Electron-electron Interaction Effects Research Articles

Modulating the Electrical Transport in Superconducting NbC Crystals by Fractal Morphology.

The self-similar fractal morphology mediated by nonequilibrium processes is widely observed in low-dimensional materials grown by various techniques. Understanding how these fractal geometries affect the physical and chemical properties of materials and devices is crucial for both fundamental studies and various applications. In particular, the interplay between superconducting phase fluctuations and disorder can give rise to intriguing phenomena depending on the dimensionality. However, current experimental studies on low-dimensional superconductors are limited to two- and one-dimensional systems, leaving fractional dimensional systems largely unexplored. Here, we use chemical vapor deposition to successfully synthesize ultrathin NbC crystals with a well-defined fractal geometry at the nanoscale. By performing electrical transport measurements, we find that both the superconducting and normal-state properties are strongly affected in the fractal samples, where the intrinsic and geometric disorder is induced. In contrast to the 2D crystal, the fractal NbC crystals show a significant low-temperature resistive upturn before the onset of superconducting transition, which can be attributed to the disorder-enhanced electron-electron interaction effect. From transport data analysis, we demonstrate that the superconducting transition in NbC is correlated to the strength of disorder and the fractional dimensions, revealing that nanoscale fractal structures can significantly modify the electronic properties of low-dimensional superconductors. Our work paves the way for the explorations of mesoscopic transport and intriguing superconducting phenomena in fractional dimensions.

Electronic Force Density Fields: Insights into Partial Bonds, Transition States, and Chemical Structure Evolution.

This paper presents the quantum-topological binding approach, in which the electrostatic and total static force density fields, Fes(r) and , together with the electron density gradient field ∇ρ(r), are simultaneously analyzed to elucidate the chemical structure of transition states and the nature of interatomic interactions for semibroken semiformed partial chemical bonds. The approach attributes the discrepancies between the force fields and Fes(r) to the nonclassical electron-electron interaction effects. The internuclear gap between the zero-flux boundaries of Fes(r) and ∇ρ(r) indicates the interatomic charge transfer phenomenon (ICT) that occurs upon the formation of a system from free atoms. Concomitantly, the mismatch of the zero-flux surfaces defined in and ∇ρ(r) can be interpreted as a phenomenon of the electron-transfer-induced quantum chemical response (QCR), which originates from the electron exchange correlation. Our study permits the assertion of parallels between partial bonds and noncovalent interactions, as both typically exhibit incomplete QCRs, indicating the partial electron sharing of the transferred density. The changes in atomic and pseudoatomic charges are employed to describe the evolution of the chemical structure upon the substitution reaction. It is observed that the acquired difference in the actual atomic electronegativity causes polarization upon the heterolytic breaking of virtually nonpolar bonds. It is further proposed that the proximity of closely related stationary states along the reaction path on a potential energy hypersurface implies their similarity in the manifestation of the ICT and sympathetic QCR. Furthermore, the involvement of an electron pair in a partial bond facilitates its delocalization through the attraction by the static forces and Fes(r) to a neighboring nucleus and through the smearing by the Pauli kinetic force FP(r).

First-order effect of electron-electron interactions on the anomalous Hall conductivity of massive Dirac fermions

We investigate the first-order correction to the anomalous Hall conductivity of 2D massive Dirac fermions arising from electron-electron interactions. In a fully gapped system in the limit of zero temperature, we find that this correction vanishes, confirming the absence of perturbative corrections to the topological Hall conductivity. At finite temperature or chemical potential, we find that the total Hall response decays faster than in the noninteracting case, depending on the strength of electron-electron interactions. These features, which could potentially be observed experimentally, show the importance of two-body interactions for anomalous Hall transport. Published by the American Physical Society 2024

Interaction- and phonon-induced topological phase transitions in double helical liquids.

Helical liquids, formed by time-reversal pairs of interacting electrons in topological edge channels, provide a platform for stabilizing topological superconductivity upon introducing local and nonlocal pairings through the proximity effect. Here, we investigate the effects of electron-electron interactions and phonons on the topological superconductivity in two parallel channels of such helical liquids. Interactions between electrons in different channels tend to reduce nonlocal pairing, suppressing the topological regime. Additionally, electron-phonon coupling breaks the self duality in the electronic subsystem and renormalizes the pairing strengths. Notably, while earlier perturbative calculations suggested that longitudinal phonons have no effect on helical liquids themselves to the leading order, our nonperturbative analysis shows that phonons can induce transitions between topological and trivial superconductivity, thereby weakening the stability of topological zero modes. Our findings highlight practical limitations in realizing topological zero modes in various systems hosting helical channels, including quantum spin Hall insulators, higher-order topological insulators, and their fractional counterparts recently observed in twisted bilayer systems.

Effect of electron-electron interaction on polarization process of exciton and biexciton in conjugated polymers

Abstract By using the one-dimensional tight-binding model modified to include electron-electric field interaction and electron-electron interaction, we theoretically explore the polarization process of exciton and biexciton in cis-polyacetylene. The dynamical simulation is performed by adopting the non-adiabatic evolution approach. The results show that under the effect of moderate electric field, when the strength of electron-electron interaction is weak, the singlet exciton is stable but its polarization presents obvious oscillation. With the enhancement of interaction, it is dissociated into polaron pair, the spin-flip of which could be observed through modulating the interaction strength. For the triplet exciton, the strong electron-electron interaction restrains its normal polarization, but it is still stable. In case of biexciton, the strong electron-electron interaction not only dissociate it, but also flip its charge distribution. We also calculate the yield of the possible states formed after the dissociation of exciton and biexciton.

Synergistic correlated states and nontrivial topology in coupled graphene-insulator heterostructures

Graphene has aroused great attention due to the intriguing properties associated with its low-energy Dirac Hamiltonian. When graphene is coupled with a correlated insulating substrate, electronic states that cannot be revealed in either individual layer may emerge in a synergistic manner. Here, we theoretically study the correlated and topological states in Coulomb-coupled and gate-tunable graphene-insulator heterostructures. By electrostatically aligning the electronic bands, charge carriers transferred between graphene and the insulator can yield a long-wavelength electronic crystal at the interface, exerting a superlattice Coulomb potential on graphene and generating topologically nontrivial subbands. This coupling can further boost electron-electron interaction effects in graphene, leading to a spontaneous bandgap formation at the Dirac point and interaction-enhanced Fermi velocity. Reciprocally, the electronic crystal at the interface is substantially stabilized with the help of cooperative interlayer Coulomb coupling. We propose a number of substrate candidates for graphene to experimentally demonstrate these effects.

Low temperature charge transport study of MWCNT/PEDOT:PSS composites: insulating to metallic regime

Multiwall carbon nanotube (MWCNT)/poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) composites have been examined for their temperature and magnetic field dependent conductivity behavior. The conductivity ratio, σ r (σ 300 K/ σ 2 K), is significantly impacted by the sulfuric acid post-treatment of the composites and a slight alteration in MWCNTs loading. By adjusting the loading of MWCNTs in the composites, the charge transport is tuned from insulating to metallic regime. For the low loading of MWCNTs (0.04 wt%), charge transport of the composite lies in the insulating regime and follows a variable range hopping model. At moderate loading of MWCNTs, the transport of the composites lies in the critical regime and the temperature dependent conductivity follows a power law model. As the MWCNTs loading increases to 4 wt%, transport of the composites shifts to the metallic regime with σ r ∼ 2.8. The temperature dependent conductivity has been explained by using electron-electron interactions and weak localization effects and the conductivity follows ∼T 1/2 and ∼T 3/4 dependence in different temperature regimes. Wave function shrinkage and forward interference effects have been used to evaluate the magnetoconductance (MC) of the samples located in the insulating regime. For the composites lying in the metallic regime, a dominant contribution from weak localization explains the behaviour of the MC. However, for those in the critical regime a combined effect of weak localization and electron-electron interactions has been observed.

Theoretical molecular spectroscopy of actinide compounds: the ThO molecule

The tiny-core generalised (Gatchina) relativistic pseudopotential (GRPP) model provides an accurate approximation for many-electron Hamiltonians of molecules containing heavy atoms, ensuring a proper description of the effects of non-Coulombian electron-electron interactions, electronic self-energy and vacuum polarisation. Combining this model with electron correlation treatment in the frames of the intermediate Hamiltonian Fock space coupled cluster theory employing incomplete main model spaces, one obtains a reliable and economical tool for excited state modelling. The performance of this method is assessed in applications to ab initio modelling of excited electronic states of the thorium monoxide molecule with term energies below 20,000 cm. Radiative lifetimes of excited states are estimated using truncated expansions of effective and metric operators in powers of cluster amplitudes.

Interaction mediated electron transport within the many-body tunnelling Hamiltonian

For a non-superconducting system and in the spirit of the transfer or tunnelling Hamiltonian formalism, an expression for the electronic tunnelling current through an insulating barrier is calculated, explicitly taking into account the effects of electron-electron interactions that can persist across the barrier. In the presence of a single tunnelling barrier, the exact Hamiltonian is projected onto the subspaces of the ‘left’ and ‘right’ conducting leads. In the weak-tunnelling limit, the well-known tunnelling Hamiltonian is recovered, along with an additional term. This additional term originates from the projection of the electron-electron interaction onto each subspace and corresponds to a correlated or interaction-mediated tunnelling. It is shown that the tunnelling current in this approximation is related to—in addition to the single-particle density of states—the two-particle spin-spin and density-density susceptibilities. The signatures of these terms have been observed in scanning tunnelling microscope experiments on atomic spin chains.

Effects of electron-electron interactions in the Yu-Shiba-Rusinov lattice model

In two-dimensional superconductors, Yu-Shiba-Rusinov bound states, induced by the magnetic impurities, extend over long distances giving rise to a long-range hopping model supporting a large number of topological phases with distinct Chern numbers. Here, we study how the electron-electron interactions affect on a mean-field level the selection of the realized Chern numbers and the magnitudes of the topological energy gaps in this model. We find that in the case of an individual choice of the model parameters the interactions can enhance or reduce the topological gap as well as cause topological phase transitions because of the complex interplay of superconductivity, magnetism, and large spatial extent of the Yu-Shiba-Rusinov states. By sampling a large number of realizations of Yu-Shiba-Rusinov lattice models with different model parameters, we show that statistically the interactions have no effect on the realized Chern numbers and typical magnitudes of the topological gaps. However, the interactions substantially increase the likelihood of the largest topological gaps in the tails of the energy gap distribution in comparison to the non-interacting case.

Low-temperature electrical transport properties of La doped BaSnO<sub>3</sub> films

A series of Ba<sub>0.94</sub>La<sub>0.06</sub>SnO<sub>3</sub> thin films are deposited on MgO(001) single crystal substrates by RF magnetron sputtering method, and their structure and electrical transport properties are systematically investigated. All films reveal degenerate semiconductor (metal) characteristics in electrical transport properties. In the high-temperature region (<inline-formula><tex-math id="M8">\begin{document}$T > {T_{\min }}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M8.png"/></alternatives></inline-formula>, where <inline-formula><tex-math id="M9">\begin{document}${T_{\min }}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M9.png"/></alternatives></inline-formula> is the temperature at which the resistivity reaches a minimum value), the resistivity of each film increases with temperature, and exhibits a linear relationship with the square of the temperature. In the low-temperature region (<inline-formula><tex-math id="M10">\begin{document}$T < {T_{\min }}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M10.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M10.png"/></alternatives></inline-formula>), the resistivity increases with decreasing temperature and varies linearly with <inline-formula><tex-math id="M11">\begin{document}$ \ln T $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M11.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M11.png"/></alternatives></inline-formula>. This temperature dependent behavior of resistivity cannot be explained by the general electron-electron interaction or weak localization effects in homogeneous disordered conductors and nor by Kondo effect. After quantitative analysis, it is found that the <inline-formula><tex-math id="M12">\begin{document}$ \ln T $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M12.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M12.png"/></alternatives></inline-formula> behavior of resistivity at low temperatures can be explained by the electron-electron Coulomb interaction effect in the presence of granularity. In addition, it is found that the Hall coefficient <inline-formula><tex-math id="M13">\begin{document}$ {R_{\text{H}}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M13.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M13.png"/></alternatives></inline-formula> also varies linearly with <inline-formula><tex-math id="M14">\begin{document}$ \ln T $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M14.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M14.png"/></alternatives></inline-formula> for the Ba<sub>0.94</sub>La<sub>0.06</sub>SnO<sub>3</sub> film, which also quantitatively accords with the theoretical prediction of the electron-electron Coulomb interaction effects in the granular metals. The results of cross-section high-resolution transmission electron microscope indicate that although the films have epitaxial structures as a whole, there are many strip-shaped amorphous regions in films, which makes the films have electrical transport properties similar to those of metal granular films. Our results provide strong support for the validity of the theory concerning the effects of Coulomb interaction on the conductivity and Hall coefficient in granular metals.

Spin-dependent scattering induced by surface states in spin-degenerate nodal-line semimetal

The topological nodal-line semimetal is characterized by the conduction band and valence band of electrons crossing along a one-dimensional line or closed loop in reciprocal space, with each nodal line carrying Π Berry phase. According to bulk-boundary correspondence, there exist drumheadlike surface states with weak dispersion at the boundary of system, surrounded by the projection of nodal loops onto the surface Brillouin zone. In most of nodal-line semimetals, the spin orbit coupling effect is weak, leading to the absence of a spin configuration for surface states under the single-particle picture. However, the featured weak dispersion of drumheadlike surface states enhances the electron-electron interaction effect, which triggers out ferromagnetic instability and causes spin splitting in the surface state. In this work, spin-dependent scattering caused by ferromagnetic surface states in spin-degenerate nodal-line semimetals is considered. It is found that both spin-splitting drumheadlike surface states can lead to resonant spin-flipped reflection. This physical process is reflected in a double-peak structure in the spin conductance spectrum. Specifically, we deal with the scattering problem induced by surface states in normal metal and nodal-line semimetal heterojunctions by using the scattering matrix and the Green’s functions theory, respectively, and obtain consistent conclusions. The result points out that spin-degenerate nodal-line semimetal surface states can still lead to spin-dependent transport, which provides a new perspective for the detection and potential application of spintronics in nodal-line semimetals.

Quasiparticle excitations in a one-dimensional interacting topological insulator: Application for dopant-based quantum simulation

We study the effects of electron-electron interactions on the charge excitation spectrum of the spinful Su-Schrieffer-Heeger (SSH) model, a prototype of a 1D bulk obstructed topological insulator. In view of recent progress in the fabrication of dopant-based quantum simulators we focus on experimentally detectable signatures of interacting topology in finite lattices. To this end we use Lanczos-based exact diagonalization to calculate the single-particle spectral function in real space which generalizes the local density of states to interacting systems. Its spatial and spectral resolution allows for the direct investigation and identification of edge states. By studying the non-interacting limit, we demonstrate that the topological in-gap states on the boundary are robust against both finite-size effects as well as random bond and onsite disorder which suggests the feasibility of simulating the SSH model in engineered dopant arrays in silicon. While edge excitations become zero-energy spin-like for any finite interaction strength, our analysis of the spectral function shows that the single-particle charge excitations are gapped out on the boundary. Despite the loss of topological protection we find that these edge excitations are quasiparticle-like as long as they remain within the bulk gap. Above a critical interaction strength of $U_c\approx 5 t$ these quasiparticles on the boundary loose their coherence which is explained by the merging of edge and bulk states. This is in contrast to the many-body edge excitations which survive the limit of strong coupling, as established in the literature. Our findings show that for moderate repulsive interactions the non-trivial phase of the interacting SSH model can be detected through remnant signatures of topological single-particle states using single-particle local measurement techniques such as scanning tunneling spectroscopy.

Corrections of band gaps and optical spectra of N-doped Sb2Se3 from G0W0 and BSE calculations

Nitrogen (N) doped Sb2Se3 semiconductor material shows potential as promising candidate for solar cell application. However, accurate calculations of its basic physical properties have been lacking. In this paper, G0W0 band gap and absorption spectrum of N-doped Sb2Se3 were studied theoretically using G0W0 approximation in combination with Bethe-Salpeter Equation (BSE), which consider the effects of electron-electron (e-e) and electron-hole (e-h) interactions. These methods are chosen and converged carefully in order to yield quantitative results in agreement with experimental data. Our band structure results revealed that apart from a rigid shift of the conduction bands in the G0W0 approach the qualitative features with that obtained within bare DFT are identical. The calculated bandgap values for Sb15N1Se24, Sb14N2Se24, Sb16Se23N1 and Sb16Se22N2 with G0W0 approximation were found to be 1.02, 0.82, 0.92 and 0.81 eV, respectively. These results showed that the bandgap values reduced by substituting N atom at Se and Sb sites in comparison to the undoped Sb2Se3. Reduction of bandgap values by substituting N atoms at Sb and Se sites are in good agreement with previous works. In the BSE calculations, inclusion of e-e and e-h interactions lead to the existence of strong exciton below the G0W0 optical band gap. The value of optical gaps which corresponding to the first peak in the BSE calculations were found to be 0.89, 0.76, 0.86 and 0.76 eV for Sb15N1Se24, Sb14N2Se24, Sb16Se23N1and Sb16Se22N2 respectively. Our theoretical study of N-doped Sb2Se3 suggests that a device fabricated from this materials can be operated on a wide range of the energy scale such as solar cell and broadband photodetector.

Phase diagram of the Su-Schrieffer-Heeger-Hubbard model on a square lattice

The Hubbard and Su-Schrieffer-Heeger Hamiltonians (SSH) are iconic models for understanding the qualitative effects of electron-electron and electron-phonon interactions respectively. In the two-dimensional square lattice Hubbard model at half filling, the on-site Coulomb repulsion, $U$, between up and down electrons induces antiferromagnetic (AF) order and a Mott insulating phase. On the other hand, for the SSH model, there is an AF phase when the electron-phonon coupling $\lambda$ is less than a critical value $\lambda_c$ and a bond order wave when $\lambda > \lambda_c$. In this work, we perform numerical studies on the square lattice optical Su-Schrieffer-Heeger-Hubbard Hamiltonian (SSHH), which combines both interactions. We use the determinant quantum Monte Carlo (DQMC) method which does not suffer from the fermionic sign problem at half filling. We map out the phase diagram and find that it exhibits a direct first-order transition between an antiferromagnetic phase and a bond-ordered wave as $\lambda$ increases. The AF phase is characterized by two different regions. At smaller $\lambda$ the behavior is similar to that of the pure Hubbard model; the other region, while maintaining long range AF order, exhibits larger kinetic energies and double occupancy, i.e. larger quantum fluctuations, similar to the AF phase found in the pure SSH model.

Sub- to super-Poissonian crossover of current noise in helical edge states coupled to a spin impurity in a magnetic field

Edge states of two-dimensional topological insulators are helical and single-particle backscattering is prohibited by time-reversal symmetry. In this paper, we show that an isotropic exchange coupling of helical edge states (HES) to a spin 1/2 impurity subjected to a magnetic field results in characteristic backscattering current noise (BCN) as a function of bias voltage and tilt angle between the direction of the magnetic field and the quantization axis of the HES. In particular, we find transitions from sub-Poissonian (antibunching) to super-Poissonian (bunching) behavior as a direct consequence of the helicity of the edge state electrons. We use the method of full counting statistics within a master equation approach treating the exchange coupling between the spin-1/2 impurity and the HES perturbatively. We express the BCN via coincidence correlation functions of scattering processes between the HES, which gives a precise interpretation of the Fano factor in terms of bunching and antibunching behavior of electron jump events. We also investigate the effect of electron-electron interactions in the HES in terms of the Tomonaga-Luttinger liquid theory.

Imaging gate-tunable Tomonaga-Luttinger liquids in 1H-MoSe2 mirror twin boundaries.

One-dimensional electron systems exhibit fundamentally different properties than higher-dimensional systems. For example, electron-electron interactions in one-dimensional electron systems have been predicted to induce Tomonaga-Luttinger liquid behaviour. Naturally occurring grain boundaries in single-layer transition metal dichalcogenides exhibit one-dimensional conducting channels that have been proposed to host Tomonaga-Luttinger liquids, but charge density wave physics has also been suggested to explain their behaviour. Clear identification of the electronic ground state of this system has been hampered by an inability to electrostatically gate such boundaries and tune their charge carrier concentration. Here we present a scanning tunnelling microscopy and spectroscopy study of gate-tunable mirror twin boundaries in single-layer 1H-MoSe2 devices. Gating enables scanning tunnelling microscopy and spectroscopy for different mirror twin boundary electron densities, thus allowing precise characterization of electron-electron interaction effects. Visualization of the resulting mirror twin boundary electronic structure allows unambiguous identification of collective density wave excitations having two velocities, in quantitative agreement with the spin-charge separation predicted by finite-length Tomonaga-Luttinger liquid theory.

Quantum simulator of extended bipartite Hubbard model with broken sublattice symmetry: Magnetism, correlations, and phase transitions

We describe here a quantum simulator of extended bipartite Hubbard model with broken sublattice symmetry. The simulator consists of a structured lateral gate confining two dimensional electrons in a quantum well into artificial minima arranged in a hexagonal lattice. The sublattice symmetry breaking is generated by forming an artificial triangular graphene quantum dot (ATGQD) with zigzag edges. The resulting extended Hubbard model generates tunable ratio of tunneling strength to electron-electron interactions and of sublattice symmetry with control over shape. The validity of the simulator is confirmed for small systems using mean-field and exact diagonalization many-body approaches which show that the ground state changes from a metallic to an antiferromagnetic (AF) phase by varying the distance between sites or depth of the confining potential. The one-electron spectrum of these triangular dots contains a macroscopically degenerate shell at the Fermi level. The shell persists at the mean-field level for weak interactions (metallic phase) but disappears for strong interactions, in the AF phase. We determine the effects of electron-electron interactions on the ground state, the total spin, and the excitation spectrum as a function of filling of the ATGQD. We find that the half-filled charge neutral shell leads to a partially spin polarized state in both metallic and AF regimes in accordance with Liebs theorem. In both regimes a relatively large gap separates the spin polarized ground state to the first excited many-body state at half filling of the degenerate shell. By adding or removing an electron, this gap drops dramatically, and alternate total spin states emerge with energies nearly degenerate to a spin polarized ground state.

Correlation strength, orbital-selective incoherence, and local moments formation in the magnetic MAX-phaseMn2GaC

We perform a theoretical study of the electronic structure and magnetic properties of the prototypical magnetic MAX-phase Mn$_2$GaC with the main focus given to the origin of magnetic interactions in this system. Using the density functional theory+dynamical mean-field theory (DFT+DMFT) method we explore the effects of electron-electron interactions and magnetic correlations on the electronic properties, magnetic state, and spectral weight coherence of paramagnetic and magnetically-ordered phases of Mn$_2$GaC. We also benchmark the DFT-based disordered local moment approach for this system by comparing the obtained electronic and magnetic properties with that of the DFT+DMFT method. Our results reveal a complex magnetic behavior characterized by a near degeneracy of the ferro- and antiferromagnetic configurations of Mn$_2$GaC, implying a high sensitivity of its magnetic state to fine details of the crystal structure and unit-cell volume, consistent with experimental observations. We observe robust local-moment behavior and orbital-selective incoherence of the spectral properties of Mn$_2$GaC, implying the importance of orbital-dependent localization of the Mn $3d$ states. We find that Mn$_2$GaC can be described in terms of local magnetic moments, which may be modeled by DFT with disordered local moments. However, the magnetic properties are dictated by the proximity to the regime of formation of local magnetic moments, in which the localization is in fact driven by the Hund's exchange interaction, and not the Coulomb interaction.

Quantum interference phenomena and electron – electron interaction in topological insulator Bi2Se3 thin polycrystalline films

The temperature (T = 2.0 - 300 K) and magnetic field (B = 0 - 14 T) dependences of the transport coefficients were obtained for the topological insulator (TI) polycrystalline Bi2Se3 films with thicknesses d = 40 and 200 nm grown on glass substrates by thermal evaporation in vacuum from a single source. In the region of weak magnetic fields, a sharp increase in magnetoresistance (MR) with increasing B was observed, indicating the presence of the weak antilocalization (WAL) effect, which is typical for 3D-TIs. However, the longitudinal resistance Rxx increases logarithmically with decreasing temperature below certain temperature Tc, which is not characteristic of the WAL effect in 2D-structures and indicates possible existence of the so-called “transport paradox”. In the temperature dependences of the Hall resistance, Hall coefficient and MR we also registered clearly pronounced kinks near Tc. The Rxx(T) dependence for a 40 nm film, with decreasing temperature in addition to the Rxx minimum near Tc, exhibits a maximum near 2.1 K. Estimates of the number of independent transport channels m and the dephasing length lφ show that as d increases from 40 nm to 200 nm, m increases from 1 to 3, while lφ changes insignificantly. The nonmonotonic behavior of the T- and B- dependences of the kinetic coefficients, the occurrence of the “transport paradox” are associated with a change in the contributions of WAL and electron-electron interaction effects when the nature and magnitude of the external influence and film thickness change. It follows from the data obtained that not only in very thin monocrystalline films, but also in rather thick polycrystalline Bi2Se3 films prepared by the simple method of the thermal evaporation in vacuum on amorphous substrates, quantum interference effects and “transport paradox” can be manifested.