Nonmonotonic Temperature Dependence Research Articles

The relationship between equations of state for small fluctuation-disperse systems and effect of gravity in macro-systems under the critical condition

The fluctuation-dispersed structure of small limited systems in critical condition is proposed on the basis of macro-systems phase transitions fluctuation theory. A distinctive feature of this structure is the equality of the number of order parameter fluctuations Nf(s) in the surface layer s = 4πl2 with the thiсkness l and the number of order parameter fluctuations Nf(v) in volume v = 4/3⋅π(l−2Rc)3. Based on this equation the linear size of a small spherical system have been evaluated, the relationship between the thermodynamic parameters has been determined – the temperature and the chemical potential with the linear size of the system which are similar to relations of M. Fisher t~1−1/ν, μ~l−1/ξ. Based on the quality similarities of the non-monotonic temperature dependences of various properties of macroinhomogeneous systems in the Earth gravity field and the non-monotonic temperature dependences of small confined systems properties under critical condition the equation of state has been proposed for small confined systems. This equation has been tested on the basis of experimental data from various properties of small confined systems.

Magnetoelectric features in the magnetic hysteresis of modified multiferroic BiFeO3: Release of latent magnetization induced by cationic modification

BiFeO3 is a well-known room temperature multiferroic material. However, the presence of antiferromagnetic magnetic order and high leakage current limit its utilization in magnetoelectric devices. This study deals with conclusive identification of the presence of magnetoelectric coupling by carefully selecting higher bond strength cationic substitution and using contactless measurement. Complementary to the magnetic studies, detailed structural analyses are carried out to determine the B-site bond length, B-site bond angle, ionic size mismatch and electronic structure of the substituent. The structural modification due to cationic substitution leads to release of latent magnetization that is locked within the cycloid. Non-monotonic temperature dependence of magnetic coercivity is observed and the results are confirmed for various compositions. These results are explained using Ginzburg-Landau Mean field model. Crossover from anomalous to usual trend of magnetic coercivity as a function of temperature is presented covering the temperature regime beyond Neel temperature of BiFeO3. Results are explained in terms of effective anisotropy which includes contribution associated with the electric polarization. This proves the existence of magnetoelectric effect. Magnetic switching is shown to be affected by the presence of ferroelectric order. The inclusion of normal ferromagnetic response in the high temperature regime makes this a unique study. The importance of this study lies in the fact that such magnetoelectric characteristic cannot be explored through the conventional electrical response measurements as the presence of high leakage current in BFO makes such measurements extremely difficult.

Effect of spin-triplet correlations on Josephson transport in atomically thin superconductor/half-metal/superconductor structures

We study the effect of the triplet proximity effect on Josephson transport in superconductor/half-metal/superconductor structures of atomic thickness beyond the quasiclassical approximation. Using the combination of microscopic ${\mathrm{Gor}}^{\ensuremath{'}}\mathrm{kov}$ formalism and tight-binding model we show that the full spin polarization inside the half-metal give rise to nonmonotonic temperature dependence of the critical current in S/HM/S Josephson junction with two spin active S/HM interfaces. We also calculate the magnetic moment inside the ${\mathrm{S}}_{2}$ layer in ${\mathrm{S}}_{1}/\mathrm{HM}/{\mathrm{S}}_{2}$ structure with spin-active ${\mathrm{S}}_{1}/\mathrm{HM}$ interface, which is induced by the spin-triplet superconducting correlations. Finally, we analyze the second-harmonic generation in ${\mathrm{S}}_{1}/\mathrm{HM}/{\mathrm{S}}_{2}$ structure with one spin-active interface.

Temperature effects in spin-dependent Hall currents in an ideal skyrmion gas

The quest for novel reliable and fast-performing logic and memory elements in classical and quantum computing requires discoveries of new effects in novel quantum materials. Skyrmions are such magnetic textures where electron scattering can essentially change the shape and location of the skyrmion, which can be used as a memory element being more efficient than domain walls as a memory device in classical computing. Because the skyrmion motion is sensitive even to very small currents, we study electron scattering by skyrmions in an ideal skyrmion gas in a ferromagnetic environment. In such systems, the direct and Hall currents become spin-dependent. For applications, it is important to consider a Hall effect in the whole range of temperatures under the assumption of the skyrmion existence. In this study, we find the nonmonotonic temperature dependence of the direct spin-up conductivity, i.e., the conductivity for the current directed along the applied electric field due to the electrons with the spin parallel to the ferromagnetic moment. Such a behavior contradicts the traditional understanding where the temperature only increases the value of the conductivity. The spin-down Hall conductivity is found to be even more dramatic exhibiting a conductivity sign change (i.e., a change in the current direction) with temperature for small skyrmion sizes. The found effects strongly depend on Fermi energy. The most pronounced dependencies take place if the Fermi energy is slightly below and above the bottom of the upper (spin-down) energy band of an ideal two-dimensional electron gas. In addition, we also find that the direct and Hall resistivities, ${\ensuremath{\rho}}_{xx}$ and ${\ensuremath{\rho}}_{xy}$, are independent of the exchange integral $J$ for $2J>{\ensuremath{\varepsilon}}_{F}$.

Universal Nucleation Behavior of Sheared Systems.

Using molecular simulations and a modified classical nucleation theory, we study the nucleation, under flow, of a variety of liquids: different water models, Lennard-Jones, and hard sphere colloids. Our approach enables us to analyze a wide range of shear rates inaccessible to brute-force simulations. Our results reveal that the variation of the nucleation rate with shear is universal. A simplified version of the theory successfully captures the nonmonotonic temperature dependence of the nucleation behavior, which is shown to originate from the violation of the Stokes-Einstein relation.

The Metastable Mpemba Effect Corresponds to a Non-monotonic Temperature Dependence of Extractable Work

The Mpemba effect refers to systems whose thermal relaxation time is a non-monotonic function of the initial temperature. Thus, a system that is initially hot cools to a bath temperature more quickly than the same system, initially warm. In the special case where the system dynamics can be described by a double-well potential with metastable and stable states, dynamics occurs in two stages: a fast relaxation to local equilibrium followed by a slow equilibration of populations in each coarse-grained state. We have recently observed the Mpemba effect experimentally in such a setting, for a colloidal particle immersed in water. Here, we show that this metastable Mpemba effect arises from a non-monotonic temperature dependence of the maximum amount of work that can be extracted from the local-equilibrium state at the end of Stage 1.

Abnormal Seebeck effect in doped conducting polymers

The Seebeck effect or thermopower relates the temperature gradient to the electric voltage drop. Seebeck coefficient α measures the transport entropy, which could either linearly increase with temperature T like metallic conducting or decrease as 1/T like semiconducting behavior. It could become more complicated in the temperature dependence for a number of disordered systems but still in a monotonic way. However, several recent experiments reported the “abnormal” non-monotonic temperature dependence of the Seebeck coefficient in doped conducting polymers, for instance, first increasing and then decreasing. Through a one-dimensional tight-binding model coupled with the Boltzmann transport equation, we investigate theoretically the doping effect for the Seebeck coefficient. We find that the abnormal behavior comes from multi bands' contribution and a two-band model (conduction or valence band plus a narrow polaronic band) can address such an abnormal Seebeck effect, namely, if there exists (i) a small bandgap accessible for thermal activation between the two bands; and (ii) a large difference in the bandwidth between the polaronic band and the conduction band (or valence band), then the Seebeck coefficient increases with temperature first, then levels off, and finally drops down.

Pressure-induced phase transition in the J1−J2 square lattice antiferromagnet RbMoOPO4Cl

We report results of magnetization and $^{31}$P NMR measurements under high pressure up to 6.4~GPa on RbMoOPO$_4$Cl, which is a frustrated square-lattice antiferromagnet with competing nearest-neighbor and next-nearest-neighbor interactions. Anomalies in the pressure dependences of the NMR shift and the transferred hyperfine coupling constants indicate a structural phase transition at 2.6~GPa, which is likely to break mirror symmetry and triggers significant change of the exchange interactions. In fact, the NMR spectra in magnetically ordered states reveal a change from the columnar antiferromagnetic (CAF) order below 3.3~GPa to the N\'{e}el antiferromagnetic (NAF) order above 3.9~GPa. The spin lattice relaxation rate $1/T_1$ also indicates a change of dominant magnetic fluctuations from CAF-type to NAF-type with pressure. Although the NMR spectra in the intermediate pressure region between 3.3 and 3.9 GPa show coexistence of the CAF and NAF phases, a certain component of $1/T_1$ shows paramagnetic behavior with persistent spin fluctuations, leaving possibility for a quantum disordered phase. The easy-plane anisotropy of spin fluctuations with unusual nonmonotonic temperature dependence at ambient pressure gets reversed to the Ising anisotropy at high pressures. This unexpected anisotropic behavior for a spin 1/2 system may be ascribed to the strong spin-orbit coupling of Mo-4$d$ electrons.

Atomistic simulation of the generation of vacancies in rapid crystallization of metals

The generation of vacancies at a crystal-liquid interface propagating under conditions of undercooling below the equilibrium melting temperature is investigated in molecular dynamics simulations performed under well-controlled temperature and pressure conditions for two representative metals, bcc Cr and fcc Ni. The results of the simulations reveal that, for both metals, vacancy concentrations produced in the course of the steady-state propagation of crystallization fronts can exceed the equilibrium values at the corresponding temperatures by orders of magnitude. Different trends in the temperature dependences of vacancy concentration observed for the two metals, namely, the continuous increase with increasing undercooling in Ni and a nonmonotonous temperature dependence in Cr, are related to the qualitatively different temperature dependences of the crystallization front velocity predicted for the two metals. The general character of the computational predictions is confirmed in simulations performed for Ni with four different interatomic potentials and for both metals with (001), (011), and (111) interface orientations. A detailed analysis of atomic rearrangements at the crystallization front suggests that the level of vacancy supersaturation is largely defined by the ability of atoms to migrate within the interfacial region and to fill the numerous vacant sites produced through the simultaneous construction of several atomic crystal planes within the interfacial region. While the majority of the transient vacancies generated during the construction of the crystal planes are annihilated by the atomic flux coming from the liquid side of the interface, a small fraction of the vacancies are trapped behind the crystallization front. Under conditions of strong undercooling, the fast movement of the interface and decreased vacancy mobility prevent the equilibration of vacancy concentration in the newly built crystalline region, thus creating a strong vacancy supersaturation. Analysis of the temperature dependence of the atomic rearrangements occurring at the crystallization front suggests incomplete relaxation of the disordered phase at and in front of the crystal-liquid interface rapidly advancing under conditions of strong undercooling.

Chirality effects on an electron transport in single-walled carbon nanotube

In our work, we investigate characteristics of conductivity for single-walled carbon nanotubes caused by spin–orbit interaction. In the case study of chirality indexes, we especially research on the three types of single-walled carbon nanotubes which are the zigzag, the chiral, and the armchair. The mathematical analysis employed for our works is the Green-Kubo Method. For the theoretical results of our work, we discover that the chirality of single-walled carbon nanotubes impacts the interaction leading to the spin polarization of conductivity. We acknowledge such asymmetry characteristics by calculating the longitudinal current–current correlation function difference between a positive and negative wave vector in which there is the typical chiral-dependent. We also find out that the temperature and the frequency of electrons affect the function producing the different characteristics of the conductivity. From particular simulations, we obtain that the correlation decrease when the temperature increase for a low frequency of electrons. For high frequency, the correlation is nonmonotonic temperature dependence. The results of the phenomena investigated from our study express different degrees of spin polarization in each chiral of single-walled carbon nanotube and significant effects on temperature-dependent charge transport according to carrier backscattering. By chiral-induced spin selectivity that produces different spin polarization, our work could be applied for intriguing optimization charge transport.

Eliashberg Theory of a Multiband Non-Phononic Spin Glass Superconductor

I solved the Eliashberg equations for a multiband non-phononic s± wave spin-glass superconductor, calculating the temperature dependence of the gaps and of superfluid density. Their behaviors were revealed to be unusual: showing non-monotonic temperature dependence and reentrant superconductivity. By considering particular input parameters values that could describe the iron pnictide EuFe2(As1−xPx)2, a rich and complex phase diagram arises, with two different ranges of temperature in which superconductivity appears.

Pinning properties of FeSeTe thin film through multifrequency measurements of the surface impedance

We present high frequency measurements of the vortex dynamics of a FeSe x Te1 − x (x = 0.5) thin film grown on a CaF2 substrate and with a critical temperature T c ≃ 18 K, performed by means of a dual frequency dielectric resonator at 16.4 GHz and 26.6 GHz. We extract and discuss various important vortex parameters related to the pinning properties of the sample, such as the characteristic frequency ν c , the pinning constant k p and the pinning barrier height U relevant for creep phenomena. We find that the vortex system is in the single-vortex regime, and that pinning attains relatively high values in terms of k p , indicating significant pinning at the high frequencies here studied. The pinning barrier energy U is quite small and exhibits a non-monotonous temperature dependence with a maximum near 12 K. This result is discussed in terms of core pinning of small portion of vortices of size jumping out of the pinning wells over very small distances, a process which is favoured in the high frequency, short ranged vortex oscillations here explored.

Elastic Properties and Glass Forming Ability of the Zr<sub>50</sub>Cu<sub>40</sub>Ag<sub>10</sub> Metallic Alloy

The elastic properties of the Zr50Cu40Ag10 metallic alloy, such as the bulk modulus B, the shear modulus G, the Young’s modulus E and the Poisson’s ratio σ, are investigated by molecular dynamics simulation in the temperature range T=250–2000 K and at an external pressure of p=1.0 bar. It is shown that the liquid–glass transition is accompanied by a considerable increase in the shear modulus G and the Young’s modulus E (by more than 50%). The temperature dependence of the Poisson’s ratio exhibits a sharp fall from typical values for metals of approximately 0.32–0.33 to low values (close to zero), which are characteristic for brittle bulk metallic glasses. Non-monotonic temperature dependence of the longitudinal and transverse sound velocity near the liquid-glass transition is also observed. The glass forming ability of the alloy is evaluated in terms of the fragility index m. Its value is m≈64 for the Zr50Cu40Ag10 metallic glass, which is in a good agreement with the experimental data for the Zr-based metallic glasses.

Equilibrium properties of penetrable soft spheres

Using the Weeks-Chandler-Andersen separation scheme, we explored equilibrium properties of fully penetrable soft spheres with an attractive tail. When the radial distribution function of the reference system is computed by means of the integral equation theory using the hyper-netted-chain closure, this separation scheme predicts pressure-density isotherms, vapour-liquid phase coexistence, and surface tension in good agreement with the results from Monte Carlo simulations even though the attractive portion of the interparticle potential has a significant effect on the radial distribution function. Despite its simplicity, the model potential we studied, with a certain parameter value, exhibits a non-monotonic temperature dependence of the liquid phase density over a wide range of pressure.

Trion induced photoluminescence of a doped MoS2 monolayer

We demonstrate that the temperature and doping dependencies of the photoluminescence (PL) spectra of a doped MoS2 monolayer have several peculiar characteristics defined by the trion radiative decay. While only zero-momentum exciton states are coupled to light, radiative recombination of non-zero momentum trions is also allowed. This leads to an asymmetric broadening of the trion spectral peak and redshift of the emitted light with increasing temperature. The lowest energy trion state is dark, which is manifested by the sharply non-monotonic temperature dependence of the PL intensity. Our calculations combine the Dirac model for the single-particle states, with parameters obtained from the first-principles calculations, and the direct solution of the three-particle problem within the Tamm-Dancoff approximation. The numerical results are well captured by a simple model that yields analytical expressions for the temperature dependencies of the PL spectra.

Long-Lasting Molecular Orientation Induced by a Single Terahertz Pulse.

We present a novel, previously unreported phenomenon appearing in a thermal gas of nonlinear polar molecules excited by a single THz pulse. We find that the induced orientation lasts long after the excitation pulse is over. In the case of symmetric-top molecules, the time-averaged orientation remains indefinitely constant, whereas in the case of asymmetric-top molecules the orientation persists for a long time after the end of the pulse. We discuss the underlying mechanism, study its nonmonotonous temperature and amplitude dependencies, and show that there exist optimal parameters for maximal residual orientation. The persistent orientation implies a long-lasting macroscopic dipole moment, which may be probed by even harmonic generation and may enable deflection by inhomogeneous electrostatic fields.

Stably and highly sensitive FIR thermometry over a wide temperature range of 303-753 K based on the GdVO4:Eu3+ and Al2O3:Cr3+ hybrid particles.

Fluorescence intensity ratio (FIR) temperature sensors provide an effective method to control or study fine variations in physical and biological research because of their high sensitivity, accuracy, and spatial resolution. However, it is difficult to maintain high sensitivity over a wide temperature range using FIR temperature sensors because of the limits of the Boltzmann distribution law. In this study, sensitivity amplification for a wide temperature range in FIR thermometry based on GdVO4:Eu3+ and Al2O3:Cr3+ hybrid particles is achieved. The mechanism of the non-monotonic temperature dependence of the relative sensitivity (Sr) is studied. The results demonstrate that the Sr stably keeps ∼2.4% per K over a wide temperature range of 303-753 K, thus providing a basis for the extensive application of FIR temperature sensors.

Diffusion in a biased washboard potential revisited.

The celebrated Sutherland-Einstein relation for systems at thermal equilibrium states that spread of trajectories of Brownian particles is an increasing function of temperature. Here, we scrutinize the diffusion of underdamped Brownian motion in a biased periodic potential and analyze regimes in which a diffusion coefficient decreases with increasing temperature within a finite temperature window. Comprehensive numerical simulations of the corresponding Langevin equation performed with unprecedented resolution allow us to construct a phase diagram for the occurrence of the nonmonotonic temperature dependence of the diffusion coefficient. We discuss the relation of the later effect with the phenomenon of giant diffusion.

Criticality of incorporating explicit in‐situ measurement of temperature‐dependent heat generation for accurate design of thermal management system for Li‐ion battery pack

Safe and efficient operation of lithium-ion batteries in electric vehicles (EVs) relies on the performance of the thermal management system. For optimum thermal management, precise estimation of heat generation rate is crucial. This paper illustrates the criticality of the inclusion of explicit in-situ temperature dependence of heat generation rate as opposed to the state of the art models. We test a cylindrical LiFePO4 (LFP) cell over a broad range (typical of EV cells) of discharge rate (2.3 C-13.6 C) and temperature (20°C-70°C) and observe a non-monotonic trend where heat rate decreases first up to 55°C to 60°C (depending upon the discharge rate) and then starts to increase. The trend does not fit into any existing heat generation model of Li-ion battery, and hence we separately include it to simulate the heat transfer behaviour of a module. Simulations confirm the substantial difference of temperature from the conventional analysis where the temperature dependency is characterized inadequately. Current work demonstrates the counter-intuitive non-monotonic temperature dependence of heat rate, the effect of the same on peak module temperature and thus assists in improving the design methodology of the thermal management system of EV battery packs. Highlights Dynamic correlation between temperature and heat generation is measured experimentally. Cylindrical Li-ion cells are tested for typical electric vehicle operation ranges (2.3 C-13.6 C and 20°C-70°C). Counter-intuitive decreasing-first-increasing-later trend is observed for all discharge rates. Temperature dependence of in-situ heat rate is explained and included in analysis. Major modification is observed for the peak cell temperature, a critical design parameter.

Temperature dependence of bend elastic constant in oblique helicoidal cholesterics

Elastic moduli of liquid crystals, known as Frank constants, are of quintessential importance for understanding fundamental properties of these materials and for the design of their applications. Although there are many methods to measure the Frank constants in the nematic phase, little is known about the elastic constants of the chiral version of the nematic, the so-called cholesteric liquid crystal, since the helicoidal structure of the cholesteric renders these methods inadequate. Here we present a technique to measure the bend modulus K33 of cholesterics that is based on the electrically tunable reflection of light at an oblique helicoidal ChOH cholesteric structure. K33 is typically smaller than 0.6 pN, showing a nonmonotonous temperature dependence with a slight increase near the transition to the twist-bend phase. K33 depends strongly on the molecular composition. In particular, chiral mixtures that contain the flexible dimer 1′′,7′′-bis(4-cyanobiphenyl-4′-yl) heptane (CB7CB) and rodlike molecules such as pentylcyanobiphenyl (5CB) show a K33 value that is 5 times smaller than K33 of pure CB7CB or of mixtures of CB7CB with chiral dopants. Furthermore, K33 in CB11CB doped with a chiral agent is noticeably smaller than K33 in a similarly doped CB7CB which is explained by the longer flexible link in CB11CB. The proposed technique allows a direct in-situ determination of how the molecular composition, molecular structure and molecular chirality affect the elastic properties of chiral liquid crystals.