156 research outputs found
An edge index for the Quantum Spin-Hall effect
Quantum Spin-Hall systems are topological insulators displaying
dissipationless spin currents flowing at the edges of the samples. In
contradistinction to the Quantum Hall systems where the charge conductance of
the edge modes is quantized, the spin conductance is not and it remained an
open problem to find the observable whose edge current is quantized. In this
paper, we define a particular observable and the edge current corresponding to
this observable. We show that this current is quantized and that the
quantization is given by the index of a certain Fredholm operator. This
provides a new topological invariant that is shown to take same values as the
Spin-Chern number previously introduced in the literature. The result gives an
effective tool for the investigation of the edge channels' structure in Quantum
Spin-Hall systems. Based on a reasonable assumption, we also show that the edge
conducting channels are not destroyed by a random edge.Comment: 4 pages, 3 figure
Simultaneous quantization of edge and bulk Hall conductivity
The edge Hall conductivity is shown to be an integer multiple of
which is almost surely independent of the choice of the disordered
configuration. Its equality to the bulk Hall conductivity given by the
Kubo-Chern formula follows from K-theoretic arguments. This leads to
quantization of the Hall conductance for any redistribution of the current in
the sample. It is argued that in experiments at most a few percent of the total
current can be carried by edge states.Comment: 6 pages Latex, 1 figur
Spontaneous edge currents for the Dirac equation in two space dimensions
Spontaneous edge currents are known to occur in systems of two space
dimensions in a strong magnetic field. The latter creates chirality and
determines the direction of the currents. Here we show that an analogous effect
occurs in a field-free situation when time reversal symmetry is broken by the
mass term of the Dirac equation in two space dimensions. On a half plane, one
sees explicitly that the strength of the edge current is proportional to the
difference between the chemical potentials at the edge and in the bulk, so that
the effect is analogous to the Hall effect, but with an internal potential. The
edge conductivity differs from the bulk (Hall) conductivity on the whole plane.
This results from the dependence of the edge conductivity on the choice of a
selfadjoint extension of the Dirac Hamiltonian. The invariance of the edge
conductivity with respect to small perturbations is studied in this example by
topological techniques.Comment: 10 pages; final versio
Scattering theory for lattice operators in dimension
This paper analyzes the scattering theory for periodic tight-binding
Hamiltonians perturbed by a finite range impurity. The classical energy
gradient flow is used to construct a conjugate (or dilation) operator to the
unperturbed Hamiltonian. For dimension the wave operator is given by
an explicit formula in terms of this dilation operator, the free resolvent and
the perturbation. From this formula the scattering and time delay operators can
be read off. Using the index theorem approach, a Levinson theorem is proved
which also holds in presence of embedded eigenvalues and threshold
singularities.Comment: Minor errors and misprints corrected; new result on absense of
embedded eigenvalues for potential scattering; to appear in RM
Anomalous Drude Model
A generalization of the Drude model is studied. On the one hand, the free
motion of the particles is allowed to be sub- or superdiffusive; on the other
hand, the distribution of the time delay between collisions is allowed to have
a long tail and even a non-vanishing first moment. The collision averaged
motion is either regular diffusive or L\'evy-flight like. The anomalous
diffusion coefficients show complex scaling laws. The conductivity can be
calculated in the diffusive regime. The model is of interest for the
phenomenological study of electronic transport in quasicrystals.Comment: 4 pages, latex, 2 figures, to be published in Physical Review Letter
Interacting fermions in self-similar potentials
We consider interacting spinless fermions in one dimension embedded in
self-similar quasiperiodic potentials. We examine generalizations of the
Fibonacci potential known as precious mean potentials. Using a bosonization
technique and a renormalization group analysis, we study the low-energy physics
of the system. We show that it undergoes a metal-insulator transition for any
filling factor, with a critical interaction that strongly depends on the
position of the Fermi level in the Fourier spectrum of the potential. For some
positions of the Fermi level the metal-insulator transition occurs at the non
interacting point. The repulsive side is an insulator with a gapped spectrum
whereas in the attractive side the spectrum is gapless and the properties of
the system are described by a Luttinger liquid. We compute the transport
properties and give the characteristic exponents associated to the frequency
and temperature dependence of the conductivity.Comment: 18 pages, 10 EPS figure
Investigation of quantum transport by means of O(N) real-space methods
Quantum transport for different systems is investigated by developing the
Kubo formula on a basis of orthogonal polynomials. Results on quantum Hall
systems are presented with particular attention to metal insulator transitions
and new universalities. Other potential applications of the present method for
RKKY mesoscopic interaction and insight for large scale computational problems,
are given.Comment: 7 pages, 8 figure
Z_2 Invariants of topological insulators as geometric obstructions
We consider a gapped periodic quantum system with time-reversal symmetry of fermionic (or odd) type, i.e. the time-reversal operator squares to ā1. We investigate the existence of periodic and time-reversal invariant Bloch frames in dimensions 2 and 3. In 2d, the obstruction to the existence of such a frame is shown to be encoded in a Z2-valued topological invariant, which can be computed by a simple algorithm. We prove that the latter agrees with the Fu-Kane index. In 3d, instead, four Z2 invariants emerge from the construction, again related to the Fu-Kane-Mele indices. When no topological obstruction is present, we provide a constructive algorithm yielding explicitly a periodic and time-reversal invariant Bloch frame. The result is formulated in an abstract setting, so that it applies both to discrete models and to continuous ones
Characterization of Rhodamine-123 as a Tracer Dye for Use In In vitro Drug Transport Assays
Fluorescent tracer dyes represent an important class of sub-cellular probes and allow the examination of cellular processes in real-time with minimal impact upon these processes. Such tracer dyes are becoming increasingly used for the examination of membrane transport processes, as they are easy-to-use, cost effective probe substrates for a number of membrane protein transporters. Rhodamine 123, a member of the rhodamine family of flurone dyes, has been used to examine membrane transport by the ABCB1 gene product, MDR1. MDR1 is viewed as the archetypal drug transport protein, and is able to efflux a large number of clinically relevant drugs. In addition, ectopic activity of MDR1 has been associated with the development of multiple drug resistance phenotype, which results in a poor patient response to therapeutic intervention. It is thus important to be able to examine the potential for novel compounds to be MDR1 substrates. Given the increasing use rhodamine 123 as a tracer dye for MDR1, a full characterisation of its spectral properties in a range of in vitro assay-relevant media is warranted. Herein, we determine Ī»max for excitation and emission or rhodamine 123 and its metabolite rhodamine 110 in commonly used solvents and extraction buffers, demonstrating that fluorescence is highly dependent on the chemical environment: Optimal parameters are 1% (v/v) methanol in HBSS, with Ī»exā=ā505 nm, Ī»emā=ā525 nm. We characterise the uptake of rhodamine 123 into cells, via both passive and active processes, and demonstrate that this occurs primarily through OATP1A2-mediated facilitated transport at concentrations below 2 ĀµM, and via micelle-mediated passive diffusion above this. Finally, we quantify the intracellular sequestration and metabolism of rhodamine 123, demonstrating that these are both cell line-dependent factors that may influence the interpretation of transport assays
Classification of Inhibitors of Hepatic Organic Anion Transporting Polypeptides (OATPs): Influence of Protein Expression on DrugāDrug Interactions
ABSTRACT: The hepatic organic anion transporting poly-peptides (OATPs) influence the pharmacokinetics of several drug classes and are involved in many clinical drugādrug interactions. Predicting potential interactions with OATPs is, therefore, of value. Here, we developed in vitro and in silico models for identification and prediction of specific and general inhibitors of OATP1B1, OATP1B3, and OATP2B1. The maximal transport activity (MTA) of each OATP in human liver was predicted from transport kinetics and protein quantification. We then used MTA to predict the effects of a subset of inhibitors on atorvastatin uptake in vivo. Using a data set of 225 drug-like compounds, 91 OATP inhibitors were identified. In silico models indicated that lipophilicity and polar surface area are key molecular features of OATP inhibition. MTA predictions identified OATP1B1 and OATP1B3 as major determinants of atorvastatin uptake in vivo. The relative contributions to overall hepatic uptake varied with isoform specificities of the inhibitors
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