Charge Transfer Effects in Naturally Occurring van der Waals Heterostructures (PbSe)_{1.16}(TiSe_{2})_{m} (m=1, 2)

Thermally Activated Photoluminescence Induced by Tunable Interlayer Interactions in Naturally Occurring van der Waals Superlattice SnS/TiS2

van der Waals (vdW) superlattices, comprising different 2D materials aligned alternately by weak interlayer interactions, offer versatile structures for the fabrication of novel semiconductor devices. Despite their potential, the precise control of optoelectronic properties with interlayer interactions remains challenging. Here, we investigate the discrepancies between the SnS/TiS2 superlattice (SnTiS3) and its subsystems by comprehensive characterization and DFT calculations. The disappearance of certain Raman modes suggests that the interactions alter the SnS subsystem structure. Specifically, such structural changes transform the band structure from indirect to direct band gap, causing a strong PL emission (∼2.18 eV) in SnTiS3. In addition, the modulation of the optoelectronic properties ultimately leads to the unique phenomenon of thermally activated photoluminescence. This phenomenon is attributed to the inhibition of charge transfer induced by tunable intralayer strains. Our findings extend the understanding of the mechanism of interlayer interactions in van der Waals superlattices and provide insights into the design of high-temperature optoelectronic devices.

Modulating Charge-Density Wave Order and Superconductivity from Two Alternative Stacked Monolayers in a Bulk 4Hb-TaSe2 Heterostructure via Pressure

Two-dimensional (2D) van der Waals heterostructures (VDWHs) containing a charge-density wave (CDW) and superconductivity (SC) have revealed rich tunability in their properties, which provide a new route for optimizing their novel exotic states. The interaction between SC and CDW is critical to its properties; however, understanding this interaction within VDWHs is very limited. A comprehensive in situ study and theoretical calculation on bulk 4Hb-TaSe₂ VDWHs consisting of alternately stacking 1T-TaSe₂ and 1H-TaSe₂ monolayers are investigated under high pressure. Surprisingly, the superconductivity competes with the intralayer and adjacent-layer CDW order in 4Hb-TaSe₂, which results in substantially and continually boosted superconductivity under compression. Upon total suppression of the CDW, the superconductivity in the individual layers responds differently to the charge transfer. Our results provide an excellent method to efficiently tune the interplay between SC and CDW in VDWHs and a new avenue for designing materials with tailored properties.

Moiré enhanced charge density wave state in twisted 1T-TiTe2/1T-TiSe2 heterostructures

Nanoscale periodic moiré patterns, for example those formed at the interface of a twisted bilayer of two-dimensional materials, provide opportunities for engineering the electronic properties of van der Waals heterostructures^1-11. In this work, we synthesized the epitaxial heterostructure of 1T-TiTe₂/1T-TiSe₂ with various twist angles using molecular beam epitaxy and investigated the moiré pattern induced/enhanced charge density wave (CDW) states with scanning tunnelling microscopy. When the twist angle is near zero degrees, 2 × 2 CDW domains are formed in 1T-TiTe₂, separated by 1 × 1 normal state domains, and trapped in the moiré pattern. The formation of the moiré-trapped CDW state is ascribed to the local strain variation due to atomic reconstruction. Furthermore, this CDW state persists at room temperature, suggesting its potential for future CDW-based applications. Such moiré-trapped CDW patterns were not observed at larger twist angles. Our study paves the way for constructing metallic twist van der Waals bilayers and tuning many-body effects via moiré engineering.

Observation of an Incommensurate Charge Density Wave in Monolayer TiSe_{2}/CuSe/Cu(111) Heterostructure

TiSe_{2} is a layered material exhibiting a commensurate (2×2×2) charge density wave (CDW) with a transition temperature of ∼200  K. Recently, incommensurate CDW in bulk TiSe_{2} draws great interest due to its close relationship with the emergence of superconductivity. Here, we report an incommensurate superstructure in monolayer TiSe_{2}/CuSe/Cu(111) heterostructure. Characterizations by low-energy electron diffraction and scanning tunneling microscopy show that the main wave vector of the superstructure is ∼0.41a^{*} or ∼0.59a^{*} (here a^{*} is in-plane reciprocal lattice constant of TiSe_{2}). After ruling out the possibility of moiré superlattices, according to the correlation of the wave vectors of the superstructure and the large indirect band gap below the Fermi level, we propose that the incommensurate superstructure is associated with an incommensurate charge density wave (I-CDW). It is noteworthy that the I-CDW is robust with a transition temperature over 600 K, much higher than that of commensurate CDW in pristine TiSe_{2}. Based on our data and analysis, we present that interface effect may play a key role in the formation of the I-CDW state.

Synthesis and Electron Microscopy Study of the Quaternary Misfit Layer Chalcogenides {(Bi,Nd)S}1+δCrS2 and {(Pb,Nd)Se}1+δ(NbSe2)2

In the present work, two compounds, in the Bi-Nd-Cr-S and Pb-Nd-Nb-Se systems, not reported to date were synthesized. The chemical composition and the structural determination of these complex compounds, at atomic resolution, was performed through conventional and aberration-corrected electron microscopy including selected area electron diffraction, high resolution (HR) transmission electron microscopy (TEM), HR scanning TEM, and the analytical associated techniques X-ray energy-dispersive spectroscopy and electron energy-loss spectroscopy. The average compositions are [(Bi_0.4,Nd_0.6)S]_1.25CrS₂ and [(Pb_0.5,Nd_0.5)Se]_1.15(Nb_1.0Se₂)₂, respectively. By using these electron microscopy techniques, we confirmed that both compounds can be described in term of two interpenetrated sublattices that fit along a but do not fit along b, giving rise to an incommensurate modulation. A closer inspection along the stacking direction of the subcell has provided an ideal structural model for [(Bi_0.4,Nd_0.6)S]_1.25CrS₂ based on the intergrowth of one layer of CrS₂, three atoms thick, (111) B1 type, and one layer of (Bi, Nd)Se, two atoms thick, (100) B1 type. In [(Pb_0.5,Nd_0.5)Se]_1.15(Nb_1.0Se₂)₂ we found that two layers of NbSe₂, which adopt the 2H-NbSe₂ polytype, alternate with one layer of (Pb, Nd)Se B1 type. In addition, crystals showing extended defects, associated with the weak interaction between the layers, were frequently found.

Ultrafast charge ordering by self-amplified exciton-phonon dynamics in TiSe2

The origin of charge density waves (CDWs) in TiSe\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{2}$$\end{document}2 has long been debated, mainly due to the difficulties in identifying the timescales of the excitonic pairing and electron–phonon coupling (EPC). Without a time-resolved and microscopic mechanism, one has to assume simultaneous appearance of CDW and periodic lattice distortions (PLD). Here, we accomplish a complete separation of ultrafast exciton and PLD dynamics and unravel their interplay in our real-time time-dependent density functional theory simulations. We find that laser pulses knock off the exciton order and induce a homogeneous bonding–antibonding transition in the initial 20 fs, then the weakened electronic order triggers ionic movements antiparallel to the original PLD. The EPC comes into play after the initial 20 fs, and the two processes mutually amplify each other leading to a complete inversion of CDW ordering. The self-amplified dynamics reproduces the evolution of band structures in agreement with photoemission experiments. Hence we resolve the key processes in the initial dynamics of CDWs that help elucidate the underlying mechanism.

The physical origins of charge density waves in 1T-TiSe2 and their response to ultrafast excitation have long been a topic of theoretical and experimental debate. Here the authors present an ab initio theory that successfully captures the observed dynamics of charge density wave formation.

Monolayer Behavior of NbS2 in Natural van der Waals Heterostructures

Monolayer transition metal dichalcogenides (TMDs) constitute an important family of materials with many intriguing properties and applications. The ability to achieve large-size and high-quality monolayer TMDs is the key prerequisite toward a deep understanding and practical application of TMDs in electronics and optoelectronics. Here, on the basis of high-resolution angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS), we find a monolayer NbS₂-dominated Fermi-level feature in a misfit compound, which is a type of natural heterostructures. Considering the infrequency of the synthesis approach and electronic properties of the NbS₂ monolayer, our results cannot only provide direct insight into the electronic structure of van der Waals heterostructures (VDWHs) but also shed light on the way toward rationally investigating  the monolayer TMDs, which are hardly obtained and studied.

Superconductivity in a misfit layered compound (SnSe)1.16(NbSe2)

The large size single crystals of (SnSe)_1.16(NbSe₂) misfit layered compound were grown and superconductivity with T _c of 3.4 K was first discovered in this system. Powder x-ray diffraction and high resolution transmission electron microscopy clearly display the misfit feature between SnSe and NbSe₂ subsystems. The Sommerfeld coefficient γ inferred from specific-heat measurements is 16.73 mJ mol^-1 K^-2, slightly larger than the usual misfit compounds. The normalized specific heat jump [Formula: see text] is about 0.98, and the electron-phonon coupling constant [Formula: see text] is estimated to be 0.80. The estimated value of the in-plane upper critical magnetic field, [Formula: see text](0), is about 7.82 T, exceeding the Pauli paramagnetic limit slightly. Both the specific-heat and H _c2 data suggest that (SnSe)_1.16(NbSe₂) is a multi-band superconductor.