BCS-BEC crossover and nodal points contribution in p-wave resonance superfluids

We solve the Leggett equations for BCS–BEC crossover of the resonance p-wave superfluid. We calculate sound velocity, specific heat and the normal density for the BCS-domain (μ > 0), BEC-domain (μ < 0) as well as for the interesting interpolation point (μ = 0) in the triplet A₁-phase in 3D. We are especially interested in the quasiparticle contribution coming from the zeroes of the superfluid gap in the A1-phase. We discuss the spectrum of orbital waves and the superfluid hydrodynamics at temperature T → 0. In this context we elucidate the difficult problem of chiral anomaly and mass-current nonconcervation appearing in the BCS-domain. We present the different approaches to solve this problem. To clarify this problem experimentally we propose an experiment for the measurement of anomalous current in superfluid A1-phase in the presence of aerogel for ³He and in the presence of Josephson tunneling structures for the ultracold gases in magnetic traps

On the Analytic "Causal" Model for the QCD Running Coupling

We discuss the model $\bar{\alpha}_{an}(Q^2)$ recently proposed for the QCD running coupling $\bar{\alpha}_s(Q^2)$ in the Euclidean domain on the basis of the "asymptotic-freedom" expression and on causality condition in the form of the $Q^2$-analyticity. The model contains no adjustable parameters and obeys the important features: (i) Finite ghost-free behavior in the "low $Q^2$" region and correspondence with the standard RG-summed UV expressions;%\item~ % (ii) The universal limiting value $\bar{\alpha}_{an}(0)$ expressed only via group symmetry factors. This value as well as the $\bar{\alpha}_{an}$ behavior in the whole IR region $Q^2 \leq \Lambda^2$ turns out to be stable with respect to higher loop corrections; %\item~ (iii) Coherence between observed $\bar{\alpha}_s(M_{\tau}^2)$ value and integral information on the IR $\bar{\alpha}_s(Q^2)$ behavior extracted from jet physics.Comment: 4 pages, LaTex with psfig.sty, 2 PostScript figure

Multiband Description of Optical Conductivity in Ferropnictide Superconductors

We study optical properties of the multiband superconductors with an s± order parameter symmetry. By comparing results of our theory with experimental data on optical conductivity for Ba0.68K0.32Fe2As2 single crystals, we show that satisfactory description of the novel superconductors can be obtained only considering a strong electron-boson coupling. We reexamine the effect of disorder and demonstrate that multiband superconductors are more robust with respect to it than naively expected by simple analogy with paramagnetic impurities in single-band superconductors. Moreover, disorder may give rise to new effects, in particular to a phase transition s±→s++. We discuss how the systematic study of disorder impact on the density of states and the optical conductivity may provide information on the underlying order parameter structur

Kohn–Luttinger effect and anomalous pairing in repulsive Fermi-systems at low density

In the large variety of models such as 3D and 2D Fermi-gas model with hard-core repulsion, 3D and 2D Hubbard model, and Shubin–Vonsovsky model we demonstrate the possibility of triplet p-wave pairing at low electron density. We show that the critical temperature of the p-wave pairing can be strongly increased in a spinpolarized case or in a two-band situation already at low density and reach experimentally observable values of (1–5) K. We also discuss briefly d-wave pairing and high-Tc superconductivity with Tc ~ 100 K which arises in the extended Hubbard model and in the generalized t-J model close to half-filling

Mechanical properties of fibroblasts depend on level of cancer transformation

AbstractRecently, it was revealed that tumor cells are significantly softer than normal cells. Although this phenomenon is well known, it is connected with many questions which are still unanswered. Among these questions are the molecular mechanisms which cause the change in stiffness and the correlation between cell mechanical properties and their metastatic potential. We studied mechanical properties of cells with different levels of cancer transformation. Transformed cells in three systems with different transformation types (monooncogenic N-RAS, viral and cells of tumor origin) were characterized according to their morphology, actin cytoskeleton and focal adhesion organization. Transformation led to reduction of cell spreading and thus decreasing the cell area, disorganization of actin cytoskeleton, lack of actin stress fibers and decline in the number and size of focal adhesions. These alterations manifested in a varying degree depending on type of transformation. Force spectroscopy by atomic force microscopy with spherical probes was carried out to measure the Young's modulus of cells. In all cases the Young's moduli were fitted well by log-normal distribution. All the transformed cell lines were found to be 40–80% softer than the corresponding normal ones. For the cell system with a low level of transformation the difference in stiffness was less pronounced than for the two other systems. This suggests that cell mechanical properties change upon transformation, and acquisition of invasive capabilities is accompanied by significant softening

Tuning the interplay between nematicity and spin fluctuations in Na1-x Li x FeAs superconductors

Strong interplay of spin and charge/orbital degrees of freedom is the fundamental characteristic of the iron-based superconductors (FeSCs), which leads to the emergence of a nematic state as a rule in the vicinity of the antiferromagnetic state. Despite intense debate for many years, however, whether nematicity is driven by spin or orbital fluctuations remains unsettled. Here, by use of transport, magnetization, and 75As nuclear magnetic resonance (NMR) measurements, we show a striking transformation of the relationship between nematicity and spin fluctuations (SFs) in Na1-x Li x FeAs; For x ≤ 0.02, the nematic transition promotes SFs. In contrast, for x ≥ 0.03, the system undergoes a non-magnetic phase transition at a temperature T 0 into a distinct nematic state that suppresses SFs. Such a drastic change of the spin fluctuation spectrum associated with nematicity by small doping is highly unusual, and provides insights into the origin and nature of nematicity in FeSCs

Nematicity and magnetism in LaFeAsO single crystals probed by As 75 nuclear magnetic resonance

We report a As75 nuclear magnetic resonance study in LaFeAsO single crystals, which undergoes nematic and antiferromagnetic transitions at Tnem∼156 K and TN∼138 K, respectively. Below Tnem, the As75 spectrum splits sharply into two for an external magnetic field parallel to the orthorhombic a or b axis in the FeAs planes. Our analysis of the data demonstrates that the NMR line splitting arises from an electronically driven rotational symmetry breaking. The As75 spin-lattice relaxation rate as a function of temperature shows that spin fluctuations are strongly enhanced just below Tnem. These NMR findings indicate that nematic order promotes spin fluctuations in magnetically ordered LaFeAsO, as observed in nonmagnetic and superconducting FeSe. We conclude that the origin of nematicity is identical in both FeSe and LaFeAsO regardless of whether or not a long-range magnetic order develops in the nematic state. © 2018 American Physical Society1

Separate tuning of nematicity and spin fluctuations to unravel the origin of superconductivity in FeSe

The interplay of orbital and spin degrees of freedom is the fundamental characteristic in numerous condensed matter phenomena, including high-temperature superconductivity, quantum spin liquids, and topological semimetals. In iron-based superconductors (FeSCs), this causes superconductivity to emerge in the vicinity of two other instabilities: nematic and magnetic. Unveiling the mutual relationship among nematic order, spin fluctuations, and superconductivity has been a major challenge for research in FeSCs, but it is still controversial. Here, by carrying out 77Se nuclear magnetic resonance (NMR) measurements on FeSe single crystals, doped by cobalt and sulfur that serve as control parameters, we demonstrate that the superconducting transition temperature Tc increases in proportion to the strength of spin fluctuations, while it is independent of the nematic transition temperature Tnem. Our observation therefore directly implies that superconductivity in FeSe is essentially driven by spin fluctuations in the intermediate coupling regime, while nematic fluctuations have a marginal impact on Tc

Collective excitations of a trapped boson-fermion mixture across demixing

We calculate the spectrum of low-lying collective excitations in a mesoscopic cloud formed by a Bose-Einstein condensate and a spin-polarized Fermi gas as a function of the boson-fermion repulsions. The cloud is under isotropic harmonic confinement and its dynamics is treated in the collisional regime by using the equations of generalized hydrodynamics with inclusion of surface effects. For large numbers of bosons we find that, as the cloud moves towards spatial separation (demixing) with increasing boson-fermion coupling, the frequencies of a set of collective modes show a softening followed by a sharp upturn. This behavior permits a clear identification of the quantum phase transition. We propose a physical interpretation for the dynamical transition point in a confined mixture, leading to a simple analytical expression for its location.Comment: revtex4, 9 pages, 8 postscript file