9 research outputs found
Freezing of molecular rotation in a paramagnetic crystal studied by P NMR
We present a detailed P nuclear magnetic resonance (NMR) study of the
molecular rotation in the compound
[Cu(pz)(2-HOpy)](PF), where pz = CHN and
2-HOpy = CHNHO. Here, a freezing of the PF 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 = 250 and 1400 K. Further, the anisotropy of the
second spectral moment of the P absorption line was calculated for the
rigid lattice, as well as in the presence of several sets of PF
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 F and P
moments in the presence of the distinct rotation modes.Comment: 9 pages, 5 figures; additionally 2 pages with 3 figures of
supplemental material; typos corrected, references added, supplemental
material adde
Field-tunable Berezinskii-Kosterlitz-Thouless correlations in a Heisenberg magnet
We report the manifestation of field-induced Berezinskii-Kosterlitz-Thouless
(BKT) correlations in the weakly coupled spin-1/2 Heisenberg layers of the
molecular-based bulk material [Cu(pz)(2-HOpy)](PF). Due to the
moderate intralayer exchange coupling of K, the
application of laboratory magnetic fields induces a substantial anisotropy
of the spin correlations. Crucially, this provides a significant BKT regime, as
the tiny interlayer exchange 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 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-HOpy)](PF) is determined by the field-tuned
anisotropy and the concomitant BKT physics.Comment: 10 pages, 7 figure
Berezinskii—Kosterlitz—Thouless correlations in copper-based quasi-2D spin systems (Review Article)
We present an overview of selected copper-based quasi-2D square-lattice spin-1/2 materials with an easy-plane anisotropy, providing the possibility to study emergent Berezinskii-Kosterlitz-Thouless (BKT) correlations. In particular, in those materials with a comparatively small exchange coupling, the effective XY anisotropy of the low-temperature spin correlations can be controlled by an applied magnetic field, yielding a systematic evolution of the BKT correlations. In cases where the residual interlayer correlations are small enough, dynamical BKT correlations in the critical regime may be observed experimentally, whereas the completion of the genuine BKT transition is preempted by the onset of long-range order. © 2023 Author(s)
Structural 130 K phase transition and emergence of a two-ion Kondo state in Ce<sub>2</sub>Rh<sub>2</sub>Ga explored by <sup>69,71</sup>Ga nuclear quadrupole resonance
We have studied the microscopic magnetic properties, the nature of the 130 K phase transition, and the ground state in the recently synthesized compound Ce2Rh2Ga by use of 69,71Ga nuclear quadrupole resonance (NQR). The NQR spectra clearly show an unusual phase transition at Tt∼130 K, yielding a splitting of the high-temperature single NQR line into two well-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/T1 shows three distinct regimes, with onset temperatures at Tt and 2 K. The temperature-independent 1/T1, observed between Tt and 2 K, crosses over to a Korringa process, 1/T1 T, below ∼2 K, which evidences a rare two-ion Kondo scenario: The system evolves into a dense Kondo coherent state below 2.0 and 0.8 K probed by the two different Ga sites. © 2022 authors. Published by the American Physical Society
Field-Tunable Berezinskii-Kosterlitz-Thouless Correlations in a Heisenberg Magnet
We report the manifestation of field-induced Berezinskii-Kosterlitz-Thouless (BKT) correlations in the weakly coupled spin-1/2 Heisenberg layers of the molecular-based bulk material [Cu(pz)2(2-HOpy)2](PF6)2. At zero field, a transition to long-range order occurs at 1.38 K, caused by a weak intrinsic easy-plane anisotropy and an interlayer exchange of J′/kB≈1 mK. Because of the moderate intralayer exchange coupling of J/kB=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′ 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 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](PF6)2 is determined by the field-tuned XY anisotropy and the concomitant BKT physics. © 2023 American Physical Society