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

Abstract: Most non-ferroelectric two-dimensional materials can be endowed with so-called sliding ferroelectricity via non-equivalent homo-bilayer stacking, which is not applicable to mono-element systems like pure graphene bilayer with inversion symmetry at any sliding vector. Herein we show first-principles evidence that multilayer graphene with N>3 can all be ferroelectric, where the polarizations of polar states stem from the symmetry breaking in stacking configurations of across-layer instead of adjacent-layer, which are electrically switchable via interlayer sliding. The non-polar states can also be electrically driven to polar states via sliding, all nearly degenerate in energy, and more diverse states with distinct polarizations will emerge in more layers. In contrast to the ferroelectric Moire domains with opposite polarization directions in twisted bilayers reported previously, the Moire pattern in some multilayer graphene systems (e.g., twisted monolayer-trilayer graphene) possess nonzero net polarizations with domains of the same direction separated by non-polar regions, which can be electrically reversed upon interlayer sliding. The distinct Moire bands of two polar states should facilitate electrical detection of such sliding Moire ferroelectricity during switching.

Abstract: The Interlayer Dzyaloshinskii-Moriya interaction (IL-DMI) chirally couples spins in different ferromagnetic layers of multilayer heterostructures. So far, samples with IL-DMI have been investigated utilizing magnetometry and magnetotransport techniques, where the interaction manifests as a tunable chiral exchange bias field. Here, we investigate the nanoscale configuration of the magnetization vector in a synthetic anti-ferromagnet (SAF) with IL-DMI, after applying demagnetizing field sequences. We add different global magnetic field offsets to the demagnetizing sequence in order to investigate the states that form when the IL-DMI exchange bias field is fully or partially compensated. For magnetic imaging and vector reconstruction of the remanent magnetic states we utilize X-ray magnetic circular dichroism photoemission electron microscopy, evidencing the formation of 360$^{\circ}$ domain wall rings of typically 0.5-3.0 $\mu m$ in diameter. These spin textures are only observed when the exchange bias field due to the IL-DMI is not perfectly compensated by the magnetic field offset. From a combination of micromagnetic simulations, magnetic charge distribution and topology arguments, we conclude that a non-zero remanent effective field with components both parallel and perpendicular to the anisotropy axis of the SAF is necessary to observe the rings. This work shows how the exchange bias field due to IL-DMI can lead to complex metastable spin states during reversal, important for the development of novel spintronic devices.

Abstract: Quantum simulations of photoexcited low-dimensional systems are pivotal for understanding how to functionalize and integrate novel two-dimensional (2D) materials in next-generation optoelectronic devices. First principles predictions are extremely challenging due to the simultaneous interplay of light-matter, electron-electron and electron-nuclear interactions. We here present an advanced ab initio many-body method which accounts for quantum coherence and non-Markovian effects while treating electrons and nuclei on equal footing, thereby preserving fundamental conservation laws like the total energy. The impact of this advancement is demonstrated through real-time simulations of the complex multivalley dynamics in a molybdenum disulfide (MoS$_{2}$) monolayer pumped above gap. Within a single framework we provide a parameter-free description of the coherent-to-incoherent crossover, elucidating the role of microscopic and collective excitations in the dephasing and thermalization processes.

Abstract: One-dimensional (1D) linear nanostructures comprising sp-hybridized carbon atoms, as derivatives of the prototypical allotrope known as carbyne, are predicted to possess outstanding mechanical, thermal and electronic properties. Despite recent advances in the synthesis, their chemical and physical properties are still poorly understood. Here, we investigate the photophysics of a prototypical polyyne (i.e., 1D chain with alternating single and triple carbon bonds), as the simplest model of finite carbon wire and as a prototype of sp-carbon based chains. We perform transient absorption experiments with high temporal resolution (<30 fs) on monodispersed hydrogen-capped hexayne H$-$(C$\equiv$C)$_6-$H synthesized by laser ablation in liquid. With the support of detailed computational studies based on ground state density functional theory (DFT) and excited state time-dependent (TD)-DFT calculations, we provide a comprehensive description of the excited state relaxation processes at early times following photoexcitation. We show that the internal conversion from a bright high-energy singlet excited state to a low-lying singlet dark state is ultrafast and takes place with a 200-fs time constant, followed by thermalization on the picosecond timescale. We also show that the timescale of these processes does not depend on the end-groups capping the sp-carbon chain. The understanding of the primary photo-induced events in polyynes is of key importance both for fundamental knowledge and for potential opto-electronic and light-harvesting applications of nanostructured carbon-based materials.

Abstract: The freestanding twisted bilayer graphene (TBG) is unstable, below a critical twist angle {\theta}_c~3.7 degrees, against a moire (2 \times 1) buckling distortion at T=0. Realistic simulations reveal the concurrent unexpected collapse of the bending rigidity, an unrelated macroscopic mechanical parameter. An analytical model connects bending and buckling anomalies at T=0, but as temperature rises the former fades, while buckling persists further. The (2 \times 1) electronic properties are also surprising. The magic twist angle narrow bands, now eight in number, fail to show zone boundary splittings despite the new periodicity. Symmetry shows how this is dictated by an effective single valley physics. These structural, critical, and electronic predictions promise to make the freestanding state of TBG especially interesting.

Abstract: Spatial topology endows topological solitons, such as skyrmions and hopfions, with fascinating dynamics. However, the temporal dimension has so far provided a passive stage on which topological solitons evolve. Here we construct spacetime magnetic hopfions: magnetic textures in two spatial dimensions that when excited by a time-periodic drive develop spacetime topology. We uncover two complementary construction routes using skyrmions by braiding their center of mass position and by controlling their internal low-energy excitations. Spacetime magnetic hopfions can be realized in nanopatterned grids to braid skyrmions and in frustrated magnets under an applied AC electric field. Their topological invariant, the spacetime Hopf index, can be tuned by the applied electric field as demonstrated by our collective coordinate modeling and micromagnetic simulations. The principles we have introduced to actively control spacetime topology are not limited to magnetic solitons, opening avenues to explore spacetime topology of general order parameters and fields.

Abstract: We investigate the second spectrum of charge carrier density fluctuations in graphene within the McWorther model, where noise is induced by electron traps in the substrate. Within this simple picture, we obtain a closed-form expression including both Gaussian and non-Gaussian fluctuations. We show that a very extended distribution of switching rates of the electron traps in the substrate leads to a carrier density power spectrum with a non-trivial structure on the scale of the measurement bandwidth. This explains the appearance of a $1/f$ component in the Gaussian part of the second spectrum, which adds up to the expected frequency-independent term. Finally, we find that the non-Gaussian part of the second spectrum can become quantitatively relevant by approaching extremely low temperatures.

Abstract: Insulators with non-trivial topology support mid-gap modes localized at the boundaries of the sample. We consider the spinless Bernevig-Hughes-Zhang (SBHZ) model, one of the simplest models of a Chern insulator, in contact with external reservoirs (metallic leads) at its opposite ends. We study scattering states formed by these edge modes using the non-equilibrium Green's function (NEGF) formalism. These special states give rise to perfect transmission from one lead to another, leading to quantized two-terminal conductance. We look at the charge and current density profiles, associated to these modes, in the insulator as well as in the leads. As expected, we find that the current inside the insulator is localized along the edges of the sample. Surprisingly, we find that even in the leads, the current density is localized and shows an interesting zigzag pattern. We also look at finite-size effects on the quantized two-terminal conductance and its dependence on system-reservoir coupling.