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1.
The magnetization M(H) in the superconducting state, dc magnetic susceptibility χ(T) in the normal state, and specific heat C(T) near the superconducting transition temperature T c have been measured for a series of fine-crystalline YBa2Cu3O y samples having nearly optimum values of y = 6.93 ± 0.3 and T c = (91.5 ± 0.5) K. The samples differ only in the degree of nanoscale structural inhomogeneity. The characteristic parameters of superconductors (the London penetration depth and the Ginzburg–Landau parameter) and the thermodynamic critical field H c are determined by the analysis of the magnetization curves M(H). It is found that the increase in the degree of nanoscale structural inhomogeneity leads to an increase in the characteristic parameters of superconductors and a decrease in H c(T) and the jump of the specific heat ΔC/T c. It is shown that the changes in the physical characteristics are caused by the suppression of the density of states near the Fermi level. The pseudogap is estimated by analyzing χ(T). It is found that the nanoscale structural inhomogeneity significantly enhances and probably even creates the pseudogap regime in the optimally doped high-T c superconductors.  相似文献   

2.
Heat capacity and resistivity of Sm0.55Sr0.45MnO3 ceramics were measured over the temperature range 80–300 K in magnetic fields of up to 26 kOe. These quantities show anomalies caused by the magnetic and structural phase transitions. The critical temperature T c and the heat capacity jump ΔC p (T c ) at T c increase with increasing applied magnetic field H, while the resistivity decreases. The temperature dependences of the measured quantities show hysteresis, which is strongly suppressed in a field of 26 kOe but is sensitive nor to the temperature range neither to the rate of temperature change. The hysteresis of the heat capacity and resistivity of Sm0.55Sr0.45MnO3 is caused by a change in T c with changing lattice parameters upon second-order structural phase transition. The results are discussed in terms of the electron phase separation model.  相似文献   

3.
The specific heat of LaAl2 and (La1-xCex)Al2 (x ? 0.0064) has been measured between 0.3 and 5 K, both in the superconducting and in the normal state. For all samples the same values for the Debye temperature as well as for the electronic specific heat coefficient have been determined. LaAl2 shows an excellent BCS behavior. A remarkable excess specific heat at low temperatures due to the Kondo effect has been observed for all superconducting as well as for the normal conducting (La1-xCex) Al2 alloys. The specific heat jump ΔC at Tc depressed rapidly with increasing Ce concentration, allows the Kondo temperature TK ? 1 K to be determined. ΔC vanishes at finite temperatures.  相似文献   

4.
The properties of the superconducting state in the amorphous Sn1?xCux thin films were characterized. The concentration of copper changes in the range from 0.08 to 0.41. The calculations were conducted in the framework of the strong-coupling formalism, wherein the Eliashberg functions determined in the tunnel experiment were used. The value of the Coulomb pseudopotential equal to 0.1 was adopted. It will be shown that the critical temperature (TC) decreases from 7 to 3.9 K. The ratio, RΔ = 2Δ(0)/kBTC, differs from the BCS value: RΔ ∈ <4.4, 3.95>, where Δ(0) represents the order parameter. Similarly behave the ratios: RC = ΔC(TC)/CN(TC) ∈ <2.2, 1.75> and RH = TCCN(TC)/H2C(0) ∈ <0.141, 0.154>. The parameter ΔC(TC) is the specific heat jump, CN(TC) denotes the specific heat of the normal state and HC(0) is the thermodynamic critical field.  相似文献   

5.
The unit cell parameters a and c of nonirradiated [N(C2H5)4]2ZnBr4 crystals in the temperature region 90–300 K and of samples irradiated with γ rays to doses of 106 and 5 × 106 R in the 270-to 300-K interval were measured using x-ray diffraction. The data obtained were used to derive the thermal expansion coefficients αa and αc. It is shown that the parameter a increases and the parameter c decreases with increasing temperature. In the vicinity of the phase transition (PT) at T = 285 K, the temperature dependences of a(T) and c(T) reveal anomalies in the form of jumps and the αa(T) and αc(T) curves have a maximum and a minimum, respectively. The heat capacity of nonirradiated and irradiated [N(C2H5)4]2ZnBr4 samples was measured by adiabatic calorimetry. A maximum was found in the C p(T) curve at T = 285 K. Both x-ray diffraction and heat capacity measurements showed that the PT temperature decreased after γ irradiation.  相似文献   

6.
在16.0°K—20.3°K之间测量了Nb3Sn样品的热容量。Nb3Sn在临界温度附近的比热跳跃值ΔC=2.21(±5%)焦耳/克分子·度。样品的临界温度Tc=17.88°K,转变宽度ΔTc≈0.2°K。ΔC值利用热力学关系式确定了Nb3Sn在0°K时的热力学临界场H0=5300奥斯特。利用本文的结果和文献上关于热膨胀系数的跳跃值Δα及?T/?P值验证了热力学关系式。扼要地描述了比热测量装置.  相似文献   

7.
The temperature dependence of heat capacity C p(T) was studied for nine rare-earth hexaborides MB6(M=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) at temperatures of 5–300 K. Using the correspondence principle for lattice heat capacities of isostructural compounds, the lattice contribution C 1(T) and the excess contribution ΔC(T) to the heat capacity of the hexaborides were determined. The lattice heat capacity C 1(T) is represented as the sum of the Debye contributions of the metal and boron sublattices: C 1(T)=C M (T)+6C B(T). The Debye temperatures πM and πB of the metal and boron sublattices were determined. The anomalies in the excess heat capacity ΔC(T)=C p (T)?C 1(T) are related to the magnetic ordering effects, the Schottky contribution, and the Jahn-Teller effect.  相似文献   

8.
《Current Applied Physics》2018,18(11):1280-1288
In this work, we pointed out that Sr substitution for Ca leads to modify the magnetic and magnetocaloric properties of Pr0.7Ca0.3-xSrxMnO3 compounds. Analyzing temperature dependence of magnetization, M(T), proves that the Curie temperature (TC) increased with increasing Sr content (x); TC value is found to be 130–260 K for x = 0.0–0.3, respectively. Using the phenomenological model and M(T,H) data measured at several applied magnetic field, the magnetocaloric effect of Pr0.7Ca0.3-xSrxMnO3 compounds has been investigated through their temperature and magnetic field dependences of magnetic entropy change ΔSm(T,H) and the change of the specific heat change ΔCP(T,H). Under an applied magnetic field change of 10 kOe, the maximum value of -ΔSm is found to be about 3 J/kg·K, and the maximum and minimum values of ΔCP(T) calculated to be about ±60 J/kg·K for x = 0.3 sample. Additionally, the critical behaviors of Pr0.7Ca0.3-xSrxMnO3 compounds around their TC have been also analyzed. Results suggested a coexistence of the ferromagnetic short- and long-range interactions in samples. Moreover, Sr-doping favors establishing the short-range interactions.  相似文献   

9.
The temperature dependences of the intense magnetocaloric effect ΔT AD(T, H) and the heat capacity C p (T) of the (La0.4Eu0.6)0.7Pb0.3MnO3 manganite are directly measured using adiabatic calorimetry. The experimental dependences ΔT AD(T) are in satisfactory agreement with those calculated from the data on the behavior of the magnetization. The factors responsible for the absence of an anomaly in the experimental temperature dependence of the heat capacity C p (T) in the range of the magnetic phase transition are discussed.  相似文献   

10.
A theory of the thermodynamic properties of a two-band superconductor with a low carrier density is developed; it is based on a phonon superconductivity mechanism with a strong electron-phonon coupling. This theory can describe the variation of the critical temperature T c, the energy gaps Δ1 and Δ2, and the relative electronic specific heat jump (C S ? C N)/C N at T = T c with the carrier density in the compound MgB2 when substitutional impurities of various valences are introduced into this system. The values of T c, Δ1, and Δ2 are shown to decrease as this compound is doped by electrons and to remain constant (or almost constant) as it is doped by holes. This behavior follows from the mechanism of filling the σ and π energy bands, which overlap at the Fermi surface. The theory agrees qualitatively with experimental data. This agreement is found to be better when intra-and interband electron scattering by an impurity potential is taken into account.  相似文献   

11.
The effects due to localized spin fluctuations (LSF) in dilute superconducting alloys are investigated. Expressions for the critical temperature Tc as function of impurity concentration nI and the specific heat jump at the transition ΔC(Tc) are obtained, both in the weak and strong magnetic limits. The results are discussed in the light of available experimental data.  相似文献   

12.
The temperature dependences of the specific heat C(T) and thermal conductivity K(T) of MgB2 were measured at low temperatures and in the neighborhood of T c . In addition to the well-known superconducting transition at T c ≈40 K, this compound was found to exhibit anomalous behavior of both the specific heat and thermal conductivity at lower temperatures, T≈10–12 K. Note that the anomalous behavior of C(T) and K(T) is observed in the same temperature region where MgB2 was found to undergo negative thermal expansion. All the observed low-temperature anomalies are assigned to the existence in MgB2 of a second group of carriers and its transition to the superconducting state at Tc2≈10?12 K.  相似文献   

13.
The superconducting state of W5SiB2 with the T2-phase structure has been investigated by a specific heat measurement and a density of state calculation. The estimated ΔCp/γTc and 2Δ(0)/kBTc values are 1.49 and 3.32, which are close to 1.43 and 3.53 within the weak coupling regime. The electronic specific heat data clearly indicates the absence of gap nodes in the superconducting order parameter, suggesting an isotropic s-wave superconductor. From the density of state calculations, we found that W d-orbital plays an important role for the superconductivity in W5SiB2.  相似文献   

14.
祝可嘉  秦晓岿  陈鸿  吴翔 《物理学报》1993,42(10):1612-1616
测量了高聚物材料聚丁二烯的比热,发现在温度Tg=178K出现玻璃转变且转变点附近的比热与降温过程有关。在降温过程中,若控制样品在某一温度Twg等待时间tw,则升温比热测量表明,Tg处的比热跃变△cp存在明显的等待时间效应,即△cp随tw的增大而增大。在Tw=169K条件下,△cp(t< 关键词:  相似文献   

15.
MgB1.9C0.1 samples are synthesized under the ambient pressure (AP) and high pressure (HP), respectively. The further studies demonstrate different field-dependence of the critical current density Jc(H) in each sample. In the view of two-gap superconductivity in these samples, δTc pinning (resulting from the spatial fluctuations of the transition temperature) is dominant in the AP sample, while in the HP sample, both δTc and δl pinning (due to the mean-free-path fluctuations) act together and their contributions vary with temperature. Besides the improvement of Hc2(0), due to the different pinning mechanism, Jc(H) of the HP sample shows a slower decay with the increasing fields than that of the AP sample in high fields, which suggests a possible method of retarding the rapid decay of Jc(H) under elevated fields.  相似文献   

16.
Using Anderson model in the non-magnetic limit, we have calculated the jump in specific heat at Tc, (ΔC), for superconducting virtual bound state alloys. It is found that the normalized (ΔC) vs normalized Tc curves deviate considerably from the BCS value. A more significant result of the present study is that the initial slope of such a curve has a maximum value of 3.638. It is the highest value reported in literature for the impurity problem and is much larger than obtained even in a large pair breaking situation like Kondo effect (2.481).  相似文献   

17.
The temperature dependence of the heat capacity at a constant pressure C p 0 = f(T) for the dimerized phase of the C60 fullerene in the temperature range 300–575 K and the thermodynamic characteristics for depolymerization of this phase under normal pressure are investigated using precision differential scanning calorimetry. It is established that thermal depolymerization is a kinetically hindered process. The final products of thermal depolymerization are identified as a partially crystalline monomer face-centered cubic phase of C60 with a degree of crystallinity α = 67 mol %. The results obtained in this study and our previous experimental data on the low-temperature heat capacity are used in the calculations of standard thermodynamic functions for the (C60)2 crystalline dimer, namely, the heat capacity C p 0 (T), the enthalpy H 0(T) ? H 0(0), the entropy S 0(T), and the Gibbs function G 0(T) ? H 0(0) in the temperature range from T → 0 to 394 K.  相似文献   

18.
Oscillations in the superconducting transition temperature ΔT c (P), in the critical magnetic field ΔH c (P), in the thermopower α / T (T 2), and in electrical resistivity ρ(T) (P is pressure) of Mo1?x -Re x alloys are observed at low temperatures against the background of specific features related to an electronic-topological transition (ETT) in these alloys. The oscillations are sensitive to the impurity concentration: they increase when the Re impurity concentration is close to the critical concentration C c at which the ETT occurs. Oscillations are also detected in the concentration dependences of the temperature coefficient of resistivity (?ρ / ?T (C)) and the thermopower derivative (?(α/T) / ?T 2 (C)) of Mo1?x -Re x alloys at low temperatures. The former and latter oscillations are shown to correlate with each other. These specific features are assumed to result from the ETT and to be related to the localization of the part of the electrons that fill a new cavity in the Fermi surface during this transition.  相似文献   

19.
The investigation of the specific heat of a RbDy(WO4)2 single crystal at temperatures 0.2–2.5 K and in magnetic fields up to 2 T are reported. The temperature dependence of the specific heat near T N=0.818 K is compared with the predictions for different models. The 2D Ising model describes satisfactorily C(T) below T N, while for T>T N none of the theoretical models agree with the behavior of C(T) of RbDy(WO4)2. The H-T phase diagram for Hc is complicated and possesses a triple point, where regions of existence of three magnetic phases converge. The magnetic ordering is analyzed from the standpoint of the Jahn-Teller nature of the structural phase transitions occurring in RbDy(WO4)2 at higher temperatures. It is shown that the form of the phase diagram depends on the direction of the vector H, for the general case of an arbitrary direction of H, two phase transitions can occur with increasing field. Fiz. Tverd. Tela (St. Petersburg) 41, 491–496 (March 1999)  相似文献   

20.
Damage region structure and property changes of YIG irradiated atD=1018?7.8×1019 n/cm2 were studied. Damage regions at 300 K were found to consist of 1) a core of Fe3+ paramagnetic phase (PP) withgΔ=0.8 mm/s; 2) a shell of Fe3+ intermediate magnetic phase with heavily distorted bond geometry and <H eff>≤300 kOe; 3) Fe3+ (a, d) surrounded by oxygen vacancies and interstitials. The dose dependence of PP concentration is given byC PP=1-exp(?βD), yielding PP core radiusr PP=12,5 Å. Magnetic ordering in PP was found to arise atT tr=90 K. NGR probabilityf′ under irradiation was found to decrease linearly according to Δf′/f′=?C PP(D). Net magnetization change was found, using the Gilleo model, to obey an analogous relationship ΔM(T)/M(T)=?C PP(D).T c dose dependence is given by ΔT c/T c=?0.5×C PP(D) and can be related to lattice parameter change to yield Δa 0/a 0=(1.42±0.04)×10?4×C PP(D). External field experiments revealed a complex dependence ofK 1 on PP concentration, elastic stress field magnitude and a with a minimum atD=1019 n/cm2.  相似文献   

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