Weak Electron-electron Interaction Research Articles

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.

Probing electron-electron interaction along with superconducting fluctuations in disordered TiN thin films

In two-dimensional (2D) disordered superconductors prior to superconducting (SC) transition, the appearance of a resistance peak in the temperature dependent resistance $R(T)$ measurements indicates the presence of weak localization (WL) and electron-electron interaction (EEI) in the diffusion channel and SC fluctuations in the Cooper channel. Here, we demonstrate an interplay between SC fluctuations and EEI by low-temperature magnetotransport measurements for a set of 2D disordered TiN thin films. While cooling the sample, a characteristic temperature ${T}^{*}$ is obtained from the $R(T)$ at which SC fluctuations start to appear. The upturn in $R(T)$ above ${T}^{*}$ corresponds to WL and/or EEI. By the temperature and field dependences of the observed resistance, we show that the upturn in $R(T)$ originates mainly from EEI with a negligible contribution from WL. Further, we have used the modified Larkin's electron-electron attraction strength $\ensuremath{\beta}(T/{T}_{\mathrm{c}})$, containing a field-induced pair-breaking parameter, in the Maki-Thompson SC fluctuation term. Here, the temperature dependence of $\ensuremath{\beta}(T/{T}_{\mathrm{c}})$ obtained from the magnetoresistance (MR) analysis shows a diverging behavior close to ${T}_{\mathrm{c}}$, and it remains almost constant at higher temperature within the limit of $\mathrm{ln}(T/{T}_{\mathrm{c}})<1$. Interestingly, the variation of $\ensuremath{\beta}(T/{T}_{\mathrm{c}})$ on the reduced temperature $(T/{T}_{\mathrm{c}})$ offers a common trend which has been closely followed by all the concerned samples presented in this paper. Finally, the temperature dependence of inverse phase scattering time (${\ensuremath{\tau}}_{\ensuremath{\phi}}^{\ensuremath{-}1}$), as obtained from the MR analysis, clearly shows two different regimes; the first one close to ${T}_{\mathrm{c}}$ follows the Ginzburg-Landau relaxation rate $({\ensuremath{\tau}}_{\mathrm{GL}}^{\ensuremath{-}1})$, whereas the second one at high temperature varies almost linearly with temperature, indicating the dominance of inelastic electron-electron scattering for the dephasing mechanism. These two regimes are followed in a generic way by all the samples despite being grown under different growth conditions.

Weak antilocalization and electron-electron interactions in topological insulator BixTey films deposited by sputtering on Si(100)

Topological insulators (TIs) which exhibit spin-momentum locking constitute an interesting class of quantum materials. In this paper, we present a systematic method to prepare ${\mathrm{Bi}}_{x}{\mathrm{Te}}_{y}$ films having tunable Bi/Te composition by employing a direct current (DC) cosputtering technique. Films of three different Te compositions, namely, the Te-deficient (BiTe), Te-stoichiometric (${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$), and Te-excessive (${\mathrm{BiTe}}_{3}$) phases, have been investigated in detail by performing the measurements of their temperature-dependent resistivity and in-plane and out-of-plane magnetoresistance responses. Clear evidence of the presence of a weak antilocalization effect and electron-electron interaction are observed in all three films having different phases of Bi-Te system. Using the Hikami-Larkin-Nagaoka and Al'tshuler-Aronov models, the topological characteristics have been evidently distinguished in these three different phases of ${\mathrm{Bi}}_{x}{\mathrm{Te}}_{y}$. The analysis of the fitting of the experimental magnetotransport data is performed to quantitatively determine the critical model parameters, viz., phase coherence length (${l}_{\ensuremath{\phi}}$), surface state penetration depth ($\ensuremath{\lambda}$), coherency factor ($\ensuremath{\alpha}$), and dephasing parameter ($p$) through which Berry phase and dimensionality of transport channels can be estimated. In this paper, we reveal a systematic correlation between the composition of the ${\mathrm{Bi}}_{x}{\mathrm{Te}}_{y}$ films and their topological properties. The parameter $\ensuremath{\kappa}$, slope of temperature dependence of conductivity correction, is studied at different magnetic fields, which suggested that the Te-stoichiometric sample shows better features of topological properties than the Te-deficient and Te-excessive samples. These experimental observations are supplemented by first-principles calculations. Additionally, we demonstrate that TIs can be grown by the sputtering technique as is desired for realizing industrial applications.

Localization and interference induced quantum effects at low magnetic fields in InGaAs/GaAs structures

The longitudinal ρxx(B, T) and Hall ρxy(B, T) resistances are experimentally investigated in n-InGaAs/GaAs nanostructures with a single and double quantum wells in the magnetic field range B = 0–2.5 T and temperatures T = 1.8–20 K. It is shown that the origin of the temperature-independent point located at ωcτ≅1 on the ρxx(B, T) curves is due to the combined action of the classical cyclotron motion and the quantum interference effects of weak localization and electron-electron interaction. The results obtained indicate that the transition from the dielectric phase to the phase of the quantum Hall effect is a crossover from weak localization (quantum interference effects in a weak magnetic field) to strong localization in quantizing magnetic fields in the quantum Hall effect regime.

Strongly correlated Chern insulators in magic-angle twisted bilayer graphene.

Interactions between electrons and the topology of their energy bands can create unusual quantum phases of matter. Most topological electronic phases appear in systems with weak electron-electron interactions. The instances in which topological phases emerge only as a result of strong interactions are rare and mostly limited to those realized in intense magnetic fields1. The discovery of flat electronic bands with topological character in magic-angle twisted bilayer graphene (MATBG) has created a unique opportunity to search for strongly correlated topological phases2-9. Here we introduce a local spectroscopic technique using a scanning tunnelling microscope to detect a sequence of topological insulators in MATBG with Chern numbers C=±1, ±2 and ±3, which form near filling factors of±3, ±2 and ±1 electrons per moiré unit cell, respectively, and are stabilized by modest magnetic fields. One of the phases detected here (C=+1) was previously observed when the sublattice symmetry of MATBG was intentionally broken by a hexagonal boron nitride substrate, with interactions having a secondary role9. We demonstrate that strong electron-electron interactions alone can produce not only the previously observed phase, but also other unexpected Chern insulating phases in MATBG. The full sequence of phases that we observe can be understood by postulating that strong correlations favour breaking time-reversal symmetry to form Chern insulators that are stabilized by weak magnetic fields. Our findings illustrate that many-body correlations can create topological phases in moiré systems beyond those anticipated from weakly interacting models.

Kohn-Luttinger Mechanism Driven Exotic Topological Superconductivity on the Penrose Lattice.

The Kohn-Luttinger mechanism for unconventional superconductivity (SC) driven by weak repulsive electron-electron interactions on a periodic lattice is generalized to the quasicrystal (QC) via a real-space perturbative approach. The repulsive Hubbard model on the Penrose lattice is studied as an example, on which a classification of the pairing symmetries is performed and a pairing phase diagram is obtained. Tworemarkable properties of these pairing states are revealed, due to the combination of the presence of the point-group symmetry and the lack of translation symmetry on this lattice. First, the spin and spacial angular momenta of a Cooper pair is decorrelated: for each pairing symmetry, both spin-singlet and spin-triplet pairings are possible even in the weak-pairing limit. Second, the pairing states belonging to the 2D irreducible representations of the D_{5} point group can be time-reversal-symmetry-breaking topological SCs carrying spontaneous bulk super current and spontaneous vortices. These two remarkable properties are general for the SCs on all QCs, and are rare on periodic lattices. Our work starts the new area of unconventional SCs driven by repulsive interactions on the QC.

Cooperative interactions govern the fermiology of the polar metal Ca3Ru2O7

The antiferromagnetic Ruddlesden-Popper ruthenate Ca$_3$Ru$_2$O$_7$ is a model polar metal, combining inversion symmetry breaking with metallic conductivity; however, its low temperature ($T < 48$ K) crystal structure and Fermi surface topology remain ambiguous despite numerous measurements and theoretical studies. Here we perform both first principles calculations with static correlations and angle resolved photoelectron spectroscopy experiments to construct a complete model of Ca$_3$Ru$_2$O$_7$, reconciling inconsistencies among interpretations of electrical transport, thermopower measurements, and momentum- and energy-resolved band dispersions. The solution relies on treating the interplay among Coulomb repulsion, magnetic ordering, spin-orbit interactions, and the RuO$_6$ octahedral degrees-of-freedom on equal footing. For temperatures $30<T < 48$ K, we propose weak electron-electron interactions produce a symmetry-preserving metal-semimetal transition with Weyl nodes in proximity to the Fermi level, whereas a new orthorhombic $Pn2_1a$ structure emerges for $T<30$ K, exhibiting charge and spin density waves from enhanced Coulombic interactions.

Theoretical design of highly correlated electron states in delafossite heterostructures

Delafossites represent natural heterostructures which can host rather different electronic characteristics in their constituting layers. The design of novel heterostructure architectures highlighting the competition between such varying layer properties is promising from the viewpoint of basic research as well as for future technological applications. By means of the combination of density functional theory and dynamical mean-field theory, we here unveil the formation of highly correlated electron states in delafossite heterostructures build from metallic PdCrO$_2$ and insulating AgCrO$_2$. Due to the sophisticated coupling between layers of strong and of weak internal electron-electron interaction, correlation-induced semimetals at ambient temperature and doped Mott-insulators at lower temperature are predicted. The unique electronic structure of delafossite heterostructures opens a door to research on novel challenging quantum matter.

Emergence of electron-phonon coupling in a dual topological insulator BiTe

A versatile van der Waals epitaxy of BiTe, a newly discovered dual TI and a predicted higher order topological insulator (HOTI) thin film on muscovite mica is demonstrated using pulsed laser deposition. Topographic, structural and XPS analyses confirm the chemical homogeneity and high crystalline quality with large-area coverage with atomically smooth surface. The magneto transport data reveals weak anti-localization and electron-electron interaction driven insulating ground state with n-type character. An elaborate thickness, temperature and magnetic field dependence of transport data indicates a transition from coupled, partially coupled and fully decoupled surface states wherein 3D electron-electron and electron-phonon scatterings play significant role in dephasing mechanism, unlike 2D electron-electron dephasing in most TIs. Also from the fitting of the resistivity upturn we found that the coulomb screening factor (F) in thicker samples turns out to be negative which indicates the presence of the electron phonon coupling in those samples.

Quantum effects in a germanium quantum well with ultrahigh mobility of charge carrier

Quantum effects in p-type Si0.2Ge0.8/Ge/Si0.2Ge0.8 heterostructure with an extremely high mobility of charge carriers μH = 1367000 cm2/(V ⋅ s) have been comprehensively studied. An analysis of Shubnikov–de Haas oscillations yielded effective mass of charge carriers, which proved to be very low, m* = 0.062m0, and the value of fluctuations of hole density along the channel δp = 3.5 ⋅ 109 cm–2. The fractional Hall effect (filling numbers 8/3, 7/3, 5/3, 4/3) observed at temperatures up to 5 K has been discovered in strong magnetic fields. The studies of quantum interference effects related to weak localization and electron-electron interaction between charge carriers, which have been conducted in such a high-mobility system for the first time, enabled calculation of spin splitting Δ = 1.07 meV and the Fermi-liquid coupling constant F0σ=−0.12, which agree with results obtained earlier.

Unconventional Temperature Dependence of the Anomalous Hall Effect in HgCr_{2}Se_{4}.

At sufficiently low temperatures, many quantum effects, such as weak localization, electron-electron interaction (EEI), and Kondo screening, can lead to pronounced corrections to the semiclassical electron transport. Although low temperature corrections to longitudinal resistivity, ordinary Hall resistivity, or anomalous Hall (AH) resistivity are often observed, the corrections to three of them have never been simultaneously detected in a single sample. Here, we report on the observation of sqrt[T]-type temperature dependences of the longitudinal, ordinary Hall and AH resistivities at temperatures down to at least 20mK in n-type HgCr_{2}Se_{4}, a half-metallic ferromagnetic semiconductor that can reach extremely low carrier densities. For the samples with moderate disorder, the longitudinal and ordinary Hall conductivities can be satisfactorily described by the EEI theory developed by Altshuler etal., whereas the large corrections to AH conductivity are inconsistent with the existing theory, which predicts vanishing and finite corrections to AH conductivity for EEI and weak localization, respectively.

Theoretical Aspects of the Cross Effect Enhancement of Nuclear Polarization under Static Dynamic Nuclear Polarization Conditions.

In this study, we perform quantum calculations of the spin dynamics of a small spin system that includes nine coupled electrons and one nucleus placed in an external magnetic field and exposed to microwave irradiation. This is an extension of a previous work in which we have demonstrated on a system of 11 coupled electron spins the dynamics of the electron polarizations composing the electron paramagnetic resonance (EPR) line during static dynamic nuclear polarization (DNP) experiments. There we have shown that the electron polarizations are determined by a spectral diffusion process, induced by the dipolar interaction and cross-relaxation. Additionally, we showed that a distinction had to be made between strong and weak dipolar-coupled systems relative to the inhomogeneity of the EPR line with only the first behaving according to the thermal mixing DNP (with two electron spin temperatures) description. The EPR spectra in the weak and strong dipolar interaction cases show different types of spectral features. In the extended spin system, we again make a distinction between weak and strong electron-electron interactions and show that the DNP spectra for the two cases are different in nature but that the DNP spectra can be derived in all cases from the EPR line shapes using the indirect cross effect.

Observation of Weak Anti-Localization and Electron-Electron Interaction on Few-Layer 1T′-MoTe2 Thin Films**Supported by the National Basic Research Program of China under Grant Nos 2014CB920904 and 2015CB921101, the Strategic Priority Research Program B of the Chinese Academy of Sciences under Grant No XDB07010200, and the Singapore National Research Foundation under Grant No NRF-RF2013-08.

Electronic transports of few-layer 1T′-MoTe2 films are measured at temperatures down to 0.26 K. The low-temperature conductivity exhibits logarithmic temperature dependence and negative magneto response. The negative magnetoconductivity can be well fitted by the two-dimensional weak anti-localization theory taking a single channel of electrons into account, with the parameters α ≈ −0.5 and lϕ ∝ T−0.5. The logarithmic temperature dependence has a positive slopeκ ≈ 0.75, indicating the dominance of electron-electron interaction over the weak anti-localization effect, with an apparently negative Coulomb screening factor F that demands future work for clarification.

A Silicon Cluster Based Single Electron Transistor with Potential Room-Temperature Switching**Supported by the National Key Research and Development Program of China under Grant No 2017YFA0303200, the National Natural Science Foundation of China under Grant Nos U1732273, U1732159, 91421109, 91622115, 11522432, 11574217 and 61774133, the Natural Science Foundation of Jiangsu Province under Grant No BK20160659.

We demonstrate the fabrication of a single electron transistor device based on a single ultra-small silicon quantum dot connected to a gold break junction with a nanometer scale separation. The gold break junction is created through a controllable electromigration process and the individual silicon quantum dot in the junction is determined to be a Si170 cluster. Differential conductance as a function of the bias and gate voltage clearly shows the Coulomb diamond which confirms that the transport is dominated by a single silicon quantum dot. It is found that the charging energy can be as large as 300 meV, which is a result of the large capacitance of a small silicon quantum dot (∼1.8 nm). This large Coulomb interaction can potentially enable a single electron transistor to work at room temperature. The level spacing of the excited state can be as large as 10 meV, which enables us to manipulate individual spin via an external magnetic field. The resulting Zeeman splitting is measured and the g factor of 2.3 is obtained, suggesting relatively weak electron-electron interaction in the silicon quantum dot which is beneficial for spin coherence time.

Superconductor-Insulator transition in sputtered amorphous MoRu and MoRuN thin films

This work shows the experimental results of the superconductor-insulator (S-I) transition for amorphous molybdenum ruthenium (MoRu) and molybdenum ruthenium nitride (MoRuN) films. These amorphous films onto c-plane sapphire substrates have been interpreted to be homogeneous by XRD and AFM measurements. Electrical and superconducting properties measurements were carried out on MoRu and MoRuN thin films deposited by reactive sputtering technique. We have analysed the data on Rsq (T) based on excess conductivity of superconducting films by the AL and MT term and weak localization and electron-electron interaction for the conductance. MoRu films which offer the most homogeneous film morphology, showed a critical sheet resistance of transition, Rc, of ∼ 2 kΩ. This values is smaller than those previously our reported for quench-condensed MoRu films on SiO underlayer held at liquid He temperature.

CHARACTERIZATIONS AND ELECTRON PARAMAGNETIC RESONANCE (EPR) QUASICRYSTALS IN AN ICOSAHEDRAL PHASE

This article deals with a study on the icosahedral phase quasicrystal of Al63Cu25Fe12 produced by casting in plasma induction furnace under an argon atmosphere to guarantee it's homogenously. But found in phase transformations from one phase to another, the structures formed by the quasicrystalline alloy having ordered icosahedral phase, leads to low electrical conductivity; but considering the location of weak electron-electron interactions. For microstructural characterization was made using the Following techniques; X-ray Diffraction, Scanning Electron Microscopy, Energy Dispersive Spectroscopy of, DSC Thermal Analysis and Spectroscopy Electron Paramagnetic Resonance (EPR), where the spectrum Obtained experimental EPR presents an amount of resonance g = 2,036. EPR spectrum provide a detailed view of magnetic equivalent sites to those of paramagnetic transition metal compositional variation in the alloy quasicrystalline and transition temperature effects on the magnetic spins in the sample ordering.

Intraband memory function and memory-function conductivity formula in doped graphene

The generalized self-consistent field method is used to describe intraband relaxation processes in a general multiband electronic system with presumably weak residual electron-electron interactions. The resulting memory-function conductivity formula is shown to have the same structure as the result of a more accurate approach based on the quantum kinetic equation. The results are applied to heavily doped and lightly doped graphene. It is shown that the scattering of conduction electron by phonons leads to the redistribution of the intraband conductivity spectral weight over a wide frequency range, however, in a way consistent with the partial transverse conductivity sum rule. The present form of the intraband memory function is found to describe correctly the scattering by quantum fluctuations of the lattice, at variance with the semiclassical Boltzmann transport equations, where this scattering channel is absent. This is shown to be of fundamental importance in quantitative understanding of the reflectivity data measured in lightly doped graphene as well as in different low-dimensional strongly correlated electronic systems, such as the cuprate superconductors.

Microstructural properties and carrier transport mechanism in Bi-doped nanocrystalline SnO2 thin films

Tin oxide (SnO2) thin films, doped with different concentrations of bismuth (Bi), are prepared by the magnetron reactive sputtering technique. Microstructural, spectroscopic and electric transport measurements are performed and the results have been correlated. The crystallite size, which is evaluated from power X-ray diffraction (XRD) studies is in the 10–20nm range. Bi is doped into SnO2 lattice as +5 valence state, leading to n type conductivity. The doped samples show the deterioration of the crystallinity with increase in Bi doping concentration, evidenced by the smaller crystallites and rougher surface compared with undoped ones. Due to highly disordered defects in the Bi-doped SnO2 films, the first order Raman-active mode inherent to SnO2 lattice shifts to lower wavenumber with the increasing of the Bi concentration. Based on Hall effect measurements, it is found that the sheet resistance dramatically increases up to 3.2×104Ω/□ with Bi doping level of 2at.%, accompanied by a sharp decrease of the electron mobility μ. The temperature dependence of the resistance presents different functional relationship relying on the doping concentration, and a possible crossover of the carrier transport mechanism from 2D to 3D behavior is established when discussed in the framework of the weak localization and electron-electron interactions.

Two-electron transfer and ionization mechanism in 80-keV/uNe8+on He collisions

Autoionization decay from doubly excited states of ${\mathrm{Ne}}^{6+}[1{s}^{2}3lnl$ ($n=3,4,5$)] (symmetric configurations) as well as the Coster-Kronig transition from doubly excited states of ${\mathrm{Ne}}^{6+}[1{s}^{2}2pnl(n\ensuremath{\geqslant}7)]$ (asymmetric configurations) are observed in the transfer ionization reaction channel of 80 keV/u ${\mathrm{Ne}}^{8+}$-He collisions. It has been predicted that the formation of symmetric configurations results from uncorrelated double-electron capture processes [Z. Chen and C. D. Lin, Phys. Rev. A 48, 1298 (1993)], and the formation of asymmetric configurations probably results from correlated double-electron capture caused by the dynamical electron-correlation effects which are attributed to small internuclear distances. However, previous experimental measurements were not able to obtain information about the dependence on internuclear distance which can be reflected in the transversal recoil-ion momentum. In this work, we measure the recoil-ion momentum in coincidence with the ejected electron velocity and find that the observed transverse recoil-ion momentum is smaller for the formation of symmetric ${\mathrm{Ne}}^{6+}[1{s}^{2}3lnl(n=3,4,5)]$ states than the formation of asymmetric ${\mathrm{Ne}}^{6+}[1{s}^{2}2pnl(n\ensuremath{\geqslant}7)]$ states. Since large momentum transfer occurs for small internuclear distances (strong electron-electron interactions) and small momentum transfer occurs for large internuclear distances (weak electron-electron interactions), the results indicate that dynamical electron correlation is important for the formation of the asymmetric states.

Magnetic catalysis and axionic charge density wave in Weyl semimetals

Three-dimensional Weyl and Dirac semimetals can support a chiral-symmetry-breaking, fully gapped, charge-density-wave order even for sufficiently weak repulsive electron-electron interactions, when placed in strong magnetic fields. In the former systems, due to the natural momentum space separation of Weyl nodes the ordered phase lacks the translational symmetry and represents an axionic phase of matter, while that in a Dirac semimetal (neglecting the Zeeman coupling) is only a trivial insulator. We present the scaling of this spectral gap for a wide range of subcritical (weak) interactions as well as that of the diamagnetic susceptibility with the magnetic field. A similar mechanism for charge-density-wave ordering at weak coupling is shown to be operative in double and triple-Weyl semimetals, where the dispersion is linear (quadratic and cubic, respectively) for the z (planar) component(s) of the momentum. We here also address the competition between the charge-density-wave and a spin-density-wave orders, both of which breaks the chiral symmetry and leads to gapped spectrum, and show that at least in the weak coupling regime the former is energetically favored. The anomalous surface Hall conductivity, role of topological defects such as axion strings, existence of one-dimensional gapless dispersive modes along the core of such defects, and anomaly cancellation through the Callan-Harvey mechanism are discussed.