Commensurate Peak Research Articles

Neutron diffraction and linear Grüneisen parameter studies of magnetism in NdFe2Ga8

We study the magnetism in ${\mathrm{NdFe}}_{2}{\mathrm{Ga}}_{8}$ by the neutron-diffraction and temperature-modulated linear Gr\"uneisen parameter measurements. Previous thermodynamical measurements have demonstrated that there are two magnetic transitions at 10 and 14.5 K, respectively. Neutron-diffraction measurements confirm that the lower one is an antiferromagnetic (AFM) transition with a commensurate magnetic structure. Both the commensurate and the incommensurate (IC) magnetic peaks are found below the higher transition but their intensities only gradually increase with decreasing temperature. Below 10 K, the commensurate peak intensity increases quickly with decreasing temperature, signaling the AFM transition, while the IC peak intensity disappears below 5 K. The linear Gr\"uneisen parameter along the $c$ axis, ${\mathrm{\ensuremath{\Gamma}}}_{c}$, shows a hysteresis behavior that is different from the hysteresis behavior for the magnetization $M$. We give a discussion of the origin of the magnetism in ${\mathrm{NdFe}}_{2}{\mathrm{Ga}}_{8}$.

Multiband Ballistic Transport and Anisotropic Commensurability Magnetoresistance in Antidot Lattices of AB-stacked Trilayer Graphene

Ballistic transport was studied in a multiple-band system consisting of an antidot lattice of AB-stacked trilayer graphene. The low temperature magnetoresistance showed commensurability peaks arising from matching of the antidot lattice period and radius of cyclotron orbits for each mono- and bilayer-like band in AB stacked trilayer graphene. The commensurability peak of the monolayer-like band appeared at a lower magnetic field than that of the bilayer-like band, which reflects the fact that the Fermi surface of the bilayer-like band is larger than that of monolayer-like band. Rotation of the antidot lattice relative to the crystallographic axes of graphene resulted in anisotropic magnetoresistance, which reflects the trigonally warped Fermi surface of the bilayer-like band. Numerical simulations of magnetoresistance that assumed ballistic transport in the mono- and bilayer-like bands approximately reproduced the observed magnetoresistance features. It was found that the monolayer-like band significantly contributes to the conductivity even though its carrier density is an order smaller than that of the bilayer-like band. These results indicate that ballistic transport experiments could be used for studying the anisotropic band structure of multiple-band systems.

Microwave-Induced Magneto-Intersubband Scattering in a Square Lattice of Antidots

The effect of microwave radiation on low-temperature electron magnetotransport in a square antidot lattice with a period of d = 0.8 micrometer based on a GaAs quantum well with two occupied energy subbands E1 and E2 is investigated. It is shown that, owing to a significant difference between the electron densities in the subbands, commensurability oscillations of the resistance in the investigated antidot lattice are observed only for the first subband. It is found that microwave irradiation under the cyclotron resonance condition results in the formation of resistance oscillations periodic in the inverse magnetic field in the region of the main commensurability peak. It is established that the period of these oscillations corresponds to the period of magneto-intersubband oscillations. The observed effect is explained by the increase in the rate of intersubband scattering caused by the difference between the electron heating in the subbands E1 and E2.

Temperature Dependence of Commensurate Magnetic Resonance in Cuprate Superconductors

Within the kinetic energy driven superconducting mechanism, we have studied the temperature dependence of commensurate magnetic resonance in cuprate superconductors. It is shown that the commensurate magnetic resonance peak at the antiferromagnetic wave vector point persists in the superconducting state until the temperature rises to the superconducting transition temperature $T_{\rm c}$. The intensity of the resonance peak decreases with increasing temperature which is just like the temperature dependence of the superconducting gap parameter. Our results are in qualitative agreement with the inelastic neutron scattering experimental data and reflect that the commensurate magnetic resonance is closely related to the creation of the charge carrier pairs and thus the superconducting mechanism of cuprate superconductors.

Spin Dynamics in the Bi-Directional Charge Density Wave State of High-T c Superconductors

In this paper, we investigate the dynamical spin susceptibility in the bi-directional charge density wave (BCDW) state by adopting a random-phase approximation. In the BCDW state, we find that no spin resonance exists and only a broad commensurate peak appears for the frequency dependence of the dynamical spin susceptibility at Q = (π,π), though a low-energy spin gap feature can also be found as in the superconducting state. While the “hourglass” type of the dispersion for the BCDW state bears some similarities with that in the superconducting state, the momentum distribution of Im χ+−(Q,ω) is just the opposite with the incommensurate peaks lying along the diagonal direction for the energy below ωc and along the axial direction above ωc. In the coexistence of SC and BCDW, the frequency dependence of the dynamical spin susceptibility at Q = (π,π) generally shows the two-peak structure, reflecting two energy scales of the spin excitations. These unique features may serve as signatures to verify whether or not the BCDW state is responsible for the formation of the Fermi arcs.

Ballistic transport in graphene antidot lattices

We observed commensurability magnetoresistance arising from ballistic electron transport in a triangular antidot lattice of high-mobility graphene. For both the monolayer and bilayer, magnetoresistance peaks were observed at the commensurability magnetic fields of the cyclotron orbit with an antidot lattice. This condition was approximately unchanged for massless and massive Dirac fermions. The peaks appeared when the carrier mean free path was roughly larger than the lattice constant of the antidot, which indicates that the effect stems from the short-range characteristics of the carrier's scattering with antidots. We also found that the magnitude of the commensurability peak diminished with changing the gate voltages to the charge neutrality point. This arose from the screening of the charged impurity in graphene, which is dependent on the carrier density.

Spin and charge order in Hg1201

We describe the Hg1201 cuprate in the pseudogap (PG) phase from a 3-band Fermi liquid point of view. The PG is modeled by a k-dependent gap parameter Δ(k), conforming with experiment. Susceptibilities are evaluated in the low-temperature metallic state without further renormalisation by interactions. The effect of strong correlations in the 2D metal is studied by coherent kinematic factors, originating from setting the Hubbard Ud→∞[1].The magnetic susceptibility shows a Y-shaped transverse spin response much like the recent experimental results [2] in the PG phase of Hg1201. The response is suppressed below the commensurate peak at ω=2Δ0. The strongly anisotropic gap Δ(k) extends the non-dispersive vertical part of the Y-shape, as seen in experiment. (Using an isotropic gap yields a very short vertical part, making the response more V-shaped.) At higher frequencies it displays isotropic upward branches from interband excitations.The SDW gap Δ(k), assumed to open at T⁎≈300K, reconstructs the Fermi surface, leaving only a pocket around the nodal points. The bands at TCDW≪T⁎nest to give a qCDW of around 0.28 r.l.u., as observed [3]. Such a gapped Fermi liquid scenario suffices to understand the charge and spin responses of underdoped Hg1201 in the experimentally observed hierarchy. The kinematic correlations caused by a large Ud change the Y-response towards the “hourglass” X-shape, thus replacing the peak at ω=2Δ0 and q=QAF with incommensurate downward branches. This result indicates that effects of strong-coupling correlations at the Fermi surface are more significant in the lanthanum than in the mercury cuprates.

Vortex states in Archimedean tiling pinning arrays

We numerically study vortex ordering and pinning in Archimedean tiling substrates composed of square and triangular plaquettes. The two different plaquettes become occupied at different vortex densities, producing commensurate peaks in the magnetization at non-integer matching fields. We find that as the field increases, in some cases the fraction of occupied pins can decrease due to the competition between fillings of the different plaquette types. We also identify a number of different types of vortex orderings as a function of the field at integer and non-integer commensurate fillings.

Magnetic Response of Pr<sub>1-<i>x</i></sub>LaCe<i><sub>x</sub></i>CuO<sub>4</sub> in Comparison with Hole-Doped Cuprates

The magnetic susceptibility of the optimally doped Pr1-xLaCexCuO4in the superconducting state is calculated using thet-Jmodel of Cu-O planes, the Mori projection operator technique, and the dispersion of electron bands derived from photoemission experiments. The electron band folding across the antiferromagnetic Brillouin zone border, which is inherent in the crystal, leads to a commensurate low-frequency response. The same band folding causes the appearance of a supplementary spin-excitation branch, which coexists with usual spin excitations. This coexistence can explain two maxima observed in the frequency dependence of the susceptibility. The two nested spin-excitation branches lead to a comb of closely spaced peaks in momentum cuts, which presumably are not resolved in experiment, being seen as a broad commensurate peak up to 100 meV. Reasons for differences in magnetic responses of electron- and hole-doped cuprates are discussed.

Vortex transport in a channel with periodic constrictions

By numerically solving the time-dependent Ginzburg–Landau equations in a type-II superconductor, characterized by a critical temperature Tc1, and the coherence length ξ1, with a channel formed by overlapping rhombuses (diamond-like channel) made of another type-II superconductor, characterized, in general, by different Tc2 and ξ2, we investigate the dynamics of driven vortex matter for varying parameters of the channel: the width of the neck connecting the diamond cells, the cell geometry, and the ratio between the coherence lengths in the bank and the channel. We analyzed samples with periodic boundary conditions (which we call ‘infinite’ samples) and finite-size samples (with boundaries for vortex entry/exit), and we found that by tuning the channel parameters, one can manipulate the vortex dynamics, e.g., change the transition from flux-pinned to flux-flow regime and tune the slope of the IV-curves. In addition, we analyzed the effect of interstitial vortices on these characteristics. The critical current of this device was studied as a function of the applied magnetic field, jc(H). The function jc(H) reveals a striking commensurability peak, in agreement with recent experimental observations. The obtained results suggest that the diamond channel, which combines the properties of pinning arrays and flux-guiding channels, can be a promising candidate for potential use in devices controlling magnetic flux motion.

Superconductivity and magnetic fluctuations in electron-doped cobaltate superconductors

We study the interplay between superconductivity and antiferromagnetic spin fluctuations in electron-doped cobaltate NaxCoO2·yH2O based on the kinetic energy driven superconductivity mechanism. We show that the superconducting state is governed by both charge carrier pairing and quasiparticle coherent, and displays a common dome-shaped phase diagram in agreement with experimental results. By calculating the dynamical spin structure factor, we theoretically find that the magnetic excitation shows a commensurate resonance peak, which locates at antiferromagnetic wave vector Q(2π/3,2π/3) for a broad range of low energies, then evolves outward into six incommensurate magnetic scattering peaks with increasing energy. Such commensurate–incommensurate spin resonance excitation should be measured by the inelastic neutron scattering (INS) technique. Our present results strongly suggest that magnetic resonance can indeed be one of the fundamental features in doped Mott insulators.

Ni-substitution effects on the spin dynamics and superconductivity in La1.85Sr0.15CuO4

Systematic inelastic neutron scattering studies have been performed on La${}_{1.85}$Sr${}_{0.15}$Cu${}_{1\ensuremath{-}y}$Ni${}_{y}$O${}_{4}$ in order to elucidate the microscopic mechanism of magnetic Ni impurity doping effects on spin dynamics and superconductivity. In contrast to Zn doping, which precipitates static or quasistatic spin correlations around dopant, Ni doping remarkably reduces ${E}_{\mathrm{comme}}$, the minimum energy of the upward branch of the hourglass-shaped magnetic dispersion where a commensurate magnetic peak is observed. Ni doping at $y\ensuremath{\sim}0.04$ further reduces ${E}_{\mathrm{comme}}$ to zero energy and broadens the distribution of ${E}_{\mathrm{comme}}$, corresponding to the appearance of the normal state at base temperature ($T\ensuremath{\sim}4$ K) as well as the broadening of the superconducting transition.

Tetrahedral Magnetic Order and the Metal-Insulator Transition in the Pyrochlore Lattice ofCd2Os2O7

Cd2Os2O7 shows a peculiar metal-insulator transition at 227 K with magnetic ordering in a frustrated pyrochlore lattice, but its magnetic structure in the ordered state and the transition origin are yet uncovered. We observed a commensurate magnetic peak by resonant x-ray scattering in a high-quality single crystal. X-ray diffraction and Raman scattering experiments confirmed that the transition is not accompanied with any spatial symmetry breaking. We propose a noncollinear all-in-all-out spin arrangement on the tetrahedral network made of Os atoms. Based on this we suggest that the transition is not caused by the Slater mechanism as believed earlier but by an alternative mechanism related to the formation of the specific tetrahedral magnetic order on the pyrochlore lattice in the presence of strong spin-orbit interactions.

Incommensurate Spin Fluctuations in Hole-Overdoped SuperconductorKFe2As2

A neutron scattering study of heavily hole-overdoped superconducting KFe2As2 revealed a well-defined low-energy incommensurate spin fluctuation at [π(1 ± 2 δ),0] with δ = 0.16. The incommensurate structure differs from the previously observed commensurate peaks in electron-doped AFe2As2 (A = Ba, Ca, or Sr) at low energies. The direction of the peak splitting is perpendicular to that observed in Fe(Te,Se) or in Ba(Fe,Co)2As2 at high energies. A band structure calculation suggests interband scattering between bands around the Γ and X points as an origin of this incommensurate peak. The perpendicular direction of the peak splitting can be understood within the framework of multiorbital band structure. The results suggest that spin fluctuation is more robust in hole-doped than in electron-doped samples, which can be responsible for the appearance of superconductivity in the heavily hole-doped samples.

Neutron-Scattering Study of Fe-Substitution Effect on the Static Spin Correlation in La2-xSrxNiO4

We have performed elastic neutron scattering measurements on the Fe-doped La 2- x Sr x NiO 4 (LSNO) with 0.12 ≤ x ≤0.50 to study the magnetic impurity effects on the static spin correlations. In the pristine LSNO system, it is well known that both spin and charge stripe orders are stabilized at low temperatures in the wide hole concentration range from 0.12 to 0.70. The incommensurability (e) increases upon Sr-doping and tends to saturate with e max ∼0.47 (r.l.u.) for x > 0.5. We found that 4% Fe-substitution for Ni ion reduces e at the fixed Sr concentration for the all measured samples with 0.12 ≤ x ≤0.5. The e in the Fe-substituted samples with x = 0.25, 0.33, 0.37, and 0.50 was 0.02–0.04 (r.l.u.) smaller than that in the pristine samples. Furthermore, we observed commensurate peak in the Fe-doped x = 0.12 sample. No well-defined incommensurate magnetic peaks were detected, unlike to the result for the pristine x = 0.12 sample. The effect of Fe-substitution on the magnetic ordering temperature ( T spin...

Superconductivity and electronic liquid-crystal states in twin-free YBa2Cu3O6+x studied by neutron scattering

In this paper, we first give a concise overview of recent experimental and theoretical work dealing with “electronic liquid-crystal states” which spontaneously break different symmetries of the CuO2 layers of high-T c cuprates, with an emphasis on evidence in the spin excitation spectrum. Then we describe the importance of using twin-free samples to look for evidence for fourfold symmetry breaking in the spectrum and explain the preparation procedure to obtain such samples. We present inelastic neutron scattering results for moderately underdoped YBa2Cu3O6.6(T c = 61 K) and nearly optimally doped YBa2Cu3O6.85(T c = 89 K). In YBa2Cu3O6.6, the dispersion topology changes when heating above T c from an hourglass shape with constricted, commensurate resonance peak to a “Y”-shape without resonance anomaly. This change, and the fact that the low-energy signal above T c can be described by an incommensurate, quasi-one-dimensional distribution, indicates a competition of superconductivity with an electronic liquid-crystal state. We then show a striking analogy between the difference signal I(5 K) − I(70 K) and the downward dispersing resonance mode in YBa2Cu3O6.85. We therefore argue that a resonance mode only emerges below T c, irrespective of the doping level. We finally discuss the implications of our results for the different scenarios invoked to explain the electronic liquid-crystal state in cuprates.

Renormalization of Commensurate Magnetic Peak in Ni-Doped La1.85Sr0.15CuO4

We have studied the magnetic excitations in impurity doped La$_{1.85}$Sr$_{0.15}$Cu$_{1-y}$A$_{y}$O$_{4}$ (A=Ni or Zn) by neutron scattering. The dispersion for Zn:$y=0.017$ is similar to that for the impurity free sample: incommensurate peaks with the incommensurability $\delta=0.12\pm0.01$ (rlu) do not change their positions up to 21 meV. On the other hand, for Ni:$y=0.029$, two incommensurate peaks observed at low energies suddenly change into a broad commensurate peak at $E_\mathrm{cross}=15$ meV. Compared to the impurity free sample with a similar Sr-concentration $x=0.16$, [B. Vignolle {\it et al.} Nature Physics {\bf 3} (2007) 163], $E_\mathrm{cross}$ for Ni:$y=0.029$ is decreased by nearly the same factor for the reduction in $T_{c}$. This is very similar to the shift of the resonance energy ($E_\mathrm{res}$) in Ni-doped YBa$_{2}$Cu$_{3}$O$_{7}$.[Y. Sidis {\it et al.}: Phys. Rev. Lett. {\bf 84} (2000) 5900]. These common impurity effects on the shift of $E_\mathrm{cross}$ and $E_\mathrm{res}$ suggest the same magnetic origin for the resonance peak in YBa$_{2}$Cu$_{3}$O$_{\delta}$ and that for a crossing point of upward and downward dispersions in the La$_{2-x}$Sr$_{x}$CuO$_{4}$. We propose that the sudden change in the dispersion is better described by a crossover from incommensurate spin fluctuations to a gapped spin wave rather than a hourglass-like dispersion.

Thermal Variations of Magnetic Excitation Spectrum in Slightly Overdoped Bi2.1Sr1.9CaCu2O8+δ

We measured the inelastic neutron scattering of slightly overdoped high- T c superconductor Bi 2.1 Sr 1.9 CaCu 2 O 8+δ (Bi2212: T c =86.2 ±3.5 K). We observed the enhancement of a commensurate peak...

Semiclassical and quantum transport in the two-dimensional electron gas in a hard-wall antidot lattice

Commensurate peaks of magnetoresistance and Shubnikov-de Haas and Aharonov-Bohm oscillations in the two-dimensional electron gas (2DEG) in a lattice of antidots with hard potential walls have been experimentally studied. The behavior of both classical magnetoresistance peaks and quantum oscillations has been shown to fundamentally depend on the lattice period and the antidot size, as well as on the smoothness of the potential at the 2DEG-antidot interface. This result indicates the necessity of revising the interpretation of all numerous experiments with antidot lattices, since this effect has been explicitly or implicitly neglected in them.

Spin-Wave Excitations in Effectively Half-Filled Mott Insulators

We study spin-wave excitations in an effectively half-filled Hubbard model, where the effectively half-filled situation is realized when all holes doped in the exactly half-filled parent compound become self-trapped lattice polarons, and their hopping rate is much slower than the neutron scattering time of magnetic excitation measurements. It is further assumed that the doped holes become stabilizing centers of spin vortices; thus, the spin texture is not necessarily that of an antiferromagnet. We derive a new spin Hamiltonian that is suitable for this situation by extending the usual antiferromagnetic Heisenberg model. It is shown that two modes of spin-wave excitations exist in this new spin Hamiltonian: the first is the one exhibits antiferromagetic dispersion in high energy excitations; the other shows a sharp commensurate peak at the excitation energy maximum, and a broadened dispersion at energies below. It is shown that the sum of these two modes form an hourglass-shaped magnetic excitation spectru...