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Rare-earth iron garnets $R_{3}$Fe$_{5}$O$_{12}$ are fascinating insulators with very diverse magnetic phases. Their strong potential in spintronic devices has encouraged a renewal of interest in the study of their low temperature spin structures and spin wave dynamics. A striking feature of rare-earth garnets with $R$-ions exhibiting strong crystal-field effects like Tb-, Dy-, Ho-, and Er-ions is the observation of low-temperature non-collinear magnetic structures featuring "umbrella-like" arrangements of the rare-earth magnetic moments. In this study, we demonstrate that such umbrella magnetic states are naturally emerging from the crystal-field anisotropies of the rare-earth ions. By means of a general model endowed with only the necessary elements from the crystal structure, we show how umbrella-like spin structures can take place and calculate the canting-angle as a result of the competition between the exchange-interaction of the rare-earth and the iron ions as well as the crystal-field anisotropy. Our results are compared to experimental data, and a study of the polarised spin wave dynamics is presented. Our study improves the understanding of umbrella-like spin structures and paves the way for more complex spin wave calculations in rare-earth iron garnets with non-collinear magnetic phases., Comment: 8 pages, 7 figures

The effect of scattering of conduction electrons by dynamical spin fluctuations on the thermopower in metals near a thermal phase transition into an antiferromagnetic phase is considered. We are interested in a transition at room temperature, as has been studied in a heterostructure involving layers of IrMn. We show that the electrical resistivity exhibits a narrow but low peak at the transition, which may be difficult to detect on top of the main contributions induced by phonons and impurities. By contrast, the thermopower is found to exhibit a prominent peak both as a function of temperature $T$ for fixed layer thickness ${t}_{\text{AFM}}$ and as a function of ${t}_{\text{AFM}}$ for fixed $T.$ We conjecture that the transition temperature ${T}_{c}$ is a function of both ${t}_{\text{AFM}}$ and the Fermi energy ${\ensuremath{\epsilon}}_{F}$. Both dependencies give rise to a sharp peak of the thermopower as a function of $T$ or ${t}_{\text{AFM}}$ near the transition. The estimated magnitude and width of the peak for the case of either three- or two-dimensional longitudinal spin fluctuations is in good agreement with experiment.

In frustrated magnetic systems with competing interactions fluctuations can lift the residual accidental degeneracy. We argue that the state selection may have different outcomes for quantum and thermal order by disorder. As an example, we consider the semiclassical Heisenberg fcc antiferromagnet with only the nearest-neighbor interactions. Zero-point oscillations select the type 3 collinear antiferromagnetic state at T=0. Thermal fluctuations favor instead the type 1 antiferromagnetic structure. The opposite tendencies result in a finite-temperature transition between the two collinear states. Competition between effects of quantum and thermal order by disorder is a general phenomenon and is also realized in the J1-J2 square-lattice antiferromagnet at the critical point J2 = 0.5 J1., 6 pages, 3 figures (accepted PRB Rapid Comm.)

By means of spin current, the flow of spin angular momentum, we find a regime of "spin treacle" in a frustrated magnetic system. To establish its existence, we have performed spin transport measurements in nanometer-scale spin glasses. At temperatures high enough that the magnetic moments fluctuate at high frequencies, the spin Hall angle, the conversion yield between spin current and charge current, is independent of temperature. The spin Hall angle starts to decrease at a certain temperature $T^{*}$ and completely vanishes at a lower temperature. We argue that the latter corresponds to the spin freezing temperature $T_{\rm f}$ of the nanometer-scale spin glass, where the direction of conduction electron spin is randomized by the exchange coupling with the localized moments. The present experiment \textit{quantitatively} verifies the existence of a distinct "spin treacle" between $T_{\rm f}$ and $T^{*}$. We have also quantified a time scale of fluctuation of local magnetic moments in the spin treacle from the spin relaxation time of conduction electrons., 8 pages, 9 figures

We measure the mode-resolved direction of the precessional motion of the magnetic order, i.e., magnon polarization, via the chiral term of inelastic polarized neutron scattering spectra. The magnon polarization is a unique and unambiguous signature of magnets and is important in spintronics, affecting thermodynamic properties such as the magnitude and sign of the spin Seebeck effect. However, it has never been directly measured in any material until this work. The observation of both signs of magnon polarization in Y_{3}Fe_{5}O_{12} also gives direct proof of its ferrimagnetic nature. The experiments agree very well with atomistic simulations of the scattering cross section.

High field magnetization measurements in pulsed fields up to 65 T have been performed on ${\mathrm{FePS}}_{3}$, which is nominally a good example of a two-dimensional Ising-like antiferromagnet on a honeycomb lattice. Measurements with the field parallel to the moment direction confirm the presence of two first-order transitions above 35 T, to $M/{M}_{\mathrm{sat}}=1/2$ and $M/{M}_{\mathrm{sat}}=1$, respectively, at 4 K. The measurements are in contradiction with published estimates for the magnetic exchange parameters, but the contradiction can be resolved by allowing for anisotropic exchange parameters in the Hamiltonian. The magnetization with the field perpendicular to the moment direction is anisotropic, with no transitions observed for fields along the a axis while a cascade of first-order transitions is observed for fields above 50 T along the b axis, the latter case also showing a strong degradation of the sample after repeated pulses. The results indicate a strong magnetolattice coupling in ${\mathrm{FePS}}_{3}$. Temperature-dependent measurements hint at a possible tricritical point.

Terbium gallium garnet (TGG), ${\mathrm{Tb}}_{3}{\mathrm{Ga}}_{5}{\mathrm{O}}_{12}$, is well known for its applications in laser optics but also exhibits complex low-temperature magnetism that is not yet fully understood. Its low-temperature magnetic order is determined by means of time-of-flight neutron powder diffraction. It is found to be a multiaxial antiferromagnet with magnetic ${\mathrm{Tb}}^{3+}$ ions forming six sublattices of magnetic moments aligned parallel and antiparallel to the $\ensuremath{\langle}100\ensuremath{\rangle}$ crystallographic directions of the cubic unit cell. The structure displays strong easy-axis anisotropy with respect to a twofold axis of symmetry in the local orthorhombic environment of the ${\mathrm{Tb}}^{3+}$ sites. The crystal-field splitting within the single-ion ground-state manifold is investigated by inelastic neutron scattering on powder samples. A strong temperature dependence of the quasidoublet ground state is observed and revised parameters of the crystal-field Hamiltonian are given. The results of bulk magnetic susceptibility and magnetization measurements are in good agreement with values based on the crystal-field model down to 20 K, where the onset of magnetic correlations is observed.

FePS$_3$ is a van der Waals compound with a honeycomb lattice that is a good example of a two-dimensional antiferromagnet with Ising-like anisotropy. Neutron spectroscopy data from FePS3 were previously analysed using a straight-forward Heisenberg Hamiltonian with a single-ion anisotropy. The analysis captured most of the elements of the data, however some significant discrepancies remained. The discrepancies were most obvious at the Brillouin zone boundaries. The data are subsequently reanalysed allowing for unequal exchange between nominally equivalent nearest-neighbours, which resolves the discrepancies. The source of the unequal exchange is attributed to a biquadratic exchange term in the Hamiltonian which most probably arises from a strong magnetolattice coupling. The new parameters show that there are features consistent with Dirac magnon nodal lines along certain Brillouin zone boundaries., 8 pages, 4 figures. The following article has been accepted by the Journal of Applied Physics. After it is published, it will be found at (https://publishing.aip.org/resources/librarians/products/journals/). The article was submitted as part of a special topic edition (https://publishing.aip.org/publications/journals/special-topics/jap/2d-quantum-materials-magnetism-and-superconductivity/)

The mineral linarite, ${\mathrm{PbCuSO}}_{4}(\mathrm{OH}{)}_{2}$, is a spin-$1/2$ chain with frustrating nearest-neighbor ferromagnetic and next-nearest-neighbor antiferromagnetic exchange interactions. Our inelastic neutron scattering experiments performed above the saturation field establish that the ratio between these exchanges is such that linarite is extremely close to the quantum critical point between spin-multipolar phases and the ferromagnetic state. We show that the predicted quantum multipolar phases are fragile and actually suppressed by a tiny orthorhombic exchange anisotropy and weak interchain interactions in favor of a dipolar fan phase. Including this anisotropy in classical simulations of a nearly critical model explains the field-dependent phase sequence of the phase diagram of linarite, its strong dependence of the magnetic field direction, and the measured variations of the wave vector as well as the staggered and the uniform magnetizations in an applied field.

The magnetic oscillations in YBCO high-temperature superconductors have been widely studied over the last decade and consist of three equidistant low frequencies with a central frequency several times more intense than its two shoulders. This remains a puzzle in spite of numerous attempts to explain the corresponding small Fermi-surface pockets. Furthermore the ARPES data indicate only four Fermi-arcs with bilayer splitting, and show no sign of such small areas in the Fermi surface. Here we argue that the magnetic oscillations measured in under-doped bilayer high temperature superconductors, in particular YBa$_{2}$Cu$_{3}$O$_{6+\delta }$, provide a measure of the interplanar electronic coupling rather than the areas of fine-grain reconstruction of the Fermi surfaces coming from induced charge density waves. This identification is based on the relative intensities of the different peaks, as well as their angular dependence, which points to an effective Fermi surface that is larger than the oscillation frequencies, and is compatible with several indications from ARPES. The dominance of such frequencies with respect to the fundamental frequencies from the Fermi surface is natural for a strongly correlated quasi-two dimensional electronic systems where non-linear mixings of frequencies are more resistant to sample inhomogeneity., Comment: 12 pages. arXiv admin note: substantial text overlap with arXiv:1606.03942

Motivated by the complex phase diagram of MnWO4, we investigate competition between anisotropy, magnetic field, and helicity for the anisotropic next-nearest-neighbor Heisenberg model. Apart from two competing exchanges, which favor a spiral magnetic structure, the model features the bi-axial single-ion anisotropy. The model is treated in the real-space mean-field approximation and the phase diagram containing various incommensurate and commensurate states is obtained for different field orientations. We discuss similarities and differences of the theoretical phase diagram and the experimental diagram of MnWO4., Comment: 5 pages, as accepted

We study the electrical transport through nanopores affected by the presence of DNA translocations. The current through the pore depends not only on the base pair inside the pore but also on what the neighbouring base pair outside of the pore is, because the neighbouring base pair possesses an electric dipole moment that exerts an electric field at the nanopore with a magnitude which is comparable to the externally applied electric field. This physical effect has not been included in previous studies, which focused on the blockage of the current by the bases inside the pore. There is much interest in extracting the base information from the current through the pore and to sequence the DNA. This extraction including our effect requires an analytical study of the phenomena so that the inverse problem is tractable. To study the electrical transport quantitatively and analytically, we formulate a rigorous analytical circuit theory for the interaction of a conducting medium with the electromagnetic field in the presence of a nanopore. We found that the base pair dipolar field only affects the total current when another base pair is in the pore and blockage occurs. It does not change the current otherwise. The effect of these dipoles is through a change of the diffusion induced by a change at the nanopore of the local charge carrier density and not by a change of the local conductivity. In addition to the effect of the dipoles, we clarify how the large capacitive response affects the current and when it will not do so.We study the electrical transport through nanopores affected by the presence of DNA translocations. The current through the pore depends not only on the base pair inside the pore but also on what the neighbouring base pair outside of the pore is, because the neighbouring base pair possesses an electric dipole moment that exerts an electric field at the nanopore with a magnitude which is comparable to the externally applied electric field. This physical effect has not been included in previous studies, which focused on the blockage of the current by the bases inside the pore. There is much interest in extracting the base information from the current through the pore and to sequence the DNA. This extraction including our effect requires an analytical study of the phenomena so that the inverse problem is tractable. To study the electrical transport quantitatively and analytically, we formulate a rigorous analytical circuit theory for the interaction of a conducting medium with the electromagnetic field in th...

The Fourier transform of the observed magnetic quantum oscillations (MQO) in YBa$_{2}$Cu$_{3}$O$_{6+\delta }$ high-temperature superconductors has a prominent low-frequency peak with two smaller neighbouring peaks. The separation and positions of these three peaks are almost independent of doping. This pattern has been explained previously by rather special, exquisitely detailed, Fermi-surface reconstruction. We propose that these MQO have a different origin, and their frequencies are related to the bilayer and inter-bilayer electron hopping rather than directly to the areas of tiny Fermi-surface pockets. Such so-called "slow oscillations" explain more naturally many features of the observed oscillations and allow us to estimate the inter-layer transfer integrals and in-plane Fermi momentum., Comment: 6 pages

We review a selection of recent applications of neutron scattering to the field of quantum magnetism. We focus on systems where, because of quantum fluctuations enhanced by frustration and low dimension, there is no long range magnetic order in the ground state. We select two examples that we treat in more depth to show how neutron studies, in conjunction with the results of other experimental techniques, can give new insights. The first is the case of the spin ladder NaV2O5, where the origin of the spin gap at low temperature is now understood in detail. Apparent contradictions between quantitative measures of the charge order from neutron inelastic scattering, resonant X-ray scattering and NMR have been resolved giving interesting insights into the correlations. The second case is that of spin dimer system Cs3Cr2 X 9 (X= Br, Cl), undergoing transitions to field induced transverse magnetic order. The Br compound is attractive as the critical fields are sufficiently low that a complete study, in different field directions, is possible. In addition, it is noteworthy in that the magnon that softens and condenses is incommensurable with the lattice. The common description in terms of Bose–Einstein condensation must be extended to include a continuous degeneracy and single ion anisotropy, and conclusions can be drawn by comparison with the Cl compound. To cite this article: B. Grenier, T. Ziman, C. R. Physique 8 (2007).

We investigate the possibility of ultracold neutron (UCN) storage in quantum states defined by the combined potentials of the Earth's gravity and the neutron optical repulsion by a horizontal surface of liquid helium. We analyse the stability of the lowest quantum state, which is most susceptible to perturbations due to surface excitations, against scattering by helium atoms in the vapor and by excitations of the liquid, comprised of ripplons, phonons and surfons. This is an unusual scattering problem since the kinetic energy of the neutron parallel to the surface may be much greater than the binding energies perpendicular. The total scattering time constant of these UCNs at 0.7 K is found to exceed one hour, and rapidly increasing with decreasing temperature. Such low scattering rates should enable high-precision measurements of the scheme of discrete energy levels, thus providing improved access to short-range gravity. The system might also be useful for neutron beta-decay experiments. We also sketch new experimental concepts for level population and trapping of UCNs above a flat horizontal mirror., 18 pages, 3 figures

We have measured spin Hall effects in spin glass metals, CuMnBi alloys, with the spin absorption method in the lateral spin valve structure. Far above the spin glass temperature Tg where the magnetic moments of Mn impurities are randomly frozen, the spin Hall angle of CuMnBi ternary alloy is as large as that of CuBi binary alloy. Surprisingly, however, it starts to decrease at about 4Tg and becomes as little as 7 times smaller at 0.5Tg. A similar tendency was also observed in anomalous Hall effects in the ternary alloys. We propose an explanation in terms of a simple model considering the relative dynamics between the localized moment and the conduction electron spin., 5 pages, 4 figures

We perform a systematical analysis of the spin Hall effect (SHE) in the Cu alloys doped with a series of 5 d elements, by the combined approach of density functional theory and Hartree–Fock approximation. We find that not only the spin orbit interactions (SOI) in both the 5 d and 6 p orbitals, but also the local correlations in the 5 d orbitals of the impurities, are decisive on the sign of the spin Hall angle (SHA). Including all of these three factors properly, we predict the SHA for each alloy in the series. The signs of CuIr and CuPt are sensitive to perturbation of the local correlations. This observation is favorable for controlling the sign of the transverse spin Hall voltage.

Using a recently developed semi-analytical method (Self-Consistent Local RPA or SC-LRPA) we study the stability of the ferromagnetic phase in diluted magnetic systems where the exchange coupling between magnetic impurities are of RKKY form. A short discussion of the relevance of these calculations with respect to the ferromagnetism observed in diluted ferromagnetic materials is provided. Then, within a two step approach, we study ferromagnetism in $Zn_{1-x}Cr_{x}Te$. In the first step of our study, we calculate the magnetic couplings between Mn impurities within the LDA. In the second step, we diagonalize the resulting effective Heisenberg Hamiltonian using the SC-LRPA. We also compare, when available, our calculations with Monte Carlo simulations and experimental measurements., Comment: Proceedings of the International Conference MSM05, Agadir Maroc September 2005'', to appear in Physica Status Solidi

We present a semi-analytic theory for the Curie temperature in diluted magnetic semi-conductors that treats disorder effects exactly in the effective Heisenberg Hamiltonian, and spin fluctuations within a local RPA. The exchange couplings are taken from concentration dependent {\it ab initio} estimates. The theory gives very good agreement with published data for well-annealed samples of Mn$_x$Ga$_{1-x}$As. We predict the critical temperatures for Mn$_x$Ga$_{1-x}$N lower than in doped GaAs, despite the stronger nearest-neighbour ferromagnetic coupling. We also predict the dependence on the hole concentration., 4 figures included

The magnetic dimer compounds Cs 3 Cr 2 Br 9 and Cs 3 Cr 2 Cl 9 are well characterized magnetic dimer compounds. In zero field the spin 3/2 moments are bound in an hexagonal lattice of magnetic singlets, but at finite magnetic fields there are transitions to field-induced magnetic phases. In the Bromide compound this transition occurs at sufficiently low fields, ∼1.5 T, that neutron diffraction results can give details of the static and dynamic magnetic structure even in the field induced phase. High magnetic field measurements have been used to determine the equation of state up to saturation. In this paper we discuss different approaches to the calculation of the magnetization and transverse magnetic order in the two compounds. Compared to spin 1/2 dimer materials, such as TlCuCl 3 , the higher spin allows for more important effects of anisotropy, and the frustration, leads to possibily incommensurate order. Two main theoretical methods will be discussed: extension of the standard theory of Tachiki and Y...

Recently a positive spin Hall angle (SHA) of 0.021, was observed experimentally in nonmagnetic CuIr alloys [Niimi et al., Phys. Rev. Lett. 106, 126601 (2011)] and attributed predominantly to an extrinsic skew scattering mechanism, while a negative SHA was obtained from ab initio calculations [Fedorov et al., Phys. Rev. B 88, 085116 (2013)], using consistent definitions of the SHA. We reconsider the SHA in CuIr alloys, with the effects of the local electron correlation U in 5d orbitals of Ir impurities, included by the quantum Monte Carlo method. We found that the SHA is negative if we ignore such local electron correlation, but becomes positive once U approaches a realistic value. This may open up a way to control the sign of the SHA by manipulating the occupation number of impurities.

Anisotropic spin-spin interactions of the symmetry described by Dzyaloshinsky and Moriya are generally considered weak, as they depend on the spin-orbit couplings. In frustrated spin systems with singlet ground states they can, however, have rather strong effects. We discuss recent results related to two gapped spin systems: $\rm CuGeO_3$ and $\rm SrCu_2(BO_3)_2$ in particular. In the first compound the Dzyaloshinsky-Moriya interactions effectively lower the symmetry of the magnetic unit cell and this leads to doubling of the low frequency mode. In the second case, the Dzyaloshinsky-Moriya interactions also split the lowest magnon mode linearly in the spin-orbit coupling. In addition, the relatively weak Dzyaloshinsky-Moriya interactions can dominate the dispersion. Consideration of the selection rules for optical transitions show that while the Dzyaloshinsky-Moriya interactions can explain much of the dynamics, they do not explain the observed transition amplitudes. This leads to a review of recent calculations of anisotropic spin-phonon couplings. We discuss how this leads to a novel mechanism to explain the ESR intensities in the spin gap systems discussed. Selection rules for this novel mechanism involving coupling to the electric field of the resonant probe are discussed and relation to polarised neutron experiments briefly mentioned., Comment: 9 pages, no figure, Proceedings of the Yukawa symposium, to appear in Progress in Theoretical Physics, 145 (2002)

We explore theoretical explanations for laser pumped experiments, that have established that muonium can be a tool in spintronics for following non-equilibrium spin polarizations of the conduction electrons. In the original experiments on Si the sensitivity was argued to be due to spin exchange with the bound electron in muonium, but in GaAs an effect is seen even in the case of negatively charged muonium. This is more problematic, as spin exchange with a third electron should be weak given the spin singlet state of the two bound electrons of the negative muonium ion. We discuss a mechanism to explain the sensitivity of negatively charged muonium ions to the spin-polarization of semiconductors in terms of the coherent mixing of charge states induced by hybridization with the semiconducting host.

The direct ESR absorption between the singlet ground and triplet excited states of the spin-gap systems is investigated. Such an absorption, which is forbidden by the conservation of the total spin quantum number in the ordinary Heisenberg-Hamiltonian, is caused by the Dzyaloshinskii–Moriya interaction. We show the selection rule in the presence of the interaction, using the exact diagonalization of the finite cluster of the quasi-one-dimensional bond-alternating spin system. The selection rule is also modified into a suitable form for the investigation of the recent experimental result for CuGeO3.

Magnetic oscillation phenomena in high magnetic fields are powerful probes of the band structure of the organics in high temperature phases and in possible low temperature density wave states. Recent calculations, which go beyond the standard theory of Falicov and Stochawski, are used to analyse apparent discrepancies between the frequencies and intensities observed in the breakdown regime, and those predicted from calculated band structures. Comparison is made to the α and κ phases of BEDT-TTF salts.

We present an inelastic neutron scattering study performed throughout the field-induced magnetic phase occurring between H c 1 ≃ 3.5 T and H c 2 ≃ 4.5 T in the spin 1/2 alternating antiferromagnetic chain compound Cu ( NO 3 ) 2 · 2.5 D 2 0 . The exchange couplings are consistently determined from the analysis of the spin dynamics both in the gapped phase, at H = 0 , and in the saturated phase, at H = 6 T , while the cancellation of the lowest energy gap in the complete field-induced magnetic phase suggests that this system is a good realization of a Bose–Einstein condensation.

The interpretation of de Haas--van Alphen oscillations in the presence of magnetic breakdown is usually based on the semiclassical theory of Falicov and Stachowiak (FS). There are now glaring discrepancies between its predictions and experiments, especially in quasi-two-dimensional organic conductors. We present an extension of the theory, using the appropriate constraints of conserved electron density, which explains the occurrence of frequencies not predicted by FS, and makes explicit the amplitudes as a function of Fermi surface parameters. The results involve a tunneling amplitude between different sheets as in FS, but other parameters as well, such as the areas of different orbits.

We use an algebraic method to compute de Haas–van Alphen oscillations in two-dimensional systems in the semiclassical approximation for cases where the Fermi surface lies on more than one sheet of the energy surface. We treat magnetic breakdown by computing the Riemann surface associated with the Bloch energy equation. The topology of this surface, in particular, its fundamental group, is used to classify electronic trajectories in the complexified Brillouin zone. Three examples taken from tight-binding models of quasi-twodimensional organic conductors show how this can be implemented to calculate frequencies and breakdown fields. @S0163-1829~98!04803-6#

We present a neutron scattering study performed in the field-induced magnetic phase occurring above a critical field H c 1 ≃ 1.5 – 2 T in Cs3Cr2Br9 for the field applied parallel and perpendicular to the dimers. The magnetic structure reveals some important differences between the two field orientations. The spin dynamics study shows that the lowest excitation is gapped above H c 1 in both cases but behaves differently close to the critical field.

We have performed extended H\"uckel tight-binding calculations to model the effects of pressure and uniaxial stress on the electronic band structure of the anisotropic organic metals \ensuremath{\alpha}-(BEDT-TTF${)}_{2}$KHg(SCN${)}_{4}$ (a) and \ensuremath{\kappa}-(BEDT-TTF${)}_{2}$Cu(NCS${)}_{2}$ (b). The calculated changes in the Fermi-surface topology are in excellent agreement with the experimental values determined from Shubnikov--de Haas measurements. In (a) the area of the closed orbits increases with pressure and decreases with uniaxial stress, while in (b) the closed orbits increase both with pressure and stress. We also report predictions of the effects of uniaxial stress in the a and c directions, and discuss the behavior of the effective mass and magnetic breakdown probability. \textcopyright{} 1996 The American Physical Society.

Spin waves in the type-III ordered antiferromagnetic state of the frustrated t- t ′ Hubbard model on the face-centred-cubic (fcc) lattice are calculated to investigate finite-U-induced competing interaction and frustration effects on magnetic excitations and instabilities. Particularly strong competing interactions generated due to the interplay of fcc lattice geometry and magnetic order result in significant spin wave softening. The calculated spin wave dispersion is found to be in qualitative agreement with the measured spin wave dispersion in the pyrite mineral MnS2 obtained from inelastic neutron scattering experiments. Instabilities to other magnetic orders (type I, type II, spiral, non-collinear), as signalled by spin wave energies turning negative, are also discussed.

The spin Hall effect is affected by the Coulomb interaction as well as spin–spin correlations in metals. Here we examine the enhancement in the effect caused by resonant skew scattering induced by electron correlations. For single-impurity scattering, local Coulomb correlations may significantly change the observed spin Hall angle. There may be additional effects because of the special atomic environment close to a surface — extra degeneracies compared to the bulk, enhanced correlations that move the relative d- or f-levels, and interference effects coming from the lower local dimension. Our results may explain the very large spin Hall angle observed in CuBi alloys. We discuss the impact on the spin Hall effect from cooperative effects, firstly in an itinerant ferromagnet where there is an anomaly near the Curie temperature originating from high-order spin fluctuations. The second case considered is a metallic spin glass, where exchange via slowly fluctuating magnetic moments may lead to the precession of...

We present a quantitative theory for ferromagnetism in diluted magnetic semiconductors under the assumption that the positions of the substitutional magnetic impurities are correlated. The theory predicts that room-temperature ferromagnetism should be possible in macroscopically homogeneous samples of MnxGa1−xAs if the probability of pairs of Mn impurities on neighboring Ga sites is sufficiently enhanced. The theory predicts similar increases in the Curie temperatures for MnxGa1−xN but the temperatures remain lower than in MnxGa1−xAs.

We give a theory of the inverse spin Hall effect (ISHE) in ferromagnetic metals based on skew scattering via collective spin fluctuations. This extends Kondo's theory of the anomalous Hall effect (AHE) to include short-range spin-spin correlations. We find a relation between the ISHE and the four-spin correlations near the Curie temperature TC. Such four-spin correlations do not contribute to the AHE, which relates to the three-spin correlations. Thus our theory shows an essential difference between the AHE and ISHE, providing an essential complement to Kondo's classic theory of the AHE in metals. We note the relation to skew-scattering mechanisms based on impurity scattering. Our theory can be compared to recent experimental results byWei et al. [Nat. Commun. 3, 1058 (2012)] for the ISHE in ferromagnetic alloys., Comment: 5 pages, 2 figures

We study the effect of a random magnetic field on the Harper problem for localization properties. The Landau subbands disappear with increasing disorder. However, in a certain range of energies there remain delocalized states at energies corresponding to the centre of these bands. For the outlying bands, we show numerical evidence that the criticality is the same as for the lowest Landau band with potential disorder. For energies close to the band centre, there is a crossover to less universal behaviour that appears to be similar to the case of random flux with zero average flux.

We study the effect of a random magnetic field on the behavior of the density of states in the Harper problem. By complete diagonalization of lattices of 100\ifmmode\times\else\texttimes\fi{}100 sites we show the manner in which the Landau subbands disappear with increasing disorder. At maximum disorder zero-counting techniques for lattices with a large number of sites, up to several million, show the density of states becoming flat and with a narrow bandwidth compared to the pure model in zero field. This allows detailed comparison with previous analytical calculations of the Lifshitz tails. The self-retracing-path approximation gives a good account of the results, except for the tail.

Spin relaxation in a quantum Hall ferromagnet, where filling is $\nu=1, 1/3, 1/5,...$, can be considered in terms of spin wave annihilation/creation processes. Hyperfine coupling with the nuclei of the GaAs matrix provides spin non-conservation in the two-dimensional electron gas and determines spin relaxation in the quantum Hall system. This mechanism competes with spin-orbit coupling channels of spin-wave decay and can even dominate in a low-temperature regime where $T$ is much smaller than the Zeeman gap. In this case the spin-wave relaxation process occurs non-exponentially with time and does not depend on the temperature. The competition of different relaxation channels results in crossovers in the dominant mechanism, leading to non-monotonic behavior of the characteristic relaxation time with the magnetic field. We predict that the relaxation times should reach maxima at $B\simeq 18\,$T in the $\nu=1$ Quantum Hall system and at $B\simeq 12\,$T for that of $\nu=1/3\,$. We estimate these times as $\sim10\,-\,30\,\mu$s and $\sim2\,-\,5\,\mu$s, respectively., Comment: 26 pages, 1 figure

We consider sweeping a system through a Landau-Zener avoided-crossing, when that system is also coupled to an environment or noise. Unsurprisingly, we find that decoherence suppresses the coherent oscillations of quantum superpositions of system states, as superpositions decohere into mixed states. However, we also find an effect we call "Lamb-assisted coherent oscillations", in which a Lamb shift exponentially enhances the coherent oscillation amplitude. This dominates for high-frequency environments such as super-Ohmic environments, where the coherent oscillations can grow exponentially as either the environment coupling or temperature are increased. The effect could be used as an experimental probe for high-frequency environments in such systems as molecular magnets, solid-state qubits, spin-polarized gases (neutrons or He3) or Bose-condensates., 4 Pages & 4 Figs - New version: introduction extended & citations added

The dynamical spectral function of a single hole moving in a two-dimensional antiferromagnet is calculated on clusters of up to 26 sites. Quasiparticle peaks in the spectral function show up unambiguously at the bottom of the continuum. In some limited region of k space vague structures in the spectral density might be reminiscent of excited string levels. Our data for the Z factor of the t-J model are in excellent agreement with the self-consistent Born approximation

By quantum Monte Carlo simulation of a realistic multiorbital Anderson impurity model, we study the spin-orbit interaction (SOI) of an Fe impurity in Au host metal. We show, for the first time, that the SOI is strongly renormalized by the quantum spin fluctuation. Based on this mechanism, we can explain why the gigantic spin Hall effect in Au with Fe impurities was observed in recent experiment, while it is not visible in the anomalous Hall effect. In addition, we show that the SOI is strongly renormalized by the Coulomb correlation U. Based on this picture, we can explain past discrepancies in the calculated orbital angular momenta for an Fe impurity in an Au host., 4 pages, 3 figures

We show, both experimentally and theoretically, a novel route to obtain giant room temperature spin Hall effect due to surface-assisted skew scattering. In the experiment, we report the spin Hall effect in Pt-doped Au films with different thicknesses $t_N$. The giant spin Hall angle $\gamma_S$ = $0.12 \pm 0.04$ is obtained for $t_N$ = 10 nm at room temperature, while it is much smaller for $t_N$ = 20 nm sample. Combined ab initio and quantum Monte Carlo calculations for the skew scattering due to a Pt impurity show $\gamma_S$ $\cong$ 0.1 on the Au (111) surface, while it is small in bulk Au. The quantum Monte Carlo results show that the spin-orbit interaction of the Pt impurity on the Au (111) surface is enhanced, because the Pt 5d levels are lifted to the Fermi level due to the valence fluctuation. In addition, there are two spin-orbit interaction channels on the Au (111) surface, while only one in bulk Au., Comment: 4 pages, 2 figures

Exact diagonalizations of small clusters up to 26 sites are used to extrapolate the ground-state energy of a single hole in the Neel state of the t-J, model or in the quantum antiferromagnetic (t-J model). A surprisingly rapid convergence with system size is observed for a wide range of parameters and for the largest clusters considered. In the Ising limit the energy dependence with J z is remarkably close to a J z 2/3 law for intermediate coupling in agreement with Brinkman-Rice type of calculations

We study the possibility of d0 ferromagnetism in the compound MgO doped with nitrogen (N). The Haldane-Anderson impurity model is formulated within the tight-binding approximation for determining the host band-structure and the impurity-host hybridization. Using the quantum Monte Carlo technique, we observe a finite local moment for an N impurity, and long-range ferromagnetic correlations between two N impurities. The ferromagnetic correlations are strongly influenced by the impurity bound state. When the ferromagnetic correlation between a pair of impurities is mapped onto the isotropic Heisenberg model for two spin-1/2 particles, the effective exchange constant J12 is found to increase with increasing temperature. Similar temperature dependence of J12 is also obtained in other diluted magnetic semiconductors, such as zincblende ZnO doped with Mn. The temperature dependence of J12 suggests that the mapping of the full Hamiltonian onto the spin Hamiltonian cannot fully describe the magnetic correlations for the diluted magnetic semiconductors at least in the limit of low impurity spin., Comment: 8 pages, 8 figures

Recent detailed first principles configurations interaction calculations with band structure wave functions for both YBa2Cu3O7−x and Tl2Ba2CaCu2O8 show that the net bare Coulomb interaction can cause superconductivity because of the anisotropic exchange interaction. Here we propose a simple model with electrons interacting with an anisotropic interaction, solve the mean field gap equation analytically, and demonstrate that, without any intermediate virtual excitations, a finite temperature superconducting transition can result because of the exchange interaction.

A recent experiment in CuBi alloys obtained a large spin Hall angle (SHA) of -0.24 (Niimi et al. Phys. Rev. Lett. 109, 156602 (2012)). We find that the SHA can be dramatically enhanced by Bi impurities close to the Cu surface. The mechanisms of this enhancement are two-fold. One is that the localized impurity state on surface has a decreased hybridization and combined with Coulomb correlation effect. The other comes from the low-dimensional state of conduction electrons on surface, which results in a further enhancement of skew scattering by impurities. Furthermore, we note that a discrepancy in sign of SHA between the experiment and previous theories is simply caused by different definitions of SHA. This re-establishes skew scattering as the essential mechanism underlying the spin Hall effect in CuBi alloys., 5 pages, 1 figure