Topological Protection Brought to Light by the Time-Reversal Symmetry Breaking.

Area of Science:

• Condensed Matter Physics
• Topological Materials Science

Background:

• Topological band theory classifies insulators by symmetries and invariants like time-reversal symmetry and Z_{2} invariant.
• Helical states at the interface of distinct topological insulators exhibit spin-momentum locking and topological protection against backscattering.

Purpose of the Study:

• To experimentally investigate the strength of topological protection in one-dimensional helical edge states.
• To provide direct evidence of topological protection in Z_{2} quantum spin-Hall insulators.

Main Methods:

• Experimental study of HgTe Z_{2} quantum spin-Hall insulator.
• Application of a small magnetic field at low temperatures.
• Measurement of electrical resistance and observation of mesoscopic fluctuations.

Main Results:

• A tiny magnetic field induced an exponential increase in resistance.
• Giant mesoscopic fluctuations and a band gap opening were observed.
• These phenomena indicate a transition to Anderson localization upon breaking time-reversal symmetry.

Conclusions:

• The experiment provides the first direct evidence for the strength of topological protection in helical edge states.
• Breaking time-reversal symmetry significantly impacts the transport properties, leading to localization.
• The findings validate theoretical predictions regarding topological protection in quantum spin-Hall systems.