Enhancement of the superconducting transition temperature of La2-xSrxCuO4 bilayers: role of pairing and phase stiffness

The superconducting transition temperature T(c) of bilayers comprising underdoped La(2-x)Sr(x)CuO(4) films capped by a thin heavily overdoped metallic La(1.65)Sr(0.35)CuO(4) layer, is found to increase with respect to T(c) of the bare underdoped films. The highest T(c) is achieved for x=0.12, close to the "anomalous" 1/8 doping level, and exceeds that of the optimally doped bare film. Our data suggest that the enhanced superconductivity is confined to the interface between the layers. We attribute the effect to a combination of the high pairing scale in the underdoped layer with an enhanced phase stiffness induced by the overdoped film.     

Two- to three-dimensional crossover in the electronic structure of (Bi, Pb)2(Sr, La)2CuO(6+delta) from angle-resolved photoemission spectroscopy

The hole-concentration (x) dependence of the three-dimensional energy-momentum dispersion in (Bi, Pb)2(Sr, La)2CuO(6+delta) has been investigated by angle-resolved photoemission spectroscopy. For a heavily overdoped sample of T(c) < or = 0.5 K, an energy dispersion of approximately 10 meV in width is observed in the vicinity of the (pi, 0) point with varying momentum along the c axis (k(z)). This k(z) dispersion is zero for underdoped, optimally doped, and slightly overdoped samples up to a doping level corresponding to T(c) = 22 k. At higher doping levels we observe significant dispersion of the order of 10 meV (sample with T(c) < or = 0.5 K). This is clear evidence that at a doping value corresponding to T(c) = 22 K, a crossover from two- to three-dimensional electronic structure occurs.     

Electrostatic tuning of the electrical properties of YBa2Cu3O7−x using an ionic liquid

Ultrathin YBa₂Cu₃O_7?x (YBCO) films were grown on SrTiO₃ (STO) substrates using the technique of high-pressure oxygen sputtering. Films were then incorporated in a field effect transistor configuration, which facilitated the control of superconductivity by electrostatic charging. While devices using STO as both the substrate and gate dielectric have produced only relatively small shifts in film electrical properties, very large changes can be realized using an electric double layer transistor configuration employing the ionic liquid DEME-TFSI as the dielectric. By depleting holes an electrostatically tuned superconductor insulator transition was studied using a finite size scaling analysis. The breakdown of scaling at the lowest temperatures suggests the presence of a mixed insulator/superconductor phase separating the two ground states. Further depletion of holes resulted in a change of the majority carriers from holes to electrons and the emergence of what appeared to be very weak re-entrant superconductivity. Also by accumulating holes an underdoped film was tuned into the overdoped regime. A two-step mechanism for electrostatic doping was revealed. Hall effect measurements suggested the presence of an electronic phase transition or a change in the Fermi surface as a function of doping near optimal doping.     

Nodal and antinodal gaps in the superconducting state of cuprates

We present an electronic Raman scattering study of a mercury-based single layer cuprate, as a function of both doping level and temperature in the superconducting state. On the deeply overdoped side, we show that the antinodal gap is a true superconducting gap. In contrast, on the underdoped side, our results reveal the existence of a break point close to optimal doping below which the antinodal gap is gradually disconnected from superconductivity.     

Same superconducting criticality for underdoped and overdoped La2-xSrxCuO4 single crystals

By measuring the superconducting diamagnetic moments for an underdoped and an overdoped La2-xSrxCuO4 single crystal with equal quality and roughly equal transition temperatures, it is found that the underdoped sample has only one transition which corresponds to H(c2), but the overdoped sample has two transitions with the higher one at H(c2). Further investigation reveals the same upper-critical field H(c2) for both samples although the overall charge densities are very different, indicating the possibility of a very direct and detailed equivalence of the superconducting condensation process in the two doping limits. The second transition for the overdoped sample can be understood as the bulk coupling between the superconducting clusters produced by macroscopic phase separation.     

Pseudogap in Bi2Sr2CaCu2O8+delta studied by measuring anisotropic susceptibilities and out-of-plane transport

We find in the Bi2Sr2CaCu2O8+delta system that the characteristic temperatures T*chi [below which the uniform susceptibilities chiabT (Hperpendicularc) and chicT (Hparallelc) decrease] and T*(rhoc) [below which the out-of-plane resistivity rhoc(T) shows typical upturn] coincide for all doping levels. We attribute the T dependence of chi's and rhoc to the anomalous (pseudogapped) density of states (DOS) in high- Tc cuprates. Furthermore, the anisotropy in the T dependence of chi's is universal, i.e., chic approximately 1.6chiab, showing that there is only a single T-dependent component in the chi's. This implies that the Curie-like behavior (dchi/dT<0) observed in overdoped samples is also caused by a DOS effect.     

Temperature dependent local Cu-O displacements from underdoped to overdoped La-Sr-Cu-O superconductor

We have studied doping evolution of the temperature dependent local Cu-O displacements in the La_2-x Sr _x CuO₄ superconductor by polarized Cu K-edge extended X-ray absorption fine structure (EXAFS) measurements. While temperature dependent Debye-Waller factor of the Cu-O bonds, measuring the local Cu-O displacements, shows an anomalous increase at low temperature for the underdoped single crystals, we do not find such a dependence for the case of the overdoped system. The results, which are discussed in the light of recent angle resolved photoemission measurements, provide an evidence for some important correlation between the doping dependent electron-lattice interaction, the charge inhomogeneities and the local Cu-O displacements in the copper oxide superconductors.Received: 9 July 2003, Published online: 19 November 2003PACS: 74.72.Dn La-based cuprates - 61.10.Ht X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc. - 74.81.-g Inhomogeneous superconductors and superconducting systems     

Doping dependence of the coupling of electrons to bosonic modes in the single-layer high-temperature Bi2Sr2CuO6 superconductor

A recent highlight in the study of high-T(c) superconductors is the observation of band renormalization or self-energy effects on the quasiparticles. This is seen in the form of kinks in the quasiparticle dispersions as measured by photoemission and interpreted as signatures of collective bosonic modes coupling to the electrons. Here we compare for the first time the self-energies in an optimally doped and strongly overdoped, nonsuperconducting single-layer Bi-cuprate (Bi2Sr2CuO6). In addition to the appearance of a strong overall weakening, we also find that the weight of the self-energy in the overdoped system shifts to higher energies. We present evidence that this is related to a change in the coupling to c-axis phonons due to the rapid change of the c-axis screening in this doping range.     

Crossover from coherent to incoherent electronic excitations in the normal state of Bi(2)Sr(2)CaCu(2)O(8+delta)

Angle resolved photoemission spectroscopy (ARPES) and resistivity measurements are used to explore the overdoped region of the high temperature superconductor Bi(2)Sr(2)CaCu(2)O(8+delta). We find evidence for a new crossover line in the phase diagram between a coherent metal phase, for lower temperatures and higher doping, and an incoherent metal phase, for higher temperatures and lower doping. The former is characterized by two well-defined spectral peaks in ARPES due to coherent bilayer splitting and superlinear behavior in the resistivity, whereas the latter is characterized by a single broad spectral feature in ARPES and a linear temperature dependence of the resistivity.     

Effect of pressure and of oxygen content on superconductivity in the cuprate HgBa2CuO4+δ. A quantitative analysis based on indirect-exchange pairing

A quantitative analysis is presented of experimental pressure and oxygen-doping effects on the critical temperature of HgBa₂CuO_4+δ over a wide range of δ, covering both under- and overdoped compositions, on the basis of indirect-exchange pairing between conduction electrons via oxygen anions. The results clearly show that for δ 0.28, the oxygen doping process is not the same as cation doping in e.g. La_2−xSr_xCuO₄: part of the doped oxygen fulfills a double role as hole dopant and as Cooper pair mediator. Furthermore, adding or extracting oxygen under experimental conditions is accompanied by a redistribution of oxygen anions in the unit cell, to or from the neighborhood of the CuO₂ layer. This effect explains the surprisingly wide range of superconducting compositions (0.03 δ 0.40) of the system. The observed, anomalous, upward shift in thermoelectric power for δ />/ 0.28 is attributed to different sites of doped oxygen beyond that limit, affecting only hole conductivity.     

What drives pseudogap physics in high-Tc cuprates? A view from the (resistance) bridge

Taking into account the evolution of the in-plane resistivity with temperature and doping, a candidate proposal for the (hole-doped) cuprate phase diagram is constructed. Many features of the phase diagram are viewed as a consequence of an anisotropic interaction that intensifies with decreasing doping from the heavily overdoped side. At a critical doping p_c, that coincides with the development of the normal state pseudogap, the interaction becomes sufficiently strong that all electronic states near the zone boundary are effectively incoherent even at T = 0.     

Tunneling into the normal state of Pr2-xCexCuO4

The temperature dependence of the tunneling conductance was measured for various doping levels of Pr(2-x)CexCuO4 using planar junctions. A normal state gap is seen at all doping levels studied, x=0.11 to x=0.19. We find it to vanish above a certain temperature T*. T* is greater than T(c) for the underdoped region and it follows T(c) on the overdoped side. This behavior suggests finite pairing amplitude above T(c) on the underdoped side.     

Discriminating the superconducting gap from the pseudogap in Bi(2)Sr(2)CaCu(2)O(8+delta) by interlayer tunneling spectroscopy

Tunneling spectroscopy using a very thin stack of intrinsic Josephson junctions has revealed that the superconducting gap is definitely different from the pseudogap in the Bi(2)Sr(2)CaCu(2)O(8+delta) system. In the underdoped region, the conductance peak arising from the superconducting gap is independently observed in the dI/dV-V curve and its position is much lower than that of the pseudogap. Near the optimum doping level and in the overdoped region, both peaks are located in close proximity. These findings are in conflict with a previous understanding of the pseudogap.     

Intermediate coupling model of cuprates: Adding fluctuations to a weak coupling model of pseudogap and superconductivity competition

We demonstrate that many features ascribed to strong correlation effects in various spectroscopies of the cuprates are captured by a calculation of the self-energy incorporating effects of spin and charge fluctuations. The self-energy is calculated over the full doping range from half-filling to the overdoped system. In the normal state, the spectral function reveals four subbands: two widely split incoherent bands representing the remnant of the two Hubbard bands, and two additional coherent, spin- and charge-dressed in-gap bands split by a spin-density wave, which collapses in the overdoped regime. The resulting coherent subbands closely resemble our earlier mean-field results. Here we present an overview of the combined results of our mean-field calculations and the newer extensions into the intermediate coupling regime.     

Zn doping of YBa2Cu3O7 in melt textured materials: peak effect and high trapped fields

The previously introduced modified melt crystallization process (MMCP) has been applied to prepare single grain YBCO bulk material with Zn partially substituting for Cu. Hole doping was controlled by an appropriate oxidizing treatment of the as-grown bulk. A field induced pinning was indicated by a well pronounced peak of the critical current density j_c in the j_c vs. H relationship for the maximal oxidized (overdoped) material containing Zn, whereas pure overdoped YBCO shows no peak effect. The peak effect for Zn-doped YBCO appearing for T=77 K at a relatively high field of about 3 T can be attributed to pair breaking by locally induced magnetic moments due to in plane Zn for Cu substitution. Besides high quality of the bulk YBCO, the peak effect is the reason for the trapped field as large as 1.12 T at 77 K in the cylinder of only 25 mm in diameter.     

Quantum critical point, scaling and universality in high Tc(CaxLa1−x)(Ba2−c−xLac+x)Cu3Oy

Using charge transport in sintered ceramic samples it is observed that at all doping, including non superconducting overdoped samples, there exists a temperature in which below it dR/dT < 0. This suggests that either the quantum critical point is not necessarily inside the superconducting dome or that the CuO₂ plane is never overdoped. Data relating experimental Cooper pair density, conductivity and T_c suggest that Homes’ relation might need a more specific definition of the conductivity σ.     

Evidence for a transition in the pairing symmetry of the electron-doped cuprates La(2-x)Ce(x)CuO(4-y) and Pr(2-x)Ce(x)CuO(4-y)

We present measurements of the magnetic penetration depth, lambda(-2)(T), in Pr(2-x)Ce(x)CuO(4-y) and La(2-x)Ce(x)CuO(4-y) films at three Ce doping levels, x, near optimal. Optimal and overdoped films are qualitatively and quantitatively different from underdoped films. For example, lambda(-2)(0) decreases rapidly with underdoping but is roughly constant above optimal doping. Also, lambda(-2)(T) at low T is exponential at optimal and overdoping but is quadratic at underdoping. In light of other studies that suggest both d- and s-wave pairing symmetry in nominal optimally doped samples, our results are evidence for a transition from d- to s-wave pairing near optimal doping.     

Random resistivity network calculations for cuprate superconductors with an electronic phase separation transition

The resistivity as a function of temperature for high temperature superconductors is very unusual and, despite its importance, lacks a unified theoretical explanation. It is linear with the temperature for overdoped compounds but it falls more quickly as the doping level decreases. The resistivity of underdoped cuprates increases like that of an insulator below a characteristic temperature where it shows a minimum. We show that this overall behavior can be explained by calculations using an electronic phase segregation into two main component phases with low and high electronic densities. The total resistance is calculated from the various contributions through several processes of random picking of the local resistivities and using a common statistical random resistor network approach.     

Robust dx2-y2 pairing symmetry in hole-doped cuprate superconductors

Although initially quite controversial, it is now widely accepted that the Cooper pairs in optimally doped cuprate superconductors have predominantly dx2-y2 wave function symmetry, and the controversy has now shifted to whether the pairing symmetry changes away from optimal doping. Here we present phase-sensitive tricrystal experiments on three cuprate systems: Y(0.7)Ca(0.3)Ba(2)Cu(3)O(7-delta) (Ca-doped Y-123), La2-xSrxCuO4 (La-214), and Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi-2212), with doping levels covering the underdoped, optimal, and overdoped regions. Our work implies that predominantly d x2-y2 pairing symmetry is robust over a large variation in doping.     

Correlation of tunneling spectra in Bi(2)Sr(2)CaCu(2)O(8+delta) with the resonance spin excitation

New break-junction tunneling data are reported in Bi(2)Sr(2)CaCu(2)O(8+delta) over a wide range of hole concentration from underdoped (T(c) = 74 K) to optimal doped (T(c) = 95 K) to overdoped (T(c) = 48 K). The conductances exhibit sharp dips at a voltage, Omega/e, measured with respect to the superconducting gap. Clear trends are found such that the dip strength is maximum at optimal doping and that Omega scales as 4.9kT(c) over the entire doping range. These features link the dip to the resonance spin excitation and suggest quasiparticle interactions with this mode are important for superconductivity.