Rajib Sarkar

Zero-field muon-spin-rotation ( μ SR ) experiments and the observation of a spontaneous Nernst effect indicate broken time-reversal symmetry (BTRS) at T c Z 2 , slightly above the superconducting transition temperature T c in Ba 1 − x K x Fe 2 As 2 , with x ∼ 0.8 . This was interpreted as evidence of a distinct state of matter where BTRS above T c arises from noncondensed Cooper pairs, which maintain long-range interband correlations. However, an inhomogeneous superconducting state competing with magnetism as the origin of the observed behavior cannot be completely ruled out to date. Here, we present detailed As 75 nuclear magnetic resonance (NMR) measurements of Ba 1 − x K x Fe 2 As 2 , with x = 0.766 , yielding T c Z 2 > T c . The NMR data demonstrate that the superconductivity is homogeneous throughout the entire sample volume despite the occurrence of nanoclustering of Ba/K atoms. We observe a suppression of the spin-lattice relaxation rate 1 / T 1 T below the characteristic temperature T * , well above T c . The specific heat yields somewhat lower values of T * , consistent with the microscopic theory of electron quadrupling. The observed monotonous suppression of T * and T c Z 2 in applied magnetic field, the overall temperature dependence of 1 / T 1 T , the NMR spectral characteristics, and the μ SR Knight shift rule out spin magnetism and the proximity to a magnetic instability. We conclude that our data indicate the appearance of pairing fluctuations below T * and that the breaking of time-reversal symmetry represents an unconventional state which is unrelated to spin magnetism.

The exploration of tunable superconductivity in strongly correlated electron systems is a central pursuit in condensed matter physics, with implications for both fundamental understanding and potential applications. The Laves phase CeRu₂, a pyrochlore compound, exhibits a three-dimensional (3D) kagome lattice type geometry giving rise to flat bands and degenerate Dirac points, where band structure features intertwine with strong multi-orbital interaction effects deriving from its correlated electronic structure. Here, we combine muon spin rotation (μSR), uniaxial in-plane stress, and hydrostatic pressure to probe the superconducting state of CeRu₂. Uniaxial stress up to 0.22 GPa induces a dome-shaped evolution of the critical temperature T_c, with an initial plateau, successively followed by enhancement and suppression without any structural phase transition. Stress is further found to drive a crossover from anisotropic to isotropic s-wave pairing. In contrast, hydrostatic pressure up to 1.9 GPa leaves T_c largely unchanged but alters the superfluid density from exponential to linear behavior at low temperatures, indicative of nodal superconductivity under hydrostatic pressure. These findings identify CeRu₂ as a prime platform for multifold tuning of superconductivity in a 3D correlated material.

The cuprate superconductor La_2−xBa_xCuO₄ (LBCO) near x = 0.125 is a striking example of intertwined electronic orders, where 3D superconductivity is anomalously suppressed, allowing spin and charge stripe order to develop. Understanding this interplay remains a key challenge in cuprates, highlighting the necessity of external tuning for deeper insight. While in-plane uniaxial stress enhances superconductivity and suppresses stripe order, the effects of c-axis compression remains largely unexplored. Here, we use muon spin rotation (μSR) and AC susceptibility with an in situ piezoelectric stress device to investigate the spin-stripe order and superconductivity in LBCO-0.115 under c-axis compression. The measurements reveal a gradual suppression of the superconducting transition temperature (T_c) with increasing c-axis stress, in stark contrast to the strong enhancement observed under in-plane stress. We further show that while in-plane stress rapidly reduces both the magnetic volume fraction (V_m) and the spin-stripe ordering temperature (T_so), c-axis compression has no effect, with V_m and T_so exhibiting an almost unchanged behavior up to the highest applied stress of 0.21 GPa. These findings demonstrate a strong anisotropy in stress response.

Physical properties of the mixed-valent tellurate of lithium and manganese, LiMn₂TeO₆, were investigated in measurements of ac and dc magnetic susceptibility χ, magnetization M, specific heat C_p, electron spin resonance (ESR), and nuclear magnetic resonance (NMR) in the temperature range 2–300 K under magnetic field up to 9 T. The title compound orders magnetically in two steps at T₁ = 20 K and T₂ = 13 K. The intermediate phase at T₂ < T < T₁ is fully suppressed by magnetic field µ₀H of about 4 T. Besides magnetic phases transitions firmly established in static measurements, relaxation-type phenomena were observed well above magnetic ordering temperature in resonant measurements.