Intrinsic Pinning in the High Field C-Phase of UPt_3

We report on the a.c. magnetic response of superconducting UPt_3 in a d.c. magnetic field. At low fields (H < H^*), the in-phase susceptibility shows a sharp drop at $T_c$ followed by a gradual decrease with decreasing temperature, while the out-of-phase component shows a large peak at T_c followed by an unusual broad peak. As the B-C phase line is crossed (H>H^*), however, both the in-phase and out-of-phase susceptibilities resemble the zero-field Meissner curves. We interpret these results in terms of a vortex pinning force which, while comparatively small in the A/B-phases, becomes large enough to effectively prevent vortex motion in the C-phase.Comment: Modified discussion, slight changes to figures, accepted in PRB Rapid Communications. RevTex file, 5 figure

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Field-dependent specific heat and multiple superconducting phases in UPt_3

We have measured the specific heat, C, of single-crystal UPt_3 in the superconducting regime as a function of temperature, T, and magnetic field, H, parallel to the c axis. We find that C(T) at fixed H<H_(c2) shows no evidence for different superconducting states. In contrast, our field-sweep data, C(H) at fixed T, have sharp changes in slope at H≊H_(c2)/2. The phase diagram deduced from these features agrees with neutron-scattering and torsional-oscillator results on the same samples. These thermodynamic measurements as a function of magnetic field constrain theories of exotic superconductivity in UPt_3

From classical to quantum glass

We study the effects of a transverse magnetic field on the dynamics of the randomly diluted, dipolar coupled, Ising magnet LiHo_(0.167)Y_(0.833)F_4. The transverse field mixes the eigenfunctions of the ground-state Ising doublet with the otherwise inaccessible excited-state levels. We observe a rapid decrease in the characteristic relaxation times, large changes in the spectral form of the relaxation, and a depression of the spin-glass transition temperature with the introduction of quantum fluctuations

Second harmonic light scattering induced by defects in the twist-bend nematic phase of liquid crystal dimers

The nematic twist-bend (NTB) phase, exhibited by certain thermotropic liquid crystalline (LC) dimers, represents a new orientationally ordered mesophase -- the first distinct nematic variant discovered in many years. The NTB phase is distinguished by a heliconical winding of the average molecular long axis (director) with a remarkably short (nanoscale) pitch and, in systems of achiral dimers, with an equal probability to form right- and left-handed domains. The NTB structure thus provides another fascinating example of spontaneous chiral symmetry breaking in nature. The order parameter driving the formation of the heliconical state has been theoretically conjectured to be a polarization field, deriving from the bent conformation of the dimers, that rotates helically with the same nanoscale pitch as the director field. It therefore presents a significant challenge for experimental detection. Here we report a second harmonic light scattering (SHLS) study on two achiral, NTB-forming LCs, which is sensitive to the polarization field due to micron-scale distortion of the helical structure associated with naturally-occurring textural defects. These defects are parabolic focal conics of smectic-like ``pseudo-layers", defined by planes of equivalent phase in a coarse-grained description of the NTB state. Our SHLS data are explained by a coarse-grained free energy density that combines a Landau-deGennes expansion of the polarization field, the elastic energy of a nematic, and a linear coupling between the two

Symmetry of the order parameter in superconducting ZrZn_2

We apply symmetry considerations to study the possible superconducting order parameters in ferromagnetic ZrZn_2. We predict that the presence and the location of the superconducting gap nodes depend on the direction of magnetization M. In particular, if M is directed along the z axis, then the order parameter should always have zeros. We also discuss how to determine the gap symmetry in ZrZn_2 using ultrasound attenuation measurements.Comment: 6 pages, submitted to PRB; some corrections and discussion adde

Coulomb blockade and quantum tunnelling in the low-conductivity phase of granular metals

We study the effects of Coulomb interaction and inter-grain quantum tunnelling in an array of metallic grains using the phase-functional approach for temperatures $T$ well below the charging energy $E_{c}$ of individual grains yet large compared to the level spacing in the grains. When the inter-grain tunnelling conductance $g\gg1$, the conductivity $\sigma$ in $d$ dimensions decreases logarithmically with temperature ($\sigma/\sigma_{0}\sim1-\frac{1}{2\pi gd}\ln(gE_{c}/T)$), while for $g\to0$, the conductivity shows simple activated behaviour ($\sigma \sim \exp(-E_c/T)$). We show, for bare tunnelling conductance $g \gtrsim 1$, that the parameter $\gamma \equiv g(1-2/(g\pi)\ln(gE_{c}/T))$ determines the competition between charging and tunnelling effects. At low enough temperatures in the regime $1\gtrsim \gamma \gg 1/\sqrt{\beta E_{c}}$, a charge is shared among a finite number $N=\sqrt{(E_{c}/T)/\ln(\pi/2\gamma z)}$ of grains, and we find a soft activation behaviour of the conductivity, $\sigma\sim z^{-1}\exp(-2\sqrt{(E_{c}/T)\ln(\pi/2\gamma z)})$, where $z$ is the effective coordination number of a grain.Comment: 11 pages REVTeX, 3 Figures. Appendix added, replaced with published versio