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First results from Mark II at SPEAR

Description: First results from the SLAC-LBL Mark II magnetic detector at SPEAR are presented. The performance of the detector is discussed and preliminary results are given on inclusive baryon production R/sub p + anti p/, R/sub ..lambda.. + anti ..lambda../, on decay modes of the D mesons and on two-photon production of eta' mesons.
Date: May 1, 1979
Creator: Abrams, G.S.; Alam, M.S. & Blocker, C.A.
Partner: UNT Libraries Government Documents Department

Coulomb correlations and optical gap in polyacetylene

Description: A model including both electron-phonon coupling (as in the SSH Hamiltonian) and electron-electron interactions (on-site term U, nearest-neighbor term V) is treated within the variational scheme of Gutswiller. It is shown that for weak electron-phonon coupling the primary effect is a bond-order wave induced by electronic correlation, whereas the lattice dimerization is a secondary effect. Correspondingly the optical gap is mainly due to electronic correlation.
Date: January 1, 1986
Creator: Baeriswyl, D. & Maki, K.
Partner: UNT Libraries Government Documents Department

Power corrections, renormalons and resummation

Description: I briefly review three topics of recent interest concerning power corrections, renormalons and Sudakov resummation: (a) 1/Q corrections to event shape observables in e(+)e(-) annihilation, (b) power corrections in Drell-Yan production and (c) factorial divergences that arise in resummation of large infrared (Sudakov) logarithms in moment or `real` space.
Date: August 1, 1996
Creator: Beneke, M.
Partner: UNT Libraries Government Documents Department

Coulomb correlation effects in quasi-one-dimensional conductors

Description: Recent results on the role of electron-electron (e-e) interactions - ''correlation effects'' - in quasi-one-dimensional conductors are reviewed. Within the Peierls-Hubbard model, the consequences of short range (on-site U and nearest neighbor V) e-e interactions for ground state properties, nonlinear excitations, and optical absorption are examined. Techniques include quantum Monte Carlo and weak and strong coupling perturbative arguments. 26 refs., 2 figs.
Date: January 1, 1986
Creator: Campbell, D.K.; Baeriswyl, D. & Mazumdar, S.
Partner: UNT Libraries Government Documents Department

Beam-beam disruption and the case for a plasma lens in e{sup {minus}}e{sup {minus}} collisions

Description: In an e{sup {minus}}e{sup {minus}} collider, the mutual disruption of the beams will significantly decrease the luminosity, different from the case of e{sup +}e{sup {minus}} colliders, where the mutual attraction of the oppositely-charged beams leads to a self-focusing effect. In this paper the authors estimate the beam disruption at SLC and NLC energies using ABEL simulations. They then give estimates of how a plasma lens might improve the luminosity in e{sup {minus}}e{sup {minus}} collisions.
Date: March 1, 1996
Creator: Chen, P.; Spitkovsky, A. & Weidemann, A.W.
Partner: UNT Libraries Government Documents Department

A step closer to visualizing the electron___phonon interplay

Description: The origin of the very high superconducting transition temperature (Tc) in ceramic copper oxide superconductors is one of the greatest mysteries in modern physics. In the superconducting state, electrons form pairs (known as Cooper pairs) and condense into the superfluid state to conduct electric current with zero resistance. For conventional superconductors, it is well established that the 2 electrons in a Cooper pair are 'bonded' by lattice vibrations (phonons), whereas in high-Tc superconductors, the 'glue' for the Cooper pairs is still under intense discussion. Although the high transition temperature and the unconventional pairing symmetry (d-wave symmetry) have led many researchers to believe that the pairing mechanism results from electron-electron interaction, increasing evidence shows that electron-phonon coupling also significantly influences the low-energy electronic structures and hence may also play an important role in high-Tc superconductivity. In a recent issue of PNAS, Carbone et al. use ultrafast electron diffraction, a recently developed experimental technique, to attack this problem from a new angle, the dynamics of the electronic relaxation process involving phonons. Their results provide fresh evidence for the strong interplay between electronic and atomic degrees of freedom in high-Tc superconductivity. In general, ultrafast spectroscopy makes use of the pump-probe method to study the dynamic process in material. In such experiments, one first shoots an ultrafast (typically 10-100 fs) 'pumping' pulse at the sample to drive its electronic system out of the equilibrium state. Then after a brief time delay ({Delta}t) of typically tens of femtoseconds to tens of picoseconds, a 'probing' pulse of either photons or electrons is sent in to probe the sample's transient state. By varying {Delta}t, one can study the process by which the system relaxes back to the equilibrium state, thus acquiring the related dynamic information. This pump-probe experiment is reminiscent of the standard method used by bell ...
Date: January 4, 2011
Creator: Chen, Y.L.; Lee, W.S.; Shen, Z.X. & /Stanford U., Appl. Phys. Dept. /Stanford U., Phys. Dept. /SLAC, PULSE
Partner: UNT Libraries Government Documents Department

Precsion Supersymmetry Measurement at the e{minus} e{minus} Collider

Description: Measurements of supersymmetric particle couplings provide important verification of supersymmetry. If some of the superpartners are at the multi-TeV scale, they will escape direct detection at planned future colliders. However, such particles induce nondecoupling corrections in processes involving the accessible superparticles through violations of the supersymmetric equivalence between gauge boson and gaugino couplings. These violations are analogous to the oblique corrections in the electroweak sector of the standard model, and can be parametrized in terms of super-oblique parameters. The e{sup -}e{sup -} collision mode of a future linear collider is shown to be an excellent environment for such high precision measurements of these SUSY parameters, which will provide an important probe of superparticles beyond reachable energies.
Date: January 1, 1998
Creator: Cheng, H.-C.
Partner: UNT Libraries Government Documents Department

Quantum Monte-Carlo Study of Electron Correlation in Heterostructure Quantum Dots

Description: The goal of this project is to study electron correlation in a confined geometry (quantum dots) within the two-dimensional quantum well in the sandwiches of two semiconductor materials. For these systems one is able to tune the electronic properties by controlling the size and the electron number, creating tremendous potential for novel applications. Much effort in this emerging field has been devoted to producing entangled states that are required for quantum information processing. At the same time, new physical phenomena have emerged from these artificial structures. Adding electrons to a quantum dot is more complicated than filling up discrete energy levels due to electron correlation. Therefore, our project is focusing on employing the state-of-the-art quantum Monte Carlo methods to study the electron-electron interaction. A close examination of the breakdown of Hund's rules and electron localization has been conducted in our simulations. The results are summarized in this report.
Date: November 12, 2006
Creator: Chou, Mei-Yin
Partner: UNT Libraries Government Documents Department

Amplitude Modulation of Atomic Wavefunctions - Final Technical Report

Description: This project developed a novel wave function modulation technique. Other modulation techniques use tailored laser pulses to directly excite a time-dependent, modulated wave function from a ground state. Our technique began with one electron already excited to a time independent eigenfunction. Then, by using excitations of a second valence electron, we modulated the other wave function. Our technique had the benefit that it was very efficient, and required low power lasers with no need for precise phase or amplitude control. On the other hand it had the difficulty of being a multi-step laser excitation with a maximum repetition rate of 10 Hz. Over the course of this project, we showed that the technique did work, and work efficiently. However, it was easy to generalize. Since the modulation depended on a difference between electron-electron interactions with the inner electron in a ground or excited state, the efficiency of the modulation was strongly state dependent. For example, we never showed any significant modulation in our tests of barium states, while our strontium measurements did show efficient modulation as long as the state to be modulated was in the 5snd group with n between 30 and 50. We completed some studies of the dependence of the amplitude modulation as we varied the time between the excitation and de-excitation pulses applied to the inner electron. The amplitude of the nearest neighbor states was well described by Multi-Channel Quantum Defect theory, but farther satellites were problematical. This may have simply reflected the low density of measurement points, since the amplitudes of the farther satellites oscillate more quickly with time. As we developed our technique, we showed that we could directly measure autoionization decay rates in the time domain, and that the net effect of a state belonging to a Rydberg series was that exponential decay ...
Date: August 1, 2001
Creator: Cooke, William E.
Partner: UNT Libraries Government Documents Department

Theory of multi-electron recoil effects on x-ray lineshapes of metals

Description: Within the change of self-consistent field approximation, x-ray spectra can be considerably richer in many-electron phenomena than once suspected. With the finite number of electrons method, these spectra can be evaluated for realistic electron-hole interactions in free electron metals. Preliminary results indicate that metals with band structure can also be treated this way. However, theories of final-state interactions in metals await the reliable determinations of the screened potential of a core hole in a metal and realistic avaluation of the effects of electron-electron interactions. (GHT)
Date: January 1, 1980
Creator: Dow, J. D.; Swarts, C. A.; Bowen, M. A.; Mehreteab, E. & Satpathy, S. S.
Partner: UNT Libraries Government Documents Department

Comments on the possible use of the SLC as an e{sup -}e{sup -} collider

Description: It has been suggested that the SLAC Linear Collider, which was designed to collide electrons with positrons, could be modified to collide electrons with electrons. A cursory examination of this suggestion leads us to conclude that such a modification is technically feasible, but by no means trivial. In this letter we outline the issues that must be addressed to realize this possibility.
Date: February 1, 1996
Creator: Erickson, R.
Partner: UNT Libraries Government Documents Department

Supersymmetry at linear colliders: The importance of being e{sup {minus}}e{sup {minus}}

Description: Advantages of the e{sup {minus}}e{sup {minus}} option at linear colliders for the study of supersymmetry are highlighted. The fermion number violating process e{sup {minus}}e{sup {minus}} {yields} {tilde e}{sup {minus}}{tilde e}{sup {minus}} provides unique opportunities for studies of slepton masses and flavor mixings. In particular, slepton mass measurements at the 100 MeV level through threshold scans of scalar pair production may be possible. Such measurements are over an order of magnitude better than those possible in e{sup +}e{sup {minus}} mode, require far less integrated luminosity, and may lead to precise, model-independent measurements of tan {beta}. Implications for studying gauginos and the importance of accurate beam polarimetry are also discussed.
Date: March 1, 1998
Creator: Feng, J.L.
Partner: UNT Libraries Government Documents Department

Dye- and Semiconductor-Sensitized Nanoparticle Solar Cell Research at NREL

Description: The major objective of this research program is to determine the operational characteristics key to efficient, low-cost, stable liquid-junction and solid-state solar cells based on sensitized nanoporous films (in collaboration with DOE's Office of Science Program). Toward this end, we are conducting experimental and theoretical studies to understand the unique physical and chemical factors governing cell performance. Current scientific issues addressed include the influence of film morphology, sensitizer, and electrolyte on the electron transport and recombination dynamics and on the light-harvesting, charge-injection, and charge-collection efficiencies. Recently, we investigated the relationship between (1) transport and recombination, (2) morphological factors of core-shell nanoparticle films and their PV properties, and (3) electron-electron interactions and their effect on the transport dynamics. In this paper, we discuss the connection between transport and recombination and its effect on cell performance.
Date: January 1, 2005
Creator: Frank, A. J.; Kopidakis, N.; Benkstein, K. D.; van de Lagemaat, J. & Neale, N. R.
Partner: UNT Libraries Government Documents Department

Physics with low-energy e/sup +/e/sup /minus// and e/sup /minus//e/sup /minus// collisions at Tau-Charm Factory

Description: The physics opportunities in e/sup /plus//e/sup /minus// collisions with /radical/s /approx equal/ 1-2 GeV and e/sup /minus//e/sup /minus// collisions with /radical/s /approximately/5 GeV at Tau-Charm Factory are being explored. The low-energy e/sup /plus//e/sup /minus// option allows precise measurements of e/sup /plus//e/sup /minus// cross sections into /pi/'s and /eta/'s and hence stringent tests of the conserved-vector-current (CVC) hypothesis in /tau/ decays. Precise measurement of the total hadronic cross section also permits a more precise calculation of the muon anomalous magnetic moment (g-2). The e/sup /minus//e/sup /minus// option provides an opportunity for a sensitive search for lepton-number violating processes e/sup /minus//e/sup /minus// /yields/ /mu//sup /minus///mu//sup /minus//, /tau//sup /minus///tau//sup /minus//, /mu//sup /minus///tau//sup /minus// .... The e/sup /minus//e/sup /minus// collider also provides an ideal laboratory for two-photon physics with no one-photon background and the direct measurement of the two-photon background in one-photon physics 19 refs., 4 figs.
Date: July 1, 1989
Creator: Gan, K.K.
Partner: UNT Libraries Government Documents Department

The stochastic radiative transfer equation: quantum damping, Kirchoff's law and NLTE

Description: A method is presented based on the theory of quantum damping, for deriving a self consistent but approximate form of the quantum transport for photons interacting with fully ionized electron plasma. Specifically, we propose in this paper a technique of approximately including the effects of background plasma on a photon distribution function without directly solving any kinetic equations for the plasma itself. The result is a quantum Langevin equation for the photon number operator; the quantum radiative transfer equation. A dissipation term appears which is the imaginary part of the dielectric function for an electron gas with photon mediated electron-electron interactions due to absorption and re-emission. It depends only on the initial state of the plasma. A quantum noise operator also appears as a result of spontaneous emission of photons from the electron plasma. The thermal expectation value of this noise operator yields the emissivity which is exactly of the form of the Kirchoff-Planck relation. This non-zero thermal expectation value is a direct consequence of a fluctuation-dissipation relation (FDR).
Date: January 24, 2005
Creator: Graziani, F R
Partner: UNT Libraries Government Documents Department

Time-resolved THz studies of carrier dynamics in semiconductors, superconductors, and strongly-correlated electron materials

Description: Perhaps the most important aspect of contemporary condensed matter physics involves understanding strong Coulomb interactions between the large number of electrons in a solid. Electronic correlations lead to the emergence of new system properties, such as metal-insulator transitions, superconductivity, magneto-resistance, Bose-Einstein condensation, the formation of excitonic gases, or the integer and fractional Quantum Hall effects. The discovery of high-Tc superconductivity in particular was a watershed event, leading to dramatic experimental and theoretical advances in the field of correlated-electron systems. Such materials often exhibit competition between the charge, lattice, spin, and orbital degrees of freedom, whose cause-effect relationships are difficult to ascertain. Experimental insight into the properties of solids is traditionally obtained by time-averaged probes, which measure e.g., linear optical spectra, electrical conduction properties, or the occupied band structure in thermal equilibrium. Many novel physical properties arise from excitations out of the ground state into energetically higher states by thermal, optical, or electrical means. This leads to fundamental interactions between the system's constituents, such as electron-phonon and electron-electron interactions, which occur on ultrafast timescales. While these interactions underlie the physical properties of solids, they are often only indirectly inferred from time-averaged measurements. Time-resolved spectroscopy, consequently, is playing an ever increasing role to provide insight into light-matter interaction, microscopic processes, or cause-effect relationships that determine the physics of complex materials. In the past, experiments using visible and near-infrared femtosecond pulses have been extensively employed, e.g. to follow relaxation and dephasing processes in metals and semiconductors. However, many basic excitations in strongly-correlated electron systems and nanoscale materials occur at lower energies. The terahertz (THz) regime is particularly rich in such fundamental resonances. This includes ubiquitous lattice vibrations and low-energy collective oscillations of conduction charges. In nanoscale materials, band structure quantization also yields novel infrared and THz transitions, including intersubband absorption in quantum wells. ...
Date: November 14, 2006
Creator: Kaindl, Robert A. & Averitt, Richard D.
Partner: UNT Libraries Government Documents Department

Progress in LTE and non-LTE transport calculations

Description: The physical processes in ionized gases are reviewed along with recent calculations of particular processes. Ongoing opacity calculations at Los Alamos are described. Some of the open issues in the calculation of physical processes relevant to opacities are discussed. 39 refs., 7 figs.
Date: January 1, 1989
Creator: Keady, J.J.; Huebner, W.F.; Abdallah, J. Jr.; Magee, N.H. Jr. (Los Alamos National Lab., NM (USA); Southwest Research Inst., San Antonio, TX (USA) & Los Alamos National Lab., NM (USA))
Partner: UNT Libraries Government Documents Department

Emergence of the Persistent Spin Helix in Semiconductor Quantum Wells

Description: According to Noether's theorem, for every symmetry in nature there is a corresponding conservation law. For example, invariance with respect to spatial translation corresponds to conservation of momentum. In another well-known example, invariance with respect to rotation of the electron's spin, or SU(2) symmetry, leads to conservation of spin polarization. For electrons in a solid, this symmetry is ordinarily broken by spin-orbit (SO) coupling, allowing spin angular momentum to flow to orbital angular momentum. However, it has recently been predicted that SU(2) can be recovered in a two-dimensional electron gas (2DEG), despite the presence of SO coupling. The corresponding conserved quantities include the amplitude and phase of a helical spin density wave termed the 'persistent spin helix' (PSH). SU(2) is restored, in principle, when the strength of two dominant SO interactions, the Rashba ({alpha}) and linear Dresselhaus ({beta}{sub 1}), are equal. This symmetry is predicted to be robust against all forms of spin-independent scattering, including electron-electron interactions, but is broken by the cubic Dresselhaus term ({beta}{sub 3}) and spin-dependent scattering. When these terms are negligible, the distance over which spin information can propagate is predicted to diverge as {alpha} {yields} {beta}{sub 1}. Here we observe experimentally the emergence of the PSH in GaAs quantum wells (QW's) by independently tuning {alpha} and {beta}{sub 1}. Using transient spin-grating spectroscopy (TSG), we find a spin-lifetime enhancement of two orders of magnitude near the symmetry point. Excellent quantitative agreement with theory across a wide range of sample parameters allows us to obtain an absolute measure of all relevant SO terms, identifying {beta}{sub 3} as the main SU(2) violating term in our samples. The tunable suppression of spin-relaxation demonstrated in this work is well-suited for application to spintronics.
Date: August 24, 2011
Creator: Koralek, Jake; Weber, Chris; Orenstein, Joe; Bernevig, Andrei; Zhang, Shoucheng; Mack, Shawn et al.
Partner: UNT Libraries Government Documents Department

Spin Transport in Semiconductor heterostructures

Description: The focus of the research performed under this grant has been the investigation of spin transport in magnetic semiconductor heterostructures. The interest in these systems is motivated both by their intriguing physical properties, as the physical embodiment of a spin-polarized Fermi liquid, as well as by their potential applications as spintronics devices. In our work we have analyzed several different problems that affect the spin dynamics in single and bi-layer spin-polarized two-dimensional (2D) systems. The topics of interests ranged from the fundamental aspects of the electron-electron interactions, to collective spin and charge density excitations and spin transport in the presence of the spin-orbit coupling. The common denominator of these subjects is the impact at the macroscopic scale of the spin-dependent electron-electron interaction, which plays a much more subtle role than in unpolarized electron systems. Our calculations of several measurable parameters, such as the excitation frequencies of magneto-plasma modes, the spin mass, and the spin transresistivity, propose realistic theoretical estimates of the opposite-spin many-body effects, in particular opposite-spin correlations, that can be directly connected with experimental measurements.
Date: February 22, 2011
Creator: Marinescu, Domnita Catalina
Partner: UNT Libraries Government Documents Department

The two-photon decay of 1s2s {sup 1}S{sup 0} states in heavy he-like atomic systems.

Description: In He-like systems the decay of the 1s2s {sup 1}S0 excited state to the 1s{sup 2} {sup 1}S0 ground state is not allowed. This excited state can only decay to the ground state via the emission of two photons. The spectral shape of the emitted continuum is determined by the complete structure of the atomic system as all bound and continuum P states contribute to the 2E1 decay. For very heavy atomic systems the {sup 3}P states ALSO have to be included and the normalized spectral shape changes with atomic number according to the relative strengths of both, the electron-electron interaction and of the relativistic effects. A brief survey on the variation of the spectral shape of the two-photon continuum with atomic number is given and compared to experiments ranging from He-like Ni to He-like Au with special emphasis on the heavy relativistic system. The data compare well with fully relativistic calculations.
Date: August 14, 2002
Creator: Mokler, P. H.; Dunford, R. W. & Kanter, E. P.
Partner: UNT Libraries Government Documents Department

Dimuon production by high energy neutrinos and antineutrinos in the Fermilab fifteen-foot bubble chamber. [60 and 89 GeV]

Description: The Fermilab fifteen-foot bubble chamber/two-plane External Muon Identifier (EMI) hybrid detector filled with Ne(47% atomic)-H/sub 2/ is exposed to the quadrupole triplet beam (< E/sub ..nu..//sub ..mu../ > = 89 GeV, < E/sub anti ..nu..//sub ..mu../ > = 60 GeV). In 326,000 pictures there are 10,148 neutrino and 1773 antineutrino charged current events with two-plane EMI muon identification and muon momenta > 4 GeV/c. In this sample there are 55 ..mu../sup -/..mu../sup +/X, 11 ..mu../sup -/..mu../sup -/X and 0 ..mu../sup +/..mu../sup +/X candidates with muon momenta > 4 GeV/c. The like-sign events are consistent with background. The opposite-sign dimuons occur at R/sub ..mu.. ..mu../ = 0.35 +- 0.10% of the single muon rate. The ..nu../sub ..mu../-induced ..mu../sup -/..mu../sup +/ rate is 0.20 +- 0.15% below E/sub nu//sub ..mu../ = 100 GeV and 0.50 +- 0.30% above 100 GeV. In the opposite-sign dimuons there are 10 candidates with an associated V/sup 0/ implying 0.76 +- 0.36 neutral strange particle per opposite-sign dimuon event. The production rate, excess of neutral strange particles, and kinematic distributions of the dimuon events are consistent with the charm model predictions. 56 references.
Date: September 1, 1979
Creator: Orthel, J.L.
Partner: UNT Libraries Government Documents Department

Tests of QCD with polarized electrons

Description: We present three measurements that exploit the highly-polarized incident electrons of the SLC facility to probe QCD and the hadronization process. We observe preliminary evidence for leading particle production in hadronic decays of the Zo to light-quark pairs. In a high-purity sample of quark jets, the momentum spectra of p, A0, and K(-) are harder than those of p(bar), A(bar)0, and K(+), supporting the hypothesis that faster particles in jets are more likely to carry the primary quark or antiquark of the jet. Second, we present an improved limit on jet handedness, which seeks to measure the transport of quark spin through the hadronization process. Finally, we search for a correlation of the three jet event orientation with the Zo spin direction, which would indicate new physics beyond the Standard model.
Date: August 1, 1996
Creator: Pavel, T.J. & Collaboration, SLD
Partner: UNT Libraries Government Documents Department