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We investigate the dc resistivity and Hall effect of the quasi-two-dimensional organic materials κ–(ET)2X, where X=Ag2(CN)3 and B(CN)4 and compare them with the results for X=Cu2(CN)3. All three compounds are considered to be quantum-spin-disordered Mott insulators. Despite high similarities in chemical composition and crystal structure, large differences in the dc resistivity and Hall coefficient are found. While around room temperature the dc transport properties are dominantly determined by the strength of the electron correlations, upon reducing the temperature, dc transport happens by hopping due to inherent disorder. The most disordered compound with X=Cu2(CN)3 turns out to have the lowest dc resistivity and the highest charge carrier density, i.e., in the phase diagram it is located closest to the metal-insulator transition. The least disordered compound with X=B(CN)4 shows the highest resistivity and the lowest carrier density, i.e., lies farthest from the metal-insulator transition. We explain such counterintuitive behavior within the theory of Mott-Anderson localization as a consequence of disorder-induced localized states within the correlation gap.

(TMTTF)2ReO4 i (TMTTF)2BF4 pripadaju u skupinu takozvanih Fabreovih soli koje pri sobnoj temperaturi većinski pokazuju kvazi 1D metalnu vodljivost. Variranjem raznih parametara poput tlaka, temperature te vrste aniona, dobiven je vrlo bogat fazni dijagram Fabreovih soli u kojem su neka od mogućih stanja: metalno stanje, stanje uređenja naboja, spin-Peierlsovo stanje, supravodljivo stanje i stanje vala gustoće spina[1]. Od prije je poznato da navedena dva spoja pri sobnoj temperaturi pokazuju kvazi 1D metalnu vodljivost te da spuštanjem temperature ispod 230 K za (TMTTF)2ReO4, odnosno 84 K za (TMTTF)2BF4, oba spoja prolaze kroz metal-izolatorski fazni prijelaz u izolatorsko stanje uređenja naboja. Također, oba spoja spadaju u skupinu soli s nesimetričnim anionima pa daljnjim spuštanjem temperature ispod 157 K za (TMTTF)2ReO4, odnosno 41.5 K za (TMTTF)2BF4, dolazi do zamrzavanja orijentacije aniona i loma simetrije pa oba spoja prolaze kroz izolator-izolatorski fazni prijelaz u stanje uređenja aniona[2]. U cilju boljeg razumijevanja transportnih mehanizama, mjerili smo otpornost i Hallov koeficijent RH u ovisnosti o temperaturi. Otpornost je mjerena u smjeru najveće vodljivosti, kao i RH (magnetsko polje je okomito na smjer struje) koji je mjeren AC kompenzacijskom metodom u magnetskim poljima do 9 T te za temperature 45 K< T < 300 K. Oba spoja pri sobnoj temperaturi imaju približno jednaku vrijednost RH koja je oko 3 puta veća od teorijski predviđene vrijednosti. RH je u oba spoja pozitivan što je u skladu s činjenicom da su nosioci naboja šupljine. Fazni prijelaz u stanje uređenja naboja posebno je naglašen u (TMTTF)2ReO4 gdje smo detektirali naglu promjenu predznaka RH u uskom intervalu temperatura. Daljnjim opadanjem temperature ispod faznog prijelaza u stanje uređenja naboja, porast RH u oba spoja prati eksponencijalni porast otpornosti što odgovara ponašanju izolatora kod kojih je ovakvo ponašanje posljedica smanjenja broja nosioca naboja sa snižavanjem temperature. Dobivene rezultate ćemo usporediti sa rezultatima srodnih spojeva te interpretirati u okviru postojećih teorijskih modela. [1] M. Dressel, Naturwissenschaften 94, 527 (2007). [2] B. Köhler, E. Rose, M. Dumm, G. Untereiner, and M. Dressel, Phys. Rev. B 84, 035124 (2011)

We have in detail characterized the anisotropic charge response of the dimer Mott insulator κ−(BEDT-TTF)2Cu2(CN)3 by dc conductivity, Hall effect, and dielectric spectroscopy. At room temperature, the Hall coefficient is positive and close to the value expected from stoichiometry; the temperature behavior follows the dc resistivity ρ(T). Within the planes the dc conductivity is well described by variable-range hopping in two dimensions; this model, however, fails for the out-of-plane direction. An unusually broad in-plane dielectric relaxation is detected below about 60 K; it slows down much faster than the dc conductivity following an Arrhenius law. At around 17 K, we can identify a pronounced dielectric anomaly concomitantly with anomalous features in the mean relaxation time and spectral broadening. The out-of-plane relaxation, on the other hand, shows a much weaker dielectric anomaly; it closely follows the temperature behavior of the respective dc resistivity. At lower temperatures, the dielectric constant becomes smaller both within and perpendicular to the planes; also, the relaxation levels off. The observed behavior bears features of relaxorlike ferroelectricity. Because heterogeneities impede its long-range development, only a weak-tunneling-like dynamics persists at low temperatures. We suggest that the random potential and domain structure gradually emerge due to the coupling to the anion network.

We present Hall-effect measurements of two-leg ladder compounds Sr14− xCaxCu24O41 (0

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.

Poznavanje Hallovog koeficijenta RH omogucuje karakterizaciju metala i poluvodica, jer odražava strukturu vrpce i tip nosilaca naboja. S druge strane, kvantitativno razumijevanje Hallovog koeficijenta jako koreliranih sustava (te visokotemperaturnih supravodica) nije još u potpunosti postignuto zbog njegove kompleksne ovisnosti o temperaturi i drugim parametrima od kojih se ocekuju kljucne informacije o osnovnim nekonvencionalnim elektronskim stanjima. Sr14-xCuxCu24O41 su kvazi-1D kuprati u kojima se naboji i spinovi uredjuju unutar podsustava lanaca i ljestvica [1]. Otkrice supervodljivosti pod tlakom (za x > 10) potaknulo je intenzivna istraživanja, jer je to prvi supervodljivi kuprat koji ne sadrži CuO2 ravnine. Pitanje dinamike naboja i raspodjele dopiranih šupljina izmedju podsustava ljestvica i lanaca je još uvijek otvoreno i razlicite eksperimentalne tehnike su dale kontradiktorne rezultate. Naši rezultati mjerenja RH i elektricne otpornosti pokazuju tipicno poluvodicko ponašanje (za x < 10). Odredili smo efektivni broj nosioca naboja i njegovu ovisnost o sastavu (x) te ga usporedili s brojem šupljina u sustavu ljestvica (dobivenim drugim mjerenjima). Za x = 11.5, koji pokazuje metalno ponašanje, naš rezultat kvalitativno odgovara ponašanju Hallovog koeficijenta za visokotemperaturne supravodice. [1] T. Vuletic, et al., Phys. Rep. 428, (2006.) 169

We have studied the current-voltage characteristics of the (TMTSF)2PF6 in the spin density state (SDW), and in zero and finite external magnetic field. For the oscillating part of the nonlinear voltage response to the applied DC electric field, the fundamental frequency distribution (as a function of this field) and a nonlinear relation between the frequency and the SDW current reveal the growth of parallel conduction channels characterized by lower velocities and larger cross-sections. The number of fundamental frequencies, their amplitude and the level of low-frequency noise as well as the depinning behaviour provide a consistent indication of the sample inhomogeneities and associated local field variations, and might be well understood within the framework of the phase slippage model. The increase of the threshold electric field with the applied magnetic filed can be explained by the Bjeli-Maki theory, if the imperfect nesting is taken into account. Finally, the electric-field dependence of the Hall resistivity is consistent with the sliding mechanism of the SDW conduction., Proučavali smo karakteristike napon – struja materijala (TMTSF)2PF6 u stanju valova spinske gustoće (SDW). Za oscilatorni dio nelinearnog naponskog odziva na istosmjerno električno polje, osnovna frekventna raspodjela (kao funkcija tog polja) i nelinearan odnos frekvencije i SDW struje pokazuju rast usporednih kanala vođenja označenih manjim brzinama i većim udarnim presjecima. Broj osnovnih frekvencija, njihove amplitude i razina niskofrekventnog šuma, kao i otkočno ponašanje sustavno pokazuju na nehomogenosti uzorka i pridružene varijacije lokalnog polja, i mogu se shvatiti u okviru modela klizne faze. Rast praga električnog polja s magnetskim poljem može se protumačiti Bjeliš-Makijevom teorijom, ako se uzme u obzir ugniježdenje. Konačno, nalazimo da je ovisnost Hallovog otpora o električnom polju u skladu s kliznim mehanizmom SDW vođenja struje.