Presence Of Charge Density Wave Research Articles

A Theoretical Study of Doping Evolution of Phonons in High-Temperature Cuprate Superconductors

Hole-doped high-temperature copper oxide-based superconductors (cuprates) exhibit complex phase diagrams where electronic orders like a charge density wave (CDW) and superconductivity (SC) appear at low temperatures. The origins of these electronic orders are still open questions due to their complex interplay and correlated nature. These electronic orders can modify the phonons in the system, which has also been experimentally found in several cuprates as a softening in the phonon frequency at the CDW vector. Recent experiments have revealed that the softening in phonons in cuprates due to CDW shows intriguing behavior with increasing hole doping. Hole doping can also change the underlying Fermi surface. Therefore, it is an interesting question whether the doping-induced change in the Fermi surface can affect the softening of phonons, which in turn can reveal the nature of the electronic orders present in the system. In this work, we investigate this question by studying the softening of phonons in the presence of CDW and SC within a perturbative approach developed in an earlier work. We compare the results obtained within the working model to some experiments.

Thermoelectric power of overdoped Tl2201 crystals: charge density waves and T 1 and T 2 resistivities

We report measurements of the in-plane thermoelectric power (TEP) for an overdoped (OD) crystal of the single layer cuprate superconductor Tl2Ba2CuO (Tl2201) at several hole concentrations (p), from 300 or 400 K to below the superconducting transition temperature (Tc ). For p = 0.192 and 0.220, small upturns in the TEP below 150 K are attributed to the presence of charge density waves (CDW) detected by resonant inelastic x-ray scattering studies. This suggests that measurement of the TEP could provide a simple and effective guide to the presence of a CDW. Over a certain temperature range, often strongly restricted by the CDW, the TEP is consistent with the Nordheim-Gorter rule and the T 1 and T 2 terms in the in-plane resistivity of similar crystals observed below 160 K. Two scenarios in which the T 1 scattering term is uniform or non-uniform around the Fermi surface are discussed. As found previously by others, for uniform scattering the T 1 terms give scattering rates (τ −1) at lower p that are somewhat larger than the Planckian value and fall to zero for heavily OD crystals. Near 160 K, τ −1 from the T 2 terms corresponds to the Planckian value.

Charge density wave and pressure-dependent superconductivity in the kagome metal CsV3Sb5 : A first-principles study

The kagome metals $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ ($A=\mathrm{K}$, Rb, and Cs) have received intensive research interest due to the presence of charge density waves (CDWs), ${\mathbb{Z}}_{2}$ topological surface states, and intriguing pressure-dependent superconductivity. Using first-principles calculations, here we study the origin of the CDW order in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ and its superconducting properties under pressure up to 45 GPa. We reveal that the momentum-dependent electron-phonon coupling (EPC) effect plays an important role in the formation of the CDW order. Upon compression, the movement of the van Hove singularity, which induces the change in the density of states at the Fermi level, as well as the redistribution of the EPC, can explain the experimentally observed double superconducting domes. The main contribution to the EPC is varied from in-plane vibrational modes in superconducting area I (2--15 GPa) to out-of-plane modes in superconducting area II (15--45 GPa). Our observations clarify the origin of the CDW order and shed some light on understanding the experimental observations of pressure-dependent superconductivity in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$.

Charge Density Waves and Magnetism in Topological Semimetal Candidates GdSbxTe2−x−δ

AbstractMany topological semimetals are known to exhibit exceptional electronic properties, which are the fundamental basis for design of novel devices and further applications. Materials containing the structural motif of a square net are known to frequently be topological semimetals. In this work, the synthesis and structural characterization of the square‐net‐based magnetic topological semimetal candidates GdSbxTe2−x−δ (0, δ indicating the vacancy level) are reported. The structural evolution of the series with Sb substitution is studied, finding a transition between a simple tetragonal square‐net structure to complex superstructure formations due to the presence of charge density waves. The structural modulations coincide with a significant modification of the magnetic order. This work thus establishes GdSbxTe2−x−δ as a platform to study the interplay between crystal symmetry, band filling, charge density wave, and magnetism in a topological semimetal candidate.

Intercalation of transition metals in aluminene bi-layers: An ab initio study.

Using first principles calculations based on density functional theory (DFT), we probe various possible stacking arrangements of bilayer aluminene and intercalate six transition metal (TM) atoms (Ti, Cr, Mn, Fe, Co, and Ni) in unique bilayer aluminene systems. Further, we calculate valence charge density and electron localization function to ascertain the nature of bonding present in both the pristine and TM-intercalated composite systems. Intercalation of Cr, Mn, and Fe is found to result in the magnetic ground state. For Ti, Co, and Ni-intercalated systems, the starting trigonal symmetry has changed to a tetragonal symmetry. Co and Ni intercalated systems exhibit much higher (negative) formation energies compared to the other composite systems. In addition, nesting of the Fermi surface has been probed for the Co and Ni intercalated systems and observations indicate the possibility of the presence of charge density wave in the systems. A dispersion-corrected DFT study suggests that the van der Waals interaction is not likely to play a crucial role in determining the properties of both the pristine and TM-intercalated systems.

Dimensional reduction and ionic gating induced enhancement of superconductivity in atomically thin crystals of 2H-TaSe2

The effects of dimensional reduction and ion intercalation on superconductivity (SC) in the presence of charge density waves (CDWs) in two-dimensional crystals of 2H-TaSe2 were characterized. We prepared atomically thin crystals by mechanical exfoliation and performed electrical transport measurements on devices made by photolithography. The superconducting transition temperature (TcSC) was found to increase monotonically as the thickness decreased, changing from 0.14 K in the bulk to higher than 1.4 K for a 3-nm-thick crystal. The temperature dependence of upper critical field was found to be anomalous. The CDW transition temperature (TcCDW) was found to decrease, but to a less extent than TcSC, from 120 K in the bulk to around 113 K for the 3-nm-thick crystal. In addition, ion intercalation was found to increase TcSC and suppress TcCDW in an atomically thin crystal of 2H-TaSe2. The implications of these findings are discussed. We suggest that dimensional reduction and ion intercalation are potentially effective ways to engineer material properties for layered transition metal chalcogenides.

Superconductivity and multiple pressure-induced phases in BaPt2As2

The newly discovered BaPt$_2$As$_2$ shows a structural distortion at around 275~K, followed by the emergence of superconductivity at lower temperatures. Here we identify the presence of charge density wave (CDW) order at room temperature and ambient pressure using single crystal x-ray diffraction, with both a superlattice and an incommensurate modulation, where there is a change of the superlattice structure below $\simeq$ 275~K. Upon applying pressure, BaPt$_2$As$_2$ shows a rich temperature-pressure phase diagram with multiple pressure-induced transitions at high temperatures, the emergence or disappearance of which are correlated with sudden changes in the superconducting transition temperature $T_c$. These findings demonstrate that BaPt$_2$As$_2$ is a promising new system for studying competing interactions and the relationship between high-temperature electronic instabilities and superconductivity.

The high temperature superconductivity in cuprates: physics of the pseudogap region

We discuss the physics of the high temperature superconductivity in hole doped copper oxide ceramics in the pseudogap region. Starting from an effective reduced Hamiltonian relevant to the dynamics of holes injected into the copper oxide layers proposed in a previous paper, we determine the superconductive condensate wavefunction. We show that the low-lying elementary condensate excitations are analogous to the rotons in superfluid $^4He$. We argue that the rotons-like excitations account for the specific heat anomaly at the critical temperature. We discuss and compare with experimental observations the London penetration length, the Abrikosov vortices, the upper and lower critical magnetic fields, and the critical current density. We give arguments to explain the origin of the Fermi arcs and Fermi pockets. We investigate the nodal gap in the cuprate superconductors and discuss both the doping and temperature dependence of the nodal gap. We suggest that the nodal gap is responsible for the doping dependence of the so-called nodal Fermi velocity detected in angle resolved photoemission spectroscopy studies. We discuss the thermodynamics of the nodal quasielectron liquid and their role in the low temperature specific heat. We propose that the ubiquitous presence of charge density wave in hole doped cuprate superconductors in the pseudogap region originates from instabilities of the nodal quasielectrons driven by the interaction with the planar $CuO_2$ lattice. We investigate the doping dependence of the charge density wave gap and the competition between charge order and superconductivity. We discuss the effects of external magnetic fields on the charge density wave gap and elucidate the interplay between charge density wave and Abrikosov vortices. Finally, we examine the physics underlying quantum oscillations in the pseudogap region.

Electronic correlations in monolayerVS2

The layered transition metal dichalcogenide vanadium disulfide (VS$_2$), which nominally has one electron in the $3d$ shell, is potent for strong correlation physics and is possibly another realization of an effective one-band model beyond the cuprates. Here monolayer VS$_2$ in both the trigonal prismatic and octahedral phases is investigated using density functional theory plus Hubbard $U$ (DFT+$U$) calculations. Trigonal prismatic VS$_2$ has an isolated low-energy band that emerges from a confluence of crystal field splitting and direct V--V hopping. Within spin density functional theory, ferromagnetism splits the isolated band of the trigonal prismatic structure, leading to a low-band-gap $S=1/2$ ferromagnetic Stoner insulator; the octahedral phase is higher in energy. Including the on-site interaction $U$ increases the band gap, leads to Mott insulating behavior, and for sufficiently high values stabilizes the ferromagnetic octahedral phase. The validity of DFT and DFT+$U$ for these two-dimensional materials with potential for strong electronic correlations is discussed. A clear benchmark is given by examining the experimentally observed charge density wave (CDW) in octahedral VS$_2$, for which DFT grossly overestimates the bond length differences compared to known experiments; the presence of CDWs is also probed for the trigonal prismatic phase. Finally, we investigate why only the octahedral phase has been observed in experiments and discuss the possibility of realizing the trigonal prismatic phase. Our work suggests trigonal prismatic VS$_2$ is a promising candidate for strongly correlated electron physics that, if realized, could be experimentally probed in an unprecedented fashion due to its monolayer nature.

In-plane and transverse superconducting fluctuation diamagnetism in the presence of charge-density waves in2H−NbSe2single crystals

The fluctuation-diamagnetism (FD) above the superconducting transition was measured in 2H-NbSe2 single crystals. The moderate uniaxial anisotropy of this compound, and some experimental improvements, allowed to measure the superconducting fluctuation effects in the two main crystallographic directions. These results reveal that the nonlocal electrodynamic effects on the FD are highly anisotropic, and they also discard a possible contribution to the FD coming from the charge-density waves (CDW) appearing below $Tcdw > Tc in 2H-NbSe2, in agreement with a phenomenological estimate.

Mixed Phase and Bound States in the Phase Diagram of the Extended Hubbard Model

The paper examines part of the ground state diagram of the extended Hubbard model, with the on-site attraction U 0 in the presence of charge density waves, superconducting and η-superconducting order parameters. We show the possibility of the stabilization of the mixed state, with all three nonzero order parameters, in the model with nearest neighbor interactions. The other result of the paper is application of the exact solution of the Schrodinger equation for the two-electron bound state, as an additional bound for the phase diagram of the model, resulting in the partial suppression of the superconducting state of the s-wave symmetry, in favor of the normal state phase.

Microscopic origin of [formula omitted] anomaly in La 2− x− y(Nd,Eu) y(Ba,Sr) xCuO 4

We adopt a t 1– t 2– t 3– J– G model for explanation of x≈ 1 8 anomaly in HTSC compounds. The calculated charge susceptibility has a maximum near ( π, π) leading to charge density wave (CDW) formation with the CDW amplitude e Q = x in very narrow band regime at intermediate temperatures and near ( π, π/2) as temperature approaches zero. The in-phase domain structure as a candidate for “stripe” picture is proposed. We propose an experiment to apply pressure, and predict fast increase of T c at anomalous value x≈ 1 8 in the presence of CDW. The enhancement of T c will decline when CDW are suppressed by pressure.

Phonon-induced contributions to superconductivity in the presence of charge-density waves

We discuss the influence of charge-density wave (CDW) order on the electron–phonon vertex and the Cooper-pair susceptibility. These quantities are investigated for different values of occupation numbers, phonon momenta and frequencies, as well as for different directions and commensurabilities of the CDW scattering vector. For the half-filled band, the onset of a CDW state results in a considerable enhancement of the electron–phonon interaction, especially when the phonon momentum is close to the scattering vector. Our results suggest that for imperfect nesting different parts of the Fermi surface contribute to a CDW and d-wave superconducting state. This feature is of particular importance for the coexistence of both phases.

Conduction-band structure and charge-density waves in 1T-TaS2.

Angle-resolved inverse photoemission spectroscopy (ARIPES) has been utilized to study the conduction-band structure of 1T-${\mathrm{TaS}}_{2}$ in the presence of charge-density waves (CDW's). In room-temperature spectra taken along the principal symmetry directions \ensuremath{\Gamma}M, \ensuremath{\Gamma}M', and \ensuremath{\Gamma}K we can clearly identify Ta 5d--derived ${\mathit{t}}_{2\mathit{g}}$ and ${\mathit{e}}_{\mathit{g}}$ manifolds, which reveal a substructure of almost dispersionless peaks attributable to CDW effects. The intensity modulation of the substructure appears to reflect the band dispersion of the undistorted 1T phase. Distinct differences in intensity are observed between the \ensuremath{\Gamma}M and \ensuremath{\Gamma}M' spectra as a consequence of the threefold symmetry of the unit cell. In temperature-dependent ARIPES between 120 and 400 K we do not, within the experimental accuracy, observe changes of the electronic structure other than a linear shift due to the electron-lattice interaction. In particular, there is no evidence of an opening of a localization gap in the unoccupied band structure as has been observed in the occupied states. From combined photoemission and inverse photoemission results the Fermi level is rather found to be pinned close to the bottom of the lowest unoccupied band.

CHARGE DENSITY WAVE PHASE TRANSITIONS OBSERVED BY X-RAY PHOTOEMISSION

X-ray photoemission (XPS) measurements of the binding energies of the Ta 4f core levels in TaS2, TaSe2, and Ta0.97Hf0.03S2 at temperatures between 14 K and 371 K are reported. The presence of charge density waves (CDW's) produces inequivalent potentials at different Ta sites which split the 4f XPS lines ; the splitting is a measure of the CDW amplitude and represents an order parameter for the CDW-lattice system. The form of the components of the split XPS lines allows the CDW-lattice configuration to be deduced, and since the potential gradients at inequivalent Ta sites are different, the unequal widths of the components yields a measure of thefluctuating nature of the CDW-lattice system. The results indicate the existence of CDW fluctuations in commensurate phases, and that in some incommensurate and quasi-commensurate phases the CDW's cannot maintain long range coherence. In 1 T-Ta0.97Hf0.03S2 the Hf impurities pin the incommensurate CDW's into domains.

Electric Field Breakdown of Charge-Density-Wave—Induced Anomalies in NbSe3

We report the suppression by electric fields of longitudinal resistivity anomalies at 145 and 59 K in the compound Nb${\mathrm{Se}}_{3}$. Sample resistance was determined by conventional four-probe dc measurement as well as with short current pulses. We attribute the observed suppression to Zener breakdown across extremely small gaps introduced by the presence of charge density waves.