47 research outputs found
Thermal history and aging effects on the physical properties of J2-polymer
Thesis (B.S.) in Chemical Engineering -- University of Illinois at Urbana-Champaign, 1987.Bibliography: leaf [21].Microfiche of typescript. [Urbana, Ill.]: Photographic Services, University of Illinois, U of I Library, [1987]. 2 microfiches (55 frames): negative
High Pressure Vibrational Properties of WS2 Nanotubes
We bring together synchrotron-based infrared and Raman spectroscopies, diamond anvil cell techniques, and an analysis of frequency shifts and lattice dynamics to unveil the vibrational properties of multiwall WS2 nanotubes under compression. While most of the vibrational modes display similar hardening trends, the Raman-active A1g breathing mode is almost twice as responsive, suggesting that the nanotube breakdown pathway under strain proceeds through this displacement. At the same time, the previously unexplored high pressure infrared response provides unexpected insight into the electronic properties of the multiwall WS2 tubes. The development of the localized absorption is fit to a percolation model, indicating that the nanotubes display a modest macroscopic conductivity due to hopping from tube to tube
High-pressure spectroscopic investigation of multiferroic Ni3TeO6
We combined diamond anvil cell techniques, infrared and Raman spectroscopies, and lattice dynamics calculations to explore the high pressure properties of multiferroic Ni3TeO6. Using a frequency trend analysis, we trace a subtle decrease in compressibility near 4 GPa to a minimum in the O-Ni2-O bond angle. This unique behavior emanates from the proximity of the Ni2 center in the Ni3-Ni2-Ni1-Te chain to a flexible pocket that is intrinsic to the crystal structure. At the same time, predicted trends in the superexchange pathways are consistent with greater antiferromagnetic character under compression, in line with both phase stability calculations and direct susceptibility measurements. These findings highlight opportunities for local structure control of corundumlike materials
Origin and stability of the charge density wave in ScVSn
Kagome metals are widely recognized as versatile platforms for exploring
novel topological properties, unconventional electronic correlations, magnetic
frustration, and superconductivity. In the VSn family of materials
( = Sc, Y, Lu), ScVSn hosts an unusual charge density wave ground
state as well as structural similarities with the VSb system ( =
K, Cs, Rb). In this work, we combine Raman scattering spectroscopy with
first-principles lattice dynamics calculations to reveal the charge density
wave state in ScVSn. In the low temperature phase, we find a five-fold
splitting of the V-containing totally symmetric mode near 240 cm
suggesting that the density wave acts to mix modes of 6/ and
symmetry - an effect that we quantify by projecting phonons of
the high symmetry state onto those of the lower symmetry structure. We also
test the stability of the density wave state under compression and find that
both physical and chemical pressure act to quench the effect. We discuss these
findings in terms of symmetry and the structure-property trends that can be
unraveled in this system
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
Pressure-Induced Magnetic Crossover Driven by Hydrogen Bonding in CuF2(H2O)2(3-chloropyridine)
Hydrogen bonding plays a foundational role in the life, earth, and chemical sciences, with its richness and strength depending on the situation. In molecular materials, these interactions determine assembly mechanisms, control superconductivity, and even permit magnetic exchange. In spite of its long-standing importance, exquisite control of hydrogen bonding in molecule-based magnets has only been realized in limited form and remains as one of the major challenges. Here, we report the discovery that pressure can tune the dimensionality of hydrogen bonding networks in CuF2(H2O)2(3-chloropyridine) to induce magnetic switching. Specifically, we reveal how the development of exchange pathways under compression combined with an enhanced ab-plane hydrogen bonding network yields a three dimensional superexchange web between copper centers that triggers a reversible magnetic crossover. Similar pressure- and strain-driven crossover mechanisms involving coordinated motion of hydrogen bond networks may play out in other quantum magnets
Vibronic coupling and band gap trends in CuGeO3 nanorods
We measured the optical response of CuGeO3 nanorods in order to reveal size effects on the electronic properties. The vibronically activated d-to-d color band excitations are activated by the 131 and 478 cm−1 phonons, with the relative contribution of the lower frequency O-Cu-O bending mode increasing with decreasing size until it dominates the process. We also uncover trends in the direct band gap, with the charge transfer edge hardening with decreasing size. These findings advance the understanding of size effects in low-dimensional copper oxides
Electron-beam induced emergence of mesoscopic ordering in layered MnPS
Ordered mesoscale structures in 2D materials induced by small misorientations
have opened pathways for a wide variety of novel electronic, ferroelectric, and
quantum phenomena. Until now, the only mechanism to induce this periodic
ordering was via mechanical rotations between the layers, with the periodicity
of the resulting moir\'e pattern being directly related to twist angle. Here we
report a fundamentally new mechanism for emergence of mesoscopic periodic
patterns in multilayer sulfur-containing metal phosphorous trichalcogenide,
MnPS, induced by the electron beam. The formation under the beam of
periodic hexagonal patterns with several characteristic length scales,
nucleation and transitions between the phases, and local dynamics are
demonstrated. The associated mechanisms are attributed to the relative
contraction of the layers caused by beam-induced sulphur vacancy formation with
subsequent ordering and lattice parameter change. As a result, the plasmonic
response of the system is locally altered, suggesting an element of control
over plasmon resonances by electron beam patterning. We pose that harnessing
this phenomenon provides both insight into fundamental physics of quantum
materials and opens a pathway towards device applications by enabling
controlled periodic potentials on the atomic scale.Comment: Electron microscopy data and analysis codes are freely available
here: https://github.com/kevinroccapriore/MnPS
Site-specific spectroscopic measurement of spin and charge in (LuFeO3)m/(LuFe2O4)1 multiferroic superlattices
Interface materials offer a means to achieve electrical control of ferrimagnetism at room temperature as was recently demonstrated in (LuFeO3)m/(LuFe2O4)1 superlattices. A challenge to understanding the inner workings of these complex magnetoelectric multiferroics is the multitude of distinct Fe centres and their associated environments. This is because macroscopic techniques characterize average responses rather than the role of individual iron centres. Here, we combine optical absorption, magnetic circular dichroism and first-principles calculations to uncover the origin of high-temperature magnetism in these superlattices and the charge-ordering pattern in the m = 3 member. In a significant conceptual advance, interface spectra establish how Lu-layer distortion selectively enhances the Fe2+ → Fe3+ charge-transfer contribution in the spin-up channel, strengthens the exchange interactions and increases the Curie temperature. Comparison of predicted and measured spectra also identifies a non-polar charge ordering arrangement in the LuFe2O4 layer. This site-specific spectroscopic approach opens the door to understanding engineered materials with multiple metal centres and strong entanglement.Fil: Fan, Shiyu. University of Tennessee; Estados UnidosFil: Das, Hena. Cornell University; Estados UnidosFil: Rébola, Alejandro Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; ArgentinaFil: Smith, Kevin. University of Tennessee; Estados UnidosFil: Mundy, Julia. Harvard University; Estados Unidos. Cornell University; Estados UnidosFil: Brooks, Charles. Cornell University; Estados UnidosFil: Holtz, Megan E.. Cornell University; Estados UnidosFil: Muller, David A.. Cornell University; Estados UnidosFil: Fennie, Craig J.. Cornell University; Estados UnidosFil: Ramesh, Ramamoorthy. Lawrence Berkeley National Laboratory; Estados Unidos. University of California at Berkeley; Estados UnidosFil: Schlom, Darrell G.. Cornell University; Estados UnidosFil: McGill, Stephen. National High Magnetic Field Laboratory; Estados UnidosFil: Musfeldt, Janice L.. University of Tennessee; Estados Unido
Linear and Nonlinear Optical constants of BiFeO_3
Using spectroscopic ellipsometry, the refractive index and absorption versus
wavelength of the ferroelectric antiferromagnet Bismuth Ferrite, BiFeO_3 is
reported. The material has a direct band-gap at 442 nm wavelength (2.81 eV).
Using optical second harmonic generation, the nonlinear optical coefficients
were determined to be d_15/d_22 = 0.20 +/- 0.01, d_31/d_22 = 0.35 +/- 0.02,
d_33/d_22 = -11.4 +/- 0.20 and |d_22| = 298.4 +/- 6.1 pm/V at a fundamental
wavelength of 800 nm.Comment: 4 pages, 3 figure