A Novel Longitudinal Mode in the Coupled Quantum Chain Compound KCuF3

Inelastic neutron scattering measurements are reported that show a new longitudinal mode in the antiferromagnetically ordered phase of the spin-1/2 quasi-one-dimensional antiferromagnet KCuF3. This mode signals the cross-over from one-dimensional to three-dimensional behavior and indicates a reduction in the ordered spin moment of a spin-1/2 antiferromagnet. The measurements are compared with recent quantum field theory results and are found to be in excellent agreement. A feature of the data not predicted by theory is a damping of the mode by decay processes to the transverse spin-wave branches.Comment: 9 pages of text plus 4 postscript figures (1 color

Multispinon continua at zero and finite temperature in a near-ideal Heisenberg chain

The space- and time-dependent response of many-body quantum systems is the most informative aspect of their emergent behaviour. The dynamical structure factor, experimentally measurable using neutron scattering, can map this response in wavevector and energy with great detail, allowing theories to be quantitatively tested to high accuracy. Here, we present a comparison between neutron scattering measurements on the one-dimensional spin-1/2 Heisenberg antiferromagnet KCuF3, and recent state-of-the-art theoretical methods based on integrability and density matrix renormalization group simulations. The unprecedented quantitative agreement shows that precise descriptions of strongly correlated states at all distance, time and temperature scales are now possible, and highlights the need to apply these novel techniques to other problems in low-dimensional magnetism

Dynamic Adaptation on Non-Stationary Visual Domains

Domain adaptation aims to learn models on a supervised source domain that perform well on an unsupervised target. Prior work has examined domain adaptation in the context of stationary domain shifts, i.e. static data sets. However, with large-scale or dynamic data sources, data from a defined domain is not usually available all at once. For instance, in a streaming data scenario, dataset statistics effectively become a function of time. We introduce a framework for adaptation over non-stationary distribution shifts applicable to large-scale and streaming data scenarios. The model is adapted sequentially over incoming unsupervised streaming data batches. This enables improvements over several batches without the need for any additionally annotated data. To demonstrate the effectiveness of our proposed framework, we modify associative domain adaptation to work well on source and target data batches with unequal class distributions. We apply our method to several adaptation benchmark datasets for classification and show improved classifier accuracy not only for the currently adapted batch, but also when applied on future stream batches. Furthermore, we show the applicability of our associative learning modifications to semantic segmentation, where we achieve competitive results

Nature of the spin dynamics and 1/3 magnetization plateau in azurite

We present a specific heat and inelastic neutron scattering study in magnetic fields up into the 1/3 magnetization plateau phase of the diamond chain compound azurite Cu$_3$(CO$_3$)$_2$(OH)$_2$. We establish that the magnetization plateau is a dimer-monomer state, {\it i.e.}, consisting of a chain of $S = 1/2$ monomers, which are separated by $S = 0$ dimers on the diamond chain backbone. The effective spin couplings $J_{mono}/k_B = 10.1(2)$ K and $J_{dimer}/k_B = 1.8(1)$ K are derived from the monomer and dimer dispersions. They are associated to microscopic couplings $J_1/k_B = 1(2)$ K, $J_2/k_B = 55(5)$ K and a ferromagnetic $J_3/k_B = -20(5)$ K, possibly as result of $d_{z^2}$ orbitals in the Cu-O bonds providing the superexchange pathways.Comment: 5 pages, 4 figure

Lifetimes of antiferromagnetic magnons in two and three dimensions: experiment, theory, and numerics

A high-resolution neutron spectroscopic technique is used to measure momentum-resolved magnon lifetimes in the prototypical two- and three-dimensional antiferromagnets Rb2MnF4 and MnF2, over the full Brillouin zone and a wide range of temperatures. We rederived theories of the lifetime resulting from magnon-magnon scattering, thereby broadening their applicability beyond asymptotically small regions of wavevector and temperature. Corresponding computations, combined with a small contribution reflecting collisions with domain boundaries, yield excellent quantitative agreement with the data.Comment: 5 pages, 4 figure

Luttinger-Liquid Behavior in the Alternating Spin-Chain System Copper Nitrate

We determine the phase diagram of copper nitrate Cu(NO$_3$)$_2\cdot$2.5D$_2$O in the context of quantum phase transitions and novel states of matter. We establish this compound as an ideal candidate to study quasi-1D Luttinger liquids, 3D Bose-Einstein-Condensation of triplons, and the crossover between 1D and 3D physics. Magnetocaloric effect, magnetization, and neutron scattering data provide clear evidence for transitions into a Luttinger liquid regime and a 3D long-range ordered phase as function of field and temperature. Theoretical simulations of this model material allow us to fully establish the phase diagram and to discuss it in the context of dimerized spin systems.Comment: 5 pages, 4 figure