Presidential Popular Constitutionalism

This Article adds a new dimension to the most important and influential strand of recent constitutional theory: popular or democratic constitutionalism, the investigation into how the U.S. Constitution is interpreted (1) as a set of defining national commitments and practices, not necessarily anchored in the text of the document, and (2) by citizens and elected politicians outside the judiciary. Wide-ranging and groundbreaking scholarship in this area has neglected the role of the President as a popular constitutional interpreter, articulating and revising normative accounts of the nation that interact dynamically with citizens’ constitutional understandings. This Article sets out a “grammar” of presidential popular constitutionalism, lays out the historical development and major transformations in its practice, proposes a set of thematic alternatives for today’s presidential popular constitutionalism, and locates presidential popular constitutionalism within the larger concerns of constitutional theory. In particular, it argues that some of the major political developments of recent decades, such as the “Reagan revolution” and the Clinton-Bush era, can be fully understood only by grasping that they are episodes in presidential popular constitutionalism

Light control of orbital domains: case of the prototypical manganite La0.5Sr1.5MnO4

Control of electronic and structural ordering in correlated materials on the ultrafast timescale with light is a new and emerging approach to disentangle the complex interplay of the charge, spin, orbital and structural degree of freedom. In this paper we present an overview of how orbital order and orbital domains can be controlled by near IR and THz radiation in the layered manganite La0.5Sr1.5MnO4. We show how near-IR pumping can efficiently and rapidly melt orbital ordering. However, the nanoscale domain structure recovers unchanged demonstrating the importance of structural defects for the orbital domain formation. On the contrary, we show that pulsed THz fields can be used to effectively orientate the domains. In this case the alignment depends on the in-plane electric field polarisation and is induced by an energy penalty that arises from THz field induced hopping of the localised charges.Peer ReviewedPostprint (author's final draft

Resonant optical control of the structural distortions that drive ultrafast demagnetization in Cr2_2O3_3

We study how the color and polarization of ultrashort pulses of visible light can be used to control the demagnetization processes of the antiferromagnetic insulator Cr$_2$O$_3$. We utilize time-resolved second harmonic generation (SHG) to probe how changes in the magnetic and structural state evolve in time. We show that, varying the pump photon-energy to excite either localized transitions within the Cr or charge transfer states, leads to markedly different dynamics. Through a full polarization analysis of the SHG signal, symmetry considerations and density functional theory calculations, we show that, in the non-equilibrium state, SHG is sensitive to {\em both} lattice displacements and changes to the magnetic order, which allows us to conclude that different excited states couple to phonon modes of different symmetries. Furthermore, the spin-scattering rate depends on the induced distortion, enabling us to control the timescale for the demagnetization process. Our results suggest that selective photoexcitation of antiferromagnetic insulators allows fast and efficient manipulation of their magnetic state.Comment: 7 pages, 5 figure

Time-Domain Separation of Optical Properties From Structural Transitions in Resonantly Bonded Materials

The extreme electro-optical contrast between crystalline and amorphous states in phase change materials is routinely exploited in optical data storage and future applications include universal memories, flexible displays, reconfigurable optical circuits, and logic devices. Optical contrast is believed to arise due to a change in crystallinity. Here we show that the connection between optical properties and structure can be broken. Using a unique combination of single-shot femtosecond electron diffraction and optical spectroscopy, we simultaneously follow the lattice dynamics and dielectric function in the phase change material Ge2Sb2Te5 during an irreversible state transformation. The dielectric function changes by 30% within 100 femtoseconds due to a rapid depletion of electrons from resonantly-bonded states. This occurs without perturbing the crystallinity of the lattice, which heats with a 2 ps time constant. The optical changes are an order-of-magnitude larger than those achievable with silicon and present new routes to manipulate light on an ultrafast timescale without structural changes

Study of second and third harmonic generation from an indium tin oxide nanolayer: Influence of nonlocal effects and hot electrons

We report comparative experimental and theoretical studies of the second and third harmonic generation from a 20 nm-thick indium tin oxide layer in proximity of the epsilon-near-zero condition. Using a tunable optical parametric amplifier, we record both spectral and angular dependence of the generated harmonic signals close to this particular point. In addition to the enhancement of the second harmonic efficiency close to the epsilon-near-zero wavelength, at oblique incidence, third harmonic generation displays an unusual behavior, predicted but not observed before. We implement a comprehensive, first-principles hydrodynamic approach able to simulate our experimental conditions. The model is unique, flexible, and able to capture all major physical mechanisms that drive the electrodynamic behavior of conductive oxide layers: nonlocal effects, which blueshift the epsilon-near-zero resonance by tens of nanometers; plasma frequency redshift due to variations of the effective mass of hot carriers; charge density distribution inside the layer, which determines the nonlinear surface and magnetic interactions; and the nonlinearity of the background medium triggered by bound electrons. We show that, by taking these contributions into account, our theoretical predictions are in very good qualitative and quantitative agreement with our experimental results. We expect that our results can be extended to other geometries where epsilon-near-zero nonlinearity plays an important role.Peer ReviewedPostprint (published version

Measurement of 10 fs pulses across the entire Visible to Near-Infrared Spectral Range

Tuneable ultrafast laser pulses are a powerful tool for measuring difficult-to-access degrees of freedom in materials science. In general these experiments require the ability to address resonances and excitations both above and below the bandgap of materials, and to probe their response at the timescale of the fastest non-trivial internal dynamics. This drives the need for ultrafast sources capable of delivering 10-15 fs duration pulses tuneable across the entire visible (VIS) and near infrared (NIR) range, 500 nm - 3000 nm, as well as the characterization of these sources. Here we present a single frequency-resolved optical gating (FROG) system capable of self-referenced characterization of pulses with 10 fs duration across the entire VIS-NIR spectral range. Our system does not require auxiliary beams and only minor reconfiguration for different wavelengths. We demonstrate the system with measurements of pulses across the entire tuning range

Does Vo2 host a transient monoclinic metallic phase?

Ultrafast phase transitions induced by femtosecond light pulses present a new opportunity for manipulating the properties of materials. Understanding how these transient states are different from, or similar to, their thermal counterparts is key to determining how materials can exhibit properties that are not found in equilibrium. In this paper, we reexamine the case of the light-induced insulator-metal phase transition in the prototypical, strongly correlated material VO2, for which a nonthermal Mott-Hubbard transition has been claimed. Here, we show that heat, even on the ultrafast timescale, plays a key role in the phase transition. When heating is properly accounted for, we find a single phase-transition threshold corresponding to the thermodynamic structural insulator-metal phase transition, and we find no evidence of a hidden transient Mott-Hubbard nonthermal phase. The interplay between the initial thermal state and the ultrafast transition may have implications for other transient states of matter.Peer ReviewedPostprint (published version

Strain-engineered diffusive atomic switching in two-dimensional crystals

Strain engineering is an emerging route for tuning the bandgap, carrier mobility, chemical reactivity and diffusivity of materials. Here we show how strain can be used to control atomic diffusion in van der Waals heterostructures of two-dimensional (2D) crystals. We use strain to increase the diffusivity of Ge and Te atoms that are confined to 5 Å thick 2D planes within an Sb[subscript 2]Te[subscript 3]–GeTe van der Waals superlattice. The number of quintuple Sb[subscript 2]Te[subscript 3] 2D crystal layers dictates the strain in the GeTe layers and consequently its diffusive atomic disordering. By identifying four critical rules for the superlattice configuration we lay the foundation for a generalizable approach to the design of switchable van der Waals heterostructures. As Sb[subscript 2]Te[subscript 3]–GeTe is a topological insulator, we envision these rules enabling methods to control spin and topological properties of materials in reversible and energy efficient ways.National Science Foundation (U.S.) (DMR-1410636 and DMR-1120901)SUTD-MIT International Design Centre (IDC) (Postdoctoral Fellowship)SUTD–MIT International Design Center (IDC) (Designer Chalcogenides IDSF1200108OH Research Project

Resilience and the (Micro-)Dynamics of Organizational Ambidexterity: Implications for Strategic HRM

In the twenty-first century, resilience has emerged as an important topic linked to calls for adaptability, well-being and organizational performance. Extant strategic human resource management (HRM) literature and practices have developed many insights into resilience. However, overall, they have a propensity to conceptualise resilience as being associated with ‘macro-’ and ‘extreme’ situations. This paper complements the prevailing perspective by developing a micro-focus on resilience through the conceptual framework of organizational ambidexterity surfacing under-examined individual resilience in connection with HRM practices. Methodologically, the paper adopts a qualitative approach presenting data from two illustrative contexts: an ‘everyday’ quasi-governmental institution and a prima facie ‘extreme’ pan-international military organization. Using template analysis, a number of valuable themes and similarities are identified. The findings and discussion underline the managerial challenges in handling organizational ambidextrous dynamics and tensions surrounding resilience, positive and sceptical approaches in relation to individual and organizational stances towards HRM practices. As such, the results point at value in HRM managers and practices recontextualising and appreciating ‘extremes’ and resilience more as an everyday (rather than exceptional) phenomenon wherein myriad micro-moments are highly significant in constructing and influencing macro-contexts. This also implies a need to see cynical resistance as normative rather than automatically negatively