Optical conductivity from local anharmonic phonons

Matsumoto, Hideki; Mori, Tatsuya; Iwamoto, Kaoru; Goshima, Shohei; Kushibiki, Syunsuke; Toyota, Naoki

doi:10.1103/physrevb.79.214306

Cited by 20 publications

(31 citation statements)

References 25 publications

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“…In this Letter, we report the first observation of the infrared-active off-center rattling phonon in the optical conductivity of -BGS by the terahertz time-domain spectroscopy (THz-TDS), and discuss the observed results based on the AP model [19]. The observed rattling phonon spectra not only show a line splitting predicted in the model, but also reveal an unexpected, drastic linewidth broadening towards low temperature, which is quite opposite from conventional thermal behavior.…”

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confidence: 78%

“…For type-I Ba 8 Ga 16 Ge 30 (BGG) which shows quasi-on-center rattling of the Bað2Þ 2þ ion, the phonon dynamics have been under intense study regarding the nature of its anharmonic vibrations, e.g., far-infrared spectroscopy [16], Raman scattering [17], and inelastic neutron scattering [18]. In our previous paper [16], we reported the optical conductivity of p-type BGG and discussed the softening and sharpening of the local anharmonic phonon toward low temperature using a one-dimensional anharmonic potential (AP) model [19]. Those characteristic behaviors are explained by nonequivalent energy level spacings resultant from the flat-bottom shape of the on-center anharmonic potential, plus the Boltzmann factor.…”

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confidence: 99%

“…For off-center rattling phonon systems the optical conductivity spectra has not been reported, only Raman scattering which has shown evidence of the off-center rattling behavior [13]. According to theoretical predictions [19], off-center rattling phonon spectra should show totally different structure PRL 106, 015501 (2011)…”

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Optical Conductivity Spectral Anomalies in the Off-Center Rattling Systemβ−Ba8Ga16Sn30

et al. 2011

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We present optical conductivity studies of the type-I clathrate Ba8Ga16Sn30, using a terahertz time-domain spectrometer (0.3-3.0 THz). The lowest-lying spectral peak at 0.72 THz due to the Ba(2) ion's off-center vibration in the oversized cage shows a drastic and anomalous temperature dependence. Below about 100 K, the single broad peak splits into two subpeaks, and with further lowering of the temperature, the spectral shape of this so-called rattling phonon shows non-Boltzmann broadening to the point that the linewidth becomes comparable to the peak frequency. Whereas the initial splitting can be understood by assuming a multiwell anharmonic potential, the strong linewidth broadening toward low temperature cannot, since the Boltzmann factor generally sharpens the low-temperature spectra. The observed behavior suggests strong interaction between the local anharmonic phonons and other excitations.

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confidence: 78%

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Optical Conductivity Spectral Anomalies in the Off-Center Rattling Systemβ−Ba8Ga16Sn30

et al. 2011

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“…Some model anharmonic potentials have been introduced to interpret experimental data on rattling Ba ion. 32 However, we have earlier found that the anharmonic potential derived from ab initio calculations is highly sensitive to details of the atomic structure. 33 As the aim of the present calculations is to aid the assignment of the experimental data we have instead applied a harmonic model also to the Ba2 atoms.…”

Section: E Theoretical Calculationsmentioning

confidence: 99%

Barium dynamics in noble-metal clathrates

et al. 2010

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Type I clathrates with noble transition metal Ba 8 TM 6 Ge 40 ͑TM= Cu, Ag, and Au͒ have been investigated by inelastic neutron scattering, single crystal and synchrotron powder x-ray diffraction and DFT calculations to probe the structure and dynamics on the atomic scale. The DFT calculation assists in the assignment of the measured low energy phonon modes, and fair agreement is obtained between experiment and theory for the phonon density of states. The Cu compound shows good agreement, whereas the Ag and Au compounds show some discrepancies. Overall, the data show that the guest atom dynamics are similar in all samples. Physical property characterization was carried out on pellets pressed by spark plasma sintering. The transport properties show large differences with Ba 8

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“…Details are discussed in Ref. [10]. For the numerical calculation of σ 1 (ω, T ) for the ν 1 mode, we fix the parameters: (1/2π) k/M =1.143 THz, λ/2k × a 2 B = 0.21 (a B is Bohr radius), and the temperature-independent constant Γ 0 /2π = 0.067 THz.…”

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Optical conductivity of rattling phonons in type-I clathrateBa8Ga16Ge30

et al. 2009

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A series of infrared-active optical phonons have been detected in type-I clathrate Ba8Ga16Ge30 by terahertz time-domain spectroscopy. The conductivity spectra with the lowest-lying peaks at 1.15 and 1.80 THz are identified with so-called rattling phonons, i.e., optical modes of the guest ion Ba 2+ (2) with T1u symmetry in the oversized tetrakaidecahedral cage. The temperature dependence of the spectra from these modes are totally consistent with calculations based on a one-dimensional anharmonic potential model that, with decreasing temperature, the shape becomes asymmetrically sharp associated with a softening for the weight to shift to lower frequency. These temperature dependences are determined, without any interaction effects, by the Bose-factor for optical excitations of anharmonic phonons with the nonequally spaced energy levels.PACS numbers: 63.20. Ry, During the past decade, thermoelectric materials such as clathrates and filled-skutterudites have renewed an interest in phonons [1]. These conductors are formed by polyhedral building blocks, where each polyhedral cage accommodates a guest ion like an alkali-earth or rareearth element. If the cage is oversized, the guest ions can vibrate with large amplitude around an on-center or off-center site in the cage potential. These Einsteinlike local modes have been called rattling phonons. The renewed interest above has been paid to interactions of these rattling phonons with acoustic phonons propagating through the cage network and carrying heat entropy, and, equally or more interestingly, with conduction electrons. However, these issues are still far from being well understood that even the charge dynamics of a single ion in rattling vibration have been little known.This paper reports on the optical conductivity of rattling phonons detected in a type-I clathrate Ba 8 Ga 16 Ge 30 (hereafter abbreviated as BGG), featuring the anharmonicity effects on the conductivity spectra in comparison with model calculations.This compound belongs to a family of ternary type-I clathrates having cubic crystal structure with space group P m3n [2]. The unit cell of the host framework consists of 6 tetrakaidecahedra and 2 dodecahedra. The latter smaller cages around 2a sites occupy the bodycentered-cubic sites, while the former oversized cages around 6d sites line up along the principal axes by sharing both the pentagonal faces with the dodecahedral cages and the hexagonal faces with neighboring tetrakaidecahedra. Every cage encapsulates one Ba 2+ ion, and the guest ions in the smaller (2a) and oversized (6d) cages are defined as Ba 2+ (1) and (2), respectively. These guest ions satisfy the Zintl rule for charge compensations that the more electro-positive guest atom donates electrons to the more electro-negative cage; 16 electrons are transferred from 8 Ba atoms to 16 Ga atoms of the cage. With finely tuned Ga/Ge concentration, therefore, the system can be a heavily doped semiconductor, in general, having both the charge-carrier's sign and density controlled.So far the electric...

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Optical conductivity from local anharmonic phonons

Cited by 20 publications

References 25 publications

Optical Conductivity Spectral Anomalies in the Off-Center Rattling Systemβ−Ba8Ga16Sn30

Optical Conductivity Spectral Anomalies in the Off-Center Rattling Systemβ−Ba8Ga16Sn30

Barium dynamics in noble-metal clathrates

Optical conductivity of rattling phonons in type-I clathrateBa8Ga16Ge30

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