Electron microscopy of frozen biological suspensions

The methodology for preparing specimens in the frozen, hydrated state has been assessed using crystals and T4 bacteriophages. The methods have also been demonstrated with lambda bacteriophages, purple membrane of Halobacterium halobium and fibres of DNA. For particles dispersed in an aqueous environment, it is shown that optimum structural preservation is obtained from a thin, quench-frozen film with the bulk aqueous medium in the vitreous state. Crystallization of the bulk water may result in solute segregation and expulsion of the specimen from the film. Contrast measurements can be used to follow directly the state of hydration of a specimen during transition from the fully hydrated to the freeze-dried state and permit direct measurement of the water content of the specimen. By changing the concentration and composition of the aqueous medium the contrast of particles in a vitreous film can be controlled and any state of negative, positive or zero contrast may be obtained. At 100 K, frozen-hydrated, freeze-dried or sugar embedded crystals can withstand a three- to four-fold increase in electron exposure for the same damage when compared with similar sugar-embedded or freeze-dried samples at room temperature

Evidence for a small hole pocket in the Fermi surface of underdoped YBa2Cu3Oy

The Fermi surface of a metal is the fundamental basis from which its properties can be understood. In underdoped cuprate superconductors, the Fermi surface undergoes a reconstruction that produces a small electron pocket, but whether there is another, as yet undetected portion to the Fermi surface is unknown. Establishing the complete topology of the Fermi surface is key to identifying the mechanism responsible for its reconstruction. Here we report the discovery of a second Fermi pocket in underdoped YBa2Cu3Oy, detected as a small quantum oscillation frequency in the thermoelectric response and in the c-axis resistance. The field-angle dependence of the frequency demonstrates that it is a distinct Fermi surface and the normal-state thermopower requires it to be a hole pocket. A Fermi surface consisting of one electron pocket and two hole pockets with the measured areas and masses is consistent with a Fermi-surface reconstruction caused by the charge-density-wave order observed in YBa2Cu3Oy, provided other parts of the reconstructed Fermi surface are removed by a separate mechanism, possibly the pseudogap.Comment: 23 pages, 5 figure

Cryo-electron microscopy of viruses

Thin vitrified layers of unfixed, unstained and unsupported virus suspensions can be prepared for observation by cryo-electron microscopy in easily controlled conditions. The viral particles appear free from the kind of damage caused by dehydration, freezing or adsorption to a support that is encountered in preparing biological samples for conventional electron microscopy. Cryo-electron microscopy of vitrified specimens offers possibilities for high resolution observations that compare favourably with any other electron microscopical method

Fermi Surface of the Electron-doped Cuprate Superconductor Nd_{2-x}Ce_xCuO_{4} Probed by High-Field Magnetotransport

We report on the study of the Fermi surface of the electron-doped cuprate superconductor Nd$_{2-x}$Ce$_x$CuO$_{4}$ by measuring the interlayer magnetoresistance as a function of the strength and orientation of the applied magnetic field. We performed experiments in both steady and pulsed magnetic fields on high-quality single crystals with Ce concentrations of $x=0.13$ to 0.17. In the overdoped regime of $x > 0.15$ we found both semiclassical angle-dependent magnetoresistance oscillations (AMRO) and Shubnikov-de Haas (SdH) oscillations. The combined AMRO and SdH data clearly show that the appearance of fast SdH oscillations in strongly overdoped samples is caused by magnetic breakdown. This observation provides clear evidence for a reconstructed multiply-connected Fermi surface up to the very end of the overdoped regime at $x\simeq 0.17$. The strength of the superlattice potential responsible for the reconstructed Fermi surface is found to decrease with increasing doping level and likely vanishes at the same carrier concentration as superconductivity, suggesting a close relation between translational symmetry breaking and superconducting pairing. A detailed analysis of the high-resolution SdH data allowed us to determine the effective cyclotron mass and Dingle temperature, as well as to estimate the magnetic breakdown field in the overdoped regime.Comment: 23 pages, 8 figure

Hall, Seebeck, and Nernst Coefficients of Underdoped HgBa2CuO4+d: Fermi-Surface Reconstruction in an Archetypal Cuprate Superconductor

Charge density-wave order has been observed in cuprate superconductors whose crystal structure breaks the square symmetry of the CuO2 planes, such as orthorhombic YBa2Cu3Oy (YBCO), but not so far in cuprates that preserve that symmetry, such as tetragonal HgBa2CuO4+d (Hg1201). We have measured the Hall (R_H), Seebeck (S), and Nernst coefficients of underdoped Hg1201 in magnetic fields large enough to suppress superconductivity. The high-field R_H(T) and S(T) are found to drop with decreasing temperature and become negative, as also observed in YBCO at comparable doping. In YBCO, the negative R_H and S are signatures of a small electron pocket caused by Fermi-surface reconstruction, attributed to charge density-wave modulations observed in the same range of doping and temperature. We deduce that a similar Fermi-surface reconstruction takes place in Hg1201, evidence that density-wave order exists in this material. A striking similarity is also found in the normal-state Nernst coefficient, further supporting this interpretation. Given the model nature of Hg1201, Fermi-surface reconstruction appears to be common to all hole-doped cuprates, suggesting that density-wave order is a fundamental property of these materials

Semiflexible chains in confined spaces

We develop an analytical method for studying the properties of a noninteracting wormlike chain (WLC) in confined geometries. The mean-field-like theory replaces the rigid constraints of confinement with average constraints, thus allowing us to develop a tractable method for treating a WLC wrapped on the surface of a sphere, and fully encapsulated within it. The efficacy of the theory is established by reproducing the exact correlation functions for a WLC confined to the surface of a sphere. In addition, the coefficients in the free energy are exactly calculated. We also describe the behavior of a surface-confined chain under external tension that is relevant for single molecule experiments on histone-DNA complexes. The force-extension curves display spatial oscillations, and the extension of the chain, whose maximum value is bounded by the sphere diameter, scales as f−1 at large forces, in contrast to the unconfined chain that approaches the contour length as f−1∕2. A WLC encapsulated in a sphere, that is relevant for the study of the viral encapsulation of DNA, can also be treated using the mean-field approach. The predictions of the theory for various correlation functions are in excellent agreement with Langevin simulations. We find that strongly confined chains are highly structured by examining the correlations using a local winding axis. The predicted pressure of the system is in excellent agreement with simulations but, as is known, is significantly lower than the pressures seen for DNA packaged in viral capsids

Etude de la reconstruction de la surface de Fermi des cuprates supraconducteurs dopés en trous

Cette thèse porte sur l'étude des propriétés électroniques des cuprates, supraconducteurs à haute température critique. Le diagramme de phase de cette famille de composé est très riche. En dopant ces composés, il est possible de changer le nombre de porteurs et ainsi d'explorer leur diagramme de phase. L'une des questions est de caractériser l'état fondamental à basse température dans la partie sous-dopée en trous du diagramme de phase. Ces dernières années, des oscillations quantiques ont été mesurées dans les cuprates, suggérant un état fondamental de type liquide de Fermi. Les oscillations observées dans le composé YBCO combinées à des mesures d'effet Hall et d'effet Seebeck montrent la présence de petites poches d'électron couvrant moins de 2 % de la première zone de Brillouin, 30 fois plus petites que la grande poche de trou prédite par les calculs de structure de bandes. L'approche de cette thèse est de considérer une reconstruction de la surface de Fermi provenant d'un ordre de charge bi-axial, comme le suggèrent de récentes mesures de RMN, de rayons X et d'ultrasons. Des mesures de transport et d'aimantation en champ magnétique pulsé jusqu'à 81 T ont été effectuées dans YBCO et dans Hg-1201à des températures allant de 1.5 K à 300 K. Cette thèse est divisée en trois parties : D'une part nous présentons des mesures d'oscillations quantiques dans YBCO sous champ magnétique pulsé pour des dopages compris entre 9.8 % et 12.3 %. L'analyse nous a permis de proposer une topologie de la surface de Fermi, qui est constituée de poches de type électron et de poche(s) de type trou. Un lien est établi entre les récentes mesures d'ordre de charge, la reconstruction de la surface de Fermi et la taille des poches mesurées. Ce scénario de reconstruction est également commenté au regard des autres sondes expérimentales et comparé aux autres scénarii envisageables. Une deuxième partie montre la restauration de la cohérence du transport interplan en dessous d'une température notée Tcoh sous champ magnétique, pour des échantillons de YBCO dont le dopage est compris entre 8.4 % et 12 %. Cette restauration de la cohérence est interprétée comme une conséquence de la reconstruction de la surface de Fermi. Enfin la dernière partie est consacrée à des mesures d'effet Hall dans Hg-1201, un système cuprate modèle. Nous avons mesuré un signe négatif de l'effet Hall à basse température et sous champ magnétique intense, mettant en évidence la présence de poches d'électron dans la surface de Fermi de ce composé. Par analogie avec YBCO, nous avons proposé qu'un mécanisme de reconstruction de la surface de Fermi par une onde de densité apparaît à basse température dans Hg-1201.This thesis focuses on the study of the electronic properties of the high temperature cuprate superconductors. The phase diagram of these compounds is very rich. By doping these compounds, it is possible to change the number of carriers and thus explore their phase diagram. One question is to characterize the ground state at low temperature in the underdoped part of the phase diagram. In recent years, quantum oscillations were measured in cuprates, suggesting a Fermi liquid-like ground state. The oscillations observed in the compound YBCO combined with Hall effect and Seebeck effect measurements show the presence of small electron pockets covering less than 2 % of the first Brillouin zone , 30 times smaller than the large hole pocket predicted by the band structure calculations. The approach of this thesis is to consider a reconstruction of the Fermi surface by a bi-axial charge density wave, as suggested by recent NMR, X-ray and ultrasound measurements. Transport and magnetization measurements in pulsed magnetic fields up to 81 T were performed in YBCO and in Hg-1201 at temperatures ranging from 1.5 K to 300 K. This thesis is divided into three parts : In the first part, we present measurements of quantum oscillations in YBCO under pulsed magnetic field for doping levels between 9.8 % and 12.3 %. The analysis allowed us to propose a particular topology of Fermi surface, which consists of electron and hole pockets. A link is established between the charge order, the reconstruction of the Fermi surface and the size of the pockets. This scenario of the reconstruction of the Fermi surface is discussed in line with other experimental probes and compared to other possible scenarios. The second part shows the restoration of the c-axis transport in YBCO with doping level between 8.4 % and 12 % below a temperature Tcoh and in a magnetic field. The restoration of the c-axis coherence is interpreted as a consequence of the reconstruction of the Fermi surface. The last part is devoted to Hall effect measurements in Hg-1201, a model cuprate. We have measured a negative Hall effect at low temperatures and in high magnetic fields, highlighting the presence of electron pockets in the Fermi surface of this compound. By analogy with YBCO, we have proposed a mechanism for reconstruction of the Fermi surface with a density wave appearing at low temperature in Hg-1201

Strain-Dependent Migration of CD4 and CD8 Lymphocyte Subsets to Lymph Nodes in NOD (Nonobese Diabetic) and Control Mice

Subpopulations of lymphoid cells were compared with respect to their ability to migrate into peripheral lymphoid organs of nonobese diabetic (NOD) mice and various strains of control mice. In short-term, in vivo homing studies, no major differences in the pattern of homing of B and T cells were observed among all mouse strains studied. On the other hand, CD4 cells localized consistently more efficiently than CD8 cells in both PP and LN of adult NOD and BALB/c mice, whereas both populations migrated roughly equivalently in LN of adult DBA/2, CBA, and C57BL/6 mice. No age-dependent differences in the homing of CD4 and CD8 cells were observed in BALB/c mice. On the contrary, in 2-week-old NOD mice, CD4 and CD8 cells migrated equally well. The preferential entry of CD4 cells in adult NOD and BALB/c did not result from increased blood transit time of CD8 cells. On the other hand, the preferential migration of CD8 cells was observed in the liver, whereas the two T-cell subsets migrated equally well in the lungs. The differences in the homing characteristics of CD4 and CD8 cells among NOD, BALB/c, and C57BL/6 mice were not related to modifications in the level of expression of adhesion molecules such as MEL-14, LFA-1, and Pgp-1