Interactions In Cuprates Research Articles

Ingredients of strong interactions in cuprates

In this article, part of the Memorial Special Issue of Physica C dedicated to the esteemed Prof. K. Alex Müller, we delve into the fundamental components of robust interactions within the cuprates. One of us (ZXS) had the privilege of spending a sabbatical year at the University of Zürich, collaborating with the experimental group led by Jürg Osterwalder in 2002. This unique opportunity facilitated direct engagement with Prof. Müller, enabling the absorption of his invaluable insights. Prof. Müller's intuition and unorthodox perspectives have indelibly shaped our approach to the cuprate conundrum. Our focus centers on the contributions personally undertaken, reflecting the trajectory of our thought process.

Magnetotransport signatures of antiferromagnetism coexisting with charge order in the trilayer cuprate HgBa2Ca2Cu3O8+\u03b4

Multilayered cuprates possess not only the highest superconducting temperature transition but also offer a unique platform to study disorder-free CuO2 planes and the interplay between competing orders with superconductivity. Here, we study the underdoped trilayer cuprate HgBa2Ca2Cu3O8+δ and we report quantum oscillation and Hall effect measurements in magnetic field up to 88 T. A careful analysis of the complex spectra of quantum oscillations strongly supports the coexistence of an antiferromagnetic order in the inner plane and a charge order in the outer planes. The presence of an ordered antiferromagnetic metallic state that extends deep in the superconducting phase is a key ingredient that supports magnetically mediated pairing interaction in cuprates.

Unmasking the Origin of Kinks in the Photoemission Spectra of Cuprate Superconductors.

The origin of a ubiquitous bosonic coupling feature in the photoemission spectra of high-T_{c} cuprates, an energy-momentum dispersion "kink" observed at ∼70 meV binding energy, remains a two-decade-old mystery. Understanding this phenomenon requires an accurate description of the coupling between the electron and some collective modes. We report here abinitio calculations based on GW perturbation theory and show that correlation-enhanced electron-phonon interaction in cuprates gives rise to the strong kinks, which not only explains quantitatively the observations but provides new understanding of experiments. Our results reveal it is the electron density of states being the predominant factor in determining the doping dependence of the kink size, manifesting the multiband nature of the cuprates, as opposed to the prevalent belief of it being a measure of the mode-coupling strength.

On the important role of the anti-Jahn-Teller effect in underdoped cuprate superconductors

In this paper it is shown that the "anti-Jahn-Teller effect" plays an essential role in giving rise to a small Fermi surface of Fermi pockets above Tc and d-wave superconductivity below Tc in underdoped cuprates. In the first part of the present paper, we review the latest developments of the model proposed by Kamimura and Suwa, which bears important characteristics born from the interplay of Jahn-Teller Physics and Mott Physics. It is shown that the feature of Fermi surfaces in underdoped LSCO is the Fermi pockets in the nodal region constructed by doped holes under the coexistence of a metallic state and of the local antiferromagnetic order. In the antinodal region in the momentum space, there are no Fermi surfaces. Then it is discussed that the phonon-involved mechanism based on the Kamimura-Suwa model leads to the d-wave superconductivity. In particular, it is shown that the origin of strong electron-phonon interactions in cuprates is due to the anti-Jahn-Teller effect. In the second part a recent theoretical result on the energy distribution curves (EDCs) of angle-resolved photoemission spectroscopy (ARPES) below Tc is discussed. It is shown that the feature of ARPES profiles of underdoped cuprates consists of a coherent peak in the nodal region and the real transitions of photoexcited electrons from occupied states below the Fermi level to a free-electron state above the vacuum level in the antinodal region, where the latter transitions form a broad hump. From this feature, the origin of the two distinct gaps observed by ARPES is elucidated without introducing the concept of the pseudogap. Finally, a remark is made on the phase diagram of underdoped cuprates.

Electrons in cuprates: A consistent ARPES view

Angle resolved photoemission spectroscopy (ARPES) has been playing a crucial role in understanding of physics behind high-temperature superconductivity. Our ARPES investigation of superconducting cuprates, performed over a decade and accomplished by very recent results, suggests a consistent view of electronic interactions in cuprates which provides natural explanation of both the origin of the pseudogap state and the mechanism for high-temperature superconductivity. Within this scenario, the spin-fluctuations play a decisive role in formation of the fermionic excitation spectrum in the normal state and are sufficient to explain the high transition temperatures to the superconducting state while the pseudogap phenomenon is a consequence of a Peierls-type intrinsic instability of electronic system to formation of an incommensurate density wave. In view of these results and their projection to numerous other materials, two general questions are arising: is the normal state in 2D metals ever stable and how does this intrinsic instability interplay with superconductivity?

Effect of covalent bonding on magnetism and the missing neutron intensity in copper oxide compounds

The importance of covalent bonding for the magnetism of 3d metal complexes was first noted by Pauling in 1931. His point became moot, however, with the success of the ionic picture of Van Vleck, where ligands influence magnetic electrons of 3d ions mainly through electrostatic fields. Anderson's theory of spin superexchange later established that covalency is at the heart of cooperative magnetism in insulators, but its energy scale was believed to be small compared to other inter-ionic interactions and therefore it was considered a small perturbation of the ionic picture. This assertion fails dramatically in copper oxides, which came to prominence following the discovery of high critical temperature superconductors (HTSC). Magnetic interactions in cuprates are remarkably strong and are often considered the origin of the unusually high superconducting transition temperature, Tc. Here we report a detailed survey of magnetic excitations in the one-dimensional cuprate Sr2CuO3 using inelastic neutron scattering (INS). We show that although the experimental dynamical spin structure factor is well described by the model S=1/2 nearest-neighbour Heisenberg Hamiltonian typically used for cuprates, the magnetic intensity is modified dramatically by strong hybridization of Cu 3d states with O p states, showing that the ionic picture of localized 3d Heisenberg spin magnetism is grossly inadequate. Our findings provide a natural explanation for the puzzle of the missing INS magnetic intensity in cuprates and have profound implications for understanding current and future experimental data on these materials.

Spin-phonon interactions in cuprates

The parameter ζ of spin-phonon coupling is obtained through the analysis of the spin-wave dynamics of magnetic systems. It is shown that the parameter of spin-phonon coupling is greater, the greater the relative electron-ion potential g 1, the smaller the exchange correlation radius r c , and the smaller the mass M of ions that form a crystalline lattice.

Angle-resolved photoemission spectroscopy of band tails and anomalous kinetics of underdoped cuprates

The electron–phonon interaction in cuprates with c-axis polarised optical phonons, which is roughly one order of magnitude stronger than superexchange, bounds holes into mobile bipolarons. Bipolarons pin the chemical potential within the charge-transfer gap of doped Mott insulators, accounting for unusual kinetics and thermodynamics of doped cuprates such as the Nernst and giant proximity effects, pseudo-gaps, and normal-state diamagnetism. We propose that “quasi-particle” peaks, “Fermi-arcs”, and high-energy “waterfalls” in the photoemission spectra of cuprates originate from the photo-ionization matrix elements of disorder-localised band-tails in the charge-transfer gap.

A note on the possible mechanisms of high-temperature superconductivity

Possible mechanisms of high-temperature superconductivity are briefly discussed. In particular, a recent paper by P W Anderson on this problem is criticized as containing a number of incorrect and unfounded statements. One of these — that the static dielectric function ε(q,0) cannot be negative — is discussed in detail, as is its consequence, a strong limit on the transition temperature Tc. Proofs are given that ε(q,0) not only can but indeed must be negative in many stable systems, including most of the conventional metals. Various types of electron–electron interaction in superconducting cuprates are discussed. The role of the electron–phonon interaction in cuprates is highlighted.

Electron–phonon interaction in cuprates with T and [formula omitted]-structure in strongly correlated limit

Electron–phonon interaction in cuprate oxides is consistently determined from realistic multi band p–d model in strong correlations limit. We consider the momenta dependence matrix elements of the EPI for modes which most coupled to electrons and analyze a possible mechanism of kink formation. By unitary transformation we obtain an effective low-energy single-band Hamiltonian that includes only electron–electron interactions renormalized by the electron–phonon coupling and depends on occupation factors.

High Temperature Superconductivity Due to a Long-Range Electron–Phonon Interaction, Application to Isotope Effects, Thermomagnetic Transport and Nanoscale Heterogeneity in Cuprates

Strong electron-phonon interactions in cuprates and other high-temperature superconductors have gathered support over the last decade in a large number of experiments. Here I briefly introduce the Froehlich-Coulomb multi-polaron model of high-temperature superconductivity, which includes strong on-site repulsive correlations and the long-range Coulomb and electron-phonon (e-ph) interactions. Our extension of the BCS theory to the strong-coupling regime with the long-range \emph{unscreened} electron-phonon interaction could naturally explain the isotope effects, unconventional thermomagnetic transport, and checkerboard modulations of the tunnelling density of states in cuprates.

Circulating-current states and ring-exchange interactions in cuprates

We consider the consequences for circulating-current states of a cyclic, four-spin, ``ring-exchange'' interaction of the type shown recently to be significant in cuprate systems. The real-space Hartree-Fock approach is used to establish the existence of charge-current and spin-current phases in a generalized Hubbard model for the CuO_2 planes in cuprates. We compare the results of the Hartree-Fock approximation with the correlated states renormalized by Gutzwiller projection factors which allows us to gauge the qualitative effects of projection to no double site occupancy. We find that charge flux states may be competitive in cuprates, whereas spin flux states are suppressed in the strongly correlated regime. We then include the ring-exchange interaction and demonstrate its effect on current-carrying states both at and away from half-filling.

Circulating-current states and ring-exchange interactions in cuprates

We consider circulating charge- and spin-current states in a generalised Hubbard model for the CuO 2 planes in cuprates. We investigate the parameter regimes for these phases in microscopic calculations on small clusters. We demonstrate that a positive ring-exchange interaction enhances (suppresses) spin- (charge-)flux phases.

Lorenz number in high T(c) superconductors: evidence for bipolarons.

The strong electron-phonon interaction in cuprates has gathered support over the last decade in a number of experiments. While phonons remain almost unrenormalized, electrons are transformed into itinerant bipolarons and thermally excited polarons when the electron-phonon interaction is strong. We calculate the Lorenz number of the system to show that the Wiedemann-Franz law breaks down because of the interference of polaron and bipolaron contributions in the heat flow. The model fits numerically the experimental Hall Lorenz number, which provides direct evidence for bipolarons in the cuprates.

Three-Spin-Polarons and Their Elastic Interaction in Cuprates

Properties of quasiparticles in doped cuprates formed by an oxygen hole and two adjacent copper holes are investigated on the basis of the extended Hubbard model. The ground state energy, wave functions and the polaron-phonon coupling are calculated. We also analyzed the polaron-polaron interaction via the phonon field. It was found that this interaction is highly anisotropic and can explain the experimentally observed phase separation in the strongly underdoped LaSrCuO:Mn system.

Tunnelling gaps and d-wave stripes in cuprates: A unified approach

Abstract The Frohlich electron-phonon interaction in cuprates and other charge-transfer oxides is shown to be much stronger than any magnetic interaction. The polaron shift due to the Frohlich interaction of about 1 eV suggests that carriers are smali (bi)polarons at all temperatures and dopings, in agreement with the oxygen isotope effect on the carrier mass, optical conductivity and other experimental observations. Two distinct energy scales, the d-wave superconducting order parameter and charge segregation in the form of stripes in cuprates, are unified in the framework of the bipolaron theory as a result of the formation of mobile bipolarons in the normal state and their Bose-Einstein condensation. Within the theory both the d-wave superconducting order parameter and the striped charge distribution result from the bipolaron (centre-of-mass) energy band dispersion rather than from any particular interaction.

Tunnelling gaps and d-wave stripes in cuprates: a unified approach

Abstract The Fröhlich electron-phonon interaction in cuprates and other charge-transfer oxides is shown to be much stronger than any magnetic interaction. The polaron shift due to the Fröhlich interaction of about 1 eV suggests that carriers are smali (bi)polarons at all temperatures and dopings, in agreement with the oxygen isotope effect on the carrier mass, optical conductivity and other experimental observations. Two distinct energy scales, the d-wave superconducting order parameter and charge segregation in the form of stripes in cuprates, are unified in the framework of the bipolaron theory as a result of the formation of mobile bipolarons in the normal state and their Bose-Einstein condensation. Within the theory both the d-wave superconducting order parameter and the striped charge distribution result from the bipolaron (centre-of-mass) energy band dispersion rather than from any particular interaction.

Coherent pairing state and high Tc superconductivity

In this paper, I explain why the coherent pairing state for high T c superconductivity must be a mixed pairing state of the extended singlet and triplet π pairs. Then, I further explore the electron interactions in cuprates that dominate the mechanism of high T c superconductivity in the coherent pairing state.

Convergence of theory and experiments on electron-lattice interactions in cuprates : An overview of SILS

A brief overview of some of main highlights of the Symposium on Localised and Itinerant States (SILS) is presented.

Polaron Dynamics and Bipolarons in Cuprates

The Frohlich electron–phonon interaction in cuprates, manganites, and other charge-transfer ionic Mott insulators is much stronger than any magnetic interaction. The polaron shift due to the Frohlich interaction, which is about 1 eV, suggests that carriers in those systems are small (bi)polarons at all temperatures and doping levels. We show both analytically and numerically that (bi)polarons exist in the itinerant Bloch states at temperatures below the characteristic phonon frequency no matter which values the parameters of the system take. The small Frohlich polaron has spectral features compatible with the single-particle tunneling and photoemission in cuprates. Whereas the band energy dispersion of intersite bipolarons is responsible for the d-wave symmetry of the condensate wave-function, the single-particle excitation spectrum is s-like in agreement with the tunneling data. Two different energy scales in Giaver tunneling and Andreev reflection experiments in cuprates can be understood in the framework of the bipolaron theory as well.