Overdoped Samples Research Articles

Thermodynamic and superconducting properties of Hg 1−xRe xBa 2CuO y

Single phase materials of Hg 1−xRe xBa 2CuO y samples were obtained for 0.00 < x < 0.15. They were studied by thermogravimetry under controlled p(O 2) to analyze the influence of Re on their chemical stability, oxygen nonstoichiometry and thermodynamic properties. Under and over-doped samples were obtained by changing the oxygen content. The oxygen content was determined by neutron diffraction and thermogravimetric measurements. The role of Re on the superconducting behavior and the improvement of the chemical stability of the Hg-1201 compounds is discussed.

Effect of doping on the linear temperature dependence of the magnetic penetration depth in cuprate superconductors

We report on measurements of the low temperature dependence of the magnetic penetration depth λ ab in several single crystals of La s− x Sr x CuO 4, Bi 2Sr 2CaCu 2O 8 and YBa 2Cu 3O 6+ x at various doping levels ranging from the under- to the overdoped regimes by using a novel single-coil technique achieving 10 pm resolution. We have found a linear dependence of λ ab in all samples with a rapidly increasing slope dλ ab / dT as doping decreases. Our analysis of the data indicates that a superconducting gap with d-wave symmetry is sufficient to quantitatively account for the above slope values in the optimally or over-doped samples. In the underdoped samples, the d-wave model predicts much smaller values than those measured by assuming realistic values for the zero-temperature λ ab and gap Δ. The experimental data are compatible with a model of thermodynamic phase fluctuations of the order parameter. Therefore, we put forward the hypothesis that the gapless properties observed in cuprates may have qualitatively different physical origins depending on the doping level.

Onset of the 0-dimensional vortex pinning behavior in the optimally doped and overdoped Bi 2Sr 2CaCu 2O 8+δ system

We report results of transverse AC magnetic permeability in the linear regime and DC magnetization loops for optimally doped and oxygen overdoped samples of Bi 2Sr 2CaCu 2O 8+δ. The data cover the low temperature (T < 25K) and low field (H a < 1000 Oe) region of the magnetic (H a -T) phase diagram. In optimally doped as well as in overdoped samples a crossover to a 0-dimensional pinning regime (Larkin length, L c c , shorter than the inter-plane CuO 2 distance) is observed at a characteristic temperature T 0 D for all investigated fields. The order-disorder transition, identified by the second peak in magnetization and a non monotonous field dependence of the AC penetration depth, is quenched below T 0 D .

Macroscopic phase separation in overdoped high temperature superconductors

Superconducting diamagnetic properties have been extensively investigated on Bi 2Sr 2-xLa xCuO 6 and La 2-xSr xCuO 4 single crystals. Strong evidence for electronic macroscopic phase separation was found in overdoped samples of both systems revealing intrinsic inhomogeneities. It is found that because of the phase separation in the overdoped region, the criticality of superconductivity becomes the same as in the underdoped region albeit very different normal state properties between these two regions. This gives a strong indication that the superconductivity in underdoped region and overdoped region may have the same underlying mechanism (pairing and phase coherence). Our conclusion here will have important implication for how we view the high temperature superconductors.

IMPORTANCE OF PHASE FLUCTUATIONS FOR THE MAGNETIC PENETRATION DEPTH OF CONVENTIONAL AND CUPRATE SUPERCONDUCTORS

In order to unveil the origin of the gapless properties of cuprate superconductors, we study the low temperature dependence of the magnetic penetration depth λ in: a) conventional NbN superconducting films with granular and epitaxial microstructure; b) single crystals of La 2-x Sr x CuO 4, Bi 2 Sr 2 CaCu 2 O 8 (BSCCO) and YBa 2 Cu 3 O 6+x at various doping levels ranging from the under- to the overdoped regimes. The measurements are performed by using a novel single-coil technique achieving 10 pm resolution. Our experimental results are as follows. a) The granular NbN films exhibit a linear low-temperature dependence of λ similar to the dependence reported earlier in cuprates. In the epitaxial NbN films, we find the characteristic exponential behaviour of conventional s-wave superconductors. b) In all single crystals of cuprates, the data confirm the linear dependence of λ ab reported earlier. In BSCCO, we find that the slope d λ ab / dT is rapidly increasing as doping decreases. As to a), our results give evidence for a gapless behaviour in conventional s-wave superconductors induced by a granular microstructure. As to b), our analysis indicates that a simple d-wave model accounts for the experimental slope values only in the optimally and over-doped samples. In the underdoped samples, the d-wave model underestimates the large slope values observed experimentally. Such large values are rather consistent with a simple model of thermodynamic phase fluctuations suited for metals with reduced dimensionality and low carrier density, as in cuprates. We argue that this fluctuation model also accounts for the linear dependence of λ found in our granular NbN films. We discuss possible mechanisms associated with the existence of weak links across the grains, which would favour thermally activated phase fluctuations of the gap in this conventional superconductor. Finally, we put forward the hypothesis that a similar phenomenon may occur in cuprates because of the intrinsic granularity associated with phase-separated metallic and insulating regions in the superconducting ab-plane.

Temperature dependence of tunneling spectra in YBa2Cu3O7−δ and Bi2Sr2CaCu2O8+δ single crystals

Abstract During past years, scanning tunneling spectroscopy (STS) investigations of high temperature superconductors have revealed unusual characteristics of the superconducting gap. We present here the temperature dependence of the tunneling conductance spectra on YBa2Cu3O7−δ and Bi2Sr2CaCu2O8+δ single crystals. We discuss similarities and differences between these two compounds. In particular we note that while in Bi2Sr2CaCu2O8+δ a pseudogap is observed above the critical temperature, even in overdoped samples, we find no indication of a pseudogap in optimally doped YBa2Cu3O7−δ above Tc. This is consistent with other striking differences between the two systems and supports the interpretation of the pseudogap in terms of strong superconducting correlations.

Specific heat of Y 0.8Ca 0.2Ba 2Cu 3O 7− δ in magnetic fields

We measured the specific heat of Y 0.8Ca 0.2Ba 2Cu 3O 7− δ samples with different oxygen content from liquid helium temperature to room temperature in magnetic fields up to 13 T. We found that there is different hole concentration dependence and field dependence of electronic specific heat between overdoped samples and underdoped samples while T≪ T c. In normal state, we observed the normal state pseudogap which is magnetic field-independent.

Effect of oxygen redistribution in Bi-based high- Tc superconductors on their normal and superconducting properties

Bi-based high- T c superconductors of three types, Bi 2Sr 2CuO 6.13 (2201), Bi 2Sr 2CaCu 2O 8.24 (2212), and Bi 1.9Pb 0.3Sr 1.8Ca 2Cu 3O 10+ x (2223), has been investigated. The temperature dependence of resistivity ρ( T) has been measured during slow heating of samples (∼2 K/h) up to 600 K in air. The maximum of ρ( T) at T max>400 K is observed for all samples. T c increases for overdoped samples (2201 and 2212) and does not change in the optimal doped sample (2223) after cooling from T max, and hole concentration decrease for all samples. ρ and T c return to initial states after the heating of samples up to 600 K. Powder X-ray diffraction is carried out during slow heating of Bi based samples.

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.

Dependences of the chemical potential shift and superconducting transition temperature on the hole concentration in La2-xSrxCuO4 and Bi2Sr2Ca1-xYxCu2O8+

Dependences of the superconducting transition temperature (Tc) on the hole concentration (nh) in La2-xSrxCuO4 and Bi2Sr2Ca1-xYxCu2O8+ have been calculated using the interband model. The phase space for pair-transfer scattering between the overlapping bands is governed by the position of the chemical potential. Downward shifts (µ) of the electron chemical potential (µ) with nh have been found. The slope is µ/ nh8 eV/hole in under-doped Bi2Sr2Ca1-xYxCu2O8+ and it becomes smaller in optimally-doped and over-doped samples (~1.2 eV/hole). In La2-xSrxCuO4 µ/ nh is equal to 1.25 eV/hole in over-doped samples and ~0.4 eV/hole in under-doped samples at T = 80 K. The pseudogap leads to the suppression of µ(nh) up to nh0.17 holes per CuO2 plane at T = 80 K. The shift of the chemical potential µ leads to the curve Tc(nh) with a maximum. The dependences µ(nh) and Tc(nh) for the systems considered agree with the experimental data.

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.

Processing and superconducting properties of Hg1-xRexBa2Ca2Cu3O8+

In this work we discuss one of the principal parameters to control for producing pieces of (Hg,Re)-1223 for device applications. The first prerequisite is the mixing of the cations which should be as good as possible and the degree of oxygen in the phase. The influence of the precursor oxygenation is discussed. It is shown that overdoped samples of (Hg,Re)-1223 have an increased irreversibility line. Finally the possibility of reprocessing sintered superconducting samples is shown, a very important aspect for the preparation of several ceramic electronic pieces.

Electronic Raman scattering and phonon self-energy effects in theR-123 system: Signatures of gap and pseudogap

Using Raman spectroscopy and transport measurements we investigate thin epitaxial films of Y{1-x}(Pr,Ca)xBa2Cu3O{6+y}. We explore the electronic Raman responses obtained after subtraction of phononic excitations, and the 2Delta peaks that form out of the electronic background below Tc. We find that the energy of the B1g 2Delta peak increases monotonically with decreasing doping until the peaks become unresolvable. In contrast, the peaks in A1g symmetry follow Tc being resolvable in the Pr-doped films. The B2g responses are weak and a 2Delta peak is only detected at the highest doping level. As a consequence of strong electron-phonon coupling, the B1g phonon at 340 cm^-1 exhibits a pronounced Fano-type line shape. We use a phenomenological model to describe the line shape that takes into account real and imaginary part of the electronic response. It allows us to obtain the self-energy corrections and the mass- enhancement factor lambda as a measure of the coupling. In the normal state we find lambda=0.015 around optimal doping and decreasing values with decreasing doping. The electron-phonon coupling increases strongly below T_c in overdoped samples in which the B1g 2Delta peaks appear in the vicinity of the phonon. Self-energy effects observed in the superconducting state can only partly be assigned to the redistributing electronic response. Anomalies with respect to frequency, linewidth, and intensity remain. They appear at increasing tempera- tures with decreasing doping and we provide evidence that they are connected to the presence of the pseudogap. We supplement our study by a comparison with single crystal data and investigate the influence of site-substitutional disorder on the electronic response and the electron-phonon interaction.

Electronic Raman scattering and phonon self‐energy effects in the R‐123 system: Signatures of gap and pseudogap

AbstractUsing Raman spectroscopy and transport measurements we investigate thin epitaxial films of Y1—x(Pr, Ca)xBa2Cu3O6+y. We explore the electronic Raman responses in A1g, B1g, and B2g symmetry obtained after subtraction of phononic excitations, and especially, the 2Δ peaks that form out of the electronic background below Tc. We find that the energy of the B1g 2Δ peak, which is a measure of the gap value, increases monotonically with decreasing doping until the peak becomes unresolvable. In contrast, the peak in A1g symmetry follows Tc being still resolvable in the Pr‐doped films. The B2g response is weak and a 2Δ peak is only detected at the highest doping level. As a consequence of strong electron‐phonon coupling, the B1g phonon at ∼340 cm—1 exhibits a pronounced Fano‐type line shape. We use a phenomenological model to describe the line shape that takes into account real and imaginary part of the electronic response. It allows us to obtain the self‐energy corrections and the mass‐enhancement factor λ as a measure of the coupling. In the normal state we find λ = 0.015 around optimal doping and decreasing values with decreasing doping. The electron‐phonon coupling increases strongly below Tc in overdoped samples in which the B1g 2Δ peaks appear in the vicinity of the phonon. Self‐energy effects observed in the superconducting state can only partly be assigned to the redistributing electronic response. Anomalies with respect to frequency, linewidth, and intensity remain. They appear at increasing temperatures with decreasing doping and we provide evidence that they are connected to the presence of the pseudogap. We supplement our study by a comparison with single crystal data and investigate the influence of site‐substitutional disorder on the electronic response and the electron‐phonon interaction.

Effects of the weak disorder on the vortex phase diagram and the second peak in untwinned YBa 2Cu 3O y single crystals

Magnetization and resistivity measurements have been performed in untwinned YBa 2Cu 3O y single crystals with slightly different oxygen contents near the optimal condition. For slightly overdoped samples, the steep increase of the magnetization at H*, which is connected with the critical point, is consistent with the field-driven disordering transition. For the slightly underdoped sample, on the other hand, the vortex phase diagram shows no remarkable feature near the critical point due to the enhanced disorder by the oxygen deficiency.

Far-infrared ellipsometric study of the spectral gap in thec-axis conductivity ofY1−xCaxBa2Cu3O7−δcrystals

The temperature (T) and doping dependence of the spectral gap in the far-infrared c-axis conductivity of ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ has been studied by ellipsometry. For underdoped crystals a T-independent spectral gap is observed which persists almost unchanged into the normal state. The normal-state gap disappears at optimum doping whereas a conventional T dependence of the spectral gap is found for overdoped samples. The analogy to the results of angle-resolved photoemission suggests that the c-axis conductivity may be dominated by the carriers at the X point of the two-dimensional Brillouin zone.

Overdoped to underdoped Y{sub 0.8}Ca{sub 0.2}Ba{sub 2}Cu{sub 3}O{sub x}: {sup 170}Yb{sup 3+} Moessbauer probe measurements of electronic states and local Cu(2) magnetic order

We have used paramagnetic170Yb3+Mossbauer probes to examine two related properties of Y0.8Ca0.2Ba2Cu3Oxas a function of the doping level. First, by measuring the low temperature thermal dependence of\(1/T_1^{el} \)of the probes in overdoped samples, we obtain information concerning the density of states below Tc. Second, by measuring the internal (molecular) field appearing on the probes in underdoped samples, we examine the fluctuating short range correlated Cu(2) magnetic order which is progressively introduced as the doping level is reduced. This magnetic order microscopically cohabits with superconductivity.

Strongly overdoped states and irreversibility-field characteristics of the Hg-1223 and Cu-1223:P superconducting cuprates

Essentially single-phase Hg-1223 and Cu-1223:P samples in wide hole-doping ranges were successfully synthesized by a high-pressure (5 GPa) technique utilizing internal and/or external oxidizing agents and by subsequent post-annealings performed in a thermobalance. The hole-doping level of the as-synthesized samples was found to be in the overdoped side for both of the phases by thermoelectric power measurements. values as low as 107 K and 67 K were obtained for the overdoped Hg-1223 and Cu-1223:P samples, respectively. The values of and were calculated from the measured M versus H data employing, respectively, the Bean model and two different criteria: (at ) and (as ). It was found that the heavier the doped hole concentration was, the more improved were the versus characteristics for both the Hg-1223 and the Cu-1223:P phases. This tendency was the same for versus characteristics calculated using either criterion. The improvement with increasing hole-doping level was especially profound in the case of Cu-1223:P: the versus characteristics of a Cu-1223:P sample with K, probably being close to the optimally doped state, were as poor as that reported for Bi-2212, while an overdoped Cu-1223:P sample with K was superior to an overdoped Hg-1223 sample with K. The mechanisms as well as the usefulness of the observed phenomena are discussed.

Investigations on the Tl-doped Hg–Ba–Ca–Cu–O high temperature superconductors in regard to hole doping and microstructural characteristics

The Tl-doped Hg-based HTSC bulk materials corresponding to Hg 1− x Tl x Ba 2Ca 2Cu 3O 8+ δ with x=0.2, 0.4, 0.5, 0.6, and 0.8 have been synthesized using the precursor route. A clear cut variation of T c( R=0) with increase in Tl concentration and hence from the underdoped to overdoped regions has been demonstrated. These variations correspond to T c variation from ∼106 to ∼133 K for underdoped samples and from ∼106 to ∼129 K for overdoped samples. The crossover from underdoped to overdoped regions is found to take place between x=0.5 and x=0.6 X-ray diffraction (XRD) and TEM studies have revealed the samples to be multiphasic with the presence of Hg, Tl:1223 as the dominant phase. Microstructural investigations reveal the presence of ordered and/or disordered stacking faults and the accompanying polytype like regions.

Evolution of vortex phase diagram with oxygen-doping in Bi 2Sr 2CaCu 2O 8+ y single crystals

We studied the vortex phase diagram in Bi 2Sr 2CaCu 2O 8+ y with oxygen-doping by using micro-Hall-probes. The field B s, where the magnetization steps occur, increases with oxygen concentrations. In addition to this, we found that oxygen doping makes the entropy change smaller. The magnetization steps were completely suppressed by further doping, while the magnetization peak was observed at high temperatures, whose temperature dependence is similar to B s( T). Moreover, the peak field B p at low temperatures shows pronounced re-entrant behavior in highly overdoped samples. All these features in the evolution of phase diagram can be explained by taking account of disorders as well as the decrease in the anisotropy due to oxygen-doping.