345 research outputs found
Ferroelectricity in the Magnetic E-Phase of Orthorhombic Perovskites
We show that the symmetry of the spin zigzag chain E phase of the
orthorhombic perovskite manganites and nickelates allows for the existence of a
finite ferroelectric polarization. The proposed microscopic mechanism is
independent of spin-orbit coupling. We predict that the polarization induced by
the E-type magnetic order can potentially be enhanced by up to two orders of
magnitude with respect to that in the spiral magnetic phases of TbMnO3 and
similar multiferroic compounds.Comment: 4 pages, 2 figures, somewhat changed emphases, accepted to PR
Insulator to Metal Transition Induced by Disorder in a Model for Manganites
The physics of manganites appears to be dominated by phase competition among
ferromagnetic metallic and charge-ordered antiferromagnetic insulating states.
Previous investigations (Burgy {\it et al.}, Phys. Rev. Lett. {\bf 87}, 277202
(2001)) have shown that quenched disorder is important to smear the first-order
transition between those competing states, and induce nanoscale inhomogeneities
that produce the colossal magnetoresistance effect. Recent studies (Motome {\it
et al.} Phys. Rev. Lett. {\bf 91}, 167204 (2003)) have provided further
evidence that disorder is important in the manganite context, unveiling an
unexpected insulator-to-metal transition triggered by disorder in a one-orbital
model with cooperative phonons. In this paper, a qualitative explanation for
this effect is presented. It is argued that the transition occurs for disorder
in the form of local random energies. Acting over an insulating states made out
of a checkerboard arrangement of charge, with ``effective'' site energies
positive and negative, this form of disorder can produce lattice sites with an
effective energy near zero, favorable for the transport of charge. This
explanation is based on Monte Carlo simulations and the study of simplified toy
models, measuring the density-of-states, cluster conductances using the
Landauer formalism, and other observables. The applicability of these ideas to
real manganites is discussed.Comment: 14 pages, 23 figures, submitted to Physical Review
Fragility of the A-type AF and CE Phases of Manganites: An Exotic Insulator-to-Metal Transition Induced by Quenched Disorder
Using Monte Carlo simulations and the two eg-orbital model for manganites,
the stability of the CE and A-type antiferromagnetic insulating states is
analyzed when quenched disorder in the superexchange JAF between the t2g
localized spins and in the on-site energies is introduced. At vanishing or
small values of the electron-(Jahn-Teller)phonon coupling, the previously
hinted "fragility" of these insulating states is studied in detail, focusing on
their charge transport properties. This fragility is here found to induce a
rapid transition from the insulator to a (poor) metallic state upon the
introduction of disorder. A possible qualitative explanation is presented based
on the close proximity in energy of ferromagnetic metallic phases, and also on
percolative ideas valid at large disorder strength. The scenario is compared
with previously discussed insulator-to-metal transitions in other contexts. It
is argued that the effect unveiled here has unique properties that may define a
new class of giant effects in complex oxides. This particularly severe effect
of disorder must be present in other materials as well, in cases involving
phases that arise as a compromise between very different tendencies, as it
occurs with striped states in the cuprates.Comment: 13 pages, 17 figures, RevTex 4, submitted for publicatio
Study of the One- and Two-Band Models for Colossal Magnetoresistive Manganites Using the Truncated Polynomial Expansion Method
Considerable progress has been recently made in the theoretical understanding
of the colossal magnetoresistance (CMR) effect in manganites. The analysis of
simple models with two competing states and a resistor network approximation to
calculate conductances has confirmed that CMR effects can be theoretically
reproduced using non-uniform clustered states. In this paper, the recently
proposed Truncated Polynomial Expansion method (TPEM) for spin-fermion systems
is tested using the double-exchange one-band, with finite Hund coupling , and two-band, with infinite , models. Two dimensional lattices
as large as 4848 are studied, far larger than those that can be handled
with standard exact diagonalization (DIAG) techniques for the fermionic sector.
The clean limit (i.e. without quenched disorder) is here analyzed in detail.
Phase diagrams are obtained, showing first-order transitions separating
ferromagnetic metallic from insulating states. A huge magnetoresistance is
found at low temperatures by including small magnetic fields, in excellent
agreement with experiments. However, at temperatures above the Curie transition
the effect is much smaller confirming that the standard finite-temperature CMR
phenomenon cannot be understood using homogeneous states. By comparing results
between the two methods, TPEM and DIAG, on small lattices, and by analyzing the
systematic behavior with increasing cluster sizes, it is concluded that the
TPEM is accurate to handle realistic manganite models on large systems. Our
results pave the way to a frontal computational attack of the colossal
magnetoresistance phenomenon using double-exchange like models, on large
clusters, and including quenched disorder.Comment: 14 pages, 17 figure
A Real Space Description of Field Induced Melting in the Charge Ordered Manganites: II. the Disordered Case
We study the effect of A site disorder on magnetic field induced melting of
charge order (CO) in half doped manganites using a Monte-Carlo technique.
Strong A-site disorder destroys CO even without an applied field. At moderate
disorder, the zero field CO state survives but has several intriguing features
in its field response. Our spatially resolved results track the broadening of
the field melting transition due to disorder and explain the unusual dependence
of the melting scales on bandwidth and disorder. In combination with our
companion paper on field melting of charge order in clean systems we provide an
unified understanding of CO melting across all half doped manganites.Comment: 9 pages, pdflatex, 10 embedded png fig
A Real Space Description of Magnetic Field Induced Melting in the Charge Ordered Manganites: I. The Clean Limit
We study the melting of charge order in the half doped manganites using a
model that incorporates double exchange, antiferromagnetic superexchange, and
Jahn-Teller coupling between electrons and phonons. We primarily use a real
space Monte Carlo technique to study the phase diagram in terms of applied
field and temperature , exploring the melting of charge order with
increasing and its recovery on decreasing . We observe hysteresis in
this response, and discover that the `field melted' high conductance state can
be spatially inhomogeneous even without extrinsic disorder. The hysteretic
response plays out in the background of field driven equilibrium phase
separation. Our results, exploring , , and the electronic parameter
space, are backed up by analysis of simpler limiting cases and a Landau
framework for the field response. This paper focuses on our results in the
`clean' systems, a companion paper studies the effect of cation disorder on the
melting phenomena.Comment: 16 pages, pdflatex, 11 png fig
Bilayer manganites: polarons in the midst of a metallic breakdown
The exact nature of the low temperature electronic phase of the manganite
materials family, and hence the origin of their colossal magnetoresistant (CMR)
effect, is still under heavy debate. By combining new photoemission and
tunneling data, we show that in La{2-2x}Sr{1+2x}Mn2O7 the polaronic degrees of
freedom win out across the CMR region of the phase diagram. This means that the
generic ground state is that of a system in which strong electron-lattice
interactions result in vanishing coherent quasi-particle spectral weight at the
Fermi level for all locations in k-space. The incoherence of the charge
carriers offers a unifying explanation for the anomalous charge-carrier
dynamics seen in transport, optics and electron spectroscopic data. The
stacking number N is the key factor for true metallic behavior, as an
intergrowth-driven breakdown of the polaronic domination to give a metal
possessing a traditional Fermi surface is seen in the bilayer system.Comment: 7 pages, 2 figures, includes supplementary informatio
In-vivo somatostatin-receptor expression in small cell lung cancer as a prognostic image biomarker and therapeutic target
Background: Given the dismal prognosis of small cell lung cancer (SCLC), novel therapeutic targets are urgently needed. We aimed to evaluate whether SSTR expression, as assessed by positron emission tomography (PET), can be applied as a prognostic image biomarker and determined subjects eligible for peptide receptor radionuclide therapy (PRRT). Methods: A total of 67 patients (26 females; age, 41–80 years) with advanced SCLC underwent SSTR-directed PET/computed tomography (somatostatin receptor imaging, SRI). SRI-avid tumor burden was quantified by maximum standardized uptake values (SUVmax) and tumor-to-liver ratios (T/L) of the most intense SCLC lesion. Scan findings were correlated with progression-free (PFS) and overall survival (OS). In addition, subjects eligible for SSTR-directed radioligand therapy were identified, and treatment outcome and toxicity profile were recorded. Results: On a patient basis, 36/67 (53.7%) subjects presented with mainly SSTR-positive SCLC lesions (>50% lesions positive); in 10/67 patients (14.9%), all lesions were positive. The median SUVmax was found to be 8.5, while the median T/L was 1.12. SRI-uptake was not associated with PFS or OS, respectively (SUVmax vs. PFS, ρ = 0.13 with p = 0.30 and vs. OS, ρ = 0.00 with p = 0.97; T/L vs. PFS, ρ = 0.07 with p = 0.58 and vs. OS, ρ = −0.05 with p = 0.70). PRRT was performed in 14 patients. One patient succumbed to treatment-independent infectious complications immediately after PRRT. In the remaining 13 subjects, disease control was achieved in 5/13 (38.5%) with a single patient achieving a partial response (stable disease in the remainder). In the sub-group of responding patients, PFS and OS were 357 days and 480 days, respectively. Conclusions: SSTR expression as detected by SRI is not predictive of outcome in patients with advanced SCLC. However, it might serve as a therapeutic target in selected patient
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