Occurrence Of High Temperature Superconductivity Research Articles

Angular Dependence of Fluctuation Conductivity in BaFe1.9Co0.1As2 Single Crystal

Fluctuation conductivity of electron-doped BaFe1.9Co0.1As2 single crystal is investigated by measurement of electrical resistivity under magnetic fields up to 13 T at different angles of θ = 0, 45 and 90 near mean-field transition temperature, where θ is the angle between the applied magnetic field and the axis perpendicular to the ab plane. The mean-field transition temperature is determined using the peak of dR/dT curves. Using the Aslamazov–Larkin theory, four main regions are identified corresponding to 3D, 2D, 1D, and SW regions, respectively, for each angle at different magnetic fields. The 1D region is a sign of conducting charge strips that are believed that these 1D conducting strips might be responsible for the occurrence of high-temperature superconductivity. Anisotropy increases as it approaches Tc, suggesting unconventional superconductivity that might be due to multiband effects in crystal. Results show that the zero-temperature coherence length in the c-direction ξ(0), the effective distance between the conducting layers d, and the channel cross section s decrease with increase in the magnetic field.

Possible high- Tc superconductivity due to incipient narrow bands originating from hidden ladders in Ruddlesden-Popper compounds

We introduce a concept of hidden ladders for the bilayer Ruddlesden-Popper type compounds: While the crystal structure is bilayer, $d_{xz}$ ($d_{yz}$) orbitals in the relevant $t_{2g}$ sector of the transition metal form a two-leg ladder along $x$ ($y$), since the $d_{xz}$ ($d_{yz}$) electrons primarily hop along the leg direction $x$ ($y$) on top of the rung $z$ direction. We predict that Sr$_3$Mo$_2$O$_7$ and Sr$_3$Cr$_2$O$_7$ are candidates for the hidden-ladder material with a right position of $E_F$, from a first-principles band calculation. Based on the analysis of Eliashberg equation for these systems, we propose a possible occurrence of high temperature superconductivity in these materials arising from the interband pair scattering processes between the wide and "incipient" narrow bands on the ladder.

Testing cellular automata interpretation of quantum mechanics in carbon nanotubes and superconductivity

Cellular Automata (CA) are represented at an effective level as intrinsic periodic phenomena, classical in the essence, reproducing the complete coherence (perfect recurrences) associated to pure quantum behaviours in condensed matter systems. By means of this approach it is possible to obtain a consistent, novel derivation of SuperConductivity (SC) essential phenomenology and of the peculiar quantum behaviour of electrons in graphene physics and Carbon Nanotubes (CNs), in which electrons cyclic dynamics simulate CA. In this way we will derive, from classical arguments, the essential electronic properties of these — or similar — graphene systems, such as energy bands and density of states. Similarly, in the second part of the paper, we will derive the fundamental phenomenology of SC by means of fundamental quantum dynamics and geometrical considerations, directly derived from the CA evolution law, rather than on empirical microscopical characteristics of the materials as in the standard approaches. This allows for a novel heuristic interpretation of the related gauge symmetry breaking and of the occurrence of high temperature superconductivity by means of simple considerations on the competition of quantum recurrence and thermal noise.

Superconductivity from repulsive electronic correlations on alternant cuprate and iron-based lattices

A key question in the theory of high-temperature superconductivity is whether off-diagonal long-range order can be induced wholly or in large part by repulsive electronic correlations. Electron pairs on cuprate and the iron-based pnictide and chalcogenide alternant lattices may interact with a strong short-range Coulomb repulsion and much weaker longer range attractive tail. Here, we show that such interacting electrons can cooperate to produce a superconducting state in which time-reversed electron pairs effectively avoid the repulsive part but reside predominantly in the attractive region of the potential. The alternant lattice structure is a key feature of such a stabilization mechanism leading to the occurrence of high-temperature superconductivity with or sign alternating s-wave or s± condensate symmetries. © 2013 Wiley Periodicals, Inc.

Fermi Glass Versus High-Temperature Superconductivity Behavior

Very recently BaSrNiO4 was reported to be a Fermi glass (Schilling et al. in J. Phys., Condens. Matter 21:015701, 2009). Its structure is essentially the one of K2NiF4 as is that of La2CuO4, in which the occurrence of high-temperature superconductivity (HTS) upon hole doping was first reported (Bednorz and Muller in Z. Phys. B 64:189, 1986; Adv. Chem. 100:757, 1988). The carriers in both have mainly eg character, and move in a stochastic potential as documented by a number of experiments. The difference of the two behaviors is mainly ascribed to the formation of intersite bipolarons (Kabanov and Mihailovic in J. Supercond. 13:950, 2000) , which is estimated to be up to two orders of magnitude larger in La2CuO4 than in BaSrNiO4. From this it follows that for HTS to occur, a large bipolaron formation energy in layered structures is required.

The delicate electronic and magnetic structure of the LaFePnO system (Pn = pnicogen)

The occurrence of high-temperature superconductivity, and the competition with magnetism, in stoichiometric and doped LaFeAsO and isostructural iron oxypnictides is raising many fundamental questions about the electronic structure and magnetic interactions in this class of materials. There are now sufficient experimental data that it may be possible to identify the important issues whose resolution will lead to the understanding of this system. In this paper, we address a number of the important issues. One important characteristic is the Fe–As distance (or more abstractly the pnicogen (Pn) height z(Pn)); we present results for the effect of z(Pn) on the electronic structure, energetics and Fe magnetic moment. We also study LaFeAsO under pressure, and investigate the effects of both electron and hole doping within the virtual crystal approximation. The electric field gradients for all atoms in the LaFeAsO compound are presented (undoped and doped) and compared with available data. The observed (π, π, π) magnetic order is studied and compared with the computationally simpler (π, π, 0) order which is probably a very good model in most respects. We investigate the crucial role of the pnicogen atom in this class, and predict the structures and properties of the N and Sb counterparts that have not yet been reported experimentally. At a certain volume a gap opens at the Fermi level in LaFeNO, separating bonding from antibonding bands. This is the first evidence that this class of materials indeed has an underlying semimetallic character, and this separation suggests directions for a better simple understanding of the seemingly intricate electronic structure of this system. Finally, we address briefly differences resulting from substitution of post-lanthanum rare earth atoms, which have been observed to enhance the superconducting critical temperature substantially.

Particle-hole symmetry in the antiferromagnetic state of the cuprates.

In the layered cuprate perovskites, the occurrence of high-temperature superconductivity seems deeply related to the unusual nature of the hole excitations. The limiting case of a very small number of holes diffusing in the antiferromagnetic (AF) background may provide important insights into this problem. We have investigated the transport properties in a series of crystals of YBa(2)Cu(3)O(y), and found that the temperature dependencies of the Hall coefficient R(H) and thermopower S change abruptly as soon as the AF phase boundary is crossed. In the AF state at low temperatures T, both R(H) and S are unexpectedly suppressed to nearly zero over a broad interval of T. We argue that this suppression arises from near-exact symmetry in the particle-hole currents. From the trends in R(H) and S, we infer that the symmetry is increasingly robust as the hole density x becomes very small (x approximately 0.01). We discuss implications for electronic properties both within the AF state and outside.

Magnets, mischief, and metals in Cobalt analogs of the superconducting cuprates

Layered Cobalt oxides provide the possibility for direct comparison with the superconducting cuprates to determine which structural, electronic or chemical factors are required for the occurrence of high temperature superconductivity. Recent results on the study of such materials are reported here.

Oxygen-isotope effect on the In-plane penetration depth in underdoped La2-xSrxCuO4 single crystals

We report measurements of the oxygen-isotope effect (OIE) on the in-plane penetration depth lambda(ab)(0) in underdoped La2-xSrxCuO4 single crystals. A highly sensitive magnetic torque sensor with a resolution of Deltatau approximately 10(-12) N m was used for the magnetic measurements on microcrystals with a mass of approximately 10 &mgr;g. The OIE on lambda(-2)(ab)(0) is found to be -10(2)% for x = 0.080 and -8(1)% for x = 0.086. It arises mainly from the oxygen-mass dependence of the in-plane effective mass m(*)(ab). The present results suggest that lattice vibrations are important for the occurrence of high temperature superconductivity.

Doping Dependence of the Effective Mass Anisotropy and Oxygen-Isotope Effect on the Magnetic Penetration Depth: The Role of Lattice Vibrations in High-Temperature Superconductivity

Studies of the critical behavior of high-temperature superconductors (HTSC) show the importance of the strong doping dependence of the effective mass anisotropy y for the generic (T, x) phase diagram of these materials. A possible microscopic model that can explain the doping dependence of y is based on polaronic charge carriers. That lattice effects may play a role for high-temperature superconductivity is further supported by measurements of the oxygen-isotope effect on the in-plane penetration depth λ ab (0) in underdoped La 2-x Sr x CuO 4 single crystals that are reported in this paper. The oxygen-isotope effect on λ al -2 (0) is found to be around -9% for the samples investigated. It arises mainly from the oxygen-mass dependence of the in-plane effective mass m ab * . The experimental facts presented in this paper suggest that lattice vibrations are important for the occurrence of high-temperature superconductivity.

V. Mechanism of Superconductivity

A possibility of phonon-mediated interaction as a mechanism of high- T c superconductivity is discussed. Because of different spatial distribution of Bloch wave function for up-spin particle and down-spin particle in the present model, the effective pair interaction which is derived from electron–phonon interaction is shown to have significant momentum- transfer dependence that is quite different from ordinary phonon-mediated interaction. This characteristic of pair interaction together with the geometric feature of Fermi-surface derived in a previous paper III leads to a dx2-y2 symmetric gap state which is consistent with experimental results. Since d-wave state has much smaller Coulomb repulsion term than s-wave state, strong electron-phonon coupling in the present model is expected to cause the occurrence of high-temperature superconductivity. However, in the present model, it is shown that a factor that reduces superconducting transition temperature appear as a result of a fluctuation effect of antiferromagnetic (AF) order of localized Cu spins. The factor λs which should be proportional to AF correlation length λ AF , determines the time scale τs which eliminates the contribution from the retarded effective pair interaction of time argument larger than τs. Existence of length scale λs explains why T c goes to zero as increasing hole carrier to some extent, and why non-magnetic impurity such as Pb reduces T c strongly. λs also explains the finite density of zero-gap states observed in overdoped regime or samples with non-magnetic impurities.

Local Kondo Temperature of Strongly Correlated Electron Liquids

The Hubbard model is studied for its paramagnetic phase. The leading-order effects in 1/ d are Kondo-effect-type local spin fluctuations, with d being the spatial dimensionality. For d →+∞ , a “local” Kondo temperature is defined to show an energy scale of the local spin fluctuations or the strength of the Kondo effect to quench magnetic moments. Even for finite d , the local Kondo temperature is still unambiguously defined through the mapping to the Anderson model. It is so renormalized as to increase by intersite and critical spin fluctuations, which are of higher order in 1/ d . The renormalization of the local Kondo temperature can play crucial roles in the suppression of magnetism, the occurrence of high-temperature superconductivity, and the occurrence of metamagnetism in the presence of magnetic fields. The above results generally apply to strongly correlated electron liquids.

Direct Evidence of the Importance of Electron-Phonon Coupling in La2CuO4: Photoinduced IR Active Vibrational Modes

Abstract We present direct evidence of the importance of the electron-phonon interaction in pure La2CuO4. Photoinduced IR active vibrational modes (IRAV) and associated bleaching of the La2CuO4 phonon modes are observed in the spectral range below 700 cm−1. The observation of photoinduced IRAV implies the existence of structural deformation around the photoexcited carriers, indicative of coupling of the photoexcitations to the lattice. We find, in addition, a broad photoinduced absorption which peaks at ≈ 0.5 eV, indicating an electronic transition deep within the energy gap. Some specific insight into the nature of the distortion around the photoexcitations can be obtained from comparison of the photoinduced bleaching at 218 cm−1 and 706 cm−1 and the doping induced bleaching (at 218 cm−1 and 704 cm−1). This remarkable similarity implies that locally in the vicinity of a photogenerated carrier, the CuO6 octahedra become tetragonal (as in the Sr-doped case). The possible importance of such a specific large local structural deformation around a carrier to the occurrence of high temperature superconductivity is quite obvious.

Enhancement of superconducting critical temperature due to metal-semiconductor transition

It is pointed out that there exists a qualitative agreement between the occurrence of high-temperature superconductivity in oxide systems (perovskite and layered perovskite structures) and the author's earlier theoretical predictions of superconductivity in the model of metal-insulator transition (partly contained in the book: High-temperature superconductivity). Here a brief account of the published results is given and attention is paid to some properties of such superconductors.

On the critical temperature of a superconducting system

Based on the assumption that in the groundbcs state the net gain in energy is equivalent to the repulsive electron-ion and electron-electron Darwin interactions, an expression forTc has been obtained which depends on only a few atomic parameters. The theory provides a criterion for the occurrence of superconductivity and yields satisfactory values ofTc for metals and alloys, and ternary chalcogenides and borides. It explains the difference inTc in the crystalline and amorphous states as well as the pressure dependence ofTc. The possibility of occurrence of high temperature superconductivity has been explored.