Field-Tunable Berezinskii-Kosterlitz-Thouless Correlations in a Heisenberg Magnet.

We report the manifestation of field-induced Berezinskii-Kosterlitz-Thouless (BKT) correlations in the weakly coupled spin-1/2 Heisenberg layers of the molecular-based bulk material [Cu(pz)_{2}(2-HOpy)_{2}](PF_{6})_{2}. 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^{'}/k_{B}≈1 mK. Because of the moderate intralayer exchange coupling of J/k_{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^{'} 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}](PF_{6})_{2} is determined by the field-tuned XY anisotropy and the concomitant BKT physics.

Field-induced quantum criticality and universal temperature dependence of the magnetization of a spin-1/2 heisenberg chain.

High-precision dc magnetization measurements have been made on Cu(C4H4N2) (NO3)2 in magnetic fields up to 14.7 T, slightly above the saturation field Hs=13.97 T, in the temperature range from 0.08 to 15 K. The magnetization curve and differential susceptibility at the lowest temperature show excellent agreement with exact theoretical results for the spin-1/2 Heisenberg antiferromagnet in one dimension. A broad peak is observed in magnetization measured as a function of temperature, signaling a crossover to a low-temperature Tomonaga-Luttinger-liquid regime. With an increasing field, the peak moves gradually to lower temperatures, compressing the regime, and, at Hs, the magnetization exhibits a strong upturn. This quantum critical behavior of the magnetization and that of the specific heat withstand quantitative tests against theory, demonstrating that the material is a practically perfect one-dimensional spin-1/2 Heisenberg antiferromagnet.

Wilson ratio of a Tomonaga-Luttinger liquid in a spin-1/2 Heisenberg ladder.

Using micromechanical force magnetometry, we have measured the magnetization of the strong-leg spin-1/2 ladder compound (C(7)H(10)N)(2)CuBr(2) at temperatures down to 45 mK. Low-temperature magnetic susceptibility as a function of field exhibits a maximum near the critical field H(c) at which the magnon gap vanishes, as expected for a gapped one-dimensional antiferromagnet. Above H(c) a clear minimum appears in the magnetization as a function of temperature, as predicted by theory. In this field region, the susceptibility in conjunction with our specific-heat data yields the Wilson ratio R(W). The result supports the relation R(W)=4K, where K is the Tomonaga-Luttinger-liquid parameter.

Field-induced Tomonaga-Luttinger liquid phase of a two-leg spin-1/2 ladder with strong leg interactions.

We study the magnetic-field-induced quantum phase transition from a gapped quantum phase that has no magnetic long-range order into a gapless phase in the spin-1/2 ladder compound bis(2,3-dimethylpyridinium) tetrabromocuprate (DIMPY). At temperatures below about 1 K, the specific heat in the gapless phase attains an asymptotic linear temperature dependence, characteristic of a Tomonaga-Luttinger liquid. Inelastic neutron scattering and the specific heat measurements in both phases are in good agreement with theoretical calculations, demonstrating that DIMPY is the first model material for an S=1/2 two-leg spin ladder in the strong-leg regime.

First-principles bottom-up study of 1D to 3D magnetic transformation in the copper pyrazine dinitrate S = (1)/(2) antiferromagnetic crystal.

On the basis of magnetic susceptibility and heat capacity data, copper pyrazine dinitrate crystal [abbreviated CuPz(NO(3))(2)] has long been considered a good prototype for S = (1)/(2) antiferromagnetic (AFM) Heisenberg chain behavior down to 0.05 K. However, a recent muon-spin rotation experiment indicated the presence of a previously unnoticed 1D to 3D magnetic transition below 0.107 K. Our aim in this work is to perform a rigorous quantitative study of the mechanism of this 1D-3D magnetic transformation, by doing a first-principles bottom-up study of the CuPz(NO(3))(2) crystal at 158 K, where the magnetic properties are clearly 1D, and at 2 K, at which the neutron structure (reported in this work) is considered nearly identical with that below 0.1 K (due to small thermal effects). A change in the magnetic topology is found between these two structures: at 158 K, there are isolated AFM spin chains (J(intra) = -5.23 cm(-1)), while at 2 K, the magnetic chains (J(intra) = -5.96 cm(-1)) weakly interact (the largest of the J(inter) parameters is -0.09 cm(-1)). This change is caused by thermal contraction upon cooling (no crystallographic phase transition is detected down to 2 K, and one will not likely occur below that temperature). The computed and experimental magnetic susceptibility chi(T) curves are nearly identical. The calculated heat capacity C(p)(T) curve has a maximum at 6.92 K, close to the 5.20 K maximum found in the experimental curve at zero external field. In spite of the 3D magnetic topology of the crystal at low temperature, the magnetic susceptibility and heat capacity curves behave as a pure 1D AFM chain in all regions because of the large J(intra)/J(inter) ratio (66.2 in absolute value) and the effect of including the J(inter) interactions will not be easily appreciated in any of these experiments. The impact of the presence of odd- and even-membered regular AFM finite chains in the CuPz(NO(3))(2) crystal has also been evaluated. Odd-membered interacting chains produce an increase in both chi(T) and C(p)(T) curves when the temperature is very close to zero, in agreement with the experimental observations, while even-membered chains produce a small shoulder in the C(p)(T) curve between 0.8 and 5 K. No changes are seen in the remaining regions. Concerning the spin gap, odd-membered chains present a quasi-zero gap but the finite even-membered chains still have a sizable one. Finally, the effect of increasing the magnitude of J(inter) was investigated by fixing the value of J(intra) to that found for the 2 K CuPz(NO(3))(2) crystal. The magnetic susceptibility and heat capacity curves remain practically unchanged.

ESR of coupled spin-1/2 chains in copper pyrazine dinitrate: unveiling geometrical frustration.

The spin dynamics of copper pyrazine dinitrate (Cu(C4H4N2)(NO3)2), a model spin-1/2 Heisenberg antiferromagnetic (AF) chain system, was investigated by means of electron spin resonance (ESR). Using the high-field ESR we evidenced the inequivalence of Cu sites belonging to adjacent spin chains in the ac planes of this compound. It was revealed that the dominating interchain interaction is of zig-zag-type. This interaction gives rise to geometrical frustration strongly affecting the character of AF ordering. Combining our experimental findings with the results of a quasiclassical approach we predict that at low temperatures the system orders in an incommensurate spiral state.

Quantum effects in a weakly frustrated s=1/2 two-dimensional heisenberg antiferromagnet in an applied magnetic field.

We have studied the two-dimensional S=1/2 square-lattice antiferromagnet Cu(pz)_{2}(ClO4)_{2} (where pz denotes pyrazine), using neutron inelastic scattering and series expansion calculations. We show that the presence of antiferromagnetic next-nearest-neighbor interactions enhances quantum fluctuations associated with resonating valence bonds. Intermediate magnetic fields lead to a selective tuning of resonating valence bonds and a spectacular inversion of the zone-boundary dispersion, providing novel insight into 2D antiferromagnetism in the quantum limit.

Hydrogen bonding and multiphonon structure in copper pyrazine coordination polymers.

We report a systematic investigation of the temperature-dependent infrared vibrational spectra of a family of chemically related coordination polymer magnets based upon bridging bifluoride (HF(2)-) and terminal fluoride (F-) ligands in copper pyrazine complexes including Cu(HF(2))(pyz)(2)BF(4), Cu(HF(2))(pyz)(2)ClO(4), and CuF(2)(H(2)O)(2)(pyz). We compare our results with several one- and two-dimensional prototype materials including Cu(pyz)(NO(3))(2) and Cu(pyz)(2)(ClO(4))(2). Unusual low-temperature hydrogen bonding, local structural transitions associated with stronger low-temperature hydrogen bonding, and striking multiphonon effects that derive from coupling of an infrared-active fundamental with strong Raman-active modes of the pyrazine building-block molecule are observed. On the basis of the spectroscopic evidence, these interactions are ubiquitous to this family of coordination polymers and may work to stabilize long-range magnetic ordering at low temperature. Similar interactions are likely to be present in other molecule-based magnets.

Extended quantum critical phase in a magnetized spin-1/2 antiferromagnetic chain.

Measurements are reported of the magnetic field dependence of excitations in the quantum critical state of the spin S=1/2 linear chain Heisenberg antiferromagnet copper pyrazine dinitrate (CuPzN). The complete spectrum was measured at k(B)T/J< or =0.025 for H=0 and H=8.7 T, where the system is approximately 30% magnetized. At H=0, the results are in agreement with exact calculations of the dynamic spin correlation function for a two-spinon continuum. At H=8.7 T, there are multiple overlapping continua with incommensurate soft modes. The boundaries of these continua confirm long-standing predictions, and the intensities are consistent with exact diagonalization and Bethe ansatz calculations.