Spontaneous symmetry-breaking in trilayer graphene

Multilagen-Graphen besteht aus mehreren atomar dünnen Schichten von Kohlenstoffatomen und weist eine Vielzahl ungewöhnlicher elektrischer Eigenschaften auf. Unter anderem wurde vorhergesagt, dass abhängig von externen elektrischen und magnetischen Feldern und je nach Lagenfolge der einzelnen Kohlenstofflagen der Grundzustand des Systems in einen korrelierten Zustand übergehen kann, der spontan Symmetrien des Systems bricht. Diese Zustände sind aber typischerweise sehr instabil und können nur in hochreinen Proben mit homogener Lagenfolge sichtbar gemacht werden. Im ersten Teil dieser Arbeit wurden die elektrischen Eigenschaften dieser korrelierten Zustände sowohl in Dreilagen-Graphen mit Bernal-Lagenfolge (ABA) als auch in Dreilagen-Graphen mit rhomboedrischer (ABC) Lagenfolgen genauer untersucht: In ABA Dreilagen-Graphen ist die Bandstruktur stark von externen elektrischen Feldern abhängig und bildet bei stärker werdenden Feldern mehrere zusätzliche Dirac-Kegel um den zentralen Dirac-Punkt der Bandstruktur aus. In dieser Arbeit wird gezeigt, dass mithilfe elektrischer und magnetischer Felder die Elektron-Elektron Wechselwirkung innerhalb der Dirac-Kegel verstärkt werden kann, bis der Grundzustand kontrolliert in einen korrelierten Zustand übergeht, der die Rotationssymmetrie des Systems spontan bricht. Anders verhält es sich in ABC Dreilagen-Graphen, dessen elektrische Struktur eine starke Berry-Krümmung und chirale Quasiteilchen aufweist. Als Folge davon wurde unter anderem vorhergesagt, dass bei verschwindender Ladungsträgerdichte mehrere spontane Quanten-Hall-Zustände auftreten können, die die chirale Symmetrie des Systems brechen. Messungen der Magnetotransporteigenschaften bei verschiedenen elektrischen und magnetischen Felder lassen ein vielfältiges Phasendiagramm der Quanten-Hall-Zustände erkennen, dass sogar Zustände mit intrinsischem orbitalem magnetischem Moment und Hall-Leitfähigkeit beinhaltet. Diese Erkenntnisse tragen zum tieferen Verständnis der korrelierten Zustände in Multilagen-Graphen und der Wechselwirkung von Ladungsträgern in zweidimensionalen Materialien bei. Im zweiten Teil der Arbeit wird der Ladungstransport in anorganischen Halogenid-Perowskit Nanodrähten untersucht. Halogenid-Perowskit Materialien haben aufgrund ihrer außergewöhnlichen optoelektronischen Eigenschaften bereits vielfach Anwendung als Basismaterial für Solarzellen und Photodetektoren gefunden. In diesem Teil der Arbeit wird der Ladungstransport in Feldeffekt-Transistoren mit CsPbBr3 Perowskit Nanodrähten in Abhängigkeit der Temperatur untersucht. Es wird gezeigt, dass ohne Beleuchtung der Probe der Ladungstransport stark von tiefen Fallzuständen dominiert wird und bei niedrigen Temperaturen komplett ausfriert. Wenn die Probe jedoch beleuchtet wird, steigt die Mobilität der Ladungsträger stark an und wird bei niedrigen Temperaturen nur von der Phononenstreuung limitiert. Diese Ergebnisse unterstreichen die Defekt-Toleranz, die häufig mit Perowskit-Materialien in Verbindung gebracht wird und liefern neue Einsichten in die elektrischen Fallenzustände in CsPbBr3 Perowskit-Nanodrähten.In recent years, multilayer graphene, a stack of several atomically thin layers of carbon atoms, has attracted growing interest due to its intriguing electronic properties and exceptional tunability. Depending on its stacking order, multilayer graphene has been predicted to be susceptible to a variety of correlated broken-symmetry ground states that can be accessed and explored upon appropriate tuning of its electronic structure via electrostatic gating and magnetic fields. However, in order to reveal these fragile states, excellent device quality and stacking order homogeneity are prerequisite. In this thesis, magnetotransport in Bernal-stacked (ABA) trilayer graphene encapsulated in hexagonal boron nitride as well as suspended rhombohedral (ABC) trilayer graphene is investigated. Depending on the stacking order, two families of correlated states that spontaneously break symmetries of the system are observed. In ABA trilayer graphene, external electric fields strongly deform the band structure and lead to the emergence of multiple off-center Dirac points (Dirac gullies). It is demonstrated that electric and magnetic fields can be used as tuning parameters to enhance electron-electron interactions within these Dirac gullies. At appropriate tuning, magnetotransport maps indicate the emergence of a new correlated ground state that spontaneously breaks the rotational symmetry of the system. In contrast, ABC trilayer graphene hosts chiral quasi-particles that exhibit a non-zero Berry phase when encircling one of the valleys of its band structure. It has been shown, that ABC trilayer graphene is susceptible to spontaneous chiral symmetry breaking due to its flat band structure at charge neutrality. Indeed, transport measurements demonstrate the emergence of several spontaneous quantum Hall phases that are driven by a giant Berry curvature. Mapping magnetostransport as a function of electric and magnetic fields reveals a rich phase diagram including states with non-zero orbital momentum and Hall conductivity. The findings of this thesis provide novel insights into the world correlated phases in multilayer graphene and interaction physics in two dimensions. In a second part of this thesis, charge transport in all-inorganic halide perovskite nanowires is investigated. In recent years, halide perovskites have emerged as promising novel materials for optoelectronic applications due to their large absorption coefficient and exceptional charge carrier lifetime. Yet, although the optical properties have been studied intensely, charge transport mechanisms and the influence of traps still remains elusive. In this thesis, temperature dependent charge transport in CsPbBr3 nanowire field-effect transistors is investigated. It is shown, that charge transport in the dark is dominated by deep traps and freezes out at low temperatures. However, illuminating the sample increases the mobility several orders of magnitude revealing even phonon-limited transport characteristics. These findings highlight and extend the notion of “defect-tolerance” of perovskite materials and provide novel insights into defect states in CsPbBr3 nanowires

Ferroelectric and anomalous quantum Hall states in bare rhombohedral trilayer graphene

Nontrivial interacting phases can emerge in elementary materials. As a prime example, continuing advances in device quality have facilitated the observation of a variety of spontaneous quantum Hall-like states, a cascade of Stoner-like magnets, and an unconventional superconductor in bilayer graphene. Its natural extension, rhombohedral trilayer graphene is predicted to be even more susceptible to interactions given its even flatter low-energy bands and larger winding number. Theoretically, five spontaneous quantum Hall phases have been proposed to be candidate ground states. Here, we provide transport evidence for observing four of the five competing ordered states in interaction-maximized, dually-gated, rhombohedral trilayer graphene. In particular, at vanishing but finite magnetic fields, two states with Chern numbers 3 and 6 can be stabilized at elevated and low electric fields, respectively, and both exhibit clear magnetic hysteresis. We also reveal that the quantum Hall ferromagnets of the zeroth Landau level are ferroelectrics with spontaneous layer polarizations even at zero electric field, as evidenced by electric hysteresis. Our findings exemplify the possible birth of rich interacting electron physics in a simple elementary material

Ketamine effects on default mode network activity and vigilance: A randomized, placebo‐controlled crossover simultaneous fMRI/EEG study

In resting-state functional connectivity experiments, a steady state (of consciousness) is commonly supposed. However, recent research has shown that the resting state is a rather dynamic than a steady state. In particular, changes of vigilance appear to play a prominent role. Accordingly, it is critical to assess the state of vigilance when conducting pharmacodynamic studies with resting-state functional magnetic resonance imaging (fMRI) using drugs that are known to affect vigilance such as (subanesthetic) ketamine. In this study, we sought to clarify whether the previously described ketamine-induced prefrontal decrease of functional connectivity is related to diminished vigilance as assessed by electroencephalography (EEG). We conducted a randomized, double-blind, placebo-controlled crossover study with subanesthetic S-Ketamine in N = 24 healthy, young subjects by simultaneous acquisition of resting-state fMRI and EEG data. We conducted seed-based default mode network functional connectivity and EEG power spectrum analyses. After ketamine administration, decreased functional connectivity was found in medial prefrontal cortex whereas increased connectivities were observed in intraparietal cortices. In EEG, a shift of energy to slow (delta, theta) and fast (gamma) wave frequencies was seen in the ketamine condition. Frontal connectivity is negatively related to EEG gamma and theta activity while a positive relationship is found for parietal connectivity and EEG delta power. Our results suggest a direct relationship between ketamine-induced functional connectivity changes and the concomitant decrease of vigilance in EEG. The observed functional changes after ketamine administration may serve as surrogate end points and provide a neurophysiological framework, for example, for the antidepressant action of ketamine (trial name: 29JN1556, EudraCT Number: 2009-012399-28)

Anisotropic Strain Induced Soliton Movement Changes Stacking Order and Bandstructure of Graphene Multilayers

The crystal structure of solid-state matter greatly affects its electronic properties. For example in multilayer graphene, precise knowledge of the lateral layer arrangement is crucial, since the most stable configurations, Bernal and rhombohedral stacking, exhibit very different electronic properties. Nevertheless, both stacking orders can coexist within one flake, separated by a strain soliton that can host topologically protected states. Clearly, accessing the transport properties of the two stackings and the soliton is of high interest. However, the stacking orders can transform into one another and therefore, the seemingly trivial question how reliable electrical contact can be made to either stacking order can a priori not be answered easily. Here, we show that manufacturing metal contacts to multilayer graphene can move solitons by several $\mu$m, unidirectionally enlarging Bernal domains due to arising mechanical strain. Furthermore, we also find that during dry transfer of multilayer graphene onto hexagonal Boron Nitride, such a transformation can happen. Using density functional theory modeling, we corroborate that anisotropic deformations of the multilayer graphene lattice decrease the rhombohedral stacking stability. Finally, we have devised systematics to avoid soliton movement, and how to reliably realize contacts to both stacking configurations

Addiction Research Consortium: Losing and regaining control over drug intake (ReCoDe)—From trajectories to mechanisms and interventions

One of the major risk factors for global death and disability is alcohol, tobacco, and illicit drug use. While there is increasing knowledge with respect to individual factors promoting the initiation and maintenance of substance use disorders (SUDs), disease trajectories involved in losing and regaining control over drug intake (ReCoDe) are still not well described. Our newly formed German Collaborative Research Centre (CRC) on ReCoDe has an interdisciplinary approach funded by the German Research Foundation (DFG) with a 12-year perspective. The main goals of our research consortium are (i) to identify triggers and modifying factors that longitudinally modulate the trajectories of losing and regaining control over drug consumption in real life, (ii) to study underlying behavioral, cognitive, and neurobiological mechanisms, and (iii) to implicate mechanism-based interventions. These goals will be achieved by: (i) using mobile health (m-health) tools to longitudinally monitor the effects of triggers (drug cues, stressors, and priming doses) and modify factors (eg, age, gender, physical activity, and cognitive control) on drug consumption patterns in real-life conditions and in animal models of addiction; (ii) the identification and computational modeling of key mechanisms mediating the effects of such triggers and modifying factors on goal-directed, habitual, and compulsive aspects of behavior from human studies and animal models; and (iii) developing and testing interventions that specifically target the underlying mechanisms for regaining control over drug intake

16p11.2 600 kb Duplications confer risk for typical and atypical Rolandic epilepsy

Rolandic epilepsy (RE) is the most common idiopathic focal childhood epilepsy. Its molecular basis is largely unknown and a complex genetic etiology is assumed in the majority of affected individuals. The present study tested whether six large recurrent copy number variants at 1q21, 15q11.2, 15q13.3, 16p11.2, 16p13.11 and 22q11.2 previously associated with neurodevelopmental disorders also increase risk of RE. Our association analyses revealed a significant excess of the 600 kb genomic duplication at the 16p11.2 locus (chr16: 29.5-30.1 Mb) in 393 unrelated patients with typical (n = 339) and atypical (ARE; n = 54) RE compared with the prevalence in 65 046 European population controls (5/393 cases versus 32/65 046 controls; Fisher's exact test P = 2.83 × 10−6, odds ratio = 26.2, 95% confidence interval: 7.9-68.2). In contrast, the 16p11.2 duplication was not detected in 1738 European epilepsy patients with either temporal lobe epilepsy (n = 330) and genetic generalized epilepsies (n = 1408), suggesting a selective enrichment of the 16p11.2 duplication in idiopathic focal childhood epilepsies (Fisher's exact test P = 2.1 × 10−4). In a subsequent screen among children carrying the 16p11.2 600 kb rearrangement we identified three patients with RE-spectrum epilepsies in 117 duplication carriers (2.6%) but none in 202 carriers of the reciprocal deletion. Our results suggest that the 16p11.2 duplication represents a significant genetic risk factor for typical and atypical R

Charge traps in all‐inorganic CsPbBr 3 perovskite nanowire field‐effect phototransistors

All-inorganic halide perovskite materials have recently emerged as outstanding materials for optoelectronic applications. However, although critical for developing novel technologies, the influence of charge traps on charge transport in all-inorganic systems still remains elusive. Here, the charge transport properties in cesium lead bromide, nanowire films are probed using a field-effect transistor geometry. Field-effect mobilities of μFET = 4 × 10−3 cm−2 V−1 s−1 and photoresponsivities in the range of R = 25 A W−1 are demonstrated. Furthermore, charge transport both with and without illumination is investigated down to cryogenic temperatures. Without illumination, deep traps dominate transport and the mobility freezes out at low temperatures. Despite the presence of deep traps, when illuminating the sample, the field-effect mobility increases by several orders of magnitude and even phonon-limited transport characteristics are visible. This can be seen as an extension to the notion of “defect tolerance” of perovskite materials that has solely been associated with shallow traps. These findings provide further insight in understanding charge transport in perovskite materials and underlines that managing deep traps can open up a route to optimizing optoelectronic devices such as solar cells or phototransistors operable also at low light intensitiesDeutsche Forschungsgemeinschaft | Ref. EXC‐2111‐390814868Deutsche Forschungsgemeinschaft | Ref. EXC 2089/1‐390776260Bayerisches Staatsministerium für Bildung und Kultus, Wissenschaft und Kunst | Ref. Solar Technologies go Hybri

Interplay between topological valley and quantum Hall edge transport

In electrostatically-gapped bilayer graphene, topologically-protected states can emerge at naturally occurring stacking domain walls even in the absence of a magnetic field. Here, the authors describe the interplay between such domain wall states and quantum Hall edge transport within the eight-fold degenerate zeroth Landau level of suspended bilayer graphene