Alexandre I. Rykov

3D-XY CRITICAL FLUCTUATIONS OF THE THERMAL EXPANSIVITY IN DETWINNED YBA2CU3O7-DELTA SINGLE CRYSTALS NEAR OPTIMAL DOPING

The strong coupling of superconductivity to the orthorhombic distortion in YBa2Cu3O7-d makes possible an analysis of the superconducting fluctuations without the necessity of subtracting any background. The present high-resolution capacitance dilatometry data unambiguously demonstrate the existence of critical, instead of Gaussian, fluctuations over a wide temperature region (+/- 10 K) around Tc. The values of the amplitude ratio A+/A-=0.9-1.1 and the leading scaling exponent |alpha|<0.018, determined via a least-squares fit of the data, are consistent with the 3D-XY universality class. Small deviations from pure 3D-XY behavior are discussed.

Dispersion of the odd magnetic resonant mode in near-optimally doped Bi2Sr2CaCu2O8+δ

We report a neutron scattering study of the spin excitation spectrum in the superconducting state of the slightly overdoped Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} system (T{sub c}=87 K). We focus on the dispersion of the resonance peak in the superconducting state that is due to a S=1 collective mode. The measured spin excitation spectrum bears a strong similarity to the spectrum of the YBa{sub 2}Cu{sub 3}O{sub 6+x} system for a similar doping level (i.e., x{approx}0.95-1), which can be described as intersecting upward- and downward-dispersing branches. A close comparison of the threshold of the electron-hole spin flip continuum, known from angle resolved photoemission measurements in the same system, indicates that the magnetic response in the superconducting state is confined, in both energy and momentum, below the gapped Stoner continuum. In contrast to YBa{sub 2}Cu{sub 3}O{sub 6+x}, the spin excitation spectrum is broader than the experimental resolution. In the framework of an itinerant-electron model, we quantitatively relate this intrinsic energy width to the superconducting gap distribution observed in scanning tunneling microscopy experiments. Our study further suggests a significant in-plane anisotropy of the magnetic response.

Anomalously large oxygen-ordering contribution to the thermal expansion of untwinned YBa2Cu3O6.95 single crystals: A glasslike transition near room temperature

We present high-resolution thermal expansion data from 5-500 K of untwinned YBa2Cu3Ox (Y123) single crystals for x approximately 6.95 and x = 7.0. Large contributions to the thermal expansivities due to O ordering are found for x approximately 6.95, which disappear below a kinetic glasslike transition near 280 K. 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 stress along the chain axis and that the lack of well-ordered chains in Nd123 and La123 is most likely a consequence of a chemical-pressure effect.

Charge transfer to the local singlet states as a function of Li content in La2Cu1−xLixO4 and La1.85Sr0.15Cu1−xLixO4

Abstract Magnetic susceptibility (χ) and NMR/NQR measurements were performed on La2Cu1−xLixO4 0≤x≤0.5) and La1.85Sr0.15Cu1−xLixO4 (0≤x≤0.425). The narrow 63Cu NQR line at νQ = 46.3 MHz (1.3 K) is detected in La2Cu0.5Li0.5O4. This frequency is higher than all 63Cu-NQR frequencies found so far in all the cuprates. Moreover, a further increase of νQ is observed in the solid solutions La2Cu1−xLixO4 with decreasing x. Corresponding electric field gradients (EFGs) are considered in terms of ionic and cluster models. The 3d hole density nx2−y2 ≈ 1 and the radial average 〈r−3〉 ≈ 9.5 a.u. are derived from the EFGs, suggesting the d9 L state of the formally trivalent copper. The antiferromagnetic ordering is rapidly suppressed as a function of x in the La2Cu1−xLixO4, and for x ≥ 0.03 we find TN ≈ 0. The evolution of χ vs. x shows a gradual decrease of Cu spin susceptibility and its disappearance in the diamagnetic La2Cu0.5Li0.5O4, in which χ = −6.7 × 10−5 cm3/mole is composed of Van Vleck and core terms. The electronic state is implied to be a local singlet (1A1g) of primarily d9 L character. The quadrupole signature of the local singlet in the range 46.3 ≤ 63 ν Q ≤ 47.0 MHz is observed for x > 0.15 in both systems, La2Cu1−xLixO4 and La1.85Sr0.15Cu1−xLixO4. The differentiation of charge between local singlet and conductivity band states is discussed.

Odd and even magnetic resonant modes in highly overdoped Bi2Sr2CaCu2O8+δ

We present inelastic neutron scattering data on highly overdoped Bi_2 Sr_2 Ca Cu_2 O_{8+\delta} single crystals with superconducting transition temperature T_C=70 K and, for comparison, a nearly optimally doped crystal with T_C=87 K. In both samples, magnetic resonant modes with odd and even symmetry under exchange of the two CuO_2 layers in the unit cell are observed. In the overdoped sample, the linewidth of the odd mode is reduced compared with the optimally doped sample. This finding is discussed in conjunction with recent evidence for intrinsic inhomogeneities in this compound. The data on odd and even resonant excitations are otherwise fully consistent with trends established on the basis of data on YBa_2Cu_3O_{6+x}. This confirms the universality of these findings and extends them into the highly overdoped regime of the phase diagram.

NMR and magnetic susceptibility in superconducting and antiferromagnetic Ga-based cuprates Y1−xCaxSr2Cu2GaO7 (0≤x≤0.3)

Abstract Magnetic susceptibility and NMR/NQR measurements were performed on Y1−xCaxSr2Cu2GaO7 (x=0, 0.1, 0.2, 0.3). The single phase samples annealed at 600°C under oxygen pressure of 30 MPa are superconductors with Tc=35 K for x=0.2 and x=0.3. In spite of the presence of a small Curie-like term, we show that the spin susceptibility in the normal state increases with Ca doping and reaches the value χspin≈0.9 cm3/Cu-mole, which is comparable to other superconducting cuprates. From the observation of Cu zero-field resonance (AFNR) and susceptibility data the parent compound is classified as 2D antiferromagnet (TN=387 K). The transition from antiferromagnetic insulator to superconductor occurs with increasing concentration of carriers, but extends over several tens percent of Ca. The superconductivity is significantly suppressed by increasing disorder within limits of solubility for Ca. The Ga NQR spectra are narrow in both antiferromagnetic and superconducting regimes, but heavily broadened in the intermediate spin-glass-like domain. From x=0 to x=0.3, the 63Cu quadrupole frequency increases from 24 to 28 MHz due to the charge transfer resulting in superconductivity. Other EFG parameters are not markedly changed from those given in YSr2Cu2GaO7 by Pieper [Physica C190(1992)261].

Magnetic-field-induced finite-size effect in the nigh-temperature superconductor YBa2Cu3O(7-delta): a comparison with rotating superfluid 4He.

The effect of strong magnetic fields (11 T) on superconductivity in YBa2Cu3O(7-delta) is investigated using high-resolution thermal expansion. We show that the field-induced broadening of the superconducting transition is due to a finite-size effect resulting from the field-induced vortex-vortex length scale. The physics of this broadening has recently been elucidated for the closely related case of rotating superfluid 4He [Phys. Rev. B 60, 12 373 (1999)]]. Our results imply that the primary effect of magnetic fields of the order of 10 T is to destroy the phase coherence; the pairing, on the other hand, appears to be quite insensitive to these fields.

Detwinning and Retwinning of YBa2Cu3O7-δ Single Crystals with Variable Oxygen Stoichiometry

The single-crystal susceptibility and magnetization hysteresis of YBa2Cu3O7 and YBaCu3O6.6 with Tc’s of 90K and 58K, respectively, were measured on the twin-free samples. The detwnning and retwinning processes are described for overdoped and underdoped regimes. The critical current in the untwinned overdoped YBa2Cu3O7 is dramatically lowered relative to both oxygen-deficient and twinned crystals.

Raman scattering in YBa2Cu3O7 single crystals: anisotropy in normal and superconductivity states

Precise temperature and polarization dependences of Raman spectra have been investigated for fully oxygenated twin-free YBa2Cu3O7 single crystals. We have found a striking superconductivity-induced x−y anisotropy in the temperature behavior of the 340 cm−1 line: the magnitudes of the softening and broadening are quite different in the xx-and yy-polarizations. This anisotropy suggests a contribution of the CuO-chain superconductivity with a pairing symmetry different from that for the CuO2 plane, or indicates that the superconducting gap amplitudes are different in the kx and ky directions. The d+s gap symmetry is the only realistic symmetry in the case of Δx≠Δy.

Raman study of untwinned YBa2Cu3O7 single crystals: new evidence for the x - y anisotropy of the superconducting gap

Abstract We present a new picture of Raman scattering in YBa 2 Cu 3 O 7 single crystals which shows both the out-of-plane and the in-plane anisotropies of the electron-phonon interactions. The x-y anisotropy in the softening and the broadening for the 340 cm −1 mode strongly suggests the anisotropy of the superconducting gap amplitude Δ x ≠ Δ y , which leads to the d+s symmetry of the gap wave function.