Angle-resolved Photoemission Spectroscopy Technique Research Articles

Recent Advances in Moiré Superlattice Systems by Angle-Resolved Photoemission Spectroscopy.

The last decade has witnessed a flourish in 2D materials including graphene and transition metal dichalcogenides (TMDs) as atomic-scale Legos. Artificial moiré superlattices via stacking 2D materials with a twist angle and/or a lattice mismatch have recently become a fertile playground exhibiting a plethora of emergent properties beyond their building blocks. These rich quantum phenomena stem from their nontrivial electronic structures that are effectively tuned by the moiré periodicity. Modern angle-resolved photoemission spectroscopy (ARPES) can directly visualize electronic structures with decent momentum, energy, and spatial resolution, thus can provide enlightening insights into fundamental physics in moiré superlattice systems and guides for designing novel devices. In this review, first, a brief introduction is given on advanced ARPES techniques and basic ideas of band structures in a moiré superlattice system. Then ARPES research results of various moiré superlattice systems are highlighted, including graphene on substrates with small lattice mismatches, twisted graphene/TMD moiré systems, and high-order moiré superlattice systems. Finally, it discusses important questions that remain open, challenges in current experimental investigations, and presents an outlook on this field of research.

Carrier Relaxation and Multiplication in Bi Doped Graphene.

By introducing different contents of Bi adatoms to the surface of monolayer graphene, the carrier concentration and their dynamics have been effectively modulated as probed directly by the time- and angle-resolved photoemission spectroscopy technique. The Bi adatoms are found to assist acoustic phonon scattering events mediated by supercollisions as the disorder effectively relaxes the momentum conservation constraint. A reduced carrier multiplication has been observed, which is related to the shrinking Fermi sea for scattering, as confirmed by time-dependent density functional theory simulation. This work gives insight into hot carrier dynamics in graphene, which is crucial for promoting the application of photoelectric devices.

Angle-resolved photoemission spectroscopy

For solid-state materials, the electronic structure is critical in determining a crystal’s physical properties. By experimentally detecting the electronic structure, the fundamental physics can be revealed. Angle-resolved photoemission spectroscopy (ARPES) is a powerful technique for directly observing the electronic structure with energy- and momentum-resolved information. Over the past few decades, major improvements in the energy and momentum resolution, alongside the extension of ARPES observables to spin (SpinARPES), micrometre or nanometre lateral dimensions (MicroARPES/NanoARPES), and femtosecond timescales (TrARPES), have led to important scientific advances. These advantages have been achieved across a wide range of quantum materials, such as high-temperature superconductors, topological materials, two-dimensional materials and heterostructures. This Primer introduces the key aspects of ARPES principles, instrumentation, data analysis and representative scientific cases to demonstrate the power of the method. We also discuss the challenges and future developments. The physical properties of a solid-state material depends on its electronic structure, which can be studied using angle-resolved photoemission spectroscopy (ARPES). This Primer introduces the ARPES technique and describes how different variants can be used for applications including superconductors, topological materials and two-dimensional materials.

Doping induced band renormalization in 122-type Fe-based superconductor

We study the electronic structure of a superconducting composition of 122-type Fe-based pnictide material, CaFe1.9Co0.1As2 employing high resolution angle resolved photoemission spectroscopy technique. The experimental results exhibit three bands close to Fermi level at Γ-point of the Brillouin zone among which only one band crosses the Fermi level. In the parent compound, CaFe2As2, all the three bands cross the Fermi level and form three hole pockets. While the destruction of Fermi pockets due to electron doping (Co-substitution dopes electrons into the system) is expected, we observe significant orbital selective band renormalization with respect to the parent compound. It appears that the effect of spin-orbit coupling is stronger in the doped compound.

Impurity band assisted carrier relaxation in Cr doped topological insulator Bi2Se3

Topological insulators (TIs) with unique band structures have wide application prospects in the fields of ultrafast optical and spintronic devices. The dynamics of hot carriers plays a key role in these TI-based devices. In this work, using the time- and angle-resolved photoemission spectroscopy technique, the relaxation process of the hot carriers in Cr-doped Bi2Se3 has been systematically studied since the ferromagnetic TI is one of the key building blocks for next-generation spintronics. It is found that electronic temperature (Te) and chemical potential (μ) decrease faster with the increase in the Cr doping concentration. Similarly, the lifetime (τ) of the excited electrons also decreases with more Cr doped into Bi2Se3. The results suggest a mechanism of impurity band-assisted carrier relaxation, where the impurity band within the bulk bandgap introduced by Cr doping provides significant recombination channels for the excited electrons. This work directly illustrates the dynamic process of the photon-generated carriers in Cr-doped Bi2Se3, which is expected to promote the applications of (Bi1-xCrx)2Se3 in photoelectric devices.

Angle-resolved photoemission spectroscopy and its application to topological materials

Angle-resolved photoemission spectroscopy (ARPES), an experimental technique based on the photoelectric effect, is arguably the most powerful method for probing the electronic structure of solids. The past decade has witnessed notable progress in ARPES, including the rapid development of soft-X-ray ARPES, time-resolved ARPES, spin-resolved ARPES and spatially resolved ARPES, as well as considerable improvements in energy and momentum resolution. Consequently , ARPES has emerged as an indispensable experimental probe in the study of topological materials, which have characteristic non-trivial bulk and surface electronic structures that can be directly detected by ARPES. Over the past few years, ARPES has had a crucial role in several landmark discoveries in topological materials, including the identification of topological insulators and topological Dirac and Weyl semimetals. In this Technical Review , we assess the latest developments in different ARPES techniques and illustrate the capabilities of these techniques with applications in the study of topological materials.

Progress of ARPES study on topological semimetals

Topological semimetal, known as a type of topological quantum materials without energy gap, has attracted lots of research interests due to its unique physical properties such as novel quasiparticles, giant magnetoresistance and large carrier mobility. Topological semimetal can be further classified into topological Dirac semimetal, topological Weyl semimetal, topological nodal-line semimetal and topological semimetals with " new fermions”. The high-resolution angle-resolved photoemission spectroscopy (ARPES) has emerged as a powerful experimental technique to directly visualize the electronic structure and identify the characteristic topological electronic states in topological semimetals. Here we would briefly introduce the ARPES technique and review some of the recent progress of ARPES study on the electronic structures of typical topological semimetals. We would focus mostly on the physics origin and ARPES signature of topological electronic structures and hope the readers would find it interesting and useful in the understanding of this material class which both is important in physics and has promising application potentials.

Scanning tunneling microscopy and photoemission studies of self-organised Ag nanostructures on the N-modified Cu(001) surface

There has been a strong interest in methods of creating nanometer scale structures and in particular forming one- and two-dimensional electron confinement structures. Self-organisation has been recognised as a promising way for growing large nanostructure domains with sufficiently regular size and spacing as required for the observation of quantum well states. We investigated the electronic properties and the morphology of Ag nano-structures on c(2 × 2)-N/Cu(001) surface. This system is an example of epitaxial growth confined on nanoscale regions due to the occurrence of an adsorbate induced reconstruction. Using a combination of Scanning Tunneling Microscopy and Angle Resolved Photoemission Spectroscopy techniques we were able to determine the morphology and the growth mode of Ag on N-modified Cu(001) surface and the occurrence of quantum size effects in the electron properties of Ag nanostripes and nanoislands, evidenced in the observation of quantum well states.

Infrared spectroscopy study of ironbased superconductor Li0.8Fe0.2 ODFeSe

We perform an in-plane optical spectroscopy measurement on iron-based superconductor Li0.8Fe0.2ODFeSe single crystal. At room temperature, the low frequency optical conductivity shows an incoherent characteristic; the Drude component is absent. With temperature decreasing, the Drude component develops and narrows rapidly. A well-defined plasma edge is observed in reflectance spectrum at temperature below 100 K, indicating a dramatically reduced scattering rate. The spectral weight contributed from free carriers is even smaller than that of FeSe single crystal. A number of phonon modes are visible in the measured spectra. We also observe clear spectral change below 160 cm-1 at 10 K, associated with the formation of superconducting energy gap in the superconducting state. The energy scale of the superconducting gap is comparable to the value measured by angle-resolved photoemission spectroscopy technique. Like FeSe and other iron pnictides, a clear temperature-induced spectral weight transfer at high energy is observed for Li0.8Fe0.2ODFeSe, indicating the presence of strong correlation effect.

Dynamics of Molecular Orientation Observed Using Angle Resolved Photoemission Spectroscopy during Deposition of Pentacene on Graphite.

A real-time method to observe both the structural and the electronic configuration of an organic molecule during deposition is reported for the model system of pentacene on graphite. Structural phase transition of the thin films as a function of coverage is monitored by using in situ angle resolved photoemission spectroscopy (ARPES) results to observe the change of the electronic configuration at the same time. A photoemission theory that uses independent atomic center approximations is introduced to identify the molecular orientation from the ARPES technique. This study provides a practical insight into interpreting ARPES data regarding dynamic changes of molecular orientation during initial growth of molecules on a well-defined surface.

Sensitivity of angle-resolved photoemission to short-range antiferromagnetic correlations

Angle-resolved photoemission spectroscopy (ARPES) is one of most powerful techniques to unravel the electronic properties of layered materials and, in recent decades, it has lead to significant progress in the understanding of the band structures of cuprates, pnictides, and other materials of current interest. On the other hand, its application to Mott-Hubbard insulating materials where a Fermi surface is absent has been more limited. Here we show that in these latter materials, where electron spins are localized, ARPES may provide significant information on the spin correlations which can be complementary to the one derived from neutron-scattering experiments. ${\mathrm{Sr}}_{2}{\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}{\mathrm{O}}_{2}{\mathrm{Cl}}_{2}$, a prototype of a diluted spin $S=1/2$ antiferromagnet (AF) on a square lattice, was chosen as a test case and a direct correspondence between the amplitude of the spectral weight beyond the AF zone boundary derived from ARPES and the spin-correlation length $\ensuremath{\xi}$ estimated from $^{35}\mathrm{Cl}$ NMR was established. It was found that even for correlation lengths of a few lattice constants, a significant spectral weight in the backbended band is present which depends markedly on $\ensuremath{\xi}$. Moreover, the temperature dependence of that spectral weight is found to scale with the $x$-dependent spin stiffness. These findings prove that the ARPES technique is very sensitive to short-range correlations and its relevance in the understanding of the electronic correlations in cuprates is discussed.

In situ engineering and characterization on the artificial heterostructures of correlated materials with integrated OMBE–ARPES

The combination of oxide molecular beam epitaxy (OMBE) and angle resolved photoemission spectroscopy (ARPES) opens a new field for studying correlated electron systems. The in situ growth of single crystalline films by OMBE increases the variety of materials available for ARPES study. Investigating the electronic structure of thin films by ARPES deepens our understanding on the interaction of different degrees of freedom at thin films and interfaces. In this paper, we review the key elements of the integrated OMBE–ARPES technique and some representative works, including the in situ ARPES investigations on thin films with cubic perovskite structure, ultra-thin films with thickness dependent metal–insulator transition, the composition and strain dependence of interfacial superconductivity, and correlated behavior in superlattices. These examples illustrate the new opportunities brought by integrating OMBE and ARPES on exploring novel physical states and functional materials.

Observing electronic structures on ex‐situ grown topological insulator thin films

Abstractmagnified imageTopological insulators represent a new state of quantum matter recently discovered with insulating bulk but conducting surface states formed by an odd number of Dirac fermions. In this Letter, we report our recent progress on the study of electronic structures of ex‐situ grown topological insulator thin films by angle resolved photoemission spectroscopy (ARPES). We successfully obtained the topological band structures of molecular beam epitaxial HgTe and vapor–solid grown Bi2Te3 thin films after proper surface cleaning procedures. This new development will not only enable us to study more topological insulators that cannot be measured by conventional in‐situ ARPES technique (e.g. by cleaving or growing samples in‐situ), but also open the door to directly characterize the electronic properties of topological insulators used in functional devices. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Unoccupied electronic structure of graphite probed by angle-resolved photoemission spectroscopy

We report the observation of anomalous bands in graphite valence band structure in angle-resolved photoemission spectroscopy (ARPES) experiments. The photon energy dependence of these bands shows a constant kinetic energy nature. Our results are supported by the very low energy electron diffraction data reported on graphite surfaces which essentially map the unoccupied states representing the photoemission final states. This suggests that the ARPES technique is capable of probing the unoccupied electronic states governed by the secondary electron emission process, along with the occupied bands of solids.

Through a Lattice Darkly: Shedding Light on Electron-Phonon Coupling in the HighTcCuprates

With its central role in conventional BCS superconductivity, electron-phonon coupling appears to play a more subtle role in the phase diagram of the high-temperature superconducting cuprates. Their added complexity due to potentially numerous competing phases, including charge, spin, orbital, and lattice ordering, makes teasing out any unique phenomena challenging. In this review, we present our work using angle-resolved photoemission spectroscopy (ARPES) exploring the role of the lattice on the valence band electronic structure of the cuprates. We introduce the ARPES technique and its unique ability to the probe the effect of bosonic renormalization (or “kink”) on near-EFband structure. Our survey begins with the establishment of the ubiquitous nodal cuprate kink leading to how isotope substitution has shed a critical new perspective on the role and strength of electron-phonon coupling. We continue with recently published work connecting the phonon dispersion seen with inelastic X-ray scattering (IXS) to the location of the kink observed by ARPES near the nodal point. Finally, we present very recent and ongoing ARPES work examining how induced strain through chemical pressure provides a potentially promising avenue for understanding the broader role of the lattice to the superconducting phase and larger cuprate phase diagram.

A brief update of angle-resolved photoemission spectroscopy on a correlated electronsystem

In this paper, we briefly summarize the capabilities of state-of-the-art angle-resolvedphotoemission spectroscopy (ARPES) in the field of experimental condensedmatter physics. Due to the advancement of the detector technology and the highflux light sources, ARPES has become a powerful tool to study the low energyexcitations of solids, especially those novel quantum materials in which many-bodyphysics are at play. To benchmark today’s state-of-the-art ARPES technique,we demonstrate that the precision of today’s ARPES has advanced to a regimecomparable to the bulk-sensitive de Haas–van Alphen (dHvA) measurements.Finally, as an example of new discoveries driven by the advancement of the ARPEStechnique, we summarize some of our recent ARPES measurements on underdopedhigh-Tc superconducting cuprates, which have provided further insight into the complex pseudogapproblem.

ARPES line shapes in FL and non-FL quasi-low-dimensional inorganic metals

Quasi-low-dimensional (quasi-low-D) inorganic materials are not only ideally suited for angle resolved photoemission spectroscopy (ARPES) but also they offer a rich ground for studying key concepts for the emerging paradigm of non-Fermi liquid (non-FL) physics. In this article, we discuss the ARPES technique applied to three quasi-low-D inorganic metals: a paradigm Fermi liquid (FL) material TiTe 2 , a well-known quasi-1D charge density wave (CDW) material K 0.3 MoO 3 and a quasi-1D non-CDW material Li 0.9 Mo 6 O 17 . With TiTe 2 , we establish that a many body theoretical interpretation of the ARPES line shape is possible. We also address the fundamental question of how to accurately determine the k F value from ARPES. Both K 0.3 MoO 3 and Li 0.9 Mo 6 O 17 show quasi-1D electronic structures with non-FL line shapes. A CDW gap opening is observed for K 0.3 MoO 3 , whereas no gap is observed for Li 0.9 Mo 6 O 17 . We show, however, that the standard CDW theory, even with strong fluctuations, is not sufficient to describe the non-FL line shapes of K 0.3 MoO 3 . We argue that a Luttinger liquid (LL) model is relevant for both bronzes, but also point out difficulties encountered in comparing data with theory. We interpret this situation to mean that a more complete and realistic theory is necessary to understand these data.

Photoemission studies of the valence bands in YBa 2Cu 3O 7-δ

Studies of the valence band electronic structure of the high T c superconductor YBa 2Cu 3O 7-δ (YBCO), obtained from photoelectron spectroscopy measurements, are reviewed. We include a brief discussion of angle integrated measurements acquired with X-ray photoemission and with ultraviolet photoemission. However, the major effort is devoted to a discussion of high resolution angle resolved photoemission spectroscopy (ARPES) studies on samples cleaved in high vacuum at cryogenic temperatures. These samples show a clear (metallic) Fermi edge cutoff, and band dispersion that is generally consistent with predictions of band theory. Mostly, electron states lying close to E F (within about 1 eV) are examined. The highly two-dimensional nature of the YBCO structure allows for reasonably straightforward measurement of the Fermi surface; consequently the Fermi surface has been measured in some detail. Evidence, from ARPES measurements, for the existence of a van Hove singularity near the Fermi level, that might play a role in high T c superconductivity, is considered. Systematic studies are presented that show changes in the band structure and Fermi surface as the oxygen stoichiometry is reduced; with oxygen depletion, YBCO changes from a high T c superconductor to a magnetically ordered insulator. ARPES studies of YBCO crystals with Co and Zn atoms substituted on Cu sites are also included. When ARPES measurements are performed at room temperature, the samples show rapid degradation; consequently, room temperature measurements are not extensively reviewed here. Despite extensive studies, some issues still remain largely unresolved or controversial. The ARPES technique is surface sensitive and the cleavage plane has not been clearly identified; consequently concerns associated with measurement of the cleaved surface are addressed. Efforts, generally unsuccessful, to observe a superconducting gap, are discussed.