Optical Probes of the Quantum-Entangled Triplet-Triplet State in a Heteroacene Dimer

The nature and extent of the spin-entanglement in the triplet-triplet biexciton with total spin zero in correlated-electron $\pi$-conjugated systems continues to be an enigma. Differences in the ultrafast transient absorption spectra of free triplets versus the triplet-triplet can give a measure of the entanglement. This, however, requires theoretical understandings of transient absorptions from the optical spin-singlet, the lowest spin-triplet exciton as well as from the triplet-triplet state, whose spectra are often overlapping and hence difficult to distinguish. We present a many-electron theory of the electronic structure of the triplet-triplet, and of complete wavelength-dependent excited state absorptions (ESAs) from all three states in a heteroacene dimer of interest in the field of intramolecular singlet fission. The theory allows direct comparisons of ESAs with existing experiments as well as experimental predictions, and gives physical understandings of transient absorptions within a pictorial exciton basis that can be carried over to other experimental systems

Diagrammatic Exciton Basis Theory of the Photophysics of Pentacene Dimers

Covalently linked acene dimers are of interest as candidates for intramolecular singlet fission. We report many-electron calculations of the energies and wavefunctions of the optical singlets, the lowest triplet exciton and the triplet-triplet biexciton, as well as the final states of excited state absorptions from these states in a family of phenyl-linked pentacene dimers. While it is difficult to distinguish between the triplet and the triplet-triplet from their transient absorptions in the 500-600 nm region, by comparing theoretical transient absorption spectra against published and unpublished experimental transient absorptions in the near and mid infrared we conclude that the end product of photoexcitation in these particular bipentacenes is the bound triplet-triplet and not free triplets. We predict additional transient absorptions at even longer wavelengths, beyond 1500 nm, to the equivalent of the classic 2$^1$A$_g^-$ in linear polyenes

Josephson Current between Triplet and Singlet Superconductors

The Josephson effect between triplet and singlet superconductors is studied. Josephson current can flow between triplet and singlet superconductors due to the spin-orbit coupling in the spin-triplet superconductor but it is finite only when triplet superconductor has $L_z=-S_z=\pm 1$, where $L_z$ and $S_z$ are the perpendicular components of orbital angular momentum and spin angular momentum of the triplet Cooper pairs, respectively. The recently observed temperature and orientational dependence of the critical current through a Josephson junction between UPt$_3$ and Nb is investigated by considering a non-unitary triplet state.Comment: 4 pages, no figure

Spin relaxation at the singlet-triplet crossing in a quantum dot

We study spin relaxation in a two-electron quantum dot in the vicinity of the singlet-triplet crossing. The spin relaxation occurs due to a combined effect of the spin-orbit, Zeeman, and electron-phonon interactions. The singlet-triplet relaxation rates exhibit strong variations as a function of the singlet-triplet splitting. We show that the Coulomb interaction between the electrons has two competing effects on the singlet-triplet spin relaxation. One effect is to enhance the relative strength of spin-orbit coupling in the quantum dot, resulting in larger spin-orbit splittings and thus in a stronger coupling of spin to charge. The other effect is to make the charge density profiles of the singlet and triplet look similar to each other, thus diminishing the ability of charge environments to discriminate between singlet and triplet states. We thus find essentially different channels of singlet-triplet relaxation for the case of strong and weak Coulomb interaction. Finally, for the linear in momentum Dresselhaus and Rashba spin-orbit interactions, we calculate the singlet-triplet relaxation rates to leading order in the spin-orbit interaction, and find that they are proportional to the second power of the Zeeman energy, in agreement with recent experiments on triplet-to-singlet relaxation in quantum dots.Comment: 29 pages, 14 figures, 1 tabl

Indirect Effect of Supersymmetric Triplets in Stop Decays

We study an extension of the minimal supersymmetric standard model with a zero hypercharge triplet, and the effect that such a particle has on stop decays. This model has the capability of predicting a 125.5 GeV Higgs even in the presence of light stops and it can modify the diphoton rate by means of the extra charged fermion triplet coupled to the Higgs. Working in the limit where the scalar triplet decouples, and with small values of mA, we find that the fermion triplet can greatly affect the branching ratios of the stops, even in the absence of a direct stop-triplet coupling. We compare the triplet extension with the MSSM and discuss how the additional fields affect the search for stop pair production.Comment: pdfLateX, 16 pages, 7 figures, 2 tables, Typos, minor changes. Version published in JHE

Strong magnetic field enhancement of spin triplet pairing arising from coexisting 2kF2k_F spin and 2kF2k_F charge fluctuations

We study the effect of the magnetic field (Zeeman splitting) on the triplet pairing. We show generally that the enhancement of spin triplet pairing mediated by coexisting $2k_F$ spin and $2k_F$ charge fluctuations can be much larger than in the case of triplet pairing mediated by ferromagnetic spin fluctuations. We propose that this may be related to the recent experiment for (TMTSF)$_2$ClO$_4$, in which a possibility of singlet to triplet pairing transition has been suggested.Comment: 5 page

Exploring Higgs Triplet Models via Vector Boson Scattering at the LHC

We present the results of a study of Higgs triplet boson production arising in the Littlest Higgs, Left-Right Symmetric, and Georgi-Machacek models in the W^\pm W^\pm, W^\pm Z, W^+ W^-, and Z Z channels at the LHC. We focus on the "gold-plated" purely leptonic decay modes and consider the irreducible electroweak, QCD, and t-quark backgrounds, applying a combination of forward-jet-tagging, central-jet-vetoing, and stringent leptonic cuts to suppress the backgrounds. We find that, given the constraints on the triplet vacuum expectation value (vev), considerable luminosity is required to observe Higgs triplet bosons in vector boson scattering. Observing a Higgs triplet at the LHC is most promising in the Georgi-Machacek model due to a weaker constraint on the triplet vev. In this model, we find that a Higgs triplet boson with a mass of 1.0 (1.5) TeV can be observed at the LHC with an integrated luminosity as low as 41 (119) fb^{-1} in the W^\pm W^\pm channel and as low as 171 (474) fb^{-1} in the W^\pm Z channel. Observation of Higgs triplet bosons in these channels would help identify the underlying theory.Comment: 14 pages, 11 figures, references adde