Multiband superconductivity arises when multiple electronic bands contribute to the formation of the superconducting state, allowing distinct pairing interactions and gap structures. Here, we present field- and temperature-dependent data on the vortex lattice structure in 2H-NbSe_{2} as a contribution to the ongoing debate as to whether the defining feature of the superconductivity is the anisotropy or the multiband nature. The field-dependent data clearly show that there are two distinct superconducting bands, and the contribution of one of them to the vortex lattice signal is completely suppressed for magnetic fields above ∼0.8  T, well below B_{c2}. By combining the temperature and field scans, we can deduce that there is a moderate degree of interband coupling. From the observed temperature dependences, we find that at low field and zero temperature, the two gaps in temperature units are 13.1±0.2 and 6.5±0.3  K (Δ_{0}=1.88 and 0.94 k_{B}T_{c}); the band with the larger gap gives just under two-thirds of the superfluid density. The penetration depth extrapolated to zero field and zero temperature is 160±2  nm.

We report on neutron scattering measurements on the vortex lattice of the noncentrosymmetric superconductor BiPd. We observe the existence of the intermediate mixed state, a region where Meissner and vortex lattice phases coexist, which is a feature of low $\kappa$ Type-II superconductors. Following this, we obtain an estimate of the value of $\kappa$ using the extended London model, which confirms the expectation that $\kappa$ should be small. Finally, we find that the temperature dependence of the vortex lattice form factor fits well to a model designed to describe singlet-triplet mixing in non-centrosymmetric superconductors, which may shed light on the question of the gap structure in BiPd.

2H−NbS2 is a classic example of an anisotropic multiband superconductor, with significant recent work focusing on the interesting responses seen when high magnetic fields are applied precisely parallel to the hexagonal niobium planes. It is often contrasted with its sister compound 2H−NbSe2 because they have similar onset temperatures for superconductivity, but 2H−NbS2 has no charge density wave whereas in 2H−NbSe2 the charge density wave order couples strongly to the superconductivity. Using small-angle neutron scattering, a bulk-sensitive probe, we have studied the vortex lattice and how it responds to the underlying superconducting anisotropy. This is done by controlling the orientation of the field with respect to the Nb planes. The superconducting anisotropy, Γac=7.07±0.2, is found to be field independent over the range measured (0.15 to 1.25 T), and the magnetic field distribution as a function of the applied magnetic field is found to be in excellent quantitative agreement with anisotropic London theory modified with a core-size cutoff correction, providing the first complete validation of this model. We find values of λab=141.9±1.5 nm for the in-plane London penetration depth, and λc∼1µm for the out-of-plane response. The field-independence indicates that we are primarily sampling the larger of the two gaps generating the superconductivity in this material. Published by the American Physical Society 2024

We present small-angle neutron scattering studies of the magnetic vortex lattice (VL) in Ca0.04Y0.96Ba2Cu3O7 up to a field of 16.7 T and Ca0.15Y0.85Ba2Cu3O7 up to 25 T to investigate the general behavior of the superconducting gap in YBCO-based compounds at high magnetic field. We find in these overdoped compounds that the series of VL structure transitions have shifted down in field relative to those reported for the undoped compound. The hole doping by calcium is expected to alter the Fermi velocity and it reduces the upper critical field of the system. However, we attribute the VL changes mainly to the weakening of the 1D superconductivity in the Cu-O chains by the disorder introduced by doping. The high-field structure of the VL is similar to recent measurements on the parent compound in even higher fields of 25 T, which indicates that the fundamental d-wave nature of the superconducting gap is unchanged by calcium doping. This is corroborated by the temperature dependence of the VL form factor, which also shows the same d-wave behavior as observed in other cuprates. We also argue that they might be the explanation of Pauli paramagnetic effects in the field dependence of the VL form factor.

We have carried out a search for a pair density wave signature using high-energy X-ray diffraction in fields up to 16 T. We do not see evidence for a signal at the predicted wavevector. This is a report on the details of our experiment, with information on where in reciprocal space we looked.

We outline the contributions and continuing influence that W. F. (Joe) Vinen has made to the science of superconductivity, with particular emphasis on the analogous behavior of superconductors and superfluids and the role of vortices/flux lines.

Some magnetic systems feature spin textures, real-space patterns in the orientation of spins that can topologically form non-trivial configurations. Among them, a vortex-like spin swirling texture known as a magnetic skyrmion has attracted particular attention. Lattices of skyrmions form in the helimagnet MnSi with a periodicity of 18 nm, which makes them amenable to investigation by Lorentz transmission electron microscopy in real space and by small-angle neutron scattering in momentum space. However, the dynamics of the skyrmion lattice are difficult to measure at the microelectronvolts energy scale at small wavevectors. Here we examine the low-energy excitations of the skyrmion state in MnSi by using the neutron spin-echo technique under small-angle neutron scattering conditions. We observe an asymmetric dispersion of the phason excitations of the lattice because of the string-like structure of the skyrmion cores.

We present small-angle neutron scattering studies of the magnetic vortex lattice (VL) in ${\mathrm{Ca}}_{0.04}{\mathrm{Y}}_{0.96}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ up to a field of 16.7 T and ${\mathrm{Ca}}_{0.15}{\mathrm{Y}}_{0.85}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ up to 25 T to investigate the general behavior of the superconducting gap in YBCO-based compounds at high magnetic field. We find in these overdoped compounds that the series of VL structure transitions have shifted down in field relative to those reported for the undoped compound. The hole doping by calcium is expected to alter the Fermi velocity and it reduces the upper critical field of the system. However, we attribute the VL changes mainly to the weakening of the 1D superconductivity in the Cu-O chains by the disorder introduced by doping. The high-field structure of the VL is similar to recent measurements on the parent compound in even higher fields of 25 T, which indicates that the fundamental $d$-wave nature of the superconducting gap is unchanged by calcium doping. This is corroborated by the temperature dependence of the VL form factor, which also shows the same $d$-wave behavior as observed in other cuprates. We also argue that they might be the explanation of Pauli paramagnetic effects in the field dependence of the VL form factor.

Small angle neutron scattering experiments have been carried out in the mixed state to determine the values and anisotropy of the penetration depth and coherence length in the superconductor beta-PdBi2. The distortion of the vortex lattice observed on changing the magnetic field direction suggests that the a-c anisotropy of penetration depth in beta-PdBi2 is small. With the magnetic field applied along the c-axis, we estimated the magnitudes of the penetration depth and coherence length by analyzing the magnetic field dependence of the neutron scattering intensity from the vortex lattice. The estimated superconductor parameters are consistent with the upper critical field obtained from the magnetization measurement.

Received 9 May 2022DOI:https://doi.org/10.1103/PhysRevB.105.219904Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasSuperconductivityVortex latticesTechniquesSmall angle neutron scatteringCondensed Matter, Materials & Applied Physics

E. Campillo, ∗ M. Bartkowiak, R. Riyat, E. Jellyman, A. S. Cameron, A. T. Holmes, O. Prokhnenko, W.-D. Stein, A. Erb, E. M. Forgan, and E. Blackburn Division of Synchrotron Radiation Research, Lund University, SE-22100 Lund, Sweden Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, United Kingdom Institut für Festkörperphysik und Materialphysik, Technische Universität Dresden, D-01069 Dresden, Germany European Spallation Source ERIC, P.O. Box 176, SE-221 00, Lund, Sweden Walther Meissner Institut, BAdW, D-85748 Garching, Germany (Dated: March 4, 2022)

Bragg diffracted intensities and q values for crystalline structures with long repeat distances may be obtained by small-angle neutron scattering (SANS) investigations. An account is given of the methods, advantages and disadvantages of obtaining such data by the multichromatic time-of-flight method, compared with the more traditional quasi-monochromatic SANS method. This is illustrated with data obtained from high-magnetic-field measurements on magnetic vortex line lattices in superconductors on the former HFM/EXED instrument at Helmholtz-Zentrum Berlin. The methods have application to other mesoscopic crystalline structures investigated by SANS instruments at pulsed sources.

The detection of variations of fundamental constants of the Standard Model would provide us with compelling evidence of new physics, and could lift the veil on the nature of dark matter and dark energy. In this work, we discuss how a network of atomic and molecular clocks can be used to look for such variations with unprecedented sensitivity over a wide range of time scales. This is precisely the goal of the recently launched QSNET project: A network of clocks for measuring the stability of fundamental constants. QSNET will include state-of-the-art atomic clocks, but will also develop next-generation molecular and highly charged ion clocks with enhanced sensitivity to variations of fundamental constants. We describe the technological and scientific aims of QSNET and evaluate its expected performance. We show that in the range of parameters probed by QSNET, either we will discover new physics, or we will impose new constraints on violations of fundamental symmetries and a range of theories beyond the Standard Model, including dark matter and dark energy models.

Specific heat and small-angle neutron scattering measurements have been performed on the superconductor fi-PdBi2. Whereas models with a uniform gap do not yield an adequate description of the temperature-dependent specific heat and the temperature and magnetic field dependence of the SANS intensity, fits to a heuristic model with two gap parameters agree well with both data sets. The fact that two parameters are required indicates a momentum-dependent gap anisotropy and=or gaps of different magnitude on different Fermi surface sheets. Fits to this model indicate that the ratio of the two gap parameters increases as a function of increasing field.

We describe designs for practical detectors of absolute rotation, which rely on the creation of magnetic fields by charged objects that are rotating with respect to an inertial frame. Our designs, motivated by an original suggestion by R.M. Brady, utilize the properties of superconductors, both to shield and confine the magnetic fields, and also as the basis of a SQUID detector of the fields produced. We show that with commercially available SQUIDs, our designs can have sufficient sensitivity and signal-to-noise ratio to measure the sidereal rate of rotation of the Earth. We consider three different designs: two of these can also be configured to provide a confirmation of the form that Maxwell's equations take in a rotating frame. We can also make a direct experimental test of whether low-frequency electromagnetic energy experiences the same inertial rest-frame as matter.

We present the results of a study of the vortex lattice in the heavy fermion superconductor CeCu2Si2, using small-angle neutron scattering (SANS). In this material at temperatures well below Tc similar to 0.6 K, the value of the upper critical field Bc2 similar to 2.2 T is strongly limited by the Pauli paramagnetism of the heavy fermions. In this temperature region, our SANS data show an increase in the magnetization of the flux line cores with field, followed by a rapid fall near Bc2. This behavior is the effect of Pauli paramagnetism and we present a theory-based model, which can be used to describe this effect in a range of materials. The pairing in CeCu2Si2 appears to arise from the effect of magnetic fluctuations, but the evidence for a d-wave order parameter is rather weak. We find that the vortex lattice structure in CeCu2Si2 is close to regular hexagonal. There are no phase transitions to square or rhombic structures; such transitions are expected for d-wave superconductors and observed in CeCoIn5; however, the temperature dependence of the SANS intensity indicates that both large and small gap values are present, most likely due to multiband s-wave superconductivity, rather than a nodal gap structure.

When the transition temperature of a continuous phase transition is tuned to absolute zero, new ordered phases and physical behaviour emerge in the vicinity of the resulting quantum critical point. Sr3Ru2O7 can be tuned through quantum criticality with magnetic field at low temperature. Near its critical field Bc it displays the hallmark T-linear resistivity and a [Formula: see text] electronic heat capacity behaviour of strange metals. However, these behaviours have not been related to any critical fluctuations. Here we use inelastic neutron scattering to reveal the presence of collective spin fluctuations whose relaxation time and strength show a nearly singular variation with magnetic field as Bc is approached. The large increase in the electronic heat capacity and entropy near Bc can be understood quantitatively in terms of the scattering of conduction electrons by these spin-fluctuations. On entering the spin-density-wave ordered phase present near Bc, the fluctuations become stronger suggesting that the order is stabilised through an "order-by-disorder" mechanism.

We present the results of a study of the vortex lattice (VL) of the nickel chalcogenide superconductor TlNi2Se2, using small angle neutron scattering. This superconductor has the same crystal symmetry as the iron arsenide materials. Previous work points to it being a two-gap superconductor, with an unknown pairing mechanism. No structural transitions in the vortex lattice are seen in the phase diagram, arguing against d-wave gap symmetry. Empirical fits of the temperature dependence of the form factor and penetration depth rule out a simple s-wave model, supporting the presence of nodes in the gap function. The variation of the VL opening angle with field is consistent with earlier reports of multiple gaps.

The charge density wave in the high-temperature superconductor YBa$_2$Cu$_3$O$_{7-x}$ (YBCO) is now known to have two different ordering tendencies differentiated by their $c$-axis correlations. These correspond to ferro- (F-CDW) and antiferro- (AF-CDW) couplings between CDW in neighbouring CuO$_2$ bilayers. This discovery has prompted a number of fundamental questions. For example, how does superconductivity adjust to two competing orders and are either of these orders responsible for the electronic reconstruction? Here we use high-energy x-ray diffraction to study YBa$_2$Cu$_3$O$_{6.67}$ as a function of magnetic field and temperature. We show that regions of the sample with F-CDW correlations suppress superconductivity more strongly than those with AF-CDW correlations. This implies that an inhomogeneous superconducting state exists, in which some regions show a weak or fragile form of superconductivity. By comparison of F-CDW and AF-CDW correlation lengths, it is furthermore concluded that F-CDW ordering is sufficiently long-range to modify the electronic structure. Our study thus suggests that F-CDW correlations have an important impact on superconducting and normal state properties of underdoped YBCO.