Heat capacity of Ln 2Cu2O5 (Ln = Tb-Lu) cuprates

The correlation between the heat capacity of heavy lanthanide cuprates and the specific features of their structure has been analyzed. It has been shown that the heat capacity regularly changes depending on the Ln ³⁺ ion radius and the global instability index (GII) within the limits of corresponding tetrads (Tb-Ho, Er-Lu).     

Interaction of binary cuprates with oxygen and water vapor at temperatures of 200–400°C

Interaction of binary cuprates with oxygen and water vapor at T = 200–400°C has been studied. It has been established that only compounds containing oxygen vacancy chains in their structure can absorb oxygen and moisture from annealing atmosphere. Absorption of oxygen brings about decrease in the lattice parameters while embedding of OH^? groups leads to their growth. In contrast to YBa₂Cu₃O _y , binary cuprates do not undergo phase transitions in interaction with the atmosphere. Saturation with water and formation of oxyhydroxides is followed by their hydrolytic decomposition involving formation of simpler oxides and hydroxides.     

Topological Scenario for High-Temperature Superconductivity in Cuprates

The structure of the joint phase diagram of high-temperature superconducting cuprates has been studied within the theory of fermion condensation. Prerequisites of the topological rearrangement of the Landau state with the formation of a flat band adjacent to the nominal Fermi surface have been established. The related non-Fermi-liquid behavior of cuprates in the normal phase has been studied with focus on the non-Fermi-liquid behavior of the resistivity ρ(T), including the observed crossover from the linear temperature behavior ρ(T, x) = A₁(x)T at doping levels x below the critical value x _c ^h corresponding to the boundary of the superconducting region to the quadratic temperature behavior at x > x _c ^h , which is incompatible with predictions of the conventional quantum-critical-point scenario. It has been demonstrated that the slope of the coefficient A₁(x) is universal and is the same on both boundaries of the joint phase diagram of cuprates in agreement with available experimental data. It has also been shown that the fermion condensate is responsible for pairing in the D-wave state in cuprates. The effective Coulomb repulsion in the Cooper channel, which prevents the existence of superconductivity in normal metals in the S channel, leads to high-temperature superconductivity in the D channel.     

Thermodynamics of textures in superfluid 3He

The contribution of the textures in the B-phase of superfluid ³He to the specific heat has been calculated. The specific heat is found to be divergent near T_c.     

Heavy fermion behaviour of the nonmagnetic CeCu4Al compound

We present measurements of the temperature dependence of the electrical resistivity, the thermopower and the specific heat of the hexagonal compound CeCu₄Al. At high temperatures, the electrical resistivity is characterized by a nearly temperature independent behaviour, followed by a continuous increase below 100K. No maximum has been found down to 1.7 K. The thermopower shows a positive maximum at about 30 K. As in CeCu₆ no negative values are observable in the range from 4.2 K up to a room temperature. The specific heat data between 7 and 15 K reveal a γ value around 280 mJ mol^-1 K^-2. Below this temperature range the specific heat c_p/T shows a rapid rise and crosses the value of 1 J mol^-1 K^-2 at about 1.45 K.     

High electronic specific heat of some cubic UX3 intermetallic compounds

The low temperature specific heat of cubic UX₃ intermetallic compounds with X = Al, Ga, In, Si, Ge and Sn have been measured. High values for the coefficient of the electronic specific heat have been found, ranging from 14 to 169 mJ/mol K².     

Specific heat of Tb x Y1−x Sb mixed crystals

The specific heat of Tb _x Y_1?x Sb mixed crystals (x=0, 0.103, 0.383, 0.428, 0.467, 0.635, 0.928 and 1.0) has been measured between 1.6 and 20°K using an adiabatic calorimeter. The crystal field potential is described well taking into account only 4th order terms, and the overall splitting of the ground multiplet⁷F₆ of the Tb³⁺ ion has been found to be 115°K independent of the concentrationx. Forx>0.46 the specific heat curves exhibit a behaviour typical for a second order phase transition. For lower concentrations a normal Schottky anomaly is found and no evidence for magnetic order was detected. The experimental results which are in agreement with magnetic measurements are compared with molecular field calculations including crystal field and exchange interaction.     

Facile synthesis and luminescence properties of uniform and monodisperse KY3F10:Ln3+ (Ln=Eu,Ce, Tb) nanospheres

Highly uniform and monodisperse KY₃F₁₀:Ln³⁺ (Ln=Eu, Ce, Tb) nanospheres, with an average diameter of 300 nm, have been successfully prepared through a simple template-free and surfactant-free stirring method under ambient conditions. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL) spectra were used to characterize the samples. The SEM images illustrate that these spheres were actually composed of randomly aggregated nanoparticles. The doped rare earth ions show their characteristic emission in the KY₃F₁₀ samples, i.e., Eu^{3+ 5}D₀–⁷F_J (J=1, 2, 3, 4), Tb^{3+ 5}D₄–⁷F_J (J=6, 5, 4, 3, 2) and Ce³⁺ 5d–4f transition emissions, respectively. An energy transfer phenomenon from Ce³⁺ to Tb³⁺ has been observed in KY₃F₁₀ nanospheres, and the energy transfer efficiency depends on the doping concentration of Tb³⁺ if the concentration of Ce³⁺ is fixed.     

Heat capacity and magnetic susceptibility of ReO3

The specific heat and magnetic susceptibility of the transition metal oxide ReO₃ have been measured. The specific heat results give a Debye temperature Θ_D = 460 ± 10 K and an electronic specific heat coefficient γ = 6.45 ± 0.07 cal/mole K² which are in good agreement with similar measurements on the cubic sodium tungsten bronzes. The magnetic susceptibility and the electronic contribution to the specific heat are within a few percent of the corresponding parameters calculated from the free electron model with one electron per unit cell. Our results show that ReO₃ behaves much like a simple metal. No experimental evidence for narrow d-band effects was observed.     

Nuclear quadrupole resonance in arsenic metal

The nuclear quadrupole resonance in arsenic metal, in the temperature range 4.2 K – 396 K, has been studied. e²qQ/h at 4.2 K is 47.123 MHz. It is in good agreement with the one predicted by the low temperature specific heat measurements.     

Antiferromagnetic linear chains in the crystalline free radical tanol

TANOL (tetramethyl-2,2,6,6, piperidinol-4, oxyl-1), which shows a broad maximum in its specific heat near 4 K was known as an example of the Heisenberg type one-dimensional antiferromagnet. The specific heat has been measured down to 0.35 K. A sharp peak appears at 0.49 K (T_N) and a T³ law is observed in the low temperature region. The interchain interactions which can be deduced are the same order of magnitude as the spin-spin dipolar interactions. The linear chain characteristics of TANOL are compared to those of some other products.     

Low temperature specific heats of erbium dihydrine. Magnetic transition and crystal field splitting effects

The specific heat of erbium dihydride, ErH_1.99, was measured in the temperature range from 1.8–230 K. A λ-type specific heat anomaly, most probably due to an antiferrómagnetic transition has been found at T_N = 2.13 ± 0.03 K. The influence of the crystal field of cubic symmetry on the splitting of the ground state ⁴I_{$\text{15}\text{2}$} of the Er³⁺ ion was analysed and the conclusion was drawn, that the doublet Γ₇ is the lowest lying crystal-field level.     

Effect of hydration on the structure of perovskite-like cuprates

The effect of hydration at T = 150 and 200°C on the structures of YBa₂Cu₃O_y (123) and a number of binary cuprates has been studied. It has been shown that the compounds containing oxygen vacancies in their structures interact with hydrogen significantly more strongly than cuprates without vacancies. Depending on the cuprate structure, hydrogen can be embedded in interstitial sites with the formation of hydrides and be attached to oxygen with the formation of hydroxides. The phase transition of the 123 phase to a defect tetragonal 124-type phase occurs only for the compounds with a high oxygen content. All the cuprates under study are more stable to reduction as compared to CuO.     

Local structure study of T′ cuprate superconductors

Undoped T′-type cuprates are believed to be charge transfer (Mott-Hubbard) insulators. Recent observation of undoped superconducting T′-type cuprates suggests half-filling weakly correlated metallic electron states (Tsukada et al.), which is against the fundamental picture. Here we report the local structures of undoped T′-(La³⁺,Y³⁺)₂CuO₄ and doped T′-(La³⁺,Ce⁴⁺)₂CuO₄ thin-film single crystals by polarized extended X-ray absorption fine structure (EXAS). The in-plane (E//ab) polarized Cu K-EXAFS data shows a remarkable Cu-O bond distance displacement. The broadened Cu-O distribution suggests a large local lattice distortion in T′-type structures, which might influence the electronic structure.     

First-principles investigation of the thermodynamic properties of two-dimensional MoS2

Density-functional perturbation theory has been applied to investigate the thermodynamic properties of two-dimensional MoS₂ within the Perdew-Burke-Ernzerhof genealized gradient approximation (PBE-GGA). The Murnaghan’s isothermal equation of state has been derived for the monolayer, giving how the pressure and the total energy of the monolayer evolve versus the volume of the unit cell. The temperature-dependent behavior of some thermodynamic quantities, including the Helmholtz free energy, total energy, Debye temperature, mean square displacements, specific heat, entropy and number of microstates, have been determined as well as their functional forms within the quasi-harmonic approximation, by calculating the partial phonon density of states as a fundamental quantity. The value of 45.6 cm${}^{?1}$ has been found for the phonon band gap in the frequency range, separating the acoustic and the optical phonon bands in fair agreement with the literature. The results broadly support the view that the validity of the Debye T³-law for low-temperature specific heat of solids is violated for the case of two-dimensional MoS₂ and therefore, a T²-law is proposed, accurately describing the behavior of the isochoric specific heat from 0 to 60 K.     

Specific heat anomaly of single phase YBa2Cu3O7-δ at superconducting transition temperature

The specific heat of single phase YBa₂Cu₃O_7-δ has been measured using non-adiabatic method between 4.2K and 120K. There is a specific heat anomaly Δc at 90K (about 3.2% of total specific heat) approximately, due to superconducting transition. From the measured value of ΔC and transition temperature T_c, the electronic density of state at Fermi level N(E_F) and Sommerfeld parameter γ calculated are 2.55±0.30states/eV.Cu-atom and 2.77±0.30 mJ/mole.K², respectively. The experimental result of N(E_F) is consistent with that of the band calculation by Mattheiss. The Debye temperature above T_c in this material deduced from Debye function is about 340K. Below 20K, the relation C=γ'T+βT³ is satisfied. But the value of γ' is smaller. That means, most of the electrons have formed superconducting Cooper pairs which give no contribution to specific heat below 20K.     

The low temperature specific heat of Lu-Cu-Y metallic glasses

The specific heat of a series of amorphous metallic alloys of the form Lu_xCu_0.37Y_0.63-x (x = 0, 0.1, 0.3 and 0.4) has been measured between 2 and 50 K, primarily in order to be able to determine the non-magnetic contributions of the specific heat in magnetic RE-Cu-Y amorphous alloys. The data at low temperature fit the simple form C_p = γT + βT³ from which values of γ and θ_D(0) have been determined. Consideration is given to the error that arises if Y is used rather than Lu or La in forming non-magnetic rare earth intermetallics for purposes of determining the non-magnetic contributions to the specific heat of magnetic samples. A simple procedure is described that allows a useful improvement in accuracy in estimating non-magnetic contributions below 20 K if Y is used. The method may also be useful if only a restricted range of compositions using Lu is possible.     

Anomalies of the low-temperature specific heat of the cuprates La2CuO4 and La2−x MxCuO4 (M = Sr, Ba)

The low-temperature specific heat of the cuprates La₂CuO₄, La_2−x M_xCuO₄ (M = Sr, Ba) in the temperature interval 2–45 K has been investigated by the technique of pulsed differential calorimetry. It is found that the coefficient of the residual linear term in the specific heat remains constant in the entire experimental temperature interval. It is shown that La atoms play a special role in the formation of the anomaly, associated with the specific nature of the interaction of these atoms with their environment, in the acoustic region of the phonon spectrum of these objects near 6 meV. Fiz. Tverd. Tela (St. Petersburg) 39, 1000–1004 (June 1997)     

The mysterious pseudogap in high temperature superconductors: an infrared view

We review the contribution of infrared spectroscopy to the study of the pseudogap in high temperature superconductors. The pseudogap appears as a depression of the frequency dependent conductivity in the c-axis direction and seems to be related to a real gap in the density of states. It can also be seen in the Knight shift, photoemission and tunneling experiments. In underdoped samples it appears near room temperature and does not close with temperature. Another related phenomenon that has been studied by infrared is the depression in the ab-plane scattering rate. Two separate effects can be discerned. At high temperatures there is broad depression of scattering below 1000 cm⁻¹ which may be related to the gap in the density of states. At a lower temperature a sharper structure is seen, which appears to be associated with scattering from a mode at 300 cm⁻¹, and which governs the carrier life time at low temperatures. This mode shows up in a number of other experiments, as a kink in angle resolved photoemission dispersion, and a resonance at 41 meV in magnetic neutron scattering. Since the infrared technique can be used on a wide range of samples it has provided evidence that the scattering mode is present in all high temperature cuprates and that its frequency in optimally doped materials scales with the superconducting transition temperature. The lanthanum and neodymium based cuprates do not follow this scaling and appear to have depressed transition temperatures.     

Low temperature heat capacity and magnetic excitation of HoAl2 in a magnetic field

The specific heat of single crystalline HoAl₂ in magnetic fields up to 7.5 T has been measured for the temperature range 1.5–16 K. In addition the energy of a magnetic excitation in a magnetic field of 5 T at 4.2 K has been determined by inelastic neutron scattering. The results have been interpreted with a cubic crystalline electric field and an exchange interaction using the same parameter set B₄=-0.85×10^-4 meV, B₆=+0.71× 10^-6 meV and T_C=31.5 K previously obtained by magnetization measurements.