The magnetic properties of the two-dimensional, S=1 honeycomb antiferromagnet BaNi2V2O8 have been comprehensively studied using DC susceptibility measurements and inelastic neutron scattering techniques. The magnetic excitation spectrum is found to be dispersionless within experimental resolution between the honeycomb layers, while it disperses strongly within the honeycomb plane where it consists of two gapped spin-wave modes. The magnetic excitations are compared to linear spin-wave theory allowing the Hamiltonian to be determined. The first-and second-neighbour magnetic exchange interactions are antiferromagnetic and lie within the ranges 10.90meV≤Jn≤13.35 meV and 0.85meV≤Jnn≤1.65 meV respectively. The interplane coupling Jout is four orders of magnitude weaker than the intraplane interactions, confirming the highly twodimensional magnetic behaviour of this compound. The sizes of the energy gaps are used to extract the magnetic anisotropies and reveal substantial easy-plane anisotropy and a very weak in-plane easy-axis anisotropy. Together these results reveal that BaNi2V2O8 is a candidate compound for the investigation of vortex excitations and Berezinsky-Kosterliz-Thouless phenomenona.

Unlike most quantum systems which rapidly become incoherent as temperature is raised, strong correlations persist at elevated temperatures in S = 1/2 dimer magnets, as revealed by the unusual asymmetric lineshape of their excitations at finite temperatures. Here we quantitatively explore and parameterize the strongly correlated magnetic excitations at finite temperatures using the high resolution inelastic neutron scattering on the model compound BaCu2V2O8 which we show to be an alternating antiferromagnetic-ferromagnetic spin−1/2 chain. Comparison to state of the art computational techniques shows excellent agreement over a wide temperature range. Our findings hence demonstrate the possibility to quantitatively predict coherent behavior at elevated temperatures in quantum magnets.In the study of unconventional states of matter, quantum magnetic materials with their strong correlations play a crucial role [1][2][3][4][5]. Quantum mechanical coherence and entanglement are intrinsic to these systems, both being relevant for potential applications in quantum devices [6,7]. However, the question arises for their persistence when increasing temperature. Intuitively, one expects temperature to suppress quantum behavior, as typically encountered in the study of quantum criticality [8]. Interestingly, this is not always the case, and in certain systems, e.g. in the presence of disorder, coherent behavior is not simply suppressed by temperature, but rather an interesting interplay develops [9,10], which can lead to counterintuitive behavior such as the increase of conductance through molecules with temperature [11].Another example is the extraordinary coherence of the magnetic excitations at elevated temperatures. This was theoretically predicted for 1-dimensional (1D) gapped quantum dimer antiferromagnets (AFM) by using integrable quantum field theory [12] and was experimentally confirmed on the strongly dimerized spin−1/2 AFM alternating chain compound copper nitrate, which has a spin-singlet ground state and gapped triplet excitations (henceforth referred to as triplons [13]) confined within a narrow band [14]. Here, the triplons interact strongly via the AFM interdimer coupling and also via an effective repulsive interaction due to the hard-core constraint. The resulting strong correlations lead to the experimentally observed asymmetric broadening of the lineshape with temperature [14,15]. So far, such experimental data was compared to exact diagonalization data from small systems and to results from low-temperature expansion around the strongly dimerized limit of Heisenberg spin-1/2 chains [16,17]. Further experimental studies revealed that the strongly correlated behavior at elevated temperatures is not restricted to 1D systems. It was recently observed that the lineshape in the 3-dimensional (3D) coupled-dimer antiferromagnet Sr 3 Cr 2 O 8 also becomes asymmetric and increasingly weighted towards the center of the band as temperature increases [18,19]. So far, no reliable theoretical approaches on the microscopic ...

The novel quantum magnet BaCu2V2O8 was recently discovered to be a rare physical realization of a one-dimensional antiferromagnetic -ferromagnetic dimerized chain which displays strongly correlated phenomena at elevated temperatures [Phys. Rev. B 93 241109(R) (2016)]. This current paper presents an extended study of the Hamiltonian of BaCu2V2O8 at base temperature. Static susceptibility and inelastic neutron scattering data are compared to several theoretical models. An analytical relation for the dynamic structure factor of the complex unit cell of BaCu2V2O8 is derived and used to identify the intrachain exchange paths. Further analysis using the first moment of the dynamic structure factor was employed to determine the exchange path responsible for the intradimer interaction. This analysis reveals that the dimer chain is formed by a dominant antiferromagnetic exchange iteraction Jintra=40.92 meV which is realized via the Cu-O-V(II)-O-Cu super-exchange path and a weak ferromagnetic coupling Jinter=-11.97 meV which arises within the