Neutron scattering study of spin and lattice dynamics in

Mignot, J.-M.; Alekseev, P. A.; Nemkovski, K. S.; Nefeodova, E. V.; Rybina, A. V.; Regnault, L.P.; Shitsevalova, N. Yu.; Iga, Fumitoshi; Takabatake, Toshiro

doi:10.1016/j.physb.2006.03.037

Cited by 7 publications

(6 citation statements)

References 18 publications

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“…As has been suggested in Ref. 13 it might be due to a continuum of additional band states which do not take place in the resonance formation. Their inclusion would require a multi-orbital conduction band model.…”

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confidence: 70%

“…It is also rapidly suppressed with increasing temperature. It has been associated with continuum excitations [13] also visible in a broad maximum in the optical conductivity [8] around 38 meV.…”

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confidence: 99%

“…Finally H C is the Coulomb interaction with an on-site hole repulsion U f f . Our model assumes the limit U f f → ∞ where doubly occupied (hole) states (4f 12 ) are excluded and the two possible Yb configurations are either 4f 14 or 4f 13 . The one without a 4f hole, i.e., 4f 14 can be accounted for by an auxiliary boson b † i [18].…”

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Theory of Spin Exciton in the Kondo SemiconductorYbB12

2009

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The Kondo semiconductor YbB12 exhibits a spin and charge gap of approximately 15 meV. Close to the gap energy narrow dispersive collective excitations were identified by previous inelastic neutron scattering experiments. We present a theoretical analysis of these excitations. Starting from a periodic Anderson model for crystalline electric field (CEF) split 4f states we derive the hybridized quasiparticle bands in slave boson mean-field approximation and calculate the momentum dependent dynamical susceptibility in random phase approximation (RPA). We show that a small difference in the hybridization of the two CEF (quasi-) quartets leads to the appearance of two dispersive spin resonance excitations at the continuum threshold. Their intensity is largest at the antiferromagnetic (AF) zone boundary point and they have an upward dispersion which merges with the continuum less than halfway into the Brillouin zone. Our theoretical analysis explains the most salient features of previously unexplained experiments on the magnetic excitations of YbB12.The so-called Kondo insulators or semiconductors like, e.g., CeNiSn, SmB 6 and YbB 12 represent a special class of strongly correlated electrons [1]. In these compounds the conduction electrons hybridize with nearly localized 4f electrons. The Coulomb repulsion of the latter results in a small energy gap[2] of order 10 meV at the Fermi level [3,4].At temperatures higher than the gap energy these materials behave like Kondo metals exhibiting their typical spin fluctuation spectrum. But a low temperatures a spin and charge gap opens indicating the formation of an insulating singlet ground state [1,5]. This may be concluded from the total suppression of the local moment in the susceptibility and from the semiconducting behavior of the resistivity, respectively [6]. The gap formation may also be seen directly in the dynamical susceptibility and finite frequency conductivity as probed in inelastic neutron scattering (INS) and optical conductivity experiments. In cubic YbB 12 the spin [7] and charge [8] gap obtained in this way are approximately equal to 15 meV but in general they need not be identical.In addition unpolarized [9] and polarized [7] INS has found an interesting dispersive fine structure around this threshold energy. Three excitation branches have been identified with energies 15, 20 and 38 meV, respectively by analyzing the spectral function of the dynamical susceptibility. Since the lower two INS peaks are narrow and mostly centered at the zone boundary L-point with Q = (π, π, π) they may be associated with the formation of a collective heavy quasiparticle spin resonance exciton appearing around the spin gap threshold [7,9] and driven by heavy quasiparticle interactions. The collective modes remain visible in the 20 meV region up to T = 159 K [10,11]. Similar spin resonance phenomena appear as result of feedback effect in unconven-tional heavy-fermion superconductors below the quasiparticle continuum threshold at 2∆ 0 where ∆ 0 is the gap amplitude[12]. The upper p...

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Theory of Spin Exciton in the Kondo SemiconductorYbB12

2009

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“…In this paper we present the results of an INS study of the phonon density of states (PhDOS) and dispersion curves in YbB 12 (partly published in [5,6]), along with possible evidence for MEC effects.…”

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“…However one should not exclude the possible interplay between electron and lattice excitations, at least in case of YbB 12 where collective magnetic excitations (characteristic for the low-temperature Kondo-insulating regime) fall in the same energy range as phonons associated with rare-earth ion vibrations. Since the structure of the magnetic excitation spectrum itself is strongly temperaturedependent such magneto-elastic coupling (MEC) effects can become especially evident when studying the temperature evolution of phonon spectra.In this paper we present the results of an INS study of the phonon density of states (PhDOS) and dispersion curves in YbB 12 (partly published in [5,6]), along with possible evidence for MEC effects. …”

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Lattice dynamics and magneto-elastic coupling in Kondo-insulator YbB₁₂

Rybina

Alekseev

Mignot

et al. 2007

J. Phys.: Conf. Ser.

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Abstract. Lattice dynamics and magneto-elastic coupling effects have been studied in the Kondo insulator YbB 12 by means of inelastic neutron scattering. The analysis of the phonon density of states, dispersion, and symmetry properties is presented in connection with a possible magneto-elastic coupling. Manifestation of such effects was found for the phonons corresponding to the vibrations of Yb atoms. IntroductionKondo-insulators form a specific class of the rare-earth compounds which behave as Kondo metals at room temperature and transform to narrow-gap semiconductors with a non-magnetic ground state on decreasing temperature [1]. YbB 12 is a typical Kondo-insulator and the only example reported so far among Yb-based compounds. It is of particular interest because of its crystal structure formed by a rigid network of boron cuboctahedra B 12 and loosely-bound Yb ions. Inelastic neutron scattering (INS) experiments on YbB 12 have revealed that the spin-gap formation on cooling below 80 K [2,3] is also accompanied by the formation of near-gap-edge dispersive magnetic excitations [4]. The conventional approach to the Kondo-insulator problem is based on the consideration of strong f-d hybridization. However one should not exclude the possible interplay between electron and lattice excitations, at least in case of YbB 12 where collective magnetic excitations (characteristic for the low-temperature Kondo-insulating regime) fall in the same energy range as phonons associated with rare-earth ion vibrations. Since the structure of the magnetic excitation spectrum itself is strongly temperaturedependent such magneto-elastic coupling (MEC) effects can become especially evident when studying the temperature evolution of phonon spectra.In this paper we present the results of an INS study of the phonon density of states (PhDOS) and dispersion curves in YbB 12 (partly published in [5,6]), along with possible evidence for MEC effects.

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Chapter 11 Kondo insulators

2008

Heavy-Fermion Systems

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Neutron scattering study of spin and lattice dynamics in

Cited by 7 publications

References 18 publications

Theory of Spin Exciton in the Kondo SemiconductorYbB12

Theory of Spin Exciton in the Kondo SemiconductorYbB12

Lattice dynamics and magneto-elastic coupling in Kondo-insulator YbB₁₂

Chapter 11 Kondo insulators

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Neutron scattering study of spin and lattice dynamics in

Cited by 7 publications

References 18 publications

Theory of Spin Exciton in the Kondo SemiconductorYbB12

Theory of Spin Exciton in the Kondo SemiconductorYbB12

Lattice dynamics and magneto-elastic coupling in Kondo-insulator YbB12

Chapter 11 Kondo insulators

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Lattice dynamics and magneto-elastic coupling in Kondo-insulator YbB₁₂