155 research outputs found
Spin-orbital polarons in electron doped copper oxides
Present work demonstrates the formation of spin-orbital polarons in electron
doped copper oxides, that arise due to doping-induced polarisation of the
oxygen orbitals in the CuO planes. The concept of such polarons is
fundamentally different from previous interpretations. The novel aspect of
spin-orbit polarons is best described by electrons becoming self-trapped in
one-dimensional channels created by polarisation of the oxygen orbitals. The
one-dimensional channels form elongated filaments with two possible
orientations, along the diagonals of the elementary CuO square plaquette.
As the density of doped electrons increases multiple filaments are formed.
These may condense into a single percollating filamentary phase. Alternatively,
the filaments may cross perpendicularly to create an interconnected conducting
quasi-one-dimensional web. At low electron doping the antiferromagnetic (AFM)
state and the polaron web coexist. As the doping is increased the web of
filaments modifies and transforms the AFM correlations leading to a series of
quantum phase transitions - which affect the normal and superconducting state
properties.Comment: Please cite this article as: A. Kusmartseva, H. Yu, K. Jin, F.V.
Kusmartsev, Spin-orbital polarons in electron doped copper oxides, Journal of
Magnetism and Magnetic Materials (2017), doi: https://doi.org/10.1016/j.jmmm.
2017.11.02
Current-Voltage Characteristics of Weyl Semimetal Semiconducting Devices, Veselago Lenses and Hyperbolic Dirac Phase
The current-voltage characteristics of a new range of devices built around
Weyl semimetals has been predicted using the Landauer formalism. The potential
step and barrier have been reconsidered for a three-dimensional Weyl
semimetals, with analogies to the two-dimensional material graphene and to
optics. With the use of our results we also show how a Veselago lens can be
made from Weyl semimetals, e.g. from NbAs and NbP. Such a lens may have many
practical applications and can be used as a probing tip in a scanning tunneling
microscope (STM). The ballistic character of Weyl fermion transport inside the
semimetal tip, combined with the ideal focusing of the Weyl fermions (by
Veselago lens) on the surface of the tip may create a very narrow electron beam
from the tip to the surface of the studied material. With a Weyl semimetal
probing tip the resolution of the present STMs can be improved significantly,
and one may image not only individual atoms but also individual electron
orbitals or chemical bonding and therewith to resolve the long-term issue of
chemical and hydrogen bond formation. We show that applying a pressure to the
Weyl semimental, having no centre of spacial inversion one may model matter at
extreme conditions such as those arising in the vicinity of a black hole. As
the materials Cd3As2 and Na3Bi show an asymmetry in their Dirac cones, a
scaling factor was used to model this asymmetry. The scaling factor created
additional regions of no propagation and condensed the appearance of
resonances. We argue that under an external pressure there may arise a
topological phase transition in Weyl semimetals, where the electron transport
changes character and becomes anisotropic. There a hyperbolic Dirac phases
occurs where there is a strong light absorption and photo-current generation
Pressure dependence of the superconducting transition temperature in CYb and CCa
We have studied the evolution, with hydrostatic pressure, of the recently
discovered superconductivity in the graphite intercalation compounds CYb
and CCa. We present pressure-temperature phase diagrams, for both
superconductors, established by electrical transport and magnetization
measurements. In the range 0-1.2 GPa the superconducting transition temperature
increases linearly with pressure in both materials with
and for CYb and CCa respectively. The
transition temperature in CYb, which has beenmeasured up to 2.3 GPa,
reaches a peak at around 1.8 GPa and then starts to drop. We also discuss how
this pressure dependence may be explained within a plasmon pairing mechanism.Comment: 4 pages, 3 figure
The emergence of quantum capacitance in epitaxial graphene
We found an intrinsic redistribution of charge arises between epitaxial
graphene, which has intrinsically n-type doping, and an undoped substrate. In
particular, we studied in detail epitaxial graphene layers thermally elaborated
on C-terminated - (- ()). We have investigated
the charge distribution in graphene-substrate systems using Raman spectroscopy.
The influence of the substrate plasmons on the longitudinal optical phonons of
the substrates has been detected. The associated charge redistribution
reveals the formation of a capacitance between the graphene and the substrate.
Thus, we give for the first time direct evidence that the excess negative
charge in epitaxial monolayer graphene could be self-compensated by the
substrate without initial doping. This induced a previously unseen
redistribution of the charge-carrier density at the substrate-graphene
interface. There a quantum capacitor appears, without resorting to any
intentional external doping, as is fundamentally required for epitaxial
graphene. Although we have determined the electric field existing inside the
capacitor and revealed the presence of a minigap () for
epitaxial graphene on - face terminated carbon, it remains small in
comparison to that obtained for graphene on face terminated . The
fundamental electronic properties found here in graphene on substrates
may be important for developing the next generation of quantum technologies and
electronic/plasmonic devices.Comment: 26 pages, 8 figures, available online as uncorrected proof, Journal
of Materials Chemistry C (2016
In-line technology for assessment of pulmonary drug delivery
The World Health Organisation estimates that 100 million people worldwide suffer from asthma. Chronic obstructive pulmonary disease (COPD) is the fourth leading cause of death worldwide. Pulmonary drug delivery is widely accepted as the firstchoice method for the treatment of respiratory diseases by glucocorticosteroids. Delivering these drugs to the lung by inhalation has many advantages in comparison to the same drug delivered orally. These include rapid onset of action, reduced dose and minimised side effects such as adrenal suppression, electrolyte imbalance, muscle weakness and growth retardation in children. Pulmonary drug delivery is also increasingly used for pain-controlling therapies and for administration of medications which are difficult to formulate orally such as proteins and peptides. The advantages of delivering drugs to the lung are undisputed, however, there are practical challenges still remaining to achieve repeatable and accurate dose delivery to the deep lung. An enabling technology for actuation-by-actuation, in-line measurement of pulmonary drug delivery is part of this greater challenge. The aerosol particles can penetrate into the deep lung only if their aerodynamic size is in the narrow range of O.5μm to 5μm. The larger particles contribute to the oropharyngeal deposition diminishing the pulmonary-delivery advantages, and the smaller particles are exhaled. Particles of this size-range agglomerate easily through adhesion/cohesion interactions. Agglomerates have to disperse in the patient's inspiration flow. Therefore, the respirable dose and therapeutic efficacy depend on the drug formulation, the inhalation device, the ambient conditions and also heavily rely on the patient's inspiratory effort, which is highly variable. An ability to assess the efficacy of the pulmonary delivery in-line with a patient will ultimately improve the effectiveness and efficiency of medical therapy. This thesis presents a novel optical technology for non-invasive and in-line measurement of the respirable cloud during pulmonary drug delivery. The technology can be used as a stand-alone instrument, in conjunction with standard laboratory analytic apparatus and ultimately in-line with a patient. It is shown how the technology concept is based on the Mie theory for light scattering by particles and on the Lambert-Beer law for light extinction by a turbid medium. A prototype device is developed to implement this concept and a series of experimental investigations are conducted to evaluate the feasibility of the approach. Comparisons between the novel in-line technology and conventional in vitro measurements using physical chemistry apparatus suggest that the approach can become a useful enabling technology in pulmonary drug delivery assessment.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Pressure Induced Changes in the Antiferromagnetic Superconductor YbPd2Sn
Low temperature ac magnetic susceptibility measurements of the coexistent
antiferromagnetic superconductor YbPd2Sn have been made in hydrostatic
pressures < 74 kbar in moissanite anvil cells. The superconducting transition
temperature is forced to T(SC) = 0 K at a pressure of 58 kbar. The initial
suppression of the superconducting transition temperature is corroborated by
lower hydrostatic pressure (p < 16 kbar) four point resisitivity measurements,
made in a piston cylinder pressure cell. At ambient pressure, in a modest
magnetic field of ~ 500 G, this compound displays reentrant superconducting
behaviour. This reentrant superconductivity is suppressed to lower temperature
and lower magnetic field as pressure is increased. The antiferromagnetic
ordering temperature, which was measured at T(N) = 0.12 K at ambient pressure
is enhanced, to reach T(N) = 0.58 K at p = 74 kbar. The reasons for the
coexistence of superconductivity and antiferromagnetism is discussed in the
light of these and previous findings. Also considered is why superconductivity
on the border of long range magnetic order is so much rarer in Yb compounds
than in Ce compounds. The presence of a new transition visible by ac magnetic
susceptibility under pressure and in magnetic fields greater than 1.5 kG is
suggested.Comment: 5 pages, 6 figure
How can Trump win?
In this paper, the McCulloch-Pitts model built on an artificial
neuron is first introduced briefly, followed by a modified model – the
coupled network model to describe social opinion network in period of the
presidential election. To illustrate the new model, its formalism and
analytical results on fixed points will be stated step by step. Then, we
investigate the dependence on the ratio of the initial conditions so that we
could find out more on relationship between current information and
preference on final results. Finally, U.S. election campaign in 2016 will be
examined comprehensively including support rates, possible preference,
time series analysis, and period analysis. Besides mathematical research,
we also take real-life activities into consideration. For example, Trump used
Twitter to help his view spreading and take advantage of the underlying
uncertainty to some extent
Manifestation of quantum rotor orbital excitations in Raman spectra of Jahn-Teller crystal LaMnO3
© Published under licence by IOP Publishing Ltd.Materials, consisting of Jahn-Teller (JT) ions, such as cuprates and manganites, display many outstanding properties, including high temperature superconductivity and colossal magnetoresistance. There, the role of JT effect, although widely recognized, is still elusive. Here we show that these materials have vibronic excitations, related to local deformations rotating around JT ions in the dynamic limit, arising from linear electron-vibrational coupling in the "Mexican hat" potential profile. Their energy depends on total angular momentum, which is quantized, as in quantum rotors. We found them in the representative JT compound of orthorhombic manganites, LaMnO3. Since the "Mexican hat" potential energy surface is double-valued, they show up near the ground and excited states of JT ions. Recently, by using spectroscopic ellipsometry technique, we showed that they appear in the excited state in the form of sidebands, accompanying the electron transition between the JT split orbitals at neighboring Mn3+ ions. Here, by using Raman scattering technique, we show that they also exist near the ground state. The found quantum rotor excitations may play an important role in many unusual properties observed in these materials
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