Charge Density Wave State Research Articles

Low temperature Raman and X-ray diffraction studies on Fe intercalated VSe2

1T-VSe2 is known to show charge density wave (CDW) below ~ 110 K. The intercalation of Fe suppresses the CDW transition and induces a metal to insulator like transition near 165 K. We have performed low temperature structural and Raman studies on the FexVSe2 (x = 0, 0.10, 0.25) compounds, besides the electronic and thermal transport studies to understand the nature of this transition. The low temperature x-ray diffraction patterns of the intercalated compound show a distinct change in c - parameter of the unit cell and the Raman spectroscopic studies shows a shift in Eg mode (~ 246–252 cm−1) in the transition regime. Absence of any anomaly in the Seebeck coefficient near the transition rules out the possibility of Fermi surface driven changes in the electronic properties. These observations suggest the importance of structural distortion in the electronic properties of the Fe intercalated VSe2. Our low temperature Raman study in the temperature range 80–300 K, highlights the fact that A1g mode (at ~ 205 cm−1) and Eg mode (at ~ 255 cm−1) are not linked to the CDW state of 1T-VSe2 and are observed for the FexVSe2 as well.

Tri-hexagonal charge order in kagome metal CsV3Sb5 revealed by 121Sb nuclear quadrupole resonance

We report 121Sb nuclear quadrupole resonance (NQR) measurements on kagome superconductor CsV3Sb5 with T c = 2.5 K. 121Sb NQR spectra split after a charge density wave (CDW) transition at 94 K, which demonstrates a commensurate CDW state. The coexistence of the high temperature phase and the CDW phase between 91 K and 94 K manifests that it is a first order phase transition. The CDW order exhibits tri-hexagonal deformation with a lateral shift between the adjacent kagome layers, which is consistent with 2 × 2 × 2 superlattice modulation. The superconducting state coexists with CDW order and shows a conventional s-wave behavior in the bulk state.

Photoinduced charge density wave phase in 1T-TaS 2 : growth and coarsening mechanisms

Recent experiments have shown that the high-temperature incommensurate (I) charge density wave (CDW) phase of 1T-TaS 2 can be photoinduced from the lower-temperature, nearly commensurate CDW state. In a first step, several independent regions exhibiting I-CDW phase modulations nucleate and grow. After coalescence, these regions form a multidomain I-CDW phase that undergoes coarsening dynamics, i.e. a progressive increase of the domain size or I-CDW correlation length. Using time-resolved X-ray diffraction, we show that the wave vector of the photoinduced I-CDW phase is shorter than in the I-CDW phase at equilibrium, and progressively increases towards its equilibrium value as the correlation length increases. We interpret this behaviour as a consequence of a self-doping of the photoinduced I-CDW, following the presence of trapped electrons in the vicinity of CDW dislocation sites. Putting together results of the present and past experiments, we develop a scenario in which the I-CDW dislocations are created during the coalescence of the I-CDW phase regions.

Emergence of New van Hove Singularities in the Charge Density Wave State of a Topological Kagome Metal RbV_{3}Sb_{5}.

Quantum materials with layered kagome structures have drawn considerable attention due to their unique lattice geometry, which gives rise to flat bands together with Dirac-like dispersions. Recently, vanadium-based materials with layered kagome structures were discovered to be topological metals, which exhibit charge density wave (CDW) properties, significant anomalous Hall effect, and unusual superconductivity at low temperatures. Here, we employ angle-resolved photoemission spectroscopy to investigate the electronic structure evolution upon the CDW transition in a vanadium-based kagome material RbV_{3}Sb_{5}. The CDW phase transition gives rise to a partial energy gap opening at the boundary of the Brillouin zone and, most importantly, the emergence of new van Hove singularities associated with large density of states, which are absent in the normal phase and might be related to the superconductivity observed at lower temperatures. Our work sheds light on the microscopic mechanisms for the formation of the CDW and superconducting states in these topological kagome metals.

Seebeck effect studies in the charge density wave state of organic conductor α-(BEDT–TTF)2KHg(SCN)4

Angular, magnetic field and temperature dependence of the interlayer Seebeck effect of the multiband organic conductor is experimentally studied at temperatures down to 0.55 K and fields up to 31 T in a wide range of angles. The background magnetic field and angular component of the Seebeck effect as well as the magnetic quantum oscillations that originate from the closed Fermi surface orbits are analyzed. The background interlayer Seebeck effect components show that above certain tilt angle of the magnetic field and above the kink field there is another CDW state in , between previously known CDW0 and CDW x states, in agreement with magnetoresistance and magnetization studies in this material. Our observations show that this state possesses some of the properties of the CDW0 state. The Fermi surface in the third CDW state is still reconstructed but less imperfectly nested as expected as this state develops above the kink field. The temperature dependence of the interlayer Seebeck effect reveals that this state is developed at temperatures below 3 K and at field orientations around the second AMRO maximum. In addition, for the first time, a detailed T − θ phase diagram of based purely on Seebeck effect measurements is presented. We find that other states and transitions, beside the CDW states, also exist in a given temperature and angular range that have not been previously reported. These observations change the whole picture about the transport processes in the organic conductor and allow to better understand the complex nature of the CDW order in this and similar compounds.

Nearly fully opened charge density wave gap in the quasi-two-dimensional conductor γ−Mo4O11 : A comparative study with η−Mo4O11

The accommodation of Luttinger liquid behavior and unique quasiparticles in the charge density wave (CDW) state has led to renewed interest in the low-dimensional molybdenum bronzes. Here, the authors report a comprehensive experimental study on the quasi-two-dimensional CDW oxide bronzes Mo${}_{4}$O${}_{11}$, in which the CDW instability is dominated by hidden Fermi surface nesting, resulting in magnetic field induced quantum oscillations. Different from the monoclinic $\ensuremath{\eta}$ phase, the orthorhombic $\ensuremath{\gamma}$ phase shows a flexible CDW transition with a nearly fully opened gap, as seen in nonlinear transport behavior related to the CDW sliding motion, which is only observed in quasi-one-dimensional systems.

Magneto-Seebeck effect and ambipolar Nernst effect in the CsV3Sb5 superconductor

We present a study of Seebeck and Nernst effect in combination with magnetoresistance and Hall measurements of the Kagome superconductor ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$. A sizable magneto-Seebeck signal appears once the charge density wave (CDW) order sets in below ${T}_{\text{CDW}}=94$ K. The Nernst signal peaks at a lower temperature ${T}^{*}\ensuremath{\sim}35\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, crossing which the Hall coefficient switches sign, which we attribute to the ambipolar transport of compensated bands due to the multiband nature of ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$. Sublinear Nernst signal and Hall resistivity also develop well below the CDW transition, which becomes apparent below ${T}^{*}$. These findings suggest that, the multiband electronic profile plays an important role in the transport properties of the CDW state in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$.

First-principles study of the double-dome superconductivity in the kagome material CsV3Sb5 under pressure

Recent high-pressure experiments discovered abnormal double-dome superconductivities in the newly synthesized kagome materials ${A\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ $(A=\mathrm{K}, \mathrm{Rb}, \mathrm{Cs})$, which also host abundant emergent quantum phenomena such as charge density wave (CDW), anomalous Hall effect, nontrivial topological property, and so on. In this work, by using first-principles electronic structure calculations, we studied the CDW state, superconductivity, and topological property in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ under pressures $(< 50 \mathrm{GPa})$. Based on the electron-phonon coupling theory, our calculated superconducting ${T}_{\text{c}}\mathrm{s}$ are consistent with the observed ones in the second superconducting dome at high pressure, but are much higher than the measured values at low pressure. The further calculations including the Hubbard $U$ indicate that with modest electron-electron correlation the magnetism on the V atoms exists at low pressure and diminishes gradually at high pressure. We thus propose that the experimentally observed superconductivity in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ at ambient/low pressures may still belong to the conventional Bardeen-Cooper-Schrieffer (BCS) type, but is partially suppressed by the V magnetism, while the superconductivity under high pressure is fully conventional without invoking the magnetism. We also predict that there are a second weak CDW state and topological phase transitions in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ under pressures. Our theoretical assertion calls for future experimental examination.

Structures and physical properties of v-based kagome metals csv6sb6 and csv8sb12 * Supported by the National Key R&D Program of China (Grant No. 2018YFE0202600), the Beijing Natural Science Foundation (Grant No. Z200005), the National Natural Science Foundation of China (Grant Nos. 11822412 and 11774423), the Fundamental Research Funds for the Central Universities and Research Funds of Renmin University of China

We report two new members of V-based kagome metals CsV6Sb6 and CsV8Sb12. The most striking structural feature of CsV6Sb6 is the V kagome bilayers. For CsV8Sb12, there is an intergrowth of two-dimensional V kagome layers and one-dimensional V chains, and the latter ones lead to the orthorhombic symmetry of this material. Further measurements indicate that these two materials exhibit metallic and Pauli paramagnetic behaviors. More importantly, different from CsV3Sb5, the charge density wave state and superconductivity do not emerge in CsV6Sb6 and CsV8Sb12 when temperature is above 2 K. Small magnetoresistance with saturation behavior and linear field dependence of Hall resistivity at high field and low temperature suggest that the carriers in both materials should be uncompensated with much different concentrations. The discovery of these two new V-based kagome metals sheds light on the exploration of correlated topological materials based on kagome lattice.

Quantum Transport Evidence of Topological Band Structures of Kagome Superconductor CsV3Sb5

We report the transport properties of kagome superconductor CsV_{3}Sb_{5} single crystals at magnetic field up to 32 T. The Shubnikov-de Haas oscillations emerge at low temperature and four frequencies of F_{α}=27 T, F_{β}=73 T, F_{ε}=727 T, and F_{η}=786 T with relatively small cyclotron masses are observed. For F_{β} and F_{ε}, the Berry phases are close to π, providing clear evidence of nontrivial topological band structures of CsV_{3}Sb_{5}. Furthermore, the consistence between theoretical calculations and experimental results implies that these frequencies can be assigned to the Fermi surfaces locating near the boundary of Brillouin zone and confirms that the structure with an inverse Star of David distortion could be the most stable structure at charge density wave state. These results will shed light on the nature of correlated topological physics in kagome material CsV_{3}Sb_{5}.

Maximal superconductivity in proximity to the charge density wave quantum critical point in CuxTiSe2

Superconductivity emerges in $1T$-TiSe$_2$ when its charge density wave (CDW) order is suppressed by Cu intercalation or pressure. Since the CDW state is thought to be an excitonic insulator, an interesting question is whether the superconductivity is also mediated by the excitonic fluctuations. We investigated this question as to the nature of doping induced superconductivity in Cu$_x$TiSe$_2$ by asking if it is consistent with the phonon-mediated pairing. We employed the {\it ab initio} density functional theory and density functional perturbation theory to compute the electron-phonon coupling Eliashberg function from which to calculate the superconducting (SC) critical temperature $T_c$. The calculated $T_c $ as a function of the doping concentration $x$ exhibits a dome shape with the maximum $T_c$ of $2-6$ K at $x \approx 0.05$ for the Coulomb pseudopotential $0 \leq \mu^* \leq 0.1$. The maximal $T_c$ was found to be pinned to the quantum critical point at which the CDW is completely suppressed and the corresponding phonon mode becomes soft. Underlying physics is that the reduced phonon frequency enhances the electron-phonon coupling constant $\lambda$ which overcompensates the frequency decrease to produce a net increase of $T_c$. The doping induced superconductivity in Cu$_x$TiSe$_2$ seems to be consistent with the phonon-mediated pairing. Comparative discussion was made with the pressure induced superconductivity in TiSe$_2$.

Interplay between charge density wave order and magnetic field in the nonmagnetic rare-earth tritelluride LaTe3

Charge density wave (CDW) states in solids bear an intimate connection to underlying fermiology. Modification of the latter by a suitable perturbation provides an attractive handle to unearth novel CDW states. Here, we combine extensive magnetotransport experiments and first-principles electronic structure calculations on a non-magnetic tritelluride LaTe$_{3}$ single crystal to uncover phenomena rare in CDW systems: $(i)$ hump-like feature in the temperature dependence of resistivity at low temperature under application of magnetic field, which moves to higher temperature with increasing field strength, $(ii)$ highly anisotropic large transverse magnetoresistance (MR) upon rotation of magnetic field about current parallel to crystallographic c-axis, (iii) anomalously large positive MR with spike-like peaks at characteristic angles when the angle between current and field is varied in the bc-plane, (iv) extreme sensitivity of the angular variation of MR on field and temperature. Moreover, our Hall measurement reveals remarkably high carrier mobility $\sim$ 33000 cm$^{2}$/Vs, which is comparable to that observed in some topological semimetals. These novel observations find a comprehensive explication in our density functional theory (DFT) and dynamical mean field theory (DMFT) calculations that capture field-induced electronic structure modification in LaTe$_{3}$. The band structure theory together with transport calculations suggest the possibility of a second field-induced CDW transition from the field-reconstructed Fermi surface, which qualitatively explains the hump in temperature dependence of resistivity at low temperature. Thus, our study exposes the novel manifestations of the interplay between CDW order and field-induced electronic structure modifications in LaTe$_{3}$, and establishes a new route to tune CDW states by perturbations like magnetic field.

Modulating charge density wave states in TaSe2 by an electride substrate

Modulating charge density wave (CDW) states in layered materials has both fundamental scientific value and application potential in future electronic devices. Based on first-principles electronic structure calculations, we have studied the modulation of the CDW states in ${\mathrm{TaSe}}_{2}$ by using a typical electride ${\mathrm{Ca}}_{2}\mathrm{N}$ as the substrate. We find that the ${\mathrm{Ca}}_{2}\mathrm{N}$ monolayer can donate 0.49 electrons/f.u. to the ${\mathrm{TaSe}}_{2}$ monolayer and meanwhile avoids the disorder effect in conventional chemical substitution approach. With the uniform electron doping from ${\mathrm{Ca}}_{2}\mathrm{N}$, the CDW order in $1H\ensuremath{-}{\mathrm{TaSe}}_{2}$ is completely suppressed; in comparison, the CDW period in $1T\ensuremath{-}{\mathrm{TaSe}}_{2}$ transforms from the Star of David pattern to a $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ triangular pattern. Our studies enrich the phase diagram of ${\mathrm{TaSe}}_{2}$ and highlight the effective manipulation of the CDW states via an electride substrate, which calls for future experimental verification.

Anomalous charge density wave state evolution and dome-like superconductivity in CuIr2Te4−x Se x chalcogenides

We report the anomalous charge density wave (CDW) state evolution and dome-like superconductivity in CuIr2Te4−x Se x (0 ⩽ x ⩽ 0.5) series. Room temperature powder x-ray diffraction (XRD) results indicate that CuIr2Te4−x Se x (0 ⩽ x ⩽ 0.5) compounds retain the same structure as the host CuIr2Te4 and the unit cell constants a and c manifest a linear decline with increasing Se content. Magnetization, resistivity and heat capacity results suggest that superconducting transition temperature (T c) exhibits a weak dome-like variation as substituting Te by Se with the maximum T c = 2.83 K for x = 0.1 followed by suppression in T c and simultaneous decrease of the superconducting volume fraction. Unexpectedly, the CDW-like transition (T CDW) is suppressed with lower Se doping (0.025 ⩽ x ⩽ 0.2) but re-emerges at higher doping (0.25 ⩽ x ⩽ 0.5). Meanwhile, the temperature-dependent XRD measurements show that the trigonal structure is stable at 20 K, 100 K and 300 K for the host sample and the doping composition with x = 0.5, thus we propose that the behavior CDW-like transition arises from the disorder effect created by chemical doping and is not related to structural transition. The lower and the upper critical fields of these compounds are also addressed.

Charge Density Wave and Electron-Phonon Interaction in Epitaxial Monolayer NbSe2 FilmsSupported by the National Natural Science Foundation of China (Grant Nos. 11774154, 11790311, and 12004172), the National Key Research and Development Program of China (Grant No. 2018YFA0306800), the Fundamental Research Funds for the Central Universities (Grant No. 0204-14380186), the Jiangsu Planned Projects for Postdoctoral Research Funds (Grant No. 2020Z172), the

Understanding the interplay between superconductivity and charge-density wave (CDW) in NbSe2 is vital for both fundamental physics and future device applications. Here, combining scanning tunneling microscopy, angle-resolved photoemission spectroscopy and Raman spectroscopy, we study the CDW phase in the monolayer NbSe2 films grown on various substrates of bilayer graphene (BLG), SrTiO3(111), and Al2O3(0001). It is found that the two stable CDW states of monolayer NbSe2 can coexist on NbSe2/BLG surface at liquid-nitrogen temperature. For the NbSe2/SrTiO3(111) sample, the unidirectional CDW regions own the kinks at ±41 meV and a wider gap at 4.2 K. It is revealed that the charge transfer from the substrates to the grown films will influence the configurations of the Fermi surface, and induce a 130 meV lift-up of the Fermi level with a shrink of the Fermi pockets in NbSe2/SrTiO3(111) compared with the NbSe2/BLG. Combining the temperature-dependent Raman experiments, we suggest that the electron-phonon coupling in monolayer NbSe2 dominates its CDW phase transition.

Ultrafast excitation and topological soliton formation in incommensurate charge density wave states

Topological soliton is a nonperturbative excitation in commensurate density wave states and connects degenerate ground states. In incommensurate density wave states, ground states are continuously degenerate and topological soliton is reckoned to be smoothly connected to the perturbative phason excitation. We study the ultrafast nonequilibrium dynamics due to photoexcited electron-hole pair in a one-dimensional chain with an incommensurate charge density wave ground state. Time-resolved evolution reveals both perturbative excitation of collective modes and nonperturbative topological phase transition due to creating novel topological solitons, where the continuous complex order parameter with amplitude and phase is essential. We identify the nontrivial phase-winding solitons in the complex plane unique to this nonequilibrium state and capture it by a low-energy effective model. The perturbative temporal gap oscillation and the solitonic in-gap states enter the optical conductivity absorption edge and the spectral density related to spectroscopic measurement, providing concrete connections to real experiments.

Observation of pressure induced charge density wave order and eightfold structure in bulk VSe2

Pressure-induced charge density wave (CDW) state can overcome the low-temperature limitation for practical application, thus seeking its traces in experiments is of great importance. Herein, we provide spectroscopic evidence for the emergence of room temperature CDW order in the narrow pressure range of 10–15 GPa in bulk VSe2. Moreover, we discovered an 8-coordination structure of VSe2 with C2/m symmetry in the pressure range of 35–65 GPa by combining the X-ray absorption spectroscopy, X-ray diffraction experiments, and the first-principles calculations. These findings are beneficial for furthering our understanding of the charge modulated structure and its behavior under high pressure.

Charge Density Wave Orders and Enhanced Superconductivity under Pressure in the Kagome Metal CsV3 Sb5.

Superconductivity in topological kagome metals has recently received great research interests. Here, charge density wave (CDW) orders and the evolution of superconductivity under various pressures in CsV3 Sb5 single crystal with V kagome lattice are investigated. By using high-resolution scanning tunneling microscopy/spectroscopy (STM/STS), two CDW orders in CsV3 Sb5 are observed which correspond to 4a× 1a and 2a× 2a superlattices. By applying pressure, the superconducting transition temperature Tc is significantly enhanced and reaches a maximum value of 8.2 K at around 1GPa. Accordingly, CDW state is gradually declined as increasing the pressure, which indicates the competing interplay between CDW and superconducting state in this material. The broad superconducting transitions around 0.4-0.8GPa can be related to the strong competition relation among two CDW states and superconductivity. These results demonstrate that CsV3 Sb5 is a new platform for exploring the interplay between superconductivity and CDW in topological kagome metals.

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 ${R}_{5}{\mathrm{Ir}}_{4}{\mathrm{Si}}_{10}$.

Aspects of symmetry and topology in the charge density wave phase of 1T–TiSe2

The charge density wave (CDW) in 1T–TiSe2 harbors a nontrivial symmetry configuration. It is important to understand this underlying symmetry both for gaining a handle on the mechanism of CDW formation and for probing the CDW experimentally. Here, based on first-principles computations within the framework of the density functional theory, we unravel the connection between the symmetries of the normal and CDW states and the electronic structure of 1T–TiSe2. Our analysis highlights the key role of irreducible representations of the electronic states and the occurrence of band gaps in the system in driving the CDW. By showing how symmetry-related topology can be obtained directly from the electronic structure, our study provides a practical pathway in search of topological CDW insulators.