Scattering Relaxation Time Research Articles

Morphological control of bundled actin networks subject to fixed-mass depletion.

Depletion interactions are thought to significantly contribute to the organization of intracellular structures in the crowded cytosol. The strength of depletion interactions depends on physical parameters such as the depletant number density and the depletant size ratio. Cells are known to dynamically regulate these two parameters by varying the copy number of proteins of a wide distribution of sizes. However, mammalian cells are also known to keep the total protein mass density remarkably constant, to within 0.5% throughout the cell cycle. We thus ask how the strength of depletion interactions varies when the total depletant mass is held fixed, a.k.a. fixed-mass depletion. We answer this question via scaling arguments, as well as by studying depletion effects on networks of reconstituted semiflexible actin in silico and in vitro. We examine the maximum strength of the depletion interaction potential U∗ as a function of q, the size ratio between the depletant and the matter being depleted. We uncover a scaling relation U∗ ∼ qζ for two cases: fixed volume fraction φ and fixed mass density ρ. For fixed volume fraction, we report ζ < 0. For the fixed mass density case, we report ζ > 0, which suggests that the depletion interaction strength increases as the depletant size ratio is increased. To test this prediction, we prepared our filament networks at fixed mass concentrations with varying sizes of the depletant molecule poly(ethylene glycol) (PEG). We characterize the depletion interaction strength in our simulations via the mesh size. In experiments, we observe two distinct actin network morphologies, which we call weakly bundled and strongly bundled. We identify a mass concentration where different PEG depletant sizes lead to weakly bundled or strongly bundled morphologies. For these conditions, we find that the mesh size and intra-bundle spacing between filaments across the different morphologies do not show significant differences, while the dynamic light scattering relaxation time and storage modulus between the two states do show significant differences. Our results demonstrate the ability to tune actin network morphology and mechanics by controlling depletant size and give insights into depletion interaction mechanisms under the fixed-depletant-mass constraint relevant to living cells.

Second sound of heat conduction in one-dimensional dielectric materials

Although recent experiment have shown that second sound can occur in graphite above 200 K, there have been no reports of second sound being observed in low-dimensional materials. In the present work, based on phonon hydrodynamics we found that second sound can occur in a single-walled carbon nanotube (SWCNT) with a length of no less than 2.1333 microns and no more than 2.1209e-4 meters for the initial temperature field with sinusoidal changes in the axial direction. The constraint conditions for relaxation times of the normal and resistive scatterings, as well as the conditions for the axial length and initial temperature distribution required for the occurrence of the second sound in dielectric nanowires are also derived from the Guyer-Krumhansl equations. For both SWCNTs and nanowires it was found that the small normal scattering relaxation time and large resistive scattering relaxation time are beneficial for the occurrence of second sound. Our results show that in comparison with two-dimensional materials, such as graphene, it is easier to experimentally excite second sound in the SWCNTs.

Sign change of intrinsic domain wall resistance in epitaxial Heusler alloy Co[formula omitted]MnAl thin films

The investigation of magnetic domain wall resistance (DWR) in epitaxial Co2MnAl (CMA) thin films, a Heusler alloy, has revealed a remarkable phenomenon of sign change. This intriguing behavior is attributed to the half-metallic band structure with high spin polarization, as verified by negative anisotropic magnetoresistance in the two-current model. In this way, the scattering relaxation time for the majority and minority electrons, which is primarily determined by the density of states of majority and minority electrons, undergoes a dramatic change in the vicinity of the half-metallic band gap. This results in an unusual upturned relaxation time ratio, leading to the sign change of intrinsic DWR. These findings deepen our understanding of the intricate dynamics of spin-dependent phenomena in half metals and shed light on the entangled magnetoresistance in Heusler alloy with high spin polarization.

High-Throughput Estimation of Phonon Thermal Conductivity from First-Principles Calculations of Elasticity.

Thermal conductivity is a crucial property for thermal management of modern electronics, thermal energy conversion, and energy sustainable development. However, it is very expensive and time-consuming to calculate the phonon thermal conductivity of materials through the fully ab initio calculations or molecular dynamics simulations. Exploiting the fundamental correlation between elastic properties (bulk and shear modulus) and phonon thermal conductivity of crystalline materials, we develop an efficient method, phonon-elasticity-thermal (PET) model to rapidly and accurately estimate the phonon thermal conductivity at the high-temperature limit based on the Born-von Karman periodic boundary condition and Umklapp phonon-phonon scattering relaxation time approximation. As a demonstration, we calculate the phonon thermal conductivities of 226 inorganic solid materials covering the whole 7 crystalline systems within a high-throughput calculation framework on account of our PET model. The high-throughput prediced phonon thermal conductivities is in good agreement with experimental measurements. Our results imply the potential application of the elasticity-based phonon thermal conductivity estimation to screen or guide the material discovery of target phonon thermal conductivity and also provide a reference for the study of phonon-elasticity-thermal relationship.

The influence of Dy concentration on the thermoelectric properties of n-type Dy-doped Bi2Te3 pellets prepared by hydrothermal and carbon burial sintering

Dy-doped Bi2Te3 pellets with three different molar ratios between the precursors BiCl3 and DyCl3∙6H2O (x = 0.05, 0.1, and 0.25) were prepared by hydrothermal and carbon burial sintering. The excess of Dy ions in the highly doped samples (x = 0.25) remarkably increased the Seebeck coefficient, reduced the thermal conductivity, and increased ZT more than twofold in the temperature range of 323–375 K compared to pristine Bi2Te3. At low Dy concentrations (x = 0.05 and 0.1), the Dy ions substituted the TeBi antisites and then intercalated between the Bi2Te3 interlayers at higher concentration. The Dy ions act as both the phonon and charge carrier scattering centers which increase the scattering factor and decrease the point defect scattering relaxation time γ. The increase in the number of Te vacancies and the higher sound velocity of Dy (2710 m/s at 20 °C) compared to that of Bi (1790 m/s) considerably contribute to the improvement of the thermoelectric performance of pristine Bi2Te3.

Why the Brillouin Light Scattering Relaxation Times of Molten Zinc Chloride Are Shorter and Weaker in Temperature Dependence than the Structural Relaxation Times from Depolarized Light and Neutron Spin Echo Spectroscopy

A longstanding problem in the Brillouin light scattering (BLS) study of polymers is the relaxation times τBLS(T) being more than an order of magnitude shorter than the α-relaxation times τα(T) determined by dielectric, depolarized light scattering (DLS), and molecular dynamics simulations. In tackling the problem, τBLS(T) was identified with the relaxation time τ0(T) of the primitive relaxation in the coupling model, which can be calculated from τα(T) and the stretch exponent βK of the Kohlrausch correlation function for the α-relaxation.. The problem was solved by finding that indeed τ0(T) is in good agreement with τBLS(T). A recent work performed the neutron spin echo study of the structural α-relaxation of the network ionic liquid ZnCl2 and found the same anomaly as polymers. The α-relaxation time τNSE(T) from neutron spin echo (NSE) as well as the α-relaxation time τDLS(T) from DLS of ZnCl2 are much longer than τBLS(T) from BLS obtained before by several research groups. The finding of the same anomaly in ZnCl2 and polymers with very different chemical and physical structures offers an opportunity to critically test the explanation given before. The test was carried out by calculating the primitive relaxation times τ0,DLS(T) and τ0,NSE(T) from τDLS(T) and τNSE(T), respectively, in zinc chloride. Good agreements of τBLS(T) from BLS with τ0,DLS(T) and τ0,NSE(T) were found and thus the explanation given for polymers remains valid for ZnCl2. The test was extended to glycerol by comparing τBLS(T) with τ0,ICNS(T) and τ0,CNS(T) calculated from the α-relaxation time τICNS(T) and τCNS(T) from incoherent and coherent neutron scattering, respectively. There is good agreement between τBLS(T) and τ0,ICNS(T) in glycerol. There is also semiquantitative agreement of τBLS(T) with τ0,DS(T) from dielectric spectroscopy as well as τ0,CNS(T). Thus, the explanation for polymers is verified in the two very different glass formers, ZnCl2 and glycerol, and it is an advancement in the application of BLS to study the dynamics of glass formers.

Enhancement of the electronic thermoelectric properties of bulk strained silicon-germanium alloys using the scattering relaxation times from first-principles calculations

We use first-principles electronic structure methods to calculate the electronic thermoelectric properties (i.e., due to electronic transport only) of single-crystalline bulk n-type silicon-germanium alloys vs Ge composition, temperature, doping concentration, and strain. We find excellent agreement to available experiments for the resistivity, mobility, and Seebeck coefficient. These results are combined with the experimental lattice thermal conductivity to calculate the thermoelectric figure of merit ZT, finding very good agreement with experiments. We predict that 3% tensile hydrostatic strain enhances the n-type ZT by 50% at carrier concentrations of n=1020cm−3 and a temperature of T=1200K. These enhancements occur at different alloy compositions due to different effects: at 50% Ge composition, the enhancements are achieved by a strain induced decrease in the Lorenz number, while the power factor remains unchanged. These characteristics are important for highly doped and high temperature materials, in which up to 50% of the heat is carried by electrons. At 70% Ge, the increase in ZT is due to a large increase in the electrical conductivity produced by populating the high mobility Γ conduction band valley, lowered in energy by strain.

Thermal conductivity and phonon hydrodynamics in transition metal dichalcogenides from first-principles

We carry out a systematic study of the thermal conductivity of four single-layer transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se) from first-principles by solving the Boltzmann transport equation (BTE). We compare three different theoretical frameworks to solve the BTE beyond the relaxation time approximation (RTA), using the same set of interatomic force constants computed within density functional theory (DFT), finding that the RTA severely underpredicts the thermal conductivity of MS2 materials. Calculations of the different phonon scattering relaxation times of the main collision mechanisms and their corresponding mean free paths (MFP) allow evaluating the expected hydrodynamic behaviour in the heat transport of such monolayers. These calculations indicate that despite of their low thermal conductivity, the present TMDs can exhibit large hydrodynamic effects, being comparable to those of graphene, especially for WSe2 at high temperatures.

Thermo-mechanical properties of boron nitride nanoribbons: A molecular dynamics simulation study

Thermo-mechanical properties of boron nitride nanoribbons (BNNRs) were computed using molecular dynamics simulation with optimized Tersoff empirical potential. Thermal conductivity (TC) and heat transport properties of BNNRs were calculated as functions of both temperature and nanoribbon’s length. The results show that TC of BNNRs decreases with raising temperature by T−1.5 up to 1000K. The phonon–phonon scattering relaxation time, mean free path of phonons, and contribution of high frequency optical phonons in TC of BNNRs were calculated at various temperatures. TC decreases as nanoribbon size increases and it converges to ∼500Wm−1K−1 for nanoribbons with length longer than 30nm. The mechanical properties, including Gruneisen parameter, stress-strain response curves, Young’s modulus, intrinsic strength, critical strain, and poisson’s ratio were calculated in the temperature range of 137–1000K. The simulation results show that Gruneisen parameter and poisson’s ratio of BNNRs are −0.092 and 0.245, respectively. The Young’s modulus of BNNRs decreases with raising temperature and its value is 630GPa at 300K. According to the results, BNNRs duo to their extraordinary thermo-mechanical properties, are the promising candidate for the future nano-device manufacturing.

Thermal conductivity and heat transport properties of nitrogen-doped graphene

In the present study, the thermal conductivity (TC) and heat transport properties of nitrogen doped graphene (N-graphene) were investigated as a function of temperature (107–400K) and N-doped concentration (0.0–7.0%) using equilibrium molecular dynamics simulation based on Green–Kubo method. According to the results, a drastic decline in TC of graphene observed at very low N-doped concentration (0.5 and 1.0%). Substitution of just 1.0% of carbon atoms with nitrogens causes a 77.2, 65.4, 59.2, and 53.7% reduction in TC at 107, 200, 300, and 400K, respectively. The values of TC of N-graphene at different temperatures approach to each other as N-doped concentration increases. The results also indicate that TC of N-graphene is much less sensitive to temperature compared with pristine graphene and the sensitivity decreases as N-doped concentration increases. The phonon–phonon scattering relaxation times and the phonon mean free path of phonons were also calculated. The contribution of high frequency optical phonons for pristine graphene and N-graphene with 7.0% N-doped concentration is 0–2% and 4–8%, respectively. These findings imply that it is potentially feasible to control heat transfer on the nanoscale when designing N-graphene based thermal devices.

Thermal conductivity and heat transport properties of graphene nanoribbons

The graphene nanoribbons (GNRs) are promising candidate for next-generation electronic materials. Herein, the thermal conductivity (TC) and heat transport properties of GNRs were investigated as a function of temperature and nanoribbon size using equilibrium molecular dynamics simulation based on Green–Kubo method. According to the results, by increasing temperature, the TC decreases monotonically by T −1.52. The size dependence of TC was studied using a series of GNRs with 2 nm in width and 2–30 nm in length. The results showed that the TC decreases with longitudinal extension to reach a plateau at ~1,500 W m−1 K−1 based on an exponential curve fitting. The phonon scattering relaxation time, the average group velocity of phonons, and the phonon mean free paths were also calculated. The contribution of high frequency optical phonons to the total TC is negligible (0.3–2.0 %) below 400 K and exponentially increases at higher temperatures. When the quantum corrections for temperature were considered to the calculated results, the maximum value for TC (~5,500 W m−1 K−1) was obtained at 130 K.

Thermal Conductivity of n-Aluminium Nitride-Added MgB2 Superconductor in Normal and Superconducting State

Thermal conductivity (k) of 0, 5, 10, and 15 wt.% aluminium nitride (n-AlN)-added polycrystalline MgB 2 superconductors, synthesized by solid reaction is discussed both in the normal and superconducting states between 20 and 300 K. The prepared samples are characterized using X-ray diffraction (XRD) and field electron gun scanning electron microscope (FEG–SEM). Resistivity measurement confirms a decrease in superconducting transition temperature of MgB 2 (T c=38.5 K) with n-AlN addition and decreases to ∼35 K in case of 15 wt.% n-AlN-added MgB 2 sample. Thermal conductivity of both MgB 2 and n-AlN-added MgB 2 pellets does not show any hump around T c, and the absolute values of k decrease with increasing n-AlN in MgB 2. Temperature dependence of the thermal conductivity of MgB 2 and n-AlN-added MgB 2 has been analyzed, assuming the role of both electrons and phonons. The Wiedemann–Franz law does not work well for the present samples, which indicates inelastic scattering (L eff < L 0). Thermal conductivity of MgB 2 and n-AlN-added MgB 2 pellets is explained by assuming effective Lorentz number, L eff= 0.1 L 0. Electronic thermal conductivity in superconducting state ( $k_{\text {el}}^{\mathrm {s}} )$ follows “two-gap model” and has been used to estimate the values of band gaps, relative contribution of each band in thermal transport, and intraband scattering relaxation time. The estimated values are fairly consistent with the previously reported results for MgB 2. We further confirm that n-AlN addition in MgB 2 introduces disorders in π bands, which reduce the π band gapsand intraband relaxation time ( $\tau _{\pi }^{\text {im}})$ . The lattice contribution of thermal conductivity in both normal and superconducting states is analyzed in the terms of Callaway’s model, assuming various phonon scatterings. Our analysis indicates that the lattice thermal conductivity of MgB 2 is dominated by phonon-sheet-like fault scattering. Addition of n-AlN in MgB 2 enhances the phonon scattering from sheet-like faults, and dislocations induced strain field scattering by >7 times compared to that for pure MgB 2 pellets.

Scaling behavior of the insulator-to-plateau transition in a topological band model

The scaling behavior of the quantum phase transition from an insulator to a quantum Hall plateau state has often been examined within systems realizing Landau levels. We study the topological transition in energy band models with nonzero Chern number, which have the same topological property as a Landau level. We find that the topological band generally realizes the same universality class as the integer quantum Hall system in magnetic field for strong enough disorder scattering. Furthermore, the symmetry of the transition characterized by the relations: $\sigma_{xy}(E)=1-\sigma_{xy}(-E)$ for the Hall conductance and $\sigma_{xx}(E)=\sigma_{xx}(-E)$ for the longitudinal conductance is observed near the transition region. We also establish that the finite temperature dependence of the Hall conductance is determined by the inelastic scattering relaxation time, while the localization exponent $\nu$ remains unchanged by such scattering.

Hall Factor Calculation for the Characterization of Transport Properties in N-Channel 4H-SiC MOSFETs

For the characterization of n-channel 4H-SiC MOSFETs, current-voltage and Hall-effect measurements were carried out at room temperature. To interpret the Hall-effect measurements, the Hall factor for the electron transport in the channel of SiC MOSFETs was evaluated, for the first time. The method of the Hall factor calculation is based on the interdependence with mobility components via the respective scattering relaxation times. The results of the calculation reveal a strong dependence of the Hall factor on the gate voltage. Depending on the gate voltage applied, the values of the Hall factor vary between 1.3 and 1.5. Sheet carrier density and drift mobility values derived from the Hall-effect measurements using our new gate-voltage-dependent Hall factor show very good agreement with simulations performed with Sentaurus Device of Synopsys.

Charge Density Wave Transition in EuAl4

Charge Density Wave Transition in EuAl₄

On the Temperature Dependence of the Hall Factor in n-Channel 4H-SiC MOSFETs

To interpret Hall-effect measurements in a range of temperatures, the Hall factor for the electron transport in the channel of a SiC MOSFETs was evaluated. The method of the Hall factor calculation is based on the interdependence with mobility components via the respective scattering relaxation times. For the first time, the temperature dependence of the Hall factor in n-channel 4H-SiC MOSFETs was calculated. The results of the calculation reveal a strong reduction of the Hall factor with increasing temperature. Depending on gate voltage and temperature, the values of the Hall factor vary between 1.2 and 1.5. In addition, the sheet carrier densities and drift mobilities derived from the Hall-effect measurements using our new temperature-dependent Hall factor show very good agreement with independent simulation results.

PHONON HEAT CONDUCTIVITY OF InSb AND CdS

The lattice thermal conductivity of InSb and CdS has been analyzed on the basis of the most acquiescent Callaway model in the temperature range 2–300.779 K and 2.296–283.565 K. To reinvigorate the effects of phonon anharmonicities, more rigorous expressions for the phonon–phonon interactions, resonance, impurity and interference scattering relaxation times have been introduced to theoretically justify the experimentally observed results. A fairly good agreement between theory and experiments has been presented.

Scattering behaviour of a two-dimensional electron gas induced by Al composition fluctuation in AlxGa1 − xN barriers in AlxGa1−xN/GaN heterostructures

This paper reports that cathodoluminescence (CL) measurements have been done to study the alloy fluctuation of the Al0.3Ga0.7N layer in Al0.3Ga0.7N/GaN heterostructures. The CL images and linescanning results demonstrate the existence of compositional fluctuation of Al in the Al0.3Ga0.7N barrier. A model using a δ-shape perturbation Hamilton function has been proposed to simulate the scattering probability of the two dimensional electron gases (2DEG) induced by Al composition fluctuation. Two factors, including conduction band fluctuation and polarization electric field variation, induced by the Al composition fluctuation have been taken into account. The scattering relaxation time induced by both factors has been estimated to be 0.31 ns and 0.0078 ns, respectively, indicating that the variation of the piezoelectric field is dominant in the scattering of the 2DEG induced by Al fluctuation.

Magnetic-Field Variations of the Pair-Breaking Effects of Superconductivity in (TMTSF)2ClO4

We have studied the onset temperature of the superconductivity Tc_onset of the organic superconductor (TMTSF)2ClO4, by precisely controlling the direction of the magnetic field H. We compare the results of two samples with nearly the same onset temperature but with different scattering relaxation time tau. We revealed a complicated interplay of a variety of pair-breaking effects and mechanisms that overcome these pair-breaking effects. In low fields, the linear temperature dependences of the onset curves in the H-T phase diagrams are governed by the orbital pair-breaking effect. The dips in the in-plane field-angle phi dependence of Tc_onset, which were only observed in the long-tau sample, provides definitive evidence that the field-induced dimensional crossover enhances the superconductivity if the field direction is more than about 19-degrees away from the a axis. In the high-field regime for H//a, the upturn of the onset curve for the long-tau sample indicates a new superconducting state that overcomes the Pauli pair-breaking effect but is easily suppressed by impurity scatterings. The Pauli effect is also overcome for H//b' by a realization of another state for which the maximum of Tc_onset(phi) occurs in a direction different from the crystalline axes. The effect on Tc_onset of tilting the applied field out of the conductive plane suggests that the Pauli effect plays a significant role in determining Tc_onset. The most plausible explanation of these results is that (TMTSF)2ClO4 is a singlet superconductor and exhibits Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states in high fields.

Photoemission from metals and cesiated surfaces

A model of photoemission from coated surfaces is significantly modified by first providing a better account of the electron scattering relaxation time that is used throughout the theory, and second by implementing a distribution function based approach (“Moments”) to the emission probability. The latter allows for the evaluation of the emittance and brightness of the electron beam at the photocathode surface. Differences with the Fowler-Dubridge model are discussed. The impact of the scattering model and the Moments approach on the estimation of quantum efficiency from metal surfaces, either bare or partially covered with cesium, are compared to experiment. The estimation of emittance and brightness is made for typical conditions, and the derivation of their asymptotic limits is given. The adaptation of the models for beam simulation codes is briefly discussed.