Excitations In Cuprate Superconductors Research Articles

Resonant inelastic X-ray scattering study of charge density waves and elementary excitations in cuprate superconductors

In the 38 years since the discovery of cuprate superconductors, the theoretical mechanism of high-temperature superconductivity remains unresolved. Recent experimental progress has focused on exploring microscopic mechanisms by using novel characterization techniques. The development of synchrotron radiation has driven significant progress in spectroscopic methods. Resonant inelastic X-ray scattering (RIXS), based on synchrotron radiation, has been widely used to study cuprate superconductors due to its ability to perform bulk measurements, provide energy-momentum resolution, and directly probe various elemental excitations. The RIXS can measure phonons, which bind Cooper pairs in the BCS theory, and magnetic fluctuations and competing orders predicted by the Hubbard model in strongly correlated systems, allowing for the study of their interrelationships. This paper reviews the progress in using RIXS to measure charge density waves and related low-energy excitations, including phonon anomalies, in cuprate superconductors. It also examines the relationship between magnetic excitation and the highest superconducting transition temperature, and provides prospects for future research directions and challenges.

Spin singlet and quasiparticle excitations in cuprate superconductors

We followed step by step the transition from an antiferromagnetic (AF) Mott insulator to a superconducting (SC) metal in the Bi$_2$Sr$_2$CaCu$_{2}$O$_{8+\delta}$ (Bi-2212) cuprate using the electronic Raman scattering spectroscopy. This was achieved by tracking the doping dependence of the spin singlet excitation originate from the AF Mott insulator, the normal state quasiparticles excitation related to the mobile charge carriers and the Bogoliubov quasiparticles related to the SC gap. We show that the signature of the pseudogap phase which develops during this transition, can be interpreted as the blocking of charge carriers by the enhancement of the antiferromagnetic correlations as the temperature drops. We find that the energy scale of the pseudogap, $\Delta_{\textrm{pg}}(p)$, closely follows the one of the spin singlet excitation, $\Delta_{\textrm{sse}}(p)$, with doping $p$. The quasiparticles lifetime considerably increases with doping when the pseudogap collapses. We reveal that the maximum amplitude of the SC gap, $\Delta_{\textrm{sc}}^{\textrm{max}}$ and the SC transition temperature \Tc are linked in an extended range of doping such as $\Delta_{\textrm{sc}}^{\textrm{max}}(p) \propto \Delta_{\textrm{sse}}(p)\, T_c(p)$. This relation suggests that the AF correlations play a key role in the mechanism of superconductivity.

Close inspection of plasmon excitations in cuprate superconductors

Recently resonant inelastic x-ray scattering experiments reported fine details of the charge excitations around the in-plane momentum ${\bf q}_{\parallel}=(0,0)$ for various doping rates in electron-doped cuprates ${\rm La_{2-x}Ce_xCuO_4}$. We find that those new experimental data are well captured by acoustic-like plasmon excitations in a microscopic study of the layered $t$-$J$ model with the long-range Coulomb interaction. The acoustic-like plasmon is not a usual plasmon typical to the two-dimensional system, but has a small gap proportional to the interlayer hopping $t_z$.

Diagram Approach to the Problem of the Normal Phase Properties of the Spin-Polaron Ensemble in Cuprate Superconductors

Taking into account the real crystalline structure of the $$\hbox {CuO}_2$$ plane within the spin-fermion model and using the diagram technique, the spin-polaron concept of the fermionic excitations in cuprate superconductors is implemented. It is shown that an account of the on-site scattering processes leads to considerable binding energy of the spin-polaron quasiparticles. An account of the two-site spin-fermion scattering processes results in the energy spectrum and spectral properties of the spin-polaron quasiparticles which agree well with experimental data on cuprate superconductors.

Doping and momentum dependence of coupling strength in cuprate superconductors

ABSTRACTSuperconductivity is caused by the interaction between electrons by the exchange of bosonic excitations, however, this glue forming electron pairs is manifested itself by the coupling strength of the electrons to bosonic excitations. Here the doping and momentum dependence of the coupling strength of the electrons to spin excitations in cuprate superconductors is studied within the kinetic-energy-driven superconducting mechanism. The normal self-energy in the particle-hole channel and pairing self-energy in the particle-pariticle channel generated by the interaction between electrons by the exchange of spin excitation are employed to extract the coupling strengths of the electrons to spin excitations in the particle-hole and particle-particle channels, respectively. It is shown that below , both the coupling strengths in the particle-hole and particle-particle channels around the antinodes consist of two peaks, with a sharp low-energy peak located at 5 meV in the optimally doped regime, and a broad-band with a weak peak centred at 40 meV. In particular, this two-peak structure in the coupling strength in the particle-hole channel can persist into the normal-state, while the coupling strength in the particle-particle channel vanishes at the nodes. However, the positions of the peaks in the underdoped regime shift towards to higher energies with the increase of doping. More specifically, although the positions of the peaks move to lower energies from the antinode to the hot spot, the weights of the peaks decrease with the move of the momentum from the antinode to the hot spot, and fade away at the hot spots.

High-energy magnetic excitations in cuprate superconductors

In this paper, the high-energy magnetic excitations in cuprate superconductors are studied based on the kinetic energy driven superconducting mechanism. The spin self-energy is evaluated explicitly in terms of the collective charge carrier modes in the particle–hole and particle–particle channels, and employed to calculate the dynamical spin structure factor. Our results show the existence of damped but well-defined dispersive spin excitations in the whole doping phase diagram. In particular, the charge carrier doping has a more modest effect on the high-energy spin excitations, and then the high-energy magnetic excitations retain roughly constant energy as a function of doping, with spectral weights and dispersion relations comparable to those found in the parent compound.

Enhanced charge excitations in electron-doped cuprates by resonant inelastic x-ray scattering

Resonant inelastic x-ray scattering (RIXS) tuned for the Cu L edge is a possible tool to detect charge excitations in cuprate superconductors. We theoretically investigate the possibility for observing a collective charge excitation by the RIXS. The RIXS process via the intermediate state inevitably makes the spectral weight of charge excitation stronger in electron doping than in hole doping. Electron-hole asymmetry also appears in the dynamical charge structure factor, showing a new enhanced small-momentum low-energy mode in electron doping. These facts indicate a possibility of detecting the new charge mode by RIXS in electron-doped systems.

Evidence for site-centered stripes from magnetic excitations in cuprate superconductors

The success of models of coupled two-leg spin ladders in describing the magnetic excitation spectrum of ${\text{La}}_{2\ensuremath{-}x}{\text{Ba}}_{x}{\text{CuO}}_{4}$ has been widely interpreted as evidence for bond-centered stripes. Here, we determine the magnetic coupling induced by the charge stripes between bond- or site-centered spin stripe modeled by two- or three-leg ladder, respectively. We find that only the site-centered models order. We further report excellent agreement of a fully consistent analysis of coupled three-leg ladders using a spin-wave theory of bond operators with the experiment.

Competing orders and the doping and momentum dependent quasiparticle excitations in cuprate superconductors

The low-energy quasiparticle excitations in hole- and electron-type cuprate superconductors are investigated via both experimental and theoretical means. It is found that the doping and momentum dependence of the empirical low-energy quasiparticle excitations is consistent with a scenario of coexisting competing orders and superconductivity in the ground state of the cuprates. This finding, based on zero-field quasiparticle spectra, is further corrobarated by the spatially resolved vortex-state scanning tunneling spectroscopy, which reveals pseudogap-like features consistent with a remaining competing order inside the vortex core upon the suppression of superconductivity. The competing orders compatible with empirical observations include the charge-density wave and spin-density wave. In contrast, spectral characteristics derived from incorporating the d-density wave as a competing order appear unfavorable in comparison with experiments.

Spin, charge, and electronic excitations in cuprate superconductors: Role of the long-range hoppings

The single-particle spectral functions of the t– t′– t″– J model are exactly calculated for a full range of the Brillouin zone by introducing the twisted boundary conditions for a 20-site cluster. By using the spectral functions, we calculate the optical conductivity σ( ω). In the hole-doped system, σ( ω) shows a good agreement with the exact ones based on the linear response theory. On the contrary, σ( ω) in the electron-doped system disagree with the exact ones. This is due to the presence of strong antiferromagnetic correlations induced by the long-range hopping t′ and t″.

Stripe order, depinning, and fluctuations inLa1.875Ba0.125CuO4andLa1.875Ba0.075Sr0.050CuO4

We present a neutron scattering study of stripe correlations measured on a single crystal of La$_{1.875}$Ba$_{0.125}$CuO$_{4}$. Within the low-temperature-tetragonal (LTT) phase, superlattice peaks indicative of spin and charge stripe order are observed below 50 K. For excitation energies $\hbar\omega\le12$ meV, we have characterized the magnetic excitations that emerge from the incommensurate magnetic superlattice peaks. In the ordered state, these excitations are similar to spin waves. Following these excitations as a function of temperature, we find that there is relatively little change in the {\bf Q}-integrated dynamical spin susceptibility for $\hbar\omega\sim10$ meV as stripe order disappears and then as the structure transforms from LTT to the low-temperature-orthorhombic (LTO) phase. The {\bf Q}-integrated signal at lower energies changes more dramatically through these transitions, as it must in a transformation from an ordered to a disordered state. We argue that the continuous evolution through the transitions provides direct evidence that the incommensurate spin excitations in the disordered state are an indicator of dynamical charge stripes. An interesting feature of the thermal evolution is a variation in the incommensurability of the magnetic scattering. Similar behavior is observed in measurements on a single crystal of La$_{1.875}$Ba$_{0.075}$Sr$_{0.050}$CuO$_{4}$; maps of the scattered intensity in a region centered on the antiferromagnetic wave vector and measured at $\hbar\omega=4$ meV are well reproduced by a model of disordered stripes with a temperature-dependent mixture of stripe spacings. We discuss the relevance of our results to understanding the magnetic excitations in cuprate superconductors.

THE DIFFERENT ENERGY SCALES OF CUPRATE SUPERCONDUCTORS INVESTIGATED BY INELASTIC LIGHT SCATTERING

Some of the results concerning the low- and high-energy excitations in cuprate superconductors obtained by Raman spectroscopy are reviewed. Raman scattering couples to both the charge and spin excitations and covers excitation energies ranging from 3 meV to 1.25 eV. There are at least two features changing the Raman spectra below the critical temperature. First, in the low-frequency regime for Raman shifts near 2Δ, a signature of the superconducting gap can be observed revealing information about the symmetry and magnitude of Δ. Second, for frequencies related to the superexchange energy J, the observation of a rearrangement of scattering intensity in the charge and spin channels has shown that superconductivity is most likely not a phenomenon dealing with the low-energy properties of the cuprates alone. We will discuss briefly the conceptual problems related to this observation.