arXiv subjects

Margarita Russina

Publications and source records attributed to Margarita Russina.

5 recordsLinked to original sources

arXiv2026

Dynamical magnetism in the disordered cubic lattice material $γ$-${\rm Ba}_{3}{\rm CoNb}_{2}{\rm O}_{9}$

$γ$-${\rm Ba}_{3}{\rm CoNb}_{2}{\rm O}_{9}$ realizes a disordered simple-cubic spin-$1/2$ lattice in which Co$^{2+}$ ions randomly occupy one third of the sites, placing the system close to the site-percolation threshold for magnetic order. Specific-heat, susceptibility, neutron spin-echo, and muon spin-rotation measurements reveal a broad thermodynamic crossover, short-range magnetic correlations, and persistent fast spin dynamics down to at least 0.1~K, with no evidence for static order or conventional spin-glass freezing. Monte Carlo simulations yield a broad distribution of orphan spins, finite clusters, and an infinite network. The calculated orphan-spin fraction ($\approx 8.8\%$) agrees well with the weakly correlated spin fraction inferred from magnetization ($\approx 8.2\%$). Exact diagonalization of a diluted $S = 1/2$ Heisenberg model captures the broad magnetic specific-heat anomaly and supports the coexistence of weakly and strongly correlated spin environments. These results support a picture in which spin-$1/2$ quantum fluctuations, together with dilution and proximity to the percolation threshold, can support a disorder-driven dynamical state with short-range correlations in three dimensions, distinct from both classical spin glasses and geometrically frustrated quantum spin liquids.

cond-mat.str-el↗

arXiv2021

Field-induced quantum spin disordered state in spin-1/2 honeycomb magnet Na2Co2TeO6

Spin-orbit coupled honeycomb magnets with the Kitaev interaction have received a lot of attention due to their potential of hosting exotic quantum states including quantum spin liquids. Thus far, the most studied Kitaev systems are 4d/5d-based honeycomb magnets. Recent theoretical studies predicted that 3d-based honeycomb magnets, including Na2Co2TeO6 (NCTO), could also be a potential Kitaev system. Here, we have used a combination of heat capacity, magnetization, electron spin resonance measurements alongside inelastic neutron scattering (INS) to study NCTO's quantum magnetism, and we have found a field-induced spin disordered state in an applied magnetic field range of 7.5 T < B (vertical to b-axis) < 10.5 T. The INS spectra were also simulated to tentatively extract the exchange interactions. As a 3d-magnet with a field-induced disordered state on an effective spin-1/2 honeycomb lattice, NCTO expands the Kitaev model to 3d compounds, promoting further interests on the spin-orbital effect in quantum magnets.

cond-mat.str-el↗

arXiv2020

Realization of the kagome spin ice state in a frustrated intermetallic compound

Spin ices are exotic phases of matter characterized by frustrated spins obeying local ice rules, in analogy with the electric dipoles in water ice. In two dimensions, one can similarly define ice rules for in-plane Ising-like spins arranged on a kagome lattice. These ice rules require each triangle plaquette to have a single monopole, and can lead to various unique orders and excitations. Using experimental and theoretical approaches including magnetometry, thermodynamic measurements, neutron scattering and Monte Carlo simulations, we establish HoAgGe as a crystalline (i.e. non-artificial) system that realizes the kagome spin ice state. The system features a variety of partially and fully ordered states and a sequence of field-induced phases at low temperatures, all consistent with the kagome ice rule.

cond-mat.str-el↗

arXiv2011

Direct Evidence for Edge-Contaminated Vortex Phase in a Nb Single Crystal using Neutron Diffraction

We report the first direct observation of a disordered vortex matter phase existing near the edge of a bulk type-II superconductor Nb using a novel position-sensitive neutron diffraction technique. This "edge-contaminated" vortex state was implicated in previous studies using transport techniques and was postulated to have played a significant role in the behavior of vortex dynamics in a wide range of type-II superconductors. It is found that upon thermal annealing, the vortex matter in the bulk undergoes re-ordering, suggesting that the edge-contaminated bulk vortex state is metastable. The edge vortex state remains disordered after repeated thermal annealing, indicating spatial coexistence of a vortex glass with a Bragg glass. This observation resolves many outstanding issues concerning the peak effect in type-II superconductors.

cond-mat.supr-con↗

arXiv2009

Magnetic excitations of the gapped quantum spin dimer antiferromagnet Sr$_{3}$Cr$_{2}$O$_{8}$

Sr$_{3}$Cr$_{2}$O$_{8}$ consist of a lattice of spin-1/2 Cr$^{5+}$ ions, which form hexagonal bilayers and which are paired into dimers by the dominant antiferromagnetic intrabilayer coupling. The dimers are coupled three-dimensionally by frustrated interdimer interactions. A structural distortion from hexagonal to monoclinic leads to orbital order and lifts the frustration giving rise to spatially anisotropic exchange interactions. We have grown large single crystals of Sr$_{3}$Cr$_{2}$O$_{8}$ and have performed DC susceptibility, high field magnetisation and inelastic neutron scattering measurements. The neutron scattering experiments reveal three gapped and dispersive singlet to triplet modes arising from the three twinned domains that form below the transition thus confirming the picture of orbital ordering. The exchange interactions are extracted by comparing the data to a Random Phase Approximation model and the dimer coupling is found to be $J_{0}=5.55$ meV, while the ratio of interdimer to intradimer exchange constants is $J'/J_{0}=0.64$. The results are compared to those for other gapped magnets.

cond-mat.str-el↗