Three Dimensionality and Orbital Characters of the Fermi Surface in (Tl,Rb)_{y}Fe_{2-x}Se_{2}

We report a comprehensive angle-resolved photoemission spectroscopy study of the tridimensional electronic bands in the recently discovered Fe selenide superconductor (Tl,Rb)_{y}Fe_{2-x}Se_{2} (T_{c}=32 K). We determined the orbital characters and the k_{z} dependence of the low energy electronic structure by tuning the polarization and the energy of the incident photons. We observed a small 3D electron Fermi surface pocket near the Brillouin zone center and a 2D like electron Fermi surface pocket near the zone boundary. The photon energy dependence, the polarization analysis and the local-density approximation calculations suggest a significant contribution from the Se 4p_{z} and Fe 3d_{xy} orbitals to the small electron pocket. We argue that the emergence of Se 4p_{z} states might be the cause of the different magnetic properties between Fe chalcogenides and Fe pnictides.     

Three-photon photoemission from Cu(100): observation of an unoccupied surface state near gG

The photon energy dependence has been measured for three-photon photoemission from Cu(100). A narrow resonance (FWHM = 0.02 eV) was observed centered at 2.02 eV photon energy. The resonance can be explained by the presence of an unoccupied surface state near the center of the surface Brillouin zone at gG and 2.0 eV above E_F.     

The energy dependence of the hard exclusive diffractive processes in pQCD as the function of momentum transfer

We predict the dependence on energy of photo (electro) production processes: γ(γ ^*)+p→V+X with large rapidity gap at small x and large momentum ?t transferred to V in pQCD. Here V is a heavy quarkonium (J/ψ,?) or longitudinally polarized light vector meson (in the electroproduction processes), etc. In the kinematics of HERA we calculate the dependence on energy of cross sections of these processes as the function of momentum transfer t, photon virtuality Q ² and/or quarkonium mass. In the kinematical region $Q_{0}^{2}\le -t\ll Q^{2}+M^{2}_{V}$ the nontrivial energy dependence of the cross section for the vector meson production due to the photon scattering off a parton follows within QCD from the summing of the double logarithmic terms. In the second regime $-t\ge Q^{2}+M^{2}_{V}$ within DGLAP approximation in all orders of perturbation theory the $q\bar{q}$ -parton elastic cross section is energy independent. We show that the correct account of the double logarithmic terms and of the gluon radiation including kinematical constraints removes the disagreement between pQCD calculations and recent HERA experimental data. The explicit formula for the dependence of the differential cross section $\frac{d^{2}\sigma}{dt\,dx_{J}}$ of these processes on $s_{\gamma^{*}N}$ is obtained. We show that perturbative Pomeron type behavior may reveal itself only at energies significantly larger than those available at HERA. In addition we evaluate the energy dependence of DVCS processes.     

Strong and complex electron-lattice correlation in optimally doped Bi2Sr2CaCu2O8+delta

We discuss the nature of electron-lattice interaction in optimally doped Bi_{2}Sr_{2}CaCu_{2}O_{8+delta} samples, using the isotope effect (IE) in angle resolved photoemission spectroscopy (ARPES) data. The IE in the ARPES linewidth and the IE in the ARPES dispersion are both quite large, implying a strong electron-lattice correlation. The strength of the electron-lattice interaction is "intermediate," i.e., stronger than the Migdal-Eliashberg regime but weaker than the small polaron regime, requiring a more general picture of the ARPES kink than the commonly used Migdal-Eliashberg picture. The two IEs also imply a complex interaction, due to their strong momentum dependence and their differing sign behaviors. In sum, we propose an intermediate-strength coupling of electrons to localized lattice vibrations via charge density fluctuations.     

Angle-resolved photoemission from Fe(110): Determination of E()

Using angle-resolved photoemission spectroscopy utilizing polarized synchrotron radiation with a clean Fe(110) sample, we identified each of the Σ₁-, Σ₃- and Σ₄-band peaks with certainty using symmetry selection rules. We also determined energy band dispersion along the γ-Σ-N direction in the bulk Brillouin zone assuming direct transitions. There are small discrepancies between the observed bands and recent ground-state band calculations possibly due to Coulomb correlation effects. A surface state with odd symmetry about the [001] crystal direction and a binding energy of 0.15 eV at γ in the surface Brillouin zone was observed. Our results show that the ground-state calculations of ferromagnetic iron based on the local exchange correlation potential are basically in agreement with experimental dispersion relationship measurements.     

Strong constraints on Lorentz violation using new γ-ray observations around PeV

The tiny modification of dispersion relation induced by Lorentz violation (LV) is an essential topic in quantum gravity (QG) theories, which can be magnified into significant effects when dealing with astrophysical observations at high energies and long propagation distances. LV would lead to photon decay at high energies; therefore, observations of high-energy photons could constrain LV or even QG theories. The Large High Altitude Air Shower Observatory (LHAASO) is the most sensitive gamma-array instrument currently operating above 100 TeV. Recently, LHAASO reported the detection of 12 sources above 100 TeV with maximum photon energy exceeding 1 PeV. According to these observations, the most stringent restriction is achieved in this study, i.e., limiting the LV energy scale to \begin{document}$1.7\times10^{33} $\end{document} eV, which is over 139,000 times that of the Planck energy, and achieving an improvement of approximately 1.9 orders of magnitude over previous limits.     

Electronic band structure of Cu3Au: An angle-resolved photoemission study along the [111]-direction

High-resolution normal photoemission (ARPE) spectra have been recorded for Cu₃Au(111) with the use of polarized synchrotron and rare-gas resonance radiation in the photon energy range from 9 to 27 eV. It is for the first time that dispersions of the gold-like bands have been found experimentally. Using a fully relativistic layer-KKR photoemission formalism, occupied and unoccupied bands as well as one-step-model photoemission spectra have been calculated. The comparison of calculated spectra with experimental ones and the observation of direct-transition resonances upon photon energy near the Brillouin zone-center reveal a shift of the unoccupied ground-state bands by about +2.5 eV (self-energy shift). The direct-transition structures in the experimental spectra have been exploited to determine the dispersions of the occupied bands along the [111] direction (A line in k space). In order to determine the wave vector of the experimental direct transitions we used as final state that calculated unoccupied band along [111], which also exists in pure copper and gold up to about 20 eV above the Fermi energy (“unfolded” band structure), shifted by + 2.5 eV. The experimental occupied bands with Cu character are in very good agreement with theory after shifting the latter by about 0.3 eV to lower energy, whereas somewhat bigger discrepancies exist for the gold-like bands.     

Fermi surface of Bi2Sr2CaCu2O8

We study the Fermi surface of Bi2Sr2CaCu2O8 using angle resolved photoemission spectroscopy (ARPES) with a momentum resolution of approximately 0.01 of the Brillouin zone. We show that, contrary to recent suggestions, the ARPES derived Fermi surface is a large hole barrel centered at (pi,pi), independent of the incident photon energy. We caution that the photon energy and k dependence of the matrix elements, if not properly accounted for, can lead to misinterpretation of ARPES intensities.     

Angle-resolved photoemission study of the MX-chain compound [Ni(chxn)(2)Br]Br(2): spin-charge separation in hybridized d-p chains

We report on the results of angle-resolved photoemission experiments on a quasi-one-dimensional (1D) MX-chain compound [Ni(chxn)2Br]Br2, which shows a gigantic nonlinear optical effect. A "band" having about 500 meV energy dispersion is found in the first half of the Brillouin zone, but disappears at kb/pi approximately 1/2. These spectral features are well reproduced by the d-p chain model with a small charge-transfer energy Delta compared with that of 1D Cu-O compounds. We propose that this smaller Delta is the origin of the absence of clear spin-charge separation in the photoemission spectra and the strong nonlinear optical effect.     

Angle-resolved photoemission from CO on Fe(110)

We report normal emission, angle-resolved photoemission measurements of the system p (1×2)Co-Fe(110) at several photon energies. The lower bind-energy feature, commonly attributed to emission from both the 1π and 5σ molecular orbitals of CO, shifts from 6.9 to 8.1 eV (relative to the Fermi energy) as the photon energy is increased from 21 to 30 eV. We attribute this dispersion to a large variation in cross section for photoionization of the 1π- and 5σ-derived orbitals and use symmetry selection rules to Identify the 6.9 eV component as the 1π and the 8.1 eV component as the 5σ. This ordering is reversed from the gas phase ordering.     

First-principles study on the electronic structure and optical properties of T12Cd2（SO4）3 and Rb2Cd2（SO4）3

With the help of ab initio full-potential linearized augmented plane wave （FPLAPW） method, calculating the electronic structure and linear optical properties is carried out for XCd2（SO4）3 （X =Tl, Rb）. The results show that Tl2Cd2（SO4）3 （TlCdS） has a larger band gap than Rb2Cd2（SO4）3 （RbCdS） and the energy bands for RbCdS are more dispersive than those of TlCdS. From their partial densities of states （PDOS）, we have observed that the hybridization between S ionic 2p and O atomic 2p orbitals forms SO4 ionic groups. The remarkable difference between RbCdS and TlCdS is, however, the degree of hybridization between cation （Tl and Rb） and its surrounding oxygen atoms. In the view of quantum chemistry, the strong p-d hybridization indicates the existence of their cation ionic bonds （Cd-O, Rb-O, and Tl-O）. The calculations of TlCdS and RbCdS show their optical properties to be less anisotropic. Their anisotropies in the optical properties mainly occur in a low photon energy region of 5-16 eV.     

Angle resolved photoemission valence band dispersions E(k) for gaP.

We experimentally determined the valence band dispersions E(k) along the symmetry directions ΓKX, ΓX, ΓL using angle-resolved photoemission. ΓX, ΓKX and ΓL have been studied by off-normal emission process with photon energy between 31 eV and 44 eV, while ΓKX has also been studied by normal emission process using photon energy from 31 to 85 eV. The resulting dispersion curves E(k) are compared to pseudopotential calculation. We used a direct transition model and a nearly-free electron final state approximation to interpret the main experimental features.     

Spin-resolved photoemission from epitaxial Au layers on Pt(111): Coverage dependence of the bandstructure and evidence of surface resonances

Au/Pt(111) has been studied by spin-, angle- and energy-resolved photoemission with normal incident circularly polarized synchrotron radiation of BESSY and normal photoelectron emission for different Au coverages. The prepared layers were characterized by LEED and Augerelectron spectroscopy and turned out to grow up two dimensional and epitaxially. In the photoemission experiments the development of the 3-dimensional bandstructure in the -direction could be observed. For a coverage of 2.6 layers the highest occupied spin-orbit split bands are located at about 0.6 eV lower binding energy than the corresponding bands for a 3D-Au crystal and show dispersion which is, however, weaker than in a 3D-Au crystal. A 5 layer Au adsorbate was found to have already the same dispersion and energetic location as a Au(111)-crystal. For thick gold layers, which behave in photoemission like Au(111)-crystals, we find structures that cannot be due to direct transitions into a free electron like final band. The coverage dependence and spin polarization of these structures show that some of them are due to surface resonances, while the origin of one strong peak could not yet be explained conclusively. In addition we find strong hybridization and two avoided crossings in the occupied part of the bandstructure.     

Spin fluctuations and the peak-dip-hump feature in the photoemission spectrum of actinides

We present first-principles multiband spin susceptibility calculations within the random-phase approximation for four isostructural superconducting PuCoIn{5}, PuCoGa{5}, PuRhGa{5}, and nonsuperconducting UCoGa{5} actinides. The results show that a strong peak in the spin-fluctuation dressed self-energy is present around 0.5 eV in all materials, which is mostly created by 5f electrons. These fluctuations couple to the single-particle spectrum and give rise to a peak-dip-hump feature, characteristic of the coexistence of itinerant and localized electronic states. Results are in quantitative agreement with photoemission spectra. Finally, we show that the studied actinides can be understood within the rigid-band filling approach, in which the spin-fluctuation coupling constant follows the same materials dependence as the superconducting transition temperature T{c}.     

Core level photoemission study of the adsorption of iodine on Ni{100}

Synchrotron radiation photoemission from the Ni 3p and I 4d core levels has been studied for iodine adsorbed on Ni{100} in a range of different phases. In the range of chemisorbed structures involving overlayer compression from coverages θ of about 0.3 to 0.4 the I 4d photoemission binding energies decrease by ~0.3 eV, while further compression to a c(2 × 2) structure leads to a further decrease by ~0.4 eV. Low temperature adsorption of molecular iodine shows a binding energy of 0.1 eV lower still. In the chemisorbed coverage range these core level shifts are found to be substantially less than the I-I repulsive energy interactions and are associated with the through-metal component of this interaction, while the residual component concealed from the photoemission experiments is attributed to through-space overlap interaction. Other processes which may contribute to the observed binding energy shifts, including Coulomb interactions in both initial and final states, are discussed in some detail.     

Observation of occupation and dispersion of 2π1 associated states for CO adsorbed on Cu{100}

Angle-resolved photoemission observations of CO adsorbed on Cu{100} to form the (√2 × √2)R45° structure show emission from a CO 2π¹ related state away from the centre of the surface Brillouin zone. These observations are shown to be compatible with the anticipated dispersion of such states and with weak 2π¹ backbonding on this substrate.     

Near-infrared downconversion through energy transfer from \mathrm{VO}_{4}^{3-} group to Yb3+ in Yb3+ doped YP0.9V0.1O4

Through the study of photoluminescence spectra, fluorescence decay curves, concentration-dependent luminescence and low temperature luminescence, the energy transfer from isolated $\mathrm{VO}_{4}^{3-}$ group to Yb³⁺ is investigated in Yb³⁺ doped YP_0.9V_0.1O₄. The experimental results show that the energy transfer from $\mathrm{VO}_{4}^{3-}$ group to Yb³⁺ is very efficient. Cooperative energy transfer, through which one high-energy photon absorbed by $\mathrm{VO}_{4}^{3-}$ group is converted to two near-infrared photons emitted by Yb³⁺ ions, is proposed to be the energy transfer process. This efficient ultra-violet to near-infrared downconversion could have potential application in improving the efficiency of silicon-based solar cell.     

High-resolution angle-resolved photoemission study of Ni(1 1 1) surface state

High-resolution angle-resolved photoemission spectroscopy (ARPES) has been conducted to study the Shockley state (SS) in ferromagnetic Ni(1 1 1) located at the point of the surface Brillouin zone. We have determined the Fermi wave vector and Fermi energy of the state with excitation photon energies of hν = 6.9-27.5 eV. On the basis of ARPES spectral shape analyses, we have found significant electron-electron interaction in the SS.     

CPT Violation in Neutrino Oscillation and Matter Effects

There is good agreement between the neutrino mass square difference determined from the solar neutrino and anti-neutrino mass square difference from the KamLAND reactor antineutrino. We consider as special case of matter density profile, which are relevant for neutrino oscillation physics. In particular, we compute to constrain a specific from of CPT violation in matter by upper bound, $|\varDelta_{21}^{m}-\overline{\varDelta_{21}^{m}}| \ll 1.098\times10^{-4}~\mathrm{eV}^{2}$ and $|\sin2\theta_{12}^{m}-\sin2\bar{\theta}_{12}^{m}|<0.0057$ . In this paper, we discuss CPT violation on neutrino oscillation in matter. The dispersion relation for the CPT violation in neutrino oscillation in matter are discussed.     

Two-body decays of gluino at full one-loop level in the quark-flavour violating MSSM

We study the two-body decays of the gluino at full one-loop level in the Minimal Supersymmetric Standard Model with quark-flavour violation (QFV) in the squark sector. The renormalisation is done in the \(\overline{\mathrm{DR}}\) scheme. The gluon and photon radiations are included by adding the corresponding three-body decay widths. We discuss the dependence of the gluino decay widths on the QFV parameters. The main dependence stems from the \(\tilde{c}_R \)–\( \tilde{t}_R\) mixing in the decays to up-type squarks, and from the \(\tilde{s}_R \)–\( \tilde{b}_R\) mixing in the decays to down-type squarks due to the strong constraints from B-physics on the other quark-flavour-mixing parameters. The full one-loop corrections to the gluino decay widths are mostly negative and of the order of about ?10%. The QFV part stays small in the total width but can vary up to ?8% for the decay width into the lightest \(\tilde{u}\) squark. For the corresponding branching ratio the effect is somehow washed out by at least a factor of two. The electroweak corrections can be as large as 35% of the SUSY QCD corrections.