arXiv subjects

N. D. Zhigadlo

Publications and source records attributed to N. D. Zhigadlo.

At least 109 records · Page 6Linked to original sources

arXiv2008

Magnetic penetration depth of single crystal SmFeAsO_{1-x}F_y: a fully gapped superconducting state

We report measurements of the in-plane magnetic penetration depth $λ$ in single crystals of SmFeAsO$_{1-x}$F$_y$ ($x\simeq y \simeq 0.2$) with $T_c \simeq 45$ K. We find that at low temperature $λ$ has an exponential temperature dependence which suggests that the Fermi surface is fully gapped. The magnitude of the minimum energy gap, $Δ_1=1.1\pm 0.1 k_BT_c$ at T=0 K, is significantly smaller than the BCS weak-coupling value suggesting that the gap is either strongly anisotropic or varies significantly between the different Fermi surface sheets. Our data fits well a two gap model with an additional larger gap of magnitude $Δ_2 = 1.8\pm0.2 k_BT_c$ which is associated with $\sim 80$% of the total superfluid density.

cond-mat.supr-con↗

arXiv2008

Structural and Superconducting Properties of RbOs2O6 Single Crystals

Single crystals of RbOs2O6 have been grown from Rb2O and Os in sealed quartz ampoules. The crystal structure has been identified at room temperature as cubic with the lattice constant a = 10.1242(12) A. The anisotropy of the tetrahedral and octahedral networks is lower and the displacement parameters of alkali metal atoms are smaller than for KOs2O6, so the "rattling" of the alkali atoms in RbOs2O6 is less pronounced. Superconducting properties of RbOs2O6 in the mixed state have been well described within the London approach and the Ginzburg-Landau parameter kappa(0) = 31 has been derived from the reversible magnetization. This parameter is field dependent and changes at low temperatures from kappa = 22 (low fields) to kappa = 31 at H_{c2}. The thermodynamic critical field H_{c}(0) = 1.3 kOe and the superconducting gap 2delta/k_{B}T_{c} = 3.2 have been estimated. These results together with slightly different H_{c2}(T) dependence obtained for crystals and polycrystalline RbOs2O6 proof evidently that this compound is a weak-coupling BCS-type superconductor close to the dirty limit.

cond-mat.supr-con↗

arXiv2008

Point-contact Andreev-reflection spectroscopy in segregation-free Mg_{1-x}Al_{x}B_2 single crystals up to x=0.32

We present new results of point-contact Andreev-reflection (PCAR) spectroscopy in single-phase Mg_{1-x}Al_{x}B_{2} single crystals with x up to 0.32. Fitting the conductance curves of our point contacts with the two-band Blonder-Tinkham-Klapwijk model allowed us to extract the gap amplitudes Delta_{sigma} and Delta_{pi}. The gap values agree rather well with other PCAR results in Al-doped crystals and polycrystals up to x=0.2 reported in literature, and extend them to higher Al contents. In the low-doping regime, however, we observed an increase in the small gap Delta_{pi} on increasing x (or decreasing the local critical temperature of the junctions, T_{c}^{A}) which is not as clearly found in other samples. On further decreasing T_{c}^{A} below 30 K, both the gaps decrease and, up to the highest doping level x=0.32 and down to T_{c}^{A}= 12 K, no gap merging is observed. A detailed analysis of the data within the two-band Eliashberg theory shows that this gap trend can be explained as being mainly due to the band filling and to an increase in the interband scattering which is necessary to account for the increase in Delta_{pi} at low Al contents (x < 0.1). We suggest to interpret the following decrease of Delta_{pi} for T_{c}^{A} < 30 K as being governed by the onset of inhomogeneity and disorder in the Al distribution that partly mask the intrinsic effects of doping and is not taken into account in standard theoretical approaches.

cond-mat.supr-con↗

arXiv2007

Synthesis and Bulk Properties of Oxychloride Superconductor Ca2-xNaxCuO2Cl2

Polycrystalline samples and submillimeter size single crystals of Na-doped Ca2CuO2Cl2 have been synthesized under high pressure. A series of experiments showed that the Na content depends not only on the pressure during the synthesis but also on the synthesis temperature and time. From a comparison of the Na-CCOC data with those of structurally related La214 cuprate superconductors we concluded that chlorine at the apical site is less effective that oxygen in supplying charge carriers to the CuO2 plans. As a result, the coupling between the CuO2 planes is weakened, the transition temperature Tc is reduced and the anisotropic nature is enhanced.

cond-mat.supr-con↗

arXiv2007

Multi-Gap Superconductivity in MgB2: Magneto-Raman Spectroscopy

Electronic Raman scattering studies on MgB2 single crystals as a function of excitation and polarization have revealed three distinct superconducting features: a clean gap below 37 cm-1 and two coherence peaks at 109 cm-1 and 78 cm-1 which we identify as the superconducting gaps in π- and σ-bands and as the Leggett's collective mode arising from the fluctuation in the relative phase between two superconducting condensates residing on corresponding bands. The temperature and field dependencies of the superconducting features have been established. A phononic Raman scattering study of the E2g boron stretching mode anharmonicity and of superconductivity induced self-energy effects is presented. We show that anharmonic two phonon decay is mainly responsible for the unusually large linewidth of the E2g mode. We observe ~2.5% hardening of the E2g phonon frequency upon cooling into the superconducting state and estimate the electron-phonon coupling strength associated with this renormalization.

cond-mat.supr-con↗

arXiv2007

Observation of Leggett's collective mode in a multi-band MgB2 superconductor

We report observation of Leggett's collective mode in a multi-band MgB2 superconductor with T_c=39K arising from the fluctuations in the relative phase between two superconducting condensates. The novel mode is observed by Raman spectroscopy at 9.4 meV in the fully symmetric scattering channel. The observed mode frequency is consistent with theoretical considerations based on the first principle computations.

cond-mat.supr-con↗

arXiv2007

Manifestations of fine features of the density of states in the transport properties of KOs2O6

We performed high-pressure transport measurements on high-quality single crystals of KOs2O6, a beta-pyrochlore superconductor. While the resistivity at high temperatures might approach saturation, there is no sign of saturation at low temperatures, down to the superconducting phase. The anomalous resistivity is accompanied by a nonmetallic behavior in the thermoelectric power (TEP) up to temperatures of at least 700 K, which also exhibits a broad hump with a maximum at 60 K. The pressure influences mostly the low-energy electronic excitations. A simple band model based on enhanced density of states in a narrow window around the Fermi energy (EF) explains the main features of this unconventional behavior in the transport coefficients and its evolution under pressure.

cond-mat.supr-con↗

arXiv2007

Study of the in-plane magnetic penetration depth in the cuprate superconductor Ca_2-xNa_xCuO_2Cl_2: role of the apical sites

A study of the in-plane magnetic penetration depth λ_ab in a series of the cuprate superconductors Ca_2-xNa_xCuO_2Cl_2 (Na-CCOC) with Na content x=0.11, 0.12, 0.15, 0.18, and 0.19 is reported. The zero temperature values of λ_ab(0) were obtained by means of the muon-spin rotation technique, as well as from measurements of the intrinsic susceptibility χ^int(0) by using the procedure developed by Kanigel et al. [Phys.Rev.B 71, 224511 (2005)]. λ_ab at T=0K was found to increase with decreasing doping from λ_ab(0)=316(19)nm for the x=0.19 sample to λ_ab(0)=430(26)nm for the x=0.11 one. From a comparison of the present Na-CCOC data with those of Bi2201 and La214 cuprate superconductors it is concluded that substitution of the apical oxygen by chlorine decreases the coupling between the superconducting CuO_2 planes, leading to an enhancement of the two-dimensional properties of Na-CCOC.

cond-mat.supr-con↗

arXiv2007

Anharmonic effects in MgB2? A comparative inelastic X-ray scattering and Raman study

We study anharmonic effects in MgB2 by comparing Inelastic X-ray and Ramanscattering together with ab-initio calculations. Using high statistics and high q resolution measurements we show that the E2g mode linewidth is independent of temperature along Gamma-A. We show, contrary to previous claims, that the Raman-peak energy decreases as a function of increasing temperature, a behaviour inconsistent with all the anharmonic ab-initio calculations of the E2g mode at Gamma available in literature. These findings and the excellent agreement between the X-ray measured and ab-initio calculated phonon spectra suggest that anharmonicity is not the main mechanism determining the temperature behaviour of the Raman-peak energy. The Raman E2g peak position and linewidth can be explained by large dynamical effects in the phonon self-energy. In light of the present findings, the commonly accepted explanation of the reduced isotope effect in terms of anharmonic effects needs to be reconsidered.

cond-mat.mtrl-sci↗

arXiv2007

Anharmonicity and self-energy effects of the E2g phonon in MgB2

We present a Raman scattering study of the E2g phonon anharmonicity and of superconductivity induced self-energy effects in MgB2 single crystals. We show that anharmonic two phonon decay is mainly responsible for the unusually large linewidth of the E2g mode. We observe ~ 2.5 % hardening of the E2g phonon frequency upon cooling into the superconducting state and estimate the electron-phonon coupling strength associated with this renormalization.

cond-mat.supr-con↗

arXiv2006

Thermal conductivity of Al-doped MgB2: Impurity scattering and the validity of the Wiedemann-Franz law

We report data on the thermal conductivity K(T,H) along the basal plane of the hexagonal crystal structure of superconducting Mg{1-y}Al{y}B2 with y=0.02 and 0.07 at temperatures between 0.5 and 50 K and in external magnetic fields between 0 and 70 kOe. The substitution of Al for Mg leads to a substantial reduction of the heat transport via electronic quasiparticles. The analysis of the K(T,H) data implies that the Al impurities provoke an enhancement of the intraband scattering rate, almost equal in magnitude for both the sigma- and the pi band of electronic excitations. This is in contrast with conclusions drawn from analogous data sets for material in which carbon replaces boron and where mainly the intraband scattering rate of the sigma band is enhanced. Our complete data set, including additional results of measurements of the low-temperature thermal conductivity of pure MgB2, confirms the validity of the Wiedemann-Franz law for both pure and doped MgB2.

cond-mat.supr-con↗

arXiv2006

Coexistence of superconductivity and incoherence in quasi 1D chains

The dimensionality of a correlated many-body system has a large impact on its electronic properties. When electrons are confined to one-dimensional chains of atoms their behavior is very different than in higher dimensional systems because they become strongly correlated, even in the case of vanishingly small interactions. The chains consisting of copper and oxygen atoms are particularly interesting, because the CuO orbitals are constituents of all known high temperature superconductors. Most of previous spectroscopic studies of CuO chain systems indicated insulating behavior[23-25]. Here we report the discovery of momentum dependent superconducting gap and hump-peak-dip structure in the spectra of the CuO chains. We demonstrate that superconductivity in the chains arises due to proximity effects and the peculiar momentum dependence of the superconducting gap shows how three dimensional coherence emerges in a layered superconductor. The presence of the hump-dip-peak structure in the spectra of the CuO chains is very unexpected as it was thought to only occur in the CuO2 planes and is frequently considered to be a signature of the d-wave pairing and the pairing boson itself.

cond-mat.supr-con↗

arXiv2006

Dual character of the electronic structure in YBa2Cu4O8: conduction bands of CuO2 planes and CuO chains

We use microprobe Angle-Resolved Photoemission Spectroscopy (muARPES) to separately investigate the electronic properties of CuO2 planes and CuO chains in the high temperature superconductor, YBa2Cu4O8. In the CuO2 planes, a two dimensional (2D) electronic structure with nearly momentum independent bilayer splitting is observed. The splitting energy is 150 meV at (pi,0), almost 50% larger than in Bi2Sr2CaCu2O(8+d) and the electron scattering at the Fermi level in the bonding band is about 1.5 times stronger than in the antibonding band. The CuO chains have a quasi one dimensional (1D) electronic structure. We observe two 1D bands separated by ~ 550meV: a conducting band and an insulating band with an energy gap of ~ 240meV. We find that the conduction electrons are well confined within the planes and chains with a non-trivial hybridization.

cond-mat.supr-con↗

arXiv2006

High-pressure synthesis and superconductivity of Ca2-xNaxCuO2Cl2

Ca2-xNaxCuO2Cl2 is a structural analogue to the La2-xSrxCuO4 superconductor. By substituting Na+ for Ca2+ mobile holes are introduced into the CuO2 planes. A series of experiments shows that the Na content x depends not only on the pressure during synthesis but also on the synthesis temperature and time. Ca2-xNaxCuO2Cl2 crystals (x=0.10 and Tc,on=13.5 K) up to 1x1x0.05 mm3 have been grown by a flux method at 35 kbar and the temperature was slowly ramped from 1250 C to 1050 C within 8 h. Applying higher synthesis pressure (up to 55 kbar) and temperature (up to 1700 C) results in polycrystalline samples with highest Tc,on=28.0 K at x=0.20. Further experiments are under way to optimize conditions to grow crystals with extended concentration of Na and Tc characteristic for optomally doped and overdoped regions.

cond-mat.supr-con↗

arXiv2006

Effect of field dependent core size on reversible magnetization of high-$κ$ superconductors

The field dependence of the vortex core size $ξ(B)$ is incorporated in the London model, in order to describe reversible magnetization $M(B,T)$ for a number of materials with large Ginzburg-Landau parameter $κ$. The dependence $ξ(B)$ is directly related to deviations in $M(\ln B)$ from linear behavior prescribed by the standard London model. A simple method to extract $ξ(B)$ from the magnetization data is proposed. For most materials examined, $ξ(B)$ so obtained decreases with increasing field and is in qualitative agreement both with behavior extracted from $μ$SR and small angle neutron scattering data and with that predicted theoretically.

cond-mat.supr-con↗

arXiv2006

The effect of magnetic impurities in a two-band superconductor: A point-contact study of Mn-substituted MgB2 single crystals

We present the first results of directional point-contact measurements in Mg_{1-x}Mn_{x}B_2 single crystals, with x up to 0.015 and bulk T_c down to 13.3 K. The order parameters Delta_{sigma} and Delta_{pi} were obtained by fitting the conductance curves with the two-band Blonder-Tinkham-Klapwijk (BTK) model. Both Delta_{pi} and Delta_{sigma} decrease with the critical temperature of the junctions T_c^A, but remain clearly distinct up to the highest Mn content. Once analyzed within the Eliashberg theory, the results indicate that spin-flip scattering is dominant in the sigma band, as also confirmed by first-principle band structure calculations.

cond-mat.supr-con↗

arXiv2006

Measuring the Penetration Depth Anisotropy in MgB$_2$ using Small-Angle Neutron Scattering

Using small-angle neutron scattering we have measured the misalignment between an applied field of 4 kOe and the flux-line lattice in MgB$_2$, as the field is rotated away from the c axis by an angle $θ$. The measurements, performed at 4.9 K, showed the vortices canting towards the c axis for all field orientations. Using a two-band/two-gap model to calculate the magnetization we are able to fit our results yielding a penetration depth anisotropy, $\glam = 1.1 \pm 0.1$.

cond-mat.supr-con↗

arXiv2006

Infrared properties of Mg$_{1-x}$Al$_x($B$_{1-y}$C$_{y}$)$_2$ single crystals in the normal and superconducting state

The reflectivity $R (ω)$ of $ab$-oriented Mg$_{1-x}$Al$_x$(B$_{1-y }$C$_y$)$_2$ single crystals has been measured by means of infrared microspectroscopy for $1300<ω<17000$ cm$^{-1}$. An increase with doping of the scattering rates in the $π$ and $σ$ bands is observed, being more pronounced in the C doped crystals. The $σ$-band plasma frequency also changes with doping due to the electron doping, while the $π$-band one is almost unchanged. Moreover, a $σ\toσ$ interband excitation, predicted by theory, is observed at $ω_{IB} \simeq 0.47$ eV in the undoped sample, and shifts to lower energies with doping. By performing theoretical calculation of the doping dependence $ω_{IB}$, the experimental observations can be explained with the increase with electron doping of the Fermi energy of the holes in the $σ$-band. On the other hand, the $σ$ band density of states seems not to change substantially. This points towards a $T_c$ reduction driven mainly by disorder, at least for the doping level studied here. The superconducting state has been also probed by infrared synchrotron radiation for $30<ω<150$ cm$^{-1}$ in one pure and one C-doped sample. In the undoped sample ($T_c$ = 38.5 K) a signature of the $π$-gap only is observed. At $y$ = 0.08 ($T_c$ = 31.9 K), the presence of the contribution of the $σ$-gap indicates dirty-limit superconductivity in both bands.

cond-mat.supr-con↗