Antiferromagnetic Regime Research Articles

Quantum Phase Diagram of an Exactly Solved Mixed Spin Ladder

We investigate the quantum phase diagram of the exactly solved mixed spin-(1/2,1) ladder via the thermodynamic Bethe ansatz (TBA). In the absence of a magnetic field the model exhibits three quantum phases associated with su(2), su(4), and su(6) symmetries. In the presence of a strong magnetic field, there is a third and full saturation magnetization plateaux within the strong antiferromagnetic rung coupling regime. Gapless and gapped phases appear in turn as the magnetic field increases. For weak rung coupling, the fractional magnetization plateau vanishs and the model undergoes new quantum phase transitions. However, in the ferromagnetic coupling regime, the system does not have a third saturation magnetization plateau. The critical behaviour in the vicinity of the critical points is also derived systematically using the TBA.

Thermal conductivity of anisotropic and frustrated spin-12chains

We analyze the thermal conductivity of anisotropic and frustrated spin-1/2 chains using analytical and numerical techniques. This includes mean-field theory based on the Jordan-Wigner transformation, bosonization, and exact diagonalization of systems with N<=18 sites. We present results for the temperature dependence of the zero-frequency weight of the conductivity for several values of the anisotropy \Delta. In the gapless regime, we show that the mean-field theory compares well to known results and that the low-temperature limit is correctly described by bosonization. In the antiferromagnetic and ferromagnetic gapped regime, we analyze the temperature dependence of the thermal conductivity numerically. The convergence of the finite-size data is remarkably good in the ferromagnetic case. Finally, we apply our numerical method and mean-field theory to the frustrated chain where we find a good agreement of these two approaches on finite systems. Our numerical data do not yield evidence for a diverging thermal conductivity in the thermodynamic limit in case of the antiferromagnetic gapped regime of the frustrated chain.

From overdoped to underdoped regime – high temperature in situ studies of Bi 2Sr 2CaCu 2O 8+ X single crystals

High temperature in situ study of the physical properties of high T c cuprates is one of the best ways to investigate the phase diagram, as hole concentration of the CuO 2 planes may alter after high temperature thermal cycles in various atmospheres. The in situ c-axis resistivity of Bi 2Sr 2CaCu 2O 8+ X crystals as a function of the thermal cycle in flowing oxygen and nitrogen gases disclosed how crystals are driven from the overdoped to the underdoped regime. High temperature in situ transmission electron microscope observations on Bi 2Sr 2CaCu 2O 8+ X crystals in vacuum revealed an orthorhombic to monoclinic phase transformation. The non-superconducting monoclinic phase may well be located at an antiferromagnetic regime of the phase diagram.

Nonperturbative real-space renormalization group scheme for the spin-12XXXHeisenberg model

In this paper we apply an analytical real-space renormalization group formulation which is based on numerical concepts of the density-matrix renormalization group. Within a rigorous mathematical framework we construct nonperturbative renormalization group transformations for the spin-$\frac{1}{2}$ $\mathrm{XXX}$ Heisenberg model in the finite-temperature regime. The developed renormalization group scheme allows for calculating the renormalization group flow behavior in the temperature-dependent coupling constant. The constructed renormalization group transformations are applied within the ferromagnetic and the antiferromagnetic regime of the Heisenberg chain. The ferromagnetic fixed point is computed and compared to results derived by other techniques.

Smirnov-Type Integral Formulae for Correlation Functions of the Bulk/Boundary XXZ Model in the Anti-Ferromagnetic Regime

Presented are the integral solutions to the quantum Knizhnik-Zamolodchikov equations for the correlation functions of both the bulk and boundary XXZ models in the anti-ferromagnetic regime. The difference equations can be derived from Smirnov-type master equations for correlation functions on the basis of the CTM bootstrap. Our integral solutions with an appropriate choice of the integral kernel reproduce the formulae previously obtained by using the bosonization of the vertex operators of the quantum affine algebra $U_q (\hat{\mathfrak{sl}_2})$.

Ferromagnetism and phase separation in one-dimensionald−pand periodic Anderson models

Using the density matrix renormalization group, we study metallic ferromagnetism in a one-dimensional copper-oxide model that contains one oxygen p orbital and one copper d orbital. The parameters for the $d\ensuremath{-}p$ model can be chosen so that it is similar to the one-dimensional periodic Anderson model. For these parameters, we compare the ground-state phase diagram with that of the Anderson model and find a ferromagnetic region analogous to one found in the Anderson model, but which is pushed to somewhat higher densities and interaction strengths. In both models, we find a region within the ferromagnetic phase in which phase separation between a localized ferromagnetic domain and a weakly antiferromagnetic regime occurs. We then choose a set of parameter values appropriate for copper-oxide materials and explore the ground-state phase diagram as a function of the oxygen-oxygen hopping strength and the electron density. We find three disconnected regions of metallic ferromagnetism and give physical pictures of the three different mechanisms for ferromagnetism in these phases.

Log-periodic oscillations for a uniform spin model on a fractal

The model of Blume-Capel on the Sierpinski gasket is investigated within the method of transfer matrices, where the thermodynamic functions are obtained after the numerical iteration of a set of discrete maps. The analysis of the T=0 transition shows that, for antiferromagnetic coupling and a finite interval of self-energy coefficient, the correlation length diverges as exp(J(eff)/T), with superimposed log-periodic oscillations in terms of the reduced temperature t=exp(-|J(eff)|/T). Both the period of oscillations and the effective interaction J(eff) depend on the strength of the actual coupling constants. In the antiferromagnetic regime, residual entropy is found for three different values of the self-energy parameter. The variation of this parameter leads, in the case of ferromagnetic coupling, to a more complex behavior for the correlation length than the already known exp[exp(J(eff)/T)] dependence observed for the Ising and Potts models.

High-pressure μSR studies on elemental rare earth metals

Muon-spin rotation data on single-crystalline samples of the heavy rare-earth metals Dy and Ho in their antiferromagnetic and ferromagnetic temperature regimes (20 K<T<T N ) have been obtained as function of hydrostatic external pressure up to 0.9 GPa using the He-gas high-pressure system installed at the decay muon beam μE1 at PSI. Due to the absence of pressure-dependent changes of the moment orientation (spin-turning) as observed in previous high-pressure measurement on ferromagnetic Gd, the pressure coefficients ( ∂B μ / ∂p) T in Dy and Ho are comparatively small, but nevertheless temperature-dependent. The volume dependence of the contact field is extracted.

Correlation functions of the XXZ Heisenberg spin- [formula omitted] chain in a magnetic field

Using the algebraic Bethe ansatz method, and the solution of the quantum inverse scattering problem for local spins, we obtain multiple integral representations of the n-point correlation functions of the XXZ Heisenberg spin- 1 2 chain in a constant magnetic field. For zero magnetic field, this result agrees, in both the massless and massive (anti-ferromagnetic) regimes, with the one obtained from the q-deformed KZ equations (massless regime) and the representation theory of the quantum affine algebra U q( sl ̂ 2) together with the corner transfer matrix approach (massive regime).

Boundary Kondo problem in the integrablet−Jmodel

We study the problem of a boundary magnetic impurity coupled with the solvable $t\ensuremath{-}J$ chain. This model provides a good starting point to understand the Kondo problem in a Luttinger liquid as well as in a strongly correlated host. As the Kondo coupling constant ${J}_{i}$ may take arbitrary values without breaking the integrability, we can study the ferromagnetic and antiferromagnetic Kondo problems simultaneously. It is shown that the boundary coupling generally splits the impurity spin into two effective spins and induces the interaction-dependent residual entropy. The absence of Kondo screening in an antiferromagnetic Kondo coupling regime is found, which indicates a genuine competing effect between the Kondo coupling ${J}_{i}$ and the impurity potential ${V}_{i}.$ A local Landau-Luttinger liquid description is proposed to calculate the specific heat of the impurity.

Dynamical incommensurability in the 2D Hubbard model

The recent discovery of incommensurate fluctuations in yttrium compounds has reopened the problem of finding new appropriate frameworks to describe the variety of experimental features presented by cuprate superconductors. According to this, the imaginary part of the dynamical spin magnetic susceptibility of the Hubbard model has been studied by means of the composite operator method. By varying the frequency, the crossover between the two antiferromagnetic regimes has been found. The incommensurability issue has been studied and discussed.

FROM T-J TO JT : SPIN-CHARGE-PHONON-COUPLING IN HIGH TEMPERATURE SUPERCONDUCTORS

Superconducting copper-oxides are modeled by a two-band Hubbard type modelHamiltonian which explicitly includes on-site and intersiteelectron–phonon interactions. The effects of both on theantiferromagnetic and superconducting state properties are investigated. Inthe antiferromagnetic regime the on-site electron–phonon interactionsinduce a squeezing of the band energies and an isotope effect on theantiferromagnetic transition temperature TN is predicted. In thesuperconducting phase the intersite electron–phonon couplings gainimportance and induce a large increase in Tc especially if the difference inband energies is large.

Spin resonance studies of the quasi-one-dimensional Heisenberg antiferromagnet Cs2CuCl4

Measurements of the magnetic-field dependent millimetre-wave response of Cs2CuCl4, a quasi-one-dimensional Heisenberg antiferromagnet, are presented. The evolution of the electron spin resonance between high temperatures (the paramagnetic regime) and temperatures below TN, at which there is an onset of long-range magnetic order (the antiferromagnetic regime), is studied for a range of crystal orientations. The magneto-optical data suggest that there is an onset of local magnetic order at temperatures considerably higher than TN as determined from neutron scattering measurements, and that the evolution towards a long-range ordered state is gradual. A phenomenological model that describes the general features is suggested.

Quantum tunneling in ferromagnetic nanoparticles interacting with a spin thermostat: Effective Hamiltonian

An effective Hamiltonian, describing quantum tunneling in ferrimagnetic nanoparticles which includes interactions between the electronic spins of nanoparticle and microscopic environmental spins (like nuclear spins or paramagnetic impurities), is obtained. Two limiting cases, describing tunneling in antiferromagnetic and ferromagnetic regimes are considered, and criterion for the transition between the two regimes is found. The validity of analytic results is verified by the exact diagonalization method.

Variational Monte Carlo Study of an Interacting Electron-Phonon Model

The phase diagram of a Hamiltonian with both on-site repulsion between electrons and with hopping probabilities which depend on dynamically varying separations of neighboring lattice sites has been studied with a new variational ansatz. A rich phase diagram is obtained as a function of density, electron-electron, and electron-phonon interaction strengths. The superconducting regime is characterized by a pattern of short and long bonds correlated with resonating pair hopping, whereas for the charge density wave phase this pattern is nearly static, and in the disordered and antiferromagnetic regimes all bond lengths are practically the same.

Metamagnetism in the two-dimensional Hubbard model with easy axis

Although the Hubbard model is widely investigated, there are surprisingly few attempts to study the behavior of such a model in an external magnetic field. Using the projector quantum Monte Carlo technique, we show that the Hubbard model with an easy axis exhibits metamagnetic behavior if an external field is turned on. For the case of intermediate correlations strength $U$, we observe a smooth transition from an antiferromagnetic regime to a paramagnetic phase. While the staggered magnetization will decrease linearly up to a critical field ${B}_{c},$ uniform magnetization develops only for fields higher than ${B}_{c}.$

Exact four-spinon dynamical correlation function of the Heisenberg model

In this paper we derive the exact expression of the four-spinon contribution to the dynamical correlation function of the spin S = 1 2 anisotropic ( XXZ) Heisenberg model in the antiferromagnetic regime. We extensively study its isotropic ( XXX) limit and derive perturbatively the Ising one. Our method relies on the quantum affine symmetry of the model, which allows for a systematic diagonalization of the Hamiltonian in the thermodynamic limit and for an exact calculation of matrix elements of local spin operators. In fact, we argue that the familiar criticism of this method related to the complication of these matrix elements is not justified. First, we give, in the form of contour integrals, an exact expression for the n-spinon contribution. After we compile recently found results concerning the two-spinon contribution, we specialize the n-spinon formula to the new case n = 4. Then we give an explicit series representation of this contribution in the isotropic limit. Finally, after we show that this representation is free of divergences, we discuss the Ising limit in which a simple expression is found up to first order in the anisotropy parameter.

Phase diagram of the non-Hermitian asymmetricXXZspin chain

The low-lying excitations of the asymmetric $XXZ$ spin chain are derived explicitly in the antiferromagnetic regime through the Bethe Ansatz. It is found that a massless and conformal invariant phase with central charge $c=1$ is separated from a massive phase by a line on which the low-lying excitations surprisingly scale with the lattice length as $\Delta E \sim N^{-\frac{1}{2}}$. The mass gap vanishes with an exponent $\frac{1}{2}$ as one approaches the massless phase. The connection with the asymmetric $6$--vertex model and some physical consequences are discussed.

Phase separation kinetics in La2CuO4+? and inhomogeneous hole doping in the antiferromagnetic regime (0 &lt;x &lt; 0.02) of La2?x Sr x CuO4

The diffusion of the excess oxygen during phase separation in La2CuO4+δ was studied using thermal history-dependent normal state magnetic susceptibilityϰ(T, t) measurements versus temperatureT and timet as a probe. A large thermal hysteresis ofϰ(T) was observed for La2CuO4.044 between data obtained after quenching to 5 K and then warming, and data obtained while or after slowly cooling from 300 K. A model for the excess oxygen diffusion is presented, from which theϰ(T, t) data yield aT-independent activation energy of 0.24(3) eV for the diffusion coefficient of the excess oxygen from 150 to 220 K. In related work, we have used139La NQR andμSR measurements to probe the antiferromagnetic (AF) region (x<0.02) of the La2−xSr x CuO4 system below the Neel temperatureTN(x), from which we extract the Cu+2 staggered magnetizationM†(x, T). M†(x, T=0), extrapolated from above 30 K, was successfully modeled with spin-wave theory, assuming that the doped holes are mobile and are situated in walls in the CuO2 plane which uncouple undoped AF domains; these domains are coupled to those in adjacent CuO2 planes. This agreement supports the previous hypothesis that microsegregation of the (mobile) doped holes into domain walls occurs above ∼30 K, consistent with the phenomenology of Emery and Kivelson. Below ∼30 K, an anomalous increase inM†(x, T) is observed, such thatM†(x, T=0) is nearly independent ofx. We interpret this effect as arising from localization of the doped holes below ∼30 K.

Magnetism in the large- U single-band Hubbard model

Using an exact expression of the t-J model in terms of fermionic holon and local spin operators, we investigate magnetism in the large- U limit of the Hubbard model as a function of doping ( x) away from half-filling. The characteristic dopant concentration x c below which the local spins remain antiferromagnetically correlated, and above which they are ferromagnetically coupled, is estimated. In the antiferromagnetic regime x < x c , a large reduction of the holon bandwidth is found and explicitly related to the zero-point motion of the spin fluctuations. The basic picture of this regime is that of a Heisenberg antiferromagnet defected by a more or less static distribution of polaronic holons. In the ferromagnetic regime, x < x c , and for a three-dimensional lattice, the model exhibits coexistence of charge itinerancy and localized magnetic moment behavior. At low temperature T the magnetization follows the Bloch T 3/2 law, while above the Curie temperature ( T c) the paramagnetic susceptibility follows the Curie-Weiss law. The Bohr magneton number is 1 − x and, hence, non integer. For T < T c ≪ T c the specific heat varies as C v/T = A + BT 1/2, where the first term is contributed by itinerant holons and the second by spin reversals. These results are consistent with the observed ferromagnetism in the 3d transition metals, suggesting that the large- U Hubbard model shares many important physical properties with the intermediate- U model. However, we point out that there are significant differences between the large- U ferromagnetism and the Stoner-Slater band ferromagnetism, and that these differences can be measured by spin polarization experiments at low temperature.