641 research outputs found
Medical ovariectomy in menopausal breast cancer patients with high testosterone levels : a further step toward tailored therapy
Five years of adjuvant therapy with anti-estrogens reduce the incidence of disease progression by about 50% in estrogen receptor-positive breast cancer patients, but late relapse can still occur after anti-estrogens have been discontinued. In these patients, excessive androgen production may account for renewed excessive estrogen formation and increased risks of late relapse. In the 50% of patients who do not benefit with anti-estrogens, the effect of therapy is limited by de novo or acquired resistance to treatment. Androgen receptor and epidermal growth factor receptor overexpression are recognized mechanisms of endocrine resistance suggesting the involvement of androgens as activators of the androgen receptor pathway and as stimulators of epidermal growth factor synthesis and function. Data from a series of prospective studies on operable breast cancer patients, showing high serum testosterone levels are associated to increased risk of recurrence, provide further support to a role for androgens in breast cancer progression. According to the above reported evidence, we proposed to counteract excessive androgen production in the adjuvant setting of estrogen receptor-positive patients and suggested selecting postmenopausal patients with elevated levels of serum testosterone, marker of ovarian hyperandrogenemia, for adjuvant treatment with a gonadotropins-releasing hormone analogue (medical oophorectomy) in addition to standard therapy with anti-estrogens. The proposed approach provides an attempt of personalized medicine that needs to be further investigated in clinical trials
Gate stability of GaN-Based HEMTs with P-Type Gate
status: publishe
Polaronic state and nanometer-scale phase separation in colossal magnetoresistive manganites
High resolution topographic images obtained by scanning tunneling microscope
in the insulating state of Pr0.68Pb0.32MnO3 single crystals showed regular
stripe-like or zigzag patterns on a width scale of 0.4 - 0.5 nm confirming a
high temperature polaronic state. Spectroscopic studies revealed inhomogeneous
maps of zero-bias conductance with small patches of metallic clusters on length
scale of 2 - 3 nm only within a narrow temperature range close to the
metal-insulator transition. The results give a direct observation of polarons
in the insulating state, phase separation of nanometer-scale metallic clusters
in the paramagnetic metallic state, and a homogeneous ferromagnetic state
Dependence of the Jahn-Teller distortion in LaMn1-xScxO3 on the isovalent Mn-site substitution
We investigated the relative importance of removing the Mn3+ Jahn-Teller distortion in driving ferromagnetism in LaMn1-xScxO3 combining x-ray powder diffraction and x-ray absorption spectroscopy at the Mn and Sc K-edges. By increasing the Sc content, the orthorhombic distortion of the Pbnm cell in LaMnO3 decreases but the unit-cell remains slightly distorted in LaScO3. Besides, the nearly tetragonal-distorted MO6 in LaMnO3 continuously evolves into a nearly regular one in LaScO3. On the other hand, x-ray absorption spectra show that the MnO6 octahedron remains Jahn-Teller distorted and the ScO6 octahedron is nearly regular along the whole series. Moreover, the ordering of the Mn3+ Jahn-Teller distortion is not disrupted in the ab plane for any Sc concentration. This contrasts with the Gasubstituted compounds, where a regular MnO6 is found for x > 0.5. However, both LaMn0.5Sc0.5O3 and LaMn0.5Ga0.5O3 show ferromagnetic behavior independently of the presence (or not) of Jahn-Teller distorted Mn3+. Thus, our results point to the Mn-sublattice dilution as the main effect in driving ferromagnetism in these manganites over local structure effects previously proposed by the spin flipping or the vibronic superexchange models
Theory of Insulator Metal Transition and Colossal Magnetoresistance in Doped Manganites
The persistent proximity of insulating and metallic phases, a puzzling
characterestic of manganites, is argued to arise from the self organization of
the twofold degenerate e_g orbitals of Mn into localized Jahn-Teller(JT)
polaronic levels and broad band states due to the large electron - JT phonon
coupling present in them. We describe a new two band model with strong
correlations and a dynamical mean-field theory calculation of equilibrium and
transport properties. These explain the insulator metal transition and colossal
magnetoresistance quantitatively, as well as other consequences of two state
coexistence
Zero Temperature Insulator-Metal Transition in Doped Manganites
We study the transition at T=0 from a ferromagnetic insulating to a
ferromagnetic metallic phase in manganites as a function of hole doping using
an effective low-energy model Hamiltonian proposed by us recently. The model
incorporates the quantum nature of the dynamic Jahn-Teller(JT) phonons strongly
coupled to orbitally degenerate electrons as well as strong Coulomb correlation
effects and leads naturally to the coexistence of localized (JT polaronic) and
band-like electronic states. We study the insulator-metal transition as a
function of doping as well as of the correlation strength U and JT gain in
energy E_{JT}, and find, for realistic values of parameters, a ground state
phase diagram in agreement with experiments. We also discuss how several other
features of manganites as well as differences in behaviour among manganites can
be understood in terms of our model.Comment: To be published in Europhysics Letter
Origin of magnetic moments and presence of a resonating valence bond state in BaYIrO
While it was speculated that 5 systems would possess non-magnetic
~=~0 ground state due to strong Spin-Orbit Coupling (SOC), all such systems
have invariably shown presence of magnetic moments so far. A puzzling case is
that of BaYIrO, which in spite of having a perfectly cubic structure
with largely separated Ir () ions, has consistently shown presence
of weak magnetic moments. Moreover, we clearly show from Muon Spin Relaxation
(SR) measurements that a change in the magnetic environment of the
implanted muons in BaYIrO occurs as temperature is lowered below 10~K.
This observation becomes counterintuitive, as the estimated value of SOC
obtained by fitting the RIXS spectrum of BaYIrO with an atomic
model is found to be as high as 0.39~eV, meaning that the system within this
model is neither expected to possess moments nor exhibit temperature dependent
magnetic response. Therefore we argue that the atomic coupling
description is not sufficient to explain the ground state of such systems,
where despite having strong SOC, presence of hopping triggers delocalisation of
holes, resulting in spontaneous generation of magnetic moments. Our theoretical
calculations further indicate that these moments favour formation of
spin-orbital singlets in the case of BaYIrO, which is manifested in
SR experiments measured down to 60~mK.Comment: 20 Pages, 7 Figure
A novel approach to breast cancer prevention: reducing excessive ovarian androgen production in elderly women
Minimizing endogenous estrogen production and activity in women at high risk for breast cancer is a prominent approach to prevention of the disease. A number of clinical trials have shown that the administration of selective-estrogen receptor modulators or aromatase inhibitors significantly reduces the incidence of breast cancer in healthy women. Unfortunately, these drugs often produce adverse effects on the quality of life and are, therefore, poorly accepted by many women, even those who are at high risk for breast cancer. We propose a novel alternative approach to decreasing estrogen production: suppression of ovarian synthesis of the androgen precursors of estrogens by administration of long-acting gonadotropin-releasing hormone analogs to women with ovarian stromal hyperplasia. The specific target population would be elderly postmenopausal women, at increased risk of breast cancer, and with high blood levels of testosterone, marker of ovarian hyperandrogenemia, and recognized factor of risk for breast cancer. Testosterone levels are measured at baseline to identify women at risk and during the follow-up to evaluate the effectiveness of therapy. The postmenopausal ovary is an important source of excessive androgen production which originates from the ovarian interstitial cell hyperplasia frequently present in breast cancer patients. We propose to counter the source of androgen excess in women with ovarian stromal hyperplasia, thus reducing the substrate for estrogen formation without completely inhibiting estrogen synthesis. Available evidence indicates that gonadotropin-releasing hormone analogs can be safely used for breast cancer prevention in postmenopausal women
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