arXiv subjects

Sergei Zvyagin

Publications and source records attributed to Sergei Zvyagin.

4 recordsLinked to original sources

arXiv2026

Quantum spin ladder with ferromagnetic rungs in Bi$_2$CuO$_3$(SO$_4$)

We introduce Bi$_2$CuO$_3$(SO$_4$) as a rare example of a spin-ladder magnet with ferromagnetic interactions on the rungs. Its magnetic response is studied through measurements of heat capacity, temperature-dependent magnetic susceptibility, and field-dependent magnetization, as well as electron spin resonance spectroscopy. These experiments are complemented by density-functional-theory calculations combined with the construction of maximally localized Wannier functions and an analysis of the relevant superexchange pathways. Quantum Monte Carlo simulations are employed to model thermodynamic properties and to quantitatively determine the magnetic exchange parameters. Our combined approach identifies Bi$_2$CuO$_3$(SO$_4$) as a two-leg spin-ladder system with ferromagnetic rungs ($J'$ $\approx -208$ K) and antiferromagnetic legs ($J$ $\approx 258$ K). These interactions of similar magnitude arise from remarkably different superexchange pathways, with the Cu--Cu distance along the leg being almost twice as long than the respective distance along the rung. The antiferromagnetic leg coupling represents the strongest oxygen-mediated long-range superexchange in a Cu$^{2+}$ compound reported to date and sets the benchmark for the role of complex superexchange pathways in quantum magnets.

cond-mat.str-el↗

arXiv2025

Influence of La-doping on the magnetic properties of the two-dimensional spin-gapped system SrCu$_2$(BO$_3$)$_2$

Aliovalent doping of the two-dimensional dimer antiferromagnet SrCu$_2$(BO$_3$)$_2$ has long been proposed as a potential route toward realizing resonating valence bond (RVB) superconductivity in this system; however, experimental progress has remained limited. This study explores the effects of La doping on the ground state of SrCu$_2$(BO$_3$)$_2$ and reports the first flux growth of Sr$_{1-x}$La$_x$Cu$_2$(BO$_3$)$_2$ single crystals with nominal doping levels up to $x$ = 0.15. Powder X-ray diffraction and energy-dispersive X-ray spectroscopy confirm the successful incorporation of La on the Sr sites within the tetragonal $I\bar{4}2m$ structure, although the effective doping was limited to approximately 50% of the nominal concentration. La doping induces systematic changes in the magnetic properties, with a reduction of the effective spin gap $Δ$ from ~28.2K to ~20.3 K at $x$ = 0.15, as determined from low-temperature magnetic susceptibility. X-band electron spin resonance measurements reveal the emergence of unpaired Cu$^{2+}$ spins in La-doped SrCu$_2$(BO$_3$)$_2$ single crystals, which develop antiferromagnetic correlations below ~5.5 K. These findings corroborate the breaking of the local spin dimers induced by La doping. Despite this, no superconductivity is observed across the entire doping range studied. The present study demonstrates that at low doping levels, electron doping locally destabilizes the spin-singlet ground state in SrCu$_2$(BO$_3$)$_2$, while the intrinsic spin dynamics of the dimer lattice remain largely preserved.

cond-mat.str-el↗

arXiv2025

Low-energy spin excitations in field-induced phases of the spin-ladder antiferromagnet BiCu$_2$PO$_6$

We report on terahertz spectroscopic measurements of quantum spin dynamics on single crystals of a spin-1/2 frustrated spin-ladder antiferromagnet BiCu$_2$PO$_6$ as a function of temperature, polarization, and applied external magnetic fields. Spin triplon excitations are observed at zero field and split in applied magnetic fields. For magnetic fields applied along the crystallographic $a$ axis, a quantum phase transition at $B_{c1}=21.4 \mathrm{T}$ is featured by a low-energy excitation mode emerging above $B_{c1}$ which indicates a gap reopening. For fields along the $b$ axis and the $c$ axis, different field dependencies are observed for the spin triplon excitations, whereas no low-lying modes could be resolved at field-induced phase transitions. We perform a theoretical analysis of the magnetic field dependence of the spin triplon modes by using continuous unitary transformations to determine an effective low energy Hamiltonian. Through an exhaustive parameter search we find numerically optimized parameters to very well describe the experimentally observed modes, which corroborate the importance of significant magnetic anisotropy in the system.

cond-mat.str-el↗

arXiv2024

Experimental observation of repulsively bound magnons

Stable composite objects, such as hadrons, nuclei, atoms, molecules and superconducting pairs, formed by attractive forces are ubiquitous in nature. By contrast, composite objects stabilized by means of repulsive forces were long thought to be theoretical constructions owing to their fragility in naturally occurring systems. Surprisingly, the formation of bound atom pairs by strong repulsive interactions has been demonstrated experimentally in optical lattices. Despite this success, repulsively bound particle pairs were believed to have no analogue in condensed matter owing to strong decay channels. Here we present spectroscopic signatures of repulsively bound three-magnon states and bound magnon pairs in the Ising-like chain antiferromagnet BaCo$_2$V$_2$O$_8$. In large transverse fields, below the quantum critical point, we identify repulsively bound magnon states by comparing terahertz spectroscopy measurements to theoretical results for the Heisenberg-Ising chain antiferromagnet, a paradigmatic quantum many-body model. Our experimental results show that these high-energy repulsively bound magnon states are well separated from continua, exhibit significant dynamical responses and, despite dissipation, are sufficiently long-lived to be identified. As the transport properties in spin chains can be altered by magnon bound states, we envision such states could serve as resources for magnonics based quantum information processing technologies.

cond-mat.str-el↗