Buried Dirac Points in Quantum Spin Hall Insulators: Implications for Majorana Kramers Pair-Based Quantum Computing

Quantum spin Hall insulators (QSHIs) host helical electronic edge states that are protected from backscattering due to time-reversal symmetry (TRS). Despite considerable work investigating QSHI edge states, there is still an open question about their unexpected resilience to large magnetic fields where TRS is undoubtedly broken. In this work, we investigate the transport properties of helical edge states in a QSHI-superconductor (QSHI-SC) junction formed by a <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mi>In</a:mi> <a:mi>As</a:mi> </a:mrow> </a:math> (15 nm)/ <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mrow> <c:mi>Ga</c:mi> <c:mi>Sb</c:mi> </c:mrow> </c:math> (5 nm) double quantum well and a superconducting tantalum ( <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mi>Ta</e:mi> </e:math> ) constriction. We observe a robust conductance plateau up to 2 T, signaling resilient edge-state transport. Using a modified Landauer-Büttiker analysis, we find that the zero-field conductance is consistent with 98% Andreev-reflection probability owing to the high transparency of the ( <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mrow> <g:mi>In</g:mi> <g:mi>As</g:mi> </g:mrow> </g:math> / <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mrow> <i:mi>Ga</i:mi> <i:mi>Sb</i:mi> </i:mrow> </i:math> )- <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:mi>Ta</k:mi> </k:math> interface. Such resilience is consistent with the Dirac point for the edge states being buried in the bulk valence band. We further theoretically show that a buried Dirac point does not affect the robustness of the quasi-one-dimensional topological superconducting phase. We find that a buried Dirac point favors the hybridization of Majorana Kramers pairs (MKPs)—predicted to exist in a QSHI-SC constriction—and fermionic modes in the QSHI vacuum edge resulting in MKP states, highlighting the subtle role of buried Dirac points in probing MKPs.