Pseudogap, charge order, and pairing density wave at the hot spots in cuprate superconductors

We address the timely issue of the presence of charge ordering at the hot-spots in the pseudo-gap phase of cuprate superconductors in the context of an emergent SU(2)-symmetry which relates the charge and pairing sectors. Performing the Hubbard-Stratonovich decoupling such that the free energy stays always real and physically meaningful we exhibit three solutions of the spin-fermion model at the hot spots. A careful examination of their stability and free energy shows that, at low temperature, the system tends towards a co-existence of charge density wave (CDW) and the composite order parameter made of diagonal quadrupolar density wave and pairing fluctuations of Ref. [Nat. Phys. $\bf{9}$, 1745 (2013)].The CDW is sensitive to the shape of the Fermi surface in contrast to the diagonal quadrupolar order, which is immune to it. SU(2) symmetry within the pseudo-gap phase also applies to the CDW state, which therefore admits a pairing density pave counterpart breaking time reversal symmetry.Comment: 15 pages, 15 figures, final version + typo corrected in Eq. (12

Charge orders, magnetism and pairings in the cuprate superconductors

We review the recent developments in the field of cuprate superconductors with the special focus on the recently observed charge order in the underdoped compounds. We introduce new theoretical developments following the study of the antiferromagnetic (AF) quantum critical point (QCP) in two dimensions, in which preemptive orders in the charge and superconducting (SC) sectors emerged, that are in turn related by an SU(2) symmetry. We consider the implications of this proliferation of orders in the underdoped region, and provide a study of the type of fluctuations which characterize the SU(2) symmetry. We identify an intermediate energy scale where the SU(2) pairing fluctuations are dominant and argue that they are unstable towards the formation of a Resonant Peierls Excitonic (RPE) state at the pseudogap (PG) temperature $T^{*}$. We discuss the implications of this scenario for a few key experiments.Comment: 16 pages, 17 figure

Quantum statistical correlations in thermal field theories: boundary effective theory

We show that the one-loop effective action at finite temperature for a scalar field with quartic interaction has the same renormalized expression as at zero temperature if written in terms of a certain classical field $\phi_c$, and if we trade free propagators at zero temperature for their finite-temperature counterparts. The result follows if we write the partition function as an integral over field eigenstates (boundary fields) of the density matrix element in the functional Schr\"{o}dinger field-representation, and perform a semiclassical expansion in two steps: first, we integrate around the saddle-point for fixed boundary fields, which is the classical field $\phi_c$, a functional of the boundary fields; then, we perform a saddle-point integration over the boundary fields, whose correlations characterize the thermal properties of the system. This procedure provides a dimensionally-reduced effective theory for the thermal system. We calculate the two-point correlation as an example.Comment: 13 pages, 1 figur

PVD-grown antibacterial Ag-TiN films on piezoelectric PVDF substrates for sensor applications

This work reports on Ti1-xAgx and Ag-TiNx electrodes deposited by dc/pulsed dc magnetron sputtering at room temperature on poly (vinylidene fluoride) (PVDF) with the purpose to develop sensors for prosthesis. In Ti1-xAgx electrodes, silver content was varied from 0 to 100 at.%; and for Ag-TiNx electrodes, the nitrogen content was varied between 34 to 43 at.% and the Ti/Ag ratio changed from 12.2 to 5.2. The antibacterial activity of the samples was assessed by agar diffusion method. Interestingly, samples from the Ag-TiNx series presented antibacterial activity, in contrast to the samples from Ti1-xAgx series. XPS results showed that the as-deposited samples from the Ag-TiNx series exhibited silver clusters smaller than 4 nm. In order to deepen the results obtained, samples were analysed after the Halo test (antibacterial test), and XPS analyses showed the disappearance of these Ag clusters in agreement with the SEM results, which displayed a visible aggregation and a significant decrease in the number of Ag clusters, suggesting that silver nanoparticles aggregated and diffused through the agar killing bacteria or inhibiting their grown in the vicinity. All results show that Ag-TiNx electrodes are more promising candidates to be used in PVDF sensor protection than Ti1-xAgx electrodesS. M. Marques gratefully acknowledges the funding from the Portuguese National funds through the FCT - Fundacao para a Ciencia e a Tecnologia, (project SFRH/BD/71259/2010). Also, the authors thank the support of FEDER through the COMPETE Programme and of the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Project PEST-C/FIS/UI607/2014 and the project Matepro-Optimizing Materials and Processes, ref. NORTE-07-0124-FEDER-000037", co-funded by the "Programa Operacional Regional do Norte" (ON.2 - O Novo Norte), under the "Quadro de Referencia Estrategico Nacional" (QREN), through the "Fundo Europeu de Desenvolvimento Regional" (FEDER).The authors thank the FCT Strategic Project of UID/BIO/04469/2013 unit, the project RECI/BBB-EBI/0179/2012 (FCOMP-01-0124-FEDER-027462) and the project "BioInd - Biotechnology and Bioengineering for improved Industrial and Agro-Food processes", REF. NORTE-07-0124-FEDER-000028 co-funded by the Programa Operacional Regional do Norte (ON.2 - O Novo Norte), QREN, FEDER.The authors thank the FCT Strategic Project of UID/BIO/04469/2013 unit, the project RECI/BBB-EBI/0179/2012 (FCOMP-01-0124-FEDER-027462) and the Project "BioHealth Biotechnology and Bioengineering approaches to improve health quality", Ref. NORTE-07-0124-FEDER-000027, co-funded by the Programa Operacional Regional do Norte (ON.2 - O Novo Norte), QREN, FEDER.The authors thank the FCT Strategic Project of UID/BIO/04469/2013 unit, the project RECI/BBB-EBI/0179/2012 (FCOMP-01-0124-FEDER-027462) and the Project "BioEnv Biotechnology and Bioengineering for a sustainable world", REF. NORTE-07-0124-FEDER-000048, co-funded by the Programa Operacional Regional do Norte (ON.2 0 Novo Norte), QREN, FEDER

PVD-grown antimicrobial thin films on PVDF substrates for sensors applications

Electroactive polymers are the most interesting class of polymers used as smart materials in various applications, such as the development of sensors and actuators for biomedical applications in areas as smart prosthesis, implantable biosensors and biomechanical signal monitoring, among others. For acquiring or applying the electrical signal from/to the piezoelectric material, suitable electrodes can be produced from Ti based coatings with tailored multifunctional properties, as conductivity and antibacterial characteristics, obtained by the inclusion of Ag. This work reports on Ti1-xAgx electrodes and Ag-TiNy electrodes deposited by d. c. and pulsed magnetron sputtering at room temperature on poly(vinylidene fluoride)(PVDF). In the first system (Ti1-xAgx electrodes), silver content was varied from 0-100 at. %. For the second system (Ag-TiNy electrodes), the nitrogen content changed between 0 to 40.3 at. % by increasing the nitrogen gas flow between 0 sccm and 15 sccm and the ratio Ti/Ag changed from 13.4 to 2.2 being clearly the visible decrease on the Ti content in the reactive mode. The X-Ray Diffraction (XRD) results revealed that the deposition conditions preserve the polymer structure and suggested the presence of crystalline Ti phase in pure titanium coating and fcc-Ag phase in pure silver coating for the Ti1-xAgx system. For the Ag-TiNy system it is possible to detect a fcc TiN structure and a fcc Ag phase. Sheet resistivity values show a typical behavior of a binary alloy system, varying between 0.12 and 28.5 /sq for the Ti1-xAgx electrodes. For the second system the sheet resistivity decrease with the nitrogen content from 12.0 /sq with 0 at. % to 2.8 /sq for 40.3 at. % of N. The piezoelectricity of the different samples show similar values, showing values from 19.6 to 27.6 pCN-1 for the Tix-1Agx system and 13.6 pCN-1 as minimum for the Ag- TiNy system, achieved for the highest N content. In order to assess the mechanical behavior of the as-sputtered films, the film/substrate system was loaded unidirectionally using a tensile machine. The stress-strain curves were analyzed and correlated with the structural data. Moreover, the antibacterial activity of the samples was assessed and it was verified that samples from the second series (Ag-TiNy) present antibacterial activity, in contrast of the first series (Ti1-xAgx)