Dynamic response of a mesoscopic capacitor in the presence of strong electron interactions

We consider a one dimensional mesoscopic capacitor in the presence of strong electron interactions and compute its admittance in order to probe the universal nature of the relaxation resistance. We use a combination of perturbation theory, renormalization group arguments, and quantum Monte Carlo calculation to treat the whole parameter range of dot-lead coupling. The relaxation resistance is universal even in the presence of strong Coulomb blockade when the interactions in the wire are sufficiently weak. We predict and observe a quantum phase transition to an incoherent regime for a Luttinger parameter $K<1/2$. Results could be tested using a quantum dot coupled to an edge state in the fractional quantum Hall effect.Comment: 4 pages, 4 figures, submitted to PR

Chiral condensate with topological degeneracy in graphene and its manifestation in edge states

Role of chiral symmetry in many-body states of graphene in strong magnetic fields is theoretically studied with the honeycomb lattice model. For a spin-split Landau level where the leading electron-electron interaction is the nearest-neighbor repulsion, a chiral condensate is shown to be, within the subspace of n = 0 Landau level, an exact many-body ground state with a finite gap, for which calculation of Chern numbers reveals that the ground state is a Hall insulator with a topological degeneracy of two. The topological nature of the ground state is shown to manifest itself as a Kekul\'ean bond order along armchair edges, while the pattern melts in the bulk due to quantum fluctuations. The whole story can be regarded as a realization of the bulk-edge correspondence peculiar to the chiral symmetry.Comment: 5 pages, 4 figures, submitted to PR

Machine learning study of single-atom platinum supported on graphene nanostructures (SAC Pt-G)

大規模計算機システム利用者研究報

Quantum phase transition of dynamical resistance in a mesoscopic capacitor

We study theoretically dynamic response of a mesoscopic capacitor, which consists of a quantum dot connected to an electron reservoir via a point contact and capacitively coupled to a gate voltage. A quantum Hall edge state with a filling factor nu is realized in a strong magnetic field applied perpendicular to the two-dimensional electron gas. We discuss a noise-driven quantum phase transition of the transport property of the edge state by taking into account an ohmic bath connected to the gate voltage. Without the noise, the charge relaxation for nu>1/2 is universally quantized at R_q=h/(2e^2), while for nu<1/2, the system undergoes the Kosterlitz-Thouless transtion, which drastically changes the nature of the dynamical resistance. The phase transition is facilitated by the noisy gate voltage, and we see that it can occur even for an integer quantum Hall edge at nu=1. When the dissipation by the noise is sufficiently small, the quantized value of R_q is shifted by the bath impedance.Comment: 5 pages, 2 figures, proceeding of the 19th International Conference on the Application of High Magnetic Fields in Semiconductor Physics and Nanotechnology (HMF-19

Spin-resolved chiral condensate as a spin-unpolarized ν=0 quantum Hall state in graphene

Motivated by the recent experiments indicating a spin-unpolarized ν=0 quantum Hall state in graphene, we theoretically investigate the ground state based on the many-body problem projected onto the n=0 Landau level. For an effective model with the on-site Coulomb repulsion and antiferromagnetic exchange couplings, we show that the ground state is a doubly degenerate spin-resolved chiral condensate in which all the zero-energy states with up spin are condensed into one chirality, while those with down spin to the other. This can be exactly shown for an Ising-type exchange interaction. The charge gap due to the on-site repulsion in the ground state is shown to grow linearly with the magnetic field, in qualitative agreement with the experiments

Machine-learned search for the stable structures of silicene on Ag(111)

Hamamoto Y., Pham T.N., Bisbo M.K., et al. Machine-learned search for the stable structures of silicene on Ag(111). Physical Review Materials 7, 124002 (2023); https://doi.org/10.1103/PhysRevMaterials.7.124002.The honeycomb lattice of silicene exhibits a variety of nontrivial reconstructions on the Ag(111) surface, whose diversity hampers the theoretical prediction of experimentally unidentified structures using computationally expensive density functional theory (DFT) calculations. We here apply an efficient method based on an evolutionary algorithm and a Gaussian process, which is trained on the fly with DFT calculations, to the search for the stable structures of silicene on Ag(111). We demonstrate that the structure search method can not only reproduce the well-known structures, but also predict the existence of metastable structures that are close in stability to the most stable ones. Detailed analyses of the obtained results reveal that such metastable structures play crucial roles in the stabilization of less ordered phases often observed experimentally. The present method can replace the conventional manual search based on intuition and is widely applicable to the investigations of new systems such as emerging two-dimensional materials