Transport and magnetic properties of Pb_{1-x}Mn_xTe (x<0.18) semiconductor alloys doped with Cr or Mo are investigated in a broad range of temperatures and magnetic fields. In PbMnTe(Cr) alloys the Fermi level may be pinned either in the conduction band or in the energy gap, depending on Mn concentration. In PbMnTe(Mo) alloys the pinning of the Fermi level is observed in the valence band as well as in the energy gap. In the latter case persistent photoconductivity is observed at low temperatures. The analysis of the temperature dependence of magnetic susceptibility shows that PbMnTe alloys doped with Cr or Mo are Curie-Weiss paramagnets revealing weak antiferromagnetic interactions between magnetic ions.
We review the results of bulk and neutron diffraction measurements on the UCu_{5-x}Au_{x} system crystallizing in the cubic AuBe_{5}-type structure. The observed properties may be understood in the light of competition between Kondo effect and RKKY exchange interactions. Effects of 5f-ligand hybridization between the U-atoms occupying the 4a-sites and the Au- or/and Cu-atoms placed at the 4c- and 16e-positions, respectively, are supposed to be different. We show that the prefential occupation of 4c-sites by Au has a marked effect on the magnetic properties of the investigated system.
We present results of transport and magnetic properties of three single-crystalline samples of the intermediate valence small-gap semiconductor SmB_{6} at low temperatures. The received resistivity dependences of the samples below 0.5 K exhibit an activated behavior with an energy gap of a few mK. The temperature dependences of the magnetic susceptibility show an increase below 15 K which can be accounted for by impurities, by bare Sm^{3+} ions or by a small amount of in-gap magnetic 4f^{5}5d^{1} states.
The experimental technique of spin polarised neutron scattering as used in magnetic form factor measurements is presented. An introduction to the interpretation and the calculation of magnetic form factors and magnetization densities is given. The experimental technique of neutron scattering theory as applied to elastic spin polarised scattering experiments is briefly introduced. The calculation of the magnetic form factor and the magnetization densities are considered for simple model systems such as a collection of localised magnetic moments or an itinerant electron system. The discussion is illustrated by an experimental investigation of the magnetic form factor in the heavy fermion superconductors UBe_{13} and UPt_{3}. Magnetization density maps and magnetic form factors are presented, and their implications for other physical quantities are briefly discussed.
A series od UFe_{2+x} materials was prepared using splat cooling. The Laves phase structure can accommodate up to 0.3 Fe excess, while T_C is enhanced from 172 K to approximately 240 K. Higher Fe concentration leads to the segregation of α-Fe. ^{57}Fe Mössbauer spectroscopy indicates higher Fe magnetic hyperfine fields on Fe nuclei occupying the U sublattice than for the regular Fe sites.
Recently new effects that are not characteristic of undoped lead telluride, such as the Fermi level pinning, giant negative magnetoresistance, were observed in Pb_{1-x}Mn_xTe alloys doped with transition and rare earth elements - Cr, Mo, Yb. We have studied transport and magnetic properties of Pb_{1-x}Mn_xTe doped with another transition element - vanadium. A series of Pb_{1-x}Mn_xTe(V) samples of different composition and degree of doping was investigated. It was observed that the resistivity demonstrates activation behavior at low temperatures for the samples with considerable amount of vanadium as well as for the samples without vanadium. The activation energy is proportional to the Mn content. In some of the samples, photoconductivity was observed at low temperatures. The results are discussed in terms of a model assuming formation of the impurity level by the vanadium impurity and the effect of the Fermi level pinning by this level.
In this paper, the electron-type heavy-fermion system and the hole-type heavy-fermion system are classified by means of the cell-volume difference between two typical valence states of f ions. Two kinds of transitions induced by pressure are studied theoretically within a unified picture presented for both two types of heavy-fermion systems. Among these transitions, the incoherence-to-coherence transition in the electron-type heavy-fermion system is confirmed intensively in experiments, while the coherence-to-incoherence transition in the hole-type heavy-fermion system is discussed as a prediction for further observations.
We study two effective models developed for description of superconductors with short-coherence length: (i) the extended Hubbard model with on-site attraction and intersite repulsion, (ii) the model of hard-core charged bosons on a lattice. The analysis is concentrated on the problem of phase separations and competition between superconductivity (SS) and charge-density-wave (CDW) orderings. The phase diagrams of the systems are shown to consist of at least seven different states, including 3 types of phase separated (PS) states: CDW-SS (PS1), CDW-normal (PS2) and the state of electron droplets (PS3). By taking into account the PS states and the effects of longer-range density-density interactions (beyond nearest neighbors) our paper substantially generalizes and modifies the conclusions of previous papers concerning the models considered.
The spin-one-half Falicov-Kimball model with spin-dependent on-site interaction between localized (f) and itinerant (d) electrons is studied by small-cluster exact-diagonalization calculations and a well-controlled approximative method in two dimensions. The results obtained are used to categorize the ground-state configurations according to common features (charge and spin ordering) for all f and d electron concentrations (n_f and n_d) on finite square lattices. It is shown that only a few configuration types form the basic structure of the charge phase diagram in the n_f-n_d plane. In particular, the largest regions of stability correspond to the phase segregated configurations and configurations that can be considered as mixtures of chessboard configurations and the full (empty) lattice. Since the magnetic phase diagram is much richer than the charge phase diagram, the magnetic superstructures are examined only at selected values of f and d electron concentrations.
The extrapolation of small-cluster exact-diagonalization calculations and the Monte Carlo method is used to study the spin-one-half Falicov-Kimball model extended by the spin-dependent Coulomb interaction (J) between the localized f and itinerant d electrons as well as the on-site Coulomb interaction (U_{ff}) between the localized f electrons. It is shown that in the symmetric case the ground-state phase diagram of the model has an extremely simple structure that consists of only two phases, and namely, the charge-density-wave phase and the spin-density-wave phase. The nonzero temperature studies showed that these phases persist also at finite temperatures. The same calculations that we performed for unsymmetric case showed that charge and spin ordering can be destroyed simultaneously or consecutively.
Coexisting low-energy scales are observed in f-electron materials. The information about some of low-energy scales is imprinted in the electron self-energy, which can be measured by angle-resolved photoemission (ARPES). Such measurements in d-electron materials over the last decade were based on high energy- and momentum- resolution ARPES techniques used to extract the self-energy information from measured spectra. Simultaneously, many-body theoretical approaches have been developed to find a link between the self-energy and many-body interactions. Here we show the transcription of such methods from d-electrons to f-electrons by presenting the first example of low energy scales in the f-electron material USb_2, measured with synchrotron-based ARPES. The proposed approach will help in answering the fundamental questions about the complex nature of the heavy fermion state.
We discuss a few models developed for description of superconductivity with very short coherence length and point out their formal equivalence to specific spin models, such as an anisotropic Heisenberg model (s = 1/2) with two- and four-spin exchange coupling and with fixed magnetization in the z-direction, an XY-Z model with single ion anisotropy and a Kondo-lattice model with anisotropic on-site exchange. New results concerning the phase diagram of the Hubbard model with strong attraction, the electromagnetic properties of systems with the first order superconducting transition and a possibility of local charge moment compensation in systems of coexisting bound pairs and itinerant electrons are presented.
We study the extended Hubbard model with on-site density-density U and intersite pair hopping J interactions, i.e. the Penson-Kolb-Hubbard model. This report focuses mainly on the properties of the model at T ≥q 0 in the case of repulsive J (J < 0) which may stabilize superconductivity with η-pairing. The analysis is performed within the (broken symmetry) Hartree-Fock approximation for arbitrary interaction parameters (J < 0 and U) and electron concentration (0 < n < 2) on the d = 2 square lattice. The phase diagrams of the model at T=0 and at finite temperatures are examined taking into account magnetic and charge-ordered phases and superconducting states with η- and s-wave pairing.
In this paper, we present room temperature unpolarized Raman scattering spectra of Ni doped PbTe single crystal sample. Crystal of PbTe(Ni) was grown by the Bridgman method. The Ni concentration in the sample used here was 1× 10^{19} at./cm^{3}. Well resolved peaks appear at about 126, 143, 181, 362 and 724 cm^{-1}. The modes at 126 and 143 cm^{-1}, which are also observed in other telluride compounds, originate from vibrations in TeO_2. We assume that the mode at about 181 cm^{-1} is connected to excitations of a local phonon mode in the vicinity of an impurity atom (donor Ni^{3+} state). Modes at about 362 cm^{-1} and 724 cm^{-1} are the second and fourth harmonic of a local phonon mode, registered here due to multiphonon emission.
We consider the excitonic correlations in the two-band solid state system composed of the valence band and conduction band electrons. We treat the phase coherence mechanism in the system by presenting the electron operator as a fermion attached to the U(1) phase-flux tube. The emergent bosonic gauge field, related to the phase variables appears to be crucial for the coherent Bose-Einstein condensation of excitons. We calculate the normal excitonic Green functions, and the single-particle density of states functions being a convolution between bosonic and fermionic counterparts. We obtain the total density of states as a sum of two independent parts. For the coherent normal fermionic density of states, there is no hybridization-gap found in the system due to strong coherence effects and phase stiffness.
We investigated the valence states of samarium ions in Sm deficient Sm_{1-x}B_{6} (x=0.03, 0.05, 0.08, 0.1 and 0.2) sintered samples, in single crystalline solid solutions Sm_{1-x}La_{x}B_{6} (x=0.16, 0.28, 0.4, 0.55 and 0.7) as well as in a SmB_{6} single crystal by X-ray absorption near edge structure (XANES) between 4.2 and 300 K. It was shown that the mixed valence state v_{Sm} of Sm-ions (v_{Sm} ≈ 2.51 for SmB_{6} at 4.2 K) in deficient samples is changed but maintained up to a concentration of 20% of vacancies and up to a concentration of 70% in samples doped with trivalent La-ions, and that in both cases it increases with temperature. On the other hand, the value of v_{Sm} increases with increasing vacancy level, whereas it decreases with increasing La^{3+} substitution. The obtained dependences of Sm-valence in SmB_{6} on vacancy as well as on La^{3+} ion concentration are in good agreement with susceptibility measurements and exact numerical calculation of the spinless Falicov-Kimball model in two dimensions.
We present first experimental results of ^{11}B-NMR of SmB_6 under applied pressure. From measurement of nuclear spin-lattice relaxation time (T_1) we find that with applied pressure the value of activation gap E_{g} is decreasing. This decrease is larger than in case of other experimental techniques. We suppose that the enhancement of 1/T_1 in temperature range 20-100 K with applied pressure reflects not only a suppression of hybridization gap, but also changes in spin correlations.
Recent improvements in two-dimensional angular correlation measurement and analysis techniques have greatly increased the power of the method to image the Fermi surfaces of electronically complex systems. That, and the further and dramatic improvement that must result when the present radioisotope positron sources are replaced by intense monoenergetic positron beams are discussed and illustrated.
The Penson-Kolb-Hubbard model, i.e. the Hubbard model with the pair-hopping interaction J is studied. We focus on the properties of the superconducting state with the Cooper-pair center-of-mass momentum q= Q (η-phase). The transition into theη-phase, which is favorized by the repulsive J (J<0) is found to occur only above some critical value |J_c|, dependent on band filling, on-site interaction U and band structure, and the system never exhibits standard BCS-like features. This is in obvious contrast with the properties of the isotropic s-wave state, stabilized by the attractive J and attractive U, which exhibit at T=0 a smooth crossover from the BCS-like limit to that of tightly bound pairs with increasing pairing strength.
The experimental manifestation of electron correlation effects in the V_2O_3 and NiS_2 systems are reviewed, and recent elementary theories for interpreting the many different types of metal-insulator transitions based on these theories are provided. Research on the critical phenomena displayed by the interacting electrons in these systems is also reviewed.
JavaScript is turned off in your web browser. Turn it on to take full advantage of this site, then refresh the page.