arXiv subjects

A. Shengelaya

Publications and source records attributed to A. Shengelaya.

At least 37 records · Page 2Linked to original sources

arXiv2010

Field Dependent Superfluid Density in the Optimally Doped SmFeAsO_(1-x)F_y Superconductor

The magnetic field dependence of the in-plane magnetic penetration depth for optimally doped SmFeAsO_(1-x)F_y was investigated by combining torque magnetometry, SQUID magnetometry, and muon-spin rotation. The results obtained from these techniques show all a pronounced decrease of the superfluid density as the field is increased up to 1.4 T. This behavior is analysed within a two-band model with self-consistently derived coupled gaps, where the superfluid density related to the larger gap is field independent and the superfluid density related to the smaller gap is strongly suppressed with increasing field.

cond-mat.supr-con↗

arXiv2010

Comparative study of the pressure effects on the magnetic penetration depth in electron- and hole-doped cuprate superconductors

The effect of pressure on the magnetic penetration depth $λ$ was tested in the hole-doped superconductor YBa2Cu3O(7-d) and in the electron-doped one Sr0.9La0.1CuO2 by means of magnetization measurements. Whereas a large change of $λ$ was found in YBa2Cu3O(7-d), confirming the non-adiabatic character of the electron-phonon coupling in hole-doped superconductors, the same quantity is not affected by pressure in the electron-doped Sr0.9La0.1CuO2, suggesting a close similarity of the latter to conventional adiabatic BCS superconductors. The present results imply a remarkable difference between the electronic properties of hole-doped cuprates and the electron-doped Sr0.9La0.1CuO2, giving a strong contribution to the long debated asymmetric consequences of hole- and electron-doping in cuprate superconductors.

cond-mat.supr-con↗

arXiv2009

Comment on ``Muon-spin rotation studies of the superconducting properties of Mo_3Sb_7'', Phys.Rev.B 78, 172505 (2008)

In a recent article Tran et al. [Phys. Rev.B 78, 172505 (2008)] report on the result of the muon-spin rotation (\muSR) measurements of Mo_3Sb_7 superconductor. Based on the analysis of the temperature and the magnetic field dependence of the Gaussian relaxation rate σ_{sc} they suggest that Mo_3Sb_7 is the superconductor with two isotropic s-wave like gaps. An additional confirmation was obtained from the specific heat data published earlier by partly the same group of authors in [Acta Mater. 56, 5694 (2008)]. The purpose of this Comment is to point out that from the analysis made by Tran et al. the presence of two superconducting energy gaps in Mo_3Sb_7 can not be justified. The analysis of \muSR data does not account for the reduction of σ_{sc} with increasing temperature, and, hence, yields inaccurate information on the magnetic penetration depth. The specific heat data can be satisfactory described within the framework of the one-gap model with the small residual specific heat component. The experimental data of Tran et al., as well as our earlier published \muSR data [Phys. Rev. B 78, 014502 (2008)] all seem to be consistent with is the presence of single isotropic superconducting energy gap in Mo_3Sb_7.

cond-mat.supr-con↗

arXiv2009

Muon-spin rotation measurements of the penetration depth of the Mo_3Sb_7 superconductor

Measurements of the magnetic field penetration depth λin superconductor Mo_3Sb_7 (T_c~2.1 K) were carried out by means of muon-spin-rotation. The absolute values of λ, the Ginzburg-Landau parameter κ, the first H_{c1} and the second H_{c2} critical fields at T=0 are λ(0)=720(100)nm, κ(0)=55(9), μ_0H_{c1}(0)=1.8(3)mT, and μ_0H_{c2}(0)=1.9(2)T. The zero temperature value of the superconducting energy gap Δ(0) was found to be 0.35(1)meV corresponding to the ratio 2Δ(0)/k_BT_c=3.83(10). At low temperatures λ^{-2}(T) saturates and becomes constant below T~0.3T_c, in agreement with what is expected for s-wave BCS superconductors. Our results suggest that Mo_3Sb_7 is a BCS superconductor with the isotropic energy gap

cond-mat.supr-con↗

arXiv2009

Comparison of different methods for analyzing $μ$SR line shapes in the vortex state of type-II superconductors

A detailed analysis of muon-spin rotation ($μ$SR) spectra in the vortex state of type-II superconductors using different theoretical models is presented. Analytical approximations of the London and Ginzburg-Landau (GL) models, as well as an exact solution of the GL model were used. The limits of the validity of these models and the reliability to extract parameters such as the magnetic penetration depth $λ$ and the coherence length $ξ$ from the experimental $μ$SR spectra were investigated. The analysis of the simulated $μ$SR spectra showed that at high magnetic fields there is a strong correlation between obtained $λ$ and $ξ$ for any value of the Ginzburg-Landau parameter $κ= λ/ξ$. The smaller the applied magnetic field is, the smaller is the possibility to find the correct value of $ξ$. A simultaneous determination of $λ$ and $ξ$ without any restrictions is very problematic, independent of the model used to describe the vortex state. It was found that for extreme type-II superconductors and low magnetic fields, the fitted value of $λ$ is practically independent of $ξ$. The second-moment method frequently used to analyze $μ$SR spectra by means of a multi-component Gaussian fit, generally yields reliable values of $λ$ in the whole range of applied fields $ H_{c1} \ll H \lesssim H_{c2}$ ($H_{c1}$ and $H_{c2}$ are the first and second critical fields, respectively). These results are also relevant for the interpretation of small-angle neutron scattering (SANS) experiments of the vortex state in type-II superconductors.

cond-mat.supr-con↗

arXiv2009

Probing the Yb$^{3+}$ spin relaxation in Y$_{0.98}$Yb$_{0.02}$Ba$_{2}$Cu$_{3}$O$_{x}$ by Electron Paramagnetic Resonance

The relaxation of Yb$^{3+}$ in YBa$_{2}$Cu$_{3}$O$_{x}$ ($6<x<7$) was studied using Electron Paramagnetic Resonance (EPR). It was found that both electronic and phononic processes contribute to the Yb$^{3+}$ relaxation. The phononic part of the relaxation has an exponential temperature dependence, which can be explained by a Raman process via the coupling to high-energy ($\sim$500 K) optical phonons or an Orbach-like process via the excited vibronic levels of the Cu$^{2+}$ ions (localized Slonczewski-modes). In a sample with a maximum oxygen doping $x$=6.98, the electronic part of the relaxation follows a Korringa law in the normal state and strongly decreases below $T_{c}$. Comparison of the samples with and without Zn doping proved that the superconducting gap opening is responsible for the sharp decrease of Yb$^{3+}$ relaxation in YBa$_{2}$Cu$_{3}$O$_{6.98}$. It was shown that the electronic part of the Yb$^{3+}$ relaxation in the superconducting state follows the same temperature dependence as $^{63}$Cu and $^{17}$O nuclear relaxations despite the huge difference between the corresponding electronic and nuclear relaxation rates.

cond-mat.supr-con↗

arXiv2008

Muon-spin rotation study of the in-plane magnetic penetration depth of FeSe_0.85: evidence for nodeless superconductivity

The in-plane magnetic penetration depth λ_{ab} of the iron selenide superconductor with the nominal composition FeSe_0.85 was studied by means of muon-spin rotation. The measurements of λ_{ab}^{-2}(T) are inconsistent with the presence of nodes in the gap as well as with a simple isotropic s-wave type of the order parameter, but can be equally well described within a two-gap (s+s) and an anisotropic s-wave gap picture. This implies that the superconducting energy gap in FeSe_0.85 contains no nodes.

cond-mat.supr-con↗

arXiv2008

Nodeless superconductivity in the infinite-layer electron-doped Sr_0.9La_0.1CuO_2 cuprate superconductor

We report on measurements of the in-plane magnetic penetration depth λ_{ab} in the infinite-layer electron-doped high-temperature cuprate superconductor Sr_0.9La_0.1CuO_2 by means of muon-spin rotation. The observed temperature and magnetic field dependences of λ_{ab} are consistent with the presence of a substantial s-wave component in the superconducting order parameter in good agreement with the results of tunneling, specific heat, and small-angle neutron scattering experiments.

cond-mat.supr-con↗

arXiv2007

Correlation between the transition temperature and the superfluid density in BCS superconductor NbB_2+x

The results of the muon-spin rotation experiments on BCS superconductors NbB_2+x (x = 0.2, 0.34) are reported. Both samples, studied in the present work, exhibit rather broad transitions to the superconducting state, suggesting a distribution of the volume fractions with different transition temperatures (T_c)'s. By taking these distributions into account, the dependence of the inverse squared zero-temperature magnetic penetration depth (λ_0^{-2}) on T_c was reconstructed for temperatures in the range 1.5K<T_c<8.0K. λ_0^{-2} was found to obey the power law dependence λ_0^{-2}\propto T_c^{3.1(1)} which appears to be common for some families of BCS superconductors as, {\it e.g.}, Al doped MgB_2 and high-temperature cuprate superconductors as underdoped YBa_2Cu_3O_{7-δ}.

cond-mat.supr-con↗

arXiv2007

Oxygen isotope effect on superconductivity and magnetism in Y_1-xPr_xBa_2Cu_3O_7-δ

Oxygen isotope (^{16}O/^{18}O) effects (OIE's) on the superconducting transition (T_c), the spin-glass ordering (T_g), and the antiferromagnetic ordering (T_N) temperatures were studied for Y_1-xPr_xBa_2Cu_3O_7-δas a function of Pr content (0.0\leq x \leq 1.0). The OIE on T_c increases with increasing x up to x\approx0.55, where superconductivity disappears. For decreasing x the OIE's on T_N and T_g increase down to x\approx 0.7 where antiferromagnetic order and down to x\approx0.3 where spin-glass behavior vanish, respectively. The OIE's on T_g and T_N are found to have {\it opposite signs} as compared to the OIE on T_c. All OIE's are suggested to arise from the isotope dependent mobility (kinetic energy) of the charge carriers.

cond-mat.supr-con↗

arXiv2007

s-wave symmetry along the c-axis and s+d in-plane superconductivity in bulk YBa_2Cu_4O_8

To clarify the order parameter symmetry of cuprates, the magnetic penetration depth $λ$ was measured along the crystallographic directions $a$, $b$, and $c$ in single crystals of YBa$_2$Cu$_4$O$_8$ via muon spin rotation. This method is direct, bulk sensitive, and unambiguous. The temperature dependences of $λ_a^{-2}$ and $λ_b^{-2}$ exhibit an inflection point at low temperatures as is typical for two-gap superconductivity (TGS) with $s+d-$wave character in the planes. Perpendicular to the planes a pure s-wave gap is observed thereby highlighting the important role of c-axis effects. We conclude that these are generic and universal features in the bulk of cuprates.

cond-mat.supr-con↗

arXiv2006

Magnetic field dependence of the oxygen isotope effect on the magnetic penetration depth in hole-doped cuprate superconductors

The magnetic field dependence of the oxygen-isotope (^{16}O/^{18}O) effect (OIE) on the in-plane magnetic field penetration depth λ_{ab} was studied in the hole-doped high-temperature cuprate superconductors YBa_2Cu_4O_8, Y_0.8Pr_0.2Ba_2Cu_3O_7-δ, and Y_0.7Pr_0.3Ba_2Cu_3O_7-δ. It was found that λ_ab for the ^{16}O substituted samples increases stronger with increasing magnetic field than for the ^{18}O ones. The OIE on λ_ab decreases by more than a factor of two with increasing magnetic field from μ_0H=0.2 T to μ_0H=0.6 T. This effect can be explained by the isotope dependence of the in-plane charge carrier mass m^\ast_{ab}.

cond-mat.supr-con↗

arXiv2006

Mixed order parameter symmetries in cuprate superconductors

The recent observation of an inflection point in the temperature dependence of the in-plane magnetic field dependence (lambda_ab) is investigated within a two-band model with coupled order parameters of different symmetries. While the dominant order parameter has d-wave symmetry, the smaller one is of s-wave symmetry. Superconductivity is robust in the d-wave channel and induced via interband interactions in the s-wave subsystem.

cond-mat.supr-con↗

arXiv2006

Correlation between oxygen isotope effects on the transition temperature and the magnetic penetration depth in high-temperature superconductors close to optimal doping

The oxygen-isotope (^{16}O/^{18}O) effect (OIE) on the in-plane magnetic penetration depth λ_{ab}(0) in optimally-doped YBa_2Cu_3O_{7-δ} and La_{1.85}Sr_{0.15}CuO_4, and in slightly underdoped YBa_2Cu_4O_8 and Y_{0.8}Pr_{0.2}Ba_2Cu_3O_{7-δ} was studied by means of muon-spin rotation. A substantial OIE on λ_{ab}(0) with an OIE exponent β_O=-d\lnλ_{ab}(0)/d\ln M_O\approx - 0.2 (M_O is the mass of the oxygen isotope), and a small OIE on the transition temperature T_c with an OIE exponent α_O=-d\ln T_{c}/d \ln M_O\simeq0.02 to 0.1 were observed. The observation of a substantial isotope effect on λ_{ab}(0), even in cuprates where the OIE on T_c is small, indicates that lattice effects play an important role in cuprate HTS.

cond-mat.supr-con↗

arXiv2006

Evidence for complex order parameter in La_{1.83}Sr_{0.17}CuO_4

The in-plane magnetic field penetration depth (λ_{ab}) in single-crystal La_{1.83}Sr_{0.17}CuO_4 was investigated by means of the muon-spin rotation (\muSR) technique. The temperature dependence of λ^{-2}_{ab} has an inflection point around 10-15K, suggesting the presence of two superconducting gaps: a large gap (Δ_1^d) with d-wave and a small gap (Δ_2^s) with s-wave symmetry. The zero-temperature values of the gaps at μ_0H=0.02T were found to be Δ_1^d(0)=8.2(2)meV and Δ_2^s(0)=1.57(8)meV.

cond-mat.supr-con↗

arXiv2005

Muon-Spin-Rotation Measurements of the Penetration Depth in the Infinite-Layer Electron-Doped Cuprate Superconductor Sr0.9La0.1CuO2

Muon spin rotation (mSR) measurements of the in-plane penetration depth lambda_ab have been performed in the electron-doped infinite layer high-Tc superconductor (HTS) Sr0.9La0.1CuO2. Absence of the magnetic rare-earth ions in this compound allowed to measure for the first time the absolute value of lambda_ab(0) in electron-doped HTS using mSR. We found lambda_ab(0)=116(2) nm. The zero-temperature depolarization rate sigma(0)?1/lambda_ab(0)^2=4.6(1) MHz is more than four times higher than expected from the Uemura line. Therefore this electron-doped HTS does not follow the Uemura relation found for hole-doped HTS.

cond-mat.supr-con↗

arXiv2005

Electron Spin-Lattice Relaxation of Er3+ ions in Er0.01Y0.99Ba2Cu3Ox

The temperature dependence of the electron spin-lattice relaxation SLR was studied in Er0.01Y0.99Ba2Cu3Ox compounds. The data derived from the electron spin resonance ESR and SLR measurements were compared to those from inelastic neutron scattering studies. SLR of Er3+ ions in the temperature range from 20 K to 65 K can be explained by the resonant phonon relaxation process with the involvement of the lowest excited crystalline-electric-field electronic states of Er3+. These results are consistent with a local phase separation effects. Possible mechanisms of the ESR line broadening at lower temperatures are discussed. Keywords: YBCO; EPR; ESR; Electron spin-lattice relaxation time, T ; Crystalline-electric-field

cond-mat.supr-con↗

arXiv2004

Study of the magnetic penetration depth in RbOs_2O_6

Measurements of the magnetic field penetration depth λin the pyrochlore superconductor RbOs_2O_6 (T_c\simeq6.3 K) were carried out by means of the muon-spin-rotation (\muSR) technique. At low temperatures λ^{-2}(T) saturates and becomes constant below T\simeq 0.2T_c, in agreement with what is expected for weak-coupled s-wave BCS superconductors. The value of λat T=0 was found to be in the range of 250 nm to 300 nm. \muSR and equilibrium magnetization measurements both reveal that at low temperatures $λ$ is almost (at the level of 10%) independent of the applied magnetic field. This result suggests that the superconducting energy gap in RbOs_2O_6 is isotropic.

cond-mat.supr-con↗