Large Pseudogap Research Articles

Checkerboard charge density wave and pseudogap of high-Tccuprate

We consider the scenario where a 4-lattice constant, rotationally symmetric charge density wave (CDW) is present in the underdoped cuprates. We prove a theorem that puts strong constraint on the possible form factor of such a CDW. We demonstrate, within mean-field theory, that a particular form factor within the allowed class describes the angle-resolved photoemission and scan tunneling spectroscopy well. We conjecture that the ``large pseudogap'' in cuprates is the consequence of this type of charge density wave.

Photoemission study of excess oxygen-doped La2CuO4.10

We have studied excess oxygen-doped La2CuO4.10 (LCO) by angle-resolved photoemission spectroscopy (ARPES) and temperature-dependent angle-integrated photoemission spectroscopy (AIPES). The observed Fermi surface and flat-band position are similar to those of optimally-doped to slightly-underdoped La2−xSrxCuO4 (LSCO). We have observed a large pseudogap of ∼70meV in the temperature-dependent AIPES spectra, which is larger than that reported for optimally-doped LSCO (∼35meV). From comparison of low-temperature AIPES spectra between different cooling rates, oxygen disorder is found to cause spectral weight transfer from higher binding energy to near EF and to make the spectral line shapes resemble those of LSCO. This explains the reduction of the large pseudogap in LSCO.

Double dispersion of the magnetic resonant mode in cuprates from the memory function approach

The magnetic excitation spectra in the vicinity of the resonant peak, as observed by inelastic neutron scattering in cuprates, are studied within the memory-function approach. It is shown that at intermediate doping the superconducting gap induces a double dispersion of the peak, with an anisotropy rotated between the downward and upward branch. Similar behavior, but with a spin-wave dispersion at higher energies, is obtained for the low-doping case assuming a large pairing pseudogap.

Temperature dependence of Fermi Arc in underdoped cuprates and restoration of the symmetry breaking

The mechanism for the evolution of the Fermi arc with increasing the temperature in high- T c cuprates has been proposed. We have discussed the electrical resistivity in the normal state and the linear-like decreasing of the spin part of the Knight shift from the large pseudogap, whose corresponding temperature is T mK, to the small pseudogap, whose corresponding temperature is T *.

Relationship between the Large Pseudogap and the Restoration of Spontaneous Symmetry Breaking in Underdoped Cuprates

We propose the mechanism for the T-evolution of the Fermi arc in high-T c cuprates. The relationship between large pseudogap (LPG) and the restoration of the spontaneous symmetry breaking has been discussed. It is suggested strongly that the restoration temperature T res (k F ) around the hot spots corresponds to T mK and T mX , below which the Knight shift and the spin susceptibility start to decrease gradually, respectively.

The Pseudogaps of HTSC

The problem of the pseudogaps in oxide HTSC is analyzed by means of the off-diagonal propagators, in analogy with the method of Nambu in superconductivity. We conjecture that the small pseudogap is obtained as the Fock diagram and the large pseudogap as the total off-diagonal self-energy (which is the sum of the Hartree diagram and the Fock diagram). We obtain real gap for undoped materials, and pseudogaps (with states in the gap) for doped and conducting materials. We find that disorder is essential for the results of the doped materials.

Spatially resolved tunneling spectroscopy on TTF-TCNQ

Local tunneling spectroscopy has been measured with low temperature UHV-STM on in-situ cleaved ab surface of organic TTF-TCNQ crystal. Due to ultra low image drift and clear molecular resolution, the spectroscopy is performed at specific molecular site either on TCNQ or TTF chains. In normal state (T= 63 K), a large pseudo-gap exists both in TTF and TCNQ chains. Above pseudo-gap local density of states differs for TTF and TCNQ chains that is in good agreement with double band model. By the signature of an anomalous in local spectroscopy measurement, a single impurity has been detected on a TTF chain. Charge density wave fluctuation is pinned by impurity above critical temperature (T=54K). Results obtained show that, Scanning Tunneling Spectroscopy can provide spatially resolved spectroscopic information at nanometer scale.

Two Pseudogap Behavior in La2-xSrxCuO4: Thermoelectric Power at High Temperature

Thermoelectric power (TEP) of high-T C superconductors has been investigated in a wide range of temperature (T C < T < 700 K) for La2−x Sr x CuO4. TEP of La2−x Sr x CuO4 shows different temperature dependences in three temperature regions. In the low-temperature region, a positive broad TEP peak is observed near T p, which shifts to lower temperature upon doping. As temperature increases, TEP decreases linearly at intermediate temperature. In the high-temperature region, TEP deviates from the linear temperature dependence at a certain temperature, T h showing a saturation behavior. As the doping concentration increases, the characteristic temperatures, T C, T p, and T h, show systematic changes. In comparison with pseudogap temperature estimated from other experiments, the large pseudogap behavior in TEP at high temperature has been discussed and distinguished from the small pseudogap observed at lower temperature. A possibility of bound pairs formation in the normal state opening the pseudogap at high temperature is discussed briefly. The coexistence of bound pairs and the normal independent carriers for T C < T < Th could be the origin of the intrinsic inhomogeneity.

Angle-Resolved Photoemission Spectroscopy of (Ca,Na)2CuO2Cl2Crystals: Fingerprints of a Magnetic Insulator in a Heavily Underdoped Superconductor

Electronic evolution from an antiferromagnet to a high-Tc superconductor is revealed by angle-resolved photoemission experiments on tetragonal Ca(1.9)Na(0.1)CuO(2)Cl(2) single crystals, which were successfully grown for the first time under high pressures. In this underdoped superconductor, we found clear fingerprints of the parent insulator: a shadow band and a large pseudo-gap. These observations are most likely described by a "chemical potential shift", which contrasts clearly with the prevailing wisdom of the "pinned chemical potential" learned from the prototype La(2-x)Sr(x)CuO(4), demonstrating that the route to a high-Tc superconductor is not unique.

Vibronic Effects in Cuprate Superconductors with a Two-Component Spectrum Created by Doping

A model [1] of the electron spectrum of a cuprate superconductor involves a new band borning with hole doping above the itinerant band. Vibronic coupling of these components leads to the renormalization of the gaps created by the interband pair transfer interaction. The phonon softening is maximal at the under/overdoped separation line where the bands begin to overlap. The vibronic hybridization determines the normal state joint pseudogap at optimal doping and one of the two pseudogaps of the underdoped material. The larger pseudogap changes its nature with doping.

Spin correlations in the algebraic spin liquid: Implications for high-Tcsuperconductors

We propose that underdoped high $T_c$ superconductors are described by an algebraic spin liquid (ASL) at high energies, which undergoes a spin-charge recombination transition at low energies. The spin correlation in the ASL is calculated via its effective theory - a system of massless Dirac fermions coupled to a U(1) gauge field. We find that without fine tuning any parameters the gauge interaction strongly enhances the staggered spin correlation even in the presence of a large single particle pseudo-gap. This allows us to show that the ASL plus spin-charge recombination picture can explain many highly unusual properties of underdoped high $T_c$ superconductors.

Bonding and physical properties of Hume-Rothery compounds with thePtHg4structure

We present a detailed experimental and theoretical study concerning bonding and structural stability of intermetallic electron compounds with the ${\mathrm{PtHg}}_{4}$ structure. Due to the simplicity of the structure these compounds represent an excellent prototype system for a more general insight into bonding and stability of the large family of $d\ensuremath{-}sp$ bonded Hume-Rothery compounds. In particular, the representatives ${\mathrm{CrGa}}_{4}$ and ${\mathrm{MnGa}}_{4}$ were synthesised and their resistivity, magnetic susceptibility, and bulk modulus measured. We find that both compounds are metallic conductors but show a remarkable large difference in their temperature independent magnetic susceptibilities. The value of the Pauli paramagnetic susceptibility of ${\mathrm{MnGa}}_{4}$ is about $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ ${\mathrm{m}}^{3}/\mathrm{mol}$ higher than that of ${\mathrm{CrGa}}_{4}.$ The ${\mathrm{PtHg}}_{4}$ structure of ${\mathrm{CrGa}}_{4}$ and ${\mathrm{MnGa}}_{4}$ is stable up to pressures of about 100 kbar. Full-potential linearized augmented plane wave calculations reproduced very well the experimental structural properties of ${\mathrm{CrGa}}_{4}$ and ${\mathrm{MnGa}}_{4}$ and showed strong directional (covalent) bonding between transition metal atoms and Ga atoms in both compounds. The directional bonding is due to a large hybridization of the narrow d bands with the Ga $\mathrm{sp}$ bands. As a consequence a large pseudogap at the Fermi level for ${\mathrm{CrGa}}_{4}$ and slightly above the Fermi level for ${\mathrm{MnGa}}_{4}$ is produced. This pseudogap is characteristic and decisive for structural stability of electron compounds with the ${\mathrm{PtHg}}_{4}$ structure. We find that structural stability appears as a competition between optimizing the pseudogap and minimizing the compound equilibrium volume. Therefore, stable electron compounds are confined to systems $T{\mathrm{Ga}}_{4}$ with T being a transition metal from group 6 or 7. A complete substitution of Ga for isovalent Al or In is not possible.

The doping dependence of T* – what is the real high- Tc phase diagram?

Underdoped high- T c superconductors are frequently characterised by a temperature, T *, below which the normal-state pseudogap opens. Two different “phase diagrams” based on the doping ( p) dependence of T * are currently considered: one where T * falls to zero at a critical doping state and the other where T * merges with T c in the overdoped region. By examining the temperature dependence of the NMR Knight shift and relaxation rate, entropy, resistivity, infrared conductivity, Raman scattering, ARPES and tunnelling data, it is concluded that the second scenario is not at all supported. Neither can one distinguish a small and a large pseudogap as is often done. T * is an energy scale, which falls abruptly to zero at p=0.19. Colour figures are available at cond-mat/0005063.

Fermi surface, pseudogap and superconducting gap in La 2− xSr xCuO 4

The electronic structure of La 2− x Sr x CuO 4 (LSCO) has been revealed by a series of photoemission spectroscopic studies. The unusual electronic states in the underdoped regime are attributed to the large and small pseudogap formation, which has been observed in other high- T C cuprates, and stripe fluctuations, which are particularly pronounced in LSCO.

Electronic Properties of the Trellis-Lattice Hubbard Model: Pseudogap and Superconductivity

We study the electronic states of the two-dimensional frustrated coupled ladder (trellis lattice) Hubbard model, which is a theoretical model of the superconducting material ${\mathrm{Sr}}_{14\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$. Temperature dependence of the density of states and magnetic susceptibility are discussed by using the fluctuation exchange method, which is applied to this model for the first time. At half filling, a large pseudogap appears in the density of states at higher temperatures. Moreover, the $d$-wave superconductivity appears at lower temperatures, where the pseudogap is well developed. These behaviors have similarities to the under-doped high- ${T}_{\mathrm{c}}$ cuprates.

One-dimensional organic superconductivity : an investigation through electron Schottky tunnelling in N/GaSb-(TMTSF)2PF6 junctions under pressure

Observation of a large pseudo-gap at the Fermi level by Schottky tunnelling of electrons from N doped-GaSb into the organic superconductor (TMTSF) 2PF6 under a pressure of 11 kbar (Tc = 1 K) establishes, on an unambiguous experimental basis the one-dimensional character of the low temperature superconducting divergence and so justifies the presence of 1-D superconducting fluctuations to high temperatures. The intrachain pairing energy 2 Δ(0) = 3.6 meV derived from tunnelling characteristics emphasizes the strength of the superconducting channel in the family of organic superconductors (TMTSF)2X.