2D Antiferromagnet Research Articles

Two-dimensional spin-1 frustrated Heisenberg model with valence-bond ground states

In this paper, we propose a method to understand the nature of the quantum disorder phase of the two-dimensional (2D) high-spin frustrated model. The ground state and excitation properties of a fully frustrated 2D spin-1 model are studied based on a model whose ground state can be found exactly. By analogy to the pseudopotential approach in the fractional quantum Hall effect, we conclude that the ground states of the fully frustrated spin-1 model are doubly degenerate valence-bond solid (VBS) states along the horizontal or vertical direction of the square lattice. We also find that the ground state could be characterized by a nonzero string order, which rarely happened in the 2D case. The method that we used reveals the connection between the fractional quantum Hall effect and the frustrated 2D antiferromagnetism system. The VBS states capture the main character of the disordered phase in the 2D spin-1 frustrated system and can be verified by a numerical method.

Impurity Induced Spin Texture in Quantum Critical 2D Antiferromagnets

We describe the uniform and staggered magnetization distributions around a vacancy in a quantum critical two-dimensional S=1/2 antiferromagnet. The distributions are delocalized across the entire sample with a universal functional form arising from an uncompensated Berry phase. The numerical results, obtained using quantum Monte Carlo simulations of the Heisenberg model on bilayer lattices with up to approximately equal to 10(5) spins, are in good agreement with the proposed scaling structure. We determine the exponent eta' = 0.40+/-0.02, which governs both the staggered and uniform magnetic structure away from the impurity and also controls the impurity spin dynamics.

Accurate Determination of the Electronic Structure Parameters of the Spin Ladder Compounds SrCu2O3, Sr2Cu3O5 and CaCu2O3

Ab initio embedded cluster calculations have been employed to calculate a large number of electronic structure parameters of three different spin ladders, namely SrCu2O3, CaCu2O3 and Sr2Cu3O5. Using the iterative difference dedicated configuration interaction methodology, magnetic couplings J and hopping amplitudes t are determined for first to fourth nearest neighbors. In addition, the four-body cyclic exchange Jring is extracted and the direct exchange K, the neutral-ionic hopping integral t0 and the on-site repulsion U are calculated for first and second nearest neighbor copper ions. The substitution of these parameters in the pertubative superexchange relation J=2K−4t 0 2 /U yields magnetic coupling parameters in close agreement with the variational estimates. The spin ladders can be considered as an interpolation between the one-dimensional (1D) spin chains and the 2D antiferromagnets. Hence, results are compared with similar parameters in the spin chain Sr2CuO3 and the two-dimensional antiferromagnet La2CuO4.

Bond‐ordered, bond‐centered stripes in cuprates

Using a combination of the bond formalism and the string picture we analyze the stability of bond-ordered, bond-centered stripe system formed in the doped two dimensional (2D) antiferromagnet (AF). Our interest in such structures is motivated by results of recent neutron scattering experiments. We concentrate especially on a stripe, in which bond order takes the shape of a two-layer brick wall, forming a domain wall (DW) between AF domains with the changing by π across each DW direction of the sublattice magnetization. By analyzing the energy of such a structure and comparing with energies of stripes with other patterns of the underlying magnetic background we draw the conclusion that at the doping level 1/8 the most stable is the system of DWs which takes the form of two-leg ladders with bond-ordered nearest parallel legs. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Appearance of a two-dimensional antiferromagnetic order in quasi-one-dimensional cobalt oxides

By means of muon spin rotation and relaxation ($\mu^+$SR) techniques, we have investigated the magnetism of quasi one-dimensional (1D) cobalt oxides $AE_{n+2}$Co$_{n+1}$O$_{3n+3}$ ($AE$=Ca, Sr and Ba, $n$=1, 2, 3, 5 and $\infty$), in which the 1D CoO$_3$ chain is surrounded by six equally spaced chains forming a triangular lattice in the $ab$-plane, using polycrystalline samples, from room temperature down to 1.8 K. For the compounds with $n$=1 - 5, transverse field $\mu^+$SR experiments showed the existence of a magnetic transition below $\sim$100 K. The onset temperature of the transition ($T_{\rm c}^{\rm on}$) was found to decrease with $n$; from 100 K for $n$=1 to 60 K for $n$=5. A damped muon spin oscillation was observed only in the sample with $n$=1 (Ca$_3$Co$_2$O$_6$), whereas only a fast relaxation obtained even at 1.8 K in the other three samples. In combination with the results of susceptibility measurements, this indicates that a two-dimensional short-range antiferromagnetic (AF) order appears below $T_{\rm c}^{\rm on}$ for all compounds with $n$=1 - 5; but quasi-static long-range AF order formed only in Ca$_3$Co$_2$O$_6$, below 25 K. For BaCoO$_3$ ($n$=$\infty$), as $T$ decreased from 300 K, 1D ferromagnetic (F) order appeared below 53 K, and a sharp 2D AF transition occurred at 15 K.

Diagonal bond ordered stripes in doped antiferromagnets: Stability analysis

Using a combination of the bond formalism and the recursion method we analyze the dynamics of a hole moving along a diagonal stripe which takes the shape of a slant stack formed by horizontal bonds which are in the spin singlet state. This work has been motivated by results of recent neutron scattering experiments on cuprates, suggesting the existence of bond order in the stripe phase which is a manifestation of the tendency towards nano-scale phase separation. The bond-ordered diagonal stripe separates antiferromagnetic domains with the opposite directions of the sublattice magnetization. The motion of a hole in the stripe is governed by the process of position exchange between the hole and the singlet on nearest bonds. Within a simplest approximation we assume that the hole propagation in 2D antiferromagnet is prevented by the string effect related to locally destroying the antiferromagnetic order by a hopping hole. In order to avoid this destruction the hole must return to the same point at which it has left the stripe. It must also retrace the same path along which it has entered the antiferromagnetic domain. We demonstrate that the diagonal stripe has higher energy than the horizontal stripe with bond order on legs, which provides additional confirmation of the suggestion that the scenario of bond-centered vertical and horizontal stripes with spin Peierls order on legs accounts for results of recent ARPES and neutron scattering measurements in 214 systems. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetism in the pseudo-two-leg ladder compound CaCu2O3

Contrary to SrCu2O3, the pseudo-ladder two-leg compound CaCu2O3 does not show a significant spin-gap behavior as expected for typical two-leg ladder systems. Magnetization measurements on single crystals show CaCu2O3 to be a three-dimensional (3D) antiferromagnet (TN=27K), showing relatively strong magnetic anisotropy and metamagnetic transitions. The 3D behavior is due to exchange between the ladders as deduced from electronic structure calculations within the local density approximation.

Coexistence of ferromagnetism and antiferromagnetism in the L0.08Ca1.92MnO4 series

The present study is devoted to the L0.08Ca1.92MnO4 series (with L=La, Pr, Ho and Y) which exhibits, at low temperature, the highest ferromagnetic components in the magnetization curves, among the electron-doped manganites belonging to the first members of the Ruddlesden–Popper family. We show that, in the whole series, coexistence of ferromagnetism and antiferromagnetism is induced, independent of the size of the L cation and that the holmium-based compound has a particular behavior with the presence of 2D antiferromagnetism instead of 3D for the other cations.

XY behaviour of the 2D antiferromagnet in a field

XY behaviour of the 2D antiferromagnet in a field

Reconciling field-theoretical and semiclassical approaches to quantum 2D antiferromagnets

We show that, starting from the field-theoretical expression for the correlation length ξ of the 2D quantum Heisenberg antiferromagnet corrected for cut-off effects and our knowledge of the classical thermodynamics of the model, the observed temperature dependence of ξ can be reproduced. We find also that the cut-off effects contains the same pure-quantum renormalizations of the pure-quantum self-consistent harmonic approximation semiclassical framework: the gap between quantum field theoretical and semiclassical results is thus bridged.

Pseudogaps in nested antiferromagnets

We analyze the fluctuation corrections to magnetic ordering in the case of a 3D antiferromagnet with flat Fermi surfaces, as physically realized in the case of chromium, and find that they are insufficient to produce a quantum critical point. This implies that the critical point observed in vandium doped chromium is due to a loss of nesting. We also derive the fermion self-energy in the paramagnetic phase and find that a pseudogap exists, though its magnitude is significantly reduced as compared to the spectral gap in the ordered state in the limit where the latter is small in comparison to the Fermi energy.

On the Hall effect in a two-dimensional doped antiferromagnet

The conductivity and the Hall coefficient of a doped 2D antiferromagnet in the normal state are considered using the Kondo lattice model in the multimoment approximation. The anomalous temperature dependence of the kinetic coefficients is explained by the strong anisotropic charge-carrier scattering from the spin subsystem and found to be in qualitative agreement with the experimental data for the normal state of high-Tc superconductors.

Reconciling field-theoretical and semiclassical approaches to quantum 2D antiferromagnets

We show that, starting from the field-theoretical expression for the correlation length ξ of the 2D quantum Heisenberg antiferromagnet corrected for cut-off effects and our knowledge of the classical thermodynamics of the model, the observed temperature dependence of ξ can be reproduced. We find also that the cut-off effects contains the same pure-quantum renormalizations of the pure-quantum self-consistent harmonic approximation semiclassical framework: the gap between quantum field theoretical and semiclassical results is thus bridged.

Heat Capacity Study of the Doping Effect of Paramagnetic Radicals on the Organic Spin–Peierls System (p-CyDOV Radical Crystal)

The doping effect of paramagnetic organic radical impurities ( p -CyDTV) on the organic spin–Peiels (SP) system of p -CyDOV (3-(4-cyanophenyl)-1,5-dimethyl-6-oxoverdazyl; the SP transition temperature T SP =15.0 K) was studied by the heat capacity measurement down to 0.1 K. At low temperatures below 1 K, quite a sharp heat capacity peak was detected for each system with small impurity concentration ( x ) at the temperature T N ( x ); T N (0)=0.135 K (±0.002 K), T N (0.01)=0.290 K and T N (0.07)=0.164 K. The appearance of the peak for the system with x =0 is considered to be due to minute and inevitable impurities in the pure p -CyDOV. From the dependence of T N ( x ), T N (0)< T N (0.01), and the heat capacity behavior on x , the peaks for x =0 and 0.01 are explained to be accompanied with the three-dimensional (3D) antiferromagnetic (AFM) transition in the SP phase. While the decrease of T N ( x ) for higher impurities x =0.07 than that for x =0.01 may be interpreted as merely a 3D AFM transition from the paramagnetic state in the quasi-1D systems with the interchain interaction | z J '/ J |=6×10 -4 . Although no anomaly of the heat capacity was found around T SP ( x )=13–15 K in the present systems, where the characteristic decrease of the magnetic susceptibility is reported, we observed a distinct hump of the heat capacity at T SP '(0)=5.6 K and T SP '(0.01)=5.3 K with magnetic entropy consumption up to 3%, suggesting that this transition may relate to SP transition.

Absorption of light by exchange-coupled ions in a 2D antiferromagnet

The optical absorption spectra of Rb2MnxCd1−xCl4 crystals are experimentally studied in the vicinity of a magnon sideband of the exciton band at a manganese content x ranging from 1.0 to 0.4. Additional absorption bands are observed with an increase in the magnetic structural disorder upon replacement of manganese ions by cadmium ions. An analysis of the evolution of the additional absorption bands in a magnetic field during the spin-flop phase transition and the change in the intensity with variations in the manganese content x demonstrates that these bands are associated with the excitation of the exchange-coupled pairs of manganese ions located in different environments in a plane square lattice. The phase boundary between the antiferromagnetic and spin-flop phases is constructed using the results of optical measurements. The manganese content corresponding to the magnetic percolation point is evaluated.

The stripe-liquid phase in cuprates and nickelates

It is well established that doped 2D antiferromagnets such as layered nickelates and certain cuprates can exhibit an ordered phase in which charge carriers are segregated to periodically-spaced domain walls separating antiferromagnetic domains. An open question concerns the nature of the state that is achieved when a stripe solid melts. Recent neutron scattering experiments on nickelates and cuprates indicate that melting leads to a stripe-liquid state; these results will be presented and discussed. The existence of the stripe-liquid phase in nickelates makes it plausible to interpret the mid-infrared absorption in terms of transitions from valence to mid-gap states associated with the stripes.

Effect of impurities on quasi-two-dimensional quantum antiferromagnet

We have studied the static and dynamic properties of quasi-two-dimensional (2D) quantum antiferromagnets (AFs) diluted with spinless impurities using spin-wave theory and T-matrix approximation. We show that the spectrum of a 2D AF at long wavelengths is overdamped at an arbitrary concentration of spinless impurities. The scattering leads to a length scale l/a∼eπ/4x, x being impurity concentration and a the lattice spacing, beyond which the influence of impurities on the spectrum is dominant. Although the dynamical properties are significantly modified we show that 2D is not the lower critical dimension for this problem. Thus, in low-dimensional systems with disorder the connection between static and dynamic quantities is not straightforward. Our results are in quantitative agreement with the recent Monte Carlo simulations and experimental data for S=1/2, S=1, and S=5/2. We have also proposed experiments which can further test the results of our theory.

A microscopic model for D-wave pairing in the cuprates: what happens when electrons somersault?

We present a microscopic model for a strongly repulsive electron gas on a 2D square lattice. We suggest that nearest-neighbor Coulomb repulsion stabilizes a state in which electrons undergo a “somersault” in their internal spin-space (spin-flux). When this spin- 1 2 antiferromagnetic (AFM) insulator is doped, the charge carriers nucleate mobile, charged, bosonic vortex solitons accompanied by unoccupied states deep inside the Mott–Hubbard charge-transfer gap. This model provides a unified microscopic basis for (i) non-Fermi-liquid transport properties, (ii) mid-infrared optical absorption, (iii) destruction of AFM long-range order with doping, (iv) angled resolved spectroscopy (ARPES), and (v) d-wave preformed charged carrier pairs. We use the configuration interaction (CI) method to study the quantum translational and rotational properties of such pairs. The CI method systematically describes fluctuation and quantum tunneling corrections to the Hartree–Fock approximation and recaptures essential features of the (Bethe ansatz) exact solution of the Hubbard model in 1D. For a single hole in the 2D AFM plane, we find a precursor to spin-charge separation. The CI ground state consists of a bound vortex–antivortex pair, one vortex carrying the charge and the other one carrying the spin of the doping hole.

Impurity spin dynamics in 2D antiferromagnets and superconductors

We discuss the universal theory of localized impurities in the paramagnetic state of 2D antiferromagnets where the spin gap is assumed to be signigicantly smaller than a typical exchange energy. We study the impurity spin susceptibility near the host quantum transition from a gapped paramagnet to a Néel state, and we compute the impurity-induced damping of the spin-1 mode of the gapped antiferromagnet. Under suitable conditions our results apply also to d-wave superconductors.

Spin-wave interaction and renormalization of magnetic anisotropy in 2D antiferromagnets.

The renormalization of the magnetic anisotropy is investigated in 2D easy-axis antiferromagnets using 1/S expansion. It was shown that in the spin-wave gap Delta appears as the 1/S2 infrared divergent correction which becomes large at T>Delta and demands analysis of the full 1/S expansion. Qualitative consequences of this gap renormalization are discussed in relation to the AFR and neutron scattering experiments.