arXiv subjects

Natalia Drichko

Publications and source records attributed to Natalia Drichko.

At least 19 recordsLinked to original sources

arXiv2026

Raman Scattering Evidence for Fluctuating Kagome-Plane Moments in $\mathrm{B} \parallel [111]$ in Pr$_2$Zr$_2$O$_7$ pyrochlore

The exotic magnetic properties of the pyrochlore Pr$_2$Zr$_2$O$_7$ are determined by the coupling of spin, orbital, and lattice degrees of freedom. Magnetism in this material originates from the non-Kramers Pr$^{3+}$ ion, and the exotic behavior has been discussed both in the framework of dipole-quadrupole properties of magnetic moments and disorder splitting the non-Kramers doublet. We use magneto-Raman spectroscopy to probe the crystal electric field (CEF) excitations of Pr$^{3+}$ in magnetic fields up to 14 T applied along the [100] and [111] crystallographic directions at 2 K. For $\mathrm{B}\parallel[100]$, the field evolution of the Raman active crystal field modes is quantitatively described by conventional Zeeman splitting of the Pr$^{3+}$ ground state doublet, including the thermally populated upper Zeeman branch at elevated temperature. For $\mathrm{}{B}\parallel[111]$, the spectra separate into responses from the triangular and kagome sublattices, and the CEF excitations of Pr$^{3+}$ on the triangular lattice exhibit the expected Zeeman shift consistent with full moments polarization. In contrast, the CEF excitations of Pr$^{3+}$ on the kagome lattice do not split as expected and instead broaden strongly with increasing field. This behavior is inconsistent with a static polarized configuration and is captured phenomenologically by a motional-narrowing description in which the kagome plane moments fluctuate between Zeeman-split levels on a meV timescale. This result offers a natural explanation for the reduced magnetization observed in $\mathrm{}{B}\parallel[111]$ and provides further evidence of the dipole-quadrupole properties of magnetic moments. It highlights Raman scattering as a sensitive probe of exotic spin-orbital dynamics in frustrated magnets.

cond-mat.str-el↗

arXiv2026

Tuning J$_1$-J$_2$ in Quasi-2D Triangular Lattice Antiferromagnet $α$-SrCr$_2$O$_4$ via Uniaxial Pressure

Triangular lattice antiferromagnets first attracted attention as a frustrated magnetic lattice which can serve as a platform to realize the resonating valence bond state. While the triangular lattice itself was shown to support classical 120 degree order, many theoretical phase diagrams suggest a quantum spin liquid state within a small range of parameters. One possible avenue to achieve such a state is to tune the anisotropy of the triangular lattice antiferromagnet by applying uniaxial pressure. This motivated our Raman scattering study of quasi-two-dimensional antiferromagnet $α$-SrCr$_2$O$_4$ under applied uniaxial pressure. Under ambient conditions, $α$-SrCr$_2$O$_4$ develops long-range helical magnetic order below T$_N$ = 43 K. We identify two-magnon excitations associated with this long-range antiferromagnetic order below T$_N$ at 15.5 meV and 40 meV by comparison with spin wave calculations. We observe the two features from the two-magnon excitation shift away from (towards) each other under applied tensile (compressive) pressure, indicating a decrease (increase) in anisotropy. Raman active phonons show a shift to higher (lower) frequencies under applied compressive (tensile) pressure, indicating efficient transmission of pressure and tuning of the lattice. We show spin wave and two-magnon density of states calculations under uniaxial pressure are consistent with our experimental results.

cond-mat.str-el↗

arXiv2026

Many-body quantum geometric effects and entanglement at the 3D metal-insulator quantum phase transition

Quantum geometry has emerged as a unifying concept across condensed matter physics, underlying phenomena from nonlinear topological response to flat-band superconductivity. While usually formulated within band theory, quantum geometry remains meaningful in disordered interacting systems~\cite{resta1999electron}. Here we show that the first negative moment of the optical conductivity -- proportional to the zero temperature quantum Fisher information as a bound on the multipartite entanglement -- provides an experimental probe of quantum geometry across the three-dimensional metal-insulator quantum phase transition in phosphorus-doped silicon. We extract a quantum geometric length $\ell$ that characterizes the local wavefunctions. Far from the transition, this length is almost coincident with the Bohr radius of the hydrogenic phosphorus donors, reflecting their atomic-scale quantum geometry. Approaching the transition, $\ell$ is enhanced, but does not diverge continuously like a correlation length; it jumps discontinuously to infinity at the critical point. This reflects the UV domination of the sum rule in three dimensions that renders it insensitive to the critical fluctuations driving the diverging dielectric constant and correlation length. Its enhancement demonstrates a ``puffing" of the donor polarizability volume of quantum geometric origin, which yields a quantum geometric corrected Clausius-Mossotti description in closer agreement with the diverging dielectric response and provides a quantum mechanical foundation for the century-old Herzfeld metallization criterion.

cond-mat.str-el↗

arXiv2026

Charge-sensitive vibrational modes in BEDT-TTF salts: Signatures of charge ordering and site charge

BEDT-TTF-based organic conductors host a number of ground states, tuned by electron repulsion from Mott and charge ordered insulators to superconductors. Knowing charge distribution on the molecular sites in the insulating state of these materials is a key to understanding the origin of these ground states. We survey and discuss the C=C stretching modes in BEDT-TTF based molecular conductors. These molecular vibrations are extremely crucial in characterization of charge-ordered insulators, and are recently linked to superconductivity in some compounds. Focusing on the known examples of BEDT-TTF$^{+0.5}$ salts, we analyse the reliability of the C=C stretching modes for the determination of charge ordering and absolute site charge. Considering the charge-ordered states, a prominent shift in frequency of 141 cm$^{-1}$ per elementary charge $e$ for $ν_{27}(b_{1u})$ and 98 cm$^{-1}$$e$ for $ν_2$($a_g$) can be clearly realised, however, the distribution resulting from different compounds span over 20 cm$^{-1}$. For nominal BEDT-TTF$^{+0.5}$ compounds, the distribution of the resonance also extends around 20 cm$^{-1}$, yielding an unexpected large uncertainty of $Δρ~\approx~(~\pm~0.045)e$, which is presumably due to the influence of small differences in the structure. This highlights the limitations of charge-frequency relations to detect small deviations in absolute charge values on molecular lattice sites, and emphasises on the use of the relations to estimate charge-ordering, rather than absolute site charge.

cond-mat.str-el↗

arXiv2026

K$_2$Co$_2$(TeO$_{3}$)$_{3}$ $\cdot$ 2.5 H$_2$O : A mineral-inspired pseudo-honeycomb cobalt dimer antiferromagnet

In recent years, magnetically-frustrated triangular and honeycomb lattice cobaltates have seen extensive study in the pursuit of a quantum spin liquid (QSL) state in a real material. In this work, we describe the hydroflux synthesis of K$_2$Co$_2$(TeO$_{3}$)$_{3}$ $\cdot$ 2.5 H$_2$O (KCoTOH), a novel zemannite-type antiferromagnet (AFM) possessing structural elements of both triangular dimer and honeycomb structural motifs. Bulk magnetometry and specific heat data support the onset of long-range AFM order below $T_\text{N}$ = 7.6(1) K, with neutron diffraction and muon spin relaxation ($μ$SR) measurements placing the majority of the ordered moment within the pseudo-honeycomb plane. We resolve three unique oscillation frequencies from the zero-field $μ$SR spectra, additionally suggesting a remarkably low level of structural disorder in as-grown KCoTOH crystals. Whereas interactions between dimerized chains of Co$^{2+}$ cations are typically observed to be negligible or ferromagnetic in nature, the largely planar ordering motif observed in KCoTOH is instead stabilized by net antiferromagnetic interactions through bridging tellurite groups. This work highlights the potential of hydroflux synthesis methods in the stabilization of magnetic materials possessing novel and potentially more frustrated lattice geometries.

cond-mat.str-el↗

arXiv2026

Tuning Charge Order in $κ$-(BEDT-TTF)$_2$Hg(SCN)$_2$X (X=Br, Cl) via Uniaxial Strain

In condensed matter physics, experimental control over material properties reflects a deep understanding of the underlying physics. In recent years, meaningful progress has been made towards a description of the physics of correlated electron systems, but examples of control of these systems remain rare. In this work, we confirm a phase diagram theoretically proposed for organic Mott insulators. We use $κ$-(BEDT-TTF)$_2$Hg(SCN)$_2$X (X=Br,Cl) (BEDT-TTF = bis(ethylenedithio)tetrathiafuvalene) materials as experimental realization of the proposed model and demonstrate the ability to tune them both ways across a phase border between a Mott insulator with a uniformly distributed charge and a charge ordered state through the application of uniaxial strain. We induce charge order at 33 K in the quantum dipole liquid material $κ$-(BEDT-TTF)$_2$Hg(SCN)$_2$Br through the application of tensile strain of 0.4% along the c-axis. We suppress charge order down to 10 K in $κ$-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl by applying a tensile strain of 1.6% along the b-axis. We use Raman scattering spectroscopy to probe the charge state through analysis of charge sensitive molecular vibrations and a low frequency mode of collective dipole fluctuations close to the phase border.

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arXiv2026

Raman scattering spectroscopic observation of a ferroelastic crossover in bond-frustrated PrCd$_3$P$_3$

2D magnetism in triangular lattices has already shown potential for hosting exotic magnetic states. Control of these magnetic states, both in terms of magnetic properties and in terms of charge doping would be the next step. This makes materials which combine triangular lattice magnetic layers with layers hosting interesting structural or electronic properties particularly useful. PrCd$_3$P$_3$, studied in this work, is one of a family of materials where triangular lattice layers of magnetic rare earth ions alternate with semiconducting hexagonal CdP layers. Using Raman scattering spectroscopy we uncover a structural instability in the CdP layers, associated with a soft mode behavior of a phonon in these layers. Raman scattering detects crystal electric field excitations, and confirms a singlet ground state for Pr$^{3+}$ and splitting of the doublet levels as a result of the structural instability in CdP layers. While Pr$^{3+}$ is non-magnetic in PrCd$_3$P$_3$ we speculate that this family of materials can realize control of the magnetic layer through the CdP layer which can become ferroelectric under strain that would relieve frustration.

cond-mat.mtrl-sci↗

arXiv2025

Much ado about MOFs: Metal-Organic-Frameworks as Quantum Materials

Metal-organic frameworks (MOFs) are a highly tunable class of crystalline materials where metal atoms or clusters are connected by organic linkers. They offer a versatile platform for exploring quantum phenomena such as entangled magnetism, superconductivity, and topology. Particularly for magnetism, their modular chemistry enables extensive control over magnetic interactions, spin magnitudes, lattice geometries, and even light-responsiveness, making them a uniquely adaptable platform. However, despite their promise, their low-temperature behavior and magnetic properties remain largely unexplored and represent an underappreciated opportunity in quantum materials research. With potential applications ranging from quantum computation to energy transfer, we believe that MOFs and particularly magnetic MOFs offer a vast and largely untapped frontier for transformative discoveries and high-impact quantum materials research.

cond-mat.str-el↗

arXiv2025

Probing scattering of Raman phonons on magnetic and electronic excitations in pyrochlores Nd$_2$Zr$_2$O$_7$ and Nd$_2$Ir$_2$O$_7$

Magnetic rare earth atoms on pyrochlore lattice can produce such exotic magnetic states as spin ice and quantum spin ice. These states are a result of the frustration in the pyrochlore lattice, as well as crystal field degrees of freedom of rare earth atoms, and their interactions with the lattice. Raman scattering spectroscopy, which possess high spectral resolution and can easily access broad energy and temperature ranges, is an optimum tool to study these excitations and their interactions. In this work we follow Raman scattering of zone center phonons and crystal field excitations of Nd$^{3+}$ in Nd$_2$Zr$_2$O$_7$ and Nd$_2$Ir$_2$O$_7$ in the temperature range where these materials are paramagnetic. A comparison between an insulating Nd$_2$Zr$_2$O$_7$ and semimetallic Nd$_2$Ir$_2$O$_7$ materials allow us to distinguish between scattering of phonons on other phonons, crystal field excitations, and electrons, highlighting interactions between these degrees of freedom.

cond-mat.str-el↗

arXiv2025

Raman spectroscopic evidence for linearly dispersed nodes and magnetic ordering in the topological semimetal V$_{1/3}$NbS$_2$

Weyl semimetals are characterized by an electronic structure with linearly dispersed nodes and distinguished chirality, protected by broken inversion or time reversal symmetry. The intercalated transition metal dichalcogenide V$_{1/3}$NbS$_2$ is proposed as a Weyl semimetal. In this study, we report polarization-resolved magnetic and electronic Raman scattering of this material, probing both the magnetic order and the electronic structure. The electronic scattering reveals a linear with frequency continuum of excitations, as the signature of electronic transitions within the proposed Weyl nodes in a two-dimensional electronic structure. Additionally, two-magnon excitations of V moments are observed near 15 meV in the magnetically ordered phase below 50 K. These excitations are well reproduced by calculations based on the Fleury-Loudon theory using spin wave exchange parameters derived from the neutron scattering data of this material and confirm the antiferromagnetic character of the order. These magnetic and electronic scattering, observed in the same spectra, provide independent spectroscopic evidence for a collinear antiferromagnetic Weyl semimetal state in V$_{1/3}$NbS$_2$.

cond-mat.str-el↗

arXiv2024

Ramification of complex magnetism in Nd$_2$Ir$_2$O$_7$ observed by Raman scattering spectroscopy

Using Raman scattering spectroscopy, we uncover a complex magnetic behavior of Nd$_2$Ir$_2$O$_7$ , which stands out among magnetic pyrochlores by the lowest temperature of the all-in-all-out (AIAO) Ir moments ordering ($T^\mathrm{N}_{\small\rm{Ir}}=33$~K) and the highest temperature at which AIAO order of rare-earth Nd ions is detected ($T^\mathrm{*}_{\small\rm{Nd}}$=15~K). Our findings suggest that in the temperature range between 15~K and 33~K, Nd magnetic moments exhibit strong fluctuations, possibly originating from spin ice behavior. This complex behavior emerges from the interplay of strong spin-orbit coupling, electronic correlations, and geometric frustration on two magnetic pyrochlore sublattices of Nd and Ir ions. The ordering of Ir magnetic moments is accompanied by an appearance of one-magnon Raman modes at 26.3 and 29.6 meV compatible with the AIAO order and of a broad mode at 14 meV, which could be associated with spinon continuum arising from Nd spin ice fluctuations. While two one-magnon excitations show minimal temperature evolution with decreasing temperature, the 14 meV mode shifts to higher frequencies as the temperature approaches a crossover to Nd AIAO order, broadens, and disappears below 15~K. An additional two-magnon excitation of the AIAO Nd order at around 33 meV appears in the spectra at low temperatures. These rather high energies of magnetic excitations of Nd moments make Nd$_2$Ir$_2$O$_7$ a particularly attractive playground to study the rare-earth magnetism on the pyrochlore lattice.

cond-mat.str-el↗

arXiv2024

Tunable magnetism of Boron Imidazolate-based Metal-Organic Frameworks

Magnetic metal-organic frameworks (MMOFs), where magnetic metal nodes are connected into a crystal structure by organic linkers, have a potential to host exotic magnetic states. We present a study of bulk magnetic properties of four metal-organic frameworks with the same boron imidazolate linkers, Cu-BIF, Co-BIF, Ni-BIF, and newly synthesized Zn-BIF, displaying a variety of lattice structures and nontrivial magnetic behaviors. While non-magnetic Zn-BIF provides an offset of magnetic response, magnetic susceptibility measurements of the other three magnetic materials demonstrate the presence of weak magnetic interactions in these MOFs, which differ between materials by sign and size. Cu-BIF, where magnetic nodes are connected into octahedral cages, shows simple paramagnetic behavior. Triangular lattice Co-MOF shows antiferromagnetic interactions on the order of 1 K, and a spin-crossover-like effect in magnetic susceptibility due to thermal depopulation of excited crystal electric field levels. Magnetic properties of Ni-BIF suggest sizable ferromagnetic interactions. Using DC/AC susceptibility and variable-field DC magnetization, we detect cluster spin-glass behavior in Ni-BIF and discuss possible microscopic origins of this behavior. This work demonstrates the variety of magnetic properties that are possible with a single organic ligand, and establishes the low energy scale of magnetic interactions through superexchange in boron imidazolate frameworks.

cond-mat.mtrl-sci↗

arXiv2024

Exfoliation and Optical Properties of S=1 Triangular Lattice Antiferromagnet NiGa$_2$S$_4$

Two-dimensional (2D) van der Waals (vdW) materials have been an exciting area of research ever since scientists first isolated a single layer of graphene. Single layer magnetic materials can provide a pathway for vdW heterostructures with magnetic properties. While most of the magnetic vdW materials exhibit ordering transitions in the bulk, here we report a successful exfoliation of a triangular lattice S=1 antiferromagnet NiGa$_2$S$_4$, which already demonstrates exotic magnetism in the bulk material. We establish the number of layers of the material by atomic force microscopy (AFM) and detail a careful characterization using Raman and optical spectroscopy to demonstrate how the optical, electronic, and structural properties of NiGa$_2$S$_4$ change as a function of sample thickness. Optical measurements and electronic structure calculations of bulk versus monolayer NiGa$_2$S$_4$ confirm the material to be a Mott insulator with an electronic gap of about 1.5 eV, which slightly increases for layers below 10 L. We conclude with a theoretical analysis of the possibility of doping monolayer NiGa$_2$S$_4$ by proximity to a metal.

cond-mat.mtrl-sci↗

arXiv2024

SiO$_2$-mediated facile hydrothermal synthesis of spiroffite-type Co$_2$Te$_3$O$_8$

The hydrothermal synthesis of novel materials typically relies on both knowledge of the redox activities of all cations present in the reaction solution and a small toolset of so-called mineralizers to tune the solution's overall chemical potential. Upon the use of a less conventional mineralizer species, SiO$_2$, we show the stabilization of spiroffite-type Co$_2$Te$_3$O$_8$ under less forceful hydrothermal conditions than in previous reports. When synthesized in the presence of both SiO$_2$ and each respective alkali carbonate as a secondary mineralizer, silicon substitution in place of tellurium in the host structure becomes apparent, and the corresponding introduced disorder gives rise to enhanced low-temperature ferromagnetism. Our results highlight the complexities of underutilized and combined mineralizer species in the stabilization and tuning of complex magnetic ground states via hydrothermal synthesis techniques.

cond-mat.mtrl-sci↗

arXiv2024

Novel Dipole-Lattice coupling in the Quantum-Spin-Liquid Material $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$

A family of molecular Mott insulators on triangular lattice provided a few S=1/2 triangular quantum spin liquid candidates, with $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ being the most studied material of this group. The large number experimental works present a conflicting set of evidence, with some suggesting spin liquid behavior, while others point towards a valence bond solid with orphan spins. In this work we use Raman scattering spectroscopy to probe both local charge on molecular sites and lattice phonons as a function of temperature down to 6~K. Based on the analysis of the line shape of the BEDT-TTF charge sensitive vibration $ν_2$ on cooling below 40 K, we suggest a development of disordered fluctuating charge disproportionation on (BEDT-TTF)$_2$ dimers of amplitude as small as 0.06$e$. The lattice phonons show strong anomalous broadening on cooling only in the (c,c) scattering channel, associated with the developing charge disproportionation. We suggest an interpretation, where the coupling of disordered charge dipoles on dimers to the lattice results in anisotropic modulation of charge transfer integrals between dimer lattice sites. Such fluctuations would result in modulation of magnetic coupling between spins which can produce fluctuating charge ordered spin-singlet pairs.

cond-mat.str-el↗

arXiv2024

Comparative Raman Scattering Study of Crystal Field Excitations in Co-based Quantum Magnets

Co-based materials have recently been explored due to potential to realise complex bond-dependent anisotropic magnetism. Prominent examples include Na$_2$Co$_2$TeO$_6$, BaCo$_2$(AsO$_4$)$_2$, Na$_2$BaCo(PO$_4$)$_2$, and CoX$_2$ (X = Cl, Br, I). In order to provide insight into the magnetic interactions in these compounds, we make a comparative analysis of their local crystal electric field excitations spectra via Raman scattering measurements. Combining these measurements with theoretical analysis confirms the validity of $j_{\rm eff} = 1/2$ single-ion ground states for all compounds, and provides accurate experimental estimates of the local crystal distortions, which play a prominent role in the magnetic couplings between spin-orbital coupled Co moments.

cond-mat.str-el↗

arXiv2023

Chemical tuning of a honeycomb magnet through a critical point

BaCo2(AsO4)2 (BCAO) has seen extensive study since its initial identification as a proximate Kitaev quantum spin liquid candidate. Thought to be described by the highly anisotropic XXZ-J_1-J_3 model, the ease with which magnetic order is suppressed in the system indicates proximity to a spin liquid phase. Upon chemical tuning via partial arsenic substitution with vanadium, we show an initial suppression of long-range incommensurate order in the BCAO system to T = 3.0 K, followed by increased spin freezing at higher substitution levels. Between these two regions, at around 10% substitution, the system is shown to pass through a critical point where the competing J_1/J_3 exchange interactions become more balanced, producing a more complex magnetic ground state, likely stabilized by quantum fluctuations. This state shows how slight compositional change in magnetically-frustrated systems may be leveraged to tune ground state degeneracies and potentially realize a quantum spin liquid state.

cond-mat.str-el↗

arXiv2023

Raman Scattering Spectra of Boron Imidazolate Frameworks Containing Different Magnetic Ions

We present a Raman scattering spectroscopic study of boron imidazolate metal-organic frameworks (BIFs) with three different magnetic metal ions and one non-magnetic in a wide frequency range from 25 to 1700 cm$^{-1}$, which covers local vibrations of the linkers and well as collective lattice vibrations. We show that the spectral region above 800 cm$^{-1}$ belongs to the local vibrations of the linkers, which have the same frequencies for the studied BIFs without any dependence on the structure of the BIFs, and are easily interpreted based on the spectra of imidazolate linkers. In contrast, collective lattice vibrations, observed below 100 cm$^{-1}$, show a distinction between cage and two-dimensional BIFs structures, with a weak dependence on the metal node. We identify the range of vibrations around 200 cm$^{-1}$, which are distinct for each MOF, depending on a metal node. Our work demonstrates the energy hierarchy in the vibrational response of BIFs.

cond-mat.mtrl-sci↗