arXiv subjects

Yurii G. Pashkevich

Publications and source records attributed to Yurii G. Pashkevich.

7 recordsLinked to original sources

arXiv2026

Strain-induced suppression of thermochromism in divalent cobalt molybdate thin films

Thermochromic oxides provide a platform for coupling lattice, electronic, and magnetic degrees of freedom, with divalent cobalt molybdate $CoMoO_4$ as a prototypical example. Despite extensive studies on powders, its thin-film behaviour - critical for device applications - remains largely unexplored. Here, we present a combined experimental and theoretical investigation of $CoMoO_4$ thin films, including the first THz-VIS-UV spectra of $β- CoMoO_4$ in thin-film form. In contrast to bulk powders, which undergo a first order $β\rightarrow α$ structural transition near 230K, the thin films retain the high-temperature $β-$phase across the entire temperature range. We show that microstrain fundamentally reshapes the phase landscape, suppressing thermochromism and stabilizing the $β-$phase. The optical spectra reveal pronounced phonon and electronic anomalies, including a softening of a low-energy, cation-dominated phonon (42cm$^{-1}$) upon cooling, in contrast to conventional mode-hardening. This behaviour indicates incipient atomic displacements analogous to those driving the bulk $β\rightarrow α$ transition, despite the absence of a structural phase change, and saturates between 200 and 150K. Temperature-dependent X-ray diffraction confirms persistent β-phase symmetry with increasing microstrain, consistent with strain-induced frustration of the first-order transition. At higher energies (0.12-3.7eV), the optical response exhibits a blue-shift of Co$^{2+}$crystal-field transitions and charge-transfer excitations, indicating a strain-enhanced ligand field. Supported by structural refinements and theoretical calculations, we identify crystal field strengthening as the key mechanism stabilizing the $β$-phase. These results establish strain as a thermodynamic lever to control phase stability and functional properties in thermochromic oxides.

cond-mat.mtrl-sci↗

arXiv2024

Composite antiferromagnetic and orbital order with altermagnetic properties at a cuprate/manganite interface

Heterostructures from complex oxides allow one to combine various electronic and magnetic orders as to induce new quantum states. A prominent example is the coupling between superconducting and magnetic orders in multilayers from high-Tc cuprates and manganites. A key role is played here by the interfacial CuO2 layer whose distinct properties remain to be fully understood. Here, we study with resonant inelastic X-ray scattering (RIXS) the magnon excitations of this interfacial CuO2 layer. In particular, we show that the underlying antiferromagnetic exchange interaction at the interface is strongly suppressed to J ~ 70 meV, as compared to J ~ 130 meV for the CuO2 layers away from the interface. Moreover, we observe an anomalous momentum dependence of the intensity of the interfacial magnon mode and show that it suggests that the antiferromagnetic order is accompanied by a particular kind of orbital order that yields a so-called altermagnetic state. Such a two-dimensional altermagnet has recently been predicted to enable new spintronic applications and superconducting proximity effects.

cond-mat.str-el↗

arXiv2021

Non-collinear and strongly asymmetric polar moments at back-gated SrTiO3 interfaces

The highly mobile electrons at the interface of SrTiO3 with other oxide insulators, such as LaAlO3 or AlOx, are of great current interest. A vertical gate voltage allows controlling a metal/superconductor-to-insulator transition, as well as electrical modulation of the spin-orbit Rashba coupling for spin-charge conversion. These findings raise important questions about the origin of the confined electrons as well as the mechanisms that govern the interfacial electric field. Here we use infrared ellipsometry and confocal Raman spectroscopy to show that an anomalous polar moment is induced at the interface that is non-collinear, highly asymmetric and hysteretic with respect to the vertical gate electric field. Our data indicate that an important role is played by the electromigration of oxygen vacancies and their clustering at the antiferrodistortive domain boundaries of SrTiO3, which generates local electric and possibly also flexoelectric fields and subsequent polar moments with a large lateral component. Our results open new perspectives for the defect engineering of lateral devices with strongly enhanced and hysteretic local electric fields that can be manipulated with various other parameters, like strain, temperature, or photons.

cond-mat.mtrl-sci↗

arXiv2021

Raman scattering of plane-wave and twisted light off chiral molecular liquids

We present an experimental study of the quasi-elastic Raman scattering (QES) of plane-wave and twisted light by liquid crystals. Depending on their temperature, these crystals can exhibit isotropic, nematic and chiral nematic phases. The question is addressed of how the phase of a crystal and the state of incident light can affect the quasi-elastic energy spectra of the scattered radiation, whose shape is usually described by the combination of Lorentzian and Gaussian components. Special attention is paid to the \textit{chiral phase}, for which the Raman QES spectrum is dominated by a Lorentzian with reduced linewidth, pointing to diminished disorder and configurational entropy. Moreover, this phase is also known for a regime of iridescence (selective backscattering) which arises when the wavelength of incident light becomes comparable with the chiral pitch length. Detailed measurements, performed in this \textit{resonant} regime and by employing twisted light, carrying various projections of the orbital angular momentum (OAM), have indicated a low-energy scattering surplus depending on OAM. We argue that this observation might indicate a transfer of angular momentum between light and the liquid crystal.

physics.optics↗

arXiv2020

Magnetism and its coexistence with superconductivity in CaK(Fe$_{0.949}$Ni$_{0.051}$)$_4$As$_4$: muon spin rotation/relaxation studies

The magnetic response of CaK(Fe$_{0.949}$Ni$_{0.051}$)$_4$As$_4$ was investigated by means of the muon-spin rotation/relaxation. The long-range commensurate magnetic order sets in below the Néel temperature $T_{\rm N}= 50.0(5)$~K. The density-functional theory calculations have identified three possible muon stopping sites. The experimental data were found to be consistent with only one type of magnetic structure, namely, the long-range magnetic spin-vortex-crystal order with the hedgehog motif within the $ab-$plane and the antiferromagnetic stacking along the $c-$direction. The value of the ordered magnetic moment at $T\approx3$ K was estimated to be $m_{\rm Fe}=0.38(11)$ $μ_{\rm B}$ ($μ_{\rm B}$ is the Bohr magneton). A microscopic coexistence of magnetic and superconducting phases accompanied by a reduction of the magnetic order parameter below the superconducting transition temperature $T_{\rm c}\simeq 9$ K is observed. Comparison with 11, 122, and 1144 families of Fe-based pnictides points to existence of correlation between the reduction of the magnetic order parameter at $T\rightarrow 0$ and the ratio of the transition temperatures $T_{\rm c}/T_{\rm N}$. Such correlations were found to be described by Machida's model for coexistence of itinerant spin-density wave magnetism and superconductivity [Machida, J. Phys. Soc. Jpn. 50, 2195 (1981) and Bud'ko et al., Phys. Rev. B 98, 144520 (2018)].

cond-mat.supr-con↗

arXiv2013

Lattice and orbital fluctuations in TiPO4

In the s = 1/2 antiferromagnetic spin chain material TiPO4 the formation of a spin gap takes place in a two step process with two characteristic temperatures, T*=111 K and TSP=74 K. We observe an unusual lattice dynamics over a large temperature regime as well as evidence for an orbital instability preceding the spin-Peierls transition. We relate different intrachain exchange interactions of the high temperature compared to the spin-Peierls phase to a modification of the orbital ordering pattern. In particular, our observation of a high energy excitation of mixed electronic and lattice origin suggests an exotic dimerization process different from other spin-Peierls materials.

cond-mat.str-el↗

arXiv2013

Interplay between lattice and spin states degree of freedom in the FeSe superconductor: dynamic spin state instabilities

Polarized Raman-scattering spectra of superconducting, single-crystalline FeSe evidence pronounced phonon anomalies with temperature reduction. A large 6.5% hardening of the B_1g(Fe) phonon mode is attributed to the suppression of local fluctuations of the iron spin state with the gradual decrease of the iron paramagnetic moment. The ab-initio lattice dynamic calculations support this conclusion. The enhancement of the low-frequency spectral weight above the structural phase transition temperature T_s and its change below T_s is discussed in relation with the opening of an energy gap between low (S=0) and higher spin states which prevents magnetic order in FeSe. The very narrow phonon line widths compared to observations in FeTe suggests the absence of intermediate spin states in the fluctuating spin state manifold in FeSe.

cond-mat.supr-con↗