Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Abstract: We investigate the effect of elastic strain on a Mach-Zehnder (MZ) interferometer created by graphene p-n junction in quantum Hall regime. We demonstrate that a Gaussian-shaped nanobubble causes detuning of the quantum Hall conductance oscillations across the p-n junction, due to the strain-induced local pseudo-magnetic fields. By performing a machine-learning-based Fourier analysis, we differentiate the nanobubble-induced Fourier component from the conductance oscillations originating from the external magnetic fields. We show that the detuning of the conductance oscillations is due to the altered pathway of quantum Hall interface channels caused by the strain-induced pseudo-magnetic fields. In the presence of the nanobubble, a new Fourier component for a magnetic flux $\Phi_{0}/2$ appears, and the corresponding MZ interferometry indicates that the enclosed area is reduced by half due to the strain-mediated pathway between two quantum Hall interface channels. Our findings suggest the potential of using graphene as a strain sensor for developments in graphene-based device fabrications and measurements technologies.

Abstract: Materials that break time-reversal or inversion symmetry possess nondegenerate electronic bands, which can touch at so-called Weyl points. The spinor eigenstates in the vicinity of a Weyl point exhibit a well-defined chirality $\pm 1$. Numerous works have studied the consequences of this chirality, for example in unconventional magnetoelectric transport. However, even a Weyl point with isotropic dispersion is not only characterized by its chirality but also by the momentum dependence of the spinor eigenstates. For a single Weyl point, this momentum-space spin structure can be brought into standard "hedgehog" form by a unitary transformation, but for two or more Weyl points, this is not possible. In this work, we show that the relative spin structure of a pair of Weyl points has strong qualitative signatures in the electromagnetic response. Specifically, we investigate the Friedel oscillations in the induced charge density due to a test charge for a centrosymmetric system consisting of two Weyl points with isotropic dispersion. The most pronounced signature is that the amplitude of the Friedel oscillations falls off as $1/r^4$ in directions in which both Weyl points exhibit the same spin structure, while for directions with inverted spin structures, the amplitude of the Friedel oscillations decreases as $1/r^3$.

Abstract: We propose a novel architecture that utilizes two 0-$\pi$ qubits based on topological Josephson junctions to implement a parity-protected superconducting qubit. The topological Josephson junctions provides protection against fabrication variations, which ensures the identical Josephson junctions required to implement the0-$\pi$ qubit. By viewing the even and odd parity ground states of a 0-$\pi$ qubit as spin-$\frac{1}{2}$ states, we construct the logic qubit states using the total parity odd subspace of two 0-$\pi$ qubits. This parity-protected qubit exhibits robustness against charge noise, similar to a singlet-triplet qubit's immunity to global magnetic field fluctuations. Meanwhile, the flux noise cannot directly couple two states with the same total parity and therefore is greatly suppressed. Benefiting from the simultaneous protection from both charge and flux noise, we demonstrate a dramatic enhancement of both $T_1$ and $T_2$ coherence times. Our work presents a new approach to engineer symmetry-protected superconducting qubits.

Abstract: Terahertz (THz) field pulses can now be applied in Scanning Tunnelling Microscopy (THz-STM) junction experiments to study time resolved dynamics. The relatively slow pulse compared to the typical electronic time-scale calls for approximations based on a time-scale separation. Here, we contrast three methods based on non-equilibrium Green's functions (NEGF): (i) the steady-state, adiabatic results, (ii) the lowest order dynamic expansion in the time-variation (DE), and (iii) the auxiliary mode (AM) propagation method without approximations in the time-variation. We consider a concrete THz-STM junction setup involving a hydrogen adsorbate on graphene where the localized spin polarization can be manipulated on/off by a local field from the tip electrode and/or a back-gate affecting the in-plane transport. We use steady-state NEGF combined with Density Functional Theory (DFT-NEGF) to obtain a Hubbard model for the study of the junction dynamics. Solving the Hubbard model in a mean-field approximation, we find that the near-adiabatic first order dynamical expansion provides a good description for STM voltage pulses up to the 1 V range.

Abstract: It is commonly believed that light cannot couple to the collective excitations of the fractional quantum Hall effect (FQHE). This assumption relies on Kohn's theorem that states that electron-electron interactions decouple from homogeneous electromagnetic fields due to galilean invariance. Here, we demonstrate that the existence of an edge breaks Kohn's theorem, and enables coupling of cavity light to the plasmonic edge modes of the FQHE. We derive the coupling using the FQHE bulk-boundary correspondence and predict the formation of experimentally detectable plasmon polaritons. We find that a single cavity mode leaves the system's topological protection intact. Interestingly, however, a multimode cavity mediates plasmon backscattering, and effectively transforms the edges of the 2D FQHE into a 1D wire. Such cavity-meditated nonlocal backscattering bodes the breakdown of the topological protection in the regime of ultra-strong photon-plasmon coupling. Our analytical framework and photoelectric findings pave the way for investigating the topological order of the FQHE via optical spectroscopic probes and provide new opportunities to control FQHE edge excitations using light.