Physical properties of magnetic domain walls in Cd2Os2O7

学位の種別: 課程博士審査委員会委員 : （主査）東京大学教授 廣井 善二, 東京大学教授 有馬 孝尚, 東京大学教授 芝内 孝禎, 東京大学教授 求 幸年, 東京大学准教授 岡本 敏宏University of Tokyo(東京大学

Fermi surface with Dirac fermions in CaFeAsF determined via quantum oscillation measurements

Despite the fact that 1111-type iron arsenides hold the record transition temperature of iron-based superconductors, their electronic structures have not been studied much because of the lack of high-quality single crystals. In this study, we completely determine the Fermi surface in the antiferromagnetic state of CaFeAsF, a 1111 iron-arsenide parent compound, by performing quantum oscillation measurements and band-structure calculations. The determined Fermi surface consists of a symmetry-related pair of Dirac electron cylinders and a normal hole cylinder. From analyses of quantum-oscillation phases, we demonstrate that the electron cylinders carry a nontrivial Berry phase $\pi$. The carrier density is of the order of 10$^{-3}$ per Fe. This unusual metallic state with the extremely small carrier density is a consequence of the previously discussed topological feature of the band structure which prevents the antiferromagnetic gap from being a full gap. We also report a nearly linear-in-$B$ magnetoresistance and an anomalous resistivity increase above about 30 T for $B \parallel c$, the latter of which is likely related to the quantum limit of the electron orbit. Intriguingly, the electrical resistivity exhibits a nonmetallic temperature dependence in the paramagnetic tetragonal phase ($T >$ 118 K), which may suggest an incoherent state. Our study provides a detailed knowledge of the Fermi surface in the antiferromagnetic state of 1111 parent compounds and moreover opens up a new possibility to explore Dirac-fermion physics in those compounds.Comment: 11 pages, 7 figures, 1 tabl

In-plane electronic anisotropy revealed by interlayer resistivity measurements on the iron-based superconductor parent compound CaFeAsF

Both cuprates and iron-based superconductors demonstrate nematicity, defined as the spontaneous breaking of rotational symmetry in electron systems. The nematic state can play a role in the high-transition-temperature superconductivity of these compounds. However, the microscopic mechanism responsible for the transport anisotropy in iron-based compounds remains debatable. Here, we investigate the electronic anisotropy of CaFeAsF by measuring its interlayer resistivity under magnetic fields with varying field directions. Counterintuitively, the interlayer resistivity was larger in the longitudinal configuration ($B \parallel I \parallel c$) than in the transverse one ($B \perp I \parallel c$). The interlayer resistivity exhibited a so-called coherence peak under in-plane fields and was highly anisotropic with respect to the in-plane field direction. At $T$ = 4 K and $B$ = 14 T, the magnetoresistance $\Delta\rho/\rho_0$ was seven times larger in the $B \parallel b_o$ than in the $B \parallel a_o$ configuration. Our theoretical calculations of the conductivity based on the first-principles electronic band structure qualitatively reproduced the above observations but underestimated the magnitudes of the observed features. The proposed methodology can be a powerful tool for probing the nematic electronic state in various materials.Comment: 25 pages, 5 figure

Magnetic gap of fe-doped BiSbTe₂Se bulk single crystals detected by tunneling spectroscopy and gate-controlled transports

Topological insulators with broken time-reversal symmetry and the Fermi level within the magnetic gap at the Dirac cone provides exotic topological magneto-electronic phenomena. Here, we introduce an improved magnetically doped topological insulator, Fe-doped BiSbTe2Se (Fe-BSTS) bulk single crystal, with an ideal Fermi level. Scanning tunneling microscopy and spectroscopy (STM/STS) measurements revealed that the surface state possesses a Dirac cone with the Dirac point just below the Fermi level by 12 meV. The normalized dI/dV spectra suggest a gap opening with Δmag ~55 meV, resulting in the Fermi level within the opened gap. Ionic-liquid gated-transport measurements also support the Dirac point just below the Fermi level and the presence of the magnetic gap. The chemical potential of the surface state can be fully tuned by ionic-liquid gating, and thus the Fe-doped BSTS provides an ideal platform to investigate exotic quantum topological phenomena.</p

Coupled spin-charge-phonon fluctuation in the all-in/all-out antiferromagnet Cd2Os2O7

We report on a spin-charge fluctuation in the all-in/all-out pyrochlore magnet Cd2Os2O7, where the spin fluctuation is driven by the conduction of thermally excited electrons/holes and associated fluctuation of Os valence. The fluctuation exhibits an activation energy significantly greater than the spin-charge excitation gap and a peculiar frequency range of 10(6)-10(10) s(-1). These features are attributed to the hopping motion of carriers as small polarons in the insulating phase, where the polaron state is presumably induced by the magnetoelastic coupling via the strong spin-orbit interaction. Such a coupled spin-charge-phonon fluctuation manifests as a part of the metal-insulator transition that is extended over a wide temperature range due to the modest electron correlation comparable with other interactions characteristic for 5d-subshell systems