Showing papers by "David A. Huse published in 1995"

Frustration induced spin freezing in a site-ordered magnet: Gadolinium gallium garnet.

Abstract: We measured the low-temperature ac magnetic susceptibilities ( ${\ensuremath{\chi}}^{\ensuremath{'}}$, ${\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}$, and ${\ensuremath{\chi}}_{3}$), static magnetization, and the specific heat of a high-quality single crystal of gadolinium gallium garnet (GGG), which is a geometrically frustrated Heisenberg magnet. We find a spin glass transition at low temperatures, in addition to unusual behavior in ${\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}$ and ${\ensuremath{\chi}}_{3}$. Comparison to nonmagnetic isomorphs and the low levels of structural and chemical disorder in GGG suggests that the transition is due to the highly frustrated geometry of the magnetic lattice.

Equilibrium behaviour of quantum Ising spin glass

Abstract: A phenomenological theory of the ordered phase of short-range quantum Ising spin glass is developed in terms of droplet excitations, and presented in detail. These excitations have free energies that provene from an interplay between the classical excitation energies ϵL, with a broad distribution whose characteristic magnitude grows with length scale L as Lθ, and quantum tunneling rates ΓL, decreasing faster than exponentially with L. At temperature T = 0, the equal-time spatial correlations due to quantum fluctuations do not show the power-law decay that occurs for T > 0 due to thermal fluctuations, but ather an exponential decay of Ornstein-Zernicke type. At finite, but still very small T, there exists a crossover length scale L ∗ (T), set by ħΓ L∗(T)⋍ kBT, below which the droplets behave quantum-mechanically and above which the behaviour is essentially classical. As for the classical spin glass, only a small fraction of droplets are thermally active at low T; it is shown that many of the low-T static properties are dominated by the thermally active droplets at the crossover length L ∗ (T) . The uniform static linear susceptibility is found to diverge at T = 0 below the lower critical dimension, d l , and to be finite above d l . The static nonlinear susceptibility diverges in all dimensions, d. The zero temperature linear ac susceptibility χ(ω) is dominated by droplets at a length scale such that ω is of order the characteristic frequency of the quantum system. The behaviour near to the quantum critical point is discussed within a conventional scaling framework if it is approached at strictly zero temperature as well as from finite T. Implications of the existence of Griffiths singularities at the critical point and the disordered phase are pointed out: In the disordered phase, Griffiths singularities dominate the low-T specific heat and the long-time correlations.

Structural evidence for a two-step process in the depinning of the superconducting flux-line lattice

Abstract: A TYPE II superconductor in a magnetic field is penetrated by a hexagonal lattice of quantized flux lines. An applied current imposes a Lorentz force on these lines, but motion of the lattice will always be inhibited by pinning to material defects. Beyond a certain 'critical' current density, the lattice can break free of its pins and flow, dissipating energy and destroying superconductivity in the sample. The microscopic nature of this process is still poorly understood; in particular, little is known about the detailed structure of the flux-line lattice as it begins to depin and flow in response to the applied current. We have used small-angle neutron scattering1a¤-3 to image the structure of the flux lattice in NbSe2 in the presence of a direct current, while also measuring the transport properties. Our observations of the structure of the flux lattice near the critical current verify theoretical predictions4 of the existence of three regimes as a function of increasing driving force (or current): first, no motion; then disordered, plastic motion; and finally, at high velocities, a coherently moving flux crystal.

Nonlocal conductivity in type-ii superconductors

Abstract: Multiterminal transport measurements on YBa[sub 2]Cu[sub 2]O[sub 7] crystals in the vortex liquid regime have shown nonlocal conductivity on length scales up to 50 microns. Motivated by these results we explore the wave vector ([bold k]) dependence of the dc conductivity tensor, [sigma][sub [mu][nu]]([bold k]), in the Meissner, vortex lattice, and disordered phases of a type-II superconductor. Our results are based on time-dependent Ginzburg-Landau (TDGL) theory and on phenomenological arguments. We find four qualitatively different types of behavior. First, in the Meissner phase, the conductivity is infinite at [ital k]=0 and is a continuous function of [ital k], monotonically decreasing with increasing [ital k]. Second, in the vortex-lattice phase, in the absence of pinning, the conductivity is finite (due to flux flow) at [ital k]=0; it is discontinuous there and remains qualitatively like the Meissner phase for [ital k][gt]0. Third, in the vortex liquid regime in a magnetic field and at low temperature, the conductivity is finite, smooth and [ital nonmonotonic], first increasing with [ital k] at small [ital k] and then decreasing at larger [ital k]. This third behavior is expected to apply at temperatures just above the melting transition of the vortex lattice, where the vortex liquid showsmore » strong short-range order and a large viscosity. Finally, at higher temperatures in the disordered phase, the conductivity is finite, smooth and again monotonically decreasing with [ital k]. This last, monotonic behavior applies in zero magnetic field for the entire disordered phase, i.e., at all temperatures above [ital T][sub [ital c]], while in a field the nonmonotonic behavior may occur in a low-temperature portion of the disordered phase.« less

Yaron et al. reply.

Abstract: Yaron et al. Reply: The Comment of Giamarchi and Le Doussal (GLD) [1] focuses on the difference between two length scales that characterize flux line lattices (FLL): the Larkin-Ovchinnikov pinning length L„derived from the critical current, and the positional correlation length L„ as measured by the linewidth of a small angle neutron scattering (SANS) experiment or by direct analysis of Bitter decorations. Our original paper [2] made the naive assertion that L, = I, GLD point out that this is not intrinsic, but that the experimental observation of the near equality of L, and L, could arise from nonequilibrium dislocations in a field-cooled experiment. Their Comment provides fresh insight into the different length scales characterizing the FLL, although there appear to be some inconsistencies with our most recent data [3]. The Larkin-Ovchinnikov theory assumes that the deformations of the FLL are purely elastic. However, we agree with GLD's assertion that topological defects are likely to be the key to the description of the FLL for the range of parameters that we have studied. While their Comment focused on SANS data, we have also used real space decoration images in our analysis of I,. For this case, we clearly find, as discussed previously [4], that at low fields (H ( 100 Oe) L, is determined by the density of free dislocations, and this density can be manipulated by the application of a dc transport current [5]. It is therefore reasonable to believe that dislocations might continue to play a significant role at the higher fields of our SANS studies (H ) 300 Oe), and that the hysteresis observed in L upon cycling the current to above the critical current I, stems from their creation and annihilation. In a recent paper [3] we have presented high resolution SANS and transport data obtained on 2H-NbSe2 at H = 1.5 kOe and T = 4.8 K. For these conditions, SANS clearly shows that cycling the current to above I, leads to a significant increase (at least one order of magnitude) in I, However, transport data in this regime show no evidence of hysteresis, even when examined in great detail; significant changes of I., do not appear to be rejected in the pinning properties. We would take this to imply that the pinning-related length I, is welL below L, for all of the histories we have examined As seen. by previous workers [6], the transport data do become history dependent for H/H, z ~ 0.5. Unfortunately, at those fields SANS suffers from insufficient resolution, so that we are precluded from measuring L in this regime. The dependence of the SANS data on field and current history raises the issue of which state represents equilibrium. In the Comment, it is assumed that the most ordered state is closest to equilibrium. However, there is evidence that under some conditions a transport current can, in fact, induce order even when the equilibrium (zero current) state is disordered [7]. We believe this assertion remains an open question. Finally, GLD assume that the range of the pinning potential rf is equal to the coherence length go. The actual value of f is unknown, but for point pins it is thought to be either a fraction of the FLL lattice spacing ao or go [8]. The analysis of our data is not definitive regarding the value of rf. Fits of equal quality can be obtained using an rf that is significantly larger than go or even one that scales with ao. Therefore rf might well be field dependent We. note that larger values of rf would decrease the estimated value of L, /L„which GLD find to be rather high. The SANS data include substantial small angle scattering from stacking faults in the NbSe2 crystal. If these are an important source of pinning, they could lead to a large value for rf. In principle, length scales that are sufficiently different should clearly be reflected in a detailed analysis of the SANS line shape. At present, however, our data are not of sufficient quality to test this assertion. We believe that understanding the roles of the different length scales must await new data, either with emphasis on the details of the line shapes or in regimes where both the transport and SANS properties can be manipulated with external currents.