Materials Science (cond-mat.mtrl-sci)

Abstract: Valleytronics, that uses the valley index or valley pseudospin to encode information, has emerged as an interesting field of research in two-dimensional (2D) systems with promising device applications. Spin-orbit coupling (SOC) and inversion symmetry breaking leads to spin-splitting of bands near the energy extrema (valleys). In order to find a new 2D material useful for valleytronics, we have designed hexagonal planar monolayers of cadmium chalcogenides (CdX, X = S, Se, Te) from the (111) surface of bulk CdX zinc blende structure. Band structure study reveals valence band local maxima at symmetry point K and its time reversal conjugate point K$\textquotesingle$. Application of SOC initiates spin-splitting in the valleys that lifts the energy degeneracy and shows strong valley-spin coupling character. We have substituted two Cd atoms in the planar monolayers by Sn atoms which increases the spin-splitting significantly. The structural, dynamic, mechanical and thermal stability of all the monolayers has been confirmed. Values of formation energies indicate that it may be feasible to synthesize the Sn-doped CdSe and CdTe monolayers using bottom-up approach. Zeeman-type spin-splitting is observed in the valley region and Rashba spin-splitting is observed at the $\Gamma$ point for Sn-doped CdSe and CdTe monolayers. Berry curvature values are more in all the Sn-doped monolayers than the pristine monolayers. These newly designed monolayers are thus found to be suitable for valleytronics applications. Sn-doped monolayers show band inversion deep in the valence and conduction bands between Sn~$s$ and $p$ and X~$p$ states but lack topological properties.

Abstract: Electric field enhancement mediated through sharp tips in scattering-type scanning near-field optical microscopy (s-SNOM) enables optical material analysis down to the 10-nm length scale, and even below. Nevertheless, mostly the out-of-plane electric field component is considered here due to the lightning rod effect of the elongated s-SNOM tip being orders of magnitude stronger as compared to any in-plane field component. Nonetheless, the fundamental understanding of resonantly excited near-field coupled systems clearly allows us to take profit from all vectorial components, especially also from the in-plane ones. In this paper, we theoretically and experimentally explore how linear polarization control of both near-field illumination and detection, can constructively be implemented to (non-)resonantly couple to selected sample permittivity tensor components, e.g. explicitly also to the in-plane directions. When applying the point-dipole model, we show that resonantly excited samples respond with a strong near-field signal, to all linear polarization angles. We then experimentally investigate the polarization-dependent responses for both non-resonant (Au) and phonon-resonant (3C-SiC) sample excitations at a 10.6~$\mu$m and 10.7~$\mu$m incident wavelength using a tabletop CO$_2$ laser. Varying the illumination polarization angle thus allows for quantitatively comparing the scattered near-field signatures for the two wavelengths. Finally, we compare our experimental data to simulation results, and thus gain the fundamental understanding of the polarization's influence on the near-field interaction. As a result, the near-field components parallel and perpendicular to the sample surface can be easily disentangled and quantified through their polarization signatures, connecting them directly to the sample's local permittivity.

Abstract: Chiral magnetic states in two-dimensional (2D) layered noncentrosymmetric magnets, which are of promising advanced spintronic applications, are usually attributed to Dzyaloshinskii-Moriya interaction (DMI), yet the role of underlying higher-order spin couplings on the emergence of chiral spin textures is rarely reported. In this work, taking lithium-decorated monolayer CrTe$_{2}$ (LiCrTe$_{2}$) as an example, we proposed a comprehensive first-principles-based spin model using the symmetry-adapted cluster expansion method. Based on this spin model, we identified the ground state of monolayer LiCrTe$_{2}$ to be a chiral labyrinth domain (LD) state, and various higher-order spin interactions are essentially responsible for stabilizing this LD state. Meanwhile, some higher-order couplings are identified to play the role of DMI due to their nontrivial spin spiral chirality selectivity. In addition, skyrmions emerge under either external magnetic field or finite temperature. Our study sheds light on the complex magnetic couplings in 2D magnets.

Abstract: The excitation of quasi-particles near the extrema of the electronic band structure is a gateway to electronic phase transitions in condensed matter. In a many-body system, quasi-particle dynamics are strongly influenced by the electronic single-particle structure and have been extensively studied in the weak optical excitation regime. Yet, under strong optical excitation, where light fields coherently drive carriers, the dynamics of many-body interactions that can lead to new quantum phases remain largely unresolved. Here, we induce such a highly non-equilibrium many-body state through strong optical excitation of charge carriers near the van Hove singularity in graphite. We investigate the system's evolution into a strongly-driven photo-excited state with attosecond soft X-ray core-level spectroscopy. Surprisingly, we find an enhancement of the optical conductivity of nearly ten times the quantum conductivity and pinpoint it to carrier excitations in flat bands. This interaction regime is robust against carrier-carrier interaction with coherent optical phonons acting as an attractive force reminiscent of superconductivity. The strongly-driven non-equilibrium state is markedly different from the single-particle structure and macroscopic conductivity and is a consequence of the non-adiabatic many-body state.

Abstract: Density-functional-theory calculations were performed to investigate the magnetism in a series of triangular-lattice monolayer MCl2 (M=V, Mn, and Ni). The magnetic stability manifests a distinct chemical trend; VCl2 and MnCl2 show the antiferromagnetic ground states and NiCl2 shows the ferromagnetic ground state. The microscopic mechanism behind the magnetic interaction is explained by the so-called Goodenough-Kanamori-Anderson rules and by the virtual-hopping process through the hopping integrals between the 3d-orbital maximally localized Wannier functions. Our result highlights the role of the direct exchange interaction and the superexchange interaction in the magnetic stabilization in two-dimensional magnets.

Abstract: The growth of monolayer hexagonal boron nitride (h-BN) on Ir(110) through low-pressure chemical vapor deposition is investigated using low energy electron diffraction and scanning tunneling microscopy. We find that the growth of aligned single hexagonal boron nitride on Ir(110) requires a growth temperature of 1500 K, whereas lower growth temperatures result in coexistence of aligned h-BN with twisted h-BN The presence of the h-BN overlayer suppresses the formation of the nano-faceted ridge pattern known from clean Ir(110). Instead, we observe the formation of a (1 $\times$ n) reconstruction, with n such that the missing rows are in registry with the h-BN/Ir(110) moir\'{e} pattern. Our moir\'{e} analysis showcases a precise methodology for determining both the moir\'{e} periodicity and the h-BN lattice parameter on an fcc(110) surface.

Abstract: Here, we report a novel AuSb 2D superstructure on Au(111) that shows agreements and discrepancies to the expected electronic features of the ideal 2D surface alloys with $\sqrt{3}\times\sqrt{3}$ periodicity. Using spin- and angle-resolved photoemission spectroscopy, we find a significant spin splitting of the alloy bands with antiparallel spin polarization. The band structure originates from the hybridization between the Sb and the Au orbitals at the 2D Sb-Au interface. Taking advantage of the good agreement between the experimental results and DFT calculations, we find the broadening of the band is due to the perturbations introduced by the 3-pointed-star-shaped defects as nonresonant impurities in the $8\times8$ superstructure. The periodic defect can properly adjust the energy position of the Rashba band while not breaking the in-plane symmetry.

Abstract: The broken inversion symmetry at the ferromagnet (FM)/heavy-metal (HM) interface leads to spin-dependent degeneracy of the energy band, forming spin-polarized surface states. As a result, the interface serves as an effective medium for converting spin accumulation into two-dimensional charge current through the inverse Rashba-Edelstein effect. Exploring and assessing this spin-to-charge conversion (SCC) phenomenon at the FM/HM interface could offer a promising avenue to surpass the presumed limits of SCC in bulk HM layers. We utilize spintronic heterostructures as a platform to measure the spin-to-charge conversion (SCC) experienced by photoexcited spin currents. These heterostructures emit terahertz electric field when illuminated by femtosecond laser pulses, enabling us to quantitatively assess the ultrafast SCC process. Our results demonstrate a robust interfacial spin-to-charge conversion (iSCC) within a synthetic antiferromagnetic heterostructure, specifically for the NiFe/Ru/NiFe configuration, by isolating the SCC contribution originating from the interface itself, separate from the bulk heavy-metal (HM) region. Moreover, the iSCC at the NiFe/Ru interface is discovered to be approximately 27% of the strength observed in the highest spin-Hall conducting heavy-metal, Pt. Our results thus highlight the significance of interfacial engineering as a promising pathway for achieving efficient ultrafast spintronic devices.