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Renew interest in the charge density wave phase of TiSe$_2$ stems from the realization of its unique driving mechanism, the so called excitonic insulator phase. Existing claims are motivated by model calculations of the band structure. In this study, angle resolved photoemsision spectroscopy and density functional theory for TiSe$_2$ are directly compared. The substantial discrepancies found between the two descriptions cast serious doubts on the exitonic insulator scenario as the correct physical mechanism underlying the periodic lattice distortion at low temperature. In particular, the formation of a valence-conduction hybridization gap in the bulk band structure is not present in the experimental data. Therefore, the origin of the structural transition in TiSe$_2$ cannot be fully explained within the existing theoretical models., Comment: 5 pages, 4 figures

The electronic origin of the structural transition in 1T-VSe$_2$ is re-evaluated through an extensive angle-resolved photoemission spectroscopy experiment. The components of the band structure, missing in previous reports, are revealed. Earlier observations, shown to be temperature independent and therefore not correlated with the phase transition, are explained in terms of the increased complexity of the band structure close to the Fermi level. Only the overall size of the Fermi surface is found to be positively correlated with the phase transition at 110 K. These observations, quite distant from the charge density wave scenario commonly considered for 1T-VSe$_2$, bring fresh perspectives toward the correct description of structural transitions in dichalcogenides materials., Comment: 7 pages, 4 figures

Kagome vanadates {\it A}V$_3$Sb$_5$ display unusual low-temperature electronic properties including charge density waves (CDW), whose microscopic origin remains unsettled. Recently, CDW order has been discovered in a new material ScV$_6$Sn$_6$, providing an opportunity to explore whether the onset of CDW leads to unusual electronic properties. Here, we study this question using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). The ARPES measurements show minimal changes to the electronic structure after the onset of CDW. However, STM quasiparticle interference (QPI) measurements show strong dispersing features related to the CDW ordering vectors. A plausible explanation is the presence of a strong momentum-dependent scattering potential peaked at the CDW wavevector, associated with the existence of competing CDW instabilities. Our STM results further indicate that the bands most affected by the CDW are near vHS, analogous to the case of {\it A}V$_3$Sb$_5$ despite very different CDW wavevectors., Comment: 33 pages, 4 figures

Kagome metals with charge density wave (CDW) order exhibit a broad spectrum of intriguing quantum phenomena. The recent discovery of the novel kagome CDW compound ScV$_6$Sn$_6$ has spurred significant interest. However, understanding the interplay between CDW and the bulk electronic structure has been obscured by a profusion of surface states and terminations in this quantum material. Here, we employ photoemission spectroscopy and potassium dosing to elucidate the complete bulk band structure of ScV$_6$Sn$_6$, revealing multiple van Hove singularities near the Fermi level. We surprisingly discover a robust spin-polarized topological Dirac surface resonance state at the M point within the two-fold van Hove singularities. Assisted by the first-principle calculations, the temperature dependence of the $k_z$- resolved ARPES spectrum provides unequivocal evidence for the proposed $\sqrt{3}$$\times$$\sqrt{3}$$\times3$ charge order over other candidates. Our work not only enhances the understanding of the CDW-dependent bulk and surface states in ScV$_6$Sn$_6$ but also establishes an essential foundation for potential manipulation of the CDW order in kagome materials., Comment: To appear in PRB

Heterostructures of topological insulator Bi$_2$Se$_3$ on transition metal dichalcogenides (TMDCs) offer a new materials platform for studying novel quantum states by exploiting the interplay among topological orders, charge orders and magnetic orders. The diverse interface attributes, such as material combination, charge re-arrangement, defect and strain, can be utilized to manipulate the quantum properties of this class of materials. Recent experiments of Bi$_2$Se$_3$/NbSe$_2$ heterostructures show signatures of strong Rashba band splitting due to the presence of a BiSe buffer layer, but the atomic level mechanism is not fully understood. We conduct first-principles studies of the Bi$_2$Se$_3$/BiSe/TMDC heterostructures with five different TMDC substrates (1T phase VSe$_2$, MoSe$_2$, TiSe$_2$, and 2H phase NbSe$_2$, MoSe$_2$). We find significant charge transfer at both BiSe/TMDC and Bi$_2$Se$_3$/BiSe interfaces driven by the work function difference, which stabilizes the BiSe layer as an electron donor and creates interface dipole. The electric field of the interface dipole breaks the inversion symmetry in the Bi$_2$Se$_3$ layer, leading to the giant Rashba band splitting in two quintuple layers and the recovery of the Dirac point in three quintuple layers, with the latter otherwise only occurring in thicker samples with at least six Bi$_2$Se$_3$ quintuple layers. Besides, we find that strain can significantly affect the charge transfer at the interfaces. Our study presents a promising avenue for tuning topological properties in heterostructures of two-dimensional materials, with potential applications in quantum devices., Comment: 10 pages, 6 figures

Electronic correlation effects are manifested in quantum materials when either the onsite Coulomb repulsion is large or the electron kinetic energy is small. The former is the dominant effect in the cuprate superconductors or heavy fermion systems while the latter in twisted bilayer graphene or geometrically frustrated metals. However, the simultaneous cooperation of both effects in the same quantum material--the design principle to produce a correlated topological flat bands pinned at the Fermi level--remains rare. Here, using angle-resolved photoemission spectroscopy, we report the observation of a flat band at the Fermi level in a 3$d$ pyrochlore metal CuV$_2$S$_4$. From a combination of first-principles calculations and slave-spin calculations, we understand the origin of this band to be a destructive quantum-interference effect associated with the V pyrochlore sublattice and further renormalization to the Fermi level by electron interactions in the partially filled V $t_{2g}$ orbitals. As a result, we find transport behavior that indicates a deviation from Fermi-liquid behavior as well as a large Sommerfeld coefficient. Our work demonstrates the pathway into correlated topology by constructing and pinning correlated flat bands near the Fermi level out of a pure $d$-electron system by the combined cooperation of local Coulomb interactions and geometric frustration in a pyrochlore lattice system., Comment: 23 pages, 4 figures, to appear in Nature Physics

Here we present the family of titanium-based kagome metals of the form LnTi$_3$Bi$_4$ (Ln: La...Gd$^{3+}$, Eu$^{2+}$, Yb$^{2+}$). Single crystal growth methods are presented alongside detailed magnetic and thermodynamic measurements. The orthorhombic (Fmmm) LnTi$_3$Bi$_4$ family of compounds exhibit slightly distorted titanium-based kagome nets interwoven with zig-zag lanthanide-based (Ln) chains. Crystals are easily exfoliated parallel to the kagome sheets and angular resolved photoemission (ARPES) measurements highlight the intricacy of the electronic structure in these compounds, with Dirac points existing at the Fermi level. The magnetic properties and the associated anisotropy emerge from the quasi-1D zig-zag chains of Ln, and impart a wide array of magnetic ground states ranging from anisotropic ferromagnetism to complex antiferromagnetism with a cascade of metamagnetic transitions. Kagome metals continue to provide a rich direction for the exploration of magnetic, topologic, and highly correlated behavior. Our work here introduces the LnTi$_3$Bi$_4$ compounds to augment the continuously expanding suite of complex and interesting kagome materials.

Here, our angle resolved photoemission spectroscopy experiment reveled that the surface band structure of the 1T-VSe2 host electronic states that was not predicted or probed before. Earlier claims to support charge density wave phase can be all explained in terms of these new findings. Its Fermi surface found to be not gaped at any point of the Brillouin zone and warping effect on the electronic structure, attributed to the lattice distortion previously, is due to the different dispersion of the multiple bands. Based on these new findings and interpretations, charge density wave induced modification on the electronic structure of 1T-VSe2 needs to be reconstructed in the future studies., Comment: 4

High-resolution angle-resolved photoemission experiments reveal subtle modifications of the surface electronic structure of VSe2. Most remarkably, we show that superconductivity can be induced in VSe2 by the right selection of substrate and growth parameters. Evidence for the superconducting state comes from the simultaneous detection of spectral kink, quasiparticle peak, Fermi gap, and their evolution with the temperature. The observation of Bogoliubov-like back-bending bands at low temperatures, signaling electron-hole pairing, further supports the presence of superconductivity in this system. The photoemission experiment also provides evidence for a formation of a pseudogap state at high temperatures, characterized by the progressive quenching of the quasiparticle peak feature coexisting with a persistent gap at the Fermi level, a behavior reminiscent of high-Tc superconductors. We attributed the origin of superconductivity in these VSe2 films to a modified Fermi surface combined with the formation of van Hove singularity points with a binding energy corresponding to the chemical potential of the system. Although Tc cannot be accurately determined from photoemission data, observations based on the survival temperature of the quasiparticle peak suggest that Tc could be as high as 100 ? 5 K and substantially higher than previous reports for any transition metal dichalcogenides., Comment: 3 figures, 8 pages

SmB6 is a strongly correlated material that has been attributed as a topological insulator and a Kondo insulator. Recent studies have found the topological surface states and low temperature insulating character to be profoundly robust against magnetic and non-magnetic impurities. Here, we use angle resolved photoemission spectroscopy to chart the evolution of topologically-linked electronic structure features versus magnetic doping and temperature in Sm$_{1-x}$M$_x$B$_6$ (M=Eu, Ce). Topological coherence phenomena are observed out to unprecedented ~30% Eu and 50% Ce concentrations that represent extreme nominal hole and electron doping, respectively. Theoretical analysis reveals that a recent re-designation of the topologically inverted band symmetries provides a natural route to reconciling the persistence of topological surface state emergence even as the insulating gap is lost through decoherence.

Dispersionless flat bands are proposed to be a fundamental ingredient to achieve the various sought after quantum states of matter including high-temperature superconductivity1-4 and fractional quantum Hall effect5-6. Materials with such peculiar electronic states, however, are very rare and often exhibit very complex band structures. Here, we report on the emergence of a flat band with a possible insulating ground state in the sub-monolayer VSe2 / Bi2Se3 heterostructure by means of angle-resolved photoemission spectroscopy and scanning tunneling microscopy. The flat band is dispersionless along the kll and kz momenta, filling the entire Brillouin zone, and it exhibits a complex circular dichroism signal reversing the sign at several points of the Brillouin zone. These properties together with the presence of a Moir\'e patterns in VSe2 suggest that the flat band is not a trivial disorder or confinement effect and could even be topologically non-trivial. Another intriguing finding is that the flat band does not modify the Dirac cone of Bi2Se3 around the Dirac point. Furthermore, we found that the flat band and the Dirac surface states of Bi2Se3 have opposite energy shifts with electron doping. This opens a novel way of controlling the spin texture of photocurrents as well as the transport properties of the heterostructure. These features make this flat band remarkably distinguishable from previous findings and our methodology can be applied to other systems opening a promising pathway to realize strongly correlated quantum effects in topological materials., Comment: 25 pages, 5 figures

Samarium hexaboride is a candidate for the topological Kondo insulator state, in which Kondo coherence is predicted to give rise to an insulating gap spanned by topological surface states. Here we investigate the surface and bulk electronic properties of magnetically alloyed Sm1-xMxB6 (M=Ce, Eu), using angle-resolved photoemission spectroscopy (ARPES) and complementary characterization techniques. Remarkably, topologically nontrivial bulk and surface band structures are found to persist in highly modified samples with up to 30% Sm substitution, and to coexist with antiferromagnetism in the case of Eu doping. The results are interpreted in terms of a hierarchy of energy scales, in which surface state emergence is linked to the formation of a direct Kondo gap, while low temperature transport trends depend on the indirect gap., Comment: Main text with 4 figures, and supplementary PDF

In this report, we demonstrate that the energy gap in Cr-doped Bi2Se3 closes and reopens in oscillatory fashion with increasing sample thickness indicating the topological quantum phase transition driven by the quantum finite size effect. The pattern of the oscillation provides an evidence that Cr-doped Bi2Se3 can have a Dirac semimetal state depending on the sample thickness and Cr-content. This resolves the puzzle of non-magnetic gap opening at the Dirac point of topological insulators. An energy gap can be opened due to the quantum finite size confinement of the bands in Dirac semimetal state. In addition, our room temperature ARPES measurements revealed a topological quantum phase transition between x = 0.06 and x = 0.08 in 7 quintuple layers Bi2-xCrxSe3 without ferromagnetism., Comment: 8 pages, 5 figures