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Population Inversion in Monolayer and Bilayer Graphene
Isabella Gierz,Matteo Mitrano,Jesse C. Petersen,Cephise Cacho,I. C. Edmond Turcu,Emma Springate,Alexander St?hr,Axel K?hler,Ulrich Starke,Andrea Cavalleri
Physics , 2014, DOI: 10.1088/0953-8984/27/16/164204
Abstract: The recent demonstration of saturable absorption and negative optical conductivity in the Terahertz range in graphene has opened up new opportunities for optoelectronic applications based on this and other low dimensional materials. Recently, population inversion across the Dirac point has been observed directly by time- and angle-resolved photoemission spectroscopy (tr-ARPES), revealing a relaxation time of only ~ 130 femtoseconds. This severely limits the applicability of single layer graphene to, for example, Terahertz light amplification. Here we use tr-ARPES to demonstrate long-lived population inversion in bilayer graphene. The effect is attributed to the small band gap found in this compound. We propose a microscopic model for these observations and speculate that an enhancement of both the pump photon energy and the pump fluence may further increase this lifetime.
Snapshots of non-equilibrium Dirac carrier distributions in graphene
Isabella Gierz,Jesse C. Petersen,Matteo Mitrano,Cephise Cacho,Edmond Turcu,Emma Springate,Alexander St?hr,Axel K?hler,Ulrich Starke,Andrea Cavalleri
Physics , 2013, DOI: 10.1038/nmat3757
Abstract: The optical properties of graphene are made unique by the linear band structure and the vanishing density of states at the Dirac point. It has been proposed that even in the absence of a semiconducting bandgap, a relaxation bottleneck at the Dirac point may allow for population inversion and lasing at arbitrarily long wavelengths. Furthermore, efficient carrier multiplication by impact ionization has been discussed in the context of light harvesting applications. However, all these effects are difficult to test quantitatively by measuring the transient optical properties alone, as these only indirectly reflect the energy and momentum dependent carrier distributions. Here, we use time- and angle-resolved photoemission spectroscopy with femtosecond extreme ultra-violet (EUV) pulses at 31.5 eV photon energy to directly probe the non-equilibrium response of Dirac electrons near the K-point of the Brillouin zone. In lightly hole-doped epitaxial graphene samples, we explore excitation in the mid- and near-infrared, both below and above the minimum photon energy for direct interband transitions. While excitation in the mid-infrared results only in heating of the equilibrium carrier distribution, interband excitations give rise to population inversion, suggesting that terahertz lasing may be possible. However, in neither excitation regime do we find indication for carrier multiplication, questioning the applicability of graphene for light harvesting. Time-resolved photoemission spectroscopy in the EUV emerges as the technique of choice to assess the suitability of new materials for optoelectronics, providing quantitatively accurate measurements of non-equilibrium carriers at all energies and wavevectors.
The Potential of Internet of Things to Improve the Quality of Healthcare Delivery
Cristina TURCU,Cornel TURCU,,Doru TILIU?E
Journal of Applied Computer Science & Mathematics , 2012,
Abstract: Radio Frequency Identification (RFID) technology is gaining significant momentum for tracking and tracing patients. The existing solutions for patient traceability are mostly centralized and closed-loop (effective only in small-scale RFID tracking applications). Lately, there has been a tendency to tag surrounding physical objects and to adopt various solutions that enable them to communicate with one another and with Information and Communications Technology (ICT) systems. This leads to a new concept named the Internet of Things (IoT), in which objects are associated with useful digital information. IoT can bring multiple benefits to healthcare through the use of information from distributed resources such as dedicated devices, RFID tags, sensors/detectors, intelligent equipment, etc. IoT is likely to have impact on many applications in healthcare subfields such as, elderly and patient care, medical supply chain, logistic, asset management, drug counterfeit detection, etc. This paper deals with aspects related to technologies used in healthcare, and with the various challenges healthcare faces during transition from RFID to IoT.
A Proposal for a Nationwide Student Gradebook Information Network
Cristina Turcu,Cornel Turcu,Evelina Graur
Computer Science , 2015,
Abstract: Todays students are encouraged to study and develop expertise in more than one national academic environment. As a matter of fact, their educational activities inevitably occur in a variety of academic settings and even span several years. Consequently students academic results and progress are expected to be easily monitored and accessed nationally. The authors of the present paper have devised a student gradebook information network to be nationally employed by all public and private universities and colleges. The papers deals with the architectural principles underlying the system and discusses aspects related to data collection, data analysis and data storage across multiple machines while providing a seamless view of entire infrastructure and service delivery system from a single Web access point. The utility of the architectural system is discussed in relationship with its major advantages: user-friendliness, security access, flexibility, transparency, distributional power, and scalability. The major beneficiary of the system is the Romanian higher education system.
Probing the structure and dynamics of molecular clusters using rotational wavepackets
Gediminas Galinis,Cephise Cacho,Richard T. Chapman,Andrew M. Ellis,Marius Lewerenz,Luis G. Mendoza Luna,Russell S. Minns,Mirjana Mladenovic,Arnaud Rouzée,Emma Springate,I. C. Edmond Turcu,Mark J. Watkins,Klaus von Haeften
Physics , 2014, DOI: 10.1103/PhysRevLett.113.043004
Abstract: The chemical and physical properties of molecular clusters can heavily depend on their size, which makes them very attractive for the design of new materials with tailored properties. Deriving the structure and dynamics of clusters is therefore of major interest in science. Weakly bound clusters can be studied using conventional spectroscopic techniques, but the number of lines observed is often too small for a comprehensive structural analysis. Impulsive alignment generates rotational wavepackets, which provides simultaneous information on structure and dynamics, as has been demonstrated successfully for isolated molecules. Here, we apply this technique for the firsttime to clusters comprising of a molecule and a single helium atom. By forcing the population of high rotational levels in intense laser fields we demonstrate the generation of rich rotational line spectra for this system, establishing the highly delocalised structure and the coherence of rotational wavepacket propagation. Our findings enable studies of clusters of different sizes and complexity as well as incipient superfluidity effects using wavepacket methods.
Momentum-dependent snapshots of a melting charge density wave
Jesse C. Petersen,Stefan Kaiser,Nicky Dean,Alberto Simoncig,Haiyun Liu,Adrian L. Cavalieri,Cephise Cacho,I. C. Edmond Turcu,Emma Springate,Fabio Frassetto,Luca Poletto,Sarnjeet S. Dhesi,Helmuth Berger,Andrea Cavalleri
Physics , 2010, DOI: 10.1103/PhysRevLett.107.177402
Abstract: Charge density waves (CDWs) underpin the electronic properties of many complex materials. Near-equilibrium CDW order is linearly coupled to a periodic, atomic-structural distortion, and the dynamics is understood in terms of amplitude and phase modes. However, at the shortest timescales lattice and charge order may become de-coupled, highlighting the electronic nature of this many-body broken symmetry ground state. Using time and angle resolved photoemission spectroscopy with sub-30-fs XUV pulses, we have mapped the time- and momentum-dependent electronic structure in photo-stimulated 1T-TaS2, a prototypical two-dimensional charge density wave compound. We find that CDW order, observed as a splitting of the uppermost electronic bands at the Brillouin zone boundary, melts well before relaxation of the underlying structural distortion. Decoupled charge and lattice modulations challenge the view of Fermi Surface nesting as a driving force for charge density wave formation in 1T-TaS2.
Direct view on the ultrafast carrier dynamics in graphene
Jens Christian Johannsen,S?ren Ulstrup,Federico Cilento,Alberto Crepaldi,Michele Zacchigna,Cephise Cacho,I. C. Edmond Turcu,Emma Springate,Felix Fromm,Christian Raidel,Thomas Seyller,Fulvio Parmigiani,Marco Grioni,Philip Hofmann
Physics , 2013, DOI: 10.1103/PhysRevLett.111.027403
Abstract: The ultrafast dynamics of excited carriers in graphene is closely linked to the Dirac spectrum and plays a central role for many electronic and optoelectronic applications. Harvesting energy from excited electron-hole pairs, for instance, is only possible if these pairs can be separated before they lose energy to vibrations, merely heating the lattice. While the hot carrier dynamics in graphene could so far only be accessed indirectly, we here present a direct time-resolved view on the Dirac cone by angle-resolved photoemission (ARPES). This allows us to show the quasi-instant thermalisation of the electron gas to a temperature of more than 2000 K; to determine the time-resolved carrier density; to disentangle the subsequent decay into excitations of optical phonons and acoustic phonons (directly and via supercollisions); and to show how the presence of the hot carrier distribution affects the lifetime of the states far below the Fermi energy.
Nitrogen Substituted Phenothiazine Derivatives: Modelling of Molecular Self-Assembling
Attila Bende,Ioan Turcu
International Journal of Molecular Sciences , 2011, DOI: 10.3390/ijms12053102
Abstract: The study aims to present a detailed theoretical investigation of noncovalent intermolecular interactions between different π–π stacking nitrogen substituted phenothiazine derivatives by applying second-order M?ller-Plesset perturbation (MP2), density functional (DFT) and semiempirical theories. The conformational stability of these molecular systems is mainly given by the dispersion-type electron correlation effects. The density functional tight-binding (DFTB) method applied for dimer structures are compared with the results obtained by the higher level theoretical methods. Additionally, the optimal configuration of the investigated supramolecular systems and their self-assembling properties are discussed.
Teachers’ Attitudes and Perceptions: Association of Teachers’ Attitudes toward Traditional and Modern Teaching Methodology According to RWCT as Well as Teachers’ Perceptions for Teaching as a Profession  [PDF]
Xhevahire Karanezi, Edmond Rapti
Creative Education (CE) , 2015, DOI: 10.4236/ce.2015.66061
Abstract: Teaching methodologies that are chosen to be used by teachers in their daily work are highly impacted by their attitudes and perceptions. According to Schoenfeld (1992) attitudes and perceptions influence teachers not only how, but what, he or she teaches. The data collection for this study was conducted using quantitative and qualitative method. There were 473 school teachers throughout Kosovo that participated in this study. Questionnaires were used to collect all quantitative data, which were compiled and adjusted in accordance with the study objectives. Focus groups discussions were conducted for the purpose of collecting the qualitative data. The data showed significant negative association between teachers’ attitudes toward traditional teaching methodologies and modern teaching methodologies according to RWCT (r = 0.446, sig = 0.000). The data also showed that there was a significant negative association between attitudes toward traditional teaching methodology and teachers’ perceptions for teaching as a profession (r = 0.092, sig = 0.046). The quantitative results were in the same line with the qualitative findings of the study.
Intelligent Device Used by an Infotmation System for Identifying and Monitoring of Patients
Ioan Ungureanu,Cristina Elena Turcu,Cornel Turcu,Vasile Gheorghita Gaitan
Computer Science , 2015,
Abstract: The aim of this paper consists in defining the hardware and software architecture of an embedded system, based on RFID technology, in order to identify patients and to achieve real time information concerning the patients biometric data, which might be used in different points of the health system (laboratory, family physician, etc.).
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