Nouvelles propriétés de transport dans les systèmes d'électrons multicouches

Ce travail de cette thèse présente les études sur l'influence du nouveau dégrée de liberté quantique, causé par le couplage tunnel entre les couches, sur les propriétés de transport des multi-puits quantiques dans un champ magnétique, à basse température, et sous irradiation micro-ondes. De nouvelles oscillations de résistance sont observées dans les systèmes d’électrons bi- et multicouches. Elles résultent d'une interférence entre les oscillations entre les sous-bandes et les oscillations induites par les micro-ondes. Des états à résistance nulle apparaissent lorsque les systèmes bicouches de haute qualité sous irradiation micro-ondes même en présence d’une diffusion additionnelle. Le mécanisme inélastique de la photorésistance est la contribution dominante à basses températures et sous un champ électronique modéré. Ce modèle confirme l'intégrité des estimations théoriques pour le temps de relaxation inélastique et mène à une explication satisfaisante de la photorésistance dans les systèmes d’électrons bi-et multicouches. \ud Dans un champ magnétique intense, la suppression de l’effet tunnel entre les couches provoque des nouveaux états corrélés à cause d’une interaction électron-électron entre les différentes couches. Dans cette thèse, les systèmes électroniques tricouches, formés par de triples puits quantiques révèlent de nouveaux états de l’effet Hall Quantique fractionnaire si l’effet tunnel est supprimé par une composante parallèle du champ magnétique aux très basses températures (mK).----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------This work is devoted to the investigation of the influence of the additional quantum degree of freedom caused by tunnel coupling on transport properties of multilayer electron systems in magnetic fields, at low temperatures and under microwave excitation. Microwave-induced resistance oscillations in bi- and multilayer electron systems are the consequence of an interference of magneto-intersubband and microwave-induced resistance oscillations which leads to peculiar oscillations in magnetoresistance. High-quality bilayer systems exposed to microwave irradiation exhibit zero-resistance states even in the presence of intersubband scattering. The inelastic mechanism of microwave photoresistance is found to be the dominant contribution at low temperatures and moderate microwave electric field. This model confirms the reliability of theoretical estimates for the inelastic relaxation time and leads to a satisfactory explanation of photoresistance in bi- and multilayer electron systems. \ud In high magnetic fields, the suppression of tunnelling between layers causes new correlated states owing to electron-electron interaction in neighboured layers. In this thesis, trilayer electron systems formed by triple quantum wells reveal new fractional quantum Hall states if tunnelling is suppressed by a parallel component of the magnetic field at mK temperatures.\ud \u

Temperature-driven transition from a semiconductor to a topological insulator

We report on a temperature-induced transition from a conventional semiconductor to a two-dimensional topological insulator investigated by means of magnetotransport experiments on HgTe/CdTe quantum well structures. At low temperatures, we are in the regime of the quantum spin Hall effect and observe an ambipolar quantized Hall resistance by tuning the Fermi energy through the bulk band gap. At room temperature, we find electron and hole conduction that can be described by a classical two-carrier model. Above the onset of quantized magnetotransport at low temperature, we observe a pronounced linear magnetoresistance that develops from a classical quadratic low-field magnetoresistance if electrons and holes coexist. Temperature-dependent bulk band structure calculations predict a transition from a conventional semiconductor to a topological insulator in the regime where the linear magnetoresistance occurs.Comment: 7 pages, 6 figure

The green brand: Explicit and implicit framing effects of ecolabelling on brand knowledge

An increasing number of brands rely on ecolabelling as associative branding strategy for communicating their sustainability performance. Specifically, associative branding aims to link a brand to an ecolabel to embed sustainability into brand knowledge to provide an added value for consumers. In this regard, the present study applies a combined measurement approach that examines both implicit and explicit information processing to assess consumers’ brand associations related to sustainability. Specifically, a pre/post-testing is conducted to evaluate the framing effect of ecolabel exposure in a brand advertisement on the enhancement of brand knowledge. The results of the current study provide evidence that in particular implicit brand sustainability is a favorable brand association that positively affects consumers’ decision-making and preferences. Furthermore, the results reveal a relevant framing effect of ecolabelling on the enhancement of brand knowledge, on both implicit and explicit levels. In addition, the study results indicate the existence of three efficacy parameters that influence brand knowledge transfer: adequate ecolabel-brand fit, early ecolabel recognition time, and distinct implicit-explicit knowledge acquisition

Orbital Fulde–Ferrell–Larkin–Ovchinnikov state in an Ising superconductor

In superconductors possessing both time and inversion symmetries, the Zeeman effect of an external magnetic field can break the time-reversal symmetry, forming a conventional Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state characterized by Cooper pairings with finite momentum1,2. In superconductors lacking (local) inversion symmetry, the Zeeman effect may still act as the underlying mechanism of FFLO states by interacting with spin–orbit coupling (SOC). Specifically, the interplay between the Zeeman effect and Rashba SOC can lead to the formation of more accessible Rashba FFLO states that cover broader regions in the phase diagram3–5. However, when the Zeeman effect is suppressed because of spin locking in the presence of Ising-type SOC, the conventional FFLO scenarios are no longer effective. Instead, an unconventional FFLO state is formed by coupling the orbital effect of magnetic fields with SOC, providing an alternative mechanism in superconductors with broken inversion symmetries6–8. Here we report the discovery of such an orbital FFLO state in the multilayer Ising superconductor 2H-NbSe2. Transport measurements show that the translational and rotational symmetries are broken in the orbital FFLO state, providing the hallmark signatures of finite-momentum Cooper pairings. We establish the entire orbital FFLO phase diagram, consisting of a normal metal, a uniform Ising superconducting phase and a six-fold orbital FFLO state. This study highlights an alternative route to achieving finite-momentum superconductivity and provides a universal mechanism to preparing orbital FFLO states in similar materials with broken inversion symmetries.</p

Outcome after stroke attributable to baseline factors-The PROSpective Cohort with Incident Stroke (PROSCIS)

Background The impact of risk factors on poor outcome after ischemic stroke is well known, but estimating the amount of poor outcome attributable to single factors is challenging in presence of multimorbidity. We aim to compare population attributable risk estimates obtained from different statistical approaches regarding their consistency. We use a real-life data set from the PROSCIS study to identify predictors for mortality and functional impairment one year after first-ever ischemic stroke and quantify their contribution to poor outcome using population attributable risks. Methods The PROSpective Cohort with Incident Stroke (PROSCIS) is a prospective observational hospital-based cohort study of patients after first-ever stroke conducted independently in Berlin (PROSCIS-B) and Munich (PROSCIS-M). The association of baseline factors with poor outcome one year after stroke in PROSCIS-B was analysed using multiple logistic regression analysis and population attributable risks were calculated, which were estimated using sequential population attributable risk based on a multiple generalized additive regression model, doubly robust estimation, as well as using average sequential population attributable risk. Findings were reproduced in an independent validation sample from PROSCIS-M. Results Out of 507 patients with available outcome information after 12 months in PROSCIS-B, 20.5% suffered from poor outcome. Factors associated with poor outcome were age, pre-stroke physical disability, stroke severity (NIHSS), education, and diabetes mellitus. The order of risk factors ranked by magnitudes of population attributable risk was almost similar for all methods, but population attributable risk estimates varied markedly between the methods. In PROSCIS-M, incidence of poor outcome and distribution of baseline parameters were comparable. The multiple logistic regression model could be reproduced for all predictors, except pre-stroke physical disability. Similar to PROSCIS-B, the order of risk factors ranked by magnitudes of population attributable risk was almost similar for all methods, but magnitudes of population attributable risk differed markedly between the methods. Conclusions Ranking of risk factors by population impact is not affected by the different statistical approaches. Thus, for a rational decision on which risk factor to target in disease interventions, population attributable risk is a supportive tool. However, population attributable risk estimates are difficult to interpret and are not comparable when they origin from studies applying different methodology. The predictors for poor outcome identified in PROSCIS-B have a relevant impact on mortality and functional impairment one year after first-ever ischemic stroke

Distinct switching of chiral transport in the kagome metals KV3_3Sb5_5 and CsV3_3Sb5_5

The kagome metals AV$_3$Sb$_5$ (A=K,Rb,Cs) present an ideal sandbox to study the interrelation between multiple coexisting correlated phases such as charge order and superconductivity. So far, no consensus on the microscopic nature of these states has been reached as the proposals struggle to explain all their exotic physical properties. Among these, field-switchable electric magneto-chiral anisotropy (eMChA) in CsV$_3$Sb$_5$ provides intriguing evidence for a rewindable electronic chirality, yet the other family members have not been likewise investigated. Here, we present a comparative study of magneto-chiral transport between CsV$_3$Sb$_5$ and KV$_3$Sb$_5$. Despite their similar electronic structure, KV$_3$Sb$_5$ displays negligible eMChA, if any, and with no field switchability. This is in stark contrast to the non-saturating eMChA in CsV$_3$Sb$_5$ even in high fields up to 35 T. In light of their similar band structures, the stark difference in eMChA suggests its origin in the correlated states. Clearly, the V kagome nets alone are not sufficient to describe the physics and the interactions with their environment are crucial in determining the nature of their low-temperature state

A dual function for chaperones SSB–RAC and the NAC nascent polypeptide–associated complex on ribosomes

In addition to assisting with protein folding, SSB and NAC also regulate ribosome biogenesis (see also companion paper from Albanèse et al. in this issue)