Superconductivity from Undressing. II. Single Particle Green's Function and Photoemission in Cuprates

Experimental evidence indicates that the superconducting transition in high $T_c$ cuprates is an 'undressing' transition. Microscopic mechanisms giving rise to this physics were discussed in the first paper of this series. Here we discuss the calculation of the single particle Green's function and spectral function for Hamiltonians describing undressing transitions in the normal and superconducting states. A single parameter, $\Upsilon$, describes the strength of the undressing process and drives the transition to superconductivity. In the normal state, the spectral function evolves from predominantly incoherent to partly coherent as the hole concentration increases. In the superconducting state, the 'normal' Green's function acquires a contribution from the anomalous Green's function when $\Upsilon$ is non-zero; the resulting contribution to the spectral function is $positive$ for hole extraction and $negative$ for hole injection. It is proposed that these results explain the observation of sharp quasiparticle states in the superconducting state of cuprates along the $(\pi,0)$ direction and their absence along the $(\pi,\pi)$ direction.Comment: figures have been condensed in fewer pages for easier readin

Superconductivity from Undressing

Photoemission experiments in high $T_c$ cuprates indicate that quasiparticles are heavily 'dressed' in the normal state, particularly in the low doping regime. Furthermore these experiments show that a gradual undressing occurs both in the normal state as the system is doped and the carrier concentration increases, as well as at fixed carrier concentration as the temperature is lowered and the system becomes superconducting. A similar picture can be inferred from optical experiments. It is argued that these experiments can be simply understood with the single assumption that the quasiparticle dressing is a function of the local carrier concentration. Microscopic Hamiltonians describing this physics are discussed. The undressing process manifests itself in both the one-particle and two-particle Green's functions, hence leads to observable consequences in photoemission and optical experiments respectively. An essential consequence of this phenomenology is that the microscopic Hamiltonians describing it break electron-hole symmetry: these Hamiltonians predict that superconductivity will only occur for carriers with hole-like character, as proposed in the theory of hole superconductivity

Mixed triplet and singlet pairing in multicomponent ultracold fermion systems with dipolar interactions

The symmetry properties of the Cooper pairing problem for multi-component ultra-cold dipolar molecular systems are investigated. The dipolar anisotropy provides a natural and robust mechanism for both triplet and singlet Cooper pairing to first order in the interaction strength. With a purely dipolar interaction, the triplet $p_z$-like polar pairing is the most dominant. A short-range attractive interaction can enhance the singlet pairing to be nearly degenerate with the triplet pairing. We point out that these two pairing channels can mix by developing a relative phase of $\pm\frac{\pi}{2}$, thus spontaneously breaking time-reversal symmetry. We also suggest the possibility of such mixing of triplet and singlet pairing in other systems.Comment: accepted by Phys. Rev.

Compressible Flows in Fluidic Oscillators

We present qualitative observations on the internal flow characteristics of fluidic oscillator geometries commonly referred to as sweeping jets in active flow control applications. This is part of the fluid dynamics videos.Comment: Videos include

Proposal for a Supersymmetric Standard Model

The fact that neutrinos are massive suggests that the minimal supersymmetric standard model (MSSM) might be extended in order to include three gauge-singlet neutrino superfields with Yukawa couplings of the type $H_2 L \nu^c$. We propose to use these superfields to solve the $\mu$ problem of the MSSM without having to introduce an extra singlet superfield as in the case of the next-to-MSSM (NMSSM). In particular, terms of the type $\nu^c H_1 H_2$ in the superpotential may carry out this task spontaneously through sneutrino vacuum expectation values. In addition, terms of the type $(\nu^c)^3$ avoid the presence of axions and generate effective Majorana masses for neutrinos at the electroweak scale. On the other hand, these terms break lepton number and R-parity explicitly implying that the phenomenology of this model is very different from the one of the MSSM or NMSSM. For example, the usual neutralinos are now mixed with the neutrinos. For Dirac masses of the latter of order $10^{-4}$ GeV, eigenvalues reproducing the correct scale of neutrino masses are obtained.Comment: 9 pages, latex, title modified. Final version published in PR

Kinetic energy driven superconductivity, the origin of the Meissner effect, and the reductionist frontier

Is superconductivity associated with a lowering or an increase of the kinetic energy of the charge carriers? Conventional BCS theory predicts that the kinetic energy of carriers increases in the transition from the normal to the superconducting state. However, substantial experimental evidence obtained in recent years indicates that in at least some superconductors the opposite occurs. Motivated in part by these experiments many novel mechanisms of superconductivity have recently been proposed where the transition to superconductivity is associated with a lowering of the kinetic energy of the carriers. However none of these proposed unconventional mechanisms explores the fundamental reason for kinetic energy lowering nor its wider implications. Here I propose that kinetic energy lowering is at the root of the Meissner effect, the most fundamental property of superconductors. The physics can be understood at the level of a single electron atom: kinetic energy lowering and enhanced diamagnetic susceptibility are intimately connected. According to the theory of hole superconductivity, superconductors expel negative charge from their interior driven by kinetic energy lowering and in the process expel any magnetic field lines present in their interior. Associated with this we predict the existence of a macroscopic electric field in the interior of superconductors and the existence of macroscopic quantum zero-point motion in the form of a spin current in the ground state of superconductors (spin Meissner effect). In turn, the understanding of the role of kinetic energy lowering in superconductivity suggests a new way to understand the fundamental origin of kinetic energy lowering in quantum mechanics quite generally

Charge-density waves in the Hubbard chain: evidence for 4k_F instability

Charge density waves in the Hubbard chain are studied by means of finite-temperature Quantum Monte Carlo simulations and Lanczos diagonalizations for the ground state. We present results both for the charge susceptibilities and for the charge structure factor at densities \rho=1/6 and 1/3; for \rho=1/2 (quarter filled) we only present results for the charge structure factor. The data are consistent with a 4k_F instability dominating over the 2k_F one, at least for sufficiently large values of the Coulomb repulsion, U. This can only be reconciled with the Luttinger liquid analyses if the amplitude of the 2k_F contribution vanishes above some U^*(\rho).Comment: RevTeX, 4 two-column pages with 7 colour figures embedded in tex

Global Shifts in Agro-Industrial Capital and the Case of Soybean Crushing: Implications for Managers and Policy Makers

Tremendous shifts are occurring in the location of agro-industrial capital around the globe. To focus discussion on this topic a session was convened at the annual meeting of the International Food and Agribusiness Management Association in Montreux, Switzerland in June of 2004. The session brought together researchers and industry leaders to better understand these dramatic shifts and the implications they hold for the agri-food system. The following article emerges from that session. The first part of the article provides the context for the discussion by looking at global shifts in soybean processing investment. The second part entails reaction by three industry panelists.Soybeans, Processing, Investment, Global strategy, Agribusiness, Agricultural and Food Policy,

Loop algorithms for quantum simulations of fermion models on lattices

Two cluster algorithms, based on constructing and flipping loops, are presented for worldline quantum Monte Carlo simulations of fermions and are tested on the one-dimensional repulsive Hubbard model. We call these algorithms the loop-flip and loop-exchange algorithms. For these two algorithms and the standard worldline algorithm, we calculated the autocorrelation times for various physical quantities and found that the ordinary worldline algorithm, which uses only local moves, suffers from very long correlation times that makes not only the estimate of the error difficult but also the estimate of the average values themselves difficult. These difficulties are especially severe in the low-temperature, large-$U$ regime. In contrast, we find that new algorithms, when used alone or in combinations with themselves and the standard algorithm, can have significantly smaller autocorrelation times, in some cases being smaller by three orders of magnitude. The new algorithms, which use non-local moves, are discussed from the point of view of a general prescription for developing cluster algorithms. The loop-flip algorithm is also shown to be ergodic and to belong to the grand canonical ensemble. Extensions to other models and higher dimensions is briefly discussed.Comment: 36 pages, RevTex ver.

The angular spin current and its physical consequences

We find that in order to completely describe the spin transport, apart from spin current (or linear spin current), one has to introduce the angular spin current. The two spin currents respectively describe the translational and rotational motion of a spin. The definitions of these spin current densities are given and their physical properties are discussed. Both spin current densities appear naturally in the spin continuity equation. Moreover we predict that the angular spin current can also induce an electric field $\vec{E}$, and in particular $\vec{E}$ scales as $1/r^2$ at large distance $r$, whereas the $\vec{E}$ field generated from the linear spin current goes as $1/r^3$.Comment: 7 pages, 2 figure