Philip L. Kuhns

Mechanism of the order–disorder phase transition, and glassy behavior in the metal-organic framework [(CH3)2NH2]Zn(HCOO)3

Transitions associated with orientational order–disorder phenomena are found in a wide range of materials and may have a significant impact on their properties. In this work, specific heat and 1H NMR measurements have been used to study the phase transition in the metal-organic framework (MOF) compound [(CH3)2NH2]Zn(HCOO)3. This compound, which possesses a perovskite-type architecture, undergoes a remarkable order–disorder phase transition at 156 K. The (DMA+) cationic moieties that are bound by hydrogen bonds to the oxygens of the formate groups (N─H⋯O ∼ 2.9 Å) are essentially trapped inside the basic perovskite cage architecture. Above 156 K, it is the orientations of these moieties that are responsible for the disorder, as each can take up three different orientations with equal probability. Below 156 K, the DMA+ is ordered within one of these sites, although the moiety still retains a considerable state of motion. Below 40 K, the rotational motions of the methyl groups start to freeze. As the temperature is increased from 4 K in the NMR measurements, different relaxation pathways can be observed in the temperature range approximately 65–150 K, as a result of a “memory effect.” This dynamic behavior is characteristic of a glass in which multiple states possess similar energies, found here for a MOF. This conclusion is strongly supported by the specific heat data.

Emergence of charge order from the vortex state of a high-temperature superconductor

Evidence is mounting that charge order competes with superconductivity in high Tc cuprates. Whether this has any relationship to the pairing mechanism is unknown as neither the universality of the competition nor its microscopic nature has been established. Here, we show using nuclear magnetic resonance that charge order in YBa2Cu3Oy has maximum strength inside the superconducting dome, similar to compounds of the La2-x(Sr,Ba)xCuO4 family. In YBa2Cu3Oy, this occurs at doping levels of p=0.11-0.12. We further show that the overlap of halos of incipient charge order around vortex cores, similar to those visualised in Bi2Sr2CaCu2O8+δ, can explain the threshold magnetic field at which long-range charge order emerges. These results reveal universal features of a competition in which charge order and superconductivity appear as joint instabilities of the same normal state, whose relative balance can be field-tuned in the vortex state.

Inductive coupling and tuning in NMR probes; Applications

Abstract Inductive coupling and tuning of NMR probes is ideally suited for certain applications because no direct electrical connections to the resonant circuit are required. The relatively transparent case of series-tuned link coupling is analyzed. It is shown that tuning and coupling adjustments are orthogonal and that most of the RF field comes from the main circuit, not the link. Applications of inductive tuning and coupling are suggested; including low-temperature NMR where Dewar space is restricted. An experimental comparison of conventional, link-coupled, and link-coupled and tuned circuits at 85 MHz is reported. Some novel VHF self-contained resonators are discussed and their performance is experimentally compared to conventional coils.

NMR hole‐burning: A study of slow molecular rotations in glassy glycerol

Chemical shift anisotropy produces inhomogeneous broadening of NMR lines in glassy and polycrystalline solids. Holes can be burned into such lines, tagging molecules which are at certain orientations. Subsequent molecular reorientations result in a spectral diffusion which is not related to spin‐spin interactions. By measuring the broadening and recovery of the hole as a function of time, detailed knowledge of the reorientations is obtained (e.g., mean jump rate and angular jump size). The reorientation rate in supercooled glycerol was followed with this technique from ∼101 to 10−2 s−1. This rate agrees well with higher frequency dielectric results and the location of the glass transition temperature. It was determined that the mean angular jump size is greater than 45°. The hole recovery curves were not exponential, but fit the Williams–Watts function exp−(t/τ)β with β = 0.5. The spin‐spin contribution to spectral diffusion is very slow, making this technique widely applicable for the study of slow motio...

Evidence for skyrmion crystallization from NMR relaxation experiments.

A resistively detected NMR technique was used to probe the two-dimensional electron gas in a GaAs/AlGaAs quantum well. The spin-lattice relaxation rate (1/T(1)) was extracted at near complete filling of the first Landau level by electrons. The nuclear spin of (75)As is found to relax much more efficiently with T --> 0 and when a well developed quantum Hall state with R(xx) approximately 0 occurs. The data show a remarkable correlation between the nuclear spin relaxation and localization. This suggests that the magnetic ground state near complete filling of the first Landau level may contain a lattice of topological spin texture, i.e., a Skyrmion crystal.

Synthesis, structure, and spectroscopic and magnetic characterization of [Mn12O12(O2CCH2But)16(MeOH)4]·MeOH, a Mn12 single-molecule magnet with true axial symmetry.

The synthesis and properties are reported of a rare example of a Mn(12) single-molecule magnet (SMM) in truly axial symmetry (tetragonal, I4). [Mn(12)O(12)(O(2)CCH(2)Bu(t))(16)(MeOH)(4)]·MeOH (3·MeOH) was synthesized by carboxylate substitution on [Mn(12)O(12)(O(2)CMe)(16)(H(2)O)(4)]·2MeCO(2)H·4H(2)O (1). The complex was found to possess an S = 10 ground state, as is typical for the Mn(12) family, and displayed both frequency-dependent out-of-phase AC susceptibility signals and hysteresis loops in single-crystal magnetization vs DC field sweeps. The loops also exhibited quantum tunneling of magnetization steps at periodic field values. Single-crystal, high-frequency electron paramagnetic resonance spectra on 3·MeOH using frequencies up to 360 GHz revealed perceptibly sharper signals than for 1. Moreover, careful studies as a function of the magnetic field orientation did not reveal any satellite peaks, as observed for 1, suggesting that the crystals of 3 are homogeneous and do not contain multiple Mn(12) environments. In the single-crystal (55)Mn NMR spectrum in zero applied field, three well-resolved peaks were observed, which yielded hyperfine and quadrupole splitting at three distinct sites. However, observation of a slight asymmetry in the Mn(4+) peak was detectable, suggesting a possible decrease in the local symmetry of the Mn(4+) site. Spin-lattice (T(1)) relaxation studies were performed on single crystals of 3·MeOH down to 400 mK in an effort to approach the quantum tunneling regime, and fitting of the data using multiple functions was employed. The present work and other recent studies continue to emphasize that the new generation of truly high-symmetry Mn(12) complexes are better models for thorough investigation of the physical properties of SMMs than their predecessors such as 1.

Superconducting fluctuations and the pseudogap in the slightly overdoped high- T(c) superconductor TlSr2CaCu2O6.8: high magnetic field NMR studies

From measurements of the 63Cu Knight shift ( K) and the nuclear spin-lattice relaxation rate ( 1/T1) under magnetic fields from zero up to 28 T in the slightly overdoped high- T(c) superconductor TlSr2CaCu2O6.8 ( T(c) = 68 K), we find that the pseudogap behavior, i.e., the reductions of 1/T1T and K above T(c) from the values expected from the normal state at high T, is strongly field dependent and follows a scaling relation. We show that this scaling is consistent with the effects of the Cooper pair density fluctuations. The present finding contrasts sharply with the pseudogap property reported previously in the underdoped regime where no field effect was seen up to 23.2 T. The implications are discussed.

NMR study of molecular motions in cyclohexanol, a glass‐forming rotor crystal

The motions in the rotor and glassy crystal phases of solid cyclohexanol are studied with proton NMR from the melt down to 5 K. Particular attention is paid to the variation of the linewidth with temperature and to the temperature and frequency dependences of T1. We find there are two distinct motions that cause minima in T1 as a function of temperature. These two motions are also responsible for the two dielectric loss peaks observed previously. From the proton line narrowing and C13 high‐resolution solid state spectra the high temperature α motion is identified as overall molecular rotation. The low temperature β motion is identified as a uniaxial internal rotation of the cyclohexyl ring about the CO bond, with the COH group remaining stationary. This explains both the strong spin relaxation and the weak dielectric relaxation associated with the β motion. Both motions have distributions of correlation times, as seen from the shallow T1 minima and the weak temperature and frequency dependences of T1. Fro...

Carrier-Concentration Dependence of the Pseudogap Ground State of Superconducting Bi(2)Sr2-xLaxCuO6+delta Revealed by Cu-63,Cu-65-Nuclear Magnetic Resonance in Very High Magnetic Fields

We report the results of the Knight shift by ⁶³,⁶⁵Cu-NMR measurements on single-layered copper-oxide Bi₂Sr(₂-x)La(x)CuO(₆+δ) conducted under very high magnetic fields up to 44 T. The magnetic field suppresses superconductivity completely, and the pseudogap ground state is revealed. The ⁶³Cu-NMR Knight shift shows that there remains a finite density of states at the Fermi level in the zero-temperature limit, which indicates that the pseudogap ground state is a metallic state with a finite volume of Fermi surface. The residual density of states in the pseudogap ground state decreases with decreasing doping (increasing x) but remains quite large even at the vicinity of the magnetically ordered phase of x ≥ 0.8, which suggests that the density of states plunges to zero upon approaching the Mott insulating phase.

Glassy spin freezing and NMR wipeout effect in the high-TcsuperconductorLa1.90Sr0.10CuO4:Critical discussion of the role of stripes

We report on 139La and 63Ccu NMR/NQR measurements in the high-Tc superconductor La1.90Sr0.10CuO4 with Tc=26.5 K. Spin fluctuations probed by 139La spin-lattice relaxation (T1), continuously slow down on cooling through Tc. We argue that spin-freezing and superconductivity are bulk effects in this sample. Thus, both phenomena have to coexist microscopically. The distribution of 139La T1 values at low temperature reveals a wide spread of spin fluctuation frequencies in CuO2 planes. A simple estimate shows that Cu nuclei at sites where electronic fluctuations are the slowest are not observable because of too short relaxation times (wipeout effect). This means that the Cu NQR wipeout, observed in this sample, can be explained primarily by slow magnetic, rather than charge, fluctuations. This result does not rule out the connection between wipeout effect and charge stripe order [Hunt et al., Phys. Rev. Lett. 82, 4300 (1999)], but it indicates that the relationship between both phenomena is not straightforward. We argue that the wipeout fraction cannot define a proper order parameter for a stripe phase, and cannot be used alone as a criterion for its existence.