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Angle-resolved photoemission measurements of electron-doped layers of tungsten diselenide reveal signatures of negative electronic compressibility that survive to much higher carrier densities than in conventional 2D electron gases. Tunable bandgaps 1 , extraordinarily large exciton-binding energies 2 , 3 , strong light–matter coupling 4 and a locking of the electron spin with layer and valley pseudospins 5 , 6 , 7 , 8 have established transition-metal dichalcogenides (TMDs) as a unique class of two-dimensional (2D) semiconductors with wide-ranging practical applications 9 , 10 . Using angle-resolved photoemission (ARPES), we show here that doping electrons at the surface of the prototypical strong spin–orbit TMD WSe 2 , akin to applying a gate voltage in a transistor-type device, induces a counterintuitive lowering of the surface chemical potential concomitant with the formation of a multivalley 2D electron gas (2DEG). These measurements provide a direct spectroscopic signature of negative electronic compressibility (NEC), a result of electron–electron interactions, which we find persists to carrier densities approximately three orders of magnitude higher than in typical semiconductor 2DEGs that exhibit this effect 11 , 12 . An accompanying tunable spin splitting of the valence bands further reveals a complex interplay between single-particle band-structure evolution and many-body interactions in electrostatically doped TMDs. Understanding and exploiting this will open up new opportunities for advanced electronic and quantum-logic devices.

The transition from a polaronic to a metallic state as the carrier density increases in strontium titanate overlaps with the onset and peak of the bulk superconducting behaviour. Surfaces and interfaces offer new possibilities for tailoring the many-body interactions that dominate the electrical and thermal properties of transition metal oxides 1 , 2 , 3 , 4 . Here, we use the prototypical two-dimensional electron liquid (2DEL) at the SrTiO 3 (001) surface 5 , 6 , 7 to reveal a remarkably complex evolution of electron–phonon coupling with the tunable carrier density of this system. At low density, where superconductivity is found in the analogous 2DEL at the LaAlO 3 /SrTiO 3 interface 8 , 9 , 10 , 11 , 12 , 13 , our angle-resolved photoemission data show replica bands separated by 100 meV from the main bands. This is a hallmark of a coherent polaronic liquid and implies long-range coupling to a single longitudinal optical phonon branch. In the overdoped regime the preferential coupling to this branch decreases and the 2DEL undergoes a crossover to a more conventional metallic state with weaker short-range electron–phonon interaction. These results place constraints on the theoretical description of superconductivity and allow a unified understanding of the transport properties in SrTiO 3 -based 2DELs.

Considerable attention has been given in the past few years to two-dimensional electron gases formed at the interface between two bulk insulators. It is now shown that a similar electronic system can be created on the surface of an oxide insulator simply by exposure to UV light. Many-body interactions in transition-metal oxides give rise to a wide range of functional properties, such as high-temperature superconductivity 1 , colossal magnetoresistance 2 or multiferroicity 3 . The seminal recent discovery of a two-dimensional electron gas (2DEG) at the interface of the insulating oxides LaAlO 3 and SrTiO 3 (ref.  4 ) represents an important milestone towards exploiting such properties in all-oxide devices 5 . This conducting interface shows a number of appealing properties, including a high electron mobility 4 , 6 , superconductivity 7 and large magnetoresistance 8 , and can be patterned on the few-nanometre length scale. However, the microscopic origin of the interface 2DEG is poorly understood. Here, we show that a similar 2DEG, with an electron density as large as 8×10 13  cm −2 , can be formed at the bare SrTiO 3 surface. Furthermore, we find that the 2DEG density can be controlled through exposure of the surface to intense ultraviolet light. Subsequent angle-resolved photoemission spectroscopy measurements reveal an unusual coexistence of a light quasiparticle mass and signatures of strong many-body interactions.

A detailed phenomenology of low energy excitations is a crucial starting point for microscopic understanding of complex materials, such as the cuprate high-temperature superconductors. Because of its unique momentum-space discrimination, angle-resolved photoemission spectroscopy (ARPES) is ideally suited for this task in the cuprates, where emergent phases, particularly superconductivity and the pseudogap, have anisotropic gap structure in momentum space. We present a comprehensive doping- and temperature-dependence ARPES study of spectral gaps in Bi ₂Sr ₂CaCu ₂O ₈₊δ, covering much of the superconducting portion of the phase diagram. In the ground state, abrupt changes in near-nodal gap phenomenology give spectroscopic evidence for two potential quantum critical points, p = 0.19 for the pseudogap phase and p = 0.076 for another competing phase. Temperature dependence reveals that the pseudogap is not static below T c and exists p > 0.19 at higher temperatures. Our data imply a revised phase diagram that reconciles conflicting reports about the endpoint of the pseudogap in the literature, incorporates phase competition between the superconducting gap and pseudogap, and highlights distinct physics at the edge of the superconducting dome.

We explore the interplay of electron-electron correlations and spin-orbit coupling in the model Fermi liquidSr2RuO4using laser-based angle-resolved photoemission spectroscopy. Our precise measurement of the Fermi surface confirms the importance of spin-orbit coupling in this material and reveals that its effective value is enhanced by a factor of about 2, due to electronic correlations. The self-energies for theβandγsheets are found to display significant angular dependence. By taking into account the multi-orbital composition of quasiparticle states, we determine self-energies associated with each orbital component directly from the experimental data. This analysis demonstrates that the perceived angular dependence does not imply momentum-dependent many-body effects but arises from a substantial orbital mixing induced by spin-orbit coupling. A comparison to single-site dynamical mean-field theory further supports the notion of dominantly local orbital self-energies and provides strong evidence for an electronic origin of the observed nonlinear frequency dependence of the self-energies, leading to “kinks” in the quasiparticle dispersion ofSr2RuO4.

The coupling between spin, valley and layer degrees of freedom in transition-metal dichalcogenides is shown to give rise to spin-polarized electron states, providing opportunities to create and manipulate spin and valley polarizations in bulk solids. Methods to generate spin-polarized electronic states in non-magnetic solids are strongly desired to enable all-electrical manipulation of electron spins for new quantum devices 1 . This is generally accepted to require breaking global structural inversion symmetry 1 , 2 , 3 , 4 , 5 . In contrast, here we report the observation from spin- and angle-resolved photoemission spectroscopy of spin-polarized bulk states in the centrosymmetric transition-metal dichalcogenide WSe 2 . Mediated by a lack of inversion symmetry in constituent structural units of the bulk crystal where the electronic states are localized 6 , we show how spin splittings up to ∼0.5 eV result, with a spin texture that is strongly modulated in both real and momentum space. Through this, our study provides direct experimental evidence for a putative locking of the spin with the layer and valley pseudospins in transition-metal dichalcogenides 7 , 8 , of key importance for using these compounds in proposed valleytronic devices.

Transition-metal dichalcogenides (TMDs) are renowned for their rich and varied bulk properties, while their single-layer variants have become one of the most prominent examples of two-dimensional materials beyond graphene. Their disparate ground states largely depend on transition metal d-electron-derived electronic states, on which the vast majority of attention has been concentrated to date. Here, we focus on the chalcogen-derived states. From density-functional theory calculations together with spin- and angle-resolved photoemission, we find that these generically host a co-existence of type-I and type-II three-dimensional bulk Dirac fermions as well as ladders of topological surface states and surface resonances. We demonstrate how these naturally arise within a single p-orbital manifold as a general consequence of a trigonal crystal field, and as such can be expected across a large number of compounds. Already, we demonstrate their existence in six separate TMDs, opening routes to tune, and ultimately exploit, their topological physics.

Two-dimensional electron gases (2DEGs) in SrTiO 3 have become model systems for engineering emergent behaviour in complex transition metal oxides. Understanding the collective interactions that enable this, however, has thus far proved elusive. Here we demonstrate that angle-resolved photoemission can directly image the quasiparticle dynamics of the d -electron subband ladder of this complex-oxide 2DEG. Combined with realistic tight-binding supercell calculations, we uncover how quantum confinement and inversion symmetry breaking collectively tune the delicate interplay of charge, spin, orbital and lattice degrees of freedom in this system. We reveal how they lead to pronounced orbital ordering, mediate an orbitally enhanced Rashba splitting with complex subband-dependent spin–orbital textures and markedly change the character of electron–phonon coupling, co-operatively shaping the low-energy electronic structure of the 2DEG. Our results allow for a unified understanding of spectroscopic and transport measurements across different classes of SrTiO 3 -based 2DEGs, and yield new microscopic insights on their functional properties. Two-dimensional electron gases in SrTiO3 offer new insights into the physics of complex oxides and offer the potential for applications in electronics. Here, King et al . show how orbital ordering, spin–orbit coupling and many-body interactions collectively shape the complex properties of these confined electron systems.

The nature of the pseudogap phase of cuprate high-temperature superconductors is a major unsolved problem in condensed matter physics. We studied the commencement of the pseudogap state at temperature T* using three different techniques (angle-resolved photoemission spectroscopy, polar Kerr effect, and time-resolved reflectivity) on the same optimally doped Bi2201 crystals. We observed the coincident, abrupt onset at T* of a particle-hole asymmetric antinodal gap in the electronic spectrum, a Kerr rotation in the reflected light polarization, and a change in the ultrafast relaxational dynamics, consistent with a phase transition. Upon further cooling, spectroscopic signatures of superconductivity begin to grow close to the superconducting transition temperature (T c ), entangled in an energy-momentum—dependent manner with the preexisting pseudogap features, ushering in a ground state with coexisting orders.

The electrical transport properties of a thin film of the diamondoid adamantane, deposited on an Au/W substrate, were investigated experimentally. The current I , in applied potential V , from the admantane-thiol/metal heterstructure to a wire lead on its surface exhibited non-symmetric (diode-like) characteristics and a signature of resistive switching (RS), an effect that is valuable to non-volatile memory applications. I ( V ) follows a hysteresis curve that passes twice through I ( 0 ) = 0 linearly, indicating RS between two states with significantly different conductances, possibly due to an exotic mechanism.