Showing 1–50 of 107 results for author: Baenitz, M

NMR/NQR and AC-susceptibility Studies in the Weyl Semimetal Superconductor 1T-MoTe$_2$ under Pressure

Authors: Takuto. Fujii, Hiroshi Yasuoka, Mukkattu Omanakuttan Ajeesh, Marcus. Schmidt, Takeshi Mito, Yu Liu, Cedomir Petrovic, Michael Baenitz

Abstract: We performed the Te-nuclear magnetic resonance, the Mo-nuclear quadrupole resonance, and the AC susceptibility in the Weyl semimetal superconductor 1T-MoTe$_2$ at pressures up to 2.17~GPa. From the temperature and pressure dependence of the AC susceptibility, the superconducting transition temperature $T_{\mathrm{c}}$ and the upper critical field $H_{\mathrm{c2}}$ were estimated. The results devia… ▽ More We performed the Te-nuclear magnetic resonance, the Mo-nuclear quadrupole resonance, and the AC susceptibility in the Weyl semimetal superconductor 1T-MoTe$_2$ at pressures up to 2.17~GPa. From the temperature and pressure dependence of the AC susceptibility, the superconducting transition temperature $T_{\mathrm{c}}$ and the upper critical field $H_{\mathrm{c2}}$ were estimated. The results deviate from the Werthamer-Helfand-Hohenberg model but are well described by $H_{\mathrm{c2}}(T)=H_{\mathrm{c2}}(0)[1-T/T_{\mathrm{c}}]^α$. The latter fit yields $H_{\mathrm{c2}}(0)=1.50$~T, $T_{\mathrm{c}}=3.81$K, and $α=1.1$ at 2.17GPa, suggesting that the superconductivity lies in a strong-coupling regime. Since the nuclear spin-lattice relaxation rate divided by temperature, $1/T_1T$, follows the Korringa relation at ambient pressure, the increase in $1/T_1T$ with pressure up to approximately 0.7~GPa indicates an increase in the density of states (DOS), $N(E_\mathrm F)$. This trend mirrors the pressure dependence of $T_{\mathrm{c}}$ in the low-pressure region, consistent with the BCS mechanism. Above 0.7~GPa, however, $N(E_\mathrm F)$ slightly decreases while $T_{\mathrm{c}}$ continues to rise, suggesting an additional pairing contribution beyond the conventional BCS picture. In the 1T$^{\prime}$ phase at 2.17~GPa, the absence of a coherence peak in $1/T_1T$ around $T_{\mathrm c}$, accompanied by a two-step decrease just below $T_{\mathrm c}$, was observed, which may be a signature of unconventional superconductivity. △ Less

Submitted 29 January, 2026; originally announced January 2026.

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

Authors: J. Grumbach, E. Häußler, S. Luther, J. Sichelschmidt, K. M. Ranjith, T. Herrmannsdörfer, M. Rotter, S. Granovsky, H. Kühne, M. Uhlarz, J. Wosnitza, H. -H. Klauß, M. Baenitz, T. Doert, M. Doerr

Abstract: 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 triang… ▽ More 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. △ Less

Submitted 11 December, 2025; originally announced December 2025.

Journal ref: Phys. Rev. B 111, 144404 (2025)

Emergent Spin-Singlet Pairing in the Frustrated Kagome Metal Sc$_3$Mn$_3$Al$_7$Si$_5$

Authors: R. Guehne, A. K. Sharma, P. Yanda, J. Noky, J. Sichelschmidt, R. Koban, W. Schnelle, C. Shekhar, M. Baenitz, C. Felser

Abstract: The metallic kagome compound Sc$_3$Mn$_3$Al$_7$Si$_5$ has attracted attention as a candidate platform where geometric frustration and itinerant electrons may cooperate to stabilize a quantum-disordered magnetic ground state. Here, we combine bulk thermodynamic probes, low-noise FIB-device transport, and comprehensive $^{55}$Mn Nuclear Magnetic Resonance (NMR) measurements to elucidate the low-temp… ▽ More The metallic kagome compound Sc$_3$Mn$_3$Al$_7$Si$_5$ has attracted attention as a candidate platform where geometric frustration and itinerant electrons may cooperate to stabilize a quantum-disordered magnetic ground state. Here, we combine bulk thermodynamic probes, low-noise FIB-device transport, and comprehensive $^{55}$Mn Nuclear Magnetic Resonance (NMR) measurements to elucidate the low-temperature spin dynamics of this system. The bulk data reveal strongly reduced magnetic entropy, a negative magnetoresistance arising from spin scattering, and field-dependent transport indicates the spin fluctuations, while showing no signatures of long-range magnetic order. NMR provides a direct local view of the correlated Mn moments: the nuclear spin-spin relaxation $T_2$ exhibits a pronounced low-temperature enhancement driven by an indirect internuclear coupling through electronic spin fluctuations, whose temperature and distance dependence point to partially gapped low-energy spin excitations. The spin-lattice relaxation rate $T_1^{-1}$ displays a Hebel-Slichter-like coherence peak near \SI{10}{K}, coincident with the resistivity crossover and a subtle heat-capacity anomaly, indicating the formation of short-range spin-singlet correlations. Together, our results demonstrate that Sc$_3$Mn$_3$Al$_7$Si$_5$ hosts an unconventional correlated state dominated by frustrated, gapped spin dynamics, placing it among the rare metallic kagome systems proximate to a quantum spin liquid. △ Less

Submitted 1 December, 2025; originally announced December 2025.

Comments: 21 pages, 13 figures including supplementary information

Journal ref: Phys. Rev. B 113, 214430 (2026)

Inhomogeneous dynamic state in the double trillium lattice antiferromagnet KBaFe$_2$(PO$_4$)$_3$

Authors: S. J. Sebastian, S. S. Islam, R. Kolay, S. Mohanty, Q. P. Ding, Y. Skourski, J. Sichelschmidt, M. Baenitz, Jonas A. Krieger, T. J. Hicken, H. Luetkens, A. A. Tsirlin, Y. Furukawa, R. Nath

Abstract: 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 ($μ$SR) experiments, supported by first principles calculations, we uncover an unconvention… ▽ More 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 ($μ$SR) experiments, supported by first principles calculations, we uncover an unconventional ground state. Despite strong antiferromagnetic interactions with a large Curie-Weiss temperature $θ_{\rm CW} = -70(2)$ 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 $μ$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. △ Less

Submitted 11 November, 2025; originally announced November 2025.

Comments: 7 pages and 3 figures

Effects of Ru-doping on the magnetism of Ag3LiIr2O6, a candidate Kitaev quantum spin liquid

Authors: Sanjay Bachhar, M. Baenitz, John Wilkinson, A. V. Mahajan

Abstract: We report our investigations on Ag3LiIr1.4Ru0.6O6, which results from the Ru substitution in the Kitaev quantum spin liquid candidate Ag3LiIr2O6. It crystallizes in the monoclinic C2/m space group like its parent compound, Ag3LiIr2O6. Our susceptibility measurements reveal an effective moment = 2.6 muB, which is higher than the moments of the parent compound and less than that of the Ru-analog (Ag… ▽ More We report our investigations on Ag3LiIr1.4Ru0.6O6, which results from the Ru substitution in the Kitaev quantum spin liquid candidate Ag3LiIr2O6. It crystallizes in the monoclinic C2/m space group like its parent compound, Ag3LiIr2O6. Our susceptibility measurements reveal an effective moment = 2.6 muB, which is higher than the moments of the parent compound and less than that of the Ru-analog (Ag3LiRu2O6), suggesting the presence of magnetic Ir4+ (Jeff= 1/2) and Ru4+ (S=1). Bulk magnetic susceptibility suggests long-range order (LRO)at T~20 K, whereas no clear signature is present in the heat capacity. Likewise, there is a loss of the 7Li NMR spectral intensity around T~20 K as expected at the onset of LRO, but a complete wipe-out is not seen in contrast to the result in Ag3LiIr2O6. There is also a T~20 K anomaly in the 7Li NMR relaxation rate and also a fall in the 7Li NMR shift with decreasing temperature. These results suggest LRO at T~20 K in Ag3LiIr1.4Ru0.6O6. However, at low-T below 10 K, we observe a power law variation in magnetic heat capacity and spin lattice relaxation rate, temperature-independent-7K, and no further loss of the 7Li NMR spectral intensity. These results might suggest the persistence or stabilisation of a quantum spin liquid-like phase, perhaps from a fraction of the sample in Ag3LiIr1.4Ru0.6O6 below 10 K. Our muon spin relaxation measurements suggest ordering around 20 K, consistent with our other probes. It appears that the main effect of Ru-substitution is to shift the LRO to a higher temperature in comparison with Ag3LiIr2O6, though there are signatures of a novel phase below about 10 K. △ Less

Submitted 6 March, 2025; originally announced March 2025.

Comments: This paper contains 7 pages and 10 figures

A novel Gapless Quantum Spin Liquid in the S = 1 4d4-honeycomb material Cu$_3$LiRu$_2$O$_6$

Authors: Sanjay Bachhar, Nashra Pistawala, S. Kundu, Maneesha Barik, M. Baenitz, Jorg Sichelschmidt, Koji Yokoyama, P. Khuntia, Surjeet Singh, A. V. Mahajan

Abstract: We report the discovery of a novel gapless quantum spin liquid in the S=1 honeycomb system Cu$_3$LiRu$_2$O$_6$ with Ru$^{4+}$ ($4d^4$) where moments remain dynamic down to 50 mK. Heat capacity measurements show no sign of magnetic ordering down to 60 mK in spite of a Curie-Weiss temperature = -222 K indicating a strong antiferromagnetic interaction. In zero field, magnetic heat capacity shows a li… ▽ More We report the discovery of a novel gapless quantum spin liquid in the S=1 honeycomb system Cu$_3$LiRu$_2$O$_6$ with Ru$^{4+}$ ($4d^4$) where moments remain dynamic down to 50 mK. Heat capacity measurements show no sign of magnetic ordering down to 60 mK in spite of a Curie-Weiss temperature = -222 K indicating a strong antiferromagnetic interaction. In zero field, magnetic heat capacity shows a linear T-dependence with Sommerfeld coefficient = 107 mJ/mol K$^2$ is much larger than that found in typical Fermi liquids. Our local probe $^7$Li nuclear magnetic resonance (NMR) measurements find a significant temperature-independent $^7$Li NMR shift (and hence a non-zero spin susceptibility) at low-T and a linear T-variation of the $^7$Li NMR spin-lattice relaxation rate 1/T$_1$ at low-T reminiscent of fermionic excitations. Muon spin relaxation measurements detect neither long-range ordering nor spin freezing down to 50 mK and the temperature variation of the muon depolarization rate shows a gradual increase with decreasing temperature and a leveling off below about 1 K evincing a persistent spin dynamics common to several spin liquid candidates. Our results provide strong signatures of a quantum spin liquid in the titled honeycomb material. △ Less

Submitted 28 January, 2025; originally announced January 2025.

Comments: 5 pages and 6 figures in main, 8 pages and 16 figures in the supplement. arXiv admin note: text overlap with arXiv:2405.06380

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

Authors: S. Luther, K. M. Ranjith, Th. Doert, J. Wosnitza, H. Kühne, M. Baenitz

Abstract: 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… ▽ More 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. △ Less

Submitted 7 January, 2025; originally announced January 2025.

Comments: 8 pages, 8 figures

Journal ref: Phys. Rev. B 110, 214440 (2024)

Magnetic frustration and weak Mn magnetic ordering in EuMn$_2$P$_2$

Authors: Sarah Krebber, Jörg Sichelschmidt, Pierre Chailloleau, Asmaa El Mard, Marvin Kopp, Michael Baenitz, Kurt Kummer, Denis V. Vyalikh, Jens Müller, Cornelius Krellner, Kristin Kliemt

Abstract: We report on the electron spin resonance (ESR), heat capacity, magnetization, nuclear magnetic resonance (NMR), magnetic circular and linear dichroism (XMCD, XMLD), as well as the electrical resistivity of EuMn$_{2}$P$_{2}$ single crystals. Antiferromagnetic order of Eu was observed in several quantities at $T^{\rm Eu}_{\rm N}\,=\,18\,\rm K$. The temperature dependencies of ESR linewidth and reson… ▽ More We report on the electron spin resonance (ESR), heat capacity, magnetization, nuclear magnetic resonance (NMR), magnetic circular and linear dichroism (XMCD, XMLD), as well as the electrical resistivity of EuMn$_{2}$P$_{2}$ single crystals. Antiferromagnetic order of Eu was observed in several quantities at $T^{\rm Eu}_{\rm N}\,=\,18\,\rm K$. The temperature dependencies of ESR linewidth and resonance shift show, when approaching the Eu-ordered state, a divergence towards $T^{\rm Eu}_{\rm N}$, indicating the growing importance of magnetic correlations and the build-up of internal magnetic fields. An additional temperature scale of $\approx 47\,\rm K$ has considerable impact on linewidth, resonance field and intensity. This points to the presence of weak Mn-based ordering. The observed ESR line is interpreted as an Eu$^{2+}$ resonance, which probes the weak magnetic background of the Mn subsystem. Such picture is suggested by the lineshape which keeps to be Lorentzian across the $47\,\rm K$ scale and by the ESR intensity which can be described by the same Curie-Weiss temperature above and below $47\,\rm K$. In the same temperature range anomalies were observed at $48.5\,\rm K$ and $51\,\rm K$ in the heat capacity data as well as a pronounced broadening of the NMR signal of the EuMn$_{2}$P$_{2}$ samples. In XMCD and XMLD measurements, this weak magnetic order could not be detected in the same temperature range which might be due to the small magnetic moment, with a potential $c$-component or frustration. △ Less

Submitted 2 January, 2025; originally announced January 2025.

Static and Dynamic Electronic Properties of Weyl Semimetal NbP -- A Single Crystal $^{93}$Nb-NMR Study

Authors: Tetsuro Kubo, Hiroshi Yasuoka, Deepa Kasinathan, K. M. Ranjith, Marcus Schmidt, Michael Baenitz

Abstract: Nuclear magnetic resonance (NMR) techniques have been used to study the static and dynamic microscopic properties of the Weyl semimetal NbP. From a complete analysis of the angular dependence of the $^{93}$Nb-NMR spectra in a single crystal, the parameters for the electric quadrupole interactions and the magnetic hyperfine interactions were determined to be $ν_{\rm Q} = 0.61$\,MHz, $η= 0.20$,… ▽ More Nuclear magnetic resonance (NMR) techniques have been used to study the static and dynamic microscopic properties of the Weyl semimetal NbP. From a complete analysis of the angular dependence of the $^{93}$Nb-NMR spectra in a single crystal, the parameters for the electric quadrupole interactions and the magnetic hyperfine interactions were determined to be $ν_{\rm Q} = 0.61$\,MHz, $η= 0.20$, $(K_{XX}, K_{YY}, K_{ZZ}) = (- 0.06, 0.11, - 0.11)$\% at 4.5\,K. The temperature and field dependence of the $^{93}$Nb Knight shift revealed a characteristic feature of the shape of the density of states with nearly massless fermions. We clearly observed a quantum oscillation of the Knight shift associated with the band structure, whose frequency was in good agreement with the previous bulk measurements. The temperature dependence of the spin-lattice relaxation rate, $1 / T_{1} T$, showed an almost constant behavior for $30 < T < 180$\,K, while a weak temperature dependence was observed below $\sim 30$\,K. This contrasts with the behavior observed in TaP and TaAs, where the $1 / T_{1} T$ measured by the $^{181}$Ta nuclear quadrupole resonance (NQR) shows $1 / T_{1} T \propto T^{2}$ and $T^{4}$ above approximately 30\,K. In TaP, the temperature dependent orbital hyperfine interaction plays a signficant role in nuclear relaxation, whereas this contribution is not observed in TaAs. Two-component spin echo oscillations were observed. The shorter-period oscillation is attributed to the origin of quadrupole coupling, while the longer-period oscillation indicates the presence of indirect nuclear spin-spin coupling, as discussed in other Weyl semimetal like TaP. △ Less

Submitted 17 October, 2024; originally announced October 2024.

Observation of the spiral spin liquid in a triangular-lattice material

Authors: N. D. Andriushin, S. E. Nikitin, Ø. S. Fjellvåg, J. S. White, A. Podlesnyak, D. S. Inosov, M. C. Rahn, M. Schmidt, M. Baenitz, A. S. Sukhanov

Abstract: The spiral spin liquid (SSL) is a highly degenerate state characterized by a continuous contour or surface in reciprocal space spanned by a spiral propagation vector. Although the SSL state has been predicted in a number of various theoretical models, very few materials are so far experimentally identified to host such a state. Via combined single-crystal wide-angle and small-angle neutron scatter… ▽ More The spiral spin liquid (SSL) is a highly degenerate state characterized by a continuous contour or surface in reciprocal space spanned by a spiral propagation vector. Although the SSL state has been predicted in a number of various theoretical models, very few materials are so far experimentally identified to host such a state. Via combined single-crystal wide-angle and small-angle neutron scattering, we report observation of the SSL in the quasi-two-dimensional delafossite AgCrSe$_2$. We show that it is a very close realization of the ideal Heisenberg $J_1$--$J_2$--$J_3$ frustrated model on the triangular lattice. By supplementing our experimental results with microscopic spin-dynamics simulations, we demonstrate how such exotic magnetic states are driven by thermal fluctuations and exchange frustration. △ Less

Submitted 29 January, 2025; v1 submitted 7 October, 2024; originally announced October 2024.

Journal ref: Nat. Commun. 16, 2619 (2025)

Concurrence of directional Kondo transport and incommensurate magnetic order in the layered material AgCrSe$_2$

Authors: José Guimarães, Dorsa S. Fartab, Michal Moravec, Marcus Schmidt, Michael Baenitz, Burkhard Schmidt, Haijing Zhang

Abstract: In this work, we report on the concurrent emergence of the directional Kondo behavior and incommensurate magnetic ordering in a layered material. We employ temperature- and magnetic field-dependent resistivity measurements, susceptibility measurements, and high resolution wavelength X-ray diffraction spectroscopy to study the electronic properties of AgCrSe$_2$. Impurity Kondo behavior with a char… ▽ More In this work, we report on the concurrent emergence of the directional Kondo behavior and incommensurate magnetic ordering in a layered material. We employ temperature- and magnetic field-dependent resistivity measurements, susceptibility measurements, and high resolution wavelength X-ray diffraction spectroscopy to study the electronic properties of AgCrSe$_2$. Impurity Kondo behavior with a characteristic temperature of $T_\text K$ = 32 K is identified through quantitative analysis of the in-plane resistivity, substantiated by magneto-transport measurements. The agreement between our experimental data and the Schlottmann's scaling theory allows us to determine the impurity spin as $S$ = 3/2. Furthermore, we discuss the origin of the Kondo behavior and its relation to the material's antiferromagnetic transition. Our study uncovers an unusual phenomenon -- the equivalence of the Néel temperature and the Kondo temperature -- paving the way for further investigations into the intricate interplay between impurity physics and magnetic phenomena in quantum materials, with potential applications in advanced electronic and magnetic devices. △ Less

Submitted 23 May, 2024; originally announced May 2024.

Comments: 11 pages, 4 figures

Journal ref: Commun Phys 7, 176 (2024)

(H,Li)$_{6}$Ru$_{2}$O$_{6}$ : a possible zero-field Ru$^{3+}$-based Kitaev Quantum Spin Liquid

Authors: Sanjay Bachhar, M. Baenitz, Hubertus Luetkens, John Wilkinson, Sumiran Pujari, A. V. Mahajan

Abstract: We report the synthesis and properties of (H,Li)$_{6}$Ru$_{2}$O$_{6}$, which is shown to be a $J_{\text{eff}}=\frac{1}{2}$ system made out of Ru$^{3+}$ moments in a honeycomb geometry. Bulk magnetization, heat capacity, nuclear magnetic resonance (NMR), and muon spin relaxation ($μ$SR) rule out the presence of static moments or any spin glass phase down to 84 mK. All techniques suggest a crossover… ▽ More We report the synthesis and properties of (H,Li)$_{6}$Ru$_{2}$O$_{6}$, which is shown to be a $J_{\text{eff}}=\frac{1}{2}$ system made out of Ru$^{3+}$ moments in a honeycomb geometry. Bulk magnetization, heat capacity, nuclear magnetic resonance (NMR), and muon spin relaxation ($μ$SR) rule out the presence of static moments or any spin glass phase down to 84 mK. All techniques suggest a crossover to a liquid-like state below about 40 K. The $^{7}$Li nuclear magnetic resonance (NMR) shift data suggest a non-zero $T$-independent spin susceptibility at low $T$. In zero field, $C_m/T$ shows $T^{-0.9}$ divergence which is consistent with vacancy-induced effects on low-energy excitations of the pristine Kitaev spin liquid. With field, power-law variations in the $^{7}$Li NMR spin-lattice relaxation rate 1/T$_{1}$ and magnetic heat capacity $C_{m}$ show quantitatively new scaling behaviors. A two-step entropy release in heat capacity is also observed putatively from $Z_{2}$ flux (low-$T$ step) and itinerant Majorana fermions (high-$T$ step). Based on these findings, we propose that (H,Li)$_{6}$Ru$_{2}$O$_{6}$ realizes a Kitaev spin liquid with no evidence of inherent magnetic ordering in zero field unlike $α$-RuCl$_{3}$ where approximately $8$ Tesla field is required to suppress magnetic order. △ Less

Submitted 4 February, 2025; v1 submitted 10 May, 2024; originally announced May 2024.

Comments: This is closest to the published version of 5 pages, 6 figures of main text + 12 pages, and 29 figures of supplementary information

Journal ref: Phys. Rev. B 110, L241102 (2024)

Orbital selective commensurate modulations of the local density of states in ScV6Sn6 probed by nuclear spins

Authors: Robin Guehne, Jonathan Noky, Changjiang Yi, Chandra Shekhar, Maia G. Vergniory, Michael Baenitz, Claudia Felser

Abstract: The Kagome network is a unique platform in solid state physics that harbors a diversity of special electronic states due to its inherent band structure features comprising Dirac cones, van-Hove singularities, and flat bands. Some Kagome-based non-magnetic metals have recently been found to exhibit favorable properties, including unconventional superconductivity, charge density waves (CDW), switcha… ▽ More The Kagome network is a unique platform in solid state physics that harbors a diversity of special electronic states due to its inherent band structure features comprising Dirac cones, van-Hove singularities, and flat bands. Some Kagome-based non-magnetic metals have recently been found to exhibit favorable properties, including unconventional superconductivity, charge density waves (CDW), switchable chiral transport, and signatures of an anomalous Hall effect (AHE). The Kagome metal ScV6Sn6 is another promising candidate for studying the emergence of an unconventional CDW and accompanying effects. We use 51V nuclear magnetic resonance (NMR) to study the local properties of the CDW phase in single crystalline ScV6Sn6, aided by density functional theory (DFT). We trace the dynamics of the local magnetic field during the CDW phase transition and determine a loss in the density of states (DOS) by a factor of $\sqrt{2}$, in excellent agreement with DFT. The local charge symmetry of the V surrounding in the CDW phase reflects the commensurate modulation of the charge density with wave vector q=(1/3,1/3,1/3). An unusual orientation dependent change in the NMR shift splitting symmetry, however, reveals orbital selective modulations of the local DOS. △ Less

Submitted 29 April, 2024; originally announced April 2024.

Comments: 25 pages, 6 figures, supplementary files

Journal ref: Nat Commun 15, 8213 (2024)

Experimental nuclear quadrupole resonance and computational study of the structurally refined topological semimetal TaSb$_2$

Authors: T. Fujii, O. Janson, H. Yasuoka, H. Rosner, Yu. Prots, U. Burkhardt, M. Schmidt, M. Baenitz

Abstract: The local electric field gradients and magnetic dynamics of TaSb$_2$ have been studied using $^{121}$Sb, $^{123}$Sb, and $^{181}$Ta nuclear quadrupole resonance (NQR) with density functional theory (DFT) calculations using XRD-determined crystal structures. By measuring all structurally expected thirteen NQR lines, the nuclear quadrupole coupling constant ($ν_Q$) and asymmetric parameter ($η$) for… ▽ More The local electric field gradients and magnetic dynamics of TaSb$_2$ have been studied using $^{121}$Sb, $^{123}$Sb, and $^{181}$Ta nuclear quadrupole resonance (NQR) with density functional theory (DFT) calculations using XRD-determined crystal structures. By measuring all structurally expected thirteen NQR lines, the nuclear quadrupole coupling constant ($ν_Q$) and asymmetric parameter ($η$) for Ta, Sb(1), and Sb(2) sites were obtained. These values are all in good agreement with the presented DFT calculations. Principal axes of the electric field gradients was determined for a single-crystal sample by measuring the angular dependencies of NMR frequency under a weak magnetic field. The unusual temperature dependence of $η$(T) of Sb(2) hints at the suppressed thermal expansion along the $a$-axis. Spin lattice relaxation rate ($1/T_1T$) measurements reveal an activated-type behavior and an upturn below 30 K. Neither the low temperature upturn nor the high temperature activation type behaviors are reproduced by the calculated $1/T_1T$ based on the calculated density of states (DOS). On the other hand, the agreement between the calculated DOS and specific heat measurements indicates that the band renormalization is small. This fact indicates that TaSb$_2$ deviates from the simple semimetal scenario, and magnetic excitations are not captured by Fermi liquid theory. △ Less

Submitted 18 December, 2023; originally announced December 2023.

Comments: 15 pages, 13 figures

Journal ref: Phys. Rev. B 109, 035116 (2024)

Repulsive Tomonaga-Luttinger Liquid in Quasi-one-dimensional Alternating Spin-$1/2$ Antiferromagnet NaVOPO$_4$

Authors: S. S. Islam, Prashanta K. Mukharjee, P. K. Biswas, Mark Telling, Y. Skourski, K. M. Ranjith, M. Baenitz, Y. Inagaki, Y. Furukawa, A. A. Tsirlin, R. Nath

Abstract: We probe the magnetic field-induced Tomonaga-Luttinger liquid (TLL) state in the bond-alternating spin-$1/2$ antiferromagnetic (AFM) chain compound NaVOPO$_4$ using thermodynamic as well as local $μ$SR and $^{31}$P NMR probes down to milli-K temperatures in magnetic fields up to 14~T. The $μ$SR and NMR relaxation rates in the gapless TLL regime decay slowly following characteristic power-law behav… ▽ More We probe the magnetic field-induced Tomonaga-Luttinger liquid (TLL) state in the bond-alternating spin-$1/2$ antiferromagnetic (AFM) chain compound NaVOPO$_4$ using thermodynamic as well as local $μ$SR and $^{31}$P NMR probes down to milli-K temperatures in magnetic fields up to 14~T. The $μ$SR and NMR relaxation rates in the gapless TLL regime decay slowly following characteristic power-law behaviour, enabling us to directly determine the interaction parameter $K$ as a function of the magnetic field. These estimates are cross-checked using magnetization and specific heat data. The field-dependent $K$ lies in the range of $0.4 < K < 1$ and indicates repulsive nature of interactions between the spinless fermions, in line with the theoretical predictions. This renders NaVOPO$_4$ the first experimental realization of TLL with repulsive fermionic interactions in hitherto studied $S=1/2$ bond-alternating AFM-AFM chain compounds. △ Less

Submitted 6 November, 2023; originally announced November 2023.

Comments: 7 pages, 2 figures, 47 references

Experimental signatures of quantum and topological states in frustrated magnetism

Authors: J. Khatua, B. Sana, A. Zorko, M. Gomilšek, K. Sethupathi M. S. Ramachandra Rao, M. Baenitz, B. Schmidt, P. Khuntia

Abstract: Frustration in magnetic materials arising from competing exchange interactions can prevent the system from adopting long-range magnetic order and can instead lead to a diverse range of novel quantum and topological states with exotic quasiparticle excitations. Here, we review prominent examples of such emergent phenomena, including magnetically-disordered and extensively degenerate spin ices, whic… ▽ More Frustration in magnetic materials arising from competing exchange interactions can prevent the system from adopting long-range magnetic order and can instead lead to a diverse range of novel quantum and topological states with exotic quasiparticle excitations. Here, we review prominent examples of such emergent phenomena, including magnetically-disordered and extensively degenerate spin ices, which feature emergent magnetic monopole excitations, highly-entangled quantum spin liquids with fractional spinon excitations, topological order and emergent gauge fields, as well as complex particle-like topological spin textures known as skyrmions. We provide an overview of recent advances in the search for magnetically-disordered candidate materials on the three-dimensional pyrochlore lattice and two-dimensional triangular, kagome and honeycomb lattices, the latter with bond-dependent Kitaev interactions, and on lattices supporting topological magnetism. We highlight experimental signatures of these often elusive phenomena and single out the most suitable experimental techniques that can be used to detect them. Our review also aims at providing a comprehensive guide for designing and investigating novel frustrated magnetic materials, with the potential of addressing some important open questions in contemporary condensed matter physics. △ Less

Submitted 15 November, 2023; v1 submitted 23 October, 2023; originally announced October 2023.

Journal ref: Physics Reports 1041, 1 (2023)

Comparative $^{181}$Ta-NQR Study of Weyl Monopnictides TaAs and TaP: Relevance of Weyl Fermion Excitations

Authors: Tetsuro Kubo, Hiroshi Yasuoka, Balázs Dóra, Deepa Kasinathan, Yurii Prots, Helge Rosner, Takuto Fujii, Marcus Schmidt, Michael Baenitz

Abstract: Based on our first detailed $^{181}$Ta nuclear quadrupole resonance (NQR) studies from 2017 on the Weyl semimetal TaP, we now extended our NQR studies to another Ta-based monopnictide TaAs. In the present work, we have determined the temperature-dependent $^{181}$Ta-NQR spectra, the spin-lattice relaxation time $T_{1}$, and the spin-spin relaxation time $T_{2}$. We found the following characterist… ▽ More Based on our first detailed $^{181}$Ta nuclear quadrupole resonance (NQR) studies from 2017 on the Weyl semimetal TaP, we now extended our NQR studies to another Ta-based monopnictide TaAs. In the present work, we have determined the temperature-dependent $^{181}$Ta-NQR spectra, the spin-lattice relaxation time $T_{1}$, and the spin-spin relaxation time $T_{2}$. We found the following characteristic features that showed great contrast to what was found in TaP: (1) The quadrupole coupling constant and asymmetry parameter of EFG, extracted from three NQR frequencies, have a strong temperature dependence above $\sim$80 K that cannot be explained by the density functional theory calculation incorporating the thermal expansion of the lattice. (2) The temperature dependence of the spin-lattice relaxation rate, $1/T_{1} T$, shows a $T^{4}$ power law behavior above $\sim$30 K. This is a great contrast with the $1/T_{1} T \propto T^{2}$ behavior found in TaP, which was ascribed to the magnetic excitations at the Weyl nodes with a temperature-dependent orbital hyperfine coupling. (3) Regarding the nuclear spin-spin interaction, we found the spin-echo signal decays with the pulse separation simply by a Lorentzian function in TaAs, but we have observed spin-echo modulations in TaP that is most likely due to the indirect nuclear spin-spin coupling via virtually excited Weyl fermions. From our experimental findings, we conclude that the present NQR results do not show dominant contributions from Weyl fermion excitations in TaAs. △ Less

Submitted 29 July, 2023; originally announced July 2023.

Comments: 8 pages, 10 figures

Journal ref: J. Phys. Soc. Jpn. 92, 094706 (2023)

Electron spin resonance study on the 4f honeycomb quantum magnet YbCl3

Authors: Jörg Sichelschmidt, Ellen Häußler, Ekaterina Vinokurova, M. Baenitz, Thomas Doert

Abstract: The local magnetic properties of Yb$^{3+}$ in the layered honeycomb material YbCl$_{3}$ were investigated by electron spin resonance on single crystals. For in-plane and out-of-plane field orientations the $g$-factor shows a clear anisotropy ($g_\|=2.97(8)$ and $g_\bot =1.53(4)$), whereas the low temperature exchange coupling and the spin relaxation display a rather isotropic character. At elevate… ▽ More The local magnetic properties of Yb$^{3+}$ in the layered honeycomb material YbCl$_{3}$ were investigated by electron spin resonance on single crystals. For in-plane and out-of-plane field orientations the $g$-factor shows a clear anisotropy ($g_\|=2.97(8)$ and $g_\bot =1.53(4)$), whereas the low temperature exchange coupling and the spin relaxation display a rather isotropic character. At elevated temperatures the contribution of the first excited crystal field level ($21\pm2$~meV) dominates the spin relaxation. △ Less

Submitted 18 July, 2023; originally announced July 2023.

Comments: 10 pages, 5 figures

Journal ref: J. Phys.: Condens. Matter 35, 425601 (2023)

Observation of the anomalous Hall effect in a layered polar semiconductor

Authors: Seo-Jin Kim, Jihang Zhu, Mario M. Piva, Marcus Schmidt, Dorsa Fartab, Andrew P. Mackenzie, Michael Baenitz, Michael Nicklas, Helge Rosner, Ashley M. Cook, Rafael González-Hernández, Libor Šmejkal, Haijing Zhang

Abstract: Progress in magnetoelectric materials is hindered by apparently contradictory requirements for time-reversal symmetry broken and polar ferroelectric electronic structure in common ferromagnets and antiferromagnets. Alternative routes could be provided by recent discoveries of a time-reversal symmetry breaking anomalous Hall effect in noncollinear magnets and altermagnets, but hitherto reported bul… ▽ More Progress in magnetoelectric materials is hindered by apparently contradictory requirements for time-reversal symmetry broken and polar ferroelectric electronic structure in common ferromagnets and antiferromagnets. Alternative routes could be provided by recent discoveries of a time-reversal symmetry breaking anomalous Hall effect in noncollinear magnets and altermagnets, but hitherto reported bulk materials are not polar. Here, we report the observation of a spontaneous anomalous Hall effect in doped AgCrSe$_2$, a layered polar semiconductor with an antiferromagnetic coupling between Cr spins in adjacent layers. The anomalous Hall resistivity 3 $μΩ$ cm is comparable to the largest observed in compensated magnetic systems to date, and is rapidly switched off when the angle of an applied magnetic field is rotated to $\sim 80^{\circ}$ from the crystalline $c$-axis. Our ionic gating experiments show that the anomalous Hall conductivity magnitude can be enhanced by modulating the $p$-type carrier density. We also present theoretical results that suggest the anomalous Hall effect is driven by Berry curvature due to noncollinear antiferromagnetic correlations among Cr spins, which are consistent with the previously suggested magnetic ordering in AgCrSe$_2$. Our results open the possibility to study the interplay of magnetic and ferroelectric-like responses in this fascinating class of materials. △ Less

Submitted 7 July, 2023; originally announced July 2023.

Comments: 8 pages, 5 figures

Quantum disordered ground state in the spin-orbit coupled Jeff = 1/2 distorted honeycomb magnet BiYbGeO5

Authors: S. Mohanty, S. S. Islam, N. Winterhalter-Stocker, A. Jesche, G. Simutis, Ch. Wang, Z. Guguchia, J. Sichelschmidt, M. Baenitz, A. A. Tsirlin, P. Gegenwart, R. Nath

Abstract: We delineate quantum magnetism in the strongly spin-orbit coupled, distorted honeycomb-lattice antiferromagnet BiYbGeO$_{5}$. Our magnetization and heat capacity measurements reveal that its low-temperature behavior is well described by an effective $J_{\rm eff}=1/2$ Kramers doublet of Yb$^{3+}$. The ground state is nonmagnetic with a tiny spin gap. Temperature-dependent magnetic susceptibility, m… ▽ More We delineate quantum magnetism in the strongly spin-orbit coupled, distorted honeycomb-lattice antiferromagnet BiYbGeO$_{5}$. Our magnetization and heat capacity measurements reveal that its low-temperature behavior is well described by an effective $J_{\rm eff}=1/2$ Kramers doublet of Yb$^{3+}$. The ground state is nonmagnetic with a tiny spin gap. Temperature-dependent magnetic susceptibility, magnetization isotherm, and heat capacity could be modeled well assuming isolated spin dimers with anisotropic exchange interactions $J_{\rm Z} \simeq 2.6$~K and $J_{\rm XY} \simeq 1.3$~K. Heat capacity measurements backed by muon spin relaxation suggest the absence of magnetic long-range order down to at least 80\,mK both in zero field and in applied fields. This sets BiYbGeO$_5$ apart from Yb$_2$Si$_2$O$_7$ with its unusual regime of magnon Bose-Einstein condensation and suggests negligible interdimer couplings, despite only a weak structural deformation of the honeycomb lattice. △ Less

Submitted 17 May, 2023; originally announced May 2023.

Comments: 6 pages, 5 figures

Possible realization of a randomness-driven quantum disordered state in an S = 1/2 antiferromagnet Sr3CuTa2O9

Authors: B. Sana, M. Barik, S. Lee, U. Jena, M. Baenitz, J. Sichelschmidt, S. Luther, H. Kuehne, K. Sethupathi, M. S. Ramachandra Rao, K. Y. Choi, P. Khuntia

Abstract: Collective behavior of spins, frustration-induced strong quantum fluctuations, and subtle interplay between competing degrees of freedom in quantum materials can lead to correlated quantum states with exotic excitations that are essential ingredients for establishing paradigmatic models and have immense potential for quantum technologies. Disorder is ubiquitous in real materials, and the detailed… ▽ More Collective behavior of spins, frustration-induced strong quantum fluctuations, and subtle interplay between competing degrees of freedom in quantum materials can lead to correlated quantum states with exotic excitations that are essential ingredients for establishing paradigmatic models and have immense potential for quantum technologies. Disorder is ubiquitous in real materials, and the detailed insights into the role of disorder on the intriguing ground state borne out of quenched randomness provide a route toward the design and discovery of functional quantum materials. Herein, we report magnetization, specific heat, electron spin resonance, and muon spin resonance studies on a 3d-electron-based antiferromagnet Sr3CuTa2O9. The negative Curie- Weiss temperature value, obtained from the Curie-Weiss fit of high-temperature magnetic susceptibility data, indicates antiferromagnetic interaction between Cu2+ moments. Specific heat data show the absence of long-range magnetic ordering down to 64 mK despite a reasonably strong exchange interaction between Cu2+ (S =1/2) spins as reflected from a Curie-Weiss temperature of -27 K. The power-law behavior and the data collapse of specific heat and magnetization data evince the emergence of a random-singlet state in Sr3CuTa2O9. The power-law-like spin auto-correlation function and the data collapse of muon polarization asymmetry with longitudinal field dependence of t(μ0H)^γ further support credence to the presence of a randomness-induced quantum disordered state. Our results suggest that randomness induced by disorder is an alternate route to realize a quantum disordered state in this antiferromagnet. △ Less

Submitted 25 September, 2024; v1 submitted 25 April, 2023; originally announced April 2023.

Journal ref: Phys. Rev. B 110, 134412 (2024)

Magnetic properties of a spin-orbit entangled Jeff=1/2 three-dimensional frustrated rare-earth hyperkagome

Authors: B. Sana, M. Barik, M. Pregelj, U. Jena, M. Baenitz, J. Sichelschmidt, K. Sethupathi, P. Khuntia

Abstract: The interplay between competing degrees of freedom can stabilize non-trivial magnetic states in correlated electron materials. Frustration-induced strong quantum fluctuations can evade long-range magnetic ordering leading to exotic quantum states such as spin liquids in two-dimensional spin-lattices such as triangular and kagome structures. However, the experimental realization of dynamic and corr… ▽ More The interplay between competing degrees of freedom can stabilize non-trivial magnetic states in correlated electron materials. Frustration-induced strong quantum fluctuations can evade long-range magnetic ordering leading to exotic quantum states such as spin liquids in two-dimensional spin-lattices such as triangular and kagome structures. However, the experimental realization of dynamic and correlated quantum states is rare in three-dimensional (3D) frustrated magnets wherein quantum fluctuations are less prominent. Herein, we report the crystal structure, magnetic susceptibility, electron spin resonance (ESR) and specific heat studies accompanied by crystal electric field (CEF) calculations on a 3D frustrated magnet Yb3Sc2Ga3O12. In this material, Yb3+ ions form a three-dimensional network of corner-sharing triangles known as hyperkagome lattice without any detectable anti-site disorder. Our results reveal a low energy state with Jeff = 1/2 degrees of freedom in the Kramers doublet state. The zero field-cooled and field cooled magnetic susceptibility taken in 0.001 T rules out the presence of spin-freezing down to 1.8K. The Curie-Weiss (CW) fit to low-T susceptibility data yields a small and negative CW temperature indicating the presence of a weak antiferromagnetic interaction between Jeff = 1/2 (Yb3+) moments. The Yb-ESR displays a broad line of non-Lorentzian shape that suggests considerable magnetic anisotropy in Yb3Sc2Ga3O12. The CEF calculations suggest that the ground state is well separated from the excited states, which are in good agreement with experimental results. The absence of long-range magnetic ordering indicates a dynamic liquid-like ground state at least down to 130 mK. Furthermore, zero field specific heat shows a broad maximum around 200 mK suggesting the presence of short-range spin correlations in this 3D frustrated antiferromagnet. △ Less

Submitted 26 August, 2023; v1 submitted 14 April, 2023; originally announced April 2023.

Journal ref: Physical Review B 108, 134413 (2023)

Absence of magnetic order and emergence of unconventional fluctuations in $J_{\rm eff} =1/2$ triangular lattice antiferromagnet YbBO$_3$

Authors: K. Somesh, S. S. Islam, S. Mohanty, G. Simutis, Z. Guguchia, Ch. Wang, J. Sichelschmidt, M. Baenitz, R. Nath

Abstract: We present the ground state properties of a new quantum antiferromagnet YbBO$_3$ in which the isotropic Yb$^{3+}$ triangular layers are separated by a non-magnetic layer of partially occupied B and O(2) sites. The magnetization and heat capacity data establish a spin-orbit entangled effective spin $J_{\rm eff} = 1/2$ state of Yb$^{3+}$ ions at low temperatures, interacting antiferromagnetically wi… ▽ More We present the ground state properties of a new quantum antiferromagnet YbBO$_3$ in which the isotropic Yb$^{3+}$ triangular layers are separated by a non-magnetic layer of partially occupied B and O(2) sites. The magnetization and heat capacity data establish a spin-orbit entangled effective spin $J_{\rm eff} = 1/2$ state of Yb$^{3+}$ ions at low temperatures, interacting antiferromagnetically with an intra-layer coupling $J/k_{\rm B} \simeq 0.53$ K. The absence of oscillations and a $1/3$ tail in the zero-field muon asymmetries rule out the onset of magnetic long-range-order as well as spin-freezing down to 20~mK. An anomalous broad maximum in the temperature dependent heat capacity with a unusually reduced value and a broad anomaly in zero-field muon depolarization rate centered at $T^*\simeq 0.7 \frac{J}{k_{\rm B}}$ provide compelling evidence for a wide fluctuating regime ($0.182 \leq T/J \leq 1.63$) with slow relaxation. We infer that the fluctuating regime is a universal feature of a highly frustrated triangular lattice antiferromagnets while the absence of magnetic long-range-order is due to perfect two-dimensionality of the spin-lattice protected by non-magnetic site disorder. △ Less

Submitted 18 October, 2022; originally announced October 2022.

Comments: 7 pages, 4 figures

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

Authors: D. Opherden, M. S. J. Tepaske, F. Bärtl, M. Weber, M. M. Turnbull, T. Lancaster, S. J. Blundell, M. Baenitz, J. Wosnitza, C. P. Landee, R. Moessner, D. J. Luitz, H. Kühne

Abstract: 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 correl… ▽ More 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. △ Less

Submitted 22 September, 2022; originally announced September 2022.

Comments: 10 pages, 7 figures

The $S=1$ dimer system K$_2$Ni(MoO$_4$)$_2$: a candidate for magnon Bose-Einstein condensation

Authors: B. Lenz, B. Koteswararao, S. Biermann, P. Khuntia, M. Baenitz, S. K. Panda

Abstract: Dimerized quantum magnets provide a unique possibility to investigate Bose-Einstein condensation of magnetic excitations in crystalline systems at low temperature. Here, we model the low-temperature magnetic properties of the recently synthesized spin $S=1$ dimer system K${}_2$Ni(MoO${}_4$)$_2$ and propose it as a new candidate material for triplon and quintuplon condensation. Based on a first pri… ▽ More Dimerized quantum magnets provide a unique possibility to investigate Bose-Einstein condensation of magnetic excitations in crystalline systems at low temperature. Here, we model the low-temperature magnetic properties of the recently synthesized spin $S=1$ dimer system K${}_2$Ni(MoO${}_4$)$_2$ and propose it as a new candidate material for triplon and quintuplon condensation. Based on a first principles analysis of its electronic structure, we derive an effective spin-dimer model that we first solve within a mean-field approximation to refine its parameters in comparison to experiment. Finally, the model is solved by employing a numerically exact quantum Monte Carlo technique which leads to magnetic properties in good agreement with experimental magnetization and thermodynamic results. We discuss the emergent spin model of K${}_2$Ni(MoO${}_4$)$_2$ in view of condensation of magnetic excitations in a broad parameter regime. Finally, we comment on a geometrical peculiarity of the proposed model and discuss how it could host a supersolid phase upon structural distortions. △ Less

Submitted 19 August, 2022; originally announced August 2022.

Comments: 7 pages, 4 figures + supplemental material of 6 pages, 6 figures. Comments are welcome

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

Authors: Sh. Yamamoto, T. Fujii, S. Luther, H. Yasuoka, H. Sakai, F. Bärtl, K. M. Ranjith, H. Rosner, J. Wosnitza, A. M. Strydom, H. Kühne, M. Baenitz

Abstract: 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 lin… ▽ More 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. △ Less

Submitted 11 July, 2022; originally announced July 2022.

Comments: 12 pages, 7 figures

Journal ref: Phys. Rev. B 106, 115125 (2022)

Diluting a triangular-lattice spin liquid: Synthesis and characterization of NaYb$_{1-x}$Lu$_{x}$S$_2$ single crystals

Authors: Ellen Häußler, Jörg Sichelschmidt, Michael Baenitz, Eric C. Andrade, Matthias Vojta, Thomas Doert

Abstract: Yb-based magnets, with a perfect triangular lattice of pseudospin-$1/2$ Yb$^{3+}$ ions, have emerged as candidates for realizing a quantum spin-liquid state, with NaYbS$_2$ being a prominent example. Here we present the solid-solution series NaYb$_{1-x}$Lu$_{x}$S$_2$ with well-defined single crystals over the entire substitution range $0\le x\le 1$. Chemical and structural analysis indicate a stat… ▽ More Yb-based magnets, with a perfect triangular lattice of pseudospin-$1/2$ Yb$^{3+}$ ions, have emerged as candidates for realizing a quantum spin-liquid state, with NaYbS$_2$ being a prominent example. Here we present the solid-solution series NaYb$_{1-x}$Lu$_{x}$S$_2$ with well-defined single crystals over the entire substitution range $0\le x\le 1$. Chemical and structural analysis indicate a statistically homogeneous replacement of Yb$^{3+}$ by Lu$^{3+}$ ions. We magnetically characterize the relatively small single crystals using electron spin resonance (ESR). Below $30$K the ESR intensity can be well described by a Curie-Weiss function for all \textit{x}, with a decreasing Weiss temperature with increasing Lu content. This reduction of the average magnetic interaction upon Lu substitution is also supported by magnetization measurements. Importantly, no signs of magnetic or spin-glass order are detected down to $2$K for any $x$. For $x>0.5$ the ESR linewidth strongly increases, indicating the breakup of the magnetic system into disconnected clusters as expected from percolation physics. The experimental magnetization data are found to be in good agreement for all $x$ with results of classical Monte-Carlo simulations for a triangular-lattice Heisenberg model, amended with a small second-neighbor interaction. Taken together, our results establish NaYb$_{1-x}$Lu$_{x}$S$_2$ as a family of diluted triangular-lattice spin liquids. △ Less

Submitted 1 April, 2022; v1 submitted 4 March, 2022; originally announced March 2022.

Comments: 9 pages, 8 figures

Journal ref: Phys. Rev. Materials 6, 046201 (2022)

Magnetic transition in the $J_\textrm{eff} = 1/2$ Kagomé system Sm$_3$Sb$_3$Zn$_2$O$_{14}$

Authors: Vinod Kumar, S. Kundu, M. Baenitz, A. V. Mahajan

Abstract: We present a study of Sm$_3$Sb$_3$Zn$_2$O$_{14}$ (SSZO) through magnetization and specific heat capacity $C_p(T)$ measurements. SSZO contains well separated planes of Sm$^{3+}$ magnetic ions on a Kagomé lattice. Though our magnetic susceptibility (where the data are limited down to 2\,K) does not show any anomaly, the $C_p(T)$ data which have been taken down to 0.35 K reveal a broad maximum at… ▽ More We present a study of Sm$_3$Sb$_3$Zn$_2$O$_{14}$ (SSZO) through magnetization and specific heat capacity $C_p(T)$ measurements. SSZO contains well separated planes of Sm$^{3+}$ magnetic ions on a Kagomé lattice. Though our magnetic susceptibility (where the data are limited down to 2\,K) does not show any anomaly, the $C_p(T)$ data which have been taken down to 0.35 K reveal a broad maximum at $T\sim1.5$\,K followed by a sharp peak at $T\sim0.5$\,K indicative of short-range correlations and long-range order, respectively. △ Less

Submitted 6 February, 2022; originally announced February 2022.

Comments: 5 pages, 6 images

Emergent 1/3 magnetization plateaus in pyroxene CoGeO$_3$

Authors: H. Guo, L. Zhao, M. Baenitz, X. Fabrèges, A. Gukasov, A. Melendez Sans, D. I. Khomskii, L. H. Tjeng, A. C. Komarek

Abstract: Despite the absence of an apparent triangular pattern in the crystal structure, we observe unusually well pronounced 1/3 magnetization plateaus in the quasi one-dimensional Ising spin chain compound CoGeO$_3$ which belongs to the class of pyroxene minerals. We succeeded in uncovering the detailed microscopic spin structure of the 1/3 magnetization plateau phase by means of neutron diffraction. We… ▽ More Despite the absence of an apparent triangular pattern in the crystal structure, we observe unusually well pronounced 1/3 magnetization plateaus in the quasi one-dimensional Ising spin chain compound CoGeO$_3$ which belongs to the class of pyroxene minerals. We succeeded in uncovering the detailed microscopic spin structure of the 1/3 magnetization plateau phase by means of neutron diffraction. We observed changes of the initial antiferromagnetic zero-field spin structure that are resembling a regular formation of antiferromagnetic "domain wall boundaries", resulting in a kind of modulated magnetic structure with 1/3-integer propagation vector. The net ferromagnetic moment emerges at these "domain walls" whereas two third of all antiferromagnetic chain alignments can be still preserved. We propose a microscopic model on the basis of an anisotropic frustrated square lattice to explain the observations. △ Less

Submitted 15 September, 2021; originally announced September 2021.

Journal ref: Physical Review RESEARCH 3, L032037 (2021)

Quantum magnetism of ferromagnetic spin dimers in $α$-KVOPO$_4$

Authors: Prashanta K. Mukharjee, K. Somesh, K. M. Ranjith, M. Baenitz, Y. Skourski, D. T. Adroja, D. Khalyavin, A. A. Tsirlin, R. Nath

Abstract: Magnetism of the spin-$\frac12$ $α$-KVOPO$_4$ is studied by thermodynamic measurements, $^{31}$P nuclear magnetic resonance (NMR), neutron diffraction, and density-functional band-structure calculations. Ferromagnetic Curie-Weiss temperature of $θ_{\rm CW}\simeq 15.9$ K and the saturation field of $μ_0H_s\simeq 11.3$ T suggest the predominant ferromagnetic coupling augmented by a weaker antiferrom… ▽ More Magnetism of the spin-$\frac12$ $α$-KVOPO$_4$ is studied by thermodynamic measurements, $^{31}$P nuclear magnetic resonance (NMR), neutron diffraction, and density-functional band-structure calculations. Ferromagnetic Curie-Weiss temperature of $θ_{\rm CW}\simeq 15.9$ K and the saturation field of $μ_0H_s\simeq 11.3$ T suggest the predominant ferromagnetic coupling augmented by a weaker antiferromagnetic exchange that leads to a short-range order below 5 K and the long-range antiferromagnetic order below $T_{\rm N}\simeq 2.7$ K in zero field. Magnetic structure with the propagation vector $\mathbf k=(0,\frac12,0)$ and the ordered magnetic moment of 0.58 $μ_B$ at 1.5 K exposes a non-trivial spin lattice where strong ferromagnetic dimers are coupled antiferromagnetically. The reduction in the ordered magnetic moment with respect to the classical value (1 $μ_{\rm B}$) indicates sizable quantum fluctuations in this setting, despite the predominance of ferromagnetic exchange. We interpret this tendency toward ferromagnetism as arising from the effective orbital order in the folded chains of the VO$_6$ octahedra. △ Less

Submitted 14 September, 2021; originally announced September 2021.

Comments: 12 pages, 14 figures, 58 references

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

Authors: M. Baenitz, M. M. Piva, S. Luther, J. Sichelschmidt, K. M. Ranjith, H. Dawczak-Dȩbicki, M. O. Ajeesh, S. -J. Kim, G. Siemann, C. Bigi, P. Manuel, D. Khalyavin, D. A. Sokolov, P. Mokhtari, H. Zhang, H. Yasuoka, P. D. C. King, G. Vinai, V. Polewczyk, P. Torelli, J. Wosnitza, U. Burkhardt, B. Schmidt, H. Rosner, S. Wirth , et al. (3 additional authors not shown)

Abstract: 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 metamagn… ▽ More 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. △ Less

Submitted 6 September, 2021; originally announced September 2021.

Yb delafossites: unique exchange frustration of 4f spin 1/2 moments on a perfect triangular lattice

Authors: B. Schmidt, J. Sichelschmidt, K. M. Ranjith, Th. Doert, M. Baenitz

Abstract: While the Heisenberg model for magnetic Mott insulators on planar lattice structures is comparatively well understood in the case of transition metal ions, the intrinsic spin-orbit entanglement of 4f magnetic ions on such lattices shows fascinating new physics largely due to corresponding strong anisotropies both in their single-ion and their exchange properties. We show here that the Yb delafossi… ▽ More While the Heisenberg model for magnetic Mott insulators on planar lattice structures is comparatively well understood in the case of transition metal ions, the intrinsic spin-orbit entanglement of 4f magnetic ions on such lattices shows fascinating new physics largely due to corresponding strong anisotropies both in their single-ion and their exchange properties. We show here that the Yb delafossites, containing perfect magnetic Yb$^{3+}$ triangular lattice planes with pseudospin $s=1/2$ at low temperatures, are an ideal platform to study these new phenomena. Competing frustrated interactions may lead to an absence of magnetic order associated to a gapless spin liquid ground state with a huge linear specific heat exceeding that of many heavy fermions, whereas the application of a magnetic field induces anisotropic magnetic order with successive transitions into different long ranged ordered structures. In this comparative study, we discuss our experimental findings in terms of a unified crystal-field and exchange model. We combine electron paramagnetic resonance (EPR) experiments and results from neutron scattering with measurements of the magnetic susceptibility, isothermal magnetization up to full polarization, and specific heat to determine the relevant model parameters. The impact of the crystal field is discussed as well as the symmetry-compatible form of the exchange tensor, and we give explicit expressions for the anisotropic g factor, the temperature dependence of the susceptibility, the exchange-narrowed EPR linewidth and the saturation field. △ Less

Submitted 14 June, 2021; originally announced June 2021.

Comments: 20 pages, 7 figures, to appear in Physical Review B

Journal ref: Physical Review B 103, 214445 (2021)

Quasi-one-dimensional magnetism in the spin-$\frac12$ antiferromagnet BaNa$_{2}$Cu(VO$_{4}$)$_{2}$

Authors: Sebin J. Sebastian, K. Somesh, M. Nandi, N. Ahmed, P. Bag, M. Baenitz, B. Koo, J. Sichelschmidt, A. A. Tsirlin, Y. Furukawa, R. Nath

Abstract: We report synthesis and magnetic properties of quasi-one-dimensional spin-$\frac{1}{2}$ Heisenberg antiferromagnetic chain compound BaNa$_2$Cu(VO$_4$)$_2$. This orthovanadate has a centrosymmetric crystal structure, $C2/c$, where the magnetic Cu$^{2+}$ ions form spin chains. These chains are arranged in layers, with the chain direction changing by 62$^0$ between the two successive layers. Alternat… ▽ More We report synthesis and magnetic properties of quasi-one-dimensional spin-$\frac{1}{2}$ Heisenberg antiferromagnetic chain compound BaNa$_2$Cu(VO$_4$)$_2$. This orthovanadate has a centrosymmetric crystal structure, $C2/c$, where the magnetic Cu$^{2+}$ ions form spin chains. These chains are arranged in layers, with the chain direction changing by 62$^0$ between the two successive layers. Alternatively, the spin lattice can be viewed as anisotropic triangular layers upon taking the inter-chain interactions into consideration. Despite this potential structural complexity, temperature-dependent magnetic susceptibility, heat capacity, ESR intensity, and NMR shift agree well with the uniform spin-$1/2$ Heisenberg chain model with an intrachain coupling of $J/k_{\rm B} \simeq 5.6$ K. The saturation field obtained from the magnetic isotherm measurement consistently reproduces the value of $J/k_{\rm B}$. Further, the $^{51}$V NMR spin-lattice relaxation rate mimics the 1D character in the intermediate temperature range, whereas magnetic long-range order sets in below $T_{\rm N} \simeq 0.25$ K. The effective interchain coupling is estimated to be $J_{\perp}/k_{\rm B} \simeq 0.1$ K. The theoretical estimation of exchange couplings using band-structure calculations reciprocate our experimental findings and unambiguously establish the 1D character of the compound. Finally, the spin lattice of BaNa$_2$Cu(VO$_4$)$_2$ is compared with the chemically similar but not isostructural compound BaAg$_2$Cu(VO$_4)_2$. △ Less

Submitted 13 January, 2021; originally announced January 2021.

Comments: 12 pages, 11 figures, 1 table, Phys. Rev. B (accepted)

Gapless quantum spin liquid in the triangular system Sr$_{3}$CuSb$_{2}$O$_{9}$

Authors: S. Kundu, Aga Shahee, Atasi Chakraborty, K. M. Ranjith, B. Koo, Jörg Sichelschmidt, Mark T. F. Telling, P. K. Biswas, M. Baenitz, I. Dasgupta, Sumiran Pujari, A. V. Mahajan

Abstract: We report gapless quantum spin liquid behavior in the layered triangular Sr$_{3}$CuSb$_{2}$O$_{9}$ (SCSO) system. X-ray diffraction shows superlattice reflections associated with atomic site ordering into triangular Cu planes well-separated by Sb planes. Muon spin relaxation ($μ$SR) measurements show that the $S = \frac{1}{2}$ moments at the magnetically active Cu sites remain dynamic down to 65 m… ▽ More We report gapless quantum spin liquid behavior in the layered triangular Sr$_{3}$CuSb$_{2}$O$_{9}$ (SCSO) system. X-ray diffraction shows superlattice reflections associated with atomic site ordering into triangular Cu planes well-separated by Sb planes. Muon spin relaxation ($μ$SR) measurements show that the $S = \frac{1}{2}$ moments at the magnetically active Cu sites remain dynamic down to 65 mK in spite of a large antiferromagnetic exchange scale evidenced by a large Curie-Weiss temperature $θ_{\mathrm{cw}} \simeq $ -143 K as extracted from the bulk susceptibility. Specific heat measurements also show no sign of long-range order down to 0.35 K. The magnetic specific heat ($\mathit{C}$$_{\mathrm{m}}$) below 5 K reveals a $\mathit{C}$$_{\mathrm{m}}$ $=$ $γT$ + $αT$$^{2}$ behavior. The significant $T$$^{2}$ contribution to the magnetic specific heat invites a phenomenology in terms of the so-called Dirac spinon excitations with a linear dispersion. From the low-$T$ specific heat data, we estimate the dominant exchange scale to be $\sim $ 36 K using a Dirac spin liquid ansatz which is not far from the values inferred from microscopic density functional theory calculations ($\sim $ 45 K) as well as high-temperature susceptibility analysis ($\sim$ 70 K). The linear specific heat coefficient is about 18 mJ/mol-K$^2$ which is somewhat larger than for typical Fermi liquids. △ Less

Submitted 28 December, 2020; v1 submitted 2 December, 2020; originally announced December 2020.

Comments: 16 pages, 21 figures, including supplementary material. A $S = \frac{1}{2}$ Dirac spin liquid scenario has been put forward to explain the field-dependent specific heat data. Comments are welcome

Journal ref: Phys. Rev. Lett. 125, 267202 (2020)

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

Authors: D. Opherden, F. Bärtl, Sh. Yamamoto, Z. T. Zhang, S. Luther, S. Molatta, J. Wosnitza, M. Baenitz, I. Heinmaa, R. Stern, C. P. Landee, H. Kühne

Abstract: 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 longitudi… ▽ More 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. △ Less

Submitted 25 January, 2021; v1 submitted 13 October, 2020; originally announced October 2020.

Comments: 9 pages, 5 figures; additionally 2 pages with 3 figures of supplemental material; typos corrected, references added, supplemental material added

Journal ref: Phys. Rev. B 103, 014428 (2021)

Signatures of a spin-1/2 cooperative paramagnet in the diluted triangular lattice of Y$_2$CuTiO$_6$

Authors: S. Kundu, Akmal Hossain, Pranava Keerthi S, Ranjan Das, M. Baenitz, Peter J. Baker, Jean-Christophe Orain, D. C. Joshi, Roland Mathieu, Priya Mahadevan, Sumiran Pujari, Subhro Bhattacharjee, A. V. Mahajan, D. D. Sarma

Abstract: We present a combination of thermodynamic and dynamic experimental signatures of a disorder driven dynamic cooperative paramagnet in a 50% site diluted triangular lattice spin-1/2 system, Y$_2$CuTiO$_6$. Magnetic ordering and spin freezing are absent down to 50 mK, far below the Curie Weiss scale of ~-134 K. We observe scaling collapses of the magnetic field- and temperature-dependent magnetic hea… ▽ More We present a combination of thermodynamic and dynamic experimental signatures of a disorder driven dynamic cooperative paramagnet in a 50% site diluted triangular lattice spin-1/2 system, Y$_2$CuTiO$_6$. Magnetic ordering and spin freezing are absent down to 50 mK, far below the Curie Weiss scale of ~-134 K. We observe scaling collapses of the magnetic field- and temperature-dependent magnetic heat capacity and magnetisation data, respectively, in conformity with expectations from the random singlet physics. Our experiments establish the suppression of any freezing scale, if at all present, by more than three orders of magnitude, opening a plethora of interesting possibilities such as disorder-stabilized long range quantum entangled ground states. △ Less

Submitted 10 September, 2020; originally announced September 2020.

Comments: 18 pages, 9 figures, published in Physical Review Letters

Journal ref: Phys. Rev. Lett. 125, 117206 - Published 9 September 2020

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

Authors: D. Opherden, N. Nizar, K. Richardson, J. C. Monroe, M. M. Turnbull, M. Polson, S. Vela, W. J. A. Blackmore, P. A. Goddard, J. Singleton, E. S. Choi, F. Xiao, R. C. Williams, T. Lancaster, F. L. Pratt, S. J. Blundell, Y. Skourski, M. Uhlarz, A. N. Ponomaryov, S. A. Zvyagin, J. Wosnitza, M. Baenitz, I. Heinmaa, R. Stern, H. Kühne , et al. (1 additional authors not shown)

Abstract: 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 squ… ▽ More 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. △ Less

Submitted 1 September, 2020; v1 submitted 25 May, 2020; originally announced May 2020.

Comments: 14 pages, 8 figures, as well as 10 pages and 18 figures of supplemental material

Journal ref: Phys. Rev. B 102, 064431 (2020)

Magnetic Properties of an Effective Spin-$\frac{1}{2}$ Triangular-Lattice Compound LiYbS$_2$

Authors: K. M. Ranjith, Ph. Schlender, Th. Doert, M. Baenitz

Abstract: Here, we report the synthesis and magnetic properties of a Yb-based triangular-lattice compound LiYbS$_2$. At low temperatures, it features an effective spin-$\frac{1}{2}$ state due to the combined effect of crystal electric field and spin orbit coupling. Magnetic susceptibility measurements and $^7$Li nuclear magnetic resonance experiments reveal the absence of magnetic long range ordering down t… ▽ More Here, we report the synthesis and magnetic properties of a Yb-based triangular-lattice compound LiYbS$_2$. At low temperatures, it features an effective spin-$\frac{1}{2}$ state due to the combined effect of crystal electric field and spin orbit coupling. Magnetic susceptibility measurements and $^7$Li nuclear magnetic resonance experiments reveal the absence of magnetic long range ordering down to 2~K, which suggests a possible quantum spin liquid ground state. A dominant antiferromagnetic nearest neighbour exchange interaction $J/k_{\rm B}\simeq$ 5.3~K could be extracted form the magnetic susceptibility. The NMR linewidth analysis yields the coupling constant between the Li nuclei and Yb$^{3+}$ ions which was found to be purely dipolar in nature. △ Less

Submitted 1 March, 2020; originally announced March 2020.

Comments: (accepted)

Two types of alternating spin-$\frac12$ chains and their field-induced transitions in $\varepsilon$-LiVOPO$_4$

Authors: Prashanta K. Mukharjee, K. M. Ranjith, M. Baenitz, Y. Skourski, A. A. Tsirlin, R. Nath

Abstract: Thermodynamic properties, $^{31}$P nuclear magnetic resonance (NMR) measurements, and density-functional band-structure calculations for $\varepsilon$-LiVOPO$_4$ are reported. This quantum magnet features a singlet ground state and comprises two types of alternating spin-$\frac12$ chains that manifest themselves by the double maxima in the susceptibility and magnetic specific heat, and by the two-… ▽ More Thermodynamic properties, $^{31}$P nuclear magnetic resonance (NMR) measurements, and density-functional band-structure calculations for $\varepsilon$-LiVOPO$_4$ are reported. This quantum magnet features a singlet ground state and comprises two types of alternating spin-$\frac12$ chains that manifest themselves by the double maxima in the susceptibility and magnetic specific heat, and by the two-step magnetization process with an intermediate $\frac12$-plateau. From thermodynamic data and band-structure calculations, we estimate the leading couplings of $J_1\simeq 20$ K and $J_2\simeq 60$ K and the alternation ratios of $α_1=J_1'/J_1\simeq 0.6$ and $α_2=J_2'/J_2\simeq 0.3$ within the two chains, respectively. The zero-field spin gap $Δ_0/k_{\rm B}\simeq 7.3$ K probed by thermodynamic and NMR measurements is caused by the $J_1$-$J_1'$ spin chains and can be closed in the applied field of $μ_{0}H_{\rm c1}\simeq 5.6$ T, giving rise to a field-induced long-range order. The NMR data reveal predominant three-dimensional spin-spin correlations at low temperatures. Field-induced magnetic ordering transition observed above $H_{c1}$ is attributed to the Bose-Einstein condensation of triplons in the sublattice formed by the $J_1$-$J_1'$ chains with weaker exchange couplings. △ Less

Submitted 2 June, 2020; v1 submitted 28 February, 2020; originally announced February 2020.

Comments: 14 pages, 14 figures

Journal ref: Phys. Rev. B 101, 224403 (2020)

Effective Spin-1/2 Moments on a Yb$^{3+}$ Triangular Lattice: an ESR Study

Authors: Jörg Sichelschmidt, Burkhard Schmidt, Philipp Schlender, Seunghyun Khim, Thomas Doert, Michael Baenitz

Abstract: We investigated the spin dynamics by electron spin resonance (ESR) of the Yb-based, effective spin-1/2 delafossites NaYbO$_{2}$, AgYbO$_{2}$, LiYbS$_{2}$, NaYbS$_{2}$, and NaYbSe$_{2}$ which all show an absence of magnetic order down to lowest reachable temperatures and thus are prime candidates to host a quantum spin-liquid ground state in the vicinity of long range magnetic order. Clearly resolv… ▽ More We investigated the spin dynamics by electron spin resonance (ESR) of the Yb-based, effective spin-1/2 delafossites NaYbO$_{2}$, AgYbO$_{2}$, LiYbS$_{2}$, NaYbS$_{2}$, and NaYbSe$_{2}$ which all show an absence of magnetic order down to lowest reachable temperatures and thus are prime candidates to host a quantum spin-liquid ground state in the vicinity of long range magnetic order. Clearly resolved ESR spectra allow to obtain well-defined $g$ values which are determined by the crystal field of the distorted octahedral surrounding of the Yb-ions in trigonal symmetry. This local crystal field information provides important input to characterize the effective $S = 1/2$ Kramers doublet as well as the anisotropic exchange coupling between the Yb ions which is crucial for the nature of the groundstate. The ESR linewidth $ΔB$ is characterised by the spin dynamics and is mainly determined by the anisotropic exchange coupling. We discuss and compare $ΔB$ of the above mentioned delafossites focussing on the low temperature behaviour which is dominated by the growing influence of spin correlations. △ Less

Submitted 4 December, 2019; originally announced December 2019.

Comments: 6 pages, 4 figures, SCES 2019 conference

Journal ref: JPS Conf. Proc. 30, 011096 (2020)

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

Authors: K. M. Ranjith, S. Luther, T. Reimann, B. Schmidt, Ph. Schlender, J. Sichelschmidt, H. Yasuoka, A. M. Strydom, Y. Skourski, J. Wosnitza, H. Kühne, Th. Doert, M. Baenitz

Abstract: 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… ▽ More 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. △ Less

Submitted 28 November, 2019; originally announced November 2019.

Comments: 6 Figures

Bose-Einstein condensation of triplons close to the quantum critical point in the quasi-one-dimensional spin-$1/2$ antiferromagnet NaVOPO$_4$

Authors: Prashanta K. Mukharjee, K. M. Ranjith, B. Koo, J. Sichelschmidt, M. Baenitz, Y. Skourski, Y. Inagaki, Y. Furukawa, A. A. Tsirlin, R. Nath

Abstract: Structural and magnetic properties of a quasi-one-dimensional spin-$1/2$ compound NaVOPO$_4$ are explored by x-ray diffraction, magnetic susceptibility, high-field magnetization, specific heat, electron spin resonance, and $^{31}$P nuclear magnetic resonance measurements, as well as complementary \textit{ab initio} calculations. Whereas magnetic susceptibility of NaVOPO$_4$ may be compatible with… ▽ More Structural and magnetic properties of a quasi-one-dimensional spin-$1/2$ compound NaVOPO$_4$ are explored by x-ray diffraction, magnetic susceptibility, high-field magnetization, specific heat, electron spin resonance, and $^{31}$P nuclear magnetic resonance measurements, as well as complementary \textit{ab initio} calculations. Whereas magnetic susceptibility of NaVOPO$_4$ may be compatible with the gapless uniform spin chain model, detailed examination of the crystal structure reveals a weak alternation of the exchange couplings with the alternation ratio $α\simeq 0.98$ and the ensuing zero-field spin gap $Δ_{0}/k_{\rm B} \simeq 2.4$~K directly probed by field-dependent magnetization measurements. No long-range order is observed down to 50\,mK in zero field. However, applied fields above the critical field $H_{c1}\simeq 1.6$\,T give rise to a magnetic ordering transition with the phase boundary $T_{\rm N} \propto {(H - H_{\rm c1})^{\frac{1}φ}}$, where $φ\simeq 1.8$ is close to the value expected for Bose-Einstein condensation of triplons. With its weak alternation of the exchange couplings and small spin gap, NaVOPO$_4$ lies close to the quantum critical point. △ Less

Submitted 30 August, 2019; originally announced September 2019.

Comments: 15 pages, 15 figures, 2 tables

Journal ref: Phys. Rev. B 100, 144433 (2019)

A case study of bilayered spin-$1/2$ square lattice compound [VO(HCOO)$_2\cdot$(H$_2$O)]

Authors: S. Guchhait, U. Arjun, P. K. Anjana, M. Sahoo, A. Thirumurugan, A. Madhi, Y. Skourski, B. Koo, J. Sichelschmidt, B. Schmidt, M. Baenitz, R. Nath

Abstract: We present the synthesis and a detail investigation of structural and magnetic properties of polycrystalline [VO(HCOO)$_2\cdot$(H$_2$O)] by means of x-ray diffraction, magnetic susceptibility, high-field magnetization, heat capacity, and electron spin resonance measurements. It crystallizes in a orthorhombic structure with space group $Pcca$. It features distorted VO$_6$ octahedra connected via HC… ▽ More We present the synthesis and a detail investigation of structural and magnetic properties of polycrystalline [VO(HCOO)$_2\cdot$(H$_2$O)] by means of x-ray diffraction, magnetic susceptibility, high-field magnetization, heat capacity, and electron spin resonance measurements. It crystallizes in a orthorhombic structure with space group $Pcca$. It features distorted VO$_6$ octahedra connected via HCOO linker (formate anions) forming a two-dimensional square lattice network with a bilayered structure. Analysis of magnetic susceptibility, high field magnetization, and heat capacity data in terms of the frustrated square lattice model unambiguously establish quasi-two-dimensional nature of the compound with nearest neighbour interaction $J_1/k_{\rm B} \simeq 11.7$~K and next-nearest-neighbour interaction $J_2/k_{\rm B} \simeq 0.02$~K. It undergoes a Néel antiferromagnetic ordering at $T_{\rm N} \simeq 1.1$~K. The ratio $θ_{\rm CW}/T_{\rm N} \simeq 10.9$ reflects excellent two-dimensionality of the spin-lattice in the compound. A strong in-plane anisotropy is inferred from the linear increase of $T_{\rm N}$ with magnetic field, consistent with the structural data. △ Less

Submitted 16 August, 2019; originally announced August 2019.

Comments: 9 pages, 7 figures, 1 table

Journal ref: Phys. Rev. Materials 3, 104409 (2019)

The spin-1/2 coupled tetramer system Ba(TiO)Cu$_4$(PO$_4$)$ _4$ probed by magnetization, specific heat, and $^{31}$P-NMR

Authors: Vinod Kumar, Aga Shahee, S. Kundu, M. Baenitz, A. V. Mahajan

Abstract: We present the synthesis and a detailed investigation of structural and magnetic properties of polycrystalline Ba(TiO)Cu$_{4}$(PO$_{4}$)$_{4}$ (BTCPO) via x-ray diffraction, magnetic susceptibility, heat capacity, and $^{31}$P Nuclear Magnetic Resonance (NMR) measurements. BTCPO has a 2D layered structure with interlinked Cu$_{4}$O$_{12}$ tetramer units. A broad maximum is observed around 16.5 K i… ▽ More We present the synthesis and a detailed investigation of structural and magnetic properties of polycrystalline Ba(TiO)Cu$_{4}$(PO$_{4}$)$_{4}$ (BTCPO) via x-ray diffraction, magnetic susceptibility, heat capacity, and $^{31}$P Nuclear Magnetic Resonance (NMR) measurements. BTCPO has a 2D layered structure with interlinked Cu$_{4}$O$_{12}$ tetramer units. A broad maximum is observed around 16.5 K in our magnetization data accompanied by a sharp anomaly around $T$ = 9.5 K in the heat capacity. An anomaly at $T$ = 9.5 K is also found in the temperature dependence of the $^{31}$P NMR spin-lattice relaxation rate $1/T_1$. A power law behavior for the heat capacity as well as for the $^{31}$P $1/T_1$ below the ordering temperature could be obtained. The $^{31}$P NMR lineshape is asymmetric and the NMR shift tracks the bulk spin-susceptibility. We estimated the isotropic and axial components of the hyperfine coupling tensor to be as the $A^{iso}_{hf} \backsimeq 6794$ $\rm{Oe/μ_{B}}$ and $A^{ax}_{hf} \backsimeq 818$ $\rm{Oe/μ_{B}}$, respectively. △ Less

Submitted 13 September, 2019; v1 submitted 23 February, 2019; originally announced February 2019.

Comments: 8 pages, 3 tables, 13 figures

Journal ref: Journal of Magnetism and Magnetic Materials Volume 492, 15 December 2019, 165600

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

Authors: K. M. Ranjith, D. Dmytriieva, S. Khim, J. Sichelschmidt, S. Luther, D. Ehlers, H. Yasuoka, J. Wosnitza, A. A. Tsirlin, H. Kühne, M. Baenitz

Abstract: 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 obser… ▽ More 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 △ Less

Submitted 1 May, 2019; v1 submitted 23 January, 2019; originally announced January 2019.

Comments: 4 figures

Journal ref: Phys. Rev. B 99, 180401 (2019)

Electron Spin Resonance on the spin-1/2 triangular magnet NaYbS2

Authors: Jörg Sichelschmidt, Philipp Schlender, Burkhard Schmidt, Michael Baenitz, Thomas Doert

Abstract: The delafossite structure of NaYbS2 contains a planar spin-1/2 triangular lattice of Yb3+ ions and features a possible realisation of a quantum spin-liquid state. We investigated the Yb3+ spin dynamics by Electron Spin Resonance (ESR) in single-crystalline samples of NaYbS2. Very clear spectra with a well-resolved and large anisotropy could be observed down to the lowest accessible temperature of… ▽ More The delafossite structure of NaYbS2 contains a planar spin-1/2 triangular lattice of Yb3+ ions and features a possible realisation of a quantum spin-liquid state. We investigated the Yb3+ spin dynamics by Electron Spin Resonance (ESR) in single-crystalline samples of NaYbS2. Very clear spectra with a well-resolved and large anisotropy could be observed down to the lowest accessible temperature of 2.7 K. In contrast to the ESR properties of other known spin-liquid candidate systems, the resonance seen in NaYbS2 is accessible at low fields (< 1T) and is narrow enough for accurate characterisation of the relaxation rate as well as the g factor of the Yb3+ spins. △ Less

Submitted 19 December, 2018; originally announced December 2018.

Comments: 8 pages

Journal ref: J. Phys.: Condens. Matter 2019

Ga$_{x}$Cu$_{4-x}$(OD)$_{6}$Cl$_{2}$: Insulating ground state in an electron doped kagome system

Authors: P. Puphal, K. M. Ranjith, A. Pustogow, M. Muller, A. Rogalev, K. Kummer, J. -C. Orain, C. Baines, M. Baenitz, M. Dressel, E. Kermarrec, F. Bert, P. Mendels, C. Krellner

Abstract: We present a successful synthesis method of the series Ga$_{x}$Cu$_{4-x}$(OD)$_{6}$Cl$_{2}$ with substitutions up to $x=0.8$. The compound remains a frustrated kagome system with an insulating ground state similar to herbertsmithite for these substitutions, as the additional charge of Ga$^{3+}$ is most likely bound to additional OH$^{-}$ and Cl$^{-}$. Besides infrared measurements, we present magn… ▽ More We present a successful synthesis method of the series Ga$_{x}$Cu$_{4-x}$(OD)$_{6}$Cl$_{2}$ with substitutions up to $x=0.8$. The compound remains a frustrated kagome system with an insulating ground state similar to herbertsmithite for these substitutions, as the additional charge of Ga$^{3+}$ is most likely bound to additional OH$^{-}$ and Cl$^{-}$. Besides infrared measurements, we present magnetic and specific-heat data down to 2 K for selected samples with $0\leq x\leq0.8$. With increasing $x$ the long-range magnetic order is suppressed, similar to what was observed in the series Zn$_{x}$Cu$_{4-x}$(OH)$_{6}$Cl$_{2}$, indicating that Ga goes predominantly to the inter-plane position of the layered crystal structure. The reduction of the frozen magnetic fraction with increasing substitution was followed by $μ$SR measurements. $^{69,71}$Ga nuclear magnetic resonance (NMR) was applied as a local probe for Ga induced disorder. One well resolved Ga NMR line of moderate width is found for each isotope across the phase diagram which indicates a rather homogeneous distribution of the Ga isotopes on a single site in the lattice. △ Less

Submitted 6 November, 2018; originally announced November 2018.

Comments: submitted to physica status solidi

Singlet ground state in the alternating spin-$1/2$ chain compound NaVOAsO$_4$

Authors: U. Arjun, K. M. Ranjith, B. Koo, J. Sichelschmidt, Y. Skourski, M. Baenitz, A. A. Tsirlin, R. Nath

Abstract: We present the synthesis and a detailed investigation of structural and magnetic properties of polycrystalline NaVOAsO$_4$ by means of x-ray diffraction, magnetization, electron spin resonance (ESR), and $^{75}$As nuclear magnetic resonance (NMR) measurements as well as density-functional band structure calculations. Temperature-dependent magnetic susceptibility, ESR intensity, and NMR line shift… ▽ More We present the synthesis and a detailed investigation of structural and magnetic properties of polycrystalline NaVOAsO$_4$ by means of x-ray diffraction, magnetization, electron spin resonance (ESR), and $^{75}$As nuclear magnetic resonance (NMR) measurements as well as density-functional band structure calculations. Temperature-dependent magnetic susceptibility, ESR intensity, and NMR line shift could be described well using an alternating spin-$1/2$ chain model with the exchange coupling $J/k_{\rm B}\simeq 52$ K and an alternation parameter $α\simeq 0.65$. From the high-field magnetic isotherm measured at $T=1.5 $ K, the critical field of the gap closing is found to be $ H_{\rm c}\simeq 16$ T, which corresponds to the zero-field spin gap of $Δ_0/k_{\rm B}\simeq 21.4$ K. Both NMR shift and spin-lattice relaxation rate show an activated behavior at low temperatures, further confirming the singlet ground state. The spin chains do not coincide with the structural chains, whereas the couplings between the spin chains are frustrated. Because of a relatively small spin gap, NaVOAsO$_4$ is a promising compound for further experimental studies under high magnetic fields. △ Less

Submitted 15 October, 2018; originally announced October 2018.

Comments: 14 pages, 10 figures, 2 tables

Journal ref: Phys. Rev. B 99, 014421 (2019)

NaYbS$_2$ - a new planar spin-$\frac{1}{2}$ triangular-lattice magnet and putative spin liquid

Authors: M. Baenitz, Ph. Schlender, J. Sichelschmidt, Y. A. Onykiienko, Z. Zangeneh, K. M. Ranjith, R. Sarkar, L. Hozoi, H. C. Walker, J. -C. Orain, H. Yasuoka, J. van den Brink, H. H. Klauss, D. S. Inosov, Th. Doert

Abstract: Platelike high-quality NaYbS$_{2}$ rhombohedral single crystals with lateral dimensions of a few mm have been grown and investigated in great detail by bulk methods like magnetization and specific heat, but also by local probes like nuclear magnetic resonance (NMR), electron-spin resonance (ESR), muon-spin relaxation ($μ$SR), and inelastic neutron scattering (INS) over a wide field and temperature… ▽ More Platelike high-quality NaYbS$_{2}$ rhombohedral single crystals with lateral dimensions of a few mm have been grown and investigated in great detail by bulk methods like magnetization and specific heat, but also by local probes like nuclear magnetic resonance (NMR), electron-spin resonance (ESR), muon-spin relaxation ($μ$SR), and inelastic neutron scattering (INS) over a wide field and temperature range. Our single-crystal studies clearly evidence a strongly anisotropic quasi-2D magnetism and an emerging spin-orbit entangled $S=1/2$ state of Yb towards low temperatures together with an absence of long-range magnetic order down to 260~mK. In particular, the clear and narrow Yb ESR lines together with narrow $^{23}$Na NMR lines evidence an absence of inherent structural distortions in the system, which is in strong contrast to the related spin-liquid candidate YbMgGaO$_{4}$ falling within the same space group $R\overline{3}m$. This identifies NaYbS$_{2}$ as a rather pure spin-1/2 triangular lattice magnet and a new putative quantum spin liquid. △ Less

Submitted 6 September, 2018; originally announced September 2018.

Journal ref: Phys. Rev. B 98, 220409 (2018)

Nuclear spin-lattice relaxation time in TaP and the Knight shift of Weyl semimetals

Authors: Z. Okvátovity, H. Yasuoka, M. Baenitz, F. Simon, B. Dóra

Abstract: We first analyze the recent experimental data on the nuclear spin-lattice relaxation rate of the Weyl semimetal TaP. We argue that its non-monotonic temperature dependence is explained by the temperature dependent chemical potential of Weyl fermions. We also develop the theory of the Knight shift in Weyl semimetals, which contains two counteracting terms. The diamagnetic term follows… ▽ More We first analyze the recent experimental data on the nuclear spin-lattice relaxation rate of the Weyl semimetal TaP. We argue that its non-monotonic temperature dependence is explained by the temperature dependent chemical potential of Weyl fermions. We also develop the theory of the Knight shift in Weyl semimetals, which contains two counteracting terms. The diamagnetic term follows $-\ln[W/\max(|μ|,k_BT)]$ with $W$, $μ$ and $T$ being the high energy cutoff, chemical potential and temperature, respectively, and is always negative. The paramagnetic term scales with $μ$ and changes sign depending on the doping level. Altogether, the Knight shift is predicted to vanish or even change sign upon changing the doping or the temperature, making it a sensitive tool to identify Weyl points. We also calculate the Korringa relation for Weyl semimetals which shows an unusual energy dependence rather than being constant as expected for a non-interacting Fermi system. △ Less

Submitted 15 February, 2019; v1 submitted 21 June, 2018; originally announced June 2018.

Comments: 9 pages, 3 figures

Journal ref: Phys. Rev. B 99, 115107 (2019)