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

Abstract: The dynamics of ultrathin membranes made of two-dimensional (2D) materials is highly susceptible to stress. Although the dynamics of such membranes under tensile stress has been thoroughly studied, their motion under compressive stress, particular in the buckled state has received less attention. Here, we study the dynamics of 2D nanomechanical resonators made of FePS$_3$, 2H-TaS$_2$ and WSe$_2$ membranes near the buckling bifurcation. Using an optomechanical method, we measure their resonant frequency and thermal transport while varying in-plane stress via membrane temperature and thermal expansion. The observed temperature dependence of the resonance frequency is well capture by a mechanical model, which allows us to extract the pre-strain, central deflection and boundary compressive stress of the membrane. Near the buckling bifurcation, we observe a remarkable enhancement of up to 7$\times$ the thermal signal in the fabricated devices, demonstrating the extremely high force sensitivity of the membranes near this point. The presented results provide insights into the effects of buckling on the dynamics of free-standing 2D materials and thereby open up opportunities for the realization of 2D resonant nanomechanical sensors with buckling-enhanced sensitivity.

Abstract: In semiconductors, the identification of doping atomic elements allowing to encode a qubit within spin states is of intense interest for quantum technologies. In transition metal dichalcogenides semiconductors, the strong spin-orbit coupling produces locked spin-valley states with expected long coherence time. Here we study the substitutional Bromine Br\textsubscript{Te} dopant in 2H-MoTe$_2$. Electron spin resonance measurements show that this dopant carries a spin with long-lived nanoseconds coherence time. Using scanning tunneling spectroscopy, we find that the hydrogenic wavefunctions associated with the dopant levels have characteristics spatial modulations that result from their hybridization to the \textbf{Q}-valleys of the conduction band. From a Fourier analysis of the conductance maps, we find that the amplitude and phase of the Fourier components change with energy according to the different irreducible representations of the impurity-site point-group symmetry. These results demonstrate that a dopant can inherit the locked spin-valley properties of the semiconductor and so exhibit long spin-coherence time.

Abstract: Recent novel topological antiferromagnetic systems have shown a strong magnetoresistance effects driven by Dirac fermion characteristics whose topology can be dynamically controlled by the N\'eel vector orientation. These new antiferromagnets are characterized by anisotropic band structures combined with complex relativistic spin structures in momentum space. While these systems have been studied in transport experiments, very little is known about their spin-dependent electronic dynamics on ultrafast timescales and far-from-equilibrium behavior. This paper investigates spin-dependent electronic dynamics due to electron-phonon scattering in a model electronic band structure that corresponds to a Dirac semimetal antiferromagnet. Following a spin-independent instantaneous excitation, we obtain a change of the antiferromagnetic spin polarization due to the scattering dynamics for the site-resolved spin expectation values. This allows us to identify fingerprints of the anisotropic band structure in the carrier dynamics on ultrashort timescales which should be observable in present experimental set-ups.

Abstract: Understanding spin dynamics on femto- and picosecond timescales offers new opportunities for faster and more efficient devices. Here, we experimentally investigate coherent spin dynamics following ultrafast all-optical excited demagnetization measured by time- resolved magneto optical Kerr effect (TR-MOKE) in ultrathin Ni80Fe20 films. On nanosecond time scales, we provide a detailed investigation of the magnetic field and pump fluence dependence of the GHz frequency precessional dynamics. We discuss how the unusual dependence of the lifetime of the coherent precession can be related to the dephasing due to nonlinear magnon interactions and the incoherent magnon background.

Abstract: Atomically-precise engineering of defects in topological quantum materials, which is essential for constructing new artificial quantum materials with exotic properties and appealing for practical quantum applications, remains challenging due to the hindrances in modifying complex lattice with atomic precision. Here, we report the atomically-precise engineering of the vacancy-localized spin-orbital polarons (SOP) in a kagome magnetic Weyl semimetal Co3Sn2S2, using scanning tunneling microscope. We achieve the repairing of the selected single vacancy and create atomically-precise sulfur quantum antidots with elaborate geometry through vacancy-by-vacancy repairing. We find that that the bound states of SOP experience a symmetry-dependent energy shift towards Fermi level with increasing vacancy size driven by the anti-bond interactions. Strikingly, as vacancy size increases, the localized magnetic moments of SOPs are tunable and ultimately extended to the negative magnetic moments resulting from spin-orbit coupling in the kagome flat band. These findings establish a new platform for engineering atomic quantum states in topological quantum materials, offering potential for kagome-lattice-based spintronics and quantum technologies.

Abstract: We construct a microscopic model based on superexchange theory for a moir\'e bilayer in chromium trihalides (Cr$X_3$, $X=$Br, I). In particular, we derive analytically the interlayer Heisenberg exchange and the interlayer Dzyaloshinskii-Moriya interaction with arbitrary distances ($\mathbf{x}$) between spins. Importantly, our model takes into account sliding and twisting geometries in the interlayer $X$-$X$ hopping processes. Unlike previous works, the $\mathbf{x}$-dependent interlayer exchange is deduced by various sliding bilayers where the geometry due to the rotation between $X$-planes is omitted. We argue that excluding twisting may lead to an incomplete interlayer exchange for a moir\'e bilayer. Using the \textit{ab initio} tight-binding Hamiltonian, we numerically evaluate the exchange interactions in CrI$_3$. We find that our analytical model agrees with the previous comprehensive density functional theory studies. Furthermore, our findings reveal the important role of the correlation effects in the $X$'s $p$ orbitals, which gives rise to a rich interlayer magnetic interaction with remarkable tunability.

Abstract: Chern insulators, which are the lattice analogs of the quantum Hall states, can potentially manifest high-temperature topological orders at zero magnetic field to enable next-generation topological quantum devices. To date, integer Chern insulators have been experimentally demonstrated in several systems at zero magnetic field, but fractional Chern insulators have been reported only in graphene-based systems under a finite magnetic field. The emergence of semiconductor moir\'e materials, which support tunable topological flat bands, opens a new opportunity to realize fractional Chern insulators. Here, we report the observation of both integer and fractional Chern insulators at zero magnetic field in small-angle twisted bilayer MoTe2 by combining the local electronic compressibility and magneto-optical measurements. At hole filling factor {\nu}=1 and 2/3, the system is incompressible and spontaneously breaks time reversal symmetry. We determine the Chern number to be 1 and 2/3 for the {\nu}=1 and {\nu}=2/3 gaps, respectively, from their dispersion in filling factor with applied magnetic field using the Streda formula. We further demonstrate electric-field-tuned topological phase transitions involving the Chern insulators. Our findings pave the way for demonstration of quantized fractional Hall conductance and anyonic excitation and braiding in semiconductor moir\'e materials.