Single-electron Spectra Research Articles

Core-resonant double photoemission from palladium films

We studied the core-resonant double photoemission process from palladium films with linearly polarized synchrotron radiation. We excited either the 3d or 4p core level and focused on the Auger transitions which leave two holes in the valence band. We find that the two-dimensional energy distributions are markedly different for the 3d and 4p decay. The 3d decay can be understood by a sequential emission of the two electrons while the 4p decay proceeds in a single step. Despite the large differences in the two-dimensional energy spectra we find the shape of the energy sum spectra rather similar. For the description of the 4p decay we propose a model which uses available single electron spectra, but suggest an alternative interpretation of these data. With this we are able to explain the range over which the available energy is shared. Key assumptions of the model are verified by our experiments on the 3d decay.

Electronic spectrum of twisted bilayer graphene

We study the electronic properties of twisted bilayers graphene in the tight-binding approximation. The interlayer hopping amplitude is modeled by a function, which depends not only on the distance between two carbon atoms, but also on the positions of neighboring atoms as well. Using the Lanczos algorithm for the numerical evaluation of eigenvalues of large sparse matrices, we calculate the bilayer single-electron spectrum for commensurate twist angles in the range $1^{\circ}\lesssim\theta\lesssim30^{\circ}$. We show that at certain angles $\theta$ greater than $\theta_{c}\approx1.89^{\circ}$ the electronic spectrum acquires a finite gap, whose value could be as large as $80$ meV. However, in an infinitely large and perfectly clean sample the gap as a function of $\theta$ behaves non-monotonously, demonstrating exponentially-large jumps for very small variations of $\theta$. This sensitivity to the angle makes it impossible to predict the gap value for a given sample, since in experiment $\theta$ is always known with certain error. To establish the connection with experiments, we demonstrate that for a system of finite size $\tilde L$ the gap becomes a smooth function of the twist angle. If the sample is infinite, but disorder is present, we expect that the electron mean-free path plays the same role as $\tilde L$. In the regime of small angles $\theta<\theta_c$, the system is a metal with a well-defined Fermi surface which is reduced to Fermi points for some values of $\theta$. The density of states in the metallic phase varies smoothly with $\theta$.

Phase-space factors and half-life predictions for Majoron-emittingβ−β−decay

A complete calculation of phase space factors (PSF) for Majoron emitting $0\nu\beta^-\beta^-$ decay modes is presented. The calculation makes use of exact Dirac wave functions with finite nuclear size and electron screening and includes life-times, single electron spectra, summed electron spectra, and angular electron correlations. Combining these results with recent interacting boson nuclear matrix elements (NME) we make half-life predictions for the the ordinary Majoron decay (spectral index $n$=1). Furthermore, comparing theoretical predictions with the obtained experimental lower bounds for this decay mode we are able to set limits on the effective Majoron-neutrino coupling constant $\langle g_{ee}^M\rangle$.

Electromagnetic probes of strongly interacting matter

The nuclear matter under extreme conditions of temperatures (T) and baryonic densities ( n B) undergoes a phase transition to quark gluon plasma (QGP). It is expected that such extreme conditions can be achieved by colliding nuclei at ultrarelativistic energies. In the present review, the suitability of photons and dileptons as diagnostic tools of QGP has been discussed. The photon and dilepton spectra originating from heavy-ion collisions at LHC energies have been explicitly displayed in this article. Results from SPS and RHIC have been discussed adequately with appropriate references. The role of single electron spectra originating from the decays of heavy flavoured mesons on QGP detection has also been discussed briefly.

Plasmons in strongly correlated systems: spectral weight transfer and renormalized dispersion.

We study the charge-density dynamics within the two-dimensional extended Hubbard model in the presence of long-range Coulomb interaction across the metal-insulator transition point. To take into account strong correlations we start from self-consistent extended dynamical mean-field theory and include nonlocal dynamical vertex corrections through a ladder approximation to the polarization operator. This is necessary to fulfill charge conservation and to describe plasmons in the correlated state. The calculated plasmon spectra are qualitatively different from those in the random-phase approximation: they exhibit a spectral density transfer and a renormalized dispersion with enhanced deviation from the canonical √q behavior. Both features are reminiscent of interaction induced changes found in single-electron spectra of strongly correlated systems.

Theory of unconventional quantum Hall effect in strained graphene

We show through both theoretical arguments and numerical calculations that graphene discerns an unconventional sequence of quantized Hall conductivity, when subject to both magnetic fields ($B$) and strain. The latter produces time-reversal symmetric pseudo/axial magnetic fields ($b$). The single-electron spectrum is composed of two interpenetrating sets of Landau levels (LLs), located at $\ifmmode\pm\else\textpm\fi{}\sqrt{2n|b\ifmmode\pm\else\textpm\fi{}B|}$, $n=0,1,2,...$. For $b&gt;B$, these two sets of LLs have opposite chiralities, resulting in oscillating Hall conductivity between 0 and $\ensuremath{\mp}2{e}^{2}/h$ in electron and hole doped systems, respectively, when the chemical potential is tuned in the vicinity of the neutrality point. The electron-electron interactions stabilize various correlated ground states, e.g., spin-polarized, quantum spin Hall insulators at and near the neutrality point, and possibly the anomalous Hall insulating phase at incommensurate filling $\ensuremath{\sim}$$B$. Such broken-symmetry ground states have similarities as well as significant differences from their counterparts in the absence of strain. For realistic strength of magnetic fields and interactions, we present scaling of the interaction-induced gap for various Hall states within the zeroth Landau level.

Absolute Kα line spectroscopy for cone-guided fast-ignition targets

Absolute Kα line spectroscopy is proposed to study laser-plasma interactions taking place in cone-guided fast ignition targets. A transmission-type spectrometer was developed, and its absolute sensitivity was measured with laser-generated x-rays and radioisotopes, showing good agreement with theoretical predictions. The absolute yield of Au Kα line was mesured in the fast ignition experimental campaign perfomed at ILE, Osaka University. Assuming a single Maxwellian electron spectrum and one-way travel of hot electrons through the cone, energy transfer efficiency of incident LFEX laser to hot electrons was derived.

Electron Localization, Tunneling and Energy Spectrum for Systems of Double Quantum Dots

ABSTRACTSemiconductor heterostructures as quantum dots demonstrate discrete atom-like energy level structure based on several hundred of electron confinement states. In the case of double QD (DQD) or double QR (DQR), there is a single electron spectrum composed of a set of quasi-doublets. We study these specific spectrum properties with their relation to the electron tunneling in DQD (DCQR) when the wave function of electron localized initially in one of the double quantum object is spread into whole system. The double InAs/GaAs quantum dots are considered within the effective approach. Tunneling in DQD is studied in connection with change of inter-dot distance and QD geometry. There are two types of such tunneling in DQD. The first is related to tunneling in the system of two identical QDs; the second one occurs in the system of non-identical QDs. The tunneling in the DQR is a tunneling in the system with non-identical quantum objects. The quasi-doublets of the DQD spectrum play an important role in the tunneling. We study effect of violation of symmetry of DQD geometry on the tunneling and show that the violation of symmetry makes difficulties for such tunneling.

Emerging coherence with unified energy, temperature, and lifetime scale in heavy fermion YbRh2Si2

We present ultrahigh resolution angle-resolved photoemission data of the prototypical Kondo lattice system YbRh${}_{2}$Si${}_{2}$, using a 7 eV laser photon source. Detailed temperature dependent measurements reveal the development of coherent states, in the form of sharp, weakly dispersing peaks at the lowest energy of single-electron spectra, below a characteristic temperature. The characteristic temperature, obtained from angle-resolved photoemission, is intriguingly of exactly the same scale as the energy and the lifetime of the coherent state.

Drag and diffusion coefficients ofBmesons in hot hadronic matter

The drag and diffusion coefficients of a hot hadronic medium consisting of pions, kaons and eta using open beauty mesons as a probe have been evaluated. The interaction of the probe with the hadronic matter has been treated in the framework of chiral perturbation theory. It is observed that the magnitude of both the transport coefficients is significant, indicating a substantial amount of interaction of the heavy mesons with the thermal bath. The results may have significant impact on the experimental observables like the suppression of single-electron spectra originating from the decays of heavy mesons produced in nuclear collisions at Relativistic Heavy Ion Collider and LHC energies.

Effects of equation of state on nuclear suppression and the initial entropy density of quark gluon plasma

We study the effects of the equation of state on the nuclear suppression of heavy flavours in quark gluon plasma and estimate the initial entropy density of the system produced at the highest RHIC energy. For this purpose we have used the experimental data on the charged particle multiplicity and the nuclear suppression of single electron spectra originating from the semi-leptonic decays of open charm and beauty mesons. We have used inputs from lattice QCD to minimize the model dependence of the results. We obtain the value of the initial entropy density which varies from 20 to 59 /fm$^3$ depending on the value of the velocity of sound that one uses for the analysis. Our investigation leads to a conservative value of the initial entropy density $\sim 20/$fm$^3$ with corresponding initial temperature $\sim 210$ MeV well above the value of the transition temperature predicted by lattice QCD.

Ionization potentials and electron affinities from reduced-density-matrix functional theory

In the recent work of S. Sharma et al. (e-print arXiv:0912.1118), a single-electron spectrum associated with the natural orbitals was defined as the derivative of the total energy with respect to the occupation numbers at half filling for the orbital of interest. This idea reproduces the bands of various periodic systems using the appropriate functional quite accurately. In the present work we apply this approximation to the calculation of the ionization potentials and electron affinities of molecular systems using various functionals within the reduced-density-matrix functional theory. We demonstrate that this approximation is very successful in general and in particular for certain functionals it performs better than the direct determination of the ionization potentials and electron affinities through the calculation of positive and negative ions respectively. The reason for this is identified to be the inaccuracy that arises from different handling of the open- and closed-shell systems.

Attosecond Two-Photon Interferometry for Doubly Excited States of Helium

We show that the correlation dynamics in coherently excited doubly excited resonances of helium can be followed in real time by two-photon interferometry. This approach promises to map the evolution of the two-electron wave packet onto experimentally easily accessible noncoincident single-electron spectra. We analyze the interferometric signal in terms of a semianalytical model which is validated by a numerical solution of the time-dependent two-electron Schrödinger equation in its full dimensionality.

Drag coefficient ofDmesons in hot hadronic matter

We evaluate the drag and diffusion coefficients of a hot hadronic medium consisting of pions, nucleons, kaons and eta using open charm mesons as a probe. The interaction of the probe with the hadronic matter has been treated in the framework of effective field theory. It is observed that the magnitude of both the transport coefficients are significant, indicating substantial amount of interaction of the heavy mesons with the thermal hadronic system. The results may have noticeable impact on the experimental observable like the suppression of single electron spectra originating form the decays of heavy mesons in nuclear collisions at relativistic energies.

Measurement of semi electronic decay of heavy flavor mesons in d+Au collision at RHIC using PHENIX detector

Abstract The PHENIX detector at RHIC has measured single electron spectra from heavy flavor decays in p+p and Au+Au collisions at s N N = 200 GeV . A strong suppression is observed compared to the binary scaling of p+p collisions for high p T single electrons in Au+Au collisions providing evidence for strong medium effects. The motivation for measuring the single electron spectra from the decay of heavy flavor mesons in d+Au collisions from the 2008 RHIC run (Run8) is to determine parton distribution modifications, the Cronin effect and possible energy loss present in cold nuclear matter. These phenomena can mask hot nuclear matter effects and therefore must be understood in order to interpret the Au+Au results. The analysis is still in progress, but the latest results and technical details are being shown in the current paper.

Probing collectivity in ultra-relativistic heavy ion collision by leptons and photons

It has been shown that the evolution of collectivity in ultra-relativistic heavy ion collision is manifested in the variation of various HBT radii with invariant mass ($M$) extracted from the correlation functions of two lepton pairs. The value of the radial velocity ($v_r$) can be estimated from the ratio of the $p_T$ distributions of single photons to lepton pairs for various $M$ windows. It has been argued that the variation of radial flow with appropriate kinematic variables can be used as an indicator of a phase transition from initially produced partons to hadrons. We also consider the elliptic flow ($v_2^{HF}$) of the matter as probed by the single electron spectra originating from the semileptonic decays of heavy mesons. The measured values of $v_2^{HF}$ and the nuclear suppression factor ($R_{\mathrm AA}$) at RHIC energy have been reproduced simultaneously by including both the collisional and radiative processes within the scope of perturbative quantum chromodynamics. The $R_{\mathrm AA}$ and $v_2^{HF}$ have been predicted for LHC energy.

LANGEVIN DYNAMICS OF HEAVY FLAVORS IN RELATIVISTIC HEAVY-ION COLLISIONS

We study the stochastic dynamics of c and b quarks, produced in hard initial processes, in the hot medium created after the collision of two relativistic heavy ions. This is done through the numerical solution of the relativistic Langevin equation. The latter requires the knowledge of the friction and diffusion coefficients, whose microscopic evaluation is performed treating separately the contribution of soft and hard collisions. The evolution of the background medium is described by ideal/viscous hydrodynamics. Below the critical temperature the heavy quarks are converted into hadrons, whose semileptonic decays provide single-electron spectra to be compared with the current experimental data measured at RHIC. We focus on the nuclear modification factor RAA and on the elliptic-flow coefficient v2, getting, for sufficiently large pT, a reasonable agreement.

Heavy quark dynamics in the QGP: [formula omitted] and [formula omitted] from RHIC to LHC

We study the stochastic dynamics of c and b quarks in the hot plasma produced in nucleus-nucleus collisions at RHIC and LHC, providing results for the nuclear modification factor R A A and the elliptic flow coefficient v 2 of the single-electron spectra arising from their semi-leptonic decays. The initial Q Q ˆ pairs are generated using the POWHEG code, implementing pQCD at NLO. For the propagation in the plasma we develop a relativistic Langevin equation (solved in a medium described by hydrodynamics) whose transport coefficients are evaluated through a first-principle calculation. Finally, at T c , the heavy quarks are made to hadronize and decay into electrons: the resulting spectra are then compared with RHIC results. Predictions for LHC are also attempted.

GENERALIZED SUPERCONDUCTING GAP IN A BOSON-FERMION MODEL

A quantum-statistical binary gas mixture model consisting of positive-energy resonant bosonic Cooper electron pairs in chemical and thermal equilibrium with single unpaired electrons yields, via two-time retarded Green functions, an analytic expression for a dimensionless coupling λ- and temperature T-dependent generalized energy gap Eg(λ, T) in the single-electron spectrum. The new gap gives a reasonable description of overdoped Bi2Sr2CuO6+δ(Bi2201).

Drag of heavy quarks in quark gluon plasma at energies available at the CERN Large Hadron Collider (LHC)

The drag and diffusion coefficients of charm and bottom quarks propagating through quark gluon plasma (QGP) have been evaluated for conditions relevant to nuclear collisions at Large Hadron Collider (LHC). The dead cone and Landau-Pomeronchuk-Migdal (LPM) effects on radiative energy loss of heavy quarks have been considered. Both radiative and collisional processes of energy loss are included in the {\it effective} drag and diffusion coefficients. With these effective transport coefficients we solve the Fokker Plank (FP) equation for the heavy quarks executing Brownian motion in the QGP. The solution of the FP equation has been used to evaluate the nuclear suppression factor, $R_{\mathrm AA}$ for the non-photonic single electron spectra resulting from the semi-leptonic decays of hadrons containing charm and bottom quarks. The effects of mass on $R_{\mathrm AA}$ has also been highlighted.