Magnetic field expulsion in driven YB2C3O6.48

Abstract

In recent years, strongly correlated materials have emerged as promising candidates for hosting next-generation technologies. Due to their rich phase diagrams featuring different competing orders, their macroscopic properties can be controlled on demand with tailored external stimuli. In particular, the development of intense ultrafast light sources in the mid-infrared and terahertz regime enabled the direct and efficient coupling to the low-energy tuning knobs of these quantum solids. Experiments in this novel field have confirmed the generation of transient ferroelectric, magnetic, topological, and superconducting orders. Cuprate superconductors are the archetypical member of this class of materials, wherein antiferromagnetism, superconductivity, charge and spin order interchange with moderate tuning of the carrier doping or crystal lattice. Remarkably, in underdoped YBa2Cu3O7, signatures of incoherent superconducting fluctuations are present above Tc, extending up to and above room temperature. The most accepted interpretation postulates that superconductivity exists locally in this pseudogap phase, and fluctuations prevent the emergence of long-range order. These observations have sparked a series of experiments aimed at resurrecting the superconducting state by directly driving the interlayer coupling of this layered compound, conjectured as the determining factor for the superconducting pairing. These expectations were positively met by experiments where transient optical properties reminiscent of equilibrium superconductivity were observed in terahertz time-domain spectroscopy. However, these findings were not conclusive since they were also compatible with a non-superconducting state with enhanced mobility. In order to clarify the nature of the photo-excited state, it was paramount to verify whether it presented the dynamical equivalent of a Meissner effect, considered a fingerprint of the macroscopic coherence characterizing the superconducting order. However, prior to this work, no experimental study had interrogated the magnetic properties of this exotic transient state. Here, we set out to address this fundamental question. The core of the investigation revolved around the development of the novel Ultrafast Magnetometry experimental technique, combining elements of magneto-optic imaging with the sampling of terahertz pulses. The diamagnetic response of the superconductor was detected by measuring the spatial profile of the magnetic field in its surroundings. Improving on previous designs, the adoption of diamagnetic detectors and advanced analysis techniques made it possible, for the first time, to follow the magnetic dynamics associated with the onset of superconductivity with 〜1 ps time-resolution and 〜1 μT sensitivity. This experimental technique revealed the presence of a sizable magnetic field expulsion in YBa2Cu3O6.48 upon photo-excitation. Quasi-static simulations confirmed that the sampled magnetic field was caused by a colossal diamagnetic response 〜-0.3, reproduced at equilibrium only by type-II superconductors. This observation is incompatible with a photo-induced increase in mobility. Rather, it underscores the onset of superconducting coherence over macroscopic length scales. Remarkably, the effect was seen to persist up to room temperature and correlated positively with the terahertz optical measurements, suggesting a common physical origin for these two different observables. These experimental findings support the picture of a pseudogap phase in which incipient superconducting correlations are enhanced or synchronized by the drive.