Metallic magnetism is both ancient and modern, occurring in such familiar settings as the lodestone in compass needles and the hard drive in computers. Surprisingly, a rigorous theoretical basis for metallic ferromagnetism is still largely missing. The Stoner approach perturbatively treates Coulomb interactions when the latter need to be large, while the Nagaoka approach incorporates thermodynamically negligible electrons into a half-filled band. Here, we show that the ferromagnetic order of the Kondo lattice is amenable to an asymptotically exact analysis over a range of interaction parameters. In this ferromagnetic phase, the conduction electrons and local moments are strongly coupled but the Fermi surface does not enclose the latter (i.e. it is small). Moreover, non-Fermi liquid behavior appears over a range of frequencies and temperatures. Our results provide the basis to understand some long-standing puzzles in the ferromagnetic heavy fermion metals, and raises the prospect for a new class of ferromagnetic quantum phase transitions.
We carefully consider the interplay between ferromagnetism and the Kondo screening effect in the conventional Kondo lattice systems at finite temperatures. Within an effective mean-field theory for small conduction electron densities, a complete phase diagram has been determined. In the ferromagnetic ordered phase, there is a characteristic temperature scale to indicate the presence of the Kondo screening effect. We further find two distinct ferromagnetic long-range ordered phases coexisting with the Kondo screening effect: spin fully polarized and partially polarized states. A continuous phas…
We propose the notion of spin-selective Kondo insulator, which provides a fundamental mechanism to describe the ferromagnetic phase of the Kondo lattice model with antiferromagnetic coupling. This unveils a remarkable feature of the ferromagnetic metallic phase: the majority-spin conduction electrons show metallic- while the minority-spin electrons show insulating-behavior. The resulting Kondo gap in the minority spin sector, which is due to the cooperation of ferromagnetism and partial Kondo screening, evidences a dynamically-induced commensurability for a combination of minority-spin electro…
Ground state ferromagnetism of the Kondo lattices is investigated within slave fermion approach by Coleman and Andrei within a mean-field approximation in the effective hybridization model. Conditions for formation of both saturated (half-metallic) and non-saturated magnetic state are obtained for various lattices. A description in terms of universal functions which depend only on bare electron density of states (DOS) is presented. A crucial role of the energy dependence of the bare DOS (especially, of DOS peaks) for the small-moment ferromagnetism formation is demonstrated.
We report on a comprehensive optical, transport and thermodynamic study of the Zintl compound Yb$_{14}$MnSb$_{11}$, demonstrating that it is the first ferromagnetic Kondo lattice compound in the underscreened limit. We propose a scenerio whereby the combination of Kondo and Jahn-Teller effects provides a consistent explanation of both transport and optical data.
It is becoming widely recognized that, contrary to earlier expectations, the usual one-band Hubbard model does not give an explanation for itinerant ferromagnetism. After reviewing the status of magnetic ordering in the one-band model, we discuss the possibility of ferromagnetism in some recently introduced two-band Hubbard models, and in generalized Anderson lattices. It is argued that these two classes of models are closely related and that it is their common feature that the ferromagnetic phase has to compete with a collective Kondo state.
The effect of binary alloy disorder on the ferromagnetic phases of f-electron materials is studied within the periodic Anderson model. We find that disorder in the conduction band can drastically enhance the Curie temperature due to an increase of the local f-moment. The effect may be explained qualitatively and even quantitatively by a simple theoretical ansatz. The emergence of an alloy Kondo insulator at non-integer filling is also pointed out.
Coexistence between ferromagnetic order and Kondo behavior has been observed in some uranium compounds. The underscreened Kondo lattice model can provide a possible description of this coexistence. Here we present a model of a lattice of S=1 spins coupled to the conduction electrons through an intra-site exchange interaction $J_{K}$ and an inter-site ferromagnetic exchange $f-f$ interaction $J_{H}$. Finite temperature results show that the Kondo temperature is larger than the Curie ordering temperature, $T_{C}$, providing a possible scenario for the coexistence of Kondo effect and magnetic ord…
We propose a modified RKKY-technique to evaluate the magnetic properties of the ferromagnetic Kondo-lattice model. Together with a previously developed selfenergy approach to the conduction electron part of the model we get a closed system of equations which can be solved self-consistently. Ferromagnetism appears for relatively low electron (hole) densities, while it is excluded around half-filling ($n=1$). For small $J$ conventional RKKY ($\sim J^2$) is reproduced; however, with strong deviations already for very moderate exchange couplings. For not too small $n$ a critical $J_c$ is needed to…
Non-collinear ferromagnetism in the Kondo lattice Ce$_5$CoGe$_2$
Zhejiang University（浙江大学）；ETH Zürich（苏黎世联邦理工学院）；PSI Center for Neutron and Muon Sciences CNM（保罗谢尔研究所中子与缪子科学中心）；Hubei Normal University（湖北师范大学）；STFC, Rutherford Appleton Laboratory（英国科学与技术设施委员会卢瑟福阿普尔顿实验室）；University of Johannesburg（约翰内斯堡大学）；Institute for Advanced Study in Physics, Zhejiang University（浙江大学物理高等研究院）；Institute of Fundamental and Transdisciplinary Research, Zhejiang University（浙江大学基础与交叉研究学院）；State Key Laboratory of Silicon and Advanced Semiconductor Materials, Zhejiang University（浙江大学硅及先进半导体材料国家重点实验室）
The structural, electronic and magnetic properties of the Kondo-lattice system CeRuSi2 are experimentally investigated and analyzed in the series of other ternary cerium compounds. This system is shown to be an excellent model system demonstrating coexistence of the Kondo effect and anomalous ferromagnetism with a small magnetic moment which is confirmed by magnetic and μSR measurements. Data on specific heat, resistivity and Seebeck coefficient are presented. Being deduced from the resistivity and specific heat data, the non-Fermi-liquid behavior is observed at low temperatures, which is unus…
Recently, nonlinear responses have been actively studied in both experiments and theory. Particularly interesting are inversion-symmetry broken systems, where an even-order nonlinear electrical conductivity can be nonzero, resulting in nonreciprocity. Second-order nonlinear conductivities attract much attention because of their sensitivity to detect inversion-symmetry breaking in materials and their functionalities. However, while the nonlinear response has been actively studied in noninteracting systems for a long time, the nonlinear response in strongly correlated materials is still poorly u…
The ferromagnetic spin exchange interaction between the itinerant electrons and localized moments on a periodic lattice, studied within the so-called Kondo lattice model (KLM), is considered for multiband situation where the hopping integral is a matrix, in general. The modified RKKY theory, wherein one can map such a model onto an effective Heisenberg-like system, is extended to a multi-band case with finite bandwidth and hybridization on a simple cubic lattice. As an input for the evaluation of the effective exchange integrals, one requires the multi-band electronic self energy which is take…
Using a canonical transformation it is possible to faithfully represent the Kondo lattice model in terms of Majorana fermions. Studying this representation we discovered an exact mapping between the Kondo lattice Hamiltonian and a Hamiltonian describing three spinless fermions interacting on a lattice. We investigate the effectiveness of this three fermion representation by performing a zero temperature mean-field study of the phase diagram at different couplings and fillings for the one-dimensional case, focusing on the appearance of ferromagnetism. The solutions agree in many respects with t…
The Kondo effect in quantum dots (QDs) - artificial magnetic impurities - attached to ferromagnetic leads is studied with the numerical renormalization group (NRG) method. It is shown that the QD level is spin-split due to presence of ferromagnetic electrodes, leading to a suppression of the Kondo effect. We find that the Kondo effect can be restored by compensating this splitting with a magnetic field. Although the resulting Kondo resonance then has an unusual spin asymmetry with a reduced Kondo temperature, the ground state is still a locally-screened state, describable by Fermi liquid theor…
Kondo hybridization in partially filled f-electron systems conveys significant amount of electronic states sharply near the Fermi energy leading to various instabilities from superconductivity to exotic electronic orders. UGe2 is a 5f heavy fermion system, where the Kondo hybridization is interrupted by the formation of two ferromagnetic phases below a 2nd order transition Tc ~ 52 K and a crossover transition Tx ~ 32 K. These two ferromagnetic phases are concomitantly related to a spin-triplet superconductivity that only emerges and persists inside the magnetically ordered phase at high pressu…
We study the effect of crystal field anisotropy in the underscreened $S=1$ Kondo lattice model. Starting from the two orbital Anderson lattice model and including a local anisotropy term, we show, through Schrieffer-Wolff transformation, that local anisotropy is equivalent to an anisotropic Kondo interaction ($J_{\parallel} \neq{J_{\perp}}$). The competition and coexistence between ferromagnetism and Kondo effect in this effective model is studied within a generalized mean-field approximation. Several regimes are obtained, depending on the parameters, exhibiting or not coexistence of magnetic …
Ferromagnetic ordering with a small magnetic moment is found below 14 K from SQUID measurements for the compound Ce4Sb1.5Ge1.5. The transport characteristics of a number of Ce4Sb3-xTx (T = Ge, Si, Sn, Pb, Al) systems were measured at room temperature, Ce4Sb1.5Ge1.5 demonstrating the highest Seebeck coefficient. Its transport properties (resistivity, thermal conductivity, thermoelectric power) are experimentally investigated in detail up to low temperatures. The Kondo-lattice behavior of resistivity and its anomaly at the ordering point are found. A comparison with the Wiedemann-Franz law is pe…
We study the ground-state properties of the SU(N)-generalization of the Kondo-lattice model in one dimension when the Kondo coupling J_K (both ferromagnetic and antiferromagnetic) is sufficiently strong. Both cases can be realized using alkaline-earth-like cold gases in optical lattices. Specifically, we first carry out the strong-coupling expansion and identify two insulating phases (one of which is the SU(N)-analogue of the well-known gapped Kondo singlet phase). We then rigorously establish that the ground state in the low-density (for J_K<0) or the high-density (for J_K>0) region is ferrom…
The Kondo lattice model has been analyzed in the presence of a random inter-site interaction among localized spins with non zero mean Jo and standard deviation J. Following the same framework previously introduced by us, the problem is formulated in the path integral formalism where the spin operators are expressed as bilinear combinations of Grassmann fields. The static approximation and the replica symmetry ansatz have allowed us to solve the problem at a mean field level. The resulting phase diagram displays several phase transitions among a ferromagnetically ordered region,a spin glass one…
We compare different approximation schemes for investigating ferromagnetism in the periodic Anderson model. The use of several approximations allows for a detailed analysis of the implications of the respective methods, and also of the mechanisms driving the ferromagnetic transition. For the Kondo limit, our results confirm a previously proposed mechanism leading to ferromagnetic order, namely an RKKY exchange mediated via the formation of Kondo screening clouds in the conduction band. The contrary case is found in the intermediate-valence regime. Here, the bandshift correction ensuring a corr…
We present a general model which includes the ferromagnetic Kondo lattice and the Hubbard model as special cases. The stability of the ferromagnetic state is investigated variationally. We discuss the mechanism of ferromagnetism in metallic nickel, emphasizing the importance of orbital degeneracy and the effect of the Hund's-rule coupling.
We report precise measurements and quantitative data analysis on the low-temperature resistivity of several ferromagnetic manganite films. We clearly show that there exists a T^{4.5} term in low-temperature resistivity, and that this term is in quantitative agreement with the quantum theory of two-magnon scattering for half metallic ferromagnets. Our present results provide the first bulk experimental evidence of half-metallic ferromagnetism in doped manganites.
Metallic ferromagnetism is in general an intermediate to strong coupling phenomenon. Since there do not exist systematic analytic methods to investigate such types of problems, the microscopic origin of metallic ferromagnetism is still not sufficiently understood. However, during the last two or three years remarkable progress was made in this field: It is now certain that even in the one-band Hubbard model metallic ferromagnetism is stable in dimensions $d=1,$ 2, and $\infty$ on regular lattices and at intermediate values of the interaction $U$ and density $n$. In this paper the basic questio…
New insights into the microscopic origin of itinerant ferromagnetism were recently gained from investigations of electronic lattice models within dynamical mean-field theory (DMFT). In particular, it is now established that even in the one-band Hubbard model metallic ferromagnetism is stable at intermediate values of the interaction U and density n on regular, frustrated lattices. Furthermore, band degeneracy along with Hund's rule couplings is very effective in stabilizing metallic ferromagnetism in a broad range of electron fillings. DMFT also permits one to investigate more complicated corr…
The competition between spin glass, ferromagnetism and Kondo effect is analysed here in a Kondo lattice model with an inter-site random coupling $J_{ij}$ between the localized magnetic moments given by a generalization of the Mattis model which represents an interpolation between ferromagnetism and a highly disordered spin glass. Functional integral techniques with Grassmann fields have been used to obtain the partition function. The static approximation and the replica symmetric ansatz have also been used. The solution of the problem is presented as a phase diagram giving $T/{J}$ {\it versus}…
We study the Kondo screening effect generated by a single-electron transistor or quantum dot embedded in a small metallic ring. When the ring circumference $L$ becomes comparable to the fundamental length scale $ξ_K^0=\hbar \upsilon _F/T_K^0$ associated with the {\it bulk} Kondo tempe the Kondo resonance is strongly affected, depending on the total number of electrons ({\it modulo} 4) and magnetic flux threading the ring. The resulting Kondo-assisted persistent currents are also calculated in both Kondo and mixed valence regimes, and the maximum values are found in the crossover region.
We construct a multi-band Hubbard model on the lattice obtained by "decorating" a closely packed $d$-dimensional lattice $\mathcal{M}$ (such as the triangular lattice) where $d\ge2$. We take the limits in which the Coulomb interaction and the band gap become infinitely large. Then there remains only a single band with finite energy, on which electrons are supported. Let the electron number be $N_\mathrm{e}=|\mathcal{M}|-N_\mathrm{h}$, where $|\mathcal{M}|$ corresponds to the electron number which makes the lowest (finite energy) band half-filled, and $N_\mathrm{h}$ is the number of "holes". It…
A Kondo lattice is often electrically insulating at low temperatures. However, several recent experiments have detected signatures of bulk metallicity within this Kondo insulating phase. Here we visualize the real-space charge landscape within a Kondo lattice with atomic resolution using a scanning tunneling microscope. We discover nanometer-scale puddles of metallic conduction electrons centered around uranium-site substitutions in the heavy-fermion compound URu$_2$Si$_2$, and around samarium-site defects in the topological Kondo insulator SmB$_6$. These defects disturb the Kondo screening cl…
We introduce a new approach to the periodic Anderson model (PAM) that allows a detailed investigation of the magnetic properties in the Kondo as well as the intermediate valence regime. Our method is based on an exact mapping of the PAM onto an effective medium strong-coupling Hubbard model. For the latter, the so-called spectral density approach (SDA) is rather well motivated since it is based on exact results in the strong coupling limit. Besides the T=0 phase diagram, magnetization curves and Curie temperatures are presented and discussed with help of temperature-dependent quasiparticle den…
Metallic spin liquid has been reported in several correlated metals, but a satisfactory theoretical description is not yet available. Here we propose a potential route to realize the metallic spin liquid and construct an effective $\mathbb{Z}_2$ gauge theory with charged fractionalized excitations on the triangular Kondo lattice. This leads to a $\mathbb{Z}_2$ metallic spin liquid featured with long-lived, heavy holon excitations of spin 0 and charge $+e$ and a partially enlarged electron Fermi surface. It differs from the weak-coupling FL$^*$ state proposed earlier and may be viewed as a frac…
Heavy fermion (HF) states emerge in correlated quantum materials due to the interplay between localized magnetic moments and itinerant electrons, but rarely appear in 3d-electron systems due to high itinerancy of d-electrons. Here, an anomalous enhancement of Kondo screening is observed at the Kondo hole of local Fe vacancies in Fe3GeTe2 which is a recently discovered 3d-HF system featuring of Kondo lattice and two-dimensional itinerant ferromagnetism. An itinerant Kondo-Ising model is established to reproduce the experimental results which provides insight of the competition between Ising fer…
A synergistic effect between strong electron correlation and spin-orbit interaction (SOI) has been theoretically predicted to result in a new topological state of quantum matter on Kondo insulators (KIs), so-called topological Kondo insulators (TKIs). One TKI candidate has been experimentally observed on the KI SmB6(001), and the origin of the surface states (SS) and the topological order of SmB6 has been actively discussed. Here, we show a metallic SS on the clean surface of another TKI candidate YbB12(001), using angle-resolved photoelectron spectroscopy. The SS showed temperature-dependent …
Starting from a spin-orbital model for doped manganites, we investigate a competition between ferromagnetic and antiferromagnetic order in a one-dimensional model at finite temperature. The magnetic and orbital order at half filling support each other and depend on a small antiferromagnetic superexchange between t_{2g} spins and on an alternating Jahn-Teller potential. The crossover to a metallic ferromagnetic phase found at finite doping is partly suppressed by the Jahn-Teller potential which may localize e_g electrons.
In the paper a possibility of metallic ferromagnetic state realization in a generalized Hubbard model with more complete accounting of electron-electron interactions, in particular, the correlated hopping and exchange interaction integrals is investigated. Recently obtained by means of mean-field approximation single electron energy spectrum is used for the description of finite temperature properties of the system. In the paper the expression for the critical temperature of ferromagnet-paramagnet transition is found, the behaviour of temperature dependencies of magnetization and paramagnetic …
Carrier-induced ferromagnetism is investigated in a diluted ferromagnetic Kondo lattice model for several ordered impurity arrangements on square and cubic lattices, allowing for quantitative comparison with different theoretical pictures. The spin stiffness is found to be optimized with respect to hole doping concentration, exchange interaction strength, as well as spin dilution due to a competition involving magnitude of carrier spin polarization and its oscillation length scale. The ferromagnetic transition temperature determined within the spin-fluctuation theory is in good agreement with …
In the band theory of ferromagnetism there is a relative shift in the position of majority and minority spin bands due to the self-consistent field due to opposite spin electrons. In the simplest realization, the Stoner model, the majority and minority spin bands are rigidly shifted with respect to each other. Here we consider models at the opposite extreme, where there is no overall shift of the energy bands. Instead, upon spin polarization one of the bands broadens relative to the other. Ferromagnetism is driven by the resulting gain in kinetic energy. A signature of this class of mechanisms…
For nearly a century, the Stoner model has dominated research in itinerant ferromagnetism, yet recent work on ultracold, optically trapped Fermi gases suggests that phase separation of spins occurs independent of long-range magnetic order. In this paper, we consider the breakdown of the Stoner criterion in a Landau-Fermi liquid with weak non-zero gauge field. Due to a process analogous to Kohn's theory of a metal-insulator transition, we find the stability of a Fermi liquid phase in the absence of non-quasiparticle contributions to the response is strongly dependent on Landau parameters of mix…
The phase diagram of the two-leg Kondo ladder is investigated using computational techniques. Ferromagnetism is present, but only at small conduction electron densities and robust Kondo coupling $J$. For densities $n\gtrsim0.4$ and any Kondo coupling, a paramagnetic phase is found. We also observed spin dimerization at densities $n$=1/4 and $n$=1/2. The spin structure factor at small $J$ peaks at $\vec{q}$=$(2n,0)π$ for $n\lesssim0.5$, and at $\vec{q}$=$(n,1)π$ for $n\gtrsim0.5$. The charge structure factor suggests that electrons behave as free particles with spin-1/2 (spin-0) for small (larg…
A theory of Kondo lattices or a $1/d$ expansion theory, with $d$ spatial dimensionality, is applied to studying itinerant-electron ferromagnetism. Two relevant multi-band models are examined: a band-edge model where the chemical potential is at one of band-edges, the top or bottom of bands, and a flat-band model where one of bands is almost flat or dispersionless and the chemical potential is at the flat band. In both the models, a novel ferromagnetic exchange interaction arises from the virtual exchange of pair excitations of quasiparticles; it has two novel properties such as its strength is…
We investigate the 2D ferromagnetic Kondo lattice model for manganites with classical corespins at Hund's rule coupling J_H=6, with antiferromagnetic superexchange 0.03 < J' < 0.05. We employ canonical and grand canonical unbiased Monte Carlo simulations and find paramagnetism, weak ferromagnetism and the Flux phase, depending on doping and on J'. The observed single particle spectrum in the flux phase differs from the idealized infinite lattice case, but agrees well with an idealized finite lattice case with thermal fluctuations.
The bilayered member of the Ruddesden-Popper family of ruthenates, Sr$_3$Ru$_2$O$_7$, has received an increasing attention due to its interesting properties and phases. By using first principle calculations we find that the ground-state is characterized by a ferromagnetic (FM) half-metallic state. This state strongly competes with an antiferromagnetic metallic phase, which indicates the possible presence of a particular state characterized by the existence of different magnetic domains. To drive the system towards a phase transition we studied the electronic and magnetic properties as a functi…
The low energy spectrum of the ferromagnetic Kondo lattice model on a N-site complete graph extended with on-site repulsion is obtained from the underlying spl(2,1) algebra properties in the strong coupling limit. The ferromagnetic ground state is realized for 1 and N+1 electrons only. We identify the large density of states to be responsible for the suppression of the ferromagnetic state and argue that a similar situation is encountered in the Kagome, pyrochlore, and other lattices with flat bands in their one-particle density of states.
We study the Hubbard model with bond-charge interaction (`correlated hopping') in terms of the Gutzwiller wave function. We show how to express the Gutzwiller expectation value of the bond-charge interaction in terms of the correlated momentum-space occupation. This relation is valid in all spatial dimensions. We find that in infinite dimensions, where the Gutzwiller approximation becomes exact, the bond-charge interaction lowers the critical Hubbard interaction for the Brinkman-Rice metal-insulator transition. The bond-charge interaction also favors ferromagnetic transitions, especially if th…
The phase diagram of the ferromagnetic Kondo model for manganites is investigated using computational techniques. In clusters of dimensions 1 and 2, Monte Carlo simulations in the limit where the localized spins are classical show a rich low temperature phase diagram with three dominant regions: (i) a ferromagnetic phase, (ii) phase separation between hole-poor antiferromagnetic and hole-rich ferromagnetic domains, and (iii) a phase with incommensurate spin correlations. Possible experimental consequences of the regime of phase separation are discussed. Studies using the Lanczos algorithm and …
We study magnetic ordering of an extended Kondo-lattice model including an additional on-site Coulomb interaction between the itinerant states. The model is solved in the dynamical mean-field theory using Wilson's numerical renormalization group approach as impurity solver. For a bipartite lattice we find at half filling the expected antiferromagnetic phase. Upon doping this phase is gradually suppressed and hints towards phase separation are observed. For large doping the model exhibits ferromagnetism, the appearance of which can at first sight be explained by Rudermann-Kittel-Kasuya-Yosida i…
Cold neutron spectroscopy was performed on CeNi_{1-x}Cu_xSn to investigate how the magnetic gap in the 'Kondo insulator' CeNiSn is destroyed by temperature (T), chemical substitution (x), and external magnetic field. Upon doping, the spin gap collapses and magnetic Bragg peaks occur initially (x = 0.13) at the commensurate wave vectors Q = (0,m/2,l) (m,l integer) where the higher-energy (4.1 meV) gap is located for x = 0. A magnetic field smooths the sharp spin gap structure in both momentum and energy, while leaving the associated static susceptibility unchanged, as does raising T.
We present a theory to model carrier mediated ferromagnetism in concentrated or diluted local moment systems. The electronic subsystem of the Kondo lattice model is described by a combined equation of motion / coherent potential approximation method. Doing this we can calculate the free energy of the system and its minimum according to the magnetization of the local moments. Thus also the Curie temperature can be determined and its dependence on important model parameters. We get qualitative agreement with the Curie temperatures' experimental values of Ga$_{1-x}$Mn$_x$As for a proper hole dens…
The 5d-electron based BaIrO3 is a nonmetallic weak ferromagnet with a Curie temperature at Tc=175 K. Its largely extended orbitals generate strong electron-lattice coupling, and magnetism and electronic structure are thus critically linked to the lattice degree of freedom. Here we report results of our transport and magnetic study on slightly Sr doped BaIrO3. It is found that dilute Sr-doping drastically suppresses Tc, and instantaneously leads to a nonmetal-metal transition at high temperatures. All results highlight the instability of the ground state and the subtle relation between magnetic…
The electrical resistivity including the Kondo resistivity increase at low temperature is calculated for thin films of dilute magnetic alloys. Assuming that in the non-magnetic host the spin-orbit interaction is strong like in Au and Cu, the magnetic impurities have a surface anisotropy calculated in part I. That anisotropy hinders the motion of the spin. Including that anisotropy the effective electron-impurity coupling is calculated by using the second order renormalization group equations. The amplitude of the Kondo resistivity contribution is reduced as the position of the impurity approac…
The first principles momentum dependent local ansatz wavefunction method (MLA) has been extended to the ferromagnetic state by introducing spin-dependent variational parameters. The theory is applied to the ferromagnetic Fe, Co, and Ni. It is shown that the MLA yields the magnetizations being comparable to the results obtained by the GGA (generalized gradient approximation) in the density functional theory. The projected momentum distribution functions as well as the mass enhancement factors are also calculated on the same footing, and are compared with those in the paramagnetic state. It is s…
We show that an interplay of double exchange and impurity randomness can explain the competition between metal-ferromagnetic and insulating charge ordered states in doped manganites. The double exchange is simplified in the Ising type, whereas the randomness is modeled by the Falicov-Kimball binary distribution. The combined model is considered in a framework of dynamical mean-field theory. Using the Kubo-Greenwood formalism, the transport coefficients are explicitly expressed in terms of single particle spectral functions. Dividing the system into two sublattices we have pointed out a direct …
We present the first rigorous example of the Hubbard model in any dimensions which exhibits metallic ferromagnetism. The model is a genuine Hubbard model with short-range hopping and on-site Coulomb repulsion, and has multi single-electron bands. In the limit where the band gap and the Coulomb repulsion become infinite, we prove that the ground states are completely ferromagnetic and at the same time conducting.
The resistance of a conventional insulator diverges as temperature approaches zero. The peculiar low temperature resistivity saturation in the 4f Kondo insulator (KI) SmB6 has spurred proposals of a correlation-driven topological Kondo insulator (TKI) with exotic ground states. However, the scarcity of model TKI material families leaves difficulties in disentangling key ingredients from irrelevant details. Here we use angle-resolved photoemission spectroscopy (ARPES) to study FeSb2, a correlated d-electron KI candidate that also exhibits a low temperature resistivity saturation. On the (010) s…
The interface between a heavy electron coherent metallic phase and an incoherent normal metal phase is discussed. The Fermi momentum mismatch between the two phases causes the carriers to scatter at the interface. The interface conductance is a monotonous increasing function of conduction electron density, n_c, and is less than 60% of that of a clean heavy fermion metal at half-filling ($n_c=1$) and can be measured experimentally. Interface experiments can be used as probe of the nature of the homogenous heavy-fermion state and provide important information on the effects of inhomogeneities in…
In this paper, we study the influence of spatial fluctuations in a two-dimentional Kondo-Lattice model (KLM) with anti-ferromagnetic couplings. To accomplish this, we first present an implementation of the dual-fermion (DF) approach based on the hybridization expansion continuous-time quantum Monte Carlo impurity solver (CT-HYB), which allows us to consistently compare the local and non-local descriptions of this model. We find that, the inclusion of non-locality restores the self-energy dispersion of the conduction electrons, {\it i.e.} the $\vec{k}$ dependence of $Σ(\vec{k}, iω_{n})$. The an…
