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Register a new userOrthogonal Magnetization and Symmetry Breaking in Pyrochlore Iridate Eu2Ir2O7
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Electrons in the pyrochore iridates experience a large interaction energy in addition to a strong spin-orbit interaction. Both features make the iridates promising for realizing novel states such as the Topological Mott Insulator. The pyrochlore iridate Eu$_2$Ir$_2$O$_7$ shows a metal-insulator transition at $T_N sim$ 120 K below which a magnetically ordered state develops. Using torque magnetometry, we uncover a highly unusual magnetic response. A magnetic field $bf H$ applied in its $a$-$b$ plane produces a nonlinear magnetization $M_perp$ orthogonal to the plane. $M_perp$ displays a $d$-wave field-angle pattern consistent with octupolar order, with a handedness dictated by field cooling, leading to symmetry breaking of the chirality $omega$. A surprise is that the lobe orientation of the $d$-wave pattern is sensitive to the direction of the field when the sample is field-cooled below $T_N$, suggestive of an additional order parameter $eta$ already present at 300 K.
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We present magnetization and Hall effect measurements on the pyrochlore iridate Nd2Ir2O7. Previous muon spin rotation measurements have shown that the system undergoes an unusual transition at T$_M$ = 110 K into a magnetic phase lacking long-range order, followed by a transition at T$_LRO$ = 6 K into a state with long-range magnetic order. We observe a small remnant magnetization when cycling through zero magnetic field at temperatures below T$_M$. Below T$_LRO$, this remnant magnetization increases sharply, and new hysteresis effects appear at a higher field B$_c$ = 2.5 T, while the Hall resistance develops a non-monotonic and hysteretic magnetic field dependence, with a maximum at B$_c$ and signatures of an anomalous Hall effect. The dependence on field sweep direction demonstrates a non-trivial transition into a magnetically ordered state with properties paralleling those of known spin-ice systems and suggests a spin reorientation transition across the metal insulator transition in the A-227 series.
A Weyl semimetallic state with pairs of nondegenerate Dirac cones in three dimensions was recently predicted to occur in the antiferromagnetic state of the pyrochlore iridates. Here, we show that the THz optical conductivity and temperature dependence of free carriers in the pyrochlore Eu2Ir2O7 match the predictions for a Weyl semimetal and suggest novel Dirac liquid behavior. The interband optical conductivity vanishes continuously at low frequencies signifying a semimetal. The metal-insulator transition at T_N = 110 K is manifested in the Drude spectral weight, which is independent of temperature in the metallic phase, and which decreases smoothly in the ordered phase. The temperature dependence of the free carrier weight below T_N is in good agreement with theoretical predictions for a Dirac material. The data yield a Fermi velocity v_F=4x10^7 cm/s, a logarithmic renormalization scale Lambda_L=600 K, and require a Fermi temperature of T_F=100 K associated with residual unintentional doping to account for the low temperature optical response and dc resistivity.
The pyrochlore Eu$_2$Ir$_2$O$_7$ has recently attracted significant attention as a candidate Weyl semimetal. The previous reports on this compound unanimously show a thermally induced metal to insulator (MI) transition, concomitant with antiferromagnetic (AFM) long-range ordering of the Ir-moments below T$_textit{N} sim $120 K. However, there are contradictory reports concerning the slope d$rho/$dT of the resistivity plots ($rho$) in the metallic state above the metal-insulator (MI) transition, and the value of $rho$ in the insulating state, both of which show significant sample dependence. Here, we explore this issue by investigating six different Eu$_2$Ir$_2$O$_7$ samples with slightly varying Eu:Ir ratio. High-resolution synchrotron powder diffraction are done to probe minor variations in the cell parameters of the various Eu$_2$Ir$_2$O$_7$ samples investigated here. Specific heat (C$ _p $) and magnetic susceptibility of all the samples showed long-range antiferromagnetic ordering upon cooling below T$ _textit{N} sim $120 K. The transitions are, however, found to be smeared out for the off-stoichiometric samples. We show that the sign of d$rho/$dT above the metal-insulator (MI) transition is highly sensitive to the unit cell length, which, in turn, depends on the level of Eu-stuffing at the Ir-site. Samples with composition close to the ideal stoichiometry (Eu : Ir $ = $ 1) showed a change of sign of d$rho/$dT from negative to positive upon cooling below a certain temperature T $^*$ $>$ T$_textit{MI}$. With increasing Eu-stuffing T$ ^* $ decreased until a negative d$rho/$dT persisted without any sign change down to T$_textit{MI}$.
By using a combination of heteroepitaxial growth, structure refinement based on synchrotron x-ray diffraction and first-principles calculations, we show that the symmetry-protected Dirac line nodes in the topological semimetallic perovskite SrIrO3 can be lifted simply by applying epitaxial constraints. In particular, the Dirac gap opens without breaking the Pbnm mirror symmetry. In virtue of a symmetry-breaking analysis, we demonstrate that the original symmetry protection is related to the n-glide operation, which can be selectively broken by different heteroepitaxial structures. This symmetry protection renders the nodal line a nonsymmorphic Dirac semimetallic state. The results highlight the vital role of crystal symmetry in spin-orbit-coupled correlated oxides and provide a foundation for experimental realization of topological insulators in iridate-based heterostructures.
We report a combined muon spin relaxation/rotation, bulk magnetization, neutron scattering, and transport study of the electronic properties of the pyrochlore iridate Nd2Ir2O7. We observe the onset of strongly hysteretic behavior in the temperature dependent magnetization below 120 K, and an abrupt increase in the temperature dependent resistivity below 8 K. Zero field muon spin relaxation measurements show that the hysteretic magnetization is driven by a transition to a magnetically disordered state, and that below 8 K a complex magnetically ordered ground state sets in, as evidenced by the onset of heavily damped spontaneous muon precession. Our measurements point toward the absence of a true metal-to-insulator phase transition in this material and suggest that Nd2Ir2O7 lies either within or on the metallic side of the boundary of the Dirac semimetal regime within its topological phase diagram.
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