A limiting factor in the ability to interpret isotope effect measurements in cuprates is the absence of sufficiently accurate data for the whole phase diagram; there is precise data for Tc, but not for the antiferromagnetic transition temperature T_N. Extreme sensitivity of T_N to small changes in the amount of oxygen in the sample is the major problem. This problem is solved here by using the novel compound (Ca 0.1 La 0.9)(Ba 1.65 La 0.35)Cu 3 O y for which there is a region where T_N is independent of oxygen doping. Meticulous measurements of T_N for samples with oxygen 16 and oxygen 18 find the absence of an oxygen isotope effect on T_N with unprecedented accuracy. A possible interpretation of our finding and existing data is that isotope substitution affects the normal state charge carrier density.
We investigate the effect of isotope substitution on the electronic spectral functions within a model where the charge carriers are coupled to bosonic charge-order (CO) fluctuations centered around some mean frequency ω_0 and with enhanced scattering at wave-vector q_c. It is shown that a mass dependence of ω_0 is not sufficient in order to account, especially at high energies, for the dispersion shifts experimentally observed in an optimally doped superconducting cuprate. We argue that isotope substitution induces a change of the spatial CO correlations which gives good account of the experim…
A recent angle resolved photoelectron spectroscopy (ARPES) study by Douglas et al. \cite {dessau-comment} on oxygen isotope exchanged Bi_2Sr_2CaCu_2O$_{8+δ}$ superconductors reported an absence of isotope effect at optimal doping, questioning the previous work by us \cite {gweon-nature}. Here, we report a new result that sheds light on this puzzling discrepancy as well as the nature of the electron lattice interaction in the cuprates: the anomalous isotope effect at optimal doping \cite {gweon-nature}, re-confirmed here, vanishes on a mere 2 % overdoping of holes. This result implies a rapid c…
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 tr…
The isotope effect provided a crucial key to the development of the BCS (Bardeen-Cooper-Schrieffer) microscopic theory of superconductivity for conventional superconductors. In superconducting cooper oxides (cuprates) showing an unconventional type of superconductivity, the oxygen isotope effect is very peculiar: the exponential coefficient strongly depends on doping level. No consensus has been reached so far on the origin of the isotope effect in the cuprates. Here we show that the oxygen isotope effect in cuprates is in agreement with the bisoliton theory of superconductivity.
Starting from the three-band $p-d$ Hubbard Hamiltonian we derive an effective $t-J$ model including electron-phonon interaction of quasiparticles with optical phonons. Within the effective Hamiltonian we analyze the influence of electronic correlations and electron-phonon interaction on the dynamical spin susceptibility in layered cuprates. We find a huge isotope effect on the resonance peak in the magnetic spin susceptibility, ${Im}χ({\bf q},ω)$, seen by inelastic neutron scattering. It results from both the electron-phonon coupling and the electronic correlation effects taken into account be…
We analyse a model where the anomalies of the bond-stretching LO phonon mode are caused by the coupling to electron dynamic response in the form of a damped oscillator and explore the possibility to reconstruct the spectrum of the latter from the phonon measurements. Preliminary estimates point to its location in the mid infrared region and we show how the required additional information can be extracted from the oxygen isotope effect on the phonon spectrum. The model predicts a significant measurable deviation from the "standard value" of the isotope effect even if the phonon frequency is far…
Within the Hubbard-Holstein model with long-range Coulomb forces, we revisit the charge-ordering scenario for the superconducting cuprates and account for the presence or the absence of a formed stripe phase in different classes of cuprates. We also evaluate the mean-field and the fluctuation-corrected critical lines for charge ordering and we relate them with the various pseudogap crossover lines occurring in the cuprates and we discuss a mechanism for their peculiar isotopic dependence. Considering the dynamical nature of the charge-ordering transition, we explain the spread of T* and of its…
We report resistivity, Hall effect, Nernst effect, and magnetoresistance measurements on T'-phase La1.85Y0.15CuO (LYCO) films prepared by pulsed laser deposition under different oxygen conditions. Our results show that superconductivity in LYCO originates from an oxygen-doped Mott-like insulator and not from a weakly correlated, half-filled band metal as proposed previously.
We have studied the oxygen-isotope effect on the superconducting transition temperature $T_{c}$ in overdoped Y$_{1-x}$Ca$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-δ}$ with $x$ = 0.10, 0.20, and 0.25. We find the oxygen-isotope exponent $α_{O}$ to be small ($\sim$0.02) for $x$ = 0.10 but substantial ($\sim$0.1) for $x$ = 0.20 and 0.25. The doping level above which $α_{O}$ increases sharply coincides with a quantum critical point where the normal-state pseudogap starts to diminish. The present isotope-effect experiments provide direct and quantitative constraints on the pairing mechanism of high-temperature su…
We critically review recent and earlier results on isotope effects in cuprate superconductors and emphasize that the sample preparation and the isotope exchange and back exchange are crucial in understanding and interpreting the data. Only extremely careful preparation techniques yield reliable results and permit differentiation between real isotope effects and marginal ones. The former are substantial and highlight the lattice vibrational importance in cuprate superconductors.
Recent experiments on Bi-based cuprate superconductors have revealed an unexpected enhancement of the pairing correlations near the interstitial oxygen dopant ions. Here we propose a possible mechanism -- based on local screening effects -- by which the oxygen dopants do modify the electronic parameters within the CuO_2 planes and strongly increase the superexchange coupling J. This enhances the spin pairing effects locally and may explain the observed spatial variations of the density of states and the pairing gap.
Within the stripe quantum critical point theory for high T_c superconductors, we point out that there is a direct contribution of charge collective fluctuations to the optical absorption and to the Raman spectra. In this latter case, we find that the critical charge collective modes can or can not be excited depending on the direction of their wavevector and on the polarization of the incoming and outgoing photons. This indicates a marked distinction between quasiparticles which are strongly and weakly coupled to critical collective modes and provides a direct confirmation that the order assoc…
We revisit a problem of Dzyaloshinsky-Moriya antisymmetric exchange coupling for a single bond in cuprates specifying the local spin-orbital contributions to Dzyaloshinsky vector focusing on the oxygen term. The Dzyaloshinsky vector and respective weak ferromagnetic moment is shown to be a superposition of comparable and, sometimes, competing local Cu and O contributions. The intermediate oxygen $^{17}$O Knight shift is shown to be an effective tool to inspect the effects of Dzyaloshinsky-Moriya coupling in an external magnetic field. We predict the effect of $strong$ oxygen weak antiferromagn…
We present extensive magnetic measurements of the (Ca_xLa_{1-x})(Ba_{1.75-x}La_{0.25+x})Cu_{3}O_{y} (CLBLCO) system with its four different families (x) having a Tc^max(x) variation of 28% and minimal structural changes. For each family we measured the Neel temperature, the anisotropies of the magnetic interactions, and the spin glass temperature. Our results exhibit a universal relation Tc=c*J*n_s for all families, where c~1, J is the in plane Heisenberg exchange, and n_s is the carrier density. This relates cuprate superconductivity to magnetism in the same sense that phonon mediated superco…
Superconductivity in high-Tc cuprates is unconventional and there is a growing acceptance that carrier pairing in these strongly correlated electronic materials is mediated mainly by bosons of electronic origin rather than the phonons. Still, a significant isotope exponent (IE) (in certain cases, much larger than 0.5, the canonical BCS value) is observed experimentally. This has led to the assumption that phonons are also involved up to a certain extent. The magnitude of the IE in cuprates depends strongly on the number of doped holes, p, in the CuO2 planes and therefore, may indicate that pho…
Anisotropy, thermal and quantum fluctuations and their dependence on dopant concentration appear to be present in all cuprate superconductors, interwoven with the microscopic mechanisms responsible for superconductivity. Here we review anisotropy, in-plane and c-axis penetration depths, isotope effect and conductivity measurements to reassess the universal behavior of cuprates as revealed by the doping dependence of these phenomena and of the transition temperature.
Differentiation of electron doping and oxygen reduction in electron-doped cuprates
Institute for Solid State Physics, The University of Tokyo（东京大学固体离子体研究所）；Department of Engineering and Applied Sciences, Sophia University（上智大学理工学部）；Department of Applied Physics, Tohoku University（东北大学应用物理系）；UVSOR Synchrotron Facility, Institute for Molecular Science（分子科学研究所UVSOR同步辐射设施）
Stripe fluctuations in La2NiO4.17 have been studied by 139La NMR using the field and temperature dependence of the linewidth and relaxation rates. In the formation process of the stripes the NMR line intensity is maximal below 230K, starts to diminish around 140K, disappears around 50K and recovers at 4K. These results are shown to be consistent with, but completely complementary to neutron measurements, and to be generic for oxygen doped nickelates and underdoped cuprates.
Both theoretical considerations and experimental data point to a more complicated nature of the valence hole states in doped cuprates than it is predicted by Zhang-Rice model. Actually, we deal with a competition of conventional hybrid Cu 3d-O 2p $b_{1g}\propto d_{x^2 -y^2}$ state and purely oxygen nonbonding state with $e_{u}x,y \propto p_{x,y}$ symmetry. The latter reveals a non-quenched Ising-like orbital moment that gives rise to a novel spinless purely oxygen scenario of the magnetic response in doped cuprates with the oxygen localized orbital magnetic moments of the order of tenths of Bo…
The dynamical mean-field theory of the three-band model of copper-oxide superconductors is used to calculate the doping dependence of the intensity of the oxygen K-edge x-ray absorption spectra of high-$T_c$ copper oxide superconductors. The model is found not to reproduce the results of a recent experiment, suggesting that at sufficiently high doping the physics beyond the conventional three-band model becomes important.
We have performed a systematic angle-resolved photoemission study of as-grown and oxygen-reduced Pr$_{2-x}$Ce$_x$CuO$_4$ and Pr$_{1-x}$LaCe$_{x}$CuO$_4$ electron-doped cuprates. In contrast to the common belief, neither the band filling nor the band parameters are significantly affected by the oxygen reduction process. Instead, we show that the main electronic role of the reduction process is to remove an anisotropic leading edge gap around the Fermi surface. While the nodal leading edge gap is induced by long-range antiferomagnetic order, the origin of the antinodal one remains unclear.
Using a large-N expansion we present and solve the linearized equation for the superconducting gap for a generalized t-J model which also contains phonons within a Holstein model. The leading Tc has d-wave symmetry with phonons giving a positive contribution to Tc. The corresponding isotope coefficient is very small at optimal doping and increases towards the classical value 1/2 with increasing dopings similar as in many cuprates.
Inelastic neutron scattering data for YBaCuO as well as for LaSrCuO indicate incommensurate neutron scattering peaks with incommensuration $δ(x)$ away from the $(π,π)$ point. $T_c(x)$ can be replotted as a linear function of the incommensuration for these materials. This linear relation implies that the constant that relates these two quantities, one being the incommensuration (momentum) and another being $T_c(x)$ (energy), has the dimension of velocity we denote $v^*$: $k_B T_c(x) = \hbar v^* δ(x)$. We argue that this experimentally derived relation can be obtained in a simple model of Joseph…
Within the framework of pure BCS(i.e. when the critical temperature is proportional to the Debye temperature), we show that the isotope coefficient is always less than 1/2 and could be extremely small in polyatomic superconductors depending on the chemical formula unit. This finding leads to quantitative explanation of observed values(correct orders of magnitude and sometimes exact numerical values) in optimally doped cuprates, magnesium diboride and alkali-doped fullerenes. Consequently, the smallness of the isotope coefficient is not only compatible with pure electron-phonon interaction, but…
The doped cuprates are considered to be a system of local spinless bosons moving in a lattice of the hole pseudo-Jahn-Teller centers CuO_{4}^{5-}. A detailed qualitative and quantitative description of the vibronic mechanism determining both the isotope substitution and the external pressure effect on T_c is given. This mechanism can properly interpret the principal peculiarities of isotopic and baric effects in cuprates except the region of the well developed percolation phenomena
Since the early days of the discovery of hole doped high-Tc cuprates, the variation of the oxygen isotope exponent (IE) with the number of doped holes, p, in the CuO2 planes has been a source of considerable debate. There is a growing acceptance in the high-Tc community that the mediating bosons leading to Cooper pairing have largely electronic character. At the same time there is an increasing acknowledgement that at some level the lattice-charge interactions might be important in cuprates. The experimentally observed substantially large IE over certain doping range always casts a shadow over…
The relatively low Tc of Bi2Sr2CuO6+D allows to study normal-state down to low temperatures and the non-substituted compound is intrinsically strongly overdoped. Hole concentration can be adjusted trough oxygen excess, but few data exist in the literature about the quantitative control of D. The synthesis, achieved in air by solid-state reaction, needs long-time annealing to obtain pure phase with stoichiometric cationic ratios. Thermogravimetric techniques were used to explore oxygen non-stoichiometry. Absolute oxygen content was determined by reduction with hydrogen, while the oxygen exchang…
In this paper, I study the oxygen isotope effect (OIE) in cuprates. I introduce a simple model that can explain experiments both qualitatively and quantitatively. In this theory, isotope substitution only affects the superfluid density, but not the pseudo-gap. Within the spin-charge separation picture, I argue that the spinon-phonon interaction is in the adiabatic limit, and therefore within the Migdal-Eliashberg theory, there is no isotope effect in the spinon mass renormalization. On the other hand, I show that the holon-phonon interaction is in the non-adiabatic limit. Therefore, the small …
The role of the oxygen degree of freedom in the cuprates' superconducting planes is analyzed in detail. Structural and photoemission results are reviewed to show that the most sparse description of the in-plane electronic states requires explicit control of the oxygens. For metallic states, the relative contributions of oxygen and copper vary along the Fermi surface, with the arc metallicity dominantly oxygen-derived. For the magnetic responses, we find that the observed incommensurability arises naturally if one keeps separate the roles of the two sites. For the charge order in LBCO, we propo…
We study oxygen K-edge x-ray absorption spectroscopy (XAS) and investigate the validity of the Zhang-Rice singlet (ZRS) picture in overdoped cuprate superconductors. Using large-scale exact diagonalization of the three-orbital Hubbard model, we observe the effect of strong correlations manifesting in a dynamical spectral weight transfer from the upper Hubbard band to the ZRS band. The quantitative agreement between theory and experiment highlights an additional spectral weight reshuffling due to core-hole interaction. Our results confirm the important correlated nature of the cuprates and eluc…
The oxygen isotope effect on the superconductivity, stripe phase and structure transition is systematically investigated in La$_{1.6-x}$Nd$_{0.4}$Sr$_{x}$CuO$_4$ with static stripe phase. Substitution of $^{16}$O by $^{18}$O leads to a decrease in superconducting transition temperature T$_C$, while enhances the temperature of the structural transition from low-temperature-orthorhombic (LTO) phase to low-temperature-tetragonal (LTT) phase. Compared to the Nd free sample, a larger isotope effect on $T_C$ is observed in La$_{1.6-x}$Nd$_{0.4}$Sr$_{x}$CuO$_4$. These results indicate that the distor…
We have studied the novel isotope effects on the pairing pseudogap in underdoped and optimally doped cuprates within the large-polaron model and two non-standard BCS-like approaches. We have shown that in the intermediate-coupling regime the precursor pairing of large polarons occurs at a mean-field temperature $T^{\ast}>T_{c}$ and the near-absent, sizable and very large oxygen and copper isotope effects on $T^{\ast}$ exist in cuprates with small and large Fermi surfaces. Our results for $T^{\ast}$, isotope shifts and exponents in slightly underdoped and optimally doped cuprates are in quantit…
The critical doping levels in cuprates, where the ground state changes its nature (from an antiferromagnet to a spin glass to superconductor to metal), are not universal. We investigate the origin of these critical doping variations by measuring the in-plane oxygen $p_σ$ hole density in the CuO$_{2}$ layers as a function of the oxygen density $y$ in (Ca$_{x}$ La$_{1-x}$)(Ba$_{1.75-x}$La$_{0.25+x}$)Cu$_{3}$O$_{y}$. This is done using the oxygen 17 nuclear quadrupole resonance parameter $ν_{Q}$. We compare compounds with $x=0.1$ and 0.4 which have significant critical $y$ variations and find tha…
We have studied the oxygen-isotope effects on T_{c} and in-plane penetration depth λ_{ab}(0) in an optimally doped 3-layer cuprate Bi_{1.6}Pb_{0.4}Sr_{2}Ca_{2}Cu_{3}O_{10+y} (T_{c} \sim 107 K). We find a small oxygen-isotope effect on T_{c} (α_{O} = 0.019), and a substantial effect on λ_{ab} (0) (Δλ_{ab} (0)/λ_{ab} (0) = 2.5\pm0.5%). The present results along with the previously observed isotope effects in single-layer and double-layer cuprates indicate that the isotope exponent α_{O} in optimally doped cuprates is small while the isotope effect on the in-plane effective supercarrier mass is s…
We have prepared oxygen isotope exchanged crystals of impurity-free YBCO with various oxygen concentents, and examined pure doping ($p$) dependance of isotope effect on superconducting transition temperature. With decreasing oxygen contents, the isotope exponent $α$ monotonously increases without any anomaly around $p = 1/8$. The monotonous increase in $α$ indicates that phonons are involved in the mechanism which causes the monotonous $T_c$ suppression with underdoping.
We show that the unusual doping dependence of the isotope effects on transition temperature and zero temperature in - plane penetration depth naturally follows from the doping driven 3D-2D crossover, the 2D quantum superconductor to insulator transition (QSI) in the underdoped limit and the change of the relative doping concentration upon isotope substitution. Close to the QSI transition both, the isotope coefficient of transition temperature and penetration depth approach the coefficient of the relative dopant concentration, and its divergence sets the scale. These predictions are fully consi…
We review various isotope effects in the high-T_c cuprate superconductors to assess the role of the electron-phonon interaction in the basic physics of these materials. Of particular interest are the unconventional isotope effects on the supercarrier mass, on the charge-stripe formation temperature, on the pseudogap formation temperature, on the EPR linewidth, on the spin-glass freezing temperature, and on the antiferromagnetic ordering temperature. The observed unconventional isotope effects strongly suggest that lattice vibrations play an important role in the microscopic pairing mechanism o…
We investigate resonant inelastic X-ray scattering (RIXS) at the O $K$-edge in insulating cuprates by means of three methods: cluster perturbation theory (CPT), Hartree--Fock approximation (HFA) and exact diagonalization (ED) method. We consider the three-band Hubbard model and show the overall momentum-dependence of the Zhang--Rice singlet (ZRS) excitation and charge-transfer excitation by the CPT combining with the perturbation scheme. A comparison of the RIXS spectra calculated using CPT and HFA reveals different momentum-dependencies through the changes in the properties of the upper Hubba…
Various unconventional isotope effects have been reported in high-temperature superconducting copper oxides which are beyond the scheme of BCS theory. Their origin is investigated within polaron theory which leads to a renormalization of the single particle energies and introduces a level shift here. It is found that the exponential squeezing of the second nearest neighbour hopping integral carries the correct isotope effect on the superconducting transition temperature Tc, as well as the one on the penetration depth. The average superconducting gap is predicted to have an isotope effect compa…
The oxygen-isotope (^{16}O/^{18}O) effect (OIE) on the zero-temperature superconducting energy gap Δ_0 was studied for a series of Y_{1-x}Pr_xBa_2Cu_3O_{7-δ} samples (0.0\leq x\leq0.45). The OIE on Δ_0 was found to scale with the one on the superconducting transition temperature. These experimental results are in quantitative agreement with predictions from a polaronic model for cuprate high-temperature superconductors and rule out approaches based on purely electronic mechanisms.
In this paper we calculate ab initially the phonon dispersion relationship of the superconductor LaFeAsO and investigate a main property in the superconductor, the oxygen isotope effect. Based on this phonon dispersion relationship, we find the fact that an important reason of the oxygen isotope effect is connected with the phonon. This result agrees well with the experimental data where the power index of the oxygen isotope effect in the superconductor LaFeAsO is small.
Correlated-electron theories of superconductivity in layered cuprates often start from the premise of a gapped spin-liquid phase proximate to the superconducting state. This assumption is justified based on analytical and numerical demonstrations of a superconducting Luther-Emery phase in the doped 2-leg one-band Hubbard ladder, and the perceived analogy between coupled ladders and the two dimensional CuO2 layer. We demonstrate from accurate density matrix renormalization group studies the absence of the superconducting Luther-Emery phase in the doped 2-leg three-band ladder consisting of both…
We use first-principles calculations to extract two essential microscopic parameters, the charge-transfer energy and the inter-cell oxygen-oxygen hopping, which correlate with the maximum superconducting transition temperature $\Tcmax$ across the cuprates. We explore the superconducting state in the three-band model of the copper-oxygen planes using cluster Dynamical Mean-Field Theory. We find that the variation in the charge-transfer energy largely accounts for the empirical trend in $\Tcmax$, resolving a long-standing contradiction with theoretical calculations.
The planar oxygen isotope effect coefficient measured as a function of hole doping in the Pr- and La-doped YBa2Cu3O7 (YBCO) and the Ni-doped La1.85Sr0.15CuO4 (LSCO) superconductors quantitatively and qualitatively follows the form originally proposed by Kresin and Wolf, which was derived for polarons perpendicular to the superconducting planes. Interestingly, the inverse oxygen isotope effect coefficient at the pseudogap temperature also follows the same formula. These findings allow the conclusion that the superconductivity in YBCO and LSCO results from polarons or rather bipolarons in the Cu…
The Fermi energy, Ef of newly discovered high-Tc superconductor MgB2 and cuprates is estimated from the measured London penetration depth using a parameter-free expression. Ef of MgB2 and more than 30 lanthanum, yttrium and mercury-based cuprates appears to be about or below 100 meV, depending on doping. There is every evidence that the remarkable low value of Ef and the strong coupling of carriers with high-frequency phonons is the cause of high Tc in all newly discovered superconductors. Taking into account that the carriers mainly reside on oxygen in cuprates, on boron in magnesium diborade…
The expressions for the superfluid density have been discussed in both weak and strong coupling approaches. The numerical calculations of temperature and isotope composition dependencies have been performed for a number of High-Tc compounds. The tight-binding parameters and corresponding Fermi surfaces are taken in accord to the available photoemission data.
Underdoped cuprates exhibit a normal-state pseudogap, and their spins and doped carriers tend to spatially separate into 1- or 2-D stripes. Some view these as central to superconductivity, others as peripheral and merely competing. Using La$_{2-x}$Sr$_x$Cu$_{1-y}$Zn$_y$O$_4$ we show that an oxygen isotope effect in $T_c$ and in the superfluid density can be used to distinguish between the roles of stripes and pseudogap and also to detect the presence of impurity scattering. We conclude that stripes and pseudogap are distinct, and both compete and coexist with superconductivity.
Electron-doped and hole-doped superconducting cuprates exhibit a symmetric phase diagram as a function of doping. This symmetry is however only approximate. Indeed, electron-doped cuprates become superconductors only after a specific annealing process: This annealing affects the oxygen content by only a tiny amount, but has a dramatic impact on the electronic properties of the sample. Here we report the occurrence of superconductivity in oxygen-deficient Nd$_{2-x}$Ce$_x$CuO$_4$ thin films grown in an oxygen-free environment, after annealing in pure argon flow. As verified by x-ray diffraction,…
Electrolyte gating is widely used to induce large carrier density modulation on solid surfaces to explore various properties. Most of past works have attributed the charge modulation to electrostatic field effect. However, some recent reports have argued that the electrolyte gating effect in VO2, TiO2 and SrTiO3 originated from field-induced oxygen vacancy formation. This gives rise to a controversy about the gating mechanism, and it is therefore vital to reveal the relationship between the role of electrolyte gating and the intrinsic properties of materials. Here, we report entirely different…
We report on resistivity and Hall measurements in thin films of the electron-doped superconducting cuprate Pr$_{2-x}$Ce$_{x}$CuO$_{4\pmδ}$. Comparisons between x = 0.17 samples subjected to either ion-irradiation or oxygenation demonstrate that changing the oxygen content has two separable effects: 1) a doping effect similar to that of cerium, and 2) a disorder effect. These results are consistent with prior speculations that apical oxygen removal is necessary to achieve superconductivity in this compound.
Muon-spin rotation (muSR) studies of the oxygen isotope (^{16}O/^{18}O) effect (OIE) on the in-plane magnetic field penetration depth lambda_{ab} in cuprate high-temperature superconductors (HTS) are presented. First, the doping dependence of the OIE on the transition temperature T_c in various HTS is briefly discussed. It is observed that different cuprate families show a similar doping dependence of the OIE on T_c. Then, bulk muSR, low-energy muSR, and magnetization studies of the total and site-selective OIE on lambda_{ab} are described in some detail. A substantial OIE on lambda_{ab} was o…
The isotope coefficient $β$ of the magnetic penetration depth in the superconducting state is studied at T=0 for a $d$-CDW and a nodal metal model. Disregarding superconductivity the Fermi surface of the first model possesses arcs whereas the second model has no arcs. We show that a large increase of $β$ in the pseudogap region is generically incompatible with Fermi arcs in the pseudogap state. Thus only the second model shows a large increase of $β$ with decreasing doping. The required electron-phonon coupling is small and compatible with first-principles calculations based on the local densi…
The magnetic field dependence of the oxygen-isotope (^{16}O/^{18}O) effect (OIE) on the in-plane magnetic field penetration depth λ_{ab} was studied in the hole-doped high-temperature cuprate superconductors YBa_2Cu_4O_8, Y_0.8Pr_0.2Ba_2Cu_3O_7-δ, and Y_0.7Pr_0.3Ba_2Cu_3O_7-δ. It was found that λ_ab for the ^{16}O substituted samples increases stronger with increasing magnetic field than for the ^{18}O ones. The OIE on λ_ab decreases by more than a factor of two with increasing magnetic field from μ_0H=0.2 T to μ_0H=0.6 T. This effect can be explained by the isotope dependence of the in-plane …
Experiments have shown that the families of cuprate superconductors that have the largest transition temperature at optimal doping also have the largest oxygen hole content at that doping. They have also shown that a large charge-transfer gap, a quantity accessible in the normal state, is detrimental to superconductivity. We solve the three-band Hubbard model with cellular dynamical mean-field theory and show that both of these observations follow from the model. Cuprates play a special role amongst doped charge-transfer insulators of transition metal oxides because copper has the largest cova…
The underdoped region of the cuprate's phase diagram displays many novel electronic phenomena both in the normal and the superconducting state. Many of these anomalous properties have found a natural explanation within the resonating valence bond spin liquid phenomenological model of Yang-Rice-Zhang (YRZ) which includes the rise of a pseudogap. This leads to Fermi surface reconstruction and profoundly changes the electronic structure. Here we extend previous work to consider the shift in critical temperature on $^{16}$O to $^{18}$O substitution, The isotope effect has been found experimentally…
Starting from the copper-oxygen Hamiltonian of the CuO2 planes, we derive analytically an extended 1-band Hubbard Hamiltonian for the electrons on copper sites, through a canonical transformation which eliminates the oxygen sites. The model sustains a variety of phases : checkerboard states, stripes, antiferromagnetism, local pairs and mixtures thereof. This approach may be helpful in understanding what is so special about the CuO2 planes, as opposed to other compounds.
