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Register a new userMottness collapse without metallisation in the domain walls of triangular-lattice Mott insulator 1T-TaS$_2$
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1T-TaS$_2$ is a charge-density-wave (CDW) compound with a Mott-insulating ground state. The metallic state obtained by doping, substitution or pulsed charge injection is characterized by an emergent CDW domain wall network, while single domain walls can be found in the pristine Mott state. Here we study whether and how the single walls become metallic. Tunneling spectroscopy reveals partial suppression of the Mott gap and the presence of in-gap states strongly localized at the domain-wall sites. Using the real-space dynamical mean field theory description of the strongly correlated quantum-paramagnet ground state we show that the local gap suppression follows from the increased hopping along the connected zig-zag chain of lattice sites forming the domain wall, and that full metallisation is preempted by the splitting of the quasiparticle band into bonding and antibonding sub-bands due to the structural dimerization of the wall, explaining the presence of the in-gap states and the low density of states at the Fermi level.
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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.
Tuning the electronic properties of a matter is of fundamental interest in scientific research as well as in applications. Recently, the Mott insulator-metal transition has been reported in a pristine layered transition metal dichalcogenides 1T-TaS$_2$, with the transition triggered by an optical excitation, a gate controlled intercalation, or a voltage pulse. However, the sudden insulator-metal transition hinders an exploration of how the transition evolves. Here, we report the strain as a possible new tuning parameter to induce Mott gap collapse in 1T-TaS$_2$. In a strain-rich area, we find a mosaic state with distinct electronic density of states within different domains. In a corrugated surface, we further observe and analyze a smooth evolution from a Mott gap state to a metallic state. Our results shed new lights on the understanding of the insulator-metal transition and promote a controllable strain engineering on the design of switching devices in the future.
In a certain regime of low carrier densities and strong correlations, electrons can crystallize into a periodic arrangement of charge known as Wigner crystal. Such phases are particularly interesting in one dimension (1D) as they display a variety of charge and spin ground states which may be harnessed in quantum devices as high-fidelity transmitters of spin information. Recent theoretical studies suggest that the strong Coulomb interactions in Mott insulators and other flat band systems, may provide an attractive higher temperature platform for Wigner crystallization, but due to materials and device constraints experimental realization has proven difficult. In this work we use scanning tunneling microscopy at liquid helium temperatures to directly image the formation of a 1D Wigner crystal in a Mott insulator, TaS$_2$. Charge density wave domain walls in TaS$_2$ create band bending and provide ideal conditions of low densities and strong interactions in 1D. STM spectroscopic maps show that once the lower Hubbard band crosses the Fermi energy, the charges rearrange to minimize Coulomb energy, forming zigzag patterns expected for a 1D Wigner crystal. The zigzag charge patterns show characteristic noise signatures signifying charge or spin fluctuations induced by the tunneling electrons, which is expected for this more fragile condensed state. The observation of a Wigner crystal at orders of magnitude higher temperatures enabled by the large Coulomb energy scales combined with the low density of electrons, makes TaS$_2$ a promising system for exploiting the charge and spin order in 1D Wigner crystals.
1T-TaS$_2$ is a cluster Mott insulator on the triangular lattice with 13 Ta atoms forming a star of David cluster as the unit cell. We derive a two dimensional XXZ spin-1/2 model with four-spin ring exchange term to describe the effective low energy physics of a monolayer 1T-TaS$_2$, where the effective spin-1/2 degrees of freedom arises from the Kramers degenerate spin-orbital states on each star of David. A large scale density matrix renormalization group simulation is further performed on this effective model and we find a gapless spin liquid phase with spinon Fermi surface at moderate to large strength region of four-spin ring exchange term. All peaks in the static spin structure factor are found to be located on the $2k_F$ surface of half-filled spinon on the triangular lattice. Experiments to detect the spinon Fermi surface phase in 1T-TaS$_2$ are discussed.
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.
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