Angle-resolved Photoemission Spectra Research Articles

Two-Dimensional Dirac Fermions Protected by Space-Time Inversion Symmetry in Black Phosphorus.

We report the realization of novel symmetry-protected Dirac fermions in a surface-doped two-dimensional (2D) semiconductor, black phosphorus. The widely tunable band gap of black phosphorus by the surface Stark effect is employed to achieve a surprisingly large band inversion up to ∼0.6 eV. High-resolution angle-resolved photoemission spectra directly reveal the pair creation of Dirac points and their movement along the axis of the glide-mirror symmetry. Unlike graphene, the Dirac point of black phosphorus is stable, as protected by space-time inversion symmetry, even in the presence of spin-orbit coupling. Our results establish black phosphorus in the inverted regime as a simple model system of 2D symmetry-protected (topological) Dirac semimetals, offering an unprecedented opportunity for the discovery of 2D Weyl semimetals.

Single-crystalline silver films on mica

Highly ordered Ag films grown on cleaved mica with a two-step procedure were characterized in detail with several techniques for surface analysis, including low-energy electron diffraction, X-ray photoemission spectra, a scanning tunneling microscope and angle-resolved photoemission spectra. Silver films of a thickness ≥100nm were deposited on mica substrates at 300K and stored under ambient conditions. The Ag/mica films became atomically flat and free of contamination when they were transferred into ultra-high vacuum and prepared with sputtering and annealing. The experimental results indicate that the surfaces of the cleaned Ag/mica films have structural and electronic properties resembling those of a Ag(111) single crystal with ordered domains aligned over a macroscopic region, but the surfaces of the Ag/mica films became rough on annealing above 623K. On the rough surfaces, the surface steps tended to bunch together, yielding flat terraces bound with steps of large heights. With the decreased thermal stability in mind, the Ag/mica film is adoptable as a cheap platform alternative to a Ag(111) single crystal for the growth of highly ordered overlayers.

Fermi surface and effective masses in photoemission response of the (Ba1\u2212xKx)Fe2As2 superconductor

The angle-resolved photoemission spectra of the superconductor (Ba1−xKx)Fe2As2 have been investigated accounting coherently for spin-orbit coupling, disorder and electron correlation effects in the valence bands combined with final state, matrix element and surface effects. Our results explain the previously obscured origins of all salient features of the ARPES response of this paradigm pnictide compound and reveal the origin of the Lifshitz transition. Comparison of calculated ARPES spectra with the underlying DMFT band structure shows an important impact of final state effects, which result for three-dimensional states in a deviation of the ARPES spectra from the true spectral function. In particular, the apparent effective mass enhancement seen in the ARPES response is not an entirely intrinsic property of the quasiparticle valence bands but may have a significant extrinsic contribution from the photoemission process and thus differ from its true value. Because this effect is more pronounced for low photoexcitation energies, soft-X-ray ARPES delivers more accurate values of the mass enhancement due to a sharp definition of the 3D electron momentum. To demonstrate this effect in addition to the theoretical study, we show here new state of the art soft-X-ray and polarisation dependent ARPES measurments.

Origin of the crossover from polarons to Fermi liquids in transition metal oxides

Transition metal oxides host a wealth of exotic phenomena ranging from charge, orbital and magnetic order to nontrivial topological phases and superconductivity. In order to translate these unique materials properties into device functionalities these materials must be doped; however, the nature of carriers and their conduction mechanism at the atomic scale remain unclear. Recent angle-resolved photoelectron spectroscopy investigations provided insight into these questions, revealing that the carriers of prototypical metal oxides undergo a transition from a polaronic liquid to a Fermi liquid regime with increasing doping. Here, by performing ab initio many-body calculations of angle-resolved photoemission spectra of titanium dioxide, we show that this transition originates from non-adiabatic polar electron–phonon coupling, and occurs when the frequency of plasma oscillations exceeds that of longitudinal-optical phonons. This finding suggests that a universal mechanism may underlie polaron formation in transition metal oxides, and provides a pathway for engineering emergent properties in quantum matter.

Angle resolved Photoemission from Ag and Au single crystals: Final state lifetimes in the attosecond range

We present angle resolved photoemission spectra for Ag(111) and Au(111) single crystals in normal emission geometry, taken at closely spaced intervals for photon energies between 8eV and 160eV. The most dominant transitions observed are attributed to f-derived final states located about 16–17eV above EF for both materials. These transitions exhibit very distinct resonance phenomena and selection rules and are reminiscent of the angular momentum characteristics of the states involved. The excited electron lifetime is in the attosecond (as) range as determined from the energy width of the observed transitions. This serves as an alternate approach to the direct determination of excited electron lifetimes by as laser spectroscopy.

Universal Mechanism of Band-Gap Engineering in Transition-Metal Dichalcogenides.

van der Waals two-dimensional (2D) semiconductors have emerged as a class of materials with promising device characteristics owing to the intrinsic band gap. For realistic applications, the ideal is to modify the band gap in a controlled manner by a mechanism that can be generally applied to this class of materials. Here, we report the observation of a universally tunable band gap in the family of bulk 2H transition metal dichalcogenides (TMDs) by in situ surface doping of Rb atoms. A series of angle-resolved photoemission spectra unexceptionally shows that the band gap of TMDs at the zone corners is modulated in the range of 0.8-2.0 eV, which covers a wide spectral range from visible to near-infrared, with a tendency from indirect to direct band gap. A key clue to understanding the mechanism of this band-gap engineering is provided by the spectroscopic signature of symmetry breaking and resultant spin-splitting, which can be explained by the formation of 2D electric dipole layers within the surface bilayer of TMDs. Our results establish the surface Stark effect as a universal mechanism of band-gap engineering on the basis of the strong 2D nature of van der Waals semiconductors.

Circular dichroism in resonant angle-resolved photoemission spectra of LaNi2Ge2

We report the soft X-ray angle-resolved photoemission study for LaNi2Ge2 to reveal the electronic structures derived from non-4f bands of the heavy fermion compound CeNi2Ge2. The photoemission spectra recorded at the La M4,5 absorption edges clearly show the enhancement of the La 5d components in the valence band spectra. The circular dichroism of photoemission spectra reveals the band-dependent dichroic response due to the orbital symmetry.

Hubbard band versus oxygen vacancy states in the correlated electron metal SrVO3

We study the effect of oxygen vacancies on the electronic structure of the model strongly correlated metal SrVO$_3$. By means of angle-resolved photoemission (ARPES) synchrotron experiments, we investigate the systematic effect of the UV dose on the measured spectra. We observe the onset of a spurious dose-dependent prominent peak at an energy range were the lower Hubbard band has been previously reported in this compound, raising questions on its previous interpretation. By a careful analysis of the dose dependent effects we succeed in disentangling the contributions coming from the oxygen vacancy states and from the lower Hubbard band. We obtain the intrinsic ARPES spectrum for the zero-vacancy limit, where a clear signal of a lower Hubbard band remains. We support our study by means of state-of-the-art ab initio calculations that include correlation effects and the presence of oxygen vacancies. Our results underscore the relevance of potential spurious states affecting ARPES experiments in correlated metals, which are associated to the ubiquitous oxygen vacancies as extensively reported in the context of a two-dimensional electron gas (2DEG) at the surface of insulating $d^0$ transition metal oxides.

First-principles approach to excitons in time-resolved and angle-resolved photoemission spectra

We show that any {\em quasi-particle} or GW approximation to the self-energy does not capture excitonic features in time-resolved (TR) photoemission spectroscopy. In this work we put forward a first-principles approach and propose a feasible diagrammatic approximation to solve this problem. We also derive an alternative formula for the TR photocurrent which involves a single time-integral of the lesser Green's function. The diagrammatic approximation applies to the {\em relaxed} regime characterized by the presence of quasi-stationary excitons and vanishing polarization. The main distinctive feature of the theory is that the diagrams must be evaluated using {\em excited} Green's functions. As this is not standard the analytic derivation is presented in detail. The final result is an expression for the lesser Green's function in terms of quantities that can all be calculated {\em ab initio}. The validity of the proposed theory is illustrated in a one-dimensional model system with a direct gap. We discuss possible scenarios and highlight some universal features of the exciton peaks. Our results indicate that the exciton dispersion can be observed in TR {\em and} angle-resolved photoemission.

Robustness of a Topologically Protected Surface State in a Sb2Te2Se Single Crystal.

A topological insulator (TI) is a quantum material in a new class with attractive properties for physical and technological applications. Here we derive the electronic structure of highly crystalline Sb2Te2Se single crystals studied with angle-resolved photoemission spectra. The result of band mapping reveals that the Sb2Te2Se compound behaves as a p-type semiconductor and has an isolated Dirac cone of a topological surface state, which is highly favored for spintronic and thermoelectric devices because of the dissipation-less surface state and the decreased scattering from bulk bands. More importantly, the topological surface state and doping level in Sb2Te2Se are difficult to alter for a cleaved surface exposed to air; the robustness of the topological surface state defined in our data indicates that this Sb2Te2Se compound has a great potential for future atmospheric applications.

Electronic Structures of Ferromagnetic CeAgSb2: Soft X-ray Absorption, Magnetic Circular Dichroism, and Angle-Resolved Photoemission Spectroscopies

We report a combined study for the electronic structures of ferromagnetic CeAgSb$_2$ using soft X-ray absorption (XAS), magnetic circular dichroism (XMCD), and angle-resolved photoemission (ARPES) spectroscopies. The Ce $M_{4, 5}$ XAS spectra show very small satellite structures, reflecting a strongly localized character of the Ce $4f$ electrons. The linear dichroism effects in the Ce $M_{4, 5}$ XAS spectra demonstrate the ground state Ce $4f$ symmetry of $\Gamma{_6}$, the spatial distribution of which is directed along the $c$-axis. The XMCD results give support to the picture of local-moment magnetism in CeAgSb$_2$. Moreover it is also found that the theoretical band dispersions for LaAgSb$_2$ provides better description of the ARPES band structures than those for CeAgSb$_2$. Nevertheless, ARPES spectra at the Ce $3d$-$4f$ resonance show the momentum dependence for the intensity ratio between Ce $4f^{1}_{5/2}$ and $4f^{1}_{7/2}$ peaks in a part of the Brillouin zone, suggesting the non-negligible momentum dependent hybridization effect between the Ce $4f$ and the conduction electrons. This is associated with the moderate mass enhancement in CeAgSb$_2$.

Anomalous transport near the Lifshitz transition at theLaAlO3/SrTiO3interface

The two-dimensional electron liquid, at the (001) interface between band insulators {L}a{A}l{O}$_3$ and {S}r{T}i{O}$_3$, undergoes Lifshitz transition as the interface is doped with carriers. At a critical carrier density, two new orbitals populate at the Fermi level, with a concomitant change in the Fermi surface topology. Using dynamical mean-field theory, formulated within a realistic three-orbital model, we study the influence of the Lifshitz transition and local electron correlations on the transport properties. We look at the thermal conductivity, optical conductivity, Seebeck coefficient and angle resolved photoemission spectra and find that at a critical density, both the thermal and dc conductivities rise sharply to higher values while the Seebeck coefficient shows a cusp. The inter-orbital electron-electron interaction transfers spectral weight near the $\Gamma$ point towards lower energy, thereby reducing the critical density. In the presence of external magnetic field, the critical density further reduces due to exchange splitting. Beyond a sufficiently large field, multiple cusps appear in the Seebeck coefficient revealing multiple Lifshitz transitions.

Determination of surface and interface magnetic properties for the multiferroic heterostructure Co/BaTiO3 using spleed and arpes

Co/BaTiO3(0 0 1) is one of the most interesting multiferroic heterostructures as it combines different ferroic phases, setting this way the fundamentals for innovative technical applications. Various theoretical approaches have been applied to investigate the electronic and magnetic properties of Co/BaTiO3(0 0 1). Here we determine the magnetic properties of 3 ML Co/BaTiO3 by calculating spin-polarized electron diffraction as well as angle-resolved photoemission spectra, with both methods being well established as surface sensitive techniques. Furthermore, we discuss the impact of altering the BaTiO3 polarization on the spectra and ascribe the observed changes to characteristic details of the electronic structure.

Phonon-assisted indirect transitions in angle-resolved photoemission spectra of graphite and graphene

Indirect transitions of electrons in graphene and graphite are investigated by means of angle-resolved photoemission spectroscopy (ARPES) with several different incident photon energies and light polarizations. The theoretical calculations of the indirect transition for graphene and graphite are compared with the experimental measurements for highly-oriented pyrolytic graphite and a single-crystal of graphite. The dispersion relations for the transverse optical (TO) and the out-of-plane longitudinal acoustic (ZA) phonon modes of graphite and the TO phonon mode of graphene can be extracted from the inelastic ARPES intensity. We find that the TO phonon mode for $k$ points along the $\Gamma$-$K$ and $K$-$M$-$K'$ directions in the Brillouin zone can be observed in the ARPES spectra of graphite and graphene by using a photon energy of about 11.1 eV. The relevant mechanism in the ARPES process for this case is the resonant indirect transition. On the other hand, the ZA phonon mode of graphite can be observed by using a photon energy of about 6.3 eV through a nonresonant indirect transition, while the ZA phonon mode of graphene within the same mechanism should not be observed.

On the combined use of GW approximation and cumulant expansion in the calculations of quasiparticle spectra: The paradigm of Si valence bands

Since the earliest implementations of the various GW approximations and cumulant expansion in the calculations of quasiparticle propagators and spectra, several attempts have been made to combine the advantageous properties and results of these two theoretical approaches. While the GW plus cumulant approach has proven successful in interpreting photoemission spectroscopy data in solids, the formal connection between the two methods has not been investigated in detail. By introducing a general bijective integral representation of the cumulants, we can rigorously identify at which point these two approximations can be connected for the paradigmatic model of quasiparticle interaction with the dielectric response of the system that has been extensively exploited in recent interpretations of the satellite structures in photoelectron spectra. We establish a protocol for consistent practical implementation of the thus established GW+cumulant scheme, and illustrate it by comprehensive state-of-the-art first-principles calculations of intrinsic angle-resolved photoemission spectra from Si valence bands.

Occurrence of a Mott-like gap in single-particle spectra of electron systems possessing flat bands

An unconventional type of the Mott’s insulators where the gap in the spectrum of single-particle excitations is associated with repulsive effective interactions between quasiparticles is shown to exist in strongly correlated electron systems of solids that possess flat bands. The occurrence of this gap is demonstrated to be the consequence of violation of particle–hole symmetry, inherent in such systems. The results obtained are applied to elucidate the Fermi arc structure observed at temperatures up to 100 K in angle-resolved photoemission spectra of the compound Sr2IrO4, not showing superconductivity down to low T.

Determining the chirality of Weyl fermions from circular dichroism spectra in time-dependent angle-resolved photoemission

We show that the intensity of pumped states near Weyl point is different when pumped with left- and right-handed circular-polarized light, which leads to a special circular dichroism (CD) in time-dependent angle-resolved photoemission spectra (ARPES). We derive the expression for the CD of time-dependent ARPES, which is directly related to the chirality of Weyl fermions. Based on the above derivation, we further propose a method to determine the chirality for a given Weyl point from the CD of time-dependent ARPES. The corresponding CD spectra for TaAs has then been calculated from first principles, which can be compared with future experiments.

Graphene Tunable Transparency to Tunneling Electrons: A Direct Tool To Measure the Local Coupling.

The local interaction between graphene and a host substrate strongly determines the actual properties of the graphene layer. Here we show that scanning tunneling microscopy (STM) can selectively help to visualize either the graphene layer or the substrate underneath, or even both at the same time, providing a comprehensive picture of this coupling with atomic precision and high energy resolution. We demonstrate this for graphene on Cu(111). Our spectroscopic data show that, in the vicinity of the Fermi level, graphene π bands are well preserved presenting a small n-doping induced by Cu(111) surface state electrons. Such results are corroborated by Angle-Resolved Photoemission Spectra (ARPES) and Density Functional Theory with van der Waals (DFT + vdW) calculations. Graphene tunable transparency also allows the investigation of the interaction between the substrate and foreign species (such as atomic H or C vacancies) on the graphene layer. Our calculations explain graphene tunable transparency in terms of the rather different decay lengths of the graphene Dirac π states and the metal surface state, suggesting that it should apply to a good number of graphene/substrate systems.

Moiré miniband features in the angle-resolved photoemission spectra of graphene/hBNheterostructures

We identify features in the angle-resolved photoemission spectra (ARPES) arising from the periodic pattern characteristic for graphene heterostructure with hexagonal boron nitride $(h\mathrm{BN})$. For this, we model ARPES spectra and intensity maps for five microscopic models used previously to describe moir\'e superlattice in graphene/$h\mathrm{BN}$ systems. We show that detailed analysis of these features can be used to pin down the microscopic mechanism of the interaction between graphene and $h\mathrm{BN}$. We also analyze how the presence of a moir\'e-periodic strain in graphene or scattering of photoemitted electrons off $h\mathrm{BN}$ can be distinguished from the miniband formation.

Evidence for a diamondlike electronic band structure of Si multilayers on Ag(111)

Silicon multilayers on Ag(111) have been suggested to exhibit the structure of silicene, a material that has been heralded as a novel basis for microelectronic applications. However, our angle-resolved photoemission spectra (ARPES) from silicon multilayers on Ag(111) and of the silver-induced reconstruction of Si(111) demonstrate, from the close match in the valence level band structures, that the films exhibit a $s{p}^{3}$ diamondlike structure. This refutes the interpretation of silicon multilayers on Ag(111) as silicene, a conclusion that is strengthened by the observation from core level photoemission that significant silver segregation occurs to the surface of these layers.