Search Results (43 total)

Magnetic resonant x-ray scattering experiments have been performed on a single crystal of EuFe₂As₂ at the Eu L₃ absorption edge. The orientation of Eu magnetic moments was directly determined: in the antiferromagnetic (AFM) ordered phase they lay parallel to the crystallographic a axis. In addition, nonresonant magnetic x-ray measurements indicate that Fe magnetic moments are aligned along the same direction in the spin-density-wave phase. As deduced by temperature dependence of integrated intensities, the Fe magnetic arrangement seems to be insensitive to the onset of Eu AFM phase. Some speculations on the low-temperature space group are reported on the base of detected resonant reflections on forbidden Bragg positions.

We report detailed measurements of composition as well as temperature dependence of the phonon density of states in a new series of FeAs compounds with composition CaFe_1−xCo_xAsF (x=0,0.06,0.12). The electronic-structure calculations for these compounds show that bands near the Fermi level are mainly formed by Fe 3d states, which is quite different from other 122 and 1111 FeAs compounds where both Fe and As are believed to be related to superconductivity. The difference in electronic structure for fluorine-based compounds may cause phonon spectra to behave differently as a function of composition and temperature in comparison with our previous phonon studies on parent and superconducting MFe₂As₂ (M=Ba,Ca,Sr). The composition as well as temperature dependence of phonon spectra for CaFe_1−xCo_xAsF (x=0,0.06,0.12) compounds have been measured using time-of-flight IN4C and IN6 spectrometers at Institut Laue Langevin, France. The comparison of phonon spectra at 300 K in these compounds shows that acoustic phonon modes up to 12 meV harden in the doped compounds in comparison to the parent CaFeAsF. While intermediate-energy phonon modes from 15 to 25 meV are also found to shift toward high energies only in the 12% Co-doped CaFeAsF compound. The experimental results for CaFe_1−xCo_xAsF (x=0,0.06,0.12) are quite different from our previous phonon studies on parent and superconducting MFe₂As₂ (M=Ba,Ca,Sr) where low-energy acoustic phonon modes do not react with doping, while the phonon spectra in the intermediate range from 15 to 25 meV are found to soften in these compounds. We argue that stronger spin phonon interaction play an important role in the emergence of superconductivity in these compounds. The lattice dynamics of CaFe_1−xCo_xAsF (x=0,0.06,0.12) compounds is also investigated using the ab initio as well as shell-model phonon calculations. We show that the nature of the interaction between the Ca and the Fe-As layers in CaFeAsF compounds is quite different compared to our previous studies on CaFe₂As₂.

We measured phonon dispersions of CaFe₂As₂ using inelastic neutron scattering and compared our results to predictions of density functional theory in the local density approximation. The calculation gives correct frequencies of most phonons if the experimental crystal structure is used, except observed linewidths/frequencies of certain modes were larger/softer than predicted. Strong temperature dependence of some phonons near the structural phase transition near 172 K may indicate strong electron-phonon coupling and/or anharmonicity, which may be important for superconductivity.

We report the pressure and temperature dependence of the phonon density-of-states in superconducting Ca_0.6Na_0.4Fe₂As₂ (T_c=21 K) and the parent compound CaFe₂As₂ using inelastic neutron scattering. We observe no significant change in the phonon spectrum for Ca_0.6Na_0.4Fe₂As₂ at 295 K up to pressures of 5 kbar. The phonon spectrum for CaFe₂As₂ shows softening of the low-energy modes by about 1 meV when decreasing the temperature from 300 to 180 K. There is no appreciable change in the phonon density of states across the structural and antiferromagnetic phase transition at 172 K. These results, combined with our earlier temperature dependent phonon density of states measurements for Ca_0.6Na_0.4Fe₂As₂, indicate that the softening of low-energy phonon modes in these compounds may be due to the interaction of phonons with electron or short-range spin fluctuations in the normal state of the superconducting compound as well as in the parent compound. The phonon spectra are analyzed with ab initio and empirical potential calculations giving partial densities of states and dispersion relations.

We have performed extensive ab initio calculations to investigate phonon dynamics and their possible role in superconductivity in BaFe₂As₂ and related systems. The calculations are compared to inelastic neutron scattering data that offer improved resolution over published data [Mittal , Phys. Rev. B 78, 104514 (2008)], in particular at low frequencies. Effects of structural phase transition and full and/or partial structural relaxations, with and without magnetic ordering, on the calculated vibrational density of states are reported. Phonons are best reproduced using either the relaxed magnetic structures or the experimental cell. Several phonon branches are affected by the subtle structural changes associated with the transition from the tetragonal to the orthorhombic phase. Effects of phonon-induced distortions on the electronic and spin structure have been investigated. It is found that for some vibrational modes, there is a significant change in the electronic distribution and spin populations around the Fermi level. A peak at 20 meV in the experimental data falls into the pseudogap region of the calculation. This was also the case reported in our recent work combined with an empirical parametric calculation [Mittal , Phys. Rev. B 78, 104514 (2008)]. The combined evidence for the coupling of electronic and spin degrees of freedom with phonons is relevant to the current interest in superconductivity in BaFe₂As₂ and related systems.

A neutron powder-diffraction experiment has been performed to investigate the structural phase transition and magnetic order in CaFe_1−xCo_xAsF superconductor compounds (x=0.00,0.06,0.12). The parent compound CaFeAsF undergoes a tetragonal to orthorhombic phase transition at 134(3) K, while the magnetic order in the form of a spin-density wave (SDW) sets in at 114(3) K. The antiferromagnetic structure of the parent compound has been determined with a unique propagation vector k=(1,0,1) and the Fe saturation moment of 0.49(5)μ_B, aligned along the long a axis. With increasing Co doping, the long-range antiferromagnetic order has been observed to coexist with superconductivity in the orthorhombic phase of the underdoped CaFe_0.94Co_0.06AsF with a reduced Fe moment [∼0.15(5)μ_B]. Magnetic order is completely suppressed in optimally doped CaFe_0.88Co_0.12AsF. We argue that the coexistence of SDW and superconductivity might be related to mesoscopic phase separation.

Antiferromagnetic ordering and structural phase transition have been investigated via comprehensive neutron and x-ray diffraction on Sn-flux-grown BaFe₂As₂ single crystals, the A-122 family of FeAs-based high-T_C superconductor compounds. The incorporation of Sn in the lattice resulted to an average composition of Ba_0.95Sn_0.05Fe₂As₂. A tetragonal-to-orthorhombic structural phase transition and a three-dimensional long-range antiferromagnetic ordering of the iron magnetic moment, with a unique magnetic propagation wave vector k=(1,0,1) have been found to take place at ∼90 K. The magnetic moments of iron are aligned along the longer a axis in the low-temperature orthorhombic phase (Fmmm with b<a<c). Our results thus demonstrate that the magnetic structure of the Sn-flux-grown BaFe₂As₂ single crystal is the same as those in the polycrystalline samples and in other A-122 iron pnictides compounds. We argue that the Sn incorporation in the lattice is responsible for a smaller orthorhombic splitting and lower Néel temperature T_N observed in the experiments.

We report inelastic neutron scattering measurements of the phonon density of states of superconducting Sr_0.6K_0.4Fe₂As₂ (T_c=32 K) and Ca_0.6Na_0.4Fe₂As₂ (T_c=21 K). Compared with the parent compound BaFe₂As₂ doping affects mainly the lower and intermediate frequency part of the vibrations. Mass effects and lattice contraction cannot solely explain these changes. Softening of phonon modes below 10 meV has been observed in both samples on cooling from 300 to 140 K. In the Ca-doped compound the softening amounts to about 1 meV while for the Sr-doped compound the softening is about 0.5 meV. There is no appreciable change in the phonon density of states on crossing T_c.

We report here extensive measurements of the temperature dependence of phonon density of states of BaFe₂As₂, the parent compound of the FeAs-based superconductors, using inelastic neutron scattering. The experiments were carried out on the thermal time-of-flight neutron spectrometer IN4 at the Institut Laue Langevin on a polycrystalline sample. There is no appreciable change in the spectra between T=10 and 200 K, although the sample undergoes a magnetic as well as a tetragonal-to-orthorhombic structural phase transition at 140 K. This indicates a rather harmonic phonon system. Shell-model lattice-dynamical calculations based on interatomic potentials are carried out to characterize the phonon data. The calculations predict a shift of the Ba phonons to higher energies at 4 GPa. The average energy of the phonons of the Ba sublattice is also predicted to increase on partial substitution of Ba by K to Ba_0.6K_0.4. The calculations show good agreement with the experimental phonon spectra and also with the specific-heat data from the literature.

We propose a model-independent scaling method to study the physical properties of high-temperature superconductors in the normal state. We have analyzed the experimental data of the c-axis resistivity, the in-plane resistivity, the Hall coefficient, the magnetic susceptibility, the spin-lattice relaxation rate, and the thermoelectric power using this method. It is shown that all these physical quantities exhibit good scaling behaviors, controlled purely by the pseudogap energy scale in the normal state. The doping dependence of the pseudogap obtained from this scaling analysis agrees with the experimental results of angle-resolved photoemission and other measurements. It sheds light on the understanding of the basic electronic structure of high-T_c oxides.

We have investigated the gamma-ray bursts (GRBs) pulses with a fast rise and an exponential decay phase, assumed to arise from relativistically expending fireballs, and found that the curvature effect influences the evolutionary curve of the corresponding hardness ratio (hereafter HRC). We find, due to the curvature effect, the evolutionary curve of the pure hardness ratio (when the background count is not included) would peak at the very beginning of the curve, and then would undergo a drop-to-rise-to-decay phase. In the case of the raw hardness ratio (when the background count is included), the curvature effect would give rise to several types of evolutionary curve, depending on the hardness of a burst. For a soft burst, an upside down pulse of its raw HRC would be observed; for a hard burst, its raw HRC shows a pulselike profile with a sinkage in its decaying phase; for a very hard burst, the raw HRC possesses a pulselike profile without a sinkage in its decaying phase. For a pulselike raw HRC as shown in the case of the hard and very hard bursts, its peak would appear in advance of that of the corresponding light curve, which was observed previously in some GRBs. For illustration, we have studied here the HRC of GRB 920216, GRB 920830, and GRB 990816 in detail. The features of the raw HRC expected in the hard burst are observed in these bursts. A fit to the three bursts shows that the curvature effect alone could indeed account for the predicted characteristics of HRCs. In addition, we find that the observed hardness ratio tends to be harder at the beginning of the pulses than what the curvature effect could predict and be softer at the late time of the pulses. We believe this is an evidence showing the existence of intrinsic hard-to-soft radiation which might be due to the acceleration-to-deceleration mode of shocks.

We have investigated the luminescence quenching of the fluorescent aromatic molecule tetracene (Tc) adsorbed on an ultrathin film of alumina on a Ni₃Al(111) substrate. The alumina film (thickness ∼5 Å) was grown by heating a Ni₃Al(111) crystal under an oxygen atmosphere. Directly after deposition of Tc molecules at low temperatures (45 K), the Tc films are disordered and no luminescence can be detected. We explain this by charge transfer to the Ni₃Al(111) substrate from mainly flat-lying Tc molecules across the alumina film. After an additional annealing cycle at 240 K, we observe a significant luminescence signal for films with nominal film thickness down to submonolayers. Low energy electron diffraction measurements reveal that the annealing cycle leads to formation of (001) oriented Tc crystallites with Tc molecules standing nearly perpendicular to the surface. We hence propose that the luminescence quenching by charge transfer is inhibited for the annealed Tc films due to the different molecule orientation and film morphology. In control experiments performed on the sapphire (0001) surface, we find a very similar behavior, except that we can detect a very low luminescence signal already for the as grown Tc films. However, the luminescence yield is very small, likely due to structural disorder.

We propose and show that the c-axis transport in high-temperature superconductors is controlled by the pseudogap energy and the c-axis resistivity satisfies a universal scaling law in the pseudogap phase. We derived approximately a scaling function for the c-axis resistivity and found that it fits well with the experimental data of Bi₂Sr₂CaCu₂O_8+δ, Bi₂Sr₂Ca₂Cu₃O_10+δ, and YBa₂Cu₃O_7−δ. Our works reveals the physical origin of the semiconductorlike behavior of the c-axis resistivity and suggests that the c-axis hopping is predominantly coherent.

Turn-by-turn beam profile data measured at the Fermilab Booster are studied. Lattice models with experimental accelerator ramping parameters are used to obtain the lattice functions for data analysis. We studied the horizontal and vertical emittance growth behavior in different stages of a booster ramping cycle and its relation to the beam intensity. The transverse and longitudinal components in the horizontal beam width are separated by a fitting model which makes use of the different scaling rules of the beam momentum. We analyze the post-transition horizontal beam size oscillation based on a model where the longitudinal phase-space mismatch has resulted from rf voltage mismatch during the transition-energy crossing. We carried out systematic multiparticle simulation to show that the source of the vertical emittance growth is a combination of the random errors in skew-quadrupole and dipole fields, and the systematic Montague resonance. The effect of random quadrupole field is small for the Fermilab Booster because the betatron envelope tunes are reasonably far away from the half-integer stop band.

Thermodynamic properties of a tetrameric bond-alternating Heisenberg spin chain with ferromagnetic-ferromagnetic-antiferromagnetic-antiferromagnetic exchange interactions are studied using the transfer-matrix renormalization group and compared to experimental measurements. The temperature dependence of the uniform susceptibility exhibits typical ferrimagnetic features. Both the uniform and staggered magnetic susceptibilities diverge in the limit T→0, indicating that the ground state has both ferromagnetic and antiferromagnetic long-range orders. A double-peak structure appears in the temperature dependence of the specific heat. Our numerical calculation gives a good account for the temperature and field dependence of the susceptibility, the magnetization, and the specific heat for Cu(3-Clpy)₂(N₃)₂ (3-Clpy=3-Chloroyridine).

A strong nonmagnetic impurity can induce a resonance state in the d-wave superconducting state. As far as magnetic properties are concerned, this resonance state behaves effectively like a free moment. It leads to a Curie-Weiss-like magnetic susceptibility in an intermediate temperature regime below T_c. From the impurity susceptibility, the effective moment of the resonance state is deduced and compared with experiments. The contribution of the resonance to the magnetic susceptibility can account for the main feature of the NMR spectra in overdoped high-T_c materials. In the underdoped regime, the contribution from the resonance to the magnetic susceptibility is also substantial, but the effective moment of the resonance is smaller than the total moment induced by a nonmagnetic impurity.

Physical effects of bilayer coupling on the tunneling spectroscopy of high-T_c cuprates are investigated. The bilayer coupling separates the bonding and antibonding bands and leads to a splitting of the coherence peaks in the tunneling differential conductance. However, the coherence peak of the bonding band is strongly suppressed and broadened by the particle-hole asymmetry in the density of states and finite quasiparticle lifetime, and is difficult to resolve by experiments. This gives a qualitative account why the bilayer splitting of the coherence peaks was not clearly observed in tunneling measurements of double-layer high-T_c oxides.

Field-tuned quantum tunneling of the supramolecular magnet [Mn₄]₂ is studied. It is a dimer constructed by two single-molecular clusters, Mn₄(S=9/2), coupled antiferromagnetically. By means of the time-dependent Schrödinger equation, The exact solution of the time dependent quantum state is numerically calculated. We obtain the curve of magnetization versus magnetic field applied along the easy axis of magnetization of [Mn₄]₂; it clearly displays steplike features separated by plateaus that are relevant to spin tunneling. Our result shows that transition of magnetization is sensitive to the sweep rate. Some resonance transitions may not occur, but they can occur at a more lower sweeping rate of the applied field. Our theoretical result agrees well with the recent experiment [W. Wernsdorfer et al., Nature (London) 416, 406 (2002)].

Al_xGa_1-xN/GaN two-dimensional electron gas structures have been grown by molecular-beam epitaxy and exhibited surface roughness defined by the faceted surface morphology of the crystal grains or subgrains, with the grain size being the obvious lateral periodicity. The roughness amplitude varied in a wide range depending on the substrate type and growth conditions. The effect of interface roughness with lateral correlation length of the order of the grain sizes (0.1–1 μm) on the scattering lifetimes is calculated theoretically and compared with experimentally observed lifetime values. We conclude that scattering by long-range interface roughness has little impact on the transport lifetime, but competes with other scattering mechanisms to be the dominant source of small-angle scattering that limits the quantum scattering lifetime.

We have derived the effective low-energy Hamiltonian for Zn- or Ni-substituted high-T_c cuprates from microscopic three-band models consisting of the most relevant Cu or impurity 3d and O 2p orbitals. We find that both the scattering potential and hopping integral induced by impurities have a finite range but decay very fast with distance from the impurity. The Zn scattering potential is very strong and attractive for electrons. The Ni scattering potential is much weaker than the Zn case, resulting from the hybridization between Ni ions and O holes. This profound difference is due to neither the electric charge nor d-level location, but rather because of the interplay between the valence state of the impurity and the strong correlation background. It gives a natural account of the unusual effect of Ni and Zn on the reduction of the superconducting transition temperature. The interlayer hopping of electrons is highly anisotropic and nonlocal, determined by the in-plane electronic structure. This leads to a quantum interference of states from different sites and affects strongly the scanning tunneling spectrum perpendicular to CuO₂ planes.

The mechanism of H migration in amorphous Si has remained an unresolved problem. The main issue is the small activation energy (1.5 eV) relative to the known strength of Si-H bonds (2–3.5 eV). We report first-principles finite-temperature simulations which demonstrate vividly that H is not released spontaneously, as proposed by most models, but awaits the arrival of a floating bond (FB). The “migrating species” is an FB-H complex, with H jumping from Si to Si and the FB literally floating around it. Migration stops when the FB veers away.

Ultrathin films of GdBa₂Cu₃O_7-δ grown by pulsed laser deposition on NdGaO₃(001) and SrTiO₃(001) were studied by grazing incidence x-ray diffraction. The critical thicknesses t_c was found to be 50 nm on SrTiO₃(001). On NdGaO₃(001), the shear distortion introduced by the substrate is released in two stages at 15 nm and 40 nm thickness by the formation of disclinations with orthogonal pseudo-Burgers vectors. The twin structure forms in the films at the first stage of strain relief, and as a consequence the twin boundaries are aligned unidirectionally. Calculated values for the critical thickness are too small for GdBCO/STO suggesting the existence of other mechanisms of strain relief, which is corroborated by our experimental findings.

The atomic structure of the group-III-rich surface of III-V semiconductor compounds has been under intense debate for many years, yet none of the models agrees with the experimental data available. Here we present a model for the three-dimensional structure of the (001)-c(8×2) reconstruction on InSb, InAs, and GaAs surfaces based on surface x-ray diffraction data that was analyzed by direct methods and subsequent least squares refinement. Contrary to common belief the main building blocks of the structure are not dimers on the surface but subsurface dimers in the second bilayer. This essential feature of the structure is accompanied by linear arrays of atoms on nonbulklike sites at the surface which, depending on the compounds, exhibit a certain degree of disorder. A tendency to group-III-dimer formation within these chains increases when descending the periodic table. We propose that all the c(8×2) reconstructions of III-V semiconductor surfaces contain the same essential building blocks.

We report direct measurements of parallel electric fields related to particle acceleration in a collisionless space plasma. The electric field is that of a monotonic potential ramp localized to ∼10 debye lengths along the magnetic field. Electrons accelerated by the parallel electric field are accompanied by intense electrostatic waves and nonlinear structures interpreted as electron phase-space holes.