arXiv subjects

K. S. Chikara

Publications and source records attributed to K. S. Chikara.

4 recordsLinked to original sources

arXiv2026

Neutron scattering evidence for two-dimensionally coupled spin-dimerized antiferromagnetic lattice in α-Cu2P2O7

The microscopic magnetic model of the low-dimensional quantum magnet alpha-Cu2P2O7 has remained controversial. We present a comprehensive study of its magnetic ground state and excitation spectrum using temperature-dependent inelastic neutron scattering, neutron diffraction, magnetization measurements, and comprehensive spin-wave modeling. Our results unambiguously establish alpha-Cu2P2O7 as a two-dimensionally coupled spin-dimerized antiferromagnetic (AF) lattice within the bc plane, with a dominant AF exchange J2 = 7.73 meV (hereafter referred to as "intradimer exchange") and weaker exchange couplings J1, J3, and J4 in the two-dimensional lattice (hereafter referred to as "interdimer exchange"), in agreement with LDA-based density functional theory and in contrast to previous GGA+U predictions. The dominant intradimer AF exchange is found between seventh-nearest-neighbor Cu-Cu ion pairs [d(Cu-Cu) = 5.125(3) A] rather than nearest-neighbor Cu-Cu ion pairs [d(Cu-Cu) = 3.014(1) A] of the structural dimers. Weak interlayer coupling (J5 = 0.03 meV) stabilizes long-range antiferromagnetic order below TN = 25 K. We further identify a weak single-ion anisotropy, associated with the distorted CuO5 polyhedra, that opens a gap in the spin-excitation spectrum and drives a field-induced metamagnetic transition. Systematic spin-wave calculations elucidate the distinct roles of interlayer coupling J5 and anisotropy term D in producing two distinct energy gaps at different antiferromagnetic zone centers. Complementary neutron diffraction and magnetization measurements as a function of applied magnetic field uncover a previously overlooked metamagnetic transition near 13 kOe and allow construction of the magnetic phase diagram in the H-T plane.

cond-mat.str-el↗

arXiv2026

Fermionic Approach to Elementary Excitations and Magnetization Plateaus in an S=1/2 XX Hybrid Trimer-Dimer Chain

We study the elementary excitations and magnetization of a one-dimensional spin-1/2 XX chain comprising trimer-dimer units (the J1-J1-J2-J3-J2 topology) under a transverse magnetic field h. Using Green's function theory and the Jordan-Wigner transformation, we map the system onto spinless fermions and focus on antiferromagnetic (AFM) interactions. At zero temperature, distinct 1/5 and 3/5 magnetization plateaus emerge, determined by the global periodicity Q=5, with the number of plateaus matching the number of excitation gaps above the Fermi level of the spinless fermions. The magnetic phase diagram in the (h-Js) plane features a Luttinger liquid (LL) state, a gapless AFM state, two magnetization plateau states, and a fully polarized gapped magnetic state. The widths of the LL and gapless AFM phases are found to be proportional to the bandwidths gamma = |E(k=0)-E(k=pi)| of the corresponding elementary excitations, whereas the widths of the magnetization plateau states are governed by the excitation gaps. Our study opens new directions for exploring interacting trimer-dimer spin chains in quantum magnetism using experimental techniques such as neutron scattering, as well as theoretical and numerical approaches including quantum Monte Carlo (QMC) and density-matrix renormalization group (DMRG) methods. Furthermore, we extend the Oshikawa-Yamanaka-Affleck (OYA) condition to generalized cluster chains, demonstrating that the allowed magnetization plateaus are governed by the global periodicity of the chain (e.g., Q=5 for a trimer-dimer chain), rather than by the local periodicity of individual units (Q=3 for a trimer or Q=2 for a dimer).

cond-mat.str-el↗

arXiv2025

Multiscale Magnetic Correlations in La2Mn2-xNixO6: Role of Crystal Structure in Double Perovskites

The magnetic correlations in double perovskites La2Mn2-xNixO6 (x = 0.5, 0.75, 1.0, 1.25 and 1.5) have been systematically investigated across macroscopic, mesoscopic, and microscopic length scales using temperature-dependent bulk DC magnetization, neutron depolarization, and neutron powder diffraction measurements, respectivitly. The magnetic properties evolve from a long-range ferromagnetic (FM) order to a cluster ferromagnetic or spin-glass (FM or SG) behavior as the Ni concentration increases. This evolution is directly linked to changes in the crystal structure, transitioning from pure orthorhombic (x=0.5) to mixed orthorhombic and monoclinic (x=0.75-1.0), and eventually to mixed trigonal and monoclinic symmetries (x=1.25-1.5). Ni substitution enhances the magnetic ordering temperature from 170 K (x=0.5) to 280 K (x=1.0), but this is accompanied by a reduction in both magnetization and ordered magnetic moment. Beyond x=1.0, any long-range magnetic ordering is absent. Additionally, all compositions exhibit a reentrant spin-glass-like phase at low temperatures (below about 50 K). Neutron diffraction analysis confirms that long-range FM order occurs only in the orthorhombic phase, while the monoclinic and trigonal phases lack such magnetic ordering. The temperature-dependent magnetic correlations are closely connected to variations in crystal structural parameters, including lattice constants and unit cell volume. The electrical conductivity behavior, following the variable range hopping (VRH) model, highlights the role of multivalence Mn and Ni ions on the electrical properties. This study elucidates the microscopic mechanisms behind the tunable magnetic and electrical properties of La2Mn2-xNixO6, offering valuable insights for the design of advanced materials for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic Ground State and Spin Excitations in the 2D Trimerized Collinear-II Lattice Antiferromagnet Li2Ni3P4O14

We report the magnetic ground state, spin excitations, and spin Hamiltonian of the 2D spin-1 trimerized Heisenberg antiferromagnet Li2Ni3P4O14. Below the magnetic ordering temperature TN = 14.5 K, the compound exhibits a canted long-range antiferromagnetic order with a propagation vector k = (0 0 0), consistent with the magnetic space group P21/c.1 (No. 14.75). The ground state magnetic structure consists of ferromagnetic spin-trimers of Ni2+ ions. The spin-trimers are coupled antiferromagnetically along the c-axis and ferromagnetically along the a-axis. Inelastic neutron scattering (INS) reveals gapped and dispersive magnon excitations below the TN, and gapless quasi-elastic scatterings at higher temperature. The linear spin-wave theory simulations reveal the essential features of the excitation spectrum; by a spin Hamiltonian composed of ferromagnetic intra-trimer exchange interaction J1 and inter-trimer exchange interactions J2 (FM) and J3(AFM) within the bc plane. The J2 and J3 along the b-axis and c-axis, respectively, with strengths of J2/J1=0.79 and J3/J1=-0.91. In addition, a weak inter planer ferromagnetic exchange interaction J4 (|J4/J1|~0.12) is found along the a-axis. The determined exchange constants reveal a 2D trimerized Collinear-II spin lattice within the bc-plane. The analysis of INS spectra by linear spin-wave theory also yields a moderate single-ion anisotropy (D/J1=0.48) which accounts for the observed spin gap below TN as well as the metamagnetic transition near 44 kOe in dc magnetization (M vs H) curves. These findings identify Li2Ni3P4O14 as a rare realization of a two-dimensional trimerized spin system and offer the direct experimental confirmation of theoretically predicted magnon excitations, unveiling the fundamental characteristics of the expected excitation spectrum.

cond-mat.str-el↗