arXiv subjects

S. H. Naqib

Publications and source records attributed to S. H. Naqib.

At least 109 records · Page 6Linked to original sources

arXiv2015

Structural, Elastic, and Electronic Properties of Recently Discovered Ternary Silicide Superconductor Li2IrSi3: An ab-initio Study

The structural, elastic, and electronic properties of the very recently discovered ternary silicide superconductor, Li2IrSi3, have been calculated using ab-initio technique. We have carried out the plane-wave pseudopotential approach within the framework of the first-principles density functional theory (DFT) implemented within the CASTEP code. The calculated structural parameters show a reasonable agreement with the experimental results. Elastic moduli of this interesting material have been calculated for the first time. The electronic band structure and electronic energy density of states indicate the strong covalent Ir-Si and Si-Si bonding which lead to the formation of the rigid structure of Li2IrSi3. Strong covalency give rise to a high Debye temperature in this system. We have discussed the theoretical results in detail in this paper.

cond-mat.supr-con↗

arXiv2015

An investigation of the in-plane dc fluctuation conductivity of optimally doped and overdoped cuprates: implication and origin of the pseudogap

In conventional superconductors the magnitude of the pairing fluctuation is primarily determined by Tc and the superconducting (SC) coherence length, ξ. In systems with strong structural and electronic anisotropies, the interlayer separation, s, plays a significant role. In cuprates, the pseudogap (PG) correlation induces a downturn in the temperature dependent resistivity. As Tc is approached from above, this downturn in the resistivity is supposed to either i) add to or ii) join smoothly to that due to paraconductivity caused by short-lived Cooper pairs. It is important to differentiate between these two possibilities since they are closely linked to the origin of the PG. It would be reasonable to assume that if the first scenario is correct then the PG has a non-SC origin, while the second scenario, if found to hold, would relate precursor pairing to the PG correlations. We have studied the in-plane fluctuation conductivity of two c-axis oriented thin films of Y0.95Ca0.05Ba2Cu3O7-d with similar hole contents (p), p = 0.165 (optimally doped) and p = 0.184 (slightly overdoped). The hole contents are fixed at these values so that the PG affects the resistivity data only at temperatures close to Tc. Analysis of paraconductivity, Δσab(T), within the mean-field Gaussian Ginzburg-Landau (MFGGL) framework reveals different features for the optimally doped (OPD) and the slightly overdoped (SOD) compounds. The excess conductivity due to Cooper pair fluctuations of the SOD sample can be described reasonably well by the MFGGL formalism. The excess conductivity of the OPD compound, on the other hand, cannot be accounted for by the MFGGL formalism with reasonable set of parameters. There is a significant added contribution to Δσab(T) for the OPD sample which appears to come from the presence of a PG. These findings point towards a non-pairing origin of the PG.

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arXiv2014

Doping dependent vortex activation energy and pseudogap in Y123

The temperature and magnetic field dependent activation energy, U(T, H), is one of the most important parameter in the field of applied superconductivity as it primarily determines both the crtical current density and the irreversibility field. Previously, we have determined the doping dependent U(T, H) from the analysis of field dependent resistive transitions in high-quality c-axis oriented crytalline thin films of Y123 (arXiv:1207.4312). In this short communication, we have showed a direct link between the characteristic field Ho that sets the magnitude of U(T, H) and the pseudogap temperature, T*. The strong dependence of Ho on the in-plane hole content, p, seems to follow from the p-dependent evolution of the pseudogap energy scale (T*, when expressed in temperature) which reduces the superconducting condensation energy as hole concentration decreases.

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arXiv2013

Zero-Field In-Plane Critical Current Density of YBa2Cu3O7-d Thin Films

Temperature dependence of the zero-field critical current density, Jc0(T), in the CuO2 planes has been investigated for c-axis oriented YBa2Cu3O7-d (Y123) thin films with different hole content, p. p-values were varied by changing the oxygen deficiency, d. Jc0(T) were calculated from the magnetic hysteresis (M-H) loops obtained at different temperatures. From the analysis of the Jc0(T) data for different sample compositions, we have extracted valuable information regarding the nature of the underlying magnetic flux pinning mechanisms in Y123 superconductors. It is found that the oxygen defects play only a secondary role in pinning of the vortices, the superfluid density, on the other hand plays a significant role. The temperature exponent, n, governing the Jc0(T) behavior for a given sample composition showed a systematic behavior as hole contents are changed.

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arXiv2013

Effects of non-magnetic defects in hole doped cuprates: exploration of the roles of the underlying electronic correlations

The effects of non-magnetic iso-valent defects inside the CuO2 plane(s) on superconducting transition temperature, Tc, dc charge transport, and the bulk magnetic susceptibility, \c{hi}(T), were investigated for the YBa2(Cu1-yZny)3O7-d (Zn-YBCO) and La2-xSrxCu1-yZnyO4 (Zn-LSCO) superconductors over a wide range of hole concentrations, p, and Zn contents (y) in the CuO2 plane(s). From the analysis of the \c{hi}(T, y) data, the pseudogap energy scale, εg, was found to be almost independent of the defect content at a given value of p. The Zn induced rate of suppression of Tc, dTc(p)/dy, was found to be strongly p-dependent and showed a systematic variation with hole concentration, except in the vicinity of p ~ 0.125, i.e., near the so-called 1/8th anomaly where the charge and spin stripe orderings are at their strongest in various families of hole doped cuprates. Near p ~ 0.125, the static striped charge ordering is largely believed to dominate the T-p electronic phase diagram. dTc(p)/dy decreased strongly around this composition, i.e., Zn abruptly became less effective in degrading Tc when p ~ 0.125. This observation, together with the facts that (i) Zn suppresses Tc most effectively and (ii) the characteristic pseudogap energy scale remains insensitive to the level of Zn substitution, provide us with important clues regarding the nature and interplay of the underlying electronic correlations present in high-Tc cuprates. We have discussed the possible connections among the stripe ordering, superconducting correlations and the pseudogap phenomenon in details in view of the above findings.

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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.

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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.

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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.

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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.

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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

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arXiv2009

Electronic band structure, phonon spectrum, and elastic properties of LaOFeAs

We present results of ab-initio calculations of the electronic band structure, lattice dynamical properties, and elastic constants of LaOFeAs, the parent compound of the recently discovered superconducting iron-oxypnictide (LaO1-xFxFeAs). The total and partial electronic density of states (EDOS) of the undoped LaOFeAs (in the insulating or metallic state) are extracted from the electronic band structure. The phonon dispersion and the phonon density of states (PDOS) are also studied. Possible implications of the band structure, EDOS, and PDOS of LaOFeAs on the eventual appearance of high-Tc superconductivity upon carrier doping are discussed. The values of various independent elastic constants for both insulating and metallic states are estimated and discussed.

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arXiv2009

Influence of the pseudogap on the Nernst coefficient of Y0.9Ca0.1Ba2Cu3Oy

We have studied the Nernst coefficient, v(T), of epitaxial thin films of the superconductor Y1-xCaxBa2Cu3Oy with x =0.05 and x = 0.1. The x = 0.1 sample has been measured at three different values of y, in the over- (OV), optimally- (OP), and under-doped (UD) states. As the doping level is reduced, v(T) starts to fall linearly with T at the temperature (T*) where we expect to see the influence of the pseudogap. The onset temperature of the superconducting fluctuation contribution to v(T) was found to vary slowly with the hole concentration (p) between p = 0.118 and 0.197. For the OP and UD samples, v(T>T*) is unusually large, being comparable with |S tan(Theta)| where S is the Seebeck coefficient and Theta the Hall angle.

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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.

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arXiv2007

Effects of Ca substitution and the pseudogap on the magnetic properties of Y1-xCaxBa2Cu3O7-delta

The effects of planar hole content, p, on the static magnetic susceptibility, chi(T), of Y1-xCaxBa2Cu3O7-delta polycrystalline samples were investigated over a wide range of Ca (x) and oxygen contents. Non-magnetic Ca2+, in the 3p6 state, induces a Curie-like contribution to chi(T) that increases systematically and non-linearly with x but is almost independent of p. We argue that this arises from statistical clusters containing two or more nearest neighbor Ca atoms. We have again found that the pseudogap in the quasi-particle spectral weight appears abruptly below a planar hole content p = 0.190 +/- 0.005.

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arXiv2007

Effect of Zn substitution on the suppression of Tc of Y1-xCaxBa2(Cu1-yZny)3O7-delta superconductors: pseudogap and the systematic shift of the optimum hole content

The effect of Zn substitution on the superconducting transition temperature, Tc, was investigated for the sintered Y1-xCaxBa2(Cu1-yZny)3O7-delta compounds over a wide range of hole concentration per CuO2 plane, p. Ca substitution enabled us to study the deeply overdoped region. p was changed by changing the oxygen deficiency (delta). A strongly p-dependent rate of suppression of Tc with Zn (dTc/dy) was found. From the analysis of the dTc(p)/dy and Tc(p,y) data, we found that the optimum hole content, popt, shifts to higher values with increasing Zn and superconductivity is at its strongest when p = 0.185 +/- 0.005. Various complementary experiments have identified this as the hole content where the pseudogap vanishes quite abruptly. We have discussed the possible relevance to these ideas with our findings.

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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.

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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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