Charge Density Wave Behavior Research Articles

Effects of lattice instability on the thermoelectric behavior of kagome metal ScV6Sn6

Kagome metal ScV6Sn6 has been a subject of interest due to the emergence of a first-order structural phase transition with intriguing charge density wave behavior below the transition temperature Tc ∼ 92 K. To explore the thermoelectric properties and provide experimental insights into the nature of the phase transition, we have carried out a combined study by means of the electrical resistivity, Seebeck coefficient, and thermal conductivity measurements on single crystalline ScV6Sn6. Pronounced features near Tc have been characterized by all measured physical quantities. In particular, the Seebeck coefficient exhibits a marked reduction as lowering temperature across Tc, attributed to an imbalance of the contribution from different type of carriers induced by the structural phase transition. From the examination of the electronic and lattice thermal conductivities, we obtained a confirmation that the observed enhancement at Tc is essentially caused by the change of the lattice thermal conductivity, demonstrating the primary importance of lattice distortions for the heat transport of ScV6Sn6. In addition, the lattice thermal conductivity above Tc was found to increase monotonically with temperature. We associated the peculiar phenomenon with lattice fluctuations, highlighting the essence of structural instability in the kagome lattice ScV6Sn6. These results add to the knowledge about the thermal transport properties in kagome materials with a hexagonal HfFe6Ge6-type structure.

Alloying effect of rare-earth tritellurides on the charge density wave and magnetic properties

Among many van der Waals materials rare-earth tritellurides (RTe3) allow studying several phenomena like magnetic, superconducting, and charge density wave (CDW). These studies show the effect of cationic alloying of antiferromagnetic RTe3 for fully tunable near room-temperature CDW properties. DyxGd1−xTe3 and DyxTb1−xTe3 alloys were synthesized through a chemical vapor transport technique, and the rare-earth element composition was controlled by changing the ratio of rare-earth metal reagents. The results show that the lattice parameters can be continuously tuned with the composition of the rare-earth cations leading to the variation of the internal chemical pressure. Temperature-dependent Raman spectroscopy and electric transport measurement show that the CDW transition temperature (TCDW) of RTe3 alloys varies with lattice parameters/chemical pressure spanning across 300–380 K. Additional magnetism studies offer the first insights into the magnetic ordering in RTe3 alloys. The emergence of multiple magnetic transitions implies complex magnetic interactions that arise from interactions between different rare-earth elements. Overall findings introduce ways to control the CDW behavior and provide valuable insights into the magnetic ordering in RTe3 alloys, contributing to further investigation and a better understanding of their properties.

The phononic and charge density wave behavior of entire rare-earth tritelluride series with chemical pressure and temperature

Here, we present comprehensive phononic and charge density wave properties (CDW) of rare-earth van der Waals tritellurides through temperature dependent angle-resolved Raman spectroscopy measurements. All the possible rare-earth tritellurides (RTe3) ranging from R = La–Nd, Sm, Gd–Tm were synthesized through a chemical vapor transport technique to achieve high quality crystals with excellent CDW characteristics. Raman spectroscopy studies successfully identify the emergence of the CDW state and transition temperature (TCDW), which offers a non-destructive method to identify their CDW response with micron spatial resolution. Temperature dependent Raman measurements further correlate how the atomic mass of metal cations and the resulting chemical pressure influence its CDW properties and offer detailed insight into the strength of CDW amplitude mode-phonon coupling during the CDW transition. Angle-resolved Raman measurements offer the first insights into the CDW-phonon symmetry interplay by monitoring the change in the symmetry of phonon mode across the CDW transition. Overall results introduce the library of RTe3 CDW materials and establish their characteristics through the non-destructive angle-resolved Raman spectroscopy technique.

Al27 NMR insight into the phase transition in BaFe2Al9

We have applied $^{27}\mathrm{Al}$ nuclear magnetic resonance (NMR) spectroscopy to investigate the iron-based aluminide of $\mathrm{Ba}{\mathrm{Fe}}_{2}{\mathrm{Al}}_{9}$. This material has been a subject of current interest due to indications of charge density wave behavior below the transition temperature ${T}_{C}\ensuremath{\simeq}100$ K. Two sets of the $^{27}\mathrm{Al}$ NMR resonance lines that are associated with two nonequivalent crystallographic aluminum sites have been well resolved. We have discussed the obtained electric field gradient and anisotropic Knight shift for each individual aluminum site, revealing the strong $ab$ plane bonding configuration for the structural properties of $\mathrm{Ba}{\mathrm{Fe}}_{2}{\mathrm{Al}}_{9}$. Pronounced features in the isotropic Knight shift and nuclear spin-lattice relaxation rate have been observed in the vicinity of ${T}_{C}$. Furthermore, the detailed $^{27}\mathrm{Al}$ NMR analyses have provided evidence for the decrease of the electronic density of states upon lowering temperature below ${T}_{C}$.

Intrinsic and doping-enhanced superconductivity in monolayer 1H−TaS2 : Critical role of charge ordering and spin-orbit coupling

The interplay of superconductivity with charge density wave (CDW) in metallic transition-metal dichalcogenides has been widely debated, and viable strategies manipulating these quantum states in the two-dimensional (2D) limit remain unclear. Using the ab initio anisotropic Migdal-Eliashberg theory, we successfully explain the superconductivity observed in monolayer $1H\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ by simultaneously determining its precise CDW structure and treating the marked modification of electron-phonon interaction and critical temperature ${T}_{\mathrm{c}}$ by spin-orbit coupling effects. With this paradigm, we further show that electron doping weakens the CDW order leading to increased ${T}_{\mathrm{c}}$ up to 11 K, along with a single-gap to two-gap superconductivity transition due to the suppression of the CDW gap. By contrast, a low hole doping barely affects the CDW but still yields a significantly enhanced superconducting order, implying their good coexistence. Combined with the synergistic behavior of CDW and superconductivity, which cooperate upon ${\mathrm{TaS}}_{2}$ thickness reduction causing an unusual rise of ${T}_{\mathrm{c}}$, our results unravel diversified interactions between the two collective orders in ultrathin ${\mathrm{TaS}}_{2}$, being competition, coexistence or cooperation depending on external stimuli, which provide key clues for controlling correlated states in devices based on 2D CDW superconductors.

Electronic states of multilayer VTe2 : Quasi-one-dimensional Fermi surface and implications for charge density waves

We have performed angle-resolved photoemission spectroscopy on epitaxial $\mathrm{V}{\mathrm{Te}}_{2}$ films to elucidate the relationship between the fermiology and charge density waves (CDWs). We found that a two-dimensional triangular pocket in 1 monolayer (ML) $\mathrm{V}{\mathrm{Te}}_{2}$ is converted to a strongly warped quasi-one-dimensional (1D) Fermi surface in the 6 ML counterpart, likely associated with the $1T$-to-$1{T}^{\ensuremath{'}}$ structural phase transition. We also revealed a metallic Fermi edge on the entire Fermi surface in 6 ML at low temperature distinct from anisotropic pseudogap in 1 ML, signifying a contrast behavior of CDW that is also supported by first-principles band-structure calculations. The present result points to the importance of simultaneously controlling the structural phase and fermiology to manipulate the CDW properties in ultrathin transition-metal dichalcogenides.

Superconductivity in a New Layered BiSe2-Based Compound LaO1-xBiSe2

We report in this contribution a new superconductor LaO1-xBiSe2 which was synthesized by solid-state reaction method. The structure analysis indicated that LaO1-xBiSe2 compound has typical CeOBiS2-type structure with P4/nmm space group. Compared with the parent phase LaOBiSe2, the lattice parameters of LaO0.9BiSe2 contracted. Physical property characterizations demonstrated that the charge density wave behavior of the parent phase was suppressed by oxygen vacancies and a superconducting transition at 2.9 K was observed in LaO0.9BiSe2 sample. The present work shows that the introduction of oxygen vacancies without extrinsic doping is an effective approach to induce superconductivity in the LaOBiSe2 system.

Angle-dependent magnetoresistance as a sensitive probe of the charge density wave in quasi-one-dimensional semimetal Ta2NiSe7

The behavior of charge density waves (CDWs) in an external magnetic field is dictated by both orbital and Pauli (Zeeman) effects. A quasi-one-dimensional (Q1D) system features Q1D Fermi surfaces that allow these effects to be distinguished, which in turn can provide a sensitive probe to the underlying electronic states. Here, we studied the field dependence of an incommensurate CDW in a transition-metal chalcogenide Ta2NiSe7 with a Q1D chain structure. The angle-dependent magnetoresistance (MR) is found to be very sensitive to the relative orientation between the magnetic field and the chain direction. With an applied current fixed along the b axis (the chain direction), the angle-dependent MR shows a striking change of the symmetry below TCDW only for a rotating magnetic field in the ac plane. In contrast, the symmetry axis remains unchanged for other configurations (H in ab and bc planes). The orbital effect conforms to the lattice symmetry, while the Pauli effect in the form of μBB/ℏvF can be responsible for such symmetry change, provided that the Fermi velocity vF is significantly anisotropic and the nesting vector changes in a magnetic field, which is corroborated by our first-principles calculations. Our results show that the angle-dependent MR is a sensitive transport probe of CDW and can be useful for the study of low-dimensional systems in general.

Charge density wave behavior and order-disorder in the antiferromagnetic metallic seriesEu(Ga1−xAlx)4

The solid solution Eu(Ga_1-xAl_x)_4 was grown in single crystal form to reveal a rich variety of crystallographic, magnetic, and electronic properties that differ from the isostructural end compounds EuGa_4 and EuAl_4, despite the similar covalent radii and electronic configurations of Ga and Al. Here we report the onset of magnetic spin reorientation and metamagnetic transitions for x = 0 - 1 evidenced by magnetization and temperature-dependent specific heat measurements. T_N changes non-monotonously with x, and it reaches a maximum around 20 K for x = 0.50, where the a lattice parameter also shows an extreme (minimum) value. Anomalies in the temperature-dependent resistivity consistent with charge density wave behavior exist for x = 0.50 and 1 only. Density functional theory calculations show increased polarization between the Ga-Al covalent bonds in the x = 0.50 structure compared to the end compounds, such that crystallographic order and chemical pressure are proposed as the causes of the charge density wave behavior.

Strongly Coupled Electron–Phonon Dynamics in Few-Layer TiSe2 Exfoliates

Ultrafast electron diffraction is used to probe the time-resolved dynamics in a few-layer TiSe2 sample. At normal incidence, the suppression of the Bragg diffraction peak intensities following photoexcitation displays strongly biexponential behavior. For tilted samples, changes in the diffraction peak positions reveal coherent acoustic vibrations that are dependent on the sample thickness and that further permit a calculation of the Young’s modulus. The complex room temperature lattice dynamics observed are attributed to strong electron–phonon coupling and electron–lattice equilibration processes, which support a Jahn–Teller origin of the charge density wave behavior in TiSe2. Additionally, the significant role that the related Kohn anomalies may play in the electron transport dynamics and transition mechanism of this material is emphasized. These results demonstrate the importance of strongly coupled electron–phonon dynamics in the relaxation of electronically excited room temperature TiSe2, which is exp...

Dispersive charge density wave excitations in Bi2Sr2CaCu2O8+δ

Experimental evidence on high-Tc cuprates reveals ubiquitous charge density wave (CDW) modulations, which coexist with superconductivity. Although the CDW had been predicted by theory, important questions remain about the extent to which the CDW influences lattice and charge degrees of freedom and its characteristics as functions of doping and temperature. These questions are intimately connected to the origin of the CDW and its relation to the mysterious cuprate pseudogap. Here, we use ultrahigh resolution resonant inelastic x-ray scattering (RIXS) to reveal new CDW character in underdoped Bi2Sr2CaCu2O8+{\delta} (Bi2212). At low temperature, we observe dispersive excitations from an incommensurate CDW that induces anomalously enhanced phonon intensity, unseen using other techniques. Near the pseudogap temperature T*, the CDW persists, but the associated excitations significantly weaken and the CDW wavevector shifts, becoming nearly commensurate with a periodicity of four lattice constants. The dispersive CDW excitations, phonon anomaly, and temperature dependent commensuration provide a comprehensive momentum space picture of complex CDW behavior and point to a closer relationship with the pseudogap state.

Effect of dimensionality on sliding charge density waves: The quasi-two-dimensionalTbTe3system probed by coherent x-ray diffraction

We report on sliding Charge Density Wave (CDW) in the quasi two-dimensional TbTe$_3$ system probed by coherent x-ray diffraction combined with {\it in-situ} transport measurements. We show that the non-Ohmic conductivity in TbTe$_3$ is made possible thanks to a strong distortion of the CDW. Our diffraction experiment versus current shows first that the CDW remains undeformed below the threshold current I$_S$ and then suddenly rotates and reorders by motion above threshold. Contrary to quasi-one dimensional systems, the CDW in TbTe$_3$ does not display any phase shifts below I$_S$ and tolerates only slow spatial variations of the phase above. This is a first observation of CDW behavior in the bulk in a quasi-two dimensional system allowing collective transport of charges at room temperature.

Observation of a spin-density wave node on antiferromagnetic Cr(110) islands

We have performed a detailed study of the evolution of the charge-density wave (CDW) and spin-density wave (SDW) on the surface of nanoscale Cr islands grown on W(110). Using low-temperature scanning tunneling microscopy (STM), we find a striking nonmonotonic thickness dependence of the CDW wavelength at 50 K. As the local island thickness $\ensuremath{\Theta}$ decreases from about 50 to 5.2 nm, the CDW wavelength gradually increases from the bulk value by approximately 30$%$. We find a gap without any CDW at coverages ${\ensuremath{\Theta}}_{\mathrm{gap}}$ between 5.2 and 3.7 nm. Spin-resolved STM data reveal that within this CDW gap the SDW modulation also disappears on the Cr island surface. At $\ensuremath{\Theta}<3.7$ nm the CDW reappears. This unusual behavior of CDW and SDW in Cr nanoislands can be understood by a reorientation of the SDW wave vector Q which is potentially driven by the pinning of the SDW node at the island surface at ${\ensuremath{\Theta}}_{\mathrm{gap}}$.

Charge-density wave condensate in charge-ordered manganites: impact of ferromagnetic order and spin-glass disorder

The behavior of charge-density wave (CDW) condensates with the systematic introduction of ferromagnetic (FM) and spin-glass phases in the charge-ordered (CO) lattice of Nd0.5Sr0.5MnO3 (NSMO) and Eu0.5Sr0.5MnO3 (ESMO) epitaxial thin films was investigated by terahertz time-domain spectroscopy. The optical conductivity of (100)-oriented NSMO film, in which the CO lattice has an insignificant volume of the FM phase, exhibits a peak with attributes of CDW collective excitation at ∼3.2 meV. This peak becomes completely submerged in the Drude continuum as the volume of the epitaxial strain-induced FM phase increases in the CO state of (110)- and (111)-oriented NSMO films. In contrast, the ESMO (100) film, having a short-range CO phase amid a spin-glass-like state, displays a subtle CDW peak in conductivity. Modeling the optical terahertz conductivity with the phenomenological Lorentz–Drude relation and extracting the optical weight due to both contributions (Lorentz and Drude), it was found that the manifestation of CDW condensates is associated with the generic strength of charge ordering and that the condensate suffers a more pronounced suppression when FM order dilutes the CO lattice than when spin-glass disorder weakens the CO state.

Anomalous behavior of ultrasonic properties near 50 K in Rb0.30MoO3 and Rb0.30(Mo1−xVx)O3

Depending on the temperature, the charge density wave (CDW) nonlinear conductivity of the blue bronzes A0.30MoO3 (A=K, Rb) shows two different regimes: a strongly damped motion above ∼50K and motion with almost no damping below ∼50K. In a search for an elastic signature of this CDW behaviour, we performed ultrasonic measurements on Rb0.30MoO3 and Rb0.30(Mo1−xVx)O3 single crystals between 4K and 300K. In Rb0.30MoO3, at T∼50K, upon cooling, a large increase of the sound velocity is observed. The ultrasonic attenuation coefficient shows an increase down to 50K followed by a plateau. In Rb0.30(Mo1−xVx)O3 (x=0.4at%) the anomaly broadens and is shifted towards higher temperatures. The results are discussed in terms of CDW glass.

Extended Phonon Collapse and the Origin of the Charge-Density Wave in2H−NbSe2

We report inelastic x-ray scattering measurements of the temperature dependence of phonon dispersion in the prototypical charge-density-wave (CDW) compound 2H-NbSe2. Surprisingly, acoustic phonons soften to zero frequency and become overdamped over an extended region around the CDW wave vector. This extended phonon collapse is dramatically different from the sharp cusp in the phonon dispersion expected from Fermi surface nesting. Instead, our experiments, combined with ab initio calculations, show that it is the wave vector dependence of the electron-phonon coupling that drives the CDW formation in 2H-NbSe2 and determines its periodicity. This mechanism explains the so far enigmatic behavior of CDW in 2H-NbSe2 and may provide a new approach to other strongly correlated systems where electron-phonon coupling is important.

Thermal expansion measurements of the quasi-one-dimensional conductor K 0.30MoO 3

The charge density wave (CDW) dynamics of the quasi-one-dimensional conductor K 0.30MoO 3 shows two different regimes depending on the temperature: a strongly damped CDW motion above ∼ 50 K and CDW motion with almost no damping below ∼ 50 K . In a search for a characterization of this CDW behaviour, we performed thermal expansion measurements on K 0.30MoO 3 single crystals in the temperature range 4–250 K. In addition to the anomaly observed at the Peierls transition at 180 K along the [102] direction, an anomaly is observed at ∼ 50 K along the [ − 201 ] and [102] directions. The results are discussed in relation with the change in the CDW rigidity at ∼ 50 K .

Ultrasonic properties near 50 K of the quasi-one-dimensional conductorsA0.30MoO3 (A = K, Rb) andRb0.30(Mo1−xVx)O3

The charge density wave (CDW) nonlinear conductivity of the blue bronzesA0.30MoO3 (A = K, Rb) shows two different regimes depending on the temperature: a strongly damped CDW motion above∼50 K and a CDW motion with almost no damping below∼50 K.In a search for an elastic signature of this CDW behaviour, we performed ultrasonic measurementson A0.30MoO3 single crystals in the temperature range 4–300 K. InRb0.30MoO3,at T∼50 K, upon cooling, a large increase of the sound velocity for the longitudinal mode measuredalong the , [102] and b directions is observed. The ultrasonic attenuation coefficient shows anincrease down to 50 K followed by a plateau. Similar results are found inK0.30MoO3. In V-dopedsamples, Rb0.30(Mo1−xVx)O3 (x = 0.4%) the anomaly broadens and is shifted towards higher temperatures. The results are discussedin relation to the changes in the CDW rigidity, disorder and dielectric response. Ascenario based on a glass transition for the CDW superstructure is proposed.

Signatures of polaronic charge ordering in optical and dc conductivity using dynamical mean field theory

We apply dynamical mean field theory to study a prototypical model that describes charge ordering in the presence of both electron-lattice interactions and intersite electrostatic repulsion between electrons. We calculate the optical and dc conductivity and derive approximate formulas valid in the limiting electron-lattice coupling regimes. In the weak-coupling regime, we recover the usual behavior of charge density waves, characterized by a transfer of spectral weight due to the opening of a gap in the excitation spectrum. In the opposite limit of very strong electron-lattice coupling, instead, the charge ordering transition is signaled by a global enhancement of the optical absorption, with no appreciable spectral weight transfer. Such behavior is related to the progressive suppression of thermally activated charge defects taking place below the critical temperature. At intermediate values of the coupling within the polaronic regime, a complex behavior is obtained where both mechanisms of transfer and enhancement of spectral weight coexist.

Charge-Stripe Order in the Electronic FerroelectricLuFe2O4

The structural features of the charge ordering states in LuFe(2)O(4) are characterized by in situ cooling transmission electron microscopy observations from 300 K down to 20 K. Two distinctive structural modulations, a major q1=(1/3,1/3,2) and a weak q2 = q1/10+(0,0,3/2), have been well determined at the temperature of approximately 20 K. Systematic analysis demonstrates that the charges at low temperatures are well crystallized in a charge-stripe phase, in which the charge-density wave behavior in a nonsinusoidal fashion results in elemental electric dipoles for ferroelectricity. It is also noted that the charge ordering and ferroelectric domains often change markedly with lowering temperatures and yield a rich variety of structural phenomena.