arXiv subjects

H. Luetkens

Publications and source records attributed to H. Luetkens.

At least 19 recordsLinked to original sources

arXiv2026

Microscopic magnetic phase evolution in the Weyl semimetal Mn$_3$Sn revealed by $\mu^+$SR

We report a comprehensive muon spin relaxation ($\mu^+$SR) and bulk magnetization study of the antiferromagnetic (AFM) Weyl semimetal Mn$_3$Sn (composition Mn${2.99}$Sn). Mn$3$Sn is reported to exhibit a commensurate inverse triangular (IT) AFM phase, an incommensurate (IC) helical AFM phase, and a proposed low-temperature spin-glass-like state. In our sample, we identify the characteristic temperatures associated with these regimes as N'eel temperature ($T\mathrm{N} = 418$ K), a macroscopic bulk transition temperature between IT-AFM to IC helical phase ($T\mathrm{t} \approx 275$ K), and low-temperature transition $T_\mathrm{f} = 21$ K. Investigating the low-temperature regime below $T_\mathrm{f}$, we find no evidence of a static spin-glass state. Instead, the sample exhibits an increasing ferromagnetic (FM) component accompanied by a localized slowing of spin fluctuations, indicating that these phenomena may be decoupled. In the IC helical AFM phase, the zero-field (ZF) spectra are best described by damped oscillations with an empirical phase offset, consistent with anharmonic and amplitude-modulated order reported by scattering studies. Upon warming above 150 K, a continuous redistribution of muon spectral weight reveals a broad, homogeneous magnetic crossover between the IC helical and IT-AFM phases. In the commensurate IT-AFM phase above $T_\mathrm{t}$, a persistent missing fraction in the initial asymmetry indicates that a subset of implanted muons, corresponding to roughly 20% of the sample-related asymmetry, undergoes unresolved ultrafast depolarization. Finally, we observe temperature-driven shifts in muon site populations above 325 K. Ultimately, our results show a highly dynamic magnetic landscape in Mn$_3$Sn, demonstrating how its complex magnetic orders often coexist and evolve continuously with temperature.

cond-mat.str-el↗

arXiv2026

Quantum spin liquid on a 3D bipartite lattice of spin trimers stabilized by enhanced effective anisotropy

Quantum spin liquids (QSLs) represent highly entangled states of matter in which frustration-induced quantum fluctuations suppress any symmetry-breaking phase transition down to absolute zero, giving rise to fractionalized excitations and emergent gauge fields. Theoretically, bond anisotropy can stabilize QSLs even on bipartite lattices, as exemplified by the Kitaev honeycomb model; however, no material has so far been established to realize such a state as its true ground state. Here we identify the three-dimensional spin-trimer magnet KBa$_3$Ca$_4$Cu$_3$V$_7$O$_{28}$ as a promising candidate for a bipartite quantum spin liquid persisting to the lowest temperatures. Strongly coupled Cu$^{2+}$ trimers form effective pseudospin-1/2 degrees of freedom upon cooling, which in turn constitute a three-dimensional bipartite network. Bulk thermodynamic measurements, neutron scattering, $\mu$SR, and NMR detect no spin freezing or symmetry-breaking phase transition down to 20 mK, but instead reveal a gapless dynamical ground state with algebraic spin autocorrelations. Complementary Monte Carlo and exact-diagonalization calculations show that this state is stabilized by a strong enhancement of effective anisotropy: a weak microscopic Cu-Cu exchange anisotropy of approximately 15 percent is generically amplified at the trimer level, producing effective pseudospin-pseudospin interaction anisotropies of 60 to 100 percent. Our results establish trimer-based networks as a promising platform for realizing anisotropy-stabilized quantum entangled states, even in three-dimensional bipartite systems with only weak microscopic anisotropy.

cond-mat.str-el↗

arXiv2026

Correlated charge order intertwined with time-reversal symmetry-breaking nodal superconductivity in the dual flat band kagome superconductor CeRu$_{3}$Si$_{2}$

Kagome materials provide a powerful platform for exploring how flat electronic bands promote symmetry-breaking quantum states, yet studies have so far focused mainly on kagome-derived $d$-electron flat bands. In this paper, we introduce CeRu$_{3}$Si$_{2}$, a kagome superconductor in which our first-principles calculations show the coexistence of Ru $d$-orbital kagome flat bands and heavy-fermion flat bands derived from Ce$^{4+}$ $4f$-states. X-ray diffraction reveals a dominant 1/2 charge order with a much weaker 1/3 component persisting up to room temperature. Theoretical calculations further highlight the correlated nature of these charge-order states. Deep within the charge-ordered state, magnetoresistance emerges below 80 K and strengthens further below 30 K. Zero-field muon spin-rotation measurements show no time-reversal symmetry (TRS) breaking in the normal state, in contrast to LaRu$_{3}$Si$_{2}$ and YRu$_{3}$Si$_{2}$. However, an applied magnetic field induces weak magnetism. Across the $A$Ru$_{3}$Si$_{2}$ family ($A$ = La, Y, and Ce), the superconducting transition temperature $T_{\rm c}$ scales linearly with the onset temperature of normal-state TRS breaking $T_{\rm {TRSB}}$ and the magnitude of the field-induced magnetic response, revealing a direct positive correlation between normal-state symmetry breaking and superconductivity. Furthermore, we identify that CeRu$_{3}$Si$_{2}$ is the first 132-type kagome compound to host nodal superconductivity together with spontaneous internal magnetic fields, providing clear evidence for intrinsic TRS breaking in the superconducting state. These results establish CeRu$_{3}$Si$_{2}$ as a unique platform where intertwined kagome $d$- and heavy fermion $f$-electron flat bands generate a rich hierarchy of electronic orders.

cond-mat.supr-con↗

arXiv2026

Unified pressure and field response across distinct charge-order regimes in Ti-doped CsV$_3$Sb$_5$

Understanding the phase diagram of kagome superconductors from a microscopic perspective is crucial for clarifying the interplay between charge order and superconductivity. Ti-doped CsV$_{3}$Sb$_{5}$ exhibits a nonmonotonic temperature-doping phase diagram in which both $T_{\rm c}$ and the charge-order temperature initially decrease with doping, followed by a crossover from long-range to short-range charge order and a subsequent increase in $T_{\rm c}$. Here, we report a muon spin rotation ($\mu$SR) study of Ti-doped CsV$_{3}$Sb$_{5}$ at two representative compositions: underdoped (Ti$_{0.05}$-CVS) and optimally doped (Ti$_{0.22}$-CVS). Using zero-field, high-field, and high-pressure $\mu$SR, we find spontaneous time-reversal-symmetry (TRS) breaking in the normal state of both compositions, strongly enhanced by an applied magnetic field and associated with long-range and short-range charge-order correlations, respectively. In the superconducting state, both samples exhibit anisotropic nodeless pairing with low superfluid density. Hydrostatic pressure substantially enhances both $T_{\rm c}$ and the superfluid density (by $\sim$2.5), revealing a linear correlation between them and pointing to unconventional pairing. Above $\sim$1 GPa, a crossover from anisotropic to isotropic nodeless pairing is observed. Despite the different nature of charge order in the two doping regimes, the superconducting responses are remarkably similar, suggesting that the competition between superconductivity and charge order occurs on a local scale, largely independent of the long-range coherence of the charge-ordered state.

cond-mat.supr-con↗

arXiv2026

The stripe state at 1/8 Ba doping hosts optimal superconductivity in La-214 cuprates under low in-plane stress

The cuprate system La$_{2-x}$Ba$_{x}$CuO$_{4}$ (LBCO) exhibits a pronounced sensitivity to in-plane uniaxial stress, particularly near the 1/8 doping anomaly, where stripe order strongly suppresses bulk superconductivity. While previous studies have focused on compositions close to 0.125, the commensurate $x$=0.125 phase remains largely unexplored under symmetry-selective lattice tuning. Here, we combine muon-spin rotation (${\mu}$SR), AC susceptibility, and electrical resistivity to investigate superconductivity, spin-stripe order, and structural response in LBCO-0.125 under in-plane uniaxial stress applied 45$^\circ$ to the Cu-O bond direction. Complementary resistivity measurements on $x$=0.115 and 0.135 track the evolution across both sides of the anomaly. We observe a giant enhancement of the bulk superconducting transition temperature in LBCO-0.125, increasing from 5 K to 37 K under 0.5 GPa. While the onset temperature of spin-stripe order decreases only modestly, the magnetic volume fraction is reduced by about a factor of two, with local order preserved. Simultaneously, the resistivity peak associated with the LTT phase is fully suppressed across all dopings. These results demonstrate that suppression of the LTT phase and reduction of the static spin-stripe-ordered volume fraction are crucial for the development of optimal three-dimensional superconductivity. Strikingly, the composition $x$=0.125, with the most robust stripe stability and the lowest ambient-pressure $T_{\rm c}$ develops the highest $T_{\rm c}$ under stress, reaching a zero-resistance state at 37 K and an onset of the superconducting transition as high as 46 K. This behavior indicates that stripe-related interactions enhance pairing strength, while static stripe order competes with superconductivity primarily at the level of phase coherence rather than pairing itself.

cond-mat.supr-con↗

arXiv2026

Crystal electric field excitations and spin dynamics in a spin-orbit coupled distorted honeycomb magnet BiErGeO$_5$

The magnetic properties and crystal electric field (CEF) scheme of BiErGeO$_5$ are investigated via magnetization, heat capacity, muon spin relaxation (muSR), and inelastic neutron scattering (INS) experiments on a polycrystalline sample. The Er$^{3+}$ ions form a quasi-two-dimensional distorted honeycomb network with a Kramers doublet ground state. Magnetic susceptibility and heat capacity reveal short-range antiferromagnetic correlations, manifested as a broad maximum around 1.4 K. Heat-capacity data further confirm the onset of a magnetic long-range order at $T_ N = 0.4$ K. The INS spectra exhibit eight CEF excitations and the CEF analysis yields the $g$-factor anisotropy with $g_{xy}/g_{z} = 1.38$ and exchange anisotropy with $J_{xy} = 2.96$ K and $J_{z} = 1.56$ K. The experimental temperature and field dependent magnetization and heat capacity are also reproduced by the simulation using CEF energy scheme. Zero-field muSR measurements down to 30 mK, do not exhibit coherent oscillations or a static 1/3 tail. The spectra are well described by two exponential relaxation components, indicating two magnetically inequivalent muon environments. The relaxation rates display a nearly temperature-independent plateau below $T_{\rm N}$ and follow an Orbach-type activated behavior at higher temperatures involving excited CEF levels, consistent with the INS results. Longitudinal-field $\mu$SR measurements reveal only weak decoupling up to 1.5 T, indicating persistent slow spin fluctuations below $T_{\rm N}$.

cond-mat.mtrl-sci↗

arXiv2026

Uniaxial stress enhanced anisotropic magnetoresistance and superconductivity in the kagome superconductor LaRu$_{3}$Si$_{2}$

Elucidating the role of the kagome electronic structure in determining the various quantum ground states is of fundamental importance. In this work, we employ in-plane uniaxial stress as a tuning parameter to probe the electronic structure and its impact on the superconducting and normal-state properties of the kagome superconductor LaRu$_{3}$Si$_{2}$, combining magnetotransport measurements with first-principles calculations. We identify a pronounced anisotropy in both the upper critical field and the normal-state magnetoresistance, indicating strong electronic anisotropy despite the three-dimensional crystal structure. Furthermore, we find that the superconducting transition temperature $T_{\rm c}$ increases under in-plane stress applied within the kagome plane, although the enhancement is modest, reaching approximately 0.3 K at 0.6 GPa. Furthermore, the absolute magnetoresistance exhibits a pronounced increase from about 22${\%}$ at zero stress to 35${\%}$ at 0.6 GPa, indicating a substantial modification of the normal state above $T_{\rm c}$. Previous studies have reported time-reversal-symmetry (TRS) breaking below a temperature scale that coincides with the onset of magnetoresistance. The simultaneous enhancement of both $T_{\rm c}$ and magnetoresistance under stress therefore suggests a positive correlation between superconductivity and normal-state electronic and magnetic properties in LaRu$_{3}$Si$_{2}$. Detailed calculations demonstrate that stress-induced changes in $T_{\rm c}$ arise from the joint evolution of the total density of states and the flat band, whereas the large magnetoresistance enhancement is dominated by the stress-driven downward shift of the Ru $dz^{2}$ kagome flat band.

cond-mat.supr-con↗

arXiv2026

Competing Ordering Modes in the Distorted Quantum Kagome Material Clinoatacamite Cu$_2$Cl(OH)$_3$

We have studied the magnetic properties of clinoatacamite Cu$_2$Cl(OH)$_3$, the parent compound of the quantum spin liquid candidate herbertsmithite and a longstanding puzzle among frustrated quantum magnets. As we reveal using density-functional theory, clinoatacamite belongs to the class of distorted kagome antiferromagnets with the kagome plane being embedded into a low-symmetry crystal structure. By means of thermodynamic measurements, muon spin rotation/relaxation as well as neutron diffraction on single crystals, we find in zero magnetic field complex behavior below $T_\mathrm{I}$ = 18.1 K which unfolds in three temperature regions I-III. We propose this complexity in multicritical clinoatacamite to arise from the competition of antiferromagnetic ordering modes from the underconstrained manifold of modes, which can lead to a metamagnetic texture in zero field.

cond-mat.str-el↗

arXiv2025

Successive magnetic transitions and multiferroicity in layered honeycomb BiCrTeO$_{6}$

Low-dimensional magnetic systems based on honeycomb lattices provide a promising platform for exploring exotic quantum phenomena that emerge from the intricate interplay of competing spin, orbital, lattice, and dipolar degrees of freedom. Here, we present a comprehensive study of the layered honeycomb lattice antiferromagnet BiCrTeO$_6$ using magnetization, specific heat, muon spin--relaxation ($\mu$SR) spectroscopy, dielectric, pyrocurrent, and high-resolution synchrotron X-ray diffraction (SXRD) measurements. Our results reveal an array of intriguing and strongly correlated phenomena, including two successive antiferromagnetic transitions at $T_{\rm N1}\approx16$ K and $T_{\rm N2}\approx11$ K, a pronounced magnetodielectric coupling effect, and ferroelectric order at $T_{\rm N2}$. Consequently, this compound emerges as a new spin-driven multiferroic system. The SXRD analysis reveals a magnetoelastic-coupling-induced structural phase transition at $T_{\rm N2}$, characterized by a symmetry lowering from P$\bar{3}$1c (163) to P31c (159), which likely triggers the onset of ferroelectricity. In addition to its low-temperature multiferroic behavior, the system exhibits dielectric relaxor characteristics at higher temperatures within the paramagnetic region ($T<50$ K), which is intrinsically linked to the antisite disorder of Cr and Te atoms.

cond-mat.str-el↗

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 ($\mu$SR) experiments, supported by first principles calculations, we uncover an unconventional ground state. Despite strong antiferromagnetic interactions with a large Curie-Weiss temperature $\theta_{\rm CW} = -70(2)$ 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 $\mu$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

Single-ion anisotropy driven chiral magnetic order in a spin-1 antiferromagnetic chain

Chirality in magnetic systems gives rise to a wide range of exotic phenomena, yet its influence in $S=1$ chains remains largely unexplored. Here, we present a comprehensive experimental study of a chiral antiferromagnetic (AFM) $S=1$ chain, [Ni(pym)(H$_{2}$O)$_{4}$]SO$_{4} \cdot$ H$_{2}$O (pym = pyrimidine), where the Ni(II) octahedral orientation exhibits a four-fold chiral periodicity. Muon spin rotation measurements indicate the onset of long-range magnetic order below $T_{\rm N} = 1.82(2)\,\mathrm{K}$. Neutron diffraction measurements reveal a chiral AFM order driven by a chiral modulation of the easy-axis anisotropy direction, rather than the typical scenario of Dzyaloshinskii-Moriya interactions, geometrical frustration or higher-order interactions. Inelastic neutron scattering (INS) measurements reveal dispersive spin-wave excitations well described by linear spin-wave theory, with Hamiltonian parameters $J_{0} = 6.81(1)\,\mathrm{K}$ (intrachain exchange), $J'_{1\rm a} = -0.091(1)\,\mathrm{K}$ (interchain exchange), and $D = -3.02(1)\,\mathrm{K}$ (easy-axis single-ion anisotropy). These parameters are further validated by Monte Carlo simulations of the magnetisation. Additionally, the INS data reveal multiple dispersionless bands, suggesting the presence of further excitations beyond the scope of our linear spin-wave theory.

cond-mat.str-el↗

arXiv2025

Distinct Uniaxial Stress and Pressure Fingerprint of Superconductivity in the 3D Kagome Lattice Compound CeRu2

The exploration of tunable superconductivity in strongly correlated electron systems is a central pursuit in condensed matter physics, with implications for both fundamental understanding and potential applications. The Laves phase CeRu$_{2}$, a pyrochlore compound, exhibits a three-dimensional (3D) Kagome lattice type geometry giving rise to flat bands and degenerate Dirac points, where band structure features intertwine with strong multi-orbital interaction effects deriving from its correlated electronic structure. Here, we combine muon spin rotation ($\mu$SR), uniaxial in-plane stress, and hydrostatic pressure to probe the superconducting state of CeRu$_{2}$. Uniaxial stress up to 0.22 GPa induces a dome-shaped evolution of the critical temperature $T_{\rm c}$, with an initial plateau, successively followed by enhancement and suppression without any structural phase transition. Stress is further found to drive a crossover from anisotropic to isotropic $s$-wave pairing. In contrast, hydrostatic pressure up to 2.2 GPa leaves $T_{\rm c}$ largely unchanged but alters the superfluid density from exponential to linear behavior at low temperatures, indicative of nodal superconductivity under hydrostatic pressure. Taken together, these results indicate that CeRu$_{2}$ occupies an ideal position in parameter space, enabling highly responsive and multifold tunability of superconductivity in this three-dimensional correlated electronic system. This warrants further quantitative analysis of the interplay between lattice geometry, electronic correlations, and pairing symmetry.

cond-mat.supr-con↗

arXiv2025

Discovery of High-Temperature Charge Order and Time-Reversal Symmetry-Breaking in the Kagome Superconductor YRu3Si2

The identification of high-temperature unconventional charge order and superconductivity in kagome quantum materials is pivotal for deepening our understanding of geometrically frustrated and correlated electron systems, and for harnessing their exotic properties in future quantum technologies. Here, we report the discovery of a remarkably rich phase diagram in the kagome superconductor YRu$_{3}$Si$_{2}$, uncovered through a unique combination of muon spin rotation (${\mu}$SR), magnetotransport, X-ray diffraction (XRD), and density functional theory (DFT) calculations. Our study reveals the emergence of a charge-ordered state with a propagation vector of (1/2, 0, 0), setting a record onset temperature of 800 K for such an order in a kagome system and for quantum materials more broadly. In addition, we observe time-reversal symmetry (TRS) breaking below $T_{2}^{*}$ ${\simeq}$ 25 K and field-induced magnetism below $T_{1}^{*}$ ${\simeq}$ 90 K, indicating the presence of a hidden magnetic state. These transitions are mirrored in the magnetoresistance data, which show a clear onset at ${\sim}$ $T_{1}^{*}$ and a pronounced increase below ${\sim}$ $T_{2}^{*}$, ultimately reaching a maximum magnetoresistance of 45${\%}$. Band structure calculations identify two van Hove singularities (VHSs) near the Fermi level, one of which resides within a flat band, suggesting a strong interplay between electronic correlations and emergent orders. At low temperatures, we find bulk superconductivity below $T_{\rm c}$ = 3.4 K, characterized by a pairing symmetry with either two isotropic full gaps or an anisotropic nodeless gap. Together, our findings point to a coexistence of high-temperature charge order, tunable magnetism, and multigap superconductivity in YRu$_{3}$Si$_{2}$, positioning it as a compelling platform for exploring correlated kagome physics.

cond-mat.supr-con↗

arXiv2025

Comment on "Neutron diffraction evidence of the 3-dimensional structure of Ba2MnTeO6 and misidentification of the triangular layers within the face-centred cubic lattice"

Frustrated magnetism continues to attract significant attention due to its potential to host novel quantum many-body phenomena and associated exotic excitations that transcend existing paradigms. Herein, we present our reply to the comment on our recent thermodynamic and muon spin relaxation studies on a frustrated double perovskite, Ba2MnTeO6 (henceforth BMTO). Previous studies by four independent groups, including our group, suggested a trigonal space group based on single-crystal and polycrystalline samples of BMTO, while the recent comment reports a cubic space group based on polycrystalline samples. We believe that the structure is fairly intricate because of the slight variations between the two space groups, refining the crystal structure of BMTO remains an unresolved problem that needs additional high-resolution XRD and neutron diffraction studies on high-quality single crystals. It is thought, however, that structural assignments will not greatly influence any of the primary findings related to the magnetism and spin dynamics of BMTO. These consist of a magnetic phase transition at around 21 K, the observation of antiferromagnetic magnon excitations exhibiting a gap of 1.4 K beneath the phase transition, the presence of short-range spin correlations well above the antiferromagnetic phase transition, and the persistence of spin dynamics even within the magnetically ordered phase. It is important to note that the magnetization, specific heat, and muon spin relaxation findings that constitute the core of our earlier study are independent; the interpretation of these findings did not rely on any specific space group. Concerning the final allocation of the symmetry of BMTO, a definitive differentiation in certain physical characteristics resulting from the symmetry is still necessary.

cond-mat.str-el↗

arXiv2025

Fluctuating magnetism in Zn-doped averievite with well-separated kagome layers

Kagome lattice decorated with S=1/2 spins is one of the most discussed ways to realize a quantum spin liquid. However, all previous material realizations of this model have suffered from additional complications, ranging from additional interactions to impurity effects. Recently, a new quantum kagome system has been identified in the form of averievite Cu(5-x)ZnxV2O10(CsCl), featuring a unique double-layer spacing between the kagome planes. Using muon spin spectroscopy we show that only a complete substitution (i.e. $x=2$) of interplanar copper ions leads to a quantum-disordered ground state. In contrast, the parent compound ($x=0$) exhibits long-range magnetic order, with a phase transition around 24 K. Experiments performed on the partially substituted material ($x=1$) show that the transformation proceeds through an intermediate disordered, partially frozen ground state, unaffected by pressures up to 23 kbar. Our study provides a microscopic view of the magnetism of the decoupling of the kagome layers and establishes the averievite as a new material platform for the experimental study of the fully-decoupled kagome layers.

cond-mat.str-el↗

arXiv2025

Competing Quantum Orders in 6R-TaS$_2$: Unconventional Superconductivity, Charge Order, and an Anomalous Hall Effect phase

The transition metal dichalcogenide 6R-TaS$_{2}$ offers a natural platform for studying the interplay among charge density wave (CDW) order, superconductivity, and transport anomalies. Recent findings reveal that, in the intermediate temperature range between charge order and superconductivity, a hidden order emerges around $T^{*}$ ${\simeq}$ 35 K-as evidenced by strong magnetoresistance and an anomalous Hall effect (AHE). However, the nature of the superconducting pairing, the hidden order, and their relationship with the CDW remain unclear. Using ${\mu}$SR, magnetotransport, susceptibility, and hydrostatic pressure techniques, we identify a nodal superconducting state with low superfluid density at ambient pressure, with no spontaneous magnetic order detected below $T^{*}$. This indicates that the AHE originates from the band structure rather than magnetism. Under pressures up to 2 GPa, the superfluid density rises markedly in correlation with the superconducting transition temperature, the nodal pairing shifts to a nodeless state, and the CDW onset is reduced by half. Notably, AHE is fully suppressed and magnetoresistance drops by 50${\%}$ within just 0.2 GPa, highlighting the fragility of the hidden order. These results reveal an unconventional superconducting pairing in 6R-TaS$_{2}$, competing with both CDW and hidden orders through weakened interlayer coupling and competition for the same electronic states. With a multifaceted approach, we establish a comprehensive phase diagram that reveals the intricate interplay and competition between the intertwined quantum orders in 6R-TaS$_{2}$.

cond-mat.supr-con↗

arXiv2025

Local probe evidence supporting altermagnetism in Co$_{1/4}$NbSe$_2$

Muon spin rotation ($μ$SR), combined with muon stopping site and local field analysis, was used to investigate the magnetic properties of cobalt intercalated 2H-NbSe$_2$ (Co$_{1/4}$NbSe$_2$). Co$_{1/4}$NbSe$_2$ is predicted to be an altermagnet, and whilst neutron diffraction has proposed its magnetic structure, microscopic details such as the magnetic volume fraction remain unclear. Therefore, a local probe investigation of its magnetism is essential. Here, we report the determination of the magnetically ordered volume fraction, ordered moment size, and magnetic structure. Our results reveal a sharp second-order transition to a full-volume-fraction, homogeneous magnetic order below $T_\mathrm{N}~=~168~$K. The moments are aligned antiparallel along the $c$-axis, consistent with neutron diffraction and altermagnetism. $μ$SR reveals that the state remains stable under a $c$-axis magnetic field up to $0.78~$T, with magnetisation measurements suggesting this robust regimes extends to at least $5$~T. Within the time resolution of $μ$SR, no precursor slow altermagnetic fluctuations were detected above $T_\mathrm{N}$, which is important for interpreting the band splitting in the paramagnetic state reported by photoemission studies. These findings support altermagnetism in Co$_{1/4}$NbSe$_2$ and motivate further experiments to explore the tunability of its magnetic and electronic structure.

cond-mat.str-el↗

arXiv2025

Contrasting $c$-axis and in-plane uniaxial stress effects on superconductivity and stripe order in La$_{1.885}$Ba$_{0.115}$CuO$_4$

The cuprate superconductor La$_{2-x}$Ba$_x$CuO$_4$ (LBCO) near $x=0.125$ is a striking example of intertwined electronic orders, where 3D superconductivity is anomalously suppressed, allowing spin and charge stripe order to develop, in a manner consistent with the emergence of a pair-density-wave (PDW) state. Understanding this interplay remains a key challenge in cuprates, highlighting the necessity of external tuning for deeper insight. While in-plane (within the CuO plane) uniaxial stress enhances superconductivity and suppresses stripe order, the effects of $c$-axis compression (perpendicular to the CuO plane) remains largely unexplored. Here, we use muon spin rotation ($\mu$SR) and AC susceptibility with an in situ piezoelectric stress device to investigate the spin-stripe order and superconductivity in LBCO-0.115 under $c$-axis compression. The measurements reveal a gradual suppression of the superconducting transition temperature ($T_{\rm c}$) with increasing $c$-axis stress, in stark contrast to the strong enhancement observed under in-plane stress. We further show that while in-plane stress rapidly reduces both the magnetic volume fraction ($V_{\rm m}$) and the spin-stripe ordering temperature ($T_{\rm so}$), $c$-axis compression has no effect, with $V_{\rm m}$ and $T_{\rm so}$ exhibiting an almost unchanged behavior up to the highest applied stress of 0.21 GPa. These findings demonstrate a strong anisotropy in stress response, underscoring the critical role of crystallographic anisotropy in governing competing electronic phases in LBCO.

cond-mat.supr-con↗