Magnetic correlations in three-dimensional ising spin glasses

By a recursive method numerically exact free energies are calculated forL×L×M Ising lattices with random bonds andL=4, 4M10, applying free boundaries in the direction where the lattice is less small and otherwise periodic boundary conditions. Both for the±J model and the gaussian model the specific heat is in fair agreement with Monte Carlo results obtained for much larger lattices. However, the correlation function [S ₀ S _{R T} ² ]_av is found to decay exponentially with distanceR [for 1R9] at temperatures far below the apparent freezing temperatures of the Monte Carlo simulations, implying that there is no nonzero Edwards-Anderson order parameter in equilibrium. This behavior is qualitatively different from Mattis spin glasses (or Ising ferromagnets) where even smaller lattices show pronounced magnetic order at low temperatures. As the Monte Carlo results give evidence for a nonzero Edwards-Anderson order parameter (for not too long observation times), which is fairly independent of lattice size down to sizes of 4³, we suggest that Edwards-Anderson ordering is a nonequilibrium phenomenon visible only in studying dynamic properties.     

Hadronic coupling constants in lattice QCD

We calculate the hadronic coupling constantsg _NN andg in QCD, including dynamical quarks in the framework of staggered fermions in the lattice approach. For the nucleon-pion coupling we obtaing _NN =13.8±5.8, to be compared with the experimental value 13.13±0.07 [1]. The coupling has been analysed for two different sets of operators with the averaged resultg =4.2±1.9 which is to be compared with the experimental value 6.06±0.01 [2].     

Study of the centrally producer ππ andK \bar K systems at 85 and 300 GeV/c

We have studied the andK systems centrally produced in proton proton collisions at 300 GeV/c and ⁺/p proton collisions at 85 GeV/c using the CERN spectrometer. Clear evidence forS ^*/f _o(975) production is observed. An analysis performed on the ^{+ –} mass spectrum in the 1.0 GeV region, using a coupled channel formalism, shows that it is possible to describe theS ^*/f _o(975) effect with one single resonance once interference of theS ^*/f _o(975) with theS-wave background is introduced. The resultingS ^*/f _o(975) parameters arem _o =979±4MeV,g =0.28±0.04,g _K =0.56±0.18 corresponding to a pole position on sheet II at (1001±2)–i(36±4) MeV. Evidence is also found for a structure having a mass of 1472±12 MeV and a width of 195±33 MeV.     

Analysis of the 33-133-133-1final state in photon-photon collisions

The reactione ⁺ e ^- e ⁺ e ^{- 00}e ⁺ e ^- 6 has been analysed using the full data sample taken with the Crystal Ball detector at the DORIS II storage ring at DESY. The ⁰⁰ invariant mass spectrum is dominated by the ', for which we determine the radiative width (') to be (4.5±0.3±0.5)keV. Near 1.9 GeV/c² we observe a second enhancement in the ⁰⁰ mass distribution. Assuming that these events are created by the production and subsequent decay of a wide resonanceX(1900), we have investigated the decay modes, the invariant mass distributions of the ⁰ and ⁰⁰ subsystems and the angular distributions of the final state mesons. We find that the data is best described byJ ^PC=2. For thisJ ^P assignment the resonance parameters are (X) BR(X)=(0.95±0.27±0.20) keV _tot (X)=(221±92±44)MeV, andM(X)=(1881±32±40) MeV/c².Deceased     

The temperatures of protons andπ − mesons in central nucleus-nucleus interactions at a momentum of 4.5 GeV/c per incident nucleon

An estimate of the temperature of protons and ^– mesons in central He–Li, He–C, C–C, C–Ne, C–Cu, C–Pb, O–Pb, Mg–Mg interactions is presented. The results indicate an increase of the proton temperature with increasing mass numbers of projectile and target nuclei (A _p ,A _T ) fromT _p =(118±3) MeV for He–Li toT _p =(141±2) MeV for C–Pb. The temperature of ^– mesons does not depend onA _P ,A _T andT 95 MeV. A satisfactory fit for ^– mesons in C–Cu, C–Pb, O–Pb, Mg–Mg collisions can be achieved by using a form involving two temperatures,T ₁ andT ₂. The relative yield of the high temperature component (T ₂) is 24% for C–Cu, C–Pb, and Mg–Mg interactions. The observed results forT _P in C–Ne, C–Cu and C–Pb collisions are consistent with the prediction of the thermodynamic hagedorn model.     

On the equivalence of different order parameters and coexistence of phases for Ising ferromagnet. II

We investigate Ising spin systems with general ferromagnetic, translation invariant interactions,H=–J _BB,J _B0. We show that the critical temperatureT _i for the order parameterp _i defined as the temperature below whichp _i>0, is independent of the way in which the symmetry breaking interactions approach zero from above. Furthermore, all the equivalent correlation functions have the same critical exponents asT T_i from below, e.g. for pair interactions all the odd correlations have the same critical index as the spontaneous magnetization. The number of fluid and crystalline phases (periodic equilibrium states) coexisting at a temperatureT at which the energy is continuous is shown to be related to the number of symmetries of the interactions. This generalizes previous results for Ising spins with even (and non-vanishing nearest-neighbour) ferromagnetic interactions. We discuss some applications of these results to the triangular lattice with three body interactions and to the Ashkin-Teller model. Our results give the answer to the question raised by R.J. Baxter et al. concerning the equality of some critical exponents.Supported by NSF Grant PHY 77-22302     

Isomeric transitions between shape coexisting states in iodine isotopes withA=115, 119 and 121

Applying delayed- r.f. coincidences and generalized centroid-shift analysis, nanosecond isomers in some odd-A iodine isotopes are studied in-beam. The reactions¹⁰⁶Cd(¹²C,p2n) and^116,118Sn(⁶Li,3n) have been used. The following half-lives were determined:T _1/2(565 keV)=0.50±0.10 ns andT _1/2(837 keV) =0.90±0.20 ns in¹¹⁵I,T _1/2(687 keV)=5.50±0.50 ns andT _1/2(2418 keV)=4.0±0.50 ns in¹¹⁹I as well asT _1/2(812 keV)=0.35±0.20 ns andT _1/2(445 keV)<0.10 ns=">¹²¹I. A systematics of the absolute probabilities for the transitions deexciting theg _9/2 9/2⁺ and _11/2 11/2 ₁ ^– band heads in^115–125I is presented and discussed.Partly supported by the Bulgarian National Research Foundation under contracts PH14, and PH31 and Kernforschungszentrum Karlsruhe, Stabsabteilung Internationale Beziehungen     

A measurement ofB meson production and lifetime usingDl? events inZ0 decays

A study ofB meson decays intoDl X final states is presented. In these events, neutral and chargedD mesons originate predominantly fromB ⁺ andB ⁰ decays, respectively. The dilution of this correlation due toD ^** production has been taken into account. From 263 700 hadronicZ ⁰ decays collected in 1991 with the DELPHI detector at the LEP collider, 92D ⁰K ^-+, 35D ⁺K ^-++ and 61D ^*0D ⁰⁺ followed byD ⁰K ^-+ orD ⁰K ^-++-, are found with an associated lepton of the same charge as the kaon. From theD ⁰ l ^- andD ^*+ l ^-, the probabilityf _d that ab quark hadronizes into aB ^– (or ) meson is found to be 0.44 ±0.08±0.09, corresponding to a total (B _s+ _b ) hadronization fraction of 0.12 _–0.12 ^+0.24 . By reconstructing the energy of eachB meson, theb quark fragmentation is directly measured for the first time. The mean value of theB meson energy fraction is:

Comparison of (π− d→nn withdσ(π+ d→pp) to test charge symmetry in the Δ-region

First results on ^– d are reported. The measurements were made using 8 specially designed neutron counters, which were carefully calibratedin situ. The differential cross sections atT =142, 180, 217, and 254 MeV were obtained at four angles between 0° and 90°, they are compared to ⁺ d pp data measured at the same energies and angles with the same setup. At every beam energy, the shape of the angular distributions of ^– d nn and ⁺ d pp is the same to ±2%. The absolute cross sections differ by 1 to 10%. The error in this comparison is ±4% implying a small violation of charge symmetry.Dedicated to Prof. I. laus on the occasion of his 60th birthdayDeceased     

Magnetic diffusion anomaly in the Fe-17·6 at.%V alloy

Results of measurement between 746 and 1231 C of the diffusion coefficient Fe-59 in the Fe-17·6 at.% V alloy, together with Mirani's et al., Lai-Borg's and Hettich's et al. data analysis, are presented. All the data show a marked ferromagnetic effect, i.e. the lnD(T) values measured at the temperaturesT< are distinctly lower than those extrapolated by the linear Arrhenius plot from the paramagnetic range. The results are interpreted in terms of the Heisenberg model, Kirkwood's and Girifalco's statistical theories. The computer fit of an analytical form, derived for the excess activation enthalpyH(T), to experimental data enabled us to calculate the values of the exchange integralJ ₀ and the paramagnetic temperatureT _p. The corresponding values for the Fe-17·6% V alloy are:J ₀=(315±11)k,k is the Boltzmann constant, andT _p=(1223±±18) K. The activation enthalpy of Fe self-diffusion in a completely ordered ferromagnetic Fe-17·6% V matrix is higher by the value ofH(0)=(55·2±1·9)kJ/mol than the value 236·35 kJ/mol corresponding to a fully paramagnetic state. Applying a three paramagnetic fit to Mirani's, Lai-Borg's and Hettichs data, we have found that the ratio of the vacancy migration enthalpy and of the formation enthalpy in -Fe, in Fe-17·6% and Fe-7·64% Si, may be expressed by the valueH _M/H _F=1·62 ± 0·18. A comparison ofJ ₀(-Fe) toJ ₀(17·6 %V) and toJ ₀(7·64% Si) shows that the magnetic dilution caused by 17·6% of vanadium is negligible withJ ₀(17·6% V)/ /J ₀(-Fe)=0·999. The dilution brought about by silicium is by about 1·24 times higher, and the ratioJ ₀(7·64% Si)/J ₀(-Fe)=0·86. The paramagnetic temperatures in the materials under study are given by the relationT _P=(gQ + 137) K, being the respective Curie-temperature. The present analysis makes it possible to calculate the Fe diffusion characteristicsD _0f aH _f in a fully ordered magnetic matrix.     

K^0 - \bar K^0 mixing and the chiral-bag model

TheB-parameter is determined by the chiralbag model calculation of the amplitude. This is correlated with theK⁺⁰ decay amplitude. The theoretical magnitude ofB-parameter depends on the final state interaction effects inK2 decays. Without the final state interaction correction one findsB(µ _B ² )0.37, with the correction includedB(µ _B ² )0.9. Similar connection between theoretical prediction of theK⁺⁰ decay amplitude and calculated value ofB parameter seems to exist in other approaches too.     

Diffractive production of ρ-mesons and of ρπ-systems by neutrinos and antineutrinos on protons

Evidence is presented for diffractive production of -mesons and of -systems invp and chargedcurrent interactions. In the (anti-)neutrino energy range 10 GeVE _v <60 gev=" the=" cross=" sections=" for=" diffractive="> and diffractive production are found to be (0.64±0.14 (stat.)±0.08 (syst.))% and (0.28±0.08 (stat.)±0.04 (syst.))% of the charged-current cross section. The diffractive signal is consistent with being entirely due to diffractivea ₁ production. However, the data cannot distinguish between diffractivea ₁ and diffractive nonresonant production. The experimental distributions ofW, Q ²,x _Bj andy _Bj for diffractive and events are consistent with model predictions.     

Observation of spatial magnetic correlations in the paramagnetic phase of chromium using polarised neutrons and polarisation analysis

Polarised neutrons and polarisation analysis has been used to investigate the paramagnetic scattering of neutrons from a single crystal of spectrographically pure chromium. The scattering was found to be strongly peaked around (100). For the three temperatures at which measurements were made, 1.18, 1.53 and 2.23T _N the peak intensity was found to decrease by 90% and the characteristic wavelength for short range magnetic order from 57. A to 36 Å. The scattering was found to be isotropically disposed about 100 and to fall to a low background level for wavevectors greater than 2/aÅ from 100. Integration of the observed scattering throughout the Brillouin zone indicated the average moment per chromium atom to be 0.1±0.05 _B independent of temperature.     

Large-xF spin asymmetry inπ0 production by 200-GeV polarized protons

The spin asymmetryA _N for inclusive ⁰ production by 200-GeV transversely-polarized protons on a liquid hydrogen target has been measured at Fermilab over a wide range ofx _F, with 0.5p _T<2 gev/c.=">x _F>0.3, the asymmetry rises with increasingx _F and reaches a value ofA _N=0.15±0.03 in the region 0.6x _F<0.8. this=" result=" provides=" new=" input=" regarding=" the=" question=" of=" the=" internal=" spin=" structure=" of=" transversely-polarized=">This work was performed at the Fermi National Accelerator Laboratory, which is operated by University Research Associates, Inc., under contract DE-AC02-76CH03000 with the U.S. Department of Energy. Work supported in part by the U.S. Department of Energy, Division of High Energy Physics, Contracts W-31-109-ENG-38, W-7405-ENG-36, DE-AC02-76ER02289, DE-AS05-76ER05096     

Precise measurement of the magnetic moment of doubly closed shell + 1 nucleon nucleus17F(I π=5/2+,T 1/2=64.5 s)

Hyperfine interactions of -emitting¹⁷F implanted in single crystals of NaF and CaF₂ were studied. The nuclear magnetic moment of theI =5/2⁺ state was determined with improved precision to be |(¹⁷F;I =5/2⁺,T _1/2=64.5s)|=4.72130±0.00025 µ_N.     

BΛF theory and flat spacetimes

We propose a reduced constrained Hamiltonian formalism for the exactly solubleBF theory of flat connections and closed two-forms over manifolds with topology ³ × (0,1). The reduced phase space variables are the holonomies of a flat connection for loops which form a basis of the first homotopy group ₁( ³), and elements of the second cohomology group of ³ with value in the Lie algebraL(G). WhenG=SO(3,1), and if the two-form can be expressed asB=ee, for some vierbein fielde, then the variables represent a flat spacetime. This is not always possible: We show that the solutions of the theory generally represent spacetimes with global torsion. We describe the dynamical evolution of spacetimes with and without global torsion, and classify the flat spacetimes which admit a locally homogeneous foliations, following thurston's classification of geometric structures.This research is supported in part by the National Science Foundation, Grant No. PHY 89-04035, by CONACyT Grant No. 400349-1714E (Mexico), and by the Association Générale pour la Coopération et le Développement (Belgium).     

Ising model on self-avoiding walk chains

The phase transitions of nearest-neighbour interacting Ising models on self-avoiding walk (SAW) chains on square and triangular lattices have been studied using Monte Carlo technique. To estimate the transition temperature (T _c) bounds, the average number of nearest-neighbours (Z _eff) of spins on SAWs have been determined using the computer simulation results, and the percolation thresholds (p _c) for site dilution on SAWs have been determined using Monte Carlo methods. We find, for SAWs on square and triangular lattices respectively,Z _eff=2.330 and 3.005 (which compare very well with our previous theoretically estimated values) andp _c=0.022±0.003 and 0.045±0.005. When put in Bethe-Peierls approximations, the above values ofZ _eff givekT _c/J<1.06 and 1.65 for Ising models on SAWs on square and triangular lattices respectively, while, using the semi-empirical relation connecting the Ising modelT _c's andp _c's for the same lattices, we findkT _c/J0.57 and 0.78 for the respective models. Using the computer simulation results for the shortest connecting path lengths in SAWs on both kinds of lattices, and integrating the spin correlations on them, we find the susceptibility exponent =1.024±0.007, for the model on SAWs on two dimensional lattices.     

On the implementability of automorphism groups

LetA be aC*-algebra andG be a locally compact group acting as strongly continuous automorphisms onA. Let be a representation ofA then we say is a covariant representation if there exists a strongly continuous unitary representation of the group acting onH which implements the automorphisms. We give necessary and sufficient conditions on a representation ofA such that a) is subrepresentation of a covariant representation and b) is subrepresentation of a covariant representation quasi-equivalent to .     

Study of the three-dimensional Ising model on film geometry with the cluster Monte Carlo method

Topological properties of clusters are used to extract critical parameters. This method is tested for the bulk properties ofd=2 percolation and thed=2, 3 Ising model. For the latter we obtain an accurate value of the critical temperatureJ/k _B T _c=0.221617(18). In the case of thed=3 Ising model with film geometry the critical value of the surface coupling at the special transitions is determined as J_1c/J=1.5004(20) together with the critical exponents ₁ ^m =0.237(5) and=0.461(15).     

Specific heat of Eu x Sr1−x Te

In this paper, we report on measurements of the specific heatC of single-crystalline Eu _x Sr_1–x Te at temperatures between 60 mK and 15 K and in magnetic fields up to 6 T. Pure antiferromagnetic EuTe shows unusual critical behavior in the vicinity of the Néel temperatureT _N=9.8 K with a positive critical exponent instead of the 3d-Heisenberg exponent =–0.12. Possible reasons for this discrepancy between theory and experiment include magnetic anisotropy effects due to magnetic dipole-dipole interactions, which may give rise to a cross-over of the critical behavior very close toT _N. This anisotropy is also seen in the specific heat below 1 K where an exponential decay ofC is observed, and in the dependence of the magnetic susceptibility on the direction of the applied field. With increasing dilution of EuTe with nonmagnetic Sr, the critical behavior changes: becomes negative and decreases continuously towards –1 atxx _c. This concentration dependence of was previously observed in the diluted ferromagnetic system Eu _x Sr_1–x S. Our data thus support that the apparent change in the critical behavior depends on the degree of disorder. Samples with concentrationx lower than the critical concentrationx _c reveal spin-glass behavior in the specific heat. In addition, the dependence ofT _N on magnetic fields is discussed. The data yield a normalized magnetic phase boundaryB _c(T)/B_c(T=0) vs.T _N(B)/T_N(B=0) which is independent of concentration.