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.     

Spontaneous SU(2) symmetry breaking in one-dimensional quantum antiferromagnet

We present a Bethe Ansatz based investigation of a one-dimensional (1D) Heisenberg spin chain in a real 3D crystal lattice. We have shown that due to an influence of the lattice distortion on a crystalline field of ligands of magnetic ions, a Heisenberg antiferromagnetic spin chain is unstable under the appearance of a magnetic anisotropy of the “easy-plane” type. The effects of an external magnetic field and nonzero temperature onto such a phase transition are studied. Received: 19 January 1998 / Revised: 16 March 1998 / Accepted: 17 March 1998     

Upper bound on the free energy of the spin 1/2 Heisenberg ferromagnet

The authors obtain an upper bound on the free energy of the spin 1/2 Heisenberg ferromagnet. The zero field bound is, at low temperature, similar to the formula given by the magnon approximation. That is, its functional dependence on temperature is the same but the constant is different.Work partially supported by U.S. National Science Foundation grants DMS 9196047 (JGC) and DMS 9002416 (JPS).     

Spin anisotropy and slow dynamics in spin glasses

We report on an extensive study of the influence of spin anisotropy on spin glass aging dynamics. New temperature cycle experiments allow us to compare quantitatively the memory effect in four Heisenberg spin glasses with various degrees of random anisotropy and one Ising spin glass. The sharpness of the memory effect appears to decrease continuously with the spin anisotropy. Besides, the spin glass coherence length is determined by magnetic field change experiments for the first time in the Ising sample. For three representative samples, from Heisenberg to Ising spin glasses, we can consistently account for both sets of experiments (temperature cycle and magnetic field change) using a single expression for the growth of the coherence length with time.     

Strong enhancement of superconducting T(c) in ferromagnetic phases

It is shown that the critical temperature for spin-triplet, p-wave superconductivity mediated by spin fluctuations is generically much higher in a Heisenberg ferromagnetic phase than in a paramagnetic one, due to the coupling of the magnons to the longitudinal magnetic susceptibility. Together with the tendency of the low-temperature ferromagnetic transition in very clean Heisenberg magnets to be of first order, this qualitatively explains the phase diagram recently observed in UGe(2).     

Linear-chain antiferromagnetism in Ni(N2H5)2(SO4)2

The compound dihydrazinium bis(sulfato) niccolate(II), Ni(N₂H₅)₂(SO₄)₂, containing sulfato-bridged chains of Ni(II) ions, can be described as an antiferromagnetic Heisenberg linear-chain system. A reasonable agreement of susceptibility measurements in the temperature region 2–80K, with a theory developed by Weng for antiferromagnetic Heisenberg linear chains with spin S=1, is obtained for a value of the intra-chain interaction $\text{J}\text{k}\text{=?3.35K}$. Preliminary results of specific heat measurements, on the other hand, do not fit quite well using this model. The origin of this discrepancy is suggested to be a zero-field splitting of the single ion.     

Persistent spin currents without spin-orbit coupling in a spin-1/2 ring

We show that a spatially dependent magnetic field can induce a persistent spin current in a spin-1/2 Heisenberg antiferromagnetic ring, proportional to the solid angle subtended by the magnetic field on a unit sphere. The magnitude of the spin current is determined by the ratio of longitudinal and transverse exchange interactions J(parallel)/J(perpendicular) and by the magnetic field magnitude.     

Field distribution and flux-line depinning in MgB (2)

We report the first observation of the field distribution and flux-line lattice (FLL) depinning in the vortex-state (VS) of a type-II superconductor probed by conduction electron spin resonance (CESR). CESR was performed in MgB (2) (T(c) approximately 39 K) at 4.1 GHz (1455 Oe) and 9.5 GHz (3390 Oe). The field distribution, n(H), and a standard deviation of sigma approximately 14 Oe (at 28 K/4.1 GHz and at 7 K/9.5 GHz) were inferred, respectively, from the distortion and broadening of the CESR in the VS. For both frequencies, the FLL depinning temperature was determined.     

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.     

Semi-fermionic representation of SU(N) Hamiltonians

We represent the generators of the SU(N) algebra as bilinear combinations of Fermi operators with imaginary chemical potential. The distribution function, consisting of a minimal set of discrete imaginary chemical potentials, is introduced to satisfy the local constraints. This representation leads to the conventional temperature diagram technique with standard Feynman codex, except that the Matsubara frequencies are determined by neither integer nor half-integer numbers. The real-time Schwinger-Keldysh formalism is formulated in the framework of complex equilibrium distribution functions for auxiliary semi-fermionic fields. We discuss the continuous large N and SU(2) large spin limits. We illustrate the application of this technique for magnetic and spin-liquid states of the Heisenberg model. Received 26 February 2001 and Received in final form 25 April 2001     

Quantification of entanglement from magnetic susceptibility for a Heisenberg spin 1/2 system

We report temperature and magnetic field dependent magnetization and quantification of entanglement from the experimental data for dichloro (thiazole) copper (II), a Heisenberg spin chain system. The plot of magnetic susceptibility vs. temperature indicates an infinite spin chain. Isothermal magnetization measurements (as functions of magnetic field) were performed at various temperatures below the antiferromagnetic (AFM) ordering, where the AFM correlations persist significantly. These magnetization curves are fitted to the Bonner–Fisher model. Magnetic susceptibility is used as an entanglement witness to quantify the amount of entanglement in the system.     

不同方向外磁场下海森堡自旋链的热纠缠

研究了两量子比特的海森堡XXX自旋链分别处于x方向和y方向均匀外磁场时系统的纠缠特性,并用负度N来度量。得到纠缠度N的解析表达式,并在此基础上进行数值计算。仔细讨论了磁场B、温度T和自旋耦合系数J对纠缠度N的影响。结果表明：纠缠度N会随着磁场|B|和温度T的增大而减小,但会随着自旋耦合系数J的增大而增大。另外,增大的J还会使临界磁场|Bc|和临界温度Tth变大。所以,我们可以通过调节B、T和J来控制热纠缠,这对固态系统中通过构建和选择参数调整系统的纠缠度具有一定的作用和意义。研究还发现,加在x方向均匀外磁场和加在y方向均匀外磁场对两量子比特的海森堡XXX自旋链的作用效果是一样的。     

Zero field muon spin relaxation in simple magnets and spin glasses

A theory is developed for the calculation of zero field muon spin relaxation function for classical simple magnets (Ising, XY, and Heisenberg) in D(=1,2,3) dimensions. The results are different from the Kubo-Toyabe theory, except for Heisenberg system in three dimension. Relation between the relaxation function and random field distribution is dicussed and a new method of analysing experimental data is suggested and discussed in the context of spin glasses.     

Study of the ferromagnetic transition in a positionally frozen Heisenberg spin system

Using extensive Monte Carlo simulations with both particle and cluster orientational moves, in conjunction with finite size scaling and histogram reweighting techniques, we have determined the Curie temperature for two models of positionally frozen Heisenberg spin systems: a system with spatial correlations corresponding to a hard sphere fluid and a spatially random system. We find that the results for the positionally frozen hard sphere Heisenberg system are fairly similar to those previously obtained for the Heisenberg spin fluid and quantitatively agree with the mean field theory estimates. The random system undergoes the ferromagnetic transition at a higher temperature since the lack of core repulsion increases the spin correlations. In this case however the mean field theory overestimates by far the critical temperature.     

Magnetic determination of H(c2) under accurate alignment in (TMTSF)2ClO4

Cantilever magnetometry has been used to measure the upper critical magnetic field H(c2) of the quasi-one-dimensional molecular organic superconductor (TMTSF)2ClO4. From simultaneous resistivity and torque magnetization experiments conducted under precise field alignment, H(c2) at low temperature is shown to reach 5 T, nearly twice the Pauli paramagnetic limit imposed on spin singlet superconductors. These results constitute the first thermodynamic evidence for a large H(c2) in this system and provide support for spin triplet pairing in this unconventional superconductor.     

Entanglement in (1/2,1) Mixed-Spin XY Model with Long-Range Interaction

In this study, we investigate entanglement in a two mixed-spin (1/2,1) XY Heisenberg spin system under an applied magnetic field by considering the long-range interaction with an inverse-square function. The spin-spin coupling constant is considered as a function of the distance between spins. We also discuss the temperature and magnetic field dependence of the thermal entanglement in this system for this interaction. The numerical results show that, in the presence of the long-range interaction, thermal entanglement between spins has a rich behavior dependent upon the interaction strength, temperature and magnetic field. We find that for less than a critical distance there are entanglement plateaus dependent upon the distance between spins, whereas above the critical distance the entanglement can exhibit sudden death.     

自旋1/2铁磁模型的相互作用淬火

本文设想了一个相互作用淬火后在 时刻开启的海森堡反铁磁模型,并对其进行了解析分析.在低温近似下,通过求解海森堡方程给出该系统随时间演化的自旋算符的解析形式.主要推算并讨论了自旋传播子,通过对传播子的讨论发现这个新体系的自旋波谱发生了变化.文末对此现象进行了讨论.     

Temperature dependence of the gap in the S=1/2 antiferromagnetic chain in the presence of a staggered field

We calculate the temperature dependent energy gap in the magnetic excitation spectrum of the one-dimensional spin 1/2 antiferromagnetic Heisenberg model in the presence of a staggered magnetic field, using the phase Hamiltonian, a self consistent harmonic approximation, and the thermal-Green function technique.     

Ising-type critical exponents of the fully frustrated spin-1/2 Heisenberg FM/AF square bilayer at a critical magnetic field

The fully frustrated spin-1/2 Heisenberg FM/AF square bilayer in a magnetic field with the ferromagnetic inter-dimer interaction and the antiferromagnetic intra-dimer interaction is explored by the use of localized many-magnon approach, which allows to connect the original purely quantum Heisenberg spin model on a square bilayer with the effective ferromagnetic Ising model on a simple square lattice. Magnetization and specific heat are investigated exactly at a field-driven phase transition from the singlet-dimer phase towards the fully saturated ferromagnetic phase, which changes from a discontinuous phase transition to a continuous one at a certain critical temperature. The mapping correspondence between the spin-1/2 Heisenberg FM/AF square bilayer and the ferromagnetic Ising square lattice suggests for this special critical point of the spin-1/2 Heisenberg FM/AF square bilayer critical exponents from the standard two-dimensional Ising universality class.     

Analytic evidence of the equivalence of the alternating Heisenberg spin chain to the mixed spin (1, 1/2) Heisenberg chain

We investigated the properties of the spin-1/2 ferromagnetic-antiferromagnetic-antiferromagnetic alternating Heisenberg chain using the spin-wave theory. The spin-wave excitation spectra, the sublattice magnetizations and the local bond energies of the model are calculated to be compared with the corresponding properties of the mixed spin (1, 1/2) chain for a range of α. The results demonstrate that all the properties show similar behaviours in the small α limit, so the properties of the mixed spin (1, 1/2) chain can be described using the spin-1/2 ferromagnetic-antiferromagnetic-antiferromagnetic alternating Heisenberg chain.