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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.

The Mott insulator κ-(BEDT-TTF)2Ag2(CN)3 forms a highly-frustrated triangular lattice of S = 1/2 dimers with a possible quantum-spin-liquid state. Our experimental and numerical studies reveal the emergence of a slight charge imbalance between crystallographically inequivalent sites, relaxor dielectric response, and hopping dc transport. In a broader perspective we conclude that the universal properties of strongly-correlated charge-transfer salts with spin liquid state are an anion-supported valence band and cyanide-induced quasidegenerate electronic configurations in the relaxed state. The generic low-energy excitations are caused by charged domain walls rather than by fluctuating electric dipoles. They give rise to glassy dynamics characteristic of dimerized Mott insulators, including the sibling compound κ-(BEDT-TTF)2Cu2(CN)3.

We have in detail characterized the anisotropic charge response of the dimer Mott insulator $\ensuremath{\kappa}\text{\ensuremath{-}}(\text{BEDT-TTF}{)}_{2}{\mathrm{Cu}}_{2}(\text{CN}{)}_{3}$ 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 $\ensuremath{\rho}(T)$. 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 have used electric-field-dependent measurements to probe the magnetic-field dependence of the phase-coherence length in the spin-density-wave (SDW) state of the organic conductor tetramethyltetraselenafulvalinium nitrate (TMTSF${)}_{2}$${\mathrm{NO}}_{3}$. The threshold electric field increases if a magnetic field is applied in the plane perpendicular to the highly conducting axis. The magnitude of the increase is angle-dependent and vanishes when the field is applied along the intermediate-conductivity axis. This behavior results from a reduction of the phase-coherence length of the SDW with strongly imperfect nesting in a magnetic field.

We present Hall-effect measurements of two-leg ladder compounds ${\text{Sr}}_{14\ensuremath{-}x}{\text{Ca}}_{x}{\text{Cu}}_{24}{\text{O}}_{41}$ $(0\ensuremath{\le}x\ensuremath{\le}11.5)$ with the aim to determine the number of carriers participating in dc transport. Distribution of holes between the ladder and chain subsystems is one of the crucial questions important for understanding the physics of these compounds. Our Hall-effect and resistivity measurements show typical semiconducting behavior for $xl11.5$. However, for $x=11.5$, the results are completely different, and the Hall coefficient and resistivity behavior are qualitatively similar to that of high-temperature copper-oxide superconductors. We have determined the effective number of carriers at room temperature and compared it to the number of holes in the ladders obtained by other experimental techniques. We propose that going from $x=0$ to $x=11.5$ less than 1 hole per f.u. is added to the ladders and is responsible for a pronounced change in resistivity with Ca doping.

${\text{Ru}}_{1\ensuremath{-}x}{\text{Sn}}_{x}{\text{Sr}}_{2}{\text{EuCu}}_{2}{\text{O}}_{8}$ and ${\text{Ru}}_{1\ensuremath{-}x}{\text{Sn}}_{x}{\text{Sr}}_{2}{\text{GdCu}}_{2}{\text{O}}_{8}$ have been comprehensively studied by microwave and dc resistivity and magnetoresistivity and by the Hall measurements. The ruthenium magnetic ordering temperature ${T}_{m}$ is considerably reduced with increasing Sn content. However, doping with Sn leads to only slight reduction of the superconducting critical temperature ${T}_{c}$ accompanied with the increase in the upper critical field ${B}_{c2}$, indicating an increased disorder in the system and a reduced scattering length of the conducting holes in ${\text{CuO}}_{2}$ 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 ${\text{CuO}}_{2}$ layers, which represent the main conductivity channel. Most of the electrons in ${\text{RuO}}_{2}$ layers are presumably localized, but the observed negative magnetoresistance and the extraordinary Hall effect lead to the conclusion that there exists a small number of itinerant electrons in ${\text{RuO}}_{2}$ layers that exhibit colossal magnetoresistance.

We present the high-temperature $(70\phantom{\rule{0.3em}{0ex}}\mathrm{K}lTl300\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ resistivity anisotropy and Hall effect measurements of the quasi-one-dimensional (1D) organic conductor $(\mathrm{TMTTF}{)}_{2}{\mathrm{AsF}}_{6}$. The temperature variations of the resistivity are pronouncedly different for the three different directions, with metalliclike at high temperatures for the $\mathbf{a}$ axis only. Above $220\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the Hall coefficient ${R}_{H}$ is constant, positive, and strongly enhanced over the expected value; and the corresponding carrier concentration is almost 100 times lower than calculated for one hole/unit cell. Our results give evidence for the existence of a high-temperature regime above $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ where the 1D Luttinger liquid features appear in the transport properties. Our measurements also give strong evidence of charge ordering in $(\mathrm{TMTTF}{)}_{2}{\mathrm{AsF}}_{6}$. At the charge-ordering transition ${T}_{\mathrm{CO}}\ensuremath{\approx}100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, ${R}_{H}(T)$ abruptly changes its behavior, switches sign, and rapidly increases with further temperature decrease.

We report on the growth of heterostructures composed of layers of the high-Curie temperature ferromagnet Co-doped $(\mathrm{La},\mathrm{Sr}){\mathrm{TiO}}_{3}$ (Co-LSTO) with high-mobility ${\mathrm{SrTiO}}_{3}$ (STO) substrates processed at low oxygen pressure. While perpendicular spin-dependent transport measurements in STO//Co-LSTO/LAO/Co tunnel junctions demonstrate the existence of a large spin polarization in Co-LSTO, planar magnetotransport experiments on STO//Co-LSTO samples evidence electronic mobilities as high as $\ensuremath{\sim}{10}^{4}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}∕\mathrm{V}\phantom{\rule{0.2em}{0ex}}\mathrm{s}$ at $T=10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. At high enough applied fields and low enough temperatures $(\ensuremath{\mu}\mathrm{H}\ensuremath{\geqslant}6\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, $T\ensuremath{\leqslant}4\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ Shubnikov--de Haas oscillations are also observed. We present an extensive analysis of these quantum oscillations and relate them with the electronic properties of STO, for which we find large scattering rates up to $\ensuremath{\approx}{10}^{\ensuremath{-}11}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$. Thus this work opens up the possibility to inject a spin-polarized current from a high-Curie temperature diluted oxide into an isostructural system with high mobility and a large spin diffusion length.

The presence of subphases in spin-density wave (SDW) phase of (TMTSF)_2PF_6 below T* \approx 4 K has been suggested by several experiments but the nature of the new phase is still controversial. We have investigated the temperature dependence of the angular dependence of the magnetoresistance in the SDW phase which shows different features for temperatures above and below T* \approx 4 K. For T > 4 K the magnetoresistance can be understood in terms of the Landau quantization of the quasiparticle spectrum in a magnetic field, where the imperfect nesting plays the crucial role. We propose that below T* \approx 4 K the new unconventional SDW (USDW) appears modifying dramatically the quasiparticle spectrum. Unlike conventional SDW the order parameter of USDW depends on the quasiparticle momentum $\Delta_1(\mvk)\propto \cos 2bk_y$. The present model describes many features of the angular dependence of magnetoresistance reasonably well. Therefore, we may conclude that the subphase in (TMTSF)2PF6 below T \approx 4 K is described as SDW plus USDW.

Comprehensive measurements of dc and ac susceptibility, dc resistance, magnetoresistance, Hall resistivity, and microwave absorption and dispersion in fields up to 8 T have been carried out on ${\mathrm{RuSr}}_{2}{\mathrm{GdCu}}_{2}{\mathrm{O}}_{8}$ with the aim to establish the properties of ${\mathrm{RuO}}_{2}$ and ${\mathrm{CuO}}_{2}$ planes. At $\ensuremath{\sim}130 \mathrm{K},$ where the magnetic order develops in the ${\mathrm{RuO}}_{2}$ planes, one observes a change in the slope of dc resistance, change in the sign of magnetoresistance, and the appearance of an extraordinary Hall effect. These features indicate that the ${\mathrm{RuO}}_{2}$ planes are conducting. A detailed analysis of the ac susceptibility and microwave data on both ceramic and powder samples show that the penetration depth remains frequency dependent and larger than the London penetration depth even at low temperatures. We conclude that the conductivity in the ${\mathrm{RuO}}_{2}$ planes remains normal even when superconducting order is developed in the ${\mathrm{CuO}}_{2}$ planes below $\ensuremath{\sim}45 \mathrm{K}.$ Thus, experimental evidence is provided in support of theoretical models which base the coexistence of superconductivity and magnetic order on decoupled ${\mathrm{CuO}}_{2}$ and ${\mathrm{RuO}}_{2}$ planes.