Andrea Cavalleri

Abstract:

We present a theoretical approach for computing and analyzing two-dimensional spectroscopy of bosonic collective excitations in disordered many-body systems. Specifically, we employ the Keldysh formalism to derive, within a nonperturbative treatment of disorder effects, the third-order nonlinear response and obtain two-dimensional spectroscopy maps. In the weak nonlinear regime of our formalism, we demonstrate the ability of the echo peak to distinguish between elastic and inelastic scattering processes, in perfect agreement with the intuition developed in isolated two-level systems. Furthermore, we discuss unique many-body effects on the echo peak signature arising from interaction-induced quantum fluctuations. In particular, we show that these quantum fluctuations induce a finite nonrephasable broadening and examine how the echo peak is influenced by the attractive or repulsive nature of the collective excitations.

Abstract:

Accessing the intrinsic critical current density in type-II superconductors can provide a means to probe microscopic superconducting properties and increase current limits in high-magnetic-field devices and electrical power systems. However, the critical current density measured using direct currents generally lies below this intrinsic limit, mainly because of vortex motion and self-heating. Here we show that picosecond electrical pulses, which act on timescales too short for vortices to move, drive supercurrents up to the intrinsic depairing limit. We probe picosecond critical currents in NbN and YBa2Cu3O7, representative s-wave and d-wave superconductors, respectively. In NbN, we find a sharp onset of the picosecond depairing at a current density of about 2.2 times the conventional critical current density, consistent with microscopic dynamics based on Bardeen–Cooper–Schrieffer theory. By contrast, YBa2Cu3O7 exhibits a gradual suppression of superconductivity with increasing current, reflecting its d-wave symmetry. These results provide a probe of superconductors beyond the reach of conventional transport measurements. Attaining the depairing current could also support superconducting electronics operating closer to intrinsic current limits.

Abstract:

Faraday waves, typically observed in driven fluids, result from the confluence of nonlinearity and parametric amplification. Here we show that optical pulses can generate analogous phenomena that persist much longer than the pump time-scales in ordered quantum solids. We present a theory of ultrafast light-matter interactions within a symmetry-broken state; dynamical nonlinear coupling between the Higgs (amplitude) and the Goldstone (phase) modes drives an emergent phason texture that oscillates in space and in time: Faraday-Goldstone waves. Calculated signatures of this spatiotemporal order compare well with measurements on K_0.3MoO₃; Higgs-Goldstone beating, associated with coherent energy exchange between these two modes, is also predicted. We show this light-generated crystalline state is robust to thermal noise, even when the original Goldstone mode is not. Our results offer a pathway for the design of periodic structures in quantum materials with ultrafast light pulses.

Abstract:

Chirality is a pervasive property of matter that underpins many important phenomena across physics1, chemistry2 and biology3. Given its broad importance, the development of protocols for rational control of chirality in solid-state systems is highly desirable, especially if this effect can be tuned continuously and in two directions. Yet, this goal has remained elusive owing to the absence of a universal conjugate field that couples linearly to this structural order4, 5–6. Here we introduce the piezochiral effect, which enables control of chirality through mechanical strain. We show by symmetry analysis that uniaxial strain induces chirality in a broad class of achiral crystals that host fragments of opposite chirality within each unit cell7,8, an effect that has so far remained unrecognized. The strain-induced handedness can be tuned either by changing the strain direction or by switching between compressive and tensile strain. We experimentally verify this effect in AgGaS2, using measurements of the optical activity under strain. Our discovery establishes a new scheme for chirality control, with potential applications that range from spintronics to asymmetric catalysis, and enantioselective interactions in biosystems.

Abstract:

High temperature superconductivity is typically associated with strong coupling and a large superconducting gap, yet these characteristics have not been demonstrated in the nickelates. Here, we provide experimental evidence that Eu substitution in the spacer layer of Nd1-xEuxNiO2 (NENO) thin films enhances the superconducting gap, driving the system toward a strong-coupling regime. This is accompanied by a magnetic-exchange-driven magnetic-field-enhanced superconductivity. We investigate the upper critical magnetic field, Hc2, and the superconducting gap of superconducting NENO thin films with x = 0.2 to 0.35. Magnetoresistance measurements reveal magnetic-field-enhanced superconductivity in NENO films. We interpret this phenomenon as a result of an interaction between magnetic Eu ions and superconducting states in the Ni dx2-y2 orbital. The upper critical magnetic field strongly violates the weak-coupling Pauli limit. Infrared spectroscopy confirms a large gap-to-Tc ratio 2Δ/kBTc≃5−6, indicating a stronger coupling pairing mechanism in NENO relative to the Sr-doped NdNiO2. The substitution of Eu in the rare-earth layer causes pronounced modifications of the superconducting gap and magnetic interactions in Nd-based nickelates, opening new pathways to engineer high-Tc superconductivity in infinite-layer nickelates.