592 research outputs found
Spin-filter effect of the europium chalcogenides: An exactly solved many-body model
A model Hamiltonian is introduced which considers the main features of the
experimental spin filter situation as s-f interaction, planar geometry and the
strong external electric field. The proposed many-body model can be solved
analytically and exactly using Green functions.
The spin polarization of the field-emitted electrons is expressed in terms of
spin-flip probabilities, which on their part are put down to the exactly known
dynamic quantities of the system.
The calculated electron spin polarization shows remarkable dependencies on
the electron velocity perpendicular to the emitting plane and the strength of
s-f coupling. Experimentally observed polarization values of about 90% are well
understood within the framework of the proposed model.Comment: accepted (Physical Review B); 10 pages, 11 figures;
http://orion.physik.hu-berlin.de
Reflections on a Measurement of the Gravitational Constant Using a Beam Balance and 13 Tons of Mercury
In 2006, a final result of a measurement of the gravitational constant
performed by researchers at the University of Z\"urich was published. A value
of G=6.674\,252(122)\times
10^{-11}\,\mbox{m}^3\,\mbox{kg}^{-1}\,\mbox{s}^{-2} was obtained after an
experimental effort that lasted over one decade. Here, we briefly summarize the
measurement and discuss the strengths and weaknesses of this approach.Comment: 13 pages, 5 figures accepted for publication in Phil. Trans. R. Soc.
Ferromagnetic Kondo-Lattice Model
We present a many-body approach to the electronic and magnetic properties of
the (multiband) Kondo-lattice model with ferromagnetic interband exchange. The
coupling between itinerant conduction electrons and localized magnetic moments
leads, on the one hand, to a distinct temperature-dependence of the electronic
quasiparticle spectrum and, on the other hand, to magnetic properties, as
e.~g.the Curie temperature T_C or the magnon dispersion, which are strongly
influenced by the band electron selfenergy and therewith in particular by the
carrier density. We present results for the single-band Kondo-lattice model in
terms of quasiparticle densities of states and quasiparticle band structures
and demonstrate the density-dependence of the self-consistently derived Curie
temperature. The transition from weak-coupling (RKKY) to strong-coupling
(double exchange) behaviour is worked out.
The multiband model is combined with a tight-binding-LMTO bandstructure
calculation to describe real magnetic materials. As an example we present
results for the archetypal ferromagnetic local-moment systems EuO and EuS. The
proposed method avoids the double counting of relevant interactions and takes
into account the correct symmetry of atomic orbitals.Comment: 15 pages, 10 figure
Quantum effects in the quasiparticle structure of the ferromagnetic Kondo lattice model
A new ``Dynamical Mean-field theory'' based approach for the Kondo lattice
model with quantum spins is introduced. The inspection of exactly solvable
limiting cases and several known approximation methods, namely the second-order
perturbation theory, the self-consistent CPA and finally a moment-conserving
decoupling of the equations of motion help in evaluating the new approach. This
comprehensive investigation gives some certainty to our results: Whereas our
method is somewhat limited in the investigation of the J<0-model, the results
for J>0 reveal important aspects of the physics of the model: The energetically
lowest states are not completely spin-polarized.A band splitting, which occurs
already for relatively low interaction strengths, can be related to distinct
elementary excitations, namely magnon emission (absorption) and the formation
of magnetic polarons. We demonstrate the properties of the ferromagnetic Kondo
lattice model in terms of spectral densities and quasiparticle densities of
states.Comment: 19 pages, 4 figure
Influence of Spin Wave Excitations on the Ferromagnetic Phase Diagram in the Hubbard-Model
The subject of the present paper is the theoretical description of collective
electronic excitations, i.e. spin waves, in the Hubbard-model. Starting with
the widely used Random-Phase-Approximation, which combines Hartree-Fock theory
with the summation of the two-particle ladder, we extend the theory to a more
sophisticated single particle approximation, namely the
Spectral-Density-Ansatz. Doing so we have to introduce a `screened`
Coulomb-interaction rather than the bare Hubbard-interaction in order to obtain
physically reasonable spinwave dispersions. The discussion following the
technical procedure shows that comparison of standard RPA with our new
approximation reduces the occurrence of a ferromagnetic phase further with
respect to the phase-diagrams delivered by the single particle theories.Comment: 8 pages, 9 figures, RevTex4, accepted for publication in Phys. Rev.
An analytical study of resonant transport of Bose-Einstein condensates
We study the stationary nonlinear Schr\"odinger equation, or Gross-Pitaevskii
equation, for a one--dimensional finite square well potential. By neglecting
the mean--field interaction outside the potential well it is possible to
discuss the transport properties of the system analytically in terms of ingoing
and outgoing waves. Resonances and bound states are obtained analytically. The
transmitted flux shows a bistable behaviour. Novel crossing scenarios of
eigenstates similar to beak--to--beak structures are observed for a repulsive
mean-field interaction. It is proven that resonances transform to bound states
due to an attractive nonlinearity and vice versa for a repulsive nonlinearity,
and the critical nonlinearity for the transformation is calculated
analytically. The bound state wavefunctions of the system satisfy an
oscillation theorem as in the case of linear quantum mechanics. Furthermore,
the implications of the eigenstates on the dymamics of the system are
discussed.Comment: RevTeX4, 16 pages, 19 figure
Element-resolved x-ray ferrimagnetic and ferromagnetic resonance spectroscopy
We report on the measurement of element-specific magnetic resonance spectra
at gigahertz frequencies using x-ray magnetic circular dichroism (XMCD). We
investigate the ferrimagnetic precession of Gd and Fe ions in Gd-substituted
Yttrium Iron Garnet, showing that the resonant field and linewidth of Gd
precisely coincide with Fe up to the nonlinear regime of parametric
excitations. The opposite sign of the Gd x-ray magnetic resonance signal with
respect to Fe is consistent with dynamic antiferromagnetic alignment of the two
ionic species. Further, we investigate a bilayer metal film,
NiFe(5 nm)/Ni(50 nm), where the coupled resonance modes of Ni and
NiFe are separately resolved, revealing shifts in the resonance
fields of individual layers but no mutual driving effects. Energy-dependent
dynamic XMCD measurements are introduced, combining x-ray absorption and
magnetic resonance spectroscopies.Comment: 16 pages, 8 figure
A Measurement of Newton's Gravitational Constant
A precision measurement of the gravitational constant has been made using
a beam balance. Special attention has been given to determining the
calibration, the effect of a possible nonlinearity of the balance and the
zero-point variation of the balance. The equipment, the measurements and the
analysis are described in detail. The value obtained for G is 6.674252(109)(54)
10^{-11} m3 kg-1 s-2. The relative statistical and systematic uncertainties of
this result are 16.3 10^{-6} and 8.1 10^{-6}, respectively.Comment: 26 pages, 20 figures, Accepted for publication by Phys. Rev.
Phase I/II study of single-agent lenvatinib in children and adolescents with refractory or relapsed solid malignancies and young adults with osteosarcoma (ITCC-050)
Osteosarcoma; Pediátrico; Tumores sólidosOsteosarcoma; Pedià trica; Tumors sòlidsOsteosarcoma; Pediatric; Solid tumorsBackground
We report results from the phase I dose-finding and phase II expansion part of a multicenter, open-label study of single-agent lenvatinib in pediatric and young adult patients with relapsed/refractory solid tumors, including osteosarcoma and radioiodine-refractory differentiated thyroid cancer (RR-DTC) (NCT02432274).
Patients and methods
The primary endpoint of phase I was to determine the recommended phase II dose (RP2D) of lenvatinib in children with relapsed/refractory solid malignant tumors. Phase II primary endpoints were progression-free survival rate at 4 months (PFS-4) for patients with relapsed/refractory osteosarcoma; and objective response rate/best overall response for patients with RR-DTC at the RP2D.
Results
In phase I, 23 patients (median age, 12 years) were enrolled. With lenvatinib 14 mg/m2, three dose-limiting toxicities (hypertension, n = 2; increased alanine aminotransferase, n = 1) were reported, establishing 14 mg/m2 as the RP2D. In phase II, 31 patients with osteosarcoma (median age, 15 years) and 1 patient with RR-DTC (age 17 years) were enrolled. For the osteosarcoma cohort, PFS-4 (binomial estimate) was 29.0% [95% confidence interval (CI) 14.2% to 48.0%; full analysis set: n = 31], PFS-4 by Kaplan–Meier estimate was 37.8% (95% CI 20.0% to 55.4%; full analysis set) and median PFS was 3.0 months (95% CI 1.8-5.4 months). The objective response rate was 6.7% (95% CI 0.8% to 22.1%). The patient with RR-DTC had a best overall response of partial response. Some 60.8% of patients in phase I and 22.6% of patients in phase II (with osteosarcoma) had treatment-related treatment-emergent adverse events of grade ≥3.
Conclusions
The lenvatinib RP2D was 14 mg/m2. Single-agent lenvatinib showed activity in osteosarcoma; however, the null hypothesis could not be rejected. The safety profile was consistent with previous tyrosine kinase inhibitor studies. Lenvatinib is currently being investigated in osteosarcoma in combination with chemotherapy as part of a randomized, controlled trial (NCT04154189), in pediatric solid tumors in combination with everolimus (NCT03245151), and as a single agent in a basket study with enrollment ongoing (NCT04447755).This work was supported by Eisai Inc., Woodcliff Lake, NJ, USA and Merck Sharp & Dohme Corp., a subsidiary of Merck & Co., Inc., Kenilworth, NJ, USA
Spin dynamics in the diluted ferromagnetic Kondo lattice model
The interplay of disorder and competing interactions is investigated in the
carrier-induced ferromagnetic state of the Kondo lattice model within a
numerical finite-size study in which disorder is treated exactly. Competition
between impurity spin couplings, stability of the ferromagnetic state, and
magnetic transition temperature are quantitatively investigated in terms of
magnon properties for different models including dilution, disorder, and
weakly-coupled spins. A strong optimization is obtained for T_c at hole doping
p << x, highlighting the importance of compensation in diluted magnetic
semiconductors. The estimated T_c is in good agreement with experimental
results for Ga_{1-x}Mn_x As for corresponding impurity concentration, hole
bandwidth, and compensation. Finite-temperature spin dynamics is quantitatively
studied within a locally self-consistent magnon renormalization scheme, which
yields a substantial enhancement in T_c due to spin clustering, and highlights
the nearly-paramagnetic spin dynamics of weakly-coupled spins. The large
enhancement in density of low-energy magnetic excitations due to disorder and
competing interactions results in a strong thermal decay of magnetization,
which fits well with the Bloch form M_0(1-BT^{3/2}) at low temperature, with B
of same order of magnitude as obtained in recent squid magnetization
measurements on Ga_{1-x}Mn_x As samples.Comment: 13 pages, 14 figure
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