Onset Of Charge Density Wave Research Articles

Charge stripes in the graphene-based materials

We present an analytical model of the charge density wave instability in graphene sheets within the intercalated graphite CaC6 compound. The instability yields the experimentally observed uniaxial charge stripes of periodically modulated electron density, coupled to the softest phonon mode of the superlattice consisting of the Ca atoms intercalated between graphene planes. The Fermi surface of the chemically doped graphene undergoes the novel type of instability driven by the mechanism that gains the condensation energy of the stripe state by the topological reconstruction of the Fermi surface. This mechanism appears to be entirely different from the one based on the Fermi surface nesting, which has been considered a paradigm in the present literature concerning the onset of charge density waves.

Complete miscibility amongst the AV3Sb5 kagome superconductors: Design of mixed A -site AV3Sb5 ( A : K, Rb, Cs) alloys

In this work we explore the chemical-property phase diagram of the AV$_3$Sb$_5$ family through A-site alloying. We demonstrate full miscibility of the alkali-site, highlighting that the three parent compounds are the terminal ends of a single solid-solution. Using both polycrystalline and single crystal methods, we map the dependence of the two primary electronic instabilities: (1) the onset of charge density wave (CDW) order ($T_\text{CDW}$) and (2) the onset of superconductivity ($T_\text{c}$) with alkali-site composition. We show continuous trends in both $T_\text{CDW}$ and $T_\text{c}$, including a region of enhanced CDW stability in K$_{1-x}$Cs$_{x}$V$_3$Sb$_5$ alloys. Together, our results open new routes for chemical perturbation and exploration of the chemical-property relationships in the class of AV$_3$Sb$_5$ kagome superconductors.

Charge Order and Fluctuations in Bi2Sr2−xLaxCuO6+δRevealed by63,65Cu-Nuclear Magnetic Resonance

The discovery of a magnetic-field-induced charge-density-wave (CDW) order in the pseudogap state via nuclear magnetic resonance (NMR) studies has highlighted the importance of “charge” in the physics of high transition-temperature (Tc) superconductivity in copper oxides (cuprates). Herein, after briefly reviewing the progress achieved in the last few years, we report new results of 63,65Cu-NMR measurements on the CDW order and its fluctuation in the single-layered cuprate Bi2Sr2−xLaxCuO6+δ. The NMR spectrum under both in- and out-of-plane magnetic fields above H = 10 T indicates that the CDW replaces the antiferromagnetic order before superconductivity appears, but disappears before superconductivity is optimized. We found that the CDW onset temperature TCDW scales with the pseudogap temperature T*. Comparison between 63Cu and 65Cu NMR indicates that the spin–lattice relaxation process is dominated by charge fluctuations in the doping regions where the CDW appears as well as at the pseudogap end point (T* = 0). These results suggest that charge orders and fluctuations exist in multiple doping regions and over a quite wide temperature range.

Restored strange metal phase through suppression of charge density waves in underdoped YBa2Cu3O7-δ.

The normal state of optimally doped cuprates is dominated by the “strange metal” phase that shows a linear temperature (T) dependence of the resistivity persisting down to the lowest T. For underdoped cuprates, this behavior is lost below the pseudogap temperature T*, where charge density waves (CDWs), together with other intertwined local orders, characterize the ground state. We found that the T-linear resistivity of highly strained, ultrathin, underdoped YBa2Cu3O7–δ films is restored when the CDW amplitude, detected by resonant inelastic x-ray scattering, is suppressed. This observation suggests an intimate connection between the onset of CDWs and the departure from T-linear resistivity in underdoped cuprates. Our results illustrate the potential of using strain control to manipulate the ground state of quantum materials.

Anomalous softening of phonon dispersion in cuprate superconductors

A softening of phonon-dispersion has been observed experimentally in under-doped cuprate superconductors at the charge-density wave (CDW) ordering wave vector. Interestingly, the softening occurs below the superconducting (SC) transition temperature T$_{c}$, in contrast to the metallic systems, where the softening occurs usually below the CDW onset temperature T$_{\text{CDW}}$. An understanding of the `anomalous' nature of the phonon-softening and its connection to the pseudo-gap phase in under-doped cuprates remain open questions. Within a perturbative approach, we show that a complex interplay among the ubiquitous CDW, SC orders and life-time of quasi-particles associated to thermal fluctuations, can explain the anomalous phonon-softening below T$_{c}$. Furthermore, our formalism captures different characteristics of the low temperature phonon-softening depending on material specificity.

Forced Diffusion of Correlated Impurities in the Peierls Conductor o-TaS3

It is shown that in orthorhombic TaS3 with quenching defects, when the temperature changes in the region below the Peierls transition temperature T < TP, forced diffusion of defects arises due to their strong interaction with the charge density wave (CDW). The relationships between the concentration of quenching defects, n, the threshold field of the onset of CDW sliding, ET, and the defect-induced shift of TP are determined: ET∝ n and ΔTP∝ n. This set of laws corresponds to the case when quenching defects positions are correlated with the CDW. The ordinary (without thermocycling) diffusion of quenching defects was detected at T ≈ 300 K, its diffusion coefficient and the height of the energy barrier were estimated. This made it possible to clarify the most probable nature of the defects. These are interstitial sulfur impurities introduced during quenching into the van der Waals gap between the chains and partially ordered at T < TP due to interaction with the CDW. This ordering significantly lowers the height of the energy barrier of forced diffusion in comparison with ordinary diffusion when the spatial configuration of the CDW changes during thermocycling. This leads to the appearance of anomalously high low-temperature forced mobility of correlated impurities.

Designed Synthesis and Structure–Property Relationships of Kinetically Stable [(PbSe)1+δ]m(VSe2)1 (m = 1, 2, 3, 4) Heterostructures

Understanding structure–function relationships is essential to guide the designed synthesis of novel materials with emergent properties. In this work, we targeted the metastable heterostructures [(PbSe)1+δ]m(VSe2)1, where m = 1–4, to test if the charge density wave (CDW) transition temperature increases as the layer thickness separating the VSe2 monolayers increases, as was observed when SnSe was the separating layer. The modulated elemental reactant approach was used to make the targeted products. This approach involves depositing elemental layers in which the number of atoms of each element per square angstrom in Pb|Se and V|Se bilayers equals the number calculated for a rock salt-structured PbSe bilayer and a CdI2-structured VSe2 slab, respectively. Layered elemental precursors with the correct composition and nanoarchitecture for each of the targeted compounds were prepared by repeatedly depositing a single V|Se bilayer followed by m Pb|Se bilayers. Precursors close to the targeted number of atoms per unit area were determined via X-ray fluorescence and the correct nanoarchitecture self-assembled to the targeted compounds during a low-temperature anneal. Resistivity measurements show that the number of PbSe layers per repeat unit (m) does not change the charge density transition onset temperature as previously reported for the analogous [(SnSe)1+δ]m(VSe2)1 compounds. The temperature dependence and absolute values of the resistivity of the m = 3 and 4 heterostructures scale as expected for composite behavior. The difference in the thickness dependence of the CDW transition between the PbSe- and SnSe-containing compounds highlights that the identity of the intervening rock salt layer plays a more important role in modifying the CDW onset temperature than the separation of the VSe2 layers.

Low-Frequency Current Fluctuations and Sliding of the Charge Density Waves in Two-Dimensional Materials.

We investigated low-frequency noise in two-dimensional (2D) charge density wave (CDW) systems, 1 T-TaS2 thin films, as they were driven from the nearly commensurate (NC) to incommensurate (IC) CDW phases by voltage and temperature stimuli. This study revealed that noise in 1 T-TaS2 has two pronounced maxima at the bias voltages, which correspond to the onset of CDW sliding and the NC-to-IC phase transition. We observed unusual Lorentzian features and exceptionally strong noise dependence on electric bias and temperature, leading to the conclusion that electronic noise in 2D CDW systems has a unique physical origin different from known fundamental noise types. We argue that noise spectroscopy can serve as a useful tool for understanding electronic transport phenomena in 2D CDW materials characterized by coexistence of different phases and strong pinning.

Inelastic x-ray study of phonon broadening and charge-density wave formation in ortho-II-ordered YBa2Cu3O6.54

Inelastic x-ray scattering is used to investigate charge density wave (CDW) formation and the low-energy lattice dynamics of the underdoped high temperature superconductor ortho-II YBa2Cu3O6.54. We find that, for a temperature ~1/3 of the CDW onset temperature (~155 K), the CDW order is static within the resolution of the experiment, that is the inverse lifetime is less than 0.3 meV. In the same temperature region, low-energy phonons near the ordering wavevector of the CDW show large increases in their linewidths. This contrasts with the usual behavior in CDW systems where the phonon anomalies are strongest near the CDW onset temperature

Charge density waves in NbSe 3: The models and the experimental evidence

Charge density wave (CDW) ordering in the prototypical low-dimensional compound NbSe 3 is reconsidered. It is shown that the model with two incommensurate modulations, q 1 = ( 0 , 0.241 , 0 ) and q 2 = ( 0.5 , 0.260 , 0.5 ) , localized on type-III and type-I bi-capped trigonal prismatic columns, does not explain details, revealed by various microscopic methods. An alternative explanation, based on the existence of modulated layered nano-domains below both CDW onset temperatures, is suggested. It accounts for the presence of the q 2 modulation in the STM images recorded above the T 2 CDW transition and for the absence of q 2 satellites in the corresponding diffraction patterns. The long periodic modulation, detected by LT STM, is attributed to a beating between the two CDWs, centered on adjacent columns of the same type. Such pairs of alternatively modulated columns represent the basic modulation units, ordered into nano-domains.

Evidence for onset of charge density wave in the La-based Perovskite superconductors

Recent studies have shown that structural instabilities play a vital role in high-T c superconductivity. We have used a local structural probe, extended X-ray absorption fine structure (EXAFS), to investigate the instability of the CuO2 plane in La-based high-T c perovskite systems. Temperature-dependent CuK-edge EXAFS spectra measured on single crystals of La1.85Sr0.15CuO4 (LSCO) and La2CuO4.1 (LCO) show a characteristic temperature (T ls ) where the local structure of the system diverges from the average one. This temperature coincides with a crossover temperature where the system shows anomalous metal-insulator transition in transport measurements. We have discussed the EXAFS results as an evidence for a particular phase transition of the system with segregation of localized and itinerant charge carriers in a short coherence charge density wave (CDW). A phase diagram for this CDW has been proposed.

Electronic Band Structure Study of the Anomalous Electrical and Superconducting Properties of Hexagonal Alkali Tungsten Bronzes AxWO3 (A = K, Rb, Cs)

The electrical and superconducting properties of hexagonal alkali tungsten bronzes AxWO3 (A = K, Rb, Cs) were examined by calculating the electronic band structure of a representative hexagonal tungsten bronze and analyzing reported crystal structures of AxWO3 (A = K, Rb, Cs). These bronzes possess one-dimensional (1D) and three-dimensional Fermi surfaces. The metal-to-semiconductor-to-metal transitions and superlattice reflections in KxWO3 and RbxWO3 are explained by a charge density wave (CDW) associated with the 1D Fermi surface. There occurs a maximum in the plots of the CDW onset temperature TB versus x for KxWO3 and RbxWO3. The presence of this maximum and the absence of a CDW in CsxWO3 reflect the balance of two opposing energy factors, the electronic instability and lattice stiffness, in forming a CDW. The dependence of the superconducting transition temperature TC on x suggests that a CDW transition removes lattice phonons conducive for superconductivity.

Applications of scanning tunneling microscopy to the study of charge density waves

Scanning tunneling microscopy (STM) studies of the surfaces of transition metal di- and tri-chalcogenides have been used to detect a variety of charge-density-wave (CDW) contributions to the surface charge modulation at 77 and 4.2K. In the 1T phases of TaSe2 and TaS2 strong charge maxima are observed which correspond to the √13 a0 × √13 a0 superlattice generated by the CDWs formed as standing waves from the conduction electrons. The charge-density contours located between the charge maxima show major contributions from the detailed arrangement of surface Se or S atom. The z-deflection observed from the total surface charge-density modulation in the 1T phases is extremely large in the range 1.0 to 2.5Å. The STM scans show the same general structure for the 1T phases at both 77 and 4.2K, but variations in the z-deflection suggest some temperature dependence of the CDW amplitude. The observations are consistent with band structure considerations and the large electron transfer associated with the CDWs. In 2H-TaSe2 at 77K and in 2H-NbSe2 at 4.2K the CDWs are much weaker than in the 1T phases and contribute only small deflections to the STM scans which are mainly dominated by the atomic modulation of the surface charge-density. The linear chain compound NbSe3 has two CDW transitions, one at 144K and one at 59K. The STM scans at 77K with only one CDW present and only 20% of the Fermi surface gapped show no detectable contribution to the surface charge modulation at the CDW wavelength. The z-deflection shows a large surface modulation and resolves the three chains per surface unit cell, but the STM pattern can be matched to the expected height and charge variations of the surface Se atoms. At 4.2K the two CDWs present in NbSe3 gap approximately 80% of the Fermi surface and a modulation at the CDW wavelength of ∼ 4b0 along the chain axis can be analyzed in terms of the band structure and CDW formation. Initial STM scans have also been performed at 77K on the linear chain compound TaS3. The CDW onset occurs at 216K with gapping of the entire Fermi surface and formation of a semiconducting phase at low temperatures. A clear CDW modulation has not yet been observed, but reasonable STM scans can be obtained and improved resolution should make it possible to detect the CDW structure. Technical problems associated with operation of the STMs at both 77 and 4.2K will also be discussed.

Positron annihilation as a probe of electronic density in the two-dimensional conductor 1T-TAS2

Tantalum disulfide is a layered compound with quasi-two-dimensional electronic conduction. The particular topology of the 2D-Fermi surface causes the onset of charge density waves which drive a metal-to-insulator phase transition in two steps. Three standard positron annihilation techniques have been used to sample the electronic density and momentum distributions in this compound as a function of temperature from 4 K to 400 K. The positron lifetime is 261(2)ps at 300 K (single component) and does not change very much with temperature. It increases to 293(2)ps in a sample containing 1% of irradiation induced defects. The anisotropy of the momentum distribution is clearly visible both in angular correlation and Doppler broadening experiments. This anisotropy is reduced but remains when positrons are trapped by irradiation defects. The momentum distributions change significantly with temperature, especially when the charge density wave phase transitions are crossed. An interpretation taking into account the possible trapping of positrons by charge density waves is proposed. The remarkable opposite temperature dependences of the positron data obtained in the in-layer and transverse directions are attributed to electron localization in a preferential orientation. Finally, an attempt is made to have access to the topology of the Fermi surface of this material by mean of the projected density of states along different axes. This procedure demonstrates that even with standard positron techniques, insight on an unusual Fermi surface can be obtained Emploi de trois techniques d'annihilation de positons pour souder la densite electronique et les distributions d'impulsion entre 4 et 400 K. Mise en evidence de l'anisotropie de la distribution d'impulsion dans les experiences de correlation angulaire ou d'elargissement Doppler, anisotropie qui diminue quand les positons sont pieges sur les defauts d'irradiation et varie de facon significative au passage des transitions de phase liees aux ondes de densite de charge. Variations thermiques remarquables et opposees selon la direction des feuillets ou perpendiculairement. Renseignements sur la surface de Fermi

Room temperature reflectivity and thermoreflectance spectra of 1T VSe2

Room temperature reflectivity and thermoreflectance measurements of 1T VSe2 below and above the charge density wave (CDW) transition temperature have been carried out. A noticeable change in the thermo-optical response at about 108K has been detected around 1.65 eV which seems to be due to the CDW onset. No evidence has been found of the commensurate CDW transition at 70K.

Anomalous electrical and magnetic properties of vanadium diselenide

Powder samples of VSe 2 have been made by direct synthesis from the elements. Single-crystal specimens have been made by vapor phase transport in the presence of iodine. X-Ray characterization indicated only the 1 T polytype. Behavior was metallic over the range 1.5 to 300°K, but there were discontinuities at about 100°K in resistivity, magnetic susceptibility, and Hall voltage. These are apparently due to the onset of charge density waves. Below 50°K, there was a clear quadratic dependence of resistivity on temperature which combined with relatively high Pauli paramagnetism to indicate Baber-type electron-electron scattering. No superconductivity was observed down to 1.5°K.

LAYER COMPOUNDS. CHARGE DENSITY WAVES IN TRANSITIONS METAL COMPOUNDS.ELECTRONIC PROPERTIES OF TRANSITION METAL DICHALCOGENIDES : CONNECTION BETWEEN STRUCTURAL INSTABILITIES AND SUPERCONDUCTIVITY

We discuss the electronic properties of the transition metal dichalcogenide layer compounds which have been investigated in detail because they undergo phase transitions accompanied by the occurence of charge density waves (CDW). The CDW state is favoured by the cylindrical nature of the Fermi surface and also by the possible existence of Van-Hove singularities in the band structure close to the Fermi level of the compounds TX2(T = Nb, Ta, X = S, Se). The anisotropy of the layer compounds is related to physical properties such as : compressibility, electrical conductivity and the superconducting properties. In the trigonal prismatic polytypes 2H-NbSe2, TaSe2, TaS2 the onset of a CDW is characterized by a weak anomaly in the resistivity ; whereas in the octahedral coordinated polytypes (1T-TaS2), the formation of a CDW induces a metal-insulator transition. A detailed study of the CDW state by NMR on 2H-NbSe2 will be presented. Below the onset temperature, a redistribution of the conduction electrons causes an inhomogeneous'broadening of the 93Nb quadrupolar satellites. From the interpretation of the line shape of the NMR lines we have deduced the existence of 3 incommensurate CDW in 2H-NbSe2 below 33 K. The amplitude of the CDW measured by NMR is proportional to the onset temperature as expected in a mean field theory. The pretransitional effects observed above the transition are related to static perturbations of the electron distribution induced by the charged impurities. A contribution of dynamical critical fluctuations above 33 K to the 93Nb spin-lattice relaxation rate has also been observed. The CDW states have been studied by resistivity measurements in other systems such as 2H-TaSe2 where a lock-in of the ICDW occurs through a 1st order phase transition between 87 and 93 K, or in the mixed polytype 4 HbTaS2 with a lock-in in 1T layers at 320 K and an onset in the 2H layers at 22 K. A pressure investigation shows a connection between the enhancement of Tc and the existence of a structural phase transition at T0 > Tc.The CDW phase transition will be discussed in terms of the phenomenological theory of McMillan. In this respect 4 HbTaS2 is of particular interest since the onset of CDW's occurs through a 1st order transition and therefore suggests that the coupling between the trigonal prismatic layers is relatively weaker in the mixed polytypes than in the pure 2H polytypes.

Experimental investigation of superconductivity in 2H-NbSe 2 single crystal

The basic quantities characterizing the superconducting behaviour of pure 2H-NbSe 2 single crystals have been determined from specific heat measurements, performed between 0.3 and 10 K in magnetic fields up to 55 KG. When Ti impurities are added, changes are observed both in the superconducting parameters and in the onset of charge density waves, strengthening the idea that a connection exists between the two phenomena.

Evidence for a connection between charge density waves and the pressure enhancement of superconductivity in 2H-NbSe 2

The pressure dependence of the onset of charge density waves (CDW) in 2H-NbSe 2 has been studied by resistivity measurements up to 35 kbar. It demonstrates that the pressure enhancement of T c is due to the presence of CDW in the crystal, and stops when T CDW becomes smaller than T c at P = 36 kbar.