arXiv subjects

Jan Ullmann

Publications and source records attributed to Jan Ullmann.

3 recordsLinked to original sources

arXiv2026

Spatial resolution and point spread function of high-resolution scanning SQUID microscopy probes

Superconducting quantum interference devices (SQUIDs) show exceptional sensitivity to magnetic flux. In scanning SQUID microscopy (SSM), SQUID size and its distance to the sample are minimized in order to map weak magnetic fields with best possible spatial resolution. SQUID-on-lever (SOL) architectures have proven especially effective as SSM probes due to their small sensor size, robustness, and ease of integration with conventional atomic force microscopy hardware. In order to optimize magnetic microscopy carried out with SOL probes and to accurately reconstruct the magnetic fields that they measure, it is essential to know their point spread function (PSF). The size and shape of this PSF are determined by magnetic flux focusing effects, which depend on the characteristic length-scales of the superconductor and the sensor geometry. By simulating the coupling to sources of magnetic flux, this work provides a mathematical description of the SOL PSF, which contains a full description of the probe's magnetic sensitivity and spatial resolution. We then use measurements of a single magnetic skyrmion to measure the magnetic flux response of a real SOL. We demonstrate excellent agreement with flux responses that are obtained from simulations, thereby confirming the calculated PSF and spatial resolution.

cond-mat.supr-con↗

arXiv2026

Structural Changes and Transport Properties of $\mathrm{YBa_2Cu_3O_7}$ Locally Modified by a He$^+$ Focused Ion Beam

Irradiation of a material with ions can cause various defects that can lead to structural phase transitions and the modification of the material's properties. Here we study the irradiation of the epitaxyally grown thin films of the high-temperature superconductor $\mathrm{YBa_2Cu_3O_7}$ with $30\,\mathrm{keV}$ He$^{+}$ ions which leads to the expansion of the crystal lattice, decrease of the critical temperature $T_c$ and eventually transition to an insulator. Fabrication of such insulating regions with a focused He-Ion beam with a spot size of $\sim 10\,\mathrm{nm}$ is a powerful technique for fabrication of superconducting nano-devices. Using low-temperature resistivity measurements, diffraction with a nanofocused X-ray beam and atomic force microscopy, we investigated how the structure and the electric transport properties of $\mathrm{YBa_2Cu_3O_7}$ depend on the irradiation dose in a range $10$--$100\,\mathrm{ions/nm^2}$ and on the lateral size of the irradiated area in a range $30$--$5000\,\mathrm{nm}$.

cond-mat.supr-con↗

arXiv2025

Advanced SQUID-on-lever scanning probe for high-sensitivity magnetic microscopy with sub-100-nm spatial resolution

Superconducting quantum interference devices (SQUIDs) are exceptionally sensitive magnetometers capable of detecting weak magnetic fields. Miniaturizing these devices and integrating them onto scanning probes enables high-resolution imaging at low-temperature. Here, we fabricate nanometer-scale niobium SQUIDs with inner-loop sizes down to 10 nm at the apex of individual planar silicon cantilevers via a combination of wafer-scale optical lithography and focused-ion-beam (FIB) milling. These robust SQUID-on-lever probes overcome many of the limitations of existing devices, achieving spatial resolution better than 100 nm, magnetic flux sensitivity of $0.3~\mu\Phi_0/\sqrt{\rm{Hz}}$, and operation in magnetic fields up to about 0.5 T at 4.2 K. Nanopatterning via Ne- or He-FIB allows for the incorporation of a modulation line for coupling magnetic flux into the SQUID or a third Josephson junction for shifting its phase. Such advanced functionality, combined with high spatial resolution, large magnetic field range, and the ease of use of a cantilever-based scanning probe, extends the applicability of scanning SQUID microscopy to a wide range of magnetic, normal conducting, superconducting, and quantum Hall systems. We demonstrate magnetic imaging of skyrmions at the surface of bulk Cu$_2$OSeO$_3$. Analysis of the point spread function determined from imaging a single skyrmion yields a full-width-half-maximum of 87 nm. Moreover, we image modulated magnetization patterns with a period of 65 nm.

cond-mat.supr-con↗