arXiv subjects

H. -J. Grafe

Publications and source records attributed to H. -J. Grafe.

At least 19 recordsLinked to original sources

arXiv2026

Microscopic NMR evidence for successive antiferroelectric and antiferromagnetic order in the van der Waals magnet CuCrP$_2$S$_6$

We present a comprehensive $^{31}$P and $^{65}$Cu nuclear magnetic resonance (NMR) study of the layered van der Waals magnet CuCrP$_2$S$_6$. The compound exhibits a sequence of structural and magnetic phase transitions: a high-temperature paraelectric state, followed by a quasi-antiferroelectric (QAFE) state near 185 K, a long-range antiferroelectric (AFE) phase below 150 K, and finally, antiferromagnetic (AFM) order below $T_\mathrm{N}$ = 30 K. The evolution of the NMR spectra, NMR shift, and spin-lattice ($T_1^{-1}$) and spin-spin ($T_2^{-1}$) relaxation rates provide direct microscopic fingerprints of these transitions. The splitting of both the NMR line and $T_1^{-1}$ below the AFE transition demonstrates the emergence of two inequivalent P sites. From $K - χ$ analysis, we extract nearly isotropic transferred hyperfine couplings and show that the NMR shift anisotropy originates primarily from the dipolar contribution, in contrast to Mn$_2$P$_2$S$_6$ and Ni$_2$P$_2$S$_6$. We determine the ferromagnetic intralayer exchange $J_{intra}\approx$ -4.9 K from the Curie Weiss temperature, consistent with ferromagnetic layers antiferromagnetically stacked along the $c$ axis, and evaluate the Moriya high temperature relaxation rate including cross correlation effects of the P P dimer. Critical divergence of $T_1^{-1}$ near $T_\mathrm{N}$ yields a critical exponent $γ\simeq$ 0.45(4), placing CuCrP$_2$S$_6$ in a three dimensional Heisenberg universality regime.

cond-mat.str-el↗

arXiv2026

Disorder-driven coexistence of distinct dynamical states in frustrated Sr$_3$CuNb$_2$O$_9$: a microscopic $μ$SR and $^{93}$Nb NMR study

Despite recent progress in identifying the exotic random singlet (RS) state in disordered frustrated magnets as a distinct correlated phase, three-dimensional (3D) realizations remain scarce. Sr$_3$CuNb$_2$O$_9$ was proposed to be one of such 3D frustrated systems with magnetic site disorder hosting an RS ground state. Here, we report a detailed microscopic investigation of Sr$_3$CuNb$_2$O$_9$ employing muon spin relaxation ($μ$SR) and $^{93}$Nb nuclear magnetic resonance (NMR) techniques. The $μ$SR zero-field relaxation rate reveals a power-law divergence of the relaxation rate as a function of temperature. Also, a power-law divergence is present in the relaxation rate as a function of applied longitudinal field, consistent with the formation of an RS phase. The $^{93}$Nb NMR spectra unambiguously resolve two components with distinct local magnetic environments, whose nature is further elucidated through spin-lattice relaxation measurements analyzed via an inverse Laplace transform (ILT) of the nuclear magnetization recovery. The relaxation-rate distribution obtained from ILT reveals two well-separated channels: a fast component, $(1/T_1)_{\mathrm{fast}}$, and a slow component, $(1/T_1)_{\mathrm{slow}}$. Both components follow distinct power-law temperature dependences ($T^α$), with $α= 0.6$ and $1.1$ for the fast and slow channels, respectively. The combined spectral and relaxation data demonstrate that the fast channel qualitatively represents an RS-like state, whereas the slow channel exhibits quantum spin liquid (QSL) like behavior, thereby establishing the microscopic coexistence of RS and QSL-like phases in Sr$_3$CuNb$_2$O$_9$.

cond-mat.str-el↗

arXiv2025

Precision magnetometry at cryogenic temperatures with gaseous 3He NMR probes

We report on compact, gaseous 3He NMR probes for precision magnetometry of magnetic fields B > 0.1 T in the temperature range from ambient temperatures down to 4 Kelvin. The gas is polarized at thermal equilibrium under pressures up to 100 bar to provide a high nuclear spin density. In order to achieve sensor readout rates of order (Hz), paramagnetic substances and/or silica gel with high specific surface area were added to reduce the otherwise long T1 relaxation time of pure 3He gas to reach thermal polarization equilibrium. Sensitivity limits, which cover the range from 10-11 < dB/B < 10-7, are accessible in a single-pulse NMR measurement and can be further improved through signal averaging in accumulated NMR scans.

physics.atom-ph↗

arXiv2025

Magnetic order and Li-diffusion in the 1/3-filled Kagome layers of antiperovskite Lithium-ion battery materials (Li$_2$Fe)SO and (Li$_2$Fe)SeO

The recently discovered lithium-rich antiperovskites (Li$_2$Fe)SeO and (Li$_2$Fe)SO host lithium and iron ions on the same atomic position which octahedrally coordinates to central oxygens. In a cubic antiperovskite these sites form Kagome planes stacked along the <111> directions which triangular motifs induce high geometric frustration in the diluted magnetic sublattice for antiferromagnetic interactions. Despite their compelling properties as high-capacity Li-ion battery cathode materials, very little is known about the electronic and magnetic properties of lithium-rich antiperovskites. We report static magnetization, Mössbauer, and NMR studies on both compounds. Our data reveal a Pauli paramagnetic-like behaviour, a long-range antiferromagnetically ordered ground state below 50 K and a regime of short-range magnetic correlations up to 100 K. Our results are consistent with a random Li-Fe distribution on the shared lattice position. In addition, Li-hopping is observed above 200 K with an activation energy of E$_a$ = 0.47 eV. Overall, our data elucidate static magnetism in a disordered magnetically frustrated and presumably semimetallic system with thermally induced ion diffusion dynamics.

cond-mat.mtrl-sci↗

arXiv2025

Ground state properties of a spin-$\frac{5}{2}$ frustrated triangular lattice antiferromagnet NH$_{4}$Fe(PO$_{3}$F)$_2$

Structural and magnetic properties of a two-dimensional spin-$\frac{5}{2}$ frustrated triangular lattice antiferromagnet NH$_{4}$Fe(PO$_{3}$F)$_2$ are explored via x-ray diffraction, magnetic susceptibility, high-field magnetization, heat capacity, and $^{31}$P nuclear magnetic resonance experiments on a polycrystalline sample. The compound portrays distorted triangular units of the Fe$^{3+}$ ions with anisotropic bond lengths. The magnetic susceptibility shows a broad maxima around $T^{\rm{max}}_χ\simeq 12$ K, mimicking the short-range antiferromagnetic order of a low-dimensional spin system. The magnetic susceptibility and NMR shift could be modeled assuming the spin-$5/2$ isotropic triangular lattice model and the average value of the exchange coupling is estimated to be $J/k_{\rm B} \simeq 1.7$ K. This value of the exchange coupling is reproduced well from the saturation field of the pulse field data. It shows the onset of a magnetic ordering at $T_{\rm N} \simeq 5.7$ K, setting the frustration ratio of $f = \frac{|θ_{\rm CW}|}{T_{\rm N}} \simeq 5.7$. Such a value of $f$ reflects moderate magnetic frustration in the compound. The d$M$/d$H$ vs $H$ plots of the low temperature magnetic isotherms exhibit a sharp peak at $H_{\rm SF} \simeq 1.45$ T, suggesting a field-induced spin-flop transition and magnetic anisotropy. The rectangular shape of the $^{31}$P NMR spectra below $T_{\rm N}$ unfolds that the ordering is commensurate antiferromagnet type. Three distinct phase regimes are clearly discerned in the $H - T$ phase diagram, redolent of a frustrated magnet with in-plane (XY-type) anisotropy.

cond-mat.mtrl-sci↗

arXiv2022

Static magnetic and ESR spectroscopic properties of the dimer-chain antiferromagnet BiCoPO$_5$

We report a comprehensive study of the static susceptibility, high-field magnetization and high-frequency/high-magnetic field electron spin resonance (HF-ESR) spectroscopy of polycrystalline samples of the bismuth cobalt oxy-phosphate BiCoPO$_5$. This compound features a peculiar spin system that can be considered as antiferromagnetic (AFM) chains built of pairs of ferromagnetically coupled Co spins and interconnected in all three spatial directions. It was previously shown that BiCoPO$_5$ orders antiferromagnetically at $T_{\rm N} \approx 10$ K and this order can be continuously suppressed by magnetic field towards the critical value $μ_0H_{\rm c} \approx 15$ T. In our experiments we find strongly enhanced magnetic moments and spectroscopic $g$ factors as compared to the expected spin-only values, suggesting a strong contribution of orbital magnetism for the Co$^{2+}$ ions. This is quantitatively confirmed by ab initio quantum chemical calculations. Within the AFM ordered phase, we observe a distinct field-induced magnetic phase transition. Its critical field rises to $\sim 6$ T at $T \ll T_{\rm N}$. The HF-ESR spectra recorded at $T\ll T_{\rm N}$ are very rich comprising up to six resonance modes possibly of the multimagnonic nature that soften towards the critical region around 6 T. Interestingly, we find that the Co moments are not yet fully polarized at $H_{\rm c}$ which supports a theoretical proposal identifying $H_{\rm c}$ as the quantum critical point for the transition of the spin system in BiCoPO$_5$ to the quantum disordered state at stronger fields.

cond-mat.str-el↗

arXiv2021

${}^{31}$P NMR investigation of quasi-two-dimensional magnetic correlations in $T_2$P$_2$S$_6$ ($T$ = Mn & Ni)

We report the anomalous breakdown in the scaling of the microscopic magnetic susceptibility, as measured via the ${}^{31}$P nuclear magnetic resonance (NMR) shift $K$, with the bulk magnetic susceptibility $χ$ in the paramagnetic state of Mn$_2$P$_2$S$_6$. This anomaly occurs near $T_\mathrm{max} \sim 117$ K the maximum in $χ(T)$ and is therefore associated with the onset of quasi-two-dimensional (quasi-2D) magnetic correlations. The spin-lattice relaxation rate divided by temperature $(T_1T)^{-1}$ in Mn$_2$P$_2$S$_6$ exhibits broad peak-like behavior as a function of temperature, qualitatively following $χ$, but displaying no evidence of critical slowing down above the Néel temperature $T_N$. In the magnetic state of Mn$_2$P$_2$S$_6$, NMR spectra provide good evidence for 60 degree rotation of stacking-fault-induced magnetic domains, as well as observation of the spin-flop transition that onsets at 4 T. The temperature-dependent critical behavior of the internal hyperfine field at the P site in Mn$_2$P$_2$S$_6$ is consistent with previous measurements and the two-dimensional anisotropic Heisenberg model. In a sample of Ni$_2$P$_2$S$_6$, we observe only two magnetically split resonances in the magnetic state, demonstrating that the multiple-peaked NMR spectra previously associated with 60 degree rotation of stacking faults is sample dependent. Finally, we report the observation of a spin-flop-induced splitting of the NMR spectra in Ni$_2$P$_2$S$_6$, with an onset spin-flop field of $H_\mathrm{sf} = 14$ T.

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

Quasi-2D magnetic correlations in Ni$_2$P$_2$S$_6$ probed by ${}^{31}$P NMR

Detailed ${}^{31}$P nuclear magnetic resonance (NMR) measurements are presented on well-characterized single crystals of antiferromagnetic van der Waals Ni$_2$P$_2$S$_6$. An anomalous breakdown is observed in the proportionality of the NMR shift $K$ with the bulk susceptibility $χ$. This so-called $K$$-$$χ$ anomaly occurs in close proximity to the broad peak in $χ(T)$, thereby implying a connection to quasi-2D magnetic correlations known to be responsible for this maximum. Quantum chemistry calculations show that crystal field energy level depopulation effects cannot be responsible for the $K$$-$$χ$ anomaly. Appreciable in-plane transferred hyperfine coupling is observed, which is consistent with the proposed Ni$-$S$-$Ni super- and Ni$-$S$-$S$-$Ni super-super-exchange coupling mechanisms. Magnetization and spin$-$lattice relaxation rate ($T_1^{-1}$) measurements indicate little to no magnetic field dependence of the Néel temperature. Finally, $T_1^{-1}(T)$ evidences relaxation driven by three-magnon scattering in the antiferromagnetic state.

cond-mat.str-el↗

arXiv2020

Ground state and low-temperature magnetism of the quasi-two-dimensional honeycomb compound InCu$_{2/3}$V$_{1/3}$O$_3$

We report a combined $^{115}$In NQR, $^{51}$V NMR and $μ$SR spectroscopic study of the low-temperature magnetic properties of InCu$_{2/3}$V$_{1/3}$O$_3$, a quasi-two dimensional (2D) compound comprising in the spin sector a honeycomb lattice of antiferromagnetically coupled spins $S=1/2$ associated with Cu$^{2+}$ ions. Despite substantial experimental and theoretical efforts, the ground state of this material was has not been ultimately identified. In particular, two characteristic temperatures of about $\sim 40$ K and $\sim 20$ K manifesting themselves as anomalies in different magnetic measurements are discussed controversially. A combined analysis of the experimental data complemented with theoretical calculations of exchange constants enabled us to identify below 39 K an ``intermediate'' quasi-2D static spin state. This spin state is characterized by a staggered magnetization with a temperature evolution that agrees with the predictions for the 2D XY model. We observe that this state gradually transforms at 15 K into a fully developed 3D antiferromagnetic Néel state. We ascribe such an extended quasi-2D static regime to an effective magnetic decoupling of the honeycomb planes due to a strong frustration of the interlayer exchange interactions which inhibits long-range spin-spin correlations across the planes. Interestingly, we find indications of the topological Berezinsky-Kosterlitz-Thouless transition in the quasi-2D static state of the honeycomb spin-1/2 planes of InCu$_{2/3}$V$_{1/3}$O$_3$.

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

Diluted paramagnetic impurities in nonmagnetic Ba$_2$YIrO$_6$

The cubic double perovskite Ba$_2$YIrO$_6$ has been investigated by the local probe techniques nuclear magnetic resonance ($^{89}$Y NMR) and muon spin rotation (muSR). Both methods confirm the absence of magnetic long-range order in this compound but find evidence for diluted localized paramagnetic moments. NMR spin-lattice relaxation rate 1/T1 measurements suggest a slowing down of localized spin moments at low temperatures. An increase of the muSR spin-lattice relaxation rate lambda confirms the presence of weak magnetism in Ba$_2$YIrO$_6$ . However, these findings cannot be explained by the recently suggested excitonic type of magnetism. Instead, they point towards tiny amounts of localized paramagnetic spin centers leading to this magnetic response on the background of a simple nonmagnetic ground state of the 5d$^4$ ($J=0)$ electronic configuration of Ir$^{5+}$.

cond-mat.str-el↗

arXiv2016

Evidence for a magnetic field-induced unconventional nematic state in the frustrated and anisotropic spin-chain cuprate LiCuSbO$_4$

Modern theories of quantum magnetism predict exotic multipolar states in weakly interacting strongly frustrated spin-1/2 Heisenberg chains with ferromagnetic nearest neighbor (NN) inchain exchange in high magnetic fields. Experimentally these states remained elusive so far. Here we report the evidence for a long-sought magnetic field-induced nematic state arising above a field of $\sim 13$ T in the edge-sharing chain cuprate LiSbCuO$_4$ $\equiv$ LiCuSbO$_4$. This interpretation is based on the observation of a field induced spin-gap in the measurements of the $^7$Li NMR spin relaxation rate $T_1^{-1}$ as well as a contrasting field-dependent power-law behavior of $T_1^{-1}$ vs. $T$ and is further supported by static magnetization and ESR data. An underlying theoretical microscopic approach favoring a nematic scenario is based essentially on the NN XYZ exchange anisotropy within a model for frustrated spin-1/2 chains. It is investigated by the DMRG technique. The employed exchange parameters are justified qualitatively by electronic structure calculations for LiCuSbO$_4$.

cond-mat.str-el↗

arXiv2016

Non-collinear antiferromagnetism of coupled spins and pseudospins in the double perovskite La2CuIrO6

We report the structural, magnetic and thermodynamic properties of the double perovskite compound La2CuIrO6 from X-ray, neutron diffraction, neutron depolarization, dc magnetization, ac susceptibility, specific heat, muon-spin-relaxation (uSR), electron-spin-resonance (ESR) and nuclear magnetic resonance (NMR) measurements. Below ~113 K, short-range spin-spin correlations occur within the Cu2+ sublattice. With decreasing temperature, the Ir4+ sublattice progressively involves in the correlation process. Below T = 74 K, the magnetic sublattices of Cu (spin s = 1/2) and Ir (pseudospin j = 1/2) in La2CuIrO6 are strongly coupled and exhibit an antiferromagnetic phase transition into a non-collinear magnetic structure accompanied by a small uncompensated transverse moment. A weak anomaly in ac-susceptibility as well as in the NMR and μSR spin lattice relaxation rates at 54 K is interpreted as a cooperative ordering of the transverse moments which is influenced by the strong spin-orbit coupled 5d ion Ir4+. We argue that the rich magnetic behaviour observed in La2CuIrO6 is related to complex magnetic interactions between the strongly correlated spin-only 3d ions with the strongly spin-orbit coupled 5d transition ions where a combination of the spin-orbit coupling and the low-symmetry of the crystal lattice plays a special role for the spin structure in the magnetically ordered state.

cond-mat.str-el↗

arXiv2015

NMR evidence for inhomogeneous nematic fluctuations in BaFe$_2$(As$_{1-x}$P$_x$)$_2$

We present evidence for nuclear spin-lattice relaxation driven by glassy nematic fluctuations in isovalent P-doped BaFe$_2$As$_2$ single crystals. Both the $^{75}$As and $^{31}$P sites exhibit stretched-exponential relaxation similar to the electron-doped systems. By comparing the hyperfine fields and the relaxation rates at these sites we find that the As relaxation cannot be explained solely in terms of magnetic spin fluctuations. We demonstrate that nematic fluctuations couple to the As nuclear quadrupolar moment and can explain the excess relaxation. These results suggest that glassy nematic dynamics are a universal phenomenon in the iron-based superconductors.

cond-mat.str-el↗

arXiv2015

NMR evidence for inhomogeneous glassy behavior driven by nematic fluctuations in iron arsenide superconductors

We present $^{75}$As nuclear magnetic resonance spin-lattice and spin-spin relaxation rate data in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Ba(Fe$_{1-x}$Cu$_x$)$_2$As$_2$ as a function of temperature, doping and magnetic field. The relaxation curves exhibit a broad distribution of relaxation rates, consistent with inhomogeneous glassy behavior up to 100 K. The doping and temperature response of the width of the dynamical heterogeneity is similar to that of the nematic susceptibility measured by elastoresistance measurements. We argue that quenched random fields which couple to the nematic order give rise to a nematic glass that is reflected in the spin dynamics.

cond-mat.str-el↗

arXiv2015

Unusual temperature evolution of superconductivity in LiFeAs

We have performed temperature dependent tunneling spectroscopy on an impurity-free surface area of a LiFeAs single crystal. Our data reveal a highly unusual temperature evolution of superconductivity: at $T_c^*=18$~K a partial superconducting gap opens, as is evidenced by subtle, yet clear features in the tunneling spectra, i.e. particle-hole symmetric coherence peaks, and a dip-hump structure which signals strong-coupling superconductivity. At $T_c=16$~K, these features substantiate dramatically and become characteristic of full superconductivity. Remarkably, this is accompanied by an almost jump-like increase of the gap energy at $T_c$ to about 87\% of its low-temperature gap value. The energy of the bosonic mode as measured by the distance between the coherence peak and the higher-energy dip remains practically constant in the whole temperature regime $T\leq T_c^*$. The comparison of these findings with established experimental and theoretical results lead us to suggest that the bosonic mode is not directly related to incommensurate spin fluctuations that have previously been observed in inelastic neutron scattering.

cond-mat.supr-con↗

arXiv2015

Spatial competition of the ground states in 1111 iron pnictides

Using nuclear quadrupole resonance, the phase diagram of 1111 $R$FeAsO$_{1-x}$F$_x$ ($R$$=$La, Ce, Sm) iron pnictides is constructed as a function of the local charge distribution in the paramagnetic state, which features low-doping-like (LD-like) and high-doping-like (HD-like) regions. Compounds based on magnetic rare earths (Ce, Sm) display a unified behavior, and comparison with La-based compounds reveals the detrimental role of static iron $3d$ magnetism on superconductivity, as well as a qualitatively different evolution of the latter at high doping. It is found that the LD-like regions fully account for the orthorhombicity of the system, and are thus the origin of any static iron magnetism. Orthorhombicity and static magnetism are not hindered by superconductivity but limited by dilution effects, in agreement with 2D (respectively 3D) nearest-neighbor square lattice site percolation when the rare earth is nonmagnetic (respectively magnetic). The LD-like regions are not intrinsically supportive of superconductivity, on the contrary of the HD-like regions, as evidenced by the well-defined Uemura relation between the superconducting transition temperature and the superfluid density when accounting for the proximity effect. This leads us to propose a complete description of the interplay of ground states in 1111 pnictides, where nanoscopic regions compete to establish the ground state through suppression of superconductivity by static magnetism, and extension of superconductivity by proximity effect.

cond-mat.str-el↗