Charge Density Wave Compound Research Articles

Experimental observation of metallic states with different dimensionality in a quasi-one-dimensional charge density wave compound

Experimental observation of metallic states with different dimensionality in a quasi-one-dimensional charge density wave compound

Tuning charge density wave of kagome metal ScV6Sn6

Compounds with a kagome lattice exhibit intriguing properties and the charge density wave (CDW) adds an additional layer of interest to research on them. In this study, we investigate the temperature and magnetic field dependent electrical properties under a chemical substitution and hydrostatic pressure of ScV6Sn6, a non-magnetic CDW compound. Substituting 5% Cr at the V site or applying 1.5 GPa of pressure shifts the CDW from 92 K to ∼ 50 K. This shift is attributed to the movement of the imaginary phonon band, as revealed by the phonon dispersion relation. The longitudinal and Hall resistivities respond differently under these stimuli. The magnetoresistance (MR) retains its quasilinear behavior under pressure, but it becomes quadratic after Cr substitution. The anomalous Hall-like behavior of the parent compound persists up to the respective CDW transition under pressure, after which it decreases sharply. In contrast, the longitudinal and Hall resistivities of Cr substituted compounds follow a two-band model and originate from the multi carrier effect. These results clearly highlight the role of phonon contributions in the CDW transition and call for further investigation into the origin of the anomalous Hall-like behavior in the parent compound.

Precursor region with full phonon softening above the charge-density-wave phase transition in 2H-TaSe2

Research on charge-density-wave (CDW) ordered transition-metal dichalcogenides continues to unravel new states of quantum matter correlated to the intertwined lattice and electronic degrees of freedom. Here, we report an inelastic x-ray scattering investigation of the lattice dynamics of the canonical CDW compound 2H-TaSe2 complemented by angle-resolved photoemission spectroscopy and density functional perturbation theory. Our results rule out the formation of a central-peak without full phonon softening for the CDW transition in 2H-TaSe2 and provide evidence for a novel precursor region above the CDW transition temperature TCDW, which is characterized by an overdamped phonon mode and not detectable in our photoemission experiments. Thus, 2H-TaSe2 exhibits structural before electronic static order and emphasizes the important lattice contribution to CDW transitions. Our ab-initio calculations explain the interplay of electron-phonon coupling and Fermi surface topology triggering the CDW phase transition and predict that the CDW soft phonon mode promotes emergent superconductivity near the pressure-driven CDW quantum critical point.

Multiple Quantum States Induced in 1T‐TaSe2 by Controlling the Stacking Order of Charge Density Waves

AbstractVan der Waals (vdW) materials afford unprecedented opportunities for control of electronic properties by utilizing the stacking degree of freedom. An intriguing frontier, largely unexplored, is the stacking of charge density wave (CDW) phases that is a broken‐symmetry state with periodically modulated charge density and the atomic lattice. Employing density functional theory, it is uncovered that the stacking order can play a significant role in the quantum phase transitions of layered 1T‐TaSe2 with a striking 2D CDW order. By controlling the vertical stacking order of CDWs, bulk 1T‐TaSe2 can host various electronic phases including quasi‐1D and 3D metals and band insulators. Particularly, the ground‐state stacking configuration shows 3D metallicity due to the enhanced intralayer and interlayer electron hopping, and the second lowest energy configuration shows band insulating behavior via interlayer dimerization, implying potential metal‐insulator transition. In ultrathin‐layer 1T‐TaSe2, not only the stacking order but also the thickness dictate the electronic properties. While the monolayer is a Mott insulator, the bilayer (trilayer) is a band insulator (metal). More interestingly, the four‐layer emerges as an insulator or a semimetal dependent on its stacking order. The wide‐tunable electronic properties of 1T‐TaSe2 CDW compound will open a new pathway for designing novel quantum devices.

Angle-resolved photoemission spectroscopy study of charge density wave order in the layered semiconductor EuTe4

Layered tellurides have been extensively studied as a platform for investigating the Fermi surface (FS) nesting-driven charge density wave (CDW) states. EuTe4, one of quasi-two-dimensional (quasi-2D) binary rare-earth tetratellurides CDW compounds, with unconventional hysteretic transition, is currently receiving much attention. Here, the CDW modulation vector, momentum and temperature dependence of CDW gaps in EuTe4 are investigated using angle-resolved photoemission spectroscopy. Our results reveal that (i) a FS nesting vector q ~ 0.67 b* drives the formation of CDW state, (ii) a large anisotropic CDW gap is fully open in the whole FS, and maintains a considerable size even at 300 K, leading to appearance of semiconductor properties, (iii) an abnormal non-monotonic increase of CDW gap in magnitude as a function of temperature, (iv) an extra, larger gap opens at a higher binding energy due to the interaction between the different orbits of the main bands.

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.

Modulated crystal structure of the atypical charge density wave state of single-crystal Lu2Ir3Si5

The three-dimensional charge density wave (CDW) compound ${\mathrm{Lu}}_{2}{\mathrm{Ir}}_{3}{\mathrm{Si}}_{5}$ undergoes a first-order CDW phase transition at around 200 K. An atypical CDW state is found, that is characterized by an incommensurate CDW with $\mathbf{q}=[0.2499(3),0.4843(4),0.2386(2)]$ at 60 K, and a large orthorhombic-to-triclinic lattice distortion with $\ensuremath{\beta}=91.945{(2)}^{\ensuremath{\circ}}$. We present the modulated crystal structure of the incommensurate CDW state. Structural analysis shows that the CDW resides on the zigzag chains of iridium atoms along $\mathbf{c}$. The structural distortions are completely similar between nonmagnetic ${\mathrm{Lu}}_{2}{\mathrm{Ir}}_{3}{\mathrm{Si}}_{5}$ and previously studied isostructural magnetic ${\mathrm{Er}}_{2}{\mathrm{Ir}}_{3}{\mathrm{Si}}_{5}$ with the small differences explained by the different values of the atomic radii of Lu and Er. Such a similarity is unique to ${R}_{2}{\mathrm{Ir}}_{3}{\mathrm{Si}}_{5}$ $(R=\mathrm{rare}\phantom{\rule{0.28em}{0ex}}\mathrm{earth})$. It differs from, for example, the rare-earth CDW compounds ${R}_{5}{\mathrm{Ir}}_{4}{\mathrm{Si}}_{10}$ for which ${\mathrm{Lu}}_{5}{\mathrm{Ir}}_{4}{\mathrm{Si}}_{10}$ and ${\mathrm{Er}}_{5}{\mathrm{Ir}}_{4}{\mathrm{Si}}_{10}$ possess entirely different CDW states. We argue that the mechanism of CDW formation, thus, is different for ${R}_{2}{\mathrm{Ir}}_{3}{\mathrm{Si}}_{5}$ and ${R}_{5}{\mathrm{Ir}}_{4}{\mathrm{Si}}_{10}$.

Angle-resolved photoemission spectroscopy study of rare-earth tritelluride charge density wave compounds: RTe3 (R = Pr, Er)

The electronic structures of layered rare-earth tritelluride RTe3 (R = Pr, Er) charge density wave (CDW) compounds have been investigated by performing R 4d → 4f resonant photoemission spectroscopy (RPES) and angle-resolved photoemission spectroscopy (ARPES) measurements for high-quality single crystals. R 4d → 4f RPES measurements reveal that the R 4f states do not contribute directly to the CDW formation in RTe3 but that the R 4f-Te 5p hybridization in PrTe3 is significantly larger than that in ErTe3. In the photon-energy maps for the Fermi-edge (E F)-crossing states in RTe3, straight vertical dispersions are observed along k c , demonstrating the 2D character for the near-E F states in both R = Pr and Er. This finding implies the weak interlayer interaction between R-Te(1) and Te(2)–Te(3) layers, which is supported by the similar linear dichroism in ARPES for PrTe3 and ErTe3. The CDW-induced Fermi surface of PrTe3 exhibits two-fold symmetric features while that of ErTe3 exhibits four-fold symmetric features. This finding reveals different CDW distortions in PrTe3 and ErTe3, the origin of which is likely to be different ionic sizes of R ions and different R 4f-Te 5p hybridization.

Prediction of the effects of rf irradiation on the I–V curves of a CDW compound

The Shapiro steps (ShSs) developing in sliding charge-density wave (CDW) under rf irradiation are considered in terms of the CDW travel in the periodic (washboard) potential. We demonstrate that treating the CDW as an inertialess object whose velocity is defined by the instantaneous voltage one can predict the positions of the ShSs in dc voltages for the given amplitude of rf voltage. Moreover, the approximation allows indicating the rf voltages at which the ShSs' magnitudes show maxima and minima. The only information necessary in advance is the I–V characteristic without rf irradiation and the cross-sectional area of the sample. Thus, one can recover the course of an I–V curve in average and the positions of ShSs on it, though not their forms.

Zoology of domain walls in quasi-2D correlated charge density wave of 1T-TaS2

Domain walls in correlated charge density wave compounds such as 1T-TaS2 can have distinct localized states which govern physical properties and functionalities of emerging quantum phases. However, detailed atomic and electronic structures of domain walls have largely been elusive. We identify using scanning tunneling microscope and density functional theory calculations the atomic and electronic structures for a plethora of discommensuration domain walls in 1T-TaS2 quenched metastably with nanoscale domain wall networks. The domain walls exhibit various in-gap states within the Mott gap but metallic states appear in only particular types of domain walls. A systematic understanding of the domain-wall electronic property requests not only the electron counting but also including various intertwined interactions such as structural relaxation, electron correlation, and charge transfer. This work guides the domain wall engineering of the functionality in correlated van der Waals materials.

Single-water-dipole-layer-driven Reversible Charge Order Transition in 1T-TaS2.

Water-solid interactions are crucial for many fundamental phenomena and technological processes. Here, we report a scanning tunneling microscopy study about the charge density wave (CDW) transition in 1T-TaS2 driven by a single water dipole layer. At low temperature, pristine 1T-TaS2 is a prototypical CDW compound with × charge order. After growing a highly ordered water adlayer, a new charge order with 3 × 3 periodicity emerges on water-covered 1T-TaS2. After water desorption, the entire 1T-TaS2 surface appears as localized × CDW domains that are separated by residual-water-cluster-pinned CDW domain walls. First-principles calculations show that the electric dipole moments in the water adlayer attract electrons to the top layer of 1T-TaS2, which shifts the phonon softening mode and induces the × to 3 × 3 charge order transition. Our results pave the way for creating new collective quantum states of matter with a molecular dipole layer.

Origin of the large magnetoresistance in the candidate chiral superconductor 4Hb−TaS2

The intriguing electronic phase diagram involving charge density wave (CDW) transitions in the $\mathrm{Ta}{\mathrm{S}}_{2}$ system has been widely investigated over the past decade, especially for the $1T$ and $2H$ phases. $4{H}_{b}\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$, regarded as the natural heterostructure that combines the characteristics of $1T$ and $2H\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$, has also been the focus recently, due to the prospects for fundamental research and device applications. Here, we have systematically investigated the electrical transport properties of $4{H}_{b}\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$ single crystals combined with the band structure calculations and found that the low-temperature phase of candidate chiral superconductor $4{H}_{b}\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$ (${T}_{c}\ensuremath{\sim}3.5$ K) at normal state is not the simple CDW but exhibits a strong magnetic field dependence. The most significant result is the emergence of the large magnetoresistance (MR), which may originate from the high mobility of holes and partial compensation. In addition, the symmetry of MR under the low magnetic field has also changed significantly in $4{H}_{b}\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$, which is closely related to the CDW structures formed in the $H$ layers at 22 K. The results are conducive to the understanding of the mechanism of MR appearing in layered CDW compounds, and the presence of MR in $4{H}_{b}\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$ suggests the potential applications for functional devices in the future.

Exotic bonding interactions and coexistence of chemically distinct periodic lattice distortions in the charge density wave compound TaTe2

The superstructure of $\mathrm{TaT}{\mathrm{e}}_{2}$ is examined by total high-energy x-ray scattering and large-scale structure modeling over a broad temperature range. At room temperature, it features double zigzag chains of Ta atoms exhibiting metallic bonding. The chains are sandwiched between planes of weakly interacting Te atoms. Below 170 K, the chains appear broken to ``butterflylike'' clusters and single zigzag chains of covalently bonded Te atoms emerge while Ta-Te interactions remain largely ionic. This is a rare example of a charge density wave compound stabilized by bonding that is a mix of covalent, ionic, and metallic character giving rise to chemically distinct periodic lattice distortions. We argue that the formation of periodic lattice distortions in the Te sublattice in addition to those in the Ta sublattice favors charge delocalization, which may explain the unusually diminished resistivity and increased magnetic susceptibility exhibited by the low-temperature charge density wave phase of $\mathrm{TaT}{\mathrm{e}}_{2}$. The effect may be common for $5d$ transition metal chalcogenides containing Te because of the extended electronic orbitals of the constituent atoms.

Eightfold fermionic excitation in a charge density wave compound

Unconventional quasiparticle excitations in condensed matter systems have become one of the most important research frontiers. Beyond two- and fourfold degenerate Weyl and Dirac fermions, three-, six- and eightfold symmetry protected degeneracies have been predicted however remain challenging to realize in solid state materials. Here, charge density wave compound TaTe4 is proposed to hold eightfold fermionic excitation and Dirac point in energy bands. High quality TaTe4 single crystals are prepared, where the charge density wave is revealed by directly imaging the atomic structure and a pseudogap of about 45 meV on the surface. Shubnikov de-Haas oscillations of TaTe4 are consistent with band structure calculation. Scanning tunneling microscopy reveals atomic step edge states on the surface of TaTe4. This work uncovers that charge density wave is able to induce new topological phases and sheds new light on the novel excitations in condensed matter materials.

Melting of charge density wave and Mott gap collapse on 1T−TaS2 induced by interfacial water

Microscopically revealing the interactions between interfacial water and the quantum states of matter is an important task from both the materials science and the physics points of view. Here we report a low-temperature scanning tunneling microscopy (STM) and spectroscopy study of water adsorption on the charge density wave compound $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, which has a Mott-insulating ground state. Interfacial water forms monolayer islands with $6\ifmmode\times\else\texttimes\fi{}6$ superstructures on the surface of $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, and the charge order under water islands can be directly imaged in STM topographies taken with negative bias voltages. Compared with the original $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}$ charge order in $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, the charge order under water islands becomes significantly disordered and denser. A $\mathsf{V}$-shaped gaplike feature emerges in water-covered $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, which may be due to the enhanced dielectric constant of interfacial water, which reduces short-range Coulomb repulsion and induces Mott gap collapse. Our observations open the way to microscopically understanding the interactions between interfacial water and the correlated quantum states of matter.

Multiple magnetic-phase transitions and critical behavior of charge-density wave compound TbTe3

We report multiple magnetic phase transitions and critical behavior of the 2D charge-density wave compound TbTe3 studied by μSR measurements and dc magnetization measurements. Zero-field μSR has shown three magnetic transitions below 7 K. The longitudinal field measurements under 50 G has confirmed the first transition at TN = 6.3 K. Scaling analysis from above TN gives the critical exponent w = 0.63(5), suggesting the Ising 3D antiferromagnetic nature of the ordering, which is likely mediated by the 2D correlations. However, the obtained w = 0.81(5) below TN indicates the ferromagnetic phase, which arises over the multiphase transitions at lower temperatures. Temperature-dependent transverse frequency shift gives a relatively smaller exponent γ = 1.0(1) than the Ising 3D model. The different transitions were also observed by dc magnetization measurements, suggesting two magnetic transitions at 7.4 K and 3.1 K, which correspond to the antiferromagnetic and ferromagnetic phases respectively.

Dynamic properties of a one-dimensional charge density wave compound in the presence of uncondensed-carriers: Insights from numerical experiences

In the present Letter, a numerical study of the dynamic properties of one-dimensional (1D) incommensurate charge density wave (CDW) compound in the presence of free carriers (not affected by the Peierls transition) is carried out. The results obtained showed that the CDW sliding state is very sensitive to this local effect. The CDW depinning threshold field, excess current normalized and amplitude of Narrow-Band-Noise (NBN) are affected by these single particle’s excitations. This latter can be viewed as a screening effect of the applied electrical field by free carriers which relax in order to reduce the CDW polarization and generate a homogeneous system response.

Charge density wave and lock-in transitions of CuV2S4

The three-dimensional charge density wave (CDW) compound ${\mathrm{CuV}}_{2}{\mathrm{S}}_{4}$ is known to undergo phase transitions at $\ensuremath{\sim}91$ and $\ensuremath{\sim}50\phantom{\rule{4pt}{0ex}}\mathrm{K}$. Employing single-crystal x-ray diffraction on an annealed crystal, we confirm the formation of an incommensurate CDW at ${T}_{\text{CDW}}\ensuremath{\approx}91\phantom{\rule{4pt}{0ex}}\mathrm{K}$, and we establish the nature of the transition at ${T}_{\text{lock-in}}\ensuremath{\approx}50\phantom{\rule{4pt}{0ex}}\mathrm{K}$ as a lock-in transition toward a threefold superstructure. As-grown crystals develop the same incommensurate CDW as the annealed crystal does, but they fail to go through the lock-in transition. Instead, the length of the modulation wave vector continues to decrease down to low temperatures in as-grown crystals. These findings are corroborated by distinct temperature dependencies of the electrical resistivity, magnetic susceptibility, and specific heat measured on as-grown and annealed crystals. A superspace model for the crystal structure of the incommensurate CDW suggests that the formation of extended vanadium clusters is at the origin of the CDW. In the lock-in phase, short and long V-V distances persist, but clusters now percolate the entire crystal. The lowering toward orthorhombic symmetry appears to be responsible for the precise pattern of short and long V-V distances. However, the orthorhombic lattice distortion is nearly zero for the annealed crystal, while it is visible for the as-grown material, again suggesting the role of lattice defects in the latter.

Competing soft phonon modes at the charge-density-wave transitions in DyTe3

The family of rare-earth tritellurides RTe$_3$ features charge-density-wave (CDW) order related to strongly momentum-dependent electron-phonon coupling. Similar to other CDW compounds, superconductivity is observed when the CDW order is suppressed via hydrostatic pressure [1]. What sets the heavier members of the RTe3 series apart is the observation of a second CDW transition at lower temperatures having an in-plane ordering wavevector $q_{CDW,2}\parallel [100]$ of almost the same magnitude but orthogonal to the ordering wavevector $q_{CDW,1}\parallel [001]$ observed at higher temperatures [2]. Here, we report an inelastic x-ray scattering investigation of the lattice dynamics of DyTe$_3$. In particular, we show that there are several phonon modes along both in-plane directions, which respond to the onset of the CDW transition at $T_{CDW,1}=308\,\rm{K}$. Surprisingly, these soft modes close to $q_{CDW,2}=(0.68,0,0)$ show strong softening near $T_{CDW,1}$ but do not exhibit any response to the lower-temperature transition at $T_{CDW,2}=68\,\rm{K}$. Our results indicate that the low-temperature CDW order is not just the 90{\deg} rotated analogue of the one appearing at high temperatures.

Local Real-Space View of the Achiral 1T-TiSe_{2} 2×2×2 Charge Density Wave.

The transition metal dichalcogenide 1T-TiSe_{2}-two-dimensional layered material undergoing a commensurate 2×2×2 charge density wave (CDW) transition with a weak periodic lattice distortion (PLD) below ≈200 K. Scanning tunneling microscopy (STM) combined with intentionally introduced interstitial Ti atoms allows us to go beyond the usual spatial resolution of STM and to intimately probe the three-dimensional character of the PLD. Furthermore, the inversion-symmetric achiral nature of the CDW in the z direction is revealed, contradicting the claimed existence of helical CDW stacking and associated chiral order. This study paves the way to a simultaneous real-space probing of both charge and structural reconstructions in CDW compounds.