DFT Calculations as a Tool to Analyse Quadrupole Splittings of Spin Crossover Fe(II) complexes

Density functional methods have been applied to calculate the quadrupole splitting of a series of iron(II) spin crossover complexes. Experimental and calculated values are in reasonable agreement. In one case spin-orbit coupling is necessary to explain the very small quadrupole splitting value of 0.77 mm/s at 293 K for a high-spin isomer

Low-temperature magnetization in geometrically frustrated Tb2Ti2O7

The nature of the low temperature ground state of the pyrochlore compound Tb2Ti2O7 remains a puzzling issue. Dynamic fluctuations and short-range correlations persist down to 50 mK, as evidenced by microscopic probes. In parallel, magnetization measurements show irreversibilities and glassy behavior below 200 mK. We have performed magnetization and AC susceptibility measurements on four single crystals down to 57 mK. We did not observe a clear plateau in the magnetization as a function of field along the [111] direction, as suggested by the quantum spin ice model. In addition to a freezing around 200 mK, slow dynamics are observed in the AC susceptibility up to 4 K. The overall frequency dependence cannot be described by a canonical spin-glass behavior.Comment: 5 pages, 4 figures + Supp. Mat (3 pages, 5 figures

Conservative Quantum Computing

Conservation laws limit the accuracy of physical implementations of elementary quantum logic gates. If the computational basis is represented by a component of spin and physical implementations obey the angular momentum conservation law, any physically realizable unitary operators with size less than n qubits cannot implement the controlled-NOT gate within the error probability 1/(4n^2), where the size is defined as the total number of the computational qubits and the ancilla qubits. An analogous limit for bosonic ancillae is also obtained to show that the lower bound of the error probability is inversely proportional to the average number of photons. Any set of universal gates inevitably obeys a related limitation with error probability O(1/n^2)$. To circumvent the above or related limitations yielded by conservation laws, it is recommended that the computational basis should be chosen as the one commuting with the additively conserved quantities.Comment: 5 pages, RevTex. Corrected to include a new statement that for bosonic ancillae the lower bound of the error probability is inversely proportional to the average number of photons, kindly suggested by Julio Gea-Banacloch

PtrA is required for coordinate regulation of gene expression during phosphate stress in a marine Synechococcus

Previous microarray analyses have shown a key role for the two-component system PhoBR (SYNW0947, SYNW0948) in the regulation of P transport and metabolism in the marine cyanobacterium Synechococcus sp. WH8102. However, there is some evidence that another regulator, SYNW1019 (PtrA), probably under the control of PhoBR, is involved in the response to P depletion. PtrA is a member of the cAMP receptor protein transcriptional regulator family that shows homology to NtcA, the global nitrogen regulator in cyanobacteria. To define the role of this regulator, we constructed a mutant by insertional inactivation and compared the physiology of wild-type Synechcococcus sp. WH8102 with the ptrA mutant under P-replete and P-stress conditions. In response to P stress the ptrA mutant failed to upregulate phosphatase activity. Microarrays and quantitative RT-PCR indicate that a subset of the Pho regulon is controlled by PtrA, including two phosphatases, a predicted phytase and a gene of unknown function psip1 (SYNW0165), all of which are highly upregulated during P limitation. Electrophoretic mobility shift assays indicate binding of overexpressed PtrA to promoter sequences upstream of the induced genes. This work suggests a two-tiered response to P depletion in this strain, the first being PhoB-dependent induction of high-affinity PO4 transporters, and the second the PtrA-dependent induction of phosphatases for scavenging organic P. The levels of numerous other transcripts are also directly or indirectly influenced by PtrA, including those involved in cell-surface modification, metal uptake, photosynthesis, stress responses and other metabolic processes, which may indicate a wider role for PtrA in cellular regulation in marine picocyanobacteria