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

Abstract: We show theoretically and experimentally the existence of topological monomodes in non-Hermitian systems created by loss engineering. This challenges the idea that edge states always come in pairs in $\mathbb{Z}_2$ symmetry-protected topological systems. We theoretically show the existence of a monomode in a non-Hermitian 1D and 2D SSH models. Furthermore, we classify the systems in terms of the (non-Hermitian) symmetries that are present and calculate the corresponding topological invariant. To corroborate the theory, we present experiments in photonic lattices in which a monomode is observed in a non-Hermitian 1D SSH chain.

Abstract: The superposition of atomic vibrations and flexoelectronic effect gives rise to a cross correlation between free charge carriers and temporal magnetic moment of phonons in conducting heterostructures under an applied strain gradient. The resulting dynamical coupling is expected to give rise to quasiparticle excitations called as magnetoelectronic electromagnon that carries electronic charge and temporal magnetic moment. Here, we report experimental evidence of magnetoelectronic electromagnon in the freestanding degenerately doped p-Si based heterostructure thin film samples. These quasiparticle excitations give rise to long-distance (>100um) spin transport; demonstrated using spatially modulated transverse magneto-thermoelectric and non-local resistance measurements. The magnetoelectronic electromagnons are non-reciprocal and give rise to large magnetochiral anisotropy (0.352 A-1T-1) that diminishes at lower temperatures. The superposition of non-reciprocal magnetoelectronic electromagnons gives rise to longitudinal and transverse modulations in charge carrier density, spin density and magnetic moment; demonstrated using the Hall effect and edge dependent magnetoresistance measurements, which can also be called as inhomogeneous magnetoelectronic multiferroic effect. These quasiparticle excitations are analogues to photons where time dependent polarization and temporal magnetic moment replaces electric and magnetic field, respectively and most likely topological because it manifests topological Nernst effect. Hence, the magnetoelectronic electromagnon can potentially give rise to quantum interference and entanglement effects in conducting solid state system at room temperature in addition to efficient spin transport.

Abstract: Magnetic droplets are nanoscale, non-topological, magnetodynamical solitons that can be nucleated in spin torque nano-oscillators (STNOs) or spin Hall nano-oscillators (SHNOs). All theoretical, numerical, and experimental droplet studies have so far focused on the free layer (FL), and any additional dynamics in the reference layer (RL) have been entirely ignored. Here we show, using all-perpendicular STNOs, that there is not only significant magnetodynamics in the RL, but the reference layer itself can host a droplet coexisting with the FL droplet. Both droplets are observed experimentally as stepwise changes and sharp peaks in the dc and differential resistance, respectively. Whereas the single FL droplet is highly stable, the coexistence state exhibits high-power broadband microwave noise. Micromagnetic simulations corroborate the experimental results and reveal a strong interaction between the droplets. Our demonstration of strongly interacting and closely spaced droplets offers a unique platform for fundamental studies of highly non-linear soliton pair dynamics.

Abstract: With the slowing down of Moore's law, augmenting complementary-metal-oxide semiconductor (CMOS) transistors with emerging nanotechnologies (X) is becoming increasingly important. In this paper, we demonstrate how stochastic magnetic tunnel junction (sMTJ)-based probabilistic bits, or p-bits, can be combined with versatile Field Programmable Gate Arrays (FPGA) to design an energy-efficient, heterogeneous CMOS + X (X = sMTJ) prototype. Our heterogeneous computer successfully performs probabilistic inference and asynchronous Boltzmann learning despite device-to-device variations in sMTJs. A comprehensive comparison using a CMOS predictive process design kit (PDK) reveals that digital CMOS-based p-bits emulating high-quality randomness use over 10,000 transistors with the energy per generated random number being roughly two orders of magnitude greater than the sMTJ-based p-bits that dissipate only 2 fJ. Scaled and integrated versions of our approach can significantly advance probabilistic computing and its applications in various domains, including probabilistic machine learning, optimization, and quantum simulation.

Abstract: Two-dimensional (2D) semiconductors possess strongly bound excitons, opening novel opportunities for engineering light-matter interaction at the nanoscale. However, their in-plane confinement leads to large non-radiative exciton-exciton annihilation (EEA) processes, setting a fundamental limit for their photonic applications. In this work, we demonstrate suppression of EEA via enhancement of light-matter interaction in hybrid 2D semiconductor-dielectric nanophotonic platforms, by coupling excitons in WS$ _2 $ monolayers with optical Mie resonances in dielectric nanoantennas. The hybrid system reaches an intermediate light-matter coupling regime, with photoluminescence enhancement factors up to 10$ ^2 $. Probing the exciton ultrafast dynamics reveal suppressed EEA for coupled excitons, even under high exciton densities $>$ 10$^{12}$ cm$^{-2} $. We extract EEA coefficients in the order of 10$^{-3} $, compared to 10$^{-2} $ for uncoupled monolayers, as well as absorption enhancement of 3.9 and a Purcell factor of 4.5. Our results highlight engineering the photonic environment as a route to achieve higher quantum efficiencies for low-power hybrid devices, and larger exciton densities, towards strongly correlated excitonic phases in 2D semiconductors.

Abstract: We experimentally investigate charge transport in In-GeTe and In-GeTe-In proximity devices, which are formed as junctions between superconducting indium leads and thick single crystal flakes of $\alpha$-GeTe topological semimetal. We observe nonmonotonic effects of the applied external magnetic field, including reentrant superconductivity in In-GeTe-In Josephson junctions: supercurrent reappears at some finite magnetic field. For a single In-GeTe Andreev junction, the superconducting gap is partially suppressed in zero magnetic field, while the gap is increased nearly to the bulk value for some finite field before its full suppression. We discuss possible reasons for the results obtained, taking into account spin polarization of Fermi arc surface states in topological semimetal $\alpha$-GeTe with a strong spin-orbit coupling. In particular, the zero-field surface state spin polarization partially suppresses the superconductivity, while it is recovered due to the modified spin-split surface state configuration in finite fields. As an alternative possible scenario, the transition into the Fulde-Ferrell-Larkin-Ovchinnikov state is discussed. However, the role of strong spin-orbit coupling in forming the nonmonotonic behavior has not been analyzed for heterostructures in the Fulde-Ferrell-Larkin-Ovchinnikov state, which is crucial for junctions involving GeTe topological semimetal.

Abstract: We consider a Josephson bijunction consisting of three superconducting reservoirs connected through two quantum dots. In equilibrium, the interdot coupling is sizable only for distances smaller than the superconducting coherence length. Application of commensurate dc voltages results in a time-periodic Hamiltonian and induces an interdot coupling at large distances. The basic mechanism of this long-range coupling is shown to be due to local multiple Andreev reflections on each dot, followed by quasiparticle propagation at energies larger than the superconducting gap. At large interdot distances we derive an effective non-Hermitian Hamiltonian describing two resonances coupled through a continuum.