Ultrahigh breakdown current density of van der Waals One Dimensional PdBr2\mathrm{PdBr_2}

One-dimensional (1D) van der Waals (vdW) materials offer nearly defect-free strands as channel material in the field-effect transistor (FET) devices and probably, a better interconnect than conventional copper with higher current density and resistance to electro-migration with sustainable down-scaling. We report a new halide based "truly" 1D few-chain atomic thread, PdBr$_2$, isolable from its bulk which crystallizes in a monoclinic space group C2/c. Liquid phase exfoliated nanowires with mean length (20$\pm$1)$\mu$m transferred onto SiO$_2$/Si wafer with a maximum aspect ratio of 5000 confirms the lower cleavage energy perpendicular to chain direction. Moreover, an isolated nanowire can also sustain current density of 200 MA/cm$^\mathrm{2}$ which is atleast one-order higher than typical copper interconnects. However, local transport measurement via conducting atomic force microscopy (CAFM) tip along the cross direction of the single chain records a much lower current density due to the anisotropic electronic band structure. While 1D nature of the nanoobject can be linked with non-trivial collective quantum behavior, vdW nature could be beneficial for the new pathways in interconnect fabrication strategy with better control of placement in an integrated circuit (IC)

Spin-crossover assisted metallization of few-layer FePS3_3 at 1.45 GPa

Magnetic insulators in reduced dimension are the ideal model systems to study spin-crossover(SCO) induced cooperative behavior under pressure. Similar to the external perturbations like light illumination or temperature, external pressure may provide new pathway to accelerate giant lattice collapse,and subsequently Mott transition in van der Waals (vdW) materials with diminishing effect of the third dimension. Here, we investigate room-temperature layer-dependent SCO and insulator-metal transition in vdW magnet,FePS3, under high pressure using micro-Raman scattering.Experimentally obtained spectra, in agreement with the computed Raman modes, indicates evidence of IMT of FePS3 started off with a spin-state transition from a high (S=2) to low spin state (S=0) with a thickness dependent critical pressure (P_c) which reduces to 1.45 GPa in 3-layer flakes compared to 10.8 GPa for the bulk counterpart. Additionally, a broad Raman mode (P*) emerges between 310 cm^{-1} and 370 cm^{-1} at elevated pressure for three different thicknesses of FePS3 flakes (3-100 layers), also corroborated with computational results which suggests the pressure dependent decrease of metal-ligand bond distance(Fe-S) with lowering of magnetic moment in FePS3. Phenomenologically, our results in few-layer flakes with strong structural anisotropy which enhances the in-plane strain with applied pressure can be understood by adopting Hubbard model and considering the spectral-range (bandwidth W) as a function of layer numbers and pressure with a power-law scaling. Reduction of the critical pressure for phase transition in few-layer vdW magnets to 1-2 GPa marks the possibility of using nano-enclosure fit for use in device electronics where the pressure is induced due to interfacial adhesion, like in vdW heterostructure or molecules trapped between layers,and thereby,avoiding the conventional use of diamond anvil cell

Raman signatures of lattice dynamics across inversion symmetry breaking phase transition in quasi-1D compound, (TaSe4_4)3_3I

Structural phase transition can occur due to complex mechanisms other than simple dynamical instability, especially when the parent and daughter structure is of low dimension. This article reports such an inversion symmetry-breaking structural phase transition in a quasi-1D compound (TaSe$_4$)$_3$I at T$_S\sim$ 141~K studied by Raman spectroscopy. Our investigation of collective lattice dynamics reveals three additional Raman active modes in the low-temperature non-centrosymmetric structure. Two vibrational modes become Raman active due to the absence of an inversion center, while the third mode is a soft phonon mode resulting from the vibration of Ta atoms along the \{-Ta-Ta-\} chains. Furthermore, the most intense Raman mode display Fano-shaped asymmetry, inferred as the signature of strong electron-phonon coupling. The group theory and symmetry analysis of Raman spectra confirm the displacive-first-order nature of the structural transition. Therefore, our results establish (TaSe$_4)_3$I as a model system with broken inversion symmetry and strong electron-phonon coupling in the quasi-1D regime.Comment: Main text - 6 figures, 11 pages, supplementary - 10 figures, 13 page

Proximitized spin-phonon coupling in topological insulator due to two-dimensional antiferromagnet

Induced magnetic order in a topological insulator (TI) can be realized either by depositing magnetic adatoms on the surface of a TI or engineering the interface with epitaxial thin film or stacked assembly of two-dimensional (2D) van der Waals (vdW) materials. Herein, we report the observation of spin-phonon coupling in the otherwise non-magnetic TI Bi$_\mathrm{2}$Te$_\mathrm{3}$, due to the proximity of FePS$_\mathrm{3}$ (an antiferromagnet (AFM), $T_\mathrm{N}$ $\sim$ 120 K), in a vdW heterostructure framework. Temperature-dependent Raman spectroscopic studies reveal deviation from the usual phonon anharmonicity at/below 60 K in the peak position (self-energy) and linewidth (lifetime) of the characteristic phonon modes of Bi$_{2}$Te$_{3}$ (106 cm$^{-1}$ and 138 cm$^{-1}$) in the stacked heterostructure. The Ginzburg-Landau (GL) formalism, where the respective phonon frequencies of Bi$_{2}$Te$_{3}$ couple to phonons of similar frequencies of FePS$_3$ in the AFM phase, has been adopted to understand the origin of the hybrid magneto-elastic modes. At the same time, the reduction of characteristic $T_\mathrm{N}$ of FePS$_3$ from 120 K in isolated flakes to 65 K in the heterostructure, possibly due to the interfacial strain, which leads to smaller Fe-S-Fe bond angles as corroborated by computational studies using density functional theory (DFT). Besides, our data suggest a double softening of phonon modes of Bi$_\mathrm{2}$Te$_\mathrm{3}$ (at 30 K and 60 K), which in turn, demonstrates Raman scattering as a possible probe for delineating the magnetic ordering in bulk and surface of a hybrid topological insulator

Emergence of a Non-van der Waals Magnetic Phase in a van der Waals Ferromagnet

International audienceManipulation of long-range order in two-dimensional (2D) van der Waals (vdW) magnetic materials (e.g., CrI$_3$, CrSiTe$_3$ etc.), exfoliated in few-atomic layer, can be achieved via application of electric field, mechanical-constraint, interface engineering, or even by chemical substitution/doping. Usually, active surface oxidation due to the exposure in the ambient condition and hydrolysis in the presence of water/moisture causes degradation in magnetic nanosheets which, in turn, affects the nanoelectronic/spintronic device performance. Counterintuitively, our current study reveals that exposure to the air at ambient atmosphere results in advent of a stable nonlayered secondary ferromagnetic phase in the form of Cr$_2$Te$_3$ (T$_{C2}$~ 160 K) in the parent vdW magnetic semiconductor Cr$_2$Ge$_2$Te$_6$ (T$_{C1}$~ 69 K). In addition, the magnetic anisotropy energy (MAE) enhances in the hybrid by an order from the weakly anisotropic pristine Cr$_2$Ge$_2$Te$_6$ crystal, increasing the stability of the FM ground state with time. Comparing with the freshly prepared Cr$_2$Ge$_2$Te$_6$, the coexistence of the two ferromagnetic phases in the time elapsed bulk crystal is confirmed through systematic investigation of crystal structure along with detailed dc/ac magnetic susceptibility, specific heat, and magnetotransport measurement. To capture the concurrence of the two ferromagnetic phases in a single material, Ginzburg-Landau theory with two independent order parameters (as magnetization) with a coupling term can be introduced. In contrast to rather common poor environmental stability of the vdW magnets, our results open possibilities of finding air-stable novel materials having multiple magnetic phases