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International Center for Materials Nanoarchitectonics (WPI-MANA)

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[[Research Highlights Vol.41] Evidence of A New Type of Quantum Effect](https://mdr.nims.go.jp/datasets/555bb675-b5f8-4685-af76-3c4cef1fe7d9)

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2022/04/04 10:05 Evidence of A New Type of Quantum Effect| MANAhttps://www.nims.go.jp/mana/research/highlights/vol41.html 1/2Previous  Index  NextResearch Highlights[Vol. 41]Evidence of A New Type of Quantum Effect28 Sep, 2018Figure: (A) Device structure. (B) Hofstadter’s butterfly, a quantum Hall effect in graphene/h-BNsuperlattice heterostructure. (C) and (D) Signature of the quantum valley Hall effect in agraphene/h-BN superlattice.The quantum Hall effect (QHE) is a phenomenon that can occur in a two-dimensional electron gas— a system in which electrons can move in a plane but not perpendicularly to it. Such a system istypically realized in a heterostructure of thinly stacked layers of different semiconductors. The QHEis usually observed at low temperatures and high magnetic fields; it manifests itself through thequantization of values of the electronic conductance (the inverse of resistance) — the values areinteger or particular fractional multiples of a fundamental conductance quantum.Recently, variants of the QHE have been discovered, such as the quantum spin Hall and theanomalous quantum Hall effect. Now, Satoshi Moriyama at WPI-MANA, NIMS, Tsukuba, Japan, andcolleagues have observed yet another relative of the QHE: the quantum valley Hall effect.The notion of ‘valleys’ refers to electronic states having the same energy but a different crystalmomentum; the states are said to lie in different valleys. A new type of electronics called‘valleytronics’, based on the valley degree of freedom, is being researched since some years.Moriyama and colleagues demonstrated the quantum valley Hall effect in a special heterostructurecapable of hosting a two-dimensional electron gas: a sheet of graphene sandwiched betweenhttps://www.nims.go.jp/mana/research/highlights/vol40.htmlhttps://www.nims.go.jp/mana/research/highlights/index.htmlhttps://www.nims.go.jp/mana/research/highlights/vol42.html2022/04/04 10:05 Evidence of A New Type of Quantum Effect| MANAhttps://www.nims.go.jp/mana/research/highlights/vol41.html 2/2hexagonal boron nitride (h-BN) layers. Graphene is a one-atom thin layer of carbon atoms forminga honeycomb pattern. h-BN is similar; it is also a monolayer with a honeycomb structure, but ithas a slightly different lattice parameter. Due to the mismatch in lattice size, the periodicity of thecombined system is much larger — overlaying the two lattices results in a moiré pattern.Importantly, the investigated electronic regime in the superlattice was ballistic: the electrons’ meanfree path (the mean distance travelled between scattering events) was estimated to be 1 to 2micrometer, which was comparable to the size of the sample.The researchers varied the magnetic field while measuring resistances, and at a small intervalcentered around zero field they observed a non-vanishing resistance — a signature of the quantumvalley Hall state.An important fundamental finding on its own, Moriyama and colleagues note that it also may leadto potential applications: “[such] unconventional magnetism should have the potential forengineering the energy-band structure [of devices] even with a weak magnetic field as well as forspintronics applications.”Reference“Observation of the quantum valley Hall state in ballistic graphene superlattices”Katsuyosih Komatsu, Yoshifumi Morita, Eiichiro Watanabe, Daiju Tsuya, Kenji Watanabe, TakashiTaniguchi, and Satoshi MoriyamaJournal : Sci. Adv. 4:eaaq0194 (2018).DOI : 10.1126/sciadv.aaq0194AffiliationsInternational Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for MaterialsScience (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, JapanContact informationInternational Center for Materials Nanoarchitectonics(WPI-MANA)National Institute for Materials Science1-1 Namiki, Tsukuba, Ibaraki 305-0044 JapanPhone: +81-29-860-4710E-mail: mana-pr[AT]ml.nims.go.jphttps://samurai.nims.go.jp/profiles/taniguchi_takashihttps://advances.sciencemag.org/content/4/5/eaaq0194