The polarized Raman scattering spectra from freshly cleaved $ab$, $ac$, and $bc$ surfaces of high quality twin free YBa$_2$Cu$_3$O$_{6.5}$ (Ortho-II) single crystals ($T_c$=57.5 K and $ΔT = 0.6$ K) were studied between 80 and 300 K. All eleven $A_g$ Raman modes expected for the Ortho-II structure as well some modes of $B_{2g}$ and $B_{3g}$ symmetry were identified in close comparison with predictions of lattice dynamical calculations. The electronic scattering from the $ab$ planes is strongly anisotropic and decreases between 200 and 100 K within the temperature range of previously reported pseudogap opening. The coupling of phonons to Raman active electronic excitations manifested by asymmetric (Fano) profiles of several modes also decreases in the same range. Among the new findings that distinguish the Raman scattering of Ortho-II from that of Ortho-I phase is the unusual relationship ($α_{xx} \approx -α_{yy}$) between the elements of the Raman tensor of the apex oxygen $A_g$ mode.
We present the transport studies of field variation of the pinning force for the flux motion across the twin boundaries in YBaCuO single crystals. It is found that the depinning current J_c decreases with an increase in the magnetic field due to reduction of the portion of vortex lines trapped by the TB's. However, at transport currents J << J_c, the vortex velocity weakly decreases with the increased field, i.e. it is determined mainly by the release of the vortex lines from the TB's.
In a recent Comment [arXiv:0711.4654], Y. Ando raises two valid issues in relation to our low-temperature heat transport study of YBa$_2$Cu$_3$O$_y$ across the insulator-to-superconductor boundary: 1) the possible role of twin boundaries in scattering phonons; 2) the best fitting procedure for extracting a fermionic (linear) term from the data. In this Reply, we show that both concerns can be laid to rest by 1) using new data from a tetragonal (twin-free) sample, and 2) using an analysis that does not involve any fitting.
We present a comprehensive inelastic neutron scattering study of the magnetic excitations in twin-free YBa(2)Cu(3)O(6.6) (Tc=61 K) for 5 K < T < 290 K. Taking full account of the instrumental resolution, we derive analytical model functions for the magnetic susceptibility chi''(Q,omega) at T = 5 K and 70 K in absolute units. Our models are supported by previous results on similar samples and are valid at least up to excitation energies of omega = 100 meV. The detailed knowledge of chi''(Q,omega) permits quantitative comparison to the results of complementary techniques including angle-resolved…
We show that rotation of the magnetic field off the plane of twin boundaries (TB's) induces transition of an ordered vortex solid phase to a disordered one. This transition arises due to appearance of transverse deformations of vortex lines near the TB's, $u_{t,TB}$, the amplitude of which satisfies the Lindemann criteria, $u_{t,rp}=c_La_0$. This order-disorder transition is accompanied by an increase in the depinning current, by crossover from an elastic creep to plastic one, and by appearance of the $S$-shaped voltage-current characteristics.
Motivated by quantum oscillations observed in highly ordered ortho-II YBa_2Cu_3O_6.5, we study the Fermi surface topology of d-density wave (ddw) or antiferromagnetic (AF) ordering in the presence of the ortho-II potential. We find that the electron pocket is unaffected by the presence of the ortho-II potential. This further strengthens the proposal that quantum oscillations observed in ortho-II-free YBa_2Cu_4O_8 arise from an electron pocket. On the other hand, the hole pocket topology is sensitive to the ortho-II potential. We show there exist generically three distinct quantum oscillations …
In this paper we explore whether the quantum oscillation signals recently observed in ortho-II YBa$_2$Cu$_3$O$_{6.5}$ may be explained by conventional density functional band-structure theory. Our calculations show that the Fermi surface of YBa$_2$Cu$_3$O$_{6.5}$ is extremely sensitive to small shifts in the relative positions of the bands. With rigid band shifts of around 30 meV small tubular pockets of Fermi surface develop around the Y point in the Brillouin zone. The cross-sectional areas and band masses of the quantum oscillatory orbits on these pockets are close to those observed. The di…
First order phase transition of the vortex lattice in twinned YBa2Cu3O7 single crystals in tilted magnetic fields
我们提出了一个详尽的分析，在孪晶YBa 2 Cu 3 O 7单晶体中进行的传输测量，其中确立了涡旋固-液转变是一阶时，施加的磁场H在一个角度θ远离孪晶面的方向。我们发现电阻转变是滞后的，V-I曲线是非线性的，在熔化线Hm（T）处显示出特征的S形，其标度为θ Hm（T，θ）。当接近临界点H*（θ）时，这些特征逐渐丧失。在H*（θ）以上，V-I特性在实验可达的V-I窗口中显示出线性响应，并且转变变得可逆。最后，我们表明，一级相变发生在一个高度相关的涡旋液体在磁场方向和未知对称性的固体状态。因此，可用的数据支持涡线熔化的情况，而不是最近提出的涡升华[T.Sasagawa等人，PRL 80，4297（1998）]。
We report on small-angle neutron scattering studies of the intrinsic vortex lattice (VL) structure in detwinned YBa2Cu3O7 at 2 K, and in fields up to 10.8 T. Because of the suppressed pinning to twin-domain boundaries, a new distorted hexagonal VL structure phase is stabilized at intermediate fields. It is separated from a low-field hexagonal phase of different orientation and distortion by a first-order transition at 2.0(2) T that is probably driven by Fermi surface effects. We argue that another first-order transition at 6.7(2) T, into a rhombic structure with a distortion of opposite sign, …
X-ray diffraction measurements show that the high-temperature superconductor YBa$_2$Cu$_3$O$_{6.54}$, with ortho-II oxygen order, has charge density wave order (CDW) in the absence of an applied magnetic field. The dominant wavevector of the CDW is $\mathbf{q}_{\mathrm{CDW}} = (0, 0.328(2), 0.5)$, with the in-plane component parallel to the $\mathbf{b}$-axis (chain direction). It has a similar incommensurability to that observed in ortho-VIII and ortho-III samples, which have different dopings and oxygen orderings. Our results for ortho-II contrast with recent high-field NMR measurements, whic…
NMR evidence for Friedel-like oscillations in the CuO chains of ortho-II YBa$_2$Cu$_3$O$_{6.5}$
对去孪晶的Ortho-II型$_2$ O $_{6.5}$（YBCO 6.5）单晶的CuO链的核磁共振（NMR）测量揭示了不寻常的和显著的性质。链Cu共振显着加宽，但逐渐，从室温冷却。其线形及其温度依赖性与常规自旋/电荷密度波（S/CDW）相变的线形及其温度依赖性显著不同。相反，线加宽归因于小振幅静态自旋和电荷密度振荡，其具有与有限长度链的末端连接的空间变化的振幅。这种CuO链现象的影响也清楚地表现在平面Cu NMR中。
The vortex lattice (VL) in the high-kappa superconductor YBa2Cu3O7, at 2 K and with the magnetic field parallel to the crystal c-axis, undergoes a sequence of transitions between different structures as a function of applied magnetic field. However, from structural studies alone, it is not possible to determine precisely the system anisotropy that governs the transitions between different structures. To address this question, here we report new small-angle neutron scattering measurements of both the VL structure at higher temperatures, and the field- and temperature-dependence of the VL form f…
Optical Conductivity of Ortho-II YBa$_2$Cu$_3$O$_6.5$
$_2$ O $_6.5$的Ortho-II相的特征在于空Cu和填充Cu-O b轴的周期性交替使晶胞在a方向上加倍。在完整的链中的额外的氧产生了一个有吸引力的潜力，在飞机上的孔。在新的布里渊区边界$k_x=\pm pi/2$，我们估计从LDA计算的间隙开口的平面带分裂成两个。使用一个平面模型，它对待的d-波超导平均场近似，我们表明，带间跃迁产生强烈的各向异性特征的光学电导率控制的区域在k空间接近（$π/2,π/2$）。该特征的边缘位置给出了该区域准粒子谱随温度变化的信息。双层分裂将显示为双边形状。
Recently, charge density wave (CDW) order in the CuO$_2$ planes of underdoped YBa$_2$Cu$_3$O$_{6+δ}$ was detected using resonant soft x-ray scattering. An important question remains: is the chain layer responsible for this charge ordering? Here, we explore the energy and polarization dependence of the resonant scattering intensity in a detwinned sample of YBa$_2$Cu$_3$O$_{6.75}$ with ortho-III oxygen ordering in the chain layer. We show that the ortho-III CDW order in the chains is distinct from the CDW order in the planes. The ortho-III structure gives rise to a commensurate superlattice refl…
Evidence for Static Magnetism in the Vortex Cores of Ortho-II YBa$_2$Cu$_3$O$_{6.50}$
报道了欠掺杂$_2$ O $_{6+x}$涡旋芯中存在静态交变磁场的证据。在$H = 1$ T和$H = 4$ T的外加磁场中测量了YBa $_2$ Cu $_3$ O $_{6.50}$涡旋态的内部磁场分布，发现了高场尾部磁场分布的一个特征，这一特征在最佳掺杂的YBa $_2$ Cu $_3$ O $_{6.95}$中不存在，并且很好地符合涡旋核中具有静态磁场的模型。场的大小估计为18（2）G，并在$T = 10$ K以上减小。我们在现有理论的背景下讨论了额外的涡核磁性的可能起源。
Static charge-density wave (CDW) and spin-density wave (SDW) order has been convincingly observed in La-based cuprates for some time. However, more recently it has been suggested by quantum oscillation, transport and thermodynamic measurements that density wave order is generic to underdoped cuprates and plays a significant role in YBa_2Cu_3O_{6+delta} (YBCO). We use resonant soft x-ray scattering at the Cu L and O K edges to search for evidence of density wave order in Ortho-II and Ortho-VIII oxygen-ordered YBCO. We report a null result -- no evidence for static CDW order -- in both Ortho-II …
We propose that the NMR line shape on the chain Cu in the stoichiometric high-$T_c$ superconductor Ortho-II YBa$_2$Cu$_3$O$_{6.5}$ is determined by the magnetization induced on Cu near O vacancies, due to strong magnetic correlations in the chains. An unrestricted Hartree-Fock calculation of a coupled chain-plane Hubbard model with nearest-neighbor d-wave pairing interaction shows that the broadening of NMR lines is consistent with disorder-induced magnetization at low temperatures. In addition, we give a possible explanation of the anomalous bimodal line shape observed at high temperatures in…
The anisotropy in the microwave conductivity of the ortho-II YBa$_2$Cu$_3$O$_{6.50}$ is studied within the kinetic energy driven superconducting mechanism. The ortho-II YBa$_2$Cu$_3$O$_{6.50}$ is characterized by a periodic alternative of filled and empty $\hat{b}$-axis CuO chains. By considering the CuO chain induced extended anisotropy impurity scattering, the main features of the anisotropy in the microwave conductivity of the ortho-II YBa$_2$Cu$_3$O$_{6.50}$ are reproduced based on the nodal approximation of the quasiparticle excitations and scattering processes, including the intensity an…
For orthorhombic superconductors we define thermodynamic anisotropy $N \equiv d T_c/d ε_{22} - dT_c/d ε_{11}$ as the difference in how superconducting $T_c$ varies with strains $ε_{ii}$, $i=(1, 2)$, along the in-plane directions. We study the hole doping ($p$) dependence of $N$ on detwinned single crystals of underdoped YBa$_2$Cu$_3$O$_y$ (YBCO) using ultrasound technique. While the structural orthorhombicity of YBCO reduces monotonically with decreasing doping over $0.065 <p<0.16$, we find that the thermodynamic anisotropy shows an intriguing enhancement at intermediate doping level, which is…
In present work the conductivity in the basis plane of YBaCuO single crystals slightly doped by Al and Pr with a pre-specified topology of twin boundaries has been investigated. The excess conductivity for the analyzed samples shows dependence like $Δσ\sim(1-T/T^*)\exp(Δ_{ab}^*/T)$ in wide temperature range $T_f<T<T^*$, where $T^*$ can be represents as mean field temperature of superconducting transition. The temperature dependence of pseudogap can be satisfactory described in terms of the BCS-BEC crossover theoretical model.
Comment on `Strong Vortex Liquid Correlation' from Multiterminal Measurements on Untwinned YBa$_2$Cu$_3$O$_{7-δ}$ Single Crystals'
Rydh和Rapp [Phys. Rev. Lett. { \bf 86}，1873（2001）.]声称非孪晶YBa $_2$ Cu $_3$ O $_{7-δ}$晶体中的涡旋液体与熔化转变上方相关，与先前的工作[D.López { \it et al.}，物理修订版信函{ \bf 76}，4034（1996）.]。在这篇评论中，我们提出了新的测量使用相同的实验技术对孪晶和非孪晶的YBa $_2$铜$_3$ O $_{7-δ}$晶体具有类似的整体特性，如Rydh和拉普报告。两种情况下涡相关响应的比较表明，他们工作的中心结论是不正确的。我们的结果再次证实了López { \it et al.}的工作。并指出了里德和拉普作品中误解的根源。
A systematic Raman study in the visible carried out on the YBa2Cu316,18Ox (x=6-7) compounds, with isotopic substitution of 18O for 16O, has detected a doping dependent deviation from harmonic behavior for the frequency shift of the in-phase mode, a smaller amount of anharmonicity for the apex mode, and almost no effect for the out-of-phase B1g-symmetry phonon. It appears that the amount of anharmonicity depends strongly on the oxygen concentration; it diminishes close to the tetragonal to orthorhombic structural phase transition and close to optimal doping, while it reaches its maximum value f…
Angle Dependence of the Transverse Thermal Conductivity in YBa$_2$Cu$_3$O$_7$ single crystals: Doppler Effect vs. Andreev scattering
在不同的磁场和温度下，测量了孪晶和非孪晶YBa $_2$ Cu $_3$ O $_7$单晶的横向热导率$κ_{xy}$随平行于CuO $_2$面的磁场与热流方向夹角$θ$的变化。对于这两种晶体，我们观察到一个明确的双重变化的场角依赖性的$κ_{xy}(θ) = - κ^0_{xy}(T,B) \sin(2θ)$。我们发现振荡幅度$κ^0_{xy}$依赖于温度和磁场。我们的结果表明，$κ^0_{xy} = a B \ln(1/(bB))$与温度和样品依赖的参数$a$和$b$。我们讨论了我们的结果的Andreev散射的准粒子的旋涡和最近提出的理论的基础上的多普勒频移的准粒子谱。
We have used resonant x-ray scattering to determine the momentum dependent charge correlations in YBa$_2$Cu$_3$O$_{6.55}$ samples with highly ordered chain arrays of oxygen acceptors (ortho-II structure). The results reveal nearly critical, biaxial charge density wave (CDW) correlations at in-plane wave vectors (0.315, 0) and (0, 0.325). The corresponding scattering intensity exhibits a strong uniaxial anisotropy. The CDW amplitude and correlation length are enhanced as superconductivity is weakened by an external magnetic field. Analogous experiments were carried out on a YBa$_2$Cu$_3$O$_{6.6…
One-dimensional nature of the magnetic fluctuations in YBa$_2$Cu$_3$O$_{6.6}$
越来越多的证据表明，电荷和自旋的不均匀分布--所谓的“条纹相”--在决定高温超导体的性质方面起着重要作用。例如，最近对YBa $_2$ Cu $_3$ O $_{7-x}$系列材料的中子散射测量表明，自旋和电荷的波动与条纹相位图一致。但是与条纹相相关的波动预计是一维的，而迄今为止观察到的磁波动似乎显示出二维对称性。在这里，我们表明，这种明显的二维结果从测量的孪生晶体，和类似的测量基本上去孪生晶体的YBa $_2$铜$_3$ O $_{6.6}$揭示了一维字符的磁波动，从而大大加强了条纹相的解释。此外，我们的研究结果还表明，超导起源于沿b晶轴延伸的电荷条纹，其中超流密度被发现是大大大于a方向。
Theory of Josephson tunneling along the c-axis of YBCO
当电流沿$_2$ O $_{7-δ}$的c轴流动时，在$_2$ O $_{7-δ}$和Pb之间存在约瑟夫森隧穿.这可能是由于$_2$ O $_{7-δ}$中的正交畸变所引起的。这一假设本身似乎是在与孪生样品的隧道电流不遵循随机统计实验相矛盾。我们提出了一个理论，这取决于相互之间的竞争的双d波耦合和相对增强的表面上的s波分量。这一理论似乎至少在定性上解释了迄今为止的所有观测结果。
Magnetic Order in YBa$_2$Cu$_3$O$_{6+x}$ Superconductors
本文用极化和非极化中子衍射方法研究了$_2$超导体中的磁有序。大部分测量是在高质量的$_2$ O $_{6.6}$晶体上进行的。结果表明，该晶体具有高度有序的邻位-Ⅱ链序，并具有明显的超导转变。非弹性散射测量显示一个非常干净的自旋间隙和赝隙，在10毫电子伏的任何强度小于50倍的共振强度。该晶体显示出复杂的磁序，似乎有三个分量。在高温下发现磁性相，其似乎源于具有在$a$ - $b$平面中但在晶格上无序的力矩的杂质。第二次排序发生在赝能隙温度附近，其具有比高温相更短的相关长度和至少部分地沿着晶体的c轴的力矩方向。它的力矩方向，温度依赖性和布拉格强度表明，它可能源于$d$ -密度波（DDW）类型的轨道排序。另外的强度增加发生在超导转变之下。在这些阶段中的磁场强度不会明显改变，在7特斯拉的磁场大致沿c轴对齐。对$_2$ Cu $_3$ O $_{7}$的磁有序的测量没有显示出信号，而在$_2$ Cu $_3$ O $_{6.45}$中发现了一个小的磁场强度，这与c轴方向的磁有序一致。结果与其他最近的中子测量。
We report measurements of the magnetic susceptibility of twinned single crystals of YBa$_{2}$Cu$_{3}$O$_{6+x}$ from just above their superconducting transition temperatures to 300 K with magnetic fields of up to 5 T applied parallel and perpendicular to the CuO$_2$ planes at 7 values of $x$. Appropriate analysis allows the relatively small, but still important, Curie terms to be separated from other contributions to the susceptibility. Our data support a picture in which the Curie terms arise from oxygen disorder in the Cu-O chains. This agrees with published work on polycrystalline samples wh…
The effect of electron irradiation with the energy of 2.5 MeV on the temperature dependences of the resistivity of an optimally doped $YBa_2Cu_3O_{7-δ}$ single crystal has been studied. The temperature dependences of both fluctuation conductivity $σ^\prime(T)$ and the pseudogap $Δ^*(T)$ on irradiation dose $ϕ$ have been calculated within the local pair model. Here we show that with an increase in $ϕ$, the value of $ρ(300 K)$ increases linearly, while $T_c$ decreases linearly. Concurrently, the value of $ρ{100 K}$ increases nonlinearly, demonstrating a feature for $ϕ_3$ = 4.3 10$^{18}$ e/$cm^2$…
Different imaging modes of low temperature laser scanning microscopy (LTLSM) have been applied to probe local optical and superconducting properties, as well as the spatial variations in thermoelectric and electronic (both dc and rf) transport, in a YBa_2Cu_3O_6.95 /LaAlO_3 (YBCO/LAO) superconducting microstrip resonator with micron-range resolution. Additionally, the local sources of microwave nonlinearity (NL) were mapped in two-dimensions simultaneously by using the LTLSM in two-tone rf intermodulation distortion contrast mode as a function of (x,y) position of the laser beam perturbation o…
The interplay between structure, magnetism and superconductivity in single crystal Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ (x=0.047) has been studied using high-resolution X-ray diffraction by monitoring charge Bragg reflections in each twin domain separately. The emergence of the superconducting state is correlated with the suppression of the orthorhombic distortion around \emph{T}$_\texttt{C}$, exhibiting competition between orthorhombicity and superconductivity. Above \emph{T}$_\texttt{S}$, the in-plane charge correlation length increases with the decrease of temperature, possibly induced by n…
