Athermal Behavior Research Articles

Asymmetric switchinglike behavior in the magnetoresistance at low fields in bulk metamagnetic Heusler alloys

A novel physical phenomenon has been observed that resembles a large asymmetric switchinglike magnetoresistance at low applied fields in bulk metamagnetic Heusler alloys. A thermally activated isothermal forward metamagnetic transition with a signature of a pronounced time-dependent relaxation was observed in a bulk B-substituted off-stoichiometric Ni-Mn-In Heusler alloy, whereas the reverse metamagnetic transition exhibited the usual athermal behavior with no thermal activation. The asymmetry between the forward and reverse metamagnetic transitions resulted in a large switchinglike, low-field magnetoresistance (\ensuremath{\sim}16% for a field change of $B$ = 0 \ensuremath{\rightarrow} 0.25 T at $T$ = 304 K) in the bulk Heusler alloys, ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{35}{\mathrm{In}}_{15\ensuremath{-}x}{\mathrm{B}}_{x}$ (1 \ensuremath{\leqslant} $x$ \ensuremath{\leqslant} 2).

Thermal-Lens-Driven Effects in $N_{\mathrm {g}}$ -Cut Yb-and Tm-Doped Monoclinic KLu(WO4)2 Crystals

The potential of N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> -cut monoclinic Yb: and Tm:KLu(WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser crystals is analyzed from the point of view of thermal lensing, including athermal behavior, unique laser mode stabilization by polarization-switching and potential for microchip operation. To this aim, we characterize dispersion and direction dependence of the generalized thermo-optic coefficients, A, for the light polarizations, E∥N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> and N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> , and calculate sensitivity factors of the thermal lens for wide range of experimental conditions, including crystal temperature, laser wavelength, pump spot radius, as well as their relation to probe beam measurements. The polarization-switching between N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> and N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> oscillating states is realized for a Tm:KLu(WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> crystal and explained on the basis of spectroscopic properties, mode-matching and cavity stability. The thermal lens properties for microchip Tm:KLu(WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser are also calculated.

All-space existence and dispersion of athermal directions in monoclinic KY(WO4)2

The analytical expressions for thermo-optic coefficients, dn/dT, for “fast” and “slow” light waves propagating along the arbitrary direction in a biaxial crystal are derived. On the basis of these expressions, the all-space analysis of existence of athermal directions is performed for monoclinic and biaxial KY(WO4)2 at 1.03μm. The calculations are performed for an arbitrary light propagation direction and polarization (not restricted to the principal planes). The appearance of directions that can be athermal for both “fast” and “slow” waves is predicted. The dispersion of athermal directions is analyzed for visible and near-IR. The existence of upper and lower dispersion limits for athermal behavior of KY(WO4)2 is shown. It is shown that the optical indicatrix axes can be athermal itself at some light wavelengths. Wavelength- and temperature-dependent position of the optical axes of KY(WO4)2 is also determined.

Athermal silicon waveguides with bridged subwavelength gratings for TE and TM polarizations

In this paper, athermal silicon waveguides using bridged subwavelength grating (BSWG) structures are proposed and investigated. The realization of temperature-independent BSWG waveguides for both polarizations is demonstrated numerically and experimentally. SU-8 polymer is used as the cladding material to compensate for the positive thermo-optic (TO) coefficient (dn/dT) of silicon. We investigate the dependence of the effective TO coefficient of BSWG waveguides on both the bridge width and grating duty cycle. The BSWG waveguides have a width of 490 nm, a height of 260 nm, and a grating pitch of 250 nm. Athermal behavior is achieved for both the transverse-magnetic (TM) and the transverse-electric (TE) polarized light for a variety of bridge width and duty cycle combinations. Furthermore, the BSWGs can be designed to be athermal for both TE and TM polarization simultaneously.

Thermal lensing and microchip laser performance of N g-cut Tm3+:KY(WO4)2 crystal

The thermal lensing effect was characterized in the diode-pumped monoclinic N g-cut Tm:KYW crystal under laser operation conditions at the wavelength of 1.94 μm. The thermal lens was found to be slightly astigmatic; its optical power D being positive for rays lying in all meridional planes. Thermal lens sensitivity factors M=dD/dP abs equal 11.8 m−1/W and 8.8 m−1/W (with respect to the absorbed pump power P abs) for principal meridional planes containing N p and N m axes. Nearly athermal behavior of N g-cut crystal is associated with the mutual compensation of different impacts to the thermal lens optical power that arise from temperature dependence of the refractive index dn/dT and anisotropic thermal expansion. It was utilized to produce passively cooled diode-pumped 0.65 W cw Tm:KYW microchip laser with slope efficiency of 44 % and low thermo-optic aberrations.

Athermal arrayed waveguide gratings in silicon-on-insulator by overlaying a polymer cladding on narrowed arrayed waveguides

Athermal arrayed waveguide gratings (AWGs) in silicon-on-insulator (SOI) are experimentally demonstrated for the first time to our knowledge. By using narrowed arrayed waveguides, and then overlaying a polymer layer, the wavelength temperature dependence of the AWGs is successfully reduced to -1.5 pm/°C, which is more than 1 order of magnitude less than that of normal SOI AWGs. The athermal behavior of the AWGs is obtained with little degradation of their performance. For the central channel, the cross talk is less than -15 dB and the insertion loss is around 2.6 dB. Good characteristics can be maintained with temperatures up to 75 °C. The total size of the device is 350 μm × 250 μm.

Elasticity of SrTiO3 perovskite under high pressure in cubic, tetragonal and orthorhombic phases

We investigated the athermal high-pressure behavior of the elastic properties of SrTiO3 (STO) up to 26 GPa in cubic, tetragonal and orthorhombic phases using the ab initio pseudo-potential method. Our results for the cubic phase are in good agreement with experiment and previous pseudo-potential calculations. There are no studies for the tetragonal and orthorhombic phases under high pressure available for comparison. To date, there are no global data on elastic parameters under high pressure for STO. We establish data when we report the results of our structural and elastic study under high pressure in the three phases. Our calculations show that the cubic–tetragonal phase transition occurs at 6 GPa and the tetragonal–orthorhombic phase transition at 14 GPa. A third and unknown phase transition from orthorhombic Cmcm to monoclinic P21 /m was observed at 24 GPa, but no study has explored it. The orthorhombic phase is unstable and this instability may be due to ferroelectricity at high pressure. The elastic properties of STO are also strongly pressure dependent with instabilities near the phase transition pressure. STO is more resistant to plastic deformation and to fracture in the cubic phase than in the tetragonal and orthorhombic phases.

Temperature-Dependent Atomic Scale Friction and Wear on PbS(100)

Friction and wear on PbS(100) surfaces have been investigated on the atomic scale as a function of temperature with atomic force microscopy. At room temperature and above, the PbS(100) surface exhibited low friction (μ < 0.05) in contact with a silicon nitride probe tip, provided that interfacial wear was not encountered. In the absence of wear, friction increased exponentially with decreasing temperature, transitioning to an athermal behavior near 200 K. An Arrhenius analysis of the temperature dependence of friction yielded an activation energy ∆E = 0.32 ± 0.02 eV for the sliding contact of a silicon nitride tip on PbS(100).

Influence of temperature and excitation procedure on the athermal behavior of Nd3+-doped phosphate glass: Thermal lens, interferometric, and calorimetric measurements

In this work, thermal and optical properties of the commercial Q-98 neodymium-doped phosphate glass have been measured at low temperature, from 50 to 300 K. The time-resolved thermal lens spectrometry together with the optical interferometry and the thermal relaxation calorimetry methods were used to investigate the glass athermal characteristics described by the temperature coefficient of the optical path length change, ds/dT. The thermal diffusivity was also determined, and the temperature coefficients of electronic polarizability, linear thermal expansion, and refractive index were calculated and used to explain ds/dT behavior. ds/dT measured via thermal lens method was found to be zero at 225 K. The results provided a complete characterization of the thermo-optical properties of the Q-98 glass, which may be useful for those using this material for diode-pumped solid-state lasers.

An improved physically based behaviour law for ferritic steels and its application to crash modelling

An improved physically based behaviour law for ferritic steels is presented in this paper, and used to perform crash simulations. Flow stress depends on three contributions: the friction stress, the work hardening at low strain rate and the effective stress derived from the viscoplastic potential. These three terms are in most existing approaches simply summed up, but an original alternative mixing solution has been proposed for bcc metals by Edgar Rauch. The viscoplastic potential comes from the most general definition of a thermally activated mechanism at the dislocation gliding scale. The law intrinsically contains a threshold stress and exhibits only two unknown physical variables (activation volume and energy). The athermal quasi-static behaviour is finally described thanks to a linear combination of a Swift and a Hockett-Sherby laws. The complete model is then validated on the available data for two commercial ferritic steel. The predictions show a very good agreement with experimental data, and allow a good description of the temperature effects with a small numbers of parameters.

Transition from Thermal to Athermal Friction under Cryogenic Conditions

Atomic scale frictional forces encountered as a function of temperature for the contact of a Si3N4 probe tip and the basal plane of MoS2 have been measured with atomic force microcopy over the temperature range 100-500 K. Friction is observed to increase exponentially with decreasing temperature from 500 to 220 K. An Arrhenius analysis of the temperature dependent friction over this range yields an effective activation energy of approximately 0.3 eV for the thermally activated stick-slip motion of the probe tip on this surface. As temperature is reduced further below 220 K, a distinct transition to a largely athermal behavior is detected and is shown to result from the onset of interfacial wear, entailing an alternative energy dissipation pathway.

A Possible Link Between Macroscopic Wear and Temperature Dependent Friction Behaviors of MoS2 Coatings

Studies to explore the nature of friction, and in particular thermally activated friction in macroscopic tribology, have lead to a series of experiments on thin coatings of molybdenum disulfide. Coatings of predominately molybdenum disulfide were selected for these experiments; five different coatings were used: MoS2/Ni, MoS2/Ti, MoS2/Sb2O3, MoS2/C/Sb2O3, and MoS2/Au/Sb2O3. The temperatures were varied over a range from −80 °C to 180 °C. The friction coefficients tended to increase with decreasing temperature. Activation energies were estimated to be between 2 and 10 kJ/mol from data fitting with an Arrhenius function. Subsequent room temperature wear rate measurements of these films under dry nitrogen conditions at ambient temperature demonstrated that the steady-state wear behavior of these coatings varied dramatically over a range of K = 7 × 10−6 to 2 × 10−8 mm3/(Nm). It was further shown that an inverse relationship between wear rate and the sensitivity of friction coefficient with temperature exists. The highest wear-rate coatings showed nearly athermal friction behavior, while the most wear resistant coatings showed thermally activated behavior. Finally, it is hypothesized that thermally activated behavior in macroscopic tribology is reserved for systems with stable interfaces and ultra-low wear, and athermal behavior is characteristic to systems experiencing gross wear.

A phenomenological model of size-dependent hardening in crystal plasticity

A phenomenological model of plastic deformation is proposed, which captures the size-dependence of plastic flow strength and work-hardening in pure FCC crystalline materials. Guided by discrete dislocation dynamics analyses, the treatment is based on two structural variables determining the mechanical state of the material. A complete description of plastic behaviour is achieved, giving two inherently different statements for the evolution of structure, supplemented by a new kinetic equation, which specifies the hardening law in differential form at fixed structure. Evolution of the first state variable is set by phenomenology; it accounts for the cardinal bulk phenomena of athermal hardening and dynamic recovery, in addition to geometric storage. The second state variable is kinematically determined so that an evolution equation need not be formulated explicitly in rate form. The model formulation leaves the classical treatment of dynamic recovery unaltered. However, since there is virtually no experimental data on the temperature and strain-rate dependence of plastic flow at the micron scale, emphasis is laid on athermal behaviour. In this limit, the model equations are integrated, following specified strain paths to give the flow strength at the current structure. Model predictions are assessed through comparison with results from discrete dislocation analyses of geometrically similar crystals subject to compression.

Decay time of polaron photoluminescence in congruent lithium niobate

AbstractThe intensity, the peak wavelength and the decay time of polaron photoluminescence in congruent lithium niobate are measured versus temperature from 77 K to 290 K. The radiative relaxation shows quasi athermal behaviour (τR ≈ 9 µs) whereas the nonradiative relaxation follows arrhenius law with activation energy of 220 meV. The crossing point between radiative and nonradiative lifetimes is about 210 K. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A Thermo-Magneto-Mechanical Free Energy Model for NiMnGa Single Crystals

AbstractThe paper extends the authors' recent model for one-dimensional rate-dependent magneto-mechanical behavior of NiMnGa single crystals to account for temperature-dependent effects including austenite/martensite and ferro-/paramagnetic phase transitions. The magneto-mechanical model is based on the Helmholtz free energy landscape constructed for a meso-scale lattice element with strain and magnetization as order parameters. This two-dimensional energy landscape includes three paraboloidal wells representing the two easy-axis and one hard-axis martensite variants relevant for the structurally one-dimensional case. Phase transformations resulting from applied stresses and magnetic fields follow from a system of evolution laws based on the Gibbs free energy equations and the theory of thermally activated processes, which in the low-thermal-activation limit appropriately reproduce the athermal transformation behavior observed in these materials. The phase fractions subsequently determine the macroscopic strain and magnetization of a sample of NiMnGa by means of a standard averaging procedure. To account for the first-order phase transitions to austenite, additional temperature-dependent wells representing the stable states of austenitic NiMnGa are introduced into the Helmholtz energy landscape. The transition from ferromagnetic to paramagnetic states is modeled as a second order transformation based on the gradual degeneration of the ferromagnetic wells with increasing temperature.

A consistent modified Zerilli-Armstrong flow stress model for BCC and FCC metals for elevated temperatures

The Zerilli-Armstrong (Z-A) physical based relations that are used in polycrystalline metals at low and high strain rates and temperatures are investigated in this work. Despite the physical bases used in the derivation process, the Z-A model exhibits certain inconsistencies and predicts inaccurate results when applied to high temperatures-related problems. In the Z-A model, the thermal stress component vanishes only when T→∞. This contradicts the thermal activation mechanism that imposes an athermal behavior for the flow stress at certain finite critical temperatures. These inconsistencies, in fact, are attributed to certain assumptions used in the Z-A model formulation that causes the model parameters to be inaccurately related to the microstructural physical quantities. New relations are, therefore, suggested and proposed in this work using the same physical bases after overcoming any inappropriate assumptions. The proposed modified relations along with the Z-A relations are evaluated using the experimental results for different bcc and fcc metals. Comparisons are also made with the available experimental results over a wide range of temperatures and strain rates. The proposed model simulations, in general, show better correlation than the Z-A model particularly at temperatures values above 300K∘. Numerical identification for the physical quantities used in the definition of the proposed model parameters is also presented.

Thermodynamic Properties of Multialkane Synthetic and Real Petroleum Mixtures

Measurements of enthalpy increments from the temperature of the ordered phases of low temperatures at 293.4 K up to the liquid phase above the melting point were carried out by differential scanning calorimetry using discontinuous mode temperature programming on 18 multiparaffinic mixtures, prepared by melting pure alkanes, and 4 commercial petroleum waxes. Temperatures and enthalpies of the solid/solid transitions and melting were determined, as well as the total enthalpy variations from the order/disorder transition onset temperature up to the melting end temperature and the heat capacity variations in the ordered solid solution and the liquid phase, as a function of temperature. The comparison of the experimental results with the values, calculated for equivalent ideal mixtures from the thermodynamic data of pure n-alkanes, allows us to highlight a deviation in relation to the ideality in the solid state for all of these mixtures: the ordered solid phase reveals a significant gap in relation to ideality, whereas the liquid mixtures show athermal behavior. The composition characteristic parameters of the mixtures (shape of the n-alkane mole fraction distribution, number of n-alkanes, and percentage of the other nonlinear hydrocarbons in the petroleum mixtures) are closely related to the degree of internal disorder in the solid solutions and thus to the deviation in relation to the ideality in the solid state.

Measurement of magnetic relaxation in the peak regime of V 3Si

Magnetization relaxation measurements are carried out in the peak effect regime of superconducting V 3Si crystal, using Quantum Design SQUID magnetometer. Relaxation in the increasing field scan is logarithmic in time, consistent with the theory of flux creep. The relaxation on the decreasing field scan however exhibits athermal behavior which is predominantly governed by the flux avalanches triggered by the small external field perturbation experienced by the superconductor during measurement scan in an inhomogeneous field.

Phase separation in suspensions of colloids, polymers and nanoparticles: Role of solvent quality, physical mesh, and nonlocal entropic repulsion

Analytic and numerical microscopic integral equation theory for polymer–particle suspensions is employed to investigate the dependence of fluid–fluid phase separation on size asymmetry, solvent quality, and higher order polymer–polymer interactions. For athermal good solvents, our prior novel prediction of enhanced miscibility with increasing (decreasing) polymer (particle) size is found not to be fundamentally tied to physical mesh formation or strong polymer-induced colloid clustering. Rather, the key is a proper treatment of the polymer second virial coefficient, which is sensitive to how chains organize in the empty space between particles. The origin of the qualitative error made by classic mean-field theories for the shifting of phase boundaries with size asymmetry is established. The phase separation behavior predicted by integral equation theory for ideal polymers is completely different than the athermal case for all size asymmetries and particle volume fractions, thereby establishing the remarkably large consequences of polymer–polymer repulsions. For large polymers or small nanoparticles under ideal solvent conditions, the suspension miscibility worsens with increasing size asymmetry, opposite to the athermal solvent behavior. However, over a significant range of intermediate size asymmetries the spinodal curves are either nearly constant, or display a nonmonotonic shifting, as size asymmetry is varied. Higher order contributions in polymer concentration modestly stabilize the miscible phase in both athermal and ideal solvents.

Phase behavior and concentration fluctuations in suspensions of hard spheres and nearly ideal polymers

The phase behavior and concentration fluctuations in suspensions of hard sphere colloids and nonadsorbing polymers under nearly ideal solvent conditions is studied experimentally. A remarkably different qualitative behavior compared to the athermal solvent case is observed for the dependence on polymer/particle size asymmetry of both the gelation and fluid–fluid phase separation boundaries. Near the theta state the effect of increasing the range of depletion attractions leads to a weak monotonic destabilization of the homogeneous phase at high particle volume fractions, with a reversal of the trend at lower volume fractions. In stark contrast to athermal solvent behavior, this nonmonotonic behavior results in multiple “curve crossings” of gel and phase separation boundaries as the polymer/particle size ratio is varied. Quantitative comparisons with no adjustable parameter PRISM integral equation theory for the fluid–fluid spinodals and osmotic compressibilities show good qualitative or semiquantitative agreement with all the experimental trends. The differences between good and ideal solvent conditions are largely attributed to changes in the polymer–polymer pair correlation functions due to the enhanced ability of coils to interpenetrate and cluster in theta solvents. Even for ideal solvent conditions the simplifying polymer model and statistical mechanical assumptions adopted by prior classic free volume and related approaches appear to miss fundamental aspects of the experimental behavior, especially for large size asymmetry ratios and/or moderate-to-high colloid volume fractions. The primary error can be identified with the approximation of a polymer chain by a phantom sphere with no conformational degrees of freedom.