Antiferromagnetic Zone Boundary Research Articles

Strain-tuned incompatible magnetic exchange-interaction in La2NiO4

Magnetic frustration is a route for novel ground states, including spin liquids and spin ices. Such frustration can be introduced through either lattice geometry or incompatible exchange interactions. Here, we find that epitaxial strain is an effective tool for tuning antiferromagnetic exchange interactions in a square-lattice system. By studying the magnon excitations in La2NiO4 films using resonant inelastic x-ray scattering, we show that the magnon displays substantial dispersion along the antiferromagnetic zone boundary, at energies that depend on the lattice of the film’s substrate. Using first principles simulations and an effective spin model, we demonstrate that the antiferromagnetic next-nearest neighbour coupling is a consequence of the two-orbital nature of La2NiO4. Altogether, we illustrate that compressive epitaxial strain enhances this coupling and, as a result, increases the level of incompatibility between exchange interactions within a model square-lattice system.

Explaining the pseudogap through damping and antidamping on the Fermi surface by imaginary spin scattering

The mechanism of the pseudogap observed in hole-doped cuprates remains one of the central puzzles in condensed matter physics. We analyze this phenomenon via a Feynman-diagrammatic inspection of the Hubbard model. Our approach captures the pivotal interplay between Mott localization and Fermi surface topology beyond weak-coupling spin fluctuations, which would open a spectral gap near hot spots. We show that strong coupling and particle-hole asymmetry trigger a very different mechanism: a large imaginary part of the spin-fermion vertex promotes damping of antinodal fermions and, at the same time, protects the nodal Fermi arcs (antidamping). Our analysis naturally explains puzzling features of the pseudogap observed in experiments, such as Fermi arcs being cut off at the antiferromagnetic zone boundary and the subordinate role of hot spots.

In-plane spin excitation anisotropy in the paramagnetic state of NaFeAs

We use unpolarized and polarized inelastic neutron scattering to study low-energy spin excitations in NaFeAs, which exhibits a tetragonal-to-orthorhombic lattice distortion at $T_s\approx 58$ K followed by a collinear antiferromagnetic (AF) order below $T_N\approx 45$ K. In the AF ordered state ($T<T_N$), spin waves are entirely c-axis polarized below $\sim$10 meV, exhibiting a gap of $\sim4$ meV at the AF zone center and disperse to $\sim$7 meV near the c-axis AF zone boundary. On warming to the paramagnetic state with orthorhombic lattice distortion ($T_N<T<T_s$), spin excitations become anisotropic within the FeAs plane. Upon further warming to the paramagnetic tetragonal state ($T>T_s$), spin excitations become more isotropic. Since similar magnetic anisotropy is also observed in the paramagnetic tetragonal phase of superconducting BaFe$_{1.904}$Ni$_{0.096}$As$_2$, our results suggest that the spin excitation anisotropy in superconducting iron pnictides originates from similar anisotropy already present in their parent compounds.

Universal disorder in Bi2Sr2CaCu2O8+x

The cuprates contain a range of nanoscale phenomena that consist of both LDOS(E) features and spatial excitations. Many of these phenomena can only be observed through the use of a SI-STM and their disorder can be mapped out through the fitting of a phenomenological model to the LDOS(E). We present a study of the nanometer scale disorder of single crystal cryogenically cleaved samples of Bi2Sr2CaCu2O8+x whose dopings range from p = 0.19 to 0.06. The phenomenological model used is the Tripartite model that has been successfully applied to the average LDOS(E) previously. The resulting energy scale maps show a structured patchwork disorder of three energy scales, which can be described by a single underlying disordered parameter. This spatial disorder structure is universal for all dopings and energy scales. It is independent of the oxygen dopant negative energy resonances and the interface between the different patches takes the form of a shortened lifetime pseudogap/superconducting gap state. The relationship between the energy scales and the spatial modulations of the dispersive QPI, static q1* modulation and the pseudogap shows that the energy scales signatures in the LDOS(E) are tied to the onset and termination of the spatial excitations. The static q1* modulations local energy range is measured and its signature in the LDOS(E) is the kink, whose number of states are modulated with a wave vector of q1*. This analysis of both the LDOS(r,E) and the spatial modulations in q-space show a picture of a single underlying disordered parameter that determines both the LDOS(E) structure as well as the energy ranges of the QPI, q1* modulation and the pseudogap states. This parameter for a single patch can be defined by the Fermi surface crossing of the parent compound anti-ferromagnetic zone boundary for a model homogeneous superconductor with the same electronic properties as the patch.

Reconstructed Fermi Surface of UnderdopedBi2Sr2CaCu2O8+δCuprate Superconductors

The Fermi surface topologies of underdoped samples the high-Tc superconductor Bi2212 have been measured with angle resolved photoemission. By examining thermally excited states above the Fermi level, we show that the Fermi surfaces in the pseudogap phase of underdoped samples are actually composed of fully enclosed hole pockets. The spectral weight of these pockets is vanishingly small at the anti-ferromagnetic zone boundary, which creates the illusion of Fermi "arcs" in standard photoemission measurements. The area of the pockets as measured in this study is consistent with the doping level, and hence carrier density, of the samples measured. Furthermore, the shape and area of the pockets is well reproduced by a phenomenological model of the pseudogap phase as a spin liquid.

Crossover region between nodal and antinodal states at the Fermi level of optimally doped and overdopedBi2Sr1.6Nd0.4CuO6+δ

We have studied Bi$_{2}$Sr$_{1.6}$Nd$_{0.4}$CuO$_{6+\delta}$ using Angle Resolved Photoemission Spectroscopy in the optimal and overdoped regions of the phase diagram. We identify a narrow crossover region in the electronic structure between the nodal and antinodal regions associated with the deviation from a pure d-wave gap function, an abrupt increase of the quasiparticle lifetime, the formation of Fermi arcs above T$_c$, and a sudden shift of the bosonic mode energy from higher energy, $\sim${60meV}, near the nodal direction, to lower energy, $\sim${20meV}, near the antinodal direction. Our work underscores the importance of a unique crossover region in the momentum space near E$_F$ for the single layered cuprates, between the nodal and antinodal points, that is independent of the antiferromagnetic zone boundary.

A momentum-dependent perspective on quasiparticle interference in Bi2Sr2CaCu2O8+δ

Scanning tunnelling spectroscopy and angle-resolved photoemission spectroscopy are complementary probes, and yet the results of recent studies using these techniques on quasiparticle excitations in the copper oxide superconductors seem to be contradictory. In fact, there is no contradiction. Angle-resolved photoemission spectroscopy (ARPES) probes the momentum-space electronic structure of materials and provides invaluable information about the high-temperature superconducting cuprates1. Likewise, scanning tunnelling spectroscopy (STS) reveals the cuprates’ real-space inhomogeneous electronic structure. Recently, researchers using STS have exploited quasiparticle interference (QPI)—wave-like electrons that scatter off impurities to produce periodic interference patterns—to infer properties of the quasiparticles in momentum space. Surprisingly, some interference peaks in Bi2Sr2CaCu2O8+δ (Bi-2212) are absent beyond the antiferromagnetic zone boundary, implying the dominance of a particular scattering process2. Here, we show that ARPES detects no evidence of quasiparticle extinction: quasiparticle-like peaks are measured everywhere on the Fermi surface, evolving smoothly across the antiferromagnetic zone boundary. This apparent contradiction stems from differences in the nature of single-particle (ARPES) and two-particle (STS) processes underlying these probes. Using a simple model, we demonstrate extinction of QPI without implying the loss of quasiparticles beyond the antiferromagnetic zone boundary.

Variational ground states of the two-dimensional Hubbard model

Recent refinements of analytical and numerical methods have improved our understanding of the ground-state phase diagram of the two-dimensional (2D) Hubbard model. Here, we focus on variational approaches, but comparisons with both quantum cluster and Gaussian Monte Carlo methods are also made. Our own ansatz leads to an antiferromagnetic ground state at half filling with a slightly reduced staggered order parameter (as compared to simple mean-field theory). Away from half filling, we find d-wave superconductivity, but confined to densities where the Fermi surface passes through the antiferromagnetic zone boundary (if hopping between both nearest-neighbour and next-nearest-neighbour sites is considered). Our results agree surprisingly well with recent numerical studies using the quantum cluster method. An interesting trend is found by comparing gap parameters Δ (antiferromagnetic or superconducting) obtained with different variational wave functions. Δ varies by an order of magnitude and thus cannot be taken as a characteristic energy scale. In contrast, the order parameter is much less sensitive to the degree of sophistication of the variational schemes, at least at and near half filling.

Extinction of quasiparticle interference in underdoped cuprates with coexisting order

Scanning tunneling spectroscopy (STS) measurements [Y. Kohsaka et al., Nature (London) 454, 1072 (2008)] have shown that dispersing quasiparticle interference (QPI) peaks in Fourier-transformed conductance maps disappear as the bias voltage exceeds a certain threshold corresponding to the coincidence of the contour of constant quasiparticle energy with the period-doubled (e.g., antiferromagnetic) zone boundary. Here we show that this may be caused by coexisting order present in the $d$-wave superconducting phase. We show explicitly how QPI peaks are extinguished in the situation with coexisting long-range spin-density wave order and discuss the connection with the more realistic case where short-range order is created by quenched disorder. Since it is the localized QPI peaks rather than the underlying antinodal states themselves which are destroyed at a critical bias, our proposal resolves a conflict between STS and photoemission spectroscopy regarding the nature of these states. We also study the momentum-summed density of states in the coexisting phase and show how the competing order produces a kink inside the ``V''-shaped $d$-wave superconducting gap in agreement with recent STS measurements [J. W. Alldredge et al., Nat. Phys. 4, 319 (2008)].

Magnon and hole excitations in the two-dimensional half-filled Hubbard model

Spin and hole excitation spectra and spectral weights are calculated for the half-filled Hubbard model, as a function of $t/U$. We find that the high energy spin spectra are sensitive to charge fluctuations. The energy difference $\Delta(\pi,0)- \Delta(\pi/2,\pi/2)$, which is negative for the Heisenberg model, changes sign at a fairly small $t/U\approx 0.053(5)$. The hole bandwidth is proportional to $J$, and considerably larger than in the t-J models. It has a minimum at ($\pi/2,\pi/2$) and a very weak dispersion along the antiferromagnetic zone boundary. A good fit to the measured spin spectra in La$_2$CuO$_4$ at $T=10K$ is obtained with the parameter values $U=3.1{\rm eV}$, $t=0.35{\rm eV}$.

Angle-Resolved Photoemission Spectroscopy of the Antiferromagnetic SuperconductorNd1.87Ce0.13CuO4: Anisotropic Spin-Correlation Gap, Pseudogap, and the Induced Quasiparticle Mass Enhancement

We performed high-resolution angle-resolved photoemission spectroscopy on Nd1.87Ce0.13CuO4, which is located at the boundary of the antiferromagnetic (AF) and the superconducting phase. We observed that the quasiparticle (QP) effective mass around (pi,0) is strongly enhanced due to the opening of the AF gap. The QP mass and the AF gap are found to be anisotropic, with the largest value near the intersecting point of the Fermi surface and the AF zone boundary. In addition, we observed that the QP peak disappears around the Néel temperature (TN) while the AF pseudogap is gradually filled up at much higher temperatures, possibly due to the short-range AF correlation.

Angle resolved photoemission studies on Sm 2− xCe xCuO 4: remnant Fermi surfaces and coupling to (π,π) scattering

We report high-resolution angle-resolved photoelectron spectroscopy studies on electron doped superconductors, Sm 2− x Ce x CuO 4 ( x=0 and 0.15). The results on the undoped ( x=0) samples show Cu–O anti-bonding states with an overall dispersion of about 0.3 eV which is similar to other insulating cases. The low energy dispersive feature quickly looses intensity before it reaches the Fermi energy near the anti-ferromagnetic zone boundary, showing a remnant behavior of the original Fermi surface (FS). In doped cases ( x=0.15), FS mapping shows suppressed intensities at the Fermi energy where the FS crosses the anti-ferromagnetic zone boundary. This is similar to what was observed on another electron doped superconductor Nd 1.85Ce 0.15CuO 4 which was ascribed being due to the formation of a high-energy pseudogap assisted by Q =( π,π) scattering. Similarities and differences in comparison to other compounds are discussed.

Theory of superconductivity coexisting with the antiferromagnetic state in UPd2Al3

We investigate the superconductivity (SC) coexisting withthe antiferromagnetic (AF) state observed in UPd2Al3.Referring to the band calculation, we consider the band structure with adual nature, that is, localized electrons, which mainly construct the AFstate, and itinerant electrons, which form the heavy fermion state andthe unconventional SC at lower temperatures. Within the random-phaseapproximation (RPA), we estimate the AF transition temperature TN, andcalculate the dynamical spin susceptibility. It possesses a collective spin wave around Q = (0, 0, 0.5) belowTN.In this situation, we evaluate the Eliashberg equation for the remaining itinerant band withinthe RPA. The most favourable even-parity pairing symmetry is cos(kx + 2ky) − cos(kx − 2ky) modulated by acos(kz)-like function. Thisis the symmetry dxywhich, in addition, possesses a tiny gap around the AF zone boundary.

Universality of the electronic structure from a half-filledCuO2plane

We present angle resolved photoemission (ARPES) data from insulating A_2CuO_2X_2 (A=Sr, Ca; X=Cl, Br), Sr_2Cu_3O_4Cl_2, and Bi_2Sr_2MCu_2O_8 (M=Er, Dy) single crystals which illustrate that the low energy electronic structure of the half-filled cuprates are independent of the apical atom. By performing a photon energy dependent study on Ca_2CuO_2Cl_2 we are able to distinguish between features which are intrinsic and those which are a result of the photoemission matrix elements. We find that the dispersion is independent of photon energy, while an asymmetry in the electron occupation probability across the antiferromagnetic zone boundary is robust to variations in photon energy. Finally, we show that the d-wave-like dispersion which exists in the insulator along the antiferromagnetic zone boundary does not precisely fit the simple d-wave functional form near the nodal direction.

Spin-wave spectrum inLa2CuO4:Double occupancy and competing interaction effects

The recently observed spin-wave energy dispersion along the antiferromagnetic zone boundary in ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ is discussed in terms of double occupancy and competing interaction effects in the $t\ensuremath{-}{t}^{\ensuremath{'}}$ Hubbard model on a square lattice.

Evolution of hole and spin dynamics in high- Tc superconductors within the small hole density limit of the t - J model

The evolution of hole and spin dynamics in high T c superconductors is studied within the self-consistent noncrossing approximation of the t - J model in the small hole density limit. As the doping concentration is increased, long-range electron correlations disappear rapidly and the quasiparticle energy band becomes considerably narrow. At a small hole density of a few percent, long-range antiferromagnetic order is destroyed leading to the inadequacy of spin wave basis approximation near small wave vectors. Spin excitations near the antiferromagnetic zone boundary are strongly renormalized and damped, but they are still well described within spin wave basis approximation.

Critical and paramagnetic spin dynamics in an antiferromagnetically coupled Heisenberg magnet: results for RbMnF3

Results are provided for the spin-spin response function of a three-dimensional, antiferromagnetically coupled Heisenberg magnet covering a range of temperatures from the critical temperature to deep in the paramagnetic phase. The wave vectors considered span the Brillouin zone. Damping rates are given at the zone centre and the antiferromagnetic-ordering wave vector, w, together with copious numerical results for the full response function. The calculations are based on the non-linear, integral-differential equations obtained from so-called coupled-mode theory. In a confrontation between experimental and theoretical findings for RbMnF3 nearly all aspects have a positive outcome. The main exception is found at T, for wave vectors close to w. Here, the measured response comprises three distinct components, reasonably ascribed to diffusive and oscillatory collective processes. In the corresponding predictions, the diffusive component is conspicuously missing. A less pronounced discrepancy if found at the antiferromagnetic zone boundary where, once again, there is more structure in the observed spectrum than in the calculated one.

Bounds for correlation functions of the Heisenberg antiferromagnet

The authors present rigorous lower bounds for spin correlations of the quantum Heisenberg antiferromagnet under the assumption of long range order, at zero as well as nonzero temperatures in the form of a crossover function. These are obtained by combining a recent inequality giving lower bounds to correlation functions with infrared upper bounds for the static susceptibility. The functional forms of the lower bounds are identical with upper (infrared) bounds for the correlation functions implying that the nature of divergence of the correlations near the antiferromagnetic zone boundary is determined. The form agrees with the prediction of the theory of spin waves. They also present a generalized inequality in quantum statistical mechanics that contains the previously known cases as special cases.

Neutron scattering study of quantum excitations in a S = ½ Heisenberg antiferromagnetic chain

Inelastic neutron scattering has been used to study the magnetic excitations of a nearly ideal realisation of a one-dimensional S =½ Heisenberg antiferromagnet, KCuF 3. The experiments were performed using the pulsed neutron source ISIS and it is seen that time of flight methods are particularly well suited to the study of low dimensional systems. The excitation spectrum consists of a quantum continuum of excited states extending up to energies double that of the usual antiferromagnetic zone boundary spin-wave mode. The scattering can be described very well by a theoretical ansatz for the dynamical correlation function.

Spectral function of holes in a quantum antiferromagnet.

By use of an effective Hamiltonian which takes into account the constraints on the motion of a hole in a quantum antiferromagnet, the spectral function of the hole is calculated. For small exchange and away from the antiferromagnetic zone boundary, it is found to be dominated by incoherent multiplespin-wave processes. The dispersion of the quasiparticle part and the possible implications for disordering of the quantum antiferromagnet are discussed.