The object of our investigation are layered and two-dimensional (2D) materials, such as cuprate and pnictide high-temperature superconductors, superconducting oxide interfaces, like LaAlO3/SrTiO3, disordered superconducting thin films, like NbN and MoGe. Upon variation of the carrier density and/or of the amount of microscopic disorder, all these systems exhibit remarkable phenomena, including a superconductor-to-insulator transition (SIT). Moreover, as a consequence of the competition between superconductivity and other phases (characterized by , e.g., spin or charge order), many of these systems behave somewhat unexpectedly, (self-) organized modulations of the carrier density may emerge and their responses to weak perturbations can be uncommonly sizable, opening the way to their exploitation in device engineering. Indeed, in many of (if not all) these materials, the occurrence of a state with intrinsic electron inhomogeneity is a major issue: In cuprates, this state is characterized by nearly statical charge-density waves (CDWs) and/or dynamical charge-density fluctuations (CDFs). The mechanism for their occurrence and their role in determining the anomalous properties of the metallic phase and in providing the glue for high-temperature superconductivity are crucial issues that will be investigated within our project. In oxide interfaces, transition metals dichalcogenides (TMDs), and disordered thin films, an inhomogeneous, likely rather filamentary, superconducting state arises, with rare regions affecting the electrodynamic, transport and non-equilibrium properties of the system near the SIT. These will be the object of systematic investigation within our project.
Our project on low-dimensional superconductors has a twofold character, aiming at both fundamental questions and potential applications. The renewed interest in this field is witnessed by several recent results. Ever increasing experimental evidence shows that CDWs/CDFs are ubiquitously present in all the families of the cuprate clan, and boosts the debate about their interplay with superconductivity and their role in the strange metallic behavior observed in these systems. In oxide interfaces, the occurrence of nanoscale inhomogeneity raised new questions and proposals. Also the field of 2D crystalline superconductors is receiving a renewed attention for the many unsolved fundamental issues and for the potential device applications. Several open questions have been posed by time-resolved spectroscopy, where the combination of high fields and time resolution allows one to probe the system while relaxing out from an out-of-equilibrium condition. Our project is at the crossroad of these crucial and intense research lines, as it focuses on the interplay between superconductivity and nanoscale inhomogeneities or charge ordering, in the form of both dynamical or nearly static density fluctuations, and investigates the nanoscale inhomogeneity as an intrinsic emergent character in low-dimensional superconductors. The proposal that nanoscale inhomogeneity is not extrinsic, but may be an emergent phenomenon in low-dimensional superconductors, opens a new perspective, which is declined in various forms in the different systems, and calls for the identification of common features. The timeliness and relevance of the our project is witnessed by the reference list, which contains several very recent papers.
Although the idea of dynamical charge ordering as a Coulomb-frustrated form of electron softness in cuprates was proposed by our group long ago [1], it found compelling experimental evidence very recently [2]. The collaborations with experimental groups active in RXS and transport experiments, which is by now well established, allow us to refine our theoretical scenario, making it ever richer and more detailed. There is a strong commitment in this field and our group is in a privileged position to analyze and explain the various open issues raised by experiments. This would make crucial progress in a field that has always been the object of intense investigation.
Experimental results on oxide interfaces and TMDs are more recent and attracted increasing attention in the last decade. Also in this field our group played a relevant role, pointing out the intrinsically inhomogeneous character of LaAlO3/SrTiO3 interfaces and TMDs [14-19,22] and providing possible microscopic explanations. Again, once the mechanisms producing this form of electron softness are understood, the way will be opened for practical exploitation [24,25]. Within this respect, the properties of vortices in thin superconducting films is deeply connected with the superfluid response of low-dimensional superconductors. In particular, the identification of the hexatic phase, not only represents a challenge for fundamental research, but also opens a new perspective on the possibility of tuning a superconductor to a resistive phase with very low dissipation by means of a magnetic field, with several possible applications.
Finally, the possibility to selectively drive collective electronic phenomena via intense THz pulses is nowadays at the frontier of the experimental research. Our group already gave a prominent contribution to the field, by establishing for the first time a general theoretical framework for understanding experiments in conventional superconductors. The next step, addressed in this project, is the analysis of unconventional systems like cuprates, where triggering non-equilibrium properties represents a privileged knob to investigate the competition and interplay among different phases.