Mechanism Of High-temperature Superconductivity Research Articles

A note on the possible mechanisms of high-temperature superconductivity

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

Electronically Driven Superlattice in the Cuprates

Upon doping the Mott–Hubbard insulator, an intermediate phase may be formed before the system reaches the Fermi-liquid phase. The pseudogap state and the spin/charge stripe phase are the possible examples of such a phase. We propose a new intermediate phase that has the form of the superlattice with \(2\sqrt{2}a\times 2\sqrt{2}b\) in the CuO2 plane, and allows the coexistence of the Mott–Hubbard insulator and the Fermi-liquid. This hypothesis is supported by neutron diffraction data, the phonon dispersion determined by neutron scattering and the recent ARPES data. The implications of such a phase on the mechanism of high-temperature superconductivity are discussed.

Magnetic ion doped Cu 1− xTl xBa 2Ca 2− yMg yCu 1.5Ni 1.5O 10− δ ( y = 0, 0.5, 1.0, 1.5) superconductors and their improved inter-plane coupling

We have synthesized Cu 0.5Tl 0.5Ba 2Ca 2Cu 3− y Ni y O 10− δ ( y = 0, 0.5, 1.0, 1.5) superconductors at normal pressure and studied the enhanced correlation among CuO 2/NiO 2 planes of Cu 0.5Tl 0.5Ba 2Ca 2− y Mg y Cu 1.5Ni 1.5O 10− δ ( y = 0.5, 1.0, 1.5) superconductors by doping Mg at the Ca sites. Surprisingly, we have not observed any substantial depression of critical temperature with Ni doping in Cu 0.5Tl 0.5Ba 2Ca 2Cu 3− y Ni y O 10− δ ( y = 0, 0.5, 1.0, 1.5) superconductors. The main objective of Mg substitution is to enhance the inter-plane coupling which may increase the interactions of the spins of Ni atoms in different planes. The increased inter-plane coupling may enhance interactions of the atomic spins with the free carriers and might be a key to understanding the effects of spin scattering and their role in the mechanism of high temperature superconductivity. We have observed a decrease of c-axis length with increased Mg concentration in Cu 0.5Tl 0.5Ba 2Ca 2− y Mg y Cu 1.5Ni 1.5O 10− δ ( y = 0.5, 1.0, 1.5) superconductors, showing an enhanced inter-plane coupling. The room temperature resistivity of the samples is decreased and the zero resistivity critical temperature [ T c( R = 0)] and the magnitude of diamagnetism are increased with higher Mg doping in Cu 0.5Tl 0.5Ba 2Ca 2− y Mg y Cu 1.5Ni 1.5O 10− δ superconductors. A maximum diamagnetism is observed in the samples when Mg atoms replace 75% of the Ca atoms, which shows that a higher Mg doping concentration enhances the inter-plane coupling which possibly promotes the de-localization of the carriers to the Ni 2+ sites and results in enhanced superconducting properties.

The Progress on Study of Isotope Effects in High T<sub>C</sub> Copper Oxide Superconductors

Through the review of the correlation on the isotope effects research and superconductivity mechanism research as well as the progress on the study of the isotope effects in the high Tc copper oxide superconductors, we have found the fundamental factors that cause the different changes of the isotope effect value in different dope region in high Tc cupper oxide superconductors. We point the high-temperature superconductivity mechanism possible origin phonons mediation electrons.

Local lattice structure in Mn-dopedLa2−xSrxCuO4studied by Cu and MnK-edge XAFS

The local lattice structures in Mn-doped La{sub 2-x}Sr{sub x}CuO{sub 4} are investigated by Cu K-edge and Mn K-edge x-ray-absorption near-edge structure and extended x-ray-absorption fine structure from 10 to 300 K. The results confirm that the doped Mn ions are located at the Cu site of La{sub 2-x}Sr{sub x}CuO{sub 4} phase and the impurity phases, such as La{sub 1-x}Sr{sub x}MnO{sub 3}, are not presented in the samples. It is found that the local lattice structures of Cu are not affected by Mn impurities and the local lattice environments of Mn are similar to those of Cu. We consider that the Mn impurities only break the Cu-O p-d hybridization locally, while the local lattice environments and the superconductivity in the regions away from the impurities remain untouched. This result implicates that local lattice inhomogeneity plays an important role in the mechanism of high-temperature superconductivity.

Lattice effects in cuprates

We describe the fine details of the dependence of the dispersion of the Cu–O bond-stretching phonon in YBa2Cu3O6+x on temperature and composition, recently determined by inelastic neutron scattering. The results suggest the presence of a localized phonon mode above the pseudogap temperature, which softens at lower temperatures. Its implications and possible role in the mechanism of high-temperature superconductivity are discussed.

Evidence for superfluidity of ultracold fermions in an optical lattice

The study of superfluid fermion pairs in a periodic potential has important ramifications for understanding superconductivity in crystalline materials. By using cold atomic gases, various models of condensed matter can be studied in a highly controllable environment. Weakly repulsive fermions in an optical lattice could undergo d-wave pairing at low temperatures, a possible mechanism for high temperature superconductivity in the copper oxides. The lattice potential could also strongly increase the critical temperature for s-wave superfluidity. Recent experimental advances in bulk atomic gases include the observation of fermion-pair condensates and high-temperature superfluidity. Experiments with fermions and bosonic bound pairs in optical lattices have been reported but have not yet addressed superfluid behaviour. Here we report the observation of distinct interference peaks when a condensate of fermionic atom pairs is released from an optical lattice, implying long-range order (a property of a superfluid). Conceptually, this means that s-wave pairing and coherence of fermion pairs have now been established in a lattice potential, in which the transport of atoms occurs by quantum mechanical tunnelling and not by simple propagation. These observations were made for interactions on both sides of a Feshbach resonance. For larger lattice depths, the coherence was lost in a reversible manner, possibly as a result of a transition from superfluid to insulator. Such strongly interacting fermions in an optical lattice can be used to study a new class of hamiltonians with interband and atom-molecule couplings.

Quantum Monte Carlo study of superconductivity in the two-leg ladder and low dimensional correlated systems

The mechanism of high-temperature superconductivity in cuprates has been studied over the last fifteen years. We do not still have reached a solution for the microscopic origin of high-temperature superconductivity. A possibility of superconductivity in correlated electron systems now becomes an important question concerning the high-Tc superconductivity. We investigate the possibility of superconductivity on the two-leg ladder and two-dimensional lattices using a recently developed Quantum Monte Carlo Optimization (QMCO) method for many fermion systems. The ground state wave function is written as a linear combination of the basis states that are generated using the auxiliary field method used in the Hubbard–Stratonovich transformation. Since the weights of bases are determined by diagonalizing the Hamiltonian to lower the ground state energy, the QMCO method is free from a difficulty known as the negative sign problem. We show that the pair correlation function is actually enhanced compared to that for the non-interacting system as the strength of the repulsive Coulomb interaction increases, indicating that the attractive interaction of electronic origin works among electrons.

The search for new high temperature superconductors

This paper reviews the present understanding of the microscopic mechanism of hightemperature superconductivity in cuprates, and considers the factors to takeinto account in the search for a new class of high temperature superconductors.

Superconductivity in the Y-Ba-Ca-Cu-O system

The search for the mechanisms of high-temperature superconductivity and novel superconducting materials are the most challenging tasks of condensed-matter physicists as well as material scientists [1–3]. The yttrium barium copper oxide (YBCO) YBa2Cu3O7 or CuBa2YCu2O7, often called “Y-123” or “Cu-1212” superconducting compound, is the first member of the homologous series of compounds of general formula Y2Ba4Cu6+nO14+n (n = 0, 1, 2). Cu-1212 phase exhibits a highest superconducting transition temperature (TC) inside YBCO family, at around 90–92 K, and till now remains the most extensively studied high-TC superconductor [4–8]. By the analogy to the Hg-based cuprates [9], we have tried to introduce additional CaO layers to the structure of CuBa2YCu2O7−δ (Cu-1212 or Cu-12102) superconductor. The purpose of the present work was to prepare new superconducting phases using the sol–gel synthesis method: (a) having a single layer of CaO between two CuO2 sheets with the nominal composition of CuBa2YCaCu3O9−δ (Cu–12113), and (b) having two CaO layers between triple CuO layers with the composition of CuBa2YCa2Cu4O11−δ (Cu–12124). The results of this study are presented herein. CuBa2YCaCu3O9−δ and CuBa2YCa2Cu4O11−δ samples were prepared by an acetate-tartrate sol–gel method developed previously for the synthesis of pure YBa2Cu4O8 superconductor [10]. As starting compounds stoichiometric amounts of analytical grade Y2O3, Cu(CH3COO)2·H2O, Ba(CH3COO)2 and Ca(CH3COO)2·H2O were used. In the sol–gel process, Y2O3 was first dissolved in 0.2 M acetic acid solution by stirring at 55–60 ◦C. After stirring the mixture for 10 hr in a beaker covered with a watch-glass a clear solution was obtained. In the next step, mixtures of the appropriate amounts of Ba(CH3COO)2, Ca(CH3COO)2·H2O, and Cu(CH3COO)2·H2O, all of them dissolved in small amounts of distilled water, were added with continuous stirring during several hours at the same temperature. An

Phonon mechanism of high-TC superconductivity

For a long time the majority view has been that phonons are irrelevant to the mechanism of high-temperature superconductivity. However, recent experimental results, including the neutron inelastic scattering measurements reviewed here, seriously challenge this view. We point out that the electron–phonon coupling in the cuprates can be substantially different from that in simple metals because of the covalency and strong electron correlation. In particular certain phonon branches induce substantial intersite charge transfer that can result in a negative electronic dielectric susceptibility. Such a strong electron–phonon coupling of certain modes could form the basis for the phonon mechanism of superconductivity in the cuprates.

Gen Shirane

Gen Shirane

Publisher's Note: Universality classes of metal-insulator transitions in strongly correlated electron systems and mechanism of high-temperature superconductivity [Phys. Rev. B72, 075113 (2005)

Publisher's Note: Universality classes of metal-insulator transitions in strongly correlated electron systems and mechanism of high-temperature superconductivity [Phys. Rev. B<b>72</b>, 075113 (2005)

Universality classes of metal-insulator transitions in strongly correlated electron systems and mechanism of high-temperature superconductivity

We study three regimes of the Mott transitions characterized by classical, marginally quantum and quantum. In the classical regime, the quantum degeneracy temperature is lower than the critical temperature of the Mott transition, Tc, below which the first-order transition occurs. The quantum regime describes the Tc=0 boundary of the continuous transition. The marginal quantum region appears sandwiched by these two regimes. The classical transition is described by the Ising universality class. However, the Ginzburg-Landau-Wilson scheme breaks down when the quantum effects dominate. The marginal quantum critical region is categorized to a new universality class, where the order parameter exponent beta, the susceptibility exponent gamma and the field exponent delta are given by beta=d/2, gamma=2-d/2 and delta=4/d, respectively, with d being the spatial dimensionality.The obtained universality classes agree with the recent experimental results for organic conductors, kappa-(ET)2Cu[N(CN)2]Cl and transition metal compounds such as V2O3. The mode coupling theory shows that the marginal quantum criticality further generates non-Fermi-liquid properties in the metallic side. A mechanism of high temperature superconductivity emerges from the density fluctuations at small wavenumber. The mode coupling theory combined with the Eliashberg equation predicts the superconductivity of the d wave symmetry with the transition temperature of the correct order of magnitude for the realistic parameters for the cuprate superconductors.

Antiferromagnetism and superconductivity of the two-dimensional extended t-J model

The mechanism of high-temperature superconductivity (HTS) and the correlation between the antiferromagnetic long-range order (AFLRO) and superconductivity (SC) are the central issues of the study of HTS theory. SC and AFLRO of the hole-doped two-dimensional extended t-J model are studied by the variational Monte Carlo method. The results show that SC is greatly enhanced by the long-range hopping terms t′ and t″ for the optimal and overdoped cases. The phase of coexisting SC and AFM in the t-J model disappears when t′ and t″ are included. It is concluded that the extended t-J model provides a more accurate description for HTS than the traditional t-J model does. The momentum distribution function n(k) and the shape of Fermi surface play critical roles for establishing the phase diagram of HTS materials.

Victor John Emery

Victor John Emery

Mechanism of high-temperature superconductivity in a striped Hubbard model

It is shown, using asymptotically exact methods, that the two dimensional repulsive Hubbard model with strongly modulated interactions exhibits ``high temperature superconductivity". Specifically, the explicit modulation, which has the same symmetry as period 4 bond-centered stripes, breaks the system into an alternating array of more and less heavily hole doped, nearly decoupled two-leg ladders. It is shown that this system exhibits a pairing scale determined by the spin-gap of the undoped two-leg ladder, and a phase ordering temperature proportional to a low positive power of the inter-ladder coupling.

Role of phonons in the mechanism of high-temperature superconductivity

While phonons are generally considered to be irrelevant to the mechanism of high-temperature superconductivity, a large number of experimental results indicate otherwise. We argue that the electron–phonon coupling in strongly correlated electron systems is unconventional, and results in a strong spin–charge–phonon coupling. It could contribute significantly to the superconductivity of the cuprates through the synergetic spin–phonon mechanism.

Synergetic Phonon–Spin–Charge Mechanism of High-Temperature Superconductivity

For a long time the majority opinion in the field of high-temperature superconductivity (HTSC) has been that it is purely an electronic phenomenon involving spin, and could be explained by a t–J Hamiltonian. Phonons and local distortion were regarded as irrelevant or harmful to HTSC. However, various experimental results indicate strong phonon involvement and ubiquitous presence of local spin–charge–lattice inhomogeneity. We suggest that the electron–phonon (e–p) coupling in the cuprate is unconventional, and a synergetic coupling of spin, charge, and phonon could explain the HTSC phenomenon. In our view the spin–charge–lattice inhomogeneity is a signature of such a coupling and an important component of the HTSC mechanism.

An alternative representation of bisoliton model of high temperature superconductivity

Davydov has clearly described in detail in his bisoliton mechanism of high temperature superconductivity [Phys. Rep. 190 (1990) 191; Phys. Status Solidi B 146 (1988) 619; Phys. Status Solidi B 148 (1988) 305]. In this article, we propose another representative form of the bisoliton mechanism simply and clearly for the compound YBa 2Cu 3O 7 by using Cu–O chain model.