Petzelt, M. Savinov,

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