An unconventional superconducting state was recently discovered in UTe2, where spin-triplet superconductivity emerges from the paramagnetic normal state of a heavy fermion material. The coexistence of magnetic fluctuations and superconductivity, together with the crystal structure of this material, suggest that a unique set of symmetries, magnetic properties, and topology underlie the superconducting state. Here, we report observations of a non-zero polar Kerr effect and of two transitions in the specific heat upon entering the superconducting state, which together suggest that the superconductivity in UTe2 is characterized by a two-component order parameter that breaks time reversal symmetry. These data place constraints on the symmetries of the order parameter and inform the discussion on the presence of topological superconductivity in UTe2.

Muon relaxation experiments reveal a slowly fluctuating magnetic field in the pseudogap phase of a cuprate superconductor.

Background: Much less is known about neutron structure than that of the proton due to the absence of free neutron targets. Neutron information is usually extracted from data on nuclear targets such as deuterium, requiring corrections for nuclear binding and nucleon off-shell effects. These corrections are model dependent and have significant uncertainties, especially for large values of the Bjorken scaling variable x. As a consequence, the same data can lead to different conclusions, for example, about the behavior of the d quark distribution in the proton at large x.Purpose: The Barely Off-shell Nucleon Structure (BONuS) experiment at Jefferson Lab measured the inelastic electron-deuteron scattering cross section, tagging spectator protons in coincidence with the scattered electrons. This method reduces nuclear binding uncertainties significantly and has allowed for the first time a (nearly) model-independent extraction of the neutron structure function F2(x, Q 2 ) in the resonance and deep-inelastic regions.Method: A novel compact radial time projection chamber was built to detect protons with momentum between 70 and 150 MeV/c and over a nearly 4π angular range. For the extraction of the free-neutron structure function F n 2 , spectator protons at backward angles (> 100• relative to the momentum transfer) and with momenta below 100 MeV/c were selected, ensuring that the scattering took place on a nearly free neutron. The scattered electrons were detected with Jefferson Lab's CLAS spectrometer, with data taken at beam energies near 2, 4 and 5 GeV. Results:The extracted neutron structure function F n 2 and its ratio to the inclusive deuteron structure function F d 2 are presented in both the resonance and deep-inelastic regions for momentum transfer squared Q 2 between 0.7 and 5 GeV 2 /c 2 , invariant mass W between 1 and 2.7 GeV/c 2 , and Bjorken x between 0.25 and 0.6 (in the DIS region). The dependence of the semi-inclusive cross section on the spectator proton momentum and angle is investigated, and tests of the spectator mechanism for different kinematics are performed.Conclusions: Our data set on the structure function ratio F n 2 /F d 2 can be used to study neutron resonance excitations, test quark-hadron duality in the neutron, develop more precise parametrizations of structure functions, as well as investigate binding effects (including possible mechanisms for the nuclear EMC effect) and provide a first glimpse of the asymptotic behavior of d/u at x → 1.

According to conventional wisdom, the extraordinary properties of the cuprate high-temperature superconductors arise from doping a strongly correlated antiferromagnetic insulator. The highly overdoped cuprates—whose doping lies beyond the dome of superconductivity—are considered to be conventional Fermi liquid metals. We report the emergence of itinerant ferromagnetic order below 4 kelvin for doping beyond the superconducting dome in thin films of electron-doped La2–xCexCuO4 (LCCO). The existence of this ferromagnetic order is evidenced by negative, anisotropic, and hysteretic magnetoresistance, hysteretic magnetization, and the polar Kerr effect, all of which are standard signatures of itinerant ferromagnetism in metals. This surprising result suggests that the overdoped cuprates are strongly influenced by electron correlations.

Bulk FeSe is superconducting with a critical temperature Tc ∼ = 8 K and SrTiO3 is insulating in nature, yet high-temperature superconductivity has been reported at the interface between a single-layer FeSe and SrTiO3. Angle resolved photoemission spectroscopy and scanning tunneling microscopy measurements observe a gap opening at the Fermi surface below ≈ 60 K. Elucidating the microscopic properties and understanding the pairing mechanism of single-layer FeSe is of utmost importance as it is a basic building block of iron-based superconductors. Here, we use the low-energy muon spin rotation/relaxation technique (LE-µSR) to detect and quantify the supercarrier density and determine the gap symmetry in FeSe grown on SrTiO3 (100). Measurements in applied field show a temperature dependent broadening of the field distribution below ∼ 60 K, reflecting the superconducting transition and formation of a vortex state. Zero field measurements rule out the presence of magnetism of static or fluctuating origin. From the inhomogeneous field distribution, we determine an effective sheet supercarrier density n 2D s 6 × 10 14 cm −2 at T → 0 K, which is a factor of 4 larger than expected from ARPES measurements of the excess electron count per Fe of 1 monolayer (ML) FeSe. The temperature dependence of the superfluid density ns(T ) can be well described down to ∼ 10 K by simple s-wave BCS, indicating a rather clean superconducting phase with a gap of 10.2(1.1) meV. The result is a clear indication of the gradual formation of a two dimensional vortex lattice existing over the entire large FeSe/STO interface and provides unambiguous evidence for robust superconductivity below 60 K in ultrathin FeSe.

Hidden magnetic order in the correlated iridate Sr 2 Ir 1−x Rh x O 4 , x = 0.05 and 0.1, has been studied using muon spin relaxation spectroscopy. In zero field (ZF) and weak longitudinal fields (LFs) ( 2 mT), the muon spin relaxation data indicate that static and dynamic local fields coexist at each muon site, and can be well described by exponentially damped static Lorentzian Kubo-Toyabe functions. The ZF relaxation rate is dominated by the static-field distribution, and a broad relaxation rate maximum at 175 K for x = 0.1 in ZF is attributed to muon diffusion and trapping. For LF 2 mT the static rate is completely decoupled, and the exponential decay is due to dynamic spin fluctuations. The temperature dependencies of the relaxation rates exhibit maxima at 215 K (x = 0.05) and 175 K (x = 0.1), in agreement with previous second harmonic generation and polarized neutron diffraction determinations of transition temperatures to a hidden-order state. The maxima are most likely due to critical slowing down of electronic spin fluctuations. The field dependencies of the dynamic spin fluctuation rates can be well described by the Redfield relation, from which the rms width B rms loc and correlation time τ c of the fluctuating field are obtained. Values of τ c are in the range of 1.5-4 ns for x = 0.1 and shorter than 2 ns for x = 0.05, suggesting an increase with increasing Rh concentration. Values of B rms loc are on the order of 1 mT, consistent with the polarized neutron diffraction cross section.

We report results of a muon spin rotation (µSR) study of the cuprate-analog nickelate La4Ni3O8, which undergoes a transition at 105 K to a low-temperature phase with charge-stripe and antiferromagnetic (AFM) order on square planar NiO2 layers. Zero-field µSR shows that the AFM transition is abrupt, commensurate and has a quasi-2D character below ∼25 K. Comparison of observed muon precession frequencies with Ni dipolar field calculations yields Ni moments 0.5µB . Dynamic muon spin relaxation above 105 K suggests critical slowing of Ni spin fluctuations, but is inconsistent with corresponding 139 La NMR results. Critical slowing and an abrupt transition are also observed in the planar cuprate AFM La2CuO 4+δ , where they are taken as evidence for weakly interplanar-coupled two-dimensional AFM spin fluctuations, but our µSR data do not agree quantitatively with theoretical predictions for this scenario when applied to the nickelate.

No abstract