Properties of dirty two-bands superconductors with repulsive interband interaction: normal modes, length scales, vortices and magnetic response

Disorder in two-band superconductors with repulsive interband interaction induces a frustrated competition between the phase-locking preferences of the various potential and kinetic terms. This frustrated interaction can result in the formation of an $s+is$ superconducting state, that breaks the time-reversal symmetry. In this paper we study the normal modes and their associated coherence lengths in such materials. We especially focus on the consequences of the soft modes stemming from the frustration and time-reversal-symmetry breakdown. We find that two-bands superconductors with such impurity-induced frustrated interactions display a rich spectrum of physical properties that are absent in their clean counterparts. It features a mixing of Leggett's and Anderson-Higgs modes, and a soft mode with diverging coherence length at the impurity-induced second order phase transition from $s_{\pm}/s_{++}$ states to the $s+is$ state. Such a soft mode generically results in long-range attractive intervortex forces that can trigger the formation of vortex clusters. We find that, if such clusters are formed, their size and internal flux density have a characteristic temperature dependence that could be probed in muon-spin-rotation experiments. We also comment on the appearance of spontaneous magnetic fields due to spatially varying impurities.Comment: Added discussion of spontaneous magnetic fields due to spatially varying impurities; Replaced with a version in print in Phys. Rev. B; 17 pages, 8 figure

Single-step preparation of inverse opal titania films by the doctor blade technique

The difficulty to infiltrate solid-state hole semiconductors within micron-thick porous titania films is one of the major limiting factors for the achievement of efficient solid-state dye-sensitized solar cells. It was already shown that through the ordered interconnected pores of an inverse opal, the large surface area of several microns thick titania film can be easily decorated with a dye and filled with a solid-state hole semiconductor. In this paper, we show that ordered inverse opal mesoporous thick films of TiO2 with these characteristics can be obtained by using a slurry of monodispersed polystyrene spheres and a titania-lactate precursor deposited by the doctor blade technique. The mechanism of formation of the inverse opal is also discussed

A fast algorithm for Direct Numerical Simulation of natural convection flows in arbitrarily-shaped periodic domains

A parallel algorithm is presented for the Direct Numerical Simulation of buoyancy-induced flows in open or partially confined periodic domains, containing immersed cylindrical bodies of arbitrary cross-section. The governing equations are discretized by means of the Finite Volume method on Cartesian grids. A semi-implicit scheme is employed for the diffusive terms, which are treated implicitly on the periodic plane and explicitly along the homogeneous direction, while all convective terms are explicit, via the second-order Adams-Bashfort scheme. The contemporary solution of velocity and pressure fields is achieved by means of a projection method. The numerical resolution of the set of linear equations resulting from discretization is carried out by means of efficient and highly parallel direct solvers. Verification and validation of the numerical procedure is reported in the paper, for the case of flow around an array of heated cylindrical rods arranged in a square lattice. Grid independence is assessed in laminar flow conditions, and DNS results in turbulent conditions are presented for two different grids and compared to available literature data, thus confirming the favorable qualities of the method

Evaluation of inorganic contaminant's mobility by leaching test on polluted environmental materials

La caratterizzazione di sedimenti contaminati è un problema complesso, in questo lavoro ci si è occupati di individuare una metodologia di caratterizzazione che tenesse conto sia delle caratteristiche della contaminazione, con analisi volte a determinare il contenuto totale di contaminanti, sia della mobilità degli inquinanti stessi. Una adeguata strategia di caratterizzazione può essere applicata per la valutazione di trattamenti di bonifica, a questo scopo si è valutato il trattamento di soil washing, andando ad indagare le caratteristiche dei sedimenti dragati e del materiale in uscita dal processo, sabbie e frazione fine, andando inoltre a confrontare le caratteristiche della sabbia in uscita con quelle delle sabbie comunemente usate per diverse applicazioni. Si è ritenuto necessario indagare la compatibilità dal punto di vista chimico, granulometrico e morfologico. Per indagare la mobilità si è scelto di applicare i test di cessione definiti sia a livello internazionale che italiano (UNI) e quindi si sono sviluppate le tecnologie necessarie alla effettuazione di test di cessione in modo efficace, automatizzando la gestione del test a pHstat UNI CEN 14997. Questo si è reso necessario a causa della difficoltà di gestire il test manualmente, per via delle tempistiche difficilmente attuabili da parte di un operatore. Le condizioni redox influenzano la mobilità degli inquinanti, in particolare l’invecchiamento all’aria di sedimenti anossici provoca variazioni sensibili nello stato d’ossidazione di alcune componenti, incrementandone la mobilità, si tratta quindi di un aspetto da considerare quando si individuano le adeguate condizioni di stoccaggio-smaltimento, si è eseguita a questo scopo una campagna sperimentale.The characterization of contaminated sediments is a daedalean problem , in this paper the focus is to identify a method of characterization that takes into account both the characteristics of the contamination, with analysis to determine the total content of contaminants, and the mobility of the pollutants themselves. An adequate characterization strategy can be applied for the evaluation of managing options, and treatments of reclamation, for this purpose , it is assessed the treatment of soil washing, taking account of the characteristics of the sediment and dredged material in the process output, sands and fine fraction, and also to comparing the characteristics of the sand in output with those of sands commonly used for different applications. It was considered necessary to investigate the compatibility from the chemical point of view, particle size and morphology. Different leaching test have been applied to investigate the mobility, they are defined in the international and Italian norms (UNI) and therefore have developed the technologies necessary to carry out tests of the sale effectively, automating the management of test pHStat CEN 14997. That has been helpful due to the type of test, which is difficult to handle in manual way. The redox conditions affect the mobility of pollutants , in particular the aging of anoxic sediments in air causes substantial changes in the oxidation state of some components , increasing mobility , it is therefore an aspect to be considered when identifying the appropriate conditions storage - disposal. With that goal a series of experiments have been carried out

Spurious Symmetry Enhancement and Interaction-Induced Topology in Magnons

Linear spin wave theory (LSWT) is the standard technique to compute the spectra of magnetic excitations in quantum materials. In this paper, we show that LSWT, even under ordinary circumstances, may fail to implement the symmetries of the underlying ordered magnetic Hamiltonian leading to spurious degeneracies. In common with pseudo-Goldstone modes in cases of quantum order-by-disorder these degeneracies tend to be lifted by magnon-magnon interactions. We show how, instead, the correct symmetries may be restored at the level of LSWT. In the process we give examples, supported by nonperturbative matrix product based time evolution calculations, where symmetries dictate that there should be a topological magnon gap but where LSWT fails to open up this gap. We also comment on possible spin split magnons in MnF$_2$ and similar rutiles by analogy to recently proposed altermagnets