Mechanisms of the Magnetic Incommensurability in p-Type Cuprate Perovskites

We use the t–J model and Mori projection operator formalism for calculating the magnetic susceptibility of p-type cuprates in the superconducting and pseudogap phases. The lack of extended tails in the peaks of the hole spectral function was shown to provide an incommensurate low-frequency response with hole dispersions derived from photoemission. The theory reproduces the hourglass dispersion of the susceptibility maxima with the upper branch reflecting the dispersion of localized spin excitations and the lower branch being due to incommensurate maxima of their damping. The intensive resonance peak appears when the hourglass waist falls below the bottom of the electron-hole continuum. In the pseudogap phase, the Fermi arcs lead to a quasielastic incommensurate response for low temperatures. This result explains the lack of the superconducting gap in the susceptibility of phase-separated underdoped lanthanum cuprates. It may also explain the strengthening of the quasielastic response by magnetic fields and impurities. The theory accounts for the magnetic stripe reorientation from the axial to diagonal direction at low hole concentrations.     

Bose–Einstein Condensation in the Pseudogap Phase of Cuprate Superconductors

We have identified the unscreened Fröhlich electron–phonon interaction (EPI) as the most essential for pairing in cuprate superconductors as now confirmed by isotope substitution, recent angle-resolved photoemission (ARPES), and some other experiments. Low-energy physics is that of mobile lattice polarons and bipolarons in the strong EPI regime. Many experimental observations have been predicted or explained in the framework of our “Coulomb–Fröhlich” model, which fully takes into account the long-range Coulomb repulsion and the Fröhlich EPI. They include pseudo-gaps, unusual isotope effects and upper critical fields, the normal state Nernst effect, diamagnetism, the Hall–Lorenz numbers, and a giant proximity effect (GPE). These experiments along with the parameter-free estimates of the Fermi energy and the critical temperature support a genuine Bose–Einstein condensation of real-space lattice bipolarons in the pseudogap phase of cuprates. On the contrary, the phase fluctuation (or vortex) scenario is incompatible with the insulating-like in-plane resistivity and the magnetic-field dependence of orbital magnetization in the resistive state of underdoped cuprates.     

Pseudogap Phenomenon in Single Phase Hg-1223 Cuprate Superconductors

The single phase Hg-1223 cuprate superconductors have been successfully fabricated. The postannealing treatment under oxygen (O₂) gas and argon (Ar) gas with different annealing time was performed in order to obtain various hole density samples. These samples were used to study the pseudogap phenomenon in the normal phase. With the plot of ln[1/(T) – 1/_N(T)] against 1/T, three characteristic temperatures T*, T _S, and T _F were obtained, which describe the role of the pseudogap phenomenon. The magnitudes of the pseudogap were extracted from the slope of the stated plots. It is found that the three characteristic temperatures T*, T _S, and T _F change with the hole density linearly. The relationship between T* and T _C goes within T* = T*_op+₀ (1–T_C/T^max _C) , where T*_op and T _C ^max are the pseudogap opening temperature and the critical temperature for the optimal sample. It is also found that the pseudogap phenomenon only exists below the optimal regime in the hole doping phase diagram.     

Charge Order and Peak-dip-hump Structure in Pseudogap Phase of Cuprate Superconductors

Within the framework of the t-J model, the nature of the peak-dip-hump structure in the electron spectrum of cuprate superconductors in the normal-state is studied by taking into account the pseudogap effect. It is shown that in analogy to the charge-order state, the peak-dip-hump structure in the electron spectrum is also generated by the momentum dependence of the pseudogap, and therefore, there is a common origin for the emergence of the charge-order state and peak-dip-hump structure. In particular, the remarkable peak-dip-hump structure in the electron spectrum appears in the entire region of the electron Fermi surface except for the hot spots.     

Fermi Arcs and Pseudogap in Cuprate Superconductors

Earlier, we have proposed [arXiv:1110.0227] the model of HTSC electronic structure modification under doping. In this model, doping by localized charges plays the key role, being responsible for local closing of the gap ??_ct between excitonic-like 3d¹⁰L^? state of cation and electronic 3d⁹L state of anion and formation (at a certain dopant concentration) of the percolation cluster with the Fermi surface located in the cation-anion band of peculiar nature. This electronic structure is favorable for the formation of diatomic negative-U centers (NUCs) and realization of an unusual mechanism of electron?Celectron interaction. Here, the nature of normal state of cuprates as well as the mechanism of pseudogap and Fermi arcs formation are considered in the framework of this model by example YBCO.     

c-Axis Transport in a Normal-State Bilayer Cuprate and Relation to Pseudogap

We study the effect of interband transitions on the normal-state optical conductivity, dc resistivity, and thermal conductivity along the c-axis, for a plane-chain bilayer cuprate coupled by a perpendicular hopping matrix element (t⊥). When t⊥ is small, the c-axis dc resistivity shows a characteristic upturn as the temperature is lowered, and the c-axis optical conductivity develops a pseudogap at low frequencies. As t⊥ is increased, intraband transitions start to dominate and a more conventional response is obtained. Similar pseudogap behavior is predicted in the thermal conductivity for which strong depression at low temperature is found. Analytical results for a simple plane-plane bilayer are also given, including the frequency sum rule of the optical conductivity.     

c-Axis Transport in a Normal-State Bilayer Cuprate and Relation to Pseudogap

We study the effect of interband transitions on the normal-state optical conductivity, dc resistivity, and thermal conductivity along the c-axis, for a plane-chain bilayer cuprate coupled by a perpendicular hopping matrix element (t). When t is small, the c-axis dc resistivity shows a characteristic upturn as the temperature is lowered, and the c-axis optical conductivity develops a pseudogap at low frequencies. As t is increased, intraband transitions start to dominate and a more conventional response is obtained. Similar pseudogap behavior is predicted in the thermal conductivity for which strong depression at low temperature is found. Analytical results for a simple plane-plane bilayer are also given, including the frequency sum rule of the optical conductivity.     

The Effect of the Phase Separation in the Phase Diagram of Cuprate Superconductors

We show that the main features of the cuprates superconductors phase diagram can be derived considering the disorder as a key property of these materials. Our basic point is that the high pseudogap line is an onset of phase separation which generates compounds made up of regions with distinct doping levels. We calculate how this continuous temperature dependent phase separation process occurs in high critical temperature superconductors (HTSC) using the Cahn–Hilliard approach. Calculations with the BdG approach and an analysis of the local density of states (LDOS), yield good agreement with the measured values of T ^* and T _c . This work has been partially supported by CAPES, CNPq and CNPq-Faperj Pronex E-26/171.168/2003.     

Origin of Strange Metallic Phase in Cuprate Superconductors

The origin of strange metallic phase is shown to exist due to these two conditions: (i) the electrons are strongly correlated such that there are neither band nor Mott–Hubbard gaps, and (ii) the electronic energy levels are crossed in such a way that there is an electronic energy gap between two energy levels associated with two different wave functions. The theory is also exploited to explain (i) the upward- and downward-shifts in the T-linear resistivity curves, and (ii) the spectral weight transfer observed in the soft X-ray absorption spectroscopic measurements of the La–Sr–Cu–O Mott insulator.     

Spin-vortices and Spin-vortex-induced Loop Currents in the Pseudogap Phase of Cuprates

We explain several anomalous phenomena observed in the pseudogap phase of hole-doped cuprates based on the recently proposed spin-vortex superconductivity theory. In this theory, doped-holes become almost immobile small polarons, and spin-vortices are formed with those small polarons as their centers. A Hartree?CFock field for conduction electrons that is optimized for the interaction energy of local moments is derived; it contains a fictitious magnetic field arising from spin-vortices, and yields current carrying states. The obtained currents are loop currents around spin-vortices, i.e., the spin-vortex-induced loop currents (SVILCs), and a collection of them produces a macroscopic current. The SVILC explains (1) nonzero Kerr rotation in zero-magnetic field after exposed in a strong magnetic field; (2) the change of the sign of the Hall coefficient with temperature change; (3) the suppression of superconductivity in the x=1/8 static-stripe ordered sample; and (4) a large anomalous Nernst signal, including its sign-change with temperature change. We show that the hourglass-shaped magnetic excitation spectrum is the evidence for the existence of spin-vortices. We further argue that the ??Fermi-arc?? in the ARPES is a support for the presence of localized moments in the bulk; a disconnected arc-shaped Fermi surface is obtained by assuming an antiferromagnetic interaction between the localized moments in the bulk and itinerant electrons in the surface region.     

Charge Variations in Cuprate Superconductors from Nuclear Magnetic Resonance

Charge inhomogeneities in the cuprates were reported early on and have been in the focus of much research recently. Nuclear magnetic resonance (NMR) is very sensitive to local charge symmetry through the electric quadrupole interaction that must detect any static charge density variation. Recent experiments in high magnetic fields that seem to induce charge density waves in some systems have rekindled the interest in static inhomogeneities. It has long been known that excessive NMR linewidths can be observed in all cuprates, but with the exception of a few materials. However, the relation of the quadrupolar linewidths with respect to variations of the charge density in the cuprates is not understood. Here, we investigate YBa₂Cu₃O₇ and we find even in a moderate magnetic field that below about 200 K, i.e., well above T _c, a temperature dependent NMR linewidth appears that must be related to incipient static charge density variations. We argue that this establishes field induced charge density variation as a more general phenomenon in the cuprates. In view of the very recent understanding of the relation between the hole distribution in the CuO₂ plane and T _c, it is argued that charge density variations are ubiquitous, but appear not related to the maximum T _c.     

A Hidden Order in the Cuprate Superconductors: The d-Density-Wave Phase

We study the electronic structure near impurities in the d-density-wave (DDW) state, a possible candidate phase for the pseudo-gap region of the high-temperature superconductors. We show that the density of states near a nonmagnetic impurity in the DDW state is qualitatively different from that in a superconductor with d_x ²-y² symmetry. Thus the electronic structure near impuritiescan provide insight into the nature of the two phases recently observed by scanning tunneling microscopy experiments in the superconducting state of underdoped Bi-2212 compounds.     

The ‘Neutron’ Magnetic Resonance in the Underdoped Cuprate Superconductor

We show analytically that the magnetic neutron resonance of nonstoichiometrically doped cuprate is a sharp plasmon excitation of the dominant d-wave band at its two-channel Kondo fixed point. Despite existing throughout the ‘normal’ state, it becomes detectable when Kondo scattering is reduced strongly below the channel transition temperature. Unlike the one-off approach of the current theory of the neutron resonance as the spin exciton, all data relevant to the resonance as well as other phenomena for the crystal are amenable to explanation in the multichannel Kondo approach.     

Effect of the External Magnetic Field on the Resonance Scattering in Cuprate Superconductors

Within the kinetic energy driven superconducting mechanism, the effect of a uniform external magnetic field on the resonance scattering in cuprate superconductors in the superconducting state is studied. It is shown that the commensurate resonance scattering at zero external magnetic field is induced into the incommensurate resonance scattering by applying a large enough external magnetic field. The part of the spin excitation dispersion seems to be an hourglass-like dispersion, which breaks down at the heavily low energy regime.     

Polaron Coherence as Origin of the Pseudogap Phase in High Temperature Superconducting Cuprates

Within a two-component approach to high T _c copper oxides including polaronic couplings, we identify the pseudogap phase as the onset of polaron ordering. This ordering persists in the superconducting phase. A huge isotope effect on the pseudogap onset temperature T ^* is predicted and in agreement with experimental data. The anomalous temperature dependence of the mean square copper–oxygen ion displacement observed above, at and below T _c , stems from an s-wave superconducting component of the order parameter, whereas a pure d-wave order parameter alone can be excluded.     

Magnetic Field Dependence of Penetration Depth in Kinetic Energy Driven Cuprate Superconductors

Within the kinetic energy driven superconductivity, the magnetic field dependent penetration depth in cuprate superconductors is studied in the linear response approach. The electromagnetic response kernel is evaluated by considering both couplings of the electron charge and electron magnetic momentum with a weak magnetic field and employed to calculate the penetration depth based on the specular reflection model, then the main features of the magnetic field dependent penetration depth are well reproduced.     

Pseudogap and (An)isotropic Scattering in the Fluctuating Charge-Density Wave Phase of Cuprates

We present a general scenario for high-temperature superconducting cuprates, based on the presence of dynamical charge density waves (CDWs) and to the occurrence of a CDW quantum critical point, which occurs, e.g., at doping p ≈ 0.16 in YBa₂Cu₃O_{6 + δ} (YBCO). In this framework, the pseudogap temperature T ^? is interpreted in terms of a reduction of the density of states due to incipient CDW and, at lower temperature to the possible formation of incoherent superconducting pairs. The dynamically fluctuating character of CDW accounts for the different temperatures at which the CDW onset revealed by X-ray scattering (T _{o n s} (p)), and the static three-dimensional CDW ordering appear. We also investigate the anisotropic character of the CDW-mediated scattering. We find that this is strongly anisotropic only close to the CDW quantum critical point (QCP) at low temperature and very low energy. It rapidly becomes nearly isotropic and marginal-Fermi-liquid-like away from the CDW QCP and at finite (even rather small) energies. This may reconcile the interpretation of Hall measurements in terms of anisotropic CDW scattering (arXiv:1604.07852v1) with recent photoemission experiments Bok, J.M., et al. Sci. Adv. 2, e1501329 (2016).     

Methods of Measuring the Low-Frequency Electric and Magnetic Fields of Ships

Methods of recording low-frequency electric and magnetic fields in an aqueous medium, generated by ships and other moving objects, are considered. The history of the development of instruments for measuring such fields is described, their characteristics are presented, and the main areas of development of future instruments for this type of measurement are determined. __________ Translated from Izmeritel'naya Tekhnika, No. 11, pp. 68–71, November, 2005.     

Linear Response Theory Around a Localized Impurity in the Pseudogap Regime of an Anisotropic Superconductor

We compare and contrast the polarizability of a d-wave superconductor in the pseudogap regime, within the precursor pairing scenario (dPG), and of a d-density-wave (dDW) state, characterized by a d-wave hidden order parameter, but no pairing. Our study is motivated by STM imaging experiments around an isolated impurity, which may in principle distinguish between precursor pairing and dDW order in the pseudogap regime of the high- superconductors. In both cases, the -dependence of the polarizability is characterized by an azimuthal modulation, consistent with the d-wave symmetry of the underlying state. However, only the dDW result shows the fingerprints of nesting, with nesting wave vector , albeit imperfect, due to a nonzero value of the hopping ratio in the band dispersion relation. As a consequence of nesting, the presence of hole pockets is also reflected by the dependence of the retarded polarizability.     

Magnetic Moments due to Orbital Currents in an Electron-Lattice Model of Cuprate Superconductors

It has been shown in an earlier paper that loop currents and magnetic moments can be generated in an electron-lattice model of superconducting cuprates if a predominant mode of vibration of the oxygen clusters is an unusual nonlinear anti-ferro pattern of 2-D vibrations called a Q ₂ mode. This makes the electron-lattice mechanism a possible candidate to explain the experimental evidence of ubiquitous strong electron-lattice interaction involving oxygen clusters and also a possible candidate to explain experimental evidence of unusual magnetic moments in the CuO ₂ planes, if confirmed. In this paper, we report a detailed numerical study of exact 2-D modes involving the Jacobian elliptic functions. The magnetic fields generated, energy of modes, frequency, and elliptic modulus are studied as the parameters of an anharmonic molecular crystal model are varied. With a suitable choice of parameters, the orders of magnitude of magnetic field and energy are consistent with the plausible estimates made in an earlier paper. The broken π/2 rotational symmetry in each CuO ₂ unit cell due to the generation of small magnetic fields makes our model a candidate to explain intra-unit-cell electronic nematicity measured recently.