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Metallic transition metal dichalcogenides (TMDs), consisting of H-NbSe$_2$, H-NbS$_2$, H-TaSe$_2$ and H-TaS$_2$, remain superconducting down to a thickness of a single layer. In these materials, thickness affects a variety of properties, including Ising protection, two-band superconductivity, and the critical temperature $T_C$, which decreases for the Nb-based, and increases for the Ta-based materials. This contradicting trend is puzzling, and has precluded the development of a unified theory. We approach the question of thickness-evolution of $T_C$ and the superconducting gap $\Delta$ by measuring high-resolution tunneling spectra in TaS$_2$-based stacked devices. Our measurements allow for simultaneous evaluation of $\Delta$, $T_C$, and the upper critical field $H_{C2}$. The latter, we find, is strongly enhanced towards the single-layer limit, following a $H_{C2} \propto \Delta^2$ proportionality ratio. Our main finding is that the same ratio holds for the entire family of metallic TMDs: TaS$_2$ and NbSe$_2$ of all thicknesses, bulk TaSe$_2$ and bulk NbS$_2$, extending over 4 orders of magnitude in $H_{C2}$ and covering both clean and dirty limits. We propose that this tunability across the TMD family is controlled by the competing charge density wave (CDW) phase. Using Gor'kov's theory, we calculate how a CDW order affects the quasiparticle density of states and the resulting $T_C$ and $H_{C2}$. Our results suggest that CDW is the key determinant factor limiting $T_C$ in the TMD family. They also show that $H_{C2}$ is universally enhanced by a factor of two orders of magnitude above the expected value, an effect that remains an open question., Comment: 28 pages, 16 figures

The synthesis of two-dimensional van der Waals magnets has paved the way for both technological applications and fundamental research on magnetism confined to ultra-small length scales. Edge magnetic moments in ferromagnets are expected to be less magnetized than in the sample interior because of the reduced amount of neighboring ferromagnetic spins at the sample edge. We recently demonstrated that CrGeTe3 (CGT) flakes thinner than 10 nm are hard ferromagnets; i.e., they exhibit an open hysteresis loop. In contrast, thicker flakes exhibit zero net remnant field in the interior, with hard ferromagnetism present only at the cleaved edges. This experimental observation suggests that a nontrivial interaction exists between the sample edge and the interior. Here, we demonstrate that artificial edges fabricated by focus ion beam etching also display hard ferromagnetism. This enables us to write magnetic nanowires in CGT directly and use this method to characterize the magnetic interaction between the interior and edge. The results indicate that the interior saturation and depolarization fields depend on the lateral dimensions of the sample. Most notably, the interior region between the edges of a sample narrower than 300 nm becomes a hard ferromagnet, suggesting an enhancement of the magnetic exchange induced by the proximity of the edges. Last, we find that the CGT regions amorphized by the gallium beam are nonmagnetic, which introduces a novel method to tune the local magnetic properties of CGT films, potentially enabling integration into spintronic devices., Comment: Main text: 13 pages, 4 figures. Supplementary information: 8 pages, 6 figures

Colloidal semiconductor quantum dots are robust emitters implemented in numerous prototype and commercial optoelectronic devices. However, active fluorescence color tuning, achieved so far by electric-field induced Stark effect, has been limited to a small spectral range, and accompanied by intensity reduction due to the electron-hole charge separation effect. Utilizing quantum dot molecules that manifest two coupled emission centers, we present a novel electric-field induced instantaneous color switching effect. Reversible emission color switching without intensity loss is achieved on a single particle level, as corroborated by correlated electron microscopy imaging. Simulations establish that this is due to the electron wavefunction toggling between the two centers dictated by the electric-field and affected by the coupling strength. The quantum dot molecules manifesting two coupled emission centers may be tailored to emit distinct colors, opening the path for sensitive field sensing and color switchable devices such as a novel pixel design for displays or an electric field color tunable single photon source., Comment: Manuscript: 27 pages, 6 figures, Supplementary: 38 pages

CrGeTe3 (CGT) is a semiconducting vdW ferromagnet shown to possess magnetism down to a two-layer thick sample. Although CGT is one of the leading candidates for spintronics devices, a comprehensive analysis of CGT thickness dependent magnetization is currently lacking. In this work, we employ scanning SQUID-on-tip (SOT) microscopy to resolve the magnetic properties of exfoliated CGT flakes at 4.2 K. Combining transport measurements of CGT/NbSe2 samples with SOT images, we present the magnetic texture and hysteretic magnetism of CGT, thereby matching the global behavior of CGT to the domain structure extracted from local SOT magnetic imaging. Using this method, we provide a thickness dependent magnetization state diagram of bare CGT films. No zero-field magnetic memory was found for films thicker than 10 nm and hard ferromagnetism was found below that critical thickness. Using scanning SOT microscopy, we identify a unique edge magnetism, contrasting the results attained in the CGT interior., Comment: Main text: 15 pages, 5 figures. Supplementary information: 9 pages, 10 figures. Supplementary videos: 5

We report on fabrication of devices integrating FeTe$_{0.55}$Se$_{0.45}$ with other van-der-Waals materials, measuring transport properties as well as tunneling spectra at variable magnetic fields and temperatures down to 35 mK. Transport measurements are reliable and repeatable, revealing temperature and magnetic field dependence in agreement with prior results, confirming that fabrication processing does not alter bulk properties. However, cross-section scanning transmission microscopy reveals oxidation of the surface, which may explain a lower yield of tunneling device fabrication. We nonetheless observe hard-gap planar tunneling into FeTe$_{0.55}$Se$_{0.45}$ through a MoS$_2$ barrier. Notably, a minimal hard gap of 0.5 meV persists up to a magnetic field of 9 T in the $ab$ plane and 3 T out of plane. This may be the result of very small junction dimensions, or a quantum-limit minimal energy spacing between vortex bound states. We also observed defect assisted tunneling, exhibiting bias-symmetric resonant states which may arise due to resonant Andreev processes., Comment: 10 pages, 5 figures