Andrea Cavalleri

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.

Abstract:

Is it feasible to alter the ground-state properties of a material by engineering its electromagnetic environment? Inspired by theoretical predictions1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11–12, experimental realizations of such cavity-controlled properties without optical excitation are beginning to emerge13, 14, 15, 16, 17, 18–19. Here we devised and implemented a new platform to realize cavity-altered materials. Single crystals of hyperbolic van der Waals (vdW) compounds provide a resonant electromagnetic environment with enhanced density of photonic states and prominent mode confinement20, 21, 22, 23–24. We interfaced hexagonal boron nitride (hBN) with the molecular superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br (κ-ET). The frequencies of infrared hyperbolic modes (HMs) of hBN (refs. 25, 26) match the infrared-active carbon–carbon (C=C) stretching molecular resonance of κ-ET implicated in superconductivity27. Nano-optical data supported by first-principles molecular Langevin dynamics simulations confirm the presence of resonant coupling between the hBN hyperbolic cavity modes and the C=C stretching mode in κ-ET. Meissner-effect measurements using magnetic force microscopy (MFM) demonstrate a strong suppression of superfluid density near the hBN/κ-ET interface. Non-resonant control heterostructures, including RuCl3/κ-ET and hBN/Bi2Sr2CaCu2O8+x (BSCCO), do not show the pronounced superfluid suppression. These observations suggest that hBN/κ-ET realizes a cavity-altered superconducting ground state. Our work highlights the potential of dark cavities devoid of external photons for engineering electronic ground-state properties of complex quantum materials.

Abstract:

Single-shot electro-optic sampling (EOS) is a powerful method enabling the measurement of weak terahertz signals that would otherwise require prohibitively long acquisition times. This is generally achieved by encoding the EOS time delay into a spatial, angular, or frequency coordinate. In general, angular-encoding techniques operate well up to 3 THz but become more challenging for larger bandwidths, due to dispersion and imaging imperfections. Here, we demonstrate a reliable angular-encoding single-shot EOS implementation that reaches frequencies beyond 6 THz. Diffraction simulations are used to design the experimental setup and adapt this technique to commercial reflection gratings, removing the need for custom-built echelon mirrors. Furthermore, we show that, contrary to earlier reports, group delay dispersion from angular dispersion does not reduce the bandwidth of single-shot EOS.