Singlet Channel Research Articles

Anti-resonance and the “0.7 anomaly” in conductance through a quantum point contact

We investigate the transmission of electrons through a quantum point contact by using a quasi-one-dimensional model with an extra local bound state staying below the band bottom. While the complete transmission in lower channels gives rise to plateaus of conductance at multiples of 2e2/h, the electrons in the lowest channel are scattered by the local bound state when it is singly occupied. This scattering produces a wide anti-resonant zero-transmittance for singlet formed by tunneling and local electrons, and has no effect on triplets, leading to an exact 0.75(2e2/h) shoulder prior to the first 2e2/h plateau. While at low temperature, the formation of Kondo cloud screens magnetic moment of the local bound state and reduces the scattering to the singlet channel, complementing the shoulder from 0.75 to 1.

Dynamics in the one-dimensional extended ionic Hubbard model

We investigate single-particle spectral densities and dynamical charge and spin structurefactors of the one-dimensional extended ionic Hubbard model in the band insulator regimeby using the perturbative continuous unitary transformations method. The one-bodystaggered potential is considered as the unperturbed part and the hopping term, on-siteelectron–electron interaction, and the nearest-neighbor repulsive interaction are treated asthe perturbations. The excitation spectrum of this model was determined in aprevious work (Hafez and Jafari 2010 Eur. Phys. J. B 78 323). It was shown thatwhen the intersite interaction is off, there are two antibound state modes and onebound state mode in the singlet channel and two bound state modes in the tripletchannel, while for finite values of intersite interaction two bound state modes werefound in each channel. Our results for dynamical charge and spin structure factorsindicate that only one of two bound/antibound state modes can be probed byelectron-energy-loss spectroscopy and inelastic neutron scattering experiments.

Factorization and Quarkonium Production

It is possible to extend the formalism for high- p T heavy quarkonium factorization beyond leading power. This extension may be helpful in interpreting the relative roles of octet and singlet channels in the formalism of nonrelativistic QCD (NRQCD). It may enable us to understand the origin of the surprisingly large results for cross sections calculated in the color singlet sector of NRQCD.

Paired phases and Bose-Einstein condensation of spin-one bosons with attractive interaction

We analyze paired phases of cold bosonic atoms with spin $S=1$ and with an attractive interaction. We derive mean-field self-consistent equations for the matrix order parameter describing such paired bosons on an optical lattice. The possible solutions are classified according to their symmetries. In particular, we find that the self-consistent equations for the SO(3) symmetric phase are of the same form as those for scalar bosons with attractive interactions. This singlet phase may exhibit either the BCS-type pairing instability (BCS phase) or the Bose-Einstein condensation (BEC) quasiparticle condensation together with the BCS-type pairing (BEC phase) for an arbitrary attraction ${U}_{0}$ in the singlet channel of the two-body interaction. We show that both condensate phases become stable if a repulsion ${U}_{2}$ in the quintet channel is above a critical value, which depends on ${U}_{0}$ and other thermodynamic parameters.

Bound states of two spin-12fermions in a synthetic non-Abelian gauge field

We study the bound states of two spin-$\half$ fermions interacting via a contact attraction (characterized by a scattering length) in the singlet channel in 3D space in presence of a uniform non-Abelian gauge field. The configuration of the gauge field that generates a Rashba type spin-orbit interaction is described by three coupling parameters $(\lambda_x, \lambda_y, \lambda_z)$. For a generic gauge field configuration, the critical scattering length required for the formation of a bound state is {\em negative}, i.e. shifts to the "BCS side" of the resonance. Interestingly, we find that there are special high-symmetry configurations (e.g., $\lambda_x = \lambda_y = \lambda_z$) for which there is a two body bound state for {\em any} scattering length however small and negative. Remarkably, the bound-state wave functions obtained for such configurations have nematic spin structure similar to those found in liquid $^3$He. Our results show that the BCS-BEC crossover is drastically affected by the presence of a non-Abelian gauge field. We discuss possible experimental signatures of our findings both at high and low temperatures.

Perturbative renormalizability of chiral two-pion exchange in nucleon-nucleon scattering

We study the perturbative renormalizability of chiral two-pion exchange for singlet and triplet channels within effective field theory, provided that the one-pion exchange piece of the interaction has been fully iterated. We determine the number of counterterms/subtractions needed to obtain finite results when the cutoff is removed, resulting in three counterterms for the singlet channel and six for the triplet. The results show that perturbative chiral two-pion exchange reproduce the data up to a center-of-mass momentum of $k~200--300$ MeV in the singlet channel and $k~300--400$ MeV in the triplet.

Mechanisms and Kinetics of the HOSO+NO Reaction

The mechanism of the reaction between HOSO and NO was investigated at the B3LYP/ 6-311 ++ G(d,p) level of theory. The geometries of the reactants, intermediates, transition states, and products were optimized. The intrinsic reaction coordinates (IRC) were traced and the connecting relationship between the transition states and the reactants, products were confirmed. The single point energies of the species were corrected at the CCSD(T) /6-311++G(d,p) level of theory. The reaction rate constants were calculated over a temperature range of 200-3000 K using classical transition state theory (TST) and canonical vibration transition state theory (CVT) combined with a small-curvature tunneling (SCT) correction. The results showed that the HOSO+NO reaction occurs in both the singlet and triplet reaction channels. The singlet reaction channel is dominant, and HNO and SO2are the main products. The chemical bond changes in the main reaction channel were analyzed by a topological analysis of the electron density.

Pauli spin blockade and lifetime-enhanced transport in a Si/SiGe double quantum dot

We analyze electron transport data through a Si/SiGe double quantum dot in terms of spin blockade and lifetime-enhanced transport (LET), which is transport through excited states that is enabled by long spin relaxation times. We present a series of low-bias voltage measurements showing the sudden appearance of a strong tail of current that we argue is an unambiguous signature of LET appearing when the bias voltage becomes greater than the singlet-triplet splitting for the (2,0) electron state. We present eight independent data sets, four in the forward bias (spin-blockade) regime and four in the reverse bias (lifetime-enhanced transport) regime, and show that all eight data sets can be fit to one consistent set of parameters. We also perform a detailed analysis of the reverse bias (LET) regime, using transport rate equations that include both singlet and triplet transport channels. The model also includes the energy dependent tunneling of electrons across the quantum barriers, and resonant and inelastic tunneling effects. In this way, we obtain excellent fits to the experimental data, and we obtain quantitative estimates for the tunneling rates and transport currents throughout the reverse bias regime. We provide a physical understanding of the different blockade regimes and present detailed predictions for the conditions under which LET may be observed.

Photodissociation of ozone in the Hartley band: Product state and angular distributions

Product state properties from the photodissociation of ozone in the ultraviolet Hartley band are investigated by trajectory surface-hopping calculations. The diabatic B and R state potential energy and coupling surfaces of Schinke and McBane [J. Chem. Phys. 132, 044305 (2010)] are employed. The properties computed include rotational and vibrational distributions in both the singlet and triplet channels, the total internal energy distribution in the triplet channel, and the photodissociation anisotropy parameter β in the singlet channel. A method for computing β from trajectories computed in internal Jacobi coordinates is described. In the singlet channel, the vibrational distribution is in good agreement with the experimental results. The observed increase in β with increasing photolysis wavelength is reproduced by the calculations and is attributed to the effects of the bending potential on the B state late in the fragmentation. The computed β values are too high with respect to experiment, and the peaks j(max) of the singlet-channel rotational distributions are too low; these discrepancies are attributed to a too steep bending potential at long O-O distances. In the triplet channel, the main part of the internal energy distribution is described well by the calculations, although the detailed structures observed in the experiment are not reproduced. The experimental rotational distributions are well reproduced, although the maxima appear at slightly too high j. The triplet state product energy distributions are shown to depend largely on the distribution of hopping points onto the R state surface. A Landau-Zener model constructed as a function of the O(2) bond distance provides a good physical description of the two-state dynamics. The high internal energy O(2) products that cannot be attributed to the excitation of the Herzberg states remain unexplained.

Baryon-Baryon Interactions in the Flavor SU(3) Limit from Full QCD Simulations on the Lattice

We investigate baryon-baryon (BB) interactions in the 3-flavor full QCD simulations with degenerate quark masses for all flavors. The BB potentials in the orbital S-wave are extracted from the Nambu-Bethe-Salpeter wave functions measured on the lattice. We observe strong flavor-spin dependences of the BB potentials at short distances. In particular, a strong repulsive core exists in the flavor-octet and spin-singlet channel (the 8s representation), while an attractive core appears in the flavor singlet channel (the 1 representation). We discuss the relation of such flavor-spin dependence with the Pauli exclusion principle at the quark level. The possible existence of an H-dibaryon resonance above the ΛΛ threshold is also discussed.

Operator product expansion and the short distance behavior of 3-flavor baryon potentials

The short distance behavior of baryon-baryon potentials defined through Nambu-Bethe-Salpeter wave functions is investigated using the operator product expansion. In a previous analysis of the nucleon-nucleon case, corresponding to the SU(3) channels 27s and $ {\overline {10}_a} $ , we argued that the potentials have a repulsive core. A new feature occurs for the case of baryons made up of three flavors: manifestly asymptotically attractive potentials appear in the singlet and octet channels. Attraction in the singlet channel was first indicated by quark model considerations, and recently been found in numerical lattice simulations. The latter have however not yet revealed asymptotic attraction in the octet channels; we give a speculative explanation for this apparent discrepancy.

Nucleon-nucleon potential in holographic QCD

We derive the nucleon-nucleon potential in the D4-D8 model of holographic QCD. The nucleon-nucleon potential emerges from one-boson exchange picture with massless pseudo-scalars and an infinite tower of spin one mesons. We observe in particular that the tensor coupling of the ρ meson with nucleon comes out naturally larger than the vector coupling, whereas for ω mesons and axial mesons, such tensor couplings are completely absent. The potential is universally repulsive ∼ 1 / r 2 at short distance, and has the usual long-distance attractive behavior ∼ − 1 / r 3 along a isospin singlet and spin triplet channel. Both the large N c form and the finite N c form are given. In the former, a shallow classical minimum of depth ∼ 0.1 M K K N c / λ forms at about r M K K ≃ 5.5 . We discuss shallow bound states of nucleons with the holographic potential.

Riemann [formula omitted]-terms in perturbative QED series, conformal symmetry and the analogies with structures of multiloop effects in [formula omitted] supersymmetric Yang–Mills theory

As was discovered recently, the 5-loop perturbative quenched QED approximation to the QED β-function consist from the rational term and the term proportional to ζ(3)-function. It is stressed, that this feature is also manifesting itself in the conformal invariant pqQED series for the 4-loop approximation to the anomalous mass dimension. The 4-loop pqQED expression for the singlet contribution into the Ellis–Jaffe polarized sum rule is obtained. It coincides with the similar approximation for the non-singlet coefficient functions of the Ellis–Jaffe sum rule and of the Bjorken polarized sum rule. It is stressed that this property is valid in all orders of perturbation theory thanks to the conformal symmetry of pqQED series and to the Crewther relation, which relates non-singlet and singlet coefficient functions of the Ellis–Jaffe sum rule with the coefficient functions of the non-singlet and singlet Adler D-functions. The basic steps of derivation of the Crewther relation in the singlet channel from the AVV triangle diagram are outlined. The similarities between analytical structures of asymptotic series for the coefficient functions in pqQED and for the anomalous dimensions in N=4 conformal invariant supersymmertic Yang–Mills theory are observed. The guess is proposed that the appearance of ζ(3)-terms in the pqQED expressions and the absence of ζ(5)-terms at the same level is the indication of absence of “wrapping” interactions in pqQED.

Three-quark static potential in perturbation theory

We study the three-quark static potential in perturbation theory in QCD. A complete next-to-leading order calculation is performed in the singlet, octets and decuplet channels and the potential exponentiation is demonstrated. The mixing of the octet representations is calculated. At next-to-next-to-leading order, the subset of diagrams producing three-body forces is identified in Coulomb gauge and its contribution to the potential calculated. Combining it with the contribution of the two-body forces, which may be extracted from the quark-antiquark static potential, we obtain the complete next-to-next-to-leading order three-quark static potential in the colour-singlet channel.

Photodissociation of ozone in the Hartley band: Potential energy surfaces, nonadiabatic couplings, and singlet/triplet branching ratio

The lowest five (1)A(') states of ozone, involved in the photodissociation with UV light, are analyzed on the basis of multireference configuration interaction electronic structure calculations with emphasis on the various avoided crossings in different regions of coordinate space. Global diabatic potential energy surfaces are constructed for the lowest four states termed X, A, B, and R. In addition, the off-diagonal potentials that couple the initially excited state B with states R and A are constructed to reflect results from additional electronic structure calculations, including the calculation of nonadiabatic coupling matrix elements. The A/X and A/R couplings are also considered, although in a less ambitious manner. The photodissociation dynamics are studied by means of trajectory surface hopping (TSH) calculations with the branching ratio between the singlet, O((1)D)+O(2)((1)Delta(g)), and triplet, O((3)P)+O(2)((3)Sigma(g) (-)), channels being the main focus. The semiclassical branching ratio agrees well with quantum mechanical results except for wavelengths close to the threshold of the singlet channel. The calculated O((1)D) quantum yield is approximately 0.90-0.95 across the main part of the Hartley band, in good agreement with experimental data. TSH calculations including all four states show that transitions B-->A are relatively unimportant and subsequent transitions A-->X/R to the triplet channel are negligible.

Femtosecond spectroscopy on the photochemistry of ortho-nitrotoluene

The photo tautomerisations of ortho-nitrotoluene (oNT) and its methylated derivative ortho-ethylnitrobenzene (oENB) have been studied by means of femtosecond spectroscopy and (TD)-DFT computations. In UV/Vis transient absorption spectroscopy a band peaking at 400 nm is seen to rise in a bi-modal manner with time constants of 1-10 ps and 1500 ps. Femtosecond stimulated Raman experiments clearly identify aci-nitro forms as the spectroscopic carriers of the 400 nm band. The assignment of the Raman spectra is based on TD-DFT computations. The quantum yields of the aci-nitro forms after 3 ns are 0.08 (oNT) and 0.3 (oENB). The aci-nitro forms are formed via a singlet channel (1-10 ps) and a triplet channel (1500 ps). There are indications that the triplet channel involves a bi-radical intermediate. In between 3 ns and 1 ms the spectrum of the aci-nitro form shifts from 400 to 390 nm. This could indicate a tautomerisation from Z-aci-nitro to an E form.

ΛNandΞ NInteractions Studied with Lattice QCD

We present our recent studies of Lambda-Nucleon (�N) as well as Cascade-Nucleon (�N) interac- tions by using lattice QCD. The equal-time Bethe-Salpeter (BS) amplitude of the lowest energy scattering state of baryon number B = 2 system (proton-� and proton-� 0 ) is calculated from lattice QCD. For the calculation of the �N potential, two different types of gauge configurations are employed: (i) 2 +1 flavor full QCD configurations generated by the PACS-CS collaboration at � = 1.9 (a = 0.0907(13) fm) on a 32 3 × 64 lattice, whose spatial vol- ume is (2.90 fm) 3 . (ii) Quenched QCD configurations at � = 5.7 (a = 0.1416(9) fm) on a 32 3 × 48 lattice, whose spatial volume is (4.5 fm) 3 . The spin-singlet central potential is calculated from the BS wave function for the spin J = 0 state, whereas the spin-triplet central potential as well as the tensor potential are deduced simultaneously from the BS wave function for the spin J = 1 state by dividing it into the S -wave and the D-wave components. For the calculation of theN potential, we employ quenched QCD configurations, at � = 5.7 (a = 0.1416(9) fm) on a 32 3 × 32 lattice, whose spatial volume is (4.5 fm) 3 . The effective central potential in the spin triplet channel as well as the central potential in the spin singlet channel are calculated for theN. The scattering lengths are obtained from the asymptotic behavior of the BS wave function by using the Luscher's formula.

Triplet annihilation exceeding spin statistical limit in highly efficient fluorescent organic light-emitting diodes

We have demonstrated that the exemplary red fluorescent organic light-emitting diodes (OLEDs) gain as much as half of their electroluminescence from annihilation of triplet states generated by recombining charge carriers. The magnitude of triplet-triplet annihilation (TTA) contribution in combination with the remarkably high total efficiencies [>11% external quantum efficiency (EQE)] indicates that the absolute amount of electroluminescence attributable to TTA substantially exceeds the limit imposed by spin statistics, which was independently confirmed by studying magnetic field effects on delayed luminescence. We determined the value of 1.3 for the ratio of the rate constants of singlet and triplet channels of annihilation, which is indeed substantially higher than the value of 0.33 expected for a purely statistical annihilation process. It is, however, in an excellent quantitative agreement with the extent of the experimental contribution of delayed luminescence to steady-state electroluminescence. The nonstatistical branching ratio of the two annihilation channels is attributed to the favorable relationship between the energies of the excited singlet and triplet states of rubrene—emissive layer host. We surmise that, with the appropriate emissive layer materials, the fluorescent OLED devices are capable of using a considerably larger fraction of triplet states than was previously believed. In principle, the upper limit for the singlet excited state yield in the TTA process is 0.5, which makes the maximum internal quantum efficiency of fluorescent OLEDs to be 25%+0.5×75%=62.5%. The estimates of maximum EQE of the fluorescent OLEDs should be revised to at least 0.2×62.5%=12.5% and, likely, even higher to account for optical outcoupling exceeding 0.2.

Forward physics with rapidity gaps at the LHC

A rapidity gap program with great potential can be realised at the Large Hadron Collider, LHC, by adding a few simple forward showercounters (FSCs) along the beam line on both sides of the main central detectors, such as CMS. Measurements of single diffractive crosssections down to the lowest masses can be made with an efficient level-1 trigger. Exceptionally, the detectorsalso make feasible the study of Central Diffractive Excitation, and in particular the reaction g+g→g+g, in the color singlet channel, effectively using the LHC as a gluon-gluon collider.

Phase diagram and critical fields of organic quasi-1d superconductors in an applied magnetic field

No consensus has been reached about the symmetry of the Bechgaard salts superconducting phase. An RG analysis is in favor of interactions dominant in the singlet channel, but very close and sub-dominant in the triplet channel. We study the properties, in a magnetic field along the b′ direction, of a d-wave singlet phase, as well as of an f-wave triplet phase. Recent data about NMR Knight shift, as well as upper critical fields have brought strong indications about the possible symmetries. We have analyzed theoretically the consequences of these experimental data: our results are in favor of a singlet phase in low field, but of a triplet phase in large fields.