Specific Heat of Mg11B2, a Two-Gap Superconductor

We report measurements of the specific heat of Mg¹¹B₂, from 1 to 50 K and in magnetic fields to 9 T, and give the values of parameters relevant to the superconductivity. The superconducting-state electron contribution is dramatically different from that of other superconductors, but the general features are consistent with predictions for a two-gap superconductor, and it can be quantitatively represented by a two-gap model based on BCS thermodynamics. Parameters characterizing the gaps are in good agreement with some spectroscopic determinations, and also with theoretical calculations. An unusually strong magnetic field dependence of the temperature-proportional term in the electron contribution to the vortex-state specific heat is evidently another manifestation of the two gaps.     

Theory of superconductivity in heavy-fermion compounds

A single-order-parameter and two-gap model (a modified Suhl two-gap model) is proposed for heavy-fermion superconductors. A simple relation between two gaps is found. The thermodynamic quantities in the superconducting state are calculated in terms of this model. It is shown that the critical values of the physical quantities are quantitatively in agreement with experiment. The behavior of the specific heat, of the ultrasonic attenuation, and of the NMR rate are also in good agreement with those of CeCu₂Si₂ and UBe₁₃ at all temperatures.     

Fe@C@Gd2O3:Eu magnetic-fluorescent composites: Facile synthesis,structure and properties

In this study, the bifunctional Fe@C@Gd₂O₃:Eu³⁺ composites were prepared via a simple co-precipitation progress followed by a subsequent heat treatment under N₂ atmosphere. The reactive carbon interlayer offers many advantages including separating photoluminescent material from magnetic material and reducing magnetite Fe₃O₄ to metal Fe. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared (FT-IR), vibrating sample magnetometer (VSM) and photoluminescence spectroscopy (PL) were used to analyze the structure, morphology, magnetic and luminescence properties of the composites. The Fe@C@Gd₂O₃:Eu³⁺ composites with well-crystallized and core–shell structure were prepared and displayed good ferrimagnetic behavior at room temperature and a strong red emission. In addition, due to the excellent magnetic and luminescence properties of the prepared composites, they may have many great potential applications in many fields, such as biological imaging, cell tracking and drug delivery.     

s -Wave Symmetry Along the c -Axis and s + d In-plane Superconductivity in Bulk YBa2Cu4O8

To clarify the order parameter symmetry of cuprates, the magnetic penetration depth λ was measured along the crystallographic directions a, b, and c in single crystals of YBa₂Cu₄O₈ via muon spin rotation. This method is direct, bulk sensitive, and unambiguous. The temperature dependences of λ _a ⁻² and λ _b ⁻² exhibit an inflection point at low temperatures as is typical for two-gap superconductivity (TGS) with s+d-wave character in the planes. Perpendicular to the planes a pure s-wave gap is observed thereby highlighting the important role of c-axis effects. We conclude that these are generic and universal features in the bulk of cuprates.     

Physical properties of a new cuprate superconductor Pr2Ba4Cu7O15−δ

We present studies of the thermal, magnetic, and electrical transport properties of reduced polycrystalline Pr₂Ba₄Cu₇O_15−δ (Pr247) showing a superconducting transition at T_c=10-16 K, and compare them with those of as-sintered non-superconducting Pr247. The electrical resistivity in the normal state exhibited T² dependence up to approximately 150 K. A clear specific heat anomaly was observed at T_c for Pr247 reduced in a vacuum for 24 h, proving the bulk nature of the superconducting state. By the reduction treatment, the magnetic ordering temperature T_N of Pr moments decreased from 16 to 11 K, and the entropy associated with the ordering increased, while the effective paramagnetic moments obtained from the DC magnetic susceptibility varied from 2.72 to 3.13μ_B. The sign of Hall coefficient changed from positive to negative with decreasing temperature in the normal state of a superconducting Pr247, while that of the as-sintered one was positive down to 5 K. The electrical resistivity under high magnetic fields was found to exhibit T^α dependence (α=0.08-0.4) at low temperatures. A possibility of superconductivity in the so-called CuO double chains is discussed.     

Specific heat, electrical resistivity and thermoelectric power of YbNi4Si

The studies of the specific heat, electrical resistivity and thermoelectric power of YbNi₄Si are reported. These studies are supported by magnetic susceptibility and X-ray photoemission spectroscopy (XPS) measurements. YbNi₄Si does not order magnetically down to 4 K. Nearly in the whole temperature range studied the magnetic susceptibility follows a Curie law with μ_eff = 4.15 μ_B/f.u. This effective magnetic moment is close to the value expected for the 4f¹³ configuration (4.54 μ_B). The Yb²⁺ and Yb³⁺ peaks observed by XPS in the valence band region confirm the domination of the Yb³⁺ valence state. Based on the specific heat measurements, the electronic specific heat coefficient γ = 25 mJ/mol/K² and the Debye temperature θ_D = 320 K were derived. A quadratic dependence of electrical resistivity at low temperatures has been observed. The Kadowaki-Woods ratio has been discussed. The thermoelectric power has been analyzed in the framework of the two band model.     

Specific heat jump of two-band superconductor KFe2As2 using Ginzburg-Landau theory

In this study specific heat jump using two-gap Ginzburg-Landau (GL) theory has been calculated. In contrast to the previous approaches, we have taken into account intergradient order parameters interaction in the GL free energy functional. The thermodynamic magnetic field revealed nonlinear temperature dependence due to interband interaction between order parameters and their gradients. The calculations showed that the specific heat jump in two-order parameter superconductors was smaller than that of single-order parameter superconductors. It has been shown that such a model is in good agreement with experimental data for KFe₂As₂ superconductors.     

Physical properties of a new cuprate superconductor Pr2Ba4Cu7O15−δ

We present studies of the thermal, magnetic, and electrical transport properties of reduced polycrystalline Pr₂Ba₄Cu₇O_15−δ (Pr247) showing a superconducting transition at T_c=10–16 K, and compare them with those of as-sintered non-superconducting Pr247. The electrical resistivity in the normal state exhibited T² dependence up to approximately 150 K. A clear specific heat anomaly was observed at T_c for Pr247 reduced in a vacuum for 24 h, proving the bulk nature of the superconducting state. By the reduction treatment, the magnetic ordering temperature T_N of Pr moments decreased from 16 to 11 K, and the entropy associated with the ordering increased, while the effective paramagnetic moments obtained from the DC magnetic susceptibility varied from 2.72 to 3.13μ_B. The sign of Hall coefficient changed from positive to negative with decreasing temperature in the normal state of a superconducting Pr247, while that of the as-sintered one was positive down to 5 K. The electrical resistivity under high magnetic fields was found to exhibit T^α dependence (α=0.08–0.4) at low temperatures. A possibility of superconductivity in the so-called CuO double chains is discussed.     

Synthesis and properties of ZnFe2O4 replica with biological hierarchical structure

ZnFe₂O₄ replica with biological hierarchical structure was synthesized from Papilio paris by a sol–gel method followed by calcination. The crystallographic structure and morphology of the obtained samples were characterized by X-ray diffraction, field-emission scanning electron microscope, and transmittance electron microscope. The results showed that the hierarchical structures were retained in the ZFO replica of spinel structure. The magnetic behavior of such novel products was measured by a vibrating sample magnetometer. A superparamagnetism-like behavior was observed due to nanostructuration size effects. In addition, the ZFO replica with “quasi-honeycomb-like structure” showed a much higher specific capacitance of 279.4 F g⁻¹ at 10 mV s⁻¹ in comparison with ZFO powder of 137.3 F g⁻¹, attributing to the significantly increased surface area. These results demonstrated that ZFO replica is a promising candidate for novel magnetic devices and supercapacitors.     

The synthesis,structure and physical properties of a non-stoichiometric sodium chromium selenide,Na0.34Cr1.15Se2

Non-stoichiometric Na_0.34Cr_1.15Se₂ was prepared from the elements. It is rhombohedral, space group R3̄m with a= 3.617, c= 38.99 A. It is a layer compound consisting of sandwiches CrSe₂ with chromium in octahedral coordination. The gaps between these sandwiches are alternatingly occupied by the extra (2 × 0.15) chromium in octahedral coordination, and sodium (2 × 0.34) in trigonal-prismatic coordination.The structural, magnetic and transport properties indicate in essence the itineracy of the chromium d electrons, strongly influenced by electron correlation and structural randomness.     

Nd2[MoC2] and RE2[WC2], RE=Ce, Pr, Nd: New carbometalates with Pr2[MoC2] structure type

Four new ternary rare-earth-metal carbometalates, Nd₂[MoC₂] and RE₂[WC₂] with RE=Ce, Pr, Nd, have been synthesized by argon arc melting and subsequent heat treatment at 1170 K for 30 days. They crystallize with the Pr₂[MoC₂] structure type with isolated C⁴⁻ species and are typical carbometalates with (i) low metal-to-carbon ratio, (ii) tetrahedral coordination of the transition metals (T) by carbon, and (iii) a polyanionic network . According to resistivity measurements the compounds are bad metals. Volume chemistry and magnetic susceptibility measurements indicate Pr³⁺, Nd³⁺, and Ce⁴⁺ species, respectively. In the latter case, the additional electron is not transferred to the polyanionic network, instead it mainly populates the Ce partial structure.     

Metals built up from ReO3 type blocks and P2O7 groups : The bronzes AxP8W32O112 (A = K,Rb, Tl)

The electron transport and magnetic properties of the A_xP₈W₃₂O₁₁₂ bronzes (A = K, Rb, Tl) have been studied as a function of temperature. These compounds are characterized by a Pauli paramagnetism. The evolution of conductivity, ranging from 1260 (Ωcm)^?1 to 1610 (Ωcm)^?1 at 300K is characteristic of a metal. Owing to the presence of ReO₃ type slabs, these properties can be interpreted by the band model conduction developed by Goodenough for Na_xWO₃ bronzes.     

Spin Density Wave Excitations in the Specific Heat of La2CuO4.11 Single Crystals

We measured the specific heat vs. temperature in single crystal samples of superconducting La₂CuO_4.11, in magnetic fields up to 15 T. After subtraction of the phonon contribution to the specific heat, obtained measuring a nonsuperconducting crystal, we found a broad anomaly centered at 50 K. This excess specific heat is attributed to fluctuations of the Cu²⁺ spins possibly enhanced by an interplay with the charge degrees of freedom.     

Magnetic Anomalies and Spin-Glass-Like Behavior in Ce2CuGe6

A Ce-based intermetallic compound Ce₂CuGe₆ and its La analogue La₂CuGe₆ were synthesized and examined by X-ray powder diffraction, electrical resistivity, magnetic susceptibility, X-ray absorption spectrum and specific-heat measurements. The results reveal that Ce₂CuGe₆ is an antiferromagnetic Kondo-lattice compound with T _N ∼15 K. The effective moment deduced from paramagnetic region is 2.58 μ _B per cerium ion, which is close to localized Ce³⁺ free ion in ²F_5/2 state. X-ray absorption measurement also precluded the existence of an intermediate valence state. The magnetic entropy calculated from heat capacity specific-heat measurement suggested a doublet ground state. In addition, we also observed a spin-glass-like behavior in this material. The spin-glass-like behavior might be due to the complex interplay between magnetic interactions.     

Electronic characterization of alkali ruthenium hollandites: KRu4O8, RbRu4O8 and Cs0.8Li0.2Ru4O8

Transport, specific heat, and magnetic measurements have been performed on three alkali hollandites: KRu₄O₈, RbRu₄O₈, and a newly synthesized Cs analog, Cs_0.8Li_0.2Ru₄O₈, which was determined to have space group I4/m (#87) and lattice parameters, a = 10.0850(4) and c = 3.12180(20). In contrast to the ruthenium perovskites, which display a wide range of electrical and magnetic behavior, the alkali hollandites are simple paramagnetic metals.     

Superconductivity and Magnetism in Pure and Substituted RuSr2GdCu2O8

We describe results of several macroscopic and local measurements of magnetic and superconducting properties for pure and Ce and Cu substituted RuSr₂GdCu₂O₈. From various experiments the physical properties phase diagram is derived as a function of the hole doping. We show that the large contribution of polarized Gd³⁺ magnetic moments to magnetization causes apparent absence of the Meissner state and augments the weak Ru ferromagnetism. This weak ferromagnetism in the superconducting state can be significantly enhanced by the external magnetic field. These results indicate that the low-temperature behavior can be qualitatively explained assuming a quasi-two-dimensional character of the superconducting regions.     

Susceptibilite magnetique et R.P.E. du Mn2+ dans les verres du systeme : MnSGa2S3GeS2

E.P.R. and a.c. susceptibility measurements results are described. For low Mn²⁺ content well resolved E.P.R. signals are observed at g = 2 (hyperfine interaction constant : A = 71.10^?4cm^?1) and poorly resolved signals at g = 4.3. For high Mn²⁺ content the latter vanishes, and the temperature and concentration dependence of the g = 2 signal are indicative of magnetic exchange interactions. The a.c. susceptibility exhibits a cusp around 2 K (T_SG). The experiments can be interpreted by a progressive freezing of the magnetic moments, with a residual contribution of isolated Mn⁺⁺ below T_SG.     

11B NMR Study of Pure and Lightly Carbon-Doped MgB2 Superconductors

We report a ¹¹B NMR line shape and spin-lattice relaxation rate (1/(T₁T))(1/(T_1T)) study of pure and lightly carbon-doped MgB_2-xC_x_{2-x}{\rm C}_x for x = 0,0.02x = 0,0.02, and 0.04, in the vortex state and in magnetic field of 23.5 kOe. We show that while pure MgB₂_2 exhibits the magnetic field distribution from superposition of the normal and the Abrikosov state, slight replacement of boron with carbon unveils the magnetic field distribution of the pure Abrikosov state. This indicates a considerable increase of H^c_c2H^c_{c2} with carbon doping with respect to pure MgB₂_2. The spin-lattice relaxation rate 1/(T₁T)1/(T_1T) demonstrates clearly the presence of a coherence peak right below T_cT_{\rm c} in pure MgB₂_2, followed by a typical BCS decrease on cooling. However, at temperatures lower than ?\approx 10 K strong deviation from the BCS behavior is observed, probably from residual contribution of the vortex dynamics. In the carbon-doped systems both the coherence peak and the BCS temperature dependence of 1/(T₁T)1/(T_1T) weaken, an effect attributed to the gradual shrinking of the σ hole cylinders of the Fermi surface with electron doping.     

Antiferromagnetism in the semiconducting lanthanide cuprates Ln2CuO4

The semiconducting lanthanide cuprates Pr₂CuO₄, Nd₂CuO₄, and Gd₂CuO₄ have been investigated using X-ray diffraction, magnetic susceptibility, and electron paramagnetic resonance techniques. The susceptibility and resonance studies indicate that the Cu²⁺ ions are ordered antiferromagnetically and that the exchange interactions involving the Ln³⁺ ions are relatively weak. The deviations from the Curie-Weiss law in Pr₂CuO₄ and Nd₂CuO₄ are in qualitative agreement with the predicted behavior for isolated Ln³⁺ ions under the influence of a crystal field. Hence these compounds provide interesting examples of planar Cu²⁺-ion antiferromagnets.     

Magnetic Specific Heat of Heavily p-Type Doped (Zn,Mn)Te:P

The magnetic contribution to the specific heat of bulk crystals of Zn_1–x Mn _x Te ( x = 0.03) heavily (up to 10¹⁹ cm^–3) p-type doped with P is studied over the temperature range 0.5–15 K and magnetic field range 0–3 T. The magnetic specific heat observed at zero magnetic field indicates that a substantial part of the magnetic ions has the degeneracy of their magnetic ground state lifted by d–d and p–d exchange interactions. The effect increases for doped and annealed samples with higher concentration of conducting holes. We have also carried out a theoretical analysis that takes into account the contributions due to small magnetic clusters, single magnetic ions in crystal field of distorted crystal lattice, and low energy excitations of the p–d exchange-coupled system of local moments and carriers.