New Josephson plasma modes in underdoped YBa2Cu3O6.6 induced by a parallel magnetic field

The c-axis reflectivity spectrum of underdoped YBa2Cu3O6.6 (YBCO) is measured below T(c)=59 K in parallel magnetic fields H parallel CuO2 up to 7 T. Upon application of a parallel field, a new peak appears at finite frequency in the optical conductivity at the expense of suppression of c-axis condensate weight. We conclude that the dramatic change originates from different Josephson coupling strengths between bilayers with and without Josephson vortices. We find that the 400 cm(-1) broad conductivity peak in YBCO gains the spectral weight under parallel magnetic field; this indicates that the condensate weight at omega=0 is distributed to this peak as well as to the new optical Josephson mode.     

磁场对La2－xSrxCuO4单晶热导的影响研究

报道了系列欠掺杂La_2-xSr_xCuO₄（x=0063,0070,0 090,0110,0125）单晶的零场和加磁场情况下ab面和c方向的热导率与温度的关系曲线，测量 温度范围从2到45K. 研究发现ab面和c方向的热导率都受到磁场的压制. 而且在磁场 的作用下，热导压制率随温度变化的关系和场致反铁磁有序的增强与温度的依赖关系有高度 相似性. 认为磁场引起的ab面的热导压制主要是电子热导的变化所致，而c方向的 压制则可能主要来源于声子热导的变化，它们均可能与磁场诱导下欠掺杂La_2-x Sr_xCuO₄的某种电荷有序和磁有序的增强密切相关. 关键词： 热导 电荷有序 磁有序     

Anisotropic field dependence of the magnetic transition in Cu2Te2O5Br2

In this paper, we present the results of measurements of the thermal conductivity of Cu2Te2O5Br2, a compound where tetrahedra of Cu2+ ions carrying S=1/2 spins form chains along the c-axis of the tetragonal crystal structure. The thermal conductivity was measured along both the c- and the a-direction as a function of temperature between 3 and 300 K and in external magnetic fields H up to 69 kOe, oriented both parallel and perpendicular to the c-axis. Distinct features of (T) were observed in the vicinity of TN=11.4 K in zero magnetic field. These features are unaltered in external fields which are parallel to the c-axis, but are more pronounced when a field is applied perpendicularly to the c-axis. The transition temperature increases upon enhancing the external field, but only if the field is oriented along the a-axis.     

Metamagnetic quantum criticality in Sr3Ru2O7 studied by thermal expansion

We report low-temperature thermal expansion measurements on the bilayer ruthenate Sr3Ru2O7 as a function of magnetic field applied perpendicular to the ruthenium-oxide planes. The field dependence of the c-axis expansion coefficient indicates the accumulation of entropy close to 8 T, related to an underlying quantum critical point. The latter is masked by two first-order metamagnetic transitions which bound a regime of enhanced entropy. Outside this region the singular thermal expansion behavior is compatible with the predictions of the itinerant theory for a two-dimensional metamagnetic quantum critical end point.     

C-axis Josephson plasma resonance observed in Tl(2)Ba(2)CaCu(2)O(8) superconducting thin films by use of terahertz time-domain spectroscopy

We have unambiguously observed the c -axis Josephson plasma resonance (JPR) in high-critical-temperature (T(c)) cuprate (Tl(2)Ba(2)CaCu(2)O(8)) superconducting thin films, employing terahertz time-domain spectroscopy in transmission as a function of temperature in zero magnetic field. These are believed to be the first measurements of the JPR temperature dependence of a high-T(c) material in transmission. With increasing temperature, the JPR shifts from 705 GHz at 10 K to ~170 GHz at 98 K, corresponding to an increase in c-axis penetration depth from 22.4+/-0.6mum to 94+/-9mum . The linewidth of the JPR peak increases with temperature, which indicates an increase in the quasi-particle scattering rate. We have probed the onset of the c -axis phase coherence to ~0.95T(c) . The JPR vanishes above T(c) as expected.     

Vortex fluctuations in underdoped Bi(2)Sr(2)CaCu(2)O(8+delta) crystals

Vortex thermal fluctuations in heavily underdoped Bi(2)Sr(2)CaCu(2)O(8+delta) (T(c)=69.4 K) are studied using Josephson plasma resonance. From the zero-field data, we obtain the c-axis penetration depth lambda(L,c)(0)=230+/-10 micrometer and the anisotropy ratio gamma(T). The low plasma frequency allows us to study phase correlations over the whole vortex solid state and to extract a wandering length r(w) of vortex pancakes. The temperature dependence of r(w) as well as its increase with dc magnetic field is explained by the renormalization of the vortex line tension by the fluctuations, suggesting that this softening is responsible for the dissociation of the vortices at the first order transition.     

添加Dy_2O_3的YBa_2Cu_3O_(7-δ)的临界电流密度特征

掺入Dy2O3的YBa2Cu3O7-δ(YBCO)粉末样品采用固相反应方法制备获得。XRD的Rietveld精修和SEM都显示Dy2O3和YBCO并存。随着添加相成分的增加,YBCO的开始转变温度Ton、超导临界温度Tc和零电阻温度Tc0都单调减小。对于某一固定的磁场,温度为77K,当x=0.05时,样品的临界电流密度Jc有最大值。磁场中,微量的Dy2O3的添加,使得YBCO临界电流密度增大。Jc的这些特征行为来自D2O3的添加导致的两方面的共同作用,一方面是超导性的变化,另一方面纳米尺寸范围内的空间分布的非均匀性。     

NON-EQUILIBRIUM MICROWAVE RESPONSE OF Yba2Cu3O7-δ GRANULAR THIN FILMS UNDER MAGNETIC FIELDS

Microwave responses of YBa₂Cu₃O_7-δ(YBCO) granular film have been studied at the microwave frequency of 30.5 GHz. In the absence of a magnetic field the dependence of a normal microwave response on the bias current is observed at a temperature close to T_c. When a magnetic field ranged from 5.0 mT to 33.0 mT is applied, the responses broaden and shift toward a lower temperature. In the superconducting state, the responses were found to be highly dependent on the magnetic field. For the current equal to 5.0mA and a magnetic field above 17.0mT the response increases and did not vanish even at a very low temperature, the fact is believed to be correlated to the anisotropic character of the structure.     

Dynamic coupling-decoupling crossover in the current-driven vortex state in Tl2Ba2CaCu2O8 probed by the Josephson plasma resonance

We have used terahertz spectroscopy to measure the Josephson plasma resonance in the superconductor Tl2Ba2CaCu2O8+delta. This allows us to probe the longitudinal ordering of pancake vortices as a function of applied ab-plane current in a 2.5 kG c-axis magnetic field. With increasing current in the low temperature vortex solid phase, we observe a decrease in the interlayer phase coherence consistent with a progressive misalignment of the pancake vortices in neighboring layers. In the high temperature vortex liquid phase, an increase in the longitudinal ordering occurs above a certain threshold current. Our results show evidence of a current-driven coupling-decoupling crossover in the pinned liquid phase.     

Proton nuclear magnetic resonance in paramagnetic CoCl2.6 H2O

The present paper deals with a study of nuclear magnetic resonance (NMR) of protons of the crystal water of paramagnetic CoCl₂.6 H₂O. The measurements were carried out on powdered samples at room temperature, for values of the external magnetic field ranging from 0.3 to 1.0 T.The NMR signals of protons of the crystal water exhibit asymmetric shape which changes with the applied external magnetic field. We found that the second moment of the resonance line shows a linear dependence on the square of the induction of the externally applied magnetic field.Both the cause of the asymmetry of the NMR line of protons of the crystal water and the dependence of the second moment of the resonance line on the induction of external magnetic field are interpreted in the paper.     

Correlations between microstructure and a.c. magnetic response of bulk superconducting YBa2Cu3O7−x

Summary The magnetic response of granular superconducting YBCO samples with different grain sizes is reported. The analysis is carried out by measuring the low-frequency a.c. magnetic susceptibility as a function of the temperature with an applied magnetic field of amplitude ranging between 0.4 and 800 A/m. The observed magnetic behaviour is interpreted in terms of grain size and weak-links properties. The experimental results evidence the influence of the sample microstructure on the magnetic susceptibility. The intergranular and intragranular onset temperatures are compared with the critical temperature of the samples obtained by means of electrical-resistivity measurements. The temperature dependence of the intergranular critical current density is derived from the susceptibility data.     

Colossal magnetodielectric effects in DyMn2O5

We have investigated the detailed magnetic field dependence of the electric polarization and dielectric constant in (Tb,Dy,Ho)Mn2O5 where magnetic and ferroelectric transitions are intimately coupled. Our fundamental discovery is the unprecedented large change of the dielectric constant with magnetic field, particularly in DyMn2O5, associated with an unusual commensurate-incommensurate magnetic transition. This extraordinary effect appears to originate from the high sensitivity of the incommensurate state to external perturbation.     

Proximity effect in gold-coated YBa(2)Cu(3)O(7-delta) films studied by scanning tunneling spectroscopy

Scanning tunneling spectroscopy on gold layers overcoating c-axis YBa(2)Cu(3)O(7-delta) (YBCO) films reveals proximity-induced gap structures. The gap size reduces exponentially with the distance from a-axis facets, indicating that the proximity effect is primarily due to the (100) YBCO facets. The penetration depth of superconductivity into the gold is approximately 30 nm, in good agreement with estimations for the dirty limit. The extrapolated gap at the interface is approximately 15 meV, similar to the value of an s-wave component of the order parameter measured at the YBCO surface in recent point-contact experiments.     

Quest for frustration driven distortion in Y2(Mo2)O(7)

We investigated the nature of the freezing in the geometrically frustrated Heisenberg spin glass Y2(Mo2)O(7) by measuring the temperature dependence of the static internal magnetic field distribution above the spin-glass temperature, Tg, using the muon spin relaxation technique. The evolution of the field distribution cannot be explained by changes in the spin susceptibility alone and suggests a lattice deformation. This possibility is addressed by numerical simulations of the Heisenberg Hamiltonian with magnetoelastic coupling at T > 0.     

Strongly enhanced flux pinning in the YBa_2Cu_3O_(7-X) films with the co-doping of Ba TiO_3 nanorod and Y_2O_3 nanoparticles at 65 K

YBa2Cu3O7-x(YBCO) films with co-doping BaTiO3(BTO) and Y2O3 nanostructures were prepared by metal organic deposition using trifluoroacetates(TFA-MOD). The properties of the BTO/Y2O3co-doped YBCO films with different excess yttrium have been systematically studied by x-ray diffraction(XRD), Raman spectra, and scanning electron microscope(SEM). The optimized content of yttrium excess in the BTO/Y2O3co-doped YBCO films is 10 mol.%, and the critical current density is as high as ~ 17 mA/cm2(self-field, 65 K) by the magnetic signal. In addition, the Y2Cu2O5 was formed when the content of yttrium excess increases to 24 mol.%, which may result in the deterioration of the superconducting properties and the microstructure. The unique combination of the different types of nanostructures of BTO and Y2O3 in the doped YBCO films, compared with the pure YBCO films and BTO doped YBCO films, enhances the critical current density(JC) not only at the self-magnetic field, but also in the applied magnetic field.     

对靶溅射技术制备的La2/3Ca1/3MnO3/YBa2Cu4O8/La2/3Ca1/3MnO3薄膜中Tc的降低

本文用对靶溅射技术制备了La2/3Ca1/3MnO3/YBa2Cu4O8/La2/3Ca1/3MnO3薄膜.与YBCO单层薄膜相比,由于超导/铁磁系统中的磁性邻近效应,三层薄膜表现出较低的超导转变温度.薄膜的R～T测量曲线显示出超磁阻(CMR)效应和超导转变,预示着超导和铁磁特性共存于LCMO/YBCO/LCMO三文治结构.     

YBa2Cu3O7－δ/SrNb0.01Ti0.99O3 p-n结的制备及其特性

采用脉冲激光沉积技术，在n型SrNb001Ti099O3(SNTO)单晶基片上生长p型YBa2Cu3O7-δ(YBCO)薄膜，制备出YBCO/SNTO p n结.YBCO薄膜是高度c轴织构的超导薄膜，且具有良好的超导电性.YBCO/SNTO p n结具有较好的整流特性和很好的温度与磁场稳定性. 关键词： YBa2Cu3O7-δ SrNb001Ti099O3 p n结     

Superconducting Phase Diagram of the Yttrium,Barium, and YBa-core in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7–δ</Subscript> by an Ising Model

We investigate the temperature dependent of magnetizations, hysteresis properties and present the superconducting phase diagrams of the YBa₂Cu₃O_7–δ (YBCO) by an Ising model within the effective-field theory. We obtain the magnetizations of the YBCO and its components (Cu1, Cu2, O1, O2, O3, O4, Ba, Y, CuO-shell, YBa-core, and total YBCO) versus the reduced temperature and external magnetic field. We find that the copper atoms have two different magnetizations (Cu1 and Cu2) and oxygen atoms have four different magnetizations (O1, O2, O3, O4) but yttrium and barium has one magnetization. The differences in the magnetizations of the copper and oxygen atoms result from the lattice location of them on the YBCO lattice. The magnetization of the Cu1 atoms is the lowest and the yttrium is highest than those of the others; hence magnetizations of the YBCO increase from the corner to inner of the orthorhombic YBCO lattice. We also find that the YBa-core, yttrium and barium atoms of the YBCO exhibit superconductivity behaviors. Therefore, Y–Ba core, yttrium and barium atoms have the superconducting phase diagram and it has Meissner, vortex, and normal states. It is also found that the upper critical coercive field (H_c2) and the critical vortex temperature (T_v) of the Ba are high than those of the Y and YBa-core.     

Observation of multiferroic properties in pyroxene NaFeGe2O6

We report the observation of multiferroicity in a clinopyroxene NaFeGe(2)O(6) polycrystal from the investigation of its electrical and magnetic properties. Following the previously known first magnetic transition at T(N1) = 13 K, a second magnetic transition appears at T(N2) = 11.8 K in the temperature dependence of the magnetization. A ferroelectric polarization starts to develop clearly at T(N2) rather than T(N1) and its magnitude increases up to ~13 μC m(-2) at 5 K, supporting the idea that the ferroelectric state in NaFeGe(2)O(6) stems from a helical spin order stabilized below T(N2). When a magnetic field of 90 kOe is applied, the electric polarization decreases to 9 μC m(-2) and T(N2) slightly increases by 0.5 K. At intermediate magnetic fields, around 28 and 78 kOe, anomalies in the magnetoelectric current, magnetoelectric susceptibility, and field derivative of magnetization curves are found, indicating field-induced spin-state transitions. Based on these electrical and magnetic properties, we provide a detailed low temperature phase diagram up to 90 kOe, and discuss the nature of each phase of NaFeGe(2)O(6).     

Quantum-classical reentrant relaxation crossover in Dy2Ti2O7 spin ice

We have studied spin relaxation in the spin ice compound Dy2Ti2O7 through measurements of the ac magnetic susceptibility. While the characteristic spin-relaxation time (tau) is thermally activated at high temperatures, it becomes almost temperature independent below T(cross) approximately 13 K. This behavior, combined with nonmonotonic magnetic field dependence of tau, indicates that quantum tunneling dominates the relaxational process below that temperature. As the low-entropy spin ice state develops below T(ice) approximately 4 K, tau increases sharply with decreasing temperature, suggesting the emergence of a collective degree of freedom for which thermal relaxation processes again become important as the spins become strongly correlated.