Artful probe for quantum magic in twisted graphene bilayer

Novel measurement of electron correlation in twisted bilayer graphene | Responsibility as student towards research supervisor | arXiv forum

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Graphene is a fascinating material with incredible properties such as high toughness, flexibility and resistance. However, two graphene layers when handled with a twist can unleash extraordinary phenomena. The material formed on stacking and twisting two graphene layers is called twisted bilayer graphene which at a “magic” twist angle has proved to be a promising platform to design versatile electronic devices, i.e., the device can be tuned as insulator, conductor and even superconductor. The magic of twisted bilayer graphene is captured in strongly and strangely correlated electronic behaviour. In this issue, I am happy to share about the novel detection of strongly correlated order in twisted bilayer graphene. Additionally, I will share a brief account on the responsibility a student must hold towards the research supervisor.

Quantum Leaps and Bounds

Novel measurement of electron correlation in twisted bilayer graphene

Imagine you have a piece of paper called graphene. Now, if you take two sheets of this graphene paper and stack them on top of each other, but twist them a little bit, something really interesting happens! They start behaving in a very special way, and at a “magic” angle two twisted layers of graphene become superconductor. What’s intriguing is that the magic is result of a piece of art called moiré pattern which drastically affects the nature of electron in twisted graphene layers.

To further the understanding, let’s understand the role of moiré! In solids, electrons move inside bands defined by the crystal structure’s potential. The movement inside these bands is significantly different from that of free electrons. Take graphene for instance. Despite electrons having a finite mass, the crystal potential in graphene allows them to move at incredibly high speeds, in Dirac bands, almost as if they were massless. Now, when we stack and twist the two graphene layers the moiré art imprints itself intricately in the coupling profile of the two layers making things drastically different in twisted bilayer graphene. It is striking that while electrons in graphene are ultra-fast, electrons in twisted bilayer graphene at the magic angle are ultra-slow hosted in extremely flat bands, and that’s where the magic begins: these slow moving electrons in flat bands strongly interact with each other.

Something interesting happens in twisted bilayer graphene system when we start to dope it, i.e., start to fill a flat band. Imagine a scenario with four containers, each with a height of H, simultaneously filling with water. As the water level in each container approaches H/4, in the blink of an eye, all the water suddenly transfers into one container, completely filling it. This filled container is then removed, resetting the system to three empty containers. The process repeats as water starts flowing again, and once more, a magical event occurs when the containers are filled to around H/3, resulting in the transfer of all the water into one container, which is then taken out. This process continues until all the containers are filled with water.

Similarly, the electronic states within the flat band of twisted bilayer graphene can be likened to the four containers; electronic states in twisted bilayer graphene have four “flavors”. Instead of water, we introduce actual electrons to fill the bands of each flavor. Initially, all flavors are equally populated. However, at specific electron fillings, a phenomenon called Dirac revivals takes place, where one band suddenly becomes completely filled owing to a first-order phase transition. This causes the electronic states of the other flavors to become empty, resembling the emptied containers. Scientists have actively explored various methods to investigate this behavior, including a recent experiment led by Professor J. I. A. Li (Brown University). In the study, they utilized electron spin resonance, an experimental technique that employs magnetic fields to induce excitations between unpaired spins. This groundbreaking work, published in Nature Physics, represents the first direct measurement that probes the collective excitations of strongly correlated electronic order in twisted bilayer graphene. Click here to read more.

Living the scientific life

Responsibility as student towards research supervisor

Both student and supervisor invest a significant amount of time and effort in making the “perfect” selection. Many aspiring researchers have asked me “What kind of supervisor should I choose?”. My answer has been consistent “Choose the person whose work attracts you”. It is understandable that students may inquire about a professor's behavior towards their research team. However, it is crucial not to take it on a personal level. A professor's behavior is shaped by years of experience, and as a new researcher, it is essential to prioritize research and output over personal work models. In my personal experience, more students seemingly give bad trips to their supervisors than the other way round. A supervisor might hold your hand, walk the way with you before letting you free or a supervisor might leave you in the ocean and you find your way to the shore. Just that as a student look up to your supervisor as a resource of experience and guidance, respect the limitations and good results will definitely follow.

Expert’s arena: cond-mat arXiv forum

Strongly-correlated electron-photon systems

The review “Strongly-correlated electron-photon systems” was published last year in Nature, and I am glad that it appeared on arXiv last week catching my eye. Controlling light-matter interaction provides a new way to manipulate strongly correlated matter, and this review enlightens with perspectives on various frontiers such as Floquet band structures in electronic materials, driven strongly correlated electrons, and cavity QED. The review strikes a perfect balance between providing essential background information and highlighting open problems in different regimes of light-matter interaction, making it a captivating and informative read. Click here to read more.

So, that’s that from this issue. Until next time, stay curious!