arXiv subjects

Aldo Coraggio

Publications and source records attributed to Aldo Coraggio.

4 recordsLinked to original sources

arXiv2026

Vanishing spin stiffness in weakly disordered two-dimensional Heisenberg ferromagnets

We show that a small fraction of antiferromagnetic bonds qualitatively alters the long-wavelength dynamics of two-dimensional Heisenberg ferromagnets. Although the classical ground state remains magnetized, weak bond frustration generates logarithmically correlated spatial fluctuations of the local spin stiffness, despite the microscopic disorder being short ranged. A replica field theory calculation shows that the effective disorder strength grows under coarse graining, while the spin stiffness decreases, yielding anomalously soft magnons with a scale-dependent dynamical exponent $z > 2$. Numerical diagonalization of the semiclassical spin-wave Hamiltonian confirms the anomalous low-energy scaling. The flow is toward an infinite-disorder, zero stiffness regime.

cond-mat.dis-nn↗

arXiv2026

Semiclassical picture of the Heisenberg spin glass in two dimensions: from weak localization to hydrodynamics

The two-dimensional Heisenberg spin-glass model is investigated by means of a semiclassical expansion around classical states. At leading order, we obtain an effective quadratic spin-wave Hamiltonian and study the localization properties of its spectrum and eigenfunctions. We find that the nature of the spin-wave excitations, whether they are hydrodynamic or localized modes, depends crucially on the relevance/irrelevance -- in the renormalization group sense -- of the correlations induced by the underlying classical order in the spin-wave Hamiltonian matrix elements: low-energy excitations around magnetically ordered states are delocalized, whereas those around spin-glass ordered states are localized, albeit weakly. We interpret this phenomenology by relating the spontaneous breaking of spin-rotation symmetry in the original Heisenberg model to the symmetry and universality class of the resulting quadratic spin-wave Hamiltonian. We conjecture that the hydrodynamic picture can be recovered through the inclusion of interactions among the spin-wave excitations at higher order in the semiclassical expansion, favoring the onset of ergodic behavior.

cond-mat.dis-nn↗

arXiv2026

Measurement-induced crossover in quantum first-detection times

The quantum first-detection problem concerns the statistics of the time at which a system, subject to repeated measurements, is observed in a prescribed target state for the first time. Unlike its classical counterpart, the measurement back action intrinsic to quantum mechanics may profoundly alter the system dynamics. Here we show that it induces a distinct change in the statistics of the first-detection time. For a quantum particle in one spatial dimension subject to stroboscopic measurements, we observe an algebraic decay of the probability of the first-detection time if the particle is free, an exponential decay in the presence of a confining potential, and a time-dependent crossover between these behaviors if the particle is partially confined. This crossover reflects the purely quantum nature of the detection process, which fundamentally distinguishes it from the first-passage problem in classical systems.

cond-mat.stat-mech↗

arXiv2025

The quantum XY chain with boundary fields: finite-size gap and phase behavior

We present a detailed study of the finite-size one-dimensional quantum XY chain in a transverse field in the presence of boundary fields coupled with the order-parameter spin operator. We consider fields located at the chain boundaries that have the same strength and that are oppositely aligned. We derive exact expressions for the gap $\Delta$ as a function of the model parameters for large values of the chain length $L$. These results allow us to characterize the nature of the ordered phases of the model. We find a magnetic (M) phase ($\Delta \sim e^{-aL}$), a magnetic-incommensurate (MI) phase ($\Delta \sim e^{-aL} f_{MI}(L)$), a kink (K) phase ($\Delta \sim L^{-2}$), and a kink-incommensurate (KI) phase ($\Delta \sim L^{-2} f_{KI}(L)$); $f_{MI}(L)$ and $f_{KI}(L)$ are bounded oscillating functions of $L$. We also analyze the behavior along the phase boundaries. In particular, we characterize the universal crossover behavior across the K-KI phase boundary. On this boundary, the dynamic critical exponent is $z=4$, i.e., $\Delta \sim L^{-4}$ for large values of $L$.

cond-mat.stat-mech↗