Professeur agrégé


Filed in: Blog.Main · Modified on : Thu, 17 Sep 20

A recent article appeared on the arXiv with the title "Experimental signatures of a three-dimensional quantum spin liquid in effective spin-1/2 Ce2Zr2O7 pyrochlore", which is similar to what we measured in our paper entitled "Quantum Spin Ice Dynamics in the Dipole-Octupole Pyrochlore Magnet Ce2Zr2O7".

Critical current is a defining parameter of a SC state. It describe the current under which there is a zero-resistance state. With no magnetic field, and under pressure variation, the critical current exhibit universal behaviour, related to the underlying critical fluctuations associated with the QCP. This paper demonstrate by studying critical current behaviour that this parameter is a powerful tool for investigating the presence of QCP in the SC state without destroying the SC state itself. This would not be possible, for say, by applying a magnetic field.

Link to article :

Recent study shows that an upward dispersion observed in a neutron spin resonance could be associated with an AFM order proximity in an superconducting phase of CeCoIn5 doped with Yb.

Magnetic excitations are commonly observed in hole doped copper oxide superconductor and pnictide Ni-underdoped. In the former, the resonance is associated with a downward dispersion as in the latter, the resonance comes with an upward dispersion. In both cases, the origin and the behaviour of this resonance is well described by the spin-exciton scenario.

Measurements made in Yb doped CeCoIn5 show, on the other hand, a dispersion associated to the resonance that cannot be explained by the spin-exciton scenario. This conclusion comes from the robustness of this feature to hole doping and the uncommensurate upward dispersion similar to a spin wave in antiferromagnetically order CeRhIn5.

Title : Theoretical and experimental study of second harmonic generation from the surface of the topological insulator Bi2Se3 Author : J. W. McIver et al. DOI : 10.1103/PhysRevB.86.035327

The recent discovery of 3D topological insulator phases in Bi1-xSbx, Bi2Se3 and related materials has generated great interest to measure their symmetry and electrical properties at an isolated surface or buried interface. However, a major experimental obstacle has been the high density of mobile electrons in the bulk of these materials, which can overwhelm the surface or interface electrical responses. Although transport results on electrically gated samples show evidence of surface carrier modulation, the contributions to the electrical response from carriers on different surfaces and in the bulk are difficult to separate and require highly insulating samples. Moreover, contacts and gates deposited on the surface may perturb the intrinsic surface electronic structure. Optical probes have been proposed as a contact-free alternative that can be focused onto a single surface.

Recently, nonlinear second harmonic generation (SHG) of light from bulk single crystals of Bi2Se3 was shown to be highly sensitive to electrons confined to the surface and accumulation region. The underlying principle for this surface sensitivity is that SHG is predominantly generated where inversion symmetry is broken, which only occurs at the surface of the bulk inversion-symmetric Bi2Se3 and in the accumulation region, where the band-bendinginduced electric field breaks the inversion symmetry.

In this work, they develop a theoretical model that describes the SHG intensity from Bi2Se3 in terms of the second- and third-order nonlinear electric susceptibilities. By performing a symmetry analysis of Bi2Se3 they identify the susceptibility tensor elements that contribute to SHG and show that their relative magnitudes can be determined by measuring the intensity and polarization of the emitted SHG as a function of crystal orientation and incident laser polarization.

Link :

When spins decide to do weird things: Skyrmions and Helimagnet

Today, we will learn about chiral magnets and their skyrmions lattices. First of all, chiral magnets are form by a long-range helimagnetic order of the magnetic moments. This order exist at zero field and below a critical temperature. The order is form by a competition between ferromagnetic exchange, Dzyaloshinsky-Moriya(DM) interaction and anisotropy. When a field is applied, the helices point along the field and this produce the conical phase. In this phase, just below Tc, a skyrmion lattice correlations are stable and metastable is a larger region of the phase diagram. I will talk about a SANS, NSE and TEM studies done on Fe_(1-x)Co_xSi.

DOI: 10.1103/PhysRevB.95.144433 DOI:10.1103/PhysRevB.89.09441 DOI:10.1038/nature09124

Title : Structural distortion-induced magnetoelastic locking in Sr2IrO4 revealed through nonlinear optical harmonic generation Author : D. H. Torchinsky et al. DOI : 10.1103/PhysRevLett.114.096404

The Sr2IrO4 was predicted to be an unconventional superconductor upon chemical doping with a pseudo gap behavior because of its crystallographic, electronic and magnetic structure to the parent compound La2CuO4 of the high-Tc cuprates. But it's unclear why no signature of superconductivity are detected. Recent experiments suggest that the structural and magnetic properties of Sr2IrO4 are not completely understood, in particular neutron diffraction studies report new Bragg peaks that challenge its long accepted crystal structure. On the other hand, resonant x-ray diffraction studies report a near perfect locking of the magnetic moment canting and oxygen octahedra rotation angles that cannot be fully explained by existing theoretical models.

In this letter, they report a global bulk structural distortion in Sr2IrO4 observed using a combination of spatially resolved optical second harmonic generation (SHG) and third harmonic generation (THG) experiments. Nonlinear optical harmonic generation is a process by which light of frequency "w" is converted into higher harmonics "nw" (n=2,3,4...) through its nonlinear interaction with a material. By Neumann's principle, the nonlinear optical susceptibility tensors that relate the incident electric field to induced electric dipole, electric quadrupole, magnetic dipole, or even higher order multiple densities, which act as sources of higher harmonic radiation and must be invariant under every symmetry operation of the crystal. The structure of the susceptibility therefore encodes the symmetries of a crystal, with higher rank allowing for more accurate levels of refinement. The components of the susceptibility can be measured through rotational anisotropy (RA) experiments, where the intensity of high harmonic light reflected from a crystal is recorded as a function of the angle subtended between the light scattering plane and a crystalline axis. Different tensor components are probed by selecting the incident and reflected light to be either P of S polarized.

Previous work have assigned Sr2IrO4 to a centrosymmetric tetragonal 4/mmm crystallographic point group (I41/acd space group). However, recent single crystal neutron diffraction studies observe additional nuclear Bragg peaks that violate the I41/acd space group. These forbidden peaks may originate from structural defects such as oxygen vacancies that distort the local symmetry or from a subtle global symmetry reduction. Although a number of alternative centrosymetric and noncentrosymetric space groups have been proposed. The RA-SHG and RA-THG patterns together show that Sr2IrO4 exhibits a globally reduced bulk structural symmetry that is best describe by the I41/a space group.

Link :

The program I will be working on is called Quanty. One of the main things it can calculate are the core level spectroscopy of correlated transition metal and rare earth compounds. Crystal field theory is used to describe the spectroscopies of those transition metal and rare earth compounds as it describes the spilttings of ionic levels. However, the energy level degeneracy are related to only the symmetry of the Hamiltonian and is thus a consequence of the symmetry of the crystal field. To determine if crystal field splitting occurs, we use group theory. In fact, determining the dimensions of the irreducible representation of a group will give the possible degeneracies. Furthermore, if we find that a representation is reducible, then we will have crystal field splittings of the energy levels.

The study of the lattice defects of A2B2O7 pyrochlores is of particular interest, especially in the fluorite-pyrochlore boundaries. This Tuesday, I will be talking about a study of the mechanisms responsible for the nonstoichiometry of the oxygen content in A2B2O7 pyrochlores. Using a Mott-Littleton like approach to calculate the displacement of the ions paired with a Dick-Overhausser shell model for polarisation, they calculated the energies associated with the the different mechanisms that the surplus of oxygen in the compound. They did so for a large variety rare-earths A and metals B, plotting contour maps for the 6 lowest energy mechanisms. They also studied the effect of defect clustering on the energy associated with the 3 most-likely mechanism responsible for the nonstoichiometry.


The Hexaboride's Family: So close yet so much different

Every family is different. The members look the same but they can be very very different. In a family, there is the main group, the one that you know the most and that you see frequently. There is always a member that likes to brag about how much he's famous, one that you can always relate to, the weird one, the three musketeers that do the same things, a soon to be famous and the other one that is close to your heart. After that, there are the distant cousins that pretty much do the same things but they live far from you. Finally, there is the lone wolf, the one that lives close to your cousins but do completely different things.

Well, the hexaboride's family is like that. The main group:

LaB6: The simple metal aka the relatable
CeB6: Quadrupolar ordering and dense Kondo system aka the weird one
PrB6, GdB6 and TbB6: Antiferromagnets aka the three musketeers
SmB6: Kondo topological insulator aka the famous one
YbB6: Predicted topological insulator aka the wannabe
EuB6: Magnetic semi-conductor and magnetic polarons aka the love one

The distant cousins:

CaB6, SrB6 and BaB6: Ferromagnets when doped

The lone wolf:

YB6: Supraconductor

Since their discovery, cobaltites (of the form LnBaCo4O7) have been studied as a class of geometrically frustrated magnets. Their structure consist of alternating layers of triangular and kagomé lattices composed of only CoO4 tetrahedra. The cobaltite studied here is CaBaCo4O7. A special property of this compound is that it shows ferrimagnetic properties at temperatures below Tc = 70 K. Furthermore, they found using neutron powered diffraction that CaBaCo4O7 has the largest structural distortion in the cobaltite series. Here is discussed how the combination of the structural distortion, the cobalt valence and the charge ordering leads to a ferrimagnetic interaction between the layers. In fact, while within a layer the different cobalt sites interacts ferromagnetically, they interact antiferromagnetically with a different cobalt site on the next layer. This explains the lifting of geometrical frustration to ferrimagnetism.

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