The Medicine Line: A Border Dividing Tribal Sovereignty, Economies and Families

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Atom-molecule coherence in Bose gases

The interaction properties of atoms are, at low temperatures, fully determined by the s-wave scattering length of the interatomic interaction potential. The magnitude and sign of this quantity strongly depend on the presence of bound states in this potential and, more precisely, on the energy of the bound state that is closest to the continuum threshold. In the multichannel case of a Feshbach resonance, the energy of the two colliding atoms in the incoming open channel is close to the energy of a bound state, i.e., a molecular state, in a coupled closed channel. Due to the different spin arrangements of the atoms in the open channel and the atoms in the molecular state, the energy difference between the bound state and the continuum threshold is experimentally accessible by means of the Zeeman coupling of the atomic spins to a magnetic field. As a result, one is able to vary the scattering length to any possible value by tuning the magnetic field. This level of experimental control has opened the road for many beautiful experiments which recently led to the demonstration of coherence between atoms and molecules, by observing coherent oscillations between atoms and molecules, analogous to coherent oscillations that are observed in ordinary two-level systems. We review the theory that describes coherence between atoms and molecules in terms of an effective quantum field theory for Feshbach-resonant interactions. The theoretical predictions resulting from this theory are in excellent agreement with experimental results

Enhanced spin conductance of a thin-Film insulating antiferromagnet

We investigate spin transport by thermally excited spin waves in an antiferromagnetic insulator. Starting from a stochastic Landau-Lifshitz-Gilbert phenomenology, we obtain the out-of-equilibrium spin-wave properties. In linear response to spin biasing and a temperature gradient, we compute the spin transport through a normal-metal-antiferromagnet-normal-metal heterostructure. We show that the spin conductance diverges as one approaches the spin-flop transition; this enhancement of the conductance should be readily observable by sweeping the magnetic field across the spin-flop transition. The results from such experiments may, on the one hand, enhance our understanding of spin transport near a phase transition, and on the other be useful for applications that require a large degree of tunability of spin currents. In contrast, the spin Seebeck coefficient does not diverge at the spin-flop transition. Furthermore, the spin Seebeck coefficient is finite even at zero magnetic field, provided that the normal metal contacts break the symmetry between the antiferromagnetic sublattices.</p

Spin-wave amplification and lasing driven by inhomogeneous spin-transfer torques

We show that an inhomogeneity in the spin-transfer torques in a metallic ferromagnet under suitable conditions strongly amplifies incoming spin waves. Moreover, at nonzero temperatures the incoming thermally occupied spin waves will be amplified such that the region with inhomogeneous spin-transfer torques emits spin waves spontaneously, thus constituting a spin-wave laser. We determine the spin-wave scattering amplitudes for a simplified model and setup, and show under which conditions the amplification and lasing occurs. Our results are interpreted in terms of a so-called black-hole laser, and could facilitate the field of magnonics, which aims to utilize spin waves in logic and data-processing devices.</p

Spin-polarized hot electron transport versus spin pumping mediated by local heating

A ‘toy model’—aimed at capturing the essential physics—is presented that jointly describes spin-polarized hot electron transport and spin pumping driven by local heating. These two processes both contribute to spin-current generation in laser-excited magnetic heterostructures. The model is used to compare the two contributions directly. The spin-polarized hot electron current is modeled as one generation of hot electrons with a spin-dependent excitation and relaxation scheme. Upon decay, the excess energy of the hot electrons is transferred to a thermalized electron bath. The elevated electron temperature leads to an increased rate of electron-magnon scattering processes and yields a local accumulation of spin. This process is dubbed as spin pumping by local heating. The built-up spin accumulation is effectively driven out of the ferromagnetic system by (interfacial) electron transport. Within our model, the injected spin current is dominated by the contribution resulting from spin pumping, while the hot electron spin current remains relatively small. We derive that this observation is related to the ratio between the Fermi temperature and Curie temperature, and we show what other fundamental parameters play a role.</p

Current-Induced Torques in Magnetic Metals: Beyond Spin Transfer

Current-induced torques on ferromagnetic nanoparticles and on domain walls in ferromagnetic nanowires are normally understood in terms of transfer of conserved spin angular momentum between spin-polarized currents and the magnetic condensate. In a series of recent articles we have discussed a microscopic picture of current-induced torques in which they are viewed as following from exchange fields produced by the misaligned spins of current carrying quasiparticles. This picture has the advantage that it can be applied to systems in which spin is not approximately conserved. More importantly, this point of view makes it clear that current-induced torques can also act on the order parameter of an antiferromagnetic metal, even though this quantity is not related to total spin. In this informal and intentionally provocative review we explain this picture and discuss its application to antiferromagnets.Comment: 5 figures, to appear in Journal of Magnetism and

Finite-size scaling at infinite-order phase transitions

Theoretical Physic

Microwave control of thermal-magnon spin transport

We observe that an rf microwave field strongly influences the transport of incoherent thermal magnons in yttrium iron garnet. Ferromagnetic resonance in the nonlinear regime suppresses thermal magnon transport by 95%. The transport is also modulated at nonresonant conditions in two cases, both related to the magnon band minimum. Firstly, a strong enhancement of the nonlocal signal appears at a static magnetic field below the resonance condition. This increase only occurs at one field polarity and can be as large as 800%. We attribute this effect to magnon kinetic processes, which give rise to band-minimum magnons and high-energy chiral surface modes. Secondly, the signal increases at a static field above the resonance condition, where the rf frequency coincides with the magnon band minimum. Our study gives insight into the interplay between coherent and incoherent spin dynamics: the rf field modifies the occupation of relevant magnon states and, via kinetic processes, the magnon spin transport.</p

Evolution of the macroscopically entangled states in optical lattices

We consider dynamics of boson condensates in finite optical lattices under a slow external perturbation which brings the system to the unstable equilibrium. It is shown that quantum fluctuations drive the condensate into the maximally entangled state. We argue that the truncated Wigner approximation being a natural generalization of the Gross-Pitaevskii classical equations of motion is adequate to correctly describe the time evolution including both collapse and revival of the condensate.Comment: 14 pages, 10 figures, Discussion of reversibility of entanglement is adde