Phonon polaritons (PPs) in van der Waals (vdW) materials can strongly enhance light-matter interactions at mid-infrared frequencies, owing to their extreme infrared field confinement and long lifetimes. PPs thus bear potential for achieving vibrational strong coupling (VSC) with molecules. Although the onset of VSC has recently been observed spectroscopically with PP nanoresonators, no experiments so far have resolved VSC in real space and with propagating modes in unstructured layers. Here, we demonstrate by real-space nanoimaging that VSC can be achieved between propagating PPs in thin vdW crystals (specifically h-BN) and molecular vibrations in adjacent thin molecular layers. To that end, we performed near-field polariton interferometry, showing that VSC leads to the formation of a propagating hybrid mode with a pronounced anti-crossing region in its dispersion, in which propagation with negative group velocity is found. Numerical calculations predict VSC for nanometer-thin molecular layers and PPs in few-layer vdW materials, which could make propagating PPs a promising platform for ultra-sensitive on-chip spectroscopy and strong coupling experiments. Main textPhonon polaritons (PPs) -light coupled to lattice vibrations -in van der Waals (vdW) crystals open up new possibilities for infrared nanophotonics, owing to their strong infrared field confinement, picosecond-long lifetimes 1-7 and tunability via thickness and dielectric environment [8][9][10][11] . Since PPs in many vdW materials spectrally coincide with molecular vibrational resonances, which abound the mid-infrared spectral range, PP are thus promising candidates for achieving vibrational strong coupling (VSC) for developing ultrasensitive infrared spectroscopy

No abstract

Atomically thin van der Waals magnetic crystals are characterized by tunable magnetic properties related to their low dimensionality. While electrostatic gating has been used to tailor their magnetic response, chemical...

a Curie temperature close to the room temperature and strong out-of-plane anisotropy. [7][8][9][10] Moreover, the magnetic properties of FGT are tunable, as they can be modified either electrically, applying a gate voltage [11] and a large electrical current, [12] or through magnetic proximity effects. [13][14][15][16] In particular, when interfaced with antiferromagnetic layered materials, FGT displays an increased coercivity and exchange bias, [13][14][15][16] which are the prototypical manifestation of magnetic proximity [17][18][19] and are key elements in spintronic devices. [20] Another attractive approach to tune the properties of a magnetic surface is molecular functionalization. [21,22] The interfaces between magnetic materials and molecules, often named spinterfaces, [21,22] host hybrid states or magnetic interactions which lead to radical changes on the magnetic properties of both the molecular layer [23][24][25][26][27][28][29][30] and the magnetic material. [27][28][29][30][31][32][33] So far, the ferromagnetic layers used for investigating spinterface effects are typically films of 3d metals or oxides with dangling bonds on the surface, which result in nonideal interfaces with molecules. Moreover, until recently, the molecular side of a spinterface has been the main target of research due to its easily tunable electronic properties, [23][24][25][26][27][28][29] whereas the possibility of tailoring the magnetism of ferromagnetic materials has yet to be fully exploited.Layered magnetic materials are excellent candidates for developing a spinterface in view of their tunable magnetism and their single crystalline nature, which offer the possibility to form highly controllable interfaces with molecules [34,35] via the so-called van der Waals epitaxy. [36][37][38][39] Indeed, hybrid heterostructures based on atomically sharp 2D material/molecule interfaces have been widely used to tailor the optoelectronic and transport properties of nonmagnetic layered materials. [40][41][42][43][44][45] However, so far the possibility to tune the properties of a layered magnetic material through the magnetic interactions at a van der Waals spinterface has not yet been experimentally demonstrated.Here, we report on the emergence of spinterface effects between molecular films of Co-phthalocyanine (CoPc) and a few-nm-thick FGT flakes. The molecular layer induces a negative magnetic exchange bias in FGT, indicating that the The exfoliation of layered magnetic materials generates atomically thin flakes characterized by an ultrahigh surface sensitivity, which makes their magnetic properties tunable via external stimuli, such as electrostatic gating and proximity effects. Another powerful approach to engineer magnetic materials is molecular functionalization, generating hybrid interfaces with tailored magnetic interactions, called spinterfaces. However, spinterface effects have not yet been explored on layered magnetic materials. Here, the emergence of spinterface effects is demonstrated at the interface between flakes of the ...

2D transition metal dichalcogenides (TMDs) represent an ideal testbench for the search of materials by design, since their optoelectronic properties can be manipulated through surface engineering and molecular functionalization. However, the impact of molecules on intrinsic physical properties of TMDs, such as superconductivity, remains largely unexplored. In this work, the critical temperature (TC) of large-area NbSe2 monolayers is manipulated employing ultra-thin molecular adlayers. Spectroscopic evidences indicate that aligned molecular dipoles within the self-assembled layers act as a fixed gate terminal, collectively generating a macroscopic electrostatic field on NbSe2. This results in a ̴ 55% increase and a 70% decrease in TC depending on the electric field polarity, which is controlled via molecular selection. The reported functionalization, which improves the air stability of NbSe2, is efficient, practical, up-scalable and suited to functionalize large-area TMDs. Our results indicate the potential of hybrid 2D materials as a novel platform for tunable superconductivity. TEXTTransition metal dichalcogenides (TMDs) are layered compounds which can be thinned down to the single-layer limit. 1,2 While mechanical exfoliation generates atomically thin TMD flakes