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Excitonic insulators host a condensate of electron-hole pairs at equilibrium, giving rise to collective many-body effects. Although several materials have emerged as excitonic insulator candidates, evidence of long-range coherence is lacking and the origin of the ordered phase in these systems remains controversial. Here, using ultrafast pump-probe microscopy, we investigate the possible excitonic insulator Ta2NiSe5 Below 328 K, we observe the anomalous micrometer-scale propagation of coherent modes at velocities of ~105 m/s, which we attribute to the hybridization between phonon modes and the phase mode of the condensate. We develop a theoretical framework to support this explanation and propose that electronic interactions provide a substantial contribution to the ordered phase in Ta2NiSe5 These results allow us to understand how the condensate's collective modes transport energy and interact with other degrees of freedom. Our study provides a unique paradigm for the investigation and manipulation of these properties in strongly correlated materials.

The layered chalcogenide Ta2NiSe5 has been proposed to host an excitonic condensate in its ground state, a phase that could offer a unique platform to study and manipulate many-body states at room temperature. However, identifying the dominant microscopic contribution to the observed spontaneous symmetry breaking remains challenging, perpetuating the debate over the ground state properties. Here, using broadband ultrafast spectroscopy we investigate the out-of-equilibrium dynamics of Ta2NiSe5 and demonstrate that the transient reflectivity in the near-infrared range is connected to the system’s low-energy physics. We track the status of the ordered phase using this optical signature, establishing that high-fluence photoexcitations can suppress this order. From the sub-50 fs quenching timescale and the behaviour of the photoinduced coherent phonon modes, we conclude that electronic correlations provide a decisive contribution to the excitonic order formation. Our results pave the way towards the ultrafast control of an exciton condensate at room temperature.

The negatively charged nitrogen-vacancy center in diamond has become a prominent room temperature spin qubit platform for quantum information and sensing applications owing to its inherent optical addressability and long spin coherence times. In particular, a major research effort has focused on exploiting the metrology capabilities of the nitrogen-vacancy center to investigate the details of biological and magnetic systems at the nanoscale using diamond micro- and nanoparticles (we will mostly refer to these as nanoparticles in what follows). These particles behave like highly localized, low-thermal-mass spin systems and can therefore be used for high spatial resolution, high sensitivity mapping of their environment. Nonetheless, some outstanding challenges limit the sensing capabilities of these nanosensors. On the one hand, the nitrogen-vacancy centers contained in nanoparticles present strongly degraded coherence properties compared to what is common in bulk diamond due to a higher density of structural defects and impurities. On the other hand, in order to investigate systems that produce a signal that decay fast with distance (e.g. single or small ensembles of spins), the qubit needs to be placed within a few nanometer of the diamond surface. Because of the presence of magnetic and electric noise resulting from electronic spin and charges at the interface, the coherence of the qubits is further diminished. As the nitrogen-vacancy center’s coherence properties ultimately limit the achievable sensitivity, improving thse properties has been a central research focus in the scientific community. In this thesis two main approaches to extend the capabilities of diamond nanoparticles as nanoscale sensors are investigated. Firstly, we discuss a fabrication process developed to produce nanodiamonds with controlled crystal properties, shape, size, and nitrogen-vacancy center placement and density. We show that these accomplishments are conducive to enhanced photoluminescence properties and bulk-like spin coherence times. We additionally demonstrate that, thanks to their precisely controlled geometry, the nanoparticles can be optically trapped within a microfluidic system and behave as highly stable, contactless nanoprobes in solution. In a separate effort, we study hybrid quantum architectures that promise to improve the sensitivity of the nitrogen-vacancy centers by enabling the amplification of the signal to be measured. In this way, the requirements on the position of the qubits with respect to the diamond’s interface can be relaxed. In particular, we first develop a technique to create portable arrays of nanoparticles embedded within a flexible and transparent matrix that can be placed in contact with a sample and provide two-dimensional mapping capabilities. We then use this sensing platform to create a ferromagnet-nanodiamond hybrid system to study the interactions between collective spin excitations (spin waves) in the ferromagnet and ensembles of nitrogen-vacancy centers in the nanodiamonds. We show that surface confined spin waves interact strongly with the qubits’ spins and can mediate and enhance their coherent magnetic interactions with a microwave source. These results pave the way to the use of spin wave mediated coupling as a way to improve the sensitivity of the nitrogen-vacancy centers both in nanoparticles and bulk material.

The layered chalcogenide Ta2NiSe5 has been proposed to host an excitonic condensate in its ground state, a phase that could offer a unique platform to study and manipulate many-body states at room temperature. However, identifying the dominant microscopic contribution to the observed spontaneous symmetry breaking remains challenging, perpetuating the debate over the ground state properties. Here, using broadband ultrafast spectroscopy we investigate the out-of-equilibrium dynamics of Ta2NiSe5 and demonstrate that the transient reflectivity in the near-infrared range is connected to the system’s low-energy physics. We track the status of the ordered phase using this optical signature, establishing that high-fluence photoexcitations can suppress this order. From the sub-50 fs quenching timescale and the behaviour of the photoinduced coherent phonon modes, we conclude that electronic correlations provide a decisive contribution to the excitonic order formation. Our results pave the way towards the ultrafast control of an exciton condensate at room temperature.

The layered chalcogenide Ta2NiSe5 has been proposed to host an excitonic condensate in its ground state, a phase that could offer a unique platform to study and manipulate many-body states at room temperature. However, identifying the dominant microscopic contribution to the observed spontaneous symmetry breaking remains challenging, perpetuating the debate over the ground state properties. Here, using broadband ultrafast spectroscopy we investigate the out-of-equilibrium dynamics of Ta2NiSe5 and demonstrate that the transient reflectivity in the near-infrared range is connected to the system’s low-energy physics. We track the status of the ordered phase using this optical signature, establishing that high-fluence photoexcitations can suppress this order. From the sub-50 fs quenching timescale and the behaviour of the photoinduced coherent phonon modes, we conclude that electronic correlations provide a decisive contribution to the excitonic order formation. Our results pave the way towards the ultrafast control of an exciton condensate at room temperature. The dominant mechanism of the excitonic insulator transition in Ta2NiSe5 and the nature of its high-temperature phase are debated. The authors report transient reflectivity measurements indicating a significant electronic contribution to the transition and a gapped state of preformed excitons at high temperatures.