3,482 research outputs found
Quantum Orders and Symmetric Spin Liquids
A concept -- quantum order -- is introduced to describe a new kind of orders
that generally appear in quantum states at zero temperature. Quantum orders
that characterize universality classes of quantum states (described by {\em
complex} ground state wave-functions) is much richer then classical orders that
characterize universality classes of finite temperature classical states
(described by {\em positive} probability distribution functions). The Landau's
theory for orders and phase transitions does not apply to quantum orders since
they cannot be described by broken symmetries and the associated order
parameters. We find projective representations of symmetry groups (which will
be called projective symmetry groups) can be used to characterize quantum
orders. With the help of quantum orders and the projective symmetry groups, we
construct hundreds of symmetric spin liquids, which have SU(2), U(1) or
gauge structures at low energies. Remarkably, some of the stable quantum phases
support gapless excitations even without any spontaneous symmetry breaking. We
propose that it is the quantum orders (instead of symmetries) that protect the
gapless excitations and make algebraic spin liquids and Fermi spin liquids
stable. Since high superconductors are likely to be described by a
gapless spin liquid, the quantum orders and their projective symmetry group
descriptions lay the foundation for spin liquid approach to high
superconductors.Comment: 58 pages, RevTeX4 home page: http://dao.mit.edu/~we
Singlets and reflection symmetric spin systems
We rigorously establish some exact properties of reflection symmetric spin
systems with antiferromagnetic crossing bonds: At least one ground state has
total spin zero and a positive semidefinite coefficient matrix. The crossing
bonds obey an ice rule. This augments some previous results which were limited
to bipartite spin systems and is of particular interest for frustrated spin
systems.Comment: 11 pages, LaTeX 2
Electron beams of cylindrically symmetric spin polarization
Cylindrically symmetric electron beams in spin polarization are reported for
the first time. They are shown to be the eigen states of total angular momentum
in the direction. But they are neither the eigen states of spin nor the
eigen states of orbital angular momentum in that direction.Comment: 10 pages and 2 figure
Collective magnetic excitations of symmetric magnetic states in iron-based superconductors
We study the collective magnetic excitations of the recently discovered
symmetric spin-density wave states of iron-based superconductors with
particular emphasis on their orbital character based on an itinerant
multiorbital approach. This is important since the symmetric
spin-density wave states exist only at moderate interaction strengths where
damping effects from a coupling to the continuum of particle-hole excitations
strongly modifies the shape of the excitation spectra compared to predictions
based on a local moment picture. We uncover a distinct orbital polarization
inherent to magnetic excitations in symmetric states, which provide a
route to identify the different commensurate magnetic states appearing in the
continuously updated phase diagram of the iron-pnictide family.Comment: 5+7 pages, 3+2 figure
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