Search

Polar distortions in solids give rise to the well-known functionality of switchable macroscopic polarisation in ferroelectrics and, when combined with strong spin-orbit coupling, can mediate giant spin splittings of electronic states. While typically found in insulators, ferroelectric-like distortions can remain robust against increasing itineracy, giving rise to so-called "polar metals". Here, we investigate the temperature-dependent electronic structure of Ca$_3$Ru$_2$O$_7$, a correlated oxide metal in which octahedral tilts and rotations combine to mediate pronounced 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 hybridisation 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 hybridisation is actually the key driver for the phase transition, reflecting a delicate interplay between spin-orbit coupling and strong electronic correlations, and revealing a new route to control magnetic ordering in solids., Comment: Contains 6+5 pages, including supplementary information

The growth of Bi on both the In-terminated (A) face and the As-terminated (B) face of InAs(1 1 1) has been investigated by low-energy electron diffraction, scanning tunnelling microscopy, and photoelectron spectroscopy using synchrotron radiation. The changes upon Bi deposition of the In 4d and Bi 5d 5/2 photoelectron signals allow to get a comprehensive picture of the Bi/InAs(1 1 1) interface. From the early stage the Bi growth on the A face is epitaxial, contrary to that on the B face that proceeds via the formation of islands. Angle-resolved photoelectron spectra show that the electronic structure of a Bi deposit of ≈10 bi-layers on the A face is identical to that of bulk Bi, while more than ≈30 bi-layers are needed for the B face. Both bulk and surface electronic states observed are well accounted for by fully relativistic ab initio calculations performed using the one-step model of photoemission. These calculations are used to analyse the dichroic photoemission data recorded in the vicinity of the Fermi level around the Γ ¯ point of the Brillouin zone.

The linewidths of the electronic bands originating from the electron-phonon coupling in graphene are analyzed based on model tight-binding calculations and experimental angle-resolved photoemission spectroscopy (ARPES) data. Our calculations confirm the prediction that the high-energy optical phonons provide the most essential contribution to the phonon-induced linewidth of the two upper occupied $\sigma$ bands near the $\bar{\Gamma}$-point. For larger binding energies of these bands, as well as for the $\pi$ band, we find evidence for a substantial lifetime broadening from interband scattering $\pi \rightarrow \sigma$ and $\sigma \rightarrow \pi$, respectively, driven by the out-of-plane ZA acoustic phonons. The essential features of the calculated $\sigma$ band linewidths are in agreement with recent published ARPES data [F. Mazzola et al., Phys.~Rev.~B. 95, 075430 (2017)] and of the $\pi$ band linewidth with ARPES data presented here., Comment: 7 pages, 4 figures

The 'double Dirac cone' 2D topological interface states found on the (001) faces of topological crystalline insulators such as Pb$_{1-x}$Sn$_{x}$Se feature degeneracies located away from time reversal invariant momenta, and are a manifestation of both mirror symmetry protection and valley interactions. Similar shifted degeneracies in 1D interface states have been highlighted as a potential basis for a topological transistor, but realizing such a device will require a detailed understanding of the intervalley physics involved. In addition, the operation of this or similar devices outside of ultra-high vacuum will require encapsulation, and the consequences of this for the topological interface state must be understood. Here we address both topics for the case of 2D surface states using angle-resolved photoemission spectroscopy. We examine bulk Pb$_{1-x}$Sn$_{x}$Se(001) crystals overgrown with PbSe, realizing trivial/topological heterostructures. We demonstrate that the valley interaction that splits the two Dirac cones at each $\bar{X}$ is extremely sensitive to atomic-scale details of the surface, exhibiting non-monotonic changes as PbSe deposition proceeds. This includes an apparent total collapse of the splitting for sub-monolayer coverage, eliminating the Lifshitz transition. For a large overlayer thickness we observe quantized PbSe states, possibly reflecting a symmetry confinement mechanism at the buried topological interface.

A comprehensive understanding of spin-polarized photoemission is crucial for accessing the electronic structure of spin-orbit coupled materials. Yet, the impact of the final state in the photoemission process on the photoelectron spin has been difficult to assess in these systems. We present experiments for the spin-orbit split states in a Bi-Ag surface alloy showing that the alteration of the final state with energy may cause a complete reversal of the photoelectron spin polarization. We explain the effect on the basis of ab initio one-step photoemission theory and describe how it originates from linear dichroism in the angular distribution of photoelectrons. Our analysis shows that the modulated photoelectron spin polarization reflects the intrinsic spin density of the surface state being sampled differently depending on the final state, and it indicates linear dichroism as a natural probe of spin-orbit coupling at surfaces., New manuscript with a changed interpretation and theoretical description of the experimental results

First-principles studies of the electron-phonon coupling in graphene predict a high coupling strength for the $\sigma$ band with $\lambda$ values of up to 0.9. Near the top of the $\sigma$ band, $\lambda$ is found to be $\approx 0.7$. This value is consistent with the recently observed kinks in the $\sigma$ band dispersion by angle-resolved photoemission. While the photoemission intensity from the $\sigma$ band is strongly influenced by matrix elements due to sub-lattice interference, these effects differ significantly for data taken in the first and neighboring Brillouin zones. This can be exploited to disentangle the influence of matrix elements and electron-phonon coupling. A rigorous analysis of the experimentally determined complex self-energy using Kramers-Kronig transformations further supports the assignment of the observed kinks to strong electron-phonon coupling and yields a coupling constant of $0.6(1)$, in excellent agreement with the calculations., Comment: 11 pages, 3 figures

A three-dimensional (3D) Dirac semimetal is a novel state of quantum matter which has recently attracted much attention as an apparent 3D version of graphene. In this paper, we report results on the electronic structure of the 3D Dirac semimetal Na3Bi at a surface that reveals its nontrivial ground state. Our studies reveal that the two 3D Dirac cones go through a topological change in the constant energy contour as a function of the binding energy, featuring a Lifshitz point, which is missing in a strict 3D analog of graphene. Our results identify an example of a band saddle-point singularity in 3D Dirac materials. This is in contrast to its two-dimensional analogs such as graphene and the Dirac surface states of a topological insulator. The observation of multiple Dirac nodes in Na3Bi connecting via a Lifshitz point along its crystalline rotational axis away from the Kramers point serves as a decisive signature for the symmetry-protected nature of the Dirac semimetal's topological bulk ground state. (Less)

Methods to generate spin-polarised electronic states in non-magnetic solids are strongly desired to enable all-electrical manipulation of electron spins for new quantum devices. This is generally accepted to require breaking global structural inversion symmetry. In contrast, here we present direct evidence from spin- and angle-resolved photoemission spectroscopy for a strong spin polarisation of bulk states in the centrosymmetric transition-metal dichalcogenide WSe$_2$. We show how this arises due to a lack of inversion symmetry in constituent structural units of the bulk crystal where the electronic states are localised, leading to enormous spin splittings up to $\sim\!0.5$ eV, with a spin texture that is strongly modulated in both real and momentum space. As well as providing the first experimental evidence for a recently-predicted `hidden' spin polarisation in inversion-symmetric materials, our study sheds new light on a putative spin-valley coupling in transition-metal dichalcogenides, of key importance for using these compounds in proposed valleytronic devices., Comment: 6 pages, 4 figures

The recent discovery of a topological phase transition in IV-VI narrow-gap semiconductors has revitalized the decades-old interest in the bulk band inversion occurring in these materials. Here we systematically study the (001) surface states of Pb1-xSnxSe mixed crystals by means of angle-resolved photoelectron spectroscopy in the parameter space 0 = T >= 9 K. Using the surface-state observations, we monitor directly the topological phase transition in this solid solution and gain valuable information on the evolution of the underlying fundamental band gap of the system. In contrast to common model expectations, the band-gap evolution appears to be nonlinear as a function of the studied parameters, resulting in the measuring of a discontinuous band-inversion process. This finding signifies that the anticipated gapless bulk state is in fact not a stable configuration and that the topological phase transition therefore exhibits features akin to a first-order transition.

Complex structure in photoemission spectra of Be(0001) surface states near the Fermi energy is observed and explained as the effect of strong electron-phonon coupling. The weak momentum dependence of the electron-phonon contribution to the electron self energy \ensuremath{\Sigma} is exploited to determine \ensuremath{\Sigma} by direct inversion of the spectra.

We report an extensive angle-resolved photoemission spectroscopy study of a set of quickly dispersing electronic states near the G(0,0) point in Bi2Sr2CaCu22xNixOy (x,1%) single crystals at photon energy hn538 eV. These states cross the Fermi surface along both the G-M and G-Y directions and are outside the well-known large Fermi surface and satellite bands due to the superstructure in Bi2212. The electronic states have a strong nonmonotonic photon energy dependence and demonstrate a symmetry different from the symmetry of the Cu-O plane. The possible origin of these electronic states is discussed. In addition, we show that the whole Fermi-surface topology in Bi2212 is photon energy dependent.

Angle-resolved photoemission spectroscopy reveals pronounced kinks in the dispersion of the σ band of graphene. Such kinks are usually caused by the combination of a strong electron-boson interaction and the cutoff in the Fermi-Dirac distribution. They are therefore not expected for the σ band of graphene that has a binding energy of more than ≈3.5 eV. We argue that the observed kinks are indeed caused by the electron-phonon interaction, but the role of the Fermi-Dirac distribution cutoff is assumed by a cutoff in the density of σ states. The existence of the effect suggests a very weak coupling of holes in the σ band not only to the π electrons of graphene but also to the substrate electronic states. This is confirmed by the presence of such kinks for graphene on several different substrates that all show a strong coupling constant of λ≈1. Funding Agencies|Lundbeck Foundation||VILLUM Foundation||Danish Council for Independent Research/Technology and Production Sciences||Swedish Research Council||Knut and Alice Wallenberg Foundations

The connection between the Fermi surface and charge-density-wave (CDW) order is revisited in 2H-TaSe2. Using angle-resolved photoemission spectroscopy, ab initio band-structure calculations, and an accurate tight-binding model, we develop the empirical k-resolved susceptibility function, which we use to highlight states that contribute to the susceptibility for a particular q vector. We show that although the Fermi surface is involved in the peaks in the susceptibility associated with CDW order, it is not through conventional Fermi surface nesting, but rather through finite energy transitions from states located far from the Fermi level. Comparison with monolayer TaSe2 illustrates the different mechanisms that are involved in the absence of bilayer splitting.

Mn5Ge3(001) thin films grown on Ge(111)-c(2 x 8) reconstructed surfaces were studied by angle-resolved photoemission using synchrotron radiation in the 14-94 eV photon energy range. The results obtained in the Gamma ALM plane and in the Gamma AHK plane are in agreement with simulations starting with band structure calculations based on the density functional theory. This provides a unique validation of band structure calculations for a proper description of the electronic properties of Mn5Ge3. Only the spectral feature very close to the Fermi level cannot be well explained by the simulation. This departure is discussed in terms of the three-dimensional nature of the sample and of correlation effects.

We study the nature of (001) surface states in Pb0.73Sn0.27Se in the newly discovered topological-crystalline-insulator (TCI) phase as well as the corresponding topologically trivial state above th ...

Understanding and control of spin degrees of freedom on the surfaces of topological materials are key to future applications as well as for realizing novel physics such as the axion electrodynamics associated with time-reversal (TR) symmetry breaking on the surface. We experimentally demonstrate magnetically induced spin reorientation phenomena simultaneous with a Dirac-metal to gapped-insulator transition on the surfaces of manganese-doped Bi2Se3 thin films. The resulting electronic groundstate exhibits unique hedgehog-like spin textures at low energies, which directly demonstrate the mechanics of TR symmetry breaking on the surface. We further show that an insulating gap induced by quantum tunnelling between surfaces exhibits spin texture modulation at low energies but respects TR invariance. These spin phenomena and the control of their Fermi surface geometrical phase first demonstrated in our experiments pave the way for the future realization of many predicted exotic magnetic phenomena of topological origin., Comment: 38 pages, 18 Figures, Includes new text, additional datasets and interpretation beyond arXiv:1206.2090, for the final published version see Nature Physics (2012)

We identify and characterize a two-dimensional phase transition in a layer of Sn on Cu(100). The stable phase at room temperature has a $(3\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2})R45\ifmmode^\circ\else\textdegree\fi{}$ structure. Above $\ensuremath{\sim}360\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, a new phase with $(\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2})R45\ifmmode^\circ\else\textdegree\fi{}$ structure is formed. The high-temperature phase exhibits a quasi-two-dimensional free-electron surface band, with Fermi surface nesting in excellent agreement with the three-times larger periodicity of the low-temperature phase. A momentum-dependent band gap opens along the nested areas of the Fermi surface in the low-temperature phase. The phase transition is a clear experimental confirmation of the role of Fermi-surface gapping and nesting in the stabilization of a commensurate two-dimensional phase, which is interpreted as a charge-density wave.

The surface electronic band structure of the Be(10(1) over bar 0) surface is experimentally determined by angle-resolved photoemission and calculated by using density-functional theory. The experim ...

First-principle calculations and angle-resolved photoemission experiments are combined to analyze the different contributions to the surface-state hole dynamics at the Be(0001) surface. The computed inelastic electron-electron scattering rate γ is in good agreement with experiment once the electron-phonon contribution is subtracted. γ is strongly influenced by details of the surface band structure. In particular, the intraband transitions within the surface state itself ignored in traditional Fermi liquid theory contribute 65%-85% to γ. This intraband contribution and the highly spatial nonlocality of the self-energy makes the energy and momentum dependence of γ to be ∝(EF-E)α (with α, We acknowledge support by the Basque Country University, Basque Hezkuntza Saila, Spanish MEC, HPRN-CT-2000-00167 (NANOPHASE), and the Max Planck Research Award funds.