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Register a new userTwo dimensional magnetic correlation in the unconventional corrugated layered oxides (Ba,Sr)$_4$Mn$_3$O$_{10}$
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Both Ba$_4$Mn$_3$O$_{10}$ and Sr$_4$Mn$_3$O$_{10}$ crystallize in an orthorhombic crystal structure consisting of corrugated layers containing Mn$_3$O$_{12}$ polydedra. The thermal variation of magnetic susceptibility of the compositions consists of a broad hump like feature indicating the presence of low dimensional magnetic correlation. We have systematically investigated the magnetic data of these compounds and found that the experimental results match quite well with the two dimensional Heisenberg model of spin-spin interaction. The two dimensional nature of the magnetic spin-spin interaction is supported by the low temperature heat capacity data of Ba$_4$Mn$_3$O$_{10}$. Interestingly, both the samples show dielectric anomaly near the magnetic ordering temperature indicating multiferroic behavior.
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We report detailed neutron scattering studies on Ba$_2$Cu$_3$O$_4$Cl$_2$. The compound consists of two interpenetrating sublattices of Cu, labeled as Cu$_{rm A}$ and Cu$_{rm B}$, each of which forms a square-lattice Heisenberg antiferromagnet. The two sublattices order at different temperatures and effective exchange couplings within the sublattices differ by an order of magnitude. This yields an inelastic neutron spectrum of the Cu$_{rm A}$ sublattice extending up to 300 meV and a much weaker dispersion of Cu$_{rm B}$ going up to around 20 meV. Using a single-band Hubbard model we derive an effective spin Hamiltonian. From this, we find that linear spin-wave theory gives a good description to the magnetic spectrum. In addition, a magnetic field of 10 T is found to produce effects on the Cu$_{rm B}$ dispersion that cannot be explained by conventional spin-wave theory.
Triple-layered ruthenate Sr$_4$Ru$_3$O$_{10}$ shows a first-order itinerant metamagnetic transition for in-plane magnetic fields. Our experiments revealed rather surprising behavior in the low-temperature transport properties near this transition. The in-plane magnetoresistivity $rho$$_{ab}$(H) exhibits ultrasharp steps as the magnetic field sweeps down through the transition. Temperature sweeps of $rho$$_{ab}$ for fields within the transition regime show non-metallic behavior in the up-sweep cycle of magnetic field, but show a significant drop in the down-sweep cycle. These observations indicate that the transition occurs via a new electronic phase separation process; a lowly polarized state is mixed with a ferromagnetic state within the transition regime.
Sr$_4$Ru$_3$O$_{10}$, the $n$ = 3 member of the Ruddlesden-Popper type ruthenate Sr$_{n+1}$Ru$_n$O$_{3n+1}$, is known to exhibit a peculiar metamagnetic transition in an in-plane magnetic field. However, the nature of both the temperature- and field-dependent phase transitions remains as a topic of debate. Here, we have investigated the magnetic transitions of Sr$_4$Ru$_3$O$_{10}$ via single-crystal neutron diffraction measurements. At zero field, we find that the system undergoes a ferromagnetic transition with both in-plane and out-of-plane magnetic components at $T_{c}$ ~ 100 K. Below $T^{*}$ ~ 50 K, the magnetic moments incline continuously toward the out-of-plane direction. At $T$ ~ 1.5 K, where the spins are nearly aligned along the $c$ axis, a spin reorientation occurs above a critical field $B_c$, giving rise to a spin component perpendicular to the plane defined by the field direction and the $c$ axis. We suggest that both the temperature- and field-driven spin reorientations are associated with a change in the magnetocrystalline anisotropy, which is strongly coupled to the lattice degrees of freedom. This study elucidates the long-standing puzzles on the zero-field magnetic orders of Sr$_4$Ru$_3$O$_{10}$ and provides new insights into the nature of the field-induced metamagnetic transition.
Relativistic massless Dirac fermions can be probed with high-energy physics experiments, but appear also as low-energy quasi-particle excitations in electronic band structures. In condensed matter systems, their massless nature can be protected by crystal symmetries. Classification of such symmetry-protected relativistic band degeneracies has been fruitful, although many of the predicted quasi-particles still await their experimental discovery. Here we reveal, using angle-resolved photoemission spectroscopy, the existence of two-dimensional type-II Dirac fermions in the high-temperature superconductor La$_{1.77}$Sr$_{0.23}$CuO$_4$. The Dirac point, constituting the crossing of $d_{x^2-y^2}$ and $d_{z^2}$ bands, is found approximately one electronvolt below the Fermi level ($E_mathrm{F}$) and is protected by mirror symmetry. If spin-orbit coupling is considered, the Dirac point degeneracy is lifted and the bands acquire a topologically non-trivial character. In certain nickelate systems, band structure calculations suggest that the same type-II Dirac fermions can be realised near $E_mathrm{F}$.
We have prepared polycrystalline samples of LaSrRh$_{1-x}$Ga$_x$O$_4$ and LaSr$_{1-x}$Ca$_x$RhO$_4$,and have measured the x-ray diffraction, resistivity, Seebeck coefficient, magnetization and electron spin resonance in order to evaluate their electronic states. The energy gap evaluated from the resistivity and the Seebeck coefficient systematically changes with the Ga concentration, and suggests that the system changes from a small polaron insulator to a band insulator. We find that all the samples show Curie-Weiss-like susceptibility with a small Weiss temperature of the order of 1 K, which is seriously incompatible with the collective wisdom that a trivalent rhodium ion is nonmagnetic. We have determined the $g$ factor to be $g$=2.3 from the electron spin resonance, and the spin number to be $S$=1 from the magnetization-field curves by fitting with a modified Brillouin function. The fraction of the $S$=1 spins is 2--5%, which depends on the degree of disorder in the La/Sr/Ca-site, which implies that disorder near the apical oxygen is related to the magnetism of this system. A possible origin for the magnetic Rh$^{3+}$ ions is discussed.
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