Commensurate Charge Density Wave Phase Research Articles

Photodoping-modified charge density wave phase transition in WS2/1T-TaS2 heterostructure

Abstract Controlling collective electronic states holds great promise for the development of innovative devices. Here, we experimentally detect the modification of the charge density wave (CDW) phase transition within a 1T-TaS2 layer in a WS2/1T-TaS2 heterostructure using time-resolved ultrafast spectroscopy. Laser-induced charge transfer doping strongly suppresses the commensurate CDW (CCDW) phase, which results in a significant decrease in both the phase transition temperature (T c) and phase transition stiffness (PTS). We interpret that photo-induced hole doping, when surpassing a critical threshold value of ~1018/cm3, sharply decreases the phase transition energy barrier. Our results provide new insights into controlling the CDW phase transition, paving the way for optical-controlled novel devices based on CDW materials.

Surface properties of 1T-TaS2 and contrasting its electron-phonon coupling with TlBiTe2 from helium atom scattering.

We present a detailed helium atom scattering study of the charge-density wave (CDW) system and transition metal dichalcogenide 1T-TaS2. In terms of energy dissipation, we determine the electron-phonon (e-ph) coupling, a quantity that is at the heart of conventional superconductivity and may even "drive" phase transitions such as CDWs. The e-ph coupling of TaS2 in the commensurate CDW phase (λ = 0.59 ± 0.12) is compared with measurements of the topo-logical insulator TlBiTe2 (λ = 0.09 ± 0.01). Furthermore, by means of elastic He diffraction and resonance/interference effects in He scattering, the thermal expansion of the surface lattice, the surface step height, and the three-dimensional atom-surface interaction potential are determined including the electronic corrugation of 1T-TaS2. The linear thermal expansion coefficient is similar to that of other transition-metal dichalcogenides. The He-TaS2 interaction is best described by a corrugated Morse potential with a relatively large well depth and supports a large number of bound states, comparable to the surface of Bi2Se3, and the surface electronic corrugation of 1T-TaS2 is similar to the ones found for semimetal surfaces.

Observation of the metallic mosaic phase in 1T−TaS2 at equilibrium

The transition-metal dichalcogenide tantalum disulphide ($1\mathit{T}\text{\ensuremath{-}}\mathrm{TaS}{}_{2}$) hosts a commensurate charge density wave (CCDW) at temperatures below 165 K where it also becomes insulating. The low temperature CCDW phase can be driven into a metastable ``mosaic'' phase by means of either laser or voltage pulses, which shows a large density of CDW domain walls as well as a closing of the electronic band gap. The exact origins of this pulse-induced metallic mosaic are not yet fully understood. Here, using scanning tunneling microscopy and spectroscopy (STM/STS), we observe the occurrence of such a metallic mosaic phase on the surface of $\mathrm{TaS}{}_{2}$ without prior pulse excitation over continuous areas larger than $100\ifmmode\times\else\texttimes\fi{}100\phantom{\rule{0.16em}{0ex}}\mathrm{nm}{}^{2}$ and macroscopic areas on the millimeter scale. We attribute the appearance of the mosaic phase to the presence of surface defects which cause the formation of the characteristic dense domain wall network. Based on our STM measurements, we further argue how the appearance of the metallic behavior in the mosaic phase could be explained by local stacking differences of the top layer. Thus we provide a potential avenue to explain the origin of the pulse-induced mosaic phase.

Incommensurate charge density wave in multiband intermetallic systems exhibiting competing orders

The appearance of an incommensurate charge density wave vector $\textbf{Q} = (Q_x,Q_y)$ on multiband intermetallic systems presenting commensurate charge density wave (CDW) and superconductivity (SC) orders is investigated. We consider a two-band model in a square lattice, where the bands have distinct effective masses. The incommensurate CDW (inCDW) and CDW phases arise from an interband Coulomb repulsive interaction, while the SC emerges due to a local intraband attractive interaction. For simplicity, all the interactions, the order parameters and hybridization between bands are considered $\textbf{k}$-independent. The multiband systems that we are interested are intermetallic systems with a $d$-band coexisting with a large $c$-band, for which a mean-field approach has proved suitable. We obtain the eigenvalues and eigenvectors of the Hamiltonian numerically and minimize the free energy density with respect to the diverse parameters of the model by means of the Hellmann-Feynman theorem. We investigate the system in real as well as momentum space and we find an inCDW phase with wave vector $\textbf{Q} = (\pi, Q_y) = (Q_x, \pi)$. Our numerical results show that the arising of an inCDW state depends on parameters, such as the magnitude of the inCDW and CDW interactions, band filling, hybridization and the relative depth of the bands. In general, inCDW tends to emerge at low temperatures, away from half-filling. We also show that, whether the CDW ordering is commensurate or incommensurate, large values of the relative depth between bands may suppress it. We discuss how each parameter of the model affects the emergence of an inCDW phase.

Charge density wave and crystalline electric field effects inTmNiC2

Single crystals of ${\mathrm{TmNiC}}_{2}$ were grown by the optical floating-zone technique and were investigated by x-ray diffraction (XRD), thermal expansion, electrical resistivity, specific heat, and magnetic susceptibility measurements. Single-crystal XRD reveals the formation of a commensurate charge density wave (CDW) characterized by a CDW modulation vector ${\mathbf{q}}_{\mathbf{2}\mathbf{c}}=(0.5,0.5,0.5)$, which is accompanied by a symmetry change from the orthorhombic space group $Amm2$ to the monoclinic space group $Cm$, i.e., to a CDW superstructure which is isostructural with that of ${\mathrm{LuNiC}}_{2}$. For all transport and thermodynamic properties, anomalies related to a second order-type thermodynamic CDW phase transition are observed at around ${T}_{\mathrm{CDW}}\ensuremath{\simeq}375\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The large specific heat anomaly at ${T}_{\mathrm{CDW}}, \mathrm{\ensuremath{\Delta}}C\ensuremath{\simeq}6.2\phantom{\rule{4pt}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}{\mathrm{mol}}^{\ensuremath{-}1}{\mathrm{K}}^{\ensuremath{-}1}$, together with noticeable changes in entropy and enthalpy related to the CDW transition, suggests that this point group symmetry breaking CDW phase transition affects more significant parts of the Fermi surface as compared to the incommensurate CDW transition of, e.g., ${\mathrm{SmNiC}}_{2}$ with no change in point group symmetry. The results on the antiferromagnetic and paramagnetic state of ${\mathrm{TmNiC}}_{2}$ obtained by the above macroscopic techniques were complemented by microscopic studies via inelastic neutron scattering. A crystalline electric field modeling of macroscopic susceptibility and magnetic specific heat and entropy contributions as well as microscopic neutron scattering data, reveal crystal field eigenstates and eigenvalues with a ground-state doublet of the Tm-$4f$ electrons, which is well separated by about 25 meV from exited states of the $J=6$ ground-state multiplet.

Distinguishing and controlling Mottness in 1T−TaS2 by ultrafast light

Distinguishing and controlling the extent of Mottness is important for materials where the energy scales of the on-site Coulomb repulsion $U$ and the bandwidth $W$ are comparable. Here, we report the ultrafast electronic dynamics of $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ by ultrafast time- and angle-resolved photoemission spectroscopy. A comparison of the electron dynamics for the intermediate phase (I-phase) as well as the low-temperature commensurate charge density wave (C-CDW) phase shows distinctive dynamics. While the I-phase is characterized by an instantaneous response and nearly time-resolution-limited fast relaxation ($\ensuremath{\sim}200$ fs), the C-CDW phase shows a delayed response and a slower relaxation (a few ps). Such distinctive dynamics reflect the different relaxation mechanisms and provide nonequilibrium signatures to distinguish the Mott insulating I-phase from the C-CDW band insulating phase. Moreover, a light-induced bandwidth reduction is observed in the C-CDW phase, pushing it toward the Mott insulating phase. Our work demonstrates the power of an ultrafast light-matter interaction in both distinguishing and controlling the extent of Mottness on the ultrafast timescale.

Nanoscale Friction across the First-Order Charge Density Wave Phase Transition of 1T-TaS2.

Nanotribology using atomic force microscopy (AFM) can be considered as a unique approach to analyze phase transition materials by localized mechanical interaction. In this work, we investigate friction on the lamellar transition metal dichalcogenide 1T-TaS2, which can undergo first-order charge density wave (CDW) phase transitions. Based on temperature-dependent atomic force microscopy under ultrahigh vacuum conditions (UHV), we can characterize the general friction levels across the first-order phase transitions and for the different phases. While structural and electronic properties for different phases appear to be of minor influence on friction, a distinct peak in friction is observed during the phase transition when cooling the sample from the nearly commensurate CDW (NC-CDW) phase to the commensurate CDW (C-CDW) phase. By performing systematic measurements as a function of load, scan velocity, and scan time, a recently proposed friction mechanism can be corroborated, where the AFM tip gradually induces local transformations of the material close to the spinodal point in a thermally activated and shear-assisted process until the surface is fully "harvested". Our results demonstrate that repeated nanomechanical stress can trigger local first-order phase transitions constituting a so far little explored mechanical energy dissipation channel.

Thickness dependent charge density wave networks on thin 1T-TaS$$_2$$

We investigate mechanically exfoliated thin 1T-TaS $$_2$$ with scanning tunneling microscopy at room temperature. Sample preparation without air exposure enables access to intrinsic charge-density-wave (CDW) phases of thin 1T-TaS $$_2$$ . At room temperature, we can observe the expected nearly commensurate CDW (NCCDW) phase on thin flakes similar to bulk 1T-TaS $$_2$$ . Further analysis reveals that CDW domains in the NCCDW phase become smaller and have a more anisotropic shape with decreasing thickness in the range of 8–28 layers. Our findings demonstrate that the anisotropic CDW nature of thin 1T-TaS $$_2$$ would be crucial to understand its exotic CDW-related phenomena and demand a systematic study on its correlation between the thickness-driven CDW domain anisotropy and the intermediate CDW states in thin 1T-TaS $$_2$$ .

Understanding the magnetic response of the Quantum Spin Liquid compound 1[formula omitted]-TaS[formula omitted]

1T-tantalum disulphide (1T-TaS2) is a promising material to realize quantum spin liquid state. Here, we take a closer look on the temperature dependent magnetic susceptibility of 1T-TaS2 at low magnetic fields to understand the formation of local magnetic moment in this system and its response as a function of temperature and field. We found that the susceptibility increases with reducing temperature even in the nearly commensurate charge density wave (CDW) phase, which exhibits metallic conductivity and hence is expected to exhibit temperature independent Pauli paramagnetic susceptibility. It indicates that local moment starts forming in the nearly commensurate CDW phase itself, which is well above the temperature where evidence of Mott insulating state appears in resistivity measurement. In the commensurate CDW phase, susceptibility significantly deviates from the Curie–Weiss law. Below T = 10 K, temperature dependence of susceptibility indicates the formation of random singlet state in 1T-TaS2.

Hidden electronic phase in strained few-layer 1T−TaS2

Layered van der Waals materials are exciting as they often host multiple, competing electronic phases. This article reports experimental observation of the co-existence of insulating and metallic phases deep within the commensurate charge density wave phase in high-quality devices of few-layer 1T-TaS2. Through detailed conductance fluctuation spectroscopy of the electronic ground state, we establish that the mixed-phase consists of insulating regions surrounded by one-dimensional metallic domain walls. We show that the electronic ground state of 1T-TaS2 can be affected drastically by strain, eventually leading to the collapse of the Mott gap in the commensurate charge density wave phase. Our study resolves an outstanding question, namely the effect of the inter-layer coupling strength on the electronic phases in layered van der Waals materials.

Observation of a topological defect lattice in the charge density wave of 1T-TaS2

Charge density wave (CDW) phases native to the transition metal dichalcogenide 1T-TaS2 are probed through a graphene layer covering its surface by using local scanning tunneling microscopy/spectroscopy and transport measurements. We show that over a wide range of temperatures, the graphene cover, while protecting the air-sensitive 1T-TaS2 from oxidation, does not obstruct access to the electronic properties of the CDW. At room temperature, where the system is in the nearly commensurate CDW phase, we observe contiguous domains of the CDW phase separated by a network of topological defects. These topological defects, dubbed CDW vortices, form a lattice with a quasi-long range translational order, which resembles the Bragg glass phase in Abrikosov vortex lattices in type-II superconductors. While the quasi-long range order in the CDW itself is not uncommon, its existence in the CDW vortex lattice is observed here for the first time. Finally, when reducing the temperature to 78 K into the commensurate CDW phase, we find that the graphene cover layer acquires properties consistent with a proximity-induced CDW.

Room Temperature Commensurate Charge Density Wave on Epitaxially Grown Bilayer 2H-Tantalum Sulfide on Hexagonal Boron Nitride.

The breaking of multiple symmetries by periodic lattice distortion at a commensurate charge density wave (CDW) state is expected to give rise to intriguing interesting properties. However, accessing the commensurate CDW state on bulk TaS2 crystals typically requires cryogenic temperatures (77 K), which precludes practical applications. Here, we found that heteroepitaxial growth of a 2H-tantalum disulfide bilayer on a hexagonal-boron nitride (h-BN) substrate produces a robust commensurate CDW order at room temperature, characterized by a Moiré superlattice of 3 × 3 TaS2 on a 4 × 4 h-BN unit cell. The CDW order is confirmed by scanning transmission electron microscopy and Raman measurements. Theoretical calculations reveal that the stabilizing energy for the CDW phase of the monolayer and bilayer 2H-TaS2-on-h-BN substrates arises primarily from interfacial electrostatic interactions and, to a lesser extent, interfacial strain. Our work shows that engineering interfacial electrostatic interactions in an ultrathin van der Waals heterostructure constitutes an effective way to enhance CDW order in two-dimensional materials.

Scanning tunneling spectroscopic study of monolayer 1T-TaS2 and 1T-TaSe2

The isostructural and isoelectronic transition-metal-dichalcogenides 1T-TaS2 and 1T-TaSe2 are layered materials with intricate electronic structures. Combining the molecular beam epitaxy growth, scanning tunneling microscopy measurements and first-principles calculations, we prepare monolayer 1T-TaS2 and TaSe2 and explore their electronic structures at the atomic scale. Both two-dimensional (2D) compounds exhibit commensurate charge density wave phase at low temperature. The conductance mapping identifies the contributions from different Ta atoms to the local density of states with spatial and energy resolution. Both 1T-TaS2 and 1T-TaSe2 monolayer are shown to be insulators, while the former has a Mott gap and the latter is a regular band insulator.

Suppression of the commensurate charge density wave phase in ultrathin 1T−TaS2 evidenced by Raman hyperspectral analysis

Using temperature dependent Raman spectroscopy we address the question of how the transition from bulk to few atomic layers affects the charge density wave (CDW) phases in 1T-TaS2. We find that for crystals with thickness larger than approx 10nm the transition temperatures between the different phases as well as the hysteresis that occurs in the thermal cycle correspond to the ones expected for a bulk sample. However, when the crystals become thinner than $\approx 10nm$, the commensurate CDW phase is suppressed down to the experimentally accessible temperatures. In addition, the nearly commensurate CDW phase is diminished below approx 4nm. These findings suggest that the interlayer coupling plays a significant role in determining the properties of CDW systems consisting of a few unit cells in the vertical direction.

Lattice Discontinuities of 1T-TaS2 across First Order Charge Density Wave Phase Transitions

Transition metal dichalcogenides are lamellar materials which can exhibit unique and remarkable electronic behavior due to effects of electron-electron and electron-phonon coupling. Among these materials, 1T-tantalum disulfide (1T-TaS2) has spurred considerable interest, due to its multiple first order phase transitions between different charge density wave (CDW) states. In general, the basic effects of charge density wave formation in 1T-TaS2 can be attributed to in plane re-orientation of Ta-atoms during the phase transitions. Only in recent years, an increasing number of studies has also emphasized the role of interlayer interaction and stacking order as a crucial aspect to understand the specific electronic behavior of 1T-TaS2, especially for technological systems with a finite number of layers. Obviously, continuously monitoring the out of plane expansion of the sample can provide direct inside into the rearrangement of the layer structure during the phase transition. In this letter, we therefore investigate the c-axis lattice discontinuities of 1T-TaS2 by atomic force microscopy (AFM) method under ultra-high vacuum conditions. We find that the c-axis lattice experiences a sudden contraction across the nearly-commensurate CDW (NC-CDW) phase to commensurate CDW (C-CDW) phase transition during cooling, while an expansion is found during the transition from the C-CDW phase to a triclinic CDW phase during heating. Thereby our measurements reveal, how higher order C-CDW phase can favor a more dense stacking. Additionally, our measurements also show subtler effects like e.g. two expansion peaks at the start of the transitions, which can provide further insight into the mechanisms at the onset of CDW phase transitions.

Modulating Charge Density Wave Order in a 1T-TaS2/Black Phosphorus Heterostructure.

Controllability of collective electron states has been a long-sought scientific and technological goal and promises development of new devices. Herein, we investigate the tuning of charge density wave (CDW) in 1T-TaS2 via a two-dimensional (2D) van der Waals heterostructure of 1T-TaS2/BP. Unusual gate-dependent conductance oscillations were observed in 1T-TaS2 nanoflake supported on BP in transport measurements. Scanning tunneling microscopy study shows that the nearly commensurate (NC) CDW phase survived to 4.5 K in this system, which is substantially lower than the NC to commensurate CDW phase transition temperature of 180 K. A Coulomb blockade model was invoked to explain the conductance oscillations, where the domain walls and domains in NC phase serve as series of quantum dot arrays and tunnelling barriers, respectively. Density functional theory calculations show that a range of interfacial interactions, including strain and charge transfer, influences the CDW stabilities. Our work sheds light on tuning CDW orders via 2D heterostructure stacking and provides new insights on the CDW phase transition and sliding mechanism.

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.

Directional sub-femtosecond charge transfer dynamics and the dimensionality of 1T-TaS2

For the layered transition metal dichalcogenide 1T-TaS2, we establish through a unique experimental approach and density functional theory, how ultrafast charge transfer in 1T-TaS2 takes on isotropic three-dimensional character or anisotropic two-dimensional character, depending on the commensurability of the charge density wave phases of 1T-TaS2. The X-ray spectroscopic core-hole-clock method prepares selectively in- and out-of-plane polarized sulfur 3p orbital occupation with respect to the 1T-TaS2 planes and monitors sub-femtosecond wave packet delocalization. Despite being a prototypical two-dimensional material, isotropic three-dimensional charge transfer is found in the commensurate charge density wave phase (CCDW), indicating strong coupling between layers. In contrast, anisotropic two-dimensional charge transfer occurs for the nearly commensurate phase (NCDW). In direct comparison, theory shows that interlayer interaction in the CCDW phase – not layer stacking variations – causes isotropic three-dimensional charge transfer. This is presumably a general mechanism for phase transitions and tailored properties of dichalcogenides with charge density waves.

Electronic structure of commensurate, nearly commensurate, and incommensurate phases of 1T−TaS2 by angle-resolved photoelectron spectroscopy, scanning tunneling spectroscopy, and density functional theory

The electronic structure of $1T\ensuremath{-}\mathrm{Ta}{\mathrm{S}}_{2}$ showing a metal-insulator transition and a sequence of different charge density wave (CDW) transformations was discussed in the frame of variable temperature angle-resolved photoelectron spectroscopy (ARPES), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations. For the commensurate charge density wave phase (CCDW) the Mott gap was estimated to be 0.4 eV and energy gaps ${\mathrm{\ensuremath{\Delta}}}_{\text{CCDW},1},\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Delta}}}_{\text{CCDW},2},\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Delta}}}_{B3\ensuremath{-}\mathit{HHB}},\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Delta}}}_{B4\ensuremath{-}B3}$ were observed. For the nearly commensurate charge density wave phase (NCCDW), the reminiscent of higher and lower Hubbard bands and a very pronounced electronic state associated with the parabolic band at the $\overline{\mathrm{\ensuremath{\Gamma}}}$ point in the Brillouin zone were identified. The incommensurate charge density wave phase (ICCDW) showed a high value of local density of states at the Fermi level and a very pronounced edge of the metallic surface state located in the range of 0.15--0.20 eV above the Fermi level. The obtained STS and ARPES results were consistent with our theoretical calculations performed within DFT formalism including spin-orbit coupling.

Photoinduced discommensuration of the commensurate charge-density wave phase in 1T−TaS2

The dynamics induced by femtosecond-laser excitation of the commensurate phase of the charge-density wave (CDW) in $1T\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$ have been studied using both time-resolved electron diffraction and the time-resolved spectroscopy of coherent-phonon dynamics. Electron diffraction results show that the commensurate CDW phase is transformed into a new phase with CDW order that is similar to the nearly commensurate phase with threshold-type transition rates; the threshold excitation density of 0.2 per 13 Ta atoms is evaluated. Coherent-phonon spectroscopy results show that, together with the amplitude mode of CDW with a frequency of 2.41 THz, two other modes with frequencies of 2.34 and 2.07 THz are excited in the photoexcited commensurate CDW phase over a timescale of several tens of picoseconds after excitation. Spectroscopic, temporal, and excitation-intensity dependent characteristics of the three coherent phonons reveal that a photoinduced decomposition of the commensurate CDW order into an ensemble of domains with different CDW orders is induced before the CDW-phase transition occurs. The physics underlying the photoinduced decomposition and evolution into discommensurations responsible for the CDW-order transformation are discussed.