arXiv subjects

Oleksandr Prokhnenko

Publications and source records attributed to Oleksandr Prokhnenko.

11 recordsLinked to original sources

arXiv2026

Resolving the Magnetic Ground State and Field-Induced Transitions in Magnetic Dirac Semimetal Candidate EuMnSb$_2$

The magnetic structure of a magnetic topological semimetal EuMnSb$_2$ is investigated in fields up to 30 T using polarized and unpolarized neutron diffraction, pulsed-field x-ray magnetic circular dichroism and pulsed-field magnetometry. We determine the zero-field magnetic structures of the Eu and Mn sublattices, and find that magnetic transitions induced by applied fields below 2 T correspond to changes in the magnetic order of the Eu spins alone without detectable perturbation to the order of the Mn spins. An additional magnetic transition is observed at fields close to the saturation field for the Eu spins. We present a mean-field model which describes key features of the magnetic behavior and allows us to estimate the dominant Eu--Eu and Eu--Mn exchange interactions responsible for the coupling between magnetism and electronic topology.

cond-mat.str-el↗

arXiv2026

Orbital-selective oxygen holes in cuprate ladders beyond the Zhang-Rice paradigm

The electronic structure of the spin-ladder cuprate Sr14Cu24O41 challenges the presumed universality of the Zhang-Rice singlet (ZRS) framework and models based exclusively on Cu-O hybridized orbitals. Combining polarization-dependent resonant soft X-ray scattering at the O K-edge with inelastic neutron scattering, we show that doped holes in the Cu2O3 ladders localize predominantly in planar non-bonding O 2pz (p{\pi}) orbitals of rung oxygen sites rather than forming conventional ZRS states. Polarization-resolved RSXS uniquely identifies this orbital assignment, while lattice and magnetic excitations reveal its coupled consequences, establishing a unified microscopic picture that excludes the conventional {\sigma}-bonded singlet. This oxygen-sublattice charge order produces an anomalous diagonal stretching phonon and explains the absence of incommensurate magnetic fluctuations and anomalous magnon splitting. These findings motivate a reassessment of hole pairing in ladder cuprates and the sufficiency of copper-centric models for cuprate superconductors.

cond-mat.str-el↗

arXiv2026

Direct High-Magnetic-Field Coupling to Stripe Order in a Cuprate Superconductor

Superconductivity in cuprates emerges out of a complex normal state that hosts density waves, pseudogap physics, and strange metal properties. Here, we access this normal state by synchronizing free-electron laser x-rays with high-magnetic-field pulses up to 44 T. We observe a linear increase in charge order amplitude and correlation length that persists far above the vortex melting transition. This behavior is incompatible with standard phase competition between charge order and superconductivity. By means of conventional hard x-ray diffraction and magnetostriction, we show that applied fields also enhance monoclinic lattice distortions. However, this magnetoelastic response is weaker and an epiphenomenon of the stripe order enhancement. Combined with recent observations of field-linear spin freezing, our results point to a direct coupling between magnetic field and the spin component of stripe order in the high-field normal state -- a mechanism independent of superconductivity suppression that has so far remained hidden from scattering probes.

cond-mat.str-el↗

arXiv2025

Spin dynamics in natural multiferroic pyroxene NaFeSi$_2$O$_6$

Spin dynamics in the natural mineral aegirine, NaFeSi$_2$O$_6$, a member of the pyroxene family, was studied by elastic and inelastic neutron scattering. Magnetization and specific heat measurements as well as single-crystal neutron diffraction maps, taken in the temperature range 2 - 20 K, confirm two successive magnetic transitions at 8.8 and 5.8 K, consistent with previous studies. The observed spin-wave excitations emerge from the incommensurate magnetic Bragg peaks corresponding to the propagation vector $k_{\rm ICM} = (0, 0.77, 0)$, and extend up to energies of about 1.5 meV. In the low-temperature helical phase, the spin dynamics of the Fe$^{3+}$ ions is well described by a simple linear spin-wave model. The observed excitations can be modeled using a spin Hamiltonian that includes three primary exchange interactions - intrachain coupling $J=0.142(2)$ meV, interchain couplings $J_1=0.083(1)$ meV and $J_2=0.186(1)$ meV - and an easy-plane anisotropy $D=0.020(6)$ meV. Our results show that no single exchange interaction dominates the spin dynamics. The similar strengths of the intrachain and interchain couplings point to the fact that the magnetic interactions in aegirine are three-dimensional rather than confined along one direction. As a result, the system cannot be considered quasi-one-dimensional, as previously suggested, and calls for further investigations.

cond-mat.str-el↗

arXiv2024

Bullet pressure-cell design for neutron scattering experiments with horizontal magnetic fields and dilution temperatures

The simultaneous application of high magnetic fields and high pressures for controlling magnetic ground states is important for testing our understanding of many-body quantum theory. However, the implementation for neutron scattering experiments presents a technical challenge. To overcome this challenge we present an optimized pressure-cell design with a novel bullet shape, which is compatible with horizontal-field magnets, in particular the high-field magnet operating at the Helmholtz-Zentrum Berlin. The cell enabled neutron diffraction and spectroscopy measurements with the combination of three extreme conditions: high pressures, high magnetic fields, and dilution temperatures, simultaneously reaching 0.7 GPa, 25.9 T, and 200 mK. Our results demonstrate the utility of informed material choices and the efficiency of finite-element analysis for future pressure-cell designs to be used in combination with magnetic fields and dilution temperatures for neutron scattering purposes.

cond-mat.str-el↗

arXiv2023

Field-induced bound-state condensation and spin-nematic phase in SrCu$_2$(BO$_3$)$_2$ revealed by neutron scattering up to 25.9 T

Bose-Einstein condensation (BEC) underpins exotic forms of order ranging from superconductivity to superfluid 4 He. In quantum magnetic materials, ordered phases induced by an applied magnetic field can be described as the BEC of magnon excitations. With sufficiently strong magnetic frustration, exemplified by the system SrCu$_2$(BO$_3$)$_2$ , no clear magnon BEC is observed and the complex spectrum of multi-magnon bound states may allow a different type of condensation, but the high fields required to probe this physics have remained a barrier to detailed investigation. Here we exploit the first purpose-built high-field neutron scattering facility to measure the spin excitations of SrCu$_2$(BO$_3$)$_2$ up to 25.9 T and use cylinder matrix-product-states (MPS) calculations to reproduce the experimental spectra with high accuracy. Multiple unconventional features point to a condensation of $S = 2$ bound states into a spin-nematic phase, including the gradients of the one-magnon branches, the presence of many novel composite two- and three-triplon excitations and the persistence of a one-magnon spin gap. This gap reflects a direct analogy with superconductivity, suggesting that the spin-nematic phase in SrCu$_2$(BO$_3$)$_2$ is best understood as a condensate of bosonic Cooper pairs. Our results underline the wealth of unconventional states yet to be found in frustrated quantum magnetic materials under extreme conditions.

cond-mat.str-el↗

arXiv2022

Prevailing charge order in overdoped cuprates beyond the superconducting dome

The extremely overdoped cuprates are generally considered to be Fermi liquid metals without exotic orders, whereas the underdoped cuprates harbor intertwined states. Contrary to this conventional wisdom, using Cu $L_3$ edge and O $K$ edge resonant x-ray scattering, we reveal a charge order (CO) in overdoped La$_{2-x}$Sr$_x$CuO$_4$ (0.35 $\leq$ x $\leq$ 0.6) beyond the superconducting dome. This CO has a periodicity of $\sim$ 6 lattice units with correlation lengths of $\sim 3 - 20$ lattice units. It shows similar in-plane momentum and polarization dependence and dispersive excitations as the CO of underdoped cuprates, but its maximum intensity differs along the c-direction and persists up to 300 K. This CO cannot be explained by either the Fermi surface instability or the doped Hubbard model and its origin remains to be understood. Our results suggest that CO is prevailing in the overdoped metallic regime and superconductivity emerges out of the CO phase upon decreasing hole carriers.

cond-mat.supr-con↗

arXiv2019

Magnetic structures and quadratic magnetoelectric effect in LiNiPO$_4$ beyond 30T

Neutron diffraction with static and pulsed magnetic fields is used to directly probe the magnetic structures in LiNiPO$_4$ up to 25T and 42T, respectively. By combining these results with magnetometry and electric polarization measurements under pulsed fields, the magnetic and magnetoelectric phases are investigated up to 56T applied along the easy $c$-axis. In addition to the already known transitions at lower fields, three new ones are reported at 37.6, 39.4 and 54T. Ordering vectors are identified with ${\bf Q}_{\mathrm{VI}}$ = (0, 1/3, 0) in the interval 37.6--39.4T and ${\bf Q}_{\mathrm{VII}}$ = (0, 0, 0) in the interval 39.4-54T. A quadratic magnetoelectric effect is discovered in the ${\bf Q}_{\mathrm{VII}}$ = (0, 0, 0) phase and the field-dependence of the induced electric polarization is described using a simple mean-field model. The observed magnetic structure and magnetoelectric tensor elements point to a change in the lattice symmetry in this phase. We speculate on the possible physical mechanism responsible for the magnetoelectric effect in LiNiPO4.

cond-mat.str-el↗

arXiv2019

Dispersions of Many-Body Bethe strings

Complex bound states of magnetic excitations, known as Bethe string, were predicted almost a century ago to exist in one-dimensional quantum magnets 1. The dispersions of the string states have so far remained the subject of intensive theoretical studies 2-7. By performing neutron scattering experiments on the one-dimensional Heisenberg-Ising antiferromagnet SrCo2V2O8 in high longitudinal magnetic fields, we reveal in detail the dispersion relations of the string states over the full Brillouin zone, as well as their magnetic field dependences. Furthermore the characteristic energy, the scattering intensity and linewidth of the observed string states exhibit excellent agreement with our precise Bethe Ansatz calculations. Our results establish the important role of string states in the quantum spin dynamics of one-dimensional systems, and will invoke studies of their dynamical properties in more general many-body systems.

cond-mat.str-el↗

arXiv2017

Magnetic order, hysteresis and phase coexistence in magnetoelectric LiCoPO$_4$

The magnetic phase diagram of magnetoelectric LiCoPO$_4$ is established using neutron diffraction and magnetometry in fields up to 25.9T applied along the crystallographic $b$-axis. For fields greater than 11.9T the magnetic unit cell triples in size with propagation vector Q = (0, 1/3, 0). A magnetized elliptic cycloid is formed with spins in the $(b,c)$-plane and the major axis oriented along $b$. Such a structure allows for the magnetoelectric effect with an electric polarization along $c$ induced by magnetic fields applied along $b$. Intriguingly, additional ordering vectors Q $\approx$ (0, 1/4, 0) and Q $\approx$ (0, 1/2, 0) appear for increasing fields in the hysteresis region below the transition field. Traces of this behavior are also observed in the magnetization. A simple model based on a mean-field approach is proposed to explain these additional ordering vectors. In the field interval 20.5-21.0T, the propagation vector Q = (0, 1/3, 0) remains but the spins orient differently compared to the cycloid phase. Above 21.0T and up until saturation a commensurate magnetic structure exists with a ferromagnetic component along $b$ and an antiferromagnetic component along $c$.

cond-mat.str-el↗

arXiv2016

Crystal growth, structure and magnetic properties of Ca10Cr7O28

A detailed diffraction study of Ca10Cr7O28 is presented which adds significant new insights into the structural and magnetic properties of this compound. A new crystal structure type was used where the a and b axes are doubled compared to previous models providing a more plausible structure where all crystallographic sites are fully occupied. The presence of two different valences of chromium was verified and the locations of the magnetic Cr5+ and non-magnetic Cr6+ ions were identified. The Cr5+ ions have spin-1/2 and form distorted kagome bilayers which are stacked in an ABC arrangement along the c axis. These results lay the foundation for understanding of the quantum spin liquid behavior in Ca10Cr7O28 which has recently been reported in [C. Balz et al., Nature Physics, 12, 942 (2016)].

cond-mat.str-el↗