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Promatrana su mikrovalna svojstva tankih filmova niobija orijentiranih paralelno i okomito s obzirom na DC magnetsko polje mjerenjem kompleksnog frekventnog pomaka za uzorak u rezonantnoj šupljini. Uspoređuju&cent ; ; i rezultate za tri različite debljine uzoraka do izražaja dolaze efekti smanjene dimenzionalnosti, što stvara preduvjete za različite teorijske pristupe. U okomitoj orijentaciji [1] najtanji uzorci (10 nm) pokazuju ponašanje slično visokotemperaturnim supravodičima (veliko gornje kritično polje Bc2 i frekvencija otpinjanja omega_0). Također, mjerenja ukazuju da analiza samo apsorpcijske komponente mikrovalnog odziva takvih uzoraka vodi na krive zaključke. Za potpune podatke o otpornosti tečenja virova rho_f potrebna su mjerenja obje komponente kompleksnog frekventnog pomaka. U paralelnoj orijentaciji [2] zanimljivo je promatrati najdeblji uzorak (160 nm) koji omogućuje ulazak dva reda virova. U mjerenju mikrovalnog odziva moguće je opaziti promjene konfiguracije rešetke virova. Ponašanje tog sustava daje informacije o međuinterakciji vorteksa te njihovoj interakciji s rubom uzorka. Rezultati su uspoređeni s razmatranjima ranijih autora. [1] D. Janjušević, Phys. Rev. B 74 (2006) 104501 [2] M. Grbić, Phy. C, u tisku, doi: 10.1016/j.physc.2007.03.415

Metalni oksidi u obliku perovskitnih struktura vjerojatno su najproučavaniji materijali posljednjih desetlje&cent ; ; ; a [1]. Jednu klasu tih materijalačine visokotemperaturni supravodiči temeljeni na ravninama bakrovih oksida, a druga vrlo proučavana klasa materijala temelji se na ravninama oksida magnetskih iona koji su najpoznatiji po kolosalnom magnetootporu. Hibridni materijali RuSr2RCu2O8 objedinjuju te dvije skupine materijala. U njima koegzistiraju dvije vrste, naizgled antagonisti&pound ; ; ; kih, dugodosežnih uredjenja: supravodljivost i feromagnetizam. Detaljnim proučavanjem mikrovalnih svojstava, magnetotransporta te Hallova efekta na nizu uzoraka raznih dopiranja, pokazali smo da se vodljiva i supravodljiva svojstva mogu korelirati s koncentracijom slobodnih šupljina u slojevima bakrovih oksida. Medjutim, za interpretaciju ponašanja magnetootpora i anomalnog Hallova otpora nužno je u razmatranja tenzora vodljivosti uvesti i delokalizirane elektrone u ravninama rutenijevih oksida. Njihov broj je vrlo malen, ali oni osje&cent ; ; ; aju veliku magnetootpornost usporedivu s onom u manganitnim strukturama poznatim po kolosalnom magnetootporu. [1] M. Imada, A. Fujimori, and Y. Tokura, Rev. Mod. Phys. 70, 1039 (1998).

Najpoznatiji visokotemperaturni supravodiči, kuprati rijetkih zemalja, uz supravodljivost pokazuju veliko bogatstvo elektronski uređenih faza čija priroda često nije do kraja razjašnjena. Posebno mjesto tu zauzimaju faze s ’prugastom’ modulacijom spina i/ili naboja ; njihova dinamika i uloga ustvaranju i razaranju supravodljivog uređenja još uvijek su predmet istraživanja i rasprava. Ovdje ćemo prezentirati rezultate istraživanja prugastih i srodnih faza pomoću nuklearne kvadrupolne rezonancije (NQR) u monokristalima reprezentativnih lantanovih kuprata, La_2-x Ba_x Cu O_4 i La_2-x-y Eu_y Sr_x CuO_4. Neočekivano, rezultati se drastično razlikuju od prijašnjih mjerenja na mikronskim prahovima. Intenzitet NQR linije bakra ispod temperatura prijelaza u prugaste faze pokazuje smanjenje koje se opaža i u prahovima(tzv. wipeout), ali je u svim promatranim monokristalima pad signala s temperaturom dramatičan i znatno nagliji nego u prahu: signal pada na razinu šuma već nekoliko K ispod prijelaza. Time dokazujemo da NQR intenzitet bakra ne može biti proporcionalan parametru uređenja - suprotno tvrdnjama na osnovi mjerenja u prahovima - s obzirom da (lokalni) parametar uređenja ne može ovisiti o veličinama kristalnih zrna. Također, iznad temperatura prijelaza određenih neutronskim i rentgenskim raspršenjem u kristalima La_2-x-y Eu_y Sr_x CuO_4 opažamo dodatnu fazu koja do sada nije viđena, u kojoj dolazi do djelomičnog gubitka NQR intenziteta. Priroda te faze je nepoznata, ali pretpostavljamo da se radi o flžuktuirajućim prugama bez dugodosežnog reda (e.g. nabojni nematik) koje su nevidljive za tehnike raspršenja. NQR kao lokalna proba nam time omogućava uvid u do sada slabo istraženu dinamiku taljenja prugastih faza i međufaza u kojima su samo neke simetrije slomljene.

Prostiranje supravodljivih flžuktuacija u području pseudoprocjepa poddopiranih visokotemperaturnih supravodiča proučava se već dulje vrijeme raznim metodama u mnogim svjetskim laboratorijima. Primjenjujući mikrovalnu tehniku(10-20 GHz), pokazali smo da se u optimalno dopiranom i blago poddopiranom supravodiču HgBa2CuO4+δ supravodljive flžuktuacije protežu oko 10 K iznad Tc. U jako poddopiranom YBa2Cu3O6+δ doseg supravodljivih flžuktuacija je oko 20 K iznad Tc. Analogna mjerenja u području THz pokazala su u nizu poddopiranih do naddopiranih tankih filmova LaSrCuO da se supravodljive flžuktuacije protežu oko 10-15 K iznad Tc. U svrhu provjere univerzalnosti dosega supravodljivih žfluktuacija, proveli smo mjerenja na monokristalima LaSrCuO raznih dopiranja sa i bez primjene vanjskog magnetskog polja, te utvrdili da flžuktuacijsko područje prati supravodljivu kupolu kao i u drugim ranije mjerenim visokotemperaturnim supravodičima.

Mjerenja mikrovalne površinske impedancije u magnetskim poljima od 0-16 T upotrijebljena su za utvrdjivanje temperaturnog dosega supravodljivih fluktuacija iznad kritiˇcne temperature Tc u monokristalima YBa2Cu3O7 d raznih dopiranja. Primijenjena metoda ima posebnu prednost u tome što odredjivanje dosega fluktuacija ne ovisi o nekim pretpostavkama o ponašanju nesupravodljivih doprinosa ukupnome mjerenom signalu, niti o pretpostavljenom teorijskom modelu temeljem kojega bi trebalo izvesti nepoznate parametre. Mjerenjem u razliˇcitim konfiguracijama uzorka i mikrovalnog polja, odredjene su zasebno fluktuacijske vodljivosti u ravnini ab i duž osi c. Fluktuacijski režim se proteže do temperature T0, koja se nalazi oko 7-8 K iznad Tc u naddopiranom uzorku i malo poddopiranom uzorku, dok se u jako poddopiranom uzorku (Tc = 57 K) supravodljive fluktuacije protežu do najviše 23 K iznad Tc, što je znatno ispod temperature T na kojoj se otvara pseudoprocjep. Uz to, važno je zapažanje da je eksperimentalno utvrdjena temperatura T0 jednaka za fluktuacijsku vodljivost u ravnini ab kao i onoj duž osi c, što ukazuje na trodimenzionalnu osobinu supravodljivih fluktuacija unatoˇc kvazi-dvodimenzionalne strukture i izrazite anizotropije u normalnoj vodljivosti poddopiranih uzoraka. [1] M. S. Grbi´c, M. Požek, D. Paar, V. Hinkov, M. Raichle, D. Haug, B. Keimer, N. Bariši´c, and A. Dulˇci´c, Phys. Rev. B 83, 144508 (2011).

Microwave absorption measurements in magnetic fields from 0 up to 16 T were used to determine the temperature range of superconducting fluctuations above the superconducting critical temperature Tc in YBa2Cu3O7−δ. Measurements were performed on deeply underdoped, slightly underdoped, and overdoped single crystals. The temperature range of the superconducting fluctuations above Tc is determined by an experimental method which is free from arbitrary assumptions about subtracting the nonsuperconducting contributions to the total measured signal and/or theoretical models to extract the unknown parameters. The superconducting fluctuations are detected in the ab plane, and c-axis conductivity, by identifying the onset temperature T′. Within the sensitivity of the method, this fluctuation regime is found only within a fairly narrow region above Tc. Its width increases from 7 K in the overdoped sample (Tc=89 K) to, at most, 23 K in the deeply underdoped sample (Tc=57 K), so that T′ falls well below the pseudogap temperature T∗. Implications of these findings are discussed in the context of other experimental probes of superconducting fluctuations in the cuprates.

We use microwave absorption technique combined with an applied magnetic field (up to 16 T) to determine the transport properties around Tc in various families of cuprates (YBCO, Hg1201, LSCO, BSCCO). From these measurements we extract the temperature range where superconducting fluctuations start to contribute. In all the measured samples the fluctuation regime was found to be confined relatively close to Tc [1, 2, 3], with a tendency to increase by lowering the doping level. We discuss the results and compare them with those obtained by other techniques. [1] M. S. Grbić, N. Barišić, A. Dulčić, I. Kupčić, Y. Li, X. Zhao, G. Yu, M. Dressel, M. Greven, M. Požek, Phys. Rev. B 80, 094511 (2009) [2] M. S. Grbić, M. Požek, D. Paar, V. Hinkov, M. Raichle, D. Haug, B. Keimer, N. Barišič, A. Dulčić, arXiv:1005.4789v1 [3] M. S. Grbić et al., unpublished

Broadband NMR spectroscopy of correlated electronic systems is nowadays a key experimental technique for determining fundamental properties of new materials applicable in the growing variety of new technologies. We present here a newly established laboratory for broadband NMR/NQR spectroscopy at the University of Zagreb (http://nmr.phy.hr) (Zagreb, Croatia). The laboratory is established and equipped within the European FP7 project SOLeNeMaR1 and with the support of Croatian Ministry of Science, Education and Sport.2 At present, the laboratory is equipped with two Tecmag Apollo spectrometers and an Oxford Instruments 12 tesla wide-bore sweepable magnet of medium homogeneity (10 ppm/cm3). The spectrometers cover frequency range 0.5 – 500 MHz and they are equipped with pulsed 1 kW amplifiers covering the whole frequency range. Cryostats cover temperature range 1.5 K – 400 K with sample space 6.25 cm in diameter, both for zero field measurements (NQR) and for measurements in magnetic field (NMR). The laboratory is open to proposals for scientific cooperation.

Mjerenja mikrovalne apsorpcije u supravodičima predstavljaju važnu metodu za određivanje temeljnih parametara supravodljivog stanja – dubine prodiranja magnetskog polja i duljine koherencije. Međutim, zbog svoje osjetljivosti, mikrovalna mjerenja pružaju mogućnost mnogo detaljnijeg proučavanja svojstava supravodljivih materijala. Ovdje pokazujemo kako uz odabir pravilne geometrije uzorka, u kojoj mikrovalovi gotovo potpuno prodiru u njegovu dubinu, mjerenja postaju iznimno osjetljiva na male promjene vodljivosti koje se javljaju prilikom otvaranja pseudoprocjepa. Takva geometrija također omogućuje opažanje najmanjih naznaka supravodljivosti u obliku fluktuacija, čak i onda kada se one ne mogu opaziti drugim metodama. Kao dodatnu potvrdu da se uistinu radi o supravodljivim fluktuacijama, pokazat ćemo njihovu iznimnu osjetljivost na uključivanje vanjskog statičkog magnetskog polja. [1] M. S. Grbić et al., poslano u tisak [2] M. Požek, I. Kupčić, A. Dulčić, A. Hamzić, D. Paar, M. Basletić, E. Tafra, G. V. M. Williams, Phys. Rev. B 77, 214514 (2008). [3] A. Narduzzo, M. S. Grbić, M. Požek, A. Dulčić, D. Paar, et al., Phys. Rev. B 78, 012507 (2008).

An approach to microwave measurements is used in order to determine both, the in-plane and out-of-plane conductivity of the high-Tc superconductor HgBa2CuO4+δ near optimal doping. Unlike the ab-plane conductivity, the c-axis conductivity is highly sensitive to superconducting fluctuations. From a single c-axis data set, we can clearly discern the opening of the pseudogap at T∗=185(15) K, the appearance of the superconducting fluctuations at a much lower temperature T′=105(2) K, and the full transition to the superconducting state at the critical temperature Tc=94.3 K. Thus, with the present high sensitivity, we establish that the extent of the superconducting fluctuations is only about 10 K above Tc.

Microwave absorption measurements have proven to be a useful tool for the determination of upper critical fields in high-Tc superconductors, even beyond the experimentally reachable dc magnetic fields. The analysis is usually based on the model for effective microwave conductivity in the mixed state of type-II superconductors, which takes into account vortex pinning mechanisms. Here we show microwave measurements of ab- and c-axis conductivity, which is particularly sensitive if the proper sample geometry is chosen. As an example, we have taken a high quality single crystal of HgBa2CuO4+delta, which can be considered as the model single-layered high-Tc superconductor known for its unique property to confine extrinsic disorder away from the superconducting layer. The upper critical fields are determined in two orientations (Bdc || c-axis and Bdc || ab-plane), and the anisotropy is extracted.

Information about the anisotropy of the high-Tc cuprate systems reveals the dimensional properties of the order parameter, i.e. the nature of the superconducting (SC) state. Microwave absorption measurements have proved to be a useful tool for the determination of upper critical fields in high-Tc superconductors, even beyond the experimentally reachable dc magnetic fields. Also, from the phase sensitive complex frequency shift one can extract the ab- and c-axis complex conductivity, which is particularly sensitive if the proper sample geometry is chosen. Here we present the measurements of a high quality single crystal of HgBa2CuO4+δ, which can be considered as the model single-layered high-Tc superconductor, known for its unique property to confine extrinsic disorder away from the superconducting layer, and the determined upper critical fields in two orientations (Bdc || c-axis and Bdc || ab-plane). From the analisys of the in-plane and out-of-plane complex conductivity we determine the onset of the superconducting fluctuations and bring new results regarding the dimensionality of the SC state in the cuprates.

New approach to microwave measurements of ab- and c-axes conductivity in nearly optimally doped HgBa2CuO4+delta single crystals evidence three characteristic temperatures. The pseudogap opens at T* approx 170K which is almost twice the superconducting critical temperature Tc = 94.3 K. The fluctuation regime of preformed Cooper pairs is observed above Tc in the c-axis conductivity, in particular by its magnetic-field dependence. It is confined to a narrow temperature range Tc < T < T' approx 105 K, far below T*, which enables unambiguous separation of T* and T'. Hence, our results support the distinction between the pseudogap formation and effects of the superconducting ordering.

An approach to microwave measurements is used in order to determine both, the in-plane and out-of-plane conductivity of the high-$T_c$ superconductor HgBa$_2$CuO$_{; ; ; 4+\delta}; ; ; $ near optimal doping. Unlike the ab-plane conductivity, the c-axis conductivity is highly sensitive to superconducting fluctuations. From a single c-axis data set, we can clearly discern the opening of the pseudogap at $T^* = 185(15) K$, the appearance of the superconducting fluctuations at a much lower temperature $T' = 105(2) K $, and the full transition to the superconducting state at the critical temperature $T_c=94.3 K$. Thus, with the present high sensitivity, we establish that the extent of the superconductiong fluctuations is only about 10 K above T_c.

Temperature and magnetic field dependent measurements of the microwave surface impedance in the novel superconductor LaO_0.9F_0.1FeAs reveal a very large upper critical field (\Bc2~$\approx 56$T) and a large value of the depinning frequency (f_0 \approx 6GHz) together with the measured effective penetration depth, \lambda_eff=200 \pm 50$nm, our results indicate a stark similarity between this system and the high-T_c superconducting cuprates.

Ru(1-x)SnxSr2EuCu2O8 and Ru(1-x)SnxSr2GdCu2O8 have been comprehensively studied by microwave and dc resistivity and magnetoresistivity and by the Hall measurements. The ruthenium magnetic ordering temperature Tm is considerably reduced with increasing Sn content. However, doping with Sn leads to only slight reduction of the superconducting critical temperature Tc accompanied with the increase of the upper critical field Bc2, indicating an increased disorder in the system and a reduced scattering length of the conducting holes in CuO2 layers. In spite of the increased scattering rate, the normal state resistivity and the Hall resistivity are reduced with respect to the pure compound, due to the increased number of itinerant holes in CuO2 layers, which represent the main conductivity channel. Most of the electrons in RuO2 layers are presumably localized, but the observed negative magnetoresistance and the extraordinary Hall e® ; ; ; ect lead to the conclusion that there exists a small number of itinerant electrons in RuO2 layers that exhibit colossal magnetoresistance.

Complex conductivity of high quality niobium thin films has been investigated by microwave technique in parallel static magnetic field. Mechanism of microwave response substantially changes from ultra-thin to thick film regime. For the thinnest film no vortices can be formed and microwave penetration is defined by strength of superconducting order parameter which varies with applied magnetic field. The thickest film measured allows formation of several rows of vortices. Microwave disipation is dominated by dynamics of vortices which is strongly affected by size effects. Results have been compared with generalised models of complex conductivity for low – dimensional superconductor in mixed state following earlier considerations of other authors. Discussion of dynamical characteristics of vortices is given for measured samples.

Microwave cavity perturbation measurements have been performed on several n-type silicon samples with different depolarization factors due to sample geometries. The general solution for the complex frequency shift in slab geometry is discussed for the specific case of semiconductors. The depolarization crossovers predicted by the theory have been experimentally observed. Their relative intensities suggest that the imaginary part of the complex conductivity of semiconductors has to be taken into account. Electron scattering time has been inferred from the microwave measurements., Načinili smo mjerenja učinka umetanja nekoliko silicijskih pločica tipa n u mikrovalni rezonator s različitim depolarizacijskim faktorima uzrokovanim oblikom pločica. Raspravljamo opće rješenje za kompleksan pomak frekvencije za poseban slučaj poluvodičke pločice. Teorijski predviđene depolarizacijske prijelaze smo eksperimentalno opazili. Njihove relativne jakosti ukazuju da se u poluvodičima mora uzeti u obzir imaginarni dio kompleksne vodljivosti. Na osnovi mikrovalnih mjerenja izveli smo vrijeme elektronskog raspršenja.

Cavity perturbation technique was used to measure temperature and magnetic field dependence of microwave complex frequency shift on high quality niobium thin films in perpendicular static magnetic field. Sample surface was oriented parallel to the microwave electric field and the static magnetic field varied up to the upper critical field. Data have been analysed by effective conductivity model for type II superconductors in mixed state which enabled us to determine depinning frequency of the sample. The analysis showed that depinning frequency is comparable to the microwave driving frequency and decreases with increasing film thickness indicating that pinning is dominated by surface deffects. Values of upper critical fields were determined by means of flow resistivity in low fields and by crossover to the normal state. Comparison of the two methods shows that the former always gives slightly higher upper critical field values, reflecting the Ginzburg– Landau coherence length of nonoverlaping vortices. We have shown that thin niobium films can serve as a model system for determination of upper critical field of high– $kappa$ superconductor from microwave measurements.

There is a general agreement that coherence length in the recently discovered binary compound MgB$_2$ is anisotropic, but a controversy arised on the question whether this anisotropy is temperature dependent or not. We have performed field and temperature dependent microwave measurements on high quality MgB$_2$ thin film in order to put more light into this controversy. The complex microwave conductivity has been analyzed both in the mean-field (MF) regime and in the fluctuation regime. The MF coherence length has been determined much below the transition to the normal state when a vortex contains many Landau levels. In contrast, the Ginzburg-Landau (GL) coherence length was obtained at the transition using the lowest-Landau-level (LLL) scaling of both, the real and imaginary parts of the complex microwave conductivity. The analysis reveals the anisotropy ratio of MF coherence lengths $\gamma_{; ; MF}; ; =\xi_{; ; MF}; ; ^{; ; ab}; ; /\xi_{; ; MF}; ; ^c\approx 2$, and the anisotropy ratio of GL coherence lengths $\gamma_{; ; GL}; ; \approx 2.8$. Both anisotropy ratios have been found to be temperature independent.

Critical fluctuations have been studied in the microwave conductivity of Bi2Sr2CaCu2O8+delta, Bi2Sr2Ca2Cu3O10+delta, and YBa2Cu3O7-delta thin films above T-c. It is found that a consistent analysis of the real and imaginary parts of the fluctuation conductivity can be achieved only if an appropriate wave vector or energy cutoff in the fluctuation spectrum is taken into account. In all of the three underdoped superconducting films one observes strong fluctuations extending far above T-c. The coherence length inferred from the imaginary part of the conductivity exhibits the static critical exponent nu=1 very close to T-c, and a crossover to the region with nu=2/3 at higher temperatures. In parallel, our analysis reveals the absence of the normal conductivity near T-c, i.e., fully opened pseudogap. Following the crossover to the region with nu=2/3, the normal conductivity is gradually recovered, i.e., the closing of the pseudogap is monitored.

Critical fluctuations have been studied in the microwave conductivity of Bi2Sr2CaCu2O8+δ, Bi2Sr2Ca2Cu3O10+δ, and YBa2Cu3O7-δ thin films above T-c. It is found that a consistent analysis of the real and imaginary parts of the fluctuation conductivity can be achieved only if an appropriate wave vector or energy cutoff in the fluctuation spectrum is taken into account. In all of the three underdoped superconducting films one observes strong fluctuations extending far above T-c. The coherence length inferred from the imaginary part of the conductivity exhibits the static critical exponent ν=1 very close to T-c, and a crossover to the region with ν=2/3 at higher temperatures. In parallel, our analysis reveals the absence of the normal conductivity near T-c, i.e., fully opened pseudogap. Following the crossover to the region with ν=2/3, the normal conductivity is gradually recovered, i.e., the closing of the pseudogap is monitored.

The temperature dependence of the coherence lengths in anisotropic and doped superconductors has been studied by the microwave method. We show that one can experimentally make distinction between the coherence lengths in the mixed state, and at the transition to the normal state. In the former case, the vortex involves many Landau levels, whereas in the latter, only the lowest Landau level is left. In MgB2 superconductor, we find that the anisotropy ratio, as determined from the coherence lengths, is not the same deeply in the mixed state and close to the transition to the normal state. These results can be interpreted in terms of the two gap structure of MgB2 which yields superposition of vortices with different radii. In YBCO superconductor, we observed variation of the coherence length with doping. In contrast, to some previous reports in the literature, our results show systematically that lower doping increases the coherence length, i.e. the pairing strength is diminished.

Mjerili smo površinsku impedanciju u optimalno dopiranim i poddopiranim monokristalima YBCO na mikrovalnoj frekvenciji 9.6 GHz. U temperaturnim mjerenjima (od 5 do 200 K) utvrđeno je da optimalno dopirani monokristal ima kritičnu temperaturu supravodljivog prijelaza 90 K, dok poddopirani uzorak ima Tc=58 K. Mjerenjem magnetskih ovisnosti (0 do 8 T) na konstantnim temperaturama odredili smo kompleksni frekventni pomak, a iz njega kompleksnu efektivnu vodljivost. Njezini realni i imaginarni dio sadrže gornje kritično polje i frekvenciju zapinjanja kao parametre koji se lako dobivaju inverzijom eksperimentalnih podataka.

Field and temperature microwave measurements have been carried out on MgB2 thin film grown on Al2O3 substrate. The analysis reveals the mean field coherence length in the mixed state and a temperature independent anisotropy ratio 2. At the superconducting transition, the scaling of the fluctuation conductivity yields the Ginzburg-Landau coherence length with a different anisotropy ratio 2.8, also temperature independent.

Mjerili smo mikrovalnu vodljivost tankog filma MgB2 u ovisnosti o temperaturi i primijenjenome vanjskome magnetskom polju. Dobivene rezultate analizirali smo s pomoću modela efektivne vodljivosti[1]. Odredili smo duljinu koherencije i frekvenciju zapinjanja za svaku vrijednost kompleksne efektivne vodljivosti izmjerenu pri određenoj temperaturi i magnetskom polju. Analiza pokazuje razliku između duljine koherencije srednjeg polja (mean field) xi_{;MF};, određene duboko u miješanom stanju, i Ginzburg-Landauove duljine koherencije $\xi_{;GL};$ u blizini prijelaza u normalno stanje[2]. \xi_{;MF}; opisuje strukturu magnetskog vira koji uključuje mnoštvo Landauovih nivoa dok \xi_{;GL}; opisuje vir koji uključuje samo najniži Landauov nivo. Mjerenjem u dvije orijentacije filma (B||c i B||ab) određivali smo anizotropiju duljine koherencije gamma=xi_{;ab};/xi_c. Dobili smo gamma_{;MF};≈ 2 i gamma_{;GL};≈ 2, 8. Ta razlika može se objasniti dvama supravodljivim procjepima[3]. Gotovo izotropan procjep pi-vrpce doprinosi virovima velikog radijusa, a anizotropan procjep sigma-vrpce virovima malog radijusa. Duboko u miješanom stanju postoje obje vrste virova, a u blizini prijelaza samo virovi manjeg radijusa. [1] A. Dulčić i M. Požek, Physica C 218 (1993) 449 ; [2] A. Dulčić et al., Phys. Rev. B 67 (2003) 020507(R). [3] S. V. Shulga et al., cond-mat/0103154 ; A. Y. Liu et al., Phys. Rev. Lett. 87 (2001) 087005 ; F. Bouquet et al., Phys. Rev. Lett. 89 (2002) 257001.

Field and temperature microwave measurements have been carried out on MgB2 thin film grown on Al2O3 substrate. The analysis reveals the mean field coherence length xi_{MF} in the mixed state and a temperature independent anisotropy ratio gamma_{MF} = xi_{MF}^{ab} / xi_{MF}^c approximately 2. At the superconducting transition, the scaling of the fluctuation conductivity yields the Ginzburg-Landau coherence length with a different anisotropy ratio gamma_{GL} = 2.8, also temperature independent., Comment: submitted to PRL

We have performed susceptibility, thermopower, dc resistance and microwave measurements on RuSr2EuCu2O8. This compound has recently been shown to display the coexistence of both superconducting and magnetic order. We find clear evidence of changes in the dc and microwave resistance near the magnetic ordering temperature (132 K). The intergranular effects were separated from the intragranular effects by performing microwave measurements on a sintered ceramic sample as well as on a powder sample dispersed in an epoxy resin. We show that the data can be interpreted in terms of the normal-state resistivity being dominated by the CuO2 layers with exchange coupling to the Ru moments in the RuO2 layers. Furthermore, most of the normal-state semiconductor-like upturn in the microwave resistance is found to arise from intergranular transport. The data in the superconducting state can be consistently interpreted in terms of intergranular weak-links and an intragranular spontaneous vortex phase due to the ferromagnetic component of the magnetization arising from the RuO2 planes., 20 pages including 6 figures in pdf format. To be published in Phys. Rev. B

Sustavno je proučavana temperaturna i magnetska ovisnost mikrovalne apsorpcije, dc i ac susceptibilnosti, dc otpora, magnetootpora i Hallovog efekta u keramičkom i praškastom uzorku supravodiljivih hibridnih rutenokuprata RuSr2EuCu2O8 i RuSr2GdCu2O8. Mikrovalna mjerenja su napravljena novom metodom za precizno određivanje Q-faktora i frekvencije rezonantne mikrovalne šupljine. U oba keramička uzorka detektiran je lokalni maksimum na temperaturi magnetskog uređenja oko 130 K. On je superponiran na maksimum širine 50 K iznad i ispod 130 K. Mikrovalni otpor u tom području pada s povećanjem vanjskog magnetskog polja. Pojave među zrnima su razdvojene od pojava u zrnima usporedbom mikrovalnih mjerenja u keramičkom i praškastom uzorku u epoxy smoli. Praškasti uzorak nema poluvodičkog porasta sniženjem temperature ispod 120 K, što dokazuje da je taj porast posljedica transporta između zrna. Lokalni maksimum na oko 130 K je prisutan i u praškastom uzorku, što dokazuje da se radi o intrinsičnom svojstvu magnetskog prijelaza. U supravodljivom stanju RuSr2EuCu2O8 (Tc=32 K) pronašli smo dokaz supravodljivih slabih veza u keramičkom uzorku koje prelaze u normalno stanje na poljima iznad 0.5 T. U supravodljivom stanju RuSr2GdCu2O8 (Tc=50 K) mikrovalna apsorpcija na 5 K u nultom polju je veća nego na 8 T. Pokazali smo da se radi o ESR liniji gadolinija koja je superponirana na mikrovalni otpor, kojim na višim poljima dominira gibanje virova u miješanom stanju. Rezultati mjerenja pokazuju da su RuO2 ravnine vodljive, ali ne postaju supravodljive.

Perovskiti su slojevite strukture s općom formulom An+1BnO3n+1 (A je alkalni element, a B prijelazni metal) koje su u posljednjem desetljeću privukle veliku pažnju fizičara čvrstog stanja zbog osebujnosti opaženih fenomena: supravodljivost, gigantski magnetootpor, uređenja spina i naboja, prijelazi metal-izolator s raznim kombinacijama paramagnetske i feromagnetske faze. U pogledu magnetskih svojstava posebno su proučavane serije s manganom (B=Mn) i rutenijem (B=Ru). Djelomične supstitucije u elementu A (npr. Sr - Ca) mogu izazvati dramatične promjene svojstava što pokazuje veliku kompetitivnost pojedinih kolektivnih uređenja. S druge strane imamo seriju bakarnih oksida koji spadaju u proslavljene visokotemperaturne supravodiče na kojima su načinjene brojne karakterizacije, ali još uvijek ostaju otvorena mnoga pitanja. Neobičnost perovskitnih struktura došla je najviše do izražaja u opažanju koegzistencije supravodljivosti i feromagnetizma u hibridnim strukturama rutenat-kuprata. Među inim odlikama visokotemperaturnih supravodiča je i opažanje velikih fluktuacija iznad i ispod faznog prijelaza te mogućnost opažanja kritičnih fluktuacija u samoj blizini kritične temperature Tc. Novija mikrovalna mjerenja pokazuju jasne dokaze o kritičnom području u kompleksnoj vodljivosti.

We have performed ac susceptibility, thermopower, dc resistance and microwave measurements on RuSr_2EuCu_2O_8. This compound has recently been shown to display the coexistence of both superconducting and magnetic order. We find clear evidence of changes in the dc and microwave resistance near the magnetic ordering temperature (132 K). The intergranular effects were separated from the intragranular effects by performing microwave measurements on a sintered ceramic sample as well as on a powder sample dispersed in an epoxy resin. We show that the data can be interpreted in terms of the normal-state resistivity being dominated by the CuO_2 layers with exchange coupling to the Ru moments in the RuO_2 layers. Furthermore, most of the normal-state semiconductor-like upturn in the microwave resistance is found to arise from intergranular transport. The data in the superconducting state can be consistently interpreted in terms of intergranular weak-links and an intragranular spontaneous vortex phase due to the ferromagnetic component of the magnetization arising from the RuO_2 planes.

The microwave response of a thin film of high temperature superconductor YBa2Cu3O7 - δ was measured for a wide region of temperatures and magnetic fields. From the measured complex frequency shift, the complex conductivity was calculated. The model for effective conductivity in the mixed state was fitted to the complex conductivity data and the values of upper critical fields Bc2(T) and depinning frequencies ω0 (T) have been obtained as fitted parameters., Mjerili smo mikrovalni odziv tankog filma visokotemperaturnog supravodiča YBa2Cu3O7−δ u širokom području temperatura i magnetskih polja. Iz izmjerenog kompleksnog frekventnog pomaka izračunali smo kompleksnu vodljivost. Numeričkom prilagodbom modela efektivne vodljivosti u miješanom stanju odredili smo vrijednosti gornjeg kritičnog polja Bc2 (T) i frekvencije opuštanja ω0(T).

Istraživan je mikrovalni odziv tankog filma visokotemperaturnog supravodiča YBCO u magnetskim poljima do 8T i temperaturama iznad 20K. Iz izmjerenog kompleksnog frekventnog pomaka izračunata je kompleksna vodljivost. Oštar minimum u frekventnom pomaku ispod kritične temperature je karakteristika uzoraka tanjih od dubine prodiranja magnetskog polja u normalnom stanju, stavljenih u mikrovalno električno polje. Kompleksna vodljivost u nultom polju pokazuje ponašanje karakteristično za dobre monokristale. Iz ovisnosti kompleksne vodljivosti o magnetskom polju pri raznim temperaturama, numeričkom prilagodbom modela efektivne vodljivosti u miješanom stanju, određene su vrijednosti gornjeg kritičnog polja B_c2(T) i frekvencije otpuštanja omega (T). Dobivena su gornja kritična polja do 70 T, premda je područje polja korištenih u eksperimentu bilo od 0-8 T. Za temperature veće od 80K numerička prilagodba nije dala odgovarajuće rezultate, što ukazuje na široko područje fluktuacija u kojem model srednjeg polja za miješano stanje ne vrijedi. B_c2 iščezava na oko 84K što odgovara minimumu u realnom frekventnom pomaku i kritičnoj temperaturi definiranoj na polovici supravodljivog prijelaza u apsorpcijskom grafu Delta(1/2Q) (T). Rezultat za frekvencije otpuštanja pokazuje da one za mjereni tanki film u cijelom temperaturnom području leže u području 10-30 GHz dokazujući pretpostavku da su mikrovalne frekvencije karakteristika dinamike magnetskih vrtloga u visokotemperaturnim supravodičima.

Using microwave methods one can determine upper critical fields which lie beyond the field of the laboratory magnet, and avoid ambiguities associated with the broadened transitions in high Tc superconductors.

In the mixed state of superconductors (H-c1 much less than H much less than H-c2) the penetration of microwaves is governed by both, complex conductivity sigma(T) and driven oscillation of vortices. In this paper, we show that an effective microwave conductivity can be derived and used to fit the field dependences of the surface resistance R(s). The fit parameter is the upper critical field. Measurements on single crystals YBa2Cu3O7-delta were made in magnetic fields (H parallel to c) up to 2.25 T, and in the temperature range from 70-100 K. The critical temperature for mean field superconductivity appears to be 89.3 K, while the apparent onset in the curve of R(s)(T) appears at about 92 K. The magnetic dependences of R(s) clearly demonstrate that one can separate the regions of mean field superconductivity from the region of fluctuations. [References: 19]

Field dependence of the microwave surface resistance in YBa_2Cu_3O_7-delta single crystal is analysed in terms of the mixed state and superconducting fluctuations. Careful fitting of the appropriate parts of the magnetoresistance curves yields the upper critical field, and mean field T_c. The microwave fluctuations conductivity is found to scale as predicted for a three dimensional (3D) superconductor.

An effective microwave conductivity is derived for the case of type-II superconductors in the mixed state. The ensuing expression for the microwave magnetoresistance predicts a nonlinear behaviour in agreement with the experiments., Izveden je izraz za efektivnu mikrovalnu vodljivost u slučaju miješanog stanja supravodiča druge vrste. Dobiveni izrazi za mikrovalni magnetootpor predviđaju nelinearno ponašanje u skladu s eksperimentalnim opažanjima.

A simple method is developed for the preparation of good quality superconducting samples of the composition Sr_2(Ln, Ce)_2MCu_2O_(10-delta), Ln = Sm, Eu ; M = Nb, Ta. Structural, microwave absorption, magnetic and superconducting properties are reported.

Pripravljena su dva niza uzoraka nominalnog sastava: Y_(1-x)Gd_xBa_2Cu_3O_(7-delta) i YBa_2Cu_(3-x)Gd_xO_(7-delta) (0 manje od x manje od 1) i uspoređena njihova svojstva. Nađeno je da u procesu termičke obrade, uz supravodljivu, nastaju i nesupravodljive faze. Rendgenogrami praha ukazuju na prisutnost BaCuO_2 u sustavu Y_(1-x)Gd_xBa_2Cu_3O_(7-delta) za x veće ili jednako 0.2. Za iste vrijednosti x u uzorcima YBa_2Cu_(3-x)Gd_xO_(7-delta) nastaje više nesupravodljivih faza, kao Y_2BaCuO_5 (ili Gd_2BaCuO_5), BaCuO_2 i treća, neidentificirana faza. Magnetska i mikrovalna mjerenja ukazuju na izraženiju degradaciju supravodljivih svojstava u YBa_2Cu_(3-x)Gd_xO_(7-delta) u usporedbi s Y_(1-x)Gd_xBa_2Cu_3O_(7-delta), u ovisnosti o sadržaju gadolinija. Mjerenje magnetske susceptibilnosti pri niskim temperaturama ukazuje na tendenciju magnetskog uređenja, koje bi potjecalo od faza s većim sadržajem gadolinija.

The effect of partial replacement of copper atoms in the superconducting orthorombic Ba2YCu3O7-delta phase with the 5a group transition metal elements has been studied. It was shown that in the Ba2YCu3-xMxO7-delta (M=V, Nb or Ta) system, the orthorombic phase is stabilized in a wide range of dopant concentrations. A degradation of superconducting properties takes places gradually by increasing the dopant level, this being accompanied by formation of the other phases, i.e. BaVO3, BaV2O8, Ba2YNbO6, Ba2YTaO6 and CuO. X-ray powder diffraction, electron paramagnetic resonance spectroscopy, low field non-resonant microwave absorption, electrical resistance and magnetic susceptibility measurements were correlated.

A paramagnetic probe was placed at various positions on the surface of sintered samples of YBa_2Cu_3O_(7-delta) superconductor. Below the superconducting transition temperature T_c, the EPR line shifts due to local fields which arise on the probe location. The analysis of the EPR spectra can provide information on the induced local fields due to Meissner-Ochsenfeld demagnetization effects and due to pinning effects after exposing the sample to the various fields.

Two different series of samples with the nominal composition Y_(1-x)Gd_xBa_2Cu_3O_(7-delta) and YBa_2Cu_(3-x)Gd_xO_(7-delta) (O less-than or equal to x less-than or equal to 1) were prepared by the solid state reaction. Pronounced sensitivity to Gd content is observed for YBa_2Cu_(3-x)Gd_xO_(7-delta). Besides formation of several impurity phases in this series the transformation of the orthorhombic phase to tetragonal symmetry is observed for x greater-than or equal to 0.1. The ac susceptibility, resistivity and field-dependent microwave absorption measurements show a rapid degradation of superconducting properties as a function of gadolinium content.

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Analizirani su slobodni radiokali u ozračenim kristalima citozina, timina i 1-metiluracila. U sva tri kristala identificirani su slobodni radikali. U kristalima 1-metiluracila i timina prepoznati su spektri parova radikala. Njihova dvospinska struktura kvantnomehanički je interpfetirana.

Radiation-induced free radicals in 1-methyl uracil, studied with the use of EPR spectroscopy, exhibit delocalized spin density in the N(1)-C(6)-C(5) region of the pyrimidine ring

Precise ESR spectra of gamma-irradiated single crystal of cytosine monohydrate demonstrate two distinct groups of the spectral lines, related to according to the crystal symmetry. Spectroscopic parameters for the radical have been determined.