arXiv subjects

L. D. Sanjeewa

Publications and source records attributed to L. D. Sanjeewa.

13 recordsLinked to original sources

arXiv2024

New insight into tuning magnetic phases of $R$Mn$_6$Sn$_6$ kagome metals

Predicting magnetic ordering in kagome compounds offers the possibility of harnessing topological or flat-band physical properties through tuning of the magnetism. Here, we examine the magnetic interactions and phases of ErMn$_6$Sn$_6$ which belongs to a family of $R$Mn$_6$Sn$_6$, $R=$ Sc, Y, Gd--Lu, compounds with magnetic kagome Mn layers, triangular $R$ layers, and signatures of topological properties. Using results from single-crystal neutron diffraction and mean-field analysis, we find that ErMn$_6$Sn$_6$ sits close to the critical boundary separating the spiral-magnetic and ferrimagnetic ordered states typical for nonmagnetic versus magnetic $R$ layers, respectively. Finding interlayer magnetic interactions and easy-plane Mn magnetic anisotropy consistent with other members of the family, we predict the existence of a number of temperature and field dependent collinear, noncollinear, and noncoplanar magnetic phases. We show that thermal fluctuations of the Er magnetic moment, which act to weaken the Mn-Er interlayer magnetic interaction and quench the Er magnetic anisotropy, dictate magnetic phase stability. Our results provide a starting point and outline a multitude of possibilities for studying the behavior of Dirac fermions in $R$Mn$_6$Sn$_6$ compounds with control of the Mn spin orientation and real-space spin chirality.

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arXiv2023

Magnetic order in the two-dimensional metal-organic framework manganese pyrazinecarboxylate with Mn-Mn dimers

The magnetic properties of [Mn(pyrazinecarboxylate)2]n (Mn-pyrazine), empirical formula C10H6MnN4O4, are investigated through susceptibility, heat capacity and neutron scattering measurements. The structure of Mn-pyrazine consists of Mn-Mn dimers linked on a distorted 2D hexagonal structure. The weak out of plane interactions create a quasi-2D magnetic material within the larger three dimensional metal organic framework (MOF) structure. We show that this material undergoes a two stage magnetic transition, related to the low dimensionality of the Mn lattice. First at 5 K, which is assigned to the initial development of short range order in the 2D layers. This is followed by long range order at 3.3 K. Applied field measurements reveal the potential to induce magnetic transitions in moderately small fields of 2 T. Neutron powder diffraction enabled the determination of a unique magnetic space group P21'/c (#14.77) at 1.5 K. This magnetic structure consists of antiferromagnetically coupled Mn-Mn dimers with spins principally along the out of plane a-axis.

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arXiv2023

Witnessing quantum criticality and entanglement in the triangular antiferromagnet KYbSe$_2$

The Heisenberg triangular lattice quantum spin liquid and the phase transitions to nearby magnetic orders have received much theoretical attention, but clear experimental manifestations of these states are rare. This work investigates a new spin-half Yb$^{3+}$ delafossite material, KYbSe$_2$, whose inelastic neutron scattering spectra reveal a diffuse continuum with a sharp lower bound. Applying entanglement witnesses to the data reveals significant multipartite entanglement spread between its neighbors, and analysis of its magnetic exchange couplings shows close proximity to the triangular lattice Heisenberg quantum spin liquid. Key features of the data are reproduced by Schwinger-boson theory and tensor network calculations with a significant second-neighbor coupling $J_2$. The strength of the dynamical structure factor at the $K$ point shows a scaling collapse in $\hbarω/k_\mathrm{B}T$ down to 0.3 K, indicating a second-order quantum phase transition. Comparing this to previous theoretical work suggests that the proximate phase at larger $J_2$ is a gapped $\mathbb{Z}_2$ spin liquid, resolving a long-debated issue. We thus show that KYbSe$_2$ is close to a spin liquid phase, which in turn sheds light on the theoretical phase diagram itself.

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arXiv2022

Zig-Zag magnetic order and potential Kitaev interactions in the spin-1 honeycomb lattice KNiAsO$_4$

Despite the exciting implications of the Kitaev spin-Hamiltonian, finding and confirming the quantum spin liquid state has proven incredibly difficult. Recently the applicability of the model has been expanded through the development of a microscopic description of a spin-1 Kitaev interaction. Here we explore a candidate spin-1 honeycomb system, KNiAsO$_4$ , which meets many of the proposed criteria to generate such an interaction. Bulk measurements reveal an antiferromagnetic transition at $\sim$ 19 K which is generally robust to applied magnetic fields. Neutron diffraction measurements show magnetic order with a $\textbf{k}=(\frac{3}{2},0,0)$ ordering vector which results in the well-known ``zig-zag" magnetic structure thought to be adjacent to the spin-liquid ground state. Field dependent diffraction shows that while the structure is robust, the field can tune the direction of the ordered moment. Inelastic neutron scattering experiments show a well defined gapped spin-wave spectrum with no evidence of the continuum expected for fractionalized excitations. Modeling of the spin waves shows that the extended Kitaev spin-Hamiltonians is generally necessary to model the spectra and reproduce the observed magnetic order. First principles calculations suggest that the substitution of Pd on the Ni sublattice may strengthen the Kitaev interactions while simultaneously weakening the exchange interactions thus pushing KNiAsO$_4$ closer to the spin-liquid ground state.

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arXiv2022

Single pair of Weyl nodes in the spin-canted structure of EuCd$_2$As$_2$

Time reversal symmetry breaking Weyl semimetals are unique among Weyl materials in allowing the minimal number of Weyl points thus offering the clearest signatures of the associated physics. Here we present neutron diffraction, density functional theory and transport measurement results which indicate that EuCd$_2$As$_2$ , under ambient field, strain and pressure, is such a material with a single pair of Weyl points. Our work reveals a magnetic structure (magnetic space group $C2'/m'$) with Eu moments pointing along the [210] direction in-plane and canted $\sim$ 30$^{\circ}$ out-of-plane. Density functional theory calculations using this structure show that the observed canting drastically alters the relevant electronic bands, relative to the in-plane order, leading to a single set of well defined Weyl points. Furthermore, we find the canting angle can tune the distance of the Weyl points above the Fermi level, with the smallest distance at low canting angles. Finally, transport measurements of the anomalous Hall Effect and longitudinal magnetoresistance exhibit properties indicative of a chiral anomaly, thus supporting the neutron scattering and DFT results suggesting EuCd$_2$As$_2$ is close to the ideal situation of the Weyl "Hydrogen atom".

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arXiv2021

Field-Induced Spin Excitations in the Spin-1/2 Triangular-Lattice Antiferromagnet CsYbSe$_2$

A layered triangular lattice with spin-1/2 ions is an ideal platform to explore highly entangled exotic states like quantum spin liquid (QSL). Here, we report a systematic in-field neutron scattering study on a perfect two-dimensional triangular-lattice antiferromagnet, CsYbSe$_2$, a member of the large QSL candidate family rare-earth chalcogenides. The elastic neutron scattering measured down to 70 mK shows that there is a short-range 120$^{\circ}$ magnetic order at zero field. In the field-induced ordered states, the spin-spin correlation lengths along the $c$ axis are relatively short, although the heat capacity results indicate long-range magnetic orders at 3 T $-$ 5 T. The inelastic neutron scattering spectra evolve from highly damped continuum-like excitations at zero field to relatively sharp spin wave modes at the plateau phase. Our extensive large-cluster density-matrix renormalization group calculations with a Heisenberg triangular-lattice nearest-neighbor antiferromagnetic model reproduce the essential features of the experimental spectra, including continuum-like excitations at zero field, series of sharp magnons at the plateau phase as well as two-magnon excitations at high energy. This work presents comprehensive experimental and theoretical overview of the unconventional field-induced spin dynamics in triangular-lattice Heisenberg antiferromagnet and thus provides valuable insight into quantum many-body phenomena.

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arXiv2021

Understanding Reentrance in Frustrated Magnets: the Case of the Er$_2$Sn$_2$O$_7$ Pyrochlore

Reentrance, the return of a system from an ordered phase to a previously encountered less-ordered one as a controlled parameter is continuously varied, is a recurring theme found in disparate physical systems, from condensed matter to black holes. While diverse in its many incarnations and generally unsuspected, the cause of reentrance at the microscopic level is often not investigated thoroughly. Here, through detailed characterization and theoretical modeling, we uncover the microscopic mechanism behind reentrance in the strongly frustrated pyrochlore antiferromagnet Er$_2$Sn$_2$O$_7$. Taking advantage of the recent advance in rare earth stannate single crystal synthesis, we use heat capacity measurements to expose that Er$_2$Sn$_2$O$_7$ exhibits multiple instances of reentrance in its magnetic field $B$ vs. temperature $T$ phase diagram for magnetic fields along three cubic high symmetry directions. Through classical Monte Carlo simulations, mean field theory and classical linear spin-wave expansions, we argue that the origins of the multiple occurrences of reentrance observed in Er$_2$Sn$_2$O$_7$ are linked to soft modes. Depending on the field direction, these arise either from a direct $T=0$ competition between the field-evolved ground states, or from a field-induced enhancement of the competition with a distinct zero-field antiferromagnetic phase. In both scenarios, the phase competition enhances thermal fluctuations which entropically stabilize a specific ordered phase. This results in an increased transition temperature for certain field values and thus the reentrant behavior. Our work represents a detailed examination into the mechanisms responsible for reentrance in a frustrated magnet and may serve as a template for the interpretation of reentrant phenomena in other physical systems.

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arXiv2020

Unravelling competing microscopic interactions at a phase boundary: a single crystal study of the metastable antiferromagnetic pyrochlore Yb$_{2}$Ge$_{2}$O$_{7}$

We report inelastic neutron scattering measurements from our newly synthesized single crystals of the structurally metastable antiferromagnetic pyrochlore Yb$_{2}$Ge$_{2}$O$_{7}$. We determine the four symmetry-allowed nearest-neighbor anisotropic exchange parameters via fits to linear spin wave theory supplemented by fits of the high-temperature specific heat. The exchange parameters so-determined are strongly correlated to the values determined for the $g$-tensor components, as previously observed for the related Yb pyrochlore Yb$_{2}$Ti$_{2}$O$_{7}$. To address this issue, we directly determined the $g$-tensor from electron paramagnetic resonance of 1% Yb-doped Lu$_{2}$Ge$_{2}$O$_{7}$, thus enabling an unambiguous determination of the exchange parameters. Our results show that Yb$_{2}$Ge$_{2}$O$_{7}$ resides extremely close to the classical phase boundary between an antiferromagnetic $Γ_5$ phase and a splayed ferromagnet phase. By juxtaposing our results with recent ones on Yb$_{2}$Ti$_{2}$O$_{7}$, our work illustrates that the Yb pyrochlore oxides represent ideal systems for studying quantum magnets in close proximity to classical phase boundaries.

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arXiv2020

Evidence of Ba substitution induced spin-canting in the magnetic Weyl semimetal EuCd$_2$As$_2$

Recently EuCd$_2$As$_2$ was predicted to be a magnetic Weyl semi-metal with a lone pair of Weyl nodes generated by A-type antiferromagnetism and protected by a rotational symmetry. However, it was soon discovered that the actual magnetic structure broke the rotational symmetry and internal pressure was later suggested as a route to stabilize the desired magnetic state. In this work we test this prediction by synthesizing a series of Eu$_{1-x}$Ba$_x$Cd$_2$As$_2$ single crystals and studying their structural, magnetic and transport properties via both experimental techniques and first-principles calculations. We find that small concentrations of Ba ($\sim 3-10\% $) lead to a small out-of-plane canting of the Eu moment. However, for higher concentrations this effect is suppressed and a nearly in-plane model is recovered. Studying the transport properties we find that all compositions show evidence of an Anomalous Hall Effect dominated by the intrinsic mechanism as well as large negative magnetoresistances in the longitudinal channel. A non-monotonic evolution of the transport properties is seen across the series which correlates to the proposed canting suggesting canting may enhance the topological effects. Careful density functional theory calculations using an all-electron approach revise prior predictions finding a purely ferromagnetic ground state with in-plane moments for both the EuCd$_2$As$_2$ and Eu$_{0.5}$Ba$_{0.5}$Cd$_2$As$_2$ compounds - corroborating our experimental findings. This work suggests that Ba substitution can tune the magnetic properties in unexpected ways which correlate to changes in measures of topological properties, encouraging future work to locate the ideal Ba concentration for Eu moment canting.

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arXiv2020

Crystal field Hamiltonian and anisotropy in KErSe2 and CsErSe2

We use neutron scattering and bulk property measurements to determine the single-ion crystal-field Hamiltonians of delafossites $\rm KErSe_2$ and $\rm CsErSe_2$. These two systems contains planar equilateral triangular Er lattices arranged in two stacking variants: rhombohedral (for K) or hexagonal (Cs). Our analysis shows that regardless the stacking order both compound exhibit an easy-plane ground state doublet with large $J_z=1/2$ terms and the potential for significant quantum effects, making them candidates for quantum spin liquid or other exotic ground states.

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arXiv2019

Tunable magnetic order in low-symmetry SeO$_3$ ligand linked $TM_3$(SeO$_3$)$_3$H$_2$O ($TM$ = Mn, Co and Ni) compounds

Generally, one has two strategies to achieve magnetic frustration: through geometric means or interactions with different length scales. As the former leads to much simpler theoretical treatments it is favored and so magnetic sublattices with geometric frustration are sought after. One approach to finding such lattices is to design them chemically by using non-magnetic linker ligands. Here we report on the magnetic properties of one such family of materials, the transition metal ($TM$) selenite hydrate compounds chemical formula $TM_3$(SeO$_3$)$_3$H$_2$O . These materials link highly distorted $TM$O$_6$ octahedra via non-magnetic [SeO$_3$]$^{2+}$ linkers. Using $TM$ = Mn, Co and Ni we report on the structural effects of changing the $TM$ site and how they may influence the magnetic structure. Using magnetic susceptibility and neutron powder diffraction we identify low temperature magnetic transitions for all three compounds characterized by the onset of long-range AFM order with moderate frustration indexes. Consideration of the magnetic structures reveal that the magnetic order is sensitive to the $TM$ site ion and is tunable as it is changed - especially from Mn to Co - with changes in both the moment direction and the ordering vector. Field dependent susceptibility and heat capacity measurements reveal metamagnetic transitions in both Mn$_3$(SeO$_3$)$_3$H$_2$O and Co$_3$(SeO$_3$)$_3$H$_2$O indicating nearby magnetic ground states accessible under relatively small applied fields. Density functional theory calculations broadly confirm these results, showing both a sensitivity of the magnetic structure to the $TM$ and its local environment. Although no spin liquid behavior is achieved, these results suggest the fruitfulness of such synthesis philosophies and encourage future work to engender higher frustration in these materials via doping, field, pressure or larger linker ligands.

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arXiv2018

The magnetic order of a manganese vanadate system with two-dimensional striped triangular lattice

Results of magnetization and neutron diffraction measurements of the manganese vanadate system Mn$_5$(VO$_4$)$_2$(OH)$_4$ are reported. The crystal structure of this compound contains triangular [Mn$_3$O$_{13}$] building blocks that produce two-dimensional Mn$^{2+}$ magnetic networks with striped triangular topologies. The Mn sheets are connected through the nonmagnetic vanadate tetrahedra extending along the $a$-axis. Magnetization measurements performed on single crystals reveal the onset of a long-range antiferromagnetic order below approximately 45 K. The magnetic structure is Néel-type with nearest-neighbor Mn atoms coupled via three or four antiferromagnetic bonds. The magnetic moments are confined within the layers and are oriented parallel to the $b$ direction. The magnitudes of ordered moments are reduced, presumably by geometrical frustration and the low-dimensionality of the lattice structure.

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arXiv2014

Complex magnetic behavior of the sawtooth Fe chains in Rb$_{2}$Fe$_{2}$O(AsO$_{4}$)$_{2}$

Results of magnetic field and temperature dependent neutron diffraction and magnetization measurements on oxy-arsenate Rb$_{2}$Fe$_{2}$O(AsO$_{4}$)$_{2}$ are reported. The crystal structure of this compound contains pseudo-one-dimensional [Fe$_{2}$O$_{6}$]$^\infty$ sawtooth-like chains, formed by corner sharing isosceles triangles of $Fe^{3+}$ ions occupying two nonequivalent crystallographic sites. The chains extend infinitely along the crystallographic $b$-axis and are structurally confined from one another via diamagnetic (AsO$_{4}$)$^{3-}$ units along the $a$-axis, and Rb$^+$ cations along the $c$-axis direction. Neutron diffraction measurements indicate the onset of a long range antiferromagnetic order below approximately 25 K. The magnetic structure consists of ferrimagnetic chains which are antiferromagnetically coupled with each other. Within each chain, one of the two Fe sites carries a moment which lies along the \emph{b}-axis, while the second site bears a canted moment in the opposite direction. Externally applied magnetic field induces a transition to a ferrimagnetic state, in which the coupling between the sawtooth chains becomes ferromagnetic. Magnetization measurements performed on optically-aligned single crystals reveal evidence for an uncompensated magnetization at low magnetic fields that could emerge from to a phase-segregated state with ferrimagnetic inclusions or from antiferromagnetic domain walls. The observed magnetic states and the competition between them is expected to arise from strongly frustrated interactions within the sawtooth chains and relatively weak coupling between them.

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