Phonon-limited valley lifetimes in single-particle bilayer graphene quantum dots

The valley degree of freedom in two-dimensional (2D) semiconductors, such as gapped bilayer graphene (BLG) and transition metal dichalcogenides, is a promising carrier of quantum information in the emerging field of valleytronics. While valley dynamics have been extensively studied for moderate band gap 2D semiconductors using optical spectroscopy techniques, very little is known about valley lifetimes in narrow band gap BLG, which is difficult to study using optical techniques. Here, we report single-particle valley relaxation times <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>T</a:mi><a:mn>1</a:mn></a:msub></a:math> exceeding several microseconds in electrostatically defined BLG quantum dots using a pulse-gating technique. The observed dependence of <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>T</b:mi><b:mn>1</b:mn></b:msub></b:math> on perpendicular magnetic field can be understood qualitatively and quantitatively by a model in which <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>T</c:mi><c:mn>1</c:mn></c:msub></c:math> is limited by electron-phonon coupling. We identify the coupling to acoustic phonons via the bond length change and via the deformation potential as the limiting mechanisms.