By means of ac magnetic-susceptibility measurements, we find evidence for a new magnetic phase of Tb$_2$Ti$_2$O$_7$ below about 140 mK in zero magnetic field. In magnetic fields parallel to [111], this phase---exhibiting frequency- and amplitude-depe
ndent susceptibility and an extremely slow spin dynamics---extends to about 70 mT, at which it gives way to another phase. The field dependence of the susceptibility of this second phase, which extends to about 0.6 T, indicates the presence of a weak magnetization plateau below 50 mK, as has been predicted by a single-tetrahedron four-spin model, giving support to the underlying proposal that the disordered low-field ground state of Tb$_2$Ti$_2$O$_7$ is a quantum spin ice.
We have carried out tilt magnetic field (B) studies of the u=12/5 fractional quantum Hall state in an ultra-high quality GaAs quantum well specimen. Its diagonal magneto-resistance Rxx shows a non-monotonic dependence on tilt angle (theta). It first
increases sharply with increasing theta, reaches a maximal value of ~ 70 ohms at theta ~ 14^o, and then decreases at higher tilt angles. Correlated with this dependence of Rxx on theta, the 12/5 activation energy (Delta_{12/5}) also shows a non-monotonic tilt dependence. Delta_{12/5} first decreases with increasing theta. Around theta = 14^{o}, Delta_{12/5} disappears as Rxx becomes non-activated. With further increasing tilt angles, Delta_{12/5} reemerges and increases with theta. This tilt B dependence at u=12/5 is strikingly different from that of the well-documented 5/2 state and calls for more investigations on the nature of its ground state.
Specific heat and ac magnetic susceptibility measurements, spanning low temperatures ($T geq 40$ mK) and high magnetic fields ($B leq 14$ T), have been performed on a two-dimensional (2D) antiferromagnet Cu(tn)Cl$_{2}$ (tn = C$_{3}$H$_{10}$N$_{2}$).
The compound represents an $S = 1/2$ spatially anisotropic triangular magnet realized by a square lattice with nearest-neighbor ($J/k_{B} = 3$ K), frustrating next-nearest-neighbor ($0 < J^{prime}/J < 0.6$), and interlayer ($|J^{prime prime}/J| approx 10^{-3}$) interactions. The absence of long-range magnetic order down to $T = $ 60 mK in $B = 0$ and the $T^{2}$ behavior of the specific heat for $T leq 0.4$ K and $B geq 0$ are considered evidence of high degree of 2D magnetic order. In fields lower than the saturation field, $B_{text{sat}} = 6.6$ T, a specific heat anomaly, appearing near 0.8 K, is ascribed to bound vortex-antivortex pairs stabilized by the applied magnetic field. The resulting magnetic phase diagram is remarkably consistent with the one predicted for the ideal square lattice, except that $B_{text{sat}}$ is shifted to values lower than expected. Potential explanations for this observation, as well as the possibility of a Berezinski-Kosterlitz-Thouless (BKT) phase transition in a spatially anisotropic triangular magnet with the N{e}el ground state, are discussed.
The quantum Hall plateau transition was studied at temperatures down to 1 mK in a random alloy disordered high mobility two-dimensional electron gas. A perfect power-law scaling with kappa=0.42 was observed from 1.2K down to 12mK. This perfect scalin
g terminates sharply at a saturation temperature of T_s~10mK. The saturation is identified as a finite-size effect when the quantum phase coherence length (L_{phi} ~ T^{-p/2}) reaches the sample size (W) of millimeter scale. From a size dependent study, T_s propto W^{-1} was observed and p=2 was obtained. The exponent of the localization length, determined directly from the measured kappa and p, is u=2.38, and the dynamic critical exponent z = 1.
The zero-field temperature-dependence of the resistivity of two-dimensional holes are observed to exhibit two qualitatively different characteristics for a fixed carrier density for which only the metallic behavior of the so-called metal-insulator tr
ansition is anticipated. As $T$ is lowered from 150 mK to 0.5 mK, the sign of the derivative of the resistivity with respect to $T$ changes from being positive to negative when the temperature is lowered below $sim$30 mK and the resistivity continuously rises with cooling down to 0.5 mK, suggesting a crossover from being metal-like to insulator-like.
