Glass-like thermal transport in symmetry-broken clathrates
We present the quantitative interpretation for the glass-like behavior of thermal conductivities κ(T) for type-I clathrate compounds involving off-centered guest ions. It is shown that the dipole-dipole interaction generated in cage/guest-ion systems is crucial to reproduce the characteristics of thermal conductivities for these symmetry-broken clathrates. The above scenario also explains well the difference of κ(T) between the p-type and the n-type β-BGS found recently by K. Suekuni et al. (Phys. Rev. B, 77 (2008) 235119.)
Off-center rattling triggers high-temperature thermal transport in thermoelectric clathrates: Nonperturbative approach
Lattice thermal conductivities $\kappa_{\rm L}$ of type-I clathrates encapsulating "off-center" guest atoms behave in a way that is sharply different from those of conventional crystals such as on-center type-I clathrates. The latter decreases with increasing temperature according to 1/$T$ above a few tens\,K. While, $\kappa_{\rm L}$ of off-center clathrates increase T-linearly above a few tens K. Further increasing temperature above $T\gtrsim$100 K, $\kappa_{\rm L}$ saturates without exhibiting appreciable T-dependence. In this temperature regime, the standard formulation of $\kappa_{\rm L}$ based on the perturbation theory is unfeasible. We herein present a theory of $\kappa_{\rm L}$ based on "non-perturbative" approach by taking into account the interaction between non-vibrational off-center rattling states of guest atoms and cage shells. It is remarkable that our theory shows excellent agreement with observed $\kappa_{\rm L}(T)$ for off-center clathrates in magnitudes, the cage volume $\Omega$-dependence and T-dependence.
Photoreflectance characterization of vanadium-doped GaAs layers grown by metalorganic vapor phase epitaxy
Abstract We characterized the vanadium-doped (GaAs:V) layers grown on GaAs substrate by metalorganic vapor phase epitaxy. The conductivity of the samples is checked by the Hall effect measurements, and it is n type with electrons concentration ranging from 2×10 13 to 2×10 17  cm −3 . We used the photoreflectance technique (PR) to study the effect of vanadium doping on the optical properties of GaAs. Our experimental results show that the built-in electric field F and the broadening parameter Γ 0 increase with V-doping concentration. Furthermore, the temperature dependence of the built-in electric field and the broadening parameter of GaAs:V samples have been investigated by PR. It has been observed that these parameters increase with increasing temperature. The decrease of the built-in electric field with decreasing temperature is due to the photovoltage effect.We characterized the vanadium-doped GaAs (GaAs:V) layers grown on GaAs substrate by metalorganic vapor phase epitaxy.