Magnetic Pseudogap Research Articles

Crucial role of thermal fluctuations and vertex corrections for the magnetic pseudogap

Crucial role of thermal fluctuations and vertex corrections for the magnetic pseudogap

Pseudogap term in the magnetic response of cuprate superconductors

This paper presents a joint analysis of inelastic neutron scattering (INS) and nuclear magnetic resonance data on superconducting cuprates, with the aim of a detailed characterization of the dynamical susceptibility ${\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}(\stackrel{P\vec}{q},\ensuremath{\omega})$ at low frequencies and at temperatures ranging from ${T}_{c}$ up to room temperature. Using the well-known relation between $1/{T}_{1}$ and ${\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}(\stackrel{P\vec}{q},\ensuremath{\omega})$, the analysis shows that the temperature dependence of the nuclear relaxation rate $1/{T}_{1}$ is controlled by a combination of dynamic spin-spin correlations between pairs of fluctuating moments in the CuO${}_{2}$ plane and a time constant proportional to the integral of ${\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}(\stackrel{P\vec}{q},\ensuremath{\omega})$ over the Brillouin zone. INS data on ${\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}(\stackrel{P\vec}{q},\ensuremath{\omega})$ for ${\text{La}}_{1.86}{\text{Sr}}_{0.14}{\text{CuO}}_{4}$ (LSCO) and ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{6.5}$ (YBCO6.5) are seen to obey $\ensuremath{\omega}/T$ scaling above a transition temperature, then fall to very low values at low temperatures. Thus, there is no evidence in such data for magnetic pseudogap effects, which are known to be quite pronounced in YBCO6.5, but somewhat muted in LSCO. Our analysis of ${T}_{1}$ data shows, however, that above the transition temperature noted above there occurs the onset of another term in ${\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}(\stackrel{P\vec}{q},\ensuremath{\omega})$, which comes to dominate $1/{T}_{1}$ at room temperature and above. We call this term ``the pseudogap term'' ${\ensuremath{\chi}}_{P}^{\ensuremath{'}\ensuremath{'}}(\stackrel{P\vec}{q},\ensuremath{\omega})$. The onset of ${\ensuremath{\chi}}_{P}^{\ensuremath{'}\ensuremath{'}}(\stackrel{P\vec}{q},\ensuremath{\omega})$ coincides with the entry into a quantum-critical regime dominated by stripes, but could also be derived from low-energy fluctuations resulting from nearby phase transitions characterized by other types of order, such as ring currents. For YBCO6.5 this onset is at ${T}_{c}\ensuremath{\sim}62\phantom{\rule{0.28em}{0ex}}\text{K}$, but for LSCO, it occurs 20--30 K higher than ${T}_{c}$. We model the $q$-space behavior of ${\ensuremath{\chi}}_{P}^{\ensuremath{'}\ensuremath{'}}(\stackrel{P\vec}{q},\ensuremath{\omega})$ and discuss its prospects for observation via INS. The occurrence of the foregoing effects is suggested to be widespread among the superconducting cuprates.

Pseudogap and its critical point in the heavily dopedBa(Fe1−xCox)2As2fromc-axis resistivity measurements

Temperature-dependent interplane resistivity, ${\ensuremath{\rho}}_{c}(T)$, was used to characterize the normal state of the iron-arsenide superconductor $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{As}}_{2}$ over a broad doping range $0\ensuremath{\le}x<0.50$. The data were compared with in-plane resistivity, ${\ensuremath{\rho}}_{a}(T)$, and magnetic susceptibility, $\ensuremath{\chi}(T)$, taken in $H\ensuremath{\perp}c$, as well as Co NMR Knight shift, $^{59}K$, and spin-relaxation rate, $1/{T}_{1}T$. The interplane resistivity data show a clear correlation with the NMR Knight shift, assigned to the formation of the pseudogap. Evolution of ${\ensuremath{\rho}}_{c}(T)$ with doping reveals two characteristic energy scales. The temperature of the crossover from nonmetallic, increasing on cooling, behavior of ${\ensuremath{\rho}}_{c}(T)$ at high temperatures to metallic behavior at low temperatures, ${T}^{\ensuremath{\ast}}$, correlates well with an anomaly in all three magnetic measurements. This characteristic temperature, equal to approximately 200 K in the parent compound, $x=0$, decreases with doping and vanishes near ${x}^{\ensuremath{\ast}}\ensuremath{\approx}0.25$. For doping levels $x\ensuremath{\ge}0.166$, an additional feature appears above ${T}^{\ensuremath{\ast}}$ with metallic behavior of ${\ensuremath{\rho}}_{c}(T)$ found above the low-temperature resistivity increase. The characteristic temperature of this charge-gap formation, ${T}_{\text{CG}}$, vanishes at ${x}_{\text{CG}}\ensuremath{\simeq}0.30$, paving the way to metallic, $T$ linear, ${\ensuremath{\rho}}_{c}(T)$ close to ${x}_{\text{CG}}$ and superlinear $T$ dependence for $x>{x}_{\text{CG}}$. None of these features are evident in the in-plane resistivity ${\ensuremath{\rho}}_{a}(T)$. For doping levels $x<{x}_{\text{CG}}$, $\ensuremath{\chi}(T)$ shows a known, anomalous, $T$-linear dependence, which disappears for $x>{x}_{\text{CG}}$. These features are consistent with the existence of a charge gap, accompanying formation of the magnetic pseudogap, and its critical suppression with doping. The inferred $c$-axis charge gap reflects the three-dimensional character of the electronic structure and of the magnetism in the iron arsenides.

Spin gaps in pseudo-one-dimensional RMn 4Al 8 compounds (R=Y, Ce and La)

LaMn 4Al 8 displays large amplitude spin fluctuations arising from linear chains of Mn atoms. Inelastic neutron scattering data for LaMn 4Al 8 shows that the dynamical magnetic response has two components: a quasielastic component and a broad inelastic peak centred near 30 meV.YMn 4Al 8 and CeMn 4Al 8 display maxima in their susceptibilities near 500 and 300 K, respectively. This has been taken as evidence for the presence of a magnetic pseudogap in YMn 4Al 8, possibly arising from one-dimensional AF correlations. We report inelastic neutron scattering data for the weak magnetic response of both YMn 4Al 8 and CeMn 4Al 8. In YMn 4Al 8 a clear spin gap of order 27 meV is evident for a limited range of wavevectors. In CeMn 4Al 8 the gap formation is less obvious, being partly masked by the intermediate-valence response of the cerium.

Pseudogap and central peak in the Emery model

The effect of antiferromagnetic (AF) correlations is studied in the framework of the three-band (Emery) model, with respect to experiments in BSCCO. We study the pseudogap regime with a central peak. Detailed dispersions of quasiparticle peaks show that one can simultaneously fit Fermi surfaces and ARPES leading-edge energy scales. The band parameter regime is a strong-coupling one: marked renormalization of the copper–oxygen overlap, making it smaller than the oxygen–oxygen overlap, while the copper–oxygen energy splitting is the largest of the three. The same regime was found previously in a zeroth-order fit of Fermi surfaces. The inclusion of AF correlations in a weak-coupling approach resolves the only qualitative discrepancy of the zeroth-order mean-field slave-boson calculation with experiment: it is argued that the observed large flat region of the dispersion around the vH point is due to the very non-dispersive central peak in the X– M direction. The sudden increase of the experimental one-particle dispersion in the X– M direction is explained by the quasiparticle strength shifting to the upper wing of the magnetic pseudogap, as one moves further away from the X (van Hove) point. Near it, the lower wing is predicted to be observed in the X– M direction, in addition to the narrow central peak, giving rise to a two-peaked structure below the Fermi level, as found experimentally.

Migdal’s theorem and the pseudogap

We study a model of quasiparticles on a two-dimensional square lattice coupled to Gaussian distributed dynamical molecular fields. The model describes quasiparticles coupled to spin or charge fluctuations, and is solved by a Monte Carlo sampling of the molecular field distributions. When the molecular field correlations are sufficiently weak, the corrections to the self-consistent Eliashberg theory do not bring about qualitative changes in the quasiparticle spectrum. But for a range of model parameters near the magnetic boundary, we find that Migdal's theorem does not apply and the quasiparticle spectrum is qualitatively different from its mean-field approximation, in that a pseudogap opens in the quasiparticle spectrum. An important feature of the magnetic pseudogap found in the present calculations is that it is strongly anisotropic. It vanishes anlong the diagonal of the Brillouin zone and is large near the zone boundary. In the case of ferromagnetic fluctuations, we also find a range of model parameters with qualitative changes in the quasiparticle spectral function not captured by the one-loop approximation, in that the quasiparticle peak splits into two. We provide intuitive arguments to explain the physical origin of the breakdown of Midgal's theorem

MAGNETISM, PHONONS AND ANISOTROPY OF HIGH TEMPERATURE CUPRATES SUPERCONDUCTORS

Phonon or electron mediated weak BCS attraction is enough to have high critical temperature if a van Hove anomaly is at work. This could apply to electron doped compounds and also to compounds with CuO 2 planes overdoped in holes, where T c decreases with increasing doping. If phonons dominate, it should lead to an anisotropic but mainly s superconductive gap, as observed recently in overdoped LaSrCuO, and probably also in electron doped compounds. If electrons dominate, a d-gap should develop as observed in a number of cases. In the underdoped range, the observed decrease of T c with hole doping can be related in all cases to the development of antiferromagnetic fluctuations which produces a magnetic pseudogap, thus lowering the density of states at the Fermi level. The observed mainly d-superconductive gap then can be due to a prevalent superconductive coupling through antiferromagnetic fluctuations; it could also possibly be attributed to the same phonon coupling as in the overdoped range, now acting on Bloch functions scattered in the magnetic pseudogap. More systematic studies of superconductive gap anisotropy and of magnetic fluctuations would be in order.

High energy secondary peak structure in tunneling spectra (hump) as possible magnetic pseudogap

It is demonstrated that tunneling spectra in various high T c cuprates display generic features. The principal conductance peaks in superconductor-insulator-normal metal (SIN) junctions indicate the superconducting gap in the density of states (DOS). Higher energy features include a dip and hump structure with a strength that is asymmetric in bias voltage. The dip and hump features follow the doping trends of the superconducting gap, Δ, with a rough scaling as ∼2Δ and ∼3Δ respectively. Tunneling spectra in underdoped Bi 2Sr 2CaCu 2O 8+δ display a more pronounced hump feature suggestive of a second gap in the DOS. It is observed that the hump feature in the tunneling density of states is consistent with other experimental observations of the so-called high energy pseudogap which may have magnetic origins.

Normal-state magnetic susceptibility in a bilayer cuprate

The magnetic susceptibility of high-${T}_{c}$ superconductors is investigated in the normal state using a coupled bilayer model. While this model describes in a natural way the normal-state pseudogaps seen in $c$-axis optical conductivity on underdoped samples, it predicts a weakly increasing susceptibility with decreasing temperature and cannot explain the magnetic pseudogaps exhibited in NMR measurements. Our result, together with some experimental evidence, suggests that the mechanism governing the $c$-axis optical pseudogap is different from that for the $a$-$b$ plane magnetic pseudogap.

Spin dynamics in YBa2(Cu0.98Zn0.02)3O6.7

Inelastic neutron scattering has been used to study the spin dynamics in underdoped zinc-substituted YBa2(Cu0.98Zn0.02)3O6.7 (T C =17 K). Zinc substitution in CuO2 planes may induce a localization of carriers at low temperature as a strong enhancement of the low energy magnetic excitations is observed below 80 K removing the “spin-gap”. In contrast, a magnetic pseudo gap is still sizeable above 100 K.

Van Hove excitons and high- Tc superconductivity VIIIA : Valence bond density waves

It has recently been shown that an important role of correlation effects is to renormalize the band structure to be more ‘Hubbard-like’ at half-filling, driving a Mott transition. Using this insight, the density wave (DW)-superconductivity competition in the high- T c oxides is reexamined. It is found that both the low-temperature orthorhombic (LTO) and the low-temperature tetragonal (LTT) transitions in La 2− x Ba x CuO 4 (LBCO) and related compounds are driven by the strong nesting of the van Hove singularity - the LTT as a static and the LTO a dynamic Jahn-Teller effect. A mean-field theory of the LTO phase is presented, in terms of a coherent superposition of charge DWs or a ‘valence bond density wave’. The resulting DW-superconductivity phase diagram strongly resembles that found in LBCO. Connections of the present model to the magnetic pseudogap and to one-dimensionality are discussed.