Vibrational properties of molecule-based multiferroics and quantum magnets across quantum phase transitions

Molecule-based materials offer unique opportunities to explore the interplay between charge, spin, and lattice across quantum phase transitions. With their flexible architectures and overall low energy scales, quantum phases can be induced at experimentally realizable conditions. In this dissertation, I present a spectroscopic study of three important families of multiferroics and quantum magnets with a variety of tuning parameters to unravel the mechanisms required to reach distinct non-equilibrium phases. The exploration of spin-lattice coupling and local lattice distortions across magnetic quantum phase transitions is the unifying theme of this work. As our first platform for investigation, we explore the coupling between ferroic orders in the metal-organic framework [(CH3)2NH2]M(HCOO)3 (M=Mn,Co,Ni) family. The formate bend links the ferroelectric and magnetic quantum phase transition in the Mn analog. Strikingly, B-site substitution drastically alters this mechanism. The Ni material behaves similarly to the Mn analog but at much higher energy scales, whereas the Co system utilizes formate stretches. B-site substitution is thus a powerful tool for developing structure-property relations within chemically analogous materials, providing control of electronic and magnetic properties as well as energy scales.Copper coordination polymers provide a second platform with which to extend our work. Magneto-infrared spectra of [Cu(pyz)2(2-HOpy)2](PF6)2 and [Cu(pyz)1.5(4-HOpy)2](ClO4)2, combined with prior work of other copper complexes, allow for the investigation of spin-lattice coupling across magnetic quantum phase transitions as a function of structural and magnetic dimensionality. Spin-phonon coupling strength versus magnetic dimensionality reveals that coupling is maximized in the ladder complex. These findings are applicable to other materials with field-induced transitions from the antiferromagnetic to fully saturated state.Multiferroic (NH4)2[FeCl5(H2O)] is our final test case, sporting a complex network of hydrogen and halogen bonds. The high-field polarization change is quenched at the quasicollinear- to collinear-sinusoidal magnetic reorientation, collapsing before magnetic saturation. Remarkably, nearly all low-frequency modes distort to facilitate the development of the magnetic quantum phase, entirely different than most other molecule-based magnets. Signatures of electron-phonon coupling emerge through magneto-infrared measurements.Together, these findings elucidate quantum phase transitions, spin-lattice coupling, and structure-property relations in molecular multiferroics and quantum magnets, motivating further exploration of non-equilibrium phases in these materials

Spectroscopic analysis of vibronic relaxation pathways in molecular spin qubit [Ho(W5O18)2]9−: sparse spectra are key

Vibrations play a prominent role in magnetic relaxation processes of molecular spin qubits as they couple to spin states, leading to the loss of quantum information. Direct experimental determination of vibronic coupling is crucial to understand and control the spin dynamics of these nano-objects, which represent the limit of miniaturization for quantum devices. Herein, we measure the magneto-infrared properties of the molecular spin qubit system Na9[Ho(W5O18)2]·35H2O. Our results place significant constraints on the pattern of crystal field levels and the vibrational excitations allowing us to unravel vibronic decoherence pathways in this system. We observe field-induced spectral changes near 63 and 370 cm-1 that are modeled in terms of odd-symmetry vibrations mixed with f-manifold crystal field excitations. The overall extent of vibronic coupling in Na9[Ho(W5O18)2]·35H2O is limited by a modest coupling constant (on the order of 0.25) and a transparency window in the phonon density of states that acts to keep the intramolecular vibrations and MJ levels apart. These findings advance the understanding of vibronic coupling in a molecular magnet with atomic clock transitions and suggest strategies for designing molecular spin qubits with improved coherence lifetimes

Spin–lattice and electron–phonon coupling in 3d/5d hybrid Sr3NiIrO6

Research at the University of Tennessee, Rutgers University, and University of Minnesota is supported by the National Science Foundation DMREF program (DMR-1629079, DMR-1629059, and DMR-1629260, respectively). The crystal growth was partially supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (No. 2016K1A4A4A01922028). We also appreciate funding from the U.S. Department of Energy, Basic Energy Sciences, contract DE-FG02-01ER45885 (Tennessee), “Science at 100 Tesla” (LANL), and “Topological phases of quantum matter and decoherence” (LANL). The NHMFL facility is supported by the U.S. National Science Foundation through Cooperative Grant DMR-1644779, the State of Florida, and the U.S. Department of Energy.While 3d-containing materials display strong electron correlations, narrow band widths, and robust magnetism, 5d systems are recognized for strong spin–orbit coupling, increased hybridization, and more diffuse orbitals. Combining these properties leads to novel behavior. Sr3NiIrO6, for example, displays complex magnetism and ultra-high coercive fields—up to an incredible 55 T. Here, we combine infrared and optical spectroscopies with high-field magnetization and first-principles calculations to explore the fundamental excitations of the lattice and related coupling processes including spin–lattice and electron–phonon mechanisms. Magneto-infrared spectroscopy reveals spin–lattice coupling of three phonons that modulate the Ir environment to reduce the energy required to modify the spin arrangement. While these modes primarily affect exchange within the chains, analysis also uncovers important inter-chain motion. This provides a mechanism by which inter-chain interactions can occur in the developing model for ultra-high coercivity. At the same time, analysis of the on-site Ir4+ excitations reveals vibronic coupling and extremely large crystal field parameters that lead to a t2g-derived low-spin state for Ir. These findings highlight the spin–charge–lattice entanglement in Sr3NiIrO6 and suggest that similar interactions may take place in other 3d/5d hybrids.Publisher PDFPeer reviewe

Search for gravitational-lensing signatures in the full third observing run of the LIGO-Virgo network

Gravitational lensing by massive objects along the line of sight to the source causes distortions of gravitational wave-signals; such distortions may reveal information about fundamental physics, cosmology and astrophysics. In this work, we have extended the search for lensing signatures to all binary black hole events from the third observing run of the LIGO--Virgo network. We search for repeated signals from strong lensing by 1) performing targeted searches for subthreshold signals, 2) calculating the degree of overlap amongst the intrinsic parameters and sky location of pairs of signals, 3) comparing the similarities of the spectrograms amongst pairs of signals, and 4) performing dual-signal Bayesian analysis that takes into account selection effects and astrophysical knowledge. We also search for distortions to the gravitational waveform caused by 1) frequency-independent phase shifts in strongly lensed images, and 2) frequency-dependent modulation of the amplitude and phase due to point masses. None of these searches yields significant evidence for lensing. Finally, we use the non-detection of gravitational-wave lensing to constrain the lensing rate based on the latest merger-rate estimates and the fraction of dark matter composed of compact objects

Search for eccentric black hole coalescences during the third observing run of LIGO and Virgo

Despite the growing number of confident binary black hole coalescences observed through gravitational waves so far, the astrophysical origin of these binaries remains uncertain. Orbital eccentricity is one of the clearest tracers of binary formation channels. Identifying binary eccentricity, however, remains challenging due to the limited availability of gravitational waveforms that include effects of eccentricity. Here, we present observational results for a waveform-independent search sensitive to eccentric black hole coalescences, covering the third observing run (O3) of the LIGO and Virgo detectors. We identified no new high-significance candidates beyond those that were already identified with searches focusing on quasi-circular binaries. We determine the sensitivity of our search to high-mass (total mass M>70 M⊙) binaries covering eccentricities up to 0.3 at 15 Hz orbital frequency, and use this to compare model predictions to search results. Assuming all detections are indeed quasi-circular, for our fiducial population model, we place an upper limit for the merger rate density of high-mass binaries with eccentricities 0<e≤0.3 at 0.33 Gpc−3 yr−1 at 90\% confidence level

Ultralight vector dark matter search using data from the KAGRA O3GK run

Among the various candidates for dark matter (DM), ultralight vector DM can be probed by laser interferometric gravitational wave detectors through the measurement of oscillating length changes in the arm cavities. In this context, KAGRA has a unique feature due to differing compositions of its mirrors, enhancing the signal of vector DM in the length change in the auxiliary channels. Here we present the result of a search for U(1)B−L gauge boson DM using the KAGRA data from auxiliary length channels during the first joint observation run together with GEO600. By applying our search pipeline, which takes into account the stochastic nature of ultralight DM, upper bounds on the coupling strength between the U(1)B−L gauge boson and ordinary matter are obtained for a range of DM masses. While our constraints are less stringent than those derived from previous experiments, this study demonstrates the applicability of our method to the lower-mass vector DM search, which is made difficult in this measurement by the short observation time compared to the auto-correlation time scale of DM

Photoluminescent and Magnetic Properties of Lanthanide Containing Apatites: Na\u3csub\u3ex\u3c/sub\u3eLn\u3csub\u3e10–x\u3c/sub\u3e(SiO\u3csub\u3e4\u3c/sub\u3e)\u3csub\u3e6\u3c/sub\u3eO\u3csub\u3e2–y\u3c/sub\u3eF\u3csub\u3ey\u3c/sub\u3e, Ca\u3csub\u3ex\u3c/sub\u3eLn\u3csub\u3e10–x\u3c/sub\u3e(SiO\u3csub\u3e4\u3c/sub\u3e)\u3csub\u3e6\u3c/sub\u3eO\u3csub\u3e2–y\u3c/sub\u3eF\u3csub\u3ey\u3c/sub\u3e (Ln = Eu, Gd, and Sm), Gd\u3csub\u3e9.34\u3c/sub\u3e(SiO\u3csub\u3e4\u3c/sub\u3e)\u3csub\u3e6\u3c/sub\u3eO\u3csub\u3e2\u3c/sub\u3e, and K\u3csub\u3e1.32\u3c/sub\u3ePr\u3csub\u3e8.68\u3c/sub\u3e(SiO\u3csub\u3e4\u3c/sub\u3e)\u3csub\u3e6\u3c/sub\u3eO\u3csub\u3e1.36\u3c/sub\u3eF\u3csub\u3e0.64\u3c/sub\u3e

Single crystals of NaEu9(SiO4)6O2, Na1.5Eu8.5(SiO4)6OF, Na1.64Gd8.36(SiO4)6O0.72F1.28, Gd9.34(SiO4)6O2, Ca2.6Eu7.4(SiO4)6O1.4F0.6, Ca4.02Sm5.98(SiO4)6F2, and K1.32Pr8.68(SiO4)6O1.36F0.64 and powders of NaEu9(SiO4)6O2, Na1.5Eu8.5(SiO4)6OF, Eu9.34(SiO4)6O2, and Gd9.34(SiO4)6O2 were synthesized via flux growth in selected alkali-fluoride melts. All of the compounds adopt the apatite structure with space group P63/m. Luminescence and magnetic data were collected on NaEu9(SiO4)6O2, Na1.5Eu8.5(SiO4)6OF, Eu9.34(SiO4)6O2, and Gd9.34(SiO4)6O2. Luminescent data indicate that changing the cations and anions that surround the lanthanide site does not change the luminescent properties, making apatites versatile structures for optical materials