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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
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Glide-plane symmetry and superconducting gap structure of iron-based superconductors
We consider the effect of glide-plane symmetry of the Fe-pnictogen/chalcogen layer in Fe-based superconductors on pairing in spin fluctuation models. Recent theories propose that so-called η-pairing states with nonzero total momentum can be realized and possess such exotic properties as odd parity spin singlet symmetry and time-reversal symmetry breaking. Here we show that when there is orbital weight at the Fermi level from orbitals with even and odd mirror reflection symmetry in z, η pairing is inevitable; however, we conclude from explicit calculation that the gap function appearing in observable quantities is identical to that found in earlier pseudocrystal momentum calculations with 1 Fe per unit cell.
Peak Locations and Relative Phase of Different Decay Modes of the a 1 Axial Vector Resonance in Diffractive Production
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Opening Up the Compressed Region of Top Squark Searches at 13 TeV LHC
Light top superpartners play a key role in stabilizing the electroweak scale in supersymmetric theories. For R-parity conserved supersymmetric models, traditional searches are not sensitive to the compressed regions. In this Letter, we propose a new method targeting this region, with top squark and neutralino mass splitting ranging from m t - m χ ≳ m t to about 20 GeV. In particular, we focus on the signal process in which a pair of top squarks are produced in association with a hard jet, and we define a new observable R M whose distribution has a peak in this compressed region. The position of the peak is closely correlated with m t . Further, we show that for the 13 TeV LHC with a luminosity of 3000 fb -1 , this analysis can extend the reach of the top squark in the compressed region to m t around 800 GeV.
Direct evidence of octupole deformation in neutron-rich 144 Ba
Here, the neutron-rich nucleus 144 Ba (t 1/2 = 11.5 s) is expected to exhibit some of the strongest octupole correlations among nuclei with mass numbers A less than 200. Until now, indirect evidence for such strong correlations has been inferred from observations such as enhanced E1 transitions and interleaving positive- and negative-parity levels in the ground-state band. In this experiment, the octupole strength was measured directly by sub-barrier, multistep Coulomb excitation of a post-accelerated 650-MeV 144 Ba beam on a 1.0–mg/cm 2 208 Pb target. The measured value of the matrix element, < 3 1 –∥M(E3)∥0 1 + >= 0.65( +17 –23 ) eb 3/2 , corresponds to a reduced B(E3) transition probability of 48( +25 –34 ) W.u. This result represents an unambiguous determination of the octupole collectivity, is larger than any available theoretical prediction, and is consistent with octupole deformation.
Neutrino masses from a pseudo-Dirac bino
We show that, in U(1) R -symmetric supersymmetric models, the bino and its Dirac partner (the singlino) can play the role of right-handed neutrinos and generate the neutrino masses and mixing, without the need for traditional bilinear or trilinear R-parity violating operators. The two particles form a pseudo-Dirac pair, the “bi ν o.” An inverse seesaw texture is generated for the neutrino-biνo sector, and the lightest neutrino is predicted to be massless. Lastly, unlike in most models with heavy right-handed neutrinos, the bi ν o can be sizably produced at the LHC through its interactions with colored particles, while respecting low energy constraints from neutrinoless double-beta decay and charged lepton flavor violation.
Experimental Investigation of the 19 Ne(p,γ) 20 Na Reaction Rate and Implications for Breakout from the Hot CNO Cycle
The 19 Ne(p,γ) 20 Na reaction is the second step of a reaction chain which breaks out from the hot CNO cycle, following the 15 O(α,γ) 19 Ne reaction at the onset of x-ray burst events. We investigate the spectrum of the lowest proton-unbound states in 20 Na in an effort to resolve contradictions in spin-parity assignments and extract reliable information about the thermal reaction rate. Here, the proton-transfer reaction 19 Ne(d,n) 20 Na is measured with a beam of the radioactive isotope 19 Ne at an energy around the Coulomb barrier and in inverse kinematics. We observe three proton resonances with the 19 Ne ground state, at 0.44, 0.66, and 0.82 MeV c.m. energies, which are assigned 3 + , 1 + , and (0 + ), respectively. In addition, we identify two resonances with the first excited state in 19 Ne, one at 0.20 MeV and one, tentatively, at 0.54 MeV. These observations allow us for the first time to experimentally quantify the astrophysical reaction rate on an excited nuclear state. Our experiment shows an efficient path for thermal proton capture in 19 Ne(p,γ) 20 Na, which proceeds through ground state and excited-state capture in almost equal parts and eliminates the possibility for this reaction to create a bottleneck in the breakout from the hot CNO cycle.
Strong Evidence for Nucleon Resonances near 1900 MeV
Data on the reaction yp→K + A from the CLAS experiments are used to derive the leading multipoles, E 0+ , M 1- , E 1+ , and M 1+ , from the production threshold to 2180 MeV in 24 slices of the invariant mass. The four multipoles are determined without any constraints. The multipoles are fitted using a multichannel L+P model that allows us to search for singularities and to extract the positions of poles on the complex energy plane in an almost model-independent method. The multipoles are also used as additional constraints in an energy-dependent analysis of a large body of pion and photoinduced reactions within the Bonn-Gatchina partial wave analysis. The study confirms the existence of poles due to nucleon resonances with spin parity J P =1/2 - , 1/2 + , and 3/2 + in the region at about 1.9 GeV.
Structure and Reactions of Be 11 : Many-Body Basis for Single-Neutron Halo
The exotic nucleus 11 Be has been extensively studied and much experimental information is available on the structure of this system. Treating, within the framework of empirically renormalised nuclear field theory in both configuration and 3D-space, the mixing of bound and continuum single-particle states through the coupling to collective particle-hole (p,h) and pairing vibrations of the 10 Be core, as well as Pauli principle acting not only between the particles explicitly considered and those participating in the collective states, but also between fermions involved in two-phonon virtual states it is possible, for the first time, to simultaneously and quantitatively account for the energies of the 1/2 + ,1/2 - low-lying states, the centroid and line shape of the 5/2 + resonance, the one-nucleon stripping and pickup absolute differential cross sections involving 11 Be as either target or residual nucleus, and the dipole transitions connecting the 1/2 + and 1/2 - parity inverted levels as well as the charge radius, thus providing a unified and exhaustive characterisation of the many-body effects which are at the basis of this paradigmatic one-neutron halo system.
Topological Phase Transitions in Multicomponent Superconductors
We study the phase transition between a trivial and a time-reversal-invariant topological superconductor in a single-band system. By analyzing the interplay of symmetry, topology, and energetics, we show that for a generic normal state band structure, the phase transition occurs via extended intermediate phases in which even- and odd-parity pairing components coexist. For inversion-symmetric systems, the coexistence phase spontaneously breaks time-reversal symmetry. For noncentrosymmetric superconductors, the low-temperature intermediate phase is time-reversal breaking, while the high-temperature phase preserves time-reversal symmetry and has topologically protected line nodes. Furthermore, with approximate rotational invariance, the system has an emergent U (1) × U (1) symmetry, and novel topological defects, such as half vortex lines binding Majorana fermions, can exist. We analytically solve for the dispersion of the Majorana fermion and show that it exhibits small and large velocities at low and high energies. Finally, relevance of our theory to superconducting pyrochlore oxide Cd 2 Re 2 O 7 and half-Heusler materials is discussed.
Novel Δ J = 1 Sequence in Ge 78 : Possible Evidence for Triaxiality
Here, a sequence of low-energy levels in 78 32 Ge 46 has been identified with spins and parity of 2 + , 3 + , 4 + , 5 + , and 6 + . Decays within this band proceed strictly through ΔJ=1 transitions, unlike similar sequences in neighboring Ge and Se nuclei. Above the 2+ level, members of this sequence do not decay into the ground-state band. Moreover, the energy staggering of this sequence has the phase that would be expected for a γ-rigid structure. The energies and branching ratios of many of the levels are described well by shell-model calculations. However, the calculated reduced transition probabilities for the ΔJ=2 in-band transitions imply that they should have been observed, in contradiction with the experiment. Within the calculations of Davydov, Filippov, and Rostovsky for rigid-triaxial rotors with γ=30°, there are sequences of higher-spin levels connected by strong ΔJ=1 transitions which decay in the same manner as those observed experimentally, yet are calculated at too high an excitation energy.
First Observation of 𝑃-odd 𝛾 Asymmetry in Polarized Neutron Capture on Hydrogen
Here, we report the first observation of the parity-violating gamma-ray asymmetry 𝐴$^{𝑛𝑝}_{𝛾}$ in neutron-proton capture using polarized cold neutrons incident on a liquid parahydrogen target at the Spallation Neutron Source at Oak Ridge National Laboratory. 𝐴$^{𝑛𝑝}_{𝛾}$ isolates the Δ𝐼 =1, 3 𝑆 1 → 3 𝑃 1 component of the weak nucleon-nucleon interaction, which is dominated by pion exchange and can be directly related to a single coupling constant in either the DDH meson exchange model or pionless effective field theory. We measured 𝐴$^{𝑛𝑝}_{𝛾}$=[−3.0±1.4(stat)±0.2(syst)]×10 −8 , which implies a DDH weak 𝜋𝑁𝑁 coupling of ℎ$^1_𝜋$=[2.6±1.2(stat)±0.2(syst)]×10 −7 and a pionless EFT constant of 𝐶 3 𝑆 1 → 3 𝑃 1 /𝐶 0 =[−7.4±3.5(stat)±0.5(syst)]×10 −11 MeV −1 . We describe the experiment, data analysis, systematic uncertainties, and implications of the result.
Observation of C P Violation in Charm Decays
A search for charge-parity $(CP)$ violation in $D^0 → K^–K^+$ and $D^0 → π^–π^+$ decays is reported, using $p p$ collision data corresponding to an integrated luminosity of 5.9 fb –1 collected at a center-of-mass energy of 13 TeV with the LHCb detector. The flavor of the charm meson is inferred from the charge of the pion in $D^*(2010)^+ → D^0π^+$ decays or from the charge of the muon in $\overline B → D^0μ^–\overline ν_μX$ decays. The difference between the $CP$ asymmetries in $D^0 → K^–K^+$ and $D^0 → π^–π^+$ decays is measured to be $ΔA_{CP} = [–18.2 ± 3.2($stat$) ± 0.9($syst$)] × 10^{–4}$ for $π$-tagged and $ΔACP = [–9 ± 8($stat$) ± 5($syst$)] × 10^{–4}$ for $μ-$ tagged $D^0$ mesons. Combining these with previous LHCb results leads to $ΔA_{CP} = (–15.4 ± 2.9) × 10^{–4}$, where the uncertainty includes both statistical and systematic contributions. The measured value differs from zero by more than 5 standard deviations. This is the first observation of $CP$ violation in the decay of charm hadrons.
Tomographic Dynamics and Scale-Dependent Viscosity in 2D Electron Systems
Fermi gases in two dimensions display collective dynamics originating from head-on collisions, a collinear carrier scattering process that dominates angular relaxation at not-too-high temperatures T<< TF. In this regime, a large family of excitations emerges, with an odd-parity angular structure of momentum distribution and exceptionally long lifetimes. This leads to “tomographic” dynamics: fast 1D spatial diffusion along the unchanging velocity direction accompanied by a slow angular dynamics that gradually randomizes velocity orientation. Here, the tomographic regime features an unusual hierarchy of timescales and scale-dependent transport coefficients with nontrivial fractional scaling dimensions, leading to fractional-power current flow profiles and unusual conductance scaling versus sample width.
Phononic Helical Nodal Lines with PT Protection in MoB 2
While condensed matter systems host both fermionic and bosonic quasiparticles, reliably predicting and empirically verifying topological states is only mature for Fermionic electronic structures, leaving topological Bosonic excitations sporadically explored. This is unfortunate, as Bosonic systems such as phonons offer the opportunity to assess spinless band structures where nodal lines can be realized without invoking special additional symetries to protect against spin-orbit coupling. Here we combine first-principles calculations and meV-resolution inelastic x-ray scattering to demonstrate the first realization of parity-time reversal symmetry protected helical nodal lines in the phonon spectrum of MoB 2 . This structure is unique to phononic systems as the spin-orbit coupling present in electronic systems tends to lift the degeneracy away from high-symmetry locations. Our study establishes a protocol to accurately identify topological Bosonic excitations, opening a new route to explore exotic topological states in crystalline materials.
Extending the Southern Shore of the Island of Inversion to F 28
Detailed spectroscopy of the neutron-unbound nucleus 28 F has been performed for the first time following proton/neutron removal from 29 Ne/ 29 F beams at energies around 230 MeV / nucleon . The invariant-mass spectra were reconstructed for both the 27 F(*) + n and 26 F(*) + 2n coincidences and revealed a series of well-defined resonances. A near-threshold state was observed in both reactions and is identified as the 28 F ground state, with S n ( 28 F) = -199(6) keV, while analysis of the 2n decay channel allowed a considerably improved S n ( 27 F) = 162(60) keV to be deduced. Comparison with shell-model predictions and eikonal-model reaction calculations have allowed spin-parity assignments to be proposed for some of the lower-lying levels of 28 F. Importantly, in the case of the ground state, the reconstructed 27 F + n momentum distribution following neutron removal from 29 F indicates that it arises mainly from the 1p 3/2 neutron intruder configuration. Finally, this demonstrates that the island of inversion around N = 20 includes 28 F, and most probably 29 F, and suggests that 28 O is not doubly magic.