Superconductivity in hcp Sc under pressure - linear response results and comparison with hcp Fe
This paper has been withdrawn due to some errors in the reported discussion, and needs a thorough revision
cond-mat.supr-con↗
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Gutzwiller-RVB Theory of High Temperature Superconductivity: Results from Renormalised Mean Field Theory and Variational Monte Carlo Calculations
We review the Resonating Valence Bond (RVB) theory of high temperatur e superconductivity using Gutzwiller projected wave functions that incorporate strong correlations. After a general overview of the phenomenon of high temperature superconductivity, we discuss Anderson's RVB picture and its implementation by renormalised mean field theory (RMFT) and variational Monte Carlo (VMC) techniques. We review RMFT and VMC results with an emphasis on recent development s in extending VMC and RMFT techniques to excited states. We compare results obtained from these methods with angle resolved photoemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM). We conclude by summarising recent successes of this approach and discuss open problems that need to be solved for a consistent and complete description of high temperature superconductivity using Gutzwiller projected wave functions.
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Possible vortex splitting in the cuprate superconductors
We propose that the observed splitting of the vortices in the cuprates into fractional vortices (partons) may be of static rather than of dynamic origin. This interpretation is backed by a study of a model with a dominant d-wave and subdominant s-wave pairing interaction. We find that the vortex may split into two partons, both of which carry one half of the magnetic flux quantum. The partons are hold together by a confining string along which the phase jumps approximately by pi and their equilibrium distance increases with lowering the energy difference epsilon between the pairing states. The partons become deconfined at the critical point where epsilon vanishes.
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A comment on the superconductivity of Sr2CuO3+delta
We have revisited data in the literature and find compelling reasons for believing that enhanced superconductivity occurs in Sr2CuO3+delta when about one third of the oxygen sites in the CuO2 layers are vacant, contrary to the almost universally accepted assumption that superconducting interactions in the high Tc cuprates occur in stoichiometric CuO2 layers.
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Basic physics of solid materials
Since the cuprate materials behave many curious properties, the mechanism that causes high-temperature superconductivity in copper oxide materials (cuprates) has been a controversial topic. To explain these curious properties, we must look out the physics what these materials include. In this letter, it is presented that there are the basic physics (1)-(9) in solid materials. I divide all solid materials into four categories, explain some properties of them qualitatively, and suggest some ways to turn a solid material into a superconductor. Particularly, this letter discusses the differences between the p-type high-Tc cuprate superconductors, the n-type superconductor Nd2-xCexCuO4, the BCS-superconductor similar MgB2, and heavy Fermions superconductors.
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Antiferromagnetic fluctuations in the superconducting phase of low- and high-temperature superconductors
Based on recent experimental results for electron-doped cuprate oxides and ferromagnetic superconductors, it is shown that antiferromagnetic fluctuations always develop in the superconducting phase of both low- and high-temperature superconductors. The relation between the magnitude of the antiferromagnetic pseudogap and the characteristic temperature of the antiferromagnetic pseudogap opening is obtained. The characteristic temperature of the antiferromagnetic pseudogap opening for metal superconductors is estimated.
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Coupled layered superconductor as a system of 2D Coulomb particles of two kinds
It is shown that the Josephson subsystem of the Lawrence-Doniach model of layered superconductors in the London approximation can be presented as a system with variable number of classical Coulomb particles. This allows us to consider the vortex system of a coupled layered superconductor as the system of these particles and 2D-vortices interacting with each other. The grand partition function of the system was written and transformed into the form of field one. Thermodynamical properties of the model obtained was studied. It is found that there is no a phase transition in the system. Instead of this the model demonstrates the crossover from a low temperature 3D behavior to high temperature 2D one which can look as a phase transition for experimental purposes.
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Flux saturation number of superconducting rings
The distributions of electrical current and magnetic field in a thin-film superconductor ring is calculated by solving the London equation. The maximum amount of flux trapped by the hole, the fluxoid saturation number, is obtained by limiting the current density by the depairing current. The results are compare it with similar results derived for the bulk case of a long hollow cylinder [Nordborg & Vinokur, Phys. Rev. B 62, 12408 (2000)]. In the limit of small holes our result reduces to the Pearl solution for an isolated vortex in a thin film. For large hole radius, the ratio between saturation numbers in bulk and film superconductors is proportional to the square root of the hole size.
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Inelastic X-ray Scattering Study of Phonons in Superconducting CaC6
We investigate the dispersion and temperature dependence of a number of phonons in the recently discovered superconductor CaC6 utilizing inelastic x-ray scattering. Four [00L] and two ab-plane phonon modes are observed, and measured at temperatures both above and below T_c. In general, our measurements of phonon dispersions are in good agreement with existing theoretical calculations of the phonon dispersion. This is significant in light of several discrepancies between experimental measurements of phonon-derived quantities and theoretical calculations. The present work suggests that the origin of these discrepancies lies in the understanding of the electron-phonon coupling in this material, rather than in the phonons themselves.
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Near-Zero Modes in Superconducting Graphene
Vortices in the simplest superconducting state of graphene contain very low energy excitations, whose existence is connected to an index theorem that applies strictly to an approximate form of the relevant Bogoliubov-deGennes equations. When Zeeman interactions are taken into account, the zero modes required by the index theorem are (slightly) displaced. Thus the vortices acquire internal structure, that plausibly supports interesting dynamical phenomena.
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Directional-dependent thermally activated motion of vortex bundles and theory of anomalous Hall effect in type-II conventional and high-Tc superconductors
The anomalous Hall effect for type-II conventional and high-Tc superconductors is studied based upon the theory of thermally activated motion of vortex bundles jumping over the directional-dependent energy barrier. It is shown that the Hall anomaly is universal for type-II conventional and high-Tc superconductors as well as for superconducting bulk materials and thin films, provided certain conditions are satisfied. We find that the directional-dependent potential barrier of the vortex bundles renormalizes the Hall and longitudinal resistivities, and Hall anomaly for superconductors is induced by the competition between the Magnus force and the random collective pinning force of the vortex bundle. We also find that the domain of anomalous Hall effect includes two regions: the region of thermally activated motion of the small vortex bundles and that of the large vortex bundles separated by the contour of the quasiorder-disorder first-order phase transition, or the peak effect of the vortex system. The Hall and longitudinal resistivities as functions of temperature as well as applied magnetic field have been calculated for type-II superconducting films and bulk materials. The conditions for occurring the double sign reversal or reentry phenomenon is also investigated. All the results are in agreement with the experiments.
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Flux line lattice state versus magnetism in electron-doped cuprate superconductor Sr_{1-x}La_{x}CuO_{2}
The microscopic details of flux line lattice state studied by muon spin rotation is reported in an electron-doped high-$T_{\rm c}$ cuprate superconductor, Sr$_{1-x}$La$_{x}$CuO$_{2}$ (SLCO, $x=0.10$--0.15). A clear sign of phase separation between magnetic and non-magnetic phases is observed, where the effective magnetic penetration depth [$λ\equivλ(T,H)$] is determined selectively for the latter phase. The extremely small value of $λ(0,0)$ %versus $T_{\rm c}$ and corresponding large superfluid density ($n_s \propto λ^{-2}$) is consistent with presence of a large Fermi surface with carrier density of $1+x$, which suggests the breakdown of the "doped Mott insulator" even at the "optimal doping" in SLCO. Moreover, a relatively weak anisotropy in the superconducting order parameter is suggested by the field dependence of $λ(0,H)$. These observations strongly suggest that the superconductivity in SLCO is of a different class from hole-doped cuprates.
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A mechanism for Fermi-surface-topology tuned superconductivity in the cuprates
Based on recent magnetic-quantum-oscillation, ARPES, neutron-scattering and other data, we propose that superconductivity in the cuprates occurs via a convenient matching of the spatial distribution of incommensurate spin fluctuations to the amplitude and phase of the $d_{x^2-y^2}$ Cooper-pair wavefunction; this establishes a robust causal relationship between the lengthscale of the fluctuations and the superconducting coherence length. It is suggested that the spin fluctuations are driven by the Fermi surface, which is prone to nesting; they couple to the itinerant holes via the on-site Coulomb correlation energy, which inhibits double occupancy of spins or holes. The maximum energy of the fluctuations gives an appropriate energy scale for the superconducting $T_{\rm c}$. Based on this model, one can specify the design of solids that will exhibit ``high $T_{\rm c}$'' superconductivity.
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Electron-phonon interaction alone does not explain the photoemission kink in cuprate superconductors
Despite over two decades of intense research efforts, the origin of high-temperature superconductivity in the copper oxides remains elusive. Angle-resolved photoemission experiments (ARPES) revealed a kink in the dispersion relations (energy vs. wavevector) of electronic states in the cuprates at binding energies of 50-80 meV, and raised the hope that this anomaly could be key to understanding high-temperature superconductivity. The kink is often interpreted in terms of interactions between the electrons and a bosonic field. While there is no consensus on the nature of the bosons or even whether a boson model is appropriate, phonons and spin fluctuations have alternatively been proposed as possible candidates. Here we report state-of-the-art first-principles calculations of the role of phonons and the electron-phonon interaction in the photoemission spectra of La2-xSrxCuO4 (LSCO). Our study demonstrates that the phonon-induced renormalization of the electron energies and the Fermi velocity is almost one order of magnitude smaller than the effect observed in photoemission experiments. Hence, the present finding rules out electron-phonon interaction in bulk LSCO as the possible origin of the measured kink. This result should bear on several proposed theories of high temperature superconductivity in addition to theories concerning the origin of kinks in cuprate photoemission data.
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Critical charge dynamics of superconducting LSCO thin films probed by complex microwave spectroscopy: Anomalous changes of the universality class by hole doping
We study the critical charge dynamics of the superconducting to the normal-state transition for LSCO thin films with a wide range of the Sr concentration, by measuring the frequency-dependent excess parts of the complex microwave conductivity, which is induced by the superconducting fluctuations. We present a dynamic scaling analysis of the complex fluctuation conductivity, which includes the information on the universality class and the dimensionality of the critical charge dynamics as a function of the Sr concentration, the film thickness and the magnetic field. In our previous study (H. Kitano et al., Phys. Rev. B 73, 092504 (2006).), the 2D-XY critical dynamics for underdoped LSCO and the 3D-XY critical dynamics for optimally doped LSCO were reported. In this study, we observed a novel two-dimensional unknown critical charge dynamics for overdoped thin films from x=0.17 to 0.20, which is clearly distinguished from the 2D-XY critical dynamics. Through the systematic measurements by changing the film thickness or by applying small magnetic field, it was confirmed that this unusual behavior, which is referred as 2D-"U" below, was not induced by the finite size effect but was intrinsic to the overdoped LSCO. Thus, it was found that the critical behavior in the phase diagram of LSCO is classified into the following three types; (i) 2D-XY for underdoped region, (ii) 3D-XY for optimally doped region, and (iii) 2D-"U" for overdoped region. In other words, the dimensionality in the critical charge dynamics is changed twice with hole doping. We discuss possible origins of such anomalous dimensional crossovers with hole doping, including an interpretation based on the possible existence of a hidden quantum critical point near the optimally doped region.
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Influence of the atomic scale inhomogeneity of the pair interaction on the local pair formation and density of states in high-T_c superconductors
The influence of the atomic-scale inhomogeneities of the pairing interaction on the superconducting order parameter distribution and the LDOS is studied in the framework of mean-field BCS theory for two-dimensional lattice model. It is found that the ratio of the local low-temperature gap in differential conductance to the local temperature of vanishing the gap $2Δ_g/T_p$ can take large enough values compared to the homogeneous case. This ratio practically does not depend on the location in the sample and is independent on the concentration of local pair interaction perturbations in wide range of concentrations. The obtained results could bear a relation to the recent measurements by Gomes {\it et. al} Nature, 447, 569 (2007).
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On the Luttinger theorem concerning number of particles in the ground states of systems of interacting fermions
We analyze the original proof by Luttinger and Ward of the Luttinger theorem, according to which for uniform ground states of systems of (interacting) fermions, which may be metallic or insulating, the number of k points corresponding to non-negative values of G_s(k;mu) is equal to the total number of particles with spin index s in these ground states. Here G_s(k;mu) is the single-particle Green function of particles with spin index s at the chemical potential mu. For the cases where the two-body interaction potential is short-range, and in particular for lattice models, we explicitly demonstrate that this theorem is unconditionally valid, irrespective of the strength of the bare interaction potential. We arrive at this conclusion by amongst other things demonstrating that the perturbation series expansion for self-energy in terms of skeleton diagrams, as encountered in the proof of the Luttinger-Ward identity, is uniformly convergent for almost all momenta and energies. We further investigate the mechanisms underlying some reported instances of failure of the Luttinger theorem. With one exception, for all the cases considered in this paper, we show that the apparent failures of the Luttinger theorem can be attributed either to shortcomings of the employed single-particle Green functions or to misapplication of this theorem. The one exceptional case brings to light the possibility of a genuine failure of the Luttinger theorem for insulating ground states, which we show to be brought about by a false limit that in principle can be reached on taking the zero-temperature limit without the value of mu coinciding with the zero-temperature limit of the chemical potential satisfying the equation of state at finite temperatures; no such ambiguity can arise for metallic states.
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Dynamical Symmetry Breaking of a Relativistic Model in Quasi-(1+1)-Dimensions. I. Formulation
The dynamical symmetry breaking in a quasi-(1+1)-dimensional relativistic model is investigated. The motions of particles in intrachain are described as a relativistic electron-hole gas, while the interchain hopping term is introduced as a 0th-component of vector in (1+1)-dimensions, a kind of chemical potential of the system. The gauge symmetry of the model is chosen as U(1) suitable for a possible situation of a real substance in condensed matter physics. We consider the BCS-type contact interactions for the s-wave fermion-pair condensates, while employ the nonlocal interactions of the generalized BCS framework to generate the $p$-, $d$- and $f$-wave condensations in the system. Especially we examine the dynamical generation of a Dirac mass term and superconductivity in the model. The phenomenon is interpreted as metal-insulator/metal-superconductor phase transitions.
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