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PubMed2004

Critical evaluation of the amino acid triplet-epitope matching concept in cadaver kidney transplantation.

BACKGROUND: A computer-based approach for determining human leukocyte antigen (HLA) compatibility between kidney donors and recipients on the basis of differences of amino acid sequences as motifs for immunogenic epitopes was proposed by Duquesnoy et al. The HLAMatchmaker algorithm focuses on HLA class I polymorphisms of serologically defined antigens encoded by the HLA-A and -B loci. HLA phenotypic mismatches that represent only a few mismatches at the amino acid triplet level are held to be not or only mildly immunogenic. This approach was proposed as being especially suitable for the allocation of donor kidneys to highly sensitized patients. METHODS: We reexamined this attractive concept using the data of the Collaborative Transplant Study. Intra- and interlocus comparisons for HLA-A and -B were performed according to the original HLAMatchmaker algorithm. To exclude the influence of HLA-DR, only transplants with no HLA-DR mismatch were considered. Patients who had one HLA-A and one HLA-B antigen mismatch were separated into subgroups, depending on the number of triplet mismatches as calculated by the HLAMatchmaker software. Separate analyses were performed for first transplants, retransplants, and patients with a panel-reactive antibody activity of 50% or more. A total of 16,997 white patients matched for HLA-DR who received a cadaver kidney transplant between 1991 and 2001 formed the basis of this analysis. RESULTS: Application of the HLAMatchmaker method could not be shown to result in any statistically significant effect on graft survival. CONCLUSIONS: The HLAMatchmaker concept is theoretically attractive; however, it could not be shown to yield useful results in this analysis. Serologic HLA typing appears to provide an insufficient basis for applying epitope matching in clinical kidney transplantation.

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PubMed2000

Singlet and triplet exciton formation rates in conjugated polymer light-emitting diodes

By applying a molecular orbital perturbation approach, we calculate the formation rates for singlet and triplet molecular excitons associated with intermolecular charge-transfer processes. It is found that the interchain bond-charge correlation has a strong influence on the relative probabilities for generating singlet and triplet excitons. Most importantly, application of our approach to a model system for poly-(paraphenylenevinylene) shows that the ratio between the electroluminescence and photoluminescence quantum yields generally exceeds the 25% spin-degeneracy statistical limit.

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PubMed2000

Spin-triplet superconductivity in Sr2RuO4 probed by andreev reflection

The superconducting gap function of Sr2RuO4 was investigated by means of quasiparticle reflection and transmission at the normal conductor-superconductor interface of Sr2RuO4-Pt point contacts. We found two distinctly different types of dV/dI vs V spectra either with a double-minimum structure or with a zero-bias conductance anomaly. Both types of spectra are expected in the limit of high and low transparency, respectively, of the interface barrier between a normal metal and a spin-triplet superconductor. Together with the temperature dependence of the spectra this result strongly supports a spin-triplet superconducting order parameter for Sr2RuO4.

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PubMed2000

Direct evidence for the localized single-triplet excitations and the dispersive multitriplet excitations in SrCu2(BO3)2.

We performed inelastic neutron scattering on the 2D Shastry-Sutherland system SrCu2(11BO3)2 with an exact dimer ground state. Three energy levels at around 3, 5, and 9 meV were observed at 1.7 K. The lowest excitation at 3.0 meV is almost dispersionless with a bandwidth of 0.2 meV at most, showing a significant constraint on a single-triplet hopping owing to the orthogonality of the neighboring dimers. In contrast, the correlated two-triplet excitations at 5 meV exhibit a more dispersive behavior.

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PubMed2000

Conduction through a quantum dot near a singlet-triplet transition.

Kondo effect in the vicinity of a singlet-triplet transition in a vertical quantum dot is considered. This system is shown to map onto a special version of the two-impurity Kondo model. At any value of the control parameter, the system has a Fermi-liquid ground state. Explicit expressions for the linear conductance as a function of the control parameter and temperature T are obtained. At T = 0, the conductance reaches the unitary limit approximately 4e(2)/h at the triplet side of the transition, and decreases with the increasing distance to the transition at the singlet side. At finite temperature, the conductance exhibits a peak near the transition point.

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PubMed2000

Spin-triplet superconductivity due to antiferromagnetic spin-fluctuation in Sr2RuO4

A mechanism leading to the spin-triplet superconductivity is proposed based on the antiferromagnetic spin-fluctuation. The effects of anisotropy in spin-fluctuation on the Cooper pairing and on the direction of d vector are examined in the one-band Hubbard model with random-phase approximation. The gap equations for the anisotropic case are derived and applied to Sr2RuO4. It is found that a nesting property of the Fermi surface together with the anisotropy leads to the triplet superconductivity with the d = &zcirc;(sink(x)+/-isink(y)), which is consistent with experiments.

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PubMed2001

Bound ro-vibronic states of triplet H3(+).

On the basis of a new, highly accurate potential energy hypersurface for the lowest triplet state of H+3, (3)Sigma(+)(u), the bound ro-vibronic states are calculated for J</=5. Since the potential has very shallow minima, those states exist only up to single vibrational excitation. The symmetry properties of the ro-vibrational states are investigated. Further, it is demonstrated that the first excited triplet state, which intersects conically with the (3)Sigma(+)(u) state, has no effect on the reported ro-vibrational energies.

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PubMed2001

Superconducting gap structure of spin-triplet superconductor Sr2RuO4 studied by thermal conductivity.

To clarify the superconducting gap structure of the spin-triplet superconductor Sr2RuO4, the in-plane thermal conductivity has been measured as a function of relative orientations of the thermal flow, the crystal axes, and a magnetic field rotating within the 2D RuO2 planes. The in-plane variation of the thermal conductivity is incompatible with any model with line nodes vertical to the 2D planes and indicates the existence of horizontal nodes. These results place strong constraints on models that attempt to explain the mechanism of the triplet superconductivity.

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PubMed2001

O2 diffusion in SiO2: triplet versus singlet.

We address the diffusion of the oxygen molecule in SiO2, using first-principles spin-polarized total-energy calculations. We find that the potential energy surfaces for the singlet and triplet states are very different in certain regions, and that the O2 molecule preserves its spin-triplet ground state not only at its most stable interstitial position inside the solid but also throughout its diffusion pathway. Therefore, the singlet state is not a good approximation to describe the behavior of O2 inside SiO2, and spin-polarization effects are fundamental to understand the properties of this system.

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PubMed2001

Tunneling through a quantum dot in local spin singlet-triplet crossover region with Kondo effect.

Tunneling conductance through a quantum dot is calculated around the local spin singlet-triplet crossover region including the Kondo effect. The calculation is carried out using the numerical renormalization group method. When the potential on the dot deepens, two electrons filling a lower energy orbital redistribute to gain Hund's coupling energy. This redistribution induces a bump in the conductance between the Coulomb peaks. The Kondo temperature on the bump is high due to the fluctuation on the singlet-triplet crossover region. The behaviors agree well with recent experiment.

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PubMed2002

Dual neutral variables and knot solitons in triplet superconductors.

We derive a dual presentation of a free energy functional for spin-triplet superconductors in terms of gauge-invariant variables. The resulting equivalent model in ferromagnetic phase has a form of a version of the Faddeev model. This allows one, in particular, to conclude that spin-triplet superconductors allow formation of stable finite-length closed vortices (knotted solitons).

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PubMed2003

Odd triplet superconductivity in superconductor-ferromagnet multilayered structures.

We demonstrate that in multilayered superconductor-ferromagnet structures a noncollinear alignment of the magnetizations of different ferromagnetic layers generates a triplet superconducting condensate which is odd in frequency. This triplet condensate coexists in the superconductors with the conventional singlet one but decays very slowly in the ferromagnet, which should lead to a large Josephson effect between the superconductors separated by the ferromagnet. Depending on the mutual direction of the ferromagnetic moments, the Josephson coupling can be both of 0 and of pi type.

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PubMed2005

Adsorption of triplet O(2) on Si(100): the crucial step in the initial oxidation of a silicon surface.

It has long been understood that a precursor mediated chemisorption is a significant part of the dynamics for the adsorption of O(2) on Si(100), which is a much studied model system of surface reaction with considerable technological relevance. However, theoretical studies on the interaction between O(2) and Si(100) have been focused on the excited singlet state of O2 and unable to explain the observations in surface scattering experiments. We demonstrate by first principles calculations that such a focus is misplaced. In reality, triplet O(2) can also react with Si(100), after overcoming small barriers, and its reaction paths provide a full account for experiments. Our results highlight the important role played by triplet O(2) in surface oxidation.

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PubMed2005

Fractional-flux vortices and spin superfluidity in triplet superconductors.

We discuss a novel type of fractional-flux vortices along with integer flux vortices in Kosterlitz-Thouless transitions in a triplet superconductor. We show that under certain conditions a spin-triplet superconductor should exhibit a novel state of spin superfluidity without superconductivity.

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PubMed2005

Electrical pump-and-probe study of spin singlet-triplet relaxation in a quantum dot.

Spin relaxation from a triplet excited state to a singlet ground state in a semiconductor quantum dot is studied by employing an electrical pump-and-probe method. Spin relaxation occurs via co-tunneling when the tunneling rate is relatively large, confirmed by a characteristic square dependence of the relaxation rate on the tunneling rate. When co-tunneling is suppressed by reducing the tunneling rate, the intrinsic spin relaxation is dominated by spin-orbit interaction. We discuss a selection rule of the spin-orbit interaction based on the observed double-exponential decay of the triplet state.

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PubMed2005

Triplet superconducting pairing and density-wave instabilities in organic conductors.

Using a renormalization group approach, we determine the phase diagram of an extended quasi-one-dimensional electron gas model that includes interchain hopping, nesting deviations, and both intrachain and interchain repulsive interactions. We find a close proximity of spin-density- and charge-density-wave phases and singlet d-wave and triplet f-wave superconducting phases. There is a striking correspondence between our results and recent puzzling experimental findings in the Bechgaard salts, including the coexistence of spin-density-wave and charge-density-wave phases and the possibility of a triplet pairing in the superconducting phase.

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PubMed2006

Gauge and Yukawa mediated supersymmetry breaking in the triplet seesaw scenario.

We propose a novel supersymmetric unified scenario of the triplet seesaw mechanism where the exchange of the heavy triplets generates both neutrino masses and soft supersymmetry breaking terms. Our framework is very predictive since it relates neutrino mass parameters, lepton-flavor-violation in the slepton sector, sparticle and Higgs spectra, and electroweak symmetry breakdown. The phenomenological viability and experimental signatures in lepton flavor-violating processes are discussed.

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PubMed2000

Triplet-phase measurements using reference-beam X-ray diffraction.

Reference-beam diffraction (RBD) is a recently developed phase-sensitive X-ray diffraction technique that incorporates the principle of multiple-beam diffraction into the standard oscillating-crystal data-collection method [Shen (1998). Phys. Rev. Lett. 80, 3268-3271]. Using this technique, a large number of multiple-beam interference profiles can be recorded simultaneously on an area detector, from which a large number of triplet phases of Bragg reflections can be determined in a crystallography experiment. In this article, both the theoretical developments and the experimental procedures of the RBD technique are described in detail. Approximate theoretical approaches for RBD are outlined and simple analytical expressions are obtained that provide the basis for an automated data-analysis procedure that can be used to extract triplet phases from a large number of measured reference-beam diffraction profiles. Experimental examples are given for a variety of crystals including GaAs, tetragonal lysozyme and AlPdMn quasicrystal, using both image plates and a charge-coupled device (CCD) as the area detector. Possible uses of the measured phases for crystal structure determination are discussed as well as future prospects of the RBD technique.

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