Measuring Hall viscosity of graphene's electron fluid Berdyugin, A I; Xu, S G; Pellegrino, F M D ...
Science (American Association for the Advancement of Science),
04/2019, Letnik:
364, Številka:
6436
Journal Article
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An electrical conductor subjected to a magnetic field exhibits the Hall effect in the presence of current flow. Here we report a qualitative deviation from the standard behavior in electron systems ...with high viscosity. We find that the viscous electron fluid in graphene responds to non-quantizing magnetic fields by producing an electric field opposite to that generated by the ordinary Hall effect. The viscous contribution is substantial and identified by studying local voltages that arise in the vicinity of current-injecting contacts. We analyze the anomaly over a wide range of temperatures and carrier densities and extract the Hall viscosity, a dissipationless transport coefficient that was long identified theoretically but remained elusive in experiments.
Electron-electron (e-e) collisions can impact transport in a variety of surprising and sometimes counterintuitive ways. Despite strong interest, experiments on the subject proved challenging because ...of the simultaneous presence of different scattering mechanisms that suppress or obscure consequences of e-e scattering. Only recently, sufficiently clean electron systems with transport dominated by e-e collisions have become available, showing behaviour characteristic of highly viscous fluids. Here we study electron transport through graphene constrictions and show that their conductance below 150 K increases with increasing temperature, in stark contrast to the metallic character of doped graphene. Notably, the measured conductance exceeds the maximum conductance possible for free electrons. This anomalous behaviour is attributed to collective movement of interacting electrons, which 'shields' individual carriers from momentum loss at sample boundaries. The measurements allow us to identify the conductance contribution arising due to electron viscosity and determine its temperature dependence. Besides fundamental interest, our work shows that viscous effects can facilitate high-mobility transport at elevated temperatures, a potentially useful behaviour for designing graphene-based devices.
Based on extensive numerical simulations, accounting for electrostatic interactions and dissipative electron-phonon scattering, we propose experimentally realizable geometries capable of sustaining ...electronic preturbulence in graphene samples. In particular, preturbulence is predicted to occur at experimentally attainable values of the Reynolds number between 10 and 50, over a broad spectrum of frequencies between 10 and 100 GHz.
The realization of equilibrium superradiant quantum phases (photon condensates) in a spatially uniform quantum cavity field is forbidden by a "no-go" theorem stemming from gauge invariance. We here ...show that the no-go theorem does not apply to spatially varying quantum cavity fields. We find a criterion for its occurrence that depends solely on the static, nonlocal orbital magnetic susceptibility χorb (q), of the electronic system (ES) evaluated at a cavity photon momentum ℏq. Only 3DESs satisfying the Condon inequality χorb (q) > 1/(4π) can harbor photon condensation. For the experimentally relevant case of two-dimensional (2D) ESs embedded in quasi-2D cavities the criterion again involves χorb (q) but also the vertical size of the cavity. We use these considerations to identify electronic properties that are ideal for photon condensation. Our theory is nonperturbative in the strength of electron-electron interaction and therefore applicable to strongly correlated ESs.
Strain effect on the optical conductivity of graphene Pellegrino, F. M. D.; Angilella, G. G. N.; Pucci, R.
Physical review. B, Condensed matter and materials physics,
01/2010, Letnik:
81, Številka:
3
Journal Article
We measure transport through a GaAlAs heterostructure at temperatures between 32 mK and 30 K. Increasing the temperature enhances the electron-electron scattering rate and viscous effects in the ...two-dimensional electron gas arise. To probe this regime we measure so-called vicinity voltages and use a voltage-biased scanning tip to induce a movable local perturbation. We find that the scanning gate images differentiate reliably between the different regimes of electron transport. Our data are in good agreement with recent theories for interacting electron liquids in the ballistic and viscous regimes stimulated by measurements in graphene. However, the range of temperatures and densities where viscous effects are observable in GaAlAs are very distinct from the graphene material system.
Introduction
A large body of literature suggests that childhood trauma exposure is a non-specific risk factor for development of eating disorders (EDs) later in life. One potential mechanism through ...which early traumatic experiences may increase the risk for EDs is represented by long-lasting changes in the body stress response system.
Objectives
We investigated the activity of the hypothalamus-pituitary-adrenal axis and of the sympathetic nervous system in adult ED patients with or without a history of childhood trauma exposure.
Methods
We recruited 35 women with EDs, admitted to the Eating Disorders Center of the Department of Psychiatry of the University of Naples “Luigi Vanvitelli”. Participants filled in the Childhood Trauma Questionnaire (CTQ), to assess exposure to childhood trauma. They were instructed to collect saliva samples at awakening and after 15, 30 and 60 minutes, in order to measure cortisol levels and salivary alpha-amylase (sAA), a marker of the sympathetic nervous system activity.
Results
According to the CTQ cut-off scores, 21 ED women were classified as maltreated (Mal) participants and 14 women as no-maltreated (noMal) ED participants. Compared to noMal ED women, Mal ED participants showed significantly decreased cortisol awakening response (CAR) and sAA morning secretion.
Conclusions
Present findings confirm that childhood trauma exposure impairs the CAR of adult patients with EDs and show that also the morning secretion of sAA is decreased in childhood maltreated adult ED patients. Therefore, our study shows for the first time a dampening in the basal activity of both components of the endogenous stress response system in childhood maltreated adult ED women.
Efficient solar water photosplitting is plagued by large overpotentials of the HER and OER. Even with a noble metal catalyst, the hydrogen evolution reaction can be limited by the strong M–H bonding ...over some metals, such as Pt, Pd, and Rh, inhibiting hydrogen desorption. H absorption is regulated by the potential at the metal nanoparticles. Through controlled periodic illumination of a Pt/TiO2 suspension, we hypothesized a fast variation of the photopotential that induced catalytic surface resonance on the metal, resulting in more than a 50% increase of the efficiency at frequencies higher than 80 Hz.
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