Existence Of Charge Density Wave Research Articles

Charge density wave transition and unusual resistance hysteresis in vanadium disulfide (1T-VS2) microflakes

We report existence of charge density wave (CDW) transition with unusual resistivity hysteresis in stable micro-flakes of 2D transition metal dichalcogenide (TMDC) vanadium disulfide (1T-VS2) as ascertained by structural studies, Raman spectroscopy, heat capacity, resistivity measurements and supported by phonon calculations based on density functional theory. The CDW transition occurs at around 296 K on cooling and manifests itself by the onset of resistivity with a negative temperature coefficient, which we identify as the transition temperature (TCDW). The transition is identified by a distinct peak in the heat capacity at T ≈ T CDW along with anomalies in temperature variation of the lattice parameters and complimentary signatures in electron diffraction and temperature-dependent Raman spectroscopy. The temperature-dependent resistivity measurements done with different ramp rates of cooling and heating show strong hysteresis and a low temperature relaxation pointing to the existence of metastable states below the transition. The Raman spectroscopy data show a hysteresis loop below the onset of the CDW transition. The phonon band structure calculations carried out on the 1T-VS2 system show the existence of the phonon mode softening along the diagonal of the Brillouin zone, which is pronounced near the transition temperature implying the prominent role of the mode softening as a driver of the transition. The results imply that the formation of CDW is sensitive to phonon mode softening rather than energy gap opening at the Fermi level.

Observation of Dirac Charge-Density Waves in Bi2Te2Se.

While parallel segments in the Fermi level contours, often found at the surfaces of topological insulators (TIs), would imply "strong" nesting conditions, the existence of charge-density waves (CDWs)-periodic modulations of the electron density-has not been verified up to now. Here, we report the observation of a CDW at the surface of the TI Bi2Te2Se(111), below ≈350K, by helium-atom scattering and, thus, experimental evidence for a CDW involving Dirac topological electrons. Deviations of the order parameter observed below 180K, and a low-temperature break of time reversal symmetry, suggest the onset of a spin-density wave with the same period as the CDW in the presence of a prominent electron-phonon interaction, originating from Rashba spin-orbit coupling.

Fingerprints of magnetoinduced charge density waves in monolayer graphene beyond half filling

A charge density wave is a condensate of fermions, whose charge density shows a long-range periodic modulation. Such charge density wave can be principally described as a macroscopic quantum state and is known to occur by various formation mechanisms. These are the lattice deforming Peierls transition, the directional, fermionic wave vector orientation prone Fermi surface nesting or the generic charge ordering, which in contrast is associated solely with the undirected effective Coulomb interaction between fermions. In two-dimensional Dirac/Weyl-like systems, the existence of charge density waves is only theoretically predicted within the ultralow energy regime at half filling. Taking graphene as host of two-dimensional fermions described by a Dirac/Weyl Hamiltonian, we tuned indirectly the effective mutual Coulomb interaction between fermions through adsorption of tetracyanoquinodimethane on top in the low coverage limit. We thereby achieved the development of a novel, low-dimensional dissipative charge density wave of Weyl-like fermions, even beyond half filling with additional magneto-induced localization and quantization. This charge density wave appears both, in the electron and the hole spectrum.

Calculations of quantum oscillations in cuprate superconductors considering the pseudogap

The observations of quantum oscillations frequencies in overdoped cuprates were in agreement with a charge density contained in a cylindrical Fermi surface but the measured frequencies of underdoped compounds were much smaller than expected. This was attributed to a topological transition into small pockets of Fermi surface associated with the existence of charge density waves. On the other hand, spectroscopic measurements suggested that the large two-dimensional Fermi surface changes continuously into a set of four disconnected arcs. Here we take into account the effect of the pseudogap that limits the available k-space area where the Landau levels are developed on the Luttinger theorem and obtain the correct total carrier densities. The calculations show how the disconnected arcs evolve into a closed Fermi surface reconciling the experiments.

Superconductivity-Insensitive Order at q∼1/4 in Electron-Doped Cuprates

One of the central questions in the cuprate research is the nature of the "normal state" which develops into high temperature superconductivity (HTSC). In the normal state of hole-doped cuprates, the existence of charge density wave (CDW) is expected to shed light on the mechanism of HTSC. With evidence emerging for CDW order in the electron-doped cuprates, the CDW would be thought to be a universal phenomenon in high-$T_c$ cuprates. However, the CDW phenomena in electron-doped cuprate are quite different than those in hole-doped cuprates. Here we study the nature of the putative CDW in an electron-doped cuprate through direct comparisons between as-grown and post-annealed Nd$_{1.86}$Ce$_{0.14}$CuO$_4$ (NCCO) single crystals using Cu $L_3$-edge resonant soft x-ray scattering (RSXS) and angle resolved photoemission spectroscopy (ARPES). The RSXS result reveals that the non-superconducting NCCO shows the same reflections at the wavevector (~1/4, 0, $l$) as like the reported superconducting NCCO. This superconductivity-insensitive signal is quite different with the characteristics of the CDW reflection in hole-doped cuprates. Moreover, the ARPES result suggests that the fermiology cannot account for such wavevector. These results call into question the universality of CDW phenomenon in the cuprates.

Stationary Josephson current in junctions involving d-wave superconductors with charge density waves: the temperature dependence and deviations from the law of corresponding states

Stationary Josephson current I c in symmetric and non-symmetric junctions involving d-wave superconductors with charge density waves (CDWs) was calculated. It was found that, if CDWs are weak or absent, there exists an approximate proportionality between I c and the product of superconducting order parameters in the electrodes (the law of corresponding states) for several factors affecting those quantities, such as the temperature, T, or one of the parameters characterizing the combined CDW superconducting phase (the degree of the Fermi surface dielectric gapping and the ratio between the parent superconducting and CDW order parameters). Otherwise, the dependences I c (T) were shown to deviate from those in the absence of CDWs, and the relevant corresponding-state dependences from linearity, the deviations being especially strong at certain rotation angles of crystalline electodes with respect to the junction plane. Hence, making use of specially designed experimental setups and analyzing the I c (T) and corresponding-state dependences, the existence of CDWs in cuprates and other non-conventional superconductors can be detected.

Charge Density Wave Accompanied by Spin Density Wave in Mn3Si

The displacement modulation due to the charge density wave (CDW) in Mn3Si, which occurs as second harmonics of the spin density wave (SDW), has been investigated by synchrotron x-ray diffraction and the existence of CDW below Néel temperature of 23 K has been confirmed. High resolution measurement revealed the splitting of the satellite peaks deviating very little from the [111] axis toward \([1\bar{1}0]\) direction, suggesting the possibility of a single or double q CDW state. The temperature dependence of the satellites indicates the existence of the phase transition at Tt = 8.5 K, in agreement with the neutron diffraction result [Phys. Rev. B 73, 224416 (2006)]. The analysis of reflection intensities using a single q model, which is a limiting case of no splitting, shows that in both phases Si and MnII have larger displacement than that of MnI. The formation of single and double q SDWs and their nesting mechanism are discussed.

Anisotropic Quantum Hall Liquids at Intermediate Magnetic Fields

The observation of significant magnetoresistance anisotropy of a two-dimensional electronic system in very clean GaAs/AlGaAs heterostructure samples in presence of moderately large perpendicular magnetic fields is a striking example of novel anisotropic behavior in the quantum Hall regime. Anisotropy appears to be the strongest at quantum Hall even-denominator filled states for filling factors ν>4 where several Landau levels are occupied. A possible explanation of these findings is due to the existence of charge density waves that are known to cause interesting phase transitions at high Landau levels. An alternative explanation of this phenomenon is to argue that the strongly correlated electronic system has stabilized in an orientationally ordered anisotropic quantum Hall liquid state. Quantum Monte Carlo calculations with a translationally invariant wave function in which rotation symmetry is broken indicate that this might be the case.

Electronic and structural properties of extended-chain compounds and polymers

Results of density functional calculations on infinite, periodic chains are reported. The method that is applied is based on linearized muffin-tin orbitals as basis functions, although the full potential and not only its muffin-tin part is included in the calculations. Special emphasis is put on analyzing the interatomic interactions by means of crystal-orbital overlap or Hamilton populations (COOP and COHP, respectively). As examples of conjugated polymers, trans-polyacetylene and polycarbonitrile are studied. Here, in particular, the existence of a bond length alternation is discussed. Subsequently, PtS{sub 2} (both without and with K counterions) and NbSe{sub 3} chains are considered. For the former, the single-chain calculations are supplemented with calculations on the crystalline compounds, and it is shown how single-chain effects are responsible for the structural properties whereas interchain effects have to be included in order to account for all the electronic properties. Parts of the results are explained through an analysis of the COOP and COHP. For NbSe{sub 3} the three different structures occurring in the crystalline material are considered, and the implications of the results for the existence of charge density waves as well as the importance of spin-orbit couplings are discussed. Finally, HF as an example of an extendedmore » hydrogen-bonded system is examined, and it is demonstrated how the electronic interactions change when the covalent and hydrogen bonds are interchanged as it occurs in charge transport via solitons.« less

EPR observations of anomalous triplet states in Ba1−x KxBiO3 and BaPbyBi1− y O3

Electron paramagnetic resonance (EPR) spectra of samples of the systems Ba1−xKxBiO3 and BaPbyBi1−yO3 are investigated over wide ranges of composition and temperature. Two main lines in the EPR spectrum with factors g1≈2.1 and g2≈4.2 are found for all compositions. It is shown that the observed EPR line with g2≈4.2 is due to oxygen ions. This probably indicates the presence of oxygen ions with different effective charges, i.e., the existence of charge density waves in the oxygen-ion sublattice in addition to charge density waves in the bismuth sublattice.

Characterization of exfoliated TaS2 thin films and the existence of charge density waves

Restacked single molecular layers of the layer compound TaS2 from suspensions was deposited on titanium substrates by using a lithium intercalation–exfoliation method. These exfoliated thin films on titanium substrates have been characterized by scanning electron microscopy and Raman spectroscopy, and x-ray diffraction techniques are also investigated for the existence of charge density wave (CDW) phases using scanning tunneling microscopy (STM). All these techniques revealed that TaS2 crystallites tend to orient mostly with their basal planes parallel to the titanium substrate. TaS2 thin films are chosen with the understanding that the CDW states could mostly be determined because of intralayer interactions in comparison to bulk single crystals where inter- and intralayer interactions are considered. STM images showed strong CDWs with a hexagonal array having periodicity of ∼12 Å and ‘‘corrugations’’ of ∼1.5 Å, respectively. The existence of a CDW superlattice, together with the surface atomic structure observed at room temperature, on these thin films (thickness, ca. 300 Å) of single layers, allows us to understand the intralayer effects on the CDWs. The existence of CDWs in these films was not observed by transmission electron microscopy even at 25 K.

Atomic force and scanning tunneling microscopy of comb-like cholesteric liquid crystalline polymer LB films

Bilayers of comb-like cholesteric liquid crystalline polymer (PChA-10) on silicon and graphite substrates are studied using AFM and STM. It is found that two kinds of two-dimensional molecular lattice can exist in this system. It is shown that Langmuir-Blodgett (LB) film perfectness depends on deposition pressure. The superiodicity observed in STM current images is consistent with the existence of charge density wave (CDW) on LB film surface.

Effect of charge-density waves on the x-ray emission edges of simple metals.

A theory of the x-ray emission edges of simple metals has been formulated in which the electrons are in a charge-density-wave ground state. The theory can explain the experimental data in the edge region of both sodium and lithium. Since the Na data were not well understood by previous theories, the present theory offers a resolution of a long-standing problem and provides further evidence of the existence of charge density waves in alkali metals.

Effect of CDW on positron annihilation characteristics in alkali metals

The existence of charge density waves in alkali metals can cause smearing at the Fermi momentum of the positron annihilation angular correlation profile. It is suggested that a significant part of the apparent non-thermalisation of positrons could be a reflection of the charge density wave ground state.

Magnetic field dependence of charge density waves in PbTe

Previous work by the authors indicated the possible existence of charge density waves in PbTe in the presence of strong magnetic fields so that the extreme quantum limit conditions prevail. The present work follows the transition as the field is lowered so that successively higher Landau levels become occupied. The transition persists at increasingly lower temperatures, having a maximum value, T cmax , identifiable with each value, N, of the last (partially) occupied Landau level.

An explanation of deviations from Matthiessen's rule in alkali metals

It is known that, in alkali metals, the large rate at which deviations from Matthiessen's rule are induced as a function of residual resistivity is comparable to that for polyvalent metals. It is suggested that this has a natural explanation if the existence of charge density waves is included in the analysis.

Surface quantum oscillations in silicon (100) inversion layers under uniaxial pressure

Surface quantum oscillations in (100) $n$-type silicon inversion layers were studied as a function of uniaxial compression. Due to an increase of the cyclotron mass the ratio of spin splitting to Landau splitting $\ensuremath{\Delta}$ increases continuously with pressure. At $p=3.6$ kbar, $\ensuremath{\Delta}$ has roughly doubled. At zero pressure only the light-mass subband ${E}_{0}$ is occupied, whereas in the high-pressure limit only the heavy-mass subband ${E}_{0}^{\ensuremath{'}}$ is populated. However, a simultaneous occupation of ${E}_{0}$ and ${E}_{0}^{\ensuremath{'}}$ at medium pressures must be excluded, since the measured valley degeneracy factor is 2 in the whole pressure range investigated. An interaction between ${E}_{0}$ and ${E}_{0}^{\ensuremath{'}}$ must be invoked, which generates a new twofold degenerate electron ground state at medium pressures. The pressure value at which the interaction becomes significant and also the cyclotron mass seem to depend on the magnetic field. Qualitatively, the experimental results can be explained by a theory of Kelly and Falicov which is based on the existence of charge-density waves in the silicon surface.

Diffuse electron scattering and vacancy ordering in VO Possible role of charge density waves

Abstract Electron diffraction patterns of disordered VO show extensive diffuse scattering and the presence of satellite spots which are incommensurate with the Bragg reflections. Annealed samples show evidence of a superstructure due to vacancy ordering with the superstructure bearing relation to the cubic lattice of the disordered phase, but no relation to the satellite spots due to diffuse scattering. The mechanism of diffuse scattering is discussed in terms of short-range order of vacancies as well as existence of CDW (charge density waves) and the latter model seems to be reasonable.