Frustrated Heisenberg antiferromagnets: fluctuation induced first order vs deconfined quantum criticality

Recently it was argued that quantum phase transitions can be radically different from classical phase transitions with as a highlight the 'deconfined critical points' exhibiting fractionalization of quantum numbers due to Berry phase effects. Such transitions are supposed to occur in frustrated ('$J_1$-$J_2$') quantum magnets. We have developed a novel renormalization approach for such systems which is fully respecting the underlying lattice structure. According to our findings, another profound phenomenon is around the corner: a fluctuation induced (order-out-of-disorder) first order transition. This has to occur for large spin and we conjecture that it is responsible for the weakly first order behavior recently observed in numerical simulations for frustrated $S=1/2$ systems.Comment: 7 pages, 3 Figures, submitted to EP

Do capital buffers mitigate volatility of bank lending? A simulation study

Critics claim that capital requirements can exacerbate credit cycles by restricting lending in an economic downturn. The introduction of Basel 2, in particular, has led to concerns that risksensitive capital charges are highly correlated with the business cycle. The Basel Committee is contemplating a revision of the Basel Accord by introducing counter-cyclical capital buffers. Others claim that capital buffers are already large enough to absorb fluctuations in credit risk. We address the question of the pro-cyclical effects of capital requirements in a general framework which takes into account banks' potential adjustment strategies. We develop a dynamic model of bank lending behavior and simulate different regulatory frameworks and macroeconomic scenarios. In particular, we address two related questions in our simulation study: How do business fluctuations affect capital requirements and bank lending? To what extent does the capital buffer absorb fluctuations in the level of mimimum required capital? --Minimum capital requirements,regulatory capital,capital buffer,cyclical lending,pro-cyclicality

Orbital Ordering and Unfrustrated (π,0)(\pi,0) Magnetism from Degenerate Double Exchange in the Iron Pnictides

The magnetic excitations of the iron pnictides are explained within a degenerate double-exchange model. The local-moment spins are coupled by superexchanges $J_1$ and $J_2$ between nearest and next-nearest neighbors, respectively, and interact with the itinerant electrons of the degenerate $d_{xz}$ and $d_{yz}$ orbitals via a ferromagnetic Hund exchange. The latter stabilizes $(\pi,0)$ stripe antiferromagnetism due to emergent ferro-orbital order and the resulting kinetic energy gain by hopping preferably along the ferromagnetic spin direction. Taking the quantum nature of the spins into account, we calculate the magnetic excitation spectra in the presence of both, super- and double-exchange. A dramatic increase of the spin-wave energies at the competing N\'eel ordering wave vector is found, in agreement with recent neutron scattering data. The spectra are fitted to a spin-only model with a strong spatial anisotropy and additional longer ranged couplings along the ferromagnetic chains. Over a realistic parameter range, the effective couplings along the chains are negative corresponding to unfrustrated stripe antiferromagnetism.Comment: 11 pages, 6 figures. Version accepted in PR

Fluctuations of spin and charge in stripe phases of layered antiferromagnets

The formation of stripes in layered cuprate high-Tc superconductors and closely related nickelate compounds is generic. Doped charge carriers condense into spontaneously generated anti-phase domain walls between antiferromagnetic insulating regions. In this thesis fluctuations of spin and charge in these two-dimensional stripe superstructures are investigated. The first part addresses the phenomenon of spin-charge separation. Experimentally charge ordering of stripes is always observed at a higher temperature than the magnetic ordering. The melting of stripes mediated by the unbinding of different types of topological defects, namely charge dislocations, charge loops, and spin vortices is studied. The phase diagram and the critical properties of the phase transitions are calculated in the framework of a renormalization-group analysis in the Coulomb-gas representation of these defects, which interact logarithmically on large scales in two dimensions. Depending on which type of defect proliferates we identify four different phases characterized by the range of charge order, spin order, and a less accessible collinear order. From the resulting phase diagram several scenarios of spin-charge separation are possible. Depending on the interaction parameters the orders can disappear at a single transition or in a sequence of two transitions. In the second part the spin dynamics of stripes is studied in the framework of a linear spin-wave theory for a minimalistic spin-only model. The magnon dispersion and the magnetic zero temperature structure factor are calculated for diagonal and vertical stripes since both configurations are realized in doped layered antiferromagnets. Acoustical as well as optical bands are included in the analysis. Incommensurate spin fluctuations and the commensurate pi-resonance at the antiferromagnetic wave vector appear as complementary features of the band structure at different energy scales. The dependence of the spin-wave velocities and the resonance frequencies on the stripe spacing and coupling is calculated. At low doping, the resonance frequency is found to scale inversely proportional to the stripe spacing. Finally, we extend our minimalistic model to a bilayer, allowing for several stripe configurations which differ by the relative location of the stripes in the layers. Again the spectral properties are calculated in linear spin-wave theory. We focus on the bilayer splitting of the magnon bands near the incommensurate low energy peaks as well as near the pi- resonance, distinguishing between the odd and even channel. We find that an x-shaped dispersion near the pi-resonance is generic for stripes. The favorable comparison of the results to experimental data suggests that the spin-only model provides a suitable and simple basis for calculating and understanding the spin dynamics of stripe

Two distinct Mott-Insulator to Bose-glass transitions and breakdown of self averaging in the disordered Bose-Hubbard model

We investigate the instabilities of the Mott-insulating phase of the weakly disordered Bose-Hubbard model within a renormalization group analysis of the replica field theory obtained by a strong-coupling expansion around the atomic limit. We identify a new order parameter and associated correlation length scale that is capable of capturing the transition from a state with zero compressibility, the Mott insulator, to one in which the compressibility is finite, the Bose glass. The order parameter is the relative variance of the disorder-induced mass distribution. In the Mott insulator, the relative variance renormalizes to zero, whereas it diverges in the Bose glass. The divergence of the relative variance signals the breakdown of self-averaging. The length scale governing the breakdown of self-averaging is the distance between rare regions. This length scale is finite in the Bose glass but diverges at the transition to the Mott insulator with an exponent of $\nu=1/D$ for incommensurate fillings. Likewise, the compressibility vanishes with an exponent of $\gamma=4/D-1$ at the transition. At commensurate fillings, the transition is controlled by a different fixed point at which both the disorder and interaction vertices are relevant.Comment: Extended, published versio

Galileo In-Situ Dust Measurements in Jupiter's Gossamer Rings

During its late orbital mission at Jupiter the Galileo spacecraft made two passages through the giant planet's gossamer ring system. The impact-ionization dust detector on board successfully recorded dust impacts during both ring passages and provided the first in-situ measurements from a dusty planetary ring. In all, a few thousand dust impacts were counted with the instrument accumulators during both ring passages, but only a total of 110 complete data sets of dust impacts were transmitted to Earth. Detected particle sizes range from about 0.2 to 5 micron, extending the known size distribution by an order of magnitude towards smaller particles than previously derived from optical imaging (Showalter et al. 2008). The grain size distribution increases towards smaller particles and shows an excess of these tiny motes in the Amalthea gossamer ring compared to the Thebe ring. The size distribution for the Amalthea ring derived from our in-situ measurements for the small grains agrees very well with the one obtained from images for large grains. Our analysis shows that particles contributing most to the optical cross-section are about 5 micron in radius, in agreement with imaging results. The measurements indicate a large drop in particle flux immediately interior to Thebe's orbit and some detected particles seem to be on highly-tilted orbits with inclinations up to 20 deg.Comment: 13 figures, 4 tables, submitted to Icaru