Strong electron correlation can induce Mott insulating behaviour and produce intriguing states of matter such as unconventional superconductivity and quantum spin liquids. Recent advances in van der Waals material synthesis enable the exploration of Mott systems in the two-dimensional limit. Here we report characterization of the local electronic properties of single- and few-layer 1T-TaSe2 via spatial- and momentum-resolved spectroscopy involving scanning tunnelling microscopy and angle-resolved photoemission. Our results indicate that electron correlation induces a robust Mott insulator state in single-layer 1T-TaSe2 that is accompanied by unusual orbital texture. Interlayer coupling weakens the insulating phase, as shown by reduction of the energy gap and quenching of the correlation-driven orbital texture in bilayer and trilayer 1T-TaSe2. This establishes single-layer 1T-TaSe2 as a useful platform for investigating strong correlation physics in two dimensions. The electrons that contribute to the Mott insulator state in single-layer 1T-TaSe2 are shown to also have a rich variation in their orbital occupation. As more layers are added, both the insulating state and orbital texture weaken.

Recent experiments have found that monolayer 1H-TaS2 grown on Au(111) lacks the charge density wave (CDW) instability exhibited by bulk 2H-TaS2. Additionally, angle-resolved photoemission spectroscopy measurements suggest that the monolayer becomes strongly electron doped by the substrate. While density functional theory (DFT) calculations have shown that electron doping can suppress the CDW instability in monolayer 1H-TaS2, it has been suggested that the actual charge transfer from the substrate may be much smaller than the apparent doping deduced from photoemission data. We present DFT calculations of monolayer 1H-TaS2 on Au(111) to explore substrate effects beyond doping. We find that the CDW instability is suppressed primarily by strong S-Au interactions rather than by doping. The S-Au interaction results in a structural distortion of the TaS2 monolayer characterized by both lateral and out-of-plane atomic displacements and a 7 x 7 periodicity dictated by the commensurate interface with Au. Simulated STM images of this 7 x 7 distorted structure are consistent with experimental STM images. In contrast, we find a robust 3 x 3 CDW phase in monolayer 1H-TaS2 on a graphene substrate with which there is minimal interaction.

33 Strong electron correlation can induce Mott insulating behavior and produce intriguing states of 34 matter such as unconventional superconductivity and quantum spin liquids. Recent advances in 35 van der Waals material synthesis enable the exploration of Mott systems in the two-dimensional 36 limit. Here we report characterization of the local electronic properties of singleand few-layer 37 1T-TaSe2 via spatialand momentum-resolved spectroscopy involving scanning tunneling 38 microscopy and angle-resolved photoemission. Our results indicate that electron correlation 39 induces a robust Mott insulator state in single-layer 1T-TaSe2 that is accompanied by unusual 40 orbital texture. Interlayer coupling weakens the insulating phase, as shown by reduction of the 41 energy gap and quenching of the correlation-driven orbital texture in bilayer and trilayer 1T42 TaSe2. This establishes single-layer 1T-TaSe2 as a useful platform for investigating strong 43 correlation physics in two dimensions. 44

: Strong electron correlation can induce Mott insulating behavior and produce intriguing states of 34 matter such as unconventional superconductivity and quantum spin liquids. Recent advances in 35 van der Waals material synthesis enable the exploration of Mott systems in the two-dimensional 36 limit. Here we report characterization of the local electronic properties of single- and few-layer 37 1T-TaSe 2 via spatial- and momentum-resolved spectroscopy involving scanning tunneling 38 microscopy and angle-resolved photoemission. Our results indicate that electron correlation 39 induces a robust Mott insulator state in single-layer 1T-TaSe 2 that is accompanied by unusual 40 orbital texture. Interlayer coupling weakens the insulating phase, as shown by reduction of the 41 energy gap and quenching of the correlation-driven orbital texture in bilayer and trilayer 1T- 42 TaSe 2 . This establishes single-layer 1T-TaSe 2 as a useful platform for investigating strong 43 correlation physics in two dimensions. Here we report a combined scanning tunneling microscopy/spectroscopy (STM/STS), angle- resolved photoemission spectroscopy (ARPES), and theoretical study of the electronic structure 72 of single-layer 1T-TaSe 2 . Our results show that in the absence of interlayer coupling single-layer 73 1T-TaSe 2 hosts a Mott-insulating ground state that exhibits a 109 ± 18 meV energy gap and 74 unusual orbital texture. Bilayer and trilayer 1T-TaSe 2 with shifted stacking order exhibit 75 successively smaller energy gaps and show no signs of the unusual orbital texture seen in the 76 single-layer limit. The single-layer band structure and density of states of 1T-TaSe 2 are found to 77 be consistent with DFT+U calculations, confirming its Mott insulator nature. The unusual single- layer orbital texture, however, is not captured by DFT+U, but is consistent with the behavior expected for a weakly screened, strongly correlated 2D insulator. Reduction of the 1T-TaSe 2 bandgap and quenching of the unusual orbital texture by the addition of new layers shows that 81 the effect of interlayer on shifted-stacked 1T-TaSe 2 is to weaken the Mott behavior.

Mott insulating behavior is induced by strong electron correlation and can lead to exotic states of matter such as unconventional superconductivity and quantum spin liquids. Recent advances in van der Waals material synthesis enable the exploration of novel Mott systems in the two-dimensional limit. Here we report characterization of the local electronic properties of single- and few-layer 1T-TaSe2 via spatial- and momentum-resolved spectroscopy involving scanning tunneling microscopy and angle-resolved photoemission. Our combined experimental and theoretical study indicates that electron correlation induces a robust Mott insulator state in single-layer 1T-TaSe2 that is accompanied by novel orbital texture. Inclusion of interlayer coupling weakens the insulating phase in 1T-TaSe2, as seen by strong reduction of its energy gap and quenching of its correlation-driven orbital texture in bilayer and trilayer 1T-TaSe2. Our results establish single-layer 1T-TaSe2 as a useful new platform for investigating strong correlation physics in two dimensions.

Recent ARPES measurements of single-layer 1H-TaS2 grown on Au(111) suggest strong electron doping from the substrate. In addition, STM/STS measurements on this system show suppression of the charge-density-wave (CDW) instability that occurs in bulk 2H-TaS2. We present results from ab initio DFT calculations of free-standing single-layer 1H-TaS2 to explore the effects of doping on the CDW. In the harmonic approximation, we find that a lattice instability along the Gamma-M line occurs in the undoped monolayer, consistent with the bulk 3x3 CDW ordering vector. Doping removes the CDW instability, in agreement with the experimental findings. The doping and momentum dependence of both the electron-phonon coupling and of the bare phonon energy (unscreened by metallic electrons) determine the stability of lattice vibrations. Electron doping also causes an expansion of the lattice, so strain is a secondary but also relevant effect.

Recent ARPES measurements of single-layer 1H-TaS2 grown on Au(111) suggest strong electron doping from the substrate. In addition, STM/STS measurements on this system show suppression of the charge-density-wave (CDW) instability that occurs in bulk 2H-TaS2. We present results from ab initio DFT calculations of free-standing single-layer 1H-TaS2 to explore the effects of doping on the CDW. In the harmonic approximation, we find that a lattice instability along the Gamma-M line occurs in the undoped monolayer, consistent with the bulk 3x3 CDW ordering vector. Doping removes the CDW instability, in agreement with the experimental findings. The doping and momentum dependence of both the electron-phonon coupling and of the bare phonon energy (unscreened by metallic electrons) determine the stability of lattice vibrations. Electron doping also causes an expansion of the lattice, so strain is a secondary but also relevant effect.

We present first-principles calculations of the vibrational properties of the transition metal dichalcogenide 1T-TaS_2 for various thicknesses in the high-temperature (undistorted) phase and the low-temperature commensurate charge density wave (CDW) phase. We also present measurements of the Raman spectra for bulk, few-layer, and monolayer samples at temperatures well below that of the bulk transition to the commensurate phase. Through our calculations, we identify the low-frequency folded-back acoustic modes as a convenient signature of the commensurate CDW wave structure in vibrational spectra. In our measured Raman spectra, this signature is clearly evident in all of the samples, indicating that the commensurate phase remains the ground state as the material is thinned, even down to a single layer. This is in contrast to some previous studies which suggest a suppression of the commensurate CDW transition in thin flakes. We also use polarized Raman spectroscopy to probe c-axis orbital texture in the low-T phase, which has recently been suggested as playing a role in the metal-insulator transition that accompanies the structural transition to the commensurate CDW phase.

We examine the adsorption of a single Ni atom on a monolayer of MgO on a Ag substrate using DFT and DFT + U computational approaches. We find that the electronic and magnetic properties vary considerably across the three binding sites of the surface. Two of the binding sites are competitive in energy, and the preferred site depends on the strength of the on-site Coulomb interaction U. These results can be understood in terms of the competition between bonding and magnetism for surface adsorbed transition metal atoms. Comparisons are made with a recent experimental and theoretical study of Co on MgO/Ag, and implications for scanning tunneling microscopy experiments on the Ni system are discussed.

Designing systems with large magnetic anisotropy is critical to realize nanoscopic magnets. Thus far, the magnetic anisotropy energy per atom in single-molecule magnets and ferromagnetic films remains typically one to two orders of magnitude below the theoretical limit imposed by the atomic spin-orbit interaction. We realized the maximum magnetic anisotropy for a 3d transition metal atom by coordinating a single Co atom to the O site of an MgO(100) surface. Scanning tunneling spectroscopy reveals a record-high zero-field splitting of 58 millielectron volts as well as slow relaxation of the Co atom's magnetization. This striking behavior originates from the dominating axial ligand field at the O adsorption site, which leads to out-of-plane uniaxial anisotropy while preserving the gas-phase orbital moment of Co, as observed with x-ray magnetic circular dichroism.