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An on/off Berry phase switch in circular graphene resonators

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Science  26 May 2017:
Vol. 356, Issue 6340, pp. 845-849
DOI: 10.1126/science.aal0212

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Flicking the Berry phase switch

When an electron completes a cycle around the Dirac point (a particular location in graphene's electronic structure), the phase of its wave function changes by π. This so-called Berry phase is tricky to observe directly in solid-state measurements. Ghahari et al. built a graphene nanostructure consisting of a central region doped with positive carriers surrounded by a negatively doped background. Scanning tunneling spectroscopy revealed sudden jumps in conductivity as the external magnetic field was increased past a threshold value. The jumps occurred when electron orbits started encompassing the Dirac point, reflecting the switch of the Berry phase from zero to π. The tunability of conductivity by such minute changes in magnetic field is promising for future applications.

Science, this issue p. 845

Abstract

The phase of a quantum state may not return to its original value after the system’s parameters cycle around a closed path; instead, the wave function may acquire a measurable phase difference called the Berry phase. Berry phases typically have been accessed through interference experiments. Here, we demonstrate an unusual Berry phase–induced spectroscopic feature: a sudden and large increase in the energy of angular-momentum states in circular graphene p-n junction resonators when a relatively small critical magnetic field is reached. This behavior results from turning on a π Berry phase associated with the topological properties of Dirac fermions in graphene. The Berry phase can be switched on and off with small magnetic field changes on the order of 10 millitesla, potentially enabling a variety of optoelectronic graphene device applications.

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