191 research outputs found
Enhanced electron correlations in FeSb
FeSb has been recently identified as a new model system for studying
many-body renormalizations in a -electron based narrow gap semiconducting
system, strongly resembling FeSi. The electron-electron correlations in
FeSb manifest themselves in a wide variety of physical properties including
electrical and thermal transport, optical conductivity, magnetic
susceptibility, specific heat and so on. We review some of the properties that
form a set of experimental evidences revealing the crucial role of correlation
effects in FeSb. The metallic state derived from slight Te doping in
FeSb, which has large quasiparticle mass, will also be introduced.Comment: 9 pages, 7 figures; submitted to Annalen der Physi
Signatures of phase transitions in the microwave response of YbRh2Si2
We used a spectroscopic microwave technique utilizing superconducting
stripline resonators at frequencies between 3 GHz and 15 GHz to examine the
charge dynamics of YbRh2Si2 at temperatures and magnetic fields close to the
quantum critical point. The different electronic phases of this heavy-fermion
compound, in particular the antiferromagnetic, Fermi-liquid, and
non-Fermi-liquid regimes, were probed with temperature-dependent microwave
measurements between 40 mK and 600 mK at a set of different magnetic fields up
to 140 mT. Signatures of phase transitions were observed, which give
information about the dynamic response of this peculiar material that exhibits
field-tuned quantum criticality and pronounced deviations from Fermi-liquid
theory.Comment: 5 pages, 3 figure
Highly Dispersive Electron Relaxation and Colossal Thermoelectricity in the Correlated Semiconductor FeSb
We show that the colossal thermoelectric power, , observed in the
correlated semiconductor FeSb below 30\,K is accompanied by a huge Nernst
coefficient and magnetoresistance MR. Markedly, the latter two
quantities are enhanced in a strikingly similar manner. While in the same
temperature range, of the reference compound FeAs, which has a
seven-times larger energy gap, amounts to nearly half of that of FeSb, its
and MR are intrinsically different to FeSb: they are smaller
by two orders of magnitude and have no common features. With the charge
transport of FeAs successfully captured by the density functional theory,
we emphasize a significantly dispersive electron-relaxation time
due to electron-electron correlations to be at the heart of
the peculiar thermoelectricity and magnetoresistance of FeSb.Comment: 8 pages, 5 figure
Microwave spectroscopy on heavy-fermion systems: probing the dynamics of charges and magnetic moments
Investigating solids with light gives direct access to charge dynamics,
electronic and magnetic excitations. For heavy fermions, one has to adjust the
frequency of the probing light to the small characteristic energy scales,
leading to spectroscopy with microwaves. We review general concepts of the
frequency-dependent conductivity of heavy fermions, including the slow Drude
relaxation and the transition to a superconducting state, which we also
demonstrate with experimental data taken on UPd2Al3. We discuss the optical
response of a Fermi liquid and how it might be observed in heavy fermions.
Microwave studies with focus on quantum criticality in heavy fermions concern
the charge response, but also the magnetic moments can be addressed via
electron spin resonance (ESR). We discuss the case of YbRh2Si2, the open
questions concerning ESR of heavy fermions, and how these might be addressed in
the future. This includes an overview of the presently available experimental
techniques for microwave studies on heavy fermions, with a focus on broadband
studies using the Corbino approach and on planar superconducting resonators.Comment: 11 pages, 6 figures, proceedings of QCnP 201
Effects of Disorder on the Competition between Antiferromagnetism and Superconductivity
Motivated by the observation of unusual magnetism in Ce_xCu_2Si_2 (), we study the effect of disorder, such as Ce vacancy, on the competition
between superconductivity (SC) and antiferromagnetism (AF) on the basis of the
phenomenological Ginzburg-Landau theory. Assuming that the AF-SC transition is
of first order in clean system, we show that a single impurity in the SC state
can induce staggered magnetization by suppressing the SC around it. For finite
concentration of impurities, the first-order AF-SC boundary in the clean case
is replaced by a finite region where the SC and the induced AF moments coexist
microscopically with spatially varying order parameters. We argue that spin
excitation spectrum in the coexistent state has a dual structure of SC gapped
mode and the low-energy spin-wave mode. In accordance with the emergence of AF
out of SC ground state, the spectral weight will be transferred from the former
mode to the latter, keeping the structure of both modes basically unchanged.Comment: 5 pages, 1 figure, submitted to J. Phys. Soc. Japa
Merging TopâDown and BottomâUp Approaches to Fabricate Artificial Photonic Nanomaterials with a Deterministic Electric and Magnetic Response
Artificial photonic nanomaterials made from densely packed scatterers are frequently realized either by top-down or bottom-up techniques. While top-down techniques offer unprecedented control over achievable geometries for the scatterers, by trend they suffer from being limited to planar and periodic structures. In contrast, materials fabricated with bottom-up techniques do not suffer from such disadvantages but, unfortunately, they offer only little control on achievable geometries for the scatterers. To overcome these limitations, a nanofabrication strategy is introduced that merges both approaches. A large number of scatterers are fabricated with a tailored optical response by fast character projection electron-beam lithography and are embedded into a membrane. By peeling-off this membrane from the substrate, scrambling, and densifying it, a bulk material comprising densely packed and randomly arranged scatterers is obtained. The fabrication of an isotropic material from these scatterers with a strong electric and magnetic response is demonstrated. The approach of this study unlocks novel opportunities to fabricate nanomaterials with a complex optical response in the bulk but also on top of arbitrarily shaped surfaces
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Novel UV-transparent 2-component polyurethane resin for chip-on-board LED micro lenses
In this work we present a novel optical polymer system based on polyurethane elastomer components, which combines excellent UV transparency with high thermal stability, good hardness, high surface tension and long pot life. The material looks very promising for encapsulation and microlensing applications for chip-on-board (CoB) light-emitting diodes (LED). The extinction coefficient k, refractive index n, and bandgap parameters were derived from transmission and reflection measurements in a wavelength range of 200-890 nm. Thermogravimetry and differential scanning calorimetry were used to provide glass transition and degradation temperatures. The surface tension was determined by means of contact angle measurements. As proof of concept, a commercial InGaN-CoB-LED is used to demonstrate the suitability of the new material for the production of microlenses. © 2020 Optical Society of America
âIch bin eine MĂ€rchenerzĂ€hlerin. So wurde ich geboren.â
Ist Tonke Dragt die groĂe Neuerin der europĂ€ischen Kinder- und Jugendliteratur? Diese sicherlich provokante Frage nimmt der Sammelband in den Fokus, der auf die Tagung â'Ich bin eine MĂ€rchenerzĂ€hlerin. So wurde ich geboren'. Tonke Dragts Jugendromane â uÌbersehene Klassiker?", die im September 2019 an der UniversitĂ€t Siegen in Kooperation mit Helma van Lierop-Debrauwer stattgefunden hat, zurĂŒckgeht
Magnon-magnon interactions in the Spin-Peierls compound CuGeO_3
In a magnetic substance the gap in the Raman spectrum, Delta_R, is
approximatively twice the value of the neutron scattering gap, Delta_S, if the
the magnetic excitations (magnons) are only weakly interacting.
But for CuGeO_3 the experimentally observed ratio Delta_R/Delta_S is
approximatively 1.49-1.78, indicating attractive magnon-magnon interactions in
the quasi-1D Spin-Peierls compound CuGe_3.
We present numerical estimates for Delta_R/Delta_S from exact diagonalization
studies for finite chains and find agreement with experiment for intermediate
values of the frustration parameter alpha.
An analysis of the numerical Raman intensity leads us to postulate a
continuum of two-magnon bound states in the Spin-Peierls phase. We discuss in
detail the numerical method used, the dependence of the results on the model
parameters and a novel matrix-element effect due to the dimerization of the
Raman-operator in the Spin-Peierls phase.Comment: submitted to PRB, Phys. Rev. B, in pres
Helmholtz Open Science Workshop âZugang zu und Nachnutzung von wissenschaftlicher Softwareâ #hgfos16, Report; November 2016
Der Report des Helmholtz Open Science Workshops âZugang zu und Nachnutzung von wissenschaftlicher Softwareâ #hgfos16 behandelt die Themen Standards und QualitĂ€tssicherung; Reproduzierbarkeit; Lizenzierung und weitere rechtliche Aspekte; Zitation und Anerkennung; Sichtbarkeit und ModularitĂ€t; GeschĂ€ftsmodelle; Personal,
Ausbildung, Karrierewege. Diese Themen sind eng miteinander verzahnt. FĂŒr jeden Themenbereich werden jeweils die Relevanz, Fragestellungen, Herausforderungen, mögliche LösungsansĂ€tze und Handlungsempfehlungen betrachtet
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