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Abstract:

The kagome lattice of spin-1/2 copper atoms in herbertsmithite is conjectured to sustain a quantum spin liquid state with spinon quasiparticles. Ideally, the kagome crystal planes are each separated by a plane of spinless zinc atoms. However, in real crystals, some spin-1/2 copper atoms substitute randomly onto these inter-kagome zinc sites. Here we reconceptualize such ‘impurity’ atoms as quantum witness spins whose dynamics is designed to probe the spin liquid state. We then introduce spin noise spectroscopy to measure the frequency and temperature dependence of witness spin dynamics, demonstrating that their phenomenology is consistent with extensive interactions between witness spins mediated by propagation of spinons through a quantum spin liquid. Ultimately, a sharp transition occurs at around 260 mK, below which the properties of both spin noise and magnetic susceptibility suggest that the witness spins form a spin glass phase. Among the theoretical models considered, we demonstrate that our observations are only consistent with spinon-mediated interactions between witness spins by either a Z2 or U(1) quantum spin liquid, with the former model more closely matching the data. Our work demonstrates that quantum mechanical witness spins may now conceivably be used as a widely applicable probe of quantum spin liquid physics.

Abstract:

Dynamical heterogeneity, in which transitory local fluctuations occur in the conformation and dynamics of constituent particles, is widely hypothesized to be essential to the evolution of supercooled liquids into the structural glass state. Yet its microscopic spatiotemporal phenomenology is challenging to detect directly in molecular glass forming liquids. Because recent theoretical advances predict that corresponding dynamical heterogeneity could occur in supercooled magnetic monopole fluids (Proc. Nat. Acad. Sci. 112, 8549 (2015)), we searched for such phenomena in Dy2Ti2O7. By measuring its microsecond-resolved spontaneous magnetization fluctuations M(t, T) we detected a sharp bifurcation in monopole noise characteristics below T≈1,500 mK, with the appearance of powerful spontaneous monopole current bursts. This intense dynamics emerges upon entering the supercooled monopole fluid regime, reaches maximum strength near T≈750 mK and then collapses along with coincident loss of ergodicity approaching Tg≈250 mK. Moreover, when the four-point dynamical susceptibility χ4(τ, T) is determined directly from temperature dependence of correlations in M(t, T), it evolves as predicted when dynamical heterogeneity is present, revealing its simultaneously and rapidly escalating length and time scales, ξ(T) and τ4(T). This overall phenomenology greatly expands our empirical knowledge of supercooled monopole fluids and, more generally, demonstrates techniques for detection of the time sequence, magnitude, statistics, and correlations of dynamical heterogeneity, access to which may greatly accelerate fundamental vitrification studies.

Abstract:

A distinctive identifier of nodal intrinsic topological superconductivity (ITS) would the appearance of an Andreev bound state on crystal surfaces parallel to the nodal axis, in the form of a topological quasiparticle surface band (QSB) appearing only for T

Abstract:

We compute the (0-11) surface spectral function, the surface density of states (DOS), and the quasiparticle interference (QPI) patterns, both in the normal state and superconducting state of UTe₂. We consider all possible nonchiral and chiral order parameters (OPs) that could, in principle, describe the superconductivity in this compound. We describe the formation of surface states whose maximum intensity energy depends on the nature of the pairing. We also study the QPI patterns resulting from the scattering of these surface states. Along the lines of [Nat. Phys. 21, 1555 (2025)1745-247310.1038/s41567-025-03000-w], we show that the main feature distinguishing between various OPs is a QPI peak that is only observed experimentally in the superconducting state. The energy dispersion and the stability of this peak is consistent among the nonchiral OPs only with a B3u pairing. Moreover, B3u is the only nonchiral pairing that shows a peak at zero energy in the DOS, consistent with the experimental observations.

Abstract:

Although no known material exhibits intrinsic topological superconductivity, where a spin-triplet electron pairing potential has odd parity, UTe2 is now the leading candidate. Generally, the parity of a superconducting order parameter can be established using Bogoliubov quasiparticle interference imaging. However, odd-parity superconductors should support a topological quasiparticle surface band at energies within the maximum superconducting energy gap. Quasiparticle interference should then be dominated by the electronic structure of the quasiparticle surface band and only reveal the characteristics of the bulk order parameter indirectly. Here we demonstrate that at the (0–11) cleave surface of UTe2, a band of Bogoliubov quasiparticles appears only in the superconducting state. Performing high-resolution quasiparticle interference measurements then allows us to explore the dispersion of states in this superconductive surface band, showing that they exist only within the range of Fermi momenta projected onto the (0–11) surface. Finally, we develop a theoretical framework to predict the quasiparticle interference signatures of this surface band at the (0–11) surface. Its predictions are consistent with the experimental results if the bulk superconducting order parameter exhibits time-reversal conserving, odd-parity, a-axis nodal, B3u symmetry.