85 research outputs found
Finite-size scaling of eigenstate thermalization
According to the eigenstate thermalization hypothesis (ETH), even isolated
quantum systems can thermalize because the eigenstate-to-eigenstate
fluctuations of typical observables vanish in the limit of large systems. Of
course, isolated systems are by nature finite, and the main way of computing
such quantities is through numerical evaluation for finite-size systems.
Therefore, the finite-size scaling of the fluctuations of eigenstate
expectation values is a central aspect of the ETH. In this work, we present
numerical evidence that for generic non-integrable systems these fluctuations
scale with a universal power law with the dimension of the
Hilbert space. We provide heuristic arguments, in the same spirit as the ETH,
to explain this universal result. Our results are based on the analysis of
three families of models, and several observables for each model. Each family
includes integrable members, and we show how the system size where the
universal power law becomes visible is affected by the proximity to
integrability.Comment: 9 pages, 8 figures; accepted for publication in Phys. Rev.
Topological phase transitions driven by next-nearest-neighbor hopping in two-dimensional lattices
For two-dimensional lattices in a tight-binding description, the intrinsic
spin-orbit coupling, acting as a complex next-nearest-neighbor hopping, opens
gaps that exhibit the quantum spin Hall effect. In this paper, we study the
effect of a real next-nearest-neighbor hopping term on the band structure of
several Dirac systems. In our model, the spin is conserved, which allows us to
analyze the spin Chern numbers. We show that in the Lieb, kagome, and T_3
lattices, variation of the amplitude of the real next-nearest-neighbor hopping
term drives interesting topological phase transitions. These transitions may be
experimentally realized in optical lattices under shaking, when the ratio
between the nearest- and next-nearest-neighbor hopping parameters can be tuned
to any possible value. Finally, we show that in the honeycomb lattice,
next-nearest-neighbor hopping only drives topological phase transitions in the
presence of a magnetic field, leading to the conjecture that these transitions
can only occur in multigap systems.Comment: 10 pages, 9 figures [erratum: corrected colors in Fig. 7(a)
Genesis of the Floquet Hofstadter butterfly
We investigate theoretically the spectrum of a graphene-like sample
(honeycomb lattice) subjected to a perpendicular magnetic field and irradiated
by circularly polarized light. This system is studied using the Floquet
formalism, and the resulting Hofstadter spectrum is analyzed for different
regimes of the driving frequency. For lower frequencies, resonances of various
copies of the spectrum lead to intricate formations of topological gaps. In the
Landau-level regime, new wing-like gaps emerge upon reducing the driving
frequency, thus revealing the possibility of dynamically tuning the formation
of the Hofstadter butterfly. In this regime, an effective model may be
analytically derived, which allows us to retrace the energy levels that exhibit
avoided crossings and ultimately lead to gap structures with a wing-like shape.
At high frequencies, we find that gaps open for various fluxes at , and
upon increasing the amplitude of the driving, gaps also close and reopen at
other energies. The topological invariants of these gaps are calculated and the
resulting spectrum is elucidated. We suggest opportunities for experimental
realization and discuss similarities with Landau-level structures in non-driven
systems.Comment: 8 pages, 4 figure
Topological phases in a two-dimensional lattice: Magnetic field versus spin-orbit coupling
In this work, we explore the rich variety of topological states that arise in
two-dimensional systems, by considering the competing effects of spin-orbit
couplings and a perpendicular magnetic field on a honeycomb lattice. Unlike
earlier approaches, we investigate minimal models in order to clarify the
effects of the intrinsic and Rashba spin-orbit couplings, and also of the
Zeeman splitting, on the quantum Hall states generated by the magnetic field.
In this sense, our work provides an interesting path connecting quantum Hall
and quantum spin Hall physics. First, we consider the properties of each term
individually and we analyze their similarities and differences. Secondly, we
investigate the subtle competitions that arise when these effects are combined.
We finally explore the various possible experimental realizations of our model.Comment: 19 pages, 15 figure
Preferential adsorption of high density lipoprotein (HDL) in blood plasma/polymer interaction
A few studies on the adsorption of plasma proteins to polymeric surfaces show that major plasma proteins: albumin (Alb), fibrinogen (Fb) and immunoglobulin (IgG) are adsorbed in much smaller quantities from plasma than from protein solutions (1,2). Present results show that this difference in adsorption is due to the preferential adsorption of high density lipoprotein from plasma onto the material surfaces studied (PVC and PS)
Dirac Cones, Topological Edge States, and Nontrivial Flat Bands in Two-Dimensional Semiconductors with a Honeycomb Nanogeometry
We study theoretically two-dimensional single-crystalline sheets of
semiconductors that form a honeycomb lattice with a period below 10 nm. These
systems could combine the usual semiconductor properties with Dirac bands.
Using atomistic tight-binding calculations, we show that both the atomic
lattice and the overall geometry influence the band structure, revealing
materials with unusual electronic properties. In rocksalt Pb chalcogenides, the
expected Dirac-type features are clouded by a complex band structure. However,
in the case of zinc-blende Cd-chalcogenide semiconductors, the honeycomb
nanogeometry leads to rich band structures, including, in the conduction band,
Dirac cones at two distinct energies and nontrivial flat bands and, in the
valence band, topological edge states. These edge states are present in several
electronic gaps opened in the valence band by the spin-orbit coupling and the
quantum confinement in the honeycomb geometry. The lowest Dirac conduction band
has S-orbital character and is equivalent to the pi-pi* band of graphene but
with renormalized couplings. The conduction bands higher in energy have no
counterpart in graphene; they combine a Dirac cone and flat bands because of
their P-orbital character. We show that the width of the Dirac bands varies
between tens and hundreds of meV. These systems emerge as remarkable platforms
for studying complex electronic phases starting from conventional
semiconductors. Recent advancements in colloidal chemistry indicate that these
materials can be synthesized from semiconductor nanocrystals.Comment: 12 pages, 12 figure
Chern-Simons theory of multi-component quantum Hall systems
The Chern-Simons approach has been widely used to explain fractional quantum
Hall states in the framework of trial wave functions. In the present paper, we
generalise the concept of Chern-Simons transformations to systems with any
number of components (spin or pseudospin degrees of freedom), extending earlier
results for systems with one or two components. We treat the density
fluctuations by adding auxiliary gauge fields and appropriate constraints. The
Hamiltonian is quadratic in these fields and hence can be treated as a harmonic
oscillator Hamiltonian, with a ground state that is connected to the Halperin
wave functions through the plasma analogy. We investigate several conditions on
the coefficients of the Chern-Simons transformation and on the filling factors
under which our model is valid. Furthermore, we discuss several singular cases,
associated with symmetric states.Comment: 11 pages, shortened version, accepted for publication in Phys. Rev.
Topological states in multi-orbital HgTe honeycomb lattices
Research on graphene has revealed remarkable phenomena arising in the
honeycomb lattice. However, the quantum spin Hall effect predicted at the K
point could not be observed in graphene and other honeycomb structures of light
elements due to an insufficiently strong spin-orbit coupling. Here we show
theoretically that 2D honeycomb lattices of HgTe can combine the effects of the
honeycomb geometry and strong spin-orbit coupling. The conduction bands,
experimentally accessible via doping, can be described by a tight-binding
lattice model as in graphene, but including multi-orbital degrees of freedom
and spin-orbit coupling. This results in very large topological gaps (up to 35
meV) and a flattened band detached from the others. Owing to this flat band and
the sizable Coulomb interaction, honeycomb structures of HgTe constitute a
promising platform for the observation of a fractional Chern insulator or a
fractional quantum spin Hall phase.Comment: includes supplementary materia
Natural and bioinspired excipients for dry powder inhalation formulations
Pulmonary drug delivery can have several advantages over other administration routes, in particular when using dry powder formulations. Such dry powder inhalation formulations generally include natural and bio-inspired excipients, which, among other purposes, are used to improve dosing reproducibility and aerosolization performance. Amino acids can enhance powder dispersibility and provide protection against moisture uptake. Sugars are used as drug-carrying diluents, stabilizers for biopharmaceuticals, and surface enrichers. Lipids and lipid-like excipients can reduce interparticle adhesive forces and are also used as constituents of liposomal drug delivery systems. Finally, biodegradable polymers are used to facilitate sustained release and targeted drug delivery. Despite their promise, pulmonary toxicity of many of the discussed excipients remains largely unknown and requires attention in future research
Large-scale purification of factor VIII by affinity chromatography: optimization of process parameters
The optimization of a new process for the extraction of human coagulation factor VIII (FVIII) from plasma with the tailor-made affinity matrix dimethylaminopropylcarbamylpentyl-Sepharose CL-4B (C3---C5 matrix) is described. First, plasma is applied to DEAE-Sephadex A-50 anion exchanger in order to separate a number of proteins, including coagulation factors II, IX and X (prothrombin complex), from FVIII. Subsequently, the unbound fraction of the ion exchanger, containing FVIII, is contacted with the C3---C5 affinity matrix. Optimization of the FVIII affinity chromatographic procedure is accomplished in terms of the ligand density of the matrix, adsorption mode (batch-wise versus column-wise adsorption and matrix to plasma ratio), and conditions of pH and conductivity to be applied on washing and desorption. In scale-up experiments, by processing 20 1 of plasma, the recovery (340 U VIII:C/kg plasma) and the specific activity (s.a.) (1.2 U VIII:C/mg protein) are better than those obtained by cryoprecipitation (recovery 300 U VIII:C/kg plasma, s.a. O.3 U VIII:C/mg protein). The newly developed process using the specially designed C3---C5 affinity matrix has potential application in the process-scale purification of FVIII
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