اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدOn the anomalous thermal evolution of the low-temperature, normal-state specific heat of various nonmagnetic intermetallic compounds
112
0
0.0
(
0
)
تأليف
M. ElMassalami
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
The low-temperature normal-state specific heat and resistivity curves of various nonmagnetic intermetallic compounds manifest an anomalous thermal evolution. Such an anomaly is exhibited as a break in the slope of the linearized C/T versus T^2 curve and as a drop in the R versus T curve, both at the same T_{beta}{gamma}. It is related, not to a thermodynamic phase transition, but to an anomaly in the density of states curves of the phonon or electron subsystems. On representing these two anomalies as additional Dirac-type delta functions, situated respectively at kB.{theta}_L (for lattice) and kB.{theta}_E (for electrons), an analytical expression for the total specific heat can be obtained. A least-square fit of this expression to experimental specific heat curves of various compounds reproduced satisfactorily all the features of the anomalous thermal evolution. The obtained fit parameters (in particular the Sommerfeld constant, {gamma}_{0}, and Debye temperatures, {theta}_D, compare favorably with the reported values. Furthermore, the analysis shows that (i) (T_{beta}{gamma}) / ({theta}_D) = 0.2(1pm1/surd6) and (ii) {gamma}_{0} {propto} ({theta}_D)^2; both relations are in a reasonable agreement with the experimental results. Finally, this analysis (based on the above arguments) justifies the often-used procedure that treats the above anomaly in terms of either a thermal variation of {theta}_D or an additional Einstein mode.
قيم البحث
اقرأ أيضاً
Helium atoms and hydrogen molecules can be strongly bound inside interstitial channels within bundles of carbon nanotubes. An exploration of the low energy and low temperature properties of He-3 atoms is presented here. Recent study of the analogous
He-4 system has shown that the effect of heterogeneity is to yield a density of states N(E) that is qualitatively different from the one-dimensional (1D) form of N(E) that would occur for an ideal set of identical channels. In particular, the functional form of N(E) is that of a 4D gas near the very lowest energies and a 2D gas at somewhat higher energies. Similar behavior is found here for He-3. The resulting thermodynamic behavior of this fermi system is computed, yielding an anomalous form of the heat capacity and its dependence on coverage.
To obtain single crystals by solution growth, an exposed primary solidification surface in the appropriate, but often unknown, equilibrium alloy phase diagram is required. Furthermore, an appropriate crucible material is needed, necessary to hold the
molten alloy during growth, without being attacked by it. Recently, we have used the comparison of realistic simulations with experimental differential thermal analysis (DTA) curves to address both these problems. We have found: 1) complex DTA curves can be interpreted to determine an appropriate heat treatment and starting composition for solution growth, without having to determine the underlying phase diagrams in detail. 2) DTA can facilitate identification of appropriate crucible materials. DTA can thus be used to make the procedure to obtain single crystals of a desired phase by solution growth more efficient. We will use some of the systems for which we have recently obtained single-crystalline samples using the combination of DTA and solution growth as examples. These systems are TbAl, Pr$_7$Ni$_2$Si$_5$, and YMn$_4$Al$_8$.
The low-temperature specific heat of a superconductor Mo3Sb7 with T_c = 2.25 (0.05) K has been measured in magnetic fields up to 5 T. In the normal state, the electronic specific heat coefficient gamma_n, and the Debye temperature Theta_D are found t
o be 34.5(2) mJ/molK^2 and 283(5) K, respectively. The enhanced gamma_n value is interpreted due to a narrow Mo-4d band pinned at the Fermi level. The electronic specific heat in the superconducting state can be analyzed in terms a phenomenological two BCS-like gap model with the gap widths 2Delta_1/k_BT_c = 4.0 and 2Delta_2/k_BT_c = 2.5, and relative weights of the mole electronic heat coefficients gamma_1/gamma_n = 0.7 and gamma_2/gamma_n = 0.3. Some characteristic thermodynamic parameters for the studied superconductor, like the specific heat jump at T_c, DeltaC_p(T_c)/gamma_nT_c, the electron-phonon coupling constant,lambda_eph, the upper H_c2 and thermodynamic critical H_c0 fields, the penetration depth, lambda, coherence length xi, and the Ginzburg-Landau parameter kappa are evaluated. The estimated values of parameters like 2Delta/k_BT_c, DeltaC_p(T_c)/gamma_nT_c, N(E_F), and lambda_eph suggest that Mo3Sb7 belongs to intermediate-coupling regime. The electronic band structure calculations indicate that the density of states near the Fermi level is formed mainly by the Mo-4d orbitals and there is no overlapping between the Mo- 4d and Sb-sp orbitals.
Using the density functional theory (DFT) formulated within the framework of the plane-wave basis projector augmented wave (PAW) method, the temperature-dependent elastic properties of MgRE (RE=Y, Dy, Pr, Sc, Tb) intermetallics with B2-type structure
are presented from first-principles. Our calculations are based on the fact that the elastic moduli as a function of temperature mainly results from thermal expansion. The comparison between the predicted results and the available experimental data for a benchmark material NiAl provides good agreements. At $T=0K$, our calculated values of lattice parameter and elastic moduli for MgRE intermetallics show excellent agreement with previous theoretical results and experimental data. While temperature increases, we find that the elastic constants decrease and approach linearity at higher temperature and zero slope around zero temperature.
We measured the low temperature specific heat of a sputtered $(Fe_{23AA}/Cr_{12AA})_{33}$ magnetic multilayer, as well as separate $1000AA$ thick Fe and Cr films. Magnetoresistance and magnetization measurements on the multilayer demonstrated antipar
allel coupling between the Fe layers. Using microcalorimeters made in our group, we measured the specific heat for $4<T<30 K$ and in magnetic fields up to $8 T$ for the multilayer. The low temperature electronic specific heat coefficient of the multilayer in the temperature range $4<T<14 K$ is $gamma_{ML}=8.4 mJ/K^{2}g-at$. This is significantly larger than that measured for the Fe or Cr films (5.4 and $3.5 mJ/K^{2}mol$ respectively). No magnetic field dependence of $gamma_{ML}$ was observed up to $8 T$. These results can be explained by a softening of the phonon modes observed in the same data and the presence of an Fe-Cr alloy phase at the interfaces.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
