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The electronic properties of graphene are described by a Dirac Hamiltonian with a four-fold symmetry of spin and valley. This symmetry may yield novel fractional quantum Hall (FQH) states at high magnetic field depending on the relative strength of symmetry-breaking interactions. However, observing such states in transport remains challenging in graphene, as they are easily destroyed by disorder. In this work, we observe in the first two Landau levels the two-flux composite-fermion sequences of FQH states between each integer filling factor. In particular, the odd-numerator fractions appear between filling factors 1 and 2, suggesting a broken-valley symmetry, consistent with our observation of a gap at charge neutrality and zero field. Contrary to our expectations, the evolution of gaps in a parallel magnetic field suggests that states in the first Landau level are not spin-polarized even up to very large out-of-plane fields.

We present the results of magnetotransport and magnetic torque measurements on the $\ensuremath{\alpha}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}\mathrm{KHg}(\mathrm{SCN}{)}_{4}$ charge-transfer salt within its high-magnetic-field phase, in magnetic fields extending to 33 T and temperatures as low as 27 mK. While the experimentally determined phase diagram closely resembles that predicted by theoretical models for charge-density waves in strong magnetic fields, the phase that occurs at fields above $\ensuremath{\sim}23 \mathrm{T},$ which is expected to be either a modulated charge-density wave phase or a charge/spin-density wave hybrid, exhibits unusual physical properties that are most atypical of a density wave ground state. Notably, the resistivity undergoes a dramatic drop below $\ensuremath{\sim}3 \mathrm{K}$ within this phase, falling in an approximately exponential fashion at low temperatures, while the magnetic torque undergoes extensive hysteresis. This hysteresis, which occurs over a broad range of fields and gives rise to a large negative differential susceptibility $\ensuremath{\partial}M/\ensuremath{\partial}B$ on reversing the direction of sweep of the magnetic field, is strongly temperature dependent and also has several of the physical characteristics predicted by critical-state models normally used to describe the pinning of vortices in type II superconductors. Such a behavior appears therefore only to be explained consistently in terms of persistent currents within the high-magnetic-field phase of $\ensuremath{\alpha}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}\mathrm{KHg}(\mathrm{SCN}{)}_{4},$ although the origin of these currents remains an open question.

The quantum Hall effect is investigated in the organic conductor (TMTSF${)}_{2}$P${\mathrm{F}}_{6}$ (TMTSF is tetramethyltetraselenafulvalene) under hydrostatic pressure. Under a pressure of $p\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}9$ kbar, the sequence of the quantum Hall numbers $L\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}{\ensuremath{\sigma}}_{\mathrm{xy}}/({\ensuremath{-}2e}^{2}/h)$ with increasing magnetic field is regular: $L\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\dots{},3,2,1,\dots{}$ as reported earlier. At $p\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}8.5$ kbar, however, the irregular sequence $L\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\dots{},3,\ensuremath{-}2,2,1,\dots{}$ is found. The temperature--magnetic field phase diagram is investigated in detail. We discuss our findings in the framework of recent theories of the quantum Hall effect in magnetic-field-induced spin-density waves.

We report magnetotransport measurements in the quasi one dimensional (Q-1-D) organic conductor (TMTTF) 2 Br at pressures up to 26 kbar, down to 0.45 K in magnetic fields up to 19 T along the c * direction. It is found that a superconducting ground state is stabilized under 26 kbar at T c =0.8 K. No magnetic field induced spin density wave (FISDW) transitions are observed below 19T unlike other Q-1-D superconductors pertaining to the selenium series. The computed amplitude of the interchain coupling along transverse directions is unable to explain the missing FISDW instability

The triple-layered ${\mathrm{Sr}}_{4}{\mathrm{Ru}}_{3}{\mathrm{O}}_{10}$ is characterized by an unexpectedly strong quasi-two dimensional characteristic, and a sharp metamagnetic transition and ferromagnetic behavior occurring within the basal plane and along the c axis, respectively. The interplane resistivity at magnetic field B, up to 32 T, exhibits very low-frequency quantum oscillations associated with the spin polarized state when B is parallel to the c axis, and a large magnetoresistivity accompanied by critical fluctuations governed by the metamagnetic transition when B is perpendicular to the c axis. The complex behavior evidenced in magnetization, specific heat, and resistivity presented is not characteristic of any obvious ground states, and points to a unusual state that shows a delicate balance between fluctuations and order. Implications of these results are discussed.

We report the interplane resistivity, ${\ensuremath{\rho}}_{c},$ at high magnetic fields B, with different orientations together with structural and magnetic properties of bilayered ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7},$ a Mott-like system with a gap of 0.1 eV. A wide array of conventionally unanticipated phenomena revealed in this work includes (1) a collapse of the c-axis lattice parameter at a metal-nonmetal transition, (2) quantum oscillations in ${\ensuremath{\rho}}_{c}$ in the gapped, nonmetallic state for the $B\ensuremath{\parallel}c$ axis, (3) interplane tunneling magnetoresistivity for the $B\ensuremath{\parallel}a$ or b axis, and yet conspicuously different anisotropies of the colossal magnetoresistivity and magnetization, and (4) a non-Fermi-liquid behavior in a metallic state fully recovered in high magnetic fields. The implications of the coexistence of these conflicting phenomena are discussed.

We present and analyze the normal phase temperature dependence of resistivity and nuclear spin-lattice relaxation of $(\mathrm{TMTSF}{)}_{2}{\mathrm{ClO}}_{4}$ under strong transverse magnetic fields. From a field-dependent scaling theory we show that the singular enhancement of the one-dimensional electron-electron umklapp scattering and antiferromagnetic spin correlations under field play a fundamental role in the large enhancement of magnetoresistance and nuclear relaxation of Bechgaard salts.

Summary form only given. We studied the Fermi surface of the organic conductor (TMTSF)/sub 2/NO/sub 3/ through the measurements of magnetic quantum oscillations. Two different families of oscillations are observed. Well defined Shuvnikov-de Haas(SdH) oscillations with frequency of ca. 60T can be observed below 8K and above 6T at the ambient pressure. The higher frequency oscillations reported elsewhere is not visible be low 12T. The angular magnetoresistance does not show any structure neither at Yamaji's magic angles nor at Lebed's resonance condition. Only the SdH oscillations for the field scaled by Cos /spl theta/ are added and easily separated. Under a pressure of 10kbar, different oscillations with frequency of ca. 200T is observed above 10T at 450mK. Even though it is the same frequency as so called rapid oscillations observed in other Bechgaard salts, angular dependence of oscillation frequency does not follow the simple cosine law contrary to that of (TMTSF)/sub 2/CIO/sub 4/. The amplitude of oscillations depend very oddly with sample orientation, too. Both suggest that the electron structure at low temperature is strongly modified upon application of pressure which may explain the absence of superconductivity and the field-induced spin-density waves. The angular magnetoresistance shows actually three maxima at angles which are not determined from the known theories.

Summary form only given. The Hall resistance as a function of the current along the conducting chains is investigated in the magnetic-field-induced spin-density wave (FISDW) phases of (TMTSF)/sub 2/PF/sub 6/, (TMTSF stands for tetramethyltetraselenafulvalene). Recently, we have reported/sub 1/ a nonlinear Hall resistance decreasing strongly with increasing current above a well-defined threshold current. Now we present strong evidence that the nonlinearity arises from the depinning of the FISDW. We show that by applying a radio-frequency excitation superimposed on a dc bias, interference features appear in the differential Hall resistance whwnever the phase change rate of the sliding FISDW is equal to the frequency of the rf excitation (Shapiro interference).

We studied the Fermi surface of the organic conductor (TMTSF) 2 NO 3 through the measurements of magnetic quantum oscillations. Well defined Shuvnikov-de Haas(SdH) oscillations with frequency of ca. 60T can be observed below 8K and above 6T at the ambient pressure. Under a pressure of 8kbar, different oscillations with frequency of ca. 180T is observed above 10T at 450mK. Electron structure at low temperature seems strongly modified upon application of pressure which may explain the absence of superconductivity and the field-induced spin-density waves. The angular magnetoresistance could not be fit neither by Yamaji's model nor by Lebed's.