Spin gap in the zigzag spin-1/2 chain cuprate Sr(0.9)Ca(0.1)CuO(2)

We report a comparative study of (63)Cu nuclear magnetic resonance spin lattice relaxation rates T(1)(-1) on undoped SrCuO(2) and Ca-doped Sr(0.9)Ca(0.1)CuO(2) spin chain compounds. A temperature independent T(1)(-1) is observed for SrCuO(2) as expected for an S=1/2 Heisenberg chain. Surprisingly, we observe an exponential decrease of T(1)(-1) for T<90 K in the Ca-doped sample evidencing the opening of a spin gap. The data analysis within the J(1)-J(2) Heisenberg model employing density-matrix renormalization group calculations suggests an impurity driven small alternation of the J(2)-exchange coupling as a possible cause of the spin gap.     

63Cu, 35Cl, and 1H NMR in the S=1/2 Kagome lattice ZnCu3(OH)6Cl2

ZnCu(3)(OH)(6)Cl(2) (S=1/2) is a promising new candidate for an ideal Kagome Heisenberg antiferromagnet, because there is no magnetic phase transition down to approximately 50 mK. We investigated its local magnetic and lattice environments with NMR techniques. We demonstrate that the intrinsic local spin susceptibility decreases toward T=0, but that slow freezing of the lattice near approximately 50 K, presumably associated with OH bonds, contributes to a large increase of local spin susceptibility and its distribution. Spin dynamics near T=0 obey a power-law behavior in high magnetic fields.     

Magnetic spin ladder (C5H12N)2CuBr4: high-field magnetization and scaling near quantum criticality

The magnetization, M(H< or =30 T,0.7< or =T< or =300 K), of (C5H12N)2CuBr4 has been used to identify this system as an S = 1/2 Heisenberg two-leg ladder in the strong-coupling limit, J( perpendicular) = 13.3 K and J( parallel) = 3.8 K, with H(c1) = 6.6 T and H(c2) = 14.6 T. An inflection point in M(H,T = 0.7 K) at half saturation, M(s)/2, is described by an effective XXZ chain. The data exhibit universal scaling behavior in the vicinity of H(c1) and H(c2), indicating that the system is near a quantum critical point.     

Competing spin phases in geometrically frustrated magnetic molecules

We identify a class of zero-dimensional classical and quantum Heisenberg spin systems exhibiting anomalous behavior in an external magnetic field B similar to that found for the geometrically frustrated kagome lattice of classical spins. Our calculations for the isotropic Heisenberg model show the emergence of a pronounced minimum in the differential susceptibility dM/dB at B(sat)/3 as the temperature T is raised from 0 K for structures based on corner-sharing triangles, specifically the octahedron, cuboctahedron, and icosidodecahedron. As the first experimental evidence we note that the giant Keplerate magnetic molecule {Mo(72)Fe(30)} (Fe(3+) ions on the 30 vertices of an icosidodecahedron) exhibits this behavior. For low T when B approximately B(sat)/3 two competing families of spin configurations exist of which one behaves magnetically "stiff" leading to a reduction of dM/dB.     

Quantum criticality and percolation in dimer-diluted two-dimensional antiferromagnets

The S = 1/2 Heisenberg model is considered on bilayer and single-layer square lattices with couplings J1, J2, with each spin belonging to one J2-coupled dimer. A transition from a Néel to disordered ground state occurs at a critical value of g = J2/J1. The systems are here studied at their dimer-dilution percolation points p*. The multicritical point (g*,p*) previously found for the bilayer is not reproduced for the single layer. Instead, there is a line of critical points (g < g*, p*) with continuously varying exponents. The uniform magnetic susceptibility diverges as T(-alpha) with alpha element of [1/2,1]. This unusual behavior is attributed to an effective free-moment density approximately T(1-alpha). The susceptibility of the bilayer is not divergent but exhibits remarkably robust quantum-critical scaling.     

Spin liquid state in an organic Mott insulator with a triangular lattice

1H NMR and static susceptibility measurements have been performed in an organic Mott insulator with a nearly isotropic triangular lattice, kappa-(BEDT-TTF)2Cu2(CN)(3), which is a model system of frustrated quantum spins. The static susceptibility is described by the spin S=1/2 antiferromagnetic triangular-lattice Heisenberg model with the exchange constant J approximately 250 K. Regardless of the large magnetic interactions, the 1H NMR spectra show no indication of long-range magnetic ordering down to 32 mK, which is 4 orders of magnitude smaller than J. These results suggest that a quantum spin liquid state is realized in the close proximity of the superconducting state appearing under pressure.     

Magnetic susceptibility and specific heat of the spin-${\frac{1}{2}}$ Heisenberg model on the kagome lattice and experimental data on ZnCu3(OH)6Cl2

We compute the magnetic susceptibility and specific heat of the spin- Heisenberg model on the kagome lattice with high-temperature expansions and exact diagonalizations. We compare the results with the experimental data on ZnCu₃(OH)₆Cl₂ obtained by Helton et al. [Phys. Rev. Lett. 98, 107204 (2007)]. Down to k_BT/J≃0.2, our calculations reproduce accurately the experimental susceptibility, with an exchange interaction J≃190 K and a contribution of 3.7% of weakly interacting impurity spins. The comparison between our calculations of the specific heat and the experiments indicate that the low-temperature entropy (below ~20 K) is smaller in ZnCu₃(OH)₆Cl₂ than in the kagome Heisenberg model, a likely signature of other interactions in the system.     

Néel temperature of quasi-low-dimensional Heisenberg antiferromagnets

The Néel temperature T(N) of quasi-one- and quasi-two-dimensional antiferromagnetic Heisenberg models on a cubic lattice is calculated by Monte Carlo simulations as a function of interchain (interlayer) to intrachain (intralayer) coupling J(')/J down to J(')/J approximately = 10(-3). We find that T(N) obeys a modified random-phase approximationlike relation for small J(')/J with an effective universal renormalized coordination number, independent of the size of the spin. Empirical formulas describing T(N) for a wide range of J(') and useful for the analysis of experimental measurements are presented.     

Magnetic order and spin dynamics in ferroelectric HoMnO3

Hexagonal HoMnO3 is a frustrated antiferromagnet (T(N)=72 K) ferroelectric (T(C)=875 K) in which these two order parameters are coupled. Our neutron measurements of the spin-wave dispersion for the S=2 Mn3+ on the layered triangular lattice are well described by a two-dimensional nearest-neighbor Heisenberg exchange J=2.44 meV, and an anisotropy D that is 0.28 meV above the spin-reorientation transition at 40 K and 0.38 meV below. For H parallel c the magnetic structures and phase diagram have been determined, and reveal additional transitions below 8 K where the ferroelectrically displaced Ho3+ ions are ordered magnetically.     

Frustrated cuprate route from antiferromagnetic to ferromagnetic spin-1/2 Heisenberg chains: Li2ZrCuO4 as a missing link near the quantum critical point

From thermodynamics, local spin density approximation+Hubbard U studies and exact diagonalizations of a five-band Hubbard model on CuO2 stripes we find that Li2ZrCuO4 (Li2CuZrO4 in traditional notation) is close to a ferromagnetic critical point. Analyzing its susceptibility chi(T) and specific heat cp(T,H) within a Heisenberg model, we show that the ratio of the 2nd to the 1st neighbor exchange integrals alpha=-J2/J1 approximately 0.3 is close to the critical value alphac=1/4. Comparing with related chain cuprates we explain the rather strong field dependence of cp, the monotonic downshift of the peak of chi(T), and its increase for alpha-->alphac+0.     

Unusual single-ion non-fermi-liquid behavior in Ce(1-x)LaxNi9Ge4

We report on specific heat, magnetic susceptibility, and resistivity measurements on the compound Ce(1-x)LaxNi9Ge4 for various concentrations ranging from the stoichiometric system with x = 0 to the dilute limit x = 0.95. Our data reveal single-ion scaling with the Ce concentration and the largest ever recorded value of the electronic specific heat Deltac/T approximately 5.5 J K-2 mol(-1) at T = 0.08 K for the stoichiometric compound x = 0 without any trace of magnetic order. While in the doped samples Deltac/T increases logarithmically below 3 K down to 50 mK, their magnetic susceptibility behaves Fermi-liquid-like below 1 K. These properties make the compound Ce(1-x)LaxNi9Ge4 a unique system on the borderline between Fermi-liquid and non-Fermi-liquid physics.     

Unconventional dynamics in triangular Heisenberg antiferromagnet NaCrO2

We report magnetization, specific heat, muon spin rotation, and Na NMR measurements on the S=3/2 rhombohedrally stacked Heisenberg antiferromagnet NaCrO2. This compound appears to be a good candidate for the study of isotropic triangular Heisenberg antiferromagnets with very weak interlayer coupling. While specific heat and magnetization measurements indicate the onset of a transition in the range Tc approximately 40-50 K, both muon spin rotation and NMR reveal a fluctuating crossover regime extending well below Tc, with a peak of relaxation rate T1(-1) around T approximately 25 K. This novel finding is discussed within the context of excitations in the triangular Heisenberg antiferromagnets.     

One-dimensional regular isotropic heisenberg antiferromagnetism in an organic radical p-diethylaminophenyl nitronyl nitroxide [DEAPNN]: Observation of spin-flop at high field

The magnetic properties of the organic radical p-diethylaminophenyl nitronyl nitroxide (DEAPNN; 2-(4'-diethylaminophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy-3-N-oxide) have been investigated. The magnetization isotherms of DEAPNN measured up to 20.5 T at 1.7 and 4.2 K show a spin-flop phenomenon characteristic of one-dimensional regular isotropic Heisenberg antiferromagnetism. The magnetic susceptibility of DEAPNN is well interpreted by the Heisenberg antiferromagnetism in the whole temperature range of 2–300 K. The exchange coupling constant is determined to be J/k = -2.45 K based on the spin Hamiltonian H = -2JΣS_iS_{i + 1}.     

Double-Cusp Susceptibility of Quantum Heisenberg Spin Glasses: Ferromagnetic Coupling and Anisotropy interaction

With the help of the generalized static replica symmetric spin-glass theory, a quantum Heisenberg S = 112 spin-glass model with the infinite-ranged random Dzyaloshinskii-Moriya (DM) interactions and ferromagnetic coupling is investigated. The dependence of the local susceptibility and the corresponding order parameters on temperature is calculated numerically for different ferromagnetic interactions and fixed anisotropy It has been found that the local susceptibility exhibits double-cusp features for different ferromagnetic coupfings (J_o). In addition, the dependence of the spontaneous moment on temperature is studied numerically, too.     

Critical spin dynamics of the 2D quantum Heisenberg antiferromagnets Sr2CuO2Cl2 and Sr2Cu3O4Cl2

We report a neutron scattering study of the long-wavelength dynamic spin correlations in the model two-dimensional S = 1/2 square lattice Heisenberg antiferromagnets Sr2CuO2Cl2 and Sr2Cu3O4Cl2. The characteristic energy scale, omega(0)(T/J), is determined by measuring the quasielastic peak width in the paramagnetic phase over a wide range of temperature ( 0.2 less similarT/J less similar0.7). The obtained values for omega(0)(T/J) agree quantitatively between the two compounds and also with values deduced from quantum Monte Carlo simulations. The combined data show scaling behavior, omega approximately xi(-z), over the entire temperature range with z = 1.0(1), in agreement with dynamic scaling theory.     

Low-temperature broken-symmetry phases of spiral antiferromagnets

We study Heisenberg antiferromagnets with nearest- (J1) and third- (J3) neighbor exchange on the square lattice. In the limit of spin S-->infinity, there is a zero temperature (T) Lifshitz point at J(3)=1/4J(1), with long-range spiral spin order at T=0 for J3>1/4J(1). We present classical Monte Carlo simulations and a theory for T>0 crossovers near the Lifshitz point: spin rotation symmetry is restored at any T>0, but there is a broken lattice reflection symmetry for 0< or =T相似文献   

二维反铁磁Heisenberg模型(s=1/2)的低能性质研究

利用一种有效的Monte Carlo簇团迭代方法进行了关于二维反铁磁Heisenberg量子模型的数值研究。比较手征微扰理论的有关解析结果,在有限温度和有限体积下精确地确定了该系统的重要低能参数:基态能量密度e_o=-0.6693(1)J/a²,参差磁化强度M_s=0.3076(4)/a²,自旋波速度?c=1.68(1)Ja和自旋块度(spin stiffness)ρ_s=0.185(5)J。计算结果与 关键词：     

Spin-peierls transition in dimerized stacks of anion-radical salt (N-Me-2,5-di-Me-Pz)(TCNQ)2, (Pz is pyrazine)

An anion-radical salt (ARS) (N-Me-2,5-di-Me-Pz)(TCNQ)₂, where Pz is pyrazine, was synthesized and its crystal structure was resolved. X-ray diffraction experiments on single crystals were performed. Heat capacity was measured in the temperature range from 2 to 300 K. Magnetisation and magnetic susceptibility were measured in the temperature range from 2 to 300 K and the low-temperature part was measured in magnetic fields from 5 mT to 5 T. The experimental results were explained in terms of dimerized Heisenberg spin chain model. Numerical calculations were performed and compared with experimental data.     

Ising phases of Heisenberg ladders in a magnetic field

We show that Dzyaloshinskii-Moriya (DM) interactions can substantially modify the phase diagram of spin-1/2 Heisenberg ladders in a magnetic field provided they compete with exchange. For nonfrustrated ladders, they induce a local magnetization along the DM vector that turns the gapless intermediate phase into an Ising phase with broken translational symmetry, while for frustrated ladders, they extend the Ising order of the half-integer plateau to the surrounding gapless phases of the purely Heisenberg case. Implications for experimental ladder and dimer systems are discussed.