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http://digitalcommons.uri.edu/phys_facpubs
Recent documents in Physics Faculty Publicationsen-usWed, 01 Jul 2015 01:43:52 PDT3600Quantum spin dynamics of the one-dimensional planar antiferromagnet
http://digitalcommons.uri.edu/phys_facpubs/158
http://digitalcommons.uri.edu/phys_facpubs/158Mon, 29 Jun 2015 09:10:56 PDT
The T=0 dynamics of the one-dimensional s=^{1}/_{2} planar antiferromagnet is studied by an approach which consists of exact analytic calculations in the Bethe formalism and numerical finite-chain calculations on rings up to 10 spins. The method makes use of well known critical exponents for the correlation functions and of exact sum rules. The authors obtain approximate analytic expressions for both the out-of-plane and the in-plane dynamic structure factors, and for related quantities such as integrated intensities, susceptibilities and autocorrelation functions. The results are discussed in relation to possible experiments on quasi-1D magnetic compounds at low T. The calculations make clear that the T=0 dynamic structure factors are dominated by two-parameter continua of excitations rather than by single branches of spin-waves as predicted by classical spin-wave theory. By varying the planar anisotropy the autocorrelation functions display interesting features in their longtime asymptotic behaviour, such as a crossover from a uniform power-law decay to an oscillatory decay and a crossover between oscillatory decays with different frequencies. They conjecture a possibility of approaching the classical limit s= infinity starting from the quantum limit s=^{1}/_{2}. This provides a qualitative, and in some aspects even quantitative, understanding of the dynamical behaviour of s>^{1}/_{2} systems in terms of a quantum approach.
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Gerhard Muller et al.Alternating linear-chain antiferromagnetism in copper nitrate Cu(NO3)2.2.5 H2O
http://digitalcommons.uri.edu/phys_facpubs/157
http://digitalcommons.uri.edu/phys_facpubs/157Mon, 29 Jun 2015 09:10:52 PDT
Current interest in the behavior of Heisenberg alternating antiferromagnetic quantum chains has been stimulated by the discovery of an unusual class of magnetoelastic spin-Peierls systems. Copper nitrate, Cu(NO3)2.2.5H2O, does not display a spin-Peierls transition, but its dominant magnetic behavior is that of a strongly alternating antiferromagnetic chain with temperature-independent alternation. A remarkable, simultaneous fit is demonstrated between theoretical studies and a wide variety of zero- (low-) field experimental measurements, including susceptibility, magnetization, and specific heat. The fitting parameters are α(degreeofalternation)=0.27, J1k=2.58 K, gb=2.31, and g⊥=2.11. Slight systematic discrepancies are attributed to weak interchain coupling. Theoretical studies also predict a rich variety of behavior in high fields, particularly in the region involving the lower and upper critical fields, Hc1=28 kOe and Hc2=44 kOe. Experimental specific-heat measurements at H=28.2 and 35.7 kOe show quantitative agreement with theory in this interesting parameter region. The fitting parameters are the same as for zero field and, again, small discrepancies between theory and experiment may be attributed to interchain coupling. The exceptional magnetic characterization of copper nitrate suggests its use for further experimental study in the vicinity of the high-field ordering region.
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Jill C. Bonner et al.Scaling behavior at zero-temperature critical points
http://digitalcommons.uri.edu/phys_facpubs/156
http://digitalcommons.uri.edu/phys_facpubs/156Mon, 22 Jun 2015 08:25:22 PDT
A scaling form for the logarithm of the partition function suitable for a zero-temperature critical point is obtained and found to hold for the spherical model in less than two dimensions and the classical n-component Heisenberg linear chain. Nevertheless, several cases are found where the critical-exponent relations involving the specific heat fail. These anomalous cases do not imply a breakdown of the scaling implicit in the basic formulation of renormalization-group theory.
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George A. Baker Jr. et al.The Spin-Wave Continuum of the S = 1/2 Linear Heisenberg Antiferromagnet
http://digitalcommons.uri.edu/phys_facpubs/155
http://digitalcommons.uri.edu/phys_facpubs/155Mon, 22 Jun 2015 08:25:20 PDTGerhard Muller et al.Magnetic Field Effects on the Spin Dynamics of the Linear Spin-1/2 Heisenberg Antiferromagnet
http://digitalcommons.uri.edu/phys_facpubs/154
http://digitalcommons.uri.edu/phys_facpubs/154Mon, 22 Jun 2015 08:25:17 PDT
We present a new approach to the low temperature dynamics of a quantum Heisenberg antiferromagnetic chain, which employs a combination of techniques: exact finite chain calculations; exact Bethe Ansatz calculations; and exact sum rules and selection rules. A striking property of the selection rules is that the contribution of several classes of states to the dynamics in non-zero field for finite systems is shown to vanish in the thermodynamic limit. Many novel quantum field-dependent effects appear such as soft modes, and multiple peak structure in the structure factor, which should be experimentally observable.
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Gerhard Muller et al.Zero-Temperature Dynamics of the s=1/2 Linear Heisenberg Antiferromagnet in a Magnetic Field
http://digitalcommons.uri.edu/phys_facpubs/153
http://digitalcommons.uri.edu/phys_facpubs/153Thu, 18 Jun 2015 05:24:54 PDT
Analytic calculations in the Bethe formalism and finite-chain calculations show that the dynamic spin-correlation function in (q, ω)-space of the s=1/2 linear Heisenberg antiferromagnet at T> = 0 in a uniform magnetic field is governed by a double continuum of excitations. Appropriate neutron scattering experiments are expected to show double-peak spectra.
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Gerhard Muller et al.Temperature and Field Dependence of Autocorrelation Functions for the One-Dimensional Heisenberg Antiferromagnet
http://digitalcommons.uri.edu/phys_facpubs/152
http://digitalcommons.uri.edu/phys_facpubs/152Thu, 18 Jun 2015 05:19:59 PDT
We present analytical and numerical results for the low frequency autocorrelation function of the 1-d s=1/2 Heisenberg antiferromagnet at low temperature and various fields. Our results are in good agreement with NMR data.
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Gerhard Muller et al.Observation of a Spin-Peierls Transition in a Heisenberg Antiferromagnetic Linear-Chain System
http://digitalcommons.uri.edu/phys_facpubs/151
http://digitalcommons.uri.edu/phys_facpubs/151Thu, 18 Jun 2015 05:13:09 PDT
Magnetic-susceptibility and EPR measurements are reported which provide the first unambiguous evidence for a spin-Peierls transition in a system of linear one-dimensional antiferromagnetic Heisenberg chains. The material studied is TTFCuS4C4(CF3)4 (TFF stands for tetrathiafulvalinium). At 12 K, the spin-lattice system undergoes a second-order phase transition to a singlet ground state.
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J. W. Bray et al.Dynamic Correlation Functions for the One-Dimensional XYZ Model: New Exact Results
http://digitalcommons.uri.edu/phys_facpubs/150
http://digitalcommons.uri.edu/phys_facpubs/150Mon, 08 Jun 2015 08:25:14 PDT
It is found that there exist special circumstances for which a rigorous relation between the three dynamic structure factors S_{μμ}(q, ω), μ=x, y, z, at T=0 of the one-dimensional spin-sXYZ model in a uniform magnetic field can be derived. This relation is used to infer new exact results for S_{xx}(q, ω) of the S= case1/2 anisotropic XY model.
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Gerhard Muller et al.Heisenberg Antiferromagnetic Chains: Quantum-Classical Crossover
http://digitalcommons.uri.edu/phys_facpubs/149
http://digitalcommons.uri.edu/phys_facpubs/149Mon, 08 Jun 2015 08:17:15 PDT
An unusual crossover mechanism has been discovered by numerical investigation of the dispersion spectrum of Heisenberg antiferromagnetic chains with various spin values in a magnetic field. This result is reflected in novel behavior of static properties such as the integrated intensity. A study of various excitation gaps using finite chain calculations extended by quantum Monte Carlo studies indicates unusual behaviour in the T=0 magnetization isotherms.
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Jill C. Bonner et al.Unusual Critical Behavior in a Bilinear‐Biquadratic Exchange Hamiltonian
http://digitalcommons.uri.edu/phys_facpubs/148
http://digitalcommons.uri.edu/phys_facpubs/148Mon, 08 Jun 2015 06:36:55 PDT
We have performed a variety of numerical studies on the general bilinear‐biquadratic spin‐1 Hamiltonian H/J=∑^{ N }_{ i=1}[S_{ i }⋅S_{ i+1} −β(S_{ i }⋅S_{ i+1})^{2}], over the range 0≤β≤∞. The model is Bethe Ansatz integrable at the special point β=1, where the spectrum is gapless, but is otherwise believed to be nonintegrable. Affleck has predicted that an excitation gap opens up linearly in the vicinity of β=1. Our studies involving spectral excitations (dispersion spectra), scaled‐gap, and finite‐size scaling calculations are not consistent with the Affleck prediction. The situation appears complex, with novel crossover effects occurring in both regimes, β<1 and β>1, complicating the analysis.
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J. C. Bonner et al.Regular and Chaotic Dynamics of Classical Spin Systems
http://digitalcommons.uri.edu/phys_facpubs/147
http://digitalcommons.uri.edu/phys_facpubs/147Mon, 01 Jun 2015 06:56:39 PDT
An international workshop on Elementary Excitations and Fluctuations in Magnetic Systems was held in Turin for five days beginning 25 May, 1987. The workshop followed much the same format as the one with the same title held in San Miniato in 1984 (proceedings: Springer Series in-Solid-State Sciences, Vol. 54), that most participants contributed talks and provided papers for the proceedings. While many of the participants had attended the first workshop, 15 of the 40 invited review papers were presented by scientists who had not. The majority of the talks reported theoretical work concerned with the introduction of new techniques. However, experimental work was also well represented, not least because many of the reported theoretical studies were motivated by experimental findings, and a highlight of the workshop was an extremely stimulating session devoted to recent neutron scattering measurements, on various systems, that exploited polarization analysis. The fine venue of the workshop, Villa Gualino, with its excellent facilities and spacious accommodation, helped to produce a delightful relaxed and friendly atmosphere. For the use of Villa Gualino and significant financial support we are indebted to our host organization, the Institute for Scientific Interchange (lSI). Additional financial support came from the Consiglio Nazionale delle Ricerche (CNR), Centro Interuniversitario di Struttura della Material del Ministero della Pubblica Istruzione (CISM-MPI) and Gruppo Nazionale di Struttura della Materia (GNSM-CNR).
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Niraj Srivastava et al.Excitations and Critical Behavior in Generalized Heisenberg Spin Chains
http://digitalcommons.uri.edu/phys_facpubs/146
http://digitalcommons.uri.edu/phys_facpubs/146Mon, 01 Jun 2015 06:49:37 PDT
An international workshop on Elementary Excitations and Fluctuations in Magnetic Systems was held in Turin for five days beginning 25 May, 1987. The workshop followed much the same format as the one with the same title held in San Miniato in 1984 (proceedings: Springer Series in-Solid-State Sciences, Vol. 54), that most participants contributed talks and provided papers for the proceedings. While many of the participants had attended the first workshop, 15 of the 40 invited review papers were presented by scientists who had not. The majority of the talks reported theoretical work concerned with the introduction of new techniques. However, experimental work was also well represented, not least because many of the reported theoretical studies were motivated by experimental findings, and a highlight of the workshop was an extremely stimulating session devoted to recent neutron scattering measurements, on various systems, that exploited polarization analysis. The fine venue of the workshop, Villa Gualino, with its excellent facilities and spacious accommodation, helped to produce a delightful relaxed and friendly atmosphere. For the use of Villa Gualino and significant financial support we are indebted to our host organization, the Institute for Scientific Interchange (lSI). Additional financial support came from the Consiglio Nazionale delle Ricerche (CNR), Centro Interuniversitario di Struttura della Material del Ministero della Pubblica Istruzione (CISM-MPI) and Gruppo Nazionale di Struttura della Materia (GNSM-CNR).
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Jill C. Bonner et al.One-Dimensional Model Systems: Theoretical Survey
http://digitalcommons.uri.edu/phys_facpubs/145
http://digitalcommons.uri.edu/phys_facpubs/145Mon, 01 Jun 2015 06:38:20 PDT
In the early 1960’s one-dimensional model systems were regarded as amusing toys with the advantage of being far more easily solvable than their ’’real’’ three-dimensional counterparts. Now essentially 1-D (quasi-1-D) magnets can be ’’tailor-made’’ in the laboratory. Even more popular is the field of organic conductors like TTF⋅TCNQ, which are naturally quasi-1-D. Currently solitons and related solutions of non-linear, dispersive 1-D differential equations are ubiquitous in physics, including the area of 1-D magnetism. These developments are discussed in the Introduction. The rest of this paper is concerned with model Hamiltonians, model comparisons, critical singularities in 1-D (quasi-1-D) systems, accuracy of numerical techniques in comparison with exact solutions, brief accounts of dilute and disordered 1-D systems, and 1-D spin dynamics. Finally, a comment is made on a variety of interesting isomorphisms between 1-D magnets and phenomena in several other areas of physics, for example 2-D ferroelectrics, field-theoretic models, and realistic fluids. Comparison of theory and experiment has been the subject of several excellent reviews and is therefore not discussed here.
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Jill C. BonnerThe Role of Electric Field and Ultrasonication in the Deposition and Alignment of Single-Walled Carbon Nanotube Networks Using Dielectrophoresis
http://digitalcommons.uri.edu/phys_facpubs/144
http://digitalcommons.uri.edu/phys_facpubs/144Wed, 27 May 2015 09:33:40 PDT
The effects of electric field and ultrasonication on the deposition and alignment of single-walled carbon nanotubes (SWCNTs) across a 10 μm electrode gap have been studied. It was found that a frequency of ~1 MHz of the applied field yields the largest current independent of the magnitude of the voltage or the ultrasonication time of the sample. Increasing the ultrasonication time of a SWCNT solution changes the I-V characteristics of the deposited nanotubes from linear to nonlinear for all the voltages and frequencies of the applied field. Even in the absence of an electric field, SWCNTs bridged the electrode gap up to a critical sonication time which depends on the concentration of nanotubes in the solution.
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S. Ammu et al.Spin Chains in a Field: Crossover from Quantum to Classical Behavior
http://digitalcommons.uri.edu/phys_facpubs/143
http://digitalcommons.uri.edu/phys_facpubs/143Tue, 26 May 2015 06:51:23 PDT
Extensive numerical studies have been performed on Heisenberg antiferromagnetic chains of spin (1/2) (up to N=20), spin 1 (N=14), spin (3/2) (N=10), and spin 2 (N=8). With use of the Lanczös technique, primarily, the two lowest-lying eigenvalues have been calculated for all values of wave vector q and all values of magnetization (SzT) up to saturation for each chain. From a knowledge of the eigenvector corresponding to the lowest eigenstate for each SzT, the T=0 spin-pair correlation functions have also been calculated as a function of field. We find a most unusual quantum-classical crossover phenomenon. It shows up in greatest detail in the field-dependent dispersion spectra, but the consequences are consistently manifested in the behavior of the T=0 magnetization isotherms and in the correlations both in real space and the Fourier transforms in q space. The additional data relevant to behavior in a field have allowed us to extend previous numerical studies of Heisenberg antiferromagnetic chains with higher spin whose purpose was to examine the validity of the Haldane conjecture. The Haldane conjecture implies that Heisenberg antiferromagnetic chains with integer spin have a gap in their excitation spectrum whereas chains with half-integer spin do not. While no feature of our extended investigations is in conflict with the conjecture, unusual features associated apparently with very slow convergence make the outcome less than conclusive. It appears that calculations on significantly longer chains are required to observe with confidence the large-N asymptotic limiting behavior.
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J. B. Parkinson et al.High-Temperature Spin Dynamics of the Classical Heisenberg Magnet in One, Two, Three and Infinite Dimensions
http://digitalcommons.uri.edu/phys_facpubs/142
http://digitalcommons.uri.edu/phys_facpubs/142Tue, 26 May 2015 06:42:54 PDT
Time-dependent spin-autocorrelation functions at T = ∞ for the classical Heisenberg magnet in dimensions d = 1,2 and 3 are investigated by computer simulations. For the equivalent-neighbor Heisenberg model, the autocorrelation function is calculated analytically.
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Gerhard MullerRegular and Chaotic Time Evolution in Spin Clusters
http://digitalcommons.uri.edu/phys_facpubs/141
http://digitalcommons.uri.edu/phys_facpubs/141Tue, 26 May 2015 06:37:19 PDT
We calculate spin-autocorrelation functions (as time averages over chaotic trajectories) and their intensity spectra for clusters of two classical spins, interacting via a nonintegrable Hamiltonian. The long-time behaviour observed includes both power-law decay and persistent oscillatory components, resulting in an intensity spectrum with power-law singularities and discrete lines.
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Niraj Srivastava et al.Energetics of Peptide (pHLIP) Binding to and Folding Across a Lipid Bilayer Membrane
http://digitalcommons.uri.edu/phys_facpubs/140
http://digitalcommons.uri.edu/phys_facpubs/140Thu, 21 May 2015 07:10:32 PDT
The pH low-insertion peptide (pHLIP) serves as a model system for peptide insertion and folding across a lipid bilayer. It has three general states: (I) soluble in water or (II) bound to the surface of a lipid bilayer as an unstructured monomer, and (III) inserted across the bilayer as a monomeric α-helix. We used fluorescence spectroscopy and isothermal titration calorimetry to study the interactions of pHLIP with a palmitoyloleoylphosphatidylcholine (POPC) lipid bilayer and to calculate the transition energies between states. We found that the Gibbs free energy of binding to a POPC surface at low pHLIP concentration (state I–state II transition) at 37°C is approximately −7 kcal/mol near neutral pH and that the free energy of insertion and folding across a lipid bilayer at low pH (state II–state III transition) is nearly −2 kcal/mol. We discuss a number of related thermodynamic parameters from our measurements. Besides its fundamental interest as a model system for the study of membrane protein folding, pHLIP has utility as an agent to target diseased tissues and translocate molecules through the membrane into the cytoplasm of cells in environments with elevated levels of extracellular acidity, as in cancer and inflammation. The results give the amount of energy that might be used to move cargo molecules across a membrane.
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Yana K. Reshetnyak et al.Translocation of Molecules into Cells by pH-Dependent Insertion of a Transmembrane Helix
http://digitalcommons.uri.edu/phys_facpubs/139
http://digitalcommons.uri.edu/phys_facpubs/139Thu, 21 May 2015 07:05:04 PDT
We have previously observed the spontaneous, pH-dependent insertion of a water-soluble peptide to form a helix across lipid bilayers [Hunt, J. F., Rath, P., Rothschild, K. J. & Engelman, D. M. (1997) Biochemistry 36, 15177–15192]. We now use a related peptide, pH (low) insertion peptide, to translocate cargo molecules attached to its C terminus across the plasma membranes of living cells. Translocation is selective for low pH, and various types of cargo molecules attached by disulfides can be released by reduction in the cytoplasm, including peptide nucleic acids, a cyclic peptide (phalloidin), and organic compounds. Because a high extracellular acidity is characteristic of a variety of pathological conditions (such as tumors, infarcts, stroke-afflicted tissue, atherosclerotic lesions, sites of inflammation or infection, or damaged tissue resulting from trauma) or might be created artificially, pH (low) insertion peptide may prove a useful tool for selective delivery of agents for drug therapy, diagnostic imaging, genetic control, or cell regulation.
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Yana K. Reshetnyak et al.