Charge Density Wave Phase Transition Research Articles

Emergent tetragonality in a fundamentally orthorhombic material.

Symmetry plays a key role in determining the physical properties of materials. By Neumann's principle, the properties of a material remain invariant under the symmetry operations of the space group to which the material belongs. Continuous phase transitions are associated with a spontaneous reduction in symmetry. Less common are examples where proximity to a continuous phase transition leads to an increase in symmetry. We find signatures of an emergent tetragonal symmetry close to a charge density wave (CDW) bicritical point in a fundamentally orthorhombic material, ErTe3, for which the two distinct CDW phase transitions are tuned via anisotropic strain. We first establish that tension along the a axis favors an abrupt rotation of the CDW wave vector from the c to a axis and infer the presence of a bicritical point where the two continuous phase transitions meet. We then observe a divergence of the nematic elastoresistivity approaching this putative bicritical point, indicating an emergent tetragonality in the critical behavior.

Volume dependence of the charge density wave phase transitions

ABSTRACT We analyse the volume dependence of the charge density wave phase transition temperature T CDW for 16 one and two dimensional materials such as the rare earth tritellurides TbTe 3 , ErTe 3 , TmTe 3 DyTe 3 , the transition metal dichalcogenide 2H-NbSe 2 , and the quasi-one dimensional conductors K 0.3 MoO 3 , NbSe 3 , ( TaSe 4 ) 2 I . The volume dependence is taken in the form of the logarithmic derivative of T CDW in function of the strain components. We point out some characteristics of this parameter. This parameter is dimensionless with an absolute value of about 10 obtained from the measurements of the thermal expansion, the longitudinal elastic constants and pressure dependence for 11 one- and two-dimensional CDW systems. In contrast a huge positive value of about 100 is found at the lower charge density wave transition in TbTe 3 , ErTe 3 , TmTe 3 and DyTe 3 .

Temperature-Driven Rotation Symmetry-Breaking States in an Atomic Kagome Metal KV3Sb5.

Kagome lattice AV3Sb5 has attracted tremendous interest because it hosts correlated and topological physics. However, an in-depth understanding of the temperature-driven electronic states in AV3Sb5 is elusive. Here we use scanning tunneling microscopy to directly capture the rotational symmetry-breaking effect in KV3Sb5. Through both topography and spectroscopic imaging of defect-free KV3Sb5, we observe a charge density wave (CDW) phase transition from an a0 × a0 atomic lattice to a robust 2a0 × 2a0 superlattice upon cooling the sample to 60 K. An individual Sb-atom vacancy in KV3Sb5 further gives rise to the local Friedel oscillation (FO), visible as periodic charge modulations in spectroscopic maps. The rotational symmetry of the FO tends to break at the temperature lower than 40 K. Moreover, the FO intensity shows an obvious competition against the intensity of the CDW. Our results reveal a tantalizing electronic nematicity in KV3Sb5, highlighting the multiorbital correlation in the kagome lattice framework.

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.

NMR study of charge density wave phase in the kagome metal RbV3Sb5

The mechanism of charge-density-wave (CDW) phase in one-dimensional lattices can be well described in theory. Whether similar mechanism works in two-dimensional systems is still a mystery. In this work, we employed 51V solid-state nuclear magnetic resonance (NMR) spectroscopy combined with first-principles computations to probe atomic structure and electronic properties at V sites in a quasi-two-dimensional crystal RbV3Sb5. Clear evidence of the CDW phase transition was observed from the temperature-dependent 51V NMR measurements. The sharp transition of 51V Knight shifts and electric field gradients (EFG) across the CDW transition supports the view that energy gap is formed by the nesting of partial Fermi surface in accompany with the superlattice formation. The calculated EFG parameters were also compared with our experiments, and the results suggest that the charge modulation of the CDW order is inverse Star-of-David pattern.

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.

Charge-Density-Wave Order And Electronic Phase Transitions in a Dilute D-Band Semiconductor.

As one of the most fundamental physical phenomena, charge density wave (CDW) order predominantly occurs in metallic systems such as quasi-one-dimensional (quasi-1D) metals, doped cuprates, and transition metal dichalcogenides, where it is well understood in terms of Fermi surface nesting and electron-phonon coupling mechanisms. On the other hand, CDW phenomena in semiconducting systems, particularly at the low carrier concentration limit, are less common and feature intricate characteristics, which often necessitate the exploration of novel mechanisms, such as electron-hole coupling or Mott physics, to explain. In this study, we combined electrical transport, synchrotron X-ray diffraction and density-functional theory (DFT) calculations to investigate CDW order and a series of hysteretic phase transitions in a dilute d-band semiconductor, BaTiS3 . Our experimental and theoretical findings suggest that the observed CDW order and phase transitions in BaTiS3 may be attributed to both electron-phonon coupling and non-negligible electron-electron interactions in the system. Our work highlights BaTiS3 as a unique platform to explore CDW physics and novel electronic phases in the dilute filling limit and could open new opportunities for developing novel electronic devices. This article is protected by copyright. All rights reserved.

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.

Charge‐Density‐Wave Resistive Switching and Voltage Oscillations in Ternary Chalcogenide BaTiS3

AbstractPhase change materials, which show different electrical characteristics across the phase transitions, have attracted considerable research attention for their potential electronic device applications. Materials with metal‐to‐insulator or charge density wave (CDW) transitions such as VO2 and 1T‐TaS2 have demonstrated voltage oscillations due to their robust bi‐state resistive switching behavior with some basic neuronal characteristics. BaTiS3 is a small bandgap ternary chalcogenide that has recently reported the emergence of CDW order below 245 K. Here, the discovery of DC voltage / current‐induced reversible threshold switching in BaTiS3 devices between a CDW phase and a room temperature semiconducting phase is reported. The resistive switching behavior is consistent with a Joule heating scheme and sustained voltage oscillations with a frequency of up to 1 kHz are demonstrated by leveraging the CDW phase transition and the associated negative differential resistance. Strategies of reducing channel sizes and improving thermal management may further improve the device's performance. The findings establish BaTiS3 as a promising CDW material for future electronic device applications, especially for energy‐efficient neuromorphic computing.

Q-dependent electron-phonon coupling induced phonon softening and non-conventional critical behavior in the CDW superconductor LaPt2Si2

This paper reports the first experimental observation of phonons and their softening on single crystalline LaPt2Si2 via inelastic neutron scattering. From the temperature dependence of the phonon frequency in close proximity to the charge density wave (CDW) q-vector, we obtain a CDW transition temperature of TCDW = 230 K and a critical exponent β = 0.28 ± 0.03. This value is suggestive of a non-conventional critical behavior for the CDW phase transition in LaPt2Si2, compatible with a scenario of CDW discommensuration (DC). The DC would be caused by the existence of two CDWs in this material, propagating separately in the non equivalent (Si1–Pt2–Si1) and (Pt1–Si2–Pt1) layers, respectively, with transition temperatures TCDW−1 = 230 K and TCDW−2 = 110 K. A strong q-dependence of the electron-phonon coupling has been identified as the driving mechanism for the CDW transition at TCDW−1 = 230 K while a CDW with 3-dimensional character, and Fermi surface quasi-nesting as a driving mechanism, is suggested for the transition at TCDW−2 = 110 K. Our results clarify some aspects of the CDW transition in LaPt2Si2 which have been so far misinterpreted by both theoretical predictions and experimental observations and give direct insight into its actual temperature dependence.

Controllable Synthesis and Charge Density Wave Phase Transitions of Two-Dimensional 1T-TaS2 Crystals.

1T-TaS2 has attracted much attention recently due to its abundant charge density wave phases. In this work, high-quality two-dimensional 1T-TaS2 crystals were successfully synthesized by a chemical vapor deposition method with controllable layer numbers, confirmed by the structural characterization. Based on the as-grown samples, their thickness-dependency nearly commensurate charge density wave/commensurate charge density wave phase transitions was revealed by the combination of the temperature-dependent resistance measurements and Raman spectra. The phase transition temperature increased with increasing thickness, but no apparent phase transition was found on the 2~3 nm thick crystals from temperature-dependent Raman spectra. The transition hysteresis loops due to temperature-dependent resistance changes of 1T-TaS2 can be used for memory devices and oscillators, making 1T-TaS2 a promising material for various electronic applications.

Doping evolution of superconductivity, charge order, and band topology in hole-doped topological kagome superconductors Cs(V1−xTix)3Sb5

The newly discovered kagome superconductors $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ ($A=\mathrm{K}$, Rb, Cs) exhibit superconductivity, charge order, and band topology simultaneously. To explore the intricate interplay between the superconducting and charge-density-wave (CDW) orders, we investigate the doping evolution of underlying electronic structures in doped topological kagome superconductors $\mathrm{Cs}{({\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x})}_{3}{\mathrm{Sb}}_{5}$ where the Ti dopant introduces hole-like charge carriers. Despite the absence of the CDW phase transition in doped compounds even for the lowest doping level of $x=0.047$, the superconductivity survives in all doped samples with enhanced critical temperatures. The high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements reveal that the Ti dopant in the kagome plane lowers the chemical potential, pushing the van Hove singularity (VHS) at $M$ point above the Fermi level. First-principle simulations corroborate the doping evolution of the band structure observed in ARPES, and affirm that the CDW instability does not occur once the VHS moves above the Fermi level, explaining the absence of the CDW ordering in our doped samples $\mathrm{Cs}{({\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x})}_{3}{\mathrm{Sb}}_{5}$. Our results also demonstrate a competition between the CDW and superconducting orders in the kagome-metal superconductor ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$, although the superconductivity is likely inconsequential of the CDW order.

Structure of the kagome superconductor CsV3Sb5 in the charge density wave state

The structure of charge density wave states in AV$_3$Sb$_5$ (A = K, Rb, Cs) kagome superconductors remains elusive, with three possible $2a\times2a\times2c$ candidates: tri-hexagonal, star-of-David, and their mixture. In this study, we conducted a systematic first-principles investigation of the nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) spectra for the $2a\times2a\times2c$ CsV$_3$Sb$_5$ structures. By comparing our simulations with experimental data, we have concluded that the NQR spectrum indicates the tri-hexagonal structure as the proper structure for CsV$_3$Sb$_5$ after its charge density wave phase transition. The NMR calculation results obtained from the tri-hexagonal structure are also consistent with the experimental data.

The Influence of Dimensionality on the Charge‐Density‐Wave Transition and its Application on Mid‐Infrared Photodetection

Abstract2D charge density wave (CDW) materials receive much attention for high responsivity and broadband photodetection in recent years due to their collective electron transport and narrow bandgap. However, the high dark current density problem hinders their real application. Here, a sharp CDW transition in quasi‐1D (TaSe4)2I is reported and applied for broadband photodetection. Especially at mid‐infrared region, the device shows both high photo responsivity of 1.18 ×103 A W−1 and large light on/off ratio of 80, which is superior to 2D CDW TaS2 and most reported low‐dimensional materials. Such high performance relies on two aspects. One is the much lower dark current density resulting from the pseudo gap associated with 1D Luttinger liquid state, which is supported by finite size scaling of nonlinear I–V at variable temperatures and occurrence of 1D structural phase transition consolidated by in situ Raman spectroscopy. The other is the high photocurrent associated with the “Fröhlich superconductivity” state, manifested by an ultrasensitive switching, which can be only accessible in 1D CDW materials, in agreement with the authors’ density functional theory calculation. This work thus reveals the pivotal role of dimensionality in CDW phase transition and paves a way for implementing highly sensitive broadband photodetector.

Correlation between charge density wave phase transition and hydrogen adsorption in 1T-TaS2 thin film devices

Thin films of tantalum disulfide in the 1T-polytype structural phase (1T-TaS2), a type of metallic two-dimensional (2D) transition metal dichalcogenides (TMDs), are reactive to H2. Interestingly, in the incommensurate charge-density wave (ICCDW) phase with a metallic state, the electrical resistance of the 1T-TaS2 thin film decreases when H2 is adsorbed on it and returns to its initial value upon desorption. In contrast, the electrical resistance of the film in the nearly commensurate CDW (NCCDW) phase, which has a subtle band overlap or a small bandgap, does not change upon H2 adsorption/desorption. This difference in H2 reactivity is a result of differences in the electronic structure of the two 1T-TaS2 phases, namely, the ICCDW and NCCDW phases. Compared to other semiconductor 2D-TMDs such as MoS2 and WS2, the metallic TaS2 has been theoretically proven to capture gas molecules more easily because Ta has a stronger positive charge than Mo or W. Our experimental results provide evidence of this. Notably, this study is the first example of H2 sensing using 1T-TaS2 thin films and demonstrates the possibility of controlling the reactivity of the sensors to the gas by changing the electronic structure via CDW phase transitions.

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.

Orbital-Hybridization-Driven Charge Density Wave Transition in CsV3 Sb5 Kagome Superconductor.

Owing to its inherent non-trivial geometry, the unique structural motif of the recently discovered kagome topological superconductor AV3 Sb5 (A = K, Rb, Cs) is an ideal host of diverse topologically non-trivial phenomena, including giant anomalous Hall conductivity, topological charge order, charge density wave (CDW), and unconventional superconductivity. Despite possessing a normal-state CDW order in the form of topological chiral charge order and diverse superconducting gaps structures, it remains unclear how fundamental atomic-level properties and many-body effects including Fermi surface nesting, electron-phonon coupling, and orbital hybridization contribute to these symmetry-breaking phenomena. Here, the direct participation of the V3d-Sb5p orbital hybridization in mediating the CDW phase transition in CsV3 Sb5 is reported. The combination of temperature-dependent X-ray absorption and first-principles studies clearly indicates the inverse Star-of-David structure as the preferred reconstruction in the low-temperature CDW phase. The results highlight the critical role that Sb orbitals play and establish orbital hybridization as the direct mediator of the CDW states and structural transition dynamics in kagome unconventional superconductors. This is a significant step toward the fundamental understanding and control of the emerging correlated phases from the kagome lattice through the orbital interactions and provides promising approaches to novel regimes in unconventional orders and topology.

Electron correlations rule the phonon-driven instability in single-layer TiSe2

We investigate the controversial case of charge-density-wave (CDW) order in single layer 1T-TiSe$_2$ by employing the density functional perturbation theory with on-site Hubbard interactions. The results emphasize the crucial role of electron correlations via Hubbard corrections in order to capture the accurate electronic structure, low- and high-temperature limits of the CDW phonon mode, and temperature-charge phase diagram. We show, in close agreement with the experiments, that total phase diagram consists of both commensurate and incommensurate CDW regions, where the latter coincide with the superconductive phase and might be instrumental for its formation. In addition to the established roles of quantum lattice fluctuations and excitonic interactions, our analysis emphasizes the overlooked crucial role of the momentum dependent electron-phonon coupling and electron correlations for the CDW phase transition in single layer TiSe$_2$.

Violation of Kohler's rule at the charge density wave transition in 1T-TiSe2

The Kohler scaling of magnetoresistance (MR) is examined across the charge-density-wave (CDW) transition of 1T-TiSe2 microflakes. The normalized MR value vs (B/ρ0)2 showed linear dependence, where B is the magnetic field and ρ0 is the resistivity at zero magnetic field. However, all MR-(B/ρ0)2 curves at various temperatures were not coincident with each other, demonstrating the violation of Kohler's rule. The Kohler slope, K = dMR/d[(B/ρ0)2], and the MR value reached a minimum at the CDW transition temperature, TCDW. The nonmonotonous relationship between the Kohler slope and the Hall coefficient was observed, indicating a nonuniform electron scattering rate on the Fermi surface. The MR value satisfied the scaling laws of ∼|T − TCDW|1.35 and ∼|T − TCDW|0.62 below and above TCDW, respectively. Our results highlight magnetotransport properties of the 1T-TiSe2 microflakes in the critical region of the CDW phase transition.

Orbital-selective band hybridisation at the charge density wave transition in monolayer TiTe2

Reducing the thickness of a material to its two-dimensional (2D) limit can have dramatic consequences for its collective electronic states, including magnetism, superconductivity, and charge and spin ordering. An extreme case is TiTe2, where a charge density wave (CDW) emerges in the single-layer, which is absent for the bulk compound, and whose origin is still poorly understood. Here, we investigate the electronic band structure evolution across this CDW transition using temperature-dependent angle-resolved photoemission spectroscopy. Our study reveals an orbital-selective band hybridisation between the backfolded conduction and valence bands occurring at the CDW phase transition, which in turn leads to a significant electronic energy gain, underpinning the CDW transition. For the bulk compound, we show how this energy gain is almost completely suppressed due to the three-dimensionality of the electronic band structure, including via a kz-dependent band inversion which switches the orbital character of the valence states. Our study thus sheds new light on how control of the electronic dimensionality can be used to trigger the emergence of new collective states in 2D materials.