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Register a new userTheoretical Modeling of the Non-equilibrium Amorphous State in 1T-TaS$_2$
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1T-TaS$_2$ is known for its remarkably complex phase diagram and its unique long-lived metastable hidden (H) state. Recently, a novel metastable state has been discovered using higher fluences for photoexcitation than in the case of the H state. The state has been dubbed as amorphous (A) due to its similarity to glass. Expanding on the work of Brazovskii and Karpov, we show that the A state can be successfully modelled with classical interacting polarons on a two-dimensional hexagonal lattice. We have found that the polaron configuration of the A state corresponds to a frustrated screened Coulomb system, where there is no order-disorder phase transition.
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1T-TaS$_2$ is a layered transition metal dichalgeonide with a very rich phase diagram. At T=180K it undergoes a metal to Mott insulator transition. Mott insulators usually display anti-ferromagnetic ordering in the insulating phase but 1T-TaS$_2$ was never shown to order magnetically. In this letter we show that 1T-TaS$_2$ has a large paramagnetic contribution to the magnetic susceptibility but it does not show any sign of magnetic ordering or freezing down to 20mK, as probed by $mu$SR, possibly indicating a quantum spin liquid ground state. Although 1T-TaS$_2$ exhibits a strong resistive behavior both in and out-of plane at low temperatures we find a linear term in the heat capacity suggesting the existence of a Fermi-surface, which has an anomalously strong magnetic field dependence.
We investigate the low-temperature charge-density-wave (CDW) state of bulk TaS$_2$ with a fully self-consistent DFT+U approach, over which the controversy has remained unresolved regarding the out-of-plane metallic band. By examining the innate structure of the Hubbard U potential, we reveal that the conventional use of atomic-orbital basis could seriously misevaluate the electron correlation in the CDW state. By adopting a generalized basis, covering the whole David star, we successfully reproduce the Mott insulating nature with the layer-by-layer antiferromagnetic order. Similar consideration should be applied for description of the electron correlation in molecular solid.
New theoretical proposals and experimental findings on transition metal dichalcogenide 1T-TaS$_2$ have revived interests in its possible Mott insulating state. We perform a comprehensive scanning tunneling microscopy and spectroscopy experiment on different single-step areas in pristine 1T-TaS$_2$. After accurately determining the relative displacement of Star-of-David super-lattices in two layers, we find different stacking orders corresponding to the different electronic states measured on the upper terrace. The center-to-center stacking corresponds to the universal large gap, while other stacking orders correspond to a reduced or suppressed gap in the electronic spectrum. Adopting a unified model, we conclude that the universal large gap is a correlation-induced Mott gap from the single-layer property. Our work provides more evidence about the surface electronic state and deepens our understanding of the Mott insulating state in 1T-TaS$_2$.
1T-TaS$_2$ is a prototypical charge-density-wave (CDW) system with a Mott insulating ground state. Usually, a Mott insulator is accompanied by an antiferromagnetic state. However, the antiferromagnetic order had never been observed in 1T-TaS$_2$. Here, we report the stabilization of the antiferromagnetic order by the intercalation of a small amount of Fe into the van der Waals gap of 1T-TaS$_2$, i.e. forming 1T-Fe$_{0.05}$TaS$_2$. Upon cooling from 300~K, the electrical resistivity increases with a decreasing temperature before reaching a maximum value at around 15~K, which is close to the Neel temperature determined from our magnetic susceptibility measurement. The antiferromagnetic state can be fully suppressed when the sample thickness is reduced, indicating that the antiferromagnetic order in Fe$_{0.05}$TaS$_2$ has a non-negligible three-dimensional character. For the bulk Fe$_{0.05}$TaS$_2$, a comparison of our high pressure electrical transport data with that of 1T-TaS$_2$ indicates that, at ambient pressure, Fe$_{0.05}$TaS$_2$ is in the nearly commensurate charge-density-wave (NCCDW) phase near the border of the Mott insulating state. The temperature-pressure phase diagram thus reveals an interesting decoupling of the antiferromagnetism from the Mott insulating state.
Strongly correlated systems exhibit intriguing properties caused by intertwined microscopic in- teractions that are hard to disentangle in equilibrium. Employing non-equilibrium time-resolved photoemission spectroscopy on the quasi-two-dimensional transition-metal dichalcogenide 1T-TaS$_2$, we identify a spectroscopic signature of double occupied sites (doublons) that are reflects fundamental Mott physics. Doublon-hole recombination is estimated to occur on time scales of one electronic hopping cycle $hbar/Japprox$ 14 fs. Despite strong electron-phonon coupling the dynamics can be explained by purely electronic effects captured by the single band Hubbard model, where thermalization is fast in the small-gap regime. Qualitative agreement with the experimental results however requires the assumption of an intrinsic hole-doping. The sensitivity of the doublon dynamics on the doping level provides a way to control ultrafast processes in such strongly correlated materials.
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