98 research outputs found
Negative electronic compressibility and nanoscale inhomogeneity in ionic-liquid gated two-dimensional superconductors
When the electron density of highly crystalline thin films is tuned by
chemical doping or ionic liq- uid gating, interesting effects appear including
unconventional superconductivity, sizeable spin-orbit coupling, competition
with charge-density waves, and a debated low-temperature metallic state that
seems to avoid the superconducting or insulating fate of standard
two-dimensional electron systems. Some experiments also find a marked tendency
to a negative electronic compressibility. We suggest that this indicates an
inclination for electronic phase separation resulting in a nanoscopic inhomo-
geneity. Although the mild modulation of the inhomogeneous landscape is
compatible with a high electron mobility in the metallic state, this
intrinsically inhomogeneous character is highlighted by the peculiar behaviour
of the metal-to-superconductor transition. Modelling the system with super-
conducting puddles embedded in a metallic matrix, we fit the peculiar
resistance vs. temperature curves of systems like TiSe2, MoS2, and ZrNCl. In
this framework also the low-temperature debated metallic state finds a natural
explanation in terms of the pristine metallic background embedding
non-percolating superconducting clusters. An intrinsically inhomogeneous
character naturally raises the question of the formation mechanism(s). We
propose a mechanism based on the interplay be- tween electrons and the charges
of the gating ionic liquid.Comment: substantially modified presentation: 12 pages 7 figure
Density inhomogeneities and Rashba spin-orbit coupling interplay in oxide interfaces
There is steadily increasing evidence that the two-dimensional electron gas
(2DEG) formed at the interface of some insulating oxides like LaAlO3/SrTiO3 and
LaTiO3/SrTiO3 is strongly inhomogeneous. The inhomogeneous distribution of
electron density is accompanied by an inhomogeneous distribution of the
(self-consistent) electric field confining the electrons at the interface. In
turn this inhomogeneous transverse electric field induces an inhomogeneous
Rashba spin-orbit coupling (RSOC). After an introductory summary on two
mechanisms possibly giving rise to an electronic phase separation accounting
for the above inhomogeneity,we introduce a phenomenological model to describe
the density-dependent RSOC and its consequences. Besides being itself a
possible source of inhomogeneity or charge-density waves, the density-dependent
RSOC gives rise to interesting physical effects like the occurrence of
inhomogeneous spin-current distributions and inhomogeneous quantum-Hall states
with chiral "edge" states taking place in the bulk of the 2DEG. The
inhomogeneous RSOC can also be exploited for spintronic devices since it can be
used to produce a disorder-robust spin Hall effect.Comment: 13 pages, 15 figure
Possible mechanisms of electronic phase separation in oxide interfaces
LaAlO3/SrTiO3 ad LaTiO3/SrTiO3 interfaces are known to host a strongly
inhomogeneous (nearly) two-dimensional electron gas (2DEG). In this work we
present three unconventional electronic mechanisms of electronic phase
separation (EPS) in a 2DEG as a possible source of inhomogeneity in oxide
interfaces. Common to all three mechanisms is the dependence of some
(interaction) potential on the 2DEG's density. We first consider a mechanism
resulting from a sizable density-dependent Rashba spin-orbit coupling. Next, we
point out that an EPS may also occur in the case of a density-dependent
superconducting pairing interaction. Finally, we show that the confinement of
the 2DEG to the interface by a density-dependent, self-consistent electrostatic
potential can by itself cause an EPS.Comment: 4 pages and 4 figures, Proceedings of the International Conference
"Superstripes 2014", 25-31 July 2015, Erice, Ital
Phase separation from electron confinement at oxide interfaces
Oxide heterostructures are of great interest both for fundamental and
applicative reasons. In particular the two-dimensional electron gas at the
LaAlO/SrTiO or LaTiO/SrTiO interfaces displays many different
physical properties and functionalities. However there are clear indications
that the interface electronic state is strongly inhomogeneous and therefore it
is crucially relevant to investigate possible intrinsic electronic mechanisms
underlying this inhomogeneity. Here the electrostatic potential confining the
electron gas at the interface is calculated self-consistently, finding that the
electron confinement at the interface may induce phase separation, to avoid a
thermodynamically unstable state with a negative compressibility. This provides
a generic robust and intrinsic mechanism for the experimentally observed
inhomogeneous character of these interfaces.Comment: 8 pages and 4 figure
Inhomogeneous multi-carrier superconductivity at LaXO3/SrTiO3 (X=Al or Ti) oxide interfaces
Several experiments reveal the inhomogeneous character of the superconducting
state that occurs when the carrier density of the two-dimensional electron gas
formed at the LaXO3/SrTiO3 (X=Al or Ti) interface is tuned above a threshold
value by means of gating. Re-analyzing previous measurements, that highlight
the presence of two kinds of carriers, with low and high mobility, we shall
provide a description of multi-carrier magneto-transport in an inhomogeneous
two-dimensional electron gas, gaining insight into the properties of the
physics of the systems under investigation. We shall then show that the
measured resistance, superfluid density, and tunneling spectra result from the
percolative connection of superconducting "puddles" with randomly distributed
critical temperatures, embedded in a weakly localizing metallic matrix. We
shall also show that this scenario is consistent with the characteristics of
the superconductor-to-metal transition driven by a magnetic field. A
multi-carrier description of the superconducting state, within a weak-coupling
BCS-like model, will be finally discussed.Comment: 12 pages 10 figure
High frequency dynamics in a monatomic glass
The high frequency dynamics of glassy Selenium has been studied by Inelastic
X-ray Scattering at beamline BL35XU (SPring-8). The high quality of the data
allows one to pinpoint the existence of a dispersing acoustic mode for
wavevectors () of nm, helping to clarify a previous
contradiction between experimental and numerical results. The sound velocity
shows a positive dispersion, exceeding the hydrodynamic value by 10%
at nm. The dependence of the sound attenuation
, reported for other glasses, is found to be the low- limit of a
more general law which applies also to the
higher region, where no longer holds.Comment: Phys. Rev. Lett. (Accepted
Complex dynamics in nanoscale phase separated supercooled liquids
The relaxation properties of supercooled AsxS100−x liquids are investigated using a combination of infrared
photon correlation spectroscopy and topological constraint theory. Results reveal two channels of relaxation
for sulfur-rich compositions that manifest by an unusual profile in the density-density autocorrelation function
involving two typical timescales. This indicates a reduced temperature-dependent dynamics for one of the
channels associated with a sulfur-rich segregated nanoscale phase that furthermore displays a low liquid fragility.
Conversely, the dynamics of the emerging cross-linked As-S network is associated with a growth of the
glass transition temperature with As content. These results can be quantitatively understood from topological
constraint theory applied to a phase separated network for which a dedicated constraint enumeration must be
achieved. The vanishing of this peculiar behavior occurs close to the reported isostatic reversibility window
observed at the glass transition
Phonon-like and single particle dynamics in liquid lithium
The dynamic structure factor, S(Q,E), of liquid lithium (T=475 K) has been
determined by inelastic x-ray scattering (IXS) in the momentum transfer region
(Q = 1.4-110 nm-1). These data allow to observe how, in a simple liquid, a
phonon-like collective mode evolves towards the single particle dynamics. As a
function of Q, one finds: i) at low Q's, a sound mode with a positive
dispersion of the sound velocity, ii) at intermediate Q's, excitations whose
energy oscillates similarly to phonons in the crystal Brillouin zones, and iii)
at high Q's, the S(Q,E) approaches a Gaussian shape, indicating that the single
particle dynamics has been reached.Comment: 3 pages and 5 figure
Observation of Umklapp processes in non-crystalline materials
Umklapp processes are known to exist in cristalline materials, where they
control important properties such as thermal conductivity, heat capacity and
electrical conductivity. In this work we report the provocative observation of
Umklapp processes in a non-periodical system, namely liquid Lithium. The lack
of a well defined periodicity seems then not to prevent the existence of these
scattering processes mechanisms provided that the local order of the systems
i.e. the maxima of the static structure factor supply the equivalent of a
reciprocal lattice vector in the case of cristalline materials.Comment: 13 pages P
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