Antiferromagnetic Regime Research Articles

Mixed azide–carboxylate bridged tri- and tetranuclear MnII clusters in coordination polymers derived from a zwitterionic dicarboxylate ligand: structures and magnetism

Two Mn(II) coordination polymers with azide and the zwitterionic dicarboxylate ligand bis(N-carboxymethyl-4-pyridinium) (bcp) were synthesized, and structurally and magnetically characterized. They are formulated as [Mn(3)(bcp)(2)(N(3))(2)(SO(4))(2)(H(2)O)(4)].6H(2)O (1) and [Mn(4)(bcp)(2)(N(3))(8)(H(2)O)(2)].4H(2)O (2). Compound 1 contains anionic linear [Mn(3)(N(3))(2)(COO)(4)(SO(4))(2)(H(2)O)(4)](4-) units with simultaneous mu(2)-EO (end-on) azide, sulfate and carboxylate bridges, while compound 2 contains [Mn(4)(COO)(4)(N(3))(8)(H(2)O)(2)](4-) clusters with mixed mu(2)-EO azide, mu(3)-EO azide and carboxylate bridges. In these compounds, the anionic tri- or tetranuclear units are linked into coordination chains by the cationic bipyridinium spacers, and are also hydrogen bonded into chains by double O-H...O bridges. Magnetic analyses were carried out on temperature-variable susceptibility data for both compounds, and also on isothermal magnetization data for 1. It is revealed that all the mixed double and triple bridges, [(EO-N(3))(COO)(SO(4))] in 1, [(COO)(EO-N(3))(2)] and [(COO)(EO-N(3))] in 2, transmit antiferromagnetic coupling between Mn(II) ions. The [(EO-N(3))(2)] bridge in , with Mn-N-Mn = 96.6 degrees, also transmits antiferromagnetic coupling, providing the first example in the antiferromagnetic regime predicted theoretically for double EO-azide bridges between Mn(II) ions. The double hydrogen bonding [(O-H...O)(2)] bridges in both compounds induce weak antiferromagnetic interactions.

Competing Large and Small Angles in a Double End‐On Azido Copper(II) Binuclear Complex: A Combined Experimental and Theoretical Study of Magnetic Interactions

AbstractThe structure and magnetic properties of double azido‐bridged CuII binuclear complex 1 with the chelating chiral ligand (S,S)‐2,2′‐isopropylidenebis(4‐phenyl‐2‐oxazoline) were analyzed by combining experimental and theoretical techniques. The CuII ions adopt a square pyramidal geometry with different degrees of distortion, whereas the two end‐on azido bridges disposed at the equatorial positions exhibit significantly different Cu–N–Cu angles, 110.6 and 97.3°, which are respectively smaller and larger than the critical 108° value distinguishing the ferromagnetic and antiferromagnetic regimes. The asymmetry in 1 arises from the use of the bulky asymmetric ligand, giving rise to two different magnetic pathways between the CuII ions. The magnetic pathway along the large Cu–N–Cu angle value dominates over the small one, resulting in a net antiferromagnetic behavior with J = –78.6 cm–1. On the basis of wavefunction calculations, we investigate the exchange interactions in synthetic compound 1 and fictitious analogs 2 and 3 holding either two large (i.e., 110.6°) or two small (i.e., 97.3°) Cu–N–Cu angles. The calculated exchange interaction in 1(–104 cm–1) is in relatively good agreement with the experimental value and corresponds precisely to the average between the antiferromagnetic value in 2 (–218 cm–1) and the ferromagnetic one in 3 (21 cm–1). The significant enhancement in the antiferromagnetic contribution accompanying the expansion of one of the Cu–N–Cu bridging angles is undoubtedly the driving force for the observed antiferromagnetic behavior in 1. The control of the local metal environments allowed us to monitor the exchange coupling interactions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Valence bond solid state induced by impurity frustration inCr8Ni

We provide a physically meaningful picture of the nature of the ground state of the ${\text{Cr}}_{8}\text{Ni}$ compound in the regime where it is a spin singlet. According to this picture, the anisotropy of the Ni atom in the Cr ring induces a dimerization in the molecule that makes the ground state to stabilize in a valence bond solid phase of virtual spins. We characterize rigorously this phase by means of a particular nonlocal order parameter that denoted the generalized string order parameter. In the completely antiferromagnetic regime, the system becomes frustrated. We have performed a numerical real-time evolution study of the correlations between the spin of the Ni impurity and the rest of the spins in order to show the reaction of the system under this frustration.

Doping evolution of the electronic specific heat coefficient in slightly-doped La2-xSrxCuO4 single crystals

Detailed doping dependence of the electronic specific heat coefficient γ is studied for La2-xSrxCuO4 (LSCO) single crystals in the slightly-doped regime. We find that γ systematically increases with doping, and furthermore, even for the samples in the antiferromagnetic (AF) regime, γ already acquires finite value and grows with x. This suggests that finite electronic density of states (DOS) is created in the AF regime where the transport shows strong localization at low temperatures, and this means the system is not a real insulator with a clear gap even though it still keeps long range AF order.

Vortex-splitting and phase-separating instabilities of coreless vortices inF=1spinor Bose-Einstein condensates

The low-lying excitations of coreless vortex states in $F=1$ spinor Bose-Einstein condensates (BECs) are theoretically investigated using the Gross-Pitaevskii and Bogoliubov--de Gennes equations. The spectra of the elementary excitations are calculated for different spin-spin interaction parameters and ratios of the number of particles in each sublevel. There exist dynamical instabilities of the vortex state which are suppressed by ferromagnetic interactions, and, conversely, enhanced by antiferromagnetic interactions. In both of the spin-spin interaction regimes, we find vortex-splitting instabilities in analogy with scalar BECs. In addition, a phase-separating instability is found in the antiferromagnetic regime.

Ground-state factorization and quantum phase transition in dimerized spin chains

We study the occurrence of ground-state factorization in dimerized $XY$ spin chains in a transverse field. Together with the usual ferromagnetic and antiferromagnetic regimes, a third case emerges, with no analogous in translationally-invariant systems, consisting of an antiferromagnetic Ne\'{e}l-type ground state where pairs of spins represent the unitary cell. Then, we calculate the exact solution of the model and show that the factorizing field represent an accidental degeneracy point of the Hamiltonian. Finally, we extend the study of the existence of ground-state factorization to a more general class of models.

The two-spinon transverse structure factor of the gapped Heisenberg antiferromagneticchain

We consider the transverse dynamical structure factor of the anisotropic Heisenbergspin-1/2 chain (XXZ model) in the gapped antiferromagnetic regime(Δ>1). Specializing to the case of zero field, we use two independent approaches based onintegrability (one valid for finite size, the other for the infinite lattice) to obtain the exacttwo-spinon part of this correlator. We discuss in particular its asymmetry with respect to theπ/2-momentum line, its overall anisotropy dependence, and its contribution to sumrules.

On string solutions of the Bethe equations in the % MathType!MTEF!2!1!+- % feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBqj3BWbIqubWexLMBb50ujbqegm0B % 1jxALjharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqr % Ffpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0F % irpepeKkFr0xfr-xfr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaa % GcbaWefv3ySLgznfgDOfdarCqr1ngBPrginfgDObYtUvgaiuaacqWF % neVtryqr1ngBPrgaiyaacqGF9aqpcqGF0aanaaa!4928! $$ \mathcal{N} = 4 $$ supersymmetric Yang-Mills theory

The Bethe equations which arise in description of the spectrum of the dilatation operator for the su(2) sector of the $$ \mathcal{N} = 4 $$ supersymmetric Yang-Mills theory are considered in the anti-ferromagnetic regime. These equations are a deformation of those for the Heisenberg XXX magnet. We prove that in the thermodynamic limit, roots of the deformed equations group into strings. We prove that the corresponding Yang's action is convex, which implies the uniqueness of a solution for centers of the strings. The state formed by strings of length (2n+1) is considered, and the density of their distribution is found. It is shown that the energy of such a state decreases as n grows. It is observed that the nonanalyticity of the left-hand sides of the Bethe equations leads to an additional contribution to the density and energy of strings of even length. We conclude that the structure of the anti-ferromagnetic vacuum is determined by the behavior of exponential corrections to string solutions in the thermodynamic limit and, possibly, involves strings of length 2. Bibliography: 14 titles.

Phase diagram of the anisotropic multichannel Kondo Hamiltonian revisited

The phase diagram of the multichannel Kondo Hamiltonian with an $XXZ$ spin-exchange anisotropy is revisited, revealing a far richer fixed-point structure than previously appreciated. For a spin-$\frac{1}{2}$ impurity and $k>2$ conduction-electron channels, a second ferromagneticlike domain is found deep inside the antiferromagnetic regime. The new domain extends above a (typically large) critical longitudinal coupling ${J}_{z}^{*}>0$ and is separated from the antiferromagnetic domain by a second Kosterlitz-Thouless line. A similar line of stable ferromagneticlike fixed points with a residual isospin-$\frac{1}{2}$ local moment is shown to exist for large ${J}_{z}⪢\ensuremath{\mid}{J}_{\ensuremath{\perp}}\ensuremath{\mid}>0$ and arbitrary $k$ and $s$ obeying $\ensuremath{\mid}k\ensuremath{-}2s\ensuremath{\mid}>1$. Here, ${J}_{z}$ is the longitudinal spin-exchange coupling, ${J}_{\ensuremath{\perp}}$ is the transverse coupling, and $s$ is the impurity spin. Near the free-impurity fixed point, spin-exchange anisotropy generates a highly relevant term for $s>1∕2$ and arbitrary $k$. Depending on the sign of ${J}_{z}^{2}\ensuremath{-}{J}_{\ensuremath{\perp}}^{2}$ and the parity of $2s$, the system flows either to a conventional Fermi liquid with no residual degeneracy or to a $k$-channel, spin-$\frac{1}{2}$ Kondo effect or to a line of ferromagneticlike fixed points with a residual isospin-$\frac{1}{2}$ local moment. These results are obtained through a combination of perturbative renormalization-group techniques, Abelian bosonization, a strong-coupling expansion in $1∕{J}_{z}$, and explicit numerical renormalization-group calculations.

Magnetocaloric effect in the frustrated square latticeJ1−J2model

We investigate the magnetocaloric properties of the two-dimensional frustrated ${J}_{1}$-${J}_{2}$ model on a square lattice. This model describes well the magnetic behavior of two classes of quasi-two-dimensional $S=1∕2$ vanadates, namely, the ${\mathrm{Li}}_{2}V\mathrm{O}X{\mathrm{O}}_{4}$ $(X=\mathrm{Si},\mathrm{Ge})$ and $A{A}^{\ensuremath{'}}\mathrm{VO}({\mathrm{PO}}_{4}{)}_{2}$ $(A,{A}^{\ensuremath{'}}=\mathrm{Pb},\mathrm{Zn},\mathrm{Sr},\mathrm{Ba})$ compounds. The magnetocaloric effect (MCE) consists of the adiabatic temperature change upon changing the external magnetic field. In frustrated systems, the MCE can be enhanced close to the saturation field because of massive degeneracies among low-lying excitations. We discuss results for the MCE in the two distinct antiferromagnetic regimes of the phase diagram. The numerical finite temperature Lanczos method as well as analytical methods based on the spin-wave expansion are employed and results are compared. We give explicit values for the saturation fields of the vanadium compounds. We predict that at subcritical fields there is first a (positive) maximum followed by a sign change of the MCE, characteristic of all magnetically ordered phases.

Phase coexistence and magnetic anisotropy inLa1−xCaxMnO3(0<x⩽0.23)studied via electron spin resonance

We report an electron-spin-resonance (ESR) study of stoichiometric ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ for $0<x\ensuremath{\leqslant}0.23$. The room-temperature ESR linewidth shows a marked decrease with $x$ that is mainly due to the variation of the magnetic susceptibility and a much slower evolution of spin relaxation, associated with the interplay of crystal-field and superexchange interactions. In the canted antiferromagnetic regime, the ESR parameters show initially an $x$-dependent ferromagnetic (FM) behavior that evolves to antiferromagnetic in the vicinity of ${T}_{N}$, reflecting the development of spin canting. A weak ferromagnetic-resonance (FMR) mode is observed below ${T}_{N}$ that becomes enhanced near the phase boundary to the ferromagnetic insulating (FMI) phase. This mode persists at higher doping levels $(x\ensuremath{\leqslant}0.16)$ and is attributed to residual FM domains due to the release of crystal strain near the structural transformation at $T<100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In the FMI regime, FMR measurements show strong uniaxial magnetic anisotropy up to $x=0.16$ with $K=(1--2)\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.3em}{0ex}}\mathrm{erg}∕{\mathrm{cm}}^{3}$ at $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The temperature dependence of the magnetic anisotropy reveals power-law scaling with the magnetization, $K(T)\ensuremath{\propto}M{(T)}^{3}$, indicative of single-ion magnetocrystalline anisotropy and the persistence of orbital order in the FMI state. As the FM metallic phase is approached, the magnetic anisotropy decreases significantly and an additional FMR mode systematically emerges in the paramagnetic regime at $T\ensuremath{\lesssim}260\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for $x=0.19--0.23$. Its relative spectral weight grows with $x$ indicating an increase of the underlying FM fraction that correlates with the enhanced metallic behavior.

Integrable models and quantum spin ladders: comparison between theory and experiment for the strong coupling ladder compounds

(abbreviated) This article considers recent advances in the investigation of the thermal and magnetic properties of integrable spin ladder models and their applicability to the physics of real compounds. The ground state properties of the integrable two-leg spin-1/2 and the mixed spin-(1/2,1) ladder models at zero temperature are analyzed by means of the Thermodynamic Bethe Ansatz. Solving the TBA equations yields exact results for the critical fields and critical behaviour. The thermal and magnetic properties of the models are investigated in terms of the recently introduced High Temperature Expansion method, which is discussed in detail. It is shown that in the strong coupling limit the integrable spin-1/2 ladder model exhibits three quantum phases: (i) a gapped phase in the regime $H<H_{c1}$, (ii) a fully polarised phase for $H>H_{c2}$, and (iii) a Luttinger liquid magnetic phase in the regime $H_{c1}<H<H_{c2}$. The critical behaviour in the vicinity of the critical points is of the Pokrovsky-Talapov type. The temperature-dependent thermal and magnetic properties are directly evaluated from the exact free energy expression and compared to known experimental results for a range of strong coupling ladder compounds. Similar analysis of the mixed spin-(1/2,1) ladder model reveals a rich phase diagram, with a 1/3 and a full saturation magnetisation plateau within the strong antiferromagnetic rung coupling regime. For weak rung coupling, the fractional magnetisation plateau is diminished and a new quantum phase transition occurs. The phase diagram can be directly deduced from the magnetisation curve obtained from the exact result derived from the HTE. The thermodynamics of the spin-orbital model with different single-ion anisotropies is also investigated.

Spin precession and real-time dynamics in the Kondo model: Time-dependent numerical renormalization-group study

A detailed derivation of the recently proposed time-dependent numerical renormalization-group (TD-NRG) approach to nonequilibrium dynamics in quantum impurity systems is presented. We demonstrate that the method is suitable for fermionic as well as bosonic baths. A comparison with exact analytical results for the charge relaxation in the resonant-level model and for dephasing in the spin-boson model establishes the accuracy of the method. The real-time dynamics of a single spin coupled to both types of baths is investigated. We use the TD-NRG to calculate the spin relaxation and spin precession of a single Kondo impurity. The short- and long-time dynamics is studied as a function of temperature in the ferromagnetic and antiferromagnetic regimes. The short-time dynamics agrees very well with analytical results obtained at second order in the exchange coupling $J$. In the ferromagnetic regime, the long-time spin decay is described by the scaling variable $x = 2\rho_F J(T) T t$. In the antiferromagnetic regime it is governed for $T < T_K$ by the Kondo time scale $1/T_K$. Here $\rho_F$ is the conduction-electron density of states and $T_K$ is the Kondo temperature. Results for spin precession are obtained by rotating the external magnetic field from the x axis to the z axis.

Electronic structure of [formula omitted] studied by Ce [formula omitted] resonant photoemission

We have performed Ce 4 d – 4 f resonant photoemission (RPE) spectroscopy on the isostructural heavy fermion system CeNiGe 2 - x Si x ( 0 ⩽ x ⩽ 1 ) . With increasing x, the unit volume decreases by about 5% and the ground state originating in Ce 4f character varies from an antiferromagnetic regime (AFM, 0 ⩽ x ⩽ 0.8 ) to a quantum critical point (QCP, x = 1 ). RPE data reveals that the cf-hybridization intensity becomes stronger as the ground state of Ce 4f electron changes from AFM to QCP, which is exclusively caused by the increase of chemical pressure due to the reduction of lattice constant in CeNiGe 2 - x Si x system.

The nuclear spin–lattice relaxation rate in the [formula omitted] materials

We examine the nuclear spin–lattice relaxation rates 1 / T 1 of PuRhGa 5 and PuCoGa 5 , in particular, in normal state. PuRhGa 5 exhibits a gradual suppression of ( T 1 T ) - 1 below 25 K far above T c ∼ 8.5 K , while measurements for PuCoGa 5 reveal a monotonic increase down to T c . We propose that this behavior is consistently understood by the crossover from the two-dimensional quantum antiferromagnetic regime of the local 5f-electron spins of Pu to the concomitant formation of the fermion pseudogap based on the two-component spin-fermion model.

The Kondo lattice model

The Kondo lattice model is one of the canonical models used to study impurity effects in strongly correlated electron systems and has been the subject of intense study for many years. The Kondo lattice model describes the interaction between a band of conduction electrons (c-electrons) and a lattice of localized magnetic moments, e.g. f-electrons. Hereafter, the ground state phases of the one-dimensional Kondo lattice are reviewed focusing on the antiferromagnetic coupling regime.

Calculation of correlation functions in the Heisenberg spin-1/2 antiferromagnetic model

We derive the exact expression of the four-spinon contribution S 4 to the dynamical correlation function S of the spin 1/2 isotropic (XXX) He isenberg model in the antiferromagnetic regime using the quantum group symmetry of the model. We first give the exact expression for the n-spinon contribution in the form of contour integrals and display known results regarding the two-spinon contribution S 2. Then we specialize the n-spinon formula to the case n = 4 and compute three sum rules for S 4 that the total S is known to satisfy exactly. These are: the total integrated intensity, the first frequency moment and the nearest-neighbor correlation function. We find that S 4 corrects only by a small amount the contribution from S 2.

Computation of dynamical correlation functions of Heisenberg chains: the gaplessanisotropic regime

We compute all dynamical spin–spin correlation functions for thespin-1/2 XXZ anisotropic Heisenberg model in the gapless antiferromagnetic regime at zerotemperature, using numerical sums of exact determinant representations for formfactors of spin operators on the lattice. Contributions from intermediate statescontaining many particles and string (bound) states are included. We present modifieddeterminant representations for the form factors valid in the general case with stringsolutions to the Bethe equations. Our results are such that the available sumrules are saturated to high precision. We Fourier transform our results back toreal space, allowing us in particular to make a comparison with known exactformulae for equal-time correlation functions for small separations in zero field,and with predictions for the zero-field asymptotics from conformal field theory.

Thermomagnetic Power and Figure of Merit for Spin-1/2 Heisenberg Chain

Transport properties in the presence of magnetic fields are numerically studied for the spin-1/2 Heisenberg XXZ chain. The breakdown of the spin-reversal symmetry due to the magnetic field induces the magnetothermal effect. In analogy with the thermoelectric effect in electron systems, the thermomagnetic power (magnetic Seebeck coefficient) is provided, and is numerically evaluated by the exact diagonalization for wide ranges of temperatures and various magnetic fields. For the antiferromagnetic regime, we find the magnetic Seebeck coefficient changes sign at certain temperatures, which is interpreted as an effect of strong correlations. We also compute the thermomagnetic figure of merit determining the efficiency of the thermomagnetic devices for cooling or power generation.

Evolution from intermediate-valence to antiferromagnetic Kondo-lattice behavior in CeNi(Ge xSi 1− x) 2

A cerium-based intermetallic compound series CeNi(Ge x Si 1− x ) 2 ( x=0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) were synthesized. Among the compounds, CeNiSi 2 is an intermediate-valence compound, while CeNiGe 2 is a Kondo-lattice with a two-step antiferromagnetic phase transition. In this work, X-ray powder diffraction, transport, magnetic and thermal measurements, as well as Ce-L III spectroscopy are presented in order to study the crossover from non-magnetic intermediate-valance regime to antiferromagnetic Kondo-lattice regime in the CeNi(Ge x Si 1− x ) 2 series. Increasing Ge concentration will depress the hybridization between 4f electrons and conducting electrons and enhance the RKKY interaction, which eventually drive the system to a magnetically ordered state.