Pseudogap In Cuprates Research Articles

Theoretical clarification of the pseudogap in cuprates

In this paper it is discussed that the large Fermi surface needs a phenomenological idea of pseudogap while the two-component Kamimura-Suwa model of small Fermi surface exhibits the non-existence of pseudogap in the antinodal region.

Universality of Single-Particle Spectra of Cuprate Superconductors

All the available data for the dispersion and linewidth of the single-particle spectra above the superconducting gap and the pseudogap in metallic cuprates for any doping have universal features. The linewidth is linear in energy below a scale omega(c) and constant above. The cusp in the linewidth at omega(c) mandates, due to causality, a waterfall, i.e., a vertical feature in the dispersion. These features are predicted by a recent microscopic theory. We find that all data can be quantitatively fitted by the theory with a coupling constant lambda(0) and an upper cutoff at omega(c), which vary by less than 50% among the different cuprates and for varying dopings. The microscopic theory also gives these values to within factors of O(2).

Phase separation and pseudogap in electron-doped cuprates: A variational cluster perturbation analysis

We study the quantum transition from an antiferromagnet to a superconductor in the single-band Hubbard model with parameters appropriate to describe electron- and hole-doped cuprates by means of variational cluster perturbation theory, a method appropriate to deal with strongly-correlated systems. In both cases, our results produce phase separation into a mixed antiferromagnetic-superconducting phase at low doping and a pure superconducting (SC) phase at higher doping. However, in the electron-doped case the energy scale for phase separation is an order of magnitude smaller than for hole doping. This behavior can be understood in terms of the different Fermi-surface evolution in p- and n-doped materials upon doping. We argue that this behavior can be related to the different pseudogap and SC transition scales in hole- and electron-doped materials.

Strong- and weak-coupling mechanisms for pseudogap in electron-doped cuprates

Using the two-particle self-consistent approach and cluster perturbation theory for the two-dimensional t-t'-t''-U Hubbard model, we discuss weak- and strong-coupling mechanisms for the pseudogap observed in recent angle resolved photoemission spectroscopy on electron-doped cuprates. In the case of the strong-coupling mechanism, which is more relevant near half-filling, the pseudogap can be mainly driven by short range correlations near the Mott insulator. In the vicinity of optimal doping, where weak-coupling physics is more relevant, large antiferromagnetic correlation lengths, seen in neutron measurements, are the origin of the pseudogap. The t-J model is not applicable in the latter case.

Evidence for Universal Signatures of Zeeman-Splitting-Limited Pseudogaps in Superconducting Electron- and Hole-Doped Cuprates

We probe the "normal" state in electron-doped (n-type) Sm2-xCexCuO4-delta through interlayer tunneling transport in magnetic fields up to 45 T. The behavior of intrinsic high-field c-axis negative magnetoresistance (MR), which is accessed in small 30 nm-high mesa structures, is characteristic of the pseudogap state. It follows a universal correlation between the excess low-energy dissipation due to the pseudogap and its closing field Hpg and is in close correspondence with the hole-doped (p-type) Bi2Sr2CaCu2O8+y. The MR in the mesas and in the bulk crystals consistently gives a Zeeman relation between the pseudogap temperature T* and its closing field, pointing to a preeminent role of spin-singlet correlations in forming the pseudogap in cuprates, regardless of their n or p type.

Two Pseudogaps in Underdoped Cuprates

Cuprate superconductivity gaps are investigated applying the interband pairing scheme to the itinerant band and a defect "band" which is created by hole doping. A pseudogap is naturally present in this model at underdoping. Vibronic hybridization of bands renormalizes the pseudogap and two superconductivity gaps. Now two pseudogaps of electronic and vibronic origin are present in the underdoped normal state as recently observed. Entering the optimally doped region (overlapping bands) there appears a joint normal state pseudogap of vibronic origin.

Pair-breaking effects in the pseudogap regime: Application to high-temperature superconductors

Abrikosov-Gor'kov (AG) theory, the foundation for understanding pair-breaking effects in conventional superconductors, is inadequate when there is an excitation gap (pseudogap) present at the onset of superconductivity. In this paper we present an extension of AG theory within two important, and diametrically opposite approaches to the cuprate pseudogap. The effects of impurities on the pseudogap onset temperature T* and on Tc, along with comparisons to experiment are addressed.

Néel transition, spin fluctuations, and pseudogap in underdoped cuprates by a Lorentz invariant four-fermion model in 2+1 dimensions

We show that the N\'eel transition and spin fluctuations near the N\'eel transition in planar cuprates can be described by an SU(2) invariant relativistic four-fermion model in 2+1 dimensions. Features of the pseudogap phenomenon are naturally described by the appearance of an anomalous dimension for the spinon propagator.

Short-range spin correlations and pseudogap in underdoped cuprates

In this paper we show that local spin-singlet amplitude with d-wave symmetry can be induced by short-range spin correlations even in the absence of pairing interactions. In the present scenario for the pseudogap, the normal state pseudogap is caused by the induced local spin-singlet amplitude due to short-range spin correlations, which compete in the low energy sector with superconducting correlations to make T c go to zero near half-filling.

Variational tests of current order parameters in the Hubbard model

Variational tests are performed for current order parameters as probable sources of the pseudogap normal state of cuprates. The calculations are carried out based on the states with correlations of the valence bond type whose formation can induce in principle both the superconducting order of the d symmetry and current phases. It is shown for the t-t′-U Hubbard models with a large value of U(∼8t) and the Hubbard splitting of the conduction band that (1) phases of alternating charge and longitudinal spin currents cannot be realized and (2) transverse spin currents are not compatible with the superconducting order and they could exist against the normal-state background only within a very narrow doping region near the optimal one. This region does not correspond to the region of existence of a pseudogap in cuprates, which refutes the above-mentioned hypothesis of the pseudogap origin. The requirements to the parameters of models for which the consideration of correlations of the valence bond type yields a reasonable phase curve. The existence of current phases in the t-t′-U-V Hubbard models with a strong interaction (V>0.25t) of particles in neighboring sites is predicted when the d-superconductivity is completely suppressed.

Possible explanation of the pseudogap in high-temperature cuprates

A concept describing the origin and some properties of the ``pseudogap phase'' of high-${T}_{c}$ superconducting cuprates is proposed based on the author's idea about resonant tunneling connection between the ${\mathrm{CuO}}_{2}$ layers. The superconducting critical temperature in this picture is defined at low doping by establishment of a three-dimensional phase correlation between the layers, and at high doping by destruction of a d-wave superconductivity by disorder. The result is a nonmonotonic behavior of ${T}_{c}$ with doping. The pseudogap phase is analyzed qualitatively on the basis of the recently discovered existence of vortices in this phase and one-dimensional fluctuations due to extended saddle points (``hot spots'').

Temperature and magnetic field-dependent tunneling study of Bi 2Sr 2CaCu 2O y in the pseudogap state

The existence of Cooper pairs above T c is a popular theory of pseudogap in cuprates. The influence of magnetic field on such Cooper pairs should be drastic, although magnitude of the field required can be quite large. We have performed tunneling measurements in the ab-plane direction on underdoped Bi 2Sr 2CaCu 2O y (Bi2212) single crystals with T c=65 K for SIS junction formed with a low-temperature STM. The gap value Δ was found to be 35 meV from low-temperature tunneling spectra. The gap stays at this value up to T c and it becomes more smeared near T c, but exists at least up to 150 K. The spectra near 100 K were studied as a function of the magnetic field and no change in the gap features was observed up to 1.1 T.

Study on the anomalies of thermoelectric power due to the normal-state pseudogap in underdoped cuprates

The thermoelectric power of underdoped Gd 1− x Pr x Ba 2Cu 3O 7− δ with different Pr contents is measured. The experiments indicate that a hump in the temperature dependence of TEP appears above T c. This is considered to be associated with the opening of the normal-state pseudogap. The present experimental results and those reported by Tallon et al . [J.L. Tallon et al., Phys. Rev. Lett. 75 (1995) 4114.] are analyzed based on the opening of the pseudogap. The pseudogap excitation energy E G is obtained. Furthermore, a phase diagram of E G versus hole concentration p is given. The analysis may provide an effective way to estimate E G.

Doping and Isotope Dependence of the Pseudogap in High-Tc Cuprates Observed by Neutron Crystal-Field Spectroscopy: A Fast Local Probe

The temperature dependence of the relaxation rate of crystal-field excitations in the high-T c superconductors HoBa2Cu4O8 and HoBa2Cu3O x (6.4 < x < 7) was investigated by inelastic neutron scattering techniques. The data show clear evidence for the opening of an electronic gap in the normal state at T* > T c in the underdoped regime as well as for slightly overdoped samples. For HoBa2Cu4O8 T* increases from 170 to 220 K upon oxygen isotope substitution (16O vs 18O). This huge isotope shift (which is absent in NMR and NQR experiments) suggests that the mechanism leading to an isotope effect on the pseudogap has to involve a time scale in the range 10−8 ≫ τ > 10−13 s.

Changes in irreversibility line, anisotropy, and condensation energy by oxygen depletion ofYBa2Cu3O7−δ

We report results of magnetization measurements on the same ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ crystal for a range of $\ensuremath{\delta}$ values $0.03&lt;~\ensuremath{\delta}&lt;~0.35,$ in magnetic fields up to 120 kOe, applied parallel or perpendicular to the ${\mathrm{CuO}}_{2}$ planes. Zero-field resistivity anisotropy measurements have also been made for the same values of $\ensuremath{\delta}.$ The irreversibility line ${H}_{\mathrm{irr}}$ falls by over a factor of 10 in the range of $\ensuremath{\delta}$ studied, and there is a similar strong decrease in superconducting condensation energy associated with the growth of the normal-state pseudogap in under-doped cuprates. On the other hand, the anisotropy in ${H}_{\mathrm{irr}}$ and the room-temperature resistivity anisotropy only increase by a factor of 2. Evidence that for our crystals the so-called ``vortex lattice melting line'' ${H}_{m}(T)\ensuremath{\approx}{H}_{\mathrm{irr}}(T)$ is presented. There is also a large field and temperature region where the reversible magnetization varies as ${H}^{\ensuremath{-}1/2}$ and its magnitude corresponds to a free-energy density of ${k}_{B}T$ in a certain field-dependent correlation volume. Many of the results are consistent with the presence of critical thermodynamic fluctuations described by the three-dimensional $\mathrm{XY}$ model; these are probably enhanced when the field along the c axis is greater than 10 kOe.

Incoherent Pair Tunneling as a Probe of the Cuprate Pseudogap

We argue that incoherent pair tunneling in a cuprate superconductor junction with an optimally doped superconducting and an underdoped normal lead can be used to detect the presence of pairing correlations in the pseudogap phase of the underdoped lead. We estimate that the junction characteristics most suitable for studying the pair tunneling current are close to recently manufactured cuprate tunneling devices.

Pseudogaps in underdoped cuprates

It has become clear in the past several years that the cuprates show many unusual properties, both in the normal and superconducting (SC) states, especially in the underdoped region. In particular, gap-like behavior is observed in magnetic properties, c-axis conductivity and photoemission, whereas in-plane transport properties are only slightly affected by the pseudogap. I shall argue that these experimental evidences must be viewed in the context of the physics of a doped Mott insulator and that they support the notion of spin charge separation. I shall review recent theoretical developments, concentrating on studies based on the t– J model. I shall describe a model based on quasiparticle (qp) excitations, which predicts the doping dependence of T c and anomalous energy-gap-to- T c ratios. Finally, I shall outline how the model may be derived from a microscopic formulation of the t– J model. After a brief review of the U(1) formulation, I shall explain some of the difficulties encountered there, and how a new SU(2) formulation can resolve some of the difficulties.

Pseudogap Regime in a BCS Bose–Einstein Crossover Scenario: Experimental Consequences and Tests

We have argued that the pseudogap regime in the cuprates is characterized by the presence of metastable, long lived pairs which form the natural intermediate state between the free fermions of the BCS and bound pairs of the Bose Einstein limits. Here we investigate the experimental consequences associated with this resonant pair scattering in the normal state. The essential feature of these pair resonances is that they are dynamic pairing fluctuations. Of particular interest in distinguishing different scenarios for the cuprate pseudogap are probes which predominantly couple to the static and dynamic pair fluctuations, respectively. We therefore contrast the behavior of the diamagnetic susceptibility with the pair tunneling spectra of Superconductor-Pseudogapped cuprate junctions.

An update on computational results for electronic models: the issue of pseudogaps in cuprates and phase separation in manganites

A brief summary of results obtained using computational techniques in the context of electronic models is provided. It is argued that the theoretically predicted photoemission spectra is compatible with the presence of a pseudogap, as observed in the experiments. New directions of research are discussed, including the development of reduced basis set algorithms, and the inclusion of realistic Coulomb interactions in the t– J model. Results for manganite models show phase separation between hole-rich ferromagnetic and hole-poor antiferromagnetic regions. The manganite normal state may be as interesting as recently observed in the cuprates with neutron scattering techniques.

THE CUPRATE PSEUDOGAP: PRECURSOR SUPERCONDUCTIVITY WITHOUT PREFORMED PAIRS

We investigate a scenario for the cuprate pseudogap which is based on strong pairing correlations above T c. This picture is motivated by the small size of the coherence length, which suggests that these superconductors are in a crossover regime between the BCS (large pair size) and Bose–Einstein (small pair size) limits. Moreover, the observed smooth evolution of the pseudogap to the superconducting gap reinforces the notion that pairing correlations may be responsible for the pseudogap. Our new theoretical approach to this well-known crossover is based on a diagrammatic theory proposed by Kadanof and Martin (and extended by Patton), which self consistently treats the coupling between the single particle and pair propagators. The pseudogap, which is present only for an intermediate pair size (or equivalently, intermediate values of the coupling constant), correlates well with the onset of a resonance in the pair propagator, as expected for a state intermediate between free and bound fermions. Our results are consolidated into a phase diagram for T c and T*. We therefore demonstrate how the onset of superconductivity occurs in a pseudogapped normal state, and contrast our approach with alternate theories in the literature.