31 research outputs found
Large phonon-drag enhancement induced by narrow quantum confinement at the LaAlO3/SrTiO3 interface
The thermoelectric power of the two-dimensional electron system (2DES) at the
LaAlO3/SrTiO3 interface is explored below room temperature, in comparison with
that of Nb-doped SrTiO3 single crystals. For the interface we find a region
below T =50 K where thermopower is dominated by phonon-drag, whose amplitude is
hugely amplified with respect to the corresponding bulk value, reaching values
~mV/K and above. The phonon-drag enhancement at the interface is traced back to
the tight carrier confinement of the 2DES, and represents a sharp signature of
strong electron-acoustic phonon coupling at the interface
Thermoelectric behavior of Ruddlesden-Popper series iridates
The goal of this work is studying the evolution of thermoelectric transport
across the members of the Ruddlesden-Popper series iridates Srn+1IrnO3n+1,
where a metal-insulator transition driven by bandwidth change occurs, from the
strongly insulating Sr2IrO4 to the metallic non Fermi liquid behavior of
SrIrO3. Sr2IrO4 (n=1), Sr3Ir2O7 (n=2) and SrIrO3 (n=inf.) polycrystals are
synthesized at high pressure and characterized by structural, magnetic,
electric and thermoelectric transport analyses. We find a complex
thermoelectric phenomenology in the three compounds. Thermal diffusion of
charge carriers accounts for the Seebeck behavior of Sr2IrO4, whereas
additional drag mechanisms come into play in determining the Seebeck
temperature dependence of Sr3Ir2O7 and SrIrO3. These findings reveal close
relationship between magnetic, electronic and thermoelectric properties, strong
coupling of charge carriers with phonons and spin fluctuations as well as
relevance of multiband description in these compounds.Comment: main paper + supplementary informatio
Giant Oscillating Thermopower at Oxide Interfaces
Understanding the nature of charge carriers at the LaAlO3/SrTiO3 interface is
one of the major open issues in the full comprehension of the charge
confinement phenomenon in oxide heterostructures. Here, we investigate
thermopower to study the electronic structure in LaAlO3/SrTiO3 at low
temperature as a function of gate field. In particular, under large negative
gate voltage, corresponding to the strongly depleted charge density regime,
thermopower displays record-high negative values of the order of 10^4 - 10^5
microV/K, oscillating at regular intervals as a function of the gate voltage.
The huge thermopower magnitude can be attributed to the phonon-drag
contribution, while the oscillations map the progressive depletion and the
Fermi level descent across a dense array of localized states lying at the
bottom of the Ti 3d conduction band. This study is the first direct evidence of
a localized Anderson tail in the two-dimensional (2D) electron liquid at the
LaAlO3/SrTiO3 interface.Comment: Main text: 28 pages and 3 figures; Supplementary information: 29
pages, 5 figures and 1 tabl
STM Study of Exfoliated Few Layer Black Phosphorus Annealed in Ultrahigh Vacuum
Black Phosphorus (bP) has emerged as an interesting addition to the category
of two-dimensional materials. Surface-science studies on this material are of
great interest, but they are hampered by bP's high reactivity to oxygen and
water, a major challenge to scanning tunneling microscopy (STM) experiments. As
a consequence, the large majority of these studies were performed by cleaving a
bulk crystal in situ. Here we present a study of surface modifications on
exfoliated bP flakes upon consecutive annealing steps, up to 550 C, well above
the sublimation temperature of bP. In particular, our attention is focused on
the temperature range 375 C - 400 C, when sublimation starts, and a controlled
desorption from the surface occurs alongside with the formation of
characteristic well-aligned craters. There is an open debate in the literature
about the crystallographic orientation of these craters, whether they align
along the zigzag or the armchair direction. Thanks to the atomic resolution
provided by STM, we are able to identify the orientation of the craters with
respect to the bP crystal: the long axis of the craters is aligned along the
zigzag direction of bP. This allows us to solve the controversy, and, moreover,
to provide insight in the underlying desorption mechanism leading to crater
formation
Study of equilibrium carrier transfer in LaAlO3/SrTiO3 from an epitaxial La1 12x Sr x MnO3 ferromagnetic layer
Using x-ray magnetic circular dichroism and ab-initio calculations, we explore the La1-xSrxMnO3/LaAlO3/SrTiO3 (001) heterostructure as a mean to induce transfer of spin polarized carriers from ferromagnetic La1-xSrxMnO3 layer into the 2DEG (two-dimensional electron gas) at the LaAlO3/SrTiO3 interface. By out-of-plane transport measurements, the tunneling across the LaAlO3 barrier is also analyzed. Our results suggest small or vanishing spin-polarization for the 2DEG: magnetic dichroism does not reveal a neat signal on Ti atoms, while calculations predict, for the pristine stoichiometric interface, a small spin-resolved mobile charge of 2.5 x 10(13) cm(-2) corresponding to a magnetic moment of 0.038 mu(B) per Ti atom, tightly confined within the single SrTiO3 layer adjacent to LaAlO3. Such a small magnetization is hard to be detected experimentally and perhaps not robust enough to survive to structural disorder, native doping, or La1-xSrxMnO3 dead-layer effects. Our analysis suggests that, while some spin-diffusion cannot be completely ruled out, the use of ferromagnetic La1-xSrxMnO3 epilayers grown on-top of LaAlO3/SrTiO3 is not effective enough to induce robust spin-transport properties in the 2DEG. The examined heterostructure is nevertheless an excellent test-case to understand some fundamental aspects of the spin-polarized charge transfer in 2D wells
Thermal Scanning-Probe Lithography for Broad-Band On-Demand Plasmonic Nanostructures on Transparent Substrates
Thermal scanning-probe lithography (t-SPL) is a high-resolution nanolithography technique that enables the nanopatterning of thermosensitive materials by means of a heated silicon tip. It does not require alignment markers and gives the possibility to assess the morphology of the sample in a noninvasive way before, during, and after the patterning. In order to exploit t-SPL at its peak performances, the writing process requires applying an electric bias between the scanning hot tip and the sample, thereby restricting its application to conductive, optically opaque, substrates. In this work, we show a t-SPL-based method, enabling the noninvasive high-resolution nanolithography of photonic nanostructures onto optically transparent substrates across a broad-band visible and near-infrared spectral range. This was possible by intercalating an ultrathin transparent conductive oxide film between the dielectric substrate and the sacrificial patterning layer. This way, nanolithography performances comparable with those typically observed on conventional semiconductor substrates are achieved without significant changes of the optical response of the final sample. We validated this innovative nanolithography approach by engineering periodic arrays of plasmonic nanoantennas and showing the capability to tune their plasmonic response over a broad-band visible and near-infrared spectral range. The optical properties of the obtained systems make them promising candidates for the fabrication of hybrid plasmonic metasurfaces supported onto fragile low-dimensional materials, thus enabling a variety of applications in nanophotonics, sensing, and thermoplasmonics