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Register a new userNeutron scattering study and analytical description of the spin excitation spectrum of twin-free YBa(2)Cu(3)O(6.6)
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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 photoemission spectroscopy (ARPES), as demonstrated in Dahm et al., Nature Phys. 5, 217, (2009). Based on accurate modeling of the effect of the resolution function on the detected intensity, we determine important intrinsic features of the spin excitation spectrum, with a focus on the differences above and below Tc. In particular, at T = 70 K the spectrum exhibits a pronounced twofold in-plane anisotropy at low energies, which evolves towards fourfold rotational symmetry at high energies, and the relation dispersion is Y-shaped. At T = 5 K, on the other hand, the spectrum develops a continuous, downward-dispersing resonant mode with weaker in-plane anisotropy. We understand this topology change as arising from the competition between superconductivity and the same electronic liquid-crystal state as observed in YBa(2)Cu(3)O(6.45). We discuss our data in the context of different theoretical scenarios suggested to explain this state.
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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 $Delta T = 0.6$ 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 ($alpha_{xx} approx -alpha_{yy}$) between the elements of the Raman tensor of the apex oxygen $A_g$ mode.
Using neutron scattering, we investigate the effect of a magnetic field on the static and dynamic spin response in heavily underdoped superconducting YBa$_{2}$Cu$_{3}$O$_{6+x}$ (YBCO$_{6+x}$) with x=0.33 (T$_{c}$=8 K) and 0.35 (T$_{c}$=18 K). In contrast to the heavily doped and superconducting monolayer cuprates, the elastic central peak characterizing static spin correlations does not respond observably to a magnetic field which suppresses superconductivity. Instead, we find a magnetic field induced resonant enhancement of the spin fluctuations. The energy scale of the enhanced fluctuations matches the Zeeman energy within both the normal and vortex phases while the momentum dependence is the same as the zero field bilayer response. The magnitude of the enhancement is very similar in both phases with a fractional intensity change of $(I/I_{0}-1) sim 0.1$. We suggest that the enhancement is not directly correlated with superconductivity but is the result of almost free spins located near hole rich regions.
Neutron Scattering measurements for YBa$_2$Cu$_3$O$_{6.6}$ have identified small magnetic moments that increase in strength as the temperature is reduced below $T^ast$ and further increase below $T_c$. An analysis of the data shows the moments are antiferromagnetic between the Cu-O planes with a correlation length of longer than 195 AA in the $a$-$b$ plane and about 35 AA along the c-axis. The origin of the moments is unknown, and their properties are discusssed both in terms of Cu spin magnetism and orbital bond currents.
We report the infrared (IR) response of Cu-O chains in the high-$T_{c}$ superconductor YBa$_{2}$Cu$_{3}$O$_{y}$ over the doping range spanning $% y=6.28-6.75$. We find evidence for a power law scaling at mid-IR frequencies consistent with predictions for Tomonaga-Luttinger liquid, thus supporting the notion of one-dimensional transport in the chains. We analyze the role of coupling to the CuO$_{2}$ planes in establishing metallicity and superconductivity in disordered chain fragments.
We report on the effects of hydrostatic pressure (HP) on the charge density wave observed in underdoped cuprates. We studied YBa$_2$Cu$_3$O$_{6.6}$ ($T_c$=61 K) using high-resolution inelastic x-ray scattering (IXS), and reveal an extreme sensitivity of the phonon anomalies related to the charge density wave (CDW) order to HP. The amplitudes of the normal state broadening and superconductivity induced phonon softening at Q$_{CDW}$ rapidly decrease as HP is applied, resulting in the complete suppression of signatures of the CDW below $sim$1 GPa. Additional IXS measurements on YBa$_2$Cu$_3$O$_{6.75}$ demonstrate that this very rapid effect cannot be explained by pressure-induced modification of the doping level and highlight the different role of external pressure and doping in tuning the phase diagram of the cuprates. Our results provide new insights into the mechanisms underlying the CDW formation and its interplay with superconductivity.
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