Fachbereichs-Kolloquium im WS 2020/21
(letzte Änderung: 12.02.2021, 09:00)
26.11.2020
Mikhail M. Glazov
Ioffe institute, st. Petersburg, russia
Einladender: Dr. Arora | PDF |
Zugangsdaten:
https://wwu.zoom.us/j/95528553602
Passwort: Phys20-21
Quantum effects in optics and transport of excitons in atomically thin semiconductors
Two-dimensional (2D) semiconductor systems based on quantum wells have revolutionized modern electronics and optoelectronics. Recently, another class of stable 2D materials starting with graphene and transition metal dichalcogenides (TMDCs) such as MoS2 have revealed novel physics at the fundamental level offering unique functionalities relevant for future applications in ‘valleytronics’ and quantum technologies. Atomically thin TMDCs possess spectacular optical and transport properties. Strong Coulomb interaction binds electrons and holes in excitons: neutral quasi-particles with considerable (up to several hundreds of meV) binding energies [1]. I present a brief overview of the key quantum effects in excitonic transport and optical properties in my talk. Particularly, I will describe the main features of the band structure and excitonic properties of TMDC monolayers and then I will focus on two phenomena including the Valley Hall effect and the charged exciton-polarons. In this talk, I will emphasize the phenomena inherent to the atomically thin semiconductors.References:
[1] Gang Wang, et al. Rev. Mod. Phys. 90, 021001 (2018)
[2] M. M. Glazov and L. E. Golub, et al. Phys. Rev. Lett. 125, 157403 (2020)
[3] A. Arora, et al. Phys. Rev. Lett. 123, 167401 (2019)
[4] K. Wagner, et al. arXiv:2007.05396 (2020)
Schematics of the VHE for excitons, with the black arrow Fd showing the drag force and arrows denoting the propagation directions of the excitons in the opposite valleys. The figure illustrates the skew scattering where the excitons are separated due to the asymmetric scattering by impurities or phonons.© M. Glazov
03.12.2020
Pd dr. Harry Mönig
WWU Münster, Physikalisches Institut
Antrittsvorlesung
Einladender: Dekan des FB Physik | PDF |
Zugangsdaten:
https://wwu.zoom.us/j/95528553602
Passwort: Phys20-21
Nanoscale interface analytics for solar cells and in surface chemistry
In this lecture, I will present experiments from scanning probe microscopy, which allows imaging the topography of a surface with an atomic-scale resolution. Furthermore, the methodology provides spectroscopic information about the local surface electronic structure or about the interatomic force interaction with piconewton sensitivity. As a complementary technique we use photoelectron spectroscopy providing not only information about the surface composition, but also about the global electronic properties and chemical binding states. With these correlating methods, we investigate the defect physics of materials used in thin film solar cells. In particular, we analyse the defect-electronic structure and dipole layer formation with a spatial resolution of a few nanometers. Our results help to develop new strategies for the efficiency optimization of these solar cells.In a different research field, we focus on the properties of various atomically defined probe tips allowing for ultrahigh resolution in atomic force microscopy. Recently, we demonstrated the outstanding imaging properties of a copper-based tip, which is terminated by a covalently bound single oxygen atom. Based on the high structural stability of this tip, we were able to image single hydrogen bonds in a complex organic network. In our latest work we perform force measurements during the controlled manipulation of single xenon atoms on a laterally anisotropic surface with various atomically defined tip structures. Our results show drastic differences in the force interaction from tip to tip providing fundamental insights in the nano-mechanical properties and chemical reactivity in atomically defined contacts.
A, Schematic representation of a force measurement at an oxide boundary during the lateral displacement of a single atom by an atomically defined tip (here: Xe-tip). B, Topography image of a single Xe atom nucleated next to the oxide domain boundary (upper panel). Vertical force field recorded during the manipulation experiment with a Xe-tip (lower panel). C, Corresponding lateral forces and energy dissipation at various tip heights.© H. Mönig
17.12.2020, 16 Uhr c.t.
laura fabbietti
TU MÜNCHEN, Physik-Department, Dense and Strange hadronic matter
Einladender: Prof. Andronic | PDF |
Zugangsdaten:
https://wwu.zoom.us/j/95528553602
Passwort: Phys20-21
The 'Alice' experiment at the lhc opens a new avenue for nuclear physics
The study of the effective strong interaction among hadrons is one of the frontier of the standard model of nuclear and particle physics. Indeed, most of interactions among stable or unstable hadrons have not been measured yet and theoretical calculations starting from first principles, such as quarks and gluons, are mostly under development.For stable nucleons, scattering experiments have been successfully employed in the past to measure two-body interactions and theoretical calculations based on chiral effective field theory are extremely successful in describing such interactions. First principle calculations on the other hand are still lacking. For hyperon-nucleon pairs such as Λ-p, Σ-p, Ξ-p the nature of the instable hyperon beams makes such measurements very difficult and consequently only scarce experimental data are available. Hyperon-hyperon interactions cannot be accessed at all with this technique.These interactions are particularly interesting also because of their connection to the physics of neutron stars. Indeed, these strong interactions drive the equation of state (EoS) of dense neutron-rich matter with strange quark content and such EoS can be tested against the measurements of neutron star masses, radii and newly detected gravitational wave signals.In this talk we show how p+p and p+Pb collisions measured by ALICE at the LHC have been exploited to study several, so far unkown, hyperon-nucleon and hyperon-hyperon interactions. Among others, we have observed for the first time the attractive pΞ- and pΩ- strong interactions. For both systems, first principle calculations based on gauge lattice models are available and could be tested for the first time. The pΞ- measurement is also relevant for the physics of neutron stars and the impact of the new results will be discussed.We will demonstrate how these new measurements open a new era for hadron physics with the possibility of measuring in the future also three-body interactions for hyperons and nucleons and any stable or unstable hadron pairs.
© L. Fabbietti
07.01.2021
stefan schÖnert
TU München, Physik-Department, experimentelle Astroteilchenphysik
Einladender: Prof. Weinheimer | PDF |
Zugangsdaten:
https://wwu.zoom.us/j/95528553602
Passwort: Phys20-21
The quest for majorana neutrinos with 'Gerda' and 'Legend'
Since neutrinos have no electric charges, they may be their own antiparticles, referred to as Majorana neutrinos, and thus violate lepton number conservation. Neutrinoless double beta decay would be a direct consequence, and the search for this decay mode is the most sensitive method to unravel the Majorana nature of neutrinos. By operating bare germanium diodes, enriched in Ge-76, in an active liquid argon shield, GERDA achieved an unprecedently low background index of 5.2 x 10−4 counts/keV kg yr in the signal region and met the design goal to collect an exposure of 100 kg yr in a background-free regime. When combined with the result of Phase I, no signal is observed after 127.2 kg yr of total exposure. A limit on the half-life of 0νββ decay in Ge-76 is set at T1/2 > 1.8 x 1026 yr at 90% C.L. [1], which coincides with the sensitivity assuming no signal. Majorana neutrino masses are therefore are constrained to mββ 79–180 meV at 90% C.L.. The new LEGEND Collaboration was founded in 2016 to develop a phased, Ge-76-based double-beta decay experimental program with discovery potential a half-life beyond 1028 years, using existing resources as appropriate to expedite physics results. Its first stage, LEGEND-200, is currently under preparation, re-purposing the GERDA experimental infrastructures at LNGS, Italy, and is scheduled to go into commissioning in 2021. In parallel, we are preparing the design for the ton-scale LEGEND-1000 stage of the experiment. In this talk, I will present the final results of GERDA and discuss the preparatory works and plans for LEGEND.[1] Final Results of GERDA on the Search for Neutrinoless Double-β Decay
14.01.2021
MARC AẞMANN
TU DORTMUND, fakultät physik, experimentelle physik 2
Einladende: Jun.-Prof. Reiter | PDF |
Zugangsdaten:
https://wwu.zoom.us/j/95528553602
Passwort: Phys20-21
An Unexpected Twist - Vortex Control in Optically Imprinted Polariton Landscapes
Exciton-polaritons are mixed light-matter quasiparticles arising due to the strong coupling of photons and excitons inside a semiconductor microcavity. As they are composite bosons of low mass, they may form a macroscopically coherent non-equilibrium condensate at elevated temperatures [1], which led to fascinating demonstrations of superfluidity [2] and quantized vortices [3] in the solid state. The photons leaking from the cavity are a part of the polariton wave function. Thus, the properties of the polariton condensate, including its energy, spin, momentum and phase may be investigated by spectroscopic means.Under non-resonant excitation, polaritons form spontaneously from free carriers and form a condensate that interacts with them. As polaritons are several orders of magnitude lighter than these carriers, the latter effectively form a static repulsive potential for the condensed polaritons. Spatially shaping the excitation beam then enables us to investigate polariton condensates in optically imprinted potential landscapes. We show how to utilize them to control the momentum and direction of flowing polaritons. Further, we discuss the creation of quantized vortices that couple directly to the orbital angular momentum of the emitted light. Finally, we demonstrate how to robustly switch the flow direction of such a vortex using a time-dependent potential and monitor the switching dynamics using a dedicated orbital angular momentum spectroscopy technique with picosecond temporal resolution [4], which opens up new perspectives for polaritonics.References:[1] J. Kasprzak et al., Nature 443, 409 (2006).
[2] A. Amo et al., Nat. Phys. 5, 805 (2009).
[3] K.G. Lagoudakis et al., Nat. Phys. 4, 706 (2008).
[4] X. Ma et al., Nat. Comm. 11, 897 (2020).
© M. Aßmann
11.02.2021
TETYANA GALATYUK
Institut für kernphysik, tu darmstadt
Einladender: Prof. Andronic | PDF |
Zugangsdaten:
https://wwu.zoom.us/j/95528553602
Passwort: Phys20-21
Shine a Light! When Matter Shatters
What happens when gold nuclei, accelerated to about 90% of the speed of light, strike gold nuclei at rest? For an extremely short time, t~10-23 seconds, states of matter at extreme temperatures (1012 K) and densities (> 280 Mt/cm3) are produced. The possibility to form and explore in the laboratory strongly interacting matter under conditions similar to those realized a few microseconds after the ”Big Bang”, or still existing today in the interior of compact stellar objects is truly fascinating.Virtual photons, the generalized form of electromagnetic radiation, materialize after short time by formation of a pair of charged leptons, e.g. an electron and a positron. Throughout the course of a heavy-ion collision such photons offer the unique opportunity to directly monitor “Roentgen-images” (in-medium electromagnetic spectral functions) and to measure “Planck-like-spectra” (temperature of the emitting source) of strongly interacting matter.This talk will discuss important experimental results on emissivity of matter obtained by the High Acceptance DiElectron Spectrometer at heavy-ion synchrotron SIS18, GSI-Darmstadt. A deeper understanding of the microscopic origin of the excess radiation requires systematic investigation of di-electron radiation emitted from baryonic resonances produced off protons in pion-induced reactions. These are studied in HADES at GSI making use of pion beams.
© Tetyana Galatyuk/TU Darmstadt