100 Matching Results

Search Results

Advanced search parameters have been applied.

Ultrafast Enhancement of Ferromagnetism via Photoexcited Holes inGaMnAs

Description: We report on the observation of ultrafast photo-enhanced ferromagnetism in GaMnAs. It is manifested as a transient magnetization increase on a 100-ps time scale, after an initial sub-ps demagnetization. The dynamic magnetization enhancement exhibits a maximum below the Curie temperature {Tc} and dominates the demagnetization component when approaching {Tc}. We attribute the observed ultrafast collective ordering to the p-d exchange interaction between photoexcited holes and Mn spins, leading to a correlation-induced peak around 20K and a transient increase in {Tc}.
Date: February 17, 2007
Creator: Wang, J.; Cotoros, I.; Dani, K.M.; Liu, X.; Furdyna, J.K. & Chemla, D.S.
Partner: UNT Libraries Government Documents Department

Hard diffraction results using D0

Description: The preliminary results on the search for hard diffraction in pp collisions with the D0 detector at Fermilab are presented. The presence of forward rapidity gaps is observed in events with high transverse momentum jet production at the center of mass energies 1800 and 630 GeV. The fraction of events with rapidity gaps is significantly higher than that expected due to multiplicity fluctuations and is consistent with predictions for hard single diffractive jet production. A class of events containing central dijets and two forward rapidity gaps is observed at {radical}s = 1800 GeV consistent with a hard double pomeron exchange event topology.
Date: June 1, 1997
Creator: Shabalina, E.
Partner: UNT Libraries Government Documents Department

Probing color-singlet exchange at D0

Description: We present latest preliminary results on hard color-singlet exchange in proton-antiproton collisions. The fraction of dijet events produced via color-singlet exchange is measured as a function of jet transverse energy, dijet pseudorapidity separation, and proton-antiproton center-of-mass energy. These results are qualitatively consistent with a color-singlet fraction that increases with increasing quark-initiated processes.
Date: July 1, 1997
Creator: Abbott, B.; Abolins, M.; Acharya, B.S. & Collaboration, D0
Partner: UNT Libraries Government Documents Department


Description: A formulation of quantum mechanical reactive scattering given by Miller is applied to the collinear H + H{sub 2} reaction. The approach is the direct analog to the Hartree-Fock method of electronic structure theory, and it obviates the need for specialized (e.g., 'natural' collision) coordinates. The rearrangement process takes place via an explicit exchange interaction (cf. electron exchange in Hartree-Fock theory), and closed channels are incorporated via a square-integrable set of correlation functions. Agreement with results obtained by others using other methods is excellent, showing this approach to quantum mechanical reactive scattering to be a viable one.
Date: October 1, 1977
Creator: Garrett, Bruce G. & Miller, William H.
Partner: UNT Libraries Government Documents Department

Di-jet production by double pomeron exchange in CDF

Description: We have studied events with a high-x{sub F} antiproton and two central jets with E{sub T} > 7 GeV in CDF, in p{anti p} collisions at {radical}s = 1800 GeV. We find an excess of events with a rapidity gap at least 3.5 units wide in the proton direction, which we interpret as di-jet production in double pomeron exchange events.
Date: May 1, 1998
Creator: Albrow, M.G.
Partner: UNT Libraries Government Documents Department

Results on hard diffraction from CDF

Description: We present results from studies of hard diffractive processes in {anti p}p collisions at {radical}s=1.8 TeV at the Tevatron using the CDF detector. Diffractive events are identified by the characteristic signature of a rapidity gap and/or by detecting a recoil antiproton with high forward momentum. Reactions studied include the diffractive production of W-bosons and of two-jet (dijet) events, diffractive heavy quark production, and dijet production by double-pomeron exchange.
Date: May 1, 1997
Creator: Goulianos, K.
Partner: UNT Libraries Government Documents Department

Charge exchange {rho}{sup zero}{pi}{sup +} photoproduction and implications for searches of exotic meson

Description: The authors analyze the processes {rvec {gamma}} + p {r_arrow} {rho}{sup 0}{pi}{sup +}n at low momentum transfer focusing on a possibility of production of an exotic J{sup PC} = 1{-+} meson state. In particular they discuss polarization observables and conclude that linear photon polarization is instrumental for separating of the exotic wave.
Date: October 1, 1999
Creator: Afanasev, A.V. & Szczepaniak, A.P.
Partner: UNT Libraries Government Documents Department

Comment on: Direct observation of spin-split electronic status of Pd at the Pd(111)/Fe(110) interface

Description: We have studied the interface formed by depositing Pd on an Fe(001) surface using spin-resolved photoemission. It is shown that the spin-resolved interface states that Weber et al. (Phys. Rev. Lett. 66, 942 (1991)) observed are probably not exchange split components. Although the Pd/Fe(001) interface photoemission spectra with polarized light appears to show an inverted exchange splitting, the two peaks show a different sensitivity to incident light polarization and hence have a different symmetry; thus the peaks do not represent an exchange split pair.
Date: September 1, 1991
Creator: Chang, Y.; Brookes, N. B.; Johnson, P. D. & Bader, S. D.
Partner: UNT Libraries Government Documents Department

Diffractive processes

Description: Recent results on elastic scattering of hadrons, single and double inelastic diffraction, and double Pomeron exchange are reviewed. Comments are made on features of the data that can be simply understood using optical models or the triple Regge model. (auth)
Date: January 1, 1975
Creator: Derrick, M.
Partner: UNT Libraries Government Documents Department

Magnetic Contribution to Heat Capacity and Entropy of Nicke Ferrite (NiFe2O4)

Description: The heat capacity of nickel ferrite was measured as a function of temperature over the range from 50 to 1200 C using a differential scanning calorimeter. A thermal anomaly was observed at 584.9 C, the expected Curie temperature, T{sub c}. The observed behavior was interpreted by recognizing the sum of three contributions: (1) lattice (vibrational), (2) a spin wave (magnetic) component and (3) a {lambda}-transition (antiferromagnetic-paramagnetic transition) at the Curie temperature. The first was modeled using vibrational frequencies derived from an experimentally-based ir absorption spectrum, while the second was modeled using a spin wave analysis that provided a T{sup 3/2} dependency in the low temperature limit, but incorporated an exchange interaction between cation spins in the octahedral and tetrahedral sites at elevated temperatures, as first suggested by Grimes [15]. The {lambda}-transition was fitted to an Inden-type model which consisted of two truncated power law series in dimensionless temperature (T/T{sub c}). Exponential equality was observed below and above T{sub c}, indicating symmetry about the Curie temperature. Application of the methodology to existing heat capacity data for other transition metal ferrites (AFe{sub 2}O{sub 4}, A = Fe, Co) revealed the same exponential equality, i.e., m = n = 5.
Date: December 15, 2005
Creator: S Ziemniak, L Anovitz, R Castelli
Partner: UNT Libraries Government Documents Department

Doping Dependent Charge Transfer Gap and Realistic Electronic Model of n-type Cuprate Superconductors

Description: Based on the analysis of the measurement data of angle-resolved photoemission spectroscopy (ARPES) and optics, we show that the charge transfer gap is significantly smaller than the optical one and is reduced by doping in electron doped cuprate superconductors. This leads to a strong charge fluctuation between the Zhang-Rice singlet and the upper Hubbard bands. The basic model for describing this system is a hybridized two-band t-J model. In the symmetric limit where the corresponding intra- and inter-band hopping integrals are equal to each other, this two-band model is equivalent to the Hubbard model with an antiferromagnetic exchange interaction (i.e. the t-U-J model). The mean-field result of the t-U-J model gives a good account for the doping evolution of the Fermi surface and the staggered magnetization.
Date: May 3, 2010
Creator: Xiang, T.
Partner: UNT Libraries Government Documents Department

Pressure-Induced Electronic Spin Transition of Iron in Magnesiow?stite-(Mg,Fe)O

Description: An electronic transition of iron in magnesiowuestite has been studied with synchrotron Moessbauer and X-ray emission spectroscopies under high pressures. Synchrotron Moessbauer studies show that the quadrupole splitting disappears and the isomer shift drops significantly across the spin-paring transition of iron in (Mg{sub 0.75},Fe{sub 0.25})O between 62 and 70 GPa, whereas X-ray emission spectroscopy of the Fe-K{sub {beta}} fluorescence lines in dilute (Mg{sub 0.95},Fe{sub 0.05})O also confirms that a high-spin to low-spin transition occurs between 46 GPa and 55 GPa. Based upon current results and percolation theory, we reexamine the high-pressure phase diagram of (Mg,Fe)O and find that iron-iron exchange interaction plays an important role in stabilizing the high-spin state of iron in FeO-rich (Mg,Fe)O.
Date: October 5, 2005
Creator: Lin, J F; Gavriliuk, A G; Struzhkin, V V; Jacobsen, S D; Sturhahn, W; Hu, M Y et al.
Partner: UNT Libraries Government Documents Department

Investigations of the R5(SixGe1-x)4 Intermetallic Compounds by X-Ray Resonant Magnetic Scattering

Description: The XRMS experiment on the Gd{sub 5}Ge{sub 4} system has shown that, below the Neel temperature, T{sub N} = 127 K, the magnetic unit cells is the same as the chemical unit cell. From azimuth scans and the Q dependence of the magnetic scattering, all three Gd sites in the structure were determined to be in the same magnetic space group Pnma. The magnetic moments are aligned along the c-axis and the c-components of the magnetic moments at the three different sites are equal. The ferromagnetic slabs are stacked antiferromagnetically along the b-direction. They found an unusual order parameter curve in Gd{sub 5}Ge{sub 4}. A spin-reorientation transition is a possibility in Gd{sub 5}Ge{sub 4}, which is similar to the Tb{sub 5}Ge{sub 4} case. Tb{sub 5}Ge{sub 4} possesses the same Sm{sub 5}Ge{sub 4}-type crystallographic structure and the same magnetic space group as Gd{sub 5}Ge{sub 4} does. The difference in magnetic structure is that Tb{sub 5}Ge{sub 4} has a canted one but Gd{sub 5}Ge{sub 4} has nearly a collinear one in the low temperature antiferromagnetic phase. The competition between the magneto-crystalline anisotropy and the nearest-neighbor magnetic exchange interactions may allow a 3-dimensional canted antiferromagnetic structure in Tb{sub 5}Ge{sub 4}. The spin-reorientation transition in both Gd{sub 5}Ge{sub 4} and Tb{sub 5}Ge{sub 4} may arise from the competition between the magnetic anisotropy from the spin-orbit coupling of the conduction electrons and the dipolar interactions anisotropy.
Date: August 18, 2008
Creator: Tan, Lizhi
Partner: UNT Libraries Government Documents Department

Polaron Coherence Condensation As the Mechanism for Colossal Magnetoresistance in Layered Manganites

Description: Angle-resolved photoemission spectroscopy data for the bilayer manganite La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} show that, upon lowering the temperature below the Curie point, a coherent polaronic metallic groundstate emerges very rapidly with well defined quasiparticles which track remarkably well the electrical conductivity, consistent with macroscopic transport properties. Our data suggest that the mechanism leading to the insulator-to-metal transition in La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} can be regarded as a polaron coherence condensation process acting in concert with the Double Exchange interaction.
Date: November 20, 2007
Creator: Mannella, N.; Yang, W. L.; Tanaka, K.; Zhou, X. J.; Zheng, H.; Mitchell, J. F. et al.
Partner: UNT Libraries Government Documents Department


Description: A large number of experiments relating to nuclear structure and materialization of energy will fall within the energy range of primary and secondary particles available from the Bevatron. Some experiments of interest are outlined below with pertinent comments. Proton-proton scattering can be extended to the high energy range using liquid hydrogen targets and hydrogenous materials by bombarding the target with the internal beam or using externally scattered protons. Proton-deuteron scattering measurements using liquid deuterium targets or solid materials can also be extended. Proton scattering from heavier nuclei may be found less difficult to interpret at these level energies as the nucleons will interact more or less independently. The wavelength of the bombarding particles will now be much smaller than nucleon diameters. Nuclear collisions represent by far the most important interaction between high energy nucleons and matter. Proton-neutron charge exchange interaction within the internal target will provide an external flux of neutrons which can be used directly for total nuclear cross section measurements and neutron-proton scattering. Low energy measurements, say below 200 Mev; show that nuclear interaction cross sections do not go down as 1/{lambda} as reactions become predominantly nuclear rather than electromagnetic. Measurements can now be extended into the Bev range to investigate the complex as opposed to the point charge concept of nuclear interactions. The external neutron beam may also provide high energy protons in charge exchange interactions with external targets. Mesons produced in the internal target will provide external meson beams of all types for cross section measurements over a large range of energies. Sufficient energy will be available for production of V particles with reasonable cross sections. The primary event leading to the production of V particles and the characteristic mode of decay can be studied. These events are best adapted to cloud chamber techniques wherein co-planer ...
Date: December 15, 1953
Creator: Karplus, Robert
Partner: UNT Libraries Government Documents Department

Characterization of the Dilute Ising Antiferromagnet

Description: A spin glass is a magnetic ground state in which ferromagnetic and antiferromagnetic exchange interactions compete, thereby creating frustration and a multidegenerate state with no long range order. An Ising system is a system where the spins are constrained to lie parallel or antiparallel to a primary axis. There has been much theoretical interest in the past ten years in the effects of applying a magnetic field transverse to the primary axis in an Ising spin glass at low temperatures and thus study phase transitions at the T=0 limit. The focus of this study is to search for and characterize a new Ising spin glass system. This is accomplished by site diluting yttrium for terbium in the crystalline material TbNi{sub 2}Ge{sub 2}. The first part of this work gives a brief overview of the physics of rare earth magnetism and an overview of experimental characteristics of spin glasses. This is followed by the methodology used to manufacture the large single crystals used in this study, as well as the measurement techniques used. Next, a summary of the results of magnetic measurements on across the dilution series from pure terbium to pure yttrium is presented. This is followed by detailed measurements on particular dilutions which demonstrate spin glass behavior. Pure TbNi{sub 2}Ge{sub 2} is an Ising antiferromagnet with a several distinct metamagnetic states below 17 K. As the terbium is alloyed with yttrium, these magnetic states are weakened in a consistent manner, as is seen in measurements of the transition temperatures and analysis of Curie-Weiss behavior at high temperature. At low concentrations of terbium, below 35%, long range order is no longer present and a spin-glass-like state emerges. This state is studied through various measurements, dc and ac susceptibility, resistivity, and specific heat. This magnetic behavior was then compared to that ...
Date: September 12, 2000
Creator: Wiener, T.
Partner: UNT Libraries Government Documents Department

The {pi}{sup +}-emission puzzle in 4 over Lambda He decay

Description: The observed {pi}{sup +} emission from the weak decay of 4 over {Lambda} He has long been an intriguing puzzle. Experimentally, the {pi}{sup +} to {pi}{sup {minus}} ratio for 4 over {Lambda} He decay is about 5%. Because mesonic decay modes of the free {Lambda} ({yields} p + {pi}{sup {minus}}, n + {pi}{sup 0}) produce no {pi}{sup +}s, more complicated mechanisms must be responsible for the {pi}{sup +} decay of 4 over {Lambda} He. Dalitz and von Hippel explored two-body decay processes of the type: (1) {Lambda} {yields} {pi}{sup 0} + n decay followed by a {pi}{sup 0} + p {yields} {pi}{sup +} + n charge-exchange reaction, and (2) {Sigma}{sup +} {yields} {pi}{sup +} + n decay following a {Lambda} + p {yields} {Sigma}{sup +} + n conversion. They concluded that neither process could account for even a 1% {pi}{sup +} + n decay as a p-wave process ruled out the promising explanation coming from von Hippel`s calculations, which had found that s-wave {Sigma}{sup +} decay might yield a sufficiently high rate. Cieply and Gal re-examined the charge-exchange contribution and concluded that, although up-to-date input parameters yield a 1.2% branching ratio, the charge-exchange mechanism cannot account for the experimental value of about 5%.
Date: October 1, 1997
Creator: Gibson, B.F. & Timmermans, R.
Partner: UNT Libraries Government Documents Department

Low energy spin-wave excitations in the bilayer manganite La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7}.

Description: Inelastic neutron scattering experiments were performed on a single crystal of the bilayer manganite La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7}. Low energy spin-wave excitations were observed along the c direction with a maximum energy of {approx} 0.5 meV at the zone boundary. The dispersion of these acoustic spin wave modes is modeled by a nearest-neighbor Heisenberg model with an inter-bilayer exchange interaction between neighboring spins in different bilayers of 0.048(1) meV and an anisotropy gap of {Delta} = 0.077(3) meV. These results confirm the two-dimensional nature of the spin-correlations in the bilayer manganites, with a ratio of the in-plane to inter-bilayer interaction of {approx}200. The temperature dependence of the energies and intensities of the spin wave excitations are in agreement with our earlier conclusion that the ferromagnetic transition is second-order.
Date: September 21, 1999
Creator: Rosenkranz, S.; Osborn, R.; Mitchell, J. F.; Vasiliu-Doloc, L.; Lynn, J. W. & Sinha, S. K.
Partner: UNT Libraries Government Documents Department

Searches for T-odd interactions in nuclear processes: Review of the theory

Description: CP-violation has been seen so far only in the neutral kaon system. Its origin is still unknown. If the CPT theorem holds, as is the case in gauge theories, CP-violating interactions violate also time reversal (T) invariance. Regarding the CPT theorem there is some experimental evidence that the interaction responsible for the observed CP-violation violates T-invariance. The observed CP-violation may just be a manifestation of the weak interaction of the Standard Model (SM), or it is due to an interaction beyond the SM. In both cases some of the new interactions may give rise to observable CP-violation where the SM contribution is invisible. This underlines the importance of searching for CP-violating and T-violating effects in many processes. In this talk we shall review what has been learned about T-violating interactions that can be probed in nuclear processes from experiments outside of and within nuclear physics, and consider the role of nuclear physics experiments in obtaining further information on such interactions.
Date: September 1995
Creator: Herczeg, P.
Partner: UNT Libraries Government Documents Department