Combined analytical and numerical approach to study magnetization plateaux in doped quasi-one-dimensional antiferromagnets

We investigate the magnetic properties of quasi-one-dimensional quantum spin-S antiferromagnets. We use a combination of analytical and numerical techniques to study the presence of plateaux in the magnetization curve. The analytical technique consists in a path integral formulation in terms of coherent states. This technique can be extended to the presence of doping and has the advantage of a much better control for large spins than the usual bosonization technique. We discuss the appearance of doping-dependent plateaux in the magnetization curves for spin-S chains and ladders. The analytical results are complemented by a density matrix renormalization group (DMRG) study for a trimerized spin-1/2 and anisotropic spin-3/2 doped chains.Comment: 12 pages, 5 figure

High-speed imaging of Strombolian explosions: The ejection velocity of pyroclasts

Explosive volcanic eruptions are defined as the violent ejection of gas and hot fragments from a vent in the Earth's crust. Knowledge of ejection velocity is crucial for understanding and modeling relevant physical processes of an eruption, and yet direct measurements are still a difficult task with largely variable results. Here we apply pioneering high-speed imaging to measure the ejection velocity of pyroclasts from Strombolian explosive eruptions with an unparalleled temporal resolution. Measured supersonic velocities, up to 405 m/s, are twice higher than previously reported for such eruptions. Individual Strombolian explosions include multiple, sub-second-lasting ejection pulses characterized by an exponential decay of velocity. When fitted with an empirical model from shock-tube experiments literature, this decay allows constraining the length of the pressurized gas pockets responsible for the ejection pulses. These results directly impact eruption modeling and related hazard assessment, as well as the interpretation of geophysical signals from monitoring networks

Universal scaling behavior of coupled chains of interacting fermions

The single-particle hopping between two chains is investigated by exact-diagonalizations techniques supplemented by finite-size scaling analysis. In the case of two coupled strongly-correlated chains of spinless fermions, the Taylor expansion of the expectation value of the single-particle interchain hopping operator of an electron at momentum k_F in powers of the interchain hopping t_perp is shown to become unstable in the thermodynamic limit. In the regime alpha<alpha_{tp} (alpha_{tp} simeq 0.41) where transverse two-particle hopping is less relevant than single-particle hopping, the finite-size effects can be described in terms of a universal scaling function. From this analysis it is found that the single-particle transverse hopping behaves as t_perp^{alpha/(1-alpha)} in agreement with a RPA-like treatment of the interchain coupling. For alpha>alpha_{tp}, the scaling law is proven to change its functional form, thus signaling, for the first time numerically, the onset of coherent transverse two-particle hopping.Comment: 12 pages, Late

Low-frequency current fluctuations in doped ladders

Charge current static and dynamical correlations are computed by exact diagonalisation methods on a 2-leg t-t'-J ladder which exhibits a sharp transition between a Luther-Emery (LE) phase of hole pairs and a phase with deconfined holes. In the LE phase, we find short-range low-energy incommensurate current fluctuations which are intrinsically connected to the internal charge dynamics within one hole pair. On the contrary, when holes unbind, the maximum of the current susceptibility moves abruptly to the commensurate wavevector $\pi$ and strongly increases for decreasing doping suggesting an instability towards a staggered flux state at sufficiently small doping.Comment: 4 pages, 5 figure

Depleted Kondo Lattices

We consider a two dimensional Kondo lattice model with exchange J and hopping t in which three out of four impurity spins are removed in a regular way. At the particle-hole symmetric point the model may be studied with auxiliary field quantum Monte Carlo methods without sign problems. To achieve the relevant energy scales on finite clusters, we introduce a simple method to reduce size effects by up to an order of magnitude in temperature. In this model, a metallic phase survives up to arbitrarily low temperatures before being disrupted by magnetic fluctuations which open a gap in the charge sector. We study the formation of the heavy-electron state with emphasis on a crossover scale T* defined by the maximum in the resistivity versus temperature curve. The behavior of thermodynamic properties such as specific heat as well as spin and charge uniform susceptibilities are studied as the temperature varies in a wide range across T*. Within our accuracy T* compares well to the Kondo scale of the related single impurity problem. Finally our QMC resuls are compared with mean-field approximations.Comment: 12 pages, 13 figures. Submitted to Phys. Rev.

Modelling hospital bed necessity for COVID-19 patients during the decline phase of the epidemic trajectory

BACKGROUND: In the present study we aimed to create a model able to predict the short-term need of hospital beds for COVID-19 patients, during SARS-CoV-2 outbreak. METHODS: We retrospectively revised data about all COVID-19 patients hospitalized at a University Hospital in Northern Italy, between March 1 and April 29, 2020. Several polynomial models (from first to fourth order) were fitted to estimate the relationship between the time and the number of occupied hospital beds during the entire period and after the local peak of the outbreak and to provide the prediction of short-term hospital beds demand. Model selection was based on the adjusted R2 (aR2) Index and likelihood ratio test (LRT). RESULTS: We included 836 hospitalizations (800 COVID-19 patients). The median length of hospital in-stay was 12 days. According to the aR2, the fourth order models best fitted the data considering the entire time period. When only the data after the peak was selected, no statistical improvement was found adding terms of order 3 and 4 and lower order polynomial models were considered for the forecasting of the hospital beds demand. Both approaches had a decreasing trend in the number of occupied beds along with time; however, the quadratic one showed a faster reduction in the predicted number of beds required by patients affected by COVID-19. CONCLUSIONS: We propose a model to predict the hospital bed requirement during the descending phase of COVID-19 outbreak, the validation of which might contribute to decision makers policy in the next weeks of pandemic

Thermodynamics of the Spin Luttinger-Liquid in a Model Ladder Material

The phase diagram in temperature and magnetic field of the metal-organic, two-leg, spin-ladder compound (C5H12N)2CuBr4 is studied by measurements of the specific heat and the magnetocaloric effect. We demonstrate the presence of an extended spin Luttinger-liquid phase between two field-induced quantum critical points and over a broad range of temperature. Based on an ideal spin-ladder Hamiltonian, comprehensive numerical modelling of the ladder specific heat yields excellent quantitative agreement with the experimental data across the complete phase diagram.Comment: 4 pages, 4 figures, updated refs and minor changes to the text, version accepted for publication in Phys. Rev. Let

Power laws in a 2-leg ladder of interacting spinless fermions

We use the Density-Matrix Renormalization Group to study the single-particle and two-particle correlation functions of spinless fermions in the ground state of a quarter-filled ladder. This ladder consists of two chains having an in-chain extended Coulomb interaction reaching to third neighbor and coupled by inter-chain hopping. Within our short numerical coherence lengths, typically reaching ten to twenty sites, we find a strong renormalization of the interchain hopping and the existence of a dimensional crossover at smaller interactions. We also find power exponents for single-particle hopping and interchain polarization consistent with the single chain. The total charge correlation function has a larger power exponent and shows signs of a crossover from incoherent fermion hopping to coherent particle-hole pair motion between chains. There are no significant excitation energies.Comment: RevTex 4 file, 10 pages, 10 eps figure

The CORONA business in modern cities

As a response to the global outbreak of the SARS-COVID-19 pandemic, authorities have enforced a number of measures including social distancing, travel restrictions that lead to the "temporary" closure of activities stemming from public services, schools, industry to local businesses. In this poster we draw the attention to the impact of such measures on urban environments and activities. For this, we use crowdsensed information available from datasets like Google Popular Times and Apple Maps to shed light on the changes undergone during the outbreak and the recovery

The Impact of SARS-COVID-19 Outbreak on European Cities Urban Mobility

The global outbreak of the SARS-COVID-19 pandemic has changed our lives, driving an unprecedented transformation of our habits. In response, the authorities have enforced several measures, including social distancing and travel restrictions that lead to the temporary closure of activities centered around schools, companies, local businesses to those pertaining to the recreation category. As such, with a mobility reduction, the life of our cities during the outbreak changed significantly. In this paper, we aim at drawing attention to this problem and perform an analysis for multiple cities through crowdsensed information available from datasets such as Apple Maps, to shed light on the changes undergone during both the outbreak and the recovery. Specifically, we exploit data characterizing many mobility modes like driving, walking, and transit. With the use of Gaussian Processes and clustering techniques, we uncover patterns of similarity between the major European cities. Further, we perform a prediction analysis that permits forecasting the trend of the recovery process and exposes the deviation of each city from the trend of the cluster. Our results unveil that clusters are not typically formed by cities with geographical ties, but rather on the spread of the infection, lockdown measures, and citizens’ reactions