La2/3Ca1/3Mn1-xCuxO3(x=4%)中的热弛豫现象

采用溶胶-凝胶法并经不同温度烧结而制备了名义组分为La2/3Ca1/3MnO3和La2/3Ca1/3Mn1-xCuxO3(x=4%)的系列样品,对它们进行了阻温关系测量.从试验结果观察到:在La2/3Ca1/3MnO3样品中,降温和升温测量的阻温曲线几乎完全重合,没有观察到明显的热弛豫现象.而在不同温度烧结的La2/3Ca1/3Mn1-xCuxO3(x=4%)样品中,阻温关系有明显的热弛豫现象.同时试验还观察到该热弛豫现象随样品烧结温度(颗粒尺寸)的增加逐渐消失.     

Galactic-cosmic-ray-produced 3He in a ferromanganese crust: any supernova 60Fe excess on earth?

An excess of 60Fe in 2.4-3.2 x 10(6) year old ferromanganese crust (237 KD) from the deep Pacific Ocean has been considered as evidence for the delivery of debris from a nearby supernova explosion to Earth. Extremely high ;{3}He/;{4}He (up to 6.12 x 10(-3)) and 3He concentrations (up to 8 x 10(9) atoms/g) measured in 237 KD cannot be supernova-derived. The helium is produced by galactic cosmic rays (GCR) and delivered in micrometeorites that have survived atmospheric entry to be trapped by the crust. 60Fe is produced by GCR reactions on Ni in extraterrestrial material. The maximum (3)He/(60)Fe of 237 KD (80-850) is comparable to the GCR (3)He/(60)Fe production ratio (400-500) predicted for Ni-bearing minerals in iron meteorites. The excess 60Fe can be plausibly explained by the presence of micrometeorites trapped by the crust, rather than injection from a supernova source.     

Computer simulation of symmetrical tilt grain boundaries in noble metals with MAEAM

This paper reports that an atomic scale study of [\bar {1}10] symmetrical tilt grain boundary (STGB) has been made with modified analytical embedded atom method (MAEAM) for 44 planes in three noble metals Au, Ag and Cu. For each metal, the energies of two crystals ideally joined together are unrealistically high due to very short distance between atoms near the grain boundary (GB) plane. A relative slide between grains in the GB plane results in a significant decrease in GB energy and a minimum value is obtained at specific translation distance. The minimum energy of Cu is much higher than that of Ag and Au, while the minimum energy of Ag is slightly higher than that of Au. For all the three metals, the three lowest energies correspond to identical (111), \mbox(113) and \mbox(331) boundary successively for two translations considered; from minimization of GB energy, these boundaries should be preferable in [\bar {1}10] STGB for noble metals. This is consistent with the experimental results. In addition, the minimum energy increases with increasing reciprocal planar coincidence density \Sigma, but decreases with increasing relative interplanar distance d /a.     

Pure gas of optically trapped molecules created from fermionic atoms

We report on the production of a pure sample of up to 3 x 10(5) optically trapped molecules from a Fermi gas of 6Li atoms. The dimers are formed by three-body recombination near a Feshbach resonance. For purification, a Stern-Gerlach selection technique is used that efficiently removes all trapped atoms from the atom-molecule mixture. The behavior of the purified molecular sample shows a striking dependence on the applied magnetic field. For very weakly bound molecules near the Feshbach resonance, the gas exhibits a remarkable stability with respect to collisional decay.     

Probe for the strong parity violation effects at RHIC with three particle correlations

In non-central relativistic heavy ion collisions, P\mathcal{P}-odd domains, which might be created in the process of the collision, are predicted to lead to charge separation along the system orbital momentum [1]. An observable, P\mathcal{P}-even, but directly sensitive to the charge separation effect, has been proposed in Ref. [2] and is based on 3-particle mixed harmonics azimuthal correlations. We report the STAR measurements using this observable for Au+Au and Cu+Cu collisions at ?{s_NN }\sqrt {s_{NN} } = 200 and 62 GeV. The results are reported as function of collision centrality, particle separation in rapidity, and particle transverse momentum. Effects that are not related to parity violation but might contribute to the signal are discussed.     

STAR measurement of system size and incident energy dependence of pt correlations at RHIC

We present the results on measurement of event-by-event p _t fluctuations and correlations for Cu+Cu collisions at ?{s_NN }\sqrt {s_{NN} } = 62.4 and 200 GeV from STAR experiment at RHIC. These results are compared with those from Au+Au collisions at ?{s_NN }\sqrt {s_{NN} } = 62.4 and 200 GeV to study the system size dependence. We find that the dynamical p _t fluctuations are finite and decrease with increasing collision centrality. The p _t correlations are studied as a function of collision centrality and are observed to decrease as we go from peripheral to central collisions. The square root of p _t correlations scaled by mean p _t are observed to be independent of beam energy as well as colliding ion size.     

Probing the quark-gluon phase transition using energy and system-size dependence of long-range multiplicity correlations in heavy ion collisions from the STAR experiment

Long-range forward-backward multiplicity correlations have been measured in heavy ion collisions at RHIC. Results for short and long-range multiplicity correlations (forward-backward) are presented for several systems (Au+Au, Cu+Cu, and pp) and energies (e.g. ?{s_NN }\sqrt {s_{NN} } = 200 and 62.4 GeV). A strong, long-range correlation is seen for central heavy ion collisions at ?{s_NN }\sqrt {s_{NN} } = 200 GeV that vanishes in semi-peripheral events and pp collisions. There is no apparent scaling with the number of participants (N _part) involved in the collision. These correlations provide information about the longitudinal behavior of the system formed in heavy ion collisions. To access the transverse behavior, the clusters produced in the same heavy ion collisions have been characterized by a study of the energy and system size dependence of the percolation density parameter (ρ). The relationship between the long-range correlation and percolation has been explored to characterize the hadron-quark/gluon phase transition and rapid thermalization of the system.     

Adsorbed layers on Au(111) — The inelastic transitions

Measurements have been made of the inelastic energy changes between layers of inert gases adsorbed on Au(111) and He atoms. The monolayers appear to show dispersionless effects while, as the layers build up in thickness, and screen the stronger forces between the He and Au(111), the vibrational modes soften. A seven layer film clearly shows dispersion. The holding potential for Au surfaces is predicted to be larger than for Ag and Cu both of which are quite similar. Comparisons between the lowest energy transition in a monolayer on Au(111) and other faces of Ag and Cu are made.     

Adsorption of Ag and Au atoms on wurtzite ZnO (0001) surface

We perform first-principles calculations to investigate various surface structures in the absorption of Ag and Au atoms on wurtzite ZnO (0001) surface. The results show that both Ag and Au atoms prefer to be absorbed on the H₃ sites (the center of Zn–O ring) of the surface, and the most favorable monolayer (ML) coverage is 1. The calculated electron structure shows that the Ag- and Au-adsorbed ZnO (0001) surfaces exhibit metallic characteristics even the ML coverage of the adatoms is very low. Finally, the work functions of Ag- and Au-adsorbed ZnO (0001) surfaces are calculated and discussed for the first time in the present work.     

重费密子系统CeCu6-xNix的极低温电阻研究

CeCu₆是一种重费密子系统,采用Ni替代Cu研究磁性原子掺杂的多晶样品CeCu_6-xNi_x(x=0.00,0.05,0.10,0.15,0.20)在极低温下的电阻随温度的变化规律.实验结果表明,随着Ni原子掺杂量的增加,电阻的规律发生变化.在500mK以下,x≈0.1的样品电阻与温度呈线性关系,表现出非费密液体行为,而其他样品皆为典型的费密液体,x≈0.1是其临界掺杂成分.对比用Au替代的情况,临界掺杂成分也是0.1,但Au掺杂是靠体积膨胀来改变合金的基态性质,而Ni掺杂则是因其磁性造成对合金基态性质的影响 关键词： 重费密子 临界掺杂 非费密液体     

Beam-energy and system-size dependence of dynamical net charge fluctuations

We present measurements of net charge fluctuations in Au + Au collisions at ?{s_NN }\sqrt {s_{NN} } = 19.6, 62.4, 130, and 200 GeV, Cu + Cu collisions at ?{s_NN }\sqrt {s_{NN} } = 62.4, 200 GeV, and p + p collisions at ?s\sqrt s = 200 GeV using the net charge dynamical fluctuations measure ν+ −,dyn. The dynamical fluctuations are non-zero at all energies and exhibit a rather modest dependence on beam energy. We find that at a given energy and collision system, net charge dynamical fluctuations violate 1/N _ch scaling, but display approximate 1/N _part scaling. We observe strong dependence of dynamical fluctuations on the azimuthal angular range and pseudorapidity widths.     

System size, energy, centrality and pseudorapidity dependence of charged-particle density in Au+Au and Cu+Cu collisions at RHIC

Charged particle pseudorapidity distributions are presented from the PHOBOS experiment at RHIC, measured in Au+Au and Cu+Cu collisions at ?{s_NN }\sqrt {s_{NN} } =19.6, 22.4, 62.4, 130 and 200 GeV, as a function of collision centrality. The presentation includes the recently analyzed Cu+Cu data at 22.4 GeV. The measurements were made by the same detector setup over a broad range in pseudorapidity, |η| < 5.4, allowing for a reliable systematic study of particle production as a function of energy, centrality and system size. Comparing Cu+Cu and Au+Au results, we find that the total number of produced charged particles and the overall shape (height and width) of the pseudorapidity distributions are determined by the number of nucleon participants, N _part. Detailed comparisons reveal that the matching of the shape of the Cu+Cu and Au+Au pseudorapidity distributions over the full range of η is better for the same N _part/2A value than for the same N _part value, where A denotes the mass number. In other words, it is the geometry of the nuclear overlap zone, rather than just the number of nucleon participants that drives the detailed shape of the pseudorapidity distribution and its centrality dependence.     

Diffusion processes of trimers on missing row surfaces: \mathrm{Cu }_{3}/\mathrm{Ag }(110) and \mathrm{Ag }_{3}/\mathrm{Cu }(110)

A semi-empirical potential according to the embedded atom, has been applied to investigate the diffusion of trimers by computing the energy barriers for different mechanisms. Our attention was more focused on the leapfrog process which is likely to occur on missing row surfaces. The activation barriers of this mechanism are calculated using drag method at 0K. These barriers are found to be 0.64 and 0.68 eV for hopping out the channel for $\mathrm{Cu }_{3}/\mathrm{Ag }(110) \mathrm{and Ag }_{3}/\mathrm{Cu }$ (110) respectively. While for hopping down at the other side they are about 0.42 and 0.32 eV. Moreover, a deep metastable position is observed during leapfrog diffusion leading to some spectacular trimer motion. At high temperature and essentially for $\mathrm{Cu }_{3}/\mathrm{Ag }$ (110), we also observed a competition between leapfrog process and concerted jump mechanism with a deformation of trimer geometry. Implications of these findings are briefly discussed.     

Exciton gas compression and metallic condensation in a single semiconductor quantum wire

We study the metal-insulator transition in individual self-assembled quantum wires and report optical evidence of metallic liquid condensation at low temperatures. First, we observe that the temperature and power dependence of the single nanowire photoluminescence follow the evolution expected for an electron-hole liquid in one dimension. Second, we find novel spectral features that suggest that in this situation the expanding liquid condensate compresses the exciton gas in real space. Finally, we estimate the critical density and critical temperature of the phase transition diagram at n{c} approximately 1 x 10;{5} cm;{-1} and T{c} approximately 35 K, respectively.     

Single-photon laser spectroscopy of cold antiprotonic helium

Some laser spectroscopy experiments carried out by the Atomic Spectroscopy and Collisions Using Slow Antiprotons (ASACUSA) collaboration to measure the single-photon transition frequencies of antiprotonic helium (\(\overline {p}\text {He}^{+}\equiv \overline {p}+\text {He}^{2+}+e^{-}\)) atoms are reviewed. The \(\overline {p}\text {He}^{+}\) were cooled to temperature T =?1.5–1.7 K by buffer-gas cooling in a cryogenic gas target, thus reducing the thermal Doppler width in the single-photon resonance lines. The antiproton-to-electron mass ratio was determined as \(M_{\overline {p}}/m_{e}=?1836.1526734(15)\) by comparisons with the results of three-body quantum electrodynamics calculations. This agreed with the known proton-to-electron mass ratio.     

On Realization of an Extremely Small Shift of MR Frequency in a Wide Range of Operating Temperatures in Rubidium Atomic Clock on 87 Rb Cell with Two Anti-Relaxation Components (Coating + Inert Gas, 40 Ar)

One of the most important problems in achieving daily frequency instability $$\sigma _{y} < 5 \cdot 10^{{ - 14}}$$ of on-board rubidium atomic clocks on absorption cell with working 87Rb atoms and mixture of buffer gases is realization of the TFS parameter — of temperature frequency shift $$\delta \nu \left( T \right)$$ at the level of $$\ \le 3 \cdot 10^{{ - 12}} /\, {^\circ } {\text{C}}.$$ The temperature dependence of the microwave “0–0” transition frequency $$\nu \left( T \right)$$ has an extremum with a small flat top ∆T ~ 0.5 °C to which the 87Rb-cell operating temperature is tuned. Significant difficulties arise in maintaining the high stability of this small ∆T zone under conditions of increased 87Rb cell operating temperature, $$T>70\, ^\circ{\rm C}$$, with an accuracy of < 0.005 °C for a day or more. To solve this problem, authors proposed a new type of 87Rb absorption cell with two dissimilar anti-relaxation (AR) components (wall coating + buffer gas, 40Ar) and created a special physical setup for optical spin pumping of 87Rb atoms at the microwave magnetic resonance frequency, $$\nu \sim \;6.834\,\;{\text{GHz}}$$, with a resolution $$0.01 \,\mathrm{H}\mathrm{z}$$. Investigations have shown TFS $$\sim 1.4 \cdot 10^{{ - 12}} /\;{{^\circ }} {\text{C}}$$ in significantly expanded (by an order of magnitude) zone, $$\Delta T$$ ≃ $$5 \left(\pm 1\right)\,\, ^\circ{\rm C} ,$$ in the operating temperature range of $$\left( {35 \div 41} \right)\;^{ \circ } {\text{C}},$$ which is ensured inside a satellite, for example. The simultaneous effect of AR-components causes the maximum mutual compensation of temperature frequency shifts in the extended ∆T zone. The experimental data show the possibility realizing daily frequency instability $$\sigma _{y} \sim 1 \cdot 10^{{ - 14}}$$ of the on-board atomic clock on 87Rb cell with two dissimilar AR-components (wall coating + inert gas, 40Ar).     

Recombination during expansion of ultracold plasma

Signals of ultracold plasma are observed by two-photon ionization of laser-cooled caesium atoms in a magneto-optical trap. Recombination of ions and electrons into Rydberg atoms during the expansion of ultracold plasma is investigated by using state-selective field ionization spectroscopy. The dependences of recombination on initial electron temperature (1--70 K) and initial ion density ($ \sim $10$^{10}$ cm$^{ - 3})$ are investigated. The measured dependence on initial ion density is $N^{1.547\pm 0.004}$ at a delay time of 5 $\mu $s. The recombination rate rapidly declines as initial electron temperature increases when delay time is increased. The distributions of Rydberg atoms on different values of principal quantum number $n$, i.e. $n=30$--60, at an initial electron temperature of 3.3 K are also investigated. The main experimental results are approximately explained by the three-body recombination theory.     

Simultaneous magneto-optical trapping of a boson-fermion mixture of metastable helium atoms

We simultaneously confine fermionic metastable 3He atoms and bosonic metastable 4He atoms in a magneto-optical trap. The trapped clouds, containing up to 1.5 x 10(8) atoms of each isotope, are characterized by measuring ions and metastable helium atoms escaping from the trap. Optical pumping of 3He atoms to a nontrapped hyperfine state is investigated and it is shown that large atom numbers can be confined without additional repumping lasers. Unique possibilities for quantum degeneracy experiments with mixtures of spin-polarized metastable 3He and 4He atoms are indicated.     

On the 2D-3D transition in epitaxial thin film growth

The theory is based on the fact that the equilibrium concentration of single atoms adsorbed on the surface of a monolayer island placed on a foreign substrate such that the substrate-deposit interaction is weaker than the deposit-deposit interaction is higher than the equilibrium concentration of atoms adsorbed on the surface of the same island now placed on a substrate of the same material. This higher adatom concentration leads to 2D nucleation on top of the monolayer island. The difference of the above equilibrium adatom concentrations appears as a driving force for the process of transformation of the initial monolayer island into a 3D island by detachment of atoms from the first monolayer island edges and their subsequent attachment to the edges of the second layer nucleus. The kinetics of this process are studied in detail, the following two cases being considered. The first case consists of breaking up and agglomeration of an initially continuous film into 3D crystals upon heating. The second case consists of change of the growth mode from layer to island mode during the vapour deposition when the substrate temperature increases. Expressions for the critical temperatures for these two phenomena to occur are derived. It is shown that they depend strongly on the substrate orientation, the critical temperature being higher for the 〈111〉 orientation in comparison with the 〈110〉 orientation, if substrate and deposit with a fcc lattice are considered. The theoretical results are compared with experimental data for deposition of Au on Mo{110}, Cu on W{110} and W{100} and Fe on Cu{111}.     

Theoretical studies of fast He atom scattering from a W{100} surface

Theoretical calculations of the scattering of fast neutral He atoms from W{100} are presented which are directly compared to the results from recent experiments of Nielsen and Delchar. The experiments which were performed for He atom energies between 150 and 1000 eV, and for incident polar angles between 0° and 65° as measured from the surface normal, displayed peaks in the polar angle distributions at 72°, 56° and ±10°. The results from classical dynamics calculations are employed here to explain the scattering phenomena that give rise to these peaks in the polar distributions. The calculations indicate that the peak at 72° is primarily due to scattering from the first layer W atoms. The peaks at ± 10° and 56° are unusual in that there are a multitude of different collision paths that result in the He atom being scattered into the same final angle. The peaks at ± 10° result from He atoms scattering mainly from the second, third and fourth layers of W atoms. The He atoms are focused on the outward path into the near normal direction by two first and two second layer W atoms. Subsurface scattering is also responsible for the peak at 56°. In this case the channel of first and second layer W atoms that focuses the outgoing He atoms is oriented at 54.7° with respect to the surface normal. It is proposed that slight variations of the experimental data from the calculated values are due to surface reconstruction of W{100} and that a more thorough analysis could reveal the microscopic nature of this structure.