641 research outputs found
Effective Operator for Transitions in Nonresonant Inelastic X-ray Scattering
Recent experiments by Larson et al. demonstrate the feasibility of measuring
local excitations using nonresonant inelastic X-ray scattering (IXS). We
establish a general framework for the interpretation where the transitions
created in the scattering process are expressed in effective one-particle
operators that follow a simple selection rule. The different operators can be
selectively probed by employing their different dependence on the direction and
magnitude of the transferred momentum. We use the operators to explain the
presence of nodal directions and the nonresonant IXS in specific directions and
planes. We demonstrate how nonresonant IXS can be used to extract valuable
ground state information for orbiton excitations in manganite
Simulation of anisotropic wet-chemical etching using a physical model
We present a method to describe the orientation dependence of the etch rate of silicon, or any other single crystalline material, in anisotropic etching solutions by analytical functions. The parameters in these functions have a simple physical meaning. Crystals have a small number of atomically smooth faces, which etch (and grow) slowly as a consequence of the removal (or addition) of atoms by rows and layers. However, smooth faces have a roughening transition (well known in statistical physics); at increasing temperature they become rougher, and accordingly the etch and growth rates increase. Consequently, the basic physical parameters of our functions are the roughness of the smooth faces and the velocity of steps on these faces. This small set of parameters describes the etch rate in the two-dimensional space of orientations (on the unit sphere). We have applied our method to the practical case of etch rate functions for silicon crystals in KOH solutions. The maximum deviation between experimental data and simulation using only nine physically meaningful parameters is less than 5% of the maximum etch rate. This method, which in this study is used to describe anisotropic etching of silicon, can easily be adjusted to describe the growth or etching process of any crysta
Determining the Electron-Phonon Coupling Strength in Correlated Electron Systems from Resonant Inelastic X-ray Scattering
We show that high resolution Resonant Inelastic X-ray Scattering (RIXS)
provides direct, element-specific and momentum-resolved information on the
electron-phonon (e-p) coupling strength. Our theoretical analysis demonstrates
that the e-p coupling can be extracted from RIXS spectra by determining the
differential phonon scattering cross section. An alternative, very direct
manner to extract the coupling is to use the one and two-phonon loss ratio,
which is governed by the e-p coupling strength and the core-hole life-time.
This allows measurement of the e-p coupling on an absolute energy scale.Comment: 4 pages, 3 figure
Magnetic circular dichroism of x-ray absorption spectroscopy at rare-earth L2,3 edges in RE2Fe14B compounds (RE = La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu)
Magnetic circular dichroism (MCD) in the x-ray absorption spectroscopy (XAS)
at the L2,3 edges for almost entire series of rare-earth (RE) elements in
RE2Fe14B, is studied experimentally and theoretically. By a quantitative
comparison of the complicated MCD spectral shapes, we find that (i) the 4f-5d
intra-atomic exchange interaction not only induces the spin and orbital
polarization of the 5d states, which is vital for the MCD spectra of the
electric dipole transition from the 2p core states to the empty 5d conduction
band, but also it accompanies a contraction of the radial part of the 5d wave
function depending on its spin and orbital state, which results in the
enhancement of the 2p-5d dipole matrix element, (ii) there are cases where the
spin polarization of the 5d states due to the hybridization with the spin
polarized 3d states of surrounding irons plays important roles, and (iii) the
electric quadrupole transition from the 2p core states to the magnetic vale!
nce 4f states is appreciable at the pre-edge region of the dipole spectrum.
Especially, our results evidence that it is important to include the
enhancement effect of the dipole matrix element in the correct interpretation
of the MCD spectra at the RE L2,3 edges.Comment: 9 pages, 5 figures, 1 table, REVTe
Liver metastases of neuroendocrine tumours; early reduction of tumour load to improve life expectancy
BACKGROUND: Neuroendocrine tumours frequently metastasize to the liver. Although generally slowly progressing, hepatic metastases are the major cause of carcinoid syndrome and ultimately lead to liver dysfunction, cardiac insufficiency and finally death. METHODS: A literature review was performed to define the optimal treatment strategy and work-up in patients with neuroendocrine hepatic metastases. Based on this, an algorithm for the management of these patients was established. RESULTS: Platelet serotonin and chromogranin A are useful biomarkers for detection and follow-up of neuroendocrine tumour. Helical computed tomography and somatostatin receptor scintigraphy are the most sensitive diagnostic modalities. Surgical debulking is an accepted approach for reducing hormonal symptoms and to establish better conditions for medical treatment, but is frequently impossible due to the extent of disease. A novel approach is the local ablation of tumour by thermal coagulation using therapies such as radiofrequency ablation (RFA) or laser induced thermotherapy (LITT). These techniques preserve normal liver tissue. There is a tendency to destroy metastases early in the course of disease, thereby postponing or eliminating the surgically untreatable stage. This can be combined with postoperative radioactive octreotide to eliminate small multiple metastases. In patients with extensive metastases who are not suitable for local destruction, systemic therapy by octreotide, (131)I-MIBG treatment or targeted chemo- and radiotherapy should be attempted. A final option for selective patients is orthotopic liver transplantation. CONCLUSION: Treatment for patients with neuroendocrine hepatic metastases must be tailored for each individual patient. When local ablative therapies are used early in the course of the disease, the occurrence of carcinoid syndrome with end stage hepatic disease can be postponed or prevented
Auxiliary fermion approach to the resonant inelastic x-ray scattering response in an underdoped cuprate
We describe a method for calculating the resonant inelastic x-ray scattering (RIXS) response—including the dynamics of the transient core hole—of many-body systems with nontrivial gap structure encoded in their single particle Green's function. Our approach introduces auxiliary fermions in order to obtain a form amenable to the determinant method of Benjamin et al., [Phys. Rev. Lett. 112, 247002 (2014)], and is applicable to systems where interactions are most strongly felt through a renormalization of the single particle propagator. As a test case we consider the Yang-Rice-Zhang ansatz for cuprate phenomena in the underdoped “pseudogap” regime, which remains a popular tool for interpreting the results of experimental probes. We show that taking the core hole dynamics into account for a system described by this ansatz pushes the RIXS peaks towards higher energy transfer, improving agreement with experiments
2-year outcome of juvenile idiopathic arthritis in current daily practice: what can we tell our patients?
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