Large quantum fluctuations in the strongly coupled spin-1/2 chains of green dioptase: a hidden message from birds and trees

The green mineral dioptase Cu6Si6O18(H2O)6 has been known since centuries and plays an important role in esoteric doctrines. In particular, the green dioptase is supposed to grant the skill to speak with trees and to understand the language of birds. Armed with natural samples of dioptase, we were able to unravel the magnetic nature of the mineral (presumably with hidden support from birds and trees) and show that strong quantum fluctuations can be realized in an essentially framework-type spin lattice of coupled chains, thus neither frustration nor low-dimensionality are prerequisites. We present a microscopic magnetic model for the green dioptase. Based on full-potential DFT calculations, we find two relevant couplings in this system: an antiferromagnetic coupling J_c, forming spiral chains along the hexagonal c axis, and an inter-chain ferromagnetic coupling J_d within structural Cu2O6 dimers. To refine the J_c and J_d values and to confirm the proposed spin model, we perform quantum Monte-Carlo simulations for the dioptase spin lattice. The derived magnetic susceptibility, the magnetic ground state, and the sublattice magnetization are in remarkably good agreement with the experimental data. The refined model parameters are J_c = 78 K and J_d = -37 K with J_d/J_c ~ -0.5. Despite the apparent three-dimensional features of the spin lattice and the lack of frustration, strong quantum fluctuations in the system are evidenced by a broad maximum in the magnetic susceptibility, a reduced value of the Neel temperature T_N ~ 15 K >> J_c, and a low value of the sublattice magnetization m = 0.55 Bohr magneton. All these features should be ascribed to the low coordination number of 3 that outbalances the three-dimensional nature of the spin lattice.Comment: Dedicated to Stefan-Ludwig Drechsler on the occasion of his 60th birthday (9 pages, 6 figures

Ultracold Bose gases in time-dependent 1D superlattices: response and quasimomentum structure

The response of ultracold atomic Bose gases in time-dependent optical lattices is discussed based on direct simulations of the time-evolution of the many-body state in the framework of the Bose-Hubbard model. We focus on small-amplitude modulations of the lattice potential as implemented in several recent experiment and study different observables in the region of the first resonance in the Mott-insulator phase. In addition to the energy transfer we investigate the quasimomentum structure of the system which is accessible via the matter-wave interference pattern after a prompt release. We identify characteristic correlations between the excitation frequency and the quasimomentum distribution and study their structure in the presence of a superlattice potential.Comment: 4 pages, 4 figure

Explaining the Effects of Government Spending Shocks on Consumption and the Real Exchange Rate

Using panel structural VAR analysis and quarterly data from four industrialized countries, we document that an increase in government purchases leads to an expansion in output and private consumption, a deterioration in the trade balance, and a depreciation of the real exchange rate (i.e., a decrease in the domestic CPI relative to the exchange-rate adjusted foreign CPI). We propose an explanation for these observed effects based on the deep habit mechanism. We estimate the key parameters of the deep-habit model employing a limited information approach. The predictions of the estimated deep-habit model fit well the observed responses of output, consumption, the trade balance, and the real exchange rate to an unanticipated government spending shock. In addition, the deep-habit model predicts that in response to an anticipated increase in government spending consumption and wages fail to increase on impact, which is consistent with the empirical evidence stemming from the narrative identification approach. In this way, the deep-habit model reconciles the findings of the SVAR and narrative literatures on the effects of government spending shocks.

The Macroeconomics of Subsistence Points

This paper explores the macroeconomic consequences of preferences displaying a subsistence point. It departs from the existing related literature by assuming that subsistence points are specific to each variety of goods rather than to the composite consumption good. We show that this simple feature makes the price elasticity of demand for individual goods procyclical. As a result, markups behave countercyclically in equilibrium. This implication is in line with the available empirical evidence.

Pricing to Habits and the Law of One Price

This paper proposes a novel international transmission mechanism based on the assumption of deep habits. The term deep habits stands for a preference specification according to which consumers form habits on a good-by-good basis. Under deep habits, firms face more elastic demand functions in markets where nonhabitual demand is high relative to habitual demand, creating an incentive to price discriminate. We refer to this type of price discrimination as pricing to habits. In the presence of pricing to habits, innovations to domestic aggregate demand induce a decline in markups in the domestic country but not abroad, leading to a departure from the law of one price. In this way, the proposed pricing-to-habit mechanism can explain the observation that prices of the same good across countries, expressed in the same currency, vary over the business cycle. Furthermore, it can account for the empirical fact that in response to a positive domestic demand shock, such as an increase in government spending, the real exchange rate depreciates, domestic consumption expands, and the trade balance deteriorates.

Explaining the Effects of Government Spending Shocks on Consumption and the Real Exchange Rate

Using panel structural VAR analysis and quarterly data from four industrialized countries, we document that an increase in government purchases leads to an expansion in output and private consumption, a deterioration in the trade balance, and a depreciation of the real exchange rate (i.e., a decrease in the domestic CPI relative to the exchange-rate adjusted foreign CPI). We propose an explanation for these observed effects based on the deep habit mechanism. We estimate the key parameters of the deep-habit model employing a limited information approach. The predictions of the estimated deep-habit model fit well the observed responses of output, consumption, the trade balance, and the real exchange rate to an unanticipated government spending shock. In addition, the deep-habit model predicts that in response to an anticipated increase in government spending consumption and wages fail to increase on impact, which is consistent with the empirical evidence stemming from the narrative identification approach. In this way, the deep-habit model reconciles the findings of the SVAR and narrative literatures on the effects of government spending shocks.

Anomalous dephasing of bosonic excitons interacting with phonons in the vicinity of the Bose-Einstein condensation

The dephasing and relaxation kinetics of bosonic excitons interacting with a thermal bath of acoustic phonons is studied after coherent pulse excitation. The kinetics of the induced excitonic polarization is calculated within Markovian equations both for subcritical and supercritical excitation with respect to a Bose-Einstein condensation (BEC). For excited densities n below the critical density n_c, an exponential polarization decay is obtained, which is characterized by a dephasing rate G=1/T_2. This dephasing rate due to phonon scattering shows a pronounced exciton-density dependence in the vicinity of the phase transition. It is well described by the power law G (n-n_c)^2 that can be understood by linearization of the equations around the equilibrium solution. Above the critical density we get a non-exponential relaxation to the final condensate value p^0 with |p(t)|-|p^0| ~1/t that holds for all densities. Furthermore we include the full self-consistent Hartree-Fock-Bogoliubov (HFB) terms due to the exciton-exciton interaction and the kinetics of the anomalous functions F_k= . The collision terms are analyzed and an approximation is used which is consistent with the existence of BEC. The inclusion of the coherent x-x interaction does not change the dephasing laws. The anomalous function F_k exhibits a clear threshold behaviour at the critical density.Comment: European Physical Journal B (in print

Magnetism of CuX2 frustrated chains (X = F, Cl, Br): the role of covalency

Periodic and cluster density-functional theory (DFT) calculations, including DFT+U and hybrid functionals, are applied to study magnetostructural correlations in spin-1/2 frustrated chain compounds CuX2: CuCl2, CuBr2, and a fictitious chain structure of CuF2. The nearest-neighbor and second-neighbor exchange integrals, J1 and J2, are evaluated as a function of the Cu-X-Cu bridging angle, theta, in the physically relevant range 80-110deg. In the ionic CuF2, J1 is ferromagnetic for theta smaller 100deg. For larger angles, the antiferromagnetic superexchange contribution becomes dominant, in accord with the Goodenough-Kanamori-Anderson rules. However, both CuCl2 and CuBr2 feature ferromagnetic J1 in the whole angular range studied. This surprising behavior is ascribed to the increased covalency in the Cl and Br compounds, which amplifies the contribution from Hund's exchange on the ligand atoms and renders J1 ferromagnetic. At the same time, the larger spatial extent of X orbitals enhances the antiferromagnetic J2, which is realized via the long-range Cu-X-X-Cu paths. Both, periodic and cluster approaches supply a consistent description of the magnetic behavior which is in good agreement with the experimental data for CuCl2 and CuBr2. Thus, owing to their simplicity, cluster calculations have excellent potential to study magnetic correlations in more involved spin lattices and facilitate application of quantum-chemical methods

Deep Habits and the Dynamic Effects of Monetary Policy Shocks

This paper introduces deep habits into a sticky-price sticky-wage economy and asks whether the countercyclical markup movements induced by deep habits is helpful for accounting for the dynamic effects of monetary policy shocks. We find that this is the case: When allowing for deep habits, the model can account very precisely for the persistent impact of monetary policy shocks on aggregate consumption and for the impact on inflation that other models have hard a time explaining. In particular, the model can account both for the price puzzle and for inflation persistence. We also show that the deep habits mechanism and nominal rigidities are complementary: The deep habits model can account for the dynamic effects of monetary policy shock at low to moderate levels of nominal rigidities. We show that the results are stable over time and are not caused by monetary policy changes.deep habits, monetary policy, price puzzle, inflation persistence, countercyclical markups