Arneil P. Reyes

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.

Magnetic Field Independence of the Spin Gap in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}

We report, for magnetic fields of 0, 8.8, and 14.8thinspthinspT, measurements of the temperature dependent {sup 63}Cu NMR spin lattice relaxation rate for near optimally doped YBa{sub 2}Cu {sub 3}O{sub 7{minus}{delta}} , near and above T{sub c} . In sharp contrast with previous work we find no magnetic field dependence. We discuss experimental issues arising in measurements of this required precision and implications of the experiment regarding issues including the spin gap or pseudogap. {copyright} {ital 1998} {ital The American Physical Society}

63Cu NMR and hole depletion in the normal state of yttrium-rich Y1-xPrxBa2Cu3O7

The {sup 63}Cu Knight shift {ital K} and spin-lattice relaxation rate 1/{ital T}{sub 1} have been measured in the superconducting cuprate system Y{sub 1{minus}{ital x}}Pr{sub {ital x}}Ba{sub 2}Cu{sub 3}O{sub 7}, 0.05{le}{ital x}{le}0.20. With Pr doping {ital K} decreases and develops a temperature dependence at both plane and chain sites. This resembles the behavior of the Cu and Y Knight shifts as well as the bulk susceptibility in oxygen-deficient YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital y}}. The orbital contribution to {ital K} and the anisotropy of the Cu hyperfine coupling remain essentially unchanged over the entire Pr concentration range. No appreciable direct effect of Pr magnetism on the conduction-band susceptibility was found. Instead, analysis of the bulk susceptibility and NMR data indicate that pair breaking and hole depletion both take part in the suppression of the superconducting transition temperature {ital T}{sub {ital c}}. The temperature dependence of 1/{ital T}{sub 1} for magnetic field parallel to the {ital c} axis is also similar to that for the oxygen-deficient compound. This agreement leads to a consistent picture of the role of antiferromagnetic fluctuations in these materials. An analysis of the data in the framework of the phenomenological theory of Millis, Monien, and Pines is given.

sup 139 La NMR study of phase separation in single-crystal La sub 2 CuO sub 4+. delta

We report a study of single-crystal La{sub 2}CuO{sub 4+{delta}}, a 38-K superconductor produced by high-pressure oxygenation. We employ {sup 139}La NMR to probe the behavior of copper spins in the CuO{sub 2} planes. At low temperatures we observe two signals, one originating from regions of the crystal rich in oxygen and a second having no excess oxygen ({delta}{approximately}0). Upon warming through 265{plus minus}5 K, the volume fraction of the crystal poor in oxygen goes to zero. The magnetic shift of the peak intensity of the line originating in the oxygen-rich portion of the crystal does not change in the vicinity of 265 K. These observations provide direct microscopic evidence for phase separation in the crystal.

Critical point and the nature of the pseudogap of single-layered copper-oxide Bi2Sr2-xLaxCuO6+δ superconductors

We apply strong magnetic fields of H=28.5 to 43 T to suppress superconductivity (SC) in the cuprates Bi2Sr2-xLaxCuO6+delta (x=0.65, 0.40, 0.25, 0.15, and 0), and investigate the low temperature (T) normal state by 63Cu nuclear spin-lattice relaxation rate (1/T1) measurements. We find that the pseudogap (PG) phase persists deep inside the overdoped region but terminates at x approximately 0.05, which corresponds to the hole doping concentration of approximately 0.21. Beyond this critical point, the normal state is a Fermi liquid that persists as the ground state when superconductivity is removed by the magnetic field. A comparison of the superconducting state with the H-induced normal state in the x=0.40 (Tc=32 K) sample indicates that there remains substantial part of the Fermi surface even in the fully developed PG state, which suggests that the PG and SC are coexisting matters.

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.

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.