Electronic Heat Capacity Research Articles

Superconductivity in non-centrosymmetric ZrNiAl and HfRhSn-type compounds

We report the discovery of superconductivity in non-centrosymmetric compounds HfNiAl, ZrNiAl, ZrNiGa, and HfPtAl by measuring their electrical transport and thermodynamic properties. HfNiAl, ZrNiAl, and ZrNiGa crystallize in the ZrNiAl-type crystal structure, whereas HfPtAl crystallizes in the HfRhSn-type crystal structure. Superconducting transitions for HfNiAl, ZrNiAl, ZrNiGa, and HfPtAl are observed at 1.0 K, 1.0 K, 0.42 K, and 0.58 K, respectively. Using the Werthamer–Helfand–Hohenberg model, the zero-temperature upper critical fields µ 0H(0) were estimated to be 0.58 T, 0.24 T, 0.08 T, and 0.34 T for HfNiAl, ZrNiAl, ZrNiGa, and HfPtAl, respectively. The observed jump in electronic heat capacity () across the superconducting transition is 1.3, 1.3, and 1.2 for HfNiAl, ZrNiAl, and HfPtAl, respectively. After the inclusion of the spin–orbit coupling in the band structure calculations, a total of six bands for ZrNiAl, HfPtAl, and ZrNiGa, and eight bands for HfNiAl were found to cross the Fermi level. Spin–orbit coupling induced maximum splitting (/k B T c) of the electronic bands near the Fermi level was found to be 1697, 517, 1138, and 4230 for HfNiAl, ZrNiAl, ZrNiGa, and HfPtAl, respectively. Large variation of the antisymmetric spin–orbit coupling (ASOC) among these compounds provides a great opportunity to study the effects of ASOC on the superconducting pairing states.

Revealing the Thermal Properties of Superconducting Magic-Angle Twisted Bilayer Graphene.

The allegedly unconventional superconducting phase of magic-angle twisted bilayer graphene (MATBG) has been predicted to possess extraordinary thermal properties, as it is formed from a highly diluted electron ensemble with a record-low carrier density (n) of ∼1011 cm-2 and electronic heat capacity (Ce) of <100kB. While these attributes position MATBG as a ground-breaking material platform for revolutionary calorimetric applications, these properties have so far not been experimentally shown. Here, we reveal the thermal properties of superconducting MATBG by monitoring its temperature dependent critical current (Ic) under continuous laser heating at 1550 nm. From the bolometric effect, we extract the temperature dependence of the electronic thermal conductance (Gth), which has a value of Gth = 0.2 pW/K at 35 mK and in the low temperature limit is consistent with a power law dependence, as expected for nodal superconductors. Our work lays the foundation for future thermal transport studies on this system.

Lattice dynamics of LiH[formula omitted]PO[formula omitted] crystal

Lattice dynamics of hydrogen bonded representative of KH2PO4 crystal family, LiH2PO4, was investigated within the first principles density functional theory. Phonon dispersion relations along symmetry directions of Brillouin zone, density of phonon states, electron density maps and heat capacity were calculated. Experimental Raman spectra (Lee et al., 2008) are interpreted in detail based on eigen-vector analysis in Brillouin zone center. We come to the conclusion that the dynamically stable phonon spectrum and a tight electrostatic coupling between the Li ions and tetrahedrally coordinated oxygen ions are the main reasons for the crystal lattice stability and for the lack of ferroelectricity in LiH2PO4 crystal.

Thermodynamic properties of perturbed monolayer PbBiI

In this paper, we theoretically investigate the influence of external electric field, Zeeman splitting field, and dilute charged impurity on the thermodynamic features of the PbBiI monolayer. Green’s function approach, the Born approximation, and the tight-binding model are employed to address these effects. We find the responses of the electronic heat capacity (HC) and Pauli spin paramagnetic susceptibility (PSPS) to the symmetry breaking originated from the presence of the perturbations. The results show that the electric (Zeeman) field leads to an increase (decrease) in the bulk gap of the system, resulting in a gapless phase at a critical Zeeman field. Correspondingly, HC and PSPS results confirm almost zero responses at the appeared critical Zeeman field. On the other hand, critical impurity characters emerge within the validity regime of the Born approximation. Finally, the combined effects of these perturbations are compared with the findings from the individual ones.

A Third Generation Calphad Description of Fe: Revisions of Fcc, Hcp and Liquid

The thermodynamic description of pure Fe was re-assessed using updated third generation Calphad models and taking into account the most recent theoretical data. In the present re-assessment, a critical evaluation was performed for the magnetic properties of the fcc and hcp phases yielding different descriptions from those accepted in the SGTE database. The selected magnetic properties enable us to model the thermodynamic properties of fcc satisfactorily without using the magnetic two-state model. A new method was applied to extrapolate the thermodynamic data for the solid phases, i.e. a single Gibbs energy expression is used for the entire temperature range for the bcc, fcc and hcp phases. The description of the liquid phase was re-evaluated following a recent suggestion on estimating the electronic heat capacity coefficient for the liquid phase. Overall, the present Calphad description can reproduce the selected experimental and theoretical data well.

A two-scale thermo-mechanically coupled model for anomalous martensite transformation and elastocaloric switching effect of shape memory alloy

Conventional shape memory alloys (SMAs) exhibit an approximately linear relationship between the critical stresses of martensite transformation (MT) and ambient temperature (Clausius-Clapeyron relationship). Meanwhile, depending on the sign of entropy difference between the parent and martensite phases, these alloys behavior either a conventional or an inverse elastocaloric effect which originates from the latent heat release/absorption during stress-induced MT. Recently, MT with a non-monotonic Clausius-Clapeyron relationship (denotes as anomalous MT) and an elastocaloric switching effect (changes from the inverse elastocaloric effect at relatively low temperature to the conventional elastocaloric effect at relatively high temperature) were reported in CoCr-based Heusler-type SMAs. In this paper, to understand and quantitatively describe these new phenomena, a two-scale thermo-mechanically coupled constitutive model is constructed. In the mesoscopic scale, the fine twinned structure and the potential correspondent variant pairs of martensite phase are analyzed based on the crystallographic symmetry and the geometrically nonlinear theory of MT. The classical Debye's model is employed to describe the lattice and electronic heat capacities of the parent and martensite phases; while, a Monte Carlo simulation for the three-dimensional (3D) Ising's model is performed and a scaling law for the magnetic heat capacity of parent phase is proposed. Then, a new constitutive model is established based on the fundamental laws of irreversible thermodynamics to describe the non-isothermal stress-induced MT between parent and martensite phases. To consider the thermo-mechanical interactions between the parent phase and correspondent martensite variant pairs, each variant pair is regarded as an inhomogeneous inclusion with a mobile interface and embedded in the parent phase. With the help of interfacial operator, the thermodynamic driving force of MT is derived from the constructed Helmholtz free energy and dissipative inequality. Internal heat generation originated from the thermo-elasticity, inelastic deformation dissipation and latent heat is derived from the conservation law of energy. In the macroscopic scale, the thermo-mechanical response of SMA specimen is obtained by solving the equations of force balance, deformation compatibility, and thermodynamic equilibrium through the full-field calculation and mean-field approximation. Finally, the proposed model is verified by comparing the predictions with the experiments. It can be found that the anomalous MT and elastocaloric switching effect of CoCrAlSi SMA can be well captured by the proposed model.

Comparison of the electronic transport of ThN against ThC

We compared the electronic heat capacity coefficient, thermal conductivity and resistivity of ThN and ThC. We found that the electronic heat capacity coefficient of both compounds was very low as evaluated using Quantum Espresso and GGA of the Perdew, Burke, and Ernzerhof (PBE) functional. The calculated electronic heat coefficient was further reduced (∼three times for ThN and four times for ThC) when the non-local hybrid functional (B3LYP) was used. QE combined with EPW code were used to calculate integrated electron-phonon coupling strength, the Eliashberg transport coupling function and electron-phonon scattering rates. The electronic resistivity was calculated using Ziman's formula for metals as implemented in EPW code and compared to the values derived from experimental data correlations. Furthermore, the effective number (neff) of electronic carriers was calculated. The electronic thermal conductivity was evaluated from the calculated electronic resistivity via Wiedemann-Franz law using the derived number of mobility electrons: nav (averaged over the temperature range 300 K–1000 K). We found that the only way to reproduce exactly the experimental electrical resistivity and electronic thermal conductivity in both ThN and ThC was to incorporate the number of electrons contributing to thermal mobility (neff), which increase with temperature. We estimated that in ThN the number of electronic carriers is two times larger and electron-phonon coupling strength lower by a factor ∼ two, which led to about five times higher electronic thermal conductivity than calculated for ThC.

Development of the temperature-dependent interatomic potential for molecular dynamics simulation of metal irradiated with an ultrashort pulse laser

Laser ablation is often explained by a two-temperature model (TTM) with different electron and lattice temperatures. To realize a classical molecular dynamics simulation of the TTM, we propose an extension of the embedded atom method to construct an interatomic potential that is dependent on the electron temperature. This method is applied to copper, and its validity is demonstrated by comparison of several physical properties, such as the energy–volume curve, phonon dispersion, electronic heat capacity, ablation threshold, and mean square displacement of atoms, with those of finite-temperature density functional theory.

Electrical resistivity of delta phase plutonium alloys

ABSTRACT I present an analysis of the temperature dependence of the electrical resistivity of δ-phase plutonium alloys. I find that the resistivity is mainly proportional to the electronic Kondo heat capacity, with an additional term, linear in temperature, that is correlated with the absolute low-temperature resistivity. This indicates a so-called Mooij correlation, which is often observed in disordered materials. These two terms can be understood together as different contributions to the resistivity, proportional to their respective electronic densities of states. The method presented in this paper provides a simple means of quantifying such resistivity data, and the data for a large number of δ-phase plutonium alloys have been fitted with only four parameters each.

Electronic and thermodynamic properties of zigzag MoS2/ MoSe2 and MoS2/ WSe2 hybrid nanoribbons: Impacts of electric and exchange fields

Using the tight-binding approach and non-equilibrium Green’s function method, we investigate the electronic band structure, total electronic heat capacity (EHC), and Pauli spin susceptibility (PSS) of zigzag MoS2/MoSe2 and MoS2/WSe2 hybrid nanoribbons in the presence of transverse electric and external exchange fields. Our results show that in the simultaneous presence of both of these fields, the zigzag MoS2/WSe2 and MoS2/MoSe2 nanoribbons are converted to half-metal systems. The total EHC increases with growing temperature until the Schottky anomaly appears at 150 K. we show that these hybrid nanoribbons become almost antiferromagnetic with the Neel temperature of 200 K. Interestingly, in the whole range temperature, the total EHC and PSS decrease by the transverse electric and external exchange fields. Our results provide an efficient and practical approach to the design of spintronic devices.

Dimer Crystallization Induced by Elemental Substitution in the Honeycomb Lattice of Ru1−xOsxCl3

Substitution effects of Os for Ru in {\alpha}-RuCl3 are investigated in a wide composition range of 0 =< x =< 0.67 in Ru1-xOsxCl3 by X-ray and electron diffraction, magnetic susceptibility, heat capacity, and Raman spectroscopy measurements. Apart from the Kitaev physics with antiferromagnetic interactions increasing with x, a rich phase diagram is obtained, which includes an antiferromagnetic long-range order below 12 K for x =< 0.15, a dome-shaped spin-singlet dimer phase below 130 K for 0.15 =< x =< 0.40, and a magnetic short-range order for x > 0.40. A dimerization as similarly observed in {\alpha}-RuCl3 under high pressure occurs in the spin-singlet phase. It is suggested that Ru-Os pairs in the solid solutions tend to form dimers with short bonds and trigger the first-order transition in the presence of pseudo-threefold rotational symmetry for dimerization around a substituted Os atom only at low substitutions. This is a rare example of molecular orbital crystallization induced by elemental substitution in a highly disordered system. The short-range order at high substitutions may be related to a random-singlet state stabilized by bond disorder in the honeycomb net.

ВЛИЯНИЕ КАЛИЯ НА ТЕПЛОЕМКОСТЬ И ТЕРМОДИНАМИЧЕСКИЕ ФУНКЦИИ АЛЮМИНИЕВОГО СПЛАВА АЖ5К10

Heat capacity is the most important characteristic of substances, and its change with temperature can determine the type of phase transformation, Debye temperature, the energy of formation of vacancies, the electron heat capacity coefficient, and other properties. In the work, the heat capacity of the AlFe5K10 alloy with potassium was determined in the “cooling” mode from the known heat capacity of the standard copper sample. In order to succeed, by processing the curves of the cooling rates of the samples of the aluminum alloy AlFe5K10 with potassium and the standard, polynomials describing their cooling rates were obtained. Further, according to the experimentally found values of the cooling rates of the samples of alloys and the standard, knowing their masses, the polynomials of the temperature dependence of the heat capacity of the alloys and the standard were established, which are described by a four-termed equation. Using integrals of specific heat, models of the temperature dependence of the change in enthalpy, entropy and Gibbs energy were established. The dependences obtained show that with increasing temperature, the heat capacity, enthalpy and entropy of the alloys increase, while the values of Gibbs energy decrease. At the same time, potassium addition reduces the heat capacity, enthalpy and entropy of the original AlFe5K10 aluminium alloy. The value of Gibbs energy increases in that case.

Electronic Heat Capacity and Lattice Softening of Partially Deuterated Compounds of κ-(BEDT-TTF)2Cu[N(CN)2]Br

Thermodynamic investigation by calorimetric measurements of the layered organic superconductors, κ-(BEDT-TTF)2Cu[N(CN)2]Br and its partially deuterated compounds of κ-(d[2,2]-BEDT-TTF)2Cu[N(CN)2]Br and κ-(d[3,3]-BEDT-TTF)2Cu[N(CN)2]Br, performed in a wide temperature range is reported. The latter two compounds were located near the metal–insulator boundary in the dimer-Mott phase diagram. From the comparison of the temperature dependences of their heat capacities, we indicated that lattice heat capacities of the partially deuterated compounds were larger than that of the pristine compound below about 40 K. This feature probably related to the lattice softening was discussed also by the sound velocity measurement, in which the dip-like structures of the Δv/v were observed. We also discussed the variation of the electronic heat capacity under magnetic fields. From the heat capacity data at magnetic fields up to 6 T, we evaluated that the normal-state γ value of the partially deuterated compound, κ-(d[3,3]-BEDT-TTF)2Cu[N(CN)2]Br, was about 3.1 mJ K−2 mol−1. Under the magnetic fields higher than 3.0 T, we observed that the magnetic-field insulating state was induced due to the instability of the mid-gap electronic state peculiar for the two-dimensional dimer-Mott system. Even though the volume fraction was much reduced, the heat capacity of κ-(d[3,3]-BEDT-TTF)2Cu[N(CN)2]Br showed a small hump structure probably related to the strong coupling feature of the superconductivity near the boundary.

The boson-based quantized dynamics of solids

Available experimental data of the Debye temperature (ΘD) and of the coefficient of the low-temperature electronic heat capacity (γ) of the condensed elements are analyzed in a rather heuristic way. Since the two associated heat capacities exhibit the same universal temperature dependence in all solids (∼T3 and ∼T, respectively), they have to be identified as the heat capacities of boson fields. In the elastic case the bosons are the well-known sound waves. Neither the dispersion relation nor the sources of the bosons of the metallic continuum are known. The bosons interact surprisingly strong with the corresponding atomistic degrees of freedom, i.e. phonons and electron-band states, and modify their dispersion relations strongly. The actual values of ΘD and γ therefore depend considerably on the interaction strength to these atomistic excitations. The experimental indications will be discussed that this interaction is according to discrete modes and appears to be quantized.

Electron–phonon coupling factor and electron heat capacity of 6H-SiC

ABSTRACT For high-energy single ion events, the relationship between electronic energy loss mechanism and atomic processes is generally described by the inelastic thermal spike model. However, the parameters required for the model are not accurately known and are often estimated from a free electron gas model. To ensure this model is more reliable and predictive, a more accurate calculation for the parameters should be taken. In this paper, the temperature dependence of the electronic heat capacity and electron–phonon coupling factor (e-ph) has been calculated, using density functional theory (DFT). The calculation results demonstrate that the effect of electron thermal excitation on the thermodynamic parameters is sensitive to the bandgap, and these parameters vary immensely with the electron temperature. Besides, to verify the accuracy of obtained parameters, both the swift heavy ions (SHI) irradiation in crystalline 6H-SiC and amorphous 6H-SiC are simulated, using the inelastic thermal spike model. And the results are more consistent with the experiment conclusion, for the obtained parameters by DFT. Hence, the calculated temperature-dependent parameters are more suitable for the inelastic thermal spike model.

Dzyaloshinskii-Moriya anisotropy effect on field-induced magnon condensation in the kagome antiferromagnet α−Cu3.26Mg0.74(OH)6Br2

We performed a comprehensive electron spin resonance, magnetization and heat capacity study on the field-induced magnetic phase transitions in the kagome antiferromagnet $\alpha-Cu_3Mg(OH)_6Br_2$. With the successful preparation of single crystals, we mapped out the magnetic phase diagrams under the $c$-axis and $ab$-plane directional magnetic fields $B$. For $B\|c$, the three-dimensional (3D) magnon Bose-Einstein condensation (BEC) is evidenced by the power law scaling of the transition temperature, $T_c\propto (B_c-B)^{2/3}$. For $B\|ab$, the transition from the canted antiferromagetic (CAFM) state to the fully polarized (FP) state is a crossover rather than phase transition, and the characteristic temperature has a significant deviation from the 3D BEC scaling. The different behaviors of the field-induced magnetic transitions for $B\|c$ and $B\|ab$ could result from the Dzyaloshinkii-Moriya (DM) interaction with the DM vector along the $c$-axis, which preserves the $c$-axis directional spin rotation symmetry and breaks the spin rotation symmetry when $B\|ab$. The 3D magnon BEC scaling for $B\|c$ is immune to the off-stoichiometric disorder in our sample $\alpha-Cu_{3.26}Mg_{0.74}(OH)_6Br_2$. Our findings have the potential to shed light on the investigations of the magnetic anisotropy and disorder effects on the field-induced magnon BEC in the quantum antiferromagnet.

Heat capacity, entropy, formation energy and spin-fluctuation behavior of U3Si5 from 2.4 to 397.4 K

• Heat capacity data of U3Si5 were obtained using a Quantum Design Physical Properties Measurement System (PPMS) from 2.4 to 397.4 K. • An upturn characteristic of spin-fluctuations in Cp/T (T) was observed below 10 K. • The Gibbs free energy of formation of U3Si5 from the elements was determined to be –45.2 ± 9.0 kJ•mol−1•atom−1. U-Si intermetallic compounds are of considerable interest for their applications as accident-tolerant nuclear fuels. Here we present low-temperature heat capacity (LTHC) measurements of one of the U-Si phases, U 3 Si 5 , using a Quantum Design Physical Properties Measurement System (PPMS) from 2.4 to 397.4 K. We observed an upturn in C p /T (T) below 10 K and have attributed this behavior to potential spin-fluctuations (SF) with an SF temperature (T sf ) of 27 K. An enhancement of LTHC was also observed, as manifested by a large electronic heat capacity coefficient (γ el ) of 342.9 mJ/mol•K 2 . From the heat capacity data, the following thermodynamic parameters were determined: the characteristic Debye temperature ( θ D ) over the temperature range 30 – 397 K is 177 ± 2 K, and the standard entropy ( Δ 0 298 . 15 S ∘ ) is 283.3 ± 5.7 J•mol −1 •K −1 (equivalent to 35.4 ± 0.7 J•mol −1 •atom −1 •K −1 ). Combined with our previously measured formation enthalpy ( Δ f H e l ∘ ) of U 3 Si 5 , the Gibbs free energy of formation of U 3 Si 5 from the elements ( Δ f G e l ∘ ) was determined to be –45.2 ± 9.0 kJ•mol −1 •atom −1 .

Evolution of non-Kramers doublets in magnetic field in PrNi2Cd20 and PrPd2Cd20

Praseodymium-based 1-2-20 cage compounds Pr$T_2X_{20}$ ($T$ is generally Ti, V, Nb, Ru, Rh, Ir; and $X$ is either Al, Zn or Cd) provide yet another platform to study non-trivial electronic states of matter ranging from topological and magnetic orders to unconventional multipolar orders and superconductivity. In this paper, we report measurements of the electronic heat capacity in two Pr-based 1-2-20 materials: PrNi$_2$Cd$_{20}$ and PrPd$_2$Cd$_{20}$. We find that the lowest energy multiplet of the Pr $4f^2$ valence configuration is a $\Gamma_3$ non-Kramers doublet and the first excited triplet is assumed to be a magnetic $\Gamma_5$. By analyzing the dependence of the energy splitting between the ground and first excited singlet states on external magnetic field, we found that the maximum in the heat capacity corresponding to the Schottky anomaly in PrPd$_2$Cd$_{20}$, unlike PrNi$_2$Cd$_{20}$, shows pronounced linear dependence on external magnetic field at higher field values. This effect is associated with the exchange interactions between the field-induced magnetic dipole moments.

Ab initio calculation of electron temperature dependent electron heat capacity and electron-phonon coupling factor of noble metals

Noble metal nanoparticles show fantastic catalytic property in various practical applications. It is a promising way to obtain nanoparticles from femtosecond laser ablation of noble metals. The calculation concentrates on electron heat capacity and electron–phonon coupling factor, which are two pivotal parameters describing the femtosecond laser-induced electron heating and the electron–phonon coupled energy transportation. The evolutions of these two parameters with increasing electron temperature are presented. It is found that the evolutions of the electron heat capacity and electron–phonon coupling factor are mainly affected by the electron density of states and the Fermi-Dirac distribution function.

Analytical modeling of thermal processes during femtosecond laser pulse action on metallic nanoparticle

The analytical approach has been proposed for the modeling of the thermal processes during the action of femtosecond laser pulse on metallic (gold) nanoparticle with allowance for the temperature dependences of the electron heat capacity and the electron-phonon coupling factor of gold based on thermal excitation of electron density. This leads to novel dependence of the electron-phonon equal temperature and the coupling factor on the maximal electron temperature. Whole process of femtosecond nanoparticle action has been divided into three stages which include firstly the heating of electron system by pulse, secondly the electron-phonon energy exchange and the establishment of the equal temperature of electrons with lattice, and thirdly the nanoparticle cooling with the corresponding analytical description of each stage. The satisfactory agreement with the computer results for femtosecond pulses validates of used analytical approach. This model can be used for the estimation of laser and nanoparticle parameters in the fields of the applications of femtosecond laser pulses.