arXiv subjects

J. Wosnitza

Publications and source records attributed to J. Wosnitza.

At least 19 recordsLinked to original sources

arXiv2026

Investigation of CeRh$_2$As$_2$ order parameters via ultrasound propagation anomalies

Unconventional superconductors with nearly degenerate pairing states are rare. CeRh$_2$As$_2$ has recently emerged as one of the few existing multi-phase superconductors. It exhibits a first-order phase transition between two distinct superconducting states when a magnetic field is applied along the crystallographic $c$-axis. While this behavior has been linked to locally broken inversion symmetry, a phase diagram based on a multi-component superconducting order parameter remains a possibility. Furthermore, superconductivity appears to coexist with an ordered state (phase I). Despite being the subject of many studies, little is known about the nature of the order parameters in both superconducting phases and phase I. Here, we use ultrasound-propagation measurements at low temperatures, in high magnetic fields and under hydrostatic pressure to address this issue. Our results strongly suggest that the superconducting order parameter in both phases is single-component, corroborating the role of local non-centrosymmetry in the development of multi-phase superconductivity in CeRh$_2$As$_2$. In addition, analysis of the elastic anomalies within the Landau framework of phase transitions strongly suggests the presence of an incommensurate magnetic order parameter in phase I.

cond-mat.supr-con↗

arXiv2026

Acoustic signatures of the field-induced electronic-topological transitions in YbNi$_4$P$_2$

We investigated the magnetoelastic properties of an YbNi$_4$P$_2$ single crystal at low temperatures under magnetic fields directed along the crystallographic [001] axis. We report a series of strong anomalies in the sound velocity, which is consistent with the cascade of electronic-topological transitions reported previously for this compound. In particular, we identify the vanishing of a small orbit on the Fermi surface, associated with a quantum-oscillation frequency of 34 T. Furthermore, the different transitions are better resolved with acoustic modes of particular symmetry. Using a microscopic model adapted to the strongly correlated electronic structure of YbNi$_4$P$_2$, we describe our results by inspecting realistic electron-phonon couplings in reciprocal space for each acoustic mode. This shows how the $k$ selectivity of ultrasound experiments allows to investigate Fermi-surface reconstructions in strongly correlated electronic systems.

cond-mat.str-el↗

arXiv2026

Magnetoelastic properties in the high-temperature magnetic phase of the skyrmion compound GdRu$_2$Si$_2$

We investigated the magnetoelastic properties of a GdRu$_2$Si$_2$ single crystal under a magnetic field applied along the crystallographic [001] and [110] directions. We report a series of strong anomalies in the sound velocity that is consistent with the complex phase diagram reported previously for this compound. In particular, in our study we focus on the recently identified magnetic phase in the high-temperature region. We show that while this phase is easily destroyed for magnetic fields applied along [001], it is rather stable for fields along [110]. Furthermore, we introduce a Landau theory and a microscopic toy model describing the elastic response at zero field. We reproduce qualitatively the observed anomalies for different acoustic modes, which allows us to propose a magnetic structure for this new high-temperature phase.

cond-mat.str-el↗

arXiv2025

Pressure-Tuned Metamagnetism and Emergent Three-Body Interactions in CsFeCl$_3$

We present a combined experimental and theoretical study of the triangular-lattice quantum antiferromagnet CsFeCl$_3$ under high magnetic fields and high pressure. Pulsed-field magnetization for the magnetic field along the symmetric $c$ direction at ambient pressure reveals a magnetization process from a nonmagnetic singlet ground state with a nearly linear increase between 3.7 and 10.7 T, a plateau-like region, and then a sharp stepwise metamagnetic transition near 32 T. Wide frequency--field range electron spin resonance indicates that the low-field regime originates from the $J = 1$ manifold, while the high-field metamagnetic transition suggests a level crossing between the $J = 1$ and $J = 2$ lowest states. Pulsed-field magnetic susceptibilities measured with a proximity detector oscillator under high pressure show that the low-field nonmagnetic singlet phase is gradually suppressed, while the high-field metamagnetic transition evolves into an increasingly rich pattern of fractional steps. While the observations at low to intermediate fields can be understood within the established spin-1 description, the high-field regime requires a new perspective, which we provide through a projected spin-1/2 framework built from Zeeman-selected crystal-field states not related by time reversal. This construction naturally allows emergent three-body interactions on triangular plaquettes and explains the asymmetric evolution of the fractional steps in the magnetization. Our findings reveal that high-field effective spin models in quantum magnets with separated yet accessible crystal-field multiplets are not constrained to even-body couplings, but can naturally host odd-body terms, opening a broader avenue for realizing field-asymmetric magnetization processes and exotic phases beyond conventional even-body physics.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic anisotropy and dipolar interactions in the frustrated triangular-lattice magnet NaGdS_2

In this comprehensive study, we present results of bulk measurements (magnetization, specific heat, ac susceptibility, thermal expansion, and magnetostriction) combined with local methods such as nuclear magnetic resonance (^23Na NMR) and electron spin resonance (ESR) and simulations (McPhase) on polycrystalline and single-crystalline NaGdS_2 samples. The rare-earth delafossite NaGdS_2 is a triangular-lattice magnet with S = 7/2 spin-only Gd^3+ moments with suppressed single-ion anisotropy. In our study, we estimate that NaGdS_2 has a weak antiferromagnetic exchange (J_H/k_B is about 52mK) and signs of long-range magnetic order are absent down to lowest temperature. However, indications of short range magnetic order are found below 180 mK in the ac susceptibility and thermal expansion. Our results indicate an interplay of Heisenberg-type and dipolar exchange. Due to the large moment of the Gd^3+ ions, one expects a strong impact of the dipolar coupling in NaGdS_2, in contrast to the related NaYbS_2. ESR and ^23Na NMR measurements, indeed, indicate the formation of short-range ferromagnetic correlations. NaGdS_2 appears to be a rare system, in which magnetic order is suppressed by a competition between Heisenberg and dipolar interactions.

cond-mat.str-el↗

arXiv2025

Contrasting magnetic behavior in MnSc_2X_4 (X = S, Se) spinel compounds investigated by magnetoelastic studies

The spinel compounds MnSc_2X_4 are highly frustrated and candidate materials for vortex-like 3q magnetic states, such as skyrmions, with propagation vectors in the [111] plane. Because of the strong magnetoelastic coupling, we could extract a refined magnetic (H, T) phase diagram for MnSc_2S_4 from ultrasound and dilatometry measurements. We found a variety of magnetic phases, including the skyrmion phase, which is stable down to lowest temperatures. In comparison, we investigated MnSc_2Se_4 , having a larger distance between the magnetic Mn^3+ ions using the same methods. Unlike in MnSc_2S_4 , we found no skyrmion phase and overall a lack of sharp anomalies indicative of phase transitions, neither in dilatometry nor ultrasound nor in specific heat and ac-susceptibility data. Motivated by our findings, we performed model calculations, which reproduced the experimentally observed magnetostriction and specific-heat results reasonably well.

cond-mat.str-el↗

arXiv2025

Large out-of-equilibrium magnetocaloric effect in rare-earth zirconate pyrochlores

We explore the magnetic properties of Nd$_2$Zr$_2$O$_7$ and Pr$_2$Zr$_2$O$_7$ single crystals subjected to pulsed magnetic fields up to 60 T using magnetization and magnetocaloric-effect (MCE) measurements, with initial temperatures ranging from 2 to 31K. The MCE data exhibit pronounced and unconventional hysteresis loops, in which the sample temperature increases during both the up-sweep and down-sweep of the field. In Nd$_2$Zr$_2$O$_7$, the MCE further displays a striking plateau as a function of time, followed by a rapid temperature rise that begins at the maximum applied field, across pulses with differing peak-field strengths. Our magnetization measurements reveal an inferred temperature of the magnetic subsystem that differs significantly from the directly measured sample temperature and exhibits opposite hysteresis: the temperature is higher on the up-sweep than the down-sweep, unlike the direct measurements. These observations indicate a breakdown of thermal equilibrium between magnetic and lattice degrees of freedom on the timescale of the pulse ($\sim 10^{-1}$s). We interpret the results using a phenomenological model involving two thermally coupled subsystems - the magnetic ions and phonons, and a thermal reservoir, which accounts well for the behavior of Pr$_2$Zr$_2$O$_7$. However, it fails to reproduce the plateau seen in Nd$_2$Zr$_2$O$_7$. Agreement with Nd$_2$Zr$_2$O$_7$ data is improved substantially if we allow the thermal coupling between the magnetic and the lattice subsystems to depend on the product $\frac{HdH}{dt}$. Our results reveal anomalously slow heat transfer between magnetic and lattice subsystems and point toward a novel mechanism for dynamically controlling the heat flow in Nd$_2$Zr$_2$O$_7$ via the rate of magnetic field variation.

cond-mat.str-el↗

arXiv2025

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

We report multi-frequency electron spin resonance spectroscopy studies of $\alpha$-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↗

arXiv2025

Spontaneous lattice distortion and crystal field effects in HoB4

The tetraboride HoB4 crystallizes in a tetragonal structure (space group P4/mbm), with the Ho atoms realizing a Shastry-Sutherland lattice. It orders antiferromagnetically at TN1 = 7.1 K and undergoes further magnetic transition at TN2 = 5.7 K. The complex magnetic structures are attributed to competing order parameters of magnetic and quadrupolar origin with significant magnetoelastic coupling. Here, we investigate the response of the lattice of HoB4 across the antiferromagnetic phase transitions by using low-temperature powder x-ray diffraction and ultrasound-velocity measurements, supported by crystal electric field (CEF) calculations. Below TN2, the crystal structure of HoB4 changes to monoclinic (space group P21/b) as a macroscopic manifestation of the quadrupolar ordering. Between 300 and 3.5 K, the total distortion amplitude is 0.46~\AA\ and the relative volume change is $3.5 \times 10^{-3}$. This structural phase transition is compatible with the huge softening of the modulus $C_{44}$ observed around TN2 due to ferroquadrupolar order. A lattice instability developing immediately below TN1 is seen consistently in x-ray and ultrasound data. CEF analysis suggests a quasi-degenerated ground state for the Ho$^{3+}$ ions in this system.

cond-mat.str-el↗

arXiv2025

Slow magnetic quantum oscillations in the c-axis magnetoresistance of UTe$_2$

Details of the electronic band structure in unconventional superconductors are key to the understanding of their fundamental ground state. The potential spin-triplet superconductor UTe$_2$, with $T_\mathrm{c}\approx 2.1\,$K, has attracted attention recently. Its main Fermi surface consists of weakly corrugated, two-dimensional Fermi-surface cylinders that run along the crystallographic $c$ axis. In addition, there is evidence for the presence of an additional small three-dimensional band. This has been discussed controversially as it may be essential for the realization of superconductivity in UTe$_2$. Here, we investigate the angle-resolved magnetoresistance and Hall effect in bulk crystalline samples with current along the $c$ axis in fields up to $60\,$T. We observe low-frequency magnetic quantum oscillations with light effective masses that are most pronounced for magnetic field applied along the $a$ axis. Two distinct frequencies indicate two separate changes in the Fermi-surface topology, likely connected with Lifshitz transitions. We discuss the origin of these oscillations in terms of magnetic breakdown, quantum interference, and other potential mechanisms.

cond-mat.str-el↗

arXiv2025

Spin-strain interactions under hydrostatic pressure in $\alpha$-RuCl$_3$

We investigate the effects of hydrostatic pressure on $\alpha$-RuCl$_3$, a prototypical material for the Kitaev spin model on a honeycomb lattice with a possible spin-liquid ground state. Using ultrasound measurements at pressures up to 1.16 GPa, we reveal significant modifications of the acoustic properties and the $H$-$T$ phase diagram of this material. Hydrostatic pressure suppresses the three-dimensional magnetic order and induces a dimerization transition at higher pressures. At low pressures, the sound attenuation exhibits a linear temperature dependence, while above 0.28 GPa, it becomes nearly temperature independent, suggesting a shift in the phonon scattering regime dominated by Majorana fermions. These findings provide new insights into spin-strain interactions in Kitaev magnets and deliver a detailed characterization of the $H$-$T$ phase diagram of $\alpha$-RuCl$_3$ under hydrostatic pressure.

cond-mat.str-el↗

arXiv2025

Rotational magnetoelastic interactions in the Dzyaloshinskii-Moriya magnet Ba$_2$CuGe$_2$O$_7$

We report the magnetoelastic properties of a Ba$_2$CuGe$_2$O$_7$ single crystal at low temperatures under a magnetic field applied along the crystallographic [001] axis. Our results extend to low temperature the $H-T$ phase diagram determined for this compound by neutron scattering. Furthermore, we observe that specific elastic modes are better sensitive to the various magnetic transitions. In particular, we observe an unusual coupling between the in-plane transverse acoustic mode and the cycloidal order at low field, which suggests a novel spin-strain mechanism originating from Dzyaloshinskii-Moriya interaction in this compound.

cond-mat.str-el↗

arXiv2025

Anisotropic magnetism and spin fluctuations in the triangular-lattice spin-liquid candidate NaYbSe$_2$: a single-crystal $^{23}$Na and $^{77}$Se NMR study

The ytterbium-based delafossite NaYbSe$_2$ is discussed as a prototype for a spin-orbit entangled, effective spin-1/2 triangular spin lattice with emerging antiferromagnetic correlations and a quantum-spin-liquid (QSL) ground state. We report on a comprehensive study of the static and dynamic anisotropic magnetism in single-crystalline samples of NaYbSe$_2$, using NMR spectroscopy as a local-probe technique. We performed $^{23}$Na and $^{77}$Se NMR measurements in magnetic fields up to 16 T, applied along the in-plane and out-of-plane crystallographic directions and at temperatures from 300 down to 0.3 K. We could determine the anisotropic hyperfine contributions from the angular dependence of the $^{23}$Na and $^{77}$Se NMR spectra. In the paramagnetic regime, we probed the temperature dependence of the $^{23}$Na and $^{77}$Se spectral shift and the hyperfine coupling constants for fields applied along the principal crystal axes. The spin-lattice relaxation-rate data indicate critical spin fluctuations and the absence of long-range magnetic order at low magnetic fields and temperatures down to 0.3 K, evidenced by a monotonic increase of $1/T_1$ and associated spectral broadening. This is a clear proof of the evolution of a critical QSL ground state with residual fluctuations down to lowest temperatures. At elevated fields, we observe the emergence of long-range order, as the temperature-dependent $1/T_1$ rate passes through a pronounced maximum at $T_N$ at given field, followed by a decrease at lower temperatures. Further, we find an inhomogeneous broadening of the $^{23}$Na spectra below $T_N$, probing the histogram of the local-field distribution in the presence of the field-induced order.

cond-mat.str-el↗

arXiv2024

Transmission of radio-frequency waves and nuclear magnetic resonance in lanthanum superhydrides

The discovery of near-room temperature superconductivity in the lanthanum hydride LaH$_{10}$ has revolutionized this field of research. However, the need to use diamond anvils for the synthesis of such superconductors severely limits the number of experimental techniques to study these materials. Nuclear magnetic resonance (NMR) is one of the key methods for probing spin systems of superconductors. In this work, we show how $^1$H NMR measurements can be realized in diamond anvil cells to study high-temperature superconductivity in lanthanum polyhydrides at pressures up to 165 GPa. In the newly discovered superhydride LaH$_{12}$, we observed a pronounced suppression of the $^1$H NMR signal intensity below $\textit{T$_{c}$(onset)}$ = 260 K in a magnetic field of 7 T, corresponding to the screening of the radio-frequency pulses. Below the critical temperature of superconductivity, all $^1$H NMR characteristics, including the spin-lattice relaxation rate $\textit{1/T$_{1}$T}$, demonstrate pronounced features, evidencing the bulk nature of the superconducting transition. In zero field, the radio-frequency signal transmission through the LaH$_{12}$ sample shows a pronounced drop below $\textit{T$_{c}$(onset)}$ = 267 K, confirming the superconducting nature of the transition. A description of the $\textit{1/T$_{1}$T}$ data with an exponential form allows the estimation of the superconducting gap $\textit{${\Delta}$(0)}$ lying between 427 and 671 K (corresponding to 36.8 to 57.8 meV), and the ratio $\textit{R$_{\Delta}$ = 2${\Delta}$(0)/k$_B$T$_c$}$ between 3.76 and 5.16 in the synthesized hydride sample.

cond-mat.supr-con↗

arXiv2024

High-field magnetic properties of the alternating ferro-antiferromagnetic spin-chain compound Cu$_2$(OH)$_3$Br

We present comprehensive high magnetic field studies of the alternating weakly coupled ferro-antiferromagnetic (FM-AFM) spin-$1/2$ chain compound Cu$_2$(OH)$_3$Br, with the structure of the natural mineral botallackite. Our measurements reveal a broad magnetization plateau at about half of the saturation value, strongly suggesting that the FM chain sublattice becomes fully polarized, while the AFM chain sublattice remains barely magnetized, in magnetic fields at least up to $50$ T. We confirm a spin-reorientation transition for magnetic fields applied in the $ac^\ast$-plane, whose angular dependence is described in the framework of the mean-field theory. Employing high-field THz spectroscopy, we reveal a complex pattern of high-frequency spinon-magnon bound-state excitations. On the other hand, at lower frequencies we observe two modes of antiferromagnetic resonance, as a consequence of the long-range magnetic ordering. We demonstrate that applied magnetic field tends to suppress the long-range magnetic ordering; the temperature-field phase diagram of the phase transition is obtained for magnetic fields up to $14$ T for three principal directions ($a$, $b$, $c^\ast$).

cond-mat.str-el↗

arXiv2024

Magnon-phonon interactions in the spinel compound MnSc$_2$Se$_4$

We investigated the magnetic and magnetoelastic properties of MnSc$_2$Se$_4$ single crystals at low temperature under a magnetic field directed along the crystallographic [111] axis. The magnetization data at low temperature show a linear increase with magnetic field, until saturation is reached above 15 T. In ultrasound, a longitudinal acoustic mode shows a softening in field, which is absent for a transverse acoustic mode. We discuss these results using a microscopic model based on the framework of linear spin-wave theory. The magnetic and magnetoelastic data are qualitatively reproduced by considering magnon-phonon interactions arising from exchange-striction coupling between the crystal lattice and spin-wave fluctuations in the zero-temperature limit.

cond-mat.str-el↗

arXiv2024

Atacamite Cu$_2$Cl(OH)$_3$ in High Magnetic Fields: Quantum Criticality and Dimensional Reduction of a Sawtooth-Chain Compound

We report an extensive high-field study of atacamite Cu$_2$Cl(OH)$_3$, a material realization of quantum sawtooth chains with weak interchain couplings, in continuous and pulsed magnetic fields up to 58 T. In particular, we have mapped the entropy landscape for fields as high as 35 T and have identified a field-induced quantum critical point at 21.9(1) T for $\mathbf{H} \parallel c$ axis. The quantum critical point separates field regions with and without magnetic order, evidenced by our thermodynamic study and $^1$H nuclear magnetic resonance spectroscopy, but lies far below full saturation of the magnetization. Corroborated by numerical results using density-matrix renormalization group (DMRG) calculations, we find this behavior associated with a dimensional reduction of the spin system: the sawtooth chain effectively decouples into an antiferromagnetic spin-$1/2$ chain (backbone of the sawtooth chain) in the presence of an exchange field produced by the remaining field-polarized spins.

cond-mat.str-el↗

arXiv2024

Origin of the non-Fermi-liquid behavior in CeRh2As2

Unconventional superconductivity in heavy-fermion systems appears often near magnetic quantum critical points (QCPs). This seems to be the case also for CeRh2As2 (Tc $\approx$ 0.31 K). CeRh2As2 shows two superconducting (SC) phases, SC1 and SC2, for a magnetic field along the c axis of the tetragonal unit cell, but only the SC1 phase is observed for a field along the basal plane. Furthermore, another ordered state (phase-I) is observed below T0 $\approx$ 0.48 K whose nature is still unclear: Thermodynamic and magnetic measurements pointed to a non magnetic multipolar state, but recent $\mu$SR and NQR/NMR experiments have clearly detected antiferromagnetic (AFM) order below T0 . Also, quasi-two-dimensional AFM fluctuations were observed in NMR and neutron-scattering experiments above T0. The proximity of a QCP is indicated by non-Fermi-liquid (NFL) behavior observed above the ordered states in both specific heat $C(T)/T \propto T^{-0.6}$ and resistivity $\rho(T) \propto T^{0.5}$. These T-dependencies are not compatible with any generic AFM QCP. Because of the strong magnetic-field anisotropy of both the SC phase and phase I, it is possible to study a field-induced SC QCP as well a phase-I QCP by varying the angle $\alpha$ between the field and the c axis. Thus, by examining the behavior of the electronic specific-heat coefficient C(T)/T across these QCPs, we can determine which phase is associated with the NFL behavior. Here, we present low-temperature specific-heat measurements taken in a magnetic field as high as 21 T applied at several angles $\alpha$. We observe that the NFL behavior does very weakly depend on the field and on the angle $\alpha$, a result that is at odd with that observations in standard magnetic QCPs. This suggests a nonmagnetic origin of the quantum critical fluctuations.

cond-mat.str-el↗