High-field Studies Research Articles

Recent high field ESR studies of low-dimensional quantum spin systems in Kobe

Recent advances in the high-field ESR studies of low-dimensional quantum spin systems in Kobe are presented. Among many systems studied, the results of S= 1 2 quasi-one-dimensional (1D) antiferromagnet BaCu 2(Si 1− x Ge x ) 2O 7 with the staggered field and S= 1 2 diamond chain system Cu 3OH 2(CO 3) 2 are discussed. As the interchain interaction is almost canceled in the BaCu 2(Si 1− x Ge x ) 2O 7 ( x=0.65) system, we have found that the field and temperature dependences of the linewidth and the resonance field obey the Oshikawa–Affleck (OA) ESR theory of 1D antiferromagnet. The effect of the interchain interaction from the x dependence measurement and the existence of the breather mode at low temperature are also discussed. On the other hand, the high-field ESR measurements at 1.8 K revealed the direct transition in Cu 3OH 2(CO 3) 2. The direct transition turned out to be anisotropic and it will be discussed in connection with the 1 3 magnetization plateau which is also anisotropic.

High-field ESR study of the dimerized-incommensurate phase transition in the spin-Peierls compound CuGeO 3

The dimerized-incommensurate phase transition in the spin-Peierls compound CuGeO 3 is probed using the tunable-frequency high-resolution electron spin resonance technique in magnetic fields up to 17 T . A field-induced development of the soliton-like incommensurate superstructure is clearly indicated as a pronounced increase of the magnon spin resonance linewidth Δ B, with a Δ B max at B c ∼13.8 T . The anomaly is explained in terms of the magnon–soliton scattering and suggests that the soliton-like phase exists close to the boundary of the dimerized-incommensurate phase transition. In addition, magnetic excitation spectra in 0.8% Si-doped CuGeO 3 are studied. Some features of the high-field high-resolution tunable-frequency ESR facility recently developed in Tallahassee are presented.

High-field cyclotron resonance studies of InMnAs-based ferromagnetic semiconductor heterostructures

We report results of experimental and theoretical studies of hole cyclotron resonance (CR) in ferromagnetic InMnAs/GaSb heterostructures. We observe two clear resonances that exhibit strong temperature dependence in position, line width, and intensity, especially near and below the Curie temperature. We attribute the two resonances to the fundamental CR transitions expected for delocalized holes in the valence band in the magnetic quantum limit. Our theoretical calculations, based on an 8×8 k · p model with s(p)–d exchange interaction taken into account, reproduced temperature-dependent CR peak shifts with decreasing temperature, in qualitative agreement with experiment. We propose that the narrowing is due to the suppression of localized spin fluctuations at low temperature.

Effect of annealing on magnetic and magnetotransport properties of Ga1−Mn As epilayers

High-field magnetic measurements performed with the use of magnetooptical Kerr effect (MOKE) in the polar configuration as well as high-field and low-field magnetotransport studies were carried out on Ga1-xMnxAs epilayers grown by low temperature molecular beam epitaxy, and subsequently annealed under various conditions. The structural investigations by means of high resolution XRD were also performed. We observe significant changes in magnetoresistivity curves, magnetization and strain introduced by the annealing.

High-field and high-pressure studies of UPdSi and UNiSi hydrides

Abstract High-field magnetization measurements have revealed an antiferromagnetic (AFM) ground state in UPdSiH1.0 and uncompensated AFM—in UNiSiH1.0. Subsequent magnetic phases in UNiSiH1.0 are ferromagnetic. Increase of TN in UPdSiH1.0 with pressure, and the estimate of the ordering temperatures in UNiSiH1.0 based on the bulk moduli indicate that the pressure effect on magnetism in the hydrides goes beyond the volume expansion.

Humor Modulates the Mesolimbic Reward Centers

Humor plays an essential role in many facets of human life including psychological, social, and somatic functioning. Recently, neuroimaging has been applied to this critical human attribute, shedding light on the affective, cognitive, and motor networks involved in humor processing. To date, however, researchers have failed to demonstrate the subcortical correlates of the most fundamental feature of humor—reward. In an effort to elucidate the neurobiological substrate that subserves the reward components of humor, we undertook a high-field (3 Tesla) event-related functional MRI study. Here we demonstrate that humor modulates activity in several cortical regions, and we present new evidence that humor engages a network of subcortical regions including the nucleus accumbens, a key component of the mesolimbic dopaminergic reward system. Further, the degree of humor intensity was positively correlated with BOLD signal intensity in these regions. Together, these findings offer new insight into the neural basis of salutary aspects of humor.

The preparation and readiness for voluntary movement: a high-field event-related fMRI study of the Bereitschafts-BOLD response

Activity within motor areas of the cortex begins to increase 1 to 2 s prior to voluntary self-initiated movement (termed the Bereitschaftspotential or readiness potential). There has been much speculation and debate over the precise source of this early premovement activity as it is important for understanding the roles of higher order motor areas in the preparation and readiness for voluntary movement. In this study, we use high-field (3-T) event-related fMRI with high temporal sampling (partial brain volumes every 250 ms) to specifically examine hemodynamic response time courses during the preparation, readiness, and execution of purely self-initiated voluntary movement. Five right-handed healthy volunteers performed a rapid sequential finger-to-thumb movement performed at self-determined times (12–15 trials). Functional images for each trial were temporally aligned and the averaged time series for each subject was iteratively correlated with a canonical hemodynamic response function progressively shifted in time. This analysis method identified areas of activation without constraining hemodynamic response timing. All subjects showed activation within frontal mesial areas, including supplementary motor area (SMA) and cingulate motor areas, as well as activation in left primary sensorimotor areas. The time courses of hemodynamic responses showed a great deal of variability in shape and timing between subjects; however, four subjects clearly showed earlier relative hemodynamic responses within SMA/cingulate motor areas compared with left primary motor areas. These results provide further evidence that the SMA and cingulate motor areas are major contributors to early stage premovement activity and play an important role in the preparation and readiness for voluntary movement.

Field-induced structural evolution in the spin-Peierls compoundCuGeO3:High-field ESR study

The dimerized-incommensurate phase transition in the spin-Peierls compound CuGeO$_3$ is probed using multifrequency high-resolution electron spin resonance (ESR) technique, in magnetic fields up to 17 T. A field-induced development of the soliton-like incommensurate superstructure is clearly indicated as a pronounced increase of the ESR linewidth $\Delta B$ (magnon excitations), with a $\Delta B_{max}$ at $B_{c}\sim$ 13.8 T. The anomaly is explained in terms of the magnon-soliton scattering, and suggests that the soliton-like phase exists close to the boundary of the dimerized-incommensurate phase transition. In addition, magnetic excitation spectra in 0.8% Si-doped CuGeO$_3$ are studied. Suppression of the $\Delta B$ anomaly observed in the doped samples suggests a collapse of the long-range-ordered soliton states upon doping, that is consistent with high-field neutron scattering experiments.

The distribution of sodium ions in aluminosilicate glasses: a high-field Na-23 MAS and 3Q MAS NMR study

The local configurations around sodium ions in silicate glasses and melts and their distributions have strong implications for the dynamic and static properties of melts and thus may play important roles in magmatic processes. The quantification of distributions among charge-balancing cations, including Na + in aluminosilicate glasses and melts, however, remains a difficult problem that is relevant to high-temperature geochemistry as well as glass science. Here, we explore the local environment around Na + in charge-balanced aluminosilicate glasses (the NaAlO 2-SiO 2 join) and its distribution using 23Na magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy at varying magnetic fields of 9.4, 14.1, and 18.8 T, as well as triple-quantum (3Q)MAS NMR spectroscopy at 9.4 T, to achieve better understanding of the extent of disorder around this cation. We quantify the extent of this disorder in terms of changes in Na-O distance ( d[Na-O]) distributions with composition and present a structural model favoring a somewhat ordered Na distribution, called a “perturbed” Na distribution model. The peak position in 23Na MAS spectra of aluminosilicate glasses moves toward lower frequencies with increasing Si/Al ratios, implying that the average d(Na-O) increases with increasing R. The peak width is significantly reduced at higher fields (14.1 and 18.8 T) because of the reduced effect of second-order quadrupolar interaction, and 23Na MAS NMR spectra thus provide relatively directly the Na chemical shift distribution and changes in atomic environment with composition. Chemical shift distributions obtained from 23Na 3Q MAS spectra are consistent with MAS NMR data, in which deshielding decreases with R. The average distances between Na and the three types of bridging oxygens (BOs) (Na-{Al-O-Al}, Na-{Si-O-Al}, and Na-{Si-O-Si}) were obtained from the correlation between d(Na-O) and isotropic chemical shift. The calculated d(Na-{Al-O-Al}) of 2.52 Å is shorter than the d(Na-{Si-O-Si}) of 2.81 Å, and d(Na-{Al-O-Al}) shows a much narrower distribution than the other types of BOs. 23Na chemical shifts in binary (Al-free) sodium silicate glasses are more deshielded and have ranges distinct from those of aluminosilicate glasses, implying that d(Na-NBO) (nonbridging oxygen) is shorter than d(Na-BO) and that d(Na-{Si-O-Si}) in binary silicates can be shorter than that in aluminosilicate glasses. The results given here demonstrate that high-field 23Na NMR is an effective probe of the Na + environment, providing not only average structural information but also chemically and topologically distinct chemical shift ranges (distributions) and their variation with composition and their effects on static and dynamic properties.

High field and low temperature X-ray study on phase segregation for Nd 0.5Sr 0.5MnO 3 powder and single crystal

It has been found that the coexistence of high-temperature ferromagnetic (FM) and low-temperature antiferromagnetic (AFM) phases was observed below the charge-order transition temperature T co for a Nd 0.5Sr 0.5MnO 3 powder sample. In order to investigate details of the phase segregation, the X-ray diffraction was carried out for the Nd 0.5Sr 0.5MnO 3 single crystal in magnetic fields up to 5 T . It was found that the phase segregation is strongly depends on the wave length of X-ray. These results suggest that the phase segregation of FM and AFM mainly occurs near the surface within a few μm in depth.

Megagauss sensors.

Magnetic fields change the way that electrons move through solids. The nature of these changes reveals information about the electronic structure of a material and, in auspicious circumstances, can be harnessed for applications. The silver chalcogenides, Ag2Se and Ag2Te, are non-magnetic materials, but their electrical resistance can be made very sensitive to magnetic field by adding small amounts--just 1 part in 10,000--of excess silver. Here we show that the resistance of Ag2Se displays a large, nearly linear increase with applied magnetic field without saturation to the highest fields available, 600,000 gauss, more than a million times the Earth's magnetic field. These characteristics of large (thousands of per cent) and near-linear response over a large field range make the silver chalcogenides attractive as magnetic-field sensors, especially in physically tiny megagauss (10(6) G) pulsed magnets where large fields have been produced but accurate calibration has proved elusive. High-field studies at low temperatures reveal both oscillations in the magnetoresistance and a universal scaling form that point to a quantum origin for this material's unprecedented behaviour.

Pulsed high field study of new partially disordered antiferromagnetic phase in Ca 3CoRhO 6

Ca 3CoRhO 6 realizes a partially disordered antiferromagnetic (PDA) state on a triangular lattice made up with ferromagnetic Ising-spin chains between 30 and 90 K. In order to investigate the spin dynamics in the 1/3 incoherent ferromagnetic chains in the PDA state through the response to external field sweeps, we carried out the pulsed high field magnetization measurements for a field-oriented sample at 40, 50 and 70 K. The response of the incoherent ferromagnetic Ising-spin chains in the PDA state to applied fields becomes slower with decreasing temperature to be below a rate of the order of 10 −3 s at 40 K. It can be considered to be consistent with the slow spin dynamics of ferromagnetic Ising-spin chains.

Gender Difference in Passive Listening to Stories: High-field fMRI Study Using Multiple Subject Analysis Based on Individual Activation Maps

Gender Difference in Passive Listening to Stories: High-field fMRI Study Using Multiple Subject Analysis Based on Individual Activation Maps

High-field transport studies of gan

A pulsed voltage input and a four-point measurement were used to determine the room temperature velocity–field characteristic of bulk gallium nitride test structures with an etched constriction. A peak electron velocity of approximately 2.5×10 7 cm/s was attained at a field of 180 kV/cm, which corresponds closely to theoretical predictions.

DNA structural perturbation induced by the CPI-derived DNA interstrand cross-linker: molecular mechanisms for the sequence specific recognition

The highly potent cytotoxic DNA-DNA cross-linker consists of two cyclopropa[c]pyrrolo[3, 4-3]indol-4(5H)-ones indoles [(+)-CPI-I] joined by a bisamido pyrrole (abbreviated to "Pyrrole"). The Pyrrole is a synthetic analog of Bizelesin, which is currently in phase II clinical trials due to its excellent in vivo antitumor activity. The Pyrrole has 10 times more potent cytotoxicity than Bizelesin and mostly form DNA-DNA interstrand cross-links through the N3 of adenines spaced 7 bp apart. The Pyrrole requires a centrally positioned GC base pair for high cross-linking reactivity (i.e., 5'-T(A/T)2G(A/T)2A*-3'), while Bizelesin prefers purely AT-rich sequences (e.g., 5'-T(A/T)4 or 5A*-3', where T represents the cross-strand adenine alkylation and A* represents an adenine alkylation) (Park et al., 1996). In this study, the high-field 1H-NMR and rMD studies are conducted on the 11-mer DNA duplex adduct of the Pyrrole where the 5'-TTAGTTA*-3' sequence is cross-linked by the drug. A severe structural perturbation is observed in the intervening sequences of cross-linking site, while a normal B-DNA structure is maintained in the region next to the drug-modified adenines. Based upon these observations, we propose that the interplay between the bisamido pyrrole unit of the drug and central G/C base pair (hydrogen-bonding interactions) is involved in the process of cross-linking reaction, and sequence specificity is the outcome of those interactions. This study suggests a mechanism for the sequence specific cross-linking reaction of the Pyrrole, and provides a further insight to develop new DNA sequence selective and distortive cross-linking agents.

Haldane-gap excitations in the low-Hcone-dimensional quantum antiferromagnetNi(C5D14N2)2N3(PF6)

Inelastic neutron scattering on deuterated single-crystal samples is used to study Haldane-gap excitations in the new $S=1$ one-dimensional quantum antiferromagnet $\mathrm{Ni}({\mathrm{C}}_{5}{\mathrm{D}}_{14}{\mathrm{N}}_{2}{)}_{2}{\mathrm{N}}_{3}({\mathrm{PF}}_{6}),$ that was recently recognized as an ideal model system for high-field studies. The Haldane gap energies ${\ensuremath{\Delta}}_{x}=0.42(3) \mathrm{meV},$ ${\ensuremath{\Delta}}_{y}=0.52(6) \mathrm{meV},$ and ${\ensuremath{\Delta}}_{z}=1.9(1) \mathrm{meV},$ for excitations polarized along the a, b, and c crystallographic axes, respectively, are measured. The dispersion relation is studied for momentum transfers both along and perpendicular to the chains' direction. The in-chain exchange constant $J=2.8 \mathrm{meV}$ is found to be much larger than interchain coupling, ${J}_{y}=1.8(4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3} \mathrm{meV}$ and ${J}_{x}=4(3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4} \mathrm{meV},$ along the b and a axes, respectively. The results are discussed in the context of future experiments in high magnetic fields.

High-frequency ESR measurements using pulsed magnetic fields in Kobe

Recent development of the high-field facility in Kobe University is presented. We installed a 3 kV, 100 kJ capacitor bank, this is four times larger in energy than the previous one, and we achieved 42 T recently using a Cu–Ag pulsed magnet. Recent high-field ESR studies of quantum spin systems such as high pressure phase (VO) 2P 2O 7 will be presented. The high-pressure phase of (VO) 2P 2O 7 is the S= 1 2 bond alternating antiferromagnet; we found an anomaly in the ESR mode of the high-pressure phase (VO) 2P 2O 7 single crystal at 20 T by observing the frequency dependence at 4.2 K. Finally, we refer to our new development of the high-field ESR system under high pressure. We developed a compact pressure cell with sapphire window which fits into our pulsed magnet. The ESR result using this pressure cell will be presented.

High-field ESR study of kagome-like substance Cu 3V 2O 7(OH) 2·2H 2O

As Cu 2+ ions in Cu 3V 2O 7(OH) 2·2H 2O form two-dimensional deformed kagome lattice structure in the ab plane, frustration is expected. To clarify the magnetic properties of this system, we performed X-band ESR measurement and high-field ESR measurements up to 16 T. The ESR signal disappeared below 5 K in X-band ESR measurement and the ESR absorption line showed clear shift below 4.2 K in our high-field ESR measurement at 80 GHz. Although no long-range order is expected around 4 K from the result of the specific heat measurement, our frequency-dependent measurement at 1.8 K showed the existence of the internal field in Cu 3V 2O 7(OH) 2·2H 2O. The results of the temperature dependences of the resonance field and the linewidth are also presented and discussed in connection with other kagome substances.

High-field ESR study on anomalous magnetization in CsFeCl 3

In CsFeCl 3, an anomalous magnetization has been observed under the magnetic field exceeding 33 T applied parallel to the crystal c-axis, which cannot be explained within the framework of the fictitious spin S=1. In order to clarify the nature of the anomalous magnetization, which is possibly due to the ground-state crossover with the excited state S=2, an ESR experiment was performed under magnetic fields up to 40 T by use of a pulse magnet. The submillimeter wave around 300 GHz was supplied by Gyrotron FU IV at Research Center of Far Infrared Region, Fukui University. In the operation frequency of 301 GHz absorption lines have been observed corresponding to the anomalous magnetization jump.

Quasi-two-dimensional spin distributions in II-VI magnetic semiconductor heterostructures: Clustering and dimensionality

Spin clustering in diluted magnetic semiconductors (DMS) arises from antiferromagnetic exchange between neighboring magnetic cations and is a strong function of reduced dimensionality. Epitaxially-grown single monolayers and abrupt interfaces of DMS are, however, never perfectly two-dimensional (2D) due to the unavoidable inter-monolayer mixing of atoms during growth. Thus the magnetization of DMS heterostructures, which is strongly modified by spin clustering, is intermediate between that of 2D and 3D spin distributions. We present an exact calculation of spin clustering applicable to arbitrary distributions of magnetic spins in the growth direction. The results reveal a surprising insensitivity of the magnetization to the form of the intermixing profile, and identify important limits on the maximum possible magnetization. High-field optical studies of heterostructures containing "quasi-2D" spin distributions are compared with calculation.