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Search Results: 1 - 10 of 31875 matches for " Thomas Maier "
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Stability of Impurities with Coulomb Potential in Graphene with Homogeneous Magnetic Field
Thomas Maier,Heinz Siedentop
Physics , 2011, DOI: 10.1063/1.4728982
Abstract: Given a 2-dimensional no-pair Weyl operator with a point nucleus of charge Z, we show that a homogeneous magnetic field does not lower the critical charge beyond which it collapses.
Superconductivity in striped and multi-Fermi-surface Hubbard models: From the cuprates to the pnictides
Thomas A. Maier
Physics , 2012, DOI: 10.1007/s10948-012-1600-7
Abstract: Single- and multi-band Hubbard models have been found to describe many of the complex phenomena that are observed in the cuprate and iron-based high-temperature superconductors. Simulations of these models therefore provide an ideal framework to study and understand the superconducting properties of these systems and the mechanisms responsible for them. Here we review recent dynamic cluster quantum Monte Carlo simulations of these models, which provide an unbiased view of the leading correlations in the system. In particular, we discuss what these simulations tell us about superconductivity in the homogeneous 2D single-orbital Hubbard model, and how charge stripes affect this behavior. We then describe recent simulations of a bilayer Hubbard model, which provides a simple model to study the type and nature of pairing in systems with multiple Fermi surfaces such as the iron-based superconductors.
Enhanced Superconductivity in Superlattices of High-$T_c$ Cuprates
Satoshi Okamoto,Thomas A. Maier
Physics , 2008, DOI: 10.1103/PhysRevLett.101.156401
Abstract: The electronic properties of multilayers of strongly correlated models for cuprate superconductors are investigated using cluster dynamical mean-field techniques. We focus on combinations of underdoped and overdoped layers and find that the superconducting order parameter in the overdoped layers is enhanced by the proximity effect of the strong pairing scale originating from the underdoped layers. The enhanced order parameter can even exceed the maximum value in uniform systems. This behavior is well reproduced in slave-boson mean-field calculations which also find higher transition temperatures than in the uniform system.
Microscopic Inhomogeneity and Superconducting Properties of a Two-dimensional Hubbard Model for High-$T_c$ Cuprate
Satoshi Okamoto,Thomas A. Maier
Physics , 2010, DOI: 10.1103/PhysRevB.81.214525
Abstract: Recent scanning tunneling microscopy measurements on cuprate superconductors have revealed remarkable spatial inhomogeneities in the single-particle energy gap. Using cellular dynamical mean-field theory, we study the zero temperature superconducting properties of a single-band Hubbard model with a spatial modulation of the electron density. We find that the inhomogeneity in the electronic structure results in a substantial spatial variation in the superconducting order parameter and single-particle energy gap, reminiscent of the experimental results. In particular, we find that the order parameter and gap amplitudes in the hole-rich regions are significantly enhanced over the corresponding quantities in a uniform system, if the hole-rich regions are embedded in regions with smaller hole density.
Development of a practical tool to measure the impact of publications on the society based on focus group discussions with scientists
Thomas Niederkrotenthaler, Thomas E Dorner, Manfred Maier
BMC Public Health , 2011, DOI: 10.1186/1471-2458-11-588
Abstract: This qualitative study presents the development of a practical tool to measure the societal impact of publications based on 8 focus group discussions with 24 biomedical scientists at the Medical University Vienna between May 2008 and May 2009. Topics focused on (1) features of an ideal tool, (2) criteria that should be considered in the assessment, and (3) the identification of practical pitfalls. In an iterative exercise involving the repeated application of the drafted tool to scientific papers, criteria for the assessment were refined. A small-scale exercise to evaluate the tool in terms of its comprehensibility, relevance and practicability was conducted using questionnaires for 6 external experts in leading positions of public health, and yielded acceptable results.The tool developed consists of three quantitative dimensions, that is (1) the aim of a publication, (2) the efforts of the authors to translate their research results, and, if translation was accomplished, (3) (a) the size of the area where translation was accomplished (regional, national or international), (b) its status (preliminary versus permanent) and (c) the target group of the translation (individuals, subgroup of population, total population).Focus group discussions with scientists suggested that the societal impact factor of a publication should consider the effect of the publication in a wide set of non-scientific areas, but also the motivation behind the publication, and efforts by the authors to translate their findings. The proposed tool provides some valuable insights for further research and practical applications in the topic area.To date, more than thousand different scales exist which aim at measuring scientific performance. The most commonly used metrics are citation-based [1], with the Institute of Scientific Information (ISI) impact factor representing the most widespread measure. The impact factor has been shown to influence editorial decision making as well as author citation b
Two-particle correlations in a dynamic cluster approximation with continuous momentum dependence: Superconductivity in the 2D Hubbard model
Peter Staar,Thomas Maier,Thomas Schulthess
Physics , 2014, DOI: 10.1103/PhysRevB.89.195133
Abstract: The DCA$^+$ algortihm was recently introduced to extend the dynamic cluster approximation (DCA) with a continuous lattice self-energy in order to achieve better convergence with cluster size. Here we extend the DCA$^+$ algorithm to the calculation of two-particle correlation functions by introducing irreducible vertex functions with continuous momentum dependence consistent with the DCA$^+$ self-energy. This enables a significantly more controlled and reliable study of phase transitions than with the DCA. We test the new method by calculating the superconducting transition temperature $T_{c}$ in the attractive Hubbard model and show that it reproduces previous high-precision determinantal quantum Monte Carlo results. We then calculate $T_c$ in the doped repulsive Hubbard model, for which previous DCA calculations could only access the weak-coupling ($U=4t$) regime for large clusters. We show that the new algorithm provides access to much larger clusters and delivers asymptotically converged results for $T_c$ for both the weak ($U=4t$) and intermediate ($U=7t$) coupling regimes, and thereby enables the accurate determination of the exact infinite cluster size result.
DCA$^+$: Dynamical Cluster Approximation with continuous lattice self-energy
Peter Staar,Thomas Maier,Thomas C. Schulthess
Physics , 2013, DOI: 10.1103/PhysRevB.88.115101
Abstract: The dynamical cluster approximation (DCA) is a systematic extension beyond the single site approximation in dynamical mean field theory (DMFT), to include spatially non-local correlations in quantum many-body simulations of strongly correlated systems. We extend the DCA with a continuous lattice self-energy in oder to achieve better convergence with cluster size. The new method, which we call DCA$^+$, cures the cluster shape dependence problems of the DCA, without suffering from causality violations of previous attempts to interpolate the cluster self-energy. A practical approach based on standard inference techniques is given to deduce the continuous lattice self-energy from an interpolated cluster self-energy. We study the pseudogap region of a hole-doped two-dimensional Hubbard model and find that in the DCA$^+$ algorithm, the self-energy and pseudo-gap temperature $T^*$ converge monotonously with cluster size. Introduction of a continuous lattice self-energy eliminates artificial long-rage correlations and thus significantly reduces the sign problem of the quantum Monte Carlo cluster solver in the DCA$^+$ algorithm compared to the normal DCA. Simulations with much larger cluster sizes thus become feasible, which, along with the improved convergence in cluster size, raises hope that precise extrapolations to the exact infinite cluster size limit can be reached for other physical quantities as well.
Epithelial-Mesenchymal Transition in Pancreatic Carcinoma
Harald J. Maier,Thomas Wirth,Hartmut Beug
Cancers , 2010, DOI: 10.3390/cancers2042058
Abstract: Pancreatic carcinoma is the fourth-leading cause of cancer death and is characterized by early invasion and metastasis. The developmental program of epithelial-mesenchymal transition (EMT) is of potential importance for this rapid tumor progression. During EMT, tumor cells lose their epithelial characteristics and gain properties of mesenchymal cells, such as enhanced motility and invasive features. This review will discuss recent findings pertinent to EMT in pancreatic carcinoma. Evidence for and molecular characteristics of EMT in pancreatic carcinoma will be outlined, as well as the connection of EMT to related topics, e.g., cancer stem cells and drug resistance.
Absence of the d-Density Wave State in 2D Hubbard Model
Alexandru Macridin,Mark Jarrell,Thomas Maier
Physics , 2004, DOI: 10.1103/PhysRevB.70.113105
Abstract: Using the Dynamical Cluster Approximation (DCA) we calculate the alternating circulating-current susceptibility and investigate the transition to the d-density wave (DDW) order in the two-dimensional Hubbard model. The 2 x 2 cluster used in the DCA calculation is the smallest that can capture d-wave order; therefore, due to the mean-field character of our calculation, we expect to overestimate d-wave transition temperatures. Despite this, we found no transition to the DDW state. In the pseudogap region the DDW susceptibility is enhanced, as predicted by the slave boson SU(2) theory, but it still is much smaller than the d-wave pairing susceptibility.
Coexistence of strong nematic and superconducting correlations in a two-dimensional Hubbard model
Shi-Quan Su,Thomas A. Maier
Physics , 2011, DOI: 10.1103/PhysRevB.84.220506
Abstract: Using a dynamic cluster quantum Monte Carlo approximation, we study a two-dimensional Hubbard model with a small orthorhombic distortion in the nearest neighbor hopping integrals. We find a large nematic response in the low-frequency single-particle scattering rate which develops with decreasing temperature and doping as the pseudogap region is entered. At the same time, the d-wave superconducting gap function develops an s-wave component and its amplitude becomes anisotropic. The strength of the pairing correlations, however, is found to be unaffected by the strong anisotropy, indicating that d-wave superconductivity can coexist with strong nematicity in the system.
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