Point-contact measurements of CeB6 and CeCu6 in high magnetic fields

Epitaxial YBa₂Cu₄O₇-thin films with thec-axis oriented perpendicular to the film plane were prepared bydc-sputtering from a single stoichiometric target on (100) SrTiO₃-substrates. Typical values of the inductively measured superconducting transitions were about 90 K with a width less than 0.5 K. Critical current densities were measured on 5 to 10 m wide strips as function of magnetic field and temperature. The temperature dependences ofj _c follow a universal functionj _c(B,T)=j _c ^* (T=0,B)·(1–T/T _c (B)) with =1.5±0.1. ForB=0 andT=77 K we obtainedj _c =4·10⁶ A/cm². The field dependence of the resistive transitions was measured with the magnetic field parallel to thec-axis. The slope of the upper critical fieldB _c2 (T) was determined for different criteria. The carrier concentration evaluated from Hall-effect measurements was found to decrease linearly from one per unit cell at 240 K with decreasing temperature extrapolating nearly through zero forT=0. Highly resolved angular dependent measurements of the critical current density withB perpendicular to the current but tilted from thec-axis show a very strong and sharp enhancement ofj _c for the magnetic field parallel to the (CuO₂)-layers (Bc). Additionally to this phenomenon, which is caused by an intrinsic pinning mechanism due to the layered structure of high-T _c -superconductors the influence of the anisotropy of the upper critical field onj _c (B, T, ) is evident nearT _c .     

Effect of grain-boundary flux pinning in MgB2 with columnar structure

We studied the flux pinning properties by grain boundaries in MgB₂ films prepared by using a hybrid physical chemical vapor deposition method on the c-axis oriented sapphire substrates. All the films we report here had the columnar grains with the growth direction perpendicular to the substrates and the grain sizes in the range of a few hundred nanometers. At very low magnetic fields, no discernable grain-boundary (GB) pinning effect was observed in all measuring temperatures, but above those fields, the effect of GB flux pinning was observed as enhanced critical current densities (J_cs) and reduced resistances when an external magnetic field (B) was aligned parallel to the c-axis. We interpret the B dependence of J_c in the terms of flux line lattice shear inside the columnar grains activated by dislocations of Frank–Read source while the flux lines pinned by GB act as anchors for dislocations. Magnetic field dependence of flux pinning force density for B parallel to the c-axis was reasonably explained by the above model.     

Current transport property in GdBCO coated conductor with artificial pinning centers in a wide range of temperature,magnetic field up to 27 T,and field angle

We have investigated current transport property in Gd₁Ba₂Cu₃O_7−δ coated conductor with artificial pinning centers in a wide range of temperature, magnetic field, B up to 27 T, and field angle. Due to the additional c-axis correlated pins, critical current density, J_c in B//c was enhanced and the improvement was observed in wide range of B. On the other hand, around B⊥c below 65 K, the angular dependence of n-value showed a valley-like behavior, although the J_c was increasing. In addition, the temperature dependence of the pinning force density defined as J_c × B was not scaled on an expected master curve. These results indicate the pinning in B⊥c is governed by different mechanism below 65 K and high magnetic field.     

In-plane magnetoresistance of electron-doped superconducting thin films Pr0.9LaCe0.1CuO4

We measured the in-plane magnetoresistance of Pr_0.9LaCe_0.1CuO₄ (PLCCO) epitaxial thin films under various magnetic fields H applied parallel to the tetragonal c-axis. The measurements were performed at the superconducting state as well as the normal state. As the magnetic field is between the low critical field H_c1 and upper critical field H_c2, a critical scaling behavior of electrical resistivity is found. We analyze the electrical transport properties and show the magnetic field H dependence of glass transition temperature T_g and the characteristic temperature T^* for the PLCCO film, which may shed some light on vortex behavior in electron-doped superconductors.     

Magneto-optical study of ferromagnetically coupled cobalt (II) pairs in cadmium bromide

Low temperature polarized absorption and magnetic circular dichroism spectra are reported for crystals of CdBr₂ doped with varying concentrations of Co²⁺ in the region near 17 700 cm^-1. Sharp zero phonon lines are identified as the trigonal field components of U_g ′(² T _1g , ² H) in dilute crystals, and as states arising from ferromagnetically coupled in-plane pairs in the concentrated ones. Measurements of the temperature dependence of the lines, and Zeeman spectra recorded with applied fields up to 50 kG parallel and perpendicular to the crystal c-axis, lead to estimates of the anisotropy of the exchange interaction in the ground state, and to both isotropic and anisotropic exchange in the excited state. The preferred direction of spin alignment in both states is parallel to the c-axis.     

A new ferromagnetic thiospinel CuCrZrS4 with re-entrant spin-glass behaviour

A new thiospinel CuCrZrS₄ has been successfully synthesized by a solid-state chemical reaction. This CuCrZrS₄ exhibits ferromagnetic properties below the Curie temperature at T _c = 60 -2 K. The appearance of irreversible effect between field-cooled and zero-field-cooled magnetization is prominent below around 5 K in a magnetic field of less than 150 Oe. The ac susceptibility χ_AC shows a rapid decrease below about 10 K. This low magnetic-field behaviour indicates the existence of a re-entrant spin-glass phase below about 10 K. The dc magnetic susceptibility above 100 K shows Curie-Weiss behaviour with an effective magnetic moment of 3.61 μ _B, which is a little less than the spin-only value of 3.87 μ _B for the Cr³⁺ ion. The asymptotic Curie temperature θ _P is approximately 65 K, which is a little higher than T _c. The valence state is confirmed to be Cu⁺Cr³⁺Zr⁴⁺S₄ ^2? on the basis of magnetic properties. The electrical resistivity ρ shows a semiconducting temperature dependence over the temperature range from 4.2 to 280 K with an activation energy of 6.84 210^?3 eV in the higher temperature range from 50 to 283 K.     

A powder neutron diffraction study of the ErCu2Ge2 compound - crystal field

The compound ErCu₂Ge₂ was studied by neutron diffraction. The diffraction diagram of this compound at 170 K agrees with its crystallographic structure. Its diagram at 1.9 K reveals the existence of superlattice lines consistent with a cell doubled in the a and c directions. The erbium magnetic moment (8.0±0.4)μ_B lies on the c-axis. Crystal field calculations on the Er³⁺ site give 7.9μ_B, with easy magnetization axis the c-axis of the crystal. Copper must contribute to the V^m_l crystal field parameters with a charge equal to 0.6⁺.     

Irreversibility line and flux pinning properties in iron-based high-Tc superconductor SmFeAsO0.85

The irreversibility line and flux pinning properties of high-T_c superconductor SmFeAsO_0.85 were studied using DC magnetization data. Polycrystalline SmFeAsO_0.85 was prepared in a high pressure synthesis apparatus under the pressure of 6 GPa. The results of DC susceptibility showed the superconducting transition at about 55 K. A critical current density J_c(B) was calculated using Bean’s critical state model. At low temperatures (20 K), J_c(B) showed a relatively high value with weak dependence on an applied magnetic field. At higher temperatures, a stronger dependence of the magnetic field was observed, which resulted from decrease in a critical current density probably due to the flux creep effect. The irreversibility line (IL) agreed well with the flux creep theory of Matsushita et al. A comparison of normalized pinning force density with the theoretical models showed that the irreversible behavior in SmFeAsO_0.85 is dominated mainly by normal point pinning (δT_c) and surface pinning mechanisms.     

Effect of a pulsed magnetic field on the transverse resistance of a layered superconductor

The field dependences of the transverse resistance of a single crystal of the layered superconductor Bi₂Sr₂CaCu₂O_y (BSCCO-2212) with T _c 0≃92 K are studied in magnetic fields up to 50 T in the perpendicular orientation H⊥(ab). It is established that in the resistive region the resistance is a power-law function of the field, and the temperature dependence of the barrier height for flux creep is obtained. It is found that in a wide temperature range, 50–125 K, the transverse magnetoresistance of the crystal in the normal state and under conditions of superconductivity suppression by a strong magnetic field is negative and can be approximated by a linear law with a temperature-dependent slope. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 420–425 (10 September 1998)     

Anisotropy of the lattice vibration of Eu and Cu in highTc superconductor Ba2EuCu3O7 analyzed by151Eu and57Fe Mössbauer effects

The anisotropy of superconductivity in highTc superconductors indicates that it is important to clarify the anisotropies of microscopic properties in these materials. In this work, we observed the anisotropy of the lattice vibration ofEu andCu inBa ₂ EuCu ₃ O ₇ by¹⁵¹ Eu and⁵⁷ Fe Mössbauer effect. Thec-axis aligned samples were prepared by healing the mixture of epoxy andBa ₂ EuCu ₃ O ₇ powder in a magnetic field. For⁵⁷ Fe Mössbauer measurement the sample doped with⁵⁷ Fe,Ba ₂ Eu(Cu _0.99 Fe _0.01)₃ O ₇, was used. The temperature dependence of Mössbauer spectrum was measured in two cases,c-axis ‖ γ-ray andc-axis ⊥ γ-ray. The Debye temperature ofEu alongc-axis (230 K) was lower than that inab-plane (265 K). The Debye temperature of⁵⁷ Fe substituted forCu(2) was lower (378 K) alongc-axis than inab-plane (417 K). The Debye temperature of⁵⁷ Fe substituted forCu(1) having no oxygen atom ina-axis was 435 K alongc-axis and 416 K inab-plane. These results are interpreted by the oxygen configuration around the Mössbauer atoms.     

AC response of vortices subjected to pinning by columnar defects in Tl2Ba2CaCu2O8 epitaxial films

A micro-Hall-probe-based ac susceptometer is used to study the dynamics of vortices in Tl₂Ba₂CaCu₂O₈ (Tl-2212) epitaxial films with and without correlated disorder in a low field (<6 kG) and high temperature (T>60 K) regime. The Bean critical state model for a thin superconducting disc in perpendicular magnetic field is used to extract the critical current density of the films. The temperature and frequency dependence of the irreversibility field (B_irr) is compared for samples with and without the linear defects. The B_irr(T) curve shows a distinct discontinuity at B≈B_φ. The frequency dependence of the screening current density J(ω), extracted from the inphase component (T^′_H) of the fundamental transmittivity, has been analyzed in the framework of Bose glass dynamics. This analysis shows that the vortex depinning temperature is ≈24 K for this Tl-2212 film and the depinning process starts via excitations of half-loops. The angle dependent studies of the susceptibility further suggest that these half-loops consist of many pancake vortices.     

<Superscript>77</Superscript>Se Low-Temperature NMR in the Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> Single Crystalline Topological Insulator

⁷⁷Se nuclear magnetic resonance (NMR) measurements in the Bi₂Se₃ topological insulator single crystal were carried out at temperatures 15.8, 88, and 293 K. Bismuth selenide single crystalline plate was studied in the orientation when the crystallographic c-axis was parallel to the external magnetic field B₀. We observed two component NMR spectra at the three temperatures. It was shown that the NMR spectrum almost did not move with decreasing temperature and the density of charge carriers did not follow the thermal activation law.     

高温退火对蓝宝石基片的表面形貌和对CeO2缓冲层以及Tl-2212超导薄膜生长的影响

研究了蓝宝石(1102)基片在不同温度和时间下退火时表面形貌和表面相结构的变化,以及它对CeO₂缓冲层和Tl-2212超导薄膜生长的影响.原子力显微镜(AFM)研究表明,在流动氧环境中1000℃温度下退火,蓝宝石(1102)的表面首先局部区域形成台阶结构,然后表面形成叠层台阶结构,随着退火时间的延长,表面发生了台阶合并现象,表面形貌最终演化为稳定的具有光滑平台的宽台阶结构.XRD测试表明,通过高温热处理可以大幅度提高蓝宝石基片表面结构的完整性.在1000℃温度下热处理20 h的蓝宝石 关键词： Tl-2212超导薄膜 蓝宝石 缓冲层     

Density of states effective mass of n-type CuInSe2 from the temperature dependence of Hall coefficient in the activation regime

The Hall coefficient R_H of n-type CuInSe₂ single crystals is measured between 10 and 300 K in pulsed magnetic field up to 35 T. The threshold field B_th, above which the magnetic freezeout starts to occur, varies linearly with temperature. From the analysis of the temperature dependence of electron concentration in the activation regime above 100 K at different field values, it is established that the density of states effective mass is independent of the magnetic field B and the activation energy E_D, above around 6 T, varies as B^1/3. Similar B^1/3 dependence of the magnetoresistance in the high magnetic field regime, reported earlier in the same material, suggests that theoretical work that could explain this coincidence is needed.     

Magnetic-Field-Controlled Quantum Critical Points in the Triangular Antiferromagnetic Cs2CuCl4-xBrx Mixed System

The triangular antiferromagnetic Cs₂CuCl_4-xBr_x mixed system is studied by neutron single-crystal diffraction in magnetic field. It shows a rich magnetic phase diagram consisting of four regimes depending on the Br concentration and is characterized by different exchange coupling mechanisms. For the investigated compositions from regime I (0 < x ≤ 1.5), a critical magnetic field B_c is found for a Br concentration x = 0.8 at B_c = 8.10(1) T and for x = 1.1 at B_c = 7.73(1) T and from regime IV (3.2 < x < 4) for x = 3.3 at B_c = 0.99(3) T. For magnetic fields larger than the respective B_c, magnetic superlattice reflections of these compounds are not found. The incommensurate magnetic wave vector q = (0, 0.470, 0) appears below the ordering temperature T_N = 0.51(1) K for Cs₂CuCl_3.2Br_0.8, and q = (0, 0.418, 0) below T_N = 1.00(6) K for Cs₂CuCl_0.3Br_3.7. Neutron diffraction experiments at around 60 mK for x = 3.7 in a magnetic field show the critical magnetic field at B_c = 7.94(16) T and the formation of the second magnetic phase at around 8.5 T depending on the temperature. Inelastic neutron scattering experiments for the compound from regime III (2 < x ≤ 3.2) with x = 2.2 show dynamical correlations at a temperature around 50 mK giving evidence for a spin liquid phase.     

3 T强磁场对真空蒸发Zn薄膜晶体结构的影响

在3 T强磁场下采用真空蒸发沉积在玻璃基片上制备了三种厚度分别为1,2,3 μm的Zn薄膜,并和无磁场下制备的薄膜进行了对比研究.对施加磁场和无磁场环境下制备的试样分别进行了X射线衍射研究.研究表明,3 T磁场下制备的Zn薄膜都是沿（002）面取向,而0 T磁场下制备的薄膜随着厚度的增加c轴取向逐渐减弱. 3 T磁场的取向作用可以维持Zn晶粒沿着c轴取向.利用扫描电子显微镜对薄膜表面形貌的研究发现,施加磁场制备的Zn薄膜表面晶粒要比无磁场条件下制备的薄膜有明显的细化.对磁场下Zn原子团形成进行了热力学分析,推导了磁场作用下的临界形核半径r^*_M和临界形核自由能ΔG^*_M.初步分析表明,r^*_M和ΔG^*_M减小从而增加临界形核浓度是Zn晶粒细化的原因. 关键词： 强磁场 晶体结构 真空蒸发沉积 薄膜     

Effective activation energy U(J,H) in epitaxial YBa2Cu3O7−δ thin films

We have measured the resistivity of epitaxial YBa₂Cu₃O_7?δ thin film as a function of temperature, current density and the magnetic field up to 8 T. The current density dependence of the effective activation energy exhibits a slight increase in the low current density range below 10³ A/cm² and a logarithmic decline in higher current density with increasing current density. The magnetic field dependence of the effective activation energy showed a crossover of vortex state from a quasi-2D for H//ab-plane to a 3D line liquid state for H//c-axis. The possible dissipation mechanisms responsible for the ln H dependence of the effective activation energy were discussed.     

Electrical conduction and critical magnetic field for superconductivity of a Nb3Se4 single crystal

With respect to the quasi-one dimensionality of single crystals of Nb₃Se₄, the electrical resistivity from 1.3 to 320 K and the critical magnetic field for superconductivity are measured. The resistivity along the Nb-chain direction is represented as a sum of a temperature independent and an intrinsic temperature dependent term. The temperature dependence of the intrinsic resistivity subjects to T³ form between 10 and 80 K above which it tends to a T linear form. The critical magnetic field is proportional to the temperature difference from the transition temperature. Its dependence is well fitted by the elliptical fluxoid model of Ginzburg-Landau theory. The ratio of the parallel and the perpendicular to the c-axis is 5.7.     

Analogous study in low magnetic field between Cu–Ge and Cu–Si ferrites

Two series of mixed copper ferrites, Cu_1+x Ge_x Fe_2−2x O₄ and Cu_1+x Si_x Fe_2−2x O₄, have been analogously investigated for x=0.0, 0.05, 0.1, 0.15, 0.2, 0.25 and 0.3. The two systems were prepared using the standard ceramic techniques. X-ray diffraction analysis indicates that both systems formed in a single phase cubic spinel structure. The lattice parameter has a constant value (0.838 nm±0.001) for the two series. The grain diameter was estimated from the scanning electron microscope micrographs for the two series. Some magnetic properties were measured at room temperature. The magnetization M was measured in the range of magnetizing field up to 5500 Am⁻¹. The relative permeability (μ_r) was calculated from the B–H relation. The B–H loops were measured at constant magnetizing current (I=2.5 A which is equivalent to 900 Am⁻¹). Also, the hystersis area and the magnetic parameters B_r, B_s, m_R (B_r/B_s) and apparent energy loss (E) were estimated from the B–H loops; μ_r, B_r, B_s and E are composition dependent.     

The sub-micrometer thickness n-InSb/i-GaAs epilayers for magnetoresistor applications at room temperatures of operation

Magnetotransport at fields up to 500 mT and LF-noise characteristics are reported for miniature magnetoresistors with ferrite concentrators based on Sn-doped n-InSb/i-GaAs heterostructures grown by MBE. The thickness of the InSb epilayers lie in the range 0.55–1.5 μm giving room temperature mobilities of 2.5–5.5 m² V⁻¹ s⁻¹ with carrier densities of (0.5–1.5)×10¹⁷ cm⁻³. The room temperature magnetoresistance (MR) for our two terminal devices could be as high as 115% at 50 mT which is comparable to the extraordinary MR (ExMR) recently reported in microscopic composite van der Pauw disks four terminal devices [Science 289 (2000) 1530]. In addition, a high signal-to-noise ratio and a good temperature stability of R(B)/R₀=0.5–0.83% K⁻¹ was observed for B<60 mT (below the saturation field B_sat for ferrite). Device resistance stability R₀(T) was equal to 0.27–0.66% K⁻¹ in zero field with a nominal device resistance R₀=197–224 Ω for DC currents in the range I=0.01–1.0 mA. The minimum detectable magnetic field is estimated from the reduced differential MR (∂R/∂B)/R=2000% T⁻¹ at B=31 mT and normalised 1/f current noise power spectral density measured at the same field. The resolution limit B_min=2.6 nT at 10² Hz and B_min=0.82 nT at 10³ Hz. These resolution limits are seven times better than those recently reported for the same material n-InSb/i-GaAs and ferrite fabricated Hall sensors [Magnetotransport and Raman characterization of n-InSb/i-GaAs epilayers, for Hall sensors applications over extremely wide ranges of temperature and magnetic field, Proceedings NGS 10, IPAP Conference Series 2, IPAP, Tokyo, 2001, pp. 151–154].