arXiv subjects

K. Hasselbach

Publications and source records attributed to K. Hasselbach.

13 recordsLinked to original sources

arXiv2025

Visualization of vortex sheets and half quantum vortices in the chiral odd-parity superconductor UPt$_{3}$

Superconductivity is characterized by vanishing electrical resistance and magnetic flux expulsion. For conventional type II superconductors, the magnetic flux expulsion is incomplete in an applied magnetic field above a critical value and magnetic flux penetrates the bulk of the superconductor in discrete quantized magnetic flux tubes (vortices), each carrying a single quantum of flux (h/2e). Investigating the unconventional superconductor UPt$_{3}$ with a scanning superconducting quantum interference device (SQUID) microscope, we observed mobile half-quantum vortices together with one quantum vortices. Cooling the material under a higher magnetic field revealed the presence of lines of magnetic contrast resembling domain walls. These observations agree with theoretical predictions for chiral superconductivity with a two dimensional complex order-parameter with sheets of half-quantum vortices separating domains of opposite order-parameter chirality.

cond-mat.supr-con↗

arXiv2020

Visualization by scanning SQUID microscopy of the intermediate state in the superconducting Dirac semimetal PdTe${}_2$

The Dirac semimetal PdTe${}_2$ becomes superconducting at a temperature $T_{c}=1.6$~K. Thermodynamic and muon spin rotation experiments support type-I superconductivity, which is unusual for a binary compound. A key property of a type-I superconductor is the intermediate state which presents a coexistence of superconducting and normal domains (flux structures) at magnetic fields lower than the thermodynamic critical field $H_{c}$. By means of scanning SQUID microscopy (SSM) we observe flux structures in the superconducting state of PdTe${}_2$. The flux structures are strongly history dependent with a transition from round shapes to laminar shapes as the magnetic field is more and more increased. The field amplitudes measured at the surface are indicative for the presence of Landau branching. The domain wall width in the intermediate state has been determined.

cond-mat.supr-con↗

arXiv2018

Quantitative magneto-optical investigation of superconductor/ferromagnet hybrid structures

We present a detailed quantitative magneto-optical imaging study of several superconductor/ferromagnet hybrid structures, including Nb deposited on top of thermomagnetically patterned NdFeB, and permalloy/niobium with erasable and tailored magnetic landscapes imprinted in the permalloy layer. The magneto-optical imaging data is complemented with and compared to scanning Hall probe microscopy measurements. Comprehensive protocols have been developed for calibrating, testing, and converting Faraday rotation data to magnetic field maps. Applied to the acquired data, they reveal the comparatively weaker magnetic response of the superconductor from the background of larger fields and field gradients generated by the magnetic layer.

cond-mat.supr-con↗

arXiv2008

Measurement of the Current-Phase Relation in Josephson Junctions Rhombi Chains

We present low temperature transport measurements in one dimensional Josephson junctions rhombi chains. We have measured the current phase relation of a chain of 8 rhombi. The junctions are either in the classical phase regime with the Josephson energy much larger than the charging energy, $E_{J}\gg E_{C}$, or in the quantum phase regime where $E_{J}/E_{C}\approx 2$. In the strong Josephson coupling regime ($E_{J}\gg E_{C} \gg k_{B}T$) we observe a sawtooth-like supercurrent as a function of the phase difference over the chain. The period of the supercurrent oscillations changes abruptly from one flux quantum $Φ_{0}$ to half the flux quantum $Φ_{0}/2$ as the rhombi are tuned in the vicinity of full frustration. The main observed features can be understood from the complex energy ground state of the chain. For $E_{J}/E_{C}\approx 2$ we do observe a dramatic suppression and rounding of the switching current dependence which we found to be consistent with the model developed by Matveev et al.(Phys. Rev. Lett. {\bf 89}, 096802(2002)) for long Josephson junctions chains.

cond-mat.mes-hall↗

arXiv2006

Observation of Vortex Coalescence, Vortex Chains and Crossing Vortices in the Anisotropic Spin-Triplet Superconductor $Sr_2 Ru O_4$

Scanning $μ$SQUID force microscopy is used to study magnetic flux structures in single crystals of the layered spin triplet superconductor Sr$\_{2}$RuO$\_{4}$. Images of the magnetic flux configuration above the $\vec{a}\vec{b}$-face of the cleaved crystal are acquired, mostly after field-cooling the sample. For low applied magnetic fields, individual vortices are observed, each carrying a single quantum of flux. Above 1 gauss, coalescence of vortices is discovered. The coalescing vortices may indicate the presence of domains of a chiral order parameter. When the applied field is tilted from the $\vec{c}$-axis, we observe a gradual transition from vortex domains to vortex chains. The in-plane component of the applied magnetic field transforms the vortex domains to vortex chains by aligning them along the field direction.This behavior and the inter-chain distance varies in qualitative agreement with the Ginzburg Landau theory of anisotropic 3D superconductors. The effective mass anisotropy of Sr$\_{2}$RuO$\_{4}$, $γ$=20, is the highest observed in three dimensional superconductors. When the applied field is closely in plane, the vortex form flux channels confined between the crystal-layers. Residual Abrikosov vortices are pinned preferentially on these channels. Thus the in-plane vortices are decorated by crossing Abrikosov vortices: two vortex orientations are apparent simultaneously, one along the layers and the other perpendicular to the layers.

cond-mat.supr-con↗

arXiv2006

Observation of two species of vortices in the anisotropic spin-triplet superconductor $Sr_2 Ru O_4$

Magnetic flux structures in single crystals of the layered spin triplet superconductor Sr$\_{2}$RuO$\_{4}$ are studied by scanning micro SQUID Force microscopy. Vortex chains appear as the applied field is tilted along the in-plane direction of the superconductor. The vortex chains align along the direction of the in-plane component of the applied magnetic field. The decoration of in-plane vortices by crossing Abrikosov vortices is observed: two vortex orientations are apparent simultaneously, one along the layers and the other perpendicular to the layers. The crossing vortices appear preferentially on the in-plane vortices.

cond-mat.supr-con↗

arXiv2004

Observation of vortex coalescence in the anisotropic spin-triplet superconductor Sr$_{2}$RuO$_{4}$

We present direct imaging of magnetic flux structures in the anisotropic, spin-triplet superconductor Sr$_{2}$RuO$_{4}$ using a scanning $μ$SQUID microscope. Individual quantized vortices were seen at low magnetic fields. Coalescing vortices forming flux domains were revealed at intermediate fields. Based on our observations we suggest that a mechanism intrinsic to the material stabilizes the flux domains against the repulsive vortex-vortex interaction. Topological defects like domain walls can provide this, implying proof for unconventional chiral superconductivity.

cond-mat.supr-con↗

arXiv2003

Electron transport in a mesoscopic superconducting / ferromagnetic hybrid conductor

We present electrical transport experiments performed on submicron hybrid devices made of a ferromagnetic conductor (Co) and a superconducting (Al) electrode. The sample was patterned in order to separate the contributions of the Co conductor and of the Co-Al interface. We observed a strong influence of the Al electrode superconductivity on the resistance of the Co conductor. This effect is large only when the interface is highly transparent. We characterized the dependence of the observed resistance decrease on temperature, bias current and magnetic field. As the differential resistance of the ferromagnet exhibits a non-trivial asymmetry, we claim that the magnetic domain structure plays an important role in the electron transport properties of superconducting / ferromagnetic conductors.

cond-mat.mes-hall↗

arXiv2001

Micro-SQUID characteristics

We report on the dependence on field and temperature of the critical current of micro-SQUIDs: SQUIDs with diameters as small as 1 micron, using Dayem bridges as weak links. We model these SQUIDs by solving the Ginzburg-Landau equations with appropriate boundary conditions to obtain the supercurrent-phase relationships. These solutions show that the phase drops and depression of the order parameter produced by supercurrent flow are often distributed throughout the micro-SQUID structure, rather than being localized in the bridge area, for typical micro-SQUID geometries and coherence lengths. The resultant highly non-sinusoidal current-phase relationships $I_c(ϕ)$ lead to reduced modulation depths and triangular dependences of the micro-SQUID critical currents on applied magnetic flux $I_c(Φ)$. Our modelling agrees well with our measurements on both Al and Nb micro-SQUIDs.

cond-mat.supr-con↗

arXiv2001

Scanning microSQUID Force Microscope

A novel scanning probe technique is presented: Scanning microSQUID Force microscopy (SSFM). The instrument features independent topographic and magnetic imaging. The SSFM operates in a dilution refrigerator in cryogenic vacuum. Sample and probe can be cooled to 0.45 K. The probe consists of a microSQUID placed at the edge of a silicon chip attached to a quartz tuning fork. A topographic vertical resolution of 0.02 micrometer is demonstrated and magnetic flux as weak as $10^{-3} Φ_{0}$ is resolved with a 1 micrometer diameter microSQUID loop.

cond-mat.supr-con↗

arXiv2000

Observation of Persistent Currents in Mesoscopic Connected Rings

We report measurements of the low temperature magnetic response of a line of 16 GaAs/GaAlAs \emph{connected} mesoscopic rings whose total length is much larger than $l_ϕ$. Using an on-chip micro-\textsc{squid} technology, we have measured a periodic response, with period $h/e$, corresponding to persistent currents in the rings of typical amplitude $0.40\pm 0.08 nA$ per ring. Direct comparison with measurements on the same rings but \emph{isolated} is presented.

cond-mat.mes-hall↗

arXiv1998

Superconducting proximity effect in a mesoscopic ferromagnetic wire

We present an experimental study of the transport properties of a ferromagnetic metallic wire (Co) in metallic contact with a superconductor (Al). As the temperature is decreased below the Al superconducting transition, the Co resistance exhibits a significant dependence on both temperature and voltage. The differential resistance data show that the decay length for the proximity effect is much larger than we would simply expect from the exchange field of the ferromagnet.

cond-mat↗

arXiv1997

Channel Interference in a Quasi Ballistic Aharonov-Bohm Experiment

New experiments are presented on the transmission of electron waves through a 2DEG (2 dimensional electron gas) ring with a gate on top of one of the branches. Magnetoconductance oscillations are observed, and the phase of the Aharanov-Bohm signal alternates between 0 and pi as the gate voltage is scanned. A Fourier transform of the data reveals a dominant period in the voltage which corresponds to the energy spacing between successive transverse modes.A theoretical model including random phase shifts between successive modes reproduces the essential features of the experiment.

cond-mat.mes-hall↗