Intrinsic suppression of topological thermal Hall effect in an exactly solvable quantum magnet

In contrast to electron (fermion) systems, topological phases of charge neutral bosons have been poorly understood despite recent extensive research on insulating magnets. The most important unresolved issue is how the inevitable inter-bosonic interactions influence the topological properties. It has been proposed that the quantum magnet SrCu$_2$(BO$_3$)$_2$ with an exact ground state serves as an ideal platform for this investigation, as the system is expected to be a magnetic analogue of a Chern insulator with electrons replaced by bosonic magnetic excitations (triplons). Here, in order to examine topologically protected triplon chiral edge modes in SrCu$_2$(BO$_3$)$_2$, we measured the thermal Hall conductivity $kappa_{xy}$ with extremely high accuracy. Contrary to the theoretical expectations, no discernible $kappa_{xy}$ was observed, which is at most $sim$ 1/20 of the prediction if present. This implies that even relatively weak inter-particle interactions seriously influence the topological transport properties at finite temperatures. These demonstrate that, in contrast to fermionic cases, the picture of non-interacting topological quasi-particles cannot be naively applied to bosonic systems, calling special attention to the interpretation of the topological bosonic excitations reported for various insulating magnets.

Half-integer quantized anomalous thermal Hall effect in the Kitaev material $alpha$-RuCl$_3$

Heat transport mediated by Majorana edge modes in a magnetic insulator leads to a half-integer thermal quantum Hall conductance, which has recently been reported for the two-dimensional honeycomb material $alpha$-RuCl$_3$. While the conventional electronic Hall effect requires a perpendicular magnetic field, we find that this is not the case in $alpha$-RuCl$_3$. Strikingly, the thermal Hall plateau appears even for a magnetic field with no out-of-plane components. The field-angular variation of the quantized thermal Hall conductance has the same sign structure of the topological Chern number, which is either $pm$1, as the Majorana band structure of the pure Kitaev spin liquid. This observation of a half-integer anomalous thermal Hall effect firmly establishes that the Kitaev interaction is primarily responsible and that the non-Abelian topological order associated with fractionalization of the local magnetic moments persists even in the presence of non-Kitaev interactions in $alpha$-RuCl$_3$.

Photon-spin qubit-conversion based on Overhauser shift of Zeeman energies in quantum dots

We propose a new method to realize a conversion of photon qubit and spin qubit using the effective magnetic field created by the nuclear polarization known as Overhauser field. We discuss its preliminary experiment on InAlAs/AlGaAs self-assembled quantum dot and also discuss effects of excitons which could destroy the conversion.