arXiv subjects

R. Khasanov

Publications and source records attributed to R. Khasanov.

At least 109 records · Page 6Linked to original sources

arXiv2012

Superconducting properties of single-crystalline A_{x}Fe_{2-y}Se_{2} (A=Rb, K) studied using muon spin spectroscopy

We report on the superconducting properties of A_{x}Fe_{2-y}Se_{2} (A = Rb, K) single crystals studied with the muon spin relaxation or rotation (\muSR) technique. At low temperatures, close to 90% of the sample volumes exhibit large-moment magnetic order which impedes the investigation of their superconducting properties by \muSR. On the other hand, about 10% of the sample volumes remain paramagnetic and clearly show a superconducting response. The temperature dependence of the superconducting carrier density was analyzed within the framework of a single s-wave gap scenario. The zero-temperature values of the in-plane magnetic penetration depths λ_{ab}(0) = 258(2) and 225(2) nm and the superconducting gaps Δ(0) = 7.7(2) and 6.3(2) meV have been determined for A = Rb and K, respectively. The microscopic coexistence and/or phase separation of superconductivity and magnetism is discussed.

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arXiv2011

Coexistence of Superconductivity and Magnetism in FeSe_1-x under Pressure

An extended investigation of the electronic phase diagram of FeSe$_{1-x}$ up to pressures of $p\simeq2.4$\,GPa by means of ac and dc magnetization, zero field muon spin rotation (ZF $μ$SR), and neutron diffraction is presented. ZF $μ$SR indicates that at pressures $p\geq0.8$\,GPa static magnetic order occurs in FeSe$_{1-x}$ and occupies the full sample volume for $p\gtrsim 1.2$\,GPa. ac magnetization measurements reveal that the superconducting volume fraction stays close to 100% up to the highest pressure investigated. In addition, above $p\geq1.2$\,GPa both the superconducting transition temperature $T_{\rm c}$ and the magnetic ordering temperature $T_{\rm N}$ increase simultaneously, and both superconductivity and magnetism are stabilized with increasing pressure. Calculations indicate only one possible muon stopping site in FeSe$_{1-x}$, located on the line connecting the Se atoms along the $c$-direction. Different magnetic structures are proposed and checked by combining the muon stopping calculations with a symmetry analysis, leading to a similar structure as in the LaFeAsO family of Fe-based superconductors. Furthermore, it is shown that the magnetic moment is pressure dependent and with a rather small value of $μ\approx 0.2\,μ_B$ at $p\simeq2.4$\,GPa.

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arXiv2011

Muon-spin rotation measurements of the magnetic penetration depth in the Fe-based superconductor Ba_(1-x)Rb_(x)Fe2As2

Measurements of the magnetic penetration depth (lambda) in the Fe-based superconductor Ba_(1-x)Rb_xFe_2As_2 (x=0.3, 0.35, 0.4) were carried out using the muon-spin rotation muSR technique. The temperature dependence of lambda is well described by a two-gap s+s-wave scenario with a small gap Delta_1 = 1 - 3 mev and a large gap Delta_2 = 7 - 9 mev. By combining the present data with those obtained for RbFe_2As_2 a decrease of the BCS ratio 2Delta_2/(k_B)(T_c) with increasing Rb content x is observed. On the other hand, the BCS ratio 2Delta_1/(k_B)(T_c) is almost independent of x. In addition, the contribution of Delta_1 to the superfluid density is found to increase with x. These results are discussed in the light of the suppression of interband processes upon hole doping.

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arXiv2011

Muon spin rotation study of the intercalated graphite superconductor CaC6 at low temperatures

Muon spin rotation (muSR) experiments were performed on the intercalated graphite CaC6 in the normal and superconducting state down to 20 mK. In addition, AC magnetization measurements were carried out resulting in an anisotropic upper critical field Hc2, from which the coherence lengths xi_ab(0)=36.3(1.5) nm and xi_c(0)=4.3(7) nm were estimated. The anisotropy parameter gamma_H= H_c2_ab/H_c2_c increases monotonically with decreasing temperature. A single isotropic s-wave description of superconductivity cannot account for this behaviour. From magnetic field dependent muSR experiments the absolute value of the in-plane magnetic penetretion depth lambda_ab=78(3) nm was determined. The temperature dependence of the superfluid density rho_s(T) is slightly better described by a two-gap than a single-gap model.

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arXiv2011

Interplay of composition, structure, magnetism, and superconductivity in SmFeAs1-xPxO1-y

Polycrystalline samples and single crystals of SmFeAs1-xPxO1-y were synthesized and grown employing different synthesis methods and annealing conditions. Depending on the phosphorus and oxygen content, the samples are either magnetic or superconducting. In the fully oxygenated compounds the main impact of phosphorus substitution is to suppress the Néel temperature TN of the spin density wave (SDW) state, and to strongly reduce the local magnetic field in the SDW state, as deduced from muon spin rotation measurements. On the other hand the superconducting state is observed in the oxygen deficient samples only after heat treatment under high pressure. Oxygen deficiency as a result of synthesis at high pressure brings the Sm-O layer closer to the superconducting As/P-Fe-As/P block and provides additional electron transfer. Interestingly, the structural modifications in response to this variation of the electron count are significantly different when phosphorus is partly substituting arsenic. Point contact spectra are well described with two superconducting gaps. Magnetic and resistance measurements on single crystals indicate an in-plane magnetic penetration depth of 200 nm and an anisotropy of the upper critical field slope of 4-5. PACS number(s): 74.70.Xa, 74.62.Bf, 74.25.-q, 81.20.-n

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arXiv2011

Tuning of competing magnetic and superconducting phase volumes in LaFeAsO$_0.945F_0.055 by hydrostatic pressure

The interplay between magnetism and superconductivity in LaFeAsO_0.945F_0.055 was studied as a function of hydrostatic pressure up to p~2.4GPa by means of muon-spin rotation (\muSR) and magnetization measurements. The application of pressure leads to a substantial decrease of the magnetic ordering temperature T_N and a reduction of the magnetic phase volume and, at the same time, to a strong increase of the superconducting transition temperature T_c and the diamagnetic susceptibility. From the volume sensitive \muSR measurements it can be concluded that the superconducting and the magnetic areas which coexist in the same sample are inclined towards spatial separation and compete for phase volume as a function of pressure.

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arXiv2011

Coexistence of Magnetism and Superconductivity in the Iron-based Compound Cs_0.8(FeSe_0.98)_2

We report on muon-spin rotation/relaxation (\muSR), electrical resistivity, magnetization and differential scanning calorimetry measurements performed on a high-quality single crystal of Cs_0.8(FeSe_0.98)_2. Whereas our transport and magnetization data confirm the bulk character of the superconducting state below T_c = 28.5(2)K, the \muSR data indicate that the system is {\it magnetic} below T_N = 478.5(3)K, where a first-order transition occurs. The first-order character of the magnetic transition is confirmed by differential scanning calorimetry data. Taken all together, these data indicate in Cs_0.8(FeSe_0.98)_2 a microscopic coexistence between the superconducting phase and a strong magnetic phase. The observed ratio between T_N and T_c is the highest one reported to date for a magnetic superconductor.

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arXiv2010

Synthesis and crystal growth of Cs0.8(FeSe0.98)2: a new iron-based superconductor with Tc=27K

We report on the synthesis of large single crystals of a new FeSe-layer superconductor Cs0.8(FeSe0.98)2. X-ray powder diffraction, neutron powder-diffraction and magnetization measurements have been used to compare the crystal structure and the magnetic properties of Cs0.8(FeSe0.98)2 with those of the recently discovered potassium intercalated system KxFe2Se2. The new compound Cs0.8(FeSe0.98)2 shows a slightly lower superconducting transition temperature (Tc=27.4 K) in comparison to 29.5 in K0.8(FeSe0.98)2). The volume of the crystal unit cell increases by replacing K by Cs - the c-parameter grows from 14.1353(13) Å to 15.2846(11) Å. For the so far known alkali metal intercalated layered compounds (K0.8Fe2Se2 and Cs0.8(FeSe0.98)2) the Tc dependence on the anion height (distance between Fe-layers and Se-layers) was found to be analogous to those reported for As-containing Fe-superconductors and Fe(Se1-xChx), where Ch=Te, S.

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arXiv2010

Anomalies in the Fermi surface and band dispersion of quasi-one-dimensional CuO chains in the high-temperature superconductor YBa2Cu4O8

We have investigated the electronic states in quasi one dimensional (1D) CuO chains by microprobe Angle Resolved Photoemission Spectroscopy. We find that the quasiparticle Fermi surface consists of six disconnected segments, consistent with recent theoretical calculations that predict the formation of narrow, elongated Fermi surface pockets for coupled CuO chains. In addition, we find a strong renormalization effect with a significant kink structure in the band dispersion. The properties of this latter effect [energy scale (~40 meV), temperature dependence and behavior with Zn-doping] are identical to those of the bosonic mode observed in CuO2 planes of high temperature superconductors, indicating they have a common origin.

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arXiv2010

Tuning the superconducting and magnetic properties in Fe_ySe_0.25Te_0.75 by varying the Fe-content

The superconducting and magnetic properties of Fe$_{y}$Se$_{0.25}$Te$_{0.75}$ single crystals ($0.9\leq y \leq1.1$) were studied by means of x-ray diffraction, SQUID magnetometry, muon spin rotation, and elastic neutron diffraction. The samples with $y<1$ exhibit coexistence of bulk superconductivity and incommensurate magnetism. The magnetic order remains incommensurate for $y\geq 1$, but with increasing Fe content superconductivity is suppressed and the magnetic correlation length increases. The results show that the superconducting and the magnetic properties of the Fe$_{y}$Se$_{1-x}$Te$_{x}$ can be tuned not only by varying the Se/Te ratio but also by changing the Fe content.

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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.

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arXiv2010

Intrinsic and structural isotope effects in Fe-based superconductors

The currently available results of the isotope effect on the superconducting transition temperature T_c in Fe-based high-temperature superconductors (HTS) are highly controversial. The values of the Fe isotope effect (Fe-IE) exponent α_Fe for various families of Fe-based HTS were found to be as well positive, as negative, or even be exceedingly larger than the BCS value α_BCS=0.5. Here we demonstrate that the Fe isotope substitution causes small structural modifications which, in turn, affect T_c. Upon correcting the isotope effect exponent for these structural effects, an almost unique value of α~0.35-0.4 is observed for at least three different families of Fe-based HTS.

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arXiv2010

Evidence for time-reversal symmetry breaking in superconducting PrPt4Ge12

Zero and longitudinal field muon spin rotation (muSR) experiments were performed on the superconductors PrPt4Ge12 and LaPt4Ge12. In PrPt4Ge12 below Tc a spontaneous magnetization with a temperature variation resembling that of the superfluid density appears. This observation implies time-reversal symmetry (TRS) breaking in PrPt4Ge12 below Tc = 7.9 K. This remarkably high Tc for an anomalous superconductor and the weak and gradual change of Tc and of the related specific heat anomaly upon La substitution in La_(1-x)Pr_xPt_4Ge_(12) suggests that the TRS breaking is due to orbital degrees of freedom of the Cooper pairs.

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arXiv2010

Iron isotope effect on the superconducting transition temperature and the crystal structure of FeSe_1-x

The Fe isotope effect (Fe-IE) on the transition temperature T_c and the crystal structure was studied in the Fe chalcogenide superconductor FeSe_1-x by means of magnetization and neutron powder diffraction (NPD). The substitution of natural Fe (containing \simeq 92% of ^{56}Fe) by its lighter ^{54}Fe isotope leads to a shift of T_c of 0.22(5)K corresponding to an Fe-IE exponent of α_Fe=0.81(15). Simultaneously, a small structural change with isotope substitution is observed by NDP which may contribute to the total Fe isotope shift of T_c.

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arXiv2010

Microscopic Study of the Superconducting State of the Iron Pnictide RbFe_2As_2

A study of the temperature and field dependence of the penetration depth λof the superconductor RbFe_2As_2 (T_c=2.52 K) was carried out by means of muon-spin rotation measurements. In addition to the zero temperature value of the penetration depth λ(0)=267(5) nm, a determination of the upper critical field B_c2(0)=2.6(2) T was obtained. The temperature dependence of the superconducting carrier concentration is discussed within the framework of a multi-gap scenario. Compared to the other "122" systems which exhibit much higher Fermi level, a strong reduction of the large gap BCS ratio 2Δ/k_B T_c is observed. This is interpreted as a consequence of the absence of interband processes. Indications of possible pair-breaking effect are also discussed.

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arXiv2010

Coherency of the superconducting state: the muon spin rotation and ARPES studies of (BiPb)_2(SrLa)_2CuO_{6+δ}

The superfluid density ρ_s in underdoped (T_c\simeq23K), optimally doped (T_c\simeq35K) and overdoped (T_c\simeq29K) single crystalline (BiPb)_2(SrLa)_2CuO_{6+δ} samples was studied by means of muon-spin rotation (\muSR). By combining the \muSR data with the results of ARPES measurements on similar samples [Nature 457, 296 (2009)] good self-consistent agreement is obtained between two techniques concerning the temperature and the doping evolution of ρ_s.

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arXiv2010

Anisotropic superconducting properties of single-crystalline FeSe0.5Te0.5

Iron-chalcogenide single crystals with the nominal composition FeSe$_{0.5}$Te$_{0.5}$ and a transition temperature of $T_{c}\simeq14.6$ K were synthesized by the Bridgman method. The structural and anisotropic superconducting properties of those crystals were investigated by means of single crystal X-ray and neutron powder diffraction, SQUID and torque magnetometry, and muon-spin rotation. Room temperature neutron powder diffraction reveals that 95% of the crystal volume is of the same tetragonal structure as PbO. The structure refinement yields a stoichiometry of Fe_1.045Se_0.406Te_0.594. Additionally, a minor hexagonal Fe_7Se_8 impurity phase was identified. The magnetic penetration depth λat zero temperature was found to be 491(8) nm in the ab-plane and 1320(14) nm along the c-axis. The zero-temperature value of the superfluid density ρ_s(0) λ^-2(0) obeys the empirical Uemura relation observed for various unconventional superconductors, including cuprates and iron-pnictides. The temperature dependences of both λ_ab and λ_c are well described by a two-gap s+s-wave model with the zero-temperature gap values of Δ_S(0)=0.51(3) meV and Δ_L(0)=2.61(9) meV for the small and the large gap, respectively. The magnetic penetration depth anisotropy parameter γ_λ(T)=λ_c(T)/λ_{ab}(T) increases with decreasing temperature, in agreement with γ_λ(T) observed in the iron-pnictide superconductors.

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arXiv2010

Magnetic excitations of Fe_{1+y}Se_xTe_{1-x} in magnetic and superconductive phases

We have used inelastic neutron scattering and muon-spin rotation to compare the low energy magnetic excitations in single crystals of superconducting Fe1.01Se0.50Te0.50 and non-superconducting Fe1.10Se0.25Te0.75. We confirm the existence of a spin resonance in the superconducting phase of Fe1.01Se0.50Te0.50, at an energy of 7 meV and a wavevector of (1/2,1/2,0). The non-superconducting sample exhibits two incommensurate magnetic excitations at (1/2,1/2,0)\pm(0.18,-0.18,0) which rise steeply in energy, but no resonance is observed at low energies. A strongly dispersive low-energy magnetic excitation is also observed in Fe1.10Se0.25Te0.75 close to the commensurate antiferromagnetic ordering wavevector (1/2-δ,0,1/2) where δ\approx 0.03. The magnetic correlations in both samples are found to be quasi-two dimensional in character and persist well above the magnetic (Fe1.10Se0.25Te0.75) and superconducting (Fe1.01Se0.50Te0.50) transition temperatures.

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