Superconducting Quantum Interference in Twisted van der Waals Heterostructures

Modern Superconducting QUantum Interference Devices (SQUIDs) are commonly fabricated from either Al or Nb electrodes, with an in-situ oxidation process to create a weak link between them. However, common problems of such planar nano- and micro-SQUIDs are hysteretic current-voltage curves, and a shallow flux modulation depth. Here, we demonstrate the formation of both Josephson junctions and SQUIDs using a dry transfer technique to stack and deterministically misalign flakes of NbSe$_{2}$; allowing one to overcome these issues. The Josephson dynamics of the resulting twisted NbSe$_{2}$-NbSe$_{2}$ junctions are found to be sensitive to the misalignment angle of the crystallographic axes. A single lithographic process was then implemented to shape the Josephson junction into a SQUID geometry with typical loop areas of $\simeq$ 25 $\mu m^{2}$ and weak links $\simeq$ 600 nm wide. These devices display large stable current and voltage modulation depths of up to $\Delta I_{c} \simeq$ 75$\%$ and $\Delta V \simeq$ 1.4 mV respectively.Comment: 12 pages, 6 figure

Suppression of superconductivity and enhanced critical field anisotropy in thin flakes of FeSe

FeSe is a unique superconductor that can be manipulated to enhance its superconductivity using different routes, while ist monolayer form grown on different substrates reaches a record high temperature for a two-dimensional system. In order to understand the role played by the substrate and the reduced dimensionality on superconductivity, we examine the superconducting properties of exfoliated FeSe thin flakes by reducing the thickness from bulk down towards 9 nm. Magnetotransport measurements performed in magnetic fields up to 16 T and temperatures down to 2 K help to build up complete superconducting phase diagrams of different thickness flakes. While the thick flakes resemble the bulk behaviour, by reducing the thickness the superconductivity of FeSe flakes is suppressed. The observation of the vortex-antivortex unbinding transition in different flakes provide a direct signature of a dominant two-dimensional pairing channel. However, the upper critical field reflects the evolution of the multi-band nature of superconductivity in FeSe becoming highly two-dimensional and strongly anisotropic only in the thin limit. Our study provides detailed insights into the evolution of the superconducting properties of a multi-band superconductor FeSe in the thin limit in the absence of a dopant substrate

Unconventional localization of electrons inside of a nematic electronic phase

The magnetotransport behavior inside the nematic phase of bulk FeSe reveals unusual multiband effects that cannot be reconciled with a simple two-band approximation proposed by surface-sensitive spectroscopic probes. In order to understand the role played by the multiband electronic structure and the degree of two-dimensionality, we have investigated the electronic properties of exfoliated flakes of FeSe by reducing their thickness. Based on magnetotransport and Hall resistivity measurements, we assess the mobility spectrum that suggests an unusual asymmetry between the mobilities of the electrons and holes, with the electron carriers becoming localized inside the nematic phase. Quantum oscillations in magnetic fields up to 38 T indicate the presence of a hole-like quasiparticle with a lighter effective mass and a quantum scattering time three times shorter, as compared with bulk FeSe. The observed localization of negative charge carriers by reducing dimensionality can be driven by orbitally dependent correlation effects, enhanced interband spin fluctuations, or a Lifshitz-like transition, which affect mainly the electron bands. The electronic localization leads to a fragile two-dimensional superconductivity in thin flakes of FeSe, in contrast to the two-dimensional high-Tc induced with electron doping via dosing or using a suitable interface.</p

Unconventional localization of electrons inside of a nematic electronic phase

The magnetotransport behaviour inside the nematic phase of bulk FeSe reveals unusual multiband effects that cannot be reconciled with a simple two-band approximation proposed by surface-sensitive spectroscopic probes. In order to understand the role played by the multiband electronic structure and the degree of two-dimensionality we have investigated the electronic properties of exfoliated flakes of FeSe by reducing their thickness. Based on magnetotransport and Hall resistivity measurements, we assess the mobility spectrum that suggests an unusual asymmetry between the mobilities of the electrons and holes with the electron carriers becoming localized inside the nematic phase. Quantum oscillations in magnetic fields up to 38 T indicate the presence of a hole-like quasiparticle with a lighter effective mass and a quantum scattering time three times shorter, as compared with bulk FeSe. The observed localization of negative charge carriers by reducing dimensionality can be driven by orbitally-dependent correlation effects, enhanced interband spin-fluctuations or a Lifshitz-like transition which affect mainly the electron bands. The electronic localization leads to a fragile two-dimensional superconductivity in thin flakes of FeSe, in contrast to the two-dimensional high-Tc induced with electron doping via dosing or using a suitable interface.Comment: 22 pages, 14 figure

BHPR research: qualitative1. Complex reasoning determines patients' perception of outcome following foot surgery in rheumatoid arhtritis

Background: Foot surgery is common in patients with RA but research into surgical outcomes is limited and conceptually flawed as current outcome measures lack face validity: to date no one has asked patients what is important to them. This study aimed to determine which factors are important to patients when evaluating the success of foot surgery in RA Methods: Semi structured interviews of RA patients who had undergone foot surgery were conducted and transcribed verbatim. Thematic analysis of interviews was conducted to explore issues that were important to patients. Results: 11 RA patients (9 ♂, mean age 59, dis dur = 22yrs, mean of 3 yrs post op) with mixed experiences of foot surgery were interviewed. Patients interpreted outcome in respect to a multitude of factors, frequently positive change in one aspect contrasted with negative opinions about another. Overall, four major themes emerged. Function: Functional ability & participation in valued activities were very important to patients. Walking ability was a key concern but patients interpreted levels of activity in light of other aspects of their disease, reflecting on change in functional ability more than overall level. Positive feelings of improved mobility were often moderated by negative self perception ("I mean, I still walk like a waddling duck”). Appearance: Appearance was important to almost all patients but perhaps the most complex theme of all. Physical appearance, foot shape, and footwear were closely interlinked, yet patients saw these as distinct separate concepts. Patients need to legitimize these feelings was clear and they frequently entered into a defensive repertoire ("it's not cosmetic surgery; it's something that's more important than that, you know?”). Clinician opinion: Surgeons' post operative evaluation of the procedure was very influential. The impact of this appraisal continued to affect patients' lasting impression irrespective of how the outcome compared to their initial goals ("when he'd done it ... he said that hasn't worked as good as he'd wanted to ... but the pain has gone”). Pain: Whilst pain was important to almost all patients, it appeared to be less important than the other themes. Pain was predominately raised when it influenced other themes, such as function; many still felt the need to legitimize their foot pain in order for health professionals to take it seriously ("in the end I went to my GP because it had happened a few times and I went to an orthopaedic surgeon who was quite dismissive of it, it was like what are you complaining about”). Conclusions: Patients interpret the outcome of foot surgery using a multitude of interrelated factors, particularly functional ability, appearance and surgeons' appraisal of the procedure. While pain was often noted, this appeared less important than other factors in the overall outcome of the surgery. Future research into foot surgery should incorporate the complexity of how patients determine their outcome Disclosure statement: All authors have declared no conflicts of interes

Dataset for "Superconducting Quantum Interference in Twisted van der Waals Heterostructures"

The dataset represents the results of electrical measurement that underpin the figures in the main manuscript and supplementary materials of "Superconducting Quantum Interference in Twisted van der Waals Heterostructures" Nano Lett. 2021, 21, 6725−6731. This manuscript describes the use of dry stamping technologies to create 2H-NbSe2 van der Waals heterostructures with twisted interfaces. We show that these behave as Josephson junctions whose I-V characteristics depend systematically on twist angle. We have developed a fabrication protocol to pattern these into Superconducting QUantum Interferometer Device (SQUID) structures containing two junctions coupled in a superconducting loop, and show that these behave as high quality DC SQUIDs with high current modulation levels in an applied an magnetic field. The archived data represent current-voltage characteristics of our Josephson Junctions and SQUIDs as a function of temperature, magnetic field and twist angle. These have been captured to establish the damping regime the junctions are operating in and to characterise the performance of SQUIDs fabricated from them.Data either represent the raw experimental datafiles as captured with our measurement apparatus, or as parameter extracted directly from these raw data files using standard fitting practices.N/AData were collected using a Keithley 2400 source measure unit and a Kepco BOP 50-8 bipolar power supply controlled with LabVIEW code. Samples were cooled in a JANIS Research SHI-4-1 crocooler system pumped with a Leybold TURBOLAB80-FF-ISO pump.All data uploaded as readily accessible text files

Suppression of superconductivity and enhanced critical field anisotropy in thin flakes of FeSe

Contains fulltext : 224975.pdf (publisher's version ) (Open Access

Dataset for "Superconducting Quantum Interference in Twisted van der Waals Heterostructures"

The dataset represents the results of electrical measurement that underpin the figures in the main manuscript and supplementary materials of "Superconducting Quantum Interference in Twisted van der Waals Heterostructures" Nano Lett. 2021, 21, 6725−6731. This manuscript describes the use of dry stamping technologies to create 2H-NbSe2 van der Waals heterostructures with twisted interfaces. We show that these behave as Josephson junctions whose I-V characteristics depend systematically on twist angle. We have developed a fabrication protocol to pattern these into Superconducting QUantum Interferometer Device (SQUID) structures containing two junctions coupled in a superconducting loop, and show that these behave as high quality DC SQUIDs with high current modulation levels in an applied an magnetic field. The archived data represent current-voltage characteristics of our Josephson Junctions and SQUIDs as a function of temperature, magnetic field and twist angle. These have been captured to establish the damping regime the junctions are operating in and to characterise the performance of SQUIDs fabricated from them

Dataset for "Superconducting Quantum Interference in Twisted van der Waals Heterostructures"

The dataset represents the results of electrical measurement that underpin the figures in the main manuscript and supplementary materials of "Superconducting Quantum Interference in Twisted van der Waals Heterostructures" Nano Lett. 2021, 21, 6725−6731. This manuscript describes the use of dry stamping technologies to create 2H-NbSe2 van der Waals heterostructures with twisted interfaces. We show that these behave as Josephson junctions whose I-V characteristics depend systematically on twist angle. We have developed a fabrication protocol to pattern these into Superconducting QUantum Interferometer Device (SQUID) structures containing two junctions coupled in a superconducting loop, and show that these behave as high quality DC SQUIDs with high current modulation levels in an applied an magnetic field. The archived data represent current-voltage characteristics of our Josephson Junctions and SQUIDs as a function of temperature, magnetic field and twist angle. These have been captured to establish the damping regime the junctions are operating in and to characterise the performance of SQUIDs fabricated from them