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.     

Magnetic spin ladder (C5H12N)2CuBr4: high-field magnetization and scaling near quantum criticality

The magnetization, M(H< or =30 T,0.7< or =T< or =300 K), of (C5H12N)2CuBr4 has been used to identify this system as an S = 1/2 Heisenberg two-leg ladder in the strong-coupling limit, J( perpendicular) = 13.3 K and J( parallel) = 3.8 K, with H(c1) = 6.6 T and H(c2) = 14.6 T. An inflection point in M(H,T = 0.7 K) at half saturation, M(s)/2, is described by an effective XXZ chain. The data exhibit universal scaling behavior in the vicinity of H(c1) and H(c2), indicating that the system is near a quantum critical point.     

High-field quasiparticle tunneling in Bi2Sr2CaCu2O(8+delta): negative magnetoresistance in the superconducting state

We report on the c-axis resistivity rho(c)(H) in Bi(2)Sr(2)CaCu(2)O(8+delta) that peaks in quasistatic magnetic fields up to 60 T. By suppressing the Josephson part of the two-channel (Cooper pair/quasiparticle) conductivity sigma(c)(H), we find that the negative slope of rho(c)(H) above the peak is due to quasiparticle tunneling conductivity sigma(q)(H) across the CuO2 layers below H(c2). At high fields (a) sigma(q)(H) grows linearly with H, and (b) rho(c)(T) tends to saturate ( sigma(c) not equal0) as T-->0, consistent with the scattering at the nodes of the d-wave gap. A superlinear sigma(q)(H) marks the normal state above T(c).     

电流磁场作用下YBCO颗粒膜的非平衡微波响应

文中从二维 Josephson弱连接网络模型出发 ,讨论了二维超导体系的磁通“涡流 -反涡流”束缚对的激发问题 ,并进一步讨论了涡旋在电流、磁场作用下的激发问题 ,给出了自由涡旋的密度分布 n(I,H ,T)随温度的分布图 ,对比高温超导体的非平稳微波响应曲线图 ,发现两者具有相似的分布特征。该结果表明 ,高温超导体的非平稳微波响应与磁通“涡旋 -反涡旋”束缚对的激发有关。     

Nernst effect and disorder in the normal state of high-T(c) cuprates

We have studied the influence of disorder induced by electron irradiation on the Nernst effect in optimally and underdoped YBa2Cu3O(7-delta) single crystals. The fluctuation regime above T(c) expands significantly with disorder, indicating that the T(c) decrease is partly due to the induced loss of phase coherence. In pure crystals the temperature extension of the Nernst signal is found to be narrow whatever the hole doping, contrary to data reported in the low-T(c) cuprate families. Our results show that the presence of intrinsic disorder can explain the enhanced range of the Nernst signal found in the pseudogap phase of the latter compounds.     

Enhanced heat capacity and a new temperature instability in superfluid 4He in the presence of a constant heat flux near t(lambda)

We present the first experimental evidence that the heat capacity of superfluid 4He, at temperatures very close to the lambda point T(lambda), is enhanced by a constant heat flux Q. The heat capacity at constant Q, C(Q), is predicted to diverge at a temperature T(c)(Q)相似文献   

Specific-heat measurements of the gap structure of the organic superconductors kappa-(ET)2Cu[N(CN)2]Br and kappa-(ET)2Cu(NCS)2

We present high resolution heat capacity measurements of the organic superconductors kappa-(ET)(2)Cu[N(CN)(2)]Br and kappa-(ET)(2)Cu(NCS)(2) in fields up to 14 T. We use the high field data to determine the normal state specific heat and hence extract the behavior of the electronic specific heat C(el) in the superconducting state in zero and finite fields. We find that in both materials for T/T(c) less or similar 0.3, C(el)(H=0) approximately T2 indicating d-wave superconductivity. The data are well described by a strong coupling d-wave model from our base temperature (T/T(c) approximately 0.1) right up to T(c). Our data help to resolve the controversy regarding the order parameter symmetry in these materials.     

Evidence for two energy scales in the superconducting state of optimally doped (Bi,Pb)2(Sr,La)2CuO6+delta

We use angle-resolved photoemission spectroscopy to investigate the energy gap(s) in (Bi,Pb)2(Sr,La)2CuO6+delta. We find that the spectral gap has two components in the superconducting state: a superconducting gap and pseudogap. Differences in their momentum and temperature dependence suggest that they represent two separate energy scales. Spectra near the node reveal a sharp peak with a small gap below T(c) that closes at T(c). Near the antinode, spectra are broad with a large energy gap of approximately 40 meV above and below T(c). The latter spectral shape and gap magnitude are almost constant across T(c), indicating that the pseudogap state coexists with the superconducting state below T(c), and it dominates spectra around the antinode. We speculate that the pseudogap state competes with the superconductivity by diminishing spectral weight in antinodal regions, where the superconducting gap is largest.     

Calculation of the bulk viscosity in SU(3) gluodynamics

We perform a lattice Monte Carlo calculation of the trace-anomaly two-point function at finite temperature in the SU(3) gauge theory. We obtain the long distance properties of the correlator in the continuum limit and extract the bulk viscosity zeta via a Kubo formula. Unlike the tensor correlator relevant to the shear viscosity, the scalar correlator depends strongly on temperature. If s is the entropy density, we find that zeta/s becomes rapidly small at high T, zeta/s<0.15 at 1.65T(c), and zeta/s<0.015 at 3.2T(c). However, zeta/s rises dramatically just above T(c), with 0.5相似文献   

Transverse Josephson plasma mode in T* phase SmLa(1-x)Sr(x)CuO(4-delta) single crystals

Far-infrared reflectivity along the c axis in T* phase SmLa(1-x)Sr(x)CuO(4-delta) single crystals is measured down to 8 cm(-1). Below T(c), the conductivity peak is observed at 25 cm(-1) for x = 0.15 ( T(c) = 30 K) along with two reflectivity edges at 13 and 27 cm(-1). The conductivity peak is attributed to the transverse Josephson plasma mode between two longitudinal Josephson plasma modes, while the oscillator strength of the peak is found to be smaller than that calculated using the Josephson-coupled multilayer model. The difference is explained by assuming that only a few junctions at the disordered (La,Sr)(2)O(2-delta) block layer take part in the plasma oscillation with omega(pI(')) = 27 cm(-1).     

Superconductivity in the Einstein solid V Al(10.1)

We used magnetic susceptibility, resistivity and heat capacity measurements to characterize the superconducting state in the Einstein solid V Al(10.1). We find that V Al(10.1) is a weak-coupling, type-II superconductor with T(c)?=?1.53?K and an upper critical field of H(c2)(0)?=?800?Oe. The heat capacity data in the range 0.07?K?相似文献   

Large local disorder in superconducting K(0.8)Fe(1.6)Se2 studied by extended x-ray absorption fine structure

We have measured the local structure of superconducting K(0.8)Fe(1.6)Se(2) chalcogenide (T(c) = 31.8 K) by temperature dependent polarized extended x-ray absorption fine structure (EXAFS) at the Fe and Se K-edges. We find that the system is characterized by a large local disorder. The Fe-Se and Fe-Fe distances are found to be shorter than the distances measured by diffraction, while the corresponding mean square relative displacements reveal large Fe-site disorder and relatively large c-axis disorder. The local force constant for the Fe-Se bondlength (k ~ 5.8 eV ?(-2)) is similar to the one found in the binary FeSe superconductor, however, the Fe-Fe bondlength appears to be flexible (k ~ 2.1 eV ?(-2)) in comparison to the binary FeSe (k ~ 3.5 eV ?(-2)), an indication of partly relaxed Fe-Fe networks in K(0.8)Fe(1.6)Se(2). The results suggest a glassy nature for the title system, with the superconductivity being similar to that in the granular materials.     

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.     

Spin Triplet Superconducting State due to Broken Inversion Symmetry in Li(2)Pt(3)B

We report (11)B and (195)Pt NMR measurements in noncentrosymmetric superconductor Li(2)Pt(3)B. We find that the spin susceptibility measured by the Knight shift remains unchanged across the superconducting transition temperature T(c). With decreasing temperature (T) below T(c), the spin-lattice relaxation rate 1/T(1) decreases with no coherence peak and is in proportion to T3. These results indicate that the Cooper pair is in the spin-triplet state and that there exist line nodes in the superconducting gap function. They are in sharp contrast to those in the isostructural Li(2)Pd(3)B which is a spin-singlet, s-wave superconductor, and are ascribed to the enhanced spin-orbit coupling due to the lack of spatial inversion symmetry. Our finding points to a new paradigm where exotic superconductivity arises in the absence of electron-electron correlations.     

Response to tilted magnetic fields in Bi(2)Sr(2)CaCu(2)O(8) with columnar defects: evidence for transverse Meissner effect

The transverse Meissner effect (TME) in the highly layered superconductor Bi(2)Sr(2)CaCu(2)O(8+y) with columnar defects is investigated by transport measurements. We present evidence for the persistence of the Bose glass phase for H(perpendicular)H(+)(perpendicular c), moving kink chains consistent with a commensurate-incommensurate transition scenario are observed. These results show the existence of the TME for H(perpendicular)相似文献   

磁场下高温超导YBCO颗粒膜的非平衡微波响应

应用二维约瑟夫逊弱连接网络模型 ,讨论了磁场引起的二维超导体系中磁通“涡旋反涡旋”束缚对的激发问题 ,给出了自由涡旋分布 n( T,H )的分析表达式 ,对比 YBCO颗粒膜样品的非平衡微波响应测量数据发现 :两者具有相似的分布特征。该结果表明 ,高温超导颗粒膜的非平衡微波响应机制与磁通“涡旋反涡旋”束缚对的激发态可能存在某种内在联系。     

Magnetic behavior of Eu(3)Ni(4)Ga(4): antiferromagnetic order and large magnetoresistance

The results of the magnetic susceptibility, isothermal magnetization, heat capacity, electrical resistivity and magnetoresistance measurements on polycrystalline Eu(3)Ni(4)Ga(4) are presented. Eu(3)Ni(4)Ga(4) forms in Na(3)Pt(4)Ge(4)-type cubic crystal structure (space group [Formula: see text]). The temperature dependence of the magnetic susceptibility of Eu(3)Ni(4)Ga(4) confirms the divalent state (Eu(2+)) of Eu ions with an effective magnetic moment μ(eff)?=?7.98?μ(B). At low fields, e.g.?at 0.01?T, a magnetic phase transition to an antiferromagnetically ordered state occurs at T(N)?=?10.9?K, which is further confirmed by the temperature dependence of the heat capacity and electrical resistivity. The field dependence of isothermal magnetization at 2?K reveals the presence of two field induced metamagnetic transitions at H(c1) and H(c2)?=?0.55 and 1.2?T, respectively and a polarized phase above H(PO)?=?1.7?T. The reduced jump in the heat capacity at the transition temperature, ΔC|(T(N))?=?13.48?J/mol-Eu?K would indicate an amplitude modulated (AM) antiferromagnetic structure. An interesting feature is that a large negative magnetoresistance, MR?=?[ρ(H)?-?ρ(0)]/ρ(0), is observed in the vicinity of magnetic transition even up to 2T(N). Similar large magnetoresistance has been observed in the paramagnetic state in some Gd and Eu based alloys and has been attributed to the magneto-polaronic effect.     

BROADENING OF RESISTIVE TRANSITION AND IRREVERSIBILITY LINE FOR EPITAXIAL YBa2Cu3O7-$\delta$ THIN FILM

The broadening of resistive transition of c axis oriented epitaxial YBCO thin film has been measured for three configurations: (1) H∥c and H⊥I; (2) H∥ab plane and H⊥I; (3) H∥ab plane and H∥I in magnetic field up to 8 Teala(T), and for different angle θ of magnetic field relative to the ab plane with H = 4T. The results obtained indicate that the broadening of resistive transition is mainly determined by the angle θ, but is hardly related to the angle α made between magnetic field and tran sport current in ab plane. This means that the broadening of resistive transition is not determined by flux motion drived by apparent Lorentz force. Au expression of angular dependence of irreveraibility line has been given.     

Metal-to-insulator crossover in YBa2Cu3Oy probed by low-temperature quasiparticle heat transport

It was recently demonstrated that in La2-xSrxCuO4 the magnetic-field (H) dependence of the low-temperature thermal conductivity kappa up to 16 T reflects whether the normal state under high magnetic field is a metal or an insulator. We measure the H dependence of kappa in YBa(2)Cu(3)O(y) (YBCO) at subkelvin temperatures for a wide doping range, and find that at low doping the kappa(H) behavior signifies the change in the ground state in this system as well. Surprisingly, the critical doping is found to be located deeply inside the underdoped region, about the hole doping of 0.07 hole/Cu; this critical doping is apparently related to the stripe correlations as revealed by the in-plane resistivity anisotropy.