Superfluid density as a guide to optimal superconductivity in doped low dimensional antiferromagnets

Following the direct observation of abrupt changes in the superconducting ground state in doped low dimensional antiferromagnets, we have identified a phase transition where superconductivity is optimal. The experiments indicate the presence of a putative quantum critical point associated with the emergence of a glassy state. This mechanism is argued to be an intrinsic property and as such largely independent of material quality and the level of disorder.Comment: Invited article in memory of D. Shoenberg; To be published by the American Inst. of Phys. and the journal of Low Temp. Phy

Two types of superconducting domes in unconventional superconductors

Uncovering the origin of unconventional superconductivity is often plagued by the overwhelming material diversity with varying normal and superconducting (SC) properties. In this article, we deliver a comprehensive study of the SC properties and phase diagrams using multiple tunings (such as disorder, pressure or magnetic field in addition to doping and vice versa) across several families of unconventional superconductors, including the copper-oxides, heavy-fermions, organics and the recently discovered iron-pnictides, iron-chalcogenides, and oxybismuthides. We discover that all these families often possess two types of SC domes, with lower and higher superconducting transition temperatures Tc, both unconventional but with distinct SC and normal states properties. The lower Tc dome arises with or without a quantum critical point (QCP), and not always associated with a non-Fermi liquid (NFL) background. On the contrary, the higher-Tc dome clearly stems from a NFL or strange metal phase, without an apparent intervening phase transition or a QCP. The two domes appear either fully separated in the phase diagram, or merged into one, or arise independently owing to their respective normal state characteristics. Our findings suggest that a QCP-related mechanism is an unlikely scenario for the NFL phase in these materials, and thereby narrows the possibility towards short-range fluctuations of various degrees of freedom in the momentum and frequency space. We also find that NFL physics may be a generic route to higher-Tc superconductivity.Comment: 34 pages. Accepted in NJP. (v2) A table of materials showing 2 SC domes, and 1 dome is added at the en

Effects of the CuO chains on the anisotropic penertration depth of YBa2Cu4O8

The temperatuer dependence of the magnetic penetration depth of grain aligned YBa2Cu4O8 has been measured along the ab plane and c-axis. Both $\lambda_ab$ and $\lambda_c$ vary as \sqrt{T} up to 0.4Tc implying a square root density of states at low energy. The results are discussed in terms of a proximity model of alternating stacked superconducting and normal layersComment: 4 pages, 3 figure

Interface superconductivity: History, development and prospects

The concept of interface superconductivity was introduced over 50 years ago. Some of the greatest physicists of that time wondered whether a quasi-two-dimensional (2D) superconductor can actually exist, what are the peculiarities of 2D superconductivity, and how does the reduced dimensionality affect the critical temperature (Tc). The discovery of high-temperature superconductors, which are composed of coupled 2D superconducting layers, further increased the interest in reduced dimensionality structures. In parallel, the advances in experimental techniques made it possible to grow epitaxial 2D structures with atomically flat surfaces and interfaces, enabling some of the experiments that were proposed decades ago to be performed finally. Now we know that interface superconductivity can occur at the junction of two different materials (metals, insulators, semiconductors). This phenomenon is being explored intensely; it is also exploited as a means to increase Tc or to study quantum critical phenomena. This research may or may not produce a superconductor with a higher Tc or a useful superconducting electronic device but it will likely bring in new insights into the physics underlying high-temperature superconductivity.Comment: http://www.simplex-academic-publishers.com/abstracts/106637257.asp

The emergence of magnetic skyrmions

This is a narrative of the basic theoretical ideas of axisymmetric two-dimensional solitons and of their connection to basic experiments on magnetic compounds. A shortened and edited version appeared in Physics Today.Comment: 5 Pages, 4 Figures, 1 Bo

A Framework for Weighted-Sum Energy Efficiency Maximization in Wireless Networks

Weighted-sum energy efficiency (WSEE) is a key performance metric in heterogeneous networks, where the nodes may have different energy efficiency (EE) requirements. Nevertheless, WSEE maximization is a challenging problem due to its nonconvex sum-of-ratios form. Unlike previous work, this paper presents a systematic approach to WSEE maximization under not only power constraints, but also data rate constraints, using a general SINR expression. In particular, the original problem is transformed into an equivalent form, and then a sequential convex optimization (SCO) algorithm is proposed. This algorithm is theoretically guaranteed to converge for any initial feasible point, and, under suitable constraint qualifications, achieves a Karush-Kuhn-Tucker (KKT) solution. Furthermore, we provide remarkable extensions to the proposed methodology, including systems with multiple resource blocks as well as a more general power consumption model which is not necessarily a convex function of the transmit powers. Finally, numerical analysis reveals that the proposed algorithm exhibits fast convergence, low complexity, and robustness (insensitivity to initial points).Comment: Accepted for publication in IEEE Wireless Communications Letter

Globally Optimal Selection of Ground Stations in Satellite Systems with Site Diversity

The availability of satellite communication systems is extremely limited by atmospheric impairments, such as rain (for radio frequencies) and cloud coverage (for optical frequencies). A solution to this problem is the site diversity technique, where a network of geographically distributed ground stations (GSs) can ensure, with high probability, that at least one GS is available for connection to the satellite at each time period. However, the installation of redundant GSs induces unnecessary additional costs for the network operator. In this context, we study an optimization problem that minimizes the number of required GSs, subject to availability constraints. First, the problem is transformed into a binary-integer-linear-programming (BILP) problem, which is proven to be NP-hard. Subsequently, we design a branch-and-bound (B&B) algorithm, with global-optimization guarantee, based on the linear-programming (LP) relaxation and a greedy method as well. Finally, numerical results show that the proposed algorithm significantly outperforms state-of-the-art methods, and has low complexity in the average case.Comment: 5 pages, 2 tables, 1 figur

On the Computation and Approximation of Outage Probability in Satellite Networks with Smart Gateway Diversity

The utilization of extremely high frequency (EHF) bands can achieve very high throughput in satellite networks (SatNets). Nevertheless, the severe rain attenuation at EHF bands imposes strict limitations on the system availability. Smart gateway diversity (SGD) is considered indispensable in order to guarantee the required availability with reasonable cost. In this context, we examine a load-sharing SGD (LS-SGD) architecture, which has been recently proposed in the literature. For this diversity scheme, we define the system outage probability (SOP) using a rigorous probabilistic analysis based on the Poisson binomial distribution (PBD), and taking into consideration the traffic demand as well as the gateway (GW) capacity. Furthermore, we provide several methods for the exact and approximate calculation of SOP. As concerns the exact computation of SOP, a closed-form expression and an efficient algorithm based on a recursive formula are given, both with quadratic worst-case complexity in the number of GWs. Finally, the proposed approximation methods include well-known probability distributions (binomial, Poisson, normal) and a Chernoff bound. According to the numerical results, binomial and Poisson distributions are by far the most accurate approximation methods.Comment: 8 pages, 2 tables, 3 figure

Energy Efficiency Optimization: A New Trade-off Between Fairness and Total System Performance

The total energy efficiency (TEE), defined as the ratio between the total data rate and the total power consumption, is considered the most meaningful performance metric in terms of energy efficiency (EE). Nevertheless, it does not depend directly on the EE of each link and its maximization leads to unfairness between the links. On the other hand, the maximization of the minimum EE (MEE), i.e., the minimum of the EEs of all links, guarantees the fairest power allocation, but it does not contain any explicit information about the total system performance. The main trend in current research is to maximize TEE and MEE separately. Unlike previous contributions, this letter presents a general multi-objective approach for EE optimization that takes into account both TEE and MEE at the same time, and thus achieves various trade-off points in the MEE-TEE plane. Due to the nonconvex form of the resulting problem, we propose a low-complexity algorithm leveraging the theory of sequential convex optimization (SCO). Last but not least, we provide a novel theoretical result for the complexity of SCO algorithms.Comment: Accepted for publication in IEEE Wireless Communications Letter

Minimizing the Installation Cost of Ground Stations in Satellite Networks: Complexity, Dynamic Programming and Approximation Algorithm

In this letter, we study the optimum selection of ground stations (GSs) in RF/optical satellite networks (SatNets) in order to minimize the overall installation cost under an outage probability requirement. First, we show that the optimization problem can be formulated as a binary-linear-programming problem, and then we give a formal proof of its NP-hardness. Furthermore, we design a dynamic-programming algorithm of pseudo-polynomial complexity with global optimization guarantee as well as an efficient (polynomial-time) approximation algorithm with provable performance guarantee on the distance of the achieved objective value from the global optimum. Finally, the performance of the proposed algorithms is verified through numerical simulations.Comment: 5 pages, 1 table, 1 figur