arXiv subjects

Stephen Nagler

Publications and source records attributed to Stephen Nagler.

4 recordsLinked to original sources

arXiv2023

Structure transition and zigzag magnetic order in Ir/Rh-substituted honeycomb lattice RuCl3

We report magnetization and neutron diffraction studies on crystal and magnetic structures of Ir- and Rh-substituted honeycomb lattice $α$-RuCl$_3$. The iridium or rhodium atoms are distributed at the Ru site with little structural modification. Both systems undergo a room-temperature monoclinic $C2/m$ to low-temperature trigonal $R\bar{3}$ phase transformation with a large recoverable hysteresis. At low temperature, a zigzag spin order is observed with the same characteristic wavevector $(0,0.5,1)$ as in the parent $α$-RuCl$_3$. Detailed magnetic structure refinement reveals an ordered moment of $\rm 0.32(5) μ_B/Ru$ and a upper boundary of canting angle of $15(4)^\circ$ away from the basal plane at 5~K for the 10\% Ir-substituted $α$-RuCl$_3$, which is different from the 0.45-0.73~$\rm μ_B/Ru$ and $32^\circ$-$48^\circ$ canting angle reported in the parent compound $α$-RuCl$_3$. The observation of unchanged RuCl$_6$ local octahedral environment, reduced magnetic moment size and canting angle highlights the potential to study quantum spin liquid behavior through non-magnetic ion doping.

cond-mat.mtrl-sci↗

arXiv2023

The sample-dependent and sample-independent thermal transport properties of $α$-RuCl$_3$

We investigated the thermal transport properties of two $α$-RuCl$_3$ crystals with different degrees of stacking disorder to understand the origin of the previously reported oscillatory feature in the field dependence of thermal conductivity. Crystal I shows only one magnetic order around 13\,K, which is near the highest T$_N$ for $α$-RuCl$_3$ with stacking faults. Crystal II has less stacking disorder, with a dominant heat capacity at 7.6\,K along with weak anomalies at 10\,K and 13\,K. In the temperature and field dependence of thermal conductivity, no obvious anomaly was observed to be associated with the magnetic order around 13\,K for either crystal or around 10\,K for crystal II. Crystal II, with less disorder, showed clear oscillations in the field dependence of thermal conductivity, while crystal I, with more disorder, did not. For crystal I, an L-shaped region in the temperature-field space was observed where thermal Hall conductivity $κ_{xy}$/T is within $\pm$20\% of the half quantized thermal Hall conductivity $κ_{HQ}$/T. While for crystal II, $κ_{xy}$/T reaches $κ_{HQ}$/T only in the high field and high temperature regime with no indication of a plateau at $κ_{HQ}$/T. Our thermal conductivity data suggest the oscillatory features are inherent to the zig-zag ordered phase with T$_N$ near 7\,K. Our planar thermal Hall effect measurements highlight the sensitivity of this phenomena to stacking disorder. Overall, our results highlight the importance of understanding and controlling crystallographic disorder for obtaining and interpreting intrinsic thermal transport properties in $α$-RuCl$_3$.

cond-mat.mtrl-sci↗

arXiv2023

Polyhedral distortions and unusual magnetic order in spinel FeMn$_{2}$O$_{4}$

Spinel compounds AB$_{2}$X$_{4}$ consist of both tetrahedral (AX$_{4}$) and octahedral (BX$_{6}$) environments with the former forming a diamond lattice and the latter a geometrically frustrated pyrochlore lattice. Exploring the fascinating properties and their correlations with structural features is critical in understanding these materials. FeMn$_{2}$O$_{4}$ has been reported to exhibit one structural transition and two successive magnetic transitions. Here, we report the polyhedral distortions and their correlations to the structural and two magnetic transitions in FeMn$_{2}$O$_{4}$ by employing the high-resolution neutron powder diffraction. While a large trigonal distortion is found even in the high-temperature cubic phase, the first-order cubic-tetragonal structural transition associated with the elongation of both tetrahedra and octahedra along the $c$ axis occurs at $T_{S} \approx$ 750 K, driven by the Jahn-Teller effect of the orbital active B-site Mn$^{3+}$ cation. A strong magnetoelastic coupling is unveiled at $T_{N1}\approx 400$ K as manifested by the appearance of Nèel-type collinear ferrimagnetic order, an anomaly in both tetrahedral and octahedral distortions, as well as an anomalous decrease of the lattice constant $c$ and a weak anomaly of $a$. Upon cooling below $T_{N2}\approx65$ K, it evolves to a noncollinear ferrimagnetic order with a canting of half B-site $Mn^{3+}$/$Fe^{3+}$ spins in the pyrochlore lattice, which is a unique magnetic order among spinels. Such a noncollinear order induces modifications of the O-B-O bond angles in the octahedra without affecting much the bond lengths of the tetrahedra/octahedra. Our study indicates that FeMn$_{2}$O$_{4}$ is a wonderful platform to unveil interesting magnetic order and to investigate their correlations to polyhedral distortions and lattice.

physics.chem-ph↗

arXiv1997

Excitations and Possible Bound States in the S=1/2 Alternating Chain Compound (VO)2P2O7

Magnetic excitations in an array of (VO)2P2O7 single crystals have been measured using inelastic neutron scattering. Until now, (VO)2P2O7 has been thought of as a two-leg antiferromagnetic Heisenberg spin ladder with chains running in the a-direction. The present results show unequivocally that (VO)2P2O7 is best described as an alternating spin-chain directed along the crystallographic b-direction. In addition to the expected magnon with magnetic zone-center energy gap DE = 3.1$ meV, a second excitation is observed at an energy just below 2DE. The higher mode may be a triplet two-magnon bound state. Numerical results in support of bound modes are presented.

cond-mat.stat-mech↗