Disconnected Arcs Research Articles

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.

Fermi surface of underdoped cuprate superconductors from interlayer magnetoresistance: Closed pockets versus open arcs

An outstanding question about the underdoped cuprates concerns the true nature of their Fermi surface which appears as a set of disconnected arcs. Theoretical models have proposed two distinct possibilities: (1) each arc is the observable part of a partially hidden closed pocket and (2) each arc is open, truncated at its apparent ends. We show that measurements of the variation in the interlayer resistance with the direction of a magnetic field parallel to the layers can qualitatively distinguish closed pockets from open arcs. This is possible because the field can be oriented such that all electrons on arcs encounter a large Lorentz force and resulting magnetoresistance whereas some electrons on pockets escape the effect by moving parallel to the field. © 2010 The American Physical Society.

Admixture of an s-Wave Component to the d-Wave Gap Symmetry in High-Temperature Superconductors

Neutron crystal-field spectroscopy experiments in the Y- and La-type high-temperature superconductors HoBa2Cu3O6.56, HoBa2Cu4O8, and La1.81Sr0.15Ho0.04- CuO4 are reviewed. By this bulk-sensitive technique, information on the gap function is obtained from the relaxation behavior of crystal-field transitions associated with the Ho3+ ions which sit as local probes close to the superconducting copper-oxide planes. The relaxation data exhibit a peculiar change from a convex to a concave shape between the superconducting transition temperature T c and the pseudogap temperature T * which can only be modeled satisfactorily if the gap function of predominantly d-wave symmetry includes an s-wave component of the order of 20–25%, independent of the doping level. Moreover, our results are compatible with an unusual temperature dependence of the gap function in the pseudogap region (T c≤T≤T *), i.e., a break up of the Fermi surface into disconnected arcs.

Isotope, pressure, and doping effects on the pseudogap in the LSCO-type compounds studied by neutron spectroscopy

Isotope, pressure, and doping effects on the pseudogap temperature T ∗ were investigated by neutron spectroscopic experiments of the relaxation rate of crystal-field excitations in La 1.96− x Sr x Ho 0.04CuO 4 ( x=0.11, 0.15, 0.20). With increasing doping, T ∗ is found to decrease from 80 K ( x=0.11) to 60 K ( x=0.15) and 50 K ( x=0.20). The application of hydrostatic pressure ( p=0.8 GPa) results in a downward shift of T ∗ by 5 K for the optimally doped compound. Upon oxygen isotope substitution ( 18O vs 16O), T ∗ is shifted upwards by 20, 10, and 5 K for x=0.11, x=0.15, and x=0.20, respectively. The large isotope and pressure effects indicate that lattice fluctuations are involved in the pseudogap phenomena. Our relaxation data are compatible with the unusual temperature dependence of the gap function in the pseudogap region, namely a breakup of the Fermi surface into disconnected arcs.

On the Pseudogap State in Y- and La-Type High-Temperature Superconductors: Doping Dependence, Isotope Effects, and Electronic Properties Studied by Neutron Spectroscopy

The doping dependence as well as the oxygen and copper isotope effects on the pseudogap temperature T* were investigated by neutron spectroscopic experiments of the relaxation rate of crystal-field excitations in rare-earth based Y- and La-type high-temperature superconductors. We found three essential results from our truly bulk-sensitive experiments: (1) For all doping levels we had T* > Tc, even in the optimally doped and overdoped regimes. (2) In bilayer high-Tc compounds we observed huge oxygen and copper isotope effects on T*, which give evidence that the pseudogap formation is governed by lattice modes involving both the oxygen and copper ions. In single layer high-Tc compounds, on the other hand, no shift of T* was found for the copper isotope substitution. (3) Our results obtained for all doping levels support the unusual temperature dependence of the gap function in the pseudogap region reported for underdoped Bi-type compounds, i.e., a breakup of the Fermi surface into disconnected arcs in the temperature range Tc < T < T*.

Truncation of a Two-Dimensional Fermi Surface due to Quasiparticle Gap Formation at the Saddle Points

We study a two-dimensional Fermi liquid with a Fermi surface containing the saddle points $(\pi,0)$ and $(0,\pi)$. Including Cooper and Peierls channel contributions leads to a one-loop renormalization group flow to strong coupling for short range repulsive interactions. In a certain parameter range the characteristics of the fixed point, opening of a spin and charge gap and dominant pairing correlations are similar to those of a 2-leg ladder at half-filling. An increase of the electron density we argue leads to a truncation of the Fermi surface with only 4 disconnected arcs remaining.