Singlet-triplet Hamiltonian for spin excitations in a Kondo insulator

In the nonsymmetric version of the periodic Anderson model for a Kondo insulator, an effective singlet-triplet Hamiltonian Ĥ _s−t with indirect antiferromagnetic f-f exchange is constructed, which makes it possible to analyze the dynamic magnetic susceptibility χ _f (k, ω) of f electrons. Hamiltonian Ĥ _s−t is used to describe the experimentally observed dispersion of the three-level spin excitation spectrum in YbB₁₂. A distinguishing feature of this analysis is the introduction of small-radius singlet and triplet collective f-d excitations that form low- and high-lying spin bands during motion over the lattice.     

Two-spinon and orbital excitations of the spin-Peierls system TiOCl

We combine high-resolution resonant inelastic x-ray scattering with cluster calculations utilizing a recently derived effective magnetic scattering operator to analyze the polarization, excitation energy, and momentum-dependent excitation spectrum of the low-dimensional quantum magnet TiOCl in the range expected for orbital and magnetic excitations (0-2.5?eV). Ti 3d orbital excitations yield complete information on the temperature-dependent crystal-field splitting. In the spin-Peierls phase we observe a dispersive two-spinon excitation and estimate the inter- and intradimer magnetic exchange coupling from a comparison to cluster calculations.     

Magnetostriction of the spin-Peierls compound CuGeO3

The longitudinal and transverse magnetostriction λ of the spin-Peierls compound CuGeO₃ in the b-c plane is measured in magnetic fields up to 20 T both above and below the transition temperature T _sp=14.3K. It is found that for a given crystallographic direction the value of magnetostriction is weakly dependent on the magnetic field direction. In the uniform U phase at T⩾T _sp, λ is negative and approximately equal in the b and c directions, while in the dimerized D phase at T<T _sp, λ is positive and λ _b >λ _c . At low temperatures, λ increases sharply at the magnetic-field-induced transition from the dimerized to the magnetic M phase. The experimental data allow estimation of the stress derivatives of the antiferromagnetic intrachain exchange interaction parameter and of the stress dependence of the critical field of the D-Mphase transition. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 156–159 (10 August 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

晶格量子涨落对spin-Peierls系统低能量激发态的影响

在非绝热近似下，研究了一维spin-Peierls系统中晶格量子涨落 对系统性质的影响，讨论了系统的二聚化相变、单粒子激发和双粒子束缚态. 结果表明，量子晶格涨落会抑制晶格的二聚化，破坏系统的spin-Peierls基态稳定性.在临界点，系统发生从二聚化spin-Peierls态向无能隙态的相变. 自旋声子耦合强度对束缚态能隙的影响比单粒子激发谱能隙显著. 关键词： sin-Peierls系统 非绝热近似 玻色化 相图     

Magnetoelectric response of the spin-Peierls compound CuGeO3

The effect of an electric field on the magnetic susceptibility in a pure CuGeO₃ single crystal at microwave frequencies is investigated. A quadratic effect of the electric field on the magnetic susceptibility, which increases with decreasing temperature, is observed in the spin-Peierls state at temperatures below 5 K. The observed effect is tentatively attributed to residual magnetism, due to distortions of the regular dimerized structure at domain walls with different values of the dimerization phase, and to the effect of the electric field on the interchain exchange interaction. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 9, 646–651 (10 November 1996)     

Thermal conductivity of the spin-Peierls compound CuGeO3

The thermal conductivity of the pure and zinc-doped spin-Peierls compound CuGeO₃ is investigated for the first time. Characteristic features reflecting the changes in the phonon spectrum of the crystals are observed in the k(T) curves at a transition into the dimerized state in CuGeO₃ at T _sp =14.2 K and in Cu_0.98Zn_0.02GeO₃ at T _sp =10.6 K. Near the spin-Peierls transition temperature T _sp the thermal conductivity of Cu_0.98Zn_0.02GeO₃ (k=3 W/m·K) is much less than the thermal conductivity of CuGeO₃ (k=21.5 W/m·K). Pis’ma Zh. éksp. Teor. Fiz. 66, No. 12, 823–826 (25 December 1997)     

Specific heat of the spin-Peierls compound CuGeO3

The specific heat of the quasi-onedimensional magnetic compound CuGeO₃ shows a sharp anomaly at the spin-Peierls transition temperatureT _sp . The experimental decrease of the magnetic specific heat belowT _sp indicates the presence of a spin gap as observed previously with inelastic neutron scattering. A magnetic field of 6T suppressesT _sp only slightly but reduces the spin gap by a factor of two.     

自旋-Peierls化合物GeCuO3电子结构的第一性原理研究

采用平面波赝势方法对自旋-Peierls化合物GecuO3的电子结构进行了第一性原理研究.计算结果表明:Cu2 的3d轨道自由度被冻结,未配对电子填充dx2-y2轨道.自旋向上和向下的x2-y2轨道间的交换劈裂导致了体系的绝缘性.费米能级附近的Cu 3d态与0(2)2p态存在很强的杂化作用,GeCuO3属于共价绝缘体.这种强共价性使得Cu2 的自旋磁矩偏离理想值,并且使得体系在Cu-O(2)-Cu键角接近90°情况下形成稳定的一维反铁磁(AFM)有序.计算得到GeCuO3不同自旋态的总能量,采用Noodleman的对称性破缺方法拟合出的自旋交换耦合常数表明GeCuO3沿c轴的一维AFM作用非常强,这是导致该体系在低温发生自旋一Peierls相变的根本原因.     

A new view of the electronic structure of the spin-Peierls compound -NaVO

The present understanding of the electronic and magnetic properties of -NaV₂O₅ is based on the hypothesis of strong charge disproportionation into V⁴⁺ and V⁵⁺, which is assumed to lead to a spin-1/2 Heisenberg chain system. A recent structure analysis shows, however, that the V-ions are in a mixed valence state and indistiguishable. We propose an explanation for the insulating state, which is not based on charge modulation, and show that strong correlations together with the Heitler-London character of the relevant intermediate states naturally lead to antiferromagnetic Heisenberg chains. The interchain coupling is weak and frustrated, and its effect on the uniform susceptibility is found to be small. Received: 28 January 1998 / Accepted: 17 March 1998     

Magnetic resonance of spin clusters and triplet excitations in a spin-Peierls magnet with impurities

The magnetic resonance spectrum of spin clusters formed in spin-Peierls magnets in the vicinity of impurity ions is investigated. The observed temperature dependences of the effective g-factor and the linewidth of the electron paramagnetic resonance (EPR) in crystals of Cu_1?x Ni_xGeO₃ are described in the model of the exchange narrowing of the two-component spectrum with one component ascribed to spin clusters and exhibiting an anomalous value of the g-factor and the other related to triplet excitations. An estimation of the size of the suppressed dimerization region around the impurity ion is obtained (this region includes about 30 copper ions). The dependence of the effective g-factor and the EPR linewidth on the impurity concentration at low temperatures indicates the interaction of clusters.     

Phase diagram of Si-doped spin-Peierls compound CuGeO3

We have performed magnetic susceptibility measurements on CuGe_1−xSi_xO₃ single crystals and obtained a detailed (T,x) phase diagram. In order to fully characterize the nature of the SP and AF phases in this diagram, we studied both order parameters by neutron diffraction as a function of temperature and Si concentration.     

Singlet-triplet dispersion reveals additional frustration in the triangular-lattice dimer compound Ba3Mn2O8

We present single crystal inelastic neutron scattering measurements of the S=1 dimerized quasi-two-dimensional antiferromagnet Ba(3)Mn(2)O(8). The singlet-triplet dispersion reveals nearest-neighbor and next-nearest-neighbor ferromagnetic interactions between adjacent bilayers that compete against each other. Although the interbilayer exchange is comparable to the intrabilayer exchange, this additional frustration reduces the effective coupling along the c axis and leads to a quasi-two-dimensional behavior. In addition, the obtained exchange values are able to reproduce the four critical fields in the phase diagram.     

Solitons in the spin-Peierls compound CuGeO <Subscript>3</Subscript>

We calculate the excitation gap, the soliton energy, and the soliton density at finite temperature, of the spin-1/2 one dimensional antiferromagnet coupled to phonons, using a self-consistent harmonic approximation, and the thermal-Green function technique. The spin degrees of freedom are represented by the phase Hamiltonian with the help of the boson representation of the spinless fermions. We estimate the critical field H_c above which begins the incommensurate phase. We also present a theoretical calculation for the specific heat in this phase. We use CuGeO₃ as an example of a compound where our theory could be applied. Received 22 October 2002 / Received in final form 21 January 2003 Published online 14 March 2003     

Evidence for unconventional s-wave superconductivity in the heavy-fermion compound UPt3

We have discussed the ultrasonic attenuation and specific heat in the superconducting state of the heavy-fermion compound UPt₃ in a quasiparticle picture. Coherence and pair-breaking effect have been taken into account for this system. Our results show that UPt₃ is most probably an unconventional s-wave superconductor.     

自旋-Peierls化合物GeCuO3电子结构的第一性原理研究

采用平面波赝势方法对自旋-Peierls化合物GeCuO₃的电子结构进行了第一性原理研究.计算结果表明：Cu²⁺的3d轨道自由度被冻结,未配对电子填充d_x²-y²轨道.自旋向上和向下的d_x²-y²轨道间的交换劈裂导致了体系的绝缘性.费米能级附近的Cu 3d态与O(2) 2p态存 关键词： 3')" href="#">GeCuO₃ 自旋-Peierls相变 第一性原理计算 共价绝缘体     

Ultrafast photoinduced melting of a spin-Peierls phase in an organic charge-transfer compound, K-tetracyanoquinodimethane

Ultrafast photoinduced phase transition in a spin-Peierls (SP) system of K-tetracyanoquinodimethane (K-TCNQ) was studied by femtosecond (fs) reflection spectroscopy. Photocarriers destabilize the SP phase, resulting in a decrease in molecular dimerization within 400 fs. Such a melting of the SP phase drives three kinds of coherent oscillations. By comparing the oscillations with the Raman bands activated by the dimerization, we show that the oscillation of 20 cm-1 is due to an LO phonon, and it plays an important role for the stabilization of the SP phase.     

Peierls transition in the quantum spin-Peierls model

We use the density matrix renormalization group method to investigate the role of longitudinal quantized phonons on the Peierls transition in the spin-Peierls model. For both the XY and Heisenberg spin-Peierls model we show that the staggered phonon order parameter scales as sqrt[lambda] (and the dimerized bond order scales as lambda) as lambda-->0 (where lambda is the electron-phonon interaction). This result is true for both linear and cyclic chains. Thus, we conclude that the Peierls transition occurs at lambda=0 in these models. Moreover, for the XY spin-Peierls model we show that the quantum predictions for the bond order follow the classical prediction as a function of inverse chain size for small lambda. We therefore conclude that the zero lambda phase transition is of the mean-field type.