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Recent documents in Physics Faculty Publicationsen-usWed, 01 Jun 2016 11:15:35 PDT3600Geometric phases in electric dipole searches with trapped spin-1/2 particles in general fields and measurement cells of arbitrary shape with smooth or rough walls
http://digitalcommons.uri.edu/phys_facpubs/269
http://digitalcommons.uri.edu/phys_facpubs/269Thu, 17 Dec 2015 06:04:50 PST
The important role of geometric phases in searches for a permanent electric dipole moment of the neutron, using Ramsey separated oscillatory field nuclear magnetic resonance, was first noted by Commins [Am. J. Phys. 59, 1077 (1991)] and investigated in detail by Pendlebury et al. [Phys. Rev. A 70, 032102 (2004)]. Their analysis was based on the Bloch equations. In subsequent work using the spin-density matrix, Lamoreaux and Golub [Phys. Rev. A 71, 032104 (2005)] showed the relation between the frequency shifts and the correlation functions of the fields seen by trapped particles in general fields (Redfield theory). More recently, we presented a solution of the Schrödinger equation for spin-1/2 particles in circular cylindrical traps with smooth walls and exposed to arbitrary fields [A. Steyerl et al., Phys. Rev. A 89, 052129 (2014)]. Here, we extend this work to show how the Redfield theory follows directly from the Schrödinger equation solution. This serves to highlight the conditions of validity of the Redfield theory, a subject of considerable discussion in the literature [e.g., M. P. Nicholas et al., Prog. Nucl. Magn. Reson. Spectrosc. 57, 111 (2010)]. Our results can be applied where the Redfield result no longer holds, such as observation times on the order of or shorter than the correlation time and nonstochastic systems, and thus we can illustrate the transient spin dynamics, i.e., the gradual development of the shift with increasing time subsequent to the start of the free precession. We consider systems with rough, diffuse reflecting walls, cylindrical trap geometry with arbitrary cross section, and field perturbations that do not, in the frame of the moving particles, average to zero in time. We show by direct, detailed, calculation the agreement of the results from the Schrödinger equation with the Redfield theory for the cases of a rectangular cell with specular walls and of a circular cell with diffuse reflecting walls.
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R. Golub et al.Ion Bombardment of Ni(110) Studied with Inverse Photoemission Spectroscopy and Low-Energy Electron Diffraction
http://digitalcommons.uri.edu/phys_facpubs/268
http://digitalcommons.uri.edu/phys_facpubs/268Mon, 12 Oct 2015 06:22:54 PDT
Inverse photoemission spectroscopy (IPES) performed on clean Ni(110) reveals an unoccupied electronic surface state with energy ~ 2.5 eV above the Fermi level for emission near the Ȳ point of the Surface Brillouin Zone. Ion bombardment of the sample creates defects that reduce the intensity of the peak in IPES spectra. Sharp, intense diffraction spots in low-energy electron diffraction (LEED) patterns taken of the clean surface become dimmer after bombardment. Results of these measurements are compared to Monte Carlo simulations of the sputtering process to ascertain the approximate size of clean patches on the sample necessary to sustain the IPES and LEED features. At 170 K, the IPES surface state peak appears closely associated with the population of surface atomic sites contained in clean circular patches of about 50 atoms. The LEED patterns persist to greater degrees of sputtering and are associated with smaller clean patches. Both measurements performed at 300 K indicate significant self-annealing of the sputtering damage.
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Benjamin Young et al.Monodisperse hard rods in external potentials
http://digitalcommons.uri.edu/phys_facpubs/267
http://digitalcommons.uri.edu/phys_facpubs/267Fri, 09 Oct 2015 09:53:36 PDT
We consider linear arrays of cells of volume Vc populated by monodisperse rods of size σVc, σ = 1,2, . . ., subject to hardcore exclusion interaction. Each rod experiences a position-dependent external potential. In one application we also examine effects of contact forces between rods. We employ two distinct methods of exact analysis with complementary strengths and different limits of spatial resolution to calculate profiles of pressure and density on mesoscopic and microscopic length scales at thermal equilibrium. One method uses density functionals and the other statistically interacting vacancy particles. The applications worked out include gravity, power-law traps, and hard walls. We identify oscillations in the profiles on a microscopic length scale and show how they are systematically averaged out on a well-defined mesoscopic length scale to establish full consistency between the two approaches. The continuum limit, realized as Vc → 0, σ →∞ at nonzero and finite σVc, connects our highest-resolution results with known exact results for monodisperse rods in a continuum. We also compare the pressure profiles obtained from density functionals with the average microscopic pressure profiles derived from the pair distribution function.
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Benaoumeur Bakhti et al.Capture 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.Occupied Surface-State Bands of Bi(1×1) Overlayers on an InAs(110) Surface Grown by Molecular-Beam Epitaxy
http://digitalcommons.uri.edu/phys_facpubs/252
http://digitalcommons.uri.edu/phys_facpubs/252Mon, 28 Sep 2015 10:24:28 PDT
The ordered p(1×1) monolayer phase of Bi on InAs(110) has been studied with the technique of angle-resolved ultraviolet photoemission spectroscopy. Three Bi-induced surface-state bands (S’,S’’, and S’’’) have been observed. Their respective band dispersions have been mapped along the high-symmetry lines of the surface Brillouin zone. The upper two bands, S’ and S’’, appear to be degenerate across most of the surface Brillouin zone except along the Γ¯-X¯’ symmetry line. The bandwidths of the Bi-induced states of Bi/InAs(110) are significantly narrower than that of Bi/GaAs(110) or Sb/GaAs(110). The polarization of these surface-state bands has been measured and compared to predictions of current theoretical models for Sb/GaAs(110).
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D. N. McIlroy et al.Effect of Criticality on Wetting Layers
http://digitalcommons.uri.edu/phys_facpubs/251
http://digitalcommons.uri.edu/phys_facpubs/251Mon, 28 Sep 2015 10:16:48 PDT
We study wetting phenomena in which the wetting layer is (nearly) critical and intrudes between two noncritical phases. Finite-size scaling theory predicts an interaction, identical in range to that due to the van der Waals forces, between the interfaces bounding the wetting layer. This finite-size interaction leads to new wetting phenomena near critical end points, e.g., in ternary mixtures. The interaction amplitude and its possible universality can be observed directly in experiment.
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M. P. Nightingale et al.Examination of the Necessity of Complete Wetting near Critical Points in Systems with Long-Range Forces
http://digitalcommons.uri.edu/phys_facpubs/250
http://digitalcommons.uri.edu/phys_facpubs/250Mon, 28 Sep 2015 10:08:17 PDT
Cahn’s general argument for complete wetting in the vicinity of critical points is critically reviewed. Critical-point wetting does occur in systems with short-range (exponentially decaying) forces. Whenever short-range forces favor wetting, while at the same time there is a tendency towards drying due to weak long-range (algebraically decaying) forces, neither critical-point wetting nor drying takes place. In this case the thickness of the partial wetting layer diverges as the bulk correlation length upon approach of the critical point.
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M. P. Nightingale et al.