The topological insulator is a new phase of matter that holds promise for a new generation of highly energy efficient electronics and highly computationally efficient information processing devices. However, it is experimentally challenging to identify topological insulators because unlike ordinary phases of matter, their properties are not described by the familiar language of local order parameters and spontaneously broken symmetries. Because conventional experimental techniques are designed to be sensitive to local order parameters, methods of measuring topological order are extremely scarce. In this work, we show that topological order in topological insulators can be revealed through measurements of the spin-polarized electronic band structure. By performing such measurements using a combination of state-of-the-art synchrotron based spin- and angle-resolved photoemission spectroscopy, we show for the first time that topological insulators are realized in nature in some remarkably simple semiconductors.

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