Itinerant Electrons Research Articles

First principles study of the effect of Cu/Ag/Au single doping and point defects on the magnetic and photocatalytic properties of ZnO

Experimental studies and theoretical calculations on the physical properties of ZnO with Cu/Ag/Au single doping have been reported, but reports on the magnetic and photocatalytic properties of ZnO by Zn vacancies and Cu/Ag/Au single doping and interstitial H coexistence are few. Moreover, locating the positions of the Zn vacancies is experimentally challenging. We used the GGA + U method within the framework of density functional theory to investigate the formation energy and magnetic and photocatalytic properties of Zn34MO36 (M = Cu/Ag/Au) and Zn34MHiO36 (M = Cu/Ag/Au) systems. Results show that the system is more stable under the O-rich condition. The Zn34CuO36, Zn34AgO36, and Zn34CuHiO36 systems all exhibit the magnetic properties of 2, −1 and 2 μB, and the O−-2p, Ag-4d, and Cu-3d states in the Zn34CuO36, Zn34AgO36 and Zn34CuHiO36 systems have local and itinerant electron properties. The Zn34AgHiO36 system has the longest electronic lifetime of 0.75 × 10−13 s and a longest hole lifetime of 0.70 × 10−13 s, with a relative maximum D value of 67.59, better carrier separation, and relatively high oxidation capacity. In the wavelength range of 400–800 nm, the Zn34AgHiO36 system has the strongest relative absorption spectrum intensity, the most pronounced red shift, the largest relative electric dipole moment of 109.19 × 10−30C·m and the largest relative activity. Therefore, the Zn34AgHiO36 system is the best photocatalyst for O production.

Chiral Kondo lattice in doped MoTe2/WSe2 bilayers.

We theoretically study the interplay between magnetism and a heavy Fermi liquid in the AB-stacked transition metal dichalcogenide bilayer system, MoTe2/WSe2, in the regime in which the Mo layer supports localized magnetic moments coupled by interlayer electron tunneling to a weakly correlated band of itinerant electrons in the W layer. We show that the interlayer electron transfer leads to a chiral Kondo exchange, with consequences including a strong dependence of the Kondo temperature on carrier concentration and anomalous Hall effect due to a topological hybridization gap. The theoretical model exhibits two phases, a small Fermi surface magnet and a large Fermi surface heavy Fermi liquid; at the mean-field level, the transition between them is first order. Our results provide concrete experimental predictions for ongoing experiments on MoTe2/WSe2 bilayer heterostructures and introduces a controlled route to observe a topological selective Mott transition.

Quantum theory of the intrinsic orbital magnetoelectric effect in itinerant electron systems at finite temperatures

Magnetization can be induced by an electric field in systems without inversion symmetry $\mathcal{P}$ and time-reversal symmetry $\mathcal{T}$. This phenomenon is called the magnetoelectric (ME) effect. The spin ME effect has been actively studied in multiferroics. The orbital ME effect also exists and has been mainly discussed in topological insulators at zero temperature. In this paper, we study the intrinsic orbital ME response in metals at finite temperature using the Kubo formula. The intrinsic response originates from the Fermi sea and does not depend on the dissipation. Especially in systems with $\mathcal{PT}$ symmetry, the extrinsic orbital ME effect becomes zero, and the intrinsic ME effect is dominant. We apply the response tensor obtained in this work to a $\mathcal{PT}$-symmetric model Hamiltonian with antiferromagnetic loop current order, demonstrating that the intrinsic ME effect is enhanced around the Dirac points.

Ferromagnetic instability in itinerant fcc lattice electron systems with higher-order van Hove singularities: Functional renormalization group study

We investigate the possibility of ferromagnetic ordering in the non-degenerate Hubbard model on the face-centered cubic lattice within the functional renormalization group technique using temperature as a scale parameter. We assume the relations between nearest, next-nearest, and next-next-nearest hopping parameters providing higher order (giant) van Hove singularity of the density of states. The ferromagnetic instability formation with lowering temperature is described consistently in the one-loop approximation for a one-particle irreducible vertex of two-particle electron interaction. The chemical potential versus temperature phase diagrams are calculated. We find ferromagnetic order only for sufficiently strong divergence of the density of states and fillings in the vicinity of van Hove singularity. The obtained Curie temperature is more than an order of magnitude smaller than the results of the random-phase approximation. The main origin of the suppression of ferromagnetism is the screening of interaction in the particle-particle channel. We also do not find the pronounced tendency towards incommensurate order when the Fermi level is moved away from a van Hove singularity, such that the first order quantum phase transitions from the ferro- to paramagnetic phase are obtained.

8Li βNMR studies of Epitaxial Thin Films of the 3D topological Dirac semimetal Sr3SnO

The inverse perovskite Sr3SnO is a 3D cubic Dirac semimetal with a very small energy gap[1]. Its unusual electronic structure confers a variety of novel properties, such as chiral topological surface states, and very strong itinerant electron orbital magnetism. Remarkably, when doped it also becomes superconducting[2]. In the lowest carrier density samples, the Fermi level lies close to the Dirac points, and orbital magnetism is maximal. Here we report the results of ion-implanted 8Li+ βNMR in Au-capped epitaxial thin films of Sr3SnO as a function of carrier content. In addition, we stop the 8Li in the Au overlayer to seek proximal evidence of the chiral surface state.In high magnetic field (6.55 T), we find remarkably little contrast in spin-lattice relaxation (SLR) between low carrier density Sr3SnO and the Au overlayer. In the inverse perovskite layer, 1/T 1 ∼ 0.14 s-1, slightly faster than Au at 300 K, while in the overlayer, there is a small but systematic enhancement in 1/T 1 compared to a control film of Au. The resonance in the Sr3SnO layer is broad with a long tail towards negative shift without resolved quadrupolar splitting.

Magnetic and electrical properties of high-entropy rare-earth manganites

Detailed investigations of structural, magnetic and electronic transport properties of hole-doped high-entropy rare-earth manganites are presented. The high-entropy samples (La0.2Nd0.2Pr0.2Sm0.2Eu0.2)1−xSrxMnO3 (0 ≤ x ≤ 0.5), synthesized using the solid-state technique, show a change in the crystal structure from Pbnm to R-3c with increasing Sr substitution, attributed to the change in the tolerance factor. Prominent ferromagnetic ordering is observed in the sample with a rhombohedral structure (x ≥ 0.3), originating from the dominant double exchange mechanism mediated by itinerant electrons. Further, the Curie temperature is smaller for the high-entropy sample with x=0.3, as compared to La0.7Sr0.3MnO3, suggesting a strong relation between the Curie temperature and the Mn–O–Mn bond angle associated with the reduced ionic radii at the rare-earth site. The electrical resistivity of the high-entropy samples is larger than those of La1−xSrxMnO3, which can be ascribed to the reduced bandwidth due to the enhanced structural distortion. A concomitant rise in magnetoresistance is observed for high-entropy samples with the increase in Sr concentration. These findings considering the configurational complexity of different rare-earths advance the understanding of high-entropy rare-earth manganites.

Scale-dependent theory of the disordered electron liquid

We review the scaling theory of disordered itinerant electrons with e−e interactions. We first show how to adjust the microscopic Fermi-liquid theory to the presence of disorder. Then we describe the non-linear sigma model (NLSM) with interactions (Finkel’stein’s model). This model is closely connected to the Fermi liquid, but is more generally applicable, since it can also be viewed as a minimal effective functional describing disordered interacting electrons. Our discussion emphasizes the general structure of the theory, and the connection of the scaling parameters to conservation laws. We then move on to discuss the metal–insulator transition (MIT) in the disordered electron liquid in two and three dimensions. Generally speaking, this MIT is a non-trivial example of a quantum phase transition. The NLSM approach allows to identify the dynamical exponent connecting the spatial and energy scales, which is central for the description of the kinetic and thermodynamic behavior of the system in the critical region of the MIT in three dimensions. In two dimensions, the system can be discussed in terms of a flow in the disorder-interaction phase plane, which is controlled by a fixed point. We demonstrate that the two-parameter RG-equations accurately describe electrons in Si-MOSFETs including the observed non-monotonic behavior of the resistance and its strong drop at low temperatures. The theory can also be applied to systems with an attractive interaction in the Cooper channel, where it describes the suppression of superconductivity in disordered amorphous films. We extend our discussion to heat transport in the two-dimensional electron liquid. Similar to the electric conductivity, the thermal conductivity also acquires logarithmic corrections induced by the interplay of the electron interaction and disorder. The resulting thermal conductivity can be calculated in the NLSM formalism after introducing so-called gravitational potentials. It may be concluded that the combined effect of disorder and e−e interactions determines all aspects of the physics of itinerant electrons in the presence of disorder.

ANi5Bi5.6+δ (A = K, Rb, and Cs): Quasi-One-Dimensional Metals Featuring [Ni5Bi5.6+δ]- Double-Walled Column with Strong Diamagnetism.

A new series of compounds, ANi5Bi5.6+δ (where A = K, Rb, and Cs) are discovered with a quasi-one-dimensional (Q1D) [Ni5Bi5.6+δ]- double-walled column and a coaxial inner one-dimensional Bi atomic chain. The columns are linked to each other by intercolumn Bi-Bi bonds and separated by an A+ cation. Typical metallic behaviors with strong correlation of itinerant electrons and the Sommerfeld coefficient enhanced with the increasing cationic radius were experimentally observed and supported by first-principles calculations. Compared to AMn6Bi5 (where A = K, Rb, and Cs), the enhanced intercolumn distances and the substitution of Ni for Mn give rise to strong diamagnetic susceptibilities in ANi5Bi5.6+δ. First-principles calculations reveal possible uncharged Ni atoms with even number of electrons in ANi5Bi5.6+δ, which may explain the emergence of diamagnetism. ANi5Bi5.6+δ, as Q1D diamagnetic metals with strong electron correlation, provide a unique platform to understand exotic magnetism and explore novel quantum effects.

Magnetic excitations in the square-lattice iridate Ba2IrO4

We report a resonant inelastic x-ray scattering investigation of ultrathin epitaxial films of ${\mathrm{Ba}}_{2}{\mathrm{IrO}}_{4}$, and compare their low-energy magnetic and spin-orbit excitations to those of their sister compound ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$. Due to the ${180}^{\ensuremath{\circ}}$ Ir-O-Ir bond, the bandwidth of the magnon and spin orbiton is significantly larger in ${\mathrm{Ba}}_{2}{\mathrm{IrO}}_{4}$, making it difficult to describe these two types of excitations as separate well-defined quasiparticles. Both types of excitations are found to be quite sensitive to the effect of epitaxial strain. In addition, we find that the $d$-level inversion observed in ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ is absent in ${\mathrm{Ba}}_{2}{\mathrm{IrO}}_{4}$, as predicted in recent theoretical studies. Our results illustrate that the magnetic properties of ${\mathrm{Ba}}_{2}{\mathrm{IrO}}_{4}$ are substantially different from those of ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$, suggesting that these materials need to be examined more carefully with electron itinerancy taken into account.

Widely-sweeping magnetic field–temperature phase diagrams for skyrmion-hosting centrosymmetric tetragonal magnets

We numerically investigate the stabilization mechanisms of skyrmion crystals under thermal fluctuations and external magnetic field in itinerant centrosymmetric tetragonal magnets. By adopting an efficient steepest descent method with a small computational cost, we systematically construct the magnetic field–temperature phase diagrams of the effective spin model derived from the itinerant electron model on a two-dimensional square lattice. As a result, we find that a square-type skyrmion crystal is stabilized by either or both of the high-harmonic wave–vector interaction and the biquadratic interaction under an external magnetic field. Especially, we discover that the former high-harmonic wave–vector interaction can stabilize the skyrmion crystal only at finite temperatures when its magnitude is small. In addition to the skyrmion crystal, we also find other stable multiple-Q states in the phase diagram. Lastly, we discuss the correspondence of the phase diagrams between the effective spin model and the skyrmion-hosting material GdRu2Si2. The present results suggest a variety of multiple-Q states could be driven by thermal fluctuations and external magnetic fields in centrosymmetric itinerant magnets.

Observation of Kondo condensation in a degenerately doped silicon metal

When a magnetic moment is embedded in a metal, it captures nearby itinerant electrons to form a so-called Kondo cloud. When magnetic impurities are sufficiently dense that their individual clouds overlap with each other they are expected to form a correlated electronic ground state. This is known as Kondo condensation and can be considered a magnetic version of Bardeen–Cooper–Schrieffer pair formation. Here, we examine this phenomenon by performing electrical transport and high-precision tunnelling density-of-states spectroscopy measurements in a highly P-doped crystalline silicon metal in which disorder-induced localized magnetic moments exist. We detect the Kondo effect in the resistivity of the Si metal at temperatures below 2 K and an unusual pseudogap in the density of states with gap edge peaks below 100 mK. The pseudogap and peaks are tuned by applying an external magnetic field and transformed into a metallic Altshuler–Aronov gap associated with a paramagnetic disordered Fermi liquid phase. We interpret these observations as evidence of Kondo condensation followed by a transition to a disordered Fermi liquid.

Signature of weakly coupled f electrons and conduction electrons in magnetic Weyl semimetal candidates PrAlSi and SmAlSi

Magnetic topological materials are a class of compounds with the underlying interplay of nontrivial band topology and magnetic spin configuration. Extensive interests have been aroused due to their application potential involved with an array of exotic quantum states. With angle-resolved photoemission spectroscopy and first-principles calculations, here we study the electronic properties of two magnetic Weyl semimetal candidates PrAlSi and SmAlSi. Though the two compounds harbor distinct magnetic ground states (ferromagnetic and antiferromagnetic for PrAlSi and SmAlSi, respectively) and 4$f$ shell fillings, we find that they share quite analogous low-energy band structure. By the measurements across the magnetic transitions, we further reveal that there is no evident evolution of the band structure in both compounds and the experimental spectra can be well reproduced by the nonmagnetic calculations, together suggesting a negligible effect of the magnetism on their electronic structures and a possibly weak coupling between the localized 4$f$ electrons and the itinerant conduction electrons. Our results offer essential insights into the interactions between magnetism, electron correlations, and topological orders in the $R$Al$X$ ($R$ = light rare earth and $X$ = Si or Ge) family.

Intertwined charge and spin orders through the exchange-interaction on a square lattice

We study exotic electronic charge orders and magnetic spin orders in a lattice model which encloses conducting electrons coupled with localized magnetic moments via the exchange-like interaction. By means of numerical simulations based on kernel polynomials method combined with Langevin dynamics, we systematically map out phase diagrams versus exchange coupling strength. At the half-filling case n=1/2 (one electron per lattice site on average), a robust antiferromagnetic insulator is formed at low temperature regime. Away from the half-filling, competition between itinerant electrons and local spin moments drives intertwined charge and spin orders. As a concrete example, at the three-quarter filling n=3/4, we identify various spin orders such as incommensurate stripe order and a long-sought double-Q state. Especially, we demonstrate that such double-Q magnetic order leads to a Dirac semi-metal phase on the square lattice, which is identified by the electronic density of states and optical conductivity. Additionally, we also uncover the spin dynamics of magnetic orders via Landau-Lifshitz method. These findings demonstrate that the interplay between itinerant electrons and preformed local spin moments via the exchange interaction is able to produce complex magnetic orders and electronic charge orders.

Multiband character revealed from weak antilocalization in platinum thin films

Platinum (Pt) has been very much used for spin-charge conversion in spintronics research due to its large intrinsic spin-orbit interaction. Magnetoconductance originating from weak antilocalization in the quantum interference regime is used as a powerful tool to obtain the microscopic information of spin-orbit interaction and the coherence phase breaking scattering process among itinerant electrons. To acquire the knowledge of different types of scattering processes, we have performed a magnetoconductance study on Pt thin films which manifests multiband (multichannel) conduction. An extensive analysis of quantum-interference-originated weak antilocalization reveals the existence of strong (weak) interband scattering between two similar (different) orbitals. Coherence phase breaking lengths $({l}_{\ensuremath{\phi}})$ and their temperature dependence are found to be significantly different for these two conducting bands. The observed effects are consistent with the theoretical prediction that there exist three Fermi sheets with one $s$ and two $d$ orbital character. This study provides the evidence of two independent nonsimilar conducting channels and the presence of anisotropic spin-orbit interaction along with $e\text{\ensuremath{-}}e$ correlation in Pt thin films.

Prediction of monolayer FeP4 with intrinsic half-metal ferrimagnetism above room temperature

The design of high-temperature ferrimagnetic materials is highly demanded for next-generation functional spintronic devices. Here, we propose that the combination of nonmetallic structural units and magnetic atoms is an effective way to achieve high-temperature magnetism in two-dimensional (2D) materials. The predicted $\mathrm{Fe}{\mathrm{P}}_{4}$ monolayer, consisting of quasisquare ${\mathrm{P}}_{4}$ units, shows intrinsic half-metal ferrimagnetism above room temperature. Each Fe atom is coordinated with four P atoms associated with the surrounding four quasisquare ${\mathrm{P}}_{4}$ units. First-principles calculations suggest that the $\mathrm{Fe}{\mathrm{P}}_{4}$ monolayer presents a Curie temperature of 460 K. More interestingly, the itinerant electrons and the unique quasisquare ${\mathrm{P}}_{4}$ units act as intermediaries and play an important role in promoting the Ruderman-Kittel-Kasuya-Yosida and superexchange interactions, respectively, which induces a robust ferrimagnetism. Our findings not only shed light on the promising future of 2D magnetic materials, but also are of interest for high-temperature spintronic applications.

Enhanced Thermoelectric Performance by Resonant Doping and Embedded Magnetic Impurity

The thermoelectric energy-conversion efficiency is significantly enhanced by introducing magnetism. Doping $\mathrm{Cr}$ atoms in a ${\mathrm{Cu}}_{2}{\mathrm{Sn}\mathrm{S}}_{3}(\mathrm{CTS})$ matrix modifies the electron density of state (e-DOS) and causes a special electron-transport mechanism by localized impurity spin moments. The localized spin moments cause a spin-spin exchange interaction with the spin of itinerant electrons of the CTS matrix. This is clearly verified in M-T measurement, which deviated from Curie's law. The figure of merit (zT) of the magnetic nanocomposite was 8 times higher than that of pristine CTS by the synergetic effect of modifying e-DOS near the Fermi level and electron transport by localized spin moments.

Magnetic molecular orbitals in MnSi.

A large body of knowledge about magnetism is attained from models of interacting spins, which usually reside on magnetic ions. Proposals beyond the ionic picture are uncommon and seldom verified by direct observations in conjunction with microscopic theory. Here, using inelastic neutron scattering to study the itinerant near-ferromagnet MnSi, we find that the system's fundamental magnetic units are interconnected, extended molecular orbitals consisting of three Mn atoms each rather than individual Mn atoms. This result is further corroborated by magnetic Wannier orbitals obtained by ab initio calculations. It contrasts the ionic picture with a concrete example and presents an unexplored regime of the spin waves where the wavelength is comparable to the spatial extent of the molecular orbitals. Our discovery brings important insights into not only the magnetism of MnSi but also a broad range of magnetic quantum materials where structural symmetry, electron itinerancy, and correlations act in concert.

Quantum transport and fractional hall effect in Moiré correlated/anticorrelated interface channels

The energy-resolved transmission of twisted circular (anticorrelated) nanoribbons depends on twisted angles, where the hybridization of local fermion momenta as f-orbitals and itinerant conduction electrons as c-orbitals play an important role.

Strongly correlated itinerant magnetism near superconductivity in NiTa4Se8

Metallic ferromagnets with strongly interacting electrons often exhibit remarkable electronic phases such as ferromagnetic superconductivity, complex spin textures, and nontrivial topology. In this report, we discuss the synthesis of a layered magnetic metal $\mathrm{Ni}{\mathrm{Ta}}_{4}{\mathrm{Se}}_{8}$ (or ${\mathrm{Ni}}_{1/4}\mathrm{Ta}{\mathrm{Se}}_{2}$) with a Curie temperature of 58 Kelvin. Magnetization data and density functional theory calculations indicate that the nickel atoms host uniaxial ferromagnetic order of about $0.7{\ensuremath{\mu}}_{B}$ per atom, while an even smaller moment is generated in the itinerant tantalum conduction electrons. Strong correlations are evident in flat bands near the Fermi level, a high heat capacity coefficient, and a high Kadowaki-Woods ratio. Density functional theory calculations suggest that electron and hole Fermi surfaces in the ferromagnetic phase are associated with opposite spin polarization. When the system is diluted of magnetic ions, the samples become superconducting below about 2 Kelvin. We discuss possible mechanisms for superconductivity in this family.

Strongly Interacting Bose Polarons in Two-Dimensional Atomic Gases and Quantum Fluids of Polaritons

Polarons are quasiparticles relevant across many fields in physics: from condensed matter to atomic physics. Here, we study the quasiparticle properties of two-dimensional strongly interacting Bose polarons in atomic Bose–Einstein condensates and polariton gases. Our studies are based on the non-self consistent T-matrix approximation adapted to these physical systems. For the atomic case, we study the spectral and quasiparticle properties of the polaron in the presence of a magnetic Feshbach resonance. We show the presence of two polaron branches: an attractive polaron, a low-lying state that appears as a well-defined quasiparticle for weak attractive interactions, and a repulsive polaron, a metastable state that becomes the dominant branch at weak repulsive interactions. In addition, we study a polaron arising from the dressing of a single itinerant electron by a quantum fluid of polaritons in a semiconductor microcavity. We demonstrate the persistence of the two polaron branches whose properties can be controlled over a wide range of parameters by tuning the cavity mode.