לוח שנה משולב

Over the past decade, the physics of low dimensional electronic systems has been revolutionized by the discovery of materials with very unusual electronic properties where the behavior of the electrons is governed by the Dirac equation. Among these, graphene has taken center stage due to its ultrarelativistic-like electron dynamics and its potential applications in nanotechnology. Moreover, recent advances in the design and nanofabrication of heterostructures based on van der Waals materials have enabled a new generation of quantum electronic transport experiments in graphene. In this talk I will describe our recent experiments exploring electron-electron interaction driven quantum phenomena in ultra-high quality graphene-based van der Waals heterostructures. In particular I will show two novel realizations of a symmetry-protected topological insulator state, specifically a quantum spin Hall state, characterized by an insulating bulk and conducting counterpropagating spin-polarized states at the system edges. Our experiments establish graphene-based heterostructures as highly tunable systems to study topological properties of condensed matter systems in the regime of strong e-e interactions and I will end my talk with an outlook of some of the exciting directions in the field.

A gas-puff z-pinch begins with the injection of a column of gas between the electrodes of a pulsed-power driver using a fast valve. This gas is preionized and then cylindrically compressed by discharge of a high current through the column. Using Argon gas, these current-driven implosions are a well-developed, high-intensity source of ~3keV, radiation [1]. Optimization of gas-puff sources depends on manipulation of the radial mass-density and gas-species profiles injected by the puff valve, achieved using multiple concentric nozzles backed by independently pressurized gas plena [2]. In particular, the Rayleigh-Taylor unstable boundary between the lower density current sheath and higher density shell of entrained gas, must be carefully controlled. We present experimental results of the response of this instability to radiative losses during the implosion phase using both Argon (Ar) and Krypton (Kr) gas puffs. Experiments are performed using a 7cm outer diameter, triple-annular nozzle designed, developed and characterized at the Weizmann Institute and fielded on the 200 ns, 1 MA COBRA generator at Cornell University. Time-gated density measurements are recorded by 532 nm small-shift shearing interferometers, acceleration of the boundary by streaked and time-gated x-ray cameras, and plasma temperatures are inferred from 527 nm Thomson scattering across the implosion region. RT amplitude e-folding times of 20 ns are recorded in Kr, 20% faster than in Ar under otherwise identical conditions. Ion temperatures of 100 eV are recorded in Kr shells, ~40% lower than in Ar, and it appears this cooling is responsible for the observed increase in RT growth rate.

* This work was supported by students and staff of the COBRA facility at Cornell University, and sponsored by the NNSA Stewardship Sciences Academic Programs.
[1] B. Jones et al. IEEE Trans. Plasma Sci. 42 1145-1152 (2014)
[2] A. Velikovich et al. Phys. Rev. Lett. 77 853-856 (1996)
[3] C. Jennings et al. Phys. Plasmas accepted (2015)

Rydberg states are electronically excited states, the spectral position of which can be described by Rydberg’s well-known formula. The physical properties of these states become very unusual at high values of the principal quantum number n: Their polarizability scales as n7, the van der Waals interaction between two Rydberg atoms or molecules as n11, the maximal electric dipole that can be induced by an electric field as n2, the threshold field for field ionization as n-4, the spacing between adjacent states of a given Rydberg series as n-3, the absorption cross section from the ground state as n-3, the transition moment between neighboring Rydberg states as n2, their lifetime as n3, etc. These physical properties and their rapid variation with n form the basis of a growing number of applications of Rydberg states in chemistry and physics.

The talk will begin with an overview of the properties of Rydberg states and their applications in physics and chemistry. I shall then present specific applications of Rydberg states my group is interested in. In the first, we use Rydberg states to carry out precision measurements of dissociation and ionization energies of molecules to test quantum electrodynamics calculations in small molecular systems such as H2 and He2. In the second, we exploit the large dipole moments of Rydberg states to control the translational motion of atoms and molecules, and develop methods to accelerate, focus, deviate and reflect beams of cold atoms and molecules, the latest experiments being carried out near the surface of chips. In the third, we study ion-molecule reactions at low temperatures within the Rydberg-electron orbit.

In this talk I will briefly review the peer review system used by most publishing houses and briefly review some alternative models. I will then move on to discuss some more advanced elements of peer review including open access, copyright and ethical issues.