We report the synthesis, structural and magnetothermal characterization down to sub-Kelvin temperatures of the polymeric complex {Ho(α-fur)3}n (α-fur = C4H3OCOO−). The α-furoate ligands bridge Ho(III) ions into one-dimensional zigzag chains running along the crystallographic c-axis. Two slightly distinct coordination environments, denoted Ho(A) and Ho(B), arise from positional disorder of one furoate ligand. Ab initio calculations were performed to estimate the zero-field splittings of both Ho at A and B sites. Magnetic susceptibility and heat capacity measurements show that at low temperatures, the compound behaves as an assembly of antiferromagnetically coupled S* = 1/2 Ising-like chains (J*/kB ≈ −0.4 K) of Ho ions with transverse anisotropy. The low-temperature susceptibility further reveals a small concentration of chain defects. Zero-field ac susceptibility measurements evidence slow relaxation of the magnetization assigned to defect-enabled single-chain magnet (SCM) dynamics in the two chain types, with activation energies EA,SCM = 2.9(3) K and EB,SCM = 4(3) K. Below 0.4 K, a crossover to a coherent quantum tunneling of the magnetization regime, assisted by hyperfine interactions, is observed. The relaxation behavior is discussed in comparison with the related compounds {Ln(α-fur)3}n (Ln = Dy, Tb) and {Ho2Ba(α-fur)8}n, highlighting the interplay between Kramers versus non-Kramers ion character, hyperfine interactions, and magnetic exchange topology in governing the low-temperature dynamics.
Vortex dynamics govern the dissipation and magneto-resistive properties of type-II superconductors. At large transport currents, the current-voltage (I-V) characteristics of a superconductor in the mixed state usually exhibit a nonlinear upturn followed by an abrupt jump occurring due to a flux-flow instability. However, other transition behaviors are also possible, including the formation of phase-slip lines and normal domains. Which mechanism dominates depends on the sample uniformity, its dimensions relative to the coherence length and penetration depth, and the rates of electron energy relaxation and heat removal. Here, based on the time-dependent Ginzburg-Landau equation, we present the results of numerical modeling of the I-V curves of superconducting films with various types of disorder. For a grain-boundary defect mesh, we find multiple voltage transitions overlaid with a nonlinear upturn of the I-V curves. For randomly arranged elongated defects, the I-V curves exhibit voltage steps, whereas for L-shaped defects oriented perpendicular to the transport current, the I-V curves show extended linear regimes separated by voltage transitions. We analyze the evolution of the order parameter along the I-V curves and discuss the experimental accessibility of the revealed dynamics regimes.
When metallic needle tips are illuminated with intense femtosecond laser pulses, the photoemitted electrons can be driven strongly in the optical field at the surface of the needle tip. In particular, electrons can elastically rescatter at the tip, leading to tell-tale features in the electron spectra, such as the plateau and the cut-off. In this work, we first review intense two-color fields driving the electron dynamics. We show how from the two-color phase-dependent spectra one can obtain the optimal phase, i.e., the phase as function of energy, for which the count rate is maximized. We demonstrate that rich information can be extracted from this optimal phase. This first, review part sets the stage for the second part of the paper, containing original data on carrier-envelope phase-dependent spectra. We show that with a similar analysis we can reveal similarly rich information from the optimal phase also in the CEP variation case. In particular, we show that the optimal phase is highly sensitive to the driving pulse duration. We expect this work to widen the tool park of strong-field physics, with direct ramifications to the fields of attosecond physics and future petahertz electronics.
