arXiv subjects

Philipp Gegenwart

Publications and source records attributed to Philipp Gegenwart.

At least 19 recordsLinked to original sources

arXiv2026

Huge Symmetric Elastoresistance in the Kondo Lattice YbRh$_2$Si$_2$

Heavy-fermion metals are prototype correlated electron systems for the study of Kondo entanglement and quantum criticality. We use the symmetry decomposed elastoresistance to uncover the fingerprints of strain-dependent Kondo scattering as function of temperature and magnetic field in the prototypical tetragonal Kondo lattice YbRh$_2$Si$_2$. By combining longitudinal and transverse resistance measurements under uniaxial strain applied along the tetragonal $[100]$ and $[110]$ directions, we obtain the elastoresistive responses in the in plane $A_{1g}$, $B_{1g}$, and $B_{2g}$ symmetry channels. While the responses in the symmetry-breaking channels are negligible, a huge enhancement of the isotropic symmetric $A_{1g}$ elastoresistance is found approaching -50 at low temperatures. Scaling analysis and comparison with linear thermal expansion measurements reveal that the symmetric elastoresistance probes the contribution of Kondo scattering to the strain dependence of magnetic entropy and signals strain- and field controlled quantum criticality upon cooling to 2 K.

cond-mat.str-el↗

arXiv2026

Low temperature thermodynamics of $S_{\mathrm{eff}}=1/2$ triangular lattice quantum spin liquid candidate TlYbS$_2$

Geometrically frustrated triangular-lattice antiferromagnets exhibit a delicate competition between magnetic order and quantum spin liquid (QSL) behavior, with the Yb-based delafossite family $A$Yb$X_2$ providing a structurally clean platform for exploring this physics. Here, we report a comprehensive study of single-crystal TlYbS$_2$ using DC magnetization, AC susceptibility, electron spin resonance (ESR), and specific heat measurements extending from room temperature to the millikelvin regime. Single-crystal X-ray diffraction confirms a trigonal $R\bar{3}m$ structure comprising well-separated triangular layers of Yb$^{3+}$ ions with no detectable site disorder. At zero field, a weak thermodynamic anomaly is observed near $530$ mK, which may indicate the onset of a weakly ordered state similar to that reported in KYbSe$_2$. Below $300$ mK, the zero-field magnetic specific heat follows $C_{\rm m}\propto T^{1.8}$, close to a quadratic temperature dependence, in contrast to the linear temperature dependence reported for the sister compound TlYbSe$_2$, which has been described in terms of the interplay between spinons and thermally excited gauge-flux excitations. Magnetization and ESR measurements establish pronounced easy-plane magnetic anisotropy, with $g_\perp/g_\parallel \approx 6.9$. The anomaly near $530$ mK exhibits a strongly anisotropic response to magnetic field. For $H\perp c$, it remains visible up to approximately $2$ T and continuously evolves toward a field-induced ordered phase above approximately $2.5$ T, followed by a sequence of field-induced phases that includes a $1/3$ magnetization plateau between approximately $5$ and $8$ T and full polarization near $17$ T. In contrast, for $H\parallel c$, the anomaly weakens and is suppressed near $3$ T, consistent with the recently reported confinement--deconfinement transition from an ordered state to a field-induced QSL.

cond-mat.str-el↗

arXiv2026

Switchable chiral antiferromagnetism through nonlinear magnetic susceptibility

Antiferromagnets (AFM) have attracted considerable attention in recent years because a number of nontrivial, technologically relevant properties associated with time-reversal symmetry (TRS) breaking are discovered in many materials. However, time-reversal (TR) partners of AFM states are generally challenging to be selected deterministically except in a few cases with nonzero net magnetization. Recently, it has been shown that the TR partners of the kagome spin ice ground state in HoAgGe with strictly zero net magnetization can be selected through a nonlinear magnetic susceptibility $χ^{(1)}$. In this work, we generalize this scenario to a broad class of chiral AFM with zero net magnetization but field-switchable ground states through $χ^{(1)}$. After a general discussion of $χ^{(1)}$ and its symmetry constraints, we first present a first-principles formalism for calculating it including both mean-field and self-consistent corrections, and then apply the approach to the noncollinear AFM family Mn$_3X$N ($X=$ Ni, Ag, Ga, Zn, Sn) in the $Γ_{5g}$ phase. An intuitive picture of the origin of $χ^{(1)}$ in similar noncollinear AFM is illustrated using 3-sublattice toy model. The model also predicts a nontrivial temperature dependence of $χ^{(1)}$ due to competitions between the longitudinal and transverse single-spin susceptibilities. Our work shows that nonlinear susceptibility can serve as a general protocol for accessing and switching TR partners in fully compensated AFM.

cond-mat.mtrl-sci↗

arXiv2026

Low-temperature magnetism and spin dynamics in the disordered triangular-lattice Yb$^{3+}$ compound LiCaYb$_5$(BO$_3$)$_6$

The interplay between geometric frustration, spin--orbit coupling, and structural disorder can give rise to unconventional magnetic ground states in rare-earth triangular-lattice magnets. We report low-temperature magnetic and $^7$Li NMR investigations of the disordered triangular-lattice compound LiCaYb$_5$(BO$_3$)$_6$ (LCYBO). Rietveld refinement confirms a hexagonal $P6_522$ structure with partial Ca/Yb antisite disorder and fractional Li occupancy. Magnetic susceptibility and magnetization measurements indicate a well-isolated effective $J_{\mathrm{eff}}=1/2$ Kramers doublet with weak antiferromagnetic interactions ($θ_{\mathrm{CW}} \approx -0.54$~K). The specific heat reveals a weak anomaly near 0.43 K, suggesting the development of short-range correlated magnetism rather than conventional long-range order. $^7$Li NMR spectra broaden strongly upon cooling, consistent with increasingly inhomogeneous internal magnetic fields. The spin-lattice relaxation exhibits two relaxation components consistent with disorder-induced distributions of local magnetic environments arising from antisite disorder and competing exchange pathways. Our results establish LCYBO as a structurally disordered frustrated triangular-lattice magnet with correlated low-energy spin dynamics.

cond-mat.dis-nn↗

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

Observation via spin Seebeck effect of macroscopic magnetic transport from emergent magnetic monopoles

Magnetic monopoles, elusive in high-energy physics, have been realised as emergent quasiparticles in solid-state systems, where their unique properties hold promise for novel spintronic applications. Magnetic monopoles have been invoked in diverse platforms, including skyrmion lattices, chiral magnets, soft ferromagnets, aritifical nanomagnets. Yet, a demonstration of their role in magnetic transport has remained elusive. Here, we report such an observation via the spin Seebeck effect in the bulk insulating pyrochlore oxide, spin ice $\mathrm{Dy_2Ti_2O_7}$. By applying a thermal gradient perpendicular to a $[111]$-oriented magnetic field, we detect a transverse spin Seebeck voltage marked by a dominant peak at the onset of monopole proliferation, accompanied by a secondary feature and frequency-dependent behavior. Our findings establish a direct link between monopole dynamics and magnetic transport in an insulating medium, establishing a new pathway for probing fractionalized excitations and advancing towards novel spintronic applications.

cond-mat.str-el↗

arXiv2026

Electrical observation via spin Seebeck effect of fractionalized excitations in a magnetic insulator

Fractionalized excitations are among the most striking signatures of emergence in quantum matter. While widely sought in frustrated magnets, their detection and characterization remain challenging, motivating the exploration of new probes. Meanwhile, Spintronics offers versatile tools for probing spin-related phenomena. In particular, the spin Seebeck effect (SSE) converts thermally driven magnetic excitations into a voltage in an adjacent metal, providing electrical access to the underlying dynamics and transport properties. Here we employ the SSE to probe emergent magnetic monopoles in the non-collinear Ising magnet Dy$_2$Ti$_2$O$_7$, a rare instance of a three-dimensional fractionalized magnet. We observe an SSE signal featuring a pronounced peak at monopole proliferation, accompanied by characteristic frequency and angular dependence. Our results broaden the scope of spintronic methods for detecting exotic excitations, provide new insights into magnetic insulators generally and monopole physics specifically, and suggest the potential of quantum materials as functional interfaces.

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

3D XY Universality and Nonlinear magnetic susceptibility in a kagome ice compound

Kagome spin ice is an intriguing class of spin systems constituted by in-plane Ising spins with ferromagnetic interaction residing on the kagome lattice, theoretically predicted to host a plethora of magnetic transitions and excitations. In particular, different variants of kagome spin ice models can exhibit different sequences of symmetry breaking upon cooling from the paramagnetic to the fully ordered ground state. Recently, it has been demonstrated that the frustrated intermetallic HoAgGe stands as a faithful solid-state realization of kagome spin ice. Here we use single crystal neutron diffuse scattering to map the spin ordering of HoAgGe at various temperatures more accurately and surprisingly find that the ordering sequence appears to be different from previously known scenarios: From the paramagnetic state, the system first enters a partially ordered state with fluctuating magnetic charges, in contrast to a charge-ordered paramagnetic phase before reaching the fully ordered state. Through state-of-the-art Monte Carlo simulations and scaling analyses using a quasi-2D model for the distorted Kagome spin ice in HoAgGe, we elucidate a single three-dimensional (3D) XY phase transition into the ground state with broken time-reversal symmetry (TRS). However, the 3D XY transition has a long crossover tail before the fluctuating magnetic charges fully order. More interestingly, we find both experimentally and theoretically that the TRS breaking phase of HoAgGe features an unusual, hysteretic response: In spite of their vanishing magnetization, the two time-reversal partners are distinguished and selected by a nonlinear magnetic susceptibility tied to the kagome ice rule. Our discovery not only unveils a new symmetry breaking hierarchy of kagome spin ice, but also demonstrates the potential of TRS-breaking frustrated spin systems for information technology applications.

cond-mat.str-el↗

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↗

arXiv2026

Absence of Long-Range Order and Magnetic Anisotropy in the Triangular Magnet NdMgAl$_{11}$O$_{19}$

We investigated the rare-earth triangular-lattice antiferromagnet NdMgAl$_{11}$O$_{19}$ using single-crystal magnetization (1.8~K $\leq T \leq$ 300~K, $μ_0 H \leq 7$~T) and specific-heat measurements down to 45~mK. The dc susceptibility confirms a well-isolated Kramers doublet ground state with pronounced Ising-type anisotropy, with $g_c \approx 3.7$ and $g_{ab} \approx 1.45$. Curie--Weiss fits yield weak, anisotropic antiferromagnetic exchange, with $θ_c = -0.54$~K and $θ_{ab} = -0.87$~K. Heat-capacity measurements show no long-range magnetic order down to 40~mK, corresponding to a frustration index $f \gtrsim 20$. Instead, $C_m/T$ exhibits a broad maximum near 0.081~K whose magnitude and field evolution are consistent with short-range correlations in an anisotropic triangular lattice. Applied magnetic fields open a Zeeman gap where the specific-heat anomaly follows $Δ= g μ_B μ_0 H$, and $M(H,T)$ is well described by a Brillouin function for an effective $J = 1/2$ moment. The field tuning of the low-temperature entropy manifold allows self-cooling from 1.8~K to 53~mK by adiabatic demagnetisation from a 9~T field. These results identify NdMgAl$_{11}$O$_{19}$ as a nearly ideal weak-exchange triangular magnet with a field-tunable correlated ground state, where two-dimensional crossover effects may emerge from frustrated XXZ interactions.

cond-mat.str-el↗

arXiv2025

Sub 1 K Adiabatic Demagnetization Refrigeration with Rare-Earth Borates Ba$_3$XB$_9$O$_{18}$ and Ba$_3$XB$_3$O$_9$, X = (Yb, Gd)

Adiabatic demagnetization refrigeration (ADR) is regaining relevance for the refrigeration to temperatures below 1 K as global helium-3 supply is increasingly strained. While ADR at these temperatures is long established with paramagnetic hydrated salts, more recently frustrated rare-earth oxides were found to offer higher entropy densities and practical advantages since they do not degrade under heating or evacuation. We report structural, magnetic and thermodynamic properties of the rare-earth borates Ba$_3$XB$_9$O$_{18}$ and Ba$_3$XB$_3$O$_9$ with X = (Yb, Gd). Except for Ba$_3$GdB$_9$O$_{18}$, which orders at 108 mK, the three other materials remain paramagnetic down to their lowest measured temperatures. ADR performance starting at 2 K in a field of 5 T is analyzed and compared to literature results.

cond-mat.str-el↗

arXiv2025

Quantum Griffiths phase in the kagome Kondo lattice CeRh$_{0.9}$Pd$_{0.1}$Sn

CeRhSn is a valence fluctuating heavy-fermion metal with a twisted Ce-kagome lattice, displaying zero-field quantum criticality, previously associated with geometrical frustration. The partial substitution of Rh by Pd in CeRh$_{1-x}$Pd$_x$Sn enlarges the unit-cell volume, suppresses valence fluctuations, decreases the Kondo temperature and stabilizes a possible long-range antiferromagnetic (AFM) ordered ground state with $T_N=0.8$ K at $x=0.5$. Previous thermodynamic and spectroscopic measurements for $x=0.1$ suggested a quantum critical spin liquid. We report low-temperature dilatometry and magnetization measurements on CeRh$_{0.9}$Pd$_{0.1}$Sn and compare with published low-$T$ specific heat data. The absence of a Grüneisen parameter divergence excludes a conventional quantum criticality scenario. Instead, the weak power-law divergences signal non-Fermi liquid (NFL) behavior, according to a disorder-driven quantum Griffiths phase scenario. At low temperatures, a negative thermal expansion is found in fields above approximately 0.5 T and the NFL scaling breaks down, probably due to the polarization of AFM correlations.

cond-mat.str-el↗

arXiv2025

Antiferromagnetic ordering and critical behavior induced giant magnetocaloric effect in distorted kagome lattice Gd$_3$BWO$_9$

We synthesize the high-quality Gd$_3$BWO$_9$ single crystal and investigate its lowtemperature magnetic and thermodynamic properties. Below $T\rm_{N}$ = 1.08 K, the anisotropic behavior of magnetic susceptibilities reveals that the Gd$^{3+}$ moments exhibit the dominant antiferromagnetic coupling along the $c$-axis, while displaying a ferromagnetic arrangement in kagome plane. With pronounced magnetic frustration, in adiabatic demagnetization refrigeration experiments starting from initial conditions of 9 T and 2 K, Gd$_3$BWO$_9$ polycrystal reaches a minimum temperature of 0.151 K, significantly lower than its $T\rm_{N}$. Due to the high density of Gd$^{3+}$ ions ($S$=7/2), the maximum magnetic entropy change reaches over 50 J kg$^{-1}$ K$^{-1}$ under fields up to 7 T in Gd$_3$BWO$_9$, nearly 1.5 times as large as commercial sub-Kelvin magnetic coolant Gd$_3$Ga$_5$O$_{12}$(GGG). The H-T phase diagram of Gd$_3$BWO$_9$ under $H$//$c$ exhibits field-induced critical behavior near the phase boundaries. This observation aligns with the theoretical scenario in which a quantum critical point acts as the endpoint of a line of classical second-order phase transitions. Such behavior suggests the importance of further investigations into the divergence of magnetic Grüneisen parameter in the vicinity of critical field at ultralow temperatures.

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

Current-linear emergent induction of pinned skyrmion textures in an oxide bilayer

Emergent electromagnetic induction (EEMI) induced through current-driven spin dynamics was recently predicted and subsequently observed in helical spin magnets, opening a new direction in spintronics and paving the way towards further miniaturization of electronic circuit elements. In contrast to conventional inductors consisting of coil-like structures whose inductance $L$ shows a linear dependence on the cross-section $A$, emergent inductors exhibit an inverse ($\propto {A}^{-1}$) proportionality, favorable for the miniaturization of electrical devices. However, the expected current-linear response of the EEMI voltage has not been demonstrated. Magnetic skyrmions hold promise as a simple platform to study the conceptual foundations of EEMI from current-driven spin dynamics. We fabricated devices of thin film bilayers of ferromagnetic SrRuO$_3$ and paramagnetic SrIrO$_3$, which are known to host interfacial Néel skyrmions detected by the appearance of a topological Hall-effect (THE). A large, positive and current-linear inductive response is found to accompany the THE. In our experiment, the current-induced dynamics of pinned magnetic skyrmions creates a voltage both parallel and perpendicular to the applied electric current flow, corresponding to longitudinal and transverse induction, respectively. This is the first report of transverse EEMI, indicating an angle of $80^{\circ}$ between skyrmion motion and the applied current. Our observation of current-linear longitudinal and transverse EEMI is a hallmark of pinned dynamics of magnetic skyrmion textures in oxide heterostructures.

cond-mat.mes-hall↗