Search Results (23 total)

We report the detailed phase diagram and anomalous transport properties of Fe-based high-T_c superconductors SmFeAsO_1-xF_x. It is found that superconductivity emerges at x∼0.07, and optimal doping takes place in the x∼0.20 sample with the highest T_c∼54  K. T_c increases monotonically with doping; the anomaly in resistivity from structural phase or spin-density-wave order is rapidly suppressed, suggesting a quantum critical point around x∼0.14. As manifestations, a linear temperature dependence of the resistivity shows up at high temperatures in the x<0.14 regime but at low temperatures just above T_c in the x>0.14 regime; a drop in carrier density evidenced by a pronounced rise in the Hall coefficient is observed below the temperature of the anomaly peak in resistivity. A scaling behavior is observed between the Hall angle and temperature: cot⁡θ_H∝T^1.5 for all samples with different x in SmFeAsO_1-xF_x system.

Pre-testing and post-testing is a commonly used method in Physics Education Research to assess student learning gains. It is well recognized in the community that timings and incentives in delivering conceptual tests can impact test results. However, it is difficult to control these variables across different studies. As a common practice, a pre-test is often administered either at or near the beginning of a course, while a post-test can be given either at or near the end of a course. Also, in conducting such tests there often is no norm as to whether incentives should be offered to students. Because these variations can significantly affect test results, it is important to study and document their impact. We analyzed five years of data that were collected at The Ohio State University from over 2100 students, who took both the pre-test and post-test of the Conceptual Survey of Electricity and Magnetism under various timings and incentives. We observed that the actual time frame for giving a test has a marked effect on the test results and that incentive granting also has a significant influence on test outcomes. These results suggest that one should carefully monitor and document the conditions under which tests are administered.

One of the important features of the littlest Higgs model with T parity, called the LHT model, is that it introduces the mirror fermions, which are the T-parity partners of the standard model fermions. In this paper, we discuss production of the mirror quark associated with a mirror neutrino via eγ and ep collisions. We find that, in a wide range of the parameter space, the mirror quark can be copiously produced at the International Linear e⁺e^- Collider and ep collider (THERA) experiments. The production rates of certain signal events, which are related the main two-body decay modes of the mirror quark, are also calculated.

We investigate the subarea-law scaling behavior of the block entropy in bipartite fermionic systems which do not have a finite Fermi surface. It is found that in gapped regimes the leading subarea term is a negative constant, whereas in critical regimes with point nodes the leading subarea law is a logarithmic additive term. At the phase boundary that separates the critical and noncritical regimes, the subarea scaling shows power-law behavior.

The specific heat C(T) of iron-based high-T_c superconductor SmO_1−xF_xFeAs (0≤x≤0.2) was systematically studied. For the undoped x=0 sample, a specific heat jump was observed at 130  K. This is attributed to the structural or spin-density-wave transition, which also manifests on resistivity as a rapid drop. However, this jump disappears with slight F doping in the x=0.05 sample, although the resistivity drop still exists. The specific heat C∕T shows clear anomaly near T_c for x=0.15 and 0.20 superconducting samples. Such anomaly has been absent in LaO_1−xF_xFeAs. For the parent compound SmOFeAs, C(T) shows a sharp peak at 4.6  K, and with electron doping in the x=0.15 sample, this peak shifts to 3.7  K. It is interpreted that such a sharp peak results from the antiferromagnetic ordering of Sm³⁺ ions in this system, which mimics the electron-doped high-T_c cuprate Sm_2−xCe_xCuO_4−δ.

Exactly solving a spinless fermionic system in two and three dimensions, we investigate the scaling behavior of the block entropy in critical and noncritical phases. The scaling of the block entropy crucially depends on the nature of the excitation spectrum of the system and on the topology of the Fermi surface. Noticeably, in the critical phases the scaling violates the area law and acquires a logarithmic correction only when a well-defined Fermi surface exists in the system. When the area law is violated, we accurately verify a conjecture for the prefactor of the logarithmic correction, proposed by D. Gioev and I. Klich [Phys. Rev. Lett. 96, 100503 (2006)].

The converse effects of spin photocurrent and current induced spin polarization are experimentally demonstrated in a two-dimensional electron gas system with Rashba spin splitting. Their consistency with the strength of the Rashba coupling as measured for the same system from beating of the Shubnikov–de Haas oscillations reveals a unified picture for the spin photocurrent, current-induced spin-polarization, and spin-orbit coupling. In addition, the observed spectral inversion of the spin photocurrent indicates a system with dominating structure inversion asymmetry.

The Brief Electricity and Magnetism Assessment (BEMA), developed by Chabay and Sherwood, was designed to assess student understanding of basic electricity and magnetism concepts covered in college-level calculus-based introductory physics courses. To evaluate the reliability and discriminatory power of this assessment tool, we performed statistical tests focusing both on item analyses (item difficulty index, item discrimination index, and item point biserial coefficient) and on the entire test (test reliability and Ferguson’s delta). The results indicate that BEMA is a reliable assessment tool.

A detailed analysis of the photoluminescence (PL) from Si nanocrystals (NCs) embedded in a silicon-rich SiO₂ matrix is reported. The PL spectra consist of three Gaussian bands (peaks A,B, and C), originated from the quantum confinement effect of Si NCs, the interface state effect between a Si NC and a SiO₂ matrix, and the localized state transitions of amorphous Si clusters, respectively. The size and the surface chemistry of Si NCs are two major factors affecting the transition of the dominant PL origin from the quantum confinement effect to the interface state recombination. The larger the size of Si NCs and the higher the interface state density (in particular, SiO bonds), the more beneficial for the interface state recombination process to surpass the quantum confinement process, in good agreement with Qin’s prediction in Qin and Li [Phys. Rev. B 68, 85309 (2003)]. The realistic model of Si NCs embedded in a SiO₂ matrix provides a firm theoretical support to explain the transition trend.

Optical properties of isolated silicon nanocrystals (nc-Si) with a mean size of ∼4 nm embedded in a SiO₂ matrix that was synthesized with an ion beam technique have been determined with spectroscopic ellipsometry in the photon energy range of 1.1–5.0 eV. The optical properties of the nc-Si are found to be well described by both the Lorentz oscillator model and the Forouhi-Bloomer (FB) model. The nc-Si exhibits a significant reduction in the dielectric functions and optical constants and a large blueshift (∼0.6 eV) in the absorption spectrum as compared with bulk crystalline silicon. The band gap of the nc-Si obtained from the FB model is ∼1.7 eV, showing a large band gap expansion of ∼0.6 eV relative to the bulk value. The band gap expansion is in very good agreement with the first-principles calculation of the nc-Si optical gap based on quantum confinement.

We report on the solar diurnal variation of the galactic cosmic-ray intensity observed by the Tibet III air shower array during the period from 1999 to 2003. In the higher-energy event samples (12 and 6.2 TeV), the variations are fairly consistent with the Compton-Getting anisotropy due to the terrestrial orbital motion around the Sun, while the variation in the lower-energy event sample (4.0 TeV) is inconsistent with this anisotropy. This suggests an additional anisotropy superposed at the multi-TeV energies, e.g., the solar modulation effect. This is the highest-precision measurement of the Compton-Getting anisotropy ever made.

Since 1996, a hybrid experiment consisting of the emulsion chamber and burst detector array and the Tibet-II air-shower array has been operated at Yangbajing (4300 m above sea level, 606 g/cm²) in Tibet. This experiment can detect air-shower cores, called burst events, accompanied by air showers in excess of about 100 TeV. We observed about 4300 burst events accompanied by air showers during 690 days of operation and selected 820 proton-induced events with its primary energy above 200 TeV using a neural network method. Using this data set, we obtained the energy spectrum of primary protons in the energy range from 200 to 1000 TeV. The differential energy spectrum obtained in this energy region can be fitted by a power law with the index of -2.97±0.06, which is steeper than that obtained by direct measurements at lower energies. We also obtained the energy spectrum of helium nuclei at particle energies around 1000 TeV.

Since 1996, a hybrid experiment consisting of an emulsion chamber and a burst detector array and the Tibet-II air-shower array has been operated at Yangbajing (4300 m above sea level) in Tibet. This experiment can detect air shower cores, called burst events, accompanied by air showers in excess of about 100 TeV. Using the burst event data observed by this experiment, we discuss the primary cosmic ray composition around the knee in comparison with the Monte Carlo simulations. In this paper, we show that all the features of burst events are wholly compatible with the heavy enriched composition in the knee energy region.

The decays of the ψ(2S) into vector plus tensor meson final states have been studied for the first time using the BES detector. We determine upper limits on branching fractions for ψ(2S) decays into ωf₂, ρa₂, K^*0K̅ ₂^*0+c.c., and φf₂^′(1525) that are, in each case, significantly smaller than the corresponding branching fractions for the J/ψ meson, scaled according to the expectations of perturbative QCD.

The processes ψ(2S)→γπ⁺π^-, γK⁺K^-, and γpp̅ have been studied using a sample of 3.79×10⁶ ψ(2S) decays. We determine the total width of the χ_c0 to be Γ_χc0^tot  =  14.3±2.0±3.0 MeV. We present the first measurement of the branching fraction B(χ_c0→pp̅ )  =  (15.9±4.3±5.3)×10^-5, where the first error is statistical and the second one is systematic. Branching fractions of χ_c0,2→π⁺π^- and K⁺K^- are also reported.

A comparison of the rates for ψ(2S)→π⁺π^-J/ψ, J/ψ→l⁺l^- and J/ψ→ anything is used to determine the J/ψ leptonic branching fractions. The results are B(J/ψ→e⁺e^-)=(5.90±0.05±0.10)% and B(J/ψ→μ⁺μ^-)=(5.84±0.06±0.10)%, where the first error is statistical and the second is systematic. Assuming lepton universality, the leptonic branching fraction of the J/ψ is B(J/ψ→l⁺l^-)=(5.87±0.04±0.09)% per species. This result is used to estimate the QCD scale factor Λ_MS¯⁽⁴⁾ and the strong coupling constant α_s.

We report first measurements of the branching fractions B(ψ_2S→γη^′)=(1.54±0.31±0.20)×10^-4 and B(ψ_2S→γη)=(0.53±0.31±0.08)×10^-4. The ψ(2S)→γη^′ result is consistent with expectations of a model that considers the possibility of η^′-η_c(2S) mixing. The ratio of the ψ(2S)→γη^′ and ψ(2S)→γη rates is used to determine the pseudoscalar octet-singlet mixing angle.

A search is performed for the production of the ψ(2S) in e⁺e^- annihilation at a center-of-mass energy of 4.03 GeV using the BES detector operated at the Beijing Electron Positron Collider (BEPC). The kinematic features of the reconstructed ψ(2S) signal are consistent with its being produced only in association with an energetic photon resulting from initial state radiation (ISR). Limits are placed on ψ(2S) production from the decay of unknown charmonia or metastable hybrids that might be produced in e⁺e^- annihilations at 4.03 GeV. Under the assumption that the observed cross section for ψ(2S) production is due entirely to ISR, the partial width Γ_ee of the ψ(2S) is measured to be 2.07±0.32 keV.

We present a detailed account of our analysis on ultrahigh-energy cosmic-ray double-core γ-family events observed in emulsion chambers at mountain levels. Extending the leading-order perturbative QCD jet calculations for hadron-hadron collisions to hadron-nucleus collisions by including nuclear effects, we performed a Monte Carlo simulation of cosmic-ray particle interaction and propagation in the atmosphere. We find a significant excess of event rates at large transverse momenta with respect to our prediction based on perturbative QCD. The excess cannot be accounted for by a compositeness model of quarks with a characteristic energy scale lower limit Λ_c>1.4 TeV or a possible value of Λ_c≃1.6 TeV from the CDF at the Fermilab Tevatron. We discuss possible onset of new physics indicated by the large discrepancy at E_lab∼10⁴-10⁵ TeV, an energy region beyond the reach of existing colliders but within the range of future hadron colliders such as the proposed Large Hadron Collider at CERN.

We find a significant excess of ultrahigh-energy cosmic-ray events at large transverse momenta with respect to the prediction by standard perturbative QCD at E_lab∼10⁴–10⁵ TeV, an energy regime beyond the reach of existing colliders but within the range of future hadron colliders. Such a large deviation cannot be accounted for by a compositeness model of quarks with a characteristic energy scale Λ_c>1.4 TeV allowed by current collider experiments, pointing towards new physics.

We have observed the photodissociation of Mg²⁺ via the repulsive 1 Σ_g²⁺ state in a reflection time-of-flight mass spectrometer. The photodissociation spectrum consists of a broad structureless continuum ranging from ≈580 to ≈690 nm and peaking near 640 nm. A spectral simulation based on theoretical ab initio potential-energy curves agrees well with the experimental profile. We have observed a triple-vector (E⁁-μ⁁-v⁁) correlation in the photofragment time-of-flight spectrum. Analysis of the correlation allows a determination of the anisotropy parameter β and the recoil velocity as a function of the laser wavelength. In the long- and short-wavelength limits of the photodissociation spectrum, the anisotropy parameter attains its maximum value, β=2, characterizing the process as a parallel (1 Σ_u²⁺→1 Σ_g²⁺) transition followed by a rapid, recoil-like fragmentation. However, excitation in the middle of the spectrum (≈620–660 nm) results in an anisotropy parameter that varies smoothly and significantly from the maximum value, indicating a second dissociation pathway for excitation in this region. Several alternatives for the competing channel are discussed. The measured recoil energy varies from E≈4100 to 7100 cm^-1 as the dissociating wavelength is tuned from λ=680 to 590 nm. This allows us to obtain an estimate for the ground-state binding energy D₀^″≈10 200±300 cm^-1, a result in very good agreement with recent theoretical predictions [G. Durand, J.-P. Daudey, and J.-P. Malrieu, J. Phys. (Paris) 47, 1335 (1986); M. Sodupe, C. W. Bauschlicher, Jr., and H. Partridge, Chem. Phys. Lett. 192, 185 (1992)].

Data from the Tibet air-shower array were used to examine the cosmic-ray shadows of the Moon and the Sun at energies around 10 TeV. The shadowing effect was clearly observed at the 5.8σ level for the Moon, while the shadow of the Sun was found in the direction away from the Sun by 0.86° to the west and 0.43° to the south. The effect of the geomagnetic field has also been observed in the shadow of cosmic rays by the Moon. The observed deflection of the Sun's shadow is briefly discussed in connection with the effect of the magnetic fields between the Sun and the Earth. This is the first observation of the effects of such magnetic fields on the cosmic-ray shadow. The maximum-likelihood analysis of the Moon data set shows that the angular resolutions of the array for showers with its mode energies 7 and 35 TeV are 0.87°_-0.10°^+0.13° and 0.54°_-0.08°^+0.11°, respectively.

The Tibet air shower array at an altitude of 4300 m has been in operation since January 1990. We have searched for continual emission of 10-TeV gamma rays from the Crab Nebula, Cygnus X-3 and Hercules X-1 during the period from June 1990 through January 1992. No dc excess is found from these three sources. Flux upper limits above 10 TeV are obtained to be 1.2×10^-12 cm^-2 s^-1 for the Crab Nebula, 1.1×10^-12 cm^-2s^-1 for Cygnus X-3, and 0.58×10^-12 cm^-2s^-1 for Hercules X-1 at the 95% C.L. This is the first observation, with an air shower array, for these sources in the 10-TeV energy region.