High-T_c Superconductivity with T_c = 52 K under Antiferromagnetic Order in Five-layered Cuprate Ba_2Ca_4Cu_5O_10(F,O)_2 with T_N = 175 K: 19F- and Cu-NMR Studies
S. Shimizu, S. -i. Tabata, H. Mukuda, Y. Kitaoka, P. M. Shirage, H. Kito, A. Iyo
备注：
4 pages, 4 figures, to be published in Journal of the Physical Society of Japan, Vol.80, No.4
发表信息：
J. Phys. Soc. Jpn. 80, 043706 (2011). [selected as Editors' Choice]
We report on the observation of high-T_c superconductivity (SC) emerging with the background of an antiferromagnetic (AFM) order in the five-layered cuprate Ba_2Ca_4Cu_5O_10(F,O)_2 through 19F-NMR and zero-field Cu-NMR studies. The measurements of spectrum and nuclear spin-lattice relaxation rates 19(1/T_1) of 19F-NMR give convincing evidence for the AFM order taking place below T_N = 175 K and for the onset of SC below T_c = 52 K, hence both coexisting. The zero-field Cu-NMR study has revealed that AFM moments at Cu sites are 0.14 mu_B at outer CuO_2 layers and 0.20 mu_B at inner ones. We remark that an intimate coupling exists between the AFM state and the SC order parameter below T_c = 52 K; the spin alignment in the AFM state is presumably changed in the SC-AFM mixed state.
The phase diagram of the superconducting cuprates is often used to show how their electronic properties change as a function of the mean doping level, i.e., the average hole content of the CuO$_2$ plane. In Nuclear Magnetic Resonance (NMR) experiments average doping, as well as the distribution of these holes between planar Cu and O reveals itself through the quadrupole splittings of the $^{63,65}$Cu and $^{17}$O NMR. Here we argue based on all published NMR data available to us in favor a new type of phase diagram that has the planar oxygen quadrupole splitting and with it the planar oxygen h…
Nuclear magnetic resonance (NMR) provides local, bulk information about the electronic properties of materials, and it has been influential for theory of high-temperature superconducting cuprates. Importantly, NMR found early that nuclear relaxation is much faster than what one expects from coupling to fermionic excitations above the critical temperature for superconductivity ($T_{\rm c}$), i.e. what one estimates from the Knight shift with the Korringa law. As a consequence, special electronic spin fluctuations have been invoked. Here, based on literature relaxation data it is shown that the …
Planar oxygen nuclear magnetic resonance (NMR) relaxation and shift data from all cuprate superconductors available in the literature are analyzed. They reveal a temperature independent pseudogap at the Fermi surface, which increases with decreasing doping in family specific ways, i.e., for some materials the pseudogap is substantial at optimal doping while for others it is nearly closed at optimal doping. The states above the pseudogap, or in its absence are similar for all cuprates and doping levels, and Fermi liquid-like. If the pseudogap is assumed exponential it can be as large as about 1…
Nuclear magnetic resonance (NMR) is a powerful quantum probe, but the early conclusions on the physics of the cuprates, based on a limited set of data, have to be revised in view of recent findings and results from extensive literature analyses of most NMR data. These show two coupled electronic spin components that influence the nuclei, most easily seen with the planar Cu shift anisotropy. One component is spin from the recently identified ubiquitous metallic excitations, the other due to the intrinsic, antiferromagnetically coupled electronic Cu spin. Both components and its intricate intera…
Nuclear magnetic resonance (NMR) shifts, if stripped off their uncertainties, must hold key information about the electronic fluid in the cuprates. The early shift interpretation that favored a single-fluid scenario will be reviewed, as well as recent experiments that reported its failure. Thereafter, based on literature shift data for planar Cu a contrasting shift phenomenology for cuprate superconductors is developed, which is very different from the early view while being in agreement with all published data. For example, it will be shown that the hitherto used hyperfine scenario is inadequ…
Nuclear magnetic resonance (NMR) is a fundamental bulk probe that provides key information about the electronic properties of materials. Very recently, the analysis of all available planar copper shift as well as relaxation data proved that while the shifts cannot be understood in terms of a single temperature dependent spin component, relaxation can be explained with one dominating Fermi liquid-like component, without enhanced electronic spin fluctuations. For the shifts, a doping dependent isotropic term, as well as doping independent anisotropic term became obvious. Here we focus on planar …
Cuprate superconductors still hold many open questions, in particular concerning their normal state properties, including the role of charge ordering. Nuclear magnetic resonance on $^{63,65}$Cu and $^{17}$O in the CuO$_2$ plane is a very sensitive bulk probe of charge variations, but not properly understood. Since early NMR, the oxygen satellite line splittings in YBa$_2$Cu$_3$O$_{6+y}$ were attributed to the orthorhombic distortion, while they could also be due to charge ordering. We employ high pressure NMR on YBa$_2$Cu$_3$O$_{6.9}$ hoping that charge order would respond to pressure differen…
We report on magnetic characteristics in four-layered high-T_c superconductors Ba_2Ca_3Cu_4O_8(F_yO_{1-y})_2 with apical fluorine through Cu- and F-NMR measurements. The substitution of oxygen for fluorine at the apical site increases the carrier density (N_h) and T_c from 55 K up to 102 K. The NMR measurements reveal that antiferromagnetic order, which can uniformly coexist with superconductivity, exists up to N_h = 0.15, which is somewhat smaller than N_h = 0.17 being the quantum critical point (QCP) for five-layered compounds. The fact that the QCP for the four-layered compounds moves to a …
A strong nonmagnetic impurity can induce a resonance state in the d-wave superconducting state. As far as magnetic properties are concerned, this resonance state behaves effectively like a free moment. It leads to a Curie-Weiss-like magnetic susceptibility in an intermediate temperature regime below $T_{c}$. From the impurity susceptibility, the effective moment of the resonance state is deduced and compared with experiments. The contribution of the resonance to the magnetic susceptibility can account for the main feature of the NMR spectra in overdoped high-$T_c$ materials. In the underdoped …
We measured $^{23}$Na-NMR spectra and nuclear spin-lattice relaxation rates $^{23}(1/T_1)$ of superconducting and non-superconducting bilayer hydrate $Na_xCoO_2-yH_2O$ (x ~ 0.3, y ~ 1.3). The central resonance frequency shows a small but various shift due to the difference in the shielding effect by intercalated $H_2O$ molecules. The different shielding effect also gives a large difference in the magnitude of $^{23}(1/T_1)$.
The electric hyperfine interaction observable in atomic spectroscopy for O and Cu ions in various configurations is used to analyze the quadrupole splitting of O and Cu nuclear magnetic resonance (NMR) in La2-xSrxCuO4 and YBa2Cu3O6+y and to determine the hole densities at both sites as a function of doping. It is found that in La2-xSrxCuO4 all doped holes (x) reside in the Cu-O plane but almost exclusively at O. For YBa2Cu3O6+y and y<0.6 doped holes are found at planar Cu as well as O. For y>0.6 further doping increases the hole content only for planar O. The phase diagram based on NMR data is…
High-Tc superconductivity in layered cuprates is described in a BCS-BEC formalism with linearly-dispersive s- and d-wave Cooper pairs moving in quasi-2D finite-width layers about the CuO_2 planes. This yields a closed formula for Tc determined by the layer width, the Debye frequency, the pairing energy, and the in-plane penetration depth. The new formula reasonably reproduces empirical values of superconducting Tc's for seven different compounds among the LSCO, YBCO, BSCCO and TBCCO layered cuprates.
Pressure-induced superconductivity was studied for a spin-ladder cuprate Sr$_2$Ca$_{12}$Cu$_{24}$O$_{41}$ using nuclear magnetic resonance (NMR) under pressures up to the optimal pressure 3.8 GPa. Pressure application leads to a transitional change from a spin-gapped state to a Fermi-liquid state at temperatures higher than $T_c$. The relaxation rate $1/T_1$ shows activated-type behavior at an onset pressure, whereas Korringa-like behavior becomes predominant at the optimal pressure, suggesting that an increase in the density of states (DOS) at the Fermi energy leads to enhancement of $T_c$. N…
We present NMR measurements of the layered nitride superconductor Li_xZrNCl. The nuclear spin-lattice relaxation rate, 1/T_1, shows that the coherence peak is strongly suppressed in Li_xZrNCl in contrast to conventional BCS superconductors. In the lightly-doped region close to the insulating state, the system shows a gap-like behavior, i.e., pseudogap, that is characterized by a reduction in the magnitude of the Knight shift and 1/T_1T. A higher superconducting (SC) transition temperature, T_c, is achieved by coexisting with the pseudogap state. These unusual behaviors, which deviate from the …
In this communication we consider the doping dependence of the strong antiferromagnetic spin fluctuations in the cuprate superconductors. We investigate the effect of an incommensurate magnetic response, as recently observed in inelastic neutron scattering (INS) experiments on several YBa_2Cu_3O_{6+x} compounds, on the spin-lattice and spin-echo relaxation rates measured in nuclear magnetic resonance (NMR) experiments. We conclude that a consistent theoretical description of INS and NMR can be reached if one assumes spatially inhomogeneous but locally commensurate spin correlations and that NM…
Very recently, there has been significant progress with establishing a common phenomenology of the superconducting cuprates in terms of nuclear magnetic resonance (NMR) shift and relaxation. Different from the old interpretation, it was shown that the shifts demand two coupled spin components with different temperature dependencies. One spin component couples isotropically to the planar Cu nucleus and is likely to reside at planar O, while the other, anisotropic component has its origin in the planar copper $3d(x^2-y^2)$ orbital. Nuclear relaxation, on the other hand, was found to be rather ub…
We calculate the rate of transverse relaxation arising from vortex motion in the mixed state of YBa_2Cu_3O_7 with the static field applied along the c axis. The vortex dynamics are described by an overdamped Langevin equation with a harmonic elastic free energy. We find that the variation of the relaxation with temperature, average magnetic field, and local field is consistent with experiments; however, the calculated time dependence is different from what has been measured and the value of the rates calculated is roughly two orders of magnitude slower than what is observed. Combined with the …
We present a theory for inelastic neutron scattering (INS) and nuclear magnetic resonance (NMR) experiments in the pseudo-gap regime of the underdoped high-$T_c$ cuprates. We show that superconducting phase fluctuations greatly affect the temperature and frequency dependence of the spin-susceptibility, $χ''$, probed by both experimental techniques. This result explains the appearance of a {\it resonance peak}, observed in INS experiments, below a temperature $T_0 > T_c$. In the same temperature regime, we find that the $^{63}$Cu spin-lattice relaxation rate, $1/T_1$, measured in NMR experiment…
Using a conventional BCS -- Fermi liquid model we calculate the c-axis optical reflectivity of the layered high temperature cuprate superconductors by obtaining the finite temperature dynamical dielectric function in a microscopic self-consistent gauge invariant formalism. We get good semi-quantitative agreement with all the existing experimental data by using the measured normal state $dc$ resistivities as the input parameters in obtaining the c-axis hopping amplitude and the normal state level broadening in our microscopic calculation.
Based on a model Hamiltonian with a d-wave pairing interaction and a competing antiferromagnetic interaction, we numerically study the site dependence of the nuclear spin resonance (NMR) relaxation rate T_1^{-1} as a function of temperature for a d-wave superconductor(DSC) with magnetic field induced spin density wave (SDW) order. In the presence of the induced SDW, we find that there exists no simple direct relationship between NMR signal rate T_1^{-1} and low energy local density of states while these two quantities are linearly proportional to each other in a pure DSC. In the vortex core re…
We consider the interlayer pair-tunneling model for layered cuprates, including an effective single particle hopping along the c-axis. A phenomenological suppression of the c-axis hopping matrix element, by the pseudogap in cuprate superconductors, is incorporated. At optimal doping, quantities characteristic to the superconducting state, such as the transition temperature and the superconducting gap are calculated. Results from our calculations are consistent with the experimental observations with the noteworthy point that, the superconducting gap as a function of temperature shows excellent…
We find systematic signatures suggesting a different superconducting nature for a triple-layered cuprate Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$ with respect to a double-layer through the properties of intrinsic Josephson junctions (IJJs). Our measurements on the current-voltage characteristics reveal that the $c$-axis maximum Josephson current density is sensitive to the superfluid density in outer planes while the critical temperature and the superconducting gap remain unaffected. Switching dynamics of stacked IJJs exhibit that the fluctuation in gauge-invariant phase difference of an IJJ implies…
We reanalyze published magnetization data and demonstrate that the conclusion of the original authors, claiming enhanced two-dimensional properties of the cuprate superconductor Tl-1234, is not supported by the experimental results. Our analysis shows that the magnetic field dependence of the mixed-state magnetization for this particular sample is amazingly close to the results of numerical calculations by E. H. Brandt for an ideal vortex lattice without fluctuations. This good agreement between experiment and theory allows for the evaluation of the absolute values of the upper critical field.
We explore the layered palladium oxides La$_2$PdO$_4$, LaPdO$_2$ and La$_4$Pd$_3$O$_8$ via ab initio calculations. La$_2$PdO$_4$, being low spin $d^8$, is quite different from its high spin nickel analog. Hypothetical LaPdO$_2$, despite its $d^9$ configuration, has a paramagnetic electronic structure very different from cuprates. On the other hand, the hypothetical trilayer compound La$_4$Pd$_3$O$_8$ ($d^{8.67}$) is more promising in that its paramagnetic electronic structure is very similar to that of overdoped cuprates. But even in the $d^9$ limit (achieved by partial substitution of La with…
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…
Recently, an analysis of all available planar oxygen shift and relaxation data for the cuprate high-temperature superconductors showed that the data can be understood with a simple spin susceptibility from a metallic density of states common to all cuprates. It carries a doping dependent but temperature independent pseudogap at the Fermi surface, which causes the deviations from normal metallic behavior, also in the specific heat. Here, a more coherent, unbiased assessment of all data, including planar Cu, is presented and consequences are discussed, since the planar Cu data were collected and…
Recent ESR data shows rotation of the antiferromagnetic (AF) easy axis in lightly doped layered cuprates upon lowering the temperature. We account for the ESR data and show that it has significant implications on spin and charge ordering according to the following scenario: In the high temperature phase AF domain walls coincide with (110) twin boundaries of an orthorhombic phase. A magnetic field leads to annihilation of neighboring domain walls resulting in antiphase boundaries. The latter are spin carriers, form ferromagnetic lines and may become charged in the doped system. However, hole or…
Light scattering by Josephson vortices is theoretically predicted. An experimental set-up for the observation of the effect is proposed. The magnetic field should be parallel to the CuO_2 layers and the crystal surface. The falling light should be at grazing angles, and the Bragg scattering must be almost backscattering. The distance between the columns of the Josephson vortices must be slightly smaller than the light-wavelength. The surface should be very clean and flat in order light scattering by surface terraces and surface roughness to give the relatively small background.
Using a mix of numerical and analytic methods, we show that recent NMR $^{17}$O measurements provide detailed information about the structure of the charge-density wave (CDW) phase in underdoped YBa$_2$Cu$_3$O$_{6+x}$. We perform Bogoliubov-de Gennes (BdG) calculations of both the local density of states and the orbitally resolved charge density, which are closely related to the magnetic and electric quadrupole contributions to the NMR spectrum, using a microscopic model that was shown previously to agree closely with x-ray experiments. The BdG results reproduce qualitative features of the exp…
We revisit the Aslamazov-Larkin theory of paraconductivity in two dimensions, to distinguish its universal features from the specific features of nearly-free paired fermions. We show that both the numerical prefactor and the temperature dependence of the experimental paraconductivity in underdoped La_{2-x}Sr_xCuO_4 are only compatible with pairing of nearly-free fermionic quasiparticles. This conclusion is strengthened by the analysis of paraconductivity data in the presence of a finite magnetic field, from which we extract a rather low value of the critical field H_{c2}(T=0).
We report $^{63}$Cu- and $^{205}$Tl-NMR studies on six-layered ($n$=6) high-$T_c$ superconducting (SC) cuprate TlBa$_2$Ca$_5$Cu$_6$O$_{14+δ}$ (Tl1256) with $T_c\sim$100 K, which reveal that antiferromagnetic (AFM) order takes place below $T_{\rm N}\sim$170 K. In this compound, four underdoped inner CuO$_2$ planes ($n$(IP)=4) sandwiched by two outer planes (OPs) are responsible for the onset of AFM order, whereas the nearly optimally-doped OPs responsible for the onset of bulk SC. It is pointed out that an increase in the out-of-plane magnetic interaction within an intra-unit-cell causes $T_{\r…
We study a model of a d-wave superconductor with strong potential scatterers in the presence of antiferromagnetic correlations and apply it to experimental nuclear magnetic resonance (NMR) results on Zn impurities in the superconducting state of YBCO. We then focus on the contribution of impurity-induced paramagnetic moments, with Hubbard correlations in the host system accounted for in Hartree approximation. We show that local magnetism around individual impurities broadens the line, but quasiparticle interference between impurity states plays an important role in smearing out impurity satell…
We argue that nuclear magnetic resonance experiments are a site-sensitive probe for the electronic spectrum in the mixed state of the high-$T_c$ cuprates. Within a spin-fermion model, we show that the Doppler-shifted electronic spectrum arising from the circulating supercurrent changes the low-frequency behavior of the imaginary part of the spin-susceptibility. For a hexagonal vortex lattice, we predict that these changes lead to {\it (a)} a unique dependence of the $^{63}$Cu spin lattice relaxation rate, $1/T_1$, on resonance frequency, and {\it (b)} a temperature dependence of $T_1$ which va…
We study angular-dependent magnetoresistance in a low $T_c$ layered cuprate Bi$_{2.15}$Sr$_{1.9}$CuO$_{6+δ}$. The low $T_c$ ~ 4 K allows complete suppression of superconductivity by modest magnetic fields and facilitate accurate analysis of the upper critical field $H_{c2}$. We observe an universal exponential decay of fluctuation conductivity in a broad range of temperatures above $T_c$ and propose a new method for extraction of $H_{c2}(T)$ from the scaling analysis of the fluctuation conductivity at $T>T_c$. Our main result is observation of a surprisingly low $H_{c2}$ anisotropy ~ 2, which …
The 63Cu nuclear quadrupole resonance (NQR) frequency, nu_q, in the multi-layered cuprates is calculated in the cluster model by the exact diagonalization method. The charge imbalance between the outer CuO_2 plane (OP) with apical oxygen (OA) and the inner plane (IP) without OA in three-layered Tl2223 is estimated by comparing our results with the experimental nu_q. In Tl-based cuprates with more than three layers, we predict a large enhancement of the splitting of nu_q between OP and IP by taking into account the reduction of bond length between Cu and OA and a resulting enhancement of the ch…
We investigate nuclear spin-lattice relaxation data in the normal state of optimally doped YBaCuO_7 by analyzing the contributions to the relaxation rate of the copper, planar oxygen and yttrium along the directions perpendicular to the applied field. In this new picture there is no contrasting temperature dependence of the copper and oxygen relaxation. We use the model of fluctuating fields to express the rates in terms of hyperfine interaction energies and an effective correlation time tau_eff characterizing the dynamics of the spin fluid. The former contain the effects of the antiferromagne…
We have performed $^{125}$Te NMR on a single crystal of the novel layered superconductor Fe$_{1.04}$Te$_{0.67}$Se$_{0.33}$ for the first time. The spin parts of the Knight shifts for both $H // a$, $H // c$ are suppressed in the superconducting state, indicating the spin singlet superconductivity. At superconducting state, $1/T_{1}$ shows the power-law behavior ($\sim T^{3}$) without any coherent peaks. Observations of the residual density of state and the $T^{3}$-law indicate the presence of the line node in the superconducting gap. In the normal state, $1/T_{1}T$ which probes the \textbf{q}-…
There is a growing compelling experimental evidence that a quantum complex matter scenario made of multiple electronic components, and competing quantum phases is needed to grab the key physics of high critical temperature (Tc) superconductivity in layered cuprates. While it is known that defects self-organization controls Tc, the mechanism remains an open issue. Here we focus on the theoretical prediction of the multi-band electronic structure and the formation of broken Fermi surfaces generated by the self-organization of oxygen interstitials O-i atomic wires in the spacer layers in HgBa2CuO…
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.
We report systematic 17O-NMR measurements on the high-Tc cuprate (Ca_xLa_{1-x})(Ba_{1.75 - x}La_{0.25 + x})Cu_3O_y, for four different families (different x). Using Knight shift data, we show that the pseudogap lines for all families are inconsistent with a quantum critical point inside the superconducting dome. In addition, at constant doping the pseudogap temperature does not vary with x, in contrast to Tc. We therefore argue that pseudogap and superconductivity are separate phenomena in these cuprates. Using Knight shift data, we show that the pseudogap opening temperature T* is much higher…
We report on the phase diagram of antiferromagnetism (AFM) and superconductivity (SC) in three-layered Ba_2Ca_2Cu_3O_6(F,O)_2 by means of Cu-NMR measurements. It is demonstrated that AFM and SC uniformly coexist in three-layered compounds as well as in four- and five-layered ones. The critical hole density p_c for the long range AFM order is determined as p_c ~ 0.075, which is larger than p_c ~ 0.02 and 0.055 in single- and bi-layered compounds, and smaller than p_c ~ 0.08-0.09 and 0.10-0.11 in four- and five-layered compounds, respectively. This variation of p_c is attributed to the magnetic …
We report detailed 17O, 139La, and 63Cu Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR) measurements in a stripe ordered La1.875Ba0.125CuO4 single crystal and in oriented powder samples of La1.8-xEu0.2SrxCuO4. We observe a partial wipeout of the 17O NMR intensity and a simultaneous drop of the 17O electric field gradient (EFG) at low temperatures where the spin stripe order sets in. In contrast, the 63Cu intensity is completely wiped out at the same temperature. The drop of the 17O quadrupole frequency is compatible with a charge stripe order. The 17O spin lattice relax…
We study the effect of thermal fluctuations of the s-wave order parameter of a quasi two dimensional superconductor on the nuclear spin relaxation rate near the transition temperature Tc. We consider both the effects of the amplitude fluctuations and the Berezinskii-Kosterlitz-Thouless (BKT) phase fluctuations in weakly coupled layered superconductors. In the treatment of the amplitude fluctuations we employ the Gaussian approximation and evaluate the longitudinal relaxation rate 1/T1 for a clean s-wave superconductor, with and without pair breaking effects, using the static pair fluctuation p…
A brief review of microscopic studies of high-Tc superconductors HgBa2CuO4+d and HgBa2CaCu2O6+d is presented. The topics concerned are the pseudo spin-gap, the antiferromagnetic spin fluctuations near the quantum critical point, and the mechanism of the high Tc via NMR. The crystal structure with the flat CuO2 plane, two-dimensional electrical resistivity, Cu NQR, NMR experimental results are discussed.
We adapt the Boson-Fermion superconductivity model to include layered systems such as underdoped cuprate superconductors. These systems are represented by an infinite layered structure containing a mixture of paired and unpaired fermions. The former, which stand for the superconducting carriers, are considered as noninteracting zero spin composite-bosons with a linear energy-momentum dispersion relation in the CuO$_2$ planes where superconduction is predominant, coexisting with the unpaired fermions in a pattern of stacked slabs. The inter-slab, penetrable, infinite planes are generated by a D…
We report ^{31}P and ^{139}La NMR studies of (La_{0.87}Ca_{0.13})FePO, which is a family member of the recently discovered superconductor LaFeAs(O_{1-x}F_x). In the normal state, Knight shift and $1/T_1T$ show that a Fermi-liquid state with moderate ferromagnetic fluctuations emerges below 30K. From 1/T_1T of ^{31}P and ^{139}La, quasi-two dimensional electronic structure is suggested, in which the FeP layer is more conductive than the LaO layer. In the superconducting (SC) state, although a clear Meissner signal was observed, 1/T_1T increases below T_c, in contrast to a decrease of 1/T_1T due…
We report that planar CuO_2 hole densities in high-T_c cuprates are consistently determined by the Cu-NMR Knight shift. In single- and bi-layered cuprates, it is demonstrated that the spin part of the Knight shift K_s(300 K) at room temperature monotonically increases with the hole density $p$ from underdoped to overdoped regions, suggesting that the relationship of K_s(300 K) vs. p is a reliable measure to determine p. The validity of this K_s(300 K)-p relationship is confirmed by the investigation of the p-dependencies of hyperfine magnetic fields and of spin susceptibility for single- and b…
The thermal fluctuations of the electric field between the CuO_2 planes of layered cuprates are considered as an origin of the electrical resistivity. The model evaluation employs a set of separate plane capacitors each having area equal to the squared in-plane lattice constant a_0^2. It is shown that the scattering of charge carriers by the fluctuation of electric charge in the conducting CuO_2 planes gives rise to the in-plane electrical resistivity rho_ab. Such a mechanism can be viewed as an analog of the Rayleigh's blue-sky law---the charge carriers are scattered by thermal fluctuations o…
Recent STM measurements have revealed the existence of periodic charge modulations at the surface of certain cuprate superconductors. Here we show that the observed patterns are compatible with the formation of a three-dimensional crystal of doped holes, with space correlations extending between different Cu-O layers. This puts severe constraints on the dynamical stability of the crystallised hole structure, resulting in a close relationship between the periodicity of the electronic modulation and the interlayer distance.
The analytical expression for dynamical charge susceptibility in layered cuprates has been derived in the frame of singlet-correlated band model beyond random-phase-approximation (RPA) scheme. Our calculations performed near optimal doping regime show that there is a peak in real part of the charge susceptibility $χ({\bf q},ω)$ at {\bf Q} = ($π$, $π$) at strong enough inter-site Coulomb repulsion. Together with the strong maximum in the Im $χ({\bf Q},ω)$ at 15 meV it confirms the formation of low-energetic plasmons or charge fluctuations. This provides a jsutification that these excitations ar…
The oxygen O2p_sigma and copper Cu4s and Cu3d_x^2-y^2 orbitals are involved in a simple LCAO model for determination of the conduction band and the oxygen-oxygen hopping is considered as a small parameter with respect to the transition amplitude between nearest neighbours. The traditional Cooper pairing is obtained by taking into account the double-electron exchange between the nearest neighbours within the two-dimensional CuO_2 plane. The equation for the superconducting gap is obtained as a result of the standard BCS treatment. It is shown that the order parameter could have either s-type or…
Evidence is presented from NMR 1/T1T and ARPES data for the sudden disappearance of 2D antiferromagnetic (AF) correlations in the lightly overdoped region (p=0.19) at the T=0 metal-insulator transition where the pseudogap energy falls to zero. AF fluctuations thus appear to be intimately associated with the pseudogap and serve primarily to weaken superconductivity, strongly reducing the condensation energy and superfluid density.
We report $^{139}$La nuclear magnetic resonance (NMR) measurements on La$_{2-x}$Sr$_x$CuO$_4$ ($0.07\leq x \leq 0.15$) and La$_{2-x}$Ba$_x$CuO$_4$ ($x=1/8$) single crystals, focusing on the spin freezing observed in 1/8-doped lanthanum cuprates. Charge stripe order seems to induce the inhomogeneous slowing down of spin fluctuations toward spin order and compete with superconductivity.
We explain recently observed linear temperature dependence of the nodal Fermi velocity $v_F (T)$ in near-optimally doped cuprates. We argue that it originates from electron-electron interaction, and is a fundamental property of an arbitrary 2D Fermi liquid. We consider a spin-fermion model with the same parameters as in earlier studies, and show that the T term is about 30% at 300K, in agreement with the data. We show that the sub-leading term in $v_F (T)$ is a regular (and small) $T^2$ correction. We also show that at a $2k_F$ quantum-critical point, temperature corrections to the dispersion …
We report systematic Cu- and F-NMR measurements of five-layered high-Tc cuprates Ba2Ca4Cu5O10(F,O)2. It is revealed that antiferromagnetism (AFM) uniformly coexists with superconductivity (SC) in underdoped regions, and that the critical hole density pc for AFM is ~ 0.11 in the five-layered compound. We present the layer-number dependence of AFM and SC phase diagrams in hole-doped cuprates, where pc for n-layered compounds, pc(n), increases from pc(1) ~ 0.02 in LSCO or pc(2) ~ 0.05 in YBCO to pc(5) ~ 0.11. The variation of pc(n) is attributed to interlayer magnetic coupling, which becomes stro…
