Pressure-tuned quantum criticality in the large-D antiferromagnet DTN.

Strongly correlated spin systems can be driven to quantum critical points via various routes. In particular, gapped quantum antiferromagnets can undergo phase transitions into a magnetically ordered state with applied pressure or magnetic field, acting as tuning parameters. These transitions are characterized by z = 1 or z = 2 dynamical critical exponents, determined by the linear and quadratic low-energy dispersion of spin excitations, respectively. Employing high-frequency susceptibility and ultrasound techniques, we demonstrate that the tetragonal easy-plane quantum antiferromagnet NiCl2 ⋅ 4SC(NH2)2 (aka DTN) undergoes a spin-gap closure transition at about 4.2 kbar, resulting in a pressure-induced magnetic ordering. The studies are complemented by high-pressure-electron spin-resonance measurements confirming the proposed scenario. Powder neutron diffraction measurements revealed that no lattice distortion occurs at this pressure and the high spin symmetry is preserved, establishing DTN as a perfect platform to investigate z = 1 quantum critical phenomena. The experimental observations are supported by DMRG calculations, allowing us to quantitatively describe the pressure-driven evolution of critical fields and spin-Hamiltonian parameters in DTN.

Nuclear Magnetic Resonance Reveals Disordered Level-Crossing Physics in the Bose-Glass Regime of the Br-Doped Ni(Cl_{1-x}Br_{x})_{2}-4SC(NH_{2})_{2} Compound at a High Magnetic Field.

By measuring the nuclear magnetic resonance (NMR) T_{1}^{-1} relaxation rate in the Br (bond) doped DTN compound, Ni(Cl_{1-x}Br_{x})_{2}-4SC(NH_{2})_{2}(DTNX), we show that the low-energy spin dynamics of its high magnetic field "Bose-glass" regime is dominated by a strong peak of spin fluctuations found at the nearly doping-independent position H^{*}≅13.6 T. From its temperature and field dependence, we conclude that this corresponds to a level crossing of the energy levels related to the doping-induced impurity states. Observation of the local NMR signal from the spin adjacent to the doped Br allowed us to fully characterize this impurity state. We have thus quantified a microscopic theoretical model that paves the way to better understanding of the Bose-glass physics in DTNX, as revealed in the related theoretical study [M. Dupont, S. Capponi, and N. Laflorencie, Phys. Rev. Lett. 118, 067204 (2017).PRLTAO0031-900710.1103/PhysRevLett.118.067204].

Bose glass and Mott glass of quasiparticles in a doped quantum magnet.

The low-temperature states of bosonic fluids exhibit fundamental quantum effects at the macroscopic scale: the best-known examples are Bose-Einstein condensation and superfluidity, which have been tested experimentally in a variety of different systems. When bosons interact, disorder can destroy condensation, leading to a 'Bose glass'. This phase has been very elusive in experiments owing to the absence of any broken symmetry and to the simultaneous absence of a finite energy gap in the spectrum. Here we report the observation of a Bose glass of field-induced magnetic quasiparticles in a doped quantum magnet (bromine-doped dichloro-tetrakis-thiourea-nickel, DTN). The physics of DTN in a magnetic field is equivalent to that of a lattice gas of bosons in the grand canonical ensemble; bromine doping introduces disorder into the hopping and interaction strength of the bosons, leading to their localization into a Bose glass down to zero field, where it becomes an incompressible Mott glass. The transition from the Bose glass (corresponding to a gapless spin liquid) to the Bose-Einstein condensate (corresponding to a magnetically ordered phase) is marked by a universal exponent that governs the scaling of the critical temperature with the applied field, in excellent agreement with theoretical predictions. Our study represents a quantitative experimental account of the universal features of disordered bosons in the grand canonical ensemble.

Magnetic tuning of all-organic binary alloys between two stable radicals.

Mixtures of 2-(4,5,6,7-tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (F4BImNN) and 2-(benzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (BImNN) crystallize as solid solutions (alloys) across a wide range of binary compositions. (F4BImNN)(x)(BImNN)((1-x)) with x < 0.8 gives orthorhombic unit cells, while x ≥ 0.9 gives monoclinic unit cells. In all crystalline samples, the dominant intermolecular packing is controlled by one-dimensional (1D) hydrogen-bonded chains that lead to quasi-1D ferromagnetic behavior. Magnetic analysis over 0.4-300 K indicates ordering with strong 1D ferromagnetic exchange along the chains (J/k = 12-22 K). Interchain exchange is estimated to be 33- to 150-fold weaker, based on antiferromagnetic ordered phase formation below Néel temperatures in the 0.4-1.2 K range for the various compositions. The ordering temperatures of the orthorhombic samples increase linearly as (1 - x) increases from 0.25 to 1.00. The variation is attributed to increased interchain distance corresponding to decreased interchain exchange, when more F4BImNN is added into the orthorhombic lattice. The monoclinic samples are not part of the same trend, due to the different interchain arrangement associated with the phase change.

Loops, chains, sheets, and networks from variable coordination of Cu(hfac)2 with a flexibly hinged aminoxyl radical ligand.

One pair of reactants, Cu(hfac)(2) = M and the hinge-flexible radical ligand 5-(3-N-tert-butyl-N-aminoxylphenyl)pyrimidine (3PPN = L), yields a diverse set of five coordination complexes: a cyclic loop M(2)L(2) dimer; a 1:1 cocrystal between an M(2)L(2) loop and an ML(2) fragment; a 1D chain of M(2)L(2) loops linked by M; two 2D M(3)L(2) networks of (M-L)(n) chains cross-linked by M with different repeat length pitches; a 3D M(3)L(2) network of M(2)L(2) loops cross-linking (M-L)(n)-type chains with connectivity different from those in the 2D networks. Most of the higher dimensional complexes exhibit reversible, temperature-dependent spin-state conversion of high-temperature paramagnetic states to lower magnetic moment states having antiferromagnetic exchange within Cu-ON bonds upon cooling, with accompanying bond contraction. The 3D complex also exhibited antiferromagnetic exchange between Cu(II) ions linked in chains through pyrimidine rings.

2-(4,5,6,7-Tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1- H-imidazole-3-oxide-1-oxyl, a hydrogen-bonded organic quasi-1D ferromagnet.

The title radical (F4BImNN) is a stable nitronylnitroxide that forms hydrogen-bonded NH... ON chains in the solid state. The chains assemble the F4BImNN molecules to form stacked contacts between the radical groups, in a geometry that is expected to exhibit ferromagnetic (FM) exchange based on spin polarization (SP) models. The experimental magnetic susceptibility of F4BImNN confirms the expectation, showing 1-D Heisenberg chain FM exchange behavior over 1.8-300 K with an intrachain exchange constant of Jchain/k = +22 K. At lower temperatures, ac magnetic susceptibility and variable field heat capacity measurements show that F4BImNN acts as a quasi-1-D ferromagnet. The dominant ferromagnetic exchange interaction is attributable to overlap between spin orbitals of molecules within the hydrogen-bonded chains, consistent with the SP model expectations. The chains appear to be antiferromagnetically exchange coupled, giving cusps in the ac susceptibility and zero field heat capacity at lower temperatures. The results indicate that the sample orders magnetically at about 0.7 K. The magnetic heat capacity ordering cusp shifts to lower temperatures as external magnetic field increases, consistent with forming a bulk antiferromagnetic phase below a Néel temperature of TN(0) = 0.72 K, with a critical field of Hc approximately 1800 Oe. The interchain exchange is estimated to be zJ/k congruent with (-)0.1 K.

Co3(RL)2(hfac)6 ladder complex of 5-[4-(N-tert-butyl-N-aminoxyl)phenyl]pyrimidine.

5-[4-(N-tert-butyl-N-aminoxyl)phenyl]pyridimine (4NITPhPyrim = RL) forms a 1-D ladder polymer complex with Co(hfac)2 of stoichiometry Co3(RL)2(hfac)6, having antiparallel [Co(II)RL]n linear chains (rails) that are cross-linked by Pyrim-Co(hfac)2-Pyrim rungs. The magnetic behavior above 100 K is consistent with contributions from one high-spin Co(II) ion (the cross-link, S = 3/2) plus two Co-ON units with strongly antiferromagnetic (AFM) metal-radical exchange (each S = 1). The chiT data show an AFM downturn as the temperature drops. Assuming weak exchange along chain portions of the polymer due to poor spin polarization across the phenyl-pyrimidine bond in RL, a linear three-spin (S = 1, 3/2, and 1) fit to the T > 18 K data yields an AFM cross-linker (rung) effective exchange of J(CL)/k = (-)5.3 K = (-)3.7 cm(-)(1). Superexchange (sigma-orbital overlap) is a likely mechanism for the effective AFM exchange between CoON and Co spin sites in the three-spin groupings.

Cyclic M2(RL)2 coordination complexes of 5-(3-[N-tert-Butyl-N-aminoxyl]phenyl)pyrimidine with paramagnetic transition metal dications.

5-(3-(N-tert-Butyl-N-aminoxyl)phenyl)pyrimidine (RL = 3NITPhPyrim) forms isostructural cyclic M2(RL)2 cyclic dimers with M(hfac)2 (M = Mn, Co, Cu; hfac = hexafluoroacetylacetonate). Mn2(hfac)4(RL)2 exhibits strong antiferromagnetic Mn-RL exchange, with weak ferromagnetic exchange (0.7 cm(-1)) between Mn-RL units that is consistent with a spin polarization exchange mechanism. The magnetic moment of Co2(hfac)4(RL)2 at higher temperatures is consistent with strongly antiferromagnetic exchange within the Co-NIT units and tends toward zero below 50 K at lower magnetic fields. Cu2(hfac)4(RL)2 shows more complex behavior, with no high-temperature plateau in chiT(T) up to 300 K but a monotonic decrease down to about 100 K. The Cu(II)-nitroxide bonds decrease by 0.2-0.3 A over the same temperature range, corresponding to a change of nitroxide coordination from axial to equatorial. This thermally reversible Jahn-Teller distortion leads to a thermally induced spin state conversion from a high-spin, paramagnetic state at higher temperature to a low-spin state at lower temperature. This spin state conversion is accompanied by a reversible solid-state thermochromic change between dull yellow-brown at room temperature and green at 77 K.

Manganese(II) and copper(II) hexafluoroacetylacetonate 1:1 complexes with 5-(4-[N-tert-butyl-N-aminoxyl]phenyl)pyrimidine: regiochemical parity analysis for exchange behavior of complexes between radicals and paramagnetic cations.

Mn(hfac)(2) and Cu(hfac)(2) form coordination complexes with 5-(4-[N-tert-butyl-N-aminoxyl]phenyl)pyrimidine, PyrimPh-NIT. (Mn[PyrimPh-NIT](hfac)(2))(2) and (Cu[PyrimPh-NIT](hfac)(2))(2), 1 and 2, respectively, are cyclic M(2)L(2) dimers that exhibit strong exchange coupling between the coordinated paramagnetic dication (M) and nitroxide (NIT) unit. The M-NIT exchange is strongly antiferromagnetic (AFM) in 1 and strongly ferromagnetic (FM) in 2. Magnetic susceptibility measurements for 1 were fitted to an AFM spin pairing model with J/k = -0.25 K between Mn-NIT spin sites units. Complex 2 also exhibits AFM spin pairing between S = 1 Cu-NIT spin units that is somewhat field dependent at low temperature. The fit of corrected paramagnetic susceptibility chi(T) to an AFM spin pairing model at 200 Oe yields J/k = (-)3.8 K, quite similar to earlier measurements at 1000 Oe yielding J/k = (-)5.0 K. At 1.40 K, the magnetization of 2 does not approach saturation until somewhat above 170 kOe, giving an S-shaped curve; at 0.55 K, the magnetization curve shows steps characteristic of field-induced crossover between the S = 0 ground state and excited spin states. From the steps in the 0.55 K data, we estimate J/k = (-)3.8-4.0 K for 2, in good agreement with the analysis of chi(T).