Evidence for even parity unconventional superconductivity in Sr2RuO4

Funding: A.C. is grateful for support from the Julian Schwinger Foundation for Physics Research. A.P. acknowledges support by the Alexander von Humboldt Foundation through the Feodor Lynen Fellowship. Work at Los Alamos was funded by Laboratory Directed Research and Development (LDRD) program, and A.P. acknowledges partial support through the LDRD. N.K. acknowledges the support by the Grants-in-Aid for Scientific Research (KAKENHI, Grant JP18K04715 and JP21H01033) from Japan Society for the Promotion of Science (JSPS). The work at Dresden was funded by the Deutsche Forschungsgemeinschaft - TRR 288 - 422213477 (projects A10 and B01). The work at University of California, Los Angeles, was supported by NSF Grants 1709304 and 2004553.Unambiguous identification of the superconducting order parameter symmetry in Sr2RuO4 has remained elusive for more than a quarter century. While a chiral p-wave ground state analogue to superfluid 3He-A was ruled out only very recently, other proposed triplet-pairing scenarios are still viable. Establishing the condensate magnetic susceptibility reveals a sharp distinction between even-parity (singlet) and odd-parity (triplet) pairing since the superconducting condensate is magnetically polarizable only in the latter case. Here field-dependent 17O Knight shift measurements, being sensitive to the spin polarization, are compared to previously reported specific heat measurements for the purpose of distinguishing the condensate contribution from that due to quasiparticles. We conclude that the shift results can be accounted for entirely by the expected field-induced quasiparticle response. An upper bound for the condensate magnetic response of < 10% of the normal state susceptibility is sufficient to exclude all purely odd-parity candidates. PostprintPeer reviewe

Vacancy Control in Acene Blends Links Exothermic Singlet Fission to Coherence

The fission of singlet excitons into triplet pairs in organic materials holds great technological promise, but the rational application of this phenomenon is hampered by a lack of understanding of its complex photophysics. Here, we use the controlled introduction of vacancies by means of spacer molecules in tetracene and pentacene thin films as a tuning parameter complementing experimental observables to identify the operating principles of different singlet fission pathways. Time-resolved spectroscopic measurements in combination with microscopic modelling enables us to demonstrate distinct scenarios, resulting from different singlet-to-triplet pair energy alignments. For pentacene, where fission is exothermic, coherent mixing between the photoexcited singlet and triplet-pair states is promoted by vibronic resonances, which drives the fission process with little sensitivity to the vacancy concentration. Such vibronic resonances do not occur for endothermic materials such as tetracene, for which we find fission to be fully incoherent; a process that is shown to slow down with increasing vacancy concentration

Vibrational Dynamics in Disordered Molecular Crystals by Picosecond Coherent Raman and Photon Echo Spectroscopies (Exciton, Low Temperature)

131 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1985.An understanding of vibrational dynamics in complex molecules is a fundamental problem in chemical physics and is important for discerning how vibrational relaxation effects actual chemical reaction rates. The natural states of a pure crystal, however, are delocalized excitons and are described by the language of solid state physics. In this thesis the important processes involved in vibrational exciton (vibron) dynamics are differentiated by investigating vibron relaxation in isotopically and chemically disordered crystals at low temperature by picosecond time-delayed Coherent Anti-Stokes Raman Scattering (psCARS) and photon echo spectroscopy (PE).Picosecond laser spectroscopy is used to observe vibrational dynamics directly in the time domain. PsCARS is very useful for the investigation of pure and heavily disordered crystals since the Raman effect has a small cross section and consequently low net absorption. The photon echo measurements are used to examine vibron relaxation above the electronic excited state and can be used on very dilute chemically mixed crystals since the laser induced transitions are dipole allowed. The experiments performed were all done at low temperature (1.5(DEGREES)K and 10(DEGREES)K) where the vibrational relaxation rates are slowed allowing the effect of crystal disorder to be investigated.The results of this study have shown that vibrational relaxation in low temperature molecular crystals depends on a variety of effects. In a pure crystal the effect of molecular symmetry has been shown to be important in determining whether fundamental or combination vibrations are involved in vibrational relaxation. The studies of isotopically mixed crystals have shown that one and two site processes can be of importance and have shown that relaxation is greatly affected by a lower energy state of the same normal coordinate on the heavier isotope. In addition, the photon echo experiments look at isolated guest molecules in a chemically different host and have shown that relaxation of the guest can be slower than the host modes of nearly equal energy, thus indicating single site decay with the emission of bulk phonons.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD

Using temperature dependent fluorescence to evaluate singlet fission pathways in tetracene single crystals

The temperature-dependent fluorescence spectrum, decay rate, and spin quantum beats are examined in single tetracene crystals to gain insight into the mechanism of singlet fission. Over the temperature range of 250 K-500 K, the vibronic lineshape of the emission indicates that the singlet exciton becomes localized at 400 K. The fission process is insensitive to this localization and exhibits Arrhenius behavior with an activation energy of 550 ± 50 cm-1. The damping rate of the triplet pair spin quantum beats in the delayed fluorescence also exhibits an Arrhenius temperature dependence with an activation energy of 165 ± 70 cm-1. All the data for T &gt; 250 K are consistent with direct production of a spatially separated 1(T⋯T) state via a thermally activated process, analogous to spontaneous parametric downconversion of photons. For temperatures in the range of 20 K-250 K, the singlet exciton continues to undergo a rapid decay on the order of 200 ps, leaving a red-shifted emission that decays on the order of 100 ns. At very long times (≈1 µs), a delayed fluorescence component corresponding to the original S1 state can still be resolved, unlike in polycrystalline films. A kinetic analysis shows that the redshifted emission seen at lower temperatures cannot be an intermediate in the triplet production. When considered in the context of other results, our data suggest that the production of triplets in tetracene for temperatures below 250 K is a complex process that is sensitive to the presence of structural defects

Pressure- and Temperature-Dependent Inelastic Neutron Scattering Study of the Phase Transition and Phonon Lattice Dynamics in Para-Terphenyl

Inelastic neutron scattering has been performed on para-terphenyl at temperatures from 10 to 200 K and under pressures from the ambient pressure to 1.51 kbar. The temperature dependence of phonons, especially low-frequency librational bands, indicates strong anharmonic phonon dynamics. The pressure- and temperature-dependence of the phonon modes suggest a lack of phase transition in the region of 0-1.51 kbar and 10-30 K. Additionally, the overall lattice dynamics remains similar up to 200 K under the ambient pressure. The results suggest that the boundary between the ordered triclinic phase and the third solid phase, reported at lower temperatures and higher pressures, is out of the pressure and temperature range of this study