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Scientific Report 2007-2009
Condensed matter physics and biophysics
(or macromolecules attached to them) is offered by the so-called optical tweezers. Optical forces
are indeed ideally suited to manipulate matter at the mesoscale which is characterised by length
scales ranging from ten nanometers to hundreds of micrometers, femtonewton to nanonewton
forces, and time scales from the microsecond on. In Rome we have built a set-up to perform
holographic optical trapping (HOT), focusing an engineered wavefront into a tiny hologram image
made of bright light spots in 3D, each spot serving as an independent point trap, providing
contactless micromanipulation technique with many body, dynamic, 3D capabilities (C30).
A large effort is also devoted to investigation of electronic properties of novel materials, mostly
semiconductor and organometallic compounds. Indeed, the synthesis of nanostructured semiconductors
is incessantly boosting the number of opportunities in the field of electronics and photonics,
as well as in the investigation of fundamental quantum phenomena in top-bench experiments. The
control and modification of the physical properties of semiconductor heterostructures at nanometre
scale lengths is thus crucial. In Rome, we presently focus on magneto-photoluminescence (m-PL)
experiments, a powerful method to investigate fundamental properties of novel semiconductor
materials such as Ga(As,N) (an example of a dilute nitrides), which feature surprising physical
properties and qualitatively new alloy phenomena, e.g., a giant negative bowing of the band gap
energy and a large deformation of the conduction band structure (C31). Moreover, we discovered
that hydrogen irradiation of GaAsN completely neutralizes the effect of N and transforms GaAsN
into virtual GaAs, with relevant changes in the energy gap and electron effective mass, among
others. This has opened a novel way to the defect engineering of dilute nitrides, where it is possible
to realize nanostructures on demand (C32).
Recently, organic molecules have been fruitfully exploited to develop devices with specific functionalities.
Engineering of these devices requires an atomic level understanding of the parameters
that control the structure and the function of these low-dimensional molecular architectures. A
crucial issue for these organic-inorganic systems is the achievement of long-range order in exotic
configurations (two-dimensional arrays, one-dimensional wires) such as to allow formation of
exemplary hybrid structures with peculiar electronic properties associated to the reduced dimensions.
We investigate organometallic molecules (like pentacene or metal-phthalocyanines, MPc)
assembled on suitable crystalline surfaces in 1D chains or 2D ordered phases, with the final goal
to design, control and optimize the electronic, transport and magnetic properties. In particular,
a model to describe the interface dipole, the electronic state diagram, the bandwidth and the
electronic state dispersion for the organic heterojunctions and organic-inorganic interfaces has
been experimentally and theoretically proved (C33,C34). Furthermore, MPc formed by a magnetic
central atoms are being used as chemical ”cage” for anchoring the magnetic ion to a metal
surface, such as the spin-state of the central atom could couple with the underlying magnetic or
non magnetic metal.
Materials are not only studied under ambient conditions, but also under extreme perturbations.
The development of modern pressure cells had made possible to investigate structural and electronic
properties of materials under high pressure. One interesting case is offered by the possibility
to modulate the electron-phonon coupling via modification of the lattice parameters, with the aim
of investigating the physics of strongly correlated systems, which as we have discussed at the
beginning represents one of the most challenging tasks of condensed-matter research (C35). Another
case is offered by low-dimensional systems where the external variables (like temperature,
magnetic field, and chemical and applied pressure) can affect the dimensionality of the interacting
electron gas, and thus the intrinsic electronic properties, as well as the interplay among different
order parameters, giving rise to rich phase diagrams (C36).
In many scientific fields considerable efforts are devoted to engeneering new materials with
specific permittivity ϵ eff and magnetic permeability µ eff , to be able to control different properties
of the electromagnetic radiation. A recent approach is based on artificial materials structures
(metamaterials, MM) constituted of a macroscopic (periodic or aperiodic) arrays of single elements:
Sapienza Università di Roma 51 Dipartimento di Fisica