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日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

IN two previous notes1, Prof. Max Born and I have shown that one can obtain a theory of superconductivity by taking account of the fact that the interaction of the electrons with the ionic lattice is appreciable only near the boundaries of Brillouin zones, and particularly strong near the corners of these. This leads to the criterion that the metal should be superconductive if a set of corners of a Brillouin zone is lying very near the Fermi surface, considered as a sphere, which limits the region in the momentum space completely filled with electrons.

On the theory of superconductivity

We show that the pseudorelativistic physics of graphene near the Fermi level can be extended to three dimensional (3D) materials. Unlike in phase transitions from inversion symmetric topological to normal insulators, we show that particular space groups also allow 3D Dirac points as symmetry protected degeneracies. We provide criteria necessary to identify these groups and, as an example, present ab initio calculations of β-cristobalite BiO(2) which exhibits three Dirac points at the Fermi level. We find that β-cristobalite BiO(2) is metastable, so it can be physically realized as a 3D analog to graphene.

Dirac Semimetal in Three Dimensions

AES, XPS and SIMS characterization of YBa2Cu3O7 superconducting high Tc thin films

Structural and electronic effects of Sr substitution for Ba in Y(Ba 1-x Sr x ) 2 Cu 3 O w at varying w

To study the origin of the magnetic transition in $\mathrm{Ba}\mathrm{V}{\mathrm{S}}_{3}$, we measured the dc magnetization on ${\mathrm{La}}_{0.1}{\mathrm{Ba}}_{0.9}\mathrm{V}{\mathrm{S}}_{3}$ sintered samples near the ferromagnetic transition. Effective critical exponents and amplitudes are obtained by means of standard modified Arrot plots and the Kouvel-Fisher method, while asymptotic critical exponents and amplitudes are obtained by means of ``correction to scaling'' analysis. The two methods yield similar results, but the latter is superior with respect to the achieved uncertainty. The obtained results indicate that the universality class of magnetic interactions driving the transition changes from short to long range when cooling through ${T}_{c}$.

Critical exponents and amplitudes of the ferromagnetic transition in La 0.1 Ba 0.9 V S 3

There has been increasing interest in materials where relativistic effects induce non-trivial electronic states with promise for spintronics applications. One example is the splitting of bands with opposite spin chirality produced by the Rashba spin-orbit coupling in asymmetric potentials. Sizable splittings have been hitherto obtained using either heavy elements, where this coupling is intrinsically strong, or large surface electric fields. Here by means of angular resolved photoemission spectroscopy and first-principles calculations, we give evidence of a large Rashba coupling of 0.25 eV A, leading to a remarkable band splitting up to 0.15 eV with hidden spin-chiral polarization in centrosymmetric BaNiS2. This is explained by a huge staggered crystal field of 1.4 V A(-1), produced by a gliding plane symmetry, that breaks inversion symmetry at the Ni site. This unexpected result in the absence of heavy elements demonstrates an effective mechanism of Rashba coupling amplification that may foster spin-orbit band engineering.

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Rashba coupling amplification by a staggered crystal field

We studied the structural and physical properties of powder samples of the high-pressure ${\text{LaMn}}_{7}{\text{O}}_{12}$ compound by means of neutron diffraction, dc magnetization, specific-heat, and dc electrical resistivity measurements. Our structural refinement shows that ${\text{LaMn}}_{7}{\text{O}}_{12}$ is the counterpart with quadruple perovskite structure of the well-known simple-perovskite ${\text{LaMnO}}_{3}$ because both compounds are single-valent ${\text{Mn}}^{3+}$ systems sharing a similar pseudocubic network of buckled corner-sharing ${\text{MnO}}_{6}$ octahedra distorted by the Jahn-Teller (JT) effect. Besides this similarity, ${\text{LaMn}}_{7}{\text{O}}_{12}$ exhibits the following structural differences: (i) a monoclinic $I2/m$, instead of orthorhombic $Pnma$, structure; (ii) a much larger buckling of the ${\text{MnO}}_{6}$ octahedra, corresponding to a Mn-O-Mn bond angle as small as $\ensuremath{\psi}=180\ifmmode^\circ\else\textdegree\fi{}\ensuremath{-}\ensuremath{\phi}=136\ifmmode^\circ\else\textdegree\fi{}$. At ${T}_{N,B}=78\text{ }\text{K}$, the neutron data show evidence of an antiferromagnetic (AFM) structure of the octahedral ${\text{Mn}}^{3+}$ ions ($B$ sites), which consists of ferromagnetically coupled antiferromagnetic $ac$ planes. This structure, commonly referred to as $C$ type, markedly differs from the $A$-type structure of ${\text{LaMnO}}_{3}$, which consists of antiferromagnetically coupled ferromagnetic $ac$ planes. We argue that this difference may be due not only to the different buckling, because the magnetic superexchange interaction is sensitive to $\ensuremath{\phi}$, but also to the existence of two distinct JT distorted $B$ sites, a characteristic feature of the quadruple perovskite structure not found in ${\text{LaMnO}}_{3}$. A further feature of this structure is the presence of a pseudocubic sublattice of JT ${\text{Mn}}^{3+}$ ions with square coordination (${A}^{\ensuremath{'}}$ site) in addition to the $B$-site sublattice. At ${T}_{N,{A}^{\ensuremath{'}}}=21\text{ }\text{K}$, the ${A}^{\ensuremath{'}}$ sublattice of ${\text{LaMn}}_{7}{\text{O}}_{12}$ is found to form a second AFM structure consisting of AFM coupled ferromagnetic planes. This additional ordering appears to occur independently of that of the $B$ sublattice. Finally, we observed a thermally activated insulating behavior of the resistivity similar to that previously reported for ${\text{LaMnO}}_{3}$, which confirms the hypothesis that the superexchange interaction is predominant in single-valent systems while the double-exchange interaction is relevant only in mixed-valence or disordered systems.

Andrea Gauzzi

Papers

AES, XPS and SIMS characterization of YBa2Cu3O7 superconducting high Tc thin films

Structural and electronic effects of Sr substitution for Ba in Y(Ba 1-x Sr x ) 2 Cu 3 O w at varying w

Critical exponents and amplitudes of the ferromagnetic transition in La 0.1 Ba 0.9 V S 3

Rashba coupling amplification by a staggered crystal field

Magnetic structure of the high-density single-valent e g Jahn-Teller system LaMn 7 O 12