738 research outputs found
Electron-phonon coupling and spin-charge separation in one-dimensional Mott insulators
We examine the single-particle excitation spectrum in the one-dimensional
Hubbard-Holstein model at half-filling by performing the dynamical density
matrix renormalization group (DDMRG) calculation. The DDMRG results are
interpreted as superposition of spectra for a spinless carrier dressed with
phonons. The superposition is a consequence of robustness of the spin-charge
separation against electron-phonon coupling. The separation is in contrast to
the coupling between phonon and spin degrees of freedom in two-dimensional
systems. We discuss implication of the results of the recent angle-resolved
photoemission spectroscopy measurements on SrCuO.Comment: 5 pages, 4 figures. submitted to the Physical Review Letter
Critical behavior of the S=3/2 antiferromagnetic Heisenberg chain
Using the density-matrix renormalization-group technique we study the
long-wavelength properties of the spin S=3/2 nearest-neighbor Heisenberg chain.
We obtain an accurate value for the spin velocity v=3.8+- 0.02, in agreement
with experiment. Our results show conclusively that the model belongs to the
same universality class as the S=1/2 Heisenberg chain, with a conformal central
charge c=1 and critical exponent eta=1Comment: RevTeX (version 3.0), 4 twocolumn pages with 4 embedded figure
spl(2,1) dynamical supersymmetry and suppression of ferromagnetism in flat band double-exchange models
The low energy spectrum of the ferromagnetic Kondo lattice model on a N-site
complete graph extended with on-site repulsion is obtained from the underlying
spl(2,1) algebra properties in the strong coupling limit. The ferromagnetic
ground state is realized for 1 and N+1 electrons only. We identify the large
density of states to be responsible for the suppression of the ferromagnetic
state and argue that a similar situation is encountered in the Kagome,
pyrochlore, and other lattices with flat bands in their one-particle density of
states.Comment: 7 pages, 1 figur
Cognitive predictors and risk factors of PTSD following stillbirth: a short-term longitudinal study.
This short-term longitudinal study investigated cognitive predictors and risk factors of posttraumatic stress disorder (PTSD) in mothers following stillbirth. After a stillbirth at ≥ 24 weeks gestational age, 65 women completed structured clinical interviews and questionnaires assessing PTSD symptoms, cognitive predictors (appraisals, dysfunctional strategies), and risk factors (perceived social support, trauma history, obstetric history) at 3 and 6 months. PTSD symptoms decreased between 3 and 6 months (Cohen's d ranged .34-.52). Regression analyses also revealed a specific positive relationship between Rumination and concurrent frequency of PTSD symptoms (β = .45). Negative Self-View and Negative World-View related positively and Self-Blame related negatively to concurrent number of PTSD symptoms (β = .48, .44, -.45, respectively). Suppression and Distraction predicted a decrease and Numbing predicted an increase in time-lagged number of PTSD symptoms (β = -.33, -.28, .30, respectively). Risk factors for PTSD symptoms were younger age (β = -.25), lower income (β = -.29), fewer previous pregnancies (β = -.31), and poorer perceived social support (β = -.26). Interventions addressing negative appraisals, dysfunctional strategies, and social support are recommended for mothers with PTSD following stillbirth. Knowledge of cognitive predictors and risk factors of PTSD may inform the development of a screening instrument
Anisotropic optical response of the mixed-valent Mott-Hubbard insulator NaCu2O2
We report the results of a comprehensive spectroscopic ellipsometry study of
NaCu2O2, a compound composed of chains of edge-sharing Cu2+O4 plaquettes and
planes of Cu1+ ions in a O-Cu1+-O dumbbell configuration, in the spectral range
0.75-6.5 eV at temperatures 7 -300 K. The spectra of the dielectric function
for light polarized parallel to the Cu1+ planes reveal a strong in-plane
anisotropy of the interband excitations. Strong and sharp absorption bands
peaked at 3.45 eV (3.7 eV) dominate the spectra for polarization along
(perpendicular) to the Cu2+O2 chains. They are superimposed on flat and
featureless plateaux above the absorption edges at 2.25 eV (2.5 eV). Based on
density-functional calculations, the anomalous absorption peaks can be assigned
to transitions between bands formed by Cu1+ 3dxz(dyz) and Cu2+ 3dxy orbitals,
strongly hybridized with O pstates. The major contribution to the background
response comes from transitions between Cu1+ 3dz2 and 4px(py) bands. This
assignment accounts for the measured in-plane anisotropy. The dielectric
response along the Cu2+O2 chains develops a weak two-peak structure centered at
2.1 and 2.65 eV upon cooling below 100 K, along with the appearance of spin
correlations along the Cu2+O2 chains. These features bear a striking
resemblance to those observed in the single-valent Cu2+O2 chain compound
LiCuVO4, which were identified as an exciton doublet associated with
transitions to the upper Hubbard band that emerges as a consequence of the
long-range Coulomb interaction between electrons on neighboring Cu2+ sites
along the chains. An analysis of the spectral weights of these features yields
the parameters characterizing the on-site and long-range Coulomb interactions.Comment: 12 pages, 12 figure
Exciton doublet in the Mott-Hubbard LiCuVO insulator identified by spectral ellipsometry
Spectroscopic ellipsometry was used to study the dielectric function of
LiCuVO, a compound comprised of chains of edge-sharing CuO
plaquettes, in the spectral range (0.75 - 6.5) eV at temperatures (7-300) K.
For photon polarization along the chains, the data reveal a weak but
well-resolved two-peak structure centered at 2.15 and 2.95 eV whose spectral
weight is strongly enhanced upon cooling near the magnetic ordering
temperature. We identify these features as an exciton doublet in the
Mott-Hubbard gap that emerges as a consequence of the Coulomb interaction
between electrons on nearest and next-nearest neighbor sites along the chains.
Our results and methodology can be used to address the role of the long-range
Coulomb repulsion for compounds with doped copper-oxide chains and planes.Comment: 4 pages with 4 figures and EPAPS supplementary online material (3
pages with 4 figures), accepted in Phys. Rev. Let
Dimerization versus Orbital Moment Ordering in the Mott insulator YVO
We use exact diagonalization combined with mean-field theory to investigate
the phase diagram of the spin-orbital model for cubic vanadates. The spin-orbit
coupling competes with Hund's exchange and triggers a novel phase, with the
ordering of orbital magnetic moments stabilized by the tilting of
VO octahedra. It explains qualitatively spin canting and reduction of
magnetization observed in YVO. At finite temperature an orbital Peierls
instability in the -type antiferromagnetic phase induces modulation of
magnetic exchange constants even in absence of lattice distortions. The
calculated spin structure factor shows a magnon splitting due to the orbital
Peierls dimerization.Comment: 4 pages, 5 figures, Revte
Mothers' experience of their contact with their stillborn infant: an interpretative phenomenological analysis.
BACKGROUND: Guidelines surrounding maternal contact with the stillborn infant have been contradictory over the past thirty years. Most studies have reported that seeing and holding the stillborn baby is associated with fewer anxiety and depressive symptoms among mothers of stillborn babies than not doing so. In contrast, others studies suggest that contact with the stillborn infant can lead to poorer maternal mental health outcomes. There is a lack of research focusing on the maternal experience of this contact. The present study aimed to investigate how mothers describe their experience of spending time with their stillborn baby and how they felt retrospectively about the decision they made to see and hold their baby or not.
METHOD: In depth interviews were conducted with twenty-one mothers three months after stillbirth. All mothers had decided to see and the majority to hold their baby. Qualitative analysis of the interview data was performed using Interpretive Phenomenological Analysis.
RESULTS: Six superordinate themes were identified: Characteristics of Contact, Physicality; Emotional Experience; Surreal Experience; Finality; and Decision. Having contact with their stillborn infant provided mothers with time to process what had happened, to build memories, and to 'say goodbye', often sharing the experience with partners and other family members. The majority of mothers felt satisfied with their decision to spend time with their stillborn baby. Several mothers talked about their fear of seeing a damaged or dead body. Some mothers experienced strong disbelief and dissociation during the contact.
CONCLUSIONS: Results indicate that preparation before contact with the baby, professional support during the contact, and professional follow-up are crucial in order to prevent the development of maternal mental health problems. Fears of seeing a damaged or dead body should be sensitively explored and ways of coping discussed. Even in cases where mothers experienced intense distress during the contact with their stillborn baby, they still described that having had this contact was important and that they had taken the right decision. This indicates a need for giving parents an informed choice by engaging in discussions about the possible benefits and risks of seeing their stillborn baby
Absence of Hole Confinement in Transition Metal Oxides with Orbital Degeneracy
We investigate the spectral properties of a hole moving in a two-dimensional
Hubbard model for strongly correlated t_2g electrons. Although superexchange
interactions are Ising-like, a quasi-one-dimensional coherent hole motion
arises due to effective three-site terms. This mechanism is fundamentally
different from the hole motion via quantum fluctuations in the conventional
spin model with SU(2) symmetry. The orbital model describes also propagation of
a hole in some e_g compounds, and we argue that orbital degeneracy alone does
not lead to hole self-localization.Comment: 4 pages, 5 figure
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