Influence of crystal defects on phase excitations in charge density waves

U ovoj disertaciji izučavan je utjecaj defekata na prijelaz u stanje vala gustoće naboja (VGN) i svojstva niskoenergetskih pobuđenja faze VGN-a, u ozračenim i dopiranim uzorcima VGN sistema o-TaS3. Smrzavanje fazne dinamike na temperaturi TG, kao dio složenih svojstva niskoenergetskih pobuđenja u VGN sistemima, posljedica je vezanja VGN-a sa defektima i slobodnim nosiocima naboja. Predloženi kriterij smrzavanja1, prema kojem je TG određena brojem slobodnih nosilaca naboja u domeni korelacije faze VGN-a je, kao glavni motiv istraživanja, u ovoj disertaciji potvrđen na uzorcima sa širokim rasponom udjela defekata. Transportna i dielektrična mjerenja, u širokom temperaturnom, naponskom i frekventnom području, pokazuju da defekti uzrokuju promjenu polja praga, niskofrekventne dinamike i dielektrične konstante, ali ne i linearne vodljivosti iznad TG, dok je ispod TG opažena samo promjena u linearnoj vodljivosti. Pokazano je da je zapinjanje faze u ozračenim uzorcima iznad TG jakog, a u dopiranim slabog karaktera. Ispod TG javlja se nelinearni kanal vodljivosti skokova varijabilnog dosega.In this dissertation the influence of defects on the transition to charge density wave (CDW) state and properties of low energy CDW phase excitations has been studied in irradiated and doped samples of CDW system o-TaS3. Phase dynamics freezing at a temperature TG, as a part of complex properties of low energy excitations in CDW systems, is a consequence of CDW coupling to defects and free charge carriers. As the main motive of this research, proposed freezing criterion1 (according to which TG is determined by a number of free charge carriers within a CDW phase correlation domain) has been confirmed in samples with a wide range of defects. Transport and dielectric measurements, over a wide temperature, voltage and frequency range, show that the defects influence the threshold field, low-frequency dynamics and dielectric constant, but not the linear conductivity above TG, while below TG only an influence on the linear conductivity has been observed. Strong and weak phase pinning mechanisms above TG are confirmed in irradiated and doped samples, respectively. Below TG variable range hopping type of nonlinear conductivity channel appears

Forest Fires and Physical Models

Hrvatska je zemlja relativno bogata šumama; 36 % od ukupne površine države pokrivaju šumske površine. Međutim, poseban problem na jadranskom području Hrvatske predstavljaju šumski požari, koji uništavaju prirodna bogatstva smanjujući biološku raznolikost, a negativan utjecaj imaju i na krajobraznu raznolikost. Sam problem je višeslojan, kompleksan i zahtijeva uključivanje brojnih grana znanosti, kao što su šumarstvo, geologija, biologija, meteorologija, a u zadnje vrijeme i fizika. Poznato je da fizika uspješno prepoznaje osnovna načela pojavnosti i djelovanja u materijalnom svijetu, a to ima vrlo široku primjenu i izvan njenih vlastitih okvira. Načini manifestacije komplesnosti i posebno zakonitosti koje fizičari sve više u suradnji s drugim disciplinama nalaze i proučavaju otvaraju vrlo široke horizonte mogućih primjena. “Korelacije u kompleksnim sustavima: od fizike do biotehnologije” je naziv interdisciplinarnog programa kojega financira Ministarstvo znanosti, obrazovanja i športa, koji je među inim povezao znanstvene pristupe fizike i šumarstva na proučavanju šumskih požara. Ovaj rad daje neke osnove o šumskim požarima specifične za svaku znanstvenu disciplinu i pokazuje mogućnosti njihove interakcije kao i važnost takvog interdisciplinarnog pristupa u rješavanju gorućih problema vezanih uz šumske požare i njihovu prevenciju.Croatia is relatively rich in forests; forested areas account for 36 % of the total territory of the country. However, forest fires occurring in the Adriatic region of Croatia are a serious threat to natural resources, since they decrease biological diversity and exert a negative impact on landscape variety. The complexity of the problem requires the involvement of a number of scientific fields, such as forestry, geology, biology, meteorology and physics. Physics is known to successfully detect some basic principles of occurrence and activity in the material world, which has a broad scope of uses beyond the boundaries of physics alone. Physics, in cooperation with other scientific disciplines, discovers and investigates ways and patterns in which complexities are manifested, thus opening the door to an array of applications. “Correlations in Complex Systems: from Physics to Biotechnology” is the title of an interdisciplinary program financed by the Ministry of Science, Education and Sport, which, among others, integrates scientific approaches of physics and forestry to the study of forest fires. This paper provides some concepts of forest fires that are specific for each scientific discipline. It also explores the possibilities of their interaction and the importance of an interdisciplinary approach to the solution of vital issues concerning forest fires and their prevention

Analysis of Distribution of Areas Affected by Forest Fires on Island of Brač, Island of Korčula and Island of Rab

Šume prekrivaju približno četvrtinu zemljine površine, i kao jedan od najvećih izvora kisika u prirodi važne su za opstanak života na Zemlji. Šumski požari kao vrlo važan fenomen za sam opstanak šuma, uz šumarstvo i ekologiju počeli su se proučavati i u fizici. U fizici je šumski požar prepoznat kao primjer kompleksnog sustava na velikim, kilometarskim skalama. Računalne simulacije omogućile su nova saznanja o šumskim požarima. U radu su korišteni podaci o broju požara i opožarenoj površini, prikupljeni u šumarijama Brač, Korčula i Rab, u razdoblju od 1991. godine do 2000. godine, koji su obrađeni primjenom fizikalnih modela, pomoću kojih se može saznati kako se požari šire, koji sve parametri i u kojoj mjeri utječu na širenje požara, te naj važnije – kako predvidjeti požare određenih razmjera. Rezultati istraživanja pokazuju da kumulativna raspodjela spaljenih površina na izabranim otocima slijedi zakon potencije u skladu s modelom Malamuda i drugih (1998). Logaritamski prikaz rezultata je pravac u najvećem dijelu. Nagib odgovara eksponentu a, jer je – dNCF/dAF~AFa. Prema navedenom modelu, ako su poznati zadani parametri nekog sustava možemo odrediti frekvenciju širenja požara, koja nam govori kolika je vjerojatnost pojave požara na nekoj površini. Skup podataka za naša tri otoka pokazuje da je s obzirom na dobiven nagib pravca za ukupan broj požara a = 1.02 ± 0.02 frekvencija širenja velika, što govori da je vjerojatnost širenja požara manja. Međutim, detaljnijom analizom dvije grupe podataka za veće požare dobije se veći nagib, što govori da je u idućih nekoliko godina rizik od požara velik, i to točno na područjima na kojima su izgorjele velike površine (na Korčuli čak do 55 km2). Iz dobivenih rezultata moguće je zaključiti da se vrijednosti nagiba pravca podudaraju za male i srednje požare, odnosno za veće frekvencije širenja kod primijenjenog modela, dok za veće požare postoje odstupanja kod primjene modela zbog konačnih dimenzija prostora. Dobiveni rezultati su poticaj za daljnja istraživanja, jer je pokazano da se poznavanjem utjecaja različitih parametara po vezanih sa širenjem požara na nekom prostoru mogu odrediti područja povećanog rizika od požara. Posebice ako je poznata raspodjela malih i srednjih požara.Forests cover approximately one fourth of the land’s surface. As one of the largest oxygen sources in the nature, they are very important for the survival of life on Earth. Forest fires have become an increasingly interesting issue not only for forestry and ecology, which study them as an important phenomenon for the survival of forests themselves, but also for physics. Physics perceives forest fires as an example of a complex system on large, kilometer-long scales. Faithful computer simulations can answer different questions, such as how fires behave, what influences their propagation, how they follow the power law and most importantly, how fires of different sizes can be predicted. In our work we used the data from the forest administrations of Brač, Korčula and Rab. The data, collected over the time period 1991 – 2000, relate to the number of fires and the size of the burned area. We began with a model in which a fire spreads in a two-dimensional (2D) grid developed by Malamud et al. (1998). There is an accurately defined number of boxes in the grid (Ng), the number of time steps (NS) and the number of fires (NF) for a given fire ignition frequency. Computer simulation modeling provides a burned area AF (AF is the number of trees destroyed in each fire). A non-cumulative number of fires in a defined time period is NF/NS and is given as a function of AF on a 2D grid of 128 x 128 for three frequencies: fS = 1/125, fS = 1/5000, fS = 1/2000. The slope of direction represents the exponent a (the power law applies) which de pends on the frequency. The number of fires for every time interval is the function of the number of trees burned in each of the fires. For every fire propagation frequency there was the NS = 1.638x 109 of time intervals. There is also a range from small to large fires, with the number of small fires far exceeding that of large ones. Small and medium fires satisfy the power law, with . = -1.02 to 1.09, while large fires exhibit bigger deviations (. = -1.16), as manifested at frequency 1/2000 due to the finite grid dimensions. This is the li mited size effect, since the fire stops after it has spread across the entire grid. In our application of the model to the data for Brač, Korčula and Rab, due to the relatively small number of data we used cumulative distribution in order to obtain qualitatively good results. By increasing the initial area interval that contains a given number of fires (A1,………A10), the fire affected area increa ses and so does the number of fires. This provided a distribution of the cumu lative area number NCF for an interval. The results of our research show that the cumulative distribution of burned areas in the selected islands follows the power law in accordance with the model by Malamud et al. (1998). A logarithmic presentation of the results is a direction in its major part. The slope corre sponds to the exponent ., because – dNCF/dAF.AF-.. According to the above model, if we know the parameters of the system we can determine fire propa gation frequency, which indicates the probability of fire occurrence in an area. A data set for the three Croatian islands shows that, in relation to the obtained slope of direction for the total number of fires . = 1.02 ± 0.02, the fire propa gation frequency is high, meaning that the probability of fire propagation is lower. However, a more detailed analysis of the two data sets for larger fires results in a greater slope, indicating a high risk of fire in the next several years, particularly in the areas that have already been severely burned (e.g. as many as 55 km2 on the island of Korčula). The obtained results allow us to conclude that in the applied model, the direction slope values coincide for small and medium fires, i.e. for higher spread frequencies, while the model used for larger fires exhibits deviations due to the finite space dimensions. The results provide a stimulus for further research, because it has been shown that if the impact of different parameters related to fire spread in an area is known, it is possible to identify areas with an increased fire risk, particularly in case of small and medium fire distribution

Temperature hysteresis in dielectric and transport properties of charge density wave system o-TaS3

We report on the temperature hysteresis observed in low frequency dielectric response and in nonlinear conductivity of charge density wave (CDW) system o-TaS$_{3}$. Between CDW transition temperature at 220 K and the glass transition temperature at 50 K both the amplitude and the relaxation time of the low frequency relaxational process are higher on heating than on cooling with similar temperature dependence as the well-known hysteresis in low field resistivity, but different hysteresis width. On the other hand, the hysteresis in the nonlinear conductivity can be seen only as the difference between the initial and subsequent I/V characteristics at given temperature implying field-induced relaxation from history dependent metastable states to stable, history independent state

Fractal nature of hard carbon prepared from C60 fullerene

International audienc

Spin-Reorientation-Driven Linear Magnetoelectric Effect in Topological Antiferromagnet Cu3TeO6

International audienceThe search for new materials for energy-efficient electronic devices has gained unprecedented importance. Among the various classes of magnetic materials driving this search are antiferromagnets, magnetoelectrics, and systems with topological spin excitations. Cu3TeO6 is a material that belongs to all three of these classes. Combining static electric polarization and magnetic torque measurements with phenomenological simulations we demonstrate that magnetic-field-induced spin reorientation needs to be taken into account to understand the linear magnetoelectric effect in Cu3TeO6. Our calculations reveal that the magnetic field pushes the system from the nonpolar ground state to the polar magnetic structures. However, nonpolar structures only weakly differing from the obtained polar ones exist due to the weak effect that the field-induced breaking of some symmetries has on the calculated structures. Among those symmetries is the PT (¯1′) symmetry, preserved for Dirac points found in Cu3TeO6. Our findings establish Cu3TeO6 as a promising playground to study the interplay of spintronics-related phenomena

Detection of Charge Density Wave Ground State in Granular Thin Films of Blue Bronze K0.3MoO3 by femtosecond spectroscopy

During the last years, femtosecond time-resolved spectroscopy (fsTRS) has become an important new tool to investigate low energy excitations in strongly correlated systems. By studying energy relaxation pathways linking various degrees of freedom (e.g., electrons, spin, or lattice), the interaction strengths between different subsystems can be deduced. Here we report on yet another application of fsTRS, where the technique is used to unambiguously determine the nature of the ground state in granular thin films of a prototype charge density wave system blue bronze, K0.3MoO3. These, potassium blue bronze, films, obtained for the first time ever, have been prepared by pulsed laser deposition and investigated by various standard characterization methods. While the results of all used methods indicate that the thin films consist of nanometer grains of K0.3MoO3, it is only the non-destructive fsTRS that demonstrates the charge density wave nature of the ground state. Furthermore, the comparison of the fsTRS data obtained in thin films and in single crystals shows the reduction of the charge density wave transition temperature and of the photoinduced signal strength in granular thin films in respect to single crystals, which is attributed to the granularity and crystal growth morphology