How the interacting electronic states and phases of layered transition-metal dichalcogenides evolve when thinned to the single-layer limit is a key open question in the study of two-dimensional ...materials. Here, we use angle-resolved photoemission to investigate the electronic structure of monolayer VSe2 grown on bilayer graphene/SiC. While the global electronic structure is similar to that of bulk VSe2, we show that, for the monolayer, pronounced energy gaps develop over the entire Fermi surface with decreasing temperature below T c = 140 ± 5 K, concomitant with the emergence of charge-order superstructures evident in low-energy electron diffraction. These observations point to a charge-density wave instability in the monolayer that is strongly enhanced over that of the bulk. Moreover, our measurements of both the electronic structure and of X-ray magnetic circular dichroism reveal no signatures of a ferromagnetic ordering, in contrast to the results of a recent experimental study as well as expectations from density functional theory. Our study thus points to a delicate balance that can be realized between competing interacting states and phases in monolayer transition-metal dichalcogenides.
Abstract
Pressure represents a clean tuning parameter for traversing the complex phase diagrams of interacting electron systems, and as such has proved of key importance in the study of quantum ...materials. Application of controlled uniaxial pressure has recently been shown to more than double the transition temperature of the unconventional superconductor Sr
2
RuO
4
, leading to a pronounced peak in
T
c
versus strain whose origin is still under active debate. Here we develop a simple and compact method to passively apply large uniaxial pressures in restricted sample environments, and utilise this to study the evolution of the electronic structure of Sr
2
RuO
4
using angle-resolved photoemission. We directly visualise how uniaxial stress drives a Lifshitz transition of the γ-band Fermi surface, pointing to the key role of strain-tuning its associated van Hove singularity to the Fermi level in mediating the peak in
T
c
. Our measurements provide stringent constraints for theoretical models of the strain-tuned electronic structure evolution of Sr
2
RuO
4
. More generally, our experimental approach opens the door to future studies of strain-tuned phase transitions not only using photoemission but also other experimental techniques where large pressure cells or piezoelectric-based devices may be difficult to implement.
There has been widespread interest in using interfaces of transition-metal oxides as a platform to control not only their electronic structure, as in semiconductor heterostructures, but also to tune ...between different collective phases. A major goal is to realize states of the quantum many-body system that are not found in the bulk phase diagrams of the constituent materials. Here, we perform a combined experimental and theoretical study of the delafossite oxide metals PdCoO
2
and PdCrO
2
, finding how electronic reconstructions at their polar surfaces drive instabilities to itinerant surface ferromagnetism. Neither compound supports ferromagnetism in bulk, with PdCrO
2
a bulk antiferromagnet, demonstrating how a delicate competition of magnetic correlations can be engineered by intrinsic self-doping at a polar surface or interface.
The ability to modulate the collective properties of correlated electron systems at their interfaces and surfaces underpins the burgeoning field of “designer” quantum materials. Here, we show how an electronic reconstruction driven by surface polarity mediates a Stoner-like magnetic instability to itinerant ferromagnetism at the Pd-terminated surface of the nonmagnetic delafossite oxide metal PdCoO
2
. Combining angle-resolved photoemission spectroscopy and density-functional theory calculations, we show how this leads to a rich multiband surface electronic structure. We find similar surface state dispersions in PdCrO
2
, suggesting surface ferromagnetism persists in this sister compound despite its bulk antiferromagnetic order.
The interplay between spin–orbit coupling and structural inversion symmetry breaking in solids has generated much interest due to the nontrivial spin and magnetic textures which can result. Such ...studies are typically focused on systems where large atomic number elements lead to strong spin–orbit coupling, in turn rendering electronic correlations weak. In contrast, here we investigate the temperature-dependent electronic structure of Ca3Ru2O7, a 4d oxide metal for which both correlations and spin–orbit coupling are pronounced and in which octahedral tilts and rotations combine to mediate both global and local inversion symmetry-breaking polar distortions. Our angle-resolved photoemission measurements reveal the destruction of a large hole-like Fermi surface upon cooling through a coupled structural and spin-reorientation transition at 48 K, accompanied by a sudden onset of quasiparticle coherence. We demonstrate how these result from band hybridization mediated by a hidden Rashba-type spin–orbit coupling. This is enabled by the bulk structural distortions and unlocked when the spin reorients perpendicular to the local symmetry-breaking potential at the Ru sites. We argue that the electronic energy gain associated with the band hybridization is actually the key driver for the phase transition, reflecting a delicate interplay between spin–orbit coupling and strong electronic correlations and revealing a route to control magnetic ordering in solids.
The transition-metal dichalcogenide VSe2 exhibits an increased charge density wave transition temperature and an emerging insulating phase when thinned to a single layer. Here, we investigate the ...interplay of electronic and lattice degrees of freedom that underpin these phases in single-layer VSe2 using ultrafast pump–probe photoemission spectroscopy. In the insulating state, we observe a light-induced closure of the energy gap, which we disentangle from the ensuing hot carrier dynamics by fitting a model spectral function to the time-dependent photoemission intensity. This procedure leads to an estimated time scale of 480 fs for the closure of the gap, which suggests that the phase transition in single-layer VSe2 is driven by electron–lattice interactions rather than by Mott-like electronic effects. The ultrafast optical switching of these interactions in SL VSe2 demonstrates the potential for controlling phase transitions in 2D materials with light.
We revisit the enduring problem of the 2 × 2 × 2 charge density wave (CDW) order in TiSe2, utilizing photon energy-dependent angle-resolved photoemission spectroscopy to probe the full ...three-dimensional high- and low-temperature electronic structure. Our measurements demonstrate how a mismatch of dimensionality between the 3D conduction bands and the quasi-2D valence bands in this system leads to a hybridization that is strongly kz dependent. While such a momentum-selective coupling can provide the energy gain required to form the CDW, we show how additional “passenger” states remain, which couple only weakly to the CDW and thus dominate the low-energy physics in the ordered phase of TiSe2.
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Abstract
Reducing the thickness of a material to its two-dimensional (2D) limit can have dramatic consequences for its collective electronic states, including magnetism, superconductivity, and charge ...and spin ordering. An extreme case is TiTe
2
, where a charge density wave (CDW) emerges in the single-layer, which is absent for the bulk compound, and whose origin is still poorly understood. Here, we investigate the electronic band structure evolution across this CDW transition using temperature-dependent angle-resolved photoemission spectroscopy. Our study reveals an orbital-selective band hybridisation between the backfolded conduction and valence bands occurring at the CDW phase transition, which in turn leads to a significant electronic energy gain, underpinning the CDW transition. For the bulk compound, we show how this energy gain is almost completely suppressed due to the three-dimensionality of the electronic band structure, including via a
k
z
-dependent band inversion which switches the orbital character of the valence states. Our study thus sheds new light on how control of the electronic dimensionality can be used to trigger the emergence of new collective states in 2D materials.
The ability to modulate the collective properties of correlated electron systems at their interfaces and surfaces underpins the burgeoning field of "designer" quantum materials. Here, we show how an ...electronic reconstruction driven by surface polarity mediates a Stoner-like magnetic instability to itinerant ferromagnetism at the Pd-terminated surface of the nonmagnetic delafossite oxide metal PdCoO
Combining angle-resolved photoemission spectroscopy and density-functional theory calculations, we show how this leads to a rich multiband surface electronic structure. We find similar surface state dispersions in PdCrO
, suggesting surface ferromagnetism persists in this sister compound despite its bulk antiferromagnetic order.
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