arXiv subjects

R. S. Islam

Publications and source records attributed to R. S. Islam.

30 records · Page 2Linked to original sources

arXiv2012

The doping and disorder dependent variation of the isotope exponent in hole doped cuprates: A non-superconducting perspective

Since the early days of the discovery of hole doped high-Tc cuprates, the variation of the oxygen isotope exponent (IE) with the number of doped holes, p, in the CuO2 planes has been a source of considerable debate. There is a growing acceptance in the high-Tc community that the mediating bosons leading to Cooper pairing have largely electronic character. At the same time there is an increasing acknowledgement that at some level the lattice-charge interactions might be important in cuprates. The experimentally observed substantially large IE over certain doping range always casts a shadow over any proposal where non-phononic mechanism is invoked. Besides, the various existing theoretical schemes, based on electron-phonon interactions, cannot describe the anomalous features shown by the IE as a function of hole concentration/disorder, either quantitatively or qualitatively. Based on a recent experiment relating the possibility of Fermi-surface reconstruction to the thermoelectric transport measurements (Nature Commun. 2:432 doi: 10.1038/ncomms1440 (2011)), we propose here a simple scenario where isotope substitutions affect the charge/spin stripe state via the coupling to the underlying lattice and thereby change the superconducting Tc. In this picture, significant part of the IE, over an extended p-range, actually originates from the isotope induced stripe modulation and is not directly related to the characteristic energy scale of the pairing phonons as is the case in conventional BCS theory. Our proposal qualitatively explains all the disorder- and p-dependent features of the IE seen in hole doped high-Tc cuprates. We also provide with an outline of some experiments that can verify the degree of validity of the proposed scenario.

cond-mat.supr-con↗

arXiv2012

Variation of the vortex activation energy, U(T, H), with hole content in YBa2Cu3O7-delta thin films

The nature of in-plane resistive transition of high-quality c-axis oriented crystalline thin films of YBa2Cu3O7-delta have been studied under magnetic fields (H) applied along the c-direction over a wide range of doped holes, p, in the CuO2 planes. The field and temperature dependent in-plane resistivity, rho_ab(T, H), below the mean field superconducting transition temperature, Tp, has been analyzed within the thermally assisted flux-flow (TAFF) scenario. We have extracted the temperature and field dependent flux activation energy, U(T, H) from this analysis. The low-T part of the rho_ab(T, H) data can be described quite well by a dimensionless activation energy having the functional form of U(T, H) = (1-t)^m(H0/H)^-β, where t = T/Tp, is the reduced temperature and H0, is a characteristic field scale that primarily determines the magnitude of the activation energy for a given sample composition. The temperature exponent, m, shows a systematic variation with the hole content, whereas the field exponent, β, is insensitive to the p-values and is close to unity irrespective of the film composition. H0, on the other hand, changes rapidly as hole concentration is varied. Possible implications of these results are discussed in this paper.

cond-mat.supr-con↗

arXiv2012

On possible reconciliation of anomalous isotope effect and non-phononic pairing mechanism in high-Tc cuprates

Superconductivity in high-Tc cuprates is unconventional and there is a growing acceptance that carrier pairing in these strongly correlated electronic materials is mediated mainly by bosons of electronic origin rather than the phonons. Still, a significant isotope exponent (IE) (in certain cases, much larger than 0.5, the canonical BCS value) is observed experimentally. This has led to the assumption that phonons are also involved up to a certain extent. The magnitude of the IE in cuprates depends strongly on the number of doped holes, p, in the CuO2 planes and therefore, may indicate that phonons play different quantitative roles as the mediating boson at different regions of the Tc-p phase diagram. Based on recent experiments on Fermi-surface reconstruction from the thermoelectric transport measurements (Nature Commun. 2:432 doi: 10.1038/ncomms1440 (2011)) and on magnetic field induced charge order in YBa2Cu3O7-delta (Nature 477, 191 (2011)), we propose a very simple scenario where isotope substitutions affect the stripe state via the coupling to the lattice and thereby change the superconducting Tc. In this picture significant part of the observed IE, over an extended p-range, originates from the isotope induced stripe modulation and is not directly related to the characteristic energy scale of the pairing phonons as is the case in the BCS framework. This simple proposal qualitatively explains all the p-dependent anomalous features of the IE observed experimentally in hole doped high-Tc cuprates.

cond-mat.supr-con↗

arXiv2011

Interplay among superconductivity, pseudogap, and stripe correlations in different high-Tc cuprates

The effect of Zn substitution in the CuO2 plane on the superconducting transition temperature, Tc, was studied for the La2-xSrxCu1-yZnyO4 and YBa2(Cu1-yZny)3O7-d compounds over a wide range of hole concentration, p, and Zn content (y). Zn induced rate of suppression of Tc, dTc(p)/dy, was found to be strongly p-dependent and showed a monotonic variation with p, except in the vicinity of p ~ 0.125, i.e., near the so-called 1/8th anomaly where the charge/spin stripe correlations are at their strongest in hole doped cuprates. The magnitude of dTc(p)/dy decreased significantly around this hole concentration implying that Zn suddenly became less effective in degrading Tc near the 1/8th anomaly for La2-xSrxCu1-yZnyO4. The same feature, somewhat at a reduced scale, was also observed for YBa2(Cu1-yZny)3O7-d compounds. This is counterintuitive since static stripe order itself degrades superconducting order. We have discussed the possible scenarios that can give rise to such a non-monotonic dTc(p)/dy near p ~ 0.125. We have also looked at the p-dependent characteristic pseudogap energy scale, εg(p), which shows a nearly linear decrease with increasing p without any noticeable feature at p ~ 0.125. Moreover, there is no significant effect of Zn content on the value of εg(p). All these observations are indicative of a complex and possibly competing interplay among the superconducting, pseudogap, and stripe correlations in the hole doped cuprates.

cond-mat.supr-con↗

arXiv2010

Effects of Zn on the grain boundary properties of La2-xSrxCu1-yZnyO4 superconductors

The properties of the grain boundaries (GBs) are of significant importance in high-Tc cuprates. Most large scale applications of cuprate superconductors involve usage of sintered compounds. The critical current density and the ability to trap high magnetic flux inside the sample depend largely on the quality of the GBs. Zn has the ability to pin vortices but it also degrades superconductivity. In this study we have investigated the effect of Zn impurity on the intergrain coupling properties in high-quality La2-xSrxCu1-yZnyO4 sintered samples with different hole concentrations, p (\equiv x), over a wide range of Zn contents (y) using field-dependent ac susceptibility (ACS) measurements. The ACS results enabled us to determine the superconducting transition temperature Tc, and the temperature Tgcp, at which the randomly oriented superconducting grains become coupled as a function of hole and disorder contents. We have analyzed the behavior of the GBs from the systematic evolution of the values of Tgcp(p, y), Tc(p, y), and from the contribution to the field-dependent ACS signal coming from the intergrain shielding current. Zn suppresses both Tc and Tgcp in a similar fashion. The hole content and the carrier localization due to Zn substitution seem to have significant effect on the coupling properties of the GBs. We have discussed the possible implications of these findings in detail in this article. PACS: 74.72.Dn; 74.62.Dh; 74.25.Sv Keywords: Zn doped La214; Critical current density; Grain boundary

cond-mat.supr-con↗

arXiv2008

Effects of Zn substitution in La2-xSrxCu1-yZnyO4: interplay among superconductivity, pseudogap, and stripe order

The effect of Zn substitution on the superconducting transition temperature, Tc, was investigated for the La2-xSrxCu1-yZnyO4 (Zn-LSCO) compounds over a wide range of hole concentration, p (= x), and Zn content (y) in the CuO2 plane. Zn induced rate of suppression of Tc, dTc/dy, was found to be strongly p-dependent and showed a systematic variation with hole concentration, except in the vicinity of p ~ 0.12, i.e., near the so-called 1/8th anomaly where the charge/spin stripe correlations are at its strongest in La2-xSrxCuO4. Near p ~ 0.12, the static striped charge ordering is widely believed to dominate the T-p phase diagram. dTc/dy decreased strongly around this hole concentration i.e., Zn became less effective in degrading Tc near p ~ 0.12. This observation is indicative of an intricate and competing interplay among the superconducting, pseudogap, and stripe correlations in Zn-LSCO.

cond-mat.supr-con↗

arXiv2008

The pseudogap and doping dependent magnetic properties of La2-xSrxCu1-yZnyO4

The effects of planar hole content, p (= x), on the static magnetic susceptibility, chi(T), of polycrystalline La2-xSrxCu1-yZnyO4 compounds were investigated over a wide range of Sr (x) and Zn (y) contents. The magnetic behavior caused by Zn was found to depend strongly on the hole content. The apparent magnetic moment induced by Zn was larger in underdoped La2-xSrxCu1-yZnyO4, decreased quite sharply around p ~ 0.19, and did not change much for further overdoping. This is interpreted in terms of the effect of the pseudogap on the Zn-induced magnetic behavior, as there is growing evidence that the pseudogap vanishes quite abruptly at p ~ 0.19 +/- 0.01. From a detailed analysis of chi(T) data the Zn-induced magnetic contribution was found to be rather complex and showed non-Curie-like features over a wide range of temperature. The observed behavior was scrutinized in terms of two scenarios (a) that of independent localized-moments and (b) low energy quasiparticle resonances associated with each Zn atom. Our study points towards the latter scenario and more generally suggests that there is a re-distribution of quasiparticle spectral weight due to Zn substitution, the features of which are greatly influenced by the presence and magnitude of the pseudogap.

cond-mat.supr-con↗

arXiv2006

On the pseudogap and doping-dependent magnetic properties of La2-xSrxCu1-yZnyO4

The effects of planar hole content, p (= x), on the uniform (q = 0) magnetic susceptibility, c(T), of La2-xSrxCu1-yZnyO4 were investigated over a wide range of Sr (x) and Zn (y) contents. A strongly p-dependent Zn-induced magnetic behavior was observed. The apparent Zn-induced magnetic moment is larger in underdoped La2-xSrxCu1-yZnyO4 and it decreases quite sharply around p ~ 0.19. It does not change much for further overdoping. This indicates a possible role of the pseudogap on the Zn induced magnetic behavior, as there is growing evidence that pseudogap vanishes quite abruptly at p ~ 0.19.

cond-mat.supr-con↗

arXiv2006

Magnetic field dependence of the temperature derivative of resistivity: a probe for distinguishing the effects of pseudogap and superconducting fluctuations in cuprates

We have studied the magnetic field dependence of the temperature derivative of the resistivity, dr(H,T)/dT, of a number of Y1-xCaxBa2(Cu1-yZny)3O7-d crystalline thin films over a wide range of sample compositions. From the analysis of the temperature derivative data we have been able to distinguish quite clearly between two characteristic temperature scales, (a) the onset of strong superconducting fluctuation temperature, Tscf and (b) the pseudogap temperature, TPG. Significantly different characteristic features of dr(H,T)/dT at Tscf and at TPG imply that Tscf and TPG have different physical origins.

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arXiv2005

Isotope Effect in the Superfluid Density of HTS Cuprates: Stripes, Pseudogap and Impurities

Underdoped cuprates exhibit a normal-state pseudogap, and their spins and doped carriers tend to spatially separate into 1- or 2-D stripes. Some view these as central to superconductivity, others as peripheral and merely competing. Using La$_{2-x}$Sr$_x$Cu$_{1-y}$Zn$_y$O$_4$ we show that an oxygen isotope effect in $T_c$ and in the superfluid density can be used to distinguish between the roles of stripes and pseudogap and also to detect the presence of impurity scattering. We conclude that stripes and pseudogap are distinct, and both compete and coexist with superconductivity.

cond-mat.supr-con↗

arXiv2004

Anomalous pseudogap and superconducting state properties of heavily disordered Y1-xCaxBa2(Cu1-yZny)3O7-delta

The role of substantial in-plane disorder (Zn) on the transport and AC susceptibility of Y1-xCaxBa2(Cu1-yZny)3O7-delta was investigated over a wide range of planar hole concentration, p. Resistivity, r(T), for a number of overdoped to underdoped samples with y equals/greater than 0.055 showed clear downturns at a characteristic temperature similar to that found at T* in Zn-free underdoped samples because of the presence of the pseudogap. Contrary to the widely observed behavior for underdoped cuprates at lower Zn contents (where the pseudogap energy increases almost linearly with decreasing p in the same way as for the Zn-free compounds), this apparent pseudogap temperature at high Zn content showed very little or no p-dependence. It also increases systematically with increasing Zn concentration in the CuO2 planes. This anomalous behavior appears quite abruptly, e.g., samples with y less than/equals 0.05 exhibit the usual T*(p) behavior. AC susceptibility of these heavily disordered samples showed the superfluid density to be extremely low. Magneto-transport, r(T,H), measurements are provisionally interpreted in terms of high-strength pinning centers for vortices in these samples. We also discuss various possible scenarios that might lead to a Zn induced pseudogap in the cuprates.

cond-mat.supr-con↗

arXiv2003

The doping phase diagram of Y1-xCaxBa2(Cu1-yZny)3O7-delta from transport measurements: tracking the pseudogap below Tc

The effects of planar hole concentration, p, and magnetic field on the resistivity, rho(T), of high-quality c-axis oriented crystalline thin films and sintered Y1-xCaxBa2(Cu1-yZny)3O7-delta samples were investigated over a wide range of Ca, Zn, and oxygen contents. Zn was used to suppress superconductivity and this enabled us to extract the characteristic pseudogap temperature, T*(p) below Tc0(p) [ = Tc (x =0, y = 0)]. We have also located the characteristic temperature, Tscf, marking the onset of significant superconducting fluctuations above Tc, from the analysis of rho(T,H,p) and rho(T,p) data. This enabled us to identify T*(p) near the optimum doping level where the values of T*(p) and Tscf(p) are very close and hard to distinguish. We again found that T*(p) depends only on the hole concentration p, and not on the level of disorder associated with Zn or Ca substitutions. We conclude that (i) T*(p) (and therefore, the pseudogap) persists below Tc0(p) on the overdoped side and does not merge with the Tc0(p) line and (ii) T*(p) and the pseudogap energy extrapolate to zero at the doping p = 0.19 +/- 0.01.

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