arXiv subjects

B. Holzapfel

Publications and source records attributed to B. Holzapfel.

At least 37 records · Page 2Linked to original sources

arXiv2013

Surface transport properties of Fe-based superconductors: The influence of degradation and inhomogeneity

Surface properties of Co-doped BaFe2As2 epitaxial superconducting thin films were inspected by X-ray photoelectron spectroscopy, scanning spreading resistance microscopy (SSRM), and point contact spectroscopy (PCS). It has been shown that surface of Fe-based superconductors degrades rapidly if being exposed to air, what results in suppression of gap-like structure on PCS spectra. Moreover, SSRM measurements revealed inhomogeneous surface conductivity, what is consistent with strong dependence of PCS spectra on contact position. Presented results suggest that fresh surface and small probing area should be assured for surface sensitive measurements like PCS to obtain intrinsic properties of Fe-based superconductors.

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arXiv2013

The order-parameter symmetry and Fermi surface topology of 122 Fe-based superconductors: a point-contact Andreev-reflection study

We report on the results of directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe_{1-x}Co_x)2As2 single crystals and epitaxial c-axis oriented films with x = 0.08 as well as in Ca(Fe_{1-x}Co_x)2As2 single crystals with x = 0.06. The PCAR spectra are analyzed within the two-band 3D version of the Blonder-Tinkham-Klapwijk model for Andreev reflection we recently developed, and that makes use of an analytical expression for the Fermi surface that mimics the one calculated within the density-functional theory (DFT). The spectra in Ca(Fe_{0.94}Co_{0.06})2As2 unambiguously demonstrate the presence of nodes or zeros in the small gap. In Ba(Fe_{0.92}Co_{0.08})2As2, the ab-plane spectra in single crystals can be fitted by assuming two nodeless gaps, but this model fails to fit the c-axis ones in epitaxial films. All these results are discussed in comparison with recent theoretical predictions about the occurrence of accidental 3D nodes and the presence of "hot spots" in the gaps of 122 compounds.

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arXiv2012

Highly responsive Y-Ba-Cu-O thin film THz detectors with picosecond time resolution

High-temperature superconducting YBa2Cu3O7-d (YBCO) thin-film detectors with improved responsivities were developed for fast time-domain measurements in the THz frequency range. YBCO thin films of 30 nm thickness were patterned to micro- and nanobridges and embedded into planar log-spiral THz antennas. The YBCO thin-film detectors were characterized with continuous wave radiation at 0.65 THz. Responsivity values as high as 710 V/W were found for the YBCO nanobridges. Pulsed measurements in the THz frequency range were performed at the electron storage ring ANKA from the Karlsruhe Institute of Technology (KIT). Due to the high responsivities of the nanobridges no biasing was required for the detection of the coherent synchrotron radiation pulses achieving very good agreement between the measured pulse shapes and simulations.

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arXiv2012

Electronic phase diagram of disordered Co doped BaFe2As2

Superconducting and normal state transport properties in iron pnictides are sensitive to disorder and impurity scattering. By investigation of Ba(Fe1-xCox)2As2 thin films with varying Co concentration, we demonstrate that in the dirty limit the superconducting dome in the electronic phase diagram of Ba(Fe1-xCox)2As2 shifts towards lower doping concentrations, which differs significantly from observations in single crystals. We show that especially in the underdoped regime superconducting transition temperatures higher than 27 K are possible.

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arXiv2012

Infrared photo-response of Fe-shunted Ba-122 thin film microstructures

We present a study of the response to pulsed infrared radiation of Fe-layer shunted pnictide thin film microstructures. The thin film multilayer consisting of 20 nm thick Fe-buffer, 50 nm thick Ba(Fe,Co)2As2 film and gold protection layer were deposited on heated MgO and MgAl2O4 substrates by pulsed-laser deposition. The multilayers were patterned into 5 to 8 um wide and 5 um long microbridges by electron-beam lithography and ion-milling technique. The microbridges show Tc up to 20 K and a critical current density up to 2.56 MA/cm2 at T = 10 K. The photo-response of Fe-shunted Ba(Fe,Co)2As2 thin film microbridges to infrared radiation was studied in a wide range of incident optical power, operation temperature and bias current. We have found that the electron energy relaxation in studied multilayers is dependent on substrate material and is 1.75 times faster in case of MgAl2O4 characterized by lattice matching to pnictide film in comparison to MgO substrate.

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arXiv2012

Non-thermal response of YBCO thin films to picosecond THz pulses

The photoresponse of YBa2Cu3O7-d thin film microbridges with thicknesses between 15 and 50 nm was studied in the optical and terahertz frequency range. The voltage transients in response to short radiation pulses were recorded in real time with a resolution of a few tens of picoseconds. The bridges were excited by either femtosecond pulses at a wavelength of 0.8 μm or broadband (0.1 - 1.5 THz) picosecond pulses of coherent synchrotron radiation. The transients in response to optical radiation are qualitatively well explained in the framework of the two-temperature model with a fast component in the picosecond range and a bolometric nanosecond component whose decay time depends on the film thickness. The transients in the THz regime showed no bolometric component and had amplitudes up to three orders of magnitude larger than the two-temperature model predicts. Additionally THz-field dependent transients in the absence of DC bias were observed. We attribute the response in the THz regime to a rearrangement of vortices caused by high-frequency currents.

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arXiv2012

KFe2As2: coexistence of superconductivity and local moment derived spin-glass

High-quality KFe2As2 single crystals have been studied by transport, magnetization and low-T specific heat measurements. Their analysis shows that superconductivity occurs (in some cases coexists) in the vicinity of disordered magnetic phases (Griffiths and spin-glass type) depending of the amount of local magnetic moments (probably excess Fe derived)and sample inhomogeneity. The achieved phenomenological description of our data including also data from the literature provides a consistent explanation of the observed non-Fermi-liquid behavior and of the nominally large experimental Sommerfeld coefficient gamma_n about 94 mJ/mol K^2. We suggest that the intrinsic value (directly related to the itinerant quasi-particles) gamma_el about 60(10)mJ/mol K^2 is significantly reduced compared with gamma_n. Then an enhanced relative jump of the electronic specific heat Delta C_el/gamma_el T_c about 0.8 and a weak total electron-boson coupling constant lambda less or about 1 follow.

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arXiv2011

Planar hybrid superconductor-normal metal-superconductor thin film junctions based on BaFe1.8Co0.2As2

To investigate the transport properties of iron based superconductors, we prepared planar hybrid superconductor-normal metal-superconductor (SNS') thin film junctions with BaFe1.8Co0.2As2 as base electrode. As counter electrode we used a lead indium alloy, while the normal metal layer was formed by thin gold films. Temperature dependent measurements of the electrical conductivity were strongly influenced by the properties of the electrodes. We developed a junction structure that allows us to characterize the electrodes, too, including the behavior of their normal state resistance in order to correct their influences on the conduction spectra. The corrected conductivity of the junction was described within an extended BTK-model and shows a behavior dominated by Andreev reflexion.

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arXiv2011

Point-contact study of ReFeAs(1-x)Fx (Re=La, Sm) superconducting films

Point-contact (PC) Andreev-reflection (AR) measurements of the superconducting gap in iron-oxipnictide ReFeAsO_{1-x}F_x (Re=La, Sm) films have been carried out. The value of the gap is distributed in the range 2Δ\simeq 5-10 meV (for Re=Sm) with a maximum in the distribution around 6 meV. Temperature dependence of the gap Δ(T) can be fitted well by BCS curve giving reduced gap ratio 2Δ/kT_c^*\simeq 3.5 (here T_c^* is the critical temperature from the BCS fit). At the same time, an expected second larger gap feature was difficult to resolve distinctly on the AR spectra making determination reliability of the second gap detection questionable. Possible reasons for this and the origin of other features like clear-cut asymmetry in the AR spectra and current regime in PCs are discussed.

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arXiv2011

Quantitative assessment of pinning forces and the superconducting gap in NbN thin films from complementary magnetic force microscopy and transport measurements

Epitaxial niobium-nitride thin films with a critical temperature of Tc=16K and a thickness of 100nm were fabricated on MgO(100) substrates by pulsed laser deposition. Low-temperature magnetic force microscopy (MFM) images of the supercurrent vortices were measured after field cooling in a magnetic field of 3mT at various temperatures. Temperature dependence of the penetration depth has been evaluated by a two-dimensional fitting of the vortex profiles in the monopole-monopole model. Its subsequent fit to a single s-wave gap function results in the superconducting gap amplitude Delta(0) = 2.9 meV = 2.1*kB*Tc, in perfect agreement with previous reports. The pinning force has been independently estimated from local depinning of individual vortices by lateral forces exerted by the MFM tip and from transport measurements. A good quantitative agreement between the two techniques shows that for low fields, B << Hc2, MFM is a powerful and reliable technique to probe the local variations of the pinning landscape. We also demonstrate that the monopole model can be successfully applied even for thin films with a thickness comparable to the penetration depth.

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arXiv2010

Influenced of Fe buffer thickness on the crystalline quality and the transport properties of Fe/Ba(Fe1-xCox)2As2 bilayers

The implementation of an Fe buffer layer is a promising way to obtain epitaxial growth of Co-doped BaFe2As2 (Ba-122). However, the crystalline quality and the superconducting properties of Co-doped Ba-122 are influenced by the Fe buffer layer thickness, dFe. The well-textured growth of the Fe/Ba-122 bilayer with dFe = 15 nm results in a high Jc of 0.45 MAcm$^{-2}$ at 12 K in self-field, whereas a low Jc value of 61000 Acm$^{-2}$ is recorded for the bilayer with dFe = 4 nm at the corresponding reduced temperature due to the presence of grain boundaries.

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arXiv2010

Point-contact study of the LuNi2B2C borocarbide superconducting film

We present point-contact (PC) Andreev-reflection measurements of a superconducting epitaxial c-axis oriented nickel borocarbide film LuNi2B2C (Tc=15.9 K). The averaged value of the superconducting gap is found to be 2.6 +/-0.2 meV in the one-gap approach, whereas the two-gap approach results in 2.14+/-0.36 meV and 3.0+/-0.27 meV. The better fit of the Andreev-reflection spectra for the LuNi2B2C - Cu PC obtained by the two-gap approach provides evidence for multiband superconductivity in LuNi2B2C. For the first time, PC electron-phonon interaction (EPI) spectra have been measured for this compound. They demonstrate pronounced phonon maximum at 8.5+/-0.4meV and a second shallow one at 15.8+/-0.6 meV. The electron-phonon coupling constant estimated from the PC EPI spectra turned out to be small (~ 0.1), like in other superconducting rare-earth nickel borocarbides. Possible reasons for this are discussed.

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arXiv2010

Highly anisotropic energy gap in superconducting Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$ from optical conductivity measurements

We have measured the complex dynamical conductivity, $σ= σ_{1} + iσ_{2}$, of superconducting Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$ ($T_{c} = 22$ K) at terahertz frequencies and temperatures 2 - 30 K. In the frequency dependence of $σ_{1}$ below $T_{c}$, we observe clear signatures of the superconducting energy gap opening. The temperature dependence of $σ_{1}$ demonstrates a pronounced coherence peak at frequencies below 15 cm$^{-1}$ (1.8 meV). The temperature dependence of the penetration depth, calculated from $σ_{2}$, shows power-law behavior at the lowest temperatures. Analysis of the conductivity data with a two-gap model, gives the smaller isotropic s-wave gap of $Δ_{A} = 3$ meV, while the larger gap is highly anisotropic with possible nodes and its rms amplitude is $Δ_{0} = 8$ meV. Overall, our results are consistent with a two-band superconductor with an $s_{\pm}$ gap symmetry.

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arXiv2010

BaFe_{1.8}Co_{0.2}As_2 thin film hybrid Josephson junctions

Josephson junctions with iron pnictides open the way for fundamental experiments on superconductivity in these materials and their application in superconducting devices. Here, we present hybrid Josephson junctions with a BaFe_{1.8}Co_{0.2}As_2 thin film electrode, an Au barrier and a PbIn counter electrode. The junctions show RSJ-like current-voltage characteristics up to the critical temperature of the counter electrode of about 7.2K. The temperature dependence of the critical current, IC, does not show an Ambegaokar-Baratoff behavior. Well-pronounced Shapiro steps are observed at microwave frequencies of 10-18GHz. Assuming an excess current, I_ex, of 200 μA at 4.2K we get an effective I_C R_N product of 6 μV.

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arXiv2010

Two-band BCS superconductivity in Ba(Fe0.9Co0.1)2As2

The conductivity and permittivity optical spectra of iron-pnictide Ba(Fe0.9Co0.1)2As2 film (Tc=20 K) are analyzed. In the superconducting state, at all temperatures up to Tc the temperature dependences of the magnetic field penetration depth and of the superconducting condensate density are well described within the generalized two-band BCS model with intraband and interband pairing interactions considered. It is shown that the smaller superconducting energy gap 2Δ = 3.7 meV develops in the electronic subsystem while the larger gap 2Δ >= 7 meV opens in the hole subsystem. The normal state parameters (plasma frequencies and scattering rates) of electron and hole conduction bands are determined. At all temperatures the obtained data are consistent with the results of electronic photoemission experiments on Ba(Fe1-x Cox)2As2.

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arXiv2010

Critical current scaling and anisotropy in oxypnictide superconductors

Investigating the anisotropy of superconductors permits an access to fundamental properties. Having succeeded in the fabrication of epitaxial superconducting LaFeAs(O,F) thin films we performed an extensive study of electrical transport properties. In face of multiband superconductivity we can demonstrate that a Blatter scaling of the angular dependent critical current densities can be adopted, although being originally developed for single band superconductors. In contrast to single band superconductors the mass anisotropy of LaFeAs(O,F) is temperature dependent. A very steep increase of the upper critical field and the irreversibility field can be observed at temperatures below 6K, indicating that the band with the smaller gap is in the dirty limit. This temperature dependence can be theoretically described by two dominating bands responsible for superconductivity. A pinning force scaling provides insight into the prevalent pinning mechanism and can be specified in terms of the Kramer model.

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arXiv2010

Direct observation of superconducting vortex clusters pinned by a periodic array of magnetic dots in ferromagnetic/superconducting hybrid structures

Strong pinning of superconducting flux quanta by a square array of 1 $μ$m-sized ferromagnetic dots in a magnetic-vortex state was visualized by low-temperature magnetic force microscopy (LT-MFM). A direct correlation of the superconducting flux lines with the positions of the dots was derived. The force that the MFM tip exerts on the individual vortex in the depinning process was used to estimate the spatial modulation of the pinning potential. It was found, that the superconducting vortices which are preferably located on top of the Py dots experience about 15 times stronger pinning forces as compared to the pinning force in the pure Nb film. The strong pinning exceeds the repulsive interaction between the superconducting vortices and allows the vortex clusters to be located at each dot. Our microscopic studies are consistent with global magnetoresistace measurements on these hybrid structures.

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arXiv2010

Scaling behaviour of the critical current in clean epitaxial Ba(Fe1-xCox)2As2 thin films

The angular-dependent critical current density, Jc(theta), and the upper critical field, Hc2(theta), of epitaxial Ba(Fe1-xCox)2As2 thin films have been investigated. No Jc(theta) peaks for H || c were observed regardless of temperatures and magnetic fields. In contrast, Jc(theta) showed a broad maximum at theta=90 degree, which arises from intrinsic pinning. All data except at theta=90 degree can be scaled by the Blatter plot. Hc2(theta) near Tc follows the anisotropic Ginzburg-Landau expression. The mass anisotropy increased from 1.5 to 2 with increasing temperature, which is an evidence for multi-band superconductivity.

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