The pseudogap in LSCO-type high-temperature superconductors as seen by neutron crystal-field spectroscopy

We studied the isotope, pressure and doping effects on the pseudogap temperature T^* by neutron spectroscopic experiments of the relaxation rate of crystal-field excitations in La_1.96−xSr_xHo_0.04CuO₄ (x = 0.11, 0.15, 0.20, 0.25) on the high-resolution time-of-flight spectrometer FOCUS at SINQ, PSI. We found clear evidence for the opening of a pseudogap in the underdoped regime at T^*(x = 0.11) = (82.2 ± 1.2) K as well as in the overdoped and the heavily overdoped compounds at T^*(x = 0.2) = (49.2 ± 0.7) K and at T^*(x = 0.25) = (46.5 ± 0.5) K, respectively. Furthermore, we investigated the effect of oxygen isotope substitution on the pseudogap, the experiments revealed ΔT^*(x = 0.11) = (21.3 ± 5.2) K and ΔT^*(x = 0.2) = (4.5 ± 1.3) K. The application of hydrostatic pressure (0.8 and 1.2 GPa) on the optimally doped compound (x = 0.15) results in a downward shift of dT^*/dp = (−5.9 ± 1.6) K/GPa.     

Raman active collective mode in underdoped cuprates

We report experimental observation of Bardasis–Schrieffer-type s-symmetry collective mode in the d-wave high T_c superconductors. The mode persists above T_c in the pseudogap phase of the underdoped superconductors — direct spectroscopic evidence of the correlations in the particle–particle channel above T_c.     

Quantum phase fluctuations responsible for pseudogap

The effect of ordering field phase fluctuations on the normal and superconducting properties of a simple 2D model with a local four-fermion attraction is studied. Neglecting the coupling between the spin and charge degrees of freedom an analytical expression has been obtained for the fermion spectral function as a single integral over a simple function. From this we show that, as the temperature increases through the 2D critical temperature and a nontrivial damping for a phase correlator develops, quantum fluctuations fill the gap in the quasiparticle spectrum. Simultaneously the quasiparticle peaks broaden significantly above the critical temperature, resembling the observed pseudogap behavior in high-T_c superconductors.     

Interplay between superconductivity and pseudogap state in bilayer cuprate superconductors

The interplay between the superconducting gap and normal-state pseudogap in the bilayer cuprate superconductors is studied based on the kinetic energy driven superconducting mechanism. It is shown that the charge carrier interaction directly from the interlayer coherent hopping in the kinetic energy by exchanging spin excitations does not provide the contribution to the normal-state pseudogap in the particle–hole channel and superconducting gap in the particle–particle channel, while only the charge carrier interaction directly from the intralayer hopping in the kinetic energy by exchanging spin excitations induces the normal-state pseudogap in the particle–hole channel and superconducting gap in the particle–particle channel, and then the two-gap behavior is a universal feature for the single layer and bilayer cuprate superconductors.     

Momentum dependence of pseudogap and superconducting gap in variation theory

To consider the origin of a pseudogap and a superconducting (SC) gap found in the high-T_c cuprates, we evaluated the momentum dependence of the singlet gap corresponding to the pseudogap and the SC gap in the t–J model, using an optimization variational Monte Carlo (VMC) method. In the underdoped regime, the singlet gap is significantly modified from the simple d_x²-y²(d)-wave gap (∝ cos k_x − cos k_y) by the contribution of long-range pairings. Its angular dependence at the quasi Fermi surface is qualitatively consistent with those experimentally observed in both hole and electron-doped cuprates. On the other hand, a SC gap is almost unchanged, preserving the original simple d-wave form. Thus, it seems that the incoherent part of the singlet gap mainly influences the forms of observed gaps.     

Optical conductivity of cuprates from Yang–Rice–Zhang ansatz: Comparison with experiment

We report the results of a theoretical investigation on charge dynamics in weakly coupled CuO₂ planes of the cuprate Ca_2 − xNa_xCuO₂Cl₂ in the pseudogap regime by using the theory of Yang, Rice, and Zhang (YRZ) [1]. With a detailed numerical analysis at various impurity scattering rates (_γimp${\gamma }_{\mathit{imp}}$), we observe that YRZ model is not able to reproduce (in magnitude) the experimentally observed frequency evolution of optical conductivity at a fixed doping level. Further, to analyze the doping evolution, we have done a detailed comparison of calculated YRZ conductivity with the experimental one using Two-Component Drude–Lorentz model. We find that YRZ model is capable of reproducing (qualitatively) the experimentally observed doping evolution of Drude processes (low energy scale) and processes at the pseudogap (intermediate energy scale). We also discuss physical reasons of the discrepancy seen in magnitudes.     

Geometrically-frustrated pseudogap phase of Coulomb liquids

We study a class of models with long-range repulsive interactions of the generalized Coulomb form V(r)∼1/r^α$V(r)\sim 1/r^{\alpha }$. We show that decreasing the interaction exponent in the regime α<d$\alpha <d$ dramatically depresses the charge ordering temperature T_c in any dimension d≥2$d\ge 2$, reflecting the strong geometric frustration produced by long-range interactions. A nearly frozen Coulomb liquid then survives in a broad pseudogap phase found at T>T_c$T>T_c$, which is characterized by an unusual temperature dependence of all quantities. In contrast, the leading critical behavior very close to the charge-ordering temperature remains identical as in models with short-range interactions.     

Electrons in cuprates: A consistent ARPES view

Angle resolved photoemission spectroscopy (ARPES) has been playing a crucial role in understanding of physics behind high-temperature superconductivity. Our ARPES investigation of superconducting cuprates, performed over a decade and accomplished by very recent results, suggests a consistent view of electronic interactions in cuprates which provides natural explanation of both the origin of the pseudogap state and the mechanism for high-temperature superconductivity. Within this scenario, the spin-fluctuations play a decisive role in formation of the fermionic excitation spectrum in the normal state and are sufficient to explain the high transition temperatures to the superconducting state while the pseudogap phenomenon is a consequence of a Peierls-type intrinsic instability of electronic system to formation of an incommensurate density wave. In view of these results and their projection to numerous other materials, two general questions are arising: is the normal state in 2D metals ever stable and how does this intrinsic instability interplay with superconductivity?     

Charge and spin inhomogeneity as a key to the physics of the high-Tc cuprates

We present a coherent scenario for the physics of cuprate superconductors, which is based on a charge-driven inhomogeneity, i.e. the “stripe phase”. We show that spin and charge critical fluctuations near the stripe instability of strongly correlated electron systems provide an effective interaction between the quasiparticles, which is strongly momentum, frequency, temperature and doping dependent. This accounts for the various phenomena occurring in the overdoped, optimally and underdoped regimes both for the normal and the superconducting phase.