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Register a new userLow-temperature specific heat of graphite and CeSb2: Validation of a quasi-adiabatic continuous method
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Added by
Tomas Perez-Casta\\~neda
Publication date
2013
fields
Physics
and research's language is
English
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We present the application of a fast quasi-adiabatic continuous method to the measurement of specific heat at 4He temperatures, which can be used for the study of a wide range of materials. The technique can be performed in the same configuration used for the relaxation method, as the typical time constants between calorimetric cell and thermal sink at 4.2 K are chosen to be of the order of tau~30 s. The accuracy in the absolute values have been tested by comparing them to relaxation-method results obtained in the same samples (performed in situ using the same set-up), with a deviation between the absolute values < 3% in the whole temperature range. This new version of the continuous calorimetric method at low temperatures allows us to completely characterize and measure a sample within a few hours with a high density of data points, whereas when employing other methods we typically need a few days. An exhaustive study has been performed for reproducibility to be tested. In the present work, we have applied this method to two different substances: CeSb2, which exhibits three magnetic transitions at 15.5 K, 11.7 K and 9.5 K, and graphite, both highly-oriented pyrolytic graphite (HOPG) and natural crystals. Our results on these graphites are discussed in comparison with previous published data on different kinds of graphite samples.
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Very recently impurity scattering effects on quasiparticles in d-wave superconductors have attracted much attention. Especially, the thermodynamic properties in magnetic fields H are of interest. We have measured the low-temperature specific heat C(T,H) of La_1.78Sr_0.22Cu_1-xNi_xO4. For the first time, the impurity scattering effects on C(T,H) of cuprate superconductors were clearly observed, and are compared with theory of d-wave superconductivity. It is found that impurity scattering leads to gamma(H)=gamma(0)(1+D((H/H_c2)(ln(H_c2/H)) in small magnetic fields. Most amazingly, the scaling of C(T,H) breaks down due to impurity scattering.
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.
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 to 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.
Both amorphous and crystalline materials frequently exhibit low temperature specific heats in excess of what is predicted using the Debye model. The signature of this excess specific heat is a peak observed in $C/T^3$ textit{versus} $T$. To understand the curious absence of long-range ordering of local distortions in the crystal structure of pyrochlore Bi$_2$Ti$_2$O$_7$, we have measured the specific heat of crystalline Bi$_2$Ti$_2$O$_7$ and related compounds. We find that the peak in $C/T^3$ versus $T$ in Bi$_2$Ti$_2$O$_7$ falls at a substantially lower temperature than other similar compounds, consistent with the presence of disorder. This thermodynamic evidence for disorder in crystalline Bi$_2$Ti$_2$O$_7$ is consistent with quenched configurational disorder among Bi lone pairs produced by geometrical frustration, which could represent a possible realization of charge ice.
The thermodynamic properties of strongly correlated system with binary type of disorder are investigated using the combination of the coherent potential approximation and dynamical mean-field theory. The specific heat has a peak at small temperatures for the concentrations close to the filling of system. This peak is associated with the local moment formation due to Coulomb interaction. The linear coefficient to the specific heat is divergent and the system stays in the non-Fermi-liquid regime.
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