arXiv subjects

H. Kühne

Publications and source records attributed to H. Kühne.

At least 19 recordsLinked to original sources

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

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↗

arXiv2025

Nuclear magnetic resonance spectroscopy in pulsed magnetic fields

This article provides an introduction to nuclear magnetic resonance spectroscopy in pulsed magnetic fields (PFNMR), focusing on its capabilities, applications, and future developments in research involving high magnetic fields. It highlights the significance of PFNMR in enhancing the understanding of solid-state materials, with particular emphasis on those exhibiting complex interactions and strong electronic correlations. Several technical aspects are discussed, including the challenges associated with high-frequency NMR experiments. The power of PFNMR is showcased through several examples, including studies on the topical materials LiCuVO$_4$, SrCu$_2$(BO$_3$)$_2$, and CeIn$_3$, offering insights into their magnetic and electronic properties at high magnetic fields. The article also discusses possible future directions for the technique, including improvements in PFNMR instrumentation and the exploration of materials under extreme conditions. This exposition underscores the role of PFNMR in advancing the frontiers of materials-science research.

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↗

arXiv2022

Field-tunable Berezinskii-Kosterlitz-Thouless correlations in a Heisenberg magnet

We report the manifestation of field-induced Berezinskii-Kosterlitz-Thouless (BKT) correlations in the weakly coupled spin-1/2 Heisenberg layers of the molecular-based bulk material [Cu(pz)$_2$(2-HOpy)$_2$](PF$_6$)$_2$. Due to the moderate intralayer exchange coupling of $J/k_\mathrm{B} = 6.8$ K, the application of laboratory magnetic fields induces a substantial $XY$ anisotropy of the spin correlations. Crucially, this provides a significant BKT regime, as the tiny interlayer exchange $J^\prime / k_\mathrm{B} \approx 1$ mK only induces 3D correlations upon close approach to the BKT transition with its exponential growth in the spin-correlation length. We employ nuclear magnetic resonance and $μ^{+}$SR measurements to probe the spin correlations that determine the critical temperatures of the BKT transition as well as that of the onset of long-range order. Further, we perform stochastic series expansion quantum Monte Carlo simulations based on the experimentally determined model parameters. Finite-size scaling of the in-plane spin stiffness yields excellent agreement of critical temperatures between theory and experiment, providing clear evidence that the nonmonotonic magnetic phase diagram of [Cu(pz)$_2$(2-HOpy)$_2$](PF$_6$)$_2$ is determined by the field-tuned $XY$ anisotropy and the concomitant BKT physics.

cond-mat.str-el↗

arXiv2022

Structural 130-K Phase Transition and Emergence of a Two-Ion Kondo State in HT-Ce$_2$Rh$_2$Ga Explored by $^{69,71}$Ga Nuclear Quadrupole Resonance

We have studied the microscopic magnetic properties, the nature of the 130-K phase transition, and the ground state in the recently synthesized compound Ce$_2$Rh$_2$Ga by use of $^{69,71}$Ga nuclear quadrupole resonance (NQR). The NQR spectra clearly show an unusual phase transition at $T_t$ $\sim$ 130 K yielding a splitting of the high-temperature single NQR line into two clearly resolved NQR lines, providing evidence for two crystallographically inequivalent Ga sites. The NQR frequencies are in good agreement with fully-relativistic calculations of the band structure. Our NQR results indicate the absence of magnetic or charge order down to 0.3 K. The temperature dependence of the spin-lattice relaxation rate, 1/$T_1$, shows three distinct regimes, with onset temperatures at $T_t$ and 2 K. The temperature-independent 1/$T_1$, observed between $T_t$ and 2 K, crosses over to a Korringa process, 1/$T_1$ $\propto$ $T$, below $\sim$ 2 K, which evidences a rare two-ion Kondo scenario: the system goes into a dense Kondo coherent state at 2.0 and 0.8 K for the two different Ga sites.

cond-mat.str-el↗

arXiv2021

The planar triangular $S=3/2$ magnet AgCrSe$_2$: magnetic frustration, short range correlations, and field tuned anisotropic cycloidal magnetic order

Our studies evidence an anisotropic magnetic order below $T_N = 32$~K. Susceptibility data in small fields of about 1~T reveal an antiferromagnetic (AFM) order for $H \perp c$, whereas for $H \parallel c$ the data are reminiscent of a field-induced ferromagnetic (FM) structure. At low temperatures and for $H \perp c$, the field-dependent magnetization and AC susceptibility data evidence a metamagnetic transition at $H^+ = 5$~T, which is absent for $H \parallel c$. We assign this to a transition from a planar cycloidal spin structure at low fields to a planar fan-like arrangement above $H^+$. A fully FM polarized state is obtained above the saturation field of $H_{\perp S} = 23.7$~T at 2~K with a magnetization of $M_s = 2.8$~$μ_{\rm B}{\rm /Cr}$. For $H \parallel c$, $M(H)$ monotonously increases and saturates at the same $M_s$ value at $H_{\parallel S} = 25.1$~T at 4.2~K. Above $T_N $, the magnetic susceptibility and specific heat indicate signatures of two dimensional (2D) frustration related to the presence of planar ferromagnetic and antiferromagnetic exchange interactions. We found a pronounced nearly isotropic maximum in both properties at about $T^* = 45$~K, which is a clear fingerprint of short-range correlations and emergent spin fluctuations. Calculations based on a planar 2D Heisenberg model support our experimental findings and suggest a predominant FM exchange among nearest and AFM exchange among third-nearest neighbors. Only a minor contribution might be assigned to the antisymmetric Dzyaloshinskii-Moriya interaction possible related to the non-centrosymmetric polar space group $R3m$. Due to these competing interactions, the magnetism in AgCrSe$_{2}$, in contrast to the oxygen based delafossites, can be tuned by relatively small, experimentally accessible, magnetic fields, allowing us to establish the complete anisotropic magnetic $H-T$ phase diagram in detail.

cond-mat.str-el↗

arXiv2021

Freezing of molecular rotation in a paramagnetic crystal studied by $^{31}$P NMR

We present a detailed $^{31}$P nuclear magnetic resonance (NMR) study of the molecular rotation in the compound [Cu(pz)$_{2}$(2-HOpy)$_{2}$](PF$_{6}$)$_{2}$, where pz = C$_4$H$_4$N$_2$ and 2-HOpy = C$_5$H$_4$NHO. Here, a freezing of the PF$_6$ rotation modes is revealed by several steplike increases of the temperature-dependent second spectral moment, with accompanying broad peaks of the longitudinal and transverse nuclear spin-relaxation rates. An analysis based on the Bloembergen-Purcell-Pound (BPP) theory quantifies the related activation energies as $E_{a}/k_{B}$ = 250 and 1400 K. Further, the anisotropy of the second spectral moment of the $^{31}$P absorption line was calculated for the rigid lattice, as well as in the presence of several sets of PF$_6$ reorientation modes, and is in excellent agreement with the experimental data. Whereas the anisotropy of the frequency shift and enhancement of nuclear spin-relaxation rates is driven by the molecular rotation with respect to the dipole fields stemming from the Cu ions, the second spectral moment is determined by the intramolecular interaction of nuclear $^{19}$F and $^{31}$P moments in the presence of the distinct rotation modes.

cond-mat.str-el↗

arXiv2020

Superconductivity of highly spin-polarized electrons in FeSe probed by $^{77}$Se NMR

A number of recent experiments indicate that the iron-chalcogenide FeSe provides the long-sought possibility to study bulk superconductivity in the cross-over regime between the weakly coupled Bardeen--Cooper--Schrieffer (BCS) pairing and the strongly coupled Bose--Einstein condensation (BEC). We report on $^{77}$Se nuclear magnetic resonance experiments of FeSe, focused on the superconducting phase for strong magnetic fields applied along the $c$ axis, where a distinct state with large spin polarization was reported. We determine this high-field state as bulk superconducting with high spatial homogeneity of the low-energy spin fluctuations. Further, we find that the static spin susceptibility becomes unusually small at temperatures approaching the superconducting state, despite the presence of pronounced spin fluctuations. Taken together, our results clearly indicate that FeSe indeed features an unusual field-induced superconducting state of a highly spin-polarized Fermi liquid in the BCS-BEC crossover regime.

cond-mat.supr-con↗

arXiv2020

Extremely well isolated 2D spin-$1/2$ antiferromagnetic Heisenberg layers with small exchange coupling in the molecular-based magnet CuPOF

We report on a comprehensive characterization of the newly synthesized Cu$^{2+}$-based molecular magnet [Cu(pz)$_2$(2-HOpy)$_2$](PF$_6$)$_2$ (CuPOF), where pz = C$_4$H$_4$N$_2$ and 2-HOpy = C$_5$H$_4$NHO. From a comparison of theoretical modeling to results of bulk magnetometry, specific heat, $μ^+$SR, ESR, and NMR spectroscopy, this material is determined as an excellent realization of the 2D square-lattice $S=1/2$ antiferromagnetic Heisenberg model with a moderate intraplane nearest-neighbor exchange coupling of $J/k_\mathrm{B} = 6.80(5)$ K, and an extremely small interlayer interaction of about 1 mK. At zero field, the bulk magnetometry reveals a temperature-driven crossover of spin correlations from isotropic to $XY$ type, caused by the presence of a weak intrinsic easy-plane anisotropy. A transition to long-range order, driven by the low-temperature $XY$ anisotropy under the influence of the interlayer coupling, occurs at $T_\mathrm{N} = 1.38(2)$ K, as revealed by $μ^+$SR. In applied magnetic fields, our $^1$H-NMR data reveal a strong increase of the magnetic anisotropy, manifested by a pronounced enhancement of the transition temperature to commensurate long-range order at $T_\mathrm{N} =2.8$ K and 7 T.

cond-mat.str-el↗

arXiv2020

Long-range magnetic order in the ${\tilde S}=1/2$ triangular lattice antiferromagnet KCeS$_2$

Recently, several putative quantum spin liquid (QSL) states were discovered in ${\tilde S} = 1/2$ rare-earth based triangular-lattice antiferromagnets (TLAF) with the delafossite structure. A way to clarify the origin of the QSL state in these systems is to identify ways to tune them from the putative QSL state towards long-range magnetic order. Here, we introduce the Ce-based TLAF KCeS$_2$ and show via low-temperature specific heat and $μ$SR investigations that it yields magnetic order below $T_{\mathrm N} = 0.38$ K despite the same delafossite structure. We identify a well separated ${\tilde S} = 1/2$ ground state for KCeS$_2$ from inelastic neutron scattering and embedded-cluster quantum chemical calculations. Magnetization and electron spin resonance measurements on single crystals indicate a strong easy-plane $g$~factor anisotropy, in agreement with the ab initio calculations. Finally, our specific-heat studies reveal an in-plane anisotropy of the magnetic field-temperature phase diagram which may indicate anisotropic magnetic interactions in KCeS$_2$.

cond-mat.str-el↗

arXiv2019

Anisotropic field-induced ordering in the triangular-lattice quantum spin liquid NaYbSe$_2$

High-quality single crystals of NaYbSe$_2$, which resembles a perfect triangular-lattice antiferromagnet without the intrinsic disorder, are investigated by magnetization and specific heat, as well as the local probe techniques nuclear magnetic resonance (NMR) and electron spin resonance (ESR). The low-field measurements confirm the absence of any spin freezing or long-range magnetic order down to 50~mK, which suggests a quantum spin liquid ground (QSL) state with gapless excitations. The instability of the QSL state is observed upon applying magnetic fields. For the $H\bot c$ direction, a field-induced magnetic phase transition is observed above 2~T from the $C_{\rm p}(T)$ data, agreeing with a clear $\frac{M_s}{3}$ plateau of $M(H)$, which is associated with an up-up-down (uud) spin arrangement. For the $H\|c$ direction, a field-induced transition could be evidenced at a much higher field range (9 - 21~T). The $^{23}$Na NMR measurements provide microscopic evidence for field-induced ordering for both directions. A reentrant behavior of $T_{\rm N}$, originating from the thermal and quantum spin fluctuations, is observed for both directions. The anisotropic exchange interactions $J_{\perp}\simeq$ 4.7~K and $J_z\simeq$2.33~K are extracted from the modified bond-dependent XXZ model for the spin-$\frac{1}{2}$ triangular-lattice antiferromagnet. The absence of magnetic long-range order at zero fields is assigned to the effect of strong bond-frustration, arising from the complex spin-orbit entangled $4f$ ground state. Finally, we derive the highly anisotropic magnetic phase diagram, which is discussed in comparison with the existing theoretical models for spin-$\frac{1}{2}$ triangular-lattice antiferromagnets.

cond-mat.str-el↗

arXiv2019

Field-induced instability of the quantum-spin-liquid ground state in the $J_{\rm eff}=\frac{1}{2}$ triangular-lattice compound NaYbO$_2$

Polycrystalline samples of NaYbO$_2$ are investigated by bulk magnetization and specific-heat measurements, as well as by nuclear magnetic resonance (NMR) and electron spin resonance (ESR) as local probes. No signatures of long-range magnetic order are found down to 0.3~K, evidencing a highly frustrated spin-liquid-like ground state in zero field. Above 2\,T, signatures of magnetic order are observed in thermodynamic measurements, suggesting the possibility of a field-induced quantum phase transition. The $^{23}$Na NMR relaxation rates reveal the absence of magnetic order and persistent fluctuations down to 0.3~K at very low fields and confirm the bulk magnetic order above 2~T. The $H$-$T$ phase diagram is obtained and discussed along with the existing theoretical concepts for layered spin-$\frac{1}{2}$ triangular-lattice antiferromagnets

cond-mat.str-el↗

arXiv2018

Increasing stripe-type fluctuations in $A$Fe$_{2}$As$_{2}$ ($A$ = K, Rb, Cs) superconductors probed by $^{75}$As NMR spectroscopy

We report $^{75}$As nuclear magnetic resonance measurements on single crystals of RbFe$_{2}$As$_{2}$ and CsFe$_{2}$As$_{2}$. Taking previously reported results for KFe$_{2}$As$_{2}$ into account, we find that the anisotropic electronic correlations evolve towards a magnetic instability in the $A$Fe$_{2}$As$_{2}$ series (with $A$ = K, Rb, Cs). Upon isovalent substitution with larger alkali ions, a drastic enhancement of the anisotropic nuclear spin-lattice relaxation rate and decreasing Knight shift reveal the formation of pronounced spin fluctuations with stripe-type modulation. Furthermore, a decreasing power-law exponent of the nuclear spin-lattice relaxation rate $(1/T_{1})_{H\parallel{ab}}$, probing the in-plane spin fluctuations, evidences an emergent deviation from Fermi-liquid behavior. All these findings clearly indicate that the expansion of the lattice in the $A$Fe$_{2}$As$_{2}$ series tunes the electronic correlations towards a quantum critical point at the transition to a yet unobserved, ordered phase.

cond-mat.str-el↗

arXiv2017

Monovacancy paramagnetism in neutron-irradiated graphite probed by $^{13}$C NMR

We report on the magnetic properties of monovacancy defects in neutron-irradiated graphite, probed by $^{13}$C nuclear magnetic resonance spectroscopy. The bulk paramagnetism of the defect moments is revealed by the temperature dependence of the NMR frequency shift and spectral linewidth, both of which follow a Curie behavior, in agreement with measurements of the macroscopic magnetization. Compared to pristine graphite, the fluctuating hyperfine fields generated by the defect moments lead to an enhancement of the $^{13}$C nuclear spin-lattice relaxation rate $1/T_{1}$ by about two orders of magnitude. With an applied magnetic field of 7.1 T, the temperature dependence of $1/T_{1}$ below about 10 K can well be described by a thermally activated form, $1/T_{1}\propto\exp(-Δ/k_{B}T)$, yielding a singular Zeeman energy of ($0.41\pm0.01$) meV, in excellent agreement with the sole presence of polarized, non-interacting defect moments.

cond-mat.mtrl-sci↗

arXiv2017

Defect-induced magnetism in SiC probed by nuclear magnetic resonance

We give evidence for intrinsic, defect-induced bulk paramagnetism in SiC by means of $^{13}$C and $^{29}$Si nuclear magnetic resonance (NMR) spectroscopy. The temperature dependence of the internal dipole-field distribution, probed by the spin part of the NMR Knight shift and the spectral linewidth, follows a Curie law and scales very well with the macroscopic DC susceptibility. In order to quantitatively analyze the NMR spectra, a microscopic model based on dipole-dipole interactions was developed. The very good agreement between these simulations and the NMR data establishes a direct relation between the frequency distribution of the spectral intensity and the corresponding real-space volumes of nuclear spins. The presented approach by NMR can be applied to a variety of similar materials and, thus, opens a new avenue for the microscopic exploration and exploitation of diluted bulk magnetism in semiconductors.

cond-mat.mtrl-sci↗

arXiv2016

Charge fluctuations and superconductivity in organic conductors: the case of $β"$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$

A $^{13}$C NMR study of the normal and superconducting states of the all-organic charge-transfer salt $β"$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$ is presented. We find that the normal state is a charge-ordered metal configured as vertical stripes, produced by a combination of 1/4-filling, correlations, and a polar counterion sublattice. The NMR properties associated with the superconducting state are consistent with gap nodes and singlet pairing, and therefore similar to other organic superconductors. Quite distinct, however, is the absence of evidence for low-energy antiferromagnetic spin fluctuations for $T>T_c=4.5$ K. Both aspects are discussed in the context of a proposal that the pairing in this compound is driven by charge fluctuations.

cond-mat.str-el↗

arXiv2015

Microscopic study of the Fulde-Ferrell-Larkin-Ovchinnikov state in an all-organic superconductor

Quasi-two dimensional superconductors with sufficiently weak interlayer coupling allow magnetic flux to penetrate in the form of Josephson vortices for in-plane applied magnetic fields. A consequence is the dominance of the Zeeman interaction over orbital effects. In the clean limit, the normal state is favored over superconductivity for fields greater than the paramagnetic limiting field, unless an intermediate, inhomogeneous state is stabilized. Presented here are nuclear magnetic resonance (NMR) studies of the inhomogeneous (FFLO) state for $β^{\prime \prime}$-(ET)$_2$SF$_5$CH$_2$CF$_2$SO$_3$. The uniform superconductivity-FFLO transition is identified at an applied field value of 9.3(0.1) T at low temperature ($T=130$ mK), and evidence for a possible second transition between inhomogeneous states at $\sim11$ T is presented. The spin polarization distribution inferred from the NMR absorption spectrum compares favorably to a single-Q modulation of the superconducting order parameter.

cond-mat.str-el↗