We investigate the magnetic excitations in view of the recent reports suggesting that the spin-wave energy may exhibit a significant dependence on the in-plane strain of a thin film of La$_2$CuO$_4$. The nature of dependence, as we find, can be explained naturally within a two-orbital model based on the $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ orbitals. In particular, as the orbital-splitting energy between the $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ orbitals increases with compressive strain, the zone-boundary spin-wave energy hardens. However, the hardening persists only until the orbital splitting reaches $\sim$ 2eV, beyond which there is no significant change. The behavior of zone-boundary spin-wave energy is explained in terms of the extent of hybridization between one of the exchange-split $d_{x^2-y^2}$ band which is nearly half filled and the $d_{3z^2-r^2}$ band. The role of second-order antiferromagnetic superexchange process involving the inter-orbital hopping is also discussed.
A planar Mott insulator with easy plane Néel order can be mapped onto a Gutzwiller-projected spin Hall state. The in-plane substitution of Zn$^{2+}$ and Li$^{+}$ for Cu$^{2+}$ brings zero modes around the spin vacancy. The Gutzwiller projection precludes single particle excitations, however, preserves the zero modes which have a local spin moment for Zn$^{2+}$ and a local charged hole for Li$^{+}$, respectively. While the local spin moment for Zn$^{2+}$ is screened by the long-range spin correlations, the active charge degree of freedom for Li$^{+}$ impurity twists the spin background with lon…
Using low-energy projection of the one-band t-t'-t"-Hubbard model we derive an effective spin-Hamiltonian and its spin-wave expansion to order 1/S. We fit the spin-wave dispersion of several parent compounds to the high-temperature superconducting cuprates: La2CuO4, Sr2CuO2Cl2 and Bi2Sr2YCu2O8. Our accurate quantitative determination of the one-band Hubbard model parameters allows prediction and comparison to experimental results of measurable quantities such as staggered moment, double occupancy density, spin-wave velocity and bimagnon excitation spectrum and density of states, which is discu…
The optical conductivity sigma(omega) is calculated at finite temperature T for CuO_2 chain clusters within a pd-Hubbard model. Data at T = 300 K for Li_2CuO_2 are reanalyzed within this approach. The relative weights of Zhang-Rice singlet and triplet charge excitations near 2.5 and 4 eV, respectively, depend strongly on T, and a rather dramatic dependence of sigma(omega) on the ratio of the first to second neighbor exchange integrals is predicted. On the basis of these results, information about exchange interactionsfor frustrated edge-shared cuprates can be obtained from T-dependent optical …
We study experimentally the Raman response of the undoped high-Tc parent compound $YBa_2Cu_3O_6$, and give a unified theory of the two-magnon Raman peak and optical conductivity based on the Hubbard-Holstein model with electron-phonon coupling (EPC). The Hubbard model without EPC can qualitatively account for the experimentally observed resonance of the Raman response, but only the Hubbard-Holstein model (i) reproduces asymmetry of the Raman spectrum, (ii) validates experimental visibility of the two-magnon peak, and (iii) predicts the correct shape and energy of the lower edge of the charge t…
We theoretically analyse the process of charge recombination in the planar Mott-Hubbard insulators with the aim to explain short picosecond-range lifetime of photoexcited carriers, experimentally studied via pump-probe experiments on the undoped cuprates. The recombination mechanism consists of two essential ingredients: the formation of a metastable s-type bound holon-doublon pair, i.e. the Mott exciton, and the decay of such an excitonic state via the multimagnon emission. In spite of the large gap that requires many bosons to be emitted, latter process is fast due to large exchange scale an…
We investigate the excitation spectrum of a two-dimensional resonating valence bond (RVB) state. Treating the $pi$-flux phase with antiferromagnetic correlations as a variational ground state, we recover the long wavelength magnon as an "RVB exciton". However, we find that this excitation does NOT exhaust the entire spectral weight and the high energy spectrum is dominated by fermionic excitations. The latter can be observed directly by inelastic neutron scattering and we predict their characteristic energy scales along different high symmetry directions in the magnetic Brillouin zone. We also…
In the framework of the generalized tight binding method we have calculated the quasiparticle band structure and the spectral functions of the undoped cuprates like $La_2 CuO_4$, $Sr_2CuO_2Cl_2$ etc. Due to spin fluctuations the in-gap state appears above the top of the valence band in the undoped antiferromagnetic insulator similar to in-gap states induced by hole doping. In the ARPES experiments the in-gap states can be detected as weak low energy satellites.
A systematic inelastic neutron scattering study of the superexchange interaction in three different undoped monolayer cuprates (La_2CuO_4, Nd_2CuO_4 and Pr_2CuO_4) has been performed using conventional triple axis technique. We deduce the in-plane antiferromagnetic (AF) superexchange coupling $J$ which actually presents no simple relation versus crystallographic parameters. The absolute spectral weight of the spin susceptibility has been obtained and it is found to be smaller than expected even when quantum corrections of the AF ground state are taken into account.
We study the magnetic excitation spectra of resonant inelastic x-ray scattering (RIXS) at the $L$-edge from undoped cuprates beyond the fast collision approximation. We analyse the effect of the symmetry breaking ground state on the RIXS process of the Heisenberg model by using a projection procedure. We derive the expressions of the scattering amplitude in both one-magnon and two-magnon excitation channels. Each of them consists of the isotropic and anisotropic contributions. The latter is a new finding and attributed to the long range order of the ground state. The presence of anisotropic te…
We develop a novel self-consistent approach for studying the angle resolved photoemission spectra (ARPES) of a hole in the t-J-Holstein model giving perfect agreement with numerically exact Diagrammatic Monte Carlo data at zero temperature for all regimes of electron-phonon coupling. Generalizing the approach to finite temperatures we find that the anomalous temperature dependence of the ARPES in undoped cuprates is explained by cooperative interplay of coupling of the hole to magnetic fluctuations and strong electron-phonon interaction.
We present angle-resolved photoemission spectroscopy (ARPES) data on undoped La2CuO4, indicating polaronic coupling between bosons and charge carriers. Using a shell model, we calculate the electron-phonon coupling and find that it is strong enough to give polarons. We develop an efficient method for calculating ARPES spectra in undoped systems. Using the calculated couplings, we find the width of the phonon side band in good agreement with experiment. We analyze reasons for the observed dependence of the width on the binding energy.
A recent observation of thermal Hall effect of magnetic origin in underdoped cuprates calls for critical re-examination of low-energy magnetic dynamics in undoped antiferromagnetic compound on square lattice, where traditional, renormalized spin-wave theory was believed to work well. Using Holstein-Primakoff boson formalism, we find that magnon-based theories can lead to finite Berry curvature in the magnon band once the Dzyaloshinskii-Moriya spin interaction is taken into account explicitly, but fail to produce non-zero thermal Hall conductivity. Assuming accidental doping by impurities and m…
Understanding similarities and differences between the cuprate and nickelate superconductors is drawing intense current interest. Competing charge orders have been observed recently in the $undoped$ infinite-layer nickelates in sharp contrast to the $undoped$ cuprates which exhibit robust antiferromagnetic insulating ground states. The microscopic mechanisms driving these differences remain unclear. Here, using in-depth first-principles and many-body theory based modeling, we show that the parent compound of the nickelate family, LaNiO$_2$, hosts a charge density wave (CDW) ground state with t…
Typical density-functional theory calculations that wrongly predict undoped cuprates to be metallic also predict Cu-O half- and full-breathing phonon energies that are significantly softer than observed, presumably because of weak on-site Coulomb repulsion on the Cu 3d orbitals. We used DFT+U calculations with antiferromagnetic supercells of La$_2$CuO$_4$ to establish correlation between the on-site repulsion strength, tuned via adjusting the value of U, and phonon dispersions. We find that breathing and half-breathing phonons reach experimental values when U is tuned to obtain the correct opt…
The optical conductivity sigma (omega) of undoped YBa2Cu3O6 is studied in detail in the mid-infrared range. Substitutions on all but the Ba site are used to identify the prominent absorption processes at 2800 and 3800 cm^-1. Experimental evidence for bimagnon-plus-phonon absorption is collected. A more critical analysis of the lineshape and the spectral weight reveals the limits of this approach. Although phonon-2-magnon multiple scattering seems to reproduce the lineshape, the necessary coupling is unrealistically large. The strong increase of high frequency spectral weight with increasing te…
In this review article, we show our recent results relating to the undoped (Ce-free) superconductivity in the electron-doped high-Tc cuprates with the so-called T' structure. For an introduction, we briefly mention the characteristics of the electron-doped T'-cuprates, including the reduction annealing, conventional phase diagram and undoped superconductivity. Then, our transport and magnetic results and results relating to the superconducting pairing symmetry of the undoped and underdoped T'-cuprates are shown. Collaborating spectroscopic and nuclear magnetic resonance results are also shown …
We study the dynamical, momentum dependent two- and four-spin response functions in doped and undoped 1D cuprates, as probed by resonant inelastic x-ray scattering, using an exact numerical diagonalization procedure. In the undoped $t-J$ system the four-spin response vanishes at $π$, whereas the two-spin correlator is peaked around $π/2$, with generally larger spectral weight. Upon doping spectra tend to soften and broaden, with a transfer of spectral weight towards higher energy. However, the total spectral weight and average peak position of either response are only weakly affected by doping…
We propose several tests of the idea that the electron is fractionalized in the underdoped and undoped cuprates. These include the ac Josephson effect, and tunneling into small superconducting grains in the Coulomb blockade regime. In both cases, we argue that the results are qualitatively modified from the conventional ones if the insulating tunnel barrier is fractionalized. These experiments directly detect the possible existence of the chargon - a charge $e$ spinless boson - in the insulator. The effects described in this paper provide a means to probing whether the undoped cuprate (despite…
The existence of a novel metal-insulator transition in the ferromagnetic state of models for undoped manganites is here discussed using numerical techniques applied to the $e_{\rm g}$-orbital degenerate Hubbard model tightly coupled with Jahn-Teller distortions. The ground-state phase diagram is presented in the plane defined by the electron-phonon coupling $λ$ and Coulomb interaction $u$. In contrast to the standard one-band Hubbard model for cuprates, the metallic phase is found to exist for finite values of both $λ$ and $u$ in the present $e_{\rm g}$-orbital Hubbard model even at half-filli…
We consider the magnetic properties of high Tc cuprates from a gauge theory point of view, with emphasis on the underdoped regime. Underdoped cuprates possess certain antiferromagnetic correlations, as evidenced, for example, by different temperature dependence of the Cu and O site NMR relaxation rates, that are not captured well by slave boson mean field theories of the t-J model. We show that the inclusion of gauge fluctuations will remedy the deficiencies of the mean field theories. As a concrete illustration of the gauge-fluctuation restoration of the antiferromangetic correlation and the …
In order to realize superconductivity in cuprates with the T'-type structure, not only chemical substitution (Ce doping) but also post-growth reduction annealing is necessary. In the case of thin films, however, well-designed reduction annealing alone without Ce doping can induce superconductivity in the T'-type cuprates. In order to unveil the origin of superconductivity in the Ce-undoped T'-type cuprates, we have performed bulk-sensitive hard x-ray photoemission and soft x-ray absorption spectroscopies on superconducting and non-superconducting Nd$_{2-x}$Ce$_x$CuO$_4$ ($x=$ 0, 0.15, and 0.19…
Doping and strain dependences of the electronic structure of the CuO6-octahedra layer within LDA+GTB method in the frameworks of six-band p-d model are calculated. Band structure and Fermi surface of the quasiparticle excitations in the effective Hubbard model are characterized by inhomogeneous distribution of the k-dependent spectral weight. Doping results in reconstruction of the band structure, redistribution of the spectral weight over dispersion surface and reconstruction of Fermi surface from small hole pockets in the underdoped region to large hole contour in the overdoped region throug…
We analyze the influence of the dimensionality on the Cu 2p_3/2 core level X-ray photoemission spectra of undoped cuprates. Both exchange splitting and delocalization properties are described within one framework using a multi-band Hubbard model. The spectral intensity is calculated by means of the Mori-Zwanzig projection technique for a CuO_4 plaquette ('zero'-dimensional), an infinite CuO_3 chain (one-dimensional), and an infinite CuO_2 plane (two-dimensional). The results are compared to the spectra of Bi_2CuO_4, Sr_2CuO_3, and Sr_2CuO_2Cl_2. Our analysis allows to distinguish between effec…
We present the first calculation of the electron-energy loss spectrum of infinite one-dimensional undoped CuO_3 chains within a multi-band Hubbard model. The results show good agreement with experimental spectra of Sr_2CuO_3. The main feature in the spectra is found to be due to the formation of Zhang-Rice singlet-like excitations. The q-dependence of these excitations is a consequence of the inner structure of the Zhang-Rice singlet. This makes the inclusion of the oxygen degrees of freedom essential for the description of the relevant excitations. We observe that no enhanced intersite Coulom…
A semi-phenomenological approach to describe the evolution of Fermi surface (FS) and electronic structure with doping is presented which is based on the spin-fermion model. The doping is simulated by a frustration term in the spin Hamiltonian and the complex internal structure of the spin-polaron quasiparticle is taken into account by a superposition of spin-polaron states with different radii. By calculating the spectrum, the spectral weights and the FS we find a rather drastic change of the electronic structure with doping which explains many photoemission data, i.e. the isotropic band botto…
A spin-fermion model that captures the charge-transfer properties of Cu-based high critical temperature superconductors is introduced and studied via Monte Carlo simulations. The strong Coulomb repulsion among $d$-electrons in the Cu orbitals is phenomenologically replaced by an exchange coupling between the spins of the itinerant electrons and localized spins at the Cu sites, formally similar to double-exchange models for manganites. This interaction induces a charge-transfer insulator gap in the undoped case (five electrons per unit cell). Adding a small antiferromagnetic Heisenberg coupling…
Spin and orbital excitations in undoped infinite layers: a comparison between superconducting PrNiO2 and insulating CaCuO2
Politecnico di Milano（米兰理工大学）；University of Cologne（科隆大学）；Université de Strasbourg（斯特拉斯堡大学）；European XFEL（欧洲X射线自由电子激光装置）；Ca’ Foscari University of Venice（威尼斯卡福斯卡里大学）；Chalmers University of Technology（查尔姆斯理工大学）；ESRF, The European Synchrotron（欧洲同步辐射装置）；Università di Roma Tor Vergata（罗马托维爾加塔大学）
The electron doping of undoped high-$T_c$ cuprates via the transfer of charge from manganites (or other oxides) using heterostructure geometries is here theoretically discussed. This possibility is mainly addressed via a detailed analysis of photoemission and diffusion voltage experiments, which locate the Fermi level of manganites above the bottom of the upper Hubbard band of some cuprate parent compounds. A diagram with the relative location of Fermi levels and gaps for several oxides is presented. The procedure discussed here is generic, allowing for the qualitative prediction of the charge…
A simple microscopic principle of `Valence bond (nearest neighbor singlet) amplitude maximization '(VBAM) is shown to be present in undoped and optimally doped cuprates and unify the very different orderings such as antiferromagnetism in the Mott insulator and the robust superconductivity accompanied by an enhanced charge and stripe correlations in the optimally doped cuprates. VBAM is nearly synonymous with the energy minimization principle. It is implicit in the RVB theory and thereby makes the predictions of RVB mean field theory of superconductivity qualitatively correct.
Evidence for strong electron-phonon coupling in high-Tc cuprates is reviewed, with emphasis on the electron and phonon spectral functions. Effects due to the interplay between the Coulomb and electron-phonon interactions are studied. For weakly doped cuprates, the phonon self-energy is strongly reduced due to correlation effects, while there is no corresponding strong reduction for the electron self-energy. Polaron formation is studied, focusing on effects of Coulomb interaction and antiferromagnetic correlations. It is argued that experimental indications of polaron formation in undoped cupra…
A universal approach is proposed to study the influence of strain (pressure) on the temperature of Bose-Einstein condensation of intersite bipolarons within the extended Holstein model. It is shown that uniaxial strain (pressure) derivatives of the temperature of such a Bose-Einstein condensation strongly depend on the arrangement of ions in the lattice. In particular, they may be positive or negative. A connection between the theoretically obtained results, along with the experimental data, on the influence of uniaxial pressure (strain) on $T_c$ of RBa$_2$Cu$_3$O$_{7-δ}$ family cuprates is di…
We investigate the influence of a cyclic spin exchange J_{cyc} on the one- and two-triplet excitations of an undoped two-leg S=1/2 ladder, using the density matrix renormalization group (DMRG). The dispersion of the S=0 two-triplet bound state is dramatically reduced by J_{cyc} due to a repulsion between triplets on neighboring rungs. In (La,Ca)_{14}Cu_{24}O_{41} a consistent description of both the spin gap and the S=0 bound state requires $J_{cyc}/J_\perp \approx 0.20-0.27$ and $J/J_{\perp} \approx 1.25-1.35$. With these coupling ratios the recently developed dynamical DMRG yields an excelle…
We present an experimental study of the dynamics of holes in the valence bands of 0, 1, and 2 dimensional undoped model cuprates, as expressed via the screening of a Cu 2p core hole. The response depends strongly upon the dimensionality and the details of the Cu-O-Cu network geometry and clearly goes beyond the present theoretical state-of-the-art description within the three-band d-p model.
We study the magnetic excitation spectra of L-edge resonant inelastic x-ray scattering (RIXS) from the spin singlet ground state in one-dimensional undoped cuprates. Analyzing the transition amplitudes of the magnetic excitations in the second-order dipole allowed process, we find that the magnetic excitations are brought about not only on the core-hole site but also on the neighboring sites. The RIXS spectra are expressed by the one-spin correlation function in the scattering channel with changing polarization, and the two-spin correlation function in the scattering channel without changing p…
Suppression of superconductivity in favour of a striped phase, and its coincidence with a structural transition from a low-temperature orthorhombic (LTO) to a low-temperature tetragonal (LTT) phase, is a ubiquitous feature of hole-doped lanthanum cuprates. We study the effect of anisotropic strain on this transition using density-functional theory on both La$_2$CuO$_4$ and the recently-synthesised surrogate La$_2$MgO$_4$ to decouple electronic and structural effects. Strikingly, we find that compressive strain applied diagonally to the in-plane metal-oxygen bonds dramatically stabilises the LT…
We report a comparative study of 63Cu Nuclear Magnetic Resonance spin lattice relaxation rates, T_1^{-1}, on undoped SrCuO_2 and Ca doped Sr_{0.9}Ca_{0.1}CuO_2 spin chain compounds. A temperature independent T_1^{-1} is observed for SrCuO_2 as expected for an S=1/2 Heisenberg chain. Surprisingly, we observe an exponential decrease of T_1^{-1} for T < 90,K in the Ca-doped sample evidencing the opening of a spin gap. The data analysis within the J_1-J_2 Heisenberg model employing density-matrix renormalization group calculations suggests an impurity driven small alternation of the J_2-exchange c…
We present a variational approach based on the string picture to analyze the internal structure and dispersion of spin polarons with different symmetries in an antiferromagnet. We then use this to discuss the properties of underdoped cuprate superconductor within the `doped insulator' picture. The theory explains the remnant Fermi surface for the undoped compunds, as well as hole pockets, Fermi arcs, high energy pseudogap and the the mid-infrared band in doped materials. Destructive interference between the phases of a photohole near $Γ$ and the internal phases of the Zhang Rice singlet combin…
In the framework of the effective low-energy model for High-$T_c$ cuprates with account for three-centers interaction terms and spin fluctuations the properties of normal and superconducting phases of p- and n-type cuprates are investigated. Microscopic model parameters were obtained from ARPES data on undoped compounds. Obtained evolution of the chemical potential with doping, Fermi Surface at optimal doping, and $T_c(x)$ phase diagram for n-type cuprates are in remarkably good agreement with the experiment. It is shown that the spin-exciton mechanism due to singlet-triplet hybridization take…
This article summarizes and extends the recent developments in the microscopic modeling of the magnetic excitations in cuprate two-leg ladder systems. The microscopic Hamiltonian comprises dominant Heisenberg exchange terms plus an additional four-spin interaction which is about five times smaller. We give an overview over the relevant energies like the one-triplon dispersion, the energies of two-triplon bound states and the positions of multi-triplon continua and over relevant spectral properties like spectral weights and spectral densities in the parameter regime appropriate for cuprate syst…
The recent observation of superconductivity in infinite-layer nickelates has brought intense debate on the established knowledge of unconventional superconductivity based on the cuprates. Despite many similarities, the nickelates differ from the cuprates in many characteristics, the most notable one among which is the magnetism. Instead of a canonical antiferromagnetic Mott insulator as the undoped cuprates, from which the superconductivity is generally believed to arise upon doping, the undoped nickelates show no sign of magnetic ordering in experiments. Through a combined density functional …
The quantitative description of correlated electron materials remains a modern computational challenge. We demonstrate a numerical strategy to simulate correlated materials at the fully ab initio level beyond the solution of effective low-energy models, and apply it to gain a detailed microscopic understanding across a family of cuprate superconducting materials in their parent undoped states. We uncover microscopic trends in the electron correlations and reveal the link between the material composition and magnetic energy scales via a many-body picture of excitation processes involving the bu…
There are two commonly discussed points of view in theoretical description of cuprate superconductors, (i) Cuprates can be described by the modified t-J model. (ii) Overdoped cuprates are close to the regime of normal Fermi liquid (NFL). We argue that recent resonant inelastic X-ray scattering data challenge both points. While the modified t-J model describes well the strongly underdoped regime, it fails to describe high energy magnetic excitations when approaching optimal doping. This probably indicates failure of the Zhang-Rice singlet picture. In the overdoped regime the momentum-integrated…
We calculate the magnetic phase diagram of the half-filled t-t'-U Hubbard model as a function of t' and U, within the Gutzwiller approximation RPA (GA+RPA). As U increases, the system first crosses over to one of a wide variety of incommensurate phases, whose origin is clarified in terms of double nesting. We evaluate the stability regime of the incommensurate phases by allowing for symmetry-breaking with regard to the formation of spin spirals, and find a crossover to commensurate phases as U increases and a full gap opens. The results are compared with a variety of other recent calculations,…
To elucidate the nature of the single-particle excitations in the undoped parent cuprates, we have performed a detailed study of Ca2CuO2Cl2 using photoemission spectroscopy. The photoemission lineshapes of the lower Hubbard band are found to be well-described by a polaron model. By comparing the lineshape and temperature dependence of the lower Hubbard band with additional O 2p and Ca 3p states, we conclude that the dominant broadening mechanism arises from the interaction between the photohole and the lattice. The strength of this interaction was observed to be strongly anisotropic and may ha…
Starting from a spin-fermion model for the cuprate superconductors, we obtain an effective interaction for the charge carriers by integrating out the spin degrees of freedom. Our model predicts a quantum critical point for the superconducting interaction coupling, which sets up a threshold for the onset of superconductivity in the system. We show that the physical value of this coupling is below this threshold, thus explaining why there is no superconducting phase for the undoped system. Then, by including doping, we find a dome-shaped dependence of the critical temperature as charge carriers …
We report studies of the electronic structure and elementary excitations of doped and undoped cuprate chains, ladders and planes. Using high energy spectroscopies such as x-ray absorption, core level photoemission and angle resolved photoemission spectroscopy, important information regarding the charge distribution and hole dynamics can be obtained. The comparison of the experimental data with suitable theoretical models sets constraints on the parameters entering into the model calculations, and offers insight into the important physical quantities governing the electronic structure of these …
Motivated by the potential for cuprate-like superconductivity in monolayer rare-earth nickelate superlattices, we study the effects of crystal field splitting, lattice distortions and strain on the charge, magnetic, and orbital order in undoped two-dimensional (2D) nickelate monolayers $R$NiO$_3$. We use a two-band Hubbard model to describe the low-energy electron states, with correlations controlled by a effective Hubbard $U$ and Hund's $J$. The electrons are coupled to the octahedral breathing-mode lattice distortions. Treating the lattice semiclassically, we apply the Hartree-Fock approxima…
Resonant inelastic x-ray scattering (RIXS) has become an important tool for studying elementary excitations in correlated materials. Here, we present a systematic theoretical investigation of the Cu L-edge RIXS spectra of undoped and doped cuprate two-leg spin-ladders in both the non-spin-conserving (NSC) and spin-conserving (SC) channels. The spectra are rich and host many exotic excitations. In the NSC-channel of the undoped case, we identify one-triplon and bound $\textit{triplet}$ two-triplon excitations in the strong-rung coupling limit, as well as confined spinons in the weak-rung coupli…
In order to investigate the low-energy antiferromagnetic Cu-spin correlation and its relation to the superconductivity, we have performed muon spin relaxation (muSR) measurements using single crystals of the electron-doped high-Tc cuprate Pr_1-x_LaCe_x_CuO_4_ in the overdoped regime. The muSR spectra have revealed that the Cu-spin correlation is developed in the overdoped samples where the superconductivity appears. The development of the Cu-spin correlation weakens with increasing x and is negligibly small in the heavily overdoped sample where the superconductivity almost disappears. Consider…
We report a Cu K-edge resonant inelastic x-ray scattering (RIXS) study of high-Tc cuprates. Momentum-resolved charge excitations in the CuO2 plane are examined from parent Mott insulators to carrier-doped superconductors. The Mott gap excitation in undoped insulators is found to commonly show a larger dispersion along the [pi,pi] direction than the [pi,0] direction. On the other hand, the resonance condition displays material dependence. Upon hole doping, the dispersion of the Mott gap excitation becomes weaker and an intraband excitation appears as a continuum intensity below the gap at the s…
The spin dynamics of high temperature superconductors measured by inelastic neutron scattering is reviewed. The spin susceptibility evolves a lot with increasing doping from the undoped insulating state to the overdoped metallic state. In the superconducting state, a strong magnetic resonance peak occurs in the spin excitation spectrum whose energy is proportional to the superconducting temperature. In underdoped and optimally doped regimes, normal state antiferromagnetic fluctuations are observed, proving the existence of strong electronic correlations in cuprates. They are characterized by a…
We present RIXS data at O K edge from La2-xSrxCuO4 vs. doping between x=0.10 and x=0.22 with attention to the magnetic excitations in the Mid-Infrared region. The sampling done by RIXS is the same as in the undoped cuprates provided the excitation is at the first pre-peak induced by doping. Note that this excitation energy is about 1.5 eV lower than that needed to see bimagnons in the parent compound. This approach allows the study of the upper region of the bimagnon continuum around 450 meV within about one third of the Brilluoin Zone around Γ. The results show the presence of damped bimagnon…
Using resonant inelastic x-ray scattering (RIXS) at the Cu $L$-absorption edge, we have observed intense, dispersive spin excitations in highly overdoped Tl$_2$Ba$_2$CuO$_{6+δ}$ (superconducting $T_c =6$ K), a compound whose normal-state charge transport and thermodynamic properties have been studied extensively and shown to exhibit canonical Fermi-liquid behavior. Complementary RIXS experiments on slightly overdoped Tl$_2$Ba$_2$CuO$_{6+δ}$ ($T_c =89$ K) and on Y$_{1-x}$Ca$_{x}$Ba$_2$Cu$_3$O$_{6+δ}$ compounds spanning a wide range of doping levels indicate that these excitations exhibit energi…
We propose a Ginzburg-Landau phenomenological model for the dependence of the critical temperature on microscopic strain in tetragonal high-Tc cuprates. Such a model is in agreement with the experimental results for LSCO under epitaxial strain, as well as with the hydrostatic pressure dependence of Tc in most cuprates. In particular, a nonmonotonic dependence of Tc on hydrostatic pressure, as well as on in-plane or apical microstrain, is derived. From a microscopic point of view, such results can be understood as due to the proximity to an electronic topological transition (ETT). In the case o…
Substitutional impurities in the CuO2 planes of the cuprates allow us to probe the electronic properties of the host material. The pseudo-gap in the underdoped regime is unmodified far from the impurities even though Tc is greatly reduced. The spin polarisation induced by magnetic impurities has an oscillatory behaviour reflecting the existing AF correlations between the Cu spins. Its influence on the NMR spectra opens a way to determine the q dependence of the static spin susceptibility and the T dependence of the AF correlation length. NMR measurements demonstrate that non-magnetic impuritie…
