Research

Mesoscopic Physics


Mesoscopic physics deals with things larger than micro, but smaller than macro, treating length scales of nm~μm. There, the system is small enough for quantum effects to play a key role, while it is large enough for us to have full control and engineering.

On top of this arena, we study a wide range of exciting quantum physics in condensed matter, including the topics below.

Kondo Effect and Kondo Cloud

When a magnetic impurity exists in a metal, conduction electrons form a spin cloud that screens the impurity spin. This basic phenomenon is called the Kondo effect. Contrary to electric charge screening, the spin screening cloud occurs quantum coherently, forming spin-singlet entanglement with the impurity. Although the spins interact locally around the impurity, the Kondo screening cloud can spread out over micrometers.

In our laboratory, we study the Kondo effect and the Kondo cloud to understand

  • how to characterize the cloud by quantum entanglement and etc.

  • how to detect the cloud by conductance and etc.

  • how to manipulate the cloud by gate voltage and etc.

See also,

  • Ivan V. Borzenets, Jeongmin Shim, Jason C. H. Chen, Arne Ludwig, Andreas D. Wieck, Seigo Tarucha, H.-S. Sim and Michihisa Yamamoto, Observation of the Kondo screening cloud, Nature (London) 579, 210(2020)[Nature]

  • Gwangsu Yoo, S.-S. B. Lee, and H.-S. Sim, Detecting Kondo Entanglement by Electron Conductance, Phys. Rev. Lett. 120, 146801 (2018)[PROLA]

  • S.-S. B. Lee, Jinhong Park, and H.-S. Sim, Macroscopic Quantum Entanglement of a Kondo Cloud at Finite Temperature, Phys. Rev. Lett. 114, 057203(2015)[PROLA]

  • Jinhong Park, S.-S. B. Lee, Yuval Oreg, and H.-S. Sim, How to Directly Measure a Kondo Cloud's Length, Phys. Rev. Lett. 110, 246603(2013)[PROLA]

Anyon and Fractional Statistics


In low-dimension, there is no prior reason for the identical particles to be fermion or boson. Instead, the exchange of the particles can induce an arbitrary phase to the wavefunction or even can transform a state to the other. These exotic quasi-particles are called anyon and can appear in strongly interacting low-dimensional condensed matter system, usually within electron fractionalization.

In our laboratory, we ask the following questions

    • What are the fundamental properties of the anyon that distinguish it from boson or fermion?

    • How the anyonic statistics is related the entanglement of the system?

    • What is the experimentally feasible setup that can detect the anyonic statistics?

See also,

  • June-Young M. Lee, Cheolhee Han, and H.-S. Sim, Fractional Mutual Statistics on Integer Quantum Hall Edges, Phys. Rev. Lett. 125, 196802 (2020) [PROLA]

  • Sunghun Park, H.-S. Sim, and Patrik Recher, Electron-Tunneling-Assisted Non-Abelian Braiding of Rotating Majorana Bound States, Phys. Rev. Lett. 125, 187702 (2020) [PROLA]

  • Byeongmok Lee, Cheolhee Han, and H.-S. Sim, Negative Excess Shot Noise by Anyon Braiding, Phys. Rev. Lett. 123, 016803(2019)[PROLA]

  • YeJe Park, Jeongmin Shim, S.-S. B. Lee, and H.-S. Sim, Nonlocal Entanglement of 1D Thermal States Induced by Fermion Exchange Statistics, Phys. Rev. Lett. 119, 210501(2017)[PROLA]

  • Cheolhee Han, Jinhong Park, Yuval Gefen, and H.-S. Sim, Topological vacuum bubbles by anyon braiding, Nature Communications 7, 11131(2016)[Nature Communications]

Topological Superconductor and Majorana Fermion

Topological superconductor(TSC) is the superconductor which has topological property in its band structure of Bogoliubov quasi particles. The result of topological nature of this system is the emergence of Majorana Fermion at the boundary or vortex. Since an adiabatic exchange of Majoranas leads to the change of quantum ground state, it is called non-Abelian anyon.

Detecting the Majorana or TSC has been hot issue, because it is expected to be a platform for fault-tolerant quantum computation due to its robustness against non-topological defects.

In this laboratory, we suggest and study a mesoscopic set-up (such as topological Josephson junction) which can give peculiar transport signature originated from Majorana fermion and topological superconductivity.

See also,

  • Sunghun Park, H.-S. Sim, and Patrik Recher, Electron-Tunneling-Assisted Non-Abelian Braiding of Rotating Majorana Bound States, Phys. Rev. Lett. 125, 187702 (2020) [PROLA]

  • Sang-Jun Choi, and H.-S. Sim, Josephson junction of finite-size superconductors on a topological insulator under a magnetic field, arXiv:1908.11403(2019)

  • Sang-Jun Choi, and H.-S. Sim, Non-Abelian Evolution of a Majorana Train in a Single Josephson Junction, arXiv:1808.08714(2018)

  • Sunghun Park, Joel Moore, and H.-S.Sim, "Absence of the Aharonov-Bohm effect of chiral Majorana fermion edge states", Phys.Rev.B 89,161208(R)(2014) [PROLA]

Quantum Pump

Quantum pump is a device that generate a DC current without DC bias. For certain condition, one can emit single-electrons periodically. It leads to a quantized current and it is applicable for fundamental standard of electrical current. These devices also be used as a fundamental building block of Fermionic quantum optics, which reveals quantum nature of electrons such as indistinguishability.

In our laboratory, we study generation and manipulation of few-electrons and find a way to measure quantum nature of emitted electrons.

See also,

  • Gento Yamahata, Sungguen Ryu, Nathan Johnson, H.-S. Sim, Akira Fujiwara, and Masaya Kataoka, Picosecond coherent electron motion in a silicon single-electron source, Nature Nanotechnology 14, 1019(2019)[Nature nanotechnology]

  • N. Johnson, C. Emary, S. Ryu, H.-S. Sim, P. See, J. D. Fletcher, J. P. Griffiths, G. A. C. Jones, I. Farrer, D. A. Ritchie, M. Pepper, T. J. B. M. Janssen, and M. Kataoka, LO-Phonon Emission Rate of Hot Electrons from an On-Demand Single-Electron Source in a GaAs/AlGaAs Heterostructure, Phys. Rev. Lett. 121, 137703(2018)[PROLA]

  • Sungguen Ryu, M. Kataoka, and H.-S. Sim, Ultrafast Emission and Detection of a Single-Electron Gaussian Wave Packet: A Theoretical Study, Phys. Rev. Lett. 117, 146802(2016)[PROLA]

Entanglement and Correlation on Quantum Dots

Quantum dot is a device confining electrons in a tiny region. Quantum dot can function as an artificial atom with high tunability of size, energy level, charge, spin, and etc. These properties of quantum dot allow to study strong correlation and entanglement in a controlled manner.

In our laboratory, we study

  • entanglement on quantum dots (Kondo effect, ...)

  • correlation between quantum dots (attractive Coulomb interaction, ...)

See also,

  • Changki Hong, Gwangsu Yoo, Jinhong Park, Min-Kyun Cho, Yunchul Chung, H.-S. Sim, Dohun Kim, Hyungkook Choi, Vladimir Umansky, and Diana Mahalu, Attractive Coulomb interactions in a triple quantum dot, Phys. Rev. B 97, 241115(R)(2018)[PROLA]

  • Gwangsu Yoo, S.-S. B. Lee, and H.-S. Sim, Detecting Kondo Entanglement by Electron Conductance, Phys. Rev. Lett. 120, 146801 (2018)[PROLA]

  • S.-S. B. Lee, Jinhong Park, and H.-S. Sim, Macroscopic Quantum Entanglement of a Kondo Cloud at Finite Temperature, Phys. Rev. Lett. 114, 057203(2015)[PROLA]

  • Gwangsu Yoo, Jinhong Park, S.-S. B. Lee, and H.-S. Sim, Anisotropic Charge Kondo Effect in a Triple Quantum Dot, Phys. Rev. Lett. 113, 236601(2014)[PROLA]