Magnetic dipolar ordering on geometrically frustrated brick-shaped lattices

The magnetic ordering of frustrated arrays of nanoscale islands can be strongly influenced by the array patterns. We theoretically present three kinds of artificial geometrically frustrated systems with different brick-shaped geometries, defined as brick-shaped lattices, and investigate their magnetic dipolar ordering at the ground state using the Monte Carlo method. The simulated results show that the magnetic ordering of three brick-shaped frustrated lattices depends strongly on the strength of dipolar interactions, depending on the lattice spacing. It is found that the long-range dipolar interactions tend to suppress the long-range ordered state and induce the short-range quasi-ice state at each vertex of the lattices. In addition, the correlations for neighboring spin pairs are closely related to not only the dipolar coupling strength but also the geometry of the lattices.     

A simulation study on the magnetic ordering in an artificial geometrically frustrated lattice

A Monte Carlo simulation on the magnetic ordering of the artificial geometrically frustrated square lattice shows that the system exhibits the spin ice state and the disorder state for strong and weak dipolar interactions. We demonstrate that the long-range dipolar interactions are significant for the short-range spin ice state.     

CeAu2In4: A candidate of quasi-one-dimensional antiferromagnetic Kondo lattice

Needle-like single crystals of CeAu₂In₄ have been grown from In flux and characterized as a new candidate of quasi-one-dimensional Kondo lattice compound by crystallographic, magnetic, transport, and specific-heat measurements down to very low temperatures. We observe an antiferromagnetic transition at T_N ≈ 0.9 K, a highly non-mean-field profile of the corresponding peak in specific heat, and a large Sommerfeld coefficient γ =369 mJ·mol^-1·K^-2. The Kondo temperature T_K is estimated to be 1.1 K, being low and comparable to T_N. While Fermi liquid behavior is observed deep into the magnetically ordered phase, the Kadowaki-Woods ratio is much reduced relative to the expected value for Ce compounds with Kramers doublet ground state. Markedly, this feature shares striking similarities to that of the prototypical quasi-one-dimensional compounds YbNi₄P₂ and CeRh₆Ge₄ with tunable ferromagnetic quantum critical point. Given the shortest Ce-Ce distance along the needle direction, CeAu₂In₄ appears to be an interesting model system for exploring antiferromagnetic quantum critical behaviors in a quasi-one-dimensional Kondo lattice with enhanced quantum fluctuations.     

Some comments on Kondo lattices and the Mott transition

The so called exhaustion problem occurs when few electrons have to screen many spins in a metal with magnetic impurities. A singlet Fermi liquid ground state is possible only if all impurities are “isotropized” in such a way as to suppress their entropy. That takes a time and the corresponding energy limits the Fermi liquid range. The present note explores that issue of time and energy scales, and it concludes that is much smaller than the single impurity Kondo temperature. Similarly the relevant energy scale is proportional to the number of electrons. Recent results on the Mott metal insulator transition in infinite dimension are reconsidered in the light of these results: controversies in that respect are shown to reduce to a simple physical question, with no firm answer as to now. Received: 5 May 1998 / Received in final form and Accepted: 29 July 1998     

On the Optical Conductivity of Kondo Insulators

The temperature-dependent optical conductivity of a Kondo insulator for different temperatures with impurity doping on the conduction band is studied within the U = ∞ Anderson lattice model in the framework of a slave-boson mean-field theory under the coherent potential approximation. The results show that the depletion of the optical conductivity in low-frequency region decreases with increasing of the temperature, and the system exhibits a transition between a low-temperature insulating phase and a high-temperature metallic phase. With increasing of the impurity concentration, the effect of the impurity scattering could result in a nearly temperature-independent gap behavior. The numerical results on the localization of the electron and its variation with temperature are also discussed with the help of optical sum rule.     

The coupling effect between ferroelectric and frustrated antiferromagnetic ordering in hexagonal ferroelectromagnet

Effects of the magnetoelectric coupling between the frustrated antiferromagnetic and ferroelectric ordering in hexagonal ferroelectromagnet are investigated by the soft-mode theory and molecular-field approximation. Applying the Heisenberg model for frustrated triangular antiferromagnets with exchange anisotropy and Diffour model for ferroelectric interaction, we discuss thermodynamic properties of the hexagonal ferroelectromagnetic system, including mean magnetization 〈s_i〉, polarization p, magnetization susceptibility χ_m, and polarization susceptibility χ_p, in a possible coupling form related to a combination of electric polarization and spin correlation. It is found that polarization induced by magnetic coupling leads to an anomaly in χ_p and a cusp in χ_m at low-temperature, which is consistent qualitatively with experimental results in hexagonal ferroelectromagnet YMnO₃.     

Geometry effect on the magnetic ordering of geometrically frustrated rectangular and triangular magnets

We investigate the effect of geometry on the ground-state ordering of artificially frustrated magnetic rectangular and triangular lattices by Monte Carlo method. By varying the vertical lattice spacing while keeping the horizontal one fixed, we show that when the ratio of vertical to horizontal lattice spacing, labeled by η, is less than 1.82, the ground state of the rectangular lattice presents long-range antiferromagnetic order and for η?1.82 the ground state changes to long-range mixed ferromagnetic and antiferromagnetic order. For the frustrated triangular lattice, the short-range ordered state as well as two long-range ordered ground states occurs transiently at η=0.87 and 2, where the energies of the two ground states with long-range order are approximately equal. In addition, the level of frustration of both frustrated lattices is found to be largely relevant to the ratio η.     

Comparison of diluted antiferromagnetic Ising models on frustrated lattices in a magnetic field

We study diluted antiferromagnetic Ising models on triangular and kagome lattices in a magnetic field, using the replica-exchange Monte Carlo method. We observe seven and five plateaus in the magnetization curve of the diluted antiferromagnetic Ising model on the triangular and kagome lattices, respectively, when a magnetic field is applied. These observations contrast with the two plateaus observed in the pure model. The origin of multiple plateaus is investigated by considering the spin configuration of triangles in the diluted models. We compare these results with those of a diluted antiferromagnetic Ising model on the three-dimensional pyrochlore lattice in a magnetic field pointing in the [111] direction, sometimes referred to as the “kagome-ice” problem. We discuss the similarity and dissimilarity of the magnetization curves of the “kagome-ice” state and the two-dimensional kagome lattice.     

Thermodynamic and magnetic properties in two artificial frustrated lattices

With the Monte Carlo simulation, we investigate the thermodynamics and magnetic properties of the artificial frustrated square and honeycomb lattices. The results from the Ising-like dipolar model show that there occurs one magnetic order transition for the square lattice while the honeycomb lattice exhibits two magnetic order phase transitions. When the magnetic field is applied perpendicular to one of sublattices, a sharp field-independent peak in the specific heat curves appears at a very low temperature for both frustrated lattices due to the occurrence of a long-range ordered state induced by the magnetic field. For the square lattice, the coercive field slightly increases with the angle of field relative to the vertical axis. For both frustrated lattices, the magnetic reversal is achieved mostly via flipping a chain of the nearest neighbor spins.     

Exact solution of the multichannel Kondo problem,scaling, and integrability

In this work we give the exact solution of the model describing the scattering of conduction electrons by an impurity in the orbital singlet state (so-calledn- channel Kondo problem). Depending on the relation between the impurity spinS and the number of electron scattering channelsn, the model behaves differently at low energies. At 2S the effective charge increases to infinity at low energies, whereas atn > 2S it tends to a finite fixed point. The model under study is the first example of the one-dimensional quantum field theory exhibiting scaling behavior.     

近藤共振现象及其在低维电子系统中的实现

文章全面、系统地介绍了近藤效应、近藤问题、近藤共振现象的起源和研究历史的发展过程,提供了一个清晰而准确的近藤物理问题的图像.同时,文章还讨论了近年来近藤共振现象在各种低维电子关联系统中的实现.     

近藤共振现象及其在低维电子系统中的实现

文章全面、系统地介绍了近藤效应、近藤问题、近藤共振现象的起源和研究历史的发展过程,提供了一个清晰而准确的近藤物理问题的图像.同时,文章还讨论了近年来近藤共振现象在各种低维电子关联系统中的实现.     

On the chiral hubbard model and the chiral Kondo lattice model

The integrability of the one-dimensional chiral Hubbard model is discussed in the limit of strong interaction,U=. The system is shown to be integrable in the sense of the existence of an infinite number of constants of motion. The system is related to a chiral Kondo lattice model at strong interactionJ=+.     

Magnetocaloric effect in the ferromagnetic Kondo lattice model

The Kondo lattice model describes a lattice of localized spins S_i interacting with the conduction electrons via a local exchange coupling J. Assuming a ferromagnetic Hund's rule coupling J>0, the model can be used to describe some itinerant magnetocaloric materials such as Gd(Si_xGe_1-x)₄, La(Fe_1-xSi_x)₁₃, and LaCa_1-xMn_xO₃, which are important for magnetic refrigeration near room temperature. The localized magnetic moments are described in the model Hamiltonian by spin operators, and the conduction electrons by fermionic operators. To study the magnetocaloric effect, a uniform external magnetic field is added through a Zeeman term. By averaging the fermionic degrees of freedom, one obtains an indirect exchange coupling between spins at sites i and j, which corresponds to the RKKY interaction. The self-consistent mean value is evaluated in the effective Heisenberg Hamiltonian within the random phase approximation (RPA). The conduction electron magnetization for a given value of is obtained from the corresponding Green's functions through the equation of motion method. The pressure and doping dependence of the Curie temperature are taken into account in the evaluation of . The magnetocaloric effect is characterized by the isothermal entropy change ΔS and the adiabatic temperature change ΔT_ad upon magnetic field variations in the neighborhood of the ferromagnetic phase transition. The results are obtained for and compared to measurements with Gd compounds.     

The Two Dimensional Kondo Model with Rashba Spin-Orbit Coupling

We investigate the effect that Rashba spin-orbit coupling has on the low energy behaviour of a two dimensional magnetic impurity system. It is shown that the Kondo effect, the screening of the magnetic impurity at temperatures T<T _K , is robust against such spin-orbit coupling, despite the fact that the spin of the conduction electrons is no longer a conserved quantity. A proposal is made for how the spin-orbit coupling may change the value of the Kondo temperature T _K in such systems and the prospects of measuring this change are discussed. We conclude that many of the assumptions made in our analysis invalidate our results as applied to recent experiments in semi-conductor quantum dots but may apply to measurements made with magnetic atoms placed on metallic surfaces.     

On the theory of liquid-crystalline ordering of polymer chains with limited flexibility

We propose a universal approximate method which enables us to study the statistical mechanics of the liquid-crystalline ordering in the solutions of stiffchain macromolecules with limited flexibility of arbitrary nature at arbitrary solution concentrations and arbitrary temperatures (solvent quality). In the variables temperature and concentration we construct the full phase diagrams for the nematic transition for the cases of long rigid rods and semiflexible macromolecules with freely jointed, persistent, and rotational-isomeric flexibility mechanisms. The method proposed can be easily generalized for the study of thermotropic polymer liquid crystals, as well as more complex polymer chain models (e.g., copolymers containing stiff and flexible fragments in the chain).     

Modified spin-wave theory for the S = 1 frustrated antiferromagnetic Heisenberg chain

We study the one-dimensional isotropic spin-1 Heisenberg magnet with antiferromagnetic nearest-neighbor (nn) and next-nearest-neighbor (nnn) interactions by using the modified spin wave theory (MSWT). The ground state energy and the singlet-triplet energy gap are obtained for several values of j, defined as the ratio of the nnn interaction constant to the nn one. We also compare two different ways of implementing the MSWT currently found in the literature, and show that, despite the remarkable differences between the equations to be solved in each procedure, the results given by both are equivalent, except for the predicted value of the j_max, the maximum value of j accessible in each treatment. Here, we suggest that j_max is related to the disorder point of the first kind. Our results show that the ground state and the gap energies increase with j, for j ≤j_max, in accordance to previous numerical results.     

Instability of Non—Fermi Liquid Behavior in the Two—Channel Kondo Model

The effects of interchannel scattering of conduction electrons by the impurity and repulsion of conduction electrons at the impurity site on the two-channel Kondo model are simultaneously considered in this paper,It is shown that these two perturbations will substantially modify the usual local non-Fermi liquid behavior of the two-channel Kondo model.With bosonization and unitary transformations we find that the system can be transformed into a single channel Kondo model with anisotropy between longitudinal and transverse exchange couplings,Whatever for originally antiferromagnetic or ferromagnetic isotropic coupling,the system always flows to strong-coupling limit,which exhibits local Fermi liquid behavior at low temperatures.     

嵌入单量子点Aharonov-Bohm环中的近藤效应

使用单杂质的Ansderson模型, 从理论上研究了一个嵌入单量子点Aharonov-Bohm环系统处在 近藤区时的基态性质, 并用slave-boson平均场方法求解了该模型.结果表明：在零温, 当介 观环内电子平均能级间隔大于近藤关联能时, 系统内仍然存在一个被减弱了的近藤效应；系 统的基态性质依赖于系统的宇称和环的大小；而尺寸效应和近藤屏蔽效应的共存导致了系统 丰富的物理性质.同时, 可以通过测量介观环中的持续电流和杂质磁化率, 达到探测近藤屏 蔽云的目的. 关键词： 持续电流 杂质磁化率 宇称效应 近藤效应 近藤屏蔽云