Itinerant Electrons Research Articles

High-mobility spin-polarized two-dimensional electron gas at the interface of EuTiO3/SrTiO3 heterostructures

Spin polarization of two-dimensional electron gas (2DEG) at the interface of EuTiO3/SrTiO3(STO) heterostructures has been theoretical predicted and experimentally observed via x-ray magnetic circular dichroism and polarized x-ray absorption spectroscopy, which, however, is lack of magnetotransport evidence. Here, we report the fabrication of high-quality EuTiO3/STO heterostructures by depositing antiferromagnetic insulating EuTiO3 thin films onto STO substrates. Shubnikov-de Haas oscillation, Hall, and magnetoresistance (MR) measurements show that the interface is not only highly conducting, with electron mobility up to cm2V−1s−1 at 1.8 K, but also shows low-field hysteretic MR effects. MR of ∼9% is observed at 1.8 K and 20 Oe, which is one order of magnitude higher than those observed in other spin-polarized 2DEG oxide systems. Moreover, the heterostructures show ferromagnetic hysteresis loops. These results demonstrate that the high-mobility 2DEG is spin polarized, whose origin is attributed to the interfacial Ti3+-3d states due to oxygen deficiency and the exchange interactions between interfacial Eu spins and itinerant Ti-3d electrons.

Clues to potential dipolar-Kondo and RKKY interactions in a polar metal

The coexistence of electric dipoles and itinerant electrons in a solid was postulated decades ago, before being experimentally established in several ‘polar metals’ during the last decade. Here, we report a concentration-driven polar-to-nonpolar phase transition in electron-doped BaTiO3. Comparing our case with other polar metals, we find a particular threshold concentration (n*) linked to the dipole density (nd). The universal ratio frac{{n}_{{{{rm{d}}}}}}{{n}^{* }}approx 8.0(6) suggests a common mechanism across different polar systems, possibly explained by a dipolar Ruderman-Kittel-Kasuya-Yosida theory. Moreover, in BaTiO3, we observe enhanced thermopower and upturn on resistivity at low temperatures near n*, resembling the Kondo effect. We argue that local electric dipoles act as two-level-systems, whose fluctuations couple with surrounding electron clouds, giving rise to a potential dipolar-counterpart of the Kondo effect. Our findings unveil a mostly uncharted territory for exploring emerging physics associated with electron-dipole correlations, encouraging further theoretical work on dipolar-RKKY and Kondo interactions.

Topological behaviors in Kondo insulators

Since Kondo insulators are simultaneously subject to time-reversal symmetry and spatial antisymmetry, topological interactions exhibit topological Kondo insulating properties. To investigate the topological behaviors, we proposed a topologically interacting Kondo model with the slave-boson mean-field approximation in a square lattice ribbon. Different phases arise in different topological interactions and itinerant electron densities. In particular, the Kondo effect disappears at the edges in the Kondo and topological Kondo insulating phases. In the topological Kondo states, the conducting edge band is non-linear.

Coexistence of Zero-Dimensional Electride State and Superconductivity in AlH2 Monolayer

Electrides, which confine “excess anionic electrons” in subnanometer-sized cavities of a lattice, are exotic ionic crystals. We propose a non-stoichiometric strategy to realize intrinsic two-dimensional (2D) superconducting electride. AlH2 monolayer, which is structurally identical to 1H-MoS2, possesses zero-dimensionally confined anionic electrons in the interstitial sites of Al triangles, corresponding to a chemical formula of [AlH2]+e−. The interaction between interstitial anionic electrons (IAEs) and host cation lattice mainly accounts for stabilization of 1H-AlH2 electride. Impressively, 1H-AlH2 monolayer is an intrinsic Bardeen–Cooper–Schrieffer superconductor with T c = 38 K, which is the direct consequence of strong coupling of the H-dominated high electronic states with Al acoustic branch vibrations and mid-frequency H-derived phonon softening modes caused by Kohn anomalies. Under tensile strain, IAEs transform into itinerant electrons, favoring the formation of stable Cooper pairs. Therefore, T c reaches up to 53 K at a biaxial fracture strain of 5%. Our findings provide valuable insights into the correlation between non-stoichiometric electrides and superconducting microscopic mechanisms at the 2D limit.

Targeted NO Oxidation and Synchronous NO2 Inhibition via Oriented 1O2 Formation Based on Lewis Acid Site Adjustment.

An appealing strategy for ensuring environmental benefits of the photocatalytic NO oxidation reaction is to convert NO into NO3- instead of NO2, yet the selectivity of products remains challenging. Here, such a scenario could be realized by tailoring the exposure of Lewis acid sites on the surface of ZrO2, aiming to precisely regulate the ROS evolution process for the selective oxidation of NO into NO3-. As evidenced by highly combined experimental characterizations and density functional theory (DFT) simulations, Lewis acid sites serving as electron acceptors could induce itinerant electron redistribution, charge-carrier transfer, and further oxidation of •O2-, which promotes the oriented formation of 1O2. As a result, monoclinic ZrO2 with more Lewis acid sites exhibited an outstanding NO conversion efficiency (56.33%) and extremely low NO2 selectivity (5.04%). The ROS-based reaction process and promotion mechanism of photocatalytic performance have been revealed on the basis of ESR analysis, ROS-quenching experiments, and in situ ROS-quenching DRIFTS. This work could provide a critical view toward oriented ROS formation and advance a unique mechanism of selective NO oxidation into NO3-.

Comparative study between charge and spin thermoelectric figure of merits in an antiferromagnetic ring

Considering the unique and diverse characteristic features of antiferromagnetic (AFM) systems, here in our work, we explore spin-dependent thermoelectric behavior in an AFM ring geometry. A reasonably large ( ≫1 ) spin figure of merit, referred to as Z S T, is obtained under suitable input conditions. Two important prerequisites are (i) breaking the symmetry among up and down spin sub-Hamiltonians and (ii) generating different asymmetric transmission line shapes across a Fermi energy for two opposite spin electrons. Describing the physical system within a tight-binding framework, where spin-dependent scattering occurs due to the interaction of itinerant electrons with local magnetic moments via the usual spin-moment exchange interaction, we compute all the thermoelectric quantities based on Landauer integrals following the Green’s function technique. The behavior of charge figure of merit, denoted as Z C T, is also discussed along with Z S T. Though Z C T reaches above unity, it is much smaller compared to Z S T. This is the key finding of our investigation. To make the present communication a self-contained one, we compare the results with another arrangement of magnetic moments in a ring-like geometry and in a chain-like one. Our analysis gives a suitable hint that in the presence of spin-dependent scattering, much more favorable energy conversion can be substantiated.

Transmission of a position-dependent mass system through a soft Coulomb potential

This research delves into the fascinating transmission characteristics of a system with variable mass, confined by a Coulomb-like potential. By utilizing an analytic approach to transmission probability, we were able to unveil the intriguing features that are observed experimentally in a specially designed two-dimensional lattice. This lattice was engineered to manipulate itinerant electrons through entanglement, resulting in variable masses as they traverse the lattice. To build our model system, we utilized the displacement operator approach to the position-dependent effective mass theory. This allowed us to investigate the effects of different configurations of leads, system geometries and lattice deformations on the transmission characteristics of the system. The resulting data showed that the model system was highly sensitive to these parameters, which led to various interesting features. First, we observed Andreev-like reflections, which occur when a particle is reflected as a hole, resulting in the transfer of both energy and charge. Another feature was reflectionless transmission, in which the transmission probability of a particle is close to unity, and almost no reflection occurs. We also observed complete localization of charge carriers, where the transmission probability is effectively zero, indicating that the charge carriers are completely confined within the lattice. Our findings demonstrate the remarkable versatility of the effective mass approach in the modeling of physical systems. By unraveling the intricate relationships between system parameters and transmission probability, we have gained new insights into the behavior of itinerant charge carriers in lattice structures. These insights may guide the design and development of novel electronic devices with enhanced performance and functionality.

Dopant concentration dependent room temperature ferromagnetism in crystalline Sc doped AlN thin films

In this study, high uniformity AlN thin films with varying Sc doping concentration levels were deposited on Ti (120 nm)/Si substrates. The impact of Sc doping ratio and annealing temperature on the magnetic properties of Al1−xScxN were studied. This work suggests that the origin of ferromagnetism of Al1−xScxN is the Al vacancies introduced by Sc ions. The amount of Al vacancies grew as the Sc doping ratio increased. According to the results of spin-polarized density of states (DOS), it is hypothesized that when the concentration of Al vacancies in Al1−xScxN grows, polarized itinerant electrons exist and result a magnetic phase change.

Tuning Oxide Properties by Oxygen Vacancy Control During Growth and Annealing.

Electrical, optical, and magnetic properties of oxide materials can often be controlled by varying the oxygen content. Here we outline two approaches for varying the oxygen content and provide concrete examples for tuning the electrical properties of SrTiO3-based heterostructures. In the first approach, the oxygen content is controlled by varying the deposition parameters during a pulsed laser deposition. In the second approach, the oxygen content is tuned by subjecting the samples to annealing in oxygen at elevated temperatures after the film growth. The approaches can be used for a wide range of oxides and nonoxide materials where the properties are sensitive to a change in the oxidation state. The approaches differ significantly from electrostatic gating, which is often used to change the electronic properties of confined electronic systems such as those observed in SrTiO3-based heterostructures. By controlling the oxygen vacancy concentration, we are able to control the carrier density over many orders of magnitude, even in nonconfined electronic systems. Moreover, properties can be controlled, which are not sensitive to the density of itinerant electrons.

Skyrmion crystal phases in Kondo lattice model on triangular lattices

Very often the magnetic skyrmions (topologically protected spin textures) form a triangular crystal in chiral magnets. Here we study the effect of itinerant electrons on the structure of skyrmion crystal (SkX) on triangular lattice using Kondo lattice model in the large coupling limit and treating the localized spins as classical vectors. To simulate the system, we employ hybrid Markov Chain Monte Carlo method (hMCMC) which includes electron diagonalization in each MCMC update for classical spins. We present the low-temperature results for system at electron density which show a sudden jump in skyrmion number reducing the size of the skyrmions when we increase the hopping strength of the itinerant electrons. We find that this high skyrmion number SkX phase is stabilized by a combined effect: lowering of density of states at electron filling and also pushing the bottom energy states further down. We show that these results hold for larger system using traveling cluster variation of hMCMC. We expect that itinerant triangular magnets might exhibit the possible transition between low-density to high-density SkX phases by applying external pressure.

Superconductivity in Ce-based cage compounds

Cerium-based ternary compounds CeNi2Cd20 and CePd2Cd20 do not exhibit long-range order down to millikelvin temperature range. Given the large separation between Ce ions which significantly reduces the super-exchange interactions and vanishingly small Ruderman–Kittel–Kasuya–Yosida interaction, here we show that nodal superconductivity mediated by the valence fluctuations must be a ground state in these materials. We propose that the critical temperature for the superconducting transition can be significantly increased by applying hydrostatic pressure. We employ an extended periodic Anderson lattice model which includes the long-range Coulomb interactions between the itinerant electrons as well as the local Coulomb interaction between the predominantly localized and itinerant electrons to compute a critical temperature of the superconducting transition. Using the slave-boson approach we show that fluctuations mediated by the repulsive electron–electron interactions lead to the emergence of d-wave superconductivity.

Dimensionality-driven metal to Mott insulator transition in two-dimensional 1T-TaSe2

Abstract Two-dimensional materials represent a major frontier for research into exotic many-body quantum phenomena. In the extreme two-dimensional limit, electron-electron interaction often dominates over other electronic energy scales, leading to strongly correlated effects such as quantum spin liquid and unconventional superconductivity. The dominance is conventionally attributed to the lack of electron screening in the third dimension. Here, we discover an intriguing metal to Mott insulator transition in 1T-TaSe2 that defies conventional wisdom. Specifically, we find that dimensionality crossover, instead of reduced screening, drives the transition in atomically thin 1T-TaSe2. A dispersive band crossing the Fermi level is found to be responsible for the bulk metallicity in the material. Reducing the dimensionality, however, effectively quenches the kinetic energy of these initially itinerant electrons, and drives the material into a Mott insulating state. The dimensionality-driven metal to Mott insulator transition resolves the long-standing dichotomy between metallic bulk and insulating surface of 1T-TaSe2. Our work further reveals a new pathway for modulating two-dimensional materials that enables exploring strongly correlated systems across uncharted parameter space.

Colossal Dielectric Permittivity in Sb/Gd Codoped Rutile TiO2 Nanoparticles Generated by Hydrogen Reduction

Abstract Ceramic TiO2 nano particles codoped with Sb/Gd ions were synthesized by a hydrothermal decomposition-precipitation technique. Some of the synthesized samples were hydrogenated at ∼550°C. The samples were characterized by investigating their structural, optical, and electrical properties by X-ray diffraction, diffuse reflection spectroscopy, and dielectric measurements, respectively. It was observed that the 3 %-Sb/Gd ionic codoping induces the growth of the rutile TiO2 phase. The different types of ionic doping of Sb5+ and Gd3+ into TiO2 nanograins found expectation to arrange core/shell sample construction that can be used to produce almost colossal permittivity material. However, the dielectric measurements showed relatively weak permittivity of order ∼370 at 1 kHz. Hydrogenation was used to create colossal permittivity, and the results show a significant increase by ∼6.63 times at 1 kHz. This result is important from the point of view of studying the influence of hydrogenation on the dielectric properties of transparent conducting oxides. The created colossal permittivity was explained within the framework of the core/shell model and doping mechanisms. Thus, the hydrogen species accumulation on the crystallite boundaries supports the creation of core/shell and colossal permittivity in addition to the generation of itinerant electrons. Such important results will be useful for future work and will support this field of research.

A first-principles study on the effect of point defects on the magnetism of the AlN:Li/Na/K system

When AIN semiconductors are grown by metal–organic chemical vapor deposition and vacuum-coating methods, interstitial H atoms are unintentionally incorporated into AlN and are often overlooked. To solve this problem, the present study adopted the first-principles hybrid functional HSE06 method under the framework of density functional theory to examine the impact of the point defects (VAl and Hi) on the magnetic properties of the AlN:Li/Na/K system. Results showed that all doped systems had single-ion spin-polarized N2− at different positions to generate magnetism, different from the source of magnetic coupling in conventional doped systems. Compared with the Al34MN36 (M = Li/Na) and Al34N36 systems, the magnetic moments of the Al34MHiN36 (M = Li/Na) and Al34HiN36 systems increased after incorporating the interstitial H. Moreover, the Al34MHiN36 (M = Li/Na) and Al34HiN36 systems unpaired itinerant electrons, and the magnetism was generated mostly by the unpaired itinerant electrons of N2− ions. The itinerant electrons exchanged electrons through Al vacancies to generate magnetism. Furthermore, the total magnetic moment of the Al34KHiN36 and Al35HiN36 systems were zero, and the use of interstitial H regulation as magnetic switches was very advantageous for these systems.

Quantification of electronic and magnetoelastic mechanisms of first-order magnetic phase transitions from first principles: application to caloric effects in La(FeSi)13

and derived quaternary compounds are well-known for their giant, tunable, magneto- and barocaloric responses around a first-order paramagnetic-ferromagnetic transition near room temperature with low hysteresis. Remarkably, such a transition shows a large spontaneous volume change together with itinerant electron metamagnetic features. While magnetovolume effects are well-established mechanisms driving first-order transitions, purely electronic sources have a long, subtle history and remain poorly understood. Here we apply a disordered local moment picture to quantify electronic and magnetoelastic effects at finite temperature in from first-principles. We obtain results in very good agreement with experiment and demonstrate that the magnetoelastic coupling, rather than purely electronic mechanisms, drives the first-order character and causes at the same time a huge electronic entropy contribution to the caloric response.

Influence of high pressure on the remarkable itinerant electron behavior in Y0.7Er0.3Fe2D4.2 compound

A monoclinic Y0.7Er0.3Fe2D4.2 compound exhibits unusual magnetic properties with different field induced magnetic transitions. The deuteride is ferrimagnetic at low temperature, and the Er and Fe sublattices present magnetic transitions at different temperatures. The Er moments are ordered below TEr = 55 K, whereas the Fe moments remain ferromagnetically coupled up to TM0 = 66 K. At TM0, the Fe moments display a sharp ferromagnetic–antiferromagnetic transition (FM–AFM) through itinerant electron metamagnetic behavior very sensitive to any volume change. Y0.7Er0.3Fe2D4.2 becomes paramagnetic above TN = 125 K. The pressure dependence of TEr and TM0 has been extracted from magnetic measurements under hydrostatic pressure up to 0.49 GPa. Both temperatures decrease linearly upon applied pressure with dTEr/dP = −126 and dTM0/dP = −140 K GPa−1 for a field of B = 0.03 T. Both magnetic Er and ferromagnetic Fe orders disappear at P = 0.44(4) GPa. However, under a larger applied field B = 5 T, dTM0/dP = −156 K GPa−1, whereas dTEr/dP = −134 K GPa−1 showing weaker sensitivity to pressure and magnetic field. At 2 K, the decrease of the saturation magnetization under pressure can be attributed to a reduction of the mean Er moment due to canting and/or a crystal field effect. Above TM0, the magnetization curves display metamagnetic behavior from an AFM to FM state, which is also very sensitive to the applied pressure. The transition field Btrans, which increases linearly upon heating, is shifted to a lower temperature upon applied pressure with ΔT = −17 K between 0 and 0.11 GPa. These results show strong decoupling of the Er and Fe magnetic sublattices vs temperature, applied field, and pressure.

Unusually strong electronic correlation and field-induced ordered phase in YbCo2

We report the first study of electrical resistivity, magnetization, and specific heat on YbCo2. The measurements on a single-phased sample of YbCo2 bring no evidence of magnetic ordering down to 0.3 K in a zero magnetic field. The manifestations of low Kondo temperature are observed. The specific heat value divided by temperature, C/T, keeps increasing logarithmically beyond 7 J/mol K2 with decreasing temperature down to 0.3 K without no sign of magnetic ordering, suggesting a very large electronic specific heat. Analysis of the magnetic specific heat indicates that the large portion of the low-temperature specific heat is not explained simply by the low Kondo temperature but is due to the strong intersite magnetic correlation in both the 3d and 4f electrons. Temperature-dependent measurements under static magnetic fields up to 7 T are carried out, which show the evolution of field-induced transition above 2 T. The transition temperature increases with increasing field, pointing to a ferromagnetic character. The extrapolation of the transition temperature to zero field suggests that YbCo2 is in the very proximity of the quantum critical point. These results indicate that in the unique case of YbCo2, the itinerant electron magnetism of Co 3d-electrons and the Kondo effect within the vicinity of quantum criticality of Yb 4f-local moments can both play a role.

Identification of a Griffiths-like phase and its evolution in the substituted pyrochlore iridates Y2Ir2−xCrxO7 (x=0.0,0.05,0.1,0.2)

We report the Griffiths phase (GP)-like state along with a cluster-glass-like state in geometrically frustrated antiferromagnetic Cr-substituted ${\text{Y}}_{2}{\text{Ir}}_{2}{\text{O}}_{7}$ pyrochlore iridates. The strength of the GP-like behavior increases with substitution. Interestingly, isothermal remanent magnetization suggests the Ising-like interaction of spins in the GP region. The GP-like state is not attributed to the structural disorder as substitution of Cr does not induce any structural change. Then the spin coupling between ${\text{Cr}}^{3+}\ensuremath{\leftrightarrow}{\text{Cr}}^{3+}$, ${\text{Ir}}^{4+}\ensuremath{\leftrightarrow}{\text{Ir}}^{4+}$, ${\text{Ir}}^{4+}\ensuremath{\leftrightarrow}{\text{Cr}}^{3+}$, and ${\text{Ir}}^{4+}\ensuremath{\leftrightarrow}{\text{Ir}}^{5+}$ leads to the competition between antiferromagnetic and ferromagnetic correlations. It gives rise to Ruderman-Kittel-Kasuya-Yosida (RKKY)-like interaction between ${\text{Cr}}^{3+}$ local magnetic moments mediated by itinerant Ir conduction electrons, hence, as a result, GP and cluster-glass-like states emerge.

Non-monotonic temperature dependence of anomalous Hall effect in a 2D van der Waals ferromagnetic material Fe3GeTe2

A search for long-range magnetic ordering in two-dimensional materials is recently providing a huge platform for the study of spin-related phenomena. However, the ferromagnetic order of FenGeTe2 (n = 3, 4, 5) still shows ambiguities and needs to be clarified. Here we report the thickness- and temperature-dependent anomalous Hall effects in h-BN/Fe3GeTe2/SiO2 van der Waals heterostructures with the thickness range of 28–175 layers of Fe3GeTe2. The anomalous Hall effects in thinner Fe3GeTe2 flakes show a square-shaped Hall signal with a strong perpendicular magnetic anisotropy, whereas bulk flakes show a gradual switching with the magnetic field due to the presence of multi- or labyrinthine domain structures, consistent with earlier reports. A surprising non-monotonic variation of the anomalous Hall remanence and saturation values with temperatures is observed to indicate a phase transition around 110–125 K, which could be due to a possible formation of an antiferromagnetic state at lower temperatures in Fe3GeTe2. Such an antiferromagnetic transition can be explained in a coupled picture of itinerant electrons and local magnetic moments in relation to the Kondo screening. An additional transition at 150–160 K is also observed in the remanence and the resistance, implying a more complicated mechanism playing a role in the magnetic phases of Fe3GeTe2. A persistence of the anomalous Hall effects observed above the Curie temperature where the remanence disappears further supports the interplay of the itinerant electrons and the local moments.

Nonlocal correlation effects due to virtual spin-flip processes in itinerant electron ferromagnets

We present an ab initio method for electronic structure calculations, which accounts for the interaction of electrons and magnons in ferromagnets. While it is based on a many-body perturbation theory we approximate numerically complex quantities with quantities from time-dependent density functional theory. This results in a simple and affordable algorithm which allows us to consider more complex materials than those usually studied in this context ($3d$ ferromagnets) while still being able to account for the nonlocality of the self-energy. Furthermore, our approach allows for a relatively simple way to incorporate self-consistency. Our results are in a good agreement with experimental and theoretical findings for iron and nickel. Especially the experimental exchange splitting of nickel is predicted accurately within our theory. Additionally, we study the half-metallic ferromagnet NiMnSb concerning its nonquasiparticle states appearing in the band gap due to spin-flip excitations.