130 research outputs found
Periodically rippled graphene: growth and spatially resolved electronic structure
We studied the growth of an epitaxial graphene monolayer on Ru(0001). The
graphene monolayer covers uniformly the Ru substrate over lateral distances
larger than several microns reproducing the structural defects of the Ru
substrate. The graphene is rippled with a periodicity dictated by the
difference in lattice parameter between C and Ru. The theoretical model predict
inhomogeneities in the electronic structure. This is confirmed by measurements
in real space by means of scanning tunnelling spectroscopy. We observe electron
pockets at the higher parts of the ripples.Comment: 5 page
Sum-over-states vs quasiparticle pictures of coherent correlation spectroscopy of excitons in semiconductors; femtosecond analogues of multidimensional NMR
Two-dimensional correlation spectroscopy (2DCS) based on the nonlinear
optical response of excitons to sequences of ultrafast pulses, has the
potential to provide some unique insights into carrier dynamics in
semiconductors. The most prominent feature of 2DCS, cross peaks, can best be
understood using a sum-over-states picture involving the many-body eigenstates.
However, the optical response of semiconductors is usually calculated by
solving truncated equations of motion for dynamical variables, which result in
a quasiparticle picture. In this work we derive Green's function expressions
for the four wave mixing signals generated in various phase-matching directions
and use them to establish the connection between the two pictures. The formal
connection with Frenkel excitons (hard-core bosons) and vibrational excitons
(soft-core bosons) is pointed out.Comment: Accepted to Phys. Rev.
Suppressed Magnetization at the Surfaces and Interfaces of Ferromagnetic Metallic Manganites
What happens to ferromagnetism at the surfaces and interfaces of manganites?
With the competition between charge, spin, and orbital degrees of freedom, it
is not surprising that the surface behavior may be profoundly different than
that of the bulk. Using a powerful combination of two surface probes, tunneling
and polarized x-ray interactions, this paper reviews our work on the nature of
the electronic and magnetic states at manganite surfaces and interfaces. The
general observation is that ferromagnetism is not the lowest energy state at
the surface or interface, which results in a suppression or even loss of
ferromagnetic order at the surface. Two cases will be discussed ranging from
the surface of the quasi-2D bilayer manganite
(LaSrMnO) to the 3D Perovskite
(LaSrMnO)/SrTiO interface. For the bilayer manganite,
that is, ferromagnetic and conducting in the bulk, these probes present clear
evidence for an intrinsic insulating non-ferromagnetic surface layer atop
adjacent subsurface layers that display the full bulk magnetization. This
abrupt intrinsic magnetic interface is attributed to the weak inter-bilayer
coupling native to these quasi-two-dimensional materials. This is in marked
contrast to the non-layered manganite system
(LaSrMnO/SrTiO), whose magnetization near the interface
is less than half the bulk value at low temperatures and decreases with
increasing temperature at a faster rate than the bulk.Comment: 15 pages, 13 figure
Image potential states of germanene
We have measured the two-dimensional image potential states (IPS) of a germanene layer synthesized on a Ge2Pt crystal using scanning tunnelling microscopy and spectroscopy. The IPS spectrum of germanene exhibits several differences as compared to the IPS spectrum of pristine Ge(001). First, the n = 1 peak of the Rydberg series of the IPS spectrum of germanene has two contributions, labelled n = 1- and n = 1+, respectively. The peak at the lower energy side is weaker and is associated to the mirror-symmetric state with opposite parity. The appearance of this peak indicates that the interaction between the germanene layer and the substrate is very weak. Second, the work function of germanene is about 0.75 eV lower in energy than the work function of Ge(001). This large difference in work function of germanene and pristine Ge(001) is in agreement with first-principles calculations. © 2020 The Author(s). Published by IOP Publishing Ltd
Lattice-stiffening transition in copolymer films of vinylidene fluoride (70%) with trifluoroethylene (30%)
We report the discovery of a compressibility phase transition at 160 K in crystalline copolymer films of vinylidene fluoride (70%) with trifluoroethylene (30%). This phase transition is distinct from the known bulk ferroelectric-paraelectric phase transition at 353 K and surface ferroelectric phase transition at 295 K. The new phase transition is characterized by an increase in the effective Debye temperature from 48 to 245 K along the 〈010〉 direction as the temperature falls below 160 K. This phase transition is evident in neutron scattering, x-ray diffraction, angle-resolved photoemission, and in the dipole active phonon modes in electron energy-loss spectroscopy. © 1999 The American Physical Society
- …