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.

Observation of Dirac Charge-Density Waves in Bi2Te2Se

While parallel segments in the Fermi level contours, often found at the surfaces of topological insulators (TIs), would imply "strong" nesting conditions, the existence of charge-density waves (CDWs)-periodic modulations of the electron density-has not been verified up to now. Here, we report the observation of a CDW at the surface of the TI Bi2Te2Se(111), below ≈350K, by helium-atom scattering and, thus, experimental evidence for a CDW involving Dirac topological electrons. Deviations of the order parameter observed below 180K, and a low-temperature break of time reversal symmetry, suggest the onset of a spin-density wave with the same period as the CDW in the presence of a prominent electron-phonon interaction, originating from Rashba spin-orbit coupling.

Nuclear Quantum Effects on the Charge-Density Wave Transition in NbX2 (X = S, Se)

Understanding the origin of charge-density wave (CDW) instability is important for manipulating novel collective electronic states. Many layered transition metal dichalcogenides (TMDs) share similarity in the structural and electronic instability, giving rise to diverse CDW phases and superconductivity. It is still puzzling that even isostructural and isoelectronic TMDs show distinct CDW features. For instance, bulk NbSe₂ exhibits CDW order at low temperature, while bulk NbS₂ displays no CDW instability. The CDW transitions in single-layer NbS₂ and NbSe₂ are also different. In the classic limit, we investigate the electron correlation effects on the dimensionality dependence of the CDW ordering. By performing ab initio path integral molecular dynamics simulations and comparative analyses, we further revealed significant nuclear quantum effects in these systems. Specifically, the quantum motion of sulfur anions significantly reduces the CDW transition temperature in both bulk and single-layer NbS₂, resulting in distinct CDW features in the NbS₂ and NbSe₂ systems.

Shape memory effect nanotools for nano-creation: examples of nanowire-based devices with charge density waves

Nanotweezers based on the shape memory effect have been developed and tested. In combination with a commercial nanomanipulator, they allow 3D nanoscale operation controlled in a scanning electron microscope. Here we apply the tweezers for the fabrication of nanostructures based on whiskers of NbS₃, a quasi one-dimensional compound with room-temperature charge density wave (CDW). The nanowhiskers were separated without damage from the growth batch, an entangled array, and safely transferred to a substrate with a preliminary deposited Au film. The contacts were fabricated with Pt sputtering on top of the whisker and the film. The high degree of synchronization of the sliding CDW under a RF field with a frequency up to 600 MHz confirms the high quality of the contacts and of the sample structure after the manipulations. The proposed technique paves the way to novel type micro- and nanostructures fabrication and their various applications.

Coherent modulation of the electron temperature and electron-phonon couplings in a 2D material

Ultrashort light pulses can selectively excite charges, spins, and phonons in materials, providing a powerful approach for manipulating their properties. Here we use femtosecond laser pulses to coherently manipulate the electron and phonon distributions, and their couplings, in the charge-density wave (CDW) material 1T-TaSe₂ After exciting the material with a femtosecond pulse, fast spatial smearing of the laser-excited electrons launches a coherent lattice breathing mode, which in turn modulates the electron temperature. This finding is in contrast to all previous observations in multiple materials to date, where the electron temperature decreases monotonically via electron-phonon scattering. By tuning the laser fluence, the magnitude of the electron temperature modulation changes from ∼200 K in the case of weak excitation, to ∼1,000 K for strong laser excitation. We also observe a phase change of π in the electron temperature modulation at a critical fluence of 0.7 mJ/cm², which suggests a switching of the dominant coupling mechanism between the coherent phonon and electrons. Our approach opens up routes for coherently manipulating the interactions and properties of two-dimensional and other quantum materials using light.

Yu-Shiba-Rusinov States in the Charge-Density Modulated Superconductor NbSe2

NbSe₂ is a remarkable superconductor in which charge-density order coexists with pairing correlations at low temperatures. Here, we study the interplay of magnetic adatoms and their Yu-Shiba-Rusinov (YSR) bound states with the charge density order. Exploiting the incommensurate nature of the charge-density wave (CDW), our measurements provide a thorough picture of how the CDW affects both the energies and the wave functions of the YSR states. Key features of the dependence of the YSR states on adsorption site relative to the CDW are explained by model calculations. Several properties make NbSe₂ a promising substrate for realizing topological nanostructures. Our results will be important in designing such systems.

Raman Spectroscopic and Dynamic Electrical Investigation of Multi-State Charge-Wave-Density Phase Transitions in 1 T-TaS2

Two-dimensional layered 1 T-TaS₂ exhibits rich charge-density-wave (CDW) states with distinct electronic structures and physical properties, leading to broad potential applications, such as phase-transition memories, electrical oscillators and photodetectors. Besides the various CDW ground states at different temperatures, multiple intermediate phases in 1 T-TaS₂ have been observed by applying optical and electrical stimulations. Here, we investigated the electric-field-driven multistate CDW phase transition by Raman spectroscopy and voltage oscillations in 1 T-TaS₂. Strong correlation was observed between electrical conductivity and intensity of fold-back acoustic and optical phonon modes in 1 T-TaS₂. This indicates that the multistate transitions arise from serial transitions, from the nearly commensurate (NC) CDW phase to out-of-equilibrium intermediate states, and finally to the incommensurate (IC) CDW phase. The dynamics of phase transition under an electric field was investigated. As the electrical field increased, the dwell time of different CDW states changed. At lower temperatures, the multistate oscillations disappeared because of higher-energy barriers between the intermediate phases and/or lower thermal excitation energies at lower temperatures.

Gate-Controlled Metal-Insulator Transition in TiS3 Nanowire Field-Effect Transistors

We explore the electrical characteristics of TiS₃ nanowire field-effect transistor (FETs), over the wide temperature range from 3 to 350 K. These nanomaterials have a quasi-one-dimensional (1D) crystal structure and exhibit a gate-controlled metal-insulator transition (MIT) in their transfer curves. Their room-temperature mobility is ∼20-30 cm²/(V s), 2 orders of magnitude smaller than predicted previously, a result that we explain quantitatively in terms of the influence of polar-optical phonon scattering in these materials. In the insulating state (<∼220 K), the transfer curves exhibit unusual mesoscopic fluctuations and a current suppression near zero bias that is common to charge-density wave (CDW) systems. The fluctuations have a nonmonotonic temperature dependence and wash out at a temperature close to that of the bulk MIT, suggesting they may be a feature of quantum interference in the CDW state. Overall, our results demonstrate that quasi-1D TiS₃ nanostructures represent a viable candidate for FET realization and that their functionality is influenced by complex phenomena.

Electronic Structure and Enhanced Charge-Density Wave Order of Monolayer VSe2

How the interacting electronic states and phases of layered transition-metal dichalcogenides evolve when thinned to the single-layer limit is a key open question in the study of two-dimensional materials. Here, we use angle-resolved photoemission to investigate the electronic structure of monolayer VSe₂ grown on bilayer graphene/SiC. While the global electronic structure is similar to that of bulk VSe₂, we show that, for the monolayer, pronounced energy gaps develop over the entire Fermi surface with decreasing temperature below T_c = 140 ± 5 K, concomitant with the emergence of charge-order superstructures evident in low-energy electron diffraction. These observations point to a charge-density wave instability in the monolayer that is strongly enhanced over that of the bulk. Moreover, our measurements of both the electronic structure and of X-ray magnetic circular dichroism reveal no signatures of a ferromagnetic ordering, in contrast to the results of a recent experimental study as well as expectations from density functional theory. Our study thus points to a delicate balance that can be realized between competing interacting states and phases in monolayer transition-metal dichalcogenides.

Unveiling Charge-Density Wave, Superconductivity, and Their Competitive Nature in Two-Dimensional NbSe2

Recently, charge-density wave (CDW) and superconductivity are observed to coexist in atomically thin metallic NbSe₂. Lacking of knowledge on the structural details of CDW, however, prevents us to explore its interplay with superconductivity. Using first-principles calculations, we identify the ground state 3 × 3 CDW atomic structure of monolayer NbSe₂, which is characterized by the formation of triangular Nb clusters and shows a scanning tunnelling microscopy (STM) image and Raman CDW modes in good agreement with experiments. We further demonstrate that from bulk to monolayer NbSe₂, as the layer thickness decreases, the CDW order is gradually enhanced with rising energy gain and strengthened Fermi surface gapping, while superconductivity is weakened due to the increasingly reduced Fermi level density of states in the CDW state. These results well explain the observed opposite thickness dependencies of CDW and superconducting transition temperatures and uncover the nature of competitive interaction between the two collective orders in two-dimensional NbSe₂.

Atomic lattice disorder in charge-density-wave phases of exfoliated dichalcogenides (1T-TaS2)

Charge-density waves (CDWs) and their concomitant periodic lattice distortions (PLDs) govern the electronic properties in several layered transition-metal dichalcogenides. In particular, 1T-TaS₂ undergoes a metal-to-insulator phase transition as the PLD becomes commensurate with the crystal lattice. Here we directly image PLDs of the nearly commensurate (NC) and commensurate (C) phases in thin, exfoliated 1T-TaS₂ using atomic resolution scanning transmission electron microscopy at room and cryogenic temperature. At low temperatures, we observe commensurate PLD superstructures, suggesting ordering of the CDWs both in- and out-of-plane. In addition, we discover stacking transitions in the atomic lattice that occur via one-bond-length shifts. Interestingly, the NC PLDs exist inside both the stacking domains and their boundaries. Transitions in stacking order are expected to create fractional shifts in the CDW between layers and may be another route to manipulate electronic phases in layered dichalcogenides.

Polytypism, polymorphism, and superconductivity in TaSe(2-x)Te(x)

Polymorphism in materials often leads to significantly different physical properties--the rutile and anatase polymorphs of TiO2 are a prime example. Polytypism is a special type of polymorphism, occurring in layered materials when the geometry of a repeating structural layer is maintained but the layer-stacking sequence of the overall crystal structure can be varied; SiC is an example of a material with many polytypes. Although polymorphs can have radically different physical properties, it is much rarer for polytypism to impact physical properties in a dramatic fashion. Here we study the effects of polytypism and polymorphism on the superconductivity of TaSe2, one of the archetypal members of the large family of layered dichalcogenides. We show that it is possible to access two stable polytypes and two stable polymorphs in the TaSe(2-x)Te(x) solid solution and find that the 3R polytype shows a superconducting transition temperature that is between 6 and 17 times higher than that of the much more commonly found 2H polytype. The reason for this dramatic change is not apparent, but we propose that it arises either from a remarkable dependence of Tc on subtle differences in the characteristics of the single layers present or from a surprising effect of the layer-stacking sequence on electronic properties that are typically expected to be dominated by the properties of a single layer in materials of this kind.