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http://scholarworks.uno.edu/phys_facpubs
Recent documents in Physics Faculty Publicationsen-usMon, 07 Aug 2017 17:21:33 PDT3600Effect of disorder on quantum phase transition in the double layered ruthenates (Sr1−xCax)3Ru2O7Effect of disorder on quantum phase transition in the double layered ruthenates (Sr1−xCax)3Ru2O7
http://scholarworks.uno.edu/phys_facpubs/67
http://scholarworks.uno.edu/phys_facpubs/67Tue, 09 Apr 2013 10:41:34 PDT
(Sr_{1−x}Ca_{x})_{3}Ru_{2}O_{7} is characterized by complex magnetic states, spanning from a long-range antiferromagnetically ordered state over an unusual heavy-mass nearly ferromagnetic (NFM) state to an itinerant metamagnetic (IMM) state. The NFM state, which occurs in the 0.4 > x > 0.08 composition range, freezes into a clusterspin glass (CSG) phase at low temperatures [Z. Qu et al., Phys. Rev. B 78, 180407(R) (2008)]. In this article, we present the scaling analyses of magnetization and the specific heat for (Sr_{1−x}Ca_{x})_{3}Ru_{2}O_{7} in the 0.4 > x > 0.08 composition range. We find that in a temperature region immediately above the spin freezing temperature Tf, the isothermal magnetization M(H) and the temperature dependence of electronic specific heat Ce(T ) exhibit anomalous power-law singularities; both quantities are controlled by a single exponent. The temperature dependence of magnetization M(T ) also displays a power-law behavior, but its exponent differs remarkably from that derived from M(H) and Ce(T ). Our analyses further reveal that the magnetization data M(H,T ) obey a phenomenological scaling law of M(H,T ) ∝ Hαf (H/T δ) in a temperature region between the spin freezing temperature Tf and the scaling temperature Tscaling. Tscaling systematically decreases with the decease of Ca content. This scaling law breaks down near the critical concentration x = 0.1 where a CSG-to-IMM phase transition occurs.We discussed these behaviors in term of the effect of disorder on the quantum phase transition.
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Leonard Spinu et al.Dynamic and temperature effects in toggle magnetic random access memory
http://scholarworks.uno.edu/phys_facpubs/66
http://scholarworks.uno.edu/phys_facpubs/66Tue, 19 Jul 2011 14:24:38 PDT
In this paper we have studied the dynamic switching in magnetic random access memory (MRAM) and its dependence on thermal effects due to a finite temperature. The model is based on the Landau-Lifshitz-Gilbert equation and the stochastic Landau-Lifshitz-Gilbert equation which are numerically integrated. The magnetic layers are assumed to be ellipsoid shaped with each magnetic layer single domain. In addition, we have taken into account the uniaxial intrinsic anisotropy. Simulations were performed for both balanced and nonbalanced synthetic antiferromagnetic elements. The switching properties are discussed as a function of applied field pulses’ length and shape. In this paper we present how the thermal fluctuations affect the switching behavior, the reliability, and the writing speed of MRAM devices.
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Dorin Cimpoesu et al.Physics of complex transverse susceptibility of magnetic particulate systems
http://scholarworks.uno.edu/phys_facpubs/65
http://scholarworks.uno.edu/phys_facpubs/65Tue, 19 Jul 2011 14:11:42 PDT
Complex transverse susceptibility is a recent proposed method for the determination of anisotropy and volume distributions in particulate magnetic media. So far, only thermal fluctuations and rate-dependent damped dynamics of the magnetic moment have been identified as reasons for the existence of the imaginary transverse susceptibility. In this paper, we apply a more general approach to derive the complex transverse susceptibility, and we show that the hysteresis phenomenon is the most general concept behind the existence of complex transverse susceptibility. In this paper, the physical origins of the imaginary part of transverse susceptibility are analyzed: rate-independent hysteresis, viscous-type rate-dependent hysteresis, and thermal relaxation effect origin. The rate-independent origin is an intrinsic contribution to the imaginary transverse susceptibility and cannot be neglected because it is a zero-temperature effect.
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Dorin Cimpoesu et al.Switching behavior of a Stoner–Wohlfarth particle subjected to spin-torque effect
http://scholarworks.uno.edu/phys_facpubs/64
http://scholarworks.uno.edu/phys_facpubs/64Tue, 19 Jul 2011 13:58:16 PDT
Recently, the current-induced spin-transfer torque has been proposed as a convenient writing process in high density magnetic random access memory. A spin-polarized current can switch the magnetization of a ferromagnetic layer more efficiently than a current induced magnetic field. Our paper discusses the switching properties of a Stoner–Wohlfarth magnetic particle for the case when spin torques and external field pulses are simultaneously present. The theoretical investigation of precessional motion is described by using Landau–Lifschitz–Gilbert equation with a spin-transfer torque term included. The main goal is to determine the parameters of field pulse for that the fast and stable switching can be achieved.
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Dorin Cimpoesu et al.Optical studies of Ge islanding on Si(111)
http://scholarworks.uno.edu/phys_facpubs/63
http://scholarworks.uno.edu/phys_facpubs/63Tue, 19 Jul 2011 13:31:52 PDT
We report an experimental study of the optical properties of island layers resulting from molecular beam epitaxial deposition of Ge on Si(111) substrates. The combination of electroreflectance spectroscopy of the E_{1} transition and Raman scattering allows us to separately determine the strain and composition of the islands. For deposition at 500 °C a deposited layer of 1.36 nm of Ge assembles into 80 nm diameter islands 11 nm thick. The average Si impurity content in the islands is 2.5% while the average in-plane strain is 0.5%. Both strain and Si impurity content in islands decrease with increasing Ge deposition
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P. D. Persans et al.Mirrorless optical bistability in a nonlinear absorbing dielectric film
http://scholarworks.uno.edu/phys_facpubs/62
http://scholarworks.uno.edu/phys_facpubs/62Tue, 19 Jul 2011 13:15:50 PDT
The^{ }optical transmissivity of a mirrorless, nonlinear, absorbing dielectric thin film^{ }is investigated numerically. The dielectric function in the film region^{ }is dependent on the intensity of the electromagnetic field. Multivalued^{ }solutions of transmissivity as a function of incident power are^{ }calculated for the steady-state wave equation. The numerical solution is^{ }applied to two different model dielectric functions. As the absorption^{ }parameter is increased, larger values of incident intensity are required^{ }to switch the systems between stable output states. Also, the^{ }peak values of transmissivity are reduced as the absorption is^{ }increased.
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Kevin L. Stokes et al.Unusual heavy-mass nearly ferromagnetic state with a surprisingly large Wilson ratio in the double layered ruthenates (Sr1−xCax)3Ru2O7
http://scholarworks.uno.edu/phys_facpubs/61
http://scholarworks.uno.edu/phys_facpubs/61Mon, 11 Jul 2011 14:10:48 PDT
We report an unusual nearly ferromagnetic heavy-mass state with a surprisingly large Wilson ratio R_{w} (e.g., R_{w}∼700 for x=0.2) in double layered ruthenates (Sr_{1−x}Ca_{x})_{3}Ru_{2}O_{7} with 0.08
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Zhe Qu et al.Calorimetric evidence of strong-coupling multiband superconductivity in Fe(Te0.57Se0.43) single crystal
http://scholarworks.uno.edu/phys_facpubs/60
http://scholarworks.uno.edu/phys_facpubs/60Mon, 11 Jul 2011 13:43:40 PDT
We have investigated the specific heat of optimally doped iron chalcogenide superconductor Fe(Te_{0.57}Se_{0.43}) with a high-quality single crystal sample. The electronic specific heat C_{e} of this sample has been successfully separated from the phonon contribution using the specific heat of a nonsuperconducting sample (Fe_{0.90}Cu_{0.10})(Te_{0.57}Se_{0.43}) as a reference. The normal-state Sommerfeld coefficient γ_{n} of the superconducting sample is found to be ~26.6 mJ/mol K^{2}, indicating intermediate electronic correlation. The temperature dependence of C_{e} in the superconducting state can be best fitted using a double-gap model with 2Δ_{s}(0)/k_{B}T_{c}=3.92 and 2Δ_{l}(0)/k_{B}T_{c}=5.84. The large gap magnitudes derived from fitting, as well as the large specific heat jump of ΔC_{e}(T_{c})/γ_{n}T_{c}~2.11, indicate strong-coupling superconductivity. Furthermore, the magnetic field dependence of specific heat shows strong evidence for multiband superconductivity
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J. Hu et al.Thermally activated transitions in a system of two single domain ferromagnetic particles
http://scholarworks.uno.edu/phys_facpubs/59
http://scholarworks.uno.edu/phys_facpubs/59Mon, 11 Jul 2011 13:28:23 PDT
Numerical simulations based on the stochastic Langevin equation are applied here to a system of two uniaxial single domain ferromagnetic particles with antiferromagnetic dipolar coupling. The hysteresis loops of a strongly coupled systems exhibit fully demagnetized, intermediate metastable configurations which separate the two fully saturated states. At small magnetostatic couplings, on the other hand, and at sufficiently weak damping, the intermediate metastable configuration becomes only partially demagnetized. This state cannot be associated with any single local minimum of the free energy function.
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Dorin Cimpoesu et al.Tribological properties of fullerenes C60 and C70 microparticles
http://scholarworks.uno.edu/phys_facpubs/58
http://scholarworks.uno.edu/phys_facpubs/58Fri, 08 Jul 2011 13:28:16 PDT
The^{ }frictional behaviors of fullerenes C_{60} and C_{70} were studied because^{ }they were speculated to be solid lubricants. For the sublimated^{ }pure C_{60} films on Si(001), a high friction coefficient (0.55–0.8)^{ }was observed under different loads and pin materials. For the^{ }C_{70} film, the friction coefficient showed a pin dependence, which^{ }changed from 0.5 with an Al_{2}O_{3} pin to about 0.9^{ }with a 440 stainless steel pin. The relatively high friction^{ }coefficients of C_{60} and C_{70} films were due to the^{ }tendency of the C_{60} and C_{70} particles to clump and^{ }compress into high shear strength layers rather than due to^{ }the impurities in the fullerenes. The benzene-solvated C_{60} · 4C_{6}H_{6} and C_{70} · xC_{6}H_{6}^{ }showed a lowered friction coefficient (0.25 for C_{60} · 4C_{6}H_{6} and 0.3^{ }for C _{70} · xC_{6}H_{6}), which might result from the lowered shear^{ }strength of the hcp structure of C_{60} · 4C_{6}H_{6} and C_{70} · xC_{6}H_{6} molecular^{ }crystals in which the benzene molecules were intercalated
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Wei Zao et al.Intrinsic chaos in a dc field biased quantum heterostructure
http://scholarworks.uno.edu/phys_facpubs/57
http://scholarworks.uno.edu/phys_facpubs/57Fri, 08 Jul 2011 13:02:39 PDT
A closed, quantum, double barrier, GaAs/AlGaAs heterostructure is made chaotic by adding a nonlinear potential term, α〈Q(t)〉, to the time-dependent Schrödinger equation, and the dynamical behavior of an electron cloud moving in the heterostructure biased by a dc electric field is examined numerically. Using phase-space diagrams, power spectrums, and Lyapunov exponents, both qualitative and quantitative measures of the chaos in the system were taken. In general, for all values of α, the nonlinearity parameter, the Lyapunov exponent, λ, increases as the applied dc field, β, increases. However, for values of α ⩽ 1.376, we notice a sharp drop in λ for the value of β = −9.2×10^{7} V/m corresponding to an average dc voltage of −.085 eV in the central well. This first order type transition to high values of λ for α>1.376 corresponds to a similar increase in the mean charge trapped in the heterostructure and in the average nonlinear potential in the central well for that dc field. This behavior is attributed to the fact that for α ⩽ 1.376 and β = −9.2×10^{7} V/m, the field effects dominate, but for α>1.376, the nonlinearity term dominates.
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A. Jason McNary et al.Digital signal propagation in dispersive media.
http://scholarworks.uno.edu/phys_facpubs/56
http://scholarworks.uno.edu/phys_facpubs/56Fri, 08 Jul 2011 12:53:40 PDT
In this article, the propagation of digital and analog signals through media which, in general, are both dissipative and dispersive is modeled using the one-dimensional telegraph equation. Input signals are represented using impulsive, Heaviside unit step, Gaussian, rectangular pulse, and both unmodulated and modulated sinusoidal pulse type boundary data. Applications to coaxial transmission lines and freshwater signal propagation, for both digital and analog signals, are included. The analysis presented here supports the finding that digital transmission in dispersive media is generally superior to that of analog. The boundary data (input signals) give rise to solutions of the telegraph equation which contain propagating discontinuities. It is shown that the magnitudes of these discontinuities, as a function of distance, can be found without the need of solving the governing equation. Thus, for digital signals in particular, signal strength at a given distance from the input source can be easily determined. Furthermore, the magnitudes of these discontinuities are found to be independent of both the dispersion coefficient k and the elastic coefficient b. In addition, it is shown that, depending on the algebraic sign of k, one of two distinct forms of dispersion is possible and that for small-time intervals, solutions are approximately independent of k
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P. M. Jordan et al.Causal implications of viscous damping in compressible fluid flows
http://scholarworks.uno.edu/phys_facpubs/55
http://scholarworks.uno.edu/phys_facpubs/55Thu, 23 Jun 2011 14:00:58 PDT
Classically, a compressible, isothermal, viscous fluid is regarded as a mathematical continuum and its motion is governed by the linearized continuity, Navier-Stokes, and state equations. Unfortunately, solutions of this system are of a diffusive nature and hence do not satisfy causality. However, in the case of a half-space of fluid set to motion by a harmonically vibrating plate the classical equation of motion can, under suitable conditions, be approximated by the damped wave equation. Since this equation is hyperbolic, the resulting solutions satisfy causal requirements. In this work the Laplace transform and other analytical and numerical tools are used to investigate this apparent contradiction. To this end the exact solutions, as well as their special and limiting cases, are found and compared for the two models. The effects of the physical parameters on the solutions and associated quantities are also studied. It is shown that propagating wave fronts are only possible under the hyperbolic model and that the concept of phase speed has different meanings in the two formulations. In addition, discontinuities and shock waves are noted and a physical system is modeled under both formulations. Overall, it is shown that the hyperbolic form gives a more realistic description of the physical problem than does the classical theory. Lastly, a simple mechanical analog is given and connections to viscoelastic fluids are noted. In particular, the research presented here supports the notion that linear compressible, isothermal, viscous fluids can, at least in terms of causality, be better characterized as a type of viscoelastic fluid.
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P. M. Jordan et al.Energy flow and fluorescence near a small metal particle
http://scholarworks.uno.edu/phys_facpubs/54
http://scholarworks.uno.edu/phys_facpubs/54Mon, 20 Jun 2011 14:24:36 PDT
We examine the classical energy-balance equation for a fluorescing system consisting of a molecule near a small, spherical metal particle capable of sustaining electromagnetic resonances and irradiated with laser light. From the energy-flow distribution in the entire system, we obtain the enhancement factor for the fluorescence emission of the adsorbed molecule. Numerical results demonstrate that the electromagnetic interactions of the molecule and the surface can be understood in terms of energy flow through the entire system and applied to investigate spectroscopic properties of adsorbates in similar systems. Absorption and emission rates of the adsorbed molecule are determined considering the energy-flow distribution and its dependence on the substrate as well as molecular parameters. Such understanding is useful in predicting spectroscopic responses of adsorbates.
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Purna C. Das et al.Cyclotron frequency coupled enhancement of Kerr rotation in low refractive index-dielectric/magneto-optic bilayer thin-film structures
http://scholarworks.uno.edu/phys_facpubs/53
http://scholarworks.uno.edu/phys_facpubs/53Mon, 20 Jun 2011 13:55:13 PDT
We investigate the enhancement of magneto-optic polar Kerr rotation over a broad range of optical frequencies. The Drude model for the dielectric tensor is considered. Resonance-like peaks in the Kerr rotation and ellipticity caused in the vicinity of a plasma edge is extended throughout the visible spectrum by deriving an expression for the cyclotron frequency such that Re(ϵ_{xx})∼1 at any given incident photon energy and plasma frequency of the material. The Kerr rotation obtained by use of this expression was studied for the case of InSb and further enhancement of Kerr rotation was achieved in the case of LiF/InSb bilayer thin-film structure, grown on a glass substrate. The numerical analysis was carried out using a 2×2 characteristic matrix, which takes into account multiple reflections and interface effects within the medium. In addition the role of various magneto-optic material parameters and layer thicknesses is investigated in determining the optical frequency at which the maximum Kerr rotation occurs, under the present cyclotron frequency condition. Enhanced Kerr rotation greater than 1.5° over a broad range of optical frequencies was obtained. Tables showing optimum figure of merit for repeated LiF/InSb bilayers at 3.1 eV for different plasma frequencies are also provided.
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A. De et al.Application of plasma resonance condition for prediction of large Kerr effects
http://scholarworks.uno.edu/phys_facpubs/52
http://scholarworks.uno.edu/phys_facpubs/52Mon, 20 Jun 2011 13:42:40 PDT
Resonance like enhancement of magneto-optic Kerr effects (MOKEs) have been attributed to numerous effects, one of them being plasma resonance of free charge carriers, which is analytically and numerically investigated here. Analytical expressions for frequency dependent enhancement of MOKE are obtained in the general framework of the Drude model, which are then applied to well known expressions describing MOKE. The derived expressions are numerically tested for various optical constants. It is known that for certain materials, the resonance like enhancements in the Kerr spectrum occur in the near vicinity of Re[ε_{xx}]=1, which is generally near the plasma edge. This is seen to be true when the plasma frequency ω_{p} is greater in magnitude as compared to the cyclotron frequency ω_{c}, i.e., if ω_{p}≫ω_{c}, whereas in the event of a large reflection edge split, i.e., if ω_{p}∼ω_{c} the resonance like peaks will occur near Re[ε_{+}.ε_{−}]≈1, which is a relatively more general condition as compared to Re[ε_{xx}]=1. Second, we see through model calculations that the spectral proximity of the MOKE resonance peak to the plasma edge is also subject to the magnitude of the background dielectric constant. Results are explained analytically and numerically. Good agreement is obtained between the expressions derived here and the numerically observed occurrence of resonance like peaks in the Kerr spectrum.
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A. De et al.Kerr-resonance-condition-coupled enhancement in magneto-optic media
http://scholarworks.uno.edu/phys_facpubs/51
http://scholarworks.uno.edu/phys_facpubs/51Mon, 20 Jun 2011 13:33:18 PDT
We derive an expression for cyclotron frequency ω_{c}, which sets Re[ϵ_{+}ϵ_{−}]=1 in a magneto-optic (MO) substrate, at any incident photon energy. Thereby, at any desired part of the optical spectrum, large Kerr effects can be obtained, which are generally known to occur either at the free-charge-carrier-plasma-resonance frequency ω_{p}, or at frequencies where active electronic transitions take place. Under these conditions, ω_{p} is seen to play a very different role; it is seen that for any ω, the magnitude of the Kerr resonance in a single MO (InSb here) substrate increases with decreasing ω_{p}. With the objective of achieving further Kerr enhancement, the effect of coating a thin film of this ω_{c}-coupled InSb layer on a Ag substrate is numerically studied. Further Kerr enhancement, at the plasma-resonance frequency of Ag, is seen, which is dependent on the thickness of the MO layer and its ω_{p}. In this configuration, giant resonances appear in the effective-dielectric-tensor spectra. The spectral locations of these resonances are dependent on the thickness of the MO layer. We interpret these resonance structures to be the effective cyclotron resonance. Our results suggest that there exists a strong correlation between the effective-cyclotron frequency and the plasma-resonance frequency of the noble
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A. De et al.Large plasma-edge broadened magneto-optic-polar-Kerr-effect-based broadband incoherent detection of terahertz spectral frequencies
http://scholarworks.uno.edu/phys_facpubs/50
http://scholarworks.uno.edu/phys_facpubs/50Wed, 15 Jun 2011 14:27:35 PDT
A magneto-optic Kerr effect (MOKE)-based spectroscopic technique is proposed in the terahertz regime. This method relies on very large reflection edge splitting effects, which occur when the cyclotron frequency (CF) is of the same order of magnitude or greater than the plasma frequency. In the event of a very large reflection edge split, the Kerr rotation (KR) no longer occurs at the plasma edge, but instead occurs at Re(ε_{+}ε_{−}) ≈ 1, within the macroscopic framework of the Drude model. This implies that one can control the spectral occurrence of the MOKE resonance, simply by tuning the magnetic field strength. This phenomenon is unheard of for much shorter wavelengths, due to practical limitations on required magnetic field strengths and hence, can only be realized in the THz regime. A 3 T magnet can easily cover the proposed 0.5–5 THz spectral range for an InSb substrate. Our calculations show that in order to achieve good spectral resolution, the InSb substrate needs to be cooled to 77 K. The Kerr rotation obtained at 77 K for a CF of 1 THz is about 6.28°, which can be increased to about 18.35° by coating a thin low refractive index material on the InSb substrate. A well established ellipsometric technique, which uses four incoherent detectors, is proposed, for fast-simultaneous measurement of KR, Kerr ellipticity and reflectivity.
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A. De et al.A search for interstellar and circumstellar C60
http://scholarworks.uno.edu/phys_facpubs/49
http://scholarworks.uno.edu/phys_facpubs/49Wed, 15 Jun 2011 14:06:47 PDT
It has recently been suggested that the diffuse interstellar bands may be formed by ionized polyhedral carbon molecules such as C60(+). While specific laboratory measurements of absorption bands of this molecular ion have not been made, a feature due to the neutral molecule C60 has been discovered at 3860A. Examination of spectra of several reddened stars, as well as one star known to have circumstellar carbonaceous dust, shows no sign of the feature, leading to upper limits of the order of 10 to the 14th/sq cm for the column density of C60. These limits are not yet sensitive enough to violate the expectations of crude predictions.
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T P. Sno et al.A new search for interstellar C3
http://scholarworks.uno.edu/phys_facpubs/48
http://scholarworks.uno.edu/phys_facpubs/48Wed, 15 Jun 2011 13:57:24 PDT
A^{ }new, very sensitive search for interstellar triatomic carbon has resulted^{ }in upper limits for a few diffuse clouds of order^{ }10^{10} cm^{ – 2}, or about 10^{ – 11} with respect to hydrogen. These^{ }limits are consistent with recent cold diffuse cloud chemistry models,^{ }but may be in conflict with shocked cloud models such^{ }as those invoked to explain CH^{ + }abundances. Our results may^{ }also be argue against linear carbon-chain molecules as carriers of^{ }the diffuse interstellar bands.
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Charles L. Joseph et al.