164 research outputs found
Quasiparticle vanishing driven by geometrical frustration
We investigate the single hole dynamics in the triangular t-J model. We study
the structure of the hole spectral function, assuming the existence of a 120
magnetic Neel order. Within the self-consistent Born approximation (SCBA) there
is a strong momentum and t sign dependence of the spectra, related to the
underlying magnetic structure and the particle-hole asymmetry of the model. For
positive t, and in the strong coupling regime, we find that the low energy
quasiparticle excitations vanish outside the neighbourhood of the magnetic
Goldstone modes; while for negative t the quasiparticle excitations are always
well defined. In the latter, we also find resonances of magnetic origin whose
energies scale as (J/t)^2/3 and can be identified with string excitations. We
argue that this complex structure of the spectra is due to the subtle interplay
between magnon-assisted and free hopping mechanisms. Our predictions are
supported by an excellent agreement between the SCBA and the exact results on
finite size clusters. We conclude that the conventional quasiparticle picture
can be broken by the effect of geometrical magnetic frustration.Comment: 6 pages, 7 figures. Published versio
Dispersion of a single hole in the t-J model
The dispersion of a single hole in the t-J model obtained by the exact result
of 32 sites and the results obtained by self-consistent Born approximation and
the Green function Monte Carlo method can be simply derived by a mean-field
theory with d-RVB and antiferromagnetic order parameters. In addition, it
offers a simple explanation for the difference observed between those results.
The presence of the extended van Hove region at (pi,0) is a consequence of the
d-RVB pairing independenct of the antiferromagnetic order. Results including t'
and t" are also presented and explained consistently in a similar way.Comment: LaTex file, 5 pages with 5 embedded eps figure
Hubbard model versus t-J model: The one-particle spectrum
The origin of the apparent discrepancies between the one-particle spectra of
the Hubbard and t-J models is revealed: Wavefunction corrections, in addition
to the three-site terms, should supplement the bare t-J. In this way a
quantitative agreement between the two models is obtained, even for the
intermediate- values appropriate for the high-Tc cuprate superconductors.
Numerical results for clusters of up to 20 sites are presented. The momentum
dependence of the observed intensities in the photoemission spectra of
Sr2CuO2Cl2 are well described by this complete strong-coupling approach.Comment: 4 two-column RevTeX pages, including 4 Postscript figures. Uses epsf.
Accepted for publication in Physical Review B, Rapid Communicatio
A numerical study of multi-soliton configurations in a doped antiferromagnetic Mott insulator
We evaluate from first principles the self-consistent Hartree-Fock energies
for multi-soliton configurations in a doped, spin-1/2, antiferromagnetic Mott
insulator on a two-dimensional square lattice. We find that nearest-neighbor
Coulomb repulsion stabilizes a regime of charged meron-antimeron vortex soliton
pairs over a region of doping from 0.05 to 0.4 holes per site for intermediate
coupling 3 < U/t <8. This stabilization is mediated through the generation of
``spin-flux'' in the mean-field antiferromagnetic (AFM) background. Holes
cloaked by a meron-vortex in the spin-flux AFM background are charged bosons.
Our static Hartree-Fock calculations provide an upper bound on the energy of a
finite density of charged vortices. This upper bound is lower than the energy
of the corresponding charged stripe configurations. A finite density of charge
carrying vortices is shown to produce a large number of unoccupied electronic
levels in the Mott-Hubbard charge transfer gap. These levels lead to
significant band tailing and a broad mid-infrared band in the optical
absorption spectrum as observed experimentally. At very low doping (below 0.05)
the doping charges create extremely tightly bound meron-antimeron pairs or even
isolated conventional spin-polarons, whereas for very high doping (above 0.4)
the spin background itself becomes unstable to formation of a conventional
Fermi liquid and the spin-flux mean-field is energetically unfavorable. Our
results point to the predominance of a quantum liquid of charged, bosonic,
vortex solitons at intermediate coupling and intermediate doping
concentrations.Comment: 12 pages, 25 figures; added references, modified/eliminated some
figure
Photoemission spectra of : a theoretical analysis
Recent angle resolved photoemission (ARPES) results for the insulating
cuprate have provided the first experimental data
which can be directly compared to the (theoretically) well--studied problem of
a single hole propagating in an antiferromagnet. The ARPES results reported a
small bandwidth, providing evidence for the existence of strong correlations in
the cuprates. However, in the same experiment some discrepancies with the
familiar 2D model were also observed. Here we discuss a comparison
between the ARPES results and the quasiparticle dispersion of both (i) the
Hamiltonian and (ii) the three--band Hubbard model in the
strong--coupling limit. Both model Hamiltonians show that the experimentally
observed one--hole band structure can be approximately reproduced using
reasonable values for , or the direct oxygen hopping amplitude .Comment: 11 pages, RevTex version 3.0, 3 postscript figures, LaTeX file and
figures have been uuencoded
A microscopic model for d-wave charge carrier pairing and non-Fermi-liquid behavior in a purely repulsive 2D electron system
We investigate a microscopic model for strongly correlated electrons with
both on-site and nearest neighbor Coulomb repulsion on a 2D square lattice.
This exhibits a state in which electrons undergo a ``somersault'' in their
internal spin-space (spin-flux) as they traverse a closed loop in external
coordinate space. When this spin-1/2 antiferromagnetic (AFM) insulator is
doped, the ground state is a liquid of charged, bosonic meron-vortices, which
for topological reasons are created in vortex-antivortex pairs. The magnetic
exchange energy of the distorted AFM background leads to a logarithmic
vortex-antivortex attraction which overcomes the direct Coulomb repulsion
between holes localized on the vortex cores. This leads to the appearance of
pre-formed charged pairs. We use the Configuration Interaction (CI) Method to
study the quantum translational and rotational motion of various charged
magnetic solitons and soliton pairs. The CI method systematically describes
fluctuation and quantum tunneling corrections to the Hartree-Fock Approximation
(HFA). We find that the lowest energy charged meron-antimeron pairs exhibit
d-wave rotational symmetry, consistent with the symmetry of the cuprate
superconducting order parameter. For a single hole in the 2D AFM plane, we find
a precursor to spin-charge separation in which a conventional charged
spin-polaron dissociates into a singly charged meron-antimeron pair. This model
provides a unified microscopic basis for (i) non-Fermi-liquid transport
properties, (ii) d-wave preformed charged carrier pairs, (iii) mid-infrared
optical absorption, (iv) destruction of AFM long range order with doping and
other magnetic properties, and (v) certain aspects of angled resolved
photo-emission spectroscopy (ARPES).Comment: 14 pages, 17 figure
Holes in the t-J_z model: a thorough study
The t-J_z model is the strongly anisotropic limit of the t-J model which
captures some general properties of the doped antiferromagnets (AF). The
absence of spin fluctuations simplifies the analytical treatment of hole motion
in an AF background and allows us to calculate the single- and two-hole spectra
with high accuracy using regular diagram technique combined with real-space
approach. At the same time, numerical studies of this model via exact
diagonalization (ED) on small clusters show negligible finite size effects for
a number of quantities, thus allowing a direct comparison between analytical
and numerical results. Both approaches demonstrate that the holes have tendency
to pair in the p- and d-wave channels at realistic values of t/J. The
interactions leading to pairing and effects selecting p and d waves are
thoroughly investigated. The role of transverse spin fluctuations is considered
using perturbation theory. Based on the results of the present study, we
discuss the pairing problem in the realistic t-J-like model. Possible
implications for preformed pairs formation and phase separation are drawn.Comment: 21 pages, 15 figure
Maternally Orphaned Children and Intergenerational Concerns Associated with Breast Cancer Deaths among Women in Sub-Saharan Africa
Importance: Low breast cancer survival in sub-Saharan Africa's young population increases the likelihood that breast cancer deaths result in maternal orphans, ie, children (<18 years) losing their mother. Objective: To estimate the number of maternal orphans and their ages for every 100 breast cancer deaths in sub-Saharan African settings during 2014-2019 and to describe family concerns about the orphaned children. Design, Setting, and Participants: Deaths occurring between September 1, 2014, and July 1, 2019, in the African Breast Cancer-Disparities in Outcomes (ABC-DO) were examined in a cohort of women diagnosed with breast cancer during 2014-2017 at major cancer treatment hospitals in Namibia, Nigeria, Uganda, and Zambia. The cohort was actively followed up for vital status via a trimonthly mobile phone call to each woman or her next of kin (typically a partner, husband, or child). Main Outcomes and Measures: The number (Poisson counts) and ages of new orphans at the time of maternal death. Results: This cohort study found that a total of 795 deaths resulted in 964 new maternal orphans, with deaths occurring in women younger than 50 years accounting for 85% of the orphans. For every 100 deaths in women younger than 50 years, there were 210 new orphans (95% CI, 196-225) overall, with country-specific estimates of 189 in Nigerian, 180 in Namibian, 222 in Ugandan, and 247 in Zambian Black women. For every 100 deaths of the women at any age, there were 121 maternal orphans, 17% of whom were younger than 5 years, 32% aged 5 to 9 years, and 51% aged 10 to 17 years at the time of maternal death. In follow-up interviews, families' concerns for children's education and childcare were reported to be exacerbated by the financial expenses associated with cancer treatment. Conclusions and Relevance: This study provides evidence that the number of maternal orphans due to breast cancer exceeds the number of breast cancer deaths among women in sub-Saharan Africa. The intergenerational consequences associated with cancer deaths in sub-Saharan Africa appear to be large and support the need for continued action to improve survival
Preexisting morbidity profile of women newly diagnosed with breast cancer in sub-Saharan Africa: African Breast Cancer—Disparities in Outcomes study
The presence of preexisting morbidities poses a challenge to cancer patient care. There is little information on the profile and prevalence of multi-morbidities in breast cancer patients across middle income countries (MIC) to lower income countries (LIC) in sub-Saharan Africa (SSA). The African Breast Cancer–Disparities in Outcomes (ABC-DO) breast cancer cohort spans upper MICs South Africa and Namibia, lower MICs Zambia and Nigeria and LIC Uganda. At cancer diagnosis, seven morbidities were assessed: obesity, hypertension, diabetes, asthma/chronic obstructive pulmonary disease, heart disease, tuberculosis and HIV. Logistic regression models were used to assess determinants of morbidities and the influence of morbidities on advanced stage (stage III/IV) breast cancer diagnosis. Among 2189 women, morbidity prevalence was the highest for obesity (35%, country-specific range 15-57%), hypertension (32%, 15-51%) and HIV (16%, 2-26%) then for diabetes (7%, 4%-10%), asthma (4%, 2%-10%), tuberculosis (4%, 0%-8%) and heart disease (3%, 1%-7%). Obesity and hypertension were more common in upper MICs and in higher socioeconomic groups. Overall, 27% of women had at least two preexisting morbidities. Older women were more likely to have obesity (odds ratio: 1.09 per 10 years, 95% CI 1.01-1.18), hypertension (1.98, 1.81-2.17), diabetes (1.51, 1.32-1.74) and heart disease (1.69, 1.37-2.09) and were less likely to be HIV positive (0.64, 0.58-0.71). Multi-morbidity was not associated with stage at diagnosis, with the exception of earlier stage in obese and hypertensive women. Breast cancer patients in higher income countries and higher social groups in SSA face the additional burden of preexisting non-communicable diseases, particularly obesity and hypertension, exacerbated by HIV in Southern/Eastern Africa
Geospatial barriers to healthcare access for breast cancer diagnosis in sub-Saharan African settings: The African Breast Cancer—Disparities in Outcomes Cohort Study
We examined the geospatial dimension of delays to diagnosis of breast cancer in a prospective study of 1541 women newly diagnosed in the African Breast Cancer—Disparities in Outcomes (ABC-DO) Study. Women were recruited at cancer treatment facilities in Namibia, Nigeria, Uganda and Zambia. The baseline interview included information used to generate the geospatial features: urban/rural residence, travel mode to treatment facility and straight-line distances from home to first-care provider and to diagnostic/treatment facility, categorized into country/ethnicity (population)-specific quartiles. These factors were investigated in relation to delay in diagnosis (≥3 months since first symptom) and late stage at diagnosis (TNM: III, IV) using logistic regression, adjusted for population group and sociodemographic characteristics. The median (interquartile range) distances to first provider and diagnostic and treatment facilities were 5 (1-37), 17 (3-105) and 62 (5-289) km, respectively. The majority had a delay in diagnosis (74%) and diagnosis at late stage (64%). Distance to first provider was not associated with delay in diagnosis or late stage at diagnosis. Rural residence was associated with delay, but the association did not persist after adjustment for sociodemographic characteristics. Distance to the diagnostic/treatment facility was associated with delay (highest vs lowest quartile: odds ratio (OR) = 1.56, 95% confidence interval (CI) = 1.08-2.27) and late stage (overall: OR = 1.47, CI = 1.05-2.06; without Nigerian hospitals where mostly local residents were treated: OR = 1.73, CI = 1.18-2.54). These findings underscore the need for measures addressing the geospatial barriers to early diagnosis in sub-Saharan African settings, including providing transport or travel allowance and decentralizing diagnostic services
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