Dimensional Antiferromagnet Research Articles

Ultrafast optical generation of antiferromagnetic meron-antimeron pairs with conservation of topological charge

We propose a new mechanism to produce a meron-antimeron pair in a two dimensional antiferromagnet with an ultrafast laser pulse via thermal Schwinger mechanism. Unlike ultrafast skyrmion nucleation, this process conserves total topological charge. We systematically show different stages of the dynamics and define proper topological invariants to characterise the configurations. The emergent structure can retain its topological structure for up to 100 ps. By introducing a topological structure factor we show that pair formation is robust against any random choice of initial magnetic configuration and can survive against disorder. Our findings demonstrate that the rich world of spin textures, which goes beyond conventional skyrmions, can be reached optically.

Abelian gauge theories on the lattice: θ-Terms and compact gauge theory with(out) monopoles

We discuss a particular lattice discretization of abelian gauge theories in arbitrary dimensions. The construction is based on gauging the center symmetry of a non-compact abelian gauge theory, which results in a Villain type action. We show that this construction has several benefits over the conventional U(1) lattice gauge theory construction, such as electric-magnetic duality, natural coupling of the theory to magnetically charged matter in four dimensions, complete control over the monopoles and their charges in three dimensions and a natural θ-term in two dimensions. Moreover we show that for bosonic matter our formulation can be mapped to a worldline/worldsheet representation where the complex action problem is solved. We illustrate our construction by explicit dualizations of the CP(N−1) and the gauge Higgs model in 2d with a θ term, as well as the gauge Higgs model in 3d with constrained monopole charges. These models are of importance in low dimensional anti-ferromagnets. Further we perform a natural construction of the θ-term in four dimensional gauge theories, and demonstrate the Witten effect which endows magnetic matter with a fractional electric charge. We extend this discussion to PSU(N)=SU(N)/ZN non-abelian gauge theories and the construction of discrete θ-terms on a cubic lattice.

High-field Magnetism of the Honeycomb-lattice Antiferromagnet Cu2(pymca)3(ClO4)

We report on the experimental results of magnetic susceptibility, specific heat, electron spin resonance (ESR), and high-field magnetization measurements on a polycrystalline sample of the spin-$1/2$ distorted honeycomb-lattice antiferromagnet Cu$_2$(pymca)$_3$(ClO$_4$). Magnetic susceptibility shows a broad peak at about 25~K, which is typical of a low dimensional antiferromagnet, and no long range magnetic order is observed down to 0.6~K in the specific heat measurements. Magnetization curve up to 70~T at 1.4~K shows triple stepwise jumps. Assuming three different exchange bonds $J_{\rm A}$, $J_{\rm B}$ and $J_{\rm C}$ from the structure, the calculated magnetization curve reproduces the observed one when $J_{\rm A}/k_{\rm B} = 43.7~{\rm K}$, $J_{\rm B}/J_{\rm A} = 1$ and $J_{\rm C}/J_{\rm A} = 0.2$ except the magnetization near 70~T, where the observed magnetization indicates another step while the calculated magnetization becomes saturated. The relationship between magnetization plateaus and exchange bonds is discussed based on the numerical calculations.

Magnetic dipole splitting of magnon bands in a two dimensional antiferromagnet

Abstract Neutron resonant spin echo spectroscopy has been applied to measure the splitting of the magnon bands due to the magnetic dipole interaction in the two dimensional antiferromagnet MnPS3. The splittings measured are more than two orders of magnitude smaller than energies of the magnons. Simulation of the magnon energies involving the calculation of the magnetic dipole interaction in MnPS3 predict splittings a little higher than those measured. The comparison confirms that the magnetic dipole interaction provides most of the anisotropy which stabilises the antiferromagnetic order in MnPS3. We present the first evidence for the splitting of magnon bands due to the magnetic dipole interaction in a two dimensional antiferromagnet.

Magnetic exchange parameters and anisotropy of the quasi-two-dimensional antiferromagnetNiPS3

Neutron inelastic scattering has been used to measure the magnetic excitations in powdered NiPS$_3$, a quasi-two dimensional antiferromagnet with spin $S = 1$ on a honeycomb lattice. The spectra show clear, dispersive magnons with a $\sim 7$ meV gap at the Brillouin zone center. The data were fitted using a Heisenberg Hamiltonian with a single-ion anisotropy assuming no magnetic exchange between the honeycomb planes. Magnetic exchange interactions up to the third intraplanar nearest-neighbour were required. The fits show robustly that NiPS$_3$ has an easy axis anisotropy with $\Delta = 0.3$ meV and that the third nearest-neighbour has a strong antiferromagnetic exchange of $J_3 = -6.90$ meV. The data can be fitted reasonably well with either $J_1 < 0$ or $J_1 > 0$, however the best quantitative agreement with high-resolution data indicate that the nearest-neighbour interaction is ferromagnetic with $J_1 = 1.9$ meV and that the second nearest-neighbour exchange is small and antiferromagnetic with $J_2 = -0.1$ meV. The dispersion has a minimum in the Brillouin zone corner that is slightly larger than that at the Brillouin zone center, indicating that the magnetic structure of NiPS$_3$ is close to being unstable.

Topological quantum phase transition in the Ising-like antiferromagnetic spin chain BaCo2V2O8

Since the seminal ideas of Berezinskii, Kosterlitz and Thouless, topological excitations are at the heart of our understanding of a whole novel class of phase transitions. In most of the cases, those transitions are controlled by a single type of topological objects. There are however some situations, still poorly understood, where two dual topological excitations fight to control the phase diagram and the transition. Finding experimental realization of such cases is thus of considerable interest. We show here that this situation occurs in BaCo$_2$V$_2$O$_8$, a spin-1/2 Ising-like quasi-one dimensional antiferromagnet when subjected to a uniform magnetic field transverse to the Ising axis. Using neutron scattering experiments, we measure a drastic modification of the quantum excitations beyond a critical value of the magnetic field. This quantum phase transition is identified, through a comparison with theoretical calculations, to be a transition between two different types of solitonic topological objects, which are captured by different components of the dynamical structure factor.

Competition between dynamic and structural disorder in a doped triangular antiferromagnet RbFe(MoO4)2

Magnetisation measurements and electron spin resonance (ESR) spectra of a doped quasi two dimensional (2D) antiferromagnet on a triangular lattice Rb1−xKxFe(MoO4)2 reveal a crucial change of the ground state spin configuration and a disappearance of a characteristic 1/3-magnetisation plateau at x = 0.15. According to theory for triangular antiferromagnets with a weak random modulation of the exchange bonds, this is a result of the competition between the structural and dynamic disorders. The dynamic zero-point or thermal fluctuations are known to lift the degeneracy of the mean field ground state of a triangular antiferromagnet and cause the spin configuration to be the most collinear, while the static disorder provides another selection of the ground state, with the least collinear structure. Low-level doping (x ≤ 0.15) was found to decrease the Néel temperature and saturation field by only few percent, while the magnetisation plateau disappears completely and the spin configuration is drastically changed. ESR spectra confirm an impurity-induced change of the so-called Y-type structure to an inverted Y-structure for x = 0.15. For x = 0.075 the intermediate regime with the decrease of width and weakening of flattening of 1/3-plateau was found.

Quasiparticle interactions in frustrated Heisenberg chains

Interactions between elementary excitations in quasi-one dimensional antiferromagnets are of experimental relevance and their quantitative theoretical treatment has been a theoretical challenge for many years. Using matrix product states, one can explicitly determine the wavefunctions of the one- and two-particle excitations, and, consequently, the contributions to dynamical correlations. We apply this framework to the (non integrable) frustrated dimerized spin-1/2 chain, a model for generic spin-Peierls systems, where low-energy quasi-particle excitations are bound states of topological solitons. The spin structure factor involving two quasi-particle scattering states is obtained in the thermodynamic limit with full momentum and frequency resolution. This allows very subtle features in the two-particle spectral function to be revealed which, we argue, could be seen, e.g., in inelastic neutron scattering of spin-Peierls compounds under a change of the external pressure.

Localized spin excitations in a disordered antiferromagnetic chain with biquadratic interactions

Dynamical theory of soliton excitation in one dimensional antiferromagnet (AFM) is studied by a revised Hamiltonian in which biquadratic interaction is taken into account in addition to the uniaxial anisotropy and exchange energy. By using Holstein-Primakoff transformation, the coherent state ansatz and the time-dependent variational principle, we obtain a set of two coupled nonlinear partial differential equations that governs the dynamics of the system. Sine-cosine function method is used to study the complete nonlinear soliton excitation and the effect of inhomogeneity in the system. The presence of inhomogeneity is found to cause a disorder in the AFM system. Finally, the evolution of Modulational Instability (MI) is analyzed in the presence of small perturbations.

High Field Magnetization of Single Crystals of the S=1/2 Quasi-1D Ising-like Antiferromagnet SrCo2V2O8

We report on the results of magnetic susceptibility and high field magnetization measurements on single crystals of SrCo2V2O8, which is regarded as an S = 1/2 quasi one-dimensional Ising-like antiferromagnet and is almost isostructural to BaCo2V2O8 that has exhibited a field-induced order-to-disorder transition. The magnetization measurements have been performed at 1.4 K below the Néel temperature TN = 4.8 K in pulsed fields of up to 50 T. The temperature dependence of magnetic susceptibility parallel to the c-axis (chain axis) has a broad maximum near 40 K, which is typical of a low dimensional antiferromagnet, while the susceptibilities perpendicular to the c-axis do not show such a maximum. The magnetization at 1.9 K along the c-axis shows a steep increase at 3.8 T as in BaCo2V2O8, corresponding to the field-induced order-to-disorder transition. The magnetization curve along the c-axis up to the saturation magnetization is well fitted to the magnetization curve calculated by the exact solution based on the Bethe ansatz for an S = 1/2 XXZ model, and this analysis provides parameter values for the exchange constant J/kB = 78 K, the anisotropic exchange parameter ɛ = 0.50, and the g-value along the c-axis gc = 6.1. In contrast, the magnetization curves perpendicular to the c-axis show gradual increase up to 50 T.

Field induced magnetic transition in low-dimensional magnets Bi(Ni,Co)PO5

We report the magnetic properties of iso-structural compounds BiNiPO5 and BiCoPO5 which undergo antiferromagnetic ordering at 17K and 11K, respectively. Positive values of θCW suggest the dominant antiferromagnetic (AFM) interactions in both the compounds. Similarly, the (TN/θCW) ratio suggests that BiCoPO5 is a frustrated one dimensional antiferromagnet while BiNiPO5 is more likely a three dimensional antiferromagnet. Magnetic specific heat Cmag was found to be extended towards high temperatures in BiCoPO5 compared to BiNiPO5 supporting the low dimensional character of BiCoPO5. The magnetic isotherm (M vs. H) at 2K shows the feature of a field induced spin-flop transition in BiCoPO5 as is expected for a strongly anisotropic AFM. We construct the field-temperature (H−T) phase diagram for BiCoPO5 from the specific heat measurements and find that the phase boundary of the PM-AFM transition can be described in terms of a mean-field power law.

High field ESR measurements of S = 1/2 low dimensional antiferromagnet (2,3-dmpyH)2CuBr4

High field ESR measurements of (2,3-dmpyH)2CuBr4 powder sample have been performed in the frequency range from 60 to 315 GHz using the pulsed magnetic field up to 16 T. Observed temperature range is 1.8 to 265 K. ESR spectrum observed at 265 K shows a typical powder pattern spectrum of Cu2+ ion, and obtained g-values are g// = 2.307 and g⊥ = 2.053. Temperature dependence measurements are also performed at 80, 160 and 315 GHz, and almost no temperature dependence of g-value is observed for all frequencies down to 1.8 K. These results are rather unusual for the low dimensional strongly correlated spin system, and possible mechanisms by spin frustration is suggested.

Resonance properties of domain boundaries in quasi-two dimensional antiferromagnets

The so-called “internal modes” localized near the domain boundaries in quasi-two dimensional antiferromagnets are investigated. The possible localized states are classified and their frequency dependences on the system discreteness parameter λ=J∕β, which describes the ratio of the magnitudes of the exchange interplane interaction and the magnetic anisotropy, are found. A sudden change in the spectrum of the local internal modes is observed at a critical value of this parameter, λ=λb=3∕4, where the domain wall shifts from a collinear to a canted shape. When λ&amp;lt;λb there are one symmetric and two antisymmetric local modes, and when λ&amp;gt;λb the modes are two symmetric, one antisymmetric, and one shear. For discreteness parameters close to the critical value, the frequencies of some of the local modes lie deep inside the gap for the linear AFM magnon spectrum and can be observed experimentally.

Magnetic properties of high-pressure phase of CaRuO3 with post-perovskite structure

CaRuO3 post-perovskite which has quasi-two-dimensional lattice shows one dimensional antiferromagnetism such as Bonner-Fisher type above 400 K. The Néel temperature, TN, is around 270 K.

Chiral gauge dynamics and dynamical supersymmetry breaking

We study the dynamics of a chiral SU(2) gauge theory with a Weyl fermion in the I=3/2 representation and of its supersymmetric generalization. In the former, we find a new and exotic mechanism of confinement, induced by topological excitations that we refer to as quintets. The supersymmetric version was examined earlier in the context of dynamical supersymmetry breaking by Intriligator, Seiberg, and Shenker, who showed that if this gauge theory confines at the origin of moduli space, one may break supersymmetry by adding a tree level superpotential. We examine the dynamics by deforming the theory on S^1 x R^3, and show that the infrared behavior of this theory is an interacting CFT at small S^1. We argue that this continues to hold at large S^1, and if so, that supersymmetry must remain unbroken. Our methods also provide the microscopic origin of various superpotentials in SQCD on S^1 x R^3 - which were previously obtained by using symmetry and holomorphy - and resolve a long standing interpretational puzzle concerning a flux operator discovered by Affleck, Harvey, and Witten. It is generated by a topological excitation, a magnetic bion, whose stability is due to fermion pair exchange between its constituents. We also briefly comment on composite monopole operators as leading effects in two dimensional anti-ferromagnets.

QUANTUM GLASSINESS AND SUPERCONDUCTIVITY IN DOPED LOW DIMENSIONAL ANTIFERROMAGNETS

Following the direct observation of abrupt changes in the superconducting ground state in doped low dimensional antiferromagnets, we have identified a phase transition where superconductivity is optimal. The experiments indicate the presence of a putative quantum critical point associated with the emergence of a glassy state. This mechanism is argued to be an intrinsic property and as such, largely independent of material quality and the level of disorder.

Is Deconfined Quantum Criticality in Frustrated Antiferromagnets Ruled Out by Generic Fluctuation Induced First Order Behavior?

Recently, it was argued that quantum phase transitions in frustrated two dimensional antiferromagnets can be radically different from their classical counterparts with as a highlight, the “deconfined critical points” exhibiting fractionalization of quantum numbers due to Berry phase effects. However, field theoretical descriptions rest on a naïve coarse graining of the microscopic lattice model assuming order parameter fluctuations on small scales to be smooth. We have developed a novel renormalization approach for such systems which incorporates fluctuations on small scales in a natural way, and is fully respecting the underlying lattice structure and the frustration mechanism. According to our findings, another profound phenomenon is around the corner: a fluctuation induced first order transition.

Erratum: “On the Theory of Spin Fluctuations around the Magnetic Instabilities Effects of Zero-Point Fluctuations”

Effects of temperature variation of the zero-point spin fluctuations on the self-consistent renormalization (SCR) theory of spin fluctuations around the magnetic instabilities are discussed. In two and three dimensional ferromagnets and three dimensional antiferromagnets the effect is mostly to renormalize the mode-mode coupling constant. The renormalized value, corresponding to the experimental value, has a finite upper bound corresponding to a divergent mean field value. In two-dimensional antiferromagnets the critical behaviors of the staggered susceptibility and the nuclear spin-lattice relaxation rate are modified by logarithmic corrections while this correction hardly appears in those of the electrical resistivity and the specific heat. The temperature variation of the mean square local amplitude of the spin density around the magnetic instability is studied by using a simple model for the dynamical susceptibility and is applied to discuss a general trend in the magneto-volume effects in weak itiner...

Pseudogap State of Underdoped Cuprate Hts as a Display of The Jahn–Teller Pseudoeffect

For underdoped quasi-two dimensional cuprate antiferromagnets within the framework of the two-component model of charge carriers (holes and Zhang–Rice polarons) it is considered two near-degenerate states of hole molecular orbitals of one-site cluster Cu2+O4 2− and two-site cluster Cu2+O4 2−+Cu2+O4 2 in CuO2 planes. It is shown that the vibronic mixing of these near-degenerate states with near energies separated by the energy barrier $$\Delta \varepsilon$$ , leads to transition of the system to the pseudogap state with new quasilocal and local states of charge carriers. The supposition that the pseudogap state is a consequence of the Jahn–Teller pseudoeffect and that barrier $$\Delta \varepsilon$$ is stipulated by magnetic interaction is discussed.

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)