NbSe2

We studied femtosecond laser driven electronic response of monolayer NbSe$_2$ using state-of-the-art computational methods, synthesis and optical characterization. Earlier studies have found distinct signatures of charge density wave (CDW) ordered phases in the ground state ($sim$ 0 K) of NbSe$_2$ monolayer, in co-existence with superconducting phase. Driving such systems with ultra-short (femtosecond) laser pulse can provide the highly sought knowledge on how to effectively control various exotic phases (e.g. CDW) of monolayer NbSe$_2$ with external control parameters such as pulse intensity. This will not only provide a fundamental understanding of non-equilibrium phase-transitions in NbSe$_2$, but also will open path-forward to revolutionize quantum information technologies such as valleytronics. Inspired by this, we have studied higher harmonic generation (HHG) in monolayer NbSe$_2$ under various intensities of femtosecond laser pulse using real-time time-dependent density functional theory (RT-TDDFT). Our computation predicted distinct signatures in HHG spectra for some higher harmonic modes in the presence of CDW orders in monolayer NbSe$_2$. Excitation energies under strong pulse, and power-law behavior of HHG spectra are also reported.

Published in: "arXiv Material Science".

Monolayer 2H-NbSe2 has recently been shown to be a 2-dimensional superconductor, with a coexisting charge-density wave (CDW). As both phenomena are intimately related to electron-lattice interaction, a natural question is how superconductivity and CDW are interrelated through electron-phonon coupling (EPC), which is important to the understanding of 2-dimensional superconductivity. This work investigates the superconductivity of monolayer NbSe2 in CDW phase using the anisotropic Migdal-Eliashberg formalism based on first principles calculations. The mechanism of the competition between and coexistence of the superconductivity and CDW is studied in detail by analyzing EPC. It is found that the intra-pocket scattering is related to superconductivity, leading to almost constant value of superconducting gaps on parts of the Fermi surface. The inter-pocket scattering is found to be responsible for CDW, leading to partial or full bandgap on the remaining Fermi surface. Recent experiment indicates that there is transitioning from regular superconductivity in thin-film NbSe2 to two-gap superconductivity in the bulk, which is shown here to have its origin in the extent of Fermi surface gapping of K and K’ pockets induced by CDW. Overall blue shifts of the phonons and sharp decrease of Eliashberg spectrum are found when the CDW forms.

Published in: "arXiv Material Science".

Dissipationless charge transport is one of the defining properties of superconductors (SC). The interplay between dimensionality and disorder in determining the onset of dissipation in SCs remains an open theoretical and experimental problem. In this work, we present measurements of the dissipation phase diagrams of SCs in the two dimensional (2D) limit, layer by layer, down to a monolayer in the presence of temperature (T), magnetic field (B), and current (I) in 2H-NbSe2. Our results show that the phase-diagram strongly depends on the SC thickness even in the 2D limit. At four layers we can define a finite region in the I-B phase diagram where dissipationless transport exists at T=0. At even smaller thicknesses, this region shrinks in area. In a monolayer, we find that the region of dissipationless transport shrinks towards a single point, defined by T=B=I=0. In applied field, we show that time-dependent-Ginzburg-Landau (TDGL) simulations that describe dissipation by vortex motion, qualitatively reproduce our experimental I-B phase diagram. Last, we show that by using non-local transport and TDGL calculations that we can engineer charge flow and create phase boundaries between dissipative and dissipationless transport regions in a single sample, demonstrating control over non-equilibrium states of matter.

Published in: "arXiv Material Science".