901 research outputs found
Duality and the vibrational modes of a Cooper-pair Wigner crystal
When quantum fluctuations in the phase of the superconducting order parameter
destroy the off-diagonal long range order, duality arguments predict the
formation of a Cooper pair crystal. This effect is thought to be responsible
for the static checkerboard patterns observed recently in various underdoped
cuprate superconductors by means of scanning tunneling spectroscopy. Breaking
of the translational symmetry in such a Cooper pair Wigner crystal may, under
certain conditions, lead to the emergence of low lying transverse vibrational
modes which could then contribute to thermodynamic and transport properties at
low temperatures. We investigate these vibrational modes using a continuum
version of the standard vortex-boson duality, calculate the speed of sound in
the Cooper pair Wigner crystal and deduce the associated specific heat and
thermal conductivity. We then suggest that these modes could be responsible for
the mysterious bosonic contribution to the thermal conductivity recently
observed in strongly underdoped ultraclean single crystals of YBCO tuned across
the superconductor-insulator transition.Comment: 14 pages; 3 figures; corrected the sample size value; version 3 to
appear in PR
Anyons in integer quantum Hall magnets
Strongly correlated fractional quantum Hall liquids support fractional
excitations, which can be understood in terms of adiabatic flux insertion
arguments. A second route to fractionalization is through the coupling of
weakly interacting electrons to topologically nontrivial backgrounds such as in
polyacetylene. Here we demonstrate that electronic fractionalization combining
features of both these mechanisms occurs in noncoplanar itinerant magnetic
systems, where integer quantum Hall physics arises from the coupling of
electrons to the magnetic background. The topologically stable magnetic
vortices in such systems carry fractional (in general irrational) electronic
quantum numbers and exhibit Abelian anyonic statistics. We analyze the
properties of these topological defects by mapping the distortions of the
magnetic texture onto effective non-Abelian vector potentials. We support our
analytical results with extensive numerical calculations.Comment: 15 pages, 12 figures, supersedes arXiv:1112.3347, to be published in
PR
An SU(2) Formulation of the t-J model: Application to Underdoped Cuprates
We develop a slave-boson theory for the t-J model at finite doping which
respect a SU(2) symmetry -- a symmetry previously known to be important at half
filling. The mean field phase diagram is found to be consistent with the phases
observed in the cuprate superconductors, which contains d-wave superconductor,
spin gap, strange metal, and Fermi liquid phases. The spin gap phase is best
understood as the staggered flux phase, which is nevertheless translationally
invariant for physical quantities. The physical electron spectral function
shows small Fermi segments at low doping which continuously evolve into the
large Fermi surface at high doping concentrations. The close relation between
the SU(2) and the U(1) slave-boson theory is discussed. The low energy
effective theory for the low lying fluctuations is derived, and new lying modes
(which were over looked in the U(1) theory) are identified.Comment: 28 pages, 8 figures, RevTe
Quantum Field Theory for the Three-Body Constrained Lattice Bose Gas -- Part II: Application to the Many-Body Problem
We analyze the ground state phase diagram of attractive lattice bosons, which
are stabilized by a three-body onsite hardcore constraint. A salient feature of
this model is an Ising type transition from a conventional atomic superfluid to
a dimer superfluid with vanishing atomic condensate. The study builds on an
exact mapping of the constrained model to a theory of coupled bosons with
polynomial interactions, proposed in a related paper [11]. In this framework,
we focus by analytical means on aspects of the phase diagram which are
intimately connected to interactions, and are thus not accessible in a mean
field plus spin wave approach. First, we determine shifts in the mean field
phase border, which are most pronounced in the low density regime. Second, the
investigation of the strong coupling limit reveals the existence of a new
collective mode, which emerges as a consequence of enhanced symmetries in this
regime. Third, we show that the Ising type phase transition, driven first order
via the competition of long wavelength modes at generic fillings, terminates
into a true Ising quantum critical point in the vicinity of half filling.Comment: 22 pages, 5 figure
Advanced single permanent magnet axipolar ironless stator ac motor for electric passenger vehicles
A program was conducted to design and develop an advanced-concept motor specifically created for propulsion of electric vehicles with increased range, reduced energy consumption, and reduced life-cycle costs in comparison with conventional systems. The motor developed is a brushless, dc, rare-earth cobalt, permanent magnet, axial air gap inductor machine that uses an ironless stator. Air cooling is inherent provided by the centrifugal-fan action of the rotor poles. An extensive design phase was conducted, which included analysis of the system performance versus the SAE J227a(D) driving cycle. A proof-of-principle model was developed and tested, and a functional model was developed and tested. Full generator-level testing was conducted on the functional model, recording electromagnetic, thermal, aerodynamic, and acoustic noise data. The machine demonstrated 20.3 kW output at 1466 rad/s and 160 dc. The novel ironless stator demonstated the capability to continuously operate at peak current. The projected system performance based on the use of a transistor inverter is 23.6 kW output power at 1466 rad/s and 83.3 percent efficiency. Design areas of concern regarding electric vehicle applications include the inherently high windage loss and rotor inertia
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