We have grown untwinned YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) films on (110) MgO substrates that were pre-annealed at high temperature in oxygen atmosphere. The annealing results in surface reconstruction with shallow facets, which induce the suppression of the YBCO twinning domains, and the preferential alignment of the CuO chains along one of the in-plane directions of the substrate. Because of the large mismatch between the in-plane lattice parameters of film and substrate, the strain induced by the MgO into the YBCO layer is strong and very peculiar. The YBCO film is compressed, with respect to the bulk, and presents a unidirectional buckling of the atomic planes, along the chains' direction, due to a deformation of the copper-oxygen octahedra. The YBCO films, which can be grown with thicknesses down to few unit cells and oxygen doping levels spanning most of the superconducting dome, are patterned into nanowires with dimensions down to 50 nm. The anisotropies due to the untwinning state are preserved in these structures; moreover, additional anisotropies appear, in ultrathin structures where strain effects become more pronounced. Such untwinned and compressively strained films can therefore be used as a platform to study the interplay between strain and the various local orders in the normal state of YBCO.
We report on an \textit{ab}-anisotropy of $J_{c \parallel b}/J_{c \parallel a}% \cong 1.8$ and $I_{c}R_{n \parallel b}/I_{c}R_{n \parallel a}\cong 1.2$ in ramp-edge junctions between untwinned YBa$_{2}$Cu$_{3}$O$_{7}$ and $s$% -wave Nb. For these junctions, the angle $θ$ with the YBa$_{2}$Cu$_{3}$O$_{7}$ crystal b-axis is varied as a single parameter. The $R_{n}$A($θ$)-dependence presents 2-fold symmetry. The minima in $I_{c}R_{n}$ at $θ\cong 50^{\circ}$ suggest a real s-wave subdominant component and negligible $d_{xy}$-wave or imaginary s-wave admixtures. The $I_{c}R_{n}$($θ$)-dependence is …
We present a study of the vortex lattice in untwinned YBa$_2$Cu$_3$O$_{7-x}$ crystals, using a combination of muon spin rotation and neutron small angle scattering measurements. Both methods show a very sharp melting temperature consistent with a first order transition. The dependence of the melting temperature on the angle of the field with respect to the crystallographic c-axis is studied. The results are compared to thermal measurements.
A.Rydh and Ö.Rapp [Phys. Rev. Lett. {\bf 86}, 1873 (2001).] claim that the vortex liquid in untwinned YBa$_2$Cu$_3$O$_{7-δ}$ crystals is correlated above the melting transition, in striking contrast to previous work [D.López {\it et al.}, Phys. Rev. Lett. {\bf 76}, 4034 (1996).]. In this Comment we present new measurements using the same experimental technique on twinned and untwinned YBa$_2$Cu$_3$O$_{7-δ}$ crystals with similar overall characteristics as those reported by Rydh and Rapp . The comparison of the vortex correlation response in both cases indicates that the central conclusion of t…
We present high-resolution capacitance dilatometry studies from 5 - 500 K of untwinned YBa2Cu3Ox (Y123) single crystals for x ~ 6.95 and x = 7.0. Large contributions to the thermal expansivities due to O-ordering are found for x ~ 6.95, which disappear below a kinetic glass-like transition near room temperature. The kinetics at this glass transition is governed by an energy barrier of 0.98 +- 0.07 eV, in very good agreement with other O-ordering studies. Using thermodynamic arguments, we show that O-ordering in the Y123 system is particularly sensitive to uniaxial pressure (stress) along the c…
We report on measurements of the doping dependence of the upper critical field $H_{c,2}$ in 50 nm thick YBa$_2$Cu$_3$O$_{7-δ}$ films. The films are untwinned and are characterized by a small in-plane compressive strain. We find that the $H_{c,2}$ shows a strong decrease in the underdoped region of the phase diagram, in agreement with what has been measured in relaxed single crystals. The origin of the decrease of $H_{c,2}$ in the underdoped regime is discussed within a scenario where charge density wave order competes with superconductivity. This demonstrates the potential of using thin films …
In a recent paper, Matl et al present a high-frequency study of the complex resistivity of a pinned vortex lattice in YBaCuO . They focus on the inductive-to-resistive transition which is investigated as a function of temperature at a constant field $B_0=2$ T, so that the transition is associated with the vanishing of vortex pinning strength. To our view, their conclusions rely on a rather brittle experimental body and the collapse of C66 results from an involved analysis of the finite frequency corrections to $ρ(ω)$. These corrections are not necessary since the complex frequency spectrum has…
With significantly improved sample quality and instrumental resolution, we clearly identify in the (pi,0) ARPES spectra from YBa2Cu3O6.993, in the superconducting state, the long-sought `peak-dip-hump' structure. This advance allows us to investigate the large a-b anisotropy of the in-plane electronic structure including, in particular, a 50% difference in the magnitude of the superconducting gap that scales with the energy position of the hump feature. This anisotropy, likely induced by the presence of the CuO chains, raises serious questions about attempts to quantitatively explain the YBa2C…
Magnetoresistance (MR) in the a-axis resistivity of untwinned YBa_{2}Cu_{3}O_{y} single crystals is measured for a wide range of doping (y = 6.45 - 7.0). The y-dependence of the in-plane coherence length ξ_{ab} estimated from the fluctuation magnetoconductance indicates that the superconductivity is anomalously weakened in the 60-K phase; this gives evidence, together with the Hall coefficient and the a-axis thermopower data that suggest the hole doping to be 12% for y = 6.65, that the origin of the 60-K plateau is the 1/8 anomaly. At high temperatures, the normal-state MR data show signatures…
We present the result of our accurate measurements of the a- and b-axis resistivity (ρ_a and ρ_b), magnetoconductivity (Δσ/ σ), Hall coefficient R_H, and the a-axis thermopower S_a in untwinned YBa_{2}Cu_{3}O_{y} single crystals in a wide range of doping (6.45 < y < 7.0). The systematics of our data reveals a number of novel 60-K-phase anomalies in the charge transport: (i) Temperature dependences of ρ_a show anomalous overlap below ~130 K for 6.65 < y < 6.80, (ii) Hall mobility μ_H shows an enhancement near y ~ 6.65, which is reflected in an anomalous y dependence of σ_{xy}, (iii) With decrea…
The complex resistivity $\hatρ(ω)$ of the vortex lattice in an untwinned crystal of 93-K $\rm YBa_2Cu_3O_7$ has been measured at frequencies $ω/2π$ from 100 kHz to 20 MHz in a 2-Tesla field $\bf H\parallel c$, using a 4-probe RF transmission technique that enables continuous measurements versus $ω$ and temperature $T$. As $T$ is increased, the inductance ${\cal L}_s(ω) ={\rm Im} \hatρ(ω)/ ω$ increases steeply to a cusp at the melting temperature $T_m$, and then undergoes a steep collapse consistent with vanishing of the shear modulus $c_{66}$. We discuss in detail the separation of the vortex-…
We have investigated the nonlinear microwave properties of electron-beam coevaporated YBaCuO films on MgO, using stripline resonators at 2.3 GHz and temperatures 1.7-80 K. The oxygen content of the films ranged from strongly underdoped to overdoped. Above 20 K, the nonlinear response of the resonators was dominated by the superconductor. We could establish clear correlations between the nonlinear surface resistance, two-tone intermodulation (IMD), and oxygen content of the films, which indicate that the superconducting order parameter is important for the nonlinearities. An exponential rather …
We have investigated the anomalous nonlinear microwave response of epitaxial electron-beam coevaporated YBa2Cu3O7-x films on MgO. The power and temperature dependent surface impedance and two-tone intermodulation distortion were measured in stripline resonators at several frequencies between 2.3 and 11.2 GHz and at temperatures between 1.7 K and Tc. All of the eight films measured to date show a decrease of the surface resistance Rs of up to one order of magnitude as the microwave current is increased up to approximately 1 mA for T < 20 K. The surface reactance Xs showed only a weak increase i…
Using a generalized RPA-type theory we calculate the in-plane anisotropy of the magnetic excitations in hole-doped high-$T_c$ superconductors. Extending our earlier Fermi-liquid based studies on the resonance peak by inclusion of orthorhombicity we still find two-dimensional spin excitations, however, being strongly anisotropic. This reflects the underlying anisotropy of the hopping matrix elements and of the resultant superconducting gap function. We compare our calculations with new experimental data on {\it fully untwinned} ${YBa}_2{Cu}_3{O}_{6.85}$ and find good agreement. Our results are …
The Hall coefficient is measured in YBa$_2$Cu$_3$O$_y$ untwinned single crystals for a wide range of doping. We show that the Hall conductivity and the Hall angle of the {\it CuO$_2$ planes} in YBa$_2$Cu$_3$O$_y$ can be extracted from measurable transport properties regardless of the conduction of the Cu-O chains nor the in-plane anisotropy of the CuO$_2$ planes. The present analysis allows us to discuss the genuine Hall effect in the CuO$_2$ planes alone in YBa$_2$Cu$_3$O$_y$ without any complications due to the Cu-O chains.
Recently, Eremin and Manske [1] presented a oneband Fermi-liquid theory for the in-plane magnetic anisotropy in untwinned high-Tc superconductor YBa2Cu3O6:85 (YBCO). They claimed that they found good agreement with inelastic neutron scattering (INS) spectra. In this Comment, we point out that their conclusion on this important problem may be questionable due to an error in logic about the orthorhombicity delta_0 characterizing the lattice structure of YBCO. In Ref. [1], a single band at delta_0>0 is proved to be in accordance with the angle resolved photoemission spectroscopy (ARPES) on untwin…
The thermal conductivity of YBa_2Cu_3O_6.9 was measured at low temperatures in untwinned single crystals with concentrations of Zn impurities from 0 to 3% of Cu. A linear term kappa_0/T = 0.19 mW/K^2.cm is clearly resolved as T -> 0, and found to be virtually independent of Zn concentration. The existence of this residual normal fluid strongly validates the basic theory of transport in unconventional superconductors. Moreover, the observed universal behavior is in quantitative agreement with calculations for a gap function of d-wave symmetry.
The nonlinear surface resistance Rs of the YBa2Cu3O7-x superconducting thin films on the MgO substrates was researched in the Hakki-Coleman dielectric resonator at the microwave signal powers from -18 dBm to +30 dBm at the ultra high frequency of 25 GHz in the range of temperatures from 12 K to 85 K. The dependences of the surface resistance on the temperature RS(T) in the YBa2Cu3O7-x superconducting thin films at the microwaves were measured. The dependence of the surface resistance on the microwave power RS(P) in the YBa2Cu3O7-x superconducting thin films at the microwaves were found at the …
We have investigated the nonlinear surface impedance and two-tone intermodulation distortion of nine epitaxial YBa2Cu3O7-d films on MgO substrates, using stripline resonators, at frequencies f=2.3-11.2GHz and temperatures T=1.7K-Tc. The power dissipation decreased by up to one order of magnitude as the microwave electric field was increased to about 100V/m for T<20 K, while the reactance Xs showed only a weak increase. The minimum of the losses correlated with a plateau in the intermodulation signal. The same features were observed for a Nb film on MgO. The anomalous response is due to nonline…
We have measured the transverse thermal conductivity $κ_{xy}$ of twinned and untwinned YBa$_2$Cu$_3$O$_7$ single crystals as a function of angle $θ$ between the magnetic field applied parallel to the CuO$_2$ planes and the heat current direction, at different magnetic fields and temperatures. For both crystals we observed a clear twofold variation in the field-angle dependence of $κ_{xy}(θ) = - κ^0_{xy}(T,B) \sin(2θ)$. We have found that the oscillation amplitude $κ^0_{xy}$ depends on temperature and magnetic field. Our results show that $κ^0_{xy} = a B \ln(1/(bB))$ with the temperature- and s…
Analysis of the a-b plane optical conductivity $σ_{ab}$ for both twinned and untwinned YBa$_{2}$Cu$_{3}$O$_{7-δ}$ as a function of temperature and doping shows that below a well defined temperature T*, a dip in the spectrum systematically appears separating the infrared charge excitation spectrum into two components with distinct energy scales. The change from monotonic behaviour in $σ_{ab}$ is found to be concurrent with the onset of phonon anomalies in Raman and infrared spectra below T*. The optical data are suggested to be evidence for the appearance of an inhomogeneous distribution of car…
Motivated by predictions of a substantial contribution of the "buckling" vibration of the CuO2 layers to d-wave superconductivity in the cuprates, we have performed an inelastic neutron scattering study of this phonon in an array of untwinned crystals of YBa2Cu3O7. The data reveal a pronounced softening of the phonon at the in-plane wave vector q = (0, 0.3) upon cooling below ~ 105 K, but no corresponding anomaly at q = (0.3, 0). Based on the observed in-plane anisotropy, we argue that the electron-phonon interaction responsible for this anomaly supports an electronic instability associated wi…
Measurements of the far infrared magnetotransmission of YBa2(ZnxCu(1-x))3O(7-d) thin film (x = 0.025) deposited on a wedged MgO substrate are reported. The application of magnetic field perpendicular to the ab plane produces at low temperature a linear increase of transmission for frequencies below 30 cm-1. We present a model of high frequency vortex dynamics which qualitatively explains these results.
A pronounced xy-anisotropy was observed in recent neutron scattering experiments for magnetic excitations in untwinned YBa_{2}Cu_{3}O_{6+y} (YBCO). The small anisotropy of the bare band structure due to the orthorhombic crystal symmetry seems to be enhanced by correlation effects. A natural possibility is that the system is close to a Pomeranchuk instability associated with a d-wave Fermi surface deformation (dFSD). We investigate this possibility in the bilayer t-J model within a self-consistent slave-boson mean-field theory. We show that the dFSD correlations drive a pronounced xy-anisotropy…
This paper reveals the origin of variation in the magnitude and temperature dependence of the normal state resistivity frequently observed in different YBCO single crystal or thin film samples with the same $T_{c}$. We investigated temperature dependence of resistivity in $YBa_{2}Cu_{3}O_{7-δ}$ thin films with 7- $δ= 6.95$ and 6.90, which were subjected to annealing in argon at 400-420 K ($120-140^{o}C$). Before annealing these films exhibited a non-linear $ρ_{ab}(T)$, with a flattening below 230 K, similar to $ρ_{b}(T)$ and $ρ_{ab}(T)$ observed in untwinned and twinned YBCO crystals, respecti…
Using 4-circle x-ray diffraction and transmission electron microscopy we have studied the microstructure and in-plane orientation of the phases present in thin film composite mixtures of YBa(2)Cu(3)O(7) and Y(2)O(3). We see a high degree of in-plane orientation and have verified a previous prediction for the in-plane order of Y(2)BaCuO(5) on (110) MgO. Transmission electron microscopy shows the composite films to be a mixture of two phases, with YBCO grain sizes of 1 micron. We have also compared our observations of the in-plane order to the predictions of a modified near coincidence site latt…
Superconducting thick films (200$μ$m-300$μ$m) of YBCO have been fabricated successfully on MgO substrates by a new approach. The precursor powders (YBa2Cu3O7-$δ$(Y123)+0.1molY2BaCuO5(Y211)) are placed between two MgO (10mm x 10mm) slices to form sandwich structure. The YBCO thick films have been obtained from the precursors by modified melt growth process. Resistance measurements of YBCO thick films show Tcon of 87.3K and $Δ$T of 4.5K. Estimates using hysteresis loops and the Bean model give a value of 2.78 x 10$^{3}$ A/cm2 (77K, 0T) for the critical current density. The observations of scanni…
We investigate the manifestation of the stripes in the in-plane resistivity anisotropy in untwinned single crystals of La_{2-x}Sr_{x}CuO_{4} (x = 0.02 - 0.04) and YBa_{2}Cu_{3}O_{y} (y = 6.35 - 7.0). It is found that both systems show strongly temperature-dependent in-plane anisotropy in the lightly hole-doped region and that the anisotropy in YBa_{2}Cu_{3}O_{y} grows with decreasing y below about 6.60 despite the decreasing orthorhombicity, which gives most direct evidence that electrons self-organize into a macroscopically anisotropic state. The transport is found to be easier along the dire…
Using a Fermi-liquid-based theory we calculate the in-plane anisotropy of the spin susceptibility for hole-doped high-T_c cuprates. Employing the two-dimensional one-band Hubbard model and a generalized RPA-type theory we consider anisotropic hopping matrix elements ($t_x \neq t_y$) and a mixing of d- and s-wave symmetry of the superconducting order parameter in order to describe orthorhombic superconductors. We compare our calculations with available inelastic neutron scattering data on untwinned YBa{2}Cu{3}O{7-δ} and find good agreement. Furthermore, we predict a strongly anisotropic in-plan…
In this work, we report a strengthened superheating effect caused by a buffering YBa2Cu3Oy (Y123 or YBCO) layer in the Nd1+xBa2-xCu3O7-y (Nd123 or NdBCO) thin film with MgO substrate (i.e., NdBCO/YBCO/MgO thin film). In the cold-seeding melt-textured (MT) growth, the NdBCO/YBCO/MgO film presented an even higher superheating level, about 20 °C higher than that of non-buffered NdBCO film (i.e., NdBCO/MgO film). Using this NdBCO/YBCO/MgO film as seeds and undergoing a maximum processing temperature (Tmax) up to 1120 °C, we succeeded in growing various RE1+xBa2-xCu3O7-y (REBCO, RE=rare elements) b…
We report on the growth and characterization of ultrathin YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) films on MgO (110) substrates, which exhibit superconducting properties at thicknesses down to 3 nm. YBCO nanowires, with thicknesses down to 10 nm and widths down to 65 nm, have been also successfully fabricated. The nanowires protected by a Au capping layer show superconducting properties close to the as-grown films, and critical current densities, which are only limited by vortex dynamics. The 10 nm thick YBCO nanowires without the Au capping present hysteretic current voltage characteristics, characteri…
Measurement of the penetration depth as a function of temperature using millimeter wave transmission in the range 130-500GHz are reported for three YBCO thin films. The experiment provides the absolute value for the penetration depth at low temperature: the derivation from the transmission data and the experimental uncertainty are discussed. We find a zero temperature penetration depth of 199+/-20nm, 218+/-20nm and 218+/-20nm, for YBCO-50nm/LaAlO3 (pristine), YBCO-130nm/MgO and YBCO-50nm/LaAlO3 (irradiated) respectively. The penetration depth exhibits a different behavior for the three films. …
We have grown and characterized 30 nm thick YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) films, deposited by pulsed laser deposition on both MgO (110) and SrTiO$_3$ (001) substrates, which induce opposite strain to the superconducting layer. By carefully tuning the in-situ post-annealing oxygen pressure, we achieved, in a reproducible way, films at different oxygen doping, spanning from the slightly overdoped down to the strongly underdoped region of the phase diagram. The transport properties of the films, investigated through resistance versus temperature measurements, are in perfect qualitative agreement …
We report the growth of NdBa2Cu3O7-d films on (100) MgO substrate by Molecular Beam Epitaxy (MBE). Large area NdBa2Cu3O7-d films with homogeneous superconducting properties were grown by precise control of stoichiometry and the optimisation of growth parameters. The stoichiometric ratio of Nd:Ba:Cu close to 1:2:3 yields films with TC of 94 K and JC values above 3.5 MA/cm2 at 77 K on bare MgO substrate. The NdBa2Cu3O7-d films grown under optimised conditions had excellent in-plane texture and good metallicity. The most significant characteristic of our MBE grown NdBa2Cu3O7-d films is the very l…
By scanning with a $30\, \mathrm{keV}$ focused He ion beam (He-FIB) across YBa$_2$Cu$_3$O$_7$ (YBCO) thin film microbridges, we create Josephson barriers with critical current density $j_\mathrm{c}$ adjustable by irradiation dose $D$. The dependence $j_\mathrm{c} (D)$ yields an exponential decay. At $4.2\, \mathrm{K}$, a transition from flux-flow to Josephson behavior occurs when $j_\mathrm{c}$ decreases below $\approx 2\, \mathrm{MA/cm^2}$. The Josephson junctions exhibit current-voltage characteristics (IVCs) that are well described by the resistively and capacitively shunted junction model,…
We present a comparative study of the angular dependent critical current density in YBa2Cu3O7 films deposited on IBAD MgO and on single crystal MgO and SrTiO3 substrates. We identify three angular regimes where pinning is dominated by different types of correlated and uncorrelated defects. We show that those regimes are present in all cases, indicating that the pinning mechanisms are the same, but their extension and characteristics are sample dependent, reflecting the quantitative differences in texture and defect density. In particular, the more defective nature of the films on IBAD turns in…
We report the results a comprehensive study of charge density wave (CDW) correlations in untwinned YBCO6+x single crystals with 0.4<x<0.99 using Cu-L3 edge resonant x-ray scattering (RXS). Evidence of CDW formation is found for 0.45<x<0.93, but not for samples with x<0.44 that exhibit incommensurate spin-density-wave order, and in slightly overdoped samples with x=0.99. This suggests the presence of two proximate zero-temperature CDW critical points at doping pc1~0.08 and pc2~0.18. The CDW reflections are observed at incommensurate in-plane wave vectors (d_a, 0) and (0, d_b). Both decrease lin…
We have developed a near-field microwave microscope to locally apply microwave frequency currents and fields to superconductors, and dielectric substrates, and measure the locally generated 2nd and 3rd harmonic responses. We measure the local nonlinear response of a Tl_2Ba_2CaCu_2O_y film grown on an MgO substrate, and observe a large response due to the enhanced current density near the edge. We also study the local nonlinear response of a YBa_2Cu_3O_7-d thin film grown on a bi-crystal SrTiO_3 (STO) substrate, and spatially identify the grain boundary through higher harmonic measurements. The…
We present a neutron triple-axis and resonant spin-echo spectroscopy study of the spin correlations in untwinned YBCO crystals with x= 0.3, 0.35, and 0.45 as a function of temperature and magnetic field. As the temperature T approaches 0, all samples exhibit static incommensurate magnetic order with propagation vector along the a-direction in the CuO2 planes. The incommensurability delta increases monotonically with hole concentration, as it does in LSCO. However, delta is generally smaller than in LSCO at the same doping level. The intensity of the incommensurate Bragg reflections increases w…
We describe and analyze selected surface impedance data recently obtained by different groups on cuprate, ruthenate and diboride superconducting films on metallic and dielectric substrates for fundamental studies and microwave applications. The discussion includes a first review of microwave data on MgB2, the weak-link behaviour of RABiTS-type YBa2Cu3O7-d tapes, and the observation of a strong anomalous power-dependence of the microwave losses in MgO at low temperatures. We demonstrate how microwave measurements can be used to investigate electronic, magnetic, and dielectric dissipation and re…
We report on transport properties of grain boundaries fabricated in YBa2Cu3O7-x thin films grown by the liquid phase epitaxy (LPE) technique on MgO asymmetrical bicrystal substrate with 45o misorientation angle. In total around 10 samples have been studied. Substantial scatter of zero field values of the critical current density at 5K has been observed. The upper limit of Jc of the order of 104 A/cm2 found in our study is close to previously reported data for 45o bicrystals grown by various physical vapour deposition methods while the minimal value of Jc for the LPE grown bicrystals in strikin…
