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Register a new userFriction Anomalies at First-Order Transition Spinodals: 1T-TaS$_2$
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Revealing phase transitions of solids through mechanical anomalies in the friction of nanotips sliding on their surfaces is an unconventional and instructive tool for continuous transitions, unexplored for first-order ones. Owing to slow nucleation, first-order structural transformations generally do not occur at the precise crossing of free energies, but hysteretically, near the spinodal temperatures where, below and above the thermodynamic transition temperature, one or the other metastable free energy branches terminates. The spinodal transformation, a collective one-shot event with no heat capacity anomaly, is easy to trigger by a weak external perturbations. Here we propose that even the gossamer mechanical action of an AFM tip may locally act as a surface trigger, narrowly preempting the spontaneous spinodal transformation, and making it observable as a nanofrictional anomaly. Confirming this expectation, the CCDW-NCCDW first-order transition of the important layer compound 1T-TaS$_2$ is shown to provide a demonstration of this effect.
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A damping-like spin orbit torque (SOT) is a prerequisite for ultralow power spin logic devices. Here, we report on the damping-like SOT in just one monolayer of the conducting transition metal dichalcogenide (TMD) TaS$_2$ interfaced with a NiFe (Py) ferromagnetic layer. The charge-spin conversion efficiency is found to be 0.25$pm$0.03 and the spin Hall conductivity (2.63 $times$ 10$^5$ $frac{hbar}{2e}$ $Omega^{-1}$ m$^{-1}$) is found to be superior to values reported for other TMDs. The origin of this large damping-like SOT can be found in the interfacial properties of the TaS$_2$/Py heterostructure, and the experimental findings are complemented by the results from density functional theory calculations. The dominance of damping-like torque demonstrated in our study provides a promising path for designing next generation conducting TMD based low-powered quantum memory devices.
1T-TaS$_2$ undergoes successive phase transitions upon cooling and eventually enters an insulating state of mysterious origin. Some consider this state to be a band insulator with interlayer stacking order, yet others attribute it to Mott physics that support a quantum spin liquid state.Here, we determine the electronic and structural properties of 1T-TaS$_2$ using angle-resolved photoemission spectroscopy and X-Ray diffraction. At low temperatures, the 2$pi$/2c-periodic band dispersion, along with half-integer-indexed diffraction peaks along the c axis, unambiguously indicates that the ground state of 1T-TaS$_2$ is a band insulator with interlayer dimerization. Upon heating, however, the system undergoes a transition into a Mott insulating state, which only exists in a narrow temperature window. Our results refute the idea of searching for quantum magnetism in 1T-TaS$_2$ only at low temperatures, and highlight the competition between on-site Coulomb repulsion and interlayer hopping as a crucial aspect for understanding the materials electronic properties.
We present a state-of-the-art density functional theory (DFT) study which models crucial features of the partially disordered orbital order stacking in the prototypical layered transition metal dichalcogenide 1T-TaS2 . Our results not only show that DFT models with realistic assumptions about the orbital order perpendicular to the layers yield band structures which agree remarkably well with experiments. They also demonstrate that DFT correctly predicts the formation of an excitation pseudo-gap which is commonly attributed to Mott-Hubbard type electron-electron correlations. These results highlight the importance of interlayer interactions in layered transition metal dichalcogenides and serve as an intriguing example of how disorder within an electronic crystal can give rise to pseudo-gap features.
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$_2$.
The ability to tune material properties using gate electric field is at the heart of modern electronic technology. It is also a driving force behind recent advances in two-dimensional systems, such as gate-electric-field induced superconductivity and metal-insulator transition. Here we describe an ionic field-effect transistor (termed iFET), which uses gate-controlled lithium ion intercalation to modulate the material property of layered atomic crystal 1T-TaS$_2$. The extreme charge doping induced by the tunable ion intercalation alters the energetics of various charge-ordered states in 1T-TaS$_2$, and produces a series of phase transitions in thin-flake samples with reduced dimensionality. We find that the charge-density-wave states in 1T-TaS$_2$ are three-dimensional in nature, and completely collapse in the two-dimensional limit defined by their critical thicknesses. Meanwhile the ionic gating induces multiple phase transitions from Mott-insulator to metal in 1T-TaS$_2$ thin flakes at low temperatures, with 5 orders of magnitude modulation in their resistance. Superconductivity emerges in a textured charge-density-wave state induced by ionic gating. Our method of gate-controlled intercalation of 2D atomic crystals in the bulk limit opens up new possibilities in searching for novel states of matter in the extreme charge-carrier-concentration limit.
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