arXiv subjects

A. A. Tsirlin

Publications and source records attributed to A. A. Tsirlin.

At least 19 recordsLinked to original sources

arXiv2026

Magnetic circular dichroism of THz modes and selection rules of Raman-active optical phonons in the polar altermagnet candidate \ce{Mn2Mo3O8}

We investigated the magnetic and vibrational excitations in the collinear altermagnet candidate \ce{Mn2Mo3O8} by temperature dependent Raman scattering and magneto-optical THz time-domain transmission spectroscopy. By comparison to \textit{ab initio} calculations accurately capturing the eigenfrequencies of the vibrational eigenmodes, we identify all optical phonons, including the lowest-lying Raman modes of $A_1$ and $E_2$ type, which had remained elusive in a previous Raman study. Moreover, we compare the selection rules for optically active phonons in the paramagnetic and the magnetically ordered phases of \ce{Mn2Mo3O8} and analyze the Raman selection rules with respect to pseudo-angular momentum conservation. No evidence of the expected splitting of the degenerate paramagnetic $E_2$ optical phonons into modes with circular polarization upon magnetic ordering could be resolved, likely due to weak spin-orbit coupling typical for Mn$^{2+}$. In contrast, we observe strong magnetic circular dichroism at a broad THz excitation band, emerging in the magnetically ordered state. This band, potentially originating from two-magnon excitations, is only electric-dipole active and features a field-dependent two-component fine structure. Its magnetic circular dichroism vanishes above the spin-flop transition at 4~T.

cond-mat.str-el↗

arXiv2026

Pseudo-Goldstone mode in altermagnetic $α$-MnTe: high-field electron spin resonance studies

We report multi-frequency electron spin resonance spectroscopy studies of $α$-MnTe in magnetic fields up to $16$ T, applied along the easy anisotropy axis. At temperatures below $T_\mathrm{N} = 310$ K, we observe a single resonance line corresponding to the pseudo-Goldstone mode of the antiferromagnetic resonance (AFMR). This mode exhibits the isotropic behavior with $g_\mathrm{eff}=2.01$, consistent with a complete quench of the orbital angular momenta for Mn$^{2+}$ ions. At low temperatures, the resonance mode is remarkably narrow ($\sim50$ mT for the full width at the half-maximum at $5$ K). The AFMR mode exhibits substantial broadening with increasing temperature, which can be understood in terms of the magnon-magnon scattering

cond-mat.str-el↗

arXiv2026

Ground-state properties of the $S=3/2$ anisotropic triangular lattice antiferromagnet Na$_3$Cr(PO$_4$)$_2$

We report the crystal structure and magnetic properties of a $S=3/2$ anisotropic triangular lattice compound Na$_3$Cr(PO$_4$)$_2$ employing single-crystal and powder x-ray diffraction, magnetization, heat capacity, and $^{31}$P nuclear magnetic resonance (NMR) experiments, supported by the band structure calculations. Magnetic susceptibility exhibits a broad maximum around 3.5 K, indicating the presence of a short-range antiferromagnetic order, typical of a low-dimensional spin system. Magnetization and heat capacity manifest an antiferromagnetic long-range ordering at around $T_{\rm N} \simeq 2.6$ K. This was further confirmed by the drastic NMR line broadening and a peak in the nuclear spin-lattice and spin-spin relaxation rates. The isothermal magnetization data exhibit a field-induced spin-flop transition at around $μ_0H_{\rm SF} \simeq 1.7$ T reminiscent of an anisotropic two-dimensional magnet, before saturating above $μ_0H_{\rm sat} \simeq 4.5$ T. The saturation field was further upheld by the field-dependent NMR relaxation measurements at low temperatures. The $^{31}$P NMR spectral shape confirms the commensurate antiferromagnetic nature of the ordering below $T_{\rm N}$. \textit{Ab initio} calculations reveal a significant deformation of the triangular spin lattice, resulting in triangles with two antiferromagnetic couplings of similar strength and a much weaker coupling along the third side of the triangle.

cond-mat.mtrl-sci↗

arXiv2025

Inhomogeneous dynamic state in the double trillium lattice antiferromagnet KBaFe$_2$(PO$_4$)$_3$

The three-dimensional (3D) magnet KBaFe$_2$(PO$_4$)$_3$ hosts a double-trillium lattice of Fe$^{3+}$ (spin, $S=5/2$) ions offering a prototypical platform to study the frustration induced effects in 3D. Through magnetization, specific heat, $^{31}$P nuclear magnetic resonance (NMR), and muon spin relaxation ($μ$SR) experiments, supported by first principles calculations, we uncover an unconventional ground state. Despite strong antiferromagnetic interactions with a large Curie-Weiss temperature $θ_{\rm CW} = -70(2)$ K, no magnetic long-range order is observed down to 30 mK. Below $T^{\ast}\simeq 3.5$ K, the NMR linewidth becomes nearly field-independent and the spin-spin relaxation rate $1/T_2$ saturates, accompanied by an inhomogeneous distribution of transverse nuclear magnetization $M_{xy}$. The latter indicates the emergence of short-range dynamical correlations, which was further corroborated by a robust and field-insensitive broad maximum in specific heat. In $μ$SR, we detect neither a static internal field nor spin freezing; instead the relaxation remains dynamic and is best described by two coexisting dynamic relaxation channels: a dominant fast (sporadic) channel and a slower Markovian component. Their differing weights and fluctuation rates suggest microscopic inhomogeneity in spin dynamics. Altogether, KBaFe$_2$(PO$_4$)$_3$ exemplifies a rare high-spin stochiometric 3D antiferromagnet that evades ordering and instead fosters a mosaic of spin dynamics driven by strong geometric frustration intrinsic to the trillium lattice.

cond-mat.mtrl-sci↗

arXiv2025

Short-range Spin Freezing State in the Double Trillium Lattice Spin-Liquid Candidate KSrFe$_2$(PO$_4$)$_3$ Revealed via $^{31}$P NMR

A comprehensive $^{31}$P nuclear magnetic resonance (NMR) study, combined with thermodynamic measurements and first-principle band-structure calculations, has been conducted to explore the ground state of the $S = 5/2$ double trillium lattice antiferromagnet KSrFe$_2$(PO$_4$)$_3$. Our experimental results indicate that the magnetic ground state is neither a conventional three-dimensional (3D) long-range order (LRO) nor a pure gapless spin-liquid state, as conjectured previously [Boya et al., APL Mater. 10, 101103 (2022)]. Specifically, the observation of a nearly field-independent NMR linewidth below $T^{*}$ = (3.5 $\pm$ 0.4) K, and a significant enhancement of spin-spin relaxation rate $1/T_2$ below $2T^{*}$ (where $T^{*}$ is the characteristic temperature identified from the magnetic susceptibility), indicate a complex magnetic ground state where spin freezing coexists with persistent dynamics. Furthermore, we argue that the lack of magnetic LRO and the persistence of strong magnetic fluctuations in KSrFe$_2$(PO$_4$)$_3$ are unlikely to originate from intersite K/Sr disorder, rather arise due to intrinsic magnetic frustration. Our findings position KSrFe$_2$(PO$_4$)$_3$ into a broader family of geometrically frustrated magnets characterized by coexisting spin freezing and pronounced antiferromagnetic fluctuations, marking it as a promising platform for investigating exotic phenomena in 3D frustrated magnets.

cond-mat.str-el↗

arXiv2025

Optical conductivity and band gap in the double-Weyl candidate SrSi2 at ambient pressure

We probe the possible double-Weyl state in cubic SrSi2 using optical spectroscopy. The complex optical conductivity was measured in a frequency range from 70 to 22 000 cm-1 at temperatures down to 10 K at ambient pressure. The optical response of SrSi2 can be well separated into the intraband (free carriers) and interband contributions. Additionally, four infrared-active phonons are detected. As follows from the optical spectra, the free-carrier density decreases with decreasing temperature, consistent with an activation behaviour. Experimental interband conductivity juxtaposed with ab initio calculations shows that conventional density-functional theory fails to describe the electronic structure of SrSi2 in the vicinity of the Fermi level. A semi-local exchange-correlation potential allows a much better agreement with the experiment, resulting in the trivial (gapped) band structure of SrSi2. The direct gap estimated from the measurements is approximately 40 meV.

cond-mat.mes-hall↗

arXiv2025

Trigonal distortion in the Kitaev candidate honeycomb magnet BaCo2(AsO4)2

We conducted x-ray absorption (XAS) and magnetic circular dichroism (XMCD) measurements at the Co $L_{2,3}$ edges on single crystals of the Kitaev candidate honeycomb lattice compound BaCo$_2$(AsO$_4$)$_2$. The measurements employed the inverse partial fluorescence yield technique, which is ideal for acquiring reliable x-ray absorption spectra from highly insulating samples, enabling precise quantitative analysis. Our experimental results revealed a significant linear dichroic signal, indicating strong trigonal distortion in the CoO$_{6}$ octahedra in BaCo$_2$(AsO$_4$)$_2$. We performed a detailed analysis of the experimental XAS and XMCD spectra using a full-multiplet configuration-interaction cluster model. This analysis unveiled that the $t_{2g}$ hole density is predominantly localized in the $a_{1g}$ orbital. Through XMCD sum rules and theoretical calculations, we quantified both the spin and orbital magnetic moments. Our study demonstrates that the local electronic structure of the CoO$_{6}$ octahedra displays an effective trigonal distortion of approximately $-0.114$ eV. This distortion is larger than the Co $3d$ spin-orbit coupling constant, emphasizing the crucial impact of local structural distortions on the electronic and magnetic properties of BaCo$_2$(AsO$_4$)$_2$.

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

Proximate spin-liquid behavior in the double trillium lattice antiferromagnet K$_2$Co$_2$(SO$_4$)$_3$

We report proximate quantum spin liquid behavior in K$_2$Co$_2$(SO$_4$)$_3$ with the magnetic Co$^{2+}$ ions embedded on a highly frustrated three-dimensional double trillium lattice. Single-crystal and high-resolution synchrotron powder x-ray diffraction experiments reveal a structural phase transition at $T_{\rm t} \simeq 125$ K from high-temperature cubic to low-temperature monoclinic phase with the three-fold superstructure. Magnetization and heat capacity consistently show the formation of the $J_{\rm eff} =1/2$ state of Co$^{2+}$ below 50 K. In zero field, K$_2$Co$_2$(SO$_4$)$_3$ shows signatures of static magnetic order formed below $T^* \simeq 0.6$ K, but muon spin relaxation experiments reveal a large fluctuating component that persists down to at least 50 mK, reminiscent of quantum spin liquid (QSL). Static order is completely suppressed in the small magnetic field of $\sim 1$ T, and low-temperature heat capacity demonstrates the $T^2$ behavior above this field, another fingerprint of QSL. Ab initio calculations show a competition of several antiferromagnetic couplings that render K$_2$Co$_2$(SO$_4$)$_3$ a promising pseudospin-$\frac12$ material for studying quantum magnetism in the double trillium lattice geometry.

cond-mat.mtrl-sci↗

arXiv2025

Interplay of $d$- and $p$-states in RbTi$_3$Bi$_5$ and CsTi$_3$Bi$_5$ flat-band kagome metals

Shifting the Fermi level of the celebrated $AM_3X_5$ (135) compounds into proximity of flat bands strongly enhances electronic correlations and severely affects the formation of density waves and superconductivity. Our broadband infrared spectroscopy measurements of RbTi$_3$Bi$_5$ and CsTi$_3$Bi$_5$ combined with density-functional band-structure calculations reveal that the correlated Ti $d$-states are intricately coupled with the Bi $p$-states that form a tilted Dirac crossing. Electron-phonon coupling manifests itself in the strong damping of itinerant carriers and in the anomalous shape of the phonon line in RbTi$_3$Bi$_5$. An anomaly in these spectral features around 150 K can be paralleled to the onset of nematicity detected by low-temperature probes. Our findings show that the materials with low band filling open unexplored directions in the physics of kagome metals and involve electronic states of different nature strongly coupled with lattice dynamics.

cond-mat.str-el↗

arXiv2025

Adiabatic demagnetization refrigeration with antiferromagnetically ordered NaGdP$_2$O$_7$

We present a comprehensive study of the structural, magnetic, and thermodynamic properties, as well as the adiabatic demagnetization refrigeration (ADR) performance of NaGdP$_2$O$_7$. Although NaGdP$_2$O$_7$ exhibits antiferromagnetic ordering at a Néel temperature of $T_{\rm N} = 570$ mK in zero field, ADR experiments achieved a minimum temperature of 220 mK starting from $T = 2$ K under an applied magnetic field of $μ_0H = 5$ T. The warm-up time back to $T = 2$ K exceeds 60 hours, which is roughly 50 times longer than that of its Yb-based analogue, underscoring the potential of NaGdP$_2$O$_7$ as an efficient precooling stage in double-stage ADR systems. We show that NaGdP$_2$O$_7$ can be seen as a network of ferromagnetic spin chains with antiferromagnetic interchain couplings and also investigate the influence of antiferromagnetic ordering on the magnetic entropy. We find that the temperature dependence of the entropy plays a more dominant role than its magnetic field dependence in the magnetically ordered state.

cond-mat.str-el↗

arXiv2025

Easy-cone state mediating the spin reorientation in topological kagome magnet Fe$_3$Sn$_2$

We investigated temperature-driven spin reorientation (SR) in the itinerant kagome magnet Fe$_3$Sn$_2$ using high-resolution synchrotron x-ray diffraction, neutron diffraction, magnetometry, and magnetic force microscopy (MFM), further supported by phenomenological analysis. Our study reveals a crossover from the state with easy-plane anisotropy to the high-temperature state with uniaxial easy-axis anisotropy taking place between $\sim40-130$~ K through an intermediate easy-cone (or tilted spin) state. This state, induced by the interplay between the anisotropy constants $K_1$ and $K_2$, is clearly manifested in the thermal evolution of the magnetic structure factor, which reveals a gradual change of the SR angle $\mathbfθ$ between $40-130$~K. We also found that the SR is accompanied by a magnetoelastic effect. Zero-field MFM images across the SR range show a transformation in surface magnetic patterns from a dendritic structure at 120~K, to domain wall dominated MFM contrast at 40~K.

cond-mat.mtrl-sci↗

arXiv2024

Ground-state properties of the double trillium lattice antiferromagnet KBaCr$_2$(PO$_4$)$_3$

Trillium lattices formed by corner-shared triangular units are the platform for magnetic frustration in three dimensions. Herein, we report structural and magnetic properties of the Cr-based double trillium lattice material KBaCr$_2$(PO$_4$)$_3$ studied by x-ray diffraction, magnetization, heat capacity, thermal conductivity, and $^{31}$P nuclear magnetic resonance (NMR) measurements complemented by density-functional band-structure calculations. Heat capacity and $^{31}$P NMR measurements reveal the magnetic transition at $T_{\rm N1} \simeq 13.5$ K in zero field followed by another transition at $T_{\rm N2} \simeq 7$ K in weak applied fields. The NMR sublattice magnetization confirms that the transition at $T_{\rm N1}$ is 3D in nature. The $^{31}$P spin-lattice relaxation rate in the ordered state follows the $T^3$ behavior indicative of the two-magnon Raman process. The spin lattice of KBaCr$_2$(PO$_4$)$_3$ comprises two crystallographically nonequivalent ferromagnetic sublattices that are coupled antiferromagnetically, thus eliminating frustration in this trillium network.

cond-mat.mtrl-sci↗

arXiv2024

Intriguing Low-Temperature Phase in the Antiferromagnetic Kagome Metal FeGe

The properties of kagome metals are governed by the interdependence of band topology and electronic correlations resulting in remarkably rich phase diagrams. Here, we study the temperature evolution of the bulk electronic structure of the antiferromagnetic kagome metal FeGe using infrared spectroscopy. We uncover drastic changes in the low-energy interband absorption at the 100 K structural phase transition that has been linked to a charge-density-wave (CDW) instability. We explain this effect by the minuscule Fe displacement in the kagome plane, which results in parallel bands in the vicinity of the Fermi level. In contrast to conventional CDW materials, however, the spectral weight shifts to low energies, ruling out the opening of a CDW gap in FeGe.

cond-mat.str-el↗

arXiv2024

Cluster-glass behaviour and large magnetocaloric effect in frustrated hyperkagome ferromagnet Li$_2$MgMn$_3$O$_8$

A detailed study of the structural and magnetic properties of the spin-$3/2$ hyperkagome lattice compound Li$_2$MgMn$_3$O$_8$ is reported. This material shows ferromagnetic response below $T_{\rm C} \simeq 20.6$ K, the temperature almost three times lower than the Curie-Weiss temperature $θ_{\rm CW} \simeq 56.6$ K. Density-functional band-structure calculations suggest that this reduction in $T_{\rm C}$ may be caused by long-range antiferromagnetic couplings that frustrate nearest-neighbor ferromagnetic couplings on the hyperkagome lattice. Large magnetocaloric effect is observed around the $T_{\rm C}$ with a maximum value of isothermal entropy change $ΔS_{\rm m}\simeq 20$ J/kg-K and a maximum relative cooling power of $RCP\simeq 840$ J/kg for the 7 T magnetic field change. Critical analysis of the magnetization data and scaling analysis of the magnetocaloric effect suggest the 3D Heisenberg/XY universality class of the transition. The DC and AC magnetization measurements further reveal glassy nature of the ferromagnetic transition. A detailed study of the non-equilibrium dynamics via magnetic relaxation and memory effect measurements demonstrates that the system evolves through a large number of intermediate metastable states and manifests significant memory effect in the cluster-glass state.

cond-mat.mtrl-sci↗

arXiv2024

Phonon and magnon dynamics across antiferromagnetic transition in 2D layered van der Waals material CrSBr

We report temperature-dependent reflectivity spectra of the layered van der Waals magnet CrSBr in the far-infrared region. Polarization-dependent measurements resolve the vibrational modes along the E$\|a$- and $b$-axes and reveal the clear structural anisotropy. While the $a$-axis phonons notably harden on cooling, the $b$-axis phonon frequencies are almost temperature-independent. A phonon splitting due to the antiferromagnetic phase transition is observed for the 180~cm$^{-1}$ $a$-axis vibrational mode, accompanied by a phonon softening below $T_N$. Furthermore, an additional mode with strong magnetic characteristics at $\sim$360~cm$^{-1}$ is identified and attributed to the magnon excitation of CrSBr.

cond-mat.mtrl-sci↗

arXiv2024

Double magnetic transition, complex field-induced phases, and large magnetocaloric effect in the frustrated garnet compound Mn$_{3}$Cr$_{2}$Ge$_{3}$O$_{12}$

A detailed study of the magnetic and magnetocaloric properties of a garnet compound Mn$_{3}$Cr$_{2}$Ge$_{3}$O$_{12}$ is carried out using x-ray diffraction, magnetization, heat capacity, and neutron diffraction measurements as well as \textit{ab initio} band-structure calculations. This compound manifests two successive magnetic transitions at $T_{\rm N1} \simeq 4.5$ K and $T_{\rm N2} \simeq 2.7$ K. Neutron powder diffraction experiments reveal that these two transitions correspond to the collinear and non-collinear antiferromagnetic ordering of the nonfrustrated Cr$^{3+}$ and frustrated Mn$^{2+}$ sublattices, respectively. The interactions within each of the Cr and Mn sublattices are antiferromagnetic, while the inter-sublattice interactions are ferromagnetic. The $H-T$ phase diagram is quite complex and displays multiple phases under magnetic field, which can be attributed to the frustrated nature of the spin lattice. Mn$_{3}$Cr$_{2}$Ge$_{3}$O$_{12}$ shows a large magnetocaloric effect with a maximum value of isothermal entropy change $ΔS_{\rm m} \simeq -23$ J/kg-K and adiabatic temperature change $ΔT_{\rm ad} \simeq 9$ K for a field change of 7 T. Further, a large value of the relative cooling power ($RCP \simeq 360$ J/kg) demonstrates the promise of using this compound in magnetic refrigeration.

cond-mat.str-el↗

arXiv2023

Disordered ground state in the spin-orbit coupled $J_{\rm eff}= 1/2$ cobalt-based metal-organic framework magnet with orthogonal spin dimers

We present the magnetic properties of a strongly spin-orbit coupled quantum dimer magnet based on Co$^{2+}$. The metal-organic framework compound Co$_2$(BDC)$_2$(DPTTZ)$_2$$\cdot$DMF features Co$^{2+}$ dimers arranged nearly orthogonal to each other, similar to the Shastry-Sutherland lattice. Our assessment based on the magnetization and heat capacity experiments reveals that the magnetic properties at low temperatures can be described by an effective $J_{\rm eff} = 1/2$ Kramers doublet and the ground state is a singlet with a tiny spin gap. Although the magnetic susceptibility could be analyzed in terms of the interacting dimer model with an isotropic intradimer coupling $J_0/k_{\rm B} \simeq 7.6$ K, this model fails to reproduce the shape of magnetization isotherm and heat capacity data. A model of isolated spin dimers with the anisotropic exchange couplings $J_{xy} \simeq 3.5$ K and $J_{z} \simeq 11$ K provides an adequate description to the magnetic susceptibility, magnetization isotherm, and heat capacity data at low temperatures. Interestingly, no field-induced quantum phase phase is detected down to 100~mK around the critical field of gap closing, suggesting the absence of Bose-Einstein condensation of triplons and establishing isolated dimers with a negligible interdimer coupling.

cond-mat.str-el↗