Relaxation Time Constant Research Articles

Application of advanced thermal analysis for characterization of crystalline and amorphous phases of carvedilol

The thermal behaviour of crystalline and amorphous carvedilol (CAR) phases was studied by advanced thermal analysis using Quantum Design Physical Property Measurement System and Differential Scanning Calorimetry. Theoretical functions describing crystalline carvedilol heat capacity at low temperatures and the Debye-Einstein function for high temperatures were obtained. Based on the experimental heat capacity values, solid and liquid baselines were established, and the state functions (H, S, G) for solid and liquid states were calculated. A comprehensive characterization of melting and glass transition processes was obtained. CAR is easily amorphizable by cooling the liquid. The residual entropy, which quantifies the extent of frozen-in disorder in the amorphous solid, for glassy CAR was estimated as 51 J·mol–1·K–1. The Kauzmann temperature (TK) was estimated based on enthalpy and entropy. Molecular motions in the amorphous phase were also studied. The activation energy for structural relaxation (Ea = 539 kJ·mol–1) and fragility parameter (m = 91) were obtained from the non-isothermal physical ageing. The isothermal physical ageing kinetics of amorphous CAR was studied by applying Kohlrausch-Williams-Watts (KWW) model. The mean molecular relaxation time constant (τKWW = 117 min) and relaxation constant (βKWW = 0.33) were obtained. CAR was classified as a fragile glass-former. Furthermore, τKWW constant for samples aged at 303.15 K is very low, thus, the physical ageing will occur during the short- and long-term storage of amorphous CAR, potentially changing its physicochemical properties during the ageing process. However, the results of molecular mobility studies (high molecular motions) show that the relationship between molecular motions in a glassy solid and its tendency to crystallization does not seem to follow an expected pattern, i.e., no crystallization occurred by thermal treatment of glassy, supercooled liquid and liquid phases of CAR as one would expect. Modern calorimetry and quantitative thermal analysis provided the fundamental kinetic and thermodynamic information about the crystalline and amorphous states of CAR.

Step Activity Monitoring in Boys with Duchenne Muscular Dystrophy and its Correlation with Magnetic Resonance Measures and Functional Performance.

Muscles of boys with Duchenne muscular dystrophy (DMD) are progressively replaced by fatty fibrous tissues, and weakness leads to loss of ambulation (LoA). Step activity (SA) monitoring is a quantitative measure of real-world ambulatory function. The relationship between quality of muscle health and SA is unknown in DMD. To determine SA in steroid treated boys with DMD across various age groups, and to evaluate the association of SA with quality of muscle health and ambulatory function. Quality of muscle health was measured by magnetic resonance (MR) imaging transverse magnetization relaxation time constant (MRI-T2) and MR spectroscopy fat fraction (MRS-FF). SA was assessed via accelerometry, and functional abilities were assessed through clinical walking tests. Correlations between SA, MR, and functional measures were determined. A threshold value of SA was determined to predict the future LoA. The greatest reduction in SA was observed in the 9- < 11years age group. SA correlated with all functional and MR measures.10m walk/run test had the highest correlation with SA. An increase in muscle MRI-T2 and MRS-FF was associated with a decline in SA. Two years prior to LoA, SA in boys with DMD was 32% lower than age matched boys with DMD who maintained ambulation for more than two-year period. SA monitoring can predict subsequent LoA in Duchenne, as a daily step count of 3200 at baseline was associated with LoA over the next two-years. SA monitoring is a feasible and accessible tool to measure functional capacity in the real-world environment.

Stress relaxation behavior of left ventricular myocardium in mice

BackgroundMyocardial tissues are known to exhibit viscoelastic behavior, i.e., the myocardial wall stress is gradually decaying when stretched at a constant level, referred to as stress relaxation behavior. The study of this behavior becomes important in structural heart diseases, where both passive and active relaxation behaviors of the myocardium may alter. As myocardial tissues display a complex, 3D micro‐architecture, the anisotropy of stress relaxation remains understudied. Our objective in this study was to investigate the anisotropy of (passive) stress relaxation behavior of the left ventricular (LV) myocardium in healthy small animals.MethodsWe used biaxial stress relaxation tests in wild‐type murine specimens (n=2). Full‐thickness myocardium LV free wall (LVFW) specimens were isolated from the mouse hearts (Fig. 1a), with the slab edges being aligned with the longitudinal (apex‐to‐outflow‐tract), circumferential, and radial directions of the LV. The specimens were mounted in a biaxial mechanical testing machine along the circumferential and longitudinal directions. The specimens were stretched in three different loading protocols with circumferential‐to‐longitudinal strain ratios of 30:30, 15:30, and 30:15%. The samples were loaded to the peak strain at a rate of 0.01 s‐1 and held at this stretch for 10 minutes to allow the relaxation. The time‐varying stress was recorded. We used an exponential decay fit to estimate the stress relaxation time constants in both directions.ResultsStress relaxation time constants showed noticeable differences between multiple loading ratios with equibiaxial loading (30:30) having the largest relaxation time constant followed by the circumferential stress in the 30:15 ratio (Fig. 1b). The larger strains led to larger stress relaxation constants (Fig. 1b‐c) indicating a strain‐dependent viscoelastic behavior. The quasi‐static equibiaxial loading of the same specimens resulted in a nearly isotropic behavior (not shown here), exhibiting insignificant contrast between the stresses in the circumferential and longitudinal directions. Interestingly, the contrast in the relaxation behavior between the circumferential and longitudinal directions were similarly insignificant (Fig. 1b). Non‐equibiaxial loadings (15:30 and 13:50) showed significantly faster relaxation for the direction with smaller strains (Fig. 1b).ConclusionsMyocardial tissue viscoelasticity is an important behavior contributing to both diastolic and systolic cardiac function, which remains understudied, especially in case of structural heart diseases associated with impaired relaxation in the LV. Our study suggests that stress relaxation behavior is prominent in the LV myocardium and may exhibit different relaxation behavior depending on strain magnitudes and strain ratios along different directions. The level of anisotropy in stress relaxation was found to be similar to the level of anisotropy in the tissue quasi‐static behavior. These results serve to motivate further studies to fully characterize the viscoelastic behavior of LV myocardium and its contribution to the LV relaxation and filling in murine models of LV diseases.

Manipulation of Left Ventricular Loading Conditions is Necessary to Identify Diastolic Dysfunction in Swine with Repetitive Pressure Overload‐Induced HFpEF

ObjectiveDiagnosis of heart failure with preserved ejection fraction (HFpEF) is often complicated by the fact that many patients exhibit normal left ventricular (LV) mechanics at rest with a reduction in diastolic compliance that is only revealed when loading conditions are altered (e.g., during exercise stress). In an effort to identify parameters of diastolic function that could be utilized to detect alterations in compliance without manipulation of LV loading conditions, we utilized invasive hemodynamic analysis and speckle‐tracking echocardiography (STE) to assess diastolic strain and stiffness in swine with repetitive pressure overload (RPO)‐induced HFpEF.MethodsSwine (n=13) were subjected to 2‐weeks of RPO via daily intravenous administration of phenylephrine (PE; 400 ug/min; 1 hour/day) through an indwelling jugular vein catheter. Indices of LV relaxation as well as LV minimum and end‐diastolic pressure were obtained invasively before and after RPO at rest and during transient PE‐induced alterations in loading conditions. STE (Vivid 9; GE) was also performed throughout a single cardiac cycle to collect circumferential diastolic strain data, which were used alongside echocardiography‐derived measurements of chamber radius and wall thickness as well as catheter‐derived measurements of LV pressure to calculate regional and global circumferential subendocardial diastolic stiffness before and after RPO.ResultsCompared with baseline measurements, 2‐weeks of RPO did not significantly increase the time constant of LV relaxation (Tau; Figure A) or decrease the rate of LV pressure change during isovolumic relaxation (dP/dtmin; Figure B). Circumferential subendocardial stiffness derived via STE across a single cardiac cycle at rest was also unaffected by RPO (Figure C), despite a significant reduction in diastolic compliance that was revealed by construction of two‐point end‐diastolic pressure‐volume curves (Figure D) after manipulation of loading conditions (Figure E). Interestingly, single‐cycle STE‐derived stiffness measurements made during acute pressure overload were also similar at baseline and after RPO due to persistent elevations in LV minimum diastolic pressure during PE infusion at both timepoints.ConclusionsAlthough reductions in LV diastolic compliance are evident after chronic RPO when loading conditions are manipulated, abnormalities in LV mechanics are not exhibited at rest even when STE is used to assess diastolic stiffness throughout a single cardiac cycle. These findings highlight the challenges associated with identification of myocardial stiffening at rest in patients with HFpEF and reinforce the importance of studying LV mechanics under variable loading conditions.

Elastic, electronic, optical and thermoelectric properties of the novel Zintl-phase Ba2ZnP2

We report and discuss the results of a detailed first-principles calculations of the structural, elastic, electronic, optical and thermoelectric properties of the new Zintl phase dibarium zinc diphosphide Ba2ZnP2. The calculated structural parameters using the GGA-PBEsol functional are in excellent agreement with the available experimental counterparts. From the monocrystalline elastic constants numerically estimated through the stress-strain technique, a set of related properties, viz., mechanical stability, elastic anisotropy, brittle/ductile character, anisotropic sound velocities, polycrystalline elastic moduli, including isotropic bulk modulus, shear modulus, Young's modulus, Poisson's ratio, average sound velocity and Debye temperature, are deduced. The electronic and optical properties are investigated through the state-of-the art FP-(L)APW + lo method with the accurate TB-mBJ potential. Ba2ZnP2 is an indirect semiconductor with a gap of 1.24 eV. The charge-carrier effective masses are calculated. The valence band maximum is less dispersive than the conduction band minimum. The microscopic origins of the electronic states composing the energy bands are determined via the PDOS diagrams. Topological analysis of the charge density shows that a covalent character is dominantly ruling the Zn–P bond inside the block ZnP4, while an ionic bonding is mainly ruling the bond between the cation Ba and the polyanion ZnP4. Frequency-dependent macroscopic linear optical functions are predicted in a wide energy range 0–30 eV. Within the visible spectra, the calculated magnitude of the absorption coefficient, reflectivity and refractive index are in the ranges ∼4−35×104cm−1, 29−36% and 3.18−3.47, respectively. The semi-classical Boltzmann transport theory within the constant relaxation time approximation is used to study the thermoelectric properties. The title compound has a figure of merit of ∼1.77 at 300 K, which makes it a potential candidate for thermoelectric applications.

First Principles Study of Janus ZrClBr Monolayer Under Biaxial Strain

We have explored the structural, electronic, and thermoelectric properties of ZrClBr monolayer using density functional theory and semi-classical Boltzmann theory. The effect of biaxial compressive and tensile strains have been studied and compared. The band gap becomes narrow (broader) with compressive (tensile) strain as compared to that of unstrained ZrClBr monolayer. The electronic band structure is sensitive to the applied strain. The total and partial density of states have been explored. The ZrClBr monolayer is dynamically stable under the applied strain. The electronic transport coefficients are evaluated using BoltzTrap2 code under the constant relaxation time approximation. The Seebeck coefficient, electrical conductivity, and the power factor have been evaluated under both compressive and tensile at the different carrier concentrations of electrons and holes. The results show that the thermoelectric properties of ZrClBr monolayer can be tuned with the application of strain.

Effect of doping titanium ions on semi‐conducting behavior, photovoltaic, and thermoelectric perovskite‐type oxides VSc 1−x Ti x O 3 : Ab‐inito study

Doping titanium's ions on perovskite-type oxides as VTixSc1−xO3; changes the semi-conducting behavior as well as magnetic, optical, and thermoelectric properties of VScO3 pervoskites. Doping titanium ions with different concentrations (x = 0.25, 0.5, 0.75, and 1) is investigated through the density functional theory (DFT) by the full potential linearized augmented plane wave (FP-LAPW) technique using GGA-mBJ potentials. All doped structures show semiconductor behavior in spin down channel, while the original structure VScO3 shows the same behavior in spin up channel. The values of energy band gap changes according the doped ions concentration Ti. The total spin magnetic moment increases as the concentration of Ti increases; the optical properties show a remarkable change during doping processes. The requirements for elastic-stability of crystal lattices are applied. All studied structures are mechanically stable, high value of the temperature of melting (Tm), and Debye temperature for VSc1−xTixO3 compounds is higher than 400°K, besides the high values of Young's modulus and bulk modulus all are implying the hardness of the perovskite. Calculated energy band structure by WIEN2k is utilized to calculate the transport coefficients by using the semi-classical Boltzmann transport theory depends on the constant relaxation time (τ) employed in the BoltzTraP package. Thermoelectric properties for all compounds are calculated and indicated that these compounds may be used in thermoelectric applications.

First-principles calculations for fundamental and spectroscopic screening of hybrid perovskite (HC(NH2)2PbI3) formamidinium lead iodide

Formamidinium lead iodide [(HC(NH2)2PbI3] material has generated significant interest in their inherent properties and extensive scope of prospective applications. Here, we have successfully computed the structural, electronic, optical, thermal, thermoelectric and elastic properties with the framework of density functional theory over the PBE-sol and WC potential approximations using the WIEN2K code. These explorations show that this material is a direct bandgap semiconductor (1.30 eV), with high absorption coefficients and better thermal stability. The dielectric function and absorption spectra peaks reveal in the IR-visible-UV energy region show the significant future of this material to be used in photovoltaic (PV) devices. Thermal properties have been calculated by GIBBS2 at different temperatures (0–800 K) and pressures (0–8 GPa). Boltztrap with constant relaxation time approximation has been used for computing the thermoelectric parameters at different temperatures (300–800 K). Seebeck coefficient decreases with increasing temperature, electrical conductivity nearly constant in the whole temperature ranges, electrical thermal conductivity increases with increasing temperature, power factor and figure of merit's are also increasing with increasing temperature at a given electron and hole concentration (1018-1019 cm−3). We have also carried out the calculations of spectroscopic limited maximum efficiency (SLME) parameter (39.09%). Moreover, for mechanical stability, we have determined the elastic properties (C11, C12, C44), bulk modulus (B), Young's modulus (Y), shear modulus (G) and stability constants B/G ratio, and Poisson's ratio (ν). These computed results show that the FAPbI3 is a relevant material to be used as an alert substance in photovoltaic device fabrications and proposed outcomes are in good acceptance with the experimental and other theoretical data.

Calculation of thermomagnetic properties using first-principles density functional theory

The BoltzWann code uses the first-principles density-functional theory to calculate the material’s response to an electric field and temperature gradient within the constant relaxation time approximation. We extend this code to include the response of the system to a magnetic field. The carrier dynamic is described by the semi-classical Boltzmann transport equation. This equation is solved in the presence of an external magnetic field within the constant relaxation time approximation and using the Jones–Zener expansion. This is done through a Wannier interpolation of the density functional theory bands using the Boltzwann code, followed by the computation of group velocities and effective masses leading to the energy-dependent transport function. This work leads to a generalized method for the calculation of thermomagnetic properties of materials. The results are validated by comparison to the analytical solutions of the thermomagnetic properties for several simple energy dispersion types and close agreements at moderate computational costs are achieved. This method can pave the path for the discovery of materials with potentially high thermomagnetic power factor.

First-principles scattering with Büttiker probes: The role of self-energies

Understanding electronic transport properties is important for designing devices for applications. Many studies rely on the semiclassical Boltzmann approach within the relaxation time approximation. This method delivers a graphic physical picture of the scattering process, but in some cases it lacks full quantum-mechanical effects. Here, we use a non-equilibrium Green's function Korringa-Kohn-Rostoker (KKR) method with phase-breaking scattering via virtual B\"uttiker terminals as a fully quantum mechanical approach to transport phenomena. With this, we assess the validity of the relation of the self-energy $\mathrm{\ensuremath{\Sigma}}$ to the scattering time $\ensuremath{\tau}$, often used in literature in the case of constant relaxation time approximation. We argue that the scattering time does not affect the thermopower in the Boltzmann approach and thus should take no effect either on the thermopower calculated via the Keldysh approach. We find a nearly linear relation for the transmission function ${T}_{S}({E}_{F},\mathrm{\ensuremath{\Sigma}})$ of free electrons and Cu with respect to $\frac{1}{\mathrm{\ensuremath{\Sigma}}}$. However, we find that this is not the case for Pd. We attribute this to neighboring states contributing due to the additional broadening via the self-energy $\mathrm{\ensuremath{\Sigma}}$. These findings suggest that a simple identification of scattering time and self-energy is not sufficient. Finally, we discuss the benefits and limits of the application of the virtual terminal approach.

Nonlinear optical properties and passively Q-switched laser application of a layered molybdenum carbide at 639 nm.

Molybdenum carbide (Mo2C) exhibits enormous potential applications in various optoelectronic and photonic fields due to its remarkably electrical and optical characteristics. Here, we fabricate a high-quality Mo2C film by the radio frequency magnetron sputtering deposition method. The nonlinear optical response and ultrafast dynamics are thoroughly studied based on open-aperture Z-scan and nondegenerate pump-probe experimental measurements. The open-aperture Z-scan experimental result exhibits a modulation depth of 8.5% and a saturation fluence of 0.28 mJ/cm2. Simultaneously, the relaxation time constant is fitted by a biexponential decay function, showing an ultrafast intraband carrier recovery time of 0.58 ps at 530 nm. Consequently, by employing the Mo2C film as a saturable absorber (SA), stable Q-switched Pr:YLF laser pulses with the shortest pulse width of 160 ns are generated at 639 nm. Our experimental results demonstrate excellent nonlinear optical properties of the layered Mo2C in the visible region and will further advance their potential applications in visible nonlinear optics.

Understanding Complex Interactions in Pediatric Diastolic Function Assessment

Diagnosing left ventricular diastolic dysfunction (DD) noninvasively in children is difficult as no validated pediatric diagnostic algorithm is available. The aim of this study is to explore the use of machine learning to develop a model that uses echocardiographic measurements to explain patterns in invasively measured markers of DD in children. Children at risk for developing DD were enrolled, including patients with Kawasaki disease, heart transplantation, aortic stenosis, and coarctation of the aorta when undergoing clinical left heart catheterization. Simultaneous invasive pressure measurements were made using a high-fidelity catheter (time constant of isovolumic relaxation [Tau, τ], left ventricular end-diastolic pressure, and maximum negative rate of pressure change) and echocardiographic DD measurements. Spearman correlations were performed for each echocardiographic feature with invasive markers to understand pairwise relationships. Separate random forest (RF) models were implemented to assess all echocardiographic features, key demographic data, and clinical diagnosis in predicting invasive markers. A backward stepwise regression model was simultaneously implemented as a comparative conventional reference model. The relative importance of all parameters was ranked in terms of accuracy reduction. Model approximation was then performed using a regression tree with the top-ranked features of each RF model to improve model interpretability. Regression coefficients of the linear models were presented. Fifty-nine children were included. Spearman correlations were generally low. The RF models' performance measures were noninferior to those of the linear model. However, the linear model's regression coefficients were unintuitive. The highest ranked important features for the RF models were propagation velocity for Tau, E/propagation velocity ratio for left ventricular end-diastolic pressure, and systolic global longitudinal strain rate for maximum negative rate of pressure change. Estimating individual components of DD can potentially improve the noninvasive assessment of pediatric DD. Although pairwise correlations measured were weak and linear regression coefficients unintuitive,approximated machine learning models aided in understanding how echocardiographic and invasive parameters of DD are related. This machine learning approach could help in further development of pediatric-specific diagnostic algorithms.

Relaxation time approximations in PAOFLOW 2.0

Regardless of its success, the constant relaxation time approximation has limited validity. Temperature and energy dependent effects are important to match experimental trends even in simple situations. We present the implementation of relaxation time approximation models in the calculation of Boltzmann transport in PAOFLOW 2.0 and apply those to model band-structures. In addition, using a self-consistent fitting of the model parameters to experimental conductivity data, we provide a flexible tool to extract scattering rates with high accuracy. We illustrate the approximations using simple models and then apply the method to GaAs, Si, hbox {Mg}_3hbox {Sb}_2, and hbox {CoSb}_3.

Dielectric Dispersion Response of Binary Mixtures of n-Butanol and Valeronitrile

Complex permittivity spectra of various concentrations (0.0 → 1.0) of n-Butanol and its mixtures with Valeronitrile were obtained in the radio and microwave frequency range (0.2 GHz to 20 GHz) using Vector network analyzer (VNA). Complex permittivity data points were fitted into Havriliak-Negami Model. CNLS fitting using LEVMW software used to obtain dielectric parameters. Excess static dielectric constant and excess inverse relaxation time for the binary mixtures were calculated and fitted in Redlich-Kister polynomial to derive the binary coefficients and standard deviations. Variation of these parameters were discussed in terms molecular interaction between molecular species.

High frequency impedance measurements of sodium solid electrolytes

Solid-state sodium batteries are currently gaining enormous interest as a lower-cost and more environmentally friendly alternative to lithium batteries. They contain significantly less critical rare elements in both the electrolyte and the active material. However, to date, there is no efficient material combination of metallic anode, cathode, and solid electrolyte for room temperature applications that does not require an additional liquid electrolyte while maintaining high energy density. NaPSiO-based glass-ceramics show high ionic conductivity at room temperature, good corrosion resistance against ambient humidity and CO2, and stability against metallic sodium. However, the conductivity mechanisms of this promising class of materials are currently poorly understood. Herein, high frequency impedance measurements up to 108 Hz shed light on the contributions of grains and grain boundaries to the total impedance, including the distribution of relaxation time constants of the fully crystallized material. In addition, analysis of the temperature dependence allows separation of electrode contributions and determination of activation energies for grain and grain boundary conductivities. Our study provides the basis for fine-tuning the stoichiometry of NaPSiO-based glass-ceramics in terms of maximizing the conductive phase fraction to optimize the performance of future solid-state sodium batteries.

In vivo measurements of lamination patterns in the human cortex.

The laminar composition of the cerebral cortex is tightly connected to the development and connectivity of the brain, as well as to function and pathology. Although most of the research on the cortical layers is done with the aid of ex vivo histology, there have been recent attempts to use magnetic resonance imaging (MRI) with potential in vivo applications. However, the high‐resolution MRI technology and protocols required for such studies are neither common nor practical. In this article, we present a clinically feasible method for assessing the laminar properties of the human cortex using standard pulse sequence available on any common MRI scanner. Using a series of low‐resolution inversion recovery (IR) MRI scans allows us to calculate multiple T1 relaxation time constants for each voxel. Based on the whole‐brain T1‐distribution, we identify six different gray matter T1 populations and their variation across the cortex. Based on this, we show age‐related differences in these population and demonstrate that this method is able to capture the difference in laminar composition across varying brain areas. We also provide comparison to ex vivo high‐resolution MRI scans. We show that this method is feasible for the estimation of layer variability across large population cohorts, which can lead to research into the links between the cortical layers and function, behavior and pathologies that was heretofore unexplorable.

Long term development of diastolic dysfunction and heart failure with preserved left ventricular ejection fraction in heart transplant recipients

Heart transplant recipients (HTX) have several risk factors for heart failure which can trigger pro-inflammatory and fibrosis factors and set into motion pathophysiologic changes leading to diastolic dysfunction and HFpEF. The objective of the study was to determine if HTX recipients with dyspnea have diastolic dysfunction and HFpEF. Twenty-five HTX were included. LV systolic and diastolic functions were evaluated using conductance catheters to obtain pressure volume loops. LV function was assessed at rest and during moderate intensity exercise of the upper extremities. A significant increase occurred in LV minimal pressure (3.7 ± 3.3 to 6.5 ± 3.5 mmHg) and end diastolic pressure or EDP (11.5 ± 4 to 18 ± 3.8 mmHg, both P < 0.01) with exercise. With exercise, the time constant of LV relaxation shortened in 2, was unchanged in 3, and increased in the remaining patients (group results: rest 40 ± 11.6 vs 46 ± 9 ms, P < 0.01). LV chamber stiffness constant was abnormally increased in all but 2 patients. Indices of LV systolic properties were normal at rest but failed to augment with exercise. In 15 who agreed to blood draw, inflammation and fibrosis markers were obtained. A significant association was observed between LV EDP and Pro-Col III N-terminal (r = 0.58, P = 0.024) and IL-1-soluble receptor (r = 0.59, P = 0.02) levels. HTX have diastolic dysfunction and can develop HFpEF several years after cardiac transplantation. The abnormally increased LV chamber stiffness and the prolongation or lack of shortening of the time constant of LV relaxation with exercise are the underlying reasons behind the observed changes in LV diastolic pressures with exercise.

Ultrafast dynamical charge-lattice coupling in rare-earth nickelate thin films studied by time-resolved terahertz spectroscopy

Rare-earth nickelates exhibit a rich phase diagram formed by the complex interplay of intertwined and competing energetics of fundamental entities. To unwind the coupling and interaction mechanisms of fundamental entities underneath, time-resolved terahertz (THz) spectroscopy was implemented to understand non-equilibrium carrier and lattice dynamics of epitaxial thin films of La x Eu1−x NiO3 (x = 0, 0.25, 0.50, 1) systems, where x = 0 is insulating while remaining are metallic at room temperature. The THz transmittance of the insulating and metallic phases exhibit contrasting photo-induced phases associated with bi-exponential and mono-exponential relaxation mechanisms, respectively. A pronounced oscillatory feature superimposed on the mono-exponential relaxation manifests only in the metallic phase. As ascribed to the acoustic phonons, the ‘x’ dependent behavior of this feature reveals an inverse relation between the strength of electron–phonon coupling and the magnitude of conductivity. In the insulating state, in contrast, the relaxation time constants are associated with the recovery of charge-ordering and electron–phonon thermalization. This dynamical lattice–charge interaction study demonstrates use of this ultrafast phenomena in nickelate thin films in new generation ultrafast photo-acoustic devices as an alternate to conventional surface acoustic wave device.

Thermal Effects on Initial Volatile Response and Relaxation Dynamics of Resistive RAM Devices

Resistive RAM (RRAM) or memristors are a class of electronic device whose resistance depends on voltage history. The changes in resistance can be divided into two categories, volatile and non-volatile. To date, the characteristics of non-volatile switching have been explored extensively with volatile switching behaviour still remaining more obscure. Here we investigate the temperature effects on TiOx based memristor volatility, and integrate these observations into a previously developed model for volatile switching. We show how device temperature affects the magnitude of the volatile resistive state in response to input stimulation, as well as the corresponding relaxation time constant. Importantly, these effects are polarity dependent. This work is part of an effort towards building a more comprehensive model of RRAM behaviour covering volatile and non-volatile phenomena as well as various environmental effects on them.

The nonlinear optical properties of Zirconium pentatelluride and its application in ultrafast solid-state lasers

The topological Dirac semimetal Zirconium pentatelluride (ZrTe5) has many special properties such as anomalous Hall effect and thermoelectric effect. A fast relaxation time constant of 250 fs indicates its great application potential in ultrafast optics, which has been investigated in a Er-doped passively mode-locking fiber laser. In this paper, the ZrTe5 was adopted as a saturable absorber (SA) to modulate Nd-doped solid-state lasers for ultrafast pulse generation. The nonlinear absorption coefficient of ZrTe5 was measured to be −0.81 cm/GW by Z-scan technique. A pulse width of 18.4 ps with a repetition frequency of 69 MHz was gained in the 1.06 μm passively mode-locking laser. For the 1.34 μm ultrafast laser, a stable pulse sequence with a pulse duration of 17.7 ps was delivered and the corresponding repetition frequency was 71 MHz. Those results revealed that ZrTe5 has an excellent laser modulation performance in ultrafast solid-state lasers. As far as we know, it is the first time the solid-state passively mode-locking laser operation by using ZrTe5 as a SA has been successfully demonstrated.