796 research outputs found
Neutron isotope reactions
The field of low temperature nuclear reactions has made slow but steady progress. Evidence has accumulated for production of energy at the level of a few watts, for production of helium in proportion to energy, for energetic particles, and for transmutations of elements. But there is no generally accepted theory for these phenomena. Progress requires a body of experimental evidence and a candidate theory through which theory and experiment can gain mutual support and acceptance. We explore the possibility that transfer of neutrons from neutron isotopes to ordinary nuclei, followed by beta decay of the neutron-enriched nuclei, facilitates a class of low-temperature transmutations. We have tested this possibility by comparing the implications of neutron isotope theory with the transmutations reported by Iwamura and associates. We find that experiment quantifies and supports the theory, and that theory clarifies and supports the experimental observations
Screening effects in superconductors
The partition function of the Hubbard model with local attraction and long
range Coulomb repulsion between electrons is written as a functional integral
with an action involving a pairing field and a local potential
. After integration over and over fluctuations in , the
final form of involves a Josephson coupling between the local phases of
and a "kinetic energy" term, representing the screened Coulomb
interaction between charge fluctuations. The competition between Josephson
coupling and charging energy allows to understand the relation between
and composition in high- materials, in particular superlattices, alloys
and bulk systems of low doping.Comment: 4 pages, revtex, no figures, submitted to Physica B (Proceedings of
SCES '96 International Conference, held in Zurich from 19th to 21st of
August
The Roton Fermi Liquid
We introduce and analyze a novel metallic phase of two-dimensional (2d)
electrons, the Roton Fermi Liquid (RFL), which, in contrast to the Landau Fermi
liquid, supports both gapless fermionic and bosonic quasiparticle excitations.
The RFL is accessed using a re-formulation of 2d electrons consisting of
fermionic quasiparticles and vortices interacting with a mutual
long-ranged statistical interaction. In the presence of a strong
vortex-antivortex (i.e. roton) hopping term, the RFL phase emerges as an exotic
yet eminently tractable new quantum ground state. The RFL phase exhibits a
``Bose surface'' of gapless roton excitations describing transverse current
fluctuations, has off-diagonal quasi-long-ranged order (ODQLRO) at zero
temperature (T=0), but is not superconducting, having zero superfluid density
and no Meissner effect. The electrical resistance {\it vanishes} as
with a power of temperature (and frequency), (with ), independent of the impurity concentration. The RFL phase also has a full
Fermi surface of quasiparticle excitations just as in a Landau Fermi liquid.
Electrons can, however, scatter anomalously from rotonic "current
fluctuations'' and "superconducting fluctuations'', leading to "hot" and "cold"
spots. Fermionic quasiparticles dominate the Hall electrical transport. We also
discuss instabilities of the RFL to a conventional Fermi liquid and a
superconductor. Precisely {\it at} the instability into the Fermi liquid state,
the exponent , so that . Upon entering the
superconducting state the anomalous quasiparticle scattering is strongly
suppressed. We discuss how the RFL phenomenology might apply to the cuprates.Comment: 43 page
Phenomenological lattice model for dynamic spin and charge fluctuations in the cuprates
Motivated by recent neutron scattering experiments on the cuprate
superconductors, we present a phenomenological framework describing the
dynamics of collective spin excitations coupled to charge/bond order
fluctuations. Our quantum lattice model contains two order parameter fields,
and can capture spin excitations both in broken-symmetry states with static
lattice modulations, as well as in homogeneous states where the charge/bond
order is fluctuating. We present results for different types of static
charge/bond order, namely site- and bond-centered stripes, and plaquette
modulation.Comment: 5 pages, 3 figures; Proceedings of SNS 2004, Sitges, Spai
The types of Mott insulator
There are two classes of Mott insulators in nature, distinguished by their
responses to weak doping. With increasing chemical potential, Type I Mott
insulators undergo a first order phase transition from the undoped to the doped
phase. In the presence of long-range Coulomb interactions, this leads to an
inhomogeneous state exhibiting ``micro-phase separation.'' In contrast, in Type
II Mott insulators charges go in continuously above a critical chemical
potential. We show that if the insulating state has a broken symmetry, this
increases the likelihood that it will be Type I. There exists a close analogy
between these two types of Mott insulators and the familiar Type I and Type II
superconductors
Alternative route to charge density wave formation in multiband systems
Charge and spin density waves, periodic modulations of the electron and
magnetization densities, respectively, are among the most abundant and
non-trivial low-temperature ordered phases in condensed matter. The ordering
direction is widely believed to result from the Fermi surface topology.
However, several recent studies indicate that this common view needs to be
supplemented. Here, we show how an enhanced electron-lattice interaction can
contribute to or even determine the selection of the ordering vector in the
model charge density wave system ErTe3. Our joint experimental and theoretical
study allows us to establish a relation between the selection rules of the
electronic light scattering spectra and the enhanced electron-phonon coupling
in the vicinity of band degeneracy points. This alternative proposal for charge
density wave formation may be of general relevance for driving phase
transitions into other broken-symmetry ground states, particularly in multiband
systems such as the iron based superconductors
One-dimensional Kondo lattice at partial band filling
An effective Hamiltonian for the localized spins in the one-dimensional Kondo
lattice model is derived via a unitary transformation involving a bosonization
of delocalized conduction electrons. The effective Hamiltonian is shown to
reproduce all the features of the model as identified in various numerical
simulations, and provides much new information on the ferro- to paramagnetic
phase transition and the paramagnetic phase.Comment: 11 pages Revtex, 1 Postscript figure. To appear in Phys. Rev. Let
Quantum vortex fluctuations in cuprate superconductors
We study the effects of quantum vortex fluctuations in two-dimensional
superconductors using a dual theory of vortices, and investigate the relevance
to underdoped cuprates where the superconductor-insulator transition (SIT) is
possibly driven by quantum vortex proliferation. We find that a broad enough
phase fluctuation regime may exist for experimental observation of the quantum
vortex fluctuations near SIT in underdoped cuprates. We propose that this
scenario can be tested via pair-tunneling experiments which measure the
characteristic resonances in the zero-temperature pair-field susceptibility in
the vortex-proliferated insulating phase.Comment: RevTex 5 pages, 2 eps figures; expanded; to appear in Phys. Rev.
A Duality Between Unidirectional Charge Density Wave Order and Superconductivity
This paper shows the existence of a duality between an unidirectional charge
density wave order and a superconducting order. This duality predicts the
existence of charge density wave near a superconducting vortex, and the
existence of superconductivity near a charge density wave dislocation.Comment: Main results are the same, but the presentation is significantly
modified. To appear in Physical Review Letter
- …