Lifetime broadening in angle-resolved photoemission

The register line formalism of angle-resolved photoemission is applied to the special case where electrons are excited from sp surface states. By considering lifetime broadening alone, it is demonstrated that it is possible to explain why photoemission linewidths increase as the initial states disperse towards the Fermi level. Favourable comparisons are made between the theory and with measurements of the surface state widths on Cu(111) and Al(001).     

Modulated photoemission spectroscopy: Application to Au

We describe modulated photoemission spectroscopy, in which an internal (sample) parameter such as temperature, or an external (apparatus) parameter such as wavelength is varied. A general formalism is developed for modulated photoemission spectroscopy and then illustrated using temperature modulated photoemission spectra and yields for Au obtained in the ～ 6 to 11.6 eV photon energy range. Modulated s-p band photoemission data are described in terms of photoemission critical points in order to explain the nature of the modulated structures in the s-p band region and relate experiment to energy band thresholds obtained from a recent band calculation for Au. Application of the formalism to modulated d-band emission leads to a method for extracting d-band deformation potentials. For example, we find that the upper d band edge moves upward with respect to E_F at a rate of 2 to 6 × 10^?4 eV/K. Modulation of the quantum yield is described and our measurements are compared with modulated optical data.     

Threshold photoemission spectroscopy in solids

Threshold photoemission spectroscopy (TPES) is used to measure the Fe 2p spectrum of a stainless steel sample. The obtained spectrum is compared with analogous spectra measured by X-ray photoemission and absorption spectroscopies. The results of this comparison suggest that resonant two-electron autoionization processes, rather than direct photoemission from the core level, are the main mechanisms contributing to the signal. Limits and applicability of this experimental approach to investigate bulk electronic properties in solids are discussed.     

Hard X-ray photoemission spectroscopy: Variable depth analysis of bulk,surface and interface electronic properties

The electronic properties of surfaces and buried interfaces can vary considerably in comparison to the bulk. In turn, analyzing bulk properties, without including those of the surface, is understandably challenging. Hard X-ray photoelectron spectroscopy (HAXPES) allows the well known ability of photoemission to interrogate the electronic structure of material systems with bulk volume sensitivity. This is achieved by tuning the kinetic energy range of the analyzed photoelectrons in the multi-keV regime. This unique ability to probe truly bulk properties strongly compliments normal photoemission, which generally probes surface electronic structure that is different than the bulk selected examples of HAXPES and possible implications towards the study of complex oxide-based interfaces and highly correlated systems are discussed.     

Exciton broadening of the angle-resolved photoemission spectra of a semiconductor

Broadening of the angle-resolved photoemission spectra caused by the electron-hole attraction is considered. For the case of a semiconductor with the model parabolic dispersion law the spectral-line shape is found at arbitrary k_e and ?ω, the energy dependence of the spectrum width is presented too. For the real crystals of silicon, germanium, and grey tin the results of numerical calculations of these values are given in some points k_e. The accounting of the electron-hole interaction gives rise to shifts and widths up to 0.1 eV order of magnitude and results in an oscillating spectrum lineshape.     

Surface chemistry: from vibrational spectroscopy to photoemission spectromicroscopy

Vibrational spectroscopy and core-level photoelectron spectroscopy are two of the most powerful tools for surface chemical analysis. We have used both techniques in conjunction with others to study hydrocarbon radical interactions with metal and semiconductor surfaces as well as thin film deposition processes. These include the investigations of CH₂ and CH₃ reactions on Cu by high resolution electron energy loss spectroscopy (HREELS), and GaN formation from Ga(CH₃)₃ and NH₃ on SiC by SR-PES. The vibrational analysis has revealed rich surface chemistry of the radicals while the photoemission study has led us to develop new approach for probing ultrafine structures. The effort involves the construction of a scanning photoemission spectromicroscope (SPEM) coupled to a synchrotron radiation light source. Recent results are presented in this brief report.     

The probing depth in photoemission and auger-electron spectroscopy

Electron scattering lengths, or escape depths, have been compiled from the literature for twenty different materials in the energy range 0–3000 eV. The energy dependence of the escape depth and its implications for interpretation of photoemission and Auger-electron data are discussed.     

Surface and bulk valence band photoemission of silicon carbide

Surface and bulk valence band photoemission spectra have been obtained for silicon carbide using Zr M-zeta (151.4eV) and Mg Kα_1,2 (1253.6eV) excitation, respectively. The data have been corrected for various broadening effects using a deconvolution procedure. Spectra for single-crystal hexagonal α-SiC and for epitaxial films of cubic β-SiC on Si (1 0 0) are compared with the results of band structure calculations. Differences in the spectra for various surface reconstructions suggest the existence of surface resonances, but no surface states are observed in the bulk optical bandgap.     

铁基超导体的角分辨光电子能谱研究进展

最近发现的超导转变温度高达55K铁基高温超导体结束了铜氧化物在超导转变温度高于40K的领域一统天下的局面.与铜氧化物高温超导体一样,超导配对对称性对于理解这一新的体系有着重要的作用.文章利用角分辨光电子能谱实验手段,全面地研究了铁基材料的能带结构和费米面以及它们随载流子掺杂浓度变化的演化过程,发现了铁基超导体中依赖费米面的无节点的超导能隙,指出了费米面间的相互作用对超导配对起着关键作用.     

Angle-resolved photoemission spectroscopy study on iron-based superconductors

Angle-resolved photoemission spectroscopy （ARPES） has played an important role in determining the band structure and the superconducting gap structure of iron-based superconductors. In this paper, from the ARPES perspective, we briefly review the main results from our group in recent years on the iron-based superconductors and their parent compounds, and depict our current understanding on the antiferromagnetism and superconductivity in these materials.     

铁基超导体的角分辨光电子能谱研究进展

最近发现的超导转变温度高达55K铁基高温超导体结束了铜氧化物在超导转变温度高于40K的领域一统天下的局面.与铜氧化物高温超导体一样,超导配对对称性对于理解这一新的体系有着重要的作用.文章利用角分辨光电子能谱实验手段,全面地研究了铁基材料的能带结构和费米面以及它们随载流子掺杂浓度变化的演化过程,发现了铁基超导体中依赖费米面的无节点的超导能隙,指出了费米面间的相互作用对超导配对起着关键作用.     

Photoemission and inverse photoemission spectroscopy of NiO

Photoemission (UPS) and inverse photoemission (BIS) spectra of NiO grown as a thin film on metallic Ni are reported. The data are discussed with respect to the electronic structure of the occupied and empty states in NiO.     

Angle-resolved photoemission spectroscopy studies of electron-electron interactions in graphene

Graphene is an emergent research topic that has attracted a huge amount of research interest ever since its experimental demonstration as a two-dimensional realization of Dirac fermions in 2005. In subsequent years, the research on graphene has rapidly expanded its field not only due to the new paradigm to study relativistic high energy physics in a condensed matter, but also due to its potential in the application for next generation devices. Most of the novel phenomena observed so far in graphene are attributed to its low-energy excitations, which is described by those of relativistic Dirac fermions. This article reviews recent progress in angle-resolved photoemission spectroscopy studies of electron-electron interactions in graphene.     

Probe-field spectroscopy in two-level systems with low Doppler broadening

The spectrum of absorption (amplification) of a weak probe field by two-level atoms colliding with atoms of a buffer gas in the field of a strong electromagnetic wave is studied theoretically. A universal, free of any collision model, solution is obtained for systems with small (compared to collision rates) Doppler broadening with no restrictions on the strong-field intensity and with the possible absence of dephasing of the light-induced dipole moment in elastic collisions of the gas particles. The relation of the collisional parameters of the problem with the characteristics of an elementary scattering event and with the macroscopic characteristics connected with the transport phenomena is established. It is found that the nontrivial features of the probe-field spectrum discovered by us previously [Zh. Éksp. Teor. Fiz. 127, 320 (2005)] in the framework of the strong-collision model are not connected with a particular model of collisions and have a universal nature.