Underdoped Research Articles

The in- and out-of-plane magnetisation of highly underdoped YBa2Cu3O6+x single crystals

Previously we have shown how measurements of static magnetic susceptibility χc(T) of YBa2Cu3O6+x single crystals for magnetic fields applied along the c-axis, and χab(T) for fields in the ab-plane, can give useful information about their thermodynamic properties which are still being hotly debated. SQUID magnetometry above the superconducting (s/c) transition temperature Tc is used for larger crystals, while piezolever torque magnetometry gives χc(T)-χab(T) for tiny crystals. Here we present new data for more heavily under-doped crystals with hole concentrations per CuO2 plane, p =0.058 to 0.073. We again find that the T-dependent anisotropy well above Tc arises from the pseudogap and the g-factor anisotropy, while at lower T there are Gaussian s/c fluctuations with a strong cut-off. This is possibly a different region where neutron scattering studies give evidence for competition between incommensurate magnetic short-range order and superconductivity. We have studied crystals with three values of x, measuring χc(T) and χab(T) immediately after fixing x by quenching on to a copper block and again after allowing sufficient time at room temperature for the Cu-O chains to order. Thus, we report data for three pairs of underdoped (UD) crystals with Tc ranging from 13 to 36 K, namely UD13 and UD20, UD15 and UD30, UD23 and UD36. As found previously for polycrystalline samples, ordering the Cu-O chains increases Tc and substantially reduces the Curie term. These are isotropic, in contrast to the Pauli-like susceptibility from the CuO2 planes where there is g factor anisotropy, and this allows us to make a powerful novel analysis. At higher T, χc(T)- χab(T) varies as a + bT where the bT term arises from the pseudogap. At lower T, deviations from this behaviour are analysed in terms of 2D or 3D Gaussian s/c fluctuations to obtain in-plane coherence lengths and upper critical fields as T→0.

Breakdown of the Fermi arcs in underdoped cuprates by incommensurate charge density waves

We studied interactions between the coherent excitations on disconnected arcs along a bare Fermi surface (the so-called Fermi arcs (FAs)) seen by angle-resolved photo emission spectroscopy (ARPES) in several underdoped (UD) cuprates and incommensurate charge density wave (IC CDW) ordering at lowering of the temperature.

Kinetics of excitations on the Fermi arcs in underdoped cuprates at low temperatures

The Fermi-liquid-like (FL) resistivity recently observed in clean Hg1201 below the pseudogap temperature was related to carriers at the nodal points on the Fermi surface (FS) (N. Bari\ifmmode \check{s}\else \v{s}\fi{}i\ifmmode \acute{c}\else \'{c}\fi{} et al., arXiv:1207.1504, doi:10.1073/pnas.13019891109). We show that this has important implications for the electronic spectrum of underdoped (UD) cuprates as a whole. Photoemission experiments (angle-resolved photoemission spectroscopy) in other cuprates picture the spectrum as ``metallic arcs'' separated from each other by regions with large energy gaps. We rigorously solve the kinetic equation in such a model. The Fermi arcs' carriers contribute to the FL resistivity, if scattering between the opposite nodal points admits the umklapp processes. The Hall coefficient defines the effective number of carriers on the arcs and at weak magnetic fields it has a positive sign. All parameters that determine the arcs' widths are measurable experimentally. We conclude that the ${T}^{2}$ resistivity gives support to the Fermi arcs' concept and argue that the idea of a reconstructed FS in UD cuprates is not consistent with the latter.

High-energy magnetic excitations in underdoped La1.90Sr0.10CuO4

We measured the magnetic excitations in underdoped (UD) La1.90Sr0.10CuO4 by using inelastic neutron scattering over a wide energy range. We succeeded in observing the high-energy excitations up to ≃160 meV. Hour-glass-like magnetic excitation, which is commonly seen in the superconducting phase of cuprate oxides, was confirmed. The E cross (the energy where the dispersion is closest to the $$\left( {\tfrac{1} {2},\tfrac{1} {2}} \right)$$ reciprocal position) of ∼40 meV follows a linear relation between E cross and the doped hole concentration in the UD region. From the dispersion of high-energy excitations above E cross, we successfully found an effective nearest neighbor interaction (J eff) of 102 ± 2 meV. This J eff is smaller than the nearest neighbor interaction J(=132 meV) in the parent compound La2CuO4, indicating a reduction of J eff by hole-doping.

Impurity effects on the d-density wave and superconductivity phase of cuprates

The effect of non-magnetic impurity-scattering is considered on the asymmetric superconducting gap (SC) and the d-density wave (DDW) phase of the high Tc cuprates. The phase diagrams for the cuprates demonstrate that Tc decreases with scattering potential and is most affected in the coexisting phase of DDW and SC orders. Moreover, the SC gap to Tc ratio increases with doping and impurity scattering. The transition temperature of the DDW state, T∗ also decreases with scattering. The density of states in the mixed SC+DDW state for the underdoped (UD) phase, SC state in the overdoped phase and the DDW state in the UD phase, also suffers a modification. Its effect on specific heat results is also discussed. The increase in the gap to Tc ratio and the specific heat are also observed in the presence of the non-magnetic impurities. A suppression in the superfluid density is found with the increase in impurity.

Spin-orbit coupling in the superconducting phase and DDW states of high-Tc cuprates

The effects of the spin-orbit coupling are considered for the high Tc cuprates with asymmetric superconducting gap (SC) and the d-density wave (DDW) phase due to its vital role in the experimental determination of the DDW state. Experiments predict an anisotropy in the DSC gap where |Δ(0,π)|>|Δ(π,0)| and the gap node deviates from the diagonal direction towards the kx axis. Measurements also demonstrate DDW to be a possible candidate for the pseudogap in the underdoped phase. Due to the spin-orbit (SO) coupling in the low temperature orthorhombic (LTO) phase, the phase diagram of the cuprates suffers a change due to the modification of the T* value, the temperature characteristic of pseudogap, although Tc remains unaltered. Moreover, for a more generalized SO coupling, the DDW gap decreases with the angle but has no effect on the SC gap. We calculate the density of states in the various regimes of doping for the mixed SC+DDW states in the underdoped (UD) phase, SC state in the overdoped phase and also the DDW state in the UD phase and compare them with various theoretical and experimental works. The temperature dependence of the specific heat does not exhibit any qualitative change due to the SO coupling.

Effect of oxygen-doping onBi2Sr2Ca2Cu3O10+δvortex matter: Crossover from electromagnetic to Josephson interlayer coupling

We study the effect of oxygen-doping on the critical temperature ${T}_{c}$, the vortex matter phase diagram, and the nature of the coupling mechanism between the Cu-O layers in the three-layer ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{Ca}}_{2}{\text{Cu}}_{3}{\text{O}}_{10+\ensuremath{\delta}}$ (Bi-2223) compound. Contrary to previous reports, in the overdoped (OD) regime, we do find a variation in ${T}_{c}$ by increasing the oxygen partial pressure of the postannealing treatment. This variation is less significant than in the bilayer compound ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{10+\ensuremath{\delta}}$ (Bi-2212) and does not follow the universal ${T}_{c}$ vs $\ensuremath{\delta}$ relation. Magnetic measurements reveal that increasing $\ensuremath{\delta}$ enlarges the field and temperature stability of the Bragg glass phase. These findings imply that the interlayer coupling between Cu-O layers enhances with $\ensuremath{\delta}$. The anisotropy parameter estimated from directional first-penetration field measurements monotonously decreases from 50 in the underdoped (UD) to 15 in the OD regimes. However, the in-plane penetration depth presents a boomerang-type behavior with $\ensuremath{\delta}$, reaching its minimum value close to optimal doping. These two facts lead to a crossover from a Josephson OD to electromagnetic UD-dominated coupling of adjacent Cu-O layers in the vicinity of optimal doping.

Nernst effect in high-Tcsuperconductors

The observation of a large Nernst signal ${e}_{N}$ in an extended region above the critical temperature ${T}_{c}$ in hole-doped cuprates provides evidence that vortex excitations survive above ${T}_{c}$. The results support the scenario that superfluidity vanishes because long-range phase coherence is destroyed by thermally created vortices (in zero field) and that the pair condensate extends high into the pseudogap state in the underdoped (UD) regime. We present a series of measurements to high fields $H$ which provide strong evidence for this phase-disordering scenario. Measurements of ${e}_{N}$ in fields $H$ up to $45\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ reveal that the vortex Nernst signal has a characteristic ``tilted-hill'' profile, which is qualitatively distinct from that of quasiparticles. The hill profile, which is observed above and below ${T}_{c}$, underscores the continuity between the vortex-liquid state below ${T}_{c}$ and the Nernst region above ${T}_{c}$. The upper critical field (depairing field) ${H}_{c2}$ determined by the hill profile (in slightly UD to overdoped samples) displays an anomalously weak $T$ dependence, which is consistent with the phase-disordering scenario. We contrast the Nernst results and ${H}_{c2}$ behavior in hole-doped and electron-doped cuprates. Contour plots of ${e}_{N}(T,H)$ in the $T$-$H$ plane clearly bring out the continuous extension of the low-$T$ vortex liquid state into the high-$T$ Nernst region in hole-doped cuprates (but not in the electron-doped cuprate). The existence of an enhanced diamagnetic magnetization $M$ that survives to intense $H$ above ${T}_{c}$ is obtained from torque magnetometry. The observed $M$ scales accurately like ${e}_{N}$ above ${T}_{c}$, confirming that the large Nernst signal is associated with local diamagnetic supercurrents that persist above ${T}_{c}$. We emphasize implications of the new features in the phase diagram implied by the high-field results and discuss relevant theories.