arXiv subjects

A. Pawbake

Publications and source records attributed to A. Pawbake.

11 recordsLinked to original sources

arXiv2026

Magnetic-field-induced magnon portfolio in a van der Waals magnet

Magnonic excitations are investigated in chromium oxychloride (CrOCl), a van der Waal (vdW) antiferromagnet prone to a multitude of magnetic phase transitions, with absorption experiments in a broad continuous energy range. At low magnetic fields, the magnon spectra show a strong bi-axial anisotropy and inform on the relative weights of the effective exchange coupling and the system anisotropies. As the magnetic field increases, magnons characteristic of a canted phase are first observed, with peculiarities attributed to in-plane anisotropies and magnon-magnon coupling. Subsequently, a hysteretic magnon spectrum appears as the system transitions to a ferrimagnetic state, with two new magnon branches partly coexisting with the lower energy canted phase branch, indicating the formation of spatially separated magnetic phases. Further changes in the magnon spectrum in higher magnetic fields accompany transitions to the different canted magnetic phases previously reported. Our experiments show that competing exchange interactions and ground states broaden the options to generate different kinds of magnonic excitations in the same vdW material upon the variation of external parameters.

cond-mat.mtrl-sci↗

arXiv2026

Magnetic field tuning of modulated magnetic orders in CrOCl at the two-dimensional limit

Chromium oxychloride is a van der Waals magnet with intrinsic competing exchange interactions, including a strong antiferromagnetic one, source of a very rich magnetic phase diagram, with ferrimagnetic, antiferromagnetic, and canted states, up to high magnetic fields. We investigate the sequence of these magnetic phases in thin layers of CrOCl using magneto-Raman scattering spectroscopy. We identify phases whose magnetic order is commensurate with the atomic lattice, and find signatures of strong magneto-striction, presumably of exchange origin. The coupling of the spin and atomic degrees of freedom in the crystal is observed down to the single-layer limit -- phonon modes significantly soften or stiffen, in a complex way due to the competition of interactions. The existence domains of the different phases change with the number of layers.

cond-mat.mes-hall↗

arXiv2026

Interplay of Charge and Magnetic Orders in SmNiC$_2$ Mediated by Electron-Phonon Interaction

We investigate the interplay between charge density wave (CDW) instabilities and ferromagnetism in SmNiC$_2$ using diffuse and inelastic x-ray scattering together with Raman spectroscopy. We identify a soft acoustic phonon driving the incommensurate CDW (I-CDW) and uncover a second Kohn anomaly at the wave vector of the commensurate CDW (C-CDW) stabilized in other $R$NiC$_2$ members ($R=$ rare earth). The marked softening of both phonons and their contrasting evolution with temperature reveal a competition between the two ordering tendencies. Alongside pronounced anomalies in the temperature dependence of the zone center and soft phonons, we observe the collective amplitude mode of the CDW, which collapses abruptly as ferromagnetism sets in. Surprisingly, the Kohn anomalies persist in the ferromagnetic state despite the degradation of Fermi-surface nesting conditions. Our experimental findings, supported by ab initio calculations, highlight the central role of the electron-phonon interaction in driving the CDW formation and tuning the balance between competing charge and magnetic orders.

cond-mat.str-el↗

arXiv2025

Optical phonons as a testing ground for spin group symmetries

Lattice vibrations are highly sensitive to crystal symmetries and their changes across phase transitions. The latter can modify irreducible (co)representations and corresponding infrared and Raman selection rules of phonons. This concept is established for relativistic magnetic point groups, simultaneously transforming spatial and spin coordinates. However, in altermagnets described by non-relativistic spin groups with disjunct symmetry operations for both vector spaces, the phonon selection rules have remained unexplored. Here, we present a detailed study of the infrared- and Raman-active modes in the collinear antiferromagnet and altermagnet candidate Co$_2$Mo$_3$O$_8$. Comparing to ab initio calculations accurately capturing the eigenfrequencies, we identify all expected phonon modes at room temperature and deduce their selection rules using both symmetry approaches. Importantly, we observe the change of selection rules upon antiferromagnetic ordering, agreeing with the relativistic symmetry approach, while the spin group formalism predicts no changes. Therefore, optical phonons can reveal the appropriate symmetry treatment.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic phases and zone-folded phonons in a frustrated van der Waals magnet

2D magnetic materials have attracted extensive research interest due to their potential application in nanospintronics, optospintronics, and in magnonics. Ferromagnetic as well as antiferromagnetic layered materials have been demonstrated and successfully inserted into van der Waals heterostructures. However, the effects of magnetic frustration in van der Waals materials and the possibilities offered by spin configurations characterized by nonlinear spin arrangements have not been fully considered yet. Herein, we establish the magnetic phase diagram of bulk CrOCl, a frustrated van der Waals magnet, using magnetization and magneto-optical spectroscopy techniques. In particular, we use the magnetic superstructures relative to the crystallographic unit cell and the associated rich zone-folded phonon series to describe the magnetic field induced phases. Theoretical calculations taking into account the competing nearest neighbors magnetic exchange interactions provide a unique insight into the lattice vibrations of this class of magnetic system. This study expands the scope of 2D magnetic materials and provides a methodology to characterize frustrated van der Waals magnets.

cond-mat.mes-hall↗

arXiv2024

Spin-Orbital-Lattice Coupling and the Phonon Zeeman Effect in the Dirac Honeycomb Magnet CoTiO$_3$

The entanglement of electronic spin and orbital degrees of freedom is often the precursor to emergent behaviors in condensed matter systems. With considerable spin-orbit coupling strength, the cobalt atom on a honeycomb lattice offers a platform that can make accessible the study of novel magnetic ground states. Using temperature-dependent Raman spectroscopy and high-magnetic field Raman and infrared (IR) spectroscopy, we studied the lattice and spin-orbital excitations in CoTiO$_3$, an antiferromagnetic material that exhibits topologically protected magnon Dirac crossings in the Brillouin zone. Under the application of an external magnetic field up to 22 T along the crystal's $c$-axis, we observed the splitting of both the spin-orbital excitations and a phonon nearby in energy. Using density functional theory (DFT), we identify a number of new modes that below the antiferromagnetic (AFM) transition become Raman-active due to the zone-folding of the Brillouin zone caused by the doubling of the magnetic unit cell. We use a model that includes both the spin and orbital degrees of freedom of the Co$^{2+}$ ions to explain the spin-orbital excitation energies and their behavior in an applied field. Our experimental observations along with several deviations from the model behavior point to significant coupling between the spin-orbital and the lattice excitations.

cond-mat.str-el↗

arXiv2024

Tuning THz magnons in a mixed van-der-Waals antiferromagnet

Alloying stands out as a pivotal technological method employed across various compounds, be they metallic, magnetic, or semiconducting, serving to fine-tune their properties to meet specific requirements. Ternary semiconductors represent a prominent example of such alloys. They offer fine-tuning of electronic bands, the band gap in particular, thus granting the technology of semiconductor heterostructures devices, key elements in current electronics and optoelectronics. In the realm of magnetically ordered systems, akin to electronic bands in solids, spin waves exhibit characteristic dispersion relations, featuring sizeable magnon gaps in many antiferromagnets. The engineering of the magnon gap constitutes a relevant direction in current research on antiferromagnets, aiming to leverage their distinct properties for THz technologies, spintronics, or magnonics. In this study, we showcase the tunability of the magnon gap across the THz spectral range within an alloy comprising representative semiconducting van-der-Waals antiferromagnets FePS$_3$ and NiPS$_3$. These constituents share identical in-plane crystal structures, magnetic unit cells and the direction of the magnetic anisotropy, but differ in the amplitude and sign of the latter. Altogether these attributes result in the wide tunability of the magnon gap in the Fe$_{1-x}$Ni$_x$PS$_3$ alloy in which the magnetic order is imposed by stronger, perpendicular anisotropy of iron.

cond-mat.mtrl-sci↗

arXiv2023

Magneto-optical sensing of the pressure driven magnetic ground states in bulk CrSBr

Competition between exchange interactions and magnetocrystalline anisotropy may bring new magnetic states that are of great current interest. An applied hydrostatic pressure can further be used to tune their balance. In this work we investigate the magnetization process of a biaxial antiferromagnet in an external magnetic field applied along the easy axis. We find that the single metamagnetic transition of the Ising type observed in this material under ambient pressure transforms under hydrostatic pressure into two transitions, a first-order spin flop transition followed by a second order transition towards a polarized ferromagnetic state near saturation. This reversible tuning into a new magnetic phase is obtained in layered bulk CrSBr at low temperature by varying the interlayer distance using high hydrostatic pressure, which efficiently acts on the interlayer magnetic exchange, and is probed by magneto-optical spectroscopy.

cond-mat.mes-hall↗

arXiv2022

Ring-Exchange Interaction Effects on Magnons in Dirac Magnet CoTiO$_3$

The magnetic interactions that determine magnetic order and magnon energies typically involve only two spins. While rare, multi-spin interactions can also appear in quantum magnets and be the driving force in the ground state selection and in the nature of its excitations. By performing time-domain terahertz and magneto-Raman spectroscopy measurements combined with theoretical modeling, we determine the origin of the magnon excitation gap in Dirac antiferromagnet CoTiO$_3$. By adding a ring-exchange interaction in a hexagonal plaquette of the honeycomb lattice to both an XXZ spin model and to a low energy spin-orbital flavor wave model, a gap is generated in the magnon spectrum at the Brillouin zone center. With this addition, the flavor wave model reproduces a large swath of experimental results including terahertz, Raman, inelastic neutron scattering, and magnetization experiments.

cond-mat.str-el↗

arXiv2022

Microscopic parameters of the van der Waals CrSBr antiferromagnet from microwave absorption experiments

Microwave absorption experiments employing a phase-sensitive external resistive detection are performed for a topical van der Waals antiferromagnet CrSBr. The field dependence of two resonance modes is measured in an applied field parallel to the three principal crystallographic directions, revealing anisotropies and magnetic transitions in this material. To account for the observed results, we formulate a microscopic spin model with a bi-axial single-ion anisotropy and inter-plane exchange. Theoretical calculations give an excellent description of full magnon spectra enabling us to precisely determine microscopic interaction parameters for CrSBr.

cond-mat.mes-hall↗

arXiv2022

Spatially resolved optical spectroscopy in extreme environment of low temperature, high magnetic fields and high pressure

We present an experimental set-up developed to perform optical spectroscopy experiments (Raman scattering and photoluminescence measurements) with a micrometer spatial resolution, in an extreme environment of low temperature, high magnetic field and high pressure. This unique experimental setup, to the best of our knowledge, allows us to explore deeply the phase diagram of condensed matter systems by tuning independently these three thermodynamic parameters, while monitoring the low-energy excitations (electronic, phononic or magnetic excitations), to spatially map the Raman scattering response or to investigate objects with low dimensions. We apply this technique to bulk FePS3, a layered antiferromagnet with a Neel temperature of T = 120 K.

cond-mat.mtrl-sci↗