Physicists have found new quantum effects in bilayer graphene

  • Theorists at the University of Texas at Dallas and colleagues in Germany have for the first time observed a rare phenomenon in a very simple material, called the quantum anomalous Hall effect. Previous experiments have only found it in complex or subtle substances.


    Fan Zhang, associate professor in the School of Physics, Natural Sciences and Mathematics, is the author of this study published in the journal Nature on October 6. The layers of carbon atoms are arranged in two honeycomb lattices stacked together.


    The quantum Hall effect is a macroscopic phenomenon in which the lateral resistance in the material gradually changes in a quantized manner. It occurs in a two-dimensional electronic system under low temperature and strong magnetic field. However, in the absence of an external magnetic field, a two-dimensional system can spontaneously generate its own magnetic field, for example, orbital ferromagnetism generated by the interaction between electrons. This behavior is called the quantum anomalous Hall effect.


    "When the rare quantum anomalous Hall effect was studied before, the materials studied were complicated," Zhang said. "In contrast, our material is relatively simple because it consists of only two layers of graphene, and it is naturally generated."


    Dr. Thomas Weitz, a professor at Göttingen University, one of the authors of the study, said: "In addition, we found that although carbon should not be magnetic or ferroelectric, the experimental characteristics we observed are consistent with these two characteristics. This is very contrary to our intuition."


    In a study published in 2011, theoretical physicist Zhang predicted that bilayer graphene would have five competing ground states, the most stable ground state of the material at temperatures close to absolute zero (-273.15 degrees Celsius or -459.67 degrees Fahrenheit). This state is driven by the interaction of electrons, whose behavior is governed by quantum mechanics and quantum statistics.


    "We predict that there will be five state groups in double-layer graphene, which compete with each other to become the ground state. It has been observed four times in the past. This is the last and most challenging observation."


    In the experiments described in the journal Nature, the researchers found eight different ground states in the fifth family that showed both quantum anomalous Hall effects, ferromagnetism, and ferroelectricity.


    "We also show that we can choose in an octave of the ground state by applying small external electric and magnetic fields, as well as controlling the sign of the carrier," Weitz says.


    Although Zhang and Weitz stated that they are mainly interested in revealing the "beauty of fundamental physics", being able to control the electronic properties of bilayer graphene to such a high degree may make it a potential candidate for future applications of low-dissipative quantum information.


    "We predict, observe, elucidate, and control a quantum anomalous Hall octave, in which three remarkable quantum phenomena—ferromagnetism, ferroelectricity, and zero-field quantum Hall effects—can coexist and even cooperate with each other in bilayer graphene," Zhang said. Now we know that we can unify ferromagnetism, ferroelectricity, and quantum anomalous Hall effects in this simple material, which is surprising and unprecedented. "