We report the electrical induction and detection of dynamic nuclear polarization in the spin-blockade regime of double GaAs vertical quantum dots. The nuclear Overhauser field measurement relies on bias voltage control of the interdot spin exchange coupling and measurement of dc current at variable external magnetic fields. The largest Overhauser field observed was about 4 T, corresponding to a nuclear polarization approximately 40% for the electronic g factor typical of these devices, |g*| approximately 0.25. A phenomenological model is proposed to explain these observations.

By using two oligonucleotide additives that bear a monophosphate group at the termini through various linkers, gap structures were formed at predetermined positions in substrate DNA, and the monophosphate groups were placed at both edges of these gaps. At pH 7.0 and 37 degrees C, the phosphodiester linkages in the gap sites were efficiently and selectively hydrolyzed by Ce(IV)/EDTA complex (EDTA = ethylenediamine-N,N,N',N'-tetraacetate). The linkages in the middle of the gaps were predominantly hydrolyzed. Compared with DNA scission using oligonucleotide additives that bear no terminal monophosphate, the present scission was much faster (22-fold for a 3-base gap and 14-fold for a 5-base gap) and more site selective. Introduction of one monophosphate group to either edge of the gaps was also effective for promotion of both site selectivity and scission rate. The monophosphate group(s) at the gap site recruits the Ce(IV) to the target site and magnifies the difference in intrinsic reactivity between the target site and the others. Even at higher reaction temperatures, the site selectivity remained satisfactorily high. Furthermore, the fragments formed by the site-selective scission were connected with various oligonucleotides by using DNA ligase, producing desired recombinant DNAs.

The ability of the hyperfine interaction to lift spin blockade was studied for vertically coupled double quantum dots in two different regimes of potential detuning between the two dots. The double dot has a triplet state, which is a sufficiently long-lived excited state to block current flow by the Pauli effect. This blockade is lifted by a spin flip transition to the singlet state, generating a leakage current. The singlettriplet separation or exchange energy decreases with increasing detuning. For small detuning, the leakage current shows a step when the Zeeman energy equals the exchange energy thus turning on the flip-flop interaction. The threshold magnetic field gradually increases with decreasing detuning. It increases more abruptly near the resonance of two singlet states reflecting the increased exchange energy. For large detuning, the leakage current is caused by singlet-triplet mixing due to the fluctuating nuclear field, and it decreases when the Zeeman energy exceeds the fluctuating nuclear field energy.

We quantify the magnitude, buildup and relaxation dynamics of dynamic nuclear polarization (DNP) in a vertical GaAs double quantum dot device. This is accomplished using electrical techniques in the two‐electron spin‐blockade regime. Measurements are based on realtime electrical control of the relevant two‐electron spindependent energy levels, and observation of DNPinduced current step features at finite external magnetic fields. A simple phenomenological picture is presented to discuss the factors that limit maximum polarization in this system. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A monophosphate group was attached to the terminus of pseudo-complementary peptide nucleic acid (pcPNA), and two of thus modified pcPNAs were combined with Ce(IV)/EDTA for site-selective hydrolysis of double-stranded DNA. The site-selective DNA scission was notably accelerated by this chemical modification of pcPNAs. These second-generation artificial restriction DNA cutters (ARCUTs) differentiated the target sequence so strictly that no scission occurred even when only one DNA base-pair was altered to another. By using two of the activated ARCUTs simultaneously, DNA substrate was selectively cut at two predetermined sites, and the desired fragment was clipped and cloned. The DNA scission by ARCUT was also successful even when the target site was methylated by methyltransferase and protected from the corresponding restriction enzyme. Furthermore, potentiality of ARCUT for manipulation of huge DNA has been substantiated by site-selective scission of genomic DNA of Escherichia coli (composed of 4,600,000 bp) at the target site. All these results indicate promising applications of ARCUTs for versatile purposes.