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Papers by Paul Seidel

Physical Review B, 2015

We report on the epitaxial growth of As-free and phase-pure thin films of the 112-pnictide compou... more We report on the epitaxial growth of As-free and phase-pure thin films of the 112-pnictide compounds LaPd x P n 2 (P n = Sb, Bi) grown on (100) MgO substrates by molecular beam epitaxy. X-ray diffraction, reflection high-energy electron diffraction, and x-ray photoelectron spectroscopy confirm the HfCuSi 2 structure of the material with a peculiar pnictogen square net layer. The superconducting transition temperature T c varies little with Pd concentration. LaPd x Sb 2 has a higher T c (3.2 K) by about 20% compared with LaPd x Bi 2 (2.7 K). Fe substitution of Pd leads to a rapid decay of superconductivity, suggesting that these superconductors are conventional type II.

IEEE Transactions on Applied Superconductivity, 2016

Journal of Physics: Conference Series, 2010

International Journal of Cardiology, 2001

A new technology has been developed which measures the magnetic field of the human heart (magneto... more A new technology has been developed which measures the magnetic field of the human heart (magnetocardiogram, MCG) by using high temperature superconducting (HTS) sensors. These sensors can be operated at the temperature of liquid nitrogen without electromagnetic shielding. We tested the reproducibility of HTS-MCG measurements in healthy volunteers. Unshielded HTS-MCG measurements were performed in 18 healthy volunteers in left precordial position in two separate sessions in a clinical environment. The heart cycles of 10 min were averaged, smoothed, the baselines were adjusted, and the data were standardized to the respective areas under the curves (AUC) of the absolute values of the QRST amplitudes. The QRS complexes and the ST-T intervals were used to assess the reproducibility of the two measurements. Ratios (R(QRS), R(STT)) were calculated by dividing the AUC of the first measurement by the ones of the second measurement. The linear correlation coefficients (CORR(QRS), CORR(STT)) of the time intervals of the two measurements were calculated, too. The HTS-MCG signal was completely concealed by the high noise level in the raw data. The averaging and smoothing algorithms unmasked the QRS complex and the ST segment. A high reproducibility was found for the QRS complex (R(QRS)=1.2+/-0.3, CORR(QRS)=0.96+/-0.06). Similarly to the shape of the ECG it was characterized by three bends, the Q, R, and S waves. In the ST-T interval, the reproducibility was considerably lower (R(STT)=0.9+/-0.2, CORR(STT)=0.66+/-0.28). In contrast to the shape of the ECG, a baseline deflection after the T wave which may belong to U wave activity was found in a number of volunteers. HTS-MCG devices can be operated in a clinical environment without shielding. Whereas the reproducibility was found to be high for the depolarization interval, it was considerably lower for the ST segment and for the T wave. Therefore, before clinically applying HTS-MCG systems to the detection of repolarization abnormalities in acute coronary syndromes, further technical development of the systems is necessary to improve the signal-to-noise ratio.

Low Temperature Physics, 2015

We have studied transport characteristics of mesoscopic multiple-mode superconducting contacts fo... more We have studied transport characteristics of mesoscopic multiple-mode superconducting contacts formed between two grains in bulk two-gap magnesium diboride. The experimental setup was realized by driving a normal-metal tip into MgB 2 polycrystalline sample and proved to be extremely stable, providing possibility to perform pressure experiments at low temperatures. It is argued that in our procedure a small piece of the superconducting electrode is captured by the tip apex and, as a result, two junctions in series are formed: a junction between a tip and MgB 2 grain and a mesoscopic disordered contact between two superconducting pellets. Although the relative weight of the first junction resistance was considerably less, its contribution is shown to be important for the comparison of measured data with expected gap values. Two hallmarks of multiple Andreev reflections inside the MgB 2 -c-MgB 2 contact (c stands for a high-transparent constriction), a zero-bias 1 / V -like singularity of the dc differential conductance and peaks connected to the two gap values, have been revealed. Finally, we report results of a hydrostatic compression experiment showing the evolution of the MgB 2 gap values with pressure. In contrast to the theoretical expectations, we have observed an increase of the smaller gap ∆ π whereas the larger gap ∆ σ decreased with increasing pressure as it should be for the electron-phonon pairing mechanism. We argue that the so-called separable model of anisotropy effects is insufficient to describe such changes and only improved two-band versions are capable to reproduce the pressure effect on the energy gaps in magnesium diboride. PACS: 74.70.Ad Metals; alloys and binary compounds (including A15, MgB 2 , etc.); 81.07.Lk Nanocontacts; 74.62.Fj Effects of pressure.

IEEE Transactions on Applied Superconductivity, 2016

Materialwissenschaft und Werkstofftechnik, 2013

Physica C: Superconductivity, 2001

We present a simple scattering approach to the charge transport across a realistic superconductor... more We present a simple scattering approach to the charge transport across a realistic superconductor±normal injector interface of a ®nite transmittance that is modeled by a double-barrier mesoscopic junction. For a d-wave pairing symmetry, our calculations combine a fully quantum-mechanical scattering formalism with a self-consistent estimation of Andreev re¯ection coecients within the quasi-classical Eilenberger equation scheme for a free specular superconducting surface. Numerical simulations con®rm experimental criteria of Cucolo for the unconventional superconducting origin of conductance anomalies in high-temperature oxides. A discussion of dephasing eects caused by inelastic scattering processes in the interlayer and their impact on the conductance spectra is given. Ó 2001 Published by Elsevier Science B.V.

Physica C: Superconductivity, 2002

We present first experimental and theoretical results demonstrating a crossover from mesoscopic (... more We present first experimental and theoretical results demonstrating a crossover from mesoscopic (phase-coherent transmission) to macroscopic (two resistors in series) behavior of conductance spectra for a normal-superconducting heterostructure. Following the lines of the B€ u uttiker's approach to normal double-barrier devices, we show that phaserandomizing events in a mesoscopic interbarrier layer together with Andreev-reflection processes drastically influence corresponding conductance vs. voltage characteristics. An anomalous temperature shift of a gap-like feature in an Ag/ LaBa 2 Cu 3 O 7Àx point contact is interpreted as an effect of the partial phase decoherence in the intermediate magnetically active region between the normal-metal tip and the bulk 123-type superconducting cuprate. Ó

Journal of Superconductivity and Novel Magnetism, 2014

We investigate the iron-based superconductor Ba(Fe 1−x Co x ) 2 As 2 (Ba-122) regarding its super... more We investigate the iron-based superconductor Ba(Fe 1−x Co x ) 2 As 2 (Ba-122) regarding its superconducting properties and possible applications. Therefore Ba-122 thin films are used as base electrode to prepare different kinds of hybrid Josephson junctions with a counter electrode of the conventional superconductor Pb. Additionally, we use both c-axis and ab-plane transport geometries and different kinds of barriers like interface-engineered surfaces, sputtered titanium oxide and gold layers. Temperature dependent I-V characteristics as well as magnetic field dependence and microwave response of the junctions are shown. The examined I-V characteristics and I c R n -T behaviours of each junction type are compared and described according to the electrical behaviour of the respective normal conducting or insulating barrier. While the I c R n product of the interface-engineered barrier junction was 12 µV and the planar junction with Au barrier showed 18 µV, we could increase the I c R n to 90 µV for planar TiO x barrier junctions.

Physics Procedia, 2012

For the investigation of the superconducting properties of iron-based superconductors Josephson j... more For the investigation of the superconducting properties of iron-based superconductors Josephson junctions as well as Andreev reflection studies are potential tools. Therefore, we prepared different kinds of junctions, based on transport in direction of c-axis and ab-plane, respectively. The first one was a planar hybrid superconductor-normal metal-superconductor (SNS') thin film junction for transport studies along the c-axis. We used a BaFe 1.8 Co 0.2 As 2 (Ba-122) thin film (thickness 80 nm) prepared by pulsed laser deposition (PLD) as base electrode and a lead indium alloy as counter electrode, while the normal metal (barrier) layer was formed by thin gold films. By tuning the thickness of the gold layer and the area of the tunneling window, we were able to switch the behavior of our junction. For a thin gold layer ( 5 nm) and greater areas, RCSJ-like Josephson behavior can be observed with a radiation dependent excess current and nearly linear temperature dependence of the critical current. Increasing the thickness of the gold layer to 10 nm leads to conductance spectra, which are dominated by Andreev reflection and which show no remaining sign of Josephson effects. Secondly, an edge-type junction for transport measurements in the ab-plane of the pnictide layer, was prepared. It also is realized with PbIn as counter electrode, but with no artificial thin film. The temperature dependent conductance spectra showed some interesting features, which cannot be described within usual theoretical models. We introduced a third geometry, phase-sensitive corner junctions, which could ultimately test the pairing symmetry of Ba-122, if it is s ± type.

In applications for high-Tc superconducting DC-SQUIDs such as biomagnetism, nondestructive evalua... more In applications for high-Tc superconducting DC-SQUIDs such as biomagnetism, nondestructive evaluation and the relaxation of magnetic nanoparticles, it is important to maintain reliable sensor performance over an extended time period. We designed and produced DC-SQUID gradiometers based on YBa2Cu3O7 x (YBCO) thin lms which are inductively coupled to a ux transformer to achieve a higher sensitivity. The gradiometers are protected

Superconductor Science and Technology, 2006

We have developed different kinds of planar high-temperature superconducting dc-SQUID gradiometer... more We have developed different kinds of planar high-temperature superconducting dc-SQUID gradiometers with performances suitable for various applications in unshielded environments. Robust small sensors were realized for non-destructive evaluation. Their long-term stability was improved by new passivation technologies. For biomagnetic measurements the required higher field sensitivity in unshielded environments gave rise to many technological problems. Some solutions for crossovers, low-ohmic contact

Physica C: Superconductivity, 1998

We have investigated I-V characteristics and their derivatives in YBa Cu O rnormal metal planar a... more We have investigated I-V characteristics and their derivatives in YBa Cu O rnormal metal planar and point contact 2 3 7 yx junctions in the voltage range up to "2 V. Both increases and decreases of the junction resistance above q0.5 V and below Ž . y0.5 V related to metal electrode , respectively, were measured. These changes were reproducibly repeated several times on the same contact, and we ascribe them to oxygen replacement in the tunneling barrier arising from degraded YBa Cu O unit cells in the vicinity of the contact. The differential conductance of these junctions showed both a linear 2 3 7 yx and a quadratic dependence with respect to applied voltage polarity. These features are described in terms of inelastic processes due to strong spin interactions in the barrier caused by oxygen replacement in unit cells of YBa Cu O . We 2 3 7 yx model the linear and the quadratic background as a superposition of elastic and inelastic transport of quasiparticles through the barrier. An abrupt change of the slope of the linear part of the differential conductance was found above temperature 150 K. q

Physical Review B, 2015

We report on the epitaxial growth of As-free and phase-pure thin films of the 112-pnictide compou... more We report on the epitaxial growth of As-free and phase-pure thin films of the 112-pnictide compounds LaPd x P n 2 (P n = Sb, Bi) grown on (100) MgO substrates by molecular beam epitaxy. X-ray diffraction, reflection high-energy electron diffraction, and x-ray photoelectron spectroscopy confirm the HfCuSi 2 structure of the material with a peculiar pnictogen square net layer. The superconducting transition temperature T c varies little with Pd concentration. LaPd x Sb 2 has a higher T c (3.2 K) by about 20% compared with LaPd x Bi 2 (2.7 K). Fe substitution of Pd leads to a rapid decay of superconductivity, suggesting that these superconductors are conventional type II.

IEEE Transactions on Applied Superconductivity, 2016

Journal of Physics: Conference Series, 2010

International Journal of Cardiology, 2001

A new technology has been developed which measures the magnetic field of the human heart (magneto... more A new technology has been developed which measures the magnetic field of the human heart (magnetocardiogram, MCG) by using high temperature superconducting (HTS) sensors. These sensors can be operated at the temperature of liquid nitrogen without electromagnetic shielding. We tested the reproducibility of HTS-MCG measurements in healthy volunteers. Unshielded HTS-MCG measurements were performed in 18 healthy volunteers in left precordial position in two separate sessions in a clinical environment. The heart cycles of 10 min were averaged, smoothed, the baselines were adjusted, and the data were standardized to the respective areas under the curves (AUC) of the absolute values of the QRST amplitudes. The QRS complexes and the ST-T intervals were used to assess the reproducibility of the two measurements. Ratios (R(QRS), R(STT)) were calculated by dividing the AUC of the first measurement by the ones of the second measurement. The linear correlation coefficients (CORR(QRS), CORR(STT)) of the time intervals of the two measurements were calculated, too. The HTS-MCG signal was completely concealed by the high noise level in the raw data. The averaging and smoothing algorithms unmasked the QRS complex and the ST segment. A high reproducibility was found for the QRS complex (R(QRS)=1.2+/-0.3, CORR(QRS)=0.96+/-0.06). Similarly to the shape of the ECG it was characterized by three bends, the Q, R, and S waves. In the ST-T interval, the reproducibility was considerably lower (R(STT)=0.9+/-0.2, CORR(STT)=0.66+/-0.28). In contrast to the shape of the ECG, a baseline deflection after the T wave which may belong to U wave activity was found in a number of volunteers. HTS-MCG devices can be operated in a clinical environment without shielding. Whereas the reproducibility was found to be high for the depolarization interval, it was considerably lower for the ST segment and for the T wave. Therefore, before clinically applying HTS-MCG systems to the detection of repolarization abnormalities in acute coronary syndromes, further technical development of the systems is necessary to improve the signal-to-noise ratio.

Low Temperature Physics, 2015

We have studied transport characteristics of mesoscopic multiple-mode superconducting contacts fo... more We have studied transport characteristics of mesoscopic multiple-mode superconducting contacts formed between two grains in bulk two-gap magnesium diboride. The experimental setup was realized by driving a normal-metal tip into MgB 2 polycrystalline sample and proved to be extremely stable, providing possibility to perform pressure experiments at low temperatures. It is argued that in our procedure a small piece of the superconducting electrode is captured by the tip apex and, as a result, two junctions in series are formed: a junction between a tip and MgB 2 grain and a mesoscopic disordered contact between two superconducting pellets. Although the relative weight of the first junction resistance was considerably less, its contribution is shown to be important for the comparison of measured data with expected gap values. Two hallmarks of multiple Andreev reflections inside the MgB 2 -c-MgB 2 contact (c stands for a high-transparent constriction), a zero-bias 1 / V -like singularity of the dc differential conductance and peaks connected to the two gap values, have been revealed. Finally, we report results of a hydrostatic compression experiment showing the evolution of the MgB 2 gap values with pressure. In contrast to the theoretical expectations, we have observed an increase of the smaller gap ∆ π whereas the larger gap ∆ σ decreased with increasing pressure as it should be for the electron-phonon pairing mechanism. We argue that the so-called separable model of anisotropy effects is insufficient to describe such changes and only improved two-band versions are capable to reproduce the pressure effect on the energy gaps in magnesium diboride. PACS: 74.70.Ad Metals; alloys and binary compounds (including A15, MgB 2 , etc.); 81.07.Lk Nanocontacts; 74.62.Fj Effects of pressure.

IEEE Transactions on Applied Superconductivity, 2016

Materialwissenschaft und Werkstofftechnik, 2013

Physica C: Superconductivity, 2001

We present a simple scattering approach to the charge transport across a realistic superconductor... more We present a simple scattering approach to the charge transport across a realistic superconductor±normal injector interface of a ®nite transmittance that is modeled by a double-barrier mesoscopic junction. For a d-wave pairing symmetry, our calculations combine a fully quantum-mechanical scattering formalism with a self-consistent estimation of Andreev re¯ection coecients within the quasi-classical Eilenberger equation scheme for a free specular superconducting surface. Numerical simulations con®rm experimental criteria of Cucolo for the unconventional superconducting origin of conductance anomalies in high-temperature oxides. A discussion of dephasing eects caused by inelastic scattering processes in the interlayer and their impact on the conductance spectra is given. Ó 2001 Published by Elsevier Science B.V.

Physica C: Superconductivity, 2002

We present first experimental and theoretical results demonstrating a crossover from mesoscopic (... more We present first experimental and theoretical results demonstrating a crossover from mesoscopic (phase-coherent transmission) to macroscopic (two resistors in series) behavior of conductance spectra for a normal-superconducting heterostructure. Following the lines of the B€ u uttiker's approach to normal double-barrier devices, we show that phaserandomizing events in a mesoscopic interbarrier layer together with Andreev-reflection processes drastically influence corresponding conductance vs. voltage characteristics. An anomalous temperature shift of a gap-like feature in an Ag/ LaBa 2 Cu 3 O 7Àx point contact is interpreted as an effect of the partial phase decoherence in the intermediate magnetically active region between the normal-metal tip and the bulk 123-type superconducting cuprate. Ó

Journal of Superconductivity and Novel Magnetism, 2014

We investigate the iron-based superconductor Ba(Fe 1−x Co x ) 2 As 2 (Ba-122) regarding its super... more We investigate the iron-based superconductor Ba(Fe 1−x Co x ) 2 As 2 (Ba-122) regarding its superconducting properties and possible applications. Therefore Ba-122 thin films are used as base electrode to prepare different kinds of hybrid Josephson junctions with a counter electrode of the conventional superconductor Pb. Additionally, we use both c-axis and ab-plane transport geometries and different kinds of barriers like interface-engineered surfaces, sputtered titanium oxide and gold layers. Temperature dependent I-V characteristics as well as magnetic field dependence and microwave response of the junctions are shown. The examined I-V characteristics and I c R n -T behaviours of each junction type are compared and described according to the electrical behaviour of the respective normal conducting or insulating barrier. While the I c R n product of the interface-engineered barrier junction was 12 µV and the planar junction with Au barrier showed 18 µV, we could increase the I c R n to 90 µV for planar TiO x barrier junctions.

Physics Procedia, 2012

For the investigation of the superconducting properties of iron-based superconductors Josephson j... more For the investigation of the superconducting properties of iron-based superconductors Josephson junctions as well as Andreev reflection studies are potential tools. Therefore, we prepared different kinds of junctions, based on transport in direction of c-axis and ab-plane, respectively. The first one was a planar hybrid superconductor-normal metal-superconductor (SNS') thin film junction for transport studies along the c-axis. We used a BaFe 1.8 Co 0.2 As 2 (Ba-122) thin film (thickness 80 nm) prepared by pulsed laser deposition (PLD) as base electrode and a lead indium alloy as counter electrode, while the normal metal (barrier) layer was formed by thin gold films. By tuning the thickness of the gold layer and the area of the tunneling window, we were able to switch the behavior of our junction. For a thin gold layer ( 5 nm) and greater areas, RCSJ-like Josephson behavior can be observed with a radiation dependent excess current and nearly linear temperature dependence of the critical current. Increasing the thickness of the gold layer to 10 nm leads to conductance spectra, which are dominated by Andreev reflection and which show no remaining sign of Josephson effects. Secondly, an edge-type junction for transport measurements in the ab-plane of the pnictide layer, was prepared. It also is realized with PbIn as counter electrode, but with no artificial thin film. The temperature dependent conductance spectra showed some interesting features, which cannot be described within usual theoretical models. We introduced a third geometry, phase-sensitive corner junctions, which could ultimately test the pairing symmetry of Ba-122, if it is s ± type.

In applications for high-Tc superconducting DC-SQUIDs such as biomagnetism, nondestructive evalua... more In applications for high-Tc superconducting DC-SQUIDs such as biomagnetism, nondestructive evaluation and the relaxation of magnetic nanoparticles, it is important to maintain reliable sensor performance over an extended time period. We designed and produced DC-SQUID gradiometers based on YBa2Cu3O7 x (YBCO) thin lms which are inductively coupled to a ux transformer to achieve a higher sensitivity. The gradiometers are protected

Superconductor Science and Technology, 2006

We have developed different kinds of planar high-temperature superconducting dc-SQUID gradiometer... more We have developed different kinds of planar high-temperature superconducting dc-SQUID gradiometers with performances suitable for various applications in unshielded environments. Robust small sensors were realized for non-destructive evaluation. Their long-term stability was improved by new passivation technologies. For biomagnetic measurements the required higher field sensitivity in unshielded environments gave rise to many technological problems. Some solutions for crossovers, low-ohmic contact

Physica C: Superconductivity, 1998

We have investigated I-V characteristics and their derivatives in YBa Cu O rnormal metal planar a... more We have investigated I-V characteristics and their derivatives in YBa Cu O rnormal metal planar and point contact 2 3 7 yx junctions in the voltage range up to "2 V. Both increases and decreases of the junction resistance above q0.5 V and below Ž . y0.5 V related to metal electrode , respectively, were measured. These changes were reproducibly repeated several times on the same contact, and we ascribe them to oxygen replacement in the tunneling barrier arising from degraded YBa Cu O unit cells in the vicinity of the contact. The differential conductance of these junctions showed both a linear 2 3 7 yx and a quadratic dependence with respect to applied voltage polarity. These features are described in terms of inelastic processes due to strong spin interactions in the barrier caused by oxygen replacement in unit cells of YBa Cu O . We 2 3 7 yx model the linear and the quadratic background as a superposition of elastic and inelastic transport of quasiparticles through the barrier. An abrupt change of the slope of the linear part of the differential conductance was found above temperature 150 K. q