Pseudogap Temperature Research Articles

Dynamic stripes and resonance in the superconducting and normal phases ofYBa2Cu3O6.5ortho-II superconductor

We describe the relation between spin fluctuations and superconductivity in a highly-ordered sample of YBaCu3O6.5 using both polarized and unpolarized neutron inelastic scattering. The spin susceptibility in the superconducting phase exhibits one-dimensional incommensurate modulations at low-energies, consistent with hydrodynamic stripes. With increasing energy the susceptibility curves upward to a commensurate, intense, well-defined and asymmetric resonance at 33 meV with a precipitous high-energy cutoff. In the normal phase, which we show is gapless, the resonance remains surprisingly strong and persists clearly in Q scans and energy scans. Its similar asymmetric spectral form above Tc=59 K suggests that incoherent superconducting pairing fluctuations are present in the normal state. On cooling, the resonance and the stripe modulations grow in well above Tc below a temperature that is comparable to the pseudogap temperature where suppression occurs in local and low-momentum properties. The spectral weight that accrues to the resonance is largely acquired by transfer from suppressed low-energy fluctuations. We find the resonance to be isotropically polarized, consistent with a triplet carrying ~2.6% of the total spectral weight of the Cu spins in the planes.

Thermodynamics and phase diagram of high temperature superconductors.

Thermodynamic quantities are derived for superconducting and pseudogap regimes by taking into account both amplitude and phase fluctuations of the pairing field. In the normal (pseudogap) state of the underdoped cuprates, two domains have to be distinguished: near the superconducting region, phase correlations are important up to temperature T(phi). Above T(phi), the pseudogap region is determined only by amplitudes, and phases are uncorrelated. Our calculations show excellent quantitative agreement with specific heat and magnetic susceptibility experiments on cuprates. We find that the mean field temperature T0 has a similar doping dependence as the pseudogap temperature T(*), whereas the pseudogap energy scale is given by the average amplitude above T(c).

Phase Fluctuations of s-Wave Superconductors on a Lattice

Based on an attractive $U$ Hubbard model on a lattice with up to second neighbor hopping we derive an effective Hamiltonian for phase fluctuations. The superconducting gap is assumed to have s-wave symmetry. The effective Hamiltonian we finally arrive at is of the extended XY type. While it correctly reduces to a simple XY in the continuum limit, in the general case, it contains higher neighbor interaction in spin space. An important feature of our Hamiltonian is that it gives a much larger fluctuation region between the Berezinskii-Kosterlitz-Thouless transition temperature identified with $T_{c}$ for superconducting and the mean field transition temperature identified with the pseudogap temperature.

Pseudogap in Gd-based 123 HTSC

The Gd(Ba 2− x Pr x )Cu 3O 7+ δ single phase polycrystalline samples with 0.00⩽ x<1.00 were investigated for pseudogap phenomenon in comparison with the (Gd 1− x Pr x )Ba 2Cu 3O 7− δ compound. The measured XRD and its Rietveld refinement show that Pr is substituted at Ba site, with its stoichiometric order, and an orthorhombic-tetragonal transition occurs at x=0.20. For x⩾0.60, the solubility limit of the system exceeds. Based on the resistivity measurements, the effect of Pr substitution in 123 structure of HTSC at Gd or Ba site is to increase the pseudogap temperature T s, although, Pr at Ba site has a stronger effect on the suppression of T s and superconductivity. Pr-doping not only reduces the carrier density and induces pseudogap, but also simultaneously increases T s and suppresses the linearity behavior of resistivity vs. temperature in the normal state. Our results imply that understanding the real suppression mechanism of superconductivity by Pr-doping in HTSC is connected unavoidably to the determination of exact position of Pr in the structure. The presented data is in good agreement with the mean field phase diagram.

Zero Bias Conductance Peak in HTS above Critical Temperature

It has recently been proposed that if phase incoherent preformed pairs exist above critical temperature (Tc), Andreev reflection should be observable above Tc too. A distinctive feature of Andreev reflection is a zero bias conductance peak (ZBCP) in the conductance/voltage (G/V) characteristics. We have performed an extensive search for the ZBCP in HTS/normal-metal planar junctions including samples which have been annealed in Argon in an attempt to induce underdoping and enhancement of the pseudogap temperature. We have observed a maximum in the G/V characteristics above Tc in a number of samples and contact configurations. In some samples this behavior persists up to room temperature, but has a very broad voltage scale, much greater than the gap voltage. In YBa 2 Cu 3 O x/ Au and YBa 2 Cu 3 O x/ Ag contacts the form and amplitude of the ZBCP is close to the theoretical prediction, but the origin of this effect could also be in sample heating. One particularly curious result comes from an Au/LSMO/YBCO sample that shows a clear sub-gap ZBCP over the temperature range 15–266 K.

Antiferromagnetic vortex core in Tl(2)Ba(2)CuO(6+delta) studied by nuclear magnetic resonance.

Spatially resolved NMR is used to probe the magnetism in and around vortex cores of nearly optimally doped Tl(2)Ba(2)CuO(6+delta) (T(c)=85 K). The NMR relaxation rate T(-1)1 at the 205Tl site provides direct evidence that the antiferromagnetic (AF) spin correlation is significantly enhanced in the vortex core region. In the core region Cu spins show a local AF ordering with moments parallel to the layers at T(N)=20 K. Above T(N) the core region is in the paramagnetic state which is a reminiscence of the state above the pseudogap temperature (T(*) approximately 120 K), indicating that the pseudogap disappears within cores.

Unified picture of Néel order destruction, d-wave superconductivity and the pseudogap phase for high Tc cuprates

Evolution from Néel order to dx2−y2-wave superconductivity in the high Tc cuprates is described in terms of the skyrmion-like spin texture induced by the doped hole. The pseudogap phase, which is associated with preformed pairs, appears when the antiferromagnetic correlation length exceeds the average hole distance. A universal formula is proposed for the pseudogap temperature versus the doped hole concentration.

Evolution of the resistivity anisotropy inBi2Sr2−xLaxCuO6+δsingle crystals for a wide range of hole doping

To elucidate how the temperature dependence of the resistivity anisotropy of the cuprate superconductors changes with hole doping, both the in-plane and the out-of-plane resistivities (\rho_{ab} and \rho_{c}) are measured in a series of high-quality Bi_{2}Sr_{2-x}La_{x}CuO_{6+\delta} (BSLCO) single crystals for a wide range of x (x = 0.23 - 1.02), which corresponds to the hole doping per Cu, p, of 0.03 - 0.18. The anisotropy ratio, \rho_{c}/\rho_{ab}, shows a systematic increase with decreasing p at moderate temperatures, except for the most underdoped composition where the localization effect enhances \rho_{ab} and thus lowers \rho_{c}/\rho_{ab}. The exact p dependence of \rho_{c}/\rho_{ab} at a fixed temperature is found to be quite peculiar, which is discussed to be due to the effect of the pseudogap that causes \rho_{c}/\rho_{ab} to be increasingly more enhanced as p is reduced. The pseudogap also causes a rapid growth of \rho_{c}/\rho_{ab} with decreasing temperature, and, as a result, the \rho_{c}/\rho_{ab} value almost reaches 10^6 in underdoped samples just above T_c. Furthermore, it is found that the temperature dependence of \rho_{c} of underdoped samples show two distinct temperature regions in the pseudogap phase, which suggests that the divergence of \rho_{c} below the pseudogap temperature is governed by two different mechanisms.

Monte Carlo study of pseudo-gap temperature [formula omitted] within JJA model

We study pseudo-gap temperature T∗ of high-Tc superconductors by a Monte Carlo simulation of anisotropic 3D Josephson junction array (JJA) model based on the Ginzburg–Landau theory. We investigate T∗ both in the cases of zero external current and finite external current I in the JJA. It is found that, the external current I depresses only a little the pseudo-gap temperature T∗, while the superconducting critical temperature Tc is much affected by I.

Theory of Nernst coefficient in high-Tc cuprates: the role of vertex corrections

We study the the Nernst coefficient (ν) in hole-doped as well as electron-doped high-Tc cuprates, which takes a much enhanced value compared with conventional metals. Especially, ν in hole-doped cuprates increases drastically below the pseudo-gap temperature, which attracts great attention in that it reflects the fundamental feature of the electronic state in the pseudo-gap region. In this article, we study these transport phenomena in terms of the fluctuation-exchange+T-matrix approximation. We focus on the role of the vertex corrections (VCs) which are necessary to keep the conservation laws, and find that both ν and Δρ/ρ are strongly enhanced by the VCs due to the antiferromagnetic (AF) and the superconducting (SC) fluctuations. In conclusion, the pseudo-gap region in high-Tc cuprates is well described by the AF and SC fluctuation scenario.

Nanoscale phase separation in colossal magnetoresistance materials: lessons for the cuprates?

A recent vast experimental and theoretical effort in manganites has shown that the colossal magnetoresistance effect can be understood based on the competition of charge-ordered and ferromagnetic phases. The general aspects of the theoretical description appear to be valid for any compound with intrinsic phase competition. In high temperature superconductors, recent experiments have shown the existence of intrinsic inhomogeneities in many materials, revealing a phenomenology quite similar to that of manganese oxides. Here, the results for manganites are briefly reviewed with emphasis on the general aspects. In addition, theoretical speculations are formulated in the context of Cu-oxides by mere analogy with manganites. This includes a tentative explanation of the spin-glass regime as a mixture of antiferromagnetic and superconducting islands, the rationalization of the pseudogap temperature T ∗ as a Griffiths temperature where clusters start forming upon cooling, the prediction of ‘colossal’ effects in cuprates, and the observation that quenched disorder may be far more relevant in Cu-oxides than previously anticipated.

Pseudogap state in overdoped Bi 2Sr 2CaCu 2O 8+ y

We use interlayer intrinsic tunneling resistivity to probe the pseudogap in Bi 2Sr 2CaCu 2O 8+ y in magnetic fields up to 60 T. A map of the field–temperature ( H– T) diagram of the pseudogap state reveals the pseudogap as a dominant feature up to very high levels of hole doping. The unconventionally large quantum fluctuation regime observed on the overdoped side is consistent with the pseudogap in the vortex cores. Unlike the characteristic fields of the superconducting state, the field that closes the pseudogap at low temperatures is temperature-independent. It is related to the pseudogap temperature T ★ through a simple Zeeman energy scale, pointing to the pseudogap controlled by the spin rather than the orbital degrees of freedom.

The role of magnetism in forming the c-axis spectral peak at 400 cm −1 in high temperature superconductors

We discuss the peak at 400 cm −1, which is seen in c-axis conductivity spectra of underdoped high temperature superconductors. The model of van der Marel and Munzar, where the peak is the result of a transverse plasmon arising from a low frequency conductivity mode between the closely spaced planes, fits our data well. Within the model we find that the temperature dependence of the peak amplitude is controlled by in-plane scattering processes. The temperature range where the mode can be seen coincides with T s, the spin gap temperature, which is lower than T ∗, the pseudogap temperature. As a function of temperature, the amplitude of the mode tracks the amplitude of the 41 meV neutron resonance and the spin lattice relaxation time, suggesting to us that the mode is controlled by magnetic processes and not by superconducting fluctuations which have temperature scale much closer to T c, the superconducting transition temperature.

Theory for high-Tcsuperconductors considering inhomogeneous charge distribution

We propose a general theory for the critical $T_c$ and pseudogap $T^*$ temperature dependence on the doping concentration for high-$T_c$ oxides, taking into account the charge inhomogeneities in the $CuO_2$ planes. The well measured experimental inhomogeneous charge density in a given compound is assumed to produce a spatial distribution of local $\rho(r)$. These differences in the local charge concentration is assumed to yield insulator and metallic regions, possibly in a stripe morphology. In the metallic region, the inhomogeneous charge density yields also spatial distributions of superconducting critical temperatures $T_c(r)$ and zero temperature gap $\Delta_0(r)$. For a given sample, the measured onset of vanishing gap temperature is identified as the pseudogap temperature, that is, $T^*$, which is the maximum of all $T_c(r)$. Below $T^*$, due to the distribution of $T_c(r)$'s, there are some superconducting regions surrounded by insulator or metallic medium. The transition to a superconducting state corresponds to the percolation threshold among the superconducting regions with different $T_c(r)$'s. To model the charge inhomogeneities we use a double branched Poisson-Gaussian distribution. To make definite calculations and compare with the experimental results, we derive phase diagrams for the BSCO, LSCO and YBCO families, with a mean field theory for superconductivity using an extended Hubbard Hamiltonian. We show also that this novel approach provides new insights on several experimental features of high-$T_c$ oxides.

高圧下ＮＭＲ ＹＢａ２Ｃｕ４Ｏ８の高圧ＮＱＲ

The temperature dependence of the 63Cu nuclear spin-lattice relaxation rate (T1T)-1 , spin-spin relaxation rate T2G-1 and frequency spectra at the Cu(2) planar site in the normal state of YBa2Cu4O8 have been measured under high pressure by the nuclear quadrupole resonance(NQR) technique. The pseudo spin gap temperature slightly decreased with increasing pressure. It seems that the pressure effects are resembled with carrier doping effects in the increase of superconducting transition temperature Tc and NQR frequency νQ(2) and decrease of pseudo spin gap temperature. If the increase of νQ(2) under pressure was caused by introducing carrier into Cu(2) planar site, the change of pseudo spin gap temperature is generally originated in carrier concentration at Cu(2) site.

Spatially-Resolved NMR Study on Antiferromagnetic Vortex Core in HTSC

Magnetism in the vortex core of nearly-optimal doped Tl2Ba2CuO6+δ (Tc=85 K) is investigated by a spatially-resolved NMR. The NMR relaxation rate T−11 at 205Tl site provides a direct evidence of the significant enhancement of the AF spin correlations in the vortex core region and shows clearly a local AF ordering of the core region Cu spins at TN=20 K. Above TN the core region is in the paramagnetic state which is a reminiscence of the state above the pseudogap temperature (T*∼−120 K), indicating that the pseudogap disappears within the vortex core.

Nernst coefficient and magnetoresistance in high-T(c) superconductors: the role of superconducting fluctuations.

In high-T(c) cuprates, the Nernst coefficient (nu) as well as the magnetoresistance (Deltarho/rho) increases drastically below the pseudogap temperature, T(*), which attracts much attention as a key phenomenon in the pseudogap region. We study these transport phenomena in terms of the fluctuation-exchange+T-matrix approximation. In this present theory, the d-wave superconducting (SC) fluctuations, which are mediated by antiferromagnetic (AF) correlations, become dominant below T(*). We especially investigate the vertex corrections both for the charge current and the heat one to keep the conservation laws. As a result, the mysterious behaviors of nu and Deltarho/rho are naturally explained as the reflection of the enhancement of the SC fluctuation, without assuming thermally excited vortices. The present result suggests that the pseudogap phenomena are well described in terms of the Fermi liquid with AF and SC fluctuations.

Angle-resolved study of density waves, superconductivity and pseudogap in two dimensions

Weakly correlated electrons on a square lattice are studied by angle-resolved functional renormalization group. Upon renormalization the interaction starts to depend on monienta and has pole-like solutions near the doping-dependent critical scale. Near half-filling this critical scale is the pseudogap temperature T*. In the overdoped regime the critical scale is the mean-field like critical temperature for d-wave superconductivity.

Magnetic order inYBa2Cu3O6+xsuperconductors

Polarized and unpolarized neutron diffraction has been used to search for magnetic order in YBa$_2$Cu$_3$O$_{6+x}$ superconductors. Most of the measurements were made on a high quality crystal of YBa$_2$Cu$_3$O$_{6.6}$. It is shown that this crystal has highly ordered ortho-II chain order, and a sharp superconducting transition. Inelastic scattering measurements display a very clean spin-gap and pseudogap with any intensity at 10 meV being 50 times smaller than the resonance intensity. The crystal shows a complicated magnetic order that appears to have three components. A magnetic phase is found at high temperatures that seems to stem from an impurity with a moment that is in the $a$-$b$ plane, but disordered on the crystal lattice. A second ordering occurs near the pseudogap temperature that has a shorter correlation length than the high temperature phase and a moment direction that is at least partly along the c-axis of the crystal. Its moment direction, temperature dependence, and Bragg intensities suggest that it may stem from orbital ordering of the $d$-density wave (DDW) type. An additional intensity increase occurs below the superconducting transition. The magnetic intensity in these phases does not change noticeably in a 7 Tesla magnetic field aligned approximately along the c-axis. Searches for magnetic order in YBa$_2$Cu$_3$O$_{7}$ show no signal while a small magnetic intensity is found in YBa$_2$Cu$_3$O$_{6.45}$ that is consistent with c-axis directed magnetic order. The results are contrasted with other recent neutron measurements.

Pressure studies on the pseudogap and critical temperatures of a high-Tcsuperconductor

We report simultaneous hydrostatic pressure studies on the critical temperature $T_c$ and on the pseudogap temperature $T^*$ performed through resistivity measurements on an optimally doped high-$T_c$ oxide $Hg_{0.82}Re_{0.18}Ba_2Ca_2Cu_3O_{8+\delta}$. The resistivity is measured as function of the temperature for several different applied pressure below 1GPa. We find that both $T_c$ and $T^*$ increases linearly with the pressure. This result demonstrate that the well known intrinsic pressure effect on $T_c$ is also present at $T^*$ and both temperatures are originated by the same superconducting mechanism.