64 research outputs found
Observation of Wannier-Stark localization at the surface of BaTiO films by photoemission
Observation of Bloch oscillations and Wannier-Stark localization of charge
carriers is typically impossible in single-crystals, because an electric field
higher than the breakdown voltage is required. In BaTiO however, high
intrinsic electric fields are present due to its ferroelectric properties. With
angle-resolved photoemission we directly probe the Wannier-Stark localized
surface states of the BaTiO film-vacuum interface and show that this effect
extends to thin SrTiO overlayers. The electrons are found to be localized
along the in-plane polarization direction of the BaTiO film
Localized vs. delocalized character of charge carriers in LaAlO3/ SrTiO3 superlattices
Understanding the nature of electrical conductivity, superconductivity and
magnetism between layers of oxides is of immense importance for the design of
electronic devices employing oxide heterostructures. We demonstrate that
resonant inelastic X-ray scattering can be applied to directly probe the
carriers in oxide heterostructures. Our investigation on epitaxially grown
LaAlO3/SrTiO3 superlattices unambiguously reveals the presence of both
localized and delocalized Ti 3d carriers. These two types of carriers are
caused by oxygen vacancies and electron transfer due to the polar discontinuity
at the interface. This result allows explaining the reported discrepancy
between theoretically calculated and experimentally measured carrier density
values in LaAlO3/SrTiO3 heterostructures.Comment: 14 pages, 3 figure
Conducting interfaces between band insulating oxides: the LaGaO3/SrTiO3
We show that the growth of the heterostructure LaGaO3/SrTiO3 yields the
formation of a highly conductive interface. Our samples were carefully analyzed
by high resolution electron microscopy, in order to assess their crystal
perfection and to evaluate the abruptness of the interface. Their carrier
density and sheet resistance are compared to the case of LaAlO3/SrTiO3 and a
superconducting transition is found. The results open the route to widening the
field of polar-non polar interfaces, pose some phenomenological constrains to
their underlying physics and highlight the chance of tailoring their properties
for future applications by adopting suitable polar materials.Comment: in press Appl. Phys. Lett. 97, 1 (2010
Terahertz displacive excitation of a coherent Raman-active phonon in V2O3
Nonlinear processes involving frequency-mixing of light fields set the basis for ultrafast coherent spectroscopy of collective modes in solids. In certain semimetals and semiconductors, generation of coherent phonon modes can occur by a displacive force on the lattice at the difference-frequency mixing of a laser pulse excitation on the electronic system. Here, as a low-frequency counterpart of this process, we demonstrate that coherent phonon excitations can be induced by the sum-frequency components of an intense terahertz light field, coupled to intraband electronic transitions. This nonlinear process leads to charge-coupled coherent dynamics of Raman-active phonon modes in the strongly correlated metal VO. Our results show an alternative up-conversion pathway for the optical control of Raman-active modes in solids mediated by terahertz-driven electronic excitation
Semiconductors: X‐Ray Writing of Metallic Conductivity and Oxygen Vacancies at Silicon/SrTiO3 Interfaces (Adv. Funct. Mater. 25/2019):Frontispiece
Direct observation of enhanced magnetism in individual size- and shape-selected 3d transition metal nanoparticles
Magnetic nanoparticles are critical building blocks for future technologies ranging from nanomedicine to spintronics. Many related applications require nanoparticles with tailored magnetic properties. However, despite significant efforts undertaken towards this goal, a broad and poorly understood dispersion of magnetic properties is reported, even within monodisperse samples of the canonical ferromagnetic 3d transition metals. We address this issue by investigating the magnetism of a large number of size- and shape-selected, individual nanoparticles of Fe, Co, and Ni using a unique set of complementary characterization techniques. At room temperature, only superparamagnetic behavior is observed in our experiments for all Ni nanoparticles within the investigated sizes, which range from 8 to 20 nm. However, Fe and Co nanoparticles can exist in two distinct magnetic states at any size in this range: (i) a superparamagnetic state, as expected from the bulk and surface anisotropies known for the respective materials and as observed for Ni, and (ii) a state with unexpected stable magnetization at room temperature. This striking state is assigned to significant modifications of the magnetic properties arising from metastable lattice defects in the core of the nanoparticles, as concluded by calculations and atomic structural characterization. Also related with the structural defects, we find that the magnetic state of Fe and Co nanoparticles can be tuned by thermal treatment enabling one to tailor their magnetic properties for applications. This paper demonstrates the importance of complementary single particle investigations for a better understanding of nanoparticle magnetism and for full exploration of their potential for applications
Magnetic-coupled electronic landscape in bilayer-distorted titanium-based kagome metals
Quantum materials whose atoms are arranged on a lattice of corner-sharing
triangles, , the kagome lattice, have recently emerged as a
captivating platform for investigating exotic correlated and topological
electronic phenomena. Here, we combine ultra-low temperature angle-resolved
photoemission spectroscopy (ARPES) with scanning tunneling microscopy and
density functional theory calculations to reveal the fascinating electronic
structure of the bilayer-distorted kagome material
TiBi, where stands for Nd and Yb.
Distinct from other kagome materials, TiBi exhibits
two-fold, rather than six-fold, symmetries, stemming from the distorted kagome
lattice, which leads to a unique electronic structure. Combining experiment and
theory we map out the electronic structure and discover double flat bands as
well as multiple van Hove singularities (VHSs), with one VHS exhibiting
higher-order characteristics near the Fermi level. Notably, in the magnetic
version NdTiBi, the ultra-low base temperature ARPES measurements
unveil an unconventional band splitting in the band dispersions which is
induced by the ferromagnetic ordering. These findings reveal the potential of
bilayer-distorted kagome metals TiBi as a promising
platform for exploring novel emergent phases of matter at the intersection of
strong correlation and magnetism
Dirac states with knobs on: interplay of external parameters and the surface electronic properties of 3D topological insulators
Topological insulators are a novel materials platform with high applications
potential in fields ranging from spintronics to quantum computation. In the
ongoing scientific effort to demonstrate controlled manipulation of their
electronic structure by external means, stoichiometric variation and surface
decoration are two effective approaches that have been followed. In ARPES
experiments, both approaches are seen to lead to electronic band structure
changes. Such approaches result in variations of the energy position of bulk
and surface-related features and the creation of two-dimensional electron
gases.The data presented here demonstrate that a third manipulation handle is
accessible by utilizing the amount of illumination a topological insulator
surface has been exposed to under typical experimental ARPES conditions. Our
results show that this new, third, knob acts on an equal footing with
stoichiometry and surface decoration as a modifier of the electronic band
structure, and that it is in continuous competition with the latter. The data
clearly point towards surface photovoltage and photo-induced desorption as the
physical phenomena behind modifications of the electronic band structure under
exposure to high-flux photons. We show that the interplay of these phenomena
can minimize and even eliminate the adsorbate-related surface band bending on
typical binary, ternary and quaternary Bi-based topological insulators.
Including the influence of the sample temperature, these data set up a
framework for the external control of the electronic band structure in
topological insulator compounds in an ARPES setting. Four external knobs are
available: bulk stoichiometry, surface decoration, temperature and photon
exposure. These knobs can be used in conjunction to tune the band energies near
the surface and consequently influence the topological properties of the
relevant electronic states.Comment: 16 pages, 8 figure
Dynamics of the photoinduced insulator-to-metal transition in a nickelate film
The control of materials properties with light is a promising approach
towards the realization of faster and smaller electronic devices. With phases
that can be controlled via strain, pressure, chemical composition or
dimensionality, nickelates are good candidates for the development of a new
generation of high performance and low consumption devices. Here we analyze the
photoinduced dynamics in a single crystalline NdNiO film upon excitation
across the electronic gap. Using time-resolved reflectivity and resonant x-ray
diffraction, we show that the pump pulse induces an insulator-to-metal
transition, accompanied by the melting of the charge order. Finally we compare
our results to similar studies in manganites and show that the same model can
be used to describe the dynamics in nickelates, hinting towards a unified
description of these photoinduced phase transitions.Comment: 17 pages, 6 figure
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