Highly anisotropic g-factor of two-dimensional hole systems

Coupling the spin degree of freedom to the anisotropic orbital motion of two-dimensional (2D) hole systems gives rise to a highly anisotropic Zeeman splitting with respect to different orientations of an in-plane magnetic field B relative to the crystal axes. This mechanism has no analog in the bulk band structure. We obtain good, qualitative agreement between theory and experimental data, taken in GaAs 2D hole systems grown on (113) substrates, showing the anisotropic depopulation of the upper spin subband as a function of in-plane B.     

Unified origin for the 3D magnetism and superconductivity in NaxCoO2

We analyze the origin of the three-dimensional (3D) magnetism observed in nonhydrated Na-rich Na(x)CoO2 within an itinerant spin picture using a 3D Hubbard model. The origin is identified as the 3D nesting between the inner and outer portions of the Fermi surface, which arise due to the local minimum structure of the a(1g) band at the Gamma-A line. The calculated spin wave dispersion strikingly resembles the neutron scattering result. We argue that this 3D magnetism and the spin fluctuations responsible for superconductivity in the hydrated systems share essentially the same origin.     

Universal behaviors of magnon-mediated spin transport in disordered nonmagnetic metal-ferromagnetic insulator heterostructures

We numerically investigate magnon-mediated spin transport through nonmagnetic metal/ferromagnetic insulator (NM/FI) heterostructures in the presence of Anderson disorder, and discover universal behaviors of the spin conductance in both one-dimensional (1D) and 2D systems. In the localized regime, the variance of logarithmic spin conductance σ²(lnG_T) shows a universal linear scaling with its average ⟨lnG_T⟩, independent of Fermi energy, temperature, and system size in both 1D and 2D cases. In 2D, the competition between disorder-enhanced density of states at the NM/FI interface and disorder-suppressed spin transport leads to a non-monotonic dependence of average spin conductance on the disorder strength. As a result, in the metallic regime, average spin conductance is enhanced by disorder, and a new linear scaling between spin conductance fluctuation rms(G_T) and average spin conductance G_T is revealed which is universal at large system width. These universal scaling behaviors suggest that spin transport mediated by magnon in disordered 2D NM/FI systems belongs to a new universality class, different from that of charge conductance in 2D normal metal systems.     

Localized electron states and magnetic properties at the interface of a two-dimensional graphene/MnO(001) system

The results of a density functional theory study of the band structure of two-dimensional (2D) graphene/MnO(001), new materials for spintronics, with an antiferromagnetic type of ordering are presented. The regularities of the change of the valence band electron structure in the 3D MnO → 2D MnO → 2D graphene/MnO(001) series have been studied in a comparison with X-ray photoelectron spectra. The stability of the system has been established, and the energy of chemical binding has been determined using the calculation of the structural energy of 2D graphene/MnO(001). The features of the spin state in the valence band—in particular, at the Fermi level—as well as interatomic interaction in 2D graphene/MnO(001) have been discussed in comparison with 2D and 3D MnO systems with antiferromagnetic ordering. The magnetic moment of the Mn atom in all considered systems has been estimated and compared with the experimental one. The effect of the spin polarization for the oxygen and carbon atoms has been detected. The nature of this effect has been considered.     

Phase transitions in quantum spin systems with isotropic and nonisotropic interactions

We prove the existence of spontaneous magnetization at sufficiently low temperature, and hence of a phase transition, in a variety of quantum spin systems in three or more dimensions. The isotropic spin 1/2x-y model and the Heisenberg antiferromagnet with spin 1, 3/2,...and with nearest neighbor interactions on a simple cubic lattice are included.Research supported by U.S. National Science Foundation under grants GP-40768X (F.J.D.), MCS 75-21684 (E.H.L.), and GP-39048 (B.S.).Alfred Sloan Fellow.     

A possible quantum spin-liquid state in antiferromagnetic 2D solid He

A possible quantum spin-liquid state in the second-layer solid ³He adsorbed on graphite is discussed based on recent specific-heat measurements. This system is known as an ideal 2D nuclear magnet with spin . Because of competition between the multiple-spin exchanges and the triangular lattice structure, the ground state of the low-density antiferromagnetic solid can be highly frustrated without any long-range order.     

Interaction effects at crossings of spin-polarized one-dimensional subbands

We report conductance measurements of ballistic one-dimensional (1D) wires defined in GaAs/AlGaAs heterostructures in an in-plane magnetic field, B. When the Zeeman energy is equal to the 1D subband energy spacing, the spin-split subband N upward arrow intersects (N+1) downward arrow, where N is the index of the spin-degenerate 1D subband. At the crossing of N=1 upward arrow and N=2 downward arrow subbands, there is a spontaneous splitting giving rise to an additional conductance structure evolving from the 1.5(2e(2)/h) plateau. With further increase in B, the structure develops into a plateau and lowers to 2e(2)/h. With increasing temperature and magnetic field the structure shows characteristics of the 0.7 structure. Our results suggest that at low densities a spontaneous spin splitting occurs whenever two 1D subbands of opposite spins cross.     

Spin freezing in the van der Waals material Mn2Ga2S5

Geometrical frustration in low-dimensional magnetic systems has been an intriguing research aspect, where the suppression of conventional magnetic order may lead to exotic ground states such as spin glass or spin liquid. In this work we report the synthesis and magnetism study of the monocrystalline Mn$_2$Ga$_2$S$_5$, featuring both the van der Waals structure and a bilayered triangular Mn lattice. Magnetic susceptibility reveals a significant antiferromagnetic interaction with a Curie-Weiss temperature $\theta_{\rm w}\sim-260$ K and a high spin $S=5/2$ Mn$^{2+}$ state. However, no long range magnetic order has been found down to 2 K, and a spin freezing transition is found to occur at around 12 K well below its $\theta_{\rm w}$. This yields a frustration index of $f = -\theta_{\rm w}/T_{\rm f} \approx 22$, an indication that the system is highly frustrated. The absence of a double-peak structure in magnetic specific heat compared with the $TM_2$S$_4$ compounds implies that the spin freezing behavior in Mn$_2$Ga$_2$S$_5$ is a result of the competition between exchange interactions and the 2D crystalline structure. Our results suggest that the layered Mn$_2$Ga$_2$S$_5$ would be an excellent candidate for investigating the physics of 2D magnetism and spin disordered state.     

Dynamics of One‐Dimensional Electrons With Broken Spin–Charge Separation

Spin–charge separation is known to be broken in many physically interesting one‐dimensional (1D) and quasi‐1D systems with spin–orbit interaction because of which spin and charge degrees of freedom are mixed in collective excitations. Mixed spin–charge modes carry an electric charge and therefore can be investigated by electrical means. We explore this possibility by studying the dynamic conductance of a 1D electron system with image‐potential‐induced spin–orbit interaction. The real part of the admittance reveals an oscillatory behavior versus frequency that reflects the collective excitation resonances for both modes at their respective transit frequencies. By analyzing the frequency dependence of the conductance the mode velocities can be found and their spin–charge structure can be determined quantitatively.     

Novel dynamic ccaling regime in hole-doped La2CuO4

Only 3% hole doping by Li is sufficient to suppress the long-range three-dimensional (3D) antiferromagnetic order in La2CuO4. The spin dynamics of such a 2D spin liquid state at T相似文献   

Transport in quasi one-dimensional spin-1/2 systems

We present numerical results for the spin and thermal conductivity of one-dimensional (1D) quantum spin systems. We contrast the properties of integrable models such as the spin-1/2 XXZ chain against nonintegrable ones such as frustrated and dimerized chains. The thermal conductivity of the XXZ chain is ballistic at finite temperatures, while in the nonintegrable models, this quantity is argued to vanish. For the case of frustrated and dimerized chains, we discuss the frequency dependence of the transport coefficients. Finally, we give an overview over related theoretical work on intrinsic and extrinsic scattering mechanisms of quasi-1D spin systems.     

Density-dependent spin polarization in ultra-low-disorder quantum wires

There is controversy as to whether a one-dimensional (1D) electron gas can spin polarize in the absence of a magnetic field. Together with a simple model, we present conductance measurements on ultra-low-disorder quantum wires supportive of a spin polarization at B=0. A spin energy gap is indicated by the presence of a feature in the range (0.5-0.7)x2e(2)/h in conductance data. Importantly, it appears that the spin gap is not constant but a function of the electron density. Data obtained using a bias spectroscopy technique are consistent with the spin gap widening further as the Fermi level is increased.     

Efficient evaluation of partition functions of inhomogeneous many-body spin systems

We present a numerical method to evaluate partition functions and associated correlation functions of inhomogeneous 2D classical spin systems and 1D quantum spin systems. The method is scalable and has a controlled error. We illustrate the algorithm by calculating the finite-temperature properties of bosonic particles in 1D optical lattices, as realized in current experiments.     

Nonequilibrium quantum dynamics of a charge carrier doped into a Mott insulator

We study real-time dynamics of a charge carrier introduced into an undoped Mott insulator propagating under a constant electric field F on the t-J ladder and a square lattice. We calculate the quasistationary current. In both systems an adiabatic regime is observed followed by a positive differential resistivity (PDR) at moderate fields where the carrier mobility is determined. Quantitative differences between the ladder and two-dimensional (2D) systems emerge when at large fields both systems enter the negative differential resistivity (NDR) regime. In the ladder system Bloch-like oscillations prevail, while in two dimensions the current remains finite, proportional to 1/F. The crossover between the PDR and NDR in two dimensions is accompanied by a change of the spatial structure of the propagating spin polaron.     

Magnetization transport and quantized spin conductance

We analyze transport of magnetization in insulating systems described by a spin Hamiltonian. The magnetization current through a quasi-one-dimensional magnetic wire of finite length suspended between two bulk magnets is determined by the spin conductance which remains finite in the ballistic limit due to contact resistance. For ferromagnetic systems, magnetization transport can be viewed as transmission of magnons, and the spin conductance depends on the temperature T. For antiferromagnetic isotropic spin-1/2 chains, the spin conductance is quantized in units of order (gmu(B))(2)/h at T=0. Magnetization currents produce an electric field and, hence, can be measured directly. For magnetization transport in electric fields, phenomena analogous to the Hall effect emerge.     

Density Matrix Simulations of the Effects of J Coupling in Spin Echo and Fast Spin Echo Imaging

A computer simulation has been used to calculate the effects of J coupling on the amplitudes of echoes produced by CPMG sequences. The program computes the evolution of the density matrix for different pulse intervals and can predict the signals obtainable from spin systems of any size and complexity. Results from the simulation confirm the prediction that a decrease in the effects of J coupling is largely responsible for the bright fat signal seen in fast spin echo imaging at high pulse rates. The effects of J coupling on CPMG echotrains are examined for A3B2 and A3B2C2 spin systems over a wide range of J coupling and chemical shift values and pulse spacings. The effects of J coupling on the point spread function obtained with fast spin echo imaging are also discussed.     

Cumulative "roof effect" in high-resolution in vivo 31P NMR spectra of human calf muscle and the Clebsch-Gordan coefficients of ATP at 1.5 T

NMR spectra of non-weakly coupled spin systems exhibit asymmetries in line intensities known as "roof effect" in 1D spectroscopy. Due to limited spectral resolution, this effect has not been paid much attention so far in in vivo spectroscopy. But when high-quality spectra are obtained, this effect should be taken into account to explain the quantum-mechanical fine structure of the system. Adenosine 5'-triphosphate (ATP) represents a 31P spin system with multiple line splittings which are caused by J-couplings of medium strength at 1.5 T. We analyzed the ATP roof effect in vivo, especially for the beta-ATP multiplet. The intensities of its outer resonances deviate by ca. 12.5% from a symmetrical triplet. As this asymmetry reflects the transition from Paschen-Back to Zeeman effect with total spin that is largely broken up, the Clebsch-Gordan coefficients of the system can be indicated in analogy to the hyperfine structure of hydrogen. Taking the roof effect into account, the chi2 of fitting in vivo ATP resonances is reduced by ca. 9% (p<0.005).     

Magnetic anisotropy,exchange coupling and Dzyaloshinskii–Moriya interaction of two-dimensional magnets

The two-dimensional (2D) magnets provide novel opportunities for understanding magnetism and investigating spin related phenomena in several atomic thickness. Multiple features of 2D magnets, such as critical temperatures, magnetoelectric/magneto-optic responses, and spin configurations, depend on the basic magnetic terms that describe various spins interactions and cooperatively determine the spin Hamiltonian of studied systems. In this review, we present a comprehensive survey of three types of basic terms, including magnetic anisotropy that is intimately related with long-range magnetic order, exchange coupling that normally dominates the spin interactions, and Dzyaloshinskii−Moriya interaction (DMI) that favors the noncollinear spin configurations, from the theoretical aspect. We introduce not only the physical features and origin of these crucial terms in 2D magnets but also many correlated phenomena, which may lead to the advance of 2D spintronics.     

Douglas Henderson: a life in statistical mechanics

In a previous study (Wang, L. S., Nicholas, J. B., Dupuis, M., Wu, H., and Colson, S. D., 1997, Phys. Rev. Lett., 78, 4450) of small silicon oxide clusters with stochiometry Si3Oy(y = 1-6), it was found that the molecule Si3O2 and its anion required special theoretical attention. This is due to a particular electronic structure feature of the neutral cluster: the s,p hybrid atomic orbitals of the two bonded silicon atoms give rise to a low lying empty π molecular orbital. As a result the ground state of Si3O2 has a triplet spin, and a state of singlet spin is a low lying excited electronic state at about 0.5 eV above the triplet state. Ab initio molecular orbital calculations show that photoelectron detachment proceeds from the 2B1 state of the anion to the 3B1 ground state of the neutral cluster. Detachment via the 1A1 state of the neutral cluster lies in the shoulder of the detachment via the triplet state.     

一种新的DENUDATINE型C20二萜生物碱结构的NMR研究

从内蒙西伯利亚乌头中分离得一个新的C20二萜生物碱,采用选择性远程DEPT,同核和异核二维NMR技术相结合进行了研究,其结构确定为Lepenine的C－11基向立体异构体,定名为11a－hydroxylapenine．结果表明,选择性远程DEPTNMR技术对于连接这类化合物中被季碳和杂原子分割的NMR自旋体系,测定基本骨架和确定信号归属都有独到之处．