TY  - JOUR
A1  - Allan, Eric
A1  - Weisser, Wolfgang W.
A1  - Fischer, Markus
A1  - Schulze, Ernst-Detlef
A1  - Weigelt, Alexandra
A1  - Roscher, Christiane
A1  - Baade, Jussi
A1  - Barnard, Romain L.
A1  - Bessler, Holger
A1  - Buchmann, Nina
A1  - Ebeling, Anne
A1  - Eisenhauer, Nico
A1  - Engels, Christof
A1  - Fergus, Alexander J. F.
A1  - Gleixner, Gerd
A1  - Gubsch, Marlen
A1  - Halle, Stefan
A1  - Klein, Alexandra-Maria
A1  - Kertscher, Ilona
A1  - Kuu, Annely
A1  - Lange, Markus
A1  - Le Roux, Xavier
A1  - Meyer, Sebastian T.
A1  - Migunova, Varvara D.
A1  - Milcu, Alexandru
A1  - Niklaus, Pascal A.
A1  - Oelmann, Yvonne
A1  - Pasalic, Esther
A1  - Petermann, Jana S.
A1  - Poly, Franck
A1  - Rottstock, Tanja
A1  - Sabais, Alexander C. W.
A1  - Scherber, Christoph
A1  - Scherer-Lorenzen, Michael
A1  - Scheu, Stefan
A1  - Steinbeiss, Sibylle
A1  - Schwichtenberg, Guido
A1  - Temperton, Vicky
A1  - Tscharntke, Teja
A1  - Voigt, Winfried
A1  - Wilcke, Wolfgang
A1  - Wirth, Christian
A1  - Schmid, Bernhard
T1  - A comparison of the strength of biodiversity effects across multiple functions
JF  - Oecologia
N2  - In order to predict which ecosystem functions are most at risk from biodiversity loss, meta-analyses have generalised results from biodiversity experiments over different sites and ecosystem types. In contrast, comparing the strength of biodiversity effects across a large number of ecosystem processes measured in a single experiment permits more direct comparisons. Here, we present an analysis of 418 separate measures of 38 ecosystem processes. Overall, 45 % of processes were significantly affected by plant species richness, suggesting that, while diversity affects a large number of processes not all respond to biodiversity. We therefore compared the strength of plant diversity effects between different categories of ecosystem processes, grouping processes according to the year of measurement, their biogeochemical cycle, trophic level and compartment (above- or belowground) and according to whether they were measures of biodiversity or other ecosystem processes, biotic or abiotic and static or dynamic. Overall, and for several individual processes, we found that biodiversity effects became stronger over time. Measures of the carbon cycle were also affected more strongly by plant species richness than were the measures associated with the nitrogen cycle. Further, we found greater plant species richness effects on measures of biodiversity than on other processes. The differential effects of plant diversity on the various types of ecosystem processes indicate that future research and political effort should shift from a general debate about whether biodiversity loss impairs ecosystem functions to focussing on the specific functions of interest and ways to preserve them individually or in combination.
KW  - Bottom-up effects
KW  - Carbon cycling
KW  - Ecological synthesis
KW  - Ecosystem processes
KW  - Grasslands
KW  - Jena experiment
KW  - Nitrogen cycling
Y1  - 2013
U6  - https://doi.org/10.1007/s00442-012-2589-0
SN  - 0029-8549
VL  - 173
IS  - 1
SP  - 223
EP  - 237
PB  - Springer
CY  - New York
ER  - 
TY  - JOUR
A1  - Casel, Katrin
A1  - Fernau, Henning
A1  - Gaspers, Serge
A1  - Gras, Benjamin
A1  - Schmid, Markus L.
T1  - On the complexity of the smallest grammar problem over fixed alphabets
JF  - Theory of computing systems
N2  - In the smallest grammar problem, we are given a word w and we want to compute a preferably small context-free grammar G for the singleton language {w} (where the size of a grammar is the sum of the sizes of its rules, and the size of a rule is measured by the length of its right side). It is known that, for unbounded alphabets, the decision variant of this problem is NP-hard and the optimisation variant does not allow a polynomial-time approximation scheme, unless P = NP. We settle the long-standing open problem whether these hardness results also hold for the more realistic case of a constant-size alphabet. More precisely, it is shown that the smallest grammar problem remains NP-complete (and its optimisation version is APX-hard), even if the alphabet is fixed and has size of at least 17. The corresponding reduction is robust in the sense that it also works for an alternative size-measure of grammars that is commonly used in the literature (i. e., a size measure also taking the number of rules into account), and it also allows to conclude that even computing the number of rules required by a smallest grammar is a hard problem. On the other hand, if the number of nonterminals (or, equivalently, the number of rules) is bounded by a constant, then the smallest grammar problem can be solved in polynomial time, which is shown by encoding it as a problem on graphs with interval structure. However, treating the number of rules as a parameter (in terms of parameterised complexity) yields W[1]-hardness. Furthermore, we present an O(3(vertical bar w vertical bar)) exact exponential-time algorithm, based on dynamic programming. These three main questions are also investigated for 1-level grammars, i. e., grammars for which only the start rule contains nonterminals on the right side; thus, investigating the impact of the "hierarchical depth" of grammars on the complexity of the smallest grammar problem. In this regard, we obtain for 1-level grammars similar, but slightly stronger results.
KW  - grammar-based compression
KW  - smallest grammar problem
KW  - straight-line
KW  - programs
KW  - NP-completeness
KW  - exact exponential-time algorithms
Y1  - 2020
U6  - https://doi.org/10.1007/s00224-020-10013-w
SN  - 1432-4350
SN  - 1433-0490
VL  - 65
IS  - 2
SP  - 344
EP  - 409
PB  - Springer
CY  - New York
ER  - 
TY  - JOUR
A1  - Casel, Katrin
A1  - Dreier, Jan
A1  - Fernau, Henning
A1  - Gobbert, Moritz
A1  - Kuinke, Philipp
A1  - Villaamil, Fernando Sánchez
A1  - Schmid, Markus L.
A1  - van Leeuwen, Erik Jan
T1  - Complexity of independency and cliquy trees
JF  - Discrete applied mathematics
N2  - An independency (cliquy) tree of an n-vertex graph G is a spanning tree of G in which the set of leaves induces an independent set (clique). We study the problems of minimizing or maximizing the number of leaves of such trees, and fully characterize their parameterized complexity. We show that all four variants of deciding if an independency/cliquy tree with at least/most l leaves exists parameterized by l are either Para-NP- or W[1]-hard. We prove that minimizing the number of leaves of a cliquy tree parameterized by the number of internal vertices is Para-NP-hard too. However, we show that minimizing the number of leaves of an independency tree parameterized by the number k of internal vertices has an O*(4(k))-time algorithm and a 2k vertex kernel. Moreover, we prove that maximizing the number of leaves of an independency/cliquy tree parameterized by the number k of internal vertices both have an O*(18(k))-time algorithm and an O(k 2(k)) vertex kernel, but no polynomial kernel unless the polynomial hierarchy collapses to the third level. Finally, we present an O(3(n) . f(n))-time algorithm to find a spanning tree where the leaf set has a property that can be decided in f (n) time and has minimum or maximum size.
KW  - independency tree
KW  - cliquy tree
KW  - parameterized complexity
KW  - Kernelization
KW  - algorithms
KW  - exact algorithms
Y1  - 2018
U6  - https://doi.org/10.1016/j.dam.2018.08.011
SN  - 0166-218X
SN  - 1872-6771
VL  - 272
SP  - 2
EP  - 15
PB  - Elsevier
CY  - Amsterdam [u.a.]
ER  - 
TY  - JOUR
A1  - Hofmann, Alexander J. L.
A1  - Züfle, Simon
A1  - Shimizu, Kohei
A1  - Schmid, Markus
A1  - Wessels, Vivien
A1  - Jäger, Lars
A1  - Altazin, Stephane
A1  - Ikegami, Keitaro
A1  - Khan, Motiur Rahman
A1  - Neher, Dieter
A1  - Ishii, Hisao
A1  - Ruhstaller, Beat
A1  - Brütting, Wolfgang
T1  - Dipolar Doping of Organic Semiconductors to Enhance Carrier Injection
JF  - Physical review applied
N2  - If not oriented perfectly isotropically, the strong dipole moment of polar organic semiconductor materials such as tris-(8-hydroxyquinolate)aluminum (Alq3) will lead to the buildup of a giant surface potential (GSP) and thus to a macroscopic dielectric polarization of the organic film. Despite this having been a known fact for years, the implications of such high potentials within an organic layer stack have only been studied recently. In this work, the influence of the GSP on hole injection into organic layers is investigated. Therefore, we apply a concept called dipolar doping to devices consisting of the prototypical organic materials N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) as nonpolar host and Alq3 as dipolar dopant with different mixing ratios to tune the GSP. The mixtures are investigated in single-layer monopolar devices as well as bilayer metal/insulator/semiconductor structures. Characterization is done electrically using current-voltage (I-V) characteristics, impedance spectroscopy, and charge extraction by linearly increasing voltage and time of flight, as well as with ultraviolet photoelectron spectroscopy. We find a maximum in device performance for moderate to low doping concentrations of the polar species in the host. The observed behavior can be described on the basis of the Schottky effect for image-force barrier lowering, if the changes in the interface dipole, the carrier mobility, and the GSP induced by dipolar doping are taken into account.
KW  - Carrier dynamics
KW  - Electric polarization
KW  - Optoelectronics
KW  - Organic electronics
KW  - Doped semiconductors
KW  - Interfaces
KW  - Organic LEDs
KW  - Organic semiconductors
Y1  - 2019
U6  - https://doi.org/10.1103/PhysRevApplied.12.064052
SN  - 2331-7019
VL  - 12
IS  - 6
PB  - American Physical Society
CY  - College Park
ER  -