Electronic Heat Capacity Research Articles

Magnetothermal Properties and Magnetocaloric Effect in the 3d Ferromagnetic Elements: Fe, Co, and Ni

We present a calculation of magnetothermal properties and magnetocaloric effect (MCE) for the ferromagnetic elements: Fe, Co, and Ni. In particular, we calculated the temperature and field dependences of magnetization, heat capacity, entropy, isothermal entropy change Delta {S}_{m}, adiabatic temperature change ΔTad, and the two figures-of-merit: the relative cooling powers RCP(S) and RCP(T). We have used the mean-field theory in calculating the magnetization, magnetic heat capacity, and magnetic entropy. The lattice and electronic contributions to the total heat capacity and entropy were calculated using standard relations to subsequently calculate Delta {T}_{ad}. Those contributions depend on the Debye temperature ϴD and the coefficient of the electronic heat capacity γe respectively. The Maxwell relation is used to calculate Delta {S}_{m} and Delta {T}_{ad}. As an example of our results, the maximum Delta {S}_{m} for the three elements, in 6 T, is between 0.17 to 0.36 J/mol K and the maximum Delta {T}_{ad} mathrm{} is between 0.46 to 1.5 K/T for the same field change. The relative cooling power RCP(S) is in the 15–36 J/mol range for the three elements in a 6 T field. Also, the relative cooling power, RCP (T), is in the 162–1044 {K}^{2} range for the same field. For Fe and Co the RCP (T) per Tesla values, i.e., 139 and 174 {K}^{2}/T respectively are comparable to that of Gd and other Gd-based magnetocaloric materials. The behavior of the magnetization, magnetic heat capacity, and magnetic entropy shows that the phase transition in these three elements is of the second order. The universal curve and Arrott plots further support this conclusion.

Theoretical and experimental study of ablation of fused silica by femtosecond laser bursts

In this paper, the ablation of fused silica by femtosecond laser burst mode and normal pulse mode was studied by three-dimensional two-temperature model and verified by experiments. The change of electron heat capacity, electron conductivity, reflectivity, and transmissivity of fused silica irradiated by femtosecond laser pulses were studied. The ablation crater shape was predicted and compared with experiments. The incubation effect induced by sub-pulses in a burst was investigated. It was found that when the sub-pulse separation time is 50 ns, the incubation effect of burst mode decreases with the increase of the number of sub-pulses, which is caused by the heat diffusion during time between sub-pulses. However, the incubation effect of burst mode increases with increase of sub-pulse number when the sub-pulse separation time is 1 ps. An optimal combination of sub-pulse number and sub-pulse separation time was found, in which the highest machining efficiency and the smallest thermal damage area could be obtained.

The Bosons of the Conventional Superconductors

For the conventional superconductors it will be shown that not only the superconducting energy gap, Egap(T=0), and the critical field, Bc(T=0), but also the London penetration depth, λL(T=0), scale in a reasonable approximation with the superconducting transition temperature, TSC, as ~TSC, ~TSC2 and ~T-1/2, respectively. From these scaling relations the conclusion obtained earlier, using a completely different method, is confirmed that the London penetration depth corresponds to the diameter of the Cooper-pairs. As a consequence, only one layer of Cooper pairs is sufficient to shield an external magnetic field completely. The large diamagnetism of the superconductors is caused by the large orbital area of the Cooper-pairs. From the fact that, in the zero-field ground state, the temperature dependence of the superconducting heat capacity is given above and below TSC by power functions of absolute temperature it follows that the only critical point is T=0. The superconducting transitions of the element superconductors, therefore, are all within the critical range at T=0. As a consequence, above and below TSC there is short-range order only. As we know from Renormalization Group (RG) theory, in the critical range the dynamics is the dynamics of a boson field, exclusively. Evidently, the Cooper-pairs have to be considered as the short-range ordered units created by this boson field. It is reasonable to assume that the relevant bosons in the superconducting state are identical with the bosons giving rise to the universal linear-in-T electronic heat capacity above TSC. Plausibility arguments will be given that these bosons must be electric quadrupole radiation generated by the non-spherical charge distributions in the soft zones between the metal atoms. The radiation field emitted by an electric quadrupole can be assumed to be essentially curled or circular. In the ordered state below TSC, the bosons are condensed in resonating spherical modes which encapsulate the two Cooper-pair electrons and shield their charge perfectly.

Modeling laser interactions with aluminum and tantalum targets using a hybrid atomistic-continuum model

A hybrid atomistic-continuum method can model the microstructure evolution of metals subjected to laser irradiation. This method combines classical molecular dynamics (MD) simulations with the two-temperature model (TTM) to account for the laser energy absorption and heat diffusion behavior. Accurate prediction of the temperature evolution in the combined MD-TTM method requires reliable accuracy in electron heat capacity, electron thermal conductivity, and electron–phonon coupling factor across the temperatures generated. This study uses the electronic density of states (DOS) obtained from first-principle calculations. The calculated electron temperature-dependent parameters are used in MD-TTM simulations to study the laser metal interactions in FCC and BCC metals and the phenomenon of laser shock loading and melting. This study uses FCC Al and BCC Ta as model systems to demonstrate this capability. When subjected to short pulsed laser shocks, the dynamic failure behavior predicted using temperature-dependent parameters is compared with the experimentally reported single-crystal and nanocrystalline Al and Ta systems. The MD-TTM simulations also investigate laser ablation and melting behavior of Ta to compare with the ablation threshold reported experimentally. This manuscript demonstrates that integrating the temperature-dependent parameters into MD-TTM simulations leads to the accurate modeling of the laser–metal interaction and allows the prediction of the kinetics of the solid–liquid interface.

Role of electronic correlations in the kagome-lattice superconductor LaRh3B2

${\mathrm{LaRh}}_{3}{\mathrm{B}}_{2}$ crystallizes in a layered structure where Rh atoms form a perfect kagome lattice. The material shows superconductivity at ${T}_{c}\ensuremath{\approx}2.6$ K and no signature for density wave instabilities. We report our measurements of electronic transport, magnetization, and heat capacity in the normal and superconducting state, and we derive normal and superconducting parameters. From first-principles calculations of the electronic band structure, we identify all features of kagome bands predominantly formed by the Rh $d$ orbitals: a flat band, Dirac cones, and van Hove singularities. The calculation of the phonon dispersions and electron-phonon coupling suggests a strong similarity between ${\mathrm{LaRh}}_{3}{\mathrm{B}}_{2}$ and ${A\text{V}}_{3}{\mathrm{Sb}}_{5}$ ($A=\text{K,Cs,Rb}$). For ${\mathrm{LaRh}}_{3}{\mathrm{B}}_{2}$, it matches quantitatively with the observed ${T}_{c}$, supporting a conventional phonon-mediated pairing mechanism. By comparison to the $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ family, we conjecture that there is a reduced importance of electron correlations in ${\mathrm{LaRh}}_{3}{\mathrm{B}}_{2}$.

Investigating the magnetic, thermoelectric, and thermodynamic properties of the GeCH3 single-layer considering external magnetic field, doping, and strain

Extensive research is ongoing to improve the performance of thermoelectric and thermodynamic properties of the material because preventing energy waste is vital in modern society. Herein, we study the thermoelectric and thermodynamic properties of the GeCH3 single-layer (SL) under the influence of an external magnetic field, electron doping, and tensile and compressive biaxial strain by using the tight-binding and equilibrium Green’s function method. We found that the electronic heat capacity, magnetic susceptibility, and electronic thermal and electrical conductivity increase by employing an external magnetic field, electron doping, and tensile biaxial strain. However, compressive biaxial strain yields a decrease in thermoelectric and thermodynamic properties. The results of our study show that the GeCH3 SL is paramagnetic. The results presented here that the GeCH3 SL is a suitable alternative for use in thermoelectric, spintronic, and valleytronics devices.

Thermodynamic properties of the Mott insulator-metal transition in a triangular lattice system without magnetic order

The organic system, $\kappa$-[(BEDT-TTF)$_{1-x}$(BEDT-STF)$_x$]$_2$Cu$_2$(CN)$_3$, showing the Mott transition between a nonmagnetic Mott insulating (NMI) state and a Fermi liquid (FL), is systematically studied by calorimetric measurements. An increase of the electronic heat capacity at the transition from the NMI state to the FL state which keeps the triangular dimer lattice demonstrates that the charge sector lost in the Mott insulating state is recovered in the FL state. We observed that the remaining low-energy spin excitations in the Mott insulating state show unique temperature dependence, and that the NMI state has a larger lattice entropy originating from the frustrated lattice, which leads to the Pomeranchuk-like effect on the electron localization. Near the Mott boundary, an unexpected enhancement and magnetic-field dependence of heat capacity are observed. This anomalous heat capacity is different from the behavior in the typical first-order Mott transition and shows similarities with quantum critical behavior. To reconcile our results with previously reported scenarios about a spin gap and the first-order Mott transition, further studies are desired.

The effect of an external magnetic field, doping, and bias voltage on the thermoelectric and thermodynamic of S-graphene monolayer

In the present report, we studied the thermoelectric and thermodynamic properties of the S-graphene (SG) monolayer under the effect of the external magnetic field, electron/hole doping, and bias voltage by the tight-binding (TB) model. The Green function method has been applied for calculating the electronic heat capacity, Pauli magnetic susceptibility, electronic thermal conductivity, Seebeck coefficient, and electrical conductivity of the SG monolayer. The results show that the electronic heat capacity increases by the magnetic field, which is in agreement with Einstein's Debye relation. It has been observed that the SG monolayer can act as a p-type or n-type semiconductor by applying the studied parameters (doping, external magnetic field, and bias voltage), which means the semiconductor type of SG is controlled by electron/hole doping and applying magnetic field, and bias voltage. Our results show that the SG monolayer is a paramagnetic material, but it depends on bias voltage and external magnetic field. Also, the total Pauli magnetic susceptibility is enhanced in the presence of an external magnetic field.

Electronic and Thermal Response of Low-Electron-Density Drude Materials to Ultrafast Optical Illumination

Many low-electron-density Drude (LEDD) materials such as transparent conductive oxides or nitrides have recently attracted interest as alternative plasmonic materials and extremely nonlinear thermal and optical materials. These materials attract research interest because they are CMOS compatible; hence, they can enable new functionalities within existing devices. However, the rapidly growing number of experimental studies has so far not been supported by a systematic theory of the electronic, thermal, and optical response of these materials. Here, we use the techniques previously derived in the context of noble metals to go beyond a simple electromagnetic modeling of LEDD materials and provide an electron-dynamics model for their electronic and thermal response. We find that the low electron density makes momentum conservation in electron-phonon interactions more important, more complex, and more sensitive to the temperatures compared with noble metals; moreover, we find that electron-electron interactions are becoming more effective due to the weaker screening. Most importantly, we show that the low electron density makes the electron heat capacity much smaller than in noble metals, such that the electrons in LEDD materials tend to heat up much more and cool down faster compared to noble metals. While here we focus on indium tin oxide (ITO), our analytical results can easily be applied to any LEDD materials.

Effect of external magnetic field and doping on electronic and thermodynamic properties of planer and buckled silicene monolayer

In this research, the electronic and thermodynamic properties of the planer and buckled silicene monolayer under an external magnetic field and doping using the tight-binding (TB) model and the Green function approach are investigated. Also, the dependence of the electronic heat capacity and magnetic susceptibility with temperature, external magnetic field, electron, and hole doping for the planer and buckled silicene monolayer is calculated. Our numerical calculation exhibits that the planer and buckled silicene monolayer have a zero band gap. We find that the electronic heat capacity increases (decreases) by applying an external magnetic field, and electron and hole doping at lower (higher) temperatures due to the increase in the thermal energy (scattering and collision) of the charge carriers. Finally, we observe that the planer and buckled silicene monolayer is antiferromagnetic, which is changed to the ferromagnetic phase when an external magnetic field and doping are applied, which makes the silicene monolayer suitable for spintronic applications.

Femtosecond laser melting NbMoTaW refractory high entropy alloy: A micro-scale thermodynamic simulation

Refractory high entropy alloys (RHEA) exhibit excellent material properties, such as high strength, high hardness, high melting point and high temperature oxidation resistance. Ultrafast laser fabricating RHEAs is potential widely used in extreme environments at high temperature. In the present study, a multiscale simulation of the femtosecond laser pulse melting NbMoTaW RHEA film was performed to observe the transient interaction process between femtosecond laser and RHEA at the atomic scale. The temperature dependent electron thermophysical parameters (electron heat capacity, electron thermal conductivity and effective electron–phonon coupling factor) were calculated from density functional theory. The two temperature model coupling molecular dynamics was used to reveal the temperature change, the density variation and the pressure evolution during the interaction of a single pulse laser with the NbMoTaW RHEA. The simulated ablation and melting depths varied differently with laser fluence parameters. The ablation depth increases with the laser fluence, while the non-removed melting depth varies little with laser energy beyond the ablation threshold. The research reveals the mechanisms in melting process at atomic-level, and the results are helpful to the fabrication of RHEA.

Metal-semiconductor transition in Ψ-graphene

The effect of hydrogenation on the density of states (DOS), band structure, Pauli magnetic susceptibility (PMS), and electronic heat capacity (EHC) of Ψ -graphene is investigated within the nearest neighbor tight-binding model. Two different hydrogenation process of Ψ -graphene is examined: half hydrogenation ( Ψ -graphone) and full hydrogenation ( Ψ -graphane). It was observed that Ψ -graphone, like Ψ -graphene, exhibits metallic behavior, but it has sharp peaks in the DOS and corresponding flat bands in the band structure near the Fermi level, due to unpaired and localized electrons in its structure. Whereas, because of the strong σ bonds in the Ψ -graphane structure, a bandgap is created in its energy spectra. It can also be seen that with the hydrogenation of Ψ -graphene, the van-Hove singularities in the DOS and the number of sub-band in the band structures increased due to the greater overlap of the orbitals. These lead to the crossovers in the PMS and the Schottky anomaly peaks in the EHC, which both of these peaks are observed in the Ψ -graphane curve at a higher temperature than the other two structures. These results indicate that the hydrogenation of Ψ -graphene leads to a metal-to-semiconductor transition.

First principles thermoelectric performance calculations of TiN, ZnS, and Ag2Se at low temperatures

In this work ab initio computations are used to explore the low temperature, thermoelectric performance of pure, single crystal TiN (Fm3‾m), ZnS (P63mc) and Ag2Se (P212121). Complimenting ab initio theory, the quasi harmonic approximation and solutions to the Boltzmann transport equation are also used to solve for the underlying structural and elastic properties, to include bulk and shear modulus, electronic band structure, phonon dispersion, heat capacity, thermal conductivity, and figure of merit. Where applicable, the results agree well with those of previously published works, and among other findings, confirm the relatively large figure of merit specifically attributable to Ag2Se. In addition to providing a viable computational approach to the solution of these aformentioned properties, this work is also intended to serve as a baseline from which additional alloy derivatives may be explored.

A terahertz detector based on superconductor-graphene-superconductor Josephson junction

Graphene is an attractive material for terahertz (THz) detectors due to its unique properties of extremely weak electron-phonon interaction and small electronic heat capacity. In this paper, we demonstrate a lens-antenna-coupled THz detector based on a diffusive superconductor-graphene-superconductor (SGS) junction, which has a sharp resistive transition at 1.4 K due to the proximity Josephson coupling. The SGS Josephson detector, measured with a dc superconducting quantum interference device (SQUID) readout, exhibits an electrical noise equivalent power (NEP) as low as 5 × 10−17 W/Hz0.5 at 0.1 K, which is still limited by the readout noise. Furthermore, the optical performance of the SGS Josephson detector is measured with a cryogenic blackbody radiation source, giving an optical NEP of 2.5–5 × 10−16 W/Hz0.5 at 1.4 THz in the temperature range from 0.1 K to 0.6 K.

Heat-conserving three-temperature model for ultrafast demagnetization in nickel

Multireservoir models are widely used for modeling and interpreting ultrafast magnetization dynamics. Here we introduce an alternative formulation to existing three-temperature models for the treatment of spin, electron, and lattice temperatures in magnetization dynamics simulations. In contrast to most existing models of calculations of energy transfer between reservoirs in these types of simulations, the heat distribution of the spin and lattice subsystems is evaluated during the simulation instead of being calculated a priori. The model is applied to investigate the demagnetization and remagnetization of fcc Ni, when subjected to a strong laser pulse. In particular, our model results in a fast interplay between the electron and spin subsystems which reproduces the main features of experimental data for fcc Ni significantly better than most reported three-temperature models. We also show that the way in which the electron, spin, and lattice heat capacities are described can have a significant impact on the simulated ultrafast dynamics. By introducing spin-lattice couplings in the simulation, it is shown that these explicit interactions only have a marginal impact on the magnetization dynamics of fcc Ni, albeit it is more pronounced for higher laser pulse powers.

Enhanced Electronic Thermal Conductivity and Heat Capacity of Biased Bilayer Boron Phosphide with Magnetic Field

Enhanced Electronic Thermal Conductivity and Heat Capacity of Biased Bilayer Boron Phosphide with Magnetic Field

Strain effect on the electronic properties of graphdiyne

In this theoretical study, within the nearest neighbor tight-binding model and Green’s function technique, the density of states (DOS), electronic heat capacity (EHC), and Pauli magnetic susceptibility (PMS) of graphdiyne system under different strain values (5%, 9%, and 15%) are calculated and the results are compared with those of a graphdiyne monolayer without any strain. It is observed that with increasing strain, the band gap near the Fermi level of graphdiyne increases monotonously due to the decreased orbital overlap between C atoms. In addition, the intra-bandgaps and the van-Hove singularities are seen in the DOS curves of all systems. These lead to the Schottky anomaly peaks in the EHC and the crossovers in the PMS curves, which are divided into two low and high temperature regions. At vicinity of zero temperatures, with increasing strain, the EHC and PMS decrease. From the theoretical stand point, this phenomenon stems from the proportional relationship of the PMS and EHC with DOS.

Anisotropic magnetic and transport properties of orthorhombic o-Pr2Co3Ge5

The crystal structure, electron energy-loss spectroscopy (EELS), heat capacity, and anisotropic magnetic and resistivity measurements are reported for Sn flux grown single crystals of orthorhombic Pr2Co3Ge5 (U2Co3Si5-type, Ibam). Our findings show that o-Pr2Co3Ge5 hosts nearly trivalent Pr ions, as evidenced by EELS and fits to temperature dependent magnetic susceptibility measurements. Complex magnetic ordering with a partially spin-polarized state emerges near T sp = 32 K, with a spin reconfiguration transition near T M = 15 K. Heat capacity measurements show that the phase transitions appear as broad peaks in the vicinity of T sp and T M. The magnetic entropy further reveals that crystal electric field splitting lifts the Hund’s rule degeneracy at low temperatures. Taken together, these measurements show that Pr2Co3Ge5 is an environment for complex f state magnetism with potential strongly correlated electron states.

Multi-temperature modeling of femtosecond laser pulse on metallic nanoparticles accounting for the temperature dependences of the parameters

This review considers the fundamental dynamical processes of metal nanoparticles during and after the impact of a femtosecond laser pulse on a nanoparticle, including the absorption of photons. Understanding the sequence of events after photon absorption and their timescales is important for many applications of nanoparticles. Various processes are discussed, starting with optical absorption by electrons, proceeding through the relaxation of the electrons due to electron–electron scattering and electron–phonon coupling, and ending with the dissipation of the nanoparticle energy into the environment. The goal is to consider the timescales, values, and temperature dependences of the electron heat capacity and the electron–phonon coupling parameter that describe these processes and how these dependences affect the electron energy relaxation. Two- and four-temperature models for describing electron–phonon relaxation are discussed. Significant emphasis is paid to the proposed analytical approach to modeling processes during the action of a femtosecond laser pulse on a metal nanoparticle. These consider the temperature dependences of the electron heat capacity and the electron–phonon coupling factor of the metal. The entire process is divided into four stages: (1) the heating of the electron system by a pulse, (2) electron thermalization, (3) electron–phonon energy exchange and the equalization of the temperature of the electrons with the lattice, and (4) cooling of the nanoparticle. There is an appropriate analytical description of each stage. The four-temperature model can estimate the parameters of the laser and nanoparticles needed for applications of femtosecond laser pulses and nanoparticles.

Structural, mechanical, optoelectronic and thermodynamic properties of bulk and film materials in Ti–Nb–C system: First-principles and experimental investigations

Both first-principles study of the random TiC–NbC solid solutions and experimental investigation of the films in Ti–Nb–C system were carried out. The mixing energy, electronic and phonon structures, elastic constants and moduli, hardness, Debye temperature, fracture toughness, dielectric function, electron energy-loss spectra and heat capacity of the random Ti1-xNbxC solid solutions were calculated and analyzed depending on composition. The Ti–C, Nb–C and Ti–Nb–C films were deposited by magnetron sputtering. The deposited films were comprehensively studied with XRD, XPS, AFM, EDS, Raman spectroscopy, indentation and tribological tests. The Ti–Nb–C films with equi-atomic composition are found to exhibit the highest Knoop hardness and lowest friction coefficient as compared to those of the parent TiC and NbC carbides. The results obtained were used to establish the mechanism of the stabilization of the TiC–NbC solid solutions, and to predict their structural, mechanical, optical and thermodynamic properties.