5,198 research outputs found
Spin-Peierls instability in a quantum spin chain with Dzyaloshinskii-Moriya interaction
We analysed the ground state energy of some dimerized spin-1/2 transverse XX
and Heisenberg chains with Dzyaloshinskii-Moriya (DM) interaction to study the
influence of the latter interaction on the spin-Peierls instability. We found
that DM interaction may act either in favour of the dimerization or against it.
The actual result depends on the dependence of DM interaction on the distortion
amplitude in comparison with such dependence for the isotropic exchange
interaction.Comment: 12 pages, latex, 3 figure
Synthesis of Graphene on Gold
Here we report chemical vapor deposition of graphene on gold surface at
ambient pressure. We studied effects of the growth temperature, pressure and
cooling process on the grown graphene layers. The Raman spectroscopy of the
samples reveals the essential properties of the graphene grown on gold surface.
In order to characterize the electrical properties of the grown graphene
layers, we have transferred them on insulating substrates and fabricated field
effect transistors. Owing to distinctive properties of gold, the ability to
grow graphene layers on gold surface could open new applications of graphene in
electrochemistry and spectroscopy.Comment: 8 pages, 4 figure
The ground state properties of the spin-1/2 transverse Ising chain with periodically varying bonds and fields
Using continued fractions we study the ground state properties of the
spin-1/2 Ising chain in a transverse field with periodically varying
interaction strengths and external fields. We consider in detail the chain
having the period of modulation of interactions equals 2 and compare the
results obtained with those corresponding to the spin-1/2 isotropic XY chain in
a transverse field. In contrast to the behaviour of the transverse XY chain,
the transverse Ising chain does not exhibit a step-like magnetization vs. field
dependence caused by the alternation of bonds, its susceptibility exhibits a
logarithmic singularity at the field determined by interaction strengths, and
it is stable with respect to spin-Peierls dimerization.Comment: 11 pages, latex, 4 figure
Magnetic properties of the quantum spin-1/2 XX diamond chain: The Jordan-Wigner approach
The Jordan-Wigner transformation is applied to study magnetic properties of
the quantum spin-1/2 model on the diamond chain. Generally, the
Hamiltonian of this quantum spin system can be represented in terms of spinless
fermions in the presence of a gauge field and different gauge-invariant ways of
assigning the spin-fermion transformation are considered. Additionally, we
analyze general properties of a free-fermion chain, where all gauge terms are
neglected and discuss their relevance for the quantum spin system. A
consideration of interaction terms in the fermionic Hamiltonian rests upon the
Hartree-Fock procedure after fixing the appropriate gauge. Finally, we discuss
the magnetic properties of this quantum spin model at zero as well as non-zero
temperatures and analyze the validity of the approximation used through a
comparison with the results of the exact diagonalization method for finite (up
to 36 spins) chains. Besides the plateau the most prominent feature of
the magnetization curve is a jump at intermediate field present for certain
values of the frustrating vertical bond.Comment: 12 pages, 9 figures, accepted for publication in Eur. Phys. J.
Regularly alternating spin-1/2 anisotropic XY chains: The ground-state and thermodynamic properties
Using the Jordan-Wigner transformation and continued fractions we calculate
rigorously the thermodynamic quantities for the spin-1/2 transverse Ising chain
with periodically varying intersite interactions and/or on-site fields. We
consider in detail the properties of the chains having a period of the
transverse field modulation equal to 3. The regularly alternating transverse
Ising chain exhibits several quantum phase transition points, where the number
of transition points for a given period of alternation strongly depends on the
specific set of the Hamiltonian parameters. The critical behavior in most cases
is the same as for the uniform chain. However, for certain sets of the
Hamiltonian parameters the critical behavior may be changed and weak
singularities in the ground-state quantities appear. Due to the regular
alternation of the Hamiltonian parameters the transverse Ising chain may
exhibit plateau-like steps in the zero-temperature dependence of the transverse
magnetization vs. transverse field and many-peak temperature profiles of the
specific heat. We compare the ground-state properties of regularly alternating
transverse Ising and transverse XX chains and of regularly alternating quantum
and classical chains.
Making use of the corresponding unitary transformations we extend the
elaborated approach to the study of thermodynamics of regularly alternating
spin-1/2 anisotropic XY chains without field. We use the exact expression for
the ground-state energy of such a chain of period 2 to discuss how the exchange
interaction anisotropy destroys the spin-Peierls dimerized phase
Band-Insulator-Metal-Mott-Insulator transition in the half--filled ionic-Hubbard chain
We investigate the ground state phase diagram of the half-filled
repulsive Hubbard model in the presence of a staggered ionic
potential , using the continuum-limit bosonization approach. We find,
that with increasing on-site-repulsion , depending on the value of the
next-nearest-hopping amplitude , the model shows three different
versions of the ground state phase diagram. For , the ground state phase diagram consists of the following
three insulating phases: Band-Insulator at , Ferroelectric Insulator
at . For
there is only one transition from a spin gapped
metallic phase at .
Finally, for intermediate values of the next-nearest-hopping amplitude
we find that with increasing
on-site repulsion, at the model undergoes a second-order
commensurate-incommensurate type transition from a band insulator into a
metallic state and at larger there is a Kosterlitz-Thouless type
transition from a metal into a ferroelectric insulator.Comment: 9 pages 3 figure
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