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http://digitalcommons.uri.edu/phys_facpubs
Recent documents in Physics Faculty Publicationsen-usTue, 06 Oct 2015 10:22:59 PDT3600Capture of Neon Atoms by 4He Clusters
http://digitalcommons.uri.edu/phys_facpubs/266
http://digitalcommons.uri.edu/phys_facpubs/266Tue, 06 Oct 2015 10:11:49 PDT
Neon atoms are captured by helium clusters in a crossed-beam experiment. The capture process depends strongly on the cluster beam source conditions. We identify a sharply defined region corresponding to expansions passing near the critical point for which the capture probability is anomalously large.
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A. Scheidemann et al.Universality of Surface Correlation Functions in Three-Dimensional Models
http://digitalcommons.uri.edu/phys_facpubs/265
http://digitalcommons.uri.edu/phys_facpubs/265Tue, 06 Oct 2015 09:58:57 PDT
Universality of surface critical behavior with respect to surface enhancement is studied for O(n) models with n=1 (Ising), n=2 (planar rotor), and n=3 (Heisenberg) on simple-cubic lattices. Finite-size methods are employed to estimate surface critical exponents for ordinary surface criticality. In addition, it is shown that universal scaling functions, independent of surface enhancement, can be constructed with all nonuniversal features of the finite-size scaling function of the spin-spin surface correlation functions incorporated in (1) a metric factor and (2) an irrelevant scaling field associated with the surface coupling strength.
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M. P. Nightingale et al.Conformal Anomaly and Critical Exponents of the XY Ising Model
http://digitalcommons.uri.edu/phys_facpubs/264
http://digitalcommons.uri.edu/phys_facpubs/264Tue, 06 Oct 2015 09:53:48 PDT
We use extensive Monte Carlo transfer-matrix calculations on infinite strips of widths L up to 30 lattice spacing and a finite-size scaling analysis to obtain critical exponents and conformal anomaly number c for the two-dimensional XY Ising model. This model is expected to describe the critical behavior of a class of systems with simultaneous U(1) and Z2 symmetries of which the fully frustrated XY model is a special case. The effective values obtained for c show a significant decrease with L at different points along the line where the transition to the ordered phase takes place in a single transition. Extrapolations based on power-law corrections give values consistent with c=3/2 although larger values cannot be ruled out. Critical exponents are obtained more accurately and are consistent with previous Monte Carlo simulations suggesting critical behavior and with recent calculations for the frustrated XY model.
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M. P. Nightingale et al.Fine Structure in the Optical-Absorption Edge of Silicon
http://digitalcommons.uri.edu/phys_facpubs/262
http://digitalcommons.uri.edu/phys_facpubs/262Tue, 06 Oct 2015 09:51:45 PDT
Details of the structure in the indirect optical-absorption edge of silicon were studied by measuring the dependence of the photocurrent in p−n junctions on the energy of the incident photons. The measurements were made at room and higher temperatures for photon energies 0.75<hν<1.08 eV. The sensitivity of the method enabled high-resolution measurements in the absorption tail. At room temperature, thresholds were found at ∼0.91, 0.99, and 1.026 eV. The derivative of the response showed extensive fine structure in this tail. The TO- and LO-phonon-assisted transitions to the ground and excited state of the exciton, previously reported in the phonon emission region, were seen here with phonon absorption occurring around 1.054 and 1.065 eV. There was additional structure of unknown origin in this region.
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C. Anagnostopoulos et al.Phase Transitions in Coupled XY-Ising Systems
http://digitalcommons.uri.edu/phys_facpubs/263
http://digitalcommons.uri.edu/phys_facpubs/263Tue, 06 Oct 2015 09:48:52 PDT
We study the critical behavior of fully frustrated XY and Josephson-junction systems by means of a coupled XY-Ising model. From Monte Carlo and transfer-matrix calculations, we find separate XY and Ising and first-order transitions, depending on the parameters. In addition, a line of continuous transitions is found, with simulataneous loss of XY and Ising order and novel critical behavior. This result is supported by Monte Carlo simulations of frustrated XY models on square and triangular lattices.
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Enzo Granato et al.Surface and Bulk Transitions in Three-Dimensional O(n) Models
http://digitalcommons.uri.edu/phys_facpubs/261
http://digitalcommons.uri.edu/phys_facpubs/261Tue, 29 Sep 2015 08:41:03 PDT
Using Monte Carlo methods and finite-size scaling, we investigate surface criticality in the O(n) models on the simple-cubic lattice with n=1, 2, and 3, i.e., the Ising, XY, and Heisenberg models. For the critical couplings we find K_{c}(n=2)=0.4541659 (10) and K_{c}(n=3)=0.693003 (2). We simulate the three models with open surfaces and determine the surface magnetic exponents at the ordinary transition to be y_{h1}^{(o)}=0.7374(15), 0.781 (2), and 0.813 (2) for n=1, 2, and 3, respectively. Then we vary the surface coupling K_{1} and locate the so-called special transition at κ_{c}(n=1)=0.50214 (8) and κ_{c}(n=2)=0.6222 (3), where κ=K_{1 }∕ K−1. The corresponding surface thermal and magnetic exponents are y(s)t1=0.715 (1) and y(s)h1=1.636 (1) for the Ising model, and y^{(s)}_{t1}=0.608 (4) and y^{(s)}_{h1}=1.675 (1) for the XY model. Finite-size corrections with an exponent close to −1 ∕ 2 occur for both models. Also for the Heisenberg model we find substantial evidence for the existence of a special surface transition.
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Youjin Deng et al.Alkali Core-Level Binding-Energy Shifts in Alkali/4d-Metal Interface Systems
http://digitalcommons.uri.edu/phys_facpubs/260
http://digitalcommons.uri.edu/phys_facpubs/260Tue, 29 Sep 2015 08:29:35 PDT
Photoemission from different alkali core levels (Na 2p, K 3p, Rb 4p, and Cs 4d) has been studied for thin alkali films deposited on four different 4d metal surfaces: Mo(110), Rh(111), Rh(110), and Pd(100). Alkali atoms at the interface, the bulk, and at the surface of the adsorbed alkali film are found to have different core-level binding energies. It is found that the core-level binding-energy shifts of the alkali metals induced by the 4d-metal substrate increase with increasing atomic number of the alkali metal and with increasing atomic number of the 4d metal. Thermodynamical quantities such as interface segregation energies and adhesion energies are deduced from the layer-resolved shifts. Estimates of the experimental binding-energy shifts are given using semiempirical calculations, and certain complications in doing that are discussed.
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E. Lundgren et al.Unified Behavior of Alkali Core-Level Binding-Energy Shifts Induced by sp Metals
http://digitalcommons.uri.edu/phys_facpubs/259
http://digitalcommons.uri.edu/phys_facpubs/259Tue, 29 Sep 2015 08:23:20 PDT
Thin overlayers of Na, K, Rb, and Cs on different sp-metal substrates have been investigated using photoelectron spectroscopy. The alkali core levels show clearly resolved binding-energy shifts between the surface layer, the intermediate layer(s), and the interface layer. The magnitude of these shifts depends on sp metal and on alkali metal. The layer-resolved core-level binding-energy shifts are well reproduced by models based on a thermodynamical description. For three-layer alkali films the core-level binding energy of the intermediate layer is found to exhibit a small but significant shift between different sp-metal substrates. A simple relationship between the core-level binding-energy shift for the interface layer and the difference in r_{s} value between the sp substrate and the adsorbate is shown to exist.
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E. Lundgren et al.Observation of Quasibound States of the Neutron in Matter
http://digitalcommons.uri.edu/phys_facpubs/258
http://digitalcommons.uri.edu/phys_facpubs/258Tue, 29 Sep 2015 07:52:54 PDT
Quasistationary states of ultracold neutrons have been observed in a double-hump potential barrier. This potential barrier was created by sandwiching a thin film of aluminum between two thin copper films. Measurements of reflection from and transmission through such composite films displayed clear resonances. The resonance positions and widths are in agreement with calculations.
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K. -A. Steinhauser et al.Finite-Size Interaction Amplitudes and their Universality: Exact, Mean-Field, and Renormalization-Group Results
http://digitalcommons.uri.edu/phys_facpubs/257
http://digitalcommons.uri.edu/phys_facpubs/257Tue, 29 Sep 2015 07:46:07 PDT
We discuss the interaction between interfaces that is mediated by critical fluctuations, and in particular the universality of the corresponding finite-size amplitudes. In the case of the two-dimensional Ising model we address the universality with respect to anisotropy. For this purpose we derive the exact free energy of a finite, anisotropic triangular lattice on a cylinder. For the rectangular Ising model we verify universality also with respect to the magnitude of the boundary fields. In mean-field theory we display the mechanism for this universality and for that with respect to the surface coupling enhancement. Numerical results, which are of experimental relevance, are obtained employing a renormalization-group approximation for three-dimensional systems.
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J. O. Indekeu et al.Field-Induced Kosterlitz-Thouless Transition in the Zero-Temperature Triangular Ising Antiferromagnet
http://digitalcommons.uri.edu/phys_facpubs/256
http://digitalcommons.uri.edu/phys_facpubs/256Tue, 29 Sep 2015 07:39:13 PDT
We investigate the zero-temperature triangular Ising antiferromagnet in a magnetic field by means of transfer matrix and Monte Carlo methods. The finite-size results are compared with predictions obtained from a mapping to the Gaussian model. The results confirm the presence of a field-induced Kosterlitz-Thouless transition to a state with long-range order.
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H. W.J. Blöte et al.Neutron Lifetime Measured with Stored Ultracold Neutrons
http://digitalcommons.uri.edu/phys_facpubs/255
http://digitalcommons.uri.edu/phys_facpubs/255Tue, 29 Sep 2015 07:22:10 PDT
The neutron lifetime has been measured by counting the neutrons remaining in a fluid-walled bottle as a function of the duration of storage. Losses of neutrons caused by the wall reflections are eliminated by varying the bottle volume-to-surface ratio. The result obtained is τβ=887.6±3 s.
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W. Mampe et al.Core-Level Satellite Excitations of K/Al(100) and K/Al(111)
http://digitalcommons.uri.edu/phys_facpubs/254
http://digitalcommons.uri.edu/phys_facpubs/254Tue, 29 Sep 2015 06:53:21 PDT
Alkali-metal-induced satellite peaks associated with the K 3p and Al 2p core levels have been measured with photoemission for K/Al(111) and K/Al(100) under both low- and room-temperature preparation conditions. For low-temperature deposition we observedddd loss peaks in good agreement with electron-energy-loss spectroscopy studies of analogous systems which we assign to the excitation of collective plasmonlike modes in the alkali-metal overlayer. For room-temperature preparation conditions, we observed significant changes in the satellite loss structure which we attribute to a decrease or loss of metallic behavior in the alkali-metal layer. We account for some of our results as a change in bonding configuration of the K atoms from on-top-of surface to substitutional adsorption for low versus room-temperature preparations.
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David R. Heskett et al.Semistochastic Projector Monte Carlo Method
http://digitalcommons.uri.edu/phys_facpubs/253
http://digitalcommons.uri.edu/phys_facpubs/253Tue, 29 Sep 2015 06:48:34 PDT
We introduce a semistochastic implementation of the power method to compute, for very large matrices, the dominant eigenvalue and expectation values involving the corresponding eigenvector. The method is semistochastic in that the matrix multiplication is partially implemented numerically exactly and partially stochastically with respect to expectation values only. Compared to a fully stochastic method, the semistochastic approach significantly reduces the computational time required to obtain the eigenvalue to a specified statistical uncertainty. This is demonstrated by the application of the semistochastic quantum Monte Carlo method to systems with a sign problem: the fermion Hubbard model and the carbon dimer.
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F. R. Petruzielo et al.