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ABSTRACT: The heat capacity of single-crystalline samples of Sr8Ga16Ge30 (SGG) and Ba8Ga16Ge30 (BGG) clathrates was measured to investigate the anharmonicity of the encapsulated atoms. At low temperatures, BGG can be
well described by a standard Debye model, and the C
p/T
3 versus T plot can be fitted with two Einstein temperatures: θ
E1=42K and θ
E2=74K. On the other hand, SGG shows deviation from the Debye model. Moreover, neither the Einstein model nor the soft potential
model (SPM) alone can fit the peak in the C
p/T
3 versus T plot, and the peak should be fitted by employing both models. Our results indicate that the effective electron mass is enhanced
by the anharmonic phonons.
KeywordsClathrates–thermoelectric–heat capacity–soft potential model
Journal of Electronic Materials 05/2012; 40(5):879-883. · 1.47 Impact Factor
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ABSTRACT: In order to quantitatively characterize the effect of motion of the guest atom in type-I clathrates X8Ga16Ge30 (X = Sr, Ba) on thermal conductivity, electrostatic potential in the crystal was visualized by synchrotron radiation x-ray diffraction. The obtained electrostatic potential clearly exhibited structural aspects of the guest atom, rattling, which is considered as a cause of suppression of thermal conductivity κ. The parameterized degree of rattling “rattling factor,” fR, being defined as a volume ratio of the electrostatic interaction region of the guest atom to the spherical volume estimated by the covalent radius of the guest atom, was found to well classify κ. The κ monotonously decreases with increasing the fR. Consequently, fR was judged as a better criterion to explore high-performance thermoelectric system of the clathrates.
Phys. Rev. B. 04/2012; 85(14).
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ABSTRACT: Low-temperature specific heat is measured on the overdoped
Ba(Fe_{1-x}Co_x)_2As_2 (x = 0.13) single crystal under magnetic fields along
three different directions. A clear anisotropy is observed on the field
dependent electronic specific heat coefficient {\gamma}(H). The value of
{\gamma}(H) is obviously larger with magnetic field along [001] (c-axis) than
that within the ab-plane of the crystal lattice, which cannot be attributed to
the effect by anisotropy of the upper critical field. Meanwhile, the data show
a rather small difference when the direction of the field is rotated from [100]
to [110] direction within the ab-plane. Our results suggest that a considerable
part of the line nodes is not excited to contribute to the quasiparticle
density of states by the field when the field is within the ab-plane. The
constraints on the topology of the gap nodes are discussed based on our
observations.
10/2011;
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ABSTRACT: Quantum transport of Dirac cone states in the iron pnictide Ba(FeAs)(2) with a d-multiband system is studied by using single crystal samples. Transverse magnetoresistance develops linearly against the magnetic field at low temperatures. The transport phenomena are interpreted in terms of the zeroth Landau level by applying the theory predicted by Abrikosov. The results of the semiclassical analyses of a two carrier system in a low magnetic field limit show that both the electron and hole reside as the high mobility states. Our results show that pairs of electron and hole Dirac cone states must be taken into account for an accurate interpretation in iron pnictides, which is in contrast with previous studies.
Physical Review Letters 05/2011; 106(21):217004. · 7.37 Impact Factor
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ABSTRACT: Low-temperature specific heat (SH) is measured on
Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ single crystals in a wide doping region under
different magnetic fields. For the overdoped sample, we find the clear evidence
for the presence of $T^2$ term in the data, which is absent both for the
underdoped and optimal doped samples, suggesting the presence of line nodes in
the energy gap of the overdoped samples. Moreover, the field induced electron
specific heat coefficient $\Delta\gamma(H)$ increases more quickly with the
field for the overdoped sample than the underdoped and optimal doped ones,
giving another support to our arguments. Our results suggest that the
superconducting gap(s) in the present system may have different structures
strongly depending on the doping regions.
03/2011;
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ABSTRACT: The quantum transport of Dirac cone states in the iron pnictide Ba(FeAs)$_2$
with a d-\,multiband system is studied by using single crystal samples. The
transverse magnetoresistance develops linearly against magnetic field at low
temperatures. The transport phenomena are interpreted in terms of the 0$^{th}$
Landau level by applying the theory predicted by Abrikosov. The results of the
semiclassical analyses of a two carrier system under low magnetic field limit
show that both electron and hole reside as the high mobility states, being
indicative to the fact that both electron- and hole Dirac cone states should be
taken into account in pairs for having the real interpretation of low
temperature electronic states in iron pnictides, being in contrast to the
previous reports.
12/2010;
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ABSTRACT: Tunneling states of Sr8Ga16Ge30 (SGG) and Ba8Ga16Ge30 (BGG) are studied in detail using single crystals with a variety of carrier concentrations from the view point of heat capacity (Cp). An important excessive contribution αT due to the tunneling states is found besides γeT of the conduction electrons. The value of α is accurately deduced to be 10.1±1.5 mJ/mol K2 for SGG while such a value is as small as 0.8±0.4 mJ/mol K2 for BGG. From the temperature evolution of Cp as a function of carrier concentration, the effective masses, one of the most important physical parameters for evaluating electron-phonon interactions λe-ph, are accurately estimated to be 1.68m0 and 1.01m0 for SGG and BGG, respectively. The λe-ph seems to be enhanced when the anharmonic rattling modes are involved.
Phys. Rev. B. 08/2010; 82(8).
