A gradual spin-state transition occurs in LaCoO3 around T approximately 80-120 K, whose detailed nature remains controversial. We studied this transition by means of inelastic neutron scattering and found that with increasing temperature an excitation at approximately 0.6 meV appears, whose intensity increases with temperature, following the bulk magnetization. Within a model including crystal-field interaction and spin-orbit coupling, we interpret this excitation as originating from a transition between thermally excited states located about 120 K above the ground state. We further discuss the nature of the magnetic excited state in terms of intermediate-spin (t(2g)(5)e(g)(1), S=1) versus high-spin (t(2g)(4)e(g)(2), S=2) states. Since the g factor obtained from the field dependence of the inelastic neutron scattering is g approximately 3, the second interpretation is definitely favored.

The kagome lattice1, which is the most prominent structural motif in quantum physics, benefits from inherent non-trivial geometry so that it can host diverse quantum phases, ranging from spin-liquid phases, to topological matter, to intertwined orders2, 3,4,5,6,7,8 and, most rarely, to unconventional su-perconductivity6,9. Recently, charge sensitive probes have indicated that the kagome superconductors AV3Sb5 (A = K, Rb, Cs)9,10,11 exhibit unconventional chiral charge order12, 13,14,15,16,17,18,19, which is analogous to the long-sought-after quantum order in the Haldane model20 or Varma model21. However, direct evidence for the time-reversal symmetry breaking of the charge order remains elusive. Here we use muon spin relaxation to probe the kagome charge order and superconductivity in KV3Sb5. We observe a noticeable enhancement of the internal field width sensed by the muon ensemble, which takes place just below the charge ordering temperature and persists into the superconducting state. Notably, the muon spin relaxation rate below the charge ordering temperature is substantially enhanced by applying an external magnetic field. We further show the multigap nature of superconductivity in KV3Sb5 and that the Tc/−2ab ratio (where Tc is the superconducting transition temperature and ab is the magnetic penetration depth in the kagome plane) is comparable to those of unconventional high-temperature superconductors. Our results point to time-reversal symmetry-breaking charge order intertwining with unconventional superconductivity in the correlated kagome lattice.

The physical mechanisms responsible for the formation of a two-dimensional electron gas at the interface between insulating srTio 3 and LaAlo 3 have remained a contentious subject since its discovery in 2004. opinion is divided between an intrinsic mechanism involving the build-up of an internal electric potential due to the polar discontinuity at the interface between srTio 3 and LaAlo 3 , and extrinsic mechanisms attributed to structural imperfections. Here we show that interface conductivity is also exhibited when the LaAlo 3 layer is diluted with srTio 3 , and that the threshold thickness required to show conductivity scales inversely with the fraction of LaAlo 3 in this solid solution, and thereby also with the layer's formal polarization. These results can be best described in terms of the intrinsic polar-catastrophe model, hence providing the most compelling evidence, to date, in favour of this mechanism.

To trace the origin of time-reversal symmetry breaking (TRSB) in Re-based superconductors, we performed comparative muon-spin rotation/relaxation (µSR) studies of superconducting noncentrosymmetric Re 0.82 Nb 0.18 (T c = 8.8 K) and centrosymmetric Re (T c = 2.7 K). In Re 0.82 Nb 0.18 , the low-temperature superfluid density and the electronic specific heat evidence a fully-gapped superconducting state, whose enhanced gap magnitude and specific-heat discontinuity suggest a moderately strong electron-phonon coupling. In both Re 0.82 Nb 0.18 and pure Re, the spontaneous magnetic fields revealed by zero-field µSR below T c indicate time-reversal symmetry breaking and thus unconventional superconductivity. The concomitant occurrence of TRSB in centrosymmetric Re and noncentrosymmetric ReT (T = transition metal), yet its preservation in the isostructural noncentrosymmetric superconductors Mg 10 Ir 19 B 16 and Nb 0.5 Os 0.5 , strongly suggests that the local electronic structure of Re is crucial for understanding the TRSB superconducting state in Re and ReT . We discuss the superconducting order parameter symmetries that are compatible with the observations. Time reversal and spatial inversion are two key symmetries which influence at a fundamental level the electron pairing in the superconducting state: on the one hand, a number of unconventional superconductors exhibit spontaneous time-reversal symmetry breaking (TRSB) on entering the superconducting state; on the other hand, the absence of inversion symmetry above T c leads to an antisymmetric spin-orbit coupling (SOC), lifting the degeneracy of the conduction-band electrons and potentially giving rise to a mixed-parity superconducting state [1,2]. Some noncentrosymmetric superconductors (NCSC), such as CePt 3 Si [3], CeIrSi 3 [4], Li 2 Pt 3 B [5, 6], and K 2 Cr 3 As 3 [7, 8], exhibit line nodes in the gap, while others such as LaNiC 2 [9] and (La,Y) 2 C 3 [10], show multiple nodeless superconducting gaps. In addition, due to the strong influence of SOC, their upper critical field can greatly exceed the Pauli limit, as has been found in CePt 3 Si [11] and very recently in (Ta,Nb)Rh 2 B 2 [12]. In general, TRSB below T c and a lack of spatial-inversion symmetry of the crystal structure are independent events. Yet, in a few cases, such as in LaNiC 2 [13], La 7 Ir 3 [14], and, in particular, in the Re-based compounds Re 6 Zr [15], Re 6 Hf [16], Re 6 Ti [17], and Re 24 Ti 5 [18], TRSB below T c is concomitant with an existing lack of crystal inversion symmetry. Such an unusually frequent occurrence of TRSB among the superconducting ReT binary alloys (T = transition metal) is rather puzzling. Its persistence independent of the particular transition metal, points to a key role played by Re. To test such a hypothesis, and to ascertain the possible relevance of the noncentrosymmetric structure to TRSB in Re-based NCSC, we proceeded with a twofold study. On one hand we synthesized and investigated an-other Re-based NCSC, Re 0.82 Nb 0.18 . On the other hand, we considered the ...