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

Abstract: The ability to transport single electrons on a quantum dot array dramatically increases the freedom in designing quantum computation schemes that can be implemented on solid-state devices. So far, however, routing schemes to precisely control the transport paths of single electrons have yet to be established. Here, we propose a silicon single-electron router that transports pumped electrons along the desired route on the branches of a T-shaped quantum dot array by inputting a synchronous phase-controlled signal to multiple gates. Notably, we show that it is possible to achieve a routing accuracy above 99% by implementing assist gates in front of the branching paths. The results suggest new possibilities for fast and accurate transport of single electrons on quantum dot arrays.

Abstract: We study the magneto-photoluminescence of an optically trapped exciton-polariton condensate in a planar semiconductor microcavity with multiple In0.08Ga0.92As quantum wells. Extremely high condensate coherence time and continuous control over the polariton confinement are amongst the advantages provided by optical trapping. This allows us to resolve magnetically induced {\mu}eV fine energy shifts in the condensate, and identify unusual dynamical regions in its parameter space. We observe polariton Zeeman splitting and, in small traps with tight confinement, demonstrate its full parametric screening when the condensate density exceeds a critical value, reminiscent of the spin- Meissner effect. For larger optical traps, we observe a complete inversion in the Zeeman splitting as a function of power, underlining the importance of condensate confinement and interactions with its background reservoir excitons.

Abstract: Higher-order topological insulators (HOTIs) are a novel type of topological phases which supports $d$-dimensional topological boundary states in $D$-dimensional systems with $D-d>1$. In this work, we theoretically predict that interlayer couplings in AB-stacked bilayer transition-metal dichalcogenides (TMDs) lead to the emergence of extrinsic second-order topological phases, where corner states are induced by the band inversion of zigzag edge bands. We find that the systems feature a quantized multiband Berry phase defined for a zigzag nanoribbon geometry, unveiling the nontrivial topological properties of its two zigzag edges. With detailed investigation into the bilayer TMDs under different geometries, we find two types of boundary-obstructed corner states arising from different corner terminations of either the same type of or heterogeneous zigzag edges. The topological nature of these corner states and their degeneracy is further analyzed with both the crystalline symmetries of different geometries, and a topological phase transition of the Berry phase induced by a layer-dependent onsite energy.

Abstract: A quantum system constrained to a degenerate energy eigenspace can undergo a nontrival time evolution upon adiabatic driving, described by a non-Abelian Berry phase. This type of dynamics may provide logical gates in quantum computing that are robust against timing errors. A strong candidate to realize such holonomic quantum gates is an electron or hole spin qubit trapped in a spin-orbit-coupled semiconductor, whose twofold Kramers degeneracy is protected by time-reversal symmetry. Here, we propose and quantitatively analyze protocols to measure the non-Abelian Berry phase by pumping a spin qubit through a loop of quantum dots. One of these protocols allows to characterize the local internal Zeeman field directions in the dots of the loop. We expect a near-term realisation of these protocols, as all key elements have been already demonstrated in spin-qubit experiments. These experiments would be important to assess the potential of holonomic quantum gates for spin-based quantum information processing.

Abstract: Electronic conductance through a single molecule is sensitive towards its structural orientation between two electrodes, owing to the distribution of molecular orbitals and their coupling to the electrode levels, that are governed by quantum confinement effects. Here, we vary the contact geometry of the porphine molecule by attaching two Au tip electrodes that resemble the mechanical break junction, via thiol anchoring groups. We investigate the current-voltage characteristics of all the contact geometries using non-equilibrium Green's function formalism along with density functional theory and tight-binding framework. We observe varying current responses with changing contact sites, originating from varied wave-function delocalization and quantum interference effect. Our calculations show asymmetric current-voltage characteristics under forward and reverse biases due to structural asymmetry of the tip electrodes in either sides of the molecule. We establish this phenomenon as a universal feature for any molecular electronic device, irrespective of the inherent structural symmetry of a molecule. This will provide fundamental insights of electronic transport through single molecule in real experimental setup. Furthermore, our observations of varying current response can further motivate the fabrication of sensor devices with porphine based biomolecules that control important physiological activities, in view of their applications in advanced diagnostics.

Abstract: The existence of spontaneous magnetization that fingerprints a ground-state ferromagnetic order was recently observed in two-dimensional (2D) van der Waals materials. Despite progress in the fabrication and manipulation of the atom-thick magnets, investigation of nanoscale magnetization properties is still challenging due to the concomitant technical issues. We propose a promising approach for a direct visualization of the domain walls formed in 2D magnetic materials. By interfacing 2D magnet with a transition metal dichalcogenide (TMD) monolayer, the strong proximity effects enable pinning the TMD excitons on the domain wall. The emergent localization stems from the proximity-induced exchange mixing between spin-dark and spin-bright TMD excitons due to the local in-plane magnetization characteristic of the domain wall in the magnetic monolayer.

Abstract: Bernal bilayer graphene hosts even denominator fractional quantum Hall states thought to be described by a Pfaffian wave function with nonabelian quasiparticle excitations. Here we report the quantitative determination of fractional quantum Hall energy gaps in bilayer graphene using both thermally activated transport and by direct measurement of the chemical potential. We find a transport activation gap of 5.1K at B = 12T for a half-filled N=1 Landau level, consistent with density matrix renormalization group calculations for the Pfaffian state. However, the measured thermodynamic gap of 11.6K is smaller than theoretical expectations for the clean limit by approximately a factor of two. We analyze the chemical potential data near fractional filling within a simplified model of a Wigner crystal of fractional quasiparticles with long-wavelength disorder, explaining this discrepancy. Our results quantitatively establish bilayer graphene as a robust platform for probing the non-Abelian anyons expected to arise as the elementary excitations of the even-denominator state.