Sea Of Conduction Electrons Research Articles

Extremely large magnetoresistance and anisotropic transport in the multipolar Kondo system PrTi2Al20

Multipolar Kondo systems offer unprecedented opportunities to design astonishing quantum phases and functionalities beyond spin-only descriptions. A model material platform of this kind is the cubic heavy fermion system PrT2Al20 (T=Ti, V), which hosts a nonmagnetic crystal-electric-field ground state and substantial Kondo entanglement of the local quadrupolar and octopolar moments with the conduction electron sea. Here, we explore the magnetoresistance (MR) and Hall effect of PrTi2Al20, which develops ferroquadrupolar (FQ) order below TQ∼2 K, and compare their behavior with that of the MR and Hall effect of the non-4f analog, LaTi2Al20. In the FQ ordered phase, PrTi2Al20 displays extremely large magnetoresistance (XMR) of ∼103%. The unsaturated, quasilinear field (B) dependence of the XMR violates Kohler's scaling and defies description based on carrier compensation alone. By comparing the MR and the Hall effect observed in PrTi2Al20 and LaTi2Al20, we conclude that the open-orbit topology on the electron-type Fermi surface sheet is key for the observed XMR. The low-temperature MR and the Hall resistivity in PrTi2Al20 display pronounced anisotropy in the [111] and [001] magnetic fields, which is absent in LaTi2Al20, suggesting that the transport anisotropy ties in with the anisotropic magnetic field response of the quadrupolar order parameter. Published by the American Physical Society 2024

A review of the Kondo insulator materials class of strongly correlated electron systems: Selected systems and anomalous behavior

Studies of strongly correlated electron systems have been at the forefront of research in condensed matter physics ever since the discovery of the co-existence of strong Pauli-paramagnetism and superconductivity in the archetypal heavy-fermion compound CeCu2Si2 in 1979. The construct of correlated electron physics typifies the behavior of thermal and electronic properties of a material when the Coulomb interaction between conduction electrons exceeds the electron kinetic energy at a given thermal energy and redefines in remarkable ways our understanding of the behavior of a metal near its ground state. While correlated electron behavior has by now been demonstrated in a variety of different types of materials, Kondo systems in particular are arguably the most intensively studied among these. The Kondo interaction is used to describe the effect that a spin-magnetic ion has on its environment when immersed in the conduction electron sea of a metal. The localized spin of the Kondo ion polarizes nearby conduction electrons to form a so-called Kondo cloud, which acts to screen and magnetically (partially) neutralize the localized spin. In Kondo systems, the low-temperature behavior is prone to the formation of heavy fermions, which is the term given to quasiparticle excitations that define the emergence of effective electron masses that can be up to three orders of magnitude greater than that of a free electron. The Kondo effect presents itself in three guises: first, the single-ion Kondo state which is found in a metal having only a small amount of magnetic ions dissolved into it; second, the incoherent Kondo state in materials where there is a Kondo ion in every crystallographic unit cell of the material, but the Kondo ions remain incoherent or uncoupled from each other; and third, the coherent Kondo lattice state which manifests itself toward low temperatures where the interaction between Kondo ions becomes comparable to the thermal energy of conduction electrons that mediate magnetic exchange between Kondo ions. In a small number of cases, the outcome of a material condensing into the Kondo state turns out to be the peculiar formation of a very narrow energy band gap at the metallic Fermi energy. Such a band gap has significant consequences in practically all of the physical properties of a material that stem from the behavior of conduction electrons in proximity of the Fermi energy. This is most readily seen in electrical resistivity, heat capacity, and magnetic susceptibility. The band gapping gives cause to the term Kondo insulator (also referred to as Kondo semimetal or heavy-fermion semiconductor) that is used to describe this exceptional variety of Kondo systems. The term Kondo insulator is in general use although most Kondo insulators have a small but finite electrical conduction in the low-temperature limit where Kondo screening may be accomplished to its full extent. While the Kondo lattice ground state is exemplified by a very high density of electronic states at the Fermi energy, Kondo insulators, on the other hand, have, by virtue of narrow band gapping, a low density of electronic states. It remains a counter-intuitive observation, therefore, that despite their low density of states, Kondo insulators have curiously strong spin polarization energy scales and accompanying high values of their Kondo temperature, being the defining quantity which acts as an organizing principle in their temperature-dependent physical properties. In this article, we review the fundamentals of the Kondo insulating state, and we discuss the theoretical principles of what is presently understood about the formation of a Kondo insulator. The experimental results of a selected number of examples that have gained prominence in this class of materials are compared to each other in order to seek out similarities that may help deepen our understanding of the Kondo insulating state.

Optically induced hybrid Bose-Fermi system in quantum wells with different charge carriers.

It is demonstrated theoretically that the circularly polarized irradiation of two-dimensional conducting systems can produce composite bosons consisting of two electrons with different effective masses (different charge carriers), which are stable due to the Fermi sea of conduction electrons. As a result, an optically induced mixture of paired electrons and normal conduction electrons (the hybrid Bose-Fermi system) appears. Elementary excitations in such a hybrid system are analyzed, and possible manifestations of the light-induced electron pairing are discussed for semiconductor quantum wells.

Anomalous Spin Precession under a Geometrical Torque.

Precession and relaxation predominantly characterize the real-time dynamics of a spin driven by a magnetic field and coupled to a large Fermi sea of conduction electrons. We demonstrate an anomalous precession with frequency higher than the Larmor frequency or with inverted orientation in the limit where the electronic motion adiabatically follows the spin dynamics. For a classical spin, the effect is traced back to a geometrical torque resulting from a finite spin Berry curvature.

Fermi-edge transmission resonance in graphene driven by a single Coulomb impurity.

The interaction between the Fermi sea of conduction electrons and a nonadiabatic attractive impurity potential can lead to a power-law divergence in the tunneling probability of charge through the impurity. The resulting effect, known as the Fermi edge singularity (FES), constitutes one of the most fundamental many-body phenomena in quantum solid state physics. Here we report the first observation of FES for Dirac fermions in graphene driven by isolated Coulomb impurities in the conduction channel. In high-mobility graphene devices on hexagonal boron nitride substrates, the FES manifests in abrupt changes in conductance with a large magnitude ≈e(2)/h at resonance, indicating total many-body screening of a local Coulomb impurity with fluctuating charge occupancy. Furthermore, we exploit the extreme sensitivity of graphene to individual Coulomb impurities and demonstrate a new defect-spectroscopy tool to investigate strongly correlated phases in graphene in the quantum Hall regime.

Extremely strong-coupling superconductivity in artificial two-dimensional Kondo lattices

When interacting electrons are confined to low-dimensions, the electron-electron correlation effect is enhanced dramatically, which often drives the system into exhibiting behaviors that are otherwise highly improbable. Superconductivity with the strongest electron correlations is achieved in heavy-fermion compounds, which contain a dense lattice of localized magnetic moments interacting with a sea of conduction electrons to form a 3D Kondo lattice. It had remained an unanswered question whether superconductivity would persist upon effectively reducing the dimensionality of these materials from three to two. Here we report on the observation of superconductivity in such an ultimately strongly-correlated system of heavy electrons confined within a 2D square-lattice of Ce-atoms (2D Kondo lattice), which was realized by fabricating epitaxial superlattices built of alternating layers of heavy-fermion CeCoIn5 and conventional metal YbCoIn5. The field-temperature phase diagram of the superlattices exhibits highly unusual behaviors, including a striking enhancement of the upper critical field relative to the transition temperature. This implies that the force holding together the superconducting electron-pairs takes on an extremely strong coupled nature as a result of two-dimensionalization.

Kondo Effect of a Magnetic Ion Vibrating in a Harmonic Potential

To discuss Kondo effects of a magnetic ion vibrating in the sea of conduction electrons, a generalized Anderson model is derived. The model includes a new channel of hybridization associated with phonon emission or absorption. In the simplest case of the localized electron orbital with the s-wave symmetry, hybridization with p-waves becomes possible. Interesting interplay among the conventional s-wave Kondo effect and the p-wave one and the Yu-Anderson type Kondo effect is found and the ground state phase diagram is determined by using the numerical renormalization group method. Two different types of stable fixed points are identified and the two-channel Kondo fixed points are generically realized on the boundary.

Heavy electrons and the symplectic symmetry of spin

The recent discovery of two heavy-fermion materials PuCoGa5 and NpPd5Al2, which transform directly from Curie paramagnets into superconductors, reveals a new class of superconductors where local moments quench directly into the superconducting condensate. Unlike other heavy-electron superconductors, where Cooper pairing is thought to be driven by spin fluctuations, these higher-transition-temperature materials do not seem to be close to a magnetic instability. Large-N expansions have been invaluable in describing heavy-fermion metals, but so far cannot treat superconductivity. Here, we introduce a new class of large-N expansion that uses symplectic symmetry to protect the odd time-reversal parity of spin and sustain Cooper pairs as well-defined singlets. We show that when a lattice of magnetic ions exchange spin with their metallic environment in two distinct symmetry channels, they can simultaneously satisfy both channels by forming a condensate of composite pairs between local moments and electrons. In the tetragonal crystalline environment relevant to PuCoGa5 and NpPd5Al2, the lattice structure selects a natural pair of spin exchange channels, predicting a unique anisotropic paired state with either d- or g-wave symmetry. This pairing mechanism also predicts a large upturn in the NMR relaxation rate above Tc and strong enhancement of Andreev reflection in tunnelling measurements. The Kondo problem—dealing with localized magnetic impurities embedded in a sea of conduction electrons—can be treated on an equal footing with superconductivity for a large system of interacting electrons.

Theory of Photoemission in Actinides

AbstractA theory is presented which describes the photoemission spectra of actinide compounds starting from the atomic limit of isolated actinide ions. The multiplets of the ion are calculated and an additional term is introduced to describe the interaction with the sea of conduction electrons. This leads to complex mixed-valent ground states, which describes well the rich spectrum observed for PuSe. In particular, the three-peak feature, which is often seen in Pu and Pu compounds in the vicinity of the Fermi level originates from f6 → f5 emission. The theory is further applied to PuSb, PuCoGa5 and Am.

Crystal-field splittings in CeX (X = N, P, As, Sb, Bi) compounds

The unusual and interesting physical properties of rare earth intermetallic compounds have their origin in the combination of strongly correlated 4f states and their hybridization with the conduction electron sea, which gives rise to their complex low temperature Kondo behavior. In particular, Ce compounds are very sensitive to the crystalline and chemical environment, as compared to other rare earth systems. The interaction of the 4f state with the conduction band plays an important role in the determination of the different magnetic, structural and transport properties of these systems. Among the cerium compounds, those of the type CeX, which crystallize in the rock salt structure, exhibit extremely unusual magnetic properties. By making use of the mixed LDA-NCA calculation technique we analyse the crystal-field splittings of CeX compounds (X=N, P, As, Sb, Bi). The obtained ab initio hybridization functions are taken as inputs to calculate the crystal-field splittings within NCA (non crossing approximation) and the tendencies are contrasted with experiments.

Anomalies at the compensation temperature in the zero-magnetization ferromagnet(Sm,Gd)Al2

The unusual zero-magnetization ferromagnet $({\mathrm{Sm}}_{1\ensuremath{-}x}{\mathrm{Gd}}_{x}){\mathrm{Al}}_{2}$ (SGA) with $x=0.01$ and 0.02 is investigated by magnetic, and magnetotransport and calorimetric measurements. Striking features observed around the compensation temperature ${T}_{\mathrm{comp}}$ are an exchange bias in a single-magnetic-lattice compound such as SGA attributable to the anisotropy of the coupling between the spin and orbital moments; an anomalous Hall effect due to the asymmetric magnetic scattering of the conduction electrons; and a field-induced specific heat transition at ${T}_{\mathrm{comp}}$ ascribable to the field-induced switching of the spin and orbital moments in a polarized conduction electron sea. The observations reveal the richness of physics in the SGA compound system.

The one-dimensional Kondo lattice model at partial band filling

The Kondo lattice model introduced in 1977 describes a lattice of localized magnetic moments interacting with a sea of conduction electrons. It is one of the most important canonical models in the study of a class of rare earth compounds, called heavy fermion systems, and as such has been studied intensively by a wide variety of techniques for more than a quarter of a century. This review focuses on the one dimensional case at partial band filling, in which the number of conduction electrons is less than the number of localized moments. The theoretical understanding, based on the bosonized solution, of the conventional Kondo lattice model is presented in great detail. This review divides naturally into two parts, the first relating to the description of the formalism, and the second to its application. After an all-inclusive description of the bosonization technique, the bosonized form of the Kondo lattice hamiltonian is constructed in detail. Next the double-exchange ordering, Kondo singlet formation, the RKKY interaction and spin polaron formation are described comprehensively. An in-depth analysis of the phase diagram follows, with special emphasis on the destruction of the ferromagnetic phase by spin-flip disorder scattering, and of recent numerical results. The results are shown to hold for both antiferromagnetic and ferromagnetic Kondo lattice. The general exposition is pedagogic in tone.

Dissipative dynamics of an extended magnetic nanostructure: spin necklace in a metallic environment.

We study theoretically the dynamics of an "xxz" spin necklace coupled to a conduction electron sea, a model system for a nanostructure in a dissipative environment. We extract the long-time behavior via a mapping to a multichannel Coulomb gas problem followed by a scaling analysis. The strong quantum fluctuations of the necklace cause a nontrivial dependence of couplings on system size which we extract via an analysis involving the "boundary condition changing operator," and confirm via a detailed numerical evaluation of one case.

Influence ofdelectrons on the dispersion relation of Ag surface plasmons for different single-crystal faces

The surface plasmon dispersion relation of Ag is calculated for different single-crystal orientations. To describe the dynamical response properties of both delocalized $5s$ electrons and more tightly bound $4d$ electrons, the jellium model is combined with the so-called dipolium model, in which the occupied Ag d bands are represented in terms of polarizable spheres located at the sites of a semi-infinite fcc lattice. The nonlocal susceptibility characterizing the s electron response in the surface region is derived using density functional theory. The screening of the Coulomb interaction between conduction electrons via the lattice of dipoles, and of the dipole interaction via the surrounding sea of conduction electrons, is treated self-consistently. Electron energy loss spectra are calculated for all three low-index faces. The surface plasmon energy is found to increase with parallel wave vector for all cases. The magnitude of the positive slope depends on the crystal orientation and, for Ag(110), on the propagation direction. These results are in qualitative agreement with electron energy loss measurements.

Screening and interaction of magnetic impurities in Luttinger liquids

The spin and charge correlations induced in the conduction electron sea by the presence of a spin-1=2 magnetic impurity are investigated for one-dimensional electrons. For correlated conduction electrons, the RKKY interaction between magnetic impurities exhibits only a slow algebraic decay with distance. Increasing the exchange coupling between conduction electrons and magnetic impurity leads to a competition between the RKKY interaction and the Kondo effect. For a two-impurity model, we study the influence of the electronic correlations on this competition. Furthermore, the Kondo screening cloud and the local spin susceptibility far away from a magnetic impurity are discussed.

RKKY-like Biquadratic Exchange in Magnetic Superlattices

Since the discovery of antiferromagnetic coupling of ferromagnetic layers across a nonmagnetic metallic spacer layer, the magnetic superlattices (SL) have become an object of intense interest of both theoretical as well as experimental studies. The problem of RKKY-reminiscent spin polarization oscillations, with a long period and giant interface magnetoresistance attracted the most attention. The recently discovered, π/2 coupling between magnetizations of the neighbouring magnetic layers [1] added new interest to the problem. This type of ordering suggests that along with the bilinear coupling of the type J 1 m1 • m2 between magnetizations of adjacent layers there is an additional multipolar interaction. There were many attempts describing the mechanisms that give rise to biquadratic exchange, being a speciflc form of coupling between magnetic quadupole moments of the ions (for details see Refs. [2, 3] and references therein). The aim of the paper is to study a new intrinsic mechanism that originates in multipolar exchange coupling. In the following we will show that the quadrupole-quadupole coupling can arise in a quite natural way from direct scattering of conduction electrons on magnetic multipole moments in the manner similar to bilinear RKKY interaction. The starting point for any description of metallic magnetic systems is the case of dilute alloys, when a few TM or RE ions are immersed in the sea of conduction electrons. The coupling of the ionic spin Sn with the itinerant electron spin σ is usually taken as the contact interaction [4]: Hex = -2JSn σδ(r). In the case of non-s magnetic ion state along with the scattering due to dipolar contact interaction there appears also scattering of conduction electrons on quadupolar moments. Kondo [5] has proved that the interaction Vqc (k) between conducting electrons and the quadrupoles is given by

Enhanced local moment formation in a chiral Luttinger liquid.

We derive here a stability condition for a local moment in the presence of an interacting sea of conduction electrons. The conduction electrons are modeled as a Luttinger liquid in which chirality and spin are coupled. We show that an Anderson $U$ defect in such an interacting system can be transformed onto a nearly Fermi-liquid problem. We find that correlations among the conduction electrons stabilize the local moment phase. A Schrieffer-Wolff transformation is then performed which results in an anisotropic exchange interaction indicative of the Kondo effect in a Luttinger liquid. The ground-state properties of this model are then equivalent to those of the Kondo model in a Luttinger liquid.

Green's function of the core electrons in a lattice model for x-ray spectra: Breakdown of perturbation theory in the long-time limit.

A model for the description of the x-ray spectra of an infinitely heavy particle embedded in a sea of conduction electrons is extended to the case of many, randomly distributed local core electrons. The mean-field thermodynamics, i.e., the exact solution in d=\ensuremath{\infty}, is presented and the Green's function of the heavy electrons is thoroughly studied. The exact asymptotic limits of this function for short and long, imaginary and real times are obtained for all interaction strengths. Two important observations are made. First, the zero-temperature function exhibits for long imaginary and real times the edge singularity independently of the ground state of the conduction electrons. Second, the finite-time approximate schemes fail to reproduce the exact long-time asymptotics at any temperature and the Green's function of the local electrons undergoes a nonanalytic crossover from short to long times. The exact long-time asymptotics are available only via the Wiener-Hopf or related nonperturbative techniques.

Overhauser shift due to dynamic nuclear polarization on the conduction electron spin resonance in fully doped polythiophene at 1.9 K

Overhauser shift ( ΔB ov ) of the electron spin resonance (ESR) line due to dynamic nuclear spin polarization (DNP) of the hyperfine coupled nuclei is observed for the fully doped, conducting form of polythiophene (PT) at 1.9 K. The DNP enhancement factor V ≈ 55 is determined via dependence of the ΔB ov on the microwave power. The proton spin relaxation time HT 1 is determined from the relaxation of the ΔB ov after switching the rf pulse. The average hyperfine coupling constant A zz is calculated to be ≈−5.6 MHz. The results will be discussed in terms of origin of the ESR signal in the fully doped polythiophene and localized moments in interaction with conduction electron sea.

Pair correlation effects in heavy fermions

We summarize progress in studying a model of two Kondo impurities interacting directly with a conduction electron sea and indirectly with each other. In the presence of particle-hole symmetry, a critical point exists at finite ratio of (antiferromagnetic) RKKY coupling to Kondo temperature. For the more general case of no particle-hole symmetry, however, indications from both a 1 N (mean field) expansion and numerical renormalization group studies of potential scattering are that the transition is washed out. We discuss the implications of these findings on pair correlations in a real (lattice) system.