Strong Pseudogap Research Articles

Collective modes and ultrasonic attenuation in a pseudogapped superconductor

We develop a theory of ultrasound decay rate $\alpha$ in a model of a superconductor with a large pseudogap $\Delta_P$, and show that at low temperatures ($T \ll T_c$) the magnitude of the decay rate $\alpha$ is controlled by the ratio of $T/\Delta$, where $\Delta \ll \Delta_P$ is a superconducting collective gap. Thus we propose new method to measure the collective gap $\Delta$ in a situation when strong pseudogap is present.

Annealing condition dependence of the superconducting property and the pseudo-gap in the protect-annealed electron-doped cuprates

Annealing as-grown electron-doped cuprates under a low oxygen-partial-pressure condition is a necessary step to achieve superconductivity. It has been recently found that the so-called protect annealing results in much better superconducting properties in terms of the superconducting transition temperature and volume fraction. In this article, we report on angle-resolved photoemission spectroscopy studies of a protect-annealed electron-doped cuprate <TEX>$Pr_{0.9}La_{1.0}Ce_{0.1}CuO_4$</TEX> on annealing condition dependent superconducting and pseudo-gap properties. Remarkably, we found that the one showing a better superconducting property possesses almost no pseudo-gap while others have strong pseudo-gap feature due to an anti-ferromagnetic order.

Fractal superconductivity near localization threshold

We develop a semi-quantitative theory of electron pairing and resulting superconductivity in bulk “poor conductors” in which Fermi energy E F is located in the region of localized states not so far from the Anderson mobility edge E c . We assume attractive interaction between electrons near the Fermi surface. We review the existing theories and experimental data and argue that a large class of disordered films is described by this model. Our theoretical analysis is based on analytical treatment of pairing correlations, described in the basis of the exact single-particle eigenstates of the 3D Anderson model, which we combine with numerical data on eigenfunction correlations. Fractal nature of critical wavefunction's correlations is shown to be crucial for the physics of these systems. We identify three distinct phases: ‘critical’ superconductive state formed at E F = E c , superconducting state with a strong pseudo-gap, realized due to pairing of weakly localized electrons and insulating state realized at E F still deeper inside a localized band. The ‘critical’ superconducting phase is characterized by the enhancement of the transition temperature with respect to BCS result, by the inhomogeneous spatial distribution of superconductive order parameter and local density of states. The major new feature of the pseudo-gapped state is the presence of two independent energy scales: superconducting gap Δ, that is due to many-body correlations and a new “pseudo-gap” energy scale Δ P which characterizes typical binding energy of localized electron pairs and leads to the insulating behavior of the resistivity as a function of temperature above superconductive T c . Two gap nature of the pseudo-gapped superconductor is shown to lead to specific features seen in scanning tunneling spectroscopy and point-contact Andreev spectroscopy. We predict that pseudo-gapped superconducting state demonstrates anomalous behavior of the optical spectral weight. The insulating state is realized due to the presence of local pairing gap but without superconducting correlations; it is characterized by a hard insulating gap in the density of single electrons and by purely activated low-temperature resistivity ln R( T) ∼ 1/ T. Based on these results we propose a new “pseudo-spin” scenario of superconductor–insulator transition and argue that it is realized in a particular class of disordered superconducting films. We conclude by the discussion of the experimental predictions of the theory and the theoretical issues that remain unsolved.

Infrared spectroscopy of pseudogaps in electron-doped superconductors

We have measured the ab-plane reflectivity spectra of n-type single-crystal cuprates R1.85Ce0.15CuO4-δ (R=Nd, Eu, Gd) at temperatures from 10 to 280K using Fourier-transform infrared (FTIR) spectroscopy. In the mid-infrared (MIR) region, Nd1.85Ce0.15CuO4-δ (NCCO) does not exhibit pseudogap-like behavior, but Eu1.85Ce0.15CuO4-δ (ECCO) and Gd1.85Ce0.15CuO4-δ (GCCO) clearly exhibit strong pseudogap absorption between 0.15eV and 0.4eV. The relevant temperature range varies with the rare-earth element R via a pressure effect. The observed pseudogap evolution is well explained in terms of the antiferromagnetic phase fluctuation.

Biordered superconductivity and strong pseudogap state

Interrelation between the two-particle and mean-field problems is used to describe the strong pseudogap and superconducting states in cuprates. We present the strong pseudogap state as an off-diagonal short-range order (ODSRO) originating from quasistationary states of the pair of repulsing particles with large total momentum ($K$ pair). Phase transition from the ODSRO state into the off-diagonal long-range ordered (ODLRO) superconducting state is associated with Bose-Einstein condensation of the $K$ pairs. A checkerboard spatial order observable in the superconducting state in the cuprates is explained by a rise of the $K$-pair density wave. A competition between the ODSRO and ODLRO states leads to the phase diagram typical of the cuprates. Biordered superconducting state of coexisting condensates of Cooper pairs with zero momentum and $K$ pairs explains some properties of the cuprates observed below ${T}_{c}$: Drude optical conductivity, unconventional isotope effect, and two-gap quasiparticle spectrum with essentially different energy scales.

Tetracritical point and staggered vortex currents in superconducting state

A phase diagram reflecting the main features of the typical phase diagram of cuprate superconductors has been studied within the framework of the Ginzburg-Landau phenomenology in the vicinity of a tetracritical point, which appears as a result of the competition of the superconducting and insulating pairing channels. The superconducting pairing under repulsive interaction corresponds to a two-component order parameter, whose relative phase is related to the orbital antiferromagnetic insulating ordering. Under weak doping, the insulating order coexists with the superconductivity at temperatures below the superconducting phase transition temperature and is manifested as a weak pseudogap above this temperature. A part of the pseudogap region adjacent to the superconducting state corresponds to developed fluctuations of the order parameter in the form of quasi-stationary states of noncoherent superconducting pairs and can be interpreted as a strong pseudogap. As the doping level is increased, the system exhibits a phase transition from the region of coexistence of the superconductivity and the orbital antiferromagnetism to the usual superconducting state. In this state, a region of developed fluctuations of the order parameter in the form of quasi-stationary states of uncorrelated orbital circular currents exists near the phase transition line.

Topology of the Fermi surface and the coexistence of orbital antiferromagnetism and superconductivity in cuprates

It has been shown that the strong coupling model taking into account a rise in the spin antiferromagnetic insulating state explains the doping dependence of the topology and shape of the Fermi contour of superconducting cuprates. Hole pockets with shadow bands in the second Brillouin zone form the Fermi contour with perfect ordinary and mirror nesting, which ensures the coexistence of orbital antiferromagnetism and superconductivity with a large pair momentum for T < TC. The weak pseudogap region (T* < T < T*) corresponds to the orbital antiferromagnetic ordering, which coexists with the incoherent state of superconducting pairs with large momenta in the strong pseudogap region (TC < T < T*).

Pairing fluctuation theory of high-Tcsuperconductivity in the presence of nonmagnetic impurities

The pseudogap phenomena in the cuprate superconductors requires a theory beyond the mean-field BCS level. A natural candidate is to include strong pairing fluctuations, and treat the two-particle and single-particle Green's functions self-consistently.At the same time, impurities are present in even the cleanest samples of the cuprates. Some impurity effects can help reveal whether the pseudogap has a superconducting origin and thus test various theories. Here we extend the pairing fluctuation theory for a clean system [Chen et al., Phys. Rev. Lett. 81, 4708 (1998)] to the case with nonmagnetic impurities. Both the pairing and the impurity T matrices are included and treated self-consistently. We obtain a set of three equations for the chemical potential \ensuremath{\mu}, ${T}_{c},$ the excitation gap $\ensuremath{\Delta}{(T}_{c})$ at ${T}_{c},$ or \ensuremath{\mu}, the order parameter ${\ensuremath{\Delta}}_{\mathrm{sc}},$ and the pseudogap ${\ensuremath{\Delta}}_{\mathrm{pg}}$ at temperature $T&lt;{T}_{c},$ and study the effects of impurity scattering on the density of states, ${T}_{c}$ and the order parameter, and the pseudogap. Both ${T}_{c}$ and the order parameter as well as the total excitation gap are suppressed, whereas the pseudogap is not for given $T&lt;~{T}_{c}.$ Born scatterers are about twice as effective as unitary scatterers in suppressing ${T}_{c}$ and the gap. In the strong pseudogap regime, pair excitations contribute a new ${T}^{3/2}$ term to the low-$T$ superfluid density. The initial rapid drop of the zero-$T$ superfluid density in the unitary limit as a function of impurity concentration ${n}_{i}$ also agrees with experiment.

The physics of inhomogeneous striped superconductors

We present a minimal model of a doped Mott insulator that simultaneously supports antiferromagnetic stripes and d-wave superconductivity. At the unrestricted mean-field level, the various phases of the cuprates, including weak and strong pseudogap phases, and two different types of superconductivity in the underdoped and the overdoped regimes, find a natural interpretation. We argue that on the underdoped side, the superconductor is intrinsically inhomogeneous — striped nanoscale coexistence of superconductivity and magnetism — and global phase coherence is achieved through Josephson-like coupling of the superconducting stripes. On the overdoped side, the state is overall homogeneous and the superconductivity is of the classical BCS type.

Microstructural evolution and stability of (Fe1−xVx)3Al alloys in relation to the electronic structure

The evolution of the microstructure of (Fe1−xVx)3Al alloys in the D03 structure has been studied as function of composition by transmission electron microscopy. The domain size of the D03 structure significantly increases as V is added to the alloy. An increase in long range order parameter is also shown. This microstructural evolution is discussed in terms of the increased second nearest neighbor interactions induced by the V additions. Electronic structure calculations are also presented in this paper. The density of states shows a strong pseudogap for the Fe2VAl compound and thus strong hybridization induced by the alloying element. This electronic structure feature is discussed in terms of interesting physical properties of the system and in relation to the density of states, stability and properties of similar compounds [Fe2MAl (M=Ti, Cr, Mn)]. Electron energy loss spectra of the Al L2–3 edge show that the strong hybridization induced by V does not affect the unoccupied Al s and d states.

Effect of Magnetic field on the Pseudogap Phenomena in High-TcCuprates

We theoretically investigate the effect of magnetic field on the pseudogap phenomena in High-Tc cuprates. The obtained results well explain the experimental results including their doping dependences. In our previous paper (J. Phys. Soc. Jpn. 68 (1999) 2999.), we have shown that the pseudogap phenomena observed in High-Tc cuprates are naturally understood as a precursor of the strong coupling superconductivity. On the other hand, there is an interpretation for the recent high field NMR measurements to be an evidence denying the pairing scenarios for the pseudogap. In this paper, we investigate the magnetic field dependence of NMR $1/T_{1}T$ on the basis of our formalism and show the interpretation to be inappropriate. The results indicate that the value of the characteristic magnetic field $B_{{\rm ch}}$ is remarkably large in case of the strong coupling superconductivity, especially near the pseudogap onset temperature $T^{*}$. Therefore, the magnetic field dependences can not be observed and $T^{*}$ does not vary when the strong pseudogap anomaly is observed. On the other hand, $B_{{\rm ch}}$ is small in the comparatively weak coupling case and $T^{*}$ varies when the weak pseudogap phenomena are observed. These results properly explain the high magnetic field NMR experiments continuously from under-doped to over-doped cuprates. Moreover, we discuss the transport phenomena in the pseudogap phase. The behaviors of the in-plane resistivity, the Hall coefficient and the c-axis resistivity in the pseudogap phase are naturally understood by considering the d-wave pseudogap.