publications

2024

1. 

   Scalar chirality interaction mediated by Yu-Shiba-Rusinov states

   K. Akkaravarawong, M. Bintz, J. I. Väyrynen, and 3 more authors

   Manuscript in preparation 2024

   Abs

   Magnetic adatoms in an s-wave superconductor can host subgap states called the Yu-Shiba-Rusinov (YSR) states, whose wave function overlap creates effective interactions between adatom spins. In this letter, we analytically derive the effective low-energy Hamiltonian of the magnetic adatom spins in an s-wave superconductor with a perpendicular magnetic field and show that third-order virtual processes involving YSR states generate a three-spin interaction \( S_i ⋅(S_j \times S_k) \)known as the scalar chirality interaction. With large-scale infinite density-matrix renormalization group (iDMRG) calculations, we show that this effective Hamiltonian can host a topological chiral spin liquid phase on the triangular lattice.

2. 

   Compressible Bosonic Integer Quantum Hall in the presence of chemical potential disorder

   K. Akkaravarawong, S. Gazit, N. Y. Yao, and 1 more author

   Manuscript in preparation 2024

   Abs

   We study the effective action of the \(U(1) \times U(1) \)model and provide the evidence for a disorder-induced compressible Bosonic integer quantum Hall state, characterized by even integer Hall conductivity and finite compressibility in analogous to its Fermionic Integer quantum Hall counterpart. Akin to Bose glass, the finite compressible is originated from rare superfluid puddles of Bosonic composite objects which are exponentially far apart. Adding/removing particles simply grows/shrinks the size of the superfluid puddles without changing the Hall conductivity \( \sigma_xy \)carried through the incompressible background. Using Monte Carlo simulation, we mapped out the phase diagrams of the model in the presence of a random chemical potential. We study the effective action of the \(U(1) \times U(1) \)model and provide the evidence for a disorder-induced compressible Bosonic integer quantum Hall state, characterized by even integer Hall conductivity and finite compressibility in analogous to its Fermionic Integer quantum Hall counterpart. Akin to Bose glass, the finite compressible is originated from rare superfluid puddles of Bosonic composite objects which are exponentially far apart. Adding/removing particles simply grows/shrinks the size of the superfluid puddles without changing the Hall conductivity \( \sigma_xy \)carried through the incompressible background. Using Monte Carlo simulation, we mapped out the phase diagrams of the model in the presence of a random chemical potential.

2019

1. 

   Probing and dressing magnetic impurities in a superconductor

   K. Akkaravarawong, J. I. Väyrynen, J. D. Sau, and 3 more authors

   Phys. Rev. Research 2019

   Abs PDF

   We propose a method to probe and control the interactions between an ensemble of magnetic impurities in a superconductor via microwave radiation. Our method relies upon the presence of subgap Yu-Shiba-Rusinov (YSR) states associated with the impurities. Depending on the sign of the detuning, radiation generates either a ferro- or antiferromagnetic contribution to the exchange interaction. This contribution can bias the statistics of the random exchange constants stemming from the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. Moreover, by measuring the microwave response at the YSR resonance, one gains information about the magnetic order of the impurities. To this end, we estimate the absorption coefficient as well as the achievable strength of the microwave-induced YSR interactions using off-resonant radiation. The ability to utilize microwave fields to both probe and control impurity spins in a superconducting host may open new paths to studying metallic spin glasses.

2015

1. 

   Ballistic Guided Electron States in Graphene

   Kamphol Akkaravarawong, Oles Shtanko, and Leonid Levitov

   arXiv preprint arXiv:1512.04185 2015

   Abs PDF

   Guiding electronic waves in a manner similar to photon transmission in optical fibers is key for developing the electron-optics toolbox. Here we outline a ‘weak guiding’ approach, in which efficient diffraction around disorder results in low-loss, high-finesse electron guiding. We describe an implementation of this scheme for guiding along a narrow-width line gate in gapless and gapped graphene. A simple model for weak guiding, which relies on the Jackiw-Rebbi midgap states, is introduced and solved. The weak-guiding modes are shown to exist for confining potential of either sign and no matter how strong or weak. Modes evolve in a cyclic manner upon varying gate potential, repeatedly sweeping the Dirac gap and becoming dispersionless (flat band) at certain magic values of gate potential. Large mode widths facilitate diffraction around disorder in the core region, enabling exceptionally large mean free paths and long-range ballistic propagation.