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Abstract:

A recent advance in the study of emergent magnetic monopoles was the discovery that monopole motion is restricted to dynamical fractal trajectories [J. N. Hallén et al., Science 378, 1218 (2022)], thus explaining the characteristics of magnetic monopole noise spectra [R. Dusad et al., Nature 571, 234 (2019); A. M. Samarakoon et al., Proc. Natl. Acad. Sci. U.S.A. 119, e2117453119 (2022)]. Here, we apply this novel theory to explore the dynamics of field-driven monopole currents, finding them composed of two quite distinct transport processes: initially swift fractal rearrangements of local monopole configurations followed by conventional monopole diffusion. This theory also predicts a characteristic frequency dependence of the dissipative loss angle for AC field–driven currents. To explore these novel perspectives on monopole transport, we introduce simultaneous monopole current control and measurement techniques using SQUID-based monopole current sensors. For the canonical material Dy2Ti2O7, we measure Φ(t), the time dependence of magnetic flux threading the sample when a net monopole current J(t) = Φ̇ (t)∕0 is generated by applying an external magnetic field B0(t). These experiments find a sharp dichotomy of monopole currents, separated by their distinct relaxation time constants before and after t ~600 μs from monopole current initiation. Application of sinusoidal magnetic fields B0(t) = Bcos(t) generates oscillating monopole currents whose loss angle ( f ) exhibits a characteristic transition at frequency f ≈ 1.8 kHz over the same temperature range. Finally, the magnetic noise power is also dichotomic, diminishing sharply after t ~600 μs. This complex phenomenology represents an unprecedented form of dynamical heterogeneity generated by the interplay of fractionalization and local spin configurational symmetry.

Abstract:

Spin-triplet topological superconductors should exhibit many unprecedented electronic properties, including fractionalized electronic states relevant to quantum information processing. Although UTe₂ may embody such bulk topological superconductivity^{1,2,3,4,5,6,7,8,9,10,11}, its superconductive order parameter Δ(k) remains unknown¹². Many diverse forms for Δ(k) are physically possible¹² in such heavy fermion materials¹³. Moreover, intertwined^14,15 density waves of spin (SDW), charge (CDW) and pair (PDW) may interpose, with the latter exhibiting spatially modulating^14,15 superconductive order parameter Δ(r), electron-pair density^16,17,18,19 and pairing energy gap^{17,20,21,22,23}. Hence, the newly discovered CDW state²⁴ in UTe₂ motivates the prospect that a PDW state may exist in this material^24,25. To search for it, we visualize the pairing energy gap with μeV-scale energy resolution using superconductive scanning tunnelling microscopy (STM) tips^{26,27,28,29,30,31}. We detect three PDWs, each with peak-to-peak gap modulations of around 10 μeV and at incommensurate wavevectors P_i=1,2,3 that are indistinguishable from the wavevectors Q_i=1,2,3 of the prevenient²⁴ CDW. Concurrent visualization of the UTe₂ superconductive PDWs and the non-superconductive CDWs shows that every P_i:Q_i pair exhibits a relative spatial phase δϕ ≈ π. From these observations, and given UTe₂ as a spin-triplet superconductor¹², this PDW state should be a spin-triplet PDW^24,25. Although such states do exist³² in superfluid ³He, for superconductors, they are unprecedented.

Abstract:

A Kondo lattice is often electrically insulating at low temperatures. However, several recent experiments have detected signatures of bulk metallicity within this Kondo insulating phase. In this study, we visualized the real-space charge landscape within a Kondo lattice with atomic resolution using a scanning tunneling microscope. We discovered nanometer-scale puddles of metallic conduction electrons centered around uranium-site substitutions in the heavy-fermion compound uranium ruthenium silicide (URu₂Si₂) and around samarium-site defects in the topological Kondo insulator samarium hexaboride (SmB₆). These defects disturbed the Kondo screening cloud, leaving behind a fingerprint of the metallic parent state. Our results suggest that the three-dimensional quantum oscillations measured in SmB₆ arise from Kondo-lattice defects, although we cannot exclude other explanations. Our imaging technique could enable the development of atomic-scale charge sensors using heavy-fermion probes.