12 research outputs found
Boundary contributions to specific heat and susceptibility in the spin-1/2 XXZ chain
Exact low-temperature asymptotic behavior of boundary contribution to
specific heat and susceptibility in the one-dimensional spin-1/2 XXZ model with
exchange anisotropy 1/2 < \Delta \le 1 is analytically obtained using the
Abelian bosonization method. The boundary spin susceptibility is divergent in
the low-temperature limit. This singular behavior is caused by the first-order
contribution of a bulk leading irrelevant operator to boundary free energy. The
result is confirmed by numerical simulations of finite-size systems. The
anomalous boundary contributions in the spin isotropic case are universal.Comment: 6 pages, 3 figures; corrected typo
Triplet superconductivity in a one-dimensional ferromagnetic t-J model
In this paper we study the ground state phase diagram of a one-dimensional
model, at 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 reduction of the bandwidth, a transition
into an insulating phase showing properties of the spin-1/2 XY model takes
place. In the case of weakly anisotropic antiferromagnetic exchange the system
shows a long range dimerized (Peierls) ordering in the ground state. The
complete weak-coupling phase diagram of the model, including effects of the
on-site Hubbard interaction, is obtained
Alternative Technique for "Complex" Spectra Analysis
. The choice of a suitable random matrix model of a complex system is very
sensitive to the nature of its complexity. The statistical spectral analysis of
various complex systems requires, therefore, a thorough probing of a wide range
of random matrix ensembles which is not an easy task. It is highly desirable,
if possible, to identify a common mathematcal structure among all the ensembles
and analyze it to gain information about the ensemble- properties. Our
successful search in this direction leads to Calogero Hamiltonian, a
one-dimensional quantum hamiltonian with inverse-square interaction, as the
common base. This is because both, the eigenvalues of the ensembles, and, a
general state of Calogero Hamiltonian, evolve in an analogous way for arbitrary
initial conditions. The varying nature of the complexity is reflected in the
different form of the evolution parameter in each case. A complete
investigation of Calogero Hamiltonian can then help us in the spectral analysis
of complex systems.Comment: 20 pages, No figures, Revised Version (Minor Changes
\eta-superconductivity in the Hubbard chain with pair hopping
The ground state phase diagram of the 1D Hubbard chain with pair-hopping
interaction is studied. The analysis of the model is performed using the
continuum-limit field theory approach and exact diagonalization studies. At
half-filling the phase diagram is shown to consist of two superconducting
states with Cooper pair center-of-mass momentum Q=0 (BCS-\eta_0 phase) and
Q=\pi (\eta_\pi-phase) and four insulating phases corresponding to the Mott
antiferromagnet, the Peierls dimerized phase, the charge-density-wave (CDW)
insulator as well as an unconventional insulating phase characterized by the
coexistence of a CDW and a bond-located staggered magnetization. Away from
half-filling the phase diagram consists of the superconducting BCS-\eta_0 and
\eta_\pi phases and the metallic Luttinger-liquid phase. The BCS-\eta_0 phase
exhibits smooth crossover from a weak-coupling BCS type to a strong coupling
local-pair regime. The \eta_\pi phase shows properties of the doublon (zero
size Cooper pair) superconductor with Cooper pair center-of-mass momentum
Q=\pi. The transition into the \eta_\pi- paired state corresponds to an abrupt
change in the groundstate structure. After the transition the conduction band
is completely destroyed and a new \eta_\pi-pair band corresponding to the
strongly correlated doublon motion is created.Comment: 15 pages Revtex, 15 embedded eps figure
Spinon and η -spinon correlation functions of the Hubbard chain
We calculate real-space static correlation functions of spin and charge degrees of freedom of the
one-dimensional Hubbard model that are described by operators related to singly occupied sites with spin
up or spin down (spinons) and unoccupied or doubly occupied sites ( η -spinons). The spatial decay of their
correlation functions is determined using density matrix renormalization group results. The nature and
spatial extent of the correlations between two sites on the Hubbard chain is studied using the eigenstates
and eigenvalues of the two-site reduced density matrix. The results show that the spinon-spinon correlation
functions decay algebraically and the η -spinon correlation functions decay exponentially, both in the half-
filling and metallic phases. The results provide evidence that these degrees of freedom are organized in
boundstates in the interacting system.Portuguese FCT both in the frame-
work of the Strategic Project PEST-C/FIS/UI607/2011 and
under SFRH/BSAB/1177 /201