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2.
Rev Sci Instrum ; 93(6): 063902, 2022 Jun 01.
Artigo em Inglês | MEDLINE | ID: mdl-35778005

RESUMO

We introduce a simple method to extract the nuclear coherent and isotope incoherent, spin incoherent, and magnetic neutron scattering cross section components from powder scattering data measured using a single neutron beam polarization direction and a position-sensitive detector with large out-of-plane coverage. The method draws inspiration from polarized small-angle neutron scattering and contrasts with conventional so-called "xyz" polarization analysis on wide-angle instruments, which requires measurements with three orthogonal polarization directions. The viability of the method is demonstrated on both simulated and experimental data for the classical "spin ice" system Ho2Ti2O7, the latter from the LET direct geometry spectrometer at the ISIS facility. The cross section components can be reproduced with good fidelity by either fitting the out-of-plane angle dependence around a Debye-Scherrer cone or grouping the data by angle and performing a matrix inversion. The limitations of the method and its practical uses are discussed.

3.
J Phys Condens Matter ; 32(37): 374007, 2020 Feb 12.
Artigo em Inglês | MEDLINE | ID: mdl-32050188

RESUMO

It is well established that in the low-temperature limit, the two-dimensional quantum Heisenberg antiferromagnet on a square lattice (2DQHAFSL) exhibits an anomaly in its spectrum at short-wavelengths on the zone-boundary. In the vicinity of the [Formula: see text] point the pole in the one-magnon response exhibits a downward dispersion, is heavily damped and attenuated, giving way to an isotropic continuum of excitations extending to high energies. The origin of the anomaly and the presence of the continuum are of current theoretical interest, with suggestions focused around the idea that the latter evidences the existence of spinons in a two-dimensional system. Here we present the results of neutron inelastic scattering experiments and Quantum Monte Carlo calculations on the metallo-organic compound Cu(DCOO)[Formula: see text]D2O (CFTD), an excellent physical realisation of the 2DQHAFSL, designed to investigate how the anomaly at [Formula: see text] evolves up to finite temperatures [Formula: see text]. Our data reveal that on warming the anomaly survives the loss of long-range, three-dimensional order, and that it is thus a robust feature of the two-dimensional system. With further increase of temperature the zone-boundary response gradually softens and broadens, washing out the [Formula: see text] anomaly. This is confirmed by a comparison of our data with the results of finite-temperature Quantum Monte Carlo simulations where the two are found to be in good accord. In the vicinity of the antiferromagnetic zone centre, there was no significant softening of the magnetic excitations over the range of temperatures investigated.

4.
Nat Commun ; 10(1): 637, 2019 02 07.
Artigo em Inglês | MEDLINE | ID: mdl-30733436

RESUMO

Pyrochlore systems are ideally suited to the exploration of geometrical frustration in three dimensions, and their rich phenomenology encompasses topological order and fractional excitations. Classical spin ices provide the first context in which it is possible to control emergent magnetic monopoles, and anisotropic exchange leads to even richer behaviour associated with large quantum fluctuations. Whether the magnetic ground state of Yb2Ti2O7 is a quantum spin liquid or a ferromagnetic phase induced by a Higgs transition appears to be sample dependent. Here we have determined the role of structural defects on the magnetic ground state via the diffuse scattering of neutrons. We find that oxygen vacancies stabilise the spin liquid phase and the stuffing of Ti sites by Yb suppresses it. Samples in which the oxygen vacancies have been eliminated by annealing in oxygen exhibit a transition to a ferromagnetic phase, and this is the true magnetic ground state.

5.
Phys Rev Lett ; 119(8): 087201, 2017 Aug 25.
Artigo em Inglês | MEDLINE | ID: mdl-28952772

RESUMO

We present the results of a combined ^{7}Li-NMR and diffraction study on LiGa_{0.95}In_{0.05}Cr_{4}O_{8}, a member of the LiGa_{1-x}In_{x}Cr_{4}O_{8} "breathing" pyrochlore family. Via specific heat and NMR measurements, we find that the complex sequence of first-order transitions observed for LiGaCr_{4}O_{8} is replaced by a single second-order transition at T_{f}=11 K. Neutron and x-ray diffraction rule out both structural symmetry lowering and magnetic long-range order as the origin of this transition. Instead, reverse Monte Carlo fitting of the magnetic diffuse scattering indicates that the low-temperature phase may be described as a collinear spin nematic state, characterized by a quadrupolar order parameter. This state also shows signs of short-range order between collinear spin arrangements on tetrahedra, revealed by mapping the reverse Monte Carlo spin configurations onto a three-state color model.

6.
Nat Phys ; 11(1): 62-68, 2015 Jan 01.
Artigo em Inglês | MEDLINE | ID: mdl-25729400

RESUMO

Quantum magnets have occupied the fertile ground between many-body theory and low-temperature experiments on real materials since the early days of quantum mechanics. However, our understanding of even deceptively simple systems of interacting spins-1/2 is far from complete. The quantum square-lattice Heisenberg antiferromagnet (QSLHAF), for example, exhibits a striking anomaly of hitherto unknown origin in its magnetic excitation spectrum. This quantum effect manifests itself for excitations propagating with the specific wave vector (π, 0). We use polarized neutron spectroscopy to fully characterize the magnetic fluctuations in the metal-organic compound CFTD, a known realization of the QSLHAF model. Our experiments reveal an isotropic excitation continuum at the anomaly, which we analyse theoretically using Gutzwiller-projected trial wavefunctions. The excitation continuum is accounted for by the existence of spatially-extended pairs of fractional S=1/2 quasiparticles, 2D analogues of 1D spinons. Away from the anomalous wave vector, these fractional excitations are bound and form conventional magnons. Our results establish the existence of fractional quasiparticles in the high-energy spectrum of a quasi-two-dimensional antiferromagnet, even in the absence of frustration.

7.
Phys Rev Lett ; 113(11): 117201, 2014 Sep 12.
Artigo em Inglês | MEDLINE | ID: mdl-25260001

RESUMO

We present new magnetic heat capacity and neutron scattering results for two magnetically frustrated molybdate pyrochlores: S=1 oxide Lu_{2}Mo_{2}O_{7} and S=1/2 oxynitride Lu_{2}Mo_{2}O_{5}N_{2}. Lu_{2}Mo_{2}O_{7} undergoes a transition to an unconventional spin glass ground state at T_{f}∼16 K. However, the preparation of the corresponding oxynitride tunes the nature of the ground state from spin glass to quantum spin liquid. The comparison of the static and dynamic spin correlations within the oxide and oxynitride phases presented here reveals the crucial role played by quantum fluctuations in the selection of a ground state. Furthermore, we estimate an upper limit for a gap in the spin excitation spectrum of the quantum spin liquid state of the oxynitride of Δ∼0.05 meV or Δ/|θ|∼0.004, in units of its antiferromagnetic Weiss constant θ∼-121 K.

8.
J Phys Condens Matter ; 25(10): 106001, 2013 Mar 13.
Artigo em Inglês | MEDLINE | ID: mdl-23388841

RESUMO

The low-energy (ε = hω < 1 meV), low-temperature (T = 0.05 K) spin dynamics of the s = 1/2 kagome candidate herbertsmithite are probed in the presence of magnetic fields up to 2.5 T. The zero-field spectra reveal a very weak continuum of scattering at T = 10 K and a broad inelastic peak centred at ε(max) = 0.2 meV at lower temperatures, T < 1 K. The broad peak is found to be strongly damped, with a liquid-like structure factor implying correlations at length scales up to r = 6 Å. The field dependence of the peak appears to follow the Zeeman splitting of s = 1/2 excitations, consistent with the weakly split 'doublets' observed in low-temperature specific heat. A possible explanation of these observations is a short-range correlated state involving defect spins between the kagome planes and moments in the kagome layers.

9.
J Phys Condens Matter ; 23(16): 164201, 2011 Apr 27.
Artigo em Inglês | MEDLINE | ID: mdl-21471624

RESUMO

Spangolite, Cu(6)Al(SO(4))(OH)(12)Cl·3H(2)O, is a hydrated layered copper sulfate mineral. The Cu(2+) ions of each layer form a systematically depleted triangular lattice which approximates a maple leaf lattice. We present details of the crystal structure, which suggest that in spangolite this lattice actually comprises two species of edge linked trimers with different exchange parameters. However, magnetic susceptibility measurements show that despite the structural trimers, the magnetic properties are dominated by dimerization. The high temperature magnetic moment is strongly reduced below that expected for the six s = 1/2 in the unit cell.

10.
Phys Rev Lett ; 103(23): 237201, 2009 Dec 04.
Artigo em Inglês | MEDLINE | ID: mdl-20366167

RESUMO

Neutron spectroscopy and diffuse neutron scattering on herbertsmithite [ZnCu(3)(OH)(6)Cl(2)], a near-ideal realization of the s=1/2 kagome antiferromagnet, reveal the hallmark property of a quantum spin liquid: instantaneous short-ranged antiferromagnetic correlations in the absence of a time-averaged ordered moment. These dynamic antiferromagnetic correlations are weakly dependent of neutron-energy transfer and temperature, and persist up to 25 meV and 120 K. At low energy transfers a shift of the magnetic scattering to low Q is observed with increasing temperature, providing evidence of gapless spinons. It is argued that these observations provide important evidence in favor of resonating-valence-bond theories of (doped) Mott insulators.

11.
Tidsskr Nor Laegeforen ; 120(12): 1451-8, 2000 May 10.
Artigo em Norueguês | MEDLINE | ID: mdl-10851944

RESUMO

Magnetic resonance imaging (MRI) of the breast is currently used for evaluation of both parenchymal disease and silicone gel implants. MRI is widely recognised as the most accurate imaging method for evaluation of breast implant integrity. Knowledge of the MRI appearance of the different types of implants and the possible complications of their use is important for diagnosis in these patients. The use of contrast-enhanced MRI in breast cancer diagnosis has been investigated thoroughly during the last decade. It is a sensitive imaging method to detect breast pathology, and may have an important diagnostic impact in carefully selected patient groups. The development of new MR techniques may improve the utility of MR in breast cancer diagnosis, treatment and research.


Assuntos
Doenças Mamárias/diagnóstico , Mama/patologia , Imageamento por Ressonância Magnética , Biópsia , Implantes de Mama/efeitos adversos , Neoplasias da Mama/diagnóstico , Feminino , Predisposição Genética para Doença , Humanos , Fatores de Risco
12.
Tidsskr Nor Laegeforen ; 119(22): 3283-7, 1999 Sep 20.
Artigo em Norueguês | MEDLINE | ID: mdl-10533411

RESUMO

Cancer patients with spinal cord compression may develop irreversible neurological deficit. The clinical picture implies back pain and subsequent neurological deficit. There is always a danger of rapid deterioration of the patient's condition. If spinal cord compression is suspected, the case is an emergency. MRI should be preferred in the diagnostic work-up, and corticosteroids be administered promptly. Radiation therapy or surgical treatment should be started as soon as possible. Patient outcome is related to the degree of neurological deficit at the start of treatment.


Assuntos
Bulbo , Metástase Neoplásica , Compressão da Medula Espinal/etiologia , Terapia Combinada , Diagnóstico Diferencial , Glucocorticoides/administração & dosagem , Humanos , Laminectomia , Imageamento por Ressonância Magnética , Bulbo/patologia , Prognóstico , Dosagem Radioterapêutica , Compressão da Medula Espinal/patologia , Compressão da Medula Espinal/terapia
13.
J Magn Reson Imaging ; 8(6): 1191-7, 1998.
Artigo em Inglês | MEDLINE | ID: mdl-9848727

RESUMO

Image-guided localized proton MR spectroscopy (MRS) of normal breasts and breast tumors (ductal and undifferentiated carcinomas) was performed using a dedicated double breast coil. In vivo 1H MR spectra from 10 normal volunteers showed signals from water and lipids only, even in breasts with small contribution of fatty breast tissue. In the spectra from 6 of the 12 examined patients, an intense signal assigned to choline compounds was detected. The signal was also detected at lower levels in the remaining patients. This study shows that in vivo 1H MRI/MRS examinations of breast tumors can be performed within an examination time of 45 to 60 minutes. Signals from breast tumor metabolites may be detected using in vivo 1H MRS.


Assuntos
Neoplasias da Mama/diagnóstico , Mama/anatomia & histologia , Carcinoma Ductal de Mama/diagnóstico , Colina/análise , Espectroscopia de Ressonância Magnética/métodos , Adulto , Idoso , Idoso de 80 Anos ou mais , Mama/química , Neoplasias da Mama/química , Carcinoma Ductal de Mama/química , Feminino , Humanos , Imageamento por Ressonância Magnética , Espectroscopia de Ressonância Magnética/instrumentação , Pessoa de Meia-Idade , Prótons
15.
Pediatr Radiol ; 28(6): 471-5, 1998 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-9634467

RESUMO

BACKGROUND: We have previously reported the results of cerebral MRI examinations in an unselected year cohort of very low birth weight (VLBW) infants at one year of corrected age. Twenty-one (78 %) of 27 infants had abnormal myelination, mainly in the central occipital white matter (COWM) and in the centrum semiovale (CS), seen on T2-weighted images. Twelve infants had irregular and dilated lateral ventricles. We speculated whether these findings indicated perinatal periventricular leukomalacia (PVL). Only two infants had completely normal MRI at age 1 year. OBJECTIVE: To determine whether the abnormal myelination seen at 1 year of age, was still present, either as delayed myelination or as gliosis caused by perinatal PVL. MATERIALS AND METHODS: In the present study, we report the results of follow-up cerebral MRI in 20 of these infants at 6 years of age. RESULTS: Most of the children with MRI deviations at 1 year still had abnormalities at 6 years. Abnormal myelination in the central occipital white matter combined with abnormalities in the CS or with ventricular dilatation at age 1 year, presented as gliosis in 12 of 13 children at 6 years of age. Abnormalities solely in the COWM at age 1 year had normalised in two of five children and persisted as delayed myelination in three at age 6 years. Gliotic changes in periventricular white matter were found in 12 of 20 children (60 %). Areas most affected were the CS (11 children) and the COWM (9 children). Delayed myelination in COWM was found in six children (30 %), combined with gliosis in CS in three children. Twelve infants had ventricular dilatation both at 1 and 6 years of age. CONCLUSIONS: The MRI correlates of PVL, i. e. gliosis and ventricular dilatation, are common findings on cerebral MRI at 6 years of age in VLBW infants.


Assuntos
Encéfalo/patologia , Recém-Nascido de muito Baixo Peso , Leucomalácia Periventricular/diagnóstico , Criança , Estudos de Coortes , Seguimentos , Humanos , Lactente , Recém-Nascido , Imageamento por Ressonância Magnética , Fatores de Tempo
16.
Dev Med Child Neurol ; 40(4): 256-62, 1998 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-9593497

RESUMO

This follow-up study reports on cerebral MRI findings in 20 very-low-birthweight (VLBW) infants without disabilities at age 1 year in relation to motor, intellectual, and perceptual function at age 6 years. MRI findings, anthropometrics, and Bayley Scales of Infant Development scores at age 1 year as predictors of psychomotor status at age 6 years are also evaluated and compared. Outcome parameters were the Peabody Developmental Motor Scales and the Wechsler Preschool and Primary Scale of Intelligence. The results show that infants with myelin hyperintensities including the centrum semiovale or with occipital hyperintensities with associated ventricular dilatation at age 1 scored lower on the Peabody Gross Motor Locomotion Scale at age 6 than infants with normal myelination or with isolated occipital hyperintensities. This may indicate damage to motor fibers caused by perinatal periventricular leukomalacia. No relation was found between abnormal MRI findings at age 1 and later fine motor, intellectual, and perceptual function. Comparing different age 1-year predictors, an abnormality score defined by MRI was used as an independent predictor of gross motor locomotion function at age 6 years. However, the Bayley Mental Development Index scores and weight at age 1 were more important predictors of later motor and intellectual outcome, respectively, than MRI findings. It is recommended that cerebral MRI should not be used routinely to examine VLBW infants without disabilities at 1 year of age.


Assuntos
Encéfalo/patologia , Recém-Nascido de muito Baixo Peso/fisiologia , Recém-Nascido de muito Baixo Peso/psicologia , Inteligência/fisiologia , Imageamento por Ressonância Magnética , Transtornos Psicomotores/patologia , Fatores Etários , Antropometria , Criança , Seguimentos , Humanos , Lactente , Transtornos da Percepção/diagnóstico , Estudos Prospectivos , Escalas de Wechsler
17.
Acta Radiol ; 38(4 Pt 2): 638-42, 1997 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-9245957

RESUMO

PURPOSE: To evaluate the diagnostic efficacy of mangafodipir trisodium (MnDPDP, Teslascan) as a new contrast agent for MR imaging of the liver based on an independent evaluation of the MR images from the European phase III studies. MATERIAL AND METHODS: MR imaging of the liver was done at 0.5-2.0 T in 17 European centres and included T1-weighted spin-echo and gradient-echo sequences before and after administration of MnDPDP to patients at a dose of 5 mumol/kg b.w. T2-weighted images were also obtained in all cases before the i.v. injection of the agent. Images of a total of 592 patients were evaluated by 4 independent experienced radiologists who were not involved in the on-site clinical trials. RESULTS: Statistically significantly more lesions were detected (p = 0.0014) in MnDPDP-enhanced T1-weighted MR images than in unenhanced images. T1-weighted gradient-echo sequences were found to be superior to T1-weighted spin-echo sequences after injection of MnDPDP. The post-contrast images were found to be statistically significantly superior to the pre-contrast images in confidence in the presence of a lesion (p < or = 0.0001), quality of lesion delineation (p < or = 0.0001), lesion conspicuity (p < or = 0.0001) and in the confidence in the diagnosis of a lesion (p = 0.001). CONCLUSION: This independent evaluation of the European phase III trials confirmed the superiority of MnDPDP-enhanced MR images over unenhanced images for lesion detection and characterization.


Assuntos
Meios de Contraste , Ácido Edético/análogos & derivados , Fígado/patologia , Imageamento por Ressonância Magnética/métodos , Manganês , Fosfato de Piridoxal/análogos & derivados , Adulto , Idoso , Idoso de 80 Anos ou mais , Meios de Contraste/administração & dosagem , Ácido Edético/administração & dosagem , Europa (Continente) , Feminino , Humanos , Neoplasias Hepáticas/diagnóstico , Neoplasias Hepáticas/secundário , Imageamento por Ressonância Magnética/estatística & dados numéricos , Masculino , Manganês/administração & dosagem , Pessoa de Meia-Idade , Fosfato de Piridoxal/administração & dosagem
18.
Neuropediatrics ; 28(3): 149-54, 1997 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-9266552

RESUMO

In this follow-up study, 20 of a geographically based year cohort of 31 surviving non-disabled VLBW (birthweight < 1500 g) children were examined at six years of age. The aim of the study was to relate cerebral MRI findings to neuro-development in these non-disabled children at six years of age. All MRI scans were evaluated for myelination pattern, periventricular gliosis, ventricular dilation and cortical atrophy. The Peabody motor test and the Wechsler Preschool and Primary Scale of Intelligence (WPPSI) were used in the evaluation of motor, mental and perceptual function. A diagnosis of attention deficit disorder with hyperactivity was made based on the examiner's impression of the child during the examination and based on the parent's history. We found that ten (50%) of the children had periventricular gliosis, mainly in centrum semiovale (CS) (nine children) and in central occipital white matter (COW) (six children). Gliosis in CS was related to lower scores on the Peabody gross motor test for locomotion, indicating involvement of corticospinal tracts. Additional gliosis in COW was related to both fine motor and gross motor impairments. We speculate that this indicates damage to both motor and visual pathways, affecting eye-hand coordination and balance function. No relationship between MRI deviations at six years and mental function based on performance, verbal and total IQ scores was found. However, there was a significant relationship between periventricular gliosis in COW and C5 and low scores on the WPPSI performance subtests: Picture completion test and Block design test. This may indicate visual and spatial perception problems, caused by damage to posterior visual pathways and occipito-thalamic tracts dealing with visuo-motor integration.


Assuntos
Córtex Cerebral/patologia , Desenvolvimento Infantil/fisiologia , Cognição/fisiologia , Recém-Nascido de muito Baixo Peso/crescimento & desenvolvimento , Desempenho Psicomotor/fisiologia , Análise de Variância , Córtex Cerebral/crescimento & desenvolvimento , Distribuição de Qui-Quadrado , Estudos de Coortes , Seguimentos , Gliose/patologia , Humanos , Recém-Nascido , Leucomalácia Periventricular/patologia , Leucomalácia Periventricular/fisiopatologia , Imageamento por Ressonância Magnética , Destreza Motora/fisiologia , Bainha de Mielina/fisiologia , Lobo Occipital/patologia , Lobo Occipital/fisiopatologia , Transtornos da Percepção/etiologia , Transtornos da Percepção/patologia , Transtornos Psicomotores/etiologia , Transtornos Psicomotores/patologia , Percepção Visual/fisiologia
19.
Radiology ; 203(1): 181-5, 1997 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-9122390

RESUMO

PURPOSE: To evaluate the feasibility of magnetic resonance (MR) perfusion imaging in the human lung to help detect perfusion defects distal to suspected pulmonary embolism. MATERIALS AND METHODS: Seven patients suspected of having pulmonary embolism first underwent ventilation-perfusion lung scintigraphy followed by MR perfusion imaging with rapid acquisition of two sets of dynamic images in the coronal and transaxial planes. A bolus of 0.05 mmol per kilogram of body weight gadopentetate dimeglumine or gadodiamide was administered. Single images obtained in each section that showed peak signal intensity from the first passage of contrast material were evaluated visually. An analysis of change in signal intensity over time was performed both on a pixel-by-pixel basis and in selected regions of interest. RESULTS: In the seven patients, a total of 18 regions of lung tissue with perfusion defects were shown on the ventilation-perfusion scans. In 16 of these regions, MR perfusion images showed a reduced peak signal intensity during first passage of the contrast agent. Perfusion defects could be detected in both the coronal and the transaxial planes on MR perfusion images. CONCLUSION: MR perfusion imaging was feasible for detection of perfusion defects distal to a pulmonary embolism.


Assuntos
Imageamento por Ressonância Magnética , Embolia Pulmonar/diagnóstico , Adulto , Idoso , Meios de Contraste , Combinação de Medicamentos , Estudos de Viabilidade , Feminino , Gadolínio , Gadolínio DTPA , Humanos , Pulmão/diagnóstico por imagem , Pulmão/patologia , Masculino , Meglumina , Pessoa de Meia-Idade , Compostos Organometálicos , Ácido Pentético/análogos & derivados , Embolia Pulmonar/diagnóstico por imagem , Cintilografia , Relação Ventilação-Perfusão
20.
Mol Plant Microbe Interact ; 10(2): 215-20, 1997 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-9057327

RESUMO

Nod factors secreted by Rhizobium leguminosarum by, viciae induce root hair deformation, the formation of nodule primordia, and the expression of early nodulin genes in Vicia sativa (vetch). Root hair deformation is induced within 3 h in a small, susceptible zone (+/-2 mm) of the root. NH4NO3, known to be a potent blocker of nodule formation, inhibits root hair deformation, initial cortical cell divisions, and infection thread formation. To test whether NH4NO3 affects the formation of a component of the Nod factor perception-transduction system, we studied Nod factor-induced gene expression. The differential display technique was used to search for marker genes, which are induced within 1 to 3 h after Nod factor application. Surprisingly, one of the isolated cDNA clones was identified as a leghemoglobin gene (VsLb1), which is induced in vetch roots within 1 h after Nod factor application. By using the drug brefeldin A, it was then shown that VsLb1 activation does not require root hair deformation. The pVsLb1 clone was used as a marker to show that in vetch plants grown in the presence of NH4NO3, Nod factor perception and transduction leading to gene expression are unaffected.


Assuntos
Fabaceae/genética , Fabaceae/microbiologia , Leghemoglobina/genética , Lipopolissacarídeos/farmacologia , Proteínas de Membrana , Plantas Medicinais , Sequência de Bases , DNA de Plantas/genética , Fabaceae/efeitos dos fármacos , Expressão Gênica/efeitos dos fármacos , Genes de Plantas/efeitos dos fármacos , Marcadores Genéticos , Dados de Sequência Molecular , Nitratos/farmacologia , Proteínas de Plantas/genética , Rhizobium leguminosarum/fisiologia
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