Temperature-dependent electron and phonon dynamics in the ternary topological insulator ( Bi 1 − x Sb x ) 2 Te 3 at terahertz frequencies

We investigate the temperature-dependent free electron conduction, terahertz phonon modes, and electron-phonon interactions, in the ternary topological insulator <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msub> <a:mrow> <a:mo>(</a:mo> <a:msub> <a:mi mathvariant="normal">Bi</a:mi> <a:mrow> <a:mn>1</a:mn> <a:mo>−</a:mo> <a:mi>x</a:mi> </a:mrow> </a:msub> <a:msub> <a:mi mathvariant="normal">Sb</a:mi> <a:mi>x</a:mi> </a:msub> <a:mo>)</a:mo> </a:mrow> <a:mn>2</a:mn> </a:msub> <a:msub> <a:mi mathvariant="normal">Te</a:mi> <a:mn>3</a:mn> </a:msub> </a:mrow> </a:math> . Terahertz time-domain spectroscopy and Raman spectroscopy on three <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mi>n</e:mi> </e:math> -type <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:mrow> <f:msub> <f:mrow> <f:mo>(</f:mo> <f:msub> <f:mi mathvariant="normal">Bi</f:mi> <f:mrow> <f:mn>1</f:mn> <f:mo>−</f:mo> <f:mi>x</f:mi> </f:mrow> </f:msub> <f:msub> <f:mi mathvariant="normal">Sb</f:mi> <f:mi>x</f:mi> </f:msub> <f:mo>)</f:mo> </f:mrow> <f:mn>2</f:mn> </f:msub> <f:msub> <f:mi mathvariant="normal">Te</f:mi> <f:mn>3</f:mn> </f:msub> </f:mrow> </f:math> samples of various thicknesses were performed. Strong phonon activity and electron–phonon coupling were experimentally revealed and found to be sensitive to temperature changes and subtle structural variations between the samples. The experimental results, well described by Drude-Lorentz-Fano model, were compared to density functional theory calculations, yielding the phonon mode assignment. Further, an additional broadened background absorption was observed and attributed to the morphology of the conductive top surface of the samples.