arXiv subjects

D. Winkler

Publications and source records attributed to D. Winkler.

At least 19 recordsLinked to original sources

arXiv2020

Properties of Grooved Dayem Bridge based YBa$_2$Cu$_3$O$_{7-δ}$ Superconducting Quantum Interference Devices and Magnetometers

The transport properties of a YBa$_2$Cu$_3$O$_{7-δ}$ superconducting quantum interference device (SQUID) based on grooved Dayem bridge weak links are studied as a function of temperature: at high temperatures ($60~$K$<T<T_\mathrm{c}=89$~K) the weak links show properties similar to SNS junctions, while at temperatures below 60~K the weak links behave like short Dayem bridges. Using these devices, we have fabricated SQUID magnetometers with galvanically coupled in-plane pick-up loops: at $T=77$~K, magnetic field white noise levels as low as $63$~fT/$\sqrt{\mathrm{Hz}}$ have been achieved.

cond-mat.supr-con↗

arXiv2019

The role of kinetic inductance on the performance of YBCO SQUID magnetometers

Inductance is a key parameter when optimizing the performance of superconducting quantum interference device (SQUID) magnetometers made from the high temperature superconductor YBa$_2$Cu$_3$O$_{7-x}$ (YBCO) because lower SQUID inductance $L$ leads to lower flux noise, but also weaker coupling to the pickup loop. In order to optimize the SQUID design, we combine inductance simulations and measurements to extract the different inductance contributions, and measure the dependence of the transfer function $V_Φ$ and flux noise $S_Φ^{1/2}$ on $L$. A comparison between two samples shows that the kinetic inductance contribution varies strongly with film quality, hence making inductance measurements a crucial part of the SQUID characterization. Thanks to the improved estimation of the kinetic inductance contribution, previously found discrepancies between theoretical estimates and measured values of $V_Φ$ and $S_Φ^{1/2}$ could to a large extent be avoided. We then use the measurements and improved theoretical estimations to optimize the SQUID geometry and reach a noise level of $S_B^{1/2}$ = 44 fT/$\sqrt{\textrm{Hz}}$ for the best SQUID magnetometer with a 8.6 mm $\times$ 9.2 mm directly coupled pickup loop. Lastly, we demonstrate a method for reliable one-time sensor calibration that is constant in a temperature range of several kelvin despite the presence of temperature dependent coupling contributions, such as the kinetic inductance. The found variability of the kinetic inductance contribution has implications not only for the design of YBCO SQUID magnetometers, but for all narrow linewidth SQUID-based devices operated close to their critical temperature.

physics.ins-det↗

arXiv2018

Magnetic field sensing with the kinetic inductance of a high-$T_\mathrm{c}$ superconductor

We carry out an experimental feasibility study of a magnetic field sensor based on the kinetic inductance of the high-$T_\mathrm{c}$ superconductor yttrium barium copper oxide. We pattern thin superconducting films into radio-frequency resonators that feature a magnetic field pick-up loop. At 77 K and for film thicknesses down to 75 nm, we observe the persistence of screening currents that modulate the loop kinetic inductance. According to the experimental results the device concept appears attractive for sensing applications in ambient magnetic field environments. We report on a device with a magnetic field sensitivity of 4 pT/Hz${}^{1/2}$, an instantaneous dynamic range of 11 $μ$T, and operability in magnetic fields up to 28 $μ$T.

physics.ins-det↗

arXiv2016

Preventing Incomplete/Hidden Requirements: Reflections on Survey Data from Austria and Brazil

Many software projects fail due to problems in requirements engineering (RE). The goal of this paper is analyzing a specific and relevant RE problem in detail: incomplete/hidden requirements. We replicated a global family of RE surveys with representatives of software organizations in Austria and Brazil. We used the data to (a) characterize the criticality of the selected RE problem, and to (b) analyze the reported main causes and mitigation actions. Based on the analysis, we discuss how to prevent the problem. The survey includes 14 different organizations in Austria and 74 in Brazil, including small, medium and large sized companies, conducting both, plan-driven and agile development processes. Respondents from both countries cited the incomplete/hidden requirements problem as one of the most critical RE problems. We identified and graphically represented the main causes and documented solution options to address these causes. Further, we compiled a list of reported mitigation actions. From a practical point of view, this paper provides further insights into common causes of incomplete/hidden requirements and on how to prevent this problem.

cs.SE↗

arXiv2015

Magnetic proximity effect at interface between a cuprate superconductor and an oxide spin valve

Heterostructures consisting of a cuprate superconductor YBa2Cu3O7x and a ruthenate/manganite (SrRuO3/La0.7Sr0.3MnO3) spin valve have been studied by SQUID magnetometry, ferromagnetic resonances and neutron reflectometry. It was shown that due to the influence of magnetic proximity effect a magnetic moment is induced in the superconducting part of heterostructure and at the same time the magnetic moment is suppressed in the ferromagnetic spin valve. The experimental value of magnetization induced in the superconductor has the same order of magnitude with the calculations based on the induced magnetic moment of Cu atoms due to orbital reconstruction at the superconductor-ferromagnetic interface. It corresponds also to the model that takes into account the change in the density of states at a distance of order of the coherence length in the superconductor. The experimentally obtained characteristic length of penetration of the magnetic moment into superconductor exceeds the coherence length for cuprate superconductor. This fact points on the dominance of the mechanism of the induced magnetic moment of Cu atoms due to orbital reconstruction.

cond-mat.supr-con↗

arXiv2014

Evidence for spin-triplet superconducting correlations in metal-oxide heterostructures with non-collinear magnetization

Heterostructures composed of ferromagnetic La0.7Sr0.3MnO3, ferromagnetic SrRuO3, and superconducting YBa2Cu3Ox were studied experimentally. Structures of composition Au/La0.7Sr0.3MnO3/SrRuO3/YBa2Cu3Ox were prepared by pulsed laser deposition, and their high quality was confirmed by X-ray diffraction and reflectometry. A non-collinear magnetic state of the heterostructures was revealed by means of SQUID magnetometry and polarized neutron reflectometry. We have further observed superconducting currents in mesa-structures fabricated by deposition of a second superconducting Nb layer on top of the heterostructure, followed by patterning with photolithography and ion-beam etching. Josephson effects observed in these mesa-structures can be explained by the penetration of a triplet component of the superconducting order parameter into the magnetic layers.

cond-mat.str-el↗

arXiv2011

Magnetoencephalography based on high-Tc superconductivity: a closer look into the brain?

Magnetoencephalography (MEG) enables the study of brain activity by recording the magnetic fields generated by neural currents and has become an important technique for neuroscientists in research and clinical settings. Unlike the liquid-helium cooled low-Tc superconducting quantum interference devices (SQUIDs) that have been at the heart of modern MEG systems since their invention, high-Tc SQUIDs can operate with liquid nitrogen cooling. The relaxation of thermal insulation requirements allows for a reduction in the stand-off distance between the sensor and the room-temperature environment from a few centimeters to less than a millimeter, where MEG signal strength is significantly higher. Despite this advantage, high-Tc SQUIDs have only been used for proof-of-principle MEG recordings of well-understood evoked activity. Here we show high-Tc SQUID-based MEG may be capable of providing novel information about brain activity due to the close proximity of the sensor to the head. We have performed single- and two-channel high-Tc SQUID MEG recordings of spontaneous brain activity in two healthy human subjects. We demonstrate modulation of the occipital alpha rhythm and the mu rhythm found in the motor cortex. Furthermore, we have discovered uncharacteristically high-amplitude activity in the theta-band from the occipital region of the brain. Our results suggest high-Tc SQUIDs can provide a closer look into the brain.

physics.med-ph↗

arXiv2010

Proximity effect and electron transport in the oxide hybrid heterostructures with superconducting/magnetic interfaces

We report on the study of electron transport in the oxide heterostructures with superconductor/magnetic matter (S/M) interfaces where anomaly large penetration of superconducting correlations in magnetic matter (proximity effect) is realized. The developed theoretical model based on multilayer magnetic structure of M-interlayer and experiment show presence of the long-range proximity effect at S/M-interface with antiferromagnetic (AF) ordering of the interlayer magnetization. The investigated hybrid heterostructures include cuprate superconductor, the AF-cuprate interlayer and conventional superconductor, Nb. The superconducting critical current with density 1-10A/cm2 and the characteristic voltage, 0.1-0.2 mV, are observed at liquid helium temperature for 15-50 nm thick M-interlayer made of AF film CaSrCuO. These heterostructures demonstrate deviation from sin-type superconducting current-phase relation and had the critical current of the second harmonic of 10-20% of the first harmonic one. The hybrid heterostructures with S/AF interface show high sensitivity to the external magnetic field that is possibly caused by the influence of an external magnetic field on the canting of magnetic moments of individual layers of AF -interlayer. Substituting the AF cuprate by a manganite film, no critical current was observed, although the M- interlayer was made very thin, down to 5 nm.

cond-mat.supr-con↗

arXiv2007

Electrical transport properties of polar heterointerface between KTaO3 and SrTiO3

Electrical transport of a polar heterointerface between two insulating perovskites, KTaO3 and SrTiO3, is studied. It is formed between a thin KTaO3 film deposited on a top of TiO2- terminated (100) SrTiO3 substrate. The resulting (KO)1-(TiO2)0 heterointerface is expected to be hole-doped according to formal valences of K (1+) and Ti (4+). We observed electrical conductivity and mobility in the KTaO3/SrTiO3 similar to values measured earlier in electron-doped LaAlO3/SrTiO3 heterointerfaces. However, the sign of the charge carriers in KTaO3/SrTiO3 obtained from the Hall measurements is negative. The result is an important clue to the true origin of the doping at perovskite oxide hetero-interfaces.

cond-mat.mtrl-sci↗

arXiv2006

Anomalous Superconducting Proximity Effect in Hybrid Oxide Heterostructures with Antiferromagnetic Layer

We report an investigation of current transport in Nb/Au/CaSrCuO/YBaCuO heterostructures including an antiferromagnetic interlayer (CaSrCuO). Epitaxial thin films of YBaCuO and CaSrCuO were grown by laser ablation on NdGaO substrates; the thicknesses of CaSrCuO films were 20 and 50 nm, and x=0.15 or 0.5. Our experimental results show that the superconducting current transport is increased in the Nb/Au/CaSrCuO/YBaCuO heterostructures in comparison with the ones observed in previously studied Nb/Au/YBaCuO heterostructures. It was found that the critical current of the structure is very sensitive to the absolute value and direction of the external magnetic field.

cond-mat.supr-con↗

arXiv2006

A single intrinsic Josephson junction with double-sided fabrication technique

We make stacks of intrinsic Josephson junctions (IJJs) imbedded in the bulk of very thin ($d\leq 100$~nm) $\mathrm{Bi_2Sr_2CaCu_2O_{8+x}}$ single crystals. By precisely controlling the etching depth during the double-sided fabrication process, the stacks can be reproducibly tailor-made to be of any microscopic height ($0-9 \mathrm{nm} <d$), i.e. enclosing a specified number of IJJ (0-6), including the important case of a single junction. We discuss reproducible gap-like features in the current-voltage characteristics of the samples at high bias.

cond-mat.supr-con↗

arXiv2005

Superconducting properties of ultrathin Bi2Sr2CaCu2O8+x single crystals

We use Ar-ion milling to thin Bi2212 single crystals down to a few nanometers or one-to-two (CuO2)2 layers. With decreasing the thickness, superconducting transition temperature gradually decreases to zero and the in-plane resistivity increases to large values indicating the existence of a superconductor-insulator transition in ultrathin Bi2212 single crystals.

cond-mat.supr-con↗

arXiv2004

Superconducting critical current of a single Cu2O4-plane in Bi2Sr2CaCu2O8+x single crystal

By feeding current into the topmost $\mathrm{Cu_2O_4}$-layer of a mesa etched into the surface of a $\mathrm{Bi_2Sr_2CaCu_2O_{8+x}}$ (BSCCO) single crystal, we measured its superconducting critical value from a sharp upturn or break in current-voltage characteristics of the mesas. From this, we estimate the sheet critical current density of a single $\mathrm{Cu_2O_4}$ plane to be $\sim$\ 0.3-0.7 A/cm at 4.5 K, corresponding to the bulk current density of $2-5\ \mathrm{MA/cm^2}$. These values are among the largest ever measured for BSCCO single crystals, thin films and tapes.

cond-mat.supr-con↗

arXiv2003

Reply to the "Comment on 'Intrinsic tunneling spectra of Bi_2(Sr_{2-x}La_x)CuO_6' ": Auxiliary information

To better address the heating issue in stacks of intrinsic Josephson junctions, we directly measure temperature of the stack by using a micron-sized thermocouple which is in direct thermal- and electrical contact to the stack, exactly in the place where the bias current is injected. Our measurements have shown that the temperature of the stack can reach 200-300K at the highest bias. Thus, we confirm experimentally that the Joule self-heating is a severe problem in intrinsic-junction-spectroscopy experiments. The pseudogap features reported in our previous paper (A.Yurgens etal, PRL 90, 147005 (2003)) are indeed an artifact of Joule heating. The detailed measurements of the temperature of the stacks at different ambient temperatures T0 allow us to deduce the unique, "heating-free" I(V)-, or dI/dV(V) curves.

cond-mat.supr-con↗

arXiv2003

Reply to the "Comment on 'Intrinsic tunneling spectra of Bi_2(Sr_{2-x}La_x)CuO_6' "

cond-mat.supr-con↗

arXiv2002

Intrinsic tunneling spectra of Bi_2(Sr_{2-x}La_x)CuO_6

We have measured intrinsic-tunneling spectra of a single CuO-layer La-doped Bi_2Sr_{2-x}La_xCuO_{6+δ} (Bi2201-La_x). Despite a difference of a factor of three in the optimal superconducting critical temperatures for Bi2201-La_{0.4} and Bi2212 (32 and 95 K, respectively) and different spectral energy scales, we find that the pseudogap vanishes at a similar characteristic temperature T*\approx 230-300K for both compounds. We find also that in Bi2201-La_x, PG humps are seen as sharp peaks and, in fact, even dominate the intrinsic spectra.

cond-mat.supr-con↗

arXiv2002

Self-heating in small mesa structures

We study analytically and numerically a problem of self-heating in small mesa structures. Our results show that the self-heating is proportional to a characteristic in-plane size of the mesa. Experimental data for small high-$T_c$ superconductor Bi2212 mesas are in qualitative agreement with our calculations. We estimate the self-heating in Bi2212 mesas with different sizes and demonstrate that the self-heating can effectively be obviated in small mesa structures.

cond-mat.supr-con↗

arXiv2000

Discrimination between the superconducting gap and the pseudo-gap in Bi2212 from intrinsic tunneling spectroscopy in magnetic field

Intrinsic tunneling spectroscopy in high magnetic field ($H$) is used for a direct test of superconducting features in a quasiparticle density of states of high-$T_c$ superconductors. We were able to distinguish with a great clarity two co-existing gaps: (i) the superconducting gap, which closes as $H \to H_{c2}(T)$ and $T\to T_c(H)$, and (ii) the $c$-axis pseudo-gap, which does not change neither with $H$, nor $T$. Strikingly different magnetic field dependencies, together with previously observed different temperature dependencies of the two gaps ~\cite{Krasnov}, speak against the superconducting origin of the pseudo-gap.

cond-mat.supr-con↗