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1.
Nat Commun ; 14(1): 7037, 2023 Nov 03.
Article in English | MEDLINE | ID: mdl-37923745

ABSTRACT

A promising route to discover exotic electronic states in correlated electron systems is to vary the hole or electron doping away from a Mott insulating state. Important examples include quantum criticality and high-temperature superconductivity in cuprates. Here, we report the surprising discovery of a quantum insulating state upon electron doping the Mott insulator CeMnAsO, which emerges below a distinct critical transition temperature, TII. The insulator-insulator transition is accompanied by a significant reduction in electron mobility as well as a colossal Seebeck effect and slow dynamics due to decoupling of the electrons from the lattice phonons. The origin of the transition is tentatively interpreted in terms of many-body localization, which has not been observed previously in a solid-state material.

2.
Dalton Trans ; 47(41): 14726-14733, 2018 Oct 23.
Article in English | MEDLINE | ID: mdl-30280742

ABSTRACT

The colossal magnetoresistance (CMR) observed in the oxypnictide NdMnAsO1-xFx has been further investigated. The magnetotransport is dominated by magnetopolarons. Magnetoresistance measurements of the series Nd(Mn1-xCox)AsO0.95F0.05 show that doping with cobalt on the manganese site pins the magnetopolarons and suppresses the CMR, which is completely destroyed by x = 0.047. The chemical doping results in non-stoichiometric samples, with both As and O vacancies. The relationship between the non-stoichiometry, magnetic order, electron doping and CMR is explored. The Nd antiferromagnetic transition and simultaneous reorientation of the Mn spins into the basal plane at 23 K (TSR) is not effected by Co doping. However, there is a significant decrease in TN(Mn) as the antiferromagnetic transition is suppressed from 360 K to 300 K as x increases from 0-0.047. The manganese moment at 10 K is also reduced from 3.86(2)µB to 3.21(2)µB over the same doping range. This reduction in the in-plane Mn moment decreases the electron-electron correlations below TSR and acts to further diminish the magnetoresistance.

3.
Sci Rep ; 6: 20705, 2016 Feb 15.
Article in English | MEDLINE | ID: mdl-26875693

ABSTRACT

The recent discovery of high temperature superconductivity in Fe arsenides has invigorated research into transition metal pnictides. Colossal magnetoresistance (CMR) has recently been reported for NdMnAsO1-xFx for x = 0.05-0.08, with a maximum magnetoresistance achieved at low temperature (MR9T(3 K)) = -95%). This appears to be a novel mechanism of CMR, which is as a result of a second order phase transition in field from an insulating antiferromagnet to a semiconducting paramagnet. Here we report a variable temperature synchrotron X-ray powder diffraction study of the CMR oxypnictide NdMnAsO0.95F0.05 between 4 K-290 K. An excellent fit to the tetragonal unit cell with space group P4/nmm is obtained over the entire temperature range, with no change in crystal structure detected down to 4 K. A coupling of the lattice and magnetic order is observed, where subtle discontinuities in the temperature variation of a and the c/a ratio are apparent as the Nd spins order antiferromagnetically and the Mn moments reorient into the basal plane at TSR. The results suggest that very small changes in lattice parameters effect the coupling between lattice, electronic and magnetic degrees of freedom.

4.
J Phys Condens Matter ; 27(11): 116001, 2015 Mar 25.
Article in English | MEDLINE | ID: mdl-25721358

ABSTRACT

A high pressure neutron diffraction study of the oxypnictide NdMnAsO0.95F0.05 has been performed at temperatures of 290-383 K and pressures up to 8.59 GPa. The results demonstrate that the antiferromagnetic order of the Mn spins is robust to pressures of up to 8.59 GPa. TN is enhanced from 360 to 383 K upon applying an external pressure of 4.97 GPa, a rate of 4.63 K GPa(-1). NdMnAsO0.95F0.05 is shown to violate Bloch's rule which would suggest that NdMnAsO0.95F0.05 is on the verge of a localized to itinerant transition. There is no evidence of a structural transition but applied pressure tends to result in more regular As-Mn-As and Nd-O-Nd tetrahedra. The unit cell is significantly more compressible along the c-axis than the a-axis, as the interlayer coupling is weaker than the intrinsic bonds contained within NdO and MnAs slabs.

5.
J Phys Condens Matter ; 23(36): 365704, 2011 Sep 14.
Article in English | MEDLINE | ID: mdl-21865639

ABSTRACT

Zero field muon spin relaxation (ZF-µSR) has been used to study the magnetic properties of the underdoped giant magnetoresistive ruthenocuprates RuSr(2)Nd(1.8-x)Y (0.2)Ce(x)Cu(2)O(10-δ) (x = 0.95, 0.80). The magnetoresistance (MR) is defined so that MR = ((ρ(H)-ρ(0))/ρ(0)) and the giant magnetoresistive ruthenocuprates RuSr(2)Nd(1.8-x)Y(0.2)Ce(x)Cu(2)O(10-δ) exhibit a large reduction in electronic resistivity upon application of a magnetic field. The ZF-µSR results show a gradual loss of initial asymmetry A(0) at the ruthenium spin transition temperature, T(Ru). At the same time the electronic relaxation rate, λ, shows a gradual increase with decreasing temperature below T(Ru). These results have been interpreted as evidence for Cu spin cluster formation below T(Ru). These magnetically ordered clusters grow as the temperature is decreased thus causing the initial asymmetry to decrease slowly. Giant magnetoresistance is observed over a wide temperature range in the materials studied and the magnitude increases as the temperature is reduced from T(Ru) to 4 K which suggests a relation between Cu spin cluster size and |-MR|.

6.
Phys Rev Lett ; 104(17): 177202, 2010 Apr 30.
Article in English | MEDLINE | ID: mdl-20482134

ABSTRACT

We report on the unconventional magnetism in the cubic B-site ordered double perovskite Ba2YMoO6, using ac and dc magnetic susceptibility, heat capacity and muon spin rotation. No magnetic order is observed down to 2 K while the Weiss temperature is approximately -160 K. This is ascribed to the geometric frustration in the lattice of edge-sharing tetrahedra with orbitally degenerate Mo5+ s=1/2 spins. Our experimental results point to a gradual freezing of the spins into a disordered pattern of spin singlets, quenching the orbital degeneracy while leaving the global cubic symmetry unaffected, and providing a rare example of a valence bond glass.

7.
Chem Commun (Camb) ; (22): 2273-4, 2007 Jun 14.
Article in English | MEDLINE | ID: mdl-17534514

ABSTRACT

An unexpected enhancement of the large negative magnetoresistance (MR) observed in RuSr(2)Nd(0.95)Y(0.15)Ce(0.9)Cu(2)O(10-delta) up to -47% at 4 K and 9 T is evidenced upon dilution of the Ru magnetic order by substitution of Ta for Ru; this enhancement of -MR scales with the cell volume.

9.
Nature ; 436(7052): 829-32, 2005 Aug 11.
Article in English | MEDLINE | ID: mdl-16094364

ABSTRACT

The mechanism of high-transition-temperature (high-T(c)) superconductivity in doped copper oxides is an enduring problem. Antiferromagnetism is established as the competing order, but the relationship between the two states in the intervening 'pseudogap' regime has become a central puzzle. The role of the crystal lattice, which is important in conventional superconductors, also remains unclear. Here we report an anomalous increase of the distance between copper oxide planes on cooling, which results in negative thermal volume expansion, for layered ruthenium copper oxides that have been doped to the boundary of antiferromagnetism and superconductivity. We propose that a crossover between these states is driven by spin ordering in the ruthenium oxide layers, revealing a novel mechanism for negative lattice expansion in solids. The differences in volume and lattice strain between the distinct superconducting and antiferromagnetic states can account for the phase segregation phenomena found extensively in low-doped copper oxides, and show that Cooper pair formation is coupled to the lattice. Unusually large variations of resistivity with magnetic field are found in these ruthenium copper oxides at low temperatures through coupling between the ordered Ru and Cu spins.

10.
Magn Reson Med ; 50(1): 99-106, 2003 Jul.
Article in English | MEDLINE | ID: mdl-12815684

ABSTRACT

The work presented here uses combined blood oxygenation level-dependent (BOLD) and arterial spin tagging (AST) approaches to study the effect of indomethacin on cerebral blood flow (CBF) and oxygen consumption (CMRO(2)) increases during motor activation. While indomethacin reduced the CBF increase during activation, it did not significantly affect the CMRO(2) increase during activation. The ratio of the activation-induced CBF increase in the presence and absence of indomethacin was 0.54 +/- 0.08 (+/-SEM, n = 8, P < 0.001), while the ratio of the CMRO(2) increase in the presence and absence of the drug was 1.02 +/- 0.08 (+/-SEM, N = 8, ns). Potential difficulties in estimating CMRO(2) changes from combined BOLD/AST data are discussed.


Subject(s)
Indomethacin/metabolism , Magnetic Resonance Imaging/methods , Motor Activity/physiology , Motor Cortex/drug effects , Motor Cortex/physiology , Somatosensory Cortex/drug effects , Somatosensory Cortex/physiology , Adult , Cerebrovascular Circulation/drug effects , Cerebrovascular Circulation/physiology , Female , Humans , Image Enhancement/methods , Male , Motor Cortex/metabolism , Oxygen Consumption/drug effects , Oxygen Consumption/physiology , Physical Stimulation , Reproducibility of Results , Sensitivity and Specificity , Somatosensory Cortex/metabolism
11.
Magn Reson Med ; 44(3): 440-9, 2000 Sep.
Article in English | MEDLINE | ID: mdl-10975897

ABSTRACT

Arterial spin tagging techniques originally used the one-compartment Kety model to describe the dynamics of tagged water in the brain. The work presented here develops a more realistic model that includes the contribution of tagged water in the capillary bed and accounts for the finite time required for water to diffuse across the blood-brain barrier. The new model was used to evaluate potential errors in cerebral blood flow values calculated using the one-compartment Kety model. The results predict that if the one-compartment Kety model is used to analyze arterial spin tagging data the observed grey matter cerebral blood flow values should be relatively insensitive to restricted diffusion of water across the capillary bed. For instance, the observed grey matter cerebral blood flow should closely approximate the true cerebral blood flow and not the product of the extraction fraction and the cerebral blood flow. This prediction is in agreement with recent experimental arterial spin tagging results.


Subject(s)
Body Water/metabolism , Cerebrovascular Circulation/physiology , Magnetic Resonance Angiography/methods , Models, Neurological , Blood Flow Velocity/physiology , Body Fluid Compartments , Cerebral Arteries/metabolism , Computer Simulation , Mathematical Computing , Reproducibility of Results , Spin Labels
12.
Magn Reson Med ; 44(3): 450-6, 2000 Sep.
Article in English | MEDLINE | ID: mdl-10975898

ABSTRACT

Steady-state arterial spin tagging approaches can provide quantitative images of CBF, but have not been validated in humans. The work presented here compared CBF values measured using steady-state arterial spin tagging with CBF values measured in the same group of human subjects using the H(2)(15)O IV bolus PET method. Blood flow values determined by H(2)(15)O PET were corrected for the known effects of incomplete extraction of water across the blood brain barrier. For a cortical strip ROI, blood flow values determined using arterial spin tagging (64+/-12 cc/100 g/min) were not statistically different from corrected blood flow values determined using H(2)(15)O PET (67+/-13 cc/100 g/min). However, for a central white matter ROI, blood flow values determined using arterial spin tagging were significantly underestimated compared to corrected blood flow values determined using H(2)(15)O PET. This underestimation could be caused by an underestimation of the arterial transit time for white matter regions.


Subject(s)
Blood Flow Velocity/physiology , Body Water/diagnostic imaging , Cerebral Arteries/physiology , Cerebrovascular Circulation/physiology , Magnetic Resonance Angiography/methods , Adult , Analysis of Variance , Body Water/metabolism , Brain/anatomy & histology , Brain/blood supply , Brain/diagnostic imaging , Cerebral Arteries/diagnostic imaging , Female , Humans , Image Processing, Computer-Assisted , Male , Oxygen Radioisotopes/analysis , Reproducibility of Results , Tomography, Emission-Computed
13.
Magn Reson Med ; 44(1): 92-100, 2000 Jul.
Article in English | MEDLINE | ID: mdl-10893526

ABSTRACT

Phase-encoded multishot SPIRAL approaches were used to acquire true 3D cerebral blood flow images of the human head using arterial spin tagging approaches. Multiple-inversion background suppression techniques, which suppress phase noise due to interacquisition fluctuations in the static magnetic field, reduced the temporal standard deviation of true 3D delta M images acquired using arterial spin tagging approaches by approximately 50%. Background suppressed arterial spin tagging (ASSIST) approaches were used to obtain high-resolution isotropic true 3D cerebral blood flow images, and to obtain true 3D activation images during cognitive (working memory) tasks. Magn Reson Med 44:92-100, 2000. Published 2000 Wiley-Liss, Inc.


Subject(s)
Cerebral Arteries/anatomy & histology , Cerebrovascular Circulation/physiology , Magnetic Resonance Imaging/methods , Adult , Artifacts , Female , Humans , Male , Phantoms, Imaging , Spin Labels , Time Factors
14.
Magn Reson Med ; 42(2): 404-7, 1999 Aug.
Article in English | MEDLINE | ID: mdl-10440966

ABSTRACT

Steady-state arterial spin tagging approaches were used to construct multislice images of relative cerebral blood flow changes during finger-tapping tasks. Statistically significant increases in cerebral blood flow were observed in primary sensorimotor cortex in all seven subjects. The mean volume of the activated region in the contralateral primary sensorimotor cortex was 0.9 cm(3), and the mean increase in cerebral blood flow in the activated area was 54% +/- 11%. Although the extended spatial coverage is advantageous for activation studies, the intrinsic sensitivity of the multislice approach is smaller than the intrinsic sensitivity of the single-slice, arterial spin tagging approach. Magn Reson Med 42:404-407, 1999. Published 1999 Wiley-Liss, Inc.


Subject(s)
Cerebral Arteries/physiology , Cerebrovascular Circulation/physiology , Magnetic Resonance Imaging/methods , Psychomotor Performance/physiology , Humans , Image Processing, Computer-Assisted , Motor Cortex/anatomy & histology , Motor Cortex/blood supply , Somatosensory Cortex/anatomy & histology , Somatosensory Cortex/blood supply , Spin Labels
15.
Neuroimage ; 8(1): 44-9, 1998 Jul.
Article in English | MEDLINE | ID: mdl-9698574

ABSTRACT

Steady-state arterial spin-tagging MRI approaches were used to quantitate regional cerebral blood flow increases in prefrontal cortex during a working memory ("two-back") task in six normal subjects. Statistically significant increases in cerebral blood flow in prefrontal cortex were observed in all six subjects: the average increase in cerebral blood flow in activated prefrontal cortex regions was 22 +/- 5 cc/100 g/min (23 +/- 7%). The results demonstrate that spin-tagging approaches can be used to follow focal activation in prefrontal cortex during cognitive tasks.


Subject(s)
Arousal/physiology , Image Processing, Computer-Assisted/instrumentation , Magnetic Resonance Imaging/instrumentation , Mental Recall/physiology , Prefrontal Cortex/blood supply , Adult , Blood Flow Velocity/physiology , Brain Mapping , Dominance, Cerebral/physiology , Female , Humans , Male , Psychomotor Performance , Regional Blood Flow/physiology
16.
Magn Reson Med ; 39(5): 825-32, 1998 May.
Article in English | MEDLINE | ID: mdl-9581614

ABSTRACT

A method is presented for multislice measurements of quantitative cerebral perfusion based on magnetic labeling of arterial spins. The method combines a pulsed arterial inversion, known as the FAIR (Flow-sensitive Alternating Inversion Recovery) experiment, with a fast spiral scan image acquisition. The short duration (22 ms) of the spiral data collection allows simultaneous measurement of up to 10 slices per labeling period, thus dramatically increasing efficiency compared to current single slice acquisition protocols. Investigation of labeling efficiency, suppression of unwanted signals from stationary as well as intraarterial spins, and the FAIR signal change as a function of inversion delay are presented. The assessment of quantitative cerebral blood flow (CBF) with the new technique is demonstrated and shown to require measurement of arterial transit time as well as suppression of intraarterial spin signals. CBF values measured on normal volunteers are consistent with results obtained from H2O15 positron emission tomography (PET) studies and other radioactive tracer approaches. In addition, the new method allows detection of activation-related perfusion changes in a finger-tapping experiment, with locations of activation corresponding well to those observed with blood oxygen level dependent (BOLD) fMRI.


Subject(s)
Brain/anatomy & histology , Cerebrovascular Circulation/physiology , Magnetic Resonance Imaging/methods , Artifacts , Brain/blood supply , Feasibility Studies , Humans , Image Processing, Computer-Assisted , Phantoms, Imaging , Sensitivity and Specificity , Time Factors
18.
Neuroimage ; 6(2): 104-12, 1997 Aug.
Article in English | MEDLINE | ID: mdl-9299384

ABSTRACT

Steady-state arterial spin tagging MRI approaches were used to quantitate regional cerebral blood flow increases during finger tapping tasks in seven normal subjects. Statistically significant increases in cerebral blood flow were observed in the contralateral primary sensorimotor cortex in all seven subjects and in the supplementary motor area in five subjects. The intrinsic spatial resolution of the cerebral blood flow images was approximately 4 mm. If no spatial filtering was applied, the average increase in cerebral blood flow in the activated primary sensorimotor cortex was 60 +/- 10 cc/100 g/min (91 +/- 32%). If the images were filtered to a spatial resolution of 15 mm, the average increase in cerebral blood flow in the activated primary sensorimotor cortex was 23 +/- 7 cc/100 g/min (42 +/- 15%), in agreement with previously reported 133Xe and PET results.


Subject(s)
Brain/anatomy & histology , Cerebral Arteries/physiology , Cerebrovascular Circulation/physiology , Magnetic Resonance Imaging/methods , Psychomotor Performance/physiology , Adult , Female , Humans , Image Processing, Computer-Assisted , Male , Motor Cortex/anatomy & histology , Motor Cortex/blood supply , Somatosensory Cortex/anatomy & histology , Somatosensory Cortex/blood supply
19.
Magn Reson Med ; 37(4): 501-10, 1997 Apr.
Article in English | MEDLINE | ID: mdl-9094071

ABSTRACT

A simple four-compartment model for magnetization transfer was used to obtain theoretical expressions for the relationship between regional cerebral blood flow and delta M, the change in longitudinal magnetization of brain water spins when arterial water spins are perturbed. The theoretical relationship can be written in two forms, depending on the approach used to normalize delta M. Using the first approach, the calculation of cerebral blood flow requires a knowledge of R1(omega 1, delta omega), the longitudinal relaxation rate observed in the presence of continuous off-resonance RF irradiation. Using the second approach, the calculation of cerebral blood flow requires a knowledge of R1(omega 1, delta omega), where R1(omega 1, delta omega) is given by the product of R1(omega 1, delta omega) and the fractional steady-state longitudinal water magnetization in the presence of off-resonance RF irradiation. If the off-resonance RF irradiation used for arterial tagging does not produce appreciable magnetization transfer effects, R1(omega 1, delta omega) can be approximated by the longitudinal relaxation rate measured in the absence of off-resonance RF irradiation, R1obs. Theoretical expressions obtained by using the four-component model for magnetization transfer are compared with equivalent expressions obtained by using two-compartment models.


Subject(s)
Blood Flow Velocity/physiology , Brain/physiology , Cerebrovascular Circulation , Magnetic Resonance Imaging/methods , Models, Theoretical , Corpus Callosum/blood supply , Humans , Mathematics , Spin Labels
20.
Magn Reson Med ; 37(2): 226-35, 1997 Feb.
Article in English | MEDLINE | ID: mdl-9001147

ABSTRACT

"Vascular" artifacts can have substantial effects on human cerebral blood flow values calculated by using arterial spin tagging approaches. One vascular artifact arises from the contribution of "tagged" arterial water spins to the observed change in brain water MR signal. This artifact can be reduced if large bipolar gradients are used to "crush" the MR signal from moving arterial water spins. A second vascular artifact arises from relaxation of "tagged" arterial blood during transit from the tagging plane to the capillary exchange site in the imaging slice. This artifact can be corrected if the arterial transit times are measured by using "dynamic" spin tagging approaches. The mean transit time from the tagging plane to capillary exchange sites in a gray matter region of interest was calculated to be approximately 0.94 s. Cerebral blood flow values calculated for seven normal volunteers agree reasonably well with values calculated by using radioactive tracer approaches.


Subject(s)
Artifacts , Cerebrovascular Circulation , Image Enhancement/methods , Magnetic Resonance Imaging/methods , Adult , Algorithms , Arteries , Blood , Body Water/metabolism , Brain/blood supply , Brain/metabolism , Capillaries/physiology , Cerebrospinal Fluid , Corpus Callosum/blood supply , Echo-Planar Imaging , Female , Humans , Male , Middle Aged
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