Charge Density Wave Superlattice Research Articles

Encoding multistate charge order and chirality in endotaxial heterostructures

High-density phase change memory (PCM) storage is proposed for materials with multiple intermediate resistance states, which have been observed in 1T-TaS2 due to charge density wave (CDW) phase transitions. However, the metastability responsible for this behavior makes the presence of multistate switching unpredictable in TaS2 devices. Here, we demonstrate the fabrication of nanothick verti-lateral H-TaS2/1T-TaS2 heterostructures in which the number of endotaxial metallic H-TaS2 monolayers dictates the number of resistance transitions in 1T-TaS2 lamellae near room temperature. Further, we also observe optically active heterochirality in the CDW superlattice structure, which is modulated in concert with the resistivity steps, and we show how strain engineering can be used to nucleate these polytype conversions. This work positions the principle of endotaxial heterostructures as a promising conceptual framework for reliable, non-volatile, and multi-level switching of structure, chirality, and resistance.

Spectrally Resolving the Phase and Amplitude of Coherent Phonons in the Charge Density Wave State of 1T‐TaSe2

AbstractThe excitation and detection of coherent phonons have given unique insights into the condensed matter, in particular for materials with strong electron–phonon coupling. A study of coherent phonons is reported in the layered charge density wave (CDW) compound 1T‐TaSe2 performed using transient broadband reflectivity spectroscopy, in the photon energy range 1.75–2.65 eV. Several intense and long‐lasting (>20 ps) oscillations, arising from the CDW superlattice reconstruction, are observed allowing for detailed analysis of the spectral dependence of their amplitude and phase. For energies above 2.4 eV, where transitions involve Ta d‐bands, the CDW amplitude mode at 2.19 THz is found to dominate the coherent response. At lower energies, instead, beating arises between additional frequencies, with a particularly intense mode at 2.95 THz. Interestingly, the spectral analysis reveals a π phase shift at 2.4 eV. Results are discussed considering the selective coupling of specific modes to energy bands involved in the optical transitions seen in steady‐state reflectivity. The work demonstrates how coherent phonon spectroscopy can distinguish and resolve optical states strongly coupled to the CDW order and provide additional information normally hidden in conventional steady‐state techniques.

Charge density wave states in phase-engineered monolayer VTe2

Charge density wave (CDW) strongly affects the electronic properties of two-dimensional (2D) materials and can be tuned by phase engineering. Among 2D transitional metal dichalcogenides (TMDs), VTe2 was predicted to require small energy for its phase transition and shows unexpected CDW states in its T-phase. However, the CDW state of H-VTe2 has been barely reported. Here, we investigate the CDW states in monolayer (ML) H-VTe2, induced by phase-engineering from T-phase VTe2. The phase transition between T- and H-VTe2 is revealed with x-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM) measurements. For H-VTe2, scanning tunneling microscope (STM) and low-energy electron diffraction (LEED) results show a robust CDW superlattice with a transition temperature above 450 K. Our findings provide a promising way for manipulating the CDWs in 2D materials and show great potential in its application of nanoelectronics.

Atomic-scale visualization of chiral charge density wave superlattices and their reversible switching

Chirality is essential for various phenomena in life and matter. However, chirality and its switching in electronic superlattices, such as charge density wave (CDW) superlattices, remain elusive. In this study, we characterize the chirality switching with atom-resolution imaging in a single-layer NbSe2 CDW superlattice by the technique of scanning tunneling microscopy. The atomic arrangement of the CDW superlattice is found continuous and intact although its chirality is switched. Several intermediate states are tracked by time-resolved imaging, revealing the fast and dynamic chirality transition. Importantly, the switching is reversibly realized with an external electric field. Our findings unveil the delicate switching process of chiral CDW superlattice in a two-dimensional (2D) crystal down to the atomic scale.

Synthesis of Atomically Thin 1T‐TaSe2 with a Strongly Enhanced Charge‐Density‐Wave Order

AbstractBulk 1T‐TaSe2 as a charge‐density‐wave (CDW) conductor is of special interest for CDW‐based nanodevice applications because of its high CDW transition temperature. Reduced dimensionality of the strongly correlated material is expected to result in significantly different collective properties. However, the growth of atomically thin 1T‐TaSe2 crystals remains elusive, thus hampering studies of dimensionality effects on the CDW of the material. Herein, chemical vapor deposition (CVD) of atomically thin TaSe2 crystals is reported with controlled 1T phase. Scanning transmission electron microscopy suggests the high crystallinity and the formation of CDW superlattice in the ultrathin 1T‐TaSe2 crystals. The commensurate–incommensurate CDW transition temperature of the grown 1T‐TaSe2 increases with decreasing film thickness and reaches a value of 570 K in a 3 nm thick layer, which is 97 K higher than that of previously reported bulk 1T‐TaSe2. This work enables the exploration of collective phenomena of 1T‐TaSe2 in the 2D limit, as well as offers the possibility of utilizing the high‐temperature CDW films in ultrathin phase‐change devices.

Excitonic and lattice contributions to the charge density wave in 1T−TiSe2 revealed by a phonon bottleneck

Understanding collective electronic states such as superconductivity and charge density waves is pivotal for fundamental science and applications. The layered transition metal dichalcogenide 1T-TiSe2 hosts a unique charge density wave (CDW) phase transition whose origins are still not fully understood. Here, we present ultrafast time- and angle-resolved photoemission spectroscopy (TR-ARPES) measurements complemented by time-resolved reflectivity (TRR) which allows us to establish the contribution of excitonic and electron-phonon interactions to the CDW. We monitor the energy shift of the valence band (VB) and coupling to coherent phonons as a function of laser fluence. The VB shift, directly related to the CDW gap closure, exhibits a markedly slower recovery dynamics at fluences above Fth = 60 microJ cm-2. This observation coincides with a shift in the relative weight of coherently coupled phonons to higher frequency modes in time-resolved reflectivity (TRR), suggesting a phonon bottleneck. Using a rate equation model, the emergence of a high-fluence bottleneck is attributed to an abrupt reduction in coupled phonon damping and an increase in exciton dissociation rate linked to the loss of CDW superlattice phonons. Thus, our work establishes the important role of both excitonic and phononic interactions in the CDW phase transition and the advantage of combining complementary femtosecond techniques to understand the complex interactions in quantum materials.

Impact of quenched oxygen disorder on charge density wave order in YBa2Cu3O6+x.

The competition between superconductivity and charge density wave (CDW) order in underdoped cuprates has now been widely reported, but the role of disorder in this competition has yet to be fully resolved. A central question is whether disorder sets the length scale of the CDW order, for instance by pinning charge density fluctuations or disrupting an otherwise long-range order. Using resonant soft x-ray scattering, we investigate the sensitivity of CDW order in YBa2Cu3O6+x (YBCO) to varying levels of oxygen disorder. We find that quench cooling YBCO6.67 (YBCO6.75) crystals to destroy their o-V and o-VIII (o-III) chains decreases the intensity of the CDW superlattice peak by a factor of 1.9 (1.3), but has little effect on the CDW correlation length, incommensurability, and temperature dependence. This reveals that while quenched oxygen disorder influences the CDW order parameter, the spatial extent of the CDW order is insensitive to the level of quenched oxygen disorder and may instead be a consequence of competition with superconductivity.

Charge-density waves in rubidium blue bronzeRb0.3MoO3observed by scanning tunneling microscopy

Using low-temperature scanning tunneling microscopy under ultrahigh vacuum, high-resolution topographical images were obtained on a cleaved $(\overline{2}01)$ surface of a rubidium blue bronze $({\mathrm{Rb}}_{0.3}\mathrm{Mo}{\mathrm{O}}_{3})$ single crystal. Only molecular lattice was observed at room temperature. At temperatures of 63 and $78\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, well below the charge density wave (CDW) transition temperature in ${\mathrm{Rb}}_{0.3}\mathrm{Mo}{\mathrm{O}}_{3}$, underlying molecular lattice and CDW superlattice were observed simultaneously in topographical constant current images. The amplitude of the CDW modulation is about $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ along $\mathbf{b}$. On the Fourier transform of these images, both main Bragg spots (molecular lattice related) and satellite spots (CDW superlattice related) coexist unambiguously. The average projection of the CDW wave vector on the $(\overline{2}01)$ surface deduced by Fourier transform is consistent with the bulk value obtained previously by other techniques. Our results show clearly that the Peierls phase of the quasi-one-dimensional blue bronze can be studied by STM.

Unexpected periodicity in the quasi-two-dimensional Mott insulator1T−TaS2revealed by angle-resolved photoemission

High-resolution angle-resolved photoemission data from the insulating phase of the charge-density-wave (CDW) material $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ reveal the backfolding of the electronic bands associated with the CDW superlattice, but also an additional, unexpected periodicity. This periodicity is consistent with the dispersion predicted at zero temperature if three-sublattice magnetic order is present, suggesting that it is the signature of fluctuations associated with magnetic ordering of a Mott insulator on a triangular lattice.

Scanning tunnelling microscopy study of the charge-density wave inHf-doped 1T-TaS2

The effect of doping with Hf atoms on the nearly commensurate (NC)charge-density-wave (CDW) structures of 1T-TaS2 wasinvestigated using scanning tunnelling microscopy (STM) at roomtemperature, for levels of substitution of Hf for Ta up to 6.9%.It was found that the Hf atom introduces a point defect to the CDWsuperlattice at the CDW domain boundary for lower contents. For Hfcontents higher than 1%, the CDW superlattice melted and adistorted structure was observed instead of the usual CDWsuperlattice. The change of the CDW structure observed by STM wascompared with the change of the ρ-T characteristics.

Glass transition from a slush to a real glass in charge-density-wave compound TaS 3

We have found by broad-band dielectric spectroscopy of charge density wave (CDW) compound TaS 3 that the main relaxation time representing CDW viscosity shows a critical slowing down in the range 55-50 K. At lower temperatures two additional processes appear that we assign to the defects in CDW superlattice. Based on very strong similarity with the glass transition in supercooled liquids, we propose a consistent scenario of the glass transition from a CDW slush to a real CDW glass. The length/time scales involved imply the existence of a new family of glasses in systems with the long-range order such as CDW.

Observation of a superlattice in silver-intercalated NbSe2 by scanning tunneling microscopy.

Scanning tunneling microscope measurements were performed on UHV cleaved samples of pure and silver intercalated 2H-${\mathrm{NbSe}}_{2}$ at room temperature. Samples intercalated to stage 2 showed a \ensuremath{\surd}13${\mathit{a}}_{0}$\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}13${\mathit{a}}_{0}$ hexagonal superlattice. We suggest that the superlattice arises from a surface charge density wave (CDW). Pure 2H-${\mathrm{NbSe}}_{2}$ does not display a CDW at room temperature but develops a 3${\mathit{a}}_{0}$\ifmmode\times\else\texttimes\fi{}3${\mathit{a}}_{0}$ CDW superlattice below 30 K. The superlattice in the intercalated samples is similar to the CDW lattices observed in many 1-T poly- morphs of the dichalcogenide materials. We suggest that the observed superlattice may result from a change in the stacking order of ${\mathrm{NbSe}}_{2}$ layers near the surface from 2H type to 1-T type in silver intercalated samples. \textcopyright{} 1996 The American Physical Society.

Characterization of exfoliated TaS2 thin films and the existence of charge density waves

Restacked single molecular layers of the layer compound TaS2 from suspensions was deposited on titanium substrates by using a lithium intercalation–exfoliation method. These exfoliated thin films on titanium substrates have been characterized by scanning electron microscopy and Raman spectroscopy, and x-ray diffraction techniques are also investigated for the existence of charge density wave (CDW) phases using scanning tunneling microscopy (STM). All these techniques revealed that TaS2 crystallites tend to orient mostly with their basal planes parallel to the titanium substrate. TaS2 thin films are chosen with the understanding that the CDW states could mostly be determined because of intralayer interactions in comparison to bulk single crystals where inter- and intralayer interactions are considered. STM images showed strong CDWs with a hexagonal array having periodicity of ∼12 Å and ‘‘corrugations’’ of ∼1.5 Å, respectively. The existence of a CDW superlattice, together with the surface atomic structure observed at room temperature, on these thin films (thickness, ca. 300 Å) of single layers, allows us to understand the intralayer effects on the CDWs. The existence of CDWs in these films was not observed by transmission electron microscopy even at 25 K.

Bias-dependent STM images of charge-density waves on TaS2.

The atomic and electronic structures of the 4Hb polytype (alternating layers of T and H types) of ${\mathrm{TaS}}_{2}$ have been studied with a scanning tunneling microscope, operating in air at room temperature. On areas with presumably the T-type layer being the surface, a commensurate \ensuremath{\surd}13\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}13 charge-density wave (CDW) superlattice is observed, independent of sample bias. On the presumably H-type surface, however, images are bias dependent. Increase of the positive sample bias from about 50 to 170 mV results in a continuous change of images from a dominant atomic pattern with a very weak superimposed CDW modulation to a fully developed commensurate \ensuremath{\surd}13\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}13 CDW superlattice. With a negative bias (down to -300 mV), however, there is hardly any indication of a CDW modulation. The bias-dependent behavior can be explained by the energy dependence of the probability of electron tunneling from the tip through top (H type) to the second (T type) layer.

Scanning-probe-microscopy studies of superlattice structures and density-wave structures in 2H-NbSe2, 2H-TaSe2, and 2H-TaS2 induced by Fe doping.

The intercalation of Fe into the van der Waals gap in the 2H phase transition-metal dichalcogenides ${\mathrm{NbSe}}_{2}$, ${\mathrm{TaSe}}_{2}$, and ${\mathrm{TaS}}_{2}$ produces many interesting electronic, magnetic, and structural effects. The scanning tunneling microscope (STM) and atomic force microscope (AFM) prove to be very sensitive to these changes and we report a wide range of results as a function of Fe concentration. All three materials support similar 3${\mathbf{a}}_{0}$\ifmmode\times\else\texttimes\fi{}3${\mathbf{a}}_{0}$ charge-density-wave (CDW) structures in the pure state at low temperatures. At low concentrations of Fe the CDW superlattice is still strong at 4.2 K and persists to high concentrations of Fe. At high concentrations, the Fe becomes ordered in the octahedral holes in the van der Waals gaps, and superlattices of the form 2${\mathbf{a}}_{0}$\ifmmode\times\else\texttimes\fi{}2${\mathbf{a}}_{0}$ and \ensuremath{\surd}3 ${\mathbf{a}}_{0}$\ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 ${\mathbf{a}}_{0}$ are observed. These can be detected at both 300 and 4.2 K. STM spectroscopy at 4.2 K shows that in 2H-${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{NbSe}}_{2}$ and 2H-${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{TaSe}}_{2}$ the energy gap in the electronic spectrum is initially reduced, but stabilizes at higher Fe concentrations and remains well defined for the ordered 2${\mathbf{a}}_{0}$\ifmmode\times\else\texttimes\fi{}2${\mathbf{a}}_{0}$ phase. A transition from a CDW to a mixed CDW and spin-density-wave state is indicated, since these high Fe concentration phases are antiferromagnetic. In 2H-${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{TaS}}_{2}$ both 2${\mathbf{a}}_{0}$\ifmmode\times\else\texttimes\fi{}2${\mathbf{a}}_{0}$ and \ensuremath{\surd}3 ${\mathbf{a}}_{0}$\ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 ${\mathbf{a}}_{0}$ superlattices are observed. The 2${\mathbf{a}}_{0}$\ifmmode\times\else\texttimes\fi{}2${\mathbf{a}}_{0}$ regions show a large energy gap, while the \ensuremath{\surd}3 ${\mathbf{a}}_{0}$\ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 ${\mathbf{a}}_{0}$ do not. The latter phase is ferromagnetic and would not be expected to exhibit a gap. The development of the electronic structures over the entire range of Fe concentrations has been followed by STM and AFM and can be tracked in detail.

Field-induced charge-density-wave deformations and phase slip in NbSe3.

We report x-ray diffraction measurements of the effects of electric fields on charge-density-wave (CDW) structure in ${\mathrm{NbSe}}_{3}$. For fields exceeding the CDW depinning field ${\mathit{E}}_{\mathit{T}}$, the (0 1+Q 0) CDW superlattice peak shifts along ${\mathit{b}}^{\mathrm{*}}$. The shift increases rapidly with decreasing temperature, is largest near the electrical contacts, and is in opposite directions in the two halves of the crystal. We show that the observed CDW deformations are consistent with those required for phase slip and CDW-to-normal carrier conversion at the contacts. From the magnitude of these deformations, we determine the CDW elastic constant.

Long range modulation of the charge density wave structure in 1T–TaS2 studied by scanning tunneling microscopy

The charge density wave (CDW) structure in 1T–TaS2 has been studied at room temperature using scanning tunneling microscopy. The CDW amplitude shows a continuous long range modulation of ∼6 CDW wavelengths giving rise to a two dimensional pattern of domain-like regions rotated with respect to the CDW superlattice.

Scanning tunneling microscopy of atoms and charge-density waves in 1T-TaS2, 1T-TaSe2, and 1T-VSe2.

The surface atomic structure and the charge-density-wave (CDW) structure in the 1T phase of ${\mathrm{TaSe}}_{2}$, ${\mathrm{TaS}}_{2}$, and ${\mathrm{VSe}}_{2}$ have been studied at 4.2, 77, and 300 K. The response of the scanning tunneling microscope (STM) to the CDW superlattice in 1T-${\mathrm{TaSe}}_{2}$ and 1T-${\mathrm{TaS}}_{2}$ is extremely strong, while in 1T-${\mathrm{VSe}}_{2}$ the CDW generates a much weaker response. In 1T-${\mathrm{TaSe}}_{2}$ and 1T-${\mathrm{TaS}}_{2}$ at 4.2 and 77 K the STM scans show a \ensuremath{\surd}13 ${\mathbf{a}}_{0}$\ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}13 ${\mathbf{a}}_{0}$ pattern characteristic of a triple-q commensurate CDW structure. The atomic lattice and CDW superlattice are simultaneously resolved, but the atomic modulation represents a small fraction of the total STM deflection leading to a substantial variation in the atomic resolution. At 300 K we continue to observe a commensurate \ensuremath{\surd}13 ${\mathbf{a}}_{0}$\ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}13 ${\mathbf{a}}_{0}$ CDW pattern in 1T-${\mathrm{TaSe}}_{2}$ while 1T-${\mathrm{TaS}}_{2}$ shows a modulated two-dimensional structure due to the incommensurate CDW wavelength characteristic of the nearly commensurate phase.The STM scans on 1T-${\mathrm{VSe}}_{2}$ show a 4${\mathbf{a}}_{0}$\ifmmode\times\else\texttimes\fi{}4${\mathbf{a}}_{0}$ CDW superlattice with a pattern that reflects a triple- to double-q transition in the CDW structure and the formation of domains. In 1T-${\mathrm{VSe}}_{2}$ the atomic modulation is dominant with a weak superimposed modulation due to the CDW superlattice. This difference in the strength of the CDW is reflected in the z deflection of the STM and in the effective barrier height as a function of tip-to-surface distance. The STM scans and profiles of z deflection have been analyzed and compared for all three materials. Defects in the CDW structure of 1T-${\mathrm{TaSe}}_{2}$ and 1T-${\mathrm{TaS}}_{2}$ have been observed which create missing CDW maxima and a variable enhancement of the surrounding CDW maxima. These appear to result from defects in the Ta layer rather than in the surface Se or S layer. Defects in the STM scans of 1T-${\mathrm{VSe}}_{2}$ are very localized and appear to be associated with Se-atom vacancies in the surface layer. The STM patterns and response will be reviewed and analyzed in terms of the band structures and Fermi surfaces of the various compounds. Results are also compared to information obtained from electron diffraction.

Scanning tunneling microscopy of the charge-density wave in orthorhombic TaS3.

Scanning tunneling microscopy has been used to study the charge-density wave (CDW) in o-${\mathrm{TaS}}_{3}$. Above the Peierls transition temperature, ${T}_{p}$=215 K, charge modulations are observed that exhibit temporal fluctuations in modulation amplitude and periodicity. As the temperature is lowered, the fluctuations diminish and a more ordered structure is observed. Below the transition temperature the CDW superlattice forms, revealing an approximate 4${c}_{0}$\ifmmode\times\else\texttimes\fi{}10${b}_{0}$ structure in good agreement with electron diffraction experiments. In addition, each unit cell is seen to contain two uniformly distributed CDW maxima along ${b}_{0}$, suggesting that the CDW in this material is not highly localized to only two of the chains of the unit cell.

Hexagonal Domain-Like Charge Density Wave Phase of TaS 2 Determined by Scanning Tunneling Microscopy

The structure of the room-temperature charge density wave (CDW) phase in octahedrally coordinated tantalum disulfide, 1T-TaS2, has been a controversial issue for over 15 years. Large-scale scanning tunneling microscope images of the intralayer structure of this phase exhibit a domain-like pattern defined by a variation in the maximum CDW amplitude. The circular domains, consisting of high-amplitude CDWs, are arranged in a regular hexagonal lattice (period 73+/-3 angstroms) that is rotated relative to the CDWs. In addition, from the analysis of atomic resolution images it was determined that there is a well-defined phase shift between the CDWs in adjacent domains, and that within a domain the CDW superlattice is commensurate with the atomic lattice. These results provide evidence for the hexagonal discommensurate CDW phase in 1T-TaS2 and also suggest an explanation for the long-standing controversy concerning the structure of this phase.