131 research outputs found
Variational and DMRG studies of the Frustrated Antiferromagnetic Heisenberg S=1 Quantum Spin Chain
We study a frustrated antiferromagnetic isotropic Heisenberg chain
using a variational ansatz and the DMRG. At , there is a
disorder point of the second kind, marking the onset of incommensurate
correlations in the chain. At there is a Lifshitz point,
at which the excitation spectrum develops a doubly degenerate structure. These
points are the quantum remnants of the transition from antiferromagnetic to
spiral order in the classical frustrated chain. At there
is a first order phase transition from an AKLT phase to a next-nearest neighbor
generalization of the AKLT model. At the transition, the string order parameter
shows a discontinuous jump of 0.085 to 0; the correlation length and the gap
are both finite at the transition. The problem of edge states in open
frustrated chains is discussed at length.Comment: 37 pages, 14 figures, submitted to Phys.Rev.
Quantum Phase Transitions in the One-Dimensional S=1 Spin-Orbital Model: Implications for Cubic Vanadates
We investigate ground-state properties and quantum phase transitions in the
one-dimensional S=1 spin-orbital model relevant to cubic vanadates. Using the
density matrix renormalization group, we compute the ground-state energy, the
magnetization and the correlation functions for different values of the Hund's
coupling and the external magnetic field. It is found that the
magnetization jumps at a certain critical field, which is a hallmark of the
field-induced first-order phase transition. The phase transition driven by
is also of first order. We also consider how the lattice-induced
ferro-type interaction between orbitals modifies the phase diagram, and discuss
the results in a context of the first-order phase transition observed in
YVO at 77K.Comment: 7 pages, 7 figur
A Unified Algebraic Approach to Few and Many-Body Correlated Systems
The present article is an extended version of the paper {\it Phys. Rev.} {\bf
B 59}, R2490 (1999), where, we have established the equivalence of the
Calogero-Sutherland model to decoupled oscillators. Here, we first employ the
same approach for finding the eigenstates of a large class of Hamiltonians,
dealing with correlated systems. A number of few and many-body interacting
models are studied and the relationship between their respective Hilbert
spaces, with that of oscillators, is found. This connection is then used to
obtain the spectrum generating algebras for these systems and make an algebraic
statement about correlated systems. The procedure to generate new solvable
interacting models is outlined. We then point out the inadequacies of the
present technique and make use of a novel method for solving linear
differential equations to diagonalize the Sutherland model and establish a
precise connection between this correlated system's wave functions, with those
of the free particles on a circle. In the process, we obtain a new expression
for the Jack polynomials. In two dimensions, we analyze the Hamiltonian having
Laughlin wave function as the ground-state and point out the natural emergence
of the underlying linear symmetry in this approach.Comment: 18 pages, Revtex format, To appear in Physical Review
Damage measurements on the NWTC direct-drive, Variable-Speed Test Bed
The NWTC (National Wind Technology Center) Variable-Speed Test Bed turbine is a three-bladed, 10-meter, downwind machine that can be run in either fixed-speed or variable-speed mode. In the variable-speed mode, the generator torque is regulated, using a discrete-stepped load bank to maximize the turbine`s power coefficient. At rated power, a second control loop that uses blade pitch to maintain rotor speed essentially as before, i.e., using the load bank to maintain either generator power or (optionally) generator torque. In this paper, the authors will use this turbine to study the effect of variable-speed operation on blade damage. Using time-series data obtained from blade flap and edge strain gauges, the load spectrum for the turbine is developed using rainflow counting techniques. Miner`s rule is then used to determine the damage rates for variable-speed and fixed-speed operation. The results illustrate that the controller algorithm used with this turbine introduces relatively large load cycles into the blade that significantly reduce its service lifetime, while power production is only marginally increased
Using the Wigner-Ibach Surmise to Analyze Terrace-Width Distributions: History, User's Guide, and Advances
A history is given of the applications of the simple expression generalized
from the surmise by Wigner and also by Ibach to extract the strength of the
interaction between steps on a vicinal surface, via the terrace width
distribution (TWD). A concise guide for use with experiments and a summary of
some recent extensions are provided.Comment: 11 pages, 4 figures, reformatted (with revtex) version of refereed
paper for special issue of Applied Physics A entitled "From Surface Science
to Device Physics", in honor of the retirements of Prof. H. Ibach and Prof.
H. L\"ut
Duality and quasiparticles in the Calogero-Sutherland model: Some exact results
The quantum-mechanical many-body system with the potential proportional to
the pairwise inverse-square distance possesses a strong-weak coupling duality.
Based on this duality, particle and/or quasiparticle states are described as
SU(1,1) coherent states. The constructed quasiparticle states are of
hierarchical nature.Comment: RevTeX, 10 page
From nodal liquid to nodal Mottness in a frustrated Hubbard model
We investigate the physics of frustrated 3-leg Hubbard ladders in the band
limit, when hopping across the ladder's rungs (t) is of the same
order as hopping along them (t) much greater than the onsite Coulomb repulsion
(U). We show that this model exhibits a striking electron-hole asymmetry close
to half-filling: the hole-doped system at low temperatures develops a
Resonating Valence Bond (RVB)-like d-wave gap (pseudogap close to (,0))
coinciding with gapless nodal excitations (nodal liquid); in contrast, the
electron-doped system is seen to develop a Mott gap at the nodes, whilst
retaining a metallic character of its majority Fermi surface. At lower
temperatures in the electron-doped case, d-wave superconducting correlations --
here, coexisting with gapped nodal excitations -- are already seen to arise.
Upon further doping the hole-doped case, the RVB-like state yields to d-wave
superconductivity. Such physics is reminiscent of that exhibited by the high
temperature cuprate superconductors--notably electron-hole asymmetry as noted
by Angle Resolved PhotoEmission Spectroscopy (ARPES) and the resistivity
exponents observed. This toy model also reinforces the importance of a more
thorough experimental investigation of the known 3-leg ladder cuprate systems,
and may have some bearing on low dimensional organic superconductors.Comment: 26 pages, 16 figure
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