arXiv subjects

Alexander A. Tsirlin

Publications and source records attributed to Alexander A. Tsirlin.

At least 19 recordsLinked to original sources

arXiv2026

Magnon band splitting without altermagnetism in CuF2

Conclusive identification of an altermagnetic state requires going beyond mere symmetry arguments. We illustrate this in a combined computational and experimental study of the rutile-like material CuF$_2$, which is on the list of predicted altermagnets. Using ab initio and linear spin-wave calculations supplied by magnetization measurements, we show that CuF$_2$ in its experimental monoclinic structure can be described by a spin-$\frac12$ model of weakly coupled square-lattice layers with the in-plane coupling $J_1\simeq 115$ K and two synergistic antiferromagnetic interplane couplings amounting to 4% and 8% of $J_1$, respectively. Driven by long-range superexchange, these interlayer couplings are oblique to the square planes, resulting in the unit-cell doubling in the magnetically ordered state, thus effectively suppressing any altermagnetic band splitting. Concurrently, we identify unusually strong Dzyaloshinskii-Moriya interactions, $|\mathbf D|/J_1\simeq 0.3$, that produce spin canting and, together with order-by-disorder effect, pin the Néel vector to the crystallographic $b$-axis. Additionally, DM anisotropy promotes magnon band splitting, but these bands remain non-chiral. Our results highlight the importance of relativistic effects even in $3d$ magnets with altermagnetic symmetries.

cond-mat.str-el↗

arXiv2026

Frustrated Gd3+ Double Perovskites as High-Performance Magnetocaloric Materials for Sub-100 mK Adiabatic Demagnetization Refrigeration

Achieving temperatures below 100 mK is essential for advancing quantum technologies and exploring fundamental quantum phenomena. While paramagnetic salts have traditionally enabled adiabatic demagnetization refrigeration (ADR), their limitations have driven the search for more effective alternatives. In this work, we present Gd3+-based double perovskites, Ba2GdSbO6 and Sr2GdSbO6, as high-performance magnetocaloric materials. Starting ADR from 2 K and 5 T, these compounds reach 67 mK and 68 mK in small finite magnetic fields, and 83 mK and 78 mK in zero field, respectively, which are the lowest reported ADR temperatures for Gd3+ magnets (S = 7/2) under these conditions. The frustrated geometry and the complex interplay of exchange and dipolar interactions suppress their antiferromagnetic ordering temperatures to 100 and 166 mK for Ba2GdSbO6 and 190 mK for Sr2GdSbO6, while maintaining an outstanding entropy density of 189 mJ K-1 cm-3 and 201 mJ K-1 cm-3, respectively. Notably, Ba2GdSbO6 sustains sub-100 mK cooling even at finite fields of 0.5 T, making it particularly promising for practical ADR applications.

cond-mat.mtrl-sci↗

arXiv2026

Band-Saddle-Point Engineering in Mixed $A$-site $A$V$_3$Sb$_5$ Kagome Metals

$A$V$_3$Sb$_5$ compounds ($A$ = K, Rb, Cs) offer an experimental platform for probing the physics of kagome metals near ideal Van Hove filling. Here, we use optical spectroscopy on $A$V$_3$Sb$_5$ with mixed $A$-site compositions to reveal the effect of the alkali metal on the low-energy electronic structure. Supported by density-functional-theory calculations, we identify a band-saddle-point inversion as a chemical consequence of Cs substitution, revealing that $A$-site composition controls the orbital character of Van Hove singularities, Fermi surface topology, and electron-phonon interactions while leaving the kagome layer intact.

cond-mat.str-el↗

arXiv2026

Raman signatures of a non-reciprocal magnetic phase transition in Ca$_2$RuO$_4$

The magnetic behavior of Ca$_2$RuO$_4$ represents a unique puzzle due to the interplay of strong electronic correlations, magneto-elastic interactions, and large spin-orbit coupling. At low temperatures, an anomalous Mott insulating state emerges, characterized by a complex antiferromagnetic order with a collective amplitude excitation of the magnetic moment which has been discussed in analogy with the Higgs mode. We report here evidence of a magnetic first-order phase transition driven by an out-of-plane magnetic field along the crystallographic $c$-axis of Ca$_2$RuO$_4$. Raman measurements in magnetic field reveal the emergence of mode of magnetic origin and a concomitant modification of the coupling between a phonon and the Higgs amplitude mode. Both the Raman features are characterized by a non-reciprocal hysteresis in magnetic field. Surprisingly, the observed phase transition does not affect the Raman scattering from the in-plane magnons, indicating that the dipolar antiferromagnetic order is preserved. This is consistent with the onset of a new quadrupolar field-controlled state, whose fluctuations structure can trigger an hybridization between the lattice and the magnetic Higgs mode resulting in the observed Raman features.

cond-mat.str-el↗

arXiv2026

Ultralow-Field Triplon Condensation in a Spin-Ladder Magnet

We realise the first ultralow-field Bose-Einstein condensation of triplons in a spin-ladder magnet, uncovering a quantum critical point at only $μ_0 H_{c1}=0.17$ T in Henmilite ($\mathrm{Ca_2Cu(OH)_4[B(OH)_4]_2}$). Unlike dimer magnets, a ladder retains extended one-dimensional correlations in its gapped parent state, making this limit strongly fluctuation dominated. Thermodynamic, magnetoelastic, $μ$SR, and neutron-diffraction measurements overturn the previous assignment of zero-field antiferromagnetic order, establishing a quantum-disordered coupled-ladder parent state with persistent low-energy dynamics. The weak low-temperature anomaly instead marks a gap-controlled crossover from the correlated ladder regime into the activated quantum-disordered state. These measurements further reveal an exceptionally asymmetric ordered dome extending to $μ_0 H_{c2}\simeq 8.2$ T. Quantum Monte Carlo simulations for the relevant spin Hamiltonian place Henmilite just on the gapped side of the zero-field ladder-ordering instability, naturally accounting for the strong separation between the exchange and residual-gap scales and the tiny critical field. Our findings extend ultralow-field triplon condensation beyond the dimer paradigm and establish Henmilite as a platform for controlled tuning across quantum criticality in a fluctuation-dominated spin ladder.

cond-mat.str-el↗

arXiv2026

Magnetic and electric properties of the metallic kagome antiferromagnet CrRhAs

CrRhAs is an antiferromagnetic kagome metal predicted to host a nontrivial spin texture with vector spin chirality [Huang \textit{et al.}, \textit{npj Quantum Mater.} \textbf{8}, 32 (2023)]. We report the synthesis and basic characterization of CrRhAs single crystals, which exhibit an antiferromagnetic transition with $T_{\rm N}$ = 150~K, evidenced by electrical transport, heat capacity, and magnetization measurements. Hall resistivity varies linearly with magnetic field, i.e., there is no nonlinear Hall contribution. Intriguingly, the Hall coefficient changes sign between the configurations of $j \parallel ab, H \perp ab$ and $j \parallel c, H \perp c$, which is likely connected to a peculiar topology of the Fermi surface. Furthermore, for $j \parallel ab$, the Hall coefficient shows a pronounced and continuous enhancement below $T_{\rm N}$, signaling a significant reconstruction of the Fermi surface or an extra scattering from the magnons. Our results offer guidance for exploring anomalous electric transport phenomena in exotic magnetic systems.

cond-mat.str-el↗

arXiv2026

Thermodynamic signatures of a field-induced ordered intermediate phase in Na$_2$Co$_2$TeO$_6$

The honeycomb cobaltate Na$_2$Co$_2$TeO$_6$ has recently been proposed as a candidate material for hosting field-induced quantum spin liquid (QSL) behavior. Here, we present a comprehensive thermodynamic study of its low-temperature, high-field phase diagram using magnetization, specific heat, and magnetocaloric-effect measurements down to 1 K. In zero field, we observe a weak residual moment that provides further insight into the nature of the magnetic ground state. For in-plane magnetic fields ($B \parallel a^*$), we identify three field-induced transitions at $B_{c1} \simeq 6$ T, $B_{c2} \simeq 7.8$ T, and $B_{c3} \simeq 10.4$ T. The magnetic Grüneisen parameter and specific heat reveal clear thermodynamic signatures of these successive phase transitions enclosing two intermediate phases. Contrary to expectations for a field-induced QSL, the phase between $B_{c2}$ and $B_{c3}$ lacks enhanced magnetic entropy but instead shows behavior consistent with a distinct ordered state. Above $B_{c3}$, the absence of additional anomalies indicates a crossover to a conventional spin-polarized regime. Our results place stringent thermodynamic constraints on the proposed QSL scenario in Na$_2$Co$_2$TeO$_6$, calling for further microscopic investigations to establish the precise nature of the field-induced phases.

cond-mat.str-el↗

arXiv2026

Quantum spin ladder with ferromagnetic rungs in Bi$_2$CuO$_3$(SO$_4$)

We introduce Bi$_2$CuO$_3$(SO$_4$) as a rare example of a spin-ladder magnet with ferromagnetic interactions on the rungs. Its magnetic response is studied through measurements of heat capacity, temperature-dependent magnetic susceptibility, and field-dependent magnetization, as well as electron spin resonance spectroscopy. These experiments are complemented by density-functional-theory calculations combined with the construction of maximally localized Wannier functions and an analysis of the relevant superexchange pathways. Quantum Monte Carlo simulations are employed to model thermodynamic properties and to quantitatively determine the magnetic exchange parameters. Our combined approach identifies Bi$_2$CuO$_3$(SO$_4$) as a two-leg spin-ladder system with ferromagnetic rungs ($J'$ $\approx -208$ K) and antiferromagnetic legs ($J$ $\approx 258$ K). These interactions of similar magnitude arise from remarkably different superexchange pathways, with the Cu--Cu distance along the leg being almost twice as long than the respective distance along the rung. The antiferromagnetic leg coupling represents the strongest oxygen-mediated long-range superexchange in a Cu$^{2+}$ compound reported to date and sets the benchmark for the role of complex superexchange pathways in quantum magnets.

cond-mat.str-el↗

arXiv2026

Revisiting the symmetry and optical phonons of altermagnetic $α$-MnTe

Using infrared (IR) and Raman spectroscopies combined with high-resolution x-ray diffraction, we address several controversial aspects of altermagnetic $α$-MnTe. We show that mechanical stress applied to crystals of this material causes a drastic broadening of Bragg peaks that conceals signatures of additional phases present in the sample. Indeed, spatially resolved Raman spectroscopy reveals that the modes around 175 cm$^{-1}$ often reported in $α$-MnTe are not reproducible across different positions and samples and originate from the secondary phase of MnTe$_2$. By combining spectroscopic probes with ab initio calculations, we establish the IR-active optical phonon of $α$-MnTe around 155 cm$^{-1}$ ($E_{1u}$) and the Raman-active optical phonon around 100 cm$^{-1}$ ($E_{2g}$) at room temperature. Two intense Raman modes around 120 and 140 cm$^{-1}$ are shown to be intrinsic, even though they can not be assigned to $Γ$-point optical phonons. These modes couple to magnetic order in $α$-MnTe and also to the transient reflectivity resulting in coherent oscillations. Both 6-fold rotation symmetry and inversion symmetry are preserved in $α$-MnTe within our experimental resolution.

cond-mat.mtrl-sci↗

arXiv2026

Anisotropic magnetoelastic coupling in the honeycomb magnet Na$_3$Co$_2$SbO$_6$

We present magnetization and dilatometry measurements on the honeycomb cobaltate Na$_3$Co$_2$SbO$_6$ and map out its detailed field-temperature phase diagram down to sub-Kelvin temperatures. Our data for in-plane magnetic fields show a strongly anisotropic $c^{*}$-axis lattice response, which is dominated by the variation of Co--O--Co bond angles according to \textit{ab initio} calculations. At $T = 0.4$~K, the magnetization $M(B)$ exhibits step-like features that are also highly anisotropic. In the case of $B \parallel b$, a small hysteresis observed around the second field-induced magnetic transition ($B_{c2}$) indicates its first-order character, whereas divergence of the magnetic Grüneisen parameter at $B_{c2}$ is suppressed upon cooling and signals the absence of quantum critical behavior upon entering the field-polarized state. None of our thermodynamic measurements provide evidence for a field-induced quantum spin liquid state near or above $B_{c2}$.

cond-mat.str-el↗

arXiv2025

One-dimensional physics of the frustrated quantum magnet PHCC

We report a comprehensive microscopic study of the frustrated quantum magnet PHCC, (C$_4$H$_{12}$N$_2$)Cu$_2$Cl$_6$, using density-functional band-structure calculations combined with numerical quantum many-body simulations of the underlying spin Hamiltonian. We show that the magnetism of PHCC is captured by a one-dimensional model of the frustrated spin chain with alternating nearest-neighbor couplings ($J_1=23.1$ K, $J_1'=7.0$ K) and uniform next-nearest-neighbor couplings ($J_2=13.9$ K). This model, which can also be thought of as a zigzag ladder, provides a quantitative description of the magnetic susceptibility and the magnetization process, and accounts for the observed dispersion of the single-triplet band and its merging into a continuum near the Brillouin zone center. We also make predictions for the existence of sharp bound (anti-bound) states of two triplets, below (above) the bottom (upper) edge of the two-particle scattering continuum.

cond-mat.str-el↗

arXiv2025

Beyond kagome: $p$-bands in kagome metals

We review recent studies on quantum materials where transition-metal atoms give rise to $d$-bands typical of kagome metals. Using examples from several material families - AV$_3$Sb$_5$, FeGe, RV$_6$Sn$_6$, and LaRu$_3$Si$_2$ - we argue that $p$-bands contributed by elements beyond the kagome network also play a crucial role in the electronic instabilities, including the charge-density-waves and superconductivity in kagome metals.

cond-mat.str-el↗

arXiv2025

Low-field magnetization processes of hexagonal easy-plane altermagnet $α$-MnTe

Single crystals of $α$-MnTe were synthesized by chemical vapor transport using iodine as the transport reagent. Structural characterization by powder x-ray diffraction confirmed the hexagonal structure (space group P6$_{3}$/mmc). Magnetization $M(T)$ and specific heat $C_p(T)$ measurements revealed an antiferromagnetic phase transition at $T_N \approx307$ K. The magnetic entropy derived from the $C_p(T)$ data is consistent with the $S = 5/2$ spin state of Mn$^{2+}$ ions. Angle- and field-dependent magnetization measurements indicate complex magnetic responses associated with domains, and show an anomaly around 1 T. These features are analyzed using a phenomenological micromagnetic model that includes higher-order anisotropic exchange interactions coupling the weak ferromagnetic component and the antiferromagnetic order parameter. The model captures the generic behavior of magnetic states and demonstrates that the observed uniaxial and unidirectional anisotropies arise from metastable domain configurations and irreversible magnetization processes.

cond-mat.mtrl-sci↗

arXiv2025

Is ruthenium dioxide altermagnet?

Ruthenium dioxide was named as one of the first and most promising altermagnetic candidates with $d$-wave symmetry. We summarize key findings for this material and critically discuss prospects of making it altermagnetic.

cond-mat.str-el↗

arXiv2025

Pressure tuning of competing interactions on a honeycomb lattice

Magnetic exchange interactions are mediated via orbital overlaps across chemical bonds. Thus, modifying the bond angles by physical pressure or strain can tune the relative strength of competing interactions. Here we present a remarkable case of such tuning between the Heisenberg (J) and Kitaev (K) exchange, which respectively establish magnetically ordered and spin liquid phases on a honeycomb lattice. We observe a rapid suppression of the Neel temperature (TN) with pressure in Ag3LiRh2O6, a spin-1/2 honeycomb lattice with both J and K couplings. Using a combined analysis of x-ray data and first-principles calculations, we find that pressure modifies the bond angles in a way that increases the |K/J| ratio and thereby suppresses TN. Consistent with this picture, we observe a spontaneous onset of muon spin relaxation (muSR) oscillations below TN at low pressure, whereas in the high-pressure phase, oscillations appear only when T < TN/2. Unlike other candidate Kitaev materials, Ag3LiRh2O6 is tuned toward a quantum critical point by pressure while avoiding a structural dimerization in the relevant pressure range.

cond-mat.str-el↗

arXiv2025

Pressure-tuned spin chains in brochantite, Cu$_4$SO$_4$(OH)$_6$

Using high-pressure single-crystal x-ray diffraction combined with thermodynamic measurements and density-functional calculations, we uncover the microscopic magnetic model of the mineral brochantite, Cu$_4$SO$_4$(OH)$_6$, and its evolution upon compression. The formation of antiferromagnetic spin chains with the effective intrachain coupling of $J\simeq 100$\,K is attributed to the occurrence of longer Cu--Cu distances and larger Cu--O--Cu bond angles between the structural chains within the layers of the brochantite structure. These zigzag spin chains are additionally stabilized by ferromagnetic couplings $J_2$ between second neighbors and moderately frustrated by several antiferromagnetic couplings that manifest themselves in the reduced Néel temperature of the material. Pressure tuning of the brochantite structure keeps its monoclinic symmetry unchanged and leads to the growth of antiferromagnetic $J$ with the rate of 3.2\,K/GPa, although this trend is primarily caused by the enhanced ferromagnetic couplings $J_2$. Our results show that the nature of magnetic couplings in brochantite and in other layered Cu$^{2+}$ minerals is controlled by the size of the lattice translation along their structural chains and by the extent of the layer buckling.

cond-mat.str-el↗

arXiv2025

Pressure-induced strange metal phase in a metallic kagome ferromagnet

Strange metallicity with $T$-linear electrical resistance preceding high-$T_c$ superconductivity remains an enigmatic, yet crucial, signature of correlation physics. Using electrical transport and magnetization measurements up to 50 GPa, we show that such a strange-metal phase is formed in pressurized kagome ferromagnet CrNiAs. In contrast to other kagome materials, a linear suppression of the Curie temperature is found, with the ferromagnetic quantum critical point at $p_{\rm{c}} \approx 12.5$ GPa. Remarkably, from $p_{\rm{c}}$ up to the highest measured pressure, characteristic strange-metal behavior is observed, whereas magnetic field reinstates the Fermi liquid. Electronic structure calculations reveal robust weakly dispersive bands persisting unchanged beyond $p_{\rm{c}}$, possibly at the origin of the $T$-linear electrical resistance. This establishes pressurized kagome ferromagnets as an intriguing platform for strange-metal behavior.

cond-mat.str-el↗

arXiv2025

High-pressure modulation of breathing kagome lattice: Cascade of Lifshitz transitions and evolution of the electronic structure

The interplay between electronic correlations, density wave orders, and magnetism gives rise to several fascinating phenomena. In recent years, kagome metals have emerged as an excellent platform for investigating these unique properties, which stem from their itinerant carriers arranged in a kagome lattice. Here, we show that electronic structure of the prototypical kagome metal, Fe$_3$Sn$_2$, can be tailored by manipulating the breathing distortion of its kagome lattice with external pressure. The breathing distortion is suppressed around 15 GPa and reversed at higher pressures. These changes lead to a series of Lifshitz transitions that we detect using broadband and transient optical spectroscopy. Remarkably, the strength of the electronic correlations and the tendency to carrier localization are enhanced as the kagome network becomes more regular, suggesting that breathing distortion can be a unique control parameter for the microscopic regime of the kagome metals and their electron dynamics.

cond-mat.str-el↗