Influence of Ag doping on the critical behavior of the heat capacity of manganites

The critical behavior of the heat capacity of Ag doped manganites is studied. Changing regularities of universal critical parameters near the critical point are determined. All investigated samples up to t _min ≈ 10^?3 correspond to 3D Heisenberg universality class of the critical behavior. The universality class of critical behavior of La_{1 – x} Ag _y MnO₃ (y ≤ x) heat capacity is shown to be Ag-concentration independent.     

Critical behavior of the heat capacity of the manganite La<Subscript>0.87</Subscript>K<Subscript>0.13</Subscript>MnO<Subscript>3</Subscript>

The heat capacity of the manganite La_0.87K_0.13MnO₃ has been measured in the temperature range 80–350 K. The nature of the ferromagnetic phase transition and the critical properties of heat capacity near the Curie temperature have been studied. The regularities of variations in the universal critical parameters near the phase transition point have been established. The calculated critical exponent and amplitudes of the heat capacity with allowance for corrections on the scaling (α = −0.13 and A ⁺/A ⁻ = 1.178) correspond to the critical behavior of the 3D Heizenberg model.     

Critical behavior of the heat capacity of Ag-doped manganites

The critical behavior of the heat capacity of silver-doped manganites has been studied. The laws of variation in the universal critical parameters near the phase transition point have been established. It has been shown that the universality class of the critical behavior of the heat capacity for the La_1−x Ag _y MnO₃ (y≤x) system does not depend on the silver concentration or the synthesis conditions.     

Direct experimental verification of the isomorphism hypothesis for critical phenomena

The heat capacity at constant volume of a mixture of methane and 0.0345 mole fractions of heptane is experimentally studied over a wide range of densities and temperatures. In the case when the transition from a three-phase state takes place in the presence of a noncritical liquid phase, it is found that the behavior of the heat capacity in the vicinity of the upper end critical point is fully isomorphic with the behavior of the heat capacity in the vicinity of the liquid-vapor critical point of one-component fluid. It is shown that the measured quantity in this experiment is the heat capacity at constant volume and constant chemical potential μ of the heavy impurity component C _{v, μ}. Thus, it has been confirmed by direct measurements that the anomaly of this heat capacity completely coincides in character with the anomaly of the heat capacity at constant volume in the vicinity of the liquid-vapor critical point of one-component fluids.     

Ferromagnetic Kondo lattice CeRuSi2 with non-Fermi-liquid behavior

The structure, electronic, thermodynamic, and magnetic properties of the CeRuSi₂ Kondo lattice with ferromagnetic ordering characterized by a small moment of the ground state are investigated. Anomalies in the temperature dependences of heat capacity and resistivity (unusual power or logarithmic behavior) observed in the low-temperature range indicate a non-Fermi-liquid behavior. The results are compared with those for other Ce_lRu _n X _m compounds and anomalous systems based on rare-earth elements and actinides that had been studied earlier.     

Heat capacity study of double perovskite-like compounds BaTi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Zr<Subscript>x</Subscript>O<Subscript>3</Subscript>

The temperature dependence of the heat capacity of two compositions in the solid solution system BaTi_1?xZr_xO₃ (x = 0.25, 0.35) was measured using adiabatic calorimetry. In the T-x phase diagram, these compounds occupy positions near the crossover from conventional ferroelectric behavior to the relaxor state. Both compounds reveal diffuse heat capacity anomalies: two anomalies in the temperature ranges 250–350 and 150–200 K at x = 0.35 and one anomaly within the range ～150–320 K at x = 0.25. The results obtained are discussed together with structural and dielectric measurements.     

Heat capacity of the La<Subscript>0.9</Subscript>Ag<Subscript>0.1</Subscript>MnO<Subscript>3</Subscript> manganite near the curie temperature

The heat capacity of the La_0.9Ag_0.1MnO₃ manganite is measured in the temperature range 77–350 K and studied in detail in the vicinity of the Curie temperature for the first time. The regularities of the variation in the universal critical parameters in the vicinity of the phase transition point are established. The critical exponent and the amplitude of the heat capacity are calculated to be α = ?0.127 and A ⁺/A ^? = 1.146 with due regard for the scaling corrections. These parameters correspond to the critical behavior within the three-dimensional Heisenberg model. The size of ferromagnetic droplets in the paramagnetic range at T > T _C is estimated as ξ ≈ 19 Å. The results obtained are analyzed thoroughly and compared with theoretical data for a number of model systems.     

Influence of an adsorbing gas on the layering transitions in crystal growth

The influence of an adsorbing gas on the layering transition of a film is studied. In agreement with an earlier publication, we find that the adsorbate lowers the critical temperature T_c(n) for the nth layer. We also find that a system which is dry near T = 0 may undergo an apparent wetting transition due to the adsorbate. Indirect evidence is given that the sequence T_c(n) ends at T_R, the roughening temperature, as conjectured by de Oliviera and Griffiths. Re-entrant behavior in the layering transitions is observed for sufficiently strong adsorption energy; similar behavior also appears in the roughening transition, for which there exists some experimental evidence.     

Improved Scaling of the Magnetic Heat Capacity in La<Subscript>0.85</Subscript>Ag<Subscript>0.15</Subscript>MnO<Subscript>3</Subscript> Manganite

The heat capacity in a La_0.8 Ag_0.15 MnO₃ manganite has been measured near the Curie temperature T _C in applied magnetic fields up to 26 kOe to study the scaling critical behavior and to obtain the universality class. The conventional scaling fails in application to the manganites with a hysteresis and the strong sensitivity of T _C to a magnetic field. However, the application of the improved scaling procedure designed by us allows yielding the good scaling the magnetic heat =0.23 capacity in La_0.85Ag_0.15MnO₃, which may belong to a new universality class for systems with the strong spin-orbital coupling of t _2g -electrons, namely, double -Heisenberg with the critical exponent of the heat capacity α = ?0.23 and the critical exponent of the correlation radius v=0.7433. This new universality class is consistent with the crystal, magnetic and orbital symmetries for the La_0.85Ag_0.15MnO₃. Scaling failure in the vicinity of T _C in the range of t/H ^1/2ν ≈ [?0.033;0.024] is understood by finite-size and other disordering effects when T →T _C. It is remarkable that finite-size effect is consistent with grain size, L ≈ 50 μm, in the La_0.85Ag_0.15MnO₃. The correlation radius, Lt ^ν ≈ 30.28 Å, estimated from the finite-size effect is of the same order of magnitude with the sizes of the ferromagnetic fluctuations and drops in manganites.     

Investigation of the electrical and magnetic properties of electron-doped Ruddlesden-Popper phases, CaO(Pr0.08Ca0.92MnO3)n (n=1, 2, 3 and ∞)

The electrical and magnetic properties of Pr^3±-doped Ruddlesden-Popper manganates with the formula, CaO(Pr_0.08Ca_0.92MnO₃)_n (n=1,2,3 and ∞) have been investigated. The electrical conductivity of the samples increases with the number of perovksite slabs sandwiched between the rock-salt-like CaO layers. Magnetic susceptibility measurements show an anomalous behavior for the n=1 composition which is attributed to the near absence of ferromagnetic correlations in a two-dimensional system. The magnetic behavior of other members can, however, be understood on the basis of a ‘ferromagnetic-droplet’ model. Since the number of electrons per perovskite slab is invariant across the series, the electronic properties are readily correlated to the phase space available for exchange interactions.     

Low temperature specific heat and magnetic properties of V/FE and V/FE/H alloys

Low temperature specific heats of V_1?xFe_x (0?x?0.34) and V_1?xFe_xH_n (x = 0.02; 0.05; 0.1; 0.15 a 0 ? n ? 0.98) were investigated between 1.5 and 16 K.In V/Fe alloys temperature independent band paramagnetism was observed at iron concentrations below 20at%; at higher iron contents local magnetic moments occur and contribute magnetic terms to the low temperature specific heat. According to Schröder [14] these contributions can be explained by thermal agitation of superparamagnetic clusters of the Fe atoms with local moments. In the temperature range of the specific heat measurements the magnetic terms are dependent on temperature (contrary to Schröder) and can be described by Planck-Einstein functions. A separation of the magnetic contributions from the lattice and electronic term of heat capacity could be obtained in a satisfying way by least squares fitting with three Planck-Einstein functions.In “nonmagnetic” V/Fe alloys with less than 20at% Fe absorption of hydrogen generates local magnetic moments and superparamagnetic contributions. These are, therefore, connected primarily more with the band electron concentration n_e, than with the iron content X_Fe At the highest n_e, values, n_e >0, large negative deviations from the regular course of the heat capacity plot are observed; these can be related to mictomagnetic behavior (freezing temperature of spin-glass state above the range of measurements, T_F > 16 K).After correction for the superparamagnetic contributions there remains in the Fe containing alloys an appreciable enhancement of the electronic heat coefficient γ, up to a factor of about 2 when compared with the iron-free V/H samples. This effect is interpreted as resulting from mass enhancements of the itinerant electrons by electron-magnon (and-paramagnon) interactions.     

Anomalies in the thermodynamic properties of mixtures near the liquid-vapor critical point

The behavior of the specific heat at constant volume and the derivative (?P/?T)_{ρ, x} has been studied experimentally near the liquid-vapor critical point of the methane-propane-pentane mixture. It has been shown that the derivative ?P/?T)_{ρ, x} for mixtures, in contrast to one-component systems, has a characteristic anomaly directly associated with an anomaly in the specific heat at constant volume. It is impossible to distinguish between the infinite increase in the specific heat or finite peak when processing of the experimental data only for the heat capacity. However, according to the joint analysis of the experimental data for both indicated quantities, both the specific heat of the mixture and the derivative ?P/?T)_{ρ, x} are singular finite quantities.     

Effect of γ irradiation on the heat capacity of (CH3)2NH2Al(SO4)2 · 6H2O crystals near the ferroelectric phase transition

The heat capacity of dimethyl ammonium-aluminum sulfate crystals (DMAAS), both nonirradiated and γ-irradiated to fluences of 10⁷, 5×10⁷, and 10⁸ R, has been measured by the adiabatic method near the ferroelectric phase transition (PT) within the 80–300 K temperature range. The C _p =f(T) curve exhibits a λ-shaped anomaly near the phase-transition point T _C =152 K. The PT temperature and the magnitude of the anomaly are shown to decrease with increasing γ-irradiation fluence. It has been established that the ferroelectric PT at T _C =152 K, which lies close to the tricritical point, shifts progressively more under γ irradiation toward the second-order PT, and that the behavior of the anomalous part of the heat capacity in the ferroelectric phase is described by the thermodynamic theory of Landau. The experimental heat-capacity data have been used to calculate the variation of the thermodynamic functions of the DMAAS crystal.     

Enhanced paramagnetism of TiBe2 and ferromagnetic transitions in TiBe2-xCux

The magnetic behavior of TiBe₂ is found to be similar to that of strongly enhanced paramagnets like Pd and Ni₃Ga. The susceptibility curves _χ(H, T = 0) and _χ(H, T = 0) both go through a smooth maximum, at 55 kOe and 10 K, respectively, which might be due to the electron-electron interaction. For TiBe_2-xCu_x compounds the transition from paramagnetism to ferromagnetism is analyzed, starting from Arrott plots of the magnetization. A critical concentration x_cr = 0.155 ± 0.005 is obtained. The Stoner-Edwards-Wohlfarth model of itinerant-electron magnetism is well followed near x_cr. The low temperature behavior of the specific heat of TiBe₂ is tentatively ascribed to the effect of the electron-phonon interaction on the electronic specific heat coefficient γ.     

Magnetic contribution to heat capacity and entropy of nickel ferrite (NiFe2O4)

The heat capacity of nickel ferrite was measured as a function of temperature from 50 to 1200 °C using a differential scanning calorimeter. A thermal anomaly was observed at 584.9 °C, the expected Curie temperature, T_C. The observed behavior was interpreted by recognizing the sum of three contributions: (1) lattice (vibrational), (2) a spin wave (magnetic) component and (3) a λ-transition (antiferromagnetic-paramagnetic transition) at the Curie temperature. The first was modeled using vibrational frequencies derived from an experimentally-based IR absorption spectrum, while the second was modeled using a spin wave analysis that provided a T^3/2 dependency in the low-temperature limit, but incorporated an exchange interaction between cation spins in the octahedral and tetrahedral sites at elevated temperatures, as first suggested by Grimes [15]. The λ-transition was fitted to an Inden-type model which consisted of two truncated power law series in dimensionless temperature (T/T_C). Exponential equality (m=n=7) was observed below and above T_C, indicating symmetry about the Curie temperature. Application of the methodology to existing heat capacity data for other transition metal ferrites (AFe₂O₄, A=Fe, Co) revealed nearly the same exponential equality, i.e., m=n=5.     

The Lorentz-Lorenz function of five gaseous and liquid saturated hydrocarbons

The refractive indices of compressed gaseous and liquid ethane, propane,n-butane,n-pentane, and liquidn-hexane were measured on up to seven isotherms at temperatures between 290 K and 500 K. Densities ranged from 0.2 g/cm³ to 0.7 g/cm³ at pressures up to 170 bar. The refractive index valuesn were combined with simultaneously measured density values ?, to compute the Lorentz-Lorenz function LL =(n ²-1)M/(n ²+2) ?=A _n+B_n(?/M), whereM is the molar mass andA _n andB _n are the first and second refractive index virial coefficients. Additionally, the functions LL/z (z=pV/nRT is the compressibility factor) of ethane, propane, andn-butane were computed to determine the second refractive index virial coefficientsB _n(T). The relative uncertainty of the measured LL functions is 0.1% in most cases. At constant temperature (LL?A _n)/A_n is ≦5·10^?3 for ethane, propane, andn-butane and ≦0.02 forn-pentane andn-hexane. The greatest deviations of the LL functions from theA _n value were measured at the temperatures near the critical points. TheA _n-values show a temperature dependence which increases with increasing molar mass (maximum value: ΔA _n/ΔT=0.0044 cm³/mol · K). The slopes of the LL-function become lower and the LL maxima become smaller with increasing distance of the temperature from the critical point. TheB _n values increase linearly with reciprocal temperature.     

Criticality in the randomness-induced second-order phase transition of the triangular Ising antiferromagnet with nearest- and next-nearest-neighbor interactions

Using a Wang-Landau entropic sampling scheme, we investigate the effects of quenched bond randomness on a particular case of a triangular Ising model with nearest- (J_nn) and next-nearest-neighbor (J_nnn) antiferromagnetic interactions. We consider the case R=J_nnn/J_nn=1, for which the pure model is known to have a columnar ground state where rows of nearest-neighbor spins up and down alternate and undergo a weak first-order phase transition from the ordered to the paramagnetic state. With the introduction of quenched bond randomness we observe the effects signaling the expected conversion of the first-order phase transition to a second-order phase transition and using the Lee-Kosterlitz method, we quantitatively verify this conversion. The emerging, under random bonds, continuous transition shows a strongly saturating specific heat behavior, corresponding to a negative exponent α, and belongs to a new distinctive universality class with ν=1.135(11), γ/ν=1.744(9), and β/ν=0.124(8). Thus, our results for the critical exponents support an extensive but weak universality and the emerged continuous transition has the same magnetic critical exponent (but a different thermal critical exponent) as a wide variety of two-dimensional (2d) systems without and with quenched disorder.     

Critical behavior of heat capacity of the SC(NH2)2 ferroelectric in the region of incommensurate phase transition

The anomalous (nonclassical) behavior of heat capacity in the region of the second-order phase transition “initial phase-incommensurate phase” was experimentally observed in the SC(NH₂)₂ ferroelectric. Such a critical behavior of heat capacity above and below the temperature of incommensurate phase transition is shown to be qualitatively consistent with the fluctuation theory of XY-type systems.     

Critical behavior in the vicinity of nematic to smectic A (N–SmA) phase transition of two polar–polar binary liquid crystalline systems

Phase diagram, critical behavior and order of the nematic (N)–smectic A (SmA) phase transition of two polar–polar binary systems (i) 4-n-heptyloxy-4′-cyanobiphenyl (7OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB); (ii) 4-n-octyloxy-4′-cyanobiphenyl (8OCB) and 4-n-nonyloxy-4′-cyanobiphenyl (9OCB) by means of a high-resolution temperature scanning measurement of birefringence have been reported in this work. A simple power law analysis has been adopted to extract the specific heat critical exponent (α′) at N–SmA transition from birefringence data. The α′ for N–SmA transition indicates a uniform crossover behavior and has appeared to be non-universal in nature. With increasing concentration of the higher homologues for both the binary systems, the N–SmA transition reveals a strong tendency to be driven towards the tricritical nature. The 3D-XY limit (i.e. α′ = ?0.007) for N–SmA transition reaches at the concentration x_8OCB = 0.28 corresponding to the McMillan ratio 0.914, whereas the tricritical point has been found to appear near x_9OCB = 1.0 corresponding to McMillan ratio 0.992.     

Specific heat and critical behavior of Tl0.3MoO3 near the Peierls phase transition

The specific heat of the quasi-one-dimensional charge-density-wave (CDW) compound Tl_0.3MoO₃ has been measured using an adiabatic continuous heating method from 100 to 220 K. A specific heat jump associated with the Peierls phase transition occurs at 172.3 K. A good scaling relation between the excess specific heat and the susceptibility is found between 140 K and 190 K. Further analysis indicates that the width of the critical region of Tl_0.3MoO₃ is about 10 K and the specific-heat critical behavior can be well described by the three-dimensional XY model.