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Register a new userCharge order dynamics in underdoped La$mathbf{_{1.6-textit{x}}}$Nd$mathbf{_{0.4}}$Sr$mathbf{{_textit{x}}}$CuO$mathbf{_{4}}$ revealed by electric pulses
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The dynamics of the charge-order domains has been investigated in La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_{4}$, a prototypical stripe-ordered cuprate, using pulsed current injection. We first identify the regime in which nonthermal effects dominate over simple Joule heating, and then demonstrate that, for small enough perturbation, pulsed current injection allows access to nonthermally-induced resistive metastable states. The results are consistent with pinning of the fluctuating charge order, with fluctuations being most pronounced at the charge-order onset temperature. The nonequilibrium effects are revealed only when the transition is approached from the charge-ordered phase. Our experiment establishes pulsed current injection as a viable and effective method for probing the charge-order dynamics in various other materials.
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The determination of the magnon diffusion length (MDL) is important for increasing the efficiency of spin Seebeck effect (SSE) based devices utilising non-metallic magnets. We extract the MDL at $50$ and $300,rm{K}$ in an $rm{Fe}_{3}rm{O}_{4}$ single crystal from the magnon dispersion obtained using inelastic neutron scattering (INS) and find them to be equal within error. We then measure the heat flux normalised SSE responses and in-plane magnetization of $rm{Fe}_{3}rm{O}_{4}$ thin films and normalise by the static magnetization contribution to the SSE before determining the MDLs from a fit of the thickness dependence. We find that the MDLs determined in this way are smaller than that measured from INS which maybe due to differences in magnon propagation between bulk and thin film $rm{Fe}_{3}rm{O}_{4}$.
CaCo$_{2-y}$As$_2$ is an unusual itinerant magnet with signatures of extreme magnetic frustration. The conditions for establishing magnetic order in such itinerant frustrated magnets, either by reducing frustration or enhancing the Stoner parameter, is an open question. Here we use results from inelastic neutron scattering and magnetic susceptibility measurements and density functional theory calculations to show that hole doping in Ca(Co$_{1-x}$Fe$_{x}$)$_{2-y}$As$_{2}$ suppresses magnetic order by quenching the associated magnetic moment while maintaining the same level of magnetic frustration. The suppression is due to tuning the Fermi energy away from a peak in the electronic density of states originating from a flat conduction band. This results in the complete elimination of the magnetic moment by $xapprox0.25$, providing a clear example of a Stoner-like transition.
Previously we have presented evidence for stripe order of holes and spins in La(1.6-x)Nd(0.4)Sr(x)CuO(4) with x=0.12. Here we show, via neutron diffraction measurements of magnetic scattering, that similar order occurs in crystals with x=0.15 and 0.20. Zero-field-cooled magnetization measurements show that all 3 compositions are also superconducting, with the superconducting transition temperature increasing as the low-temperature staggered magnetization decreases.
The ground-state orbital occupancy of the Ru$^{4+}$ ion in Ca$_{2-x}$La$_x$RuO$_4$ [x=0, 0.05(1), 0.07(1) and 0.12(1)] was investigated by performing X-ray absorption spectroscopy (XAS) in the vicinity of the O K edge as a function of angle between the incident beam and the surface of the crystals. A minimal model of the hybridization between the O 2p states probed at the K edge and the Ru 4d orbitals was used to analyze the XAS data, allowing the ratio of hole occupancies $n_{xy}/n_{yz,zx}$ to be determined as a function of doping and temperature. For the samples displaying a low-temperature insulating ground-state ($xleq0.07$), $n_{xy}/n_{yz,zx}$ is found to increase significantly with increasing doping. For x=0.12, which has a metallic ground-state, the XAS spectra are found to be independent of temperature, and not to be describable by the minimal hybridization model. To understand the origin of the evolution of the electronic structure across the phase diagram, we have performed theoretical calculations based on a model Hamiltonian, comprising electron-electron correlations, crystal field ($Delta$) and spin-orbit coupling ($lambda$), of a Ru-O-Ru cluster. Our calculations of the Ru hole occupancy as a function of $Delta/lambda$ establish that the enhancement of $n_{xy}/n_{yz,zx}$ is driven by significant changes to the crystal field as the tetragonal distortion of the RuO$_6$ octahedral changes from compressive to tensile with La doping. It also shows that the hole occupancy of the O 2p and Ru 4d orbitals display the same trend as a function of $Delta/lambda$, thus validating the minimal hybridization model. In essence, our results suggest that the predominant mechanism driving the emergence of the low-temperature metallic phase in La doped Ca$_2$RuO$_4$ is the structurally induced redistribution of holes within the t2g orbitals, rather that the injection of free carriers.
The dynamics of charge-ordered states is one of the key issues in underdoped cuprate high-temperature superconductors, but static short-range charge-order (CO) domains have been detected in almost all cuprates. We probe the dynamics across the CO (and structural) transition in La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_{4}$ by measuring nonequilibrium charge transport, or resistance $R$ as the system responds to a change in temperature and to an applied magnetic field. We find evidence for metastable states, collective behavior, and criticality. The collective dynamics in the critical regime indicates strong pinning by disorder. Surprisingly, nonequilibrium effects, such as avalanches in $R$, are revealed only when the critical region is approached from the charge-ordered phase. Our results on La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_{4}$ provide the long-sought evidence for the fluctuating order across the CO transition, and also set important constraints on theories of dynamic stripes.
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