High-field Studies Research Articles

High angle annular dark field and electron energy loss spectroscopy study on the substitution and distribution of cobalt in ZnO by multilayer growth

Co-doped ZnO films were synthesized by ion beam sputtering using multilayer (ZnO/Co) growth. Both the distribution and the chemical states of Co in ZnO can be well controlled by varying the ratio of the nominal layer thickness of ZnO to Co. Transmission electron microscopy indicated that all of the as-deposited Zn1−x(Co)xO films were polycrystalline with a (0002) preferred orientation. The local microstructures and chemical states were identified by Z-contrast imaging and electron energy loss spectroscopy. In ZnO (1.5 nm)/Co (0.1 nm), homogeneous Co-doped ZnO was observed to have been formed through interdiffusion. However, decreasing or increasing the thickness of ZnO leads to the formation of Co clusters in the ZnO matrix or Zn1−x(Co)xO multilayers, respectively. For ZnO thickness≧1.5 nm, Co is substituted for Zn, and its valence state is 2+. All Co-doped ZnO films show room-temperature ferromagnetic behavior, which appears to depend strongly on the Co distribution.

High-Field EPR and ESEEM Investigation of the Nitrogen Quadrupole Interaction of Nitroxide Spin Labels in Disordered Solids: Toward Differentiation between Polarity and Proticity Matrix Effects on Protein Function

The combination of high-field electron paramagnetic resonance (EPR) with site-directed spin labeling (SDSL) techniques employing nitroxide radicals has turned out to be particularly powerful in revealing subtle changes of the polarity and proticity profiles in proteins enbedded in membranes. This information can be obtained by orientation-selective high-field EPR resolving principal components of the nitroxide Zeeman (g) and hyperfine ( A) tensors of the spin labels attached to specific molecular sites. In contrast to the g- and A-tensors, the (14)N ( I = 1) quadrupole interaction tensor of the nitroxide spin label has not been exploited in EPR for probing effects of the microenvironment of functional protein sites. In this work it is shown that the W-band (95 GHz) high-field electron spin echo envelope modulation (ESEEM) method is well suited for determining with high accuracy the (14)N quadrupole tensor principal components of a nitroxide spin label in disordered frozen solution. By W-band ESEEM the quadrupole components of a five-ring pyrroline-type nitroxide radical in glassy ortho-terphenyl and glycerol solutions have been determined. This radical is the headgroup of the MTS spin label widely used in SDSL protein studies. By DFT calulations and W-band ESEEM experiments it is demonstrated that the Q(yy) value is especially sensitive to the proticity and polarity of the nitroxide environment in H-bonding and nonbonding situations. The quadrupole tensor is shown to be rather insensitive to structural variations of the nitroxide label itself. When using Q(yy) as a testing probe of the environment, its ruggedness toward temperature changes represents an important advantage over the g xx and A(zz) parameters which are usually employed for probing matrix effects on the spin labeled molecular site. Thus, beyond measurenments of g xx and A(zz) of spin labeled protein sites in disordered solids, W-band high-field ESEEM studies of (14)N quadrupole interactions open a new avenue to reliably probe subtle environmental effects on the electronic structure. This is a significant step forward on the way to differentiate between effects from matrix polarity and hydrogen-bond formation.

High-field study of the magnetization in Ti doped ErCo2 compound

The magnetization of powdered MgCu2 structure-type intermetallic ErCo1.95Ti0.05 was studied in the temperature range of 4.2–123K in magnetic fields up to 23T. A field-cooled magnetization jump at certain temperatures (TC) corresponds to a first-order magnetic phase transition from a paramagnetic to a ferrimagnetic state indicating cobalt moment formation. We observe that TC approach asymptotically T=50K with the cooling field Hc. The magnetization above TC stays constant (temperature independent), but does not saturate even in the field of 23T. The moment per unit formula was found to be approximately 7.25μB for this field. This behavior may be attributed to domain structures with the enhanced magnetocrystalline anisotropy field with titanium impurities. Magnetization versus magnetic field curves has also been recorded for both zero-field-cooled and field-cooled cases at various temperatures below and above TC. These curves below TC show typical ferrimagnetic phase; in the intermediate temperature range of 34–50K metamagnetism was observed and the sample becomes pure paramagnetic above 50K. In addition, the resistivity measurement was performed in the temperature range of 4.2–273K. A jumplike drop in the resistivity was observed at TC. In comparison with that of ErCo2, this transition temperature shifts to higher temperature as much as 2K, consistent with the slight increase of the lattice parameter. This drop in resistivity becomes much deeper presumably due to the internal magnetoresistance within the large domains. Analysis of the resistivity data in terms of spin fluctuations including s-d electron scattering and electron-phonon interaction reveals that T2 behavior becomes dominant in the temperature range of 34–50K where the magnetization data exhibit metamagnetism. Therefore, we assert based on both magnetization and resistivity results that the spin-fluctuation effects are driving force for the cobalt moment formations.

High-field MRS study of GABA, glutamate and glutamine in social anxiety disorder: Response to treatment with levetiracetam

Objective Abnormalities in brain gamma-aminobutyric acid (GABA) and glutamate may be relevant to the underlying pathophysiology of anxiety disorders including social anxiety disorder (SAD). Methods We used proton magnetic resonance spectroscopy (pMRS) to examine whole brain and regional GABA, glutamate and glutamine in patients ( N = 10) with SAD at baseline compared to a matched group of healthy controls (HC), and changes following 8 weeks of pharmacotherapy with levetiracetam. Results For SAD subjects, there were significantly higher whole brain levels of glutamate and glutamine, though no significant differences in GABA. In the thalamus, glutamine was higher and GABA lower for SAD subjects. There was a significant reduction in thalamic glutamine with levetiracetam treatment. Conclusion Our findings provide preliminary support for impaired GABAergic and overactive glutamatergic function in social anxiety disorder and the potential relevance of changes in these systems for the anxiolytic response to levetiracetam.

Tuning Electronic Ground States by Using Chemical Pressure on Quasi-Two Dimensional β′′-(BEDT-TTF)4[(H3O)M(C2O4)3]·Y

We report high-field magnetotransport studies on quasi-two dimensional β″-(BEDT-TTF)4[(H3O)M(C2O4)3]·Y where Y is a solvent in the anionic layer. By changing the size of the solvent the low temperatures electronic behaviour varies from superconducting (for larger solvents, Y=C6H5NO2 and C6H5CN) to metallic (for smaller solvents, Y=C5H5N and CH2Cl2). These changes in the ground state are connected with modifications of the Fermi surface, which varies from having one or two pockets for the superconducting charge-transfer salts to at least four pockets in the case of metallic ones. When superconducting, the materials have very large in-plane critical fields (up to 32 T) and enhanced effective masses compared with the metallic compounds. The role of the charge-order fluctuations in stabilizing the superconducting ground state and the effects of intrinsic local disorder is discussed.

High-field ESR study of one-dimensional [formula omitted] Heisenberg antiferromagnet with next-nearest exchange interaction

High-field ESR measurements of Cu[2-(2-aminomethyl)pyridine]Br 2 (Cu(ampy)Br 2) powder sample, which is a model substance of the S = 1 2 one-dimensional Heisenberg antiferromagnet (1DHAF) with a frustration, have been performed in the frequency region from 40 to 480 GHz using the pulsed magnetic field up to 16 T. The obtained g-values are g / / = 2.06 and g ⊥=2.06, which suggest that the orbital ground state of Cu 2+ in Cu(ampy)Br 2 is the x 2– y 2 orbital state. Distinct frequency dependent g-shift is observed below 4.2 K from the temperature dependence measurements. The possibility of the phase transition below 4.2 K is discussed from the frequency dependence results at 1.7 and 4.2 K.

Quantum critical behaviour of the plateau-insulator transition in the quantum Hall regime

High-field magnetotransport experiments provide an excellent tool to investigate the plateau-insulator phase transition in the integral quantum Hall effect. Here we review recent low-temperature high-field magnetotransport studies carried out on several InGaAs/InP heterostructures and an InGaAs/GaAs quantum well. We find that the longitudinal resistivity ρxx near the critical filling factor νc ≈ 0.5 follows the universal scaling law ρxx(ν, T) ∝ exp(-Δν/(T/T0)κ), where Δν = ν-νc. The critical exponent κ equals 0.56 ± 0.02, which indicates that the plateau-insulator transition falls in a non-Fermi liquid universality class.

High-field studies of the slow thermal death of interlayer coherence in quasi-two-dimensional metals

The interlayer magnetoresistance ρzz of the organic metal κ-(BEDTTTF)2Cu(NCS)2 has been studied in fields B of up to 45 T and at temperatures T from 0.5 K to 50 K. The peak in ρzz seen in exactly in-plane fields, a definitive signature of interlayer coherence, remains to Ts exceeding the Anderson criterion for incoherent transport by a factor ∼ 25. Angle-dependent magnetoresistance oscillations (AMROs) due to Fermi-surface orbits are suppressed by rising T, with a T2dependence suggesting electron-electron scattering.

Infrared study in high magnetic fields of the crystal-field excitations in PrMnO 3

Pr 3+ ion crystal field (CF) excitations in PrMnO 3 single crystals have been studied by infrared transmission, in the 1800–8000 cm −1 range, as a function of temperature and applied magnetic field up to 13 T. No noticeable frequency shifts which might occur below T N∼100 K, as a result of the antiferromagnetic transition, are observed in the Pr 3+ CF levels. A set of CF parameters that fit the experimental levels as well as the low temperature Pr 3+ magnetic moment in PrMnO 3 has been determined.

Low- and high-field transport studies for semiconducting carbon nanotubes

We calculate low- and high-field electron transport characteristics for semiconducting, single-wall, zigzag carbon nanotubes with diameters ranging from small to relatively large. The Boltzmann transport equation is solved directly using an iterative technique and indirectly by the ensemble Monte Carlo method. The basis for the transport calculation is provided by electronic structure calculations within the framework of a simple tight-binding model. Scattering mechanisms considered are due to the electron–phonon interactions involving longitudinal acoustic, longitudinal optic, and radial breathing-mode phonons. Results show significant effects of tube diameter on both low- and high-field transport characteristics.

Dynamic shim updating on the human brain

Dynamic alteration of shim settings during a multi-slice imaging experiment can improve static magnetic-field homogeneity over extended volumes. In this report, a pre-emphasized dynamic shim updating (DSU) system capable of rapidly updating all non-degenerate zeroth through second-order shims is presented and applied to high-field multi-slice imaging studies on the human brain. DSU is utilized in both non-oblique and oblique slicing geometries while updating in-plane and through-slice shims. Image-based magnetic-field maps are used to quantify homogeneity improvements and comparisons are made on a slice-specific basis between static global shimming and increasing orders of shim inclusion utilized DSU. The influence of oblique slicing geometry on DSU-utilized global homogeneity is also quantified computationally. Finally, the effect of DSU on susceptibility artifact reduction in single-shot axial-sliced EPI is analyzed using experimental acquisitions.

Field-dependent Hall effect in single-crystal heavy-fermion YbAgGe below1K

We report the results of a low temperature (T >= 50 mK) and high field (H <= 180 kOe) study of the Hall resistivity in single crystals of YbAgGe, a heavy fermion compound that demonstrates field-induced non-Fermi-liquid behavior near its field-induced quantum critical point. Distinct features in the anisotropic, field-dependent Hall resistivity sharpen on cooling down and at the base temperature are close to the respective critical fields for the field-induced quantum critical point. The field range of the non-Fermi-liquid region decreases on cooling but remains finite at the base temperature with no indication of its conversion to a point for T -> 0. At the base temperature, the functional form of the field-dependent Hall coefficient is field direction dependent and complex beyond existing simple models thus reflecting the multi-component Fermi surface of the material and its non-trivial modification at the quantum critical point.

QUASIPARTICLE SPECTROSCOPY AND HIGH-FIELD PHASE DIAGRAMS OF CUPRATE SUPERCONDUCTORS – AN INVESTIGATION OF COMPETING ORDERS AND QUANTUM CRITICALITY

We present scanning tunneling spectroscopic and high-field thermodynamic studies of hole- and electron-doped (p- and n-type) cuprate superconductors. Our experimental results are consistent with the notion that the ground state of cuprates is in proximity to a quantum critical point (QCP) that separates a pure superconducting (SC) phase from a phase comprised of coexisting SC and a competing order, and the competing order is likely a spin-density wave (SDW). The effect of applied magnetic field, tunneling current, and disorder on the revelation of competing orders and on the low-energy excitations of the cuprates is discussed.

Prelude to Everest: Alexander M. Kellas and the 1920 high altitude scientific expedition to Kamet.

Following his untimely death due to illness during the early stages of the first Mount Everest Reconnaissance Expedition in 1921, Alexander M. Kellas has received relatively little attention in either mountaineering or scientific literature. He remains an obscure figure despite his noteworthy contributions to high altitude physiology and exploration. He can be considered not only one of the finest exploratory Himalayan mountaineers in history, but also the first person to apply state-of-the-art knowledge of high altitude physiology to field investigations at altitudes over 6000 m. By the time of his death, it is extremely likely that Kellas had spent more time above 6000 m than anyone on Earth, undertaking no fewer than eight Himalayan expeditions between 1907 and 1921. This article revisits and examines in some detail the most ambitious high altitude physiological field study undertaken through the second decade of the 20th century, A. M. Kellas and Henry T. Morshead's 1920 Kamet Expedition. This undertaking by Kellas and Morshead was unique because it specifically emphasized investigation of the practical difficulties inherent in climbing at very high altitudes. During this endeavor, Kellas carried out the first rigorous tests of the value of supplementary oxygen for climbing at high altitude. The results of the field studies conducted during the 1920 Kamet Expedition provided strong support for the use of supplementary oxygen at high altitude. However, after Kellas died on the approach march to Everest the following year, the British mountaineering establishment did not again have a similar proponent or exponent of extreme altitude field research until physiologist Griffith Pugh once again took up the challenge in the early 1950s.

High electric field study of the superconducting transition ofY Ba2Cu3O7−x

The superconducting transitions ofY Ba2Cu3O7−x (YBCO) films of different thicknesses in high current densities and high electric fields areinvestigated. A pulsed-current technique allows for true four-probe measurements with a pulseduration as short as 50 ns, resulting in a significant reduction of self-heating. Electric fields up to900 V cm−1 can be applied without sample damage. The data are compared to a theoreticalmodel that describes the superconducting transition by the Hartree approximationof the Lawrence–Doniach model for a layered superconductor, together with atemperature correction in order to account for the self-heating. Although this modelfits the electric-field dependence of the resistivity of a 330 nm thick YBCO filmwell, it fails for a 36 nm ultra-thin film, where self-heating is much smaller. Thisobservation is a direct indication of intrinsic non-ohmic behaviour, resulting fromthe suppression of superconducting order parameter fluctuations in high electricfields.

Experimental Examination of Chain-Dimer Model in Ca1-xCuO2(x=0.164) by High Field ESR Measurements

Intensive high-field ESR studies have been performed to investigate the peculiar ground state of a quasi-one dimensional cupric oxide Ca 1- x CuO 2 ( x =0.164), to which a two-sublattice model (chain-dimer model) composed of a Heisenberg antiferromagnetic chain and dimer state with low spin density is proposed from the results of magnetic susceptibility and specific heat measurements. Using the powder sample and high frequency sources from 60 to 490 GHz, we observed antiferromagnetic resonance mode with a typical frequency–field relation of the easy-axis type at 1.8 K. Moreover, weak ESR signals are also observed in the ordered state. From the analysis of the integrated intensity for those weak signals, it is found that the dimer model reproduces the temperature dependent behaviors quite well. These results support the chain-dimer model from the microscopic point of view.

X-ray diffraction study in high magnetic fields of magnetovolume effect in itinerant-electron metamagnetic La(Fe0.88Si0.12)13 compound

Magnetovolume effect due to the itinerant-electron metamagnetic (IEM) transition in La(Fe0.88Si0.12)13 has been investigated by x-ray diffraction measurements in high magnetic fields. No crystal structure change was confirmed after the IEM transition. The relative change in the lattice constant measured perpendicular to the magnetic field direction is similar to the longitudinal linear magnetostriction, and hence the volume change due to the IEM transition is isotropic. The volume change in both the ferromagnetic and the paramagnetic states is proportional to the square of the magnetization, though the magnetovolume coupling constant in the former is smaller than that in the latter.

High-field ESR study on frustrated spin chain system KCu 5V 3O 13

High-field ESR technique has been applied to the novel frustrated spin chain system KCu 5V 3O 13, to reveal the peculiar ground state proposed by the previous results of the magnetic susceptibility and the magnetization measurements. We successfully observed the antiferromagnetic resonance with the easy axis type anisotropy below 7 K . In addition, small shifts of the resonance field probably due to the re-arrangements of spin configuration in the ordered phase were also observed at about 4 K in the detailed temperature dependence measurements. This result confirmed the observation of double peaks at 3.9 and 7 K in the specific heat measurements performed in advance to the high-field ESR measurements. We will discuss about the ground state of KCu 5V 3O 13 based on these results together with those obtained in the KMgCu 4V 3O 13, where no antiferromagnetic resonance was observed.

A high-field functional MRI study of quadri-lingual subjects

We assessed six multilingual subjects by functional MRI using a Noun Verb Generation task in four different languages. We hypothesised that the degree of proficiency in each language would be related to the extent of functional activity measured in a region of interest analysis. Proficiency in each language was quantified using two neuropsychological tests. All four languages activated overlapping brain areas, corresponding to the major language regions. The number of activated voxels correlated with proficiency, so that the activated volume increased for languages in which a subject had poorer proficiency. Activation did not appear to be dependent on the age at which the language was learnt.

High-field ultrasonic study on PrPtBi with non-Kramers doublet ground state

We have measured the elastic constants of PrPtBi single crystal with the cubic MgAgAs-type structure as functions of temperatures and magnetic fields. A remarkable softening of 10% in ( c 11− c 12)/2 was observed below 20 K with decreasing temperatures, while c 44 shows a monotonous increase. These behaviors are attributed to the crystal-field ground state non-Kramers doublet Γ 3. The enormous softening in ( c 11− c 12)/2 prefers a ferro-quadrupolar order of O 2 0 or O 2 2, which brings about a structural phase transition of cubic–tetragonal or cubic-orthorhombic. The magnetic field dependences of c 11 and ( c 11− c 12)/2 up to 25 T were explained qualitatively in terms of a quadrupolar susceptibility assuming the crystal-field level proposed by inelastic neutron scattering. Above 1.8 K the field dependence shows no clear indication of phase transition, which mean paramagnetic phase is stable above 1.8 K up to 25 T .