Charge Density Wave Instability Research Articles

Superconductivity and charge density wave formation in TlxV6S8

We report on specific heat and resistivity measurements under hydrostatic pressure on the quasi-one dimensional metal TlxV6S8. We studied the interplay between the low temperature superconducting (SC) ground state and a high temperature charge density wave (CDW) instability. We observed a clear dependency of the physical properties of TlxV6S8 on the Tl concentration x. The CDW anomaly is present in all investigated samples that are strongly enhanced at half Tl filling, x = 0.47. This is also the only composition for which no signature of superconductivity is observed. The specific heat results regarding the SC phase in Tl0.63V6S8 suggest that this compound is a highly anisotropic, weak coupling superconductor. Pressure suppresses both SC and CDW transitions to lower temperatures. Nevertheless, as the CDW gap is closed at a critical pressure pc, the increase in the density of states leads to a small enhancement of Tc suggesting that SC and CDW compete for parts of the Fermi-surface.

Temperature-driven phase transformation in Y3Co: Neutron scattering and first-principles studies

Contrary to previous studies that identified the ground state crystal structure of the entire ${R}_{3}$Co series ($R$ is a rare earth) as orthorhombic $Pnma$, we show that Y${}_{3}$Co undergoes a structural phase transition at ${T}_{t}\ensuremath{\simeq}160$ K. Single crystal neutron diffraction data reveal that at ${T}_{t}$ the trigonal prisms formed by a cobalt atom and its six nearest-neighbor yttrium atoms experience distortions accompanied by notable changes of the Y-Co distances. The formation of the low-temperature phase is accompanied by a pronounced lattice distortion and anomalies seen in heat capacity and resistivity measurements. Density functional theory calculations reveal a dynamical instability of the $Pnma$ structure of Y${}_{3}$Co. In particular, a transversal acoustic phonon mode along the $(00\ensuremath{\xi})$ direction has imaginary frequencies at $\ensuremath{\xi}<1/4$. Employing inelastic neutron scattering measurements we find a strong damping of the $(00\ensuremath{\xi})$ phonon mode below a critical temperature ${T}_{t}$. The observed structural transformation causes the reduction of dimensionality of electronic bands and decreases the electronic density of states at the Fermi level that identifies Y${}_{3}$Co as a system with the charge density wave instability.

Possible quantum phase manipulation of a two-leg ladder in mixed-dimensional fermionic cold atoms

The recent realization of mixed-dimensional systems of cold atoms has attracted much attention from both experimentalists and theorists. In this article we investigate a two-species Fermi atom mixture: one species of atom exists in two hyperfine states and is confined to move in a two-leg ladder, interacting with an on-site interaction, and the other moves freely in a two-dimensional square lattice that contains the two-leg ladder. The two species of atoms interact via an on-site interaction on the ladder. In the limit of weak interspecies interactions, the two-dimensional gas can be integrated out, leading to an effective long-range mediated interaction in the ladder, generated by the on-site interspecies interaction. We show that the form of the mediated interaction can be controlled by the density of the two-dimensional gas and that it enhances the charge-density wave instability in the two-leg ladder after the renormalization-group transformation. Parametrizing the phase diagram with various experimentally controllable quantities, we discuss the possible tuning of the macroscopic quantum many-body phases of the two-leg ladder in this mixed-dimensional fermionic cold atom system.

Electron-phonon superconductivity near charge-density-wave instability in LaO0.5F0.5BiS2: Density-functional calculations

We discuss the electronic structure, lattice dynamics and electron-phonon interaction of newly discovered superconductor LaO$_{0.5}$F$_{0.5}$BiS$_{2}$ using density functional based calculations. A strong Fermi surface nesting at $\mathbf{k}$=($\pi $,$\pi $,0) suggests a proximity to charge density wave instability and leads to imaginary harmonic phonons at this $\mathbf{k}$ point associated with in-plane displacements of S atoms. Total energy analysis resolves only a shallow double-well potential well preventing the appearance of static long-range order. Both harmonic and anharmonic contributions to electron-phonon coupling are evaluated and give a total coupling constant $\lambda \simeq 0.85$ prompting this material to be a conventional superconductor contrary to structurally similar FeAs materials.

Anharmonic suppression of charge density waves in 2H-NbS2

The temperature dependence of the phonon spectrum in the superconducting transition metal dichalcogenide 2H-NbS$_2$ is measured by diffuse and inelastic x-ray scattering. A deep, wide and strongly temperature dependent softening, of the two lowest energy longitudinal phonons bands, appears along the $\mathrm{\Gamma M}$ symmetry line in reciprocal space. In sharp contrast to the iso-electronic compounds 2H-NbSe$_2$, the soft phonons energies are finite, even at very low temperature, and no charge density wave instability occurs, in disagreement with harmonic ab-initio calculations. We show that 2H-NbS$_2$ is at the verge of the charge density wave transition and its occurrence is only suppressed by the large anharmonic effects. Moreover, the anharmonicity and the electron phonon coupling both show a strong in-plane anisotropy.

Plasmon evolution and charge-density wave suppression in potassium intercalated 2H-TaSe2

We have investigated the influence of potassium intercalation on the formation of the charge-density wave (CDW) instability in 2H-TaSe2 by means of electron energy-loss spectroscopy and density functional theory. Our observations are consistent with a filling of the conduction band as indicated by a substantial decrease of the plasma frequency in experiment and theory. In addition, elastic scattering clearly points to a destruction of the CDW upon intercalation as can be seen by a vanishing of the corresponding superstructures. This is accompanied by a new superstructure, which can be attributed to the intercalated potassium. Based on the behavior of the c-axis upon intercalation we argue in favor of interlayer sites for the alkali metal and that the lattice remains in the 2H modification.

Effect of dimensionality and spin-orbit coupling on charge-density-wave transition in 2H-TaSe2

A first-principles investigation of the charge-density-wave (CDW) instability in bulk and single-layer 2H-TaSe${}_{2}$ is presented, focusing on the origin of the CDW instability, the role of the interlayer interactions, and the effect of spin-orbit coupling. While interlayer interactions and spin-orbit coupling have a nontrivial effect on the electronic structure and Fermi surface, the CDW instability is predicted to remain robust, with little or no change in the ordering wave vector. This is in contrast to the closely related 2H-NbSe${}_{2}$ material, where the CDW wave vector depends on dimensionality. The results are analyzed in terms of the interplay between the momentum dependence of the electron-phonon coupling and that of the electronic response function.

Electron–hole instability in 1T-TiSe2

In this paper, we address the question of the origin of the charge density wave instability in 1T-TiSe2. We develop a model considering the direct Coulomb interaction between electrons and holes in the valence and conduction bands near the Fermi energy. Using the Bethe–Salpeter equation, we calculate the electron–hole correlator, which reveals an instability at low temperature leading to a transition toward a commensurate superstructure mediated by the electron–phonon coupling, in agreement with experiments. On the basis of this correlator, the electron self-energies are then calculated and the corresponding photoemission spectra are compared with the experimental ones, revealing good agreement. The signature of electron–hole fluctuations in photoemission is emphasized. Furthermore, we calculate the spectral function of the phonon mode, whose softening is experimentally observed at the transition.

Origin of the charge density wave in 1T-TiSe2

All-electron ab initio calculations are used to study the microscopic origin of the charge density wave (CDW) in 1$T$-TiSe${}_{2}$. A purely electronic picture is ruled out as a possible scenario, indicating that the CDW transition in the present system is merely a structural phase transition. The CDW instability is the result of a symmetry lowering by electron correlations occurring with electron localization. Suppression of the CDW in pressurized and in Cu-intercalated 1$T$-TiSe${}_{2}$ is explained by a delocalization of the electrons, which weakens the correlations and counteracts the symmetry lowering.

Thermoelectric power of TTF[Ni(dmit)2

The 1D organic salt TTF[Ni(dmit)2]2 becomes superconductor with Tc=1.6K under an applied hydrostatic pressure of 7kbar. Structural determinations in this system lead us to suspect that superconductivity (SC) coexists with a charge density wave (CDW) instability at low pressure. In order to better understand how SC emerge from a CDW and to revisit the pressure–temperature phase diagram of the TTF[Ni(dmit)2]2 we performed transport and thermoelectric power measurements under pressure.

Optical study of charge instability in CeRu2Al10in comparison with CeOs2Al10and CeFe2Al10

The anisotropic electronic structure responsible for the antiferromagnetic transition in CeRu$_2$Al$_{10}$ at the unusually high temperature of $T_0$ = 28 K was studied using optical conductivity spectra, Ce 3d X-ray photoemission spectra, and band calculation. It was found that the electronic structure in the $ac$ plane is that of a Kondo semiconductor, whereas that along the b axis has a nesting below 32 K (slightly higher than $T_0$). These characteristics are the same as those of CeOs$_2$Al$_{10}$ [S. Kimura {\it et al.}, Phys. Rev. Lett. 106, 056404 (2011).]. The $c$-$f$ hybridization intensities between the conduction and $4f$ electrons of CeRu$_2$Al$_{10}$ and CeOs$_2$Al$_{10}$ are weaker than that of CeFe$_2$Al$_{10}$, showing no magnetic ordering. These results suggest that the electronic structure with one-dimensional weak $c$-$f$ hybridization along the b axis combined with two-dimensional strong hybridization in the $ac$ plane causes charge-density wave (CDW) instability, and the CDW state then induces magnetic ordering.

Field-induced charge-density-wave transitions in the organic metal α-(BEDT-TTF)2KHg(SCN)4 under pressure

Successive magnetic-field-induced charge-density-wave transitions in the layered molecular conductor α-(BEDT-TTF)2KHg(SCN)4 are studied in a hydrostatic pressure regime in which the zero field charge-density- wave (CDW) state is completely suppressed. It is shown that the orbital effect of the magnetic field restores the density wave, while orbital quantization induces transitions between different CDW states as the field strength is varied. The latter show up as distinct anomalies in the magnetoresistance as a function of field. The interplay between the orbital and Pauli paramagnetic effects, which act, respectively, to enhance and to suppress the CDW instability, is particularly manifest in the angular dependence of the field-induced anomalies.

Charge-Density Wave and Superconducting Dome inTiSe2from Electron-Phonon Interaction

At low temperature TiSe2 undergoes a charge-density wave instability. Superconductivity is stabilized either by pressure or by Cu intercalation. We show that the pressure phase diagram of TiSe2 is well described by first-principles calculations. At pressures smaller than 4 GPa charge-density wave ordering occurs, in agreement with experiments. At larger pressures the disappearing of the charge-density wave is due to a stiffening of the short-range force constants and not to the variation of nesting with pressure. Finally, we show that the behavior of T(c) as a function of pressure is entirely determined by the electron-phonon interaction without need of invoking excitonic mechanisms. Our work demonstrates that phase diagrams with competing orders and a superconducting dome are also obtained in the framework of the electron-phonon interaction.

Ab initiotheory of the pseudogap in cuprate superconductors driven byC4 symmetry breaking

Understanding the origin of the pseudogap is an essential step toward elucidating the pairing mechanism in the cuprate superconductors. Recently there has been strong experimental evidence showing that $C$4 symmetry breaking occurs on the formation of the pseudogap. This form of symmetry breaking was predicted by the fluctuating bond model (FBM), an empirical model based on a strong, local coupling of electrons to the square of the planar oxygen vibrator amplitudes. In this paper we approach the FBM theory from a new direction, starting from ab initio molecular dynamics simulations. The simulations demonstrate a doping-dependent instability of the in-plane oxygens toward displacement off the Cu-O-Cu bond axis. From these results and perturbation theory we derive an improved and quantitative form of the FBM. A mean-field solution of the FBM leads to $C$4 symmetry breaking in the oxygen vibrational amplitudes and to a $d$-type pseudogap in the electronic spectrum, the features linked by recent experimental data. The phase diagram of the pseudogap derived from mean-field theory, its doping and temperature dependences, including the phase boundary ${T}^{*}$, agree well with experimental data. We extend the theory to include the long-range Coulomb interaction on the same basis as the FBM interaction. When the long-range Coulomb interaction is included in the FBM, a charge density wave (CDW) instability in the charge channel is predicted, which explains the nanoscale, rather than spatially uniform, behavior of the $C$4 symmetry breaking. Taking the CDW into account, with the theoretical $k$ dependence of the pseudogap, enables the Fermi surface arc phenomenon to be understood.

Exciton-phonon interactions and superconductivity bordering charge order in TiSe2

The strong coupling between lattice modes and charges which leads to the formation of charge density waves in materials such as the transition-metal dichalcogenides may also give rise to superconductivity in the same materials, mediated by the same exciton or phonon modes that dominate the charge ordered state. Such a superconducting phase has recently been observed for example in TiSe2, both upon intercalation with Copper and in the pristine material under pressure. Here we investigate the interplay of exciton formation and electron-phonon coupling within a simplified model description. We find that the combined exciton-phonon modes previously suggested to drive the charge density wave instability in TiSe2 are also responsible for the pairing of electrons in its superconducting regions. Based on these results, it is suggested that both of the observed domes form part of a single superconducting phase. We also study the effect of the quantum critical fluctuations emerging from the suppressed charge order on the transport properties directly above the superconducting region, and compare our finding with reported experimental results.

Density-wave instability inα−(BEDT-TTF)2KHg(SCN)4studied by x-ray diffuse scattering and by first-principles calculations

$\ensuremath{\alpha}\text{\ensuremath{-}}{(\text{BEDT-TTF})}_{2}\text{KHg}{(\text{SCN})}_{4}$ develops a density wave ground state below 8 K whose origin is still debated. Here we report a combined x-ray diffuse scattering and first-principles density functional theory study supporting the charge density wave (CDW) scenario. In particular, we observe a triply incommensurate anharmonic lattice modulation with intralayer wave vector components which coincide within experimental errors to the maximum of the calculated Lindhard response function. A detailed study of the structural aspects of the modulation shows that the CDW instability in $\ensuremath{\alpha}\text{\ensuremath{-}}{(\text{BEDT-TTF})}_{2}\text{KHg}{(\text{SCN})}_{4}$ is considerably more involved than those following a standard Peierls mechanism. We thus propose a microscopic mechanism where the CDW instability of the BEDT-TTF layer is triggered by the anion sublattice. Our mechanism also emphasizes the key role of the coupling of the BEDT-TTF and anion layers via the hydrogen bond network to set the global modulation.

Effect of dimensionality on the charge-density wave in few-layer2H-NbSe2

We investigate the charge-density wave (CDW) instability in single and double layers, as well as in the bulk $2{\text{H-NbSe}}_{2}$. We demonstrate that the density functional theory correctly describes the metallic CDW state in the bulk $2{\text{H-NbSe}}_{2}$. We predict that both monolayer and bilayer ${\text{NbSe}}_{2}$ undergo a CDW instability. However, while in the bulk the instability occurs at a momentum ${\mathbf{q}}_{\text{CDW}}\ensuremath{\approx}\frac{2}{3}\ensuremath{\Gamma}M$, in freestanding layers it occurs at ${\mathbf{q}}_{\text{CDW}}\ensuremath{\approx}\frac{1}{2}\ensuremath{\Gamma}M$. Furthermore, while in the bulk the CDW leads to a metallic state, in a monolayer the ground state becomes semimetallic, in agreement with recent experimental data. We elucidate the key role that an enhancement of the electron-phonon matrix element at $\mathbf{q}\ensuremath{\approx}{\mathbf{q}}_{\text{CDW}}$ plays in forming the CDW ground state.

Spin Polarization Transition in the Symmetric Electron-Electron and Electron-Hole Bilayers

We explore the feasibility of the existence of a spin-polarization transition in the symmetric electron-electron and electron-hole bilayers. We accomplish this task by drawing a comparison between the ground-state energies of the unpolarized and fully polarized phases of the bilayer system. The ground-state energy calculation is performed by using the q-STLS (quantum Singwi, Tosi, Land, and Sjolander) theory. In addition, the numerical results are studied for the static density susceptibility and pair-correlation function over a wide range of layer parameters viz. particle density rsl, and layer spacing d. Interestingly enough, a spin-polarization transition is found to take place in both the electron-electron and electron-hole bilayers from the unpolarized to the polarized liquid well before the unpolarized liquid could actually make transition to the Wigner crystal (WC) state. The polarized electron-electron and electron-hole bilayers too support the charge density wave (CDW) and WC instabilities, but the crossover density is now lowered in comparison with their respective unpolarized counterparts.

Electron-phonon coupling in compressed1T-TaS2: Stability and superconductivity from first principles

The coexistence of charge-density waves and superconductivity in compressed $1T{\text{-TaS}}_{2}$ is investigated through density-functional calculations. It has been speculated that this coexistence arises from a real-space phase separation of insulating domains of the commensurate-charge-density-wave phase and metallic interdomain regions. Approximating the structure of the interdomain regions with the undistorted $1T$ structure, the present calculations confirm that pressure suppresses the charge--density-wave instability and stabilizes the interdomain phase. The electron-phonon coupling constants of the compressed $1T$ phase are found to have strong wave-vector dependence, with large contributions from soft modes associated with the incipient charge-density wave instability. The pressure dependence of the coupling parameters is discussed in relation to the measured insensitivity of the superconducting transition temperature to pressure.

Peierls distortion as a route to high thermoelectric performance in In4Se3-δ crystals

Thermoelectric energy harvesting-the transformation of waste heat into useful electricity-is of great interest for energy sustainability. The main obstacle is the low thermoelectric efficiency of materials for converting heat to electricity, quantified by the thermoelectric figure of merit, ZT. The best available n-type materials for use in mid-temperature (500-900 K) thermoelectric generators have a relatively low ZT of 1 or less, and so there is much interest in finding avenues for increasing this figure of merit. Here we report a binary crystalline n-type material, In(4)Se(3-delta), which achieves the ZT value of 1.48 at 705 K-very high for a bulk material. Using high-resolution transmission electron microscopy, electron diffraction, and first-principles calculations, we demonstrate that this material supports a charge density wave instability which is responsible for the large anisotropy observed in the electric and thermal transport. The high ZT value is the result of the high Seebeck coefficient and the low thermal conductivity in the plane of the charge density wave. Our results suggest a new direction in the search for high-performance thermoelectric materials, exploiting intrinsic nanostructural bulk properties induced by charge density waves.