847 research outputs found
Phase Diagram of the spin S=1/2 Extended XY model
The quantum phase transition in the ground state of the Extended spin S=1/2
XY model is studied in detail. Using the exact solution of the model the low
temperature thermodynamics, as well as the ground state phase diagram of the
model in the presence of applied uniform and/or staggered magnetic field are
discussed.Comment: 12 pages, 12 figure
Phase Diagram of the Extended Hubbard Model with Pair Hopping Interaction
A one-dimensional model of interacting electrons with on-site ,
nearest-neighbor , and pair-hopping interaction is studied at
half-filling using the continuum limit field theory approach. The ground state
phase diagram is obtained for a wide range of coupling constants. In addition
to the insulating spin- and charge-density wave phases for large and ,
respectively, we identify bond-located ordered phases corresponding to an
enhanced Peierls instability in the system for , , and to a
staggered magnetization located on bonds between sites for , .
The general ground state phase diagram including insulating, metallic, and
superconducting phases is discussed. A transition to the
-superconducting phase at is briefly
discussed.Comment: 6 pages in revtex format, 2 fig files in ep
The taming of recurrences in computability logic through cirquent calculus, Part I
This paper constructs a cirquent calculus system and proves its soundness and
completeness with respect to the semantics of computability logic (see
http://www.cis.upenn.edu/~giorgi/cl.html). The logical vocabulary of the system
consists of negation, parallel conjunction, parallel disjunction, branching
recurrence, and branching corecurrence. The article is published in two parts,
with (the present) Part I containing preliminaries and a soundness proof, and
(the forthcoming) Part II containing a completeness proof
Magnetization plateau in the S=1/2 spin ladder with alternating rung exchange
We have studied the ground state phase diagram of a spin ladder with
alternating rung exchange in
a magnetic filed, in the limit where the rung coupling is dominant. In this
limit the model is mapped onto an Heisenberg chain in a uniform and
staggered longitudinal magnetic fields, where the amplitude of the staggered
field is . We have shown that the magnetization curve of the
system exhibits a plateau at magnetization equal to the half of the saturation
value. The width of a plateau scales as , where in the
case of ladder with isotropic antiferromagnetic legs and in the case
of ladder with isotropic ferromagnetic legs. We have calculated four critical
fields ( and ) corresponding to transitions between
different magnetic phases of the system. We have shown that these transitions
belong to the universality class of the commensurate-incommensurate transition.Comment: 6 pages, 2 figure
Competing effects of interactions and spin-orbit coupling in a quantum wire
We study the interplay of electron-electron interactions and Rashba
spin-orbit coupling in one-dimensional ballistic wires. Using the
renormalization group approach we construct the phase diagram in terms of
Rashba coupling, Tomonaga-Luttinger stiffness and backward scattering strength.
We identify the parameter regimes with a dynamically generated spin gap and
show where the Luttinger liquid prevails. We also discuss the consequences for
the operation of the Datta-Das transistor.Comment: 4 pages, 2 figure
Triplet superconductivity in a 1D itinerant electron system with transverse spin anisotropy
In this paper we study the ground state phase diagram of a one-dimensional
t-J-U model away from half-filling. In the large-bandwidth limit and for
ferromagnetic exchange with easy-plane anisotropy a phase with gapless charge
and massive spin excitations, characterized by the coexistence of triplet
superconducting and spin density wave instabilities is realized in the ground
state. With increasing ferromagnetic exchange transitions into a ferrometallic
and then a spin gapped triplet superconducting phase take place.Comment: 11 pages, 10 figures, accepted for publication in Eur. Phys. J.
Dislocation-mediated melting of one-dimensional Rydberg crystals
We consider cold Rydberg atoms in a one-dimensional optical lattice in the
Mott regime with a single atom per site at zero temperature. An external laser
drive with Rabi frequency \Omega and laser detuning \Delta, creates Rydberg
excitations whose dynamics is governed by an effective spin-chain model with
(quasi) long-range interactions. This system possesses intrinsically a large
degree of frustration resulting in a ground-state phase diagram in the
(\Delta,\Omega) plane with a rich topology. As a function of \Delta, the
Rydberg blockade effect gives rise to a series of crystalline phases
commensurate with the optical lattice that form a so-called devil's staircase.
The Rabi frequency, \Omega, on the other hand, creates quantum fluctuations
that eventually lead to a quantum melting of the crystalline states. Upon
increasing \Omega, we find that generically a commensurate-incommensurate
transition to a floating Rydberg crystal occurs first, that supports gapless
phonon excitations. For even larger \Omega, dislocations within the floating
Rydberg crystal start to proliferate and a second,
Kosterlitz-Thouless-Nelson-Halperin-Young dislocation-mediated melting
transition finally destroys the crystalline arrangement of Rydberg excitations.
This latter melting transition is generic for one-dimensional Rydberg crystals
and persists even in the absence of an optical lattice. The floating phase and
the concomitant transitions can, in principle, be detected by Bragg scattering
of light.Comment: 21 pages, 9 figures; minor changes, published versio
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