Magnetoacoustics of the Low-Dimensional Quantum Antiferromagnet Cs2CuCl4 with Spin Frustration

We report on results of sound-velocity and sound-attenuation measurements in the triangular-lattice spin-1/2 antiferromagnet Cs₂CuCl₄ (T _N =0.6 K), in external magnetic fields up to 14 T, applied along the b axis, and at temperatures down to 300 mK. The results are analyzed with a quasi-two-dimensional hard-core boson theory based on exchange-striction coupling. There is a good qualitative agreement between theoretical and experimental results.     

Vibrational spectrum and X-ray diffraction studies of CeZrO4 phase with an ordered arrangement of Ce and Zr ions prepared by graphite reduction

A series of binary CeO₂–ZrO₂ compounds with the composition of Ce/Zr = 1:1 (molar ratio) were reduced by graphite powders at various reducing temperatures (1173 ≤ T _red. ≤ 1573 K) for 24 h in a self-made reducing device. The reduced products obtained at various T _red. were characterized by X-ray diffraction and Vibration Spectrum. In the case of 1223 ≤ T _red. ≤ 1573 K, the reduced phase is identified as the k-CeZrO₄ phase with the space group of P2₁3, in which Ce and Zr ions are in an ordered arrangement similar to that of the pyrochlore Ce₂Zr₂O_7, and the ordering level between Ce and Zr ions in the pyrochlore-type k-CeZrO₄ phase decreased with the decreasing T _red.. The phase is stabilized without decomposition down to room temperature; the reason is attributed to the reducing atmosphere retention caused by graphite until the end of experiment. In the case of T _red. < 1223 K, a phase transition from k-CeZrO₄ phase to t′-Ce_0.5Zr_0.5O₂ phase was observed. The formation of t′ phase is due to the fact that the reducing atmosphere is too weak to severe enough to yield Zr⁴⁺/Ce⁴⁺ cation ordered configurations. The graphite reducing mechanism is discussed in the paper.     

Critical behavior in Fe-doped manganites La0.67Ba0.22Sr0.11Mn1−x Fe x O3 (0 ≤ x ≤ 0.2)

The critical properties of the manganese perovskite La_0.67Ba_0.22Sr_0.11Mn_1?x Fe _x O₃ (0 ≤ x ≤ 0.2) around the paramagnetic (PM)–ferromagnetic (FM) phase transition were investigated using modified Arrott plots and Kouvel–Fisher method based on the data of static magnetic measurements recorded around the curie temperature T _C. Based on the evaluated critical exponents for the compounds with different x values (0.378 ≤ β ≤ 0.411, 1.247 ≤ γ ≤ 1.393, and 4.018 ≤ δ ≤ 4.73) we concluded that the PM–FM phase transition in the compound with x = 0 belongs to the three-dimensional Heisenberg universality. However, the substitution of Fe³⁺ for Mn³⁺ resulted in shifting the value of γ toward the 3D-Ising model value and a slight increase in β. This behavior was discussed and correlated with the random distribution of Fe³⁺ at the Mn³⁺ sites and the consequent suppression the double exchange interaction.     

Isothermal solid–liquid transitions in the (Ni,B)/ZrB2 system as revealed by sessile drop experiments

In the framework of joining processes of ultrahigh temperature ceramics (UHTCs), sessile drop experiments were performed in the Ni–B/ZrB₂ system in the range 1110° ≤ T ≤ 1200 °C. They show that, at temperatures between 1110 and 1150 °C, isothermal solid–liquid transitions occur in a sequence; while in fact at T ≥ 1200 °C, the drop melts without any further phase transition, at lower temperatures, complete melting is followed by a solidification stage and final remelting. This complex behavior, which can be very relevant when utilizing Ni–B alloys for brazing processes (e.g., by the transient liquid phase bonding technique), is successfully interpreted on the basis of the complete B–Ni–Zr phase diagram newly computed by CALPHAD: Isothermal sections, calculated between 1110 and 1150 °C, show that the composition of the drop enters, crosses, and leaves the primary solidification region of the Zr₂Ni₂₁B₆ ternary compound. The use of thermodynamic modeling for the explanation of experimental data sets a link between wetting and joining experiments and phase diagrams assessment.     

Enhanced multiferroic and dielectric properties of Sr2+-doped BiFe0.94(Mn0.04Cr0.02)O3 thin films

Sr²⁺-doped B_1?xSr_xFe_0.94(Mn_0.04Cr_0.02)O₃ (B_1?xSr_xFMC, x = 0.00, 0.05 and 0.09) thin films were prepared on FTO/glass (SnO₂: F) substrates by using a sol–gel spin-coating method. X-ray diffraction (XRD), Rietveld refined XRD data and Raman scattering spectra illustrate a structural evolution from trigonal (R3c: H) to tetragonal (P4) occurs in B_1?xSr_xFMC thin films with the increase of Sr²⁺ concent. Superiorly multiferroic and dielectric properties were obtained in the B_1?xSr_xFMC thin films, e.g., large remanent polarization value, a high dielectric constant (P _r = 139.21 μC/cm² and ε_r = 396.7 for x = 0.09) and large saturation magnetization (M _s = 2.08 emu/cm³ for x = 0.05). The leakage current density of B_1?xSr_xFMC thin films is increased with increasing Sr²⁺ concentration, wherein the leakage current density of all the B_1?xSr_xFMC films is of the order of magnitude of 10^?5 A/cm², which are still lower than that in the pure BFO film (10^?3 A/cm²).     

Magnetic flux penetration into superconductor in vortex-liquid and vortex-glass regimes

Magnetic flux penetration into a half-space occupied by a type-II superconductor occurring in a vortex-liquid or vortex-glass state is considered under the assumption that magnetic field exhibits a sharp increase with time (in the blow-up regime) at the interface. It is shown that each phase admits effective localization of magnetic flux in the form of a magnetic wave front, which is stopped at a depth of x _s ∝e^−σσ in a vortex-liquid regime and at x _s ∝(2σ)^1/(σ+2) in a vortex-glass regime. Here, the parameter σ = U ₀/T depends on the thermal activation barrier height U ₀ and on the temperature T in the vicinity of the melting curve T _m (B) at a given magnetic field induction B.     

Evidence for a Self-bound Liquid State and the Commensurate–Incommensurate Coexistence in 2D 3He on Graphite

We made heat-capacity measurements of two dimensional (2D) ³He adsorbed on graphite preplated with monolayer ⁴He in a wide temperature range (0.1≤T≤80 mK) at densities higher than that for the 4/7 phase (=6.8 nm^?2). In the density range of 6.8≤ρ≤8.1 nm^?2, the 4/7 phase is stable against additional ³He atoms up to 20% and they are promoted into the third layer. We found evidence that such promoted atoms form a self-bound 2D Fermi liquid with an approximate density of 1 nm^?2 from the measured density dependence of the γ-coefficient of heat capacity. We also show evidence for the first-order transition between the commensurate 4/7 phase and the ferromagnetic incommensurate phase in the second layer in the density range of 8.1≤ρ≤9.5 nm^?2.     

Three-Dimensional Network Polymeric Structure of (2-Amino-5 Ammoniopyridinium)[CuCl4]: Crystal Supramolecularity and Magnetic Properties

A single crystal structure determination of the complex (C₅H₉N₃)CuCl₄ [henceforth I] has been carried out. It crystallizes in the orthorhombic space group Cmca, with the cell parameters a=6.7053(5) Å, b=22.4171(19) Å, c=13.4173(9) Å, V=2016.8(3) Å³, and Z=8. In the complex, the anion is comprised of infinite zigzag chains of monochloro-bridged $\mathrm{CuCl}_{4}^{2-}$ units ?–CuCl₃–μ–Cl–CuCl₃–μ–Cl–CuCl₃–? surrounded by the pyridinium-ammonium cations. The Cu ions are five-coordinate to chloride ions in a distorted square-pyramidal geometry with the zigzag chains formed by corner-sharing polyhedra. The Cu ion is located 0.151 Å above the basal plane. In the crystal, there are four intermolecular hydrogen bonding interactions, linking the CuCl chains to the cations to form a 2D-network. The resulting 2D-networks are further linked by aryl?aryl (π?π) interactions within the cationic chains leading to a 3D-network. Variable temperature magnetic susceptibility data were fit to a one-dimensional S=1/2 antiferromagnetic chain model yielding C=0.44(1) emu-K/mol-Oe and J=?37(1) K.     

Superconductivity in ZrB12 and LuB12 with Various Boron Isotopes

In BCS-type superconductors Zr ^N B₁₂ (T _C≈6 K) and Lu ^N B₁₂ (T _C≈0.42 K) heat capacity C(T) and magnetization measurements have been carried out on high quality single crystals with various boron isotopes (with N=10, 11, and with natural composition of ¹⁰B and ¹¹B). Parameters of the superconducting and normal states have been deduced from this study, allowing comparison between these two dodecaborides. It was shown that ZrB₁₂ is a type-II superconductor in which the Ginzburg–Landau parameter varies in the range κ=0.8–1.12. A detailed analysis of specific heat in the normal state of R¹⁰B₁₂, R¹¹B₁₂ and R^natB₁₂ (where R=Zr, Lu) has revealed three Einstein type vibration modes of Zr⁴⁺-ions in ZrB₁₂ with characteristic energies θ _E1(Zr ^N B₁₂)≈200 K and θ _E2,3(Zr ^N B₁₂)≈450 K, whereas only one quasi-local mode of Lu³⁺-ions in LuB₁₂ with θ _E1(Lu ^N B₁₂)≈160–170 K was detected. This difference of vibration spectra has been found to be the main reason for the dramatic change of T _C for ZrB₁₂ and LuB₁₂. Using strong coupling approximation, moreover, the Coulomb pseudopotential μ ^?, electron–phonon interaction constant λ ^? and logarithmic average frequency ω _ln were determined.     

Superconducting behavior of high-T c YBa2Cu3O7 thin films with BaO impurity produced by pulsed laser deposition

We have investigated the superconducting behavior of high-T _c YBa₂Cu₃O₇ (YBCO) thin films containing BaO impure phase produced by pulsed laser deposition. The thin films were characterized by the standard four-probe method, X-ray diffraction (XRD), and scanning electron microscopy (SEM). XRD showed that all these thin films contained BaO impurity, with thec-axis normal to the surface of the substrates. The presence of impurity existed from substrate temperatureT _s of 727 to 796°C. When these thin films with BaO impurity were measured under the magnetic fields, it was found that the critical current densityJ _c increased slightly with increase in magnetic fieldB within the range ofB≤500 G, in the case ofB perpendicular to thec-axis of the film.     

Effect of Short-Range Ferromagnetic Ordering on the Electrical Transport Properties of (La0.9Bi0.1)2/3Ca1/3MnO3 in the Magnetic Field up to 40 T

The temperature and magnetic field dependence of the resistance of polycrystalline sample (La_0.9Bi_0.1)_2/3Ca_1/3MnO₃ was investigated in pulsed high magnetic field up to 40 T over a wide temperature region. In order to describe the magnetotransport properties of the sample, an extended Mott hopping conduction model was employed. Instead of B _J (gμ _B J(T)B/k _B T) in the ferromagnetic (FM) state and $B_{J}^{2}(g\mu_{B}J(T)B/k_{B}T)$ in the paramagnetic (PM) state, the magnetic field dependence of the resistance could scale nicely with the Brillouin function B _J (gμ _B J(T)B/k _B T) in all the given temperatures (above and below the Curie temperature T _c ). The deduced value of the average spin moment J(T) increases with the increasing of the temperature and reaches the maximum value (～35) around T _c . The large value of J(T) indicate the presence of magnetic clusters in the sample which is induced by the short-range ferromagnetic ordering, however, compared to the un-doped compound La_2/3Ca_1/3MnO₃, the value of J(T) is much smaller, which might be ascribe to the location of the 6s² lone pair electrons of Bi³⁺ ions in the sample (La_0.9Bi_0.1)_2/3Ca_1/3MnO₃.     

Fish-Tail Effect and Its Evolution Under Neutron Irradiation in Li-Doped YBa2Cu3O7−x

The evolution of the critical current density of Li-doped YBa₂Cu₃O_7?x polycrystalline samples submitted to neutron irradiation is investigated as function of magnetic field (0 ≤ B ≤ 6 T) temperature (5 ≤ T ≤ 85 K) and neutron fluence (0 ≤ Φ ≤ 9.98 × 10¹⁷ cm^?2). At fluences lower than 10¹⁷ cm^?2, a second peak in j _s vs. B dependence is present (fish-tail effect). Its magnitude decreases with increasing the fluence. Above 10¹⁷ cm^?2, the second peak of current density completely disappears; instead, the logarithmic susceptibility shows a second peak at a certain field B _infl. A dependence of B _infl on fluence is proposed.     

Cooperative Jahn–Teller transition in Li[Li x Mn2–x ]O4: a muon-spin rotaion/relaxation (μSR) view

The magnetic nature of the spinel antiferromagnet Li[Li _x Mn_2–x ]O₄ with x = 0?0.15 has been studied with muon-spin-rotation and relaxation (μ⁺SR) spectroscopy. Both weak transverse field and zero field μ⁺SR measurements indicate that the whole sample enters into a static disordered magnetic phase below T _N for all the samples measured; T _N = 61 K for LiMn₂O₄, whereas 27–23 K for the x = 0.05?0.15 samples. It was also clarified that both the field distribution width and the field fluctuation rate show a clear change at the Jahn–Teller transition temperature (T _JT = 280 K) for LiMn₂O₄, and a short-range cooperative JT distortion appears below 280 K even for Li[Li_0.15Mn_1.85]O₄.     

Magnetization Study in CuCr2O4 Spinel Oxide

We report magnetization and ac susceptibility as functions of the temperature and frequency for CuCr₂O₄ spinel oxide from 2 K to 300 K. Bulk CuCr₂O₄ crystallizes at room temperature in a tetragonal distorted spinel and above 865 K its structure is cubic spinel; distortion is caused by Jahn–Teller Cu²⁺ ions. The magnetization data of the polycrystalline sample indicates ferrimagnetic order below T _C =122 K. Magnetization isotherm resulted in an average magnetic moment of 0.08 μ_B/f.u. at 2 K values lowest to expected value. This discrepancy can be explained assuming a triangular configuration of spins Cr³⁺ and Cu²⁺. The ferromagnetic phase of the sample does not show glassy behavior. Its magnetic response can be explained simply from the domain wall dynamics of otherwise homogeneous ferrimagnet.     

Anisotropy of acousto–optic figure of merit in lithium tetraborate crystals

We analyse anisotropy of acousto–optic figure of merit (AOFM) for Li₂B₄O₇ crystals in order to estimate the prospects of these crystals in acousto–optics. We find that the maximal AOFM, 3.44 × 10^?15 s³/kg, is peculiar for the isotropic acousto–optic interaction of the incident ordinary optical wave with the quasi-longitudinal acoustic wave. For the case of anisotropic diffraction in Li₂B₄O₇, the maximum 1.87 × 10^?15 s³/kg can be reached using the interaction of the extraordinary optical wave with the quasi-longitudinal acoustic wave. The case of collinear diffraction is characterized by small AOFMs, with the largest value 0.26 × 10^?15 s³/kg.     

Weak ferromagnetic polar phase in the BiFe1−xTixO3 multiferroics

Neutron powder diffraction and magnetization measurements of selected samples of the BiFe_1?xTi_xO₃ series were performed. Ti⁴⁺ substitution was shown to induce the appearance of weak ferromagnetism in the initial polar R3c phase stable at x ≤ 0.1. In the concentration range 0 ≤ x ≤ 0.1, room-temperature residual magnetization increases from 0 to 0.25 emu/g (the latter is characteristic of the field-induced weak ferromagnetic state in pure BiFeO₃). The calculated ferroelectric polarization decreases from ~70 μC/cm² (x = 0) to ~60 μC/cm² (x = 0.1) at room temperature. Magnetic ordering coexists with the large spontaneous polarization in a broad temperature range to make the BiFe_1?xTi_xO₃ (x → 0.1) perovskites promising for multiferroic applications.     

Superconducting Properties of Ag and Sb Substitution on Low-Density YBa2Cu3O δ Superconductor

The effects of silver (Ag) and antimony (Sb) substitution on low-density YBa₂Cu₃O _δ (YBCO) superconductor were investigated. Two series of sample with a nominal composition of YBa_2?x Ag _x Cu₃O _δ and YBa_2?x Sb _x Cu₃O _δ where x=0.05, 0.10, 0.15, 0.20, 0.30, 0.40 and 0.50 were synthesized and characterized. All Ag-doped samples showed metallic behavior at the normal state and T _C?onset was found at 90 K. T _C?zero decreased as the Ag concentration increased. Optimum Ag concentration was achieved at x=0.20 where T _C?zero has the highest value of 87 K and J _C at 70 K is 16.50 A/cm². For Sb-doped case with x≤0.30, the samples showed metallic behavior above T _C?onset while semiconducting behavior was shown for x≥0.40. The optimum Sb concentration was achieved at x=0.15 where T _C?zero is 85 K and J _C value measured at 70 K is 2.75 A/cm². T _C?onset and T _C?zero were found to decrease toward higher Sb concentration. The crystallographic structure transformed to tetragonal in Sb-doped samples of x≥0.30 while other samples remain orthorhombic.     

3He Effect on 2D Superfluidity in 3He–4He Mixture Films on Planar Gold

There have been a number of experiments exploring the nature of 2D superfluidity and the configuration of ³He–⁴He mixture films on various substrates. To date, a possible film-structure at T=0 is that of a simple layer model, ³He/superfluid ⁴He/solid-like ⁴He/substrate, in which the submonolayer superfluidity is strongly affected by the coverage of the ³He overlayer. Yet the mechanism is not been fully understood. In this paper, we report a QCM study at 60 MHz for the ³He effect on the superfluidity of mixture films on flat gold, mainly focusing on the anomalous depletion of the temperature dependence of the superfluid density σ _s. In the measurements, we kept the ³He coverage constant (n ₃= 0, 3.6, 7.2, 19.0, 57.2, or 92.8 μmol/m²) and then incrementally added ⁴He. We observed the evolution of the ³He effect on σ _s(T) with increasing ³He coverage; this depletion of σ _s(T) rapidly increases and then saturates near n ₃～1 layer. From the analysis of the linear-temperature region in the plot of the dissipation peak temperature T _p as a function of the superfluid ⁴He coverage n _4s and comparison with previous studies on Mylar and porous gold, we found a universal function for the strength of the ³He effect for all substrates.     

Microwave dielectric properties and low temperature firing of (1 − x)Li2Zn3Ti4O12 –xLi2TiO3 (0.2 ≤ x ≤ 0.8) ceramics with B2O3–CuO addition

In the present work, the (1 ? x)Li₂Zn₃Ti₄O₁₂–xLi₂TiO₃ (0.2 ≤ x ≤0.8) ceramics were prepared via the solid state reaction method. The 0.8Li₂Zn₃Ti₄O₁₂–0.2Li₂TiO₃ ceramic sample sintered at 1,160 °C for 2 h demonstrated high microwave dielectric property with a relative permittivity of 18.0, a high quality factor (Q × f) ~ 100,000 GHz (at 7.2 GHz), and a temperature coefficient of resonant frequency about ?47.8 ppm/°C. With 2.0 wt% 0.4B₂O₃–0.6CuO addition, a relative permittivity of 17.5, a Q × f value of 71,000 GHz and a temperature coefficient of resonant frequency of ?44.4 ppm/^oC can be obtained in 0.8Li₂Zn₃Ti₄O₁₂–0.2Li₂TiO₃ ceramic sintered at 925 °C for 5 h and the chemical compatibility with silver electrode indicates that the ceramics may be a suitable candidate for the low temperature co-fired ceramic technology application.     

(K0.5Na0.5)NbO3–Bi(Mg0.5Ti0.5)O3 solid solution: phase evolution, microstructure and electrical properties

Lead-free piezoelectric ceramics with the composition of (1 ? x)(K_0.5Na_0.5)NbO₃–xBi(Mg_0.5Ti_0.5)O₃ [(1 ? x)KNN–xBMT, 0 ≤ x ≤ 0.04] were synthesized via solid-state reaction method. X-ray diffraction patterns revealed that the orthorhombic—tetragonal phase transition was present for (1 ? x)KNN–xBMT with increasing the content of BMT. The study of dielectric properties illustrated that both peaks of orthorhombic—tetragonal (T _O–T ) and tetragonal—cubic (T _T–C ) phase transitions shifted to lower temperature. Through adding BMT, the electrical properties of KNN ceramics were obviously improved. The optimized piezoelectric and ferroelectric properties with d ₃₃  = 127 pC/N, k _p  = 36.58 %, P _r  = 22.1 μC/cm² were obtained as x = 0.01.