7,002 research outputs found
Quantum FFLO state in clean layered superconductors
We investigate the influence of Landau quantization on the superconducting
instability for a pure layered superconductor in the magnetic field directed
perpendicular to the layers. We demonstrate that the quantization corrections
to the Cooper-pairing kernel with finite Zeeman spin splitting promote the
formation of the nonuniform state in which the order parameter is periodically
modulated along the magnetic field, i.e., between the layers
(Fulde-Ferrell-Larkin-Ovchinnikov [FFLO] state). The conventional uniform state
experiences such a quantization-induced FFLO instability at low temperatures
even in a common case of predominantly orbital suppression of superconductivity
when the Zeeman spin splitting is expected to have a relatively weak effect.
The maximum relative FFLO temperature is given by the ratio of the
superconducting transition temperature and the Fermi energy. This maximum is
realized when the ratio of the spin-spitting energy and the Landau-level
separation is half-integer. These results imply that the FFLO states may exist
not only in the Pauli-limited superconductors but also in very clean materials
with small Zeeman spin-splitting energy. We expect that the described
quantization-promoted FFLO instability is a general phenomenon, which may be
found in materials with different electronic spectra and order-parameter
symmetries.Comment: 18 pages, 10 figures, minor correction
Interplay between orbital-quantization effects and the Fulde-Ferrell-Larkin-Ovchinnikov instability in multiple-band layered superconductors
We explore superconducting instability for a clean two-band layered
superconductor with deep and shallow bands in the magnetic field applied
perpendicular to the layers. In the shallow band, the quasiclassical
approximation is not applicable, and Landau quantization has to be accounted
for exactly. The electronic spectrum of this band in the magnetic field is
composed of the one-dimensional Landau-level minibands. With increasing
magnetic field the system experiences series of Lifshitz transitions when the
chemical potential enters and exits the minibands. These transitions profoundly
influence the shape of the upper critical field at low temperatures. In
addition, the Zeeman spin splitting may cause the nonuniform state with
interlayer modulation of the superconducting order parameter
(Fulde-Ferrell-Larkin-Ovchinnikov state). Typically, the quantization effects
in the shallow band strongly promote the formation of this state. The uniform
state remains favorable only in the exceptional resonance cases when the
spin-splitting energy exactly matches the Landau-level spacing. Furthermore,
for specific relations between electronic spectrum parameters, the alternating
FFLO state may realize, in which the order parameter changes sign between the
neighboring layers. For all above cases, the reentrant high-field
superconducting states may emerge at low temperatures if the shallow band has
significant contribution to the Cooper pairing.Comment: 25 pages, 14 figures, minor revisions and more references adde
Strong Landau-quantization effects in high-magnetic-field superconductivity of a two-dimensional multiple-band metal near the Lifshitz transition
We investigate the onset of superconductivity in magnetic field for a clean
two-dimensional multiple-band superconductor in the vicinity of the Lifshitz
transition when one of the bands is very shallow. Due to small number of
carriers in this band, the quasiclassical Werthamer-Helfand approximation
breaks down and Landau quantization has to be taken into account. We found that
the transition temperature TC2(H) has giant oscillations and is resonantly
enhanced at the magnetic fields corresponding to full occupancy of the Landau
levels in the shallow band. This enhancement is especially pronounced for the
lowest Landau level. As a consequence, the reentrant superconducting regions in
the temperature-field phase diagram emerge at low temperatures near the
magnetic fields at which the chemical potential matches the Landau levels. The
specific behavior depends on the relative strength of the intraband and
interband pairing interactions and the reentrance is most pronounced in the
purely interband coupling scenario. The reentrant behavior is suppressed by the
Zeeman spin splitting in the shallow band, the separated regions disappear
already for very small spin-splitting factors. On the other hand, the
reentrance is restored in the resonance cases when the spin-splitting energy
exactly matches the separation between the Landau levels. The predicted
behavior may realize in the gate-tuned FeSe monolayer.Comment: 23 pages, 9 figures, more references added and one figure adde
Applications of atomic ensembles in distributed quantum computing
Thesis chapter. The fragility of quantum information is a fundamental constraint faced by anyone trying to build a quantum computer. A truly useful and powerful quantum computer has to be a robust and scalable machine. In the case of many qubits which may interact with the environment and their neighbors, protection against decoherence becomes quite a challenging task. The scalability and decoherence issues are the main difficulties addressed by the distributed model of quantum computation. A distributed quantum computer consists of a large quantum network of distant nodes - stationary qubits which communicate via flying qubits. Quantum information can be transferred, stored, processed and retrieved in decoherence-free fashion by nodes of a quantum network realized by an atomic medium - an atomic quantum memory. Atomic quantum memories have been developed and demonstrated experimentally in recent years. With the help of linear optics and laser pulses, one is able to manipulate quantum information stored inside an atomic quantum memory by means of electromagnetically induced transparency and associated propagation phenomena. Any quantum computation or communication necessarily involves entanglement. Therefore, one must be able to entangle distant nodes of a distributed network. In this article, we focus on the probabilistic entanglement generation procedures such as well-known DLCZ protocol. We also demonstrate theoretically a scheme based on atomic ensembles and the dipole blockade mechanism for generation of inherently distributed quantum states so-called cluster states. In the protocol, atomic ensembles serve as single qubit systems. Hence, we review single-qubit operations on qubit defined as collective states of atomic ensemble. Our entangling protocol requires nearly identical single-photon sources, one ultra-cold ensemble per physical qubit, and regular photodetectors. The general entangling procedure is presented, as well as a procedure that generates in a single step Q-qubit GHZ states with success probability p(success) similar to eta(Q/2), where eta is the combined detection and source efficiency. This is signifcantly more efficient than any known robust probabilistic entangling operation. The GHZ states form the basic building block for universal cluster states, a resource for the one-way quantum computer
From meadows to milk to mucosa – adaptation of Streptococcus and Lactococcus species to their nutritional environments
Lactic acid bacteria (LAB) are indigenous to food-related habitats as well as associated with the mucosal surfaces of animals. The LAB family Streptococcaceae consists of the genera Lactococcus and Streptococcus. Members of the family include the industrially important species Lactococcus lactis, which has a long history safe use in the fermentative food industry, and the disease-causing streptococci Streptococcus pneumoniae and Streptococcus pyogenes. The central metabolic pathways of the Streptococcaceae family have been extensively studied because of their relevance in the industrial use of some species, as well as their influence on virulence of others. Recent developments in high-throughput proteomic and DNA-microarray techniques, in in vivo NMR studies, and importantly in whole-genome sequencing have resulted in new insights into the metabolism of the Streptococcaceae family. The development of cost-effective high-throughput sequencing has resulted in the publication of numerous whole-genome sequences of lactococcal and streptococcal species. Comparative genomic analysis of these closely related but environmentally diverse species provides insight into the evolution of this family of LAB and shows that the relatively small genomes of members of the Streptococcaceae family have been largely shaped by the nutritionally rich environments they inhabit.
Fitting Weibull ACD Models to High Frequency Transactions Data: A Semi-parametric Approach based on Estimating Functions
Autoregressive conditional duration (ACD) models play an important role in financial modeling. This paper considers the estimation of the Weibull ACD model using a semi-parametric approach based on the theory of estimating functions (EF). We apply the EF and the maximum likelihood (ML) methods to a data set given in Tsay (2003, p203) to compare these two methods. It is shown that the EF approach is easier to apply in practice and gives better estimates than the MLE. Results show that the EF approach is compatible with the ML method in parameter estimation. Furthermore, the computation speed for the EF approach is much faster than for the MLE and therefore offers a significant reduction of the completion time.Volatility, Option pricing, Volatility of volatility, Forecasting
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